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V6 engine

A V6 engine is a V engine with six cylinders mounted on the crankcase in two banks of three cylinders, usually set at either a right angle or an acute angle to each other, with all six pistons driving a common crankshaft. It is the second most common engine configuration in modern cars after the inline four.

The V6 is one of the most compact engine configurations, shorter than the straight 4 and in many designs narrower than the V8 engine, and is well suited to the popular transverse engine front-wheel drive layout. It is becoming more common as the space allowed for engines in modern cars is reduced at the same time as power requirements increase, and has largely replaced the inline-6, which is too long to fit in many modern engine compartments. Although it is more complicated and not as smooth as the inline 6, the V6 is more compact, more rigid, and less prone to torsional vibrations in the crankshaft. The V6 engine has become widely adopted for medium-sized cars with engine displacements between 2.5 and 4.0 litres (146-242 CID), often as an optional engine where a straight-4 is standard, or as an economy engine where a V8 is a higher-cost option.

The first V6 was introduced by Lancia in 1950 with the Lancia Aurelia. Other manufacturers took note and soon other V6 engines were in use. In 1959, GM introduced a heavy duty 305 cubic inch (5 L) 60-degree V6 for use in their pickup trucks and Suburbans, an engine design that was later enlarged to 478 cubic inches (7.8 L) for heavy truck and bus use.

The design really took off after the 1962 introduction of the Buick Special, which offered a 90 degree V6 with uneven firing intervals that shared some parts commonality with a small Buick V8 of the period. Though the Buick Special met consumer resistance due to its excessive vibration, it was the first instance of a mass-produced V6 engine designed specifically for passenger automobiles. In 1983 Nissan produced Japan's first V6 engine with the VG series. A very popular V6 engine with a large fanbase is the Alfa Romeo Arese V6 designed by Giuseppe Busso.

Modern V6 engines commonly range in displacement from 2.5 L to 4.3 L (150-262 CID), though larger and smaller examples have been produced.

Balance and smoothness

Due to the odd number of cylinders in each bank, V6 designs are inherently unbalanced and can benefit from some auxiliary counterbalancing. A V6 is basically two straight-3 engines running on the same crankshaft, and since the straight-3 suffers from a primary dynamic imbalance which causes an end-to-end rocking motion, the V6 also suffers from the same problem unless steps are taken to mitigate it.

Unlike a 90° V8 with crossplane crankshaft, a V6 cannot be laid out so that the vibrations from the two banks completely cancel each other. This makes designing a smooth engine a much more complicated affair. When Lancia pioneered the V6 in 1950, they used a 60° angle between the cylinder banks and a six-throw crankshaft to achieve equally spaced firing intervals of 120°. This still has some balance and secondary vibration problems. When Buick designed a 90° V6 based on their 90° V8, they initially used a simpler three-throw crankshaft laid out in the same manner as the V8 with pairs of connecting rods sharing the same crankpin, which resulted in firing intervals alternating between 90° and 150°. This produced a rough-running design which was unacceptable to many customers. Later, Buick and other manufacturers refined the design by using a split-pin crankshaft which achieved a regular 120° firing interval by staggering adjacent crankpins by 15° in opposite directions to eliminate the uneven firing and make the engine reasonably smooth. Some manufacturers such as Mercedes Benz have taken the 90° design a step further by adding a balancing shaft to offset the primary vibrations and produce an almost fully balanced engine.

Some designers have reverted to a 60° angle between cylinder banks, which produces a more compact engine, but have used three-throw crankshafts with flying arms between the crankpins of each throw to achieve even 120° angles between firing intervals. This has the additional advantage that the flying arms can be weighted for balancing purposes. This still leaves an unbalanced primary couple, which is offset by counterweights on the crankshaft and flywheel to leave a small secondary couple, which can be absorbed by carefully designed engine mounts.

