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.
The majority of automotive four-cylinder engines use a straight-four layout[1]: pp. 13–16 (with the exceptions of the flat-four engines produced by Subaru and Porsche)[2] and the layout is also very common in motorcycles and other machinery. Therefore the term "four-cylinder engine" is usually synonymous with straight-four engines. When a straight-four engine is installed at an inclined angle (instead of with the cylinders oriented vertically), it is sometimes called a slant-four.
Between 2005 and 2008, the proportion of new vehicles sold in the United States with four-cylinder engines rose from 30% to 47%.[3][4] By the 2020 model year, the share for light-duty vehicles had risen to 59%.[5]
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