A W12 engine is a twelve-cylinder piston engine where either three banks of four cylinders, or four banks of three cylinders are arranged in a W configuration around a common crankshaft.

The Napier Lion is a 12-cylinder, petrol-fueled 'broad arrow' W12 configuration aircraft engine built by D. Napier & Son from 1917 until the 1930s. A number of advanced features made it the most powerful engine of its day and kept it in production long after other contemporary designs had been superseded. It is particularly well known for its use in a number of racing designs, for aircraft, boats and cars.