2-stroke tuning, squish velocity for race fuel and methanol
2-stroke tuning, squish velocity for race fuel and methanol
(OP)
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.
RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
Some empirical info suggests for reduced octane requirements this is correct for 2 and 4 smokes.
RE: 2-stroke tuning, squish velocity for race fuel and 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.
RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
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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RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
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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RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
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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RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
Cheers
Greg Locock
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RE: 2-stroke tuning, squish velocity for race fuel and methanol
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
RE: 2-stroke tuning, squish velocity for race fuel and methanol
Regards
Pat
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RE: 2-stroke tuning, squish velocity for race fuel and methanol
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
RE: 2-stroke tuning, squish velocity for race fuel and methanol
Regards
Pat
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RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
Quiet obviously, the right controlled lab testing of fuels could help a BUNCH in the R&D process but still would not exclude many hours of R&D for the right setup.
RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
RE: 2-stroke tuning, squish velocity for race fuel and methanol
It is my belief (but I hae no back to back data) that with really good squish, you can cut a degree or two from timing from maximum power, but there are many variables and when increasing squish, I would also normally add some compression and de bur the chamber and smooth off all sharp edges. This is why I have no back to back data.
I also have no back to back on petrol vs methanol flame speeds, but a great number of variables also make tests under real world conditions very difficult. That might explain the variations from text to text.
Methanol will tolerate a lot more compression than most grades of petrol based racing fuel, so engines optimised to methanol might have very high dome pistons that interfere with flame travel and in fact could cause some turbulence that divides the chamber with a squish area in the middle of the camber. I have seen a SBC that needed 60 deg advance until we installed flame slots in the dome. The slots with no other change allowed the timing to be set at 42 deg. Of course the slots slightly reduced compression by removing a few cc from the dome. This was 20 or 30 years ago, but if you want to do the sums, say 2 cc per slot, 16:1 CR with full domes, 4" bore, 3" stroke.
Racing fuel is a different composition to common petrol purchased at the bowser.
Common petrol varies quite a bit according to brand, season, region and grade, so what is the standard composition to be used for comparison in test results you might obtain, and how might that relate to your petrol.
Other variables are latent heat of vaporisation, specific heat, mass of fuel used, surface energy, viscosity, boiling point, vapour pressure, effect of compression on the above, size of jets or nozzles. These all effect evaporation rate, and therefore actual burn rate in the chamber of a running engine.
Regards
Pat
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RE: 2-stroke tuning, squish velocity for race fuel and methanol
However, if using alcohols the knock limit may well be beyond MBT so an increase in squish, in this instance, will allow you to increase the compression ratio in an attempt to regain some BSFC.
An increase in turbulence within the combustion chamber is going to cause autoignition in a positive ignition engine with a premixed charge.
In terms of comparison of flame speed for different fuels one must be careful when comparing laminar flame speeds when the majority of combustion, within a positive ignition engine, is turbulent.
The only real way to make any sense out of all of this is through thermo analysis using incylinder pressure measurements & PV/Pcrank diagrams because, as Pat has already said, there are so many variables.
On another note I must say that I am a little surprised that the OP is not familiar with John Heywoods book, perhaps not enough time spent on the 'R' part of 'R&D'???
MS
RE: 2-stroke tuning, squish velocity for race fuel and methanol
I would say the "R" part of our R&D consists of plenty of dyno time working with different chamber designs. Sometimes theories only get you so far.
RE: 2-stroke tuning, squish velocity for race fuel and methanol
No, of course its not...but I would say that it is, arguably, the text most often used by professional engineers as reference on the subject matter.
In terms of " Sometimes theories only get you so far." I would tend to agree but I'm afraid that the majority of 'garden shed' engineers I have come into contact with do not have even the most basic understanding of the fundamentals & theories of the IC engine.
That may sound arrogant but I'm afraid that the companies that employ professional engineers, such as some of the members of this forum, spend an awful lot of money on their equipment, training & people to try to get an edge over competitors in an industry that is (was???) worth billions - its as simple as that.
By the way I have just noticed an ommision in my last post: -
" An increase in turbulence within the combustion chamber is going to cause autoignition in a positive ignition engine with a premixed charge.
should say ....NOT cause autoignition...
