JP2006250065A - Air-fuel ratio control device - Google Patents

Abstract

Dispersion of detection values of an air-fuel ratio sensor is suppressed and accuracy of air-fuel ratio control is improved.
SOLUTION: A catalyst 5 having an oxygen storage capability for taking in or releasing oxygen in an exhaust gas in accordance with an exhaust air / fuel ratio of an engine 1 and an upstream air / fuel ratio sensor 8 for detecting an exhaust air / fuel ratio upstream of the catalyst 5. A downstream air-fuel ratio sensor 9 that detects a downstream exhaust air-fuel ratio, an oxygen storage amount estimation means 11 that estimates an oxygen storage amount of the catalyst 5 based on the output of the upstream air-fuel ratio sensor 8, and an oxygen storage amount The element temperature of the upstream air-fuel ratio sensor 8 is controlled on the basis of the means 11 for controlling the exhaust air-fuel ratio of the engine so that becomes the target value, the estimated value of the oxygen storage amount and the output of the downstream air-fuel ratio sensor 9. Upstream air-fuel ratio sensor element temperature control means 11.
[Selection] Figure 1

Description

  The present invention relates to engine air-fuel ratio control, and more particularly to control for improving detection accuracy of a sensor that detects an air-fuel ratio.

  In order to simultaneously purify HC, CO, and NOx in the engine exhaust with a three-way catalyst, the catalyst atmosphere needs to be the stoichiometric air-fuel ratio (hereinafter referred to as stoichiometric), and when the air-fuel ratio deviates even slightly from stoichiometric. The catalyst is provided with an oxygen storage capacity so that the purification rate does not decrease.

  As a result, when exhaust that is leaner than stoichiometric is given, the catalyst takes in oxygen in the exhaust, and the catalyst atmosphere can be maintained stoichiometric until the oxygen storage amount is saturated. Further, when exhaust richer than stoichiometric is given, oxygen retained by the catalyst is released, and the catalyst atmosphere can be maintained stoichiometric until all of the retained oxygen is released. In this way, the catalyst compensates for the excess or deficiency of oxygen resulting from a temporary air-fuel ratio shift, and the catalyst atmosphere can be kept substantially stoichiometric.

  In this case, if the air-fuel ratio is controlled so that the oxygen storage amount of the catalyst always becomes a target value, for example, about half of the maximum storage amount, the catalyst intake and release capacities are equalized, and the rich from the air-fuel ratio stoichiometry, Absorption capacity is enhanced against fluctuations on either side of the lean, and the exhaust purification efficiency can be kept at its best.

  For this reason, the oxygen storage amount of the catalyst is obtained by integrating the oxygen excess / deficiency amount (converted from the air / fuel ratio) of the exhaust gas flowing into the catalyst based on the output of the air / fuel ratio sensor, and this oxygen storage amount matches the target value. Thus, a method for feedback control of the air-fuel ratio is known.

  By the way, the output of the air-fuel ratio sensor installed on the upstream side of the catalyst may cause an error (shift to rich or lean side) due to deterioration, quality variation at the time of manufacture, or the like.

  If an error occurs in the detected air-fuel ratio, the oxygen storage amount of the catalyst cannot be accurately calculated based on the output of the air-fuel ratio sensor, and as a result, the oxygen storage amount of the catalyst cannot be controlled as a target value. There is a problem that the purification efficiency of exhaust gas decreases.

In order to solve this problem, in Patent Document 1, it is determined whether the output of the downstream air-fuel ratio sensor has become the lean side or the rich side for a certain time or more, and the downstream air-fuel ratio has become the lean side or the rich side for a certain time or more. When it becomes, the output of the upstream air-fuel ratio sensor is corrected based on the output of the downstream air-fuel ratio sensor.
JP 2001-234789 A

  By the way, when the oxygen concentration sensor is used to detect the oxygen concentration, the output voltage (slice level) corresponding to the air-fuel ratio at the time of stoichiometry between the lean output voltage and the rich output voltage of the oxygen concentration sensor. Is set as a threshold value for reversing the air-fuel ratio (changing the fuel injection amount). That is, the control point for reversing the air-fuel ratio is when the output voltage reaches the slice level in each of the lean-rich and rich-lean changes. This slice level is normally a fixed value, but it is also known that the slice level is variably set according to the operating state.

