JP6617569B2 - NOx sensor diagnostic device - Google Patents

Description

  The present disclosure relates to a technique for diagnosing the responsiveness of a NOx sensor installed downstream of a reduction catalyst that selectively reduces NOx in exhaust gas of an internal combustion engine with a reducing agent.

  2. Description of the Related Art Conventionally, as an exhaust sensor that detects a gas state in exhaust gas of an internal combustion engine, a NOx sensor that detects NOx concentration in exhaust gas is known. Based on the output of the NOx sensor, the engine ECU appropriately controls the engine operating state and the exhaust gas purification state by controlling, for example, the fuel injection amount, the EGR gas amount, the amount of reducing agent injected into the SCR catalyst, and the like. .

Note that ECU is an abbreviation for Electronic Control Unit, EGR is an abbreviation for Exhaust Gas Recirculation, and SCR is an abbreviation for Selective Catalytic Reduction.
The responsiveness of the output of the NOx sensor may be lower than that of a NOx sensor with normal responsiveness due to deterioration of the sensor element or the like.

  When the engine operating state is a steady state and the injection amount from the fuel injection valve is constant, the NOx amount discharged from the cylinder does not change. In this case, the delay in response of the NOx sensor does not matter. However, when the engine operating state changes from a steady state to a transient state and the injection amount from the fuel injection valve increases or decreases, the NOx concentration acquired from the output of the NOx sensor with reduced responsiveness is the responsive normal NOx sensor. It becomes a state later than the NOx concentration acquired from the output of.

  Controlling the fuel injection amount, the EGR gas amount, the urea water injection amount, and the like based on the NOx concentration indicated by the output of the NOx sensor with reduced responsiveness may cause an increase in combustion noise and emission.

  Therefore, in Patent Document 1, the normal output of a normal exhaust sensor having a responsiveness and the reduced output of an exhaust sensor having a responsiveness lower than the normal exhaust sensor by a predetermined value are estimated, and the normal output and the reduced output And responsiveness of the exhaust sensor is to be diagnosed based on the actual output of the exhaust sensor.

JP 2010-275952 A

  The actual output of the NOx sensor installed on the downstream side of the SCR catalyst varies depending on the amount of purification by which the SCR catalyst reduces NOx. That is, the actual output of the NOx sensor is a value reflecting the amount of NOx purification by the SCR catalyst. If the normal output and the reduced output are estimated without considering the purification amount of the SCR catalyst with respect to the actual output, there is a difference between the estimated value of the normal output and the reduced output and the actual output depending on the presence or absence of the purification amount.

  Therefore, when the technique described in Patent Document 1 that diagnoses the responsiveness of the exhaust sensor based on the normal output, the reduced output, and the actual output without considering the purification amount of the SCR catalyst is applied to the NOx sensor, The responsiveness may not be properly diagnosed.

  An object of one aspect of the present disclosure is to provide a technique for appropriately diagnosing the responsiveness of a NOx sensor installed downstream of a reduction catalyst that selectively reduces NOx in exhaust gas with a reducing agent.

  One aspect of the present disclosure diagnoses the responsiveness of the NOx sensor (36) installed downstream of the reduction catalyst (20) that selectively reduces NOx in the exhaust gas of the internal combustion engine (2) with a reducing agent. In the sensor diagnostic device (40), the upstream acquisition unit (46, S402, S406), the NOx amount estimation unit (48, S408, S410), the normal estimation unit (50, S412), and the decrease estimation unit (52, S414). ), An actual output acquisition unit (54, S402, S416), and a diagnosis unit (58, S422-S430).

  The upstream acquisition unit acquires the NOx amount on the upstream side of the reduction catalyst. The NOx amount estimation unit estimates the NOx amount obtained by removing the NOx purification amount that is reduced and purified by the reduction catalyst from the upstream NOx amount acquired by the upstream acquisition unit. The normal estimation unit estimates the normal output of the NOx sensor with normal responsiveness based on the NOx amount estimated by the NOx amount estimation unit. The decrease estimation unit estimates a decrease output that is less responsive than the normal output estimated by the normal estimation unit.

  The actual output acquisition unit acquires the actual output of the NOx sensor. The diagnosis unit diagnoses the responsiveness of the NOx sensor based on the normal output, the decrease output estimated by the decrease estimation unit, and the actual output acquired by the actual output acquisition unit.

  According to this configuration, based on the NOx amount obtained by subtracting the NOx purification amount purified by the reduction catalyst from the upstream NOx amount, the normal output of the responsive normal NOx sensor and the responsiveness higher than the normal output are obtained. Estimate low drop power.

