JPH11132031A - Catalyst deterioration detection device for internal combustion engine - Google Patents

Abstract

(57) [Summary] [Problem] To accurately detect catalyst deterioration without reducing the frequency of execution of deterioration determination. SOLUTION: An air-fuel ratio sensor 13 is arranged in an exhaust passage on an upstream side of a three-way catalyst 20 of an internal combustion engine 1, and an O ₂ sensor 15 is arranged on a downstream side. The control circuit 10 performs feedback control of the engine air-fuel ratio to the target air-fuel ratio based on the outputs of the sensors 13 and 15 and also controls the sensors 13 and 15 during the feedback control.
Is determined based on the output trajectory length. The control circuit estimates the current catalyst temperature from the engine intake air amount, sets the allowable range for executing the catalyst deterioration determination based on the catalyst temperature, and deteriorates only when the exhaust air-fuel ratio flowing into the catalyst is within this allowable range. Make a decision. As a result, accurate deterioration determination can be performed regardless of the catalyst temperature, and unnecessary execution of the catalyst deterioration determination is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst deterioration judging device for judging whether or not an exhaust purification catalyst of an internal combustion engine has deteriorated.

[0002]

2. Description of the Related Art There is known a catalyst deterioration judging device for judging whether or not a catalyst has deteriorated based on the output of a downstream air-fuel ratio sensor disposed in an exhaust passage downstream of an exhaust purification catalyst installed in an exhaust passage of an internal combustion engine. ing. As an example of this type of catalyst deterioration determination device, there is one described in, for example, Japanese Patent Application Laid-Open No. Hei 9-119309.

In the apparatus disclosed in the publication, when determining whether the catalyst has deteriorated based on the length of the trajectory of the output of the downstream air-fuel ratio sensor, the exhaust air-fuel ratio detected by the upstream air-fuel ratio sensor disposed upstream of the catalyst is determined. That is, the determination of catalyst deterioration is made only when the exhaust air-fuel ratio flowing into the catalyst is within a range between a predetermined upper limit value and a predetermined lower limit value. As is well known, the three-way catalyst performs a so-called O ₂ storage function of absorbing oxygen in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean, and releasing the absorbed oxygen when the exhaust air-fuel ratio is rich. For this reason, if the three-way catalyst is normal, even if the air-fuel ratio fluctuates slightly on the upstream side of the catalyst, the fluctuation of the air-fuel ratio of the exhaust gas on the downstream side of the catalyst becomes small due to the O ₂ storage action of the catalyst, and The trajectory length of the output of the air-fuel ratio sensor is relatively short. However, since the O ₂ storage function of the catalyst decreases with the deterioration of the catalyst, when the catalyst deteriorates, the amount of oxygen absorbed and released by the O ₂ storage function decreases, and the fluctuation of the exhaust air-fuel ratio on the downstream side of the catalyst increases, and the catalyst gradually increases upstream. The exhaust gas air-fuel ratio fluctuates in the same manner. For this reason,
If the catalyst deteriorates, the locus length of the downstream air-fuel ratio sensor output also increases. Therefore, the degree of deterioration of the catalyst can be known by monitoring the trajectory length of the output of the air-fuel ratio sensor on the downstream side of the catalyst.

However, the fluctuation of the air-fuel ratio on the downstream side of the catalyst varies depending on the change width of the air-fuel ratio of the exhaust gas flowing into the catalyst even if the O ₂ storage action of the catalyst is the same (even if the degree of deterioration of the catalyst is the same). May be. That is, the catalyst O ₂
The amount of oxygen absorbed and released per unit time by the storage action changes according to the deviation of the air-fuel ratio of the exhaust gas flowing into the catalyst from the stoichiometric air-fuel ratio. For example, when the exhaust air-fuel ratio flowing into the catalyst is significantly richer than the stoichiometric air-fuel ratio, the amount of oxygen released from the catalyst per unit time increases. In this case, even if the catalyst is not degraded, oxygen is exhausted in a short time, and the exhaust air-fuel ratio downstream of the catalyst also fluctuates greatly to the rich side. On the other hand, when the exhaust air-fuel ratio on the upstream side of the catalyst is significantly leaner than the stoichiometric air-fuel ratio, the catalyst absorbs oxygen to a saturated amount in a short time, and the exhaust air-fuel ratio on the downstream side of the catalyst is normal. Even if it does, it will greatly fluctuate to the lean side. Therefore, if the variation range of the air-fuel ratio of the exhaust gas flowing into the catalyst air-fuel ratio allowable larger the normal catalyst than the range is erroneously determined to be deteriorated in O ₂ storage operation of the normal catalyst occurs.

In order to prevent the above-mentioned erroneous determination, the deterioration detecting device disclosed in Japanese Patent Application Laid-Open No. 9-130909 discloses a method in which the exhaust air-fuel ratio detected by the air-fuel ratio sensor on the upstream side of the catalyst is within a certain range (that is, the O.D. _The calculation of the trajectory length of the output of the downstream air-fuel ratio sensor is stopped when the value is out of the allowable air-fuel ratio range in the _two storage operation).

