Title:
TROUBLE DIAGNOSIS DEVICE FOR EXHAUST GAS PURIFICATION SYSTEM AND TROUBLE DIAGNOSIS METHOD FOR EXHAUST GAS PURIFICATION SYSTEM
Kind Code:
A1


Abstract:
There are provided a trouble diagnosis device and a trouble diagnosis method for an exhaust gas purification system in which the presence or absence of a trouble of the exhaust gas purification system such as abnormality of a control system for the exhaust gas purification system, catalyst deterioration or the like can be determined with high precision, and the reliability of the exhaust gas purification system can be enhanced.

The device includes upstream-side NOx flow rate calculating means, downstream-side NOx flow rate calculating means, reduction condition determining means for determining whether at least one condition for performing normal reduction of NOx is satisfied, upstream-side NOx flow amount calculating means for integrating the upstream-side NOx flow amount to calculate the amount of NOx passing through the upstream side of the catalyst within a predetermined time when the condition concerned is determined to be satisfied, downstream-side NOx amount calculating means for integrating the downstream-side NOx flow amount to calculate the amount of NOx passing through the downstream side of the catalyst within a predetermined time when the condition concerned is determined to be satisfied, and trouble determining means for comparing the upstream-side NOx amount and the downstream-side NOx amount to determine whether the exhaust gas purification system operates normally.




Inventors:
Kitazawa, Eiichi (Saitama, JP)
Application Number:
12/525401
Publication Date:
02/04/2010
Filing Date:
01/25/2008
Primary Class:
Other Classes:
60/299
International Classes:
F01N11/00; F01N3/10
View Patent Images:
Related US Applications:
20070151228Fixed nozzle thrust augmentation systemJuly, 2007Smith
20100024407Emissions control filter assembly and systemFebruary, 2010Merchant et al.
20050115221Model vehicle exhaust deflectorJune, 2005Kulis II
20070245707Fuel control systemOctober, 2007Pashley
20090017741CHIMNEY CAP WITH REPLACEABLE OR RECYCLABLE CERAMIC CATALYTIC FILTER INSERTJanuary, 2009Arnold Jr. et al.
20080295486Exhaust treatment system implementing temporary engine controlDecember, 2008Crowell et al.
20070240420INTEGRATED ENGINE GENERATOR RANKINE CYCLE POWER SYSTEMOctober, 2007Bronicki
20100011750EXHAUST EMISSION PURIFICATION METHOD AND EXHAUST EMISSION PURIFICATION SYSTEMJanuary, 2010Onodera et al.
20100095678Heat Shield Sealing for Gas Turbine Engine CombustorApril, 2010Hawie et al.
20090301081AMBIENT-AIR PULSED VALVE FOR INTERNAL COMBUSTION ENGINES EQUIPPED WITH A TURBOCHARGERDecember, 2009Thiery et al.
20070144149CONTROLLED REGENERATION SYSTEMJune, 2007Kolavennu et al.



Primary Examiner:
LEE, BRANDON DONGPA
Attorney, Agent or Firm:
MICHAEL BEST & FRIEDRICH LLP (Bosch) (MILWAUKEE, WI, US)
Claims:
1. 1-7. (canceled)

8. A trouble diagnosis device for an exhaust gas purification system for passing exhaust gas discharged from an internal combustion engine through NOx catalyst to reduce NOx contained in the exhaust gas, the trouble diagnosis device comprising: upstream-side NOx flow rate calculating means for calculating the flow amount of upstream-side NOx flow amount per unit time at an upstream side of the NOx catalyst; downstream-side NOx flow rate calculating means for calculating a downstream-side NOx flow amount per unit time at a downstream side of the NOx catalyst; reduction condition determining means for determining whether at least one condition for performing normal reduction of NOx is satisfied; upstream-side NOx flow amount calculating means for integrating the upstream-side NOx flow amount to calculate the amount of upstream-side NOx passing through the upstream side of the NOx catalyst within a predetermined time when a condition concerned is determined to be satisfied; downstream-side NOx amount calculating means for integrating the downstream-side NOx flow amount to calculate the amount of downstream-side NOx passing through the downstream side of the NOx catalyst within a predetermined time when the condition concerned is determined to he satisfied; and trouble determining means for comparing the upstream-side NOx amount and the downstream-side NOx amount to determine whether the exhaust gas purification system operates normally.

9. The trouble diagnosis device for the exhaust gas purification system according to claim 8, wherein the upstream-side NOx amount calculating means and the downstream-side NOx amount calculating means store integration values which have been obtained till now when integration of the NOx flow amount is interrupted, and resume the integration from the stored integration values when the condition concerned is satisfied again.

10. The trouble diagnosis device for the exhaust gas purification system according to claim 8, wherein the upstream-side NOx amount calculating means and the downstream-side NOx amount calculating means store integration values which have been obtained till now when integration of the NOx is interrupted, and reset the integration values when the condition concerned is not satisfied again within a predetermined time.

11. The trouble diagnosis device for the exhaust gas purification system according to claim 9, wherein the upstream-side NOx amount calculating means and the downstream-side NOx amount calculating means store the integration values which have been obtained till now when the integration of the NOx is interrupted, and reset the integration values when the condition concerned is not satisfied again within a predetermined time.

12. The trouble diagnosis device for the exhaust gas purification system according to claims 9, wherein the trouble determining means finishes the integration of the upstream-side NOx flow amount and the downstream-sidle NOx flow amount when the upstream-side NOx amount reaches a predetermined value, and compares the upstream-side NOx amount and the downstream-side NOx amount.

13. The trouble diagnosis device for the exhaust gas purification system according to claims 10, wherein the trouble determining means finishes the integration of the upstream-side NOx flow amount and the downstream-side NOx flow amount when the upstream-side NOx amount reaches a predetermined value, and compares the upstream-side NOx amount and the downstream-side NOx amount.

14. The trouble diagnosis device for the exhaust gas purification system according to claim 8, wherein the trouble determining means compares the ratio between the upstream-side NOx amount and the downstream-side NOx amount with a threshold value which is determined in accordance with a condition under which reduction of NOx is normally performed for determination.

15. The trouble diagnosis device for the exhaust gas purification system according to claim 9, wherein the trouble determining means compares the ratio between the upstream-side NOx amount and the downstream-side NOx amount with a threshold value which is determined in accordance with a condition under which reduction of NOx is normally performed for determination.

16. The trouble diagnosis device for the exhaust gas purification system according to claim 10, wherein the trouble determining means compares the ratio between the upstream-side NOx amount and the downstream-side NOx amount with a threshold value which is determined in accordance with a condition under which reduction of NOx is normally performed for determination.

