Abstract:
A phaser comprising a housing, a rotor, and first and second passages. The housing has at least one chamber defined by an advance wall, an arcuate outer wall, and a retard wall. The rotor has at least one vane projecting from an outer circumference, separating the chamber in the housing into advance and retard chambers. The first passage facilitates fluid communication to a first port in the advance or retard chamber and a second passage to a second port in the other advance or retard chambers. Each port is spaced apart form the first wall or second wall of the vane, such that when the vane is moved towards the advance or retard wall of the chamber far enough, the passages are obstructed by the housing and fluid flow to the passages is restricted, such that impact of the vane with the walls of the chamber is cushioned.

Description:
REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of parent application Ser. No. 10/376,876, filed Feb. 28, 2003 now U.S. Pat. No. 6,866,013 entitled “Hydraulic Cushioning Of A Variable Valve Timing Mechanism” which claims priority from an invention which was disclosed in Provisional Application No. 60/374,241, filed Apr. 19, 2002, entitled “Hydraulic Cushioning of a Variable Valve Timing Mechanism.” The aforementioned applications are hereby incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention pertains to the field of variable valve timing (VCT) systems. More particularly, the invention pertains to a VCT mechanism having hydraulic cushioning. 
   2. Description of the Related Art 
   The performance of an internal combustion engine can be improved by the use of dual 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. Engine performance in an engine with dual camshafts can be further improved, in terms of idle quality, fuel economy, reduced emissions or increased torque, by changing the positional relationship of one of the camshafts, usually the camshaft which operates the intake valves of the engine, relative to the other camshaft and relative to the crankshaft, to thereby vary the timing of the engine in terms of the operation of intake valves relative to its exhaust valves or in terms of the operation of its valves relative to the position of the crankshaft. 
   Consideration of information disclosed by the following U.S. Patents, which are all hereby incorporated by reference, is useful when exploring the background of the present invention. 
   U.S. Pat. No. 4,601,231 shows a rotary actuator that restricts oil flow to prevent leakage without using the sealing members of the vane and reduces the velocity of the vane as it approaches impact with the housing, thus cushioning the impact of the vane against the stops. Multiple passages and switches are used to move oil to separate passages as the vane nears the stop. The vane is used to block the primary passage and switch to the bypass circuit. Specifically, the vane blocks a main path and oil is supplied to a subpath. A small amount of oil flows into the bypath and the majority of the oil flows and pushes against a ball valve, which is in opposition to a spring. The force of the oil overcomes the spring and pushes the ball valve from its seat and oil behind the ball valve and moves into a passage which leads the oil to bear against a cutout of the vane. When the pressure is great enough, the vane separates from the stopper so that a new oil chamber is formed therebetween. At the same time, oil passes through a path and reaches a boundary between the other vane and stopper and effects a similar separation when the vane advances, opening a main oil path. The oil from the main path is fed directly into a second diametrically opposed newly opened oil chamber from the main path. Simultaneously, oil the other chamber is discharged from the oil port via the main path as the rotation of the vanes takes place. A cushion effect is created so that the impact or shock of the vane against the spring is moderate. 
   U.S. Pat. No. 5,002,023 describes a VCT system within the field of the invention in which the system hydraulics includes a pair of oppositely acting hydraulic cylinders with appropriate hydraulic flow elements to selectively transfer hydraulic fluid from one of the cylinders to the other, or vice versa, to thereby advance or retard the circumferential position on of a camshaft relative to a crankshaft. The control system utilizes a control valve in which the exhaustion of hydraulic fluid from one or another of the oppositely acting cylinders is permitted by moving a spool within the valve one way or another from its centered or null position. The movement of the spool occurs in response to an increase or decrease in control hydraulic pressure, P C , on one end of the spool and the relationship between the hydraulic force on such end and an oppositely direct mechanical force on the other end which results from a compression spring that acts thereon. 
   U.S. Pat. No. 5,107,804 describes an alternate type of VCT system within the field of the invention in which the system hydraulics include a vane having lobes within an enclosed housing which replace the oppositely acting cylinders disclosed by the aforementioned U.S. Pat. No. 5,002,023. The vane is oscillatable with respect to the housing, with appropriate hydraulic flow elements to transfer hydraulic fluid within the housing from one side of a lobe to the other, or vice versa, to thereby oscillate the vane with respect to the housing in one direction or the other, an action which is effective to advance or retard the position of the camshaft relative to the crankshaft. The control system of this VCT system is identical to that divulged in U.S. Pat. No. 5,002,023, using the same type of spool valve responding to the same type of forces acting thereon. 
