A turboprop engine is a turbine engine that drives an aircraft propeller. In the simplest form, a turboprop consists of an intake, compressor, combustor, turbine and a propelling nozzle.
Air is drawn into the intake and compressed by the compressor. Fuel is then added to the compressed air in the combustor, where the fuel-air mixture then combusts in the combustor. The hot combustion gases expand through the turbine. Some of the power generated by the turbine is used to drive the compressor of the turbine engine. The rest of energy is transmitted through the reduction gearing to the propeller. Further expansion of the gases occurs in the propelling nozzle, where the gases exhaust to atmospheric pressure. The propelling nozzle provides a relatively small proportion of the thrust generated by a turboprop.
Turboprop engine attempts to increase the air flow by using propeller-driven by the turbine in addition to a small thrust produced by the exhaust nozzle.
In turboprop engine turbine is to provide power for both the compressor and the propeller and here is a little energy left for producing jet thrust. Thus, the turboprop engine derives most of its propulsive thrust from the propeller and derives only a small portion (10 to 20%) from the exhaust nozzle. Since the shaft speed of gas turbine engine is very much higher than that of a propeller, a reduction gear must be placed between the turbine shaft and the propeller to enable the propeller to operate efficiently. The propulsive thrust developed due to teh two reasons, first, the propeller increases the air momentum. Second, the overall engine provides and internal momentum increases.