Patent Publication Number: US-2020277903-A1

Title: Cylinder deactivation system and cylinder deactivation method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-035503 filed on Feb. 28, 2019 and Japanese Patent Application No. 2020-000119 filed on Jan. 6, 2020, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to a cylinder deactivation system and a cylinder deactivation method for deactivating an operation of an internal combustion engine. 
     Description of the Related Art 
     As this type of apparatus, there have been known apparatuses that when predetermined conditions are satisfied during deceleration of the vehicle, sequentially stop fuel injection to the multiple cylinders of an engine with time. Such an apparatus is described in, for example, Japanese Unexamined Patent Application Publication No. 2003-049684 (JP2003-049684A). The apparatus of JP2003-049684A sequentially stops fuel injection to the cylinders in accordance with the order of ignition of the cylinders. 
     However, if an apparatus that sequentially stops fuel injection to cylinders in accordance with the ignition order, such as JP2003-049684A, is disposed in a system in which catalyst devices for cleaning up emissions are disposed on multiple exhaust passages connected to an engine, the catalyst devices may not be able to sufficiently clean up emissions. 
     SUMMARY OF THE INVENTION 
     A cylinder deactivation system includes an internal combustion engine including a plurality of cylinders having a plurality of first group cylinders belonging to a first group and a plurality of second group cylinders belonging to a second group, a first catalyst device and a second catalyst device respectively disposed in an exhaust passage of the first group and an exhaust passage of the second group, a fuel supply part configured to individually supply a fuel to each of the plurality of cylinders, and an electronic control unit having a microprocessor and a memory connected to the microprocessor. The microprocessor is configured to perform outputting a mode switch instruction from a first mode in which a fuel supply to the plurality of cylinders is performed to a second mode in which the fuel supply to the plurality of cylinders is stopped, and when the mode switch instruction is output, controlling the fuel supply part so as to stop the fuel supply to the plurality of cylinders in stages. The microprocessor is configured to perform the controlling including controlling the fuel supply part so as to stop a fuel supply to the plurality of second group cylinders after a fuel supply to the plurality of first group cylinders is stop. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features, and advantages of the present invention will become clearer from the following description of embodiments in relation to the attached drawings, in which: 
         FIG. 1  is a drawing showing a position of multiple cylinders of an engine to which a cylinder deactivation system according to an embodiment of the present invention is applied; 
         FIG. 2  is a drawing schematically showing a configuration of main components of an engine to which a cylinder deactivation system according to an embodiment of the present invention is applied; 
         FIG. 3  is a diagram showing an example of the operation as a comparative example; 
         FIG. 4  is a block diagram showing a configuration of main components of a cylinder deactivation system according to an embodiment of the present invention; 
         FIG. 5  is a diagram showing an example of characteristics set by the controller in  FIG. 4 ; 
         FIG. 6  is a diagram showing an example of delay times of each cylinder calculated by the controller in  FIG. 4 ; 
         FIG. 7  is a flowchart showing an example of a process performed by the controller in  FIG. 4 ; 
         FIG. 8  is a diagram showing an example of the operation of a cylinder deactivation system according to an embodiment of the present invention; and 
         FIG. 9  is showing another example of characteristics set by the controller in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, an embodiment of the present invention will be described with reference to  FIGS. 1 to 9 . A cylinder deactivation system according to the embodiment of the present invention is applied to an engine that is a spark-ignition internal combustion engine having a fuel cut function of stopping fuel supply to multiple cylinders during decelerated travel or the like of the vehicle. For example, this engine is a V-6 engine where multiple cylinders are disposed in a V-shape in a side view and a pair of front and rear banks are formed and is also a four-cycle engine that undergoes four strokes of intake, expansion, compression and exhaust in one operation cycle. Note that the engine may be an engine where a pair of left and right banks are formed. 
       FIG. 1  is a drawing showing the position of multiple (six) cylinders  1 to  6 of an engine  1 . The engine  1  includes three cylinders  1 to  3 belonging to a front side bank (front bank)  1   a  and three cylinders  4 to  6 belonging to a rear side bank (rear bank)  1   b . Hereafter, the three cylinders  1 to  3 belonging to the front bank (first group)  1   a  may be referred to as the “front-bank cylinders (or first group cylinders),” and the three cylinders  4 to  6 belonging to the rear bank (second group)  1   b  as the “rear-bank cylinders (or second group cylinder).” The cylinders  1 to  6 have the same configuration. 
