GB2294556A - Exhaust emission control system for internal combustion engine - Google Patents

Description

EXHAUST EMISSION CONTROL SYSTEM FOR INTERNAL COMBUSTION ENGINE 2294556 The

present invention relates to improvements in an exhaust emission control system for an internal combustion engines, and more particularly to an exhaust emission control system provided with a partially two- branch exhaust conduit and two catalytic converters.

Japanese Utility Model Provisional Publication No. 1-66420 discloses a typical ex ' haust emission control system as shown in Fig. 16. Ibis conventional exhaust emission control system is provided with an exhaust gas passage 21 which is branched into first and second passages 22 and 23. Two main catalytic converters 24 and 25 are installed in the first and second passages 22 and 23, respectively. An auxiliary catalytic converter 26 is installed upstream of the main catalytic converter 25 in the second passage 23. A connecting passage 27 connects the first and second passages 22 and 23 so as to communicate a portion between the auxiliary catalytic converter 26 and the second main catalytic converter 25 and a portion upstream of the first main catalytic converter 24. A selector valve 28 is disposed at a branch portion to the first and second passages 22 and 23 in order to select one passage from the first and second passages 22 and 23 as an exhaust gas passage. A switching valve 29 is disposed in the connecting passage 27 to select one of open and close conditions of the connecting passage 27. The selector valve 28 and the switching valve 29 are controlled according to an exhaust gas temperature detected by a temperature sensor. When the temperature of exhaust gases is lower than a predetermined temperature, the selector valve 28 is set to open the second passage 23 and to close the first passage 22, and the switching valve 29 is closed so that the exhaust gases flow through the auxiliary catalytic converter 26 and the main catalytic converter 25. When the temperature of the exhaust gases becomes higher than the predetermined value, the selector valve 28 is set to open the first passage 22 and to close the second passage 23, and the switching valve 29 is opened so that the exhaust gases flow through the main catalytic converters 24 and 25.

However, the conventional exhaust emission control system is arranged to select one of the first state where the exhaust gases flow through the auxiliary catalytic converter 26 and the second main catalytic converter 25 and the second state where the exhaust gases flow through both the first and second main catalytic converters 24 and 25 while bypassing the auxiliary catalytic converter 26, by independently operating the selector valve 28 and the switching valve 29. Therefore, a system for operating the valves 28 and 29 is complicated in structure and operation.

It would therefore be desirable to be able to provide an improved exhaust emission control system which simplifies structure and valve operation.

It would also be desirable to be able to provide an exhaust emission control system which further improves emission characteristics thereof and suppresses the degradation of elements thereof.

An exhaust emission control system according to the present invention is for an internal combustion engine and comprises an exhaust passage which includes an upstream portion, first and second branch portions connected to the upstream portion, and a downstream portion connected to the first and second branch portions. A first catalytic converter is installed in the first branch portion of the exhaust passage. A second catalytic converter is installed in the downstream portion of the exhaust passage. A selector valve is disposed at inlets of the first and second branch portions. The selector valve is operated to be set in one of a first state that the first branch portion is opened and the second branch portion is closed and a second state that the first branch portion is closed and the second branch portion is opened. A first oxygen sensor is installed upstream of the first catalytic converter in the first branch portion. A second oxygen sensor is installed upstream of the second catalytic converter in the downstream portion. An engine condition judging means judges a thermal condition of the engine. A control means controls the selector valve in one of the first and second states according to the judged thermal condition. A selecting means selects one of the outputs from the first and second oxygen sensors according the state of the selector valve as a factor for calculating a fuel injection amount of the engine.

In the drawings, like reference numerals designate like parts and elements throughout all figures, in which:

Fig. 1 is a schematic view of an exhaust emission control system according to the present invention; Fig. 2 is a flowchart which shows an operation manner of a first embodiment of the exhaust emission control system according to the present invention; Fig. 3 is a flowchart following to the flowchart of Fig. 2; Fig. 4 is a flowchart which shows the content of a subroutine A in the flowchart of Fig. 2; Fig. 5 is a flowchart which shows an operation procedure of a second embodiment of the exhaust emission control system according to the present invention; Fig. 6 is a flowchart which shows an operation procedure of a third embodiment of the exhaust emission control system according to the present invention Fig. 7 is a flowchart following to the flowchart of Fig. 6; Fig. 8 is a flowchart which shows the content of a subroutine B in Fig. 7; Fig. 9 is a flowchart which shows the content of a subroutine C in Fig. 8; Fig. 10 is a flowchart which shows an operation procedure of a fourth embodiment of the exhaust emission control system according to the present invention; Fig. 11 is a flowchart following to the flowchart of Fig. 10; Fig. 12 is a flowchart following to the flowchart of Fig. 10; Fig. 13 is a flowchart following to the flowchart of Fig. 12; Fig. 14 is a flowchart which shows the content of a subroutine D in Fig. 10; Fig. 15 is a view which shows a diagnosis process pattern of the fourth embodiment; and Fig. 16 is a schematic view of a conventional exhaust emission control system.

Referring to Figs. I to 4, there is shown a first embodiment of an exhaust emission control system of an internal combustion engine according to the present invention.

The exhaust emission control system comprises an exhaust gas conduit I connected with an internal combustion engine 100. The exhaust gas conduit I includes an upstream portion lU whose upstream end is connected to the internal combustion engine 100. A downstream end of the upstream portion IU is connected to first and second branch portions IA and IB. Downstream ends of the first and second branch portions 1A and 1B are connected with a downstream portion ID. An auxiliary catalytic converter 2 is installed in the first branch portion IA. A main catalytic converter 3 is installed in the downstream portion ID. A selector valve unit 4, which switchingly selects one of the first and second branch portions as an exhaust gas passage, is disposed in the first and second branch portions IA and IB. The selector valve unit 4 comprises a first valve disc 4A disposed in the first branch portion IA and a second valve disc 4B disposed in the second branch portion IB. The first and second valve discs 4A and 4B are connected to an operation shaft 5 so that when one of the first and second valve discs 4A and 4B is set in an open condition, the other one is set in a closed condition. The operation shaft 5 is rotated by a rotating device 6 such as a motor unit so as to selectively open the first and second branch portions IA and IB through the first and second valve discs 4A and 4B. That is, the selector valve unit 4 takes one of first and second states. When the first valve disc 4A is opened and the second valve disc is closed, that is, when the selector value unit 4 is set in the first state, the exhaust gases from the engine 100 flow to the first branch portion IA. When the first valve disc 4A is closed and the second valve disc 4B is opened, that is, when the selector valve unit 4 is set in the second state, the exhaust gases flow, to the second branch portion 1B.

A first exhaust oxygen sensor (first 02 sensor) 7 is disposed upstream of the auxiliary catalytic converter 2 in the first branch portion IA. A second exhaust oxygen sensor 8 is disposed upstream of the main catalytic converter 3 in the downstream portion 1D. A third exhaust oxygen sensor 14 is disposed downstream of the main catalytic converter 3.

The internal combustion engine 100 is controlled by a control unit 9. More particularly, the fuel injection amount to be supplied to the engine 100 is determined by the control unit 9.

The control unit 9 is electrically connected with an air flow meter 10 which outputs an intake air amount signal Qa indicative of an intake air amount, a crank angle sensor I I which outputs a signal indicative of the rotation speed Ne of the engine 100, a starter switch 12 which outputs a signal indicative of the turned-on condition of the starter switch 12, an engine coolant temperature sensor 13 which outputs a signal TWO indicative of a coolant temperature of the engine 100, and the first, second and third exhaust oxygen sensors 7, 8 and 14 which respectively outputs signals indicative of the oxygen density, that is, indicative of rich or lean mixture condition of the air/fuel ratio.

