Development and future of the hottest aircraft eng

The main function of aircraft engine is to provide propulsion or support for aircraft, and it is the heart of aircraft. In the decades since the advent of aircraft, engines have developed rapidly, from piston engines used in early low-speed aircraft to jet engines that can drive aircraft to fly at supersonic speed, and rocket engines that can work in outer space on carrier rockets. Up to now, aircraft engines have formed a large family with a wide variety of uses. There are two common classification principles for aircraft engines: according to whether the air participates in the engine work and the principle that the engine generates propulsion power. According to whether the engine needs air to work, aircraft engines can be divided into two categories, which are roughly as follows:

air suction engines are referred to as aspirated engines. They must suck in air as the oxidant (combustion promoter) of the "ready" fuel of their competitors. Therefore, they cannot work in space outside the dense atmosphere and can only be used as aircraft engines. Generally speaking, aeroengine refers to this kind of engine. Including piston engine, gas turbine engine, ramjet engine and pulse jet engine

rocket jet engine is an engine that does not rely on air. Spacecraft must be installed with this engine because it needs to fly outside the atmosphere. It can also be used as the booster power of aircraft. According to the different energy sources that form jet flow energy, rocket engines are divided into chemical rocket engines, electric rocket engines and nuclear rocket engines

according to the different principles of generating propulsion power, aircraft engines can be divided into direct reaction engine and indirect reaction engine. The direct reaction engine uses the backward injection of high-speed air flow to generate forward reaction to propel the aircraft. Direct reaction engine is also called jet engine, which includes turbojet engine, ramjet engine, pulse jet engine, rocket jet engine, etc

the indirect reaction engine is driven by the engine to rotate the propeller of the aircraft and the rotor of the helicopter to work on the air. When the air accelerates to flow backward (downward), the air generates reaction force on the propeller (rotor) to promote the aircraft. Such engines include piston engine, turboprop engine, turboshaft engine, turboprop fan engine, etc. Turbofan engine has both direct reaction and indirect reaction, but it is often classified as direct reaction engine, so it is also called turbofan jet engine

1. Piston engine

aviation piston engine is a machine that uses gasoline and air to mix, burn in a closed container (cylinder), and expand to work. The piston engine must drive the propeller, which generates push (pull) force. Therefore, as the power unit of the aircraft, the engine and propeller are inseparable. It is usually 1:15, that is, 15 kg of air is needed to burn a kilogram of gasoline

before the Second World War, all aircraft used piston engines as the power of the aircraft. This engine itself could not generate forward power, but needed to drive a pair of propellers to rotate in the air, so as to push the aircraft forward. This combination of piston engine and propeller has always been a fixed propulsion mode of aircraft, and few people have questioned it

by the end of the 1930s, especially in World War II, due to the needs of the war, the performance of the aircraft had developed rapidly, with a flight speed of 700-800 kilometers per hour and an altitude of more than 10000 meters. However, people suddenly found that the propeller aircraft seemed to have reached the limit, although engineers increased the power of the engine more and more to traverse the curve: after the experiment was completed, it was higher, from 1000 kW to 2000 kW or even 3000 kW, However, the speed of the aircraft has not been significantly improved, and the engine obviously feels "powerless"

the problem lies in the propeller. When the speed of the aircraft reaches 800 kilometers per hour, because the propeller is always rotating at high speed, the tip part of the propeller is actually close to the speed of sound. The direct consequence of this transonic flow field is that the efficiency of the propeller drops sharply and the thrust decreases. At the same time, due to the large windward area of the propeller, the resistance is also large, and with the rise of the flight altitude, the atmosphere becomes thinner, The power of piston engine will also drop sharply. These factors together determine that the propulsion mode of piston engine + propeller has come to an end. In order to further improve flight performance, a new propulsion mode must be adopted, and jet engine came into being

