EGT, Bleeds & N1 ?

[LOKE]

Assuming a - 30º C – the Turbo Jet Engine produces Max Thrust with Bleeds Off – right? Now if we go Bleeds On we get less thrust and lower N1 – 100.1% Bleeds On versus 101.0% Bleeds Off at Sea Level on the engine that I’m looking at.

Why doesn’t the engine turn the same RPM – or higher – with the Bleeds Off and produce less thrust – assuming some power is extracted to the Bleeds. Is the EGT the limiting factor?

In any case given the above 2 conditions – can we assume that the EGT is going to be higher in the Bleeds Off condition due to the higher N1.

I’ve probably mixed a bunch of apples and Oranges here – but I hope that someone can discern the question.

BTW – this is a “Thrust Bump” situation so it is not possible to go to Bleeds Off and a higher assumed temp and reduce EGT – there are no Reduced Thrust TO with a Bump.


Any information would be appreciated,

LK

[idg]

LOKE
Indeed it is (probably) the EGT limit. When the Bleed is off there is a greater mass airflow through the engine. To heat this to the same (max EGT) limit we have to pump more fuel into the engine. Thus we get more energy at the turbine which equals more RPMs and thus more thrust.

Actually we should be careful when talking about this kind of situation with a FADEC controlled engine since it will be the FADEC that sets an RPM (or EPR) and accepts whatever EGT that occurs. As the engine gets older the EGT required to achieve rated thrust will increase and thus reduce the EGT 'margin' in the engine.

When the engine is new however there will be a large EGT margin available and so it would be entirely possible (without the FADEC getting in the way, as it were) to achieve the same thrust bleeds on to bleeds off.

[80/20]

No engine bleed = lower EGT.
Best example is to do a bleeds on takeoff in a classic with a 24K engine (without derate) on a hot day and note how the EGT peaks around 1500 feet agl. during climbout. You might even have to reduce power to avoid over-temp. Many operators have a policy to allways do engine bleeds off takeoff when OAT is above 28C with non-derated engines on the classic.