  Thus, by considering the NOx purification amount by the reduction catalyst, it is possible to estimate the normal output and the reduced output with higher accuracy than when the NOx purification amount is not considered. Thereby, the responsiveness of the NOx sensor can be properly diagnosed based on the normal output, the reduced output, and the actual output of the NOx sensor.

  Note that the reference numerals in parentheses described in this column and in the claims indicate the correspondence with the specific means described in the embodiment described later as one aspect, and the technical scope of the present invention. It is not limited.

The block diagram which shows the exhaust gas purification system by this embodiment. The time chart which shows the relationship between an engine speed and the NOx density | concentration of the downstream of the SCR catalyst which does not consider NOx purification amount. The time chart which shows the relationship between an engine speed and the NOx density | concentration of the downstream of the SCR catalyst which considered NOx purification amount. The time chart which shows integration of the deviation of fall output and normal output, and integration of the deviation of actual output and normal output. The flowchart which shows a NOx sensor diagnostic process. The characteristic view which shows the relationship between ammonia adsorption amount, catalyst temperature, and a purification rate. The data flow figure showing NOx sensor diagnostic processing.

Embodiments to which the present invention is applied will be described below with reference to the drawings.
[1. Constitution]
An exhaust purification system 10 of the present embodiment shown in FIG. 1 is a system that purifies exhaust gas discharged from, for example, a multi-cylinder diesel engine 2. Hereinafter, the diesel engine 2 is also simply referred to as the engine 2.

  The exhaust purification system 10 includes a throttle valve 12, an EGR valve 16, an SCR catalyst 20, a urea water injection valve 30, an exhaust temperature sensor 32, two NOx sensors 34 and 36, an ECU 40, and the like. Fuel accumulated in a common rail (not shown) is injected into the engine 2 from the fuel injection valve 4.

  A turbocharger compressor (not shown) is rotationally driven by a turbocharger turbine 14 installed in the exhaust passage 110 via a shaft (not shown). Then, the intake air of the intake passage 100 compressed by the compressor of the turbocharger passes through an intercooler (not shown), the flow rate is adjusted by the throttle valve 12, and is sucked into each cylinder of the engine 2.

  The throttle valve 12 is throttled in order to allow more EGR gas to enter in the low load region, but is kept almost fully open in the high load region in order to increase the intake air amount and reduce pumping loss. The intake air amount sucked into the engine 2 is detected by an intake air amount sensor (not shown).

The EGR valve 16 is installed in an EGR passage 120 that connects the intake passage 100 and the exhaust passage 110 of the engine 2 and controls the amount of EGR that is circulated from the exhaust side to the intake side.
In the exhaust passage 110, an SCR catalyst 20, a urea water injection valve 30, an exhaust temperature sensor 32, and two NOx sensors 34 and 36 are installed.

  The SCR catalyst 20 adsorbs urea water injected from the urea water injection valve 30. The urea water adsorbed by the SCR catalyst 20 is decomposed into ammonia and carbon dioxide by being hydrolyzed when the exhaust gas temperature exceeds a predetermined temperature. Then, ammonia generated by hydrolysis acts as a reducing agent in the SCR catalyst 20, and reduces and purifies NOx.

  The urea water injection valve 30 is installed on the upstream side of the SCR catalyst 20. The urea water injection valve 30 is an electromagnetic valve whose opening and closing is controlled by the ECU 40, and injects urea water in a tank (not shown) by opening the valve. The two NOx sensors 34 and 36 are installed on the upstream side and the downstream side of the SCR catalyst 20, respectively, and detect the NOx concentrations on the upstream side and the downstream side of the SCR catalyst 20, respectively.

  The ECU 40 includes a microcomputer including a CPU and a semiconductor memory such as a RAM, a ROM, and a flash memory. The number of microcomputers constituting the ECU 40 may be one or more.

  Each function of the ECU 40 is realized by the CPU executing a program stored in a non-transitional tangible recording medium such as a ROM or a flash memory. By executing this program, a method corresponding to the program is executed.

  The ECU 40 includes a temperature acquisition unit 42, a reducing agent estimation unit 44, an upstream acquisition unit 46, a NOx amount estimation unit 48, and a normal estimation unit 50 as functional configurations realized by the CPU executing the program. , A decrease estimation unit 52, an actual output acquisition unit 54, an integration unit 56, and a diagnosis unit 58.

  The technique for realizing these elements constituting the ECU 40 is not limited to software, and hardware in which some or all of the elements are combined with a logic circuit, an analog circuit, or the like may be used. A temperature acquisition unit 42, a reducing agent estimation unit 44, an upstream acquisition unit 46, a NOx amount estimation unit 48, a normal estimation unit 50, a decrease estimation unit 52, an actual output acquisition unit 54, an integration unit 56, The functions executed by the diagnosis unit 58 will be described later.

