None of the intake, compression, power, or exhaust strokes have anything to do with gravity or atmospheric pressure.
Initially, a secondary motor causes the rotating components to rotate, which causes the pistons to move in the cylinder. Later, either inertia or the power stroke from another cylinder maintains the rotation.
The piston movement itself is caused by the piston arm, which is offset from the axis of the crank shaft.
Initially, a secondary motor causes the rotating components to rotate, which causes the pistons to move in the cylinder. Later, either inertia or the power stroke from another cylinder maintains the rotation.
The piston movement itself is caused by the piston arm, which is offset from the axis of the crank shaft.