JP4419150B2 - NOx catalyst abnormality diagnosis device and abnormality diagnosis method - Google Patents

Description

  The present invention relates to an apparatus and a method for diagnosing abnormality of a NOx catalyst provided in an exhaust passage of an internal combustion engine.

  In general, a NOx catalyst for purifying NOx (nitrogen oxide) contained in exhaust gas is known as an exhaust purification device disposed in an exhaust system of an internal combustion engine such as a diesel engine. On the other hand, if an abnormality such as deterioration or failure occurs in the NOx catalyst, the NOx purification ability deteriorates, and more NOx is discharged to the atmosphere than in the normal state. Diagnosis is done. In particular, in the case of an engine mounted on an automobile, there is a request for performing a catalyst abnormality diagnosis in an on-board state (onboard).

  For example, Patent Literature 1 discloses a selective reduction type NOx catalyst deterioration diagnosis device that reduces NOx when a reducing agent is supplied. According to this, the NOx concentration on the downstream side of the catalyst when the reducing agent is supplied to the NOx catalyst and the NOx concentration on the downstream side of the catalyst when the reducing agent is not supplied to the NOx catalyst are detected, and these NOx concentrations are detected. Based on this, the deterioration of the catalyst is diagnosed.

  Patent Document 2 discloses a technique for adding a reducing agent when the NOx storage reduction catalyst reaches the light-off determination temperature and determining deterioration of the NOx catalyst based on the exhaust gas temperature difference between the upstream and downstream of the catalyst. It is disclosed. Patent Document 3 discloses a technique for determining catalyst deterioration by comparing a catalyst deterioration determination criterion corresponding to an engine operating state and a deterioration degree calculated from the catalyst temperature.

JP-A-11-93647 JP 2003-214153 A JP-A-7-26944

  By the way, in the apparatus described in Patent Document 1, there is a problem that exhaust emission deteriorates at the time of diagnosis of deterioration of the NOx catalyst. That is, a state in which a reducing agent is not supplied in order to diagnose deterioration of the NOx catalyst is created. In this case, the NOx catalyst cannot reduce NOx when the reducing agent is not supplied, and NOx is discharged.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an NOx catalyst abnormality diagnosis device and abnormality diagnosis method that can prevent exhaust emission deterioration during abnormality diagnosis.

In order to achieve the above object, according to the first aspect of the present invention,
A NOx catalyst provided in the exhaust passage of the internal combustion engine;
Measuring means for measuring an index value related to the NOx purification ability of the NOx catalyst;
Based on the index value measured by the measuring means at a timing when the activity of the NOx catalyst is relatively low and the index value measured by the measuring means at the timing when the activity of the NOx catalyst is relatively high, An abnormality diagnosis device for a NOx catalyst is provided, comprising abnormality determination means for determining abnormality of the catalyst.

  There is a difference or change in the NOx purification ability of the NOx catalyst between the timing when the activity of the NOx catalyst is relatively low and the timing when the activity is relatively high. The difference or change amount changes according to the degree of deterioration of the NOx catalyst. Therefore, it is possible to determine whether the NOx catalyst is normal or abnormal by measuring the index value regarding the NOx purification ability of the NOx catalyst at both timings and determining the difference between these index values or the amount of change. In particular, an index value measured at a timing when the activity of the NOx catalyst is relatively low, in other words, a timing at which the catalyst temperature has not yet sufficiently reached the activation temperature range is used. At such timing, the NOx catalyst originally does not have sufficient NOx purification ability. Therefore, by using the index value measured at this timing, there is no need to create a state in which the NOx purification ability is low even though the activity of the NOx catalyst is high as in Patent Document 1, for example. Therefore, it is possible to execute an abnormality diagnosis of the NOx catalyst without intentionally deteriorating the exhaust emission.

Further, according to the second aspect of the present invention, in the first aspect, the timing at which the activity of the NOx catalyst is relatively low is the first timing,
The timing at which the activity of the NOx catalyst is relatively high includes a second timing at which the catalyst activity is higher than the first timing, and a third timing at which the catalyst activity is higher than the second timing,
The abnormality determination means is measured at the first timing when the amount of change between the index value measured at the first timing and the index value measured at the second timing is less than or equal to a predetermined value. The abnormality of the NOx catalyst is determined based on a change amount between the measured index value and the index value measured at the third timing.

Moreover, the 3rd form of this invention is a 2nd form,
Catalyst temperature estimating means for estimating the catalyst temperature of the NOx catalyst;
Reductant addition control means for controlling the addition of the reducing agent to the NOx catalyst based on the catalyst temperature estimated by the catalyst temperature estimating means,
The reducing agent addition control means adds the reducing agent when an amount of change between the index value measured at the first timing and the index value measured at the second timing is larger than a predetermined value. The catalyst temperature to be started is changed to a lower temperature side.

Moreover, the 4th form of this invention is a 3rd form,
The reducing agent addition control means adds the reducing agent when an amount of change between the index value measured at the first timing and the index value measured at the third timing is larger than a predetermined value. The starting catalyst temperature is changed to a higher temperature side.

Further, a fifth aspect of the present invention is any one of the first to fourth aspects,
The timing at which the activity of the NOx catalyst is relatively low is a timing at which the catalyst temperature becomes lower than the activation start temperature, and the timing at which the activity of the NOx catalyst is relatively high is that the catalyst temperature is equal to or higher than the activation start temperature. It is characterized by the following timing.

The sixth aspect of the present invention is the second aspect,
The first timing is a timing at which the catalyst temperature becomes lower than the activation start temperature, and the second timing is a timing at which the catalyst temperature is equal to or higher than the activation start temperature and lower than the activation end temperature, The third timing is a timing at which the catalyst temperature becomes equal to or higher than the activation end temperature.

Further, a seventh aspect of the present invention is any one of the first to sixth aspects,
The index value is a NOx purification rate.

