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turbojet
Before World War II, all planes used piston engines as power. This kind of engine can't produce forward power by itself, so it needs to drive a propeller to rotate in the air, so as to push the plane forward. This combination of piston engine and propeller has always been a fixed propulsion mode of aircraft, and few people question it.
By the end of 1930s, especially during World War II, due to the needs of the war, the performance of aircraft developed rapidly, with the flying speed reaching 700-800 km/h and the altitude reaching 10000 meters. However, people suddenly found that the propeller plane seems to have reached its limit, even if engineers increase the engine power from 1000 kW to 2000 kW or even 30000 m.
The problem is the propeller. When the plane reaches 800 kilometers per hour, the propeller tip is actually close to the speed of sound because it has been rotating at high speed. The direct consequence of this transonic flow field is that the propeller efficiency drops sharply and the thrust drops. At the same time, the windward area of the propeller is large, which brings greater resistance. Moreover, with the increase of flying height, the atmosphere becomes thinner and the power of piston engine will drop sharply. These factors together determine that the propulsion mode of piston engine+propeller has come to an end. In order to further improve the flight performance, a new propulsion mode must be adopted, and the jet engine came into being.
The principle of jet propulsion is familiar to everyone. According to Newton's third law, all forces acting on an object have equal and opposite reactions. When the jet engine is working, a large amount of air is inhaled from the front end, and then it is ejected at high speed after combustion. In this process, the engine exerts a force on the gas to accelerate it backwards, and the gas also gives the engine a reaction force to push the plane forward. In fact, this principle has been applied to practice for a long time. The firecrackers we play fly to the sky by the reaction of gunpowder gas ejected from the tail.
As early as 19 13, French engineer Rennes Loran obtained the patent of jet engine, but it was a ramjet engine, which could not work at the low speed at that time and lacked the required high-temperature heat-resistant materials. 1930, Frank Whittle obtained his first patent for gas turbine engine, but it was not until 1 1 year later that his engine made its first flight. This kind of engine in whittle formed the foundation of modern turbojet engine.
The structure of modern turbojet engine consists of inlet, compressor, combustion chamber, turbine and nozzle, and there is an afterburner between the turbine and nozzle of fighter. Turbojet engine is also a kind of heat engine, which must follow the principle of heat engine doing work: high pressure inputs energy and low pressure releases energy. Therefore, from the principle of generating output energy, jet engines and piston engines are the same, and they all need four stages: intake, pressurization, combustion and exhaust. The difference is that in the piston engine, these four stages are carried out in time-sharing sequence, while in the jet engine, they are continuous. The gas flows through all parts of the jet engine in turn, corresponding to the four working positions of the piston engine.
Air first enters the air intake of the engine. When an airplane flies, it can be regarded as the airflow flowing to the engine at the speed of flight. Because the flight speed of the aircraft is changing and the incoming flow speed of the compressor is within a certain range, the function of the inlet is to adjust the future flow to a suitable speed through the adjustable pipeline. When flying at supersonic speed, the airflow speed in front of and inside the inlet decreases to subsonic speed, and the stagnation of airflow at this time can increase the pressure by ten times or even dozens of times, which greatly exceeds the pressurization multiple in the compressor, thus producing a ramjet that only relies on speed stamping without compressor.
The compressor behind the air inlet is specially used to increase the pressure of air flow. When air flows through the compressor, the working blades of the compressor do work on the air flow, which increases the pressure and temperature of the air flow. At subsonic speed, compressor is the main component of airflow pressurization.
The high-temperature and high-pressure gas flowing out of the combustion chamber flows through the turbine installed on the same shaft as the compressor. Part of the gas can be expanded in the turbine and converted into mechanical energy to drive the compressor to rotate. In a turbojet engine, the work done by the expansion of the airflow in the turbine is exactly equal to the work consumed by the compressor to compress air and the work required by the transmission accessories to overcome friction. After combustion, the energy of gas in front of the turbine is greatly increased, so the expansion ratio in the turbine is much smaller than the compression ratio in the compressor. The pressure and temperature at the turbine outlet are much higher than those at the compressor inlet, and the thrust of the engine comes from this part of gas energy.
The high-temperature and high-pressure gas flowing out of the turbine continues to expand in the tail nozzle and is discharged backward from the nozzle at high speed along the axial direction of the engine. This speed is much faster than the speed of airflow entering the engine, which makes the engine get reactive thrust.
Generally speaking, the higher the temperature when the airflow comes 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, it can only reach about 65,438+650 K at present. Modern fighters sometimes need to increase thrust in a short time, and then add an afterburner behind the turbine to inject fuel, so that unburned gas can be mixed with the injected fuel for re-combustion. Because the afterburner has no rotating parts, the temperature can reach 2000K, which can increase the thrust of the engine to about 1.5 times. Its disadvantage is that the fuel consumption increases sharply, and the high temperature also affects the life of the engine. Therefore, the afterburner of the engine is generally limited, only ten seconds at low altitude, mostly used for take-off or combat, and can be driven for a long time at high altitude.
With the development of aviation gas turbine technology, people have developed various jet engines such as ramjet engine and pulse engine based on turbojet engine and different supercharging technologies. According to the different output energy, there are turbofan engine, turboprop engine, turboshaft engine and propeller fan engine.
Although the fuel consumption of jet engine is higher than that of piston engine at low speed, its excellent high-speed performance makes it quickly replace the latter and become the mainstream of aero-engine.