Can jet engines be operated at slow speeds?

Jet engine

Lexicon> Letter S> Jet engine

Definition: an engine that generates thrust (propulsion) by ejecting a gas jet

More specific terms: turbine jet engine, turbojet, deflagration jet engine, turbofan engine, turbofan, propeller turbines, turboprop, ramjet, ramjet

English: jet engine

Category: Basic Concepts

Author: Dr. Rüdiger Paschotta

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Original creation: January 19, 2020; last change: 11.08.2020


Jet engines are air-breathing engines that generate thrust (a propulsive force) by ejecting a jet of gas. There are the following types of this:

  • Turbine jet engines are those that contain a rotating system with a turbine and usually a compressor driven by it.
  • Ramjet engines do not require a compressor or a turbine, because the incoming air is compressed by the inflow at high speed. They are of particular interest for very high speeds well above the speed of sound, but have hardly been used so far.
  • Detonator jet engines are a technology that was tried and tested in the early days of jet engines, which is based on a pulsating deflagration and is no longer used today.

Turbine jet engines are now the common propulsion technology for most aircraft with the exception of propeller-driven aircraft, also for most helicopters (helicopters). They usually offer travel speeds in the region of 800 to 900 km / h, so not too far below the speed of sound. The thrust of a turbofan engine in a large airliner is of the order of 100 kN (kilonewtons) when cruising; at the start it can be a lot more. Mainly combat aircraft can also reach supersonic speeds with appropriately optimized engines.

Most jet engines use kerosene as their fuel.

Rocket engines do not belong to the jet engines; although they also generate thrust from an ejected jet, they are not air-breathing.

Turbine jet engines

Single-flow jet engine (turbojet)

The earliest developed and simplest type of jet engine is that Single flow jet engine, also as Turbojet designated. This can be viewed as a type of gas turbine that is optimized for generating propulsion. It works on the following principle:

  • At the front end (see Figure 1), atmospheric air flows in, usually through an aerodynamically shaped inlet, and then meets a turbine compressor with rotating blades, which compresses the air to a significant extent. This increases the temperature of the air massively, e.g. B. to 600 ° C.
  • The air then enters a combustion chamber, where fuel is continuously injected through nozzles with the aid of a fuel pump. As the fuel burns, the temperature rises massively and the hot gas expands, so that its speed increases considerably as it passes through the combustion chamber. The pressure, on the other hand, drops slightly again.
  • The hot gas then flows through a turbine, which removes some of the energy from it and greatly reduces the pressure in the process. The mechanical energy supplied by the turbine is mainly used to drive the compressor; The turbine and compressor rotate on a common shaft that transmits this power. To a lesser extent, energy is also taken to operate auxiliary units, for example for a fuel pump and to generate electrical energy for various purposes, including outside the engine.
  • Finally, the exhaust gas, which is still quite high in energy, escapes to the outside through a thrust nozzle. This is shaped in such a way that the exhaust gas flows out at a significantly increased speed, but no longer massively increased pressure. (The previously more common term Jet propulsion strongly emphasized the role of the nozzle and to a certain extent hid the more complicated, but also more important technical details of the jet engines.)
  • The thrust (propulsion) of the engine occurs according to the recoil principle: the exhaust gas is expelled at a higher speed than the air entering the front end. This effectively accelerates air backwards and pushes the engine forward accordingly.

Especially in combat aircraft, the engine can also have one Afterburner contain. With this, fuel is injected again behind the turbine, which then also burns and the speed of the exhaust gas is further increased. This is mostly only used for a short-term increase in thrust, e.g. B. in fast climbing maneuvers. The price for this increased thrust, which is achieved with relatively little technical effort and with little additional weight, is a massive increase in fuel consumption and an enormous amount of noise.