       FIG. 2  is a drawing schematically showing the configuration of main components of the engine  1 .  FIG. 2  shows the configuration of one of the cylinders  1 to  6. As shown in  FIG. 2 , the engine  1  includes a cylinder  3  formed in a cylinder block  2 , a piston  4  disposed slidably in the cylinder  3 , and a combustion chamber  6  formed between the piston  4  and a cylinder head  5 . The piston  4  is coupled to a crankshaft  8  through a connecting rod  7 . Reciprocation of the piston  4  along the inner wall of the cylinder  3  causes rotation of the crankshaft  8 . 
     The cylinder head  5  is provided with an intake port  11  and an exhaust port  12 . An intake passage  13  communicates with the combustion chamber  6  through the intake port  11 , while an exhaust passage  14  communicates with the combustion chamber  6  through the exhaust port  12 . The intake port  11  is opened and closed by an intake valve  15 , and the exhaust port  12  is opened and closed by an exhaust valve  16 . A throttle valve  19  is disposed on the intake passage  13  located at the upstream side of the intake valve  15 . The throttle valve  19  consists of, for example, a butterfly valve. The throttle valve  19  controls the amount of intake air supplied to the combustion chamber  6 . The intake valve  15  and exhaust valve  16  are open/close driven by a valve train  20 . 
     An ignition plug  17  and a direct-injection injector  18  are mounted on the cylinder head  5  and cylinder block  2 , respectively, so as to face the combustion chamber  6 . The ignition plug  17  is disposed between the intake port  11  and exhaust port  12 . The ignition plug  17  generates a spark by electrical energy to ignite a fuel-air mixture in the combustion chamber  6 . The injector  18  is disposed adjacent to the intake valve  15 . The injector  18  is driven by electrical energy and injects the fuel downward into the combustion chamber  6 . Note that the injector  18  may be disposed otherwise and may be disposed, for example, near the ignition plug  17 . 
     The valve train  20  includes an intake cam shaft  21  and an exhaust cam shaft  22 . The intake cam shaft  21  is integrally provided with intake cams  21   a  corresponding to the respective cylinders  3 . The exhaust cam shaft  22  is integrally provided with exhaust cams  22   a  corresponding to the cylinders  3 . The intake cam shaft  21  and exhaust cam shaft  22  are coupled to the crankshaft  8  through timing belts (not shown) and rotate once each time the crankshaft  8  rotates twice. The intake valve  15  is opened and closed by rotation of the intake cam shaft  21  through an intake rocker arm (not shown) at a predetermined timing corresponding to the profile of the intake cam  21   a . The exhaust valve  16  is opened and closed by rotation of the exhaust cam shaft  22  through an exhaust rocker arm (not shown) at a predetermined timing corresponding to the profile of the exhaust cam  22   a.    
     The output torque of the engine  1 , that is, the torque generated by rotation of the crankshaft  8  is inputted to a transmission (not shown). The transmission is a stepped transmission, which is able to change the speed ratio in stages so as to correspond to multiple shift positions (e.g., six positions). Note that the transmission may be a continuously variable transmission (CVT), which is able to change the speed ratio continuously. Rotation from the engine  1  is speed-changed by the transmission and then transmitted to the drive wheels. Thus, the vehicle travels. 
     As shown in  FIG. 1 , the exhaust passages  14  of the front-bank cylinders  1 to  3 are connected to a common exhaust passage  141 , and the exhaust passages  14  of the rear-bank cylinders  4 to  6 are connected to a common exhaust passage  142 . Note that the exhaust passages of the front-bank cylinders  1 to  3 and the exhaust passages of the rear-bank cylinders  4 to  6 may be represented by  14 A and  14 B, respectively, for distinction. Catalyst devices  23  and  24  for cleaning up emissions are disposed in the exhaust passages  141  and  142 , respectively. The catalyst devices  23  and  24  are three-way catalysts having a function of eliminating and cleaning up HC, CO, and NOx included in emissions by oxidation and reduction and have the same configuration. The clean-up efficiency of the catalyst devices  23  and  24  is high when the air fuel ratio is the stoichiometric air fuel ratio. The clean-up efficiency of HC and CO is low in a fuel excess state (a rich state), and the clean-up efficiency of NOx is low in an air excess state (a lean state). 
