Straight-three engine
A straight-three engine is a three-cylinder piston engine where cylinders are arranged in a line along a common crankshaft.
Less common than straight-four engine, 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 and secondary balance, 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.
Usage in cars
One of the first 3 cylinder cars include the 3 cylinder 15hp Rolls Royce produced in 1905. The 1956–1960 Saab 93 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 replaced by the Ford Taunus V4 engine.The Wartburg cars and FSO Syrena 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 Justy and the Sumo.
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.
| Years | Name | Fuel | Notes |
| 1977–1993 | Daihatsu C-series | Petrol | Used in the Daihatsu Charade and Daihatsu Mira/Cuore |
| 1983–2001 | Suzuki G10 | Petrol | Debuted in the Suzuki Cultus/Swift |
| 1986–1995 | VM Motori R392 | Diesel | Turbocharged, used in the Alfa Romeo 33 |
| 1987–present | Mitsubishi 3G8 | Petrol | Debuted in the Mitsubishi Minica |
| 1991–1992 | Elsbett ELKO 3.82.92t | Diesel | Small-scale production engine for biodiesel conversions |
| 1991–2008 | Daewoo S-TEC | Petrol | Used in the Daewoo Tico and Daewoo Matiz |
| 1996–2002 | GM X10XE | Petrol | Debuted in the Opel Corsa |
| 1998–2007 | Mercedes-Benz M160 | Petrol | Turbocharged, used by Smart |
| 1998–2005 | Volkswagen R3 PD TDI 3L | Diesel | Turbocharged, used in the Volkswagen Lupo and Audi A2 |
| 1999–2014 | Mercedes-Benz OM660 | Diesel | Turbocharged, used by Smart |
| 1999–2005 | VM Motori R 315 | Diesel | Turbocharged, debuted in the Hyundai Accent |
| 2000–2006 | Honda ECA1 | Petrol | Used by the Honda Insight hybrid car |
| 2003–present | Mitsubishi 3A9 | Petrol | Debuted in the Mitsubishi Mirage |
| 2004–2011 | Hyundai U engine | Diesel | Turbocharged, debuted in the Kia Picanto |
| 2004–present | Volkswagen R3 | Petrol | Debuted in the Volkswagen Fox |
| 2004–2009 | Mercedes-Benz OM639 | Diesel | Turbocharged, used by the Smart Forfour and Mitsubishi Colt |
| 2004–present | Toyota 1KR-FE | Petrol | Debuted in the Toyota Aygo |
| 2010–present | Nissan HR | Petrol | Some versions supercharged, debuted in the Nissan Micra |
| 2011–2017 | Fiat XSDE | Diesel | Debuted in the India-market Chevrolet Beat |
| 2012–present | Ford EcoBoost | Petrol | Some versions turbocharged, debuted in the Ford Focus |
| 2012–present | BMW B37 | Diesel | Turbocharged, debuted in the Mini |
| 2012–present | Renault TCe | Petrol | Naturally aspirated and Turbocharged, debuted in the Renault Clio IV |
| 2013–present | BMW B38 | Petrol | Turbocharged, debuted in the BMW i8 |
| 2013–present | GM small gasoline engine | Petrol | Turbocharged, debuted in the Opel Adam |
| 2014–present | PSA Group PureTech | Petrol | Naturally aspirated and Turbocharged, debuted in the Peugeot 308 |
| 2018–present | GM E-Turbo | Petrol | Turbocharged; debuted in the Chevrolet Orlando |
| 2018–present | Ingenium AJ150 | Petrol | Turbocharged mild hybrid and plug-in hybrid; debuted in the Range Rover Evoque and Land Rover Discovery |
| 2020–present | Toyota G16E | Petrol | Turbocharged, debuted in the Toyota GR Yaris |
| 2020-present | Koenigsegg TFG engine | Petrol | Turbocharged, Freevalve engine debuted in Koenigsegg Gemera. |
| 2016-present | FCA Fiat Global Small Engine "FireFly" | Petrol | Naturally aspirated and Turbocharged, debuted in the Brazilian version of the 2017 Fiat Uno#Fiat Novo Uno |
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
straight-three engines have been used in road bikes and racing bikes by several companies.From 1985–1995, the BMW K75 was produced with a straight-three engine.
British company Triumph is particularly renowned for a transversely-mounted straight-three engine. Variants have been used in their Speed Triple, Trident, Sprint, and Tiger series.
In addition Triumph makes the Rocket III model, various variants of which have held the record for motorcycle with the largest engine displacement.
In 2019, the Moto2 class in the MotoGP World Championship switched to using Triumph 765 cc triple engines.
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. 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 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.