Six-cylinder designs are also more suitable for larger displacement engines than four-cylinder ones because power strokes of pistons overlap. In a four-cylinder engine, only one piston is on a power stroke at any given time. Each piston comes to a complete stop and reverses direction before the next one starts its power stroke, which results in a gap between power strokes and noticeable vibrations. In a six-cylinder engine (other than odd-firing V6s), the next piston starts its power stroke 60° before the previous one finishes, which results in smoother delivery of power to the flywheel. In addition, because inertial forces are proportional to the cube of the piston mass and the square of the piston speed, high-speed six-cylinder engines will suffer less stress and vibration than four-cylinder ones of equal displacement.

Comparing engines on the dynamometer, a typical even-fire V6 shows instantaneous torque peaks of 150% above mean torque and valleys of 125% below mean torque, with a small amount of negative torque (engine torque reversals) between power strokes. On the other hand, a typical four-cylinder engine' shows peaks of nearly 300% above mean torque and valleys of 200% below mean torque, with 100% negative torque being delivered between strokes. In contrast, a V8 engine shows peaks of less than 100% above and valleys of less than 100% below mean torque, and torque never goes negative. The even-fire V6 thus ranks between the four and the V8, but closer to the V8, in smoothness of power delivery. An odd-fire V6, on the other hand, shows highly irregular torque variations of 200% above and 175% below mean torque, which is significantly worse than an even-fire V6, and in addition the power delivery shows large harmonic vibrations that have been known to destroy the dynamometer.

V angles

60 degrees

The most efficient cylinder bank angle for a V6 is 60 degrees, which design is very compact and tends to minimize vibrations. While they are not as well balanced as inline-6 and flat-6 engines, modern techniques for designing and mounting engines have largely disguised their vibrations. Unlike most other angles, 60 degree V6 engines can be made acceptably smooth without the need for balance shafts. When Lancia pioneered the 60° V6 in 1950, they used a 6-throw crankshaft to give equal firing intervals of 120°. However, more modern designs often use a 3-throw crankshaft with what are termed flying arms between the crankpins, which not only give the required 120° separation but also can be used for balancing purposes. Combined with a pair of heavy counterweights on the crankshaft ends, these can eliminate all but a modest secondary imbalance which can easily be damped out by the engine mounts.

This configuration is a good fit in cars which are too big to be powered by four-cylinder engines, but for which compactness and low cost are important. The most common 60° V6s were built by General Motors (the heavy duty commercial models, as well as a design used in many GM front wheel drive cars) and Ford European subsidiaries (Essex V6, Cologne V6 and the more recent Duratec V6). Other 60° V6 engines are the Chrysler 3.3 V6 Engine, Nissan VQ engine and the Alfa Romeo V6 engine.

90 degrees

90-degree V6 engines are also produced, usually so they can use the same production-line tooling set up to produce V8 engines (which normally have a 90-degree V angle). Although it is relatively easy to derive a 90-degree V6 from an existing V8 design by simply cutting two cylinders off the engine, this tends to make it wider and more vibration-prone than a 60-degree V6. The design was first used by Buick when it introduced its 198 CID Fireball V6 as the standard engine in the 1962 Special. Other examples include the Maserati V6 used in the Citroën SM, the PRV V6, Chevrolet's 4.3 L Vortec 4300 and Chrysler's 3.9 L (238 CID) Magnum V6 and 3.7 L (226 CID) PowerTech V6. The Buick V6 was notable because it introduced the concept of uneven firing, as a result of using the 90 degree V8 cylinder angle without adjusting the crankshaft design for the V6 configuration. These engines were often referred to by mechanics as "shakers," due to the tendency of the engine to bounce around at idle speed. More modern 90-degree V6 engine designs avoid these vibration problems by using crankshafts with offset split crankpins to make the firing intervals even, and often add balancing shafts to eliminate the other vibration problems. An example is the 90-degree Mercedes-Benz V6 which, although designed to be built on the same assembly lines as the V8, uses split crankpins, a counter-rotating balancing shaft, and careful acoustic design to make it as smooth and quiet as the inline-6 it replaced.

