# Unusual variations on two-stroke head design.



## Owen_N (Dec 26, 2021)

I want to try a head design of my own.
The base engine is a TP60 aero engine, two stroke, fixed head.
I an going to convert in to a separate 4-stud head.

I am looking to divide the combustion area up into 3 parts, based on a shaped trench containing the sparkplug,
and two main squish collector areas.

The objective is to try and separate out two areas of varying exhaust gas content for part throttle running.
I can then fire the sub-chamber away from the exhaust side, and attempt to jet-ignite the sub-chamber on the opposite side.
The end result I am after is an engine that fires a bit more like a four-stroke in firing consistency, and reduction of misfires.
I would also like this to have minimal effect on my peak torque area of around 6000 rpm.
Questions:
Has anyone seen head projects like this before?
What is the likely advance BTC and the angle range for combustion?
Where should peak pressure occur?

I am looking for the main effect between 2500 and 5000 rpm.
How much will the piston move in the "burn" range?
bore is 45 mm, stroke is 36.2 mm.

Should I move the sparkplug off-centre to the first sub-chamber side?
Is the cross-jet of burning gas likely to occur?
Is 6mm enough clearance for sparkplug nose to piston?
Is 9:1 basic CR ok for these engines?
Is 1.5mm OK for squish clearance, or should I use 0.6mm, tapering out.?

There are 4 tapered regions shown.
The central spark plug region was originally chosen to capture a small part of the loop charge without full squish, for good flame initiation.
A passive pre-chamber sparkplug would be good, if I had one.

Are there any changes I should make that would advance my underlying aims?

My basic tools should be enough to make the head and fins. I don't really need to mill anything.
I have a small lathe I can use for mating surfaces.


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## Nerd1000 (Dec 26, 2021)

It sounds like you want a stratified charge combustion system. These are no simple matter, in the past there have been various implementations for SI engines and most of them rely on direct injection to achieve the desired rich mixture near the plug that can then ignite a lean mixture elsewhere.


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## Owen_N (Dec 26, 2021)

The motor seems to run really smoothly at fast idle after a fast run-up.
This seems to indicate that fairly minor changes will give the same effect.
I put this effect down to better vaporisation and mixture heating before ignition.

I am inclining to two spark plugs, and a slotted head running towards the exhaust, plus dividing up the squish areas.
I think a two-stroke is quite stratified before the final "squish".
I shall , with some experimentation, find out.

The idea of two plugs is to rely on partial jet ignition at lower speed, but direct spark ignition at full throttle.
This helps avoid any performance dropoff due to slow burn speed.

Really blending and pre-heating the mixture, re: Smokey Yunik, would help.

I need to try to do something similar at a lower temperature, and without "blowing out" the spark.
Extreme local squish turbulence is not good for ignition.
Capture of a "cleaner" mixture portion closer to the chamber rear wall, opposite the the exhaust, seems a likely possibility.


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## Owen_N (Dec 26, 2021)

I did have another question:
I am looking at getting a block of 6061 or 6063 aluminium, 80x80x30mm.

Where is this forum mainly based?  am I using the correct units for you?

I can find a cut block of 6063 on AliExpress, at 100x30x200 at $ 78 nz including freight.
This is $55.70 US.
Is this a good price?  can I do better?

I am a bit reluctant to use a USA - based metal supplier, as freight can be a problem.

Occasionally I can get some good USA deals.  Amazon is not too bad right now, and I ordered a CDI ignition  set ex USA.
I think this soft extrusion grade (6063)  should be OK for a hand-made two-stroke cylinder head.
The fins are fairly easy to cut.


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## Owen_N (Dec 27, 2021)

Another question:
What is a good tool to cut the sealing seat for a spark plug?
I don't want to have to mount and spin the whole head at a funny angle.
There was a russian align-facing tool in 22 and 50mm diameter on aliexpress, but they don't ship to here.
I haven't seen any ads for similar tools.
can I get an adjustable one?
What are they called? - piloted align facer?? - I will try that in Google.


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## Bazzer (Dec 27, 2021)

Owen_N said:


> I did have another question:
> I am looking at getting a block of 6061 or 6063 aluminium, 80x80x30mm.
> 
> Where is this forum mainly based?  am I using the correct units for you?
> ...



The forum is slightly biased to North America but with healthy inputs from Europe and other geographical areas.

You should be able to find 60 series alu in NZ, there are a mass of suppliers on Ebay but shipping to NZ will be the problem, better to get it shipped to a private address in say the UK or USA and then to forward ship to you, that will work out cheaper.


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## Steamchick (Dec 27, 2021)

Just a note on "modern" combustion chamber design...
As the combustion chamber expands, the charge should attain an approximate spherical form until it reaches cylinder walls and head and piston faces simultaneously.  Gases that are unburnt before the flame front reaches the cold surfaces will burn the CO, but as soon as the flame front reaches code metal it cools below CO combustion temp. A lot of shaping of head causes a lot of cold patches (below 350 deg. C.). Hence dirty exhaust and wasted fuel = performance.
From the spark, the flame propagates as a tiny, rapidly expanding sphere, disorted by the turbulent gas flow introducing  fresh mixture into the ball of flame. The squish generates the turbulent gas. 
Enjoy your development!
K2


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## lohring (Dec 27, 2021)

I wrote a series of articles on nitro engine design.  The third is on glow ignition head design and includes a design for a head to be used on a stunt engine.  The idea was to enable a 2-4-2 pattern where the engine alternates between firing every and firing every other revolution.  Below is the article and pictures of the head designs tried.

Lohring Miller


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## Steamchick (Dec 27, 2021)

Thanks Lohring. This is completely beyond my expertise. Although I read Phil Irvine - Tuning for speed - a few decades ago, and have owned a few 2 stroke motorcycles (some race tuned - e.g. A 350LC Yamaha with stage 3 tuning- 17.5 :1 compression showing on kick-over, WOT. Needed 105 octane petrol! Had simple symmetrical squish bands in the flat toroidal heads. But the transfer ports were HUGE! Wide exhaust port 3mm higher than Mr Yamaha's design...), I have never done the head design/tuning myself. And all my industrial engine work was on production tuned, low emission 4-strokes (1980s to present decade).
But you have done some interesting and clever stuff!
K2


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## Owen_N (Dec 27, 2021)

Bazzer said:


> The forum is slightly biased to North America but with healthy inputs from Europe and other geographical areas.
> 
> You should be able to find 60 series alu in NZ, there are a mass of suppliers on Ebay but shipping to NZ will be the problem, better to get it shipped to a private address in say the UK or USA and then to forward ship to you, that will work out cheaper.


Yes, I have found some unspecified bar on ebay at 80 diameter, 50 long, for about $42nz ex china, which should be OK for freight.
I already ordered some 100 sq by 30 thick for $108 nz, so I will save this for further ref.
Basic material is $1.50/kg, cut-to-size bulk $6.50/kg, but short pieces around 1 kg  cut-to-size and delivered can easily get 10x+ this price.
Cutting charges are not cheap!


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## Owen_N (Dec 27, 2021)

lohring said:


> I wrote a series of articles on nitro engine design.  The third is on glow ignition head design and includes a design for a head to be used on a stunt engine.  The idea was to enable a 2-4-2 pattern where the engine alternates between firing every and firing every other revolution.  Below is the article and pictures of the head designs tried.
> 
> Lohring Miller
> 
> ...


Thank you for that. I like the twin-plug glow layout.  I will read your PDF.
I see you have quite a few mounting holes.
For a metal-to-metal seal and 4 studs as per most common motorcycle engines,
how deep do I need for bosses , and backing material, to ensure a good seal.
I was thinking 8mm??


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## Steamchick (Dec 28, 2021)

Hi Owen,
For the engine design I strongly urge 2 to 2 1/2 times the diameter of the stud/bolt for the length of hole the thread goes into, in the cylinder threaded boss. The last thing you want is a stripped thread in that hole... The bosses in the cylinder head should be at least 1 diameter thick. Aluminium will distort as the material contact pressure increases - as the engine expands and the steel studs/bolts expand less. Use of THICK steel washers, more than twice the stud diameter will reduce this surface distortion.
With a gasketed joint, the design should have LONG studs so the variation of load on the gasket is reduced between hot and cold conditions. Which is why many motorcycle air-cooled engines have such long head bolts. And why many engines have studs the full length of the barrel, going into the crankcases. But if you have a metal to metal joint at the cylinder head, this is just down to the coefficient of expansion of the barrel  and head metal, versus the stud metal coefficient, versus 0.2% yield stress of the alloy head. 
So you can do the sums as you know your design.
Torque control of the fixings during assembly is about setting the correct (adequate) pre-load (cold) but without overstressing the stud loading when hot. So calculations of torque are needed to get the correct pre-load on the studs.
(been there, 30 odd years ago, and forgotten most of it now!).
K2


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## Steamchick (Dec 28, 2021)

Owen, maybe you can show your design (drawings) for the rest of us to suggest answers to your questions, and  improvements if necessary? Words don't really express everything..
K2


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## awake (Dec 28, 2021)

Owen_N said:


> I think this soft extrusion grade (6063)  should be OK for a hand-made two-stroke cylinder head.
> The fins are fairly easy to cut.



According to Wikipedia, aka the-source-of-all-knowledge, 6063 comes in a variety of different tempers. In general, dead soft aluminum is unpleasant to machine, as it gums up the cutting tools. Using a higher grade / harder temper will make life much easier - still easy to cut, but cuts much more cleanly.

I don't think 6063 would have any advantages over 6061 for your purpose, and indeed 6061 might be better and easier / cheaper to source. But same thing applies - the temper makes all the difference.


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## lohring (Dec 28, 2021)

Thanks, but most of the things in the article were the ideas of others.  We did a lot of testing on 26 cc engines including some head button changes, and I've made head buttons for several 2.5 to 11 cc nitro engines.  The article is an overview of the work a lot of people have done over the last 60 years or so.

Lohring Miller


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## lohring (Dec 28, 2021)

I've always used 6061 T6 for head buttons.  The typical nitro engine uses 6 #6-32 hex head grade 8 cap screws.  One of my biggest shocks came when reassembling a Chinese engine, I broke the head off the cap screw.  Even the lowest cost Allen head screws in my local hardware store are grade 8.

Lohring Miller


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## Owen_N (Dec 28, 2021)

I did post some rough sketches with my first post.
I have revised my ideas since then, to two semi-hemispherical areas, centred plugs, a 7x11 channel in the centre, and squish from the sides.
I have some long bolts, so I will use them instead of studs.

There is a lot of thread at the bottom end. 
I just need some thick, large diameter washers.
Material doesn't arrive until February, so I have plenty of time.

I am doing tests on the finning of the original head/barrel, and a steady 160 degrees C seems to be a good aim.

I find that the vertical height of fins can be really short, and a large sector can just be the top thick horizontal fin.
The top of the plug boss is looking like a good height for the head.
It is well cooled by the prop flow.


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## Owen_N (Dec 30, 2021)

Here is a proper drawing of the twin-plug proposal.

The arrangement of the twin chambers should assist clearance of exhaust gas.
the finning has been extended a lot, as some cool squish areas have been lost at the cylinder walls, and  over the exhaust.
Possibly the finning can be shorter opposite the exhaust.


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## Steamchick (Dec 30, 2021)

Hi Owen,
While I see you are trying to get 2 plugs and a conjoined pair of combustion chambers into the head, and I am just learning about this from never having seen these ideas before, I am confused by how the 2 combustion chambers are supposed to be beftter than 1? I don't know if the racing Yamaha motorcycle engines that dominated that world for a decade were anything but a single hemi-head with single plug and large squish... as my Production racing LC350 (Quicker than a road going 500-4) had such a simple design.... with a single plug. One cylinder did have a preference for eating pistons, and while I could not measure any differences, as soon as I used higher octane fuel than the best street fuel (@ 97octane), it was OK. So I made a thin spacer and reduced the compression from 17:1 at kick-over to 15:1 at kick-over and that fixed it. (A friend let me try some of his 105 Octane racing fuel 50:50 in the 97 octane road fuel to see how that affected the engine). The whole basis for the higher performance (according to the local racing specialists - who supplied me pistons frequently) was the huge porting for the transfer ports, which contributed to the higher compression due to the larger charge passing through the engine. I feel you may suffer from the "larger, oddly shaped" combustion chamber and not gain any real preformance advantage, so I am interested to follow your ideas. A friend who tuned and raced go-karts in the 1980s also advised that the "spherical" combustion chamber with blarge squish was his preferred shape, with maximum transfer port cross-section, multi-ported (5 transfer ports)  with well flowed shapes. he said simply: "draw a sphere around the plug-gap at TDC (inside the head and piston shape but not including squish) - this should maximise the combustion - then draw a sphere at just before exhaust opening (Including squish), and this should maximise the finish of the burn before exhaust. He did his doctorate in conjunction with Ricardo (where he worked for a few years before I worked with him) and explained how the combustion "ball" of gas needs to ignite as a sphere - actually in the centre of a toroid developing into a sphere - to avoid more multiple flame-fronts (shock-wave) colliding, that causes detonation. Perhaps your engine will cause detonation from the twin ignition "spheres" - meeting between the 2 chambers in the connecting corridor? Perhaps the idea of connecting the 2 combustion chambers is not so appropriate? - I don't know, but maybe you can explain a bit more of your ideas? 
I am confused by your comments about "twin chambers assisting clearance of exhaust gases"? If "clearing the exhaust" appears to be a problem with your engines, then I should draw the combustion chamber at exhaust opening, and at transfer port opening to maybe estimate what is happening there? The exhaust needs adequate time (and cross-sectional area) to let the gases escape prior to transfer port opening so that the pressure and temperature of the residual gases in the combustion chamber do not ignite the incoming gases through the transfer ports. This is modelled by some very large and high speed computers in universities and research institutes, and by many racing engine tuners in their brains! Mine isn't good enough though! The flame front travels at sonic speed, for the pressure encountered, which is changing rapidly as combustion progresses and the piston moves...
I find this very interesting, but I am NOT an expert in this subject, so just learning from you all!
I look forward to some interesting results?
K2


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## Ken I (Dec 30, 2021)

While fooling around with cylinder head design you also need to consider what the variables are related to fuel, octane ratings and a whole bunch of other stuff that might otherwise thwart your plans and damage your engine - link to an article I have posted previously which is germaine to this.
Especially apropos to Lohring's excellent article on head design.

Fuels, Octane Rating etc.

This is a compilation of articles I cobbled together.
Regards, Ken


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## Owen_N (Dec 30, 2021)

Regarding Steamchick post:
This is not a performance mod at the top end.
The main objective is to smooth out bottom end and mid range. This mod is no use if you are turning 5000 rpms plus.
I surmise that the exhaust gas is not fully mixed with the incoming charge, for most transfer port layouts.
This may not apply to all loop charged engines.
I want to squish and fire in sequence from best mix to worst.

It would be nice to have access to a high end simulation system such as AVL Fire to verify this stratification, and adjust the transfer ports to suit.
The centre throat may not survive adjustment of the clearance volume- I have a couple of CCs to find.
There may be a "Jet Firing" effect if the second chamber fails to fire, but if the second chamber contains a good burning mix,
the sparkplug will help eliminate the slow burn and tendency to detonate.
It would be nice if a model aircraft two-stroke ran more like a four-stroke, for firing smoothness.

I think the lifespan of these things is limited- in 10 years new internal combustion engines of all kinds will not be available to most people.
There may even be a lot of limitations on running existing engines, and a severe restriction of suitable liquid fuel.
I don't know how this will play out for the current automotive fleet, with a 20-30+ year life expectancy.

Re: octane ratings: I think many normal motorcycle motors were run at 6.5:1 nominal ratio, as the oil mix tends to promote knock.

- It is a bit "suck it and see" to see if it will run on 91 octane (NZ) at 9:1 comp. I think 91 corresponds to a lower test number in USA.


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## Bazzer (Dec 30, 2021)

_"It would be nice if a model aircraft two-stroke ran more like a four-stroke, for firing smoothness."
_
That of course is a matter of personal opinion !!

_"There may even be a lot of limitations on running existing engines, and a severe restriction of suitable liquid fuel."_

Alcohol burning model aircraft engines will be running for decades yet, the beauty is that you can potentially make the alcohol fuel and vegetable based oils yourself.


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## Owen_N (Dec 30, 2021)

Maybe this is peculiar to single cylinder model aircraft two-strokes.

Misfiring causes massive torsional vibration with a light airframe.

I have ordered a two-stroke twin after dissatisfaction with the TP60 single. 

Hopefully having two cylinders evens out power delivery a bit, even though they fire at the same time,
and it has better balance, though it does have a large rocking couple.

The TP60 is also father fragile. It will make a good test bed for a cylinder head, though.

I want to use it in a contra-rotating hovering platform similar to the Hiller one, and this requires running at part throttle.
A 3-cylinder Saito radial 80cc four stroke would be better, but they are fairly heavy and much more expensive.

This got me interested in two-strokes, and I saw an opportunity to fire in the order of best to worst mixture at part throttle.

Most people don't really need the smoother low-speed running, and motorcycle installations seem to absorb the vibration.

The application is unusual in that weight saving and thrust to weight ratio is more important than usual for
model aero engines.

I have moved to IC from electric to get a sensible flight duration. 2 minutes is getting a bit marginal.

I can go up to 30 minutes with an IC engine, though 10 minutes is long enough.

Electric is OK for Helicopters, but I wanted to do something different.

Regarding liquid fuel, I think car fuel will be available for a while to support the aging fleet.

It is only in certain areas that IC cars and motorcycles will be banned, like city centres.
I see that motorcycles are already banned in some cities.

Use of small IC engines may be penalised more easily, though, depending on the whims of politicians, and the opinions of the
majority.

It is possible that we will be forced to stop running existing small IC engines and buy electric powered devices such as lawnmowers,
model airplanes, motor scooters-light aircraft??

In theory, small petrol engines could be converted to alcohol, depending on "which way the wind blows".


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## Owen_N (Dec 31, 2021)

Twin plug head- Revision 1 31-12-21.
--------------------------------------------
1) Chambers adjusted to 30 degrees conic.
This is very close to spherical, adjusted for plug clearance.

2) angled plug moved inboard.

3) vertical plug raised for piston clearance.

4) fin arrangement revised - one more full width next to plug,  loop inflow side fins shortened,

    tiny fins removed, exhaust side fin adjusted to clear bolt bosses.

5) centre channel dimensions adjusted to line up with other views.

6) more dimension lines added.


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## xpylonracer (Dec 31, 2021)

I maybe wrong but I understood Owen, the OP, was not seeking more performance in power output but he wanted to change the characteristics of the engine to make it sound like a 4 stroke.

xpylonracer


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## Owen_N (Dec 31, 2021)

The sound will not be the same as a 4-stroke, because you don't get the contrast between intake and exhaust cycles,
and two-valve poppet setups have quite a unique "tone" as well.

It may sound a little closer to a wankel engine at low revs, but more even firing and less torsional vibration are the main aims.
It is really only of interest to RC model airplane people.

It would make a nicer running car engine as well, but those are dying out now.

Direct airblast head injection would work, but  is complex and expensive.

High pressure finely atomised direct injection without the air could give smoother low speed running, and allows
use of jet turbine fuel or diesel with spark ignition.

With proper controls, this could also prevent detonation. Possibly rapidly pulsed injection.
The problem with staged injection is that it takes some time, and is more suitable to a constant pressure diesel cycle.

There have been trials blowing metered air into the transfer ports as well, but this sounds mechanically tricky.
Maybe a displacement blower and a separate valving system could do it?? You might as well just use a four-stroke.


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## lohring (Jan 1, 2022)

As was pointed out in the article I posted, the goal of one of the head designs was to enable a 2-4-2 pattern where the engine alternates between firing every and firing every other revolution.  Model four strokes are also widely used.  

I agree that electric power is rapidly replacing IC engine power starting with the smaller power plants.  IC engines will always be built by modelers.  Steam power is only used in very special applications where it is unlikely to be replaced; nuclear powered ships and submarines.  Land based steam power plants seem to be more expensive than greener alternatives and are fading.  Otherwise only model builders preserve the heritage.

Lohring Miller


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## Owen_N (Jan 2, 2022)

Proposed twin-spark head.
A compromise  head design with one plug could be a continuation of the first chamber with an extension based on a slanted cylindrical section.
Volume is biased towards the first chamber, but the majority of the squish is directed to a line extending away from the sparkplug.
This still has quite a long flame front, similar to a normal offset chamber, but does not direct all the mixture straight at the plug.
This allows the possibility of a good firing mix close to the plug.

This variation could be worth a tryout after the first version.


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## Owen_N (Jan 5, 2022)

Variation 3., revision 2.

Adjustment to get 6.5cc head clearance, align second plug to be square with piston top.
this gives 5mm sparkplug electrode clearance to the piston.

The cross-chamber now has vertical sides, and 12mm width, and about 10 to 11 mm depth at the plugs.

there are some questions:
1)  Is there enough side clearance on the firing zones?

2) Can the electrodes go closer to the piston?
Say, 4mm clearance?

3) Presumably a bit of outwards taper around the sparkplugs is desirable to reduce wall quenching?

Using a rounded form both sides ( cool side and exhaust side) will reduce volume a little.

