Turbojet engines use jet propulsion to avoid the inherent weaknesses of rockets and ramjet engines. Because it uses a turbine-driven compressor, the engine has enough pressure to generate strong thrust at low speeds. Turbojet engines work on a "duty cycle". It sucks air from the atmosphere, compresses and heats it, and the air gains energy and momentum and moves from it at speeds of up to 2,000 feet per second (610 meters per second), or about 1,400 miles per hour (2,253 kilometers per hour). Push into the nozzle and discharge. When the high-speed jet flows out of the engine, it simultaneously drives the compressor and turbine to continue rotating, maintaining the "working cycle". The mechanical layout of a turbine engine is relatively simple because it only contains two main rotating parts, the compressor and the turbine, as well as one or several combustion chambers. However, not all aspects of this engine are of this simplicity, as thermal and aerodynamic issues are more complex. These problems are caused by the high operating temperatures of the combustion chamber and turbine, the changing airflow through the compressor and turbine blades, and the design of the exhaust system that removes the gases and creates the propelling jet.
The propulsion efficiency of an engine depends largely on its flight speed. At aircraft speeds below approximately 450 mph (724 km/h), pure jet engines are less efficient than propeller-type engines due to airflow disturbances caused by the propeller's high tip speed. At 350 mph (563 km/h) km/h) or above) the propeller efficiency decreases rapidly. Thus, pure turbojet engines are best suited for higher flight speeds. These characteristics allow some medium-speed aircraft to use a combination of a propeller and a gas turbine engine instead of a pure turbojet - a turboprop engine.
Propulsion Efficiency
A turboprop engine is most efficient at a speed of Mach number Malt; 0.6. When the speed increases to Mach number 0.6-0.9, the superiority of the propeller/turbine combination is replaced to a certain extent by the internal and external engines, ducted fan engines and propeller fan engines. The exhaust flow of these engines is larger than that of pure jet turbojet engines but the jet speed is lower. Therefore, their propulsion efficiency is equivalent to that of turboprop engines and exceeds the propulsion efficiency of pure jet engines. Under subsonic (Malt; 1.0) conditions, the propulsion efficiency of turbojet engines is the lowest. When the flight speed of the aircraft exceeds the speed of sound (Magt; 1.0), the propulsion efficiency of the turbofan engine begins to decrease due to the large windward area; in contrast, the propulsion efficiency of the turbojet engine increases rapidly, even in the Mach number range of 2.5-3.0 , the propulsion efficiency of the turbojet engine can still reach 90. Because of this, compared with the medium bypass ratio turbofan engine with a bypass ratio of 0.5-0.8 commonly used by third-generation aircraft, the F-119 turbofan used by the F-22 The engine reduces the bypass ratio back to 0.29 in order to achieve supersonic cruise (Ma1.4).
Each engine has its optimal flight envelope - (xy coordinate system composed of speed x/height y). It does not mean that turbofan engines are necessarily more fuel-efficient than turbojet engines. At the speed of sound, a turbofan engine with afterburner also consumes more fuel than a turbojet engine.
Adjustable inlet
The turboramjet engine combines a turbojet engine (which is commonly used at various speeds below Mach 3) with a ramjet engine to operate at high It has good performance at Mach number. This engine is surrounded by a duct with an adjustable inlet at the front and an afterburner nozzle with an adjustable nozzle at the rear. During takeoff and acceleration, and in flight conditions below Mach 3, the engine works like a conventional turbojet engine; when the aircraft accelerates to above Mach 3, its turbojet mechanism is closed, and the airway air passes through the guide blades Bypassing the compressor, it flows directly into the afterburner nozzle, which then becomes the combustion chamber of the ramjet engine.
This engine is suitable for aircraft that require high speed flight and maintain high Mach number cruising conditions. Under these conditions, the engine works as a ramjet engine.
Turbine rocket engine
The structure of the turbine/rocket engine is similar to that of the turbine/ramjet engine. An important difference is that it provides its own oxygen for combustion. This engine has a multi-stage turbine-driven low-pressure compressor, and the power to drive the turbine is generated by burning fuel and liquid oxygen in a rocket-type combustion chamber. Because the gas temperature can be as high as 3500 degrees, additional fuel needs to be injected into the combustion chamber for cooling before the gas enters the turbine. This rich mixture (gas) is then diluted with air from the compressor, and the remaining fuel is burned in a conventional afterburner system. Although this engine is smaller and lighter than a turbo/ramjet engine, it consumes more fuel. This trend makes it more suitable for interceptors or spacecraft launch carriers. These aircraft require high-altitude and high-speed performance, often requiring high acceleration performance without long endurance.
