JP4348543B2 - Catalyst degradation detector - Google Patents

Description

  The present invention relates to a catalyst deterioration detection device that detects deterioration of an exhaust purification catalyst of an internal combustion engine.

Prior art
In this type of conventional catalyst deterioration detection device, it is assumed that the deterioration of the three-way catalyst (exhaust gas purification catalyst) means that the O ₂ storage function has decreased, and the absolute amount of oxygen that is adsorbed and held (or released) by the three-way catalyst The amount accurately represents the degree of deterioration of the three-way catalyst. Therefore, the absolute amount of oxygen adsorbed and held by the three-way catalyst is the deviation of the air-fuel ratio detected by the air-fuel ratio sensor disposed in the exhaust passage downstream of the internal combustion engine and downstream of the exhaust purification catalyst from the stoichiometric air-fuel ratio. And the product of the catalyst circulation gas amount. The equation for calculating the oxygen adsorption amount is α · (ΔA / F) · Ga · Δt. Where α is the oxygen content ratio, ΔA / F is the deviation of the air-fuel ratio detected by the air-fuel ratio sensor from the stoichiometric air-fuel ratio, Ga is the amount of gas flowing through the catalyst, and Δt is the oxygen adsorption (or release) action. Is the time.

  Here, when an oxygen sensor using zirconia is used as the air-fuel ratio sensor disposed in the exhaust passage downstream of the exhaust purification catalyst, the oxygen concentration on the exhaust side of the internal combustion engine is measured, Based on the combustion model, the air-fuel ratio before combustion in the internal combustion engine when the oxygen concentration is shown is estimated. Then, in the air-fuel ratio gas, oxygen corresponding to the stoichiometric air-fuel ratio reacts entirely with the internal combustion engine and the exhaust purification catalyst, and the remainder is treated as an amount adsorbed and held by the exhaust purification catalyst (see, for example, Patent Document 1). ).

Prior art 2.
In addition, it is possible to adsorb and release oxygen within the range of the maximum oxygen adsorption amount due to the oxygen adsorption capacity. However, all the adsorbed oxygen can be released instantly, or the oxygen can be absorbed instantly to the full capacity. Can not be adsorbed. There is a limit to the amount of oxygen that can be adsorbed and released instantaneously, and a method of detecting deterioration using this instantaneously adsorbable oxygen amount or instantaneously releasable oxygen amount is also disclosed. For example, when the exhaust purification catalyst is not deteriorated, lean oxygen is adsorbed to the exhaust purification catalyst. Oxygen that is not produced is generated and detected by an air-fuel ratio sensor downstream of the catalyst. In this way, it is possible to detect deterioration, and because deterioration detection is based on the amount of oxygen that is instantaneously adsorbed and released, air-fuel ratio control for detecting deterioration can be done in a short period of time, and exhaust emission deterioration and drivability It is said that the deterioration of the above is not induced (see, for example, Patent Document 2).

Prior art 3.
In addition, as a method of directly using the output voltage of the air-fuel ratio sensor without using the combustion model, the output (voltage) of the air-fuel ratio sensor disposed downstream of the exhaust purification catalyst is rich from lean to the catalyst deterioration detection. The degree of catalyst deterioration is determined using a change gradient (ΔV = V 1 −V 2) from immediately after changing to (voltage V 1) to immediately before changing from rich to lean (voltage V 2) (see, for example, Patent Document 3).

JP-A-5-133264 (26th to 32nd paragraphs, FIGS. 1 and 6) Japanese Patent Laying-Open No. 2002-4930 (paragraphs 201 to 211, FIG. 24) JP 2002-4930 A (paragraphs 158 to 159, FIGS. 20 and 21)

  When the catalyst deterioration is detected by calculating the absolute value of the oxygen adsorption amount, assuming that the catalyst deterioration is a deterioration of the oxygen adsorption capacity.In many cases, the decrease in the oxygen adsorption capacity is caused by the deterioration of the catalyst noble metal and the oxygen adsorption material. Both results are included, but not always. For example, when the heat resistance of the catalyst noble metal and the oxygen adsorbing substance are different in the thermal history, the catalyst noble metal is deteriorated but the oxygen adsorbing substance is not deteriorated. Alternatively, partial degradation also occurs due to physical peeling or dropping. In these cases, even if the oxygen adsorption amount is not reduced, the exhaust purification rate is reduced due to deterioration of the catalyst noble metals, but there is a problem that it cannot be detected as catalyst deterioration.

