Difference between Pulse jet, Ramjet, Scramjet, Turbojet, Turbofan, Turboprop & Rocket Engines
Jet propulsion is the basic physics associated with all the airplanes, jets, missiles and rockets. All these vehicles except rockets run with the help of jet engines which are air breathing engines. However, rockets will not belong to the category of air breathing engines, rather they are self-contained engines. The jet engines are further classified into
- Turbojets
- Turbofans
- Turboprops
- Pulsejets
- Ramjets
- Scramjets
This page will give you the basic differences between all these engines so that the Mechanical engineer or the Aeronautical engineer in you can identify the basic concepts behind these engines. So fasten your seat belts, let's take off.
Before going into the basics of these engines, one must be very thorough with the concept of Mach Number. Mach Number is a dimensionless number which manifests the compressible nature of the flow field. It is defined as the ratio velocity of flow to the acoustic velocity (Velocity of sound) at the given temperature of the flow field. The given condition of the flow field is a very important consideration for Mach Number. Mathematically, M=V/a.
- For a Subsonic flow - M<1
- For Sonic flow - M=1
- For Supersonic flow - 1<M<5
- For Hypersonic flow - M>5
The airplanes which we used to travel is below Mach 1 or nearly to Mach 1 and is in the subsonic velocity. Which means Airplane is traveling at a velocity less than the speed of sound at the altitude temperature where it is flying. The speed of sound at that altitude is roughly 300m/s. That means 1080 Km/hr. But airplanes or passenger flights used to travel at a speed below 1000 Km/hr. So that is all about Mach Numbers.
Now let us discuss jet engines. Jet engines are also known as reaction engines because the reaction produced by the engines propels the aircraft or vehicle. They are basically internal combustion engines discharging hot exhaust gases at high velocities that generated thrust. Jet engines produce power by sucking air at the front side and discharging it at the backside, unlike the automobile IC engine which produces power by the reciprocating motion of the piston. The thrust force is the force which propels all these engines which can be better explained by Newton's third law of motion. Some of the important components of conventional jet engines or a turbojet engine are compressor, combustor, turbine and a nozzle. However, there can be additional elements attached like fans in case of a turbofan engine and propeller in case of a turboprop engine. Pulsejet, Ramjet and Scramjet engines which are modern developments in the field of jet engines which don't require compressor and turbine.
Turbofan engines comprise of a fan at the inlet whose blades are mostly made of Titanium and rest of the design is similar to turbojet engine. These fans suck large quantity of air compared to turbojet engines and there is a reduction in fuel requirements and an additional propulsive force which is obtained by bypassing the air through the duct which goes to the back of the engine rather than going to the core of the engine. So turbofan engines are more efficient than turbojet engines of the same capacity. Due to this reason, most of the modern aircraft are using turbofan engines.
In a Turboprop engine, a propeller is attached to the inlet instead of a fan. The propeller is turned with the help of a gas turbine. Even though fan and propeller work behind the same physics, a fan basically an enclosed element which allows a bulk movement of air and will run at higher speeds and have simpler cross sections than that of a propeller. A propeller, on the other hand, is primarily used to produce thrust and operate at a comparatively lower speed and have complex cross sections compared to a fan. So that was all about the design and physics. However, here in turboprop engines, instead of using all the gases to produce thrust, most of the energy is used to drive the turbine. The turbine then rotates the drive shaft which rotates the propeller. The exhaust velocity of a turboprop is low and contribute to low thrust as most of the energy is utilized to turn the drive shaft. Many low speed modes of transport and small aircraft use turbo prop engines.
Now let us discuss jet engines. Jet engines are also known as reaction engines because the reaction produced by the engines propels the aircraft or vehicle. They are basically internal combustion engines discharging hot exhaust gases at high velocities that generated thrust. Jet engines produce power by sucking air at the front side and discharging it at the backside, unlike the automobile IC engine which produces power by the reciprocating motion of the piston. The thrust force is the force which propels all these engines which can be better explained by Newton's third law of motion. Some of the important components of conventional jet engines or a turbojet engine are compressor, combustor, turbine and a nozzle. However, there can be additional elements attached like fans in case of a turbofan engine and propeller in case of a turboprop engine. Pulsejet, Ramjet and Scramjet engines which are modern developments in the field of jet engines which don't require compressor and turbine.
Turbofan engines comprise of a fan at the inlet whose blades are mostly made of Titanium and rest of the design is similar to turbojet engine. These fans suck large quantity of air compared to turbojet engines and there is a reduction in fuel requirements and an additional propulsive force which is obtained by bypassing the air through the duct which goes to the back of the engine rather than going to the core of the engine. So turbofan engines are more efficient than turbojet engines of the same capacity. Due to this reason, most of the modern aircraft are using turbofan engines.
