thumb|[[Engine block of an Elsbett straight-three diesel engine]]
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.
Historically less common than straight-four engines, straight-three engines have been used in small and mid-sized vehicles, motorcycles, and agricultural machinery. Their use has increased since the 2020s as part of a broader industry trend toward smaller, turbocharged engines to meet fuel economy and emissions requirements. By the mid-2020s, they accounted for a small but growing share of new vehicles, after being rare prior to 2020.
Compared to straight-four engines, straight-three engines are typically lighter and have fewer moving parts, which can improve efficiency, but have limitations in performance and are prone to vibration, which have largely been overcome by advances in engine and vehicle design.
Design
thumb|[[Four-stroke engine|Four-stroke straight-three engine with firing order 1-3-2]]
Compared with straight-four engines, which always have a cylinder on its power stroke, straight-three engines have intervals in which no cylinder is on its power stroke. As a result, individual combustion events must be stronger to achieve comparable output. producing evenly spaced firing intervals. This configuration provides perfect primary and secondary balance in the reciprocating-plane, but primary and secondary rotating-plane imbalances are present. This manifests as an end-to-end rocking couple due to asymmetry in piston motion about the center cylinder, resulting in pronounced low-frequency vibrations that can be difficult to isolate. These can be mitigated through measures such as balance shafts and engine mounts designed to limit their transmission to the vehicle structure.
Straight-three engines can offer improved fuel efficiency due to their smaller size, lower mass, and reduced internal friction from having fewer moving parts. For a given displacement, each cylinder is also larger than in a straight-four engine, allowing larger valves and improved airflow. 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.
Historically, straight-three engines have been less popular than straight-four engines in small and mid-sized vehicles. However, their use increased in the 2020s as part of a broader industry trend toward smaller, turbocharged engines to meet fuel economy and emissions requirements. In the United States, three-cylinder engines grew from less than 1% of new vehicles before 2020 to about 5% by the 2025 model year, roughly matching the share of eight-cylinder gasoline engines, while four-cylinder engines remained dominant at 60% of the market in 2025.
{| class="wikitable"
|+ Notable straight-three engines
|-
! Years !! Name !! Fuel!! Notes
!Ref.
|-
| 1977–1993 || Daihatsu C-series || Petrol ||
|
|-
| 1983–2001 || Suzuki G10 || Petrol ||
|
|-
| 1986–1995 || VM Motori R series|| Diesel || Turbocharged
|
|-
| 1987–present || Mitsubishi 3G8 || Petrol ||
|
|-
|1988–present
|Honda E0 series
|Petrol
|Some versions turbocharged
|
|-
| 1991–2008 || Daewoo S-TEC || Petrol ||
|
|-
| 1996–2002 || GM X10XE || Petrol ||
|
|-
| 1998–2007 || Mercedes-Benz M160 || Petrol || Turbocharged
|
|-
| 1998–2005 || Volkswagen R3 PD TDI 3L || Diesel || Turbocharged
|
|-
| 1999–2014 || Mercedes-Benz OM660 || Diesel || Turbocharged
|
|-
| 1999–2005 || VM Motori R 315 || Diesel || Turbocharged
|
|-
| 2000–2006 || Honda ECA1 || Petrol || Mated to IMA hybrid system
|
|-
|2003–present
|Honda P series
|Petrol
|Some versions turbocharged
|
|-
| 2003–present || Mitsubishi 3A9 || Petrol ||
|
|-
| 2004–2011 || Hyundai U engine || Diesel || Turbocharged
|
|-
| 2004–present || Volkswagen R3 (EA111) || Petrol ||
|
|-
| 2004–2009 || Mercedes-Benz OM639 || Diesel || Turbocharged
|
|-
| 2004–present || Toyota 1KR-FE || Petrol ||
|
|-
| 2010–present || Nissan HR || Petrol || Some versions supercharged
|
|-
| 2011–2017 || Fiat XSDE || Diesel ||
|
|-
| 2012–present || BMW B37 || Diesel || Turbocharged
|
|-
| 2012–present
| Ford EcoBoost
| Petrol
| Some versions turbocharged
|
|-
|2012–present
|Honda S series
|Petrol
|Some versions turbocharged
|
|-
| 2012–present || Renault TCe || Petrol ||Some versions turbocharged
|
|-
| 2013–present || BMW B38 || Petrol || Turbocharged
|
|-
| 2013–present || GM small gasoline engine || Petrol || Turbocharged
|
|-
| 2014–present || PSA Group PureTech || Petrol ||Some versions turbocharged
|
|-
|2016–present
|Fiat Global Small Engine
|Petrol
|Some versions turbocharged
|
|-
| 2018–present || GM E-Turbo || Petrol || Turbocharged
|
|-
| 2018–present || Ingenium AJ150 || Petrol || Turbocharged
|
|-
| 2020–present || Toyota G16E|| Petrol || Turbocharged
|
|-
| 2020–present || Koenigsegg TFG || Petrol || Turbocharged
|
|}
Usage in motorcycles
thumb|right|1969–1975 [[Kawasaki H1 Mach III ]]
thumb|right|2004–present [[Triumph Rocket III engine ]]
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.
From 1985–1995, the BMW K75 was produced with a straight-three engine (based on the straight-four engine from the BMW K100).
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. <!-- not sure, but it looks like a longitudinal inline. Therefore split off the sentence. -->
In 2019, the Moto2 class in the MotoGP World Championship switched to using Triumph 765 cc (46.7 cu in) triple engines.
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. 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.
See also
- Straight engine
- V3 engine