Napier Sabre - Design and Development - Genesis


During the 1930s, designers were looking to future engine developments; for example, studies showed the need for engines capable of developing one horsepower per cubic inch of displacement (about 45 kW/liter). This specific power output was needed to propel aircraft large enough to carry large fuel loads for long range flights. It was clear that this sort of performance would not be easy to achieve. A typical large engine of the era, the Pratt & Whitney R-1830 Twin Wasp, developed about 1,200 hp (895 kW) from 1,820 cubic inches (30 liters), so an advance of some 50 percent would be needed. This called for radical changes, and while many companies tried to build such an engine, none of them were successful.

In 1927, Harry Ricardo published a study on the concept of the sleeve valve engine. In it, he stated traditional poppet valve engines would likely have a hard time producing much more than 1,500 hp (1,100 kW), a figure that many companies were eyeing for next generation engines. In order to pass this limit, the sleeve valve would have to be used in order to increase volumetric efficiency, as well as to decrease the engine's sensitivity to detonation prevalent with the poor quality fuels in use at the time. Halford's office was next to Ricardo's in London, and while Ricardo started work with Bristol Engines on a whole line of sleeve-valve designs, Halford started work with Napier, using the Dagger as the basis. The layout of the H-block, with its inherent balance, and the Sabre's relatively short stroke, allowed it to run at a higher rate of rotation in order to deliver more power from a smaller displacement, provided that good volumetric efficiency could be maintained (with better breathing), which sleeve valves could do. Another important effect of increasing the number of cylinders is that the piston area increases (for a given capacity and bore/stroke ratio) and this also aids higher power.

The Napier company decided first to develop a large 24 cylinder, liquid cooled engine, capable of producing at least 2,000 hp (1,491 kW) in late 1935. Although the company continued with the opposed H layout of the Dagger, this new design positioned the cylinder blocks horizontally, and it was to use sleeve valves. All of the accessories were grouped accessibly above and below the cylinder blocks, rather than being located at the front and rear of the engine as in most contemporary designs.

The first Sabre engines were ready for testing in January 1938, although they were limited to 1,350 hp (1,000 kW). By March, they were already passing tests at 2,050 hp (1,500 kW), and by June 1940, when the Sabre passed the Air Ministry's 100-hour test, the first production versions were delivering 2,200 hp (1,640 kW) from their 2,238 cubic inch (37 liter) displacements. By the end of the year, they were producing 2,400 hp (1,800 kW). To put this in perspective, the contemporary 1940 Rolls-Royce Merlin II was generating just over 1,000 hp (750 kW).

Read more about this topic:  Napier Sabre, Design and Development

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