VWGoA - Ex. 1008
`Volkswagen Group of America, Inc. - Petitioner
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`1
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`

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`U. S. Patent Nov. 24, 1987
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`Sheet1of28
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`4,707,984
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`Fig. I
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`up = SINGLE O2 SENSOR SYSTEM
`(WORST CASE)
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`I,O '- DOUBLE O2 SENSOR SYSTEM
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`2
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`

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`U. S. Patent Nov. 24, 1987
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`Sheet2 of28
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`4,707,984 ‘
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`T
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`_
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`_CO_NTROL C_IRCUIT
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`3
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`U. S. Patent Nov. 24, 1937
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`Sheet 3 of28
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`4,707,984
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`Fig. 3A
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`1I2(1l3)
`I
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`13(15)
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`4
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`U. S. Patent Nov. 24, 1987
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`Sheet4 of28
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`4,707,984
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`Fig. 4/—\
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`5
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`U. S. Patent Nov. 24, 1937
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`Sheet5 of28
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`4,707,984
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`Fig_5
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`FIRST FEEDBACK
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`U. S. Patent Nov. 24, 1987
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`Sheet6 of28
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`4,707,984
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`, Fig. 5B
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`516
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`7
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`U. S. Patent Nov. 24, 1987
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`Sheet7 of28
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`4,707,984[
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`Fig. 5 c:
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`

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`U. S. Patent Nov. 24, 1987
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`Sheet8 of28
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`4,707,984‘
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`A/F
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`9
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`U. S. Patent Nov. 24, 1987
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`Sheet9 of28
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`4,707,984
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`Fig. 7
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`ACTIVATION/DEACTIVATIO
`DET_ ROUTINE
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`701
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`10
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`10
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`U. S. Patent Nov. 24, 1987
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`Sheet 10 of28 4,707,984
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`Fig. 8A
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`DETERIORATION
`DETECTING
`ROUTIN E
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`P
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`11
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`11
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`U. S. Patent Nov. 24, 1987
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`Sheet 11 of28 4,707,984
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`TFX__ 31 TFX+TF
`32
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`STORE TFX
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`IN B-RAM
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`12
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`12
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`V U.S. Patent Nov. 24, 1987
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`Sheet 12 cm 4,707,984 I
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`Fig. 9A
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`o ‘
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`TIME
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`Fig. 9B
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`TIME
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`13
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`13
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`U. S. Patent Nov. 24, 1937
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`Sheet 13 of28 4,707,984
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`Fig. /0
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`14
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`ACTIVATION/
`DEACT1 VATION
`DET, ROUTINE
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`14
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`S. Patent Nov. 24, 1937
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`Sheet 14 of28 4,707,984
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`Fig. //
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`ACTIVATIO N/
`DEACTIVAT IO N
`DET. ROUTINE
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`15
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`15
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`U. S. Patent Nov. 24, 1987
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`Sheet 15 of28 4,707,984
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`/.214
`SECOND FEEDBACK
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`Fig.12
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`16
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`17
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`U. S. Patent Nov. 24, 1987
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`Sheet 16 cm 4,707,984
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`Fig. /2B
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`A
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`17
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`U. S. Patent Nov. 24, 1987
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`Sheet 17 of28 4,707,984
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`Fig. l2C
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`21
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`U. S. Patent Nov. 24, 1937
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`Sheet 13 qf28 4,707,984
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`U. S. Patent Nov. 24, 1987
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`Sheet 19 of28 4,707,984
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`21
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`U. S. Patent Nov. 24, 1987
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`Sheet 20 of28 4,707,984
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`Fi§,15
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`Fig. /54
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`21
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`_ U. S. Patent Nov. 24, 1937
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`Sheet2l of28 4,707,934
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`RETURN 1533
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`22
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`U. S. Patent Nov. 24, 1987
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`Sheet 22 of28 4,707,984 '
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`Fig. /6
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`TAU
`ROUTINE
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`1605
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`23
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`

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`U. S. Patent Nov. 24, 1987
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`sheet 23 of28 4,707,984
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`% Fig-/74
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`25
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`

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`U. S. Patent Nov. 24, 1987
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`Sheet 25 cm 4,707,984
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`COLD START
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`HOT START
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`Fig. l9A ‘
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`SPD
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`l : ‘
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`A
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`V-» PRIOR ART
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`Fig. /‘FAF2
`~ ?PRESENT INVENTION
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`PRESENT INVENTION
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`26
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`

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`U. S. Patent Nov. 24, 1987
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`Sheet 26 of28 4,707,984‘
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`FROM STEP 502
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`Fig. 20
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`TO STEP 516
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`27
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`U. S. Patent Nov. 24, 1987
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`Sheet 27 of28 4,707,984 '
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`A/F1
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`28
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`U. S. Patent Nov. 24, 1987
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`Sheet 28 of28 4,707,984
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`Fig. 22
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`FROM STEP 1203 (1503)
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`TO STEP l217(15l7)
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`29
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`

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`1
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`4,707,984
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`2
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`DOUBLE AIR-FUEL RATIO SENSOR SYSTEM
`HAVING IMPROVED RESPONSE
`CHARACTERISTICS
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`
`The present invention relates to a method and appara-
`tus for feedback control of an air-fuel ratio in an internal
`combustion engine having two air-fuel ratio sensors
`upstream and downstream of a catalyst converter dis-
`posed within an exhaust gas passage.
