Apparatus For Diagnosing Deterioration Of Nox Absorption-reduction Catalyst

Abstract

An apparatus for diagnosing deterioration of a NOx absorption-reduction catalyst provided at an exhaust path of an engine includes a sensor disposed upstream of the catalyst to sense a NOx concentration in emission gas, a calculating unit calculating a first ratio of emission of NOx to inflow of NOx or a second ratio of absorption of NOx to inflow of NOx, and a diagnosing unit which diagnoses deterioration of the catalyst using the first or second ratio as an indicator. The calculating unit calculates inflow of NOx based on an output of the sensor and either the flow volume of the emission gas or a correlation value of the flow volume of the emission gas, calculates the absorption of NOx based on the amount of rich components required for reducing the NOx, and calculates the emission of NOx based on the difference between the inflow and absorption of NOx.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2009-029220 filed Feb. 11, 2009, the description of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Technical Field of the Invention

[0003] The present invention relates to an apparatus for diagnosing deterioration of a NOx absorption-reduction catalyst (hereinafter referred to as an "NOx catalyst"), which apparatus diagnoses deterioration of a NOx catalyst provided at an exhaust path of an internal combustion engine (engine).

[0004] 2. Related Art

[0005] In recent years, so-called lean-burn engines or cylinder injection engines, which control air-fuel ratio to be leaner than a stoichiometric air-fuel ratio (theoretical air-fuel ratio), have been in practical use for the purposes of improving fuel consumption of vehicles. These engines tend to produce more NOx (nitrogen oxides) than normally used engines. Therefore, some of such engines use a NOx absorption-reduction catalyst (NOx catalyst) to ensure decrease of the amount of emission of NOx (hereinafter referred to as "NOx emission").

[0006] The NOx catalyst functions in such a way that it absorbs NOx when the air-fuel ratio of the exhaust gas is lean, and purges (discharges) NOx by reducing the absorbed NOx when the air-fuel ratio has been enriched (or has reached the stoichiometric air-fuel ratio). Therefore, in the case where lean-burn operation continues for a long time, NOx purge control (also called "rich purge control" or "rich spike control") is ensured to be performed, so that the amount of absorption of NOx (hereinafter referred to as "NOx absorption") in the NOx catalyst is prevented from reaching saturation levels. With the NOx purge control, a target air-fuel ratio is intermittently switched to a rich air-fuel ratio during the lean-burn operation to purge NOx by reducing the NOx absorbed in the NOx catalyst.

[0007] When the NOx catalyst has been deteriorated and thus the performance of absorbing NOx is degraded, the NOx emission in the atmospheric air will increase. Therefore, deterioration of the NOx catalyst (degradation in the performance of absorbing NOx) is required to be detected at an earlier occasion.

[0008] In this regard, some techniques have been suggested recently, with which deterioration of NOx catalyst can be diagnosed.

[0009] For example, JP-A-2008-057404 discloses an apparatus for diagnosing deterioration of catalyst, in which a NOx sensor is disposed downstream of a NOx catalyst to sense the NOx concentration in the gas emitted (hereinafter referred to as "emission gas") from the NOx catalyst. This document further discloses that the output of the NOx sensor is adapted to be accumulated in a predetermined time period including the period in the vicinity of completing the NOx purge (rich spike). Then, in this apparatus, deterioration of the NOx catalyst is diagnosed based on whether or not the sum of the outputs of the NOx sensor (amount of NOx emission) has exceeded a predetermined deterioration determining threshold.

[0010] Further, for example, JP-A-2008-064075 discloses an apparatus for diagnosing deterioration of catalyst, in which a NOx sensor is disposed upstream of a NOx catalyst to accumulate the outputs of the NOx sensor. This document further discloses that the total amount of O.sub.2 and NOx that have been absorbed in the NOx catalyst (total amount of absorption (hereinafter referred to as "total absorption")) before start of NOx purge (rich spike) is calculated while the NOx purge is performed, based on the output of an O.sub.2 sensor disposed downstream of the NOx catalyst. Then, in this apparatus, deterioration of the NOx catalyst is diagnosed based on the sum of the outputs (amount of inflow of NOx (hereinafter referred to as "NOx inflow") into the NOx catalyst) of the NOx sensor and the total absorption.

[0011] Generally, as the size of a NOx catalyst (catalytic capacity) increases, the NOx absorption and the total absorption will increase. Also, as the flow volume of emission gas (hereinafter referred to as "emission gas flow") that flows into a NOx catalyst increases, the NOx emission from the NOx catalyst will increase.

[0012] Deterioration diagnosis of a NOx catalyst may be ensured to be conducted based on the sum of the outputs of a NOx sensor (NOx emission) as disclosed in JP-A-2008-057404. Also, deterioration diagnosis of a NOx catalyst may be ensured to be conducted based on the sum of the outputs of a NOx sensor (NOx inflow into the NOx catalyst) and a total absorption as disclosed in JP-A-2008-064075. However, with these configurations, the accuracy in the deterioration diagnosis will be impaired unless the deterioration determining threshold is set according to the catalytic capacity or the engine operating condition (emission gas flow).

[0013] However, the configuration of setting the deterioration determining threshold according to the catalytic capacity or the engine operating condition (emission gas flow) may create a drawback. Specifically, with such a configuration, developing and designing the systems for diagnosing deterioration of NOx catalyst will have to involve time-consuming processes of checking the deterioration determining thresholds, leading to low productivity. As a countermeasure against this drawback, the number of the processes of checking deterioration determining thresholds may be decreased by limiting the engine operating condition, under which deterioration diagnosis is conducted, so that the emission gas flow will fall on a predetermined certain value. However, with this countermeasure, the frequency of conducting deterioration diagnosis may fall off and thus the required frequency of conducting deterioration diagnosis is unlikely to be ensured.

SUMMARY OF THE INVENTION

[0014] The present invention has been made in light of the problem set forth above and has as its object to provide an apparatus for diagnosing deterioration of a NOx catalyst, which is able to readily enhance the accuracy in the deterioration diagnosis of a NOx catalyst, readily enhance productivity (readily decrease the number of checking processes) and readily ensure the frequency of conducting deterioration diagnosis, by mitigating the influences that may be exerted by the size of the NOx catalyst (catalytic capacity) or by the operational states upon the deterioration diagnosis of the NOx catalyst.

[0015] In order to achieve the object, the present invention provides, as one aspect, an apparatus for diagnosing deterioration of a NOx absorption-reduction catalyst provided at an exhaust path of an internal combustion engine, comprising: a NOx sensor disposed upstream of the catalyst to sense a NOx concentration in emission gas that flows into the catalyst; a deterioration diagnostic indicator calculating unit which calculates a first ratio of the amount of emission of NOx from the catalyst, to the amount of inflow of NOx into the catalyst, or a second ratio of the amount of absorption of NOx in the catalyst, to the amount of inflow of NOx into the catalyst; and a deterioration diagnosing unit which diagnoses deterioration of the catalyst by using the first ratio or the second ratio as a deterioration diagnostic indicator, wherein the deterioration diagnostic indicator calculating unit calculates the amount of inflow of NOx into the catalyst based on an output of the NOx sensor and either the flow volume of the emission gas into the catalyst or a correlation value of the flow volume of the emission gas, calculates the amount of absorption of NOx in the catalyst based on the amount of rich components required for reducing the NOx absorbed by the NOx catalyst, and calculates the amount of emission of NOx from the catalyst based on the difference between the amount of inflow of NOx into the catalyst and the amount of absorption of NOx in the catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the accompanying drawings:

[0017] FIG. 1 is a schematic diagram illustrating an engine control system in general, according to a first embodiment of the present invention;

[0018] FIG. 2 is a flow diagram illustrating a process flow of a NOx catalyst deterioration diagnostic routine, according to the first embodiment;

[0019] FIG. 3 is a time diagram illustrating fuel injection quantity and output behaviors of individual sensors at the time of conducting deterioration diagnosis of a NOx catalyst, according to the first embodiment;

[0020] FIG. 4 illustrates a relationship between deterioration factor of a NOx catalyst and non-purification factor of the NOx catalyst;

[0021] FIG. 5 is a schematic diagram illustrating an engine control system in general, according to a second embodiment of the present invention;

[0022] FIG. 6 is a flow diagram illustrating a process flow of a NOx catalyst deterioration diagnostic routine, according to the second embodiment;

[0023] FIG. 7 is a flow diagram illustrating a process flow of a NOx emission summing routine, according to the second embodiment;

[0024] FIG. 8 is a time diagram illustrating fuel injection quantity and output behaviors of individual sensors at the time of conducting deterioration diagnosis of a NOx catalyst, according to the second embodiment;

[0025] FIG. 9 is a flow diagram illustrating a process flow of a NOx emission summing routine, according to a third embodiment of the present invention; and

[0026] FIG. 10 is a time diagram illustrating fuel injection quantity and output behaviors of individual sensors at the time of conducting deterioration diagnosis of a NOx catalyst, according to the third embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] With reference to the accompanying drawings, hereinafter will be specifically described three embodiments of the present invention applied to a lean-burn engine.

