You know what, Dave, Brian?


Thinking about this more and while knowing that both facts are true; a lean mixture ignites more spontaneously than a rich mixture and also burns more quickly, it doesn't really matter...


What does matter, for maximum power, is that the timing of maximum cylinder pressure occurs just a split milli-second after the piston has surpassed TDC.

What we have to control the timing of this event, in a given spark ignition engine, with a given C/R, running at a given RPM and temperature, under a given load, is the spark advance (the mixture strength is nearly a constant, due to emission laws).

It really doesn't matter if this maximum pressure is achieved just after TDC, by utilizing 10 degrees of ignition advance, or 50.
Just that the maximum cylinder pressure comes on time, every time.

In our glow engines, with all the other 'givens' from the above paragraph and with a given glow plug and fuel (you cannot change them while the engine is running, can you?), the only control we have over the timing of this maximum pressure, is the mixture strength, which controls the propagation of the fuel burn.

So, it doesn't really matter, like with spark ignition, when the glow ignition actually occurred.
Just that the maximum cylinder pressure comes on time, every time.