Optimal crank angle for maximum combustion pressure

Hello HPA. You have mentioned that the optimal ignition advance is such that the combustion pressure is maximized when the cranckshaft is 16-18 degrees after TDC. Is this specification universal, or is it dependent on the the relative lengths of the piston stroke and crankshaft radius? Thanks.

Good question - a short rod-stroke ratio has higher acc'n from TDC, so the charge will be pressurising a more rapidly expanding volume and so have a potentially lower peak, but the rod angle will be appling the force at a better angle to the crankshaft pin. On the other hand, a long rod will have more dwell around TDC and so the pressure may be expected to be higher but not being as well applied to the crank' pin.

Traditionally, a long rod has been considered better for high rpm, a short rod for low rpm - however, this may in part be due to flame propogation (how fast it's burned in the cylinder) as longer dwell may allow the pressure to build more and a long rod will reduce the side load of the piston on the cylinder wall, which may reduce frictional losses.

That's all very well and good, though - every engine is going to have unique properties and variations, and the only real way to find out exactly what the engine needs is to set the timing for best torque/power and let the point of peak pressure lie where it will - which would be expected to occur in the range given.

I have found many different topics calling different crank angles optimal but all of them are within 5 to 20 degrees frame.

Was that for peak cylinder pressure, Shota? Quite a wide range if so, but different engines and rpm ranges can be expected to vary a fair bit.

Yes, it was for peak pressure. I've seen so many different numbers, but as I already said they all are within 15 degrees range ( from 5 degrees atdc to 20 degrees atdc depending on engine geometry)...