17. The trouble diagnosis device for the exhaust gas purification system according to claim 8, wherein the upstream-side NOx flow rate calculating means performs its calculation on a basis of an NOx concentration discharged from the internal combustion engine which is calculated from an operation state of the internal combustion engine.

18. The trouble diagnosis device for the exhaust gas purification system according to claim 9, wherein the upstream-side NOx flow rate calculating means performs its calculation on a basis of an NOx concentration discharged from the internal combustion engine which is calculated from an operation state of the internal combustion engine.

19. The trouble diagnosis device for the exhaust gas purification system according to claim 10, wherein the upstream-side NOx flow rate calculating means performs its calculation on a basis of an NOx concentration discharged from the internal combustion engine which is calculated from an operation state of the internal combustion engine.

20. A trouble diagnosis method for an exhaust gas purification system for diagnosing the presence or absence of a trouble of an exhaust gas purification system in which exhaust gas discharged from an internal combustion engine is passed through NOx catalyst to reduce NOx contained in the exhaust gas, the method comprising: calculating all upstream-side NOx flow amount and a downstream-side NOx flow amount per unit time at upstream and downstream sides of the NOx catalyst; determining whether at least one condition under which the reduction of NOx is normally performed is satisfied or not; integrating the upstream-side NOx flow amount and the downstream-side NOx flow amount when a condition concerned is satisfied, and calculating an upstream-side NOx amount and the downstream-side NOx amount passing through the upstream side and the downstream side of the NOx catalyst within a predetermined time; and comparing the upstream-side NOx amount with the downstream-side NOx amount to determine whether the exhaust gas purification system operates normally.

Description:

TECHNICAL FIELD

The present invention relates to a trouble diagnosis device for an exhaust gas purification system and a trouble diagnosis device for an exhaust gas purification system. Particularly, the present invention relates to trouble diagnosis device and method for an exhaust gas purification system that can perform trouble determination by comparing an NOx amount at the upstream side of NOx catalyst and an NOx amount at the downstream side of the NOx catalyst.

BACKGROUND ART

Particular matter (hereinafter referred to as PM), NOx (NO or NO2), etc. which may induce a risk of affecting the environment are contained in exhaust gas discharged from an internal combustion engine such as a diesel engine or the like. An exhaust gas purification system using NOx catalyst disposed in an exhaust gas passage is known as an exhaust gas purification system used to purify NOx out of the above materials.

An exhaust gas purification system using NOx storage catalyst and an SCR (Selective Catalytic Reduction) system using selective reduction catalyst are known as such an exhaust gas purification system. Accordingly to the exhaust gas purification system using the NOx storage catalyst, NOx in exhaust gas is absorbed under the state that the air-fuel ratio of the exhaust gas is under a lean state, and when the air-fuel ratio of the exhaust gas is changed to a rich state, NOx is subjected to reductive reaction with hydro carbon (HC) and carbon oxide (CO) in the exhaust gas while NOx is discharged, thereby purifying the exhaust gas. Furthermore, the SCR system uses catalyst for selectively reducing NOx in exhaust gas and supplies reducing agent mainly containing urea or HC into the exhaust gas so that NOx is subjected to reductive reaction with the catalyst, thereby purifying the exhaust gas.

In these exhaust gas purification systems, there is a case where originally-expected reduction of NOx does not occur due to some abnormality of the system itself, deterioration of the catalyst, defect of the reducing agent or the like. However, even when the purification efficiency of NOx is lowered, no trouble occurs on operation. Therefore, the operation is continued without taking any action. Accordingly, NOx is discharged into atmospheric air, which affects the environment.

Therefore, a technique of self-diagnosing reduction of the purification coefficient of NOx in the exhaust gas purification system has been proposed. For example, a catalyst deterioration diagnosis device for an internal combustion engine which diagnoses reduction of NOx conversion performance due to deterioration of catalyst has been proposed (see Patent Document 1). More specifically, there is disclosed a catalyst deterioration diagnosis device for an internal combustion engine in which NOx sensors are provided at the upstream side and downstream side of the catalyst, the output of each NOx sensor is measured, the ratio in NOx concentration between the upstream and downstream sides of the catalyst, that is, the NOx conversion rate is calculated, and a reference value of the NOx concentration ratio is set on the basis of the rotational number of the internal combustion engine and the basic fuel injection amount. The NOx concentration ratio is compared with the reference value, and the reduction of the NOx conversion rate is determined on the basis of the comparison result.

Furthermore, there has been disclosed a method of diagnosing the deterioration of the NOx catalyst by estimating the amount of NOx captured by the NOx catalyst (the NOx amount at the upstream side of the NOx catalyst) from the operation state of the internal combustion engine, etc. and using an integration value of the NOx concentration detected by NOx sensor at the downstream side without using the NOx concentration itself detected by the NOx sensor of the internal combustion engine (for example, see Patent Document 2).

Patent Document 1: JP-A-7-54641 (Scope of Claim for Patent, FIG. 5)

Patent Document 2: JP-A-2005-54604 (Claims 7, 12, paragraphs [0047] to [0048])

DISCLOSURE OF THE INVENTION

Problem to be Solved by the Invention

The purification efficiency of NOx may be reduced irrespective of the presence or absence of the catalyst deterioration in accordance with conditions such as the temperature of the NOx catalyst, the NOx concentration at the upstream side of the NOx catalyst, the flow rate of exhaust gas, etc. Furthermore, the purification efficiency of NOx may be reduced due to abnormality of the control system for the exhaust gas purification system, defect of reducing agent, deterioration of oxidation catalyst when the oxidation catalyst is disposed at the upstream side of the NOx catalyst or the like.

However, the catalyst deterioration diagnosis disclosed in the Patent Document 1 and the Patent document 2 is the deterioration diagnosis which pays no attention to the condition as described above. Therefore, there is a risk that the deterioration is determined to progress more bitterly as compared with the actual catalyst deterioration degree, so that the reliability of the diagnosis result may be lowered.

Therefore, the inventors of this invention have made an earnest effort, and found that the problem as described above can be solved by performing trouble determination on the basis of integrated NOx amount only when a predetermined condition is satisfied in the case where the NOx amounts at the upstream and downstream sides of NOx catalyst are compared with each other to perform the trouble determination of an exhaust gas purification system, thereby implementing this invention. That is, an object of the present invention is to provide a trouble diagnosis device for an exhaust gas purification system and a trouble diagnosis method for an exhaust gas purification system in which the presence or absence of a trouble of the exhaust gas purification system such as abnormality of a control system for the exhaust gas purification system, catalyst deterioration or the like can be determined with high precision, and the reliability of the exhaust gas purification system can be enhanced.

