Sunday, October 3, 2021

Turboprop engine | How it works?

Turboprop engine is basically another variant of a turbojet engine except that one more turbine is present in a turboprop engine, towards the back of the combustion chamber, and it works through a shaft and gear to turn a propeller at the front of the engine. Turboprop is used in many low speed transport aircraft. It basically uses a gas turbine core to turn a propeller. Propeller engines develop thrust by moving a large mass of air through a small change in velocity.

Turboprop engines are very efficient at flight speeds less than Mach 0.6. Turboprop powered aircraft requires less runway for takeoff and landing than a turbojet or turbofan powered aircraft of the same size. Some examples of turboprop powered aircraft are Tupolev Tu-95 Bear, Consolidated Vultee XP-81, Bombardier Dash 8 and the Alenia ATR 42.

Parts of a turboprop engine
!Original: EmoscopesVector: M0tty, CC BY 2.5, via Wikimedia Commons

Basically there are two main parts of a turboprop engine i.e., the core engine and the propeller. The core of turboprop engine is very similar to a turbojet engine, except a difference that is instead of expanding all the hot exhaust through the nozzle to produce thrust, most of the energy of the exhaust in a turboprop engine is used to turn the turbine.

In some turboprop engine, there may be an additional turbine stage present, which is connected to a drive shaft. This drive shaft is then connected to a gear box, which is connected to a propeller that produces most of the thrust. Since most of the energy of the core exhaust is used for turning the drive shaft, therefore the exhaust velocity of a turboprop engine is very low and it contributes very little thrust.

One important thing to note here is that they are used only for low speed aircraft such as cargo planes, because propellers become less efficient as the speed of the aircraft increases. And the high speed aircrafts usually use high bypass turbofans because of their high fuel efficiency and high speed capability.

There is another type of turboprop engine known as turboshaft engine, in which the gear box is not connected to the propeller but to some other drive device and are mainly used in helicopters.

How it works?

A turboprop engine uses the same principle as a turbojet engine to produce energy. One main difference between the turboprop and the turbojet engine is that additional turbines, a power shaft and a reduction gearbox is used into the design to drive the propeller. So basically a turboprop engine consists of an intake, reduction gearbox, compressor, combustor, turbine, and a propelling nozzle.

Talking about the working principle of turboprop engine, air is drawn into the intake and then compressed by the compressor. After that fuel is added to the compressed air in the combustor, where the fuel-air mixture combusts. The hot combustion gases then expand through the turbine.

The gearbox is driven by the same turbines and shaft that drive the engine compressor, and sometimes the turbines may be separate with the power turbine driving a concentric and isolated shaft to power the gearbox. So the turbines extract almost all of the energy from the exhaust stream and use some of it to power the engine compressor and the rest is transmitted to drive the propeller.

Further, more expansion of the gases occurs in the propelling nozzle, and finally the gases exhaust to atmospheric pressure. Note that the propelling nozzle provides a very low thrust as compared to the thrust generated by a turboprop, since almost all of the engine’s power is used to drive the propeller.

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