United States Patent [19J
`Hitomi et al.
`
`111111111111111111111111111111111111111111111111111111111111111111111111111
`US005233831A
`5,233,831
`[II] Patent Number:
`[45] Date of Patent: Aug. 10, 1993
`
`[54] EXHAUST CONTROL SYSTEM FOR
`INTERNAL COMBUSTION ENGINE
`
`[75]
`
`Inventors: Mitsuto Hitomi; Kenji Kashiyama,
`both of Hiroshima; Ken Umehara,
`Higashihiroshima, all of Japan
`[73] Assignee: Mazda Motor Corporation,
`Hiroshima, Japan
`
`[21] Appl. No.: 906,290
`[22] Filed:
`Jun.29, 1992
`[30]
`Foreign Application Priority Data
`Jun. 28, 1991 [JP]
`Japan .................................. 3-159114
`Jan. 31, 1992 [JP]
`Japan .................................. 4-042114
`Feb. S, 1992 [JP]
`Japan .................................. 4-054249
`Int. CI.s ................................................ FOIN 3/28
`[51]
`[52] U.S. CI ......................................... 60/284; 60/292;
`60/300; 123/90.15; 123/323; 123/339
`[58] Field of Search ......................... 60/284, 292, 300;
`123/339, 323, 90.15
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`2,906,253 9/1959 Nallinger ............................ 123/339
`
`3,503,716 3/1970 Berger ................................... 60/284
`3,723,070 3/1973 Houdry ................................. 60/284
`
`FOREIGN PATENT DOCUMENTS
`62-667 1/1987 Japan .
`1-159431 6/1989 Japan .
`2-115537 4/1990 Japan .
`2-256815 10/1990 Japan .
`Primary Examiner-Douglas Hart
`Attorney, Agent, or Firm-Keck, Mahin & Cate
`[57]
`ABSTRACT
`An exhaust control system has a first catalytic device
`disposed in an exhaust line and a second catalytic de(cid:173)
`vice, which is smaller in capacity than the first catalytic
`device, disposed in the exhaust line upstream from the
`first catalytic device. The system includes a shutter
`device disposed in the exhaust line upstream from the
`first catalytic device. The shutter device is caused, by a
`controller, to close the exhaust line during a predeter(cid:173)
`mined period from a start of engine cranking, before an
`engine is warmed up, and open the exhaust line after the
`engine is warmed up, thereby promoting activation ,of
`the catalytic devices.
`
`21 Claims, 9 Drawing Sheets
`
`CONTHOL UNIT
`
`BOSCH-DAIMLER EXHIBIT 1002
`
`Page 1 of 19
`
`

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`Page 2 of 19
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`PROVIDE
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`
`Page 3 of 19
`
`

`

`U.S. Patent
`
`Aug. 10, 1993
`
`Sheet 3 of 9
`
`5,233,831
`
`YES
`
`CALCULATE ZC
`
`P3
`
`P4
`
`FIG.3
`
`Page 4 of 19
`
`

`

`U.S. Patent
`
`Aug. 10, 1993
`
`Sheet 4 of 9
`
`.
`
`5,233,831
`
`T2
`
`,.-
`
`-
`
`VALVLE
`LIFT
`
`B. D.C.
`
`T3
`
`I
`__..--.._....._
`...... '(.
`
`\ ""'
`'
`
`B. D.C.
`
`CRANK
`ANGLE
`
`TC
`
`0/L
`FIG.4
`
`PRESSURE
`
`B.D.C.
`
`rTW
`I
`,ry'
`
`I I /"
`
`"
`
`T.D.C.
`
`FIG.S
`
`BURNING
`
`CRANK
`ANGLE
`
`Page 5 of 19
`
`

`

`U.S. Patent
`
`Aug. 10, 1993
`
`Sheet 5 of 9
`
`5,233,831
`
`VALVE
`UFT
`
`T3
`\
`........
`_,.-~---....
`
`'
`-"""'
`'
`
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`
`B.D. C.
`
`CRANK
`ANGlE
`
`TC
`
`0/l
`FIG.6
`
`1/1
`
`II
`
`ENGINE
`lOAD
`
`III
`
`I
`
`1500
`
`(RPM}
`
`ENGINE SPEED
`
`FIG.7
`
`Page 6 of 19
`
`