According to the eighth aspect of the present invention,
A method for diagnosing an abnormality of a NOx catalyst provided in an exhaust passage of an internal combustion engine,
Measuring an index value relating to the purification performance of the NOx catalyst at a timing when the activity of the NOx catalyst is relatively low;
Measuring the index value at a timing when the activity of the NOx catalyst is relatively high;
And determining the abnormality of the NOx catalyst based on the measured index values. A method for diagnosing abnormality of the NOx catalyst is provided.

The ninth aspect of the present invention is the eighth aspect,
The timing when the activity of the NOx catalyst is relatively low is the first timing,
The timing at which the activity of the NOx catalyst is relatively high includes a second timing at which the catalyst activity is higher than the first timing, and a third timing at which the catalyst activity is higher than the second timing,
The abnormality determination step is measured at the first timing when an amount of change between the index value measured at the first timing and the index value measured at the second timing is equal to or less than a predetermined value. The abnormality of the NOx catalyst is determined based on a change amount between the measured index value and the index value measured at the third timing.

The tenth aspect of the present invention is the ninth aspect,
Estimating a catalyst temperature of the NOx catalyst;
Controlling the addition of a reducing agent to the NOx catalyst based on the estimated catalyst temperature, and
The reducing agent addition control step adds the reducing agent when the amount of change between the index value measured at the first timing and the index value measured at the second timing is greater than a predetermined value. Including changing the starting catalyst temperature to a lower temperature side.

The eleventh aspect of the present invention is the tenth aspect,
The reducing agent addition control step adds the reducing agent when the amount of change between the index value measured at the first timing and the index value measured at the third timing is larger than a predetermined value. It includes changing the catalyst temperature to be started to a higher temperature side.

The twelfth aspect of the present invention is any one of the eighth to eleventh aspects.
The timing at which the activity of the NOx catalyst is relatively low is a timing at which the catalyst temperature is lower than the activation start temperature, and the timing at which the activity of the NOx catalyst is relatively high is at or above the activation start temperature. It is characterized by such timing.

The thirteenth aspect of the present invention is the ninth aspect,
The first timing is a timing at which the catalyst temperature becomes lower than the activation start temperature, and the second timing is a timing at which the catalyst temperature is equal to or higher than the activation start temperature and lower than the activation end temperature, The third timing is a timing at which the catalyst temperature becomes equal to or higher than the activation end temperature.

Further, a fourteenth aspect of the present invention is any one of the eighth to thirteenth aspects,
The index value is a NOx purification rate.

  According to the present invention, it is possible to provide an NOx catalyst abnormality diagnosis device and abnormality diagnosis method capable of preventing deterioration of exhaust emission during abnormality diagnosis.

  The best mode for carrying out the present invention will be described below with reference to the accompanying drawings.

  FIG. 1 is a schematic system diagram of an internal combustion engine according to an embodiment of the present invention. In the figure, 10 is a compression ignition type internal combustion engine or diesel engine for automobiles, 11 is an intake manifold communicated with an intake port, 12 is an exhaust manifold communicated with an exhaust port, and 13 is a combustion chamber. . In the present embodiment, fuel supplied from a fuel tank (not shown) to the high pressure pump 17 is pumped to the common rail 18 by the high pressure pump 17 and accumulated in a high pressure state, and the high pressure fuel in the common rail 18 is transferred from the injector 14 to the combustion chamber. 13 is directly injected and supplied. Exhaust gas from the engine 10 passes from the exhaust manifold 12 through the turbocharger 19 and then flows into the exhaust passage 15 downstream thereof. After being purified as described later, the exhaust gas is discharged to the atmosphere. In addition, as a form of a diesel engine, it is not restricted to the thing provided with such a common rail type fuel injection device. It is also optional to include other exhaust purification devices such as EGR devices.

  On the other hand, the intake air introduced from the air cleaner 20 into the intake passage 21 passes through the air flow meter 22, the turbocharger 19, the intercooler 23, and the throttle valve 24 in order to reach the intake manifold 11. The air flow meter 22 is a sensor for detecting the intake air amount, and specifically outputs a signal corresponding to the flow rate of the intake air. The throttle valve 24 is an electronically controlled type.

  The exhaust passage 15 is provided with a NOx catalyst 34 that reduces and purifies NOx contained in the exhaust gas in the passage. The NOx catalyst 34 of the present embodiment is a selective reduction type NOx catalyst, and can continuously reduce NOx when a reducing agent is added.

  In the exhaust passage 15 upstream of the NOx catalyst 34, an addition valve 40 for selectively adding urea as a reducing agent to the NOx catalyst 34 is provided. Urea is used in the form of a urea aqueous solution, and is injected and supplied into the exhaust passage 15 from the addition valve 40 toward the NOx catalyst 34 on the downstream side. A supply device 42 for supplying the urea aqueous solution is connected to the addition valve 40, and a tank 44 for storing the urea aqueous solution is connected to the supply device 42.

  Further, an electronic control unit (hereinafter referred to as ECU) 100 is provided as a control means for controlling the entire engine. The ECU 100 includes a CPU, a ROM, a RAM, an input / output port, a storage device, and the like. The ECU 100 controls the injector 14, the high-pressure pump 17, the throttle valve 24, and the like so that desired engine control is executed based on detection values of various sensors. The ECU 100 also controls the addition valve 40 and the supply device 42 to control the urea addition amount. As sensors connected to the ECU 100, in addition to the air flow meter 22 described above, a NOx sensor provided on the downstream side of the NOx catalyst 34, that is, a post-catalyst NOx sensor 50, and an upstream side and a downstream side of the NOx catalyst 34 are provided. In addition, a pre-catalyst exhaust temperature sensor 52 and a post-catalyst exhaust temperature sensor 54 are included. The post-catalyst NOx sensor 50 outputs to the ECU 100 a signal corresponding to the NOx concentration of the exhaust gas at the installation position, that is, the post-catalyst NOx concentration. The pre-catalyst exhaust temperature sensor 52 and the post-catalyst exhaust temperature sensor 54 output a signal corresponding to the exhaust gas temperature at the installation position to the ECU 100.

