Internal combustion engine having solenoid-operated valves and control method

An internal combustion engine including solenoid-operated intake and exhaust valves is operable in a partial operating mode with at least one cylinder being deactivated, without suffering from vibration of the engine. Where particular cylinders are deactivated under cylinder deactivation control, the exhaust valve of each deactivated or inactive cylinder is opened for a certain period of time which starts ahead of the bottom dead center. The timing of opening the exhaust valve is determined so that the pressure within the inactive cylinder is lower than the atmospheric pressure when the exhaust valve is opened. The timing of closing the exhaust valve is determined so that the peak value of the pressure within the inactive cylinder becomes almost equal to the peak value of the pressure within active cylinders. Where the cylinders are successively deactivated with the engine operating on a six-stroke cycle, opening and closing actions of the exhaust valve are controlled so that an increase in the cylinder pressure due to compression of gas in the currently deactivated cylinder and an increase in the cylinder pressure due to combustion in an active cylinder do not occur at the same time.

INCORPORATION BY REFERENCE
 The disclosure of Japanese Patent Application No. HEI 11-142382 filed on
 May 21, 1999 including the specification, drawings and abstract is
 incorporated herein by reference in its entirety.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to an internal combustion engine having
 solenoid-operated valves, and in particular to such an engine having
 solenoid-operated valves, which is operable in a partial operating mode
 with at least one of the cylinders being deactivated by inhibiting
 combustion therein under cylinder deactivation control. The invention also
 relates to a control method for controlling such an internal combustion
 engine.
 2. Description of Related Art
 Internal combustion engines capable of performing cylinder deactivation
 control under which a selected one or more of cylinders is/are deactivated
 or disabled are known in the art, and one example of such engines is
 disclosed in Japanese Patent Laid-Open Publication No. 7-279697. In this
 type of engine, fuel injection and ignition are stopped with respect to
 one or more cylinders selected from a plurality of cylinders of the
 engine, and cylinder deactivation control is performed by keeping an
 intake valve and an exhaust valve in their closed positions. Here,
 "deactivating a cylinder" means inhibiting any of suction of an air/fuel
 mixture, combustion, and exhaust of combustion gas, while allowing
 reciprocating motion of the piston. In the following description,
 cylinders that are deactivated or disabled under cylinder deactivation
 control will be called "inactive cylinders" or "deactivated cylinders"
 when appropriate, and cylinders in which combustion takes place even when
 the engine is in a partial operating mode are called "active cylinders" or
 "activated cylinders." With the cylinder deactivation control performed,
 the fuel injection is inhibited in the inactive cylinders, and pumping
 loss can be reduced, with a result of improved fuel efficiency.
 To perform cylinder deactivation control, only a particular cylinder or
 cylinders may be deactivated or disabled, or the cylinders of the engine
 may be successively deactivated in a certain order.
 In the case where particular cylinders are selected as inactive cylinders,
 gas contained in the combustion chamber of each inactive cylinder slowly
 leaks into the crankcase through a sliding surface of the piston, and the
 pressure within the inactive cylinder becomes lower than the pressure of
 active cylinders. Consequently, torque variations arise from a difference
 between the pressure within the active cylinders and the pressure within
 the inactive cylinders, resulting in increased vibration of the engine.
 Where the cylinders of the engine are successively deactivated in a certain
 order, there arises almost no reduction in the pressure within the
 currently deactivated cylinder due to the gas leakage as described above.
 When the piston of the inactive cylinder reaches the top dead center,
 therefore, burnt gas contained in the combustion chamber is compressed so
 that the pressure within the inactive cylinder increases to substantially
 the same level as that achieved upon combustion. Here, the sum of the
 cylinder pressures in the engine as a whole differs depending upon whether
 or not the timing of an increase in the pressure within the inactive
 cylinder coincides with the timing of an increase in the cylinder pressure
 due to combustion in an active cylinder. Accordingly, even if the
 cylinders are successively deactivated one after another, the engine still
 suffers from increased vibration due to variations in the output torque of
 the engine.
 SUMMARY OF THE INVENTION
 It is therefore an aspect of the invention to control an increase in
 vibration of the internal combustion engine caused by execution of
 cylinder deactivation control.
 To control an increase in vibration of the engine, the invention provides
 an internal combustion engine having solenoid-operated valves, which
 engine includes a plurality of cylinders and a controller. The controller
 causes at least one of the plurality of cylinders to operate in a cylinder
 deactivation mode in which the intake stroke, combustion stroke and
 exhaust stroke are inhibited, and controls the pressure within each
 inactive cylinder that operates in the cylinder deactivation mode.
 With the internal combustion engine constructed as described above or a
 control method according to the invention, it is possible to suppress
 vibration of the engine due to variations in the cylinder pressures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 Preferred embodiments of the invention will be described with reference to
 the drawings.
