Cold weather MAP/RPM question

[jetay1]

I have a total brain block here, could you help me out please.

Say you are cruising with a set MAP+RPM at 65% power combination, leaned to best economy in ISA conditions at 4000ft. Naturally aspirated engine.

Suddenly the temperature of the air is decreased by 30C. None of the engine controls are touched.

What happens to MAP and engine output in % ? Increase or decrease.

How does the picture change if the engine is carburated or fuel injected?

[plugster]

With a constant fuel flow the power outpout shouldn't change as you would be operating in the lean of peak range where fuel flow translates directly into power output.

[jetay1]

Hi

I didn't quite get the meaning of this part "lean of peak range where fuel flow translates directly into power output".

If the temperature of the air decreases, doesn't mass flow of air through the carburator increase? As it does, fuel flow would also increase, which would cause a slight power output increase, indicated by a MAP increase?

I know this may sound odd, but I can't get my head around it and I'll soon tell why

[Piltdown Man]

I'll have a guess: MAP would increase (more dense air), FF would increase, Power would increase, IAS would increase, TAS would reduce, Drag would increase, SFC would improve.

PM

[plugster]

I assumed constant fuel flow which wouldn't happen on a carbureted engine when the OAT changes. It may happen however on injected engines where there are corrective measures for varying density but we all know how good it works from time to time...
E.g. You are operating peak egt. You increase MAP with const FF. Would the power change? No. There is simply no fuel left to be burned. Same figure for decreasing temperature. More mass with at constant MAP put into the engine thus operating on the lean side. Now if the fuel controller manages to compensate for the change in density altitude (changing temperature on a constant pressure alt) the power would increase.
I wouldn't be too sure about decreasing TAS, I'm not sure whether increased drag would weigh out increased power (condition: not operating on constant percentage of T/O power as in the POH).
My assumption for the given conditions on a fixed pitch prop would be increasing rpm, decreasing MAP as the power increases and the lower deck pressure drops with increasing RPM and volumeflow.

[Ocampo]

I'll have a guess: MAP would increase (more dense air), FF would increase, Power would increase, IAS would increase, TAS would reduce, Drag would increase, SFC would improve.

Shouldn't the FF remain constant since the mix control is regulating a valve and that valve will remain in that position whether it's cold or hot? Or does the increased density of air "drags" more fuel with it? It would be sort of the same effect as when you don't enrich the mixture through the descent. So...maybe the engine would detonate?

[plugster]

The engine driven fuel pump forces more fuel through the mixer on higher power settings. Otherwise your engine would be deliberatly flooded when idling on the ground with the mixture set to rich. The mixture only controls a ratio ( in ideal conditions).
The engine would only tend to detonation on very high power settings with a too lean mixture ( t/o). You won't get any engine to detonation on a 50% ( or even higher) power setting whatever the mixture is.

[Ocampo]

Thanks for that clarification plugster.

I still think PM's got it right though

[sycamore]

Since it`s a manual controlled engine, if the temp changes the air density will change,so the mixture will be weaker if it gets colder,weaker if it got warmer..

[Microburst2002]

If temperature decreases but altitude is maintained (constant pressure) then density increases. But MAP should remain the same since pressure hasn't changed and the throttle valve remains in the same position ¿Right?. % power would be increased, since you would be burning more fuel per stroke, now (more fuel flow).
What would happen with EGT? I don't know. On the one hande, less OAT, less EGT. On the other, more FF, more EGT.

[rudderrudderrat]

Hi jetay1,

I think:

What happens to MAP and engine output in % ? Increase or decrease.

Pressure remains the same - so does MAP. Greater mass of air into the engine, so power will increase. If fixed prop, air density has a bigger effect on the prop so RPM will fall. If variable pitch prop, RPM will remain the same but pitch will absorb the power. Both will see increase in airspeed.

How does the picture change if the engine is carburated or fuel injected?

Carburettor engine will need to be enriched (denser air) to maintain mixture, fuel injected engine will automatically adjust to maintain mixture (mass flow sensor).

[dirkdj]

it would make a difference if you have a constant speed prop or fixed pitch. With constant speed prop, the air mass would increase while the mixture would remain the same, so the mixture ratio would become leaner.

With a fixed pitch prop, there might be an increase in RPM which could trigger other effects. Sorry, long time since I last flew fixed pitch prop or carbs. Search for the John Deakin series on engine management.

[plugster]

Search for the John Deakin series on engine management

I can only second that!