Abstract:
A variable camshaft timing apparatus ( 10 ) includes a pulse actuating circuit ( 24 A, 50,44,56,60,24 R and  24 R, 52,46,54,58,24 A) for oscillating the variable camshaft timing apparatus in reaction to fluid under pulsation, and includes a pressure actuating circuit ( 30,34,36,40,44,50,24 A, 24 R, 52,46,66,80/180,32  and  30,34,38,42,46,52,24 R, 24 A, 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.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based, in part, on provisional U.S. patent application No. Ser. 60/260,309, which was filed on Jan. 8, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an internal combustion engine having a control system for controlling the operation of a variable camshaft timing mechanism (VCT) of the type in which the position of a camshaft is circumferentially varied relative to the position of a crankshaft. More specifically, this invention relates to control systems for operating VCT devices in response to fluid under continuous pressure and fluid under pulsation to selectively advance, retard, or maintain the position of the camshaft. 
     2. Description of the Prior Art 
     It is known that the performance of an internal combustion engine can be improved by the use of dual overhead camshafts, one to operate the intake valves of the various cylinders of the engine and the other to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both of the camshafts can be driven by a single crankshaft-powered chain drive or belt drive. It is also known that the performance of an internal combustion engine having dual overhead camshafts, or but a single camshaft, can be improved by changing the positional relationship of a camshaft relative to the crankshaft. 
     It is also known that engine performance in an engine having one or more camshafts can be improved by varying camshaft timing, specifically in terms of idle quality, fuel economy, reduced emissions, or increased torque. For example, the camshaft can be “retarded” for delayed closing of intake valves at idle for stability purposes and at high engine speed for enhanced output. Likewise, the camshaft can be “advanced” for premature closing of intake valves during mid-range operation to achieve higher volumetric efficiency with correspondingly higher levels of torque. In a dual overhead camshaft engine, retarding or advancing the camshaft is accomplished by changing the positional relationship of one of the camshafts, usually the camshaft that operates the intake valves of the engine, relative to the other camshaft and the crankshaft. Accordingly, retarding or advancing the camshaft varies the timing of the engine in terms of the operation of the intake valves relative to the exhaust valves, or in terms of the operation of the valves relative to the position of the crankshaft. 
     There are a multitude of VCT architectures using actuating components that include piston-cylinder devices, hub and vanes, single lobe vanes, and opposed lobe vanes. Similarly, there are at least three distinct styles of VCT actuation in the prior art. The first style is referred to hereafter as an Oil Pressure Actuated (OPA) VCT. The OPA system includes a VCT responsive to fluid under continuous pressure generated by an engine oil pump. The second style is referred to hereafter as a Camshaft Torque Actuated (CTA) VCT. The CTA system includes a VCT responsive to fluid under pulsations generated by torque pulses in the camshaft. The third style is referred to hereafter as a multi-mode VCT. The multi-mode system includes a VCT responsive to both fluid under pressure and under pulsation to oscillate the camshaft. 
     With OPA devices, the VCT uses fluid output of an engine oil pump where the actuation rate of the VCT is limited by the available hydraulic power supplied by the pump. Many such VCT systems incorporate hydraulics including a hub having multiple circumferentially spaced vanes cooperating within an enclosed housing having multiple circumferentially opposed walls. The vanes and the walls cooperate to define multiple fluid chambers, and the vanes divide the chambers into first and second sections. For example, U.S. Pat. No. 4,858,572 (Shirai et al.) teaches use of such a system for adjusting an angular phase difference between an engine crankshaft and an engine camshaft using oil pressure from a pump. Shirai et al. discloses fluid circuits having check valves, a spool valve and springs, and electromechanical valves. Fluid is transferred from the first section to the second section, or vice versa, to thereby oscillate the vanes and hub with respect to the housing in one direction or the other. Each branch of the fluid flow path runs from one section to the other through a drainage clearance between the hub and the camshaft, back through the oil pump, and then through the spool valve and a check valve. The check valve prevents fluid from flowing out of each section back to the spool valve. 
