Engine control unit

Abstract

To improve the purification efficiency of ammonia produced in a three-way catalyst.SOLUTION: An ECU 20 performs, in an engine 10 in which a three-way catalyst 12, a second air-fuel ratio sensor 15, and an oxygen catalyst 13 are installed on an exhaust passage 11 in this order from the upstream side, air-fuel ratio sub-feedback control for switching a target air-fuel ratio from a rich air-fuel ratio to a lean air-fuel ratio when a rear air-fuel ratio detected by the second air-fuel ratio sensor 15 becomes equal to or less than a rich determination value, and for switching the target air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio when the rear air-fuel ratio becomes equal to or greater than a lean determination value. The ECU 20, at the time of performing the sub-feedback control, when an amount of overshoot of the rear air-fuel ratio to a richer side is larger than a stoichiometric air-fuel ratio after switching the target air-fuel ratio from the rich air-fuel ratio to the lean air-fuel ratio, variably sets the lean determination value to a value indicating a leaner air-fuel ratio than that when the amount of overshoot is small.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an engine control device. [Background technology]

[0002] Some vehicle engines are equipped with a three-way catalyst in the exhaust passage that oxidizes and purifies hydrocarbons (HC) and carbon monoxide (CO) in the exhaust, and reduces and purifies nitrogen oxides (NOx) in the exhaust. Three-way catalysts generate ammonia during the NOx reduction process.

[0003] Patent Document 1 describes an engine that has an oxidation catalyst installed downstream of a three-way catalyst and an injector that supplies oxygen to the oxidation catalyst. The oxidation catalyst of such an engine oxidizes and purifies ammonia produced by the three-way catalyst using the oxygen supplied from the injector. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2014-515701 Summary of the Invention [Problem to be solved by the invention]

[0005] The amount of ammonia produced in a three-way catalyst varies depending on the engine's operating conditions. As the amount of ammonia produced changes, the amount of oxygen required to purify it also changes. Therefore, it is necessary to precisely control the amount of oxygen in the oxidation catalyst. [Means for solving the problem]

[0006] The engine control device for solving the above problem is applied to an engine including a three-way catalyst installed in an exhaust passage, an oxidation catalyst installed in a portion of the exhaust passage downstream of the three-way catalyst, and an air-fuel ratio sensor installed in a portion of the exhaust passage downstream of the three-way catalyst and upstream of the oxidation catalyst, and when an air-fuel ratio detection value of the air-fuel ratio sensor becomes a value richer than a rich determination value set to a value indicating an air-fuel ratio richer than the stoichiometric air-fuel ratio, the air-fuel ratio of the mixture combusted in the engine is switched from a rich air-fuel ratio richer than the stoichiometric air-fuel ratio to a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio. and, when the air-fuel ratio detection value becomes leaner than a lean determination value which is set to a value indicating an air-fuel ratio leaner than the stoichiometric air-fuel ratio, performs air-fuel ratio sub-feedback control to switch the air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio, and when the air-fuel ratio detection value becomes leaner than a lean determination value which is set to a value indicating an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the engine control device is configured to variably set the lean determination value so that the air-fuel ratio becomes a value indicating a leaner air-fuel ratio than when the overshoot amount is small when the air-fuel ratio detection value overshoots to a value richer than the stoichiometric air-fuel ratio after the air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio. [Effects of the Invention]

