A turboshaft engine is a variant of a jet engine that has been optimised to produce shaft power to drive machinery instead of producing thrust. Turboshaft engines are most commonly used in applications that require a small, but powerful, light weight engine, inclusive of helicopters and auxiliary power units.
A turboshaft engine uses the same principles as a turbojet to produce energy, that is, it incorporates a compressor, combustor and turbine within the gas generator of the engine. The primary difference between the turboshaft and the turbojet is that an additional power section, consisting of turbines and an output shaft, has been incorporated into the design. In most cases, the power turbine is not mechanically linked to the gas generator. This design, which is referred to as a "free power turbine", allows the speed of the power turbine to be optimised for the machinery that it will energize without the need for an additional reduction gearbox within the engine. The power turbine extracts almost all of the energy from the exhaust stream and transmits it via the output shaft to the machinery it is intended to drive.
A turboshaft engine is very similar to a turboprop and many engines are available in both variants. The principal difference between the two is that the turboprop version must be designed to support the loads of the attached propeller whereas a turboshaft engine need not be as robust as it normally drives a transmission which is structurally supported by the vehicle and not by the engine itself.