Exhaust gas purification apparatus for internal combustion engine

An exhaust gas purification apparatus for an internal combustion engine is designed to prevent a decrease in the accuracy of measurement by a PM sensor (17). The apparatus includes an NOx catalyst (7) provided in an exhaust passage (3) of an internal combustion engine (1) to reduce NOx by reducing agent supplied to it, a supplying device (6) that supplies urea to the NOx catalyst (7) from the upstream of the NOx catalyst (7), the PM sensor (17) that detects the amount of particulate matter in the exhaust gas in the downstream of the NOx catalyst (7), and a restriction part (10) that restricts supply of urea by the supplying device (6) when the amount of particulate matter adhering to the NOx catalyst (7) is equal to or larger than a threshold.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification apparatus for an internal combustion engine.

BACKGROUND ART

Patent document 1 describes an NOx selective reduction catalyst (which will be also referred to simply as “NOx catalyst” hereinafter) and a PM sensor provided downstream of a filter for trapping particulate matter (which will be sometimes referred to as “PM” hereinafter) in exhaust gas.

Patent document 2 describes that deposition of PM contained in exhaust gas on a catalyst deteriorates the catalytic effect.

Patent document 3 describes that when it is determined that a catalyst is in a poisoned condition, a determination as to the degree of deterioration of the catalyst is disabled, or outputting of a result of a determination as to the degree of deterioration of the catalyst is disabled.

Patent document 4 describes that adhesion of HC, SOF, and PM etc. to an exhaust gas purification catalyst leads to a deterioration (poisoning) of the purifying function.

Patent document 5 describes that since liquids such as condensed water have electrical conductivities higher than PM, whose main ingredient is carbon, a change in the capacitance caused by adhesion of condensed water to an electrode part of an PM sensor is significantly larger than a change in the capacity caused by adhesion of PM to the electrode part of the PM sensor.

Even if a filter is provided upstream of an NOx catalyst, a certain amount of PM passes through the filter, and consequently PM adheres to the NOx catalyst in some cases. For example, if the filter has a crack, a large amount of PM adheres to the NOx catalyst. If the surface of the NOx catalyst is covered with PM adhering to the NOx catalyst, the hydrolysis of urea is hindered. In consequence, urea passes through the NOx catalyst without being used to remove NOx in the NOx catalyst. If such urea adheres to the PM sensor, the urea affects the output value of the PM sensor, possibly leading to a deterioration in the accuracy of PM detection. In the case where a cover for protecting a PM sensor element is provided, if reducing agent adheres to the cover, PM is hard to reach the element, possibly leading to a deterioration in the accuracy of PM detection. Although a failure of the filter can be determined using the PM sensor, adhesion of urea to the PM sensor makes the determination of filter failure difficult.

PRIOR ART DOCUMENTS

Patent Documents

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

The present invention has been made in view of the above-described problems, and its object is to reduce the deterioration in the accuracy of determination as to a failure of a filter.

Means for Solving the Problems

To achieve the above object, an exhaust gas purification apparatus for an internal combustion engine according to the present invention comprises:

an NOx selective reduction catalyst provided in an exhaust passage of an internal combustion engine to reduce NOx by reducing agent supplied to it;

a supplying device that supplies urea to said NOx selective reduction catalyst from the upstream of said NOx selective reduction catalyst;

a filter provided in the exhaust passage upstream of said supplying device to trap particulate matter in exhaust gas;

a PM sensor that measures the amount of particulate matter in exhaust gas in the downstream of said NOx selective reduction catalyst; and

a disabling part that disables determination as to a failure of said filter using a measure value of said PM sensor when the amount of particulate matter adhering to said NOx selective reduction catalyst is equal to or larger than a threshold.

Urea supplied by the supplying device to the NOx catalyst is hydrolyzed in the NOx catalyst to become ammonia. This ammonia is used as reducing agent in the NOx catalyst. In other words, NOx is reduced by the presence of ammonia. The amount of particulate matter (PM) adhering to the NOx selective reduction catalyst can be estimated or measured based on, for example, the amount PM discharged from the internal combustion engine or the amount of PM measured by a sensor in the upstream of the NOx catalyst.

If a large amount PM adheres to the NOx catalyst when urea is supplied by the supplying device, hydrolysis of urea is hindered by the PM. A portion of the urea that is not hydrolyzed does not reduce NOx in the NOx catalyst and flows out downstream of the NOx catalyst. Urea sometimes passes through the NOx catalyst in this way to adhere to a PM sensor. If urea adheres to the PM sensor, values output from the PM sensor change, making accurate measurement of PM difficult.

A determination as to a failure of the filter can be made using a measurement value of the PM sensor. Since the PM measured by the PM sensor is the PM having passed through the filter, the larger the measurement value of the PM sensor is, the higher the degree of failure of the filter considered to be. For example, if the filter has a crack, the larger the area of the crack opening is, the larger the amount of PM passing through the filter is. The determination as to a failure of the filter may include a determination as to whether or not the filter is faulty and a determination as to the degree of failure of the filter.

