JP7605147B2 - Exhaust gas purification device for internal combustion engines - Google Patents

Description

The present invention relates to an exhaust gas purification device for an internal combustion engine.

It is known that a catalyst is placed in the exhaust passage of an internal combustion engine to purify harmful substances (HC, NOx, etc.) in the exhaust gas. Patent Document 1 describes how a reducing agent is supplied to the catalyst to improve the NOx purification performance.

JP 2002-349249 A

However, when a reducing agent is supplied to the catalyst, ammonia is produced from NOx through a chemical reaction in the catalyst, and the produced ammonia may leak out of the catalyst.

In view of the above problems, the object of the present invention is to promote purification of NOx in the catalyst while suppressing the outflow of ammonia from the catalyst.

The gist of this disclosure is as follows:

(1) An exhaust purification device for an internal combustion engine, comprising: a catalyst arranged in an exhaust passage of an internal combustion engine and capable of storing oxygen; an oxygen storage amount calculation unit that calculates the amount of oxygen stored downstream of the catalyst; and an air-fuel ratio control unit that controls the air-fuel ratio of the inflowing exhaust gas flowing into the catalyst, the air-fuel ratio control unit executing rich control that makes the air-fuel ratio of the inflowing exhaust gas richer than the theoretical air-fuel ratio, and controlling the air-fuel ratio of the inflowing exhaust gas so that when the amount of oxygen stored is relatively large, the amount of reducing agent supplied to the catalyst in the rich control is greater than when the amount of oxygen stored is relatively small.

(2) The exhaust gas purification device for an internal combustion engine described in (1) above, in which the air-fuel ratio control unit increases the richness of the air-fuel ratio of the inflowing exhaust gas in the rich control when the oxygen storage amount is relatively large, compared to when the oxygen storage amount is relatively small.

(3) An exhaust gas purification device for an internal combustion engine as described in (1) or (2) above, in which the air-fuel ratio control unit extends the execution time of the rich control when the amount of oxygen stored is relatively large, compared to when the amount of oxygen stored is relatively small.

(4) An exhaust gas purification device for an internal combustion engine described in any one of (1) to (3) above, in which the air-fuel ratio control unit controls the air-fuel ratio of the inflowing exhaust gas so as to increase the amount of oxygen stored when the amount of oxygen stored is equal to or less than a predetermined threshold value.

(5) An exhaust gas purification device for an internal combustion engine according to any one of (1) to (4) above, further comprising an output control unit that controls the output of the internal combustion engine and the output of a motor provided in a vehicle equipped with the internal combustion engine as a power source of the vehicle, and when the output of the internal combustion engine falls short of a required output due to a decrease in the supply amount of the reducing agent, the output control unit causes the motor to output the shortfall in the required output.

The present invention can promote purification of NOx in the catalyst while suppressing the outflow of ammonia from the catalyst.

FIG. 1 is a diagram that shows a schematic diagram of an internal combustion engine to which an exhaust gas purification device for an internal combustion engine according to a first embodiment of the present invention is applied. FIG. 2 is a graph showing an example of the purification characteristic of a three-way catalyst. FIG. 3 is a functional block diagram of the ECU in the first embodiment. FIG. 4 is a diagram showing the amounts of NOx and ammonia flowing out from the catalyst during rich control when the amount of oxygen stored downstream of the catalyst is changed. FIG. 5 is a flowchart showing a control routine for the reducing agent supply process in the first embodiment. FIG. 6 is a diagram showing the relationship between the amount of oxygen stored downstream of the catalyst and the amount of reducing agent supplied to the catalyst. FIG. 7 is a flowchart showing a control routine for the reducing agent supply process in the second embodiment. FIG. 8 is a diagram showing the relationship between the amount of oxygen stored downstream of the catalyst and the amount of oxygen supplied to the catalyst. FIG. 9 is a functional block diagram of an ECU in the third embodiment. FIG. 10 is a flowchart showing a control routine for the reducing agent supply process in the third embodiment.

Embodiments of the present invention will be described in detail below with reference to the drawings. In the following description, similar components are given the same reference numbers.

First Embodiment
First, a first embodiment of the present invention will be described with reference to FIGS.

