Abstract:
A method for coordinating engine throttle position with camshaft phaser motion during transient engine operation such that desired internal residual dilution is maintained. A dilution model for residual mass fraction and a table of desired dilution values are embedded in the engine control algorithm. The dilution model is applied to calculate the desired throttle and camshaft phaser positions for the next intake event. In a first method, if the throttle is capable of changing the airflow into the engine cylinders faster than the camshaft phasers can respond, the throttle is modulated to maintain desired dilution levels while the phasers are allowed to move as fast as they can. In a second method, if the phaser response faster than the engine intake port airflow response to a throttle position change, the throttle is allowed to move as fast as it can while phaser motion is modulated to maintain desired dilution levels.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to internal combustion engines; more particularly, to such engines having capabilities for variable camshaft phasing; and most particularly, to a method for controlling fuel mixture dilution during speed and load transients in such engines. 
       BACKGROUND OF THE INVENTION 
       [0002]    Internal combustion engines having variable camshaft phasing are well known. Camshaft phasers are used to improve fuel economy and reduce formation of nitrogen oxides (NOx) by adjusting the timing of Intake Valve Opening (IVO), Exhaust Valve Closing (EVC), or both IVO and EVC independently so that the engine cylinders are held at their dilution limit during operating conditions of partial load. Cylinder dilution is defined as the fraction of burned gas left over from the previous combustion event that is contained in the cylinder during the compression stroke. 
         [0003]    Under steady state operating conditions, the optimum positions of the cam phaser or cam phasers as a function of engine speed and load is readily determined by mapping of engine performance on an engine dynamometer. The test results are tabulated in look-up tables which are programmed into an Engine Control Module (ECM) that governs the position of the cam phasers for any given condition of engine speed and load. This procedure yields appropriate dilution control for steady-state engine operation. 
         [0004]    A problem arises, however, during transient periods between different engine states of speed and load requiring movement of each phaser from a first position to a second position, during which time it is necessary to carefully coordinate phaser positions to the load response of the engine. The load response rate of an engine following a driver input to the accelerator pedal depends on several engine design features, and cam phaser response depends upon both its design as well as the particular engine operating condition. Depending on the response rate of the cam phasers relative to the load response of the engine, periods of either excessive dilution or sub-optimal dilution can result. If phaser response is relatively fast, the result will be too much dilution when engine load is increasing, and too little dilution when load is decreasing. If phaser response is relatively slow, the opposite occurs. Excessive dilution causes unstable combustion, while sub-optimal dilution reduces fuel economy and increases NOx emissions. 
         [0005]    In prior art practice, to avoid excessive dilution and unstable combustion, the phaser position is calibrated conservatively, resulting in sub-optimum fuel economy and increased NOx under real vehicle driving conditions. 
         [0006]    What is needed in the art is a method for maintaining desired optimum dilution levels during transient engine operation. 
         [0007]    It is a principal object of the present invention permit optimum fuel mixture dilution to be maintained during transient engine operation, resulting in improved overall fuel economy and NOx emission control. 
       SUMMARY OF THE INVENTION 
       [0008]    Two embodiments are described. It is understood that the embodiments described herein may include more than one phaser such as, for example, where two phasers are used—one in conjunction with the intake valve cam shaft and one in conjunction with an exhaust valve cam shaft. Also, unless specifically identified in the description as being one or the other, when the term “phaser” is used, it should be taken to mean either an intake valve phaser or an exhaust valve phaser, or both. 
         [0009]    For the first embodiment, it is assumed that the ECM and the throttle valve are capable of changing airflow to the engine cylinders faster than the phaser can respond, in which case the intake air flow may be adjusted to follow optimally the progress of the phaser during its movement from a first position to a second position by calculating and setting an optimal intake air flow at a sequential plurality of phaser transient positions. 
         [0010]    In some cases, movement of the phaser is faster than the engine intake port airflow response to a throttle position change. Thus, in another embodiment, the intake and exhaust cam phaser positions may be adjusted to follow optimally the progress of the throttle. 