  However, the output voltage level of the oxygen concentration sensor has a characteristic of decreasing as the element temperature increases.

  Accordingly, when the slice level is a fixed value, a shift occurs in the position of the control point between the lean output voltage and the rich output voltage due to a change in the element temperature. Even when the output voltage characteristic is set variably in accordance with the operating state, the output voltage characteristic has a slight inclination near the stoichiometric range, so that the control point is shifted similarly.

  This control point deviation causes the air-fuel ratio to be reversed in a lean or rich state, so that the estimation accuracy of the oxygen storage amount decreases, and as a result, the oxygen storage of the catalyst becomes the target value. It becomes impossible to control the exhaust gas, and the exhaust purification efficiency decreases.

  In the control described in Patent Document 1, as described above, the output value of the upstream oxygen concentration sensor is corrected based on the output value of the downstream oxygen concentration sensor, but changes in the element temperature are taken into consideration. Absent.

  Therefore, even if the deviation of the control point due to deterioration or quality error of the oxygen concentration sensor is eliminated by correcting the output value of the upstream oxygen concentration sensor, it is necessary to perform correction again when the element temperature changes after that. In addition, since correction is not performed until the downstream oxygen concentration sensor detects a shift to the lean or rich side for a certain period of time or longer, the air-fuel ratio band (hereinafter referred to as the catalyst window) with the optimum purification performance of the catalyst. ), And the exhaust gas purification efficiency decreases as a result. This is the same even when the slice level is a fixed value or is variably set according to the operating state.

  Therefore, an object of the present invention is to control the air-fuel ratio with high accuracy by suppressing variations in detection accuracy of the air-fuel ratio sensor.

  An air-fuel ratio control apparatus according to the present invention includes a catalyst having an oxygen storage capability of taking in or releasing oxygen in exhaust gas in accordance with an exhaust air-fuel ratio of an engine, and an upstream air-fuel ratio that detects an exhaust air-fuel ratio upstream of the catalyst. A fuel ratio sensor; a downstream air-fuel ratio sensor that detects an exhaust air-fuel ratio downstream of the catalyst; an oxygen storage amount estimation means that estimates an oxygen storage amount of the catalyst based on an output of the upstream air-fuel ratio sensor; Means for controlling the exhaust air-fuel ratio of the engine so that the oxygen storage amount becomes a target value, element temperature adjusting means for adjusting the element temperature of the upstream air-fuel ratio sensor, the estimated value of the oxygen storage amount, and the downstream side The element temperature adjusting means is controlled based on the output of the air-fuel ratio sensor.

  According to the present invention, by controlling the element temperature of the upstream air-fuel ratio sensor, it is possible to suppress changes in the element temperature that cause variations in the detected value of the sensor, so the air-fuel ratio detection accuracy is improved. In addition, highly accurate air-fuel ratio control becomes possible.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic diagram showing the configuration of a system to which this embodiment is applied.

  1 is an engine (internal combustion engine), 2 is an intake manifold that supplies intake air to the engine 1, 3 is an exhaust manifold that collects exhaust from the engine 1, 4 is an exhaust passage that is connected downstream of the exhaust manifold 3 and discharges exhaust to the atmosphere (Hereinafter, the exhaust manifold 3 and the exhaust passage 4 are collectively referred to as an exhaust passage 4).

  The intake manifold 2 is provided with a fuel injection valve 6 and an atmospheric pressure sensor 7.

  The exhaust passage 4 is provided with an upstream air-fuel ratio sensor 8 that detects the oxygen concentration of the exhaust, and an exhaust temperature sensor 10 that detects the exhaust temperature. In addition, an EGR pipe 14 that recirculates a part of the exhaust gas as EGR gas to the intake passage 2 is provided, and the EGR pipe 14 is provided with an EGR valve 12 that adjusts the flow rate of the EGR gas.