`

`U.S. Patent
`
`Aug. 10, 1993
`
`Sheet 6 of 9
`
`5,233,831
`
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`Page 7 of 19
`
`

`

`U.S. Patent
`US. Patent
`
`Aug. 10, 1993
`Aug. 10, 1993
`
`Sheet 7 of 9
`Sheet 7 of 9
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`
`5,233,831
`5,233,831
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`Page 8 0f 19
`
`Page 8 of 19
`
`

`

`U.S. Patent
`
`Aug. 10, 1993
`
`Sheet 8 of 9
`
`5,233,831
`
`-20
`
`IGNmON
`TIMING -10
`
`TOC 0
`
`UMIT
`
`10
`
`1000
`
`EXHAUST
`PRESSURE
`500
`(mmHg)
`
`UMIT
`-----+-
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`AIR CHARGING MAGNIACAllON
`
`2.0
`
`FIG.1 0
`
`Page 9 of 19
`
`

`

`U.S. Patent
`
`Aug. 10, 1993
`
`Sheet 9 of 9
`
`5,233,831
`
`ENGINE ·
`SPEED
`
`AIR CHARGING
`RATE
`
`THROTILE
`OPENING
`
`TORQUE
`
`I
`I
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`TIME
`
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`START
`FIG .11
`
`I ENGINE
`SPEED
`
`FIG.12
`
`Page 10 of 19
`
`

