The W engine is a specific type of reciprocating / piston internal combustion engine configuration. The cylinder banks resemble the letter W, in the same way a V engine resembles the letter V. There have been three entirely different implementations of this concept: one with three banks of cylinders, one with four banks, and one with two banks of cylinders and two crankshafts.
The original "three-bank" design
The classical W engine uses three banks of cylinders, all connected to one crankshaft.
One of the first W engine was a W3, built by Anzani in 1906, to be used in their motorbikes. It is this W3 engine which also powered the Blériot XI, the aircraft used by Louis Blériot, when on 25 July 1909, he made the first ever successful flight across the English Channel. Shortly afterwards, the W3 configuration was changed to a 120° angle, three cylinder radial engine configuration as the original W3 engine's replacement.
The 1917 Napier Lion aircraft engine was a first W12 engine. Lorraine built the 12Ed and 18Ka aero-engines of 450 horsepower (336 kW; 456 PS) and 650 horsepower (485 kW; 659 PS) in the early 1920s, while Isotta-Fraschini built the 18 cylinder Asso 750 and Asso 1000 of 820 horsepower (611 kW; 831 PS) and 1,100 horsepower (820 kW; 1,115 PS) in the late 1920s.
Later, a three-bank W12 design was also pursued by Audi, who later abandoned the project. However, Volkswagen Group later built an experimental W18 engine for Bugatti's EB 118 and EB 218 concept cars, but the design was determined to be impractical because of the irregular firing order required by the three rows of six cylinders.
The modern "four-bank" design
Volkswagen Group created the first successful automotive W engine - with the introduction of their W12. It combined two narrow-angle VR6 engines around a single crankshaft for a total of four banks of cylinders. For this reason, the four-bank configuration is sometimes, and more accurately, referred to as a "VV" ("vee-vee" or "double-vee") or "VR", to distinguish it from the traditional three-bank "W" design.
The W12 is used in the Volkswagen Phaeton, the Volkswagen Touareg, the Audi A8, and the Bentley Continental GT — though in the latter application, the engine has been highly modified by Bentley, and fitted with twin turbochargers. As a result, it produces considerably more power than the original version. The narrow (15°) angle between bank pairs makes this resemble a V12 engine, in that it has just two cylinder heads and two sets of camshafts. The W12 engine has bore-stroke of 84.0 millimetres (3.31 in) and 90.2 millimetres (3.55 in).
Volkswagen Group went on to produce a W16 engine prototype which produced 465 kilowatts (632 PS; 624 bhp) for the Bentley Hunaudières concept car. A quad-turbocharged version of this engine went into production in 2005 powering the 736 kilowatts (1,001 PS; 987 bhp) Bugatti Veyron EB16.4. A W8 engine was also produced for use in the Volkswagen Passat.
The major advantage of these engines is packaging; that is, they contain high numbers of cylinders but are relatively compact in their external dimensions.
The W-engine in the Bugatti Veyron
In 2006, the Volkswagen Group-owned Bugatti produced the Bugatti Veyron EB16.4; with an 8.0 litre W16 engine. This had four turbochargers, and it produces DIN rated motive power output of 736 kilowatts (1,001 PS; 987 bhp) at 6,000 revolutions per minute (rpm). It utilises four valves per cylinder, 64 valves total, with four overhead camshafts arranged in a 2x double overhead camshaft (2xDOHC - two overhead camshafts per cylinder bank - sometimes referred to as a 'quad cam') layout, and a bore-stroke ratio 1:1 (both bore and stroke are 86.0 millimetres (3.39 in)).