Patent Publication Number: US-10323585-B2

Title: Method for controlling of valve timing of continuous variable valve duration engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 15/258,035, filed on Sep. 6, 2016, and claims priority to and the benefit of Korean Patent Application Nos. 10-2015-0177464, filed on Dec. 11, 2015, and 10-2017-0154705, filed on Nov. 20, 2017, the entirety of each of which are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to a system and a method for controlling valve timing of a continuous variable valve duration engine. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     An internal combustion engine combusts mixed gas in which fuel and air are mixed at a predetermined ratio through a set ignition mode to generate power by using explosion pressure. 
     Generally, a camshaft is driven by a timing belt connected with a crankshaft that converts linear motion of a cylinder by the explosion pressure into rotating motion to actuate an intake valve and an exhaust valve, and while the intake valve is opened, air is suctioned into a combustion chamber, and while an exhaust valve is opened, gas which is combusted in the combustion chamber is exhausted. 
     To improve the operations of the intake valve and the exhaust valve and thereby improve engine performance, a valve lift and a valve opening/closing time (timing) should be controlled according to a rotational speed or load of an engine. Therefore, a continuous variable valve duration (CVVD) device controlling the opening duration of an intake valve and an exhaust valve of the engine and a continuous variable valve timing (CVVT) device controlling the opening and closing timing of the intake valve and the exhaust valve of the engine have been developed. 
     The CVVD device may control opening duration of the valves. In addition, the CVVT device may advance or retard the opening or closing timing of the valves in a state that the opening duration of the valve is fixed. That is, if the opening timing of the valve is determined, the closing timing is automatically determined according to the opening duration of the valve. 
     However, in case of combining the CVVD device and the CVVT device, both the opening duration and timing of the valve should be simultaneously controlled. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. 
     SUMMARY 
     The present disclosure provides a system and a method for controlling valve timing of a continuous variable valve duration engine that simultaneously controls duration and timing of the valve being equipped with a continuous variable valve duration device and a continuous variable valve timing device disposed on an intake valve side and equipped with a continuous variable valve duration device on an exhaust valve side of a turbo engine vehicle by independently controlling an opening and closing timing of an intake valve and an exhaust valve. 
     One form of the present disclosure, a method for controlling intake and exhaust valves of an engine may include: controlling, by an intake continuous variable valve timing (CVVT) device, opening and closing timings of the intake valve; determining, by a controller, a target opening duration of the intake valve, a target opening duration of the exhaust valve, and at least one of a target opening timing or a target closing timing of the intake valve and the exhaust valve, based on an engine load and an engine speed; modifying, by an intake continuous variable valve duration (CVVD) device, current opening and closing timings of the intake valve based on the target opening duration of the intake valve; modifying, by an exhaust CVVD device, current opening and closing timings of the exhaust valve based on the target opening duration of the exhaust valve; advancing, by the intake CVVD device, the current opening timing of the intake valve while simultaneously retarding the current closing timing of the intake valve by a predetermined value, or retarding the current opening timing of the intake valve while simultaneously advancing the current closing timing of the intake valve by a predetermined value, based on the target opening duration of the intake valve; and advancing, by the exhaust CVVD device, the current opening timing of the exhaust valve while simultaneously retarding the current closing timing of the exhaust valve by a predetermined value, or retarding the current opening timing of the exhaust valve while simultaneously advancing the current closing timing of the exhaust valve by a predetermined value, based on the target opening duration of the exhaust valve. 
     In particular, the intake CVVD device advances the current opening timing of the intake valve while simultaneously retarding the current closing timing of the intake valve when the target opening duration of the intake valve is longer than a duration between the current opening timing and current closing timing of the intake valve. 
     The intake CVVD device retards the current opening timing of the intake valve while simultaneously advancing the current closing timing of the intake valve when the target opening duration of the intake valve is shorter than a duration between the current opening timing and current closing timing of the intake valve. 
     The exhaust CVVD device advances the current opening timing of the exhaust valve while simultaneously retarding the current closing timing of the exhaust valve when the target opening duration of the exhaust valve is longer than a duration between the current opening timing and current closing timing of the exhaust valve. 
     The exhaust CVVD device retards the current opening timing of the exhaust valve while simultaneously advancing the current closing timing of the exhaust valve when the target opening duration of the exhaust valve is shorter than a duration between the current opening timing and current closing timing of the exhaust valve. 
     The method further includes the step of adjusting, by the intake CVVT device, the current opening and closing timings of the intake valve to the target opening and closing timings of the intake valve, respectively. 
     The method of further includes the step of adjusting, by the exhaust CVVT device, the current opening and closing timings of the exhaust valve to the target opening and closing timings of the exhaust valve, respectively. 
