How the turbojet engine works …? Working Principle and Advantages and Disadvantages
Schematic diagram of turbojet with afterburner with station numbering:
- The thrust of a turbojet engine is developed by compressing the free stream air in the diffuser or inlet and compressor. The diffuser converts the kinetic energy of the entering air into pressure rise which is achieved by ram effect. Diffusion in the inlet occurs due the geometric shaping of the inlet.
- 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. Compressing the air increases its pressure and temperature
- Compressor types used in turbojets were typically axial or centrifugal.
- Use of axial flow compressors enable high pressure ratios. Modern axial compressors are split into low pressure and high pressure spools, driven by corresponding two stage turbine. High compressor ratios of 15:1 or more can be achieved while improving stability of operation at off-design conditions. The high pressure air is then mixed with fuel and burnt in the combustion chamber under constant pressure condition.
- The combustion gasses at high temperature and pressure are expanded in the turbine and the exhaust nozzle. The expansion of gasses in the turbine provides power to drive the compressor while the exhaust nozzle expands the gasses to atmospheric pressure, thereby producing propulsive force, thrust.
- The net thrust delivered by the engine is the result of converting internal energy to kinetic energy.
- The exhaust products downstream of the turbine still contain adequate amount of oxygen. Additional thrust augmentation can be achieved by providing an afterburner in the jet pipe in which additional amounts of fuel can be burnt.
- Turbojet engines are most suitable for speeds above 800 km/hr and up to 3.0 mach numberAdvantages of Turbojet:
- Power to Weight ratio is about 4 times that of Piston-Prop combination
- Simple, easy to maintain, requires lower lubricating oil consumption. Complete absence of liquid cooling reduces frontal area
- Allows faster speeds, even at supersonic speeds.
- There is no limit to power output while piston engines reached their peak power, beyond which any increase will result in high complexity and greater weight/frontal area.
- Speed of turbojet is not limited by the propeller. Turbojets can attain higher speeds than turboprop aircraft
- Fuel economy at low operational speeds is very poor
- It has low take-off thrust and hence poor starting characteristics
- High operating temperatures and engine parts are subjected to thermal stresses
- Turbojets are the second oldest type; it has a high, usually supersonic, exhaust speed and low frontal cross-section, and so is best suited to high-speed, usually supersonic, flight. Although once widely used, they are relatively inefficient compared to turboprop and turbofans for subsonic flight.
- Low bypass turbofans have a lower exhaust speed than turbojets and are mostly used for high sonic and transonic and low supersonic speeds. High bypass turbofans are used for subsonic aircraft and are quite efficient and are widely used for airliners.
Application: Turbojet engine is highly suited for aircraft at speeds above 800 km/hr.