The control unit 9 receives these signals and calculates a basic fuel injection amount Tp by the equation Tp=K x Qa/Ne (K is constant). Further, the control unit 9 calculates a feedback correction coefficient a of an air/fuel ratio by varying it by means of a known proportional-plus-integral control according to an output of either the first exhaust oxygen sensor 7 and the second exhaust oxygen sensor 8.

The fuel injection amount Te is obtained by multiplying the basic fuel injection amount Tp with the airlfuel ratio feedback correction coefficient (x. The control unit 9 outputs a pulse signal which has a pulse width corresponding to the fuel injection amount Te. Further, the control unit 9 executes a feedback control of the airlfuel ratio by keeping the actual airlfuel ratio within a theoretical air/fuel ratio (stoichiometric ratio).

The control unit 9 further executes a switching control of the selector valve unit 4 according to a thermal condition of the engine 100 and a selecting control of one of the outputs of the first and second oxygen sensors 7 and 8. A flowchart of Figs. 2 and 3 shows the switching control procedure of the selector valve unit 4 and the selecting control of the outputs from the first and second oxygen sensors 7 and 8.

In a step S1, the control unit 9 judges whether a starter switch 12 is turned on or not. When the starter switch 12 is turned on, the routine proceeds to a step S2. When it is not turned on, the routine jumps to a step S3.

In the step S2, the first valve disc 4A is opened and the second valve disc 4B is closed, that is, the selector valve unit 4 is set in the first state so that the exhaust gases flow, through the auxiliary catalytic converter 2 and then through the main catalytic converter 3.

In a step S4, the control unit 9 resets a flag F1 (Fl=0). The state of Fl=0 indicates that the valve unit 4 is set in the first state.

In a step S5, the control unit 9 reads the signal TWO indicative of the coolant (water) temperature at a starting time of the engine.

In a step S6, the control unit 9 calculates a time period T1 according to the read coolant temperature indicative signal TWO (T 1 =f(TWO)).

In a step S7, a time period TIMERI counted by a first timer in the control unit 9 is reset (TIMER1=0).

In the step S3, the control unit 9 reads the output OSI of the first exhaust oxygen sensor 7 upon converting into digital signals by A/D conversion.

In a step S8, the control unit 9 read-in the output OS2 of the first oxygen sensor 8 upon converting into digital signals by A/I) conversion.

In a step S9, the control unit 9 judges whether Fl=0 or not.

When Fl=O, that is, when the selector valve unit 4 is set in the first state while the starter switch 12 is turned on, the routine proceeds to a step S10. When F Ir- 0, the routine proceeds to a step S 11.

In a step SIO, the control unit 9 compares the basic fuel injection amount Tp with a predetermined value AI. When Tp < AI, the routine proceeds to a step S12. When Tp!'!= AI, the routine proceeds to a step S13.

In the step S12, the control unit 9 compares the time period TIMERI with the predetermined value TI. When TIMERI<TI, the routine proceeds to a step S14. When TIMERI ik TI, the routine proceeds to the step S13.

In the step S14, the time period TIMERI is incremented by DT (TIMER1= TIMER1 +DT), then the routine proceeds to a step S15 at which a subroutine A is started. A detail explanation of the subroutine A will be discussed later with reference to a flowchart of Fig. 5. After the execution of the subroutine A, the routine proceeds to a step S16 wherein the control unit 9 calculates the fuel injection amount Te.

In a step S17, the control unit 9 operates fuel injection valves to inject the calculated fuel injection amount Te.

After the execution of the step S17, the routine returns to a start.

The steps SIO and S12 are steps for judging the thermal condition of the engine 100. That is, when Tp tl: AI (NO) or TIMERI h- T1 (NO), the routine proceeds to the step S13 wherein the control unit 9 controls the valve unit 4 to set in the second state where the first valve disc 4A is closed and the second valve disc 4B is opened so that the exhaust gases flow through the second branch portion 1B and the main catalytic converter 3.

In a step S18, the control unit 9 sets the flag F1 at 1 (Fl=l).

In a step S19, the time period TIMER1 of the first timer is reset (TIMER 1 =0).

In a step S20, the time period TIMER2 of a second timer in the control unit 9 is reset (TIMER2=0).

In a step S21, the control unit 9 determines a clump value ctO at an air/fuel ratio feedback correction coefficient (x (ctO=(X).

In a step S22, a value C2, indicative of the number of times that Tp->A2 or Netl=NE is satisfied, is reset (C2=0).

On the other hand, in the step 11 executed when the control unit 9 judges that F1 1; 0 in the step S9, the control unit 9 judges whether the flag F1 is set at 2 or not. When F1 x. 2, the routine proceeds to a step S23. When Fl=2, the routine proceeds to a step S24. The state Fl=2 indicates that the selector valve unit 4 is set in the first state when the engine 100 is set in a warmed, low speed, and low load condition.

In the step S23, the control unit 9 compares the basic fuel injection amount Tp with a second predetermined value A2.

When Tp<A2, the routine proceeds to a step S25. When Tp:k A2, the routine proceeds to the step S15.

In the step S25, the control unit 9 compares the engine rotation speed Ne with a predetermined value NE. When Ne<NE, the routine proceeds to a step S26. When Ne tl: NE, the routine proceeds to the step S15.

In the step S26, the control unit 9 counts up a value Cl indicative of the number of times that Tp<A2 and Ne<NE are satisfied (Ci=Ci+l).

In a step S27, the control unit 9 compares the value Cl with a predetermined value D. When Cl>I), the routine proceeds to a step S28. When Cl: D, the routine proceeds to the step S15.

In the step S28, the control unit 9 sets the selector valve unit 4 in the first state where the first valve disc 4A is opened and the second valve disc 413 is closed so that the exhaust gases flow through the auxiliary catalytic converter 2 and the main catalytic converter 3.

In a step S29, the flag F1 is set at 2 (Fl=2), that is, the control unit 9 judges that the selector valve unit 4 is set in the first state when the engine is operated in a warmed, low rotation speed, and low load condition.

In a step S30, the time period TIMER2 is reset (TIMER2=0).

In a step 531, the control unit 9 sets the clamp value (xo at the air/fuel ratio feedback correction coefficient (x (eco=(x).

On the other hand, in the step 24 which is executed when the control unit 9 judges in the step S 11 that F 1 = 2, the control unit 9 compares the basic fuel injection amount Tp with a predetermined value A2. When Tp<A2, the routine proceeds to step S31 When Tp! A2, the routine proceeds to a step S34.

In the step S33, the control unit 9 compares the engine rotation speed Ne with the predetermined value NE. When Ne<NE, the routine proceeds to the step S15. When Ne:k NE, the routine proceeds to a step S34.

In the step S34, the control unit 9 counts up the value C2 indicative of the number of times that Tp<A2 or Ne<NE is satisfied (C2=C2+1).

In a step S35, the control unit 9 compares the value C2 with a predetermined value E. When C2< E, the routine proceeds to the step S15. When C2:lf= E, the routine proceeds to the step S13 wherein the control unit 9 sets the selector valve unit 4 in the second state, as mentioned above.

Next, the subroutine A executed in the step S15 will be discussed hereinafter with reference to a flowchart of Fig. 4.

In a step S41, the control unit 9 judges whether the engine is controlled by an airlfuel ratio feedback control (1 control) or not. When the engine 100 is not controlled by the 1 control, the routine proceeds to a step S42 wherein the feedback correction coefficient cc of the airlfuel ratio is set at 1 ((x= 1).

When the engine 100 is controlled by the 1 control, the routine proceeds to a step S43 wherein the control unit 9 judges whether Fl=1 or not. When Fl=l, that is, when the engine 100 is operated in the second state, the routine proceeds to a step S44 wherein OS=OS2 so that the output OS2 of the second exhaust oxygen sensor 8 is used as an output value OS in the 1 control. When Fl=0 or 2, that is, when the engine 100 is operated in the first state, or first state during the warmed, low engine rotation speed, and low load condition, the routine proceeds to a step S45 wherein OS=OS1 so that the output OS1 of the first oxygen sensor 7 is used as an output OS in the X control.

