W18 engine

A W18 engine is an eighteen cylinder piston engine in a W configuration. Recent W18 engines powered the Bugatti EB 118, Bugatti EB 218 and Bugatti EB 18/3 Veyron concept cars in the late 1990s. This engine featured three banks of six cylinders set 60 degrees apart.

VAG also developed a variation of this engine - it 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 band 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.