The present invention relates to a glow plug for an internal combustion engine and more particularly to a glow plug suitable for a diesel engine.
In diesel engines, a glow plug is now used as a part for starting at low temperatures, and in order to improve the starting characteristic of diesel engines, a small-sized, fast-heating glow plug is now desired.
Most conventional glow plugs are sheathed type glow plugs, in which a heating wire made of Ni-Cr alloy or like material is wound in the form of a coil and placed in a sheath which is formed of a heat- and corrosion-resistant alloy such as stainless steel or inconel and whose one end is closed, and the interior portion of the sheath around the heating wire is filled with an insulating material such as magnesium oxide. In glow plugs of this type, therefore, since the heat conduction between the heating wire and the sheath is effected through the insulating material, it takes time for the surface of the sheath to be heated to redness for igniting a fuel-air mixture, that is, the preheating time is prolonged.
In an effort to solve such problems, that is for attaining rapid heating in sheathed type glow plugs, there is adopted a method in which the rated voltage is set at a low level, and during preheating, a battery voltage higher than the rated voltage is temporarily applied directly for rapid glowing, while during after-glow after start-up of the engine, the battery voltage is dropped by means of a resistor and the rated voltage is applied. In such method, however, it is necessary that a resistor having a fairly large capacity and hence involving generation of heat should be provided separately or as a built-in component, thus causing problems such as an increase in installation space required and a more complicated structure of glow plug, as well as wasteful power consumption in the resistor.