Why jet engine has to have high rpm?
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Why jet engine has to have high rpm?
Hi aviators,
Flying the big jets and ace the technical pilot interview are saying,
A jet engine has to have high rpm to be efficient and this efficiency can be only achieved at high altitude where drag is low.
And why jet engine has to have high rpm to be efficient ?
Why these books always explain half way and stop, or only explain the end story within basic explanation?
Not complaining but it isn't the first time lol
Lol thanks a lot!
Flying the big jets and ace the technical pilot interview are saying,
A jet engine has to have high rpm to be efficient and this efficiency can be only achieved at high altitude where drag is low.
And why jet engine has to have high rpm to be efficient ?
Why these books always explain half way and stop, or only explain the end story within basic explanation?
Not complaining but it isn't the first time lol
Lol thanks a lot!
What You need is not those refresher books You mentioned. Try getting a training book on gas turbine engine technology instead; this will help You get a reasonable understanding.
The turbine section does not work well at low rpm.
The IP and HP compressor sections don't either.
Consider a turbocharged motor car. How must pressure rise does it give with the engine a low rpm ?
The IP and HP compressor sections don't either.
Consider a turbocharged motor car. How must pressure rise does it give with the engine a low rpm ?
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I went to Roll Royce's jet engine,
32. With increasing altitude the ambient air
pressure and temperature are reduced. This affects
the engine in two interrelated ways:
The fall of pressure reduces the air density and
hence the mass airflow into the engine for a
given engine speed. This causes the thrust or
s.h.p. to fall. The fuel control system, as
described in Part 10, adjusts the fuel pump
output to match the reduced mass airflow, so
maintaining a constant engine speed.
The fall in air temperature increases the density
of the air, so that the mass of air entering the
compressor for a given engine speed is greater.
This causes the mass airflow to reduce at a
lower rate and so compensates to some extent
for the loss of thrust due to the fall in atmospheric
pressure. At altitudes above 36,089 feet and up
to 65,617 feet, however, the temperature
remains constant, and the thrust or s.h.p. is
affected by pressure only.
But it is still not clear enough.
So it means engine rpm has to be high to suck in air effectively, at high altitude, less air so less drag, so rpm higher. However, less air for it to suck in as well is it ?
Thanks a lot for helping!
32. With increasing altitude the ambient air
pressure and temperature are reduced. This affects
the engine in two interrelated ways:
The fall of pressure reduces the air density and
hence the mass airflow into the engine for a
given engine speed. This causes the thrust or
s.h.p. to fall. The fuel control system, as
described in Part 10, adjusts the fuel pump
output to match the reduced mass airflow, so
maintaining a constant engine speed.
The fall in air temperature increases the density
of the air, so that the mass of air entering the
compressor for a given engine speed is greater.
This causes the mass airflow to reduce at a
lower rate and so compensates to some extent
for the loss of thrust due to the fall in atmospheric
pressure. At altitudes above 36,089 feet and up
to 65,617 feet, however, the temperature
remains constant, and the thrust or s.h.p. is
affected by pressure only.
But it is still not clear enough.
So it means engine rpm has to be high to suck in air effectively, at high altitude, less air so less drag, so rpm higher. However, less air for it to suck in as well is it ?
Thanks a lot for helping!
A compressor converts kinetic energy into pressure energy.
Kinetic energy varies with the square of the rpm.
Therefore low rpm = very little energy.
The turbine converts pressure (and temperature) energy into kinetic energy.
Therefore low rpm = very little energy.
Kinetic energy varies with the square of the rpm.
Therefore low rpm = very little energy.
The turbine converts pressure (and temperature) energy into kinetic energy.
Therefore low rpm = very little energy.
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A Simplistic View
One simple reason for high RPM,
Thrust formulae is: T = 1/2MxV2
Therefore if we want to increase the thrust we can increase the mass.
Two ways to achieve this are:
1) Increase the size of the compressor
2) Increase the speed of the compressor
Both would increase the mass air flow through the engine but increasing the size of the compressor would increase the weight and drag of the engine.
As stated,this is a simplistic view of one concept. I am sure there are more complex reasons outside my remit as a fixer and not a designer.
Thrust formulae is: T = 1/2MxV2
Therefore if we want to increase the thrust we can increase the mass.
Two ways to achieve this are:
1) Increase the size of the compressor
2) Increase the speed of the compressor
Both would increase the mass air flow through the engine but increasing the size of the compressor would increase the weight and drag of the engine.
As stated,this is a simplistic view of one concept. I am sure there are more complex reasons outside my remit as a fixer and not a designer.
Last edited by Shiny10; 12th Nov 2013 at 15:38.
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Maybe the answer is BECAUSE THEY CAN... A piston engine has lumps of metal called pistons which move up, stop, then move down and stop, several thousand times a minute. If you hold a brick in one hand and try to move it up and down, you use a lot of energy just doing that. At high rpm the con-rods will soon break and cause a failure.
The turbine has no 'up and down' parts, only a well balanced shaft that revolves smoothly. The only limit to the rpm achieved is the maximum rpm that it can still hold onto its turbine blades against the centrifugal forces.
So there we have it... Piston engines can't exceed say 10,000 rpm and turbines can't exceed, lets say 100,000 rpm, before they fail.
