Straight-five engine

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.^[1] 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.^[2][3]

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.^[4] 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.^[5]

Characteristics

The MAN B&W 5S50MC five-cylinder two-stroke diesel engine. This particular engine is found aboard a 29000 tonne chemical carrier.

Smoothness

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.

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 cylinders 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, which 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 (Audi 2.5, VW 2.5 R5) five-cylinder engines have the firing order 1-2-4-5-3^[6] 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 and in some V6 engines.^[7][8] 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.

Packaging and performance

A five-cylinder engine is longer and more expensive to manufacture than a comparable four-cylinder engine, but some manufacturers like Volvo feel these costs are outweighed by its greater capacity in a smaller space than a six-cylinder.

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 Ford Focus RS Mk2 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.

Balance

A disadvantage of a straight-five over a straight-six engine is that a straight-five engine is not inherently balanced. Any even-firing 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.^[9]

Honda's RC149 GP motorcycle of 1966 used a 125 cc straight 5 engine which was configured like a straight six with one of the middle cylinders missing; this eliminated any rocking couple (and reduced crank torsion due to piston inertia, ideal at the high engine speeds it would be run at) whilst leaving free forces equivalent to a 25 cc single cylinder of similar construction.

Fueling

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 fueling issue was never present in diesel engines (except the Volvo D5) which used fuel injection from the very start, which is why large five-cylinder diesels were commonly seen decades before the type's adoption for automotive use.

Firing order

Four stroke straight-five engine

Common firing orders for inline-fives are 1-2-4-5-3 and 1-5-2-3-4, or their reverses.

Considering even-firing engines only, unlike a four-stroke straight-four, -six or -eight cylinder engine, each five-cylinder crank configuration will support only a single firing order (plus reverse).

By contrast, the standard straight-six crankshaft design, on account of its six throws being distributed at only three unique angles, or phases, can be run on any of four different firing orders, assuming the necessary changes to valve, ignition and fueling (where appropriate) timing systems have been made. That said, very few deviate from 1-5-3-6-2-4 because of the tidy intake and exhaust packaging it offers (see the fueling section above).

A four-stroke inline-five cannot have shared piston phases amongst its cylinders without also being uneven firing, because 720 degrees (4 piston strokes) divided by 5 is 144 which is not a divisor of 360, so it must always have five crankpin phases. In this way, the balance of an inline-five engine is especially and intrinsically linked to its firing order, and vice versa, in that they cannot be chosen separately.

In the case of two-stroke engines, all inline engines are similarly limited in having a single firing order for a given crank configuration; because a complete cycle occurs every 360 degrees, there is no chance to share piston phases without having simultaneous ignitions, so the inline-five is at no disadvantage in this case.

To calculate the total number of firing orders possible requires only simple combinatorics; specifically: circular permutations. Firing orders are cyclic, or circular; so 1-2-3 is the same as 2-3-1 etc. This means the total number of permutations can be reduced by a factor of the number of cylinders; five in this case.

The total number of potential firing orders is therefore:

$\frac{5!}{5} \; = \; 4! \; = \; 4 \times 3 \times 2 \times 1 \; = \; 24$

Where 5! means the factorial of 5.

This also corresponds in a sense to the 24 unique orderings of crank pin phases {0°, 72°, 144°, 216°, 288°} along the crankshaft's length. Remembering that the crankshaft is free to rotate, these phase figures are only relative; using one as a datum, there are only four possible other phases, yielding the same result of 24 permutations.

With no intuitive sense as to which of the possible 24 crankshaft configurations will yield optimal results in terms of balance or crankshaft torsion etc. it was left for engineers to consider each one in turn, at least at the initial theoretical level. However, the possibility space of 24 firing orders has effectively been reduced to just two in practical use.

For four-strokes:

• 1-2-4-5-3 results in the minimal primary (crank speed) rocking couple, and so is the one chosen for inline-fives destined for road cars such as those made by Volvo, VW / Audi, GM and Honda.
• 1-5-2-3-4 results in the maximal primary rocking couple, but minimal secondary imbalance, and rarely finds use in inline-fives. However, it is the order sometimes used for each bank of some V10s e.g. the Dodge Viper and Audi R8.

