Abstract:
A standard cam phasing OCV may be employed as a virtual check valve to choke the backflow of oil during negative cam torque conditions, including execution of a duty cycle command in an event-based manner. Normally, OCV duty cycle commands are made on a time basis, but for VCV the duty cycle output change must be synchronized with engine events. A method is disclosed for calculating and delivering the VCV duty cycle so that both time-based and event-based controls are maintained and work together. Phase alignment of response time of the OCV solenoid is based upon cam target wheel edges and is event-based. An initial phase rate vs. phase angle is monitored by the Engine Control Module (ECM). Adjustment of the phase angle is provided to achieve maximum cam position phase rate.

Description:
TECHNICAL FIELD 
     The present invention relates generally to an oil flow control valve for a camshaft phaser; more particularly, to the operation of an oil flow control valve (OCV) as a virtual check valve (VCV) against undesired back flow of oil from a camshaft phaser during camshaft torque reversal events; and most particularly to a method for synchronizing the actuation of such an oil control valve as a VCV. 
     BACKGROUND OF THE INVENTION 
     Camshaft phasers (also referred to herein as “cam phasers”) are used to control the angular phase relationship of a camshaft to a pulley or sprocket driven by a crankshaft of an internal combustion engine. A variable cam phaser (VCP) allows changing the phase relationship while the engine is running. Typically, a VCP is used to phase shift an intake cam on a dual cam engine to broaden the torque curve of the engine, to increase peak power at high revolution speeds, and to improve the idle quality. Also, an exhaust cam can be phase shifted by a cam phaser to provide internal charge dilution control, which can significantly reduce HC and NOx emissions, and/or to improve fuel economy. The above objectives are known in the art as “combustion demands”. With this definition, a VCP is used to account for combustion demands of an internal combustion engine. 
     Cam phasers typically are either “vane-type” or “spline-type” and are controlled by hydraulic systems that use pressurized lubrication oil from the engine. An “advance” or “retard” position of the camshaft is commanded via an oil flow control valve (OCV) that controls the oil flow to different ports entering a VCP. A typical OCV is a spool valve comprising a spool slidably disposed in a cylindrical body, the spool being driven by attachment to an electrical solenoid, and the position of the spool within the body serving to open and close appropriate porting in the body connected to the VCP ports. 
     A known problem in cam phaser operation is that valve-opening and valve-closing events of a camshaft can give rise to torque reversals resulting in pressure peaks in the oil contained in the chambers of the VCP, which peaks can be higher than the oil control supply pressure, i.e., the oil pressure supplied by the engine. The camshaft experiences a measurable resistance to rotation as a cam follower climbs the opening ramp of each cam lobe; and similarly, a measurable assistance to rotation as the cam follower descends the closing ramp. Thus, the measurable torque on the camshaft follows a predictable variation, the amplitude and frequency (period) of which are governed by the particular size and shape of an engine&#39;s cam lobes, drive train kinematics and the number of cylinders. 
     This phenomenon can lead to a certain amount of undesired reverse oil flow out of the phaser, diminishing the phasing performance of the cam phasing system by causing the rotational position of the phaser rotor within the phaser stator to be changed. This problem is exacerbated by any air bubbles in the oil supply system, which act as pneumatic cushions preventing hydraulic rigidity, and further by conditions of high oil temperature and/or low oil pressure such as may be experienced by an engine under heavy load and thermal stress. 
     To avoid such reverse oil flow under the above mentioned circumstances, it is known in the art to employ a check valve (CV) in the oil supply passage of either the cylinder head or the crankcase. Such a check valve also ensures that the cam phaser does not empty out in cases when the oil pressure is reduced, for example when the engine is stopped. However, this approach adds significant cost to the cylinder head or engine block. Also, the implementation of the check valve can be difficult because of oil routing. Further, the check valve should not be placed too far away from the cam phaser in order to be effective. Perhaps most important, a mechanical check valve itself acts as a flow restriction in the allowed direction of oil flow, thus diminishing the inherent phasing rate (response rate) performance of a cam phaser system. 
     Another approach, as disclosed in Published US Patent Application No. 2007/0175425, published Aug. 2, 2007, the relevant disclosure of which is incorporated herein by reference, is to vary the action of the solenoid driving the OCV to effectively block such reverse flow by appropriate positioning of the OCV spool within the valve body. The OCV can be cycled full on and full off (preferably “dithered” at an intermediate spool position by application of an AC signal to the solenoid) in synchronization with cam torque reversals, which function serves to accept all positive hydraulic power into the VCP, which enhances the phasing rate, while rejecting all negative hydraulic power, which diminishes the phasing rate, thus effectively rectifying oil pressure in the VCP. The OCV thus can function as a “virtual check valve” (VCV) when a constant phase angle of the rotor is desired, allowing omission of an additional mechanical check valve, in addition to its nominal function of providing oil selectively to the advance and retard ports of the phaser when a change in phase angle is desired. 
     The required frequency of OCV activation is readily calculated from the engine speed and the number of engine cylinders as follows:
 
