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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-6.
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. Consequently, 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.
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 its cylinders once 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. 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 stokes for each revolution of the crankshaft.
A given power stroke can last no more than 180 degrees of crankshaft rotation, so the power strokes of a four-cylinder engine are sequential, with no overlap. At the end of one cylinder's power stroke another cylinder fires.
In a one, two, or three cylinder engine there are times when no power stroke is occurring. In a three-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 three-cylinder engine has 60 degrees of "silence" when no power stroke takes place.
Five-cylinder engines have a crank with 72 degree angles (except for the VW V5, that has an offset in the crank that corresponds to the angle between the cylinders. Despite the V configuration the engine has even firing intervals.). Most five cylinder engines have the firing order 1-2-4-5-3. Firing of one cylinder after another (e.g. 1-2-3-4-5 in case of a five cylinder engine) is never used except in 3 cylinder engines where there is no alternative. The reason for this is that the resulting engine will have a strong tendency of rocking from end to end, as well as having generally poor balance.
A five-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 five cylinders, there are 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 power strokes have some overlap, a five-cylinder engine may run more smoothly than a non-overlapping four-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-six gets a power stroke every 120 degrees. So there is more overlap (180° - 120° = 60°) than in a five-cylinder engine (180° - 144° = 36°). However, this increase in smoothness of a six-cylinder engine over a five-cylinder engine is not as pronounced as that of a five-cylinder engine over a four-cylinder engine. The inline-five loses less power to friction as compared to an inline-six. It also uses fewer parts, and it is physically shorter, so it requires less room in the engine bay, allowing for transverse mounting.
A five-cylinder engine is longer and more expensive to manufacture than a comparable four-cylinder engine, but some manufacturers feel these costs are outweighed by its greater capacity in a smaller space than a six-cylinder.
From the standpoint of driving experience, five-cylinder engines are noted for combining the best aspects of four- and six-cylinder engines. They generate more power and torque than four-cylinder engines, while maintaining better fuel economy and "pep" than six-cylinder engines. Five-cylinder turbos have been used on more than one occasion in sport and racing applications for their balance of performance qualities. The Volvo S60 R has a 2.5 litre turbocharged inline-five–cylinder engine which is capable of generating 300 brake horsepower (224 kW) and 295 lbf·ft (400 N·m) of torque across a large amount of its rpm ranges. The new Ford Focus RS performance car uses the same Volvo 5-cylinder engine, developed (by Ford) to very similar power levels, and is one of the most powerful FWD production cars ever created. Another example of a high power 5 cylinder car is the Audi RS2, with its 2.2 turbocharged engine making 311 hp.
A disadvantage of a straight-five over a straight-six engine is that a straight-five engine is not inherently balanced. A straight-five design has free moments (vibrations) of the first and second order, while a straight-six has zero free moments. This means that no additional balance shafts are needed in a straight-six. By comparison an inline-four engine 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-four designs.
The use of straight-five petrol engines in mass production cars only became truly viable with the advent of reliable fuel injection. A five-cylinder engine using a carburetor fuel system has an unavoidable problem in that the length of the inlet manifold between the carburetor varies too greatly between cylinders at the ends of the engine and those nearer the carburetor for reliable and consistent fuel delivery. Using multiple carburetors (two or three) always results in one carburetor feeding more cylinders than the other, which also produces running and tuning problems. In theory individual carburetors could be used for each cylinder, but this approach is expensive and still brings with it the attendant difficulties in balancing the multiple carbs. Multi-point fuel injection circumvents all the above problems by feeding each cylinder individually from a central, single pump. This fuelling issue was never present in diesel engines (except the Volvo D5) which used direct injection from the very start, which is why large five-cylinder diesels were commonly seen decades before the type's adoption for automotive use.
The inline-five is known for its characteristic sound. Unlike other engine configurations, the inline-five's sound can be approximated by doubling the cylinders.
The first production straight-five engine for a passenger vehicle was the Mercedes-Benz OM617, a 3.0 L diesel engine introduced in 1974 and used in the Mercedes-Benz 300D. It was first turbocharged in 1978. Mercedez-Benz continued to use five-cylinder diesel engines until 2006, when the OM612 and OM647 engines ended production.
Since 2006, Ford has produced a turbocharged 3.2 L five-cylinder engine under the Duratorq name for the Ford Transit, Ford Ranger, and Mazda BT-50. The same engine will be sold in the North American Transit under the Power Stroke name.
Audi produced the first petrol straight-five, a 2.1 L SOHC engine, in the 1976 Audi 100. In 1981, the Audi 100 also became available with the smallest production straight-five, a 1.9 L variant. Audi used straight-five engines in many of their cars between 1976 and the early 1990s, including the Audi Quattro and the Audi RS2 Avant. An Audi five-cylinder engine was also used in the Italdesign Aztec concept car. In 2009, Audi began using a new turbocharged five-cylinder engine in the Audi TT RS and later the Audi RS3 and the Audi quattro concept.
Since Volvo introduced their Volvo 850 in 1992, much of their line-up has switched to their five-cylinder Modular engine, with their engines also seeing use in Ford's Focus ST and RS models. All of the inline five engines used by Volvo and Ford are built at the Volvo Skövde engine plant in Skövde, Sweden.
General Motors's Atlas family of inline engines included two straight-fives, a 3.5 and a 3.7 L, used in their GMT 355 mid-size truck family (including the Chevrolet Colorado and the Hummer H3) from 2004 to 2012.
The smallest straight-five was found on the Honda racing motorcycle, the 125-cc-class RC149, raced in 1966. The five-cylinder configuration was used because it shared components (and hence simultaneous development) with the two-cylinder engine, 50 cc, RC116.[better source needed]