4) That volume for the taper form  has to removed from some other region.
5) The cross chamber could be necked more in the middle.
Will this make scavenging significantly worse?


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## Steamchick (Jan 6, 2022)

From my amateur seat.... I would site the spark centrally between head surfaces and piston surface at TDC.... to permit max sphere of flame to develop before the gases hit cold metal, and where shock waves can reflect. But swirl will dramatically distort the "natural centre" location, so your guess is better than mine! You can only guess at where the swirl will be centred, and how the flames will distort as a result. I understand the expert view is for high swirl and flame turbulence to prevent (deter) detonation and feed fresh cool charge into the combustion zone for optimum combustion. But the chamber is shaped differently at the ignition point (advanced) to that at TDC and thereafter, as the piston approaches and recedes. So modelling what really goes on at max revs is not a simple task....
K2


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## Owen_N (Jan 6, 2022)

Part of the problem is that the chamber is an odd shape.

"hemispherical" chambers do not seem to be half a sphere.

I would have thought that part hemisphere, part straight sides would be better.
This makes the potential flame front more like a ball.
A downside of this is the flame has to turn round sharper corners to spread out into the squish zone at 20 deg atdc.
and travel a bit further.
However, without high end simulation, I just have to try it.

I can pull the plug up a bit and keep the chamber diameter at say 18mm,
or push the plug down a bit and open the chamber out to a full hemisphere at 24mm
Possibly the higher plug is more efficient in this case.

I can reshape the centre of the twin chamber to a more triangular shape, which gives me material to go from 12mm out to 18mm
around the plugs, I think.

It is hard to calculate the actual volume, so I will leave the centre fillets in, keep the 5mm electrode tip clearance, and
measure and adjust to the final shape.


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## Steamchick (Jan 6, 2022)

I agree that "hemi" heads are not spherical, nor even half of that, but usually more like 40%-ish of the sphere. but considering (with a single combustion chambered head) the squish band forms a dough-nut shaped ring of swirling gases around the central point, the chamber contains the large majority of gases (Less than 10% remain in the squish zone?). So igniting this - before TDC so the ball of flame achieves max pressure soon after TDC - the flame tends to be a jagged and haphazard flame curling around the "surface of the doughnut" (from what I have seen from simiulations and high speed film off the web). Then as the piston travels down the stroke, towards the "cylindrical shape" (not exactly, because of the head shaping) the ball of flame expands out so in theory at least, it reaches all the cooler surfaces simultaneously. (Hence the desirability in modern 4-stroke engines of a bore = stroke, and near-flat head-shape). for the 2-stroke you are studying, the exhaust opening is determined by othe parameters, so you should consider how the flame can expand from a doughnut around each spark-plug, in 10 degree crank intervals from ignition to exhaust opening....
If you can estimate the max rpm of the engine, you can estimate the time for each 10 degree step, and crudely estimate the chamber pressure, - hence speed of flame propagation for each step, and guess how the "doghnuts" of gas will burn at each stage.
This is a very complex problem, - and I don't envy you the work involved to get something like sensible results - but this is a way to manage the steps required...
I hope my amateur fumblings intoi your expertise don't disturb your thoughts, but help to clarify them?
Do tell me to "shut-up!" if you think I am "off my trolley" on this one...
I won't be offended,
K2


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## Owen_N (Jan 6, 2022)

I have seem mention of a package called AVL Fire which does engine simulation, but that is supercomputer stuff, I think.

I think I will just make something. It will either sort of work, or give me more ideas.

There is the prospect of deliberately stratifying the chamber at part throttle, and limiting exhaust gas mixing,
to help the two-stage burn.

It is likely that the flame behaviour will be less than optimum for full throttle running, though.


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## Steamchick (Jan 6, 2022)

Had a browse of the AVL cover page...








						AVL FIRE™ M
					

30 years of automotive simulation experience has been distilled into AVL FIRE™ M, our new, unique computational fluid dynamics (CFD) simulation solution supporting the development of both conventional and electrified powertrains. FIRE M offers cutting edge turbulence modelling accounting f...




					www.avl.com
				



It looks like interesting software for your study? But do AVL sell this FIRE-M software? - Can it be used on a "simple" PC?
It wasn't clear to me how you get it.
Have fun!
K2


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## Owen_N (Jan 6, 2022)

AVL Fire
If you have to ask, you can't afford it!

This looks like a four valve two stroke simulation! -image-
I would suppose this engine would be intended to run as a 4-stroke, then go to two-stroke for full power.
This could be tricky to manage as a changeover, and emissions management could be an issue.
This could boost output by 35% at least at the same rpm.
With timed valve control it could be a lot more.

A downside is you want an advanced valve system similar to what Koenigsegg is looking at.
(electro-pneumatic)
It is difficult to go from 4-cycle to two-cycle otherwise.
I wonder what the power penalty is using pneumatic power to operate the valves?

You end up blowing all that compressed air out the exhaust.
 Maybe 5 or 6 hp for a 4 cylinder engine at high rpm??
This is more suited to a high performance vehicle, where the extra cost does not matter.

Presumably power losses can be recovered at lower rpm by varying the inlet vs exhaust duration (Atkinson cycle).
and otherwise adapting the valve operating cycle.


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## Owen_N (Jan 6, 2022)

The head metal has turned up, so I measured the existing head.
the distance from the plug seat to the piston  is 17mm, vs my 16 and 14mm.
The volume is 5ml just about exactly, vs my estimate of 6.5ml.

This gives a nominal CR of 12:1 !

This seems quite high, for a two-stroke on regular gas.

Possibly the propeller load/rpm characteristic allows them to get away with this.

Trying to pull full torque at 3500 rpm would likely  cause detonation.

This motor seems to settle at 5500-5800 rpm at present with the 23x10 prop.
it would rev higher with a 23x8 prop.

I will need to revise my twin plug layout quite a bit to get to this point.
I will have a look at this tonight.


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## Steamchick (Jan 7, 2022)

High Owen, 
I have just been reading some earlier discussions and trying to get "back to basics" of what you are trying to do. I am really clueless on the idea of 2-stroke alternating with 4-stroke firing. I don't have any pearls of advice there, except I don't think you can do that sensibly. I have driven motorcycle 2 strokes that would 4-stroke under certain throttle settings, - even "8 or 12-stroke" on over-run with tiny throttle openings above idle. (Who knows what is happening when the engine is not firing every stroke, anyway!). But those were just engine-carburettor quirks, not planned running modes.
On Octane vs. compression.  On many of the motorcycles I have owned, the compression with wide open throttle and kicked-over was anything up to 12:1 for regular production engines running on 95 to 99 octane fuel. readings of 8:1 to 11: 1 were typical on engines made from 1960 through to 1980-ish... 
But the actual compression is achieved by sucking in a charge from beneath the piston, with stroke from TDC down to transfer port opening, then this is pumped into the top end at transfer when some of the charge escapes down the exhaust port before that closes, when the remainder is compressed to TDC. This means that "calculated" compression is likely to be wrong, and actual compression is somewhere between the area of piston x stoke from transfer port closure to TDC, and the area of piston x stoke from exhaust port closure to TDC.. Or so I reckon?
In fact, I had great difficulty starting one bike (worn rings) that had compression as low as 6:1 on kick-over. - But once running, it kept running until I let the revs drop too low, although the power was well below proper power levels. (maybe 1/3rd or less?). New rings fitted and it was back to FUN again! (~12:1 on my gauge). I only experienced detonation issues at part throttle (~5000rpm) on one engine - 17:1 race tuned but with 99 octane fuel.... That was OK when I dropped the compression to 13:1 by using a double-thickness cylinder head gasket... which made it OK for road-use, even through max power and revs were reduced.

Probably none of this is of any use, but I look forward to reading your next ideas...
K2


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## Owen_N (Jan 7, 2022)

version R4 with 12:1 CR.
Note: 30 degree plug angle and 5mm spacing of electrode to piston retained.

To get more hemispherical, the plug angle would need to be 45 degrees.

For now, the chamber sides are quite straight.

An advantage of this is that squish is not directed straight at the first plug, so there is less mixing
of exhaust gas at part throttle.
I will try this layout, and see if the partly restrained flame has a negative effect on top end power.

Squish clearance is an even 1mm.
I should get away with no squish radius, as the chamber is an extension of the cylinder wall on the cool side.


Steamchick said:


> High Owen,
> I have just been reading some earlier discussions and trying to get "back to basics" of what you are trying to do. I am really clueless on the idea of 2-stroke alternating with 4-stroke firing. I don't have any pearls of advice there, except I don't think you can do that sensibly. I have driven motorcycle 2 strokes that would 4-stroke under certain throttle settings, - even "8 or 12-stroke" on over-run with tiny throttle openings above idle. (Who knows what is happening when the engine is not firing every stroke, anyway!). But those were just engine-carburettor quirks, not planned running modes.
> On Octane vs. compression.  On many of the motorcycles I have owned, the compression with wide open throttle and kicked-over was anything up to 12:1 for regular production engines running on 95 to 99 octane fuel. readings of 8:1 to 11: 1 were typical on engines made from 1960 through to 1980-ish...
> But the actual compression is achieved by sucking in a charge from beneath the piston, with stroke from TDC down to transfer port opening, then this is pumped into the top end at transfer when some of the charge escapes down the exhaust port before that closes, when the remainder is compressed to TDC. This means that "calculated" compression is likely to be wrong, and actual compression is somewhere between the area of piston x stoke from transfer port closure to TDC, and the area of piston x stoke from exhaust port closure to TDC.. Or so I reckon?
> ...


A fairly long timed 4 stroke closes inlet 60 deg abdc
vs 2-stroke closing the exhaust 86 deg abdc, so there is a relative comp ratio going on there
0f 1+ 0.5 vs1+ 0.0675, 1.5 ,1.07, % = 75%,  53.5%
<edit> bad trig.
So a 4 stroke nominally 12:1 actually is 9:1, A 2 stroke 12: 1 is actually 6.42:1, a big difference.

 The 94 deg atdc is quite late for a two-stroke, probably not intended to use a tuned pipe.

A modern 4-valve motorbike  multi uses quite high compression, but some of the older English bikes were much lower.
8.5:1 seems a common number, but probably shorter timing??
8.5:1 was common on older V8 truck  engines, too.- 1960s and 1970s...

The Triumph  500 twin and 750 triples had quite high compression.

The engine alternating 2 and 4-stroke timing has never gained popularity to the point of production, as it needs a turbocharger to run, and you can get the extra power just by winding up the boost, but less efficiently.

Possibly turbo plus variable compression is a better target?

Anyhow, I am not trying to do this, - my version is an "enhanced stratification two-stage burn" in a normal two stroke.
This is intended to give smoother running at part loading and lower revs, without taking much off the top end.

The poor running is quite noticeable with a model aero two-stroke with throttle control.

I could take further measures to calm chamber turbulence if needed, by:
1) blocking the boost ports.
2) re-angling the side transfer ports, with a short "sweeper section" to blow flat across the top of the piston.
this probably only has to be 1/4 or less of the whole port width.
This part has a flatter roof angle.
The idea is to ensure exhaust gas does not get sucked up the back of the cylinder and mixed into the good mixture.


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## Steamchick (Jan 7, 2022)

Thanks Owen. Interesting.
Post WW2, Although tetra-ethyl lead was available (most aviation engines of WW2 needed it!), it wasn't commercially cheap enough for the limited gasoline in Europe for decades....
I remember in 1960s compressions were around 6:1, or 7:1 in car engines, for "Regular" fuel ~88 ~91octane. And most 4 stroke motorcycles, we're the same,  until late 1960s when the benefits of performance of 8.5:1 compression - with 96~99 octane Premium fuel became more commonly used. By early 1970s, some performance cars & motorcycles were available up to 10:1 compression and needed super 100+ octane fuel.
When the star rating was introduced, I thinK 4 star was 99 octane, 5 star 105 octane, and 2 star 92 octane. After lead was removed... 1992? - compression ratios were dropped from 10:1 to 8.5 or 9: 1,  before fuel injection became more widespread, and knock sensors could retard ignition automamically.... and 4 star became 95 octane fuel!
But this is my memory, so don't quote me,  do your research on what you can get today!
Keep up the good work!
K2


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## Owen_N (Jan 20, 2022)

Update on new head: 21-01-22

1)The engine has been built and test run on one plug.
There is a smoothness benefit between 3000 and 4500 rpm with this engine.

It really needs two sparkplugs.
The first plug location is not that good over 5000 rpm.

I am waiting for my twin-fire CDI unit, so I can run both plugs.

2) I would like to add an offset skullcap insert, for some reshaping.
The sparkplug entry paths seem to be OK, but the chamber could be shallower and rounder.
Can I just glue this in place, or does it need screws?  I can probably add screw seating on the outside.
<> Part of the sparkplug thread stays in the outer head.  I will have a go at drilling and retapping it like this.
I think there is enough head material to recess like this, otherwise I may need to add a taper to the insert and the outer head.
The material in place is fairly dome-shaped.

3) I have a new idea on adaptable transfer ports.

The low speed ports need less exhaust blowdown timing, but can be aimed further towards the rear of the cylinder and higher up.

I was thinking of using reed valve blocks cut in half and remounted, but a downside is increased crankcase/transfer volume.

I can cut the whole sides of the cylinders out down to the base flange and use a plain 6063 aluminium insert.
This should take the basic ring loading, no combustion pressure.
the main transfers can have reduced timing.
I will need to order some bigger blocks of aluminium, I think.
30x30x30 blocks may be big enough.

There are two problems that I see:
3a) the rear transfer location does not get much air cooling. (relative to the propeller).
3b) the reed valve flow will drop off at higher rpm, leading to a lack of reed  and reed block cooling.
Is this a real problem? are there any cures?

I have been following a two-stroke builder on You Tube, titled "Two Stroke Stuffing", and he uses this idea
on his "blown", disc valve exhaust , engine.
He has better mixture cooling of the reed valves, however.

I am going to trial deflector vanes to get more airflow behind the cylinder.


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## Steamchick (Jan 21, 2022)

Just one "worry" springs to mind... re: "
I can cut the whole sides of the cylinders out down to the base flange and use a plain 6063 aluminium insert.
This should take the basic ring loading, no combustion pressure. " - What happens when the aluminium rises from cold (20C) to operating temp of the cylinder - maybe 150C? Differential expansion of the aluminium may spring the cylinder apart and even split it?
K2


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## Owen_N (Jan 21, 2022)

Steamchick said:


> Just one "worry" springs to mind... re: "
> I can cut the whole sides of the cylinders out down to the base flange and use a plain 6063 aluminium insert.
> This should take the basic ring loading, no combustion pressure. " - What happens when the aluminium rises from cold (20C) to operating temp of the cylinder - maybe 150C? Differential expansion of the aluminium may spring the cylinder apart and even split it?
> K2


I don't think that will be a problem. I use a jointing compound between the pieces.
The existing steel block I used on the boost side has given no problems.


I suspect the outer cylinder is a fairly high silicon content, but seems more ductile than the inner lining.

That destroyed one of my high speed steel cutters when I was trimming the top of the cylinder.

An hss steel cutter  will still nick the surface.

Maybe they have figured  out a way to get aluminium carbides in there without nikasil.
It looks like an aluminium finish, but is slightly lighter in colour than the rest of the cylinder.
You can see the contrast at the bottom of the cylinder.
It looks like there are black speckles in it.  Maybe hot sputter with carbon particles??
Laser in vacuum?
There is a sharp delineation at the ports.


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## Steamchick (Jan 22, 2022)

Owen, I can only say "suck it and see". Your machining capability looks to be of a very high quality, so I have no doubt parts will fit well. But thermal expansion of the 2 different materials may form a distortion in the cylindrical shape when the motor gets hot. As the engine is so small, it may be in the realms of "less than the tolerance of the engine". By that I mean, how much distortion the engine can tolerate without seizing, wearing badly, or causing blow-by at the rings. Good luck! I am impressed with the head development posts so please continue.
K2.


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## Owen_N (Jan 22, 2022)

Barrel revision R2:

Ports are now staggered slightly, and a cut-down reed block is added,

The drawings  are posted here.
there is a need to make the top port roofline steeper than the bottom one.
possibly twisting the ports along their span relative to the barrel may work.
This orientation of the reed valve was chosen because of dimension constraints, and to
clear the bottom transfer duct.
It also gives a straight shot at the top port.
Dimensions are taken from the OEM reed block, but the one selected is from a Yamaha Jog.
I will vary dimensions to suit.
There should be enough clearance for the prop.

There is some revision to the OEM port timing shown.

The actual angles are a bit off due to conrod angularity.
Towards the top of the stroke distance to degrees is more, and towards the bottom, it is less.
The slight lift of 2.5mm in the exhaust should be OK.

There should be little or no drop in power at 6000 rpm, my main load point.
Possibly I should bump the compression ratio up a little.

Non-pipe engines are exhaust open 90 deg atdc +,
Tuned Pipe engines tend to be about 85 degrees atdc.

The transfer blow-down allowance is reduced by 1mm.

The reed valve is to prevent excess blowback of exhaust gas into the crankcase at higher revs.

Reminder of objective:
To provide porting that adapts to rpm and throttle openings.
The top transfer is more ideal for lower rpms and loads.
This should improve engine smoothness at part throttle with a propeller.
This could make a significant difference in vibration with a light airframe.

You probably wouldn't notice much difference with a motorbike- a little less popping, maybe?
road mufflers are good at keeping that quiet currently.


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## Owen_N (Jan 22, 2022)

Steamchick said:


> Owen, I can only say "suck it and see". Your machining capability looks to be of a very high quality, so I have no doubt parts will fit well. But thermal expansion of the 2 different materials may form a distortion in the cylindrical shape when the motor gets hot. As the engine is so small, it may be in the realms of "less than the tolerance of the engine". By that I mean, how much distortion the engine can tolerate without seizing, wearing badly, or causing blow-by at the rings. Good luck! I am impressed with the head development posts so please continue.
> K2.


The alterations are below the level of any compression and expansion activity. and any distortion should not affect these activities.
Possibly there may be a little unevenness in piston support during the bottom part of the cycle.

The area between the ports also help with load-bearing during the expansion stroke.

I have reversed the exhaust side for this reason, as there is more wall and piston area on the exhaust side, which is now also the thrust side.
The vast holes cut in pistons nowdays is not good for durability.

Race engines don't have them. 
They have disc valves and bridge exhausts overlapping the transfers.

This requires a bigger spacing from the pin to the crown, and long skirts all round.
The transfers are fully outside the barrel as well, and studs are further apart.
You will see this with chainsaw engines as well.


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## lohring (Jan 22, 2022)

Two stroke port layouts have been extensively studied for over 40 years.  The pioneer work was done for Yamaha by Gordon Blair at Queens University at Belfast.  His #12 cylinder had the worst scavenging while #14 was the best.  The final refinement was done by Jan Thiel at Aprilia.  Pictures are below.  My articles are also attached.


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## Owen_N (Jan 22, 2022)

lohring said:


> Two stroke port layouts have been extensively studied for over 40 years.  The pioneer work was done for Yamaha by Gordon Blair at Queens University at Belfast.  His #12 cylinder had the worst scavenging while #14 was the best.  The final refinement was done by Jan Thiel at Aprilia.  Pictures are below.  My articles are also attached.
> 
> View attachment 133342
> View attachment 133347
> ...


Thank you for your articles and drawings.

My exercise is not intended as a high power two stroke, but possibly equivalent to a chainsaw motor, but with smoother low power, low speed running mode, as well as reasonable power at 6000-6500 rpm- around 75-90 hp/litre,
with a standard can-type model aircraft muffler and header pipe, about 60mm x 250mm.

I think 4.5 hp at 6000 rpm is about in the range for 60cc.

I don't have a direct standard for this wooden prop , but I think 4.5 hp is a good estimate looking at specs for similar APC props.

If I choose a smaller prop, I can get over 7500 rpm- ie a 20x8 inch instead of a 24x10 inch.

This exhaust does seem to provide a degree of suction, but no tuned pressure pulse.

This kind of running is not a priority with motorcycles or chainsaws, but is significant for model aircraft.

If I could get a 5 hp air-cooled wankel or similar rotary engine, at 1300 to 1800 gram basic weight, I would use that, and this line of inquiry would not have arisen.

This power and weight range in rotaries is not represented in the market at the moment.

Some of the older Sachs units from the 1970's  were in that range.
I think one I have seen was about 8 hp.

There have been some made for military drones, but sort of POA, or they have folded.

The military spec setup is somewhat more expensive than a model aircraft spec.


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## Bentwings (Jan 23, 2022)

Steamchick said:


> Owen, I can only say "suck it and see". Your machining capability looks to be of a very high quality, so I have no doubt parts will fit well. But thermal expansion of the 2 different materials may form a distortion in the cylindrical shape when the motor gets hot. As the engine is so small, it may be in the realms of "less than the tolerance of the engine". By that I mean, how much distortion the engine can tolerate without seizing, wearing badly, or causing blow-by at the rings. Good luck! I am impressed with the head development posts so please continue.
> K2.


