The straight-6 or inline-6 engine (often abbreviated I6 or L6) is a six cylinder internal combustion engine with all six cylinders mounted in a straight line along the crankcase. The single bank of cylinders may be oriented in either a vertical or an inclined plane with all the pistons driving a common crankshaft. Where it is inclined, it is sometimes called a slant-6. The straight-6 layout is the simplest engine layout that possesses both primary and secondary mechanical engine balance, resulting in relatively low manufacturing cost combined with much less vibration than engines with fewer cylinders.
Exceptions to the shift to V6 engines include BMW, which specializes in high-performance straight-6s, Volvo, which designed a compact straight-6 engine/transmission package to fit transversely in its larger cars, and the Australian Ford Falcon, which still uses a straight-6 configuration. Straight-6s also continue to be commonly used in medium to large trucks, and sport utility vehicles, where engine length is less of a concern. In 2002 General Motors introduced the Vortec 4200 as part of the modular straight-4, straight-5 and straight-6 GM Atlas engine line.
An inline four cylinder or V6 engine without a balance shaft will experience secondary dynamic imbalance, resulting in engine vibration. As a general rule, the forces arising from any dynamic imbalance increase as the square of the engine speed—that is, if the speed doubles, vibration will increase by a factor of four. In contrast, inline six engines have no primary or secondary imbalances, and with carefully designed crankshaft vibration dampers to absorb torsional vibration, will run more smoothly at the same crankshaft speed (RPM). This characteristic has made the inline six popular in some European sports-luxury cars, where smooth high-speed performance and good fuel economy are desirable. As engine reciprocating forces increase with the cube of piston mass, inline six is a preferred configuration for large truck engines.
However many of the more sporty high performance engines use the 4 bearing design because of better torsional stiffness (eg BMW small straight 6's, Ford's Zephyr 6). In a 7 main bearing design the crank has two throws between each cylinder. The accumulated length of main bearing journals and 12 crank throws, gives a relatively torsionally flexible crankshaft. The 4 main bearing design has only 6 crank throws and 4 main journals so is much stiffer in the torsional domain. At high rpm the lack of torsional stiffness can make the 7 main bearing design susceptible to torsional flex and potential breakage. Note that a V12 engine can be made with the same number of crank throws as the 7 main bearing straight 6. Another factor affecting large straight 6 engines is the front mounted timing chain which connects the camshaft(s) to the crank. The camshafts are also quite long and subject to torsional flex as they in turn operate valves alternately near the front of the engine and near the rear. At high rpm the camshaft(s) can flex torsionally while the crank is doing likewise. This results in valve timing for the rear most cylinders becoming inaccurate and erratic, losing power and in extreme cases resulting in mechanical interference between valve and piston with catastrophic results. Some designers have experimented with installing the timing chain/gears in the middle of the engine (between cylinders 3 and 4) or adding a second timing chain at the rear of the engine. Either method can solve the problem.
Another factor affecting the ability of the large 6 cylinder engines to achieve high rpm is the simple geometric reality of a relatively long stroke (undersquare) design. A straight 6 is a long engine and the designer is usually encouraged to make it as short as possible while height is not usually a problem. Hence the tendency to use a longer stroke and smaller bore than in a V engine to achieve a given capacity. By contrast, a long-stroke V engine tends to become too wide, which encourages increasing the bore rather than the stroke to increase displacement. The typically longer stroke of the straight-6 increases crank throw and piston speed and so tends to reduce the rpm rating of the engine.
Mercedes-Benz has used straight-6 engines in its cars for around 100 years, starting in the 1900s with a monstrous 10 L engine producing . Before and after the merger of Daimler and Benz in 1926, the combined company produced a variety of powerful straight-6 engines, culminating in a 7 L supercharged unit producing up to . Mercedes-Benz began the post-war era by producing straight-4s, but resumed making straight-6s in 1951 with the M130, which were the beginning of the modern era of MB straight-6s. Following that introduction, the company produced two lines of gasoline (petrol) straight-6s at any one time, a small six and a larger six, in addition to its straight-4s, straight-5s, and later V8s. Although the company has used diesel engines in its cars since 1934, it introduced its first straight-6 OM603 a 3.0 L diesel in 1985. In 1996 the company replaced its gasoline straight-6s with a series of 90-degree M112 V6 engines, although it continued to produce diesel straight-6s.
