Spark Plug
A spark plug is an insulated electrical device that fits into an engine combustion chamber. A high voltage spark is passed across a gap between a pair of terminals at just the right moment to ignite fuel sprayed into the combustion chamber.
Spark plugs use an insulated central electrode connected to a magneto circuit or ignition coil by a heavily insulated high tension lead. The voltage difference is created by earthing or grounding the base terminal on the spark plug.
Depending upon the way fuel is ignited; internal combustion engines are divided into spark-ignition engines, and compression-ignition engines (diesel engines), which use heated glow plugs to improve cold starting.
Spark Plug Operation
As charge is built up in the ignition coil, electrons flow from the coil generating a voltage difference across the spark gap. No current flows until the voltage rises until it surpasses the dielectric strength of the gases.
At this moment, the gases in the gap become ionised and electrons start to flow across the gap.
There is a massive surge across the gap raising the temperature of the spark to about 60,000 C. The hot ionised gases expand explosively generating a small fireball as the fuel and oxygen react.
Spark plug construction
Spark Plug Terminal
This is the metal terminal that connects the plug to the ignition system.
Ribs
The ceramic ribs provide electrical insulation to prevent the increasing electrical energy from leaking to the metal case.
Insulator
Ceramic Aluminium Oxide is a great insulator and can withstand high temperatures and voltages. The spark plug temperature rating largely depends upon the ceramic used for the insulator tip surrounding the centre electrode.
Spark Plug Head Seal
Spark plugs screw into the cylinder head and protrude into the combustion chamber. The spark plug prevents the high pressure combustion gases from exiting the chamber.
Metal Jacket
The spark plug jacket is threaded to screw into the cylinder head and acts as the ground dissipating the current carried by the side electrode.
Ground electrode
Ground Electrodes are normally welded to the metal jacket. Sometimes called side electrodes; they are made from high nickel steel.
Centre electrode
The centre electrode acts as the cathode and carries the current to initiate sparking.
It needs to withstand high temperatures and oxidisation so the tip can be made from a variety of alloying metals: copper, chromium, ferro-nickel.
High quality plugs often have electrodes of precious metal alloys of iridium, palladium, platinum, tungsten, yttrium, silver, and gold.
Spark plug gap
Spark plugs can be adjusted to have the optimal gap for a particular engine. Different engine manufactures can specify a different optimal spark gap even though the same plug may be used. A badly adjusted spark plug may give only intermittent ignition that show as a marked reduction in engine power and fuel efficiency.
Heat range
If the spark plug tip gets too hot then it can cause pre-ignition or knocking. If too cold, then carbon based electrically conductive deposits can coat the insulator shorting-out the spark current.
The number specifying the heat range indicates whether the plug is designed to run Hot or Cold.
A hot spark plug is a better heat insulator,
A cold spark plug conducts more heat lowering the tip temperature.
Selecting the correct spark plug heat range is all about keeping the tip hot enough at engine idle to avoid carbon build up and cold enough at full power to avoid pre-ignition.
Spark Plug Inspection
The spark plug that has been running for a while provides a record of how the engine is performing at peak power.
- Normal ignition produces a light brown discoloration of the tip
- Persistent under ignition produces a pock marked sandblasted appearance
- Over ignition can damage the insulating case around the central electrode
- A plug running too cold will build up deposits around the plug
- A plug running too hot will make the ceramic look granular or porous