Here is a list of eight major types or classification of jet engines.
Jet Engine Type # 1. Thermojet:
A thermojet is a rudimentary type of jet engine. At its heart is an ordinary piston engine, but instead of this driving a propeller, it drives a compressor. The compressed air is channelled into a combustion chamber, where fuel is injected and ignited. The high temperatures generated by the combustion cause the gases in the chamber to expand and escape at high pressure from the exhaust, creating a reactive force that drives the engine.
Thermojet engines provide greater thrust than a propeller mounted on a piston engine; this has been demonstrated in a number of different aircrafts.
Jet Engine Type # 2. Rocket:
It carries all the propellants on board and does not require air intake. Oxidizers are provided on board for combustion. It is used as space shuttle vehicle.
These are suitable for high altitudes, even in vacuum. Efficient at very high speeds. Disadvantages include risk of carrying oxidizers, extreme thermal stresses of combustion chamber, not reusable, extraordinarily noise.
Jet Engine Type # 3. Pulsejet:
Air is compressed and combusted intermittently instead of continuously. Some designs use valves. Very simple design, commonly used on model aircraft. Noisy, inefficient (low compression ratio), works poorly on a large scale, valves on valved designs wear out quickly.
Pulse jet is very similar to ram jet in construction except that in addition to diffuser at intake, combustion chamber and exhaust nozzle, it has mechanically operated flapper valves which can allow or stop air flow in the combustion chamber.
Thus pulse jet is an intermittent flow, compressorless type of device with minimum number of moving parts. Pulsejet was the power plant of German V-1 bomb popularly known as ‘Buzz bomb’ or flying Bomb first used in World War II in 1939-45.
The operation of the pulse-jet is as follows- During starting compressed air is forced into the inlet which opens the spring loaded flapper valves; the air enters combustion chamber into which fuel is injected and burnt with the help of a spark plug. Combustion occurs with a sudden explosion (process 2-3) i.e., combustion is at constant volume instead of constant pressure as in other propulsive devices.
The pulse-jet cycle is more near to Otto-cycle. Ram action can also be used to increase the pressure of the cycle. Due to increase in pressure in combustion chamber, the flapper-valves close stopping the air supply and at the same time the gases are forced out of the nozzle with a very high velocity. Because of this high velocity, the pressure continues to fall since the vacuum is created in the combustion chamber.
This allows the flapper valves to open and fresh air enters the combustion chamber. Some air also reenters from the tailpipe because the pressure in the tailpipe is also very low, and the next cycle starts. This increase in pressure due to back flow helps in better combustion as it will be hot by the time reaches combustion chamber and also increases the thrust due to additional filling. The frequency of cycles depends upon the duct shape and working temperature. In V-1 rocket it was about 40 cycles per second which corresponds to about 2400 rpm of a two-stroke reciprocating engine.
Once the cycle is started neither the initial flow of compressed air nor the spark-plug is necessary as the inlet diffusion will produce sufficient compression and any residual flame or even a hop-spot in the combustion chamber will ignite the fuel. Though the air flow is intermittent, the fuel is continuously supplied.
The pulse jet has low thermal efficiency and limited speed range. Speed is limited by (1) design of diffuser at high speeds and (2) the flapper valves, the only mechanical part in the pulse jet, also have certain natural frequency and if resonance with the cycle frequency occurs and the valves may remain open and no compression will take place.
Also as the speed increases it is difficult for the air to flow back. This reduces the total compression pressure as well as the mass flow air which results in incomplete combustion and lower thrust. The reduction thrust and efficiency is quite sharp as the speed increases. At subsonic speeds it might not to operate as the speed is not sufficient to raise the air pressure to combustion pressure.
1. This is very simple device only next to ramjet and is light in weight. It requires very small and occasional maintenance.
2. Unlike ramjet, it has static thrust because of the compressed air starting; the static thrust is even more than the cruise thrust.
3. It can run on almost any type of liquid fuels without any much effect on the performance. It can also operate on gaseous fuel with a little modifications.
4. Pulse jet is relatively cheap.
1. The biggest disadvantage is very short life of flapper valves and high rates of fuel consumption. The SFC is as high as that of ramjet.
