[Berkut]

Yep, rich mixtures burn more slowly than lean ones*, and that's why we use a very rich mixture on takeoff.

The ideal point for pressure to peak inside the cylinder is about 15 degrees after top dead center. That point is where we have the best mechanical advantage between the connecting rod and the crankshaft. The problem with an aircraft engine is keeping the point of peak pressure (PPP) happening at 15 degrees after TDC, because it's affected by several different things:

1. High manifold pressure tends to advance the PPP, moving it closer to top dead center.

2. Low RPM tends to advance the PPP, moving it closer to TDC.

3. Rich mixtures tend to retard the PPP, moving it farther from TDC.*

4. Advancing or retarding the ignition timing also advances or retards the PPP. Since most aircraft engines have magnetos with fixed timing, we can't really do anything about this one.

When you are taking off, you are operating at high power, high manifold pressure, and low airflow (less cooling). Higher power requires higher mean pressure inside the cylinder. High MP is pushing the PPP closer to TDC, which isn't good. To counter this, your carburetor or fuel injection system will enrichen the mixture at full throttle, slowing down the combustion and retarding the PPP so that it moves back to around 15 degrees after TDC where it belongs. The extra fuel is there for timing purposes, to retard the PPP. This is also cooler, which is nice since we have less airflow at takeoff speeds and the higher mean pressure drives up CHTs.

So why doesn't your car have to go through all this crap? Because your car has variable ignition timing, and it knows how to use it. Your car also strives to maintain that ~15 degree after TDC PPP, but it has tools that the aircraft engine doesn't. Ideally, your car could sense pressure inside the cylinder, see where it peaks, and adjust the ignition timing as required to advance or retard the PPP. But it can't do that, or at least none that I know of can. What it can do is look at all the factors that affect PPP. It knows the manifold pressure from the MAP sensor. It knows the RPM and crank position from the crank angle sensor. The fuel injection maintains an ideal mixture using the mass airflow and oxygen sensors. The computer can use all of that information to calculate the PPP and retard or advance the ignition timing accordingly.

Want to know where the old "don't operate oversquare" myth has its roots? Consider the effect on the PPP when you have low RPM and a high manifold pressure.

*There's more to these statements, but that's another post.