  The upstream air-fuel ratio sensor 8 is provided with a heater as element temperature adjusting means, and a control unit 11 described later controls the heater as upstream air-fuel ratio sensor element temperature control means.

  A catalyst 5 for purifying exhaust gas is interposed in the exhaust passage 4, and a downstream air-fuel ratio sensor 9 for detecting the oxygen concentration of the exhaust gas that has passed through the catalyst 5 is provided downstream of the catalyst 5.

  A crank angle sensor 13 for detecting the rotation speed of the crankshaft is provided in the vicinity of the crankshaft (not shown) of the engine 1.

  Detection values of the upstream air-fuel ratio sensor 8, the downstream air-fuel ratio sensor 9, the atmospheric pressure sensor 7, the exhaust temperature sensor 10, the crank angle sensor 13, and the like are read into the control unit 11.

  The control unit 11 calculates the engine speed based on the detection value of the crank angle sensor 13, and determines the fuel injection amount from the engine speed and the intake air amount detected by an air flow meter (not shown). Further, the air-fuel ratio upstream of the catalyst 5 and the air-fuel ratio downstream of the catalyst 5 are calculated from the detected values of the upstream air-fuel ratio sensor 8 and the downstream air-fuel ratio sensor 9, respectively, and the above-mentioned air-fuel ratio on the upstream side and downstream side are calculated based on the above-mentioned air-fuel ratio. The fuel injection amount is corrected by air-fuel ratio feedback. The element temperature of the upstream air-fuel ratio sensor 8 is also read.

  FIG. 2 shows output characteristics of oxygen concentration sensors used as the upstream air-fuel ratio sensor 8 and the downstream air-fuel ratio sensor 9. As shown in FIG. 2, the sensor output has a substantially constant high electromotive force level in the vicinity of rich to stoichiometric, the electromotive force level rapidly decreases in the vicinity of stoichiometric, and a substantially constant low electromotive force level in the vicinity of stoichiometric to lean. Become. That is, the binary output is richer and leaner than stoichiometric. Then, an output voltage (slice level) corresponding to the stoichiometric air-fuel ratio is set between the lean output voltage and the rich output voltage of the oxygen concentration sensor, and the air-fuel ratio is inverted (the fuel injection amount is changed). Threshold).

  FIG. 3 is a diagram for explaining general air-fuel ratio feedback control in the case where an oxygen concentration sensor is used, and specifically shows the relationship between the output of the oxygen concentration sensor and a correction signal for the fuel injection amount.

  As described above, since the output of the oxygen concentration sensor is a binary output indicating whether the air-fuel ratio is richer or leaner than the stoichiometric value, the air-fuel ratio control is basically ON-OFF control, and the output of the oxygen concentration sensor is shown in FIG. As shown, the control point is moved up and down.

  In addition, since the air-fuel ratio feedback control is generally performed by PI control, when the fuel injection amount reaches the control point, it first changes stepwise by P (proportional component) and then by I (integral component) a predetermined amount. It changes with inclination. For example, when the fuel injection amount is increased when the control point is reached, the air-fuel ratio gradually changes in a rich direction and reaches the control point again. This time, the fuel injection amount is controlled to be reduced. Since this process is repeated, the fuel injection amount goes back and forth between lean and rich with respect to the stoichiometry as shown in FIG.

  By the way, the oxygen concentration sensor has a characteristic of shifting in the direction of the low electromotive force level with the above output characteristic when the element temperature increases.

  Therefore, when the slice level is fixed, the control point shifts to the rich side when the element temperature rises. This is because the output characteristic of the oxygen concentration sensor is not a perfect step change, but has a slope near the stoichiometry, so that the output characteristic diagram in FIG. This is because the relative position of the slice level between the output at the rich time and the output at the lean time becomes higher than that in the state where the element temperature is low.

  When this control point shift occurs, when the air-fuel ratio changes from rich to stoichiometric, the fuel injection amount is reduced even though the air-fuel ratio is still rich. The estimation accuracy of the oxygen storage amount will decrease.

