Why jet engine has to have high rpm?
Fascinating the amount of depth in knowledge some have here, not too sure one needs to know that whist flying but hey some of it is 'nice to know' & comforting to some
Last edited by Wally Mk2; 13th Nov 2013 at 05:00.
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Hi dreamlinerwannabee
You have some complicated responses below. But it is easy really. On a car compression ratio (ESP diesel) gets more air and therefore more fuel into the chamber. The higher the better. A turbo does even more.
And on a car the faster you can rev it the more power
The jet does the same thing and archives very high compression ratios. It does this by running at very high rpm with multiple compressors. A jet won't even start below 25% rpm. And the faster you spin the thing the more power- but there are physical limits to how much you can do this of course.
Hope this helps
You have some complicated responses below. But it is easy really. On a car compression ratio (ESP diesel) gets more air and therefore more fuel into the chamber. The higher the better. A turbo does even more.
And on a car the faster you can rev it the more power
The jet does the same thing and archives very high compression ratios. It does this by running at very high rpm with multiple compressors. A jet won't even start below 25% rpm. And the faster you spin the thing the more power- but there are physical limits to how much you can do this of course.
Hope this helps
The question related to why jet engines need high RPM to be efficient. Answers about high mass airflow and high power and low weight are all well and good, but don't answer the original question.
The efficiency of an engine is nothing to do with having to produce lots of power/thrust, it's about maximising the output/input ratio.
A jet engine requires high RPM to be efficient because the efficiency of a heat engine relates directly to the compression ratio.
It's why a diesel engine is ALWAYS going to have better thermal efficiency than a gasoline engine - because diesel engines are higher compression. A similar size/weight gasoline engine may be much more powerful, and have a better power to weight ratio, but it's not as efficient.
Jet engines require high RPM to achieve satisfactory compression ratios. While a piston engine design can achieve very high compression at very, very low RPM, a turbine engine can not, because it is an open design. All of those design features which make a turbine engine really reliable and well suited to aviation (constant pressure, few moving parts, inherent cooling etc.) also mean that at low RPM it develops essentially zero compression.
If you want to see the difference (and would like to chop off your arm at the same time), go and spin a small turbine engine by hand and you will easily get it up to 300-500rpm with little or no resistance. A similar output piston engine would be impossible to rotate by hand at more than 10 rpm.
In summary:
Jet engines require very high RPM compared to piston engines to achieve satisfactory compression ratios. Compression ratio is directly proportional to brake specific fuel consumption (thermal efficiency).
Therefore high thermal efficiency requires high RPM.
The efficiency of an engine is nothing to do with having to produce lots of power/thrust, it's about maximising the output/input ratio.
A jet engine requires high RPM to be efficient because the efficiency of a heat engine relates directly to the compression ratio.
It's why a diesel engine is ALWAYS going to have better thermal efficiency than a gasoline engine - because diesel engines are higher compression. A similar size/weight gasoline engine may be much more powerful, and have a better power to weight ratio, but it's not as efficient.
Jet engines require high RPM to achieve satisfactory compression ratios. While a piston engine design can achieve very high compression at very, very low RPM, a turbine engine can not, because it is an open design. All of those design features which make a turbine engine really reliable and well suited to aviation (constant pressure, few moving parts, inherent cooling etc.) also mean that at low RPM it develops essentially zero compression.
If you want to see the difference (and would like to chop off your arm at the same time), go and spin a small turbine engine by hand and you will easily get it up to 300-500rpm with little or no resistance. A similar output piston engine would be impossible to rotate by hand at more than 10 rpm.
In summary:
Jet engines require very high RPM compared to piston engines to achieve satisfactory compression ratios. Compression ratio is directly proportional to brake specific fuel consumption (thermal efficiency).
Therefore high thermal efficiency requires high RPM.
http://www.pprune.org/tech-log/10655...-altitude.html
The higher the RPM the higher the compression.
Ahh, the simplest questions always generate the most complex answers and vice-versa!
The efficiency of a heat engine is largely governed by the compression ratio. A heat engine is one that converts heat in a gas (added by a fuel) to do work. All combustion engines, like the Otto cycle (four stroke) engine, Diesel engine and Brayton Cycle (gas turbine) engine are heat engines.
How does the amount the air/fuel mix is compressed effect the efficiency? Imagine a piston travelling up a cylinder and compressing the gas in that cylinder (like pushing in the handle of a bike pump with your thumb over the exit hole). If released, the piston would now travel back down the cylinder until the pressure inside equalled the atmospheric pressure. The work done in compressing the gas is recovered as the piston moves back down the cylinder under the pressure of the compressed gas (less friction losses etc.)
If the piston compresses the gas to half its original volume, then it has a compression ratio of 2:1, as the piston recovers it can do work based on the compression it started with. The more the air is compressed, the more work can be done as it recovers. Otto cycle piston engines have a compression ratio of around 8:1.
