Absolute Redline!

[Vortex347]

Hi guys, I remember reading somewhere (will try and find the site url) that the rate of combustion of fuel is limited to 24300 rpm. Anything more than this and the fuel can't combust quick enough for the stroke. Does this mean that the absolute redline is limited by the rate of combustion of the fuel they use at 24300 rpm?

[riff_raff]

Not exactly the case. What really matters is the heat release rate from combustion.

Consider this theoretical example where the piston is fixed at TDC during combustion (quiescent combustion). The spark initiates combustion of the fuel/air charge, and a spherical flame front propagates outward from the spark until all the fuel/air mixture in the chamber volume has been combusted. In this chamber space with no turbulence the flame front progresses at a fairly consistent velocity, and thus requires a certain amount of time to complete combustion.

Now consider the practical example where the piston is moving, the chamber volume is constantly changing, and there is lots of turbulence created by the intake port flow and radial inflow from squish. All this turbulence causes the flame front to progress more rapidly thru the combustion space. But at some point with high rpm engines, even with lots of turbulence, the combustion is not fast enough to produce the rate of heat release necessary for satisfactory thermal efficiency. While it is possible for an SI recip piston engine to operate in excess of 24300 rpm, there is a point of diminishing returns with regards to power, rpm and efficiency.

[Vortex347]

there is a point of diminishing returns with regards to power, rpm and efficiency.

What would you think this point is in terms of rpm at a guess (would have to be below 24300 right)?

[PlatinumZealot]

Around 22k.. Williams did a release on this.. Can't find it though... this is due to friction.
There are new tribology technologies since then so maybe this can be even higher now.

[flynfrog]

For an idea of how turbulent it is in there.

[riff_raff]

What would you think this point is in terms of rpm at a guess (would have to be below 24300 right)?

Without any design constraints, it would be possible to build an engine with small cylinder dimensions that performs very nicely at well over 24,300.

[toraabe]

OK , but due to higher inner friction the power would probably not be higher. Remember that you always want to get the power at as low rpm as possible. That is also why the current engines is so much more efficient + all the accessories of course

What would you think this point is in terms of rpm at a guess (would have to be below 24300 right)?

Without any design constraints, it would be possible to build an engine with small cylinder dimensions that performs very nicely at well over 24,300.

[flynfrog]

What would you think this point is in terms of rpm at a guess (would have to be below 24300 right)?

Without any design constraints, it would be possible to build an engine with small cylinder dimensions that performs very nicely at well over 24,300.

RC engines do double that without much trouble.

Search for the honda 5 it was a 125cc gp bike with a 21500 red line in the 60's

[J.A.W.]

Honda also put huge effort into a V4 500cc 4T ( defacto V8) designed to spin up to ~22,000rpm 35 years back,
in an attempt to compete with 2T 500cc Grand Prix racebikes.. unsucessfully..

Harry Ryger has recently made claims about a developmental 125cc ( 2T) racing kart engine good for 70hp @ 17,000rpm,
& yet is reckoned to able to spin out to ~30, 000rpm..

[flynfrog]

JAW do you have a link for that engine?

[J.A.W.]

JAW do you have a link for that engine?

'LucF' has posted a few semi-mysterious details on the "2-stroke" thread elsewhere in this section, F-F..

You could try contacting him, to ask if any further data releases are available.

[gautam_2006k]

Yes it depends on fuel type. For eg nitro engines can rev upto 35000rpm. This is because nitromethane is more stable than Ron 98 fuel.

[gautam_2006k]

V8 in F1 revved upto 21000rpm. In 2005 McLaren Mercedes under Kimi's driving touched upto 370.1kph at Monza. There is a guy who has modified his ninja to rev upto 20000rpm and it sounds insane. YouTube - Ninja ZX2R 20000rpm.

[Juzh]

V8 in F1 revved upto 21000rpm.

20k was max ever. 2006 cosworth.