Method and device for controlling oxygen storage amount of three-way catalyst

Abstract

An exhaust passage (14) of an internal combustion engine (1) is provided with a three-way catalyst (15), and the air-fuel ratio is feedback controlled so as to maintain the oxygen storage amount in the three-way catalyst (15) at a target oxygen storage amount. An estimated oxygen storage amount (b1) is obtained from an intake air amount and an exhaust air-fuel ratio detected by an air-fuel ratio sensor (19), and a target air-fuel ratio is set in accordance with the difference between the target oxygen storage amount (b2) and the estimated oxygen storage amount (b1). The target oxygen storage amount (b2) is set with the characteristic of becoming lower as the intake air amount becomes higher, on the basis of the intake air amount corresponding to the flow rate of gas flowing into the three-way catalyst (15). The foregoing cancels out an increase in the NOx slip ratio when the gas flow rate is high.

Description

[Technical Field]

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

[0002] Three-way catalysts are capable of oxidizing CO and HC in exhaust gas and reducing NOx, but in order to achieve a high level of both oxidation and reduction through catalytic action, the catalyst's ability to store and release oxygen, or so-called oxygen storage capacity, is important. For this reason, a known technique is to monitor the oxygen storage amount of the three-way catalyst and variably control the target air-fuel ratio in air-fuel ratio feedback control so that this oxygen storage amount is maintained at an intermediate target oxygen storage amount (for example, 50%).

[0003] Furthermore, Patent Document 1 discloses that, based on the load and rotation speed of the internal combustion engine, the target oxygen storage amount is set to be relatively small under operating conditions that result in high NOx emissions, and set to be relatively large under operating conditions that result in high CO and HC emissions.

[0004] According to the inventor's new findings, the optimal target oxygen storage amount correlates with the gas flow rate flowing into the three-way catalyst. When the gas flow rate through the three-way catalyst is high, the flow rate of the gas passing through the catalyst layer of the three-way catalyst increases, and the NOx slip rate (the proportion of NOx that passes through without being converted) increases. Therefore, it is desirable to lower the oxygen storage amount of the three-way catalyst as the gas flow rate increases.

[0005] Patent Document 1 does not disclose any control of the amount of oxygen storage related to the gas flow rate. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-008921 Summary of the Invention

[0007] The present invention provides a three-way catalyst oxygen storage amount control method for an internal combustion engine equipped with a three-way catalyst in an exhaust passage, which performs feedback control to adjust the air-fuel ratio of the engine to a target air-fuel ratio near the stoichiometric air-fuel ratio, and controls the target air-fuel ratio so that the oxygen storage amount of the three-way catalyst becomes the target oxygen storage amount, comprising: The target oxygen storage amount is set in accordance with the flow rate of gas flowing into the three-way catalyst so that the target oxygen storage amount decreases as the gas flow rate increases.

[0008] In this way, by lowering the target oxygen storage amount as the gas flow rate flowing into the three-way catalyst increases, the increase in the NOx slip rate caused by the increase in gas flow velocity is offset, and NOx can be purified more reliably. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram illustrating the configuration of an internal combustion engine equipped with a three-way catalyst according to an embodiment of the present invention; [Figure 2] FIG. 3 is an explanatory diagram showing a control flow according to an embodiment. [Figure 3] FIG. 4 is a characteristic diagram showing the characteristic of the target oxygen storage amount relative to the intake air amount. [Figure 4] 1 is a time chart showing an example of changes in (a) intake air amount, (b) oxygen storage amount, (c) target air-fuel ratio, and (d) actual air-fuel ratio. DETAILED DESCRIPTION OF THE INVENTION

[0010] An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is an explanatory diagram showing a schematic configuration of an internal combustion engine 1 of one embodiment to which the present invention is applied. The internal combustion engine 1 of the one embodiment is a four-stroke spark-ignition internal combustion engine (a so-called gasoline engine), and each cylinder is provided with an intake valve 2, an exhaust valve 3, and an ignition plug 4. The illustrated example is configured as a direct injection type engine, and a fuel injection valve 5 that injects fuel into the cylinder is disposed, for example, on the intake valve 2 side. Note that the engine may also be configured as a port injection type that injects fuel toward an intake port 6.

[0011] An electronically controlled throttle valve 10, the opening of which is controlled by a control signal from an engine controller 9, is installed upstream of a collector section 8 in an intake passage 7 connected to the intake port 6 of each cylinder. An air flow meter 11, which detects the amount of intake air, is disposed upstream of the throttle valve 10, and an air cleaner 12 is disposed further upstream.