[0007] The engine control device has the effect of increasing the efficiency of purifying ammonia produced by the three-way catalyst. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram illustrating a schematic configuration of an embodiment of an engine control device; [Figure 2] 4 is a flowchart of an air-fuel ratio sub-feedback control routine executed by the engine control device. [Figure 3] 4 is a graph showing the relationship between the amount of ammonia produced and a rich determination value. [Figure 4]10A is a time chart showing the transition of the target air-fuel ratio, (B) the transition of the oxygen storage amount of the three-way catalyst, (C) the transition of the rear air-fuel ratio, (D) the transition of the ammonia production amount of the three-way catalyst, and (E) the transition of the oxygen storage amount of the oxidation catalyst during execution of the air-fuel ratio sub-feedback control by the engine control device. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of an engine control device will be described in detail below with reference to FIGS. (Configuration of engine control device) An engine 10 shown in FIG. 1 is equipped with a three-way catalyst 12 and an oxidation catalyst 13 installed in an exhaust passage 11. The oxidation catalyst 13 is installed downstream of the three-way catalyst 12 in the exhaust passage 11. The three-way catalyst 12 is a catalytic device that oxidizes hydrocarbons and carbon monoxide in the exhaust gas and reduces nitrogen oxides in the exhaust gas. The oxidation catalyst 13 is a catalytic device that oxidizes ammonia in the exhaust gas. Both the three-way catalyst 12 and the oxidation catalyst 13 have oxygen storage capacity.

[0010] The engine 10 also has two air-fuel ratio sensors: a first air-fuel ratio sensor 14 and a second air-fuel ratio sensor 15. The first air-fuel ratio sensor 14 is installed in a portion of the exhaust passage 11 upstream of the three-way catalyst 12. The second air-fuel ratio sensor 15 is installed in a portion of the exhaust passage 11 downstream of the three-way catalyst 12 and upstream of the oxidation catalyst 13.

[0011] The ECU 20, which serves as an engine control device for controlling the engine 10, includes a processor 21 and a memory 22. The memory 22 stores programs and data for engine control. The processor 21 reads and executes the programs stored in the memory 22, thereby causing the ECU 20 to perform various processes related to engine control. The ECU 20 receives detection signals from various sensors that detect the operating state of the engine 10, including a first air-fuel ratio sensor 14 and a second air-fuel ratio sensor 15. The ECU 20 controls the intake air amount GA, fuel injection amount, ignition timing, and the like of the engine 10 based on the detection results of these sensors.

[0012] (Air-fuel ratio control) The ECU 20 controls the air-fuel ratio of the mixture burned in the engine 10 based on the detection results of the first air-fuel ratio sensor 14 and the second air-fuel ratio sensor 15. The air-fuel ratio control is performed through two types of feedback control: main feedback control based on the detection result of the first air-fuel ratio sensor 14, and sub-feedback control based on the detection result of the second air-fuel ratio sensor 15. The ECU 20 performs the main feedback control by adjusting the fuel injection amount based on the deviation between the air-fuel ratio detection value by the first air-fuel ratio sensor 14 and a target air-fuel ratio so as to reduce the deviation. In the sub-feedback control, the ECU 20 alternately switches the target air-fuel ratio of the main feedback control between a lean air-fuel ratio, which is an air-fuel ratio leaner than the stoichiometric air-fuel ratio, and a rich air-fuel ratio, which is an air-fuel ratio richer than the stoichiometric air-fuel ratio.

[0013] 2 shows a flowchart of a sub-feedback control routine executed by the ECU 20 for sub-feedback control. The ECU 20 repeatedly executes this routine at predetermined control intervals while the engine 10 is running. Note that "F / B" in FIG. 2 represents feedback. Also, "A / F" in FIG. 2 represents air-fuel ratio. Furthermore, in the following description, the detected value of the air-fuel ratio of the second air-fuel ratio sensor 15 will be referred to as the rear air-fuel ratio.

[0014] When this routine starts, the ECU 20 first determines in step S100 whether a lean air-fuel ratio is set as the target air-fuel ratio. If a lean air-fuel ratio is not set as the target air-fuel ratio (NO), that is, if a rich air-fuel ratio is set as the target air-fuel ratio, the ECU 20 determines in step S110 whether the rear air-fuel ratio is equal to or less than a predetermined rich determination value. If the rear air-fuel ratio exceeds the rich determination value (NO), the ECU 20 ends the processing of this routine in the current control cycle. On the other hand, if the rear air-fuel ratio is equal to or greater than the rich determination value (YES), the ECU 20 switches the target air-fuel ratio from the rich air-fuel ratio to the lean air-fuel ratio in step S120, and then ends the processing of this routine in the current control cycle.

