Control method and control device for internal combustion engine

Abstract

An internal combustion engine (1) has a three-way catalyst (15). An engine controller (9) is provided with: an oxygen amount calculation unit (102) for determining the amount of oxygen per unit time flowing into a three-way catalyst (15); and an oxygen storage amount calculation unit (108) that estimates the current oxygen storage amount by means of the cumulative calculation of the positive and negative oxygen amounts. A reference oxygen storage capacity calculation unit (107) outputs a reference oxygen storage capacity as a value that decreases as the intake air amount increases, on the basis of the intake air amount output by the intake air amount calculation unit (101). A catalyst diagnosis unit (104) determines catalyst deterioration by comparing a reference oxygen storage capacity taking into account the intake air amount with the current oxygen storage capacity.

Description

Technical Field

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

[0002] The three-way catalyst can oxidize CO and HC in exhaust gas and reduce NOx. In order to achieve both oxidation and reduction at a high level through catalytic action, the so-called oxygen storage capacity of the catalyst, that is, the storage and release of oxygen, is important.

[0003] Here, the capacity of the three-way catalyst provided in the exhaust passage of the internal combustion engine to store oxygen to the maximum extent, i.e., the oxygen storage capacity, decreases as the catalyst deteriorates (permanent degradation and temporary degradation). Therefore, various technologies are provided, that is, during the operation of the internal combustion engine, the oxygen storage capacity of the three-way catalyst is obtained in some way, and the catalyst degradation is diagnosed based on the degree to which the actual oxygen storage capacity decreases relative to the initial oxygen storage capacity (for example, Patent Document 1). As disclosed in Patent Document 1, the initial oxygen storage capacity as a comparison object becomes a value set corresponding to the initial state of the three-way catalyst, i.e., the characteristics of a new three-way catalyst.

[0004] According to the new knowledge of the inventors of the present invention, the actual oxygen storage capacity of the three-way catalyst in operation is affected by 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 oxygen and NOx flow out to the downstream side at a stage where the oxygen storage amount is relatively small compared to a case where the gas flow rate is small.

[0005] In the prior art such as Patent Document 1, such substantial change in oxygen storage capacity associated with the gas flow rate is not taken into consideration, and, for example, erroneous determination is likely to occur when performing degradation diagnosis under operating conditions with a large gas flow rate.

[0006] Patent Document 1: Japanese Patent Application Publication No. 2001-329832 Summary of the invention

[0007] The present invention relates to a control method for an internal combustion engine having a three-way catalyst in an exhaust passage, wherein catalyst degradation is diagnosed based on a decrease in oxygen storage capacity of the three-way catalyst relative to a reference oxygen storage capacity, wherein:

[0008] The reference oxygen storage capacity is set according to a flow rate of gas flowing into the three-way catalyst such that the reference oxygen storage capacity decreases as the gas flow rate increases.

[0009] Thus, as the gas flow rate flowing into the three-way catalyst increases, the reference oxygen storage capacity is set smaller, thereby offsetting the decrease in the actual oxygen storage capacity due to the increase in the gas flow rate, making it difficult to make an erroneous judgment in the degradation diagnosis. 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 1 is a characteristic diagram showing an example of an operating point of an internal combustion engine according to an embodiment.

[0012] Figure 3 is a functional block diagram of a control of an embodiment.

[0013] Figure 4 This is a characteristic diagram showing the characteristic of the reference oxygen storage capacity with respect to the intake air amount. DETAILED DESCRIPTION

[0014] Hereinafter, one embodiment of the present invention will be described in detail based on the drawings. Figure 1 This is an explanatory diagram showing the 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 used as a power generation internal combustion engine in a series hybrid vehicle to drive a power generation electric generator that mainly acts as a generator according to a power request, and is composed of a spark ignition type internal combustion engine (so-called gasoline internal combustion engine) of a 4-stroke cycle. In one example, it is a series 3-cylinder internal combustion engine, and each cylinder has an intake valve 2 and an exhaust valve 3 and a spark plug 4. In addition, the illustrated example is configured as an in-cylinder direct injection type internal combustion engine, and the fuel injection valve 5 that injects fuel into the cylinder is, for example, arranged on the side of the intake valve 2. In addition, it can also be a port injection type structure that injects fuel into the intake port 6.

