Hyper engine
Air-cooled engines from a variety of US companies were delivering similar power ratings by the early 1940s, and the licensed production of the Rolls-Royce Merlin as the Packard V-1650 provided hyper-like performance from an inline while the Allison V-1710 did the same from a US design, one produced as a private effort outside the hyper program.
Increasing the compression ratio is an easy change that improves the mean effective pressure (MEP), but leads to engine knocking from inconsistent detonation.
In the UK, Harry Ricardo had written an influential paper on the sleeve valve system for exactly these reasons, claiming it was the only way forward.
He had some success in selling this idea, most notably to Bristol Aeroplane Company Engines, where Roy Fedden became "a believer".
Ironically it was one of Ricardo's papers on the sleeve valve design that led to the USAAC's hyper engine efforts.
Sam Heron, head of development at Wright Field and a former colleague of Ricardo while Heron had been working at the Royal Aircraft Factory, Farnborough, started working on the problem with a single-cylinder test engine that he converted to liquid cooling, using a Liberty L-12 engine cylinder.
He pushed the power to 480 psi Brake Mean Effective Pressure, and the coolant temperature to 300 °F (149 °C) before reaching the magic numbers.
The contract limited Continental's role to construction and testing, leaving the actual engineering development to the Army.
[6] They used the L-12's overhead camshaft to operate multiple valves of smaller size, which would improve charging and scavenging efficiency.
They eventually determined that exhaust valves could run cooler when a hollow core filled with sodium is used — the sodium liquefies and considerably increases the heat transfer from the valve's head to its stem and then to the relatively cooler cylinder head where the liquid coolant picks it up.
This placed its performance on a par with newer experimental engines from Europe like the Rolls-Royce PV-12, at least when running on the higher-octane fuels the Army planned to use.
By then other engines had already passed its 1,600 hp (1,200 kW) rating, and although the IV-1430 had a better power-to-weight ratio, there was little else to suggest setting up production in the middle of the war was worthwhile.
[7] The project was eventually guided by the requirements in the "Request for data R40-C", which was included as a part of the Financial Year (FY) 1940 aircraft procurement program.
The Allison's in-line vee cylinder arrangement allowed for a narrow aerodynamic shape that had less drag than the air-cooled radial engine fighters that predominated in America at the time.
[10] The fighter aircraft procurement program for FY 1940 was contained in a document that was approved by Assistant Secretary of War Louis K. Johnson on 9 June 1939.
Ironically, engines that were not considered under the program; the Allison V-1710, Pratt & Whitney R-2800 Double Wasp, Wright R-3350 Duplex-Cyclone and Pratt & Whitney R-4360 Wasp Major, all surpassed the USAAC requirements, and continue flying into the 21st century, primarily flying restored warbird aircraft.