   U.S. Pat. Nos. 5,172,659 and 5,184,578 both address the problems of the aforementioned types of VCT systems created by the attempt to balance the hydraulic force exerted against one end of the spool and the mechanical force exerted against the other end. The improved control system disclosed in both U.S. Pat. Nos. 5,172,659 and 5,184,578 utilizes hydraulic force on both ends of the spool. The hydraulic force on one end results from the directly applied hydraulic fluid from the engine oil gallery at full hydraulic pressure, P S . The hydraulic force on the other end of the spool results from a hydraulic cylinder or other force multiplier which acts thereon in response to system hydraulic fluid at reduced pressure, P C , from a PWM solenoid. Because the force at each of the opposed ends of the spool is hydraulic in origin, based on the same hydraulic fluid, changes in pressure or viscosity of the hydraulic fluid will be self-negating, and will not affect the centered or null position of the spool. 
   U.S. Pat. No. 5,289,805 provides an improved VCT method which utilizes a hydraulic PWM spool position control and an advanced control algorithm that yields a prescribed set point tracking behavior with a high degree of robustness. 
   In U.S. Pat. No. 5,361,735, a camshaft has a vane secured to an end for non-oscillating rotation. The camshaft also carries a timing belt driven pulley which can rotate with the camshaft but which is oscillatable with respect to the camshaft. The vane has opposed lobes which are received in opposed recesses, respectively, of the pulley. The camshaft tends to change in reaction to torque pulses which it experiences during its normal operation and it is permitted to advance or retard by selectively blocking or permitting the flow of engine oil from the recesses by controlling the position of a spool within a valve body of a control valve in response to a signal from an engine control unit. The spool is urged in a given direction by rotary linear motion translating means which is rotated by an electric motor, preferably of the stepper motor type. 
   U.S. Pat. No. 5,497,738 shows a control system which eliminates the hydraulic force on one end of a spool resulting from directly applied hydraulic fluid from the engine oil gallery at full hydraulic pressure, P S , utilized by previous embodiments of the VCT system. The force on the other end of the vented spool results from an electromechanical actuator, preferably of the variable force solenoid type, which acts directly upon the vented spool in response to an electronic signal issued from an engine control unit (“ECU”) which monitors various engine parameters. The ECU receives signals from sensors corresponding to camshaft and crankshaft positions and utilizes this information to calculate a relative phase angle. A closed-loop feedback system which corrects for any phase angle error is preferably employed. The use of a variable force solenoid solves the problem of sluggish dynamic response. Such a device can be designed to be as fast as the mechanical response of the spool valve, and certainly much faster than the conventional (fully hydraulic) differential pressure control system. The faster response allows the use of increased closed-loop gain, making the system less sensitive to component tolerances and operating environment. 
   U.S. Pat. No. 5,657,725 shows a control system which utilizes engine oil pressure for actuation. The system includes A camshaft has a vane secured to an end thereof for non-oscillating rotation therewith. The camshaft also carries a housing which can rotate with the camshaft but which is oscillatable with the camshaft. The vane has opposed lobes which are received in opposed recesses, respectively, of the housing. The recesses have greater circumferential extent than the lobes to permit the vane and housing to oscillate with respect to one another, and thereby permit the camshaft to change in phase relative to a crankshaft. The camshaft tends to change direction in reaction to engine oil pressure and/or camshaft torque pulses which it experiences during its normal operation, and it is permitted to either advance or retard by selectively blocking or permitting the flow of engine oil through the return lines from the recesses by controlling the position of a spool within a spool valve body in response to a signal indicative of an engine operating condition from an engine control unit. The spool is selectively positioned by controlling hydraulic loads on its opposed end in response to a signal from an engine control unit. The vane can be biased to an extreme position to provide a counteractive force to a unidirectionally acting frictional torque experienced by the camshaft during rotation. 
