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 inline-4 and in many designs narrower than the V8. Owing to its compact length, the V6 lends itself well to the widely used 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 straight-six engine, which is too long to fit in many modern engine compartments. The V6 engine has become widely adopted for medium-sized cars, often as an optional engine where an inline-4 is standard, or as a base engine where a V8 is a higher-cost performance option.
Recent forced induction V6 engines have delivered horsepower and torque output comparable to contemporary larger displacement, naturally aspirated V8 engines, while reducing fuel consumption and emissions, such as the Volkswagen Group's 3.0 TFSI which is supercharged and directly injected, and Ford Motor Company's turbocharged and directly injected EcoBoost V6, both of which have been compared to Volkswagen's 4.2 V8 engine.
Some of the first V6-cars were built in 1905 by Marmon. Marmon was something of a V-Specialist which began with V2-engines, then built V4's and V6's, later V8's and in the 1930s Marmon was one of the few car-makers of the world which ever built a V16 car.
Another V6-car was designed in 1918 by Leo Goosen for Buick Chief Engineer Walter L. Marr. Only one prototype Buick V6 car was built in 1918 and was long used by the Marr family.
The first series production 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 GMC introduced a unique 60-degree heavy-duty 305 in3 (5 L) 60° V6 for use in their pickup trucks and Suburbans, an engine design that was later enlarged to 478 in3 (7.8 L) for heavy truck and bus use. The discovery of the sweet spot of 60 degrees maximized power while minimizing vibration and size. In short, GMC chanced on an optimal design at a time when the straight-six engine was considered the pinnacle of 6-cylinder design.
1962 saw the introduction of the Buick Special, which offered a 90° V6 with uneven firing intervals that shared some parts commonality with a small Buick V8 of the period. GM sold the engine tooling to Kaiser-Jeep in 1967, then repurchased it in 1974. In 1977, Buick introduced a split pin crankshaft to implement an even-fire version of the engine.
Balance and smoothness
Due to the odd number of cylinders in each bank, V6 designs are inherently unbalanced, regardless of their V-angle.[dubious ] Each cylinder bank in a V6 has an odd number of pistons, so the V6 also suffers from the same problem unless steps are taken to mitigate it. In the horizontally opposed flat-6 layout, the rocking motions of the two straight cylinder banks offset each other, while in the straight-six engine layout, the two ends of engine are mirror images of each other and compensate every rocking motion. Concentrating on the first order rocking motion, the V6 can be modeled as two separate straight-3 engines where counterweights on the crankshaft and a counter rotating balance shaft compensate the first order rocking motion. At mating, the angle between the banks and the angle between the crankshafts can be varied so that the balancer shafts cancel each other in the 90° V6 (larger counter weights) and in the even firing 60° V6 with 60° flying arms (smaller counter weights, second order rocking motion balanced by a single co-rotating balancer shaft).
A 90° V6 can use almost the same technique that balances an even firing 90° crossplane V8 in primary and secondary order. A flatplane V8 is in primary balance because each 4-cylinder bank is in primary balance. In a crossplane V8, balance is achieved at each cylinder pair, since the primary imbalance of a 90° pair is a special case that can be cancelled with a crankshaft counterweight. Secondary balance is achieved by the staggered arrangement of the crossplane crank. A simple 90° V6 with crankshaft counterweights achieves good balance for similar reasons, although the uneven firing intervals will be perceived as roughness at low RPM, making this an unpopular solution. Therefore, designing a smooth V6 engine is a much more complicated problem than the straight-6, flat-6, and V8 layouts. Although the use of offset crankpins, counterweights, and flying arms has reduced the problem to a minor second-order vibration in modern designs, all V6s can benefit from the addition of auxiliary balance shafts to make them completely smooth.
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. Arguably, the roughness is in the exhaust note, rather than noticeable vibration, so the perceived smoothness is rather good at higher RPM. 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 Buick in later versions of their V6 and 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 piston displacement, high-speed six-cylinder engines will suffer less stress and vibration per piston than an equal displacement engine with fewer cylinders.
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.