`2. Description of the Related Art
`Generally, in a feedback control of the air-fuel ratio
`sensor (0; sensor) system, a base fuel amount TAUP is
`calculated in accordance with the detected intake air
`amount and detected engine speed, and the base fuel
`amount TAUP is corrected by an air-fuel ratio correc-
`tion coefficient FAF which is calculated in accordance
`with the output signal of an air-fuel ratio sensor (for
`example, an 0; sensor) for detecting the concentration
`of a specific component such as the oxygen component
`in the exhaust gas. Thus, an actual fuel amount is con-
`trolled in accordance with the corrected fuel amount.
`The above-mentioned process is repeated so that the
`air-fuel ratio of the engine is brought close to a stoichio-
`metric air-fuel ratio. According to this feedback con-
`trol, the center of the controlled air-fuel ratio can be
`within a very small range of ajr-fuel ratios around the
`stoichiometric ratio required for three-way reducing
`and oxidizing catalysts (catalyst converter) which can
`remove three pollutants CO, HC, and NO; simulta-
`neously from the exhaust gas.
`In the above-mentioned O2 sensor system where the
`O2 sensor is disposed at a location near the concentra-
`tion portion of an exhaust manifold, i.e., upstream of the
`catalyst converter, the accuracy of the controlled air-
`fuel ratio is affected by individual differences in the
`characteristics of the parts of the engine, such as the O2
`sensor, the fuel injection valves, the exhaust gas recircu-
`lation (EGR) valve, the valve lifters, individual changes
`due to the aging of these parts, environmental changes,
`and the like. That is, if the characteristics of the 0;
`sensor fluctuate, or if the uniformity of the exhaust gas
`fluctuates, the accuracy of the air-fuel ratio feedback
`correction amount FAF is also fluctuated, thereby caus-
`ing fluctuations in the controlled air-fuel ratio.
`To compensate for the fluctuation of the controlled
`air-fuel ratio, double 0; sensor systems have been sug-
`gested (see: U.S. Pat. Nos.“ 3,939,654, 4,027,477,
`4,130,095, 4,235,204). In a double O2 sensor system,
`another 0; sensor is provided downstream of the cata-
`lyst converter, and thus an air-fuel ratio control opera-
`tion is carried out by the downstream-side O2 sensor is
`addition to an air-fuel ratio control operation carried
`out by the upstream-side O2 sensor. In the double O2
`sensor system, although the downstream-side 0; sensor
`has lower response speed characteristics when com-
`pared with the upstream-side O2 sensor,
`the down-
`stream-side O2 sensor has an advantage in that the out-
`put fluctuation characteristics are small when compared
`‘ with those of the upstream-side O2 sensor, for the fol-
`.lowing reasons:
`(1) On the downstream side of the catalyst converter,
`the temperature of the exhaust gas is low, so that the
`downstream-side O2 sensor is not affected by a high
`temperature exhaust gas.
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`5
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`l0
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`(2) On the downstream side of the catalyst converter,
`although various kinds of pollutants are trapped in
`the catalyst converter,
`these pollutants have little
`affect on the downstream side 0; sensor.