   U.S. Pat. No. 5,979,380 discloses a valve timing control device with a locking mechanism for connecting the housing member and the rotor and a canceling device that cancels the locking mechanism. A vane divides a chamber into a first pressure chamber and a second pressure chamber. At least one of the walls defining the chambers has a bump or bulge into the chamber and a tapered cut bottom near the rotor. When the vane is flat or flush against the bulge, a small chamber is created with the sides being defined by the vane and the housing, the top of the chamber being the bulge and the bottom of the chamber being the rotor. The fluid flows from this chamber to passages leading to the advance or retard chambers. The ports of the passages are large and the tapered cut of the housing ensures that no obstruction of fluid can occur as the vane moves and becomes flush with the bulge of the housing. 
   For damping, the rotor has a receiving hole that becomes aligned with the canceling hole in the housing that receives the lock pin. The lock pin has a small diameter and a large diameter portion and is biased from the housing towards the rotor by a spring. When the rotor is rotated relative to the housing at the maximum retard condition the canceling hole and the receiving hole align and prevents the vane from colliding with the housing. 
   U.S. Pat. No. 6,247,434 shows a multi-position variable camshaft timing system actuated by engine oil. Within the system, a hub is secured to a camshaft for rotation synchronous with the camshaft, and a housing circumscribes the hub and is rotatable with the hub and the camshaft and is further oscillatable with respect to the hub and the camshaft within a predetermined angle of rotation. Driving vanes are radially disposed within the housing and cooperate with an external surface on the hub, while driven vanes are radially disposed in the hub and cooperate with an internal surface of the housing. A locking device, reactive to oil pressure, prevents relative motion between the housing and the hub. A controlling device controls the oscillation of the housing relative to the hub. 
   U.S. Pat. No. 6,250,265 shows a variable valve timing system with actuator locking for internal combustion engine. The system comprising a variable camshaft timing system comprising a camshaft with a vane secured to the camshaft for rotation with the camshaft but not for oscillation with respect to the camshaft. The vane has a circumferentially extending plurality of lobes projecting radially outwardly therefrom and is surrounded by an annular housing that has a corresponding plurality of recesses each of which receives one of the lobes and has a circumferential extent greater than the circumferential extent of the lobe received therein to permit oscillation of the housing relative to the vane and the camshaft while the housing rotates with the camshaft and the vane. Oscillation of the housing relative to the vane and the camshaft is actuated by pressurized engine oil in each of the recesses on opposed sides of the lobe therein, the oil pressure in such recess being preferably derived in part from a torque pulse in the camshaft as it rotates during its operation. An annular locking plate is positioned coaxially with the camshaft and the annular housing and is moveable relative to the annular housing along a longitudinal central axis of the camshaft between a first position, where the locking plate engages the annular housing to prevent its circumferential movement relative to the vane and a second position where circumferential movement of the annular housing relative to the vane is permitted. The locking plate is biased by a spring toward its first position and is urged away from its first position toward its second position by engine oil pressure, to which it is exposed by a passage leading through the camshaft, when engine oil pressure is sufficiently high to overcome the spring biasing force, which is the only time when it is desired to change the relative positions of the annular housing and the vane. The movement of the locking plate is controlled by an engine electronic control unit either through a closed loop control system or an open loop control system. 
   U.S. Pat. No. 6,263,846 shows a control valve strategy for vane-type variable camshaft timing system. The strategy involves an internal combustion engine that includes a camshaft and hub secured to the camshaft for rotation therewith, where a housing circumscribes the hub and is rotatable with the hub and the camshaft, and is further oscillatable with respect to the hub and camshaft. Driving vanes are radially inwardly disposed in the housing and cooperate with the hub, while driven vanes are radially outwardly disposed in the hub to cooperate with the housing and also circumferentially alternate with the driving vanes to define circumferentially alternating advance and retard chambers. A configuration for controlling the oscillation of the housing relative to the hub includes an electronic engine control unit, and an advancing control valve that is responsive to the electronic engine control unit and that regulates engine oil pressure to and from the advance chambers. A retarding control valve responsive to the electronic engine control unit regulates engine oil pressure to and from the retard chambers. An advancing passage communicates engine oil pressure between the advancing control valve and the advance chambers, while a retarding passage communicates engine oil pressure between the retarding control valve and the retard chambers. 
