A straight-12 engine or inline-12 engine is a twelve-cylinder internal combustion engine with all twelve cylinders mounted in a straight line along the crankcase. A straight-12 is a very long engine, and therefore straight-12 engines are not common in automobiles, but it has been used for large military trucks. Also some Russian firms built straight-12s for use in boats in the 1960s and 1970s.
The Wärtsilä-Sulzer RTA96-C, the largest reciprocating engine in the world As of 2006, is a straight-12. It is used on container ships.