<Overall explanation of the internal combustion engine>
Fig. 1 is a schematic diagram of an internal combustion engine to which an exhaust gas purification device for an internal combustion engine according to a first embodiment of the present invention is applied. The internal combustion engine shown in Fig. 1 is a spark ignition internal combustion engine, specifically, a gasoline engine that uses gasoline as fuel. The internal combustion engine is mounted on a vehicle and functions as a power source for the vehicle.

The internal combustion engine has an engine body 1 including a cylinder block 2 and a cylinder head 4. A plurality of cylinders (e.g., four) are formed inside the cylinder block 2. A piston 3 that reciprocates in the axial direction of the cylinder is disposed in each cylinder. A combustion chamber 5 is formed between the piston 3 and the cylinder head 4.

The cylinder head 4 is formed with an intake port 7 and an exhaust port 9. The intake port 7 and the exhaust port 9 are each connected to the combustion chamber 5.

The internal combustion engine also includes an intake valve 6 and an exhaust valve 8 arranged in the cylinder head 4. The intake valve 6 opens and closes the intake port 7, and the exhaust valve 8 opens and closes the exhaust port 9.

The internal combustion engine also includes a spark plug 10 and a fuel injection valve 11. The spark plug 10 is disposed in the center of the inner wall surface of the cylinder head 4, and generates a spark in response to an ignition signal. The fuel injection valve 11 is disposed on the periphery of the inner wall surface of the cylinder head 4, and injects fuel into the combustion chamber 5 in response to an injection signal. In this embodiment, gasoline with a theoretical air-fuel ratio of 14.6 is used as the fuel supplied to the fuel injection valve 11.

The internal combustion engine also includes an intake manifold 13, a surge tank 14, an intake pipe 15, an air cleaner 16, and a throttle valve 18. The intake ports 7 of each cylinder are connected to the surge tank 14 via the corresponding intake manifold 13, and the surge tank 14 is connected to the air cleaner 16 via the intake pipe 15. The intake ports 7, the intake manifold 13, the surge tank 14, the intake pipe 15, etc. form an intake passage that guides air to the combustion chamber 5. The throttle valve 18 is disposed in the intake pipe 15 between the surge tank 14 and the air cleaner 16, and is driven by a throttle valve drive actuator 17 (e.g., a DC motor). The throttle valve 18 is rotated by the throttle valve drive actuator 17, so that the opening area of the intake passage can be changed according to its opening degree.

The internal combustion engine also includes an exhaust manifold 19, an exhaust pipe 22, and a catalyst 20. The exhaust port 9 of each cylinder is connected to the exhaust manifold 19. The exhaust manifold 19 has multiple branches connected to each exhaust port 9, and a collection section where these branches are gathered. The collection section of the exhaust manifold 19 is connected to a casing 21 that contains a catalyst 20. The casing 21 is connected to an exhaust pipe 22. The exhaust port 9, exhaust manifold 19, casing 21, exhaust pipe 22, etc. form an exhaust passage that discharges exhaust gas generated by the combustion of the mixture in the combustion chamber 5.

The vehicle equipped with the internal combustion engine is also provided with an electronic control unit (ECU) 31. As shown in FIG. 1, the ECU 31 is a digital computer, and includes a RAM (random access memory) 33, a ROM (read only memory) 34, a CPU (microprocessor) 35, an input port 36, and an output port 37, all of which are interconnected via a bidirectional bus 32.

The ECU 31 executes various controls of the vehicle and the internal combustion engine based on the outputs of various sensors provided in the vehicle or the internal combustion engine. To this end, the outputs of the various sensors are transmitted to the ECU 31. In this embodiment, the outputs of the air flow meter 40, the air-fuel ratio sensor 41, the temperature sensor 42, the load sensor 43, and the crank angle sensor 44 are transmitted to the ECU 31.

The air flow meter 40 is disposed in the intake passage of the internal combustion engine, specifically, in the intake pipe 15 upstream of the throttle valve 18. The air flow meter 40 detects the flow rate of air flowing through the intake passage. The air flow meter 40 is electrically connected to the ECU 31, and the output of the air flow meter 40 is input to the input port 36 via a corresponding AD converter 38.

The air-fuel ratio sensor 41 is disposed in the exhaust passage upstream of the catalyst 20, specifically in the assembly portion of the exhaust manifold 19. The air-fuel ratio sensor 41 detects the air-fuel ratio of the exhaust gas discharged from the cylinders of the internal combustion engine and flowing into the catalyst 20. The air-fuel ratio sensor 41 is electrically connected to the ECU 31, and the output of the air-fuel ratio sensor 41 is input to the input port 36 via the corresponding AD converter 38.

