In other words, in a piston engine, the best-power mixture is richer than that producing the EGT peak that signals stoichiometric combustion.

Why is that, physically?

Lycoming representation of EGT/CHT versus power

  • $\begingroup$ Welcome to Av.SE! $\endgroup$ – Ralph J Sep 1 '18 at 0:53
  • $\begingroup$ Is it actually true that peak EGT corresponds to a stoichiometric ratio? Bear in mind the indicated EGT depends on the speed of combustion as well as the chemical energy released, since it's sensed at a particular point in the tailpipe. $\endgroup$ – pericynthion Sep 28 at 8:10
  • $\begingroup$ Related, on space.SE: Pro/cons of burning propellant in stochiometric ratio? $\endgroup$ – Manu H Sep 28 at 10:52

It's because, although the Best Power mixture dumps more fuel than can be consumed by the available oxygen, the extra fuel increases the combustion speed in the charge, increasing the buildup up of pressure in the combustion chamber, and this increases the total potential energy in the air/fuel charge that can be converted to work, even though not all of fuel is burned. You could say that the excess fuel has kind of a catalytic effect, or maybe a turbocharging effect... sort of... up to a point.

Beyond a certain point, adding more excess fuel becomes counterproductive, so Best Power mixture is the sweet spot where the speed of combustion and pressures generated are maximized and the cylinder makes the most horsepower it can make (the highest possible combustion pressure) with the available displacement and airflow.

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    $\begingroup$ I don't know if by turbocharging effect you are referring this but: evaporation of the fuel cools down the air charge on its way to the cylinder. More fuel, cooler air charge, more air going to the cylinder. Up to a certain point (max power) this might be of significance. Might, because I don't remember and can't find any sources about this to confirm if the difference is significant between max power and stoich in gasoline engines. When comparing gasoline and ethanol engines, the difference is significant. $\endgroup$ – Jpe61 Sep 28 at 7:24
  • $\begingroup$ By difference you mean the amount of evaporative temperature drop? To the extent it does, I would think it's only a factor in carbureted engines where the droplet stream has a long way to go. $\endgroup$ – John K Sep 28 at 17:27
  • $\begingroup$ Yes. That might very well be the case. Difference in the amount of gasoline between stoich and max power is not that big after all. $\endgroup$ – Jpe61 Sep 28 at 18:00
  • $\begingroup$ I am pretty sure the rich mixture "decreases" combustion speed (increases RON), allowing spark advance and favouring any forced induction boost. Water can do the same. Takeoff power was traditionally hard on pistons, which are really only ring carriers. The heat and stress is considerably higher. A rich mixture is less "knocky" as the combustion speed slows. $\endgroup$ – mckenzm Dec 1 at 9:03

in practical terms, running rich means that despite the unevenness of the fuel-air mix both in a single cylinder firing and across all cylinders firing and across all firings in the engine, all cylinders are guaranteed to NOT be running lean on ANY firing, which means all of them will be pulling best power on EVERY firing.

In addition, there is an effect that I know about from motorcycle engines which is probably true of airplane engines and that is the "quality" of combustion as a function of richness: If you run rich, preignition and lean misfire (both of which reduce combustion efficiency) are prevented, and the delivered power is smoother. You can hear this: a motorcycle engine sounds "happier" when running rich than it does running lean- and it pulls harder, even though the mix is nonstochiometric.


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