Now lets introduce some combustion after the piston has compressed the air, as happens in an Otto cycle engine (a "suck, squeeze, bang, blow four stroke). If the charge was only compressed at a 2:1 ratio, then the effect of the combustion can only do work until the piston recovers to ambient pressure. The more the piston compresses the charge, the further away from ambient pressure it is and thus more work can be done by the piston as it recovers. So the higher the fuel/air mix is compressed, the more efficiently that fuel can expend its energy.
Why not compress the charge more in a four stroke, and get even better efficiency? Because as the charge is compressed it heats up, until at higher compressions it heats enough to explode at the wrong time ("known as "pinging", "knocking" or detonation) which may create enough pressure to exceed the strength of the engine components. Typically you put holes in pistons or cylinder heads - not considered a good thing.
Diesel engines actually take advantage of this effect and use "compression ignition" instead of a spark plug, so they can achieve double the compression of Otto engines and are consequentially more efficient. An aviation diesel engine, burning kerosene (Jet fuel) is being developed and should be available on the Socata Trinidad in a year or so. It is lighter, has less moving parts and burns less fuel with more power than your current Lycoming or Continental.
All this can be proved mathematically from first principles through the laws of thermodynamics, a branch of Physics usually studied at second year degree level in Mechanical Engineering. I won't bore you with the proof, but if you are really interested you can click here.
The same principle applies in gas turbine engines, although technically you don't talk about compression ratios, but rather cycle pressure ratios. Known as the thermal efficiency, or internal efficiency of the engine the higher the pressure ratios, the higher the efficiency of combustion. As the compressor is not a cylinder travelling a fixed distance, but a "fan blowing air into a small space" the pressure ratio is governed by the engine RPM.
So gas turbine engines like to run at high RPM.
Next you need to look at the propulsive efficiency, or external efficiency of the engine (the study in Physics known as mechanics). The amount of thrust provided by an engine (propeller or jet) is related to the amount of air they throw out the back, and the speed at which they throw that air. In symbols, if m kilograms is the mass of air affected per second, and if it is given an extra velocity of v meters per second by the propulsion device, then the momentum given to the air per second is mv, so Thrust = mv (per second).
A propeller engine uses a large m and a small v, a gas turbine engine uses a small m and a large v. 10 kg of air given a velocity of 1 m/s has the same thrust as 1 kg/s of air given a velocity of 10 m/s. Which is most efficient? Well, the rate at which kinetic energy is given to the air (the work done) is ½mv² watts. So the first case requires 5 watts of work and the latter requires 50 watts of work. Clearly the piston is more efficient. Problems occur as the speed increases, and the propeller efficiency breaks down.
An easier way to think of it is by considering the waste energy in the flow. Stand behind a propeller engine at takeoff, and it will knock your hat off. Stand behind a jet giving the same thrust and it will knock you off your feet! Waste energy dissipated in the jet wake, which represents a loss, can be expressed as [W(vj-V)²]÷2g (W is the mass flow, vj is the jet velocity, V is the aircraft velocity, so (vj-V) is the waste velocity). As the jet exhaust leaves the gas turbine at roughly the same speed whether it is standing still or moving, the faster the engine is moving the less waste energy lost. Assuming an aircraft speed of 375 mph and a jet velocity of 1,230 mph the efficiency of a turbo-jet is approx. 47%. At 600 mph the efficiency is approx. 66%. Propeller efficiency at these speeds is approx. 82% and 55% respectively.
So gas turbine engines like to fly at high TAS.
The problem is that at high RPM at sea level turbo-jets are sucking in very thick air, so when they add a heap of fuel and burn it they make tremendous amounts of thrust - good for take-off but way too much for level flight at low level (with the associated high IAS).
Aircraft generate too much drag at high Indicated Air Speed and keeping the engine turning at an efficiently high RPM at sea level would quickly exceed the aircraft speed limitations.
As you increase altitude, however, the amount of thrust the engine can produce reduces (as it is sucking in "thinner" air) even though it is still operating at high compression ratios (for good thermal efficiency). Also you can have a high TAS (for good propulsive efficiency) at a low IAS (for lower drag on the airframe). There are a few other advantages, like the cold temperature, which keeps the turbine temperatures down as well.
So jet engines like to fly at high power and at high TAS, while aeroplanes like a moderate IAS, and the regime were all this can be achieved together is at high altitude.
The higher the RPM the higher the compression.
Ahh, the simplest questions always generate the most complex answers and vice-versa!
The efficiency of a heat engine is largely governed by the compression ratio. A heat engine is one that converts heat in a gas (added by a fuel) to do work. All combustion engines, like the Otto cycle (four stroke) engine, Diesel engine and Brayton Cycle (gas turbine) engine are heat engines.