The temperature sensor 42 is disposed in the casing 21 that houses the catalyst 20. The temperature sensor 42 detects the temperature (bed temperature) of the catalyst 20. The temperature sensor 42 is electrically connected to the ECU 31, and the output of the temperature sensor 42 is input to the input port 36 via the corresponding AD converter 38.

The load sensor 43 is connected to an accelerator pedal 50 provided on a vehicle equipped with an internal combustion engine, and detects the amount of depression of the accelerator pedal 50. The load sensor 43 is electrically connected to the ECU 31, and the output of the load sensor 43 is input to the input port 36 via a corresponding AD converter 38. The ECU 31 calculates the engine load based on the output of the load sensor 43.

The crank angle sensor 44 generates an output pulse each time the crankshaft of the internal combustion engine rotates a predetermined angle (e.g., 10 degrees). The crank angle sensor 44 is electrically connected to the ECU 31, and the output of the crank angle sensor 44 is input to the input port 36. The ECU 31 calculates the engine speed based on the output of the crank angle sensor 44.

Meanwhile, the output port 37 of the ECU 31 is connected to the spark plug 10, the fuel injection valve 11, and the throttle valve drive actuator 17 via corresponding drive circuits 39, and the ECU 31 controls these. Specifically, the ECU 31 controls the ignition timing of the spark plug 10, the injection timing and injection amount of fuel injected from the fuel injection valve 11, and the opening of the throttle valve 18.

The internal combustion engine described above is a non-supercharged internal combustion engine that uses gasoline as fuel, but the configuration of the internal combustion engine is not limited to the above configuration. Therefore, the specific configuration of the internal combustion engine, such as the cylinder arrangement, fuel injection mode, intake and exhaust system configuration, valve mechanism configuration, and the presence or absence of a supercharger, may be different from the configuration shown in FIG. 1. For example, the fuel injection valve 11 may be arranged to inject fuel into the intake port 7. Also, a configuration for recirculating EGR gas from the exhaust passage to the intake passage may be provided.

<Exhaust gas purification device for internal combustion engine>
An exhaust gas purification device for an internal combustion engine (hereinafter simply referred to as the “exhaust gas purification device”) according to an embodiment of the present invention will be described below. The exhaust gas purification device includes a catalyst 20 and an ECU 31.

The catalyst 20 is disposed in an exhaust passage of an internal combustion engine and configured to purify exhaust gas flowing through the exhaust passage. In this embodiment, the catalyst 20 is a three-way catalyst capable of simultaneously purifying hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). The catalyst 20 has a support (substrate) made of ceramic or metal, a precious metal having catalytic action (e.g., platinum (Pt), palladium (Pd), rhodium (Rh), etc.), and a promoter having oxygen storage capacity (e.g., ceria (CeO ₂ ) etc.). The precious metal and the promoter are supported on the support.

Figure 2 is a diagram showing an example of the purification characteristics of a three-way catalyst. As shown in Figure 2, the purification rate of HC, CO, and NOx by the three-way catalyst is very high when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is in the region near the stoichiometric air-fuel ratio (purification window A in Figure 2). Therefore, the catalyst 20 can effectively purify HC, CO, and NOx when the air-fuel ratio of the exhaust gas is maintained near the stoichiometric air-fuel ratio.

The catalyst 20 also stores or releases oxygen depending on the air-fuel ratio of the exhaust gas by the auxiliary catalyst. Specifically, when the air-fuel ratio of the exhaust gas is leaner than the stoichiometric air-fuel ratio, the catalyst 20 stores excess oxygen in the exhaust gas. On the other hand, when the air-fuel ratio of the exhaust gas is richer than the stoichiometric air-fuel ratio, the catalyst 20 releases oxygen that is insufficient to oxidize HC and CO. As a result, even if the air-fuel ratio of the exhaust gas deviates slightly from the stoichiometric air-fuel ratio, the air-fuel ratio on the surface of the catalyst 20 is maintained close to the stoichiometric air-fuel ratio, and HC, CO, and NOx are effectively purified by the catalyst 20.

