CFM56 flat rated

[barrichello72]

Hi folks
One question i have that i am having a hard time wrapping my head around is why the EGT remains constant beyond the Flat Rated Temperature as the OAT increases- I understand why the N1 decreases being the thrust output T dependent upon the air density which in turn is affected amongst other factors by the OAT but i can not visualize the reason why the EGT remains constant.

Many thanks for your inputs

[Dave Therhino]

That's the intentional design of the rating table. When you are in the higher OAT range above the "knee" of the rating curve, the maximum engine thrust capability of the engine is limited by the temperature the hot section can withstand due to material temperature limitations. The control system and its rating tables are designed to keep the hot section temperatures below the material limits, and that is why the thrust is rolled off with rising OAT above the "flat rated" range to achieve a roughly constant EGT above the knee of the thrust vs OAT rating curve.

The engines are built to be as light weight as possible while still delivering adequate thrust as marketed to the airframer and operator. Setting aside more subtle limiting aspects of engine design such as blade pressure loading and compressor stage pressure ratio limits, the maximum thrust a turbine engine can produce can be limited by the internal gas pressure the engine case can withstand, the rotational speed capability of the shaft system structures, or the temperature capability of the high turbine. Below the knee of the takeoff rating curve, the engine is typically case pressure limited, leading to that part of the curve being designed to provide relatively constant thrust vs temperature, with shaft speed increasing with temperature. Above the knee it is hot section temperature limited, leading to that part of the curve being designed to provide relatively constant EGT vs OAT. The MCT rating at high altitude is typically fan shaft speed limited.

Tdracer may be able to offer more as turbine engine control system integration with aircraft is his area of expertise.

[vilas]

As pointed out the power an Engine produces is limited by it's core strength, rotational forces and ability to withstand high temperature. As the atmospheric temperature increases beyond the flat rated temperature the density decreases and if the fuel flow is kept constant the EGT starts rising beyond the limit EGT so fuel flow is progressively reduced to keep EGT at limit. The reverse is happening at temperatures below the flat rated temperature. As the air density is increasing the EGT is not limiting but more fuel cannot be added as the core strength is limiting. Therefore fuel flow is kept constant to limit thrust below that.

[TopSwiss 737]

This image might help to illustrate what Dave Therhino and vilas are explaining.


https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/576x415/corner_point_dd7faa11b95ed07c916fc404c9f4fd0794cc07d1.png


The figure above shows that the N1 is increasing with increasing OAT to the Corner Point. The EGT is consequently increasing to the Corner Point to remain constant beyond it. The margin to the maximum EGT-value (Red Line) is also displayed.

The EGT Margin is the difference between the EGT Red Line and the EGT observed on an engine at TOGA with a temperature greater than the Corner Point OAT.

[tdracer]

Tdracer may be able to offer more as turbine engine control system integration with aircraft is his area of expertise.

Sorry, a bit late to the party here (I was off line over the weekend and just catching up). I think the previous replies have covered it rather well - but in a nutshell once you get to the corner point temperature, the intent is to hold the burner exit/turbine inlet temperature constant and the max rated thrust (N1/EPR) is reduced as necessary to maintain the turbine inlet temp as the outside temperature increases.
Sometimes other factors come into play - e.g. max rotor speed margins - but as a general rule it's EGT which relates directly to the turbine inlet temp.