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Flying LOP in a carbureted Lycoming

I dare to go on one of the most controversially discussed topics in the GA scene, but since I own a PA28 Archer II with a Lycoming O-360 and recently had a longer international flight, where I could try various engine settings but without Engine Monitor (only EGT display) and have already read some articles by Mike Bush about leaning like this I wonder how you see this. But first of all my procedure:

1.) Start and then set 1000 RPM and lean Mixture immediately until the engine is about to starve. This has the advantage that the spark plugs get less deposits, less AVGAS is burned and besides that it will be determined at the runup or at the latest at the T/O also in spite of forgetting the line-up check that full rich has to be set, because otherwise the engine only coughs and doesn’t even reach 1500 RPM.
2.) T/O and cruise climb full rich and don’t exceed the max. RPM of 2650, but I think I never got above 2500 even at heights just above sea level.
3) At 6000 ft at the latest, start with leaning to counteract the oxygen deficiency, even more if the take-off airfield is at these altitudes it is also recommended to perform the runup at 2000 RPM, then to lean until max. RPM is reached, and then to leave the red lever there for T/O. I prefer to climb with something from Vy to Cruise climb speed rather than anything closer to Vx.
4.) In the cruise the power setting is 55-60% (according to settings on the Sun Shade in the PA28) and then lean until the engine gets rough. Then I’m already very LOP with the leanest of the four cylinders but the other three are still running somewhere between LOP and maybe almost ROP, which I don’t know without engine monitor. But what I do know is that at below 60% power setting I can’t do anything wrong and can’t get the CHT above 400° and the EGT above 1400° and see that when the red lever is pushed back the EGT display is peaked and then the engine starts to run rough and the EGT drops off a bit. Where exactly the critical temperature limit in the power setting of the O-360 is can’t be determined in the AFM, but on most engines it’s somewhere between 60-75% where it can’t be exceeded. I’m a bit on the lean side of peak EGT from this moment on and more LOP doesn’t work because of the uneven distribution of the amount of the fuel/air spray from the carburetor to the respective cylinders, because the leanest cylinder doesn’t want to work anymore and the engine runs much too rough and vibrates. And now comes the trick.
5.) When the engine is running just rough, move the carburettor heating slowly forwards on and observe the RPM. I’ll peak somewhere in the position of the carburettor heating lever (with me this is approx. 3/4 on) and this is the position, which you can note down, in order to be able to fly with your carburettor engine LOP and then the engine runs smoothly again.

You can adapt the power setting again because the RPM has risen and try again if you can do even more RPM with the carburetor preheating. But generally this is a setting that always stays the same and whose lever position you can mark, no matter which OAT and regardless of altitude and DA.

Switching on the carburetor preheating brings warmer air into the combustion chamber and therefore a significant loss of power, which you can compensate with the mixer/throttle in the cruise, because then full power is not needed. The effect, which the carburettor heating brings on the engine, comes about by the fact that with not fully switched on or off position a valve stands diagonally in the incoming air to the carburettor, which produces additional turbulence and thus the incoming air/fuel mixture is distributed more homogeneously over the 4 cylinders.

Unfortunately, there is a disadvantage to this: By partially switching on the carburetor preheating, a part of the air goes directly from the engine compartment without air filter into the combustion chamber. But if I don’t fly over a coal-fired power station at 1000 ft. AGL is that a problem? That’s the point I can’t judge.

LSPG, LSZC, Switzerland

No problem :-) Even coal fired power station in your region would not emit particles that would affect your Lycoming. The engine will run a little hotter, and obviously you also loose some power with carb heat.
But there is one thing you have to watch out for if running with partial carb heat, and that is raising the temperature from “to cold for carb ice” into “possible icing”. The best (only?) way to judge that is with a carburettor thermometer.

Partial carb heat is an old trick from the time of the big radial engines, and I think it worked on the previous O-540 engine on my Piper Dakota. The present engine runs fine LOP without it (as far as I can judge with only a single channel EGT).

Carb engines are individually quite different when it comes to mixture distribution among cylinders. One PA-28 O-360 Lycoming I knew could hardly be leaned at all on cruise power, without starting to run rough. After engine overhaul it was much better.

The leaning instruction in most carb engine manuals (and airplane POH’s) is to lean until the engines run a little rough, and then enrich a little so it runs smooth again. This instruction is obviously based on not having proper engine instrumentation. Following it will lead to a wide variety of mixture settings (and fuel consumptions!) because of the individual differences in mixture distribution in engines. In fact thousands of pilots and students may be running their engine LOP without ever having heard of it, because their engine does not run rough until well into LOP. And there’s no harm in that, because all pressures and temp’s are lower LOP than peak EGT or best power (not sure about exhaust valve temperatures though).

