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Usually found in four- and six-cylinder configurations, the straight engine, or inline engine is an internal-combustion engine with all cylinders aligned in one row or plane, with no offset.

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 much smaller in overall physical dimensions than designs like 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, the four-cylinder models are inherently off balance and rough, unlike 90 degree V fours and horizontally opposed 'boxer' 4 cylinders.

Automobile use

The inline-four engine is by far 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, notably Acura, Audi, Mercedes-Benz, Toyota, Volkswagen and Volvo, have also used straight-5 configurations. The General Motors Atlas family includes straight-4, straight-5, and straight-6 engines.

Once, the straight-8 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 an aluminium alloy cylinder block 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-8 in 1933. However, following the Second World War, the straight-8 was supplanted by the lighter and more compact V8 engine.

Many manufacturers mount straight engines at an angle from the vertical, referring to them as slant engines. Chrysler's famous Slant 6 was used in many models in the 1960s and 1970s. Honda also often mounts their straight-4 and straight-5 engines at a slant, as on the Honda S2000 and Acura Vigor. SAAB first used an inline-4 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-6 in automotive applications. Lanchester balance shafts, an old idea reintroduced by Mitsubishi in the 1980s to overcome the natural harshness of the straight-4 engine and rapidly adopted by many other manufacturers, have made both straight-4 and V6 engines much more refined than used to be the case. The inherent smoothness of the straight-6 is no longer as great an advantage as it used to be. Secondly, at around the same time, fuel consumption became a much more important factor. Cars became smaller and much more space-efficient. The engine bay of a modern small or medium car, typically designed for a straight-4, often does not have room for a straight-6, 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.

Straight-6 engines are used in some models from the manufacturers BMW, Ford Australia, Chevrolet, GMC, Toyota, Suzuki and Volvo Cars.

Aviation, bus and rail use

Renault produced an inverted air-cooled straight-6 for airplanes, this was used on the Stampe. 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.

Engines of this type in some buses and trains have been built in a horizontal form. 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) are commonly seen in this design. Such engines may be based on a conventional upright engine with alterations to make it suitable for horizontal mounting.


See also

Piston engine configurations
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Inline types H · U · Square four · VR · Opposed · X
Stroke cycles Two-stroke cycleTemplate:· Four-stroke cycleTemplate:· Six-stroke cycle
Straight Single · 2 · 3 · 4 · 5 · 6 · 8 · 10 · 12 · 14
Flat 2 · 4 · 6 · 8 · 10 · 12 · 16
V 4 · 5 · 6 · 8 · 10 · 12 · 16 · 20 · 24
W 8 · 12 · 16 · 18
Valves Cylinder head portingTemplate:· CorlissTemplate:· SlideTemplate:· ManifoldTemplate:· MultiTemplate:· PistonTemplate:· PoppetTemplate:· SleeveTemplate:· Rotary valveTemplate:· Variable valve timingTemplate:· Camless
Mechanisms CamTemplate:· Connecting rodTemplate:· CrankTemplate:· Crank substituteTemplate:· CrankshaftTemplate:· Scotch YokeTemplate:· SwashplateTemplate:· Rhombic drive
Linkages EvansTemplate:· Peaucellier–LipkinTemplate:· Sector straight-lineTemplate:· Watt's (parallel)
Other HemiTemplate:· RecuperatorTemplate:· Turbo-compounding

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