The turbine has no 'up and down' parts, only a well balanced shaft that revolves smoothly. The only limit to the rpm achieved is the maximum rpm that it can still hold onto its turbine blades against the centrifugal forces.
So there we have it... Piston engines can't exceed say 10,000 rpm and turbines can't exceed, lets say 100,000 rpm, before they fail.
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Just as an afterthought... How much of a leap of faith did Frank Wittle have when he first pushed the throttle open on his new turbine engine.
You can only imagine the thought process... They all knew that large engines could run at 4000 rpm, so maybe they decided to start off their tests at that rpm. That went Ok, so lets try 6000rpm... again Ok... Try 8000 still Ok... And all the way up to whatever figure they reached before it went bang.... maybe 30,000 rpm, and the exposed combustion chambers shining red-hot.
Only one word for that.... Brilliant.
You can only imagine the thought process... They all knew that large engines could run at 4000 rpm, so maybe they decided to start off their tests at that rpm. That went Ok, so lets try 6000rpm... again Ok... Try 8000 still Ok... And all the way up to whatever figure they reached before it went bang.... maybe 30,000 rpm, and the exposed combustion chambers shining red-hot.
Only one word for that.... Brilliant.
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Jet Engine Efffiency
In simple terms, thermal efficiency is derived from the ratio of power out divided by power in. For a jet engine, power out is the rate of change of kinetic energy of the air passing through the engine, and power in is the fuel flow rate x specific energy of the fuel:
KE in per second = (TAS x air mass flow at inlet)/2
KE out per second = (jet velocity/2) x (air mass flow + fuel mass flow)
Heat energy in per second = fuel mass flow rate x QR
(QR=42MJ/kg is a reasonably good heating value for jet fuel)
If you neglect fuel mass flow in the KE calculation, because it is small compared to air mass flow for a high bypass turbofan engine, you get:
Thermal efficiency = air mass flow rate x (Vjet^2 - TAS^2) / (2 x Mdotf x QR)
Propulsive efficiency is about maximising thrust by having a small change in speed of air as it goes through the engine, but a large mass flow. Multiply thermal efficiency and propulsive efficiency together and you get overall efficiency. for a large modern highbypass fan it is difficult to get overall efficiency much above 45%.
Thus to get good overall effiency you need to make the air mass flow rate large and the fuel flow rate low in comparison. You do this by having a massive fan with a high bypass ratio and spinning it fast. The faster it spins, the greater the KE imparted to the air because the air mass flow increases.
KE in per second = (TAS x air mass flow at inlet)/2
KE out per second = (jet velocity/2) x (air mass flow + fuel mass flow)
Heat energy in per second = fuel mass flow rate x QR
(QR=42MJ/kg is a reasonably good heating value for jet fuel)
If you neglect fuel mass flow in the KE calculation, because it is small compared to air mass flow for a high bypass turbofan engine, you get:
Thermal efficiency = air mass flow rate x (Vjet^2 - TAS^2) / (2 x Mdotf x QR)
Propulsive efficiency is about maximising thrust by having a small change in speed of air as it goes through the engine, but a large mass flow. Multiply thermal efficiency and propulsive efficiency together and you get overall efficiency. for a large modern highbypass fan it is difficult to get overall efficiency much above 45%.
Thus to get good overall effiency you need to make the air mass flow rate large and the fuel flow rate low in comparison. You do this by having a massive fan with a high bypass ratio and spinning it fast. The faster it spins, the greater the KE imparted to the air because the air mass flow increases.
...massive fan with a high bypass ratio and spinning it fast
....and the poster was asking a simple question.
Some of us have not only flown with gas turbines, but one of us has lectured as a consultant in jet engine construction and operation.
I think I'll leave now.
SUCK, SQUEEZE, BURN, BLOW.
That is all.
That is all.
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Blade tip speed
LM, no need for that you know! Was just trying to condense a fairly complex bit of theory down to a simple, memorable conclusion for the OP.
Well aware of tip speed limitations, I won't go into it here in great detail but clearly once the blade tips approach Mach 1 then things get complicated. Tip speed is related to fan radius and fan RPM, hence to get a larger diameter fan you have to reduce the RPM. P&W embarked on geared turbofans for exactly this reason.
Well aware of tip speed limitations, I won't go into it here in great detail but clearly once the blade tips approach Mach 1 then things get complicated. Tip speed is related to fan radius and fan RPM, hence to get a larger diameter fan you have to reduce the RPM. P&W embarked on geared turbofans for exactly this reason.
Suck Squeeze Bang and Blow
so nowhere does it say 'spin"
Fair point about limitations in RPM speed being limited by sonic velocities
But the other parts like squeeze and bang work best at maximum energy conditions or higher RPM. If you've got it, then flaunt it?
So would this need an operating cycle condition to explain this?
cue a gas turbine lecturer
so nowhere does it say 'spin"
Fair point about limitations in RPM speed being limited by sonic velocities
But the other parts like squeeze and bang work best at maximum energy conditions or higher RPM. If you've got it, then flaunt it?
So would this need an operating cycle condition to explain this?
cue a gas turbine lecturer
As you wish.
