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Flying LOP in a carbureted Lycoming

If you’re flying through air which is visibly free of contamination (not a sand storm), no harm will come to your engine flying with any amount of carb heat. Once carb heat is applied, assure that the engine is re leaned, which you’re obviously doing, so that’s fine. Be aware that in certain very uncommon cold air operation, the use of partial carb heat can actually induce carb ice, by melting ice which would otherwise not have accumulated in the venturi, so having a carb air temperature gauge is wise. And yes, I have seem fuel economy improvement resulting from the use of partial carb heat in cruise flight, most obviously effective in a carburetted Continental 550 which I used to fly. I’m confident it works in other engine models, though would require observant experimentation. Different RMP settings will also affect fuel distribution between cylinders.

Bear in mind that the carburetted versions of these engines were designed more than a half century ago, when fuel economy was hardly a consideration. And, at the time in history when these engines were designed and approved, there was very limited instrumentation, we have the benefit of incredible instrumentation now, which makes precise operation much easier, while still assuring that limitations are not exceeded. Thus there are certainly efficiencies to be found which were never intended, nor described in manufacturer’s operating instructions. You are still bound by engine and airplane limitations, but within those, there is room for experimentation for fuel economy.

Home runway, in central Ontario, Canada

Pilot_DAR wrote:

Be aware that in certain very uncommon cold air operation, the use of partial carb heat can actually induce carb ice, by melting ice which would otherwise not have accumulated in the venturi, so having a carb air temperature gauge is wise.

As I don’t have a carb air temp. gauge what are those temperatures at which that can occur?

Last Edited by Neal at 02 Sep 13:10
LSPG, LSZC, Switzerland

Neal wrote:

As I don’t have a carb air temp. gauge what are those temperatures at which that can occur?

I don’t have an exact number, but I would be cautious when air temperature is between 0C and -5C. Moisture in air at -5C (and colder) will be frozen, and could be expected to pass through the venturi without melting on the way, after that, it does not matter for the engine to run. I estimate that between -5C and 0C the moisture, which could have been frozen, and passed through harmlessly, could be thawed by the partial carb heat, and instantly refreeze as carb ice in the venturi. Again, the numbers I have presented are my guess, I’m not even sure there are authoritative values, it’s the concept I want to illuminate. This it the reason behind flight manual warnings to not apply partial carb heat without a carb air temperature gauge. It’s okay, if you are able to observe carb air temperature, as you can then purposefully prevent a venturi temperature in the freezing range. This is a low risk, but high criticality problem, awareness, and caution are appropriate. If you’re operating partial carb heat, and suspect carb ice, go to full carb heat, and re lean the engine for peak EGT until you believe that the ice has been cleared.

Home runway, in central Ontario, Canada

Rwy20 wrote:

“when full rich, the engine is cooled by the excess fuel”

So why do we run the engine on full rich mixture at max power?

ESKC (Uppsala/Sundbro), Sweden

Full rich mixture retards the flamefront propagation, bringing the peak ICP later in the downgoing cycle of the cylinder, reducing pressure and temps.
Same with tetraethyl lead, it doesn’t lubricate but it slows down the combustion event.

EBKT

dirkdj wrote:

Full rich mixture retards the flamefront propagation, bringing the peak ICP later in the downgoing cycle of the cylinder, reducing pressure and temps.

Makes sense. That happens with LOP mixtures, as well, doesn’t it?

ESKC (Uppsala/Sundbro), Sweden

correct, that is the reason LOP gives lower temps and better efficiency due to more mechanical advantage.

Last Edited by dirkdj at 02 Sep 16:16
EBKT

So in principle a FADEC system could do full power with LOP mixture rather than full rich?

ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

So in principle a FADEC system could do full power with LOP mixture rather than full rich?

I don’t think it can do full power LOP, because the limiting factor for full power is how much air (oxygen) is pumped into the engine. LOP, you mix/inject less fuel than the oxygen can burn, so you end up with less power.

But it can do full power on less rich than full rich :)

ELLX

I’ve read before that using carb heat allows leaner mixture settings without misfire. I’ve never tried it, but will do so on my O-320. I will admit to a suspicion that the effect of heated inlet air is to richen the mixture by reducing air mass flow, and thereby stop the lean misfire. If that were true, when you open the throttle or increase rpm slightly to restore mass flow and power to its previous level, the fuel flow goes where it would be if you had left the mixture just on the rich side of misfire with cold air.

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