[0006]

However, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 9-119309, a problem may occur because the allowable air-fuel ratio range for determining deterioration is always set to a constant range. That is, even with the same catalyst, as the catalyst temperature increases, the O ₂ storage action increases, and the amount of oxygen that can be adsorbed and held by the catalyst increases.
The allowable upstream air-fuel ratio range is larger than when the catalyst temperature is low. For this reason, Japanese Unexamined Patent Application Publication No.
If the allowable air-fuel ratio range is always set to be constant as in the device of Japanese Patent Application Laid-Open No. 9, there is a case where the set allowable air-fuel ratio range becomes wider than the actual allowable air-fuel ratio range depending on the catalyst temperature, and erroneous determination of catalyst deterioration occurs. there is a possibility. In this case, the erroneous determination can be prevented by setting the allowable air-fuel ratio range to a narrow range based on the case where the catalyst temperature is low. However, if the allowable air-fuel ratio range is set to a narrow range according to the case where the catalyst temperature is low, the allowable air-fuel ratio range is set unnecessarily narrow when the catalyst temperature is high, and the catalyst deterioration determination is executed. There is a problem that the frequency is reduced.

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide a catalyst deterioration detection device for an internal combustion engine which enables accurate catalyst deterioration determination while maintaining a high catalyst deterioration determination execution frequency.

[0008]

According to the first aspect of the present invention, there is provided a three-way catalyst having an O ₂ storage function disposed in an exhaust passage of an internal combustion engine, and an exhaust gas downstream of the three-way catalyst. A downstream air-fuel ratio sensor that is disposed in a passage and detects an exhaust air-fuel ratio on the downstream side of the three-way catalyst; an air-fuel ratio control unit that controls an air-fuel ratio of the internal combustion engine to a target air-fuel ratio; and the air-fuel ratio control unit. When the air-fuel ratio of the internal combustion engine is controlled, a deterioration determination unit that determines whether the three-way catalyst has deteriorated based on the output of the downstream air-fuel ratio sensor, and an air-fuel ratio of exhaust gas flowing into the three-way catalyst. Determination permission means for permitting the deterioration determination means to determine whether or not the three-way catalyst has deteriorated when the air-fuel ratio is within a predetermined diagnosis execution air-fuel ratio range; and the diagnosis execution air-fuel ratio range according to the temperature of the three-way catalyst. Setting means for setting Catalyst deterioration detecting apparatus is provided for an internal combustion engine.

That is, according to the first aspect of the present invention, a diagnosis execution air-fuel ratio range for permitting the catalyst deterioration determination is set according to the catalyst temperature. As a result, the diagnosis execution range can be set to the permissible air-fuel ratio range of the O ₂ storage operation at each catalyst temperature, so that the frequency of execution of the catalyst deterioration determination is not unnecessarily reduced, and the catalyst deterioration erroneous regardless of the catalyst temperature. Judgment is prevented.

[0010]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a schematic diagram showing the entire configuration when the present invention is applied to an internal combustion engine for a vehicle. In FIG. 1, 1 is an internal combustion engine main body, 2 is a surge tank provided in an intake passage of the engine 1, 2a is an intake manifold connecting the surge tank 2 and an intake port of each cylinder, and 16 is an upstream side of the surge tank 2. A throttle valve arranged in the intake passage and having an opening corresponding to the amount of operation of the accelerator pedal 21 by the driver;
Reference numeral 7 denotes a fuel injection valve that injects pressurized fuel into an intake port of each cylinder of the engine 1. The throttle valve 16 is provided with a throttle opening sensor 17 for generating a voltage signal corresponding to the operation amount (opening) of the throttle valve.

In FIG. 1, reference numeral 11 denotes an exhaust manifold for connecting the exhaust ports of the cylinders to a common exhaust pipe 14, 20 a three-way catalyst disposed on the exhaust pipe 14, and 13 an exhaust junction of the exhaust manifold 11. (Upstream of the three-way catalyst 20)
The upstream air-fuel ratio sensor 15 is disposed at the three-way catalyst 20.
This is a downstream air-fuel ratio sensor arranged in the downstream exhaust pipe 14. The three-way catalyst 20 can exhaust air-fuel ratio flowing to purify HC in the exhaust gas when in the vicinity of the stoichiometric air-fuel ratio, CO, three components of the NO _X at the same time.

In this embodiment, a so-called linear air-fuel ratio sensor which generates a continuous voltage signal corresponding to the exhaust air-fuel ratio on a one-to-one basis is used as the upstream air-fuel ratio sensor 13.
As the downstream air-fuel ratio sensor 15, a so-called Z-type output O ₂ sensor whose output voltage rapidly changes near the stoichiometric air-fuel ratio is used. FIG. 2A shows general output characteristics of a linear air-fuel ratio sensor, and FIG. 2B shows general output characteristics of a Z-type output O ₂ sensor. As shown in FIG. 2A, the linear air-fuel ratio sensor outputs a continuous voltage signal substantially proportional to the air-fuel ratio, whereas the O ₂ sensor outputs a stoichiometric air-fuel ratio as shown in FIG. 2B. The output voltage changes suddenly in the vicinity, and the output voltage is not proportional to the exhaust air-fuel ratio except in a narrow range near the stoichiometric air-fuel ratio.

