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An Alfa Romeo 2600 engine.

The straight-6 (also inline 6, I-6, or I6) is an internal combustion engine with six cylinders aligned in a single row. The name slant-6 is sometimes used when the cylinders are at an angle from the vertical.

Straight-6 engines have perfect primary and secondary balance and require no balance shaft.

Usually a straight-6 was used for engine displacements between about 2.5 and 4.0 L. Sometimes this configuration is used to make smaller engines which tend to be powerful and very smooth running, but also rather expensive to manufacture and physically longer than alternative layouts. The smallest production straight-6 was found in the 1927 Alfa Romeo 1500, displacing just 1.5 L. The largest are used in tractor-trailer combinations and some low speed diesels in cargo and passenger ships. These engines have a displacement of 1,000 L or more.

Straight-six engines were historically more common than V6s, mainly because the length of such engines was not such a concern in rear wheel drive vehicles but also because V6s (unlike the crossplane V8) were somewhat difficult to make smooth-running. The widespread use of front-wheel-drive and transverse ("east-west") engine configurations in smaller cars saw that the shorter engine length of the V6 became highly desirable, and these days most six-cylinder engines are made in the V configuration.

Straight 6 engines in Europe

Many manufacturers build cars equipped with straight six engines. Manufacturers BMW and Volvo both produce multiple models with straight sixes. BMWs are rear wheel drive, but Volvo builds cars equipped with front wheel drive and a transverse straight six.

Although Mercedes-Benz used to build many straight sixes, it has recently abandoned the layout and now only engineers V6 engines (they have retained the former for medium duty diesel applications such as the MBE 906).

BMW, on the other hand, is one of the few remaining manufacturers to persist with the I6 configuration, making petrol and turbo-diesel engines ranging from 2.0 to 3.2 L in displacement (as of 2005). It has also updated its 3.0 I-6 engine for the 2006 model year line-up for the BMW 6-series and BMW 7-series vehicles.

Opel has also used a straight-6 engine in the 1970s until the early 90s, ranging between 2.5 and 4.0 L. They powered Opel's top of the line models, including the Monza, the Omega and the Commodore.

In 1959, Saab had an experimental car with two transverse straight-3 engines bolted together—the Saab Monster.

Straight-6 engines in Britain

The straight 6 was the archetypal British engine for sports and luxury cars for many years. Rolls-Royce used straight-6 engines until changes in their design make the shorter V8 layout more suitable.

Jaguar used them, from 1949 until the mid 1990s in form of the legendary twin cam Jaguar XK6 engine, until, reputedly at Ford's insistence, they adopted a V8. Aston Martin used a straight 6 for many years as well.

Bristol produced a straight 6 until 1961, based on BMW plans, that was also used in many small manufacturers' cars.

The compact Triumph straight six powered their high-end saloon and sports cars from the mid 1950s to the mid 1970s.

British sports car company TVR has designed its own straight six, known as the Speed Six, and now uses it exclusively in all of their models.

Straight 6 engines in the United States

Engines of this type were popular before World War II in mid-range cars. Most manufacturers started building straight 6 engines when cars grew too large for the straight-4, although Ford went straight to the V8.

After the war, larger cars required larger engines, and the straight-6 became the base engine model used on economy cars only. The vast majority of American cars during this period had V8s.

The Chrysler Corporation had noteworthy slant six engines, used in the Plymouth Valiant and Dodge Dart models of the 1960s and 1970s.

When cars began to get smaller again in the 1970s, the trend was towards the greater compactness enabled by the V6 layout, and straight 6 engines became rare in American cars except for trucks and vans. Jeeps were an exception to the rule, getting the AMC Straight 6 engine as the base engine option in 1972, and getting a high-performance 4.0 L option in 1987. Usage of the AMC 4.0 has been declining in Jeep vehicles since the 2002 replacement of the Jeep Cherokee with the Liberty, which features the Chrysler 3.7 L V6 instead. It has declined further since the 2005 introduction of the third generation Jeep Grand Cherokee, which also uses the 3.7 L V6. It is currently only available in the Jeep Wrangler.

In 2001 General Motors introduced a new family of straight engines, the Atlas, for use in the newly-introduced Chevrolet TrailBlazer/GMC Envoy. The I6 was chosen for development because of the desirable operating characteristics of its self balanced design.

As far as passenger vehicles are concerned, inline six engines might be making a comeback in some larger vehicle types such as trucks and SUVs. An example is the 5.9 L Cummins Turbo Diesel engine used in Dodge Rams.

Ford still uses a straight six engine, a 24-valve 4.0 L plant in atmospheric and supercharged variants, in its Australia-only product, the Ford Falcon.

Straight-6 engines in Asia

The Japanese have used the straight 6 with great success since the 1960s in a wide range of applications, from passenger vehicles, to sports cars, to SUV's. Both Datsun and Toyota were among the first in this trend, though Prince Motors (later acquired by Datsun) and others offered straight sixes in that time too.

Toyota started with their M-series engine and later the F, FZ, G, and JZ engines, and Datsun started with their H-series and later the L and RB engines. In 1990's Toyota offered representatives of all 5 families in their vehicles: the G in the Altezza (and others); the M and its replacement, the JZ, in the Toyota Supra (and others); and the F and its replacement, the FZ, in the Land Cruiser. In the 2000's, Toyota's still offers the FZ-series, G-series and the JZ-series engines.

In Korea, GM Daewoo's Magnus (sold abroad as the Chevrolet Evanda, Chevrolet Epica or Suzuki Verona) comes with a Daewoo-designed straight 6.

Diesel straight-6 engines

The inline 6 in diesel form with a much larger displacement is commonly used for various industrial applications. These range from various types of heavy equipment to power generation. As with everyday passenger vehicles, the smooth running characteristics of the I6 engine is what makes it desirable for industrial use. In addition, an I6 engine is mechanically simpler than a V6 or V8. It has only one cylinder head and half as many camshafts as a V engine.

See also

Straight engine

Piston engine configurations
Straight Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14
V 2, 4, 5, 6, 8, 10, 12, 16, 20, 24
Flat 2, 4, 6, 8, 10, 12, 16, H
W 8, 9, 12, 16, 18
Other inline H, VR, Opposed, U (Square), X
Other Hemi, Radial, Rotary, Pistonless, Deltic, (Wankel)

Heat engines
Stroke cycles
Engine types
Gas turbinePistonJetRocket engineSteam engineStirling engineTschudiTwingle
Cylinder head portingD slideFour-strokeManifoldMultiPistonPoppetSleeve
Piston layouts
Single cylinderStraightOpposedFlatVWHDelticRadialRocket engine nozzleRotaryStelzerControlled CombustionBourke
Motion mechanisms
CamConnecting rodCoomber rotaryCrankCrank substituteCrankshaftLinkages (EvansPeaucellier-LipkinSector straight-lineWatt) • Double acting/differential cylinder
Thermodynamic cycle