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Straight-six engine

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Engine block of aBMW M20straight-six engine (top view)

Thestraight-six engine(also referred to as aninline-six engine;abbreviated I6 or L6) is apiston enginewith six cylinders arranged in a straight line along thecrankshaft.A straight-six engine has perfect primary and secondaryengine balance,resulting in fewer vibrations than other designs of six or fewer cylinders.

Until the mid-20th century, the straight-six layout was the most common design for engines with six cylinders. However,V6 enginesbecame more common in the 1970s. By the 2000s, V6 engines replaced most straight-six engines.

Characteristics

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In terms of packaging, straight-six engines are almost always narrower than aV6 engineorV8 engine,but longer thanstraight-four engines,V6s, and most V8s.[1][2][3]

Straight-six engines are typically produced indisplacementsranging from 2 to 4 L (122 to 244 cu in),[4]: p209 however engines ranging in size from the 0.7 L (43 cu in)Benelli 750 Seimotorcycle engine[5]to the 6.7 L (409 cu in)Cummins B Series enginepickup truck engine have also been produced.[6]Due to its well-balanced configuration, the straight-six can be scaled up to substantial sizes for heavy trucks, locomotives, industrial and marine use.[7]: p4 

Engine balance and vibration

[edit]
Typical firing order of 1-5-3-6-2-4
3890-litreMAN B&W 6S60MCmarine diesel engine

If an appropriatefiring orderis used, a straight-six engine has perfect primary and secondaryengine balance.The primary balance is due to the front and rear trio of cylinders moving in pairs (albeit 360° out of phase), thus canceling out the rocking motion present in astraight-three engine.The secondary balance is due to thecrank throwsbeing arranged in three planes offset at 120°, resulting in the non-sinusoidal forces summing to zero for all free forces until the sixth order.

The engine balance characteristics of a straight-six engine compare favorably with the more commonstraight-four enginesandV6 engines,which experience significant secondary dynamic imbalance, resulting in engine vibration.[7]: pp12-13 As engine reciprocating forces increase with the cube of piston bore, the straight-six is a preferred configuration for large truck engines.[7]: pp40-47 

Two-stroke engines

[edit]

An even-firing six-cylindertwo-strokeengine requires ignitions at 60° intervals, or else it would run with simultaneous ignitions and be no smoother than atriplein power delivery. As such, it also requires crank throws at 60° –. Such designs appear to have been limited to diesel engines such as the Detroit71 series,marine engines, and outboard motors.

Some of the 120 possible crankshaft configurations have useful properties. Still, all of them have a rocking imbalance that may or may not require a balance shaft, depending on the application. The six pistons with six unique phases cannot be "paired" as in the four-stroke case. The Detroit engines used a configuration that, once the primaryrocking couplewas balanced out, was also perfectly balanced at all other rocking couples until the 6th order.[8]Mercurycame to use a configuration that canceled only the primary rocking couple and was run without a balancer.[9]

The reciprocating masses of all configurations are still imbalanced at only 6th-order and up in their plane of motion. Still, the balance of kinetic energy exchange between pistons has improved to a residual 6th-order-and-up inertial torque oscillation compared with the four-stroke design being imbalanced at 3rd-order and up.

Crankshaft

[edit]
Crankshaft with four main bearings

Crankshaftsfor straight-six engines usually have either fourmain bearings(i.e., a bearing in between each pair ofcrankpinsand one at each end) or seven main bearings (i.e., a bearing between every crankpin):

  • Large displacement and diesel engines typically use seven bearings to minimize crankshaft flex. When an engine is subject to high loads at low speeds, the greater distance between main bearings causes increased crankshaft flex. Modern high-compression engines subject the crankshaft to greater bending loads from higher peak gas pressures, requiring the crank throws to have more significant support from adjacent bearings. So, it is common for straight-six engines to use seven main bearings.[7]: p38 
  • Smaller and high-performance engines[when?]typically use four bearings since having fewer main journals increases the torsional stiffness of the crankshaft. The lack of torsional stiffness can make the seven main bearing designs susceptible to torsional flex and potential breakage at high engine speeds. Any torsional flex in the crankshaft is compounded by the torsional flex of thecamshaftsfor the rear cylinders since the camshafts are rather long and subject to torsional flex. At high engine speeds, the combination of camshaft and crankshaft flex results in inaccurate timing of the valve openings, which, in the worst case, can cause the valves and pistons to collide with catastrophic results.

