Hispano-Suiza 24Y and 24Z

In 1936, the French Air Ministry issued a requirement for a new aircraft engine in the 2000 horsepower range, for a transatlantic flying boat project. At this point in time, Hispano-Suiza was already producing its 12Y, a 1000 hp V12 used mostly in fighters. Thus, the company decided to use the 12Y as a basis for a 24-cylinder design. However, unlike Allison with its V-3420 and Daimler-Benz with its DB 606 a bit later, Hispano-Suiza engineers didn't choose to assemble two V12's side-by-side on a common gearbox. Rather, while the cylinder banks were reused from the 12Y, and many parts remained interchangeable, the whole engine was rearranged into a compact H layout. The engine is composed of two vertical Flat-12's, each having its own crankshaft, connected on a common gearbox. Superchargers, cooling systems, pumps, and other systems were directly borrowed from the 12Y. A prototype was tested, but the development was suspended in favor of more pressing military need. The prototype was rediscovered in Poland after WWII. The circumstances in which it was transported to Poland are unclear.^[1]

This version was developed in parallel with the Type 82. The difference was that it didn't join the power from both crankshafts into a single propeller. Rather, each crankshaft drove one of two contra-rotating propellers, making the two parts of the engines quite independent, without any mechanical connection. The prototype disappeared during WWII.^[1]