Patent Publication Number: US-2007113807-A1

Title: Internal combustion engine comprising a variable valve lift system and a variable valve timing system, and a method for such an engine

Description:
TECHNICAL FIELD  
      The present invention relates to an internal combustion engine comprising a variable valve lift system and a variable valve timing (VVT) systems, and more particularly to operating the engine selectively in either of at least two valve lift modes.  
     BACKGROUND AND SUMMARY OF THE PRESENT INVENTION  
      An internal combustion engine can be provided with a variable valve lift system, for example a cam profile shift (CPS) system, and a variable valve timing (VVT) system. Such an engine is described in for example US6581564B2. In engines where one oil feed system is provided for actuation in both the CPS system and the VVT system, the physical connection between the VVT system and the CPS system via the oil system can cause the following problem: A mode shift in the CPS system might cause a pressure pulse, which can affect the VVT-position. The VVT angle can due to this effect have a control error of up to 10 CA (crankshaft angles).  
      Also in engines in which the CPS system and the VVT system have separate systems for their respective actuation, whether hydraulic or not, a mode shift in the CPS system might affect the VVT-position. The cause could be a friction change due to the different camlobes used in the CPS modes. Further, where both systems are electrically actuated, a CPS mode shift can cause a voltage shift affecting the VVT position.  
      This is undesirable, since the related change in the VVT position can cause a change of the volumetric efficiency of the engine, which in turn may affect vehicle driveability and driver comfort.  
      Accordingly, this invention is directed to a method for minimizing torque steps when shifting modes in a variable valve lift system of an internal combustion engine.  
      This is accomplished by a method for controlling valve timing in an internal combustion engine capable of operating selectively in either of at least two valve lift modes, the engine having a variable valve timing (VVT) system, the method comprising: sending a control pulsing signal to a VVT system actuator in response to a valve lift mode shift.  
      Thereby, an effect on the VVT position by a valve lift mode shift can be counteracted or compensated for, so that the VVT position remains essentially the same before, during and after the mode shift.  
      The VVT actuator could be controllable by at least one control valve of a hydraulic system, the control valve being controllable by an engine control unit, whereby the control pulse is sent to the control valve. In case the variable valve lift system comprises a valve lift shift actuator controllable by the same hydraulic system, a pressure pulse, positive or negative, caused by changing the valve lift mode and the physical connection by the hydraulic system between the VVT actuator and the valve lift shift actuator, can be counteracted by the provided control pulse, so that the VVT position is not affected.  
      However, it should be noted that the VVT actuator could be controlled by some other type of system. For example, the VVT actuator could use an electric power supply which is controlled by an engine control unit.  
      Preferably, the control pulse is sent after a time interval (Dt) following a signal for the valve lift mode shift. The signal for the valve lift mode shift could be a signal from which a valve lift mode shift can be predicted. For example, the signal could be in the form of an internal command in an engine control unit. Also, the control pulse could be terminated by a decrement function.  
      The above advantages and other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      In the following text, the invention will be described in detail with reference to the attached drawings. These drawings are used for illustration only and do not in any way limit the scope of the invention. In the drawings:  
       FIG. 1  is a schematic view of parts of an internal combustion engine, including 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, and  
       FIGS. 3 and 4  are diagrams of engine control signals as functions of time. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)  
       FIG. 1  shows schematically parts of an internal combustion engine. A camshaft I 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 in the form of a cam profile shifting (CPS) system  4 , adapted to shift 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 , in the form of valve tappets, located between the camlobes and the valve. As depicted in the schematic presentation of  FIG. 1  and  2 , the valve actuators can be fixed to each other by a valve lift shift actuator  4   c , here also referred to as a CPS actuator  4   c , in turn controllable by a hydraulic system  9 , indicated in  FIG. 1  with broken lines. The hydraulic system  9  can be, as is known in the art, used for manoeuvring a plurality of units of the engine, and comprises a hydraulic pump  9   a . For control of the CPS actuator(s)  4   c , the CPS system  4  comprises a CPS actuator control valve  9   b  in the hydraulic system  9 . The CPS actuator control valve  9   b  is controllable by an engine control unit (ECU)  7 . The CPS actuator control valve  9   b  can be of a known type, for example a solenoid valve, not further described here. The ECU  7  has, as known, computational capabilities and storage capabilities, and can be formed by one device, or more than one physically separate, but logically connected devices.  
      In a low lift mode, shown in  FIG. 1 , the valve actuators  4   a ,  4   b  are not fixed to each other by the CPS actuator  4   c , 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 by the CPS actuator  4   c , 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 US5950583A. 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.  
      The engine also comprises a variable valve timing (VVT) system  8 , which is adapted to adjust the VVT position, i.e. set the camshaft I at desired angular positions in relation to the camshaft wheel  3   b . The VVT system comprises a VVT actuator  8   a , for example of the type described in US6135077A, at the camshaft wheel  3   b . The VVT actuator  8   a  is adapted to mechanically change the VVT position, and is controllable by the hydraulic system  9 . For control of the VVT actuator  8   a , the VVT system comprises a VVT actuator control valve  9   c  in the hydraulic system  9 , which could be of the same type as the CPS actuator control valve  9   b . The VVT actuator control valve  9   c  is also 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.  