     With CTA devices, the VCT uses the energy of reactive torques in the camshaft to power the VCT hydraulically through a check-valve fluid circuit. The camshaft is subjected cyclically to resistant torques when the rising profiles of the cam lobes open the valves against the action of the valve springs, and then to driving torques when the valve springs close the valves by causing them to follow along the descending profiles of the cam lobes. The alternating resistant and driving torques in the camshaft translate into slight pulsations in the vane. These pulsations result in alternating pressure differentials across the vane that alternately compress the fluid in the advance and retard fluid chambers. To retard the camshaft, fluid is allowed to escape during the pulsations from the advance chamber and flow to the retard chamber through one branch of a one-way fluid circuit. Alternately, to advance the camshaft, fluid is allowed to escape during the pulsations from the retard chamber to the advance chamber through another branch of a one-way fluid circuit. Accordingly, the VCT changes phase by exchanging fluid from one fluid chamber to the other using the differential in pressure of the fluid in the fluid chambers to increase the volume of one fluid chamber at the expense of the other. 
     For example, U.S. Pat. 5,645,017 to (Melchior) teaches use of a torque pulse actuated VCT to change phase of a camshaft. The &#39;017 patent discloses a vane type VCT having a vane within a housing that delimits opposing antagonistic chambers that are interconnected by two unidirectional circuits having opposite flow directions. A valve communicates with the two unidirectional circuits so as to transfer fluid from one antagonistic chamber to the other in response to alternating pressure differentials between the antagonistic chambers, where the pressure differentials result solely from torque pulsations in the camshaft and vane. 
     In the systems described above, VCT actuation is accomplished in response to torque pulsation in the camshaft or in response to engine oil pressure from an engine oil pump, but not both. This presents a significant disadvantage. 
     First, there are shortcomings to using only the CTA powered VCT. The CTA device has a significantly lower frequency response than the OPA device, even though the potential actuation rate of the CTA device is substantially higher than the OPA device due to the larger amount of energy in the cam torque inputs. For example, inline four cylinder engines typically operate at relatively high speeds and therefore generate very high frequency torque pulses to which CTA systems do not respond quickly enough to cause actuation of the VCT. Thus, the relatively low frequency response of the CTA system results in a dramatic drop in CTA performance at the higher engine speeds of the inline four cylinder engines. Similarly, inline six cylinder engines typically exhibit low amplitude camshaft torque pulses that are also inadequate to actuate the VCT. 
     In contrast, the OPA systems have nearly the opposite problem. Since the actuation rate of the OPA device is strongly dependent on engine oil pressure, the device performs well at higher engine speeds, when the oil pump is producing an abundance of oil pressure. At lower engine speeds, however, particularly when the engine is running hot, the performance suffers because the oil pump is producing relatively little oil pressure. 
     Because the OPA device performs well at high speed and the CTA performs well at lower speeds, it would be advantageous to combine both strategies and architectures into one multi-mode VCT device and be able to selectively switch between the two independently and/or use both simultaneously. For example, U.S. Pat. 5,657,725 (Butterfield et al.), which is assigned to the assignee hereof teaches uses of a dual-mode VCT system to change phase of a camshaft. The &#39;725 patent discloses a dual-mode device responsive to torque pulses and/or engine oil pump pressure for actuation. In the &#39;725 patent there is disclosed a VCT apparatus having a vane within a housing that delimits opposing advance and retard chambers that are interconnected by an hydraulic circuit having two check valves and a spool valve therein. Here, fluid flows from one chamber to the other, through one check valve and then through the spool valve, in response to sufficiently strong torque pulsations in the vane. When there are not sufficiently strong pulsations present in the vane, fluid flows from the one chamber, not through the check valve, but directly through the spool valve to exhaust. Simultaneously, make-up fluid from the engine oil pump flows through the spool valve both directly to the other chamber and indirectly to the other chamber, by cycling in parallel through the other check valve back through the spool valve. 
     While the &#39;725 patent discloses a significant improvement upon the prior art, there are still some disadvantages. For example, the system is two-position only and is not capable of maintaining position between fully advanced and fully retarded positions. Additionally, the system uses a relatively complicated hydraulic circuit and spool valve system. 
     Accordingly, what is needed is a multi-mode VCT system that is capable of advancing, retarding, and maintaining a camshaft in intermediate positions over the entire speed range of an engine and uses relatively inexpensive and uncomplicated and hydraulic circuitry and components. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a multi-mode VCT system that is capable of advancing, retarding, and maintaining a camshaft in intermediate positions over the entire speed range of an engine and uses relatively inexpensive and uncomplicated and hydraulic circuitry and components. 