  The ECU 40 determines an engine operation state and an exhaust purification state from output signals of various sensors such as an exhaust temperature sensor 32, NOx sensors 34, 36, an intake air temperature sensor (not shown), an intake air amount sensor, an engine speed sensor, and an accelerator opening sensor. get. Then, the ECU 40 controls the operation of the actuators such as the fuel injection valve 4, the EGR valve 16, and the urea water injection valve 30 based on the acquired engine operation state and exhaust purification state.

(Response of NOx sensor 36)
Next, the responsiveness of the NOx sensor 36 will be described. In a steady state where the vehicle travels at a constant speed and the engine load does not change, the amount of NOx discharged from the engine 2 is constant, and the amount of NOx reduced and purified by the SCR catalyst 20 is also constant. Therefore, the NOx concentration detected by the NOx sensor 36 installed on the downstream side of the SCR catalyst 20 is also constant.

  In FIG. 2, a two-dot chain line denoted by reference numeral 200 represents NOx when NOx generated in the engine 2 passes through the SCR catalyst 20 from the engine 2 to the NOx sensor 36 on the downstream side of the SCR catalyst 20 is not considered. This is the NOx concentration detected by the sensor 36.

  Next, when the accelerator pedal is turned off and the vehicle enters a deceleration operation state as a transient state, the ECU 40 cuts off the fuel injection from the fuel injection valve 4 as shown in FIG. When the injection amount from the fuel injection valve 4 becomes 0 due to the fuel cut, combustion does not occur in the cylinder of the engine 2. As a result, as indicated by reference numeral 200, the NOx concentration detected by the NOx sensor 36 becomes zero.

  When shifting from fuel cut to idle operation, fuel is injected from the fuel injection valve 4 and burned in the cylinder, so that NOx is discharged from the engine 2. If the amount of NOx purified by the SCR catalyst 20 is not taken into account, as indicated by reference numeral 200, the NOx concentration on the downstream side of the SCR catalyst 20 increases and becomes a constant value.

  However, since the ammonia produced by the decomposition of the urea water is adsorbed on the SCR catalyst 20, NOx discharged from the engine 2 after shifting from the fuel cut to the idle operation is reduced by the SCR catalyst 20. .

  Therefore, the actual output 210 of the NOx concentration on the downstream side of the SCR catalyst 20 detected by the NOx sensor 36 converges to 0 from the NOx concentration that was a constant value before the fuel cut. The actual output 210 also reflects the delay time for NOx generated in the engine 2 to reach the NOx sensor 36.

  There is a time delay until the exhaust gas discharged from the engine 2 reaches the NOx sensor 36 due to the piping length or the like. Therefore, as indicated by the actual output 210, the NOx concentration on the downstream side of the SCR catalyst 20 in which the NOx sensor 36 is installed changes with a delay from the change in the NOx concentration in the cylinder.

  This delay time varies depending on the flow rate of the exhaust gas. The flow rate of the exhaust gas changes depending on the engine operating state using the engine speed, the fuel injection amount, the intake air amount, and the like as parameters. Therefore, the delay time in which the NOx concentration at the position where the NOx sensor 36 is installed is delayed with respect to the NOx concentration in the cylinder can be calculated and estimated based on the engine operating state.

  Here, if the amount of NOx purification reduced by the SCR catalyst 20 is not taken into consideration, the normal output 212 of the NOx sensor 36 with normal responsiveness and the NOx sensor 36 with lower responsiveness than the NOx sensor 36 with normal responsiveness. The reduced output 214 is estimated to change as shown in FIG.

  The normal output 212 and the reduced output 214 shown in FIG. 2 take into account the delay time for the NOx generated in the engine 2 to reach the NOx sensor 36. However, since the NOx purification amount reduced by the SCR catalyst 20 is not considered in the normal output 212 and the reduced output 214 as in the case of the reference numeral 200, the normal output 212 and the reduced output 214 are values indicated by the reference numeral 200. Converge to.

  In FIG. 2, when the responsiveness of the actual output 210, the normal output 212, and the lowered output 214 is compared, the responsiveness of the actual output 210 is faster than that of the normal output 212. This is because the actual output 210 detected by the NOx sensor 36 indicates the NOx concentration reflecting the NOx purification amount reduced by the SCR catalyst 20, whereas the normal output 212 and the reduced output 214 indicate the SCR catalyst. This is because the NOx purification amount reduced at 20 is not considered.

  Therefore, when the estimated value of the normal output 212 and the reduced output 214 estimated without considering the NOx purification amount reduced by the SCR catalyst 20 is compared with the actual output 210, the responsiveness of the NOx sensor 36 is diagnosed. There is a possibility of misdiagnosis.