  As other sensors, a crank angle sensor 26 and an accelerator opening sensor 27 are connected to the ECU 100. The crank angle sensor 26 outputs a crank pulse signal to the ECU 100 when the crank angle rotates, and the ECU 100 detects the crank angle of the engine 10 based on the crank pulse signal and calculates the rotational speed of the engine 10. The accelerator opening sensor 27 outputs a signal corresponding to the accelerator pedal opening (accelerator opening) operated by the user to the ECU 100.

Selective reduction type NOx catalyst (SCR: Selective Catalytic Reduction) 34 has a base material such as zeolite or alumina supporting a noble metal such as Pt, or a transition metal such as Cu supported on the surface of the base material by ion exchange. Examples thereof include those obtained by carrying a titania / vanadium catalyst (V ₂ O ₅ / WO ₃ / TiO ₂ ) on the surface of the substrate. The selective reduction type NOx catalyst 34 reduces and purifies NOx when the catalyst temperature (catalyst bed temperature) is in the active temperature range and urea as a reducing agent is added. When urea is added to the catalyst, ammonia is produced on the catalyst and this ammonia reacts with NOx to reduce NOx.

The amount of urea added to the NOx catalyst 34 is controlled by the ECU 100 based on the post-catalyst NOx concentration detected by the post-catalyst NOx sensor 50. Specifically, the urea injection amount from the addition valve 40 is controlled so that the detected value of the post-catalyst NOx concentration is always zero. In this case, the urea injection amount may be set based only on the detected value of the post-catalyst NOx concentration, or the basic urea injection amount based on the engine operating state (for example, the engine speed and the accelerator opening) Feedback correction may be performed based on the detection value of the sensor 50. Since the NOx catalyst 34 can reduce NOx only when urea is added, usually urea is always added. Further, control is performed so that urea is added only in a minimum amount necessary for reducing NOx discharged from the engine. This is because when urea is added excessively, ammonia is discharged downstream of the catalyst (so-called NH ₃ slip), which causes a strange odor and the like.

  Here, if the minimum urea amount required to reduce the total amount of NOx discharged from the engine is A and the urea amount actually added is B, these ratios B / A are called equivalent ratios. . Although the urea addition control is executed so that the equivalence ratio is as close to 1 as possible, the actual equivalence ratio is not necessarily 1 because the operation state of the engine changes every moment in practice. When the equivalence ratio is less than 1, the urea supply amount is insufficient, and NOx is discharged to the downstream side of the catalyst. This is detected by the post-catalyst NOx sensor 50 to increase the urea supply amount. When the equivalence ratio is greater than 1, the urea supply amount becomes excessive. The added urea may adhere to the NOx catalyst 34. In this case, even if the addition of urea is stopped, NOx can be reduced by the attached urea for a while.

  The temperature of the NOx catalyst 34 can be directly detected by a temperature sensor embedded in the catalyst, but in the present embodiment, this is estimated. Specifically, the ECU 100 estimates the catalyst temperature based on the pre-catalyst exhaust temperature and the post-catalyst exhaust temperature detected by the pre-catalyst exhaust temperature sensor 52 and the post-catalyst exhaust temperature sensor 54, respectively.

  Here, a method for estimating the catalyst temperature will be described with reference to FIG. The temperature of the exhaust gas upstream of the catalyst flowing into the NOx catalyst 34 is Tf (° C.), and the amount of the exhaust gas is Ga (g / s). Here, since the amount of exhaust gas can be considered to be equal to the amount of air taken into the engine, the amount of intake air Ga is defined as the amount of exhaust gas. This amount of exhaust gas is the amount of exhaust gas flowing into the catalyst per unit time (here 1 second). On the other hand, the catalyst temperature of the NOx catalyst 34 is Tc (° C.), and the weight of the NOx catalyst 34 is Mc (g). The temperature of the exhaust gas on the downstream side of the catalyst discharged from the NOx catalyst 34 is defined as Tr (° C.).

The thermal energy of the exhaust gas upstream of the catalyst is Ef, and the thermal energy of the NOx catalyst 34 is Ec. These thermal energies Ef and Ec can be expressed by the following equations (1) and (2). However, Cg is the specific heat ratio of the exhaust gas, Cc is the specific heat ratio of the NOx catalyst 34, and both are constant values.
Ef = Ga × Tf × Cg (J / s) (1)
Ec = Mc × Tc × Cc (J) (2)

By the way, thermal equilibrium is considered when exhaust gas having thermal energy Ef is supplied to the NOx catalyst 34 having thermal energy Ec. Considering that the NOx catalyst 34 and the exhaust gas are in a thermal equilibrium state after a lapse of unit time from the start of the supply of exhaust gas, and assuming that the temperature of both after the thermal equilibrium is Tm, the thermal equilibrium equation is expressed by the following equation (3).
Ef + Ec = Ga * Tm * Cg + Mc * Tm * Cc (3)

  This temperature Tm is a basic value of the estimated temperature of the NOx catalyst 34. However, in reality, the exhaust gas and the NOx catalyst 34 do not reach a complete thermal equilibrium state, and the exhaust gas at the temperature Tr is exhausted downstream of the NOx catalyst 34 and the thermal energy escapes. Therefore, based on the temperature Tr, the thermal energy Er that has escaped to the downstream side is calculated, whereby the basic estimated temperature Tm of the NOx catalyst 34 is feedback-corrected, and the final estimated catalyst temperature is calculated.

  As understood from the above description, in the present embodiment, the pre-catalyst exhaust temperature Tf, which is the exhaust gas temperature upstream of the NOx catalyst, is detected by the pre-catalyst exhaust temperature sensor 52, and the exhaust gas temperature downstream of the NOx catalyst is detected. A post-catalyst exhaust temperature Tr is detected by a post-catalyst exhaust temperature sensor 54. Then, an intake air amount Ga that can be regarded as equivalent to the exhaust gas amount is detected by the air flow meter 22. Based on these detection values, the ECU 100 estimates the catalyst temperature Tc of the NOx catalyst 34.

