Abnormality detection system of engine exhaust system

In an internal combustion engine, a hydrocarbon feed valve (15), NOx storage catalyst (13), particulate filter (14), and electric resistance type sensor (29) are arranged in an engine exhaust passage in this order from an upstream side. The electric resistance type sensor (29) generates an output value corresponding to the amounts of deposition of particulate matter and hydrocarbons which are contained in the exhaust gas and deposited at the sensor part thereof. From the change of the output value of the electric resistance type sensor (29), it is judged if the hydrocarbons have slipped through the NOx storage catalyst (13) and if the particulate matter has slipped through the particulate filter (14).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of International Application No. PCT/JP2012/079814, filed Nov. 16, 2012, the content of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an abnormality detection system of an engine exhaust system.

BACKGROUND ART

Known in the art is an internal combustion engine which arranges a particulate filter in an engine exhaust passage and which arranges a particulate matter sensor in the engine exhaust passage downstream of the particulate filter so as to detect particulate matter contained in the exhaust gas which slips through the particulate filter without being trapped by the particulate filter (for example, see PTL 1). In this internal combustion engine, this particulate matter detection sensor is used for example to detect if the particulate filter has cracked and thereby a large amount of particulate matter slips through the particulate filter, that is, the particulate filter has become abnormal.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

On the other hand, in case where an NOxstorage catalyst able to store NOxwhen an air-fuel ratio of exhaust gas is lean and able to release stored NOxby making the air-fuel ratio of the exhaust gas rich is arranged in an engine exhaust passage, a hydrocarbon feed valve is arranged in the engine exhaust passage upstream of the NOxstorage catalyst, and hydrocarbons are injected from the hydrocarbon feed valve to make the air-fuel ratio of the exhaust gas flowing into the NOxstorage catalyst rich when NOxshould be released from the NOxstorage catalyst, if the NOxstorage catalyst deteriorates, the hydrocarbons injected from the hydrocarbon feed valve will slip through the NOxstorage catalyst. In this case, if it were possible to detect that hydrocarbons had slipped through the NOxstorage catalyst, it would be possible to detect that the NOxstorage catalyst deteriorates.

In this regard, in this case, if viewed from the viewpoint of simplification of the detection system and reduction of the manufacturing cost, it can be said to be desirable to detect the particulate matter which slips through the particulate filter and the hydrocarbons which slip through the NOxstorage catalyst by a single sensor. However, the particulate matter which slips through the particulate filter and the hydrocarbons which slip through the NOxstorage catalyst differ in properties. Therefore, up to now, it was never considered at all to simultaneously detect these particulate matter and hydrocarbons by a single sensor.

Therefore, the inventors engaged in repeated studies on the differences in properties between the particulate matter which slips through the particulate filter and the hydrocarbons which slip through the NOxstorage catalyst and as a result discovered it is possible to detect these particulate matter and hydrocarbons by a single sensor Therefore, an object of the present invention is to provide an abnormality detection system of an engine exhaust system which is able to detect the particulate matter which slips through the particulate filter and the hydrocarbons which slip through the NOxstorage catalyst by a single sensor.

Solution to Problem

According to the present invention, there is provided an abnormality detection system of an engine exhaust system in an internal combustion engine in which an NOxstorage catalyst able to store NOxwhen an air-fuel ratio of exhaust gas is lean and able to release stored NOxby making the air-fuel ratio of the exhaust as rich is arranged in an engine exhaust passage, a hydrocarbon feed valve is arranged in the engine exhaust passage upstream of the NOxstorage catalyst, a particulate filter for trapping particulate matter contained in the exhaust gas is arranged in the engine exhaust passage downstream of the NOxstorage catalyst, and hydrocarbons are injected from the hydrocarbon feed valve to make the air-fuel ratio of the exhaust gas flowing into the NOxstorage catalyst rich when NOxshould be released from the NOxstorage catalyst, wherein an electric resistance type sensor having a sensor part to which particulate matter and hydrocarbons which are contained in exhaust gas deposit and generating an output value corresponding to an amount of deposition of the particulate matter and hydrocarbons to the sensor part is arranged in the engine exhaust passage downstream of the particulate filter, the output value of the electric resistance type sensor when hydrocarbons are injected from the hydrocarbon feed valve to release NOxfrom the NOxstorage catalyst and when hydrocarbons slip through the NOxstorage catalyst exhibits a behavior which changes by a faster speed compared with when particulate matter slips through the particulate filter, then changes in direction of change to an opposite direction, and, when the output value of the electric resistance type sensor changes, it is judged if hydrocarbons have slipped through the NOxstorage catalyst when hydrocarbons are injected from the hydrocarbon feed valve or particulate matter has slipped through the particulate filter from the difference in behavior of the output value of the electric resistance type sensor.

Advantageous Effects of Invention

It is possible to detect the particulate matter which slips through the particulate filter and the hydrocarbons which slip through the NOxstorage catalyst by a single electric resistance type sensor and therefore it is possible to simplify the detection system and possible to reduce the manufacturing cost.

DESCRIPTION OF EMBODIMENTS

FIG. 1is an overall view of a compression ignition type internal combustion engine. Referring toFIG. 1, 1indicates an engine body,2a combustion chamber of each cylinder,3an electronically controlled fuel injector for injecting fuel into each combustion chamber2,4an intake manifold, and5en exhaust manifold. The intake manifold4is connected through an intake duct6to the outlet of a compressor7aof an exhaust turbocharger7, while the inlet of the compressor7ais connected through an intake air amount detector8to an air cleaner9. Inside the intake duct6, a throttle valve10which is driven by an actuator is arranged. Around the intake duct6, a cooling device11is arranged for cooling the intake air which flows through the inside of the intake duct6. In the embodiment which is shown inFIG. 1, the engine cooling water is guided to the inside of the cooling device11were the engine cooling water is used to cool the intake air.