 F   Hz =(RPM×# of cylinders)/120  (Eq. 1)
 
     However, correct synchronization timing between OCV spool motions and cam torque reversals has been not at all easy to accomplish in the prior art, nor is a method for accurate synchronization disclosed in detail in the incorporated reference. 
     What is needed in the art is a method for providing accurate synchronization between OCV spool motion and cam torque reversals in an internal combustion engine to effectively block oil flow reversals from a camshaft phaser during camshaft torque reversals. 
     It is a principal object of the present invention to improve operation of a camshaft phaser and of an engine incorporating a camshaft phaser. 
     SUMMARY OF THE INVENTION 
     Briefly described, US Patent Application No. 2007/0175425, published Aug. 2, 2007, discloses how a standard cam phasing OCV may be commanded between two spool positions to choke the reverse flow of oil through the valve during negative cam torque conditions. A critical part of the VCV concept is the execution of a duty cycle command in an event-based manner. Normally, duty cycle commands to the OCV are made on a time basis, but for VCV, the duty cycle output change must be synchronized with engine events. The present invention is directed to a method for calculating and delivering the VCV duty cycle so that both time-based and event-based controls are maintained and work together. Phase alignment of electromechanical response time of the OCV and solenoid is based upon cam target wheel edges and therefore is event-based. An initial phase rate vs. phase angle is calculated and monitored by the Engine Control Module (ECM), and adjustment of the phase angle is provided by a self-learning algorithm to achieve maximum cam position phase rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a graph showing the operating parameters for an exemplary virtual check valve; 
         FIG. 2  is a graph of a full engine cycle (two crankshaft turns) illustrating definitions of cam torque cycle, percent of period that the OCV activation is full on (OCV duty cycle %=1), and period of cam torque reversal frequency; 
         FIG. 3  is a graph showing a typical synchronization between OCV activation, cam target wheel, and cam torque; 
         FIG. 4  is a schematic flow diagram showing structure of a duty cycle calculation and command in accordance with the present invention; 
         FIG. 5  is a schematic flow diagram showing structure of the switch point learning function; and 
         FIG. 6  is a graph showing the operating benefit provided by a virtual check valve when operated in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As disclosed in the incorporated published patent application, reverse flow on account of the pressure ratio on the output side of the OCV and a corresponding input side can be prevented by synchronizing the movement of the OCV spool with oil pressure characteristics on the output side of the OCV, such synchronization resulting in the spool&#39;s being moved to a position where the OCV&#39;s oil ports are closed once the oil pressure on the output side of the OCV is too high for feeding oil to the cam phaser, and in the spool&#39;s being moved to a position where the oil ports are open once the oil pressure on the output side of the OCV allows for feeding oil to the cam phaser. 
     Note that such movement control may be applied to either or both of the phase-advance and phase-retard supply/return ports of a camshaft phaser. In a presently preferred embodiment of a method in accordance with the present invention, the spool is positioned at an intermediate location within the valve body to close both ports, thus capturing a hydraulically rigid, incompressible oil volume within the phaser stator and thereby assuring against any movement of the rotor within the stator. 
     Further, rather than sensing the required oil pressures directly, the torque reversals during camshaft rotation can be predicted from knowledge of the camshaft position with respect to the crankshaft rotational cycle. Thus, all that is required is to monitor the camshaft position, which typically is indicated in a prior art engine via a camshaft target wheel or other such rotary encoder. 
     The correct phasing of the cyclic pair of cam torque and OCV spool motion as a VCV is the subject of the present invention. 
     Referring to  FIG. 1 , various operating parameters are shown for an exemplary virtual check valve in accordance with the present invention. A is the amplitude (0.68%) of the OCV duty cycle above the duty constant B (O.32%). C is the pulse width (65%) of the duty cycle. D is the period (0.04 seconds) of the duty cycle. E is the phase delay (0.025 seconds). 
     Characterizing the electrical response time of an electrical solenoid device is a well-established science. This technique results in defining a mechanical spool motion versus time when its electrical coil is energized with an electrical impulse. 