  On the other hand, urea addition to the NOx catalyst 34 is controlled by the ECU 100 based on the estimated catalyst temperature Tc. As will be described in detail later, urea addition is performed when the estimated catalyst temperature Tc reaches or exceeds a predetermined addition start temperature Tcst (for example, about 200 ° C.) at which the NOx catalyst 34 becomes active, and the estimated catalyst temperature Tc is When the addition start temperature is lower than Tcst, urea addition is stopped. This is because NOx cannot be reduced even if urea is added before the activation of the NOx catalyst. The urea addition is also stopped when the estimated catalyst temperature Tc becomes equal to or higher than a predetermined upper limit temperature Tmax (for example, about 600 ° C.) that is much higher than the addition start temperature Tcst. Also in this case, NOx cannot be reduced efficiently even if urea is added. In general, however, the exhaust temperature of a diesel engine is lower than that of a gasoline engine, and the frequency at which the catalyst temperature reaches such an upper limit temperature is relatively low. Eventually, urea addition is executed when the catalyst temperature Tc is equal to or higher than the addition start temperature Tcst and lower than the upper limit temperature Tmax, and when it is not in this temperature range, urea addition is stopped.

  Note that, when the engine is warmed up, the NOx catalyst 34 is heated by the exhaust heat from the engine, whereas the post-catalyst NOx sensor 50 is heated relatively quickly by the heating of the built-in heater. Therefore, normally, the post-catalyst NOx sensor 50 is activated earlier than the NOx catalyst 34. The ECU 100 detects the impedance of the post-catalyst NOx sensor 50 and controls the heater so that the impedance becomes a predetermined value corresponding to the activation temperature of the post-catalyst NOx sensor 50.

  Next, abnormality diagnosis of the NOx catalyst 34 will be described.

  In general, the characteristic feature of the abnormality diagnosis of the NOx catalyst 34 in the present embodiment is that each of the index values related to the NOx purification ability of the NOx catalyst 34 is measured at a timing when the activity of the NOx catalyst 34 is relatively low and at a relatively high timing. The point is that the abnormality of the NOx catalyst 34 is determined based on these measured index values. Here, the activity of the NOx catalyst can be expressed by the catalyst temperature which is the correlation value. In this embodiment, the NOx purification rate is used as an index value related to the NOx purification capacity of the NOx catalyst 34. However, other values can be used as these values.

  There is a difference or change in the NOx purification rate of the NOx catalyst 34 between the timing when the activity of the NOx catalyst 34 is relatively low and the timing when it is relatively high. The difference or amount of change varies according to the degree of deterioration of the NOx catalyst 34. Therefore, it is possible to determine whether the NOx catalyst 34 is normal or abnormal by determining the difference or the magnitude of the change amount. In particular, the measured value of the NOx purification rate at the timing when the activity of the NOx catalyst 34 is relatively low, in other words, the timing at which the catalyst temperature has not yet sufficiently reached the activation temperature range is used. At such timing, even if a reducing agent is added, the NOx catalyst 34 originally does not have sufficient NOx purification ability. Therefore, by using the measured value of the NOx purification rate at this timing, it is not necessary to create a state where the NOx purification ability is low even though the activity of the NOx catalyst 34 is high as in Patent Document 1, for example. That's it. Therefore, it is possible to execute the abnormality diagnosis without intentionally deteriorating the exhaust emission.

  The NOx purification rate R of the NOx catalyst 34 is expressed by R = N2 / N1, where N1 is the upstream NOx amount flowing into the NOx catalyst 34, and N2 is the downstream NOx amount discharged from the NOx catalyst 34. Is done. The NOx amount N2 on the downstream side of the catalyst is calculated by the ECU 100 based on the after-catalyst NOx concentration Cr detected by the after-catalyst NOx sensor 50.

  On the other hand, the value estimated by the ECU 100 is used as the NOx amount N1 on the upstream side of the catalyst in this embodiment. That is, the ECU 100 calculates the catalyst upstream side NOx amount N1 as an estimated value based on the detected values of the parameters representing the engine operating state (for example, the engine rotational speed NE and the accelerator opening degree AC) according to a predetermined map or function. Additionally or alternatively, a pre-catalyst NOx sensor may be provided on the upstream side of the NOx catalyst 34, and the NOx amount N1 on the upstream side of the catalyst may be calculated based on the NOx concentration detected by the pre-catalyst NOx sensor. The ECU 100 calculates the NOx purification rate R using the NOx amount N1 on the upstream side of the catalyst and the NOx amount N2 on the downstream side of the catalyst thus obtained.

  Here, details of the catalyst abnormality diagnosis in the present embodiment will be described with reference to FIG. 3A to 3C show the relationship of the NOx purification rate R (vertical axis) of the NOx catalyst 34 to the estimated catalyst temperature Tc (horizontal axis), respectively. The solid line is the normal catalyst, and the alternate long and short dash line is the abnormal catalyst. In particular, in the warm-up process of the NOx catalyst 34, the NOx catalyst is gradually activated as the catalyst temperature rises, and the NOx purification rate gradually increases. In the figure, the value enclosed by a square frame is the estimated catalyst temperature Tc, the value enclosed by an ellipse frame is the true catalyst temperature Tc ', and the value enclosed by a circle is the NOx purification rate R. The figure shows a case where urea is always added, and the NOx purification rate R rises smoothly from 0 as the catalyst temperature rises, and eventually converges to the maximum purification rate according to the degree of deterioration of the catalyst. However, in actual control, urea addition is started when the estimated catalyst temperature Tc reaches a predetermined addition start temperature Tcst, and the purification rate R = 0 before that. Therefore, in practice, as shown in FIG. 4, the NOx purification rate R rises rapidly at the same time when urea addition starts.

  As shown in FIG. 3, in the case of a normal catalyst indicated by a solid line, a large NOx purification rate R can be obtained after catalyst activation. On the other hand, in the case of the abnormal catalyst indicated by the alternate long and short dash line, only a NOx purification rate R smaller than that of the normal catalyst can be obtained even if the catalyst is activated.

