Patent Publication Number: US-2007113808-A1

Title: Internal combustion engine comprising a variable valve lift system and a method for controlling valve lift shifting

Description:
TECHNICAL FIELD  
      The present invention relates to a method for controlling valve lift shifting in an internal combustion engine having a variable valve lift system whereby the engine can operate selectively in either of at least two valve lift modes, and a variable valve timing system, and more particularly, to minimizing torque changes when shifting lift modes.  
     BACKGROUND AND SUMMARY OF THE INVENTION  
      In an internal combustion engine with variable valve lift system, for example a cam profile shifting (CPS) system as described in U.S. Pat. No. 5,287,830A, switching valve lift mode can introduce a step in the output torque of the engine, which can cause discomfort to persons in a vehicle in which the engine is operating.  
      In that regard, the inventors herein have recognized that the engine output torque is related to engine volumetric efficiency and load. In particular, at an operating condition where the torque is the same for more than one valve lift mode, volumetric efficiency and load are also the same for the same valve lift modes, under the condition that the air fuel ratio, engine friction and combustion efficiency remain the same in the valve lift modes. The volumetric efficiency is dependent mainly on the engine speed, the VVT position, the valve lift mode, and the pressure ratio across the cylinders, i.e. the intake/exhaust relative pressure. Therefore, the inventors herein determined that it is in principle possible to find, for any given engine speed and any given intake/exhaust relative pressure, a VVT position value that gives the same volumetric efficiency, and therefore the same torque, in both valve lift modes. In other words, by finding this balance value of the VVT position, it is possible to obtain the same volumetric efficiency in more than one valve lift mode, and thereby to obtain the same output torque in more than one valve lift mode. Still differently worded, it is desirable to find a VVT position where the torque curves for two valve lift modes intersect. Thus, torque steps can be minimised when shifting modes in a variable valve lift system of an internal combustion engine.  
      Accordingly, the present invention is directed to minimizing torque steps when shifting modes in a variable valve lift system of an internal combustion engine by controlling the variable valve timing system so as to reduce a difference between two of the valve lift modes regarding the output torque of the engine.  
      In one embodiment of the present invention, the variable valve timing system is controlled based at least partially on the engine speed and a pressure parameter of the engine. Regarding the pressure parameter, as explained further below, the pressure ratio across the cylinders gives an accurate result, but can be approximated by a number of alternative parameters. More specifically, as explained closer below, in one alternative the pressure parameter is at least partly based on the pressure in an intake manifold of the engine, and in another alternative, the pressure parameter is at least partly based on the air flow in the intake manifold. In yet another alternative the variable valve timing system is controlled based at least partially on the engine speed and the position of a throttle valve of the engine.  
      It will be appreciated that features of the invention are susceptible to being combined in any combination without departing from the scope of the invention as defined by the accompanying claims.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      By way of example only, embodiments of the present invention will now be described with reference to the accompanying drawings wherein:  
       FIG. 1  is a schematic view of parts of an internal combustion engine, comprising a variable valve timing system and a variable valve lift system, the latter in a low lift position;  
       FIG. 2  is a detail of the view in  FIG. 1 , where the cam profiles have been shifted to a high lift position;  
       FIG. 3  is a block diagram depicting a method according to a first embodiment of the invention;  
       FIGS. 4   a - 4   d  are diagrams of variable valve timing values and load; and  
       FIG. 5  is a block diagram depicting methods according to an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)  
       FIG. 1  shows schematically parts of a naturally aspirating internal combustion engine. A camshaft  1  is driven by a crankshaft  2  via a belt (or chain)  3 , a crankshaft wheel  3   a,  and a camshaft wheel  3   b  in a manner known in the art. The engine comprises a variable valve lift system in the form of a cam profile shifting (CPS) system  4 , adapted to change the lift profile of intake and/or exhaust valves of the engine, by changing the valve lift provided by camlobes  6   a,    6   b  on the camshaft  1 . In  FIG. 1  only one valve, in the form of an intake valve  5  at a cylinder  5   a  is shown. In this example, for each valve  5  there is provided one low lift camlobe  6   a  and two high lift camlobes  6   b.    
      In operation each of the camlobes  6   a,    6   b  acts on valve actuators  4   a,    4   b,  located between the camlobes and the valve. The valve actuators are connectable by connecting means (not shown), in turn controllable by a CPS hydraulic control system  4   c.  In turn the CPS hydraulic control system  4   c  in controllable by an engine control unit (ECU)  7 . The ECU  7  has computational capabilities and storage capabilities, and can be formed by one or more physically separate, but logically connected devices. The current CPS system  4  setting is known to the ECU  7 . In a low lift mode, shown in  FIG. 1 , the valve actuators  4   a,    4   b  are not fixed to each other, which results in the valve lift being controlled by the low lift camlobe  6   a.    
      As can be seen in  FIG. 2 , in a high lift mode the valve actuators  4   a ,  4   b  are fixed to each other, so that the valve motion is controlled by the high lift camlobes  6   b.    
