Petrol engine. Computer artwork of an internal combustion engine cylinder. The piston (lower right) is moved up and down by the combustion (burning) of fuel. At the beginning of the cycle, the intake valve (left) opens, allowing fuel into the chamber. The piston moves down, drawing in air and fuel vapour. As the piston returns, this is compressed and ignited by the spark plug (upper left). The resulting explosion drives the piston back down for another cycle. The exhaust valve (upper right) opens to vent waste gases as the piston returns again. Diesel engines use compression, rather than a spark plug, to trigger combustion.