What type of engine has cylinders in a single row?

An engine with cylinders in a single row is an inline engine.

Inline engines, also called straight engines, place every cylinder along one bank beside the crankshaft. This layout usually needs only one cylinder head, making it simpler to manufacture and service than many V or flat designs. The trade-off is that an inline engine can be longer or taller than an engine with multiple cylinder banks.

Inline-four engines became especially common in passenger cars because they are compact, economical, and relatively inexpensive. Inline-six engines are known for naturally smooth operation, while straight-eight engines were used in some luxury cars during the early twentieth century before shorter V8 designs became more practical.

A common mix-up is confusing cylinder arrangement with engine mounting direction. An inline engine may be mounted longitudinally or transversely, and it may even be tilted in a slant configuration. “Straight engine” and “inline engine” are the usual automotive terms for the same basic single-row layout.