  Further, in the prior art 1, when calculating the oxygen adsorption amount, the oxygen concentration adsorbed in the exhaust purification catalyst is calculated using the deviation of the air-fuel ratio estimated from the value of the air-fuel ratio sensor with respect to the theoretical air-fuel ratio. However, in order to estimate the air-fuel ratio from the measured value of the air-fuel ratio sensor, it is necessary to perform a complicated calculation via the combustion model of the internal combustion engine, which causes a calculation error depending on the combustion state and the deterioration state of the exhaust purification catalyst. There is.

  Further, as in the prior art 2, there is the following problem when judging the deterioration of the exhaust purification catalyst from the measurement of the instantaneous oxygen adsorption amount and the instantaneous oxygen release amount. In other words, even when only the catalyst noble metal is deteriorated (including deterioration due to physical peeling and dropping, the same applies hereinafter unless otherwise specified), the catalyst is used when the oxygen adsorbent is healthy and oxygen is adsorbed quickly. There is a problem that deterioration cannot be detected.

  Further, the method of detecting deterioration from the air-fuel ratio change gradient using the output voltage of the direct air-fuel ratio sensor as in the prior art 3 is straightforward because a combustion model is not used, but the instantaneous oxygen adsorption amount and instantaneous Similar to the oxygen release method, there are the following problems. That is, even when only the catalyst noble metal deteriorates, there is a problem that catalyst deterioration may not be detected, for example, when oxygen is adsorbed quickly.

  The present invention has been made in order to solve the above-described problems of the prior art, and without performing complicated calculations via a combustion model of an internal combustion engine, the catalyst noble metal and oxygen constituting the exhaust purification catalyst It is an object of the present invention to provide a catalyst deterioration detection device that can detect catalyst deterioration even when only one of the adsorbed substances deteriorates.

  An apparatus for detecting catalyst deterioration according to the present invention includes an upstream oxygen gas disposed in an exhaust passage upstream of an exhaust purification catalyst for detecting an oxygen concentration upstream of an exhaust gas purification catalyst disposed in an exhaust passage of an internal combustion engine. A sensor, a downstream oxygen sensor disposed in an exhaust passage downstream of the exhaust purification catalyst to detect an oxygen concentration downstream of the exhaust purification catalyst, and an oxygen concentration upstream of the exhaust purification catalyst. An air-fuel ratio control means for setting the value, an inflow gas amount detection means for detecting the amount of gas flowing into the exhaust purification catalyst, an oxygen concentration upstream and downstream of the exhaust purification catalyst, and an inflow gas amount, respectively. The amount of released oxygen calculated using the calculated expression expressed as a function of the amount of adsorbed oxygen and the oxygen adsorption rate, or the upstream and downstream oxygen concentrations and the amount of inflow gas, respectively, of the exhaust purification catalyst Formula represented by a function of the iodine release rate, with those having a deterioration detecting means for detecting the above exhaust gas purifying catalyst.

  According to the present invention, even when only one of the catalyst precious metal and the oxygen adsorbing substance constituting the exhaust purification catalyst is deteriorated, it can be detected as catalyst deterioration, and even when the deterioration types are different, the leakage is not detected. There is an unprecedented remarkable effect that it can be detected without any problems. In addition, there is no need to perform complicated calculations via a combustion model of an internal combustion engine.