On the other hand, the exhaust manifold5is connected to the inlet of an exhaust turbine7bof the exhaust turbocharger7, and the outlet of the exhaust turbine7bis connected through an exhaust pipe12ato the inlet of an NOxstorage catalyst13. A particulate filter14for trapping the particulate matter PM contained in the exhaust gas is arranged downstream of the NOxstorage catalyst13, and the outlet of the particulate filter14is connected to an exhaust pipe12b.Upstream of the NOxstorage catalyst13inside the exhaust pipe12a,a hydrocarbon feed valve15is arranged for feeding hydrocarbons comprised of diesel oil or other fuel used as fuel for a compression ignition type internal combustion engine. In the embodiment shown inFIG. 1, diesel oil is used as the hydrocarbons which are fed from the hydrocarbon feed valve15.

On the other hand, the exhaust manifold5and the intake manifold4are connected with each other through an exhaust gas recirculation (hereinafter referred to as an “EGR”) passage16. An electronically controlled EGR control valve17is arranged in the EGR passage16, and around the EGR passage16, a cooling device18is arranged for cooling the exhaust gas which flows through the inside of the EGR passage16. In the embodiment which is shown inFIG. 1, the engine cooling water is guided to the inside of the cooling device18where the engine cooling water is used to cool the exhaust gas. Further, each fuel injector3is connected through a fuel feed tube19to a common rail20. This common rail20is connected through an electronically controlled variable discharge fuel pump21to a fuel tank22. The fuel which is stored inside of the fuel tank22is fed by the fuel pump21to the inside of the common rail20. The fuel which is fed to the inside of the common rail20is fed through each fuel feed tube19to the fuel injector3.

An electronic control unit30is comprised of a digital computer provided with a ROM (read only memory)32, a RAM (random access memory)33, a CPU (microprocessor)34, an input port35, and an output port36, which are connected with each other by a bidirectional bus31. An air-fuel ratio sensor23is arranged in the exhaust pipe12aupstream of the NOxstorage catalyst13, and a temperature sensor24is arranged at the inlet portion of the NOxstorage catalyst13. In addition, a temperature sensor25is arranged also at the outlet portion of the NOxstorage catalyst13. Furthermore, a pressure difference sensor26for detecting the pressure difference between before and after the particulate filter14is attached to the particulate filter14. On the hand, a temperature sensor27, an air-fuel ratio sensor28and an electric resistance type sensor29for detecting the particulate matter PM and the hydrocarbon HC are arranged in the exhaust pipe12bdownstream of the particular filter14.

The output signals of the air-fuel ratio sensors23,28, the temperature sensors24,25,27, the pressure difference sensor26and the intake air amount detector8are input through respectively corresponding AD converters37to the input port35. In addition, the output signal of a detection circuit39of the electric resistance type sensor29is also input through a corresponding AD converter37to the input port35. Further, the accelerator pedal40has a load sensor41connected to it which generates an output voltage proportional to the amount of depression L of the accelerator pedal40. The output voltage of the load sensor41is input, through a corresponding AD converter37to the input port35. Furthermore, at the input port35, a crank angle sensor42is connected which generates an output pulse every time a crankshaft rotates by, for example, 15°. On the other hand, the output port36is connected through corresponding drive circuits38to each fuel injector3, actuator for driving the throttle valve10, hydrocarbon feed valve15, EGR control valve17and fuel pump21.

First, referring toFIGS. 2A and 2B, the electric resistance type sensor29which is arranged in the exhaust pipe12bwill be explained.FIG. 2Ais a disassembled perspective view of a sensor part of an electric resistance type sensor29. As shown inFIG. 2A, the sensor part of the electric resistance type sensor29is, for example, comprised of a pair of plate-shaped electrical insulators50,51made of alumina. The surface52of the electrical insulator50which is positioned at the opposite side to the electrical insulator51is exposed to the exhaust gas which flows through the inside of the exhaust pipe12b.The electrical insulator51is made to closely contact the back surface of the electrical insulator50which is positioned at the electrical insulator51side. On the surface52of the electrical insulator50which is exposed to the exhaust gas, strip-shaped positive electrodes53and strip-shaped negative electrodes54are alternately arranged at equal intervals. First end parts of the positive electrodes53are connected to a common electrode terminal55which extends in the long direction of the electrical insulator50, while first end parts of the negative electrodes54are connected to a common electrode terminal56which extends in the long direction of the electrical insulator50. Therefore, the overall shape of the positive electrodes53and the overall shape of the negative electrodes54are both comb shapes. On the other hand, a thin film electric heater57is formed on the surface of the electrical insulator51at the electrical insulator50side.

