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Napier Lion

The Lion was a 12-cylinder W-block inline aircraft engine built by Napier & Son starting in 1917, and ending in the 1930s. A number of advanced features made it the most powerful engine of its day, and kept it in production long after contemporary designs had stopped production. It is particularily well known for its use on a number of racing designs, both in aircraft and racing boats.

Early in World War I, Napier were contracted to build aero engines from other companies designs: initially a Royal Aircraft Factory model and then Sunbeams. Both proved to be rather unreliable, and in 1916 Napier decided to design their own instead. Reasoning that the key design criteria were high power, light weight, and low frontal area, the engine was laid out with its 12 cylinders in what they called a "broad arrow" - three banks of four cylinders sharing a common crankcase. This suggested the designs first name, the Triple-Four. Today these designs, of which there were only a few, are referred to as a W-block. The engine was also advanced in form, using four valve heads with twin overhead camshafts, and a single block milled from aluminum instead of the more common separate-cylinder steel construction used on almost all other designs.

The newly-renamed Lion's design was completed in 1917, and the first hand-built prototypes ran late that year. It was fitted to a deHavilland D.H.9 in early 1918, proving to have many cooling problems. In addtion the milled block turned out to be difficult to build with any accuracy and they reverted to separate cylinders, although they remained aluminum. Both of these problems were worked out by the middle of the year and the engine entered production in June. The first Lion I versions delivered 450 horsepower from their 25 litres. It then took the crown of the most powerful engine from the Liberty L-12, the excellent US wartime design of 400hp.

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

In highly-tuned racing forms the engine could reach 1,300hp, and it broke a host of world records: height, air speed, and long distance in aircraft, water speed (delivering 1,375hp in a highly tuned Lion for 100 miles per hour in 1933) and even land speed: Lions powered many of Sir Malcom Campbell's record breakers (including over 250 MPH in 1932) and John Cobb's 394 MPH Railton in 1947 - a record that came well after the Lion had passed its prime and yet was to stand for 32 years. Lions powered successful entrants in the most prestigious event in air racing, the Schneider Cup, in 1922 and 1927, but were then dropped by Supermarine in favour of a new, especially designed for racing, engine from Rolls-Royce.

In the 1930s a new generation of much larger and more powerful engines started to appear, and the Lion was clearly past its prime. Gradually, they fell further and further behind. By the time the Bristol Hercules and the Rolls-Royce Merlin arrived in the late 1930s, the Lion was too small and old-fashioned.

In order to address this problem, Napier started the design of two new engines using the even more compact H-block layout. The 16-cylinder Rapier produced 400hp, the 24-cylinder Dagger delivered just under to 1000hp. However these were both smaller than contemporary designs from other companies, and Napier had to start fresh with a new sleeve valve design, which eventually matured into the superb Sabre.


Layout: twelve-cylinder, W-block, water-cooled inline
Bore/Stroke/Displacement: 5.5"/5 1/8"/1462 cu in (25l)
Compression ratio: (unknown)
Power: 500hp at 2050 RPM
Weight: 858lbs (290 kg)