When urea adheres to the PM sensor, the measurement value of the PM sensor changes due to the urea. When this is the case, making a determination as to a failure of the filter using a value measured by the PM sensor can lead to an erroneous determination. However, since the disabling part disables the determination as to a failure of the filter using the measurement value of the PM sensor, errors in determination as to a failure of the filter can be reduced.

The determination as to a failure of the filter may be made using other methods without using a measurement value of the PM sensor. The aforementioned threshold is an amount of PM that hinders hydrolysis of urea in the NOx catalyst. The threshold may be the amount of PM adhering to the NOx catalyst at which the amount of urea that passes through the NOx catalyst exceeds an allowable range.

The apparatus according to the present invention may further include a restriction part that restricts supply of urea by said supplying device when the amount of particulate matter adhering to said NOx selective reduction catalyst is equal to or larger than a threshold.

The restriction part restricts supply of urea when in a state in which urea passes through the NOx catalyst. Specifically, when the amount of particulate matter adhering to the NOx catalyst is equal to or larger than the threshold, the restriction part restricts supply of urea. The restriction of supply of urea may include disabling (suspending) supply of urea and reducing the quantity of supplied urea. By restricting supply of urea in this way, the amount of urea passing through the NOx catalyst can be decreased, and therefore the adhesion of urea to the PM sensor can be prevented. In consequence, a decrease in the accuracy of measurement by the PM sensor can be prevented.

The apparatus according to the present invention may include a removing part that removes particulate matter adhering to said NOx selective reduction catalyst when the amount of particulate matter adhering to said NOx selective reduction catalyst is equal to or larger than the threshold.

The PM adhering to the NOx catalyst can be removed by, for example, raising the temperature of the exhaust gas flowing into the NOx catalyst or heating the NOx catalyst. If PM is removed from the NOx catalyst, hydrolysis of urea is promoted, and therefore adhesion of urea to the PM sensor can be reduced.

In the apparatus according to the present invention, when an integrated value of the amount of particulate matter passing through said filter is equal to or larger than a predetermined value, the amount of particulate matter adhering to said NOx selective reduction catalyst may be equal to or larger than the threshold.

In the case where a filter is provided upstream of the NOx catalyst, PM is trapped by the filter. Consequently, so long as the filter is normal, the amount of PM adhering to the NOx catalyst is little. On the other hand, if a failure such as cracking occurs with the filter, PM passes through the filter and adheres to the NOx catalyst. Since the integrated value of the amount of PM passing through the filter and the amount of PM adhering to the NOx catalyst correlate with each other, the amount of PM adhering to the NOx catalyst can be estimated based on the integrated value of PM passing through the filter. Therefore, when the integrated value of the amount of PM passing through the filter is equal to or larger than a predetermined value, it may be concluded that the amount of PM adhering to the NOx catalyst is equal to or larger than the threshold. The predetermined value mentioned above is a value at which hydrolysis of urea in the NOx catalyst is hindered. The predetermined value may be a value at which the amount of urea passing through the NOx catalyst just exceeds an allowable range. The amount of PM passing through the filter may be determined based on an presumed degree of failure of the filter.

In the apparatus according to the present invention, when a measurement value of said PM sensor deviates from a presumed normal range by a margin equal to or larger than a predetermined value, the amount of particulate matter adhering to said NOx selective reduction catalyst may be equal to or larger than the threshold.

The measurement value of the PM sensor changes with the amount of PM discharged from the internal combustion engine. The amount of PM discharged from the internal combustion engine is determined in accordance with the operation state of the internal combustion engine. In the case where the apparatus has a filter, the measurement value of the PM sensor also changes with the amount of PM passing through the filter. If the PM sensor is normal, the measurement values of the PM sensor fall within a specific range. The presumed normal range is this specific range. On the other hand, when urea adheres to the PM sensor, the measurement value of the PM sensor changes due to effects of urea. Consequently, measurement values of the PM sensor deviate from the presumed normal range. If the deviation is equal to or larger than a predetermined value, it is considered that urea adheres to the PM sensor. In other words, it may be concluded that amount of PM adhering to the NOx catalyst is equal to or larger than the threshold. The predetermined value mentioned above may be the smallest absolute value of the difference between the presumed normal range of the measurement value of the PM sensor and the measurement values of the PM sensor in the state in which urea adheres to the PM sensor.

In the apparatus according to the present invention, said restriction part may estimate the amount of particulate matter passing through said filter on the assumption that the degree of failure of said filter is a predetermined degree.

Since the amount of PM passing through the filter changes in accordance with the degree of failure of the filter, the amount of PM adhering to the NOx catalyst also changes. In other words, the degree of failure of the filter and the amount of PM adhering to the NOx catalyst correlate with each other. Therefore, on the assumption that the degree of failure of the filter is a predetermined degree, the amount of particular matter adhering to the NOx catalyst can be estimated. The degree of failure of the filter may be the degree of decrease in the PM trapping rate, or the crack rate of the filter. The degree of failure of the filter may be a ratio of the amount of PM flowing out of the filter to the amount of PM flowing into the filter in a predetermined operation state. The predetermined value of the degree of failure of the filter may be specified arbitrary, but it may be determined in the following manner.

