(1) Field of the Invention
The present invention relates to a control apparatus for a glow plug-type preheating device provided for a diesel engine for vehicles, and in particular, relates to an apparatus for controlling the preheating time of a diesel engine by a glow plug-type preheating device.
(2) Description of the Prior Art
In a conventional glow plug-type preheating device for a diesel engine, the time for preheating by glow plugs, that is, the time for applying an electric current to glow plugs, is adjusted by a driver based on the red heat state of an indicator including an electric heating wire. In this case, however, influences of the ambient temperature or the warming-up state of the engine can hardly be judged by an inexperienced driver and therefore, control of the preheating time is difficult for ordinary drivers.
As means for overcoming this disadvantage, there has been proposed a method in which the time of preheating by glow plugs is controlled depending on the engine temperature or battery voltage. In a control apparatus where the preheating time is controlled according to the engine temperature, for example, the temperature of engine cooling water is detected by a thermistor or the like and the preheating time is set by a timer which is arranged so that the set time is changed according to a detection voltage from the thermistor or the like.
However, since the change of the electric resistance according to the change of the temperature is not so great in the thermistor, the relation between the temperature of engine cooling water and the preheating time is as indicated by characteristic curve "a" of FIG. 1, and when the engine is warmed up, the preheating time is unnecessarily prolonged. As means for eliminating this defect, there has been adopted a method in which the detection voltage of the termistor is applied to the timer through an amplifier to obtain a characteristic as indicated by curve "b" of FIG. 1 or a method in which a comparator is additionally disposed to cut the characteristic "a" at the point "c" in FIG. 1. If such method is adopted, however, an amplifier or comparator has to be additionally disposed, and the cost of the control apparatus is inevitably increased.