Largest Aircraft Engine

The Lycoming XR-7755 was the largest piston-driven aircraft engine ever produced, with 36 cylinders totaling about 7,750 in³ (127 L) of displacement and a power output of 5,000 horsepower (3,700 kilowatts). It was originally intended to be used in the "European bomber" that eventually emerged as the Convair B-36. Only two examples were built before the project was terminated in 1946.

The engine first started testing at 5,000 hp (3,700 kW) in 1944 with the XR-7755-3. A second example was provided, as planned, to the United States Army Air Forces at Wright Field in 1946. However, by this time the Air Force had lost interest in new piston designs due to the introduction of jet engines. The original test engine was later delivered to the Smithsonian Institution, where it was recently restored.

General characteristics

• Type: 36-cylinder turbosupercharged liquid-cooled "star" (9 banks at 40º angles, 4 cylinders in each bank) aircraft piston engine
• Bore: 6 ³⁄₈ in (161.9 mm)
• Stroke: 6 ³⁄₄ in (171.4 mm)
• Displacement: 7,756.3 in³ (127.1 L)
• Length: 120 in (3,050 mm)
• Diameter: 60 in (1,525 mm)
• Dry weight: 6,050 lb (2,745 kg)

Components

• Valvetrain: Single overhead camshaft with separate cams for takeoff and economical cruise (Variable valve timing)
• Cooling system: Liquid-cooled

Performance

• Power output:
• 5,000 hp (3,730 kW) at 2,600 rpm takeoff
• 4,000 hp (2,985 kW) at 2,300 rpm cruise
• Specific power: 0.64 hp/in³ (29.3 kW/L)
• Specific fuel consumption:
• 0.70 lb/(hp·h) (0.43 kg/(kW·h)) at takeoff power
• 0.485 lb/(hp·h) (0.29 kg/(kW·h)) at 70% power
• 0.37 lb/(hp·h) (0.22 kg/(kW·h)) at minimum cruise power
• Power-to-weight ratio: 0.82 hp/lb (1.36 kW/kg)