   U.S. Pat. No. 6,311,655 shows multi-position variable cam timing system having a vane-mounted locking-piston device. An internal combustion engine having a camshaft and variable camshaft timing system, wherein a rotor is secured to the camshaft and is rotatable but non-oscillatable with respect to the camshaft is described. A housing circumscribes the rotor, is rotatable with both the rotor and the camshaft, and is further oscillatable with respect to both the rotor and the camshaft between a fully retarded position and a fully advanced position. A locking configuration prevents relative motion between the rotor and the housing, and is mounted within either the rotor or the housing, and is respectively and releasably engageable with the other of either the rotor and the housing in the fully retarded position, the fully advanced position, and in positions therebetween. The locking device includes a locking piston having keys terminating one end thereof, and serrations mounted opposite the keys on the locking piston for interlocking the rotor to the housing. A controlling configuration controls oscillation of the rotor relative to the housing. 
   U.S. Pat. No. 6,374,787 shows a multi-position variable camshaft timing system actuated by engine oil pressure. A hub is secured to a camshaft for rotation synchronous with the camshaft, and a housing circumscribes the hub and is rotatable with the hub and the camshaft and is further oscillatable with respect to the hub and the camshaft within a predetermined angle of rotation. Driving vanes are radially disposed within the housing and cooperate with an external surface on the hub, while driven vanes are radially disposed in the hub and cooperate with an internal surface of the housing. A locking device, reactive to oil pressure, prevents relative motion between the housing and the hub. A controlling device controls the oscillation of the housing relative to the hub. 
   It has became more common for variable camshaft timing mechanisms to be made in a vane/housing format. Working hydraulic chambers are created by imposing either single or multiple vanes of a rotor attached to the camshaft into a cavity in a housing that is attached to the camshaft sprocket. The circumferential length of the pocket or cavity in the housing determines the relative phase travel of the camshaft relative to the sprocket/housing. The control is accomplished by exhausting fluid such as oil from one chamber while simultaneously filling the opposing chamber. This causes the variable camshaft timing mechanism to move the camshaft relative to the crankshaft manifested in a phase position. 
   The rate of change of the camshaft is determined in part by how fast the oil can exhaust from the resisting or draining hydraulic chamber. As the rotor of the VCT reaches the end of its travel limited by the cavity of the housing, the rotor will impact the housing and cause undesirable noise. As can be seen, there is need in a phaser to reduce the noise at the end of travel and keeping suitable rate of change in the phase position of the camshaft. 
   SUMMARY OF THE INVENTION 
   A phaser comprising a housing, a rotor, and first and second passages. The housing has at least one chamber defined by an advance wall, an arcuate outer wall, and a retard wall. The rotor has at least one vane projecting from an outer circumference, separating the chamber in the housing into advance and retard chambers. The first passage facilitates fluid communication to a first port in the advance or retard chamber and a second passage to a second port in the other advance or retard chambers. Each port is spaced apart form the first wall or second wall of the vane, such that when the vane is moved towards the advance or retard wall of the chamber far enough, the passages are obstructed by the housing and fluid flow to the passages is restricted, such that impact of the vane with the walls of the chamber is cushioned. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows a vane-type VCT phaser. 
       FIG. 2A  shows the vane in a central or null position. 
       FIG. 2B  shows the vane in a retard position. 
       FIG. 3  shows an alternative embodiment of the present invention. 
       FIG. 4  shows VCT system suitable for the present invention. 
       FIG. 5  shows a Cam Torque Actuated (CTA) VCT system applicable to the present invention. 
       FIG. 6  shows a close-up of  FIG. 2B . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1 through 2B , a vane-type VCT phaser comprises a housing  201 , the outside of which has sprocket teeth  208  that mesh with and are driven by timing chain  209 . Coaxially located within the housing  201 , free to rotate relative to the housing  201 , is a rotor  202  with vanes  205  projecting from the rotor&#39;s circumference  220 . The vanes  205  separate chambers in the housing defined by an arcuate outer wall, an advance wall  216  and a retard wall  218  into advance and retard chambers  206 ,  207 . The vanes  205  are formed of a first wall  215 , a second wall  214 , and top  223 . Along the circumference of the rotor  202 , a distance from the first and second walls  215 ,  214  of vanes  205 , are advance and retard passages  213 ,  212  extending from chambers  206  and  207  to a central control valve  204 . The central control valve  204  routes pressurized fluid to the advance and retard chambers  206 ,  207  by the advance and retard passages  213 ,  212 . A small fluid passage  224  is also present between the housing  201  and the circumference  220  of the rotor  202  not exposed to the chambers  206 ,  207 . 
