Method and device for controlling oxygen reserve of three-way catalyst

Abstract

A three-way catalyst (15) is provided in an exhaust passage (14) of an internal combustion engine (1), and the air-fuel ratio is feedback-controlled so that the oxygen storage amount of the three-way catalyst (15) is maintained at a target oxygen storage amount. An estimated oxygen storage amount (b1) is obtained on the basis of the exhaust air-fuel ratio and the intake air amount detected by the air-fuel ratio sensor (19), and the target air-fuel ratio is set on the basis of the difference between the target oxygen storage amount (b2) and the estimated oxygen storage amount (b1). The target oxygen storage amount (b2) is set on the basis of the intake air amount corresponding to the flow rate of the gas flowing into the three-way catalyst (15) in such a manner that the larger the intake air amount is, the lower the target oxygen storage amount (b2) is. As a result, an increase in the NOx escape rate when the gas flow rate is large is cancelled out.

Description

Technical Field

[0001] The present invention relates to a control method and device for appropriately controlling the oxygen storage amount of a three-way catalyst provided in an exhaust passage of an internal combustion engine. Background Art

[0002] The three-way catalyst can be used to oxidize CO and HC in exhaust gas and reduce NOx, but in order to achieve both the oxidation and reduction based on the catalytic action at a high level, the so-called oxygen storage capacity of the catalyst for storing and releasing oxygen is important. Therefore, the following technology is known, that is, monitoring the oxygen storage amount of the three-way catalyst and variably controlling the target air-fuel ratio in the air-fuel ratio feedback control so that the oxygen storage amount is maintained at an intermediate target oxygen storage amount (for example, 50%).

[0003] Patent Document 1 discloses a scheme in which, based on the load and rotation speed of the internal combustion engine, the target oxygen storage amount is set to be smaller under operating conditions where NOx emissions increase, and the target oxygen storage amount is set to be larger under operating conditions where CO and HC emissions increase.

[0004] According to the new insights of the inventors of the present invention, the optimal target oxygen storage capacity is related to the gas flow rate flowing into the three-way catalyst. If the gas flow rate flowing in the three-way catalyst is large, the flow rate of the gas passing through the catalyst layer of the three-way catalyst increases, and the slip rate (proportion of NOx passing through without conversion) increases. Therefore, it is preferred that the oxygen storage capacity of the three-way catalyst is set lower as the gas flow rate increases.

[0005] Patent Document 1 does not disclose such control of the oxygen storage amount in relation to the gas flow rate.

[0006] Patent Document 1: Japanese Patent Application Publication No. 2000-008921 Summary of the invention

[0007] The present invention is a method for controlling the oxygen storage amount of a three-way catalyst, wherein the air-fuel ratio of an internal combustion engine having a three-way catalyst in an exhaust passage is feedback-controlled in a manner such that the air-fuel ratio is a target air-fuel ratio close to the theoretical air-fuel ratio, and the target air-fuel ratio is controlled in such a manner that the oxygen storage amount of the three-way catalyst reaches the target oxygen storage amount.

[0008] The target oxygen storage amount is set according to the flow rate of gas flowing into the three-way catalyst so that the target oxygen storage amount becomes smaller as the gas flow rate increases.

[0009] By setting the target oxygen storage amount to a lower level as the flow rate of gas flowing into the three-way catalyst increases, an increase in the escape rate of NOx due to an increase in the gas flow rate is offset, thereby achieving more reliable purification of NOx. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 It is a diagram for explaining the structure of an internal combustion engine having an embodiment of a three-way catalyst.

[0011] Figure 2 It is an explanatory diagram showing a control flow of one embodiment.

[0012] Figure 3 It is a characteristic diagram showing the characteristic of the target oxygen storage amount with respect to the intake air amount.