In FIG. 3, the amount of NOx that is reduced and purified by the SCR catalyst 20 is considered in FIG.
If the NOx purification amount reduced and purified by the SCR catalyst 20 is used as a correction value and the NOx concentration indicated by the reference numeral 200 in FIG. 3 that is the same as the reference numeral 200 in FIG. 2 is corrected, the NOx concentration after the fuel cut becomes zero.

  In the present embodiment, the correction value of the NOx purification amount is such that the engine operating state starts transitioning from the steady state to the transient state, and the time change amount of the fuel injection amount from the fuel injection valve 4 exceeds a predetermined value. It is set when the diagnosis process of the NOx sensor 36 is started. Therefore, in FIG. 3, the correction value in the steady state before the fuel cut is started is not set and is the initial value of zero.

  Similarly, when the normal output 212 and the decrease output 214 in FIG. 2 are corrected using the NOx purification amount as a correction value, the normal output 220 and the decrease output 222 in FIG. 3 are obtained. In FIG. 2 and FIG. 3, the responsiveness shown by the actual output 210 is the same.

  The normal output 220 reaches the position where the NOx amount on the upstream side of the SCR catalyst 20, the purification amount of NOx reduced by the SCR catalyst 20, and the exhaust gas passes through the SCR catalyst 20 and the NOx sensor 36 is installed. The time required until this time and the response characteristic of the normal NOx sensor 36 are estimated as parameters.

  If the NOx sensor 34 is installed upstream of the SCR catalyst 20 as in this embodiment, the NOx amount upstream of the SCR catalyst 20 can be obtained from the NOx concentration detected by the NOx sensor 34. .

  On the other hand, when the NOx sensor 34 is not installed on the upstream side of the SCR catalyst 20, the NOx amount on the upstream side of the SCR catalyst 20 is exhausted from the engine 2 to the SCR catalyst 20 to the NOx amount discharged from the engine 2. It can be estimated in consideration of the delay time for the arrival of gas. The NOx amount discharged from the engine 2 is calculated based on the intake air amount, the injection amount, the EGR gas amount, and the like.

  The reduced output 222 uses, for example, the response characteristic of the NOx sensor whose responsiveness is reduced by a predetermined value, instead of the response characteristic of the normal NOx sensor among the parameters described above when estimating the normal output 220. Presumed. For example, if the response delay of the normal output 220 is 1, the response delay of the reduced output 222 is set to about 5 times.

  The response delay of the reduced output 222 set to about five times as a predetermined value with respect to the response delay of the normal output 220 is an example. The predetermined value is appropriately set depending on how much the delay in response of the NOx sensor 36 is diagnosed.

The reduced output 222 may be estimated by performing a first-order lag process on the normal output 220.
Thus, as a result of estimating the normal output 220 and the reduced output 222 in consideration of the purification amount of NOx reduced by the SCR catalyst 20 and the delay in responsiveness, the responsiveness of the reduced output 222 in FIG. On the other hand, the response of the actual output 210 is degraded. In this case, it can be appropriately diagnosed that the responsiveness of the NOx sensor 36 is lower than the responsiveness of the lowered output 222.

Next, the diagnosis of the reduction in the actual output response will be described in more detail.
In FIG. 4, while the engine operating state shifts from a steady state to a transient state due to fuel cut, and all of the normal output 300, the reduced output 302, and the actual output 304 converge, the reduced output 302 and the normal output 300 An integrated value S1 of the deviation and an integrated value S2 of the deviation between the actual output 304 and the normal output 300 are respectively calculated.

  Based on the value of S2 / S1, the degree of responsiveness reduction of the actual output 304 is diagnosed. Instead of the deviation between the actual output 304 and the normal output 300, the integrated value of the deviation between the actual output 304 and the reduced output 302 may be calculated as S2.

  When the responsiveness of the NOx sensor 36 is normal and the actual output 304 matches the normal output 300, since S2 = 0, S2 / S1 = 0. When the actual output 304 is equal to the reduced output 302, S2 / S1 = 1. Therefore, the degree of decrease in the NOx sensor 36 can be diagnosed based on S2 / S1.

[2. processing]
Next, the NOx sensor diagnostic process executed by the ECU 40 will be described with reference to FIG. In FIG. 5, “S” represents a step. The flowchart of FIG. 5 is always executed.

  In S400 of FIG. 5, the temperature acquisition unit 42 acquires the catalyst temperature of the SCR catalyst 20. The temperature acquisition unit 42 acquires the exhaust temperature detected by the exhaust temperature sensor 32 installed on the upstream side of the SCR catalyst 20 as the catalyst temperature.