FIG. 2Bis a partial cross-sectional view of the surface52of the electrical insulator50as seen along the B-B section ofFIG. 2A. Note that, inFIG. 25, the black dots illustrate the particulate matter PM which deposits on the surface52of the electrical insulator50. The particulate matter PM is comprised of a various of substances including carbon. This particulate matter PM has electroconductivity and has tackiness. Therefore, if exhaust gas contains particulate matter PM, the particulate matter PM gradually builds up on the surface52of the electrical insulator50. If the surface52of the electrical insulator50between the strip-shaped positive electrodes53and the strip-shaped negative electrodes54is buried by the particulate matter PM, the resistance value between the positive electrodes53and the negative electrodes54will fall. That is, if the exhaust gas contains particulate matter PM, the resistance value between the positive electrodes53and the negative electrodes54will fall along with the elapse of time. Therefore, it becomes possible to detect the cumulative value of the particulate matter PM which is contained in the exhaust gas from the change in the resistance value between the positive electrodes53and the negative electrodes54.

FIG. 3shows the detection circuit39of the electric resistance type sensor29. As shown inFIG. 3, the detection circuit39has a power source58and a fixed resistance59. On the other hand, inFIG. 3, 57shows a variable resistance which is formed by particulate matter PM between the positive electrodes53and negative electrodes54. This variable resistance57and fixed resistance59are serially connected to the power source58. If the amount of deposition of particulate matter PM to the surface52of the electrical insulator50increases, the resistance value of the variable resistance57becomes lower and the current which flows through the variable resistance57increases, so the voltage across the two ends of the fixed resistance59increases. The voltage across the two ends of the fixed resistance59is output from the detection circuit39as the output voltage V. Below, this output voltage V will be called the output voltage V of the electric resistance type sensor29. Note that, the change of the resistance value of the variable resistance57can be taken out as the output current. Therefore, these output voltage V and output current will be referred to together as the output value of the electric resistance type sensor29.

FIG. 4Ashows the relationship between the amount of particulate matter PM which deposits on the sensor part of the electric resistance type sensor29and the resistance value R between the positive electrodes53and the negative electrodes54, whileFIG. 4Bshows the relationship between the amount of particulate matter PM which is deposited on the sensor part of the electric resistance type sensor29and the output voltage V of the electric resistance type sensor29. As will be understood fromFIG. 4A, the more the amount of particulate matter PM which deposits on the sensor part of the electric resistance type sensor29increases, the more the resistance value R between the positive electrodes53and the negative electrodes54fall and, as will be understood fromFIG. 4B, the more the amount of particulate matter PM which deposits on the sensor part of the electric resistance type sensor29increases, the more the output voltage V of the electric resistance type sensor29increases.

Next, a function of the NOxstorage catalyst13will be explained.FIGS. 5A and 5Bschematically shows a surface part of a catalyst carrier which is carried on a substrate of the NOxstorage catalyst13. At this NOxstorage catalyst13, as shown inFIGS. 5A and 5B, for example, there is provided a catalyst carrier60made of alumina on which precious metal catalysts61and62are carried. Furthermore, on this catalyst carrier60, a basic layer63is formed which includes at least one element selected from potassium K, sodium Na, cesium Cs, or another such alkali metal, barium Ba, calcium Ca, or another such alkali earth metal, a lanthanide or another such rare earth and silver Ag, copper Cu, iron Fe, iridium Ir, or another metal able to donate electrons to NOx.

On the other hand, inFIG. 5A and 5B, the precious metal catalyst61is comprised of platinum Pt, while the precious metal catalyst62is comprised of rhodium Rh. Note that, in this case, both the precious metal catalysts61and62may be comprised of platinum Pt. Further, on the catalyst carrier60of the NOxstorage catalyst13, in addition to platinum Pt and rhodium Rh, palladium Pd may be further carried or, instead of rhodium Rh, palladium Pd may be carried. That is, the precious metal catalysts61and62which are carried on the catalyst carrier60are comprised, of at least one of platinum Pt, rhodium Rh and palladium Pd.

Now, when the air-fuel ratio of the exhaust gas which flows into the NOxstorage catalyst13is lean, as shown inFIG. 5A, part of the NO which is contained in the exhaust gas is oxidized on the platinum61and becomes NO2. Next, this NO2is further oxidized and absorbed in the basic layer63in the form of nitrate ions NO3−. Next, the nitrate ions NO3−diffuses in the basic layer63and becomes nitrates. That is, at this time, the NOxin the exhaust gas is absorbed in the form of nitrates inside of the basic layer63. However, if the amount of the NOxabsorbed in the form of nitrates inside of the basic layer63is increased, an NOxpurification rate drops. Accordingly, when the amount of the NOxabsorbed in the form of nitrates inside of the basic layer63is increased, it :is necessary to release the NOxabsorbed inside of the basic layer63from the basic layer63.

In this case, if making the air-fuel ratio of the exhaust gas which flows into the NOxstorage catalyst13rich by feeding hydrocarbons from the hydrocarbon feed valve15, it is possible to release the NOxabsorbed inside of the basic layer63from the basic layer63.FIG. 7Bshows the case where the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst13is made rich when the NOxis absorbed in the form of nitrates inside of the basic layer63. In this case, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the opposite direction (NO3−→NO2), and consequently the nitrates absorbed in the basic layer63successively become nitrate ions NO3−and, as shown inFIG. 5B, are released from the basic layer63in the form of NO2. Next, the released NO2is reduced by the hydrocarbons HC and CO contained in the exhaust gas.

