General information:

On a diesel engine, combustion is affected by self-ignition of the fuel sprayed into the highly compressed and thereby highly heated combustion air. In a cold engine, the self-ignition temperature is not attained by compression alone. A pre-glow system is therefore required. The pre-glow system serves the purpose of increasing the temperature of the compressed air to facilitate the firing of the cold engine by the use of a glow plug. The duration of pre-glowing depends on temperature of the engine and on ambient temperature.

Pencil element glow plugs consist essentially of a housing with screw-in threads and a pencil element pressed into the housing. The single-pole connecting pin is glued to the housing by means of a non-releasable round aluminum nut.

The pencil element glow plugs are designed for a current of 12 volts and are operated in parallel. On some older diesels, the glow plugs operate on a current of 6 volts. A dropping resistor is used to reduce the voltage to 6 volts. After a glow period of 9 seconds, a "Quick-Start" pencil element temperature of approximately 1,652°F is attained, after 30 seconds the maximum temperature amounts to 1,976°F.

Quick-Start pencil element glow plugs are usually used in passenger vehicles while trucks use the slower pencil element glow plugs.

The pencil element is heated indirectly by means of a heater element. This heater element, a coil made of a resistance wire, is embedded and insulated in a ceramic compound. When the glow system is switched on, each glow plug is subject to a current of approximately 20 amps, peak impulse of approximately 40 amps. Under the influence of increasing heat, the inherent resistance of the glow plug increases and will limit the current to approximately 8 amps.

After a glow period of approximately 20 seconds a heater pencil element temperature of 1,652°F will be attained, after approximately 50 seconds the maximum temperature will be 1,976°F.