Such single-flow engines have a relatively simple and compact structure and achieve high thrust compared to their relatively low weight. So they have a very low power-to-weight ratio, which is of course cheap for airplanes. However, their propulsion efficiency, and even more so the effective propulsion efficiency, is quite low, and the noise emissions are very high. Both are related to the very high ejection speed of the exhaust gases, which, however, requires the generation of a strong thrust. In this regard, the turbofan engines described below are significantly cheaper - at least in the subsonic range. However, single-flow engines are still sometimes used in military aircraft, e.g. B. where the disadvantages of turbofan engines (larger dimensions, higher weight, lower dynamics) would be too significant.


Turbofan engines, also called Twin-flow jet engines, Bypass engines or Turbofans (Figure 2), are the most common type of jet engines today. They have the following differences from single-flow engines:

  • After the inlet, the air first flows through a fan (fan) with a significantly larger diameter than the following turbine. This fan pushes a large part of the air past the side of the following turbine (the core engine) to the outside; this creates a large part of the thrust. The area of ​​the fan is usually much shorter than the entire engine.
  • The drive power of the blower must in turn be provided by the turbine. This means that much more energy is extracted from the exhaust gas than with the single-flow engine. A significantly higher shaft power is generated, and the thrust generated directly by the core engine is correspondingly lower. In this respect, the core engine resembles a stationary gas turbine, as it is, for. B. could be used in a small gas power plant; there too, of course, the wave power is maximized, which is why one also uses one Shaft power turbine speaks.
Accelerating a larger amount of air less strongly is better in terms of energy efficiency and noise generation.

Overall, a significantly larger amount of air is accelerated compared to the single-flow engine, but at a lower exhaust speed. This brings about a significant improvement in the propulsion efficiency and thus also the overall energy efficiency of the engine, as a lower specific fuel consumption. (A smaller part of this advantage is, however, compensated for by the fact that this design increases the power-to-weight ratio by, for example, 20%.) Another important advantage is a significant reduction in noise emissions, because the average speed of the gases is significantly lower and the particularly loud inner part is Is shielded to a certain extent by the quieter external air flow.

An important parameter is the so-called bypass ratio, defined as the ratio of the air mass flows in the bypass (i.e. past the turbine) and in the core flow (i.e. through the turbine). In engines for modern commercial aircraft, the bypass ratio sometimes already reaches values ​​above 10, while older engines still work with values ​​in the region of 5.

In early turbofan engines, all rotating parts (fan, compressor, turbine) moved on a common axis, i.e. rotated at the same speed. However, this is not optimal at all; the large fan can only be operated at a limited speed in order to avoid disadvantageous vortex formation at the blade tips at supersonic speeds, while, on the other hand, higher speeds are more advantageous for the smaller turbine.

Turbine turbines have been continuously optimized for decades - which has significantly reduced their fuel consumption and noise generation.

That is why ingenious constructions were developed. For example, the fan can be mounted on an inner shaft (Low pressure wave) sit together with the last turbine stage, while the compressor and the main part of the turbine via an outer shaft (High pressure wave, arranged coaxially around the inner shaft) are connected. Further optimization is possible through Geared turbofans, where the speed of the fan is reduced by a gearbox. Some jet engines already have three different shafts. In addition, engines with a larger fan and thus a further increased bypass flow ratio have also been developed, for which geared turbofans are also an essential requirement because larger fans have to rotate correspondingly more slowly. However, there are limits to this development. B. because with further enlargement of the fan the entire engine would be quite large and heavy.

The strongest driving force for such developments is the reduction of fuel consumption and thus both operating costs and climate-damaging emissions. The further reduction of noise emissions is also an important goal, especially for use in urban areas. For this, a certain increase in the complexity of the engine is accepted - of course, taking into account other aspects such as operational safety, construction and maintenance costs.

For smaller aircraft with jet engines, such as private jets, turbofan engines are also used, but with a lower bypass ratio z. B. in the region of 3, with correspondingly moderate improvements in fuel consumption and noise emissions.

In the meantime, turbofan engines are also common in military aircraft; however, they were introduced later in this area and often have low bypass ratios, e.g. B. in combat aircraft mostly below 1.5. In the case of fighter planes for supersonic speeds, the values ​​can even be far below 1. This is due to the fact that the specific disadvantages of the sheath current principle are more significant in such aircraft, while the fuel consumption (in relatively few operating hours per year), exhaust emissions and noise are given less attention.

