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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).
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. They have a support bearing between each piston as compared to "flat and V" engines which have support bearings between every two pistons. Although six-cylinder engines are inherently balanced,[vague] the four-cylinder models are inherently off balance and rough, unlike 90-degree V fours and horizontally opposed 'boxer' four cylinders.
The inline-four engine is the most common four-cylinder configuration, whereas the straight-6 has largely given way to the V6 engine, which although not as naturally smooth-running is smaller in both length and height and easier to fit into the engine bay of smaller modern cars. Some manufacturers, including Acura, Audi, Mercedes-Benz, Toyota, Volkswagen and Volvo, have also used straight-five configurations. The General Motors Atlas family includes straight-four, straight-five, and straight-six engines. Some small cars have inline three engines.
Once, the straight-eight was the prestige engine arrangement; it could be made more cheaply than a V-engine by luxury car makers, who would focus on other specifics than the geometric ones, and even built engines more powerful than any V8 engine. In the 1930s, Duesenberg used a cylinder block made from aluminium alloy, with four valves per cylinder and hemispherical heads to produce the most powerful engine on the market. It was thus a selling point for Pontiac to introduce the cheapest straight-eight in 1933. However, following World War II, the straight-eight was supplanted by the lighter and more compact V8 engine, which allowed shorter engine bays to be used in the design.
When a straight engine is mounted at an angle from the vertical it is called a slant engine.  Chrysler's Slant 6 was used in many models in the 1960s and 1970s. Honda also often mounts its straight-four and straight-five engines at a slant, as on the Honda S2000 and Acura Vigor. SAAB initially used the Triumph Slant-4 engine tilted at 45 degrees for the Saab 99, but later versions of the engine were less tilted.
Two main factors have led to the recent decline of the straight-six in automotive applications. First, Lanchester balance shafts, an old idea reintroduced by Mitsubishi in the 1980s to overcome the natural imbalance of the inline-four engine and rapidly adopted by many other manufacturers, have made both inline-four and V6 engines smoother-running; the greater smoothness of the straight-six layout is no longer as great an advantage. Second, fuel consumption became more important, as cars became smaller and more space-efficient. The engine bay of a modern small or medium car, typically designed for an inline-four, often does not have room for a straight-six, but can fit a V6 with only minor modifications.
Some manufacturers (originally Lancia, and more recently Volkswagen with the VR6 engine) have attempted to combine advantages of the straight and V configurations by producing a narrow-angle V; this is more compact than either configuration, but is less smooth (without balancing) than either.
Bus and rail use
Some buses and trains with straight engines have their engines mounted with the row of cylinders horizontal. This differs from a flat engine because it is essentially an inline engine laid on its side. Underfloor engines for buses and diesel multiple units (DMUs) commonly use this design. Such engines may be based on a conventional upright engine with alterations to make it suitable for horizontal mounting.
Some straight engines, in the strict sense, have been produced for aircraft. Renault produced a straight-four inverted air-cooled motor, which was used on the Stampe SV.4. A similar design was the de Havilland Gipsy series of engines, used on the Tiger Moth and other aircraft. Advantages include improved visibility for the pilot in single-engined craft, and lower center of gravity.
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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.
- Today's Technician: Automotive Engine Performance. Douglas Vidler. Cengage Learning, 1 Jul 2003
- Johnson, E. R. (2011-04-20). "Glossary". United States Naval Aviation, 1919-1941: Aircraft, Airships and Ships Between the Wars (illustrated ed.). McFarland. p. 326. ISBN 978-0-78644-550-9. "INLINE ENGINE–A type of reciprocating piston engine in which an even (4-6-8-12) number of cylinders are arranged either in a straight line or in a V-type configuration directly above (or below) the crankcase. Most early inline aircraft engines were water-cooled via a radiator system, though air-cooled types began to appear during the 1930s."
- Wilson, Hugo (1995). "Glossary". The Encyclopedia of the Motorcycle (in UK English). London: Dorling Kindersley. pp. 309 – 310. ISBN 0-7513-0206-6. "in-line Engine layout in which the cylinders are arranged in a row, and in-line with the wheels of the machine."
- Hunt, Phil; McKay, Malcolm; Wilson, Hugo; Robinson, James (2012), Duckworth, Mick, ed., Motorcycle: The Definitive Visual History, DK Publishing, Penguin Group, pp. 126, 210, ISBN 9781465400888
- Tuttle, Mark, Jr. (December 2005), "BMW F800S", Rider (General OneFile. Web. 29 June 2012. ): 15