JP4585971B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine that controls an air-fuel ratio so as to recover the NOx trapping ability of the NOx purification catalyst in an internal combustion engine equipped with a NOx purification catalyst that traps NOx in exhaust gas.

  Conventionally, a diesel engine type is known as an internal combustion engine provided with a NOx purification catalyst in an exhaust passage. In this type of internal combustion engine, the air-fuel ratio of the air-fuel mixture is normally lean-controlled. At that time, NOx in the exhaust gas in the lean atmosphere is captured (adsorbed) by the NOx purification catalyst. Thereby, exhaust gas is purified. Further, when the NOx trapping amount of the NOx purifying catalyst approaches a limit value and the NOx trapping capacity decreases, the air-fuel ratio of the air-fuel mixture is temporarily controlled from the lean side to the rich side in order to recover the NOx purifying capacity. So-called rich spike control is executed. Thereby, exhaust gas in a rich atmosphere is supplied to the NOx purification catalyst, and the captured NOx is reduced in the NOx purification catalyst, whereby the NOx trapping ability is recovered.

  In such an internal combustion engine, when rich spike control is executed, a part of the NOx trapped by the NOx purification catalyst due to deterioration of the NOx purification catalyst is released into the exhaust passage without being reduced. As a result, the NOx concentration of the exhaust gas temporarily increases, and the exhaust gas characteristics may deteriorate. Hereinafter, this phenomenon is referred to as “slip NOx”. As an air-fuel ratio control device for an internal combustion engine that executes rich spike control while suppressing the generation of such slip NOx, for example, a device described in Patent Document 1 is known.

  In the air-fuel ratio control device of Patent Document 1, the NOx purification rate of the NOx purification catalyst is calculated based on the detected NOx concentration in the exhaust gas during lean control, and when this becomes lower than a predetermined determination value, In order to recover the NOx trapping ability of the NOx purification catalyst, rich spike control is executed. After the start of the rich spike control, the difference between the NOx concentration on the downstream side of the NOx purification catalyst and the NOx concentration on the upstream side, that is, the NOx concentration difference indicating the degree of occurrence of slip NOx is continuously calculated, and the maximum value is calculated. Ask. Further, the threshold value is set by searching the map according to the execution time of lean control immediately before the rich spike control, the catalyst temperature, and the oxygen concentration on the upstream side of the catalyst. When the maximum value of the NOx concentration difference is larger than the threshold value and slip NOx occurs, a correction coefficient is calculated according to the deviation between the maximum value and the threshold value, that is, the degree of occurrence of slip NOx. The next lean control execution time is calculated by multiplying the correction coefficient by the previous lean control execution time. That is, the execution time of lean control is corrected according to the degree of occurrence of slip NOx.

  As described above, since the execution time of the lean control is corrected according to the deviation between the maximum value of the NOx concentration difference and the threshold value, the slip NOx is reduced due to the decrease in the NOx trapping capacity resulting from the deterioration of the NOx purification catalyst. Even when the generated amount is increased, the amount of NOx trapped in the NOx purification catalyst is reduced by the next rich spike control by correcting the execution time of the lean control to be shorter accordingly. be able to. As a result, when the next rich spike control is executed, the NOx trapping ability of the NOx purification catalyst can be recovered while suppressing the generation of slip NOx.

JP 2005-194927 A

  According to the above-described conventional air-fuel ratio control device for an internal combustion engine, the greater the amount of slip NOx generated, the shorter the lean control execution time is corrected. Accordingly, the rich spike control execution frequency increases. As a result, there is a problem that the fuel consumption deteriorates. Further, in the determination of the degree of occurrence of slip NOx, the deviation between the NOx concentration on the downstream side of the NOx purification catalyst and the NOx concentration on the upstream side is used, so in addition to the actual occurrence state of slip NOx, Exhaust gas containing unburned fuel as a reducing agent and the NOx purification ability of the NOx purification catalyst are affected. As a result, since the determination accuracy of the degree of occurrence of slip NOx is low, the slip NOx cannot be appropriately suppressed, and the exhaust gas characteristics may be deteriorated.

  The present invention has been made to solve the above-described problems. In the case of controlling the air-fuel ratio so as to recover the NOx purification ability of the NOx purification catalyst while suppressing the generation of slip NOx, the fuel consumption and exhaust gas characteristics are achieved. An object of the present invention is to provide an air-fuel ratio control apparatus for an internal combustion engine that can improve the engine.

In order to achieve the above object, the invention according to claim 1 includes a NOx purification catalyst 12 that captures NOx in exhaust gas in a lean atmosphere and reduces captured NOx when exhaust gas in a rich atmosphere is supplied. The air-fuel ratio control apparatus 1 for the internal combustion engine 3 includes NOx concentration detection means (NOx sensor 23) for detecting NOx concentration CNOx in the exhaust gas downstream of the NOx purification catalyst 12, and exhaust gas in a rich atmosphere in the NOx purification catalyst 12. When the rich control condition for supplying the fuel is satisfied (when the determination result in step 60 is YES), the air-fuel ratio AF of the air-fuel mixture supplied to the internal combustion engine 3 is changed from the lean side to the rich side with respect to the stoichiometric air-fuel ratio. Air-fuel ratio control means (ECU2, steps 69 to 75) for controlling the air-fuel ratio to be switched, and air-fuel ratio AF of the air-fuel mixture by the air-fuel ratio control means is switched to the rich side Slip NOx parameter calculating means (ECU2, step) for calculating slip NOx parameters (slip value SLIP_CNOx, average value of NOx concentration CNOx, total NOx amount) indicating the degree of occurrence of slip NOx in accordance with the NOx concentration CNOx detected in descending 8, 26), and the air-fuel ratio control means is estimated that the NOx reduction operation in the NOx purification catalyst after the calculated slip NOx parameter and switching to the rich side of the air-fuel ratio has ended. Changes in the air-fuel ratio AF when switching from lean to rich according to the slip NOx ratio (slip-through ratio R_SLTH), which is the ratio to the NOx concentration (through value TH_CNOx) detected at the timing, and the slip NOx parameter Determine the speed (steps 52, 56, 85, 94, 14,115) that characterized.

According to this air-fuel ratio control apparatus for an internal combustion engine, the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine when a rich control condition for supplying exhaust gas in a rich atmosphere to the NOx purification catalyst is satisfied by the air-fuel ratio control means. However, control is performed so as to switch from the lean side to the rich side with respect to the stoichiometric air-fuel ratio, and a slip NOx parameter representing the degree of occurrence of slip NOx is calculated according to the NOx concentration detected after the switching. The rate of change of the air-fuel ratio when switching from the lean side to the rich side is estimated that the NOx reduction operation in the NOx purification catalyst after the slip NOx parameter and the switching to the rich side of the air-fuel ratio has been completed. It is determined according to the slip NOx ratio, which is the ratio with the NOx concentration detected at the timing, and the slip NOx parameter . Thus, since the change rate of the air-fuel ratio is determined according to the slip NOx parameter indicating the degree of occurrence of slip NOx and the slip NOx ratio , the NOx of the NOx purification catalyst is appropriately suppressed while suppressing the generation of slip NOx. Capturing ability can be restored.

  That is, since the slip NOx can be suppressed only by changing the change rate of the air-fuel ratio when the air-fuel ratio is switched from the lean side to the rich side, the NOx purification catalyst deteriorates unlike the conventional case where the execution time of the lean control is changed. Even at this time, it is possible to avoid an increase in the execution frequency of the rich control, and the fuel efficiency can be improved accordingly. Further, since the slip NOx parameter is calculated only in accordance with the NOx concentration in the exhaust gas downstream of the NOx purification catalyst, the slip NOx parameter uses a deviation between the downstream NOx concentration and the upstream NOx concentration of the NOx purification catalyst. Unlike the case, the NOx purification ability of the exhaust gas containing unburned fuel as the reducing agent and the NOx purification catalyst can avoid affecting the determination result of the slip NOx, and the determination accuracy of the generation degree of the slip NOx can be improved. Can do. As a result, the exhaust gas characteristics can be improved by more reliably suppressing the slip NOx.

According to a second aspect of the present invention, in the air-fuel ratio control device 1 for the internal combustion engine 3 according to the first aspect, the air-fuel ratio control means sets a plurality of change rates of the air-fuel ratio AF according to the slip NOx ratio and the slip NOx parameter. It is characterized in that it is determined to switch at the stage (steps 52, 56, 85, 94, 114, 115).

According to the air-fuel ratio control apparatus for an internal combustion engine, the change rate of the air-fuel ratio is determined so as to be switched at a plurality of stages according to the slip NOx ratio and the slip NOx parameter, and accordingly, according to the degree of occurrence of slip NOx. The air-fuel ratio can be controlled more finely, and the generation of slip NOx can be more reliably suppressed.

  According to a third aspect of the present invention, in the air-fuel ratio control apparatus 1 for the internal combustion engine 3 according to the first or second aspect, the air-fuel ratio control means includes a slip NOx in which the change rate of the air-fuel ratio AF is represented by a slip NOx parameter. It is characterized in that it is determined so as to be smaller as the degree of occurrence of (steps 52, 56, 85, 94, 114, 115).

  According to this air-fuel ratio control apparatus for an internal combustion engine, the rate of change of the air-fuel ratio is determined to be smaller as the degree of occurrence of slip NOx represented by the slip NOx parameter is larger. It can suppress more reliably.