Means of Solving the Problem

According to the present invention, there is provided a trouble diagnosis device for an exhaust gas purification system for passing exhaust gas discharged from an internal combustion engine through NOx catalyst to reduce NOx contained in the exhaust gas that comprises: upstream-side NOx flow rate calculating means for calculating the upstream-side NOx flow amount per unit time at the upstream side of the NOx catalyst; downstream-side NOx flow rate calculating means for calculating a downstream-side NOx flow amount per unit time at the downstream side of the NOx catalyst; reduction condition determining means for determining whether at least one condition for performing normal reduction of NOx is satisfied; upstream-side NOx flow amount calculating means for integrating the upstream-side NOx flow amount to calculate the amount of upstream-side NOx passing through the upstream side of the NOx catalyst within a predetermined time when the condition concerned is determined to be satisfied; downstream-side NOx amount calculating means for integrating the downstream-side NOx flow amount to calculate the amount of downstream-side NOx passing through the downstream side of the NOx catalyst within a predetermined time when the condition concerned is determined to be satisfied; and trouble determining means for comparing the upstream-side NOx amount and the downstream-side NOx amount to determine whether the exhaust gas purification system operates normally, whereby the above problem can be solved.

In the construction of the trouble diagnosis device for the exhaust gas purification system according to the present invention, it is preferable that the upstream-side NOx amount calculating means and the downstream-side NOx amount calculating means store the integration values which have been obtained till now when the integration of the NOx flow amount is interrupted, and resume the integration from the stored integration values when the condition concerned is satisfied again.

In the construction of the trouble diagnosis device for the exhaust gas purification system according to the present invention, it is preferred that the upstream-side NOx amount calculating means and the downstream-side NOx amount calculating means store the integration values which have been obtained till now when the integration of the NOx is interrupted, and reset the integration values when the condition concerned is not satisfied again within a predetermined time.

In the construction of the trouble diagnosis device for the exhaust gas purification system according to the present invention, it is preferred that the trouble determining means finishes the integration of the upstream-side NOx flow amount and the downstream-side NOx flow amount when the upstream-side NOx amount reaches a predetermined value, and compares the upstream-side NOx amount and the downstream-side NOx amount.

Furthermore, in the construction of the trouble diagnosis device for the exhaust gas purification system according to the present invention, it is preferable that the trouble determining means compares the ratio between the upstream-side NOx amount and the downstream-side NOx amount with a threshold value which is determined in accordance with a condition under which reduction of NOx is normally performed.

Still furthermore, in the construction of the trouble diagnosis device for the exhaust gas purification system according to the present invention, it is preferable that the upstream-side NOx flow rate calculating means performs the calculation on the basis of an NOx concentration discharged from the internal combustion engine which is calculated from an operation state of the internal combustion engine.

Furthermore, according to another aspect of the present invention, a trouble diagnosis method for an exhaust gas purification system for diagnosing the presence or absence of a trouble of an exhaust gas purification system in which exhaust gas discharged from an internal combustion engine is passed through NOx catalyst to reduce NOx contained in the exhaust gas, comprises: calculating an upstream-side NOx flow amount and a downstream-side NOx flow amount per unit time at the upstream and downstream sides of the NOx catalyst; determining whether at least one condition under which the reduction of NOx is normally performed is satisfied or not; integrating the upstream-side NOx flow amount and the downstream-side NOx flow amount when the condition concerned is satisfied, and calculating an upstream-side NOx amount and the downstream-side NOx amount passing through the upstream side and the downstream side of the NOx catalyst within a predetermined time; and determining comparing the upstream-side NOx amount with the downstream-side NOx amount to determine whether the exhaust gas purification system operates normally.

Effect of the Invention

According to the trouble diagnosis device for the exhaust gas purification system according to the present invention, the NOx amounts at the upstream side and the downstream side of the NOx catalyst which are integrated only when the condition for performing the normal reductive reaction of NOx is satisfied are compared with each other, whereby a reduction efficiency when the condition concerned is not satisfied can be excluded from materials for performing the determination. Accordingly, there can be provided the trouble diagnosis device for the exhaust gas purification system that can determine the presence or absence of a trouble of the exhaust gas purification system such as abnormality of a control system for the exhaust gas purification system, deterioration of NOx catalyst, reduction of the quality of reducing agent or the like.

Furthermore, in the trouble diagnosis device for the exhaust gas purification system according to the present invention, when the integration of the NOx flow amount is interrupted and then resumed, the integration is resumed from the integration value at the interruption time, whereby the trouble diagnosis can be efficiently performed without starting the trouble diagnosis from the beginning even when the operation state of the internal combustion engine is unstable.

Still furthermore, in the trouble diagnosis device for the exhaust gas purification system according to the present invention, the integration value is reset when the time period for which the integration of the NOx flow amount is interrupted is continued for a predetermined time or more, whereby reliability of a determination result can be enhanced.

Still furthermore, in the trouble diagnosis device for the exhaust gas purification system according to the present invention, the integration is finished when he upstream-side NOx amount reaches a predetermined value, and the upstream-side NOx amount and the downstream-side NOx amount are compared with each other, whereby a time required for the trouble diagnosis can be prevented from being excessively lengthened, so that the efficient trouble diagnosis can be performed.

Still furthermore, in the trouble diagnosis device for the exhaust gas purification system according to the present invention, the trouble determination is carried out by comparing the reduction efficiency of NOx with a predetermined threshold value, whereby the determination can be properly performed in accordance with the operation state of the internal combustion engine or the like.

Still furthermore, in the trouble diagnosis method for the exhaust gas purification system according to the present invention, the upstream-side NOx flow amount is calculated, not on the basis of a detection value obtained by an NOx sensor, but on the basis of an estimation value estimated from the operation state of the internal combustion engine, whereby the number of NOx sensors is reduced and the increase of the cost can be suppressed.

Still furthermore, according to the trouble diagnosis method for the exhaust gas purification system according to the present invention, the reduction efficiency under the state that the condition for performing the normal reductive reaction of NOx is not satisfied is excluded from materials for performing the determination. Accordingly, the presence or absence of a trouble of the exhaust gas purification system such as abnormality of the control system for the exhaust gas purification system, deterioration of the NOx catalyst, reduction of the quality of reducing agent or the like can be determined with high precision.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] is a diagram showing an example of the construction of an exhaust gas purification system according to an embodiment of the present invention.

[FIG. 2] is a block diagram showing an example of the construction of a trouble diagnosis device for the exhaust gas purification system.

[FIG. 3] is timing chart showing how to integrate an NO flow amount.

[FIG. 4] is a flowchart (part 1) showing an example of a trouble diagnosis method for the exhaust gas purification system according to the embodiment of the present invention.