`

`1
`
`5,233,831
`
`EXHAUST CONTROL SYSTEM FOR INTERNAL
`COMBUSTION ENGINE
`
`i
`gine which eliminates harmful emissions in exhaust gas
`even before the activation of a catalytic device.
`The foregoing objects of the present invention are
`accomplished by providing an exhaust control system
`S for an internal combustion engine having a first cata(cid:173)
`lytic device disposed in an exhaust passage, and a sec(cid:173)
`ond catalytic device disposed in the exhaust passage
`upstream of the frrst catalytic device. The second cata(cid:173)
`lytic device has a capacity smaller than that of the frrst
`10 catalytic device. The exhaust control system includes a
`shutter valve, disposed in the exhaust passage upstream
`of the fU"St catalytic device, which is controlled to open
`and close the exhaust passage. When the engine is
`cranking at a temperature lower than a predetermined
`temperature, the shutter valve closes the exhaust pas·
`sage during a predetermined· specific period so as to
`hold exhaust gases in the exhaust passage and prevent
`the engine from stalling. This promotes the activation of
`the second catalytic device sufficiently so that it per(cid:173)
`forms its inherent function. The exhaust gases passed
`through and heated by the second catalytic device pro-
`mote the activation of the fU"St catalytic device.
`
`15
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`This invention relates to an exhaust control system
`having a catalytic device in an exhaust line for purifying
`exhaust gases.
`2. Description of Related Art
`In an internal combustion engine of an automotive
`vehicle, a catalytic device, such as a catalytic converter,
`is typically disposed in an exhaust passage for purifying
`exhaust gases discharged from the vehicle through the
`exhaust passage. It is desirable to promptly activate the
`catalytic device after the engine starts so that its inher(cid:173)
`ent gas purifying function can be effectively performed.
`Therefore, the catalytic device is provided with a
`heater which receives electric power from an electric 20
`power supply of the vehicle and heats the catalytic
`device in order to promptly activate the catalyst of the
`catalytic device. In a vehicle equipped with an engine
`which has an exhaust system including such a catalytic
`device and a heater, a large amount of electric power is 25
`conventionally needed for the heater. Accordingly, a
`large battery having a large capacity must be installed in
`an engine compartment or the like. Such a large size
`battery may cause a shortage of space for various neces(cid:173)
`sary elements which must be installed in the engine 30
`compartment.
`In some engines of this kind, a primary catalytic de(cid:173)
`vice, disposed in the exhaust passage, and a secondary
`catalytic device, which has a smaller capacity than the
`primary catalytic device, disposed in the exhaust pas- 3S
`sage upstream from the primary catalytic device, are
`provided. When the engine cranks after it starts at a low
`temperature, the secondary catalytic device is initially
`heated by exhaust gas to its active temperature so as to
`purify the exhaust gas. The exhaust gas, purified and 40
`heated by the secondary catalytK: device, is then led to
`the primary catalytic device, and promotes activation of
`the primary catalytic device. As a result, a time period
`before the primary catalytic device is heated and acti·
`vated sufficiently to purify the exhaust gas is shortened. 45
`Such an engine and catalytic device arrangement is
`known from, for instance, Japanese Unexamined Patent
`Publication No. 2-256,815.
`Even in an exhaust system which has a primary cata(cid:173)
`lytic device and a secondary catalytic device which has so
`a smaller capacity than the primary catalytic device and
`is disposed upstream from the primary catalytic device,
`exhaust gas is allowed to be discharged to the outside of
`the vehicle without being purified by both the second(cid:173)
`ary catalytic device and the primary catalytic device ss
`until the secondary catalytic device reaches its active
`temperature during warming up of the engine. Such a
`problem also occurs even in an exhaust system which
`has a heater for the primary catalytic device.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`The above and other objects and features of the pres(cid:173)
`ent invention will be apparent to those skilled in the art
`from the following description of a preferred embodi(cid:173)
`ment thereof when considered in conjunction with the
`drawings, in which:
`FIG. 1 is a schematic illustration showing an internal
`combustion engine with an exhaust system according to
`the preferred embodiment of the present invention;
`FIG. 2 is a flow chart illustrating an exhaust control
`routine for a microcomputer of an exhaust control unit;
`FIG. 3 is a flow chart illustrating an ignition timing
`control routine for the microcomputer;
`FIG. 4 is a diagram illustrating valve lifts of intake
`and exhaust valves;
`FIG. 5 is a diagram representing exhaust pressure in
`a combustion chamber of the engine;
`FIG. 6 is a diagram illustrating valve lifts of the in(cid:173)
`take and exhaust valves for a varied valve timing;
`FIG. 7 is a diagram illustrating various ranges of
`engine operating conditions;
`FIG. 8 is a diagram representing exhaust pressure
`with respect to shutter valve opening;
`FIG. 9 is a diagram showing engine output torque;
`FIG. 10 is a diagram representing exhaust pressure
`with respect to air charging magnification;
`FIG. 11 is a diagram showing engine speed, air charg(cid:173)
`ing rate and throttle opening in comparisons of the
`exhaust system of this invention with ideal and conven(cid:173)
`tional exhaust systems; and
`FIG. 12 is a diagram showing engine output torque
`with respect to engine speed.
`
`SUMMARY OF THE INVENTION
`It is an object of the present invention to provide an
`exhaust control system for an internal combustion en(cid:173)
`gine which promotes the activation of a catalytic device
`sufficiently to purify exhaust gases, even when the en- 65
`gine is operating at a low temperature.
`It is another object of the present invention to pro(cid:173)
`vide an exhaust control for an internal combustion en-
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`Referring to the drawings in detail and, in particular,
`60 to. FIG. 1, showing an exhaust control system according
`to a preferred embodiment of the present invention, an
`engine body 1 has fU"St and second cylinder banks 2 and
`3 arranged in a V -type formation and at a predeter-
`mined angle relative to each other. The first cylinder
`bank 2 is formed with a plurality of, for instance three,
`cylinders 5 arranged in a row. Similarly, the second of
`the cylinders 5 is represented by chained line for each of
`the first and second cylinder rows. A piston 6 is snugly
`
`Page 11 of 19
`
`

