The radial engine has cylinders arranged in a circle, making it lightweight and powerful for aircraft. It differs from the rotary engine and was used in WWII bombers, but was superseded by liquid-cooled designs in later years. Its design influenced modern Harley-Davidson V-twin motorcycle engines.
The radial engine has its banks of cylinders arranged in a circle around the crankshaft. The typical radial engine has three to nine cylinders and is air-cooled. This type of engine is used primarily in aircraft due in part to its light weight and enormous power potential. The relatively slow turning radial motor allows the motor to drive a propeller without the use of a gear reduction accessory, making it the perfect fit for an aircraft.
Unlike the rotary engine which normally works by making the cylinders rotate around the crankshaft, the radial engine works by making the crankshaft rotate inside the engine block. Unlike an automobile engine that uses a long crankshaft for its long straight engine block, the radial engine uses a short crankshaft within the circular or round engine design. Also, all piston connecting rods are attached to the crankshaft in a single or daily shot. A piston rod is permanently attached to the crankshaft and is known as the master rod. The other piston rods are free floating and rotate around the crankshaft in bearings attached to the master rod pins.
The radial engine design was used exclusively in World War II bombers and can be found in the United States’ B-17 and B-25 aircraft. Many European countries also operated aircraft equipped with radial engines with great success. While the radial-design engine produces copious amounts of horsepower, the B-17 bomber used four supercharged radial versions, each capable of producing 1,200 horsepower from its 1,800 cubic inches (approximately 29.5 liters). Fighter aircraft of the time operated primarily with inline liquid-cooled engines due to the ability to mount the engines within the body panels, making the aircraft more aerodynamic.
While enjoying great success in World War II, the radial engine had been superseded by liquid-cooled designs in nearly all aircraft by the time of the Korean War. Advances in engine design, as well as engine components, led to lighter and more reliable liquid-cooled power plants. The radial has seen glory pass, and evolution made it an outdated design in just a few short years. However, radial design influences can be found in modern Harley-Davidson V-twin motorcycle engines. With the two piston rods attached to the crankshaft in a single trunnion, the Harley-Davidson air-cooled engine runs on many of the same major designs as the radial engine.
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