JP5690182B2 - Control device for internal combustion engine - Google Patents

Description

  The present invention relates to a control apparatus for an internal combustion engine in which a NOx catalyst that captures NOx in exhaust gas is provided in an exhaust system, and a lean burn operation that burns an air-fuel mixture that is leaner than a stoichiometric air-fuel ratio is possible.

  In this type of internal combustion engine, lean burn operation is performed for the purpose of improving fuel consumption. During the lean burn operation, since the oxygen concentration in the exhaust gas is relatively high, the NOx in the exhaust gas cannot be properly reduced by the three-way catalyst and may be discharged without being purified. In order to prevent such a problem, a NOx purification NOx catalyst has been conventionally provided in an exhaust system of an internal combustion engine.

  For example, a NOx occlusion-type NOx catalyst captures (adsorbs and holds) NOx in exhaust gas in an oxidizing atmosphere in which the oxygen concentration in the exhaust gas is relatively high because the air-fuel ratio of the air-fuel mixture is lean. In a reducing atmosphere in which the concentration of the reducing agent such as HC and CO in the exhaust gas is relatively high due to the rich air-fuel ratio, the trapped NOx is released and the exhaust gas is purified by reducing with the reducing agent.

  Further, when the NOx catalyst deteriorates over time, there is a possibility that NOx cannot be properly purified. For this reason, conventionally, for example, Patent Document 1 discloses that the deterioration of the NOx catalyst is determined as follows. In other words, an air-fuel ratio sensor for detecting the exhaust air-fuel ratio (the ratio of oxygen to the reducing agent in the exhaust gas) is provided downstream of the NOx catalyst in the exhaust system, and the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is lean. The elapsed time until the exhaust air-fuel ratio detected by the air-fuel ratio sensor is switched from lean to rich after the switch from rich to rich is counted. Then, the trapped amount of NOx trapped by the NOx catalyst is calculated based on the elapsed time, and the deterioration of the NOx catalyst is determined based on the calculated trapped amount of NOx.

Japanese Patent No. 2692380

  FIG. 5 shows a general relationship between the temperature of the NOx catalyst and the NOx trapping ability. In the same figure, L1 is a case where the NOx catalyst is a new product which has not deteriorated at all, L2 is a case where the NOx catalyst is deteriorated more than a new product, and L3 is a case where the NOx catalyst is further deteriorated than L2. Each case is shown. L4 indicates a case where the NOx catalyst is abnormal. In the present specification and claims, “deterioration” refers to a state where NOx trapping ability is trapped to some extent although the NOx trapping ability is lower than that of a new product. Further, “abnormal” refers to a state in which almost no NOx can be captured with the progress of deterioration of the NOx catalyst. As shown by L1 to L3 in the figure, the NOx catalyst has a lower NOx trapping ability, that is, the ability to adsorb and retain NOx, even when the temperature is relatively high, even if it is normal without any abnormality. Has characteristics.

  Therefore, for example, when it is determined that the NOx catalyst is abnormal when the NOx trapping capacity is smaller than the predetermined threshold value NGC, when the temperature of the NOx catalyst is equal to or higher than the predetermined temperature TLNCREF, L3 is shown in FIG. As described above, even when the NOx catalyst is normal, the NOx trapping ability becomes smaller than the threshold value NGC, and as a result, there is a possibility that the NOx catalyst is erroneously determined to be abnormal. In order to avoid such erroneous determination, for example, when the temperature of the NOx catalyst is equal to or higher than a predetermined temperature TLNCREF, the abnormality determination of the NOx catalyst is prohibited, and the predetermined temperature TLNCREF is set only when the NOx catalyst is abnormal. It is conceivable to set a lower temperature so that the NOx trapping capacity falls below the threshold value NGC. Thereby, even when the NOx catalyst is normal, the NOx catalyst is not erroneously determined to be abnormal.

  Further, when the abnormality determination is prohibited as described above, it is unknown whether the NOx catalyst is abnormal. Therefore, when the lean burn operation is executed in such a situation, when the NOx catalyst is actually abnormal, , NOx is discharged without being purified. In order to prevent such a problem, it is conceivable that the lean burn operation is also prohibited when the abnormality determination is prohibited when the temperature of the NOx catalyst is equal to or higher than the predetermined temperature TLNCREF. However, in that case, even if the NOx catalyst is actually normal, as long as the temperature of the NOx catalyst is equal to or higher than the predetermined temperature TLNCREF, the lean burn operation is prohibited unnecessarily together with the prohibition of abnormality determination of the NOx catalyst. As a result, the opportunity for performing lean burn operation is reduced, and as a result, the fuel consumption of the internal combustion engine is deteriorated.

  The present invention has been made in order to solve the above-described problems, and can accurately determine the abnormality of the NOx catalyst and appropriately ensure the execution opportunity of the lean burn operation. It is an object of the present invention to provide a control device for an internal combustion engine that can improve the efficiency.

