Abstract:
A camshaft (26) has a vane (60) secured to an end thereof for  non-oscillag rotation therewith. The camshaft also carries a sprocket (32) which can rotate with the camshaft but which is oscillatable with the camshaft. The vane has opposed lobes (60a, 60b) which are received in opposed recesses (32a, 32b), respectively, of the sprocket. The recesses have greater circumferential extent than the lobes to permit the vane and sprocket to oscillate with respect to one another, and thereby permit the camshaft to change in phase relative to a crankshaft whose phase relative to the sprocket is fixed by virtue of a chain drive extending therebetween. The camshaft tends to change in reaction to pulses which it experiences during its normal operation, and it is permitted to change only in a given direction, either to advance or retard, by selectively blocking or permitting the flow of hydraulic fluid, preferably engine oil, through the return lines (94, 96) from the recesses by controlling the position of a spool (100) within a valve body (98) of a control valve in response to a signal indicative of an engine operating condition determined from a closed loop feedback system.

Description:
FIELD OF THE INVENTION 
     This invention relates to an internal combustion engine in which the timing of the camshaft of a single camshaft engine, or the timing of one or both of the camshafts of a dual camshaft engine, relative to the crankshaft is varied to improve one or more of the operating characteristics of the engine. 
     BACKGROUND OF THE INVENTION 
     It is known that 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. It is also known that 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. Heretofore, such changes in engine valve timing have been accomplished by a separate hydraulic motor operated by engine lubricating oil. However, this actuating arrangement consumes significant additional energy and it increases the required size of the engine lubricating pump because of the required rapid response time for proper operation of the camshaft phasing actuator. Further, these arrangements are typically limited to a total of 20° of phase adjustment between crankshaft position and camshaft position, and typically such arrangements are two-position arrangements, that is, on, or fully phase adjusted as one position, or off, or no phase adjustment, as a second position. The present invention is designed to overcome these problems associated with prior art variable camshaft timing arrangements by providing a self-actuating, variable camshaft timing arrangement which does not require external energy for the operation thereof, which does not add to the required size of the engine lubricating pump to meet transient hydraulic operation requirements of such variable camshaft timing arrangement, which provides for continuously variable camshaft to crankshaft phase relationship within its operating limits, and which provides substantially more than 20° of phase adjustment between the crankshaft position and the camshaft position. Prior U.S. Pat. Nos. which describe various systems of the foregoing type are 5,046,460 and 5,002,023, the disclosures of each of which are incorporated by reference. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for phase adjustment of an internal combustion engine in which the position of the camshaft, or the positions of one or both of the camshafts in a dual camshaft system, is phase adjusted relative to the crankshaft by an actuating arrangement which is controlled by a robust closed loop system having a hydraulic pilot stage with a pulse width modulated (PWM) solenoid. A predetermined set point dictates the desired camshaft phase angle for certain engine performance criteria. This variable camshaft timing (VCT) system can be used to improve important engine operating characteristics such as idle quality, fuel economy, emissions or torque A preferred embodiment of a camshaft mounted hydraulic VCT mechanism uses one or more radially extending vanes which are circumferentially fixed relative to the camshaft and which are receivable in cavities of a sprocket housing that is oscillatable on the camshaft Hydraulic fluid is selectively pumped through a proportional (spool) valve to one side or another of each vane to advance or retard the position of the camshaft relative to the sprocket. A pumping action occurs in reaction to a signal generated by a closed loop feedback system. Closed loop feedback control is imperative for any but the &#34;two-position&#34; case, i.e., fully advanced or fully retarded. This is because camshaft phase is controlled by the integral of the spool valve position. That is, spool position corresponds not to camshaft phase, but to its rate of change. Thus, any steady state spool position other than null (centered) will cause the VCT to eventually go to one of its physical limits in phase. Closed loop control allows the spool to be returned to null as the camshaft phase reaches its commanded position or set point. An additional result of using feedback control is that the system performance is desensitized to mechanical and environmental variations. This results in a reduction of the effects of short term changes, such as changes in oil pressure or temperature, or long term variations due to tolerances or wear. In addition, set point tracking error in the presence of unanticipated disturbances (e.g. torque shifts) is reduced. A degree of sensitivity reduction and disturbance rejection is referred to as the &#34;robustness&#34; of the control system. Closed loop control can thus provide stable set point tracking with some degree of robustness. 