     Referring to  FIG. 2 , the terminology of the present invention is further defined. Curve  10  is the torque curve for an individual cam lobe of an engine camshaft of a V-6 engine, the period  12  of the curve being ⅓ of two rotations (720°) of the crankshaft, or 240°. The time length of period  12  is the reciprocal of the torque reversal frequency F Hz  as expressed above in Equation 1. In the spool position curve  14 , ideally the spool position  16  that counters the maximum torque reversal  18  is centered under the point of maximum reversal  18 . The OCV activation curve is expressed as 0% duty cycle portions  20  and 100% duty cycle portions  22 . Note that duty cycle portions  22  anticipate slightly the desired spool positions, and also note that the spool position is maintained for some time after the OCV is electrically deactivated. This is because of lag in both directions. In one known OCV, this lag amounts to about 8 mseconds in the activation direction and about 25 mseconds in the deactivation direction. 
     Referring to  FIG. 3 , the synchronization timing is added to the curves shown in  FIG. 2 . Square wave  24  is the electrical signal resulting from sensing of a tooth edge or other signal-generating means of a camshaft target wheel or other encoder as is well known in the automotive arts. Instead of measuring the camshaft torque directly, the target wheel signal is directly and consistently indicative of the angular position of the camshaft and therefore of the torque curve, as noted above. At a designated rotary event defined by change  26  in this signal, which may be either rising or falling, a time-based offset  28  is performed which is based upon the speed of the engine, after which activation  22  of the OCV solenoid is initiated as described above to drive the OCV spool to a position  16  blocking the appropriate port(s) of the spool valve, as described and illustrated in the incorporated reference, during the peak  18  of the torque reversal  10 . The OCV thus is made to function as a virtual check valve during torque reversals in accordance with the invention. 
     It has been found that the impact of torque reversals is most severe under conditions of low oil pressure and/or low oil viscosity. These conditions typically correspond to engine speeds of less than about 1000 rpm and/or engine oil temperatures in excess of about 110° C. Fortuitously, a current-technology OCV cannot respond rapidly enough for VCV operation at engine speeds much above 1000 rpm, and attempts to use it under such adverse conditions can actually worsen the apparent torque reversals through incorrect timing of the OCV duty cycle. 
     Referring now to  FIGS. 3 and 4 , the structure of the duty cycle calculation and command in accordance with the present invention is shown. The sequence and description of this control  25  is as follows:
         1. At initialization, the switch points  30  in crank angle, defined as the points in crank angle where cam torque changes from positive to negative, are determined from calibration settings. The calibrations define the location of key cam torque events as a function of engine speed (RPM). These switch points will call the SetVCPC_VCV_FreqAndDutyCycle function  32  by an interrupt  26  scheduled on a target wheel tooth or by a timer.   2. When RPM conditions are met (preferably less than about 1000 rpm), the SetVCPC_VCV_FreqAndDutyCycle function will set the duty cycle between the normal PID duty cycle (VCXXDDC), which is non-VCV for positive cam torque, and an adjusted integral duty cycle (VCXXINGV+offset), for negative cam torque. The adjustment is based on the expected choked-flow duty cycle, which is a function of valve temperature. The offset is determined based on the integral duty cycle and the expected choked-flow duty cycle, to ensure rationality between the two (i.e. the integral duty cycle should not be less than the choked-flow duty cycle). If this occurs, a default offset shall be used. The SetVCPC_PWM_FreqAndDutyCycle function  34  is bypassed for this case.   3. When RPM conditions are not met, the SetVCPC_VCV_FreqAndDutyCycle function returns immediately. In this case, SetVCPC_PWM_FreqAndDutyCycle is executed as normal.   4. The function SetIO_PWM_FrequencyAndDutyCycle 36 is called from either SetVCPC_PWM_FreqAndDutyCycle or SetVCPC_VCV_FreqAndDutyCycle. SetIO_PWM_FrequencyAndDutyCycle is the function that delivers the final duty cycle to the valve.   5. Conditions for enabling VCV are determined at a time-based rate of 15.6 ms.   6. PID duty cycle is determined at a time-based rate of 15.6 ms, regardless of VCV operation.   7. Determination and delivery of the VCV duty cycle is at an engine-event-based rate, driven by interrupts at the crank-angle locations determined in step 1.       