2. Turbojet engine

turbojet engine is still a kind of heat engine, so we must follow the work principle of heat engine: input energy under high pressure and release energy under low pressure. 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 internal energy of the gas is expanded in the turbine and converted into mechanical energy to drive the compressor to rotate. In the turbojet engine, the work done by the expansion of the gas flow in the turbine is exactly equal to the work consumed by the compressed air of the compressor and the work required by the transmission accessories to overcome friction. After combustion, the fuel gas energy 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 that at the compressor inlet. The thrust of the engine is derived from this part of fuel 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 along the axial direction of the engine at high speed. This speed is much higher than the speed at which the air flow enters the engine, so that the engine obtains the reaction 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 limitations of turbine materials, it can only reach about 1650k at present. Modern fighters sometimes need to increase the thrust in a short time, and then add an afterburner to inject fuel after the turbine, so that the underburned gas and the injected fuel can be mixed and 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 about 1.5 times. Its disadvantage is that the fuel consumption increases sharply, and the excessive temperature also affects the service life of the engine. Therefore, the engine afterburner is generally time limited, which is less than a dozen seconds at low altitude. It is mostly used for takeoff or combat, but it can be operated for a long time at high altitude

with the progress of aviation gas turbine technology, people have developed a variety of jet engines on the basis of turbojet engines, such as ramjet and pulse engine according to different supercharging technologies; According to the different energy output, 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 aeroengine

3 Turbofan engine

since whittle invented the first turbojet engine, the turbojet engine soon replaced the piston engine with its powerful power and excellent high-speed performance, becoming the preferred power unit of fighter aircraft, and began to be applied in other aircraft. However, with the development of jet technology, the shortcomings of turbojet engine are becoming more and more prominent, that is, at low speed, the fuel consumption is large, the efficiency is low, and the flight range of the aircraft becomes very short. Although this is not very serious for high-speed fighters performing air defense missions, it is unacceptable to use it on subsonic civil transport aircraft with strict economic requirements

at present, aviation turbofan engines are mainly divided into two categories, namely, non afterburner turbofan engines and afterburner turbofan engines. The former is mainly used for high subsonic transport aircraft, and the latter is mainly used for fighter aircraft. Due to different purposes, the structural parameters of these two types of engines are also very different

the non afterburner turbofan engine not only has a higher temperature in front of the turbine, but also has a larger fan diameter, and the bypass ratio can reach more than 8. This engine is more economical than the turbojet engine, and the available flight speed is higher than the piston engine. It is widely used in modern large trunk airliners, military transport aircraft and other aircraft with a maximum speed of about m0.9. According to the principle of the heat engine, when the power of the engine is certain, the more working medium participating in the propulsion, the greater the thrust obtained. Due to the large fan diameter, the air flow of the non afterburner turbofan engine is large, so the thrust is also large. At the same time, due to the low exhaust speed, the noise of this engine is also small

the afterburner turbofan engine does not turn on afterburner during aircraft cruise. At this time, it is equivalent to an afterburner turbofan engine. However, in order to pursue a high thrust weight ratio and reduce resistance, the bypass ratio of this engine is generally below 1.0. In high-speed flight, the afterburner of the engine is turned on, and the air in the outer culvert and the gas behind the turbine enter the afterburner together, and then burn again after fuel injection, so that the thrust can be greatly increased, even more than that of the afterburner turbojet engine. With the increase of speed, the afterburner ratio of this engine will rise, and the fuel consumption rate will decrease. Afterburner turbofan engine has been widely used in the new generation of fighter aircraft because of its characteristics of low fuel consumption at low speed and large thrust to weight ratio when afterburner is turned on with the increase of functions

4. Turbine shaft engine

in the type of turbine engine with compressor, turbine shaft engine "she added that the engine appeared later, but it has been widely used in helicopters and vertical/short takeoff and landing aircraft

in terms of work and structure, the turbine shaft engine is similar to the turbine propeller engine. They all evolved from the principle of turbofan engine, but the latter turned the fan into a propeller, while the former turned the fan into a helicopter rotor. In addition, the turboshaft engine also has its own characteristics: it is generally equipped with a free turbine (that is, a turbine that does not drive the compressor and is specially used for power output), and it is mainly used in helicopters and vertical/short takeoff and landing aircraft. Turboshaft engine is used in helicopters. It cooperates with the rotor to form the power unit of helicopters. According to the theory of turbofan engine, theoretically speaking, the larger the rotor diameter, the better. The same core engine produces the same cycle power, and the larger the rotor diameter, the greater the lift generated on the rotor. In fact, due to the loss in the process of energy conversion, the rotor can not be made infinite, so the diameter of the rotor is limited—— Generally speaking, the air flow through the rotor is times that through the turboshaft engine

compared with piston engine, another power device commonly used in turboshaft engine and helicopter, the power weight ratio of turboshaft engine is much larger, more than 5. Moreover, in terms of the power generated by the engine, the turboshaft engine is also much larger,