     If, in the engine  1  thus configured, fuel supply from the injectors  18  to the cylinders  1 to  6 is simultaneously stopped (that is, fuel cut is performed simultaneously on the cylinders  1 to  6) when a predetermined fuel cut condition is satisfied, the engine output torque is suddenly reduced and shock to a driver of the vehicle is caused. To reduce such shock, it is conceivable that fuel cut will be performed on the cylinders on a cylinder-by-cylinder basis in a predetermined order with a lapse of time. 
     However, if fuel cut is performed on a cylinder-by-cylinder basis in a configuration in which the catalyst devices  23  and  24  are disposed so as to correspond to the front-bank cylinders  1 to  3 and the rear-bank cylinders  4 to  6, respectively, such as the present embodiment, the combustion time in a lean state may become longer. This may increase the amount of stored oxygen and thus make the reduction of NOx difficult, which may lead to emission deterioration.  FIG. 3  is a diagram showing this problem and shows an example of the operation during fuel cut. In  FIG. 3 , the horizontal axis represents the time, and the vertical axis represents the engine output torque. A characteristic f 1  set such that the torque is gradually reduced with time is a characteristic for determining the fuel cut timing (a fuel cut characteristic). 
     In  FIG. 3 , a fuel cut instruction is output and then fuel cut is performed in stages in the order of  4→ 1→ 5→ 2→ 3→ 6 (e.g., in the combustion order) from time point t 0 . For example, note the rear-bank cylinders  4 to  6 shown by thick lines in  FIG. 3 . In the period from time point t 0  to time point t 1 , combustion is performed in the two cylinders and  6 of the rear-bank cylinders  4 to  6 (two-cylinder combustion); in the period from time point t 1  to time point t 2 , combustion is performed in the one cylinder  6 thereof (one-cylinder combustion). For this reason, the rear-bank cylinders  4 to  6 as a whole become a lean state at time point t 0  and later. Particularly, at time point t 1 , when one-cylinder combustion is started, and later, the level of leanness and thus the amount of oxygen stored in the catalyst device  24  are increased. 
     The lean-state allowable time, that is, the time in which the amount of stored oxygen is not saturated and the catalyst device  24  is able to exhibit NOx clean-up ability (leanness allowable time Δta) depends on the ability of the catalyst device  24 . If the actual lean-state time (e.g., the one-cylinder combustion time Δtb) is longer than the leanness allowable time Δta, Nox is not cleaned up, leading to emission deterioration. To prevent such emission deterioration, the cylinder deactivation system according to the present embodiment is configured as follows. 
       FIG. 4  is a block diagram showing the configuration of main components of a cylinder deactivation system  100  according to the present embodiment. As shown in  FIG. 4 , the cylinder deactivation system  100  is formed centered on a controller  30  for controlling the engine. A rotational speed sensor  31 , an accelerator opening angle sensor  32 , a vehicle speed sensor  33 , a shift position sensor  34 , AF sensors  35 , a torque sensor  36 , the multiple injectors  18  disposed on the cylinders  1 to  6 (only one is shown in  FIG. 4 ) are connected to the controller  30 . 
     The rotational speed sensor  31  is a sensor that detects the engine rotational speed and consists of, for example, a crank angle sensor that is disposed on the crankshaft  8  and outputs a pulse signal in association with rotation of the crankshaft  8 . The accelerator opening angle sensor  32  is disposed on the accelerator pedal (not shown) of the vehicle and detects the manipulated variable of the accelerator pedal (accelerator opening angle). The vehicle speed sensor  33  detects the vehicle speed. The shift position sensor  34  detects the current shift position of the transmission. The AF sensors  35  are disposed on the respective exhaust passages  14 A and  14 B and detect the emission air fuel ratio in the exhaust passages  14 A and  14 B. The torque sensor  36  is a sensor that detects the output torque of the engine  1  or a physical quantity having a correlation with the output torque and consists of, for example, an air-flow sensor that detects the amount of intake air of the engine  1 . The output torque (estimated torque) of the engine  1  is obtained on the basis of the value detected by the torque sensor  36 . 