Embodiment 1 FIG.
1 to 5 are diagrams for explaining a catalyst deterioration detection apparatus according to Embodiment 1 of the present invention. More specifically, FIG. 1 is a diagram showing an overall configuration of the catalyst deterioration detection apparatus, and FIG. FIG. 3 is a characteristic diagram showing the relationship between the detected oxygen concentration value and the adsorbed oxygen concentration of the upstream and downstream air-fuel ratio sensors when the limit current type air-fuel ratio sensor is used. FIG. 4 is a characteristic diagram illustrating the relationship between the oxygen concentration detection value of the side air-fuel ratio sensor and the released oxygen concentration, FIG. 4 is a characteristic diagram illustrating the frequency factors of the normal catalyst and the deteriorated catalyst, and FIG. 5 is the catalyst deterioration degree and frequency factor A. It is a characteristic view which shows the relationship.

The catalyst deterioration detection device according to the present embodiment is a catalyst for detecting an oxygen concentration upstream of an exhaust purification catalyst (hereinafter referred to simply as catalyst) 3 disposed in an exhaust passage 2 of the internal combustion engine 1. 3, an upstream oxygen sensor (air-fuel ratio sensor) 41 disposed in the upstream exhaust passage 2, and a downstream exhaust passage 2 of the catalyst 3 for detecting the oxygen concentration downstream of the catalyst 3. And a downstream oxygen sensor (air-fuel ratio sensor) 42.
Further, an air flow meter 6 is provided as an inflow gas amount detecting means for detecting the amount of gas flowing into the catalyst 3.

Furthermore, the upstream and downstream oxygen concentrations detected by the upstream oxygen sensor (air-fuel ratio sensor) 41 and the downstream oxygen sensor (air-fuel ratio sensor) 42, respectively, and the amount of inflow gas detected by the air flow meter 6 are detected. Released oxygen, which is calculated using a function of an adsorbed oxygen amount and an oxygen adsorption rate, or an oxygen concentration upstream and downstream of the catalyst 3 and an inflow gas amount, respectively. An electronic control unit 7 is provided as a deterioration detecting means for detecting deterioration of the catalyst 3 using an expression expressed by a function of the amount and the oxygen release rate.
Furthermore, the electronic control unit 7 constitutes an air-fuel ratio control means together with the injector 5, and controls the injector 5 to set the oxygen concentration upstream of the catalyst 3 to a predetermined value.

  The electronic control unit 7 includes a CPU for performing calculations, a RAM for storing various information such as calculation results, a backup RAM in which the stored contents are held by a battery, a ROM for storing each control and calculation program, and the like. ing. The electronic control unit 7 controls the engine 1 and the injector 5 based on the air-fuel ratio, and makes a determination on the deterioration of the catalyst 3.

For example, a three-way catalyst is used as the catalyst 3.
As the oxygen sensors (air-fuel ratio sensors) 41 and 42, zirconia type limiting current type is used. A zirconia-type limiting current oxygen sensor (air-fuel ratio sensor) has platinum electrodes on both sides of an electrolyte formed of zirconia, and a diffusion-controlling layer made of a porous layer on the opposite side of the electrolyte of one electrode. ing. The other electrode is in contact with the atmosphere having a constant oxygen concentration, and the diffusion-controlled layer is in contact with the measurement gas.

Zirconia has oxygen ion conductivity. When a voltage is applied between both electrodes, when the measurement gas is in an oxygen atmosphere, oxygen that has passed through the diffusion-controlled layer becomes oxygen ions at the outer electrode and passes through the zirconia. The current value at this time is proportional to the oxygen concentration. When the measurement gas is a reducing atmosphere, the reducing gas that has passed through the diffusion-controlled layer reacts with oxygen that has passed through the zirconia by the oxygen pump effect. This amount of oxygen is the amount of oxygen necessary to oxidize the reducing gas. The pump current in an oxygen atmosphere and a reducing atmosphere can be directly calculated from the following theoretical formula.
In addition, such a theoretical formula is generally known. For example, in publications (Hiroyuki Kinoshita, “Zirconia Oxygen Meter-Solid Electrolyte”, Horiba Technical Report, March 1994, No. 8) P.55).