In this embodiment, the surge tank 2 in the intake passage is provided with an intake pressure sensor 3 for generating a voltage signal corresponding to the intake pressure (absolute pressure) in the surge tank. The water jacket 8 of the cylinder block is provided with a water temperature sensor 9 that generates an analog voltage electric signal corresponding to the temperature of the cooling water. The output signals of the throttle valve opening sensor 17, the intake pressure sensor 3, the water temperature sensor 9, the air-fuel ratio sensor 13, and the O ₂ sensor 15 are input to an A / D converter 101 with a built-in multiplexer of the control circuit 10 described later. Is done.

In FIG. 1, reference numerals 5 and 6 denote crank angle sensors disposed near the camshaft and the crankshaft (not shown) of the engine 1, respectively. For example, the crank angle sensor 5 generates a reference position detection pulse signal every 720 ° in terms of a crank angle, and the crank angle sensor 6 generates a crank angle detection pulse signal every 30 ° of the crank angle. The pulse signals of these crank angle sensors 5 and 6 are transmitted to the control circuit 1
0, the output of the crank angle sensor 6 is supplied to the CPU 1 of the control circuit 10.
03 is supplied to the interrupt terminal. The control circuit 10 calculates the number of revolutions (rotational speed) of the engine 1 based on the crank angle pulse signal interval from the crank angle sensor 6 and uses it for various controls.

The control circuit 10 is configured as a microcomputer, for example, and includes an A / D converter 101 with a built-in multiplexer, an input / output interface 102, a CPU 10
In addition to 3, a ROM 104, a RAM 105, a backup RAM 106 capable of holding data even when the main switch is turned off, a clock generation circuit 107, and the like are provided.

The control circuit 10 controls the fuel injection amount of the engine 1 based on the intake pressure, the throttle valve opening and the engine speed.
Addition to performing basic control of the engine 1 of the ignition timing control, etc., in the present embodiment, the air-fuel ratio sensor 13 as described later, O ₂ sensor 15 performs the deterioration determination of the three-way catalyst 20 based on the output.
That is, the control circuit 10 functions as each unit of the deterioration determination unit, the determination permission unit, and the setting unit described in the claims.

In order to perform the above control, the control circuit 10 executes an A / D conversion routine executed at regular intervals to execute an intake pressure (PM) signal from the intake pressure sensor 3 and a throttle opening (throttle opening) from the throttle opening sensor 17. TA) signal, cooling water temperature (THW) signal from the water temperature sensor 9, air-fuel ratio sensor 13
A / D conversion is performed on the exhaust air-fuel ratio signal (VAF) from the O2 sensor 15 and the exhaust air-fuel ratio signal (VOS) from the O ₂ sensor 15.

The input / output interface 102 of the control circuit 10 is connected to the fuel injection valve 7 via a drive circuit 108, and controls the fuel injection amount and the injection timing from the fuel injection valve 7. Next, calculation of the fuel injection amount of the engine according to the present embodiment will be described. In the present embodiment, the fuel injection amount FI to the engine 1 is calculated by the following equation.

FI = FCR × α + DF + β (1) where FCR is a fuel injection amount (basic fuel injection amount) required to maintain the engine air-fuel ratio at a target air-fuel ratio (stoichiometric air-fuel ratio), and DF is an upstream side. This is a feedback correction amount determined by feedback control based on the output VAF of the air-fuel ratio sensor 13 and the output VOS of the downstream O ₂ sensor 15. Also,
α and β are coefficients for correcting the fuel injection amount at the time of a cold start or a transient operation of the engine, and α = 1, β in a normal steady operation.
= 0 is set.

In this embodiment, the basic fuel injection amount FCR is calculated by the following equation. FCR = MC / AFT (2) Here, MC is the amount of air actually sucked into the cylinder, and is previously determined by the control circuit 10 based on the intake pressure PM detected by the intake pressure sensor 3 and the engine speed NE. The value is obtained by adding a correction for the transient operation state based on the change in the throttle opening TA to the value calculated from the numerical map stored in the ROM 104. AFT is a target air-fuel ratio (in this embodiment, a stoichiometric air-fuel ratio). That is, the basic fuel injection amount FCR represents a fuel injection amount required for setting the engine air-fuel ratio to the stoichiometric air-fuel ratio with respect to the amount of air actually sucked into the cylinder.

The feedback correction amount DF is calculated by the following equation. _{DF = K FP × FD + K} FI × ΣFD ... (3) where, FD is the difference between the actually supplied to the engine fuel amount and the basic fuel injection amount FCR, ΣFD represents the integrated value of FD. That is, the feedback correction amount DF is a proportional term K _FP × FD of a deviation FD between the fuel amount actually supplied to the engine and the fuel amount FCR required for setting the engine to the target air-fuel ratio.
And the integral term K _FI × ΣFD. K _FP ,
K _FI is a proportional coefficient and an integral coefficient, respectively.

