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 a plane with all the pistons driving a common crankshaft. Where it is inclined, it is called a slant-four. In a specification chart or when an abbreviation is used, an engine is listed either as I4 or L4. The inline-four layout is in primary balance and confers a degree of mechanical simplicity which makes it popular for economy cars. However, despite its simplicity, it suffers from an imbalance which causes minor vibrations in smaller engines. These vibrations become more powerful as engine size and power increase, the inline-four is the most common engine configuration in modern cars, while the V6 engine is the second most popular. 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, however, there are some notable exceptions. Early vehicles also tended to have engines with larger displacements to develop horsepower, the Model A Ford was built with a 3.3 L inline-four engine. Inline-four diesel engines, which are lower revving than gasoline engines, Mitsubishi still employs a 3.0 L inline-four diesel. Generally, European and Asian manufacturers of trucks with a vehicle weight rating between 7.5 and 18 tonnes use inline four-cylinder diesel engines with displacements around 5 L. The MAN D0834 engine is a 4.6 L inline-4 with 220 hp and 627 lb·ft, the Isuzu Forward is a medium-duty truck which is available with a 5.2 L inline-four engine that delivers 210 hp and 470 lb·ft. The Hino Ranger is a medium-duty truck which is available with a 5.1 L inline-four engine that delivers 175 hp and 465 lb·ft, the earlier Hino Ranger even had a 5.3 L inline-four engine. The Kubota M135X is a tractor with a 6.1 L inline-four and this turbo-diesel engine has a bore of 118 mm and a relatively long stroke of 140 mm. One of the strongest Powerboat-4-cylinders is the Volvo Penta D4-300 turbodiesel and this is a 3.7 L-inline-4 with 300 hp and 516 lb·ft. Brunswick Marine built a 127 kW3.7 L 4-cylinder gasoline engine for their Mercruiser Inboard/outboard line, the block was formed from one half of a Ford 460 cubic inch V8 engine. This engine was produced in the 1970s and 1980s, One of the largest inline-four engines is the MAN B&W 4K90 marine engine
Ford inline-four engine with cylinder head removed
A cutaway Renault-Nissan M9R 2.0 L Straight-4 DOHC Common rail diesel engine
Computer generated image showing the major internal moving parts of an inline-four engine with belt-driven double overhead camshafts and 4 valves per cylinder.