FF vs EGT vs N1

[SirJ]

I just flew some kind of training in Caucasus an observed some interesting behaviour of the engine panel.

 

 

During cruise (@ 50,000ft) the gauge showed different EGT but the same FF and N2 RPM. The oil pressure seeems to vari a bit but quite suttle.

 

 

Question:

 

 

How can the engine achieve different EGT with same FF and N2?

If there would be weare to the engine in order to achieve same RPM you would need more fuel (-> higher EGT due to lower efficency of compressor and turbine).

 

 

Best regards

 

 

edit: just found out the display shows N2 instead of N1...but N1 is also the same on both engines

Edited February 2, 2019 by SirJ
detail

[Ahmed]

Because IRL wear and tear is different between engines and the situation you describe is quite usual.

 

In simulations this is normally not modeled but ED took the step of modelling it in the F/A-18.

[SirJ]

right you are!

 

but shouldn't there be a difference in FF as well?

 

 

 

The (thermodynamical) reason for higher EGT is lower turbin efficency which gives you lower N1&N2 - FF fixed.

 

 

Since the controlled parameter by FADEC is N1 - it tries to counteract the lower N1 speed by higher FF -> higher EGT.

 

 

So at least one parameter should change too...

[AKarhu]

What FADEC? :smilewink:

[mvsgas]

right you are!

but shouldn't there be a difference in FF as well?

The (thermodynamical) reason for higher EGT is lower turbin efficency which gives you lower N1&N2 - FF fixed.

Since the controlled parameter by FADEC is N1 - it tries to counteract the lower N1 speed by higher FF -> higher EGT.

So at least one parameter should change too...

 

Keep in mind in real life, there is also the gauge to consider. The Some engine use the generator to tell RPM, some use a sensor ( something else to add to the mix). Same goes for the EGT temp sensor, some are old, some new, etc. It is normal for jet engines to have different number in the air ( i.e. RPM, oil pressure, EGT/FTIT, etc.) That is why most manuals only refer to maximums and minimums while in the air. Most manuals would say: " RPM not higher that ____". Or;" Oil pressure no less than ___". But they won't tell you specific numbers because those change depending on ambient condition, speed altitude, etc. Also there are normal fluctuations on the gauges.