A variable camshaft timing apparatus (10) includes a pulse actuating
circuit (24A,50,44,56,60,24R and 24R,52,46,54,58,24A) for oscillating the
variable camshaft timing apparatus in reaction to fluid under pulsation,
and includes a pressure actuating circuit
(30,34,36,40,44,50,24A,24R,52,46,66,80/180,32 and
30,34,38,42,46,52,24R,24A,50,44,64,80/180,32) for oscillating the variable
camshaft timing device in reaction to fluid under pressure. Advance and
retard valves (44,46) are interconnected with the pulse and pressure
actuating circuits for independently and simultaneously activating the
pulse and pressure actuating circuits. Finally, an exhaust valve (80,180,
280) is positioned in fluid communication with the pulse and pressure
actuating circuits, such that the variable camshaft timing device may be
oscillated using one or both of the pulse actuating and pressure actuating
circuits, and may be maintained in position using one or both of the pulse
actuating and pressure actuating circuits.