Most of my model aero engines had hemi head or chamber. The old K&B .40 had a bar or deflector on the Ustinov then just a lot in the head . I’m not so sure this didn’t cause detonation. I blew u a number of these while running excess nitro  there was a thin gasket that we often took out too that probably didn’t help longevity. All my current gas motors have hemi shape chambers pistons are generally flat top . I don’t have an 4 stroke engines . 
Byron


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## Owen_N (Jan 23, 2022)

Bentwings said:


> Most of my model aero engines had hemi head or chamber. The old K&B .40 had a bar or deflector on the Ustinov then just a lot in the head . I’m not so sure this didn’t cause detonation. I blew u a number of these while running excess nitro  there was a thin gasket that we often took out too that probably didn’t help longevity. All my current gas motors have hemi shape chambers pistons are generally flat top . I don’t have an 4 stroke engines .
> Byron


Actual profile of the combustion chamber seems to be relatively insensitive, if it has about 50%  squish, and a bell-shaped cross-section with rounded edges.

A longer chamber likely benefits from twin plugs, for faster burn. I need to trial that, when my twin-spark CDI arrives.
You don't seem to need squish all the way round.

Some chambers work with sharper sides and bevelled ends, in the boost side and exhaust side.
these are machined round to start with.

My next exercise, a skullcap, would likely benefit from a heat-conductive , rigid, jointing compound.
Any ideas? possibly metal powder in a silicate matrix??- waterglass??  Sodium Silicate?
It needs to be stable at 350 degrees C, I think.

The VHT exhaust paint may be heading in the right direction, but you don't want much shrinkage, otherwise it will form voids.
Exhaust sealant looks fairly insulating.


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## Owen_N (Jan 23, 2022)

I am still having trouble finding aluminium at reasonable prices.
Freight is a killer, at about $60 us per piece.
In NZ, no-one is offering to sell small quantities cut-to-size.
I suppose the hobby market is too small, here.

Ex China, I got one piece 6061 dia 60mm x 100mm, $62.84 incl frt and tax (nz) m - about $45 US.
and one piece 7075 40mm x 100mm x 100mm, $104.25 nz, or $74.50 US incl tax (15%)
This is $66.51 usd per kg for the bigger piece, and $56 usd per kg delivered for the smaller piece.
weight-wise?

Considering commodity value is $3.12 usd/kg??
Mind you, alloy, plus retail quantity, will bump this up a lot.
kgs are 0.8 and 1.12.

I think ex shop, cut one-off , 1.5" x 2.5" x 6"  was USD $7.20, but USD  $60.08 for freight! (ex USA)
total = 67.28 x 1.15 for local tax = $77.32 usd

In kgs this is 38mm x 63 x 150 @ 2800 kg/ cu mt = 3.6 x 10^5 x 2.8 x 10 ^3 = 1.008 kgs approx.
This is about  $77 usd per kg delivered (correction)

The China deal seems better, probably due to their cheaper freight rates.

This is delivering mid-march, though, so hardly UPS!
Days roughly 6 + 28 + 6 = 40 days to deliver.
Quite often this is an upper limit, and they can arrive weeks earlier.

-----------------------------------------------------------------------------------
Machining properties:

what are 6061 and 7075 aluminium like to work with?
Do you use this alloy numbering system?

It is common on China sites, but I see the the EU is using something different now.

7075 (alloyed with zinc) seems a lot harder and stronger.
Possibly easier to get a good lathe surface finish?
Does it work-harden?
What is it like for heat resistance?

Some alloys and heat treatments are not good for continued higher temperature use, say 200 degrees C plus?

6061 is usually used for extrusions, and tends to be quite soft.

It is probably difficult to machine nicely  on a lathe, and gets bad built-up edge and tearing with HSS tools.
Are tungsten tips any better with this?

I have some tool-grade silver steel and stainless steel here, and it is a bit tricky.
Possibly full flooding with coolant, and tungsten tools, may help.
It hardens very quickly if you drill it with HSS drills.

That stuff can wait until I am better set up with a bigger lathe, a coolant spray system,
and a coolant collection tray.
The coolant probably shoots all over the place, too.

I saw a fully flooded cnc mill setup on you tube, and he had it in a clear acrylic box.

<edit>
my calcs were a bit off:
actual paypal bills were $41.64 for the smaller piece, and $55.43 for the larger piece.
I probably will incur currency charges on top, though.
I will check.
<edit>
back- converting at 1.49,
smaller = 43.40 USD, larger = $54 USD
my initial estimate: 45 and 74.50 was a bit off . maybe I double applied tax at 1.15?
74/1.15 = 64??  
so $54/1.12kg = $48 USD/kg, and the smaller piece is 43.40/0.8 = $54 USD per kg.

Still an improvement over $77 usd per kg, total, delivered.

I would have to get someone to collect and re-export it at a lower shipping rate if possible.

Is this very complex re: customs, postage, correct stickers on the package, type of packaging?

Usually this stuff is wrapped in bubble wrap, then put in a self-sealing plastic bag, with a fancy sticker on it.

Then if you want to send it by mail, there is a heavy/bulky goods charge, registered mail charge, etc.
Would it be overall cheaper than $60 USD per kg?

I saw an article on you tube, xjet, where he got a book sent to him by mail, and postage was over $60.
It was a big, heavy book, though.


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## Owen_N (Jan 23, 2022)

Two-Stroke engine skullcap jointing compound:

I have chosen Rocol Steamseal. this is Manganese dioxide, clay, silicates, naptha, linseed oil.

An alumina ceramic or graphite-based compound would be better for thermal conductivity, but it is not easily available online.
This stuff sets harder than graphite-based compounds., and is less likely to drop into the engine.

It is probably too hot an app for epoxy-type adhesives.

I will try running it ,then strip and check for thermal damage, and I can monitor sparkplug barrel temp.

The sparkplugs will be partly threaded into the skullcap.
I will also thread in a few M5 capscrews to help hold it securely in place.
The head really needs more fins, but this setup will do for tests.
I was thinking  a 100mm x 100mm x 40mm block to get more finning.
I bought a spare one, but may use it for other things.

Prolonged heat soak of 7075-t7 aluminium plate  just drops strength and hardness- possibly getting back to a state similar to T0??

I don't need a lot of strength in 7075, so this seems OK.

I found some comments  on embrittlement, but I don't know what they mean by this.  Loss of ductility??
Under what conditions?
It seems to be fairly resistant to inter-granular penetration in corrosive conditions.


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## Owen_N (Jan 29, 2022)

More transfer ideas:
1) side-boost port extended from the main transfer, and with 2mm less blowdown and a steep angle.
This would blow close in to the rear cylinder wall.
The shape gives enough support to the ring with a wider port.
It needs a smooth curved transition into the main transfer, and possibly a bit more chamfer.

2) Full reed valve transfers.
This is on the theory that exhaust gas blowback and mixing in the crankcase is causing most of the rough running at off- idle and medium-slow running, below 3000 rpm.

Reed valves have been shown to be better than piston-port, but worse than rotary disc,
in valving efficiency.

This should not have a lot of effect on overall power through most of the range,
but may cause some residual crankcase charge at the top end.
On the plus side, the charge cannot reverse-flow back into the crankcase.

It would not work well on a fully piped engine, as pipe suction helps keep the transfers flowing,
then supercharges the mixture back into the cylinder on the blocking pulse.

3) Make the transfer blocks more easily removable, with just a thin sealing coat of J-B Weld.
This means adding bolting wings, making stud passages, and gluing material between the fins to pick up bolt threads. This does not need to take up much of the fins.
This would allow me to trial several layouts more easily.

The exhaust timing seems to be quite long already, at 180 degrees.
I can maybe lift it 1 mm, increase standard blowdown slightly, and add the offset side boost, with 2mm less blowdown.

I am setting the corrected CR at around 9.5, up from 8.0:1

This changes head clearance volume from 5 ml to 4 ml, stroke 36.5, bore 46mm, swept volume 60.7 cc

This is good for 3% torque gain, but going from 21mm to20mm for the exhaust opening loses 4.8 % of the effective volume,
so there will be no gains from the compression change alone.

the exhaust lift could be handy to trial different transfer layouts.
For the main transfer, I can stick with 5mm blowdown spacing for now.

It will be interesting to see if the anti-blowback for the exhaust gas makes it run more smoothly at the bottom end, and
whether more drag in the transfer ports knocks a lot off the 6000 rpm test range.

This is where I need to be for this prop I have selected.
If I go to a 20x8 prop, I can get a lot more revs, but I don't think it will give me more thrust that the original prop.

I have seem 8200 rpm with the 20x8, but the engine seems to overheat at that rpm.

I am looking at a 40 x 100 x 100 head design, up from 75x75 at the top fins.
A better cooled head seems to keep the barrel much cooler round the back, too.
The 30mm tall head I have made doesn't have enough fins.

I don't know why the OEMs run the engines so hot, Peaking at 250 degrees C can't be good for them.
(measured hot spots).
They tend to seize rather easily.
Also, many installations are worse for cooling than having the cylinder in open air.

Apologies for the drawings looking like they were on the back of a napkin.
Actual dimensioned drawings to follow, once my parts arrive.

My working drawings still look a bit rough compared with modern CAD, but they do the job.


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## Owen_N (Feb 8, 2022)

Progress update:

The skullcap variation of the head has  been overmodified, and now will not start.

I suspect 30 degree plugs with the electrodes only 1.5mm from the piston causes a poor burn on startup attempts.
-A type of flame quench.
I will wait for more head material to arrive, and try a design with a 10:1 CR and 45 degree plugs.

I have lifted the exhaust 1mm, and changed the transfers slightly, which I hope does not also cause starting problems.

I can compare it with my spare barrel/head, which starts well with the current piston and ring.
I may need to convert the spare to swappable heads to be sure, if my next head is also hard starting.

I was also wanting to try low speed running with a compression release restrictor about 5mm up the barrel,
to load up the engine at low speed, which smooths out engine firing.
The hole is there, but temporarily blocked until I get the head sorted.


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## Owen_N (Feb 9, 2022)

Here are the new head drawings.
The head clearance volume should be about 4.4 mls, but these shapes are hard to estimate.

It is sort of egg-shaped in the top view, but fairly triangular in the side view.
How close would I be if I took a segment of a sphere overlaying the side view?
I can extend it out a bit to match the top view more?

The objective here is to reduce the quenching effect of the earlier head, so that it can actually be started.
Do the sparkplugs need to be more out of the squish zone as well?
Usually the single plug is lifted a few more mms.
There have been twin plug heads before, but the head is generally semi-spherical for spark plugs,
and I think the plugs may be slightly further apart, but still about 40 to 45 degrees off vertical.
Glowplug engines are not so touchy.
<edit>
This would require a half sphere with a radius of 13mm.
By slightly squashing the half sphere it would fit.
Minor axis is 18mm, major axis is 28mm.
Anyone know any formulas for elliptical spheroids, to check that the volume is similar?
I will google that.


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## Steamchick (Feb 10, 2022)

I find this fascinating, but beyond my knowledge....
Good luck, but keep trying. That is how breakthroughs happen. I'm sure I am not the only one enjoying your posts, so please don't stop writing.
2 steps forward... One back, is usual in this sort of development.
To check volume, fill cavity with wax or clay, then drop clay into a measuring cylinder of water.... or melt wax into a simple shape You can measure, like a rectangular mould?, then calculate volume.
K2


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## Owen_N (Feb 10, 2022)

Steamchick said:


> I find this fascinating, but beyond my knowledge....
> Good luck, but keep trying. That is how breakthroughs happen. I'm sure I am not the only one enjoying your posts, so please don't stop writing.
> 2 steps forward... One back, is usual in this sort of development.
> To check volume, fill cavity with wax or clay, then drop clay into a measuring cylinder of water.... or melt wax into a simple shape You can measure, like a rectangular mould?, then calculate volume.
> K2


I just syringe kero in there.  It has only one free surface.

I found the formula for a spherical ellipsoid- quite simple..

This looks like an appropriate shape, so I will stick close to this.
"Hopefully" the quenching problem will go away with the different proportions.
with a larger volume to sparkplug size ratio, you can lay the plug over more.
This is what they do in car engines.
Plugs can be about 30 degrees off horizontal.
I have seen single plug two-stroke heads with the plug laid over that far.


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## Owen_N (Mar 15, 2022)

I am back to this engine again.

My block of aluminium has turned up, and I am working on a new head.

I am concerned that this forum may be a little "high quality work" for my bodgie engines tacked together with J-B Weld, though.

What do you think?


I just fixed a bad sparkplug thread  (trying to drive a tap in a drill press- overdid it)
I can't be bothered to wait until mid april for proper parts, so I have used a swaged bush, pinned in place with cut-down
1mm nails.
This is slightly marginal.  I will see if it stays put.

After the CR check, I will test run to see if it starts OK, then cut the head fins.

I also want to modify the piston and barrel to eliminate the boost ports, using sections cut from old parts.

My calcs show that the piston patch-plugs will stay in place, supported mainly by the skirt hole bridges.
at 6000 rpm, vertical force is about 40N per insert patch, and shear load on the bridges is tiny- about 2 MPa.
support is also via tapering the piston cutouts to resist inwards forces, plus J-B weld.

Most of the piston skirt is cool enough so that the J-B Weld shouldn't deteriorate substantially.

I have used it around the transfer ports, and, apart from slight yellowing, it stands up well.

The objective is to arrest the rapid piston wear a bit, and get the engine durable enough for my next series of mods.

I think the current port mods plus a little higher CR means that no power will be lost without boost ports.

If I carefully fit the piston inserts, they should be at the correct diameter.

The barrel inserts can be levelled with 2 straight edges for support while the initial  J-B layer is setting.
The ring passes over J-B bridges OK.

I am assuming that the J_B Weld is around 15 MPa max, but may be a bit lower.
it is flexible and crack-resistant.

18 MPa is high-end for heat cured epoxy.
Most epoxy compounds will be lower than this.

I am working around the 6000-6500 rpm range because it suits the  model aircraft propellers.
They would take a more powerful engine to rev faster, and even then, it is more efficient to just increase the prop diameter and pitch.

I wonder if I should go more to small motorbikes, like the 2Stroke Stuffing guy?? (YouTube)
I don't have a great urge to build and fly large rc model planes.

These engines are no good to me unless I can build a contra-rotating prop drive, for a vertical takeoff aircraft.

For "normal" RC planes, I am quite happy with the 200-500 watt range of electric motors.
I would also rather build odd engines and run them rather than go fly any kind of RC model plane, for now.


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## Bazzer (Mar 15, 2022)

_"These engines are no good to me unless I can build a contra-rotating prop drive, for a vertical takeoff aircraft."_

You do not need a contra rotating prop for a VTOL prop driven model aircraft, just go on the web and see the 1000's of youngsters prop hanging electric RC models .

I would suggest that you stand less chance of VTOL with a contra rotating prop just due to the weight complexities and complications in the system.


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## Bentwings (Mar 15, 2022)

Owen_N said:


> I just syringe kero in there.  It has only one free surface.
> 
> I found the formula for a spherical ellipsoid- quite simple..
> 
> ...


I’ve done giant scale Rc models for years some very high performance some just for boring holes in the sky.

spark plug holes can generally be repaired with heli coils. Mc master Carr has then many auto parts stores have then unless you are into radically different sizes. I have Tig welded some heads years ago but unless it’s not replaceable it’s not a cheap or easy fix. It maybe that it’s just easier to make a new custom head with your mods built into the design.
Most of my larger gas motors ran about 7k rpm adjusting a little by prop size generally the top is running at a stalled speed so it takes a lot of power to get much rpm change on the ground other than pitch or diameter change.  My smaller glow engine ran a lot faster but much smaller props. But the same factor applied. It took a lot of nitro to get much rpm change on the ground. In the air the props began to act more like a screw snd you could get some real speed or pulling power. 
mid bevinterested in seeing your counter rotating device. I’ve seen it done but not for a long time. There have been a number of constant speed devices planned but I’ve not seen one that really worked as the real ones do. 
byron


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## Steamchick (Mar 15, 2022)

I have used the modern aluminium solder to make aluminium items. It needs care so you don't  melt the parent components, but seems as strong as annealed aluminium when made. Very good - I guess - for filling spark plugs holes for re-machining?
Or previously, I would have used zinc alloy repair rods for aluminium.
K2


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## Owen_N (Mar 15, 2022)

Bazzer said:


> _"These engines are no good to me unless I can build a contra-rotating prop drive, for a vertical takeoff aircraft."_
> 
> You do not need a contra rotating prop for a VTOL prop driven model aircraft, just go on the web and see the 1000's of youngsters prop hanging electric RC models .
> 
> I would suggest that you stand less chance of VTOL with a contra rotating prop just due to the weight complexities and complications in the system.


If I built a tailsitter, then 2x 250 watt electric motors running 7 inch props would be sufficient.

Using these largish petrol engines, the prop torque is quite high.
Possibly a "Monocopter" layout would work, with large deflector vanes.
I have seen it done in electric.

Altitude control would be problematic, as the petrol engine does not provide  the rpm control
that electrics are quite good at.
Additional fast-acting thrust deflectors would be needed.

An engine under 2 kgs has over 10 kgs static thrust, so there is a lot of reserve.

Possibly a reversing system with some  acetyl gears would be light enough- maybe another 2 kgs.

I have one designed, but I need a better lathe and possibly a mill to make it.

For satisfactory lift performance, at least 1.5 to one thrust to weight is needed to gain altitude.
1.8:1 would be better.
Contra-rotation is more efficient at thrust to power ratio so that makes up for some weight gain.

The front support bearing runs at 12-13,000 rpm, and possibly needs a circulating oil system to control heat build up.
I would also want to fit an electric start, at about 500g .

The whole assembly extends about 160mm out from the normal prop position, so the gear case needs a lot of bracing back to the engine.

I have an electric start system on hand, but it is not a simple fit to the engine, and it gets in the way of the main case for the contra-rotating gearbox, and makes a larger diameter casing necessary.
There is a large steel gear that sits behind the prop drive flange, on a one-way freewheel.
The gear has large holes in it and is quite narrow, so it is not that heavy, though.

You don't want to handle an engine setup with a vertical axis with 20 inch props while it is running!


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## Owen_N (Mar 15, 2022)

Bentwings said:


> I’ve done giant scale Rc models for years some very high performance some just for boring holes in the sky.
> 
> spark plug holes can generally be repaired with heli coils. Mc master Carr has then many auto parts stores have then unless you are into radically different sizes. I have Tig welded some heads years ago but unless it’s not replaceable it’s not a cheap or easy fix. It maybe that it’s just easier to make a new custom head with your mods built into the design.
> Most of my larger gas motors ran about 7k rpm adjusting a little by prop size generally the top is running at a stalled speed so it takes a lot of power to get much rpm change on the ground other than pitch or diameter change.  My smaller glow engine ran a lot faster but much smaller props. But the same factor applied. It took a lot of nitro to get much rpm change on the ground. In the air the props began to act more like a screw snd you could get some real speed or pulling power.
> ...


I couldn't find heli coils in the 10mm size.

The nearest was 10mm to 14mm brass adapters, but it has to be bought overseas , with long lead times.

I also need a suitable tap. I have ordered the parts, but the longest lead time is mid april.

In the mean time, my 11.5mm bush, swaged to 12.7mm (half inch, ) and pinned out to 12.7mm, could be later repaired if needed.

A problem with this is that there is only about 13mm diameter clearance in the chamber, and a tap will leave cuts in the chamber wall.
It may work OK.
I should swage around the pins, to make sure they don't drop into the cylinder.


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## kf2qd (Mar 15, 2022)

Schnuerle porting with a hemispherical head with squish band all the way around. You adjust the size of the hemispherical section and the squish band to fine tune how it runs. Schnuerle porting means you use a flat top piston and simplify the design. The more complex, the more variables, the more variables, the less chance of control.


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## Owen_N (Mar 15, 2022)

kf2qd said:


> Schnuerle porting with a hemispherical head with squish band all the way around. You adjust the size of the hemispherical section and the squish band to fine tune how it runs. Schnuerle porting means you use a flat top piston and simplify the design. The more complex, the more variables, the more variables, the less chance of control.


1) an offset chamber seems to work OK for these lower revving engines.

2) there is a 2mm dome across 45mm bore. on the piston.

3) there are a lot of variations on Schnuerle  loop layouts. Very wide inlet curves on the side transfers seem to be beneficial.
I am limited by the standard model aeroplane case layout, and close-in base studs.

For high revving engines, wrapping the exhaust ports around the sides with bridges, and dropping the front side transfers down, seems to work.

Look at the  "2Stroke stuffing" engine, YouTube.  It makes about 20 HP at the crank at 17500 rpm, 50cc, reasonably wide power band.

This is as good as any GP engines have ever done., and the port layout seems biased to rear ports and not so much loop.

My mods are much milder than that, and I am aiming at a much lower rpm level.

Later mods are to examine the effects of raised transfers and side reed valves, as well as a controlled compression release bleed for low rpm
engine loading.  These may well have no effect, or just damage peak torque at the rpm range of interest.
I have to try it to see. Transfer upper edge profile can be adjusted back with J_B weld, as long as I make sure the piston ring
doesn't hang up in the port.
So far, an angled top and upper widening at the rear seems to work for transfers.
The bad head was causing problems, so I have changed that.

Plug height or angle away from the piston top seems to be fairly critical. - I guess there is a  quenching effect that makes starting difficult.


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## Owen_N (Mar 16, 2022)

I was looking at getting a race-capable 2-stroke 125 motorcycle engine, and building on that, but they seem to a bit like hens teeth-hard to find.