[0015] On the other hand, if a lean air-fuel ratio is set as the target air-fuel ratio (S100: YES), the ECU 20 determines in step S130 whether the rear air-fuel ratio is less than the stoichiometric air-fuel ratio. That is, in step S130, the ECU 20 determines whether the rear air-fuel ratio is a value indicating an air-fuel ratio richer than the stoichiometric air-fuel ratio. If the rear air-fuel ratio is less than the stoichiometric air-fuel ratio (YES), the ECU 20 updates the value of the ammonia generation amount OV in step S140 and then ends the processing of this routine for the current control cycle. The ammonia generation amount OV represents an estimated value of the amount of ammonia generated in the three-way catalyst 12 after switching the target air-fuel ratio from a rich air-fuel ratio to a lean air-fuel ratio. In step S140, the ECU 20 updates the ammonia generation amount OV by setting a value that satisfies the relationship of equation (1) as the updated value. In equation (1), "GA" represents the intake air amount of the engine 10, and "K" represents a predetermined coefficient.

[0016]

number

[0017] On the other hand, if the rear air-fuel ratio is equal to or higher than the stoichiometric air-fuel ratio (S130: NO), that is, if the rear air-fuel ratio is the stoichiometric air-fuel ratio or an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the ECU 20 proceeds to step S150. In step S150, the ECU 20 sets a lean determination value based on the amount of ammonia generated OV. In this embodiment, the ECU 20 sets the lean determination value using a map stored in advance in the memory 22.

[0018] The relationship between the amount of ammonia generated OV and the lean determination value in the map is shown in Figure 3. The lean determination value is set to be a larger value when the amount of ammonia generated OV is large than when it is small, within a range of values ​​indicating an air-fuel ratio leaner than the stoichiometric air-fuel ratio.

[0019] In the following step S160, the ECU 20 determines whether the rear air-fuel ratio is equal to or greater than the lean determination value. If the rear air-fuel ratio is less than the lean determination value (NO), the ECU 20 ends the processing of this routine for the current control cycle. On the other hand, if the rear air-fuel ratio is equal to or greater than the lean determination value (YES), the ECU 20 switches the target air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio in step S170. Furthermore, the ECU 20 clears the value of the ammonia generation amount OV to "0" in the same step S170, and then ends the processing of this routine for the current control cycle.

[0020] (Actions and Effects of the Embodiments) The operation and effects of this embodiment will be described. Figure 4 shows an example of an embodiment of sub-feedback control. Figure 4(A) shows the change in the target air-fuel ratio, Figure 4(B) shows the change in the oxygen storage amount (OSA) of the three-way catalyst 12, and Figure 4(C) shows the change in the rear air-fuel ratio. Figure 4(D) shows the change in the amount of ammonia produced OV of the three-way catalyst 12, and Figure 4(E) shows the change in the oxygen storage amount of the oxidation catalyst 13. In the following explanation, combustion at an air-fuel ratio leaner than the stoichiometric air-fuel ratio will be referred to as lean combustion, and combustion at an air-fuel ratio richer than the stoichiometric air-fuel ratio will be referred to as rich combustion.

[0021] In the sub-feedback control, when the rear air-fuel ratio becomes equal to or greater than the lean determination value during lean combustion, the ECU 20 switches the target air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio and starts rich combustion. In the case of Figure 4, rich combustion starts at times t1, t4, t7, and t10.

[0022] During rich combustion, exhaust gas with high concentrations of unburned fuel components such as carbon monoxide (CO) and hydrocarbons (HC) flows into the three-way catalyst 12. The three-way catalyst 12 purifies the unburned fuel components in the exhaust gas by releasing the oxygen stored during lean combustion. After rich combustion begins, the rear air-fuel ratio indicates a value close to the stoichiometric air-fuel ratio while the three-way catalyst 12 is releasing the stored oxygen to purify the unburned fuel components. Thereafter, as the amount of oxygen stored in the three-way catalyst 12 approaches zero, the rear air-fuel ratio changes to the richer side of the stoichiometric air-fuel ratio.