[0012] The exhaust ports 13 of each cylinder are joined together to form a single exhaust passage 14, which is provided with a three-way catalyst 15 for purifying exhaust gases. The three-way catalyst 15 is, for example, a monolithic ceramic catalyst in which a catalyst layer containing catalytic metal is coated on the surface of a monolithic ceramic body having fine passages formed therein. The three-way catalyst 15 may also be configured to include multiple catalysts (for example, a manifold catalyst and an underfloor catalyst) arranged in series.

[0013] An air-fuel ratio sensor 19 for detecting the exhaust air-fuel ratio is disposed in the exhaust passage 14 on the inlet side of the three-way catalyst 15, i.e., on the upstream side of the three-way catalyst 15. This air-fuel ratio sensor 19 is a so-called wide-range air-fuel ratio sensor that obtains an output according to the exhaust air-fuel ratio. Note that a second air-fuel ratio sensor, such as an O2 sensor, may be provided downstream of the three-way catalyst 15 for purposes such as calibrating an air-fuel ratio feedback control system including the air-fuel ratio sensor 19 and diagnosing deterioration of the three-way catalyst 15.

[0014] Detection signals from the air-fuel ratio sensor 19 and air flow meter 11 are input to the engine controller 9. Detection signals from a number of sensors, such as a crank angle sensor 21 for detecting the engine speed, a water temperature sensor 22 for detecting the coolant temperature, and an accelerator position 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 these input signals, the engine controller 9 optimally controls the amount and timing of fuel injection by the fuel injection valve 5, the ignition timing by the spark plug 4, the opening of the throttle valve 10, etc.

[0015] As one of various controls of the internal combustion engine 1, the engine controller 9 performs air-fuel ratio control to optimize the exhaust purification performance of the three-way catalyst 15. The air-fuel ratio control controls the fuel injection amount by feedback control (e.g., PID control) based on the exhaust air-fuel ratio detected by the air-fuel ratio sensor 19 so that the fuel injection amount follows a target air-fuel ratio near the stoichiometric air-fuel ratio. Here, the target air-fuel ratio is controlled so that the oxygen storage amount of the three-way catalyst 15, estimated from the exhaust air-fuel ratio, becomes the target oxygen storage amount.

[0016] FIG. 2 is an explanatory diagram, in the form of a flowchart, illustrating the flow of air-fuel ratio control based on the oxygen storage amount. The intake air amount detected by the air flow meter 11 is input to the processes shown as steps S1, S2, and S4 as a parameter corresponding to the gas flow rate flowing into the three-way catalyst 15. The "intake air amount" does not refer to the air amount per cylinder cycle, but rather to the air flow rate per unit time taken into the internal combustion engine 1 (i.e., passing through the air flow meter 11). The exhaust air-fuel ratio (catalyst inlet air-fuel ratio) detected by the air-fuel ratio sensor 19 is input to the processes shown as steps S1 and S5. 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 adding or subtracting the oxygen storage amount based on the exhaust air-fuel ratio at that time for each calculation cycle of the engine controller 9. In other words, simply put, 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 both positive and negative values. Hereinafter, this will be referred to as the "estimated oxygen storage amount."

[0017] In step S2, the target oxygen storage amount is set based on the gas flow rate flowing into the three-way catalyst 15, i.e., the intake air amount. Figure 3 shows the characteristics of the target oxygen storage amount relative to the intake air amount. For example, the characteristics shown in Figure 3 are provided to the engine controller 9 in the form of a table, and the target oxygen storage amount is output relative to the input intake air amount. As shown in Figure 3, the target oxygen storage amount has the characteristic of becoming smaller as the intake air amount increases. More specifically, the correlation between the intake air amount and the target oxygen storage amount is such that in a region where the intake air amount is relatively small, the target oxygen storage amount decreases rapidly with increasing intake air amount, and in a region where the intake air amount is relatively large (the region on the right in Figure 3), the target oxygen storage amount decreases gradually with increasing intake air amount.

[0018] In step S3, the target oxygen storage amount obtained in step S2 is compared with the estimated oxygen storage amount obtained in step S1 to determine the difference between them.

[0019] This difference value is one of the inputs for the processing of step S4. In step S4, the target air-fuel ratio (target air-fuel ratio on the inlet side of the three-way catalyst 15) is calculated using this difference value and the gas flow rate flowing into the three-way catalyst 15, i.e., the intake air amount. For example, if the estimated oxygen storage amount is larger than the target oxygen storage amount, the target air-fuel ratio is controlled to be richer than the stoichiometric air-fuel ratio. If the gas flow rate flowing into the three-way catalyst 15 is large, the decrease in the oxygen storage amount due to enriching the air-fuel ratio will be relatively rapid, so the target air-fuel ratio is set taking into account the gas flow rate, i.e., the intake air amount, so that the oxygen storage amount changes at an appropriate speed.

[0020] 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 calculated, and a feedback correction amount for the fuel injection amount is output by feedback control such as PID control. Finally, the fuel injection amount injected from the fuel injection valve 5 for each cycle is corrected using this feedback correction amount.

