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A straight-3 or inline-3 is an internal combustion engine with three cylinders arranged in a straight line side by side.

Most straight-3 engines employ a crank angle of 120° and are thus 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. The use of a balance shaft in antiphase to that vibration produces a smoothly running engine.

An exception to the 120° crankshaft can be found in some of the straight-3 engines made by motorcycle manufacturer Laverda. In these engines, sometimes referred to as 180° triples, the outer pistons rise and fall together like a 360° straight-2. The inner cylinder is offset 180° from the outer cylinders. In these engines cylinder 1 fires, 180° later cylinder 2 fires, and then 180° later cylinder 3 fires. There's no power stroke on the final 180° of rotation--it's like a 4-cylinder engine that's missing the fourth cylinder.

Automobile use

The straight-3 is the smallest engine in use in modern cars, generally for engine displacements of around 1.0L. Cars in the 'super-mini' class often use them on base models, as do the Japanese Keicars. A straight-3 is also used in the Suzuki Swift, its American cousin the Geo Metro, the Subaru Justy, the Honda Insight, and also two stroke Saabs (93 - 96). One experimental Saab had two transverse straight-3 engines, the Saab Monster.

The smallest straight-3 engine was the 543 cc Suzuki F5A used in the 1982 Cervo. Smart currently produces a diminutive 799 cc Diesel straight-3, the smallest ever. Most straight-3 engines fall below 1.2 liters, with a 1198 cc Volkswagen unit seen as the largest petrol unit. A 1.8 L (1779 cc) Diesel was produced by Alfa Romeo for their 1984 33 1.8 TD, the largest straight-3 ever produced.

Motorcycle use

4-stroke :

Triumph, Laverda, Yamaha, BMW.

2-stroke :

Kawasaki, Suzuki

Piston engine configurations
Straight Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14
V 2, 4, 5, 6, 8, 10, 12, 16, 20, 24
Flat 2, 4, 6, 8, 10, 12, 16, H
W 8, 9, 12, 16, 18
Other inline H, VR, Opposed, U (Square), X
Other Hemi, Radial, Rotary, Pistonless, Deltic, (Wankel)

Heat engines
Stroke cycles
Engine types
Gas turbinePistonJetRocket engineSteam engineStirling engineTschudiTwingle
Cylinder head portingD slideFour-strokeManifoldMultiPistonPoppetSleeve
Piston layouts
Single cylinderStraightOpposedFlatVWHDelticRadialRocket engine nozzleRotaryStelzerControlled CombustionBourke
Motion mechanisms
CamConnecting rodCoomber rotaryCrankCrank substituteCrankshaftLinkages (EvansPeaucellier-LipkinSector straight-lineWatt) • Double acting/differential cylinder
Thermodynamic cycle