Specifically, the apparatus according to the present invention may be provided with a differential pressure sensor that measures difference between the pressure in the exhaust passage upstream of said filter and the pressure in the exhaust passage downstream of said filter, and the predetermined degree of failure of said filter may be the lowest degree of failure at which it is possible to determine that the filter is faulty by means of the differential pressure sensor.

If the predetermined degree of failure of the filter is too low, the amount of PM actually adhering to the NOx catalyst is larger than the estimated amount of PM adhering to the NOx catalyst in cases where the degree of failure of the filter is higher than assumed, as is the case, for example, when the filter is removed. Then, the PM sensor becomes abnormal before supply of urea is disabled. On the other hand, if the predetermined degree of failure of the filter is too high, supply of urea is disabled frequently, possibly leading to a decrease in the removal rate of NOx. Moreover, there is a possibility that the number of times of making a determination as to a failure of the filter may decrease.

When the degree of failure of the filter is low, as is the case when the filter is has a small crack, the measurement value of the differential pressure sensor is almost equal to that in the case where the filter is normal. Therefore, a failure of the filter can be determined by means of the differential pressure sensor only when the degree of failure of the filter is relatively high. On the other hand, when the degree of failure of the filter is relatively high, the accuracy of measurement by the PM sensor is deteriorated because the amount of urea adhering to the PM sensor is large.

If the predetermined value of failure of the filter is the lowest degree of failure at which it is possible to determine that the filter is faulty by means of the differential pressure sensor, supply of urea can be prevented from restricted frequently. On the other hand, in cases where the degree of failure of the filter is high, as is the case, for example, when the filter is removed, a determination as to a failure of the filter can be made using the differential pressure sensor.

As above, in the apparatus according to the present invention, when the degree of failure of said filter is equal to or higher than the predetermined degree, a determination as to a failure of said filter may be made based on a measurement value of said differential pressure sensor, and when the degree of failure of said filter is lower than the predetermined degree, a determination as to a failure of said filter may be made based on a measurement value of said PM sensor.

As described above, even when the PM sensor is in trouble, a determination as to a failure of the filter can be made by means of the differential pressure sensor. Therefore, a decrease in the accuracy of determination as to a failure of the filter can be prevented.

Advantageous Effect of the Invention

The present invention can prevent a decrease in the accuracy of determination as to a failure of a filter.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

In the following, specific embodiments of the exhaust gas purification apparatus for an internal combustion engine according to the present invention will be described with reference to the drawings.

FIG. 1is a diagram showing the general configuration of an exhaust gas purification apparatus for an internal combustion engine according to the embodiment. The internal combustion engine1shown inFIG. 1may be either a gasoline engine or a diesel engine.

The internal combustion engine1is connected with an intake passage2and an exhaust passage3. The intake passage2is provided with an air flow meter11that measures the quantity of intake air flowing in the intake passage2. On the other hand, the exhaust passage3is provided with, in order from the upstream of exhaust gas flow, an oxidation catalyst4, a filter5, an injection valve6, an NOx selective reduction catalyst7(which will be hereinafter referred to as the NOx catalyst7).

The oxidation catalyst4may be a catalyst having an oxidative effect. For example, it may be a three way catalyst. The oxidation catalyst4may be supported on the filter5.

The filter5traps PM in the exhaust gas. The filter5may support a catalyst. As the filter5traps PM, PM is gradually deposited on the filter5. The PM deposited on the filter5can be removed by oxidizing it by performing what is called a filter regeneration process, which is the processing of forcibly raising the temperature of the filter5. For example, the temperature of the filter5can be raised by supplying HC to the oxidation catalyst4. Alternatively, the oxidation catalyst4may be eliminated, and equipment for raising the temperature of the filter5may be provided. Still alternatively, the temperature of the filter5may be raised by discharging high temperature gas from the internal combustion engine1.

The injection valve6injects aqueous urea solution. The aqueous urea solution injected through the injection valve6is hydrolyzed in the NOx catalyst7to become ammonia (NH3), all or a portion of which adheres to the NOx catalyst7. This ammonia is used as reducing agent in the NOx catalyst7. In this embodiment, the injection valve6corresponds to the supplying device in the present invention. Alternatively, a device that supplies solid state urea instead of the injected aqueous urea solution may be employed.

The NOx catalyst7reduces NOx in the exhaust gas when reducing agent is present. Therefore, if ammonia is adsorbed to the NOx catalyst7beforehand, NOx can be reduced by ammonia in the NOx catalyst7.