     The controller  30  consists of an electronic control unit (ECU) and includes a computer including an arithmetic processing unit, such as a CPU, a storage unit, such as a ROM or RAM, and other peripheral circuits. The controller  30  includes a drive mode instructing unit  301  as instructing unit, a characteristic setting unit  302  as setting unit, an order determination unit  303 , and an injector control unit  304  as functional elements. 
     The drive mode instructing unit  301  determines whether a predetermined fuel cut condition is satisfied, on the basis of signals from the rotational speed sensor  31 , accelerator opening angle sensor  32  and vehicle speed sensor  33 . If it determines that the fuel cut condition is satisfied, the drive mode instructing unit  301  outputs an instruction (mode switch instruction) to switch the drive mode from a normal mode in which fuel cut is not performed on the cylinders  1 to  6 to a stop mode in which fuel cut is performed thereon. Specifically, if, in a non-fuel cut state, the accelerator opening angle is equal to or smaller than a predetermined value; the engine rotational speed is equal to or greater than a predetermined value; and the vehicle speed is equal to or greater than a predetermined value, the drive mode instructing unit  301  determines that the fuel cut condition is satisfied. For example, during deceleration travel, the drive mode instructing unit  301  determines that the fuel cut condition is satisfied. 
     The characteristic setting unit  302  sets a fuel cut characteristic for determining the fuel cut timing, in accordance with the drive state of the vehicle.  FIG. 5  is a diagram showing an example of fuel cut characteristics. Fuel cut characteristics are set so as to correspond to the shift positions of the transmission and are also set such that the torque is gradually reduced with time. More specifically, fuel cut characteristics are set such that the amount of reduction in the torque is gradually reduced with time from the initial value which is the output torque detected by the torque sensor  36  (the negative inclination is gradually reduced). For example, characteristics f 1  and f 2  in  FIG. 5  are characteristics in different shift positions at a predetermined engine rotational speed, and the characteristic f 1  is a characteristic in a lower shift position than that of the characteristic f 2 . That is, to reduce shock caused by fuel cut, fuel cut characteristics are set such that the torque is reduced more gently as the shift position is lower (as the speed ratio is greater). 
     Although not shown, fuel cut characteristics are set considering not only the shift position but also the engine rotational speed. That is, fuel cut characteristics are set such that, in the same shift position, the torque is reduced more gently as the engine rotational speed is lower. When the drive mode instructing unit  301  outputs a mode switch instruction to switch to the stop mode, the characteristic setting unit  302  sets a fuel cut characteristic on the basis of the output torque detected by the torque sensor  36 . For example, the characteristic setting unit  302  selects a fuel cut characteristic corresponding to the current shift position and engine rotational speed from among multiple fuel cut characteristics previously stored in the storage unit of the controller  30  and sets this characteristic. 
     The order determination unit  303  determines the order of fuel cut of the cylinders. Specifically, the order determination unit  303  determines a cylinder to which the fuel is to be injected immediately after a stop mode instruction is output, as a cylinder on which fuel cut is to be performed first (the first cylinder). The order determination unit  303  then determines two cylinders belonging to the same group (bank) as that of the first cylinder, as a cylinder on which fuel cut is to be performed secondly (the second cylinder) and a cylinder on which fuel cut is to be performed thirdly (the third cylinder). The order determination unit  303  then determines three cylinders belonging to a group (bank) different from that of the first cylinder, as cylinders on which fuel cut is to be performed fourthly, fifthly, and sixthly (the fourth cylinder, fifth cylinder, and sixth cylinder). 
     For example, if the first cylinder is the front-bank cylinder  1, the second and third cylinders are the front-bank cylinders  2 and  3 and the fourth, fifth, and sixth cylinders are the rear-bank cylinders  4,  5, and  6. On the other hand, if the first cylinder is the rear-bank cylinder  4, the second and third cylinders are the rear-bank cylinders  5 and  6 and the fourth, fifth, and sixth cylinders are the front-bank cylinders  1,  2, and  3. That is, the order determination unit  303  determines the order of fuel cut of the cylinders such that the order of fuel cut of multiple cylinders in the same group becomes sequential order. 