[FIG. 5] is a flowchart (part 2) showing an example of the trouble diagnosis method for the exhaust gas purification system according to the embodiment of the present invention.

[FIG. 6] is a flowchart (part 3) showing an example of the trouble diagnosis method for the exhaust gas purification system according to the embodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Embodiments concerning a trouble diagnosis device for an exhaust gas purification system and a trouble diagnosis method for an exhaust gas purification system according to the present invention will be described specifically with reference to the drawings. However, these embodiments are examples of the present invention. Accordingly, the present invention is not limited to these embodiments, and thus these embodiments may be arbitrarily altered within the scope of the present invention.

In the figures, the same reference numerals represent the same members, and the description is fitly omitted.

1. Trouble Diagnosis Device for Exhaust Gas Purification System

(1) Whole Construction of Exhaust Gas Purification System

First, an example of the construction of an exhaust gas purification system having a trouble diagnosis device for an exhaust gas purification system according to an embodiment (hereinafter referred to as “system” in some cases) will described with reference to FIG. 1.

An exhaust gas purification system 10 shown in FIG. 1 uses urea water solution as reducing agent, and passes exhaust gas through NOx catalyst 13 together with the reducing agent to selectively reduce NOx. The exhaust gas purification system 10 has a NOx catalyst 13 which is disposed at some point of an exhaust gas passage 11 connected to an internal combustion engine and selectively reduces NOx contained in exhaust gas, and a reducing agent supply device 20 containing a reducing agent injection valve 31 for injecting reducing agent into the exhaust gas passage 11 at the upstream side of the NOx catalyst 13. Temperature sensors 15 and 16 are disposed at the upstream side and the downstream side of the NOx catalyst 13 of the exhaust gas passage 11 respectively, and also an NOx sensor 17 as downstream-side NOx concentration calculating means is disposed at the downstream side of the NOx catalyst 13. The constructions of the NOx catalyst 13, the temperature sensors 15, 16 and the NOx sensor 17 are not limited to specific ones, and well-known members may be used.

However, in the exhaust gas purification system of this embodiment, at least the NOx sensor 17 has a trouble diagnosis function for the sensor itself, and outputs error information to CAN (Controller Area Network) 65 described later when an abnormal state is detected.

The reducing agent supply device 20 has an injection module 30 containing a reducing agent injection valve 31, a storage tank 50 in which reducing agent is stocked, a pump module 40 containing a pump 41 for pressure-feeding the reducing agent in the storage tank 50 to the reducing agent injection valve 31, and a control unit (hereinafter referred to as “DCU: Dosing Control unit”) 60 for controlling the injection module 30 and the pump module 40 to control the injection amount of the reducing agent to be injected from the reducing agent injection valve 31. The storage tank 50 and the pump module 40 are connected to each other through a first supply passage 57, the pump module 40 and the injection module 30 are connected to each other through a second supply passage 58, and the injection module 30 and the storage tank 50 are connected to each other through a circulation passage 59.

In the example of the exhaust gas purification system 10 shown in FIG. 1, DCU 60 is connected to CAN 65. CAN 65 is connected to a control unit (hereinafter referred to as “ECU: Engine Control Unit” in some cases) 70 for controlling the operation state of the internal combustion engine. Not only information concerning the operation state of the internal combustion engine such as a fuel injection amount, an injection timing, a rotational number, etc. are written in CAN 65, but also information of all sensors, etc. provided to the exhaust gas purification system 10 are written in CAN 65. In CAN 65, it can be determined whether an input signal value is within a standard range or not of CAN 65. DCU 60 connected to CAN 65 can read information on CAN 65, and also output information onto CAN 65.

In this embodiment, ECU 70 and DCU 60 comprise separate control units, and can communicate information with each other through CAN 65. However, ECU 70 and DCU 60 may be constructed as a single control unit.

The storage tank 50 is provided with a temperature sensor 51 for detecting the temperature of reducing agent in the tank, a level sensor 55 for detecting the residual amount of reducing agent and a quality sensor 53 for detecting quality such as viscosity, concentration, etc. of reducing agent. Values detected by these sensors are output as signals and written onto CAN 65. Well-know parts may be properly used as these sensors.

Urea water solution, hydro carbon (RC) is mainly used as the reducing agent to be stocked, and the exhaust gas purification system of this embodiment is constructed to use urea water solution.

The pump module 40 has a pump 41, a pressure sensor 43 as pressure detecting means for detecting the pressure in the second supply passage 58 at the downstream of the pump 41 (hereinafter referred to as “pressure of reducing agent” in some cases), a temperature sensor 45 for detecting the temperature of reducing agent to be pressure-fed, a foreign material collecting filter 47 disposed at some position of the second supply passage 58 at the downstream side of the pump 41, and a pressure control valve 49 for returning a part of the reducing agent from the downstream side of the pump 41 to the upstream side of the pump 41 to reduce the pressure when the pressure of the reducing agent at the downstream side of the pump 41 exceeds a predetermined value.

The pump 41 comprises an electrically-operated pump, for example, and it is driven on the basis of a signal transmitted from DCU 60. Well-known sensors may be properly used as the pressure sensor 43 and the temperature sensor 45. Values detected by these sensors are output as signals, and written onto CAN 65. Furthermore, a well-known check valve or the like may be used as the pressure control valve 49.

The injection module 30 has a pressure chamber 33 in which reducing agent pressure-fed from the pump module 40 side is stocked, a reducing agent injection valve 31 connected to the pressure chamber 33, an orifice 35 disposed at some position of a passage extending from the pressure chamber 33 and intercommunicating with the circulation passage 59, and a temperature sensor 37 disposed just before the orifice 35.

The reducing agent injection valve 31 comprises an ON-OFF valve for controlling ON-OFF of valve opening through duty control, for example. Furthermore, in the pressure chamber 33, the reducing agent pressure-fed from the pump module 40 is stocked under a predetermined pressure, and the reducing agent is injected into the exhaust gas passage 11 when the reducing agent injection valve 31 is opened on the basis of a control signal transmitted from DCU 60. The orifice 35 is disposed in the passage at the downstream side of the pressure chamber 33, so that the internal pressure of the pressure chamber 33, the second supply passage 58 at the upstream side of the orifice 35 is not lowered easily, and thus the output of the pump module 40 can be suppressed to a low value. As not shown, a valve for performing circulation control of reducing agent may be provided at some position of the circulation passage 59 in place of arranging the orifice 35.