Not many disc valve water-cooled 2-stroke 125cc motorcycle engines around.

The idea of building something like Mr "2stroke-stuffing" (M2SS)  seems a bit weird.
At least start with a motorcycle transmission?

125 seems a better starting point than a 50cc.  - It gives better performance, but is still a small engine.
It is also capable of up to 55 hp, which is better than a lot of mid-sized motorbikes.

"over-the-counter" engines were more like 35 hp, though.

It takes some extreme tuning to get to 55 hp, and lots of replacement parts.

The bearings don't last that long, and you need a top end rebuild every couple of race meetings?

Sounds like drop-in dry sleeves might be a good idea??
- and a ready source of pistons and rings.

I was wondering about SG iron as a barrel material.
This stuff machines like mild steel- wouldn't a steel liner be just as good?

I would be iffy about running aluminium pistons on plain steel, if the built-up edge I get on tool steel is any indication.

Those gold tungsten tool tips seem to be better for non-stick.
Is that a kind of electroplated titanium compound on them?
That would be good on a cylinder liner.
I will google that.

If you want it to rev to 18,000 rpm, the initial drive ratio could be tricky.- A centrifugal starter clutch could be good there.
otherwise you could need a large gear spread.

You also don't want to slip a manual clutch that much.
M2SS's  vario drive is pretty much 2-speed as he uses it on the dyno.

 With road use it could give better better ratio variations.

Starting with a road transmission also would give problems with the clutch speed.

You want a huge step down ratio to the clutch.
This is difficult to fit on a 125-sized clutch. A 500 GP clutch gear ring is a lot bigger.
- maybe a 2-stage gear reduction?

Also, the engine would be pretty much unusable below 8000 or so rpm, as you would be fighting the pipe,
and possibly blowing a lot of exhaust gas back into the cylinder.

These engines don't seem to start pulling smoothly until they have been loaded up- maybe why the riders like
constantly "zooming" the engine up , to clear out any misplaced exhaust gas.
It is amazing they don't backfire into the crankcase more-not good for reed valve engines!


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## Bentwings (Mar 16, 2022)

Owen_N said:


> If I built a tailsitter, then 2x 250 watt electric motors running 7 inch props would be sufficient.
> 
> Using these largish petrol engines, the prop torque is quite high.
> Possibly a "Monocopter" layout would work, with large deflector vanes.
> ...


if you can get JB to do the job great . Tell us about it , pictures.
You might look at a site called project farm . He tests every thing . You may even get a direct response from him  I know he has tested all sorts of glues . His tests are worth watching , he tells what’s happening  and shows test results compare results against claims very well done videos

byron


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## Owen_N (Mar 16, 2022)

Bentwings said:


> if you can get JB to do the job great . Tell us about it , pictures.
> You might look at a site called project farm . He tests every thing . You may even get a direct response from him  I know he has tested all sorts of glues . His tests are worth watching , he tells what’s happening  and shows test results compare results against claims very well done videos
> 
> byron


(233) The Ultimate Epoxy Competition--Which Epoxy is the Best? - YouTube
J-B weld stands up OK in this test.
I quite like the other one, too -Devcon-, but the smaller quantities are not available in NZ.

They claim J-B is good up to 330 degrees F , or 165 degrees C.

I find air cooled cylinder head temperatures get above this, and J-B becomes soft and powdery.

I presume the particular item you want to know about is the piston inserts.

On 4-stroke engines, I have seem nylon and PTFE buttons used in the thrust faces , and nylon is good up to 150 degrees C continuous,
or 175 degrees C intermittent.
I expect the edge right next to the ring land will soften, but most of the J-B should be good.

I have used it inside the combustion chamber, right out at the edge of the squish area, to fill an accidental cut,  and it slowly erodes.
Closer in to the sparkplug, it vanishes quite quickly.


I will post before run and after run photos.
<edit>
J-B original formula -sustained 500 deg F, 3960 psi- 27 MPa. Ultimate.
This is higher strength than most epoxies, and may be under special conditions, such as a tensile machined test piece.
500 F = 260 deg C.
I would suspect that 180 deg C is more realistic, according to surface temperature infra-red readings.

Devcon is only rated to 120 degrees C, as are most of the others.


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## Owen_N (Mar 17, 2022)

The TP-60 mod engine lives again!

the poor head chamber shape was the problem.

It maxes around 5500 -5800 rpm with my test prop, fast run-ups, and no sustained running.

A change in behaviour from the original porting is a jump from idle up to around 4000 rpm with a very small throttle advance.
I suspect draw-through from low pressure in the exhaust.

There is about a 1.5mm lift in the exhaust port, and bigger, re-shaped transfers- more trapezoidal-shaped
to help stop ring catching.
Blowdown offset between exhaust and transfer ports is slightly smaller.

These are similar results to the 70cc twin, once run in, so this can be my benchmark for ongoing mods.

No obvious improvements compared to  standard porting.


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## Steamchick (Mar 18, 2022)

Hi Owen,
I would be interested to see a porting diagram (360degree timing would be OK) to understand these changes better. - But I understand if you don't want to publish "intellectual property" on this "theft ridden" means of communicating! (Interweb: www = "What's Worth-it Walks!").
My only real experience of re-porting 2-strokes was to raise the (theoretical) power of my 250 MZ by 8Hp. Made a huge difference to top end on the motorway, but I didn't notice any change to "town-riding" acceleration torque. The bike was very good at mid-range revs before and after. Before I had the mod (a tuning company's design) Itook a transfer of the poerting, and again afterwards, and the changes were quite small and subtle. But included a very small transfer port through the piston, just beneath the rings, opposite to the exhaust port, directing a 3rd jet of intake gases up into the combustion chamber.
I also owned a "stage 3 race tuned" LC350 Yamaha. Good for over 12500rpm... and over 125mph ... but had 17:1 compression measured at tick-over. It eroded the pistons to seizure if I ran at 55mph for more than a moment (a big flat spot that entailed a gear change to get past). I was told I needed 105 octane fuel. But otherwise trouble free ("it went like stink!"). When I reduced the compression to ~15:1 with addition of a second head gasket, the problem disappeared using pump 98 octane fuel. That engine had more than double the transfer port CSA, exhaust port "squared-out" compared to original. The exhaust port timing was 1~2mm higher (top edge) that original, or so I was told by my local tuning shop, who told me that the engine had been modified (by one of their track rivals) for Production racing... and the "stage 3" tuning should not be on the road! It was as quick as the 500cc 4-cylinder road bike (race replica) that Yamaha made later.
Thanks 
K2


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## Owen_N (Mar 18, 2022)

Steamchick said:


> Hi Owen,
> I would be interested to see a porting diagram (360degree timing would be OK) to understand these changes better. - But I understand if you don't want to publish "intellectual property" on this "theft ridden" means of communicating! (Interweb: www = "What's Worth-it Walks!").
> My only real experience of re-porting 2-strokes was to raise the (theoretical) power of my 250 MZ by 8Hp. Made a huge difference to top end on the motorway, but I didn't notice any change to "town-riding" acceleration torque. The bike was very good at mid-range revs before and after. Before I had the mod (a tuning company's design) Itook a transfer of the poerting, and again afterwards, and the changes were quite small and subtle. But included a very small transfer port through the piston, just beneath the rings, opposite to the exhaust port, directing a 3rd jet of intake gases up into the combustion chamber.
> I also owned a "stage 3 race tuned" LC350 Yamaha. Good for over 12500rpm... and over 125mph ... but had 17:1 compression measured at tick-over. It eroded the pistons to seizure if I ran at 55mph for more than a moment (a big flat spot that entailed a gear change to get past). I was told I needed 105 octane fuel. But otherwise trouble free ("it went like stink!"). When I reduced the compression to ~15:1 with addition of a second head gasket, the problem disappeared using pump 98 octane fuel. That engine had more than double the transfer port CSA, exhaust port "squared-out" compared to original. The exhaust port timing was 1~2mm higher (top edge) that original, or so I was told by my local tuning shop, who told me that the engine had been modified (by one of their track rivals) for Production racing... and the "stage 3" tuning should not be on the road! It was as quick as the 500cc 4-cylinder road bike (race replica) that Yamaha made later.
> ...


I can take a print off the barrel on the next strip-down, and draw round it.
I am not sure how that corresponds to angle. The rod ratio is about 1.8:1,
exhaust is around 180 degrees, transfer about 120 degrees about bottom centre - pretty conventional.
the only notable change are the wider trapezoidal ports.
I have a diagram I can tidy up a bit.

I doubt if there is anything really unusual.

I thought the idea of the inlet-side power-valve was neat!  "2stroke-stuffing" has 2 separate tracts on a modified disc valve case.
Accidently discovered by  breaking and jamming the intake disc during a dyno run.
No-one else seems to have documented it.

Most reed valve mounts are on the removable barrel, but the top of the case is accessible if the cylinder leans forward a bit.

I have seen, on YouTube, a re-made Kawa Mach 4 upper case with reed valve mounts straight on the case.

It may be possible to get 2 reasonable tracts in there using the same carb.
A deliberately smaller reed valve block may be needed- maybe one half of a big one?

A blocking mechanism that rotates like a front power valve could open up the direct tract.
This would be a "Pipe only" type mod.
<edit>
This modified print looks fairly accurate.


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## Steamchick (Mar 18, 2022)

I always liked the idea of a disc valve as it makes the intake timing assymetrical, which is what the reed valve is doing I suppose. Always thought an additional disc valve between the crankcase and transfer port could improve transfer timing... but never had enough time to spend trying to work it all out! Glad to be in touch with someone who is trying some variations!
K2


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## Owen_N (Mar 18, 2022)

Looking at a 125 gp or carting engine, with cast pistons.
these are normally limited to 25 m/s, but some production engines run to 27 m/s average piston speed.
this is 14,000 to 15,120 rpm with 54.5mm stroke.

The Vortex 125  engine is claimed to be 43 HP at 13,900 rpm, and maximum 14,000 rpm.
27 m/s probably shouldn't be used for sustained running.

A 50cc 2-stroke with 36 mm stroke could run to 21,194 rpm as a maximum power point.
2SS uses 17,500 rpm, so he has some leeway.
If his output of 20 hp was applied to a 125, that is 2.5x the volume, a 125 should make 40 hp exactly at 14,000
rpm at the same level of volume efficiency.
I have heard of disc valve engines running at 55 hp for 125cc- I will check that out.
<edit>
I think Aprilia had 55 hp at 13,000 rpm.
quote ex Kevin Cameron, cycle world.

I would suppose everyone else matched that fairly quickly.

This I think would include draw-through by pipe suction, and "supercharging" from the pipe pressure pulses.

Pipes were generally 36mm inlet, 5.1 inches maximum diameter , or 130mm.
This also shows what  a difference disc valves makes over reed valves- 30% more power at 94% rpm.
The favoured exhaust is T type with centre bridge.
This is not easy to use with a power valve, but smaller side exhaust ports are not that efficient.
The Honda CR 250 has a complex power valve system that blocks off the two side ports.
The main port valve has a rocking action.

I would like to see if the 2stroke-stuffing guy can match this kind of "average piston pressure" and
capacity to power ratio.
It is early days yet.
I will see if I can find a few port-print type maps.
I have a couple of port cross-section drawings, showing the way a "full" transfer port swings out and back in, top edge angles, and
inside curvature.
I am not sure whether the boost port should be 60 degrees of the horizontal.

A bit of up-angle is good to get the transfer streams to merge centrally, and not chaotically.
Visualising this experimentally is difficult.

That fancy GT Power and AVL-Fire simulation would have to emulate this effect.
I think they dyno-ed the original engine to benchmark the simulations.
energies-11-02739.pdf  ex google.


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## Owen_N (Mar 19, 2022)

Steamchick said:


> Hi Owen,
> I would be interested to see a porting diagram (360degree timing would be OK) to understand these changes better. - But I understand if you don't want to publish "intellectual property" on this "theft ridden" means of communicating! (Interweb: www = "What's Worth-it Walks!").
> My only real experience of re-porting 2-strokes was to raise the (theoretical) power of my 250 MZ by 8Hp. Made a huge difference to top end on the motorway, but I didn't notice any change to "town-riding" acceleration torque. The bike was very good at mid-range revs before and after. Before I had the mod (a tuning company's design) I took a transfer of the porting, and again afterwards, and the changes were quite small and subtle. But included a very small transfer port through the piston, just beneath the rings, opposite to the exhaust port, directing a 3rd jet of intake gases up into the combustion chamber.
> I also owned a "stage 3 race tuned" LC350 Yamaha. Good for over 12500rpm... and over 125mph ... but had 17:1 compression measured at tick-over. It eroded the pistons to seizure if I ran at 55mph for more than a moment (a big flat spot that entailed a gear change to get past). I was told I needed 105 octane fuel. But otherwise trouble free ("it went like stink!"). When I reduced the compression to ~15:1 with addition of a second head gasket, the problem disappeared using pump 98 octane fuel. That engine had more than double the transfer port CSA, exhaust port "squared-out" compared to original. The exhaust port timing was 1~2mm higher (top edge) that original, or so I was told by my local tuning shop, who told me that the engine had been modified (by one of their track rivals) for Production racing... and the "stage 3" tuning should not be on the road! It was as quick as the 500cc 4-cylinder road bike (race replica) that Yamaha made later.
> ...


OOO!
I am jealous! Those LC 350 Yamahas were very desirable!
They are probably on the same price list as restored Kawasaki Mach 4s nowdays!
 I saw one of those that went for $50,000 USD.


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## Steamchick (Mar 19, 2022)

$50,000 USD !! A crazy price! Mine sold for £1300 in 1998? It was a "cheap and cheerful" commuter back then. Clever "one-touch" indicator buttons, very light, easy riding position, and OOOOHHHH!!! All that power with light and easy gearbox and clutch. etc. But it was going to either kill me, lose my licence for excessive speed, kill someone I hit, or something I didn't want to risk.... so I sold it and bought a Guzzi V50II, to tame the adrenalin and save my nech/sanity/licence! (I still have it). Crude by any comparison with the LC350... (But I have grown to like it).
K2


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## Bentwings (Mar 20, 2022)

Steamchick said:


> Hi Owen,
> I would be interested to see a porting diagram (360degree timing would be OK) to understand these changes better. - But I understand if you don't want to publish "intellectual property" on this "theft ridden" means of communicating! (Interweb: www = "What's Worth-it Walks!").
> My only real experience of re-porting 2-strokes was to raise the (theoretical) power of my 250 MZ by 8Hp. Made a huge difference to top end on the motorway, but I didn't notice any change to "town-riding" acceleration torque. The bike was very good at mid-range revs before and after. Before I had the mod (a tuning company's design) Itook a transfer of the poerting, and again afterwards, and the changes were quite small and subtle. But included a very small transfer port through the piston, just beneath the rings, opposite to the exhaust port, directing a 3rd jet of intake gases up into the combustion chamber.
> I also owned a "stage 3 race tuned" LC350 Yamaha. Good for over 12500rpm... and over 125mph ... but had 17:1 compression measured at tick-over. It eroded the pistons to seizure if I ran at 55mph for more than a moment (a big flat spot that entailed a gear change to get past). I was told I needed 105 octane fuel. But otherwise trouble free ("it went like stink!"). When I reduced the compression to ~15:1 with addition of a second head gasket, the problem disappeared using pump 98 octane fuel. That engine had more than double the transfer port CSA, exhaust port "squared-out" compared to original. The exhaust port timing was 1~2mm higher (top edge) that original, or so I was told by my local tuning shop, who told me that the engine had been modified (by one of their track rivals) for Production racing... and the "stage 3" tuning should not be on the road! It was as quick as the 500cc 4-cylinder road bike (race replica) that Yamaha made later.
> ...


There is a guy that tests everything on you tube.

Project farm you tube I YHINK is his site he did a test of  job weld and several other adhesives JB. Did pretty well . 
I think you could get away with it on glow motors as they run cooler gas motors get pretty warm EGT are often 800 + deg F cyl heads around 350 deg F so I YHINK you are pushing the limit there but try it . I certainly would like to see it work . I can’t help much as to port design . I’ve done tuned pipes but even there I’d have to do some searching for data. 
Byron


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## Owen_N (Mar 21, 2022)

Bentwings said:


> There is a guy that tests everything on you tube.
> 
> Project farm you tube I YHINK is his site he did a test of  job weld and several other adhesives JB. Did pretty well .
> I think you could get away with it on glow motors as they run cooler gas motors get pretty warm EGT are often 800 + deg F cyl heads around 350 deg F so I YHINK you are pushing the limit there but try it . I certainly would like to see it work . I can’t help much as to port design . I’ve done tuned pipes but even there I’d have to do some searching for data.
> Byron


This particular engine may suit a semi-tuned pipe.
The conical expanding part is ok out to around 60-70 mm in diameter, but a 6000 rpm pipe would be too long.
If I got a good torque build from 5000 rpm, I could spin up to possibly  7000 rpm, depending on the torque curve.

Prop torque climbs very quickly. I think static thrust is proportional to the cube of the power, and 
has a square relationship to rpm and velocity. T = dens x effy x V sq x A  (thrust)
- similar format to drag, only no 0.5 factor.

A pipe on this engine may give 30% more torque, = 30 % more power at the same rpm,

and possibly up to 10% more thrust..

To get from 6000 to 6500 is am 8.3 % increase, 1.083^2 = 1.17 or 17% more thrust.
at say 10% more thrust, velocity increases by 1.05, or 6300 rpm.
This is not exactly correct, but gives some idea.
if I increase speed  by 1.05, then power is x 36.5 % more.

Is there any way I can get a correct tuned response into a pipe that is only 320-400 mm long?
The idea is that this length would fit into a typical model aircraft, which is maybe 800mm long in the body.
If a short tailless design is considered, the body length would be less.
I am considering wingspan from 60-80 inches, or 1.5 m to 2m.


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## Steamchick (Mar 21, 2022)

I suggest you consider how a contra-bassoon or trombone works, compared to a clarinet or post horn. In musical instruments, columns of air work fine if doubled back on themselves. Or even wrapped in coils - like a French horn or Susaphone! Suppose you take reflecting compression-cone and tail pipe and silencer part of the traditional 2-cone expansion box, then the large end feeds into an outer spiral chamber, that returns along the outside but the spiral is sized for length and cross-section to mimic the length and cross-section of the longer expansion cone of the traditional expansion box. The engine exhaust would then feed into the longer outer expansion spiral chamber and expanded gases would then be compressed and reflected when they pass into the central part. The whole silencer should be less than half the length of a traditional straight expansion box, and fit within a fuselage.
You'll need to cut some cardboard parts and work out the construction, but first decide the "plot" of CSA versus length, and do some sums to work out the spiral and outside can shape to make the chamber around the inner cone simulate the expansion cone.
Ask if you don't understand, and I'll do some more work to explain.
K2


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## Owen_N (Mar 21, 2022)

Steamchick said:


> I suggest you consider how a contra-bassoon or trombone works, compared to a clarinet or post horn. In musical instruments, columns of air work fine if doubled back on themselves. Or even wrapped in coils - like a French horn or Susaphone! Suppose you take reflecting compression-cone and tail pipe and silencer part of the traditional 2-cone expansion box, then the large end feeds into an outer spiral chamber, that returns along the outside but the spiral is sized for length and cross-section to mimic the length and cross-section of the longer expansion cone of the traditional expansion box. The engine exhaust would then feed into the longer outer expansion spiral chamber and expanded gases would then be compressed and reflected when they pass into the central part. The whole silencer should be less than half the length of a traditional straight expansion box, and fit within a fuselage.
> You'll need to cut some cardboard parts and work out the construction, but first decide the "plot" of CSA versus length, and do some sums to work out the spiral and outside can shape to make the chamber around the inner cone simulate the expansion cone.
> Ask if you don't understand, and I'll do some more work to explain.
> K2


You might have to draw me a picture.
I am thinking Archimedean screw, but to mimic the cross-section it would be very fat!
You could change the pitch of the screw as well.
Weren't there mufflers that has an internal screw like this?

Wouldn't making the gas pass though this kind of passage cause more resistance?

I was thinking that the tail cone could be inverted for half its length. What effect would this have?
I think all that does is reduce the crossways travel time, but the lengthways spreading effect would be similar but halved.
The reverse pulse building at the exhaust may be more effective over a narrower rev range.

Some alternate "stinger" inlets are set at the half cone position.
what does the end cone actually do?

1) it decelerates some of the expanded gas from the front cone.
2) it progressively reflects and spreads out the sonic front.
3) the spread out sonic front then reforms back in the front cone, and acts as a compressor at the engine outlet.
This can block further outflow, and even reverse some flow.

There is possibly a sweeping effect back down the front cone.

Do the two cones have to have the same angle?
How long is a 6000 rpm exhaust?  I see that the main cones add up to about 1.5 feet on a 17500 rpm cone.
(see 2stroke-stuffing).
is a slower setup proportionately longer? 17.5/6 = 2.9, or 4.4 feet. , 53 inches.
Plus the short header, the stinger, and the muffler.
That is enormous! most motorbikes only have room for about 25 inches of exhaust!
I have see pipes bent at the stinger with the muffler going back the other way.


