Any hard core 2-stroke tuners here? We are doing some extensive research on head designs for high performance 2-strokes. Mostly looking at MSV numbers, squish band dims, etc. We are looking to tune for specific fuels and though the MSV can be calculated, it really means nothing if you don't look at a specific fuel and I can find now good data for say target head specs for methanol. We will mostly be looking concentric squish designs with near hemi bowls.
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2-stroke tuning, squish velocity for race fuel and methanol 1
- Thread starter bob1111
- Start date
- Thread starter
- #2
For methanol:
The highest compression ratio you can stand before permanent damage. (from the time the intake port closes)
Squish design is pretty basic with a center plug but clearance may become empirical. With a steel rod and light weight piston and you may get by with .030".But if it touches the head at high revs it is probably close enough.
Cheers
I don't know anything but the people that do.
The highest compression ratio you can stand before permanent damage. (from the time the intake port closes)
Squish design is pretty basic with a center plug but clearance may become empirical. With a steel rod and light weight piston and you may get by with .030".But if it touches the head at high revs it is probably close enough.
Cheers
I don't know anything but the people that do.
- Thread starter
- #5
Thanks, but I am really not looking at the squish clearance but rather the max velocity of gasses from the "squish" action of the outside gasses working towards the center. This is calculation is affected my piston speeds, squish clearance, squish angle, squish/bore area, etc. We have done extensive work with gasoline and race fuels but cannot seem to find much data on the alcohols. Squish can largely affect the performance of an engine. Was just hoping someone has experience with this.
patprimmer
New member
A whole bunch of factors determine minimum piston to head clearance. Off the top of my head they are:-
Co-efficient of expansion of materials used in piston, block, rods and crank.
Temperature differences between piston, block, rods and crank.
Deck height of block.
Pin height in piston.
Bore size.
Piston to bore clearance.
Piston skirt length and rigidity.
Longitudinal rigidity in crank and crank support webs or bulkheads.
stretch in rods.
Maximum RPM.
None of the above mentions total strokes vs power strokes.
As others have mentioned, a common method is to reduce it until there are just witness marks on the pistons, but make sure the ring lands have not closed down and clamped the top ring i the grove.
Give it 0.002 or 0.003" extra deck clearance if you want long life with no maintenance. You can still get long life if you are prepared to pull it down from time to time and scrape the piston top where the witness marks appear as the skirts and bores wear and allow the piston to rock more at TDC.
This is not so critical if you have a high octane vs the dynamic compression ratio as pre ignition will not be an issue, but it still increases flame speed and allows maximum power with less ignition advance.
Regards
Pat
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Co-efficient of expansion of materials used in piston, block, rods and crank.
Temperature differences between piston, block, rods and crank.
Deck height of block.
Pin height in piston.
Bore size.
Piston to bore clearance.
Piston skirt length and rigidity.
Longitudinal rigidity in crank and crank support webs or bulkheads.
stretch in rods.
Maximum RPM.
None of the above mentions total strokes vs power strokes.
As others have mentioned, a common method is to reduce it until there are just witness marks on the pistons, but make sure the ring lands have not closed down and clamped the top ring i the grove.
Give it 0.002 or 0.003" extra deck clearance if you want long life with no maintenance. You can still get long life if you are prepared to pull it down from time to time and scrape the piston top where the witness marks appear as the skirts and bores wear and allow the piston to rock more at TDC.
This is not so critical if you have a high octane vs the dynamic compression ratio as pre ignition will not be an issue, but it still increases flame speed and allows maximum power with less ignition advance.
Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
for site rules
- Thread starter
- #7
Again, I am in no way asking for a preference on squish clearances. We know where we need to be. I am asking about the squish "velocity" of combustion gasses BTDC. This will occur approx 10-20* BTDC. I fully understand all the mechanics involved in avoiding mechanic contact of the piston and head.
patprimmer
New member
Viper
I think we posted simultaneously.
I think those who have the data you require might not be willing to share.
On 4 strokes it is difficult to get to much squish, but I can see where a 2 stroke gives so much potential area and the option to place the area where you like that it might be worthwhile to pursue.
Regards
Pat
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I think we posted simultaneously.
I think those who have the data you require might not be willing to share.
On 4 strokes it is difficult to get to much squish, but I can see where a 2 stroke gives so much potential area and the option to place the area where you like that it might be worthwhile to pursue.
Regards
Pat
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BrianPetersen
Mechanical
Maybe this is a dumb question, but with a 2-stroke engine, is there *ever* a set of circumstances under which the squish velocity could be too much?? I can't think of what those circumstances might be.
patprimmer
New member
I have my doubts, but I don't actually know.
