The spark plug is the device that provides the electrical spark that ignites your car’s engine. As the name suggests, the spark plug generates a “spark” of electricity which, like a lightning bolt, arcs across a gap. To accomplish this, the spark has to be a very high voltage of electricity, usually between 40,000 and 100,000 volts.
The battery supplies this electricity. It travels through wires into the ignition coil or ‘magneto coil’ and into the spark plug via electric terminals.
Each spark plug has an insulated corridor for the electricity to travel down to the tip of the electrode. The ignition coil sends high voltages through the spark plug and its composition doesn't release the voltage. A difference in voltage builds up between the centre electrode and the side electrode, ionising the air around them. A spark then jumps across the space in between the centre and side electrode, and in doing so heats the surrounding air and ignites the air-fuel mix.
Spark plugs have a ceramic insulator insert surrounding the metal conductor. This has the dual effect of preventing sparks from occurring anywhere else except the tip of the electrode, and keeping the heat from spreading. The electrode must stay white hot to burn off deposits of fuel additive that may build up.
Interestingly, whatever happens inside a car engine leaves marks on the spark plug. It is the epicentre of activity. Engines operating normally will leave a light brownish discoloration at the tip of the spark plug. Anything other than this could mean problems with the engine, and a mechanic can ‘read’ the spark plug to diagnose a malfunction.
As a car owner you should have your spark plugs inspected during your car service. If the spark plug is being replaced, be sure to get a reading on your engine from the mechanic before it is disposed of. It’s cheaper to replace it as part of your vehicle’s service than to increase fuel costs through poorer fuel consumption or repairs on the magneto coil.
The standard type of spark plug is a ‘hot plug’. The ceramic insert has only little contact with the metal part of the plug, making it difficult for heat to escape. However, high performance vehicles, which generate much more heat anyway, often have ‘cold plugs’ to prevent the excess heat from setting light to the fuel before the spark is created. In cold plugs, the ceramic/metal contact area is much greater, so more heat can escape.
If you are considering upgrading your car’s spark plug with an after-market high performance plug, your owner’s manual should advise you whether you need a hot plug or a cold plug. It’s important to use the right plug for your car to prevent engine fires.