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## Steamchick (Mar 21, 2022)

Hi Owen,
Yes, I have been thinking about something like an Archimedian screw - with variable pitch as the passage progresses along the pipe.
First I need to understand - what length and diameter of header pipe are you planning? Do you have a design of expansion box already worked out?
If not, this guide seems straightforward to design one.
https://www.google.com/search?sxsrf=APq-WBs99ZFZpTf5aO8_20cBuomRxG4QvA:1647859534465&source=univ&tbm=isch&q=design+of+model+aircraft+tuned+exhaust+pipe&client=tablet-android-samsung&fir=3T4uBYf2jAL5zM%2CVKzv8LPCrn-n4M%2C_%3B7-QXKn5d0hRVhM%2CvdFz9FknV5LBkM%2C_%3BdHymh39mLdmBRM%2CVKzv8LPCrn-n4M%2C_%3BHexMFsLilZAHiM%2CC8ecOuVs0cqiWM%2C_%3BZXt_MDn2QLBiDM%2CFhISyNWfxPUUsM%2C_%3BAay5ldBGI4lK9M%2CId6_vQh5tDojIM%2C_%3BmFX5vIkmYaCIbM%2CC8ecOuVs0cqiWM%2C_%3BOqqOK4MKaRRfnM%2CAJHN6HjXjTU5mM%2C_&usg=AI4_-kQ0UuUO8oMkG_6tHn_NjiWxE4v5zQ&sa=X&ved=2ahUKEwjdxMORg9f2AhVGecAKHXA4AWwQ7Al6BAhOEEY&biw=962&bih=601&dpr=1.33#imgrc=IW6z9R50RGvJhM

As gases do not really appreciate cones and tubes, just flat planes that they reflect off, the "spiral" tube should be pretty much OK as would a rectangular tube. In simple terms, the mean diameter of a spiral tube would be akin to the length of the middle of the spiral chamber - that is really just a rectangular tube wrapped into a spiral. But as the cross-section needs to change, the change of pitch of the screw alters the length of one side of the rectangle - thus increasing or reducing the CSA, and simulating the expanding and reducing cones of a twin-cone expansion box.
So I understand you are planning on a 60cc, engine, 6300rpm as the tuned speed. (I may get this completely wrong, but please bear with me?).
So:
we can calculate closely the pipe length. The formula for determining the length is:

Lt = (Eo x Vs) / N English OR (83.3(Eo x Vs)) / N Metric

Where:

Lt = tuned pipe length, in inches OR millimeters

Eo = exhaust open period, in degrees

Vs = wave speed (1700 ft/sec OR 518.16 Meters/sec at sea level)

N = crankshaft speed, in RPM
I.E.
83.3 x (120 x 518.16)/6300 = 822.1mm. But supposing that is around a spiral of mean diameter 50mm: Then we will need 822mm to the Widest point of the divergent and convergent "cones", including the pipe from the engine port to expansion box.
So assuming 100mm from exhaust port to feeding in tangentially to the expansion box, the divergent cone will need to be 722.1mm long (4.6 revolutions of the spiral)... the convergent cone say 362mm long (2.3revolutions of the spiral.
Now supposing the divergent cone has "an expansion angle of 10degrees. - Tan 10 = 0.176: You exhaust port is about 495sq.mm. CSA. so the header (assumed 100mm long) shall be about 25mm diameter. Therefore the widest part of a divergent cone from 25mm dia at 362mm long shall become ~139mm diameter (Bigger than my guess of 100mm diameter, so totally illogical!), or a CSA of 15175sq.mm. On a spiral, mean width 50 mm this would become a rectangle of  303mm long.... (one revolution at mean diameter of the spiral chamber). The converging cone would be half of that. SO: Just the diverging spiral on dia 100mm tube would be longer than the straight cone!
This is becoming ridiculous, so don't bother trying to design that one! It would need to be at least 150mm diameter to start to shorten the overall 1.08m long regular twin cone design...
I think anything larger than a regular twin-cone- design will create more wind drag, as it will increase the CSA of the aircraft.
Back to the head-scratching stage? e.g. - Make a longer fuselage, 6inches diameter, capable of holding a regular twin-cone expansion chamber 40inches long?

K2


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## lohring (Mar 21, 2022)

I wrote a series of articles that cover the development of the high power two stroke.  The emphasis is the application of these principles to model size 26 & 35 cc engines that we have a lot of experience with.

Lohring Miller


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## Owen_N (Mar 21, 2022)

Steamchick said:


> Hi Owen,
> Yes, I have been thinking about something like an Archimedian screw - with variable pitch as the passage progresses along the pipe.
> First I need to understand - what length and diameter of header pipe are you planning? Do you have a design of expansion box already worked out?
> If not, this guide seems straightforward to design one.
> ...


here is a possible sleeved folding scheme, where the tail cone is a convergent annulus around the centre cone.
I see the side view shows a much shallower convergent angle for the length, even though the annulus starting
area should be very close at 1/2 r wide.
A fairly long slotted or perforated area is needed for the transfer to the muffler stage.

How do you think this would work?

Will it actually reflect the main sonic pulse?

I will calculate some actual lengths.

Thanks for the google search-I downloaded or copied  lots of images from this.

If I can keep the centre cone to 72 mm, then the overall diameter will be 110 mm., which is probably OK.

Thinking about return pulses- unless 45 degree cones are used, the return pulses will bounce around quite a bit.

*****************  TP60 engine mods are coming along well.

I will turn the barrel and piston around again, to make the Cool-side the combustion thrust side.

The ring land above the exhaust is overloading, and wiping aluminium above the exhaust.

This may be caused by insufficient piston support beside the exhaust, or piston rocking.
Fixing the cold side should reduce rocking.

It will be interesting to see how the inserts stand up, and whether J-B can be used as a piston skirt coating.

The alternate Devcon etc "plastic metal" options seem to be only rated to 120 degrees, vs around 200 degrees C for J-B.
J-B seems to have a gritty surface texture.
It doesn't seem to erode piston rings though.


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## Owen_N (Mar 22, 2022)

I ran a pipe spreadsheet.
The conic sections look ok.
inlet chosen at 1 inch.

the sticking point is the centre parallel section, of 645mm

front cone = 271mm, tail cone = 226mm, and diameter = 63.5.
This is a very strange looking pipe. 
I will compare it with the 66 cc torque pipe, which looks more "normal".
overall length = 58.3 inches.


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## Owen_N (Mar 22, 2022)

1) I think this spreadsheet has been 'fiddled" with.
The example calc i did, and an actual similar example, are wildly different.
2) are there any advantages in using shallow angles?
Are 8 degrees and 16 degrees OK?

You were suggesting that a conventional pipe would be just over 1 metre in total.
Mine shows 1.3m just over the cones, disregarding the header pipe and the stinger.
the example pipe shows 580mm over the cones, but he uses a 400 mm header instead of a 250mm one, which is the recommended maximum length.
This 580 dimension would fit nicely into the everted cone layout from the previous drawing.

What effect does a very long header pipe have? This is the size across the bendy bit supplied. about 320mm.
how would this affect the other dimensions?

Is the length from piston to the end of the tail cone the dimension that is maintained, and take the extra length out of the centre
section.

The only difference is the example pipe uses 160 degrees exhaust duration, whereas mine is 180 degrees.
- a difference of + 12.5% on time.  - would that make the pipe 12.5 % longer?

My transfers are 154 degrees, which is a little to close to the exhaust for good blowdown.

I checked it on a rod angularity diagram. The rod length is 66 mm.

160 degrees  seems quite short in duration.

The comment was that  he raised the exhaust 3mm, so it probably wasn't a model aero engine.
- you might find that timing on a moped??
Degrees is not very useful in calculating volumetric efficiency, as the rod angularity makes a big difference.
you wouldn't think that 21mm was half way to 36.5mm.
the balance is only 15.5 mm


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## Steamchick (Mar 22, 2022)

Hi Owen, Of course, your knowledge is a bit beyond mine, so I am likely to be wrong in a lot of what I suggest, based on your better expertise. But I am enjoying bouncing ideas around, if it helps you work out a good solution. (But please feel free to tell me where to get off, if the journey goes the wrong way! - I'll not be offended at all).
Ignore my dimensions, I am just as likely to be completely "off-beam" with my assumptions, that will have affected the numbers considerably.
But to  reply to your questions:

_What effect does a very long header pipe have? This is the size across the bendy bit supplied. about 320mm.
how would this affect the other dimensions_? As far as I understand, the reflected wave relies on the total length from exhaust port to reflection point. This has been advised as the joint between the diverging cone and the converging cone.... SO: A Long header makes for a shorter diverging cone, and a very short header makes for a longer diverging cone.
I don't understand how to calculate the diameter of the large end of the diverging cone.
In my head, I think the Concentric cone arrangement you have drawn is good. BUT - I think the reflection point would be the end face where the gases double back on themselves.
I have been trying to consider what difference a flat end makes, compared to a short converging cone. My memory is inaccurate, but in the Motorcycle club at Uni, we had some pretty intelligent discussions (some were studying this in Mech Eng as their projects). The idea was that the diverging cone expands, cools and slows the pressure wave and exhaust gases. This gives a shorter pipe than if the header CSA alone was continuous to the end of the exhaust. (Simple straight pipe exhausts of pre-WW2 racing). The "reverse cone" - the Converging cone - generates the reflection of the pressure wave - to stuff some gases back into the cylinder by getting back to the exhaust port before the port closes. A Cone, will give some variation (caused by revs, throttle, Ambient air cooling of the diverging cone, etc.) so the "tuned pipe" was more functional across of a range of engine speed and load and not so "Peaky" = exactly tuned to a specific load and engine speed.
Considering the concentric conical chambers: (A thought experiment). The shock wave is travelling at "sonic" velocity for the residual pressure in the exhaust from the previous exhaust gas pulse (I.E. gases that are still exiting via the tail-pipe: => Faster than in "Air"). As it expands into the diverging cone, it slows, as the pressure drops along the length of the cone. When it reaches the reversing end of the concentric cones, there is a clear reflection wave that will travel back to the exhaust port. If the inner conical part is made a bit longer, this will be less peaky in tuning. Because of this, I can see no reason to have the "reversed cone" of the converging outer cone. So I would make the outer chamber parallel OD at 110mm diameter. In effect, this outer annular chamber would then be a continuation of the diverging cone, thus dou=bling the length of the diverging gas column, and giving a second reflection wave when the gases reach the "front" end, where the discharge pipe is fitted. This second reflection wave, should travel back to the exhaust port at something like twice the time of the first.... so if the engine is running at "half speed" the second pressure wave will "tune-in"...(?). Thus giving a possible tuned peak of performance at half of the 6300rpm...? = 3150rpm? - Any use for "cruising" engine speed and load?
Of course, I may be completely off the rails with these ideas...
K2


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## CaneSelvatico (Mar 22, 2022)

Hi, based on my knowledge and experience I think the folded cone won't work very well.

To explain my conclusion I'd like to clarify the internal physics of a tuned pipe
First, you have to consider two properties as separated: gas pressure and waves.
You want to get rid of all the exhaust gas pressure before the next cycle begins and that is controlled by the diameter and length of the stinger and muffler (and you can place it almost everywhere without much effect on performance), so basically it only depend on your target rpm. the pressure in the exhaust should be back to the starting point every 360deg.
The waves are a bit more complicate to manage, and travel at the speed of sound which is affected bay both the gas temperature (450-600C ) and pressure (pressure is usually ignored).
The waves are what do the work of extracting more exhaust gas and push back the fresh mixture that get overblown out of the exhaust port, so de duration of the waves effect depend on the time duration of the exhaust port opening

Ex. rpm=6000 
exhaust timing=150deg
6000rpm= 10ms*360deg
exhaust opening time = 4.16ms
so all the last positive pressure wave should reach back to the exhaust port 4.16ms after it started from it, using the sound speed of 570m/s (@~530C) or 57 cm*ms the total length would be 118.5cm (3.8ft)
Higher rpm = shorter pipe
wider port timing = longer pipe

Cones and lengths.
The header length determines how long before the first negative wave (generated by the diverging cone) start to have effect in the port area.  Short header= faster action / Long header = Slower action
The diverging cone generate a negative pressure wave that lower the pressure pulling the gases out of the cylinder faster.
Steep cone=higher pull / narrow cone = lower pull //
long cone = longer effect duration / short cone = short duration
usually here you use a longer and narrow cone that gives you a bigger working band and more pull duration.

The converging cone work the same way but generate a positive wave that push the gases back, so you want its action to be of a shorter duration and at the last moments of port opening (short and steep cone at the max distance from the port)

The cylindrical middle section is only used for space requirement as you could have the two cones back to back but usually as the cone widens the wave intensity decrease so after a certain point is not as effective. It also help to reduce the sharp change in wall angle

--------
 Hence my conclusion is that a a folded back cone will act almost as a straight wall giving a very powerful wave but of an extremely short duration (that could be totally ineffective due to gas inertia).
But maybe some experimentation would prove me wrong

Hope this would be of help
Cheers, Riccardo


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## Bentwings (Mar 22, 2022)

Owen_N said:


> I ran a pipe spreadsheet.
> The conic sections look ok.
> inlet chosen at 1 inch.
> 
> ...





Owen_N said:


> This particular engine may suit a semi-tuned pipe.
> The conical expanding part is ok out to around 60-70 mm in diameter, but a 6000 rpm pipe would be too long.
> If I got a good torque build from 5000 rpm, I could spin up to possibly  7000 rpm, depending on the torque curve.
> 
> ...


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## Steamchick (Mar 22, 2022)

CaneSelvatico said:


> Hi, based on my knowledge and experience I think the folded cone won't work very well.
> 
> To explain my conclusion I'd like to clarify the internal physics of a tuned pipe
> First, you have to consider two properties as separated: gas pressure and waves.
> ...


I tend to agree with you, from my limited understanding of what is really going on. The thoughts were that the doubled-back gas flow would enable the overall length to be effectively halved. - The >1m designed system is just too long for the fuselage. so 1/2 ~1/3rd of that physical length is what Owen wants to design.
Having read your explanation, what concerns me a bit is the idea of a small tail-pipe (as shown by Owen's diagram?)? Considering that the gas will have expanded from (say 60ccs at 3 bar at exhaust port) to less than 0.4bar above atmosphere, then surely it needs a big hole to escape in a short time through such a small hole? I would have expected a larger hole that the header pipe? - even something like 4~5cm dia? - Then the pipe would be able to get to atmospheric before the next pulse of exhaust when the port opens after the next bang?
I'll admit to never having designed IC engine exhausts, but I am curious, as I have designed an effective silencer for a 6l cylinder evacuating to atmosphere from 28bar. (A deep "Boom!" at 136dBa at 1m from the exhaust with the previous silencer/expansion box - my silencer achieved below 125dBa. at 1m from the exhaust, with a higher frequency "crack" sound.). But that was for a High Voltage circuit breaker that was 1.5 miles from the nearest habitation...
K2


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## Owen_N (Mar 22, 2022)

Combined reply:

1) The offset double-reverse-cone reflector is intended to reflect a plane-like  sound wave into the outer chamber.
The main gas flow is moving a lot slower, but may create a weak reflecting wave at that point.

2) I have come up with a double-fold with a centre section that has a constant cross-section.

3) the very narrow "V" between the 2 shells probably makes a very poor reflector, but may be good at accelerating the body of the main gas flow.
The outer chamber of 3 should end with a short cone about 32-45 degrees to the inner shell.

4) the gas will be getting quite cool by the time it gets to the outer shells, so velocity of sound will be slower, and effective length will be more.
The current cone-part length of the double- folded system is around 750mm.
It wouldn't take much to get it to 1000mm.

Predicting what it should be is tricky, but probably less than 1330 mm.

5) I have found that the shell temperature of a sudden exhaust expansion settles around 140 degrees C, so the bulk of the chamber with a controlled expansion will be much cooler.
What temperature and pressure combination is contributing to 1700 ft/sec speed of sound?

6) The exhaust chamber has a full half-cycle to evacuate. With an controlled expansion  of 20x, the chamber pressure
and density should already be pretty low, plus it retains a fair amount of velocity, so it should shoot out of a 14mm hole ok.
The velocity disappears if the gas is not channelled out, but extended and offset stingers seem to work, so
it is probably not a big problem.  It will reduce evacuation efficiency.

I calculated that the discharge pressure from 10:1 compression should only be around 20 to 30 psi.
(adiabatic expansion formula).

From the amount of noise it makes you would think pressure would be over 100 psi- rather like a .22 short machine gun??

A 400cc 4-stroke has no problem with about a 20mm outlet choke point, and the velocity is quite high.

A 60cc two-stroke should be quite happy with a 14mm outlet tube.

I don't know why they want a 6 inch stinger tube, though.  Maybe there is some gas slug inertia effect,
helping to get a lower evacuation pressure.


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## Owen_N (Mar 22, 2022)

Here is the new version of the folded pipe. this gives about 830mm over the cones, plus the 320mm header pipe.
any ideas whether this would work at 6300 rpm, 180 degrees duration?
Any reverse pulse just before transfer close would be beneficial, as long as the pulse lasts for at least 30 degrees, until the exhaust closes.
I presume a standard reverse pulse is either very precise, or fairly long duration.
Maybe worth a try?
I will check for jobbing sheet rolling services.
I have some 0.2mm stainless , but I couldn't roll it up.


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## Owen_N (Mar 22, 2022)

Improved reflector angles to  stop back-reflection from initial wave front.
The previous designs allowed some of the wave to reflect back instantly.
looking at it, two stages in the outer cone would be better- 45 degrees, then 30 degrees.
What do you think?
Can any direct reflections escape back down the centre cone?


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## CaneSelvatico (Mar 23, 2022)

Steamchick said:


> I tend to agree with you, from my limited understanding of what is really going on. The thoughts were that the doubled-back gas flow would enable the overall length to be effectively halved. - The >1m designed system is just too long for the fuselage. so 1/2 ~1/3rd of that physical length is what Owen wants to design.
> Having read your explanation, what concerns me a bit is the idea of a small tail-pipe (as shown by Owen's diagram?)? Considering that the gas will have expanded from (say 60ccs at 3 bar at exhaust port) to less than 0.4bar above atmosphere, then surely it needs a big hole to escape in a short time through such a small hole? I would have expected a larger hole that the header pipe? - even something like 4~5cm dia? - Then the pipe would be able to get to atmospheric before the next pulse of exhaust when the port opens after the next bang?
> I'll admit to never having designed IC engine exhausts, but I am curious, as I have designed an effective silencer for a 6l cylinder evacuating to atmosphere from 28bar. (A deep "Boom!" at 136dBa at 1m from the exhaust with the previous silencer/expansion box - my silencer achieved below 125dBa. at 1m from the exhaust, with a higher frequency "crack" sound.). But that was for a High Voltage circuit breaker that was 1.5 miles from the nearest habitation...
> K2



I understand the limit that Owen have, as space requirement has always been an issue for 2stroke motorcycles (look at the crazy design of some multi-cylinder motorcycles)
As for the tailpipe I didn't explained myself very well, given also that I never gave to much of a thought about that, making the stinger diameter always between 1/2 to 3/4 of the header diameter only basing it on "feel".
But the idea is to keep a backpressure higher than atmospheric but lower than the exhausting pressure, permitting an almost constant flow keeping the pressure and temperature of the gasses inside also more constant (helping also with silencing).


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## CaneSelvatico (Mar 23, 2022)

Owen_N said:


> Combined reply:
> 
> 1) The offset double-reverse-cone reflector is intended to reflect a plane-like  sound wave into the outer chamber.
> The main gas flow is moving a lot slower, but may create a weak reflecting wave at that point.
> ...



I'll try to give my view on your points.
1)the double reflecting cone will cause a lot of diffraction nullifying the effect of the waves (the interaction is the same of waves on a river, that reflect and move in a completely different way than the water flow).
the gas flow will easily double back  into the annular passage even with a flat end as it follow the pressure difference and inertia.
2)the constant cross section could be useful for helping the gas expand and slow but wont affect the wave
3)as before the flow would have slowed considerably in the first cone but the local flow speed is not much influent on the pumping effect
4/5) 1700ft/s is at around 740F and is considered as average gas temperature. Obviously the pipe wall will be at a much lower temperature because while the gas it pretty hot is not very dense and will be expanded and expelled before having enough time to heat up the exhaust (same as inside the cylinder, the burning gas will be exceeding 2800F and aluminium melt at 1200F but the time is very short limiting the amount of heat transferred)
6) I think it might interesting to try eliminating the stinger altogether and just have one hole (or many small holes), but the sound might be too loud

I put here a few link of expansion pipes for Italian Vespas and moped so it might give you an idea of space constrained tuned pipes
the original are just baffled pot that give a lot of backpressure (10mm final diameter for a 50cc)
vespa original
this is for the 200cc and the header diameter is 40mm while the stinger is just 16mm
"tuned" original
this is an original modified to give a little more power but look inconspicuous
vespa tuned
racing exhaust for the same engine, and is all bent over itself in order to fit it under the frame

ciao moped original
pot muffler for 50cc  header 22mm stinger 10mm
ciao moped tuned
same engine but "snake" pipe to fit under the frame


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## Owen_N (Mar 24, 2022)

Here are the "before run" photo of the J-B weld changes.

This also shows the new head.

The small steel pins seem to have escaped, but top end damage was minimal. A few slight piston dings.
The swaged bush seems to be holding well.
I put a new ring in, so it will take a while to settle down.
It got through 7 minutes running with nothing alarming happening.
I will strip it after 5 x 7 minute tankfulls., and see how things are going.