     FIG. 2A  shows a close-up of the vane  205  and the advance and retard chambers  206 ,  207  in a central or null position. In this position, restrictions on the fluid in the chambers  206 ,  207  is not present. 
     FIG. 2B  shows the phaser in the retard position. In this position, fluid from the central control valve  204  is supplied to the retard chamber  207  from retard passage  212 . The fluid entering the retard chamber  207  forces the vane  205  to move to the left as shown by the figure, to a retard position, where the first wall  215  of the vane  205  and the advance wall  216  of the housing  201  forms a fluid pocket  228 . Fluid moves from the fluid pocket  228  to newly formed passage  230 , where the fluid is restricted or obstructed between the housing  201  and the circumference  220  of the rotor  201  as shown by the diagonal portion of  FIG. 6 . From the restricted passage  230 , fluid exits back to central control valve  204  through advance passage  213 . Fluid in the fluid pocket  228 , shown by the crosshatched portion is not under any restriction. Since fluid is restricted upon exit from the advance chamber  206  in the restricted passage  230 , the movement of the vane  205  is also slowed as it nears the advance wall  216  of the chamber, cushioning the vane from impact with the advance wall  216 . 
   As discussed in the background, undesirable noise occurs when the vane  205  slams into the advance wall  216  or the retard wall  218  of the chamber in the housing  201 . The restricted passage  230  prevents the vane  205  from slamming against the advance wall  216  of the chamber of the housing  201  and similarly the retard wall  218  on the opposite side of the chamber of the housing  201  by decelerating the amount of fluid that can exit the advance chamber  206  through the restricted passage  230  as described above. 
   While it is not shown, the same restriction occurs when the phaser is in the advance position, such that fluid is supplied from the central control valve  204  to the advance chamber  206  forcing the vane  205  to the right, to an advance position, where the second wall  214  of the vane  215  and the retard wall  218  of the housing  201  forms a fluid pocket  228 . Fluid moves from the fluid pocket  228  to newly formed passage  230 , where the fluid is restricted between the housing  201  and the circumference  220  of the rotor  202  not exposed to the chamber, similar to that shown by the diagonal portion of  FIG. 6 . From the restricted passage  230 , fluid exits back to central control valve  204  through retard passage  212 . Fluid in the fluid pocket  228 , similar to that shown by the crosshatched portion in  FIG. 6  is not under any restriction. Since fluid is restricted upon exit from the retard chamber  207  in the restricted passage  230 , the movement of the vane  205  is also slowed as it nears the retard wall  218  of the chamber. 
   It is noted that the present invention contemplates application in any type VCT phaser including Cam Torque Actuated (CTA), oil pressure actuated (OPA), or torsion assist (TA) phasers. It is further noted that normal phasing operation is defined as the rate of change of the camshaft when passages are fully within the cavity of the housing  1 . 
   It will be recognized by one skilled in the art that this description is common to vane phasers in general, and the specific arrangement of vanes, chambers, passages and valves shown in  FIG. 1  may be varied within the teachings of the invention. For example, the number of vanes and their location can be changed, some phasers have only a single vane, others as many as a dozen, and the vanes might be located on the housing and reciprocate within chambers on the rotor. The housing might be driven by a chain or belt or gears, and the sprocket teeth might be gear teeth or a toothed pulley for a belt. 
   Referring to  FIG. 3 , an alternative embodiment of the present invention is shown. A pair of separate inlet sources  28 ,  30  is introduced each with a check valve  32  and a separate exhaust port  12 ,  13  respectively. As can be seen, the phaser of VCT system would have an unlimited supply of fluid to fill the chambers  6 ,  7  and their respective exhaust ports  12 ,  13  thereby limiting the velocity of the rotor  2  near the end of travel. Thus good VCT response in all directions is achieved while limiting the velocity and thus the impact energy as the vane  5  approaches its mechanical stops due to the physical limitations of the housing cavity. 