  Therefore, in this embodiment, in order to eliminate the control point shift caused by the change in the element temperature, the heater is used for the element temperature itself based on the output of the downstream air-fuel ratio sensor 9, that is, the actual air-fuel ratio of the exhaust gas. Control.

  Specifically, the control shown in FIG. 4 is performed. Hereinafter, description will be given according to each step of FIG.

  In step S1, it is determined whether the upstream air-fuel ratio sensor 8 has been activated. If not activated, the determination is repeated until activated, and if activated, the process proceeds to step S2. The reference temperature for this activation determination is set to about 350 degrees, for example.

  In step S2, the element temperature of the upstream air-fuel ratio sensor 8 is controlled to a standard temperature, for example, approximately 600 degrees using a heater. For controlling the element temperature, a so-called element resistance feedback method or a duty map method according to the operating state is used.

  In step S3, it is determined whether or not the exhaust gas temperature is lower than the element temperature set in step S2.

  If the exhaust gas temperature is higher, the process proceeds to step S4, the heater duty is set to a fixed value, and steps S3 and S4 are repeated until the element temperature becomes higher.

  If the element temperature becomes higher in step S3, the process proceeds to step S5, and the downstream air-fuel ratio sensor 9 is inverted from rich to lean or from lean to rich when the oxygen storage amount limit value of the catalyst 5 is exceeded. It is determined whether or not The oxygen storage amount of the catalyst 5 can be obtained by integrating the amount of oxygen absorbed or released into the catalyst 5 from the intake air amount and the output of the upstream air-fuel ratio sensor 8 at that time, and the limit value is the catalyst used. It is preset according to the capacity of 5.

  If not reversed, the process proceeds to step S6. If reversed, the process proceeds to step S7.

  In step S6, the difference (QA integrated value excess) between the oxygen storage amount (QA integrated value) and the oxygen storage amount limit value at t1 is calculated, and the element temperature correction amount is determined by searching the correction amount table shown in FIG. The element temperature of the upstream air-fuel ratio sensor 8 is lowered by setting and stopping the heater.

  In the correction amount table of FIG. 5, the element temperature correction amount increases as the value of the excess QA integrated value increases.

  In step S7, before the limit value of the oxygen storage amount of the catalyst 5 is exceeded, it is determined whether or not the downstream air-fuel ratio sensor 9 has been reversed from rich to lean or from lean to rich.

  If reversed, the process proceeds to step S8, the difference (QA integrated value surplus) between the oxygen storage amount and the oxygen storage amount limit value when reversed is calculated, and the element is obtained by searching the correction amount table shown in FIG. The temperature correction amount is set and the heater is activated to increase the element temperature of the upstream air-fuel ratio sensor 8.

  If not reversed, the process returns to step S2.

  If the element temperature of the upstream air-fuel ratio sensor 8 is changed in steps S6 and S8, the process proceeds to step S9 to determine whether or not the operation of the engine 1 has stopped. If the engine 1 does not stop, the process returns to step S5. If stopped, the process is terminated.

  The flow from step S5 to step S6 and from step S7 to step S8 will be described in detail.

  If the output of the downstream air-fuel ratio sensor 9 does not reverse even though the QA integrated value exceeds the oxygen storage capacity limit value in step S5, the oxygen storage amount calculated from the intake air amount and the output of the upstream air-fuel ratio sensor 8 Although the limit value has been reached, the actual storage amount has not reached the limit value. Since the downstream air-fuel ratio sensor 9 detects the actual air-fuel ratio of the exhaust gas, the difference between the calculated oxygen storage amount and the actual value causes the output of the upstream air-fuel ratio sensor 8 to shift. That is, the upstream air-fuel ratio sensor 8 detects that the air-fuel ratio is richer than the actual air-fuel ratio.

