Brotherhood steam engine. Diagram showing a longitudinal section through a Brotherhood steam engine, showing the cylinder and valve gear. This is a double-acting engine with the two pistons at upper centre (vertical). The crankshaft runs horizontally from left to right below the pistons, driving the rotary gear at far right. At far left is an area (labelled steam) where the steam enters the engine. Arrows show how the steam passes through valves to drive the pistons from above. Brotherhood engines were used in the British Royal Navy in the early 20th century.