Straight-three engine

A straight-three engine, also known as an inline-triple, or inline-three (abbreviated I3 or L3), 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°. This gives perfect first and second order balance on reciprocating mass, but an end-to-end rocking motion is induced because there is no symmetry in the piston velocities about the middle piston. The use of a balance shaft reduces this undesirable effect.

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 that utilized 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. 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 inline-three petrol engine. A 1,779 cubic centimetres (108.6 cu in) diesel engine was produced by VM Motori for the 1984 Alfa Romeo 33 1.8 TD, the largest inline-three produced for automotive use. Detroit Diesel Series 71 and Series 53 engines were available in 3 cylinder form for trucks and farm tractors.

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