A car spark plug firing. Spark plugs are used in petrol engines to ignite a compressed fuel/air mixture in the combustion cylinders. This provides the force required to propel the car. A supply of electricity, normally at 6 or 12 volts, is fed via a distributor to an induction coil. There the voltage is stepped up to about 18,000V, enough to create a spark in the dense gas in the cylinder. At normal engine speeds, a spark plug may fire between 300 and 3,000 times each minute.