  In order to achieve the above object, according to the first aspect of the present invention, an NOx catalyst 9 for capturing NOx in exhaust gas is provided in the exhaust system (exhaust passage 7), and the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. It is the control device 1 of the internal combustion engine 3 capable of lean burn operation for burning the air-fuel mixture, and it is determined whether or not the NOx catalyst 9 is abnormal based on the NOx trapping ability (second output SVO2) of the NOx catalyst 9. Temporary determination means (ECU2, step 18) for temporary determination, temperature acquisition means (exhaust gas temperature sensor 27, ECU2) for acquiring the temperature of the NOx catalyst 9, and the acquired temperature of the NOx catalyst 9 (NOx catalyst temperature TLNC) Temporary determination by the temporary determination means is prohibited when the temperature is equal to or higher than the upper limit temperature TLMTH having a predetermined value as an initial value, and is temporarily determined when the temperature of the NOx catalyst 9 is lower than the upper limit temperature TLMTH. Temporary determination prohibiting means (ECU2, steps 11 to 13) to be permitted and upper limit temperature updating means (ECU2, step 22) for updating the upper limit temperature TLMTH to a lower side when it is temporarily determined that the NOx catalyst 9 is abnormal. And lean burn prohibiting means (ECU2, step 1) for prohibiting lean burn operation when temporary determination is prohibited by the temporary determination prohibiting means and permitting lean burn operation when temporary determination is permitted. When the NOx catalyst 9 is tentatively determined to be abnormal and the upper limit temperature TLMTH updated by the upper limit temperature updating means becomes equal to or lower than a predetermined determination temperature TJUD lower than the initial value, the NOx catalyst 9 is abnormal. And an abnormality confirmation means (ECU2, step 26) for confirming that there is.

  According to this configuration, whether or not the NOx catalyst is abnormal is temporarily determined by the temporary determination unit based on the NOx trapping ability of the NOx catalyst, and the temporary determination prohibiting unit temporarily determines whether the NOx catalyst is abnormal. The determination is prohibited when the acquired temperature of the NOx catalyst is equal to or higher than the upper limit temperature having a predetermined value as an initial value, and is allowed otherwise. When it is temporarily determined that the NOx catalyst is abnormal, the upper limit temperature is updated to the lower side by the upper limit temperature updating means. Then, when it is temporarily determined that the NOx catalyst is abnormal and the updated upper limit temperature is equal to or lower than a predetermined determination temperature lower than its initial value, the NOx catalyst is abnormal by the abnormality determination means. It is decided. In this case, since the temporary determination of the abnormality of the NOx catalyst is permitted when the temperature of the NOx catalyst is lower than the upper limit temperature, the relationship between the temperature of the NOx catalyst being lower than the determination temperature when the upper limit temperature is lower than the determination temperature. It is in.

  From the above, as apparent from the characteristics of the NOx catalyst described with reference to FIG. 5, in a situation where the temperature of the NOx catalyst is lower than the determination temperature and there is no possibility of erroneously determining that the NOx catalyst is abnormal, NOx When it is temporarily determined that the catalyst is abnormal, it can be determined that the NOx catalyst is abnormal, and therefore the abnormality of the NOx catalyst can be accurately determined.

  Further, according to the configuration described above, when the temporary determination of the abnormality of the NOx catalyst is prohibited, that is, when the temperature of the NOx catalyst is equal to or higher than the upper limit temperature, the lean burn operation is prohibited by the lean burn prohibiting means, Otherwise, it is allowed. As described above, the upper limit temperature that prescribes prohibition of temporary determination and lean burn operation is not set to a predetermined fixed value, but is updated to a lower side when it is temporarily determined that the NOx catalyst is abnormal. In addition, the initial value of the upper limit temperature can be set to a relatively high temperature, so that the lean burn operation permission region defined by the upper limit temperature can be expanded. As a result, unlike the above-described conventional case, unnecessary prohibition of lean burn operation when the NOx catalyst is not abnormal can be suppressed, so that the execution opportunity can be appropriately secured, thereby improving the fuel efficiency of the internal combustion engine. Can be obtained.

  According to a second aspect of the present invention, in the control device 1 for the internal combustion engine 3 according to the first aspect, the upper limit temperature update means gradually increases the upper limit temperature TLMTH every time it is temporarily determined that the NOx catalyst 9 is abnormal. It is characterized by updating so as to decrease.

  According to this configuration, every time it is temporarily determined that the NOx catalyst is abnormal, the upper limit temperature that regulates the prohibition of lean burn operation is updated so as to gradually decrease. Therefore, unlike the case where the upper limit temperature is greatly reduced at one time, it is possible to prevent the lean burn operation permission area specified by the upper limit temperature from being suddenly narrowed, and until the upper limit temperature falls below the judgment temperature. The lean burn operation can be performed, and the execution opportunity of the lean burn operation can be secured more appropriately.

It is a figure which shows roughly the control apparatus by embodiment of this invention with the internal combustion engine to which this is applied. It is a flowchart which shows the engine control process for controlling an internal combustion engine. It is a flowchart which shows the abnormality determination process for determining abnormality of a NOx catalyst. It is a figure for demonstrating the method of the temporary determination of abnormality of the NOx catalyst performed in abnormality determination processing. It is a figure which shows the general relationship between the temperature of a NOx catalyst, and the trapping ability of NOx.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. An internal combustion engine (hereinafter referred to as “engine”) 3 shown in FIG. 1 is a four-cycle type gasoline engine having four cylinders (not shown), and is mounted on a vehicle (not shown) as a power source. A crank angle sensor 21 is provided on a crankshaft (not shown) of the engine 3. The crank angle sensor 21 outputs a CRK signal and a TDC signal, which are pulse signals, to an ECU 2 (to be described later) of the control device 1 as the crankshaft rotates.