         [0011]    Briefly described, a method in accordance with a first embodiment of the invention commands phaser positions according to the desired engine load as indicated by the driver&#39;s accelerator pedal command and the phaser is allowed to move as fast as it can, while the throttle is dynamically adjusted to properly coordinate the airflow to achieve a desired residual dilution fraction dynamically. 
         [0012]    In a second embodiment of the invention, the throttle is allowed to move as fast as it can in response to the driver&#39;s accelerator pedal command, while the phaser position commands are modulated by the ECM to deliver the desired residual dilution fraction dynamically. 
     
     
       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 schematic diagram of a feedback control system for an internal combustion engine; 
           [0015]      FIG. 2  is a schematic logic diagram for operating the control system shown in  FIG. 1  in accordance with the invention; 
           [0016]      FIG. 3  is a schematic logic diagram for operating the control system specific to the first embodiment; and 
           [0017]      FIG. 4  is a schematic logic diagram for operating the control system specific to the second embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Referring to  FIG. 1 , a throttle and cam phaser control system  10  for use in accordance with the invention comprises an internal combustion engine  12  having an intake manifold  14  for supplying intake air to engine  12  and a throttle valve  16  for regulating the flow of air into manifold  14 . The position of throttle valve  16  is regulated by a signal  18  sent from an electronic throttle controller (ETC)  20 . 
         [0019]    Engine  12  includes at least an intake valve camshaft phaser  22 , or an exhaust valve camshaft phaser  24  and optionally both an intake and exhaust valve camshaft phaser  22 ,  24 . Each phaser  22 ,  24  sends a position signal  26 ,  28  to an engine control module (ECM)  30  defining a programmable controller or computer as is known in the engine control arts; and each phaser  22 ,  24  receives a position command  32 ,  34  from ECM  30 . (For engines not equipped with an exhaust valve camshaft phaser  24 , signals  28 ,  34  obviously are omitted. For engines not equipped with an intake valve camshaft phaser  22 , signals  26 ,  32  obviously are omitted.) 
         [0020]    An engine speed (RPM) signal  36  is sent to ECM  30  from engine  12 . A manifold air pressure (MAP) signal  38  is sent to ECM  30  from manifold  14 . A throttle valve position signal  40  is sent to ECM  30  and ETC  20  from throttle valve  16 . An accelerator pedal input signal  44  received from a driver&#39;s actuation of the accelerator pedal is normally interpreted by the ECM  30  as a desired torque output or load from the engine  12 . As shown in  FIG. 1 , ECM  30  calculates a desired throttle position and sends a signal  42  to ETC  20 , which adjusts the throttle position via signal  18 . 
         [0021]    Referring to  FIG. 2 , a schematic logic diagram  100  for operating throttle and cam phaser control system  10  shown in  FIG. 1  includes an engine state estimator  102 , table of desired dilution values  104  and a calculated dilution value model  106 , all of which preferably reside within ECM  30 . 
         [0022]    Estimator  102  comprises lock-up tables, equations, algorithms, or combinations of these three that; (1) can received signals of various current engine parameters  101   a - f , such as for example, throttle position  101   a , engine speed  101   b , the opening time of the intake valve  101   c  and of the exhaust valve  102   d  by detecting the intake and exhaust cam phaser positions, ambient temperature  101   e  and engine coolant temperature  101   f ; and (2) predicts certain engine conditions  103   a - h  that will exist at the next engine event in the combustion cycle. For example, the “lead” engine speed  103   a , manifold air pressure  103   b , exhaust manifold pressure  103   c , manifold absolute temperature  103   d , intake valve cam phaser position  103   e , exhaust valve cam phaser position  103   f , intake port flow  103   g  and engine output (MEP, mean effective pressure)  103   h  are predicted. 
