In automotive engineering, a longitudinal engine is an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicle, front to back. An engine similarly mounted along the frame of a motorcycle is called an in-line engine.
- Pickerill, Ken (Jun 26, 2009). "Glossary". In Main, Larry. Automotive Engine Performance. Today's Technician (5th ed.). Clifton Park, NY USA: Cengage Learning. p. 464. ISBN 978-1-43544-520-8. "Longitudinal engine mounting An engine mounted lengthways in the chassis."
- Duffy, James E.; Scharff, Robert (Mar 1, 2003) . "Chapter 2: Vehicle Construction Technology". In Clark, Sandy. Auto Body Repair Technology (4th ed.). Clifton Park, NY USA: Cengage Learning. pp. 25–26. ISBN 0-7668-6272-0. "A longitudinal engine mounts the crankshaft centerline front to rear when viewed from the top."
- Wilson, Hugo (1995). "Glossary". The Encyclopedia of the Motorcycle (in UK English). London: Dorling Kindersley. pp. 309–310. ISBN 0-7513-0206-6. "in-line Engine layout in which the cylinders are arranged in a row, and in-line with the wheels of the machine."
- Henshaw, Peter (Jun 15, 2008). "Super Bantam". The BSA Bantam Bible: All Models 1948 to 1971. Veloce Publishing. p. 42. ISBN 978-1-84584-159-1. "...including industrial engines and the Sunbeam S7, that civilised but low-powered shaft-drive in-line twin that BSA hoped would fill a niche as a gentleman's machine..."