A first exhaust gas temperature sensor12that measures the temperature of the exhaust gas is provided in the exhaust passage3upstream of the oxidation catalyst4. A second exhaust gas temperature sensor13that measures the temperature of the exhaust gas is provided in the exhaust passage3downstream of the oxidation catalyst4and upstream of the filter5. A third exhaust gas temperature sensor14that measures the temperature of the exhaust gas and a first NOx sensor15that measures the NOx concentration in the exhaust gas are provided in the exhaust passage3downstream of the filter5and upstream of the injection valve6. A second NOx sensor16that measures the NOx concentration in the exhaust gas and a PM sensor17that measures the amount of PM in the exhaust gas are provided in the exhaust passage3downstream of the NOx catalyst7. Moreover, the exhaust passage3is provided with a differential pressure sensor20that measures the difference between the pressure in the exhaust passage3downstream of the oxidation catalyst4and upstream of the filter5and the pressure in the exhaust passage3downstream of the filter5and the upstream of the NOx catalyst7. With the differential pressure sensor20, the differential pressure across the filter5(which will be also referred to as the filter differential pressure hereinafter) can be measured. Not all of the above-described sensors are essential, but they may be provided as necessary.

The internal combustion engine1having the above-described construction is equipped with an ECU10, which is an electronic control unit for controlling the internal combustion engine1. The ECU10controls the internal combustion engine1in response to operation conditions of the internal combustion engine1and driver's requests.

The ECU10is connected through electrical wiring with an accelerator opening degree sensor18that outputs an electrical signal representing the amount of depression of the accelerator pedal to enable measurement of the engine load and a crank position sensor19that measures the engine speed. Signals output from these sensors are input to the ECU10. Moreover, the ECU10is connected through electrical wiring with the injection valve6, and the injection valve6is controlled by the ECU10.

The ECU10executes the aforementioned filter regeneration process when the amount of PM deposited on the filter5reaches or exceeds a predetermined amount. The filter regeneration process may be performed when the travel distance of a vehicle equipped with the internal combustion engine1reaches or exceeds a predetermined distance. Alternatively, the filter regeneration process may be performed at regular intervals.

The ECU10makes a determination as to a failure of the filter5on the basis of the amount of PM measured by the PM sensor17. When a failure of the filter5such as cracking occurs, the amount of PM passing through the filter5increases. By detecting this increase in the amount of PM by the PM sensor17, a failure of the filter5can be determined.

For example, a determination as to a failure of the filter5is made by comparing the integrated value of the amount of PM over a predetermined period of time calculated based on the measurement value of the PM sensor17and the integrated value of the amount of PM over the predetermined period of time calculated on the assumption that the filter5is in a predetermined condition.

FIG. 2is a schematic diagram showing the structure of the PM sensor17. The PM sensor17is a sensor that outputs an electrical signal representing the amount of PM deposited on itself. The PM sensor17has a pair of electrodes171and an insulator172arranged between the two electrodes171. As PM adheres between the two electrodes171, the electrical resistance between the two electrodes171changes. Because this change in the electrical resistance correlates with the amount of PM in the exhaust gas, the amount of PM in the exhaust gas can be measured based on the change in the electrical resistance. This amount of PM may be the mass of PM per unit time. Alternatively, it may be the mass of PM over a predetermined time. The structure of the PM sensor17is not limited to the one shown inFIG. 2. What is essential is that the PM sensor used be one which measures PM and whose measurement value is changed by effects of urea.

The PM sensor17is arranged downstream of the filter5. This means that the PM adhering to the PM sensor17is the PM that has passed through the filter5without being trapped by the filter5. Therefore, the amount of PM deposited in the PM sensor17correlates with the integrated value of the amount of PM having passed through the filter5.

FIG. 3is a time chart showing the change in the measurement value of the PM sensor17over time in a case where the filter5is normal and in a case where the filter5is faulty. In the case where the filter5is faulty, PM is deposited on the PM sensor17faster, and accordingly the time E at which the measurement value starts to increase is earlier than that in the case where the filter5is normal. Therefore, if, for example, the measurement value at the time when the elapsed time from the start of the internal combustion engine1reaches a predetermined time F is equal to or larger than a threshold, it may be determined that the filter5is faulty. The predetermined time F is a period of time that does not make the measurement value of the PM sensor increase in the case where the filter5is normal and makes the measurement value of the PM sensor increase in the case where the filter5is faulty. The predetermined time F is determined based on, for example, experiments. The threshold is determined in advance as the smallest value of the measurement values of the PM sensor17in cases where the filter5is faulty, on the basis of, for example, experiments.

It might appear that the PM sensor17may be provided downstream of the filter5and upstream of the NOx catalyst7. However, arranging the PM sensor17at such a location makes the distance from the filter5to the PM sensor17short. If this is the case, there is a possibility that PM having passed through a cracked portion of the filter5may reach the location around the PM sensor17without being diffused in the exhaust gas. Then, there may be cases, depending on the location of the crack on the filter5, where PM hardly adheres to the PM sensor17, and PM is not detected. This may lead to a deterioration in the accuracy of the failure determination.

In contrast to the above, in this embodiment, since the PM sensor17is arranged downstream of the NOx catalyst7, the distance from the filter5to the PM sensor17is long. Therefore, PM having passed through the filter5has been diffused at the location around the PM sensor17. Therefore, it is possible to detect PM irrespective of the location of the crack on the filter5.