  The CRK signal is output every predetermined crank angle (for example, 30 °). The ECU 2 obtains the rotational speed NE (hereinafter referred to as “engine rotational speed”) NE of the engine 3 based on the CRK signal. The TDC signal is a signal indicating that one of the pistons of the four cylinders is near the TDC (top dead center) at the start of the intake stroke. In this example of the four-cylinder type, the TDC signal is output at every crank angle of 180 °. The The engine 3 is provided with a cylinder discrimination sensor (not shown), and the cylinder discrimination sensor outputs a cylinder discrimination signal, which is a pulse signal for discriminating the cylinder, to the ECU 2. The ECU 2 calculates the crank angle position for each cylinder based on the cylinder discrimination signal, the CRK signal, and the TDC signal.

  In the intake passage 4 of the engine 3, a throttle valve 5, an intake pressure sensor 22, and a fuel injection valve 6 are provided in order from the upstream side. The opening degree of the throttle valve 5 (hereinafter referred to as “throttle valve opening degree”) is controlled by the ECU 2, whereby the intake air amount taken into the combustion chambers of the respective cylinders via the intake passage 4 is controlled. Further, the throttle valve opening TH is detected by a throttle valve opening sensor 23, and the detection signal is output to the ECU 2.

  The intake pressure sensor 22 detects the pressure (hereinafter referred to as “intake pressure”) PBA in the intake passage 4 as an absolute pressure, and outputs a detection signal to the ECU 2. The ECU 2 calculates the intake air amount GIN according to the engine speed NE and the intake pressure PBA. The fuel injection valve 6 is provided for each cylinder so as to face the intake port (only one is shown). The valve opening time and valve opening timing of the fuel injection valve 6 are controlled by the ECU 2, whereby the fuel injection operation by the fuel injection valve 6 is controlled.

  The engine 3 is further provided with an ignition plug (not shown) for igniting the air-fuel mixture in the combustion chamber for each cylinder. The ignition operation of the spark plug is controlled by the ECU 2.

  Further, in the exhaust passage 7 (exhaust system) for exhausting exhaust gas from the engine 3, there are a LAF sensor 24, a three-way catalyst 8, a first O2 sensor 25, a NOx catalyst 9, and a second O2 sensor 26 in order from the upstream side. Is provided. The LAF sensor 24 is composed of zirconia or a platinum electrode, and the air-fuel ratio region of the air-fuel mixture burned in the combustion chamber is in a wide range from the rich region to the lean region than the stoichiometric air-fuel ratio. While detecting the oxygen concentration linearly, the detection signal is output to the ECU 2. Based on the oxygen concentration detected by the LAF sensor 24, the ECU 2 calculates an actual air-fuel ratio A / FACT representing the air-fuel ratio of the actual burned air-fuel mixture.

  The three-way catalyst 8 purifies the exhaust gas by oxidizing HC and CO in the exhaust gas and reducing NOx. The ability of the three-way catalyst 8 to purify HC, CO, and NOx is that when the exhaust gas is in a stoichiometric atmosphere, that is, the exhaust air-fuel ratio (the ratio of oxygen to the reducing agent such as HC and CO in the exhaust gas) is the stoichiometric air-fuel ratio. Both are high when they are in a relatively narrow region including. Further, the NOx catalyst 9 is of a so-called NOx storage type, and captures (adsorbs and holds) NOx in the exhaust gas in an oxidizing atmosphere where the oxygen concentration in the exhaust gas is relatively high, while reducing the exhaust gas in the exhaust gas. In a reducing atmosphere where the concentration of the agent is relatively high, the trapped NOx is released, and the exhaust gas is purified by reducing with a reducing agent.

  The first O2 sensor 25 is composed of zirconia or a platinum electrode, and outputs to the ECU 2 an output (hereinafter referred to as “first output”) FVO2 based on the oxygen concentration in the exhaust gas immediately downstream of the three-way catalyst 8. The first output FVO2 becomes Hi level when the exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio, becomes Lo level when it is lean (see FIG. 4), and changes rapidly before and after the stoichiometric air-fuel ratio. . The second O2 sensor 26 is configured in the same manner as the first O2 sensor 25, and outputs to the ECU 2 an output (hereinafter referred to as “second output”) SVO2 based on the oxygen concentration in the exhaust gas immediately downstream of the NOx catalyst 9. . The second output SVO2 also changes in the same manner as the first output FVO2 according to the exhaust air / fuel ratio.

  Further, an exhaust gas temperature sensor 27 (temperature acquisition means) is provided immediately upstream of the NOx catalyst 9 in the exhaust passage 7. The exhaust gas temperature sensor 27 detects the temperature of the exhaust gas flowing into the NOx catalyst 9 and outputs a detection signal to the ECU 2. The ECU 2 calculates the temperature of the NOx catalyst 9 (hereinafter referred to as “NOx catalyst temperature”) TLNC according to the detected temperature of the exhaust gas.