 FIG. 1 shows the construction of an internal combustion engine of the first
 embodiment of the invention. While the internal combustion engine takes
 the form of a four-cylinder engine having four cylinders, only one
 cylinder 14 is illustrated in FIG. 1 for the purpose of further
 explanation. The engine is controlled by an electronic control unit
 (hereinafter abbreviated to "ECU") 10. The cylinder 14 and water jacket 16
 are formed within a cylinder block 12. A piston 18 is located in the inner
 bore of the cylinder 14 such that the piston 18 is slidable along the side
 wall of the cylinder 14 in the vertical direction as viewed in FIG. 1. A
 cylinder head 20 is fixed in position on the cylinder block 12. For each
 cylinder, an intake port 22 and an exhaust port 24 are formed in the
 cylinder head 20.
 A combustion chamber 26 of the engine is defined by the bottom face of the
 cylinder head 20, the top face of the piston 18, and the side wall of the
 cylinder 14. The intake port 22 and exhaust port 24 are both open to the
 combustion chamber 26. Valve seats 28, 30 are formed at an opening end
 portion of the intake port 22 which faces the combustion chamber 26, and
 an opening end portion of the exhaust port 24 which faces the combustion
 chamber 26. Also, a distal end portion of a spark plug 32 is exposed to
 the combustion chamber 26.
 Solenoid-operated valves 38, 40 are incorporated in the cylinder head 20.
 The solenoid-operated valve 38 includes an intake valve 42. The intake
 valve 42 may be seated against the valve seat 28, thereby to block off the
 intake port 22 from the combustion chamber 26. When the intake valve 42 is
 separated or lifted away from the valve seat 28, the intake port 22 and
 the combustion chamber 26 are brought into communication with each other.
 Similarly, the exhaust valve 44 mounted in the solenoid-operated valve 40
 may be seated against the valve seat 30, thereby to block off the exhaust
 port 24 from the combustion chamber 26. When the exhaust valve 44 is
 separated or lifted away from the valve seat 30, the exhaust port 24 and
 the combustion chamber 26 are brought into communication with each other.
 The construction and operation of the solenoid-operated valves 38 and 40
 will be now described. FIG. 2 is a cross-sectional view showing the whole
 construction of the solenoid-operated valve 40. The construction of the
 solenoid-operated valve 38 is similar to that of the solenoid-operated
 valve 40, and therefore no description of the valve 38 will be provided
 herein.
 As shown in FIG. 2, the exhaust valve 44 includes a valve shaft 45 that
 extends upwards. The valve shaft 45 is supported by a valve guide 46 that
 is fixed within the cylinder head 20 such that the valve shaft 45 is
 movable in the axial direction. An armature shaft 48 in the form of a
 rod-like member made of a non-magnetic material is provided on the valve
 shaft 45, such that the lower end face of the armature shaft 48 abuts on
 the upper end face of the valve shaft 45.
 A lower retainer 50 is fixed to the upper end portion of the valve shaft
 45. A lower spring 52 is mounted on the lower portion of the lower
 retainer 50. The lower end of the lower spring 52 abuts on the cylinder
 head 20. The lower spring 52 serves to bias the lower retainer 50 and the
 armature shaft 48 upwards as viewed in FIG. 2.
 An upper retainer 54 is fixed to the upper end portion of the armature
 shaft 48. An upper spring 56 is located such that its lower end portion
 abuts on the upper portion of the upper retainer 54. A cylindrical upper
 cap 57 is disposed around the upper spring 56 so as to surround its outer
 periphery. The upper end portion of the upper spring 56 abuts on an
 adjuster bolt 58 that is screwed on the upper cap 57. The upper spring 56
 serves to bias the upper retainer 54 and the armature shaft 48 downwards
 as viewed in FIG. 2.
 An armature 60 in the form of an annular member made of a soft, magnetic
 material is joined to the outer periphery of the armature shaft 48. An
 upper coil 62 and an upper core 64 are disposed above the armature 60.
 Also, a lower coil 66 and a lower core 68, both of which are formed of a
 magnetic material, are disposed below the armature 60. The armature shaft
 48 is slidably supported by central portions of the upper core 64 and
 lower core 68. The upper coil 62 and lower coil 66 are connected to the
 ECU 10. Under control of the ECU 10, certain exciting current is supplied
 to the upper coil 62 and lower coil 66.
 The upper core 64 and the lower core 68 are supported by an outer sleeve 74
 such that the cores 64, 68 are spaced at a predetermined distance from
 each other. The upper cap 57 is fixed to the upper end face of the upper
 core 64. Also, the position of the adjuster bolt 58 is adjusted so that
 the armature 60 is located intermediate between the upper core 64 and the
 lower core 68 when it is in the neutral position.
 Next, the operation of the solenoid-operated valve 40 will be described.
 In the solenoid-operated valve 40, the exhaust valve 44 is seated against
 the valve seat 28 when the armature 60 abuts on the upper core 64. This
 state is maintained with certain exciting current being supplied to the
 upper coil 62. In the following description, the position at which the
 exhaust valve 44 is seated against the valve seat 30 will be called "fully
 closed position" of the exhaust valve 44.
