1. Field of the Invention
This invention relates to an evaporative emission control system for an internal combustion engine, and more particularly to an evaporative emission control system of this kind equipped with a function of detecting abnormality of the system.
2. Prior Art
Conventionally, an evaporative emission control system has been used for an internal combustion engine installed on an automotive vehicle, which prevents evaporative fuel generated from a fuel tank of the engine from being emitted into the atmosphere. This system temporarily stores evaporative fuel in a canister thereof, and purges the stored evaporative fuel into the intake system of the engine via a purging passage thereof.
Japanese Provisional Utility Model Publication (Kokai) No. 2-26754 discloses an evaporative emission control system of this kind as shown in FIG. 1, which is comprised of a canister 101, a purging passage 102 connecting between the canister 101 and an intake pipe of the engine, a purge control valve 103 arranged across the purging passage 102, and a pressure sensor 104 arranged in communication with the purging passage at an intermediate location thereof for detecting pressure within the purging passage, thereby detecting abnormality of the system based on pressure within the purging passage detected by the pressure sensor 104 and pressure within the intake pipe.
In the conventional system, however, the pressure sensor 104 is required to detect even a subtle change in the pressure in the purging passage caused by opening and closing of the purge control valve 103, due to a small flow resistance of the canister 101, and hence high detecting accuracy is required of the pressure sensor 104.
Further, in a variation of the conventional system where the pressure sensor 104 is arranged in the purging passage 2 at a location between the purging control valve 103 and the intake pipe of the engine, a pressure level detected when the purge control valve 103 is closed is lower than one detected when the pressure control valve 103 is open. However, in the course of use of the canister 101, the flow resistance thereof increases, so that a change in the pressure caused by opening and closing of the purge control valve 103 progressively decreases, and hence in this variation as well, the pressure sensor 104 is required to have high detecting accuracy.