Spark Plugs

The sparking plug ignites the compressed fuel-and-air mixture in the cylinder of a petrol engine by means of a spark which leaps from the central electrode to the so-called earth electrode.

The sparking plug consists essentially of the two electrodes, the insulator and the body with its screw thread and connecting nut. The ignition current coming from the distributor flows through the central electrode and produces the spark between this electrode and the earth electrode.

The ignition voltage is about 25,000 volts, the spark gap between the electrodes being about 0.60.7 mm (0.025-0.03 inches). In engines with a low compression ratio, i.e., in which the combustion mixture is not so greatly compressed, the distance between the electrodes is made larger: this produces a longer spark, which can more readily ignite the mixture than a shorter one can. Since a highly compressed gas presents a higher resistance to the spark, a smaller spark gap is employed with high compression ratios.

The deciding factor in the choice of a sparking plug is its thermal value, which characterises the thermal behaviour of the plug.

It is determined in a test engine under accurately defined conditions. For instance, a thermal value of 95 means that incandescent surface ignition occurs in the test engine after 95 seconds.

A plug with a high thermal value, i.e., a plug which can be subjected to a high thermal load and yet remain cold, has a smaller insulator - so as to provide better heat dissipation - than a plug with a low thermal value.