In a Turboprop engine, a propeller is attached to the inlet instead of a fan. The propeller is turned with the help of a gas turbine. Even though fan and propeller work behind the same physics, a fan basically an enclosed element which allows a bulk movement of air and will run at higher speeds and have simpler cross sections than that of a propeller. A propeller, on the other hand, is primarily used to produce thrust and operate at a comparatively lower speed and have complex cross sections compared to a fan. So that was all about the design and physics. However, here in turboprop engines, instead of using all the gases to produce thrust, most of the energy is used to drive the turbine. The turbine then rotates the drive shaft which rotates the propeller. The exhaust velocity of a turboprop is low and contribute to low thrust as most of the energy is utilized to turn the drive shaft. Many low speed modes of transport and small aircraft use turbo prop engines.
Now let us make a gear shift to our discussion and move to the Jets or fighter planes from the aircraft. If you take the case of a Jet or a fighter plane, it usually moves in a Mach number range of up to Mach 3. They can start the jet from 0 velocities (that is from rest) or Mach 0 to a velocity range which is 3 times higher than the sound velocity at the attitude they are flying or up to Mach 3. So these jets can fly up to 3280 Km/hr at the max which uses a jet engine. Note that all these jet engines, whether it is a turbofan or turbojet or turboprop, they all use a compressor and turbine which are rotating elements.
A Pulsejet engine is another special class of jet engines which do not use compressor and turbine and usable at the subsonic speeds. Pulsejet engines are having a valve system in the front (unlike fans or compressors in front of turbo jet engines). They are spring loaded valves.To start the process, it is necessary to initiate air flow through the valve, which is usually done with the help of a high-pressure air source. This valve allows the combustion gases or pulse of detonation after combustion flow in one direction, which is through the long exhaust tube. Pulsejet engines use a spark plug for initial ignition of fuel only. After combustion, due to high pressure, the inlet valve will close and a higher momentum flow of gases is obtained at the end of the exhaust tube which is greater than the inlet air momentum ultimately creating a thrust which helps in the propulsion of the system. However, the end of the exhaust tube, we will get a reverse of flow. This is because the flow of energy inside the exhaust tube creates a vacuum inside the combustion chamber which introduces a negative pressure gradient in the flow direction and reverses the flow. This is known as Kadenacy effect. These constant pulsing back and forth movement of the detonation wave or energy which keeps the engine running and the system ignited continues to happen and so the engine is named as pulse jet engines.
The frequency of pulses depends up on the length of the exhaust pipe and the volume of the combustion chamber. This negative pressure also induces the inlet valve to open and suck the fresh air from outside to the combustion chamber. Some amount of fire or residual flame which exists in the exhaust tube will reignite the fresh charge with the air. So a steady flow and sustained ignition are accomplished by the residual flame. So there is no need of spark plug further. Pulsejets are having lower overall propulsion efficiency compared to turbojets even if it possesses the advantage of the lack of moving parts. So in present state of development, turbojets are preferred over pulsejets.
Turbojet and pulse jet engines are not suitable to operate at hypersonic speed, say over Mach 3. So ultimately, the question is what could we do to make jet engines faster or what are the limitations of jet engines? In jet engines, we have low speeds in the combustion chamber. You can also say the speed is very much subsonic. So for flying at supersonic speed, we must slow the flow down at the intake and the compressor so that it moves at a low or subsonic speed as it passes through the combustor. But this transition of velocities will create shock and is detrimental to the engines. So there should be some other technique so that we can use jet engines at higher speeds above Mach 3.
A Pulsejet engine is another special class of jet engines which do not use compressor and turbine and usable at the subsonic speeds. Pulsejet engines are having a valve system in the front (unlike fans or compressors in front of turbo jet engines). They are spring loaded valves.To start the process, it is necessary to initiate air flow through the valve, which is usually done with the help of a high-pressure air source. This valve allows the combustion gases or pulse of detonation after combustion flow in one direction, which is through the long exhaust tube. Pulsejet engines use a spark plug for initial ignition of fuel only. After combustion, due to high pressure, the inlet valve will close and a higher momentum flow of gases is obtained at the end of the exhaust tube which is greater than the inlet air momentum ultimately creating a thrust which helps in the propulsion of the system. However, the end of the exhaust tube, we will get a reverse of flow. This is because the flow of energy inside the exhaust tube creates a vacuum inside the combustion chamber which introduces a negative pressure gradient in the flow direction and reverses the flow. This is known as Kadenacy effect. These constant pulsing back and forth movement of the detonation wave or energy which keeps the engine running and the system ignited continues to happen and so the engine is named as pulse jet engines.
