EGT rise on TO Roll..

[Gold Leader]

Every time we start a Jet Engine after we push the start lever in we get a EGT increase… now the EGT rises to what ever temperature for that time of the day and then rolls back to a stable temperature…(I presume this roll back is due to the increased airflow in the engine). Now as the thrust is advanced for taxi… the EGT rises and then drops back again…(also this time I guess, the temp. increases as the fuel flow has increased, but as the air flow over the eng has also increased in a few seconds the temp. subsequently rolls back again)
But has any one noticed, that when we set Take Off Thrust the EGT RISES AND KEEPS RISING as the aircraft accelerates… WHY ??? Shouldn’t the EGT drop as the airflow over the engine has increased ???
And if I have noticed correctly the temp. keeps rising on the Take Off Roll and even after Take Off and only comes down as the thrust is reduced at acceleration altitude.. :confused:

[Streamline]

Obviously your ATPL courses does not cover the subject of jet engines extensively enough.
Just ask yourself what happens to the pressures/ temp in your engine as you accellerate down the runway with constant N1 and you have your answer.
However the fact that you did notice this is interesting, EGT is to the jest engine what body temp is to the human, higher that normal is a sign of.......
The key thing is to take note of the margin you have over the max EGT you will then have a good idea when you are going to make a critical Take Off as far as EGT is concerned.
With a bit of experiance you will be able to predict your peak EGT during Take Off as a function of OAT.
Part of our job is to know what is normal in order to detect early enough wat isn't.

[Gold Leader]

Thanx Streamline... but is there any thumb rule to predict the EGT on OAT ?? im sure it varies engine to engine but is there any generic rule ???

[Streamline]

There is no general rule, you will have to build your own mental database with experiance.

[barit1]

The EGT excursion is the result of many factors, but one has nothing to do with the airspeed increasing:

If you've been at idle (or near idle, as in a long taxi) the rotor and casings have stabilized at a moderate temperature. Now pour in the fuel for takeoff, and the first thing that happens is the engine temporarily is at a rich fuel/air ratio to develop the torque to wind it up to speed. Once on-speed, the fuel flow drops back to a value determined by the governor (which may be either mechanical or electronic).

But now the lightweight casing of the engine starts heating up and expanding, meaning that the rotor-to-stator clearances increase, meaning the gas leakage past the blade tips is greater. This inefficiency calls for a bit more fuel, and more EGT.

Finally the massive rotor starts to catch up, and as it heats, the clearances once again close down to a normal stabilized level. This restores the design efficiency, not as much fuel is required to hold RPM, and the EGT cools down a bit.

All this can be observed in a static runup. Add in the airspeed increase during takeoff, and this is exaggerated.

:ok: OK?

[White Bear]

Interesting question, but some of the answers have only confused me.
I have no experience with jet engines at all, so please, no abuse.

Might I ask the following question given this scenario?

Like any internal combustion engine that does not have a throttled air intake, the power produced is controlled only by the amount of fuel introduced into the combustion chamber.

So at idle, a jet engine has much more air passing through it than can be used by the fuel introduced, so it is in effect running lean.

Increasing the amount of fuel will allow the engine to make use of more of that air and turn it into useful work, by increasing the rpm of the rotors, which brings in more air etc.

As the amount of fuel introduced brings the air/fuel ratio more towards adiabatic, the exhaust temp will rise, because combustion temp is rising.

Then the fuel supply is held constant, but the induction continues to increase (as the engine is still spooling up, and from increased ram intake pressure) once again leaning the mixture and tail pipe temps drop.

It would seem to me that rpm is controlled only by the amount of fuel introduced, because the amount of air available for combustion always exceeds the amount of fuel available.

If fuel were not restricted then would the engine continue to accelerate until it destroys itself?

Am I completely off base?
Regards,
W.B.

[barit1]

...
Like any internal combustion engine that does not have a throttled air intake, the power produced is controlled only by the amount of fuel introduced into the combustion chamber.
So at idle, a jet engine has much more air passing through it than can be used by the fuel introduced, so it is in effect running lean.

Not on most engines. The efficiencies of the compressor and turbine are optimized for high power, and thus off-design at idle. Thus the steady-state fuel/air ratio is probably higher than you'd expect. In fact - if you make a very slow accel on the throttle, stop every 5% or so and let it stabilize, note the EGT and then move on to the next point, you may well find that idle is NOT the coolest EGT. There may be a dip or "bucket" at 75% or so.


Increasing the amount of fuel will allow the engine to make use of more of that air and turn it into useful work, by increasing the rpm of the rotors, which brings in more air etc.
As the amount of fuel introduced brings the air/fuel ratio more towards adiabatic, the exhaust temp will rise, because combustion temp is rising.
Then the fuel supply is held constant, but the induction continues to increase (as the engine is still spooling up, and from increased ram intake pressure) once again leaning the mixture and tail pipe temps drop.

Agreed.
[EDIT: Ooops - I just reread this - On modern engines there's never a "fuel supply is held constant" condition; it's always being modulated to hold some RPM or other parameter to a constant value. Once the RPM stabilizes the FF may LOOK pretty constant, but it's constantly being trimmed because of changing externals (Temp & Pressure) and internals (component efficiencies). Think closed loop control.]


It would seem to me that rpm is controlled only by the amount of fuel introduced, because the amount of air available for combustion always exceeds the amount of fuel available.
If fuel were not restricted then would the engine continue to accelerate until it destroys itself?

Yes - and for this reason the governor is probably double- or triple- redundant.



Am I completely off base?
Regards,
W.B.

No, you have a good grasp of gas turbines, except for the (admittedly esoteric) idle case. :ok: