Straight-6
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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. It was also sometimes used for smaller engines but these, although very smooth running, tended to be rather expensive to manufacture and they where inevitably physically longer than alternative layouts. The smallest production straight-6 was found in the Benelli 750 Sei motorcycle, displacing 747.7 cc (0.75 L / 45.6 cu in). The largest are used to power ships and have displacements of 1,000 L or more.
Straight-6 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 difficult to run smoothly. 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.
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[edit] Straight-6 engines in Europe
Volvo produced straight-6 engines like the Volvo B30 engine for the Volvo 164 (1969-1975) and the B6304 engine for the 960/S90 (RWD) models from 1992-1998. In the following model year, the same engine was further refined as the B6294 (stock/turbo) for transverse mounting and VVT in the front-wheel drive Volvo S80. For 2007 a new inline-6 was introduced. It is referred to as the "Short Inline-6" engine since it is only 3 mm longer than the inline-5 Volvo engines. The Short I6 engine's remarkable compactness is achieved by moving the timing and ancillary drives to the rear end of the engine and moving the crankshaft torsional vibration damper inside the crankcase.
Mercedes-Benz built straight-6 engines as gas and diesel, but now prefers the V6 layout for its line of gasoline engines, while reserving the straight-6 for medium duty diesel applications, such as the MBE 906. (See List of MB-engines)
BMW, on the other-hand, is one of the few remaining manufacturers to persist with the straight-6 configuration, making petrol and turbo-diesel engines ranging from 2.0 to 3.2 L (122–195 cu in) in displacement (as of 2005). In 2006, they announced that the upcoming 335i model of their E92 3-series coupé will have a 3.0 L twin-turbo straight-6, showing the company's continued dedication to this configuration.
Opel has also used a straight-6 engine in the 1970s until the early 1990s, ranging between 2.5 and 4.0 L (153–242 cu in). 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.
Alfa Romeo has used one of smallest straight-6 engine (1500 cc) in Alfa Romeo 1500 model (1927). Also RL model used straight-6 and Alfa Romeo 2600 (1961–1969).
[edit] 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 made the shorter V8 layout more suitable.
Jaguar used them, from 1949 until the mid 1990s in the form of the legendary twin-camshaft Jaguar XK6 engine, until, at Ford's insistence, they adopted a V8. Aston Martin used a straight-6 for many years, as did Austin-Healey in their Austin-Healey 3000. MG also used a straight-6 in their MGC.
Bristol produced a straight-6 until 1961, based on a BMW design, that was also used by many small automakers.
The compact Triumph straight-6 powered their high-end saloon and sports cars from the mid-1950s to the mid-1970s. It was available in 2.0L and 2.5L capacities. Triumph claimed that their TR5 model was the first car in the UK to come with fuel injection as standard; the TR5 has a 2.5L Triumph straight-6. Other Triumph vehicles to use the Triumph straight-6 are:-
- GT6 1966-73
- Vitesse 1962–71
- The 2000 Range 1963-77
- TR6 1969-76
British sports car company TVR designed its own straight-6, known as the Speed Six, which powers its current range of cars.
Land Rover used a 2.6 L (~159 cu in) straight-6 from 1967 in certain series Land Rover models.
[edit] 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.
After WWII, larger cars required larger engines, and buyers of larger cars tended to prefer V8s; performance sixes such as the Hudson Hornet engine were exceptions to the rule, and were not often top sellers. After Chevrolet introduced its V8 in 1955, the straight-6 became almost exclusively a base engine model pitched to economy-minded customers. Trucks (both light and heavy duty) also incorporated the straight-6 until the mid-1950s where passenger car V8s were optioned. The new wave of compact cars that started in the late 1950s provided a suitable home for straight-6 designs.
The Chrysler Corporation had noteworthy slant-6 engines, used in the Plymouth Valiant and Dodge Dart A-body models of the 1960s and 1970s.
Another significant straight-6 engine family was introduced by American Motors (AMC) in 1964. These engines were used in a variety of AMC passenger and Jeep utility vehicles. They were also assembled and marketed internationally. Some markets (such as Mexico - by VAM) built their own specialized versions. This engine is considered to be one of the best ever made and it received modifications and upgrades as engine control technology improved. It is noteworthy that this "modern era" I-6 was produced continuously for 42 years (even after Chrysler's buyout of AMC in 1987) all the way through 2006.
Ford and General Motors straight-6s of the 1960s and 1970s were generally nondescript, except for the overhead cam Pontiac six of the late '60s. Although the Pontiac six was one of the few straight-6s of its era to be advertised as exceeding 200 horsepower, it wooed few performance buyers away from V8s in the muscle car era and was eventually discontinued in favor of a less costly design.
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. The decline of the straight-6 was in response to lighter weight of the V6 layout to which the compactness of the V-design led to the straight-6's demise as a passenger car motor.
Jeeps were an exception to the rule, aside from AMC's 2.5L, Jeep CJ's began offering AMC's 232 and 258, both known as "High Torque," straight-6's as a common engine option in 1972. These engines continued to receive upgrades with an advanced for its time, high-performance 4.0 L (242 cu in) 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. The last application of the 4.0 was in the 2006 Jeep Wrangler; for 2007 the engine has been replaced with a 3.8 L V6.
Ford used a straight-6 in baseline Mustangs and in its other models for many decades. They were also found in F150 pickups (most notably the 300 cu in 4.9L inline six) until 1997 when they were replaced with a V6.
In 2001 General Motors introduced a new family of straight engines, the Atlas, for use in the newly-introduced Chevrolet TrailBlazer/GMC Envoy. The straight-6 was chosen for development because of the desirable operating characteristics of its self balanced design.
As far as passenger vehicles are concerned, straight-6 engines might be making a comeback.
[edit] 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 Nissan and Toyota were among the first in this trend.
Toyota started with their M-series engine and later the F, FZ, G, and JZ engines, and Nissan started with their H-series and later the L as well as the RB series engines (in the R31-R34 Skyline). Honda built the Honda CBX 1000 motorcycle from 1978 till 1981. 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.
[edit] Straight-6 engines in Australia
Historically, all three manufacturers in Australia used straight-6s. Chrysler had built the Slant 6 in Australia and the unique to Australia Hemi straight-6. These engines were used in the Chrysler Valiant and the Valiant Charger producing up to 320hp. Chrysler no longer owns any factories in Australia.
Holden built 161, 186 and 202 cu in straight-6s from 1968-1984. They were used in the Kingswood, Torana and in the early Commodores. Modern Commodores use V6s.
Ford Australia has been producing straight-6s since 1960 and is the only manufacturer in Australia to still build them. Ford has built 144, 200, 240 and 250 cu in engines, with the 240 being called the 3.9L or 4.0L and the 200 being called the 3.3L. They have been used since 1960 in the Falcon, 1970-1982 in the Cortina and from 2004 in the Ford Territory. The current straight-6 engines in the Falcon and Territory are called the Barra.
The high-performance division of Ford Australia, Ford Performance Vehicles, produce vehicles equipped with the 4.0 litre DOHC 24-valve turbocharged straight-6 with variable cam timing, which produces 270 kW (362 hp) @ 5250 rpm and 550Nm (406 ft·lbf) @ 2000 - 4250 rpm — the highest level of torque in any Australian production car to date (along with the HSV E Series).
[edit] Diesel straight-6 engines
The straight-6 in diesel form with a much larger displacement is commonly used for industrial applications. These include various types of heavy equipment, power generation, as well as transit buses or coaches. Virtually every medium-duty to large over-the-road truck employs an inline six diesel engine. Its virtues are superior low-end torque, very long service life, smooth operation and dependability. On-highway vehicle operators look for straight-6 diesels, which are smooth-operating and quiet. Off-highway applications such as tractors, marine engines, and electric generators need a motor that is rugged and powerful.
As with everyday passenger vehicles, the smooth running characteristics of the straight-6 engine are what make it desirable for industrial use. In addition, a straight-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.
Notable versions include the 5.9 liter I-6 Cummins found in the Dodge Ram and the 3.2 liter straight-6 used in the Mercedes-Benz E320 CDI sold in America from 2004 through 2006.
[edit] See also
| 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 |
| W | 8, 9, 12, 16, 18 |
| Other inline | H, VR, Opposed, U (Square), X |
| Other | Radial, Rotary, Pistonless (Wankel) |