The amount of fuel actually supplied to the engine is obtained by calculating the in-cylinder air amount MC used in the above-mentioned equation (2) from the upstream air-fuel ratio sensor 13 output VAF. It is calculated by dividing by the fuel ratio ABF. That is, the deviation FD is given by FD = MC / ABF-FCR (4). The fuel injection amount increases as the deviation FD increases and as the integral value ΣFD of the deviation FD increases.

Next, the calculation of the air-fuel ratio ABF on the upstream side of the catalyst 20 in the equation (4) will be described. Upstream air-fuel ratio ABF
Is calculated based on the output VAF of the upstream air-fuel ratio sensor 13. The output of the upstream air-fuel ratio sensor 13 is, for example, a change over time of the stoichiometric air-fuel ratio equivalent output, a variation in output characteristics of each sensor, and furthermore, an upstream air-fuel ratio. In some cases, the mixture state of the exhaust gas from each cylinder is not uniform at the position of the sensor 13, so that it may not always be exactly equal to the engine air-fuel ratio. On the other hand, at the position of the downstream O ₂ sensor 15, the mixed state of the exhaust gas is uniform and the exhaust gas temperature is also low, so that the O ₂ sensor output changes relatively little over time. Therefore, in the present embodiment, sub-feedback correction is performed based on the deviation VD between the actual output of the downstream O ₂ sensor 15 and the target air-fuel ratio equivalent output VOST (in the present embodiment, the stoichiometric air-fuel ratio equivalent output of the O ₂ sensor 15). And calculates the upstream air-fuel ratio sensor output VAF using the feedback correction amount DV.
Is corrected by the following equation.

VAF '= VAF + DV (5) where VAF' is the output of the upstream air-fuel ratio sensor after sub feedback correction. Further, the sub-feedback correction amount DV is, O ₂ sensor 15 outputs a deviation between the target air-fuel ratio corresponding output VOST VD (VD = VOS-VOST ... (6)
) Is calculated as DV = K _XP × VD + K _XI × ΣVD (7)

In the equation (7), ΔVD represents an integrated value of the deviation VD, and K _XP and K _XI represent a proportional constant and an integral constant, respectively. That is, the sub-feedback correction amount DV is obtained by dividing the proportional term (K _XP × VD) and the integral term (K _XI × ΣV) of the deviation VD.
D). In the present embodiment, the air-fuel ratio on the upstream side of the catalyst 20 is obtained from the relationship shown in FIG. 2A using the upstream-side air-fuel ratio sensor output VAF 'after the above correction.

As described above, in this embodiment, the output of the upstream air-fuel ratio sensor 13 is corrected by the sub-feedback control based on the output of the downstream O ₂ sensor 15, and the engine output is corrected based on the corrected output of the upstream air-fuel ratio sensor 13. The air-fuel ratio (that is, the air-fuel ratio of the exhaust flowing into the catalyst 20) is controlled to the target air-fuel ratio. Next, the principle of the catalyst deterioration determination in the present embodiment will be described.

In the present embodiment, the presence or absence of deterioration of the catalyst 20 is determined based on the length of the trajectory of the output of the upstream air-fuel ratio sensor 13 and the length of the trajectory of the output of the downstream O ₂ sensor 15 as described later. As is known, the three-way catalyst adsorbs oxygen in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean, and releases oxygen adsorbed and held when the air-fuel ratio of the inflowing exhaust gas is rich.
₂ Perform storage action. Due to this O ₂ storage action, even when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst fluctuates slightly near the stoichiometric air-fuel ratio, the three-way catalyst is maintained in the stoichiometric air-fuel ratio atmosphere, and the function of the three-way catalyst is maximized. Exhibited to the limit.

Further, as described above, when the three-way catalyst is sufficiently exerting the O ₂ storage function, the air-fuel ratio of the exhaust gas downstream of the catalyst 20 is maintained substantially in the vicinity of the stoichiometric air-fuel ratio. Less. For this reason, when the three-way catalyst is sufficiently exhibiting the O ₂ storage function, the locus length LVOS of the output of the downstream O ₂ sensor 15 (FIG. 3) is compared with the locus length LVAF of the output of the upstream air-fuel ratio sensor. And become smaller. However, when the three-way catalyst deteriorates, the catalyst O ₂
The storage function also decreases, and the exhaust air-fuel ratio on the downstream side of the catalyst 20 also changes according to the air-fuel ratio of the exhaust gas on the upstream side of the catalyst 20. Thereby, the downstream O ₂ sensor 1
The trajectory length LVOS of the five outputs approaches the trajectory length LVAF of the output of the upstream air-fuel ratio sensor 13 as the catalyst deteriorates. Therefore, the trajectory length LV of the output of the downstream O ₂ sensor 15
The degree of deterioration of the catalyst can be determined by monitoring the OS. However, the fluctuation cycle and amplitude of the output of the downstream O ₂ sensor 15 vary depending on the cycle and amplitude of the air-fuel ratio fluctuation of the exhaust gas flowing into the catalyst 20 even within the allowable air-fuel ratio range of the O ₂ storage operation of the catalyst. Therefore, in this embodiment,
In order to eliminate the influence of the difference in the state of the exhaust air-fuel ratio fluctuation on the upstream side of the catalyst 20, the fluctuation state of the output VAF 'after the sub feedback correction of the upstream air-fuel ratio sensor 13 (that is, the trajectory length LVAF of the output VAF') and Downstream O ₂ sensor 15
Accurate deterioration determination is made by comparing the locus length LVOS of the output VOS. That is, in the present embodiment, the trajectory length L of the output VOS of the downstream O ₂ sensor 15 is
VOS and output VAF ′ of sub-feedback correction of upstream air-fuel ratio sensor 13 (in the following description, output VAF ′ of sub-feedback correction of upstream air-fuel ratio sensor 13)
Is simply referred to as VAF).
When the value of / LVAF becomes larger than a certain value, it can be determined that the three-way catalyst has deteriorated.