2. The speed of the pulse jet is limited within a very narrow range of 650-800 km/hr because of the limitations in the aerodynamic design.
3. The operational range of the pulse jet is also limited in altitude range.
4. The high degree of vibrations due to intermittent nature of the cycle and the buzzing noise has made it suitable for pilotless craft only.
5. It has lower propulsive efficiency than turbojet engine.
Jet Engine Type # 4. Ramjet:
A ramjet, sometimes referred to as a stovepipe jet, is a type of jet engine. It contains no (major) moving parts and can be particularly useful in applications requiring a small and simple engine for high speed use; such as missiles. They have also been used successfully, though not efficiently, as tipjets on helicopter rotors.
In its simplest form a turbojet consists of an air intake, compressor, combustor, turbine and nozzle. In a ramjet, owing to the high flight speed, the ram compression is sufficient to dispense with the need for a compressor and a turbine to drive it. So a ramjet is virtually a ‘flying stovepipe’, a very simple device comprising of an air intake, a combustor, and a nozzle. Normally the only moving parts are those within the turbopump, which pumps the fuel to the combustor, in a liquid fuel ramjet. Solid fuel ramjets are even simpler.
Ramjets try to exploit the very high total pressure within the streamtube approaching the air intake lip. A reasonably efficient intake will recover much of the freestream stagnation pressure, to support the combustion and expansion processes.
Most ramjets operate at supersonic flight speeds and use one or more conical (or oblique) shock waves, terminated by a strong normal shock, to decelerate the airflow to a subsonic velocity at intake exit. Further diffusion is then required to get the air velocity down to level suitable for the combustor.
Since there is no downstream turbine, a ramjet combustor can safely operate at stoichiometric fuel: air ratios, which implies a combustor exit stagnation temperature of the order of 2400 K for kerosene. Normally the combustor must be capable of operating over a wide range of throttle settings, for a range of flight speeds/altitudes. Usually a sheltered pilot region enables combustion to continue when the vehicle intake undergoes high yaw/pitch, during turns.
Other flame stabilisation techniques make use of flame holders, which vary in design from combustor cans to simple flat plates, to shelter the flame and improve fuel mixing. Overfuelling the combustor can cause the normal shock within a supersonic intake system to be pushed forward beyond the intake lip, resulting in a substantial drop in engine airflow and net thrust.
Because nozzle pressure ratios are relatively high, ramjet engines are normally fitted with a convergent/divergent propelling nozzle. Given sufficient initial flight velocity, a ramjet will be self-sustaining. Indeed, unless the vehicle drag is extremely high, the engine/airframe combination will tend to accelerate to higher and higher flight speeds, substantially increasing the air intake temperature.
As this could have a detrimental effect on the integrity of the engine and/or airframe, the fuel control system must reduce engine fuel flow to stabilize the flight Mach number and, thereby, air intake temperature to sensible levels.
As a ramjet contains no (major) moving parts, it is lighter than a turbojet and can be particularly useful in applications requiring a small and simple engine for high speed use; such as missiles. They have also been used successfully, though not efficiently, as tipjets on helicopter rotors.
Ramjets generally give little or no thrust below about half the speed of sound, and they are inefficient (less than 600 seconds due to low compression ratios) until the airspeed exceeds 1000 km/h (600 mph). Even above the minimum speed a wide flight envelope (range of flight conditions), such as low to high speeds and low to high altitudes, can force significant design compromises, and they tend to work best optimised for one designed speed and altitude (point designs). However, ramjets generally outperform gas turbine based jet engine designs at supersonic speeds (Mach 2-4). Although inefficient at the slower speeds they are more fuel-efficient than rockets over their entire useful working range.
Jet Engine Type # 5. Scramjet:
A scramjet (supersonic combustion ramjet) is a variation of a ramjet with the key difference being that the flow in the combustor is supersonic. At higher speeds it is necessary to combust supersonically to maximize the efficiency of the combustion process. Like a ramjet, a scramjet essentially consists of a constricted tube through which inlet air is compressed by the high speed of the vehicle, fuel is combusted, and then the exhaust jet leaves at higher speed than the inlet air. Also like a ramjet, there are few or no moving parts.
