Wright R-3350 engine

The R-3350 Duplex-Cyclone was one of the most powerful radial aircraft engines produced in the United States. It was a twin row, supercharged, air-cooled, radial engine with 18 cylinders. Power ranged from 2,200 to over 3,700 hp (1,640 to 2,760 kW), depending on the model. First developed prior to World War II, the R-3350's design required a long time to mature before finally being used to power the B-29 Superfortress. After the war, the engine had matured sufficiently to become a major civilian airliner design, notably in its Turbo-Compound forms.

In 1927 Wright Aeronautical introduced their famous Cyclone engine, which powered a number of designs in the 1930s. After merging with Curtiss to become Curtiss-Wright in 1929, an effort was started to redesign the engine to the 1,000 hp (750 kW) class. The new Wright R-1820 Cyclone 9 first ran successfully in 1935, and would become one of the most-used aircraft engines in the 1930s and WWII.

At about the same time Pratt & Whitney had started a development of their equally famous Wasp design into a larger and much more powerful two-row design that would easily compete with this larger Cyclone. In 1935 Wright decided to follow P&W's lead, and started to develop much larger engines based on the mechanicals of the Cyclone. The result were two designs with a somewhat shorter stroke, a 14 cylinder design that would evolve into the Wright R-2600, and a much larger 18 cylinder design that became the R-3350.

The first R-3350 was run in May 1937, but proved to be rather temperamental. Continued development was slow, both due to the complex nature of the engine, as well as the R-2600 receiving considerably more attention. The R-3350 didn't fly until 1941, after the prototype Douglas XB-19 had been re-designed from the Allison V-3420 to the R-3350.

Things changed dramatically in 1940 with the introduction of a new contract by the USAAC to develop a long-range bomber capable of flying from the US to Germany with a 2,000 lb (900 kg) bomb load. Although smaller than the Bomber D designs that led to the B-19, the new designs required roughly the same sort of power. When preliminary designs were returned in the summer of 1940, three of the four designs were based on the R-3350. Suddenly the engine was seen as the future of Army aviation, and serious efforts to get the design into production started.

By 1943 the ultimate development of the new bomber program, the B-29, was flying. However the engines remained temperamental, and showed an alarming tendency to overheat. A number of changes were introduced into the aircraft production line in order to provide more cooling at low speeds, and the planes were rushed to operate in the Pacific in 1944. This proved unwise, as the overheating problems were not completely solved, and the engines had a tendency to burst into flame after takeoff.

Early versions of the R-3350 were equipped with carburetors, which led to serious problems with inadequate fuel mixture distribution. Near the end of World War II, in late 1944, the system was changed to use direct fuel injection, where fuel was injected directly into the combustion chamber. This change improved engine reliability immediately. After the war the engine became a favourite of large aircraft of all designs, most notably the Lockheed Constellation and Douglas DC-7.

Following the war, in order to better serve the civilian market, the Turbo-Compound system was developed in order to deliver better "gas milage". In these versions of the engine, three separate power recovery turbines were attached to the exhaust piping of each group of 6 cylinders, using the power not to deliver additional boost as in a normal turbocharger, but geared directly to the engine crankshaft by fluid drives in order to deliver more power. This recovered about 20% of the heat of the exhaust, (something around 500 hp) which would otherwise be wasted. This is not without cost, however, for those devices are also nicknamed "Parts Recovery Turbines" (and worse), and were another source of failures.

By this point reliability had improved, with the mean time between overhauls at 3,500 hours, and specific fuel consumption on the order of 0.4 lb/hp.hour (243 g/kWh). Engines still in use are now limited to 52 inches of manifold pressure and 2,880 HP with 100 octane fuel (100LL) instead of the 59.5 inches and 3,400 HP possible with 115/145 fuels, which are no longer available.