An O-320 172 running LOP at 6.5 gph (25Lph) for 65% power :
Apparently, the powerflow exhaust helps (the guy is paid by them so no idea).
Peter wrote:
Best power is somewhere around 80F ROP.The reason these air cooled engines cannot do high power at/around peak EGT is because they cannot get rid of the heat then generated, so one has to run them quite rich to get the pressure pulse profile more favourable.
Yes, the air/fuel mixture acts on both intake and exhaust temperatures, in theory best power is achieved at peak of (Exhaust Temp – Intake Temp), in practice, this is achieved at max EGT*(1 – ratio air/fuel of specific heat per mol) which is roughly at peak EGT*95%
You have to go very rich to induce noticeable misfire, whereas misfire arrives abruptly when you go too lean and at that point power falls off a cliff.
Carb heat is intended to delay the onset of lean misfire to an even leaner mixture setting, which aids in making sure that no fuel in even the richest part of the charge is left unburnt. It is useful to imagine how differing effects of more and less fuel result from stoichiometric being mostly air with a little fuel vapor.
Best power is somewhere around 80F ROP.
The reason these air cooled engines cannot do high power at/around peak EGT is because they cannot get rid of the heat then generated, so one has to run them quite rich to get the pressure pulse profile more favourable.
FADEC can’t change this much. It could change it a bit because it could vary the ignition timing.
We have a nice old thread here.
Having a carburettor doesn’t change the way the engine burns the stuff, but it does make it harder to achieve even power from all cylinders i.e. acceptable vibration levels.
Slightly richer than stoichiometric mixture provides maximum power. This accounts for imperfect mixture distribution by ensuring that all the air has fuel nearby to burn.
lionel wrote:
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.
The engine would give maximum power with the stoichiometrically optimal mixture, would it not? Both richer and leaner would give less power. A surplus of oxygen is as bad as a surplus of fuel.
By using partial carb heat, the fuel vaporizes better, this may make the difference between running smoothly or not on non-injected engines when LOP. Same with a slightly closed throttle, it improves fuel/air mixing.
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.
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 :)
So in principle a FADEC system could do full power with LOP mixture rather than full rich?