      Alternatively, still within the scope of the present invention, the CPS system can be provided in a variety of manners known in the art, for example as described in U.S. Pat. No. 5,950,583A. In particular, the CPS actuator can be provided in alternative manners, and the CPS system can be adapted to assume more than two valve lift modes, or cam profile modes.  
      The engine also comprises a variable valve timing (VVT) system  8 , which is adapted to set the camshaft  1  at desired angular positions in relation to the camshaft wheel  3   b . The VVT system comprises a timing actuator  8   a , for example of the type described in U.S. Pat. No. 6,135,077A, at the camshaft wheel  3   b . The timing actuator  8   a  is controllable by a VVT hydraulic control system  8   b . In turn the VVT hydraulic control system  8   b  in controllable by the ECU  7 . Also, by means of a connection  8   c  to a cam phase detector and a connection  8   d  to a crankshaft position detector, the ECU can determine a current value of the VVT position.  
      Also provided is a capability (not shown) to establish the engine speed, in any manner known in the art.  
      Besides the engine speed, the method according to this embodiment of the invention uses a pressure parameter, regarding which the following should be noted: The pressure ratio across the cylinder(s), i.e. the intake/exhaust relative pressure, is an accurate parameter for the method, but the exhaust pressure is difficult to measure in practice.  
      In the embodiment presented here, the intake manifold relative pressure is used as an approximation of the intake/exhaust relative pressure. In a naturally aspirating engine, the intake manifold relative pressure, i.e. intake/atmospheric pressure ratio, is a good parameter to use, since it is a good approximation of said intake/exhaust relative pressure, and since it is easy to establish in practice. As an alternative approximation for a naturally aspirating engine, the air flow, detected by means of a suitable sensor in the intake manifold, could be used instead of, or in combination with, the manifold relative pressure. As a further alternative for a naturally aspirated engine, the position of the throttle valve  10  can be used to obtain an approximate value of the intake/exhaust relative pressure.  
      For a turbo charged engine, and for the intake/exhaust relative pressure, the exhaust pressure can be approximated as a function of the air flow added to the atmospheric pressure. Alternatively, in a turbo charged engine, the exhaust pressure can be approximated as a function of the turbo charge pressure.  
      As can be seen in  FIG. 1 , in an intake manifold  9  a throttle valve  10  is controllable by the ECU  7 . Downstream of the throttle valve  10  is a first pressure sensor  11 , for providing the ECU  7  with values of the manifold absolute pressure (MAP). Also, a second pressure sensor  12  is connected to the ECU for providing the latter with values of the atmospheric pressure (ATM). The ECU  7  can determine values of an intake manifold relative pressure as a ratio between the MAP and the ATM.  
      The invention is based on the realisation that the volumetric efficiency and the load, on which the output torque (brake torque) is partly dependent, are in turn dependent mainly on the engine speed, the intake/exhaust relative pressure, the value of the VVT system  8 , and the setting of the CPS system  4 . Based on said realisation, it is in principle possible to find, for any given engine speed and any given intake/exhaust relative pressure, a VVT system setting value that gives the same volumetric efficiency, and therefore the same torque, in both valve lift modes, herein also referred to as CPS modes. Here, the VVT system setting value that gives the same volumetric efficiency in both CPS modes is referred to as the VVT balance value BV VVT .  
      As can be seen in  FIG. 3 , in this embodiment of the invention, MAP and ATM are determined  301 ,  302 , and based thereon, the intake relative pressure, IRP, is determined  303 . Based on the relative pressure IRP and the engine speed, n,  304 , the VVT balance value BV VVT  is determined  305 , as specified below:  
      Referring to  FIGS. 4   a  and  4   b , the ECU  7  stores predetermined data correlating, for each of the CPS modes, values of the engine speed, n, the intake relative pressure, IRP, the VVT position, and the load.  FIG. 4   a  depicts an example of a part of this correlation data, showing the load as a function of the VVT position at a certain engine speed n 1 . In this example, the load refers to the air load per engine revolution or per combustion, or the engine air mass flow per revolution or per combustion, but alternatively other corresponding parameters known to the skilled person could be used. Further, in this example there are two possible CPS modes, a first mode CPS 1  and a second mode CPS 2 , but the method is applicable also the engines having more than two such modes. For each mode CPS 1 , CPS 2 , there is for a number of values of the intake relative pressure IRP 1 , IRP 2 , IRP 3  a curve giving a correlation between VVT positions and the load. Circles  41 ,  42 ,  43  indicate positions where curves for different CPS modes and for the same intake relative pressure intersect, i.e. where the load is the same at both CPS modes and at respective intake relative pressures IRP 1 , IRP 2 , IRP 3 , respectively. Thus, the VVT balance value BV VVT  is the VVT position at such a point  41 ,  42 ,  43  of intersection.  
      Similarly, as shown in  FIG. 4   b , for some other engine speed n 2 , the VVT balance value BV VVT  is determined as the VVT position at a point  44 ,  45 ,  46  of intersection of curves for different CPS modes and for the same intake relative pressure.  
