Napier Lion

Early in the First World War, Napier were contracted to build aero engines to designs from other companies, initially a Royal Aircraft Factory model and then Sunbeams. Both engines proved to be unreliable and in 1916 Napier decided to design an engine with high power, light weight and low frontal area. Napier's engineers laid out the engine with its 12 cylinders in what they called a "broad arrow"—three banks of four cylinders sharing a common crankcase. This suggested the design's first name, the Triple-Four. The configuration is also known as a W engine.[1] The engine was also advanced in form, the heads using four valves per cylinder with twin overhead camshafts on each bank of cylinders and a single block being milled from aluminium instead of the common separate-cylinder steel construction used on almost all other designs.

Cutaway view showing the double overhead camshaft arrangement

Under Arthur Rowledge, the design of the engine, renamed Lion, was completed in 1917; hand-built prototypes ran later that year. It was fitted to an Airco DH.9 in early 1918, and many cooling problems were observed during testing. The milled block was difficult to build with the required accuracy and the design reverted to separate aluminium cylinders. Both problems were solved by the middle of the year and the engine entered production in June 1918. The first Lion I versions delivered 450 horsepower (340 kW) from their 24 litres. The power output made the Lion the most powerful Allied aircraft engine, which had previously been the Liberty L-12, producing 400 horsepower (300 kW).

As the most powerful engine available (particularly after a turbocharger became an option in 1922), the Lion went on to commercial success. Through the years between the wars the Lion was ubiquitous and Napier manufactured little else. They stopped making cars in 1925 and little thought was given to replacing their world-famous product. Between the wars the Lion engine powered over 160 different aircraft types.

The Napier Lion installed in the Napier-Railton car

In highly tuned racing versions, the engine could reach 1,300 hp (970 kW) and it was used to break many world height, air speed and distance records in aircraft and boats, delivering 1,375 hp (1,025 kW) in a highly tuned Lion for a water speed record of 100 mph (160 km/h) in 1933. In land speed records, Lion engines powered many of Sir Malcolm Campbell's record breakers including a record of over 250 mph (400 km/h) in 1932 and John Cobb's 394 mph (634 km/h) Railton Mobil Special in 1947—a record that came well after the Lion had passed its prime and stood until the 1960s. The record had been held by British drivers for 32 years. Lions powered successful entrants in the most prestigious event in air racing, the Schneider Trophy, in 1922 and 1927 but were dropped by Supermarine in favour of a new engine, the Rolls-Royce R, which had been designed for racing.

During the 1930s a new generation of much larger and more powerful engines appeared and the Lion became uncompetitive. By the time the Bristol Hercules and the Rolls-Royce Merlin arrived in the late 1930s, the Lion was obsolete. The Sea Lion, a marine version of the Lion, was used to power high speed RAF Rescue Launches. The Lion aero engine was also adapted to power propeller-driven motor sleighs, which were used for high-speed transport and SAR duties on sea ice by the Finnish Air Force and Navy.

Turning away from the broad arrow layout, Napier designed new engines using the more compact H engine layout. The 16-cylinder Napier Rapier produced 400 hp (300 kW) and the 24-cylinder Napier Dagger delivered just under 1,000 hp (750 kW). The engines were smaller than contemporary designs from other companies and Napier started afresh with a new sleeve valve design, which evolved into the Napier Sabre.

Napier Lion on display at Imperial War Museum Duxford

Preserved Napier Lion engines are on static display at

• Brooklands Museum
• Canada Aviation Museum
• Imperial War Museum Duxford
• National Maritime Museum[6]
• Solent Sky

Data from Lumsden[7]

• Napier-Bentley
• Vehicles powered by Napier Lion engines

Comparable engines

• Farman 12We
• Hispano-Suiza 18R
• Lorraine 12E Courlis

Related lists

• List of aircraft engines

Wikimedia Commons has media related to Napier Lion.

• "The Napier Lion Aeromotor". Flight. XI (13): 397–402. 27 March 1919. No. 535. Retrieved 12 January 2011. Contemporary technical description of the Lion with photographs and drawings.