Therefore, in the present invention, the amount of NOxwhich is adsorbed in the form of nitrates in the basic layer63is estimated by for example calculation. When the amount of NOxwhich is adsorbed in the form of nitrates in the basic layer63is estimated as exceeding the allowable value, hydrocarbons are fed from the hydrocarbon feed valve15to make the air-fuel ratio of the exhaust gas flowing into the NOxstorage catalyst13rich. In this case, even if making the air-fuel ratio of the exhaust gas flowing into the NOxstorage catalyst13just slightly rich, it is not possible to make NOxbe released well from the basic layer63. To make NOxbe released well from the basic layer63, it is necessary to make the air-fuel ratio of the exhaust gas flowing into the NOxstorage catalyst13rich to an extent required for releasing the NOxwell. The injection amount of the hydrocarbons from the hydrocarbon feed valve15which makes the air-fuel ratio of the exhaust gas flowing into the NOxstorage catalyst13rich to an extent required for good release of NOxis stored as a function of for example the injection amount Q from the fuel injector3and the engine speed N in the form of a map in advance in the ROM32.

Now then, usually, if injecting hydrocarbons from the hydrocarbon feed valve15by an injection amount stored in the map, NOxcan be made to be released from the basic layer63well. At this time, the air-fuel ratio (A/F) of the exhaust gas which flows into the NOxstorage catalyst13and the amount of hydrocarbons which slips through the NOxstorage catalyst13are shown inFIG. 6(A). FromFIG. 6(A), it will be understood that at this time, almost no hydrocarbons slip through the NOxstorage catalyst13. On the other hand, if the NOxstorage catalyst13deteriorates, even if injecting hydrocarbons from hydrocarbon feed valve15by the injection amount which is stored in the map, it is not possible to sufficiently utilize all of the injected hydrocarbons for releasing NOx. Therefore, in this case, as shown inFIG. 6(B), the amount of hydrocarbons which slip through the NOxstorage catalyst13increases. Therefore, if able to detect the amount of hydrocarbons which slips through at this time, it is possible to judge if the NOxstorage catalyst13deteriorates.

In this regard, when hydrocarbons are fed from the hydrocarbon feed valve15, if the hydrocarbons slip through the NOxstorage catalyst13, the hydrocarbons which slip through deposit on the surface52of the electrical insulator50of the electric resistance type sensor29. In this regard, the surface52of the electrical insulator50is exposed to the exhaust gas, so is high in temperature. Therefore, when there is little slipthrough of the hydrocarbons, the hydrocarbons end up burning upon depositing on the surface52of the electrical insulator50. As a result, in this case, hydrocarbons will not deposit on the surface52of the electrical insulator50. However, if the amount of hydrocarbons which slip through the NOxstorage catalyst13is large, the hydrocarbons temporarily build up on the surface52of the electrical insulator50. In this case, since the hydrocarbons also have electroconductivity, if a large amount of hydrocarbons deposit on the surface52of the electrical insulator50, the resistance value between the positive electrodes53and negative electrodes54will fall.

On the other hand, the hydrocarbons which build up on the surface52of the electrical insulator50immediately burn upon building up and are eliminated from the surface52of the electrical insulator50. Therefore, when a large amount of hydrocarbons build up on the surface52of the electrical insulator50, the resistance value between the positive electrodes53and negative electrodes54temporarily falls. Therefore, when feeding hydrocarbons from the hydrocarbon feed valve15, slipthrough of the hydrocarbons through the NOxstorage catalyst13can be detected.

PIG.7A shows the relationship between the amount of hydrocarbons HC which temporarily deposit on the sensor part of the electric resistance type sensor29and the resistance value R between the positive electrodes53and negative electrodes54at this time, whileFIG. 7Bshows the relationship between the amount of hydrocarbons HC which temporarily deposit on the sensor part of the electric resistance type sensor29and the output voltage V of the electric resistance type sensor29at this time. As will be understood fromFIG. 7A, the more the amount of temporary deposition of hydrocarbons HC at the sensor part of the electric resistance type sensor29increases, the more the resistance value R between the positive electrodes53and the negative electrodes54falls. As will be understood fromFIG. 7B, the more the amount of temporary deposition of hydrocarbons HC at the sensor part of the electric resistance type sensor29increases, the more the output voltage V of the electric resistance type sensor29increases.

Next, referring toFIGS. 8A and 8B, slipthrough of particulate matter PM in the particulate filter14will be explained. Note that,FIGS. 8A and 8Bshow the relationship between the output voltage V of the electric resistance type sensor29and the running distance of the vehicle. Now then, almost all of the particulate matter PM which is contained in the exhaust gas exhausted from the engine is usually trapped by the particulate filter14. Therefore, almost no particulate matter PM slips through the particulate filter14. Therefore, the output voltage V of the electric resistance type sensor29is usually zero or, as shown inFIG. 8A, is maintained at an extremely low value.

On the other hand, when the particulate filter14should be regenerated, the particulate matter PM which is trapped by the particulate filter14is made to burn. At this time, if a situation arises at which the temperature of the particulate filter14becomes extremely high and after the particulate matter PM finishes being burned, the temperature of the particulate filter14is made to rapidly fall, the particulate filter14sometimes fractures, that is, cracks. If the particulate filter14cracks, particulate matter PM will slip through the particulate filter14.FIG. 8Bshows the case where the particulate filter14cracks and particulate matter PM slips through the particulate filter14. In this case, the output voltage V of the electric resistance type sensor29rises to the predetermined allowable value VX in several minutes to tens of minutes in accordance with the extent of the crack.

