A straight-three engine, also known as inline-three engine, or a triple, (abbreviated I3 or R3) is an reciprocating piston internal combustion engine with three cylinders arranged in a straight line or plane, side by side.
Most inline-three 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, 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 exception to the 120° crankshaft can be found in some of the inline-three 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-two engine. 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 is no power stroke on the final 180° of rotation.
The smallest inline-three engine was the 543 cubic centimetres (33.1 cu in) Suzuki F5A, which was used in the 1982 Cervo. Smart currently produces a diminutive 799 cubic centimetres (48.8 cu in) inline-three diesel engine, the smallest automotive diesel engine in production. 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/Forsa and related Geo/Chevy Metro used an inline-three.
Some Daihatsu cars use inline-three engines. The Charade and the Mira/Cuore used (or are still fitted with) this engine type. Three-cylinder 1.0 litre diesel and turbo diesel 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 was known for using three cylinder petrol- and diesel engines; in the Audi A2, Volkswagen Polo, SEAT Ibiza and Škoda Fabia. The engines in these cars ranged from 1.2 litre petrols with four valves per cylinder that delivered 47 to 65 kilowatts (64 to 88 PS; 63 to 87 bhp), to 1.4 TDI diesels that delivered 51 to 66 kilowatts (69 to 90 PS; 68 to 89 bhp) and had turbo's with variable vane geometry and delivered outstanding economy, this particular engine was used in small cars of all marques of the Volkswagen Group. The most innovative three cylinder engine the Volkswagen Group released was the 1.2 TDI diesel, it was one of the first all-aluminium diesel engines, and at the time of release it was the lightest and most economic engine in production. It won many awards and was used in the "3L" versions (from its diminutive fuel consumption of only 3 litres per 100 kilometres (94.2 mpg-imp; 78.4 mpg-US)) of the Audi A2 and the Volkswagen Lupo.
In the 1950s and 1960s, the Saab 93, Saab 95, Saab 96, and certain Dampf-Kraft-Wagen (DKW) automobiles were powered by inline-three-cylinder, two-stroke engines. Also, the Wartburg automobiles manufactured in Eastern Germany used this kind of engine.
Currently, the only new three-cylinder car available in North America is the Smart fortwo.
| Piston engine configurations|
|Type|| Bourke • Controlled combustion • Deltic •Orbital • Piston • Pistonless (Wankel) •|
Radial • Rotary • Single • Split cycle • Stelzer • Tschudi
|Inline types||H · U · Square four · VR · Opposed · X|
|Stroke cycles||Two-stroke cycle • Four-stroke cycle • Six-stroke cycle|
|Straight||Single · 2 · 3 · 4 · 5 · 6 · 8 · 10 · 12 · 14|
|Flat||2 · 4 · 6 · 8 · 10 · 12 · 16|
|V||4 · 5 · 6 · 8 · 10 · 12 · 16 · 20 · 24|
|W||8 · 12 · 16 · 18|
|Valves|| Cylinder head porting • Corliss • Slide • Manifold • Multi • Piston • Poppet •|
Sleeve • Rotary valve • Variable valve timing • Camless
|Mechanisms|| Cam • Connecting rod • Crank • Crank substitute • Crankshaft •|
Scotch Yoke • Swashplate • Rhombic drive
|Linkages||Evans • Peaucellier–Lipkin • Sector straight-line • Watt's (parallel)|
|Other||Hemi • Recuperator • Turbo-compounding|