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

I think the reason the GA scene gets away with this is because weakening the metal (by excessive temperature) doesn’t matter if the internal cylinder pressure is not much.

So the various scenarios e.g.

  • in most PPL training and most PPL flying, a full rich climb doesn’t get the CHT hot enough because one is only going to say 1500-2000ft
  • in cruise, say 60%, it doesn’t matter what you do, because the cylinder pressure is not high enough
  • most training and rental (which includes most “aeroclub” activity) the rental is “wet” so few care about fuel economy
  • shock loading cracks, via rapid power reductions, don’t happen in most PPL scenarios because most people cruise full-rich i.e. with low CHTs (verdict: “shock cooling is a myth”)

Put all this together and you end up with just a fairly small vulnerable population of engines. Most of them are the bigger engines which are used to climb higher, etc, and the owners are (nowadays) more likely to be clued-up, or at least have the instrumentation.

And most turbo engines are used for “heavy duty” flying so they get the worst of it.

It is like so much in GA. It is easy to get a situation where the various pilot populations don’t encounter something, because they are all doing something which via good fortune doesn’t expose them to it, but somebody who does something a bit different, will. It’s like social research – a real bugger to do right

Administrator
Shoreham EGKA, United Kingdom

@complex-pilot wrote:

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?

By this you must mean that all fuel in the cylinder will combust — even at full rich mixture. Or put another way, with the same throttle setting, less fuel will burn when you lean the mixture. (Note that I am talking about fuel actually burned, not fuel consumed — it is obvious that fuel consumption goes down then you lean.)

All the energy released when the fuel burns is converted into heat and/or mechanical work. The speed of combustion doesn’t matter other than affecting the proportion of heat and mechanical work.

So according you what you write less fuel will burn and thus less energy will be released when you lean the mixture. But leaning initially causes both the temperature to increase and the amount of mechanical work to increase. So something is wrong here. Could you straighten this out for me?

ESKC (Uppsala/Sundbro), Sweden

@Airborne_Again

This is about fuel not cooling a cylinder? Or CHT when ROP or LOP?

Highest CHT is about 50F EGT ROP. So if CHT is directly proportional to ICP’s, then ICP’s will also be hightest.
Now imagine 50F EGT LOP. This will result in a much cooler CHT and low ICP.
100F EGT ROP will result in a much hotter CHT than 50F EGT LOP.
Again ICP’s will move equally with CHT’s.

Normal for every spark ignition engine, no exceptions. Including your lawn mower.

Another point to consider:
When 50F EGT LOP, all fossile fuel is burned. Virtually no carbon monoxide in your exhaust.
Also other nasty exhaust emissions are as good as killed off at such an EGT LOP.
With the heater on in winter time, I feel it is a very good place to be.
When 50F EGT ROP, the exhaust is full of unburned fossile fuel (hydrocarbons), one measure known to us pilots is carbon monoxide. There is lots of it at this setting.
When 100F EGT ROP, the exhaust is full of even more unburned fossile fuel and higher volume of carbon monoxide particles.

A very hot and dirty affair.

Last Edited by complex-pilot at 27 Oct 17:18

@complex-pilot wrote:

When 100F EGT ROP, the exhaust is full of even more unburned fossile fuel and higher volume of carbon monoxide particles.

So we are in agreement that when the engine operates on the rich side of the stoichiometric optimal mixture, then there will be unburnt fuel in the exhaust. Very good.

But can you then explain what you meant when you wrote

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?

It did not make sense to me then and it still does not make sense.

If there is some unburnt fuel (which we now are in agreement that there is) then that fuel must have a cooling effect – just like water would.

ESKC (Uppsala/Sundbro), Sweden

It is not the fuel itself that cools the CHT but the fact that very rich or lean mixture burns slower than stoichiometric, causing the peak combustion pressure to peak later in the cycle, with the Piston moving further down, having less compression than at TDC.
Remember, the spark came at about 22° BTC, the slower the air/fuel mixture burns, the lesser the final ICP/CHT will be.

EGT is a quick reacting proxy for ICP, offset about 40° on the rich side. Peak CHT will come at about 40° ROP.