Usage in cars

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The first production straight-six engine was introduced in the DutchSpyker60 HPracing car in 1903.[10]Straight-six engines increased in popularity in the years after and by 1909, approximately 80 manufacturers were using them (including 62 in the United Kingdom).[11]Prior to the 1950s,V6 engineswere rarely used, due to the poorerengine balanceof V6 engines compared to straight-six engines. Since the 1980s, however, the shorter length of V6 engines has seen most manufacturers replace straight-six engines with V6 engines. An exception to this trend is the German brandBMW,which has always used a straight-six layout for its six-cylinder engines.[12]

Since 2017, the trend of switching to V6 engines has reversed due to the ability to create a modular engine family of straight engines sharing many components.[13][14]Examples include the 2017–presentMercedes-Benz M256 engine,the 2019–presentJaguar Land Rover Ingenium engines(AJ300 and AJ300D versions), the 2021–presentStellantis Hurricane engineand the 2022–present MazdaSkyactiv-XandSkyactiv-Dengines.[15][14][16]

Europe

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1978-1981BMW M88engine
2014-2016Volvo SI6engine (transversely mounted)

Alfa Romeo's first production straight-six engine - 6.3 Lflatheadpetrol engine - was introduced in 1921 in theAlfa Romeo G1luxury car. Aoverhead valvedesign was introduced in the 1922Alfa Romeo RLsports car, and anoverhead camshaftdesign was used in the 1927Alfa Romeo 6Csports car and various racing cars from 1927 until 1954. The last Alfa Romeo model using a straight-six was the 1961–1969Alfa Romeo 2600executive car before the company switched to V6 engines.

Mercedes-Benz's history of straight-six engines began with the 1913Mercedes D.Iaircraft engine. The first automotive straight-six engine was the 1924–1929Daimler M8363.9 L petrol engine.[17]Following World War 2, Mercedes resumed production of straight-six engines with the 1951 introduction of theMercedes-Benz M180overhead camshaft engine. In 1985, theMercedes-Benz OM6033.0 L diesel straight-six engine was introduced. In 1996, the company replaced its petrol straight-sixes with a series of V6 engines, although it continued producing diesel straight-six engines. Production of petrol straight-six engines resumed in 2017 with the introduction of theMercedes-Benz M256turbocharged DOHC engine.

Opel began production of straight-six engines in 1927 with a 1.8 L flathead petrol engine used by theOpel 8/40 PS.The displacement of this engine was expanded as it was used in later models such as theOpel KapitänandOpel Admiral,with later versions switching to an overhead valve (pushrod) design. In 1968, the straight-six versions of theOpel CIH enginewere introduced, initially using a single overhead camshaft (SOHC) with some later versions using double overhead camshafts (DOHC). Production of the Opel CIH engine continued until 1993, when a V6 engine replaced it.

Volvo began production of straight-six engines with the 1929–1958Penta DBflathead petrol engines. The company resumed production in 1969 with theVolvo B30overhead valve petrol engine, followed by the straight-six versions of theVolvo Modular Engineintroduced in 1995 and then theVolvo SI6 engineintroduced in 2006. Several models (such as the 1998–2006Volvo S80) used the uncommon design of a transversely-mounted straight-six engine. Production of Volvo straight-six engines ceased in 2015.