Working Principle Edit this paragraph
The structure of a modern turbojet engine consists of an inlet, a compressor, a combustion chamber, a turbine and a tail nozzle. The turbine and tail nozzle of a fighter jet There is also an afterburner. Turbojet engines are still a type of heat engine and must follow the work principle of a heat engine: input energy under high pressure and release energy under low pressure. Therefore, from the principle of generating output energy, jet engines and piston engines are the same. They both require four stages of air intake, pressurization, combustion and exhaust. The difference is that in piston engines, these The four stages are carried out in sequence in a time-sharing manner, but in a jet engine they are carried out continuously. The gas flows through each part of the jet engine in sequence, corresponding to the four working positions of the piston engine.
The first thing the air enters is the inlet of the engine. When the aircraft is flying, it can be seen as the airflow flowing to the engine at the flight speed. Since the speed of the aircraft changes, the compressor adapts to the incoming flow speed. There is a certain range, so the function of the inlet is to adjust the incoming flow to a suitable speed through the adjustable pipe. During supersonic flight, the airflow speed in front of and in the inlet duct is reduced to subsonic speed. At this time, the stagnation of the airflow can increase the pressure by ten or even dozens of times, which greatly exceeds the pressure increase in the compressor. , thus producing a ramjet engine that relies solely on speed ramming and does not require a compressor.
The compressor behind the air inlet is specially used to increase the pressure of the air flow. When the air flows through the compressor, the working blades of the compressor do work on the air flow, causing the pressure and temperature of the air flow to increase. At subsonic speeds, the compressor is the main component for airflow pressurization.
The high-temperature and high-pressure gas flowing out from the combustion chamber flows through the turbine mounted on the same shaft as the compressor. Part of the internal energy of the gas expands in the turbine and is converted into mechanical energy, which drives the compressor to rotate. In a turbojet engine, the work done by the expansion of the airflow in the turbine under equilibrium conditions is equal to the work consumed by the compressor to compress the air and the transmission accessories overcome friction. the work required. After combustion, the energy of the gas in front of the turbine increases greatly, so the expansion ratio in the turbine is much greater than the compression ratio in the compressor. The pressure and temperature at the turbine outlet are much higher than the compressor inlet. The thrust of the engine is this part of the gas of energy.
The high-temperature and high-pressure gas flowing out of the turbine continues to expand in the tail nozzle and is discharged rearward from the nozzle along the engine axial direction at high speed. This speed is much greater than the speed at which the airflow enters the engine, allowing the engine to obtain reaction thrust.
Generally speaking, the higher the temperature of the airflow when it comes out of the combustion chamber, the greater the energy input and the greater the thrust of the engine. However, due to limitations of turbine materials, etc., it can only reach about 1650K. Modern fighter jets sometimes need to increase thrust in a short period of time. An afterburner is added after the turbine to inject fuel, allowing the incompletely burned gas to mix with the injected fuel. The fuel is mixed and burned again. Since there are no rotating parts in the afterburner chamber, the temperature can reach 2000K, which can increase the thrust of the engine to about 1.5 times.
Its disadvantage is that fuel consumption increases sharply, and excessively high temperatures also affect the life of the engine. Therefore, the engine afterburner is generally turned on for a time limit. It only lasts for more than ten seconds at low altitudes. It is mostly used for takeoff or combat, and it can be turned on for a longer time at high altitudes. time.
Edit this section of development history
War needs
Before World War II, all aircraft used piston engines as the power of the aircraft. The engine itself cannot generate forward power, but needs to drive a propeller to rotate in the air to propel the aircraft forward. This combination of piston engine and propeller has always been the fixed propulsion mode of aircraft, and few people have questioned it.
By the end of the 1930s, especially during World War II, due to the needs of the war, the performance of aircraft had developed rapidly. The flight speed reached 700-800 kilometers per hour, and the altitude reached more than 10,000 meters. But people suddenly discovered that the propeller aircraft seemed to have reached its limit. Although engineers increased the power of the engine higher and higher, from 1,000 kilowatts, to 2,000 kilowatts or even 3,000 kilowatts, the speed of the aircraft still did not increase significantly, and the engine clearly felt "powerful". It can’t be used”.
The key issue
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 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 larger windward area of ??the propeller, the resistance is also larger. Moreover, as the flight altitude increases, the atmosphere becomes thinner. Piston engines also experience a drastic drop in power. The combination of these factors determines that the propulsion mode of piston engines and propellers has come to an end. To further improve flight performance, a new propulsion mode must be adopted, and jet engines came into being.