First Embodiment

[0028] Referring to FIGS. 1 to 4, a first embodiment of the present invention will be described.

[0029] First, referring to FIG. 1, the configuration of an engine control system in general of the first embodiment will be described.

[0030] The engine control system includes an engine 11, i.e. an internal combustion engine. The intake pipe 12 of the engine 11 is provided with an air cleaner 13 disposed most upstream thereof, and an air flow meter 14 disposed downstream of the air cleaner 13 to sense the flow volume of intake air (hereinafter referred to as "intake air flow"). A throttle valve 15 and a throttle position sensor 16 that detects a throttle position are disposed downstream of the air flow meter 14.

[0031] Further, a surge tank 17 is disposed downstream of the throttle valve 15. An intake-pipe pressure sensor 18 is disposed at the surge tank 17 to sense the pressure in the intake pipe. The surge tank 17 is provided with an intake manifold 19 which introduces air to the individual cylinders of the engine 11. Fuel injection valves 20 are attached to the intake manifold 19 so as to be close to the intake ports of the respective cylinders to inject fuel toward the intake ports.

[0032] Also, a three-way catalyst 22 and a NOx catalyst 23 (NOx absorption-reduction catalyst) are disposed in series midway in the exhaust pipe 21 (exhaust path) of the engine 11. The three-way catalyst 22 purges HC, CO, and the like, contained in the emission gas. The NOx catalyst 23 purges NOx contained in the emission gas. The three-way catalyst 22 disposed upstream of the NOx catalyst 23 is formed to have a relatively small capacity, so that warming up can be completed at an earlier occasion of the startup to decrease emission of the exhaust gas at the startup.

[0033] The NOx catalyst 23 on the downstream side absorbs NOx when the air-fuel ratio of the emission gas is leaner than a stoichiometric air-fuel ratio (theoretical air-fuel ratio). When the air-fuel ratio has been enriched (or has reached the stoichiometric level), the NOx catalyst 23 discharges NOx by reducing and purging the absorbed NOx. The NOx catalyst 23 is formed to have a relatively large capacity, so that NOx can be well absorbed in a high-load zone where the amount of NOx in the emission gas becomes large.

[0034] The exhaust pipe 21 is also provided with an air-fuel ratio sensor 24 (A/F sensor) disposed upstream of the three-way catalyst 22 to sense the air-fuel ratio of the emission gas, a NOx sensor 25 disposed upstream of the NOx catalyst 23 (downstream of the three-way catalyst 22) to sense the NOx concentration in the emission gas that flows into the NOx catalyst 23, and an O.sub.2 sensor 26 disposed downstream of the NOx catalyst 23 to sense the O.sub.2 concentration in the emission gas emitted from the NOx catalyst 23. The output voltage of the O.sub.2 sensor 26 is reversed depending on whether the air-fuel ratio of the emission gas is richer or leaner than the stoichiometric level. The output of the air-fuel sensor 24, on the other hand, substantially linearly changes according to the air-fuel ratio of the emission gas.

[0035] An O.sub.2 sensor may be disposed upstream of the three-way catalyst 22, replacing the air-fuel ratio sensor 24. Also, an air-fuel ratio sensor may be disposed downstream of the NOx catalyst 23, replacing the O.sub.2 sensor 26.

[0036] The NOx sensor 25 upstream of the NOx catalyst 23 is incorporated with a function of sensing O.sub.2 or a function of detecting an air-fuel ratio in addition to the function of sensing NOx.

[0037] The engine 11 has a cylinder block which is attached with a cooling water temperature sensor 27 that senses the temperature of the cooling water, and a crank angle sensor 28 that senses the engine speed.

[0038] The outputs of these various sensors are inputted to an engine control circuit (hereinafter referred to as an "ECU") 29. The ECU 29 is principally configured by a microcomputer incorporating a ROM (storage medium) that stores engine control programs. The ignition timing, fuel injection quantity and the like are controlled according to the engine operational states by executing these programs.

[0039] The NOx catalyst 23 absorbs NOx when the air-fuel ratio of the emission gas is lean, and purges (discharges) NOx by reducing the absorbed NOx when the air-fuel ratio has been enriched (has reached the stoichiometric level). Accordingly, in the case where lean-burn operation continues for a long time, NOx purge control (also called "rich purge control" or "rich spike control") is ensured to be performed, so that the NOx absorption in the NOx catalyst 23 can be prevented from reaching the saturation level. In the NOx purge control, a target air-fuel ratio is intermittently switched to a rich air-fuel ratio during the lean-burn operation to purge NOx by reducing the NOx absorbed in the NOx catalyst 23.

[0040] When the NOx catalyst 23 is deteriorated to degrade the function of absorbing NOx, the NOx emission into the atmospheric air will be increased. For this reason, the deterioration of the NOx catalyst 23 (degradation in the function of absorbing NOx) is required to be detected at an earlier occasion.

[0041] In this regard, an approach taken in the first embodiment is to calculate the ratio of the NOx emission from the NOx catalyst 23, to the NOx inflow into the NOx catalyst 23 (hereinafter referred to as "non-purification factor" (first ratio)) and to use the non-purification factor as a deterioration diagnostic indicator to thereby diagnose deterioration of the NOx catalyst 23. The non-purification factor is calculated from the following Formula (1):

Non-purification factor = NOx emission/NOx inflow = ( NOx inflow-NOx absorption ) / NOx inflow = { NOx inflow - ( Rich component inflow-Rich component emission ) } / NOx inflow ( 1 ) ##EQU00001##

(where, NOx emission=NOx inflow-NOx absorption; and NOx absorption=Rich component inflow-Rich component emission)

[0042] The "NOx inflow" in Formula (1) corresponds to the amount of NOx that flows into the NOx catalyst 23. The NOx inflow may be calculated by multiplying the output of the NOx sensor 25 (NOx concentration sensed in the emission gas that flows into the NOx catalyst 23) disposed upstream of the NOx 23, by the emission gas flow. Then, the products may be summed to update the sum of the NOx inflow. This process of calculation may be repeated at a predetermined operation cycle. The emission gas flow may be calculated based on the intake air flow sensed by the air flow meter 14, taking into account the flow delay of the air system present from the position of the air flow meter 14 to the position of the NOx sensor 25.

[0043] The "NOx emission" in Formula (1) corresponds to the amount of NOx emitted from the NOx catalyst 23. The NOx emission may be calculated by obtaining the difference between the NOx inflow into the NOx catalyst 23 and the NOx absorption in the NOx catalyst 23.

NOx emission=NOx inflow-NOx absorption (2)

[0044] The NOx absorption in the NOx catalyst 23 may be calculated based on the amount of rich components (hereinafter referred to as "rich component amount") required for reducing NOx absorbed by the NOx catalyst 23. Specifically, the NOx absorption in the NOx catalyst 23 may be calculated by obtaining the difference between the amount of rich components flowing into the NOx catalyst 23 (hereinafter referred to as "rich component inflow" in the NOx catalyst 23) and the amount of rich components emitted from the NOx catalyst 23 (hereinafter referred to as "rich component emission" of the NOx catalyst 23), when reducing the NOx absorbed in the NOx catalyst 23 by performing NOx purge (rich purge).

