V6 engine


A V6 engine is a six-cylinder piston engine where the cylinders and cylinder blocks share a common crankshaft and are arranged in a V configuration.
The first V6 engines were designed and produced independently by Marmon Motor Car Company, Deutz Gasmotoren Fabrik and Delahaye. Engines built after World War II include the Lancia V6 engine in 1950 for the Lancia Aurelia, and the Buick V6 engine in 1962 for the Buick Special. The V6 layout has become the most common layout for six-cylinder automotive engines.

Design

Due to their short length, V6 engines are often used as the larger engine option for vehicles which are otherwise produced with inline-four engines, especially in transverse engine vehicles. A downside for luxury cars is that V6 engines produce more vibrations than straight-six engines. Some sports cars like the Porsche 911 use flat-six engines instead of V6 engines, due to their near perfect primary engine balance and lower centre of gravity.
The displacement of modern V6 engines is typically between, though larger and smaller examples have been produced, such as the Mazda V6 used in the 1991–1998 Mazda MX-3, or the Mitsubishi V6 engine used in the 1992–1998 Mirage/Lancer, while the largest gasoline V6 built was the GMC V6 used in the 1962 GMC C/K series 6500.

Balance and smoothness

All V6 engines with even firing spacing—regardless of the V-angle between the cylinder banks—are subject to a primary imbalance caused by each bank consisting of an straight-three engine, due to the odd number of cylinders in each bank. Straight-six engines and flat-six engines do not experience this imbalance. To reduce the vibrations caused by this imbalance, most V6 engines use a harmonic damper on the crankshaft and/or a counter-rotating balance shaft.
Six-cylinder designs have less pulsation in the power delivery than four-cylinder engines, due to the overlap in the power strokes of the six-cylinder engine. In a four-cylinder, four-stroke 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, especially at lower engine speeds. In a six-cylinder engine with an even firing interval, the next piston starts its power stroke 60° before the previous one finishes, which results in smoother delivery of power to the flywheel.
Comparing engines on a dynamometer, a V6 engine shows instantaneous torque peaks of 154% above mean torque and valleys of 139% below mean torque, with a small amount of negative torque between power strokes. In the case of a four-cylinder engine, the peaks are approximately 270% above mean torque and 210% below mean torque, with 100% negative torque being delivered between strokes. However, a V6 with uneven firing intervals of 90° and 150° shows large torque variations of 185% above and 172% below mean torque.

Cylinder bank angles

10 to 15 degrees

Since 1991, Volkswagen has produced narrow angle VR6 engines with V-angles of 10.5 and 15 degrees shared by both banks of cylinders, in a design similar to the 1922–1976 Lancia V4 engine. These engines use a single cylinder head so are technically a straight engine with the name "VR6" coming from the combination of German words "Verkürzt" and "Reihenmotor" meaning "shortened inline engine". The VR6 engines were used in transverse engine front-wheel drive cars which were originally designed for inline-four engines. Due to the minimal extra length and width of the VR6 engine, it could be fitted to the engine compartments relatively easily, in order to provide a displacement increase of 60 percent.
Since there is no room in the V between the cylinder banks for an intake system, all the intakes are on one side of the engine, and all the exhausts are on the other side. It uses a firing order of 1-5-3-6-2-4, rather than the common V6 firing order of 1-2-3-4-5-6 or 1-6-5-4-3-2.

60 degrees

A V-angle of 60 degrees is the optimal configuration for V6 engines regarding engine balance. When individual crank pins are used for each cylinder, an even firing interval of 120 degrees can be used. This firing interval is a multiple of the 60-degree V-angle; therefore, the combustion forces can be balanced through the use of the appropriate firing order.
The inline-three engine that forms each cylinder bank, however, produces unbalanced rotating and reciprocal forces. These forces remain unbalanced in all V6 engines, often leading to the use of a balance shaft to reduce the vibration.
The 1950 Lancia V6 engine was pioneering in its use of a six-throw crankshaft to reduce vibration. More recent designs often use a three-throw crankshaft with 'flying arms' between the crankpins allowing for an even firing interval of 120 degrees to be achieved. A pair of counterweights on the crankshaft can then be used to almost perfectly cancel out the primary forces and reduce the secondary vibrations to acceptable levels. The engine mounts can be designed to absorb these remaining vibrations.
A 60-degree V-angle results in a narrower engine overall than V6 engines with larger V-angles. This angle often results in the overall more compact engine, making the engine easier to fit either longitudinally or transversely in the engine compartment.