[0013] Figure 4 1 is a time chart showing an example of changes in (a) the intake air amount, (b) the oxygen storage amount, (c) the target air-fuel ratio, and (d) the actual air-fuel ratio. DETAILED DESCRIPTION

[0014] Hereinafter, one embodiment of the present invention will be described in detail based on the drawings. Figure 1 1 is an explanatory diagram showing a schematic structure of an internal combustion engine 1 to which an embodiment of the present invention is applied. The internal combustion engine 1 of one embodiment is a spark ignition type internal combustion engine (so-called gasoline internal combustion engine) of a four-stroke cycle, and has an intake valve 2, an exhaust valve 3, and a spark plug 4 in each cylinder. In addition, the illustrated example is configured as a direct-injection type internal combustion engine, and a fuel injection valve 5 that injects fuel into the cylinder is, for example, arranged on the side of the intake valve 2. In addition, a port injection type structure that injects fuel into the intake port 6 may also be used.

[0015] An electronically controlled throttle valve 10 whose opening is controlled by a control signal from an engine controller 9 is mounted on the upstream side of a manifold section 8 of an intake passage 7 connected to an intake port 6 of each cylinder. An air flow meter 11 for detecting the amount of intake air is arranged on the upstream side of the throttle valve 10, and an air filter 12 is arranged further upstream.

[0016] The exhaust ports 13 of the cylinders are joined as one exhaust passage 14, and a three-way catalyst 15 for purifying exhaust gas is provided in the exhaust passage 14. The three-way catalyst 15 is, for example, a so-called monolithic ceramic catalyst in which a catalyst layer including a catalyst metal is coated on the surface of a monolithic ceramic body having fine passages. In addition, the three-way catalyst 15 is a structure including a plurality of catalysts (for example, a manifold catalyst and an underfloor catalyst) arranged in series.

[0017] An air-fuel ratio sensor 19 for detecting the exhaust air-fuel ratio is arranged at the inlet side of the three-way catalyst 15 of the exhaust passage 14, i.e., at a position upstream of the three-way catalyst 15. The air-fuel ratio sensor 19 is a so-called wide-range air-fuel ratio sensor that can obtain an output corresponding to the exhaust air-fuel ratio. In addition, in order to calibrate the air-fuel ratio feedback control system including the air-fuel ratio sensor 19, diagnose the deterioration of the three-way catalyst 15, etc., a second air-fuel ratio sensor such as an O2 sensor may be provided on the downstream side of the three-way catalyst 15.

[0018] The detection signals of the air-fuel ratio sensor 19 and the air flow meter 11 are input to the engine controller 9. The detection signals of a plurality of sensors such as the crank angle sensor 21 for detecting the rotation speed of the internal combustion engine, the water temperature sensor 22 for detecting the cooling water temperature, and the accelerator opening sensor 23 for detecting the amount of depression of the accelerator pedal operated by the driver are also input to the engine controller 9. Based on the above input signals, the engine controller 9 controls the fuel injection amount and injection timing by the fuel injection valve 5, the ignition timing by the spark plug 4, the opening of the throttle valve 10, etc. to be optimal.

[0019] As one of the various controls of the internal combustion engine 1, the engine controller 9 performs air-fuel ratio control for optimizing the exhaust purification performance based on the three-way catalyst 15. The air-fuel ratio control is a control in which the fuel injection amount is controlled by feedback control (for example, PID control) in a manner along a target air-fuel ratio near the theoretical air-fuel ratio based on the exhaust air-fuel ratio detected by the air-fuel ratio sensor 19. Here, the target air-fuel ratio is controlled in a manner such that the oxygen storage amount of the three-way catalyst 15 estimated from the exhaust air-fuel ratio reaches the target oxygen storage amount.

