[curtis73]

The energy comes from two places; the combustion of other cylinders, and the energy stored in the flywheel.

The explosion of fuel pushes the cylinder down. Inertia carries it through the other three cycles. The weight of the flywheel, the crankshaft, and other rotating parts keep things going. In a multi-cylinder engine, there is always another cylinder on the combustion stroke pushing another cylinder on the other strokes.

Its a bit wasteful, but it works. The combustion "throws" the weight around enough to get it back to another compression stroke

The battery basically only runs the starter motor. After the engine is running, the only electricity used by the engine is the ignition system to spark combustion which is more than covered by the alternator.