The most efficient cylinder bank angle for a V6 is 60 degrees, minimizing size and vibration. While 60° V6 engines 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, a 6-throw crankshaft was used 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, the Nissan VQ engine, the Mazda K engine, the Alfa Romeo V6 engine, many Toyota V6 engines, and later versions of the Mercedes-Benz V6 engine.
90° 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° V angle). Although it is relatively easy to derive a 90° 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° 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, the Rover KV6 (2.0litre and 2.5litre, the Honda C engine used in the NSX, Chevrolet's 4.3 L Vortec 4300 and Chrysler's 3.9 L (238 in3) Magnum V6 and 3.7 L (226 in3) PowerTech V6. The Buick V6 was notable because it introduced the concept of uneven firing, as a result of using the 90° cylinder bank angle and shared-crankpin crankshaft design found in the V8 engine (although the V6 crankshaft does have 3 crank throws set at 120° apart, rather than 90° apart as found in the V8) . Rather than firing every 120° of crankshaft rotation, the cylinders would fire alternately at 90° and 150°, resulting in strong harmonic vibrations at certain engine speeds. 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° 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. Examples include the later versions of the Buick V6, and earlier versions of the Mercedes-Benz V6. The Mercedes V6, although designed to be built on the same assembly lines as the V8, used split crankpins, a counter-rotating balancing shaft, and careful acoustic design to make it almost as smooth as the inline-6 it replaced. However, in later versions Mercedes changed to a 60° angle, making the engine more compact and allowing elimination of the balancing shaft. Despite the difference in V angles, the Mercedes 60° V6s are built on the same assembly lines as 90° V8s.
120° might be described as the natural angle for a V6 since the cylinders fire every 120° of crankshaft rotation. Unlike the 60° or 90° 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, unlike the crossplane crankshaft V8, there is no way to arrange a V6 so that unbalanced forces from the two cylinder banks will completely cancel each other. As a result, the 120° V6 acts like two straight-3s running on the same crankshaft and, like the straight-3, suffers from a primary dynamic imbalance which requires a balance shaft to offset.
The 120° layout also produces an engine which is too wide for most automobile engine compartments, so 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. By comparison, the 180° flat-6 boxer engine is only moderately wider than the 120° V6, 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.
Spanish truck manufacturer Pegaso built the first production 120° V6 for the Z-207 mid size truck in 1955. The engine, a 7.5-litre alloy Diesel designed under the direction of engineer Wifredo Ricart uses a single balance shaft rotating at the speed of the crankshaft
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 One 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 designed 120° V220/V300T V6 engines for use in light aircraft. The ignition sequence was symmetrical, with each cylinder firing 120° after the previous cylinder resulting in smooth power delivery. A balance shaft on the bottom of the engine offset the primary dynamic imbalance. The straight, pin-type crankshaft journals in the 120° V-6 layout allowed a shorter and stiffer crankshaft than competing flat-6 engines, while water cooling resulted in better temperature control than air cooling. These engines could run on automotive gasoline rather than avgas. However, the design was shelved in 2006 and there are no plans for production.
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 engine, which is such a narrow angle it can use a single cylinder head and double overhead camshafts for both cylinder banks. With seven 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-, 8-, 12-, 16- and 20-cylinder versions of their 567 Series, 645 Series and 710 Series locomotive, marine and stationary Diesel engines. This angle is optimum for the more common 8- and 16-cylinder versions. In all of these engines, directly opposite cylinders always fire 45 degrees apart, so engines other than 8- and 16-cylinder versions are uneven firing. 6-cylinder engines were only made in the 567 and 645 Series; 20-cylinder engines were only made in the 645 and 710 Series.
- The 54° GM/Opel V6, designed to be narrower than normal for use in small front-wheel drive cars.
- The 65° Ferrari Dino V6, allowing larger carburetors (for potentially higher power in race tuning) than a 60° angle, while suffering a slight increase in vibrations.
- The 72° Mercedes-Benz Bluetec Diesel V6 utilizes a counter-rotating balance shaft and crankpins offset by 48° to eliminate vibration problems and make the engine even-firing.
- The 75° Isuzu Rodeo and Isuzu Trooper V6 of 3.2 and 3.5 L in both SOHC and DOHC versions.
- The 80° Honda RA168-E Formula One engine in the McLaren MP4/4.
