The present invention relates to a pressure control apparatus for the fuel tank of the motor vehicle and more particularly to a pressure control apparatus for controlling the pressure of gases within the fuel tank during an engine operation.
Emission control standards for vehicle fuel systems have been in force for these years and a conventional production motor vehicle now has an evaporative emission control system that includes a vapor storage canister. The vapor storage canister stores fuel vapors generated in the fuel tank and discharges fuel vapors to the induction system of the engine so as to make those vapors burned together with mixture gases in the combustion chamber of the engine. In the near future, the emission control standards are going to be tightened such that evaporative emissions are restricted during broader operational conditions including a normal running. Particularly, the tightened emission standards contain a rule that the pressure in the fuel tank should be restricted to be controlled below a specified value through a greater part of vehicle running conditions.
To meet these requirements of the rule, some countermeasures for reducing the pressure within the fuel tank during a vehicle running are needed. There have been proposed several techniques for controlling the pressure within the fuel tank to date. For example, Japanese application Laid open No. 1991-222855 discloses a technique for controlling the pressure within the fuel tank especially after an engine stop. In this prior art, a breather line for delivering a fuel vapor runs from the fuel tank to the canister and in parallel with the breather line a by-pass line is provided on the way of the breather line. A check valve is disposed on the breather line and on the other hand a solenoid valve is provided on the by-pass line. The solenoid valve is designed to be opened or closed by a timer. During an engine stop, the solenoid valve is opened for a predetermined time in order to relieve the pressure in the recess occupied above the fuel level.
However, in this prior art the pressure during an engine operation is restricted by the check valve so that the pressure within the fuel tank is not reduced to a level below the one determined by the check valve while an engine is running.