Trade of Motor Mechanic - eCollege
Trade of Motor Mechanic - eCollege
Trade of Motor Mechanic - eCollege
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Module 4 - Unit 1<br />
<strong>Trade</strong> <strong>of</strong> <strong>Motor</strong> <strong>Mechanic</strong> - Phase 2 Course Notes<br />
3.10 Spark Plugs<br />
18<br />
Basic Ignition Systems<br />
A spark plug is an electrical device that fits into the cylinder head<br />
<strong>of</strong> some internal combustion engines and ignites compressed air<br />
and petrol by means <strong>of</strong> an electric spark.<br />
Spark Plug Construction<br />
A spark plug is composed <strong>of</strong> a shell, insulator and the conductor.<br />
It pierces the wall <strong>of</strong> the combustion chamber and therefore must<br />
also seal the combustion chamber against high pressures and<br />
temperatures, without deteriorating over long periods <strong>of</strong> time and<br />
extended use. The spark plug consists <strong>of</strong> a plated metal shell with a<br />
ceramic insulator and an electrode extending through the centre <strong>of</strong><br />
the insulator. Threads on the metal shell allow it to be screwed into<br />
the cylinder head and a short earth electrode attached to one side is<br />
bent in towards the centre electrode. The electrodes are <strong>of</strong> special<br />
alloy wire with a set recommended gap between them. The spark<br />
bridges this gap to ignite the air-fuel mixture in the combustion<br />
chamber. Spark plugs are identified by three different features.<br />
These are:<br />
•<br />
•<br />
•<br />
•<br />
Thread size or diameter.<br />
Reach or length <strong>of</strong> the thread.<br />
Heat range or operating temperature.<br />
Plug firing voltage<br />
Revision 2.0 July 2007
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