BMW's first product was the 1917BMW IIIastraight-six aircraft engine. The company began production of automotive straight-six engines in 1933 with theBMW M78petrol engine, a 1.2 L overhead valve design that evolved over the years into theBMW M337(produced until 1958). Production of straight-six engines resumed in 1968 with theBMW M30single overhead camshaft engine, built for 27 years and used in various models. The 1978–1989BMW M88engine was a double overhead camshaft design that was introduced in theBMW M1mid-engine sport car. BMW's introduction of turbocharged straight-six engines (aside from the low-volume variants of the M30 engine in the 1980s) was in 2006BMW N54and the production of naturally aspirated engines ceased in 2015. As of 2022,theBMW B58turbocharged straight-six engine remains in production,[18]along with its higher performanceBMW S58variants.

United Kingdom

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Colored dice with white background
Rolls-Royce Silver Ghostflathead engine
1969Jaguar XKDOHCengine

Rolls-Royce's first straight-six engine was a 6.0 LIOEpetrol engine, which was used in the 1905Rolls-Royce 30 hpluxury car. This car was replaced by the 1906–1926Rolls-Royce Silver Ghost,which switched to a flathead (side-valve) design for its straight-six engine.

In 1906, theStandardSixluxury car was introduced, powered by a 20 hp (15 kW; 20 PS) I6 petrol engine.[19]Standard's engines were also used in several cars built by SS Cars and its successor Jaguar, such as the 1932SS 1sports car, the 1936SS Jaguar 100and the 1938Jaguar 3½ Litresports saloon/coupe and the 1948Jaguar Mark Vluxury car.

The 1927Rover Two-litreluxury car introduced the company'sIOEstraight-six petrol engine. This engine was used in various Rover models until theRover P5was discontinued in 1973, and in various Land Rover models from the 1961Land Rover Series IIAuntil 1980[citation needed]Land Rover Series III.

The 1928Austin 20/6luxury car introduced Austin's flathead straight-six petrol engine. The 1938-1939 Austin Twenty-Eight used an enlarged version of this engine. This was replaced by theAustin D-Series engine,an overhead valve engine initially designed for trucks, which was used in passenger cars from 1947 until 1968 (along with severalJensen Motorsmodels from 1946 to 1962). The overhead valveBMC C-Serieswas used by various BMC brands from 1954 to 1971, followed by the 2.2 L version of theBMC E-Seriesoverhead camshaft engine, which was produced from 1970 until 1982.[20]

The 1930–1936Wolseley Hornet sixlightweight car was powered by a 1.3-1.6 L overhead camshaft straight-six petrol engine.

The 1931–1932MG F-typetourers, 1932–1934MG K-typesports cars, and 1934–1936MG N-typesports cars were powered by an overhead camshaft straight-six petrol engine.

During the mid-1930s, theRiley MPHsports car andRiley Kestrel 6[21]saloon were produced in small numbers and were powered by dual overhead camshaft straight-six petrol engine.

The 1947Bristol 400luxury car was powered by an overhead valve straight-six petrol engine based on the design of theBMW M328engine. This engine remained in use until theBristol 406was discontinued in 1961.

The dual overhead camshaftJaguar XK6 enginepetrol engine was produced from 1948 to 1992 in theJaguar XK120sports car. Introduced as a 3.4 L, it was used in passenger and racing cars, produced in displacements of 2.4 to 4.2 L. The XK6 engine was followed by theAJ6 and AJ16engines, produced from 1984 to 1996, before being replaced by a Ford-derived V6 engine.

The 1948–1959Lagonda straight-6dual overhead camshaft petrol engine was used in various Aston Martin and Lagonda cars. This engine's successor was the Tadek Marek-designed straight-six used in the DB4 (1958), DB5 (1963), DB6 (1965) and DBS (1967).

TheFord Zephyr 6overhead valve engine was used in theFord Zephyrexecutive car and several other models from 1951 to 1966.

TheTriumph I6overhead valve straight-six petrol engine was produced from 1960 to 1977 and debuted in the Standard Vanguard Six sports saloon. TheLeyland PE166engine was loosely based on the Triumph design and was produced from 1977 to 1986.

The 1972–1977TVR 2500Msports car was powered by the Triumph I6 engine. Then, from 1999 to 2007, TVR's ownTVR Speed Sixdual overhead camshaft engine was used in several of the company's sports cars.