Volvo produced straight-6s, the Volvo B30 engine (1969-1975) also the B6304 and the B6254 engines during late 90's. As Volvo developed front-wheel drive models, they mounted their inline-6 engine transversely by using a short transaxle package and relocated engine-driven accessories. The 3.2 L straight-6 introduced in 2006 was only slightly longer than its straight-5, achieved by moving the camshaft drive to the back of the engine and sharing the same gear train with ancillaries mounted in otherwise unused space over top of the transmission. It was short for a straight-6 and also very narrow. Volvo claims a transversely mounted inline engine leaves more crush space to protect against frontal impacts than a (shorter) transverse V6 or a longitudinally mounted inline-6.
Opel has also used a straight-6 engine since 1930s 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 Admiral, Kapitän, Monza, Senator, Omega, and Commodore.
Alfa Romeo used straight-6 engine in G1 and G2 models (1921-1923), RL model (1922–1927) and between 1925–1954 in Alfa Romeo 6C series road and racing cars, the 1500 version had one of the smallest straight-6 engines (1487 cc). The last Alfa Romeo model using straight-6 was Alfa Romeo 2600 (1961–1969).
The most prominent of these was the Jaguar XK6 engine, which reportedly was developed during long nights during World War II when Jaguar founder William Lyons and his staff were on fire watch duty in the Jaguar factory in Coventry and had nothing better to do than design a new engine. The result was displayed in the Jaguar XK120 at the London Motor Show in 1948. The 3.4 L twin overhead camshaft XK6 engine engine was highly advanced compared to previous British engines, most of which were side-valve units. The Jaguar XK120 and the XK-powered Jaguar C-Type and Jaguar D-type, went on to score victories in races and rallies in the UK, Europe and North America. They dominated the 24 Hours of Le Mans during the 1950s, where Jaguar C-Types won in 1951 and 1953, and the D-Types had three more wins in 1955, 1956 and 1957. The engine design, enlarged to 3.8 L, reached its apogee in the Jaguar E-type introduced in 1961, which was capable of . In 1964, the XK engine was again enlarged to 4.2 L, which was considered the most powerful and refined of the series. The last XK-engined Jaguar went out of production in 1986, but some XK engined cars such as the Daimler DS420 limousine were still available into 1990s. The XK6 engine was followed by the AJ6 and AJ16 engines. After Jaguar was acquired by Ford, these engines were replaced these engines with the Ford Duratec-derived Jaguar AJ-V6 engine.
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 1.6, 2.0, and 2.5 L 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 2.5 L Triumph straight-6. The TR6 fitted with fuel injection straight-6 (inline six) engines only had a limited production due to financial problems by which then Triumph sadly went bust. Other Triumph vehicles that use the Triumph straight-6 are:
The Rover SD1 saloon used a Triumph designed straight-6 of 2.3 and 2.6 L capacities. The SD1 saloon was then given the the GM based V8 engine Rover used. Its capacities came in 3.5, 3.9, 4.0, 4.5 and 5.0 it was 3.5 commonly found in the SD1.
British sports car company TVR designed its own straight-6, known as the Speed Six, which was also used in the Sagaris, with its capacity of 4 liters. Land Rover used a 2.6 L, straight-6 from 1967 in certain series Land Rover models.
Ford UK produced a straight 6 engine for the Zephyr and Zodiac range of passenger cars from the Mk 1 of 1951 (2262 cc) through the Mk 2 (2553 cc) and Mk 3 until 1966. The straight-6 was a 4 main bearing 12 overhead valve design with a short stroke. Rated output grew from just in the Mk 1 to in the Mk 3 Zodaic.
After World War II, 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 although it became one of the hottest cars on the road and dominated stock car racing (NASCAR) in the early Fifties..