`(3) On the downstream side of the catalyst converter,
`the exhaust gas is mixed so that the concentration of
`oxygen in the exhaust gas is approximately in an equi-
`librium state.
`
`Therefore, according to the double O2 sensor system,
`the fluctuation of the output of the upstream-side O2
`sensor is compensated for by a feedback control using
`the output of the downstream-side O2 sensor. Actually,
`as illustrated in FIG. 1, in the worst case, the deteriora-
`tion of the output characteristics of the 0; sensor in a
`single 0; sensor system directly effects a deterioration
`in the emission characteristics. On the other hand, in a
`double O2 sensor system, even when the output charac-
`teristics of the upstream-side O2 sensor are deteriorated,
`the emission characteristics are not deteriorated. That
`is, in a double O2 sensor system, even if only the output
`characteristics of the downstream-side 02 are stable,
`good emission characteristics are still obtained.
`In the above-mentioned double O2 sensor system,
`however, since the downstream-side air-fuel ratio sen-
`sor is located on a low temperature side when compared
`with the upstream-side air-fuel ratio sensor, it will take
`a relatively long time for the downstream-side air-fuel
`ratio sensor to be activated. Therefore, when a feedback
`control by the downstream-side air-fuel ratio sensor is
`carried out before the downstream-side air-fuel ratio
`sensor is activated, the controlled air-fuel ratio again
`becomes overrich or overlean due to the inclination of
`the output of the downstream-side air-fuel ratio sensor
`thus deteriorating the fuel consumption, the drivability,
`and the conditions of the exhaust emission characteris-
`tics for the HC, CO, and NOx components thereof.
`Note that, if the activation/deactivation of the down-
`stream-side O2 sensor is carried out by determining
`whether or not the output of the downstream-side O2
`sensor is swung from the lean side to the rich side or
`vice versa, it is impossible to obtain the determination of
`activation of the downstream-side O2 sensor when the
`output thereof is inclined to the rich side or the lean
`side. Also,
`the determination of deactivation of the
`downstream-side O2 sensor cannot be obtained, even
`when the temperature thereof is reduced at an interme-
`diate state of driving the engine.
`
`SUMMARY OF THE INVENTION
`
`It is an object of the present invention to provide a
`double air-fuel ratio sensor system in an internal com-
`bustion engine with which the fuel consumption, the
`drivability, and the exhaust emission characteristics are
`improved when the downstream-side O2 sensor is in an
`deactivation state.
`
`According to the present invention, in a double air-
`fuel ratio sensor system including two 02 sensors up-
`stream and downstream of a catalyst converter pro-
`vided in an exhaust passage, an actual air-fuel ratio is
`adjusted in accordance with the outputs of the up-
`stream-side air-fuel ratio sensor and the downstream-
`side air-fuel ratio sensor. The adjustment of the actual
`air-fuel ratio by the downstream-side air-fuel ratio sen-
`sor is prohibited in accordance with a coolant tempera-
`ture of the engine. Since the sensor temperature of the
`downstream-side air-fuel ratio sensor can be detected
`indirectly by the temperature of the coolant, the activa-
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`30
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`

`3
`tion/deactivation of the downstream-side air-fuel ratio
`sensor can be properly carried out.
`BRIEF DESCRIPTION OF THE DRAWINGS
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`4,707,984
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`5
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`10
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`The present invention will be more clearly under-
`stood from the description as set forth below with refer-
`ence to the accompanying drawings, wherein:
`FIG. 1 is a graph showing the emission characteris-
`tics of a single 02 sensor system (worst case) and a
`double O2 sensor system;
`FIG. 2 is a schematic view of an internal combustion
`engine according to the present invention;
`FIGS. 3A and 3B are circuit diagrams of the signal
`processing circuits of FIG. 2;
`.
`FIGS. 4A and 4B are graphs showing the output 15
`characteristics of the signal processing circuits of FIGS.
