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.

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