Aircraft Gas Turbine Engines

    Hello! In this article I want to talk about how aircraft gas turbine engines (GTEs) work. I will try to make this the most simple and understandable language.

    Aviation GTE can be divided into:

    • turbojet engines (turbojet engines)
    • double-circuit turbojet engines (turbojet engines)
    • Turboprop engines
    • Turbojet engines (TVAD)

    Moreover, turbofan engines and turbofan engines may contain an afterburner, in which case they will be turbofan engines and turbofan engines respectively. In this article we will not consider them.

    Let's start with turbojet engines.

    Turbojet engines

    This type of engine was created in the first half of the 20th century and began to find widespread use by the end of World War II. The world's first production turbojet aircraft was the German Me.262. Turbojet engines were popular until the 60s, after which they began to be replaced by turbojet engines.

    Modern Me-262 photo taken in 2016.

    The simplest turbojet engine includes the following elements:

    • Input device
    • Compressor
    • Combustion chamber
    • Turbine
    • Jet nozzle (hereinafter simply nozzle)

    We can say that this is the minimum set for normal engine operation.

    Now consider what you need and why.

    The input device is an expanding * channel in which air is supplied to the compressor and pre-compressed. In it, the kinetic energy of the incoming air is partially converted to pressure.

    * hereinafter we will talk about subsonic speeds. At supersonic speed, physics is changing, and everything is completely different there.

    A compressor is a device in which air pressure rises. The compressor can be characterized by such a value as the degree of pressure increase. In modern engines, it is already starting to step beyond 40 units. In addition, the temperature in it increases (maybe somewhere up to 400 degrees Celsius).

    Combustion chamber - a device in which heat is supplied to compressed air (after the compressor) due to fuel combustion. The temperature in the combustion chamber is very high, can reach 2000 degrees Celsius. It may seem to you that the gas pressure in the chamber also increases greatly, but this is not so. It is theoretically assumed that heat is supplied at constant pressure. In reality, it falls a little due to losses (the problem of imperfect construction).

    A turbine is a device that converts part of the energy of the gas after the combustion chamber into the energy of the compressor drive. Since turbines are used not only in aviation, a more general definition can be given: this is a device that converts the internal energy of the working fluid (in our case, the working fluid is gas) into mechanical work on the shaft. As you can understand, the turbine and compressor are on the same shaft and are rigidly interconnected. If in the compressor there is an increase in gas pressure, then in the turbine, on the contrary, a decrease, that is, the gas expands.

    A nozzle is a narrowing channel in which the potential energy of the gas is converted into kinetic (the remaining energy reserve of the gas after the turbine). As in a turbine, gas expansion occurs in the nozzle. A jet forms, which, emerging from the nozzle, moves the plane.

    With the basic elements sorted out. But still it’s not very clear how it works? Then again, briefly.

    Air from the atmosphere enters the inlet, where it is slightly compressed and enters the compressor. In the compressor, the air pressure rises even stronger, and the temperature rises. After the compressor, air enters the combustion chamber and, being mixed there with fuel, ignites, which leads to a strong increase in temperature, at, you can say, constant pressure. After the combustion chamber, hot compressed gas enters the turbine. Part of the energy of the gas is spent on the rotation of the compressor by the turbine (so that it can perform its function described above), another part of the energy is spent on the movement of the aircraft we need, due to the fact that the gas passing through the turbine turns into a jet stream in the nozzle and escapes from it (nozzle) into the atmosphere. This completes the cycle. Of course, in reality, all the processes of the cycle are continuous.

    Such a cycle is called a Brighton cycle, or a thermodynamic cycle with a continuous nature of the working process and the supply of heat at constant pressure. In this cycle, all gas turbine engines work.

    Brayton cycle in PV coordinates

    H, V - the compression process in the input device
    in the K - compression process in the compressor
    K-F - isobaric heat input
    F-T - gas expansion process in turbine
    T-C - gas expansion process in the nozzle
    P-H - isobaric heat removal to the atmosphere.

    Schematic design of a turbojet engine, where 0-0 is the axis of the engine

    Turbojet engine can have two shafts. In this case, the compressor consists of a low pressure compressor (KND) and a high pressure compressor (KVD), and the supply of work will be carried out by a low pressure turbine (HPH) and a high pressure turbine (HPT), respectively. Such a scheme is more advantageous gasdynamically.

    Sectional view of a real engine of this kind.

    We examined the principle of operation of the simplest scheme of an aircraft gas turbine engine. Naturally, turbofan engines are installed on modern Airbus and Boeing engines, the design of which is much more complicated, but it works according to the same laws. Let's look at them.

    Bypass turbojet engine

    The turbofan engine, first of all, differs from the turbofan engine in that it has two circuits: external and internal. The internal circuit contains the same as the turbojet engine: a compressor (divided into low pressure and high pressure pumps), a combustion chamber, a turbine (divided into high pressure and high pressure pumps) and a nozzle. The external circuit is a channel, with a nozzle at the end. It has neither a combustion chamber nor a turbine. In front of both circuits (immediately after the input device of the engine) is a compressor stage operating on both circuits.