120 degrees

120 degrees might be described as the natural angle for a V6 since the cylinders fire every 120 degrees of crankshaft rotation. Unlike the 60 degree or 90 degree configuration, it allows pairs of pistons to share crank pins in a three-throw crankshaft without requiring flying arms or split crankpins to be even-firing. However, it has few other advantages in a road car. The 120° layout produces an engine which is too wide for most automobile engine compartments, and it suffers from a primary dynamic imbalance which requires a balance shaft to offset. It is more often used in racing cars where the car is designed around the engine rather than vice-versa, and vibration is not as important. On the other hand, the flat-6 boxer engine is only moderately wider, and unlike the V6 is a fully-balanced configuration with no vibration problems, so it is more commonly used in aircraft and in sports/luxury cars where space is not a constraint and smoothness is important.

Ferrari introduced a very successful 120° V6 racing engine in 1961. The Ferrari Dino 156 engine was shorter and lighter than the 65 °Ferrari V6 engines that preceded it, and the simplicity and low center of gravity of the engine was an advantage in racing. It won a large number of Formula 1 races between 1961 and 1964. However, Enzo Ferrari had a personal dislike of the 120° V6 layout, preferring a 65° angle, and after that time it was replaced by other engines.

Bombardier has designed 120° V220/V300T V6 engines for use in light aircraft. The ignition sequence is symmetrical, with each cylinder firing 120 degree after the previous cylinder resulting in smooth power delivery. A balance shaft on the bottom of the engine offsets the primary dynamic imbalance intrinsic in any V6 layout. The straight, pin-type crankshaft journals in the 120° V-6 layout allow a shorter and stiffer crankshaft than competing flat-6 engines, while water cooling results in better temperature control than air cooling. These engines have the additional advantage that they can run on automotive gasoline rather than avgas, but have been produced only in limited quantities to date.

Other angles

Narrower angle V6 engines are very compact but can suffer from severe vibration problems unless very carefully designed. Notable V6 bank angles include:

  • The 10.6° and 15° Volkswagen VR6, which is such a narrow angle it can use a single cylinder head and double overhead camshafts for both cylinder banks. With 7 main bearings, it is more like a staggered-bank in-line six rather than a normal V6, but is only slightly longer and wider than a straight-4.
  • The 45° Electro-Motive 6 cylinder version of their model 567 Diesel locomotive engine.
  • The 54° GM/Opel V6, designed to be narrower than normal for use in small front-wheel drive cars.
  • The 65° Ferrari Dino V6. A 60° angle limited the size of the carburetors which originally used in the engine, while a 65° angle allowed larger carburetors at the expense of a slight increase in vibrations.
  • The 75° Isuzu Rodeo and Isuzu Trooper V6 of 3.2 and 3.5 liters in both SOHC and DOHC versions.

Odd and even firing

Many older V6 engines were based on V8 engine designs, without altering the V angle or using a more sophisticated crankshaft to even out the firing interval. One characteristic of these engines was a notorious odd-firing behavior.

Purpose-built V6 engines use one crankpin per cylinder for an even 120° ignition pattern. In contrast, most V8 engines share a common crankpin between opposite cylinders in each bank. That is, the crankshaft has just four pins for eight cylinders, and a cylinder fires every 90° for smooth operation.

V6 engines derived from V8 engines often have three shared crankpins arranged at 120° from each other, similar to an inline 3-cylinder, with two pistons per crankpin. If the cylinder banks are arranged at 90° (as they commonly are in V8-derived V6s), this leads to a firing pattern with groups of two cylinders separated by 90° of rotation, and groups separated by 150° of rotation.

An example is the Buick 231 odd-fire, which has a firing order 1-6-5-4-3-2. As the crankshaft is rotated through the 720° required for all cylinders to fire, the following events occur on 30° boundaries:

Angle 90° 180° 270° 360° 450° 540° 630°
Odd firing 1 6 5 4 3 2
Even firing 1 6 5 4 3 2

Nissan uses the firing order 1-2-3-4-5-6 in some of the V6 engines they make.

