The power generated by a single piston engine is very limited, so people connect several piston engines in parallel to form a star or V-shaped piston engine. The picture below shows a typical star piston engine.
Modern high-speed aircraft mostly use jet engines. The principle is that air is inhaled, mixed with fuel, ignited, and the air after explosion and expansion is ejected backward, and its reaction force pushes the plane forward. In the lower part of the engine, each compressor fan sucks air from the air inlet and compresses the air once and twice, so that the air can better participate in combustion. The orange-red cavity behind the fans is the combustion chamber, where the mixture of air and engine oil is ignited, and after the combustion is expanded, it is ejected backward, driving the last two fans to rotate and finally being discharged from the engine. The latter two fans and the front compressor fan are installed on the same shaft, so they will drive the compressor fan to continue to suck air, thus completing a working cycle.
turbojet
The principle of this engine is basically the same as the above-mentioned air injection principle, and it has the advantages of fast acceleration and simple design. In order to improve the thrust of turbojet engine, it is necessary to increase the temperature and pressure ratio of gas in front of the turbine, which will increase the exhaust speed and lose more kinetic energy, thus resulting in the contradiction between improving thrust and reducing fuel consumption. Therefore, the fuel consumption of turbojet engine is high, which is the fatal weakness of commercial civil aircraft.
Part of the air inhaled by the turbofan engine is blown out from the outer duct, and part of it is sent to the inner duct core engine (equivalent to a pure turbojet engine). The front-end "fan" acts like a propeller, and improves the propulsion efficiency of the jet engine by reducing the exhaust speed. At the same time, through accurate design, more gas energy is transferred to the external duct through the fan, which also solves the problem of too fast exhaust speed, thus reducing the fuel consumption of the engine. Because the design of this kind of fan should consider the needs of internal and external ducts, it is much more difficult than turbojet engine.
ramjet, stovepipe jet, athodyd
This kind of engine has no fan and other devices, and relies entirely on the principles of compressing inhaled air, ignition, combustion and post-injection. Therefore, it has the advantages of simple structure, small volume, large thrust and fast acceleration. The disadvantage is that it needs external energy to start (usually rocket boost) and is not suitable for recycling.
The birth of turbofan jet engine
After World War II, with the passage of time and technological innovation, turbojet engines have been insufficient to meet the power demand of new aircraft. Especially the subsonic civil aircraft and large transport aircraft developed rapidly after World War II, the flight speed should reach high subsonic speed and the fuel consumption should be small, so the engine efficiency should be high. The efficiency of turbojet engine can no longer meet this demand, which shortens the range of the above aircraft. Therefore, for some time, there have been more and more large aircraft using turboprop engines.
In fact, as early as 1930s, there were some rough early designs of jet engines with external bypass. In the 1940s and 1950s, early turbofan engines began to be tested. However, due to the very high requirements for the design and manufacture of fan blades. Therefore, it was not until the 1960s that people were able to manufacture fan blades that met the requirements of turbofan engines, thus opening the practical stage of turbofan engines.
In 1950s, NACA (the predecessor of NASA) carried out very important scientific research on turbofan engine. 1955-56, the research results were transferred to General Electric Company (GE) for further development. GE successfully launched CJ805-23 turbofan engine at 1957, and then broke a lot of records of supersonic jet engine. But the earliest practical turbofan engine was Pratt? Whitney (Pratt &; Whitney JT3D turbofan engine. Actually, pu? Hui company started the development project of turbofan engine later than General Electric Company. After learning the secret of GE's research and development of CJ805, they quickly stepped up their work and first launched the practical JT3D.
1960, Rolls? Rolls-Royce's "Conway" turbofan engine began to be adopted by Boeing 707 large-scale long-range jet aircraft, becoming the first turbofan engine used by civil airliners. In the 1960s, Lockheed's "Samsung" passenger plane and Boeing 747 "Treasure" passenger plane adopted Luo? Romania's large turbofan engine RB2 1 1-22B marks the full maturity of turbofan engine. Since then, turbojet engines have been quickly abandoned by the western civil aviation industry.