  According to a fourth aspect of the present invention, in the air-fuel ratio control device 1 for the internal combustion engine 3 according to any one of the first to third aspects, the slip NOx parameter calculating means sets the slip NOx parameter to the rich side of the air-fuel ratio AF. It is calculated as a deviation (MX_CNOx−ST_CNOx) between the maximum value MX_CNOx of the NOx concentration detected after switching and the NOx concentration (initial value ST_CNOx) detected at the start of switching.

  According to this air-fuel ratio control apparatus for an internal combustion engine, the slip NOx parameter is calculated as a deviation between the maximum value of the NOx concentration detected after switching to the rich side of the air-fuel ratio and the NOx concentration detected at the start of switching. Therefore, as described above, unlike the conventional case in which the deviation between the NOx concentration on the downstream side of the NOx purification catalyst and the NOx concentration on the upstream side is used, the exhaust gas containing unburned fuel as the reducing agent and the NOx in the NOx purification catalyst It is possible to avoid the purification ability from affecting the determination of slip NOx and improve the determination accuracy of the degree of occurrence of slip NOx.

  According to a fifth aspect of the present invention, in the air-fuel ratio control device 1 for the internal combustion engine 3 according to any one of the first to third aspects, the slip NOx parameter calculating means converts the slip NOx parameter to the rich side of the air-fuel ratio AF. It is calculated as an average value of NOx concentration CNOx from a switching start to a predetermined timing (time t2, t3 or t4) after switching.

  When the air-fuel ratio of the air-fuel mixture is controlled to be switched from the lean side to the rich side with respect to the stoichiometric air-fuel ratio in order to supply exhaust gas in a rich atmosphere to the NOx purification catalyst, the state of occurrence of slip NOx after switching to the rich side In general, the NOx concentration changes so as to show a temporary peak. In addition, for example, when the amount of slip NOx is small, the NOx concentration repeats up and down. There are various patterns of change. On the other hand, according to the air-fuel ratio control apparatus for an internal combustion engine, the slip NOx parameter is calculated as an average value of the NOx concentration from the start of switching to the rich side of the air-fuel ratio until the predetermined timing after switching. Regardless of the change pattern of the NOx concentration, the slip NOx parameter can be calculated as a value that appropriately represents the degree of occurrence of slip NOx without being affected by the change pattern, thereby determining the accuracy of determining the degree of occurrence of slip NOx. Can be further improved.

  According to a sixth aspect of the present invention, in the air-fuel ratio control device 1 for the internal combustion engine 3 according to any one of the first to third aspects, the slip NOx parameter calculating means sets the slip NOx parameter to the rich side of the air-fuel ratio AF. The total NOx amount discharged from the NOx purification catalyst 12 is calculated from the start of switching to the predetermined timing after switching (time t2, t3 or t4).

  According to this air-fuel ratio control apparatus for an internal combustion engine, the slip NOx parameter is calculated as the total NOx amount discharged from the NOx purification catalyst between the start of switching to the rich side of the air-fuel ratio and the predetermined timing after switching. Therefore, as described above, regardless of the change pattern of the NOx concentration, the slip NOx parameter can be calculated as a value that appropriately represents the degree of occurrence of the slip NOx without being affected by the change pattern. The determination accuracy of the degree of NOx generation can be further improved.

  Hereinafter, an air-fuel ratio control apparatus for an internal combustion engine according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic configuration of an internal combustion engine (hereinafter referred to as “engine”) 3 to which the air-fuel ratio control device 1 of the present embodiment is applied, and FIG. 2 shows a schematic configuration of the air-fuel ratio control device 1. . As shown in FIG. 2, the air-fuel ratio control device 1 includes an ECU 2. The ECU 2 executes various control processes such as air-fuel ratio control according to the operating state of the engine 3, as will be described later. To do.

  The engine 3 is an in-line four-cylinder diesel engine mounted on a vehicle (not shown), and includes four sets of cylinders 3a and pistons 3b (only one set is shown), a crankshaft 3c, and the like. The engine 3 is provided with a crank angle sensor 20. The crank angle sensor 20 includes a magnet rotor and an MRE pickup, and outputs a CRK signal and a TDC signal, which are pulse signals, to the ECU 2 as the crankshaft 3c rotates.

  The CRK signal is output at one pulse every predetermined crank angle (for example, 30 °), and the ECU 2 calculates the engine speed NE (hereinafter referred to as “engine speed”) NE based on the CRK signal. The TDC signal is a signal indicating that the piston 3b of each cylinder 3a is at a predetermined crank angle position slightly ahead of the TDC position of the intake stroke, and one pulse is output for each predetermined crank angle.

  The engine 3 is provided with a fuel injection valve 4 for each cylinder 3a (only one is shown), and each fuel injection valve 4 is electrically connected to the ECU 2. As will be described later, the valve opening time and the valve opening timing of the fuel injection valve 4 are controlled by the ECU 2, whereby the fuel injection amount and the fuel injection timing are controlled.

  On the other hand, an air flow sensor 21, a turbocharger 7, a throttle valve mechanism 8, and a swirl valve mechanism 9 are provided in the intake passage 6 of the engine 3 in order from the upstream side. The air flow sensor 21 is composed of a hot-wire air flow meter, detects the flow rate of fresh air that passes through a throttle valve 8a described later, and outputs a detection signal representing it to the ECU 2. Based on the detection signal of the air flow sensor 21, the ECU 2 calculates a fresh air amount M_ACT that is estimated to be actually taken into the cylinder 3a.

  The turbocharger 7 includes a compressor blade 7a provided on the downstream side of the air flow sensor 21 in the intake passage 6, a turbine blade 7b provided in the middle of the exhaust passage 11, and rotating integrally with the compressor blade 7a. Variable vanes 7c (only two are shown), a vane actuator 7d for driving the variable vanes 7c, and the like.

  In the turbocharger 7, when the turbine blade 7 b is rotationally driven by the exhaust gas in the exhaust passage 11, the compressor blade 7 a integrated therewith is also rotated at the same time, so that fresh air in the intake passage 6 is pressurized. That is, the supercharging operation is executed.

  The variable vane 7c is for changing the supercharging pressure generated by the turbocharger 7, and is rotatably attached to the wall of the portion of the housing that houses the turbine blade 7b. The variable vane 7c is mechanically coupled to a vane actuator 7d connected to the ECU 2. The ECU 2 changes the rotational speed of the turbine blade 7b, that is, the rotational speed of the compressor blade 7a by changing the opening of the variable vane 7c via the vane actuator 7d and changing the amount of exhaust gas blown to the turbine blade 7b. Thereby, the supercharging pressure is controlled.

  On the other hand, the throttle valve mechanism 8 includes a throttle valve 8a and a TH actuator 8b for driving the throttle valve 8a. The throttle valve 8a is rotatably provided in the middle of the intake passage 6, and changes the flow rate of fresh air passing through the throttle valve 8a by the change in the opening degree accompanying the rotation. The TH actuator 8b is a combination of a motor and a reduction gear mechanism (both not shown), and is electrically connected to the ECU 2. The ECU 2 controls the opening degree of the throttle valve 8a via the TH actuator 8b.

  Further, the portion of the intake passage 6 on the downstream side of the throttle valve mechanism 8 is an intake manifold 6a including one collecting portion and four branch portions branched therefrom. The passage in the intake manifold 6a is divided into a swirl passage 6b and a bypass passage 6c from the collecting portion to each branch portion, and these passages 6b and 6c communicate with the cylinder 3a through two intake ports, respectively. ing.

  The above-described swirl valve mechanism 9 stirs the air-fuel mixture in the cylinder 3a by generating a swirl in the cylinder 3a. The swirl valve 9a provided in the swirl passage 6b and a swirl actuator that drives the swirl valve 9a. 9b and the like. The swirl actuator 9b is a combination of a motor and a reduction gear mechanism (both not shown), and is electrically connected to the ECU 2. The ECU 2 changes the opening degree of the swirl valve 9a via the swirl actuator 9b, thereby controlling the state of occurrence of the swirl.

  Further, the engine 3 is provided with an exhaust gas recirculation device 10. The exhaust gas recirculation device 10 recirculates a part of the exhaust gas in the exhaust passage 11 to the intake passage 6 side, an EGR passage 10a connected between the intake passage 6 and the exhaust passage 11, and the EGR passage 10a. And an EGR control valve 10b for opening and closing the valve. One end of the EGR passage 10 a opens to a portion upstream of the turbine blade 7 b of the exhaust passage 11, and the other end opens to a bypass passage 6 c of the intake passage 6.

  The EGR control valve 10b is a linear electromagnetic valve whose lift changes linearly between a maximum value and a minimum value, and is electrically connected to the ECU 2. The ECU 2 changes the opening degree of the EGR passage 10a via the EGR control valve 10b, thereby controlling the recirculation amount of exhaust gas, that is, the EGR amount.