In a step S46, the control unit 9 compares the output OS of one of the first and second oxygen sensors 7 and 8 with a slice level SI, in order to judge whether the output OS of one of the exhaust oxygen sensors 7 and 8 indicates a rich condition or lean condition. When the control unit 9 judges that the output OS of one of the oxygen sensor 7 and 8 indicates the lean condition, the routine proceeds to a step S47 wherein the control unit 9 sets the flag F2 at 0 (F2=0). When the control unit 9 judges that the output of the oxygen sensor 7, 8 indicates the rich condition, the routine proceeds to a step S48 wherein the control unit 9 sets the flag F2 at 1 (F2=1).

In a step S49, the control unit 9 judges whether FI=0 or not. When the control unit 9 judges that 171=0 indicative of the first state, the routine proceeds to a step S50 wherein the control unit 9 judges that the change (invert) of the output OS of the exhaust oxygen sensor 7, 8 between the rich and lean conditions.

When the control unit 9 judges in the step S51 that F2=0 indicative of the lean condition, the routine proceeds to a step S52 wherein the feedback correction coefficient (x of the air/fuel ratio is incremented by PL ((X=(X+PL). When the control unit 9 judges in the step S51 that F2=1 indicative of the rich condition, the routine proceeds to a step S53 wherein the feedback correction coefficient (x of the air/fuel ratio is decremented by PR (cc-- ct-PR). When the output OS of the exhaust oxygen sensor 7, 8 is not inverted between rich and lean conditions in the step S50, the routine proceeds to a step S54 wherein the control unit 9 checks the flag F2 as to whether the output OS of the exhaust oxygen sensor 7, 8 indicates rich or lean condition. When F2=0, that is, when the output OS of the exhaust oxygen sensor 7, 8 indicates the lean condition, the routine proceeds to the step S55 wherein the feedback correction coefficient (x of the air/fuel ratio is incremented by IL ((X=(X+IL). When the control unit 9 judges that F2=1 indicative of the rich condition, the routine proceeds to a step S56 wherein the correction coefficient a of the air/fuel ratio feedback is decremented by IR CX=(X-W. On the other hand, when the control unit 9 judges in the step S49 that F1 is not 0 (FI 1; 0), that is, when the thermal condition of the engine 100 is changed to the second state, the routine proceeds to the step S57 wherein the control unit 9 compares the time period TIMER2 with a predetermined value 20 T2. When TIMER2>T2, the routine proceeds to the step S50. When TIMER2 T2, the routine proceeds to a step S58. In the step S58, the time period TIMER2 is incremented by DT (TIMER2=TIMER2+DT). Following this, the routine proceeds to a step S59 wherein the feedback correction coefficient cc of the 25 air/fuel ratio is set at the clamp value cio. The processing of the steps S49, S57, S58 and S59 is for clumping the feedback correction coefficient a of the air/fuel ratio during a time delay which is caused by the switching of the output of the oxygen sensors 7 and 8 from that of the first 30 oxygen sensor 7 to that of the second oxygen sensor 8. With the thus arranged exhaust emission control system according to the present invention, the switching of the exhaust gas passage is easily carried out by controlling the selector valve unit 4. Furthermore, since the exhaust gases under the engine 35 warmed condition are supplied to the main catalytic converter 3 and not supplied to the auxiliary catalytic converter 2 and the first exhaust oxygen sensor 7, the degradation of the auxiliary catalytic converter 2 and the first exhaust oxygen converter 7 is suppressed.

Furthermore, as clear from the processing of the steps S23, S25, S26, S27 and S28, when the engine 100 is in a warmed, low speed and low load condition, the selector valve unit 4 is set in the first state so that the low temperature exhaust gases flow through auxiliary catalytic converter 2. This further improves emission characteristics. In addition, during a predetermined period for changing the utilizing output of the first and second exhaust oxygen sensors 7 and 8, the feedback correction coefficient a of the airlfuel ratio is clamped. Therefore, the 1 control of the engine 100 is stably executed even if a time delay of the output from the exhaust oxygen sensors 7 and 8 is occurred during the switching from the output of the first exhaust oxygen sensor 7 to the output of the second exhaust oxygen sensor 8.

Referring to Fig. 5, there is shown a control manner of a second embodiment of the exhaust emission control system according to the present invention. The construction of the second embodiment is the same as that of the first embodiment shown in Fig. 1. In this second embodiment, the judgement at the switching of the selector valve unit 4 is executed upon taking account of an activated condition of the second exhaust oxygen sensor 8. That is, when the engine 100 becomes warm and the second exhaust oxygen sensor 8 is active, the selector valve unit 4 is set in the second state where the exhaust gases flow through the second branch portion 1B and the main catalytic converter 3.

Further, in this second embodiment, during a condition that the engine is warmed and is operated in low engine speed and low load, a control for flowing the exhaust gases through the auxiliary catalytic converter 2 and the main catalytic convert 3 is not executed.

The control manner of the second embodiment will be discussed hereinafter with reference to Fig. 5.

In this flowchart of Fig. 5, steps S61 to S68 are the same as the steps S1 to S8 in the flowchart of Fig. 2.

Following these steps, in a step S69, the control unit 9 judges whether Fl=0 or not. When Fl=0, that is, when the engine 100 is operated in the first state, the routine proceeds to a step S70. When FI l: 0, the routine proceeds to a step S71 wherein the subroutine A of Fig. 5 is executed.

In the steps S70 and S72, the control unit 9 judges the thermal condition of the engine 100. That is, when the judgement in the step S70 is no (Tp cl A1) or when the judgement in the step S72 is no (Tp<AI and TIMERI:E' T1), the routine proceeds to a step S73 wherein the control unit 9 judges the activity of the second exhaust oxygen sensor 8. The judgement of the activity of the second exhaust oxygen sensor 8 is executed by detecting whether a predetermined time period has passed in a condition that the output voltage of the second exhaust sensor 8 is larger than a preset voltage in a predetermined engine operating condition. That is, the control unit 9 judges whether or not the second exhaust oxygen sensor 8 is active when the above-mentioned predetermined time period has passed. Also, when the engine 100 has been operated for a predetermined time period T1 which is determined from a engine coolant temperature TWO at an engine start time, the control unit 9 judges that the second exhaust oxygen sensor 8 is active.

On the other hand, when the control unit 9 judges in the step S73 that the second exhaust oxygen sensor 8 is not active, the routine proceeds to a step S74 wherein the time period TIMERI is incremented by DT (TIMERI=TIMER1 +DT). When the control unit 9 judges in the step S73 that the second exhaust oxygen sensor 8 is active, the routine proceeds to a step S75 wherein the control unit 9 sets the selector value unit 4 in the second state so that the exhaust gases flows through the second branch portion IB and the main catalytic converter 3.

In a step S76, the flag F1 is set at 1 (Fl=l) indicative the selector valve unit 4 is set in the second state.

In a step S77, the time period TIMER1 is reset (TIMER1=0).

After the execution of the subroutine A in the step S7 1, the routine proceeds to a step S78 wherein the control unit 9 calculates the fuel injection amount Te.

In a step S79, the control unit 9 operates fuel injectors to inject the calculated fuel injection amount Te.

With the thus arrange exhaust emission control system of the second embodiment according to the present invention, when the engine 100 is warmed and the second exhaust oxygen sensor 8 is active, the selector valve unit 4 is set so that the exhaust gases flow through the second branch portion 1B and the main catalytic converter 3. Therefore, the X control of the engine 100 by using the output OS2 of the second exhaust oxygen sensor 8 is not executed in the condition that the second exhaust oxygen sensor 8 is not active. This prevents the degradation of the exhaust emission.

