Straight-four engine

The inline-four engine or straight-four engine is a type of inline internal combustion four-cylinder engine with all four cylinders mounted in a straight line, or plane along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft. Where it is inclined, it is sometimes called a slant-four. In a specification chart or when an abbreviation is used, an inline-four engine is listed either as I4 or L4 (for longitudinal, to avoid confusion between the digit 1 and the letter I).

The inline-four layout is in perfect primary balance and confers a degree of mechanical simplicity which makes it popular for economy cars. However, despite its simplicity, it suffers from a secondary imbalance which causes minor vibrations in smaller engines. These vibrations become more powerful as engine size and power increase, so the more powerful engines used in larger cars generally are more complex designs with more than four cylinders.

Today almost all manufacturers of four-cylinder engines for automobiles produce the inline-four layout, with Subaru and Porsche 718flat-four engines being notable exceptions, and so four-cylinder is usually synonymous with and a more widely used term than inline-four. The inline-four is the most common engine configuration in modern cars, while the V6 engine is the second most popular. In the late 2000s (decade), due to stringent government regulations mandating reduced vehicle emissions and increased fuel efficiency, the proportion of new vehicles sold in the U.S. with four-cylinder engines (largely of the inline-four type) rose from 30 percent to 47 percent between 2005 and 2008, particularly in mid-size vehicles where a decreasing number of buyers have chosen the V6 performance option.

This inline engine configuration is the most common in cars with a displacement up to 3.0 L. In practice, the displacement of inline-four petrol engines in cars rarely exceeds this figure. For example, the largest engine of this form on the U.S. market in model year 2015 is the Toyota 2TR-FE, which is available in the Toyota Tacoma. However, there are some notable exceptions. Porsche used a 3.0 L four in its 944 S2 and 968 sports cars, the Chevrolet Colorado came with a 2.9 L inline four, the International Harvester Scout was available with a 3.2 L inline four from 1965 until 1980 and Rolls-Royce produced several inline-four engines of 2,838 cc with basic cylinder dimensions of 3.5 in (89 mm) diameter and 4.5 in (110 mm) stroke (Rolls-Royce B40). Early vehicles also tended to have engines with larger displacements to develop horsepower and torque. The Model A Ford was built with a 3.3 L inline-four engine.

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