Working Principle of Turbojet Engine

The structure of modern turbojet engine consists of intake port, compressor, combustion chamber, turbine and tail nozzle. There is an afterburner between the turbine and tail nozzle of fighter. Turbojet engine is still a kind of heat engine. It must follow the working principle of heat engine: input energy at high pressure and release energy at low pressure. Therefore, in terms of the principle of generating output energy, jet engine and piston engine are the same. They all need four stages: intake, pressurization, combustion and exhaust. The difference is that in piston engine, these four stages are carried out in turn in time, but in jet engine, they are carried out continuously, and the gas flows through each part of the jet engine in turn. The four working positions of the piston engine should be located.

Air first enters the intake port of the engine. When an aircraft is flying, it can be seen that the airflow flows to the engine at the flying speed. Because the flight speed of the aircraft varies, and the inflow speed adapted by the compressor has a certain range, the function of the intake port is to adjust the future flow to the appropriate speed through an adjustable pipe. In supersonic flight, the airflow velocity in front of the intake port and in the intake port decreases to subsonic speed. At this time, the stagnation of the airflow can increase the pressure by more than ten times or even tens of times, which greatly exceeds the pressure increase multiple in the compressor. Thus, a ramjet engine with only velocity ramming and no compressor is produced.

The compressor behind the intake port is designed to increase the pressure of the air flow. When the air flows through the compressor, the compressor working blade works on the air flow, which increases the pressure and temperature of the air flow. At subsonic speed, compressor is the main component of air turbocharging.

The high temperature and high pressure gas from the combustion chamber flows through the turbine installed on the same axis as the compressor. In a turbojet engine, the work done by the expansion of the gas flow in the turbine under the equilibrium state is equal to the work consumed by the compressed air of the compressor and the work needed by the transmission accessories to overcome friction. After combustion, the gas energy in front of the turbine increases greatly, so the expansion ratio in the turbine is much larger than the compression ratio in the compressor. The pressure and temperature at the outlet of the turbine are much higher than that at the inlet of the compressor. The thrust of the engine comes from this part of the gas energy.

The high temperature and high pressure gas flowing out of the turbine continues to expand in the tail nozzle, and discharges from the nozzle to the rear along the engine axis at high speed. This speed is much greater than the speed at which the airflow enters the engine, which gives the engine a counteracting thrust.

Generally speaking, the higher the temperature of the air flow coming out of the combustion chamber, the greater the input energy and the greater the thrust of the engine. However, due to the limitation of turbine materials, only about 1650K can be achieved. Modern fighters sometimes need to increase thrust in a short time. After the turbine, an afterburner is added to inject fuel, so that the mixture of insufficiently burned gas and injected fuel can be burned again. Because there are no rotating parts in the afterburner, the temperature can reach 2000K, which can increase the thrust of the engine to 1.5 times left. Right. Its disadvantage is that the fuel consumption increases sharply, while the high temperature also affects the life of the engine, so the engine start-up afterburner is generally time-limited, low altitude is only a dozen seconds, mostly used for take-off or combat, high altitude can be opened for a longer time.