     Accordingly, it is an object of the present invention to provide an improved VCT method and apparatus 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. Further, it is an object of the present invention to provide a VCT method and apparatus of the foregoing type which maintains substantially unchanged performance over a wide range of parameter variations. Specifically, it is an object of the present invention to provide a VCT method and apparatus of the foregoing type which is substantially insensitive to fluctuations in engine oil pressure, and changes in system parameters such as component tolerances, spring rate, air entrainment and leakage which utilizes a predetermined set point to dictate the desired camshaft phase angle to effectuate certain engine performance criteria. 
     For a further understanding of the present invention and the objects thereof, attention is directed to the drawings and to the following brief descriptions thereof, to the detailed description of the preferred embodiment, and to the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1a is a block diagram of a basic closed loop feedback system for a VCT system; 
     FIG. 1b is a block diagram of the robust VCT control law of a preferred embodiment of the present invention used in a closed loop feedback system; 
     FIG. 1c is a block diagram of the digital implementation of the robust VCT control law illustrated in FIG. 1b.; 
     FIG. 1d is a block diagram of the robust VCT control law of an alternate embodiment of the present invention utilizing a single-loop configuration and filtered set point; 
     FIG. 1e is a block diagram of the robust VCT control law of an alternate embodiment of the present invention including variation compensation and disturbance feed-forward; 
     FIG. 1f is a block diagram illustrating the component stages of a synchronous feedback filter; 
     FIG. 2 is an end elevational view of a camshaft with an embodiment of a variable camshaft timing system applied thereto; 
     FIG. 3 is a view similar to FIG. 2 with a portion of the structure thereof removed to more clearly illustrate other portions thereof; 
     FIG. 4 is a sectional view taken on line 4--4 of FIG. 3; 
     FIG. 5 is a sectional view taken on line 5--5 of FIG. 3; 
     FIG. 6 is a sectional view taken on line 6--6 of FIG. 3; 
     FIG. 7 is a end elevational view of an element of the variable camshaft timing system of FIGS. 2-6; 
     FIG. 8 is an elevational view of the element of FIG. 7 from the opposite end thereof; 
     FIG. 9 is a side elevational view of the element of FIGS. 7 and 8; 
     FIG. 10 is an elevational view of the element of FIG. 9 from the opposite side thereof; and 
     FIG. 11 is a simplified schematic view of the variable camshaft timing arrangement of FIGS. 2-10. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As is described in the aforesaid U.S. Pat. Nos. 5,046,460 and 5,002,023 and as is schematically shown in FIG. 1a, camshaft and crankshaft measurement pulses in a VCT system are generated by camshaft and crankshaft pulse wheels 27 and 28, respectively, as the crankshaft (not shown) and camshaft 26 are rotated, and these can be used to actuate the operation of one or more hydraulic elements of a hydraulically operated VCT system. According to the present invention, in a VCT system, for example, a system according to the embodiment of FIGS. 2-10, as hereinafter described, the measurement pulses are detected by camshaft and crankshaft measurement pulse sensors 27a and 28a, respectively, and issued to a phase measurement device 107. The time from successive crank-to-cam pulses, divided by the time for an entire revolution and multiplied by 360°, gives the measured phase difference, Θ o  20. The phase measurement Θ o  20 is then supplied to a synchronous filter 25, schematically shown in FIG. 1f. As the camshaft rotates, the torque pulses 10 superimpose a high frequency disturbance on the VCT phase, Θ o  20. Thus, there is an exact synchronization between the torque pulses 10 and the high frequency disturbance. Likewise, the camshaft measurement pulses 27a are also synchronized with the disturbance. According to the present invention it is possible to take advantage of this synchronization to efficiently filter the phase measurement, Θ o  20, so that the high frequency disturbance is isolated from the control action. As the camshaft speed varies, the filter frequency automatically tracks the disturbance frequency. The filter itself is a discrete-time notch filter with a sampling frequency equal to that of the camshaft measurement pulse frequency 27a. The filtered phase measurement, Θ F  30, is supplied to a control law 108. Since the high frequency disturbance is isolated, the control law 108 does not attempt to compensate for it. This further makes it possible to save actuation power, reduce wear and enhance signal linearity by such a filtering step herein described. 
     FIG. 1f illustrates an embodiment for the filter in the case when the number of camshaft measurement pulses per revolution (n) is greater than twice the number of torque pulses per revolution (m). The filter eliminates the fundamental frequency of the torque disturbance. In the case when n&lt;2m, the disturbance is &#34;aliased&#34; to a lower frequency and this is the frequency addressed by the filter. Further stages can also be added to eliminate harmonics of the disturbance frequency. 