   As discussed supra, the rate of change of the camshaft is determined, in part, by how fast fluid can exhaust from the resisting hydraulic chamber. As the rotor  2  of the VCT reaches the end of its travel, as limited by the housing  1 , the rotor  2  will impact the housing  1  and cause undesirable noise. The present invention permits the fluid to exhaust normally from the hydraulic chamber and thus does not limit the actuation rate of the VCT during normal phasing until the rotor nears the end of its travel. At this point the exhaust port would be restricted by the close clearance between the rotor  2  and the housing  1  by the provision of the distances  20 ,  22  at each end of the housing cavity respectively. In order to facilitate the normal fluid flow, separate inlet passages  28 ,  30  cures the possible defect of insufficient flow out of the exhaust chamber to the inlet chamber (see  FIG. 3 ). Without the separate inlet passage, fluid might not be exhaust sufficiently during the end of travel time segments. The end result may be insufficient fluid flow out of the exhaust chamber into the opposite chamber. However, the vane still moves in that the volume of the opposite chamber is increasing. This increase may cause the opposite chamber to draw undesirable material such as ambient air around the phaser. 
   The present invention gradually decelerates the VCT rotor  2  to a stop, thus limiting the impact energy with which the rotor  2  impacts the housing  1 . The present invention contemplates application in any type VCT phaser. 
   For example, in  FIG. 3  when fluid is exhausting from chamber  6  via passage  13 , at the end of travel of vane  5  the fluid flow rate may be decreased due to the structure of the present invention. At this juncture, chamber  7  still needs to be filled with sufficient fluid flow of a suitable rate. If the flow is below a threshold value, undesirable effects including entry of ambient air may get into chamber  7 . The introduction of inflow passage  30  reduces or solves the undesirable effect problem by introducing sufficient fluid flow rate thereby resulting in sufficient fluid flow into chamber  7 . Similar results occur at the opposite end of travel of the vane. 
   It is noted that only a portion of the phaser is shown here. The phaser may have more than one similar structure as shown in  FIGS. 2A ,  2 B,  3 , or  6 . For example, the phaser may have 2, 4, or 8 similar structures. 
     FIG. 4  is a schematic depiction that shows, in part, the VCT system of the present invention. A null position is shown in  FIG. 4 . Solenoid  120  engages spool valve  114  by exerting a first force upon the same on a first end  29 . The first force is met by a force of equal strength exerted by spring  21  upon a second end  17  of spool valve  114  thereby maintaining the null position. The spool valve  114  includes a first block  19  and a second block  23  each of which blocks fluid flow respectively. 
   The phaser  542  includes a vane  558 , a housing  57  using the vane  558  to delimit an advance chamber A and a retard chamber R therein. Typically, the housing  57  and the vane  558  are coupled to crankshaft (not shown) and camshaft (also not shown) respectively. Vane  558  is permitted to move relative to the phaser housing by adjusting the fluid quantity of advance and retard chambers A and R. If it is desirous to move vane  558  toward the retard side, solenoid  120  pushes spool valve  114  further right from the original null position such that fluid in chamber A drains out along duct  40  through duct  180 . The fluid is in fluid communication with an outside sink (not shown) by means of having block  19  sliding further right to allow said fluid communication to occur. Simultaneously, fluid from a source passes through duct  51  and is in one-way fluid communication with duct  70  by means of one-way valve  150 , thereby supplying fluid to chamber R via duct  50 . This can occur because block  23  moved further right causing the above one-way fluid communication to occur. When the desired vane position is reached, the spool valve is commanded to move back left to its null position, thereby maintaining a new phase relationship of the crank and cam shaft. 
   Referring to  FIG. 5 , a Cam Torque Actuated (CTA) VCT system applicable to the present invention is shown. The CTA system uses torque reversals in camshaft caused by the forces of opening and closing engine valves to move vane  942 . The control valve in a CTA system allows fluid flow from advance chamber  92  to retard chamber  93  or vice versa, allowing vane  942  to move, or stops fluid flow, locking vane  942  in position. CTA phaser may also have oil input  913  to make up for losses due to leakage, but does not use engine oil pressure to move phaser. 