United States

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1954Hudson Waspengine
1977AMC straight-6 engine

The 1906–1908Ford Model Kluxury car used a 405 cu in (6.6 L) straight-six petrol engine and was the only Ford six-cylinder passenger car engine until the 1940s. TheFord flathead I6was produced from 1941 until 1951, followed by theFord OHV I6overhead valve engine from 1952 through 1964, then theFord Thriftpower Sixoverhead valve engine from 1960 until 1982, and theFord 240 I6from 1965 through 1972. TheFord 300 cu in (4.9 L) versionof the straight-six engine was built from 1965 until 1996, with notable uses in trucks, SUVs, and vans. However, Ford straight-sixes were replaced by V6 engines in passenger cars during the mid-1970s.

In 1908, theOldsmobile Model Zwas powered by a flathead straight-six petrol engine, which was produced until 1912 (in theOldsmobile Limitedluxury car) in displacements of 453 cu in (7.4 L), 505 cu in (8.3 L) and 706 cu in (11.6 L). Oldsmobile's next straight-six engine was introduced in the 1913Oldsmobile Sixluxury car, initially with a displacement of 380 cu in (6.2 L), followed by a displacement of 177 cu in (2.9 L) for the 1917-1921Oldsmobile Model 37luxury car. The later generations of theOldsmobile Straight-6also used a flathead design from its introduction in the 1923Oldsmobile Model 30luxury car until it was replaced in 1950 by Oldsmobile's V8 engine.

The 1913–1929Oakland Sixluxury car was powered by a flathead petrol engine produced in displacements of 177 cu in (2.9 L) and 334 cu in (5.5 L). In 1926, thePontiac Sixwas introduced as a lower-cost version of the Oakland six, powered by thePontiac Split-Head Sixflathead engine, which used two cylinder heads. This engine was replaced by the 1941–1954Pontiac flathead six.The unrelatedPontiac OHV 6overhead valve engine was produced in 1964–1965, based on a Chevrolet design. Pontiac's final straight-six engine was the 1966–1969Pontiac OHC 6overhead camshaft engine, which was replaced by Chevrolet's straight-six engine and Buick's V6 engine. a The overhead valveBuick Straight-6petrol engine was introduced in the 1914Buick Sixluxury car and was produced until 1930. Buick did not make another six-cylinder engine until they introduced a V6 engine in 1962.

The 1916 through 1926Hudson Super Sixwas powered by a 289 cu in (4.7 L) straight-six petrol engine. This was followed by anIOEversion in 1916. The 1951Hudson Hornetintroduced a 308 cu in (5.0 L) flathead straight-six engine.[22]These engines dominated dirt track and NASCAR racing at the time.[23][24][25]Production of the Hudson I6 continued after the 1954 merger formingAmerican Motors Corporationand was eventually replaced by a newRambler V8after 1956.

In 1924, Chrysler began production of a straight six version of theChrysler flatheadpetrol engine. This was replaced by the 1959–2000Chrysler Slant-6overhead valve straight-six petrol engine, which was so named due to the 30-degree angle used to reduce the height of the engine (with the trade-off of a wider engine). The Slant-6 was released in theDodge Darteconomy car and used in many models until a V6 engine replaced it after 30 years.

TheChevrolet Stoveboltoverhead valve straight-six petrol engine was introduced in 1929 as a replacement for the brand's straight-four engines and was produced in displacements of 181 cu in (3.0 L), 194 cu in (3.2 L) and 207 cu in (3.4 L). The second generation of this engine family - often referred to as theBlue Flameengine - was produced from 1937 to 1962 in displacements of 216 cu in (3.5 L), 235 cu in (3.9 L) and 261 cu in (4.3 L). This was followed by the 1962–1988Chevrolet Turbo-Thrift engine(also using an overhead valve design), which was replaced by various General Motors V6 engines.

