Metal rod glow plugs
Metal rod glow plugs are equipped with a heating coil in a glow rod made of heat-resistant metal. A ceramic material - magnesium oxide - in the interior of this glow rod protects the heating coil from vibrations and shocks. Because magnesium oxide is also an outstanding thermal conductor, the heat that is produced can quickly be dissipated outward.
There are four different types of rod glow plugs: standard, quick-start, QGS and self regulating glow plugs. QGS glow plugs are equipped with "Quick Glow System". Very short pre-glowing times and improved cold-start properties are the result. They require a special control unit for regulation of the power supply. Further development of the QGS resulted in the AQGS (Advanced Quick Glow System) glow plug.
Self Regulating Metal rod glow plugs (SRM glow plugs) are equipped with a heating coil and a regulating coil. The heating coil heats very quickly, and the regulating coil increases its resistance as the temperature increases to control the current flow. Therefore, this type of glow plug requires almost no external control.
The task of the heating coil
Heating and control coils are enclosed in a glow rod made of heat-resistant light-metal alloy. They form a common resistor element, whereby the heating coil combined with the front part of the glow rod forms the heat zone and the control coil is fastened to the current-carrying connecting bolt. The insulating powder (magnesium oxide) is packed between the coil and rod.
The heating coil is not exposed directly to the combustion and the shockwaves which arise through the rapid expansion of the mixture. The glow rod provides the heating coil a certain amount of protection against the carbon arising from the combustion. It does not deposit there, so no short circuit occurs.