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, and they are still used in light trucks available today. 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. This engine was reliable and achieved some success in racing after engineers discovered that the 30-degree slant of the engine allowed them to use very long intake runners to boost horsepower by tuning the intake system. Part of the reason for its reliability was that it was originally designed to be built of aluminum, but after Chrysler had problems with manufacturing the engines in aluminum, the rest were built in cast iron without changing the design to compensate for the stronger metal. Although it only had four crankshaft main bearings instead of the seven used by its competitors, they were the same size as those on the Hemi V8. After 30 years of production, it was discontinued in favor of V6 engines because it was too long to mount transversely in front wheel drive cars.
Another noteworthy 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. AMC also sold their straight-6's to International Harvester to be used in International's "Light Line" vehicles: Scouts, pickups, and Travelalls. These engines 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. It featured a durable design with a cast iron block and cylinder head, hydraulic lifters (with non-adjustable rockers), and seven main bearings. Since the cars were designed to take the weight of an optional V8, AMC was able to make their straight-sixes much stronger and heavier than they needed to be. As a result, the engine blocks were so sturdy that some were used in race cars in the Indianapolis 500. In the 1978 race, an AMC engine produced at 8,500 RPM with 80-inches of manifold pressure.
A significant step was taken by Kaiser Jeep with the 1963 Tornado straight-6, the first U.S. designed mass-produced overhead cam (OHC) automobile engine. However, it was complex (by 1960s standards) for civilian vehicles in the U.S., but continued to be installed in military Jeeps and was also produced through 1982 by IKA in Argentina.
Ford and General Motors straight-6s of the 1960s and 1970s were generally nondescript, except for the overhead cam Pontiac six of the late-1960s. Although the Pontiac six was one of the few straight-6s of its era to be advertised as exceeding , it wooed few performance buyers away from V8s in the muscle car era and was eventually discontinued in favor of a less costly design.
American automakers found it more profitable to sell slow-speed straight-6s as "economy" engines and V8s as "performance" engines regardless of their horsepower potential, since big, unsophisticated, overhead valve engines were relatively cheap to manufacture, and fuel economy was not a concern prior to the 1973 oil crisis.
The trend after the fuel crises in the 1970s was towards smaller cars with better fuel economy. Despite this, straight-6 engines became rare in American cars although they continued to be used in trucks and vans. The decline of the straight-6 was in response to the more compact size of the V6 layout. The straight-6 required a longer engine compartment that was more appropriate to a larger car. The shorter V6 could be used in a shorter engine compartment and therefore fit better in a more compact car. It was also relatively easy to cut two cylinders off a V8 design to produce a V6 that could be manufactured on the same assembly line as the V8, which was convenient for American manufacturers.
Jeeps were an exception to the trend to V6s, and began offering AMC's , known as "High Torque," straight-6s 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 CID) option in 1987. Usage of the AMC 4.0 declined in Jeep vehicles after the Jeep Cherokee (in North America) was replaced by the Liberty in 2002, which featured Chrysler's 3.7 L (226 CID) V6 instead. It declined further after the 2005 introduction of the third generation Jeep Grand Cherokee, which also used 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 (231 CID) 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 inline six) until 1997 when they were replaced with a V6.
In 1989 Chrysler introduced the 5.9 L Cummins B Series engine as an option on its pickup trucks. Displacing nearly 1 litre per cylinder, this straight-6 turbocharged diesel engine was an attractive alternative to the big gasoline V8s normally used on full-sized pickups because its better fuel economy and nearly twice as much low-speed torque. The usual marketing cachet of competing V8s from GM and Ford was offset by the "real" truck origin of the Cummins engine because earlier diesel V8s derived from gasoline engines had reliability problems. The current 6.7 L version is the largest straight-6 engine ever produced for a passenger vehicle.
In 2001 General Motors introduced a new family of straight engines, the Atlas, for use in the Chevrolet TrailBlazer/GMC Envoy. The straight-6 was chosen for development because of the desirable operating characteristics of its self-balanced design.
Toyota started with their F-series engine and later the M, FZ, G, and JZ engines, and Nissan started with their H-series and later the L of the early Fairlady Zs as well as the RB series engines (in the R31-R34 Skyline). Honda built the Honda CBX 1000 motorcycle from 1978 to 1981. In the 1990s Toyota offered straight-6s in all their lines: 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 2000s, Toyota still offers the FZ-series, G-series, and the JZ-series engines.
