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

LeSving wrote:

The only thing that can be done is to increase fuel flow to more than what is needed for combustion. […] the excess fuel is, for all intents and purposes, cooling down the engine […]
This is what I’ve been trying to say, too, but you did it better!

ESKC (Uppsala/Sundbro), Sweden

@LeSving

The first solution is expensive, heavy and bulky, and isn’t needed 95-99% of the time, so what about the second one. What can be done? Not much in fact. The only thing that can be done is to increase fuel flow to more than what is needed for combustion. That excess fuel is nevertheless converted to heat. Heat that is exhausted through the pipes. There is no other way this will work. You have to remove excess heat somehow. This is the exact same thing that is happening in an afterburner on a jet engine, only not so extreme. You sacrifice a large amount of efficiency to achieve more power, and the excess heat is thrown out with the exhaust. This means the excess fuel is, for all intents and purposes, cooling down the engine. There is no other correct way to look at this. In other words; you achieve more power because the decrease in efficiency is OK since the excess heat is mostly just being exhausted through the exhaust pipes, and does not heat up the engine itself. (Keep in mind, there is no need for the exhaust temperature to increase, it’s purely a function of the amount of heat going out the exhaust. Heat capacity of unburned fuel is orders of magnitude higher than gas)

There are a few points that are incorrect.
1) A spark ignition piston engine is nothing like a jet engine, neither is the combustion process anything like it
2) Excess fuel does not cool cylinder CHT. It would perhaps if the cylinder would be filled full of it, but a spark ignition engine will not ignite a flame with a mixture richer richer than 1 part of fuel to 7 parts of air.
That is not that much fuel really, however put more in and the mixture will not ignite, now the fuel is cooling a cylinder, but the engine is now longer doing any work.
3) ICP’s are not affected by outside cooling air. Control ICP’s and you control CHT. Cooling air has a secondary function cooling CHT’s.
For example one can operate an air cooled spark ignition engine at -20c and one will observe very low CHT’s. Low OAT, too much cooling, can mask ICP’s that are way too high, the engine is perhaps operating in the RED Box.
4) The aircraft we use have been designed with 3 knobs, all are on the panel, hopefully not behind it.
All 3 knobs where meant to be used, it is simply that some operators do not know enough to make best use of it.
LOP is the fastest safe SOP for an Avgas fuel injected engine, by staying outside the RED Box.
When operating LOP, up to 20 parts of air to every part of fuel is used to keep the fire going. CHT’s and ICP’s are now at the lowest possible before the engine stalls. Fuel is not cooling CHT’s. It is low ICP’s that give low CHT’s.

However, carburetted engines are so limitted due to very poor mixture distribution that proper LOP SOP is not achievable.
These engines can perhaps just achieve PEAK EGT and then performance falls off a cliff.
That does not reverse the proven concept of controlling ICP’s and therby controlling CHT’s though.
A badly set up engine is simply too limiting to experience it properly.

Last Edited by complex-pilot at 28 Oct 14:31

Another way to slow the flame front and reduce peak cylinder pressure is to partially close the throttle, as you do on a car or motorcycle engine for most of its service life. Interestingly those engines massively advance the spark timing when operating at partial throttle. Fifty degrees of spark timing advance (versus for example 25 degrees at full throttle) brings the peak cylinder pressure back to the appropriate potion of the power stroke, as a opposed to a fixed timing timing engine like an aircraft engine which because of its high power service cannot often benefit much from advancing the ignition timing, except at high altitude. In that case electronic ignition which advances spark timing at low manifold pressure (with full throttle) can be used to increase efficiency, combatting the slower flame front.

@Silvaire
Sure, a closed thottle can reduce ICP’s, too.
Thinking about a closed throttle though, generally it will also create pumping losses, as the piston is moving against a much closed throttle, creating negative pressure, or a partial vacuum during one of the 4 strokes of the piston required for power output from the engine.
Ring flutter can be induced that way, too. But nothing wrong with a slightly reduced throttle setting when required, of course.

So a slightly reduced throttle opening and thus reduced air pressure into the cylinder can do a lot to reduce CHT’s and ICP’s during the initial climb out phase when still full rich. (Not so when LOP!)
This is most noticable with an engine that still produces full rated power and is not flying on reduced power already.