`

`5,233,831
`
`4
`3
`branched exhaust passage 35. The secondary catalytic
`received in each cylinder 5 so as to slide up and down.
`The engine body 1 is formed with a combustion cham-
`device 37 has a smaller capacity than that of the primary
`catalytic device 32 and is equipped with a heater 36
`ber, which is provided with an ignition plug 8 above the
`disposed upstream therefrom. The shutter valve 40,
`piston 6. Further, the engine body 1 is formed with an
`individual intake passage 14 and an individual exhaust 5 which is positioned downstream from the secondary
`catalytic device 32, is actuated by an electric actuator
`passage 15. The engine body is formed with two intake
`38. An exhaust gas reservoir 41 having a large capacity
`ports and two exhaust ports, opening into the combus-
`tion chamber of each cylinder, which are opened and
`is disposed upstream from each secondary catalytic
`closed at an appropriate timing by intake valves 10 and
`device 37. Since the secondary catalytic device 37 dis-
`exhaust valves 11, respectively. The intake valve 10 is 10 posed in the branched exhaust passage 35 has a rela-
`tively small capacity, it promptly reaches an active
`driven by an intake camshaft (not shown) in synchro-
`nism with rotation of an output shaft of the engine, such
`temperature due to exhaust gas produced during warm-
`as an engine crankshaft. Similarly, the exhaust valve 11
`ing-up of the engine, even though the exhaust gas is at
`is driven by an exhaust camshaft (not shown) in syn-
`a low temperature after the engine starts. Therefore,
`chronism with rotation of the engine output shaft. The 15 during the warming-up of the engine, the secondary
`catalytic device 37 is activated first so as to perform
`exhaust camshaft is associated with a variable valve
`timing mechanism 9 so· that the exhaust valve 11 is
`exhaust gas purification. Then, the exhaust gas heated
`driven at a first valve timing T1 or at a second valve
`by the secondary catalytic device 37 is led to the pri-
`mary catalytic device 32 so as to promote activation of
`timing T2, shown by a solid line and a broken line,
`respectively, in FIG. 4. The exhaust valve operation is 20 the primary catalytic device 32.
`retarded so that the exhaust port when operated at the
`Shutter valve 40 is controlled by the actuator 38 to
`second valve timing T2, is closed later than when oper-
`.
`t.
`
`
`close the branch exhaust passage 35 when the engme
`1 10 . e va ve iS · T1 Th · tak
`a e a
`e irS v ve 1mmg
`xh
`.
`.
`e m
`
`t al
`t d t th fi
`t
`d ·
`· T3 nven o open an c ose a a va ve immg
`d 1
`t
`t"
`
`starts sc as to cause e aust gas to stagnate m the
`h
`1
`as s own
`h
`xh
`t 1
`(OIL)
`· d TC ·
`t 25 branc ed e aust passage 35 upstream from the shutter
`m. FIG 4 A fi
`1
`irs va ve over ap
`peno
`iS se
`·
`·
`40 Thi
`.
`.
`. s promotes act~vatlOn ~f the secondary
`to be longer when the exhaust valve 11 is operated at
`valve . .
`catalytic device 37, due to an mcrease m tempe!ature of
`the second valve timing T2 than a second valve overlap
`t~e exha_ust gas. Thereaf~er, when a predetermmed spe-
`(OIL) period TW when the exhaust valve 11 is oper-
`ated at the first valve timing T1. The valve overlap
`cific penod after the engme starts has passed, the aetna-
`period is that period in which the intake valve 10 and 30 tor 38 causes the shutter valve_40 to open the branched
`exhaust passage 35 so as to discharge the exhaust gas
`the exhaust valve 11 are simultaneously open.
`A fuel injector 16 is disposed in an individual intake
`through the branched exhaust passage 35 and the com-
`passage 14 so as to inject fuel into a respective cylinder
`mon exhaust passage 31. When the shutter valve 40
`closes the ~xh~u_st passage, the exhaus~ ~as passed
`through an individual intake passage 14. The individual