NOx absorption=Rich component inflow-Rich component emission (3)

[0045] In the case where the NOx sensor 25 disposed upstream of the NOx catalyst 23 is incorporated with an air-fuel ratio detecting function, the air-fuel ratio upstream of the NOx catalyst 23 (air-fuel ratio of the emission gas that flows into the NOx catalyst 23) may be detected by the air-fuel ratio detecting function of the NOx sensor 25. Then, the rich component inflow into the NOx catalyst 23 may be calculated based on the detected upstream air-fuel ratio and the emission gas flow (intake airflow), using the following Formula (4):

Rich component inflow = Emission gas flow/Upstream air-fuel ratio - Emission gas flow/Theoretica l air-fuel ratio ( 4 ) ##EQU00002##

[0046] In Formula (4), the "Rich component inflow" is calculated by subtracting the "Emission gas flow/Theoretical air-fuel ratio" from the "Emission gas flow/Upstream air-fuel ratio" to thereby calculate the rich component inflow exceeding the theoretical air-fuel ratio.

[0047] In the case where the NOx sensor 25 disposed upstream of the NOx catalyst 23 is not incorporated with the air-fuel ratio detecting function (or in the case where there is no sensor that senses the air-fuel ratio upstream of the NOx catalyst 23), the air-fuel ratio upstream of the NOx catalyst 23 cannot be directly detected. Therefore, the air-fuel ratio detected by the air-fuel ratio sensor 24 upstream of the three-way catalyst 22 may be used as the air-fuel ratio upstream of the NOx catalyst 23.

[0048] The three-way catalyst 22 exerts high emission gas purification efficiency when the air-fuel ratio of the emission gas fails within the purification window approximate to the stoichiometric level. This is because a target air-fuel ratio of the emission gas is set to be richer than the purification window of the three-way catalyst 22 while NOx purge (rich purge) is performed, and thus because the purification factor of the three-way catalyst 22 is drastically lowered, resulting in that the rich components in the emission gas are hardly purified (consumed) in the three-way catalyst 22 but flow into the NOx catalyst 23, and further resulting in that the air-fuel ratio upstream of the three-way catalyst 22 becomes substantially the same as the air-fuel ratio upstream of the NOx catalyst 23.

[0049] The rich component inflow into the NOx catalyst 23 may be estimated based on at least one of the fuel injection quantity, target air-fuel ratio, air-fuel ratio correction amount, and the like.

[0050] The rich component emission of the NOx catalyst 23 may be calculated based on the emission gas flow from the NOx catalyst 23 (intake air flow) and the air-fuel ratio downstream of the NOx catalyst 23 (air-fuel ratio of the emission gas from the NOx catalyst 23), using the following Formula (5):

Rich component emission = Emission gas flow/Downstream air-fuel ratio - Emission gas flow/Theoretical air-fuel ratio ( 5 ) ##EQU00003##

[0051] In Formula (5), the "Rich component emission" is obtained by subtracting the "Emission gas flow/Theoretical air-fuel ratio" from the "Emission gas flow/Downstream air-fuel ratio" to thereby calculate the rich component emission exceeding the theoretical air-fuel ratio.

[0052] In the case where an air-fuel sensor is disposed downstream of the NOx catalyst 23, the air-fuel ratio downstream of the NOx catalyst 23 may be detected by this air-fuel ratio sensor. In the example of the configuration shown in FIG. 1, since the gas sensor downstream of the NOx catalyst 23 is the O.sub.2 sensor 26, the air-fuel ratio downstream of the NOx catalyst 23 cannot be directly detected. In this case, the air-fuel ratio downstream of the NOx catalyst 23 may be estimated by multiplying the O.sub.2 concentration sensed by the O.sub.2 sensor 26 downstream of the NOx catalyst 23, by an air-fuel ratio conversion coefficient k, using the following Formula (6):

Downstream air-fuel ratio=O.sub.2concentration.times.k (6)

[0053] Substituting the NOx inflow and the NOx absorption (=Rich component inflow-Rich component emission) calculated in this way into Formula (1), the non-purification factor of the NOx catalyst 23 can be calculated. Further, using the non-purification factor as a deterioration diagnostic indicator, deterioration diagnosis of the NOx catalyst 23 can be conducted.

[0054] The NOx sensor 25 has a property of sensing not only NOx but also an ammonia component (NH.sub.3) when the air-fuel ratio of the emission gas is rich. For this reason, during the rich period when the ammonia component increases, the NOx concentration sensed by the NOx sensor 25 may result in a larger value than will be obtained from an actual amount of NOx. Specifically, the NOx concentration will become larger by the degree corresponding to the concentration of the ammonia component.

[0055] Taking this property into consideration in the first embodiment, the NOx concentration to be sensed is ensured to be set to "0" during the rich period when the emission gas that flows around the NOx sensor 25 becomes richer than the stoichiometric level, so that the NOx inflow into the NOx catalyst 23 will be inhibited from being summed. According to this configuration, the NOx inflow into the NOx catalyst 23 can be prevented from being overestimated due to the presence of the ammonia component. As a result, the accuracy of calculating the NOx inflow into the NOx catalyst 23 can be prevented from being degraded.

[0056] A larger value of the non-purification factor of the NOx catalyst 23 calculated from Formula (1) means that the degree of deterioration of the NOx catalyst 23 is larger by that much (see FIG. 4). Therefore, deterioration of the NOx catalyst 23 is determined based on whether or not the non-purification factor is equal to or larger than a predetermined deterioration determining threshold. FIG. 2 shows a NOx catalyst deterioration diagnostic routine, with which a deterioration diagnostic process for the NOx catalyst 23 is performed under the control of the ECU 29 as will be described below.

[0057] The NOx catalyst deterioration diagnostic routine shown in FIG. 2 is repeatedly executed at a predetermined calculation cycle during engine operation. This routine plays a roll of the deterioration diagnostic indicator calculating means and the deterioration diagnosing means. Upon start of the present routine, it is determined, in step 101, first, whether or not deterioration diagnosis execution conditions have been met. For example, it is determined whether or not the following conditions (1) to (4) have been met.

[0058] (1) That the temperature of the NOx catalyst 23 falls in a predetermined temperature range suitable for purging (absorbing/reducing) NOx.

[0059] (2) That a predetermined period has expired (or predetermined summed traveling distance, predetermined summed fuel consumption, etc. has been reached) from the completion of the previous deterioration diagnosis.

[0060] (3) That the engine operational state is a steady operational state.

[0061] (4) That no malfunction has been detected in the engine control system, the sensor system or the like by the self-diagnostic function loaded on the vehicle.

[0062] If any one of the conditions (1) to (4) has not been met, the deterioration diagnosis execution conditions will not be satisfied, and thus the present routine is ended without performing the subsequent processes.

[0063] On the other hand, if the conditions (1) to (4) have been met, the deterioration diagnosis execution conditions will be satisfied and the process of step 102 and the subsequent processes will be performed to conduct deterioration diagnosis of the NOx catalyst 23 as follows. First, in step 102, the output of the NOx sensor 25 upstream of the NOx catalyst 23 is obtained to detect the NOx concentration upstream of the NOx catalyst 23 (the NOx concentration in the emission gas that flows into the NOx catalyst 23). Then, control proceeds to step 103 where the output of the air flow meter 14 (intake air flow Ga) is obtained as information (correlation value) correlated to the emission gas flow into the NOx catalyst 23 to thereby estimate the emission gas flow Ga into the NOx catalyst 23. In this regard, the emission gas flow Ga may be estimated taking into account the flow delay of the air system present from the position of the air flow meter 14 to the position of the NOx catalyst 23.

[0064] Subsequently, control proceeds to step 104 where it is determined whether or not the air-fuel ratio of the emission gas upstream of the NOx catalyst 23 (air-fuel ratio of the emission gas that flows around the NOx sensor 25) is lean, based on the results obtained from the O.sub.2 sensing function or the air-fuel ratio detecting function of the NOx sensor 25 upstream of the NOx catalyst 23. As a result, when the air-fuel ratio of the emission gas upstream of the NOx catalyst 23 is determined to be rich, it is determined that the NOx concentration sensed by the NOx sensor 25 may be larger than will be obtained from an actual amount of NOx. Specifically, the NOx concentration may be larger by the degree corresponding to the concentration of the ammonia component. Then, control proceeds to step 105. In step 105, the NOx concentration to be sensed is set to "0" to inhibit the NOx inflow into the NOx catalyst 23 from being summed, and then control proceeds to step 106.

