Oil Temp Vs Engine RPM

[Mr.Vortex]

Hi all pprune members,

I have notice that most of the time when jet engine has raised their RPM
, the engine oil temperature will decrease regardless of flight condition. But
when I look in piston engine like Cessna 172, the oil temp rise as the engine RPM
increase. Why is that happened? Is it due to the fact that the jet engine has an
oil cooler that cool by the fuel and hence more throttle open = more fuel flow
through the oil cooler and hence the oil temp drop is that right or it has the
other thing to do with it? I have tried to search in this forum but have no luck
so far.

Thanks for all of your reply.

Best regards.

[Admiral346]

Hi!
I don't have a definite answer, but I believe you are on the right track.

In a piston, oil is not only used for lubrication, a huge job it has to do is the internal cooling of the engine.
So more RPM, more work, more cooling.

In a jet, the oil is also used for fuel heating. So more fuel flow, more heat is transfered from oil to fuel. But then it still does some cooling inside the engine, so I am not sure my answer is correct.

Nic

[DERG]

Piston engine has more surface to surface interfaces than a jet engine per energy transferred. Piston engine has less oil too per the output in kW. It has less oil in the sump per output.

Oil in a good gas turbine really SHOULD have an easy life compared to the piston engine, less fuel contamination and hence less degradation.

Most aircraft engines are air cooled just as the jet engine is. If you check out the T972 QF32 investigation thread we go into some detail about oil temps on jet engines.

If you are interested check out "tribology in aerospace gas turbines".

[Turbine D]

Mr.Vortex
I have notice that most of the time when jet engine has raised their RPM
, the engine oil temperature will decrease regardless of flight condition.

That is partially true, but only for a very short while. In a high speed bearing application such as that in a jet engine, a significant source of heat generation is that of the energy required to pump the oil from the bearing itself. At very high speeds this is generally the most significant source of heat. In a jet engine, as oil flow is increased, say from ground idle to TO thrust, the bearing temperatures will at first fall to some lower value, but then will steadily increase. They reason is a drag loss caused by the bearing ploughing through oil that is momentarily trapped in the bearing before being flung out by centrifugal force. The term is called "churning loss". The temperature rises because the churning losses overpowers the oil's ability to remove the heat generated. The temperature generally begins to settle to a norm once the climb out phase of the flight is accomplished. Oil from bearing sump returns is pumped through a cooler where it is cooled by main engine fuel flow coming from the aircraft wings.

I should add, at high altitudes during the cruise portion of the flight, oil cooling is switched to the air oil cooler for those engines that have both oil cooling systems.

Turbine D