  Since the zirconia oxygen sensors 41 and 42 utilize oxygen ion conductivity, they are used at a high temperature of about 300 ° C. or higher. Therefore, reducing gas and oxygen react near the electrode. Therefore, the oxygen concentration in the vicinity of the electrodes of the zirconia oxygen sensors 41 and 42 is detected. Similarly, on the catalyst 3, the reducing gas reacts with oxygen on the catalyst 3 to adsorb excess oxygen.

  FIG. 2 shows the relationship between the oxygen concentration upstream and downstream of the catalyst 3 and the air-fuel ratio, and a method for calculating the amount of adsorbed oxygen in an oxygen atmosphere will be described. FIG. 3 shows the relationship between the oxygen concentration upstream and downstream of the catalyst and the air-fuel ratio, and a method for calculating the amount of released oxygen in the reducing atmosphere will be described. 2 and 3, the horizontal axis represents the air-fuel ratio, the vertical axis represents the oxygen concentration, and the air-fuel ratio at the intersection of the straight line representing the upstream oxygen concentration and the horizontal axis corresponds to the theoretical air-fuel ratio.

  If the oxygen concentration is calculated from the current values detected by the upstream and downstream air-fuel ratio sensors 41 and 42 by the above relational expression and the difference in oxygen concentration is calculated, the oxygen adsorbed or released by the catalyst 3 (in FIG. 2) The adsorbed oxygen concentration or the released oxygen concentration in FIG. 3) can be calculated. As in the prior art 1, the air-fuel ratio before the internal combustion engine is estimated from the oxygen concentration value obtained from the downstream air-fuel ratio sensor using the combustion model in the internal combustion engine, and the difference from the stoichiometric air-fuel ratio is further taken. Compared with the method of estimating the oxygen adsorption amount, the oxygen concentration at the inlet (upstream) and outlet (downstream) of the catalyst 3 is directly compared, so that it is easy to operate regardless of the combustion state of the internal combustion engine and the deterioration of the catalyst. It is possible to directly detect actual phenomena.

When the exhaust gas is in an oxygen atmosphere, the amount of adsorbed oxygen per unit time (adsorption reaction rate) by this method is
Adsorption reaction rate = {Xo (front) −Xo (rear)} × Qa (1)
Can be expressed as However, Xo (front) is the oxygen concentration calculated from the upstream air-fuel ratio sensor 41, Xo (rear) is the oxygen concentration calculated from the downstream air-fuel ratio sensor 42, and Qa is the gas flow rate actually measured from the air flow meter 6. is there. The air flow meter 6 generates an output voltage proportional to the intake air amount Ga (g / sec) supplied into the engine cylinder per unit time. The gas flow rate Qa may be estimated from the engine operating state without using the air flow meter 6.

On the other hand, a chemical reaction is generally a function of temperature and concentration, and is expressed in the form of a power of the concentration of a component involved in the reaction (for example, a publication (“Chemical Engineering III” by Nobuo Otake, Iwanami Zensho, p.12) reference).
Therefore, the oxygen adsorption rate (adsorption reaction rate) is
Oxygen adsorption rate = k × Xo (front) ^α × Y ^β (2)
Can be expressed as
Here, k is a reaction rate constant and a function of temperature, and is an Arrhenius equation shown in the following equation.
k = A × exp (−Ea / RT) (3)
Here, the parameter A is a frequency factor, Ea is the activation energy of the catalyst 3, R is a gas constant, and T is the temperature of the catalyst 3.
Substituting equation (3) into equation (2),
Adsorption reaction rate = A × exp (−Ea / RT) × Xo (front) ^α × Y ^β (4)
It can be expressed as.

  Since the reduction in the catalyst purification rate is due to the reduction in the reaction rate, it depends on the frequency factor A, the activation energy Ea, the temperature T, the catalyst upstream oxygen concentration Xo (front), and the adsorption site concentration Y from Equation (4). Thermal deterioration, which is the main deterioration of catalyst deterioration, is a serious deterioration that is difficult to recover and must be detected. The degree to which the thermal history is affected by temperature conditions differs between the catalytic noble metal and the oxygen adsorbing material, but the specific surface area of the catalytic noble metal and oxygen adsorbing material decreases as the thermal history progresses, and the frequency factor in the reaction rate equation A is affected by the decrease. The activation energy Ea is a value unique to the catalyst. Since the temperature T, the catalyst upstream oxygen concentration Xo (front), and the adsorption site concentration Y are controllable parameters, the deterioration of the catalyst can be determined by evaluating the frequency factor A.