 If exciting current supplied to the upper coil 62 is cut off while the
 exhaust valve 44 is being kept in the fully closed position,
 electromagnetic force that has acted on the armature 60 disappears. As a
 result, the armature 60 displaces or moves downwards in FIG. 2 under the
 biasing force of the upper spring 56. If suitable exciting current is
 supplied to the lower coil 66 at a point of time when the amount of
 displacement of the armature 60 reaches a predetermined value, a force is
 then generated to pull or attract the armature 60 toward the lower core
 68. As a result, the armature 60 moves downwards in FIG. 2 along with the
 exhaust valve 44, against the biasing force of the lower spring 52. The
 exhaust valve 44 continues to be moved until the armature 60 abuts on the
 lower core 68. In the following description, the position of the exhaust
 valve 44 at which the armature 60 abuts on the lower core 68 will be
 called "fully open position". This state is maintained with certain
 exciting current being supplied to the lower coil 66.
 If exciting current supplied to the lower coil 66 is cut off while the
 exhaust valve 44 is being kept in the fully open position, electromagnetic
 force that has acted upon the armature 60 disappears. If the
 electromagnetic force that has acted upon the armature 60 disappears, the
 armature 60 displaces or moves upwards in FIG. 2 under the biasing force
 of the lower spring 52. If suitable exciting current is supplied to the
 upper coil 62 at a point of time when the amount of displacement of the
 armature 60 reaches a predetermined value, a force to pull or attract the
 armature 60 toward the upper core 64 is then generated, namely, a force to
 move the exhaust valve 44 upwards in FIG. 2 is generated.
 If the above-described force acts upon the armature 60, the armature 60 is
 caused to displace or move upwards in FIG. 2 along with the exhaust valve
 44, against the biasing force of the upper spring 56. The exhaust valve 44
 continues to be moved until the armature 60 abuts on the upper core 64,
 namely, until the exhaust valve 44 reaches the fully closed position.
 As described above, the exhaust valve 44 can be brought into the fully
 closed position by supplying suitable exciting current to the upper coil
 62, and can be also brought into the fully open position by supplying
 suitable exciting current to the lower coil 66. Thus, the
 solenoid-operated valve 40 makes it possible to repeatedly reciprocate the
 exhaust valve 44 between the fully open position and the fully closed
 position, by alternately supplying exciting current to the upper coil 62
 and the lower coil 66.
 The solenoid-operated valve 38 including the intake valve 42 operates in a
 similar manner to the solenoid-operated valve 40 as described above. In
 the first embodiment, the ECU 10 causes exciting current to be supplied
 alternately to the upper coil 62 and lower coil 66 of the respective
 solenoid-operated valves 38, 40 at a suitable timing, thereby to open and
 close the intake valve 42 and the exhaust valve 44 at a desired timing.
 Referring back to FIG. 1, the internal combustion engine includes an intake
 manifold 80. The intake manifold 80 includes a plurality of branch pipes
 each of which communicates a surge tank 82 with a corresponding intake
 port 22. A fuel injection valve 83 is mounted in each of the branch pipes.
 The fuel injection valve 83 operates to inject fuel into the branch pipe
 in response to a command signal from the ECU 10.
 An intake pipe 84 provided with a throttle valve 86 is connected to the
 upstream end of the surge tank 82 for communication therewith. An air flow
 meter 87 is provided in a portion of the intake pipe 84 that is located
 upstream of the throttle valve 86. The air flow meter 87 generates to the
 ECU 10 a signal representing the flow rate (which will be called "specific
 volume of intake air") of the air introduced by suction into the intake
 pipe 84. The ECU 10 then determines the specific volume of intake air
 based on the output signal of the air flow meter 87. An air cleaner 88 is
 connected to an upstream-side end portion of the intake pipe 84. Thus,
 ambient air that has passed through the air cleaner 88 flows into the
 intake pipe 84. In the meantime, an exhaust passage 90 communicates with
 the exhaust port 24 of the engine.
 The engine is also provided with a crank angle sensor 94. The ECU 10
 receives an output signal from the crank angle sensor 94, and determines a
 crank angle CA on the basis of the output signal from the crank angle
 sensor 94.
 The internal combustion engine of the first embodiment is operable in a
 partial operating mode in which fuel injection and ignition are inhibited
 in a selected one or more cylinders, out of the four cylinders of the
 engine, so that the selected cylinders are deactivated or disabled
 (cylinder deactivation control). Under the cylinder deactivation control,
 fuel injection is stopped in the deactivated or inactive cylinders, and
 pumping loss is reduced, thus assuring improved fuel efficiency. For
 example, cylinder #1 and cylinder #4 are predetermined or pre-selected as
 such cylinders that are to be deactivated under cylinder deactivation
 control.
 Under conventional cylinder deactivation control, the intake valve 42 and
 exhaust valve 44 of each inactive cylinder are kept closed. Where a
 particular cylinder is predetermined or pre-selected as an inactive
 cylinder, and the intake valve 42 and exhaust valve 44 of the inactive
 cylinder are always kept closed, the pressure within the inactive cylinder
 is reduced since gas contained in the combustion chamber 26 of the
 inactive cylinder slowly leaks into the crankcase through a sliding
 surface of the piston 18. As a result, a difference in the peak value of
 the cylinder pressure arises between the active cylinders and the inactive
 cylinders, resulting in increased vibration of the engine as compared with
 the case where the engine operates in a full operating mode (as opposed to
 the partial operating mode) with all of the four cylinders being
 activated.