      Referring to  FIGS. 3 and 4   c : When the VVT balance value BV VVT  is determined, a VVT position range, MIN VVT -MAX VVT , comprising the balance value BV VVT  is determined. In general, the VVT position range is determined based on at least one predetermined offset value in relation to the balance value of the valve timing. More particularly, based the VVT balance value BV VVT , a predetermined positive offset value  306  and a predetermined negative offset value  307 , a minimum limit MIN VVT  and a maximum limit MAX VVT  for the VVT position is determined  308 .  
      A special case is shown in  FIG. 4   d . Here the determined VVT balance value BV VVT , is outside an operational range, MINOPER-MAXOPER, of the VVT system. Thereby, the VVT position is controlled so as to minimize the torque difference in a CPS mode change. More specifically, a desired VVT position is determined within the operational range, MINOPER-MAXOPER. In the example in  FIG. 4   d , the desired VVT position is at the lower VVT limit MINOPER. Subsequently, a VVT position range, MIN VVT -MAX VVT , comprising the desired VVT position is determined. In this example, based the desired VVT position and a predetermined positive offset value, a maximum limit MAX VVT  for the VVT position is determined.  
      As can be seen in  FIG. 3 , after determining a current VVT position value CURR VVT , it is determined whether the current VVT position value is within the range delimited by the minimum limit MIN VVT  and the maximum limit MAX VVT  for the VVT position,  309 . If the current VVT position value is outside said range, the VVT position is changed so as to fall within the range,  310 . This could be done regardless of a request to shift the CPS mode in order to prepare the system for a CPS mode shift request.  
      The ECU  7  is also adapted to determine a requested CPS mode REQ CPS    311  and a current CPS mode CURR CPS    312 , and determine if the requested CPS mode REQ CPS  and the current CPS mode CURR CPS  are identical  313 . If they are identical, no CPS mode change is carried out  314 . If the requested CPS mode REQ CPS  and the current CPS mode CURR CPS  are not identical, and the current VVT position value is within the range MIN VVT -MAX VVT    309 , the CPS mode is changed to the requested CPS mode REQ CPS    315 . Also, the CPS mode is changed to the requested CPS mode REQ CPS    315  if the current VVT position has been adjusted to fall within this range MIN VVT -MAX VVT    310 .  
      Alternatively, if it is determined  309  that the current VVT value is outside the range delimited by the minimum limit MIN VVT  and the maximum limit MAX VVT  for the VVT position, simply no change of the CPS system mode is carried out. Also, if it is determined  309  that the current VVT value is outside said range, a CPS mode shift could be based on other criterias.  
      As mentioned, in the determination of the VVT balance value BV VVT , alternative parameters can be used instead of the load. For example, the volumetric efficiency can be used since it is in a way similar to the load dependent upon the CPS modes, engine speed, the intake/exhaust relative pressure and the VVT position. Also, instead of the load, the CPS modes, engine speed, the intake/exhaust relative pressure and the VVT position can be mapped against the output torque (brake torque). In the latter case, the air/fuel ratio, the combustion efficiency, and the friction loss (partly dependent on engine oil temperature) can be, for the method described here, assumed to be independent of the CPS modes. Alternatively, one or more of the air/fuel ratio, the combustion efficiency, and the friction loss can be determined for the respective CPS modes, so that consideration is given to such a parameter as well. For example, when determining whether to change the CPS mode, consideration can be given to a determined combustion efficiency in the required CPS mode. If it is determined, based on the calculated combustion efficiency that there is no risk of knocking within the VVT range MIN VVT -MAX VVT  in the required CPS mode, the CPS mode can be changed. However, if it is determined that there is a risk of knocking in said range in the required CPS mode, the VVT position, or the range, can be offset to counteract this effect.  
       FIG. 5  shows some of the steps in a method according an alternative embodiment of the invention. Differing from the embodiment in  FIG. 3 , consideration is made to transient torque requests. A transient torque request can for example be the result the driver of a vehicle, in which the engine is operating, pushing the accelerator pedal quickly. Is such a case, the focus is shifted from creating a smooth CPS mode change, to accomplishing a fast response to the torque request. In this embodiment, the ECU determines the degree of requested transient torque  501 . The degree of requested transient torque is partly dependent on the pace at which the accelerator pedal is pushed. Based on the degree of requested transient torque a maximum positive offset value for the VVT position is determined  507 . If the degree of requested transient torque is relatively large, a relatively large maximum positive offset value for the VVT position is allowed. The CPS mode change takes place in the same manner as described above with reference to  FIG. 3 .  
      As an alternative to the embodiment described with reference to  FIG. 5 , the method could comprise determining whether the requested transient torque is higher than a predetermined threshold value, and if it is, allowing a CPS mode change regardless of whether the VVT position is within the VVT position range, MIN VVT -MAX VVT .  
      It should be noted that the invention is equally applicable to engines with variable valve lift systems providing more than two valve lift modes. It should also be noted that the method according to the invention is equally applicable to engines with spark ignition and engines with compression ignition.