Referring to Figs. 6 and 7, there is shown a control manner of a third embodiment of the exhaust emission control system according to the present invention. The construction of the third embodiment is the same as that of the first embodiment shown in Fig. 1. This third embodiment is arranged to determine the switching of the selector valve unit 4 upon taking account of the activity of the main catalytic converter 3. Tliat is, when the control unit 9 judges that the engine is warmed and the main catalytic converter 3 is active, the selector valve unit 4 is controlled so that the exhaust gases flow through the second branch portion IB and the main catalytic converter 3. The activity of the main catalytic converter 3 is judged according to a frequency ratio between the output of the second exhaust oxygen sensor 8 and the output of the third oxygen sensor 14.

The control manner of the third embodiment of the exhaust emission control system according to the present invention will be discussed hereinafter with reference to Figs. 6 and 7.

Steps S81, S83 and S84 are the same as the steps SI, S2 and S4 of the flowchart in Fig. 2.

In a step S85, the control unit 9 resets a flag F6 (F6=0). The state of F6=0 indicates that the read-in of the control frequency of the second exhaust oxygen sensor 8 is not finished.

In a step S86, the control unit 9 resets a flag F7 (F7=0). The state of F7=0 indicates that the read-in of the control frequency of the thirdexhaust oxygen sensor 14 is not finished.

Steps S87, S88 and S89 are the same as the steps S5, S6 and S7 of the flowchart in Figs. 2 and 3.

In a step S90, the control unit 9 resets the time period TIMER3 of a third timer (TIMER3=0).

In a step S91, the control unit 9 resets the time period TIMER4 of a fourth timer (TIMER4=0).

In a step S82, the control unit 9 reads the output OS1 of the first exhaust oxygen sensor 7 upon converting it into digital signals by AID conversion.

In a step S92, the control unit 9 reads the output OS2 of the second oxygen sensor 8 upon converting it into digital signals by AID conversion.

In a step S93, the control unit 9 reads the output OS3 of the third oxygen sensor 14 upon converting it into digital signals by AID conversion.

In a step S94, the control unit 9 judges whether Fl=0 or not. When F=O, that is, when the selector valve unit 4 is set in the first state, the routine proceeds to a step S95. When aO, the routine proceeds to a step S96 wherein a subroutine B is executed.

In the step S95, the control unit 9 compares the basic fuel injection amount Tp with the predetermined value Al. When Tp<A1, the routine proceeds to a step S98. When Tp tl= Al, the routine proceeds to a step S97.

In the step S98, the control unit 9 compares the time period TIMER1 and the predetermined value TI. When TIMERI<T1, the routine proceeds to a step S99. When TIMERI:k TI, the routine proceeds to the step S97.

In the step S99, the time period TIMERI is incremented by DT (TIMIER1=TIMIERI+DT). Following this, in the step S96, the subroutine B is executed according to a flowchart shown in Fig. 8.

Then, in a step S100, the control unit 9 calculates the fuel injection amount Te.

In a step S101, the control unit 9 operates fuel injectors to inject the calculated fuel injection amount Te.

In this flowchart in Figs. 6 and 7, the judgement of the thermal condition of the engine 100 is executed in the steps S95 and S98. That is, when Tp k AI or TIMER1 k TI, the routine proceeds to the step S97 wherein the control unit 9 judges whether all of the first, second, and third exhaust oxygen sensors 7, 8 and 14 are active or not. When the judgement in the step S97 is no, that is, when either of the first, second, and third exhaust oxygen sensors7, 8, and 14 is not active, the routine proceeds to a step S99. When all of the exhaust oxygen sensors 7, 8 and 14 are active, the routine proceeds to a step S 102.

In the step S102, the control unit 9 judges whether a control frequency ND2 of the second exhaust oxygen sensor 8 has been read or not, that whether F6= 1 or not. When F6= 1 indicative that the read-in of the control frequency ND2 has been finished, the routine proceeds to a step S103. When F6=0 indicative that such read-in has not been finished, the routine proceeds to a step S99.

In the step S104, the control unit 9 calculates a frequency ratio (or hertz rate: HZR) between the control frequency ND2 of the second exhaust oxygen sensor 8 and a control frequency ND3 of the third exhaust oxygen sensor 14 (HZR=ND31ND2).

In a step S105, the control unit 9 compares the herts rate HZR with a predetermined value B. When HZR<B, that is, when the control unit 9 judges that the main catalytic converter 3 is activated, the routine proceeds to a step S106. When HZR tl:B, that is, when the control unit 9 judges that the main catalytic converter 3 is not activated, the routine proceeds to the step S99.

In the step S106, the control unit 9 controls the selector valve unit 4 in the second state so that the exhaust gases flow through the second branch portion 1B and the main catalytic converter 3. That is, when the control unit 9 judges that the engine 100 is warmed and the main catalytic converter 3 is activated, the exhaust gases bypass the first branch portion IA including the auxiliary catalytic converter 2 and flow through the 10 second branch portion IB and the main catalytic converter 3.

In a step S107, the control unit 9 sets the flag F1 at 1 (Fl=l) and stores it. The state of Fl=1 indicatives that the selector valve unit 4 is set in the second state.

In a stem S108, the control unit 9 resets the time period 15 TIMERI of the first timer (TIMER1=0).

Next, the subroutine B in the step S96 will be discussed with reference to a flowchart of Fig. 8.

In a step S111, the control unit 9 judges whether the engine 100 is in the 1 control condition or not. When the engine 20 100 is in the 1 control condition, the routine proceeds to a step S113. When it is not in the 1 control condition, the routine proceeds to a step S112 wherein the correction coefficient cc is set at 1 (ct= 1).

In the step S113, the control unit 9 compares the output OSI of the first exhaust oxygen sensor 7 with a slice level SLI.

When OSI<M, that is, when the output OSI indicates the lean condition, the routine proceeds to a step S114 wherein the flag F3 is reset (F3=0). When OSI ik SL1, that is, when the output OS1 indicates the rich condition, the routine proceeds to a step S115 wherein the flag F3 is set at 1 (F3=1).

In a step S116, the control unit 9 compares the output OS2 of the second exhaust oxygen sensor 8 with a slice level SL2. When OS2<SU, that is, when the output OS2 indicates the lean condition, the routine proceeds to a step S117 wherein the flag F4 is reset (F3=0). When OS2 tl- SL2, that is, when the output OS2 indicates the rich condition, the routine proceeds to a step S118 wherein the flag F4 is set at 1 (F4=1).

In a step S119, the control unit 9 judges whether Fl=0 or not. When Fl=l, that is, when the selector valve unit 4 is set in the second state, the routing proceeds to a step S120 wherein a flag F5 indicative of the oxygen rich-lean condition of the oxygen sensor used in the 1 control is set at the flag F4 (F5=F4). When Fl=0, that is, when the selector valve unit 4 is set in the first state, the routine proceeds to a step S121 wherein the flag F5 is set at the flag F3 (F5=F3).

In a step S122, the control unit 9 judges whether Fl=0 or not. When Fl=0, the routine proceeds to a step S123 wherein the control unit 9 judges.whether the output of the oxygen sensor 7, 8 used in the 1 control is inverted or not, that is, whether the flag F5 is inverted or not. When it is judged in the step S 123 that the output is inverted, the routine proceeds to a step S124 wherein the control unit 9 judges whether F5=0 or not. When F5=0, that is, when the output of the oxygen sensor for the 1 control indicates the lean condition, the routine proceeds to a step S125 wherein the feedback correction coefficient cc of the airlfuel ratio is incremented by PL (cc=cc+PL)., When F5=1, that is, when the output of the oxygen sensor for the 1 control indicates the rich condition, the routine proceeds to a step S126 wherein the feedback correction coefficient cc is decremented by PR (a=ct-PR).