Principle of turbofan jet engine
The limited thrust of turboprop engine affects the flight speed of aircraft. Therefore, it is necessary to improve the efficiency of jet engines. The efficiency of engine includes thermal efficiency and propulsion efficiency. The thermal efficiency can be improved by increasing the temperature of the gas in front of the turbine and the pressure increasing ratio of the compressor. Because high temperature, high density gas contains more energy. However, under the condition of constant flight speed, increasing the temperature in front of the turbine will naturally increase the exhaust speed. The gas with fast flow rate loses a lot of kinetic energy when it is discharged. Therefore, unilaterally increasing thermal power, that is, increasing the temperature in front of the turbine, will lead to a decline in propulsion efficiency. To improve engine efficiency in an all-round way, it is necessary to solve the contradiction between thermal efficiency and propulsion efficiency.
The beauty of turbofan engine lies in increasing the temperature in front of the turbine without increasing the exhaust speed. The structure of turbofan engine is actually to add several stages of turbines in front of turbojet engine, and these turbines drive a certain number of fans. Like a common jet engine, a part of the airflow inhaled by the fan is sent to the compressor (the term is called "inner duct"), and the other part is directly discharged from the periphery of the turbojet engine casing ("outer duct"). Therefore, the gas energy of turbofan engine is divided into two kinds of exhaust streams generated by fan and combustion chamber respectively. At this time, in order to improve the thermal efficiency and increase the temperature before the turbine, more gas energy can be transferred to the external pipeline through the fan by appropriate turbine structure and increasing the diameter of the fan, thus avoiding a significant increase in exhaust speed. In this way, thermal efficiency and propulsion efficiency are balanced, and the efficiency of the engine is greatly improved. High efficiency means low fuel consumption and longer range.
Afterburner turbofan engine
Unreinforced turbofan engine
As mentioned above, the turbofan engine has high efficiency, low fuel consumption and long flight range.
However, the technology of turbofan engine is complex, especially how to correctly distribute the airflow inhaled by the fan to the outer duct and the inner duct is a great technical problem. Therefore, few countries can develop turbofan engines, and China has not made domestic turbofan engines in batches. Turbofan engine is relatively expensive, which is not suitable for the birth of low-cost aircraft turbojet engine.
Before World War II, the combination of piston engine and propeller has made great achievements, enabling human beings to challenge the sky. But by the end of 1930s, the development of aviation technology made this combination reach its limit. When the flight speed of the propeller reaches 800 km/h, the blade tip is actually close to the speed of sound. The transonic flow field makes the propeller efficiency decrease sharply, and the thrust decreases instead of increasing. The windward area of propeller is large, and the resistance is also large, which greatly hinders the improvement of flight speed. At the same time, with the increase of flying height and thin atmosphere, the power of piston engine will also decrease.
This gave birth to a brand-new jet engine propulsion system. The jet engine sucks in a lot of air, and after burning, it ejects at a high speed, which produces a reaction force on the engine and pushes the plane forward.
As early as 19 13, French engineer Rennes? Roland put forward the design of ramjet and obtained a patent. But at that time, there was no corresponding boosting means and materials, and jet propulsion was just a fantasy. 1930, British Frank? Whittle obtained the patent of gas turbine engine, which was the first practical jet engine design. 1 1 years later, the engine he designed made its first flight, thus becoming the originator of turbojet engines.
Principle of turbojet engine
Turbojet engine is called turbojet engine for short, and usually consists of inlet, compressor, combustion chamber, turbine and nozzle. Some military engines have afterburner between turbine and tail nozzle.
The turbojet engine is a heat engine, and the principle of doing work is the same: high pressure inputs energy and low pressure releases energy.
When working, the engine first sucks air from the air inlet. This process is not simply to open an inlet, because the flight speed is variable and the compressor has strict requirements on the inlet speed, so the inlet must be able to control the inlet speed within a suitable range.
As the name implies, the compressor is used to increase the pressure of the inhaled air. The compressor is mainly in the form of fan blades, and the rotation of the blades does work to the airflow, increasing the pressure and temperature of the airflow.
Then the high-pressure airflow enters the combustion chamber. The fuel nozzle of the combustion chamber injects oil, which is ignited after being mixed with air, generating high-temperature and high-pressure fuel gas, which is discharged backward.
The high-temperature and high-pressure gas flows back through the high-temperature turbine, and part of the energy can expand in the turbine and be converted into mechanical energy to drive the turbine to rotate. Because the high-temperature turbine and the compressor are installed on the same shaft, it also drives the compressor to rotate, thus repeatedly compressing the sucked air.
The high-temperature and high-pressure gas flowing out of the high-temperature turbine continues to expand in the tail nozzle and is discharged backward at high speed from the tail nozzle. This speed is much greater than the speed of airflow entering the engine, resulting in
In order to react to the thrust of the engine, the plane was driven forward.