Turboprop engines
Turboprop engines combine the best features of turbojet and piston engines. The former is more efficient at high speeds and high altitudes. Consequently, commuter aircraft and military transports tend to feature turboprop engines. A turboprop engine differs from a turbojet engine in that the design is optimized to produce rotating shaft power to drive a propeller,
instead of thrust from the exhaust gas. The turboprop uses a gas turbine to turn a large propeller.
The shaft that connects the propeller to the turbine is also linked to a gearbox that controls the propeller’s speed. The propeller is most efficient and quiet when the tips are spinning at just under supersonic speed.
Turboprop engines may be further classified into two groups, depending on the turbine driving the propeller.
In the first group, the propeller is driven by the same gas turbine driving the compressor.
In the second group, an additional turbine (normally denoted as a free-power turbine) turns the propeller.
The turbofan engines
The turbofan engine was designed as a compromise between the turboprop and turbojet engines. It includes a large internal propeller and two streams of air flowing through the engine. The primary stream travels through all the components like a turbojet engine, while the secondary air passes through the fan and is either ducted outside through a second nozzle identified as the cold nozzle or mixes with the hot gases leaving the turbine(s) and both are expelled from a single nozzle.
Turbofan engines have better performance and fuel economy than turbojet at low power settings, low speeds, and low altitudes.
