chamber 22 it will be appreciated that there is no hydraulic communication between the advance volume and the retard volume and that both these volumes are constant during operation of the spool 19 as it will further explained below a control actuator 30 for example a step motor acts on one end of the spool 19 and controls movement of the spool 19 relative to the valve sleeve 17 under the control of an engine control unit ecu 31 this control actuator 30 is able to lock the spool 19 in position in to thereby lock the phaser vane 13 in position in operation the spool 19 can be moved to various positions between the advanced position shown in fig2 and the retarded position shown in fig3 based upon various engine parameters monitored by the ecu 31 which utilizes this information to operate the control actuator 30 hence as shown fig2 when the ecu 31 controls the control actuator 30 i e the step motor to pull the spool 19 to the left from its null position fig2 the oil in the valve advance chamber 21 is pushed to the phaser advance chamber 15 while the oil in the phaser retard chamber 16 is pulled to the valve retard chamber 22 causing the vane 13 to advance the control actuator 30 is then locked in position in order to avoid any further motion of the vane 13 toward the advance or retard direction similarly referring now to fig3 when the ecu 31 controls the control actuator 30 i e the step motor to pushes the spool 19 to the right from its null position.