  On the other hand, on the downstream side of the turbine blade 7b in the exhaust passage 11, a LAF sensor 22, a NOx purification catalyst 12, and a NOx sensor 23 are provided in order from the upstream side. The LAF sensor 22 is composed of zirconia and a platinum electrode, and the oxygen concentration in the exhaust gas flowing in the exhaust passage 11 is measured in a wide range of air-fuel ratios from a rich region richer than the stoichiometric air-fuel ratio to an extremely lean region. It detects linearly and outputs a detection signal representing it to the ECU 2. The ECU 2 calculates the air-fuel ratio of the exhaust gas, that is, the air-fuel ratio AF of the burned mixture based on the detection signal of the LAF sensor 22.

  Further, the NOx purification catalyst 12 captures NOx in the exhaust gas when the exhaust gas in the lean atmosphere, that is, the exhaust gas whose oxygen concentration is higher than the oxygen concentration corresponding to the stoichiometric air-fuel ratio flows, and the rich spike control described later. Thus, when exhaust gas in a rich atmosphere flows in, the captured NOx is reduced.

  On the other hand, the NOx sensor 23 (NOx concentration detection means) detects the NOx concentration CNOx in the exhaust gas that has passed through the NOx purification catalyst 12, and outputs a detection signal representing it to the ECU 2.

  Further, an accelerator opening sensor 24 is connected to the ECU 2. The accelerator opening sensor 24 detects a depression amount (hereinafter referred to as “accelerator opening”) AP of an accelerator pedal (not shown) of the vehicle, and outputs a detection signal indicating the detected amount to the ECU 2.

  The ECU 2 includes a microcomputer including a CPU, a RAM, a ROM, an I / O interface (all not shown), and the engine 2 according to the detection signals of the various sensors 20 to 24 described above. 3 is determined, and air-fuel ratio control processing or the like is executed as described below according to the operating state. As a result, the air-fuel ratio AF of the air-fuel mixture is controlled to a lean value during normal operation, and during rich spike control, a predetermined rich spike value, which will be described later, is used to reduce NOx trapped by the NOx purification catalyst 12. Controlled to AF_RICH. Further, when slip NOx occurs due to deterioration of the NOx purification catalyst 12 during the rich spike control, the slip NOx is not generated at the air-fuel ratio AF at the start of the next rich spike control. Control is performed up to the rich spike value AF_RICH at a change rate smaller than the case. Hereinafter, this control is referred to as “slip correction control”.

  In the present embodiment, the ECU 2 corresponds to air-fuel ratio control means and slip NOx parameter calculation means.

  Further, the ECU 2 includes a RAM with a backup power source capable of holding stored data even when the engine is stopped when the ignition switch is turned off, and a general RAM without a backup power source. A NOx trapping amount S_QNOx, which will be described later, is empty. Values such as the fuel ratio correction coefficient K_NOx, the previous intermediate value AF_MIDZ, the correction coefficient calculated flag F_KNOx, and the slip corrected flag F_AFSLIP are stored in a RAM with a backup power source.

  Next, a rich spike control execution condition determination process executed by the ECU 2 will be described with reference to FIG. As will be described below, this process determines whether or not the execution condition of the rich spike control for reducing the NOx trapped by the NOx purification catalyst 12 is satisfied, and the air-fuel ratio correction coefficient K_NOx and the air-fuel ratio The correction value AF_CORR is calculated, and is executed at a predetermined control cycle (for example, 10 msec).

  In this process, first, in step 1 (abbreviated as “S1” in the figure, the same applies hereinafter), it is determined whether or not the rich condition flag F_RICH is “1”. If the determination result is NO and the execution condition of the rich spike control is not satisfied, the process proceeds to step 2 to calculate the NOx trapping amount S_QNOx.

  This NOx trapping amount S_QNOx corresponds to an estimated value of the NOx amount trapped by the NOx purification catalyst 12, and is specifically calculated as an integrated value of the NOx emission amount QNOx as described below. . First, the required torque PMCMD is calculated by searching a map (not shown) according to the engine speed NE and the accelerator pedal opening AP, and then the map (not shown) is searched according to the required torque PMCMD and the engine speed NE. Thus, the NOx emission amount QNOx is calculated. Thereafter, the NOx trapping amount S_QNOx is calculated by adding the calculated NOx emission amount QNOx to the previous value of the NOx trapping amount.

  Next, the routine proceeds to step 3 where it is determined whether or not the NOx trapping amount S_QNOx is equal to or greater than a predetermined value SQNOx1. When the determination result is NO, it is determined that the execution condition of the rich spike control is not satisfied, and the process proceeds to step 12, and the rich condition flag F_RICH is set to “0” to indicate that, and then this process is performed. finish.

  On the other hand, when the determination result in step 3 is YES, it is determined that the execution condition of the rich spike control is satisfied. In step 4, the NOx trapping amount S_QNOx is reset to 0, and then the process proceeds to step 5 where the rich spike control is performed. Is set to “1” to indicate that the execution condition is satisfied. Thereby, in the loop after the next time, the determination result of step 1 becomes YES, and in that case, the process proceeds to step 6.

  In step 6 following step 1 or 5, the total reducing agent amount S_QRA is calculated. This total reducing agent amount S_QRA represents an integrated value of the reducing agent amount estimated to have been supplied to the NOx purification catalyst 12, that is, an integrated value of the unburned fuel amount, and is specifically calculated as follows. The First, a reducing agent amount QRA is calculated by searching a map (not shown) according to the fresh air amount M_ACT, the engine speed NE, and a fuel injection amount QINJ, which will be described later, and then this is the previous value of the total reducing agent amount. Is added to the total reducing agent amount S_QRA.

  Next, the routine proceeds to step 7, where it is determined whether or not the total reducing agent amount S_QRA is equal to or greater than a predetermined end determination value SQRA1. When the determination result is NO, it is determined that the reduction of NOx in the NOx purification catalyst 12 has not been completed and the rich spike control should be continued, and the routine proceeds to step 8 where the slip value SLIP_CNOx ( Slip NOx parameter) is calculated.

  This slip value SLIP_CNOx represents the degree of occurrence of slip NOx, and its calculation process is specifically executed as shown in FIG. In this process, first, in step 20, it is determined whether or not the slip value calculated flag F_SLIP is “1”. If the determination result is NO and the slip value SLIP_CNOx has not been calculated, the process proceeds to step 21 to determine whether or not the previous value F_RICHZ of the rich condition flag is “0”.

  When this determination result is YES and the current loop is the first time when the execution condition of the rich spike control is switched from the state where it is not satisfied to the state where it is satisfied, in step 22, the NOx concentration CNOx detected by the NOx sensor 23 is After setting as the initial value ST_CNOx, the process proceeds to step 23. On the other hand, when the determination result of step 21 is NO, the process proceeds to step 23 as it is.

In step 23 following step 21 or 22, it is determined whether or not the following two conditions are both satisfied.
The current value CNOx of NOx concentration is larger than the previous value CNOxZ.
The current value CNOx of the NOx concentration is larger than the initial value ST_CNOx.

  If the decision result in the step 23 is YES and both the above two conditions are established, the process proceeds to a step 24, in which the NOx concentration CNOx is set as the maximum value MX_CNOx.

  Next, at step 25, it is determined whether or not the maximum value MX_CNOx calculated at step 24 is larger than the previous value MX_CNOxZ. When this determination result is NO, this process is terminated as it is.

  On the other hand, when the determination result in step 25 is YES, it is determined that the slip value SLIP_CNOx should be calculated, and the process proceeds to step 26 where the deviation (MX_CNOx−ST_CNOx) between the maximum value and the initial value is set as the slip value SLIP_CNOx. Set.

  Next, the routine proceeds to step 27, where the count value CT_SL of the slip value counter is set to a value (CT_SLZ + 1) obtained by adding the value 1 to the previous value CT_SLZ. That is, the count value CT_SL of the slip value counter is incremented by 1. Thereafter, this process is terminated.

  On the other hand, if the determination result in step 23 is NO and CNOx ≦ CNOxZ or CNOx ≦ ST_CNOx, it is determined in step 28 whether or not the count value CT_SL of the slip value counter is greater than or equal to a predetermined value CTSL1. When the determination result is NO, this process is terminated.

  On the other hand, when the determination result in step 28 is YES, it is determined that the number of calculations of the slip value SLIP_CNOx has reached a sufficient value, and the slip value SLIP_CNOx is appropriately calculated, the process proceeds to step 29, where CNOx <CNOxZ. Whether or not is established is determined. When the determination result is YES and the NOx concentration tends to decrease, the process proceeds to step 30 where the count value CT_DW of the decreasing tendency counter is set to a value (CT_DWZ + 1) obtained by adding the value 1 to the previous value CT_DWZ. That is, the count value CT_DW of the decreasing tendency counter is incremented by 1.

  Next, the routine proceeds to step 31, where it is determined whether or not the count value CT_DW of the downward tendency counter is equal to or greater than a predetermined value CTDW1. When the determination result is NO, this process is terminated. On the other hand, when the determination result is YES, it is determined that the slip NOx is in an end state, it is determined that the calculation of the slip value SLIP_CNOx should be ended, the process proceeds to step 32, and the slip value calculated flag is displayed to represent it. After F_SLIP is set to “1” and the count values CT_SL and CT_DW of the two counters are both reset to 0, this process is terminated.

  As described above, when the slip value calculated flag F_SLIP is set to “1” in step 32, the determination result in step 20 is YES in the loop after the next time, and in this case, the present process is ended as it is. .

