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?
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.
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.
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).