High-compression, internal-combustion engine used extensively for powering drilling rigs. In a diesel engine, air is drawn into the cylinders and compressed to very high pressures; ignition occurs as fuel is injected into the compressed and heated air. Combustion takes place within the cylinder above the piston, and expansion of the combustion products imparts power to the piston. Diesel automatically ignites and burns when it is compressed to a very high pressure. The released energy is contained by the engine and powers the vehicle. The key difference between diesel and petrol engines is auto-ignition. A spark plug ignites the fuel in a petrol engine whereas a diesel engine auto-ignites. Diesel has a Cetane Index and Number rather than the Research Octane Number (RON) that petrol has.