Hyper engine
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The hyper engine was a hypothetical aircraft engine design, an engine that would be able to deliver 1 horsepower per cubic inch (about 46 kW/L) of engine displacement . This was the power-to-weight ratio sweet spot for long-range airliners, which needed to be able to lift huge fuel loads and still have extra capability left over for cargo and passengers. A number of companies attempted to build hyper engines in the 1930s, but they were not ready for service until late in the decade. By this point World War II had started and production was needed for military uses. After the war most designs turned to the jet engine, and the term had already fallen from use.
During the 1930s, designers were looking to the future of engine development, and the need for 1 hp/in³. It was clear that this sort of performance would not be easy to achieve. A typical large engine of the era, the Pratt & Whitney Twin Wasp, developed about 1,200 hp (895 kW) from 1,820 in³ (30 L), so an advance of some 50% would be needed. Although scaling up one of these designs would increase the total available power, it would not have any dramatic effect on the power-to-weight; for that more radical changes were needed.
One way to increase the power-to-weight would be to use lighter materials. While this would help it would only deliver a few percent, not the 50% increase needed. For the rest of the needed performance the power extracted from the fuel would have to improve by increasing the compression ratio. A number of common problems hampered this; much higher-octane fuels were needed to reduce detonation, improved superchargers or turbochargers were needed that would not rob power at lower altitudes and overheat the fuel, and some system was needed to cool the poppet valves, which otherwise heated up so much they ignited the fuel on their own.
The term "hyper" was most commonly used in the United States, and it was there that most dedicated attempts to build such a design took place. Continental worked from 1932 on their I-1430, finally getting to production quality in 1943. By this point other engines had passed its 1,600 hp (1,200 kW) ratings, and while the I-1430 had a better power-to-weight ratio, there was little else to suggest it was worthwhile setting up production in the middle of the war. Lycoming spent almost as much time on their 1,275 hp O-1230, and found much the same reception when it was ready for production in 1939. They tried again by bolting two O-1230's together into the H-2470 H engine, and while this engine was planned for use in the P-53, this plane did not go into production, and neither did the engine.
In fact the first successful hyper engine was not designed specifically to be one. The Napier Sabre was running at lower ratings in 1938, and became a hyper when it reached 2,400 hp (1,800 kW) in 1940, from 2,238 in³ (36.7 L). Unlike the US designs, the Sabre attacked the performance problem by running at higher RPM instead of increasing the power of any one stroke. This was only possible in the Sabre due to the use of sleeve valves, which can operate at much higher speeds than poppet valves. Whereas most engines would run at about 2,400 RPM, the Sabre ran at 3850 RPM, thereby explaining it's 50% increase in performance. The Sabre saw widespread use in the later half of World War II, after numerous production quality problems had been addressed.