Straight-5

The straight-five engine or inline-five engine is an internal combustion engine with five cylinders aligned in one row or plane, sharing a single engine block and crankcase. This configuration is a compromise between the smaller inline-four engine and the larger straight-six.

Henry Ford had an inline-five engine developed in the late 1930s to early 1940s for a compact economy car design, which never saw production due to lack of demand for small cars in the United States.Lancia of Italy developed a 5-cylinder diesel engine in the late 1930s for use in their RO truck series to replace the earlier 2-cylinder diesel and 3-cylinder petrol engines used. The subsequent model became known as the 3RO and was used by both Italian and German armed forces during WW2. This truck remained in production until 1950.

A straight-five engine did not see production for passenger cars until Mercedes-Benz introduced the OM617 diesel in 1974. The first production petrol straight-five was the 2.1 R5 introduced by Volkswagen Group in the Audi 100 towards the end of the 1970s, developments of which powered the Audi Quattro rally racer. An analysis of their use shows they were often used by middle-market brands seeking to extend their engine ranges into higher capacities without going for the six-cylinder option. In recent years the engine has been falling out of favour, with Volvo announcing in 2014 it would discontinue building them, for example.

The five-cylinder engine's advantage over a comparable four-cylinder engine is best understood by considering power strokes and their frequency. A four-stroke cycle engine fires each cylinder once every 720 degrees — the crankshaft makes two complete rotations. Assuming an even firing engine, divide 720 degrees by the number of cylinders to determine how often a power stroke occurs. For a four-cylinder engine, 720° ÷ 4 = 180° so there is a power stroke every 180 degrees, which is two power strokes per revolution of the crankshaft. A V8 engine gets a power stroke every 90 degrees: 720° ÷ 8 = 90°, which is four power strokes for each revolution of the crankshaft.

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Wikipedia