Abstract:
An internal combustion engine including a variable valve lift system whereby the engine can operate selectively in either of at least two valve lift modes, as well as a method for controlling valve lift shifting in such an engine are disclosed. The method comprises comparing the fuel supply with a fuel supply threshold value, and at least partly based on the result of the comparison, determining whether a valve lift mode shift should be performed. Preferably, the fuel supply threshold value corresponds to the fuel supply being cut off.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a method for controlling valve lift shifting in an internal combustion engine comprising a variable valve lift system whereby the engine can operate selectively in either of at least two valve lift modes, and more particularly to minimizing torque changes when shifting modes in a variable valve lift system of an internal combustion engine.  
       BACKGROUND AND SUMMARY OF THE INVENTION  
       [0002]     In an internal combustion engine with a variable valve lift system, in particular a cam profile shifting (CPS) system, for instance as described in U.S. Pat. No.  5 , 287 , 830 A, switching cam profiles can introduce a change in the output torque of the engine, which can cause discomfort to persons in a vehicle in which the engine is operating. The reason for such a change in the output torque could be that in the amount of air trapped in the cylinders differ from one valve lift mode to another.  
         [0003]     Accordingly, the present invention is directed to a method for minimizing a change in engine output torque when shifting modes in a variable valve lift system of an internal combustion engine, the method including comparing an actual fuel supply with a fuel supply threshold value, and at least partly based on the result of the comparison, determining whether a valve lift mode shift should be performed.  
         [0004]     Thereby, based on the comparison, it can be determined that if the fuel supply is equal to or below the fuel supply threshold value, a valve lift mode shift is performed. As is closer described below, if the engine fuel supply is above the fuel supply threshold value, it can be determined that a valve lift mode shift is not performed. The fuel supply threshold value should be chosen so that the energy released at individual combustions, regardless of the air supply, is low. Thereby, at, or under the fuel supply threshold value, a difference of the air supply caused by a valve lift mode shift will not cause a noticeable step in the output torque of the engine.  
         [0005]     It should be noted that the invention does not exclude the possibility that criteria other than the fuel supply, such as requested transient torque, can be considered when determining whether a valve lift mode shift should be performed.  
         [0006]     Preferably, the fuel supply threshold value corresponds to the fuel supply being cut off. Thereby, it is secured that a difference of the air supply caused by a valve lift mode shift will not cause a step in the output torque of the engine.  
         [0007]     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 de-fined by the accompanying claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     By way of example only, embodiments of the present invention will now be de-scribed with reference to the accompanying drawings wherein:  
         [0009]      FIG. 1  is a schematic view of parts of an internal combustion engine, comprising a variable valve timing system and a cam profile shifting system, the latter in a low lift position;  
         [0010]      FIG. 2  is a detail of the view in  FIG. 1 , where the cam profiles have been shifted to a high lift position; and  
         [0011]      FIG. 3  is a block diagram depicting a method according to an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  shows schematically parts of an 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 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 , located between the camlobes and the valve. The valve actuators are fixable to each other by a CPS actuator  4   c , here in the form of a locking pin  4   c , in turn controllable by a hydraulic system  9 , indicated in FIG. I 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 cam profile shifting (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.  
         [0013]     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.    
         [0014]     Alternatively, still within the scope of the present invention, the variable valve lift 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 cam profile modes.  
         [0015]     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 U.S. Pat. No. 6,135,077A, 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  8  comprises a VVT actuator control valve  9   c  in the hydraulic system  9 . The VVT actuator control valve  9   c  is also controllable by the ECU  7 , and could be of the same type as the CPS actuator control valve  9   b.    
         [0016]     Also provided are means (not shown) to establish the engine speed, in a manner known in the art.  
         [0017]     As can be seen in  FIG. 1 , in an intake manifold  10  a fuel injector  11  is controllable by the ECU  7 . Alternatively, the fuel injector can be provided in the cylinder  5 .  
         [0018]     Here reference is made to  FIG. 3 . The ECU  7  is adapted to determine a required CPS mode  311 , and a current CPS mode  312 , and also to determine whether the required and the current CPS modes are identical  313 . If they are identical, no cam profile shift is performed  314 . If the required and the current CPS modes are not identical, it is determined  315  whether the current engine fuel supply CF, provided by the fuel injector(s)  11 , is below a fuel supply threshold value FT, described closer below. Here, fuel supply refers to the total supply of fuel in the engine. Alternatively, it could refer to a mean fuel supply to the respective cylinder, i.e. the total supply in the engine divided by the number of cylinders.  
         [0019]     Based on the result of the determination  315  regarding the engine fuel supply CF, it is determined whether a valve lift mode shift should be performed. If the engine fuel supply CF is equal to or below the fuel supply threshold value FT, a valve lift mode shift is performed  316 . If the engine fuel supply CF is above the fuel supply threshold value FT, it is determined  317  whether the CPS mode should be changed according to an alternative CPS mode shifting strategy. This alternative strategy could be a strategy that takes the VVT position into account for cam profile shifts, as described in the European Patent Application No. 05110938.7, filed by the same applicant on the same day as this application. If it is determined  317  according to the alternative CPS mode shifting strategy that a valve lift mode shift should be performed, a valve lift mode shift is performed  316 . However, if it is determined  317  according to the alternative CPS mode shifting strategy that a valve lift mode shift should not be performed, a required CPS mode  311  and a current CPS mode  312  are again determined.  
         [0020]     Alternatively, if it is determined  317  according to the alternative CPS mode shifting strategy that a valve lift mode shift should not be performed, simply no cam profile shift is performed. As a further alternative, no consideration for any alternative CPS mode shifting strategy could be made. In such a case, if it is determined  315  that the engine fuel supply CF is above the fuel supply threshold value FT, a required CPS mode  311  and a current CPS mode  312  are again determined, or simply no cam profile shift is performed.  
         [0021]     The fuel supply threshold value FT could be predetermined, or it could be determined based on at least one engine operation related parameter, for example the engine speed. Such determinations could be based on predetermined functions or tables mapping the fuel supply threshold value FT to the engine operation related parameter(s). Also, the fuel supply threshold value FT could be calibratable. In a preferred embodiment, the fuel supply threshold value FT is zero, i.e. the fuel supply CF is cut off.  
         [0022]     It should be noted that the invention is equally applicable to engines with variable valve lift systems providing more than two valve lift modes.  
         [0023]     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.