In this embodiment according to the present invention, if the output voltage V of the detection circuit39rises to the allowable value VX, the electric heater57of the electric resistance type sensor29starts to be powered and the heating action of the electrical, insulator50is started. If the heating action of the electrical insulator50is started, the particulate matter PM which deposited on the surface52of the electrical insulator50is made to burn and the particulate matter PM gradually disappears from the surface52of the electrical insulator50. As a result, as shown inFIG. 8B, the output voltage V of the electric resistance type sensor29gradually fails. Next, if the output voltage V of the electric resistance type sensor29becomes zero, the acting of powering the electric heater57is stopped. Note that, if the particulate filter14cracks, in the particulate filter14, even after that, the particulate matter PM continues to slip through. As a result, the output voltage V of the electric resistance type sensor29again rises to the allowable value VX.

In this way, in this embodiment according to the present invention, the electric resistance type sensor29is provided with the electric heater57for heating the sensor part of the electric resistance type sensor29. When the output value of the electric resistance type sensor29exceeds the predetermined allowable value VX, the heating action by the electric heater57is performed to burn off the particulate matter PM which deposits on this sensor part. Note that, even when the particulate filter14does not crack, if the vehicle is run for several thousand kilometers or more, the output voltage V of the electric resistance type sensor29sometimes reaches the allowable value TX. In this case as well, in the same way as the case which is shown inFIG. 8B, the electric heater57of the electric resistance type sensor29is powered and the heating action of the electrical insulator50is performed.

As shown inFIG. 8B, when the particulate filter14cracks, the output voltage V of the electric resistance type sensor29rises relatively slowly. Therefore, when it is judged that the output voltage V of the electric resistance type sensor29is gently rising as shown inFIG. 8B, it, is possible to judge that the particulate filter14cracks, that is, the particulate filter14becomes abnormal. Note that, in this embodiment according to the present invention, to reliably detect that the particulate filter14becomes abnormal, when the period Δt (FIG. 8B) at which the heating action by the electric heater57is performed is shorter than a predetermined period, it is judged that the particulate filter14cracks, that is, that the particulate filter14becomes abnormal.

Next, referring toFIG. 9, the method for judging if slipthrough of the hydrocarbons is occurring at the NOxstorage catalyst13when hydrocarbons are injected from the hydrocarbon feed valve15to release NOxfrom the NOxstorage catalyst13will be explained.FIG. 9shows the change in the air-fuel ratio (A/F) of the exhaust gas flowing into the NOxstorage catalyst13, the change in the output voltage V of the electric resistance type sensor29, and the change in the injection completion flag when hydrocarbons are injected from the hydrocarbon feed valve15to release NOxfrom the NOxstorage catalyst13. Note that,FIG. 9(A)shows the case where the NOxstorage catalyst13is not deteriorating, whileFIG. 9(B)shows the case where the NOxstorage catalyst13is deteriorating. Note that, the output voltage of the electric resistance type sensor29before the output voltage V of the electric resistance type sensor29rises due to the hydrocarbon feed valve15injecting hydrocarbons will be referred to below as the “reference voltage V0”. This reference voltage V0becomes zero when particulate matter PM is not building up on the sensor part of the electric resistance type sensor29. On the other hand, as will he understood fromFIGS. 9(A) and 9(B), the injection completion flag is set when the action of injection of hydrocarbons from the hydrocarbon feed valve15is completed.

As explained above, when the NOxstorage catalyst13is not deteriorating, almost no hydrocarbons slip through the NOxstorage catalyst13when hydrocarbons are injected from the hydrocarbon feed valve15tto release NOxfrom the NOxstorage catalyst13. Therefore, in this case, as shown inFIG. 9(A), the output voltage V of the electric resistance type sensor29only changes a little from the reference voltage V0. As opposed to this, when the NOxstorage catalyst13deteriorates, as explained above, a large amount of hydrocarbons slips through the NOxstorage catalyst13and, as a result, as shown inFIG. 9(B), the output voltage V of the electric resistance type sensor29rapidly increases from the5reference voltage V0in from 1 second to several seconds and then decreases as the hydrocarbons which deposit on the surface52of the electrical insulator50burn.

That is, as shown inFIG. 8B, the output value of the electric resistance type sensor29when the particulate matter PM slips through the particulate filter14changes continuously toward the same direction of change. As opposed to this, as shown inFIG. 9(B), the output value of the electric resistance type sensor29when hydrocarbons are injected from the hydrocarbon feed valve15and slip through the NOxstorage catalyst13changes by a speed faster than the speed of change of the output value of the electric resistance type sensor29when the particulate matter PM slips through the particulate filter14, then changes in direction of change to the opposite direction and returns to the original output value. Further, the speed of change of the output value of the electric resistance type sensor29in this case is extremely fast compared with the case where the particulate filter14cracks.

In this way, the output value of the electric resistance type sensor29when hydrocarbons are injected from the hydrocarbon feed valve15to release NOxfrom the NOxstorage catalyst.13and when the hydrocarbons HC slip through the NOxstorage catalyst13exhibits a behavior which changes by a speed faster than when particulate matter PM slips through the particulate filter14, then changes in direction of change to the opposite direction. Therefore, the behavior of the output value of the electric resistance type sensor29completely differs between when the hydrocarbons HC slip through the NOxstorage catalyst13and the particulate matter PM slips through the particulate filter14. Therefore, when the output value of the electric resistance type sensor29changes, it is possible to judge if cracking of the particulate filter14causes the output value of the electric resistance type sensor29to change or deterioration of the NOxstorage catalyst13causes the output value of the electric resistance type sensor29to change from the difference of behavior of the output value of the electric resistance type sensor29.