Furthermore, the circulation passage 59 disposed between the injection module 30 and the storage tank 50 is provided so that reducing agent other than reducing agent injected from the reducing agent injection valve 31 of the injection module 30 is made to reflow into the storage tank 50 out of reducing agent pressure-fed from the pump module 40 in order to prevent the reducing agent from being affected by exhaust gas heat or the like and exposed to high temperature.

DCU 60 performs the operation control of the reducing agent injection valve 31 on the basis of various information existing on CAN 65 so that a proper amount of reducing agent is injected into the exhaust gas passage 11. Furthermore, DCU 60 in the embodiment of the present invention has a function as a trouble diagnosis device for the exhaust gas purification system 10 (hereinafter referred to as “trouble diagnosis device”).

DCU 60 mainly comprises a microcomputer having a well-known construction, and in FIG. 1, constructions represented by functional blocks are shown with respect to parts concerning the operation control of the reducing agent injection valve 31, the driving control of the pump 41 and the trouble diagnosis of the exhaust gas purification system 10.

That is, DCU 60 in the embodiment of the present invention comprises, as main constituent elements, a CAN information taking and generating unit (represented as “CAN information take-out and generation” in FIG. 1), a trouble diagnosis unit of the exhaust gas purification system (represented by “system trouble diagnosis” in FIG. 1), a pump driving control unit (represented as “pump driving control” in FIG. 1), a reducing agent injection valve operation controller (represented as “Udv operation control” in FIG. 1), etc. Specifically, these parts are implemented by executing programs through the microcomputer (not shown).

The CAN information taking and generating unit reads information concerning the driving state of the internal combustion engine output from ECU 70, sensor information output from the NOx sensor 17 and information existing on CAN 65, and outputs these information onto the respective parts.

Furthermore, the pump driving controller continually reads information concerning the pressure of reducing agent in the second supply passage 58 which is output from the CAN information taking and generating unit, and executes feedback control on the pump 41 on the basis of the pressure information, so that the pressure of the reducing agent in the second supply passage 58 and the pressure chamber 33 is kept to a substantially fixed state. For example, in the case where the pump 41 is an electrically-operated pump, when the output pressure value is lower than a target value, the pump 41 is controlled so that the duty ratio of the electrically-operated pump is increased to increase the pressure. Conversely, when the output pressure value exceeds the target value, the pump 41 is controlled so that the duty ratio of the electrically-operated pump is reduced to lower the pressure.

The reducing agent injection valve operation controller reads the information concerning the reducing agent in the storage tank 50, the information concerning the exhaust gas temperature, the NOx catalyst temperature and the NOx concentration at the downstream side of the NOx catalyst, the information concerning the operation state of the internal combustion engine, etc. which are output from the CAN information taking and generating unit, generates a control signal for injecting from the reducing agent injection valve 31 reducing agent whose amount is required to reduce NOx contained in exhaust gas, and outputs the control signal to a reducing agent injection valve operation device (represented by “Udv operation device” in FIG. 1) 67 for operating the reducing agent injection valve 31.

Purification of exhaust gas by the exhaust gas purification system 10 constructed as shown in FIG. 1 is carried out as follows.

Under the operation of the internal combustion engine, the reducing agent in the storage tank 50 is pumped up by the pump 41, and pressure-fed to the injection module 30 side. At this time, the detection value of the pressure sensor 45 at the downstream side of the pump 41 provided to the pump module 40 is fed back. When the detection value is less than a predetermined value, the output of the pump 41 is increased. When the pressure value exceeds a predetermined value, the pressure is reduced by the pressure control valve 49. Accordingly, the pressure of the reducing agent pressure-fed to the injection module 30 side is controlled to be kept to a substantially fixed value.

Furthermore, the reducing agent pressure-fed from the pump module 40 to the injection module 30 flows into the pressure chamber 33 of the reducing agent and it is kept to substantially fixed pressure, whereby the reducing agent is injected into the exhaust gas passage 11 at all times when the reducing agent injection valve 31 is opened. Furthermore, the reducing agent reflows through the circulation passage 59 into the storage tank 50. Therefore, the reducing agent which is not injected into the exhaust gas passage 11 is stocked in the pressure chamber 33, and thus it is prevented from being exposed to high temperature with exhaust gas heat.

Under the state that the reducing agent is stocked in the pressure chamber 33 under a substantially fixed pressure value, DCU 60 determines the amount of reducing agent to be injected on the basis of information such as the operation state and exhaust gas temperature of the internal combustion engine, the temperature of the NOx catalyst 13 and the amount of NOx which is passed through the NOx catalyst 13 without being reduced and measured at the downstream side of the NOx catalyst 13, etc., generates the control signal corresponding to the determined reducing agent amount and outputs the control signal to the reducing agent injection valve operation device (not shown). The duty control of the reducing agent injection valve 31 is performed by the reducing agent injection valve operation device, and a proper amount of reducing agent is injected into the exhaust gas passage 11. The reducing agent injected into the exhaust gas passage 11 flows into the NOx catalyst 13 while mixed in the exhaust gas, and used for the reductive reaction of NOx contained in the exhaust gas, whereby the purification of the exhaust gas is performed.

(2) Trouble Diagnosis Device

Here, DCU 60 of the embodiment of the present invention is provided with a trouble diagnosis unit of the exhaust gas purification system 10. The trouble diagnosis unit of the exhaust gas purification system 10 compares the upstream-side NOx amount and downstream-side NOx amount of the NOx catalyst which pass within a predetermined time under the state that a predetermined condition is satisfied, and diagnoses whether the exhaust gas purification system operates normally.

As shown in FIG. 2, the trouble diagnosis unit of the exhaust gas purification system has upstream-side NOx concentration calculating means for detecting the NOx concentration at the upstream side of the NOx catalyst (represented as “upstream-side NOx concentration calculation” in FIG. 2), upstream-side NOx flow rate calculating means for calculating the NOx flow amount at the upstream side of the NOx catalyst per unit time (represented as “upstream-side NOx flow rate calculation” in FIG. 2), upstream-side NOx amount calculating means for calculating the amount of NOx passing through the upstream side of the NOx catalyst within a predetermined time (represented as “upstream-side NOx amount calculation” in FIG. 2), downstream-side NOx flow rate calculating means for calculating the NOx flow rate at the downstream side of the NOx catalyst (represented as “downstream-side NOx flow rate calculation” in FIG. 2), and downstream-side NOx amount calculating means for calculating the amount of NOx passing through the downstream side of the NOx catalyst within a predetermined time (represented as “downstream-side NOx amount calculation” in FIG. 2).

