Figure shows a standard spark plug. The centre electrode is connected to the top terminal by a stud. The electrode is constructed of a nickel based alloy. Silver and platinum are also used for some applications. If a copper core is used in the electrode this improves the thermal conduction properties.
The insulating material is ceramic based and of a very high grade. The electrically conductive glass seal between the electrode and terminal stud is also used as a resistor. This resistor has two functions: firstly to prevent burn off of the centre electrode; and secondly to reduce radio interference. In both cases the desired effect is achieved because the resistor damps the current at the instant of ignition. Flash-over or tracking down the outside of the plug insulation is prevented by ribs. These effectively increase the surface distance from the terminal to the metal fixing bolt, which is of course earthed to the engine.
Due to the many and varied constructional features involved in the design of an engine, the range of temperatures a spark plug is exposed to can vary significantly. The operating temperature of the centre electrode of a spark plug is critical. If the temperature becomes too high then preignition may occur as the fuel air mixture may become ignited due to the incandescence of the plug electrode. On the other hand if the electrode temperature is too low then carbon and oil fouling can occur, as deposits are not burnt off. Fouling of the plug nose can cause shunts (a circuit in parallel with the spark gap). It has been shown through experimentation and experience that the ideal operating temperature of the plug electrode is between 400 and 900°C.
The heat range of a spark plug then is a measure of its ability to transfer heat away from the centre electrode. A hot running engine will require plugs with a higher thermal loading ability than a colder running engine. Note that hot and cold running of an engine in this sense refers to the combustion temperature and not to the efficiency of the cooling system.
The following factors determine the thermal capacity of a spark plug.
- insulator nose length.
- electrode material.
- thread contact length.
- projection of the electrode.
It has been found that a longer projection of the electrode helps to reduce fouling problems due to low power operation, stop go driving and high altitude conditions. In order to use greater projection of the electrode better quality thermal conduction is required to allow suitable heat transfer at higher power outputs. Figure shows the heat conducting paths of a spark plug together with changes in design for heat ranges. Also shown is the range of part numbers for NGK plugs.
"Hot’ and ‘Cold’ spark plugs.
For normal applications alloys of nickel are used for the electrode material. Chromium, manganese, silicon and magnesium are examples of the alloying constituents. These alloys exhibit excellent properties with respect to corrosion and burn off resistance. To improve on the thermal conductivity compound electrodes are used. This allows a greater nose projection for the same temperature range as discussed in the last section. A common example of this type of plug is the copper core spark plug.
Silver electrodes are used for specialist applications as silver has very good thermal and electrical properties. Again with these plugs nose length can be increased within the same temperature range. Platinum tips are used for some spark plug applications due to the very high burn off resistance of this material. It is also possible because of this to use much smaller diameter electrodes thus increasing mixture accessibility. Platinum also has a catalytic effect further accelerating the combustion process. Spark plug electrode gaps in general have increased as the power of the ignition systems driving the spark has increased. The simple relationship between plug gap and voltage required is that as the gap increases so must the voltage (leaving aside engine operating conditions). Further, the energy available to form a spark at a fixed engine speed is constant, which means that a larger gap using higher voltage will result in a shorter duration spark. A smaller gap will allow a longer duration spark. For cold starting an engine and for igniting weak mixtures the duration of the spark is critical. Likewise the plug gap must be as large as possible to allow easy access for the mixture to prevent quenching of the flame.
The final choice is therefore a compromise reached through testing and development of a particular application. Plug gaps in the region of 0.6 to 1.2 mm seem to be the norm at present.