How does the amount the air/fuel mix is compressed effect the efficiency? Imagine a piston travelling up a cylinder and compressing the gas in that cylinder (like pushing in the handle of a bike pump with your thumb over the exit hole). If released, the piston would now travel back down the cylinder until the pressure inside equalled the atmospheric pressure. The work done in compressing the gas is recovered as the piston moves back down the cylinder under the pressure of the compressed gas (less friction losses etc.)
If the piston compresses the gas to half its original volume, then it has a compression ratio of 2:1, as the piston recovers it can do work based on the compression it started with. The more the air is compressed, the more work can be done as it recovers. Otto cycle piston engines have a compression ratio of around 8:1.
Now lets introduce some combustion after the piston has compressed the air, as happens in an Otto cycle engine (a "suck, squeeze, bang, blow four stroke). If the charge was only compressed at a 2:1 ratio, then the effect of the combustion can only do work until the piston recovers to ambient pressure. The more the piston compresses the charge, the further away from ambient pressure it is and thus more work can be done by the piston as it recovers. So the higher the fuel/air mix is compressed, the more efficiently that fuel can expend its energy.
Why not compress the charge more in a four stroke, and get even better efficiency? Because as the charge is compressed it heats up, until at higher compressions it heats enough to explode at the wrong time ("known as "pinging", "knocking" or detonation) which may create enough pressure to exceed the strength of the engine components. Typically you put holes in pistons or cylinder heads - not considered a good thing.
Diesel engines actually take advantage of this effect and use "compression ignition" instead of a spark plug, so they can achieve double the compression of Otto engines and are consequentially more efficient. An aviation diesel engine, burning kerosene (Jet fuel) is being developed and should be available on the Socata Trinidad in a year or so. It is lighter, has less moving parts and burns less fuel with more power than your current Lycoming or Continental.
All this can be proved mathematically from first principles through the laws of thermodynamics, a branch of Physics usually studied at second year degree level in Mechanical Engineering. I won't bore you with the proof, but if you are really interested you can click here.
The same principle applies in gas turbine engines, although technically you don't talk about compression ratios, but rather cycle pressure ratios. Known as the thermal efficiency, or internal efficiency of the engine the higher the pressure ratios, the higher the efficiency of combustion. As the compressor is not a cylinder travelling a fixed distance, but a "fan blowing air into a small space" the pressure ratio is governed by the engine RPM.
So gas turbine engines like to run at high RPM.
Next you need to look at the propulsive efficiency, or external efficiency of the engine (the study in Physics known as mechanics). The amount of thrust provided by an engine (propeller or jet) is related to the amount of air they throw out the back, and the speed at which they throw that air. In symbols, if m kilograms is the mass of air affected per second, and if it is given an extra velocity of v meters per second by the propulsion device, then the momentum given to the air per second is mv, so Thrust = mv (per second).
A propeller engine uses a large m and a small v, a gas turbine engine uses a small m and a large v. 10 kg of air given a velocity of 1 m/s has the same thrust as 1 kg/s of air given a velocity of 10 m/s. Which is most efficient? Well, the rate at which kinetic energy is given to the air (the work done) is ½mv² watts. So the first case requires 5 watts of work and the latter requires 50 watts of work. Clearly the piston is more efficient. Problems occur as the speed increases, and the propeller efficiency breaks down.
An easier way to think of it is by considering the waste energy in the flow. Stand behind a propeller engine at takeoff, and it will knock your hat off. Stand behind a jet giving the same thrust and it will knock you off your feet! Waste energy dissipated in the jet wake, which represents a loss, can be expressed as [W(vj-V)²]÷2g (W is the mass flow, vj is the jet velocity, V is the aircraft velocity, so (vj-V) is the waste velocity). As the jet exhaust leaves the gas turbine at roughly the same speed whether it is standing still or moving, the faster the engine is moving the less waste energy lost. Assuming an aircraft speed of 375 mph and a jet velocity of 1,230 mph the efficiency of a turbo-jet is approx. 47%. At 600 mph the efficiency is approx. 66%. Propeller efficiency at these speeds is approx. 82% and 55% respectively.
So gas turbine engines like to fly at high TAS.
The problem is that at high RPM at sea level turbo-jets are sucking in very thick air, so when they add a heap of fuel and burn it they make tremendous amounts of thrust - good for take-off but way too much for level flight at low level (with the associated high IAS).
Aircraft generate too much drag at high Indicated Air Speed and keeping the engine turning at an efficiently high RPM at sea level would quickly exceed the aircraft speed limitations.
As you increase altitude, however, the amount of thrust the engine can produce reduces (as it is sucking in "thinner" air) even though it is still operating at high compression ratios (for good thermal efficiency). Also you can have a high TAS (for good propulsive efficiency) at a low IAS (for lower drag on the airframe). There are a few other advantages, like the cold temperature, which keeps the turbine temperatures down as well.
So jet engines like to fly at high power and at high TAS, while aeroplanes like a moderate IAS, and the regime were all this can be achieved together is at high altitude.
