The piston-type engine used in cars has cylinders (where the combustion occurs), pistons that slide up and down inside the cylinders, connecting rods that turn the crankshaft, and a head (metal cover) where the spark plugs create the ignition. Two additional openings allow for the intake of the gasoline mixture and for the discharge of exhaust. Some vehicles use diesel engines that were invented by Rudolf Diesel in 1892. The diesel relies on the heat produced by compression for its power instead of ignition from the spark plugs. This type of engine is generally used in heavier vehicles that require large amounts of power such as trains, ships, and trucks. Both types of engines are lubricated by oil to reduce friction and overheating.