The 1-5-2-3-4 has also found use in two-stroke outboard engines, e.g. Mercury's Force 150.^[10]

Note that a given crankshaft configuration begets a different firing order for a two-stroke application from that which it dictates in the four-stroke case, and vice-versa. The crankshaft for a firing order of 1-2-4-5-3 in a four-stroke would yield a firing order of 1-5-2-3-4 as a two-stroke. So the crankshaft would have crankpin phases of {0°, 144°, 216°, 288°, 72°} in both cases. Reverse firing orders require a different, mirror-image crankshaft (unless the engine can be permitted to spin backwards).

See the balance section for more.

Sound

The four-stroke inline-five's 5 firing pulses for every two crank rotations generates a distinctive sound, which can be represented by the musical interval 5:2. Engine sounds (even single cylinders), being periodic in nature, occupy a spread of frequencies that can be represented as ascending multiples, as in a Fourier series; this starts with the fundamental frequency (half crank speed) and rises in integer multiples, with each multiple being present in differing amounts in the final sound. Taking the octave of one pitch in an interval results in a related interval (i.e. through inversion); this is like selecting a higher multiple in the previously described series of frequencies. Such an inverted interval might be 5:4, via 5:2; i.e. a major third of the just kind. This is purely harmonic, but it is not too far removed from the 12-tone-equal-tempered major third found in much of western music.

Other intervals for other even-firing engines of differing cylinder counts can similarly be discovered. E.g. triples and derivatives (6, 12 cylinders etc): 3:2; inline-four and derivatives: 2:1; inline-seven: 7:4; and so on.

Automobile use

An Audi 2.3 NG engine, mounted longitudinally

A Volvo B5244S engine, mounted transversely

Diesel

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. Mercedes-Benz continued to use five-cylinder diesel engines until 2006, when the OM612 and OM647 engines ended production, however SsangYong Motor Company continues to use them in their SUVs.

Audi produced a number of five-cylinder diesel engines for the Audi 100 and Audi A6 from 1978 to 1997. This engine is used in several Volvos and Volkswagen vans.

Volvo designed their own D5 engine which is available since 2001 in most of their vehicle line.

The Jeep Grand Cherokee was available with straight-five diesels in Europe. From 1999 till 2001 with a 3.1l VM Motori engine and from 2002 till 2004 with a 2.7 liter Mercedes-Benz engine.

In the late 1990s, Rover Group developed an in-house straight-five diesel engine, the Td5, for the Land Rover Discovery and Defender.

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.

Fiat produced a turbocharged 2.4 L straight-five diesel engine in the 90's. This engine was used in the Fiat Marea, Lancia Kappa, Lancia Lybra, Lancia Thesis, Alfa Romeo 156 and Alfa Romeo 159.

Petrol

The first moves towards a petrol straight-five were made by Rover, which developed a 2.5-litre five-cylinder engine in 1966 as a means of supplying more power for the Rover P6 saloon. However the design, fuelled by three carburettors (one of a different size to the other pair), suffered fuelling problems and did not proceed past the prototype stage.

Audi unveiled the first production 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.^[11] Audi used 2.2L straight-five engines in many of their cars between 1976 and the early 1990s, including the 305 hp Audi Quattro and the Audi RS2 Avant with 315 hp. Audi Inline 5 engine is used extensive in Motorsport, particularly in Rally in the famous Group B with 650 hp Audi S1 Sport Quattro E2 and in IMSA GT Championship with 2.1 20V inline 5 720 hp Audi 90 quattro IMSA GTO.^{[12][13][14][15][16]}

For the year 1987 factory team tested a 1000 hp version of the inline-5 powered Audi S1 Sport Quattro,^[17] but that prototype was never used in racing followed by abolition of Group B.