The edges of the rear wall insert look sunken, but they are flush to the touch.
The side transfer length increase has given no problems.
You can go up to 90degrees of port if there are sloping edge details in place.
The slopes I am using seem to be enough.
The profile around one end of the transfer is larger, but that part of the port is blocked with J-B weld.
the slight chamfer there is good for ring anti-snag.

The compression release hole has opened up again. It was only blocked with zap fast glue.
The engine has a slight sound of 4-stroking at 5,000 rpm, but I think the odd sound could be due to the
compression release hole.
This 2mm hole probably doesn't waste a significant amount of power around 5-6000 rpm.
I checked the area for heating, and it seemed steady around 155 degrees C . Not unusually hot.

It runs a little oddly at present, but I expect it to be more consistent and tuneable once the ring is run in.
It needs a counter weight on the offset muffler mount.
It was quite steady before I cut the fins out of the head, making it about half the weight.
the muffler is angled up about 60 degrees to horizontal, and has its own board.
The previous muffler mount had a counter weight as well- about half a pound,- a spare panel beating backing block??
the twin didn't need a counterweight, but its vibration is in a different plane.
I expect this one has mainly a rotating imbalance. around the propeller axis.


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## Owen_N (Mar 24, 2022)

CaneSelvatico said:


> I'll try to give my view on your points.
> 1)the double reflecting cone will cause a lot of diffraction nullifying the effect of the waves (the interaction is the same of waves on a river, that reflect and move in a completely different way than the water flow).
> the gas flow will easily double back  into the annular passage even with a flat end as it follow the pressure difference and inertia.
> 2)the constant cross section could be useful for helping the gas expand and slow but wont affect the wave
> ...


I worked my way through the adiabatic calcs, based on 70 bar at 10:1 cr,
and the peak temp seemed to be around 1300 deg c,
discharge temp 140 deg c,
discharge pressure 20 psi. (gauge).

Where do the higher pressures and temperatures come from?

You won't get excess heating and erosion of four stroke exhaust valves at these conditions.


I am sure you won't get 393 deg C in the muffler unless you are running on the limiter, and have a lot of unburned fuel,
or excess fuel is burning in the pipe.

Under adverse conditions, temperature can get over 650 degrees C - a nice red glow for steel.

Other factors causing non-adiabatic behaviour- condensation of ionised mixtures, formation of compounds at lower temperature and pressures, strongly delayed flame front- usually only seen in Wankel engines at very high rpms.

Does this effect actually reduce output power, or just spread chamber pressure over a wider range of piston motion?
- rather like a steam engine.

I have grabbed you references, and will have a look.

It looks like I will have to buy my own rolling machine- fairly cheap, but the centre roller looks too large.
I will look on YouTube.
How do I make cones to such small opening diameters?


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## Owen_N (Mar 24, 2022)

Well, still adiabatic, but the heat capacities given are not so good for actual combustion.
Cp and Cv  are not constants.
There is probably a lot more heat energy than indicated by the peak pressure.
This could be found independently by measuring the amount of petrol in grams and using the calorific value.
I think around 10:1 is the usual mixture in mass ratios?
I could then compare the values for adiabatic work, torque, and theoretical efficiency.
Does the 30% power, 30% cooling, 30% out the exhaust, really hold in this case?
Specific fuel consumption comes into this, too.  2-strokes use about 30% more fuel than 4-strokes for the same power.

Back to the current engine mods.
Maximum RPM seems to have settled at 5000 rpm, so I will reinstate the boost ports in a different way.
I think I can feed them from the transfer chests, in passages under the barrel internal surface.
The rear of the side transfer chests can be extended a bit, as well.
The actual bore of the boost ports outlets will be restricted to 5mm diameter, angled 60 degrees.

I will strip the engine tonight, cut the new passages, J-B on some new passage covers, 
and photograph the J-B barrel and piston wear tomorrow morning.

My Notebook screen seems to be failing, so I am using an old notebook and my one-drive hookup
for now.


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## Owen_N (Mar 25, 2022)

Here are the "after" photos.
There are no problems with adhesion with the J-B weld.
The main problem is that aluminium wears too quickly sliding over the J-B Weld.
I will sand back the affected parts of the piston, and build it up with many coats of Caliper paint.
This may stay put, is hard, solvent proof, and will protect the aluminium.
This is a matte ceramic-based coating, and may wear badly on the J-B weld as well.
I really need a spray-on adhesive dry lube coating. 
I will look for some.


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## Owen_N (Mar 26, 2022)

Now here is a totally speculative mod:

I have embedded 3 pieces of raw PTFE sheet in the piston thrust face.
This stuff is quite soft and easy to cut with a scalpel blade.
Dimensions 3mmx13mm.
These are not enough to take the full piston thrust, but I am hoping for a "Burnish-in"
effect, where the PTFE gets burnished into the rest of the piston face and the cylinder wall, including a bit of 
J-B Weld surfacing.

"Before" photos to follow, tomorrow, after I shape and finish the piston skirt.

I will arrange for slight insert protrusion, and a light push fit for the piston into the barrel.

These should stay in place much like o-rings in a groove.

The piston recess holes have 45 degree sides, and the inserts are an arc of a circle.

I noticed that they have low bridging strength, so I filled in behind then with J-B weld as well.


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## Owen_N (Mar 27, 2022)

Here are the "before" photos.

I ran it, and it didn't blow up in 7 minutes running, so this mod appears to be safe.
Lets see if the piston mods are good for multiple hours running!

The engine didn't run so well, but symptoms are of poor ring to cylinder seal, so that should improve on the next run.
(inconsistent mixture requirements, not wanting to rev continually).
The photos also show how I am feeding the boost ports.
There are a couple of oil leaks through the J-B weld edges, but no serious air leaks, so I will leave that alone for now.
There are places on the piston inserts where I can add some more PTFE strips.
I will refresh the strips on the next rebuild, see if it made any difference to wear, and add more strips.
If the lower cylinder wall on the cold side was more flush and not built up with J-B, then that would cut
down on wear.

However, just refreshing the PTFE strips each rebuild may keep this engine running well enough for more mod tests.
I want to see if higher transfers combined with reed valves makes any difference, and if it will make enough power
at the 6000 rpm point.
<edit>
After run 2, it seems to be running tight and seizing, from about 4000 rpm continuous.

That is bad for the exhaust side skirt wear. I can feel tightness on the first turn, then it frees up.

Seizure may be caused by the expansion of the PTFE with heat.
Also, possibly the tightish initial fit is being made worse by piston expansion as well, and a lack of "give" in the PTFE.

It will develop very high internal pressure before extruding into the clearance gap.
This does show that the PTFE  is staying put, and acting as a bearing.
The rotation resistance will be coming from excess pressure on the exhaust side skirt.
Very likely there will be seize marks. 

I will try running in more gradually around 3500 rpm for several runs, before trying for more rpms.
Hopefully it will settle down, then I can do a check-dismantle.


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## Owen_N (Mar 28, 2022)

Plan for a modification to the piston PTFE sliders.

1) PTFE on steel, lubricated ; Cf is around 0.04.( static and dynamic).
This compares well with the aluminium on steel-style lubrication of the piston skirt.
This is mixed boundary, dynamic, and squeeze lubrication, which varies from 0.1 to 0.001
for the dynamic coefficient of friction.

Maximum piston side thrust, assume 15 degrees rod angle, 50bar, 45mm diameter piston
sin 15 = 0.26
area = 1.6 x 10^-3 sq mt.
F = PA = 50 x 10^5 x 1.6 x 10^-3 x 0.26  = 21x 10^2 N, or 2 kN
PTFE area = 26 x 3 x 10^-6 = 78 x 10^-6
PTFE pressure = F/A = 2 x 10^3/ 78 x 10^-6 = 25 MPa
this is pretty high.  Recommended maximum is 3 MPa.
<> With 4 strips, this drops to 12.5 MPa

I would suppose that PTFE on steel is only limited by excess heating and the pressure needed to start extruding the PTFE .
This example would be towards the low end, say 2 MPa compressive yield strength,
for general specs.

However, to extrude the material into the clearance gap would be similar to the failure of a rubber o-ring,
which would be into the 100s of MPa, I think.
They are quite happy at 5000 psi, or 35 mPa.
The UTS of nitrile rubber is probably a lot higher, but general softness or resistance to indentation, is lower for the rubber. Modulus of elasticity is also higher for PTFE.

This is sort of apples and oranges, as PTFE takes a permanent indentation, and rubber does not,
with similar pressure applied.

I will run the engine for 5 more cycles, and check for permanent deformation , wiping,
and wear.
(I can increase actual support by 2 more inserts 3x13 area.)

On the exhaust side, the inserts need to be oriented vertically, with square ends.
Otherwise they will fall out into the exhaust port.
They are spread about 50 degrees apart.

I can fit 4 vertically, plus one as a skirt bottom support.

This gives maybe about 6MPa, as it is the low thrust side of the piston.

the J-B weld will take  ult = 27 MPa, but not for normal loading.


I will report back on the "after" state of the inserts.

If it is looking good, I will take it another stage.
.
The current problem is that the exhaust side skirt is not lubricating well with a constant pressure on it.
It needs a little clearance to bring in lubricating oil.

This raises the problem in that clearances are fairly ideal without making room for the inserts.

Both flanks need to be reduced by 0.3 or so mm to allow the inserts to stand out a bit, and still
fit in the bore.

However, the outer perimeter of the exhaust side skirt is important in sealing crankcase pressure, so
should be left in place. A flexible rim would be nice, to take load off the lip, but
I can't figure out how it should be attached.
However, excessive lip wear can probably be repaired.

An o-ring format for the inserts is not good, as they need to be vertically restrained.
I presume that end location of 3x2mm area is enough given potential side forces of
1 kN (say)  x 0,04 =40N, area = 6 x 10^-6, <> x 4 ;P = F/A = 1.8  MPa.

Shear stress in the insert at 3 x 13 = 39 x 10^-6 , F/A =  40/39 x 10^6 = 1 MPa, <> /4  = 0.25 MPa.

I shall just have to see how the inspection goes.

Drag engines seem to get away with 3 round buttons of a harder material in the thrust face, which must have quite high stress in the insert holes. These are larger pistons, too, -around 75mm diameter.
and greater con-rod angularity, plus running nitro and/or supercharging.
These are not intended for continuous output, however.
<> means a later edit.


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## Owen_N (Mar 29, 2022)

I Found the cause of the engine problem:
a damaged ring land and a tiny crack in the piston in the same area.
Just enough to overheat the piston.

Seizing is on the cold side, where the damage is. 
The exhaust side skirt looks good.
It doesn't look like the PTFE inserts have any effect.

This engine is retired until further notice.


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## Owen_N (Mar 29, 2022)

Further notice:

The engine looks fairly OK, but the ring is a bit scored.
Ring gap is still around 0.3mm, so I gave it another run.
I discovered that the carb pulse tube had a hole in it, so that could be causing lean running, more seize-up heating, and hard starting.
That also seems to cause the rpm racing effect.

Minimum idle seems to be about 2300 rpm. It used to be lower before any mods.
I will run it at that for a while and see if the ring improves.
I honed the bore, and it is looking OK.

I chamfered the bottom of the ring groove about 15 degrees each side of the land damage.
I must have dropped a tool tip in there. It only goes halfway into the groove.
this stops any blowby being channelled to a specific spot.

I have flooded it now, so I pulled the plugs and and letting it dry out a bit.


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## Owen_N (Mar 30, 2022)

More problems- J-B weld getting into the crankcase is a bit like grinding paste.- probably the source of rapid piston and ring wear.
All my rings are too gappy now, and won't start.

I will order some more rings  and some pistons. They are fairly cheap.

The  wrist pin, thrust washers, needle rollers seem to be doing ok.
They take a while to get here ex Singapore.
I ran a couple of tissues around the crank cavity, and one removed heaps of fine powder.
I will give it a rinse out before more running.
I think barrel tolerances will still be OK , despite several hones.

That J_B weld on the back of the piston was put on with too much haste.
Possible sandblasting or some kind of surface strip should be used.
I will look for a micro jet sandblaster??


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## Owen_N (Mar 30, 2022)

It looks like Torque-pro is not supplying parts anymore, so I have bought a chainsaw top end that is also 45mm bore, 52cc.
This looks like it could be adapted to my crankcase.

The TP60 will have slightly more stroke, so a base spacer will be needed.

Not to bad a price- ex Canada. This should arrive about 10 April.
This also gets around TP-60 dodgy piston cutouts.

I also get two rings, so it is not so sensitive to ring wear.
Here are the buy details.
the stud centres are similar.

This is a flat-top piston, rather than my dome piston, so the head swap may be a problem.
There is plenty of material I can cut out of the head.
I will need longer studs.

This looks like a desirable direction to move. There are Husqvarna models that are similar, so plenty of parts.
I can also switch to a chainsaw bottom end if I want to, to get a double-ended crank.

I was thinking about moving to 125cc motorbike engines, but an old bike would be $4000 nzd, plus I would need a complete road bike as a donor.- 
rather expensive.
Engines by themselves are only on ebay uk or us, which has high freight- probably $500 nzd shipping cost.
for something that size and weight.  - likely 15-20 KGs??

Chainsaw parts are a lot cheaper, but I need a clutch and gearbox.


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## CaneSelvatico (Mar 30, 2022)

Sometime you need to start off a blank page... could be the good one.
As the PTFE and JB-weld insert I think they could have contributed to overheating because of the very poor heat conductivity compared to aluminium, as the piston get mostly cooled by its contact with the cylinder wall (lacking the full cycle of exhaust and intake of 4 strokes)


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## Owen_N (Mar 30, 2022)

This looks like it could be a little tricky, as the chainsaw piston appear to have different wrist pin boss details:
11mm diameter,  not 10mm, and bottom-located rod, not top-located rod.

Some messing about is needed to locate the top pin and space out the piston.
possibly I can bore the pin bosses out a bit more and machine up some inserts.

Also, stud holes are at 47mm spacing, not 45mm, so hole ovalling is needed.

Stroke goes from 32.7(52cc)  to 36.5 (60cc), so there is a bit of fingers crossed that there is enough upper bore length for exhaust timing.
I want to keep just below 180 degrees.
Around 178 degrees seems to be normal for this bore size and capacity.
Normal street motorcycle engines go up to 180 degrees for 80-100 cc capacity.

The lower transfer pockets need some changes, and the barrel extension is likely too big in diameter as well.
- full dismantle and machine of cases may be needed.

Possibly a 5mm spacer and some exhaust port adjustment may do.

The piston has side holes, so a similar setup to the TP60 engine may work, where transfers are fully in the barrel section.
In any case, the side transfer pockets are limited to the depth of the main base flange on the crankcase.

This barrel seems to be a lower tune than the TP-60 one, so some mods may be needed.
I can run it fairly standard as a benchmark. 
Probably will turn more like 5000 rpm rather than 6000 rpm with my test prop.


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## Owen_N (Mar 30, 2022)

CaneSelvatico said:


> Sometime you need to start off a blank page... could be the good one.
> As the PTFE and JB-weld insert I think they could have contributed to overheating because of the very poor heat conductivity compared to aluminium, as the piston get mostly cooled by its contact with the cylinder wall (lacking the full cycle of exhaust and intake of 4 strokes)


It seemed to be doing OK on the exhaust side.
Gritty bits in the oil possibly increases heating by blowby and poor ring seating.
This may have contributed to more piston and barrel wear on the cool side, even when it wasn't seizing up.

A totally new piston and barrel ex the chainsaw plus a good rinse-out should help.

The actual brand of the chainsaw that the parts are for is a bit of a mystery.- generic Chinese, with several brand names.
It  looks to be as good as the Husqvarna and Stihl rebuild kits, though.

Engine Builders don't like putting 2 rings on race engines, but Hus and Stihl only use one ring, to be cheap.

If you have one ring, it shouldn't be 1.5mm or 2mm wide, high spring tension.

1.2mm rings are adequate.

The 2-ring piston probably uses 1.2mm rings.

Carburettor problems didn't help.

I don't think the pulse pumping was working at all, and it was running on the low end jet only.

Me blowing a bit of fuel through the carb with a syringe as a prime got all the air out of the carb passages.

This meant the engine was chronically lean at any significant revs.
Not good for an air cooled two-stroke.
<edit>
The insulating effect could be in play with the rear wall insert. It is fairly isolated from the rest of the cylinder by J-B Weld.

This is normally two long slots, and not an area of conductive cooling.

It could have been mixture-cooled as for most of the piston underside.

The holes at the sides of the piston help with this.

With me filling in the piston slots and rear wall slots, more heat could be coming from the piston skirt, and making the area too hot for good lubrication- hence the severe wear. There is no external finning at that spot.

Quite a few engines have "full" skirts, and they rely more on conduction through the skirt to the cylinder wall for cooling.

Other engines have a "Skeleton skirt" for the bottom half of the rear skirt.

The top half acts as the thrust face above the transfer ports, and the bottom is cooled by incoming carburettor mixture.

The skeleton extension helps counter piston rocking, and helps maintain the exhaust-side minimal clearance.

This clearance is not too critical, as the exhaust side is the thrust side on compression.

The engine can become more noisy with noticeable piston slap, if general clearances are over about  0.3 mm.
0.1 to 0.2mm is a better range.
It is more of a metallic tapping sound.

That won't stop it running. The piston ring seal and ring gap are more important.

You do want some actual suction at the carburettor.
It the engine is not so good , it may only start with fast electric cranking.

Mine has been like that for the last few starts, and now the ring has "gapped up" to the point even that does not work.
"New Ring Time".


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## Owen_N (Mar 31, 2022)

Possible schemes for reworking the piston pin bore:
--------------------------------------------------------------
1) I don't have tooling to face off the bosses inside the piston, accurately.
2) any bore should be reamed to size. I think I have 10mm-12mm adjustable reamers.
I can do up to 15mm if needed.

3) what side thrust is to be expected from top rod location?

4) scheme 1: 
 Shouldered bushes pushed in from outside, with  separate cap plates, and 2 sets  2x m3 retaining screws.
The cap plate can also locate the piston pin.

The retaining screws can be locked with J-B weld. That should take the heat.
I am a bit iffy about high temperature locking fluids.

The separate caps allow some lateral clearance in the main bush without passing loading on to the screws.

There looks to be enough material for the screw threads.
Then I can put the side shims back in the usual place.
These just take up wear from the needle roller cage.
I can hand-fit the side bushes for a good fit on the rod eye.

I think I can turn the bushes on my micro-lathe.

The chuck is a bit miserable, though. It doesn't have a lot of grip.
I will need to make up rod stock from larger rectangular pieces, and turn both ends to get it to grip properly.
The chuck is not all that central, so turn, bore, part have to be in the same set.

Getting a nice bore on the piston pin hole is a bit trickier.

I need to make up an adjustable fixture I can bolt to the existing lathe chuck.
I have done similar face plate fixtures before, but this one needs to be fully adjustable and really square.
maybe 3-way radial flats and clamp bars.
The radial flex in the micro-lathe should be small enough for fairly good results.
The thing is a bit springy, but nowhere near as bad as my drill press-that is pretty chronic!


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## Owen_N (Apr 1, 2022)

Topic variation:
Building a 125cc watercolled twostroke engine.
1) I can cut the gearbox off a 125 honda 4-stroke engine.
*******I need to get a tig set that can handle aluminium.****
Engine cases can be tig welded, if they are  lined up correctly.
Mr Millyard has demonstrated this.

2) I can probably line up a gear primary drive well enough to tig weld it.

It is best if case parts can be lined up against and existing built-up crankshaft with large ballraces,.

3) A chainsaw engine case set could be used, and the chainsaw engine crank can be adapted to the existing crank gear, which
I think is not machined as part of the crank.
Entry point of the carburettor can be a bit odd on some chainsaws, which affects how I can mount the engine.

4) can eutectic alumium be tig welded?

5) what is a good material to make the barrel and waterjacket from?
It does not need to be high silicon eutectic, and uses Nikasil coating as a bearing surface.

6) I need to get my lathe/mill combo first, for barrel boring and other machining.

8) big blocks of aluminium are expensive, and I would have to pay $50+ freight to get them.

A 150 x 100 x 150 block is 2.25 x 10^-3 cu mt, den = 2.7, mass = 6.075  kg.
I have found that freight coasts about $50nzd per kg, material $10 nzd/kg, adds up to $364.50 nzd.
This may be a lot cheaper rough-cast from ingot in nz, and freight will be a lot lower.

9) I don't want to get into "lost wax casting", as it needs a lot of equipment, and you make a lot of bad castings learning how to do it right.
Patterns can be 3d-printed, if you cook the mould hot enough to get all the plastic out.- maybe 350 deg C??

10) could the exhaust passage, barrel, and water jacket be welded together from individual parts?
The would need to be machined as "split" and joined.

11) I can get rough-cast machinable blocks done using sand casting, and machine them out.
They are likely to be a bit porous.  Is that important?

12) I can design my own water-cooled head, "puck" and barrel set, but I needed a real example to copy.
Maybe buy an old engine off ebay?  - freight is a problem.
There are lots of fine details that need to be measured and examined.