[0023] When the rear air-fuel ratio becomes equal to or less than the rich determination value after the start of rich combustion, the ECU 20 switches the target air-fuel ratio from the rich air-fuel ratio to the lean air-fuel ratio to start lean combustion. In the case of FIG. 4, lean combustion starts at times t2, t5, and t8. During lean combustion, exhaust gas with high concentrations of NOx and oxygen flows into the three-way catalyst 12. At this time, the three-way catalyst 12 purifies the NOx in the exhaust by storing oxygen in the exhaust and forming a reducing atmosphere.

[0024] There is a limit to the amount of oxygen that the three-way catalyst 12 can store. In the following description, the upper limit of the amount of oxygen that the three-way catalyst 12 can store is referred to as the maximum storage amount. When the oxygen storage amount of the three-way catalyst 12 approaches the maximum storage amount, the remaining oxygen flows out of the three-way catalyst 12. As a result, the rear air-fuel ratio changes from a value indicating an air-fuel ratio richer than the stoichiometric air-fuel ratio to a value indicating an air-fuel ratio leaner than the stoichiometric air-fuel ratio. In the case of FIG. 4, the rear air-fuel ratio changes to a value indicating an air-fuel ratio leaner than the stoichiometric air-fuel ratio at times t3, t6, and t9. If the rear air-fuel ratio subsequently becomes equal to or greater than the lean determination value, the ECU 20 switches the target air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio, as described above.

[0025] Immediately after the start of lean combustion, unburned fuel components that flowed in during rich combustion remain in the three-way catalyst 12. During the period after the start of lean combustion when unburned fuel components remain in the three-way catalyst 12, ammonia is produced by the reaction shown in formula (2) during the reduction process of NOx in the exhaust gas. Ammonia is produced using carbon monoxide, an unburned fuel component. Therefore, if a large amount of unburned fuel components remain in the three-way catalyst 12 at the start of lean combustion, the amount of ammonia produced in the three-way catalyst 12 also increases.

[0026] [ka]

[0027] The ECU 20 estimates the amount of ammonia produced in the three-way catalyst 12. The ECU 20 estimates the amount of ammonia produced OV in the following manner. Immediately after the start of lean combustion, unburned fuel components remaining inside the three-way catalyst 12 flow out. Therefore, while the outflow of unburned fuel components continues after the start of lean combustion, the rear air-fuel ratio indicates a value that is richer than the stoichiometric air-fuel ratio. In other words, the rear air-fuel ratio immediately after switching the air-fuel ratio from a rich air-fuel ratio to a lean air-fuel ratio overshoots to a value richer than the stoichiometric air-fuel ratio. The deviation amount of the rear air-fuel ratio to a value richer than the stoichiometric air-fuel ratio at this time (= stoichiometric air-fuel ratio - rear air-fuel ratio) indicates the concentration of unburned fuel components in the exhaust gas flowing out of the three-way catalyst 12. In the following description, this deviation amount, i.e., the difference between the stoichiometric air-fuel ratio and the rear air-fuel ratio, is referred to as the rich deviation amount. The amount of unburned fuel components flowing out of the three-way catalyst 12 is calculated as the product of the rich deviation amount and the exhaust flow rate. The exhaust flow rate is approximately proportional to the intake air amount GA. In step S140 of FIG. 2, the ECU 20 calculates a value proportional to the amount of unburned fuel components flowing out from the three-way catalyst 12 as an update amount for the amount of ammonia production OV.

[0028] When all the unburned fuel components remaining in the three-way catalyst 12 have flowed out, the rear air-fuel ratio becomes a value indicating an air-fuel ratio leaner than the stoichiometric air-fuel ratio. In the following description, the period from the time when the air-fuel ratio is switched from a rich air-fuel ratio to a lean air-fuel ratio to the time when the rear air-fuel ratio becomes a value leaner than the stoichiometric air-fuel ratio is referred to as the overshoot period. The integrated value of the update amount during the overshoot period is proportional to the total amount of unburned fuel components that flowed out from the three-way catalyst 12 after the start of lean combustion. This value is proportional to the amount of unburned fuel components remaining in the three-way catalyst 12 at the start of lean combustion. In the following description, the amount of unburned fuel components remaining in the three-way catalyst 12 at the start of lean combustion is referred to as the amount of residual unburned fuel components.