Returning to FIG. 3, after the slip value SLIP_CNOx calculation process in step 8 is executed as described above, the process proceeds to step 9 to calculate the slip-through ratio R_SLTH (slip NOx ratio) . Specifically, the slip-through ratio R_SLTH calculation process is executed as shown in FIG.

  In this process, first, in step 40, it is determined whether or not the slip value calculated flag F_SLIP is “1”. When this determination result is NO, this process is terminated as it is. On the other hand, if the determination result is YES and the slip value SLIP_CNOx has been calculated, the process proceeds to step 41 to determine whether or not the slip-through ratio calculated flag F_RATIO is “1”.

  When the determination result is NO, it is determined that the slip-through ratio R_SLTH should be calculated, the process proceeds to step 42, and it is determined whether or not the total reducing agent amount S_QRA described above is equal to or greater than a predetermined through determination value SQRA2. Determine. The predetermined through determination value SQRA2 is a value for determining that the reduction of NOx in the NOx purification catalyst 12 is almost completed, and is set to a value that satisfies SQRA2 <SQRA1.

When the determination result of step 42 is NO, this process is ended as it is. On the other hand, when the determination result of step 42 is YES, it is determined that the reduction of NOx in the NOx purification catalyst 12 is almost finished, and the process proceeds to step 43, where the NOx concentration CNOx is set as the through value TH_CNOx. That is, through value TH_CNOx is, NOx reduction of the NOx purifying catalyst 12 has almost completed, Ru is calculated as a value when the NOx concentration CNOx became almost unchanged state. In the present embodiment, the through value TH_CNOx corresponds to the NOx concentration detected at the timing when it is estimated that the NOx reduction operation in the NOx purification catalyst has ended after the air-fuel ratio is switched to the rich side. .

  Thereafter, the process proceeds to step 44, where the slip-through ratio R_SLTH is set to a value obtained by dividing the slip value by the through value (SLIP_CNOx / TH_CNOx). Next, the process proceeds to step 45, where the slip-through ratio calculated flag F_RATIO is set to “1” in order to indicate that the slip-through ratio R_SLTH has been calculated, and then this process is terminated. As a result, the determination result in step 41 is YES in the loop after the next time, and in this case, the present process is terminated as it is.

  Returning to FIG. 3, after the slip-through ratio R_SLTH calculation process of step 9 is executed as described above, the process proceeds to step 10, and the calculation process of the air-fuel ratio correction coefficient K_NOx and the air-fuel ratio correction value AF_CORR is executed. These values K_NOx and AF_CORR are used for calculating the target air-fuel ratio AF_CMD and the low gain values KP2, KI2, and KD2 in the rich spike control process described later. Specifically, the calculation process is shown in FIG. Run as shown.

  As shown in the figure, first, at step 50, it is determined whether or not a slip-through ratio calculated flag F_RATIO is “1”. If the determination result is NO and the slip-through ratio R_SLTH has not been calculated, the present process is terminated as it is.

  On the other hand, if the determination result in step 50 is YES and the slip-through ratio R_SLTH is calculated, the process proceeds to step 51 to determine whether or not the correction coefficient calculated flag F_KNOx is “1”. If the determination result is NO, it is determined that the air-fuel ratio correction coefficient K_NOx should be calculated, the process proceeds to step 52, and the map shown in FIG. 7 is searched according to the slip value SLIP_CNOx and the slip-through ratio R_SLTH. Thus, the air-fuel ratio correction coefficient K_NOx is calculated.

  In the drawing, R_SLTHm represents a predetermined value of m (m is an integer of 2 or more) slip-through ratios R_SLTH, and these values are set so that R_SLTH1 <R_SLTH2 <...... <R_SLTHm is established. Has been. CNOxn represents a predetermined value of n (n is an integer of 2 or more) NOx concentrations, and these values are set so that CNOx1 <CNOx2 <... <CNOxn. The above setting is the same in FIG. 8 described later.

  In this map, the air-fuel ratio correction coefficient K_NOx is set to a value that satisfies 0 <K_NOx ≦ 1, and is set to a smaller value as the slip-through ratio R_SLTH is larger or the slip value SLIP_CNOx is larger. ing. This is due to the following reason. First, regarding the slip-through ratio R_SLTH, the greater the slip-through ratio R_SLTH, that is, the greater the ratio of the slip value SLIP_CNOx to the through value TH_CNOx, the greater the degree of influence that the slip NOx has on the exhaust gas characteristics. Accordingly, the change rate of the air-fuel ratio AF to the rich spike value AR_RICH in slip correction control, which will be described later, is set to a smaller value to more reliably suppress the generation of slip NOx.

  Further, regarding the slip value SLIP_CNOx, the change rate of the air-fuel ratio AF to the rich spike value AR_RICH is set to a smaller value so as to suppress the slip value SLIP_CNOx as the slip value SLIP_CNOx increases.

  In step 53 following step 52, a correction coefficient calculated flag F_KNOx is set to “1” to indicate that the air-fuel ratio correction coefficient K_NOx has been calculated. As a result, in the subsequent loop, the determination result in step 51 is YES, and in this case, the process proceeds to step 54.

  In step 54 following step 51 or 53, it is determined whether or not the slip corrected flag F_AFSLIP is “1”. The slip correction completed flag F_AFSLIP has its initial value set to “0” and, as will be described later, after the occurrence of slip NOx, in the rich spike control after the next time, the air / fuel ratio correction coefficient K_NOx or the air / fuel ratio correction value is set. It is set to “1” when slip correction control using AF_CORR is executed.

  When the determination result of step 54 is NO, this process is ended as it is. On the other hand, if the determination result in step 54 is YES and the slip correction control is being executed, but slip NOx occurs, it is determined that the air-fuel ratio correction value AF_CORR should be calculated, and the process proceeds to step 55. Then, it is determined whether or not the correction value calculated flag F_CORR is “1”. If the determination result is YES and the air-fuel ratio correction value AF_CORR has been calculated, the present process is terminated as it is.

  On the other hand, if the determination result in step 55 is NO and the air-fuel ratio correction value AF_CORR has not been calculated, the process proceeds to step 56 to search the map shown in FIG. 8 according to the slip value SLIP_CNOx and the slip-through ratio R_SLTH. Thus, the air-fuel ratio correction value AF_CORR is calculated.

  In this map, the air-fuel ratio correction value AF_CORR is set to a larger value as the slip-through ratio R_SLTH is larger or as the slip value SLIP_CNOx is larger. As described above, the larger the slip-through ratio R_SLTH, the greater the degree of influence of slip NOx on the exhaust gas characteristics, and accordingly, for the rich spike of the air-fuel ratio AF in slip correction control described later. This is because the generation of slip NOx is more reliably suppressed by setting the rate of change to the value AR_RICH to a smaller value. Further, as described above, the larger the slip value SLIP_CNOx is, the smaller the change rate of the air-fuel ratio AF to the rich spike value AR_RICH is set to a smaller value so as to suppress the slip value SLIP_CNOx.

  Next, the routine proceeds to step 57, where the correction value calculated flag F_CORR is set to “1” to indicate that the air-fuel ratio correction value AF_CORR has been calculated, and then this processing is terminated. As a result, the determination result in step 55 is YES in the next and subsequent loops, and in this case, the present process is terminated as it is.

  Returning to FIG. 3, in step 10, the calculation process of the air-fuel ratio correction coefficient K_NOx and the air-fuel ratio correction value AF_CORR is executed as described above, and then this process ends.

  On the other hand, when the determination result in step 7 is YES, it is determined that the NOx reduction in the NOx purification catalyst 12 is finished and the rich spike control should be finished, the process proceeds to step 11 and the total reducing agent amount S_QRA is set. In addition to resetting to 0, all three flags F_SLIP, F_RATIO, and F_CORR are reset to “0”. Next, in order to indicate that the rich spike control should be terminated, the rich condition flag F_RICH is set to “0” in step 12 described above, and then the present process is terminated.

  Next, the air-fuel ratio control process executed by the ECU 2 will be described with reference to FIG. As described below, this process controls the air-fuel ratio AF of the air-fuel mixture supplied into the cylinder 3a, and is executed at a predetermined control cycle (for example, the generation timing of the TDC signal).

  In this process, first, in step 60, it is determined whether or not the rich condition flag F_RICH described above is “1”. If the determination result is NO and the execution condition of the rich spike control is not satisfied, it is determined that the air-fuel ratio AF should be controlled to the lean side, and the routine proceeds to step 61 where the engine speed NE and the accelerator opening degree are determined. The required torque PMCMD is calculated by searching a map for lean control (not shown) according to the AP.

  In step 62 following step 61, the fuel injection amount QINJ is calculated by searching a map for lean control (not shown) according to the engine speed NE and the required torque PMCMD.

  Next, the routine proceeds to step 63, where a fuel injection end timing φINJ is calculated by searching a map for lean control (not shown) according to the engine speed NE and the fuel injection amount QINJ. As a result, fuel is injected from the fuel injection valve 4 into the cylinder 3a in accordance with the fuel injection amount QINJ and the fuel injection end timing φINJ calculated as described above.

  In step 64 following step 63, three flags F_AFRICH, F_AFMID1, and F_AFMID2 described later are all reset to “0”.

  Next, the routine proceeds to step 65 where a lean control process is executed. Specifically, as described below, throttle valve control processing, supercharging pressure control processing, swirl control processing, and EGR control processing are executed.

