Horsepower increased from 1200 per engine on the B17 to 2200 horsepower for the B29.
Development of engines for the massive new B29 was an ongoing process in design. The engines for the B29 Superfortress underwent over 6,400 design changes from their initial introduction of the experimental models flown in 1942. Such innovation eventually led to engines that produced 2200 horsepower each as compared to 1200 horsepower engines on the earlier B17. This represented such a great improvement in efficiency that the planes reached nearly one horsepower per pound of weight for the engines.
Introduction of the Wright Cyclone R-3350
The B29 was powered by four Wright Cyclone R-3350 18 cylinder air-cooled engines. Wright Aeronautical Corporation begin work on the R-3350 in January 1936 and succeeded in running the first one in May 1937. The R-3350 design was similar to the earlier R-2600 14-cylinder engine. The R-3350 had two cylinders per row of increased displacement but kept the same bore and stroke as the R-2600. Early models suffered inconsistencies between front and rear cylinder rows due to downdraft carburetion. Addition of a direct fuel-injection system in later models corrected this.
Dodge Corporation was selected to contribute to the effort of producing the necessary engines for the the B29 during World War II. The Dodge-produced engines could be used interchangeably with those produced by Wright. By the end of the war, Dodge had produced 18,413 engines for the fleet of 3,628 B29s. Many of the innovations to the R-3350 occurred thanks to 120 graduate engineers and their assistants at Dodge. These innovations included shot-peened connecting rods, prestressed pistons, high-pressure polishing of engine parts, increased oil pressure and supercharging. Dodge improved tolerances for fuel injection to within thousandths of an inch, improving engine performance.
When equipped with two General Electric turbosuperchargers, the the R-3350 delivered 2200 horsepower at takeoff. The engine could delivered 2300 horsepower at 25,000 feet in case of emergency. The standard output was 1800 horsepower at 2400 rpm. The engines were equipped with propellers with a diameter of more than 5 meters. These propellers operated at 2600 rpm and could deliver airspeed of 550 k/hour.
The R-3350 had a displacement volume of 54,860 cc and an engine diameter of 1, 413 mm. The cylinder diameter was 155.4 mm and the piston stroke was 160.2 mm. The engine had a compression ratio of 6.85. In order to maximize engine life, the flight engineer could regulate engine temperature. The engine temperature at the cylinder head was 260 degrees Celsius at takeoff. This was reduced to 218 degrees in continuous operation. Lubricant temperature was 85 degrees Celsius during continuous operation. In order to avoid freezing, carburetor intake temperature was 30 degrees Celsius.