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.

 

[searcyfarms]

i seriously doubt you would notice the difference in 2600 vs 2800 in seat sensation on the tractor or rotary propulsion on a 50hp tractor even less on a tractor of lesser hp

agreed, weight and traction determine way more than 200 rpms

Here is an example on a farmall engine D282 used in 560/656/706 in the 560 it was rated at 65hp @ 1800rpm = 59.49pto + 656 = 61.52pto @ 1800rpm + 706 =72.42pto @2100rpm same engine in combine i believe was @2300 and close to 80hp if i remember right

 

[nards444]

yeah not sure RPMs matter, my truck spins the same RPMs as my challenger but my challenger has about 100 more HP and TQ, comes down to many thing, torque and HP curves, bore stroke, displacement, weight, tires etc.

 

[Threepoint]

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.

Yep, the concept of claimed HP simply as a function of engine rpm is all I was trying to explore with my initial question. Specifically, whether a manufacturer might buff up its marketing merely by rating an engine at a higher rpm to achieve the same power output as another manufacturer who rates a slightly more powerful engine at a lower rpm. :scratchchin:

After seeing the comments in this thread from folks that know a lot more about engine design than I do, I'm concluding that the answer may be "yes", in theory, but that the question is really meaningless as a practical matter without knowing much more about the design characteristics of the two engines being compared. And that information is probably never going to be readily available in the published marketing data. At least for someone like myself to decipher. For example, all of the tractor brands I've taken a quick look at online for this inquiry give the HP and displacement of the engines for their various models. But as one poster points out, even if two engines have the same displacement, one might be achieving that with a shorter stroke but larger bore, thus turning at higher rpms.

Seems to me another big variable complicating this with today's CUTs is that, even with identical engines within the same series of the same brand, the exact same engine may be used in all models, with the HP determined merely by the fuel delivery to the injectors programmed into the ECU by the brand's proprietary software. One example is my Kioti NX4510HST cab. The same engine is used in the rest of the NX series, up to the NX6010HST, but programmed for more fuel delivery. Other brands do this as well, presumably to make manufacturing more efficient and enable them to hit different price points with changes that are very easy for them to make.

 

[Industrial Toys]

Our JD 6200 was available as a 6300 and 6400 all identical except for HP and the injector pump, but quite a price difference. I always like a machine working as easy as possible and hopefully longer. Never chipped my truck or any such thing. Then you might as well be on steroids as well! lol

 

[finn1]

Yep, the concept of claimed HP simply as a function of engine rpm is all I was trying to explore with my initial question. Specifically, whether a manufacturer might buff up its marketing merely by rating an engine at a higher rpm to achieve the same power output as another manufacturer who rates a slightly more powerful engine at a lower rpm. :scratchchin:

After seeing the comments in this thread from folks that know a lot more about engine design than I do, I'm concluding that the answer may be "yes", in theory, but that the question is really meaningless as a practical matter without knowing much more about the design characteristics of the two engines being compared. And that information is probably never going to be readily available in the published marketing data. At least for someone like myself to decipher. For example, all of the tractor brands I've taken a quick look at online for this inquiry give the HP and displacement of the engines for their various models. But as one poster points out, even if two engines have the same displacement, one might be achieving that with a shorter stroke but larger bore, thus turning at higher rpms.

Seems to me another big variable complicating this with today's CUTs is that, even with identical engines within the same series of the same brand, the exact same engine may be used in all models, with the HP determined merely by the fuel delivery to the injectors programmed into the ECU by the brand's proprietary software. One example is my Kioti NX4510HST cab. The same engine is used in the rest of the NX series, up to the NX6010HST, but programmed for more fuel delivery. Other brands do this as well, presumably to make manufacturing more efficient and enable them to hit different price points with changes that are very easy for them to make.

There are two ways to get to a target horsepower on a given engine. First, one can spin it at a higher rpm, but that approach is not without issues, as there are piston speed parameters (feet per minute of piston travel) and pumping and friction losses that hurt breathing and, ultimately, fuel consumption.

