Comparison Engine/PTO HP and "rated RPM"

[Threepoint]

I haven't paid much attention to this up to now, but I've noticed that for CUTs with similar frame sizes, the published specs of one tractor brand might state gross (or net) engine HP and PTO HP at, say, 2600 "rated rpm", while another might state it at 2800 "rated rpm".


I know only enough about power and torque curves at this point to be dangerous to myself and others. But I do know that engine and PTP hp is typically measured with a dynamometer, thus measuring rotational force, and that HP = Torque x RPM div by 5252.


If that is so, then unless the torque for the specific engines being measured actually drops as the rpm rises above 2600, then the measured HP should rise as the rpm increases, no? Conversely, it should drop slightly as the rpm decreases.


My specific question is this: If we have two CUTs in the same frame class, and both claim gross engine horsepower of 50 HP, but the first brand is rated at 2600 rpm and the second at 2800, does the second actually have less HP if rated at 2600? There may be more to this than I'm taking into account, but if a buyer is simply trying to compare HP in a particular instance, is using a higher rated rpm in the published specifications perhaps a way for a brand to enhance the marketing appeal?

 

[oosik]

Too many are fooled by the ever so slight differences in Hp at slightly higher/lower rpm's. All else being equal - it's the weight of the tractor & type of tires that will get the job done. Weight & pull/grunt will do the job.

 

[radios1]

the one rated at the lower RPM has a larger engine, so it don't have to rev so high!. it's like why do formula 1 cars rev so high, 7000 RPM+ well, that's to get more HP with the same size engine!..

 

[Croomrider]

Most engines do lose torque if they go above a certain RPM. There can be a lot of different reasons for it, but it basically comes down to the design criteria. When you see rpm's listed on tractor HP, that rpm is most likely the highest torque rpm for that engine and is usually also where the tractor is set to make 540 rpm's at the PTO.

 

[Croomrider]

the one rated at the lower RPM has a larger engine, so it don't have to rev so high!. it's like why do formula 1 cars rev so high, 7000 RPM+ well, that's to get more HP with the same size engine!..

That would generally be true if the engines are not turbo charged. A smaller turbocharged engine could make a higher rated HP at a lower RPM!

 

[ptsg]

Most engines do lose torque if they go above a certain RPM. There can be a lot of different reasons for it, but it basically comes down to the design criteria. When you see rpm's listed on tractor HP, that rpm is most likely the highest torque rpm for that engine and is usually also where the tractor is set to make 540 rpm's at the PTO.

Not entirely true. Here is the power and torque graph for the Kukje/Cummins A1700 on my tractor. The 540 RPM on the PTO is around 2500 engine RPM when the HP maxes out. While the 540E is around 1900 engine RPM, right at the torque peak.

 

[ericm979]

Tractor designers often aim for "torque rise" (sometimes called torque backup). That's where as the RPMS drop from the operating RPMS, torque rises. That makes the tractor hold RPMS better under load and be less likely to stall. Obviously torque can't rise too much or over to far an RPM drop. But since it's often the case that tuning for good torque at low RPMS will naturally result in torque peaking at lower RPM than the mechanical redline or the HP peak, it's common to have torque rise.

Torque backup | Perkins Engines

 

[finn1]

The governor on the fuel system, either electronic or old style mechanical systems pulls back fuel delivery above rated speed of the engine. High idle, ie max unloaded engine rpm, unloaded is anywhere from 3% to 10% above the speed that the engine produces max power. The governor / power curve above rated is called the overrun curve.

Stationary engines, for example genset engines have a very steep overrun curve, sometimes using an isochronous governor strategy.

Road vehicles usually have broader regulation curves for driveability reasons, ie hitting the governor at shift points is like hitting a wall, as power drops off seemingly instantaneously with a “tight” governor.

Torque rise is defined as the increase in torque output as the engine is lugged down from rated speed/ max power. 15% is a descent torque rise target. You sometimes hear reference to “constant power” lug curves. Extremely high torque rise settings effectively result in constant power as the engine speed decreases.

It’s all math.

 

[Industrial Toys]

Am I wrong in thinking, Speed makes HP by math alone. An Engine going 1800 will make "about" half the HP as one going 3600, give or take. Why you pay so much more for an 1800 RPM generator as you are buying twice as much engine as one could get away with.

But Displacement makes real HP.

 

[Katahdin]

I think you (the OP) overlooked bore/stroke ratio and displacement. If the engines are of the same displacement, but one has a shorter stroke, that engine has to spin faster to make the same power. And it can spin faster, because the pistons have less travel distance and friction per stroke.