However, since the PM sensor17is provided downstream of the injection valve6, there is a possibility that reducing agent (urea) injected through the injection valve6may adhere to the PM sensor17. The adhesion of reducing agent to the PM sensor17can cause a change in the measurement value of the PM sensor17. If PM adheres to the NOx catalyst7, the hydrolysis of urea is hindered by the PM. A portion of urea that is not hydrolyzed flows out of the NOx catalyst7without reacting in the NOx catalyst7. In other words, if the NOx catalyst7is covered with PM, urea passes through the NOx catalyst7. If such urea adheres to the PM sensor17, a change in the measurement value of the PM sensor17will result.

FIG. 4is a time chart showing the change in the measurement value of the PM sensor17over time in a case where it is normal and in a case where it is abnormal. The case where the measurement value is abnormal is a case where reducing agent adheres to the PM sensor17to cause a change in the measurement value.

When normal, the measurement value increases with the lapse of time. Specifically, the measurement value increases with the amount of PM adhering to the PM sensor17. On the other hand, when abnormal, the measurement value may not only increase but decrease in some cases. Moreover, when abnormal, it may take some time for the measurement value to start to increase in some cases. If urea adheres to the PM sensor17and the deposited amount thereof reaches or exceeds a predetermined amount, the measurement value of the PM sensor17increases in the same manner as in the case where PM is deposited on it. However, since urea evaporates at a temperature lower than PM. In consequence, the urea adhering to the PM sensor17evaporates when the temperature of the exhaust gas of the internal combustion engine1is high. Then, the deposited amount of urea decreases, leading to a decrease in the measurement value of the PM sensor17. This phenomenon will not occur when only PM is deposited on the PM sensor17.

If adhering urea is deposited on the cover of the PM sensor17, holes provided on the cover may be stopped in some cases. If the holes are stopped, PM cannot reach the pair of electrodes171, so that PM is not detected. Then, it takes some time for the measurement value to start to increase.

As described above, if urea passes through the NOx catalyst7because of the adhesion of PM to the NOx catalyst7, the measurement value of the PM sensor17changes to make the determination as to a failure of the filter5difficult. In view of this, in this embodiment, the amount of PM adhering to the NOx catalyst7(which will also be referred to as the PM deposition amount hereinafter) is estimated or measured, and if the PM deposition amount reaches or exceeds a threshold, it is concluded that the urea passes through the NOx catalyst7because of the PM adhering to the NOx catalyst7. The PM deposition amount correlates with the integrated value of the amount of PM passing through the filter5(which will also be referred to as the passing PM amount). Therefore, the passing PM amount is estimated or measured, and if the integrated value of the passing PM amount reaches or exceeds a predetermined value, it is concluded that the PM deposition amount is equal to or higher than the threshold.

Referring toFIG. 4, if the actual measurement value of the PM sensor17deviates from a presumed normal range by a margin equal to or larger than a predetermined value, it may be concluded that the amount of PM adhering to the NOx catalyst7is equal to or larger than the threshold. The presumed normal range may be determined based on the amount of PM discharged from the internal combustion engine1. The predetermined value mentioned above may be set to be the smallest absolute value of the difference between the presumed normal range of the measurement value of the PM sensor17and the measurement values of the PM sensor17in the state in which urea adheres to the PM sensor17. As seen fromFIG. 4, when urea adheres to the PM sensor17, the measurement value of the PM sensor17can decrease. Therefore, if the measurement value of the PM sensor17decreases, it may be concluded that the measurement value of the PM sensor17deviates from the presumed normal range by a margin equal to or larger than the predetermined value.

Moreover, when the PM deposition amount is equal to or larger than the threshold, the determination as to a failure of the filter5using the measurement value of the PM sensor17is disabled. Then, in addition, injection of aqueous urea solution through the injection valve6may be restricted. Alternatively, use of the measurement value of the PM sensor17may be disabled. The restriction of injection of aqueous urea solution through the injection valve6may include disabling injection of aqueous urea solution or reducing the quantity of injected aqueous urea solution. In the case where the PM deposition amount is equal to or larger than the threshold, if the amount of reducing agent adsorbed in the NOx catalyst7is large enough to allow removal of NOx, injection of aqueous urea solution through the injection valve6may be disabled. If the amount of reducing agent adsorbed in the NOx catalyst7is not large enough to allow removal of NOx, the determination as to a failure of the filter5may be disabled while enabling injection of aqueous urea solution through the injection valve6.

The PM deposition amount increases in accordance with the integrated value of the passing PM amount. Therefore, it is assumed that the PM deposition amount exceeds an allowable range when the integrated value of the passing PM amount reaches a predetermined value. The allowable range of the PM deposition amount is determined in such a way that the amount of urea passing through the NOx catalyst7falls within an allowable range. The integrated value of the passing PM amount is estimated in the following way.