The principle of jet propulsion is familiar to everyone. According to Newton's third law, every force acting on an object has an equal and opposite reaction force. When a jet engine is working, it inhales a large amount of air from the front end, burns it, and ejects it at high speed. During this process, the engine exerts force on the gas to accelerate it backwards, and the gas also gives the engine a reaction force to push the aircraft forward. In fact, this principle has been applied in practice for a long time. The firecrackers we have played with fly into the sky relying on the reaction force of the gunpowder gas ejected from the tail.
Breakthrough
As early as 1913, French engineer Rennes. Lorraine received a patent for a jet engine. It was a ramjet engine that simply didn't work at the low speeds of the time and lacked the high-temperature heat-resistant materials needed. In 1930, Frank. Whittle obtained his first patent for the use of a gas turbine engine, but it was not until 11 years later that his engine made its first flight. Whittle's engine formed the basis of the modern turbojet engine.
Progress
With the advancement of aviation gas turbine technology, people have developed a variety of jet engines based on turbojet engines. For example, according to different supercharging technologies, there are Ramjet and pulse engines; depending on the energy output, there are turbofan engines, turboprop engines, turboshaft engines and propeller fan engines.
Although jet engines consume more fuel than piston engines at low speeds, their excellent high-speed performance quickly replaced the latter and became the mainstream of aero engines.
Related structures edit this paragraph
Inlet duct
The main structure of the axial flow turbojet engine is as shown in the figure. The air first enters the inlet duct, because the aircraft The flight status changes, and the air inlet needs to ensure that the air can finally smoothly enter the next structure: the compressor (compressor). The main function of the intake duct is to adjust the air to a state where the engine can operate normally before entering the compressor.
When flying at supersonic speeds, both the nose and the inlet will generate shock waves (shock waves, also known as shock waves). The pressure of the air will increase after passing through the shock wave. Therefore, the inlet can play a certain pre-compression effect, but the location of the shock wave Improper use will cause uneven local pressure and may even damage the compressor. Therefore, generally, the inlet opening of a supersonic aircraft has a shock wave adjustment cone to adjust the position of the shock wave according to the airspeed.
Aircraft with side air intake or belly air intake will be affected by the fuselage boundary layer (boundary layer) because the air inlet is close to the fuselage, and there will also be an attached Surface layer adjustment device. The so-called boundary layer refers to a layer of air flowing close to the surface of the fuselage. Its flow rate is much lower than the surrounding air, but its static pressure is higher than the surrounding air, forming a pressure gradient. Because of its low energy, it is not suitable for entering the engine and needs to be removed. When the aircraft has a certain angle of attack (AOA, or angle of attack), due to changes in pressure gradient, boundary layer separation will occur in the part where the pressure gradient increases (such as the leeward side), that is, the boundary layer that was originally close to the aircraft will The boundary layer of the fuselage suddenly breaks away at a certain point, forming turbulence. Turbulent flow is relative to laminar flow. Simply put, it is a fluid that moves irregularly. Strictly speaking, all flows are turbulent flows. The generation mechanism of turbulence and the modeling of the process are not very clear. But this does not mean that turbulence is bad. Turbulence must be fully utilized in many places in the engine, such as the combustion process.
Compressor
The compressor is composed of stator plates and rotor plates. A pair of stator plates and rotor plates are called the first stage. The stator Fixed on the engine frame, the rotor is connected to the turbine by the rotor shaft. Current turbojet engines generally have 8-12 stage compressors. The more stages there are, the greater the pressure is toward the rear. When a fighter jet suddenly performs a high-g maneuver, the air pressure flowing into the front stage of the compressor drops sharply, while the pressure in the rear stage is very high. At this time, reverse expansion of the high-pressure air in the rear stage occurs, and the engine The extremely unstable working condition is called "surge" in engineering. This is the most fatal accident of the engine and is likely to cause shutdown or even structural damage. There are several ways to prevent "surge" from occurring. Experience shows that surge mostly occurs between the 5th and 6th stages of the compressor. A bleed ring is installed in the sub-interval to allow timely pressure relief when abnormal pressure occurs to avoid the occurrence of surge. Or the rotor shaft can be made into two layers of concentric hollow cylinders, respectively connecting the front-stage low-pressure compressor and the turbine, and the rear-stage high-pressure compressor and another set of turbines. The two sets of rotor sets are independent of each other and can automatically adjust the speed when the pressure is abnormal. Avoid surge.
