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A straight-three engine, also known as an inline-triple, or inline-three (abbreviated I3, L3 or R3), is a reciprocating piston internal combustion engine with three cylinders arranged in a straight line or plane, side by side.
Straight-three engines generally employ a crank angle of 120°. 120° cranks are rotationally balanced; however, since the three cylinders are offset from each other, the firing of the end cylinders induces a rocking motion from end to end, since there is no opposing cylinder moving in the opposite direction as in a rotationally balanced straight-six engine. The use of a balance shaft in an antiphase to that vibration produces a smoothly running engine.
An inline three cylinder engine with 180° crankshaft can be found in early examples of the Laverda Jota motorcycle made by Italian manufacturer Laverda. In these engines, the outer pistons rise and fall together like a 360° straight-two engine. The inner cylinder is offset 180° from the outer cylinders. In these engines, cylinder number one fires, then 180° later cylinder number two fires, and then 180° later cylinder number three fires. There is no power stroke on the final 180° of rotation. This unusual crank angle came to be due to lack of proper tooling at the factory, which also made vertical twin engines which also utilizes a 180° crankshaft. After 1982, this engine had the regular 120° crank angle.
The smallest inline-three, four-stroke automobile engine was the 543 cubic centimetres (33.1 cu in) Suzuki F5A, which was first used in the 1979 Suzuki Alto/Fronte. Smart currently produces a diminutive 799 cubic centimetres (48.8 cu in) inline-three diesel engine, the smallest automotive diesel engine yet. Most inline-three engines fall below 1.2 litres, with a 1,198 cubic centimetres (73.1 cu in) Volkswagen Group unit seen as the largest petrol engine. A 1,779 cubic centimetres (108.6 cu in) diesel engine was produced by VM Motori to the 1984 Alfa Romeo 33 1.8 TD, the largest inline-three produced for automotive use.
Basic versions of the Suzuki Swift (and its related badge-engineered versions) used a 993 cc inline-three-cylinder engine, with single-point injection. The same engine was also used in the Suzuki Wagon R+, with multi-point injection. This was replaced from 2003, with an Opel-sourced 998 cc unit, with four valves per cylinder and variable-length intake manifold, also used on the similar Opel Agila and the third generation of the Opel Corsa, and later on the Suzuki Splash and the fifth generation of the Opel Corsa. The Opel models previously used a 973 cc engine, which was initially featured on the second generation of the Opel Corsa.
Some Daihatsu cars use straight-three engines. The Charade used and the Mira/Cuore uses this engine type. Three-cylinder 1.0 litre diesel and turbodiesel engines were also offered in Daihatsu Charades. Korean cars Daewoo Tico, based on the 1988 Suzuki Alto, and later base versions of Daewoo Matiz also used inline-three 796 cubic centimetres (48.6 cu in) 41 horsepower (31 kW; 42 PS) S-TEC petrol engine.
The Volkswagen Group is known for using three-cylinder petrol and diesel engines, in the Audi A2, Volkswagen Polo, Volkswagen Fox, Volkswagen Lupo, SEAT Ibiza, SEAT Córdoba and Škoda Fabia. These engines range from 1.2 litre petrol, with four valves per cylinder, that delivers 47 to 65 kilowatts (64 to 88 PS; 63 to 87 bhp), to 1.4 TDI diesel, that delivers 51 to 66 kilowatts (69 to 90 PS; 68 to 89 bhp) and has turbocharger with variable vane geometry. The most innovative three-cylinder engine the Volkswagen Group released was the 1.2 TDI diesel (launched in 1998), that was one of the first all-aluminium diesel engines and at the time of release was the lightest and most economic engine in production. It was used in the "3L" versions (from its diminutive fuel consumption of 3 litres per 100 kilometres or 94 miles per imperial gallon) of the Audi A2 and the Volkswagen Lupo. Another three-cylinder engine is the 1.2 TDI diesel launched with the fifth generation of Volkswagen Polo (in 2009), that uses common rail injection (unlike the previous 1.2 TDI engine, which used unit injectors).
Mitsubishi has made extensive use of three-cylinder engines, which have also been used in the Smart ForTwo, since 2004 on the Mitsubishi Colt, and from 2012 on the Mitsubishi Mirage. A three-cylinder diesel engine was also used on the Mitsubishi Colt and the Smart Forfour.