     During the step of determining the target opening duration of the intake valve, the controller sets the target opening duration of the intake valve to a first intake opening duration in a first control region where the engine load is between first and second predetermined loads, and the controller controls the intake CVVD device to adjust a current intake opening duration to the first opening duration and controls the exhaust valve to limit a valve overlap in the first control region. 
     In one form, a valve overlap is limited by fixing the current opening and closing timings of the intake valve and by setting the closing timing of the exhaust valve to be a maximum value to maintain combustion stability in the first control region 
     During the step of determining the target opening duration of the intake valve, the controller sets the target opening duration of the intake valve to a second intake opening duration in a second control region where the engine load is greater than the second predetermined load and equal to or less than a third predetermined load, and the controller controls the intake CVVD device to adjust the current intake opening duration to the second intake opening duration, and wherein the second intake opening duration is equal to or longer than the first intake opening duration. 
     The method further includes the step of determining, by the controller, a third control region where the engine load is greater than the third predetermined load and less than a fourth predetermined load and the engine speed is between first and second predetermined speeds, or where the engine load is greater than the third predetermined load and equal to or less than a fifth predetermined load and the engine speed is between the second predetermined speed and a third predetermined speed; advancing, by the intake CVVT device, the current closing timing of the intake valve by a predetermined angle in the third control region; and advancing, by the exhaust CVVD device, the current closing timing of the exhaust valve by a predetermined angle in the third control region. 
     The method further includes the step of advancing the current closing timing of the intake valve to be approximately at a bottom dead center (BDC) when the engine speed less than or equal to a predetermined speed; and advancing the current closing timing of the intake valve to be after the BDC when the engine speed is greater than the predetermined speed in the third control region. 
     The method may further have the step of determining a fourth control region, by the controller, where the engine load is greater than a fourth predetermined load and equal to or less than a fifth predetermined load and the engine speed is equal to or greater than a first predetermined speed and equal to or less than a second predetermined speed; and approaching, by the intake CVVT device, the current closing timing of the intake valve to be approximately at a bottom dead center (BDC) in the fourth control region. 
     The method further includes the step of approaching the current closing timing of the intake valve to a top dead center (TDC) by the intake CVVT device, and approaching the current closing timing of the exhaust valve to the TDC by the exhaust CVVD device in the fourth control region. 
     The method further includes the step of determining, by the controller, a fifth control region where the engine load is greater than a fifth predetermined load and equal to or less than a maximum engine load and the engine speed is between first and second predetermined speeds; controlling, by the controller, a throttle valve to be fully opened; and advancing, by the intake CVVT device, the current opening timing of the intake valve to be before a top dead center (TDC), and controlling the current closing timing of the intake valve to be a predetermined value after a bottom dead center in the fifth control region. 
     The method further includes the step of determining, by the controller, a sixth control region where the engine load is greater than a fifth predetermined load and equal to or less than a maximum engine load and the engine speed is greater than a second predetermined speed and equal to or less than a third predetermined speed; controlling, by the controller, a throttle valve to be fully opened; and advancing, by the intake CVVT device, the current closing timing of the intake valve by a predetermined angle in the sixth control region. 
     The method further includes the step of approaching the current closing timing of the exhaust valve to a top dead center to inhibit a valve overlap in the sixth control region. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram showing a system for controlling valve timing of a continuous variable valve duration engine according to one form of the present disclosure; 
         FIG. 2  is a perspective view showing a continuous variable valve duration device and a continuous variable valve timing device which are disposed on an intake valve side, and a continuous variable valve duration device disposed on an exhaust valve side according to one form of the present disclosure; 
         FIG. 3  is a side view of a continuous variable valve duration device assembled with a continuous variable valve timing device which is disposed on intake valve and exhaust valve sides; 
         FIG. 4  is a partial view of an inner wheel and a cam unit of a continuous variable valve duration device in one form; 
         FIGS. 5A-5C  are views illustrating the operation of a continuous variable valve duration device in  FIG. 4 ; 
         FIGS. 6A and 6B  are views illustrating a cam slot of a continuous variable valve duration device in exemplary forms; 
         FIGS. 7A-7C  are valve profiles of a continuous variable valve duration device in one form; 
         FIGS. 8A-8D  illustrate a change of an opening duration and opening and closing timings of a valve; 
         FIGS. 9A and 9B  are flowcharts showing a method for controlling valve timing of a continuous variable valve duration engine according to one form of the present disclosure; 
         FIG. 10  is a schematic diagram illustrating control regions in one form; 
         FIGS. 11A-11C  are drawings showing duration, opening timing, and closing timing of an intake valve depending on an engine load and an engine speed according to the present disclosure; and 
         FIGS. 12A-12C  are drawings showing duration, opening timing, and closing timing of an exhaust valve depending on an engine load and an engine speed according to the present disclosure. 
