Glow plug technologies

Glow plugs are basically differentiated between metal rod glow plugs and ceramic glow plugs. The two technologies have special characteristics in regard to thermal behavior.

Metal rod glow plugs

Metal rod glow plugs from NGK for the pre-glowing in diesel engines, even at the lowest outside temperatures.

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.

Ceramic glow plugs

Ceramic glow plugs from NGK – for higher temperatures and shorter pre-glowing times.

As opposed to a metal rod glow plug, the heating coil of a ceramic glow plug has an especially high melting point. It is also sheathed in silicon nitrite, an extremely rugged ceramic material. The combination of the heating coil and ceramic sheath enable higher temperatures and extremely short preheating times due to excellent thermal conductivity. Also, ceramic glow plugs have a more compact design. This is important, because there is very little free space in today’s engines.

In an SRC (Self Regulating Ceramic) glow plug the heating element is made of ceramic with a sintered metal heating coil. An HTC (High Temperature Ceramic) or NHTC (New High Temperature Ceramic) glow plug has a heating element and a heater made of ceramic.

The NHTC ceramic glow plug was specially developed by NGK to enable automobile manufacturers to comply with the Euro 4 and Euro 5 emissions standards. One of the goals of these standards is to reduce the compression ratio in diesel engines. NHTC glow plugs reach a temperature of 1,000 °C in less than two seconds and can after-glow for more than ten minutes at temperatures of up to 1,350 °C. Optimal combustion is assured even with low compression ratios. In addition, the NHTC glow plug can glow intermediately to prevent cooling of the particle filter in deceleration phases.