The second method is to run the engine at a higher bmep (brake mean effective pressure, a parameter that normalizes a flywheel power output for displacement). Higher bmep ratings can be achieved by tuning combustion chamber geometry, fuel system parameters (spray patterns, pressures, rate of injection, etc), and advanced turbocharging, etc.

 

[nards444]

Yep, the concept of claimed HP simply as a function of engine rpm is all I was trying to explore with my initial question. Specifically, whether a manufacturer might buff up its marketing merely by rating an engine at a higher rpm to achieve the same power output as another manufacturer who rates a slightly more powerful engine at a lower rpm. :scratchchin:

After seeing the comments in this thread from folks that know a lot more about engine design than I do, I'm concluding that the answer may be "yes", in theory, but that the question is really meaningless as a practical matter without knowing much more about the design characteristics of the two engines being compared. And that information is probably never going to be readily available in the published marketing data. At least for someone like myself to decipher. For example, all of the tractor brands I've taken a quick look at online for this inquiry give the HP and displacement of the engines for their various models. But as one poster points out, even if two engines have the same displacement, one might be achieving that with a shorter stroke but larger bore, thus turning at higher rpms.

Seems to me another big variable complicating this with today's CUTs is that, even with identical engines within the same series of the same brand, the exact same engine may be used in all models, with the HP determined merely by the fuel delivery to the injectors programmed into the ECU by the brand's proprietary software. One example is my Kioti NX4510HST cab. The same engine is used in the rest of the NX series, up to the NX6010HST, but programmed for more fuel delivery. Other brands do this as well, presumably to make manufacturing more efficient and enable them to hit different price points with changes that are very easy for them to make.

If tractors are like cars theres probably not set method to determine HP, Torque lifting capacity, etc. Bottom line is I could take my truck to dyno and it would be different every day of the week. My guess is most ratings are pretty close, but are not spot on.

 

[CobyRupert]

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.

I think the simple answer is: Well, yes....but no. That is:
Hp = Torque x speed (rpm). So twice the rpm would be twice the horsepower if the torque output stayed constant at all rpms. But it doesn't, this is the "torque curve". The flatter the torque curve then the more true your statement is. Similarly, if the torque is 1/2 at twice the rpm, horsepower would be the same at both rpms.
Others can explain why torque varies with rpms.

 

[Raspy]

Naturally aspirated engines have natural torque and HP curves based on the engine's ability to breathe. This is the result of cam timing, valve lift, intake and exhaust port design, manifolds, etc. This generally works out to maximum torque arriving below maximum HP. And max torque is where the engine is running most efficiently under load. Where it can breath the best while working. Every different engine, with a given HP will have a little different torque/HP curve. So, if a bunch of different brands of engines are all rated at 50 HP, they certainly will have a few hundred RPM difference in RPM at maximum torque. Only discussing non-turboed engines here.

If you are trying to do the maximum amount of work possible for a given tractor, you can run it at the maximum HP RPM and select a gear that will keep it there. Just set it at full throttle, and select the highest gear it can pull without dropping down in RPM below the max HP number. That gives you the maximum amount of work and won't hurt the engine, but it does stress it as hard as it is designed to run.

For PTO use, you have one set gear ratio, 540 at the PTO, that is geared through the transmission and not selectable. It's nice to run the engine at maximum torque RPM here because it is not revving so high and it will hold a load without dropping RPM better than at max HP RPM. Maximum PTO HP will show as a lower number than rated engine HP because the engine output is different at the crankshaft than at the PTO shaft, because of the transmission losses, and maximum torque occurs below maximum HP. Drawbar HP should be different again.