13) I don't really need an exhaust power valve, as this is not for track racing.
If I make the exhaust with two bridges, and timed at 190 degrees ref the stroke bottom, it doesn't really need a power valve.

14) I can copy parts of the "2Stroke-Stuffing" 50cc barrel as a design source.  The exhaust bridges can be very narrow if they are recut after adding the nikasil coating and boring. This has some good ideas on exhaust and transfers.
in particular, the front transfers are angled at the top and quite low, to allow more exhaust area.

His initial drawing of the rear transfers is not quite right. both sides of the port should be parallel as they enter the cylinder.

"He" has not fully tested and developed the wide-port barrel yet, so there will be lessons to learn.

I will need a chain-driven dyno-engine load. An eddy current system sounds good, but 50-60 hp raping means it would be quite large,
and costly.

15) it is probably best to use a piston with a full skirt, but with external piston pin holes.
This limits the exhaust a little, but the fancy screw-together pistons are only being made by Mark Atkinson, and they are quite experimental.

Under-piston cooling could also  be rather experimental, as many race engines use flow-through pistons for cooling in this area.

I suppose I can start shopping for required equipment.


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## Steamchick (Apr 3, 2022)

On a 250 cc MZ motorcycle , that I in a 'tuned' non- standard condition, there was a pair of very narrow rings high on the piston, and a 3rd transfer port through a hole high on the back of the piston. This gave a jet of transfer gas from the back to cause more turbulence in the combustion chamber - added to the transfer draughts from the symmetrical side transfer ports. It was also suppose to aid piston cooling.
K2


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## Owen_N (Apr 4, 2022)

Steamchick said:


> On a 250 cc MZ motorcycle , that I in a 'tuned' non- standard condition, there was a pair of very narrow rings high on the piston, and a 3rd transfer port through a hole high on the back of the piston. This gave a jet of transfer gas from the back to cause more turbulence in the combustion chamber - added to the transfer draughts from the symmetrical side transfer ports. It was also suppose to aid piston cooling.
> K2


Race-car rings seem to be getting down to 0.8mm.
I wonder if they are spring steel with chrome facing?  It seems very thin for cast iron.
Thinner rings are better, as ring surface force and friction depends on the ring width.

With a reed valve engine, the boost port can run from the reed chamber, and the hole in the piston is not needed.

Rotary valve engines can run a circular transfer gallery around the back side of the cylinder.
The boost port jet seems to be desirable.

With simulations, it seems to get a long way towards the cylinder head centre, and beats the main scavenge loop jets.
Have I posted the article with the simulations and AVL Fire yet?

I watched a YouTube article that found no difference in power output between one ring and two, when the rings are new.

I like the idea of two rings, as the period between ring replacements is longer, as the bottom ring wears slower than the top ring.

A downside of two rings is that if the top ring breaks, you are more likely to wreck the barrel surface, and mark up the head puck.

Broken rings do not seem to be too common, though.
If you have a race-prepped or motocross two-stroke engine, frequent stripping and inspection would be a good idea.

I have collected a vast number of You Tube videos to watch on diagnosing piston carbon patterns, 2-stroke wear and faults,
and general building up oddball engines from parts- mixing 2 and 4 stroke parts, making multi-cylinder engine from single-cylinder parts,
etc.
Regarding TIG, 300A ac seems to be a good target size, to do up to 8mm aluminium.
I suppose you turn it right down to do thin stainless.

I see I haven't had any replies on the slip rollers.  I will search more for roller diameter specs. That may be my next purchase.


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## Steamchick (Apr 4, 2022)

HI Owen.
Back in the late 80s when I worked in Engine Design, we had a couple of engines with cast iron rings, but the "new" engines had steel rings - made from wire, drawn and rolled to "ring" profile, then coiled into long "springs" - slit - ends re-shaped - Sized and gapped then Nitrided. The steel was NOT really hard like spring steel (120 ton), more like an 80-ton silver steel than a mild (40 ton). BUT I don't remember the spec. Try the "Perfect Circle" website for information... they are USA expert ring makers.
The outer face of the rings were slightly curved, so a point contact on the bore as new. The ends (last 30 degrees or so?) were slightly bent off round, as the intention was to maintain a constant radial pressure on the bore.... But I can't remember if this is a slight outward bend, or inward bend. I discussed the maths (algorithm) with the engineer... but my brain has forgotten some stuff in 40+ years.... I think a "square" rule...? Without this change of curvature, the spring radial pressure on the end of the rings drops-off and blow-by occurs.... I think? But get it wrong and the ends wear rapidly with the "end-of-ring" pressure being too high? => SO BOUGHT STEEL RINGS from a "Proper" ring manufacturer, achieve something you cannot replicate in the workshop. They are more efficient, unlikely to break, nitrided for hardness, and have curved faces which improves the ring lubrication and gives a smaller contact point. And the tan load (radial pressure) is more uniform around the rings, especially at the ring ends (and lower) than is traditional for cast iron rings. (Ring=> bore pressure and friction is significantly lower... of the order of "percentages"...).
Hope this helps?
K2


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## Owen_N (Apr 4, 2022)

1) Unlikely that I will use them for now, as I am using standard pistons designed for wider rings.
The next engine may use some Yamaha YZ parts, so I will see what they have.
The YZ125 crank looks fairly well thought out and proven.

2) I heard back from the low volume vehicle people, and a 2-stroke engine is OK in a low volume motorbike.

3) I have ordered a slip roll set ex UK - freight costs about the same as the price, but not to bad at $490nz in total.

4) This current engine layout will keep me going for a while, but eventually I want to get a suitable used top end and barrel unit to measure up.
I need to find the correct base stud pattern and what kinds of top-end gaskets are used.

they seem to use about 4x m8 at the cylinder base, and 6x m6 at the top, I think.
There is quite a step out to the base stud ring.

If copper ring gaskets are used, a lot of bolt pressure is required.
I would have that an inner "C" ring and an outer o-ring would work, with o-rings on each stud.
The centre sparkplug tower need one or more o-rings to seal the water jacket.
The "c" ring doesn't need as much pressure.

This is a big step up from the model airplane engines, which use 4x m5 on the barrels, 45mm bore diameter.
I am using exhaust cement on the top head joint, to avoid continually lapping in the head and barrel, and losing barrel height.


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## Steamchick (Apr 5, 2022)

Motorcycle engines - for normal road use - need low speed torque to pull-away, and mid-range torque for cruising speed. Although the adrenalin pushes us to ride as fast as we can, we cannot do so for traffic... etc. So the engines have lots of scope for tuning, but un-tuned, the low speed torque may be useful for you? In fact, semi -off- road bikes have better low speed tuning than "regular" road bikes...
Chainsaw and strimmer engines etc. on the other hand, are aimed at running at full speed, when the power is being used. So perhaps not so suitable for the variable power and torque running you need?
Hope this helps?
K2


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## Owen_N (Apr 5, 2022)

Steamchick said:


> Motorcycle engines - for normal road use - need low speed torque to pull-away, and mid-range torque for cruising speed. Although the adrenalin pushes us to ride as fast as we can, we cannot do so for traffic... etc. So the engines have lots of scope for tuning, but un-tuned, the low speed torque may be useful for you? In fact, semi -off- road bikes have better low speed tuning than "regular" road bikes...
> 
> Chainsaw and strimmer engines etc. on the other hand, are aimed at running at full speed, when the power is being used. So perhaps not so suitable for the variable power and torque running you need?
> Hope this helps?
> K2


Interesting idea-open a sector of the converging chamber cone at lower engine revs, plus  a larger secondary chamber.
This should reduce the blocking pulse. This is instead of having a power valve.
I think I read that Honda used a similar scheme?.
The errant blocking pulse tends to make them splutter and drop in power.

Good use of model airplane servos, arduino processor  to monitor rpms and send out a pwm signal for the servos.
(pulse width modulated)
-do the servo and other line need opto-isolators?
One of those grunty 10kg-cm servos may do the job?
what kind of rpm sensing can I use?  Possibly if I buy a small honda, all the dashboard stuff will need replacing as well if I have a different ignition module.
-I have all that  arduino and servo stuff here- I haven't learned how to use it all yet, though.

Mr 2Stroke-Stuffing thinks that his engines are too peaky to use standard gearboxes- though they seem to pull from 8000 through to 17500 rpms ok-
a bit of clutch slipping needed.
- that was my estimate just listening to the engine on the motorbike dyno, with the vario rubber belt drive.
A normal 5 speed box would work OK if quite a bit of clutch slipping was used. A wet clutch would stand up to that.
I don't know why racers like dry clutches- more precise slip characteristics?  They overheat very quickly.
Also, higher coefficient of friction- less plates- shorter engagement range?? 

You could stack a centrifugal clutch in front of the standard clutch, or just slam-shift at speed.- needs a solid dog-box.
Those small honda-style "flat" engines work like that.- up to 110cc.



A regular road engine will pull away 2500-3000 rpms fairly well. I had a hot cam in a 750 honda which needed these kind of revs to get going.
It wasn't all that noticeable.

The engine never really performed any better. It needed high-comp pistons as well to benefit from the cam.


----------



## Owen_N (Apr 6, 2022)

Discussion of adjustable geometry expansion chamber:

I want to investigate closable hatches in the expansion chamber.

1) Are there noticeable pressure pulses in the chamber?
2) Is a good chamber seal important? What percentage of seal can be sacrificed and still get a good reflecting wave?
I would guess a maximum of 2-3 sq mm could be tolerated- maybe a bit more.
A sound wave depends on a substantial pressure fluctuation over a very short time.
This wave is quite directional, so radial pressure leaks don't make a big difference.

3) What temperature are the parts subjected to?  - this could vary from 130 degrees C up to 350 degrees C intermittent.
4) Cross-section geometry is not that important. Parts of the pipe could have flats on them. Would a full square section chamber still work?
5) Would round butterfly valves in short tubular sections be adequate?  angled butterfly valves give a fairly good seal in a machined bore.
Half-circle overlap plates could also be considered.
The bearings could be stainless steel in bored PTFE bushes.  Stainless is not that stainless if heated and subjected to exhaust gasses.
It does make good car exhausts, though, so it maintains fair oxide stability.
6) How much of the surface area needs to move? does this directly relate to the chamber maximum cross-sectional area?
A 125 engine has a nominal 5.1 inch chamber maximum diameter, but it could go up to 6 inches or more.
Would opening up 1/4 of the perimeter in a rectangular section, or a round area, be enough to substantially disrupt the blocking pulse?
Would a smaller opening do the disruption as well?
7) Where should the opening be? a) in the straight section? b) half way down the end cone?
9) what is the effect of half the valve protruding into the chamber?  This will have a wave and flow diverting effect.

I did some Google searches, and only found one article from Thumpertalk, about having a secondary inner cone.
I would have thought that more people would have discussed this online.

Regarding the exhaust power valve, this would only have a minimal effect of the rpm when the blocking pulse would have its effect.
If you drop the timing from 195 degrees to 180 degrees, at 13,000 rpm, this is about 8% difference, or 1040 rpm.
Over what range is the blocking pulse normally effective? I have heard that power bands can be as narrow as 200 rpm for extreme tunes.
Opening the exhaust removes the blocking pulse, but also stops it interfering with the transfer process, so may eliminate the characteristic
"choking misfire" that these engines get.  You get a similar effect with long overlap 4-stroke engines.

I will try a few more different google queries.  Maybe the reference I saw was on 4-stroke exhaust tuning valves, but was unclear.


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## Owen_N (Apr 6, 2022)

B-port top profiles.

This is normally called the secondary transfer port, but  on some engines, they are larger than the A-port.
Here is a diagram.  The right-hand side is the one under discussion.
I would have thought converging the sides of the port to this extent would be bad, leading to unstable flow.
Note: the top of the port may be angle up to 25 degrees as well. This may be relative to a domed piston, about 8 degrees start of dome.
My idea is to have no more than 8 degrees total convergence for both sides in the plan view, and the cold-side wall of the port should be twisted to suit.
What is the general opinion on this?
There seem to be many ideas on all the top view (plan) angles for the A and B ports.

2Stroke-Stuffing seems to direct the leading edge of the b-port almost straight across the piston, and the a-port angles are parallel, and a lot flatter than those shown.
The leading edge of the A-port crosses just before the piston centre.
The objective is that all port flow converges a fair bit above the piston, and is biased towards the rear of the cylinder.
I would have thought that a sweep at a fairly shallow angle to the piston, from the A-port, would be good, but is does not seem to be necessary.
If the top angle of the transfer duct is too shallow, the rising flow is very unstable.
<edit>
the B-port form will make the stream fan out and get thinner.

We are spending energy on the fanning process, and losing it from velocity.

This means that the stream direction of interest is the average of the two port sides.

As drawn, this seems to intersect the line from the front of the A-transfer port.

The fanning action may be of advantage to:
1) thin out the vertical stream;
2) filling in to the sides and bottom of the chamber.
These shapes are the results of simulation and testing, so there must be a positive benefit.

Contrary to the A-port shapes shown, most other examples have a rounded top leading edge, and I see the
Aprilia port A is slightly lower than the other ports.

The Aprilia top view seems to be slightly biased more forward, as well.

I will go over a few port maps I have here, and compare timing, using a 2:1 rod to stroke ratio.
I would think that 2 exhaust bridges would give the straightest exhaust top edge, if we are going to cover about 160 degrees
with the exhaust ports.
The outer ports would be t-shaped.
I will post the last Aprilia image as well.
26mm out of 54.5mm for exhaust opening.
 I will check my diagram.
at 1.88 rod ratio, this is 206 degrees exhaust. this is a fair bit.
inlet is 142 degrees.
The port bottom shows at 56mm, and I would expect 54.5mm, so the top of the cylinder may be higher than the top of the piston.

Bore is shown as 53.8 rather than 54mm, so I am not sure whether this reflects that stroke will be more than 54.5mm.
anyhow, this gives me a rough idea.

2Stroke-Stuffing(2SS) is only using 190 degrees exhaust, but the difference in volume may need more timing.- he is 49ccs vs 125ccs.

Anyhow, Aprilia timing is only a rough guide. Similar output may be gained at slightly different timing.

I want to see how 2SS gets on with his wide-exhaust barrel.
His secondary barrel does 20 hp at 17,500 rpm, which corresponds to 40hp at 13,000 rpm for 125cc on pump gas.
Aprilia claim 55, so he has some way to go.

He is working on the supercharged 50cc for now, so it may be quite a while before he gets back to the other engine.

I would think turbocharging would be better and easier for a two-stroke, as the supercharger is robbing a lot of power.
If you tune for 12,000 rpm on the turbo, you may not get a big power band.
Finding a very small turbo could be a problem.  The IHI ones  go down to 600 cc- quite a lot larger!.

The back-pressure of the turbo helps the engine run as normal, but at higher pressure.
Boost and power are fairly unlimited with a turbo, but strictly limited with a roots-type blower.
- limitations on intercool , detonation, heat, strength of engine.


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## Owen_N (Apr 8, 2022)

Cost contributions and solutions for a high-rpm 125 two-stroke:

1) As a crate engine, these are really expensive! - maybe $3500 usd??  Also, USA sources seem to have unusually high freight costs.

2) crank, con-rod, piston pin seem to be up-rated a lot from equivalent chainsaw parts to handle higher rpm loads.
They still only pull 25m/s avg at 14,000 rpm, though.
These parts should be bought ready made, as they are proven components.

the piston can be bought with a skirt coating included, which is good.
Quite a bit pricier than a chainsaw piston.

3) a gearbox off a 125 honda 4-stroke could be used, if the two-stroke crankcase was modified to suit.
A 125 race 2ST engine has much more torque than a 4-stroke road engine- so the standard clutch will not do.
They are probably both rated at around 13,000 rpm.
the 4-stroke has a shorter stroke.

the 4-stroke is 10 hp.- this seems a bit low for a fast-spinning engine? I would have expected 15 hp.
Maybe they have de-rated these a bit since earlier versions.
Possibly emissions is a problem if you go for higher power, plus low end pull is not good.

A 250 4-stroke race engine is good for around 45 hp at maybe 16,000 rpm.
They used to do 18,000rpm, but needed a lot more servicing.

The modified crankcase does seem possible with tig welding, and still preserve the ballrace housings.
Possibly the barrel mating surface should be skimmed.
 The whole thing could be machined from solid with relatively simple equipment,
but horrendous material costs- $600 plus if using imported slab aluminium.

The difficulty is getting existing cheap cases that fit a proper crankshaft.

4) Aftermarket aluminium radiators are not very expensive.

5) Purchase of a complete set of second-hand parts is desirable to get a close look at finer construction details.
Some parts could be tig welded and re-machined and redone in Nikasil.
- freight looks like a problem, though.

ex TradeMe are quite expensive. I have heard of them changing hands "privately" for under $900 nz.

6) Carburettors seem to be very expensive new. 
What would be a suitable size?  36mm?  38mm?  40mm?- possibly seek some advice, and look for cheaper options.

7) I need to get one of those pricey right-angled porting tools.
2 Stroke Porting and Polishing Kit (ccspecialtytoolstore.com) 
This one seems to be best, but pricy at over $1000 USD.
are there cheaper ones which are just as good?

1/8" 90 Deg. Right Angle Pencil Die Grinder : Amazon.ca: Tools & Home Improvement 
this is $389.95 USD.  Is it any good? what does it fit onto?
2 stroke porting kit for smaller 2 stroke engines (ccspecialtytoolstore.com) 

Here is an $871 USD version.
it seems to have a compact head size.

Dremel 575 Right Angle Attachment - Power Rotary Tool Attachments - Amazon.com 
The Dremel one is cheap, but is not compact.

Dremel 3000-2/30 130W Rotary Tool Kit - Bunnings New Zealand 
Full Dremel kit- I will get one of these- more durable that the Ozito equivalent.

I have 3 Ozito sets, and the flex shafts die quickly, then the speed control gets dodgy- maybe worn brushes?
The extendable hanger is no good, either.

IC-300 | 90° Angle Attachment (NAKANISHI) | NAKANISHI | MISUMI (misumi-ec.com) 
body unit temporarily not available.
Mold Shop Tools - TELESCOPING RIGHT ANGLE ROTARY HANDPIECE , $660 handpiece only,
Mold Shop Tools - FOREDOM FLEXIBLE SHAFT MOTORS & KITS , $239 USD for flex shaft tool.
These look to be 110V units, with no 220v option.
Were are 230V, 50Hz here.


Re: motorbike layout.  - possibly redo the frame in 15mm,20mm, 12.5mm ms tube.  They have a basically steel frame.
The engine needs to be mounted higher to clear the big fat exhaust pipe.

This means more swingarm down-angle, possibly totally replace the swingarm structure, rear shocks, longer swingarm.

I also like a fairly high seat. Can this be done with the standard forks, fuel tank shell?

I have fairly long legs, and I should be just able to "flat-foot" it when stopped.

The Honda has a racy-looking fuel tank.
The original shocks are nowhere near race-grade, but I don't want to race it.

It has to be safe when cornering at 90+ mph, though.
- the original is flat out at 75mph, if you duck your head a bit.

I hear the "LC" 350 Yamahas were a bit dodgy at speed, and they are bigger and heavier.

This all sounds incredibly expensive, so it is on the Long Term plan.

It the meantime, I can amuse myself grafting the chainsaw and model aeroplane engines together, and modifying that.
There is plenty of scope for upgrading.
This is more in the spirit of MODEL engines!  

I had a thought of adding extra transfer channels opposite the exhaust. This means adding on to the crankcase,
and creating new mating faces.
Some chainsaws are built like this. The main barrel studs can be sleeved, and pass through the secondary transfer channels.


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## Owen_N (Apr 10, 2022)

Possible alternative:

Chinese water-cooled 49cc 2-stroke motorcycle engine, including carb, plus separate set of radiators.
The header pipe can be parted-off from an aftermarket cheap expansion chamber exhaust??
- images to follow-
I can then build a water-brake dyno, and play around with modifying the engine.
It looks like it has quite a small carb.  Maybe a 35mm carb is better for a hotted-up version.
I can try out the intake-side power valve, major transfer port mods, wide exhaust ports, total cylinder porting rework.
This one looks like it is rated for about 13,000 rpm, and 15 hp.  - that can be taken up to 17-18,000, I would suppose, and 20 hp.
That normally would require a disc valve, but may be possible with the resonant power valve- possibly a separate resonant chamber-stub
(helmholtz-style) to open at the same time as the bypass to the reed valve.
The reed block can be opened up to wrap around a centre tract.

Very likely a 250+ amp ac tig and a lathe-mill combo would be prerequisites to doing a proper job.

Again, this is getting away from the usual model-engines concept and into roadable motorcycle powerplants, albeit quite small ones.

- maybe doing somewhat similar to mr 2Stroke-stuffing, but stick to pump gas!

I don't think I would be getting so much into automated milling or aluminium casting, though.

I would be quite happy if I can match his 20 hp on pump gas.

-hone dyno, fume extractor, good silencing system!!


Regarding carbs- possibly an SU-style 1.5 inch would be suitable.
It doesn't have to have a snappy pickup.
Are these still made? Can I find plans for a home-made version?
A good Lectron carb would be maybe $500 by itself!.

Selling this style engine on AliExpress seems to be something new.  Maybe they will go to proper 125 motocross engines eventually??
<edit>
Keihin PWK 35, 36 seem to be vey cheap.