     When the drive mode instructing unit  301  outputs a mode switch instruction to switch to the stop mode, the injector control unit  304  outputs control signals to the injectors  18  of the cylinders  1 to  6 to perform fuel cut on the cylinders  1 to  6. In this case, the injector control unit  304  first calculates the times from when fuel cut on the first cylinder is started until fuel cut is performed on the respective remaining cylinders (the second to sixth cylinders), that is, the fuel cut delay times of the respective cylinders in accordance with the fuel cut characteristic set by the characteristic setting unit  302 .  FIG. 6  is a diagram showing an example of the delay times calculated in accordance with the fuel cut characteristic f 1 . Note that the delay time Δt 1  of the first cylinder is 0. 
     Specifically, as shown in  FIG. 6 , assuming that the engine output torque is reduced from the initial value T 1  to T 2 , T 3 , T 4 , T 5 , T 6 , and 0 at equal intervals each time fuel cut is performed on one cylinder, the injector control unit  304  sets target points P 1  to P 6  corresponding to the torques T 1  to T 6  on the fuel cut characteristic f 1  and calculates the times from the time point at which fuel cut is performed on the first cylinder (the target point P 1 ) to the target points P 2  to P 6 , as the respective delay times Δt 2 , Δt 3 , Δt 4 , Δt 5 , and Δt 6  of the second, third, fourth, fifth, and sixth cylinders. The injector control unit  304  then counts the time elapsed since the fuel cut of the first cylinder. When the elapsed time reaches the delay times Δt 2  to Δt 6 , the injector control unit  304  sequentially performs fuel cut on the second to sixth cylinders. 
     Before the drive mode instructing unit  301  outputs a mode switch instruction to switch to the stop mode, the injector control unit  304  controls the amount of injected fuel by outputting control signals to the injectors  18  so that the air fuel ratio becomes the stoichiometric air fuel ratio. That is, the injector control unit  304  performs AF feedback control on the basis of signals from the AF sensors  35 . On the other hand, when the drive mode instructing unit  301  outputs a mode switch instruction to switch to the stop mode, the injector control unit  304  stops the AF feedback control over the bank on which fuel cut has been started. For example, if fuel cut on the front bank  1   a  is started first, the injector control unit  304  stops the AF feedback control over the front bank  1   a  and continues the AF feedback control over the rear bank  1   b . Subsequently, when fuel cut on the rear bank  1   b  is started, the AF feedback control over the rear bank  1   b  is also stopped. 
       FIG. 7  is a flowchart showing an example of a process (a fuel cut process) performed by the CPU of the controller  30  in  FIG. 4  in accordance with a program stored in memory in advance. The process shown by this flowchart is started, for example, when the engine  1  is operating in the normal mode. Subsequently, this process is repeated in a predetermined cycle until the change to the stop mode is complete, that is, until fuel cut on all the cylinders  1 to  6 is complete. 
     As shown in  FIG. 7 , first, in S 1  (S means a process step), it is determined whether the flag is 0 or 1. In the initial state before the fuel cut condition is satisfied, the flag is 0. If it is determined in S 1  that the flag is 0, the process proceeds to S 2 ; if it is determined in S 1  that the flag is 1, the process proceeds to S 8 . In S 2 , signals are read from the sensors  31  to  36 . Then, in S 3 , it is determined whether the fuel cut condition is satisfied, on the basis of the signals from the sensors  31  to  33 . If the determination in S 3  is YES, the process proceeds to S 4 . If the determination in S 3  is NO, the process ends. In this case, the amount of injected fuel is controlled (feedback control) so that the air fuel ratio detected by the AF sensors  35  becomes the stoichiometric air fuel ratio. On the other hand, in S 4 , a fuel cut characteristic having the output torque detected by the torque sensor  36  as the initial value is set on the basis of the current shift position detected by the shift position sensor  34  and the engine rotational speed detected by the rotational speed sensor  31 . 
