Turbo jet engine
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Why the fuel consumption of turbojet engine is higher than a turbofan engine?


A turbojet engine is a kind of turbo engine. It is characterized by completely relying on gas flow to generate thrust. Usually used as the power for high-speed aircraft. Fuel consumption is higher than that of turbofan engines.

There are two types of turbojet engines: centrifugal and axial. The centrifugal engine was patented by Sir Frank Whittle in 1930, but it was not until 1941 that an airplane equipped with this engine went into the sky for the first time. Participating in the Second World War, the axial flow was born in Germany, and as the first practical jet fighter Me-262 powering force participated in the battle at the end of 1944.

Compared with the centrifugal turbojet engine, the axial flow type has the advantages of small cross-section and high compression ratio, but it requires higher quality materials-which did not exist around 1945. Today’s turbojet engines are all axial flow.

Animation of turbo jet engine
Animation of turbo jet engine

Inlet

The main structure of the axial turbojet engine is shown in the figure. The air first enters the intake duct. Because the flight state of the aircraft is changing, the intake duct needs to ensure that the air can finally enter the next structure smoothly: the compressor. The main function of the intake duct is to adjust the air to the state where the engine can operate normally before entering the compressor. When flying at supersonic speeds, shock waves will be generated at the nose and intake ports. The pressure of the air will increase after the shock waves. Therefore, the intake ports can play a role in pre-compression, but the improper shock wave position will cause local pressure failure. Even, it may even damage the compressor. Therefore, the intake port of a supersonic aircraft generally has a shock wave adjustment cone, which adjusts the position of the shock wave according to the airspeed.

Turbo jet engine
Picture annotations: 1-suction, 2-low pressure compression, 3-high pressure compression, 4-combustion, 5-exhaust,

Airplanes with the air intake on both sides or belly, because the air inlet is close to the fuselage, will be affected by the surface layer, and a surface layer adjustment device will also be attached. 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 that of the surrounding air, but its static pressure is higher than that of the surroundings, forming a pressure gradient. Because of its low energy, it is not suitable for entering the engine and needs to be eliminated. When the aircraft has a certain angle of attack, due to changes in the pressure gradient, the separation of the boundary layer 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 fuselage suddenly detaches at a certain point. Form turbulence.

Pratt & Whitney F135 Jet engine
Pratt & Whitney F135 Jet engine

Turbulent flow is relative to laminar flow. Simply put, it is a fluid with irregular motion. Strictly speaking, all flows are turbulent. The generation mechanism of turbulence and the modeling of the process are still not clear. However, non-referential turbulence is not good. In many places in the engine, such as the combustion process, it is necessary to make full use of turbulence.

Compressor

The turbine blades of the compressor are composed of stator blades and rotor blades. A pair of stator blades and rotor blades are called a first stage. The stator is fixed on the engine frame, and the rotor is connected to the turbine by the rotor shaft. The current turbojet engines are generally 8-12 stage compressors. The higher the number of stages, the greater the backward pressure. When the fighter suddenly makes a high maneuver, the pressure of the air flowing into the front stage of the compressor drops sharply, and the pressure of the latter stage is very high. At this time, the latter stage high pressure air will reversely expand and the engine will work. The extremely unstable condition is called “surge” in engineering. This is the most deadly accident of the engine, which is likely to cause flameout or even structural damage. There are several ways to prevent “surge” from happening. Experience has shown that surge mostly occurs between stages 5 and 6 of the compressor, and a bleed ring is set in the secondary interval so that when the pressure is abnormal, the pressure can be relieved in time to avoid the occurrence of surge. Or make the rotor shaft into two concentric hollow cylinders, connect the front low-pressure compressor and the turbine respectively, and the back high-pressure compressor and another set of turbines. The two sets of rotors are independent of each other, and the speed can be automatically adjusted when the pressure is abnormal. Avoid surge.

Schematic diagram of the principle of an axial turbojet engine
Schematic diagram of the principle of an axial turbojet engine

Combustion chamber and turbine

After being compressed by the compressor, the air enters the combustion chamber and mixes with kerosene for combustion, expands to do work; then flows through the turbine, pushing the turbine to rotate at a high speed. Because the turbine and the compressor rotor are connected to a shaft, the speed of the compressor, the compressor and the turbine are the same. Finally, the high-temperature and high-speed gas is sprayed out through the nozzle, and the reaction force is used to provide power. The initial form of the combustion chamber was several small cylindrical combustion chambers arranged in a ring around the rotor shaft. Each cylinder was not sealed, but had holes in appropriate places, so the entire combustion chamber was connected, and later developed to an annular shape. The combustion chamber is compact, but the entire fluid environment is not as good as the cylindrical combustion chamber. There is also a combined combustion chamber that combines the advantages of the two.

