straight engine

The straight or inline engine is an internal combustion engine with all cylinders aligned in one row and having no offset. Usually found in four, six and eight cylinder configurations, they have been used in automobiles, locomotives and aircraft, although the term in-line has a broader meaning when applied to aircraft engines, see Inline engine (aviation).

Design

A straight engine is considerably easier to build than an otherwise equivalent horizontally opposed or V engine, because both the cylinder bank and crankshaft can be milled from a single metal casting, and it requires fewer cylinder heads and camshafts. In-line engines are also smaller in overall physical dimensions than designs such as the radial, and can be mounted in any direction. Straight configurations are simpler than their V-shaped counterparts. Although six-cylinder engines are inherently balanced, the four-cylinder models are inherently off balance and rough, unlike 90-degree V fours and horizontally opposed 'boxer' four cylinders.

Slant engines and horizontally-mounted engines

When a straight engine is mounted at an angle from the vertical it is called a ''slant engine''.Today's Technician: Automotive Engine Performance. Douglas Vidler. Cengage Learning, 1 Jul 2003 Notable slant engines include the 1959-2000 Chrysler Slant-6 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.

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

In aviation, the term "inline engine" is used more broadly, for any non- radial reciprocating engine, including V engines, W engines, H engines and horizontally opposed engines.

Many straight engines, in the stricter sense, 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 in France, Walter in Czechoslovakia, and Hirth in Germany all built a similar range of engines which were popular in their respective markets.

Some straight aircraft engines have been inverted, with the crankshaft at the top of the engine, and the pistons hanging down rather than the reverse. 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. A two-cylinder straight engine mounted across the frame is sometimes called a parallel twin. Other times, motorcycling experts treat the terms parallel, straight, and inline as equivalent, and use them interchangeably.

References

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Straight