Abstract:
A vane-type camshaft phaser includes a vaned camshaft rotor disposed within a lobed stator, defining phase advance and retard chambers filled with oil. The height of the rotor is less than the height of the stator, providing space for a vaned brake rotor alongside the camshaft rotor. The brake rotor is free to rotate independently of the camshaft rotor. The volume of each advance and retard chamber is a function of the rotational position of both the camshaft rotor and the brake rotor, and the volume of each chamber is constant. Rotation of the brake rotor in one direction causes rotation of the camshaft rotor in the opposite direction. The brake rotor is connected to a controllable brake mechanism. By sensing of the camshaft rotor position and feedback control of the braking mechanism, the camshaft rotor may be maintained at any position.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to camshaft phasers for varying the timing of combustion valves in internal combustion engines; more particularly, to mechanism for varying the phaser relationship between an engine crankshaft and engine camshaft within a camshaft phaser; and most particularly, to a camshaft phaser actuated by a variable braking mechanism. 
       BACKGROUND OF THE INVENTION 
       [0002]    Vane-type camshaft phasers for varying the timing of combustion valves in an internal combustion engines are well known. In a vane-type phaser, timing advance and retard chambers are formed within the phaser between inwardly-extending lobes of a generally cylindrical stator and outwardly-extending vanes of a rotor concentrically disposed within the stator. The stator is mechanically coupled and indexed to the rotational position of the engine crankshaft, and the rotor is mechanically coupled to the camshaft. 
         [0003]    Typically, a camshaft phaser includes an oil control valve for controlling oil flow into and out of the advance and retard chambers to rotate the rotor with respect to the stator. The valve receives pressurized oil from an oil gallery in the engine block and selectively distributes oil to controllably vary the phase relationship between the engine&#39;s camshaft and crankshaft. By using pulse width modulated (PWM) control of the oil valve, cam timing is altered by command from an engine control module (ECM). In this manner, the oil control valve is a throttle and direction control valve that modulate cam position and the speed with which it changes from one position to another. 
         [0004]    Several problems are known to exist with prior art oil-pressure actuated vane-type phasers. 
         [0005]    First, engine oil pressure typically is relatively low at low engine speeds, and therefore at low engine speeds the response of a prior art camshaft phaser can be sluggish and not predictable. 
         [0006]    Second, oil viscosity is temperature dependent, and therefore phaser operation at low ambient temperatures and high oil viscosity can be slow and unreliable. At high engine temperatures, as may occur in warm climates, engine viscosity can be undesirably low, resulting as above in low oil pressure. 
         [0007]    Third, for fast phaser actuation a larger engine oil pump may be required, at a cost of additional parasitic energy drain on the engine and increased engine manufacturing cost. 
         [0008]    What is needed in the art is a camshaft phaser system that does not rely on dynamic supply of engine oil under pressure for actuation of a camshaft rotor. 
         [0009]    It is a principal object of the present invention to provide camshaft phasing that is independent of a dynamic supply of engine oil to the phaser. 
         [0010]    It is a further object of the invention to provide reliable camshaft phasing over a wide range of engine speeds and operating temperatures. 
       SUMMARY OF THE INVENTION 
       [0011]    Briefly described, a vane-type camshaft phasing system includes a camshaft rotor disposed conventionally within a chamber formed in a lobed stator, defining phase advance and retard chambers therebetween filled with oil. The rotor and stator each have a plurality of respective vanes and lobes. The height of the rotor is less than the height of the stator, providing space for a vaned brake rotor alongside the vaned camshaft rotor within the stator chamber, the brake rotor being free to rotate independently of the camshaft rotor. Thus, the volume of each advance and retard chamber at any given time is a function of the rotational position of both the camshaft rotor and the brake rotor. Further, the volume of each advance and retard chamber is constant, so that rotation of the brake rotor in one direction causes rotation of the camshaft rotor in the opposite direction. Manipulation of the brake rotor is used to vary the phase of the camshaft with respect to the stator, which is operationally connected to the engine crankshaft. The brake rotor is connected to a brake mechanism, such as a hysteresis brake, eddy current brake, friction brake, or the like. 
