A straight-three engine (also called an inline-triple or inline-three) is a three-cylinder

piston engine A reciprocating engine, also often known as a piston engine, is typically a heat engine that uses one or more reciprocating pistons to convert high temperature and high pressure into a rotating motion. This article describes the common featu ...

where cylinders are arranged in a line along a common crankshaft. Less common than straight-four engines, straight-three engines have nonetheless been used in various motorcycles, cars and agricultural machinery.

Design

crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...

angle of 120 degrees is typically used by straight-three engines, since this results in an evenly spaced firing interval. Another benefit of this configuration is perfect primary balance and secondary balance, however an end-to-end

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is induced because there is no symmetry in the piston velocities about the middle piston. A

balance shaft Balance shafts are used in piston engines to reduce vibration by cancelling out unbalanced dynamic forces. The counter balance shafts have eccentric weights and rotate in opposite direction to each other, which generates a net vertical force. Th ...

is sometimes used to reduce the vibrations caused by the rocking couple. Other crankshaft angles have been used occasionally. The 1976-1981 Laverda Jota motorcycle used a 180 degree crankshaft, where the outer pistons rise and fall together and inner cylinder is offset from them by 180 degrees. This results in three power strokes evenly-spaced at 180 degrees each, and then no power strokes during the final 180 degrees of crankshaft rotation. The 2020 Triumph Tiger 900 motorcycle uses a "T-Plane" crankshaft where the crankshaft throws are at 90 intervals, such that the throws for cylinders 1 and 3 are separated by 180 degrees (therefore the three throws together forming a "T" shape when viewed from the end).

Usage in cars

Among the first cars to use a straight-three engine is the 1953-1955 DKW F91, powered by a two-stroke engine. The 1956-1960

Saab 93 The Saab 93 (pronounced ''ninety-three'') is the second production automobile that was manufactured by Saab. Styled by Sixten Sason, it was first presented on December 1, 1955. The 93 was powered by a longitudinally-mounted three-cylinder 748&nbs ...

saw the introduction of Saab's two-stroke engine, which was also used in the Saab 95, Saab 96 and Saab Sonett until 1968 after which it was replace by the Ford Taunus V4 engine. The Wartburg cars (manufactured in East Germany) and FSO Syrena (manufactured in Poland) also used straight-three engines. The 1967 Suzuki Fronte 360 uses a two-stroke engine. In 1980, Suzuki began production of a four-stroke engine, which was introduced in the Alto and Fronte models. The Subaru EF engine is a 4-stroke petrol engine which was introduced in 1984 and used in the

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and the

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(the export version of the Sambar). The straight-three versions of the Ford EcoBoost engine - a turbocharged 1.0-litre petrol engine - was introduced in the 2012 Ford Focus. It uses an unbalanced flywheel to shift the inherent three-cylinder imbalance to the horizontal plane where it is more easily managed by engine mounts, and so remove the need to use balance shafts. In 2016, cylinder deactivation was added, claimed to be a world first for three-cylinder engines.

Usage in motorcycles

The advantages of a straight-three engine for motorcycles are that it has a shorter length than an inline-four engine and produces less vibration than a straight-twin engine.

Four-stroke

Four-stroke A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either directio ...

straight-three engines have been used in road bikes and racing bikes by several companies.365 Motorcycles You Must Ride. Dain Gingerelli, Charles Everitt, James Manning Michels. MBI Publishing Company, 10 Jan 2011 From 1985-1995, the

BMW K75 The BMW K100 is a family of four-cylinder 987 cc motorcycles that were manufactured by BMW from 1983 to 1992. Background As the 1970s came to an end, BMW faced three problems from developing its flat-twin boxer engine further: *Emissions r ...

was produced with a straight-three engine (based on the straight-four engine from the BMW K100). The British company Triumph has produced several models with transversely-mounted straight-three engines, such as the 1994-present

Triumph Speed Triple The Triumph Speed Triple is a series of motorcycles produced by Triumph Motorcycles. The 1994 Hinckley Triumph was one of the first motorcycles produced in the streetfighter style (a modern sport bike or race replica motorcycle without an aer ...

and the 2004-present Triumph Rocket III. In

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, the Moto2 class in the

MotoGP World Championship Grand Prix motorcycle racing is the premier class of motorcycle road racing events held on road circuits sanctioned by the Fédération Internationale de Motocyclisme (FIM). Independent motorcycle racing events have been held since the start of ...

switched to using Triumph 765 cc (46.7 cu in) triple engines.Triumph enters Grand Prix racing as Moto2 engine supplier
'' The Telegraph'', 18 December 2018. Retrieved 15 December 2019.

Two-stroke

Two-stroke designs are less common in straight-three engines than four-stroke designs, however several were produced by Japanese manufacturers in the late 1960s through to 1980s. The Kawasaki triple engine was produced from 1968 to 1980 and was used in various road bikes and racing bikes. Most versions were air-cooled, however several were water-cooled.Japanese Production Racing Motorcycles. Mick Walker. Redline Books, Sep 2004 Similarly, the 1972-1980

Suzuki GT series The GT series is a series of two-stroke chain drive motorcycles manufactured and marketed by Suzuki for model years 1972-1977, with a range of engine capacities and cylinder counts. Certain markets received the GT380 until model year 1980. GT3 ...

engines were used for both road bike and racing bikes, and were available in both air-cooled and water-cooled versions.

Other uses

Agriculture

An example of an agricultural application is the '' Fairbanks-Morse 32E14'' low-speed diesel engine. The straight-three layout is common for diesel tractor engines, such as the '' Perkins AD3.152''. This engine was used in the Massey Ferguson 35 and Fordson Dextra tractors, as well as for marine and stationary applications.

Aviation

The Hewland AE75 is a 750 cc two-stroke aircraft engine that was produced in the mid-1980s. It was an inverted three-cylinder design with liquid-cooling that produced .“Pilot” magazine February 1986 page 32

References

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