     The present invention includes a variable camshaft timing device including a pulse actuating circuit for oscillating the variable camshaft timing device in reaction to fluid under pulsation. A pressure actuating circuit is included for oscillating the variable camshaft timing device in reaction to fluid under pressure. Advance and retard valves are interconnected with the pulse and pressure actuating circuits for independently and simultaneously activating the pulse and pressure actuating circuits. Finally, an exhaust valve is positioned in fluid communication with the pulse and pressure actuating circuits, whereby 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. 
     Accordingly, it is an object of the present invention to provide an improved variable camshaft timing device for varying camshaft timing in an internal combustion engine. 
     It is another object to provide a multi-mode variable camshaft timing device that is capable of operating in response to fluid under pressure from a pump and fluid under pulsations from alternating camshaft torques. 
     It is yet another object to provide a multi-mode variable camshaft timing device that is capable of maintaining position anywhere between a fully advanced and fully retarded position over the full range of engine speed, and does not necessarily require use of a spool valve, but may as an option. 
     These objects and other features, aspects, and advantages of this invention will be more apparent after a reading of the following detailed description, appended claims, and accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a VCT device according to the preferred embodiment of the present invention; 
     FIG. 1A is an end view of the device of FIG. 1 in its assembled state; 
     FIG. 2 is a schematic view of a VCT control system according to the preferred embodiment of the present invention, where the VCT is maintaining position; 
     FIG. 3 is a schematic view of a VCT control system of the present invention showing an alternative valve, where the VCT is advancing under cam torque actuation; 
     FIG. 4 is a schematic view of the VCT control system of FIG. 3, where the VCT is retarding under cam torque actuation; 
     FIG. 5 is a schematic view of a VCT control system according to the present invention showing an oil pressure actuated exhaust valve, where the VCT is advancing under oil pressure actuation; 
     FIG. 6 is a schematic view of a VCT control system according to the present invention showing an electro-hydraulic actuated exhaust valve, where the VCT is retarding under oil pressure actuation; 
     FIG. 7 is a schematic view of a VCT control system according to an alternative and the presently preferred embodiment of the present invention where the VCT is maintaining position; 
     FIG. 8 is a schematic view of another and the secondarily preferred embodiment of a VCT control system according to the present invention operating in a CTA mode during a phase shift to an advance position; 
     FIG. 9 is a view like FIG. 8 during a phase shift to a retard position; 
     FIG. 10 is a view like FIGS. 8 and 9 in which the VCT is not operating to shift phase either to an advance position or to a retard position; 
     FIG. 11 is a schematic view of the embodiment of a VCT control system of FIGS. 8-10 operating in an OPA mode during a phase shift to an advance position; 
     FIG. 12 is a view like FIG. 11 during a phase shift to a retard position; and 
     FIG. 13 is a view like FIGS. 11 and 12 in which the VCT is not operating to shift phase either to an advance position or to a retard position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In general, an hydraulic timing system is provided for varying the phase of one rotary member relative to another rotary member. More particularly, the present invention provides a multi-mode Variable Camshaft Timing system (VCT) that is powered by, or is responsive to, engine oil under pressure from a pump and/or from engine oil under pressure pulsations inherent as a result of the tongue pulsations that occur in a rotating camshaft. While the present invention will be described in detail with respect to internal combustion engines, the VCT system is also well suited to other environments using hydraulic timing devices. Similarly, the fluid medium described herein is preferably engine oil, but any other standard hydraulic fluid may be used. Accordingly, the present invention is not limited to only internal combustion engines. 
     Referring now in detail to the Figures, there is shown in FIG. 1 a VCT apparatus  10  according to the preferred embodiment of the present invention. It is contemplated that the VCT apparatus  10  operates under control of an engine control module as is commonly known in the art. The VCT apparatus  10  includes a housing  12  having sprocket teeth  14  circumferentially disposed around its periphery. The housing  12  circumscribes a hub  16  to define fluid chambers  24  therebetween. The hub  16  is mechanically connected to a camshaft  26  to be rotatable therewith but not oscillatable with respect thereto. The hub  16  is in fluid communication with the camshaft  26  via passages (not shown) as is commonly known in the art. The hub  16  includes circumferentially spaced lobes  18  extending radially outwardly to divide each fluid chamber  24  into advance and retard chambers  24 A and  24 R, as shown in FIG.  1 A. Each lobe  18  includes a slot  20  therein for housing a vane  22 . The vane  22  cooperates with the inside of the housing  12  to seal the advance and retard chambers  24 A and  24 R so that they are fluid tightly separated from one another. 