     
    
    
     The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
     As those skilled in the art would realize, the described forms may be modified in various different ways, all without departing from the spirit or scope of the present disclosure. 
     Throughout this specification and the claims which follow, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising” will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. 
     It is understood that the term “vehicle” or “vehicular” or other similar terms as used herein is inclusive of motor vehicles in general including hybrid vehicles, plug-in hybrid electric vehicles, and other alternative fuel vehicles (e.g., fuels derived from resources other than petroleum). As referred to herein, a hybrid electric vehicle is a vehicle that has two or more sources of power, for example a gasoline-powered and electric-powered vehicle. 
     Additionally, it is understood that some of the methods may be executed by at least one controller. The term controller refers to a hardware device that includes a memory and a processor configured to execute one or more steps that should be interpreted as its algorithmic structure. The memory is configured to store algorithmic steps, and the processor is specifically configured to execute said algorithmic steps to perform one or more processes which are described further below. 
     Furthermore, the control logic of the present disclosure may be embodied as non-transitory computer readable media on a computer readable medium containing executable program instructions executed by a processor, a controller, or the like. Examples of computer readable media include, but are not limited to, ROM, RAM, compact disc (CD)-ROMs, magnetic tapes, floppy disks, flash drives, smart cards, and optical data storage devices. The computer readable recording medium can also be distributed in network coupled computer systems so that the computer readable media is stored and executed in a distributed fashion, e.g., by a telematics server or a controller area network (CAN). 
       FIG. 1  is a schematic block diagram showing a system for controlling valve timing of a continuous variable valve duration engine according to one form of the present disclosure. 
     As shown in  FIG. 1 , a system for controlling valve timing of a continuous variable valve duration engine includes: a data detector  100 , a camshaft position sensor  120 , a controller  300 , an intake continuous variable valve duration (CVVD) device  400 , an intake continuous variable valve timing (CVVT) device  450 , and an exhaust continuous variable valve duration (CVVD) device  500 . 
     The data detector  100  detects data related to a running state of the vehicle for controlling the CVVD devices and the CVVT devices, and includes a vehicle speed sensor  111 , an engine speed sensor  112 , an oil temperature sensor  113 , an air flow sensor  114 , and an accelerator pedal position sensor (APS)  115 , although other sensors or systems may be employed to detect or determine the desired data. 
     The vehicle speed sensor  111  detects a vehicle speed, transmits a corresponding signal to the controller  300 , and may be mounted at a wheel of the vehicle. 
     The engine speed sensor  112  detects a rotation speed of the engine from a change in phase of a crankshaft or camshaft, and transmits a corresponding signal to the controller  300 . 
     The oil temperature sensor (OTS)  113  detects temperature of oil flowing through an oil control valve (OCV), and transmits a corresponding signal to the controller  300 . 
     The oil temperature detected by the oil temperature sensor  113  may be determined by measuring a coolant temperature using a coolant temperature sensor mounted at a coolant passage of an intake manifold. Therefore, in one form, the oil temperature sensor  113  may include a coolant temperature sensor, and the oil temperature should be understood to include the coolant temperature. 
     The air flow sensor  114  detects an air amount drawn into the intake manifold, and transmits a corresponding signal to the controller  300 . 
     The accelerator pedal position sensor (APS)  115  detects a degree in which a driver pushes an accelerator pedal, and transmits a corresponding signal to the controller  300 . The position value of the accelerator pedal may be 100% when the accelerator pedal is pressed fully, and the position value of the accelerator pedal may be 0% when the accelerator pedal is not pressed at all. 
     A throttle valve position sensor (TPS) that is mounted on an intake passage may be used instead of the accelerator pedal position sensor  115 . Therefore, in one form, the accelerator pedal position sensor  115  may include a throttle valve position sensor, and the position value of the accelerator pedal should be understood to include an opening value of the throttle valve. 
     The camshaft position sensor  120  detects a change of a camshaft angle, and transmits a corresponding signal to the controller  300 . 
     Referring to  FIG. 2 , a continuous variable valve duration device and a continuous variable valve timing device are mounted on the intake valve side, and the continuous variable valve duration device is mounted on the exhaust valve side through a fixed opening device. Therefore, an exhaust valve opening (EVO) timing is fixed in the one form of the present disclosure. For example, the EVO timing may be fixed at an angle a before bottom dead center of approximately 40 to 50 degrees at most of part load regions so as to be advantageous fuel efficiency. 