`intake passages 14 form a downstream portion of an 35 ~hrough the mdividual exha~st passa~e 15 iS mtroduced
`~to th~ exhaust gas res~rvmr 41 havi~g a large ca~ac-
`intake system 18 of the engine. A common intake pas-
`ity. This pr~vents stalhng of the engme a short _tim_e
`sage 19 forms an upstream portion of the intake system
`a~ter the engme starts wh~m the te~peratw:e of.engm~ iS
`18. The common intake passage 19 is provided, in order
`still low. The pred~termmed specific penod ~ which
`from its upstream end, with an air cleaner 20, an air flow
`sensor 21, a throttle valve 22 and a surge tank 23. The 40 the shutter valve 40 iS kept close~, af!er t?e engi?e s~rts
`surge tank 23 is connected to upstream ends of the indi-
`and_ when the temperature ?f engme iS_ stdllow, iS a time
`penod Tv ~eces~ to activate suffici~ntly the second-
`vidual intake passages 14. The common intake passage
`19 is provided with a bypass passage 28, having an idle
`ary c~talytic _device _37 so tha~ the engme does not stall.
`speed control valve 29, which is well known in the art
`The time penod Tv 1S set, for mstance, between 5 and 10
`for controlling idle speed of the engine, so as to allow 45 seconds. The shutter valve 40 is opened when predeter-
`intake air to bypass the throttle valve 22.
`mined engine operating conditions, which will be de-
`The second cylinder bank 3 has a distributer 25 at-
`scribed later, are present. The secondary catalytic de-
`tached to it. The distributor 25 is electrically connected
`vice 37 is heated, by its associated heater 36, during the
`to the ignition plug 8 for each cylinder and is driven by
`predetermined specific period after the shutter valve 40
`the engine crankshaft 24. The fuel mixture supplied to so has closed the branched exhaust passage 35. Activation
`of the secondary catalytic device, therefore, is pro-
`the cylinder 5 is ignited and burned by an ignition sys-
`tem including the ignition plugs 8, the distributer 25, an
`moted more quickly. When the shutter valve 40 closes
`ignition coil 26 electrically connected to the distributer
`the branched exhaust passage 35 during the time period
`25, an ignition timing controller 27, and so forth.
`Tv after the engine starts, the temperature of engine
`Burned gases produced in the combustion chambers are 55 and, hence, of exhaust gas is still low. Consequently, the
`exhaust gas stagnates in the branched exhaust passage
`discharged into each individual exhaust passage 15.
`Individual exhaust passages 15 are respectively con-
`35 disposed upstream from the shutter valve 40. There-
`nected to the cylinders 5 and form an upstream portion
`fore, activation of the secondary catalytic device 37 is
`of an exhaust system 30. A downstream portion of the
`promoted more effectively after the engine starts when
`exhaust system 30 is formed by a common exhaust pas- 60 the temperature of the engine and, hence, of exhaust gas
`sage 31. A primary three-way catalytic device 32, such
`is still low, because of an increase in temperature of the
`as a catalytic converter including rhodium, and a si-
`exhaust gas. The shutter valve control mentioned above
`lencer 33 are disposed in the common exhaust passage
`prevents exhaust gas from being purified by the second-
`31. A pair of branched exhaust passages 35 are respec-
`ary catalytic device 37 or by the primary catalytic de-
`tively connected to the plurality of the individual ex- 65 vice 32 and then discharged outside the vehicle through
`haust passages 15. A secondary three-way catalytic
`the exhaust system within the time period Tv. Such
`device 37, such as another catalytic converter including
`discharge is prevented until the secondary catalytic
`rhodium, and a shutter valve 40 are disposed in each
`device 37 is activated sufficiently to purify the exhaust
`
`Page 12 of 19
`
`