When it is judged in the step S123 that the output of the oxygen sensor for the 1 control is not inverted, the routine proceeds to a S127 wherein the control unit 9 judges whether the flag F5 is set at 0 or not. When F5=0, that is, when the output of the oxygen sensor indicates the lean condition, the routine proceeds to a step S128 wherein the feedback correction coefficient (x of the air/fuel ratio is incremented by IL ((x=ct+IL).

When F5=1, that is, when the output of the oxygen sensor indicates the rich condition, the routine proceeds to a step S129 wherein the feedback correction coefficient cc is decremented by IR (cc=(X-IR).

On the other hand, when it is judged in the step S 122 that F 1A0, that is, when the state of the selector valve unit 4 is changed to the second state, the routine proceeds to a step S130 wherein the control unit 9 compares TIMER2 with the predetermined value T2. When TIMER2>T2, the routine proceeds to the step S123. When TIMER2 =5 T2, the routine proceeds to a step S131.

In the step S131, the time period TIMER2 is incremented by DT (TIMER2=TIMER2+DT).

In a step S132, the airlfuel ratio feedback correction coefficient (x is set at the clump value (xo (ec=ao).

In a step S134, the control unit 9 judges whether Fl=0 or not. When Fl=O, that is, when the selector valve unit 4 is set in the first state, the routine proceeds to a step S135 wherein a subroutine C is executed. This subroutine C is for reading the hertz rate (HZR) and will be discussed with reference to a flowchart of Fig. 9.

In a step S141, the control unit 9 judges whether the rich-lean condition (mixture condition) is inverted according to the content of the flag 4 indicative of one of the rich and the lean conditions of the output OS2 of the second exhaust oxygen sensor 8. When it is judged that the rich or lean condition is not inverted, the routine proceeds to a S142 wherein the time period TIMER3 is incremented by DT (TIMER3=TIMER3+DT). Following this, the routine proceeds to a step S143. When it is judged in the step S141 that the rich-lean condition is inverted, the routine proceeds to a step S144 wherein a value N2 indicative of the number of times that the output OS2 of the second exhaust oxygen sensor 8 is inverted, is incremented by 1 (N2=N2+1).

Following this, the routine proceeds to a step S145 wherein the control unit 9 judges whether the time period TIMER3 becomes larger than a predetermined time period such as 20 sec. or not. When TIMER120 sec., the routine proceeds to the step S142. When TIMER3>20 sec., the routine proceeds to a step S146 wherein the value N2 indicative of the number of times of the inversion of the output OS2 is read as the control frequency ND2.

In a step S147, the flag F6 is set at 1 so as to indicate that the reading of the control frequency of the second exhaust oxygen sensor 8 is finished (F6=1).

In a step S 148, the time period TIMER3 is reset (TIMER3=0).

In a step S149, the value N2, indicative of the number of times that the output OS2 of the second exhaust oxygen sensor 8 is inverted, is reset (N2=0).

Following to one of the steps S142 and S149, in the step S143, the control unit 9 judges whether or not the rich-lean condition is changed according to the flag F8 which indicates the rich-lean condition of the output OS3 of the third exhaust oxygen sensor 14. When it is judged that the condition is not changed, the routine proceeds to a step S150 wherein the time period TIMER4 is incremented by DT (TIMER4= TIMER4+DT). When it is judged that the rich-lean condition is changed, the routine proceeds to a step S151 wherein a value N3, indicative of the number of times that the inversion of the output OS3 of the third exhaust oxygen sensor 14 is inverted, is incremented by 1 (N3=N3+1).

In a step S152, the control unit 9 judges whether or not the time period TIMER4 becomes larger than a predetermined time period such as 20 sec. When TIMER4--"::,20sec, the routine proceeds to a step S150. When TIMER4>2Osec, the routine proceeds to a step S153 wherein the control unit 9 reads a parameter N3 indicative of the number of times of the inversion of the output OS3 of the third exhaust oxygen sensor 14 as the control frequency ND3.

In a step S154, a flag F7 is set at 1 (F7=1) so as to indicate that the reading of the control frequency ND3 of the third exhaust oxygen sensor 14 is finished.

In a step S155, the time period TIMER4 is reset 3 5 (TIMER4=0).

In a step S156, the value N3 is reset (N3=0).

With the thus arranged exhaust emission control system of the third embodiment according to the present invention, the selector valve unit 4 is set in the second state when the control unit 9 judges that the engine 100 is in a warmed condition and the main catalytic converter 3 is activated. This prevents the degradation of the exhaust emission.

Referring to Figs. 10 to 15, the fourth embodiment of the exhaust emission control system according to the present invention will be discussed. The construction of the fourth embodiment is the same as that of the first embodiment shown in Fig. 1. The fourth embodiment is arranged to diagnose a normality of operation of the selector valve unit 4. The control manner of the fourth embodiment of the exhaust emission control system according to the present invention will be discussed with reference to flowcharts in Figs. 10 to 14.

In a step S201, the control unit 9 judges whether the starter switch 12 is turned on or not. When the starter switch 12 is turned on, the routine proceeds to a step 202. When the starter switch 12 is not turned on, the routine proceeds to a step S203.

In the step S202, the control unit 9 sets the selector valve 4 in the first state where the first valve disc 4A is opened and the second valve disc 4B is closed so that the exhaust gases from the engine 100 flow through a first branch portion IA and the auxiliary catalytic converter 2 and to the main catalytic converter 3.

In a step S204, all of flags F11 to F18 and values C, N1, N2 and N are reset (F11=0, F12=0, F13=0, F14=0, F15=0, F16=0, F17=0, F18 =0, C=O, N1=0, N2=0, N=O).

In a step S205, the time period TIMERI is reset (TIMER1=0).

In a step S206, the time period TIMER2 is set infinite (TIMER2= oo).

In the step S203, the control unit 9 judges whether the engine 100 is put in a cool condition or warmed condition. When it is judged that the engine 100 is yet cool, the routine proceeds to a step S207. When it is judged that the engine 100 is warmed, the routine proceeds to a step S208.

In the step S207, the flag F17 is reset (F17=0) so as to indicate that the output OSI of the first exhaust oxygen sensor 7 is used in the 1 control.

In a step S209, the control unit 9 judges whether FI 1=0 or not. The state of FI 1=0 indicates that the first diagnosis is not finished. When Fl l=0, that is, when it is judged that the first diagnosis is not yet finished, the routine proceeds to a step S210. When F11=1, that is, when it is judged that the first diagnosis is finished, the routine proceeds to a step S211 wherein a subroutine D is executed.

In the step S210, the control unit 9 compares the time period TIMER1 with a predetermined value Ta. When TIMERI<Ta, the routine proceeds to a step S212. When TIMERIkTa, the routine proceeds to a step S213.

In the step S212, the control unit 9 judges whether the engine 100 is put in the 1 control condition or not. When it is judged that the engine 100 is put in the X control condition, the routine proceeds to a step S214. When it is judged that the engine 100 is not put in the 1 control condition, the routine directly proceeds to the step S211.

After the execution of the subroutine D in the step S211, the routine proceeds to a step S215 wherein the fuel injection amount is calculated.

In a step S216, the fuel injection is executed by operating 30 the fuel injectors.

In the step S213, the first diagnosis is executed by comparing a value NI indicative of the number of times of inversion of the rich-lean condition obtained from the first exhaust oxygen sensor 7 with a standard value A.

That is, when the starter switch 12 is turned on and the engine 100 is in the cool condition, the control unit 9 sets the selector valve unit 4 in the first state where the first valve disc 4A is opened and the second valve disc 4B is closed so that the exhaust gases flow through the first branch portion IA, the auxiliary catalytic converter 2, and the main catalytic converter 3. However, if the value N1 is smaller than the standard value A, it is afraid that the selector valve unit 4 may be in trouble and is not correctly operated. Accordingly, when N1:kA in the step S213, that is, when it is judged that the selector valve unit 4 is correctly (normally) operated, the routine proceeds to a step S217 wherein a flag F13 is reset (F13=0) so as to indicate that the selector valve unit 4 is correctly set in the first state. Following this, the routine proceeds to a step S218 wherein the control unit 9 diagnoses that the selector valve unit 4 is correctly set in the first state, that is, the control unit 9 diagnoses the normality of the operation of the selector valve unit 4 to the first state.

