Abstract: A hydraulic control unit is configured to switch among a first state where a discharge passage of a pump driven by an internal combustion engine communicates with both a phase-advance passage and a lock passage and simultaneously a phase-retard passage communicates with a drain passage, a second state where the discharge passage communicates with both the phase-retard passage and the lock passage and simultaneously the phase-advance passage communicates with the drain passage, and a third state where the phase-advance passage, the phase-retard passage, and the lock passage all communicate with the discharge passage. The hydraulic control unit is further switchable to a fourth state where the discharge passage communicates with both the phase-advance passage and the phase-retard passage and simultaneously the lock passage communicates with the drain passage.

Abstract: A directional control valve is configured to switch among a first position at which a discharge passage communicates with a phase-advance passage and a phase-retard passage and a lock passage communicates with a drain passage, a second position at which the discharge passage communicates with the phase-advance passage and the lock passage and the phase-retard passage communicates with the drain passage, a third position at which the discharge passage communicates with the phase-retard passage and the lock passage and the phase-advance passage communicates with the drain passage, and a fourth position at which the discharge passage communicates with the lock passage and fluid-communication between the discharge passage and each of the phase-advance passage and the phase-retard passage is blocked. The directional control valve is further switchable to a sixth position at which the phase-advance passage, the phase-retard passage, and the lock passage all communicate with the discharge passage.