[0015] An electronically controlled throttle valve 10 is installed on the upstream side of the collecting portion 8 of the intake passage 7 connected to the intake port 6 of each cylinder, and the opening degree of the throttle valve 10 is controlled according to a control signal from the engine controller 9. An air flow meter 11 for detecting the intake air amount 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 each cylinder are gathered into an exhaust passage 14, and a three-way catalyst 15 for exhaust purification is provided in the exhaust passage 14. The three-way catalyst 15 is, for example, a so-called single ceramic catalyst in which a catalyst layer including a catalyst metal is coated on the surface of a single ceramic body having a fine passage. In addition, the three-way catalyst 15 may also be a structure including a plurality of catalysts (such as a composite catalyst and an underfloor catalyst) arranged in series.

[0017] An upstream 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, that is, at a position upstream of the three-way catalyst 15. The upstream 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, a downstream air-fuel ratio sensor 20 that responds to the composition of the exhaust gas passing through the three-way catalyst 15 is arranged on the downstream side of the three-way catalyst 15 for the purpose of correction of the air-fuel ratio feedback control system including the upstream air-fuel ratio sensor 19, and for the deterioration diagnosis of the three-way catalyst 15. The downstream air-fuel ratio sensor 20 may be, for example, an O2 sensor, but in one example, a so-called wide-range air-fuel ratio sensor that is the same as the upstream air-fuel ratio sensor 19 is adopted.

[0018] The detection signals of the air-fuel ratio sensors 19, 20 and the air flow meter 11 are input to the engine controller 9. The detection signals of multiple sensors such as the crank angle sensor 21 for detecting the rotation speed of the internal combustion engine and the water temperature sensor 22 for detecting the cooling water temperature 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 of the fuel injection valve 5, the ignition timing of the spark plug 4, the opening of the throttle valve 10, etc. to be optimal. The engine controller 9 is connected to the integrated controller 31 that controls the entire series hybrid vehicle including the electric generator for driving, etc. via the in-vehicle network 32 such as CAN communication, and receives various requests or instructions including start / stop from the integrated controller 31.

[0019] The internal combustion engine 1 of one embodiment is a power generation internal combustion engine of a series hybrid vehicle, and therefore is basically started when the SOC of the battery for driving the vehicle decreases, such as Figure 2 As shown in the example, the system operates at several predetermined operating points (combinations of load and rotation speed) according to the magnitude of the power generation request. Figure 2 4 operating points P1 to P4 are shown in the figure. When the power generation request is small, the engine is operated at the operating point P1 set near the optimal fuel consumption point. When the power generation request is large, the operating points P2, P3, and P4 closer to the high speed and high load side are selected. In addition, the above operating points are not strictly one point, but each includes a small range but an appropriate range for the rotation speed and load.

[0020] The engine controller 9 performs air-fuel ratio control for optimizing the exhaust purification performance of the three-way catalyst 15 as one of various controls of the internal combustion engine 1. The air-fuel ratio control performs feedback control on the fuel injection amount (injection pulse width) of the fuel injection valve 5 in the following manner, that is, the oxygen storage amount of the three-way catalyst 15 is estimated based on the exhaust air-fuel ratio detected by the upstream air-fuel ratio sensor 19 so that the oxygen storage amount reaches the target oxygen storage amount (set to the middle value of the reference oxygen storage capacity as described later).

[0021] Furthermore, the engine controller 9 performs a degradation diagnosis of the three-way catalyst 15 during the operation of the internal combustion engine 1. As described above, the capacity of the three-way catalyst 15 to store oxygen to the maximum extent, that is, the oxygen storage capacity, decreases as the catalyst deteriorates (permanent degradation and temporary degradation). Therefore, during the operation of the internal combustion engine 1, the current oxygen storage capacity of the three-way catalyst 15 can be obtained, and the catalyst degradation can be diagnosed based on the degree to which the oxygen storage capacity decreases relative to the reference oxygen storage capacity (for example, the oxygen storage capacity of the three-way catalyst 15 equivalent to a new product). For example, when the exhaust gas air-fuel ratio on the downstream side of the three-way catalyst 15 detected by the downstream air-fuel ratio sensor 20 reverses from lean to rich, the oxygen storage amount can be regarded as 0, and conversely, when it reverses from rich to lean, it can be regarded as saturated. Therefore, the current oxygen storage capacity can be obtained based on the relationship with the oxygen storage amount estimated based on the exhaust gas air-fuel ratio detected by the upstream air-fuel ratio sensor 19. The current oxygen storage capacity is compared with the reference oxygen storage capacity to determine whether the catalyst is degraded. For example, if the current oxygen storage capacity is lower than the reference oxygen storage capacity by a predetermined ratio, it is determined to be degraded.

