The perfect engine?

In this occasional series on interesting engine variants, we’ve covered the glamour engines – the virtuoso V12s, that Bruce Springsteen of powerplants that is the V8, and the Teutonic knight known as the flat six. But let’s put emotion aside and put on our most coldly boring and analytical engineer’s hat. Is any of these actually the perfect automotive powerplant? Er, no. Not even close I’m afraid. So what is?

Well, to start with, it would have to be a four. Sorry. Why so? Well, a four-cylinder motor is a commonsense compromise: enough cylinders to avoid the basic NVH (noise, vibration and harshness) issues of a single or twin, but still mechanically simple (read ‘cheap’) to manufacture compared to a six or an eight.

But let’s dig further. A four-cylinder engine of around about 2-litres means that each cylinder’s swept volume is around 500cc – a slightly short-measure Imperial pint, if you will. Now, 500ccs happens to be a Goldilocks cylinder capacity. With a reasonable stroke, so we don’t have to have any wacky conrods or crankshaft geometries, the diameter of the piston is small enough that the flame-front from a single spark plug can propagate fast enough to reach the cylinder walls and thus burn fully during the power stroke.