The Napier Sabre was a 24-cylinder four-stroke sleeve valve piston aircraft engine designed by Major Frank Halford and built by Napier & Son during WWII. It was one of the most powerful piston aircraft engines in the world, especially for inline designs, developing over 3,500 horsepower (2,200 kW) in its later versions and 5,500 hp in late-model prototypes. However, the rapid conversion to jet engines after the war led to the quick demise of the Sabre, as Napier also turned to jets.
They followed the Lion with two new H-block designs: an H-16 engine (Rapier) and an H-24 (Dagger). The H-block has a compact layout, as it essentially consists of two horizontally opposed inline engines lying one atop or beside another. Since the cylinders are opposed, the motion in one is balanced by the opposite motion in the one on the opposite side, leading to smooth running. However, in these new designs, Napier oddly decided to use air cooling; in service, the rear cylinders proved to be impossible to cool properly, leading to terrible reliability problems.
In 1927, Harry Ricardo published a seminal 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 beyond 1,500 hp (1,100 kW), a figure 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 decrease the engine's sensitivity to detonation on 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 of what would become the most powerful engine in the world. The H-block layout's inherent balance, and the Sabre's relatively short stroke, allowed it to run at higher RPM to deliver more power from a smaller displacement, provided good volumetric efficiency could be maintained (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 first Sabre engines were ready for testing in January 1938, although at a severely limited 1,350 hp (1,000 kW). By March they were already passing tests at 2,050 hp (1,500 kW), and by June 1940 the first production-ready versions were delivering 2,200 hp (1,640 kW) from their 2,238 in³ (37 L). 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).
Problems started to appear as soon as volume production started. Up to that point the engines had been hand-assembled by Napier craftsmen, and it proved to be rather difficult to adapt it to assembly line production techniques. In particular, the sleeves tended to fail quite often, seizing the engine in the process. At that time Bristol were well on the way to delivering their first production sleeve valve engines, the Taurus. At first Bristol refused to work with Napier, but eventually, under intense pressure from the Air Ministry, they relented, and the problems soon disappeared with the addition of Bristol's sleeve manufacturing techniques (which had been hard come by and hence carefully guarded).
Quality control also proved to be a serious problem. Engines were often delivered with improperly cleaned castings, broken piston rings, and machine cuttings left inside the engine. Mechanics were constantly overworked trying to keep Sabres running, and during cold weather they had to run them every two hours during the night so that the engine oil would not congeal and prevent the engine starting the next day (unlike 'multigrades' today, the oils available tended to become thick at low temperatures, preventing the Sabre from 'picking-up' when started). These problems took too long to straighten out, and for many the engine started to attain a bad reputation. To make matters worse, mechanics and pilots unfamiliar with the very different nature of this engine tended to blame the Sabre for problems which were caused by incorrect handling. This was all exacerbated by the representatives of the competing Rolls-Royce company, who had their own agenda. Rolls-Royce eventually built their own interpretation called the Eagle.
Napier did not seem to be particularly interested in these sorts of problems, however, and instead continued to tinker with the design for better performance. In 1942 they started a series of projects to improve altitude performance with the addition of a three-speed, two-stage supercharger, but at this point the basic engine was still not running reliably. In December 1942 the company was purchased by English Electric, who immediately ended the supercharger project and focused the entire company on the production problems. The situation quickly improved.
By 1944, the Sabre V was delivering 2,400 hp (1,800 kW) consistently, and the reputation of the engine started to improve. This was the last version to see service, however, seeing service on the Hawker Typhoon and its derivative, the Tempest. Without the advanced supercharger, the engine's performance over 20,000 ft fell off rapidly, and pilots flying Sabre-powered aircraft were generally instructed to enter fights only below this altitude. At low altitude, both planes were extremely formidable, the Typhoon easily outpacing its German counterpart, the Focke-Wulf Fw 190. With the destruction of the Luftwaffe during early 1944, Typhoons were increasingly used as strike fighters, notably by the famed Second TAF.
Development continued, and the later Sabre VII delivered 3,500 hp (2,600 kW) with a new supercharger; the final test examples delivered 5,500 hp (4.100 kW) at 45lb/sq in boost. By the end of the war there were several engines of the same power class; the Pratt & Whitney R-4360 Wasp Major was producing about 3,055 hp (2,280 kW), with over twice the displacement, 4,360 in³ (71 L).