  In S402, the reducing agent estimation unit 44, for example, in the steady state of the engine operation state, the urea water injection amount that the urea water injection valve 30 injects to the upstream side of the SCR catalyst 20, and the NOx amount on the upstream side of the SCR catalyst 20. And the amount of ammonia adsorbed on the SCR catalyst 20 is estimated based on the NOx concentration detected by the NOx sensor 36 on the downstream side of the SCR catalyst 20.

  Specifically, the reducing agent estimation unit 44 determines the NOx amount reduced by the SCR catalyst 20 from the NOx amount upstream of the SCR catalyst 20 and the NOx amount detected by the NOx sensor 36 downstream of the SCR catalyst 20. And the amount of ammonia consumed is calculated from the reduced amount of NOx. Then, the reducing agent estimation unit 44 estimates the ammonia amount adsorbed by the SCR catalyst 20 from the injection amount of urea water injected by the urea water injection valve 30 and the consumed ammonia amount.

  In the steady state of the engine operating state, the delay in response of the NOx sensor 36 does not affect the detected NOx concentration, and therefore, the NOx amount on the downstream side of the SCR catalyst 20 can be acquired from the output of the NOx sensor 36.

  Further, when the NOx sensor 34 is installed upstream of the SCR catalyst 20 as in the present embodiment, the reducing agent estimation unit 44 calculates the NOx amount upstream of the SCR catalyst 20 from the output of the NOx sensor 34. You can get it.

  In the processing of S400 and S402, the determination of S404 is Yes, the absolute value of the time change amount of the fuel injection amount from the fuel injection valve 4 is larger than a predetermined value, and the engine operating state is changed from the steady state to the transient state. It runs until the migration starts. The transient state of the engine operating state represents both the deceleration state and the acceleration state.

  When the determination in S404 is Yes and the engine operating state shifts from the steady state to the transient state, diagnosis of the NOx sensor 36 is started. In S <b> 406, the upstream acquisition unit 46 acquires the NOx amount on the upstream side of the SCR catalyst 20 from the output of the NOx sensor 34.

  In S408, the NOx amount estimating unit 48 reduces NOx in the SCR catalyst 20 based on the catalyst temperature acquired from the exhaust temperature sensor 32 by the temperature acquiring unit 42 and the ammonia adsorption amount estimated by the reducing agent estimating unit 44. Estimate the amount of purification to be purified.

  The amount of purification is estimated as follows, for example. First, from the purification rate map of FIG. 6 showing the relationship between the catalyst temperature, the ammonia adsorption amount, and the NOx purification rate, the purification rate is acquired using the catalyst temperature and the ammonia adsorption amount as parameters. As shown in FIG. 6, the purification rate, that is, the NOx purification amount increases as the catalyst temperature increases and the ammonia adsorption amount in the SCR catalyst 20 increases.

  Next, as shown in FIG. 7, the NOx purification amount is estimated by multiplying the upstream NOx amount by the purification rate acquired from the purification rate map. The purification rate acquired from the purification rate map indicates the rate at which NOx is purified during one loop process constituted by S406 to S426. Therefore, the NOx purification amount estimated in S408 represents the amount by which NOx is purified during one loop process.

  The ammonia adsorption amount is calculated from the ammonia adsorption amount at the start of diagnosis acquired in S402, the injection amount injected from the urea water injection valve 30 during the diagnosis, and the NOx purification amount estimated in the previous loop processing. The reducing agent estimation unit 44 estimates from the amount of ammonia consumed.

  In S410, the NOx amount estimation unit 48 subtracts the purification amount estimated in S408 from the upstream NOx amount of the SCR catalyst 20 acquired in S406, as shown in FIG. presume. This NOx amount is a value that does not take into account the delay time for the exhaust gas to pass through the SCR catalyst 20.

  In S412, the normal estimation unit 50 sets a delay time for the exhaust gas to pass through the SCR catalyst 20 with respect to the NOx amount downstream of the SCR catalyst 20 estimated by the NOx amount estimation unit 48 in S410, as shown in FIG. The normal output is estimated in consideration.

  In S414, the drop estimation unit 52 sets the responsiveness of the drop output to a constant multiple, for example, a delay of about 5 times, or normal if the normal output delay estimated in S412 is 1. First-order lag processing is performed on the output and set, and the reduced output is estimated as shown in FIG.

In S <b> 416, the actual output acquisition unit 54 acquires an actual output indicating the NOx concentration on the downstream side of the SCR catalyst 20 from the NOx sensor 36.
In S418, the integrating unit 56 calculates a deviation tS1 between the reduced output estimated in S414 and the normal output estimated in S412 and a deviation tS2 between the actual output acquired in S416 and the normal output estimated in S412. In S420, the integrating unit 56 adds the deviations tS1 and tS2 calculated in S418 to the integrated values S1 and S2 of the deviations.

