Why jet engine has to have high rpm?

[Dreamlinerwannabe]

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!

[Tu.114]

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.

[Lightning Mate]

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 ?

[Dreamlinerwannabe]

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!

[Lightning Mate]

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.

[Shiny10]

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.

[phiggsbroadband]

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.

[Lightning Mate]

So it's got nothing to do with pressure ratio then !!

BTW, Formula 1 reciprocating piston engines achieve 18,000 - 19,000 rpm.

[phiggsbroadband]

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.

[Lightning Mate]

I know of Frank Whittle, but who is Frank Wittle ?

[Onceapilot]

Mostly wrong here. As Tu said, read the theory books:ok:.

[Lightning Mate]

Mostly wrong here.

Am I ?

The floor is yours.

[2 LOW]

Did anyone see this brilliant doco on BBC2 the other day?

All thanks to someone called Frank Whittle

Cold War Hot Jets (1) - YouTube

[WeekendFlyer]

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.

[Lightning Mate]

...massive fan with a high bypass ratio and spinning it fast

...and blade tip speed ?

....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.

[TURIN]

SUCK, SQUEEZE, BURN, BLOW.
That is all.

[WeekendFlyer]

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.

[lomapaseo]

Suck Squeeze Bang and Blow

so nowhere does it say 'spin" :)

Fair point about limitations in RPM speed being limited by sonic velocities :ok:

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 :)

[TURIN]

http://upload.wikimedia.org/wikipedia/commons/thumb/3/3c/Brayton_cycle.svg/745px-Brayton_cycle.svg.png

[HazelNuts39]

May be it has to do with weight. A slowly rotating engine would be heavier per unit thrust than a higher rpm engine.

[Wally Mk2]

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:ok:

[TheChitterneFlyer]

Here we go... the Bandstand grows and grows!

[yanrair]

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

[Slippery_Pete]

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.

[Checkboard]

http://www.pprune.org/tech-log/10655-why-jet-engines-more-fuel-efficient-high-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.