Next to contact breaker, spark plug is probably the weakest part of the ignition system. It is mounted in the combustion chamber of the engine, where working conditions are- severe. During peak combustion conditions the temperature in the combustion chamber in a modern car engine may be around 2500 ºC and the pressure about 7 Mpa. Moreover a spark plug is also exposed to thermal and load cycling fatigue due to sudden changes in temperature and pressure-from the high temperature of burnt gas to the relatively low temperature of the air/fuel mixture and from the high pressure at the time of explosion of the air/fuel mixture to low pressure during induction. In addition, the spark plug has to endure high voltage, mechanical vibration and the corrosive atmosphere of combustion gages. A modern spark plug has an economical life of about 10,000-16,000 km.
The requirements of a good sparking plug are:
1. Very high resistance to current leakage.
2. Continued maintenance of the proper gap under all conditions.
3. Gas tightness-Any leakage of hot gas will upset the normal steady state of the plug, causing such high temperatures to be reached that the insulator and the electrode disintegrate und full into the cylinder.
4. Resistance to corrosion-corrosion is detrimental because it increases gap length and thus raises the sparking potential required.
5. sufficient ‘reach’ into the combustion chamber-Reach is defined as the length of the thread portion that enters the cylinder. If a long-reach plug is fitted into a short-reach hole, combustion chamber space is reduced which increases the compression ratio, more heat is produced and the exposed edges of the threaded portion may be heated to incandescence’ resulting in preignition. Moreover the protruding end of a long reach plug in a short reach hole may also damage the piston or valve. If a short-reach plug, on the other han