Unusual variations on two-stroke head design.

[Owen_N]

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.

 

[Nerd1000]

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.

 

[Owen_N]

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.

 

[Owen_N]

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.

 

[Owen_N]

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.

 

[Bazzer]

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.

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.

 

[Steamchick]

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

 

[lohring]

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

 

[Steamchick]

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

 

[Owen_N]

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!

 

[Owen_N]

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

View attachment 132218View attachment 132219

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??

 

[Steamchick]

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

 

[Steamchick]

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

 

[awake]

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.

 

[lohring]

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

 

[lohring]

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

 

[Owen_N]

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.

 

[Owen_N]

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.

 

[Steamchick]

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

 

[Ken I]

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