     Then, in S 5 , the order of fuel cut of the multiple cylinders  1 to  6 (the first to sixth cylinders) is determined. Specifically, a cylinder into which the fuel is to be injected immediately after the fuel cut condition is satisfied is determined as the first cylinder on which fuel cut is to be performed first; the remaining two cylinders in the same group as that of the first cylinder are determined as the second and third cylinders; and the three cylinders in the group different from that of the first cylinder are determined as the fourth to sixth cylinders. Then, in S 6 , the delay times Δt 2  to Δt 6  from when fuel cut on the first cylinder is started until fuel cut is performed on the second to sixth cylinders are calculated in accordance with the fuel cut characteristic set in S 4 . Then, in S 7 , the flag is set to 1. 
     Then, in S 8 , it is determined whether the fuel cut delay time of one of the cylinders  1 to  6 has been reached. If the determination in S 8  is YES, the process proceeds to S 9 ; if the determination in S 8  is NO, the process ends. In S 9 , fuel cut is sequentially performed on the cylinders whose delay time has been determined to have been reached, ending the process. For the first cylinder, fuel cut is performed thereon with the delay time Δt 1  of 0 (in other words, without setting the delay time) in S 9 . For the second to sixth cylinders, when the corresponding delay times Δt 2  to Δt 6  are determined to have been reached in S 8  after the flag is set to 1, fuel cut is performed thereon in S 9 . When performing fuel cut, there is stopped AF feedback control over the bank to which the cylinder to be subjected to fuel cut belongs. 
       FIG. 8  is a diagram showing an example of the operation of the cylinder deactivation system  100  according to the present embodiment. The characteristic f 1  in  FIG. 8  is a fuel cut characteristic set by the characteristic setting unit  302 . A characteristic shown by a stepwise solid line is a characteristic showing time-dependent torque reductions after outputting a mode switch instruction to switch to the stop mode. Note that a characteristic shown by a stepwise dotted line is a torque reduction characteristic shown when fuel cut is performed in accordance with the order of ignition of the cylinders  1 to  6. Specifically, in the characteristic shown by the dotted line, fuel cut is performed at the target points P 1  to P 6  in the order of  4,  1,  5,  2,  3, and  6. 
     In the example of  FIG. 8 , after having output the mode switch instruction to switch to the stop mode, first, fuel cut is performed on the front-bank cylinder  1 at the target point P 1 . Then, fuel cut is performed on the remaining front-bank cylinders  2 and  3 at the target points P 2  and P 3 . Then, fuel cut is sequentially performed on the rear-bank cylinders  4,  5, and  6 at the target points P 4 , P 5 , and P 6 . Note that in  FIG. 8 , the torque reduction timings lag behind the fuel cut timings (the target points P 2  to P 6 ). This is because there are time lags between fuel cut and the subsequent operation of the cylinders  2 to  6. 
     The cylinder deactivation system  100  according to the present embodiment sequentially performs fuel cut on the multiple cylinders  1 to  6 on a bank basis. Thus, the cylinder deactivation system  100  is able to reduce the time Δtc from when fuel cut on one (e.g.,  1) of the front-bank cylinders  1 to  3 is started until fuel cut on the three cylinders ( 1 to  3) is complete and the time Δtd from when fuel cut on one (e.g.,  4) of the rear-bank cylinders  4 to  6 is started until fuel cut on the three cylinders ( 4 to  6) is complete. As a result, the cylinder deactivation system  100  is able to confine the respective lean-state times Δtc and Δtd of the banks  1   a  and  1   b  within the leanness allowable time Δta of the catalyst devices  23  and  24  and thus to reliably prevent emission deterioration. 
     The cylinder deactivation system  100  according to the present embodiment is able to achieve advantages and effects such as the following. 