An Audi five-cylinder engine as a power choice is very popular in European Drag Racing Championship, particularly in Scandinavian countries. With extreme modification for Drag Racing, the Audi produced 2.2 20V Inline 5 engines (like 3B, AAN, ABY, ADU) is shown to be capable of power in excess of 1 megawatt (1,340 hp)^[18][19]

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 being used in Ford's Focus ST and RS models. All of the inline-5 petrol engines used by Volvo and Ford are built at the Volvo Skövde engine plant in Skövde, Sweden.

Honda produced the G series five-cylinder engine beginning in 1989 and used it in the Vigor, Inspire, Ascot, Rafaga, and Acura TL.

Volkswagen has used straight-five engines in their Eurovan, and have recently developed a different straight-five engine which is used in the Jetta, Passat, Golf, Rabbit, and New Beetle in North America. In South Africa, the well known VW Microbus/Caravelle (a passenger carrier) was later equipped with an Audi 5-cylinder 2.5 L (1991-1994) and still later a 2.6 L (1995-2002) water cooled engine, replacing the old air cooled Volkswagen 1,800 and 2,100 cc power plants that gave substandard performance to the large family bus. The tall straight-five Audi engine had to be slanted to fit below a rear engine deck, employing an extra electric cooling blower on shut-down, so as to prevent warping of the hot aluminium head and inlet manifold.

Fiat also makes use of his own straight-fives petrol engines in European Fiat Marea (2.0 L, 20v, 155 hp), Fiat Bravo, Fiat Coupé (2.0 L, 20v, turbocharged, 220 hp) and Fiat Stilo (2.4 L, 20v, 170 hp) and in Brazilian Fiat Marea with 3 variants: 2.0L, 20v, N/A, 142 hp; 2.0L, 20v, turbocharged, 182 hp and 2.4 L, 20v, N/A, 162 hp.

Lancia also uses the straight-five Fiat engines in late Lancia Kappa, Lancia Lybra and Lancia Thesis.

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.

Motorcycle use

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.^[20]

References

1. "Henry Ford's Weird Old Engines", Popular Science, August 1960, p. 195
2. Storia Illustrata del Camion Italiano (in Italian). Edizione Neri. ISBN 978-88-900 955-8-0.
3. Storia Illustrata del Camion Italiano Edizione Neri ISBN 978-88-900 955-8-0
4. Graham Robson. Audi Quattro. p. 12.
5. http://driventowrite.com/2014/08/21/theme-of-the-month-engines-throbby-thrummy-quints/
6. Firing Order of Cylinders
7. Z32 300ZX service manual, , p53, 05/10/14
8. Article on Chevrolet engine firing orders , 05/10/14
9. Robert Bosch GmbH, Bosch Automotive Handbook, 6th edition p. 459-463, Professional Engineering Publishing 2004, ISBN 1-86058-474-8.
10. http://www.boats.net/parts/search/Merc2/Force/150%20H.P.%20%281989-1992%29/1989-A%20THRU%201992-C/CRANKSHAFT%20-%20PISTON/parts.html
11. "Cars++ 1985 Audi 100 1.9 page". Carsplusplus.com. Retrieved 2011-01-17.
12. de:Audi 90 quattro IMSA GTO
13. http://www.classicdriver.com/de/article/audi-90-quattro-imsa-gto
14. http://www.automobile-catalog.com/car/1989/1588910/audi_90_quattro_imsa-gto.html
15. http://www.ableitet.no/mod/audi-90-imsa-gto_en.htm
16. http://www.speedhunters.com/2009/05/retrospective_gt_gt_quattro_firepower_the_imsa_audi_90_gto/
17. http://www.thesmokingtire.com/2012/secret-rally-car-audi-group-s-prototype/
18. http://www.musclecarszone.com/best-sleepers-ever-1100hp-16vampir-vw-golf-vs-audi-90-1250-hp-crazy-drag-race/
19. http://audi-motorsport-blog.blogspot.com/2012/09/worlds-fastest-audi-rs4-with-1100-hp-is.html
20. "Joep Kortekaas' Honda Race History - 1966". Vf750fd.com. Retrieved 2011-01-17.