FIGS. 3B and 3C show that the change in the output of the downstream O ₂ sensor 15 due to the deterioration of the three-way catalyst described above corresponds to the output of the upstream air-fuel ratio sensor 13 (FIG. 3A). FIG. As shown in FIG. 3A, in this embodiment, the air-fuel ratio on the upstream side of the catalyst fluctuates in a relatively small range around the stoichiometric air-fuel ratio by the air-fuel ratio feedback control. For this reason,
The trajectory length LVAF of the output of the upstream air-fuel ratio sensor is shown in FIG.
As shown in FIG.

FIG. 3B shows the output waveform of the downstream O ₂ sensor 15 when the catalyst has not deteriorated. As described above, when the catalyst has not deteriorated, the exhaust air-fuel ratio on the downstream side of the catalyst is maintained near the stoichiometric air-fuel ratio.
₂ sensor for the air-fuel ratio of exhaust gas to generate different output voltages depending on whether rich or lean, the output of the downstream O ₂ sensor 15 varies in a relatively long period between the rich level and a lean level. Therefore, as shown in FIG. 3B, the trajectory length LVOS of the output of the downstream O ₂ sensor 15 becomes an extremely small value, and the value of the ratio of these trajectory lengths LVOS / LVAF also becomes small.

FIG. 3C shows an output waveform of the downstream O ₂ sensor 15 when the catalyst is deteriorated. When the three-way catalyst is deteriorated, the output of the downstream O ₂ sensor is changed due to the decrease of the O ₂ storage function accompanying the fluctuation of the exhaust air-fuel ratio on the upstream side of the catalyst (FIG. 3).
(A)) because it fluctuates with almost the same cycle as in FIG.
As shown in (C), the locus length LVO of the downstream O ₂ sensor 15
S increases, and the trajectory length ratio LVOS / LVAF takes a large value.

FIG. 4 shows a determination value map used for detecting catalyst deterioration in the present embodiment. In this embodiment,
The determination value of the trajectory length ratio LVOS / LVAF at the time of the deterioration determination is set according to each value of the trajectory length LVAF of the upstream air-fuel ratio sensor 13. For this reason, the determination value map in the present embodiment has a vertical axis on which the trajectory length LV of the output of the downstream O ₂ sensor 15 is plotted.
FIG. 4 shows the OS, and the horizontal axis represents the trajectory length LVAF of the output of the upstream air-fuel ratio sensor 13.

As shown in FIG. 4, the trajectory length ratio LVOS / LVAF
The determination using the map of LVOS and LVAF instead of itself is made, for example, when LVAF is an extremely large value or a small value (that is, when the air-fuel ratio fluctuation on the upstream side is extremely large or small). If the determination is performed using the trajectory length ratio, accurate determination may not be performed.

Further, as shown in FIG. 4, in this embodiment, a region where the catalyst is determined to be deteriorated (deteriorated region) is separated from a region where the catalyst is determined to be normal (normal region). Area (dead zone) that does not belong to any of them.
As described later, in the present embodiment, the trajectory lengths LVOS and LOS
When the combination with the VAF enters the deteriorated region in FIG. 4, it is determined that the catalyst has deteriorated. When the combination enters the normal region in FIG. 4, it is determined that the catalyst is normal. Do not do. The reason why the dead zone is provided in the catalyst deterioration determination map is to improve the reliability of the catalyst deterioration and normality determination.

By the way, the downstream O ₂ sensor 1
When the 5 based on the trajectory length of the output performs deterioration determination of the catalyst, such as air-fuel ratio of the exhaust gas flowing into the catalyst as described above varies beyond the allowable air-fuel ratio range in O ₂ storage operation of the normal catalyst In this state, even if the catalyst has not deteriorated, the fluctuation of the exhaust air-fuel ratio on the downstream side of the catalyst increases, and the trajectory length LVOS of the output of the downstream O ₂ sensor 15 increases.
Becomes large, and it is erroneously determined that the normal catalyst has deteriorated.

