Napier Sabre

The Sabre was a 24-cylinder sleeve valve aircraft engine built by Napier & Son during WWII. It was one of the most powerful engines in the world, notably for inline designs, developing over 3,000 horsepower in its later versions. The rapid turn to jet engines after the war led to the quick demise of the Sabre however, and Napier turned to jets as well.

Prior to the Sabre Napier had been working on large engines for some time. Their most famous was the Lion, which had been a very successful engine between the wars and which, in modified form, powered several of the Supermarine designs to the Schneider Trophy in 1923 and 1927. They followed this with two new designs, an H-16 engine known as the Rapier[?], and a H-24 known as the Dagger[?]. The H-block has a compact layout, essentially it consists of two horizontally opposed inline engines lying one on top of the other. Another advantage is that since the cylinders are opposed, the motion of one is smoothed out by the opposite motion in another on the opposite side, leading to very smooth running. Oddly however, in these new designs Napier decided to use air cooling. In practice the rear banks proved to be impossible to cool properly, leading to terrible reliability problems in service.

During the 1930s designers were looking to the future of engine development. Many studies showed the need for engines that could produce 1hp per cubic inch of displacement, in order to be able to provide the power needed to equip large aircraft that could carry enough fuel for long-range use. This design goal became known as the Hyper engine, and it was clear 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 1,200hp from 1820ci, so an advance of some 50% would be needed. This called for radical changes, and while many companies tried to build the Hyper engine, none of them came to fruition.

In 1927 the RAE published a seminal study on the concept of the sleeve valve engine. In it they essentially stated that traditional poppet valve engines would likely have a hard time producing much beyond 1,500hp, a figure many companies were eyeing for next generation engines. In the mid-1930s Napier took the opportunity to set about developing the Delta into the most powerful engine in the world, by redesigning it with the sleeve valve system, and adding adding water cooling. The H-block layout's inherent balance allowed it to run at higher RPM to deliver more power from a smaller design (more bangs per second means more power delivered), while the sleeve valve would allow these RPMs to be reached.

The first Sabre engines were ready for running in January 1938, although at a severly limited 1,350hp. By March they were already passing tests at 2,050, and by June 1940 the first production-ready versions were delivering 2,200hp from their 2,238ci, close enough to 1hp/ci to be the first Hyper engine to actually work. By the end of the year they were producing 2,400hp. To put this in perspective, the contemporary 1940 Rolls Royce Merlin II was generating just over 1,000hp, and the most powerful engines in the world all developed around 1,200hp.

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 to assembly line techniques. In particular the sleeves tended to fail quite often, seizing the engine in the process. At the time Bristol[?] were developing their own sleeve valve designs, and their Taurus engine had the same bore. 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 well-machined sleeves.

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. These took too long to clean up, and for many the engine started to receive a bad name. Making matters worse, mechanics and pilots were unfamiliar with this very different engine, and tended to blame the Sabre for problems which were caused by incorrect handling, particularly so as the competing Rolls-Royce company's representatives had their own agenda. Mechanics were constantly overworked trying to keep Sabres running, and during cold weather had to run them every two hours during the night so they wouldn't seize.

The problems were eventually addressed, however, and the engine started to allow higher and higher boost settings. By 1944 the Sabre V was delivering 2,400hp reliably, and the reputation of the engine started to improve. This was the last to see service however. The later Sabre VII delivered 3500 with a new supercharger, and the final test articles delivered 4,000hp. By the end of the war there were several engines of the same power class, the Pratt & Whitney R-4360[?] Wasp Major was at that time about 3,055hp, but used over twice the displacement at 4360ci.

The Sabre's primary use was in the Hawker Typhoon and it's development, the Tempest designs. While the former was not the fastest plane in the air, the engine drove it past anything flying at lower altitudes, where it could reach about 412mph. At higher altitudes the thick wing of the Typhoon made it slower, and it found itself being used primarily as a strike fighter. The later Tempest added a new low-drag wing, and the otherwise similar plane became the fastest prop fighter of the war, at least for a short time.

Specifications

For Napier Sabre II, the first production version:

Bore/Stroke/Displacement: 5.0"/6.5"/2,238cu in (24.9l)

Compression ratio: (unknown)

Power: 2,180hp at 3700 RPM

Weight: 2000 lbs

References:

Sabre Sleeve Valve Engine (http://www.cpmac.com/~cmcgarry/napier/napierwelc)

LJK Setright: The power to fly: the development of the piston engine in aviation. Allen & Unwin, 1971.