  In S422, the diagnosis unit 58 determines that the determination in S404 is Yes, the engine operating state shifts from the steady state to the transient state, calculates S1 and S2, and starts diagnosing responsiveness to the NOx sensor 36. It is determined whether the elapsed time is less than a predetermined limit time.

  If the determination in S422 is No and the elapsed time after starting the diagnosis of the NOx sensor 36 exceeds the limit time, the determination in S426 does not become Yes before the elapsed time exceeds the limit time. This is when at least one of the output, the reduced output, and the actual output does not converge.

  In this case, the diagnosis unit 58 determines that there may be an abnormality in which at least one of the normal output, the reduced output, and the actual output does not converge within the limit time. Possible causes of the abnormality include abnormalities in the fuel injection valve 4, the EGR valve 16, the intake air amount sensor, the NOx sensors 34, 36, and the like.

  Even if no abnormality has occurred, if the integrated value S1 and the integrated value S2 are calculated over the limit time in a state where noise is generated in the normal output, the reduced output, and the actual output, the diagnosis unit 58 determines that the integrated value It is determined that an error is likely to occur between S1 and the integrated value S2.

  When the elapsed time is equal to or longer than the limit time, the responsiveness of the NOx sensor 36 is normally diagnosed based on the integrated value calculated in S420 due to noise included in the normal output, the reduced output, and the actual output. It is a time set in advance as impossible.

  If the determination in S422 is No and the elapsed time exceeds the limit time before all of the normal output, the reduced output, and the actual output converge, the diagnosis unit 58 appropriately determines the degree of decrease in the responsiveness of the NOx sensor 36. It cannot be diagnosed, and it is determined that there is a possibility of misdiagnosis. Therefore, in S424, the diagnosis unit 58 stops the diagnosis of the NOx sensor 36 and ends this routine. Thereby, it is possible to suppress erroneous diagnosis of the degree of decrease in the responsiveness of the NOx sensor 36.

  If the determination in S422 is Yes and the elapsed time from the start of diagnosis is less than the limit time, in S426, the diagnosis unit 58 determines whether all of the normal output, the reduced output, and the actual output have converged to a constant value. Determine.

If the determination in S426 is No and at least one of the normal output, the reduced output, and the actual output has not converged, the process proceeds to S406.
If the determination in S426 is Yes, and the normal output, the reduced output, and the actual output all converge to a certain value before the elapsed time from the start of diagnosis reaches the limit time or more, in S428, the diagnosis unit 58 determines that S2 It is determined whether / S1 is larger than a predetermined value. The predetermined value is set to 1, for example. If the determination in S428 is Yes and S2 / S1 is greater than the predetermined value, the diagnosis unit 58 determines in S430 that the responsiveness of the NOx sensor 36 has decreased.

  In this case, since appropriate engine control cannot be performed based on the NOx concentration detected by the NOx sensor 36, the ECU 40 executes appropriate processing with respect to a decrease in responsiveness of the NOx sensor 36 in S432.

  For example, the ECU 40 notifies the driver that the responsiveness of the NOx sensor 36 is reduced by at least one of voice, a display, and a warning light. Further, based on the correspondence relationship between the engine operating state so far and the output of the NOx sensor 36 and the delay in the response of the NOx sensor 36 diagnosed, the current NOx sensor 36 outputs NOx from the current engine operating state. The concentration may be estimated.

In the above embodiment, the engine 2 corresponds to the internal combustion engine, the SCR catalyst 20 corresponds to the reduction catalyst, ammonia corresponds to the reducing agent, and the ECU 40 corresponds to the NOx sensor diagnostic device.
In the above embodiment, S400 and S408 correspond to the processing as the temperature acquisition unit 42, S402 and S408 correspond to the processing as the reducing agent estimation unit 44, and S402 and S406 correspond to the processing as the upstream acquisition unit 46. S408 and S410 correspond to the processing as the NOx amount estimating unit 48, S412 corresponds to the processing as the normal estimating unit 50, S414 corresponds to the processing as the decrease estimating unit 52, and S402 and S416 correspond to the actual processing. Corresponding to the processing as the output acquisition unit 54, S418 and S420 correspond to the processing as the integration unit 56, and S422 to S430 correspond to the processing as the diagnosis unit 58.

[3. effect]
In the embodiment described above, the following effects (1) to (7) can be obtained.
(1) Excluding the estimated value of the purification amount of NOx purified by the SCR catalyst 20 from the upstream NOx amount, the normal output of the responsive normal NOx sensor, and the responsiveness to the normal output is a predetermined value The reduced output of the NOx sensor that is decreasing is estimated.