     The intake continuous variable valve duration (CVVD) device  400  controls an intake valve opening duration of the engine according to a signal of the controller  300 , and the exhaust continuous variable valve duration (CVVD) device  500  controls an exhaust valve opening duration of the engine according to a signal of the controller  300 . 
     The intake continuously variable valve timing (CVVT) device  450  controls opening and closing timing of the intake valve according to a signal of the controller  300 . 
       FIG. 3  is a side view of the CVVD device ( 400 ,  500 ) applied to the intake and exhaust valves in another form as assembled with valves  200  operating with cylinders  201 ,  202 ,  203 ,  204 . In particular, the intake CVVD device  400  is assembled with the intake CVVT device  450 . In one form, two cams  71  and  72  may be formed on first and second cam portions  70   a  and  70   b , and a cam cap engaging portion  76  may be formed between the cams  71  and  72  and supported by a cam cap  40 . The valve  200  is opened and closed by being in contact with the cams  71  and  72 . 
     As illustrated in  FIG. 3 , the CVVD device includes: a cam unit  70  in which a cam  71  is formed and into which a cam shaft  30  is inserted; an inner wheel  80  to transfer the rotation of the cam shaft  30  to the cam unit  70  (See, in  FIG. 4 ); a wheel housing  90  in which the inner wheel  80  rotates and movable in a direction perpendicular to the camshaft  30 ; a guide shaft  132  having a guide thread and provided in a direction perpendicular to the camshaft  30 , the guide shaft mounted by a guide bracket  134 ; a worm wheel  50  having an inner thread engaged with the guide thread and disposed inside the wheel housing  90 ; and a control shaft  102  formed with a control worm  104  meshing with the worm wheel  50 . The control worm  104  is engaged with an outer thread formed on the outer side of the worm wheel  50 . The CVVD device further includes a sliding shaft  135  fixed to the guide bracket  134  and guiding the movement of the wheel housing  90 . 
       FIG. 4  is a partial view of the inner wheel  80  and the cam unit  70  of the CVVD device of the  FIG. 3 . Referring to  FIG. 4 , First and second sliding holes  86  and  88  are formed in the inner wheel  80 , and a cam slot  74  is formed in the cam unit  70 . 
     The CVVD device further includes: a roller wheel  60  inserted into the first sliding hole  86  allowing the roller wheel  60  to rotate; and a roller cam  82  inserted into the cam slot  74  and the second sliding hole  88 . The roller cam  82  may slide in the cam slot  74  and rotate in the second sliding hole  88 . 
     The roller cam  82  includes: a roller cam body  82   a  slidably inserted into the cam slot  74  and a roller cam head  82   b  rotatably inserted into the second sliding hole  88 . 
     The roller wheel  60  includes: a wheel body  62  slidably inserted into the camshaft  30  and a wheel head  64  rotatably inserted into the first sliding hole  86 . A cam shaft hole  34  is formed in the camshaft  30  and a wheel body  62  of the roller wheel  60  is movably inserted into the camshaft hole  34 . The structure and operation of the CVVD device discussed above applies to both the intake and exhaust CVVD devices  400 ,  500 . 
       FIGS. 5A-5C  illustrate the operation of the CVVD device.  FIG. 5A  illustrates a neutral state in which the rotational center of the camshaft  30  and the cam unit  70  coincide with each other. In this case, the cams  71  and  72  rotate at the same speed as the camshaft  30 . When the controller  300  applies a control signal based on engine load and/or engine speed, a control motor  106  rotates the control shaft  102 . Then, the control worm  104  rotates the worm wheel  50  which in turn rotates and moves along the guide thread formed on the guide shaft  132 . 
     As a result, the worm wheel  50  causes a change to a position of the wheel housing  90  relative to the cam shaft  30 . As illustrated in  FIGS. 5B and 5C , when the position of the wheel housing  90  moves in one direction with respect to the center of rotation of the camshaft  30 , the rotational speed of the cams  71 ,  72  with respect to the camshaft  30  are changed in accordance with their phases.  FIG. 7A  and  FIG. 8B  are drawings showing a valve profile illustrating valve opening duration change by the operation of the CVVD device (i.e., intake CVVD device, exhaust CVVD device). The solid line represents a general valve profile (e.g., a current opening duration), and the dotted line shows the valve profile as a short opening duration (e.g., a target opening duration in  FIG. 8B ) is applied.  FIG. 8A  illustrates a changed valve profile when the long opening duration is applied by the CVVD device. The controller  300  determines a target opening duration based on an engine load and an engine speed and controls the CVVD device (i.e., the intake CVVD device, the exhaust CVVD device) to modify current opening and closing timings of the valve based on the target opening duration. 
