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The crankshaft converts the linear motion of the piston into rotation of the crankshaft.
It turns the stroke movement of the piston(s) into a rotation.
In the case of a single cylinder engine it would require a LARGE flywheel, so that rotary momentum is maintained. In the case of a multi-cylinder engine there are multiple firings of individual cylinders in 360 degrees of crankshaft rotation. Each cylinder as it fires helps the crankshaft maintain rotary momentum, thus a lighter flywheel is required.
Crankshaft
Gudgeon Pin/Piston Pin
Both con-rod length and shape of the crankshaft affect piston stroke
Before Top Dead Center (BTDC) is a term used to describe the position of a piston inside an internal combustion engine during its upward stroke. It is a value, measured in degrees, that represents the amount of crankshaft rotation prior to the apex of this stroke.
In essence, the the displacement of the piston is porportional to the sine of the phase of the crankshaft. this isn't quite true but it gives a rough idea of what is happening. if you meant what happens in one cycle of a piston/crankshaft then it is essentially this: as the piston moves down it forces the crankshaft through one half of a revolution and then on the way back up the piston forces the crankshaft through the remaining half of a revolution - back to its start position to start again
The sound is created by the 45 degree angle of the pistons on a single crankpin. Both pistons rise and fall in the cylinders at nearly the same time (45 degrees of crankshaft rotation apart). Most V engines have separate crankpins for each piston.
crankshaft
The con rods
The piston includes the piston, rings, rod, rod bearing, and crankshaft bearing.