The trouble diagnosis unit contains exhaust gas mass flow rate calculating means for calculating the mass flow rate of exhaust gas (represented as “exhaust gas mass flow rate calculation” in FIG. 2), catalyst temperature calculating means for calculating the temperature of the NOx catalyst from detection values of the temperature sensors at the upstream side and downstream side of the NOx catalyst (represented as “catalyst temperature calculation” in FIG. 2), reduction condition determining means for determining whether at least one condition for performing reduction of NOx is satisfied or not (represented as “reduction condition determination” in FIG. 2), and trouble determining means for determining the presence or absence of a trouble of the exhaust gas purification system by comparing the upstream-side NOx amount and the downstream-side NOx amount (represented as “trouble determination” in FIG. 2).

The exhaust gas mass flow rate calculating means reads information concerning the operation state of the internal combustion engine which is output from the CAN information taking and generating unit, and calculates the mass flow rate of the exhaust gas discharged from the internal combustion engine.

Furthermore, as in the case of the exhaust gas flow rate calculating means, the upstream-side NOx concentration calculating means reads information concerning the operation state of the internal combustion engine which is output from the CAN information taking and generating unit, and calculates the concentration of NOx discharged from the internal combustion engine.

A fuel injection amount, a rotational number, the status of an exhaust circulation device (EGR: Exhaust Gas Recirculation), an exhaust circulation amount, an air suction amount, a cooling water temperature, etc. are used as the information concerning the operation state of the internal combustion engine which is used to calculate the exhaust gas mass flow rate and calculate the concentration of NOx discharged from the internal combustion engine and exists on CAN. The calculation of the exhaust gas mass flow rate and the NOx concentration on the basis of these information can be performed by a well-known method.

In DCU 60 of this embodiment, the NOx concentration at the upstream side of the NOx catalyst is determined by the calculation. However, as in the case of the NOx concentration at the downstream side of the NOx catalyst, an NOx sensor may be disposed at the upstream side of the NOx catalyst and the detection value of the NOx sensor concerned is used although the cost may rise up.

The upstream-side NOx flow rate calculating means calculates the NOx flow amount at the upstream side of the NOx catalyst per unit time on the basis of the upstream-side NOx concentration calculated by the upstream-side NOx concentration calculating means described above and the exhaust gas mass flow rate. The upstream-side NOx amount calculating means integrates the NOx flow rate in a time zone satisfying a predetermined condition out of the NOx flow rate calculated by the upstream-side NOx flow rate calculating means, thereby integrating the NOx amount passing through the upstream side of the NOx catalyst within a predetermined time.

Furthermore, the downstream-side NOx flow rate calculating means calculates the NOx flow amount at the downstream side of the NOx catalyst per unit time on the basis of the NOx concentration detected by the NOx sensor disposed at the downstream side of the NOx catalyst and the exhaust gas mass flow rate, which are output from the CAN information taking and generating unit. The downstream-side NOx amount calculating means integrates the NOx flow rate in a time zone satisfying a predetermined condition out of the NOx flow rate calculated by the downstream-side NOx flow rate calculating means, thereby calculating the NOx amount passing through the downstream-side of the NOx catalyst within a predetermined time.

In DCU 60 of this embodiment, the upstream-side NOx amount calculating means and the downstream-side NOx amount calculating means integrate the NOx flow rate only when it is determined in reduction condition determining means that the system satisfies various conditions under which the reduction of NOx is normally performed. The temperature of the NOx catalyst, the concentration of exhausted NOx, the flow rate of the exhaust gas, etc. are used as the conditions for normally performing the reduction of NOx. It is an important element to the activation state of the catalyst whether the temperature of the NOx catalyst is within a predetermined range or not.

Whether the exhausted NOx concentration and the flow rate of the exhaust gas are within predetermined ranges is an important element to whether the flow rate of NOx flowing into the NOx catalyst is within a processing capacity range of the catalyst or not. That is, when the NOx flow rate is integrated although these conditions are not satisfied, the system is diagnosed to be broken down in spite of a situation that the reduction efficiently is merely low, and thus the reliability of the diagnosis result is lowered. Therefore, the integration is performed only when the above conditions are satisfied.

The reduction condition determining means determines whether the exhaust gas purification system satisfies the various conditions for normally performing the reduction of NOx, and outputs signals to the upstream-side NOx amount calculating means and the downstream-side NOx amount calculating means. These conditions contain the temperature of the NOx catalyst calculated by the catalyst temperature calculating means, the NOx concentration at the upstream side of the NOx catalyst calculated by the upstream-side NOx concentration calculating means, the exhaust gas mass flow rate calculated by the exhaust gas flow rate calculating means, etc. A condition range for normally performing the reduction of NOx in the NOx catalyst is defined in advance, and when these conditions are within the prescribed range, signals are output so that the upstream-side NOx amount calculating means and the downstream-side NOx calculating means integrate the NOx flow rate.

The catalyst temperature calculating means estimates the temperature of the NOx catalyst by using map or the like on the basis of the temperature information detected by the temperature sensors at the upstream side and downstream side of the NOx catalyst output from the CAN information taking and generating unit. Here, the estimated temperature information of the NOx catalyst is used as one of the conditions for normally performing the reduction of NOx.

Furthermore, RAM (Random Access Memory) is connected to the trouble diagnosis unit of DCU 60 of this embodiment, and the integration values of the NOx amounts which are calculated by the upstream-side NOx amount calculating means and the downstream-side NOx amount calculating means are stored on a case-by-case basis.

The trouble determining means reads the upstream-side NOx amount and the downstream-side NOx amount stored in RAM, and determines the ratio of the downstream-side NOx amount to the upstream-side NOx amount. In addition, the trouble determining means compares the reduction efficiency of NOx represented by this ratio with a predetermined threshold value to determine whether the system operates normally.

Furthermore, the trouble determining means is provided with an NOx flow rate counter. When the condition for normally performing the reduction of NOx is satisfied, the counter is subjected to addition processing. When the condition concerned is not satisfied, the counter is subjected to subtraction processing. The counter value is used to determine whether the integration value of the NOx flow rate is reliable to the extent that it can be used for the trouble diagnosis of the system.

Furthermore, in order to define the maximum time until the state that the system does not satisfy the above condition is continued and the integration value of the NOx flow rate is reset, the value of the NOx flow rate counter is prevented from increasing to a predetermined value MAX or more. The addition rate of the NOx flow rate counter is increased when the NOx flow rate is high, and it is reduced when the NOx flow rate is low. This is because the reliability of the trouble diagnosis using the integration value of the NOx flow rate is more enhanced as the NOx flow rate is higher, and the time taken until the counter value is equal to zero by subjecting the counter to the subtraction processing can be lengthened.

For descriptive purposes, the timing chart of FIG. 3 is illustrated while the addition rate is set to be fixed.