On engines I have worked on you could never get to much, but I never worked on any thing really extreme, like say a very big bore, very short stroke very high compression engine with no valves in the head and a very compact fairly spherical shaped combustion chamber on one side of the piston and head.
Regards
Pat
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On engines I have worked on you could never get to much, but I never worked on any thing really extreme, like say a very big bore, very short stroke very high compression engine with no valves in the head and a very compact fairly spherical shaped combustion chamber on one side of the piston and head.
Regards
Pat
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- Thread starter
- #11
Actually, a high msv will induce autoignition in the same way a high CR will. MSV comes up with RPM so it can get rather tricky to get the right values. We have some of our own methods for working with MSV but really thought there would be other data out there on the subject.
GregLocock
Automotive
So are you intending to publish your results when you've finished?
Cheers
Greg Locock
SIG
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Cheers
Greg Locock
SIG
lease see FAQ731-376 for tips on how to make the best use of Eng-Tips."Actually, a high msv will induce autoignition in the same way a high CR will."
I've never heard of this phenomena before - through what mechanism does the autoignition occur?
All of my experience to date suggests that for an increase in charge motion (especially squish) detonation will be reduced - not increased.
MS
I've never heard of this phenomena before - through what mechanism does the autoignition occur?
All of my experience to date suggests that for an increase in charge motion (especially squish) detonation will be reduced - not increased.
MS
patprimmer
New member
My understanding has always been less detonation, but faster flame spread which equals faster pressure build up which might allow later timing to achieve the same peak pressure and same or slightly earlier peak pressure point.
Regards
Pat
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Regards
Pat
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I agree with the idea that you cannot get to much 'squish'...
On my current engine I am changing the CR from 13.87 to ~14.8 without altering the existing clearance from piston top to cylinder head, ~0.035" (piston down 0.005" and 0.030" compressed gasket). If all goes well, I will probably just 'run with it' as it is...However, I do have one option (aside an unacceptable thinner gasket) to "dry deck" the engine using "o" rings. That would bring clearance down to something akin to 0.010". We shall see what happens next month. I'm using 110 octane now and I have available a ~116 octane at a higher $$$$$$$$! I'd rather stick to the $8/gal 110, though.
"Witness marks" Pat! I don't get witness marks when I screw up, I get "dents"!!! ;o)
Rod
On my current engine I am changing the CR from 13.87 to ~14.8 without altering the existing clearance from piston top to cylinder head, ~0.035" (piston down 0.005" and 0.030" compressed gasket). If all goes well, I will probably just 'run with it' as it is...However, I do have one option (aside an unacceptable thinner gasket) to "dry deck" the engine using "o" rings. That would bring clearance down to something akin to 0.010". We shall see what happens next month. I'm using 110 octane now and I have available a ~116 octane at a higher $$$$$$$$! I'd rather stick to the $8/gal 110, though.
"Witness marks" Pat! I don't get witness marks when I screw up, I get "dents"!!! ;o)
Rod
patprimmer
New member
I don't call them dents if they don't reduce the top ring side clearance in the grove ;-)
Regards
Pat
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Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &
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black2003cobra
Automotive
I'm also wondering about the MSV vs knock comment. I would normally think that the faster the end gas is consumed, the less chance of knock. So could it be that with extreme amounts of turbulence, that there is significant charge heating so as to actually increase the tendency to knock?
BrianPetersen
Mechanical
I suspect what's happening is that the combustion gets completed faster and ends up finishing "too soon" so it's more prone to knock, but if the ignition timing were retarded a little to bring the point of maximum cylinder pressure back to a normal range (~15 degrees ATDC same as a 4 stroke? don't know) the outcome might be better overall ... But, everything I've ever fiddled with has been 4-stroke, so if anyone with specific 2-stroke experience says otherwise, I'll defer to that opinion.
black2003cobra
Automotive
I ran a quick simulation. Baseline vs two cases with shorter burn durations w/o and w/ retarded spark. In the third case, I adjusted spark to get peak pressure to occur at the same crank angle, for the case with less advance. Peak cylinder pressures and temperatures in both second and third cases were higher than that of the baseline case.
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- #20
That is accurate Cobra. basically if flame front propogates too fast, it will increase cylinder pressures too fast and induce auto-ignition or an un-controlled burn. Pushing the limits of MSV can be rewarding though in that you can reduce spark advance thus reduce negative work the engine does. The problem with two stroke work heads is there are too many variables. We have some theories here that seem to work but we cannot exactly defy the physics laws though.
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