[0021] 4 is a time chart showing an example of changes in the target oxygen storage amount and other parameters under the control of the above embodiment. From top to bottom, the chart shows (a) intake air amount, (b) oxygen storage amount, (c) target air-fuel ratio, and (d) actual air-fuel ratio. In the (b) oxygen storage amount column, the estimated oxygen storage amount b1 and the target oxygen storage amount b2 are shown overlapping each other.

[0022] In this example time chart, the intake air amount is constant until time t1, and then gradually increases from time t1 to t3. In the oxygen storage amount column, the estimated oxygen storage amount b1 and the target oxygen storage amount b2 are the same until time t1. When the intake air amount begins to increase at time t1, the target oxygen storage amount b2 begins to decrease and deviates from the estimated oxygen storage amount b1.

[0023] Therefore, based on the difference between the target oxygen storage amount b2 and the estimated oxygen storage amount b1, the target air-fuel ratio changes to the rich side at time t2 as shown in (c). Until time t2, the target air-fuel ratio is a constant value, for example, near the stoichiometric air-fuel ratio. As the target air-fuel ratio changes to the rich side at time t2, feedback control is performed to increase the fuel injection amount, and the actual air-fuel ratio shown in (d) changes to the rich side. Therefore, the estimated oxygen storage amount b1 also gradually decreases.

[0024] 4 is an explanatory time chart for explaining the behavior of the above embodiment, and does not necessarily accurately depict the actual waveforms. For example, the period from time t1 to time t2 is depicted in an exaggerated manner to show the delay due to the calculation cycle.

[0025] According to the above embodiment, the target oxygen storage amount of the three-way catalyst 15 is controlled based on the intake air amount, i.e., the gas flow rate flowing into the three-way catalyst 15, and is controlled to a relatively low oxygen storage amount when the gas flow rate is large. When the gas flow rate flowing into the three-way catalyst 15 is large, the flow rate of gas passing through the catalyst layer of the three-way catalyst 15 increases, and the NOx slip rate tends to increase. In the above embodiment, by controlling the oxygen storage amount to a relatively low value, the increase in the NOx slip rate caused by the increase in gas flow rate is offset, and more reliable purification of NOx is achieved.

[0026] Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and various modifications are possible. For example, in the above embodiment, the intake air amount is used as a parameter corresponding to the gas flow rate flowing into the three-way catalyst 15, but the exhaust gas flow rate may be calculated based on the intake air amount and taking combustion into consideration, or the exhaust gas flow rate flowing through the exhaust passage may be detected by some means.

[0027] In the above embodiment, the "intake air amount" may be either a mass flow rate or a volume flow rate, and the relationship between the target oxygen storage amount and the intake air amount as shown in FIG. 3 may be set in a form appropriate for each.

Claims

1. A method for controlling an oxygen storage amount of a three-way catalyst, which performs feedback control of an air-fuel ratio of an internal combustion engine equipped with a three-way catalyst in an exhaust passage so that the air-fuel ratio is in line with a target air-fuel ratio near the stoichiometric air-fuel ratio, and controls the target air-fuel ratio so that the oxygen storage amount of the three-way catalyst becomes the target oxygen storage amount, The target oxygen storage amount is set in accordance with the flow rate of gas flowing into the three-way catalyst such that the target oxygen storage amount decreases as the flow rate of gas increases. A method for controlling the amount of oxygen storage in a three-way catalyst.

2. As a correlation between the gas flow rate and the target oxygen storage amount, in a region where the gas flow rate is relatively low, the target oxygen storage amount decreases rapidly with increasing gas flow rate, and in a region where the gas flow rate is relatively high, the target oxygen storage amount decreases gradually with increasing gas flow rate. The method for controlling an oxygen storage amount of a three-way catalyst according to claim 1 .

3. The flow rate of air drawn into the internal combustion engine is regarded as the gas flow rate. The method for controlling an oxygen storage amount of a three-way catalyst according to claim 1 .

4. An oxygen storage amount control device for a three-way catalyst, comprising: a three-way catalyst provided in an exhaust passage of an internal combustion engine; an air-fuel ratio sensor provided on the inlet side of the three-way catalyst; and a controller for controlling a fuel injection amount, wherein the controller performs feedback control to adjust the air-fuel ratio of the internal combustion engine to a target air-fuel ratio near the stoichiometric air-fuel ratio, and controls the target air-fuel ratio so that the oxygen storage amount of the three-way catalyst becomes the target oxygen storage amount, The above controller is The target oxygen storage amount is set in accordance with the flow rate of gas flowing into the three-way catalyst such that the target oxygen storage amount decreases as the flow rate of gas increases. Oxygen storage amount control device for three-way catalyst.

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