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What is a piston, and what does it do

What is a piston, and what does it do

A piston is at the heart of a reciprocating engine. It consists of a moving circular peice of metal with piston rings to achieve an air-tight seal once it is installed within the engine cylinder. The piston is attached via a piston/gudgeon pin to a connecting rod, which in turn is connected to the crankshaft. 

In four-stroke (gasoline and diesel) car engines, the intake, compression, combustion and exhaust process takes place above the piston in the cylinder head, which forces the piston to move up and down (or in and out in a flat engine) within the cylinder, thereby causing the crankshaft to turn.

What is a piston made of?

Engine components need to be hardwearing for longevity and lightweight to improve efficiency.

As a result, pistons are usually made from an aluminium alloy but the piston rings (usually comprising, from top to bottom, a compression ring, a wiper ring and an oil ring) are made from cast iron or steel. 

The oil ring wipes oil from the cylinder wall when the piston is moving but over time it and the other rings can wear, allowing oil from the crankcase to move into the combustion chamber. 

Excessive oil consumption and white smoke from the exhaust tailpipes indicate piston ring wear.

Internal combustion engines can operate with a single cylinder - and therefore one piston (motorbikes and petrol lawnmowers) or as many as 12, but most automobiles have four or six. 

Radial engines, commonly used in propeller-driven planes, have an odd number of cylinders and pistons for a smoother operation.

Pistons also feature in external combustion engines, otherwise known as steam engines, where water is heated in a boiler and the resulting steam is used to propel a pair of pistons (typically) in external cylinders, which then drive the wheels. Rotary engines do not have cylinders or pistons.