Straight-5

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The straight-5 or inline-5 is an internal combustion engine with five cylinders aligned in one row, sharing a single engine block and crankcase. This configuration is a compromise between the smaller straight-4 and the larger straight-6.

Straight-5 engines have been used rarely but continuously since their invention. In recent years they have become increasingly popular. They are used in Audis, Volvos and on some diesel-powered Mercedes-Benz models, as well as on the Acura Vigor. Recently, General Motors launched a popular straight-5 engine for its compact trucks (Chevrolet Colorado, GMC Canyon, and the Isuzu i-350). It also powers the Hummer H3. The engine is part of its Atlas family that also includes 4- and 6-cylinder versions. Since the Volvo 850 was introduced in 1991, many Volvos make use of straight-5 engines, often turbocharged. Most of Volvo's current model lineup are powered by inline-5 motors (naturally aspirated-5, T5, and even D5) with exceptions marked otherwise (I6, T6, V8). Volvo engines recently found an application in the Ford Focus ST in which the Ford Focus shares its platform structure with the Volvo S40. Volkswagen has used straight-five engines in their Eurovan, and have recently developed a different straight-five engine which is used in the Jetta and Golf/Rabbit. Fiat also makes use of a 5-cylinder engine (both petrol and diesel) in larger Fiat and Lancia models.

[edit] Characteristics

The 5-cylinder engine's advantage over a comparable 4-cylinder engine is best understood by considering power strokes and their frequency. A 4-cycle engine fires all its cylinders every 720 degrees — the crankshaft makes two complete rotations. If we assume an even firing engine, we can divide 720 degrees by the number of cylinders to determine how often a power stroke occurs. 720 degrees ÷ 4 = 180 degrees, so a 4-cylinder engine gets a power stroke every 180 degrees. A V8 engine gets a power stroke every 90 degrees, (720° ÷ 8 = 90°).

A given power stroke can last no more than 180 degrees of crankshaft rotation, so the power strokes of a 4-cylinder engine are sequential, with no overlap. At the end of one cylinder's power stroke another cylinder fires.

In a 1-, 2-, or 3-cylinder engine there are times when no power stroke is occurring. In a 3-cylinder engine a power stroke occurs every 240 degrees, (720° ÷ 3 = 240°). Since a power stroke cannot last longer than 180 degrees, this means that a 3-cylinder engine has 60 degrees of "silence" when no power stroke takes place.

A 5-cylinder engine gets a power stroke every 144 degrees, (720° ÷ 5 = 144°). Since each power stroke lasts 180 degrees, this means that the power strokes have some overlap and the engine runs more smoothly than a non-overlapping 4-cylinder engine.

A 5-cylinder engine also gains smoothness over a 4-cylinder engine because, with the latter, there is no point when all pistons reverse direction at the same time. A typical inline-4 has the center pistons rising and falling together and the outer pistons rising and falling together. When pistons #1 and #4 are at top dead centre (TDC), #2 and #3 are at bottom dead centre (BDC).

Every cylinder added beyond five increases the overlap of firing strokes and makes for a smoother engine. An inline-6 gets a power stroke every 120 degrees. So there is more overlap (180° - 120° = 60°) than in a 5-cylinder engine (180° - 144° = 36°). However, this increase in smoothness of a 6-cylinder engine over a 5-cylinder engine is not as pronounced as that of a 5-cylinder engine over a 4-cylinder engine. The inline-5 loses less power to friction as compared to an inline-6. It also uses fewer parts, and it is physically shorter, so it requires less room in the engine bay, allowing for transverse mounting.

A 5-cylinder engine is longer and more expensive to manufacture than a comparable 4-cylinder engine, but some manufacturers feel these costs are outweighed by its greater smoothness.

A disadvantage of a straight-5 over a straight-6 is that a straight-5 engine is not inherently balanced. A straight-5 design has free moments (vibrations) of the first and second order, while a straight-6 has zero free moments. This means that no additional balance shafts are needed in a straight-6. By comparison a straight-4 has no free moments of the first or second order, but it does have a large free force of the second order which contributes to the vibration found in unbalanced straight-4 designs. (Information from the 6th edition of the Bosch Automotive Handbook, pages 459-463.)

[edit] Automobile use

The smallest production straight-5 was found on the 1981 Audi 100, with 1921 cc of displacement. The GM Atlas L5R 3700 is the largest at 3,654 cc.

[edit] Motorcycle use

The smallest straight-5 was found on the Honda racing motorcycle, the 125cc RC149, raced in 1966. The five cylinder configuration was used because it shared components (and hence simultaneous development) with the 2-cylinder engine, 50cc, RC116.

Piston engine configurations
Straight	Single, 2, 3, 4, 5, 6, 8, 9, 10, 12, 14
V	2, 4, 5, 6, 8, 10, 12, 16, 20, 24
Flat	2, 4, 6, 8, 10, 12, 16
W	8, 9, 12, 16, 18
Other inline	H, VR, Opposed, U (Square), X
Other	Radial, Rotary, Pistonless (Wankel)