How A Turboprop Engine Works

A turboprop engine is a gas turbine engine that runs a propeller. Unlike a pure turbojet engine which produces thrust, a turboprop engine relies almost entirely on its propeller to generate forward motion. The turbine section of the engine is simply used to turn the propeller, and this turning action of the propeller makes it generate the required propeller thrust which pulls the aircraft forward.

The advantage of a turboprop engine compared to a piston engine-powered propeller aircraft is the ability of the former to create a larger amount of power that can be sustained at higher altitudes. Turboprop engines are also simpler and more reliable compared to piston-engine aircraft.

Photo: ATR

The inside a turboprop engine?

A turboprop engine on the inside looks very similar to a jet engine if we were to ignore the propeller. It has a compressor stage, a combustion chamber, a turbine stage, and finally an exhaust. The working of the engine is also similar. Air is sucked by the engine intake and directed to the compressor assembly. Here, the air is slowed down and its pressure increased. From the compressor, the air then goes to the combustion chamber where it is burnt with fuel which increases its velocity and temperature. The air is then passed to the turbine stages and this is where the major difference between a jet engine and a turboprop engine is seen.

The inside workings of a turboprop engine. Photo: Emoscopes by Wikimedia

In a jet engine, the turbines add kinetic energy to the flow, and this flow is then expelled out of the exhaust to generate the thrust. However, in a turboprop engine, the turbines are connected to a propeller shaft and thus almost all the energy extracted by the turbines is used to rotate the propeller. Due to a turboprop being in effect a turbine engine, a small amount of jet thrust is generated (about 5%).

The other major difference is the presence of a reduction gearbox. Propellers become highly inefficient at higher RPMs (Revolutions Per Minute) and hence rotating it at the speed of the turbine at tens of thousands of RPM is not an option. This is where the reduction gearbox comes in. It is located in between the propeller and the propeller turbine shaft. The gearbox reduces the RPM of the propeller so that it rotates at an acceptable rate. Most turboprop propellers operate at about 2000 to 3000 RPM. This is a significant reduction as the engine turbine rotates at about 20,000 to 30,000 RPM.