At the beginning of the 20th century, the innovation of radial engines came from the conversion of a rotary engine by C. M. Manly. Following this, a few years later, Jacob Ellehammer made the world's first air-cooled radial engine. Before World War I, radial engines competed with rotary engines and inline water-cooled engines.
One of the best visual explanations of the operation of a radial engine can be seen in the above spectacular video by Ian Jimmerson who takes us through some interesting math and a great explanation of a full-scale wooden prototype engine, which would have taken months to create.
If you ever wondered just how all those connecting rods get attached to the crank, this video will demonstrate the assembly of radial engines with the removal of each individual engine part. For all the lovers of radial engines, you may even need some popcorn and a comfortable chair.
The power produced by a radial engine comes from the combustion of all individual cylinders. Each individual cylinder is connected either directly or indirectly to the central crankshaft. One cylinder serves as the master rod which directly connects to the crankshaft, whereas all other cylinder's rods connect to pivoting points around the master rod.
Every radial engine has an odd number of cylinders, and the firing order alternates i.e. cylinders 1, 3, 5, 2, 4. Each revolution of the crankshaft will hit every cylinder, with each cylinder having an intake, compression, power and exhaust stroke. As the cylinders fire, the rods rotate around the crankshaft and cause it to spin. A counterbalance spins in the opposite direction to prevent vibration.
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The competition between rotary, radial, and water-cooled engines continued through to the end of World War I, where some engine designs had reached the limits of design and were superseded by others.
The endurance of radial engines from those designed into the 1920s proved a decisive factor in their feasibility for military use and was adopted as the standard by the U.S. Navy. The ability to fly air-cooled radial engine powered airplanes an unprecedented 300 hours showcased the superiority of radial engines over other models. World War II saw a massive number of aircraft fitted with 14-cylinder and 18-cylinder radial engines. Some tanks faced with problems of how to power them also adopted radial engine designs.
A radial engine works like any other internal combustion engine. The benefits are that radial engines weigh less than liquid-cooled engines, are less vulnerable to damage, and run smoother. The cons are a lack of airflow and cooling, increased drag, and their size is larger than other conventional engines, which would also reduce the visibility for pilots for example.
The math behind all this, from the reason for the odd number of cylinders to the firing order to how the cam works, can all be seen in the spectacular video by Ian Jimmerson of a full-scale wooden prototype engine. You may want to get the popcorn.