By the way, the only reason to have tetraethyl lead in the fuel is also to slow down the flame front, nothing else.

Last Edited by dirkdj at 27 Oct 18:16
EBKT

@Airborn_Again

I thought there was perhaps some more to this.

If there is some unburnt fuel (which we now are in agreement that there is) then that fuel must have a cooling effect – just like water would.

Fuel does not cool CHT’s. It is as Dirk wrote earlier.

So, when say 100F EGT ROP, lots of fuel swirling around in a cylinder, CHT’s are much hotter than when 50F EGT LOP.
When LOP there is excess air, not excess fuel swirling around.

Fuel does not cool cylinders, period.

Lower ICP’s cool cylinders, then as a secondary function there is an effect from cooling air, air density, OAT, etc.

If LOP v ROP, and corresponding CHT’s are all a muddle in our mind, then there are fantastic specialists availalble to cange that for us.

Dirk posted a free link to read up on, but an online course might be a better place to start from for the more initiated.

Last Edited by complex-pilot at 27 Oct 18:29

As written further back, the unburnt fuel (say 30% of total fuel, if really far ROP) does cool the engine but the effect (= the latent heat of evaporation of avgas) is negligible compared to the heat generated by burning the other 70%.

IMHO this “engine stuff” is actually a whole lot less complicated than is made out by those running the $1000 courses The issue is really that a lot of GA (less now than say 20 years ago) doesn’t put in the relatively slight effort to understand even the simple bit, and most mechanics (who a lot of owners take guidance from!) know even less.

Administrator
Shoreham EGKA, United Kingdom

Peter, the online course is free to try and if one likes to continue it is only $395.00, not as much as written earlier.

Last Edited by complex-pilot at 27 Oct 22:29

Graham wrote:

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.

The fallacy here is failure to take into account that these ancient engines actually were designed to be operated that way. “Full rich” when on full throttle, lean in cruise. Overly simplified this is also how a FADEC would operate the engine. The only difference really is the precision of how it’s done. Max power and max efficiency are very different and contradicting objectives for an engine. You cannot have both at the same time, and no matter how you play with the numbers, max power will produce most heat. The reason for this is when burning more fuel, you produce more heat.

Faster or slower flame fronts are secondary effects, and no cause of anything (for an otherwise well functional and sized engine in an aircraft) . The main effect is more power = more heat, by orders of magnitude. At max power the objective is:

  • To get as much power as possible while also being able to remove the excess heat.

At max efficiency (in cruise) the objective is:

  • To get as much work as possible for the least amount of fuel.

For the first objective, the main problem is removal of heat. How is heat removed from an engine? Roughly speaking 1/3 of the heat is converted to work (driving the propeller), 1/3 is cooled by the cooling system and 1/3 is exhausted in the exhaust pipes. To increase cooling you have to:

  1. make a bigger and better cooling system, or
  2. somehow push more heat out through the exhaust pipes, or
  3. increase efficiency (increase the 1/3 converted to work), but by doing so you also work against the main objective, which is max power. Thus, this is no solution to the main objective. It’s utterly useless to think in terms of max efficiency when the objective is max power. An after burner on a jet engine increases the power by a factor 1.5-2, but the fuel consumption is increased by a factor insane. This is no problem though, since all the excess heat is just thrown out into the atmosphere.

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)

The second objective, max efficiency, is straight forward. Here the cooling system is adequately sized for cruising conditions. If heat is a problem, then

  1. the cooling system needs to be fixed or re-designed.
  2. You are pushing the engine too hard, creating too much heat by burning too much fuel

If the second point is a problem, then you have already lost the objective of maximizing the efficiency (which is another way of saying you need a bigger engine for the cruise performance you want).

Last Edited by LeSving at 28 Oct 10:10
ENVA ENOP ENMO, Norway

Overly simplified this is also how a FADEC would operate the engine. The only difference really is the precision of how it’s done.

Exactly right.

And this is why we don’t have FADEC on these air cooled engines. It would not do very much. So nobody (who understands engines) bothers to develop it because, if they did, the first customers would find it doesn’t save them any significant fuel, so the R&D and certification cost would have been wasted.

Administrator
Shoreham EGKA, United Kingdom
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