Spark Plugs......

A?spark plug?is an electrical device used in an?internal combustion engine?to produce a spark which ignites the air-fuel mixture in the?combustion chamber. As part of the engine's?ignition system, the spark plug receives high-voltage electricity (generated by an?ignition coil?in modern engines and transmitted via a?spark plug wire) which it uses to generate a spark in the small gap between the positive and negative?electrodes. The?timing of the spark?is a key factor in the engine's behaviour, and the spark plug usually operates shortly before the?combustion stroke?commences.

The spark plug was invented in 1860, however its use only became widespread after the invention of the?ignition magneto?in 1902.?Diesel engines?use compression ignition (instead of?spark ignition), therefore they do not normally use spark plugs.

Design

The main elements of a spark plug are the?shell,?insulator,?central electrode?and?side electrode?(also known as "ground strap"). The main part of the insulator is typically made from?sintered?alumina?, a hard ceramic material with high?dielectric strength.[1][2][3]?In marine engines, the shell of the spark plug is often a double-dipped, zinc-chromate coated metal

A spark plug passes through the wall of the?combustion chamber, therefore it must also form part of the seal for the high-pressure gasses within the combustion chamber.