   The detailed operation of CTA phaser system is as follows.  FIG. 5  depicts a null position in that ideally no fluid flow occurs because the spool valve  140  stops fluid circulation at both advance end  98  and retard end  910 . When cam angular relationship is required to be changed, vane  942  necessarily needs to move. Solenoid  920 , which engages spool valve  140 , is commanded to move spool  140  away from the null position thereby causing fluid within the CTA circulation to flow. It is pointed out that the CTA circulation ideally uses only local fluid without any fluid coming from source  913 . However, during normal operation, some fluid leakage occurs and the fluid deficit needs to be replenished by the source  913  via a one way valve  914 . The fluid in this case may be engine oil. The source  913  may be the oil pan. 
   There are two scenarios for the CTA phaser system. First, there is the Advance scenario, wherein an Advance chamber  92  needs to be filled with more fluid than in the null position. In other words, the size or volume of chamber  92  is increased. The advance scenario is accomplished by way of the following. 
   Solenoid  920  pushes the spool valve  140  toward right such that the left portion  919  of the spool valve  140  still stops fluid flow at the advance end  98 . But simultaneously the right portion  917  moved further right leaving retard portion  910  in fluid communication with duct  99 . Because of the inherent torque reversals in camshaft, drained fluid from the retard chamber  93  feeds the same into advance chamber  92  via one-way valve  96  and duct  94 . 
   Similarly, for the second scenario, a retard chamber  93  needs to be filled with more fluid than in the null position. In other words, the size or volume of chamber  93  is increased. The retard scenario is accomplished by way of the following. 
   Solenoid  920  reduces its engaging force with the spool valve  140  such that an elastic member  921  forces spool  140  to move left. The right portion  920  of the spool valve  140  stops fluid flow at the retard end  910 . But simultaneously the left portion  919  moves further right leaving Advance portion  98  in fluid communication with duct  99 . Because of the inherent torque reversals in camshaft, drained fluid from the Advance chamber  92  feeds the same into Retard chamber  93  via one-way valve  97  and duct  95 . 
   As can be appreciated, with the CTA cam phaser, the inherent cam torque energy is used as the motive force to re-circulate oil between the chambers  92 ,  93  in the phaser. This varying cam torque arises from alternately compressing, then releasing, each valve spring, as the camshaft rotates. 
   It should be noted that  FIGS. 4 and 5  are used to show different types of VCT system suitable for the present invention. Some structures are not depicted in detail. For these details, refer to  FIGS. 2–3 . 
   The following are terms and concepts relating to the present invention. 
   It is noted the hydraulic fluid or fluid referred to supra are actuating fluids. Actuating fluid is the fluid which moves the vanes in a vane phaser. Typically the actuating fluid includes engine oil, but could be separate hydraulic fluid. The VCT system of the present invention may be a Cam Torque Actuated (CTA) VCT system in which a VCT system that uses torque reversals in camshaft caused by the forces of opening and closing engine valves to move the vane. The control valve in a CTA system allows fluid flow from advance chamber to retard chamber, allowing vane to move, or stops flow, locking vane in position. The CTA phaser may also have oil input to make up for losses due to leakage, but does not use engine oil pressure to move phaser. A vane is a radial element actuating fluid acts upon, housed in chamber. A vane phaser is a phaser which is actuated by vanes moving in chambers. 
   There may be one or more camshaft per engine. The camshaft may be driven by a belt or chain or gears or another camshaft. Lobes may exist on camshaft to push on valves. In a multiple camshaft engine, most often has one shaft for exhaust valves, one shaft for intake valves. A “V” type engine usually has two camshafts (one for each bank) or four (intake and exhaust for each bank). 
   A chamber or cavity is defined as a space within which vane rotates. Chamber may be divided into advance chamber (makes valves open sooner relative to crankshaft) and retard chamber (makes valves open later relative to crankshaft). Check valve is defined as a valve which permits fluid flow in only one direction. A closed loop is defined as a control system which changes one characteristic in response to another, then checks to, see if the change was made correctly and adjusts the action to achieve the desired result (e.g. moves a valve to change phaser position in response to a command from the ECU, then checks the actual phaser position and moves valve again to correct position). The control valve is a valve, which controls flow of fluid to phaser. The control valve may exist within the phaser in CTA system. The control valve may be actuated by oil pressure or solenoid. Crankshaft takes power from pistons and drives transmission and camshaft. Spool valve is defined as the control valve of spool type. Typically the spool rides in bore, connects one passage to another. Most often the spool is most often located on center axis of rotor of a phaser. 