  The ECU 2 further receives from the accelerator opening sensor 28 a detection signal indicating the accelerator opening AP, which is an operation amount of an accelerator pedal (not shown) of the vehicle, and from the vehicle speed sensor 29 a detection signal indicating the vehicle speed VP of the vehicle. Each is output.

  The ECU 2 includes a microcomputer including a CPU, RAM, ROM, EEPROM, an I / O interface (all not shown), and the like. The ECU 2 executes the engine control process and the NOx catalyst 9 abnormality determination process shown in FIGS. 2 and 3, respectively, according to the control program stored in the ROM based on the detection signals of the various sensors 21 to 29 described above. In the present embodiment, the ECU 2 corresponds to temporary determination means, temperature acquisition means, temporary determination prohibition means, upper limit temperature update means, lean burn prohibition means, and abnormality determination means in the present invention.

This engine control process is a process for operating and controlling the engine 3 in any of the following stoichiometric combustion operation, lean burn operation and rich spike operation, and is executed for each cylinder in synchronization with the generation of the TDC signal described above. Is done.
-Stoichiometric combustion operation : By controlling the air-fuel ratio of the air-fuel mixture supplied to the engine 3 to be the stoichiometric air-fuel ratio, the stoichiometric air-fuel mixture is combusted in the combustion chamber, thereby executing the stoichiometric combustion operation. Driving mode.
-Lean burn operation : By controlling the air-fuel ratio to be leaner than the stoichiometric air-fuel ratio, the lean burn operation is performed by burning the air-fuel mixture leaner than the stoichiometric air-fuel ratio in the combustion chamber. Driving mode.
Rich spike operation : An operation mode in which the air-fuel ratio is controlled to be slightly richer than the stoichiometric air-fuel ratio in order to release and reduce NOx trapped by the NOx catalyst 9.

  First, in step 1 of FIG. 2 (illustrated as “S1”, the same applies hereinafter), it is determined whether or not the prohibition flag F_FORBID is “1”. This prohibition flag F_FORBID is set to “1” when temporary abnormality determination described later for temporarily determining abnormality of the NOx catalyst 9 is prohibited, and will be described later in the abnormality determination processing of FIG. Set to If the answer to step 1 is YES (F_FORBID = 1) and the abnormal temporary determination of the NOx catalyst 9 is prohibited, the lean burn operation is prohibited and the stoichiometric combustion operation is executed (step 2). finish.

  During this stoichiometric combustion operation, the intake air amount GIN is controlled via the throttle valve opening TH in accordance with the engine speed NE and the required torque TREQ, and the calculated actual air-fuel ratio A / FACT becomes the stoichiometric air-fuel ratio. Next, the injection operation of the fuel injection valve 6 is controlled. The required torque TREQ is calculated by searching a predetermined map (not shown) according to the engine speed NE and the detected accelerator pedal opening AP. As described above, the stoichiometric air-fuel mixture is burned in the combustion chamber during the stoichiometric combustion operation.

On the other hand, if the answer to step 1 is NO (F_FORBID = 0) and the abnormal temporary determination of the NOx catalyst 9 is not prohibited, it is determined whether or not the lean burn flag F_LEAN is “1” (step 3). . The lean burn flag F_LEAN indicates that all of the following predetermined execution conditions (a) to (d) for executing the lean burn operation are satisfied by “1”.
(A) The engine speed NE is within a predetermined range.
(B) The vehicle speed VP is within a predetermined range.
(C) The required torque TREQ is below a predetermined value and is in a low load region.
(D) The throttle valve opening TH is not more than a predetermined value.

  When the answer to step 3 is YES (F_LEAN = 1), the lean burn operation is executed (step 4), and this process is terminated. During the lean burn operation, the throttle valve opening TH is controlled to be almost fully open, and the injection operation of the fuel injection valve 6 is controlled so that the actual air-fuel ratio A / FACT becomes the target air-fuel ratio A / FOBJ. In this case, the target air-fuel ratio A / FOBJ is set to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio in accordance with the engine speed NE and the required torque TREQ. As described above, during the lean burn operation, the air / fuel mixture having an air / fuel ratio leaner than the stoichiometric air / fuel ratio burns in the combustion chamber.

  On the other hand, when the answer to step 3 is NO (F_LEAN = 0), it is determined whether or not the rich spike flag F_RICH is “1” (step 5). The rich spike flag F_RICH is set to “1” when a predetermined execution condition for executing the rich spike operation is satisfied. This execution condition is determined to be satisfied when it is determined that the trapped amount of NOx trapped by the NOx catalyst 9 is in a saturated state. The trapped amount of NOx is calculated according to the engine speed NE, the intake air amount GIN, the first output SVO1 of the first O2 sensor 25, and the like.

  When the answer to step 5 is YES (F_RICH = 1), rich spike operation is executed (step 6), and the process is terminated. During the rich spike operation, as in the case of the stoichiometric combustion operation, the intake air amount GIN is controlled via the throttle valve opening TH in accordance with the engine speed NE and the required torque TREQ. Further, the injection operation of the fuel injection valve 6 is controlled so that the actual air-fuel ratio A / FACT is slightly richer than the stoichiometric air-fuel ratio. As described above, during the rich spike operation, the air / fuel mixture slightly richer than the stoichiometric air / fuel ratio burns in the combustion chamber.