Here, the determination method of each parameter in Formula (4) is demonstrated.
Since the activation energy Ea is a value inherent to the catalyst, it can be cited from literature values. It can also be obtained in advance by actually changing the temperature and measuring the reaction rate. Specifically, steady operation is performed at a predetermined air-fuel ratio, and the time change of the oxygen adsorption site concentration Y is measured. In the case of the primary reaction, if the initial concentration of the oxygen adsorption site concentration Y is Y ₀ , the following relationship is established.
ln (Y ₀ / Y) = kt (5)
Here, t is an operation time. Plotting ln (Y ₀ / Y) against time, the slope is the rate constant k. The same measurement is performed at different temperatures T, and the rate constant k at each temperature T is obtained.
The relationship between the rate constant k and the temperature T is Equation (3). Taking the logarithm of both sides of Equation (3),
lnk = lnA-Ea / RT (6)
It becomes. When lnk is plotted against 1 / T, the intercept is lnA and the slope is -Ea / R, and the activation energy Ea can be obtained from the slope.
The temperature T of the catalyst 3 may be actually measured by attaching a temperature sensor, or the relationship between the operating state of the engine 1 and the gas temperature can be estimated in advance and estimated from the actual operating state.

α and β are reaction orders, respectively. These orders are obtained in advance by experiments. The order α with respect to the oxygen concentration can be obtained by measuring the oxygen adsorption rate by changing Xo (front) under the condition that the oxygen adsorption site concentration Y is constant. For equation (2)
If k ′ = k × Y ^β (7),
Oxygen adsorption rate = k 'x Xo (front) ^α (8)
It is expressed.
Taking the log of both sides,
log oxygen adsorption rate = logk '+ α x logXo (front) (9)
When y = intercept + gradient × x is plotted as the vertical axis log oxygen adsorption rate and the horizontal axis logXo (front), the order α is obtained from the slope.

Similarly, β can be obtained by measuring the oxygen adsorption rate at different Y under the condition that the inflowing oxygen concentration is constant.
If k ″ = k × X (front) ^α (10),
Oxygen adsorption rate = k ″ × Y ^β (11)
It is expressed.
Taking the log of both sides,
log oxygen adsorption rate = logk ″ + β × Y (12)
When the vertical axis log oxygen adsorption rate and the horizontal axis logY are plotted, the order β is obtained from the slope.

  In addition, the maximum value of the oxygen adsorption site concentration Y varies depending on the design of the catalyst, and is measured in advance. For example, the engine 1 is operated at a predetermined air-fuel ratio, and the operation is continued while monitoring the oxygen concentration upstream and downstream of the catalyst 3. And it can obtain by integrating Formula (1) until the oxygen concentration downstream of the catalyst 3 shows the same value as the oxygen concentration upstream of the catalyst 3. The current value of the oxygen adsorption site concentration Y can be obtained by integrating the equation (1) in the period up to the present and the difference between the integrated value up to the present and the maximum value.

Since (1) = (4), the equations represented by the functions of the oxygen adsorption rate and the adsorbed oxygen amount, respectively calculated using the oxygen concentration upstream and downstream of the catalyst 3 and the inflow gas amount,

Is obtained.
y = {Xo (front) -Xo (rear)} (14)
z = exp (−Ea / RT) × Xo (front) ^α / Qa (15)
x = ^Yβ (16)
Then,
y = A * z * x (17)
Thus, the frequency factor A can be obtained from the slope of the y and x plots. That is, it is possible to determine the frequency factor A from the slope of a plot of {Xo (front) -Xo (rear )} and Y ^beta.