     More specifically, as illustrated in  FIG. 8B , the CVVD device may retard the current opening timing of the intake valve while simultaneously advancing the current closing timing of the intake valve to shorten the opening duration according to a predetermined value provided by the controller  300 . When the controller applies a longer opening duration (i.e., a target opening duration) than the current opening duration, as illustrated in  FIG. 8A , the CVVD device may advance the current opening timing of the intake valve while simultaneously retarding the current closing timing of the intake vale so that the modified opening duration becomes longer than the current opening duration. The same operation discussed above applies to the exhaust valve to control an opening duration of the exhaust valve. 
       FIGS. 8C and 8D  illustrate the relationship between the operation of the CVVD device and the CVVT device. As discussed above, the CVVD device may change the opening duration of a valve (e.g., intake or exhaust valve) whereas the CVVT device may shift a valve profile according to a target opening and/or a target closing timings without change to the period of the valve opening duration. It should be noted that the changing opening duration by the CVVD device may occur after changing valve opening and/or closing timings of intake or exhaust valves by the CVVT device. In another form, the operation of the CVVT device to change the opening and closing timings may occur after the operation of the CVVD device. In still another form, the operation of the CVVD and CVVT devices may perform simultaneously to change the opening duration and the timing of opening and closing of intake or exhaust valves. 
       FIGS. 6A and 6B  illustrate a view of the cam slot  74   a ,  74   b  of the CVVD device, and  FIGS. 7A-7C  illustrate valve profiles of the CVVD device in exemplary forms of the present disclosure. 
     Referring to  FIGS. 6A-6B , the cam slot  74   a  may be formed in an advanced position relative to the cam  71 ,  72 , or in another form the cam slot  74   b  may be formed in a retarded position relative to the cam  71 ,  72 . In another form, the cam slot  74   a ,  74   b  may be formed to have the same phase as the lobe of the cam  71 ,  72 . These variations are enable to realize various valve profiles. Based on the position of the cam slot  74   a ,  74   b , and a contact position between the cam and the corresponding valve (i.e., the intake valve, the exhaust valve), the opening and closing timings of the intake valve (or exhaust valve) may vary.  FIG. 7B  shows that the CVVD device may advance (for a short opening duration) or retard the current closing timing (for a long opening duration) of the corresponding valve (i.e., intake valve, exhaust valve) by a predetermined value based on the target opening duration of the intake valve or exhaust valve while maintaining the current opening timing of the intake valve or the exhaust valve. In another form, as illustrated in  FIG. 7C , the CVVD device may advance (for a long opening duration) or retard (for a short opening duration) the current opening timing of the intake valve or the exhaust valve by a predetermined value based on the target opening duration of the intake valve or the exhaust valve while maintaining the current closing timing of the intake valve or the exhaust valve. 
     The controller  30  may classify a plurality of control regions depending on an engine speed and an engine load based on signals from the data detector  100  and camshaft position sensor  120 , and control the intake CVVD device  400 , the exhaust CVVD device  500  and the intake CVVT device  450 . Herein, the plurality of control regions may be classified into six regions. 
     The controller  300  may apply a maximum duration (i.e., a target opening duration) to the intake valve and control to limit a valve overlap by using the exhaust valve in a first region, apply the maximum duration to the intake and exhaust valves in a second region, and advance an intake valve closing (IVC) timing and exhaust valve closing (EVC) timing in the third region. And the controller  300  may approach the intake valve closing (IVC) timing to a bottom dead center (BDC) in a fourth region, control a wide open throttle valve (WOT) and advance an intake valve opening (IVO) timing before a top dead center (TDC) and control an intake valve closing (IVC) timing after the bottom dead center (BDC) in a fifth region, and control a wide open throttle valve (WOT) and advance an intake valve closing (IVC) timing in a sixth region. 
     For these purposes, the controller  300  may be implemented as at least one processor that is operated by a predetermined program, and the predetermined program may be programmed in order to perform each step of a method for controlling valve timing of a continuous variable valve duration engine according to one form of the present disclosure. 
     Various forms described herein may be implemented within a recording medium that may be read by a computer or a similar device by using software, hardware, or a combination thereof, for example. 
     The hardware of the forms described herein may be implemented by using at least one of application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, and electrical units designed to perform any other functions. 
     The software such as procedures and functions of the forms described in the present disclosure may be implemented by separate software modules. Each of the software modules may perform one or more functions and operations described in the present disclosure. A software code may be implemented by a software application written in an appropriate program language. 
     Hereinafter, a method for controlling valve timing of a continuous variable valve duration engine according to one form of the present disclosure will be described in detail with reference to  FIG. 9A  to  FIG. 12C . 