  First, in the throttle valve control process, the target throttle valve opening TH_CMD is set to a predetermined fully open value TH_WOT. Thereby, a control input corresponding to the target throttle valve opening TH_CMD is input to the TH actuator 8b, and as a result, the throttle valve 8a is controlled to be fully opened.

  In the supercharging pressure control process, a target opening VAN_CMD of the variable vane 7c is calculated by searching a map for lean control (not shown) according to the fuel injection amount QINJ. Thereby, a supercharging pressure is controlled by inputting a control input corresponding to the target opening VANE_CMD to the vane actuator 7d.

  Further, in the swirl control process, a target swirl opening SW_CMD of the swirl valve 9a is calculated by searching a map for lean control (not shown) according to the fuel injection amount QINJ. Thereby, the swirl is controlled by inputting a control input corresponding to the target swirl opening degree SW_CMD to the swirl actuator 9b.

  On the other hand, in the EGR control process, a target fresh air amount M_CMD is calculated by searching a lean control map (not shown) according to the fuel injection amount QINJ. Then, the EGR control valve 10b is controlled by a predetermined feedback control algorithm so that the fresh air amount M_ACT converges to the target fresh air amount M_CMD. As described above, after executing the lean control, the present process is terminated.

  On the other hand, if the determination result in step 60 is YES and the rich spike control execution condition is satisfied, the routine proceeds to step 66, where a map for rich spike control (not shown) is made according to the engine speed NE and the accelerator pedal opening AP. Is calculated to calculate the required torque PMCMD. In this rich spike control map, the required torque PMCMD is set to a larger value as the engine speed NE is higher or the accelerator pedal opening AP is larger.

  In step 67 following step 66, the fuel injection amount QINJ is calculated by searching a map for rich spike control (not shown) according to the engine speed NE and the required torque PMCMD. In this rich spike control map, the fuel injection amount QINJ is set to a larger value as the engine speed NE is higher or as the required torque PMCMD is larger.

  Next, the routine proceeds to step 68, where a fuel injection end timing φINJ is calculated by searching a map for rich spike control (not shown) according to the engine speed NE and the fuel injection amount QINJ. As a result, fuel is injected from the fuel injection valve 4 into the cylinder 3a in accordance with the fuel injection amount QINJ and the fuel injection end timing φINJ calculated as described above.

  In step 69 following step 68, as described below, a rich spike control process is executed to reduce NOx adsorbed on the NOx purification catalyst 12. Thereafter, this process is terminated.

  Next, the rich spike control process will be described with reference to FIG. As shown in the figure, in this process, first, in step 70, a target air-fuel ratio AF_CMD calculation process is executed. Specifically, the target air-fuel ratio AF_CMD calculation process is executed as shown in FIG.

  First, in step 80, it is determined whether or not the rich set flag F_AFRICH is “1”. When the determination result is NO, the process proceeds to step 81 to determine whether or not the correction coefficient calculated flag F_KNOx is “1”.

  When the determination result is NO, that is, when the air-fuel ratio correction coefficient K_NOx has not been calculated because the slip NOx has never occurred in the rich spike control before the previous time, the target air-fuel ratio AF_CMD is set in step 82. It is set to a predetermined rich spike value AF_RICH (for example, value 14). Next, the process proceeds to step 83 where the rich set flag F_AFRICH is set to “1” to indicate this, and then this process is terminated. As a result, in the loop after the next time, the determination result in step 80 is YES, and in this case, the present process is terminated as it is.

  On the other hand, when the determination result in step 81 is YES, that is, when the air-fuel ratio correction coefficient K_NOx is calculated by the occurrence of slip NOx in the rich spike control before the previous time, the process proceeds to step 84 and the slip corrected flag It is determined whether or not F_AFSLIP is “1”.

When the determination result is NO, it is determined that slip correction control using the air-fuel ratio correction coefficient K_NOx should be executed in the current rich spike control because slip NOx has occurred for the first time during the previous rich spike control. In step 85, the intermediate value AF_MID is calculated by the following equation (1). Note that AF_STO in the following equation (1) is a theoretical air-fuel ratio.
AF_MID = AF_STO− (AF_STO−AF_RICH) · K_NOx
...... (1)

  Next, the routine proceeds to step 86, where it is determined whether or not the air-fuel ratio AF is equal to or less than the intermediate value AF_MID. When the determination result is NO, the process proceeds to step 87 to determine whether or not the intermediate value set flag F_AFMID1 is “1”.

  When the determination result is NO, the process proceeds to step 88, and the target air-fuel ratio AF_CMD is set to the intermediate value AF_MID. Next, in step 89, the first intermediate value set flag F_AFMID1 is set to “1” to indicate this, and then the present process is terminated. As a result, the determination result in step 87 is YES in the next and subsequent loops, and in this case, the present process is terminated as it is.

  On the other hand, if the determination result in step 86 is YES and the air-fuel ratio AF has decreased to the intermediate value AF_MID or less, the routine proceeds to step 90 where the slip correction flag F_AFSLIP is set to “ Set to “1”.

  Next, the process proceeds to step 91, where the intermediate value AF_MID is set as the previous value AF_MIDZ. Next, as described above, after executing Steps 82 and 83, the present process is terminated.

  On the other hand, if the determination result in the step 84 is YES, that is, if the slip correction control has been executed in the rich spike control before the previous time, the process proceeds to a step 92 to determine whether or not the correction value calculated flag F_CORR is “1”. Is determined. If the determination result is NO and slip NOx has not occurred in the previous rich spike control, as described above, the process is terminated after step 86 and subsequent steps are executed.

  On the other hand, if the determination result in step 92 is YES, and slip NOx occurs in spite of the slip correction control being executed in the previous rich spike control, the process proceeds to step 93 and the second intermediate value set flag It is determined whether or not F_AFMID2 is “1”.

  When the determination result is NO, the process proceeds to step 94, where the intermediate value AF_MID is set to the sum of the previous value and the air-fuel ratio correction value AF_MIDZ + AF_CORR. Next, in order to represent this, in step 95, the second intermediate value set flag F_AFMID2 is set to "1". Thereby, in the loop after the next time, the determination result of step 93 becomes YES, and in this case, the process proceeds to step 96.

  In step 96 following step 93 or 95, it is determined whether or not the air-fuel ratio AF is equal to or less than the intermediate value AF_MID. When the determination result is NO, as described above, after step 87 and subsequent steps are executed, the present process is terminated.

  On the other hand, when the determination result of step 96 is YES, as described above, after executing steps 91, 82, and 83, the present process is terminated.

  Returning to FIG. 10, the target air-fuel ratio AF_CMD calculation process in step 70 is executed as described above, and then the process proceeds to step 71 to calculate the target fresh air amount M_CMD. Specifically, the target fresh air amount M_CMD is calculated by multiplying the fuel injection amount QINJ by the target air-fuel ratio AF_CMD.

  Next, the routine proceeds to step 72, where a target throttle valve opening TH_CMD is calculated by searching a map for rich spike control (not shown) according to the target fresh air amount M_CMD and the engine speed NE. As a result, a control input corresponding to the target throttle valve opening TH_CMD is input to the TH actuator 8b, and the throttle valve 8a is controlled to be throttled from the fully open state.

  In step 73 following step 72, the target opening degree VANE_CMD of the variable vane 7c is calculated by searching a map for rich spike control (not shown) according to the target fresh air amount M_CMD and the engine speed NE. Thereby, a supercharging pressure is controlled by inputting a control input corresponding to the target opening VANE_CMD to the vane actuator 7d.

  Next, in step 74, a target swirl opening SW_CMD of the swirl valve 9a is calculated by searching a map for rich spike control (not shown) according to the target fresh air amount M_CMD and the engine speed NE. Thereby, the swirl is controlled by inputting a control input corresponding to the target swirl opening degree SW_CMD to the swirl actuator 9b.

  Next, the process proceeds to step 75, and the target EGR amount EGR_CMD calculation process is executed as described below, and then the present process ends.

  Next, the calculation process of the target EGR amount EGR_CMD will be described with reference to FIG. As shown in the figure, in this process, first, in step 100, the basic value EGR_FF is calculated by searching a map (not shown) according to the target fresh air amount M_CMD and the engine speed NE.

  Next, proceeding to step 101, a correction value EGR_FB is calculated. Specifically, the calculation of the correction value EGR_FB is executed by a method to which a PID control algorithm is applied, as shown in FIG.

  First, in step 110, it is determined whether or not the first and second intermediate value set flags F_AFMID1 and F_AFMID2 described above are both “0”. When the determination result is YES, that is, when slip correction control is not executed in the current rich spike control, the routine proceeds to step 111 where the P-term gain KP is set to a predetermined high gain value KP1 and the I-term gain KI is set to a predetermined high gain. The D term gain KD is set to a predetermined high gain value KD1 for the value KI1. These three high gain values KP1, KI1, and KD1 are all set to positive predetermined values.

  On the other hand, when the determination result in step 110 is NO, that is, when slip correction control is being executed in the current rich spike control and the target air-fuel ratio AF_CMD is set to the intermediate value AF_MID, the routine proceeds to step 112 and rich setting has been completed. It is determined whether or not the flag F_AFRICH is “0”. When the determination result is YES, the above-described step 111 is executed.