  The deviation of the control point will be described with reference to FIG. 9 in which the vicinity of the stoichiometry in the output characteristic diagram of FIG. 2 is enlarged. FrS in FIG. 9 is an output characteristic at the reference element temperature of the upstream air-fuel ratio sensor 8. Based on this output characteristic, an output approximately halfway between the rich output and the lean output is set as a slice level. The intersection of the level and FrS is used as a control point. FrH represents output characteristics in a state where the element temperature is higher than that of FrS, and FrL represents output characteristics in a state where the element temperature is lower than that of FrS.

  The state in which the upstream air-fuel ratio sensor 8 detects that the air-fuel ratio is richer than the actual air-fuel ratio is a state represented by FrH in FIG. In other words, although the output between the rich time and the output at the lean time is ContB in the figure, the intersection with the slice level, that is, the control point is ContA, and Δ ( This is a state shifted by (ContB-ContA). Therefore, the element temperature is decreased by the element temperature correction amount TMPHOSL obtained from the correction amount table of FIG. 5 in step S6 so as to eliminate the control point deviation Δ (ContB−ContA).

  If the output of the downstream air-fuel ratio sensor 9 reverses before the QA integrated value reaches the oxygen storage capacity limit value in step S7, the upstream air-fuel ratio sensor 8 is less than the actual air-fuel ratio, contrary to step S5. It is detected to be lean.

  This is the state of FrL in FIG. 9, and although the middle of the rich output and the lean output is ContC in the figure, the intersection with the slice level, that is, the control point is ContD, and ContD In this state, Δ (ContD−ContC) is shifted to the output side during lean. Therefore, in step S8, the element temperature is increased by the element temperature correction amount TMPHOSH obtained from the correction amount table of FIG.

  Here, the effect of correcting the deviation of the control point by controlling the element temperature of the upstream air-fuel ratio sensor 8 will be described in comparison with a conventionally known method of correcting the sensor output itself.

  For example, when the output of the upstream air-fuel ratio sensor 8 is corrected based on the output of the downstream air-fuel ratio sensor 9, the output value of ContA is set to the output of ContB while the output characteristic remains FrH in the state of FrH in FIG. It corrects so that it may recognize as a value. In this method, since the change in the element temperature is not taken into consideration, if the output value changes due to the change in the element temperature after correcting the output value, the correction must be performed again. Therefore, the frequency of correction, that is, the frequency of deviating from the purification performance window increases, and the exhaust performance deteriorates.

  On the other hand, in the present embodiment, the element temperature itself is controlled, so that the factor that causes the deviation of the control point itself is suppressed. Therefore, the frequency of deviating from the exhaust performance window of the catalyst 5 is reduced, and as a result, deterioration of exhaust performance due to a shift in the control point can be suppressed. Further, by suppressing the factor that causes the control point shift, the air-fuel ratio detection accuracy is improved, and appropriate air-fuel ratio control can be performed.

  Although the technology for variably controlling the slice level according to the operating state is also known, even if the slice level is variably controlled, the element temperature remains the cause of the control point deviation, As described above, there is a problem that the correction frequency increases and the exhaust performance deteriorates.

  Next, FIGS. 7 and 8 show time charts regarding changes in the output signal of the downstream air-fuel ratio sensor 9 and changes in the element temperature when the control of FIG. 4 is performed.

  FIG. 7 shows a case where the downstream air-fuel ratio sensor 9 does not reverse from rich to lean even when the oxygen storage amount limit value is exceeded.

  The QA integrated value increases with time, and the element temperature of the upstream air-fuel ratio sensor 8 is kept constant at approximately 700 degrees. When the oxygen storage amount limit value is exceeded at t1, the output of the downstream air-fuel ratio sensor 9 does not reverse from rich to lean but remains rich. At t2, the output of the downstream air-fuel ratio sensor 9 is reversed lean. The difference between the QA integrated value and the oxygen storage amount limit value at t1 is an excess amount of the QA integrated value. Using this, the correction amount table of FIG. 5 is searched to set the element temperature correction amount, and at t1, the upstream side The element temperature of the air-fuel ratio sensor 8 is lowered by a correction amount (here, approximately 100 degrees). Thereafter, this control is repeated.

