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 hand, is fitted into-a long reach hole the combustion-chamber space is increased, compression ratio is reduced and carbon is deposited, in the threads of the hole which makes difficult the fitting of a correct reach plug at a later date.
6. Electrode temperature must be maintained between certain limits. At higher temperature (850ºC or above) there is danger of preignition and also corrosion of electrodes. At temperatures lower than 300ºC, however, carbon or even oil deposits are formed causing the leakage of electricity to earth instead of sparking across the plug electrodes. The optimum temperature range is 500-800ºC.
The plug has three main Parts, the centre electrode, the ground electrode and the insulation separating them. Besides these, these are the body shell, the sealing ring and the gasket washer. The upper end of the centre electrode is connected to the spark plug terminal, where H.T. cable from the ignition coil in case of single cylinder engines (or from distributor in case of multicylinder engines) is connected. The lower end of the centre electrode projects beyond the insulator to form a gap with the ground electrode. The insulator is meant to fulfill the following functions :
(i) to insulate the centre electrode from body shell, thereby preventing the leakage of high voltage surge from leaking to earth within the shell.
(ii) to control the working temperature of the centre electrode by suitably adopting the thermal conductivity of the insulating material, its shape and the length of the heat path, while designing the spark plug for a given engine. The body shell serves to house the electrodes and the insulator. Gas-tight seal is necessary to prevent the hot gas from leaking between the insulator and the body shell and between the insulator and the centre electrode. Such seals may be of different types, e.g. solid ring, dry powder, metal powder fused into glass, etc. Besides above the hot gas from the combustion chamber may also leak between the plug and the cylinder head. To seal this flat ring gasket washer is commonly used. In some modern spark plugs, the centre electrode is made in two pieces. By doing so the designer can use different metals to suit best the different requirements of the upper piece which has to the connected to the h.t. cable and the lower piece which has to go into (he combustion chamber. In some spark plugs, centre electrode contains a built-in resistance in series with the electrode. This resistance reduces the number of high voltage surges accompanying the full voltage surge for the spark, thereby reducing the radio and television interference due to ignition system. The resistance also increases the plug life by cutting down peak current that would burn the electrodes. Such plugs are called resistor spark plugs and their life is about double the life of ordinary plugs.