[0065] On the other hand, in step 104, when the air-fuel ratio of the emission gas upstream of the NOx catalyst 23 is determined to be lean, control proceeds to step 106 without performing the process in step 105.

[0066] Then, in step 106, the NOx concentration ((B) in FIG. 3) upstream of the NOx catalyst 23 sensed by the NOx sensor 25 is multiplied by the emission gas flow Ga and by a calculation time interval dt to calculate the NOx inflow (=NOx concentrationGadt) into the NOx catalyst 23 during the calculation time interval dt this time. Then, the resultant value is added to the previously calculated sum of the NOx inflow to thereby update the sum of the NOx inflow.

[0067] After that, control proceeds to step 107 where the NOx absorption in the NOx catalyst 23 is summed based on the rich component amount required for completely reducing the NOx absorbed in the NOx catalyst 23. Specifically, the difference is obtained between the rich component inflow ((A) in FIG. 3) into the NOx catalyst 23 and the rich component emission ((C) in FIG. 3) from the NOx catalyst 23, when reducing the NOx absorbed in the NOx catalyst 23 by performing NOx purge (rich purge). The difference is then multiplied by the operation time interval dt to calculate the NOx absorption in the NOx catalyst 23 during the operation time interval dt this time. Then, the resultant value is added to the sum of the previously calculated NOx absorption to update the sum of the NOx absorption.

[0068] In this regard, the rich component inflow ((A) in FIG. 3) into the NOx catalyst 23 may be calculated by dividing the emission gas flow Ga into the NOx catalyst 23 by the air-fuel ratio upstream of the NOx catalyst 23, using the following Formula (7):

Rich component inflow=Ga/Upstream air-fuel ratio-Ga/Theoretical air-fuel ratio (7)

[0069] The air-fuel ratio detected by the air-fuel ratio sensor 24 upstream of the three-way catalyst 22 may be used as the air-fuel ratio upstream of the NOx catalyst 23.

[0070] The rich component emission ((C) in FIG. 3) from the NOx catalyst 23 may be calculated by dividing the emission gas flow Ga from the NOx catalyst 23 by the air-fuel ratio downstream of the NOx catalyst 23, using the following Formula (8):

Rich component emission=Ga/Downstream air-fuel ratio-Ga/Theoretical air-fuel ratio (8)

[0071] As shown in the example of configuration of FIG. 1, when the gas sensor downstream of the NOx catalyst 23 is the O.sub.2 sensor 26, the air-fuel ratio downstream of the NOx catalyst 23 cannot be directly detected. Therefore, the air-fuel ratio downstream of the NOx catalyst 23 may be estimated by multiplying the O.sub.2 concentration sensed by the O.sub.2 sensor 26 downstream of the NOx catalyst 23, by the air-fuel ratio conversion coefficient k, using the following Formula (9).

Downstream air-fuel ratio=O.sub.2concentration.times.k (9)

[0072] Subsequently, control proceeds to step 108 where the non-purification factor of the NOx catalyst 23 is calculating by substituting the NOx inflow summed in step 106 and the NOx absorption summed in step 107 into the following Formula (10):

Non-purification factor=(NOx inflow-NOx absorption)/NOx inflow (10)

[0073] Then, control proceeds to step 109 where the non-purification factor of the NOx catalyst 23 is compared with the predetermined deterioration determining threshold. When the non-purification factor is equal to or less than the deterioration determining threshold, the NOx catalyst 23 is determined, in step 111, not to have been deteriorated (determined to be normal). When the non-purification factor has exceeded the deterioration determining threshold, the NOx catalyst 23 is determined, in step 110, to have been deteriorated.

[0074] According to the first embodiment described above, deterioration diagnosis of the NOx catalyst 23 is conducted using the non-purification factor of the NOx catalyst 23 as a deterioration diagnostic indicator, which factor is calculated based on the output of the NOx sensor 25, for example, disposed upstream of the NOx catalyst 23. Therefore, compared with the case where the output sum of the NOx sensor 25 or the total absorption is used as a deterioration diagnostic indicator as disclosed in JP-A-2008-057404 or JP-A-2008-064075, the influences can be mitigated, which may be exerted by the size of the NOx catalyst 23 (catalytic capacity) or by the engine operational states upon the deterioration diagnosis of the NOx catalyst. Thus, the accuracy in the deterioration diagnosis of the NOx catalyst 23 as well as the productivity (decrease in the number of checking processes) can be readily enhanced. Also, the frequency of conducting deterioration diagnosis can be readily ensured.

[0075] In the first embodiment, the non-purification factor has been calculated, which is a ratio of the NOx emission from the NOx catalyst 23 to the NOx inflow into the NOx catalyst 23. Alternative to this, a purification factor (second ratio) may be calculated, which is a ratio of the NOx absorption in the NOx catalyst 23 to the NOx inflow into the NOx catalyst 23. Then the purification factor may be used as a deterioration diagnostic indicator to determine deterioration of the NOx catalyst 23, based on whether or not the purification factor is equal to or less than the predetermined determining threshold.

[0076] The purification factor and the non-purification factor of the NOx catalyst 23 have a relationship as expressed by the following Formula (11):

Purification factor=1-Non-purification factor (11)

Substituting the NOx absorption and the NOx inflow calculated in the same manner as in the first embodiment into the following Formula (12), the purification factor may be calculated.

Purification factor=NOx absorption/NOx inflow (12)

[0077] The purification factor of the NOx catalyst 23 calculated by Formula (12) may be used as a deterioration diagnostic indicator to conduct deterioration diagnosis of the NOx catalyst 23. According to this deterioration diagnosis, completely the same effect as in the first embodiment can be obtained.

Second Embodiment

[0078] Referring to FIGS. 5 to 8 hereinafter will be described a second embodiment of the present invention. In the second and the subsequent embodiments, the components identical with or similar to those in the first embodiment are given the same reference numerals for the sake of omitting explanation.

[0079] In the first embodiment, the NOx sensor 25 has been disposed upstream of the NOx catalyst 23. However, as shown in FIGS. 5 to 8, in the present embodiment, a NOx sensor 31 is disposed downstream of the NOx catalyst 23, and an O.sub.2 sensor 32 or an air-fuel ratio sensor is disposed upstream of the NOx catalyst 23 (downstream of the three-way catalyst 22). In the present embodiment, the NOx sensor 31 is incorporated with an air-fuel ratio detecting function as well as a NOx sensing function. Other hardware configurations are similar to the first embodiments.

[0080] Similar to the first embodiment, in the second embodiment as well, the non-purification factor is calculated, which is a ratio of the NOx emission from the NOx catalyst 23 to the NOx inflow into the NOx catalyst 23. Then, using the calculated non-purification factor as a deterioration diagnostic indicator, deterioration diagnosis of the NOx catalyst 23 is conducted. In the present embodiment, however, since the NOx sensor 31 is positioned downstream of the NOx catalyst 23, the NOx inflow into the NOx catalyst 23 cannot be calculated from the output of the NOx sensor 31.

[0081] Therefore, in the second embodiment, the non-purification factor of the NOx catalyst 23 is calculated by the following Formula (13):

Non-purification factor = NOx emission/NOx inflow = NOx emission/(NOx absorption + NOx emission ) = NOx emission / { ( Rich component inflow-Rich component emission ) + NOx emission } ( 13 ) ##EQU00004##

(where, NOx inflow=NOx absorption+NOx emission; and NOx absorption=Rich component inflow-Rich component emission)

[0082] In Formula (13), the "NOx emission" corresponds to an amount of NOx emitted from the NOx catalyst 23. The NOx emission may be calculated by multiplying the output of the NOx sensor 31 disposed downstream of the NOx catalyst 23 (the NOx concentration sensed in the emission gas emitted from the NOx catalyst 23), by the emission gas flow. Then, the products may be summed to update the sum of the NOx emission. This process of calculation may be repeated at a predetermined operation cycle. The emission gas flow may be calculated based on the intake air flow sensed by the air flow meter 14, taking into account the flow delay of the air system present from the position of the air flow meter 14 to the position of the NOx sensor 31.