  FIG. 8 shows a case where the downstream side air-fuel ratio sensor 9 reverses from rich to lean before reaching the oxygen storage amount limit value.

  The QA integrated value increases with time, and the element temperature of the upstream air-fuel ratio sensor 8 is kept constant at approximately 600 degrees. The output of the downstream air-fuel ratio sensor 9 is reversed from rich to lean at t1, but the QA integrated value does not reach the oxygen storage amount limit value. The surplus with respect to the oxygen storage amount limit value of the QA integrated value up to t1 is the QA integrated value surplus, and the device temperature correction amount is set by searching the correction amount table of FIG. The element temperature of the side air-fuel ratio sensor 8 is increased by a correction amount (here, approximately 50 degrees).

  At t2, the output of the downstream side air-fuel ratio sensor 9 is reversed from lean to rich. At this time, due to the effect of the element temperature correction at t1, the QA integrated value coincides with the oxygen storage amount limit value, so that the surplus QA integrated value is zero. Therefore, the element temperature of the upstream air-fuel ratio sensor 8 is not corrected here.

  At t3, the output of the downstream air-fuel ratio sensor 9 is reversed from rich to lean. However, the control point shift occurs due to factors such as changes in the operating state, and the QA integrated value becomes the oxygen storage amount limit value as in t1. Not reached. Accordingly, the QA integrated value surplus up to t3 is obtained in the same manner as t1, and the element temperature correction amount is set by searching the correction amount table of FIG. 6 using this, and the element temperature of the upstream air-fuel ratio sensor 8 is corrected. Increase by an amount (approximately 50 degrees here). This control is repeated after t3.

  As described above, in the present embodiment, the output of the upstream air-fuel ratio sensor 9 is corrected based on the estimated value of the oxygen storage amount and the output of the downstream air-fuel ratio sensor 9, and the element temperature of the upstream air-fuel ratio sensor 8 is controlled. Therefore, it is possible to detect the air-fuel ratio with high accuracy by suppressing variations in sensor output due to changes in element temperature.

  If the output of the downstream air-fuel ratio sensor 9 does not reverse from rich to lean or lean to rich even if the estimated value of the oxygen storage amount exceeds a preset limit value, the element temperature of the upstream air-fuel ratio sensor 8 is determined. When the output of the downstream air-fuel ratio sensor 9 is changed from rich to lean or from lean to rich before the estimated value of the oxygen storage amount exceeds the preset limit value, the upstream side empty is changed. Since the element temperature of the fuel ratio sensor 8 is changed to a high temperature side by a predetermined amount, the deviation of the output value of the upstream air fuel ratio sensor 8 can be accurately detected.

  Since the predetermined amount is set according to the difference between the estimated value of the oxygen storage amount and the preset limit value, an appropriate correction amount can be set according to the deviation of the output value.

  Note that the present invention is also applicable when a so-called A / F sensor is used in place of the oxygen sensor as a sensor for detecting the air-fuel ratio.

  The A / F sensor can detect the air-fuel ratio from rich to lean, unlike an oxygen concentration sensor that provides binary output of rich and lean. Then, as shown in FIG. 13, the inclination changes according to the change in the element temperature, and the control point shifts due to the change in the inclination. However, by controlling the element temperature as in the present embodiment, The deviation can be corrected.

  A second embodiment will be described.

  Since the system configuration of the present embodiment and the element temperature control of the upstream air-fuel ratio sensor 8 are the same as those of the first embodiment, the description thereof is omitted.

  However, in addition to the upstream air-fuel ratio sensor 8, the element temperature of the downstream air-fuel ratio sensor 9 is also controlled.

  Specifically, the control is performed according to the flowchart shown in FIG. Steps S1 to S8 in FIG. 10 are the same as those in FIG.

  After the element temperature of the upstream side air-fuel ratio sensor 8 is changed in step S6 or S8, it is determined in step S10 whether or not the operation region (EGR region) in which EGR is performed.

  In step S11, the element temperature of the downstream air-fuel ratio sensor 9 is controlled according to the EGR amount.