Figure 3 is a functional block diagram of the ECU 31 in the first embodiment. In this embodiment, the ECU 31 has an oxygen storage amount calculation unit 61 and an air-fuel ratio control unit 62. The oxygen storage amount calculation unit 61 and the air-fuel ratio control unit 62 are functional modules that are realized by the CPU 35 of the ECU 31 executing a program stored in the ROM 34 of the ECU 31.

The oxygen storage amount calculation unit 61 calculates the amount of oxygen stored in the catalyst 20, i.e., the oxygen storage amount of the catalyst 20. In particular, in this embodiment, as will be described in detail later, the storage amount calculation unit 61 calculates the oxygen storage amount in the downstream portion of the catalyst 20.

The air-fuel ratio control unit 62 controls the air-fuel ratio of the exhaust gas (hereinafter referred to as "inflow exhaust gas") flowing into the catalyst 20. Specifically, the air-fuel ratio control unit 62 sets a target air-fuel ratio and controls the amount of fuel supplied to the combustion chamber 5 so that the air-fuel ratio of the inflow exhaust gas matches the target air-fuel ratio. For example, the air-fuel ratio control unit 62 feedback controls the amount of fuel supplied to the combustion chamber 5 so that the air-fuel ratio detected by the air-fuel ratio sensor 41 matches the target air-fuel ratio.

The air-fuel ratio control unit 62 may control the amount of fuel supplied to the combustion chamber 5 so that the air-fuel ratio of the inflowing exhaust gas matches the target air-fuel ratio without using the air-fuel ratio sensor 41. In this case, the air-fuel ratio control unit 62 calculates the amount of fuel supplied to the combustion chamber 5 from the intake air amount, engine speed, and target air-fuel ratio so that the ratio of fuel and air supplied to the combustion chamber 5 matches the target air-fuel ratio.

In this embodiment, the air-fuel ratio control unit 62 purifies the NOx in the exhaust gas by creating a reducing atmosphere in the catalyst 20. That is, in order to purify the NOx in the exhaust gas, the air-fuel ratio control unit 62 executes rich control to make the air-fuel ratio of the inflowing exhaust gas richer than the theoretical air-fuel ratio. This makes it possible to suppress the deterioration of exhaust emissions due to the outflow of NOx.

However, when a reducing agent is supplied to the catalyst 20 by rich control, ammonia is generated from NOx by a chemical reaction in the catalyst 20, and the generated ammonia may flow out from the catalyst 20. The catalyst 20 is composed of an upstream portion and a downstream portion, and ammonia is generated in the upstream portion of the catalyst 20 before it flows out from the downstream portion of the catalyst 20. In this regard, the inventor of the present application has found, as a result of intensive research, that even if ammonia is generated in the catalyst 20, if oxygen is stored in the downstream portion of the catalyst 20, the flow out of ammonia from the catalyst 20 is suppressed. Note that the downstream portion of the catalyst 20 (hereinafter referred to as the "catalyst downstream portion") is located downstream of the upstream portion of the catalyst 20 (hereinafter referred to as the "catalyst upstream portion") in the exhaust passage of the internal combustion engine, and the boundary between the catalyst upstream portion and the catalyst downstream portion is set, for example, at the center position in the longitudinal direction of the catalyst 20.

4 is a diagram showing the amounts (integrated values) of NOx and ammonia ( _NH3 ) flowing out from the catalyst 20 during rich control when the amount of oxygen stored downstream of the catalyst is changed. In this experiment, the amount of oxygen stored downstream of the catalyst was adjusted to three stages (large, medium, and small) before supplying reducing agent to the catalyst 20 by rich control.

As shown in FIG. 4, when the amount of oxygen stored downstream of the catalyst is the greatest, the amount of ammonia flowing out from the catalyst 20 during rich control is reduced by 48% compared to when the amount of oxygen stored downstream of the catalyst is the least. On the other hand, the amount of NOx flowing out from the catalyst 20 during rich control is almost constant regardless of the amount of oxygen stored downstream of the catalyst. This phenomenon is thought to occur when ammonia generated upstream of the catalyst is oxidized and eliminated by the oxygen stored downstream of the catalyst. For this reason, in order to promote purification of NOx while suppressing the outflow of ammonia, it is desirable to supply a reducing agent to the catalyst 20 in an amount that does not deplete the oxygen stored downstream of the catalyst.