Next, the case where the exhaust gas is in a reducing atmosphere will be described. When the exhaust gas is in a reducing atmosphere, the amount of released oxygen per unit time (oxygen release rate) is
Oxygen release rate = − {Xo (front) −Xo (rear)} × Qa (18)
Can be expressed as
On the other hand, the reaction rate for oxygen release (oxygen release rate) depends on the rate constant k = A × exp (−Ea / RT), the reducing agent concentration X _Red (front), and the oxygen adsorption concentration O.
Release reaction rate = A × exp (−Ea / RT) × X _Red (front) ^γ × O ^ω (19)
Can be expressed as O is obtained by integrating the equation (18) up to the present time. γ and ω are reaction orders.

Since (18) = (19), an expression expressed as a function of the amount of released oxygen and the rate of oxygen release calculated using the oxygen concentration upstream and downstream of the catalyst 3 and the amount of inflow gas, respectively,

Is obtained.
In this case, - it can be obtained {Xo (front) -Xo (rear )} and O ^omega frequency factor A from the slope of a plot of.

Next, the deterioration determination method will be described.
The relationship between the frequency factor A calculated from the relational expressions (13) and (20) and the actually measured exhaust gas purification rate is obtained in advance, and the catalyst is determined from the relationship between the frequency factor A and the exhaust gas purification rate obtained in advance. 3 is determined.
More specifically, for example, when the exhaust gas is in an oxygen atmosphere, as shown in FIG. 4, in the relationship between Xo (front) -Xo (rear) and the adsorption site concentration Y, the deteriorated catalyst is compared with the normal catalyst. Since the frequency factor A takes a small value, deterioration can be detected. That is, based on the relationship between the frequency factor A and the actually measured exhaust purification rate, a threshold value of the frequency factor A is determined, and when the value of the frequency factor A obtained from the equation (13) is smaller than the threshold value, the catalyst Judge that it is deteriorated.
Further, based on the relationship between the frequency factor A and the actually measured exhaust purification rate, the relationship between the degree of deterioration of the catalyst and the frequency factor A is prepared in advance as shown in FIG. 5, and the frequency obtained from the equation (13). The degree of deterioration of the catalyst may be detected from the value of factor A using FIG.

  In the case where the exhaust gas is in a reducing atmosphere, the frequency factor A is smaller for the deteriorated catalyst than for the normal catalyst in the relationship between Xo (front) -Xo (rear) and the oxygen adsorption concentration O from the equation (20). Since the value is taken, it is possible to detect the deterioration of the catalyst 3 or the degree of deterioration as in the case where the exhaust gas is in an oxygen atmosphere.

As the deterioration determination timing, if the adsorption site concentration Y and the oxygen adsorption concentration O are always integrated and the oxygen adsorption amount in the catalyst 3 is constantly monitored, the frequency factor A can be calculated at all times. , Can always be determined.
Alternatively, the frequency at which the adsorption site concentration Y or the oxygen adsorption concentration O reaches a predetermined value by setting a predetermined oxygen adsorption amount and performing pre-control for setting the adsorption site concentration Y or the oxygen adsorption concentration O to a predetermined value. Factor A can also be used as a judgment value.
Further, if the relationship between the adsorption site concentration Y or the oxygen adsorption concentration O, the frequency factor A, and the catalyst purification rate is given in advance in the map, and the frequency factor A that is less than the allowable value of the catalyst purification rate is calculated, the catalyst It can also be determined that the deterioration has occurred.

  In the above description, the output current values of the oxygen sensors (air-fuel ratio sensors) 41 and 42 are converted into oxygen concentration values. However, in the same sensor, since the oxygen concentration and the current value are in a proportional relationship, they are not converted into oxygen concentrations. A similar determination can be made using the direct current value. In this case, instead of {Xo (front) −Xo (rear)} in the expressions (13) and (20), {current value (front) −current value (rear)} is obtained, and the absolute value of the frequency factor A is calculated. However, it is possible to detect catalyst deterioration as a relative value.