[0083] The "NOx inflow" in Formula (13) corresponds to an amount of NOx that flows into the NOx catalyst 23, and may be calculated by adding the NOx absorption in the NOx catalyst 23 to the NOx emission from the NOx catalyst 23.

NOx inflow=NOx absorption+NOx emission (14)

[0084] The NOx absorption of the NOx catalyst 23 may be calculated based on the rich component amount required for reducing the NOx absorbed in the NOx catalyst 23. Specifically, the NOx absorption may be calculated by obtaining the difference between the rich component inflow into the NOx catalyst 23 and the rich component emission from the NOx catalyst 23, when reducing the NOx absorbed in the NOx catalyst 23 by performing the NOx purge (rich purge).

NOx absorption=Rich component inflow-Rich component emission (15)

[0085] In the case where the air-fuel ratio sensor is disposed upstream of the NOx catalyst 23 (downstream of the three-way catalyst 22), the air-fuel ratio upstream of the NOx catalyst 23 (the air-fuel ratio of the emission gas that flows into the NOx catalyst 23) may be detected by the air-fuel ratio sensor. Then, the rich component inflow into the NOx catalyst 23 may be calculated based on the detected upstream air-fuel ratio and the emission gas flow (intake air flow), using the following Formula (16).

Rich component inflow = Emission gas flow/Upstream air-fuel ratio - Emission gas flow/Theoretical air-fuel ratio ( 16 ) ##EQU00005##

[0086] In the example of the configuration shown in FIG. 5, since the gas sensor upstream of the NOx catalyst 23 (downstream of the three-way catalyst 22) is the O.sub.2 sensor 32, the air-fuel ratio upstream of the NOx catalyst 23 cannot be directly detected. Therefore, as has been described in the first embodiment, the air-fuel ratio detected by the air-fuel ratio sensor 24 upstream of the three-way catalyst 22 may be used as the air-fuel ratio upstream of the NOx catalyst 23. It should be noted that the rich component inflow into the NOx catalyst 23 may be estimated based on at least one of the fuel injection quantity, target air-fuel ratio, air-fuel ratio correction amount, and the like.

[0087] The rich component emission from the NOx catalyst 23 may be calculated based on the emission gas flow (intake air flow) from the NOx catalyst 23 and the air-fuel ratio downstream of the NOx catalyst 23 (the air-fuel ratio of the emission gas from the NOx catalyst 23), using the following Formula (17).

Rich component emission = Emission gas flow/Downstream air-fuel ratio - Emission gas flow/Theoretical air-fuel ratio ( 17 ) ##EQU00006##

[0088] In the present embodiment, the NOx sensor 31 downstream of the NOx catalyst 23 is incorporated with the air-fuel ratio detecting function. Therefore, the air-fuel ratio downstream of the NOx catalyst 23 may be detected using the air-fuel ratio detecting function of the NOx sensor 31. In the case where the NOx sensor 31 downstream of the NOx catalyst 23 is incorporated with the O.sub.2 sensing function instead of the air-fuel ratio detecting function, the air-fuel ratio may be estimated by multiplying the O.sub.2 concentration sensed by the O.sub.2 sensing function, by the air-fuel ratio conversion coefficient k.

[0089] The NOx emission and the NOx absorption (=Rich component inflow-Rich component emission) calculated as described above may be substituted into Formula (13) to calculate the non-purification factor of the NOx catalyst 23. The obtained non-purification factor may be used as a deterioration diagnostic indicator to conduct deterioration diagnosis of the NOx catalyst 23.

[0090] Further, in the present embodiment, consideration has been given to the fact that the NOx concentration sensed by the NOx sensor 31 becomes larger than will be obtained from the actual amount of NOx during the rich period when the ammonia component increases. Specifically, the NOx concentration will become larger by the degree corresponding to the concentration of the ammonia component. Thus, the NOx concentration to be sensed is set to "0" in the present embodiment during the rich period when the emission gas that flows into so the NOx catalyst 23 becomes richer than the stoichiometric level. In this way, the NOx emission from the NOx catalyst 23 is ensured to be inhibited from being summed.

[0091] When the NOx sensor 31 is disposed downstream of the NOx catalyst 23 as in the present embodiment, the concentration of the ammonia component may become high even when the emission gas that flows around the NOx sensor 31 is not rich. To explain in detail, when the emission gas that flows into the NOx catalyst 23 has been enriched, the concentration of the ammonia component in the emission gas is estimated to have reached a high level. In such a case, the rich components are consumed with the reductive reaction against the absorbed NOx in the course that the emission gas flows through the NOx catalyst 23, while the ammonia component passes through the NOx catalyst 23. For this reason, the concentration of the ammonia component may become high even when the emission gas that has flowed out of the NOx catalyst 23 and flows around the NOx sensor 31 is not rich.

[0092] Considering the above, the NOx emission from the NOx catalyst 23 may be ensured to be inhibited from being summed, as in the present embodiment, during the rich period when the emission gas that flows into the NOx catalyst 23 is enriched. According to this configuration, the NOx emission from the NOx catalyst 23 can be prevented from being overestimated due to the presence of the ammonia component. In this way, the accuracy of calculating the NOx emission from the NOx catalyst 23 can be prevented from being degraded.

[0093] In the present embodiment, summing the NOx emission from the NOx catalyst 23 is ensured to be inhibited during the period when the output of the O.sub.2 sensor 32 upstream of the NOx catalyst 23 is rich. Also, summing the NOx emission is ensured to be inhibited even after completing the NOx purge (rich purge) up until the expiration of a predetermined period of time. Inhibition of summing the NOx emission for a while after completing the NOx purge (rich purge) is based on an idea of considering the flow delay of the enriched emission gas, which delay is caused up until the emission gas reaches the NOx sensor 31 downstream of the NOx catalyst 23.

[0094] The deterioration diagnostic process for the NOx catalyst 23 of the present embodiment described above is performed by the ECU 29 according to a NOx catalyst deterioration diagnostic routine shown in FIG. 6 as will be described below.

[0095] The NOx catalyst deterioration diagnostic routine shown in FIG. 6 is repeatedly performed at a predetermined operation cycle during the engine operation. This routine plays the role of the deterioration diagnostic indicator calculating means and the deterioration diagnosing means. Upon start of the present routine, it is determined, in step 201, first, whether or not deterioration diagnosis execution conditions have been met, in the same manner as in the first embodiment. When the deterioration diagnosis execution conditions have not been met, the present routine is ended without performing the subsequent processes.

[0096] Conversely, when it is determined, in step 201, that the deterioration diagnosis execution conditions have been met, the process in step 202 and the subsequent processes are performed to conduct deterioration diagnosis of the NOx catalyst 23 as follows. First, in step 202, a NOx emission summing routine shown in FIG. 7, which will be described later, is performed to sum the NOx emission from the NOx catalyst 23.

[0097] After that, control proceeds to step 203 where the NOx absorption in the NOx catalyst 23 is summed based the rich component amount required for completely reducing the NOx absorbed in the NOx catalyst 23. Specifically, the difference is obtained between the rich component inflow ((A) in FIG. 8) into the NOx catalyst 23 and the rich component emission ((C) in FIG. 8) from the NOx catalyst 23, when reducing the NOx absorbed in the NOx catalyst 23 by performing the NOx purge (rich purge). The difference is multiplied by the calculation time interval dt to calculate the NOx absorption in the NOx catalyst 23 during the calculation time interval dt this time. The resultant value is then added to the previously calculated sum of the NOx absorption to update the sum of the NOx absorption.

[0098] In this regard, the rich component inflow ((A) in FIG. 8) into the NOx catalyst 23 may be calculated by dividing the emission gas flow Ga into the NOx catalyst 23 by the air-fuel ratio upstream of the NOx catalyst 23, using the following Formula (18).