     Referring again to FIG. 1, the assembly that includes the camshaft  26  with the hub  16  and housing  12  is caused to rotate by torque applied to the housing  12  by an endless chain (not shown) that engages the sprocket teeth  14  so that rotation is imparted to the endless chain by a rotating crankshaft (also not shown). The use of a cogged timing belt to drive the housing  12  is also contemplated. Rotation, in turn, is imparted from the housing  12  to the hub  16  through fluid in the fluid chambers  24 A and  24 R. 
     The hub  16  can be circumferentially retarded or advanced in position with respect to the housing  12 . Therefore, the housing  12  rotates with the camshaft  26  and is oscillatable with respect to the camshaft  26  to change the phase of the camshaft  26  relative to the crankshaft. The VCT hardware, as opposed to the VCT  10  as a system, may be of any architecture that is well known in the art. Accordingly, examples of well known VCT hardware architectures include those of commonly assigned U.S. Pat. No. 5,107,804 (Becker et al.) and the aforesaid &#39;725 patent, which are also incorporated by reference herein. In addition to the VCT hardware, an oscillation control configuration is required to oscillate the VCT apparatus  10 , and is described below. 
     To complement the hardware example shown in FIG. 1, FIG. 2 illustrates a schematic of the VCT apparatus  10  of the present invention. It is contemplated, and is well known in the art, that VCT control systems include fluid circuits that are drilled or otherwise machined or formed into the hardware components of the VCT apparatus  10 . The exact location of passages and interconnections of such fluid circuitry is not critical to the present invention and is therefore only schematically illustrated. 
     Structurally, the control system for the VCT apparatus  10  can be described in terms of passages, valves, etc. A fluid pressure source such as an engine oil pump  30  is located upstream and is in fluid communication with the downstream advance and retard chambers  24 A and  24 R that are separated by the lobe  18 . The engine oil pump  30  includes an inlet side  301  that communicates with a sump  32  of the engine oil system, and includes an opposite outlet side  300  that supplies oil to the advance and retard chambers  24 A and  24 R. The sump  32  collects oil from various parts of the control system to complete the circuits thereof An oil supply passage  34  fluidly communicates the outlet side  300  of the pump and branches into an advance branch passage  36  and a retard branch passage  38 . The branch passages  36  and  38  include supply check valves  40  and  42 , respectively, for permitting oil flow in a downstream direction from the pump  30  but prevents oil flow in an upstream direction back toward the pump  30 . In other words, the check valves  40  and  42  prevent counterflow back to the pump  30 . 
     Downstream of each check valve, each branch passage  36  and  38  terminates in an advance or retard valve  44  or  46 , respectively. Preferably, the valves  44  and  46  are pulse width modulated (PWM) valves, having a supply port  44 S or  46 S in fluid communication with the oil supply passage  34 . Each of the valves  44  and  46  also include a control port  44 C or  46 C in fluid communication with one end of an advance or retard chamber passage  50  or  52 . An opposite end of the chamber passage  50  or  52  fluidly communicates with one of the advance or retard chambers  24 A and  24 R. Each valve  44  or  46  finally includes an exhaust port  44 E or  46 E communicable with the control port  44 C or  46 C and in fluid communication with both a pulse passage  54  or  56  and an exhaust passage  64  or  66 . Each pulse passage  54  or  56  includes one end in communication with the valve  44  or  46 , and an opposite end in communication with one of the advance or retard chambers  24 A and  24 R and with one of the corresponding chamber passages  50  and  52 . Each pulse passage  54  and  56  includes a pulse check valve  58  and  60 , respectively, just upstream of the connection with the chamber passage  50  or  52  to prevent upstream oil flow through the pulse passage  54  or  56 , or in other words, to prevent counterflow from the chamber  24 A or  24 R toward the valve  44  or  46 . Each exhaust passage  64  and  66  includes one end in communication with the exhaust port  44 E or  46 E, respectively, of the valve  44  or  46  and with an exhaust valve  80 , such that the exhaust valve  80  terminates each of the exhaust passages  64  and  66 . Accordingly, the exhaust valve  80 , as shown in FIG. 2 includes a piston  82  that is radially disposed within a radial valve passage  84  within the hub  16 . 
