The spark plug is fitted to the cylinder head. Its function is to conduct the high potential into the combustion chamber and produce a spark. The cross section of a typical spark plug can be seen in the picture The spark plug has two electrodes, a steel shell assembly and an insulator unit.
Working of a Spark Plug
The steel shell assembly houses the core insulator unit. The lower portion of the shell is threaded. The ground electrode is welded to the lower portion of the threaded section. The upper portion of the shell is hexagonal is shape, like a bolt head. This hexagonal portion provides a surface for the spark plug spanner. With the help of the spanner, the plug can be screwed into and out of the threaded hole in the cylinder head.
The core insulator is designed to withstand high voltages, pressures as high as 45 kscm and temperatures approximating 2000°C.
The center electrode is housed within the insulator. This is so made without causing distortion, shrinkage or cracking. The electrode is cemented or screwed into the insulator. Sometime, the electrode is secured within the insulator by a fused vitreous seal. This seal ensures perfect sealing under expansion and contraction. The upper end of the center electrode is connected to the terminal of the spark plug wire. The lower end of the center electrode projects beyond the insulator. This end forms a small gap with the ground electrode.
Sealing gaskets are incorporated between the insulator and the shell of the spark plug. These gaskets prevent the escape of gases under various temperature and pressure conditions. A copper gasket is incorporated between the plug shell and the cylinder head. This gasket provides a seal between the two units and helps in the conduction of heat.
The size of the electrode must be fairly substantial to resist burning. The size of the electrode also determines heat transfer. The electrode temperature must be sufficiently high to burn off rapidly any sooty or oily deposit. A compromise is made in deciding the size of the electrodes. As such, the electrode size will vary for different types of engines.
The shape of the electrode should be such as to allow any oil to drain away from the spark plug. This is important in the case of two stroke engines which use petroil (petrol mixed with lubricating oil) lubrication.
The spark plug electrodes are made of platinum tungsten or iridium alloys or more usually of nickel chromium barium alloys.
In the past, porcelain or mica were generally used for the spark plug insulator. Presently, oxides of aluminium and silicon are used. The insulator is glazed with a silica coating, expect near the tip. This coating resists adherence of carbon on the surfaces exposed to the combustion gases.