[0029] As shown in the above equation (2), ammonia is produced in the three-way catalyst 12 using CO, an unburned fuel component. Therefore, when the amount of remaining unburned fuel components is large, the amount of ammonia produced OV in the three-way catalyst 12 also increases. In the sub-feedback control routine of FIG. 2, the ECU 20 calculates a value proportional to the amount of remaining unburned components as the value of the amount of ammonia produced OV. In this way, the ECU 20 estimates the amount of ammonia produced OV in the three-way catalyst 12 based on the amount of overshoot of the rear air-fuel ratio richer than the stoichiometric air-fuel ratio after lean combustion begins.

[0030] The ammonia produced in the three-way catalyst 12 flows into the oxidation catalyst 13 installed downstream. If the oxidation catalyst 13 has stored a sufficient amount of oxygen, it purifies the ammonia through the reactions shown in formulas (3) and (4).

[0031] [ka]

[0032] If the amount of oxygen stored in the oxidation catalyst 13 is insufficient compared to the amount of ammonia produced by the three-way catalyst 12, the ammonia that was not purified will be released into the outside air. The period during which the oxidation catalyst 13 stores oxygen when exhaust gas with a high oxygen concentration flows in is the period from when the three-way catalyst 12 can no longer store all the oxygen in the exhaust gas after lean combustion begins until lean combustion ends. In the following explanation, this period will be referred to as the oxygen storage period of the oxidation catalyst 13. In the case of Figure 4, the periods from time t3 to time t4, the period from time t6 to time t7, and the period from time t9 to time t10 correspond to the oxygen storage periods of the oxidation catalyst 13.

[0033] Here, the period from time t1 to time t4 in Figure 4 is defined as the first period, the period from time t4 to time t7 as the second period, and the period from time t7 to time t10 as the third period. Here, it is assumed that the exhaust flow rate is constant during the periods shown in Figure 4. In this case, the amount of ammonia generated OV in the three-way catalyst 12 during each period is proportional to the area of ​​the hatched portion in Figure 4(B). In Figure 4, the amount of ammonia generated OV during the second period is greater than that during the first period, and the amount of ammonia generated OV during the third period is even greater than that during the second period.

[0034] In the sub-feedback control routine of FIG. 2, when the ammonia generation amount OV is large, the ECU 20 sets the lean determination value to a value leaner than when the ammonia generation amount OV is small. In the example of FIG. 4, the lean determination value is set to a value leaner than that of the first period in the second period, and to a value even leaner than that of the second period in the third period. When the lean determination value is changed to the lean side, the oxygen storage period of the oxidation catalyst 13 is lengthened, and the amount of oxygen stored by the oxidation catalyst 13 during that period increases. As a result, the amount of ammonia that the oxidation catalyst 13 can purify in the next period increases. In this way, the ECU 20 variably sets the lean determination value according to the ammonia generation amount OV of the three-way catalyst 12, thereby adjusting the oxygen storage amount of the oxidation catalyst 13 to the amount necessary to purify ammonia.

[0035] In the engine control device of this embodiment, the second air-fuel ratio sensor 15 corresponds to an air-fuel ratio sensor installed in a portion of the exhaust passage 11 downstream of the three-way catalyst 12 and upstream of the oxidation catalyst 13. In addition, the rear air-fuel ratio, which is the air-fuel ratio detection value by the second air-fuel ratio sensor 15, corresponds to the air-fuel ratio detection value.

[0036] According to the engine control device of the present embodiment described above, the following effects can be achieved. (1) In the engine control device of this embodiment, the ECU 20 estimates the amount of ammonia generated by the three-way catalyst 12. The ECU 20 then variably sets the lean determination value in the sub-feedback control of the air-fuel ratio so that when the amount of ammonia generated is large, the value is leaner than when the amount of ammonia generated is small. The amount of oxygen storage in the oxidation catalyst 13 can be adjusted to ensure the amount necessary for purifying the ammonia generated by the three-way catalyst 12, depending on the amount of ammonia generated. This improves the efficiency of purifying the ammonia generated by the three-way catalyst 12.

