Parts of a Reciprocating Engine

by Ndung'u Patrick

A reciprocating engine is a heat engine that utilizes one or more reciprocating pistons to convert pressure into rotating motion. A reciprocating engine is also referred to as an internal combustion engine. The naming criterion derives from the fuel mixture burned within the engine. Major parts of a reciprocating engine include the cylinders, pistons, connecting rods, crankshaft, valves, spark plugs and a valve operating mechanism. These are all used to power conventional vehicles.


A cylinder in a reciprocating engine refers to the confined space in which combustion takes place. Cylinders are arranged in several ways. These include: a single row arrangement, a V-shape arrangement, a W-shape arrangement and a horizontal or flat arrangement.


Pistons in a reciprocating engine are usually attached to each cylinder. In a reciprocating engine, a piston slides up and down to create a rotary motion. A piston's wall is usually grooved to hold rings that fit tightly against a cylinder wall, preventing gases from escaping the combustion chamber.

Connecting Rod

A connecting rod in a reciprocating engine links a piston and the crankcase held by a crankshaft. The connecting rod in a reciprocating engine, while connected to a rotary motion piston, is used to turn a propeller. This results in the rotary motion of the crankshaft.


A crankshaft in a reciprocating engine transforms the up and down movement of a piston into rotary motion. While connected to a piston with a connecting rod, a crankshaft yields a rotary motion as the piston moves up and down. During an intake stroke in a piston engine, a piston is pulled downward, creating a vacuum in the cylinder chamber. During a compression stroke in a reciprocating engine, a crankshaft drives a piston upward in the cylinder. This back and forth motion yields a rotary movement in the crankshaft while varying pressure levels in a cylinder.


A reciprocating engine has an intake and an exhaust valve. These are located adjacent to the fuel-air mixture inlet and exhaust outlet at the top of a cylinder, respectively. An intake valve in a reciprocating engine regulates entry of the air and fuel mixtures while an exhaust valve lets out exhaust and burned gases from the combustion chamber.

Spark Plugs

Spark plugs in a reciprocating engine are usually located on top of a cylinder above the valves. They serve to ignite the compressed air and fuel mixture during the compression and ignition strokes in a reciprocating engine. Ignition takes place just before a piston reaches its top position. This results in very hot gases expanding rapidly to drive a piston down while turning the crankshaft to yield rotary motion.

About the Author

Ndung'u Patrick has been an academic and news writer across various disciplines since 2003. An accredited journalist, he has worked with Kenya News Agency since 2009 as a press officer and holds a Bachelor of Science in communications and public relations.