Illustration of the internal workings of a petrol engine, an internal combustion engine. At the beginning of the cycle, the intake valve (upper right) opens, allowing fuel into the cylinder (centre). The piston moves down, drawing in air and fuel vapour. As the piston returns, this is compressed and ignited by the spark plug (upper centre). The resulting explosion drives the piston down, which turns the crankshaft that powers the car. The exhaust valve (upper left) opens to vent waste gases as the piston returns again. The flow of fuel and exhaust gases is controlled by spring-mounted valves (rods at upper left and right).