The Wright R-3350 Turbo Compound is an air-cooled, double-row, 18-cylinder, radial aircraft engine displacing 3,350 cubic inches. Turbo Compounds could produce up to 3,700 take-off horsepower when using the highest grade fuels available. Wright Aeronautical Division (WAD), a subsidiary of the Curtiss-Wright Corporation, produced many models of R-3350s between 1940 and 1960. However, the Turbo Compound was the most powerful and fuel-efficient R-3350 engine.
In 1927, Wright Aeronautical introduced its 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 became one of the most used aircraft engines in the 1930s and World War II, powering all examples of the B-17 Flying Fortress, each powerplant assisted by a General Electric-designed turbocharger for maximum power output at high altitudes.
In 1935 Wright 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. 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, in the prototype Douglas XB-19.
By 1943 the Boeing B-29 Superfortress, powered by the R-3350 was flying. However the engines remained temperamental, and showed an alarming tendency of the rear cylinders to overheat. A number of changes were introduced into the aircraft production line in order to provide more cooling at low speeds, with the aircraft rushed into operational use in the Pacific in 1944. This proved unwise, as the early B-29 tactics of maximum weights combined with high temperature airfields produced overheating problems that were not completely solved, and the engines had a tendency to swallow their own valves.
After the war the engine was redesigned, and became a favorite for large aircraft, notably the Lockheed Constellation, the Douglas DC-7 and the Lockheed P2V-7 used by the RCAF. It is still used on the Martin Mars transport (flying as a forestry tanker with Coulson Flying Tankers on Vancouver Island) and the Douglas Skyraider flown in Bellingham, WA.
The goal of high horsepower and excellent fuel efficiency was achieved with the use of a process called turbocompounding, which uses the high velocity exhaust gases to drive a turbine that transfers energy to the crankshaft. Wright was the only aircraft engine manufacturer to put a Turbo Compound engine into production.
To build an R-3350 Turbo Compound engine, Wright took an improved post-war model engine and sandwiched three power recovery turbines (PRTs) spaced at 120º intervals around the rear of the engine. The addition of the three PRTs between the power and supercharger sections added only eleven extra inches overall length compared to a non-turbocompounded engine. Each PRT is driven the by the exhaust gases of three front- and three rear-row cylinders. The power is transferred to the engine crankshaft through a fluid coupling. Turbocompounding added about 550 horsepower at take-off power and 240 horsepower at cruise settings over a similar non-turbocompounded R-3350. These power increases were achieved with a weight penalty of about 500 pounds. Additional power could be extracted from the engine for short times by using an anti-detonant injection (ADI) - a water/alcohol mixture sprayed into the induction system.
The fuel burn for the PRT equipped aircraft was nearly the same as the older Pratt and Whitney R-2800, while producing more horsepower. By this point, reliability had improved with the mean time between overhauls at 3,500 hours and specific fuel consumption in the order of 0.4 lb/hp/hour giving it a 34% fuel efficiency. Engines still in use are now limited to 52 inches of manifold pressure, giving 2,880 hp with 100LL aviation fuel. Specially developed engines for air racing use Nitrous Oxide injection and at 3,200 rpm and 80 inches of manifold pressure produce 4,500 horsepower.
The engine on display is from an RCAF Canadair CP-107 Argus anti-submarine patrol bomber. This engine drove a Curtiss Electric 3-blade 15 ft 6 in (4.7 m) diameter feathering and reversible propeller through a .4375:1 reduction gear.