Propeller turbine (turboprop)

As explained above, there are limits to increasing the bypass ratio of turbofan engines. In principle, however, these limits can be overcome in a fairly simple way: The fan - the fan inside the engine housing - is replaced by a propeller whose diameter is significantly larger than that of the rest of the engine. This design is called Propeller turbine or Turboprop designated.

The propeller can be correspondingly large, so that bypass flow ratios (defined here as the propeller flow in relation to the mass flow through the turbine) are possible well above 10 - around 50 or even 100.

The speed of the propeller must of course be adjusted accordingly via a gearbox. Ideally, the propeller should have adjustable blades (i.e. with a variable angle of attack) in order to achieve optimum efficiency in different operating situations.

Turboprops are in principle not suitable for the highest flight speeds.

A significant reduction in fuel consumption is achieved with this approach. However, the possible flight speeds with the propeller are significantly lower than with pure jet engines, since the high peripheral speed of the propeller in connection with the high speeds of the incoming air leads to problematic supersonic conditions; the resulting strong eddies can massively reduce efficiency. For this reason, turboprops are mainly used in short-haul aircraft, where travel speed is less important, but hardly for medium-haul or long-haul flights. Typically, the cruising speed of a turboprop aircraft is about Mach 0.6, i.e. H. in the area of ​​700 km / h. The flight altitude is also more limited than with turbofans.

The technical development of turboprop aircraft is also continuing to reduce both fuel consumption and noise emissions and to increase the quality of the exhaust gas. Other developments aim to increase the possible travel speeds in order to be able to use more turboprops. A technical approach is, for example, the use of two propellers rotating in opposite directions, which can reduce the swirl in the air jet.

Ramjet engines

At speeds well in the supersonic range, the use of a compressor in the engine no longer makes sense; it is sufficient to compress the incoming air through a suitably shaped inlet. If drive power is no longer required for a compressor, a turbine is no longer required. This leads to a very simple type of engine than Ramjet designated.

A crucial problem with such engines, however, is that they can only work at all from a very high airspeed. That is why they have never been used, for example, in commercial aircraft - even those for speeds above the speed of sound - but rather for special military applications.

In view of the ecological problems, dreams of the future of ramjet supersonic flights must remain dreams.

In the future, too, ramjet engines are unlikely to become more widespread, at least in the civilian sector, as conventional air traffic at speeds below the speed of sound is increasingly coming under criticism, mainly because of ecological problems (keyword: climate protection), and supersonic flights are hardly any would be possible without another massive increase in fuel consumption.

A distinction is made between ramjet engines Ramjets and Scramjets. With the former, the incoming air in the engine is slowed down to a speed below the speed of sound (in relation to the engine). The combustion chamber can therefore be operated in the subsonic range. Only after combustion is the escaping gas accelerated again to supersonic speed.

With the scramjet, on the other hand, the speed of the air remains above the speed of sound everywhere in the engine. This is structurally very difficult to cope with because it implies, for example, that the materials used are exposed to extremely high temperatures. Such problems can only be overcome with ingenious cooling concepts, for example using the heat of vaporization of the fuel and by achieving a suitable temperature stratification via a correspondingly optimized design. Such engines have only been demonstrated in a few cases and little is known about them because of military secrecy. The applications are also likely to be primarily in the military sector, for example for extremely fast attack weapons with nuclear weapons.

Turbine engines for helicopters

Most helicopters (helicopters) are also powered by a kerosene turbine. As with the turboprop, this is not optimized for high thrust, but for high shaft power, which is largely required to drive the rotor. The difference to the turbo-prop is actually only in the quantitative range: The speed of the rotor is of course much lower than that of a propeller, which is why a gear with a correspondingly stronger reduction is required.