  On the other hand, when the determination result in step 112 is NO, that is, when slip correction control is being executed and the target air-fuel ratio AF_CMD is set to the rich spike value AF_RICH, the routine proceeds to step 113, where the rich spike value and the empty spike value are set. It is determined whether or not the absolute value | AF_RICH-AF | of the deviation from the fuel ratio is smaller than a predetermined value DAF1. The predetermined value DAF1 is set to a positive value close to the value 0.

  If the determination result is NO and the air-fuel ratio AF has not converged to the vicinity of the rich spike value AF_RICH, the routine proceeds to step 114, and the three low gain values KP2, KI2, and KD2 are calculated by the method described below.

  First, when F_KNOx = 1 & F_AFSLIP = 0, that is, when slip NOx occurs in the state where slip correction control is not executed in the previous rich spike control, three unillustrated values are selected according to the air-fuel ratio correction coefficient K_NOx. By searching the map, three low gain values KP2, KI2, and KD2 are respectively calculated.

  In these maps, the three low gain values KP2, KI2, and KD2 are set to values considerably smaller than the three high gain values KP1, KI1, and KD1, respectively, and the smaller the air-fuel ratio correction coefficient K_NOx, that is, It is set to be a smaller value as the generation degree of slip NOx is larger. This is to suppress the generation of slip NOx by controlling the air-fuel ratio AF so that the change speed thereof becomes a smaller value in the slip correction control as the generation degree of slip NOx is larger.

  In addition, when F_KNOx = 1 & F_AFSLIP = 1 & F_CORR = 0, that is, when slip NOx does not occur in the state in which slip correction control is being executed in the previous rich spike control, it is calculated by the previous slip correction control. The three low gain values KP2, KI2, and KD2 are used as they are.

  Further, when F_KNOx = 1 & F_AFSLIP = 1 & F_CORR = 1, that is, when slip NOx occurs in spite of executing slip correction control in the previous rich spike control, according to the air-fuel ratio correction value AF_CORR, By searching three maps (not shown), three low gain values KP2, KI2, and KD2 are respectively calculated.

  In these maps, the three low gain values KP2, KI2, and KD2 are set to values considerably smaller than the three high gain values KP1, KI1, and KD1, respectively, and the larger the air-fuel ratio correction value AF_CORR, that is, It is set to be a smaller value as the generation degree of slip NOx is larger. As described above, as the degree of occurrence of slip NOx increases, the slip correction control is performed so that the change rate of the air-fuel ratio AF becomes smaller in the slip correction control, thereby suppressing the occurrence of slip NOx. Because.

  Next, the routine proceeds to step 115, where the P-term gain KP is set to the low gain value KP2, the I-term gain KI is set to the low gain value KI2, and the D-term gain KD is set to the low gain value KD2.

  On the other hand, when the determination result in step 113 is YES and AF≈AF_RICH, step 111 described above is executed.

  In step 116 following step 111 or 115, the fresh air amount deviation DM and the I-term integral value SUMI stored in the RAM are respectively set as the previous value DMZ of the new air quantity deviation and the previous value SUMIZ of the I-term integral value. Set. Thereafter, in step 117, the fresh air amount deviation DM is set to a deviation (M_ACT-M_CMD) between the fresh air amount and the target fresh air amount.

  Next, in step 118, the proportional term FBP is set to the product KP · DM of the P term gain and the fresh air quantity deviation. Thereafter, the process proceeds to step 119, where the integral term FBI is set to the product KI · DM of the I term gain and the fresh air amount deviation.

  Next, the routine proceeds to step 120, where the I-term integral value SUMI is set to the sum of its previous value and integral term SUMIZ + FBI. Thereafter, in step 121, the differential term FBD is set to a value KD (DM-DMZ) obtained by multiplying the deviation between the current value of the fresh air amount deviation and the previous value by the D term gain.

  Next, the process proceeds to step 122, where the correction value EGR_FB is set to the sum of the proportional term, the I-term integral value, and the derivative term (FBP + SUMI + FBD), and then the present process is terminated.

  As described above, in the calculation process of the correction value EGR_FB, during the slip correction control, when the target air-fuel ratio AF_CMD is set to the intermediate value AF_MID, the three feedback gains KP, KI, KD are set to the predetermined high gain value KP1, Since KI1 and KD1 are set, the air-fuel ratio AF can be quickly converged to the intermediate value AF_MID. Further, when the target air-fuel ratio AF_CMD is switched from the intermediate value AF_MID to the rich spike value AF_RICH, the three feedback gains KP, KI, KD are low gain values KP2, KI2, which are considerably smaller than the high gain values KP1, KI1, KD1. Since these low gain values KP2, KI2, and KD2 are calculated according to the air-fuel ratio correction coefficient K_NOx or the air-fuel ratio correction value AF_CORR, the air-fuel ratio AF is set according to the degree of occurrence of slip NOx. It is possible to gently converge to the rich spike value AF_RICH.

  In particular, since the air-fuel ratio AF is controlled so that the change rate of the air-fuel ratio AF becomes smaller as the generation degree of slip NOx becomes larger, the rich-spike value AF_RICH is changed from the value during lean control to the air-fuel ratio AF. The generation of slip NOx can be more reliably suppressed as compared with the case where the change is made at once. Further, after the air-fuel ratio AF has converged to the rich spike value AF_RICH, the three feedback gains KP, KI, KD are set to the predetermined high gain values KP1, KI1, KD1, so the air-fuel ratio AF is set to the rich spike value. It is possible to reliably hold the AF_RICH.

  Returning to FIG. 12, after the calculation process of the correction value EGR_FB is executed in step 101 as described above, the process proceeds to step 102, where the target EGR amount EGR_CMD is set to the sum of the basic value and the correction value (EGR_FF + EGR_FB).

  Next, in step 103, it is determined whether or not the target EGR amount EGR_CMD is equal to or smaller than a predetermined lower limit value EGR_LIMIT_L. When the determination result is NO, the process proceeds to step 104 to determine whether or not the target EGR amount EGR_CMD is equal to or greater than a predetermined upper limit value EGR_LIMIT_H. If this determination result is NO and EGR_LIMIT_L <EGR_CMD <EGR_LIMIT_H, this processing is ended as it is.

  On the other hand, if the determination result in step 104 is YES and EGR_CMD ≧ EGR_LIMIT_H, the process proceeds to step 105, the target EGR amount EGR_CMD is set to a predetermined upper limit value EGR_LIMIT_H, and then this process is terminated.

  On the other hand, if the determination result in step 103 is YES and EGR_CMD ≦ EGR_LIMIT_L, the process proceeds to step 106, the target EGR amount EGR_CMD is set to a predetermined lower limit value EGR_LIMIT_L, and then this process ends. As described above, by calculating the target EGR amount EGR_CMD, feedback control is performed so that the fresh air amount M_ACT converges to the target fresh air amount M_CMD. As a result, feedback control is performed so that the air-fuel ratio AF converges to the target air-fuel ratio AF_CMD.

  Next, a control result when the air-fuel ratio control process of the present embodiment as described above is executed will be described with reference to FIGS. FIG. 14 shows an example of the result of air-fuel ratio control, and FIG. 15 is an enlarged view of a part of the NOx concentration CNOx curve in FIG. 14 for easy understanding.

  In the control result example shown in FIG. 14, when the execution condition of rich spike control is satisfied and F_RICH = 1 is satisfied (time t1), F_KNOx = 0, and the determination results of steps 80 and 81 are both NO. As a result, the target air-fuel ratio AF_CMD is set to the rich spike value AF_RICH. Thus, the air-fuel ratio AF is controlled so as to quickly converge to the rich spike value AF_RICH by the PID control algorithm using the high gain values KP1, KI1, and KD1. That is, rich spike control is executed.

  After the start of the rich spike control, the NOx concentration CNOx continues to increase from the initial value ST_NOx until time t2, and decreases after time t2. That is, the occurrence of slip NOx causes the NOx concentration CNOx at time t2 to be set as the maximum value MX_CNOx. Then, when S_QRA ≧ S_QRA2 is satisfied at time t3 and the determination result in step 42 is YES, a through value TH_CNOx is calculated (step 43), and then a slip-through ratio R_SLTH is calculated (step 44). In addition, the air-fuel ratio correction coefficient K_NOx is calculated, and the correction coefficient calculated flag F_KNOx is set to “1” (steps 52 and 53).

  Next, when S_QRA ≧ S_QRA1 is satisfied at time t4 and the determination result in step 7 is YES, the rich condition flag F_RICH is set to “0”, thereby switching from rich spike control to lean control.

  During the lean control, when the execution condition of the rich spike control is satisfied and F_RICH = 1 is satisfied (time t5), F_KNOx = 1 is satisfied, and the determination result in step 81 is YES, Thereafter, slip correction control is executed. That is, the intermediate value AF_MID is calculated based on the air / fuel ratio correction coefficient K_NOx, the target air / fuel ratio AF_CMD is set to the intermediate value AF_MID (steps 85 and 88), and the air / fuel ratio AF is set to the target air / fuel ratio AF_CMD by the PID control algorithm. Controlled to converge.