Rich component inflow=Ga/Upstream air-fuel ratio-Ga/Theoretical air-fuel ratio (18)

[0099] The air-fuel ratio detected by the air-fuel ratio sensor 24 upstream of the three-way catalyst 22 may be used as the air-fuel ratio upstream of the NOx catalyst 23.

[0100] Further, the rich component emission ((C) in FIG. 8) from the NOx catalyst 23 may be calculated by dividing the emission flow Ga from the NOx catalyst 23 by the air-fuel ratio downstream of the NOx catalyst 23, using the following Formula (19).

Rich component emission=Ga/Downstream air-fuel ratio-Ga/Theoretical air-fuel ratio (19)

[0101] The air-fuel ratio detected by the air-fuel ratio detecting function of the NOx sensor 31 downstream of the NOx catalyst 23 may be used as the air-fuel ratio downstream of the NOx catalyst 23.

[0102] Subsequently, control proceeds to step 204 where the non-purification factor of the NOx catalyst 23 is calculated by substituting the NOx emission summed in step 202 and the NOx absorption summed in step 203 into the following Formula (20):

Non-purification factor=NOx emission/(NOx absorption+NOx emission) (20)

[0103] Then, control proceeds to step 205 where the non-purification factor of the NOx catalyst 23 is compared with a predetermined deterioration determining threshold. When the non-purification factor is equal to or less than the deterioration determining threshold, the NOx catalyst 23 is determined, in step 207, not to have been deteriorated (to be normal). When the non-purification factor has exceeded the deterioration determining threshold, the NOx catalyst 23 is determined, in step 206, to have been deteriorated.

[0104] The NOx emission summing routine shown in FIG. 7 is a sub-routine executed in step 202 of the NOx catalyst deterioration diagnostic routine shown in FIG. 6. Upon start of the NOx emission summing routine, the output of the NOx sensor 31 downstream of the NOx catalyst 23 is obtained, first, in step 301, to detect the NOx concentration downstream of the NOx catalyst 23 (the NOx concentration of the emission gas from the NOx catalyst 23).

[0105] After that, control proceeds to step 302 where the output (intake air flow Ga) of the air flow meter 14 is obtained as information correlated to the emission gas flow from the NOx catalyst 23, so that the emission gas flow Ga from the NOx catalyst 23 can be estimated. In this regard, the emission gas flow Ga may be estimated taking into account the flow delay of the air system present from the position of the air flow meter 14 to the position of the NOx sensor 31.

[0106] Then, control proceeds to step 303 where it is determined whether or not the current time falls in the period when the output of the O.sub.2 sensor 32 upstream of the NOx catalyst 23 is enriched, or the period within a predetermined time from the completion of the NOx purge (rich purge). As a result, if the current time is determined to fall in the period when the output of the O.sub.2 sensor 32 upstream of the NOx catalyst 23 is enriched, or the period within a predetermined time from the completion of the NOx purge (rich purge), control proceeds to step 304. In step 304, the NOx concentration sensed by the NOx sensor 31 downstream of the NOx catalyst 23 is set to "0" (the NOx emission from the NOx catalyst 23 is inhibited from being summed). Then, control proceeds to step 305.

[0107] Conversely, when a "No" determination is made in step 303 (when the current time falls in neither the period when the output of the O.sub.2 sensor 32 upstream of the NOx catalyst 23 is enriched, nor the period within a predetermined time from the completion of the NOx purge), control proceeds to the subsequent step 305 without performing the process of step 304.

[0108] In step 305, the NOx concentration ((B) in FIG. 8) downstream of the NOx catalyst 23 sensed by the NOx sensor 31 is multiplied by the emission gas flow Ga and the calculation time interval dt to calculate the NOx emission (=NOx concentrationGadt) from the NOx catalyst 23 during the calculation time interval dt this time. The resultant value is then added to the summed value of the previously calculated NOx emission to update the summed value of the NOx emission to thereby end the present routine.

[0109] In the second embodiment described above as well, the same effects as in the first embodiment can be obtained.

Third Embodiment

[0110] With reference to FIGS. 9 and 10, hereinafter will be described a third embodiment of the present invention.

[0111] In the second embodiment described above, the NOx concentration to be sensed downstream of the NOx catalyst 23 has been set to "0" during the period when the output of the O.sub.2 sensor 32 upstream of the NOx catalyst 23 is enriched, or the period within a predetermined time from the completion of the NOx purge (rich purge). In this way, the NOx emission from the NOx catalyst 23 has been ensured to be inhibited from being summed.

[0112] The third embodiment shown in FIGS. 9 and 10 makes use of a rich period when the emission gas into the NOx catalyst 23 is richer than the stoichiometric level. Specifically, in this rich period, the output of the NOx sensor 31 during this rich period is subjected to an upper limit guard process with the output of the NOx sensor 31 immediately before the rich period. Using the value resulting from the upper limit guard process, the NOx emission from the NOx catalyst 23 is ensured to be calculated.

[0113] The present embodiment has a configuration similar to the second embodiment. Specifically, the present embodiment also takes into account the flow delay of the enriched emission gas, the delay being caused up until the enriched emission gas reaches the NOx sensor 31 downstream of the NOx catalyst 23. Therefore, the NOx emission from the NOx catalyst 23 is calculated using the value resulting from the upper limit guard process performed with the output of the NOx sensor 31 immediately before the rich period. This calculation is performed not only during the period when the output of the O.sub.2 sensor 32 upstream of the NOx catalyst 23 is enriched, but also during the period within a predetermined time from the completion of the NOx purge (rich purge). Other configurations are the same as in the second embodiment.

[0114] In the present embodiment, a NOx emission summing routine shown in FIG. 9 is performed. In this routine, the output of the NOx sensor 31 downstream of the NOx catalyst 23 is obtained, first, in step 401, so that the NOx concentration downstream of the NOx catalyst 23 can be detected. In the subsequent step 402, the output (intake air flow Ga) of the air flow meter 14 is obtained, so that the emission gas flow Ga from the NOx catalyst 23 can be estimated.

[0115] After that, control proceeds to step 403 where it is determined whether or not the output from the O.sub.2 sensor 32 upstream of the NOx catalyst 23 has just been reversed from lean to rich. When the output has just been reversed from lean to rich, control proceeds to step 404. In step 404, the NOx concentration sensed by the NOx sensor 31 at the time is stored in the memory, such as a RAM, as an upper limit guard value, and then control proceeds to step 405.

[0116] When it is determined, in step 403, that the output from the O.sub.2 sensor 32 is not in the state of having just been reversed from lean to rich, control proceeds to step 405 without performing the process, in step 404, of storing an upper limit guard value.

[0117] In step 405, it is determined whether or not the current time falls in the period when the output of the O.sub.2 sensor 32 upstream of the NOx catalyst 23 is enriched, or the period within a predetermined time from the completion of the NOx purge (rich purge). As a result, when it is determined that the current time fails in the period when the output of the O.sub.2 sensor 32 upstream of the NOx catalyst 23 is enriched, or the period within a predetermined time from the completion of the NOx purge (rich purge), control proceeds to step 406. In step 406, the upper limit guard value stored in step 404 is used to perform the upper limit guard process (detected NOx concentrations upper limit guard value) for the detected NOx concentration obtained in step 401, and then control proceeds to step 407.

[0118] Conversely, when a "No" determination is made in step 405 (the current time falls in neither the period when the output of the O.sub.2 sensor 32 upstream of the NOx catalyst 23 is enriched, nor the period within a predetermined time from the completion of the NOx purge), control proceeds to the subsequent step 407 without performing the upper limit guard process, in step 406, for the detected NOx concentration.

[0119] In step 407, the NOx emission from the NOx catalyst 23 is summed in the same manner as in the second embodiment.

[0120] In the third embodiment described above, the NOx emission from the NOx catalyst 23 can be summed in the period when the output of the O.sub.2 sensor 32 upstream of the NOx catalyst 23 is enriched, regarding the output of the NOx sensor 31 immediately before the rich period as being the NOx concentration in the rich period. In this case, the output of the NOx sensor 31 immediately before the rich period corresponds to the NOx concentration sensed last, which has been less influenced by the ammonia component. Therefore, the NOx emission from the NOx catalyst 23 can be prevented from being overestimated due to the presence of the ammonia component. Thus, the accuracy of calculating the NOx emission from the NOx catalyst 23 can be prevented from being degraded.

