TURBOJET ENGINE

TURBOJET ENGINE

The turbojet is an airbreathing jet engine, typically used in aircraft. It resembles the simplest form of gas turbine. It was separately coinvented by the two fathers of jet engines: Frank Whittle from Britain and von Ohain from Germany. The first airplane powered by a turbojet engine was the He178 German aircraft powered by the He S-3 engine on August 27, 1939.

Types of Turbojet Engine

1. According to Power
1. Nuclear Turbojet Engine
2. Non-Nuclear Turbojet Engine

2. According to reused of exhaust gases
1. Afterburner Turbojet Engine
2. Non-afterburner Turbojet Engine

3. According to Spool
1. Single Spool
(a) Axial flow compressor types
(b) Centrifugal flow compressor types
(i) Single entry types 
(ii) Double entry types
(c) Axial-Centrifugal flow compressor types
2. Double Spool

Construction

It consists of:
1. Diffuser.
2. Compressor.
3. Combustion chamber.
4. Turbine 
5. Exhaust nozzle.

1. Diffuser -
The function of the diffuser is to convert the kinetic energy of the entering air into pressure energy.

2. Compressor -
Compressor is a mechanical device which compressed the air out from diffuser & increases its pressure and temperature. The compressor is driven by the turbine. It rotates at high speed, adding energy to the airflow and at the same time squeezing (compressing) it into a smaller space. Axial or centrifugal types compressor are used in turbojet engine.

3. Combustion chamber-
In a turbojet, the air and fuel mixture burn in the combustion chamber and pass through to the turbine in a continuous flowing process with no pressure build-up. Instead, a small pressure loss occurs in the combustion chamber.

4. Turbine-
A turbine is a device that harnesses the kinetic energy of some fluid - such as water, steam, air, or combustion gases - and turns this into the rotational motion of the device itself. Hot gases leaving the combustor expand through the turbine. Typical materials for turbines include inconel and Nimonic.

5. Exhaust nozzle-
The function of the nozzle is to convert the pressure energy of the combustion gases into kinetic energy.

Working

Air from the atmosphere enters into turbojet engine. The air velocity gets reduced and its static pressure is increased by diffuser, Then the air passes through the rotary compressor in which the air is further compressed. Then the high pressure air flows into the combustion chamber. In the combustion chamber, the fuel is injected by suitable injectors and the air-fuel mixture is burnt. Heat is supplied at constant pressure.

The highly heated products of combustion gases are then enters the turbine and partially expanded. The power produced by the turbine is just sufficient to drive the compressor, fuel pump and other auxiliaries. The hot gases from the turbine are then allowed to expand in the exhaust nozzle section. In the nozzle, pressure energy of the gas is converted into kinetic energy. So the gases coming out from the unit with very high velocity.

Due to high velocity of gases coming out from the unit, a reaction or thrust is produced in the opposite direction. This thrust propels the air craft. Like ramjet engine, the turbojet engine is a continuous flow
engine.

Advantages

1. Construction is simple.
2. Less wear and tear.
3. Less maintenance cost.
4. It runs smoothly because continuous thrust is produced by continuous combustion of fuel.
5. The speed of a turbojet is not limited by the propeller and it can attain higher flight speed than turbo propeller air crafts.
6. Low grade fuels like kerosene, paraffin, etc., can be used. This reduces the fuel cost.
7. Reheat is possible to increase the thrust.
8. Since turbojet engine has a compressor, it can be operated under static conditions.

Disadvantages

1. It has low take-off thrust and hence poor starting characteristics.
2. Fuel consumption is high.
3. Costly materials are used.
4. The fuel economy at low operational speed is extremely poor.
5. Sudden decrease of speed is difficult.
6. Propulsive efficiency and thrust are lower at lower speeds.

Applications

It is best suited for Piloted air-crafts, Military aircrafts, etc.

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