In this article we will discuss about:- 1. Introduction to Jet Engines 2. History of Jet Engines 3. Thermal Efficiency 4. Propulsive Efficiency 5. Overall Efficiency 6. Thrust Specific Fuel Consumption (TSFC) 7. Cycle Improvements 8. Advantages and Disadvantages of Jet Propulsion over the Other System 9. Application of Various Propulsive Engines.
Contents:
- Introduction to Jet Engines
- History of Jet Engines
- Thermal Efficiency of a Turbojet Engine
- Propulsive Efficiency of Jet Engines
- Overall Efficiency of Propulsive System
- Thrust Specific Fuel Consumption (TSFC) of Jet Engines
- Cycle Improvements of Jet Engines
- Advantages and Disadvantages of Jet Propulsion over the Other System
- Application of Various Propulsive Engines
1. Introduction to Jet Engines:
A jet engine is an engine that discharges a fast moving jet of fluid to generate thrust in accordance with Newton’s third law of motion. This broad definition of jet engines includes turbojets, turbofans, rockets and ramjets and water jets, but in common usage, the term generally refers to a gas turbine Brayton cycle engine used to produce a jet of high speed exhaust gases for special propulsive purposes in Illustration 1.
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The principle of jet propulsion is based on Newton’s second and third laws of motion. Momentum is imported to a mass of fluid in such a manner that the reaction of the imparted momentum gives a propulsive force. This is done by expanding the gas which is at high pressure and temperature through a nozzle so that the gas with markedly increased velocity in the form of jet comes out into the atmosphere and its reaction in the opposite direction gives the propulsive force.
The open cycle gas turbine is most suited for jet propulsion. If the working fluid is expanded in the turbine such that the power developed is only sufficient to drive the compressor and accessories and the rest expansion is achieved in a nozzle which is placed just after the turbine. This unit will serve as a jet propulsion system.
The gas from the turbine while passing through the nozzle will be accelerated and will come out in the form of jet with a tremendous velocity. The reaction of this jet propels the unit in the forward direction (i.e., opposite direction of the jet).
The jet propulsion engines are classified as follows:
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1. Atmospheric jet engines.
2. Rocket engines.
1. Atmospheric Jet Engines:
Atmospheric jet engines require oxygen from atmospheric air for combustion of the fuel. As a result their performance depends to a great extent on the forward speed of the engine and on the atmospheric pressure and temperature.
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2. Rocket Engines:
The rocket engine carries its own oxidizer for the combustion of the fuel and is, therefore, independent of atmospheric air. It requires maximum specific fuel consumption among all types of propulsion systems.
Atmospheric jet engines are further classified as follows:
(a) Steady Combustion Systems (Continuous Air Flow):
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(i) Turbojet
(ii) Turbo jet with after burner (also known as turboram jet, turbojet with tail-pipe burning and turbojet with reheater).
(iii) Turboprop (also known as prop-jet)
(iv) Ram jet (also known as athodyds and Lorin tube)
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(b) Intermittent Combustion System-Intermittent Air Flow:
(i) Pulse jet (also known as aeropulse, resojet, schmidt tube and intermittent jet). The turbojet, the turbojet with after burner and turbo-prop are all modified forms of simple open-cycle gas turbine. The ramjet and pulse jet are athodyds (aero-thermodynamic ducts) i.e., straight duct-type of jet engines without compressor and turbine wheels.
Rocket engines are further classified as:
(i) Liquid-propellent and
(ii) Solid propellent.
Rocket propulsion should be regarded as a source of power for achieving objectives obtainable by other methods.
Some of the rocket applications are:
(a) Artillery barrage rockets
(b) Anti-tank ‘bazcoka’ rockets
(c) All types of guided missiles
(d) Aircraft launched rockets
(e) Jet assisted take-off for air planes
(f) Engines for long range, high speed guided missiles and pilot less aircraft
(g) Transonic airplanes such as D-558.
2. History
of Jet Engines:
Since the beginning of aviation in the early 1900s, the speed and flight endurance of powered aircraft have been determined by the power and efficiency of their propulsion systems—piston engines at first and now powerful jet engines.
Jet engines are the most widely used form of propulsion on commercial and military aircraft. They generate power when a mixture of compressed air and fuel is ignited and the resulting hot gases escape through an exhaust nozzle. The reaction of the hot expanding gases exiting the exhaust nozzle produces thrust in the opposite direction and moves the aircraft forward.