       FIG. 9A  and  FIG. 9B  are flowcharts showing a method for controlling valve timing of a continuous variable valve duration engine, and  FIG. 10  is a schematic block diagram of showing control regions based on engine load (e.g., engine torque) and engine speed. 
     In addition,  FIGS. 11A-11C  are drawings showing duration, opening timing, and closing timing of an intake valve depending on an engine load and an engine speed, and  FIGS. 12A-12C  are drawings showing duration, opening timing, and closing timing of an exhaust valve depending on an engine load and an engine speed. 
     In the  FIGS. 11A-11C  and  FIGS. 12A-12C , an IVD map and an EVD map indicate a crank angle, an IVO timing map indicates an angle before a top dead center, an IVC timing map indicates an angle after a bottom dead center, an EVO timing map indicates an angle before a bottom dead center, and an EVC timing map indicates an angle after a top dead center. 
     As shown in  FIG. 9A  and  FIG. 9B , a method for controlling valve timing of a continuous variable valve duration engine starts with classifying a plurality of control regions depending on an engine speed and an engine load by the controller  30   o  at step S 100 . The control region may be divided into six control regions (e.g., first, second, third, fourth, fifth and sixth control regions). 
     The first to sixth control regions are indicated in  FIG. 10 , and  FIG. 11A  to  FIG. 12C  in more detail.  FIG. 10  schematically describes the control regions based on the engine load (e.g., engine torque) and engine speed (e.g., revolutions per minutes “rpm”). However, the control regions may vary based on engine type or engine size. Mixed control may be performed at the boundary of each region to minimize the control impact of the engine. Accordingly, the range of each region shown in the present application is exemplary, and the classification of each region may be varied. 
     The controller  300  may determine control regions as a first control region (namely, {circle around (1)} an idling region or a low-load condition) when the engine load is between a first predetermined load (e.g., a minimum engine torque) and a second predetermined load, a second control region (namely, {circle around (2)} an mid-load condition) when the engine load is greater than the second predetermined load and equal to or less than a third predetermined load, and a third control region (namely, {circle around (3)} a high-load condition) where the engine load is greater than the third predetermined load and less than a fourth predetermined load and the engine speed is between a first predetermined speed (e.g., an idle rpm) and a second predetermined speed, or where the engine load is greater than the third predetermined load and equal to or less than a fifth predetermined load and the engine speed is between the second predetermined speed and a third predetermined speed (i.e., an engine maximum rpm). 
     In addition, the controller  300  may determine a fourth control region (namely, {circle around (4)} a low-speed and high-load condition) when the engine load is greater than the fourth predetermined load and equal to or less than a fifth predetermined load and the engine speed is equal to or greater than the first predetermined speed and equal to or less than the second predetermined speed, a fifth control region (namely, {circle around (5)} a low speed-wide open throttle “WOT” condition) when the engine load is greater than the fifth predetermined load and equal to or less than a maximum engine load and the engine speed is between the first and second predetermined speeds, and a sixth control region (namely, {circle around (6)} an mid-high speed-WOT condition) when the engine load is greater than the fifth predetermined load and equal to or less than the maximum engine load and the engine speed is greater than the second predetermined speed and equal to or less than a third predetermined speed (e.g., an engine maximum rpm). 
     Referring to  FIG. 10 , the first predetermined load (e.g., a minimum engine torque) is measured when a input from the APS is zero “0,” and the second to fifth predetermined loads, and the second and third predetermined engine speeds may be calculated by the following equations:
 
Second predetermined load=min_ L +(⅕)×(max_ L −min_ L );
 
Third predetermined load=min_ L +(⅖)×(max_ L −min_ L );
 
Fourth predetermine load=min_ L +(½)×(max_ L −min_ L );
 
Fifth predetermined load=min_ L +(⅘)×(max_ L −min_ L );
 
Second predetermined engine speed=min_ S +( 3/10)×(max_ S −min_ S ); and
 
Third predetermined engine speed=max_ S,  
 
     where, min_L is the minimum engine torque; max_L is a maximum engine torque; min_S is a minimum engine rpm (e.g., Idle rpm); and max_S is a maximum engine rpm. 
     Meanwhile, referring to  FIGS. 11A-11C  and  FIGS. 12A-12C , a crank angle is marked in an intake valve duration (IVD) map and an exhaust valve duration (EVD) map, which indicating the opening time of the intake valve and exhaust valve. For example, regarding the IVD map in the  FIG. 11A , a curved line written as a number  200  at inner side of the fourth region means that the crank angle is approximately 200 degrees, a curved lined marked as a number  220  at outer side of the number  200  means that the crank angle is approximately 220 degrees. Although not shown in the drawing, the crank angle which is more than approximately 200 degrees and less than approximately 220 degrees is positioned between the curved line of the number  200  and the curved line of the number  220 . 