On the other hand, when M<A in the step S213, that is, when it is judged that the selector valve unit 4 is not correctly set in the first state, the routine proceeds to a step S219 wherein the flag F13 is set at 1 (F13=1) so as to indicate that the selector valve unit 4 is not correctly set in the first state. Following this, the routine proceeds to a step S220 wherein the control unit 4 diagnoses that the selector valve unit 4 is not correctly set in the first state, that is, the selector valve unit 4 is in trouble as to the operation of the selector valve unit 4 into the first state.

In a step S221, the flag F11 is set at 1 (Fll=l) so as to indicate that the first diagnosis is finished.

In a step S222, the control unit 9 finishes the first diagnosis. Following this, the routine proceeds to the step S211.

When the control unit 9 judges in the step S203 that the engine 100 is in a warmed condition, the routine proceeds to the step S208 shown in Fig. 11 wherein it is judged whether F12=0 or F12=1. When F12=0, that is, when it is judged that it is not necessary to execute a third diagnosis, the routine proceeds to a step S224. When F12=1, that is, when it is judged that it is necessary to execute the third diagnosis, the routine proceeds to a step S223.

In the step S224, the control unit 9 judges whether the thermal condition of the engine 100 is changed from the cool condition to the warmed condition or not. When the judgement in the step S224 is no, the routine proceeds to a step S 225. When the judgement in the step S224 is yes, the routine proceeds to a step S226.

In the step S226, the control unit 9 sets the selector valve 4 in the second state where the first valve disc 1A is closed and the second valve disc 113 is opened so that the exhaust gases flow through the second branch portion IB and the main catalytic converter 3.

In a step S227, the flag F17 is set at 1 (F17=1) so as to indicate that the output OS2 of the second exhaust oxygen sensor 8 is used in the 1 control.

In a step S228, the time period TIMERI is reset (TIMER 1 =0).

In a step S229, the time period TIMER2 is reset (TIMER2=0).

In a step S230, the value NI, indicative of the number of times that the output OSI of the first exhaust oxygen sensor 7 is inverted, is reset (N1=0).

In the step S225, wherein it is judged whether F15=0 or F15=1. When F15=0, that is, when it is judged that a second diagnosis is not finished, the routine proceeds to a step S231.

When F15=1, that is, when it is judged that the second diagnosis is finished, the routine proceeds to the step S211.

In the step S231, the control unit 9 compares the time period TIMERI with a predetermined value Tb. When TIMER1<Tb, the routine proceeds to a step S232. When TIMERLL12-Th, the routine proceeds to a step S233.

In the step S232, the control unit 9 judges whether the engine 100 is in the 7, control condition or not. When the judgement in the step S232 is yes, the routine proceeds to a step S234 wherein the time period TIMER1 is incremented by DT (TIMERI= TIMER1+DT). Following this, the routine proceeds to the step S21 1. When the judgement in the step S232 is no, the routine proceeds to the step S21 1.

In the step S233, the control unit 9 executes the second diagnosis by comparing the value N2 indicative of the number of times of the inversion between the rich and lean conditions of the output OS2 of the second, exhaust oxygen sensor 8 with a 10 standard value B. That is, in addition to the first diagnosis, when the selector valve unit 4 is set in the second state where the first valve disc 1A is closed and the second valve disc IB is opened, the control unit 9 checks the value N2. When the value N2 is smaller than a standard value B (N2<B), it is afraid that the selector valve unit 4 may not be correctly set in the second state. When N2tkB in the step S233, that is, when it is judged that the selector valve unit 4 is correctly set in the second state, the routine proceeds to a step S235 wherein a flag F14 is reset (F14--0) so as to indicate that the 20 selector valve unit 4 is correctly set in the second state. Following this, the routine proceeds to a step S236 wherein the control unit 9 judges whether F13=0 or F13=1. When F13=0, that is, when the selector valve unit 4 is correctly set in the first state, the routine proceeds to a step S237 wherein the control unit 9 25 finishes the diagnosis. Further, the routine proceeds to a step S239. When F13=1, that is, when the selector valve unit 4 is not correctly set in the first state, the routine proceeds to a step S238 wherein the flag F12 is set at 1 (F12=1) so as to indicate that it is necessary to execute the third diagnosis. Following this, the 30 routine proceeds to a step S239.

On the other hand, when N2<B in the step S233, that is, when it is afraid that the selector valve unit 4 may not be correctly set in the second state, the routine proceeds to a step S240 wherein the flag F14 is set at 1 (F14=1) so as to indicate that the selector valve unit 4 is not correctly set in the second state. Following this, the routine proceeds to a step S241 wherein the diagnosis is finished. Following this, the routine proceeds to a step S242 wherein the control unit 9 judges that the selector valve unit 2 goes wrong and turns on some alarming means such as an alarm lamp. Then, the routine proceeds to the step S239. In the step S239, the flag F15 is set at 1 (F15=1) in order to indicate that the second diagnosis is finished. Following this, the routine proceeds to a step S211. As clear from the execution of the steps S233, S240, S241 10 and S242, regardless of the result of the first diagnosis, when the result of the second diagnosis indicates that the selector valve unit 4 is not correctly operated, the control unit 9 judges that the selector valve unit 4 goes wrong. Also, as clear from the execution of the steps S233, S235, S236 and S237, when both of the first diagnosis and the second diagnosis indicate a normal condition that the correct or normal operation of the selector valve unit 4 is executed, the control unit 9 judges that the selector valve unit 4 is correctly operated. Further, as clear from the execution of the steps S233, 20 S235, S236 and S238, when the result of the first diagnosis indicates abnormal condition of the selector valve unit 4 and the result of the second diagnosis indicates the normal condition, the control unit 9 executes the third diagnosis. That is, in the step S238, the flag F12 is set at 1 (F12=1). Then, the routine proceeds to the step S21 1 through the step S239. When the judgement in the step S208 is no, the routine proceeds to the step S223 of Fig. 12 wherein the control unit 9 judges whether F16=0 or F16=1. When F16=0, that is, when it is judged that the third diagnosis is not executed, the routine 30 proceeds to a step S243. When F16=1, that is, when it is judged that the third diagnosis has been executed, the routine proceeds to a step S244. In the step S243, the control unit 9 compares the basic fuel injection amount Tp with a predetermined value Tpl. When Tp<Tpl, the routine proceeds to a step S245. When Tp:Lk-Tpl, the routine proceeds to a step S246.

In the step S245, the control unit 9 compares the engine rotation speed Ne with a predetermined value Nel. When Ne<Nel, the routine proceeds to a step S247. When Ne:k-Nel, the routine proceeds to the step S246.

In the step S247, the control unit 9 counts up a value C indicative of the number of times that Tp<Tpl and Ne<Nel are satisfied. (C;zC-1) In a step S248, a value E, indicative of the number of times that Tp<Tpl, or Ne<Nel is satisfied, is reset (E=O).

In a step S249, the control unit 9 compares the value C with a predetermined value' Cl. When C>C1, the routine proceeds to a step S250. When C =r. Cl, the routine proceeds to the step S211.

In the step S250, the control unit 9 sets the selector valve unit 4 in the first state.

In a step S251, the flag F17 is reset (F17=0) in order to indicate the output OSI of the first exhaust oxygen sensor 7 is used as an output value in the 1 control.