Therefore, in the present invention, in an internal combustion engine, an NOxstorage catalyst13able to store NOxwhen an air-fuel ratio of exhaust gas is lean and able to release stored NOxby making the air-fuel ratio of the exhaust gas rich is arranged in an engine exhaust passage, a hydrocarbon feed valve15is arranged in the engine exhaust passage upstream of the NOxstorage catalyst13, a particulate filter14for trapping particulate matter PM contained in the exhaust gas is arranged in the engine exhaust passage downstream of the NOxstorage catalyst13, and hydrocarbons are injected from the hydrocarbon feed valve15to make the air-fuel ratio of the exhaust gas flowing into the NOxstorage catalyst13rich when NOxshould be released from the NOxstorage catalyst13,

an electric resistance type sensor29having a sensor part to which particulate matter PM and hydrocarbons HC which are contained in exhaust gas deposit and generating an output value corresponding to an amount of deposition of the particulate matter PM and hydrocarbons HC to the sensor part is arranged in the engine exhaust passage downstream of the particulate filter14, the output value of the electric resistance type sensor29when hydrocarbons are injected from the hydrocarbon feed valve15to release NOxfrom the NOxstorage catalyst13and when hydrocarbons slip through the NOxstorage catalyst13exhibits a behavior which changes by a faster speed compared with when particulate matter slips through the particulate filter14, then changes in direction of change to an opposite direction, and, when the output value of the electric resistance type sensor29changes, it is judged if hydrocarbons have slipped through the NOxstorage catalyst13when hydrocarbons are injected from the hydrocarbon feed valve15or particulate matter has slipped through the particulate filter14from the difference in behavior of the output value of the electric resistance type sensor29.

In this regard, the hydrocarbon feed valve15and the electric resistance type sensor29are separated in distance, so when the hydrocarbons HC which are injected from the hydrocarbon feed valve15slip through the NOxstorage catalyst13, it takes time for the hydrocarbons HC to reach the electric resistance type sensor29. In this case, the hydrocarbons which are injected from the hydrocarbon feed valve15reach the electric resistance type sensor29within a time period determined from the engine operating state after injection of hydrocarbons. In this case, to remove as much as possible the effects of outside disturbance and accurately detect the amount of hydrocarbons which slip through the NOxstorage catalyst13by the electric resistance type sensor29, it is preferable to find the amount of hydrocarbons which slips through from the change of the output value of the electric resistance type sensor29in a time period which is determined from this engine operating state.

Therefore, in the embodiment according to the present invention, when hydrocarbons are injected from the hydrocarbon feed valve15, it is judged if hydrocarbons have slipped through the NOxstorage catalyst13from the change in the output value of the electric resistance type sensor29within a time period after the injection of hydrocarbons determined from the engine operating state, that is, within a predetermined time period. Note that, the predetermined time period after injecting the hydrocarbons is the time period until the hydrocarbons injected from the hydrocarbon feed valve15reach and deposit on the sensor part of the electric resistance type sensor29. Specifically speaking, the predetermined time period after injecting the hydrocarbons, as shown inFIGS. 9(A)and (B), is the time period from when the time t1until a little before the hydrocarbons injected from the hydrocarbon feed valve15reach the electric resistance type sensor29after the injection of hydrocarbons elapses or from when the time t1until reaching the electric resistance type sensor29elapses to when the time t2from the injection of Hydrocarbons until the hydrocarbons deposited on the sensor part of the electric resistance type sensor29are burned away elapses. These times t1and t2are respectively stored as functions of for example the injection amount Q from the fuel injector3and the engine speed N in the form of maps in advance in the ROM32.

Next, referring toFIGS. 10A, 10B, and 10Cwhich take out and show only the changes in the output voltage V which is shown inFIG. 9(B), various methods for judging slipthrough of the hydrocarbons will be explained. Now then, as explained above, the speed of change of the output value of the electric resistance type sensor29when hydrocarbons are injected from the hydrocarbon feed valve15and the hydrocarbons HC slip through the NOxstorage catalyst13is much faster than the speed of change of the output value of the electric resistance type sensor29when particulate matter PM slips through the particulate filter14. Therefore, as shown inFIG. 10A, in the first example, when the speed of change dv/dt of the output voltage V of the detection circuit39when rising from the reference voltage V0exceeds a set value XD, it is judged that the hydrocarbons have slipped through the NOxstorage catalyst13.

In this case, this set value XD is larger than the speed of change of the output voltage V of the electric resistance type sensor29when particulate matter PM slips through the particulate filter14. Therefore, in other words, in a predetermined time period after injection of hydrocarbons (time period from elapse of t1to elapse of t2), when the output value changes by a speed of change faster compared with the speed of change of the output value of the electric resistance type sensor29when particulate matter PM slips through the particulate filter14, it is judged that the hydrocarbons have slipped through the NOxstorage catalyst13.

On the other hand, when the amount of rise of the output voltage V of the electric resistance type sensor29from the reference voltage V exceeds a predetermined amount ΔVZ, it is possible to judge that hydrocarbons have slipped through the NOxstorage catalyst13. Therefore, in the example which is shown inFIG. 10B, when the amount of change of the output value of the electric resistance type sensor29exceeds the predetermined amount of change ΔVZ in a predetermined time period after injection of hydrocarbons (time period from elapse of t1to elapse of t2), it is judged that the hydrocarbons have slipped through the NOxstorage catalyst13.