`

`5
`gas after the engine starts when the temperature of
`exhaust gas is still low.
`Since the capacity of the secondary catalytic device
`37 is smaller than that of the primary catalytic device
`32, electric power needed by the heater 36 is smalL In 5
`addition, a battery, forming part of the electric power
`system, is allowed to have a small capacity and, there(cid:173)
`fore, a small size. This helps to avoid a shortage of space
`from occurring in the engine compartment.
`When combustibility of an air-fuel mixture in the 10
`engine is impaired in an engine operating condition in
`which the shutter valve 40 closes the branched exhaust
`passage 3S or when the vehicle is accelerating, the pre(cid:173)
`determined specific period for which the shutter valve
`40 should be kept closed is altered. More particularly, 15
`the predetermined specific period is changed to a period
`from a time at which the engine starts when the temper(cid:173)
`ature of exhaust gas is still low to a time at which im(cid:173)
`pairment of combustibility of an air-fuel mixture in the
`engine occurs or at which the acceleration of the vehi- 20
`cle starts. In this manner, the shutter valve 40 is put into
`an open condition when impairment of the combustibil-
`ity of the engine occurs or the acceleration of the vehi(cid:173)
`cle starts. In this case, since the secondary catalytic
`device 37 becomes sufficiently active before or after the 25
`shutter valve 40 is opened, the exhaust gas is prevented
`from being discharged through the individual exhaust
`passage 15, the branch exhaust passage 35 and the com(cid:173)
`mon exhaust passage 31 before being purified by the
`secondary catalytic device 37 or the primary catalytic 30
`device 32.
`Actuator 38 is disposed in the branched exhaust pas(cid:173)
`sage 35 and actuates the shutter valve 40. The actuator
`38 is activated by an actuator control signal Ca from a
`control unit SO. The control unit SO receives various 35
`signals, such as a air flow rate signal Sa from the air
`flow sensor 21, an engine speed signal Sn from an en(cid:173)
`gine speed sensor 42 disposed at the distributer 25, a
`throttle opening signal St from a throttle sensor 43, a
`temperature signal Sg, indicative of the temperature of 40
`the secondary catalytic device 37, from a temperature
`sensor 44 associated with one of the two secondary
`catalytic devices 37, an ignition signal Si from an igni(cid:173)
`tion key switch 4S, and an engine operating condition
`signal Sx, indicative of operating conditions of the en- 45
`gine which include the temperature of cooling water
`and the like. Based on these signals, the control unit SO
`controls the actuators 38 to actuate the shutter valves
`40, the heaters 36 for the secondary catalytic devices 37,
`the variable valve timing mechanism 9 for changing the 50
`valve timing of the exhaust valves 11 and the idle speed
`control valve 29 for adjusting idling speed of the engine.
`The control unit 50 also controls ignition timing of the
`engine. Each ofthese sensors may be of any well known
`type.
`In the control of the actuator 38 by the control unit
`50, when the ignition key switch 4S provides an ignition
`signal Si, indicating engine cranking, a temperature
`signal Sg from the temperature sensor 44 indicates that
`a temperature Tc of the secondary catalytic device 37 is 60
`still below an active temperature Teo. Then, when the
`engine starts and the exhaust gas temperature is still
`low, the control unit SO provides an actuator control
`signal Ca tc each actuator 38 so as to cause the shutter
`valve 40 to close the branched exhaust passage 3S. The 65
`duration of the time period Tv is set at a time between,
`for instance, 5 and 10 seconds. For the duration of this
`time period Tv from a time at which the ignition signal
`
`55
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`5,233,831
`
`6
`Si is provided, the control unit SO interrupts the actua(cid:173)
`tor control signal Ca to the actuator 38 so as to cause the
`shutter valves 40 to open the branched exhaust passage
`35. However, even if the time period Th has not
`elapsed, the control unit SO interrupts the actuator con(cid:173)
`trol signal Ca so as to also open the branched exhaust
`passage 3S when an engine speed signal Sn is received
`from the engine speed sensor 42 representing change
`AN in engine speed N less than a predetermined minus
`value -A. This indicates a sudden reduction in engine
`speed N due to an impairment in combustibility of an
`air-fuel mixture in the engine or receiving a throttle