This looks to be a somewhat illegal knockoff, though.
The Chinese seem to have a loose attitude to brand marks.

- would customs seize these on entry into the country??

A 38 (1.5") is the size for the 125 yamaha YZ engine, so likely way too big for a 50 cc.
21mm seems to be a common scooter hot-up size.

SU carbs can be got second-hand, but very expensive for "as new" condition.
maybe the Keihin is the way to go...


At least they are very common, and there are lots of spare jets available.
The Lectron ones are $750 + USD!!!


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## Owen_N (Apr 11, 2022)

The rebuild of the 60cc two-stroke is under way.

It has a problem with the exhaust side piston skirt  of the chainsaw parts being too short.
I have raised the piston about 2 mm to give me a later exhaust opening, and there is an equivalent gap under the piston in the exhaust port.
I will screw an aluminium "blocker plate" to the floor of the exhaust.
there is very little ring load at this point, so plain 6061 aluminium should be OK.
It has to be shaped to match the cylinder shape, but does not have to be super accurate.

I think I can screw in from inside the cylinder at an angle. I will have a go.

The exhaust port is a lot narrower that the TP60 barrel.- about 65 degrees in rotation around the bore instead of 75 degrees, and has a square top.
the transfers also have a square shape.  They could have an angled top insert up to 25 degrees.

I have ordered another kit for a different engine, a Husqvarna 154, which seems to have a longer piston skirt.
That has the 45mm bore, but 34mm stroke instead of my 36.5mm.

I wonder if I could build up the piston skirt width so that I can use a wider exhaust port?

The J-B Weld could hold extra skirt pieces in shear, if the surfaces are clean and sanded.
I have also ordered a spare piston. 
That extra part would get well crunched if it dropped into the cases.

I have had a chunk of piston drop off before in another engine, and the engine seemed to survive it OK. The piston parts tend to fracture rather than 
jam in place, and don't get kicked out through the side of the case.
The engine does make a rather odd jangling noise.

All the nominally 45mm pistons vary a bit in base size.  the TP60 piston will not fit in the new barrel, which is a fraction of a mm smaller.

The next job is to make  adapter bushes for the pistons.
I think I can take the piston pin bore out from 11 mm to 13 mm.
This is better than 12mm, as I am more able to make a 13-10 bush with a small shoulder.
I should bore before doing the outer size, as the material may be more stable while cutting.
A good job for tonight!

I will see if it can be done on my micro-lathe. The chuck does not seem to have much grip.

If that works, then I can make a fixture for the piston, and do  the piston pin hole machining.
I can bolt a plate fixture to the chuck, and have done this before for the head recess machining.


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## Owen_N (Apr 20, 2022)

Looking for 250 or so amp tig for welding aluminium.
I think a can run 250A out for 230V ac, 10A in??
300A would be better, though.

I need to find out how aluminium welding works.
Do I need inverter rectification, then high-low pulsing over that??

This gear would feed back to my two-stroke barrel project plans.

I think even "2Stroke-Stuffing" gave up on aluminium casting, and got one 3d metal printed.
Casting intricate "lost-wax" 2-stroke barrels takes lots of practice and knowhow.

Welding and heat treating could be a bit of a learning curve, too, especially if parts are made with high-silicon alloys.

This needs a certain level of heat treatment to work with head studs, otherwise the barrel tends to go out-of-round.
- what do they call those honing rigs you bolt on the barrel-?-might need one of those.
Do I need a ceramics-style furnace for heat treatment?

I need to find out about removing nikasil. Then I can welds, heat treat, and send the barrel and piston away to be redone in Nikasil.
I presume I will be shifting exhaust port bridges around.

Two bridges looks good for the straightest exhaust top over 120 degrees of exhaust.
*** correction- 180-20 = 160 degrees exhaust port.  This should clear the piston pin.
Full-skirt pistons are a bit experimental.
I think 65 degrees is about maximum for square-top ports.

Is a full straight port even the most desirable?  most have a slight curve, or step in the middle, to go with pipe block-supercharge.
************************************************************************************************************************

This chainsaw barrel I am working on now looks suspiciously like  eutectic aluminium-silicon. It is fairly brittle.

It also seems to have a Nikasil coating as well- I have accidently flaked a few bits off- fortunately not in critical areas.

**************************************************************************************************************************
I am keeping this in engine discussion, as it is not really about hobby engines that I have built or am building,
but tends to get more wide-ranging.

A left a message on KiwiBiker, but this kind of "Mad Engine Science" doesn't seem to be that popular there.

I suppose it is a bit far-fetched even for Bucket Bike builders.

I wonder if two-stroke hotup is a bit of a dying art- there doesn't seem to be a lot on-line at the top end , like
"record" engine level, or even mucking about with old Grand Prix bike engines.
I suppose those are "collectible" and " restored" nowdays.

There is plenty of action with "bicycle engines"- but you don't want to get over 25 mph with those things-they vibrate badly when wound up,
and the bikes are too flexible and under-braked.


These things are still used for Motocross, Bonneville-type record racing, and drag bikes.

I suppose some people have their "Trade Secrets", still.

The hotted-up model RC engines tend to be smaller.
Even 15cc is quite rare as a hotted-up engine nowdays.

I think those would run better if converted to spark, but still run nitro glo fuel.

On a "Plus", my slip-roll set arrived today, from the UK.- maybe a pipe for the "chainsaw" engine, when I get it going.
I am slowly getting the piston bushes sorted out.

<edit>
Regarding full-skirt pistons:
I am not sure screwing the skirt on to the rest of the piston is the right way to go.

I would prefer recessing hex socket screws into the piston head. Actual steel nuts would be good, too, but there may not be enough room.
for a 40mm dia piston, 4x m5 grade 8 screws would do, with 12mm thread engaged.
Screw head dimensions are 8mm wide and 5mm tall, approx.
Exhaust cement would make a good locking compound. It is not very strong, but keys into the surfaces quite well.

A slightly bumpy piston top makes no difference to the engine output power.

Should the split be at the ring groove, for single ring pistons, or offset?

2Stroke-stuffing was talking about stepped rings for wider ports, but I don't know if he will carry on with this.
vertical pin-lugs in rings doesn't seem to work.
I don't think the HP penalty of using ring bridges and losing exhaust width is as high as he was saying.

Bridges can be put in as airfoil webs, with quite a narrow leading edge, which reduces port obstruction.
He did tests with airfoil fins in the exhaust flow, and difference was minimal compared with no fins.

- that was with water at much lower velocity, and for a different reason, but it is a good indication.

He also runs an elliptical exhaust channel out quite a distance. I think the two side-branches can be merged into a circle over a much shorter distance,
at maybe 8-10 + degrees convergence at the merge. I will draw it up.

Some car collectors can come in at up to 30 degrees merge angle, but they are not all firing at once.
I was thinking 30mm exhaust port exit diameter.

The lower-speed engines are 22 mm, and a fast 125 single  is around 36mm, with 38mm carb.
I shall compare these cross-section areas with the actual port area at the cylinder.


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## Steamchick (May 1, 2022)

This guy seems to know a lot about modifying transfer ports and timing... May be of interest?

K2


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## Owen_N (May 2, 2022)

Steamchick said:


> This guy seems to know a lot about modifying transfer ports and timing... May be of interest?
> 
> K2





			https://vannik.co.za/EngMod2T.htm
		


I will be getting a copy of engmod2T and will be  working with that.

This allows engine simulation, and shows  the effects of changing engine features.

I quite like some of the changes "2strokeStuffing" has done.
I will watch the video you have referred to.

The interesting features I would like to consider are:

1) how to support the upper and lower halves of the barrel when they are filled with ports.
Are the blades between the ports also the main barrel supports?
Should the exhaust and outer ports shell be part of the supporting structure?

If these are changed, will the barrel still line up enough?

What gear do I need to rebore it?  How do I remove the nikasil?  Can I just farm out some operations?

Will getting nikasil in a weld make for a poor weld?- you are alloying in extra nickel.

2) can I space out the base studs more on a standard engine?. Can I lift the deck join up and widen it?

3) can a water-cooled barrel be reworked by tig welding it?  - if it is made from 4032 eutectic aluminium-silicon, it is
low ductility, and probably needs preheating, and maybe a careful choice of filler rod.

4) can I weld in new sections of port straight on to an existing water jacket?- and still hold water?

5) do I need to buy a heat treat oven or a pottery kiln to stabilise the welding?

6) can I add a "water" cooling circuit to the lower barrel, under the "teacup" port layout?

7) will an "always open" inlet work using an existing crankcase?  Does it need an extended carb tube?
Can the carb be in the middle of the tube?  What theory and formulae apply here?

The structural problems are the most important.  I think actual port layout examples are available.

Possibly a feature could be designing a 360 degree skirt piston that bolts together.

I need to buy a fair bit of expensive gear before I can do anything like this- lathe, mill, tig welder....

Maybe an $800 usd angle-tip port-grinder set?? - there doesn't seem to be any cheap stuff around that is good for ports.

- I just whip the back off the transfer ports, and glue  new backs on using J-B weld  - it works well.

Special bench grinder just for tungsten tips?


I could buy the engine and just strip it, measure it, and work out some alterations before that.
The engine I have my eye on does not really support high-grade porting, hence all the alterations.


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## Owen_N (May 2, 2022)

Here is the style of barrel I would like to work towards<image>
it is from the "2strokeStuffing" series of videos.
Notable points:
1) wide-spread base studs; - maybe M8?- however, m6 would be sufficient, and only 5 x m6 in the head.
2) wrap-around exhaust manifold built into the barrel.
I would not have such a long exhaust extension, but have an earlier assembly break line.

I an also interested in exhaust stacked over inlet, with the exhaust being half height, about 200 degrees around the cylinder, and
inlet ports around 360 degrees of the cylinder.

I think this could be practical to build, with substantial between-port webs added before cutting the main barrel supports.
these can be narrowed to about 4mm inside the barrel.
the exhaust manifold takes the form of 2 scroll cases, one each side.
Inlet flow is still biased to the cool side.

Possibly a horizontal web needs to be added and integrated into the nikasil coating.
A down-side is that crank-case volume tends to be larger, to feed all the inlet ports.
there would be 2 inlets front and back, around 65 degrees wide, with the rear port at a higher angle, and the front port around 20-25 degrees.
there would be two more side transfers each side, around 60-65 degrees, and exhausts in 3x 65 degrees and 2x a little narrower, to add to 210 degrees including dividers.
another issue is: heat transfer into the top of the transfers- should a thin water- passage area be included?
: will the exhaust reflection work well with a) all ports the same height; b) ports at different distances from the exhaust chamber??

Similar layouts have been tested in simulation. I  will have a go at simulating this.
The idea with port widths is the keep all vertical webs lined up for good barrel support. They can diverge in shape, further away from the barrel.
This also allows a complete ring in the "teacup inner" transfer port shape, for a water passage.
The barrel has to be jacked up a bit from the flywheels to allow free transfer access.- you don't want barrel cutouts.
This may require a slightly different piston-pin location, or a longer rod.

I think mounting cheese-head socket screws sunk into the piston top would be acceptable, and the piston pin mount would be a separate piece.
Retaining rings would not be needed, as the pin would bear against the inside of the skirt.


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## Owen_N (May 8, 2022)

My attempt at bushing a  piston to a smaller pin size is coming along.
I am not sure whether this will work ok, now. 

I wonder how thick the actual piston crown is? If I have 8mm, I can recess m5 hex socket screws into the crown, and make a component to
hold the piston pin more securely.

My current Micro-lathe is not up to making an entire piston with crown, ring grooves, skirt, and inner recess.

I will carry on with the current bushed setup. 
The pin and bush are not fully supported by adequate material in the piston pin boss, and the boss may
shatter. 
 1.5mm may be a bit low for the bush walls if they are overhung, too.
I also haven't found nice spots to put the m3 screws holding the side retainers on.

I have fitted one bush, but it is not in deep enough, so I will push it out again once I have bored the other side to 13mm- actually around 12.9mm.

Once I have both holes bored slightly undersize, I can ream them as a pair to improve alignment, and I can also ream both final holes as a pair.

Any change in overall squareness to the piston will be minimal, - in the order of 0.2 mm over 35mm, or 1 in 175, 0.3 degrees.
I will finish it off, run it, and inspect it for damage.  I have a spare piston and barrel off a slightly different engine if I trash this set.


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## Steamchick (May 9, 2022)

Hi Owen, re: _"I wonder how thick the actual piston crown is? If I have 8mm, I can recess m5 hex socket screws into the crown, and make a component to hold the piston pin more securely._ "
If the crown is very thin, you can consider a nut-and-bolt arrangement, if you use countersunk screws in the crown, as these are reasonably easy to seal (cone in cone) - even maybe a drop of Loctite or silicon sealer will survive the alloy temperatures? The nuts on the underside of the crown/part you are installing to reinforce the pin bosses can then take the tensile load of the assembly, without risking femur threads in the crown?
Or maybe I misunderstand the item?
Cheers!
K2


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## Steamchick (May 9, 2022)

Hi again Owen. Can you clarify the small end arrangement? The industry standards (if I remember correctly!) are either a pin fixed in the piston and small end bearing in the rod (so called "floating pin"), or a pin fixed in the rod and the piston pin  bores become the bearings for the small end motion (so called "fixed pin"). My experience of 2-stroke engines has only been "floating pin" types, often with needle roller bearings to reduce heat loading here (the nomenclature relates to the con-rod joint). Bosses are basically the same strength, but different finished sizes for the applications.  I guess that you are going to a smaller pin - fixed in the piston - with bearing in the con-rod?
K2


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## Owen_N (May 9, 2022)

Steamchick said:


> Hi Owen, re: _"I wonder how thick the actual piston crown is? If I have 8mm, I can recess m5 hex socket screws into the crown, and make a component to hold the piston pin more securely._ "
> If the crown is very thin, you can consider a nut-and-bolt arrangement, if you use countersunk screws in the crown, as these are reasonably easy to seal (cone in cone) - even maybe a drop of Loctite or silicon sealer will survive the alloy temperatures? The nuts on the underside of the crown/part you are installing to reinforce the pin bosses can then take the tensile load of the assembly, without risking femur threads in the crown?
> Or maybe I misunderstand the item?
> Cheers!
> K2


the underside of the crown is hollowed, but I can machine that more smoothly and add a domed plate.
Thank you for the suggestion. I will have a look for grade 8 hex socket head countersink screws, and order some.


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## Owen_N (May 9, 2022)

Steamchick said:


> Hi again Owen. Can you clarify the small end arrangement? The industry standards (if I remember correctly!) are either a pin fixed in the piston and small end bearing in the rod (so called "floating pin"), or a pin fixed in the rod and the piston pin  bores become the bearings for the small end motion (so called "fixed pin"). My experience of 2-stroke engines has only been "floating pin" types, often with needle roller bearings to reduce heat loading here (the nomenclature relates to the con-rod joint). Bosses are basically the same strength, but different finished sizes for the applications.  I guess that you are going to a smaller pin - fixed in the piston - with bearing in the con-rod?
> K2


No, this is a conventional 2-stroke top end, with needle roller in the centre. The pin is standard, retained by wire circlips.
The problem is the RC model engine uses a smaller piston pin diameter. I can make a smaller cage "piston" that fits inside a larger outer piston, and remove 
everything outboard of the piston pin end. there is about 4mm further to the cylinder wall each side. This can have enough material to thread to take the crown screws- about 10mm thread should do.


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## Owen_N (May 15, 2022)

I was successful with my second go at putting stepped bushes in a piston. 
This one has better bosses tor my app.

However, the piston skirt is too narrow to cover the exhaust port, and the ring locating pin enters the inlet port, so I will try some addons with J-B weld.
I will get to see if the surface adhesion and peel holds up with heat and oil.
  I think the J-B is porous to oil.  
Previous use on piston inserts and transfer buildup looks promising. 
 I don't think we will get the full 18 MPa working load. 
 UTS is 24 MPa. I haven't worked out the relative volume of aluminium I can support with various lengths of J-B Weld joints.

I will look up the acceleration formula, and try and work out some stresses. rpm will be limited to 8000 rpm max, so average piston speed is below 12 m/s.

I have ordered a couple of carburettors for future use on a watercooled engine.
A PWK 30mm and a PVK 32mm
they are copies of Keihin carbs, but were quite cheap.

The PWK is a flat-slide type, and the PVK is a constant velocity carb.  I want to extract the butterfly section for the inlet reed valve bypass, or inlet power valve.


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## Owen_N (May 15, 2022)

Piston peak acceleration:

This is in the order of 16,142 m/s(sq) or 1647g at 8000 rpm, 36.5mm stroke.
This is proportional to the square of the rotational speed.

I derived this from plots on Wikipedia for 3.6" stroke giving 2.3 inches/rad(sq) acceleration.
This is actually at the top of the stroke- bottom stroke acceleration is a bit less.
The bottom acceleration is the worst loading case, being partly tensile, and mostly shear.

Typical hanging part = 24x10x2 = 1.3 grams.
glue area = 10x2mm
f= Ma = 16,142 x 1.3 x 10**-3 = 21N
stress = f/a = 21/20 x 10**6 = 1 MPa.
This is acceptable. - up to 18 MPa is allowed under ideal conditions.

Actual glue area is along the top in tension, plus down the side at 24x1.5 mm, so load on the glue appears to be sustainable.

We shall see if the effect of heat, oil, and cycling load, will undermine the adhesion.

Actual component prep is OK, with basic degreasing with acetone wipes, roughened by Dremel/milling cutter, new parts.
Light sand blasting would be better.
thrust and impact loading should be fairly isolated from the add-on parts.

I have also transferred the bridging parts at piston radius, under the boss, from one piston to the other. This is to avoid having corners of the
addon wings hanging free, and takes some potential load off the glue.


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## Owen_N (May 21, 2022)

The engine seems to run OK.
No sign of things coming apart, and performance is on par with the original and modified TP  parts.
I now have to get my optical  rev-counter running properly again.- recharge the batteries, paint a white patch on one prop blade?
I will do a disassembly check after an hours running.

I accidentally fouled the propeller with the cam-out starter, and smashed all my engine mounts. No injuries.
You have to be careful about leaning in to the starter, and it is in a poor position, below waist height.
The prop was a bit dinged, so I swapped it for a lower pitch prop. The restart went well, after a carb cleanout.
I added two boost ports, but of a more sensible size than the original, with smaller piston holes. This should wear better than the original.

It doesn't seem to matter about the extra side holes in the piston.
I thought that the vent holes for the boost ports would help with circulation under the piston.
125 motocross engines seem to have no under-piston venting at all.

here are some photos of the parts:


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## Owen_N (May 23, 2022)

progress report:  sudden suspicious stoppages.
I pulled it down, and there is no obvious seize marks, but higher than normal bore wear than expected, including the ring area.
It just looks polished, with no scoring.
I am presuming that the engine cannot do without the side vents, so I have added some - see photos.
I also put the screws back with red Permatex thread lock- they were not as tight as expected.

Possibly air-cooled two-strokes need more under-piston venting. - any history on this?  The Suzuki 50s don't seem to have any side vents
<Edit>  yes, the 41mm bore Suzukis don't have vents. I had an AC 50 as my first bike, and it has no piston vents.
Maybe it is a feature of the 45mm bore engines??
There is a double hit of more top area to absorb heat, and less piston skirt to dispose of heat.


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## Owen_N (May 27, 2022)

We are back to "engines built"  but I will stay in general discussion for now.
I had my first full tank high speed run.
The carb is behaving oddly, in that if I turn down the low speed mix a bit, it stalls after a while at high speed.
If I keep low speed at 1.5 turns, it rich misfires around 5,000 rpm, but runs smoothly around 7,000 rpm.
I left the high speed needle at 1.5
My tacho is playing up- it seems to flatten the batteries too much on standby, and doesn't have a positive "off".
I shall have to leave the batteries out when I am not using it.
There is a sudden engine tone change above 7,000 rpm, which I suspect is the start of transfer blowback.
I stop winding it up at this point.

I am abandoning my initial goal of smooth low speed running and gradual speed increases on the throttle, as this would need radical head changes.

Possibly the primary plug should be more recessed, and it could take a variable transfer setup. Reducing transfer area makes the engine run more smoothly at lower rpms.

 I have also accidently blown up my twin-spark cdi unit, but this could just be a blasted electrolytic capacitor, which could be fixed.  It is not keen on 16 volts!
I may come up with some schemes for this engine, but it is running as well as it is going to go on a prop, and without a pipe.

Any more revs would blow up my fabricated piston, and it need the prop for cooling.

I am looking at getting a cheaper 400 mm floor-type high flow blower for water cooling, but I could dunk the radiator in a big tub of water.
-a large round rubbish bucket?  - I will look for one.
That could need a thermostat added to the water circuit. I will check when the other engine arrives.

I will wait to start on my water brake until the aluminium tube arrives.  I will try fabrication with the aluminium solder, but to get a really well-wicked
joint may take extra wet flux, not just flux-cored rod.  I can see how it stands up for strength and heat stability, too.

I also need to start collecting other materials for the dyno, and dummy up the Arduino, and the Notebook serial interface, and display/recording software.
I have an Arduino board, but I haven't had a go at using it yet.  This one runs on a regulated 5V supply. or off the USB plug.
I need to look for a load cell module that is readable by the Arduino.


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