Firstly, the passing PM amount is estimated on the assumption that the filter5has a failure such as crack. The passing PM amount as such is calculated by multiplying the amount of PM discharged from the internal combustion engine1, which can be determined based on the operation state of the internal combustion engine1, by a predetermined value. The predetermined value mentioned above is a ratio of the amount of PM flowing out of the filter5to the amount of PM flowing into the filter5in a state in which the filter5is faulty. This ratio will be hereinafter referred to as the “passing rate”.

The passing rate changes in accordance with the size of the crack of the filter5or the degree of cracking (which will be hereinafter referred to as the crack rate) of the filter5. The crack rate is a value representing the degree of failure of the filter5and can be determined based on the PM trapping efficiency. For example, when the filter5is in a brand new condition, the crack rate is 0%, and when the filter5cannot trap PM at all, the crack rate is 100%. Moreover, when the filter5is removed, the crack rate may be assumed to be 100%.

In this embodiment, the passing PM amount is specified on the assumption that the filter5has a predetermined crack rate. The passing PM amount is calculated by multiplying the amount of PM discharged from the internal combustion engine1by the passing rate. Since the amount of PM discharged from the internal combustion engine1correlates with the engine speed and the fuel injection quantity, the relationship between them is determined by, for example, experiments and prepared in advance as a map. The amount of PM discharged from the internal combustion engine1is calculated from the engine speed and the fuel injection quantity using this map.

The passing rate changes in conjunction with the filter differential pressure or the quantity of exhaust gas flowing into the filter5. Therefore, the relationship between the passing rate and the filter differential pressure or the relationship between the passing rate and the quantity of exhaust gas flowing into the filter5is determined by, for example, experiments and prepared in advance as a map, which is stored in the ECU10. For example, when the crack rate of the filter5is somewhat large, the higher the filter differential pressure is, the larger the passing rate is. The quantity of exhaust gas can be calculated based on the intake air quantity of the internal combustion engine1and the quantity of fuel supplied to the internal combustion engine1. The integrated value of the passing PM amount can be obtained by calculating the passing PM amount per a predetermined time repeatedly and adding up the results.

FIG. 5is a graph showing a relationship between the travel time of the vehicle equipped with the internal combustion engine1and the integrated value of the amount of PM passing through the filter5(the passing PM amount). The travel time of the vehicle may be replaced by the travel distance of the vehicle. The integrated value of the passing PM amount correlates with the amount of PM adhering to the NOx catalyst7. The relationship between them can be determined in advance by, for example, experiments. When the integrated value of the passing PM amount reaches or exceeds the predetermined value, the amount of PM adhering to the NOx catalyst7reaches or exceeds the threshold. Then, processing for removing the PM adhering to the NOx catalyst7is carried out. For example, a heater may be attached to the NOx catalyst7, and PM may be removed by raising the temperature of the NOx catalyst7. Alternatively, the temperature of the exhaust gas flowing into the NOx catalyst7may be raised, as is the case with the regeneration process for the filter5. Still alternatively, the temperature of the exhaust gas flowing into the NOx catalyst7may be raised by performing the regeneration process for the filter5. As the PM adhering to the NOx catalyst7is removed, the integrated value of the passing PM amount decreases to zero. In this embodiment, the ECU10that executes the process of removing PM from the NOx catalyst7corresponds to the removing part in the present invention.

When the integrated value of the passing PM amount is larger than the predetermined value, and when the process of removing PM from the NOx catalyst7is performed, the determination as to a failure of the filter5using the measurement value of the PM sensor17is disabled Moreover, the quantity of supplied urea is restricted, or use of the measurement value of the PM sensor17is disabled.

FIG. 6is a flow chart of the process of determination as to a failure of the filter5in this embodiment. This routine is executed at regular intervals by the ECU10.

In step S101, it is determined whether or not the regeneration process for the filter5has been performed.

Performing the regeneration process for the filter5removes PM adhering to the NOx catalyst7and urea adhering to the PM sensor17. In other words, this process makes the integrated value of the passing PM amount equal to zero. This condition is a precondition.

If the determination made in step S101is affirmative, the process proceeds to step S102. On the other hand, if the determination is negative, this routine is terminated, because the precondition is not met.

In step S102, the passing PM amount is integrated. The passing PM amount is calculated as a value correlating with the amount of PM adhering to the NOx catalyst7(i.e. the PM deposition amount). The passing PM amount is calculated by multiplying the amount of PM discharged from the internal combustion engine1by the passing rate. The integrated value of the passing PM amount is obtained by calculating the passing PM amount per a predetermined time repeatedly and adding up the results. After the completion of the processing of step S102, the process proceeds to step S103.

In step S103, it is determined whether or not the integrated value of the passing PM amount is smaller than a predetermined value. This predetermined value is determined in advance as the smallest integrated value of such a passing PM amount that prevents hydrolysis of urea by, for example, experiments. In other words, what is determined in this step is whether or not hydrolysis of urea normally proceeds in the NOx catalyst7. If the determination made in step S103is affirmative, the process proceeds to step S104. On the other hand, if the determination is negative, the process proceeds to step S105.

