Think acceleration rate...
Since acceleration itself is a rate , specifically the rate at which velocity changes versus time, I assume by this you are referring to the rate at which acceleration changes versus time.
Well I started thinking about this the other day when posting on the other thread ... ..........
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......My fingers are now tired.
For years I've had a lot of similar stuff rattling around in my poor brain. I've always assumed that the engineers somewhere must have written the necessary formulas to relate the expansion rate of the burning gas to piston position and speed, to cylinder pressure, to crank angle, etc, etc. Got to be a big Kray somewhere capable of running simulations, right?