Odd and even firing
Many older V6 engines were based on V8 engine designs, in which a pair of cylinders was cut off the front of V8 without altering the V angle or using a more sophisticated crankshaft to even out the firing interval. Most V8 engines share a common crankpin between opposite cylinders in each bank, and a 90° V8 crankshaft has just four pins shared by eight cylinders, with two pistons per crankpin, allowing a cylinder to fire every 90° to achieve smooth operation.
Early 90° V6 engines derived from V8 engines had three shared crankpins arranged at 120° from each other. Since the cylinder banks were arranged at 90° to each other, this resulted in a firing pattern with groups of two cylinders separated by 90° of rotation, and groups separated by 150° of rotation, causing a notorious odd-firing behavior, with cylinders firing at alternating 90° and 150° intervals. The uneven firing intervals resulting in rough-running engines with unpleasant harmonic vibrations at certain engine speeds.
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:
More modern 90° V6 engines avoid this problem by using split crankpins, with adjacent crankpins offset by 15° in opposite directions to achieve an even 120° ignition pattern. Such a 'split' crankpin is weaker than a straight one, but modern metallurgical techniques can produce a crankshaft that is adequately strong.
In 1977, Buick introduced the 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. In 1988, Buick introduced a V6 engine that not only had split crankpins, but had a counter-rotating balancing shaft between the cylinder banks to eliminate almost all primary and secondary vibrations, resulting in a very smooth-running engine.
The V6 engine was introduced into racing by Lancia in the early 1950s. 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 had 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 3,102 cc (189 cu in) V6 making 230 PS (170 kW) won the 1953 Carrera Panamericana with Juan Manuel Fangio at the wheel.
After that came the Ferrari Dino V6. Alfredo Ferrari (nicknamed Dino), 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. The Dino V6 underwent several evolutions, including an increased engine displacement to 2,417 cc (147 cu in), 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° 2.0 L (120 cu in) V6 with an aluminum block but was replaced in 1969 by a 2.4 L (150 cu in) 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. The 164 introduced a 3.0 L (180 cu in) V6, a 2.0 V6 turbocharged in 1991 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 3.2 L (200 cu in), where it found application in the 156 GTA, 147 GTA, 166, GT, GTV and Spider 916. Production was discontinued in 2005.
A notable racing use of the V6 engine was the Alfa Romeo 155 V6 TI, designed for the 1993 Deutsche Tourenwagen Meisterschaft season and equipped with a 2.5 L (150 cu in) engine making a peak power of 490 PS (360 kW; 480 hp) at 11,900 rpm.
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 turbocharged 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 optimal for the wing cars of the era and later engines used V angles of 90° or less.
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 cu in) V6 (not yet offered to the general public) as the powerplant in a special racing configuration making 255 hp (190 kW). This was the same year that the Viper concept was shown to the public.
Originally the plan was to produce two versions of this race car, a 255 hp (190 kW) version and a 500 hp (370 kW) model, the 255 hp (190 kW) 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 250 hp (190 kW) (stock versions introduced in 1990 produced 150 hp or 110 kW) and hitting 160 mph (260 km/h) on the track. The engine itself was not that far from a standard-production 3.3. The Shelby engine is only making about 50 hp (37 kW) 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 quite successful history of using V6's for racing in both IMSA and the JGTC. Development of their V6s for sports cars began in the early 1980s 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 230 PS (169 kW). The VG30ET was later revised into the VG30DETT for the Z32 300ZX in 1989. The VG30DETT sported both an additional turbocharger and an extra pair of camshafts, making the engine a genuine DOHC twin-turbo V6 producing 300 PS (221 kW). Nissan used both of these engines in its IMSA racing program throughout the 1980s and 1990s each producing well over 800 hp (600 kW). In the Japan Grand Touring Car Championship, or JGTC, Nissan opted for a turbocharged version of its VQ30 making upwards of 500 hp (370 kW) to compete in the GT500 class.
The V6 turbo engine is to be revived for the 2014 Formula One season, and V6 turbos have been used in the IndyCar Series since 2012, with Chevrolet and Honda currently supplying the engines. Lotus also made engines in the 2012 season, but pulled out at the end of the year.
V6 engines are popular powerplants in medium to large outboard motors.
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