2-stroke, 2-cylinder engine?

[ShedBoy]

SHEDBOY

I would like to see the PDF you mention.

PM sent

Brock

 

[Gedeon Spilett]

hi
have a look at this one too, from my "to do" list, an old timer 2 strokes (spark ignition !) 2 cylinders in line, as 2 independent engines.
the pdf for the complete blue prints and instructions build (in french) is here
http://ti1ca.com/jumac77l-bicylindres-Sablier2-bicylindres-Sablier2.pdf.html

18 mm bore & stroke, a decent size, cheap and not that difficult to make IMO
cheers

 

[lohring]

Below are a couple pictures of the Aprilia 250 V twin. It's two 125 cylinder engines with their own crankshafts geared together. Their 125 is considered to be the most highly developed simple two stroke so far.

Lohring Miller

 

[Niels Abildgaard]

Have always dreamed of better mousetraps and light sport aircraft engines.Let me try to scheme the ultimate Rotax 912 beater.
We need about 60 horsepower and the prop shall turn not more than 2400 rpm to avoid to much noise.
To find the nessecary two stroke volume (Fourstrokes are undelicious) i have used the formula

Moving effective pressure (bars)
times
Volume (litres)
times
working beats per second

will equal

horsepower
times
7,5

Mep for crankcase scavenging can be 5 and for very mild turbocharging 10
(Normal fourstrokes can be 8 to10 but do not merit consideration among decent people.)
Let me asume 10 bar Mep and 40 rounds per second and target 60 horsepower
This gives 570 ccm per cylinder for my inversed 90 degree V2.
A stroke of 120 mm gives a mean piston speed of 10 meter per second and thats where piston engines run best.
Bore is thus 78 let us say 80 mm.
Pistons are not really good for controling exhaust.They go unround and may loose temper.Valves are much better.
A 2400 rpm two stroke exhaust valve behaves like a 4800 fourstroke and can be pushrod.
WW1 or 2 directcoupled aircraftengines were about 16 to 22 kg mass per littre so can be very competitive here.
It is now time for drawing

 

[dman]

Using a common crankpin arrangement as in the classical four stroke V twin however - although it might run, won't be of much use unless you use some sort of blower arrangement.

i hope not to start a flame war but what makes you believe this? i'm actually liking the idea of a common crank pin v twin, or better a v4. why use a common pin? well i don't see the point in a v2 if the cylinders are phased the same. may as well make a boxer or big single.

a common pin twin may need more timing and more tuning of port angles and may idle unevenly but by my quick math, not considering the rod:stroke ratio the displaced lower end volume is about .707x the total volume of the two cylinders. once you factor in port height that should be enough to evacuate the exhaust from the cylinders but it may need a little more timing than a single cylinder and it may be more sensitive to port angles.

 

[petertha]

3. super charging....usually positive displacement type with a displacement a bit more than the combined cylinder volume. vane style is popular for models and roots is used on full size engines... a centrifugal may work on full scale but on model scale I doubt it (centrifugal chargers don't scale nice and are harder to engineer because they have a pressure per tip velocity relationship rather than a volume per revolution relationship.) super charing has been done on model radial engines

dman, I am going to start a new inquiry post specifically related to SC/boost for model radials, so I don't want to derail this 2-stroke post.

But in the meantime, can you provide any links or references to your reference "vane style is popular for models". I'd be interested to see details of what work has been done on models, how much boost they provide, how they are constructed etc.

I've seen pics of the Hodgson 'centrifugal device' applied to his radials. (I've callled it device as I'm not sure its a centrifugal SC. Ive heard it called a combination mild booster / flow disperser, but I'm not qualified to say in any event).

whirlywheel construction & assembly near bottom of build pics
http://picasaweb.google.com/18.cyl.radial.engine/HodgsonRadial18#5293775425552828066

cad sketch under crankshaft assembly
http://thebloughs.net/hobbies/metalworking/hodgson9/

 

[Niels Abildgaard]

Blower Charged Two Stroke Engines
Model engineer 1971 issue 34(12 13 14)
M.Hollick

 

[dman]

dman, I am going to start a new inquiry post specifically related to SC/boost for model radials, so I don't want to derail this 2-stroke post.

But in the meantime, can you provide any links or references to your reference "vane style is popular for models". I'd be interested to see details of what work has been done on models, how much boost they provide, how they are constructed etc.