    Not very clear picture comes out, right? Let's see how it works.

    Schematic design of a twin-shaft twin-circuit turbojet engine

    The air entering the engine, passing through the first stage of the low-pressure compressor, is divided into two flows. One part of the air goes along the internal circuit, where the same processes occur that were described when we disassembled the turbojet engine. The second part of the air enters the external circuit, receiving energy from the first stage of the low pressure switch (the one that works on two circuits). In the external circuit, air energy is spent only on overcoming hydraulic losses (due to friction). In the end, this air enters the nozzle of the external circuit, creating tremendous traction. The thrust created by the external circuit can account for 80% of the thrust of the entire engine.

    One of the most important characteristics of a turbofan engine is the bypass ratio. The bypass ratio is the ratio of the air flow in the external circuit to the air flow in the internal circuit. This number can be either greater or less than one. On modern engines, this number goes beyond the value of 12 units.
    Engines, the bypass ratio of which is more than two, is usually called turbofan, and the first stage of the compressor (the one that works on both circuits) is a fan.

    Turbojet aircraft Boeing 757-200. In the foreground you can see the input device and the fan

    On some engines, the fan is driven by a separate turbine, which is placed closest to the nozzle of the internal circuit. Then the engine turns out to be three-shaft. For example, according to such a scheme, Rolls Royce RB211 engines were installed (installed on L1011, B747, B757, B767), D-18T (An-124), D-36 (Yak-42)

    D-18T in the context of the inside

    The main advantage of the turbofan engine is the possibility of creating great traction and good efficiency, compared with turbojet engines.

    On this, I would like to finish about the turbofan engine and move on to the next type of engine - a theater of operations.

    Turboprop engines

    A turboprop engine, like a turbojet, belongs to gas turbine engines. And it works almost like a turbojet. An elementary turboprop engine consists of elements already familiar to us: a compressor, a combustion chamber, a turbine and a nozzle. To them are added a gearbox and a screw.


    The principle of operation is the same as that of a turbojet one, with the difference that almost all of the gas energy is spent on the turbine to rotate the compressor and to rotate the screw through the gearbox (here the screw and gearbox are on the same shaft as the compressor). The screw creates the bulk of the thrust. The remaining, after the turbine, part of the energy is directed into the nozzle, forming a jet thrust, but it is small, it can be a tenth of the total. The gearbox in this circuit is needed in order to lower the revolutions and transmit the moment, since the turbine can rotate at a very high frequency, for example, 10,000 revolutions per minute, and the screw only needs 1,500. And the screw is quite heavy.

    Schematic design of

    a turboprop engine But there is another scheme of turboprop engines: with a free turbine.
    Its essence is that a separate turbine is placed behind a conventional compressor turbine, which is not mechanically connected to the compressor turbine. Such a turbine is called free. The connection between the compressor turbine and the free turbine is only gas-dynamic. A separate shaft goes from a free turbine onto which a gearbox with a screw is mounted. Everything else works the same way as in the first case. Most modern engines perform exactly this way. One of the advantages of such a scheme is the ability to use the engine on the ground as an auxiliary power unit (APU) without moving the propeller.

    Schematic design of a turbine engine with a free turbine

    I want to note that it is not necessary to look at turboprop engines as an ineffective relic of the past. I have heard such statements several times, but they are incorrect.
    A turboprop engine in some cases has the highest efficiency, as a rule, on airplanes with not very high speeds (for example, at 500 km / h), moreover, the aircraft can be of impressive size. In this case, a turboprop engine can be several times more profitable than the turbojet engine considered earlier.

    On this about turboprop engines you can finish. We slowly approached the concept of a turboshaft engine.

    Turbo engine

    Most readers here must first hear such a name. This type of engine is mounted on helicopters.

    The turboshaft engine is very similar to a turbo-prop engine with a free turbine. It also consists of a compressor, a combustion chamber, a compressor turbine, then comes a free turbine, connected with everything previous only gasdynamically. But such an engine does not create jet thrust, it does not have a jet nozzle, only the exhaust. A free turbine has its own shaft, which is connected to the main gearbox of the helicopter (rotor). Yes, all the helicopters I know have such a gearbox, and, as a rule, it is of impressive size. The fact is that the rotor speed of the helicopter is very low. If in an airplane, as I wrote above, they can reach 1,500 rpm, then in a helicopter, for example, in the Mi-8, only 193 rpm.
    A helicopter engine speed is often very high (due to the small size), and they have to be reduced a hundred or more times. It happens that the gearbox is on the engine and on the helicopter itself, for example, the Mi-2 and its engine GTD-350.

    Schematic design of a turboshaft engine

    TV3-117 engine from a Mi-8 helicopter. On the right you can see the exhaust pipe and drive shaft.

    So, we examined four types of gas turbine engines. I hope my text was clear and useful to you. All questions and comments can be written in the comments.

    Thanks for attention.

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