High vs Low RPM motors

[Dforesi]

So a former team member and I were having a discussion on why it is that high rpm engines seem to be ideal for racing applications. I understand the fact that the longer you can maintain your torque into the rev range the better. But with all else being equal it seems to be that short stroke, lower torque, high rpm engines seem to be the ideal choice. I've always understood the fact that for towing applications you would want a long stroke so that each power cycle produces a higher amount of torque and your engine doesn't have to be screaming to get moving. However I've never been able to wrap my head around why it goes the other way.

Does it come down to gearing restrictions where if the high torque/low rev motor is geared high enough then power delivery becomes very rough and throttle response is decreased?

Just interested to hear other members thoughts on this point.

[Slim]

A high rpm engine will of course be smoother, but I think in the range of engines you find in FSAE that you wouldn't really get to the point where the driver feels a choppy power delivery. However, singles and (low speed engines?) can be harder to start and idle because there is a much greater speed and power variation throughout one cycle. Which I've heard can become significant to combustion dynamics at low speeds.

It might be because at low rpm you lose a much larger percentage of combustion heat to your engine block. Your heat rate is roughly constant and so the longer a stroke takes the more heat will be lost. If your engine goes fast enough, this affect will be reduced but you will begin to lose more energy to friction loses. Peak thermal efficiency is somewhere between these two extremes.

Longer stroke motors also see higher piston accelerations and higher forces. Which means they need to be build heavier and are likely to accelerate more slowly than a lighter high rpm engine. Although, I don't know what happens when you apply a suitable gear reduction and the fact that your longer stroke engine is operating at lower speeds/accelerations.

[Slim]

Actually, another idea,for a given engine volume because Power=Torque*RPM,you make more power at higher rpm because RPM increases faster than torque drops off from having to shorten the stroke. But for towing you just need lot of torque to get it moving so you go for the low rpm long stroke. For FSAE we want to convert lots of power into kinetic energy, so high RPM is good. Of course then there is the restrictor problem at high rpm....

I'm not an engine guy so could be wrong on stuff, but it was an interesting question and got me thinking.

[Charlie]

Horsepower is what accelerates the vehicle.

For the same displacement and torque, you get more horsepower with increasing engine speed. Therefore generally, an engine with high power-to-mass ratio will run at a higher engine speed.

It really is that simple generally, although lots of semantics can be discussed.

[PatClarke]

long stroke so that each power cycle produces a higher amount of torque

Diforesi,
Explain to me please how a long stroke engine produces more torque?

Pat

[Owen Thomas]

Like others have said, it is primarily because you will get more usable power at higher revs(P = T*omega).

I believe the phenomenon of high RPM racing engines is mostly because of head geometries. A high RPM engine like found in F1 is finely crafted to operate perfectly in a very small range. A lower RPM engine has a much broader powerband, but is not extracting the peak potential of the engine cycle.

In higher levels of motorsports, you can afford to have a peaky engine because you can also afford a driver capable of using it. For towing a trailer, you do not want to be flitting between gears every 3 seconds. Also, like you said, they have to have some get up and go in the lower range just to get rolling (no double clutch launch ).

Just my take on it, there's tons of finer details beyond my own understanding.

[Racer-X]

Originally posted by PatClarke:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">long stroke so that each power cycle produces a higher amount of torque

Diforesi,
Explain to me please how a long stroke engine produces more torque?

Pat </div></BLOCKQUOTE>

I think he's talking about a stroker. You know same bore, more stroke, more displacement.


As for the OP's question. For the same displacement the motor that revs higher (if properly tuned) will make more power. More power means if you gear it right you can put torque to the wheels for longer and have more force to accelerate. This is because power is a function of RPM and torque. If a racing series is displacement limited without restrictors you will almost always see motors being pushed to the highest RPM possible to get as much power out as they can.

[Jan_Dressler]

Originally posted by PatClarke:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">long stroke so that each power cycle produces a higher amount of torque

Diforesi,
Explain to me please how a long stroke engine produces more torque?

Pat </div></BLOCKQUOTE>

long stroke so that each power cycle produces a higher amount of torque and your engine doesn't have to be screaming to get moving

I think he is talking about more torque in the lower rpm range. Which can be true:

Longer stroke --> smaller bore --> smaller valves and ports --> higher speed of air/fuel mixture at a given speed --> more tumble/swirl --> more turbulence --> (together with more compact shape of the combustion chamber; and less wall heat transfer) faster/more efficient combustion --> ignition timing can be adjusted accordingly --> more torque

[Owen Thomas]

Longer stroke --> smaller bore --> smaller valves and ports --> higher speed of air/fuel mixture at a given speed --> more tumble/swirl --> more turbulence --> (together with more compact shape of the combustion chamber; and less wall heat transfer) faster/more efficient combustion --> ignition timing can be adjusted accordingly --> more torque

Well put Mr. Dressler. You can also see how these characteristics would limit torque at higher rpms, affecting peak power.

It's probably worth mentioning too that a fundamental limitation for IC engines is piston speed. This is why you can't get the cylinder filling benefits of a long stroke and still have high rpm/power.

[Jan_Dressler]

Originally posted by Owen Thomas:
Well put Mr. Dressler. You can also see how these characteristics would limit torque at higher rpms, affecting peak power.

Thanks. And yes, it certainly affects peak power

Originally posted by Owen Thomas:
It's probably worth mentioning too that a fundamental limitation for IC engines is piston speed. This is why you can't get the cylinder filling benefits of a long stroke and still have high rpm/power.

Well, you wouldn't get very high performance out of a long stroke engine at high rpm anyway, even if it could do that high rpm, as you mentioned.

The other way round, you will get into problems performance-wise if you design your bore too large - At a bore-stroke ratio of 2:1 and plus, the tumble flow field of your ordinary 4-valve pentroof combustion chamber will have increasing problems to develop properly. That's where valve designs other than the conventional poppet valve might come into play...

Additionally it has to be mentioned that a rather large valve area and ports for a given swept capacity (and therefore larger bore/stroke ratio) not only comes with higher engine speed, but also with turbo-/supercharging.