I've seen pics of the Hodgson 'centrifugal device' applied to his radials. (I've callled it device as I'm not sure its a centrifugal SC. Ive heard it called a combination mild booster / flow disperser, but I'm not qualified to say in any event).

whirlywheel construction & assembly near bottom of build pics
http://picasaweb.google.com/18.cyl.radial.engine/HodgsonRadial18#5293775425552828066

cad sketch under crankshaft assembly
http://thebloughs.net/hobbies/metalworking/hodgson9/

i'm not sure i can say it's popular. maybe that was inaccurate, it's not as common to see radial 2 strokes for the reasons of it needing a supercharger so i can only say ive seen a couple examples. but i've seen it, it's a logical way to go because it fits nicely on the back of the crankcase and is easy to build.

radial 2 strok plans

also note the super charger doesn't produce boost it's just an air pump, conventional wisdom is that it needs to displace 1.3-1.5x the volume of the cylinders above the exhaust port. boost can't happen on a 2 stroke untill the exhaust starts working. on a typical 2 stroke the exhaust port has more timing than the intake so blowing more into it just pushes more out the exhaust. once the exhaust starts working there is a designed in reflection and/or pendulum effect that provide reversion after the cylinder is over scavenged (so the reversion is unburned fuel/air mixture) and can raise the cylinder pressure which is why the exhaust needs to be timed wider than the intake.

 

[Jordan]

Not a Freak!
An interesting early V twin 2 stroke engine is at the Sammy Miller Museum in England.
Search for images in Google etc.
A brochure can be seen here: http://reddevilmotors.blogspot.com.au/2012/06/stanger-two-stroke-v-twin-1919.html
It has two separate crank chambers.
I think it could be possible to have a common chamber on a V twin, if the crankpins were staggered so the pistons went up and down in unison?
My wife made me sell my MZ - she said it was ugly!
Jordan

 

[petertha]

Blower Charged Two Stroke Engines
Model engineer 1971 issue 34(12 13 14)
M.Hollick

Now how am I going to get my hands on those magazine articles?

 

[Niels Abildgaard]

Not a Freak!
An interesting early V twin 2 stroke engine is at the Sammy Miller Museum in England.
Search for images in Google etc.
A brochure can be seen here: http://reddevilmotors.blogspot.com.au/2012/06/stanger-two-stroke-v-twin-1919.html
It has two separate crank chambers.
I think it could be possible to have a common chamber on a V twin, if the crankpins were staggered so the pistons went up and down in unison?
My wife made me sell my MZ - she said it was ugly!
Jordan

Hello Jordan

Can we old men have a picture of Mz and wife and let us judge if You choosed wisely?

Regards

Niels

 

[wildun]

i hope not to start a flame war but what makes you believe this? i'm actually liking the idea of a common crank pin v twin, or better a v4. why use a common pin? well i don't see the point in a v2 if the cylinders are phased the same. may as well make a boxer or big single.
.

Sorry Dman,
I have just seen your post so I have to sit here a while and digest it all and if I can't come up with a logical answer, then I'll tell you that I can't!:rant:

 

[wildun]

.
Pistons are not really good for controling exhaust.They go unround and may loose temper.Valves are much better.
A 2400 rpm two stroke exhaust valve behaves like a 4800 fourstroke and can be pushrod.
It is now time for drawing

Niels,
An italian company in the seventies built a kart engine with DOHC, (and I mean DOHC TWO STROKE, from memory the camshafts went at half crank speed, the valves opening alternately (keeping them from over doing things at high revs) - maybe not a great idea for a racing Go Kart because it disappeared quickly from the scene! but maybe quite a good idea for a light aircraft engine however, - possibly less piston seizures.

Now I have to sit down and figure out Dman's theories! - maybe he's trying to wind me up!

 

[mikegw1961]

Petertha

Old model engineer magazines are available on eBay

Normally they are about £4.00 / copy
I have just got 5 old issues for £19.95

Mike

 

[wildun]

i hope not to start a flame war but what makes you believe this? i'm actually liking the idea of a common crank pin v twin, or better a v4. why use a common pin? well i don't see the point in a v2 if the cylinders are phased the same. may as well make a boxer or big single.

a common pin twin may need more timing and more tuning of port angles and may idle unevenly but by my quick math, not considering the rod:stroke ratio the displaced lower end volume is about .707x the total volume of the two cylinders. once you factor in port height that should be enough to evacuate the exhaust from the cylinders but it may need a little more timing than a single cylinder and it may be more sensitive to port angles.

Dman,
don't worry about a flame war, I have a hide like a Rhino and I do like to -shall we say 'debate things' but I will concede defeat if I see that you are talking from an enlightened perspective!
As I see it, (and you may have a bit of a jump on me from the point of view of sitting down and nutting it out) and I did mention earlier that it would work - possibly not well, but i think it all boils down to the application it is to be used for, - eg is it to sit on a wooden plinth and burble along looking pretty, just to prove that it will run? or is it going to produce some useful horsepower with sophisticated computer designed expansion chambers laboriously welded together, (or perhaps done by robot welder), - Gordon Jennings would turn in his grave if he heard you!

Truth is mate, I don't know, I haven't looked at this stuff for about 30 years, but my advice is, "suck it and see" - more stuff has been designed by trial and error, educated guesses and doing outrageous things in the past, than many people would ever admit!