`3A and 3B, respectively;
`FIGS. 5, 5A—5C, 7, 8, 8A—8B, 10, 11, 12, 12A—12C,
`13, 14, 16, 20, and 22 are flow charts showing the opera-
`tion of the control circuit of FIG. 2;
`FIGS. 6A through 6D are timing diagrams explaining
`the flow charts of FIG. 5;
`FIGS. 9A and 9B are timing diagrams of examples of
`the output of an O2 sensor;
`'
`FIGS. 14A through 14H are timing diagrams explain- 25
`ing the flow charts of FIGS. 5, 7 (8, 10, 11) 12, and 13;
`FIGS. 17A through 17H are timing diagrams explain-
`. ing the flow charts of FIGS. 5, 7 (8, 10, 11), 15, and 16;
`FIGS. 18A, 18B, and 18C, and FIGS. 19A through
`19F are timing diagrams for explaining the effect of the 30
`present invention; and
`FIGS. 21A through 21D are timing diagrams for
`explaining the flow chart of FIG. 20.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`20
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`35
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`In FIG. 2, which illustrates an internal combustion
`‘engine according to the present invention, reference
`numeral 1 designates a four-cycle spark ignition engine
`disposed in an automotive vehicle. Provided in an air- 40
`"intake passage 2 of the engine 1 is a potentiometer-type
`' - airflow meter 3 for detecting the amount of air taken
`into the engine 1 to generate an analog voltage signal in
`proportion to the amount of air flowing therethrough.
`The signal of the airflow meter 3 is transmitted to a 45
`multiplexer-incorporating analog-to-digital (A/D) con-
`verter 101 of a control circuit 10.
`Disposed in a distributor 4 are crank angle sensors 5
`and 6 for detecting the angle of the crankshaft (not
`shown) of the engine 1. In this case, the crank-angle 50
`sensor 5 generates a pulse signal at every 720° crank
`angle (CA) while the crank-angle sensor 6 generates a
`pulse signal at every 30° CA. The pulse signals of the
`crank angle sensors 5 and 6 are supplied to an input/
`output (I/O) interface 102 of the control circuit 10. In 55
`addition, the pulse signal of the crank angle sensor 6 is
`then supplied to an interruption terminal of a central
`processing unit (CPU) 103.
`Additionally provided in the air-intake passage 2 is a
`fuel injection valve 7 for supplying pressurized fuel 60
`from the fuel system to the air-intake port of the cylin-
`der of the engine 1. In this case, other fuel injection
`valves are also provided for other cylinders, though not
`shown in FIG. 2.
`
`Disposed in a cylinder block 8 of the engine 1 is a 65
`coolant temperature sensor 9 for detecting the tempera-
`ture of the coolant. The coolant temperature sensor 9
`generates an analog voltage signal in response to the
`
`4
`temperature of the coolant and transmits it to the A/D
`converter 101 of the control circuit 10.
`Provided in an exhaust system on the downstream-
`side of an exhaust manifold 11 is a three-way reducing
`and oxidizing catalyst converter 12 which removes
`three pollutants CO, HC, and No, simultaneously from
`the exhaust gas.
`Provided on the concentration portion of the exhaust
`manifold 11, i.e., upstream of the catalyst converter 12,
`is a first 02 sensor 13 for detecting the concentration of
`oxygen composition in the exhaust gas. Further, pro-
`vided in an exhaust pipe 14 downstream of the catalyst
`converter 12 is a second 02 sensor 15 for detecting the .
`concentration of oxygen composition in the exhaust gas.
`The O2 sensors 13 and 15 generate output voltage sig-
`nals and transmit them via signal processing circuits 112
`and 113 to the A/D converter 101 of the control circuit
`10.
`
`Reference numeral 16 designates a starter switch
`which generates and transmits an output STA to the
`I/O interface 102 of the control circuit 10.
`The control circuit 10, which may be constructed by
`a microcomputer, further comprises a central process-
`ing unit (CPU) 103, a freerun converter 104, a read-only
`memory (ROM) 105 for storing a main routine, inter-
`rupt routines such as a fuel injection routine, an ignition
`timing routine, tables (maps), constants, etc., a random
`access memory 106 (RAM) for storing temporary data,
`a backup RAM 107, a clock generator 108 for generat-
`ing various clock signals, a down counter 109, a flip-
`flop 110, a driver circuit 111, and the like.
`Note that the battery (not shown) is connected di-
`rectly to the backup RAM 107 and, therefore, the con-
`tent thereof is never erased even when the ignition
`switch (not shown) is turned off.
`The down counter 109,
`the flip-flop 110, and the
`driver circuit 111 are used for controlling the fuel injec-
`tion valve 7. That is, when a fuel injection amount TAU
`is calculated in a TAU routine, which will be later
`explained,
`the amount TAU is preset
`in the down
`counter 109, and simultaneously, the flip-flop 110 is set.
