Straight-6

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 litres. 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 was the Cummins B series turbodiesel at 5.9 L, available in Dodge Ram trucks since 1989.

Straight-six engines were historically more common than V6's, mainly because the length of such engines was not such a concern in rear-drive vehicles but also because V6's (unlike the 90-degree 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.

Many manufacturers build cars equipped with straight six engines. Manufacturers BMW and Volvo both produce multiple models with straight sixes and several automakers have isolated straight-six offering's, such as Suzuki's Verona and Ford's Falcon. Most of these cars are rear wheel drive, but both Volvo and Suzuki build cars equipped with front wheel drive and a transverse straight six. Although Mercedes used to build many straight sixes, it has recently given them up and now only engineers V6 engines. BMW, on the other hand, is one of the few remaining manufacturers to persist with the I-6 configuration, making petrol and turbo-diesel engines ranging from 2.0 to 3.2 litres in displacement (as of 2005). Toyota also uses straight sixes in some of their sportier cars and large displacement I-6 engines in their Prado off-road vehicles.

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.

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 liter option in 1987.

In 2001 General Motors introduced a new family of straight engines, the Atlas, for use in the newly-introduced Chevrolet TrailBlazer/GMC Envoy. The I-6 was chosen for development because of the desirable operating characteristics of its self balanced design. This engine is also to be used in the new Saab 9-7. In 1959 Saab had an experimental car with two transverse straight-3 engines bolted together—the Saab Monster.

British straight-6 engines

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 until, 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. TVR also uses a straight 6 called Speed Six in some of their coupes and convertibles such as the Tuscan.

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 I-6 engine is what makes it desirable for industrial use. In addition, an I-6 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