  As described above, during the stoichiometric combustion operation, the air-fuel ratio of the air-fuel mixture is controlled to the stoichiometric air-fuel ratio. Therefore, the NOx in the exhaust gas is reduced (purified) by the three-way catalyst 8 upstream of the NOx catalyst 9. Therefore, NOx hardly flows into the NOx catalyst 9. On the other hand, during lean burn operation, since the air-fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio, the oxygen concentration of the exhaust gas is high, so that NOx in the exhaust gas is hardly reduced by the three-way catalyst 8. , Captured by the NOx catalyst 9. For this reason, the execution condition of the rich spike operation based on the NOx trapping amount of the NOx catalyst 9 described above is established during the lean burn operation, whereby the operation mode is switched from the lean burn operation to the rich spike operation.

  When the rich spike flag F_RICH is “1”, that is, during the rich spike operation, the lean burn flag F_LEAN described above is set to “0” regardless of whether the execution conditions (a) to (d) are satisfied. Retained. As a result, when the answer to step 3 is NO, the lean burn operation of step 4 is not executed. Further, the rich spike flag F_RICH is reset to “0” when it is determined that the NOx of the NOx catalyst 9 is sufficiently released and reduced by executing the rich spike operation. This determination is made according to the integrated value of the exhaust gas flow rate calculated according to the intake air amount GIN.

  On the other hand, if the answer to step 5 is NO (F_RICH = 0), the above step 2 is executed and the process is terminated.

  Next, the abnormality determination process will be described with reference to FIG. This process is executed in synchronization with the generation of the TDC signal as in the engine control process described above, and is executed in parallel with the engine control process. First, in step 11, it is determined whether or not the calculated NOx catalyst temperature TLNC is lower than the upper limit temperature TLMTH.

  This upper limit temperature TLMTH is set to a predetermined initial value when the vehicle is shipped or when the NOx catalyst 9 is replaced with a new one. This initial value is set to the highest temperature among the temperatures at which it is not erroneously determined to be abnormal in the temporary abnormality determination of the NOx catalyst 9 when the NOx catalyst 9 is a new product that has not deteriorated. 690 ° C. Since the upper limit temperature TLMTH prescribes prohibition and permission such as abnormal provisional determination, the upper limit temperature TLMTH is set to a value with a predetermined hysteresis in order to prevent this prohibition and permission hunting.

  If the answer to step 11 is NO and the NOx catalyst temperature TLNC ≧ the upper limit temperature TLMTH, there is a possibility that the abnormality of the NOx catalyst 9 may not be properly determined. Therefore, to prohibit the temporary abnormality determination, the prohibition flag F_FORBID is set to “ 1 ”(step 12), and this process is terminated without determining whether the NOx catalyst 9 is abnormal.

  On the other hand, if the answer to step 11 is YES and the NOx catalyst temperature TLNC <the upper limit temperature TLMTH, the prohibition flag F_FORBID is set to “0” in order to permit the temporary abnormality determination (step 13). Next, it is determined whether or not the rich spike flag F_RICH is “1” (step 14).

  If the answer is NO (F_RICH = 0) and the rich spike operation is not being performed, the present process is terminated without determining whether the NOx catalyst 9 is abnormal. This is because, in the temporary abnormality determination, since the abnormality of the NOx catalyst 9 is determined based on the second output SVO2 of the second OS sensor 26 immediately after the start of the rich spike operation, the abnormality cannot be determined when the rich spike operation is not being performed. Because.

  On the other hand, when the answer to step 14 is YES (F_RICH = 1), it is determined whether or not the previous value F_RICHZ of the rich spike flag is “0” (step 15). When the answer is YES, that is, when this is a loop immediately after the start of the rich spike operation, an updated flag F_DONE described later is reset to “0” (step 16), and the process proceeds to step 17. On the other hand, when the answer to step 15 is NO, step 16 is skipped and the process proceeds to step 17.

  In step 17, it is determined whether or not the updated flag F_DONE is “1”. When this answer is NO, a temporary abnormality determination is executed (step 18). This abnormal temporary determination will be described below with reference to FIG.

  When the operation mode is switched from the lean burn operation to the rich spike operation (time point t1 in FIG. 4), the first output FVO2 of the first O2 sensor 25 that has been in the Lo level state until then because the exhaust air-fuel ratio has been lean. (Solid line) suddenly changes to the Hi level when the exhaust air-fuel ratio changes to the rich side by the rich spike operation. In this case, immediately after switching from the lean burn operation to the rich spike operation, the reducing agent such as HC and CO in the exhaust gas is oxidized by the oxygen stored in the three-way catalyst 8. Since the exhaust air-fuel ratio of the exhaust gas immediately downstream does not immediately change to the rich side, the first output FVO2 of the first O2 sensor 25 provided on the downstream side of the three-way catalyst 8 has a time delay corresponding to that. , Change to Hi level.