     In addition, a unit of number designated in an intake valve opening (IVO) timing map is before a top dead center (TDC), a unit of number designated in an intake valve closing (IVC) timing map is after a bottom dead center (BDC), a unit of number designated in an exhaust valve opening (EVO) timing map is before BDC, and a unit of number designated in an exhaust valve closing (EVC) map is after TDC. 
     Each region and curved line in the  FIGS. 11A-11C  and  FIGS. 12A-12C  are an exemplary form of the present disclosure, it may be modified within the technical idea and scope of the present disclosure. 
     Referring to  FIG. 9A  to  FIG. 12C , the control regions are determined according to the engine speed and load in the step of S 100 . After that, the controller  300  determines whether the engine state is under the first region at step S 110 . 
     In the step of S 110 , if the engine load is between first and second predetermined loads, the controller  300  determines that the engine state is under the first region. At this time, the controller  300  applies a maximum duration or a first intake opening duration to the intake valve and controls the valve overlap between the exhaust valve and intake valve at step S 120 . The valve overlap is a state where the intake valve is opened and the exhaust valve is not closed yet. 
     In other words, when the engine is under low load, then the controller  300  may control both the intake valve opening (IVO) timing and the intake valve closing (IVC) timing being fixed such that the intake valve has a maximum duration value. In other words, the controller  300  controls the intake CVVD device to adjust a current opening duration to the first intake opening duration by advancing the IVO timing and retarding the IVC timing. 
     As shown in  FIGS. 11A-11C , the first region may be fixed approximately 0 to 10 degrees before TDC in the IVO timing map and fixed approximately 100 to 110 degrees after BDC in the IVC timing map. 
     In addition, the controller  300  may limit the overlap by setting the EVC timing as a maximum value to maintain combustion stability. 
     When the current engine state does not belong to the first region at the step S 110 , the controller  300  determines whether the current engine state belongs to the second region at step S 130 . However, each of the control regions may be determined immediately by the controller  300  based on the engine load and/or engine speed. 
     When the current engine state belongs to the second region where the engine load is greater than the second predetermined load and equal to or less than the third predetermined load, the controller  300  controls the intake valve and the exhaust valve to maintain the maximum duration at step S 140 . In another form, the controller  300  may set the target opening duration of the intake valve to a second intake opening duration in the second control region, and the controller  300  controls the intake CVVD device to adjust the current opening duration to the second intake opening duration. The second opening duration may be set to be equal to or longer than the first intake opening duration. 
     The controller  300  may maintain the maximum duration of the exhaust valve by retarding the EVC timing as the engine load increases. Herein, the controller  300  may use the maximum duration of the exhaust by fixing the IVO timing and the IVC timing with the maximum duration of the intake used in the first region. 
     Meanwhile, a manifold absolute pressure (MAP) of the intake which is a pressure difference between atmospheric pressure and pressure of the intake manifold should be constantly maintained in the natural aspirated engine, on the contrary, it may not need to control in the turbo engine according to one form of the present disclosure because pressure of the intake manifold is greater than atmospheric pressure due to boost. 
     When the engine load is greater than the third predetermined load and less than a fourth predetermined load and the engine speed is between first and second predetermined speeds, or when the engine load is greater than the third predetermined load and equal to or less than a fifth predetermined load and the engine speed is between the second predetermined speed and a third predetermined speed, the controller  300  determines whether the current engine state belongs to the third region at step S 150 . 
     When the current engine state belongs to the third region at the step S 150 , the controller  300  advances the IVC timing and the EVC timing at step S 160 . 
     Since the IVC timing is controlled at the LIVC position (for example, an angle approximately 100-110 of degrees after a bottom dead center) in the first and second regions, knocking may be generated as the engine load is increased. Accordingly, fuel efficiency may be deteriorated as boost pressure is increased and knocking is deteriorated. In order to inhibit or prevent effect as described above, the controller  300  advances the IVC timing. 
     At this time, as shown in  FIGS. 11A-11C , the controller  300  may rapidly advance the IVC timing close to a bottom dead center when the engine speed is less than or equal to a predetermined speed, and may slowly advance the IVC timing at an angle of approximately 30 to 50 degrees after the bottom dead center when the engine speed is greater than the predetermined speed so as to reflect characteristic of the turbo engine. The predetermined speed may be approximately 1500 rpm. 
     In addition, as shown in  FIGS. 12A-120 , the controller  300  may advance the EVC timing close to a top dead center since the EVC timing is positioned at the maximum value of the overlap in the first and second regions. 