A scramjet requires supersonic airflow through the engine, thus, similar to a ramjet, scramjets have a minimum functional speed. This speed is uncertain due to the low number of working scramjets, relative youth of the field, and the largely classified nature of research using complete scramjet engines. However it is likely to be at least Mach 5 for a pure scramjet, with higher Mach numbers 7-9 more likely. Thus scramjets require acceleration to hypersonic speed via other means.
The first working scramjet was developed by a team from the University of Queensland, Australia led by Professor Allan Paull.
A scramjet is a type of engine which is designed to operate at the high speeds normally associated with rocket propulsion. It differs from a classic rocket by using air collected from the atmosphere to burn its fuel, as opposed to an oxidizer carried with the vehicle. Normal jet engines and ramjet engines also use air collected from the atmosphere in this way. The problem is that collecting air from the atmosphere causes drag, which increases quickly as the speed increases. Also, at high speed, the air collected becomes so hot that the fuel doesn’t burn properly any more.
The scramjet is a proposed solution to both of these problems, by modifications of the ramjet design. The main change is that the blockage inside the engine is reduced, so that the air is not slowed down as much.
This means that the air is cooler, so that the fuel can bum properly. Unfortunately the higher speed of the air means that the fuel has to mix and burn in a very short time, which is difficult to achieve.
To keep the combustion of the fuel going at the same rate, the pressure and temperature in the engine need to be kept constant. Unfortunately, the blockages which were removed from the ramjet were useful to control the air in the engine, and so the scramjet is forced to fly at a particular speed for each altitude. This is called a “constant dynamic pressure path” because the wind that the scramjet feels in its face is constant, making the scramjet fly faster at higher altitude and slower at lower altitude.
The inside of a very simple scramjet would look like two kitchen funnels attached by their small ends. The first funnel is the intake, and the air is pushed through, becoming compressed and hot. In the small section, where the two funnels join, fuel is added, and the combustion makes the gas become even hotter and more compressed. Finally, the second funnel is a nozzle, like the nozzle of a rocket, and thrust is produced.
Note that most artists’ impressions of scramjet-powered vehicle designs depict waveriders where the underside of the vehicle forms the intake and nozzle of the engine. This means that the intake and nozzle of the engine are asymmetric and contribute directly to the lift of the aircraft. A waverider is the required form for a hypersonic lifting body.
Seeing its potential, organisations around the world are researching scramjet technology. Scramjets will likely propel missiles first, since that application requires only cruise operation instead of net thrust production. Much of the money for the current research comes from governmental defence research contracts.
Jet Engine Type # 6. J-58 Combined Ramjet/Turbojet:
The SR-71’s Pratt and Whitney J58 engines were rather unusual. They could convert in flight from being largely a turbojet to being largely a compressor-assisted ramjet. At high speeds (above Mach 2.4), the engine used variable geometry vanes to direct excess air through 6 bypass pipes from downstream of the fourth compressor stage into the afterburner. 80% of the SR-71’s thrust at high speed was generated in this way, giving much higher thrust, improving specific impulse by 10-15%, and permitting continuous operation at Mach 3.2. The name coined for this configuration is turbo-ramjet.
Jet Engine Type # 7. Pre-Cooled Turbojets:
Engines that may need to operate at low hypersonic speeds could theoretically have much higher performance if a heat exchanger is used to cool the incoming air. The low temperature allows lighter materials to be used and combustors to inject more fuel (ordinarily, fuel flow must be reduced at high speed to prevent the turbines from melting, but doing so greatly reduces thrust—precooling the air avoids this.)
This idea leads to plausible designs like SABRE, that might permit single-stage-to-orbit, and ATREX, that might permit jet engines to be used as boosters for space vehicles.
Jet Engine Type # 8. Nuclear-Powered Ramjets:
Project Pluto was a nuclear-powered ramjet, intended for use in a cruise missile. Rather than combusting fuel as in regular jet engines, air was heated using a high-temperature, unshielded nuclear reactor. This raised the specific impulse of the engine by stupendous amounts, and the ramjet was predicted to be able to fly for months at supersonic speeds (Mach 3 at tree-top height).
However, there was no obvious way to stop it once it had taken off, which is a great disadvantage. Unfortunately, because the reactor was unshielded, it was dangerous to be in or around the flight path of the vehicle (although the exhaust itself was not radioactive).