reciprocating engine

reciprocating engine, a form of steam engine in which a piston moves back and forth inside a cylinder, transmitting its motion by connecting rod and crank to a driving shaft. Reciprocating engines used at sea to drive a propeller shaft were usually installed so that the pistons moved vertically up and down the cylinders, but in some cases were installed to give a horizontal movement. After the introduction of the steam turbine at the end of the 19th century, the commonest form of ship's reciprocating engines, which had their cylinders in line above the crankshaft, were called by their crews the ‘up and downers’.

Reciprocating engines were ‘double-acting’ which means that steam would be admitted on the top of the piston to force it downwards while the space below the piston would be open to exhaust so that the exhaust steam would go to the condenser. When the piston reached the bottom of the stroke, steam would be admitted to the space below the piston to force it upwards, and the space above the piston would be open to exhaust.

See also compound engine; side lever engine; steam propulsion; triple expansion engine.