Mumbai University > Mechanical Engineering > SEM 6 > Thermal and Fluid Power Engineering

Marks: 10M

Main components of turboprop engine are (i) a propeller, (ii) a gear reduction unit, (iii) a compressor, (iv) a gas turbine and the nozzles.

Out of the total propulsive thrust generated, around 80% of the thrust is generated by the gas turbine and the remaining 10-20% is generated by the expansion of gases in nozzles.

The power required to drive a compressor and the propeller is obtained from the gas turbine.

The propeller is driven through the gear reduction unit by the gas turbine which draws a large amount of air. Large part of the air drawn by the propeller is passed through the ducts around the engine and the remainder is compressed in the diffuser by ram compression and again in the compressor.

The high temperature gases, generated by burning the fuel in combustor, are expanded in the turbine and eventually over the nozzles.

Total thrust developed is the sum of thrusts developed by the propeller and the nozzle.

The pressure ratio for expansion in the turbine is much higher than that of turbojet engine.

$\text{Bypass ratio} = \frac{\text{mass flow bypassing the combustor}}{\text{mass flow through the combustor}}$

Bypass ratio is of 100:1 or more.

The propellers are best suited for low speeds( <300 MPH) flight.

The turboprop engines can use either the same turbine to run both the compressor and the propeller or in some systems it may use two turbines to drive the compressor and the propeller separately.

Advantages:

1. Low specific fuel consumption
2. Low frontal area and low weight.
3. High efficiencies at wide ranges of speeds below 750-800 kmph.
4. Higher thrust and power at the time of takeoff and climb.

Limitations:

1. Reduction gear unit increases weight and cost.
2. Propulsive efficiency decreases rapidly.

Applications:

1. These are suitable for applications at subsonic speeds.
2. Application in bomber and transport aircrafts for military applications and for civial aviation aircrafts.