`As a result, the driver circuit 111 initiates the activation
`of the fuel injection valve 7. On the other hand, the
`down counter 109 counts up the clock signal from the
`clock generator 108, and finally generates a logic “1”
`signal from the carry-out terminal of the down counter
`109, to reset the flip-flop 110, so that the driver circuit
`111 stops the activation of the fuel injection valve 7.
`Thus,
`the amount of fuel corresponding to the fuel
`injection amount TAU is injected into the fuel injection
`valve 7.
`
`Interruptions occur at the CPU 103, when the A/D
`converter 101 completes an A/D conversion and gener-
`ates an interrupt signal; when the crank angle sensor 6
`generates a pulse signal; and when the clock generator
`108 generates a special clock signal.
`The intake air amount data Q of the airflow meter 3
`and the coolant temperature data THW of the coolant
`sensor 9 are fetched by an A/D conversion routine(s)
`executed at every predetermined time period and are
`then stored in the RAM 105. That is, the data Q and
`THW in the RAM 106 are renewed at every predeter-
`mined time period. The engine speed Ne is calculated
`by an interrupt routine executed at 30" CA, i.e., at every
`pulse signal of the crank angle sensor 6, and is then
`stored in the RAM 106.
`There are two types of signal processing circuits 118
`and 113, i.e., the flow-out type and the flow-in type. As
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`31
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`31
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`

`

`5
`illustrated in FIG. 3A, the flow-out type signal process-
`ing circuit comprises a grounded resistor R1 and a volt-
`age buffer OP. Therefore, as shown in FIG. 4A, when
`the temperature of the O2 sensor 13 (or 15) is low and
`the O2 sensor 13 (or 15) is in a nonactive state, the out-
`put of the signal processing circuit 112 (or 113) is low,
`due to sink currents by the resistor R1, regardless of the
`rich or lean state of the O2 sensor 13 (or 15). Contrary
`to this, when the O2 sensor 13 (or 15) is activated by an
`increase of the temperature of the signal processing
`circuit 112 (or 113) generates a rich signal which has a
`high potential or a lean signal which has a low potential.
`Therefore, in this case, the activation/deactivation state
`of the O2 sensor 13 (or 15) can be determined by
`whether a rich signal is low or high. On the other hand,
`as illustrated in FIG. 3B, the flow-in type signal pro-
`cessing circuit comprises a resistor R2 connected to a
`power supply Vcc and a voltage buffer OP. Therefore,
`when the temperature of the O2 sensor 13 (or 15) is low
`and the O2 sensor 13 (or 15) is in a nonactive state, the
`output of the signal processing circuit 112 (or 113) is
`high, due to source currents by the resistor R2, regard-
`less of the rich or lean stage of the O2 sensor 13 (or 15).
`Contrary to this, when the O2 sensor 13 (or 15) is acti-
`vated by an increase of the temperature thereof, the
`signal processing circuit 112 (or 113) generates a high
`potential rich signal or a low potential
`lean signal.
`’ Therefore, in this case, the activation/deactivation state
`of the O2 sensor 13 (or 15) can be determined by
`whether a lean signal is low or high.
`Note that, hereinafter, the signal processing circuits
`112 and 113 are the flow-out type.,
`The operation of the control circuit 10 of FIG. 2 will
`be now explained.
`FIG. 5 is a routine for calculating a first air-fuel ratio
`feedback correction amount FAF1 in accordance with
`the output of the upstream-side O2 sensor 13 executed at
`every predetermined time period such as 4 ms.
`At step 501, it is determined whether or not all the
`‘.fCCdb3.Cl( control (closed-loop control) conditions by
`the upstream-side O2 sensor 13 are satisfied. The feed-
`back control conditions are as follows:
`(i) the engine is not in a starting state;
`(ii) the coolant temperature THW is higher than 50° C.;
`(iii) the power fuel incremental amount FPOWER is 0;
`and
`
`(iv) the upstream-side O2 sensor 13 is in an activated
`state.
`Note that the determination of activation/non-activa-
`tion of the upstream-side O2 sensor 13 is carried out by
`determining whether or not the coolant temperature
`TI-IWé70° C., or by whether or not the output of the
`upstream-side O2 sensor 13 is once swung,
`i.e., one
`changed from the rich side to the lean side, or vice
`versa. Of course, other feedback control conditions are
`introduced as occasion demands. However, an explana-
`tion of such other feedback control conditions is omit-
`ted.