  Further, after the operation mode is switched to the rich spike operation, when all the oxygen of the three-way catalyst 8 is consumed, the first output FVO2 changes in a Hi level state. In addition, the reducing agent in the exhaust gas flows into the NOx catalyst 9, whereby NOx trapped by the NOx catalyst 9 is released and reduced. Therefore, on the downstream side of the NOx catalyst 9, the exhaust air-fuel ratio changes in a lean state. As a result, the second output SVO2 (one-dot chain line) of the second O2 sensor 26 provided on the downstream side of the NOx catalyst 9 is Unlike the first output FVO2, it changes in the Lo level state.

  When all of the NOx from the NOx catalyst 9 is released and reduced (time t2 in FIG. 4), the reducing agent in the exhaust gas passes through the NOx catalyst 9 as it is without being used for NOx reduction. 9, the exhaust air-fuel ratio on the downstream side changes to the rich side, whereby the second output SVO2 changes to the Hi level.

  Since NOx is hardly trapped when the NOx catalyst 9 is abnormal, the exhaust air-fuel ratio downstream of the NOx catalyst 9 after the start of the rich spike operation is earlier than when the NOx catalyst 9 is normal, Change to rich side. As a result, as indicated by a broken line in FIG. 4, the second output SVO2 'when the NOx catalyst 9 is abnormal changes to the Hi level at a timing earlier than the normal second output SVO2 indicated by the one-dot chain line. As is apparent from the above, the second output SVO2 in this case represents the NOx trapping ability of the NOx catalyst 9.

  From the above, the temporary abnormality determination is performed as follows. That is, after the start of the rich spike operation, the integrated value QEGSUM of the exhaust gas flow rate after the first output FVO2 exceeds the predetermined value VREF (see FIG. 4), that is, the exhaust air-fuel ratio of the exhaust gas flowing into the NOx catalyst 9 becomes richer. An integrated value QEGSUM of the exhaust gas flow rate after the change is calculated according to the intake air amount GIN. When the second output SVO2 obtained when the integrated value QEGSUM of the exhaust gas flow rate reaches the predetermined value QREF is greater than the predetermined determination value VJUD (see FIG. 4), the NOx capturing ability of the NOx catalyst 9 is high. Assuming that the NOx catalyst 9 is abnormal, the temporary abnormality flag F_NGTEM is set to “1” to indicate that the NOx catalyst 9 is abnormal.

  On the other hand, when the second output SVO2 obtained when the integrated value QEGSUM of the exhaust gas flow rate reaches the predetermined value QREF is equal to or less than the determination value VJUD, it is temporarily determined that the NOx catalyst 9 is not abnormal (normal) and In order to express, the temporary abnormality flag F_NGTEM is set to “0”.

  The predetermined value QREF is used to determine whether or not the reducing agent necessary for determining the abnormality of the NOx catalyst 9 is supplied to the NOx catalyst 9 without excess or deficiency, and is set by an experiment or the like. Yes. Further, the determination value VJUD is set to a value approximately halfway between the Hi level and the Lo level. In the present embodiment, the second output SVO2 when the integrated value QEGSUM of the exhaust gas flow rate reaches the predetermined value QREF corresponds to the capturing ability of the NOx catalyst 9 in the present invention.

  Further, after the determination result by the abnormal temporary determination in step 18 is obtained during the rich spike operation, the abnormal temporary determination is not executed until the rich spike operation is finished and then executed again. That is, the temporary abnormality determination is performed only once during one rich spike operation. Further, as is apparent from the above-described determination method for temporary tentative abnormality, the predetermined value QREF defines the end timing of the temporary tentative determination, and is set to a value at which the abnormal tentative determination ends during the rich spike operation. ing.

  In step 19 following step 18 in FIG. 3, it is determined whether or not the current temporary abnormality determination has been completed. When this answer is NO, the present process is finished as it is, while when YES, the current temporary abnormality determination is completed, and when the determination result is obtained, the temporary abnormality flag F_NGTEM indicating this determination result is “ It is determined whether it is “1” (step 20).

  When the answer to step 20 is NO, the present process is terminated as it is. On the other hand, when YES (F_NGTEM = 1) and the NOx catalyst 9 is provisionally determined to be abnormal, the upper limit temperature TLMTH is higher than the predetermined determination temperature TJUD. It is determined whether or not it is high (step 21). The determination temperature TJUD is set to a predetermined temperature lower than the initial value of the upper limit temperature TLMTH described above. More specifically, when the NOx catalyst 9 is normal, when the NOx catalyst 9 is normal, the determination temperature TJUD is greater than the determination value VJUD when the second output SVO2 described above is normal when the NOx catalyst 9 is normal. For example, the maximum temperature is set so that it is not erroneously determined to be abnormal, for example, 570 ° C.

  When the answer to step 21 is YES (TLMTH> TJUD), the upper limit temperature TLMTH is updated to a lower side to a value obtained by subtracting the predetermined temperature TREF from the upper limit temperature TLMTH obtained at that time (step 22). The updated upper limit temperature TLMTH is stored in the aforementioned EEPROM of the ECU 2. As described above, after the upper limit temperature TLMTH is updated, when the engine 3 is stopped and restarted, the updated upper limit temperature TLMTH is used for this process unless the NOx catalyst 9 is replaced with a new one. In the present embodiment, the storage device for storing the upper limit temperature TLMTH is an EEPROM, but may be another nonvolatile storage device such as a flash memory.