[0020] Figure 2This is an explanatory diagram showing the flow of the air-fuel ratio control based on the oxygen storage amount in the form of a flowchart. In the processing represented as steps S1, S2, and S4, the intake air amount detected by the air flow meter 11 is input as a parameter equivalent to the gas flow rate flowing into the three-way catalyst 15. In addition, the "intake air amount" does not refer to the air amount per cycle of the cylinder, but refers to the flow rate of air per unit time sucked into the internal combustion engine 1 (i.e., passing through the air flow meter 11). In addition, in the processing represented as steps S1 and S5, the exhaust air-fuel ratio (catalyst inlet air-fuel ratio) detected by the air-fuel ratio sensor 19 is input. In step S1, the oxygen storage amount of the three-way catalyst 15 is estimated based on the exhaust air-fuel ratio detected by the air-fuel ratio sensor 19 and the gas flow rate flowing into the three-way catalyst 15, i.e., the intake air amount. This estimation is performed by performing addition and subtraction operations of the oxygen storage amount based on the exhaust air-fuel ratio at that time in each calculation cycle of the engine controller 9. In short, if the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 15 is lean, the oxygen storage amount increases, and if it is rich, the oxygen storage amount decreases, so the oxygen storage amount at that time is estimated by integrating in both positive and negative directions. Hereinafter, this is referred to as "estimated oxygen storage amount".

[0021] In step S2 , the target oxygen storage amount is set based on the gas flow rate that flows into the three-way catalyst 15 , that is, the intake air amount. Figure 3 The characteristic of the target oxygen storage amount relative to the intake air amount is given to the engine controller 9 in the form of a table. Figure 3 The characteristic shown in FIG. 1 outputs the target oxygen storage amount relative to the intake air amount as input. Figure 3 As shown in FIG. 1 , the target oxygen storage amount has a characteristic that the larger the intake air amount, the smaller the value. More specifically, as the correlation between the intake air amount and the target oxygen storage amount, the target oxygen storage amount decreases sharply with an increase in the intake air amount in an area where the intake air amount is relatively small, and increases sharply with an increase in the intake air amount in an area where the intake air amount is relatively large. Figure 3 The target oxygen storage capacity decreases slowly with respect to the increase in the intake air amount.

[0022] In step S3, the target oxygen storage amount obtained in step S2 and the estimated oxygen storage amount obtained in step S1 are compared to obtain a difference therebetween.

[0023] This difference becomes one of the inputs of the processing of step S4. In step S4, the target air-fuel ratio (target air-fuel ratio at the inlet side of the three-way catalyst 15) is calculated using the difference and the gas flow rate, i.e., the intake air amount, flowing into the three-way catalyst 15. For example, if the estimated oxygen storage amount is greater than the target oxygen storage amount, the target air-fuel ratio is controlled to be richer than the theoretical air-fuel ratio. If the gas flow rate flowing into the three-way catalyst 15 is large, the decrease in the oxygen storage amount accompanying the enrichment of the air-fuel ratio becomes relatively rapid, so the target air-fuel ratio is set in consideration of the gas flow rate, i.e., the intake air amount, so that the oxygen storage amount changes at an appropriate speed.

[0024] In step S5, the difference between the target air-fuel ratio obtained in step S4 and the exhaust air-fuel ratio (i.e., the actual air-fuel ratio) detected by the air-fuel ratio sensor 19 is obtained, and the feedback correction amount of the fuel injection amount is outputted by feedback control such as PID control. Finally, the fuel injection amount injected from the fuel injection valve 5 in each cycle is corrected using the feedback correction amount.

[0025] Figure 4 1 is a timing chart showing an example of changes in the target oxygen storage amount and the like achieved by the control of the above-described embodiment. From the top of the figure, (a) the intake air amount, (b) the oxygen storage amount, (c) the target air-fuel ratio, and (d) the actual air-fuel ratio are shown in order. The column (b) of oxygen storage amount shows the estimated oxygen storage amount b1 and the target oxygen storage amount b2 in an overlapping manner.

[0026] In the example of this time chart, the intake air amount is constant until time t1, and the intake air amount gradually increases during the period from time t1 to t3. The estimated oxygen storage amount b1 and the target oxygen storage amount b2 in the oxygen storage amount column coincide with each other until time t1. If the intake air amount starts to increase at time t1, the target oxygen storage amount b2 shows a decreasing trend and deviates from the estimated oxygen storage amount b1.