`opening signal St from the throttie opening sensor 43
`representative of a change in throttle opening Th of the
`throttle valve 22 greater than a predetermined plus
`value A. Such a change in the throttle opening Th indi(cid:173)
`cates that the engine or vehicle has been put under
`acceleration. This control of the shutter valve 40 is
`performed after a period, defmed between a time the
`ignition signal Si is provided and a time at which com(cid:173)
`bustibility of the engine is impaired or vehicle accelera(cid:173)
`tion occurs, has elapsed. This period is substituted for
`the predetermined specific period. •
`In addition to the actuator control signal Ca, the
`control unit 50 provides a heater driver 46 for the heat(cid:173)
`ers 36 with a heater control signal Cb. The heater con(cid:173)
`trol signal Cb is maintained until a time period Th has
`elapsed. The time period Th may, for instance be more
`than ten seconds, and follows a time period Ts neces(cid:173)
`sary for the engine to crank up from the provision of an
`ignition signal Si from the ignition key switch 45. The
`time period Th is longer than the time period Tv estab(cid:173)
`lished for keeping the shutter valve 40 closed after an
`engine ignition at a low temperature. In the presence of
`the heater control signal Cb, the heater driver 46
`supplies an electric power to each heater 36 so as to heat
`the corresponding secondary catalytic device 37. In this
`control, the control unit SO controls the heater driver 46
`to begin the supply of electric power to both the heaters
`36 once the time period Ts necessary for the engine to
`crank up after the provision of the ignition signal Si has
`elapsed. This preyents coincidental supply of electric
`power to the engine and the heater driver 46, thereby
`reducing the load on an electric power system for an
`engine starter.
`Control unit 50 controls an ignition timing for the
`engine by a normal ignition control when the shutter
`valves 40 open the branched exhaust passages 3S and by
`a warming-up ignition control when the shutter valves
`40 close the branched exhaust passages 3S. In the nor(cid:173)
`mal ignition control, a standard advance angle is ob(cid:173)
`tained based on an engine speed N, represented by an
`engine speed signal Sn, and a quantity Q of intake air,
`represented by an air flow rate signal Sa. The standard
`advance angle is subjected to an advancing correction
`or a retarding correction based on the temperature of
`cooling water, represented by a temperature signal Sx.
`After this correction, the advance angle is used as a
`substantial advance angle. Then, an ignition control
`signal Ci, corresponding to the substantial advance
`angle, is generated and sent to the ignition timing con(cid:173)
`troller 27. The ignition timing controller 27 cuts off a
`primary current in the primary coil of the ignition coil
`26 at a timing determined by the ignition control signal
`Ci. Consequently, a high pulse voltage generated at a
`secondary coil of the ignition coil 26 is distributed to the
`ignition plugs 8 through the distributer 2S in a predeter-
`
`Page 13 of 19
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`7
`mined order. This voltage sparks the ignition plugs 8 at
`timings determined by the ignition control signal Ci.
`During the warming-up ignition control, the standard
`advance angle is computed based on the engine speed
`signal Sn. The temperature signal Sa is subjected to a 5
`retard correction with a predetermined value of retar(cid:173)
`dation, which is an angle between 10 and 20 degrees
`from a top dead center position of the piston in a com(cid:173)
`pression stroke. An ignition control signal Ci, corre(cid:173)
`sponding to a substantial advance angle thus obtained, is 10
`provided and sent to the ignition timing controller 27 so
`as to spark the ignition plug 8 at an ignition timing
`determined by the ignition control signal Ci. When the
`shutter valve 40 closes the branched exhaust passage 3S,
`the temperature of exhaust gas is increased, even when 15
`the engine operates at low speeds. The secondary cata(cid:173)
`lytic device 37, therefore, is activated in a short period.
`During this control, an engine stall should surely be
`avoided.
`Further, the control unit SO manages valve overlap 20
`control and idle speed control. The control unit SO
`provides a valve timing control signal Cc to the variable
`valve timing mechanism 9 so as to change a valve over(cid:173)
`lap (0/L) between the intake valve 10 and the exhaust