[0022] Here, the reference oxygen storage capacity that serves as a reference for the degradation judgment is set in this embodiment in consideration of the gas flow rate flowing into the three-way catalyst 15. As described above, the actual oxygen storage capacity of the three-way catalyst 15 in operation is affected by the gas flow rate flowing into the three-way catalyst 15. If the gas flow rate flowing in 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 oxygen and NOx flow out to the downstream side at a stage where the oxygen storage amount is relatively small compared to the case where the gas flow rate is small. Therefore, if the reference oxygen storage capacity is fixedly specified corresponding to the oxygen storage capacity at the time of the new product without considering the influence of the gas flow rate, an erroneous judgment may occur when the gas flow rate at which the actual oxygen storage capacity is reduced is large.

[0023] In this embodiment, the reference oxygen storage capacity is set according to the gas flow rate in such a manner that the reference oxygen storage capacity becomes smaller as the gas flow rate increases. Here, in one embodiment, the intake air amount detected by the air flow meter 11 is used as a parameter corresponding to the gas flow rate flowing into the three-way catalyst 15. In addition, the "intake air amount" is not the amount of air per cycle of the cylinder, but represents the flow rate of air sucked into the internal combustion engine 1 (i.e., passing through the air flow meter 11) per unit time.

[0024] In addition, one of the controls performed by the engine controller 9 includes a disturbance control for suppressing or eliminating the temporary degradation (also called temporary poisoning) of the catalyst performance of the three-way catalyst 15. Regarding the disturbance control, rich combustion with a larger equivalence ratio and lean combustion with a smaller equivalence ratio are periodically repeated. For example, in an internal combustion engine 1 having multiple cylinders, N rich combustions are performed continuously in accordance with the combustion sequence, followed by N lean combustions, and the above process is periodically repeated. Temporary degradation of the catalyst is a phenomenon in which oxygen, HC, etc. adhere to the surface of the catalyst metal, thereby reducing the surface area of ​​the catalyst metal and reducing the catalyst performance. If the air-fuel ratio of the gas in contact with the catalyst is periodically changed by a large amplitude, the poisoning substances covering the surface of the catalyst metal are peeled off and the catalyst performance is restored. In addition, in this disturbance control, the average air-fuel ratio is also maintained near the theoretical air-fuel ratio.

[0025] As described above, the internal combustion engine 1 of one embodiment Figure 2 The four operating points P1 to P4 are shown as examples, but at the operating point P4 on the highest speed and highest load side, temporary degradation of the catalyst is likely to occur. In particular, in the power generation application of a series hybrid vehicle, the operation at the same operating point is continued for a long time. Therefore, in one embodiment, when the internal combustion engine 1 is operated at the operating point P4, disturbance control is performed in parallel. Thereby, temporary degradation of the catalyst is suppressed or eliminated.

[0026] Here, the disturbance control needs to be performed within the range of ensuring the conversion rate of HC, CO, and NOx of the three-way catalyst 15. For example, if the lean combustion period in the disturbance control (in other words, the number of consecutive lean combustions N) is too long, the lean combustion is performed even after the oxygen storage amount of the three-way catalyst 15 is saturated, and as a result, NOx flows out to the downstream of the three-way catalyst 15. Similarly, if the rich combustion period (the number of consecutive rich combustions N) is too long, the rich combustion is performed even after the oxygen storage amount of the three-way catalyst 15 becomes 0, and as a result, HC and the like flow out to the downstream of the three-way catalyst 15. Therefore, in order to prevent the conversion rate from being reduced on both the lean side and the rich side, the optimal rich / lean variation period of the disturbance control is set to a period corresponding to the oxygen storage capacity of the three-way catalyst 15. As mentioned above, the actual oxygen storage capacity is affected by the size of the gas flow rate flowing into the three-way catalyst 15. Therefore, in one embodiment, the rich / lean cycle of the disturbance control is set based on the above-mentioned reference oxygen storage capacity corresponding to the gas flow rate flowing into the three-way catalyst 15, that is, the intake air amount. Specifically, if the gas flow rate is large and the reference oxygen storage capacity is small, the cycle is set to be relatively short. For example Figure 2The operating point P4 has a larger gas flow rate (intake air amount) than the other operating points P1 to P3, but the period of disturbance control is set to a relatively short value based on the baseline oxygen storage capacity at this time. Therefore, the reduction in conversion rate caused by the excessively long rich combustion period or lean combustion period as mentioned above can be suppressed, and temporary degradation can be eliminated.