  As described above, in the present embodiment, an equation represented by a function of the amount of adsorbed oxygen and the oxygen adsorption rate calculated using the upstream and downstream oxygen concentrations and the inflow gas amount of the catalyst 3, respectively, or the catalyst 3, the deterioration of the catalyst 3 is detected by using the expressions expressed by the functions of the released oxygen amount and the oxygen release rate, which are calculated using the upstream and downstream oxygen concentrations and the inflow gas amount, respectively. 3 Even if only one of the catalytic noble metal and oxygen adsorbing material deteriorates, such as the catalytic noble metal and oxygen adsorbing material of the catalyst 3 deteriorate or peel off or fall off, it is detected as catalyst deterioration. Can do.

  In other words, the equation used to detect catalyst degradation expressed by equation (13) or equation (20) is a function of both the amount of adsorbed oxygen and the oxygen adsorption rate or both the amount of released oxygen and the rate of oxygen release. Since it consists of functions, it is possible to separate and evaluate the reaction rate parameters in the reaction rate of the catalyst 3 determined by various factors. Since the reaction rate parameter directly represents the exhaust gas purification performance of the catalyst 3, for example, even when the oxygen adsorbing material is healthy but only the noble metal catalyst is deteriorated, the reaction rate parameter is evaluated, so that the exhaust gas purification performance is deteriorated. Detection is possible. Moreover, since the amount of adsorbed oxygen can be evaluated by integrating the oxygen adsorption rate or the oxygen release rate, it is possible to detect the deterioration of the oxygen adsorbing substance. Therefore, even when only one of the noble metal catalyst and the oxygen adsorbing material is deteriorated, the catalyst deterioration can be detected.

Embodiment 2. FIG.
FIG. 6 is a diagram for explaining a catalyst deterioration detection apparatus according to Embodiment 2 of the present invention. More specifically, oxygen concentration detection of upstream and downstream air-fuel ratio sensors when using a concentration cell type air-fuel ratio sensor. It is a characteristic view which shows the relationship between a value and adsorption oxygen concentration. In FIG. 6, the horizontal axis represents the air-fuel ratio, the vertical axis represents the oxygen concentration, and the air-fuel ratio at the intersection of the straight line representing the upstream oxygen concentration and the horizontal axis corresponds to the stoichiometric air-fuel ratio.

  In the first embodiment, the case where zirconia-type limiting current type sensors are used as the oxygen sensors (air-fuel ratio sensors) 41 and 42 has been described. However, in this embodiment, zirconia-type concentration cell type ones are used. To do. Since the other configuration is the same as that of the first embodiment, differences from the first embodiment will be mainly described below.

  The zirconia concentration cell type oxygen sensor operates on the principle of a concentration cell. The electrolyte of the battery is made of zirconia and has platinum electrodes on both sides of the electrolyte. One electrode is in contact with the exhaust gas (measurement gas), and the other electrode is in contact with the atmosphere (reference gas) having a constant oxygen concentration. When both electrodes are exposed to atmospheres having different oxygen concentrations, an electromotive force represented by the Nernst equation is generated. The oxygen concentration can be calculated from the measured electromotive force by the theoretical formula (21). In addition, such a theoretical formula is generally known. For example, in publications (Hiroyuki Kinoshita, “Zirconia Oxygen Meter-Solid Electrolyte”, Horiba Technical Report, March 1994, No. 8) P.55).

  When a concentration cell type is used as the oxygen sensors 41 and 42, as shown in FIG. 6, measurement can be performed only by lean operation from the stoichiometric air-fuel ratio where oxygen is present in the exhaust gas. Using the oxygen concentration detected during the lean operation by the oxygen sensors 41 and 42, catalyst deterioration can be detected in the same manner as in the first embodiment, and the same effect as in the first embodiment can be obtained.

  Although the output voltage values of the oxygen sensors 41 and 42 are converted into oxygen concentration values using the equation (21), since the logarithm of the oxygen concentration and the voltage value are proportional to each other in the same sensor, they are not converted to the oxygen concentration. The same determination can be made using the voltage value directly. In this case, instead of {Xo (front) −Xo (rear)} in the equations (13) and (20), {voltage value (front) −voltage value (rear)} is obtained, and the absolute value of the frequency factor A is calculated. However, it is possible to detect catalyst deterioration as a relative value.

