Straight-5

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.

Historically, straight-5 engines have been used rarely but continuously since their invention, though they have become increasingly popular in recent years. They are used in Audis, VWs, Volvos and on some diesel-powered Mercedes-Benz models, as well as on the Acura 2.5 TL and Vigor. Recently, General Motors launched a straight-5 engine for its popular compact trucks which is a derivative of the inline 5 cylinder engine used in top of the range Fiat models (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 Rabbit in North America. Audi's inline-5 powered many of the company's bigger models in the 80s, and was most notably used (in a turbocharged format) in the Audi Quattro racing car. Fiat also makes use of a 5-cylinder engine (both petrol and diesel) in larger Fiat and Lancia models.

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 a power stroke is always in effect. Because of uneven levels of torque during the expansion strokes divided among the 5 cylinders, there is increased secondary-order vibrations. At higher engine speeds, there is an uneven third-order vibration from the crankshaft which occurs every 144 degrees. Because the the power strokes have some overlap, a 5-cylinder engine may run more smoothly than a non-overlapping 4-cylinder engine, but only at limited mid-range speeds where second and third-order vibrations are lower.

Every cylinder added beyond five increases the overlap of firing strokes and makes for less primary order vibration. 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 capacity in a smaller space than a 6-cylinder.

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.Robert Bosch GmbH, "Bosch Automotive Handbook, 6th edition" p. 459-463, Professional Engineering Publishing 2004, ISBN 1860584748.]

Automobile use

The smallest production straight-5 was found on the 1981 Audi 100, with 1921 cc of displacement. [http://www.carsplusplus.com/specs1985/audi_100_19.php Cars++ 1985 Audi 100 1.9 page] ] Audi used straight-5 engines in many of their cars between 1980 and the early 1990s. Mercedes used 5 cyl configuration for their diesels in the 70s all the way to the 90s. The GM "Atlas L5R 3700" is the largest at 3,654 cc. Currently Volvo Cars uses the straight 5 cylinder engine as their mainstay engine in every vehicle in the lineup. All of the inline 5 engines used by Volvo and Ford are built at the Volvo Skövde engine plant in Skövde, Sweden.

Motorcycle use

The smallest straight-5 was found on the Honda racing motorcycle, the 125cc-class RC149, raced in 1966. The five cylinder configuration was used because it shared components (and hence simultaneous development) with the 2-cylinder engine, 50 cc, RC116. [http://www.vf750fd.com/Joep_Kortekaas/1966.html#RC149 Joep Kortekaas' Honda Race History - 1966] ]

References