Spark plugs
7.7.10 Spark plugs
Figure 7.18 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.
Flashover or tracking down the outside of
Figure 7.18 Construction of a typical spark plug
the plug insulation is prevented by ribs. These effectively increase the surface distance from the terminal to the metal fixing bolt, which is of
combustion temperature and not to the efficiency course earthed to the engine.
of the cooling system.
Due to the many and varied constructional The following factors determine the thermal features involved in the design of an engine, the
capacity of a spark plug.
range of temperatures a spark plug is exposed to can vary significantly. The operating temperature
● insulator nose length
of the centre electrode of a spark plug is critical.
● electrode material
If the temperature becomes too high then pre-
● thread contact length
ignition may occur as the fuel air mixture may ● projection of the electrode become ignited due to the incandescence of the plug electrode. On the other hand if the electrode
It has been found that a longer projection of temperature is too low then carbon and oil foul-
the electrode helps to reduce fouling problems ing can occur, as deposits are not burnt off. Fouling
due to low power operation, stop go driving and of the plug nose can cause shunts (a circuit in
high altitude conditions. In order to use greater parallel with the spark gap). It has been shown
projection of the electrode better quality thermal through experimentation and experience that the
conduction is required to allow suitable heat trans- ideal operating temperature of the plug electrode
fer at higher power outputs. Figure 7.19 shows the is between 400 and 900°C.
heat conducting paths of a spark plug together with The heat range of a spark plug then is a meas-
changes in design for heat ranges. Also shown is ure of its ability to transfer heat away from the
the range of part numbers for NGK plugs. centre electrode. A hot running engine will require
For normal applications alloys of nickel are plugs with a higher thermal loading ability than a
used for the electrode material. Chromium, man- colder running engine. Note that hot and cold
ganese, silicon and magnesium are examples of running of an engine in this sense refers to the
the alloying constituents. These alloys exhibit
116 Advanced automotive fault diagnosis particular application. Plug gaps in the region of
0.6 to 1.2 mm seem to be the norm at present.