 In the internal combustion engine of the first embodiment, on the other
 hand, the exhaust valve 44 of each inactive cylinder is opened at an
 appropriate timing, so as to avoid reduction in the pressure within the
 inactive cylinder, and suppress the vibration of the engine.
 Initially, some explanation will be made on changes in the pressure within
 each cylinder when the intake valve 42 and exhaust valve 44 of each
 inactive cylinder are kept closed under cylinder deactivation control, and
 the resulting vibration of the engine, as compared with pressure changes
 and vibration that occur when the engine is in a full operating mode with
 all of the cylinders being activated.
 FIG. 3 shows the opening and closing actions (A) of the exhaust valve 44 of
 cylinder #4, changes (B and C) in the pressures within cylinders #3 and
 #4, respectively, and vibration waveform (D) of the engine when the engine
 is in a full operating mode with all cylinders being activated. FIG. 4
 shows the case where the intake valve 42 and exhaust valve 44 of each
 deactivated or inactive cylinder are kept closed during execution of the
 above-described cylinder deactivation control.
 While FIG. 3 and FIG. 4 show the pressures within the cylinders #3 and #4
 as typical examples, the pressures within the cylinders #1 and #2 show
 substantially the same changes. The vibration waveform (D) of the engine
 as shown in FIG. 3 and FIG. 4 represents an output signal of an
 acceleration pickup that is mounted on the cylinder head 20 for measuring
 rolling vibration around the crankshaft, and the vertical axis indicates
 the acceleration. In FIG. 3 and FIG. 4, the horizontal axis indicates the
 crank angle of the engine, and is calibrated on the basis of the top dead
 center (TDC) and bottom dead center (BDC) of the piston 18 of the cylinder
 #4.
 As shown in FIG. 3, the pressure within the cylinder #3 (B) and the
 pressure within the cylinder #4 (C) increase up to their peak values in
 synchronization with combustion timing of each cylinder while the engine
 is operating in a full operating mode with all of the cylinders being
 activated. Furthermore, the respective peak values are substantially equal
 to each other. In this case, the vibration of the engine can be suppressed
 to be relatively small, as is understood from the waveform (D) of FIG. 3.
 Where the intake valve 42 and exhaust valve 44 of each inactive cylinder
 are kept closed during execution of cylinder deactivation control, on the
 other hand, the pressure within the inactive cylinder is reduced because
 of leakage of gas from the combustion chamber 26 of the inactive cylinder
 into the crankcase. Accordingly, the pressure within the cylinder #4 as an
 inactive cylinder increases only by a small degree when the piston 18
 almost reaches the top dead center, as shown in FIG. 4, and its peak value
 is considerably small. While the engine is operating in a partial
 operating mode with at least one of the cylinders being deactivated, each
 of the active cylinders is required to generate a larger magnitude of
 torque than the torque generated by each cylinder in a full operating mode
 (with all of the cylinders activated), in order to provide the same output
 torque as that obtained in the full operating mode. To this end, the peak
 value of the pressure (B) within the cylinder #3 as an active cylinder is
 larger than the peak value that is achieved when the engine is in a full
 operating mode with all of the cylinders activated (FIG. 3).
 As described above, where the intake valve 42 and exhaust valve 44 of each
 inactive cylinder are kept closed under cylinder deactivation control, the
 pressure within the inactive cylinder is reduced while the pressure within
 each active cylinder is increased, resulting in an increased difference in
 the cylinder pressure between the inactive cylinders and the active
 cylinders. With the four-cylinder engine taken as a whole, four peaks
 appear in the cylinder pressures at intervals of 180 CA (crank angle) when
 the engine is in a full operating mode with all of the cylinders
 activated, whereas two peaks larger than those in the full operating mode
 appear in the cylinder pressures at intervals of 360 CA when the engine is
 in a partial operating mode with part of the cylinders deactivated.
 Namely, larger peaks of the cylinder pressures appear at longer intervals
 during execution of cylinder deactivation control, as compared with the
 peaks appearing in the full operating mode of the engine. Consequently,
 the output torque of the engine varies to a greater extent, and the
 vibration (D in FIG. 4) of the engine during execution of cylinder
 deactivation control is increased as compared with the vibration (D in
 FIG. 3) that occur in the full engine operating mode with all cylinders
 activated.
 FIG. 5 is a time chart showing the opening and closing actions of the
 intake valve 42 and exhaust valve 44 of each of the cylinders #1 through
 #4 when the engine of the first embodiment shifts from a full operating
 mode with all of the cylinders activated, to a partial operating mode with
 part of the cylinders deactivated. In FIG. 5 and similar time charts as
 shown later, "IN" and "EX" represent the intake valve 42 and the exhaust
 valve 44, respectively. The horizontal axis indicates the crank angle of
 the engine, and is calibrated based on the top dead center (TDC) and
 bottom dead center (BDC) of the piston 18 of each cylinder, and the top
 row in the figure indicates the number of the cylinder in which combustion
 and expansion takes place. FIG. 6 shows changes in the pressures within
 the cylinders #3 and #4 and the vibration of the engine when the intake
 valve 42 and the exhaust valve 44 of each inactive cylinder are opened and
 closed according to the time charts of FIG. 5.