`If one or more of the feedback control conditions is
`not satisfied, the control proceeds to step 527, in which
`the amount FAF1 is caused to be 1.0 (FAF1=1.0),
`thereby carrying out an open-loop control operation.
`Note that, in this case, the correction amount FAF1 can
`be a learning value or a value immediately before the
`feedback control by the upsteam O2 sensor 13 is
`stopped.
`
`.
`
`10.
`
`4,707,984
`
`6
`Contrary to the above, at step 501, if all of the feed-
`back control conditions are satisfied, the control pro-
`ceeds to step 402.
`At step 502, an A/D conversion is performed upon
`the output voltage V1 of the upstream-side O2 sensor 13,
`and the A/D converted value thereof is then fetched
`from the A/D converter 101. Then at step 403, the
`voltage V1 is compared with a reference voltage VR1
`such as 0.45 V, thereby determining whether the cur-
`rent air-fuel ratio detected by the upstream-side O2
`sensor 13 is on the rich side or on the lean side with
`respect to the stoichiometric air-fuel ratio.
`If V1_5.VR1, which means that the current air-fuel
`ratio is lean, the control proceeds to step 504, which
`determines whether or not the value of a first delay
`counter CDLY1 is positive. If CDLY1>0, the control
`proceeds to step 505, which clears the first delay
`counter CDLY1, and then proceeds to step 506. If
`CDLY1-E0, the control proceeds directly to step 506.
`At step 506, the first delay counter CDLY1 is counted
`down by 1, and at step 507, it is determined whether or
`not CDLY1<TDL1. Note that TDL1 is a lean delay
`time period for which a rich state is maintained even
`after the output of the upsteam-side O2 sensor 13 is
`changed from the rich side to the lean side, and is de-
`fined by a negative value. Therefore, at step 507, only
`when CDLY1 <TDL1 does the control proceed to step
`508, which causes CDLY1 to be TDL1, and then to
`step 509, which causes a first air-fuel ratio flag F1 to be
`“O” (lean state). On the other hand, if V1>VR1, which
`means that the current air-fuel ratio is rich, the control
`proceeds to step 510, which determines whether or not
`the value of the first delay counter CDLY1 is negative.
`If CDLY1 <0, the control proceeds to step 511, which
`clears the first delay counter CDLY1, and then pro-
`ceeds to step 512. If CDLY1é0, the control directly
`proceeds to 512. At step 512, the first delay counter
`CDLY1 is counted up by 1, and at step 513, it is deter-
`mined whether or not CDLY1>TDR1. Note that
`TDR1 is a rich delay time period for which a lean state
`is maintained even after the output of the upstream-side
`O2 sensor 13 is changed from the lean side to the rich
`side, and is defined by a positive value. Therefore, at
`step 513, only when CDLY1 >TDR1 does the control
`proceed to step 514, which causes CDLY1 to be TDR1,
`and then to step 515, which causes the first air-fuel ratio
`flag F1 to be “1” (rich state).
`Next, at step 516, it is determined whether or not the
`first air-fuel ratio flag F1 is reversed, i.e., whether or not
`the delayed air-fuel ratio detected by the upstream-side
`0; sensor 13 is reversed. If the first air-fuel ratio flag F1
`is reversed, the control proceeds to steps 517 to 519,
`which carry out a skip operation. That is, if the flag F1
`is “O” (lean) at step 517, the control proceeds to step
`518, which remarkably increases the correction amount
`FAF by a skip amount RSR. Also, if the flag F1 is “1”
`(rich) at step 517, the control proceeds to step 519,
`which remarkably decreases the correction amount
`FAF by the skip amount RSL. On the other hand, if the
`first air-fuel ratio flag F1 is not reversed at step 516, the
`control proceeds to step 520 to 522, which carries out
`an integration operation. That is, if the flag F1 is “O”
`(lean) at step 520, the control proceeds to step 521,
`which gradually increases the correction amount FAF1
`by a rich integration amount KIR. Also, if the flag F1 is
`“1” (rich) at step 520, the control proceeds to step 522,
`which gradually decreases the correction amount
`FAF1 by a lean integration amount KIL.
