Ariel motorcycle engine, 1934.
© Science Museum / Science & Society Picture Library
The 'square four' engine was designed in 1930 by Edward Turner of Ariel Motors Ltd. It was known as the square four as the four cylinders were arranged vertically at the four corners of the square. The pistons drove two separate crankshafts geared together, with the valves located in the cylinder head and operated by a chain-driven, overhead camshaft. The square four was Ariel's foremost design achievement and was produced, with improvements and modifications, from 1931 until 1958.