[0121] In the second and the third embodiments described above, the non-purification factor has been calculated, which is a ratio of the NOx emission from the NOx catalyst 23 to the NOx inflow into the NOx catalyst 23. Alternative to this, a purification factor may be calculated, which is a ratio of the NOx absorption in the NOx catalyst 23 to the NOx inflow into the NOx catalyst 23. The purification factor may be used as a deterioration diagnostic indicator to determine deterioration of the NOx catalyst 23 based on whether or not the purification factor is equal to or less than a predetermined deterioration determining threshold.

[0122] The purification factor of the NOx catalyst 23 also establishes a relationship expressed by Formula (11) provided in the first embodiment. Thus, the purification factor may be calculated by substituting the NOx absorption and the NOx emission calculated in the same manner as in the second and the third embodiments into the following Formula (21):

Purification factor=NOx absorption/(NOx absorption+NOx emission) (21)

[0123] The purification factor of the NOx catalyst 23 calculated by Formula (21) may be used as a deterioration diagnostic indicator to conduct deterioration diagnosis of the NOx catalyst 23. According to this deterioration diagnosis, completely the same effects as in the second and the third embodiments can be obtained.

[0124] The application of the present invention is not limited to the lean-burn engines. The present invention may be applied to those engines, such as cylinder-injection engines and dual-injection engines combining intake-port injection and cylinder injection, in which a NOx catalyst is installed. As a matter of course, the present invention may be variously modified for application to any engines, irrespective of the presence of or the type of a catalyst, if any, upstream of the NOx catalyst 23, within the scope not departing from the spirit of the present invention.

[0125] Hereinafter, aspects of the above-described embodiments will be summarized.

[0126] The above embodiments provide, as one aspect, an apparatus for diagnosing deterioration of a NOx absorption-reduction catalyst provided at an exhaust path of an internal combustion engine, including: a NOx sensor disposed upstream of the catalyst to sense a NOx concentration in emission gas that flows into the catalyst; a deterioration diagnostic indicator calculating unit which calculates a first ratio (non-purification factor) of the amount of emission of NOx from the catalyst, to the amount of inflow of NOx into the catalyst, or a second ratio (purification factor) of the amount of absorption of NOx in the catalyst, to the amount of inflow of NOx into the catalyst; and a deterioration diagnosing unit which diagnoses deterioration of the catalyst by using the first ratio or the second ratio as a deterioration diagnostic indicator, wherein the deterioration diagnostic indicator calculating unit calculates the amount of inflow of NOx into the catalyst based on an output of the NOx sensor and either the flow volume of the emission gas into the catalyst or a correlation value of the flow volume of the emission gas, calculates the amount of absorption of NOx in the catalyst based on the amount of rich components required for reducing the NOx absorbed by the NOx catalyst, and calculates the amount of emission of NOx from the catalyst based on the difference between the amount of inflow of NOx into the catalyst and the amount of absorption of NOx in the catalyst.

[0127] According to the embodiment, the non-purification factor or the purification factor of the NOx catalyst is calculated from the output, for example, of the NOx sensor disposed upstream of the NOx catalyst. The calculated non-purification factor or the purification factor is used as a deterioration diagnostic indicator to conduct deterioration diagnosis of the NOx catalyst. Therefore, compared with the case where the output sum of the NOx sensor or the total absorption is used as a deterioration diagnostic indicator as disclosed in JP-A-2008-057404 or JP-A-2008-064075, the influences that may be exerted by the size of the NOx catalyst (catalytic capacity) or by the operational states upon the deterioration diagnosis of the NOx catalyst can be mitigated. Thus, the accuracy in the deterioration diagnosis of the NOx catalyst as well as the productivity (decrease in the number of checking processes) can be readily enhanced. Also, the frequency of conducting deterioration diagnosis can be readily ensured.

[0128] The NOx sensor has a property of sensing not only NOx but also an ammonia component (NH.sub.3) when the air-fuel ratio of the emission gas is rich. For this reason, during the period when the ammonia component increases, the NOx concentration sensed by the NOx sensor may result in a larger value than will be obtained from an actual amount of NOx. Specifically, the NOx concentration will become larger by the degree corresponding to the concentration of the ammonia component.

[0129] Considering the property, the deterioration diagnostic indicator calculating unit may inhibit the calculation of the amount of inflow of NOx into the catalyst during a rich period when the emission gas flowing around the NOx sensor is richer than a theoretical air-fuel ratio (stoichiometric air-fuel ratio). According to this configuration, the NOx inflow into the NOx catalyst can be prevented from being overestimated due to the presence of the ammonia component. As a result, the accuracy of calculating the NOx inflow into the NOx catalyst can be prevented from being degraded.

[0130] The NOx sensor senses an O.sub.2 concentration or an air-fuel ratio in the emission gas, and the deterioration diagnostic indicator calculating unit determines whether or not the emission gas flowing around the NOx sensor is richer than the theoretical air-fuel ratio, based on the O.sub.2 concentration or the air-fuel ratio sensed by the NOx sensor.

[0131] According to this configuration, there is no need of providing a sensor upstream of the NOx catalyst other than the NOx sensor so that O.sub.2 concentration or air-fuel ratio can be detected. Thus, this configuration has such advantages as saving space, reducing the number of parts, and the like. However, the present apparatus may be configured to have a sensor upstream of the NOx catalyst in addition to the NOx sensor so that O.sub.2 concentration or air-fuel ratio can be detected.

[0132] The embodiment described above has used the NOx sensor which is provided upstream of the NOx catalyst. However, the embodiment may be applied to a system in which the NOx sensor is provided downstream of the NOx catalyst.

[0133] In this case, the apparatus may include a NOx sensor disposed downstream of the catalyst to sense a NOx concentration in emission gas that is emitted from the catalyst; a deterioration diagnostic indicator calculating unit which calculates a first ratio of the amount of emission of NOx from the catalyst, to the amount of inflow of NOx into the catalyst, or a second ratio of the amount of absorption of NOx in the catalyst, to the amount of inflow of NOx into the catalyst; and a deterioration diagnosing unit which diagnoses deterioration of the catalyst by using the first ratio or the second ratio as a deterioration diagnostic indicator, wherein the deterioration diagnostic indicator calculating unit calculates the amount of emission of NOx from the catalyst based on an output of the NOx sensor and either the flow volume of the emission gas from the catalyst or a correlation value of the flow volume of the emission gas, calculates the amount of absorption of NOx in the catalyst based on the amount of rich components required for reducing the NOx absorbed in the NOx catalyst, and calculates the amount of inflow of NOx into the catalyst by adding the amount of absorption of NOx in the catalyst to the amount of emission of NOx from the catalyst.

[0134] In the embodiment, the non-purification factor or the purification factor of the NOx catalyst is calculated from the output, for example, of the NOx sensor disposed downstream of the NOx catalyst. The calculated non-purification factor or the purification factor is used as a deterioration diagnostic indicator to conduct deterioration diagnosis of the NOx catalyst. Therefore, the influences that may be exerted by the size of the NOx catalyst (catalytic capacity) or by the operational states upon the deterioration diagnosis of the NOx catalyst can be mitigated. Thus, the accuracy in the deterioration diagnosis of the NOx catalyst as well as the productivity (decrease in the number of checking processes) can be readily enhanced. Also, the frequency of conducting deterioration diagnosis can be readily ensured.

[0135] In this case as well, consideration is given to the fact that the NOx concentration sensed by the NOx sensor becomes larger than will be obtained from the actual amount of NOx. Specifically, the NOx concentration becomes larger by the degree corresponding to the ammonia component concentration during the rich period when the ammonia component increases. Thus, calculation of the NOx emission from the NOx catalyst is ensured to be inhibited during the rich period when the emission gas that flows into the NOx catalyst becomes richer than the theoretical air-fuel ratio.