`
`10
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`32
`
`

`

`7
`The correction amount FAF1 is guarded by a mini-
`mum value 0.8 at steps 523 and 524, and by a maximum
`value 1.2 at steps 525 and 526, thereby also preventing
`the controlled air-fuel ratio from becoming overrich or
`overlean.
`The correction amount FAF1 is then stored in the
`RAM 105, thus completing this routine of FIG. 5 at step
`528.
`
`The operation by the flow chart of FIG. 5 will be
`further explained with reference to FIGS. 6A through
`6D. As illustrated in FIG. 6A, when the air-fuel ratio
`A/F is obtained by the output of the upstream-side O2
`sensor 13, the first delay counter CDLY1 is counted up
`during a rich state, and is counted down during a lean
`state, as illustrated in FIG. 6B. As a result, a delayed
`air-fuel ratio corresponding to the first air-fuel ratio flag
`F1 is obtained as illustrated in FIG. 6C. For example, at
`time t1, even when the air-fuel ratio A/F is changed
`from the lean side to the rich side, the delayed air-fuel
`ratio F1 is changed at time t2 after the rich delay time
`period TDR1. Similarly, at time t3, even when the air-
`fuel ratio A/F is changed from the rich side to the lean
`side, the delayed air-fuel ratio F1 is changed at time t4
`after the lean delay time period TDL1. However, at
`time t5, t5, or t7, when the air-fuel ratio A/F is reversed
`within a smaller time period than the rich delay time
`period TDR1 or the lean delay time period TDL1, the
`delayed air-fuel ratio F1 is reversed at time tg. That is,
`the delayed air-fuel ratio F1 is stable when compared
`with the air-fuel ratio A/F. Further, as illustrated in
`FIG. 6D, at every change of the delayed air-fuel ratio
`F1 from the rich side to the lean side, or vice versa, the
`correction amount FAF1 is skipped by the skip amount
`RSR or RSL, and also, the correction amount FAF1 is
`gradually increased or decreased in accordance with
`the delayed air-fuel ratio F1.
`Air-fuel ratio feedback control operations by the
`downstream-side O2 sensor 15 will be explained. There
`are two types of air-fuel ratio feedback control opera-
`tions by the downstream-side O2 sensor 15,
`i.e.,
`the
`’ operation type in which a second air-fuel ratio correc-
`tion amount FAF2 is introduced thereinto, and the
`operation type in which an air-fuel ratio feedback con-
`trol parameter in the air-fuel ratio feedback control
`operation by the upstream-side O2 sensor 13 is variable.
`Further, as the air fuel ratio feedback control parame-
`ter, there are nominated a delay time period TD (in
`more detail, the rich delay time period TDR1 and the
`lean delay time period TDL1), a skip amount RS (in
`more detail, the rich skip amount RSR and the lean skip
`amount RSL), and an integration amount K1 (in more
`detail, the rich integration amount KIR and the lean
`integration amount KIL).
`For example, if the rich delay time period becomes
`larger
`than
`the
`lean
`delay
`time
`period
`(TDR1>(’TDL1)),
`the controlled air-fuel ratio be-
`comes richer, and if the lean delay time period becomes
`larger
`than
`the
`rich
`delay
`time
`period
`((—-TDL1)>TDR1), the controlled air-fuel ratio be-
`comes leaner. Thus the air-fuel ratio can be controlled
`by changing the rich delay time period TDR1 and the
`lean delay time period (—TDL1) in accordance with
`the output of the downstream-side O2 sensor 15. Also, if
`the rich skip amount RSR is increased or if the lean skip
`amount RSL is decreased, the controlled airfuel ratio
`becomes richer, and if the lean skip amount RSL is
`increased or if the rich skip amount RSR is decreased,
`the controlled air-fuel ratio becomes leaner. Thus, the
`
`l0
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4,707,984
`
`8
`air-fuel ratio can be controlled by changing the rich skip
`amount RSR and the lean skip amount RSL in accor-
`dance with the output of the downstream-side O2 sensor
`15. Further, if the rich integration amount KIR is in-
`creased or if the lean integration amount KIL is de-
`creased, the controlled air-fuel ratio becomes richer,
`and if the lean integration amount KIL is increased or if
`the rich integration amount KIR is decreased, the con-
`trolled air-fuel ratio becomes leaner. Thus, the air-fuel
`ratio can be controlled by changing the rich integration
`amount KIR and the lean integration amount KIL in
`accordance with the output of the downstream-side O2
`sensor 15. Still further, if the reference voltage VR1 is
`increased, the controlled air-fuel ratio becomes richer,
`and if the reference voltage VR1 is decreased, the con-
`trolled air-fuel ratio becomes leaner. Thus, the air-fuel
`ratio can be controlled by changing the reference volt-
`age VR1 in accordance with the output of the down-
`stream-side O2 sensor 15.