  The catalyst temperature at which the NOx catalyst 34 changes from inactive to active, in other words, the catalyst temperature when the NOx purification rate R becomes greater than 0 to 0 is referred to as the activation start temperature and is represented by Tc0. Further, the catalyst temperature when the activation of the NOx catalyst 34 is almost finished, in other words, the catalyst temperature when the NOx purification rate R has converged to the maximum purification rate is referred to as the activation end temperature, and is represented by Tced.

  FIG. 3A shows a case where the estimated catalyst temperature Tc is not shifted from the true catalyst temperature Tc ′ and is equal. The timing when the estimated catalyst temperature Tc = Tc0 and the timing when the true catalyst temperature Tc ′ = Tc0 are the same. On the other hand, FIGS. 3B and 3C show a case where the estimated catalyst temperature Tc is deviated from the true catalyst temperature Tc ′. FIG. 3B shows a case where the estimated catalyst temperature Tc is deviated by α to a lower temperature side than the true catalyst temperature Tc ′. For example, the timing when Tc = Tc0 becomes Tc ′ = Tc0−α. Are the same. The diagram itself is shifted to the right by α relative to that of FIG. On the other hand, FIG. 3C shows a case where the estimated catalyst temperature Tc is deviated by α higher than the true catalyst temperature Tc ′. For example, the timing when Tc = Tc0 becomes Tc ′ = Tc0 + α. Are the same. The diagram itself is shifted to the left by α relative to that of FIG.

  Thus, the estimated catalyst temperature Tc may deviate from the true catalyst temperature Tc ′. As described above, the estimated catalyst temperature Tc is obtained from the detection values of the pre-catalyst exhaust temperature sensor 52, the post-catalyst exhaust temperature sensor 54, and the air flow meter 22, but these detection values are true for reasons such as sensor deterioration. May deviate from the value. As a result, the estimated catalyst temperature Tc may deviate from the true catalyst temperature Tc ′.

  Now, with reference to FIG. 3 (A), in the present embodiment, the addition of urea is started when the estimated catalyst temperature Tc reaches the addition start temperature Tcst. The addition start temperature Tcst is set in advance to a value such that the catalyst temperature (true catalyst temperature) is not less than the activation start temperature Tc0 and less than the activation end temperature Tced. In particular, since it is advantageous in terms of emission to start adding urea as soon as possible in the warm-up process and to operate the NOx catalyst as soon as possible, control is performed to change the addition start temperature Tcst as low as possible. This point will be described later.

  In order to diagnose the abnormality of the NOx catalyst, the first NOx purification rate R1 is measured when the estimated catalyst temperature Tc reaches a predetermined value Tc1. This measurement timing is referred to as a first timing, and the predetermined value Tc1 is referred to as a first predetermined value. In the present embodiment, the first timing is set to a timing at which the estimated catalyst temperature Tc becomes lower than the predetermined activation start temperature Tc0, that is, a timing at which the NOx purification rate R becomes zero. The first predetermined value Tc1 is determined based on the changeable addition start temperature Tcst, and is determined as Tc1 = Tcst−T1 (T1 is a predetermined constant value, for example, 30 ° C.). The first predetermined value Tc1 is determined such that the NOx purification rate R at the first timing is zero even when the variation in the estimated value Tc of the catalyst temperature is taken into consideration.

  Alternatively, the first timing is a timing at which the estimated catalyst temperature Tc is equal to or higher than the activation start temperature Tc0, for example, slightly higher than the activation start temperature Tc0 (that is, a timing at which the NOx purification rate is slightly higher than 0). May be set.

  Next, when the estimated catalyst temperature Tc reaches the predetermined value Tc2, the second NOx purification rate R2 is measured. This timing is referred to as a second timing, and the predetermined value Tc2 is referred to as a second predetermined value (Tc2> Tc1). In the present embodiment, the second timing is set to a timing at which the estimated catalyst temperature Tc is equal to or higher than the changeable addition start temperature Tcst and lower than the predetermined activation end temperature Tced. In other words, the second timing is set such that the NOx purification rate R2 is equal to or higher than the NOx purification rate Rst at the start of urea addition and less than the maximum purification rate. The second predetermined value Tc2 is also determined based on the changeable addition start temperature Tcst, and is determined as Tc2 = Tcst + T2 (T2 is a predetermined constant value, for example, 30 ° C.).

  Thus, the NOx purification rates R1 and R2 are measured at the first timing when the activity of the NOx catalyst 34 is relatively low and the second timing when it is relatively high. Basically, the difference ΔR12 = R2−R1 between these purification rates is compared with a predetermined value ΔR12s. If the difference ΔR12 is larger than the predetermined value ΔR12s, the catalyst is normal (see the solid line in the figure), and the difference ΔR12 is equal to or smaller than the predetermined value ΔR12s. If so, it can be determined that the catalyst is abnormal (see the dashed line in the figure). In addition, there exists an advantage which can absorb the offset variation of a sensor by taking a difference in this way.

  By the way, when there is an estimated deviation of the catalyst temperature as described above, it is not always possible to accurately determine whether the catalyst is normal or abnormal only by this method. That is, as shown in FIG. 3B, when the estimated catalyst temperature Tc is shifted to a lower temperature side than the true catalyst temperature Tc ′, the NOx purification rate R2 at the second timing (Tc = Tc2) is apparently reduced. As a result of the difference ΔR12 from the NOx purification rate R1 at the first timing (Tc = Tc1) being small, there is a possibility of erroneously determining that the catalyst is abnormal even if the catalyst is normal.

  Therefore, in the present embodiment, the NOx purification rate R3 is also measured at the third timing when the estimated catalyst temperature Tc becomes the third predetermined value Tc3 (Tc3> Tc2). The third timing is set to a timing at which the estimated catalyst temperature Tc becomes equal to or higher than the activation end temperature Tced. In other words, the NOx purification rate R is set to a timing near the maximum purification rate. The third predetermined value Tc3 is also determined on the basis of the changeable addition start temperature Tcst, and is determined as Tc3 = Tcst + T3 (T3 is a predetermined constant value, for example, 60 ° C.).