     The variables for FIG. 1f are as follows: ##EQU1## 
     the feedback control law 108 is described in detail in FIG. 1b. The filtered phase angle measurement, Θ F  30, is sent to a proportional-integral control block 208 where the filtered phase angle measurement, Θ F  30, is subtracted from a set point r 35 to give the tracking error, e o  32. The tracking error e o  32 is then processed by a proportional-integral (PI) control block 208 to give infinite DC gain as well as phase lead to compensate for integrator lag. The integral action assures that the steady-state tracking error goes to zero. 
     The output of the PI control block 208 is then used to control the &#34;inner loop&#34; of the system. The filtered phase angle measurement 30 is subtracted from it, resulting in an inner loop error, e 1  33. This loop error e 1  33 is multiplied by a loop gain, K 2  , and subjected to the effect of a phase-lead compensation 308. Such phase-lead compensation 308 gives a quick response by substantially canceling the low frequency phase lag of the PWM pilot stage 106 (shown in FIGS. 1a and 11). The gains and phase-lead frequencies provide enough freedom to achieve independent control of closed-loop dynamics and robustness. 
     FIG. 1c shows the identical feedback control law 108 for digital implementation. The variables for the PI control block 408 are: 
     
         T.sub.s =0.02 sec. 
    
     
         Z.sup.-1 =unit delay 
    
     The variables for the phase-lead compensation block 508 are: 
     
         c=w.sub.lag /W.sub.lead 
    
     
         b=exp -W.sub.lag T.sub.s 
    
     In FIG. 1d, an alternate embodiment of the VCT control law 108 is shown utilizing a single-loop configuration. The set point, r 35 is pre-processed by a filter, F(s) 35a prior to subtracting the feedback signal Θ f  30. The resulting error, e 2  34 is then processed by the PI control block 218 and phase-lead block 318, resulting in the PWM duty cycle. Thus, it is an object of this alternate embodiment of the present invention to incorporate the advantages of the control law shown in FIGS. 1b and c into a single-loop configuration. 
     FIG. 1e is an alternate embodiment of the present invention which illustrates an expanded closed loop feedback system including variation compensation and disturbance feed forward 608. The gain of this hydromechanical system depends on a number of variables such as hydraulic supply pressure, engine speed, oil temperature and natural crankshaft/camshaft orientation. In order to counteract the phenomena in the controller 208, the net effect of all the variables is estimated and the proportional gain, K p , is increased as response decreases. The controller 208 anticipates disturbance phenomena by adjusting the null duty cycle, U null  611, according to an estimate of the net effect. An estimate, Δ null 609, is determined as a nonlinear function of pressure, temperature and the predetermined set point 35. It is then subtracted from a nominal null, U o  610, to give an overall value, U null  611, used in the control loop. 
     FIGS. 2-10 illustrate an embodiment of a hydraulic vane system in which a housing in the form of a sprocket 32 is oscillatingly journalled on a camshaft 26. The camshaft 26 may be considered to be the only camshaft of a single camshaft engine, either of the overhead camshaft type or the inblock camshaft type. Alternatively, the camshaft 26 may be considered to be either the intake valve operating camshaft or the exhaust valve operating camshaft of the dual camshaft engine In any case, the sprocket 32 and the camshaft 26 are rotatable together, and are caused to rotate by the application of torque to the sprocket 32 by an endless roller chain 38, shown fragmentarily, which is trained around the sprocket 32 and also around a crankshaft not shown. As will be here after described in greater detail, the sprocket 32 is oscillatingly journalled on the camshaft 26 so that it is oscillatable at least through a limited arc with respect to the camshaft 26 during the rotation of the camshaft, an action which will adjust the phase of the camshaft 26 relative to the crankshaft. 
     An annular pumping vane 60 is fixedly positioned on the camshaft 26, the vane 60 having a diametrically opposed pair of radially outwardly projecting lobes 60a, 60b and being attached to an enlarged end portion 26a of the camshaft by bolts 62 which pass through the vane 60 into the end portion 26a. In that regard, the camshaft 26 is also provided with a thrust shoulder 26b to permit the camshaft to be accurately positioned relative to an associated engine block, not shown. The pumping vane 60 is also precisely positioned relative to the end portion 26a by a dowel pin 64 which extends therebetween. The lobes 60a, 60b are received in radially outwardly projecting recesses 32a, 32b, respectively, of the sprocket 32, the circumferential extent of each of the recesses 32a, 32b being somewhat greater than the circumferential extent of the vane lobes 60a, 60b which are received in such recesses to permit limited oscillating movement of the sprocket 32 relative to the vane 60. The recesses 32a, 32b are closed around the lobes 60a, 60b, respectively, by spaced apart, transversely extending annular plates 66, 68 which are fixed relative to the vane 60, and, thus, relative to the camshaft 60, by bolts 70 which extend from one to the other through the same lobe, 60a or 60b. Further, the inside diameter 32c of the sprocket 32 is sealed with respect to the outside diameter of the portion 60d of the vane 60 which is between the lobe 60a, 60b, and the tips of the lobes 60a, 60b of the vane 60 are provided with sealed receiving slots 60e, 60f, respectively. Thus, each of the recesses 32a, 32b of the sprocket 32 is capable of sustaining hydraulic pressure, and within each recess 32a, 32b, the portion on each side of the lobe 60a, 60b, respectively, is capable of sustaining hydraulic pressure. 
