Basically, a turboprop engine is a different type of turbojet machine which is used to drive a propeller. It has specially been designed to draw a lot more energy from the hot gas than is necessary in order to be able to drive a propeller. This energy extraction is done in the turbine section of the gas generator. Here, thrust is generated by a gas jet which flows from a jet nozzle.
Although, a turboprop is very similar to a turbojet engine as they have the same layout, three new features is added to the former to perform its function: 1) an additional turbine to drive the propeller; 2) a two-spool arrangement of the rotational machinery; 3) a mechanical reduction gear to convert the high rotatory speed of the turbine to the more moderate speed of the propeller.
If the turbojet engine was conceived to accelerate a relatively low air mass flow to a high exhaust velocity, a turboprop was designed to accelerate a high mass flow to a low velocity. This gives the turboprop engine a great fuel efficiency (and cabin noise), but the aircraft it propels cannot develop high flight speed. Thus, it is used in transport aircraft and in helicopters.
At flight velocity around Mach 0.8 (1000 km/h at 11,000 m of altitude) turboprop and turbojet engines operate at low propulsive efficiency, because this flight speed is too high for a turboprop, but low for a turbojet. To fill the gap, turbofan engines were designed, performing extremely efficiently at high subsonic cruise velocities. That is why they are used in civil aviation aircraft.
Diagram of turboprop engine
Diagram of propulsive efficiency characteristics of turboprop, turbofan, and turbjet engines
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