Most jet engines today fall into three categories — turbojet, turbofan and turboprop. Turbojets incorporate a turbine-driven compressor to pull air into the engine and compress it before fuel is injected into the combustion chamber and ignited.
Turbofan engines are turbojets in which additional power is generated by compressor blades that extend beyond the exterior of the main engine casing. Turboprop engines are turbojets that have propellers which provide extra thrust.
Today’s jet engines have succeeded in flying faster than speed of sound, making best use of sophisticated intake systems that minimize drag even at hypersonic speeds, efficient fuel control systems, newly invented materials that can sustain very high temperatures, auxiliary power units that ensure starting in worst conditions and vectored thrust nozzle that can direct thrust in required direction.
Here, we focus on modern jet engines which are used for military and civil purposes, their major components, and systems. Type comparison helps to identify the best engine for a particular application and set of situations. It also focuses on advanced designs like Ramjet, Scramjet, etc.
Jet engines can be dated back to the first century AD, when Hero of Alexandria invented the aeolipile. This used steam power directed through two jet nozzles so as to cause a sphere to spin rapidly on its axis. So far as is known, it was never used for supplying mechanical power, and the potential practical applications of Hero’s invention of the jet engine were not recognised. It was simply considered a curiosity.
Jet propulsion only literally and figuratively took off with the invention of the rocket by the Chinese in the 11th century. Rocket exhaust was initially used in a modest way for fireworks but gradually progressed to propel some quite fearsome weaponry; and there the technology stalled for hundreds of years.
The problem was that rockets are simply too inefficient to be useful for general aviation. Instead, by the 1930s, the piston engine in its many different forms (rotary and static radial, air-cooled and liquid-cooled inline) was the only type of power-plant available to aircraft designers. This was acceptable as long as only low performance aircraft were required, and indeed all that were available.
However, engineers were beginning to realise conceptually that the piston engine was self-limiting in terms of the maximum performance which could be attained; the limit was essentially one of the propeller efficiency. This seemed to peak as blade tips approached the speed of sound. If engine, and thus aircraft, performance were ever to increase beyond such a barrier, a way would have to be found to radically improve the design of the piston engine, or a wholly new type of power-plant would have to be developed.
This was the motivation behind the development of the gas turbine engine, commonly called a “jet” engine, which would become almost as revolutionary to aviation as the Wright brothers’ first flight.
The earliest attempts at jet engines were hybrid designs in which an external power source supplied the compression. In this system (called a thermojet by Secondo Campini) the air is first compressed by a fan driven by a conventional piston engine, then it is mixed with fuel and burned for jet thrust.
The examples of this type of design were the Henri Coanda’s Coanda-1910 aircraft, and the much later Campini Caproni CC.2, and the Japanese Tsu- 11 engine intended to power Ohka kamikaze planes towards the end of World War II. None were entirely successful and the CC.2 ended up being slower than the same design with a traditional engine and propeller combination.
The key to a practical jet engine was the gas turbine, used to extract energy to drive the compressor from the engine itself. The gas turbine was not an idea developed in the 1930s- the patent for a stationary turbine was granted to John Barber in England in 1791.
The first gas turbine to successfully run self-sustaining was built in 1903 by Norwegian engineer Aegidius Elling. The first patents for jet propulsion were issued in 1917. Limitations in design and practical engineering and metallurgy prevented such engines reaching manufacture. The main problems were safety, reliability, weight and, especially, sustained operation.
3. Thermal Efficiency of a Turbojet Engine:
Thermal efficiency is defined as the ratio of propulsive power developed at the exhaust nozzle to the heat supplied by the fuel. This ratio indicates the degree of utilisation of fuel in accelerating the fluid flow.
4. Propulsive Efficiency
of Jet Engines:
Propulsive efficiency is defined as the ratio of useful propulsive power or thrust power to the sum of thrust power and unused kinetic energy of the jet. It is generally denoted by ηp. Here the kinetic energy of the jet is relative to
5. Overall Efficiency of Propulsive System:
Overall efficiency of the propulsive system gives the performance of the propulsive efficiency. It indicates the extent to which the system utilizers the energy supplied. Also, the overall efficiency is defined as the ratio of the rate of doing useful propulsive work and the rate at which energy is supplied to the system. It is indicated by η0.
