Continental XI-1430 - Development

Development

In the late 1920s Harry Ricardo wrote a paper on the sleeve valve design that led to the USAAC's hyper engine efforts. He claimed that the 1 hp/in³ goal was impossible to achieve with poppet valve type engines. The USAAC engineering team at Wright Field decided to test this claim by beating it. The I-1430 was the result of an experimental effort at Wright Field to build a high-power cylinder using conventional poppet valves. The engineers, led by Sam Heron, used a variety of techniques to raise the allowable RPM, which was the key to increased power without requiring a larger engine.

The USAAC was interested in very large bomber designs, and were most interested in engines that could be buried in the wings in order to improve streamlining. From this requirement they designed a 12-cylinder horizontally opposed engine using twelve separate "hyper" cylinders. Although this sort of arrangement, with entirely separate cylinders from each other and the crankcase, was common for liquid-cooled Central Powers World War I-era inline-6 aviation engines, as in the German Mercedes D.III of nearly two decades earlier - and had been used for the 1918-era Allied Liberty L-12 liquid-cooled aviation engine with significant success - it had fallen from use in favor of engines featuring a monobloc engine design philosophy, with a cylinder block that combined the cylinders and the crankcase, leading to much stiffer engines, that were better able to handle increased power.

The USAAC proposed an engine of about 1200 cubic inches (20 L), hoping the engine's smaller size would lead to reduced drag and hence improved range. By 1932, the USAAC's encouraging efforts led the Army to sign a development contract with Continental Motors Company for the continued development of the engine design. The contract limited Continental's role to construction and testing, leaving the actual engineering development to the Army.

A second cylinder was added to Hyper No. 1 to make a horizontal-opposed engine for evaluation of an opposed-piston 12-cylinder engine. After running the modified engine with different combinations of cylinder bore and stroke, it was found that the high coolant temperatures required to maintain the required output was impractical. A third high-performance single-cylinder engine was then constructed with lower operating parameters. This one-cylinder engine was designated "Hyper No. 2", and became the test bed for developing the cylinders that would become the Continental O-1430 ("O" for "opposed") engine. It would require a ten year development period which changed the layout to first an upright V-12 engine and later, an inverted V-12 engine, before becoming reliable enough to consider for full production as the Continental I-1430 in 1943.

During development, interest in the "buried engine" concept faded. Improvements in conventional streamlining, notably the NACA cowling, eliminated the need for a buried engine for improved performance. Additionally, with bomber designs like the B-17, using radial engines for power, starting to enter production, the need for new bomber designs became less pressing and the Army turned its attention to new pursuit models. For this role the O-1430 was not terribly useful, so Continental modified the basic design into a V-12, and then into an inverted-V-12, the I-1430, in the same basic format that the German Luftwaffe of World War II would use in their own, monobloc-design and significantly larger displacement Daimler-Benz DB 601 of 33.9 liter (2,075 cubic inch) displacement and Junkers Jumo 211 (with a slightly larger 35 liter, 2,135 cubic inch displacement) liquid-cooled inverted V-12 engines, with each type built in the thousands.