  In step S12, it is determined whether or not the engine 1 has stopped. If it has not stopped, the process returns to step S5, and if it has stopped, the process ends.

  Here, a control method will be described with reference to FIG. FIG. 11 is a map showing the relationship between the output level of the downstream air-fuel ratio sensor 9 and the residual ratio of the exhaust gas component. The vertical axis is the residual ratio of the exhaust gas component, and the horizontal axis is the output of the downstream air-fuel ratio sensor 9. is there. The solid line in the figure is the residual rate in the reference EGR amount.

  From FIG. 11, it can be seen that the remaining ratio of the exhaust gas changes by changing the slice level. Specifically, when the slice level is increased, the NOx residual rate decreases and the HC and CO residual rates increase. Conversely, when the slice level is lowered, the NOx residual rate increases and the HC and CO residual rates decrease. Note that the slice level at the reference time is set to an output in which the NOx remaining rate is equal to the HC and CO remaining rates.

  A dotted line 1 represents the remaining rate when the flow rate of EGR gas is increased, and a dotted line 2 represents the remaining rate when the flow rate of EGR gas is decreased.

  The flow rate of the EGR gas is determined by operating conditions such as the engine speed and the fuel injection amount, and is set by searching for a map that has been mapped beforehand.

  For example, when the flow rate of EGR gas increases, the amount of NOx contained in the exhaust gas before flowing into the catalyst 5 decreases, but conversely, rich gases such as HC and CO increase. That is, the remaining rate balance changes according to the EGR gas flow rate, and as shown in FIG. 11, the point where the remaining rates of NOx, HC, and CO are equal shifts to the low output side, and when the EGR gas flow rate decreases. Shift to higher output.

  Therefore, in order to improve the accuracy of air-fuel ratio control, it is necessary to correct the slice level to a level at which the remaining ratios of NOx, HC, and CO are equal.

  However, even if the slice level itself is variably controlled, there is a problem that the air-fuel ratio detected due to variations in element temperature varies as described above.

  Further, as described in the first embodiment, when the element temperature is changed, it is possible to obtain the same effect as changing the slice level, and to directly control the element temperature which is a factor of variation in the air-fuel ratio detection output. Can suppress the cause of variation.

  Therefore, in the present embodiment, the element temperature of the downstream air-fuel ratio sensor 9 is controlled instead of changing the slice level. Since the relationship between the EGR flow rate and the exhaust gas residual ratio balance and the relationship between the element temperature and the change amount of the sensor output can be obtained by experiments and simulations, if the EGR amount is set from the operating state, the element is based on the EGR amount. The amount of change in temperature can be set. Specifically, a table of EGR gas flow rate and element temperature as shown in FIG. 12 is prepared, and the element temperature is set by searching this table. In the table of FIG. 12, the set element temperature TMPRRO decreases as the EGR flow rate EGRVOL increases.

  As described above, when the EGR gas flow rate is set instead of the feedback control according to the change in the exhaust gas remaining rate, the deviation of the control point caused by the EGR gas flow rate is calculated to change the element temperature in advance. Let That is, conventionally, the residual ratio balance of NOx, HC, and CO is detected and feedback control based on the balance is performed, whereas in the present embodiment, feedforward control is performed.

  Therefore, the frequency of deviating from the catalyst purification performance window is reduced, and as a result, the air-fuel ratio detection accuracy is improved, and appropriate air-fuel ratio control can be performed.

  As described above, in this embodiment, in addition to the same effects as those of the first embodiment, the EGR device is further provided, the EGR gas amount is set according to the operating state of the engine, and the downstream air-fuel ratio is set according to the set EGR gas amount. Since the element temperature of the sensor 9 is changed by a predetermined amount, the deviation of the control point of the downstream air-fuel ratio sensor 9 can be corrected even when the remaining ratio of the exhaust gas changes according to the EGR gas amount.

  The predetermined amount that is the amount of change in the element temperature of the downstream side air-fuel ratio sensor 9 is set based on the amount of change in the remaining ratio of the exhaust gas that changes according to the amount of EGR gas. Control point deviation can be set optimally.

