A straight-two engine, parallel twin or inline twin is a two-cylinder piston engine that has its cylinders arranged in a single row. There are two primary crankshaft configurations for this engine with a new third configuration. One has both pistons moving identically (360° offset) and is commonly called parallel twin. An inline twin or straight-two has one piston moving up as the other moves down (180° offset). The third and new concept offsets the pistons 90° from each other similar to a V-Twin or a crossplane (American) V8.
Straight-two engines regardless of configuration do not use a common crank pin for both cylinders. Even in a parallel twin each cylinder has its own crank pin, they are just set at the same angle.
In the past, straight-two engines have been used in very small cars (e.g. Microcars, "light" cars, and City cars like the Fiat 500 and Mitsubishi Minica) and in farm equipment, notably by John Deere whose large two-cylinder engines were used in their line of farm tractors up until 1960.
From 1967 to 1972 Honda produced the N360 and its successors N400 and N600 with straight-two engines in 360cc, 400cc, and 600cc sizes. The Z600 was produced from 1970 to 1972. From 1958 to 1971, Subaru produced the 360 with a rear mounted, rear drive 358cc air-cooled engine.
The only current production car to use an inline twin engine is the Tata Nano, announced 10 January 2008, which has a 623 cc engine with a single balance shaft. Most cars now use at least a straight-3 because of its better power characteristics.
Ferrari briefly considered creating an inline two-cylinder engine for Formula One use in the 1950s. Aurelio Lampredi worked with Enzo and Dino Ferrari on this design but abandoned development due to unsatisfactory balance. It is believed that all the prototypes built simply exploded during tests.
- A detailed analysis of the effect of different crankshaft offset angles on the engine balance of a straight twin engine.