The catalyst O_TwoStorage effect is catalyst temperature
The higher the degree, the higher _TwoFor storage function
The permissible air-fuel ratio range increases as the catalyst temperature increases.
You. For this reason, the above-mentioned permit
If the catalyst temperature is low when the air-fuel ratio range is set
The catalyst deterioration judgment is permitted in the state where an erroneous judgment occurs.
May occur. Also suitable for low catalyst temperatures
If the allowable air-fuel ratio range is set, the catalyst temperature
If it is high, the air-fuel
The catalyst deterioration determination is not performed even in the ratio range,
In short, there is a problem that the frequency of deterioration determination is reduced.

Therefore, in this embodiment, the allowable air-fuel ratio range of the exhaust gas flowing into the catalyst is set according to the catalyst temperature, and the allowable air-fuel ratio range is set wider as the catalyst temperature increases. In the present embodiment, as described above, the output VAF after the sub feedback correction of the upstream air-fuel ratio sensor 13 is performed.
The exhaust air-fuel ratio ABF flowing into the catalyst is calculated based on the following equation. Therefore, in the present embodiment, the allowable air-fuel ratio range in the O ₂ storage operation is previously determined by experiment using a normal catalyst, and the upstream-side air-fuel ratio sensor output VAF is set within the output range corresponding to the allowable air-fuel ratio range. , The catalyst deterioration determination operation is performed only in the case of. FIG. 5 shows an allowable air-fuel ratio range in the present embodiment. As shown in FIG. 5, the allowable air-fuel ratio range in the present embodiment is set around the target air-fuel ratio AFT (stoichiometric air-fuel ratio), the upper limit of the allowable air-fuel ratio range as the catalyst temperature T _CAT rises VAF
H is set to a large value and the lower limit value VAFL is set to a small value.

Incidentally, in order to set the allowable air-fuel ratio range according to the catalyst temperature T _CAT as described above, it is necessary to know the catalyst temperature T _CAT during operation of the engine accurately. Catalyst temperature T
_{Although CAT} can be detected by arranging a temperature sensor on the carrier of the catalyst 20, in this embodiment, the catalyst temperature is estimated based on the engine operating state without installing a temperature sensor on the catalyst by the method described below. I am trying to do it.

Hereinafter, the catalyst temperature detecting method of the present embodiment will be described. During actual operation, the catalyst temperature fluctuates according to the engine operating state.However, for example, when the engine air-fuel ratio is feedback-controlled to the stoichiometric air-fuel ratio, the combustion temperature in the engine combustion chamber does not change significantly. The rising speed and the falling speed of the catalyst temperature per unit time are determined by the exhaust gas flow rate (engine intake air amount) and the catalyst temperature at that time.

FIGS. 6 and 7 show the catalyst temperature T._CATEngine suction empty
FIG. 6 is a graph showing a time change according to the air volume MC, and FIG.
Temperature rise (heating), and FIG. 7 shows the case of temperature decrease (cooling).
ing. FIGS. 6 and 7 show the unit time on the horizontal axis, which will be described later.
The execution interval t of the medium temperature detection routine is shown.
As can be seen from FIG. 6, the catalyst temperature is lower than the current catalyst temperature.
The more the engine intake air volume is large,
As the catalyst temperature increases, the rate of increase decreases
Is constant by the amount of engine intake air
Converges to the temperature. For example, in FIG.
Volume MC_TwoLooking at the case of, the initial temperature is T₀In
The catalyst temperature T_CATIs initially per unit time t
T_Two-T ₀But the initial temperature is T_FourIn Case of
T per unit time_Five-T_FourOnly rises and the initial temperature
Is T_Five(Engine intake air amount MC_TwoFinal temperature in case of
In this case, the temperature rise becomes zero.

In the case of cooling, as shown in FIG.
The temperature increases as the current catalyst temperature increases and the engine intake air
The smaller the amount, the quicker it drops, and as the catalyst temperature falls,
After a certain period of time,
The temperature converges to a certain temperature determined by the amount of intake air. example
For example, in FIG._TwoIn case of
The initial temperature is T₉, The catalyst temperature T
_CATIs T per unit time at first₉-T₇Descend only
But the initial temperature is T₇In case of T ₇-T
₆Only falls and the initial temperature is T_Five(Engine intake air volume
MC_TwoTemperature drop is 0 in the case of
Becomes

[0042] In the present embodiment, the catalyst temperature T change per unit time in the _CAT [Delta] T _CAT and engine intake air amount M
The relationship between FIG. 6 and FIG. 7 for the combination with C is obtained in advance by experiments or the like, and stored in the ROM 104 of the control circuit 10 as a numerical value table (map) as shown in FIG. 6 to 8 show the change in the catalyst temperature during the execution of the air-fuel ratio feedback control. For example, the same relationship as that in FIG. 6 or FIG. In the present embodiment, a numerical value table having the same format as that shown in FIG. 8 is separately created in advance by an experiment or the like and stored in the ROM 104 for the change in the catalyst temperature per unit time during fuel increase and fuel cut. circuit 10 is constantly calculates the current catalyst temperature by integrating the change [Delta] T _{CA T} per unit of catalyst temperature T _CAT time using these maps during engine operation.