  This third timing is a sufficiently high temperature side so that the influence of the estimated deviation of the catalyst temperature is eliminated. At this third timing, there is a clear difference in the NOx purification rate R3 between the normal catalyst (see the solid line in the figure) and the abnormal catalyst (see the dashed line in the figure). When the difference ΔR12 in the NOx purification rate between the first timing and the second timing is equal to or less than the predetermined value ΔR12s, the difference in the NOx purification rates R1, R3 between the first timing and the third timing ΔR13 (= R3−R1) is calculated, and this difference ΔR13 is compared with a predetermined value ΔR13s. If the difference ΔR13 is larger than the predetermined value ΔR13s, it is finally determined that the catalyst is normal. If the difference ΔR13 is equal to or smaller than the predetermined value ΔR13s, it is finally determined that the catalyst is abnormal.

  By the way, as shown in FIG. 3C, when the estimated catalyst temperature Tc is shifted to a higher temperature side than the true catalyst temperature Tc ′, the NOx purification rate R2 at the second timing (Tc = Tc2) has an estimated deviation. Compared to the case where there is not (FIG. 3A), the difference ΔR12 from the NOx purification rate R1 at the first timing (Tc = Tc1) is also increased. Therefore, since it is correctly determined that the catalyst is normal, there is no problem. What should be considered as a problem is a case where the estimated catalyst temperature Tc is shifted to a lower temperature side than the true catalyst temperature Tc ′ as shown in FIG.

  Next, a process for executing the abnormality diagnosis as described above will be described with reference to FIG. The illustrated routine is repeatedly executed by the ECU 100 at predetermined intervals (for example, 16 msec).

  In the first step S101, it is determined whether or not the present time has reached the first timing, specifically, whether or not the estimated catalyst temperature Tc has reached a first predetermined value Tc1 (= Tcst−T1) or more. . If it is determined that it has not been reached, this routine is terminated. On the other hand, if it is determined that it has reached, in step S102, the NOx purification rate is measured, and the measured value is stored as R1. In this embodiment, R1 = 0 (%).

  In the next step S103, it is determined whether or not the present time has reached the urea addition start timing, specifically, whether or not the estimated catalyst temperature Tc has reached the addition start temperature Tcst or higher. If it is determined that it has not been reached, this routine is terminated. On the other hand, if it is determined that it has reached, the addition of urea (reducing agent) is started in step S104.

  In the next step S105, it is determined whether or not the present time has reached the second timing, specifically, whether or not the estimated catalyst temperature Tc has reached or exceeded a second predetermined value Tc2 (= Tcst + T2). If it is determined that it has not been reached, this routine is terminated. On the other hand, if it is determined that it has reached, in step S106, the NOx purification rate is measured, and the measured value is stored as R2.

  Thereafter, in step S107, a difference ΔR12 (= R2−R1) between the NOx purification rates of the first timing and the second timing is calculated. In step S108, the difference ΔR12 is compared with a predetermined value ΔR12s.

  If the difference ΔR12 is greater than the predetermined value ΔR12s, it is determined in step S109 that the NOx catalyst is normal, and the NOx catalyst abnormality flag is turned off. In step S110, a predetermined constant value Ta (eg, 1 ° C.) is subtracted from the current addition start temperature Tcst, and the result is stored as a new addition start temperature Tcst. As a result, the addition start temperature Tcst is changed or updated to a lower temperature value, and the urea addition start and the catalyst operation start timing are advanced. As a result, the emission, particularly the emission after start-up, is improved. This routine is thus completed. When the addition start temperature Tcst is changed by Ta in this way, the first to third predetermined values Tc1 to Tc3 defining the first to third timings are also changed by Ta following this. It will be. When this step S110 is repeatedly executed, the addition start temperature Tcst is gradually moved to the low temperature side by a predetermined value Ta.

  On the other hand, if the difference ΔR12 is equal to or smaller than the predetermined value ΔR12s in step S108, there is a possibility that the NOx catalyst is abnormal, but there is also a possibility that the catalyst temperature is deviated. Proceed to S111. In step S111, it is determined whether or not the present time has reached the third timing, specifically, whether or not the estimated catalyst temperature Tc has reached a third predetermined value Tc3 (= Tcst + T3) or more. If it is determined that it has not been reached, this routine is terminated. On the other hand, if it is determined that it has reached, the NOx purification rate is measured in step S112, and the measured value is stored as R3.

  Thereafter, in step S113, a difference ΔR13 (= R3−R1) between the NOx purification rates of the first timing and the third timing is calculated. In step S114, the difference ΔR13 is compared with a predetermined value ΔR13s.

  If the difference ΔR13 is larger than the predetermined value ΔR13s, it is determined in step S115 that the NOx catalyst is normal and the NOx catalyst abnormality flag is turned off. That is, the difference ΔR12 in the NOx purification rate between the first timing and the second timing is small because the NOx catalyst is not abnormal but the catalyst temperature estimation deviation is the main cause. Judge as normal.

  In step S116, a predetermined constant value Tb (for example, 5 ° C.) is added from the current addition start temperature Tcst, and the result is stored as a new addition start temperature Tcst. Thereby, the addition start temperature Tcst is changed or updated to a higher temperature side value. The reason for this is as follows. As performed in step S110, when the addition start temperature Tcst is gradually advanced to the low temperature side, the second predetermined value Tc2 moves to the low temperature side accordingly, and the NOx purification rate R2 at the second timing is increased. It becomes low, and the difference ΔR12 between the NOx purification rates of the first and second timings becomes small. That is, if the addition start temperature Tcst is shifted too much to the low temperature side, the purification rate difference ΔR12 becomes too small due to this, and there is a possibility that the abnormality is erroneously determined. Further, if the addition start temperature Tcst is shifted too much to the low temperature side, the reducing agent is added in a state where the NOx catalyst does not have sufficient NOx purification ability, and the reducing agent may be unreacted and pass through the catalyst. Therefore, in order to prevent such a situation, the addition start temperature Tcst is changed to a higher temperature value. As in this embodiment, the step width Tb to the high temperature side of the addition start temperature Tcst is preferably larger than the step width Ta to the low temperature side. In this way, the addition start temperature Tcst is gradually and gradually moved to the low temperature side, and when the addition start temperature Tcst is shifted too much to the low temperature side, the addition start temperature Tcst can be immediately returned to the high temperature side. When the addition start temperature Tcst is changed to the high temperature side by Tb in this way, the first to third predetermined values Tc1 to Tc3 that define the first to third timings follow this, and Tb is also only Tb. It will be changed to the high temperature side. When the execution of step S116 is thus completed, this routine is terminated.