(3) Timing Chart

The upstream-side NOx flow rate and the downstream-side NOx flow rate are integrated in the trouble diagnosis device of this embodiment when the system satisfies various conditions for normally performing the reduction of NOx. Next, this will be described in detail with reference to the timing chart shown in FIG. 3.

First, when the state that the condition of normally performing the reduction of NOx is satisfied (the state that Condition is set to a state indicating True) is set at a time point of t1, the NOx flow rate counter is subjected to the addition processing (NOx flow rate counter Inc). The integration of the NOx flow rate at the upstream side of the NOx catalyst is started by Pre-Integrator of the upstream-side NOx amount calculating means at the time point of t1, and the integration is continued insofar as Condition is set to the state of True.

Thereafter, since the condition for normally performing the reduction of NOx is not satisfied at a time point of t2 (Condition is set to a state indicating False), the NOx flow rate counter is subjected to the subtraction processing (NOx flow rate counter Dec). The integration of the NOx flow rate at the upstream side which is continually performed is interrupted at the time point of t2. Subsequently, the NOx flow rate counter is also subjected to the subtraction processing, and the integration value which is integrated by Pre-Integrator is reset at a time point of t3 at which the counter value of the NOx flow rate counter is set to zero without exceeding a prescribed value START.

Subsequently, when Condition is set to True at a time point of t4 again, the addition processing of the NOx flow rate counter is resumed, and also the integration of the NOx flow rate at the upstream side of the NOx catalyst by Pre-Integrator is resumed. The state that Condition is set to True is continued between t4 and t5, and thus the integration of the NOx flow rate at the upstream side of the NOx catalyst is also continued. Thereafter, at a time point of t5, Condition is set to False. Therefore, as in the case of the time point of t2, the NOx flow rate counter is subjected to the subtraction processing, and also the integration of the NOx flow rate at the upstream side is interrupted.

Subsequently, at a time point of t6, Condition is set to True again before the NOx flow rate counter is equal to zero. Therefore, the NOx flow rate counter is subjected to the addition processing again, and also the integration of the upstream-side NOx flow rate by Pre-Integrator is resumed.

At this time, the NOx flow rate counter exceeds the prescribed value START at a time point of t7. Therefore, the integration value which is integrated by Pre-Integrator is added to Main-Integrator, and also the switching operation is carried out so that the subsequent integration is performed by Main-Integrator. In the time chart of FIG. 3, Main-Integrator is set so that the previous integration value has been already stored therein. By selectively using Pre-Integrator and Main-Integrator, the storage of the integration value is carried out by Main-Integrator, and the integration at the initial stage is carried out by Pre-Integrator. Therefore, when the NOx flow rate counter is reset at some midpoint at the initial stage of the integration, it can be returned to zero at all times.

Subsequently, at a time point of t8, the NOx flow rate counter exceeds the prescribed value MAX, and thus the NOx flow rate counter is fixed to MAX insofar as Condition is set to True. As described above, the value of the NOx flow rate counter is fixed to MAX at maximum in order to prevent such a situation that the time taken until the state that Condition is set to False is continued at some midpoint and thus the NOx flow rate counter is set to zero and thus reset and the time taken until the integration is finally finished are excessively lengthened, so that the determination precision is lowered and the time required for the trouble determination is lengthened.

Subsequently, the integration is interrupted under the state that Condition is set to False, and the integration of Main-Integrator is continued under the state that Condition is set to True.

Subsequently, a timer 1 is actuated at a time point of t9 at which the value of the integrated upstream-side NOx amount exceeds the prescribed value MIN. The integration of Main-Integrator is finished at a time point of t10 at which the period of the timer 1 is finished. With respect to the timing at which the integration of Main-Integrator is finished, even before the timer 1 is finished, the integration is finished when the period for which Condition indicates False is continued for a predetermined time after the value of the upstream NOx amount exceeds the prescribed value MIN.

As described above, The integration is finished after a predetermined time elapses from the time when the integration value of the upstream-side NOx flow rate exceeds the prescribed value MIN because after the prescribed value MIN corresponding to the lowest NOx integration amount is secured when the trouble determination is performed, the NOx integration amount is further accumulated so that a required time is not remarkably lengthened, thereby enhancing the determination precision.

2. Trouble Diagnosis Method for Exhaust Gas Purification System

Next, a specific routine of the trouble diagnosis method for the exhaust gas purification system will be described with reference to the flowcharts of FIGS. 4 to 6. This routine may be executed at all times, or it may be executed by interrupting every fixed time.

First, after the routine is started, the mass flow rate Gf of the exhaust gas discharged from the internal combustion engine is calculated in step S100, and then the NOx concentration in the exhaust gas discharged from the internal combustion engine, that is, the NOx concentration Nu at the upstream side of the NOx catalyst is calculated in step S101. Subsequently, in step S102, the NOx flow rate Nfu per unit time at the upstream side of the NOx catalyst is calculated on the basis of the mass flow rate Cf of the exhaust gas and the upstream-side NOx concentration Nu which are calculated in step S100 and step S101, and then the processing goes to step S103.

In step S103, it is determined whether there is no error information from the NOx sensor provided at the downstream side of the NOx catalyst and the input value from the NOx sensor is within a standard range of CAN. When these conditions are not satisfied, the processing is returned to the start position. When it is determined that both the conditions are satisfied, the processing goes to step S104.

In step S104, the NOx concentration Nd at the downstream side of the NOx catalyst which exists on CAN and is detected by the NOx sensor is read out, the NOx flow amount Nfd per unit time at the downstream side of the NOx catalyst is calculated on the basis of the NOx concentration Nd at the downstream side of the NOx catalyst read in step S104 and the mass flow rate Gf of the exhaust gas calculated in step S100 in step S105, and then the processing goes to step S106.

In step S106, as in the case of the step S103 described above, it is determined whether there is no error information from the NOx sensor and the input value from the NOx sensor is within the standard range of CAN, and also it is determined whether the system is set to an reducing agent-injection possible state (hereinafter referred to as “test environment condition TE”). When it is determined that the test environment condition TE is not satisfied, the system is not set to the test-possible state, and thus the processing is returned to the start position. On the other hand, when it is determined that the test environment condition TE is satisfied, the processing goes to step S107 to calculate the increase amount Inc or the decrease amount Dec of the NOx flow rate counter at the upstream side of the NOx catalyst, and then the processing goes to step S108 (FIG. 5).