[0027] Figure 3 1 is a functional block diagram of control related to the reference oxygen storage capacity of one embodiment. In addition, the functions shown in the block diagram are implemented by software or hardware executed by the engine controller 9. As shown in the figure, the functional block diagram includes an intake air amount calculation unit 101, an oxygen amount calculation unit 102, an oxygen storage estimation unit 103, a catalyst diagnosis unit 104, an air-fuel ratio control unit 105, and a cycle setting unit 106. The oxygen storage estimation unit 103 has a reference oxygen storage capacity calculation unit 107, an oxygen storage amount calculation unit 108, a lean side reset unit 109, and a rich side reset unit 110.

[0028] The intake air amount calculation unit 101 calculates the intake air amount sucked into the internal combustion engine 1 based on the detection signal of the air flow meter 11. The intake air amount is regarded as the gas flow rate flowing into the three-way catalyst 15. The oxygen amount calculation unit 102 calculates the oxygen amount per unit time flowing into the three-way catalyst 15 based on the exhaust air-fuel ratio detected by the upstream air-fuel ratio sensor 19 and the gas flow rate flowing into the three-way catalyst 15, that is, the intake air amount. In addition, the oxygen amount is given as both a positive value and a negative value. That is, if it is a rich air-fuel ratio, the oxygen amount is a negative value. The oxygen storage amount calculation unit 108 calculates the current oxygen storage amount by accumulating the oxygen amount per unit time output by the oxygen amount calculation unit 102. The value of the current oxygen storage amount is input to the catalyst diagnosis unit 104 and the air-fuel ratio control unit 105 respectively.

[0029] The reference oxygen storage capacity calculation unit 107 calculates the reference oxygen storage capacity based on the intake air volume (i.e., the gas flow rate flowing into the three-way catalyst 15) output by the intake air volume calculation unit 101. In one embodiment, a reference oxygen storage capacity value is assigned to the intake air volume as a parameter. Figure 4 The corresponding graph of the characteristics shown in FIG. 1 is used to obtain the reference oxygen storage capacity corresponding to the intake air amount. Figure 4 As shown, the larger the intake air volume is, the smaller the reference oxygen storage capacity is.

[0030] In addition, both the oxygen storage amount and the reference oxygen storage capacity can be processed according to the mass of oxygen (unit: g). According to convention, the oxygen storage amount when the three-way catalyst 15 is new and the gas flow rate is the smallest is set to 100 (%), and the percentage relative to the oxygen storage amount is expressed. Therefore, the reference oxygen storage capacity is the following percentage, that is, when the intake air amount is the smallest, it is 100 (%), and the larger the intake air amount, the smaller the percentage.

[0031] Thus, the value of the reference oxygen storage capacity calculated by the reference oxygen storage capacity calculation unit 107 is given as an upper limit value to the oxygen storage capacity calculation unit 108. The oxygen storage capacity output by the oxygen storage capacity calculation unit 108 is limited by the upper limit value. That is, the oxygen storage capacity estimated by the cumulative calculation of the oxygen storage capacity calculation unit 108 does not exceed the reference oxygen storage capacity taking into account the intake air amount.

[0032] In addition, the value of the reference oxygen storage capacity obtained by the reference oxygen storage capacity calculation unit 107 is input to the catalyst diagnosis unit 104 , the air-fuel ratio control unit 105 , and the cycle setting unit 106 .

[0033] As described above, the catalyst diagnosis unit 104 compares the current oxygen storage capacity with the reference oxygen storage capacity to determine whether the catalyst is deteriorated. The reference oxygen storage capacity used as the comparison object is a value of the reference oxygen storage capacity that takes the intake air amount into consideration. This can suppress, for example, erroneous determination when the gas flow rate flowing into the three-way catalyst 15 is large.

[0034] In the air-fuel ratio control unit 105, the target oxygen storage amount in the air-fuel ratio control is set using the reference oxygen storage capacity taking into account the intake air amount. For example, a value of 1 / 2 of the reference oxygen storage capacity is set as the target oxygen storage amount. In this way, air-fuel ratio control suitable for the actual oxygen storage capacity that changes according to the gas flow rate is achieved.

[0035] In the cycle setting unit 106, the rich / lean cycle in the above-mentioned disturbance control is set based on the reference oxygen storage capacity corresponding to the intake air amount. The smaller the reference oxygen storage capacity is, the shorter the cycle is set. In addition, when the disturbance control is performed only at a specific operating point (load and rotation speed), the intake air amount basically becomes a specific value, so the cycle becomes a fixed value.