`.
`FIG. 7 is a routine for determining whether the
`downstream-side 0; sensor 15 is activated or deacti-
`vated, and is executed at a predetermined time period
`such as every 50 ms. At step 701, the coolant tempera-
`ture THW is fetched, and it is determined whether or
`not the coolant temperature THW is higher than a pre-
`determined temperature such as 50° C. As a result, if
`THW>50° C., the control proceeds to step 702 which
`sets an air-fuel ratio feedback control execution flag
`FB2, and if THW§50° C., the control proceeds to step
`703 which clears the air-fuel ratio feedback control
`execution flag FB2. The air-fuel ratio feedback control
`execution flag FB2 is then stored in the RAM 106,
`thereby completing this routine at step 704.
`Thus, according to the routine of FIG. 7, the activa-
`tion/deactivation of the downstream-side O2 sensor 15
`is determined by the coolant temperature THW.
`FIGS. 8 and 10 are also routines for determining
`whether the downstream-side O2 sensor 15 is activated
`or deactivated. In more detail, FIG. 8 is a routine for
`detecting the deterioration degree of the downstream-
`side 02 sensor 15, executed at a predetermined time
`period such as every 4 ins, and FIG. 10 is a routine for
`determining whether the downstream-side 0; sensor 15
`is activated or deactivated, executed at a predetermined
`crank angle such as every 360° CA. Note that the rou-
`tine of FIG. 8 is carried out only when the feedback
`conditions by the downstream-side O2 sensor 15 are
`satisfied. That is, in the three-way reducing and oxidiz-
`ing catalysts, when a lean air-fuel ratio atmosphere
`prevails, O2 is absorbed thereinto, and when a rich air-
`fuel ratio atmosphere prevails, HC and C0 are absorbed
`thereinto, and are reacted with the absorbed 02 This is
`a so-called O2 storage effect. An air-fuel feedback con-
`trol operation provides an optimum frequency and am-
`plitude of the air-fuel ratio thereby positively making
`use of such an 0; storage effect. Therefore, according
`to the air-fuel feedback control, if an 0; sensor such as
`the downstream-side O2 sensor 15 is not deteriorated,
`the output thereof is swung as shown in FIG. 9A. Con-
`trary to the above, when the O2 sensor is deteriorated,
`only a little oxygen penetrates the zirconia elements of
`the O2 sensor. As a result, when the exhaust gas is
`changed from a rich state to a lean state, the_ change of
`the output of the O2 sensor from a rich signal to a lean
`signal is delayed, so that a time period of change of the
`output of the 0; sensor from maximum to minimum
`becomes long. That is, before the output of the O2 sen-
`sor becomes sufficiently low,
`the controlled air-fuel
`
`33
`
`

`

`4,707,984
`
`9
`ratio is reversed. As a result, the frequency of the con-
`trolled air-fuel ratio is reduced as shown in FIG. 9B,
`thereby reducing the O2 storage effect of the three way
`catalysts. Thus, when the O2 sensor is further deterio-
`rated, the frequency and amplitude of the O2 sensor are
`both reduced.
`In FIG. 8, at step 801, an A/D conversion is per-
`formed upon the output V2 of the downstream-side O2
`sensor 15, and at step 802, it is determined whether or
`not V2>V2o is satisfied. Here, V2o is a value of the
`output V2 previously fetched by this
`routine.
`If
`V2>V2o (positive slope), the control proceeds to step
`803 which determines whether or not a slope flag Y is
`“O”, and if V2—§V2o (negative slope), the control pro-
`ceeds to step 808 which determines whether or not the
`slope flag Y is “1”. Here, the slope flag Y (=“1”) shows
`that the output V2 of the downstream-side O2 sensor