  On the other hand, if the difference ΔR13 is equal to or smaller than the predetermined value ΔR13s in step S114, it is determined in step S117 that the NOx catalyst is abnormal, and the NOx catalyst abnormality flag is turned on. That is, when the purification rate difference ΔR13 is small even at the third timing, the abnormality of the NOx catalyst is determined for the first time, and it is finally determined that the NOx catalyst is abnormal. This routine is thus completed.

  As described above, according to the abnormality diagnosis of the present embodiment, it is possible to determine the abnormality of the NOx catalyst separately from the estimated deviation of the catalyst temperature, and it is possible to improve the reliability of the abnormality diagnosis. Further, the timing at which the NOx catalyst starts NOx purification, that is, the timing at which urea addition is started can be gradually moved to the low temperature side, which is very advantageous for improving emissions, particularly for improving cold emissions after starting the engine. . In general, the reducing agent addition start timing is set to a constant value on the safe side, that is, the high temperature side in consideration of various variations such as sensor errors, but in the case of this embodiment, the reducing agent is reduced to the optimal timing according to the individual. Agent addition start timing can be adjusted. Furthermore, when the timing is advanced too much to the low temperature side, the timing can be returned to the high temperature side, and it is possible to prevent the timing from being advanced excessively and to distinguish between the excessive advance and the catalyst abnormality. The reliability of catalyst abnormality diagnosis can be improved. In addition, since abnormality diagnosis can be executed while the engine and the NOx catalyst are warming up, there is an advantage that the diagnosis frequency can be secured.

  As understood from the above description, in this embodiment, the NOx purification rate measuring means is configured by the post-catalyst NOx sensor 50, the ECU 100, the accelerator opening sensor 27, and the crank angle sensor 26, and the abnormality determining means is configured by the ECU 100. Is done. Further, the catalyst temperature estimating means is constituted by the ECU 100, the accelerator opening sensor 27 and the crank angle sensor 26, and the reducing agent addition control means is constituted by the addition valve 40, the supply device 42 and the ECU 100.

  As mentioned above, although embodiment of this invention was described, this invention can also take other embodiment. For example, a reducing agent other than urea can be used. For example, ammonia, hydrocarbon (HC), alcohol, hydrogen, carbon monoxide, or the like can be used.

  In the above embodiment, the first abnormality determination is performed based on the NOx purification rate R1 at the first timing and the NOx purification rate R2 at the second timing, and when the result may be abnormal, Based on the NOx purification rate R1 at the timing and the NOx purification rate R3 at the third timing, the final abnormality determination is performed. However, the present invention is not limited to this. For example, the abnormality determination may be performed based only on the NOx purification rate R1 at the first timing and the NOx purification rate R3 at the third timing. Further, the catalyst activity (catalyst temperature) at the first timing is the same as that at the first timing when the comparison is performed at the first timing and the second timing, and when the comparison is performed at the first timing and the third timing. However, it is not always necessary to be the same, and they may be different.

  In the embodiment, the abnormality determination is performed by comparing the difference in the NOx purification rate between the timings with a predetermined value. However, the comparison method is not limited to this, for example, the ratio of the NOx purification rate between the timings. May be compared with a predetermined value to determine abnormality.

  The present invention stores NOx in exhaust gas when the inflowing exhaust gas has an excessive oxygen concentration (lean), and stores NOx stored when the inflowing exhaust gas has a lean oxygen concentration (rich). It can also be applied to a catalyst. In this case, since the NOx purification start timing of the NOx catalyst mainly depends on the catalyst temperature, the control of the NOx purification start timing (steps S110 and S116) as in the above embodiment cannot be performed, but the catalyst activity is still high. It is possible to determine the abnormality of the NOx catalyst based on the NOx purification rate between the relatively low timing and the relatively high timing.

  It is also optional to provide another exhaust purification device, for example, an oxidation catalyst or a diesel particulate filter (DPF) in the exhaust passage. The present invention can be applied to an internal combustion engine other than the compression ignition type internal combustion engine, for example, a spark ignition type internal combustion engine, particularly a direct injection lean burn gasoline engine.

  The embodiment of the present invention is not limited to the above-described embodiment, and includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

1 is a schematic system diagram of an internal combustion engine according to an embodiment of the present invention. It is a reference figure for demonstrating the estimation method of a catalyst temperature. It is a graph which shows the relationship between an estimated catalyst temperature and the NOx purification rate of a NOx catalyst, (A) is a case where an estimated catalyst temperature is equal to a true catalyst temperature, (B) is a case where an estimated catalyst temperature is lower than a true catalyst temperature. (C) shows the case where the estimated catalyst temperature is higher than the true catalyst temperature. It is a graph which shows the actual rise of NOx purification rate. It is a flowchart of an abnormality diagnosis process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Engine 15 Exhaust passage 26 Crank angle sensor 27 Accelerator opening degree sensor 34 NOx catalyst 40 Addition valve 42 Supply apparatus 50 After-catalyst NOx sensor 52 Before-catalyst exhaust temperature sensor 54 After-catalyst exhaust temperature sensor 100 Electronic control unit (ECU)
R NOx purification rate R1 NOx purification rate R2 at the first timing NOx purification rate R3 at the second timing NOx purification rate Tc at the third timing Catalyst temperature, estimated catalyst temperature Tc0 Activation start temperature Tced Activation end temperature Tc1 First predetermined Value Tc2 Second predetermined value Tc3 Third predetermined value Tcst Addition start temperature

Claims (14)