     When the current engine state does not belong to the third region at the step S 150 , the controller  300  determines whether the current engine state belongs to the fourth region at step S 170 . In another form, the controller  300  may determine the condition for the fourth control region without performing the step of determining the first, second and third control regions. 
     If the engine state is under the fourth region in the S 170 , the controller  300  controls the IVC timing close to the BDC at step S 180 . 
     The fourth region may be a low boost region (or, a low-speed and high-load region) that the engine load is greater than the fourth predetermined load and equal to or less than the fifth predetermined load and the engine speed is greater than or equal to the first predetermined speed and less than the second predetermined speed. For example, the first predetermined speed (i.e., an idle rpm) may be approximately 1500 rpm or less, and the second predetermined speed may be approximately 2500 rpm. 
     The controller  300  controls the IVC timing close to BDC in the fourth region, thereby improving fuel efficiency. In addition, the controller  300  may inhibit or prevent the overlap by controlling the IVO timing and the EVC timing close to top dead center. Accordingly, a short intake duration (e.g., approximately 180 degrees) may be used in the fourth region. 
     When the engine load is greater than the fifth predetermined load and equal to or less than a maximum engine load and the engine speed is between the first and second predetermined speeds, the controller  300  determines that the current engine state belongs to the fifth region at step S 190 . 
     When the current engine state belongs to the fifth region at the step S 190 , the controller  300  fully opens a throttle valve, advances the IVO timing before the TDC and controls the IVC timing after the BDC at step S 200 . 
     In the turbo engine, if the throttle valve is controlled to be wide open (Wide Open Throttle “WOT”) when the engine speed is equal to or greater than the first predetermined speed (e.g., an idling rpm) and less than the second predetermined speed (e.g., 2500 rpm), intake port pressure becomes higher than exhaust port pressure by boosting. Therefore, effect of a scavenging phenomenon which emits combustion gas of the exhaust is prominent in the turbo engine compared to a natural aspirated engine. Accordingly, as shown in  FIGS. 11A-11C , the controller  300  may advance the IVO timing at an angle of approximately 20 to 40 degrees before the TDC to generate the scavenging, and control the IVC timing at angle of approximately 0 to 20 degrees after the BDC. More specifically, the fresh air at a higher pressure than that of the burned gases (combustion gas) scavenges the burned gases and evacuates them through the exhaust valve, thus filling the space freed by these gases. 
     Although the EVO timing is retarded after the BDC in order to increase the scavenging phenomenon through exhaust interference reduction, the EVO timing is fixed before the BDC in one form of the present disclosure. Accordingly, the present disclosure may be appropriate for a three-cylinder engine insignificantly influenced by the exhaust interference than a four-cylinder greatly influenced by the exhaust interference. 
     When the current engine state is greater than the fifth predetermined load and equal to or less than the maximum engine load and the engine speed is greater than the second predetermined speed and less than a third predetermined speed (e.g., a maximum rpm of an engine), the controller  300  determines whether the current engine state belongs to the sixth region at step S 210 . 
     When the current engine state belongs to the sixth region at the step S 210 , the controller  300  fully opens a throttle valve and advances the IVC timing at step S 220 . 
     When the engine speed is greater than a predetermined speed (e.g., approximately 3500 rpm) in the sixth region, the scavenging phenomenon disappears because exhaust port pressure is much higher than intake port pressure. In this case, since the EVO timing is fixed to increase the exhaust pumping, the valve overlap can be inhibited or prevented by approaching the EVC timing to the TDC, as shown in  FIGS. 12A-12C . 
     Meanwhile, when WOT control is performed at a high speed condition, the knocking is rarely generated in the natural aspiration engine, on the contrary, the knocking may be deteriorated in the turbo engine. Thus, as shown in  FIGS. 11A-11C , the controller  300  may advance the IVC timing within an angle of approximately 50 degrees after BDC to reduce knocking by decreasing boost pressure. 
     As described above, according to the present disclosure, duration and timing of the continuous variable valve are simultaneously controlled, so the engine may be controlled under desired conditions. 
     That is, since opening timing and closing timing of the intake valve and the exhaust valve are appropriately controlled, the fuel efficiency under a partial load condition and engine performance under a high load condition are improved. In addition, a starting fuel amount may be reduced by increasing a valid compression ratio, and exhaust gas may be reduced by shortening time for heating a catalyst. 
     In addition, by omitting the continuously variable valve timing device at the exhaust valve side, it is possible to reduce cost and avoid the problem of valve timing control. 
     While this present disclosure has been described in connection with what is presently considered to be practical exemplary forms, it is to be understood that the present disclosure is not limited to the disclosed forms. On the contrary, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the present disclosure.