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Leaning - will a correctly leaned engine always end up with the same fuel consumption?

That diagram is for the atmosphere in which the aircraft is flying. Not the air reaching the carb. Raising the temperature of the air, without adding water, will leave the dewpoint the same.

Maoraigh
EGPE, United Kingdom

Yes, which is why one gets a fuel injected engine stoppage at an OAT of -15C, with the air being heated to -5C. But nobody seems to believe this, despite it having been verified by actual airborne measurements.

Administrator
Shoreham EGKA, United Kingdom

Maoraigh wrote:

Raising the temperature of the air, without adding water, will leave the dewpoint the same

Yes, If you move directly right on the chart you maintain constant dew point.

@pilot dar

Fuel is the cheapest, and most reliable cooling possible to reduce the risk of engine overheat damage.

Oh, dear. Unfortunately that is one of the many old wive’s tales that is still being kicked about.

Fuel does not cool CHT’s. If one thinks about that carefully, it is not possible. In terms of combustion, fuel makes big bangs whenever ignited, not comparable with water which on the other hand does cool when added to the mix.
Has anyone ever seen a fire engine squirting fuel to put a fire out?

Just to add quickly here an explanation:
A richer mixture (more than 150F ROP) slows the flame front, thereby delaying the combustion to a later point in the combustion cycle due to the mechanical advantage gained by the rotating piston. A lower Internal Cylinder Pressure (ICP) is achieved. ICP’s are directly proportional to CHT’s, thereby lower ICP’s result in lower CHT’s.

Exactly the same occurs with a lean mixture when on the lean side from peak EGT’s.
It is exactly as Dirk wrote.

Come on, it is time for you guys to brush up your knowledge here, time has moved on and data is available to view for all.

Perhaps try the online course from APS, then you can view the data for yourselves and with the newly gained knowledge, the whole combustion saga will become suddenly much clearer.

Best money you will ever spend.

Last Edited by complex-pilot at 13 Oct 22:27

complex-pilot wrote:

In terms of combustion, fuel makes big bangs whenever ignited, not comparable with water which on the other hand does cool when added to the mix.

As long as there is enough oxygen, yes. If not then it will behave like water.

ESKC (Uppsala/Sundbro), Sweden

Well, I wrote in terms of combustion, right?

That assumes 3 basic ingredients:

Enough oxygen
Enough fuel
Enough of a spark for ignition

The point here is that fuel does not cool a cylinder.
Also it is very bad thing to assume fuel is cheap and the more we poor into a cylinder the better.
Fuel is very expensive and we owe it to ourselves to understand that a very rich mixture is only to be used during take off & climb.At any other time mixture should be leaned out as much as practicable for a given engine installation.

BTW; lead in 100LL also does not cool or protect valve seats!

Last Edited by complex-pilot at 14 Oct 13:10

It is true that the evaporation of excess fuel does not cool the combustion. The latent heat of evaporation of avgas is of the order of 1% of the energy released by burning it (I recall reading a proper analysis a few years ago, but it will be out there on the internet). What happens however is that excess fuel (i.e. operating rich of peak; ROP) does reduce the average temperature of the process.

ROP operation is required in Lyco/Conti engines during climb, because they are not able to generate max rated power while burning stochiometrically (peak EGT, let alone LOP) without excessive CHTs. The thin metal sections, and air cooling, and the low airflow during climb, cannot do it. Water cooling might be able to but even car engines run ROP when you floor the accelerator.

Administrator
Shoreham EGKA, United Kingdom

The point is that during the combustion fuel does not have a cooling effect on cylinder CHT. Period.

It is by controlling ICP’s that one can control CHT’s. This is done ROP and LOP, the relative CHT can be the same in both cases!

By controlling the speed of the flamefront we can control ICP’s in our engines, thereby affecting CHT’s and oil temps.

It is as Dirk wrote, by varying the start of combustion in relation to the mechanical advantage of the piston, which is due to the piston connected to the crankshaft continuing to move, one can control ICP’s.

Everything else such as air cooling, size of cyllinder fins and baffle installation, etc. are all secondary in function.

Last Edited by complex-pilot at 14 Oct 13:44

Let’s start from the basic premise that we’re trying to avoid excessive cylinder head temperatures, since that is what causes damage.

Then consider that the oft-taught technique of leaning to peak EGT (whether by instrumentation or reference to rough running) and then enriching ‘a bit’ tends to put you on the part of the curve (rich-of-peak) which closely corresponds to the highest CHTs.

This is ok in a simple world, since below the magic POH-prescribed line of 75% power you can safely run at just about any mixture that will burn. This simple world is propagated further by an instruction to only lean when below 75% power.

Of course those of us who understand the science know that it’s ok to lean above 75% power, but that you have to be more careful about it and if you don’t understand what you’re doing (i.e. understand the temperature curves) then it’s probably best not to.

EGLM & EGTN

Graham wrote:

Then consider that the oft-taught technique of leaning to peak EGT (whether by instrumentation or reference to rough running) and then enriching ‘a bit’ tends to put you on the part of the curve (rich-of-peak) which closely corresponds to the highest CHTs.
This is ok in a simple world, since below the magic POH-prescribed line of 75% power you can safely run at just about any mixture that will burn. This simple world is propagated further by an instruction to only lean when below 75% power.

Well, yes and no. Running 50 degrees ROP at 75% (or even just 70%, or even just 65%) will put you right inside the famous red box. So that’s definitely discouraged.

The thing is that is red box is not only black and white . In practice, thousands of aircraft are run there, day in day out, at cylinder temps right around 400 degrees. And they don’t suffer too much it seems.

So, whilst 50 degrees ROP is – comparatively speaking – the worst place to be, it is still doesn’t seem to be “bad” enough, in absolute terms. At least for normally aspirated engines.

Last Edited by boscomantico at 17 Oct 10:08
Frankfurt (EDFZ, EDFE), Germany
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