Gasoline powered engines rely on spark plugs to ignite the air/fuel mixture in the combustion chamber during the power stroke. Most spark plugs fire with 30,000 volts of electricity. This amount of voltage is achieved from a transformer system often called a coil or coil pack. Voltage comes into the coil(s) from the 12.6v battery and is then stepped up to the appropriate voltage required to fire the spark plugs.
While the engine is running, the voltage in the battery is replaced by the alternator. An alternator is a belt-driven device that is mounted on the front of the engine and produces a charge for the battery.