In 1977, Buick introduced a new "split-pin crankshaft" in the 231. Using a crankpin that is 'split' and offset by 30° of rotation resulted in smooth, even firing every 120°. However, in 1978 Chevrolet introduced a 90° 200/229 V6, which had a compromise 'semi-even firing' design using a crankpin that was offset by only 18°. This resulted in cylinders firing at 108° and 132°, which had the advantage of reducing vibrations to a more acceptable level and did not require strengthening the crankshaft. In 1985 Chevrolet's 4.3 (later the Vortec 4300) changed it to a true even-firing V6 with a 30° offset, requiring larger crank journals to make them adequately strong.

In 1986 the similarly-designed 90° PRV engine adopted the same 30° crankshaft offset design to even out its firing. Most recent 90° V6 designs use split crankpins to even out the firing intervals. Such a 'split' crankpin is weaker than a straight one, but modern metallurgical techniques can produce a crankshaft that is adequately strong.

Racing use

The V6 engine was introduced into racing by Lancia in the early '50s. After good results with privately entered Aurelia saloons Lancia set a works competition department in 1951. Four B20 Coupes were entered in the '51 Mille Miglia and the one driven by Giovanni Bracco and Umberto Maglioli caused quite a stir by finishing second overally after the 4.1-litre Ferrari driven by Villoresi and Cassani, a car which three times more power than the Lancia. After that encouraging start Lancia decided to carry on with the endurance racing program, first with specially prepared Aurelias (called Da Corsa) and then with specially built prototypes. A D24 with a V6 making won the 1953 Carrera Panamericana with Juan Manuel Fangio at the wheel.

After that came the Ferrari Dino V6. Alfredo Ferrari (nicknamed Dino), the only legitimate son of Enzo Ferrari, suggested to him the development of a 1.5 L DOHC V6 engine for Formula Two at the end of 1955. Soon afterwards, Alfredo fell ill, suffering from muscular dystrophy. While in hospital, he discussed technical details with the engineer Vittorio Jano. Dino would never see the engine; he died on June 30, 1956 at the age of 24.

The Dino V6 underwent several evolutions, including an increased engine displacement to , for use in the Ferrari 246 Formula One car in 1958.

The use of a wide 120° bank angle is appealing for racing engine designers as it permits a low center of gravity. This design is even considered superior to the flat-6 in that it leaves more space under the engine for exhaust pipes; thus the crankshaft can be placed lower in the car. The Ferrari 156 built for new Formula One 1.5 L regulations used a Dino V6 engine with this configuration.

The Dino V6 engine saw a new evolution in 1966 when it was adapted to road use and produced by a Ferrari-Fiat joint-venture for the Fiat Dino and Dino 206 GT (this car was made by Ferrari but sold under the brand Dino). This new version was redesigned by Aurelio Lampredi initially as a 65° V6 with an aluminum block but was replaced in 1969 by a cast-iron block version (the Dino car was renamed the 246GT).

The Fiat Dino and Dino 246GT were phased out in 1974, but 500 engines among the last built were delivered to Lancia, who was like Ferrari already under the control of Fiat. Lancia used them for the Lancia Stratos which would become one of the most successful rally cars of the decade.

The Alfa Romeo V6 was designed in the 1970s by Giuseppe Busso, the first car to use them being the Alfa Romeo 6. The over-square V6, with aluminium alloy block and heads, has seen continuous use in road vehicles, from the Alfetta GTV6 onwards. A notable use of the Bussone Sei (Busso's Little Six) V6 was the Alfa Romeo 155 V6 TI. Turbocharged, it had a peak power of at 11,900 rpm. The 164 introduced a V6, and in 1992, a 3.0 L DOHC 24 valve version. The Alfa 156 introduced a 2.5 L DOHC 24 valve version in 1997. The engine capacity was later increased to , where it found application in the 156 GTA, 147 GTA, 166, GT and GTV. Production was discontinued in 2005.

Another influential V6 design was the Renault-Gordini CH1 V6, designed by François Castaing and Jean-Pierre Boudy, and introduced in 1973 in the Alpine-Renault A440. The CH1 was a 90° cast iron block V6, similar to the mass produced PRV engine in those two respects but otherwise dissimilar. It has been suggested that marketing purposes made the Renault-Gordini V6 adopt those characteristics of the PRV in the hope of associating the two in the public's mind.