Secondary functions of aircraft engines

In an aircraft, an engine primarily serves to generate propulsion, but it also has important secondary functions:

  • As already mentioned above, a turbine can drive a generator to generate electrical energy, which not only covers the low internal requirements of the engine, but also the requirements of the entire aircraft.
  • Another essential function is the supply of the air conditioning system, including the system for ensuring adequate cabin pressure, even at high altitudes. So far, this system has only been operated with electrical energy in rare cases. The usual technical approach is to use Bleed air from the compressors of the engines. Despite the low air temperature at high altitudes (e.g. −50 ° C), the bleed air is quite hot (up to 200 ° C) due to the compression to several bar. It is cooled in the air conditioning system with outside air, i.e. suitably tempered, and is fed to the aircraft cabin in the front area (mixed with recirculated cabin air from the rear).Used air is discharged in the rear area of ​​the cabin through an adjustable valve. Unfortunately, this process is relatively inefficient in terms of energy.

Comparison of jet engines with aircraft engines

Most jet engines work without a propeller, but only with an internal fan. You can therefore achieve significantly higher speeds. With turbo jets (without a fan), speeds well above the speed of sound are possible, and even more so with ramjet engines.

Another advantage of jet engines is the relatively less complex structure of a jet engine and the significantly higher reliability achieved with it, which is of course particularly important for aircraft. In this regard, aircraft engines, which are usually designed as reciprocating piston engines, are not optimal. However, the principle of the jet engine is difficult to implement with very low powers, or only with very low energy efficiency. But there are even very small jet engines for use in model airplanes.

The power-to-weight ratio of jet engines is also quite low and helps to minimize the overall weight of an aircraft.

However, the energy efficiency of jet engines is not too high, and at the same time the need for propulsion power is correspondingly high at the high flight speeds practiced with them. Lower fuel consumption is possible with slower aircraft that are operated by a propeller with a correspondingly lower ejection speed - the propeller being either an aircraft engine or a turbine (→Turboprop) can be driven.

In contrast to an internal combustion engine (with internal combustion), combustion in a jet engine takes place continuously, which in principle favors complete and clean combustion. However, the conditions in the combustion chamber of an engine are not ideal. In particular, the flow speed in the engine is far above the flame speed; Correct combustion is only possible through recirculation in the combustion chamber. The combustion temperature is also quite high, which, in conjunction with the high flow rate, favors the formation of nitrogen oxides in particular. In addition, it is not practical to improve the exhaust quality with a downstream catalytic converter. For this reason, jet engines do not achieve particularly good exhaust gas quality, which is particularly problematic in view of the high exhaust gas quantities (e.g. per kilometer and person).

Numerical example for thrust and drive power

A turbofan engine in a modern commercial aircraft generates a thrust in the order of 100 kN (kilonewtons) during normal operation. The drive power in the sense of the drive energy transferred per unit of time is the product of thrust and speed. At a cruising speed of 850 km / h (= 236 m / s) the drive power is approx. 23.6 MW. If the engine has a specific fuel consumption of 1.8 · 10−5 kg / s / N, i.e. 1.8 kg kerosene consumed per second, this corresponds to a fuel output of 77.4 MW, calculated on the basis of the calorific value of 43 MJ / kg. The drive efficiency is then 23.6 MW / 77.4 MW = 30.5%.

At low speeds, for example on the runway, an engine can generate considerably more thrust. With the same thrust, it consumes roughly half as much fuel per second as at cruising speed due to the low flow velocity of the air. Nevertheless, this leads to a significantly lower drive efficiency. A larger proportion of the energy used is transferred to the exhaust gas emitted and less to the aircraft.

The thrust of the engines of a commercial aircraft is usually much less than their weight. For this reason, vertical ascent is only possible for relatively light military aircraft with nevertheless high engine power. A commercial aircraft is kept in the air mainly by the lift on the wings. The generation of this lift causes some of the thrust requirement, but only a fraction of the aircraft's weight.

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See also: engine, gas turbine, kerosene, thrust
as well as other articles in the Basic Concepts category