The 1952 through 2006AMC Straight-6petrol engine initially used a flathead design before being upgraded to an overhead valve design in 1956. A new I6 design with a short stroke and sevenmain bearingcrankshaft was introduced in 1964.[26]The engine was rugged, reliable, and became noted for longevity. A turbocharged racing engine based on the AMC Straight-6 engine block produced 875 hp (652 kW) and competed in the 1978 Indianapolis 500 race.[27][28]The final application for the AMC Straight-6 engine was the 2006Jeep Wrangler (TJ),after which a V6 replaced it.[29]

The 1962 through 1973Jeep Tornadooverhead camshaft straight-six engine was introduced in theWillys Jeep Station Wagon.[30]At the time of its introduction, the Tornado engine had the lowest specific fuel consumption of an American gasoline (petrol) engine.[31]The Tornado engine was replaced by the AMC I6 engine.

In 2001, General Motors resumed production of straight-six engines with theVortec 4200dual overhead camshaft petrol engine. This engine was used in various SUV models until 2009.[32]Also, theDuramax Straight-6turbocharged diesel engine has been available in several General Motors SUV and light truck models since 2020.[33]

On March 25, 2022, Stellantis announced their new turbocharged straight-six engine, called Hurricane.[34]Two outputs are available, a standard 400 or 500 hp (298 or 373 kW; 406 or 507 PS) high performance rated at 450 or 475 pound force-feet (610 or 644 N⋅m) of torque. The turbos on the Hurricane Standard Output deliver a peak boost of 22 psi, while the Hurricane High Output turbos deliver 26 psi of peak boost. The 3.0 L Hurricane is produced at Stellantis’ Saltillo Engine Plant in Mexico.

Asia

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1970sNissan S20 engine
1990sNissan RB engine

Toyota's first straight-six engine was theToyota Type A,produced from 1935 through 1947. The Type A was an overhead-valve petrol engine based on theChevrolet Stoveboltengine. This was followed by the first generationToyota Foverhead valve engine, which was produced from 1949 to 1975, which in turn was followed by the2F versionfrom 1975 to 1988 and the fuel-injected3F / 3FE versionfrom 1988 to 1992. This was replaced by the dual overhead camshaftToyota FZ engine,produced from 1993 until 2008. Produced alongside these engines was the single overhead camshaftToyota M engine,which was introduced in 1965 and produced over seven generations until 1993 (with the7M-GTEbeing the final version of the M engine). The M engine was replaced by the dual overhead camshaftToyota JZ engine,which was produced from 1990 to 2007 and is arguably known as Toyota's best straight-six engine. Toyota's third line of straight-six engines was the 2.0 LToyota G engine,which was released as a single overhead camshaft engine in 1979 and upgraded to dual overhead camshafts before production ended in 2008 (with the 1G-FE being the final variant).

Nissan's first straight-six engine was the 1950–1952Nissan NAKflathead petrol engine, which continued in various forms until production of theNissan Pengine ended in 2003. The 1963-1965Nissan Koverhead valve petrol engine was used in theNissan Cedric Special 50luxury car. Using a similar design, theNissan H30engine was used in several luxury cars from 1965 through 1989. In 1966, Nissan began production of the six-cylinder versions of theNissan Lsingle overhead camshaft engine, which was produced until 2009. The 1985–2004Nissan RB engine,used in theNissan Skylineand several other cars, was produced in single overhead camshaft and dual overhead camshaft configurations until a V6 engine replaced it. TheNissan TBoverhead valve engine was introduced in 1987 and produced alongside the other straight-six engines.

The six-cylinder versions of thePrince G enginewere introduced in 1963 and remained in production until 1969, three years after Prince's merger with Nissan. The 1969–1973Nissan S20dual overhead camshaft engine (used in theNissan Fairladyand first generation of theNissan Skyline GT-R) was based on the Prince G engine.

Mitsubishi produced six-cylinder versions of theMitsubishi KE enginefrom 1963 to 1970, as well as the rare six-cylinder versions of theMitsubishi 6G34version of the single overhead camshaft "Saturn" engine from 1970 to 1976.

The 2000–2006Daewoo Magnus(also called the Chevrolet Evanda, Chevrolet Epica, Holden Epica, or Suzuki Verona) was powered by theDaewoo XK6straight-six petrol engine, which is one of the few straight-six engines to be used in atransverse enginefront-wheel drive car.