MS
RE: 2-stroke tuning, squish velocity for race fuel and methanol
Guess I was not paying attention. I get set in my own ideas and sometimes only 'scan' some of the other contributors posts. I'll try to do better, Matt.
Rod
RE: 2-stroke tuning, squish velocity for race fuel and methanol
cheers malbeare
ht
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RE: 2-stroke tuning, squish velocity for race fuel and methanol
I realize many companies spend bottomless penny banks doing these things but they are also not designing chambers for exotic fuels that I am aware of. The two stroke chamber is simple in appearance, yet complex in design because of all the variables that can be applied. Dome height, squish band area ratio, angle, MSV, spark kernel height, etc.
Main question in mind is if alky propagates faster than gasoline, why do most engines require additional timing to achieve max power?
Also, regarding squish band width, it has been taught for years that wider gives more bottom end while narrower gives better top end performance. Our testing has not definitively proved that theory either. One thing that is worth investigating is squish clearance compared to squish width. It would seem that the fuel charge trapped between the piston and squish band will not contribute to the performance of the engine, but rather burn MUCH later (well after 20*ATDC) and contribute to destructive burn patterns. Shrinking this distance to near zero seems a no brainer yet many insist that higher clearance is recommended.
Looking at one test engine we use, rod stretch is calculated to only .005" yet we hear this engine should have .040-.050" clearance. Puzzling at times. Our test engine has run MUCH tighter tolerance but with a totally different chamber design so results there are questimates at best.
RE: 2-stroke tuning, squish velocity for race fuel and methanol
" I realize many companies spend bottomless penny banks doing these things " Yes, right up until the recession !!!
Not sure what you mean by 'BMT' but if you mean that you are looking for complete heat release to occur at TDC then I think you are on the right track!
" Main question in mind is if alky propagates faster than gasoline, why do most engines require additional timing to achieve max power? "
I would suggest that a contributory factor is that the stoic AFR of alcohols is relatively low & the latent heat of evaporation relatively high such that the charge has a much reduced enthalpy, during compression, compare to an equivalent gasoline/air charge.
" It would seem that the fuel charge trapped between the piston and squish band will not contribute to the performance of the engine "
A classic example of quench and is of detriment to any chamber design and an obvious HC emissions raiser.
MS
RE: 2-stroke tuning, squish velocity for race fuel and methanol
"I would suggest that a contributory factor is that the stoic AFR of alcohols is relatively low & the latent heat of evaporation relatively high such that the charge has a much reduced enthalpy, during compression, compare to an equivalent gasoline/air charge."
I agree with that! It just makes me wonder what we can do to improve the performance. WE have always run into mechanic limits to obtain ridiculous CRs but often wonder about trying difference chamber designs with CRs as high as 18:1. We currently have only ran 16:1 here with promising results.
RE: 2-stroke tuning, squish velocity for race fuel and methanol
Robin Sipe.
RE: 2-stroke tuning, squish velocity for race fuel and methanol
you need the "gap" to expell the mixture across to the combution chamber. contry to populare belief retarding the timming increases the power for two reasons ,it alows the power stroke to be still active with greater angle on the crank ,and it delivers hoter gas to the pipe giving faster sound wave reflection allowing higher effective revs(more power)
Methanols main function is its cold burn to protect the engine under high loads .needs captive CR of +16 to 1 above the exhaust port or it wont burn properly.
Hope this helps
RE: 2-stroke tuning, squish velocity for race fuel and methanol
1 the compressed alcohol mixture will have a higher density
2 the higher heat of vaporization will cool the compressed gas, piston crown and cylinder head more, thus creating more density
3 the mixture will be harder to light off
therefore we can assume that;
1 higher CR will be needed to heat the gas more to create a comparable ignition point between the gasoline charge and alcohol charge
2 MSV should not change because as i understand it msv's are tailored to operating range (this may be wrong, as i have never seen an actual equation that determines MSV, and the parameters may be changed slightly by the flow of different gas densities, seems the equation is a closely guarded secret and those with the secret have been less than forthcoming with their information sources, any help pointing me in the right direction would be greatly appreciated)
back to my "R"
Tom