FIG. 9 is a flowchart showing a catalyst temperature detection routine according to this embodiment. This routine is executed by the control circuit 10 at regular intervals (unit time t in FIGS. 6 and 7). When the routine starts in FIG. 9, the current engine intake air amount MC is read in step 901, and the catalyst temperature T _CAT calculated in the previous execution of the routine is read in step 903. As the value of the engine intake air amount MC, a value separately calculated for use in the above equation (2) is used. Also, at the time of engine starting, or high temperature of the intake air temperature or the cooling water temperature is used as the initial value of T _{CA T.} In step 905, it is determined whether or not the fuel increase is currently being performed. In step 907, it is determined whether or not the fuel cut is currently being performed. If neither the fuel increase nor the fuel cut is currently being performed (that is, the air-fuel ratio feedback control is being performed). Step 90
The process proceeds to step _S9, and the catalyst temperature change amount ΔT _CAT per unit time is determined from the map shown in FIG. 8 using the current engine intake air amount MC and the catalyst temperature T _CAT .

Next, at step 911, the catalyst temperature at the end of the current routine is calculated as T _CAT + Δ using ΔT _CAT.
Calculated as T _CAT . In step 913,
The value of the calculated T _CAT is stored in RAM105 predetermined region, executing this routine is terminated. Step 9
Currently fuel increase at 05, or if a current fuel cut carried out in step 907, [Delta] T _CAT respectively, using a map of [Delta] T _CAT during separate fuel increase or during fuel cut in step 915 or step 917
Is calculated, and the value of ΔT _CAT is used to calculate step 911.
The following is performed: By executing this routine, the RAM 10
The predetermined area 5 always stores the current catalyst temperature T _CAT .

FIG. 10 and FIG. 11 are flowcharts showing the catalyst deterioration determining operation of this embodiment. This operation is performed as a routine executed by the control circuit 10 at regular intervals. In FIG. 10, in step 1001, first, the engine speed NE and the intake pressure PM are read from the respective sensors, and the output VAF of the upstream air-fuel ratio sensor 13 and the output VOS of the downstream O ₂ sensor are read.

In step 1003, it is determined whether or not a condition for executing catalyst deterioration determination is satisfied. Here, the deterioration determination execution conditions include, for example, that the air-fuel ratio feedback control is currently being executed, that the engine operating state is stable (for example, that the engine speed NE and the rate of change of the intake pressure PM are smaller than predetermined values). ). If the deterioration determination condition is not satisfied, this routine ends immediately after execution of step 1003.

If the catalyst deterioration determination condition is satisfied in step 1003, then in steps 1005 and 1007, the current engine speed NE and intake pressure P
M (ie, engine load) is within a predetermined range (NEL <NE)
It is determined whether <NEH and PML <PM <PMH). NE in steps 1005 and 1007
If any of PM and PM is not within the predetermined range, the routine ends without executing the catalyst deterioration determination. That is, the catalyst deterioration determination is not performed in an operation state in which one of the engine speed and the load is extremely high or low.

If both the engine speed NE and the intake pressure PM are within the predetermined ranges in steps 1005 and 1007, then in step 1009 the current catalyst temperature T _CAT calculated by the routine of FIG. 9 is read. Step 1
011 In the basis of this catalyst temperature T _{CA T,} an allowable air-fuel ratio range VAFL and VAFH of O ₂ storage operation in the current catalyst temperature T _CAT from the relationship shown in FIG. 5 is determined. Then, in step 1013, step 100
It is determined whether or not the current catalyst upstream-side exhaust air-fuel ratio read in step 1 is within the allowable air-fuel ratio range (VAFL ≦ VAF ≦ VAFH). If the current catalyst upstream air-fuel ratio is not within the allowable range in step 1013 (VAF <VAF or VAF> VAFH), the operation after step 1015 is not performed, the routine ends, and the current catalyst upstream air-fuel ratio is reduced. In the allowable range (VAFL ≦ VAF ≦
(VAFH) only, the catalyst deterioration determination operation of step 1015 and thereafter (FIG. 11) is performed. That is, in this embodiment, the catalyst temperature T _CAT is estimated, and the catalyst deterioration diagnosis execution air-fuel ratio range is set according to the catalyst temperature T _CAT .

If the current catalyst upstream air-fuel ratio is within the diagnosis execution air-fuel ratio range at step 1013, then at step 1015 the trajectory length L of the upstream air-fuel ratio sensor output VAF is determined.
Locus length LVOS between VAF and downstream O ₂ sensor output VOS
Is calculated. In this embodiment, the trajectory length of the sensor output is approximately obtained by integrating the absolute value of the difference between the sensor output at the time of executing the previous routine and the sensor output at the time of executing the current routine (see FIG. 12). Note that VAF ₀ , VAF
OS ₀ is the value of VAF and VOS at the time of the previous execution of the routine, and is updated in step 1017 every time the routine is executed.