[0136] When the NOx sensor is disposed downstream of the NOx catalyst, the concentration of the ammonia component may become high even when the emission gas that flows around the NOx sensor is not rich. To explain in detail, when the emission gas that flows into the NOx catalyst has been enriched, the concentration of the ammonia component in the emission gas is estimated to have reached a high level. In such a case, the rich components are consumed with the reductive reaction against the absorbed NOx in the course that the emission gas flows through the NOx catalyst, while the ammonia component passes through the NOx catalyst. For this reason, the concentration of the ammonia component may become high even when the emission gas that has flowed out of the NOx catalyst and flows around the NOx sensor is not rich.

[0137] Accordingly, the NOx emission from the NOx catalyst may be ensured to be inhibited from being calculated during the period when the emission gas that flows into the NOx catalyst is enriched. According to this configuration, the NOx emission from the NOx catalyst can be prevented from being overestimated due to the presence of the ammonia component. In this way, the accuracy in calculating the NOx emission from the NOx catalyst can be prevented from being degraded.

[0138] The deterioration diagnostic indicator calculating unit may calculate, during a rich period when the emission gas flowing into the catalyst is richer than a theoretical air-fuel ratio, the amount of emission of NOx from the catalyst by using a value resulting from an upper limit guard process to which an output of the NOx sensor during the rich period is subjected with an output of the NOx sensor immediately before the rich period.

[0139] According to this configuration, the NOx emission from the NOx catalyst can be calculated in the rich period, regarding the output of the NOx sensor immediately before the rich period as being the NOx concentration in the rich period. In this case, the output of the NOx sensor immediately before the rich period corresponds to the NOx concentration sensed last, which has been less influenced by the ammonia component. Therefore, the NOx emission from the NOx catalyst can be prevented from being overestimated due to the presence of the ammonia component. Thus, the accuracy of calculating the NOx emission from the NOx catalyst can be prevented from being degraded.

[0140] The apparatus may include an O.sub.2 sensor disposed upstream of the catalyst to sense an O.sub.2 concentration in the emission gas, wherein the deterioration diagnostic indicator calculating unit determines whether or not the emission gas flowing into the catalyst is richer than the theoretical air-fuel ratio, based on the O.sub.2 concentration sensed by the O.sub.2 sensor. Alternatively, the apparatus may include an air-fuel ratio sensor disposed upstream of the catalyst to sense an air-fuel ratio in the emission gas, wherein the deterioration diagnostic indicator calculating unit determines whether or not the emission gas flowing into the catalyst is richer than the theoretical air-fuel ratio, based on the air-fuel ratio sensed by the air-fuel ratio sensor.

[0141] Thus, the O.sub.2 sensor or the air-fuel ratio sensor that senses the O.sub.2 concentration or air-fuel ratio in the emission gas may be disposed upstream of the NOx catalyst. According to this configuration, an accurate determination can be made as to whether or not the emission gas that flows into the NOx catalyst has been enriched.

[0142] The apparatus may further include an air-fuel ratio sensor disposed upstream of the catalyst to sense an air-fuel ratio in the emission gas, wherein the deterioration diagnostic indicator calculating unit determines whether or not the emission gas flowing into the catalyst is richer than the theoretical air-fuel ratio, based on the air-fuel ratio sensed by the air-fuel ratio sensor.

[0143] In short, the difference between the rich component inflow and the rich component emission in the NOx catalyst corresponds to the rich component amount consumed with the reductive reaction against the NOx absorbed in the NOx catalyst. Therefore, the NOx absorption of the NOx catalyst can be calculated based on the difference between the rich component inflow and the rich component emission.

[0144] It will be appreciated that the present invention is not limited to the configurations described above, but any and all modifications, variations or equivalents, which may occur to those who are skilled in the art, should be considered to fall within the scope of the present invention.

Claims

1. An apparatus for diagnosing deterioration of a NOx absorption-reduction catalyst provided at an exhaust path of an internal combustion engine, comprising: a NOx sensor disposed upstream of the catalyst to sense a NOx concentration in emission gas that flows into the catalyst; a deterioration diagnostic indicator calculating unit which calculates a first ratio of the amount of emission of NOx from the catalyst, to the amount of inflow of NOx into the catalyst, or a second ratio of the amount of absorption of NOx in the catalyst, to the amount of inflow of NOx into the catalyst; and a deterioration diagnosing unit which diagnoses deterioration of the catalyst by using the first ratio or the second ratio as a deterioration diagnostic indicator, wherein the deterioration diagnostic indicator calculating unit calculates the amount of inflow of NOx into the catalyst based on an output of the NOx sensor and either the flow volume of the emission gas into the catalyst or a correlation value of the flow volume of the emission gas, calculates the amount of absorption of NOx in the catalyst based on the amount of rich components required for reducing the NOx absorbed by the NOx catalyst, and calculates the amount of emission of NOx from the catalyst based on the difference between the amount of inflow of NOx into the catalyst and the amount of absorption of NOx in the catalyst.

2. The apparatus according to claim 1, wherein the deterioration diagnostic indicator calculating unit inhibits the calculation of the amount of inflow of NOx into the catalyst during a rich period when the emission gas flowing around the NOx sensor is richer than a theoretical air-fuel ratio.

3. The apparatus according to claim 2, wherein the NOx sensor senses an O.sub.2 concentration or an air-fuel ratio in the emission gas, and the deterioration diagnostic indicator calculating unit determines whether or not the emission gas flowing around the NOx sensor is richer than the theoretical air-fuel ratio, based on the O.sub.2 concentration or the air-fuel ratio sensed by the NOx sensor.

4. An apparatus for diagnosing deterioration of a NOx absorption-reduction catalyst provided at an exhaust path of an internal combustion engine, comprising: a NOx sensor disposed downstream of the catalyst to sense a NOx concentration in emission gas that is emitted from the catalyst; a deterioration diagnostic indicator calculating unit which calculates a first ratio of the amount of emission of NOx from the catalyst, to the amount of inflow of NOx into the catalyst, or a second ratio of the amount of absorption of NOx in the catalyst, to the amount of inflow of NOx into the catalyst; and a deterioration diagnosing unit which diagnoses deterioration of the catalyst by using the first ratio or the second ratio as a deterioration diagnostic indicator, wherein the deterioration diagnostic indicator calculating unit calculates the amount of emission of NOx from the catalyst based on an output of the NOx sensor and either the flow volume of the emission gas from the catalyst or a correlation value of the flow volume of the emission gas, calculates the amount of absorption of NOx in the catalyst based on the amount of rich components required for reducing the NOx absorbed in the NOx catalyst, and calculates the amount of inflow of NOx into the catalyst by adding the amount of absorption of NOx in the catalyst to the amount of emission of NOx from the catalyst.

5. The apparatus according to claim 4, wherein the deterioration diagnostic indicator calculating unit inhibits the calculation of the amount of emission of NOx from the catalyst during a rich period when the emission gas flowing into the catalyst is richer than a theoretical air-fuel ratio.

6. The apparatus according to claim 4, wherein the deterioration diagnostic indicator calculating unit calculates, during a rich period when the emission gas flowing into the catalyst is richer than a theoretical air-fuel ratio, the amount of emission of NOx from the catalyst by using a value resulting from an upper limit guard process to which an output of the NOx sensor during the rich period is subjected with an output of the NOx sensor immediately before the rich period.

7. The apparatus according to claim 5, further comprising an O.sub.2 sensor disposed upstream of the catalyst to sense an O.sub.2 concentration in the emission gas, wherein the deterioration diagnostic indicator calculating unit determines whether or not the emission gas flowing into the catalyst is richer than the theoretical air-fuel ratio, based on the O.sub.2 concentration sensed by the O.sub.2 sensor.

8. The apparatus according to claim 5, further comprising an air-fuel ratio sensor disposed upstream of the catalyst to sense an air-fuel ratio in the emission gas, wherein the deterioration diagnostic indicator calculating unit determines whether or not the emission gas flowing into the catalyst is richer than the theoretical air-fuel ratio, based on the air-fuel ratio sensed by the air-fuel ratio sensor.

9. The apparatus according to claim 1, wherein the deterioration diagnostic indicator calculating unit calculates the amount of absorption of NOx in the catalyst based on the difference between the amount of rich components flowing into the catalyst and the amount of rich components emitted from the catalyst, when reducing the NOx absorbed in the catalyst.