A straight-three engine (also called an inline-triple or inline-three) is a three-cylinder piston engine where cylinders are arranged in a line along a common crankshaft. Less common than

straight-four engines A straight-four engine (also called an inline-four) is a four-cylinder piston engine where cylinders are arranged in a line along a common crankshaft. The vast majority of automotive four-cylinder engines use a straight-four layout (with the e ...

, straight-three engines have nonetheless been used in various motorcycles, cars and agricultural machinery.

Design

A crankshaft 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 Engine balance refers to how the forces (resulting from combustion or rotating/reciprocating components) are balanced within an internal combustion engine or steam engine. The most commonly used terms are ''primary balance'' and ''secondary bala ...

and

secondary balance Engine balance refers to how the forces (resulting from combustion or rotating/reciprocating components) are balanced within an internal combustion engine or steam engine. The most commonly used terms are ''primary balance'' and ''secondary ba ...

, however an end-to-end rocking couple is induced because there is no symmetry in the piston velocities about the middle piston. A balance shaft 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 F91 may refer to : * F91 Dudelange, a football club, based in Dudelange in southern Luxembourg * ''Mobile Suit Gundam F91'', a 1991 animated film * HMS Brazen (F91), a 1981 British Royal Navy Type 22 frigate * HMS Murray (F91), a British Royal Navy ...

, powered by a

two-stroke engine A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being completed in one revolution of ...

. The 1956-1960 Saab 93 saw the introduction of Saab's two-stroke engine, which was also used in the

Saab 95 The Saab 95 is a seven-seater, two-door station wagon which was produced by Saab from 1959 to 1978. Initially it was based on the Saab 93 sedan, but the model's development throughout the years followed closely that of the Saab 96 after the 93 ...

Saab 96 The Saab 96 is an automobile manufactured and marketed by Saab from 1960 to January 1980, replacing the 93. The 96 featured aerodynamic two-door bodywork, four-passenger seating and at first a two-stroke, three-cylinder engine, later a four-stro ...

and

Saab Sonett The Saab Sonett is an automobile manufactured between 1955 and 1957 and again between 1966 and 1974 by Saab of Sweden. Sonetts share engines and other components with Saab 93, 95 and 96 of the same era. It was mainly intended for the lucrati ...

until 1968 after which it was replace by the

Ford Taunus V4 engine The Ford Taunus V4 engine is a 60° V4 piston engine with one balance shaft, introduced by Ford Motor Company in Germany in 1962. The German V4 was built in the Cologne plant and powered the Ford Taunus and German versions of the Consul, Capri, an ...

. The

Wartburg The Wartburg () is a castle originally built in the Middle Ages. It is situated on a precipice of to the southwest of and overlooking the town of Eisenach, in the state of Thuringia, Germany. It was the home of St. Elisabeth of Hungary, the ...

cars (manufactured in East Germany) and

FSO Syrena The Syrena was a Polish automobile model first exhibited at the Poznań Trade Fair in 1955 and manufactured from 1957 to 1972 by the '' Fabryka Samochodów Osobowych'' (FSO) in Warsaw and from 1972 until 1983 by ''Fabryka Samochodów Małoli ...

(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 The EF is an automotive engine made by Subaru, introduced in 1984 for the Subaru Justy. It is a water-cooled, straight-three, four-stroke engine with a belt-driven single overhead camshaft, and a chain-driven balance shaft. It has an alloy cylinde ...

is a 4-stroke petrol engine which was introduced in 1984 and used in the Justy and the Sumo (the export version of the Sambar). The straight-three versions of the

Ford EcoBoost engine EcoBoost is a series of turbocharged, direct-injection gasoline engines produced by Ford and originally co-developed by FEV Inc. (now named FEV North America Inc.). EcoBoost engines are designed to deliver power and torque consistent with th ...

- 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 Variable displacement is an automobile engine technology that allows the engine displacement to change, usually by deactivating cylinders, for improved fuel economy. The technology is primarily used in large, multi-cylinder engines. Many automobile ...

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 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 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 and the 2004-present

Triumph Rocket III The Triumph Rocket III is a three-cylinder motorcycle made by Triumph Motorcycles Ltd. At it had the largest- displacement engine of any production motorcycle until the release of the Triumph Rocket 3. The name "Rocket III" is derived from ...

. In

2019 File:2019 collage v1.png, From top left, clockwise: Hong Kong protests turn to widespread riots and civil disobedience; House of Representatives votes to adopt articles of impeachment against Donald Trump; CRISPR gene editing first used to experim ...

, the Moto2 class in the MotoGP World Championship 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 A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being completed in one revolution of ...

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 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 Fairbanks, Morse and Company was an American manufacturing company in the late 19th and early 20th century. Originally a weighing scale manufacturer, it later diversified into pumps, engines, windmills, coffee grinders, radios, farm tractors, fee ...

32E14'' low-speed diesel engine. The straight-three layout is common for diesel

tractor A tractor is an engineering vehicle specifically designed to deliver a high tractive effort (or torque) at slow speeds, for the purposes of hauling a trailer or machinery such as that used in agriculture, mining or construction. Most commo ...

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 Hewland is a British engineering company, founded in 1957 by Mike Hewland, which specialises in racing-car gearboxes. Hewland currently employ 130 people at their Maidenhead facility and have diversified into a variety of markets being particul ...

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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