  At that time, until the target air-fuel ratio AF_CMD is set to the rich spike value AF_RICH, F_AFMID1 = 1 & F_AFRICH = 0 is established, so that the three feedback gains KP, KI, KD have predetermined high gain values KP1, KI1. , KD1 (steps 112 and 111). As a result, the air-fuel ratio AF is controlled so as to converge quickly to the intermediate value AF_MID, and after AF ≦ AF_MID is established, F_AFRICH = 1, so that the three feedback gains KP, KI, KD are empty. The low gain values KP2, KI2, and KD2 calculated according to the fuel ratio correction coefficient K_NOx are set (steps 112, 114, and 115). Thereby, the air-fuel ratio AF is controlled so as to converge gradually to the rich spike value AF_RICH.

  Thereafter, at the time when AF≈AF_RICH (time t6), the three feedback gains KP, KI, KD are set to predetermined high gain values KP1, KI1, KD1 (steps 113, 111), whereby the air-fuel ratio AF Is held near the rich spike value AF_RICH. Next, at time t7, when S_QRA ≧ S_QRA1 is satisfied and the determination result in step 7 is YES, F_RICH = 0 is satisfied, whereby the rich spike control is switched to the lean control.

  As described above, according to the air-fuel ratio control apparatus 1 of the present embodiment, when the slip NOx occurs during the rich spike control, the slip value SLIP_CNOx and the slip-through ratio R_SLTH are calculated, and these slip value SLIP_CNOx and slip The air-fuel ratio correction coefficient K_NOx is calculated according to the through ratio R_SLTH, and the slip correction control is executed during the next rich spike control.

  In this slip correction control, first, the target air-fuel ratio AF_CMD is set to the intermediate value AF_MID, and the feedback gains KP, KI, and KD are set to predetermined high gain values KP1, KI1, and KD1, respectively. Is controlled to converge quickly to the intermediate value AF_MID. After the air-fuel ratio AF converges to the intermediate value AF_MID, the target air-fuel ratio AF_CMD is set to the rich spike value AF_RICH, and the feedback gains KP, KI, KD are higher than the high gain values KP1, KI1, KD1, respectively. By setting the gain values KP2, KI2, and KD2 to be very small, the air-fuel ratio AF is controlled so as to gradually converge to the rich spike value AF_RICH. As described above, when the air-fuel ratio AF is switched from the lean side to the rich side, the air-fuel ratio AF is controlled so that the change speed is switched in two stages, so that the air-fuel ratio AF changes at a stretch to the rich spike value AF_RICH. Unlike the case where the air-fuel ratio AF is performed, the NOx trapping ability of the NOx purification catalyst can be recovered while suppressing the generation of slip NOx because the air-fuel ratio AF can be finely controlled so that the change speed thereof becomes smaller as a whole.

  In other words, slip NOx can be suppressed only by changing the change rate of the air-fuel ratio AF when the rich spike control is executed. Therefore, unlike the conventional case where the execution time of the lean control is changed, even when the NOx purification catalyst 12 deteriorates. Thus, it is possible to avoid an increase in the execution frequency of the rich spike control, and to improve the fuel efficiency accordingly.

  Further, since both the slip value SLIP_CNOx and the slip-through ratio R_SLTH used for calculating the intermediate value AF_MID are calculated based only on the NOx concentration in the exhaust gas downstream of the NOx purification catalyst 12, the downstream side of the NOx purification catalyst Unlike the conventional case of using the deviation between the NOx concentration of NOx and the upstream NOx concentration, the NOx purification ability of the exhaust gas containing unburned fuel as the reducing agent and the NOx purification catalyst 12 affects the determination result of the slip NOx. Can be avoided, and the determination accuracy of the degree of occurrence of slip NOx can be improved. As a result, the exhaust gas characteristics can be improved by more reliably suppressing the slip NOx.

  Further, as the slip value SLIP_CNOx is larger, the air-fuel ratio correction coefficient K_NOx is set to a smaller value, whereby the intermediate value AF_MID is calculated as a value on the theoretical air-fuel ratio side, and the low gain values KP2, KI2, and KD2 are Calculated as a smaller value. As a result, the air-fuel ratio AF is controlled such that the change rate of the air-fuel ratio AF from the start of the rich spike control until reaching the rich spike value AF_RICH becomes smaller as the degree of occurrence of slip NOx increases. Generation of slip NOx can be more reliably suppressed.

  In addition to this, the slip value SLIP_CNOx is calculated as a deviation between the maximum value MX_CNOx of the NOx concentration detected after the start of the rich spike control and the initial value ST_CNOx of the NOx concentration detected at the start of switching. Thus, unlike the conventional case where the deviation between the NOx concentration on the downstream side of the NOx purification catalyst and the NOx concentration on the upstream side is used, the NOx purification capability of the exhaust gas containing unburned fuel as the reducing agent and the NOx purification catalyst The influence on the determination of NOx can be avoided, and the determination accuracy of the degree of occurrence of slip NOx can be improved.

  Further, since the air-fuel ratio correction coefficient K_NOx is calculated according to the slip-through ratio R_SLTH in addition to the slip value SLIP_CNOx, the NOx exhausted from the NOx purification catalyst 12 at the point when NOx reduction is almost completed. The change rate of the air-fuel ratio AF can be determined while reflecting the concentration CNOx, that is, the operating state of the engine 3. Thereby, generation | occurrence | production of slip NOx can be suppressed further reliably. For the same reason, the rate of change of the air-fuel ratio AF can be determined while taking into consideration the effect on the exhaust gas characteristics of NOx discharged from the NOx purification catalyst 12 when the reduction of NOx is almost completed. Thereby, the exhaust gas characteristics can be further improved.

  In addition to this, when slip NOx occurs due to deterioration of the NOx purification catalyst 12 in spite of executing the slip correction control during the rich spike control, the slip value SLIP_CNOx and the slip- An air-fuel ratio correction value AF_CORR is calculated according to the through ratio R_SLTH, and the intermediate value AF_MID is a value closer to the theoretical air-fuel ratio side by using this air-fuel ratio correction value AF_CORR at the time of slip correction control in the next rich spike control. And the low gain values KP2, KI2, and KD2 are calculated in accordance with the air-fuel ratio correction value AF_CORR. As a result, the greater the degree of occurrence of slip NOx, the smaller the speed of change of the air-fuel ratio AF from the start of the rich spike control until the rich spike value AF_RICH is reduced in the next slip correction control. Since the fuel ratio AF is controlled, the generation of slip NOx can be more reliably suppressed.

  Further, since the air-fuel ratio correction value AF_CORR is calculated according to the slip value SLIP_CNOx and the slip-through ratio R_SLTH in the same manner as the air-fuel ratio correction coefficient K_NOx, the same as described above when using the air-fuel ratio correction coefficient K_NOx. An effect can be obtained.

  In the embodiment, in the slip correction control, the air-fuel ratio AF is controlled so that the change speed from the intermediate value AF_MID to the rich spike value AF_RICH is controlled so that the change speed from the intermediate value AF_MID becomes smaller. However, the air-fuel ratio control method for suppressing the slip NOx in the present invention is not limited to this, and by controlling the air-fuel ratio AF so that the changing speed is switched at a plurality of stages. The control method may be any control method that can control the change speed to the rich spike value AF_RICH to be smaller as a whole, thereby suppressing the slip NOx.

  For example, as shown in FIG. 16, in the slip correction control, the air-fuel ratio AF is controlled so that the changing speed to the intermediate value AF_MID is relatively small, and the change from the intermediate value AF_MID to the rich spike value AF_RICH is controlled. You may control so that speed may become larger. Further, as shown in FIG. 17, control may be performed so that the change speed of the air-fuel ratio AF changes in three stages. Further, as shown in FIG. 18, the air-fuel ratio AF may be controlled so as to change in a stepped manner in multiple steps, so that the changing speed to the rich spike value AF_RICH may be reduced as a whole.

  The embodiment is an example in which the air-fuel ratio AF is feedback controlled during the slip correction control, but the control algorithm used when controlling the air-fuel ratio in order to suppress the slip NOx in the present invention is not limited to this. Needless to say. For example, from the value during lean control to the intermediate value AF_MID, the air-fuel ratio AF is quickly controlled by the feedforward control algorithm, and thereafter, the air-fuel ratio AF is gradually reduced to the rich spike value AF_RICH by the feedback control algorithm. You may control so that it may converge.

  On the other hand, contrary to the above, from the value during lean control to the intermediate value AF_MID, the air-fuel ratio AF is quickly controlled by the feedback control algorithm, and thereafter, it is gradually reduced to the rich spike value AF_RICH by the feedforward control algorithm. You may control to. In this case, the target air-fuel ratio AF_CMD may be set so as to gradually decrease from the intermediate value AF_MID to the rich spike value AF_RICH. Further, the air-fuel ratio AF may be controlled from a value during lean control to a rich spike value AF_RICH by a feedforward control algorithm. Also in this case, the target air-fuel ratio AF_CMD may be set so as to gradually decrease from the intermediate value AF_MID to the rich spike value AF_RICH.

  The embodiment is an example in which the slip value SLIP_CNOx as the slip NOx parameter is calculated as a deviation between the maximum value MX_CNOx of the NOx concentration and the initial value ST_CNOx of the NOx concentration detected at the start of switching. The slip NOx parameter is not limited to this, and any parameter that represents the degree of occurrence of slip NOx may be used.

