Radial Steam Engines

Updated: 30 March 2013

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Radial steam engines have three or more cylinders arranged at an equal spacing on a circle around the crankshaft. The cylinders stay still while the crankshaft revolves; this may seem a crashingly obvious statement but there were such things as rotary engines, in which the crankshaft stayed still and the cylinder assembly revolved around it. These should not be confused with the other sort of rotary engine, in which some kind of a piston rotated in a fixed housing.


The Brotherhood engine was a popular and successful three-cylinder radial design that was adapted for various operating fluids. A large number were used as water engines, for driving hydraulic capstans in docks and so on, but they were also widely used as steam engines. Versions for steam use are shown here; it is notable that the steam versions show considerable design differences from the water-driven version.

Left: The Brotherhood Radial Engine: axial section

A intriguing feature of this engine is the use of large spherical pivots instead of a conventional "little end" bearing inside the pistons. The main exhaust valve is a port uncovered by the sphere as the angle of the connecting rod changes. There are also uniflow-like supplementary exhaust ports that are uncovered as the piston reaches the bottom of its stroke. The crankcase is used as an exhaust manifold.

From Modern Power Generators

Left: The Brotherhood Radial Engine: longitudinal section

This shows the steam inlet arrangements. Steam enters at left and passes through some rather convoluted passages to a piston-type inlet valve, driven from an eccentric on the crankshaft. There appears to be no means of varying the cut-off.

Power is taken off from the flexible coupling on the right.

From Modern Power Generators

Above: The Brotherhood Radial Engine: alternative valvegear

This image is believed to show an earlier version of the Brotherhood engine. There are no spherical ends to the connecting rods, and the piston valves are driven from extensions on the next piston going anti-clockwise. There appears to be no means of varying the cut-off.

Left: The Brotherhood Radial Engine: account of a test

From Model Engineer & Electrician for 23 June 1904, p593

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