     (1) The cylinder deactivation system  100  includes the engine  1  that includes the multiple cylinders  1 to  6 including the multiple front-bank cylinders  1 to  3 belonging to the one bank  1   a  and the multiple rear-bank cylinder cylinders  4 to  6 belonging to the other bank  1   b , the catalyst devices  23  and  24  disposed in the exhaust passage  141  of the cylinders of the bank  1   a  and the exhaust passage  142  of the cylinders of the bank  1   b , the injectors  18  that individually supply the fuel to the cylinders  1 to  6, the drive mode instructing unit  301  that outputs a mode switch instruction to switch from the normal mode in which the fuel is supplied to the cylinders  1 to  6 and the stop mode in which fuel supply to the cylinders  1 to  6 is stopped, and the controller  30  that when the drive mode instructing unit  301  outputs a mode switch instruction to switch from the normal mode to the stop mode, controls the injectors  18  so that fuel supply to the cylinders  1 to  6 is stopped in stages ( FIGS. 1 and 4 ). The controller  30  controls the injectors  18  so that fuel supply to the front-bank cylinders  1 to  3 is stopped and then fuel supply to the rear-bank cylinders  4 to  6 is stopped or so that fuel supply to the rear-bank cylinders  4 to  6 is stopped and then fuel supply to the front-bank cylinders  1 to  3 is stopped. 
     As seen above, when sequentially performing fuel cut on the cylinders  1 to  6, the cylinder deactivation system  100  sequentially performs fuel cut on each of the front-bank cylinders  1 to  3 and the rear-bank cylinders  4 to  6. Thus, the cylinder deactivation system  100  is able to reduce the time Δtc from when fuel cut on one cylinder of the front bank  1   a  is started until fuel cut on the three cylinders thereof is complete and the time Δtd from when fuel cut on one cylinder of the rear bank  1   b  is started until fuel cut on the three cylinders thereof is complete. As a result, the cylinder deactivation system  100  is able to confine the respective lean-state times Δtc and Δtd of the banks  1   a  and  1   b  with the leanness allowable time Δta of the catalyst devices  23  and  24  and thus to reliably prevent emission deterioration while suppressing shock caused by torque reductions during fuel cut. 
     (2) The cylinder deactivation system  100  also includes the characteristic setting unit  302  that sets a fuel cut characteristic in which the engine output torque is gradually reduced with time ( FIG. 4 ). The controller  30  controls the injectors  18  so that fuel supply to the cylinders  1 to  6 is sequentially stopped, in accordance with the characteristic (e.g., the characteristic f 1 ) set by the characteristic setting unit  302  ( FIG. 8 ). Thus, the cylinder deactivation system  100  is able to perform fuel cut on the cylinders  1 to  6 in stages at the optimum timings such that shock caused by torque reductions is reduced.
 
(3) The characteristic setting unit  302  sets the multiple fuel cut characteristics f 1  and f 2  in accordance with the shift positions (speed ratios) of the transmission configured to change rotation speed input from the engine  1  and to output the resulting rotation speed ( FIG. 5 ). Although the magnitude of shock caused by fuel cut varies with the speed ratio, the cylinder deactivation system  100  according to the present embodiment determines the fuel cut timing in accordance with a characteristic corresponding to the speed ratio and thus is able to perform fuel cut at the optimum timings.
 
(4) The cylinder deactivation system  100  also includes the AF sensors  35  that detect the emission air fuel ratio. Before the drive mode instructing unit  301  outputs a mode switch instruction to switch from the normal mode to the stop mode, the injector control unit  304  controls the injectors  18  by AF feedback control so that the air fuel ratios detected by the AF sensors  35  become the predetermined air fuel ratio (e.g., the stoichiometric air fuel ratio). When the drive mode instructing unit  301  outputs a mode switch instruction to switch to the stop mode, the injector control unit  304  controls the injectors  18  so that a processing to stop of fuel supply to the front-bank cylinders  1 to  3 is started; AF feedback control over the front-bank cylinders  1 to  3 is stopped until the processing is complete; and AF feedback control over the rear-bank cylinders  4 to  6 is continued. As seen above, the injector control unit  304  stops AF feedback control over the bank on which fuel cut has been started and thus is able to prevent the amount of injected fuel from being excessively corrected to the rich side.
 
     Fuel cut characteristics set by the characteristic setting unit  302  are not limited to the above-mentioned characteristics f 1  and f 2 .  FIG. 9  is a diagram showing an example of another fuel cut characteristic f 3 .  FIG. 9  also shows, by a dotted line, a characteristic f 4  showing changes in the output torque in the period from when the accelerator pedal is released (time point t 11 ) until fuel cut is started (time point t 12 ), which is a characteristic immediately before fuel cut is started. There is a delay time between when the accelerator pedal is released (t 11 ) and when fuel cut is started (t 12 ). The reasons include that there is a delay in reducing the amount of intake air of the engine  1 , that is, air intake into the engine  1  is behind the operation of the throttle valve  19 ; the ignition timing is retarded in ignition control, that is, a delay is caused in the ignition timing retarding process; and the like. Although multiple characteristics f 3  are set in accordance with the shift position and the engine rotational speed as described above,  FIG. 9  shows only one characteristic (solid line) corresponding to the above-mentioned characteristic f 1 . 