 In the first embodiment, ignition is inhibited in the cylinder #1 and
 cylinder #4 as inactive cylinders during execution of cylinder
 deactivation control, and the exhaust valve 44 is opened for a certain
 period of time that starts ahead of the bottom dead center (BDC), as
 indicated by hatched areas in FIG. 5. During the operation of the engine,
 the exhaust passage 90 is filled with exhaust gas whose pressure is
 substantially equal to the atmospheric pressure. If the exhaust valve 44
 is opened, therefore, the exhaust gas is reintroduced from the exhaust
 passage 90 into the combustion chamber 26, so that the pressure within the
 inactive cylinder increases to be substantially equal to the atmospheric
 pressure. If the exhaust valve 44 is then closed in the course of
 displacement of the piston 18 toward the top dead center, the exhaust gas
 that has been reintroduced into the combustion chamber 26 is compressed.
 Thus, the pressure within the cylinder #4 as an inactive cylinder is
 increased after the exhaust valve 44 is closed, as shown in FIG. 6, so
 that a difference between the pressure within the cylinder #3 and that
 within the cylinder #4 is reduced. Consequently, the vibration (as
 represented by waveform D in FIG. 6) of the engine is suppressed or
 reduced as compared with the case of the cylinder deactivation control as
 shown in FIG. 4.
 In this connection, the pressure of the inactive cylinder may exceed the
 atmospheric pressure at around the top dead center location. Even if the
 exhaust valve 44 is opened in this state, exhaust gas cannot be
 reintroduced from the exhaust passage 90 into the combustion chamber 26.
 Accordingly, the valve-opening timing of the exhaust valve 44 associated
 with the inactive cylinder is set to a point of time when the piston 18
 moves from the top dead center toward the bottom dead center until the
 cylinder pressure becomes equal to or smaller than the atmospheric
 pressure.
 The peak value of the pressure within the inactive cylinder depends upon
 the timing in which the exhaust valve 44 is closed. Namely, as the
 valve-closing timing of the exhaust valve 44 is closer to the bottom dead
 center, the peak value of the cylinder pressure is increased because of a
 large compression ratio of the reintroduced exhaust gas. The valve-closing
 timing of the exhaust valve 44 is determined depending upon the engine
 load, so that a change in the acceleration of the engine vibration due to
 the pressure within the inactive cylinder approximately coincides with a
 change in the acceleration of the engine vibration due to the pressure
 within the active cylinder.
 A control routine to be executed by the ECU 10 in the first embodiment will
 be now explained.
 FIG. 7 is a flowchart of a cylinder deactivation control routine to be
 executed by the ECU 10 in the first embodiment. This routine is started in
 a repeated manner.
 Once the first routine is started, step 100 is initially executed. In step
 100, the presence of a request for start of cylinder deactivation control
 is determined, namely, whether cylinder deactivation control is requested
 to be started or not is determined. If no request for start of cylinder
 deactivation control is generated, namely, if a negative decision (NO) is
 obtained in step 100, the current cycle of the control routine is
 terminated. If a request for start of cylinder deactivation control is
 generated, namely, if an affirmative decision (YES) is obtained in step
 100, step 102 is then executed.
 In step 102, an operation for inhibiting fuel injection and ignition in the
 cylinder #1 and cylinder #4 is performed.
 In step 104, the intake valves 42 and exhaust valves 44 of the cylinder #1
 and cylinder #4 are closed. This valve-closing operation is performed at a
 point of time when the combustion stroke and exhaust stroke are finished.
 In step 106, a peak value P.sub.MAX of the pressure within the active
 cylinders is estimated. The pressure within the active cylinders is
 substantially proportional to the load of the engine. In this step 106,
 therefore, the peak value P.sub.MAX is estimated based upon the specific
 volume of intake air. Here, the peak value P.sub.MAX of the cylinder
 pressure may be directly detected by a cylinder pressure sensor that is
 provided in the cylinder #2 or cylinder #3 as an active cylinder.
 In step 108, the valve-closing timing T.sub.close of the exhaust valves 44
 of the inactive cylinders is determined based on the peak value P.sub.MAX
 of the pressure within the active cylinders.
 Step 110 is then executed to determine whether the current point of time is
 the valve-opening timing T.sub.open of the exhaust valves 44 of the
 inactive cylinders or not. As described above, the valve-opening timing
 T.sub.open is set in advance to a point of time at which the pressure
 within the inactive cylinders falls below the atmospheric pressure. If the
 current point of time is the valve-opening timing T.sub.open, namely, if
 an affirmative decision (YES) is obtained in step 110, step 112 is
 executed to open the exhaust valves 44 of the cylinder #1 and cylinder #4,
 and step 114 is then executed. If the current point of time is not the
 valve-opening timing T.sub.open, namely, if a negative decision (NO) is
 obtained in step S110, on the other hand, step 116 is executed.
