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A W engine is a type of reciprocating engine arranged with its cylinders in a configuration in which the cylinder banks resemble the letter W, in the same way those of a V engine resemble the letter V.
Three different configurations have been called W engines:
- Three banks of cylinders sharing a common crankshaft, a configuration also known as broad arrow configuration due to its shape resembling the British government broad arrow property mark.
- Four banks of cylinders sharing a common crankshaft
- Two banks of cylinders with two crankshafts.
The original "three-bank" design
One of the first W engines was the Anzani 3-cylinder, built in 1906, to be used in Anzani motorcycles. 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 flight across the English Channel. Shortly afterward 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 an early 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.
A three-bank W12 design was also pursued by Audi, which later abandoned the project. Volkswagen Group 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 Feuling W
Similar to the W3 built by Anzani in 1906, the Feuling W3 is a 180 horsepower (134 kW; 182 PS), three-cylinder air-cooled engine for motorcycle cruisers. Like radial aircraft engines it has a master connecting rod and two slave rods connected to the three 101.6 millimetres (4.00 in) pistons. Motorcycle Cruiser Magazine reviewed Feulings's "Warlock" powered motorcycle in the October edition of 2000. Cory Ness built his chopper using a Feuling W3 engine during a Biker Build Off episode.
The original "four-bank" design
Early in World War II, Daimler-Benz built the DB 606A/B "power system", weighing 1.5 tonnes apiece. Based on the Daimler-Benz DB 601 V12 aircraft engine. It was a pair of liquid-cooled inverted V12s coupled to work on a single gear shaft. Derided as "welded-together engines" when given inadequately-designed engine nacelles as with the He 177A German heavy bomber, they were in essence W24 engines of the 4-bank arrangement of 6 cylinders each. The inboard rows of cylinders oriented downward and almost parallel to each other.
The modern "four-bank" design
Volkswagen Group created the first successful automotive W engine, with the introduction of its W8 (as a testbed for the W12). The W12 combines two narrow-angle VR6 engine cylinder heads 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 "WR", to distinguish it from the traditional three-bank "W" design (the earlier W8 combined two VR4 engines.)
The W8 was used in the B5.5 Volkswagen Passat and 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. 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 uses four valves per cylinder, 64 valves total, with four overhead camshafts arranged in a double overhead camshaft (two overhead camshafts per cylinder bank) layout, and a bore-stroke ratio 1:1 (both bore and stroke are 86.0 millimetres (3.39 in)).
The motorcycle "two-bank" design
A very rare type of W engine is found in motorcycles of the MotoGP class. These are two-stroke, 500 cubic centimetres (30.5 cu in) V engines with two banks of two cylinders and two separate crankshafts, one per bank of cylinders, thus constituting a sort of "W" form. The angle between the banks varies from 60 to 75 degrees.
There are two major advantages of this engine over the more traditional inline-four engine or I4 engine. The first is the width of the engine: a I4 engine will be narrower than an inline-four engine with the same displacement, but a W4 with its two crankshafts will be even smaller. The second advantage is that the W4 lacks the need for a balance shaft; it will run smoothly if the two crankshafts rotate in opposite directions. This is a weight advantage over the I4 engine, which will need a balance shaft.
This type of engine should not be confused with the U engine, which also has two banks of cylinders and two crankshafts, but which is made by combining two similar straight engines. The U engine lacks the advantages the "W" form of the engine has in terms of width and weight.
- "The New Sunbeam Overhead Valve Type Engines", Aviation Week and Space Technology (McGraw-Hill) 3, 1917: 32
- Domonoske, Arthur Boquer; Finch, Volney Cecil (1936). Aircraft engines: theory, analysis, design, and operation (Engineering textbook). J. Wiley & Sons. p. 7. Retrieved 2014-04-25. "The W, or broad arrow engine, has three rows of cylinders of which the central row is vertical with the other two rows forming equal angles with the vertical."
- Taylor, Charles Fayette (1985) . The Internal-combustion Engine in Theory and Practice: Combustion, fuels, materials, design. Massachusetts Institute of Technology. ISBN 0-262-70027-1. LCCN 84028885. Retrieved 2013-12-17. "W engine is similar to a V engine but with three banks of cylinders. The two V angles are usually equal."
- Rajput, R. K. Internal Combustion Engines. New Delhi, India: Laxmi Publications. ISBN 817008637X. Retrieved 2013-12-17. "W-engine Same as V-engine except with three banks of cylinders on the same crankshaft."
- "Make Mine a Triple: The Feuling W3". Interlink Media. MotorcycleCruiser.com. October 2000. Retrieved 31 December 2009.
- Photo of W3-engined Cory Ness chopper