(i) Field of the Invention
The present invention relates to an electrical heating type honeycomb heater which can suitably be used in an exhaust gas purification system for effectively purifying harmful components such as hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x) in an exhaust gas exhausted from an internal combustion engine, particularly HC generated in large quantities at the time of an engine start (at a cold start).
(ii) Description of the Prior Art
Heretofore, the research and development of an automobile exhaust gas purification system for purifying harmful components such as HC, CO and NO.sub.x in an exhaust gas of a car have been actively carried out, but particularly in recent years, the purification of these harmful substances at a cold start of a car has been an important technical theme with the intensification of exhaust gas regulations.
That is to say, when the temperature of such an exhaust gas as exhausted immediately after the start of an engine is low, a catalyst does not reach its functional temperature, and so the purification ability of the catalyst is poor. In addition, at this time, HC are exhausted in a larger amount than at the time of a steady running.
As one means for solving such a technical problem, much attention has been paid to a technique in which an electrical heating type heater is electrified before or simultaneously with the start of the engine to quickly heat the catalyst supported on the heater, the so-called light-off catalyst and main catalyst and the like closely arranged in the rear of the heater up to the functional temperature of the catalysts.
For example, Japanese Patent Application Laid-open No. 295184/1991 which has been filed by the present applicants discloses a resistance adjustable type heater in which at least two electrodes for electrification are attached to a honeycomb structure having a number of passages, and a resistance adjusting means comprising slits or the like is provided between the electrodes.
However, in the resistance adjustable type heater described in Japanese Patent Application Laid-open No. 295184/1991, a heating rate is low, when the whole heater is heated and a low electric power is applied. Thus, the catalyst supported on the heater cannot reach an ignition temperature in a short period of time, with the result that most of HC generated in large quantities at the time of a cold start is passed through the catalyst without being purified. On the other hand, if a large electric power is applied to quickly heat the heater, a large-capacity battery, heavy cables and a control device are required.