Why do you want to run LOP? There is very little fuel economy benefit compared to peak EGT. The engine will run a little cooler, but that is somewhat offset by the use of carb heat. The only real advantage of LOP that I see is when it can keep the engine out of the “red box”, i.e. when you can combine high power with a low sfc, without TIT or CHT going high, as when climbing or cruising at high power with a turbocharged engine.

Last Edited by huv at 02 Sep 08:04
huv
EKRK, Denmark

Neal wrote:

By partially switching on the carburetor preheating, a part of the air goes directly from the engine compartment without air filter into the combustion chamber

So, why wouldn’t the manufacturer introduce some kind of device that just disrupts the airflow and improves the mixing?

Noe wrote:

device that just disrupts the airflow and improves the mixing

They did. It’s called fuel injection. Doing it on a carb’ed engine would probably reduce the engine’s max power.

huv
EKRK, Denmark

Neal wrote:

The effect, which the carburettor heating brings on the engine, comes about by the fact that with not fully switched on or off position a valve stands diagonally in the incoming air to the carburettor, which produces additional turbulence and thus the incoming air/fuel mixture is distributed more homogeneously over the 4 cylinders.

I doubt that benefit can be measured, the fuel/air mix is highly turbulent already with or without carburetor heat action (mainly due to relative airflow speeds to the small venturi intakes size but some is also attributed to temperature differentials), so the carburetor valve mechanical orientation may not matter that much apart from controlling intake temperature, but I guess that temperature gradient does have a positive impact on the level of air/fuel mixing?

Better have small size injectors

Last Edited by Ibra at 02 Sep 08:49
ESSEX, United Kingdom

Ibra wrote:

I doubt that benefit can be measured, t

I believe Neal is right about the turbulence. At least his is the common explanation of how partial carb heat “works” on the engine.

Last Edited by huv at 02 Sep 09:01
huv
EKRK, Denmark

The other way is to pull the throttle just back from WOT the tiniest amount so that the throttle plate is not completely perpendicular to the airflow and it introduces a bit of disruption into the airflow.

Gains are likely to be marginal.

All you can really do with a carb is lean it until it is rough and then enrich again to make it smooth. Depending on how well balanced it happens to be this may not be very much at all, or it may be well past peak EGT, or more likely somewhere in the middle. Instrumentation helps a bit (in that you at least know if you’re getting past peak EGT or not) but if the engine won’t get to peak before running roughly then this won’t suddenly improve just because you get instrumentation.

On the 0-360 I fly it will go past peak EGT before getting rough. I tend to go with peak as long as I’m high enough that I’m not developing enough power to have to worry about the red box. The TB10 needs every knot it can get.

EGLM & EGTN

Neal wrote:

3) At 6000 ft at the latest, start with leaning to counteract the oxygen deficiency

As you have an EGT gauge, I would suggest to set the red needle on the current EGT at around 1000’ AAL, then lean during the climb to keep it there. If the engine is properly adjusted for take-off mixture, then doing this gives you a good indication of a safe place to lean to during climb.

I have tried exactly what you describe on our club Archer II around two weeks ago on a longer flight at FL85. Leaned past peak EGT until roughness occurred, with the throttle not yet fully open, then tried to smooth out again by playing with the carb heat. Didn’t change a thing on that plane. Then my friend on the right seat, also an instructor, became nervous and I set the engine back to 75°F ROP as per club procedure…

In the HPA course I recently completed, there is a chapter on reciprocating piston engines, and it regurgitates all the old wives tales about how to operate an engine. E.g. “over-leaning is bad”, “don’t over-square”, “when full rich, the engine is cooled by the excess fuel” and I don’t remember the numerous other nonsense. There was even a test question about “If you weaken the mixture, CHT will a) increase, b) decrease, c) something something and d) some other implausible choice”. Obviously, a) was the right answer. It was really painful to read, but explains the current state of knowledge about engine operation by most flight instructors in Europe.

huv wrote:

I believe Neal is right about the turbulence. At least his is the common explanation of how partial carb heat “works” on the engine.

I was referring if the explanation is “thermal” or “aerodynamic”…

ESSEX, United Kingdom

Ibra wrote:

I doubt that benefit can be measured

It is quite obvious as having leaned so much the engine runs rough and then applying partial carb heat the RPM goes up and the roughness stops.Rwy20 wrote:

As you have an EGT gauge, I would suggest to set the red needle on the current EGT at around 1000’ AAL, then lean during the climb to keep it there. If the engine is properly adjusted for take-off mixture, then doing this gives you a good indication of a safe place to lean to during climb.

Thanks for that good idea!

Rwy20 wrote:

Didn’t change a thing on that plane.

I know it doesn’t work on all engines even if it’s the same O-360.

LSPG, LSZC, Switzerland
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