Turbines always work under extreme conditions, and have extremely demanding requirements on their materials and manufacturing processes. Currently, powder metallurgy hollow blades are mostly used for integral casting, that is, all blades and blisks are cast at once. Compared with the early days, each blade and blisk were cast separately, and then jointed by tenon, which saves a lot of joint quality. The manufacturing materials are mostly high-temperature alloy materials, and the hollow blades can be passed with cold air to cool down. The new engine developed for the fourth-generation fighter will be equipped with ceramic powder metallurgy blades with more outstanding high-temperature performance. These methods are all to improve one of the most important parameters of turbojet engines: the temperature in front of the turbine. High pre-vortex temperature means high efficiency and high power.

Nozzle and afterburner

The shape and structure of the nozzle (or nozzle) determines the state of the final discharged air flow. Early low-speed engines used simple convergent nozzles to achieve the purpose of speed increase. According to Newton’s third law, the greater the gas ejection speed, the greater the reaction force the aircraft will obtain. However, the growth rate in this way is limited, because the final gas velocity will reach the speed of sound, and then a shock wave will appear to prevent the gas velocity from increasing. The use of convergent-expansion nozzles (also called Laval nozzles) can obtain supersonic jets.

The maneuverability of the 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 used. Installing a gas rudder surface at the nozzle or directly using a deflectable nozzle (also known as a thrust vector nozzle, or a vector thrust nozzle) are two historical solutions, the latter of which has entered the practical application stage. The superb maneuverability of the famous Russian Su-30 and Su-37 fighters is due to the AL-31 thrust vector engine of the Lulica Design Bureau. -22 has developed this technology to a more mature and reliable level, which also has better performance than the former two. The representative of the gas rudder surface is the X-31 technical demonstrator of the United States.

Euro fighter typhoon Afterburn
Euro fighter typhoon Afterburn

After passing through the turbine, the high-temperature fuel gas still contains part of the oxygen that has not been consumed in time. If kerosene is continuously injected into such fuel gas, it can still be burned and generate additional thrust. Therefore, the engines of most modern fighters are equipped with afterburners behind the turbines to achieve the purpose of greatly increasing the thrust of the engines in a short time. Generally speaking, the afterburner can increase the maximum thrust by 50% in a short time, but the fuel consumption is amazing. It is generally only used for take-off or coping with fierce aerial fights. The only thing that can be opened during supersonic cruising, and reach the best The efficient one is SR-71.

Usage

Turbojet engines are suitable for a wide range of navigation, from low-altitude low subsonic speed to high-altitude supersonic aircraft are widely used. The fighter MiG-25 high-altitude supersonic fighter of the former Soviet Union is powered by the turbojet engine of the Mikulin-Tumansk Design Bureau, which has set a fighter speed record of 3.3 Mach and a ceiling of 37,250 meters. (This record is unlikely to be broken in a period of time)

Compared with turbofan engines, turbojet engines have worse fuel economy, but their high-speed performance is better than turbofans, especially high-altitude and high-speed performance.

Basic parameters

  • Thrust-to-weight ratio : Thrust to weight ratio, the ratio of the weight of the engine itself representative of engine thrust, the greater are the better the performance.
  • Compressor stages : it represents how many stages of the compressor’s compression blades. Generally, the larger the stage, the larger the compression ratio.
  • Turbine stages : it represents how many stages of the turbine blades of the turbine.
  • Compression ratio : The ratio of the pressure after the intake air is compressed by the compressor to the pressure before compression. Generally, the larger the better, the better the performance.
  • Maximum net thrust at sea level: The thrust generated by the engine at full speed when the speed difference between the engine and the outside air (airspeed) is zero at sea level and conditions. The units used include kN (kilonewtons) and kg (kg) , Lb (pounds), etc.
  • Thrust hour fuel consumption : also known as the ratio of pushing (specific thrust), the ratio of fuel consumption rate and thrust, metric units of kg / Nh, smaller by more fuel-efficient.
  • The temperature before the turbine : the temperature before the high-temperature and high-pressure gas flow after combustion enters the turbine. Generally, the larger the temperature, the better the performance.
  • Gas outlet temperature : the temperature of exhaust gas as it leaves the turbine.
  • Mean time to failure : The total average time between two failures for each engine. The longer the engine, the less likely it is to fail and the lower the maintenance cost.

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