  Thus, since the amount of NOx purification in the SCR catalyst 20 is taken into account, the normal output and the reduced output of the NOx sensor installed on the downstream side of the SCR catalyst 20 can be estimated with high accuracy. Therefore, the responsiveness of the NOx sensor can be properly diagnosed based on the normal output, the reduced output, and the actual output of the NOx sensor.

  (2) Since the responsiveness of the NOx sensor is diagnosed based on the ratio between the integrated value of the deviation between the actual output and the normal output and the integrated value of the deviation between the reduced output and the normal output, either normal output or reduced output The degree of decrease in the responsiveness of the NOx sensor can be diagnosed, not just the magnitude relationship between the actual output and the actual output.

  (3) Even if output variation due to noise or the like occurs in any of normal output, reduced output, and actual output of the NOx sensor, by integrating the deviation, if the elapsed time from the start of diagnosis is less than the limit time, The influence of output variation on the integrated value can be reduced.

  (4) Since calculation of S1 and S2 is started when the operating state of the engine 2 shifts from the steady state to the transient state, normal output, reduced output, and actual output before the operating state of the engine 2 shifts to the transient state The time for performing integration in a steady state in which and do not change can be shortened as much as possible.

  (5) In the configuration in which the NOx sensor 34 is not installed on the upstream side of the SCR catalyst 20, the gas state in the cylinder estimated from the operating state of the engine 2 and the exhaust gas reach the SCR catalyst 20 from the engine 2. The amount of NOx on the upstream side of the SCR catalyst 20 is calculated and acquired based on the time required until the time. Thereby, even in a configuration in which the NOx sensor 34 is not installed on the upstream side of the SCR catalyst 20, the NOx amount on the upstream side of the SCR catalyst 20 can be acquired.

  (6) A normal output is estimated based on a parameter including at least the response characteristic of a normal NOx sensor, and the responsiveness decreases by a predetermined value with respect to the response characteristic of the normal NOx sensor instead of the response characteristic of the normal NOx sensor. The reduced output is estimated based on the same parameters as the normal output estimation including at least the response characteristic of the NOx sensor being used. Except for the response characteristics, the reduced output is estimated using the same parameters as those used when estimating the normal output, so the reduced output can be easily estimated.

  (7) The elapsed time after the engine operating state shifts from the steady state to the transient state and the diagnosis process for calculating S1 and S2 is started before all of the normal output, the reduced output, and the actual output converge. If the time limit is exceeded, the diagnosis is stopped. Thereby, it is possible to avoid misdiagnosing the responsiveness of the NOx sensor 36.

[4. Other Embodiments]
(1) In the above embodiment, the NOx purification rate is acquired from the ammonia adsorption amount in the SCR catalyst 20 and the catalyst temperature of the SCR catalyst 20. On the other hand, when the engine 2 is started and the catalyst temperature reaches a predetermined temperature, the purification amount may be constant even if the catalyst temperature changes, and the purification rate may be acquired from the ammonia adsorption amount in the SCR catalyst 20.

  (2) In the above embodiment, in order to remove the NOx purification amount purified by the SCR catalyst 20 from the upstream NOx amount of the SCR catalyst 20, the upstream NOx amount is multiplied by the purification rate to reduce the NOx purification amount. And the purification amount was subtracted from the NOx amount upstream of the SCR catalyst 20. In contrast, the NOx purification amount purified by the SCR catalyst 20 may be removed from the upstream NOx amount by multiplying the upstream NOx amount by the non-purification rate instead of the purification rate.

  (3) A plurality of functions of one constituent element in the above embodiment may be realized by a plurality of constituent elements, or a single function of one constituent element may be realized by a plurality of constituent elements. Further, a plurality of functions possessed by a plurality of constituent elements may be realized by one constituent element, or a single function possessed by a plurality of constituent elements may be realized by one constituent element. Moreover, you may abbreviate | omit a part of structure of the said embodiment. Further, at least a part of the configuration of the above embodiment may be added to or replaced with the configuration of the other embodiment. In addition, all the aspects included in the technical idea specified only by the wording described in the claims are embodiments of the present invention.

  (4) In addition to the NOx sensor diagnostic device described above, an exhaust purification system including the NOx sensor diagnostic device as a constituent element, a NOx sensor diagnostic program for causing a computer to function as the NOx sensor diagnostic device, and the NOx sensor diagnostic program are recorded. The present invention can also be realized in various forms such as a recording medium and a NOx sensor diagnostic method.