         [0023]    Table  104  comprises a map of optimal dilution values for dilute limit operation (dilution tolerance) as a function of various engine operating conditions (for example, engine speed, manifold absolute temperate, intake cam phaser position, engine load and coolant temperature) as may be generated in known fashion either by a well-calibrated engine simulation or by direct-cylinder gas sampling during the compression stroke of the engine on a test stand during operation under dilution-limited conditions. The engine operating condition inputs to Table  104 , for purposes of selecting a specific optimal dilution value for the inputted operating conditions, may include predicted engine speed  103   a , predicted manifold absolute temperate  103   d , predicted opening time of the intake valve  103   e  and an engine load parameter (manifold air pressure  103   b  or MEP  103   h ) as received from engine state estimator  102 , and a coolant temperature signal  101   f  from an engine sensor. The output of Table  104  is the desired dilution value  108  for the next engine intake event (DV_des_lead). 
         [0024]    Calculated dilution value model  106  comprises a map of actual dilution values in a cylinder of engine  12  as a function of predicted RPM  103   a , predicted manifold air pressure  103   b , predicted exhaust manifold pressure  103   c , predicted manifold absolute temperature  103   d , predicted intake cam phaser position  103   e  and predicted exhaust valve cam phaser position  103   f . The dilution value model  106  is derived from actual dilution values which may be generated in known fashion either by a well-calibrated engine simulation model or by direct-cylinder gas sampling during the compression stroke of the engine on a test stand. The output of the dilution model  106  is a predicted value of actual dilution for the next engine intake event  110  (DV_lead). 
         [0025]    Referring to  FIGS. 2 and 3 , In a first embodiment, throttle valve position is adjusted to follow optimally the progress of the phasers from a first position to a second position by calculating and setting an optimal intake air flow (throttle valve position) at a sequential plurality of phaser transient positions. A method in accordance with the first embodiment of the invention for setting an optimal throttle position during transient engine operating conditions comprises the following steps: 
         [0026]    a) provide an engine state estimator  102  for receiving current engine parameters such as throttle position  101   a , engine speed  101   b , intake valve cam phaser position  101   c , exhaust valve cam phaser position  101   d , ambient temperature  101   e  and engine coolant temperature  101   f  and for predicting, for the next engine intake event, engine speed  103   a , manifold air pressure  103   b , exhaust manifold pressure  103   c , manifold absolute temperature  103   d , intake valve cam phaser position  103   e , exhaust valve cam phaser position  103   f , intake port flow  103   g  and mean effective pressure  103   h;    
         [0027]    b) generate a table of optimal dilution values  104  for dilute limit operation as a function of predicted engine speed, manifold absolute temperature, intake valve cam phaser position, and engine load and current (coolant temperature) engine operating conditions; 
         [0028]    c) provide a model of calculated dilution values  106  as a function of engine operating conditions (engine speed, manifold absolute pressures and temperatures) and phaser positions; 
         [0029]    d) calculate an optimal dilution value  108  from Table  104  using predicted engine speed  103   a , manifold air temperature  103   d , intake valve cam phaser position  101   e  and the engine load parameter  103   b  or  103   h  and current (coolant temperature) engine operating conditions  101   f;    
         [0030]    e) calculate an actual dilution value  110  from Model  106  using the predicted engine operating conditions  103   a - f  received from engine state estimator  102 . (This is the expected dilution level for the next cylinder event if no corrective action is taken by the controller); 
         [0031]    f) use the optimal dilution value  108  and the calculated actual dilution value  110  to calculate  112  a desired intake port flow (PtFlow_des)  114  for the next intake event that will cause calculated actual dilution value  110  to equal optimal dilution value  108 ; PtFlow_des may be calculated according to the equation: 
         [0000]      PtFlow —   des =(calculated actual dilution value/optimal dilution value)×PtFlow_lead 
         [0032]    g) compare  120  the desired intake port flow  114  to the predicted intake port flow  118  and generate an error signal (PtFlow_err)  122 : the difference or error is an estimate of how much the intake port airflow must be increased or decreased by throttle  16  for the next intake event; and 
         [0033]    h) calculate a new corrected throttle position  124  and set a new, corrected throttle position  126 / 42  based on the error signal  122  generated in g). 
         [0034]    An alternative method, as shown by a dotted line in  FIG. 3 , is to input PtFlow_des  114  into the engine state estimator  102  and calculate a new corrected throttle position. Since there is a lower limit to the throttle response rate that drivers will accept, a rate-limiting step  116  should be included. 