     A spring  86  supports the valve  80  in a valve closed position, such that a combined exhaust passage  88  is blocked by the valve  80 . The spring force may be chosen in accordance with a calculation of the rotational speed of the engine, to establish the desired valve opening condition, as is well known. In the valve open position, the exhaust valve  80  and combined exhaust passage  88  communicate with the sump  32  of the engine either via passageways or by draining down through gaps between engine components, which is consistent with designs well known in the art. The PWM valves  44  and  46  and the exhaust valve  80  are preferably controlled by a central source such as an engine control unit or the like, as is well known in the art. 
     Systemically, the VCT control system can be described in terms of circuits defined from the structure described above. The VCT control system includes a pulse actuating circuit and a pressure actuating circuit. The pulse actuating circuit is further divided into a retard pulsing path, an advance pulsing path, and a make-up oil circuit. The retard pulsing path includes in fluid communication, the advance chamber  24 A, the advance chamber passage  50 , the advance PWM valve  44 , the retard pulse passage  56 , and the retard chamber  24 R. Similarly, the advance pulsing path includes in fluid communication, the retard chamber  24 R, the retard chamber passage  52 , the retard PWM valve  46 , the advance pulse passage  54 , and the advance chamber  24 A. Additionally, since the system is not perfectly sealed against oil loss, the make-up oil circuit is necessary and is defined by the oil supply passage  34 , the valve  44  or  46 , the chamber passage  50  or  52 , and the chamber  24 A or  24 R. 
     Similarly, the pressure actuating circuit is further divided into a pressure supply path and a pressure exhaust path. The pressure supply path includes in fluid communication, the oil supply passage  34 , one check valve  40  or  42 , one valve  44  or  46 , the chamber passage  50  or  52 , and the chamber  24 A or  24 R. The pressure exhaust path includes in fluid communication, the other chamber  24 A or  24 R, the other chamber passage  50  or  52 , the other valve  44  or  46 , the exhaust passage  64  or  66 , and the exhaust valve  80 . 
     In operation, the VCT apparatus  10  oscillates or maintains position anywhere in and between a fully retarded position and a fully advanced position. In the fully retarded position, the volume of the advance chamber  24 A would be approximately zero, while the volume of the retard chamber  24 R would be at a maximum. The reverse is true for the VCT apparatus  10  in the fully advanced position. To maintain any position intermediate the fully advanced and fully retarded positions, the VCT apparatus  10  of the present invention operates under closed loop control. In other words, as is well known, the VCT system communicates with position feedback sensors that monitor the relative position of the camshaft. The position feedback is used by the VCT system in further controlling the phase of the VCT apparatus  10 . 
     In FIG. 2, the VCT apparatus  10  is shown maintaining position halfway between the fully advanced and retarded positions. To achieve this result, the pressure actuating circuit is activated to supply oil to both the advance and retard chambers  24 A and  24 R simultaneously. Accordingly, oil flows from the pump  30  through the oil supply passage  34  into each oil supply branch  36  and  38 . The oil continues through each check valve  40  and  42  and into the supply port  44 S or  46 S of each valve  44  or  46 . Each valve  44  or  46  is positioned in an exhaust port-closed position to direct oil out of the control port  44 C and  46 C and through the chamber passage  50  or  52  into the respective chamber  24 A or  24 R. The pulse check valves  58  and  60  remain closed against their seats under fluid pressure from the chamber passage  50  or  52 . Thus each chamber  24 A or  24 R experiences the same fluid pressure from the pump  30  through each respective branch of the control system. Here, no fluid pressure from the pump  30  reaches the exhaust passages  64  or  66 . Accordingly, the exhaust valve  80  may remain closed, or may be open, because the state of the exhaust valve  80  will have no significant effect in this control system state. 
     FIG. 3 illustrates the control system in an advancing state under cam torque actuation. Cam torque actuation operates in response to reactive camshaft torques as previously described in the Background section above. Here, the advance valve  44  remains in the exhaust-closed position, while the retard valve  46  is moved to a source closed position. An exhaust valve  180  takes a closed position. Accordingly, each torque pulsation of the VCT apparatus  10  in the advancing direction acts to momentarily compress the oil in the retard chamber  24 R. This compression causes the oil in the retard chamber  24 R to escape therefrom into the advancing pulsing path: through the retard chamber passage  52 , into the control port  46 C of the advance valve  46  and out the exhaust port  46 E, through the advance pulse passage  54 , past the check valve  58 , and into the advance chamber  24 A. Check valve  60  prevents pulsing oil from circumventing the advance valve  44 . Make up oil flows from the pump  30 , up through the advance valve  44  and into the advance chamber  24 A. The supply check valve  40  prevents oil under pulsation from discharging back to the pump  30 . 
