A W18 engine is an eighteen cylinder piston engine which usually has a single crankshaft with three banks of six cylinders arranged in an inverted broad arrow configuration. Recent W18 engines powered the Bugatti EB 118, Bugatti EB 218 and Bugatti 18/3 Chiron concept cars in the late 1990s. This engine featured three banks of six cylinders set 60 degrees apart. The W18 engine was abandoned due to shifting problems.
The variation developed by Volkswagen retained the 18 cylinders but instead of three cylinder banks, it had four. This confused many who saw the specification in print at motor shows since 18 isn't evenly divisible by four. The design was loosely based on the concept Volkswagen used for its V5 engines which had one bank with three cylinders and one bank with only two. Essentially the four-bank W18 was two V9 engines (one bank of five cylinders and one bank of four) mated to a common crankshaft. Ultimately the design proved to be impractical.
An earlier example is the Isotta-Fraschini Asso 750 used to power the Italian Savoia-Marchetti S.55 seaplanes flown by Italo Balbo into Chicago in 1933. This engine used the same layout as the above-mentioned Bugatti - basically a vertical six with two adjacent banks set at 60 degrees each to make a W-18.
In 1967 Scuderia Ferrari built an experimental 500cc W3 engine in order to assess the potential for a 3-litre W18 Formula 1 engine. Although the engine developed 80 bhp at 11000rpm and 160 bhp per litre, the idea was not proceeded with and subsequently in 1972 the rules were changed to outlaw the use of engines in Formula 1 with more than 12 cylinders.