The straight engine (also called inline engine) is a configuration of multi-cylinder

piston engine A reciprocating engine, more often known as a piston engine, is a heat engine that uses one or more Reciprocating motion, reciprocating pistons to convert high temperature and high pressure into a Circular motion, rotating motion. This article ...

where all of the cylinders are arranged in a single row, rather than radially or in two or more cylinder banks.

Design

A straight engine is easier to build than an equivalent

flat engine A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, ...

V engine A V engine, sometimes called a Vee engine, is a common configuration for internal combustion engines. It consists of two cylinder banks—usually with the same number of cylinders in each bank—connected to a common crankshaft. These cylinder ...

, as it uses one cylinder head rather than two. Inline engines are also narrower than flat engines or V engines; however, they are longer and can be taller. The

engine balance Engine balance refers to how the inertial forces produced by moving parts in an internal combustion engine or steam engine are neutralised with counterweights and Balance shaft#Overview, balance shafts, to prevent unpleasant and potentially dam ...

characteristics of a straight engine depend on the number of cylinders and the firing interval.

Slant engines and horizontally-mounted engines

When a straight engine is mounted at an angle from the vertical it is called a ''slant engine''. Notable slant engines include the 1959-2000 Chrysler Slant-6 engine, 1961-63 Pontiac Trophy 4 engine and the 1968-1981 Triumph Slant-4 engine. Some buses and diesel multiple unit trains take this concept further by mounting the engines horizontally (i.e. with a slant angle of 90 degrees). This is used to reduce the height of the engine, so that it can be located under the floor of the train or bus.

Number of cylinders

* I-twin (also known as "I2") * I3 * I4 * I5 * I6 * I7 * I8 * I9 * I12 * I14

Usage in automobiles

The straight-three and straight-four configurations are the most common layouts for three- and four-cylinder engines respectively. Straight-five engines are occasionally used, most recently by Audi and Volvo. Straight-six engines were common prior to the 1990s, however most six-cylinder engines now use a V6 layout. Similarly, straight-eight engines were popular in the 1920s to 1940s, however they were replaced by the more compact V8 layout.

Aviation use

Many straight engines have been produced for aircraft, particularly from the early years of aviation and through the interwar period leading up to the Second World War. Straight engines were simpler and had low frontal area, reducing drag, and provided better cockpit visibility. Straight sixes were especially popular in the First World War, and most German and Italian and some British aircraft used descendants of Daimler's pre-war inline six. Prominent examples include the German Mercedes D.III and BMW IIIa, Italian Isotta Fraschini V.4 and British Siddeley Puma. The British de Havilland Gipsy family of engines and their descendants included straight-four and straight-six upright and inverted air-cooled engines which were used on a wide range of smaller aircraft around the world, including on the Tiger Moth biplane, and helped made the configuration popular for light aircraft. Menasco and Fairchild-Ranger in the United States,

Renault Renault S.A., commonly referred to as Groupe Renault ( , , , also known as the Renault Group in English), is a French Multinational corporation, multinational Automotive industry, automobile manufacturer established in 1899. The company curr ...

in France, Walter in Czechoslovakia, and Hirth in Germany all built a similar range of engines which were popular in their respective markets. The aviation use of term " inline engine" is used more broadly than for straight engines, since it also applies to other configurations where the cylinders are located in rows (e.g.

W engine A W engine is a type of piston engine where three or four cylinder banks share the same crankshaft, resembling the letter "W" when viewed from the front. W engines with three banks of cylinders are also called "broad arrow" engines, due to thei ...

H engine An H engine is a piston engine comprising two separate flat engine, flat engines (complete with separate crankshafts), most often geared to a common output shaft. The name "H engine" is due to the engine blocks resembling a letter "H" when viewe ...

s and horizontally opposed engines).

Inverted engines

Some straight aircraft engines have used an ''inverted engine'' configuration, whereby the crankshaft is at the top of the engine and the pistons hang downwards from it. Advantages of the inverted arrangement include a raised thrust line for improved clearance for the propeller, which either allows for the use of a larger, more efficient propeller, or for shorter undercarriage. Since the thrust line is higher, the engine can be mounted lower in the airframe, improving visibility forward, which is no longer blocked by the cylinder heads. It also allows for a simpler exhaust to keep gasses clear from the cockpit.

Motorcycle use

In motorcycling, the term "in-line" is sometimes used narrowly, for a straight engine mounted in line with the frame. straight, and inline as equivalent, and use them interchangeably.

References

{{Piston engine configurations Straight