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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made it the most powerful engine of its day and kept it in production long after other contemporary designs had been superseded.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.
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 from their 24 . The power output made the Lion the most powerful Allied aircraft engine, which had previously been the Liberty L-12, producing .
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.
In engine tuning racing versions, the engine could reach and it was used to break many world height, air speed and distance records in aircraft and boats, delivering in a highly tuned Lion for a water speed record of in 1933. In land speed records, Lion engines powered many of Sir Malcolm Campbell's record breakers including a record of over in 1932 and John Cobb's 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 and the 24-cylinder Napier Dagger delivered just under . 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.
| + Lion modelsVessey 1997 |
| Vickers Vernon Mk.III |
| Gloster Gorcock |
| Mainstay engine of the RAF in the late 1920s, replaced by Lion XI |
| Turbocharged, intercooled |
| Gloster Guan |
| Gloster III (Schneider Trophy entrant) Supermarine S.4 |
| Golden Arrow Blue Bird (1927) Miss England I Supermarine S.5 Gloster IV |
| Supermarine S.5 Gloster IV |
| Blue Bird (1931) Fred H Stewart's Enterprise Betty Carstairs' Estelle V powerboat Miss Britain III Gloster VI (Schneider Trophy entrant) Railton Special (John Cobb's land speed record car) |
| Gloster Gorcock |
| Napier-Railton |
| British Power Boat Company Type Two 63 ft HSL |
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