In the 1950s and 1960s, the Saab 93, Saab 95, Saab 96, and certain DKW automobiles were powered by inline-three-cylinder, two-stroke engines. Also, the Wartburg automobiles manufactured in Eastern Germany, and FSO Syrena manufactured in Poland, used this kind of engine.
The Toyota Peugeot Citroën Automobile (TPCA) joint venture is using a common 996 cc inline-three-cylinder engine in models of the Aygo, 107 and C1 respectively, which is derived from Daihatsu. The same engine has been provided for the Toyota Passo, the Toyota Yaris, the Toyota Agya, the Toyota Belta, the Toyota iQ, and for the Daihatsu Boon, the Daihatsu Cuore, the Daihatsu Ayla and the Subaru Justy (see Toyota KR engine).
Nissan has developed a 1.2-litre three-cylinder version of their HR series of petrol engines, which is used since 2010 for the Nissan Micra, the Nissan Almera (Thailand) and the Nissan Note. There is also a supercharged version, with direct injection and variable valve timing.
The Hyundai Motor Group used a 1.5-litre three-cylinder diesel engine (developed by VM Motori) on the second generation of the Hyundai Accent, the Hyundai Getz and the Hyundai Matrix, and developed its own 1.1-litre three-cylinder diesel engine (part of their U-Line), which has been used in the Kia Picanto and the Hyundai i10. Another 1.0-litre three-cylinder petrol engine (part of the Kappa series) has been used in the Hyundai i10 and the Hyundai HB20.
Exclusively for the Indian market, the Chevrolet Beat is offered with a 1.0-litre (936 cc) three-cylinder diesel engine, called XSDE (Xtra Smart Diesel Engine). It was co-developed by GM Powertrain Torino and the GM Technical Center India, and uses the Fiat MultiJet diesel technology.
Ford started offering their new turbocharged 1.0-litre EcoBoost unit in the Focus from 2012. This new engine features several innovations, including the use of an unbalanced flywheel to remove the need to use balancer shafts.
In September 2012, Renault introduced a new turbocharged 0.9-liter TCe engine, that is used in the fourth generation of the Renault Clio, the Renault Captur and in the second generation of the Dacia Sandero and Dacia Logan.
In October 2013, the PSA Peugeot Citroën group inaugurated the production line of their new turbocharged three-cylinder 1.2-litre petrol engine, in Douvrin, France. The engine, branded EB Turbo PureTech, will be first fitted on the Citroën C4 and the Peugeot 308, in the first half of 2014.
Starting from 2014, the Opel Adam will be offered with a new turbocharged three-cylinder engine, mated to a six-speed manual gearbox. This engine, branded SIDI (Spark Ignition Direct Injection), uses direct injection, continuously variable valve timing and has a lightweight aluminium cylinder block.
The Triumph Rocket III, has a 2,294 cc (140.0 cu in) straight-three engine and the company has produced a number of other transversely mounted straight-three engines, such as the water-cooled Daytona 955i, which was also the company's first fuel injected machine; and 1,050 cc Speed Triple.
Following a comparative study at Osaka University's Faculty of Engineering, between in-line and L-shaped motors, Kawasaki Motors decided to develop a range of high performance, two stroke straight-three engines. Between 1969 and 1978, the company produced air-cooled inline, triple engines with capacities of 250 cc, 350 cc, 400 cc, 500 cc, and 750 cc, and the H1-R 500cc and H2-R or KR750 750 cc racing models in both air-cooled and water-cooled forms.
Inline three-cylinder engines are not limited to propelling motor vehicles. The may also be used in general industrial applications. An example is the Fairbanks-Morse 32E14 slow-speed diesel engine which is shown coupled to a water pump.
Inline 3 engines are very common in diesel engined tractors, as well as other agricultural machinery. Nearly all manufacturers of diesel tractors have or have had models with three-cylinder engines on their program. The perhaps best known example of any kind of three-cylinder diesel engine is the Perkins AD3.152 that were used in Massey Ferguson 35 tractors, as well as in Fordson Dexta and several other tractors. This engine was also used for marine and stationary applications. Other examples of manufacturers include Nuffield(BMC), Bolinder-Munktell/Volvo BM, International Harvester, John Deere, Deutz-Fahr, Ford and many others.
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