In a step S252, the control unit 9 judges whether the flag F17 is inverted or not. When the flag F17 is inverted, the routine proceeds to a step S253. When the flag F17 is not inverted, the routine proceeds to a step S254.

In the step S253, the time period TIMER2 is reset (TIMER2=0).

In a step S255, the value N2, indicative of the number of times that the output OS2 of the second exhaust oxygen sensor 8 is inverted, is reset (N2=0).

In a step S256, the time period TIMERI is reset (TIMER1=0).

In the step S254, the control unit 9 compares the time period TIMERI with a predetermined value Tc. When TIMERI<Tc, the routine proceeds to a step S257. When TIMERItl:Tc, the routine proceeds to a step S258.

In the step S257, the control unit 9 judges whether the engine 100 is in the 1 control condition or not. When the engine is in the 1 control condition, the routine proceeds to a step S259 wherein the time period TIMERI is incremented by DT (TIMER 1 =TIMER 1 +DT). When the engine 100 is not in the 1 control condition, the routine directly proceeds to the step S211.

In the step S258, the control unit 9 executes the third diagnosis by comparing the value NI with the standard value D.

That is, in case that the result of the second diagnosis is not clear, when the engine is warmed and operated in lower speed and low load for a predetermined time period, the control unit 9 checks the value NI by forcibly setting the selector valve unit 4 in the first state. If the value NI is smaller than the standard value D, it is afraid that the exhaust gases may not flow through the auxiliary catalytic converter 2 due to the trouble of the selector valve unit 4. Accordingly, in the step S258, the value NI is compared with the standard value D. When NlkI), that is, when the selector valve unit 4 is normally operated, the routine proceeds to a step S260 wherein the control unit 9 judges that the selector valve unit 4 is correctly operated. Then, the routine proceeds to a step S261.

On the other hand, when NI<J), that is, when it is afraid that the selector valve unit 4 may be incorrectly operated, the routine proceeds to a step S262 wherein the control unit 9 judges that the selector valve unit 4 is incorrectly operated. Following this, the routine proceeds to a step S263 wherein the alarm lamp is turned on. Then, the routine proceeds to the step S261 wherein the diagnosis is finished.

In the step S261, the control unit 9 finishes the third diagnosis.

Following this, the routine proceeds to a step S264 wherein the flag F16 is set at 1 (F16=1) in order to indicate that the third diagnosis has been finished. Then, the routine proceeds to the step S21 1.

When it is judged in the step S243 that Tpt':TpI or judged in the step S245 that NeNel, the routine proceeds to a step S246 wherein the value E, indicative of the number of times of a condition that Tp->TpI or Neel:Nel, is counted up (E--E+I).

In a step S265, the control unit 9 compares the value E with a predetermined value El. When R5El, the routine proceeds to a step S266 where the value C is reset (C=O). Then, the routine proceeds to a step S267 wherein the value N2 is reset (N2=0).

Further, in a step S268, the time period TIMERI is reset (TIMER1=0). Following this, the routine proceeds to the step S211.

When E>E1 in the step S265, the routine proceeds to a step 269 wherein the control unit 9 sets the selector valve unit 4 in the second state.

In a step S270, the flag F17 is set at 1 (F17=1) in order to indicate that the output OS2 of the second exhaust oxygen sensor 8 is used as an output value in the 1 control.

In a step S271, the control unit 9 judges whether the flag F17 is inverted or not. When the flag F17 is inverted, the routine proceeds to a step S271 wherein the time period TIMER2 is reset (TIMER2=0). Then, the routine proceeds to a step S266. On the other hand, when it is judged in the step S223 that F16=1, the

routine proceeds to a step S244 wherein the control unit 9 judges whether the flag F16 is inverted or not. When the flag F16 is inverted, the routine proceeds to a step S273 wherein the control unit 9 sets the selector valve unit 4 in the second state.

In a step S274, the flag F17 is set at 1 (F17=1).

In a step S275, the time period TIMER2 is reset (TIMER2=0). Then, the routine proceeds to the step S211.

When the flag F16 is not inverted in the step S244, the routine directly proceeds to the step S211.

Next, the manner of operation by the subroutine D will be discussed hereinafter with reference to the flowchart in Fig. 14.

In a step S281, the control unit 9 judges whether the engine is in the 1 control condition or not. When the engine 100 is not in the 1 control condition, the routine proceeds to a step S228 wherein the correction coefficient (x of the airlfuel ratio feedback is set at 1 (cc = 1).

When the engine 100 is in the 1 control condition, the routine proceeds to a step S283 wherein the control unit 9 judges whether F17=0 or not. When F17=0, that is, when the output OSI of the fiist exhaust oxygen sensor 7 is used as an output value OS for the 1 control, the routine proceeds to a step S284 wherein the value OS is set at OSI (OS=OSI) in order to use the output OSI as the output value OS. When F17=1, that is, when the output OS2 of the second exhaust oxygen sensor 8 is used as the output value in the X control, the routine proceeds to a step S285 wherein OS is set at OS2 (OS=OS2) in order to use the output OS 1 as the output value OS.

In a step S286, the control unit 9 compares the output OS of the oxygen sensor with a slice level SL in order to judge the rich or lean condition of the output of the oxygen sensor. When it is judged that the output indicates the lean condition, the routine proceeds to a step S287 wherein the flag F18 is reset (F18=0) in order to indicate that the output of the oxygen sensor indicates the lean condition. When it is judged that the output of the oxygen sensor indicates the rich condition, the routine proceeds to a step S288 wherein the flag F18 is set at 1 (F18=1) in order to indicate that the output value OS of the 1 control indicates the rich condition.

In a step S289, the control unit 9 compares the time period TIMER2 with a predetermined value T2. When TIMER2T2, that is, when the time period TIMER2 is within a clamping time of the feedback correction coefficient (x of the airlfuel ratio, the routine proceeds to a step S291 wherein the time period TIMER2 is incremented by DT (TIMER2=TIMER2+DT). Then, the routine proceeds to a step S282.

When TIMER2>T2 in the step S289, that is, when the time period is over the clamping time of the feedback correction coefficient (x, the routine proceeds to a step S290 wherein the control unit 9 judges whether the flag F18 is inverted or not.

When it is judged that the flag F18 is inverted, the routine proceeds to a step S292 wherein the value N indicative of the number of the inversion times of the output OS for the 1 control is counted up (N=N+l). Then, the routine proceeds to a step S293.

In the step S293, the control unit 9 judges whether F18=0 or not, in order to judge the rich-lean condition of the output of the oxygen sensor. When F18=0, that is, when it is judged that the output of the oxygen sensor indicates the lean condition, the routine proceeds to a. step S294 wherein the correction coefficient (x is incremented by PL (0t=ct+PL). When F18=1, that is, when it is judged that the output of the oxygen sensor indicates the rich condition, the routine proceeds to a step S295 wherein the correction coefficient (x is decremented by PR ((x=cc-PR).

In a step S296, the control unit 9 judges whether F18=0 or not, in order to judge the rich-lean condition of the output of the oxygen sensor. When F18=0, that is, when it is judged that the output OS for the 1 control indicates the lean condition, the routine proceeds to a step S297 wherein the correction coefficient cc is incremented by IL (cc=()'+IL). When F18=1., that is, when it is judged that the output OS of the 1 control indicates the rich condition, the routine proceeds to a step S298 wherein the correction coefficient a is decremented by IR (ct=(x-IR).

With the thus arranged exhaust emission control system of the fourth embodiment according to the present invention, there is provided with a function for diagnosing the operation of the selector valve unit 4 according to at least one of the number of the inversion times of the output of the first exhaust oxygen sensor and the number of the inversion times of the output of the second exhaust oxygen sensor 8. Accordingly, even if the selector valve unit 4 goes wrong and generates some incorrect operations, it becomes possible to detect such incorrect operation of the selector valve unit 4. Therefore, it becomes possible to previously prevent the degradation of the catalytic converters and the exhaust oxygen sensors. Consequently, the degradation of the characteristics of the exhaust emission is prevented.