     Preferably, a PID/VCV control system in accordance with the present invention can analyze its own performance and automatically improve upon and maximize it (known in the art as “learning”). Referring to  FIG. 5 , the structure of such switch point learning is shown. This algorithm  37  observes the phase rate that is achieved by the VCV operation, and uses a comparison to adjust the torque switch points and the choked-flow duty cycle, since these parameters contain uncertainty. 
     The following describes the sequence of VCV learning: 
     1. The phaser response diagnostic  38  (an existing diagnostic that measures real-time phasing rate) continuously monitors phasing control. Phasing responses that are captured in the VCV operating range (i.e. low RPM) are delivered separately to a VCV learning function. 
     2. The VCV learning function consists of two steps:
         a) compare  40  the measured VCV phase rate  41  with the non-VCV phase rate  42 , which is stored in calibration. If the VCV measured phase rate is found to be less than the non-VCV phase rate, VCV parameter update is required (VCV_ParamUpdtRqrd=TRUE). The VCV phase rate may be required to exceed the non-VCV phase rate by a calibratible amount.   b) To improve VCV phase rate, there are four parameter update options:
           i) Shift cam torque switch points to advance.   ii) Shift cam torque switch points to retard.   iii) Shift choked-flow duty cycle higher (toward 100%).   iv) Shift choked-flow duty cycle lower (toward 0%).   
               

     The magnitude of each parameter adjustment is calibratible. The parameter update function attempts each of these steps in turn. When a parameter update adjustment has been made, no further adjustments can be made until new VCV phase rate data are available. The effect of the parameter adjustment must be observed through the phaser response diagnostic  44 , with new sample data generated. A minimum number of data points are required to assess the success or failure of the parameter adjustment. 
     If the parameter adjustment is determined to be successful (i.e. phase rate was increased), then that parameter adjustment is retained. If the phase rate is still less than the non-VCV phase rate, then that same parameter adjustment technique is repeated until the VCV phase rate is improved beyond the non-VCV phase rate, or until it reaches the desired excess defined in a). 
     If the parameter adjustment is not successful (i.e. no improvement in phase rate), then the parameter adjustment is eliminated and the next option is exercised. Each parameter adjustment must be confirmed by new data from the phaser response diagnostic, in the VCV range. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Phasing Rates (Crank angle degrees per second) 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
               
               
                   
                 1000 RPM 
                 750 RPM 
                 750 RPM 
               
               
                   
                 120° C. 
                 120° C. 
                 150° C. 
               
               
                   
                 0% Air 
                 0% Air 
                 5% Air 
               
               
                   
                   
               
             
          
           
               
                 Virtual Check Valve System 
                 83 
                 76 
                 56 
               
               
                 Mechanical Check Valve System 
                 96 
                 76 
                 59 
               
               
                 No Check Valve, 100% OCV 
               
               
                 Duty Cycle 
                 94 
                 61 
                 39 
               
               
                   
               
             
          
         
       
     
     Referring now to Table 1, phasing rates are shown for three different engine operating conditions, illustrating the benefits and limitations of the use of a VCV. 
     Condition 1 is an engine having a hydraulically rigid (air-free) oil supply system, operating at a medium speed near the upper end of an acceptable thermal range. It is seen that a mechanical check valve provides only marginal improvement in phase rate over a non-check valve system, and that the VCV system is inferior. This is because the limit of electromechanical response of the OCV has been reached. 
     Condition 2 is an engine having a hydraulically rigid (air-free) oil supply system, but operating at a lower speed, again near the upper end of an acceptable thermal range. It is seen that both a mechanical check valve and a VCV provide significant improvement in phase rate over a non-check valve system. This is because the oil pressure, being a function of engine speed, is sufficiently low that cam torque reversals have a significant impact on performance in a non-checked system. 
     Condition 3 is the same as Condition 2 except that the operating temperature is raised from 120° C. to 150° C. and the oil system contains 5% air. It is seen that the use of either form of check valve is a distinct improvement over a non-checked system because the combined deleterious effects of high temperature, low oil pressure, and hydraulic non-rigidity are at least partially overcome. 
     These results also are shown graphically in  FIG. 6 . 
     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.