     The exhaust valve  180  of FIG. 3 is actuated by engine oil pressure, and includes a spring-loaded piston  182  that is preferably axially disposed within an axial passage  184  within the hub  16 . A spring  86  supports the valve  180  in a valve closed position, such that a combined exhaust passage  88  is blocked by the valve  180 . As shown, the engine oil pressure is insufficient to displace the valve  180  for OPA operation. 
     FIG. 4 illustrates the mirror image of FIG. 3, the control system in a retarding state under cam torque actuation. Here, the retard valve  46  remains in the exhaust-closed position, while the advance chamber valve  44  is moved to a source closed position. Accordingly, each torque pulsation of the VCT apparatus  10  in the retarding direction acts to momentarily compress oil in each advance chamber  24 A. This compression causes the oil in the advance chamber  24 A to discharge therefrom into the retard pulsing path through the advance chamber passage  50 , into the control port  44 C of the valve  46  and out the exhaust port  44 E of the valve  44 , through the retarding pulse passage  56 , past the check valve  60 , and into the retard chamber  24 R. The check valve  58  prevents pulsing oil from circumventing the pulsing path. Make-up oil flows from the pump  30 , up through the retard valve  46  and into the retard chamber  24 R. The supply check valve  42  prevents oil under pulsation from discharging back to the pump  30 . The exhaust valve  180  of FIG. 4 is the same as that shown in FIG.  3 . 
     FIG. 5 illustrates the control system in an advancing state under oil pressure actuation. Oil pressure actuation operates in response to available hydraulic power of the engine as previously described in the Background section above. Here, oil flows under pressure from the pump  30  through the pressure actuating circuit. Specifically, oil flows through the check valve  40 , into the supply port  44 S of the valve  44  and out the control port  44 C thereof, through the advance chamber passage  50 , and into the for advance chamber  24 A. Simultaneously, oil flows out of the retard chamber  24 R, through the retard pulse passage  52 , into the control port  46 C of the valve  46  and out the exhaust port  46 E thereof, through the exhaust passage  66 , through the exhaust valve  180 , and into the sump  32  to be recycled through the pump  30 . 
     The exhaust valve  180  of FIG. 5 is the same as that of FIGS. 3 and 4 and is used as a switching means to invoke oil pressure actuation of the VCT apparatus  10 . Here, the exhaust valve  180  is opened under fluid pressure from the engine oil pump  30  at higher engine speeds when CTA loses effectiveness. The exhaust valve  180  opens when sufficient engine oil pressure acts upon the valve  180  to overcome a predetermined spring force. An exhaust actuation passage  190  fluidly communicates an exhaust valve chamber  192  with the oil supply passage  34 . Accordingly, oil constantly flows to the exhaust valve  180  but only acts to open the valve  180  under a minimum oil pressure in correlation with a predetermined engine speed sufficient to generate the minimum oil pressure. Therefore, the spring force is selected in accordance with a calculation of the oil pressure of the engine as balanced against the spring force to establish the desired valve opening condition. As shown in the valve open position, the exhaust valve  180  and a combined exhaust passage  188  communicate with the sump  32  of the engine either via passageways or by draining down and over components of the engine consistent with designs well known in the art. 
     FIG. 6 illustrates the mirror image of FIG. 5, the control system in a retarding state under oil pressure actuation. Oil flows under pressure from the pump  30  through the pressure actuating circuit. Oil flows thorough the check valve  42 , into the supply port  46 S of the retard valve  46  out the control port  46 C thereof, through the retard chamber passage  52 , and into the retard chamber  24 R. Simultaneously, oil flows out of the advance chamber  24 A, through the advance chamber passage  50  into the control port  44 C of the advance valve  44  and out the exhaust port  44 E thereof, through the exhaust passage  64 , through the exhaust valve  180 , and into the sump  32  to be recycled. 
     FIG. 6 also illustrates the exhaust valve  180  alternatively actuated by engine oil pressure controlled by a solenoid valve  194 . Here, the exhaust valve  180  is actuated similar to that the exhaust valve  180  of FIG. 5, except the solenoid valve  194  controls actuation. Accordingly, a much lighter spring force may be selected such that the exhaust valve  180  will open under a relatively low engine speed and oil pressure, but only when the solenoid valve  194  is open. This will permit a much broader range of engine speed over which the exhaust valve  180  may open. Again, placement of hardware such as the solenoid valve  194  is not critical to the present invention and is engineered in accordance with techniques already well known in the art. 
