A magnetic, whole affect, optical or magnetic oh resistive sensor usually mounted internally in the engine to inform that ECM or PCM of piston position on the intake stroke for timing synchronization of sequential fuel injection.
The bearing areas of a crankshaft are the main rod journals.
The second stroke of the four stroke engine cycle, in which the piston moves from the bottom dead center in the intake valve is closed, trapping and compressing the air/fuel mixture in the cylinder.
And ignition system that uses a distributor, electronically controlled or not.