In step S104, a determination as to a failure of the filter5is made using the PM sensor17. At this time, supply of urea is enabled. After the completion of the processing of step S104, this routine is terminated.

In step S105, processing for removing the PM adhering to the NOx catalyst7is executed. For example, HC is supplied to the oxidation catalyst4to raise the temperature of the exhaust gas flowing into the NOx catalyst7, thereby oxidizing the PM. At this time, detection of failure of the filter5is disabled, and supply of urea through the injection valve6is restricted. After the completion of the processing of step S105, the process returns to step S102. In this embodiment, the ECU10that executes the processing of step S105corresponds the restriction part or the disabling part in the present invention.

As described above, in this embodiment, when there is a possibility that the accuracy of the measurement value of the PM sensor17is decreased by urea passing through the NOx catalyst7, it is possible to restrict supply of urea, disable the determination as to a failure of the filter5using the measurement value of the PM sensor17, and/or disable use of the measurement value of the PM sensor17. In consequence, a decrease in the accuracy of measurement by the PM sensor17can be prevented, or an erroneous determination can be prevented from being made in the determination as to a failure of the filter5. In other words, the accuracy of the determination as to a failure of the filter5can be prevented from being deteriorated.

This embodiment can also be applied to the case where the system does not have a filter5. In the case where the system does not have a filter5, the passing PM amount may be considered to be equal to the amount of PM discharged from the internal combustion engine1. Alternatively, the passing rate and the crack rate may be considered to be equal to 100%.

This embodiment employs a method of specifying the passing rate different from the method employed in the first embodiment. Moreover, the differential pressure sensor20is additionally used in the determination as to a failure of the filter5. The other features such as the apparatus used are the same as those in the first embodiment and will not be described accordingly.

In this embodiment, the passing rate or the passing PM amount is specified on the assumption that the degree of failure (crack rate) of the filter5is the lowest degree of failure (crack rate) at which it is possible to determine that the filter5is faulty by means of the differential pressure sensor20. When the crack rate is low, the change in the differential pressure is small, and therefore, the crack of the filter5cannot be detected by means of the differential pressure sensor20. If the passing rate is specified on the assumption that the crack rate of the filter5is at the boundary between an allowable range and an unallowable range, the actual passing rate is higher than the estimated passing rate in cases where the a large amount of PM reaches the NOx catalyst7and the PM sensor17, as is the case, for example, when the filter5is removed by a user. Then, the measurement value of the PM sensor17becomes abnormal before the determination as to a failure of the filter5is disabled. This can lead to a deterioration in the accuracy of the determination as to a failure of the filter5.

On the other hand, if the passing rate is specified taking the removal of the filter5etc. into consideration, the passing PM amount is estimated to be large. Then, the integrated value of the estimated passing PM amount reaches the predetermined amount frequently, and therefore supply of urea is restricted frequently. Supply of urea is restricted when the integrated value of the passing PM the filter5is faulty be made. The crack rate of the filter5indicated as B is the lowest crack rate at which it is possible to determine that the filter5is faulty by means of differential pressure sensor20.

When the actual crack rate of the filter5is equal to or higher than A and lower than B, although the filter5is faulty, it is not possible to determine that the filter5is faulty by means of the differential pressure sensor20. Therefore, when the crack rate of the filter5is equal to or higher than A and lower than B, a determination as to a failure of the filter5is made using the measurement value of the PM sensor17. On the other hand, when the crack rate of the filter5is equal to or higher than B, the integrated value of the actual passing PM amount is larger than the integrated value of the estimated passing PM amount. Consequently, there is a possibility that the measurement value of the PM sensor7may be abnormal. In this case, the accuracy of the failure determination can be improved by making a determination as to a failure of the filter5using the differential pressure sensor20instead of the PM sensor17.

As above, since the estimated value of the passing PM amount is smaller than that in the case where the passing rate is specified taking removal of filter5etc. into consideration, supply of urea can be prevented from being restricted frequently. In addition, the determination as to a failure of the filter5can be prevented from being disabled frequently. Moreover, the measurement value of the PM sensor17is less liable to become abnormal due to adhesion of a large amount of urea to the PM sensor17. Furthermore, if a crack larger than expected such as removal of the filter5occurs, the failure of the filter5can be determined by means of the differential pressure sensor20.

As above, if the passing rate is specified on the assumption that the degree of failure of the filter5is the lowest degree of failure at which it is possible to determine that the filter5is faulty by means of the differential pressure sensor20, a failure of the filter5can be determined by means of the differential pressure sensor20even if the actual passing rate is higher than the passing rate as specified. Specifically, if the crack rate of the filter5is so large that it is possible to determine that the filter5is faulty by means of the differential pressure sensor20, the actual passing PM amount is larger than the estimated passing PM amount, and consequently the amount of PM adhering to the NOx catalyst is larger than estimated. In consequence, urea passes through the NOx catalyst7to adhere to the PM sensor17, thereby deteriorating the accuracy of measurement by the PM sensor17. Even in this case, the failure of the filter5can be determined by means of the differential pressure sensor20because the crack rate is high.

