Page 264 - 05. Subyek Teknik Mesin - Automobile Mechanical and Electrical Systems Automotive Technology Vehicle Maintenance and Repair (Vehicle Maintenance Repr Nv2) by Tom Denton

2
248 Automobile mechanical and electrical systems

Figure 2.333 Modern high-performance spark plug

Figure 2.334 Cutaway section of a standard spark plug

the plug has to withstand a number of severe conditions. It must withstand severe
vibration and a harsh chemical environment. Finally, but perhaps most importantly,
the insulation properties must withstand voltages pressures up to 40 kV.
Figure 2.334 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. Flash over or tracking down the outside
of the plug insulation is prevented by ribs which effectively increase the surface
distance from the terminal to the metal fi xing bolt, which is earthed to the engine.
Because of 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
Key fact signifi cantly. The operating temperature of the centre electrode of a spark plug is
critical. If the temperature becomes too high then preignition may occur, where
The ideal operating temperature of a
spark plug electrode is between 400 the fuel/air mixture may be ignited owing to the incandescence of the plug
and 900°C. electrode. If the electrode temperature is too low, then carbon and oil fouling can
occur as deposits are not burnt off. The ideal operating temperature of the plug
electrode is between 400 and 900°C.