 In step 116, it is determined whether the current point of time is the
 valve-closing timing T.sub.close of the exhaust valves 44 of the cylinder
 #1 and cylinder #4. If the current point of time is the valve-closing
 timing T.sub.close, namely, if an affirmative decision (YES) is obtained
 in step 116, step 118 is executed to close the exhaust valves 44 of the
 cylinder #1 and cylinder #4, and step 114 is then executed. If the current
 point of time is not the valve-closing timing T.sub.close, namely, if a
 negative decision (NO) is obtained in step S116, step 114 is immediately
 executed.
 In step 114, the presence of a request for termination of cylinder
 deactivation control is determined, namely, whether cylinder deactivation
 control is requested to be terminated or not is determined. If no request
 for termination is generated, namely, a negative decision (NO) is obtained
 in step 114, the control flow returns to step 106. If a request for
 termination is generated, namely, if an affirmative decision (YES) is
 obtained in step 114, step 120 is executed to perform an operation to
 cancel inhibition of fuel injection and ignition of the cylinder #1 and
 cylinder #4, and then the current cycle of the routine is terminated.
 As described above, in the internal combustion engine of the first
 embodiment, the exhaust valves 44 of the inactive cylinders are opened
 during cylinder deactivation control, thereby to increase the pressure
 within the inactive cylinders. During cylinder deactivation control,
 therefore, a difference between the pressure within the inactive cylinders
 and the pressure within the active cylinders can be reduced, and vibration
 of the engine can be suppressed.
 In the above description of the first embodiment, the exhaust valve 44
 associated with each of the cylinder #1 and cylinder #4 as inactive
 cylinders is opened only once during a single reciprocating motion of the
 piston 18. It is, however, unnecessary to open the exhaust valve 44 upon
 every reciprocation of the piston 18 since the pressure within the
 inactive cylinders is not immediately reduced after these cylinders are
 deactivated or disabled. For instance, the exhaust valves 44 associated
 with the cylinder #1 and cylinder #4 may be alternately opened each time
 the piston 18 reciprocates, as shown in FIG. 8. It is also possible to
 open the exhaust valve 44 each time the piston 18 reciprocates several
 times.
 It may also be proposed to open the intake valve 42 so as to introduce new
 air into the combustion chamber 26. If the new air is introduced by
 suction into the combustion chamber 26 of an inactive cylinder, however,
 the new air is discharged directly into the exhaust passage 90 when the
 operation of the inactive cylinder is restarted. As a result, the air/fuel
 ratio measured in the exhaust passage 90 may be changed to the lean side,
 which may result in deterioration of the performance of catalyst for
 purifying exhaust gas. Also, where the new air is introduced into the
 combustion chamber 26 of the inactive cylinder, the interior of the
 cylinder is cooled, which may prevent the inactive cylinder from smoothly
 restarting its operation when it is activated again. In the first
 embodiment, since the exhaust valve 44 is opened so that burnt gas is
 re-introduced into the combustion chamber 26, the burnt gas is discharged
 again into the exhaust passage 90 when the operation of the inactive
 cylinder is restarted, thus causing no influence on the air/fuel ratio in
 the exhaust passage 90. Also, since high-temperature burnt gas is
 introduced into the combustion chamber 26, the inactive cylinder is
 prevented from being cooled. Thus, the first embodiment makes it possible
 to suppress vibration of the engine during cylinder deactivation control,
 while avoiding problems that would otherwise occur upon restart of the
 operations of the inactive cylinders after termination of cylinder
 deactivation control.
 Next, a second embodiment of the invention will be described. While the
 construction of the internal combustion engine of the second embodiment is
 similar to that of the first embodiment, the second embodiment is
 different from the first embodiment in that the cylinders of the engine
 are successively selected as deactivated or inactive cylinders, namely,
 the cylinders are successively deactivated under cylinder deactivation
 control.
 To successively deactivate or disable all of the cylinders under cylinder
 deactivation control, the engine may perform six-stroke cycle operations
 (six-stroke cycle cylinder deactivation control). The second embodiment is
 adapted to suppress vibration of the engine during execution of the
 six-stroke cycle cylinder deactivation control.
 Initially, the operation of the engine under six-stroke cycle cylinder
 deactivation control will be described.
 FIG. 9 is a time chart showing the opening and closing actions of the
 intake valves 42 and exhaust valves 44 of the cylinder #1 through cylinder
 #4, where the intake valve 42 and exhaust valve 44 associated with each
 cylinder are kept closed during the time in which the piston 18
 reciprocates once from the bottom dead center at which the combustion
 stroke ends to the next bottom dead center so that the four cylinders are
 successively deactivated or disabled.
 In the cylinder #1 as shown in FIG. 9, for example, the intake stroke and
 the compression stroke take place in the periods T1 and T2, respectively,
 and the combustion stroke takes place in the following period T3. The
 intake valve 42 and exhaust valve 44 are kept closed in the periods T4 and
 T5 subsequent to the period T3, so that the cylinder #1 is deactivated.
