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 inline-6, 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.
Modern V6 engines commonly range in displacement from 2.5 to 4.0 L (150 to 240 cu in), though larger and smaller examples have been produced.
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.
From 1908 to 1913 the Deutz Gasmotoren Fabrik produced benzene electric trainsets (Hybrid) which used a V6 as generator-engine. 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, GM introduced a 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. 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. 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 inline-6 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 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, 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.