     FIG. 7, illustrates an alternative and the presently preferred embodiment of the present invention that uses a purely mechanical valving arrangement instead of the electro-mechanical valve arrangement of FIGS. 2 through 6. A VCT apparatus  110  is shown maintaining position halfway between the fully advanced and retarded positions. To achieve this result, the pressure actuating circuit is activated to supply oil to both advance and retard chambers  124 A and  124 R simultaneously. Accordingly, oil flows from a pump  130  through an oil supply passage  134  into an oil supply branch  136 . The oil continues through a check valve  140  and into a supply port  145 S of a spool valve  145 . 
     The spool valve  145  is positioned in an exhaust port-closed position to direct oil through pulse passages  154  and  156  into the respective chambers  124 A and  124 R. The pulse check valves  158  and  160  open under fluid pressure from the oil supply branch  136 . Thus each chamber  124 A or  124 R experiences the same fluid pressure from the pump  130  through each respective branch of the control system. Here, no fluid pressure from the pump  130  reaches an exhaust passage  165 , because an exhaust check valve  170  blocks flow into the exhaust passage  165 , and the spool valve  145  blocks flow from the chamber passages  150  and  152 . 
     To advance in CTA mode, the spool valve  145  shifts to the left to open a retard chamber passage  152  to the exhaust passage  165 , which is blocked by an exhaust valve  180  near a retard exhaust port  145 R. Accordingly, oil pulsing from the retard chamber  124 R deadheads at the retarding check valve  160 , flows through the retard chamber passage  152  around the spool valve  145  on the right side, deadheads against the spool valve  145  in the advance chamber passage  150  on the left side, flows through the exhaust check valve  170  around the spool valve  145  into the advance pulse passage  154  past the advance check valve  158  and into the advance chamber  124 A. Here, source oil alone may or may not be sufficient to change phase of the VCT apparatus  110 , and, therefore, oil under pulsation is used to change phase of the VCT apparatus  110 . To advance in OPA mode, the spool valve shifts to the left to open the retard chamber passage  152  to the exhaust passage  165 , which would be open to a sump  132 . 
     To retard in CTA mode, the spool valve  145  shifts to the right to open an advance chamber passage  150  to the exhaust passage  165 , which is blocked by the exhaust valve  180  near an advance exhaust port  145 A. Accordingly, oil pulsing from the advance chamber  124 A deadheads at the advance check valve  158 , flows through the advance chamber passage  150  around the spool valve  145  on the left side, deadheads against the spool valve  145  in the retard chamber passage  152  on the right side, flows through the exhaust check valve  170  around the spool valve  145  into the retard pulse passage  156  past the retard check valve  160  and into the retard chamber  124 R. To retard in OPA mode, the spool valve shifts to the right to open the advance chamber passage  150  to the exhaust passage  165 , which would be open to the sump  132 . The shifting of the spool valve  145  to the left or right from the position in FIG. 7 may be controllably actuated in any suitable manner, for example, by a variable force solenoid (not shown). 
     FIGS. 8-13 illustrate an alternative embodiment of the present invention in which the change from a CTA mode (FIGS. 8-10) to an OPA mode (FIGS. 11-13) is responsive to a position of a centrifugally operated, and, therefore, radially extending control valve  288 . The valve  288  moves to and fro within a valve body  280 , which may be considered to extend radially within a rotating camshaft  226 . At low rotational speeds of the camshaft  226 , the valve  288  will be radially inwardly biased, to the left as shown in FIGS. 8-13, by a spring  286 , and in the position of the valve  288  in FIGS. 8-10, no oil will be able to flow through the valve  288  to an exhaust line  232  that leads to an engine oil sump. In this position of the valve  288 , oil will flow either from a retard chamber  224 R of a fluid chamber  224  in a housing  212  to an advance chamber  224 A of the chamber  224  (FIG. 8) or oil will flow from the advance chamber  224 A to the retard chamber  224 R (FIG.  9 ), or no oil will flow between the advance chamber  224 A and the retard chamber  224 R (FIG.  10 ), depending on the position of a spool element  290  that slides to and fro within a valve body  292 . In that regard, the spool element  290  has spaced lands  290 A,  290 B that are adapted to block flow into or out of chambers  224 A,  224 R through lines  254 ,  256 , respectively (FIG.  10 ), or to permit flow out of chamber  224 R into chamber  224 A (FIG. 8) through the valve body  292 , or to permit flow out of chamber  224 A into chamber  224 R (FIG. 9) through the valve body  292 , depending on the axial position of the spool  290  within the valve body  292 . In that regard, the spool  290  is resiliently biased to its FIG. 8 position, one of its end positions, by a spring  294 , which is positioned within the camshaft  226 , the spring  294  acting on an end of the spool  290 . The spool  290  is also urged to its FIGS. 9 and 10 positions by a variable force solenoid  290 , which acts on an opposed end of the spool  290 , the solenoid  296  being controlled in its operation by an electronic engine control unit  298 , in a known manner. 