6. Thrust Specific Fuel Consumption (TSFC)
of Jet Engines:
Similar to engines and turbines, the specific fuel consumption in case of jet propulsive system is on the basis of thrust produced.
Hence, the thrust specific fuel consumption (TSFC) is defined as the amount of fuel required to produced unit thrust per hour. Thrust take here is Newton. Therefore,
Sometimes, specific fuel consumption is based on thrust power. Then, Thrust Power Specific Fuel Consumption (TPSFC) is defined as the ratio of fuel consumed per hour per unit thrust power.
The nature of the variation curves for thrust, thrust power, TSFC and TPSFC against air or flight speed are shown in Fig. 35.7.
The performance of the jet propulsion system is affected mainly by two parameters as:
1. Forward speed of the air-craft and
2. Altitude of the air-craft.
7. Cycle Improvements
of Jet Engines:
Increasing the overall pressure ratio of the compression system raises the combustor entry temperature. Therefore, at a fixed fuel flow and airflow, there is an increase in turbine inlet temperature. Although the higher temperature rise across the compression system implies a larger temperature drop over the turbine system, the nozzle temperature is unaffected, because the same amount of heat is being added to the system.
There is, however, a rise in nozzle pressure, because overall pressure ratio increases faster than the turbine expansion ratio. Consequently, net thrust increases, whilst specific fuel consumption (fuel flow/net thrust) decreases.
Thus turbojets can be made more fuel efficient by raising overall pressure ratio and turbine inlet temperature in unison. However, better turbine materials and/or improved vane/blade cooling are required to cope with increases in both turbine inlet temperature and compressor delivery temperature. Increasing the latter requires better compressor materials.
8. Advantages and Disadvantages of Jet Propulsion over the Other System
:
Following are the advantages of jet propulsion:
1. Low-Specific Weight:
The specific weight of the jet propulsion is one-fourth to one-half of the reciprocating engine.
2. No Unbalance Force:
There are no reciprocating parts and so jet propulsion is free from unbalanced forces. Greater reliability is thus achieved.
3. Small Frontal Area:
The frontal area of jet propulsion is less than one-fourth the frontal area of the reciprocating engines which decreases drag greatly and hence makes available greater power, particularly at high loads. This also reduces the air cooling problem.
4. No Restriction in Power Output:
Compared to reciprocating engine the jet propulsion with greatly increased power output can be built because the power is not limited by detonation. The unit can work/operate over a large range of mixture strength.
5. High-Speed:
The speed of jet propulsion is not limited by propeller. High speed can be obtained.
6. Neither Lubrication Nor Radiators:
Jet propulsion requires neither internal lubrication nor radiators as reciprocating engines require.
7. At high speed greater than 900 kmph and at an altitude greater than 10,000 metres, the efficiency of the jet is much higher than that of a propeller.
8. Combustion and delivery of power is continuous, whilst peak and fluctuating pressures do not occur.
9. There is no ship stream loss, the drag is reduced, and warm compressed air is available for cabin heating.
10. The unit permits of a better position of the pilot whilst the absence of a propeller permits a smaller under carriage.
Disadvantages:
The disadvantages from which jet propulsion suffers are:
1. Particularly at low pressure, the thermal efficiency is lower. At low altitude and speed upto 150 m/sec/540 kmph, the fuel consumption is 2 to 3 times that of a reciprocating engine.
2. The plant is very noisy, materials costly and life short.
3. The compression-pressure ratio is not constant as in the reciprocator, but varies approximately with the square of the speed.
4. There are certain difficulties which are encountered in the running of the propulsive unit.
9. Application of Various Propulsive Engines
:
(a) Turbo-Prop:
Turbo-prop jet engines are used for medium and long range transport and bomber aircraft. They fly with subsonic speed.
(b) Turbojet:
Turbojets are mainly used in military as fighters, bombers and for transport applications. Generally they fly with supersonic speeds, e.g. MIGS, Mirage, Knat, Jaugar etc. The only turbojet used for civil aviation is concord.
(c) Turbofan:
Used mainly for civil aviation. Flies at subsonic speed.
(d) Ramjet:
Ramjets are used for misses, as pilotless aircraft. They fly at supersonic speed.
(e) Pulsejet:
They are used for missiles, applications and fly at supersonic speed.