Further, the cumulative value of the amount of change of the output voltage V of the electric resistance type sensor29with respect to the reference voltage V0is proportional to the amount of hydrocarbons which slips through the NOxstorage catalyst13, therefore, when this cumulative amount exceeds a predetermined amount MV, it can be judged that hydrocarbons have slipped through the NOxstorage catalyst13. Therefore, in the example which is shown inFIG. 10C, in a predetermined time period after injection of hydrocarbons (time period after t1elapses, then t2elapses), the amount of change of the output value of the electric resistance type sensor29from the reference voltage V0is cumulatively added. When the cumulative value ΣV of the amount of change of the output value exceeds the predetermined value MV, it is judged that hydrocarbons have slipped through the NOxstorage catalyst.

On the other hand, when there is a request for injection of hydrocarbons from the hydrocarbon feed valve15, if a sufficient amount of hydrocarbons is not injected due to clogging or some other such reason, even if the NOxstorage catalyst13deteriorates, the hydrocarbons injected from the hydrocarbon feed valve15are only oxidized, and thus almost no hydrocarbons are exhausted from the NOxstorage catalyst13. Therefore, in this case, if using the output value of the electric resistance type sensor29as the basis to judge if the NOxstorage catalyst13is deteriorating, it is mistakenly judged that the NOxstorage catalyst13is not deteriorating. Therefore, the judgment of whether the NOxstorage catalyst13is deteriorating has to be performed when the hydrocarbon feed valve15is normally injecting hydrocarbons.

In this regard, if hydrocarbons are normally injected from the hydrocarbon feed valve15, as shown inFIGS. 9(A)and (B), the air-fuel ratio of the exhaust gas which flows out from the particulate filter14, that is, the air-fuel ratio (A/F) which is detected by the air-fuel ratio sensor28, becomes smaller than the predetermined air-fuel ratio XAF. Therefore, if judging whether the NOxstorage catalyst13is deteriorating when the air-fuel ratio (A/F) which is detected by the air-fuel ratio sensor28becomes smaller than the predetermined air-fuel ratio XAF, there is no longer a danger of mistaken judgment. Therefore, in this embodiment according to the present invention, it is judged if hydrocarbons have slipped through the NOxstorage catalyst13when hydrocarbons are injected from the hydrocarbon feed valve15and when the air-fuel ratio which is detected by the air-fuel ratio sensor28becomes smaller than the predetermined air-fuel ratio.

Next, referring toFIG. 11andFIG. 12, a routine for malfunction diagnosis for performing the example which is shown inFIG. 10Awill be explained. Note that, this routine is executed by interruption every predetermined time

Referring toFIG. 11, at step70, the output voltage V of the electric resistance type sensor29is read. Next, at step71, it is judged if the electric heater57of the electric resistance type sensor29is being powered. When the electric heater57is not being powered, the routine proceeds to step72where it is judged if the output voltage V of the electric resistance type sensor29exceeds the allowable value VX which is shown inFIG. 8B. When the output voltage V of the electric resistance type sensor29exceeds the allowable value VX, the routine proceeds to step73where the electric heater57of the electric resistance type sensor29starts to be powered. Next, the routine proceeds to step74. If the electric heater57starts to be powered, at the time of the next interruption, the routine jumps from step71to step74.

At step74, it is judged if the output voltage V of the electric resistance type sensor29falls to zero or a minimum value MIN close to zero. When the output voltage V of the electric resistance type sensor29does not fall to the minimum value MIN, the processing cycle is ended, while when the output voltage V of the electric resistance type sensor29falls to the minimum value MIN, the routine proceeds to step75. At step75, the electric heater57is stops being powered. Next, at step76, it is judged if the period Δt (FIG. 8B) at which the heating action by the electric heater57of the electric resistance type sensor29is performed is shorter than a predetermined period Xt. If the period Δt at which the heating action by the electric heater57of the electric resistance type sensor29is performed is shorter than the predetermined period Xt, it is provisionally judged that the particulate filter14is abnormal and the routine proceeds to step77.

At step77, it is judged if Δt<Xt stands continuously for N times or more (N being an integer of 2 or more) when the routine proceeds to step76. When it is not judged at step76that Δt<Xt stands continuously for N times or more, the processing cycle is ended. As opposed to this, when it is judged at step76that Δt<Xt stands continuously for N times or more, the routine proceeds to step78where it is judged if the particulate filter14is abnormal. If it is judged that the particulate filter14is abnormal, for example, a warning lamp is turned on.

On the other hand, when, at step72, it is judged that the output voltage V of the electric resistance type sensor29does not exceed the allowable value VX which is shown inFIG. 8E, the routine proceeds to step79where, as shown inFIG. 9, it is judged if an injection completion flag which is set when the hydrocarbon feed valve15finishes injecting hydrocarbons is set. When the injection completion flag is not set, the processing cycle is ended. As opposed to this, when the injection completion flag is set, the routine proceeds to step80where it is judged if the injection completion flag has now been set. When the injection completion flag has now been set, the routine proceeds to step81where the output voltage V of the electric resistance type sensor29is made the reference voltage V0. Next, at step82, the times t1and t2corresponding to the engine operating state are calculated. Next, the routine proceeds to step83. On the other hand, when it is judged at step80that the injection completion flag has not been set now, the routine jumps to step83. That is, when an injection of hydrocarbons from the hydrocarbon feed valve15is completed, the reference voltage V0is found and the times t1and t2are calculated.