  The present invention is not limited to the above-described embodiments, and it goes without saying that various modifications can be made within the scope of the technical idea described in the claims.

  The present invention is applicable to engine air-fuel ratio control.

It is a figure showing the structure of the system of 1st Embodiment. It is an output characteristic figure of an oxygen concentration sensor. It is a figure for demonstrating air fuel ratio feedback control. It is a control flowchart of a 1st embodiment. It is an element temperature correction amount table (for excessive QA integrated value). It is an element temperature correction amount table (for QA integrated value surplus). It is a time chart when the control of 1st Embodiment is performed (when rich-lean does not invert even if a QA integrated value exceeds an oxygen storage amount limit value). It is a time chart when the control of the first embodiment is executed (when the rich-lean is reversed before the QA integrated value exceeds the oxygen storage amount limit value). It is an enlarged view of the output characteristic figure of an oxygen concentration sensor. It is a control flowchart of a 2nd embodiment. It is a figure showing the relationship between the output level of a downstream air-fuel-ratio sensor, and an exhaust-gas residual rate. It is a setting element temperature table for setting the element temperature according to an EGR gas flow rate. It is an output characteristic figure of an A / F sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine 2 Intake manifold 3 Exhaust manifold 4 Exhaust passage 5 Catalyst 6 Fuel injection valve 7 Atmospheric pressure sensor 8 Upstream air-fuel ratio sensor 9 Downstream air-fuel ratio sensor 10 Exhaust temperature sensor 11 Control unit 12 EGR valve 13 Crank angle sensor

Claims (5)

A catalyst having an oxygen storage capacity for taking in or releasing oxygen in the exhaust gas according to the exhaust air-fuel ratio of the engine;
An upstream air-fuel ratio sensor for detecting an exhaust air-fuel ratio upstream of the catalyst;
A downstream air-fuel ratio sensor for detecting an exhaust air-fuel ratio downstream of the catalyst;
Oxygen storage amount estimation means for estimating the oxygen storage amount of the catalyst based on the output of the upstream air-fuel ratio sensor;
Means for controlling the exhaust air-fuel ratio of the engine so that the oxygen storage amount becomes a target value;
Element temperature adjusting means for adjusting the element temperature of the upstream air-fuel ratio sensor;
An air-fuel ratio control device comprising upstream air-fuel ratio sensor element temperature control means for controlling the element temperature adjusting means based on the estimated value of the oxygen storage amount and the output of the downstream air-fuel ratio sensor . If the estimated value of the oxygen storage amount exceeds a preset limit value, the element temperature control means determines that the downstream side air-fuel ratio sensor output does not reverse from rich to lean or from lean to rich. Change the element temperature of the air-fuel ratio sensor low by a predetermined change amount,
If the output of the downstream air-fuel ratio sensor is reversed from rich to lean or lean to rich before the estimated value of the oxygen storage amount exceeds a preset limit value, the element temperature of the upstream air-fuel ratio sensor is changed. The air-fuel ratio control apparatus according to claim 1, wherein the air-fuel ratio control apparatus is changed higher by a predetermined change amount.   The air-fuel ratio control device according to claim 2, wherein the element temperature control means sets the predetermined change amount according to a difference between the estimated value of the oxygen storage amount and the limit value. An EGR device that recirculates a portion of the exhaust gas as EGR gas to the intake passage;
An operating state detecting means for detecting the operating state of the engine;
EGR gas amount setting means for setting the amount of the EGR gas to be recirculated according to the operating state;
The downstream air-fuel ratio sensor element temperature control means for changing the element temperature of the downstream air-fuel ratio sensor by a predetermined amount in accordance with the EGR gas amount set by the EGR gas amount setting means. The air-fuel ratio control apparatus according to any one of the above.   5. The air-fuel ratio control apparatus according to claim 4, wherein the downstream air-fuel ratio sensor element temperature control means sets the predetermined amount based on a change amount of an exhaust gas remaining rate balance that changes in accordance with the EGR gas amount.

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