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 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. It has also recently found an application in the Ford Focus ST. 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 fits under the bonnet better, 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, 12, 16, 18
Other inline	H, VR, Opposed, U (Square), X
Other	Radial, Rotary, Pistonless (Wankel)