In this embodiment, the oxygen storage amount calculation unit 61 calculates the oxygen storage amount downstream of the catalyst, and the air-fuel ratio control unit 62 controls the air-fuel ratio of the inflowing exhaust gas so that the amount of reducing agent supplied to the catalyst 20 in rich control is greater when the oxygen storage amount downstream of the catalyst is relatively large, compared to when the oxygen storage amount downstream of the catalyst is relatively small. This makes it possible to supply an appropriate amount of reducing agent according to the oxygen storage amount downstream of the catalyst to the catalyst 20, thereby promoting the purification of NOx in the catalyst 20 while suppressing the outflow of ammonia from the catalyst 20.

<Reducing Agent Supply Process>
The above-mentioned control flow will be described below with reference to the flowchart of Fig. 5. Fig. 5 is a flowchart showing a control routine for the reducing agent supply process in the first embodiment. This control routine is repeatedly executed by the ECU 31 at predetermined time intervals.

First, in step S101, the oxygen storage amount calculation unit 61 calculates the oxygen storage amount OSAd downstream of the catalyst. For example, the oxygen storage amount calculation unit 61 uses a map or a formula to calculate the amount of oxygen absorbed and released downstream of the catalyst (the amount of change in the oxygen storage amount) based on predetermined parameters, and calculates the oxygen storage amount OSAd downstream of the catalyst by integrating this amount of absorption and release. The predetermined parameters are, for example, the temperature of the catalyst 20, the flow rate of the inflowing exhaust gas, and the air-fuel ratio of the inflowing exhaust gas.

The oxygen storage amount calculation unit 61 estimates the temperature of the catalyst 20 based on, for example, the output of the temperature sensor 42. The temperature sensor 42 may be disposed in the exhaust passage upstream of the catalyst 20 to detect the temperature of the inflowing exhaust gas, or may be disposed in the exhaust passage downstream of the catalyst 20 to detect the temperature of the exhaust gas flowing out from the catalyst 20. The oxygen storage amount calculation unit 61 may also estimate the temperature of the catalyst 20 based on a predetermined state quantity of the internal combustion engine (for example, engine water temperature, intake air volume, engine load, etc.) without using the temperature sensor 42.

The oxygen storage amount calculation unit 61 also estimates the flow rate of the inflowing exhaust gas based on, for example, the output of the air flow meter 40. Note that a flow rate sensor that detects the flow rate of the exhaust gas may be provided in the exhaust passage upstream of the catalyst 20, and the oxygen storage amount calculation unit 61 may estimate the flow rate of the inflowing exhaust gas based on the output of this flow rate sensor.

The oxygen storage amount calculation unit 61 estimates the air-fuel ratio of the inflowing exhaust gas based on the output of the air-fuel ratio sensor 41, for example. Note that, as the air-fuel ratio of the inflowing exhaust gas, a target air-fuel ratio of the inflowing exhaust gas determined by the air-fuel ratio control unit 62 may be used instead of the air-fuel ratio detected by the air-fuel ratio sensor 41. Also, an air-fuel ratio sensor (catalyst air-fuel ratio sensor) that detects the air-fuel ratio of the exhaust gas flowing into the catalyst downstream portion may be provided at the boundary between the catalyst upstream portion and the catalyst downstream portion, and the air-fuel ratio detected by the catalyst air-fuel ratio sensor may be used as the air-fuel ratio of the inflowing exhaust gas instead of the air-fuel ratio detected by the air-fuel ratio sensor 41.

In addition, in the internal combustion engine, when fuel cut control is executed to stop the fuel supply to the combustion chamber 5, the oxygen storage amount calculation unit 61 increases the oxygen storage amount OSAd downstream of the catalyst according to the execution time of the fuel cut control, etc. The air-fuel ratio control unit 62 executes the fuel cut control, for example, when the depression amount of the accelerator pedal 50 is zero or almost zero (i.e., the engine load is zero or almost zero) and the engine speed is equal to or higher than a predetermined speed that is higher than the idling speed.

