Turboprop and turboshaft engines do not provide significant thrust through their jet pipes, so EPR would not be of any use in determining the thrust being produced by the engine. Engine torque is used to indicate the power that is developed by these engines, and the indicator is known as a torquemeter. The engine torque or turning moment is proportional to the horsepower and is transmitted through the propeller or rotor reduction gear.
A torquemeter system is shown in fig. In this system, the axial thrust produced by the helical gears is opposed by oil pressure acting on a number of pistons; the pressure required to resist the axial thrust is transmitted to the indicator.
In addition to providing an indication of engine power, the torquemeter system may also be used to automatically operate the propeller feathering system if the torquemeter oil pressure falls due to a power failure. It is also used, on some installations, to assist in the automatic operation of the water injection system to restore or boost the take-off power at high ambient temperatures or at high altitude airports.