[0037] (2) The ECU 20 calculates the amount of ammonia generated OV in the three-way catalyst 12 based on the amount of overshoot of the rear air-fuel ratio to a value richer than the stoichiometric air-fuel ratio after switching the air-fuel ratio from a rich air-fuel ratio to a lean air-fuel ratio. Specifically, the ECU 20 calculates the amount of ammonia generated OV as the integrated value of the product of the amount of rich deviation of the rear air-fuel ratio and the exhaust flow rate during the overshoot period after the start of lean combustion. Therefore, the amount of ammonia generated OV in the three-way catalyst 12 can be estimated with high accuracy.

[0038] (3) The ammonia production amount OV of the three-way catalyst 12 changes depending on the state of the catalyst. For example, the amount of ammonia produced increases. In contrast, a three-way catalyst 12 that has deteriorated over time has lower catalytic activity than a three-way catalyst 12 immediately after production, and therefore the ammonia production amount OV decreases. The engine control device of this embodiment can maintain an appropriate ammonia purification capacity in accordance with such changes in the activity state of the three-way catalyst 12 over time.

[0039] (Other embodiments) This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

[0040] The ammonia generation amount OV may be calculated in a manner different from the above embodiment as long as it is based on the amount of overshoot of the rear air-fuel ratio richer than the stoichiometric air-fuel ratio. For example, the ammonia generation amount OV may be calculated based on the peak value of the rich deviation amount during the overshoot period. Alternatively, the ammonia generation amount OV may be calculated based on the length of the overshoot period.

[0041] The engine control device of the above embodiment and modified examples can also be applied to engines having a configuration different from that shown in FIG. 1, as long as the engine has a three-way catalyst 12, an oxidation catalyst 13, and a second air-fuel ratio sensor 15 installed in this order from the upstream side of the exhaust passage 11. [Explanation of symbols]

[0042] 10 Engine 11 Exhaust passage 12 Three-way catalyst 13 Oxidation catalyst 14 First air-fuel ratio sensor 15 Second air-fuel ratio sensor 20 ECU (Engine Control Unit) 21 processors 22 Memory

Claims

1. The present invention is applied to an engine including a three-way catalyst installed in an exhaust passage, an oxidation catalyst installed in a portion of the exhaust passage downstream of the three-way catalyst, and an air-fuel ratio sensor installed in a portion of the exhaust passage downstream of the three-way catalyst and upstream of the oxidation catalyst, an engine control device that performs sub-feedback control of an air-fuel ratio, wherein when an air-fuel ratio detection value of the air-fuel ratio sensor becomes richer than a rich determination value set to a value indicating an air-fuel ratio richer than a stoichiometric air-fuel ratio, the air-fuel ratio of the mixture burned in the engine is switched from a rich air-fuel ratio richer than the stoichiometric air-fuel ratio to a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio, and when the air-fuel ratio detection value becomes leaner than a lean determination value set to a value indicating an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the air-fuel ratio is switched from the lean air-fuel ratio to the rich air-fuel ratio, An amount of ammonia produced by the three-way catalyst is calculated based on an amount of overshoot of the air-fuel ratio detection value to a value richer than the stoichiometric air-fuel ratio after switching the air-fuel ratio from the rich air-fuel ratio to the lean air-fuel ratio, and when the amount of ammonia produced is large, the lean determination value is variably set to a value indicating a leaner air-fuel ratio than when the amount of ammonia produced is small. Engine control device.

2. The amount of ammonia produced is calculated as a value obtained by integrating the product of the rich deviation amount and the exhaust flow rate during an overshoot period, the overshoot period is a period from a point in time when the air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio to a point in time when the detected air-fuel ratio value becomes a value leaner than the stoichiometric air-fuel ratio, The rich deviation amount is the difference obtained by subtracting the detected air-fuel ratio value from the stoichiometric air-fuel ratio. The engine control device according to claim 1 .

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