`valve 11. The valve overlap is changed to the second 25
`valve overlap time T2 from the first valve timing T1 as
`shown in FIG. 4.
`When the engine temperature is still low, the control
`unit SO controls the idle speed control valve 29 so that
`it opens wide and increases a rate of fuel injection in 30
`order to compensate for increased work required of the
`engine for discharging exhaust gases together with clos(cid:173)
`ing the shutter valve 40. As shown in FIG. S, when the
`control unit SO operates to extend the valve overlap
`period after the engine starts at a low temperature, 35
`combustion in the combustion chamber becomes slow,
`due to an increase of "reprimarying" gas in the combus(cid:173)
`tion chamber in accordance with extension of the over(cid:173)
`lap period. In such a combustion condition, combustion
`occurs at a later half period of the extension stroke. At 40
`the same time, carburetion of fuel is also promoted due
`to an increase of reprimarying gas in the combustion
`chamber in accordance with extension of the overlap
`period.
`By this control, the temperature of the gas exhausted 45
`from the engine becomes high. As a result, the second·
`ary and primary three-way catalytic devices are acti·
`vated in a short time period. Because these catalytic
`devices are activated in a only short period, purification
`of exhaust gas starts quickly.
`Referring to FIG. 6, a variation of overlap period of
`the exhaust valve 11 and the intake valve 10 is shown.
`The exhaust valve opens earlier at a second valve time
`T2' than at a first valve timing T1' so that exhaust gas at
`a high temperature in an extension stroke of the engine 55
`is passed to the exhaust passage 30. As a result, the
`temperature of the secondary three-way catalytic de(cid:173)
`vice is elevated in a shorter period by the high tempera(cid:173)
`ture exhaust gas.
`The operation of the exhaust system shown in FIG. 1 60
`is best understood by reviewing FIGS. 2 and 3, which
`are flow charts illustrating a main routine and a subrou(cid:173)
`tine for a microcomputer of the control unit SO. Pro(cid:173)
`gramming a computer is a skill well understood in the
`art. The following description is written to enable a 65
`programmer having ordinary skill in the art to prepare
`an appropriate program for the microcomputer. The
`particular details of any such program would, of course,
`
`50
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`5,233,831
`
`8
`depend upon the architecture of the particular com·
`puter selected.
`Referring to FIG. 2, which is a flow chart of the
`exhaust system control routine, four flags, such as an
`ignition flag FS, a shutter flag FS, a heater flag FH and
`an acceleration flag FA, are used. The ignition flag FS
`is set to a state of .. 1," which indicates that the ignition
`key switch 4S is turned on, or to a state of "0," which
`indicates that the ignition key switch 4S is turned off.
`The shutter flag FV is set to a state of "1," which indi(cid:173)
`cates that the shutter valve 40 closes the branched ex·
`haust passage 3S, or to a state of "0," which indicates
`that the shutter valve 40 opens the branched exhaust
`passage 3S. The heater flag FH is set to a state of "1,"
`which indicates that the heaters 36 are activated, or to a
`state of "0," which indicates that the heaters 36 are
`inactive. The acceleration flag FA is set to a state of
`"1," which indicates that the engine is under accelera·
`tion, or to a state of "0," which indicates that the engine
`is not under acceleration.
`After initializing or resetting the ignition flag FS and
`the acceleration flag FA to their initial states of "0" at
`step S1, various necessary signals Si, Sg, etc., are read at
`step S2. Then, a decision is made st step S3 as to
`whether or not the ignition flag FS has been set to "1."
`If the answer is "NO," another decision is made at step
`S4 as to whether or not an ignition signal Si is provided.
`The ignition signal indicates that the ignition key switch
`4S is turned on to start the engine or that the engine
`starter is actuated to start engine cranking. When an
`ignition switch Si is detected, after setting the ignition
`flag FS to "1" at step SS, the sequence proceeds to step
`S6. If the answer to the decision made at step 63 is
`"YES," the sequence proceeds directly to step S6. On
`the other hand, if the decision made at step S4 is "NO,"
`then, the signals are renewed at step S2.
`At step S6, a decision is made as to whether or not the
`temperature qf the secondary catalytic dev