     The functioning of the structure of the embodiment of FIGS. 2-10, as thus far described, may be understood by reference to FIG. 11. Hydraulic fluid, illustratively in the form of engine lubricating oil, flows into the recesses 32a, 32b by way of a common inlet line 82. The inlet line 82 terminates at a juncture between opposed check valves 84 and 86 which are connected to the recesses 32a, 32b, respectively, by branch lines 88, 90, respectively. The check valves 84, 86 have annular seats 84a, 86a, respectively, to permit the flow of hydraulic fluid through the check valves 84, 86 into the recesses 32a, 32b, respectively. The flow of hydraulic fluids through the check valves 84, 86, is blocked by floating balls 84b, 86b, respectively, which are resiliently urged against the seats 84a, 86a, respectively, by springs 84c, 86c, respectively. The check valves 84, 86, thus, permit the initial filling of the recesses 32a, 32b and provide for a continuous supply of makeup hydraulic fluid to compensate for leakage therefrom. Hydraulic fluid enters the line 82 by way of a spool valve 92, which is incorporated within the camshaft 26, and hydraulic fluid is returned to the spool valve 92 from the recesses 32a, 32b by return lines 94, 96, respectively. Because of the location of the check valves 84 and 86 which block the backflow of hydraulic fluid, the need for the spool valve 100 to return to the null (centered) position to prevent such backflow is eliminated. 
     The spool valve 92 is made up of a cylindrical member 98 and a spool 100 which is slidable to and fro within the member 98. The spool 100 has cylindrical lands 100a and 100b on opposed ends thereof, and the lands 100a and 100b, which fit snugly within the member 98, are positioned so that the land 100b will block the exit of hydraulic fluid from the return line 96, or the land 100a will block the exit of hydraulic fluid from the return line 94, or the lands 100a and 100b will block the exit of hydraulic fluid from both return lines 94 and 96, as is shown in FIG. 11, where the camshaft 26 is being maintained in a selective intermediate position relative to the crankshaft of the associated engine. 
     The position of the spool 100 within the member 98 is influenced by an opposed pair of springs 102, 104 which act on the ends of the lands 100a, 100b respectively. Thus, the spring 102 resiliently urges the spool 100 to the left, in the orientation illustrated in FIG. 11, and the spring 104 resiliently urges the spool 100 to the right in such orientation. The position of the spool 100 within the member 98 is further influenced by supply of pressurized hydraulic fluid within a portion 98a of the member 98, on the outside of the land 100a, which urges the spool 100 to the left. The portion 98a of the member 98 receives its pressurized fluid (engine oil) directly from the main oil gallery (&#34;MOG&#34;) 130 of the engine, and this oil is also used to lubricate a bearing 132 in which the camshaft 26 of the engine rotates. 
     The control of the position of the spool 100 within the member 98 is in response to hydraulic pressure within a control pressure cylinder 134 whose piston 134a bears against an extension 100c of the spool 100. The surface area of the piston 134a is greater than the surface area of the end of the spool 100 which is exposed to hydraulic pressure within the portion 98a, and is preferably twice as great. Thus, the hydraulic pressures which act in opposite directions on the spool 100 will be in balance when the pressure within the cylinder 134 is one-half that of the pressure within the portion 98a. This facilitates the control of the position of the spool 100 in that, if the springs 102 and 104 are balanced, the spool 100 will remain in its null or centered position, as illustrated in FIG. 11, with less than full engine oil pressure in the cylinder 134, thus allowing the spool 100 to be moved in either direction by increasing or decreasing the pressure in the cylinder 134, as the case may be. 