Despite such considerations, this engine won the European 2 L prototype championship in 1974 and several European Formula Two titles. This engine was further developed in a tubocharged 2 L version that competed in Sports car and finally won the 24 Hours of Le Mans in 1978 with a Renault-Alpine A 442 chassis.

The capacity of this engine was reduced to 1.5 L to power the Formula One Renault RS01. Despite frequent breakdowns that resulted in the nickname of the 'Little Yellow Teapot', the 1.5 L finally saw good results in 1979.

Ferrari followed Renault in the turbo revolution by introducing a turbocharged derivative of the Dino design (a 1.5 L 120° V6) with the Ferrari 126. However, the 120° design was not considered optimum for the wing cars of the era and later engines used V angles of 90° or less.

Both Renault and Ferrari failed in their attempt to win the Drivers' Championship with V6 Turbo engines. The first turbocharged engine to win the championship was the Straight-4 BMW.

They were followed by a new generation of Formula One engines, the most successful of these being the TAG V6 (designed by Porsche) and the Honda V6. This new generation of engines were characterized by odd V angles (around 80°). The choice of these angles was mainly driven by aerodynamic consideration. Despite their unbalanced designs these engines were both quickly reliable and competitive; this is generally viewed as a consequence of the quick progress of CAD techniques in that era.

In 1989 Shelby tried to bring back the Can-Am series, using the Chrysler 3.3 L (201 CID) V6 (not yet offered to the general public) as the powerplant in a special racing configuration making . This was the same year that the Viper concept was showed to the public.

Originally the plan was to produce two versions of this race car, a version and a model, the version being the entry circuit. The cars were designed to be a cheap way for more people to enter auto racing. Since all the cars were identical, the winners were to be the people with the best talent, not the team with the biggest pockets. The engines had Shelby seals on them and could only be repaired by Shelby's shop, ensuring that all the engines are mechanically identical.

Only 100 of these 3.3s were ever built. Of these 100, 76 were put into Shelby Can-Am cars (the only 76 that were ever sold). No significant amount of spare parts were produced, and the unsold engines were used for parts/spares. The Shelby specific parts, such as the upper intake manifold, were never made available to the general public. According to a small article in the USA Today (in 1989), these cars were making [stock versions introduced in 1990 produced 150 hp) and hitting on the track. The engine itself was not that far from a standard-production 3.3. The Shelby engine is only making about more than the newest 3.3 factory engines from Chrysler. The Can-Am engine has a special Shelby Dodge upper intake manifold, a special Shelby Dodge throttle body, and a special version of the Mopar 3.3 PCM (which had this engine redlining at 6800 rpm).

Nissan also has a brief history of using V6's for racing in the JGTC. Though their first V6 race car did not appear until 2003, development of V6s for sports cars began with the VG engine initially used in the Z31 300ZX. The engine began life as a SOHC, turbocharged 3.0L power plant with electronic fuel injection, delivering . This was revised in 1989 with the Z32 300ZX. The VG30ET was upgraded into the VG30DETT, sporting both an additional turbocharger and an extra pair of camshafts, making the engine a genuine DOHC producing . In spite of this, the engine was less preferential to Nissan's RB26DETT, which powered the Nissan Skyline GT-R, which was also favoured for racing over the 300ZX.

In 2002, Nissan canceled the Skyline GT-R's production run permanently. Though the Skyline GT-R was still used for JGTC by Nismo, the 2003 race car used a new engine: the VQ30DETT. It was a V6 derived from Nissan's VQ family, purpose built for the track. Though similar to the VG30DETT, the VQ30DETT produced a staggering . After 2003, the engine was used in Nismo's Fairlady Z JGTC cars. In 2008, Nismo replaced the Fairlady Z JGTC with the new Nissan GT-R, but powered by a normally-aspirated V8 instead of the road car's twin-turbo V6 VR38DETT.

Notes

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