  A limiting current type may be used for one of the upstream and downstream air-fuel ratio sensors (oxygen sensors) 41 and 42, and a concentration cell type may be used for the other. In this case as well, measurement can be performed only in the lean operation, and catalyst deterioration can be detected in the same manner as in the first embodiment using the oxygen concentration measured during the lean operation. The same effect can be obtained.

It is a figure which shows the whole structure of the catalyst deterioration detection apparatus by Embodiment 1 of this invention. FIG. 5 is a characteristic diagram illustrating a relationship between the detected oxygen concentration values of the upstream and downstream air-fuel ratio sensors and the adsorbed oxygen concentration when the limit current type air-fuel ratio sensor is used according to the first embodiment of the present invention. FIG. 6 is a characteristic diagram illustrating a relationship between the detected oxygen concentration values of the upstream and downstream air-fuel ratio sensors and the released oxygen concentration when the limit current type air-fuel ratio sensor is used according to the first embodiment of the present invention. FIG. 6 is a characteristic diagram illustrating frequency factors of a normal catalyst and a deteriorated catalyst according to the first embodiment of the present invention. FIG. 5 is a characteristic diagram illustrating a relationship between a catalyst deterioration degree and a frequency factor A according to the first embodiment of the present invention. FIG. 6 is a characteristic diagram illustrating a relationship between an oxygen concentration detection value and an adsorbed oxygen concentration of upstream and downstream air-fuel ratio sensors when using an electromotive force type air-fuel ratio sensor according to Embodiment 2 of the present invention.

Explanation of symbols

  1 engine, 2 exhaust passage, 3 catalyst, 41, 42 oxygen (air-fuel ratio) sensor, 5 injector, 6 air flow meter, 7 electronic control unit.

Claims (1)

An upstream oxygen sensor disposed in the exhaust passage upstream of the exhaust purification catalyst for detecting an oxygen concentration upstream of the exhaust purification catalyst disposed in the exhaust passage of the internal combustion engine;
A downstream oxygen sensor disposed in an exhaust passage downstream of the exhaust purification catalyst for detecting an oxygen concentration downstream of the exhaust purification catalyst;
Air-fuel ratio control means for setting the oxygen concentration upstream of the exhaust purification catalyst to a predetermined value; inflow gas amount detection means for detecting the amount of gas flowing into the exhaust purification catalyst;
Using the oxygen adsorption rate equation calculated using the oxygen concentration and the inflow gas amount upstream and downstream of the exhaust purification catalyst, or the oxygen concentration and the inflow gas amount upstream and downstream of the exhaust purification catalyst, respectively. In a catalyst deterioration detection device comprising a deterioration detection means for detecting deterioration of the exhaust purification catalyst using an oxygen release rate equation calculated respectively.
Each of which is calculated using the upstream and downstream oxygen concentration and the inflow gas amount of the exhaust purification catalyst, wherein the oxygen adsorption rate is formula (i), the upstream and downstream oxygen concentration of the exhaust gas purifying catalyst each of which is calculated by using the gas amount, the formula of the oxygen release rate is an expression (ii), the formula (i) or seeking frequency factor a from the equation (ii), the frequency factor a was previously obtained and the relationship between the exhaust gas purification rate of the exhaust gas purifying catalyst, the catalyst deterioration detector you and detects the deterioration of the exhaust purification catalyst.

However,
Xo (front) is the oxygen concentration upstream of the exhaust purification catalyst,
Xo (rear) is the oxygen concentration downstream of the exhaust purification catalyst,
X _Red (front) is the reducing agent concentration upstream of the exhaust purification catalyst,
Qa is the amount of gas flowing into the exhaust purification catalyst,
A is a frequency factor,
Ea is the activation energy of the exhaust purification catalyst,
R is a gas constant,
T is the temperature of the exhaust purification catalyst,
Y is the oxygen adsorption site concentration,
O is the oxygen adsorption concentration,
α, β, γ, and ω are reaction orders.

Images