Straight-four engine

A straight-four engine (also referred to as an inline-four engine) is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.

Most automotive four-cylinder engines use a straight-four layout, and the term "four-cylinder engine" is often used synonymously with straight-four engines. However, less popular configurations also exist, including the flat-four and V4, while inclined variants of the inline layout are sometimes referred to as slant-four engines. The layout is also used in motorcycles and other machinery.

In the United States, beginning in 2005, four-cylinder engines became increasingly prevalent as part of a broader industry trend toward smaller, turbocharged engines to meet fuel economy and emissions requirements. As of the 2025 model year, they were used in about 60% of new vehicles.