Interestingly, the C172S (most recent C172, presently in production) POH states that “recommended lean mixture” is 50°F ROP while “economy mixture” is peak EGT.
Interestingly, the C172S (most recent C172, presently in production) POH states that “recommended lean mixture” is 50°F ROP while “economy mixture” is peak EGT.
The POHs have the habit of showing you how to get good power out of the aircraft, assuming that fuel does not cost much. The “economy” settings are for people that have to fly very long legs, not people that want to save money traditionally.
I never cruise richer than peak EGT. Why waste fuel?
It’s immediately obvious on speed at any altitude, and the difference between 50-75F ROP and 120F ROP is probably a 1000-2000ft difference in how high you can climb.
It would be interesting to investigate/learn if peak power is achieved with lesser spread from EGT peak on engines with smaller cylinders, fuel injection etc – things that improve uniformity of fuel distribution. The reason for the extra fuel to make peak power is to make up for imperfect distribution of fuel through the charge, making sure that all the air the engine can breathe has enough fuel to burn.
LOP means all the fuel gets burned, ROP means all the air gets burned. It seems to me that a perfect engine would do both at peak.
(I have to say that in the absence of a good EGT, mine is poor and one cylinder only, I enjoy setting mixture with rpm and electric CS prop turned off. Obviously you can find peak power/rpm mixture very easily, and then the onset of lean roughness too. The only problem is knowing where you are in between those points, if it matters to you!)
The reason for the extra fuel to make peak power is to make up for imperfect distribution of fuel through the charge, making sure that all the air the engine can breathe has enough fuel to burn.
I’ve not heard that before… I thought the ~120F ROP point is where you get the best alignment between the cylinder pressure profile and the crankshaft angle, consistent with a lower CHT which enables the extra heat to be dissipated into the air, oil and exhaust.
I thought it was partly because the fuel would evaporate, absorbing energy and increasing the pressure whilst cooling the mix?
That’s what I thought, too….
I read somewhere that the theory about the evaporation of the fuel cooling the combustion is wrong (despite appearing in all the books and all over the internet) because the latent heat of evaporation of avgas is of the order of 0.5% of the energy produced by burning avgas.
God forbid putting basic electronics and a proper cooling system on the engine.
Doing it by trial and error is so much fun!
Then we would be having debates about which oxygen sensor lasts more than 100hrs at 65% cruise 
I think electronic ignition, with a timing advance a function of the rpm, would be nice, would not require any gas sensors, and would allow 91UL in perhaps all Lyco/Conti engines, but nobody would get more MPG in cruise…
How often do you change O2 sensors on your car? Less often than a CHT plug I bet :-)
It’s not necessarily about MPG – though I’m sure an ECU can get it right way more often than we can. It’s about ease of use.
Liquid cooling would also have the added advantage of a gas-driven engine preheat as opposed to the ground-breaking idea of negative piston / cylinder clearances when it’s cold and the requirement of carrying a camping heater and assorted tubing to unfreeze your engine USSR Gaz66 ca 1967 style.