OTH, not so good is it potentially for the engine to have only the RPM reduced during climb out, with a view to reduce ICP’s.
Because reduced RPM, will increase ICP’s and CHT unless other factors are changed, too.
So reducing only RPM after take off is not a good procedure, especially considering secondary cooling is not good either during the climb out phase due to angle of attack and low airflow due to low airspeed. A potentially very bad situation!
But we all have to do it, to be careful with noise sensitive neighbours. Best is it perhaps to do it the right way, which I have been lucky enough to be shown.

However, during cruise a fully open throttle and lots of ram air when far enough LOP provide a much more efficient SOP, because pumping losses are much removed, ICP’s are low, thus CHT’s are low and operation is outside the RED box and thereby the aircraft is flying very fast and with cool CHT’s, no matter what the OAT or condition of engine baffling.

Spark advance now works a little different in a certified spark ignition aero piston engine, when compared to say a car engine.
A certified engine installation in an aircraft has required always a minimum of at least one mag to fire at a fixed timing, whereas only the second mag can be advanced.
For what it is worth, I have not yet seen an installation where there is much gained in efficiency or speed with this method, especially when compared to gains by LOP and WOT (wide open throttle) for cruise operation, without having to spend much on new kit.
I know there are other systems being designed for our engines that potentially promise huge gains, but those will have to be certified first, which is a big hurdle to climb for the manufacturers.

Last Edited by complex-pilot at 28 Oct 16:33

This (leaner mixture burns slower so needs an earlier spark) is one of the two reasons why lower RPM gives you slightly better MPG (the other is lower mechanical losses) – so long as you can get enough power to maintain slight at a reasonable low-drag AoA.

At say FL100, I get a few % more MPG at 2200rpm over 2400rpm.

Administrator
Shoreham EGKA, United Kingdom

@Peter

Sure, but low RPM settings with high MP (WOT) should be only done LOP to avoid heavy wear on your long stroke Lycoming engine.

Yes, sure. I always fly peak EGT or slightly LOP.

Administrator
Shoreham EGKA, United Kingdom

@Peter

I have learned that peak EGT is only the starting point of efficiency, and between peak EGT and about 25F EGT LOP there are other undesirable exhaust gases produced.

Also ICP’s at peak EGT are not changed a lot from say ICP’s at 25F ROP EGT.

A NA piston engine flying at very high altitude produces very little max power and ICP’s are therefore lower anyway.
However, this is when engine cooling is not so good, due to the the thin air and CHT’s need to be watched carefully and basically one is limited by CHT’s now.

The latest data I have seen is for a NA engine to keep all CHT’s during cruise below 360F, and for a turbo engine installation below 380F.

Last Edited by complex-pilot at 28 Oct 17:01

Yes. There was a thread on why CHTs go high at high altitude / high RPM. One has to use a high RPM to get enough power.

Administrator
Shoreham EGKA, United Kingdom

complex-pilot wrote:

1) A spark ignition piston engine is nothing like a jet engine, neither is the combustion process anything like it

They are both thermal machines. The laws of thermodynamics does not work differently depending on the machine. They need fuel and air to work, to produce heat and pressure and cooling is needed to prevent melting. Only the cycle is different. They all burn liquid fuel.

complex-pilot wrote:

2) Excess fuel does not cool cylinder CHT. It would perhaps if the cylinder would be filled full of it, but a spark ignition engine will not ignite a flame with a mixture richer richer than 1 part of fuel to 7 parts of air.
That is not that much fuel really, however put more in and the mixture will not ignite, now the fuel is cooling a cylinder, but the engine is now longer doing any work.

Well, what I said was that using a richer mixture enables you to produce more power at the cost of reduced efficiency. Now, reducing the efficiency would normally mean increase in production of unused heat (laws of thermodynamics). There simple is no way you physically can reduce efficiency and produce more power without also producing more heat. Much more heat than a simple linear relation with the increase in power. The reason it works nonetheless is that the excess fuel carries the heat away through the exhaust without heating the engine itself + some evaporation effect. This is the exact same thing that happens in an afterburner, thermodynamically speaking at least.

The net effect is, as you say, slower burning and a longer maintained pressure. But the reason it is possible is the excess fuel. It is highly inefficient due to:

  1. lots of unused fuel
  2. lots of unused heat and pressure is exhausted with the unused fuel (you are far away from an ideal process, both in terms of thermodynamics, cycle and combustion (chemistry))

Simply put, excess fuel cools the engine at max power.

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