The circular arrangement of cylinders with valves and pistons and the hub shape of the crankshaft, distinguish a radial engine from other internal combustion engines. Other engines have cylinders in a straight line, in a V shape, or in 2 banks facing each other.
The unique circular arrangement of valves makes a radial engine into a particularly powerful kind of internal combustion machine. It does not need a system of reduction gears, as the maximum rotations per minute are relatively low. A radial engine can be adequately cooled by air, eliminating the need for water. This is because all the valves are in the same plane.
A radial engine may have up to 9 cylinders and a minimum of 3 cylinders surrounding a crankshaft in the center. Each cylinder has a piston and valves to control the inlet of a fuel cum air mixture and for outlet of exhaust after compression and combustion. Spark plugs ignite the fuel and air mixture. Each cylinder can have two spark plugs and may be powered by a magneto instead of a battery.
This design improves reliability and this is an important consideration in applications such as aviation. The pistons have sleeves to ensure full compression and sealing. Combustion produces high energy that is transmitted to the crankshaft. Each cylinder is connected to the crankshaft. One rod is fixed while the others move with the cylinders and the crankshaft. The pistons, valves, and spark plugs are similar to those used in other internal combustion engines.
Radial engines were popular for aircraft during the first half of the 20th century. They had high displacement and were therefore extremely powerful internal combustion engines compared to the ones used in automobiles in those days. They were highly reliable, as even a crack in the engine block could not stop all the cylinders from working.
They were therefore particularly popular for long flights over water. Both sides in the Second World War used radial engines for their air forces. However, lighter gas turbine engines and flat arrays of cylinders began to replace radial engines in the early 1950s. Important drawbacks have led to the gradual eclipse of radial engines over the past 50 years.
Radial engines tend to develop hydraulic locks after a pilot switches off. This is because radial engines use relatively large amount of oil. Residual oil can leak in to cylinders as they point downwards in an aircraft engine. The residual oil must be drained out before the engine is restarted.
Radial engine design has a high amount of drag and is not suitable for high-speed aircraft such as fighters. Radial engines do not have any widespread aviation or industrial use anymore. They are limited to a few special forms of transport and applications. Designer motorcycles for example still use a form of radial engine for distinctive thrust and noise.
Radial engines are still used in some light and hobby aircraft, as they are durable, simple to maintain and to operate. Radial engine design, because of its memorable hub and spoke pattern, is used occasionally for decoration and as part of artifacts.