         [0012]    In operation, when the brake mechanism is de-energized, frictional torque of the camshaft and valves will automatically urge the camshaft rotor in the retard direction, thus driving the brake rotor in the advance direction. As the brake is progressively actuated, the retarding force on the camshaft rotor is progressively countered. When brake friction exceeds camshaft friction, the camshaft rotor begins to move in the phase-advance direction. By appropriate sensing of the camshaft rotor position and corresponding feedback control of the braking mechanism, the camshaft rotor may be stopped and maintained at any desired position in its range of authority. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0014]      FIG. 1  is a cross-sectional view of a prior art camshaft phaser, showing a three-vaned rotor operative within a three-lobed stator; 
           [0015]      FIG. 2  is a cross-sectional view of a first embodiment of a camshaft phaser improved in accordance with the present invention, showing a four-vaned camshaft rotor and a four-vaned brake rotor operative within a four-lobed stator; 
           [0016]      FIG. 3  is a schematic cross-sectional view of the camshaft phaser shown in  FIG. 2 , taken along line  3 - 3 , and showing the phaser in full camshaft phase retard mode; 
           [0017]      FIG. 4  is a schematic cross-sectional view like that shown in  FIG. 3 , showing the phaser in full camshaft phase advance mode; 
           [0018]      FIG. 5  is a schematic cross-sectional view like that shown in  FIG. 3 , showing the phaser in a camshaft phase position intermediary between full retard and full advance modes; 
           [0019]      FIG. 6  is a schematic cross-sectional view of a second embodiment of a camshaft phaser improved in accordance with the present invention; and 
           [0020]      FIG. 7  is a schematic cross-sectional view of a camshaft phaser in accordance with the invention, showing an exemplary braking apparatus for rotary positioning of the brake rotor. 
       
    
    
       [0021]    The exemplifications set out herein illustrate currently preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    The benefits and advantages of a camshaft phaser system in accordance with the invention may be better appreciated by first considering a prior art phaser having pressurized oil actuation from an engine oil supply. 
         [0023]    Referring to  FIG. 1 , in a prior art camshaft phaser  10 , a conventional stator  12  has a generally cylindrical shape and a plurality of angularly spaced-apart radial lobes  14  extending inwardly. Stator  12  is adapted to be driven rotationally by the crankshaft assembly (not shown) of an internal combustion engine  16  via a conventional sprocket wheel  18 . Concentrically disposed within stator  12  is a rotor  20  having a plurality of conventional radial vanes  22  extending outwardly from a central hub  24 , vanes  22  being interspersed with lobes  14  such that conventional first and second chambers  26 , 28  are formed on either side of each vane  22  for respectively advancing or retarding the position of the rotor with respect to the stator. Chambers  26 , 28  are closed axially by sprocket wheel  18  and a cover plate (not visible in  FIG. 1 ). All first and second chambers  26 , 28  are filled with oil. Prior art phaser assembly  10  may optionally include a locking pin subassembly  30  disposed in a vane  22  for rotationally immobilizing the rotor with respect to the stator at a specific predetermined relative angle, for example, full retard of the valve timing. Pressurized actuating oil is provided to first chambers  26  via first passages  32  in hub  24 , and to second chambers  28  via second passages  34  in hub  24 . 
         [0024]    Referring to  FIG. 2 , a first embodiment  100  of a camshaft phaser improved in accordance with the present invention comprises a stator  112  similar to prior art stator  12  and having a generally cylindrical shape and a plurality of angularly spaced-apart radial lobes  114  (in the present example, four lobes) extending inwardly. Stator  112  is adapted to be driven rotationally by the crankshaft assembly (not shown) of an internal combustion engine  16  via a conventional sprocket wheel (not shown) similar to prior art sprocket wheel  18 . Concentrically disposed within stator  112  is a camshaft rotor  120  similar to prior art rotor  20  and having a plurality of conventional radial vanes  122  extending outwardly from a central hub  124 , vanes  122  being interspersed with lobes  114  such that first and second chambers  126 , 128  are formed on either side of each vane  122  for respectively advancing or retarding the position of the rotor with respect to the stator. (For discussion purposes herein, phaser  100  is being driven clockwise  101 , thereby defining chambers  126  as phase advance chambers and chambers  128  as phase retard chambers. Chambers  126 , 128  are closed axially by the sprocket wheel and a cover plate (also not visible in  FIG. 2 ). All first and second chambers  126 , 128  are filled with oil. Camshaft rotor  120  in operation is attached to a camshaft  152  (see  FIG. 7 ) of engine  16  and rotates therewith in known fashion. 