The frequency of pulses depends up on the length of the exhaust pipe and the volume of the combustion chamber. This negative pressure also induces the inlet valve to open and suck the fresh air from outside to the combustion chamber. Some amount of fire or residual flame which exists in the exhaust tube will reignite the fresh charge with the air. So a steady flow and sustained ignition are accomplished by the residual flame. So there is no need of spark plug further. Pulsejets are having lower overall propulsion efficiency compared to turbojets even if it possesses the advantage of the lack of moving parts. So in present state of development, turbojets are preferred over pulsejets.
Turbojet and pulse jet engines are not suitable to operate at hypersonic speed, say over Mach 3. So ultimately, the question is what could we do to make jet engines faster or what are the limitations of jet engines? In jet engines, we have low speeds in the combustion chamber. You can also say the speed is very much subsonic. So for flying at supersonic speed, we must slow the flow down at the intake and the compressor so that it moves at a low or subsonic speed as it passes through the combustor. But this transition of velocities will create shock and is detrimental to the engines. So there should be some other technique so that we can use jet engines at higher speeds above Mach 3.
In jet engines, it was the role of the compressor to raise the pressure and temperature of the gas. But for a jet engine traveling at very high speed, we really don't need a compressor. It is because just slowing the flow down to subsonic conditions itself will raise the pressure and temperature to a level which we can do the combustion. So we can just throw away the compressor and also the turbine as the turbine was used to drive the compressor. So ultimately what remains? It is just a hollow passages with converging and diverging sections. And such an engines is called Ramjet engine.
Ramjet is a jet engine without a compressor or a turbine. Ramjet has an inlet diffuser to take the air which is greater than Mach 1 and it is slowed down to Mach less than 1. Combustion takes place at Ma<1 by adding the fuel and finally we exhaust the gas through a convergent-divergent nozzle to a supersonic velocity (ie Ma>1). And we can produce a high momentum at the outlet of the nozzle higher than the air which is breathed at the inlet, we can potentially produce thrust from the engine. So by the application of newtons third law, the system cruise in the opposite direction of thrust.
However, ramjets have some limitations. Ramjet can't start from 0 speed. Because they need the system to be moving in order to intake air at supersonic speed. Condition around Mach 3 will be a good starting point for a ramjet engine. It can go up to Mach 6 range. But it we try to cruise the ramjet at speeds higher than Mach 6, because of the ram effect (forcing the air into the engine), there is a rise in temperature and pressure to a level which is too high for not enabling good combustion. So at these speeds, that is greater than Mach 6, we don't need air to slow down to subsonic conditions. Instead, we can just slow the flow little bit, thus raising its temperature and pressure slightly and leave at supersonic state in the combustor. An engine which does the same is called a Scramjet.
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Scramjet engine has an inlet (conical intake) where we slow the flow down, raising its pressure and temperature, it has combustors (around 6 combustion chambers) where we burn some fuel. The gas travel at supersonic speed in the combustion chamber. Then we exhaust the gas through divergent nozzles (around 6 nozzles). So if we get a gas which is coming out of the engine with a high momentum than the gas which is coming in, we will get a thrust force to propel the system forward. They also don't have any rotating parts or moving parts like compressors and turbines. So a scramjet engines can be used at hypersonic speeds. It could be possible to run Scramjet at Mach 10 and Mach 12. Experiments are going on to find the upper Mach limit of Scramjets. The main difference of scramjet and ramjet is that scramjet is a supersonic combustion ramjet, that is ramjet which can operate at supersonic flow regimes throughout. Some of the problems faced by scramjet engines are problems of combustion at very high Mach numbers. The effect of drag is substantial in scramjet engines and the heating problems become severe.
Rockets are not air breathing engines like ramjet and scramjets. They are completely self-contained. They must carry fuel and oxidizer during its travel. Rockets are designed in such a way primarily because of the restrictions in the operating conditions of the rocket, that is it has to cruise in the outer space where the air is absent. Since they are carrying both fuel and oxidizer, which is a liquid or solid substance, especially the huge requirement of oxidizer, makes the rocket heavier and bulky. The good thing about rocket is that it can operate in any environment. They are versatile engines. However, they are less efficient in the air regimes compared to ramjet and scramjet.
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| Fig: Comparison of Jet Engines and its operational range |
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| Comparison table of Turbojet, Pulsejet, Ramjet and Scramjet engines |
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Tremendous, Excellent information. I loved your hard work for this information about rocket engines,
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