A longitudinal engine is a type of internal combustion engine that uses a crankshaft to convert the linear motion of the pistons into rotational motion. Unlike a transverse engine, where the pistons move horizontally, a longitudinal engine has the pistons move vertically, parallel to the crankshaft. This design allows for a more compact engine layout, making it ideal for smaller vehicles such as cars and motorcycles. Longitudinal engines are commonly used in front-engine, rear-wheel drive configurations, and are known for their smooth operation and efficient power delivery.