Straight-6
The straight-6 or inline 6 (I-6) is an internal combustion engine with six cylinders aligned in one vertical 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 balancing shafts.
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.
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.
The front wheel drive Suzuki Verona and rear wheel drive Ford Falcon remain some of the very few straight sixes still in use in contemporary passenger vehicles, along with some Mercedes and BMW rear wheel drive luxury cars. Mercedes has recently given up the staight six, 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 excellent petrol and turbo-diesel engines ranging from 2,0 to 3,0 litres in displacement (as of 2004). Toyota also uses 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. 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 an I-6 Vortec engine. 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 larger cars required a V8, and Jaguar used them until relatively recently. 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 in some of their coupes and convertables 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.
See also: straight engine
