By William Pearce
In 1936, the Ministère de l’Air (French Air Ministry) issued a specification for a 2,000 hp (1,491 kW) engine intended to power a flying boat for transatlantic service. The aircraft was to carry at least 40 passengers and 1,100 lb (500 kg) of cargo 3,725 miles (6,000 km) against a 37 mph (60 km/h) headwind. Hispano-Suiza already had its 12Y engine of 1,000 hp (746 kW) in production and was investigating ways to effectively double that engine. Their design efforts led to the 24-cylinder Hispano-Suiza 24Y aircraft engine.
The idea behind the 24Y engine was to utilize as many 12Y engine components as possible. The Hispano-Suiza 12Y engine was a liquid-cooled V-12. Each bank of six cylinders was cast en bloc with an integral cylinder head. The 12Y had a 5.91 in (150 mm) bore, a 6.69 in (170 mm) stroke, and a total displacement of 2,200 cu in (36.05 L). The 12Y-50 was one of the last and most powerful versions of the engine; it produced 1,100 hp (820 kW) at 2,500 rpm.
The 24Y engine’s configuration was a vertical H-24: two cylinder banks were mounted vertically above the crankcase, and two cylinder banks were below. A crankshaft served each upper and lower cylinder bank pair. Four aluminum 12Y-50* cylinder blocks were mounted on the 24Y’s crankcase. Each cylinder block included two valves per cylinder, a single overhead camshaft, and the camshaft’s vertical drive shaft. The 7 to 1 compression pistons were connected to the hollow, one-piece crankshaft via fork-and-blade connecting rods, and all components were from the 12Y engine. Each crankshaft had six throws and was supported by seven main bearings. The two-piece, aluminum crankcase was formed by an upper and lower half and was unique to the 24Y.
At the rear of the engine, each crankshaft drove a single-speed supercharger at 10 times crankshaft speed. The superchargers gave the engine 2.3 psi (.16 bar) of boost. Separate intake manifolds led from each supercharger to the upper and lower cylinder banks on one side of the engine. Three carburetors were positioned along each intake manifold. Each of the engine’s 12 carburetors supplied the air/fuel mixture to a pair of cylinders.
The two spark plugs per cylinder were fired by four magnetos driven from the rear of the engine. Two magnetos were located above each supercharger. Four fuel pumps were mounted below and between the superchargers. The left and right sides of the engine had separate coolant systems, and a coolant pump was located below each supercharger.
At the front of the engine, each crankshaft had a 28-tooth gear that engaged a 55-tooth propeller gear. This combination created a .509 to 1 gear reduction for the propeller shaft. Between each crankshaft and its power gear was a Sarazin torsional vibration damper. Two versions of the 24Y were built, and they differed in their propeller drive. The 24Y Type 82 was designed to power contra-rotating propellers. In this engine, one crankshaft drove the inner propeller shaft while the other crankshaft drove the outer propeller shaft. The 24Y Type 90 was designed to power a single-rotation propeller and was available with either a normal length or extended gear reduction nose case. Some sources state the Type 90 had accommodations for a cannon to fire through the propeller shaft, but photos indicate this was unlikely.
The Hispano-Suiza 24Y had a 5.91 in (150 mm) bore and a 6.69 in (170 mm) stroke. The engine’s total displacement was 4,400 cu in (72.10 L). The 24Y produced 2,200 hp (1,641 kW) at 2,500 rpm for takeoff. Max power was 2,000 hp (1,491 kW) at 2,400 rpm at 10,827 ft (3,300 m), and cruising power was 1,500 hp (1,119 kW) at 2,250 rpm at 10,827 ft (3,300 m). The engine had a specific fuel consumption of .50 lb/hp/hr (304 g/kW/hr). The Type 82 was 6.46 ft (1.97 m) long, 3.05 ft (.93 m) wide, and 4.27 ft (1.30 m) tall. The engine weighed 2,204 lb (1,000 kg). The Type 90 had the same width and height as the Type 82 but was 3.38 ft (1.03 m) longer with the extended gear reduction case, for a total length of 9.84 ft (3.00 m). The Type 90’s weight was listed as 2,161 lb (980 kg).
Exactly when the 24Y was first run has been lost to history. The engine made its public debut in November 1938 at the Salon de l’Aéronautique (Air Show) in Paris, France. A Type 90 engine was displayed there, and it attracted a lot of attention. Unfortunately for Hispano-Suiza, that attention did not translate into sales. War in Europe was imminent by 1939, and Hispano-Suiza had turned its attention to developing the new 12Z engine. The 12Z was the next evolutionary step beyond the 12Y for Hispano-Suiza’s V-12 engines. War would interrupt the 12Z’s development, but the 12Z would later inspire another 24-cylinder engine known as the 24Z, which was configured like the 24Y. It is doubtful that the 24Y was ever flown.
Only a small number of 24Y engines were built—probably just one Type 82 and one Type 90 with an extended gear reduction case. Having disappeared during World War II, the disposition of the Type 90 is not known. The Type 82 wound up in Poland at the end of World War II. Most likely, it was part of Herman Goering’s aviation collection that was moved to Poland late in the war to keep it from being damaged during Allied bombing raids. The Hispano-Suiza 24Y Type 82 engine is currently preserved (without its original propeller shaft) and on display in the Polish Aviation Museum in Krakow.
*Some sources state that 12Y-51 cylinder blocks were used on the 24Y. The 12Y-50 and 12Y-51 were basically the same engine, the only difference being the crankshaft rotation. When viewed from the rear, the 12Y-50 rotated counter clockwise; the 12Y-51 rotated clockwise. The cylinder blocks of the 12Y-50 and 12Y-51 engines were the same.
Aircraft Engines of the World 1941 by Paul H. Wilkinson (1941)
Hispano Suiza in Aeronautics by Manuel Lage (2004)
Jane’s All the World’s Aircraft 1939 by C. G. Grey and Leonard Bridgman (1939)
“Some Trends in Engine Design” Flight (8 December 1938)