 While the piston 18 moves toward the top dead center and the bottom dead
 center during the inactive periods T4 and T5, gas contained in the
 combustion chamber 26 is compressed and expanded while being disconnected
 or shut off from the intake port 22 and the exhaust port 24. After the
 exhaust stroke takes place with the exhaust valve 44 opened in the period
 T6, the intake stroke, compression stroke, combustion stroke, inactive
 compression stroke, inactive expansion stroke, and exhaust stroke are
 effected again. Similar operations are performed in the cylinders #2
 through #4, as shown in FIGS. 9B-9D. Thus, under six-stroke cycle cylinder
 deactivation control, the engine operates on the six-stroke cycle
 consisting of the intake, compression, combustion, inactive compression,
 inactive expansion, and exhaust strokes, and the cylinder #1, cylinder #3,
 cylinder #2, and cylinder #4 are successively selected as an inactive
 cylinder in this order, i.e., in the order in which combustion takes
 place.
 It will be understood from FIG. 9 that combustion in the #3 cylinder is
 delayed 180 CA (crank angle) relative to that in the cylinder #1, and
 combustion in the cylinder #2 is delayed 360 CA relative to that in the
 cylinder #3. Also, combustion in the cylinder #4 is delayed 180 CA
 relative to that in the #2 cylinder, and the combustion in the cylinder #1
 is delayed 360 relative to that in the cylinder #4. In comparison with a
 normal engine operation in which combustion successively occurs in the
 four cylinders at intervals of 180 CA, the engine operation as shown in
 FIG. 9 is characterized in that a pause exists between combustion in the
 cylinder #3 and that in the cylinder #2, and between combustion in the
 cylinder #4 and that in the cylinder #1 while combustion successively
 takes place in the order of #1 cylinder.fwdarw.#3 cylinder.fwdarw.#2
 cylinder.fwdarw.#4 cylinder.
 FIG. 10A shows changes in the pressures within the cylinders #1 through #4
 over the six-stroke cycle, as indicated by a solid line, one-dot chain
 line, two-dot chain line, and a broken line, respectively, where the
 engine operates according to the time chart of FIG. 9. FIG. 10B shows
 changes in the sum of the pressures within all of the cylinders (cylinders
 #1 through #4) (which will be called "composite cylinder pressure").
 As described above with respect to the first embodiment, gas leaks little
 by little from the combustion chamber 26 of the inactive cylinder toward
 the crankcase. Where each cylinder is deactivated immediately after the
 combustion stroke under the above six-stroke cycle cylinder deactivation
 control, for example, almost all the burnt gas remains in the combustion
 chamber 26 of the currently deactivated cylinder without leaking. In this
 case, if the piston 18 moves toward the top dead center in the currently
 deactivated cylinder, the burnt gas is compressed, whereby the cylinder
 pressure increases to substantially the same level as that achieved at the
 time of combustion when the piston 18 reaches the top dead center. As
 shown in FIG. 10A, therefore, the pressure within each cylinder reaches
 its maximum level at the top dead center at which the combustion stroke
 begins, and the following top dead center. Namely, two peaks per
 six-stroke cycle appear in the pressure within each cylinder.
 As is understood from FIG. 9, combustion occurs in the cylinder #2 when the
 piston of the cylinder #3 as a currently deactivated cylinder is located
 close to the top dead center. As is also understood from FIG. 9,
 combustion occurs in the cylinder #1 when the piston of the cylinder #4 as
 a currently deactivated cylinder is located close to the top dead center.
 Thus, a peak of the cylinder pressure resulting from the combustion in the
 cylinder #2 and a peak of the cylinder pressure resulting from compression
 of gas in the cylinder #3 as a currently deactivated cylinder appear at
 almost the same time at around time "t1" in FIG. 10A. Similarly, a peak of
 the cylinder pressure resulting from the combustion in the cylinder #1 and
 a peak of the cylinder pressure resulting from compression of gas in the
 cylinder #4 as a currently deactivated cylinder appear at almost the same
 time at around time "t4" in FIG. 10A. On the other hand, a single peak of
 the cylinder pressure appears at around time "t2", "t3", "t5" and "t6",
 which peak results from the combustion in the cylinder #4 as an active
 cylinder, compression of gas in the cylinder #2 as an inactive cylinder,
 combustion in the cylinder #3 as an active cylinder, and compression of
 gas in the cylinder #1 as an inactive cylinder, respectively. As shown in
 FIG. 10B, therefore, the peak value of the composite cylinder pressure at
 around time "t1" and "t4" at which the pressures within two cylinders
 increase at almost the same time differs from the peak value of the
 composite cylinder pressure at around time "t2", "t3", "t5" and "t6" at
 which the pressure within only one cylinder increases. Consequently, the
 vibration of the engine is increased.