  In step 23 following step 22, the updated flag F_DONE is set to “1” in order to indicate that the upper limit temperature TLMTH has been updated based on the determination result of the current temporary abnormality determination. As a result, the answer to step 17 becomes YES (F_DONE = 1). In this case, the present process ends.

  As described above, the updated flag F_DONE is reset to “0” at the start of the rich spike operation (step 16), and the abnormality temporary determination is performed only once during one rich spike operation. Further, when it is temporarily determined that the NOx catalyst 9 is abnormal based on this temporary abnormality determination (step 20: YES), the upper limit temperature TLMTH is updated in step 22, and then the rich spike is started again. At the same time, unless the NOx catalyst 9 is provisionally determined to be abnormal, the updated flag F_DONE is held at “1”. As a result, the present process is terminated at step 17, and the upper limit temperature TLMTH is not updated.

  As apparent from the above, the upper limit temperature TLMTH is updated every time the NOx catalyst 9 is determined to be abnormal. Further, the predetermined temperature TREF used as a subtraction term for updating the upper limit temperature TLMTH to the lower side is set to a temperature at which the upper limit temperature TLMTH gradually decreases, for example, 30 ° C.

  In step 24 following step 23, it is determined whether or not the upper limit temperature TLMTH updated in step 22 is lower than the above-described determination temperature TJUD. When this answer is NO, the present process is terminated as it is, whereas when YES (TLMTH <TJUD), the upper limit temperature TLMTH is updated to the determination temperature TJUD (step 25), and the present process is terminated.

  On the other hand, if the answer to step 21 is NO and the upper limit temperature TLMTH is equal to or lower than the determination temperature TJUD, it is determined that the NOx catalyst 9 is abnormal, and an abnormality confirmation flag F_NG is set to “1” to indicate this. (Step 26), and this process ends.

  The reason for determining (determining) the abnormality of the NOx catalyst 9 as described above is as follows. That is, step 21 is executed when the NOx catalyst temperature TLNC is lower than the upper limit temperature TLMTH in step 11, so when the answer to step 21 is NO (TLMTH ≦ TJUD), the NOx catalyst temperature TLNC is The relationship is lower than the determination temperature TJUD. This clearly indicates that the NOx catalyst 9 is in a temperature state in which there is no possibility of erroneously determining that it is abnormal (state lower than the predetermined temperature TLNCREF in FIG. 5), as is apparent from the above-described setting method of the determination temperature TJUD. ). This is because, in such a situation, the NOx catalyst 9 is provisionally determined to be abnormal by the abnormal temporary determination based on the NOx trapping ability of the NOx catalyst 9 described above (step 20: YES).

  When the abnormality confirmation flag F_NG is set to “1”, a warning light (not shown) for notifying the driver of the abnormality of the NOx catalyst 9 is turned on. Thereafter, the abnormality confirmation flag F_NG is held at “1” unless the NOx catalyst 9 is replaced with a new one, and is reset to “0” when the NOx catalyst 9 is replaced. When the abnormality confirmation flag F_NG is “1”, the lean burn operation is always prohibited and the stoichiometric combustion operation is executed.

  As described above, according to the present embodiment, the temporary abnormality determination of the NOx catalyst 9 is executed based on the NOx trapping ability of the NOx catalyst 9 (step 18 in FIG. 3), and this temporary abnormality determination is performed as follows. While the NOx catalyst temperature TLNC is higher than the upper limit temperature TLMTH having a predetermined value as an initial value, the NOx catalyst temperature TLNC is prohibited, but otherwise it is permitted (steps 11 to 13). When it is temporarily determined that the NOx catalyst 9 is abnormal (step 20: YES), the upper limit temperature TLMTH is updated to the lower side (step 22).

  Then, the upper limit temperature TLMTH updated to the lower side is a predetermined determination temperature TJUD (for example, 570 ° C.) lower than the initial value (step 21: NO), and the NOx catalyst temperature TLNC is lower than the determination temperature TJUD. When the NOx catalyst 9 is temporarily determined to be abnormal, it is determined that the NOx catalyst 9 is abnormal (step 26). The determination temperature TJUD is set to the highest temperature at which there is no possibility that the NOx catalyst 9 is erroneously determined to be abnormal. As described above, the abnormality of the NOx catalyst 9 can be accurately determined.

  Further, when the temporary abnormality determination is prohibited, that is, when the NOx catalyst temperature TLNC is equal to or higher than the upper limit temperature TLMTH, the lean burn operation is prohibited, but is permitted otherwise (step 1 in FIG. 2). Since the upper limit temperature TLMTH that regulates prohibition of abnormal provisional determination and lean burn operation is not set to a predetermined fixed value, it is updated to a lower side when the NOx catalyst 9 is provisionally determined to be abnormal. The initial value of the upper limit temperature TLMTH is set to a maximum temperature that is not erroneously determined as abnormal in the temporary abnormality determination when the NOx catalyst 9 is new, that is, a relatively high temperature (for example, 690 ° C.). . Therefore, it is possible to expand the permitted range of the lean burn operation defined by the upper limit temperature TLMTH. As a result, unlike the above-described conventional case, an unnecessary prohibition of the lean burn operation when the NOx catalyst 9 is not abnormal can be suppressed, so that the execution opportunity can be appropriately ensured, thereby improving the fuel efficiency of the engine 3. Can be obtained.