A NOx catalyst provided in the exhaust passage of the internal combustion engine;
Measuring means for measuring an index value related to the NOx purification ability of the NOx catalyst;
Based on the index value measured by the measuring means at a timing when the activity of the NOx catalyst is relatively low and the index value measured by the measuring means at the timing when the activity of the NOx catalyst is relatively high, An abnormality determination means for determining an abnormality of the catalyst ;
Catalyst temperature estimating means for estimating the catalyst temperature of the NOx catalyst;
Reducing agent addition control means for controlling the addition of the reducing agent to the NOx catalyst based on the catalyst temperature estimated by the catalyst temperature estimating means;
With
The reducing agent addition control means has a change amount between an index value measured at a timing when the activity of the NOx catalyst is relatively low and an index value measured at a timing when the activity of the NOx catalyst is relatively high. An NOx catalyst abnormality diagnosis device, characterized in that, when larger than a predetermined value, the catalyst temperature at which addition of the reducing agent is started is changed to a lower temperature side . The timing when the activity of the NOx catalyst is relatively low is the first timing,
The timing at which the activity of the NOx catalyst is relatively high includes a second timing at which the catalyst activity is higher than the first timing, and a third timing at which the catalyst activity is higher than the second timing,
The abnormality determination means is measured at the first timing when the amount of change between the index value measured at the first timing and the index value measured at the second timing is less than or equal to a predetermined value. An abnormality of the NOx catalyst is determined based on an amount of change between the measured index value and the index value measured at the third timing ;
The reducing agent addition control means adds the reducing agent when an amount of change between the index value measured at the first timing and the index value measured at the second timing is larger than a predetermined value. The abnormality diagnosis device for a NOx catalyst according to claim 1, wherein the catalyst temperature to be started is changed to a lower temperature side . The reducing agent addition control means adds the reducing agent when an amount of change between the index value measured at the first timing and the index value measured at the third timing is larger than a predetermined value. The abnormality diagnosis device for a NOx catalyst according to claim 2, wherein the catalyst temperature to be started is changed to a higher temperature side .   The step width when the catalyst temperature for starting the addition of the reducing agent is changed to a higher temperature side is larger than the step width when the catalyst temperature for starting the addition of the reducing agent is changed to a lower temperature side.
  The abnormality diagnosis apparatus for a NOx catalyst according to claim 3. The timing at which the activity of the NOx catalyst is relatively low is a timing at which the catalyst temperature becomes lower than the activation start temperature, and the timing at which the activity of the NOx catalyst is relatively high is that the catalyst temperature is equal to or higher than the activation start temperature. The NOx catalyst abnormality diagnosis device according to claim 1, wherein the timing is such that The first timing is a timing at which the catalyst temperature becomes lower than the activation start temperature, and the second timing is a timing at which the catalyst temperature is equal to or higher than the activation start temperature and lower than the activation end temperature, The NOx catalyst abnormality diagnosis apparatus according to any one of claims 2 to 4 , wherein the third timing is a timing at which the catalyst temperature becomes equal to or higher than the activation end temperature.   The abnormality diagnosis device for a NOx catalyst according to any one of claims 1 to 6, wherein the index value is a NOx purification rate. A method for diagnosing an abnormality of a NOx catalyst provided in an exhaust passage of an internal combustion engine,
Measuring an index value relating to the purification performance of the NOx catalyst at a timing when the activity of the NOx catalyst is relatively low;
Measuring the index value at a timing when the activity of the NOx catalyst is relatively high;
Determining an abnormality of the NOx catalyst based on the measured index values ;
Estimating a catalyst temperature of the NOx catalyst;
Controlling the addition of a reducing agent to the NOx catalyst based on the estimated catalyst temperature;
With
In the reducing agent addition control step, an amount of change between an index value measured at a timing when the activity of the NOx catalyst is relatively low and an index value measured at a timing when the activity of the NOx catalyst is relatively high is calculated. A method for diagnosing abnormality of a NOx catalyst, comprising changing a catalyst temperature at which addition of the reducing agent is started to a lower temperature side when the value is larger than a predetermined value . The timing when the activity of the NOx catalyst is relatively low is the first timing,
The timing at which the activity of the NOx catalyst is relatively high includes a second timing at which the catalyst activity is higher than the first timing, and a third timing at which the catalyst activity is higher than the second timing,
The abnormality determination step is measured at the first timing when an amount of change between the index value measured at the first timing and the index value measured at the second timing is equal to or less than a predetermined value. An abnormality of the NOx catalyst is determined based on an amount of change between the measured index value and the index value measured at the third timing ;
The reducing agent addition control step adds the reducing agent when the amount of change between the index value measured at the first timing and the index value measured at the second timing is greater than a predetermined value. 9. The NOx catalyst abnormality diagnosis method according to claim 8, wherein the catalyst temperature to be started is changed to a lower temperature side . The reducing agent addition control step adds the reducing agent when the amount of change between the index value measured at the first timing and the index value measured at the third timing is larger than a predetermined value. 10. The abnormality diagnosis method for a NOx catalyst according to claim 9 , comprising changing the catalyst temperature to be started to a higher temperature side .   The step width when the catalyst temperature for starting the addition of the reducing agent is changed to a higher temperature side is larger than the step width when the catalyst temperature for starting the addition of the reducing agent is changed to a lower temperature side.
  The method for diagnosing abnormality of a NOx catalyst according to claim 10. The timing at which the activity of the NOx catalyst is relatively low is a timing at which the catalyst temperature is lower than the activation start temperature, and the timing at which the activity of the NOx catalyst is relatively high is at or above the activation start temperature. The NOx catalyst abnormality diagnosis method according to any one of claims 8 to 11, wherein the timing is as follows. The first timing is a timing at which the catalyst temperature becomes lower than the activation start temperature, and the second timing is a timing at which the catalyst temperature is equal to or higher than the activation start temperature and lower than the activation end temperature, The abnormality diagnosis method for a NOx catalyst according to any one of claims 9 to 11 , wherein the third timing is a timing at which the catalyst temperature becomes equal to or higher than the activation end temperature.   The method for diagnosing abnormality of a NOx catalyst according to any one of claims 8 to 13, wherein the index value is a NOx purification rate.

Images