     The pressure within the cylinder 134 is controlled by a solenoid 106, preferably of the pulse width modulated type (PWM), in response to a control signal from a closed loop feedback system 108, shown schematically, in FIG. 11. The feedback control system processes a signal sent from a sensing device 107 which measures the phase angle between the camshaft 26 and the crankshaft, not shown. The closed loop feedback system 108 compares the phase angle to a predetermined set point and issues a signal to the solenoid 106. With the spool 100 in its null position when the pressure in the cylinder 134 is equal to one-half the pressure in the portion 98a, as heretofore described, the on-off pulses of the solenoid 106 will be of equal duration; by increasing or decreasing the on duration relative to the off duration, the pressure in the cylinder 134 will increased or decreased relative to such one-half level, thereby moving the spool 100 to the right or to the left, respectively. The solenoid 106 receives engine oil from the main engine oil gallery (MOG) 130 through an inlet line 114 and selectively delivers engine oil from such source to the cylinder 134 through a supply line 138. As is shown in FIGS. 4 and 5, the cylinder 134 may be mounted at an exposed end of the camshaft 26 so that the piston 134a bears against an exposed free end 100c of the spool 100. In this case, the solenoid 108 is preferably mounted in a housing 134b which also houses the cylinder 134a. 
     Makeup oil for the recesses 32a, 32b of the sprocket 32 to compensate for leakage therefrom is provided by way of a small, internal passage 120 within the spool 100, from the passage 98a to annular space 98b of the cylindrical member 98, from which it can flow into the inlet line 82. A check valve 122 is positioned within the passage 120 to block the flow of oil from the annular space 98b to the portion 98a of the cylindrical member 98. 
     The vane 60 is alternating urged in clockwise and counter clockwise directions by the torque pulsation in the camshaft 26 and these torque pulsations tend to oscillate the vane 60, and, thus, the camshaft 26, relative to the sprocket 32. However, in the FIG. 11 position of the spool 100 within the cylindrical member 98, such oscillation is prevented by the hydraulic fluid within the recesses 32a, 32b of the sprocket 32 on opposite sides of the lobes 60a, 60b, respectively, of the vane 60, because no hydraulic fluid can leave either of the recesses 32a, 32b, since both return lines 94, 98 are blocked by the position of the spool 100. If, for example, it is desired to permit the camshaft 26 and vane 60 to move in a counter clockwise with respect to the sprocket 32, it is only necessary to increase the pressure within the cylinder 134 to a level greater than one-half that in the portion 98a of the cylindrical member. This will urge the spool 100 to right and thereby unblock the return line 94. In this condition of the apparatus, counter clockwise torque pulsations in the camshaft 26 will put fluid out of the portion of the recess 32a and allow the lobe 60a of vane 60 to move into the portion of the recess which has been emptied of hydraulic fluid. However, reverse movement of the vane will not occur as the pulsations in the camshaft become oppositely directed unless and until the spool 100 moves to the left, because of the blockage of the fluid flow through the return line 96 by the land 100b of the spool 100. Thus, large pressure variations induced by camshaft torque pulses will not affect the condition of the system, eliminating the need to synchronize the opening and closing of the spool valve 92 with individual torque pulses. While illustrated as a separate closed passage in FIG. 11, the periphery of the vane 60 actually has an open oil passage slot, element 60c in FIGS. 2-10, which permits the transfer of oil between the portion of the recess 32a on the right side of the lobe 60a and the portion of the recess 32b on the right side of the lobe 60b, which are the nonactive sides of the lobes 60a and 60b; thus, counter clockwise movement of the vane 60 relative to the sprocket 32 will occur when flow is permitted through return line 94 and clockwise movement will occur when flow is permitted through return line 96. 
     Further, the passage 82 is provided with an extension 82a to the nonactive side of one of the lobes 60a or 60b, shown as the lobe 60b, to permit a continuous supply of makeup oil to the nonactive sides of the lobes 62a and 62b for better rotational balance, improved damping of vane motion, and improved lubrication of the bearing surfaces of the vane 60. 
     The elements of the structure of FIGS. 2-10 which correspond to the elements of FIG. 11, as described above, are identified in FIGS. 2-10 by the referenced numerals which were used in FIG. 11, it being noted that the check valves 84 and 86 are disc type check valves in FIGS. 2-10 as opposed to the ball type check valves of FIG. 11. While this type check valves are preferred for the embodiment of FIGS. 2-10, it is to be understood that other types of check valves can also be used. 
     Although the best mode contemplated by the inventors for carrying out the present invention as of the filing date hereof has been shown and described herein, it will be apparent to those skilled in the art that suitable modifications, variations, and equivalents may be made without departing from the scope of the invention, such scope being limited solely by the terms of the following claims.