Australia

[edit]
1970Chrysler Hemi-6 Engine
2004–2009Ford Barra engine

From the 1950s to the 2010s, many cars produced in Australia were powered by a straight-six engine.

Holden's first car, the 1948Holden 48-215sedan, was powered by theHolden 'grey' motor,an overhead valve petrol engine. This engine was replaced by theHolden 'red' motor,which was produced from 1963 to 1980 and in turn followed by the 1980–1984Holden 'blue' motor.The final locally produced Holden straight-six was the 1984–1986Holden 'black' motor,which was initially replaced by the Japanese-builtNissan RB30engine before Holden switched to a locally-built V6 engine.

Ford produced straight-six engines for the longest time of any Australian manufacturer. In 1960, theFord Falconlarge sedan was introduced with a locally-built version of the AmericanFord Straight-6overhead valve engine. In 1998, these engines were upgraded to a single overhead camshaft design. This was followed in 2002 by theFord Barradual overhead camshaft engine in 2002, which was produced in both naturally aspirated and turbocharged versions. The Ford Barra engine remained in use until Ford Australia ceased local production in 2016.

TheChrysler Valiantwas introduced in 1962, powered by the AmericanChrysler Slant 6.In 1970, the Valiant switched to theChrysler Hemi-6overhead valve engine, an Australia-only engine that was produced until Chrysler Australia ceased production of large cars in 1981.[35]

The 1962–1965Austin FreewayandWolseley 24/80large sedans were powered by BMC Australia'sBlue Streakoverhead valve petrol engine. This was followed by the six-cylinder versions of theBMC E-seriesoverhead camshaft engines, which were introduced in the 1970Austin Kimberley / Austin Tasmanfront-wheel-drive sedans, which were produced until 1972.[36]This engine was upsized to a 2.6 L displacement in 1973 and usedLeyland P76and theMorris Marinalarge cars until 1975.[37]

Usage in motorcycles

[edit]
1978-1982Honda CBXengine

From 1964 to 1967, Honda produced severalGrand Prix motorcycle racingmotorcycles powered by straight-six versions of theHonda RCpetrol racing engine; these versions were designated the '3RC164', 'RC165', 'RC166' andHonda RC174.[38]The unrelated road-useHonda CBXwas produced from 1978 to 1982 and was powered by a 1,047 cc (63.9 cu in) dual overhead camshaft petrol engine.[39][40]

The 1973–1989Benelli Seiseries was the first road-use motorcycle to use a straight-six engine. Initially released with a 747 cc (45.6 cu in) single overhead camshaft petrol engine, in 1979 it was upgraded to a 906 cc (55.3 cu in) engine.[41]

The 1979–1989Kawasaki Z1300roadster motorcycle was powered by a 1,286 cc (78.5 cu in) dual overhead camshaft petrol engine.[42]

From 2011–present, theBMW K1600series of touring motorcycles have been powered by a 1,649 cc (100.6 cu in) dual overhead camshaft petrol engine.[43][44]

Usage in trucks

[edit]

Straight-six engines used in trucks include:

  • 5.9 and 6.7 L versions of the 1984–presentCummins B Seriesturbocharged diesel engine[45]
  • General Motors' 2002–2009Vortec 4200naturally-aspirated petrol engine

Diesel engines

[edit]

Automotive engines

[edit]

Straight-six engines used in automobiles include:

  • 1978-1995Volkswagen D24naturally-aspirated engine and the 1982–1992Volkswagen D24Tturbocharged engine
  • 1986-1997Mercedes-Benz OM603engine - produced in naturally aspirated and turbocharged variants
  • BMW's line of diesel engines consists of theBMW M21(introduced in 1981) through to the currentBMW B57engine
  • 1985-2009Nissan RD(naturally-aspirated and turbocharged variants) and 1987-2007 Nissan TD42 (turbocharged) engines

Industrial-use and truck engines

[edit]

See also

[edit]
Wikimedia Commons has media related toStraight-6 engines.

References

[edit]
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