  For example, the average value of the NOx concentration CNOx from the start of the rich spike control to the predetermined timing after the start (for example, the timing at time t2, t3, or t4 in FIG. 14 in the embodiment) may be used as the slip NOx parameter. Good. In such a configuration, the following effects can be obtained. In other words, when rich spike control is executed, the state of occurrence of slip NOx after the start of control is generally the case where the NOx concentration changes so as to show a temporary peak. When the amount of NOx is small, the NOx concentration repeatedly increases and decreases, and various change patterns exist. Therefore, as described above, by using the average value of the NOx concentration CNOx as the slip NOx parameter, the slip NOx parameter can be set to the slip NOx without being affected by any change pattern of the NOx concentration CNOx. Can be calculated as a value that appropriately represents the degree of occurrence of slip NOx, thereby further improving the accuracy of determining the degree of occurrence of slip NOx.

  Further, as the slip NOx parameter, the total amount discharged from the NOx purification catalyst 12 from the start of the rich spike control to the predetermined timing after the start (for example, the timing at time t2, t3 or t4 in FIG. 14 of the embodiment). A NOx amount may be used. For example, when this predetermined timing is the timing at time t4 in FIG. 14, the total NOx amount is the average of the space velocity of exhaust gas in the NOx purification catalyst 12 and the NOx concentration CNOx between time t1 and time t4 in FIG. And based on the value. In such a configuration, as described above, the slip NOx parameter is calculated as a value that appropriately represents the degree of occurrence of slip NOx without being influenced by any change pattern of the NOx concentration CNOx. This can further improve the accuracy of determining the degree of occurrence of slip NOx.

  Further, the embodiment determines the change rate of the air-fuel ratio AF by searching the map of FIG. 7 according to the slip value SLIP_CNOx as the slip NOx parameter and the slip-through ratio R_SLTH as the operation state parameter. In this example, the air-fuel ratio correction coefficient K_NOx is calculated. However, the method of determining the change rate of the air-fuel ratio AF according to the present invention is not limited to this, and in addition to the slip NOx parameter, the operating state of the engine 3 during the rich spike control is determined. Any method may be used as long as it determines the change rate of the air-fuel ratio AF in accordance with the represented operating condition parameter.

  For example, instead of the slip-through ratio R_SLTH, an integrated value of the space velocity of the exhaust gas supplied to the NOx purification catalyst 12 between time t1 and time t4 in FIG. 14 may be used as the operating state parameter. That is, the air-fuel ratio correction coefficient K_NOx may be calculated by searching the map according to the integrated value of the space velocity and the slip value SLIP_CNOx. When configured in this manner, the change rate of the air-fuel ratio AF can be determined while reflecting the total amount of NOx discharged from the NOx purification catalyst 12 between time t1 and time t4 in FIG. Thereby, the exhaust gas characteristics can be further improved.

  In the embodiment, when the air-fuel ratio AF is feedback controlled from the intermediate value AF_MID to the rich spike value AF_RICH in the slip correction control, the feedback gains KP, KI, and KD are respectively set to the air-fuel ratio correction coefficient K_NOx or the air-fuel ratio correction. In this example, the low gain values KP2, KI2, and KD2 searched for the map according to the value AF_CORR are set. However, the feedback gains KP, KI, and KD may be set to predetermined values, respectively.

  Further, in the embodiment, when slip NOx occurs during the execution of slip correction control using the air-fuel ratio correction coefficient K_NOx, the air-fuel ratio correction value AF_CORR is calculated and used to change the air-fuel ratio AF to the change rate thereof. However, when slip NOx occurs during execution of slip correction control using the air-fuel ratio correction coefficient K_NOx, the air-fuel ratio correction coefficient is determined according to the degree of occurrence of slip NOx. By correcting K_NOx itself, the air-fuel ratio AF may be controlled so that the changing speed becomes a smaller value.

  On the other hand, the embodiment is an example in which the through value TH_CNOx used for calculating the slip-through ratio R_SLTH is calculated as the NOx concentration CNOx at the timing when S_QRA ≧ SQRA2 is established. The through value TH_CNOx is calculated after the rich spike control is started. Further, the NOx concentration CNOx detected at a timing when a predetermined time has elapsed may be calculated, or a value estimated according to the operating state of the engine 3 at the timing (for example, the space velocity of exhaust gas in the NOx purification catalyst 12). May be calculated as

  Further, in the embodiment, when slip NOx occurs during execution of rich spike control, control is performed so that the change speed of the air-fuel ratio AF is switched in two stages at the time of slip correction control in the next rich spike control. However, the air-fuel ratio control method of the present invention is not limited to this, and any method may be used as long as it determines the rate of change of the air-fuel ratio from the lean side to the rich side according to the slip NOx parameter. For example, during the slip correction control in the rich spike control, the control is performed so that the change speed of the air-fuel ratio AF from the lean side to the rich side becomes a constant gradient, and the gradient is set to the slip value SLIP_CNOx as the slip NOx parameter. It may be determined accordingly.

1 is a diagram schematically showing a schematic configuration of an internal combustion engine to which an air-fuel ratio control apparatus according to an embodiment of the present invention is applied. It is a block diagram which shows schematic structure of an air fuel ratio control apparatus. It is a flowchart which shows the execution condition determination process of rich spike control. It is a flowchart which shows the calculation process of slip value SLIP_CNOx. It is a flowchart which shows the calculation process of slip through ratio RATIO. It is a flowchart which shows the calculation process of the air fuel ratio correction coefficient K_NOx and the air fuel ratio correction value AF_CORR. It is a figure which shows an example of the map used for calculation of the air fuel ratio correction coefficient K_NOx. It is a figure which shows an example of the map used for calculation of the air fuel ratio correction value AF_CORR. It is a flowchart which shows an air fuel ratio control process. It is a flowchart which shows a rich spike control process. It is a flowchart which shows the calculation process of target air fuel ratio AF_CMD. It is a flowchart which shows the calculation process of the target EGR amount EGR_CMD. It is a flowchart which shows the calculation process of correction value EGR_FB. It is a timing chart which shows the example of a result of air fuel ratio control. It is the figure which expanded a part of timing chart of FIG. It is a timing chart which shows the control result of the modification of slip correction control processing. It is a timing chart which shows the control result of the other modification of slip correction control processing. It is a timing chart which shows the control result of the other modification of slip correction control processing.

Explanation of symbols

1 Air-fuel ratio control device
2 ECU (air-fuel ratio control means, slip NOx parameter calculation means)
3 Internal combustion engine
12 NOx purification catalyst
23 NOx sensor (NOx concentration detection means)
AF air-fuel ratio
NOx concentration in exhaust gas downstream of CNOx NOx purification catalyst ST_CNOx initial value (NOx concentration detected at start of switching)
MX_CNOx maximum value (maximum value of NOx concentration detected after switching)
TH_CNOx through value (after switching to the rich side air-fuel ratio, the detected NOx concentration at the time that is estimated to NOx reduction operation that put into the NOx purification catalyst is completed)
SLIP_CNOx Slip value (slip NOx parameter)
R_SLTH Slip-through ratio ( slip NOx ratio )

Claims (6)

An air-fuel ratio control apparatus for an internal combustion engine comprising a NOx purification catalyst that captures NOx in exhaust gas in a lean atmosphere and reduces the captured NOx when exhaust gas in a rich atmosphere is supplied,
NOx concentration detection means for detecting the NOx concentration in the exhaust gas downstream of the NOx purification catalyst;
When the rich control condition for supplying exhaust gas in a rich atmosphere to the NOx purification catalyst is satisfied, the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine is switched from the lean side to the rich side with respect to the stoichiometric air-fuel ratio. Air-fuel ratio control means to control;
Slip NOx parameter calculating means for calculating a slip NOx parameter representing the degree of occurrence of slip NOx according to the NOx concentration detected after the air-fuel ratio control means switches to the rich side of the air-fuel ratio of the air-fuel ratio;
With
The air-fuel ratio control means is detected at the timing when the calculated slip NOx parameter and the NOx reduction operation in the NOx purification catalyst after the rich switching of the air-fuel ratio is estimated to have ended. An air-fuel ratio control apparatus for an internal combustion engine, wherein a change speed of the air-fuel ratio when switching from a lean side to a rich side is determined according to a slip NOx ratio that is a ratio to a NOx concentration and the slip NOx parameter . 2. The internal combustion engine according to claim 1, wherein the air-fuel ratio control unit determines the change speed of the air-fuel ratio to be switched in a plurality of stages according to the slip NOx ratio and the slip NOx parameter. Air-fuel ratio control device.   The air-fuel ratio control means determines the air-fuel ratio change rate so as to be smaller as the degree of occurrence of slip NOx represented by the slip NOx parameter is larger. An air-fuel ratio control apparatus for an internal combustion engine as described.   The slip NOx parameter calculating means calculates the slip NOx parameter as a deviation between a maximum value of the NOx concentration detected after switching to the rich side of the air-fuel ratio and a NOx concentration detected at the start of switching. The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 3,   4. The slip NOx parameter calculating means calculates the slip NOx parameter as an average value of NOx concentration from a start of switching to the rich side of the air-fuel ratio to a predetermined timing after switching. An air-fuel ratio control apparatus for an internal combustion engine according to any one of the above.   The slip NOx parameter calculating means calculates the slip NOx parameter as a total NOx amount discharged from the NOx purification catalyst between a start of switching to the rich side of the air-fuel ratio and a predetermined timing after switching. The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 3.

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