If the actual crack rate is lower than the lowest crack rate at which it is possible to determine that the filter5is faulty by means of differential pressure sensor20, the actual passing PM amount is smaller than the estimated passing PM amount. Consequently, the integrated value of the actual passing PM amount reaches the predetermined value after the integrated value of the estimated passing PM amount reaches the predetermined value. Moreover, since the process of removing the PM adhering to the NOx catalyst7is performed at the time when the integrated value of the estimated passing PM amount reaches the predetermined value, the PM adhering to the NOx catalyst7is removed before the actual passing PM amount reaches the predetermined value. In other words, PM can be removed from the NOx catalyst7before the measurement value of the PM sensor17becomes abnormal due to hindrance of hydrolysis of urea by effects of PM adhering to the NOx catalyst7. In consequence, the accuracy of the determination as to a failure of the filter5can be improved.

Furthermore, when the integrated value of the estimated passing PM amount is equal to or larger than the predetermined value, supply of urea is restricted. Consequently, the passing of urea through the NOx catalyst7can be controlled, and therefore the adhesion of urea to the PM sensor17can be controlled. In consequence, the accuracy of the determination as to a failure of the filter5can be improved.

Moreover, the determination as to a failure of the filter5that is made using the PM sensor17may be disabled when the integrated value of the estimated passing PM amount is equal to or larger than the predetermined value. This can prevent an erroneous determination from being made. The determination as to a failure of the filter5may be disabled when the integrated value of the estimated passing PM amount is equal to or larger than the predetermined value. This can prevent an erroneous determination from being made.

FIG. 8is a flow chart of the process of determination as to a failure of the filter5in this embodiment. This routine is executed at regular intervals by the ECU10. The steps in which processing the same as that in the flow chart ofFIG. 6is executed are denoted by the same step numbers and will not be described further.

If the determination made in step S101is affirmative, the process proceeds to step S201. In step S201, a determination as to a failure of the filter5is made using the differential pressure sensor20. In this step, a determination as to a failure of the filter5is made using the differential pressure sensor20. In the case where the differential pressure sensor20is used, a failure of the filter5cannot be determined unless the crack rate of the filter5is relatively high. Therefore, what is determined in this step is whether or not the filter5has a relatively large crack. In this step, the situation in which the filter5has been removed is also determined as a failure of the filter5. After the completion of the processing of step S201, the process proceeds to step S202.

In step S202, it is determined whether or not the filter5is normal. In other words, it is determined whether or not it has been concluded that the filter5is normal in step S201. When the crack rate of the filter5is relatively low, it is determined in step S201that the filter5is normal. Therefore, a determination as to a failure of the filter5using the measurement value of the PM sensor17is executed in a later step. If the determination made in step S202is affirmative, the process proceeds to step S102. On the other hand, if the determination is negative, which means that it is determined that the filter5is faulty, this routine is terminated.

If the determination made in step S103is negative, the process proceeds to step S203. In step S203, the execution of the determination as to a failure of the filter5using the PM sensor17is disabled. Alternatively, use of the measurement value of the PM sensor17may be disabled. Moreover, supply of urea through the injection valve6is restricted. In this case, the determination as to a failure of the filter5using the differential pressure sensor20has already been made in steps S201and S202. Therefore, even if the filter5is faulty, its crack rate is low. Then, if PM is removed from the NOx catalyst7, urea may be supplied and the determination as to a failure of the filter5using the PM sensor17may be made. For example, after the regeneration process for the filter5is carried out, supply of urea may be enabled, or the determination as to a failure of the filter5using the PM sensor17may be enabled. Moreover, removal of PM may be carried out in the same manner as step S105. In this embodiment, the ECU10that executes the processing of step S203corresponds to the restriction part or the disabling part in the present invention.

As described in the foregoing, in this embodiment, a determination as to a failure of the filter5can be made selectively using the differential pressure sensor20and the PM sensor17. Furthermore, the passing rate is specified on the assumption that the degree of failure (crack rate) of the filter5is the lowest degree of failure (crack rate) at which it is possible to determine that the filter5is faulty by means of the differential pressure sensor20. This can prevent supply of urea from being restricted frequently or prevent the determination as to a failure of the filter5from being disabled frequently. Furthermore, the determination as to a failure of the filter5can be prevented from being performed in a state in which PM sensor17is abnormal.

DESCRIPTION OF THE REFERENCE SIGNS

1: internal combustion engine2: intake passage3: exhaust passage4: oxidation catalyst5: filter6: injection valve7: NOx selective reduction catalyst10: ECU11: air flow meter12: first exhaust gas temperature sensor13: second exhaust gas temperature sensor14: third exhaust gas temperature sensor15: first NOx sensor16: second NOx sensor17: PM sensor18: accelerator opening degree sensor19: crank position sensor20: differential pressure sensor