In the step S108 to which the processing is shifted when the increase amount Inc or the decrease amount Dec of the NOx flow rate Nfu at the upstream side of the NOx catalyst is calculated, it is determined whether the temperature Tc of the NOx catalyst is within a prescribed range, the NOx concentration Nu at the upstream side of the NOx catalyst is within a prescribed range and the NOx flow rate Nfu at the upstream side of the NOx catalyst is within a prescribed range. When all of these conditions are satisfied, the processing goes to step S109, and the NOx flow rate counter is added by the amount corresponding to the increase amount Inc determined in step S107. Subsequently, the NOx flow rate Nfu at the upstream side of the NOx catalyst is integrated in step S110, the NOx flow rate Nfd at the downstream side of the NOx catalyst is integrated in step S111, and they are stored in RAM.

After the NOx flow rate Nfu at the upstream side of the NOx catalyst and the NOx flow rate Nfd at the downstream side of the NOx catalyst are stored, it is determined in step S112 whether the NOx flow rate counter reaches the prescribed value START and also a recording value addition flag RcrdGf is equal to zero. When all of them are satisfied, the processing goes to step S113 to read the stored integration value of the NOx flow rate Nfu at the upstream side of the NOx catalyst, add the read-out integration value to the previously accumulated integration value and also store the addition result into RAM again. Furthermore, in step S114, the stored integration value of the NOx flow rate Nfd at the downstream side of the NOx catalyst is read, added to the previously accumulated integration value and then stored into RAM again. Thereafter, the recording value addition flag RcrdGf is set to 1 in step S115, and then the processing goes to step S116.

On the other hand, when it is determined in step S112 that the NOx flow rate counter does not reach the prescribed value START or the recording value addition flag RcrdGf is not equal to zero, the processing goes to step S116, and it is determined whether the recording value addition flag RcrdGf is put up. When the recording value addition flag RcrdGf is not put up, the processing is returned to the start position. On the other hand, when the recording value addition flag RcrdGf is put up, the processing goes to step S117 to store the integration value Nnu of the NOx flow rate Nfu at the upstream side of the NOx catalyst, the integration value Nnd of the NOx flow rate Nfd at the downstream side of the NOx catalyst is stored in step S118, and then the processing goes to step S119.

In step S119 to which the processing is shifted when the recording value addition flag RcrdGf is put up, it is determined whether the integration value Nnu of the NOx flow rate Nfu at the upstream side of the NOx catalyst is equal to the prescribed value MIN or more. When the integration value Nnu of the NOx flow rate Nfu is less than the prescribed value MIN, the processing goes to step S120 to determine whether the NOx flow rate counter reaches the prescribed MAX. When the NOx flow rate counter does not reach the prescribed value MAX, the processing is directly returned to the start position. On the other hand, when the NOx flow rate counter reaches the prescribed value MAX, the counter value is fixed to MAX in step S121, and the processing is returned to the start position.

On the other hand, when the integration value Nnu of the NOx flow rate Nfu is equal to MIN or more, the processing goes to step S122 to determine whether the NOx flow rate counter reaches the prescribed value MAX. When the NOx flow rate counter does not reach the prescribed value MAX, the processing is returned to the start position again. On the other hand, when the NOx flow rate counter reaches the prescribed value MAX, the NOx flow rate counter is fixed to the prescribed value MAX in step S123, and the processing goes to step S124.

In step S124 to which the processing is shifted when the integration value Nnu of the NOx flow rate Nfu is equal to MIN or more and the NOx flow rate counter is fixed to MAX, it is determined whether the timer 1 is under operation. When the timer 1 is stopped, the timer 1 is actuated in step S125, and then the processing is returned to the start position. On the other hand, when the timer 1 is under operation, the processing goes to step S126.

Subsequently, in step S126, it is determined whether the timer 1 is finished. When the timer 1 is not finished, the processing is returned to the start position. On the other hand, when the timer 1 is finished, the NOx flow rate counter is reset in step S127, and then the processing goes to step S135 (FIG. 6).

On the other hand, when all the conditions are not satisfied in step S108, the processing goes to step S128 to subject the NOx flow rate counter to the subtraction processing by the amount corresponding to Dec determined in step S107.

Subsequently, in step S129, it is determined whether the NOx flow rate counter is equal to zero or not. When the NOx flow rate counter is not equal to zero, the integration value is fixed in step S130, and it is determined whether the timer 1 is under operation or not in step S131. When the timer 1 is stopped, the processing is returned to the start position, and when the timer 1 is under operation, the processing goes to step S126. On the other hand, when the NOx flow rate counter is equal to zero, the processing goes to step S132, and it is determined whether the timer 1 is under operation. When the timer 1 is stopped, the integration values Nnu and Nud are reset in step S133, and then the processing is returned to the start position. When the timer 1 is under operation, the timer 1 is reset in step S134, and then the processing goes to step S135 (FIG. 6).

In step S135, the integration value Nnu of the NOx flow rate Nfu at the upstream side of the NOx catalyst is compared with the integration value Nnd of the NOx flow rate Nfd at the downstream side of the NOx catalyst to calculate the actual NOx reduction coefficient PE. Subsequently, in step S136, a threshold value PEt for the NOx purification coefficient which will be originally obtained is calculated and set on the basis of parameter values concerning the reduction of NOx under a condition under which the integration has been performed, a reduction coefficient calculation completion flag is put up in step S137, and then the processing goes to step S138.

In step S138, it is determined whether the threshold value PEt of the NOx reduction coefficient determined in step S136 is equal to a prescribed value PE0 or more. When PEt is smaller than the prescribed value PE0, the processing is returned to the start position. On the other hand, when PEt is larger than the prescribed value PE0, the processing goes to step S139 to determine whether the actual NOx reduction coefficient PE is smaller than the threshold value determined in step S136. When PE is smaller than the threshold value PEt, TestError is set because the NOx reduction coefficient is lowered, and the diagnosis is finished. On the other hand, when PE is larger than the threshold value PEt, TestOK is set because the NOx purification coefficient is kept to a predetermined value or more, and the diagnosis is finished.

According to the trouble diagnosis method for the exhaust gas purification system based on the flowchart described above, the purification efficiency can be calculated by integrating the NOx amounts at the upstream side and downstream side of the NOx catalyst under the state that the condition for normally performing the reductive reaction of NOx is satisfied. Accordingly, the presence or absence of a trouble of the exhaust gas purification system such as the abnormality of the control system of the exhaust gas purification system, the deterioration of the NOx catalyst, the reduction in quality of the reducing agent, etc. can be accurately determined.

The construction of the exhaust gas purification system shown in FIG. 1 is an example, and the exhaust gas purification system which can implement the trouble diagnosis method of the present invention is not limited to the thus-constructed exhaust gas purification system. For example, CAN may be omitted or DCU may be constructed to be integral with the engine ECU. Furthermore, as another example, the exhaust gas purification system may be constructed so that the circulation passage provided for the purpose of the temperature control of the reducing agent is omitted.