     As shown in  FIG. 9 , after the accelerator pedal is released (after time point t 11 ), the output torque is linearly reduced (a characteristic f 4 ). The characteristic setting unit  302  calculates the inclination of the characteristic f 4  on the basis of a signal from the torque sensor  36  and sets the fuel cut characteristic f 3  in accordance with the calculated inclination of the characteristic f 4 . That is, the characteristic setting unit  302  sets the fuel cut characteristic f 3  such that it becomes a characteristic having a constant inclination matching the inclination of the characteristic f 4 . Thus, the torque is reduced at a constant rate before and after fuel cut is started (before and after time point t 12 ), allowing for a further reduction in shock caused by fuel cut. 
     The above-mentioned embodiment can be modified into various forms. Hereafter, modifications will be described. While, in the above embodiment, a V-6 engine having a pair of front and rear banks is use as an example of an internal combustion engine, other internal combustions such as an engine horizontally opposed engine may be used as an example of an internal combustion engine as long as the other internal combustion have a plurality of (a first group and second group). While, in the above embodiment, the front-bank cylinders and the rear-bank cylinders respectively are configured of three cylinders, the number of cylinders of a first group and a second group cylinders may be otherwise. 
     While, in the above embodiment, the catalyst device (first catalyst device)  23  and the catalyst device (second catalyst device)  24  are respectively disposed in the exhaust passage  141  connected to the front bank  1   a  and the exhaust passage  142  connected to the rear bank  1   b , a pair of catalyst devices may be disposed otherwise as long as the pair of catalyst devices disposed in exhaust passages of a first group and exhaust passages of a second group. A pair of catalyst devices may be configured otherwise as long as the pair of catalyst devices have an oxygen storage capacity and also have a reduction function. While, in the above embodiment, the direct-injection injector  18  is used in each of the cylinders  1 to  6, a fuel supply part may be configured otherwise as long as the fuel supply part individually supplies fuel to each of a plurality of cylinders. 
     While, in the above embodiment, the drive mode instructing unit  301  output the mode switch instruction from the normal mode (first mode) to the stop mode (second mode) on the basis of sensors  31  to  33 , a drive mode instructing unit may be configured otherwise. The controller  30  as a controller may be configured otherwise as long as the controller controls a fuel supply part such as the injector  18  so as to stop a fuel supply to a plurality of second group cylinders (e.g., the rear-bank cylinders  4 to  6) after stopping a fuel supply to a plurality of first group cylinders (e.g., the front-bank cylinders  1 to  3) when performing a fuel cut is instructed. Further, a fuel cut can be performed without setting a fuel cut characteristic by the characteristic setting unit  302  (a setting unit). While, in the above embodiment, the emission air fuel ratio is detected by the AF sensors  35  detects, an air fuel ratio detection part may be configured otherwise. 
     The present invention can be used as a cylinder deactivation method of an internal combustion engine in a cylinder deactivation system in which the internal combustion engine includes a plurality of cylinders having a plurality of first group cylinders belonging to a first group and a plurality of second group cylinders belonging to a second group, a fuel supply part is configured to individually supply a fuel to each of the plurality of cylinders, and a first catalyst device and a second catalyst device are disposed respectively in an exhaust passage of the first group and an exhaust passage of the second group. 
     The above embodiment can be combined as desired with one or more of the above modifications. The modifications can also be combined with one another. 
     According to the present invention, it is possible to sufficiently obtain effect of cleaning up emissions by a catalyst device even if fuel supply is stopped in stages in a system in which a catalyst device is individually disposed in each of a plurality of exhaust passages connected to an engine. 
     Above, while the present invention has been described with reference to the preferred embodiments thereof, it will be understood, by those skilled in the art, that various changes and modifications may be made thereto without departing from the scope of the appended claims.