In Korea, GM Daewoo's FWD Magnus (sold abroad as the Chevrolet Evanda, Chevrolet Epica, Holden Epica or Suzuki Verona) comes with a Daewoo-designed straight-6. The Daewoo engine is one of the few straight-6s designed to be installed transversely in front wheel drive cars and it is an extremely short engine in its configuration.
BMC developed a straight six cylinder engine based on the B-series engine in the late 1950s. It appeared in the Austin Freeway and Wolseley 24/80. Although successful in Australia and tried successfully in the prototype MGC the cost of retooling meant that the engine remained indigenous to Australia. In the early 1970s a six-cylinder engine derived from the E-series was used in the Australian market for the P76 and Marina.
Chrysler had built the Slant 6 in Australia and the unique to Australia Hemi straight-6. These engines, made in 215, 245, and 265 CID capacity, were used in the Chrysler Valiant and the Valiant Charger producing up to . Chrysler no longer owns any factories in Australia.
Holden up until 1986 built their own straight-6s, adapted from a Chevrolet design. A '132.5' cubic inch, 2.171 L unit (known as the 'grey' motor) was used until 1963, with a minor increase in displacement in 1960 to '138' cubic inch, 2.261 L, when it was replaced by a newer Chevrolet based design (known as the 'red' engine) which was offered in different capacities. Holden engine sizes included the '138' 2.261 L '149' 2.441 L '161' 2.638 L '173' (2.85 L - 1971-1984), '186' (3.0 L - 1968-1971) and '202' (3.3 L - 1971-1986) - the largest and most popular of the series. This motor was firstly replaced by an imported RB20/30Nissan straight-6, offered in 2.0L (in New Zealand) and 3.0L forms, until Holden's Buick designed 3.8L V6 replaced it outright in 1988. Holden now make and use the new global HFV6 in their local and export passenger cars.
Ford Australia has been producing straight-6s since 1960 and is the only manufacturer in Australia to still build straight sixes, however, production is expected to be discontinued in 2010 due to difficulty in meeting Euro-IV standards with the current configuration. Ford has built 144, 170, 188, 200, 221, 240 and 250 cu in engines, with the 240 being called the 3.9 L or 4.0 L and the 200 being called the 3.3 L. They have been used since 1960 in the Falcon, 1972-1981 in the Cortina and from 2004 in the Ford Territory. The current straight-6 engines in the Falcon and Territory are called the Barra and have a 4.0 L displacement.
The high-performance division of Ford Australia, Ford Performance Vehicles, produce vehicles equipped with the 4.0 L DOHC 24-valve turbocharged straight-6 with variable cam timing, which produces at 5250 rpm and at 2000 - 4250 rpm — the highest level of torque in any Australian production car to date (along with the HSV E Series).
For road use, Honda introduced the Honda CBX 1000 in 1978. Kawasaki introduced the 1300cc Kz1300 in 1979. Benelli introduced the 750 Sei in 1976, which was later enlarged to 900cc to become the 900 Sei.
As with everyday passenger vehicles, the smooth running characteristics of the straight-6 engine are what make it desirable for industrial use. The fact that the straight-6 is the simplest engine that is in both primary and secondary balance means it can be scaled up to very large sizes without causing excessive vibration, and the fact that most of the engine components and accessories can be conveniently located along both sides, rather than on top of or underneath the cylinder banks, means that access and maintenance is easier than on a V-type engine in a truck or industrial configuration. In addition, a straight-6 engine is mechanically simpler than a V6 or V8 since it has only one cylinder head and in the overhead camshaft configuration has only half as many camshafts.
Notable versions include the 5.9 and 6.7 liter I-6 Cummins found in the Dodge Ram and the "DT" series Navistar DT Engine of inline 6-cylinder medium-duty diesel engines by International Truck and Engine Corporation, which are widely held as the gold standard engines for the medium-duty market.
Diesel straight-6's are also found in passenger cars, most notably those made by BMW. The twin turbo M57 produces up to from its 3 litre capacity, and has won numerous International Engine of the Year awards. Another notable diesel is the 3.2 liter straight-6 used in the Mercedes-Benz E320 CDI sold in America from 2004 through 2006.