I realize torque and horsepower are essentially the same measurement since horsepower is simply torque over time. But I'm curious as to why diesel engines or electric motors always provide torque ratings whereas turboprop engines are only rated in horsepower. Is there any reason for this?
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$\begingroup$ If the turboshaft engine is not in an aircraft, they actually do provide torque numbers. Example AGT1500 (engine of the M1 Abrams) has 3700Nm at 3000rpm. $\endgroup$– juwensCommented Jul 20 at 23:26
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$\begingroup$ You’ll even find values for the “chrysler turbine car” from Wikipedia “The 1963 Turbine's engine generated 130 brake horsepower (97 kW; 132 PS) and an instant 425 pound force-feet (576 N⋅m) of torque at stall speed”. $\endgroup$– juwensCommented Jul 20 at 23:40
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3 Answers
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The reason a Diesel engine has a torque figure and a horsepower power figure is because max torque and max horsepower occur at different engine RPMs. Have a look at any performance curves of a piston engine and this will be evident. Hence, to define in a few numbers the maximum performance of the engine, you need to give both figures.
However, turboprop engines for propeller aircraft, and turboshaft for helicopters, operate differently. Maximum efficiency for a rotor or propeller occurs at a certain RPM. Operating at different RPMs is inefficient. Hence, propellers and helicopter rotors are designed to operate at a fixed RPM. As load changes, the blade pitch angle is changed so it “bites” into more or less air. Hence, RPM can be kept constant, despite the load (and hence torque demand) changing. Now, since the power turbine is running at a constant fixed RPM, only 1 performance figure is needed (SHP or TQ) because the other can simply be derived from it.
Torque is just as important in a turboshaft or turboprop as a piston engine. These engines all have a torque sensor that usually works by measuring the twist in the power turbine shaft. Most helicopters do an engine performance check daily that involves reading TQ of the gauge prior to flight to ensure adequate performance.
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$\begingroup$ One can compare it to these 50ccm ICE scooters (like a vespa). There CVT is tuned so that the engine always runs at relatively high rpm, hence delivering nearly max hp. That’s the reason why they can start so fast at traffic lights, despite having only 7kw. The CVT is similar to the self adjusting blade pitch. $\endgroup$– juwensCommented Jul 20 at 23:51
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Because it's less useful for aviation application.
For diesel and electric, or ground vehicle and industrial motors in general, lack of peak torque is a problem (engine/motor stall), but for aviation, there's never a condition that you can stall a turbine. Usually you stall the propeller way before you stall a turbine.
Or, put it another way, an aviation turbine is rarely found short of torque.
However torque is still an important performance metric and will be stated in the data sheet, but simply isn't actively marketed as much as diesel and electric motor.
answered Dec 17, 2018 at 3:54
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$\begingroup$ Thanks for the answer. But I'm still curious as to why torque would be a more important metric say for a truck engine but less important for powering a low RPM helicopter rotor for example. Wouldn't torque be as important a metric for a turboprop or turboshaft, considering the RPM is low which requires a lot of torque? Maybe I'm just getting confused around the old torque vs horsepower debate. $\endgroup$ Commented Dec 17, 2018 at 4:08
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$\begingroup$ @ChristophePochari Problem with truck is after you've dropped to 1st gear, or starting with 1st gear, the highest force the wheel can put on to the road is limited by the engine toque. And if the engine can not provide enough torque, then the result is usually very bad (stalling, rolling down hill, or can not start to move at all). For an airplane, as long as the system (prop, gear box) are well designed, the engine never face such difficult situation. $\endgroup$ Commented Dec 17, 2018 at 4:19
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$\begingroup$ is it possible to compare the power requirements of a truck compared to spinning a prop or rotor? $\endgroup$ Commented Dec 17, 2018 at 4:40
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1$\begingroup$ @ChristophePochari Simply put, there's no minimum torque to spin the prop or rotor - it's going to spin however small the torque (within reason). Static friction is very small, so less torque just means longer spin-up (energy limited) or slower maximum speed (drag limited). This is adequately covered by the power metric. $\endgroup$– TheracCommented Dec 17, 2018 at 6:33
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Torque and horsepower are different measurements.
Horsepower is a measure of power, where one horsepower equals 746 watts.
Torque is a rotational force, which might be akin to voltage. One can have large amounts of torque, but if a shaft will not turn, no work is done.
Work is accomplished by power.
One may frequently see torque specifications for diesels, because they tend have different operating characteristics. For example, in common diesel engines, max torque may appear at substantially lower RPMs than on a comparable gasoline (Otto cycle) engine. With similarly sized gas and diesel engines, the gas engine usually has a higher horsepower rating, and always at a higher RPM.
The torque rating of a diesel engine or an electric motor are merely forces which the device can apply, at a certain RPM. They are not power ratings.
Horsepower and torque are important attributes when designing propulsion systems, so that the drive requirements (eg. propeller, fan, or wheels on the road) can be matched effectively to the power plant (eg. engine, motor or rubberband).