The HP/torque curve for modern emissions and turbo engines are a different story. With a turbo, they can program the torque curve because the engine can be pushed much harder. It gets force fed more air. So the designers will select how much torque they will allow and design where that number comes out in the RPM range. Or they can hold that number over a wide range of RPM. For example: Cummins engines around 2004, had full rated torque from 1400 RPM up to near redline. The torque curve was programmed and not based on volumetric efficiency. Just remember that the chart you see with a turbo engine, with a common rail injection system, is a programmed chart, not a chart based on the engines ability to naturally breath.

When you see a max torque RPM on a non-turbo engine, that is very near the point of maximum efficiency while working. It is the point where you'll see a tan/brown haze from the exhaust, but not black smoke. It's the happy zone for the engine.

One small nitpick here: RPM means Revolutions Per Minute. It is already plural. There is no such thing as RPMS. Putting an S after RPM would mean that "RPM" was a singular event that had to be made plural. A revolution is singular. Revolutions per minute, is plural. RPMs means you are discussing various engine speeds, not a particular engine speed. Such as: "What are the two different RPMs you like to run your tractor at while plowing or discing?"

 

[Threepoint]

[snip]
The HP/torque curve for modern emissions and turbo engines are a different story. With a turbo, they can program the torque curve because the engine can be pushed much harder. It gets force fed more air. So the designers will select how much torque they will allow and design where that number comes out in the RPM range. Or they can hold that number over a wide range of RPM.

John - This is interesting. That makes me wonder whether, when rpm changes in turbocharged, common-rail injector diesels due to throttle setting or load, the ECU is actually modulating the air input from the turbocharger relative to the fuel delivered by the injectors, instead of modulating the fuel delivered by the injectors relative to the air input from the turbocharger. I had thought of it as the latter. I suppose the result is the same: the delivery of the air-fuel mixture modulates the torque and HP curves as a function of engine rpm, and the relationship is not necessarily linear. (I think ) Either way, it would allow the same engine to be used in several models of CUTs, each claiming a different HP.

But this raises another question in my mind, going back to my initial post. If modern engine manufacturers have such flexibility to program for HP and torque this way, relative to rated rpm, why might one choose to use 2800 as their "rated" rpm, rather than, say, 2600? I doubt that I'm alone in associating increased engine rpm with increased noise and vibration, as a general rule. Maybe that's not correct, but from a vendor's marketing perspective, it seems to me being able to claim 50 HP at 2600 rated rpm would be more appealing than 50 HP at 2800 rated rpm. :confused3:

 

[Raspy]

Threepoint,

In diesels they don't control the air intake. The boost is a direct result of HP being produced, which drives the turbo with exhaust energy. Boost pressure is limited by the waste gate in the turbo, which opens to bypass exhaust around the turbine, to limit the intake manifold pressure. Fuel is controlled, but it's not the mixture they are controlling, it's the fuel volume. They can keep ramping up the fuel quantity as long as there is enough air and time to burn the fuel efficiently. The fuel map knows the amount of air entering the engine, or the intake manifold pressure and controls the fuel to give the HP desired at that throttle position and RPM, or the max fuel that can be burned with that amount of air, or the max HP they will allow the engine to produce, or to limit the temperature in the cooling system.

The turbo is a feedback loop. The more power you make, the more air the turbo pumps in. Which means you can make still more power, which spins the turbo even faster, etc. But it's not just the quantity of air that determines the maximum amount of fuel, the fuel takes time to inject and time to burn. The power stroke is very short, so eventually, fuel is still being injected as the exhaust valve is beginning to open. Lots of smoke and no more power produced.

The advertised HP and torque specs are decided on, I guess, for various reasons. The engine has to survive at the rated HP, without any failures, and has to be happy to work long hours at it's rated torque, at the PTO RPM, or max HP RPM. All this while meeting emissions standards.

The turbo simply makes a small displacement engine act like a large displacement engine. But it's still physically small, with small bearings, pistons, etc. So it needs to have excellent cooling and lubricating systems to put up with all the strain. Turbos and large fuel delivery can easily overload engines. Before turbos, engine were much heavier for their HP and the limit was how much atmospheric air was available.