piston speed 2

[golfpin]

A very happy new year to all,

I picked up on thread that Greg Alcock replied to, referring to my hero of engineers, Lanchester, he , I understand put forward a theory in about 1905, that engine speed was not limited by Revs but by piston speed, and that the upper limits for the time, based on then known materials was 4000 feet per min. However one had to divide the piston speed by the square root of the Stroke: to Bore ratio to get a better measure of the stresses involved. My Question is does this theory hold true for today,s engines given the huge advancement in materials. I did this calc some years ago on the specs of my 1100 Suzuki and was very interested to see that the suzuki redline translated into 3999 feet per min.
Your thoughts, comments,
All the best Golfpin

 

[ivymike]

Hmm...piston speed is a figure worth considering, but there is not a hard limit. Big slow engines will be in the 8m/s neighborhood, on highway truck diesels in the 10 to 14 neighborhood, little engines in the 15 to 20 neighborhood, and racing engines up to about 27 m/s.

There is always a specific thing that goes wrong first when you over rev an engine, and there a few candidates for what that might be. Only a couple of those know what piston speed is. Snapping a rod is a possibility, rod bearing failure, valve to piston clash, etc.

A good indicator of how heavily loaded an engine is is BMEP.

 

[ivymike]

thread71-85349

 

[Komodo86]

Accepted maximums are closer to 5000fpm (25m/s) nowadays. You won't find many, if any, production cars exceeding that level. You'll not find many racing engines designed with any degree of endurance surpassing it either. Drag engines are another matter.

 

[golfpin]

Thanks for the input chaps but I need to ask further so to speak, is the increase in the piston speed purely as a result of better piston material? Was this move forward perhaps as result of I think Yamaha,s breakthrough in 2 stroke piston{ I think silicon was part of the answer] design and material that lead to the increase in revs, On reflection, I don,t have the specs to hand but before the rev limit on F1 engines was introduced 20,000, at a guess what was the P/S then? What was the nature of engine failures at that higher rev limit, I seem to recall lots of smoke indicating a hole in the piston perhaps? This is conjecture on my part.
Be interested in comments,
Thanks Golfpin

 

[ornerynorsk]

I dunno, the valve train seems to be a much more limiting factor. I've seen Ford small blocks rev to 14,000 plus with a highly modified valve train.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[tbuelna]

Mean piston velocity is relatively meaningless in terms of dynamic forces on crank components. What matters is acceleration of the recip piston/crank masses. And with regards to fatigue life, the number of load cycles and the relative degree of load reversal also matter greatly.

 

[golfpin]

Thanks for the replies chaps am very intrigued ornerynorsk you mention a small block Ford reving to 14K what Ford was this? I was having an informal chat to colleagues the other day and mentioned I had seen a Spike to 14200 on a toyota 4aeg motor and there appeared to be no harm lot of scepticism to.
Would be very interested in what that highly modified valve train was. F1 type motors we know of because of the pneumatic systems used.

tbuelna I must agree that the load reversals because of the Otto cycle system are a greater issue and that is perhaps, as in the aircraft industry parts are"Lifed" and then turfed out before they can break. Hence replacement of certain parts at the end of one run down the drag strip by the top teams, this I have only heard of.
Cheers Golfpin

 

[Komodo86]

I'm not so sure it's to do with materials, and more to do with the mach index in the inlet ports nowadays. The theory is that engines effectively choke above 25-26m/s, without a LOT of work. A drag engine, as I assume a 14000rpm SB Ford to be, are not really comparable to anything designed to run for longer than a few minutes before rebuild.

F1 can rev to 20,000rpm, but with 39mm strokes, that's still only a piston speed of 26m/s.

Motorbike engines, all around the same limit too.

 

[ornerynorsk]

Look up Coates Engineering. I have no association with this company and I'm not sure what they're up to these days. This was back in the mid-1990's that they were building heads for the sb Ford.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[GregLocock]

The dragon has the right answer I think, if you exceed M0.3 then you run into compressibility effects and eventually mach choking, and at some point the incoming charge will not be able to keep up with the retreating piston. Say your intake valve area is 33% of the piston area, then 30 m/s seems a likely limit, 5000 fpm roughly

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[Komodo86]

Another plausible reason is flame front speed. I've seen speeds of 100ft/sec mooted. That's 30m/s.

So a combination of inlet mach index and 'outrunning the flame' is the most likely answer. If the mach index doesn't get you, the flame speed will.

 

[Tmoose]

http://i1015.photobucket.com/albums/af280/mrvedit/Camaro Tech/Blown Engine/BlownEngine009.jpg

http://cache.gawker.com/assets/images/12/2010/02/engine_explosion_1.jpg

 

[golfpin]

Thanks to all who contributed must be one of the most informative I have had the pleasure to read.
Dragon, I like, that says it all! Greg and yourself and all others must consider yourselves as "mythbusters"
Golfpin

 

[SomptingGuy]

... I don't buy the "outrunning the flame" idea. The flame is moving in a gas body which is itself moving with the piston. Besides, piston speed isn't that high during the actual combustion period.

- Steve

 

[ornerynorsk]

I agree. Technically speaking, how would it be possible that an engine could sustain an RPM higher than what will cause a piston to outrun the flame? Not unless we can temporarily suspend the laws of physics. Detonation rate is x times higher than flame propagation.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[Lou Scannon]

In my experience with more or less conventional engines, increasing rpm typically does not significantly dilate combustion phasing in terms of crank degrees. If it did, we would see noticable and detrimental lengthening of heat release in the crank domain, and consequent loss of thermal efficiency due to reduction of effective expansion ratio.
The posited reason that the combustion rate more or less keeps pace with rpm, is that the intensities of the bulk charge motion and turbulence in the cylinder are quite linked to the piston speed, which drives both the velocity of charge that is brought into the cylinder through the intake port(s), and subsequent modification of the charge motion after it has entered the cylinder, i.e. mainly during the upper part of the compression stroke. While I am not a combustion analyst at the 3D CFD and combustion kinetics level, I believe that the experts in these fields have satisfied themselves that the foregoing explanation is valid to some extent.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

I agree with hemi. The burn time decreases with rpm due to increased speed of turbulent charge motion. The shortening burn time compensates for the shorter time available to the extent that the increase in spark advance required in principle is on the order of only about 1/2 to 1 degree per 1,000 rpm, or about 2.5 to 5% per 1,000 rpm. In practice the increased advance is less than that.

 

[Komodo86]

That might be a trend over the majority of the RPM range, and it makes sense.

Is it definitely the case as the piston is approaching/passing around 27m/s though? Have there been any studies to specifically study this aspect?

 

[Komodo86]

What I should have said above is, has that theory/observation been done at piston speeds significantly over the speeds mention here?