     Control of oil flow into or out of the chambers  224 A,  224 R in an OPA mode of the embodiment of FIGS. 8-13 is illustrated in FIGS. 11,  12 , the flow being out of the chamber  224 R and into the chamber  224 A in FIG. 11, or there will be no flow into or out of either chamber  224 A or  224 R, in FIG. 13 except for some leakage of make-up oil across the spool  290 , depending on the axial position of the spool  290  within the valve body. 
     In FIG. 11, the land  290 B is positioned to allow flow out of the chamber  224 R through the line  256  and the valve body  292 , but this flow now passes into the exhaust line  232  because of the position of the valve  280  within the valve body  280 . At the same time, engine oil with flow into the chamber  224 A from a source  230  through a line  234 , the valve body  292  and the line  254 , the land  290 A being positioned to open the line  254  to inflow. In the FIG. 12 position of the spool  290 , oil will flow from the source  230  through the line  234 , the valve body  292  and the line  256  into the chamber  224 R; at the same time, oil will flow out of the chamber  224 A through the line  254 , the valve body  292  and the valve body  280  into the exhaust line  232 . 
     In FIG. 12, the land  290 B is positioned to allow flow from the source  230  through the valve body and the line  256  into the chamber  224 R, and the land  290 A is positioned to allow flow out of the chamber  224 A through the line  254 , the valve body  292  and the valve body  288  into the exhaust line,  232 , a line  266  with branches  266 A,  266 B extending between the valve body  288  and the valve body  292  to provide flow either from the chamber  224 R to the valve body  288  through the branch line  266 B and the line  266  (FIG.  11 ), or from the chamber  224 A to the valve body  288  through the branch lines  266 A and the line  266  (FIG.  12 ). In any case, the land  290 A is positioned to block oil flow through the valve body  292  into the branch line  266 A in the FIG. 11 condition of operating, and the land  290 B is position to block oil flow from the valve body  292  into the branch line  266 B in the FIG. 12 condition of operation. 
     The to and fro movement of the spool  290  in the valve body  292  in the OPA mode of operation of FIGS. 11-13 is the same as in the CTA mode of operation of FIGS. 8-10, namely under a variable force imposed on an end of the spool  290  by the variable force solenoid  296 , which is opposed by a force imposed on an opposed end of the spool  290  by the spring  294 . Likewise, the force imposed on the spool  290  by the solenoid  296  is controlled by the engine oil controller  298 . 
     In the FIG. 13 condition of operation, there will be no oil flow into or out of the chamber  224 R because the land  290 B of the spool  290  is positioned to block flow through the line  256 . Likewise, in this condition of operation there will be no oil into or out of the chamber  224 A because the land  290 A of the spool  290  is positioned to block flow through the line  254 . In any case, it is to be understood that the solenoid  296  can be operated with some dither in either the FIG. 10 or the FIG. 13 conditions of the embodiment of FIGS. 8-13 to permit some small flow of make-up oil into the chambers  224 A,  224 R to replace any oil lost by leakage thereform. 
     From the above, it can be appreciated that a significant advantage of the present invention is that the camshaft may be advanced or retarded with respect to an engine crankshaft reliably over the entire speed range of any engine, regardless of either a lack of sufficient oil pump capacity or an absence of sufficient pulsations in the camshaft. 
     An additional advantage is that the VCT of the present invention involves inexpensive modifications to the control systems of already well known VCT hardware having oil passages therethrough. 
     While the present invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Accordingly, the scope of the present invention is to be limited only by the following claims.