CFM56 EGT variation
 

Why does the CFM56-5C as fitted to the A340 run normal cruise EGT's around 700 degrees C, where the -5A/B will run similar N1 speeds but EGT's around 100-130 degrees C lower? I know that on the CFM56 the EGT is technically the ITT, but I was just pondering the difference. Is it due to the increased thrust rating?

Also, does EGT vary according to altitude at similar N1 speeds?

What is the maximum EGT you would see at TOGA thrust under normal circumstances on both the -5C and -5A/B? I know what the -5C limits are (950C redline and 725C when starting)

Hi Tom,

I'm no CFM56 expert but I'd there are a number of possibilities....

1. The higher temperature on the 5C could be result of the increased thrust rating....i.e. pushing the engine harder for a given fan and core size.

2. The temps on the 5A/B cycle could have been lowered in the performance design phase to boost the disc and shaft lives (lower temps = longer life). While not optimal from an efficiency point of view, the A320 doesn't sit in the cruise very long relative to an A340 and so a higher core temp may not provide much benefit in terms of fuel consumption but it would significantly extend the life of the discs and shafts which are life limited by cycles.

I suspect it's a combination of the two.

Cheers,

P2C

Cycle life limits on disk and shafts are not very sensitive to cruise temps.

However, turbine blade life is

I will be interested in what Barit1 has to say on this

Not an absolute answer just food for thought, EGT is only a reference temperature and different depending on the location of the sensors. It is possible they moved the EGT probes.

Sorry, lomapaseo, my experience on CFM-5A/B/C's is zero. Any of the above could be true, although I'm leaning more to the turbine blade life theory.

I'll also propose that the different fan rotor/stator and the mixed exhaust of the -5C are contributors - i.e. different thrust-to-N1 relationship. Thus simply running the same physical N1 can lead to very different core parameters.

But all of these are merely speculation; I've seen similar effects on other engines.

Could it be, that same N1 indication in % in fact translates to different N1 speed / compressor output to core and the delta EGT is just a result?

Yours, FD.

i am not an cfm56 expert but afaik the 5C differs much from the 5B. its the only engine in the family with a 5 stage LPT and has a significant bigger fan. it has also a higher thrust- especially climb and cruise.

pushing this all from a similar core may result in higher temps.

correct, the -5C2/3/4/P is equipped with 5LPT-stages (instead of 4 at other CFM56 variants) and also with an additional Booster-Section stage (or LPC) so the pressure level (or combustion chamber inlet pressure/temperature) inside the engine is a little bit higher -> higher EGT.
The position of the EGT-Probes should be similar (2nd stage LPT-stator).

IMHO the CFM56 is a little bit to small for the A340 runs always close to the limits (and sometimes exceeding those), so the engines deteriorate faster and the turbine-efficiency decreases which results in a higher EGT.

The reason for the EGT altitude variance is air density - high altitude air is thin, so there is less rotational resistance on the rotors and less fuel is burnt for a given N1/N2

Whippersnapper:
But EGT is related to fuel/air ratio. Less fuel; less air; ratio remains (roughly) the same! Besides, does the 343 cruise much higher than a 320?

No, EGT is more related to how hard you thrash the donk.

This is a fairly complex performance question but it's related to "corrected N1".

In brief - as TAT falls with altitude, the corrected N1 increases, even at the same indicated N1. This means the engine is working harder, relative to its capability at that altitude. Working harder = higher EGT (of course Tom355uk, you, & I know it's really ITT) :8