      The VVT actuator  8   a  with the VVT actuator control valve  9   c  is integrating which means that a constant value of the signal from the ECU  7  to the VVT actuator control valve  9   c  gives a constant flow of oil through the valve if the pressure drop over the valve is constant. Since the CPS system and the VTT system use the same hydraulic system, a CPS mode shift can result in a change of the oil pressure at the VVT actuator control valve  9   c . As described closer below, in order to compensate for this change of pressure, the VVT actuator control valve  9   c  is activated so as to counteract the effect of the pressure change.  
      Here reference is made also to  FIG. 3 , which shows signals of the ECU  7  in the form of a VVT actuator control valve signal  11 , and a CPS actuator control valve signal  12 . In connection to a valve lift mode shift, here also referred to as a cam profile mode shift, the ECU  7  sends a control pulse  14  for the VVT actuator in this example. In this embodiment, at the time t 0 , a signal is identified from which a valve lift mode shift is predicted. The mode shift involves changing the cam profile mode from the low lift mode to the high lift mode. The signal is in the form of an internal command in the ECU  7 . At least partly based on the internal command, the ECU sends a signal to the CPS actuator control valve  9   b . Also, according to this embodiment of the invention, at the end of a time interval Dt following the time t 0  at which the signal or internal command was identified, the ECU  7  sends a control pulse  14  to the VVT actuator control valve  9   c.    
      Alternatively, the ECU  7  could send the control pulse  14  to the VVT actuator control valve  9   c  at the end of a time interval following a time at which the ECU sends a signal to the CPS actuator control valve  9   b.    
      The control pulse  14  in this example includes a step to its maximum amplitude A, and is terminated by a decrement function F. For this presentation, the control pulse  14  in  FIG. 3  is defined as positive. The control pulse  14  will result in a movement of the VVT actuator control valve  9   c  so as for the pressure at the VVT actuator to be constant despite the pressure change in the hydraulic system caused by the CPS mode shift. Thus, the control pulse  14  will counteract the effect of the cam profile shift on the VVT position.  
      Thus, the effect of the control pulse  14  is to counteract the influence of the cam profile mode shift on the VVT position. The time interval Dt, and the amplitude A and the decrement function F of the control pulse  14  could be predetermined. Alternatively, one or more of these parameters, Dt, A, F could be determined based on at least one engine operation related parameter, for example the engine speed and/or the oil temperature. Such determinations could be based on predetermined functions or tables mapping the time interval Dt, the pulse amplitude A and the pulse decrement function F to the engine operation related parameter(s). The time interval Dt, the pulse amplitude A, and the decrement function F, or the functions for their determination, could be calibratable, i.e., they could be adjusted before operation so that an optimal effect of the control pulse  14  is achieved. More specifically, the shape of the control pulse should adapted to a shape of a pressure pulse in the hydraulic system caused by a CPS mode shift.  
      The time interval Dt is preferably chosen so that the VVT actuator  8   a  response to the control pulse  14  occurs at the same time as the CPS mode shift affects the VVT position. The delay between the CPS mode shift control signal and the time at which the CPS mode shift affects the VVT position can be due to, for example, a pressure pulse travelling through a hydraulic system adapted to control the CPS system as well as the VVT system.  
      Here reference is made also to  FIG. 4 . A cam profile shift is made in the opposite direction in relation to the shift described with reference to in  FIG. 3 . Thus, at the time t 0 , the ECU sends a signal to the CPS actuator control valve  9   b  in order to change the cam profile mode from the high lift mode to the low lift mode. After a time interval Dt, the ECU  7  sends a control pulse  14  to the VVT actuator control valve  9   c . The control pulse  14  in  FIG. 4  is negative instead of positive. Therefore, the pressure pulse sent to the VVT actuator  8   a , will result in the VVT actuator  8   a  acting in the opposite direction in relation to the actuator action following the control pulse in  FIG. 3 . However, for certain CPS systems, not described closer here, it can be suitable to provide control pulses to the VVT system having the same sign (positive or negative) for both types of CPS mode shifts, i.e. low lift to high lift and vice versa.  
      Alternatives are possible to the embodiment described above. For example, the control pulse can have alternative shapes, for example it can be in the form of a constant amplitude pulse. Also, the control pulse can be sent simultaneously as the ECU signal  12  for the cam profile shift.  
      The invention is equally applicable to engines where the VVT system and the CPS system are actuated by two separate hydraulic systems, or where any or both of the systems are actuated by some other type of system, for example including electromagnetic actuators. Further, 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.  
      The invention is not limited to the embodiments described above and may be varied freely within the scope of the appended claims.