         [0025]    The axial height, or thickness, of camshaft rotor  120  is less than the axial height, or thickness of stator  112 , defining a thickness difference therebetween. A brake rotor  140 , comprising a general hub region  142  and a plurality of radially extending vanes  144 , has a thickness substantially equal to the rotor/stator thickness difference. Brake rotor  140  is disposed, like camshaft rotor  120 , within stator  112  between camshaft rotor  120  and the phaser cover plate  121  ( FIG. 7 ). Camshaft rotor  120  and brake rotor  140  are free to rotate independently of one another about phaser axis  145 . 
         [0026]    Camshaft rotor vanes  122  and brake rotor vanes  144  are slidingly sealed radially against the cylindrical inner wall  146  of stator  112  and are substantially sealed against leakage between chambers  126  and  128 . Thus, it will be seen that the volume of each chamber  126  and each chamber  128  is unique and defined by the size and shape of the stator lobes  114  and the rotor vanes  122 , 144 . It will be further seen that rotation of either of rotors  120 , 140  in a first direction must cause the other of rotors  120 , 140  to rotate in the opposite direction due to displacement of oil within the constant-volume chambers  126 , 128 . Thus, when brake means are provided for controlling the rotational position of brake rotor  140 , the rotational position of camshaft rotor  120  will be similarly controlled (and thus the camshaft phase angle). 
         [0027]    This dynamic relationship is shown schematically in  FIGS. 3 through 5 . 
         [0028]    Referring to  FIG. 3 , respective vanes of camshaft rotor  120  and brake rotor  140  are shown disposed within stator  112 , defining phaser advance chamber  126  and phaser retard chamber  128 . Brake  150 , which exerts a rotation-restraining torque on brake rotor  140  when energized, is de-energized, as for example at engine start-up. The frictional resistance to rotation experienced by the camshaft  152  within the engine is expressed as a camshaft friction torque  154  that drives the camshaft rotor  120  to a fully retarded position. Oil in advance chamber  126  is displaced by camshaft rotor  120  into the brake rotor portion of chamber  126 , and simultaneously oil in retard chamber  128  is displaced by brake rotor  140  into the camshaft rotor portion of chamber  128 , causing brake rotor  140  to be rotated to a fully advanced position, in the absence of resistance from brake  150 . 
         [0029]    Referring to  FIG. 4 , it must be remembered that both camshaft rotor  120  and brake rotor  140  are rotating, with stator  112 , under the action of engine sprocket torque  153 , all in the same direction  101  about mutual axis  145  ( FIG. 2 ) with respect to engine  16 . Brake  150  is grounded to non-rotating engine  16  and is able to exert a rotation-restraining brake torque  156  on brake rotor  140 . When brake torque  156  exceeds camshaft friction torque  154 , brake rotor  140  is moved in the retard direction within chamber  126  and camshaft rotor  120  is moved in the advance direction within chamber  128 . Thus it is possible to control the relative advance and retard positions of camshaft rotor  120  simply by controlling drag on rotation of brake rotor  140 . 
         [0030]    Referring to  FIG. 5 , when brake torque  156  equals camshaft friction torque  154 , the angular position of camshaft rotor  120 , and thus the phase angle of camshaft  152 , is set at whatever position is desired between full retard and full advance. The set position of camshaft rotor  120  will remain fixed until brake torque  156  is increased or decreased, as desired to advance or retard, respectively, the phase of camshaft  152  with respect to stator  112 . 
         [0031]    Note that the operation of improved camshaft phaser  100  is independent of the oil supply system for engine  16 , although some replenishment connection thereto is desirable to compensate for leakage and thereby maintain voidless oil fill in chambers  126 , 128 . A check valve (not shown) may be desirable to maintain oil pressure within the phaser at a predetermined value. 