90 degrees

Many manufacturers, particularly American ones, built V6 engines with a V-angle of 90 degrees based on their existing 90-degree V8 engines. Such configurations were easy to design by removing two cylinders and replacing the V8 engine's four-throw crankshaft with a three-throw crankshaft. This reduced design costs, allowed the new V6 to share components with the V8 engine, and sometimes allowed manufacturers to build the V6 and V8 engines on the same production line.
The downsides of a 90 degree design are a wider engine which is more vibration-prone than a 60 degree V6. The initial 90 degree V6 engines had three shared crankpins arranged at 120 degrees from each other, due to their origins from the V8 engines. This resulted in an uneven firing order, with half of the cylinders using a firing interval of 90 degrees and other half using an interval of 150 degrees. The uneven firing intervals resulted in rough-running engines with "unpleasant" vibrations at low engine speeds.
Several modern 90 degree V6 engines reduce the vibrations using split crankpins offset by 30 degrees between piston pairs, which creates an even firing interval of 120 degrees for all cylinders. For example, the 1977 Buick 231 "even-fire" V6 engine was an upgraded version of the Buick Fireball engine with a split-pin crankshaft to reduce vibration by achieving an even firing order. Such a 'split' crankpin is weaker than a straight one, but modern metallurgical techniques can produce a crankshaft that is adequately strong.
A balance shaft and/or crankshaft counterweights can be used to reduce vibrations in 90 degree V6 engines.

120 degrees

At first glance, 120 degrees might seem to be the optimal V-angle for a V6 engine, since pairs of pistons in alternate banks can share crank pins in a three-throw crankshaft and the combustion forces are balanced by the firing interval being equal to the angle between the cylinder banks. A 120 degree configuration, unlike the 60 degree or 90 degree configurations, would not require crankshafts with flying arms, split crankpins, or seven main bearings to be even-firing. However, the primary imbalance caused by odd number of cylinders in each bank still remains in a 120 degree V6 engine. This differs from the perfect balance achieved by a 90 degree V8 engine with a commonly used crossplane crankshaft, because the inline-four engine in each bank of the V8 engine does not have this primary imbalance.
A 120 degree design also results in a large width for the engine, being only slightly narrower than a flat-six engine. Therefore, the flat-six engine has been used in various automobiles, whereas use of the 120 degree V6 engine has been limited to a few truck and racing car engines, with the exception of the McLaren M630 engine, which uses a 120 degree bank angle with a single balance shaft to eliminate all primary couples. The M630 also takes advantage of the wide angle by placing the turbochargers inside the vee, commonly referred to as a 'hot vee' configuration. The Ferrari 296 GTB is the first Ferrari road car to sport a V6 turbo with a vee angle of 120 degrees between the cylinder banks.

Other angles

Other angle V6 engines are possible but can suffer from severe vibration problems unless very carefully designed. Notable V-angles include:
  • 45 degrees: EMD 567 and EMD 645 locomotive, marine and stationary Diesel engines. These engines were based on V8 and V16 engines which also used a V-angle of 45 degrees.
  • 54 degrees: 1994–2004 General Motors 54-degree automotive engine. A slightly smaller than usual V-angle was used to reduce the width of the engine, allowing it to be used in small transverse-engine front-wheel drive cars.
  • 65 degrees: 1956–1975 Ferrari Dino automobile engine. The V-angle was increased from the then-common 60 degree angle to allow larger carburetors to be used. Crankpins with an offset of 55 degrees within every pair of cylinders were used to achieve the even firing interval of a 60 degree V6 engine. The 2009–2017 Nissan-Renault V9X automobile engine also used a 65 degree bank angle, to allow a turbocharger to fit between the cylinder banks.
  • 72 degrees: Mercedes-Benz OM642 BlueTEC diesel engine. This engine uses crank pins offset by 48 degrees, to achieve an even firing interval.
  • 75 degrees: 1992–2004 Isuzu V engine used in the Isuzu Rodeo and Isuzu Trooper. These engines were produced in both SOHC and DOHC versions. A 75 degree V6 engine is also used by the 2016–2022 Honda NSX.
  • 80 degrees: 1988 Honda RA168-E engine used in the McLaren MP4/4 Formula One racing car.