B. Glow Plug Design
Operation of Preglow SystemB. Glow plug design
a) Filament-type glow plugs
The filament-type glow plugs are essentially comprising the plug body (5) with threads M 18 x 1.5, one center electrode with filament wire and the outer electrode. The current is carried by conductor (1) ora connecting cable, depending on location of plug, to center electrode (13) or to ring element (3) of outer electrode (11). The center electrode (13) and the outer electrode (11) are connected to each other by means of the glow filament (6). The two electrodes are insulated in relation to each other by the insulating compound (12) and in relation to the plug body (5) by the insulation compound (4).
In addition, a plastic insulator (2) is installed at upper end of glow plug between outer and center electrode. The connection insulator (10) insulates the two current connections. The domed washer (8) installed in insulator serves as a lock for nut (7).
The life of a glow plug depends to a considerable extent on the condition of the injection nozzle and the combustion sequence. Injection pressures which are too low, binding nozzle needles, coked and dripping nozzles as well as a too early begin of inlection may result in early breaking of filament. In addition, oil carbon bridges may lead to a ground connection and subsequent burning of filament.
If a glow plug is burnt out, it is absolutely necessary to check the glow plug which comes next in circuit, since a short-circuit between the threaded stem (1 and the outer pole (2) or the outer pole and cylinder head by metallic foreign particles or soot is possible. In addition, any residue in threaded bore of cylinder head must be removed.
The temperature of the glow filament during preheating may amount to approx. 900-1000 C. The combustion temperature of the engine at operating temperature amounts to approx. 800 °C. Temperatures above 900 °C during continuous operation will shorten the life of glow plugs. The glow plugs have an input of approx. 60 amps at a voltage drop of approx. 0.9 volt per glow plug. They are connected in series with resistance conductors.
b) Quick-start pencil element glow plugs
The quick-start pencil element glow plugs are essentially comprising a housing with M 12 x 1.25 threads and a heater rod pressed into housing.
The single-pole connecting pin is screwed to housing by means of a round brass nut which cannot be released.
The pencil element glow plugs are designed for a voltage of 11 volts and are operated in parallel.
The heater eiement comprises a heating and control coil connected in series.
b Current curve of quick-start pencil element glow plug
When the glow system is switched on a current of approx. 30 amps will flow per glow plug.
The heater coil heats the glow plug very quickly.
The control coil increases its resistance with increasing temperature and restricts the current to approx. 14 amps. This will protect the glow plug against overload.
b Temperatu(e curve of quick-start pencil element glow plug.
After a glow period of 9 seconds, a pencil element temperature of 900 °C/1652 °F is attained, after 30 seconds the max. temperature of 1180 °C/2147 °F.