  Further, every time it is temporarily determined that the NOx catalyst 9 is abnormal, the upper limit temperature TLMTH that defines prohibition of lean burn operation is updated so as to gradually decrease. Therefore, unlike the case where the upper limit temperature TLTTH is greatly reduced at once, the lean burn operation permission region defined by the upper limit temperature TLMTH can be prevented from being suddenly narrowed, so that the upper limit temperature TLMTH can be made lower than the determination temperature TJUD. Until this time, the lean burn operation can be performed, and the execution opportunity of the lean burn operation can be secured more appropriately.

  In addition, this invention can be implemented in various aspects, without being limited to the described embodiment. For example, in this embodiment, the temporary abnormality determination of the NOx catalyst 9 is performed by the above-described method using the second output SVO2 during the rich spike operation, but if it is based on the NOx trapping ability of the NOx catalyst 9 Other methods may be used. For example, a NOx sensor for detecting the concentration of NOx in the exhaust gas is provided on the upstream side and the downstream side of the NOx catalyst 9 in the exhaust passage 7, and the detection results of these upstream and downstream NOx sensors during lean burn operation Based on the above, the trap amount of NOx of the NOx catalyst 9 may be calculated, and the temporary abnormality determination may be performed based on the calculated trap amount of NOx.

  In the embodiment, the NOx catalyst temperature TLNC is calculated (estimated) based on the temperature of the exhaust gas detected by the exhaust gas temperature sensor 27. However, even if the NOx catalyst temperature TLNC is directly detected by the temperature sensor attached to the NOx catalyst 9. Of course it is good. Furthermore, in the embodiment, the initial value of the upper limit temperature TLMTH is set to the highest temperature among the temperatures at which it is not erroneously determined to be abnormal when the NOx catalyst 9 is new, but when the NOx catalyst 9 is new As long as the temperature is not erroneously determined as abnormal, the maximum temperature is not necessarily required. In the embodiment, the determination temperature TJUD is set to the highest temperature among the temperatures that are not erroneously determined as abnormal when the NOx catalyst 9 is normal, but is lower than the initial value of the upper limit temperature TLMTH, And if it is the temperature which is not mistakenly determined as abnormal when the NOx catalyst 9 is normal, it may not necessarily be the highest temperature.

  Furthermore, although embodiment is an example which applied this invention to the engine 3 which is a gasoline engine for vehicles, this invention is not restricted to this, Various engines using fuels other than gasoline, for example, a diesel engine, It can also be applied to LPG engines, and can be applied to various engines other than those for vehicles, for example, engines for marine propulsion devices such as outboard motors whose crankshafts are arranged in the vertical direction, and other industrial engines. Is also applicable. In addition, it is possible to appropriately change the detailed configuration within the scope of the gist of the present invention.

1 control device 2 ECU (temporary determination means, temperature acquisition means, temporary determination prohibition means, upper limit temperature update means, lean burn prohibition means, abnormality determination means)
3 Engine 7 Exhaust passage (exhaust system)
9 NOx catalyst 27 Exhaust gas temperature sensor (temperature acquisition means)
SVO2 second output (NOx capture capacity of NOx catalyst)
TLNC NOx catalyst temperature TLMTH upper limit temperature TJUD judgment temperature

Claims (2)

A control device for an internal combustion engine capable of performing a lean burn operation in which a NOx catalyst for capturing NOx in exhaust gas is provided in an exhaust system and burns an air-fuel mixture leaner than a stoichiometric air-fuel ratio,
Provisional determination means for tentatively determining whether or not the NOx catalyst is abnormal based on the NOx capturing ability of the NOx catalyst;
Temperature acquisition means for acquiring the temperature of the NOx catalyst;
The temporary determination by the temporary determination means is prohibited when the acquired temperature of the NOx catalyst is equal to or higher than an upper limit temperature having a predetermined value as an initial value, and when the temperature of the NOx catalyst is lower than the upper limit temperature Provisional judgment prohibition means for permitting provisional judgment;
Upper limit temperature update means for updating the upper limit temperature to a lower side when it is temporarily determined that the NOx catalyst is abnormal;
Lean burn prohibiting means for prohibiting the lean burn operation when the temporary determination is prohibited by the temporary determination prohibiting means, and permitting the lean burn operation when the temporary determination is permitted;
When it is temporarily determined that the NOx catalyst is abnormal and the upper limit temperature updated by the upper limit temperature update means is below a predetermined determination temperature lower than the initial value, the NOx catalyst is abnormal. An abnormality confirmation means for confirming that there is,
A control device for an internal combustion engine, comprising:   2. The control of the internal combustion engine according to claim 1, wherein the upper limit temperature update unit updates the upper limit temperature to be gradually decreased every time it is temporarily determined that the NOx catalyst is abnormal. apparatus.

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