A reciprocating internal combustion engine in which the power necessary to produce motion of the mechanism is obtained by igniting a compressed fuel and air mixture within the cylinder or cylinders by extreme heat resulting from compression. the engine consists of a cylinder block assembly complete with cylinder head(s) and oil pan(s). it may include attachments or accessories, such as starter motor, transmission for auxiliary equipment, clutch, oil filter, flywheel, flywheel housing or battery charging generator. a fuel tank is not included. for a complete operating power unit mounted on a base, see power unit (1), diesel.