More particularly, the exhaust emission control system is provided with the first, second, and third diagnoses for checking the normality of the selector valve unit 4. In the first diagnosis, the normality of the selector valve unit 4 is judged according to the number of the inversion times of the output of the first exhaust oxygen sensor 7, in case that the starter switch is turned on, the engine 100 is in the cool condition, and the selector valve unit 4 is set in the first state. In the second diagnosis, the normality of the selector valve unit 4 is judged according to the number of the inversion times of the output of the second exhaust oxygen sensor 8, in case that the engine 100 is warmed and the selector valve unit 4 is set in the second state. In the third diagnosis, the normality of the selector valve unit 4 is judged according to the number of the inversion times of the output of the first exhaust oxygen sensor 7 by forcibly setting the selector valve unit 4 in the first state when the engine 100 is kept for a predetermined time period in the warmed, low rotation speed,and low load condition.

Therefore, as shown in Fig. 15, regardless of the result of the first diagnosis, when it is diagnosed in the second diagnosis that the selector valve unit 4 goes wrong, the control unit 9 obtains a final diagnosis result that the selector valve unit 4 goes wrong (NG). Further, when it is diagnosed in both of the first and second diagnoses that the selector valve unit 4 is correctly operated (OK), the control unit 9 obtains a final result that the selector valve unit 4 is correctly operated (OK). These double diagnoses further increases the liability of the exhaust emission control system. Additionally, in case that the result of the first diagnosis is that the selector valve unit 4 goes wrong (NG) and the result of the second diagnosis is that the selector valve unit 4 is correctly operated (OK), it is impossible to obtain a final diagnosis result as it is. Therefore, in this case, the third diagnosis is executed to obtain final result as to whether the selector valve unit 4 is correctly operated or not. This diagnosis method including the first, second, and third diagnoses largely improves the diagnosis ability.

3

Claims (1)

1. Claims: 1 1. An exhaust emission control system for an internal 2

   combustion engine, comprising: an exhaust passage including an upstream portion, first and 4 second branch portions connected to the upstream portion, and a 5 downstream portion connected to the first and second branch 6 portions; 7 a first catalytic converter installed in the first branch 8 portion of said exhaust passage;

   9 11 a second catalytic converter installed in the downstream portion of said exhaust passage; a selector valve disposed at inlets of the first and second 12 branch portions, said.selector valve being operated to be set in 13 one of a first state that the first branch portion is opened and the 14 second branch portion is closed and a second state that the first branch portion is closed and the second branch portion is opened; 16 a first oxygen sensor installed between said selector valve 17 and said first catalytic converter, in the first branch portion; 18 a second oxygen sensor installed upstream of said second 19 catalytic converter, in the downstream portion; an engine -condition judging means for judging a thermal 21 condition of the engine;

   22 3 a control means for controlling said selector valve in one of 23 the first and second states according to the judged thermal 24 condition; and a selecting means for selecting one of the outputs from said 26 first and second oxygen sensors according the state of said 27 selector valve as a factor for calculating a fuel injection amount of 28 the engine.

   1 2. An exhaust emission control system as claimed in Claim 1, 2 wherein the airlfuel ratio of the internal combustion engine is controlled by feedback control according to a theoretical air/fuel 4 ratio by calculating a basic fuel injection amount from an intake air amount and a rotation speed of the internal combustion 6 engine, by calculating a correction coefficient of the basic fuel 7 injection amount from the output of one of said first and second 8 oxygen sensors, and by calculating a fuel injection amount by 9 multiplying the correction coefficient with the basic fuel injection amount.

   1 3. An exhaust emission control system as claimed in Claim 1 or 2, 2 wherein said selecting means selects the output from said first 3 oxygen sensor when said selector valve unit is set in the first 4 state, and selects the output from said second oxygen sensor when said selector valve unit is set in the second state.

   1 4.

   2 An exhaust emission control system as claimed in any preceding claim, comprising a second engine- condition judging means 3 for judging that the engine is in a low speed and low load 4 condition, said control means controlling said selector valve into the first state when the first azineconditionjudging means judges 6 that the engine is in a warmed condition and the second 7 engine- condition judging means judges that the engine is in the 8 low speed and low load condition.

   1 5. An exhaust emission control system as claimed in-any-preceding 2 claim. comprising a sensor activity judging means for judging 3 the activity of said second oxygen sensor, said control means 4 controlling said selector valve unit into the second state when the 5 (first) engindition j means j that the engine is in a 6 warmed condition and said sensor activity judging means judges that said second oxygen sensor is active.

   2 6. An exhaust emission control system as claimed in any preceding claim comprising a converter activity judging means for judging 3 that said second catalytic converter is active, said controlling 4 means controlling said selector valve unit into the second state when the (first) engine-condition judgiry mmns Judges that the engine is in a 6 warmed condition and said converter activity judging means 7 judges that said second catalytic converter is active.

   3 4 7. An exhaust emission control system as claimed in Claim 6, 2 further comprising a third oxygen sensor disposed downstream of said second catalytic converter in the downstream portion of said exhaust passage, said converter activity judging means judging the activity of said second catalytic converter according to a frequency ratio between the output of said second exhaust 7 oxygen sensor and the output of the third oxygen sensor.

   1 8. An exhaust emission control system as claimed in any 3 4 7 preceding claim, further comprising:

   a first comparing means for comparing the output of said first oxygen sensor with a first standard value; a second comparing means for comparing the output of said 6 second oxygen sensor with a second standard value; and a valve operation diagnosing means for diagnosing 8 operation of said selector valve according to one of the results of 9 said first comparing means and said second comparing means.

   2 3 7 8 9 1 9. An exhaust emission control system as claimed in Claim 8, wherein said first comparing means compares the actual output of said first oxygen sensor during the actual operation of said 4 selector valve with the first standard value, which is an output of 5 said first oxygen sensor during normal operation of said 6 selector valve, and said second comparing means compares the actual output of said second oxygen sensor during the actual operation of said selector valve with the second standard value, which is an output of said second oxygen sensor during normal operation of said selector valve.

   1 2 10. An exhaust emission control system as claimed in Claim 8, wherein said valve operation diagnosing means comprises a first 11 12 3 diagnosing means, a second diagnosing means, and a third 4 diagnosing means, said first diagnosing means diagnosing the 5 normality of the operation of said selector valve according to the 6 output of said first oxygen sensor when the engine is in a 7 warmed condition and said selector valve is to be set in the first 8 state, said second diagnosing means diagnosing the normality of 9 the operation of said selector valve according to the output of said second oxygen sensor when the engine is in a warmed condition and said selector valve is to be set in the second state, said third diagnosing means diagnosing the normality of the 13 operation of said selector valve according to the output of said 14 first oxygen sensor when the engine is in a warmed, low speed, 15 and low load condition and when said selector valve is forcibly 16 set in the first state.

   11. An exhaust emission control system as claimed in Claim 10, 2 wherein said valve operation diagnosing means diagnoses that 3 said selector valve is normally operated when both of said first 4 diagnosing means and said second diagnosing means diagnose 5 that said selector valve is normally operated, said valve 6 operation diagnosing means diagnosing that said selector valve is 7 not normally operated when said second diagnosing means 8 diagnoses that said selector valve is not normally operated, or 9 when both of said first diagnosing means and said third 10 diagnosing means diagnose that said selector valve is not 11 normally operated.

   1 12. An et emission control system as claimed in any preceding claim, wherein 2 - the (fi-st) engine -condition judging means judges the engine 3 thermal condition on the basis of engine coolant temperature 4 at a start time of the engine.

   1 13. An exhaust control system substantially as described with reference to any of the embodiments illustrated in Figures 1 to 15 of the accompanying drawings.