At step83, it is judged if the elapsed time from when hydrocarbons have finished being injected exceeds the time t1. When the elapsed time “t” from when hydrocarbons have finished being injected does not exceeds the time t1, the processing cycle is ended. As opposed to this, when the elapsed time “t” from when hydrocarbons finish being injected exceeds the time t1, the routine proceeds to step84where it is judged if a permit flag which permits judgment of whether the NOxstorage catalyst13deteriorates is set. When the routine first proceeds to step84after hydrocarbons have finished being injected, the permit flag is not set, so the routine proceeds to step85where the air-fuel ratio (A/F) which is detected by the air-fuel ratio sensor28is read.

Next, at step86, it is judged it the air-fuel ratio (A/F) detected by the air-fuel ratio sensor28becomes smaller than the predetermined air-fuel ratio XAF. When the air-fuel ratio (A/F) detected by the air-fuel ratio sensor28becomes smaller than the predetermined air-fuel ratio XAF, the routine proceeds to step87where the permit flag is set. Next, the routine proceeds to step88. If the permit flag is set, in the next processing cycle, the routine jumps from step84to step88. At step88to step90, it is judged if the NOxstorage catalyst13is deteriorating. Therefore, when the permit flag is set, it is learned that judgment of whether the NOxstorage catalyst13is deteriorating is performed.

That is, at step88, the speed of change dV/dt of the output voltage V of the electric resistance type sensor29is calculated. Next, at step89, it is judged if the speed of change dV/dt of the output voltage V of the electric resistance type sensor29is larger than the set value XD. When the speed of change dV/dt of the output voltage V of the electric resistance type sensor29is larger than the set value XD, it is judged that the NOxstorage catalyst13is deteriorating and then the routine proceeds to step90where it is judged that the NOxstorage catalyst13is abnormal. If it is judged that the NOxstorage catalyst13is abnormal, for example, a warning lamp is turned on. Next, at step91, it is judged if the elapsed time “t” from when the hydrocarbons have finished being injected exceeds the time t2. When the elapsed time “t” from when the hydrocarbons have finished being injected exceeds the time t2, the routine proceeds to step92where the injection completion flag is reset. Next, the routine proceeds to step93where the permit flag is reset.

On the other hand, at step89, when it is judged that the speed of change dV/dt of the output voltage V of the electric resistance type sensor29is smaller than the set value XD, the routine jumps to step91. When the speed of change dV/dt of the output voltage V of the electric resistance type sensor29up to when the elapsed time “t” from when hydrocarbons finished being injected exceeds the time t2does not become larger than set value XD, the routine does not proceed to step90. Therefore, it is judged that the NOxstorage catalyst13is not deteriorating. Note that, when it is judged at step86that the air-fuel ratio (A/F) which is detected by the air-fuel ratio sensor28does not become smaller than the predetermined air-fuel ratio XAF, the routine jumps to step91, so at this time, judgment whether the NOxstorage catalyst13is deteriorating is not performed.

Next, the routine for malfunction diagnosis for performing the example which is shown inFIG. 10Bwill be explained. In this case, as the routine for malfunction diagnosis, a routine in which steps88to90which are shown in the part surrounded by the broken line A of FIG.12are replaced with steps88to90of the part surrounded by the broken line A ofFIG. 13is used. Therefore, the routine for performing the example which is shown inFIG. 10Bdiffers from the routine which is shown inFIG. 11andFIG. 12in only the part which is shown inFIG. 13, so below only the part which is shown inFIG. 13will be explained.

That is, in the example which is shown inFIG. 10B, as shown inFIG. 13, first, at step88, the predetermined amount of change ΔVZ is added to the reference voltage V0to calculate a set value VE(=V0+ΔVZ). Next, at step89, it is judged if the output voltage V of the electric resistance type sensor29exceeds the set value VE. When the output voltage V of the electric resistance type sensor29exceeds the set value VE, it is judged that the NOxstorage catalyst13is deteriorating and the routine proceeds to step90where it is judged that the NOxstorage catalyst13is abnormal. As opposed to this, when the output voltage V of the electric resistance type sensor29does not exceed the set value VE, the routine jumps to step91ofFIG. 12.

Next, the routine for malfunction diagnosis for performing the example which is shown inFIG. 10Cwill be explained. In this case, as the routine for malfunction diagnosis, a routine in which steps88to90which are shown in the part surrounded by the broken line A ofFIG. 12are replaced with steps88to90of the part surrounded by the broken line A ofFIG. 14is used Therefore, the routine for performing the example which is shown inFIG. 10Cdiffers from the routine which is shown inFIG. 11andFIG. 12in only the part which is shown inFIG. 14, so below only the part which is shown inFIG. 14will be explained.

That is, in the example which is shown inFIG. 100, as shown inFIG. 14, first, at step88, the value (V−V0) of the output voltage V of the electric resistance type sensor29minus the reference voltage V0is cumulatively added and the result of cumulative addition is made the cumulative value ΣV. Next, at step89, it is judged if the cumulative value ΣV exceeds the set value MV. When the cumulative value ΣV exceeds the set value MV, it is judged that the NOxstorage catalyst13is deteriorating and the routine proceeds to step90where it is judged that the NOxstorage catalyst13is abnormal. As opposed to this, when the cumulative value ΣV does not exceed the set value MV, the routine jumps to step91ofFIG. 12.

EXPLANATION OF REFERENCES

15hydrocarbon feed valve

29electric resistance type sensor