 In the internal combustion engine of the second embodiment, where the
 piston of the currently deactivated cylinder reaches the top dead center
 at the same time that combustion takes place in another cylinder, the
 exhaust valve 44 associated with the deactivated cylinder is opened so as
 to avoid an increase in the pressure within the deactivated cylinder,
 thereby to suppress vibration of the engine.
 FIG. 11 is a time chart showing opening and closing actions of the intake
 valves 42 and exhaust valves 44 of the cylinders #1 through #4 in the
 second embodiment. In the cylinder #3 and cylinder #4, the exhaust valve
 44 is opened during a rest period or inactive period after completion of
 each combustion stroke, as indicated by hatched areas in FIG. 11.
 FIG. 12A shows changes in the pressure within each cylinder where the
 engine operates according to the time chart of FIG. 11. FIG. 12B shows
 changes in the composite cylinder pressure. The exhaust valve 44
 associated with a currently deactivated cylinder is opened during the time
 in which the cylinder #3 and cylinder #4 are deactivated, and therefore
 the relevant cylinder pressure does not increase even if the piston 18
 moves toward the top dead center. Thus, the pressure within the cylinder
 #3 that is being deactivated is kept at a relatively low level when the
 pressure within the cylinder #2 increases due to combustion in the
 cylinder #2 at around time "t1", as shown in FIG. 12A. Similarly, the
 pressure within the currently deactivated cylinder #4 is kept at a
 relatively low level when the pressure within the cylinder #1 increases
 due to combustion in the cylinder #1 at around time "t4". In other words,
 a peak of the pressure within each cylinder appears alone or separately.
 Consequently, a peak value of the composite cylinder pressure of the
 engine is kept generally constant, and the vibration of the engine can be
 suppressed.
 In the second embodiment, the exhaust valve 44 of the currently deactivated
 cylinder is opened when the piston of the deactivated cylinder reaches the
 top dead center at the same time that combustion occurs in another
 cylinder, under six-stroke cycle cylinder deactivation control. With this
 arrangement, an increase in the cylinder pressure due to combustion in an
 active cylinder and an increase in the pressure within a currently
 deactivated cylinder do not take place at the same time. It is thus
 possible to suppress vibration of the engine during execution of
 six-stroke cycle cylinder deactivation control.
 In the case where particular cylinders are selected or predetermined as
 inactive cylinders, the inactive cylinders suffer from reduction in the
 temperature and a lack of lubricating oil, and may not be re-activated
 smoothly. According to the six-stroke cycle cylinder deactivation control,
 on the other hand, all of the cylinders are successively caused to operate
 as an inactive or deactivated cylinder, and are therefore free from the
 above-described problems.
 In the second embodiment, the ECU 10 causes the exhaust valves 44
 associated with the cylinder #1 and cylinder #4 to be opened in such
 timing as indicated in FIG. 11, thereby to control the pressures within
 the inactive cylinders.
 While the exhaust valve 44 associated with the cylinder #3 or cylinder #4
 is caused to open over the entire inactive period of the relevant cylinder
 as shown in the time charts of FIG. 11, the valve opening/closing timing
 of the exhaust valve 44 of the inactive cylinder is preferably determined
 experimentally, so that the vibration of the engine can be minimized.
 It may be proposed to open the intake valve 42 as means for reducing the
 pressure within the deactivated cylinder. If the intake valve 42 is
 opened, however, burnt gas in the inactive or deactivated cylinder flows
 into active cylinders through the intake pipe 84, and adversely affects
 the operation of the active cylinders. The second embodiment is free from
 such a problem since the pressure within the inactive cylinder is reduced
 by opening the exhaust valve 44 rather than the intake valve 42.
 In the illustrated embodiment, the engine controller (ECU 10) is
 implemented as a programmed general purpose computer. It will be
 appreciated by those skilled in the art that the controller can be
 implemented using a single special purpose integrated circuit (e.g., ASIC)
 having a main or central processor section for overall, system-level
 control, and separate sections dedicated to performing various different
 specific computations, functions and other processes under control of the
 central processor section. The controller also can be a plurality of
 separate dedicated or programmable integrated or other electronic circuits
 or devices (e.g., hardwired electronic or logic circuits such as discrete
 element circuits, or programmable logic devices such as PLDs, PLAs, s
 or the like). The controller can be implemented using a suitably
 programmed general purpose computer, e.g., a microprocessor,
 microcontroller or other processor device (CPU or MPU), either alone or in
 conjunction with one or more peripheral (e.g., integrated circuit) data
 and signal processing devices. In general, any device or assembly of
 devices on which a finite state machine capable of implementing the
 procedures described herein and in the flowchart shown in FIG. 7 can be
 used as the controller. A distributed processing architecture can be used
 for maximum data/signal processing capability and speed.
 While the invention has been described with reference to preferred
 embodiments thereof, it is to be understood that the invention is not
 limited to the disclosed embodiments or constructions. To the contrary,
 the invention is intended to cover various modifications and equivalent
 arrangements. In addition, while the various elements of the disclosed
 invention are shown in various combinations and configurations, which are
 exemplary, other combinations and configurations, including more, less or
 only a single element, are also within the spirit and scope of the
 invention.