 

[wildun]

"i hope not to start a flame war but what makes you believe this? i'm actually liking the idea of a common crank pin v twin, or better a v4. why use a common pin? well i don't see the point in a v2 if the cylinders are phased the same. may as well make a boxer or big single."]

That would just be a matter of preference I guess - maybe a boxer or big single would have nicer sound or better looks, - but no doubt there is some very good reason why a common pin 'V' arrangement is never seen in use on a two stroke, so you should perhaps try it and find out why.
The 'V' angle will have a big effect on your volume calculations too, - I was envisaging a 90 deg. angle but a much smaller V angle would be getting closer to single cylinder territory.
Also, I'm working on the assumption that you would be using normal transfers, but with a narrow angle V the closeness of the cylinders would cause the transfer passages to "tangle" (food for thought).
It probably will be necessary to have seperate bulky exhaust chambers, - (always a problem on two strokes), but again, it depends on the application it is to be used for.

[ "a common pin twin may need more timing and more tuning of port angles and may idle unevenly but by my quick math, not considering the rod:stroke ratio the displaced lower end volume is about .707x the total volume of the two cylinders. once you factor in port height that should be enough to evacuate the exhaust from the cylinders but it may need a little more timing than a single cylinder and it may be more sensitive to port angles".

Not sure what you mean by "more timing" - to me that means longer duration, ie. earlier opening and later closing? - (depends on where we come from I guess!).
That I think it would be extremely difficult to calculate and it will depend very much on the design of the exhaust chamber. There will need to be a huge amount of trial and error experimentation to get it right!

By "port angles", do you mean in a directional sense or crank angles/port timing?

And some people think that the two stroke engine is simple!

BTW, - Not saying you are wrong at all,- no contest! - just trying to get my head around the concept by bringing some points, which I consider to be valid into the discussion and I would like your comments.

 

[Niels Abildgaard]

A 90 degree 60 horsepower/2400 rpm aircraft engine .
80 mm bore and 120 stroke.
Mass as shown (uncut cylinder units) 16 kg.

 

[dman]

Not sure what you mean by "more timing" - to me that means longer duration, ie. earlier opening and later closing? - (depends on where we come from I guess!).
That I think it would be extremely difficult to calculate and it will depend very much on the design of the exhaust chamber. There will need to be a huge amount of trial and error experimentation to get it right!

By "port angles", do you mean in a directional sense or crank angles/port timing?

And some people think that the two stroke engine is simple!

BTW, - Not saying you are wrong at all,- no contest! - just trying to get my head around the concept by bringing some points, which I consider to be valid into the discussion and I would like your comments.

well "more timing" is just an idea i had of how you'd deal with 29.3% less pumping volume. the ratio of the crankcase displacement and the compressed volume has everything to do with the port height which also controls the timing. but it might not matter. the idea of having a surplus of pumping volume might just be because it was inherent to the single cylinder design and worked. i think with some time put into flow patterns you may be able to get closer to 1:1 pumping volume to compression volume and have it run good.

there would also need to be time put into examining how you'd deal with the pumping being out of phase from the cylinders. it would trail one and lead the other. and there would be overlap with the pistons at different heights and different rates of flow so restriction on one cylinder may over fill the other. i think it could be done but it would take time with a physics simulator or a lot of thought and some experimentation or all 3.

most of my theoretical and applied knowledge is with american v-8 racing engines. so i'm delving into mostly theory here. i used to play with 2 strokes but mostly just as rebuilds and maintenance. not so much designing exhaust or altering timing.

this is as far as i want to go into this on this thread. maybe a new thread could be started on the ideas of v-twin 2 strokes and what's best, seperate crank pins and timed together, sperate crank case areas and timed 180 apart, or single pin, single crank case. they all have points. the single pin would be the biggest engineering nightmare but would balance well and be small and light and although odd fire would be smoother than a single. the 2 pin 1 crankcase would be simple to design and be light but with both pistons having the same firing angle it's kinda like just having one piston. and separate crank cases would have even firing and no design problems but adds a bit more weight though that wouldn't matter much at all. i admit a common pin v twin 2 stroke is messy. i wouldn't expect it to be considered a good idea but i kinda want to try it.

 

[wildun]

Niels,
How many exhaust valves per cylinder? - pushrod? - normally aspirated as per Dman's recipie?
I can see that the OHV design will allow shorter piston skirts and I guess this will compensate for the longer stroke. - I see you are going for torque.
Looking forward to watching it progress.

 

[wildun]

i admit a common pin v twin 2 stroke is messy. i wouldn't expect it to be considered a good idea but i kinda want to try it.

Dman,
Yes, but I would be inclined to give it a go, can't do any harm. - you are probably way more qualified than I am, - these days I tend to work more on hunches and maybe the odd educated guess, I probably would be capable of working it out on paper, but I'm just too lazy now!
I'm afraid my engineering ambitions are a bit limited since moving to a smaller home, and then there's the age thing..........just excuses I guess! - waiting to see what Niels is planning!
Thanks for the discussion