Combustion chamber and turbine
After being compressed by the compressor, the air enters the combustion chamber and mixes with kerosene for combustion, expands and performs work; then flows through the turbine, pushing the turbine to rotate at high speed. Because the turbine and compressor rotors are connected to the same shaft, the rotational speeds of the compressor and turbine are the same. Finally, the high-temperature and high-speed gas is ejected through the nozzle to provide power with reaction force. The initial form of the combustion chamber was several small cylindrical combustion chambers arranged annularly around the rotor axis. Each cylinder was not sealed, but had holes in appropriate places, so the entire combustion chamber was connected. Later it developed into an annular shape. The combustion chamber has a compact structure, but the entire fluid environment is not as good as a cylindrical combustion chamber. There are also combined combustion chambers that combine the advantages of both.
Turbines always work under extreme conditions and have extremely demanding requirements on their materials and manufacturing processes. Most of the hollow plates are made of powder metallurgy and cast as a whole, that is, all the plates and the plate are cast and formed at one time. Compared with the early days, each page and page plate were cast separately and then connected with tenons, which saved a lot of joint mass. The manufacturing materials are mostly high-temperature resistant alloy materials, and the hollow plates can be passed through cold air to cool down. The new engine developed for the fourth-generation fighter aircraft will be equipped with ceramic powder metallurgy plates with better high-temperature performance. These means are all aimed at increasing one of the most important parameters of the turbojet engine: the temperature in front of the turbine. High vortex front temperature means high efficiency and high power.
Nozzle
The shape and structure of the nozzle (nozzle) determines the state of the final airflow. Early low-speed engines used simple convergent nozzles to achieve growth purpose. According to Newton's third law, the greater the gas ejection speed, the greater the reaction force the aircraft will obtain.
However, the speed increase in this way is limited, because eventually the air flow speed will reach the speed of sound, and then a shock wave will appear to prevent the gas speed from increasing. The use of convergent-diverging nozzles (also called Laval nozzles) can achieve supersonic jet flow. The maneuverability of an aircraft mainly comes from the aerodynamic force provided by the wing surface, and when the maneuverability requirements are high, the thrust of the jet stream can be directly utilized. Installing a gas rudder at the nozzle mouth or directly using a deflectable nozzle (also called a thrust vector nozzle, or a vector thrust nozzle) are two solutions in history, of which the latter has entered the practical application stage. The superb maneuverability of the famous Russian Su-30 and Su-37 fighters benefit from the AL-31 thrust vector engine of the Lyulika Design Bureau. The representative of the gas steering surface is the American X-31 technology demonstrator.
Afterburner
The high-temperature gas after passing through the turbine still contains some oxygen that will not be consumed in the future. If you continue to inject kerosene into such gas, it can still burn and produce additional thrust. Therefore, some high-performance fighter engines have an afterburner (or afterburner) added after the turbine to significantly increase the engine thrust in a short period of time. Generally speaking, afterburning can increase the maximum thrust by 50% in a short period of time, but the fuel consumption is alarming. It is generally only used for takeoff or dealing with fierce aerial dogfights, and cannot be used for long-term supersonic cruise.
Edit this paragraph for basic parameters
Thrust to weight ratio: Thrust to weight ratio, which represents the ratio of the engine thrust to the weight of the engine itself. The larger the better the performance.
Compressor stage number: represents the number of stages of compression blades of the compressor. Generally, the larger the stage number, the greater the compression ratio.
Number of turbine stages: represents the number of stages of turbine blades in the turbine.
Compression ratio: The ratio of the pressure of the intake air after being compressed by the compressor to the pressure before compression. Generally, the larger the better the performance.
Maximum net thrust at sea level: The thrust generated by the engine operating at full speed at sea level altitude and conditions when the speed difference (airspeed) with the outside air is zero. The units used include kN (thousands) Newton), kg (kilogram), lb (pound), etc.
Fuel consumption rate per unit thrust per hour: also called specific thrust, the ratio of fuel consumption rate to thrust. The metric unit is kg/N-h. The smaller the fuel consumption rate, the more fuel-efficient it is.
Turbine front temperature: The temperature before the high-temperature and high-pressure airflow enters the turbine after combustion. Generally, the higher the temperature, the better the performance.
Gas outlet temperature: The temperature of the exhaust gases as they leave the turbine.
Mean time to failure: The total average time between two failures of each engine. The longer the engine, the less likely it is to fail and generally the lower the maintenance cost.
Usage Edit this paragraph
Turbojet engines are suitable for a wide range of navigation, ranging from low-altitude and low-subsonic speeds to high-altitude supersonic aircraft. The legendary MiG-25 high-altitude supersonic fighter of the former Soviet Union is powered by the turbojet engine of the Ryulika Design Bureau. It once set a fighter speed record of Mach 3.3 and a ceiling record of 37,250 meters. (This record is unlikely to be broken for some time)
Compared with turbofan engines, turbojet engines have worse fuel economy, but their high-speed performance is better than that of turbofans, especially at high altitudes. High speed performance.