A turboprop engine is a variant of a jet engine that has been optimised to drive a propeller. Turboprop equipped aircraft are very efficient at lower flight speeds (less than mach 0.6), burning less fuel per seat-mile and requiring significantly less runway for takeoff and landing than a turbojet or turbofan powered aircraft of the same size. When the aircraft is used over relatively short distances, these cost and performance benefits offset the lower speed making turboprops the engine of choice for most commuter aircraft. Examples of turboprop powered aircraft include the Bombardier Dash 8, the Alenia ATR 42 and the Pilatus PC-12.
A turboprop 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 turboprop and the turbojet is that additional turbines, a power shaft and a reduction gearbox have been incorporated into the design to drive the propeller. The gearbox may be driven by the same turbines and shaft that drive the engine compressor, mechanically linking the propeller and the engine, or the turbines may be separate with the power turbine driving a concentric, mechanically isolated shaft to power the gearbox. The latter design is referred to as a "free power turbine" or, more simply, a "free turbine" engine. In either case, the turbines extract almost all of the energy from the exhaust stream using some of it to power the engine compressor and the rest to drive the propeller.
A turboprop engine is very similar to a turboshaft 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.