         [0035]    Referring to  FIGS. 2 and 4 , in a second embodiment, where the reaction time of the phaser is fast relative to the change of airflow to the engine cylinders following a movement of a transient throttle valve, the phaser positions are adjusted to follow optimally the progress of engine airflow resulting from the throttle valve&#39;s movement from a first position to a second position by calculating and setting an optimal position of the phasers at a sequential plurality of throttle valve transient positions. A method in accordance with the second embodiment of the invention for setting a phaser position during transient engine operating conditions comprises the following steps (steps a-e in the first embodiment are the same as steps a-e in the second embodiment): 
         [0036]    a) provide an engine state estimator  102  for receiving current engine parameters such as throttle position  101   a , engine speed  101   b , intake valve cam phaser position  101   c , exhaust valve cam phaser position  101   d , ambient temperature  101   e  and engine coolant temperature  101   f  and for predicting, for the next engine intake event, engine speed  103   a , manifold air pressure  103   b , exhaust manifold pressure  103   c , manifold absolute temperature  103   d , intake valve cam phaser position  103   e , exhaust valve cam phaser position  103   f , intake port flow  103   g  and mean effective pressure  103   h;    
         [0037]    b) generate a table of optimal dilution values  104  for dilute limit operation as a function of predicted engine speed, manifold absolute temperature, intake valve cam phaser position, and engine load and current (coolant temperature) engine operating conditions; 
         [0038]    c) provide a model of calculated dilution values  106  as a function of engine operating conditions (engine speed, manifold absolute pressures and temperatures) and phaser positions; 
         [0039]    d) calculate an optimal dilution value  108  from Table  104  using predicted engine speed  103   a , manifold air temperature  103   d , intake valve cam phaser position  101   e  and the engine load parameter  103   b  or  103   h  and current (coolant temperature) engine operating conditions  101   f;    
         [0040]    e) calculate an actual dilution value  110  from Model  106  using the predicted engine operating conditions  103   a - f  received from engine state estimator  102 . (This is the expected dilution level for the next cylinder event if no corrective action is taken by the controller); 
         [0041]    f) compare the optimal dilution value  108  with the calculated actual dilution value  110  and generate a dilution value error signal (DV_err)  130  which is an estimate of the amount by which the dilution value must be increased or decreased for the next intake event, by changing the position of the intake valve cam phaser; 
         [0042]    g) compare the predicted value for dilution  110  to the current value  111  to generate a value for the expected change of dilution, d(DV)  134 ; 
         [0043]    h) compare the predicted intake valve cam phaser position  103   e  to the current intake valve cam phaser position  101   c  to generate an expected change of phaser position, d(IVO)  132 ; 
         [0044]    i) calculate a desired intake valve cam phaser position (IVO_des)  144  using the following equation: 
         [0000]        IVO   —   des =predicted intake valve cam phaser position+ DV   —   err/[d ( DV )/ d ( IVO )], 
         [0045]    j) generate a Table  136  of exhaust valve cam phaser position/intake valve cam phaser position ratios (EVO/IVO) that yield optimal dilution values over steady-state engine operating conditions; 
         [0046]    k) calculate  138  a desired exhaust valve cam phaser position (EVO_des) using the following equation: 
         [0000]    
       
      
       EVO 
       — 
       des=[EVO/IVO]×IVO 
       — 
       des  
      
     
         [0047]    l) set a new, corrected intake valve cam phaser position  140  and a new, corrected exhaust valve cam phaser position  142  based on the calculation made in step i, j and k 
         [0048]    Since on a given vehicle application the response rate of the valve cam phasers relative to the airflow response due to a throttle change may vary depending on operating conditions, a practical controller may use a combination of the above two embodiments. 
         [0049]    While the invention has been described by reference to an engine having both intake valve and exhaust valve cam phasers, it is understood that the system in accordance with the invention is also applicable to an engine having only an intake valve cam phaser or only an exhaust valve cam phaser. 
         [0050]    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.