The radial engine has its cylinder banks 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 tremendous horsepower potential. The relatively slow-turning radial engine allows the engine to power a propeller without the use of a gear reduction attachment, making it the perfect fit for an aircraft.
Unlike the rotary engine that typically operates by having the cylinders rotate around the crankshaft, the radial engine operates by having the crankshaft rotate inside of the engine block. Unlike an automobile engine that utilizes a long crankshaft for its long straight engine block, the radial engine uses a short crankshaft inside of the circular or round engine design. Also, all of the piston's connecting rods attach to the crankshaft on a single throw or journal. One piston rod is attached to the crankshaft permanently and is known as the master rod. The other piston rods are free floating and revolve around the crankshaft on bearings attached to pins on the master rod.
The radial engine design was used in World War II bombers exclusively and can be found on 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 great amounts of horsepower, the B-17 bomber utilized four supercharged radial versions, each capable of producing 1,200 horsepower from its 1,800 cubic inches (about 29.5 liters). Fighter aircraft of the era operated mainly on inline liquid-cooled engines due to the ability to mount the engines within the body panels, which made the planes more aerodynamic.
While enjoying great success in the second World War, the radial engine had been replaced by liquid-cooled designs in nearly all of the aircraft by the time of the Korean War. Advances in engine design as well as engine components led to lighter weight, more reliable, liquid-cooled power plants. The radial has seen glory pass it by, and evolution made it an obsolete design in just a few short years. Influences of the radial design can be found in modern-day Harley-Davidson V-twin motorcycle engines, however. With the two piston rods attaching to the crankshaft on a single rod journal, the Harley-Davidson air-cooled engine operates with many of the same principal designs as the radial engine.