[0027] Therefore, based on the difference between the target oxygen storage amount b2 and the estimated oxygen storage amount b1, the target air-fuel ratio is changed to the rich side at time t2 as shown in column (c). Until time t2, the target air-fuel ratio is, for example, a constant value near the stoichiometric air-fuel ratio. The target air-fuel ratio is changed to the rich side at time t2, so that the fuel injection amount is feedback-controlled in the increasing direction, and the actual air-fuel ratio shown in column (d) is changed to the rich side. Therefore, the estimated oxygen storage amount b1 is also gradually reduced.

[0028] also, Figure 4 This is a timing chart for explaining the behavior of the above-mentioned embodiment, and does not necessarily accurately depict the actual waveform. For example, the delay of the operation cycle is exaggerated during the period from time t1 to time t2.

[0029] According to the above embodiment, the target oxygen storage amount of the three-way catalyst 15 is controlled based on the intake air amount, that is, the gas flow rate flowing into the three-way catalyst 15, and when the gas flow rate is large, it is controlled to a relatively low oxygen storage amount. If the gas flow rate flowing into the three-way catalyst 15 is large, the flow rate of the gas passing through the catalyst layer of the three-way catalyst 15 increases, and the NOx escape rate shows an increasing trend. In the above embodiment, the oxygen storage amount is controlled to be relatively low, thereby offsetting the increase in the NOx escape rate caused by the increase in the gas flow rate, and achieving more reliable purification of NOx.

[0030] One embodiment of the present invention has been described above, but the present invention is not limited to the above embodiment, and various changes can be made. For example, in the above embodiment, the intake air volume is used as a parameter corresponding to the gas flow rate flowing into the three-way catalyst 15, but the exhaust gas flow rate can be calculated based on the intake air volume in consideration of combustion, or the exhaust gas flow rate flowing in the exhaust passage can be detected by any method.

[0031] In addition, in the above embodiment, the “intake air amount” may be either a mass flow rate or a volume flow rate, for example, as long as they are set in appropriate forms. Figure 3 The relationship between the target oxygen storage capacity and the intake air volume shown is sufficient.

Claims

1. A method for controlling the oxygen storage amount of a three-way catalyst, wherein the air-fuel ratio of an internal combustion engine having a three-way catalyst in an exhaust passage is feedback-controlled in a manner such that the air-fuel ratio is a target air-fuel ratio close to the theoretical air-fuel ratio, and the target air-fuel ratio is controlled in a manner such that the oxygen storage amount of the three-way catalyst reaches the target oxygen storage amount, wherein: The target oxygen storage amount is set according to the flow rate of gas flowing into the three-way catalyst so that the target oxygen storage amount becomes smaller as the gas flow rate increases.

2. The method for controlling the oxygen storage amount of a three-way catalyst according to claim 1, wherein: The correlation between the gas flow rate and the target oxygen storage amount has the following characteristics: in an area with a relatively low gas flow rate, the target oxygen storage amount decreases sharply with respect to an increase in the gas flow rate; and in an area with a relatively high gas flow rate, the target oxygen storage amount decreases slowly with respect to an increase in the gas flow rate.

3. The method for controlling the oxygen storage amount of a three-way catalyst according to claim 1, wherein: The flow rate of air sucked into the internal combustion engine is regarded as the above-mentioned gas flow rate.

4. An oxygen storage amount control device for a three-way catalyst, comprising: a three-way catalyst, which is arranged in an exhaust passage of an internal combustion engine; an air-fuel ratio sensor, which is arranged at the inlet side of the three-way catalyst; and a controller, which controls the fuel injection amount, and the controller performs feedback control on the air-fuel ratio of the internal combustion engine in a manner along a target air-fuel ratio near the theoretical air-fuel ratio, and controls the target air-fuel ratio in a manner that the oxygen storage amount of the three-way catalyst reaches the target oxygen storage amount, wherein: The controller sets the target oxygen storage amount according to a flow rate of gas flowing into the three-way catalyst so that the target oxygen storage amount becomes smaller as the gas flow rate increases.

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