After integrating the trajectory lengths LVAF and LVOS as described above, in step 1019, the value of the time counter CT is increased by one. That is, the value of the counter CT increases by one each time the routine is executed while the conditions of steps 1003 to 1007 and step 1013 are satisfied. Further, CT ₀ is set to a value of about 20 / Δt (Δt is an execution interval (second) of this routine). And step 1
Step 1023 following degradation determining operation is performed when the value of the timing counter CT of any diagnostic area 021 reaches the CT _0. That is, in the present embodiment, if the trajectory lengths LVAF and LVOS can be integrated for about 20 seconds even intermittently, the deterioration determination operation of step 1023 and subsequent steps is executed.

Next, a description will be given of the deterioration determination operation of step 1023 and subsequent steps. In step 1023, it is determined whether or not the trajectory lengths LVAF and LVOS of the upstream air-fuel ratio sensor and the downstream O ₂ sensor integrated as described above fall within the deterioration area (see FIG. 4) of the above-described deterioration determination map. If it is in the deterioration determination area, the value of the deterioration flag XF is set to 1 (deterioration) in step 1025. Also, LVA
If the values of F and LVOS are not in the degraded area, then in step 1027, the values of LVAF and LVOS are in the normal area (FIG. 4).
Is determined. If it is in the normal area, the value of the deterioration flag XF is set to 0 (normal) in step 1029. If neither deterioration nor normal is determined in steps 1023 and 1027, the value of the deterioration flag XF is held as it is without being changed. The value of the deterioration flag XF is stored in the backup RAM 106 of the control circuit 10 to prepare for the next inspection and repair, and when the value of the deterioration flag XF is set to 1, a routine executed by the control circuit 10 separately is used. The warning light is turned on to notify the driver that the catalyst has deteriorated.

Step 1031 shows the operation after the end of the deterioration judgment. That is, in step 1031, the clock counter CT and the trajectory lengths LVAF and LVOS calculated so far are calculated.
Are all cleared. Thus, the integration of the trajectory length LVAF and LVOS is newly started from the next execution of the routine. As described above, according to the present embodiment,
During diagnosis, the air-fuel ratio range for diagnosis is set appropriately according to the current catalyst temperature, so that deterioration determination is always performed under appropriate conditions regardless of catalyst temperature, and execution of catalyst deterioration determination is unnecessarily limited. Is prevented.

[0053]

According to the present invention, an appropriate diagnosis execution air-fuel ratio range is always set in accordance with the catalyst temperature, so that accurate catalyst deterioration determination can be performed while maintaining a high catalyst deterioration determination execution frequency. It has an excellent effect.

[Brief description of the drawings]

FIG. 1 is a diagram showing a schematic configuration of an embodiment in which the present invention is applied to an internal combustion engine for a vehicle.

FIG. 2 is a diagram showing general output characteristics of a linear air-fuel ratio sensor (FIG. 2 (A)) and an O ₂ sensor (FIG. 2 (B)).

FIG. 3 is a diagram illustrating a change in output of a downstream O ₂ sensor due to deterioration of a catalyst.

FIG. 4 is a diagram showing a catalyst deterioration determination map based on a trajectory length between an upstream air-fuel ratio sensor output and a downstream air-fuel ratio sensor output.

FIG. 5 is a diagram showing an example of a catalyst deterioration diagnosis execution air-fuel ratio range.

FIG. 6 is a diagram showing a relationship between a catalyst temperature change and an exhaust gas flow rate.

FIG. 7 is a diagram showing a relationship between a catalyst temperature change and an exhaust gas flow rate.

FIG. 8 is a diagram showing a format of a numerical map used for catalyst temperature calculation.

FIG. 9 is a flowchart illustrating a catalyst temperature detection routine.

FIG. 10 is a part of a flowchart showing a catalyst deterioration determination routine.

FIG. 11 is a part of a flowchart showing a catalyst deterioration determination routine.

FIG. 12 is a diagram illustrating an approximate calculation method of a trajectory length of a sensor output.

[Explanation of symbols]

1 ... engine body 3 ... intake pressure sensor 10 ... control circuit 13 ... upstream air-fuel ratio sensor 15 ... downstream O ₂ sensor 20 ... three-way catalyst

Claims (1)

[Claims] 1. A disposed in an exhaust passage of an internal combustion engine, O ₂
A three-way catalyst having a storage function, a downstream air-fuel ratio sensor disposed in an exhaust passage downstream of the three-way catalyst and detecting an exhaust air-fuel ratio downstream of the three-way catalyst, and an air-fuel ratio of the internal combustion engine. Air-fuel ratio control means for controlling to a target air-fuel ratio, and when the air-fuel ratio of the internal combustion engine is controlled by the air-fuel ratio control means, the presence or absence of deterioration of the three-way catalyst based on the output of the downstream air-fuel ratio sensor. Degradation determination means for determining, and determination permission for allowing the determination of deterioration of the three-way catalyst by the deterioration determination means when the air-fuel ratio of exhaust gas flowing into the three-way catalyst is within a predetermined diagnostic execution air-fuel ratio range. Means for setting the diagnostic execution air-fuel ratio range in accordance with the temperature of the three-way catalyst.