         [0032]    Note further that improvements in accordance with the present invention may be applied to a prior art camshaft phaser actuated by pressurized engine oil, defining thereby a hybrid oil/brake actuated phaser (not shown). 
         [0033]    Note still further that the term “oil” as used herein should be taken to mean any suitable working fluid in chambers  126 , 128 . Synthetic fluids other than petroleum oil, and having a lesser temperature/viscosity dependence, may be preferred in some applications. 
         [0034]    Note yet further that a spring (not shown) may be added to the proposed cam phaser to augment the camshaft friction torque  154 , and to provide a motive force to drive camshaft rotor  120  to a default position when the engine is off, or in the event of a phaser malfunction. A torsional spring is preferred. 
         [0035]    Note also that the proposed phaser assembly may optionally include a locking pin subassembly or any other mechanism for rotationally immobilizing camshaft rotor  120  with respect to stator  112  at a specific predetermined relative angle, for example, full retard of the valve timing, in a way similar to locking pin  30  in prior art phaser  10 . 
         [0036]    Referring to  FIG. 6 , in a second embodiment of a camshaft phaser  200  improved in accordance with the invention, a septum plate  280  is installed between camshaft rotor  220  and brake rotor  240 . In this embodiment, both the advance chamber and the retard chamber are thus composed of respective sub-chambers  226   a , 226   b  and  228   a , 228   b , the subchambers being connected by openings  282 ,  284 , respectively, in plate  280 . Septum plate  280  can facilitate an optimized configuration of camshaft rotor  220  and brake rotor  240  to avoid leakage and friction between the two rotors as they move relative to one another in operation of the phaser. Further, openings  282 , 284  may be fitted with check valve(s) and other apparatus (not shown) to further control the flow of oil between respective sub-chambers  226   a , 226   b  and  228   a , 228   b.    
         [0037]    Referring now to  FIG. 7 , an exemplary brake  150  is shown for actuating a brake rotor  140  in a camshaft phaser  100  improved in accordance with the invention. Various brake mechanisms are envisioned within the scope of the invention, for example, mechanical friction brakes actuated with an electromagnetic actuator (neither is shown) or a known electromagnetic eddy current brake  160 . 
         [0038]    A presently preferred type of brake is an electromagnetic hysteresis brake  162 , such as is available from Magtrol, Inc., West Seneca, N.Y. These types of brakes are commonly used as loads in dynamometers and have three advantages: they are contact-less, producing torque through a magnetic air gap without the use of magnetic particles or friction components, and hence little wear is to be expected; they are easy to control, since the amount of torque is a direct, monotonous function of current, which is generally linear until magnetic saturation; and the torque they produce is generally independent of rotational speed. 
         [0039]    The hysteresis effect in magnetism is applied to torque control by the use of two basic components: a reticulated pole structure  164  and a specialty steel rotor/shaft assembly  166  fastened together but not in physical contact with pole structure  164 . Pole structure  164  may be formed of any soft magnetic steel, either laminated or not laminated. Until a field coil  168  is energized, a drag cup  170  mounted on shaft assembly  166  can spin freely with the shaft assembly with only minimal friction from the associated bearings. Drag cup  170  is preferably formed of a semi-hard alloy, for example, Alnico, cobalt alloys  26  or  17 , Fe—Cr—Co alloys, Fe—Mn alloys, or the like. When a magnetizing force from field coil  168  is applied to drag cup  170 , the air gap  172  in pole structure  164  becomes a flux field. Drag cup  170 , and hence brake rotor  140 , is magnetically restrained from rotation. As would be obvious to one of ordinary skill in the art, the rotational position of camshaft  152  and camshaft rotor  120  may be monitored and appropriate current supplied to field coil  168  to cause a desired level of braking of brake rotor  140  to position camshaft rotor  120  at any desired position within its range of authority between full advance and full retard. 
         [0040]    Although a brake is preferred to move phaser rotor  140 , because of low electric energy draw, one skilled in the art will recognize that other actuation mechanisms, including electric motors, could be considered as well. 
         [0041]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.