Straight-two

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A straight-two engine, parallel twin or inline twin is a two-cylinder piston engine that has its cylinders arranged in a single row. It is often improperly referred to as a "straight twin". A true straight twin (or parallel twin) engine uses a common crank pin for both cylinders.

Straight-two engine designs are mostly used on motorcycles, eg. Edward Turner's 1937 Triumph Speed Twin and later British twins. In the past they have also been used in very small cars (eg. Microcars like the Fiat 500) and in farm equipment, notably by John Deere whose large two-cylinder engines were used in their line of farm tractors up until 1960.

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 the better power characteristics.

An example of a British four stroke "parallel twin" engine, as fitted to a 1974 Norton Commando MKIIA 850 Interstate, photographed outside the workshop of a well known Commando racer and specialist, adjacent to the Thruxton race circuit

Most of the British four-stroke cycle inline two-cylinder engines had a crank angle of 360°, which means that both pistons are always in the same position as each other and move in same direction. This leads to a working cycle every 360°. The mechanical balance of this design is no better than that of a similar displacement single-cylinder engine, because the forces of both cylinders are compounded; however, the advantage is that the firing is regular, with one cylinder firing each revolution of the crankshaft rather than every second revolution.

Japanese motorcycles, with the exception of Yamaha, use a crank angle of 180°. This leads to fewer vibrations but uneven firing, resulting in these engines having a distinctive exhaust note.

In two-stroke cycle engines, the crank angle is generally 180° with a working cycle every 180°. Such an engine will produce fewer vibrations.

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.

[edit] External links

• A detailed analysis of the effect of different crankshaft offset angles on the engine balance of a straight twin engine.