Internal combustion engines are used to produce mechanical power from the chemical energy contained in hydrocarbon fuels. There are two commonly used internal combustion engine cycles: the two-stroke cycle and the four-stroke cycle. Both cycles are based on the preceding description of the essential power-producing sequence: compression of the unburned mixture, its combustion, and then expansion as burned gases. There are two basic types of internal combustion engine: the spark-ignition engine (the Otto-cycle engine), and the compression-ignition engine, which is often called the diesel. In the spark-ignition engine, the fuel–air mixture is essentially premixed prior to combustion, which is initiated with a spark discharge. In the diesel, air alone is inducted and compressed; fuel is injected into the cylinder just before combustion commences. In one of the simplest and most frequently used types of two-stroke engine designs, the bottom surface of the piston in conjunction with that portion of the crankcase beneath each cylinder is used as the scavenging pump.