   A Differential Pressure Control System (DPCS) is a system for moving a spool valve, which uses actuating fluid pressure on each end of the spool. One end of the spool is larger than the other, and fluid on that end is controlled (usually by a Pulse Width Modulated (PWM) valve on the oil pressure), full supply pressure is supplied to the other end of the spool (hence differential pressure). The Valve Control Unit (VCU) is a control circuitry for controlling the VCT system. Typically the VCU acts in response to commands from ECU. 
   A driven shaft is any shaft, which receives power (in VCT, most often camshaft). Driving shaft is any shaft which supplies power (in VCT, most often crankshaft, but could drive one camshaft from another camshaft). ECU is Engine Control Unit that is the car&#39;s computer. Engine Oil is the oil used to lubricate engine, pressure can be tapped to actuate phaser through control valve. 
   The housing is defined as the outer part of phaser with chambers. The outside of housing can be pulley (for timing belt), sprocket (for timing chain) or gear (for timing gear). Hydraulic fluid is any special kind of oil used in hydraulic cylinders, similar to brake fluid or power steering fluid. Hydraulic fluid is not necessarily the same as engine oil. Typically the present invention uses “actuating fluid.” The lock pin is disposed to lock a phaser in position. Usually lock pin is used when oil pressure is too low to hold phaser, as during engine start or shutdown. 
   An Oil Pressure Actuated (OPA) VCT system uses a conventional phaser, where engine oil pressure is applied to one side of the vane or the other to move the vane. 
   An open loop is used in a control system which changes one characteristic in response to another (say, moves a valve in response to a command from the ECU) without feedback to confirm the action. 
   Phase is defined as the relative angular position of camshaft and crankshaft (or camshaft and another camshaft, if phaser is driven by another cam). A phaser is defined as the entire part which mounts to cam. The phaser is typically made up of rotor and housing and possibly spool valve and check valves. A piston phaser is a phaser actuated by pistons in cylinders of an internal combustion engine. The rotor is the inner part of the phaser, which is attached to a camshaft. 
   Pulse-width Modulation (PWM) provides a varying force or pressure by changing the timing of on/off pulses of voltage or fluid pressure. The solenoid is an electrical actuator, which uses electrical current flowing in coil to move a mechanical arm. A Variable force solenoid (VFS) is a solenoid whose actuating force can be varied, usually by PWM of supply voltage or with a current controller. A VFS is an alternative to an on/off (all or nothing) solenoid. 
   The sprocket is a member used with chains such as engine timing chains. Timing is defined as the relationship between the time a piston reaches a defined position (usually top dead center (TDC)) and the time something else happens. For example, in VCT or VVT systems, timing usually relates to when a valve opens or closes. Ignition timing relates to when the spark plug fires. 
   A Torsion Assist (TA) or Torque Assisted phaser is a variation on the OPA phaser, which adds a check valve in the oil supply line (i.e. a single check valve embodiment) or a check valve in the supply line to each chamber (i.e. two check valve embodiment). The check valve blocks oil pressure pulses due to torque reversals from propagating back into the oil system, and stop the vane from moving backward due to torque reversals. In the TA system, motion of the vane due to forward torque effects is permitted; hence the expression “torsion assist” is used. Graph of vane movement is step function. 
   A VCT system includes a phaser, control valve(s), control valve actuator(s) and control circuitry. Variable Cam Timing (VCT) is a process, not a thing, that refers to controlling and/or varying the angular relationship (phase) between one or more camshafts, which drive the engine&#39;s intake and/or exhaust valves. The angular relationship also includes phase relationship between cam and the crankshafts, in which the crank shaft is connected to the pistons. 
   Variable Valve Timing (VVT) is any process which changes the valve timing. VVT could be associated with VCT, or could be achieved by varying the shape of the cam or the relationship of cam lobes to cam or valve actuators to cam or valves, or by individually controlling the valves themselves using electrical or hydraulic actuators. In other words, all VCT is VVT, but not all VVT is VCT. 
   Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.