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piston speed
2

piston speed

piston speed

(OP)
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

RE: piston speed

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.

RE: piston speed

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.

RE: piston speed

(OP)
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

RE: piston speed

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.

RE: piston speed

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.

RE: piston speed

(OP)
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

RE: piston speed

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.

RE: piston speed

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.

RE: piston speed

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


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RE: piston speed

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.

RE: piston speed

(OP)
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

RE: piston speed

... 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

RE: piston speed

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.

RE: piston speed

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

RE: piston speed

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.

RE: piston speed

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?

RE: piston speed

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

RE: piston speed

what good is mean piston speed as a measure over say actual peak piston speed?

RE: piston speed

i think it will depend on specifics of geometry and therefore not directly related but i have not worked it out yet

RE: piston speed

rod length will affect peak piston speed

RE: piston speed

almost certainly the engine with higher MPS will have a higher peak speed but i also imagine a few % would be significant if you are pushing the envelope on what is considered a extremely high "mean piston speed"

RE: piston speed

A 100mm stroke at 7500rpm has an MPS of 25m/s.

The peak piston speed with a 150mm rod is 41.22m/s. With a 200mm rod is 40.42m/s.

A change of about 1.9%. Is that significant?

RE: piston speed

this thread is about MPS limits, i cant see how ignoring a 2% higher peak piston speed for a given MPS makes sense. close enough is not good enough in some applications.

RE: piston speed

Hi Golfpin,

I enjoyed reading the responses to your post. I agree fully with Ivymike (very first response), and I can back it up with a little anecdotal data. The Teledyne Continental horizontally opposed 6 cylinder 520 cid engine with 4 inch stroke is offered as a recommended 1600 hour TBO engine when rated at 2700 rpm (uncorrected mean piston speed of 1800 fpm or 9.1 mps), but the recommended TBO drops to 1200 hour when the engine is rated at 3400 rpm (2267 fpm or 11.5 mps) and about the same BMEP.

(TSIO-520-UB versus the GTSIO-520-L).

But the real point of my response is to share some of Sir Harry's comments about piston speed and the relationship between piston speed and engine breathing (and power).

Regards,

Dick


RE: piston speed

Greg Locock, i was merely alluding to that when considering port Mach number and flame speed it would be more relevant to look at actual velocity, peak velocity or average over a more narrow crank angle of the when those things are occurring rather than washing away some details away by using MPS.

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