The crank in question is the crankshaft, a concentric cast metal shaft running through the bottom of your internal combustion engine.
As it spins, the pistons which are clamped to it rise and fall within their cylinders. On the up strokes they compress a fuel/air mixture in the combustion chamber which explodes and drives the piston down to spin the crank which spins the cogs in your gearbox which, in turn, spins the propeller.
The big difference between petrol and diesel power is the means of igniting the compressed air/fuel mixture in the combustion chamber.
Petrol engines take a small zap of electricity from the battery, enhance it in the coil
