Valve timing control system

A valve timing control system installed in a drive transmission system that transmits a drive force from a drive shaft of an internal combustion engine to a follower shaft, which opens and closes at least one of an intake valve and an exhaust valve of the internal combustion engine, so that the valve timing control system adjusts opening and closing timing of the at least one of the intake valve and the exhaust valve, the valve timing control system comprising:

• a housing that is rotated together with one of the drive shaft and the follower shaft and has at least one receiving chamber, each of which is formed within a predetermined angular range in a rotational direction;
• a vane rotor that is rotated together with the other one of the drive shaft and the follower shaft and has at least one vane, each of which is received in a corresponding one of the at least one receiving chamber to partition the receiving chamber into a corresponding retard chamber and a corresponding advance chamber, wherein the vane rotor is driven to rotate in a corresponding one of a retard side and an advance side relative to the housing by a pressure of working fluid in a corresponding one of the retard chamber and the advance chamber of the at least one receiving chamber, so that a relative phase of the vane rotor relative to the housing is controlled;
• a phase switch valve that is switched to implement:
• one of supplying of the working fluid from a fluid source to the retard chamber of the at least one receiving chamber and discharging of the working fluid from the retard chamber of the at least one receiving chamber;
• and
• one of supplying of the working fluid from the fluid source to the advance chamber of the at least one receiving chamber and discharging of the working fluid from the advance chamber of the at least one receiving chamber;
• at least one phase check valve, each of which is installed in a corresponding one of:
• at least one retard passage, each of which connects between the phase switch valve and the retard chamber of a corresponding one of the at least one receiving chamber;
• and
• at least one advance passage, each of which connects between the phase switch valve and the advance chamber of a corresponding of the at least one receiving chamber, wherein each of the at least one phase check valve limits the working fluid to flow from a corresponding check valve connecting chamber, which is a corresponding one the retard chamber and the advance chamber of the corresponding one of the at least one receiving chamber connected to the phase check valve, to the phase switch valve and permits the working fluid to flow from the phase switch valve to the corresponding check valve connecting chamber;
• at least one drain control valve, each of which is driven by a pilot pressure and is installed in a corresponding one of at least one bypass discharge passage, which bypasses a corresponding one of the at least one phase check valve and connects between the corresponding check valve connecting chamber and the phase switch valve, wherein:
• each drain control valve blocks the corresponding bypass discharge passage when the working fluid is supplied from the fluid source to the corresponding check valve connecting chamber to rotate the vane rotor in a corresponding one of the retard side and the advance side relative to the housing;
• and
• the drain control valve opens the corresponding bypass discharge passage when the working fluid is discharged from the corresponding check valve connecting chamber to rotate the vane rotor in the other one of the retard side and the advance side relative to the housing;
• a drain switch valve that is installed in at least one pilot passage, which is branched from a supply passage that supplies the working fluid from the fluid source to the phase switch valve, and which is connected to the at least one drain control valve to apply the pilot pressure to the at least one drain control valve by the working fluid supplied from the fluid source, wherein the drain switch valve is switched to implement one of supplying of the working fluid from a corresponding one of the at least one pilot passage to a corresponding one of the at least one drain control valve and discharging of the working fluid from the corresponding one of the at least one pilot passage;
• and
• a pulsation reducing means that is installed in a fluid passage, which extends from the phase switch valve to the at least one drain control valve through the supply passage, a branch point between the supply passage and the at least one pilot passage, and the at least one pilot passage, wherein the pulsation reducing means is for permitting supplying of the working fluid from the fluid source to the at least one drain control valve and is also for reducing pressure pulsation, which is transmitted from the phase switch valve to the at least one drain control valve through the fluid passage.