2: engine (internal combustion engine), 10: exhaust purification system, 40: ECU (NOx sensor diagnostic device), 42: temperature acquisition unit, 44: reducing agent estimation unit, 46: upstream acquisition unit, 48: NOx amount estimation unit, 50: Normal estimation unit, 52: Decrease estimation unit, 54: Actual output acquisition unit, 56: Integration unit, 58: Diagnosis unit

Claims (8)

The NOx sensor (36), which is installed downstream of the reduction catalyst (20) that selectively reduces NOx in the exhaust gas of the internal combustion engine (2) with a reducing agent, detects the NOx amount in the exhaust gas. In the NOx sensor diagnostic device (40) for diagnosis,
An upstream acquisition unit (46, S402, S406) for acquiring the amount of NOx upstream of the reduction catalyst;
A NOx amount estimating unit (48, S408, S410) for estimating a NOx amount obtained by removing a NOx purification amount that is reduced and purified by the reduction catalyst from the upstream NOx amount acquired by the upstream acquisition unit;
A normal estimation unit (50, S412) for estimating a normal output of a NOx sensor having normal responsiveness based on the NOx amount estimated by the NOx amount estimation unit;
A drop estimation unit (52, S414) for estimating a drop output having lower responsiveness than the normal output estimated by the normal estimation unit;
An actual output acquisition unit (54, S402, S416) for acquiring the actual output of the NOx sensor;
A diagnosis unit (58, S422 to S430) for diagnosing the responsiveness of the NOx sensor based on the normal output, the reduced output estimated by the decrease estimation unit, and the actual output acquired by the actual output acquisition unit; ,
NOx sensor diagnostic device. In the NOx sensor diagnostic device according to claim 1,
At least the above among a reducing agent estimation unit (44, S402, S408) for estimating the amount of the reducing agent in the reduction catalyst and a temperature acquisition unit (42, S402, S408) for acquiring the catalyst temperature of the reduction catalyst A reducing agent estimation unit;
The NOx amount estimating unit includes at least the upstream NOx amount among the upstream NOx amount, the reducing agent amount estimated by the reducing agent estimating unit, and the catalyst temperature acquired by the temperature acquiring unit. The amount of NOx is estimated based on the amount of the reducing agent and the amount of the reducing agent, and the more the amount of the reducing agent in the reduction catalyst, the higher the catalyst temperature when the temperature acquisition unit is provided. Increasing the amount of purification,
NOx sensor diagnostic device. In the NOx sensor diagnostic device according to claim 1 or 2,
The upstream acquisition unit determines the upstream NOx amount based on a gas state in the cylinder estimated from an operating state of the internal combustion engine and a time required for exhaust gas to reach the reduction catalyst from the cylinder. To get an estimate,
NOx sensor diagnostic device. In the NOx sensor diagnostic device according to any one of claims 1 to 3,
The normal estimation unit estimates the normal output based on a parameter including at least the NOx amount estimated by the NOx amount estimation unit and a response characteristic of the normal NOx sensor;
The decrease estimation unit replaces the response characteristic of the normal NOx sensor with the parameter including at least the response characteristic of the NOx sensor whose responsiveness has decreased by the predetermined value with respect to the response characteristic of the normal NOx sensor. Estimating the reduced output based on:
NOx sensor diagnostic device. In the NOx sensor diagnostic device according to any one of claims 1 to 3,
The decrease estimation unit performs first-order lag processing on the normal output estimated by the normal estimation unit to estimate the decrease output.
NOx sensor diagnostic device. In the NOx sensor diagnostic device according to any one of claims 1 to 5,
An integrated value (56) for calculating S1 and S2 with S1 being an integrated value of deviation between the reduced output and the normal output, and S2 being an integrated value of deviation between the actual output and the normal output or the reduced output. , S418, S420),
The diagnosis unit (S428) diagnoses the responsiveness of the NOx sensor based on the S1 and the S2.
NOx sensor diagnostic device. The NOx sensor diagnostic device according to claim 6,
The integrating unit starts calculating S1 and S2 when the operating state of the internal combustion engine shifts from a steady state to a transient state.
NOx sensor diagnostic device. In the NOx sensor diagnostic device according to claim 7,
The integrating unit calculates S1 and S2 until all of the normal output, the reduced output, and the actual output converge after the operating state of the internal combustion engine changes from a steady state to a transient state,
The diagnostic unit (S422, S424) determines whether the elapsed time after the operation state of the internal combustion engine has shifted from a steady state to a transient state and starts calculating S1 and S2 is the normal output and the reduced output. If the NOx sensor response is exceeded for a predetermined time or more before all of the actual output converges, the NOx sensor response diagnosis is stopped.
NOx sensor diagnostic device.

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