Design Factors for ICE Peak HP versus RPM?

[NTL2009]

TL;DR Summary

What factors influence the RPM at which an ICE develops peak HP?

I'm thinking about the large (for the 1960's) farm tractors I drove, that produced lots of low end torque, and max RPM was ~ 1800 ~2200 (from memory). Are there certain design factors to maximize horsepower at these low RPMs, versus a common auto engine that might peak around 5,000-6,000 RPM?

I realize the farm tractor is designed for low RPM to extend life, as it is often run all day at that RPM, versus a typical auto that hits high RPM for acceleration, then settles down to that 1800~ 2200 RPM for cruising. Does bore x stroke play into it? Valve timing, 'breathing' (I guess the size of the air intake?), or other things? Or am I looking at it wrong, and they limit the engine to ~ 2000 RPM for long life, and the power you get it is the power you get, due to limits of torque x RPM? Though I would think it could be optimized.

I ask paritally out of pure curiosity, and partially thinking about a series/parallel hybrid car, where it could be designed such that the ICE never needs to rev so high, the acceleration would come from the motor/battery, and the engine would cruise along like that farm tractor. That ICE could be designed differently if it never needed to hit high RPM (stroke would not be so limited by piston speed, less issue with valve float, etc).

 

[Tom.G]

Summary:: What factors influence the RPM at which an ICE develops peak HP?

Are there certain design factors to maximize horsepower at these low RPMs,

Yup.

Largely a large bore to get large surface area for the combustion pressure to act on.

RPM would be restricted by how much air/fuel can be 'inhaled' (limited cross-section of intake and exhaust, and by stroke)

Cheers,
Tom

 

[Baluncore]

I'm thinking about the large (for the 1960's) farm tractors I drove, that produced lots of low end torque, and max RPM was ~ 1800 ~2200 (from memory).

Was the tractor you drove diesel? What make and model?

High speed engines need to change speed rapidly, so they have lightweight flywheels and 5 or more cylinders.

Tractors need weight for traction. A 3 or 4 cylinder engine with a heavy flywheel suits a tractor better than a car in stop/start traffic.

 

[NTL2009]

It was a John Deere 4010 Diesel (they made a gas version as well), but it was generally true of any farm tractor, that they ran at fairly low RPMs (I think this was the only one we had with a tach at the time - it was new in 1963, our others were pre-war models). Here are some specs, 6 cylinder, and rated HP is at 2200 RPM. Gas version has a "square" Bore/Stroke (same rated HP and RPM) . Diesel a longer stroke, expected I guess, for the higher compression ratio.

Diesel is higher displacement also - 380 ci [6.2 L] versus 302 ci [4.9 L] for gasoline.
I'd need to look at an auto engine curve, but in very round numbers, 80 HP @ 2200 RPM would probably be high for a 302 ci [4.9 L] auto engine, since their peak is around 5,500 RPM?

OK, did some searching - this 226 CI in-line 6 auto engine produces ~ 95 HP at 2200, rated peak is at 4,000 RPM. So not so very different at 2200 RPM. and it is undersquare configuration -- a 3.34-inch bore and 4.48-inch stroke, so that would seem to back up @Tom.G comment that a higher bore would provide more force for those low RPMs.

https://cars.typepad.com/.a/6a00d83451b3c669e201630667f0ac970d-800wi

So while lower RPM would allow for a longer stroke (which allows more time for the expanding gas to work on the piston), since piston speed would still be relatively low, I guess the surface area of the wider bore wins out at low RPM? Other than that, I don't think there are any big differences, just optimization of a lot of little things?

http://www.tractordata.com/farm-tractors/000/0/6/61-john-deere-4010-engine.html

 

[jack action]

Does bore x stroke play into it?

It is only a question of mean piston speed, so only the stroke matters in the RPM output. Your engines have typical low mean piston speeds for 'endurance' engines.

Valve timing,

Valve timing is more or less linked to the mean piston speed.

'breathing' (I guess the size of the air intake?)

You adjust the valve size / bore ratio according to the mean piston speed range where you want to perform.

they limit the engine to ~ 2000 RPM for long life, and the power you get it is the power you get,

• You determine the desired mean piston depending on how hard you want your engine to work;
• Then you determine your total piston area according to the power output you want;
• Then you determine your stroke according to the compromise you wish to make:
  • Shorter stroke: Small displacement but high RPM (Lighter, but need a more complex transmission and valvetrain; usually bad performance at lower speeds)
  • Longer stroke: Low RPM but large displacement (Heavier)
• Then you determine the number of cylinders according to the desired bore and/or combustion chamber design and/or smoothness of the engine

where it could be designed such that the ICE never needs to rev so high

That engine would necessarily be huge. You have a choice: For a given mean piston speed, either the power comes from high RPM or a large displacement.

 

[NTL2009]

...

NTL2009 said: where it could be designed such that the ICE never needs to rev so high

That engine would necessarily be huge. You have a choice: For a given mean piston speed, either the power comes from high RPM or a large displacement.

Thanks, especially for that last part, I guess there really is no substitute (all else being equal) for more power strokes per minute! That has to mean... more power (over a minute)! And yes, that John Deere 80 HP engine was HUGE (to keep RPM low to extend life).

 

[Lnewqban]

With the increased size of the engine, the problem of increased internal forces of heavier reciprocating parts due to higher values of acceleration and deceleration becomes more important.
Those increased forces don't mix well with reduced dynamic lubrication due to slower sliding speeds between surfaces of two parts.
Ship engines that have huge pistons rotate very slowly.

 

[jack action]

With the increased size of the engine, the problem of increased internal forces of heavier reciprocating parts due to higher values of acceleration and deceleration becomes more important.
Those increased forces don't mix well with reduced dynamic lubrication due to slower sliding speeds between surfaces of two parts.
Ship engines that have huge pistons rotate very slowly.

Slower RPMs doesn't mean slower sliding speed. That is why you must look at mean piston speed instead. This is what influences accelerations & decelerations. The largest engine in the world (stroke: 2.5 m; rpm range: 22-120 rpm; MPS: 8.5 m/s) has a mean piston speed comparable to the ones of the tractors discussed in this thread.

The RPM range is always determined by the desired mean piston speed.