[0036] The lean side reset unit 109 and the rich side reset unit 110 included in the oxygen storage estimation unit 103 reset the current oxygen storage amount cumulatively calculated or estimated by the oxygen storage amount calculation unit 108 based on the inversion of the detected air-fuel ratio of the downstream air-fuel ratio sensor 20 to lean / rich. For example, when the detected air-fuel ratio of the downstream air-fuel ratio sensor 20 is lean, the lean side reset unit 109 considers that the oxygen storage capacity of the three-way catalyst 15 is saturated, and resets the estimated value of the oxygen storage amount using the value of the reference oxygen storage capacity. Here, as the reference oxygen storage capacity, the reference oxygen storage capacity that takes into account the intake air amount output by the reference oxygen storage capacity calculation unit 107 is used. As an example, if the reference oxygen storage capacity that takes into account the intake air amount is set to 90(%), the current oxygen storage amount that is considered to be saturated is reset to 90(%). When the air-fuel ratio detected by the downstream air-fuel ratio sensor 20 is rich, the rich side reset unit 110 regards the oxygen storage amount of the three-way catalyst 15 as 0 and resets the estimated value of the oxygen storage amount to 0. By providing the lean side reset unit 109 and the rich side reset unit 110, the accuracy of the oxygen storage amount estimated by cumulative calculation is improved.

[0037] 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 amount is used as a parameter equivalent 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 amount and considering combustion, or the exhaust gas flow rate flowing in the exhaust passage can be detected by any method.

[0038] In addition, in the above embodiment, the "intake air amount" can be either a mass flow rate or a volume flow rate. For example, as long as the mass flow rate is set in an appropriate form, Figure 4 The relationship between the target oxygen storage capacity and the intake air volume shown is sufficient.

[0039] In the above-mentioned embodiment, an example of applying the present invention to a power generation internal combustion engine of a series hybrid vehicle is described, but the present invention is not limited to the power generation internal combustion engine and can be widely applied to internal combustion engines that drive vehicles.

[0040] Furthermore, in the above-described embodiment, the disturbance control is always executed at a specific operating point. However, the disturbance control may be executed when temporary degradation of the catalyst is detected or estimated regardless of the operating point.

[0041] Furthermore, in the above-mentioned embodiment, the "reference oxygen storage capacity" is treated as the maximum oxygen storage capacity at which the oxygen storage capacity reaches saturation under the respective intake air amounts, but in the present invention, the threshold value itself, which is compared with the current oxygen storage capacity, may be used as the "reference oxygen storage capacity" and solved according to the intake air amount or the gas flow rate.

Claims

1. A control method for an internal combustion engine having a three-way catalyst in an exhaust passage, wherein catalyst degradation is diagnosed based on a decrease in oxygen storage capacity of the three-way catalyst relative to a reference oxygen storage capacity, wherein: The reference oxygen storage capacity is set according to a flow rate of gas flowing into the three-way catalyst such that the reference oxygen storage capacity decreases as the gas flow rate increases.

2. The internal combustion engine control method 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.

3. The internal combustion engine control method according to claim 2, wherein: The reference oxygen storage capacity is obtained from a table using the flow rate of air sucked into the internal combustion engine as a parameter.

4. The internal combustion engine control method according to claim 1, wherein: A disturbance control is performed to periodically repeat rich combustion with a large equivalence ratio and lean combustion with a small equivalence ratio in order to relieve temporary degradation of the catalyst, and a rich / lean variation period of the disturbance control is set based on a reference oxygen storage capacity taking the gas flow rate into consideration.

5. The control method of the internal combustion engine according to claim 1, wherein: The air-fuel ratio is controlled so that the oxygen storage amount of the three-way catalyst reaches a target oxygen storage amount, and the target oxygen storage amount is set based on a reference oxygen storage capacity taking into account the gas flow rate.

6. The control method of an internal combustion engine according to claim 1, wherein: The oxygen storage amount of the three-way catalyst is estimated based on the gas flowing into the three-way catalyst, and when the air-fuel ratio downstream of the three-way catalyst is reversed to lean, the oxygen storage amount estimated value is reset using a reference oxygen storage capacity taking into account the gas flow rate.

7. A control device for an internal combustion engine, comprising: a three-way catalyst disposed in an exhaust passage of the internal combustion engine; an upstream air-fuel ratio sensor and a downstream air-fuel ratio sensor disposed on the upstream side and the downstream side of the three-way catalyst, respectively; and a controller for controlling a fuel injection amount, wherein the controller diagnoses catalyst degradation based on a decrease in oxygen storage capacity of the three-way catalyst relative to a reference oxygen storage capacity, wherein: The controller sets the reference oxygen storage capacity according to a flow rate of gas flowing into the three-way catalyst such that the reference oxygen storage capacity decreases as the gas flow rate increases.

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