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 (46 kW/L) of engine displacement. The term was generally used only in the United States, where the Army Air Corps funded development of several hyper engines of about 1200 cubic inches (20 L) in the 1930's, hoping the engine's small size would lead to better streamlining and improved range. None of these designs entered production, by the time they were ready new aircraft designs were demanding much larger engines.
During the 1930s designers were looking to the future of engine development, and the need for engines of about 1 hp/in³ became an obvious target. 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.
As the ultimate design goal was a favourable power-to-weight ratio for long-range airliners and bombers, which needed to be able to lift huge fuel loads and still have extra capability left over for cargo and passengers, simply scaling up an existing design was no solution. While it would have increased the total available power, it would not have any dramatic effect on the power-to-weight; for that more radical changes were needed.
Improvements in construction and lighter materials had already delivered some benefits. Aluminum cast-block engines were rapidly improving in quality in the early 1930s and had lowered engine weight noticeably. However another 50% savings simply wasn't there. To reach that goal engines would either have to extract more power from the fuel, or alternately run at a higher speed (more "bangs" deliver more power).
Engines were already fairly efficient, so in order to extract more power they would have to increase the amount of fuel/air mixture, the charge, either by growing larger or using a supercharger to force more charge into the engine. The whole idea of the hyper concept was to remain small, so only the second solution would help in this case. However at the time the entire industry used 87 octane fuels and were already running near the compression ratio limit for that fuel (about 6.5 to 1). Increasing compression, although mechanically possible, would lead to ping and potentially damage the engine.
Increasing RPM looked considerably more attractive. At the time a number of problems hampered this way forward, notably problems with the poppet valves used in most engines. At higher operating speeds they simply could not open and close fast enough to keep up with the engine. Additionally the exhaust valves tended to heat up so much they created hot-spots inside the cylinder that could also lead to ping.
Several solutions to the valve problem were known in the 1930s. In England, Harry Ricardo had become a proponent of the sleeve valve system for exactly these reasons, and had some success convincing British engine companies to invest in the idea, notably Bristol Engines, where Roy Fedden became "a believer". His competitor, Frank Halford, designed his own sleeve valve engine, which was picked up by Napier & Son.
Sleeve valves were not the only solution, however. Another was to use overhead cams pushing multiple smaller valves per cylinder to improve airflow, and to cool the valves with liquid sodium to prevent hot spots. Several developments based on these solutions were started in the late 1930s.
Continental Motors worked from 1932 on their IV-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 (951 kW) 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.
Ford also worked on their V-1650 based on the hyper principles, but this saw almost no interest.
Generally the problems with the designs were not so much technical as that they were simply providing too little total power. By the time they were ready for production the aircraft industry was demanding much more power from any new design than they could yield. Re-engining existing airframes designed for 1,200 hp (890 kW) class engines would have resulted in a notable performance increase, but even for that a major airframe re-design would have been required to take the full advantage of a buried engine. Such a redesign was not considered worthwhile, mostly because much larger aircraft with considerably more performance would be available at about the same time.
In fact the first successful hyper engine was British. 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). Using sleeve valves, the Sabre ran at 3,800 RPM, in an era when 2,400 to 2,600 rpm was normal, thereby explaining its 50% increase in performance. Bristol's series of air-cooled radials matured earlier, and went on to become popular engines for many late-war designs.
Ironically, by the end of the war most engines were technically hypers due to improved charge - the very solution considered not viable in the 1930s. This was due to ever increasing octane ratings of the available fuels. Pre-war engines ran on 87 octane fuel, but the British were able to buy up most of the world's supply of light sweet crude just before the war started, and with improvements to their distilation factories they were able to guarantee supply of 100 octane fuels. This led to an immediate boost in power for all supercharged engines then in service. By the end of the war the Rolls-Royce Merlin could deliver bursts of up to 2,000 hp (1.5 MW) from 1650 in³ (27 L), running on 150 octane fuel and dramatically increasing supercharger boost levels. Of course the same was also true of the Sabre, who's late-war prototypes delivered a whopping 3,500 hp (2.6 MW).