In addition, an air-fuel ratio sensor (downstream air-fuel ratio sensor) that detects the air-fuel ratio of the exhaust gas flowing out from the catalyst 20 may be provided in the exhaust passage downstream of the catalyst 20, and the oxygen storage amount calculation unit 61 may correct the oxygen storage amount OSAd downstream of the catalyst based on the output of the downstream air-fuel ratio sensor. In this case, for example, when the output air-fuel ratio of the downstream air-fuel ratio sensor reaches a rich judgment air-fuel ratio (e.g., 14.55) that is slightly richer than the stoichiometric air-fuel ratio, the oxygen storage amount OSAd is corrected to zero, and when the output air-fuel ratio of the downstream air-fuel ratio sensor reaches a lean judgment air-fuel ratio (e.g., 14.65) that is slightly leaner than the stoichiometric air-fuel ratio, the oxygen storage amount OSAd is corrected to the maximum value. Note that the output air-fuel ratio of the downstream air-fuel ratio sensor means the air-fuel ratio corresponding to the output current of the downstream air-fuel ratio sensor, that is, the air-fuel ratio detected by the downstream air-fuel ratio sensor.

Next, in step S102, the air-fuel ratio control unit 62 determines the amount of reducing agent to be supplied to the catalyst 20 based on the oxygen storage amount OSAd downstream of the catalyst calculated by the oxygen storage amount calculation unit 61. At this time, when the oxygen storage amount OSAd is relatively large, the air-fuel ratio control unit 62 increases the amount of reducing agent to be supplied to the catalyst 20 compared to when the oxygen storage amount OSAd is relatively small. For example, the air-fuel ratio control unit 62 determines the amount of reducing agent to be supplied based on the oxygen storage amount OSAd using a map such as that shown in FIG. 6. Specifically, as shown by the solid line in FIG. 6, the air-fuel ratio control unit 62 linearly increases the amount of reducing agent to be supplied as the oxygen storage amount OSAd increases. Note that, as shown by the dashed line in FIG. 6, the air-fuel ratio control unit 62 may increase the amount of reducing agent to be supplied in stages as the oxygen storage amount OSAd increases.

Next, in step S103, the air-fuel ratio control unit 62 executes rich control to make the air-fuel ratio of the inflowing exhaust gas richer than the theoretical air-fuel ratio so that the supply amount of reducing agent determined in step S102 is supplied to the catalyst 20. At this time, the air-fuel ratio control unit 62 increases the richness degree of the air-fuel ratio of the inflowing exhaust gas in the rich control as the supply amount of reducing agent increases. That is, when the oxygen storage amount OSAd is relatively large, the air-fuel ratio control unit 62 increases the richness degree of the air-fuel ratio of the inflowing exhaust gas in the rich control compared to when the oxygen storage amount OSAd is relatively small. Note that the richness degree means the difference between the air-fuel ratio richer than the theoretical air-fuel ratio and the theoretical air-fuel ratio. In addition, the air-fuel ratio control unit 62 may extend the execution time (continuation time) of the rich control as the supply amount of reducing agent increases. That is, when the oxygen storage amount OSAd is relatively large, the air-fuel ratio control unit 62 may extend the execution time of the rich control compared to when the oxygen storage amount OSAd is relatively small. After step S103, this control routine ends.

Second Embodiment
The configuration and control of the exhaust purification system in the second embodiment are basically the same as those of the exhaust purification system in the first embodiment, except for the points described below. Therefore, the second embodiment of the present invention will be described below, focusing on the differences from the first embodiment.

If the amount of oxygen stored downstream of the catalyst remains low, even if the amount of reducing agent supplied to the catalyst 20 is limited, the oxygen downstream of the catalyst may be depleted, and ammonia may flow out of the catalyst 20. For this reason, in the second embodiment, when the amount of oxygen stored downstream of the catalyst is equal to or less than a predetermined threshold, the air-fuel ratio control unit 62 controls the air-fuel ratio of the inflowing exhaust gas so that the amount of oxygen stored downstream of the catalyst increases. This makes it possible to restore the amount of oxygen stored downstream of the catalyst, and thus further suppresses the outflow of ammonia from the catalyst 20.

Figure 7 is a flowchart showing a control routine for the reducing agent supply process in the second embodiment. This control routine is repeatedly executed by the ECU 31 at predetermined time intervals.

First, in step S201, similar to step S101 in FIG. 5, the oxygen storage amount calculation unit 61 calculates the oxygen storage amount OSAd downstream of the catalyst.

Next, in step S202, the air-fuel ratio control unit 62 determines whether the oxygen storage amount OSAd downstream of the catalyst calculated by the oxygen storage amount calculation unit 61 is equal to or less than the threshold value TH. The threshold value TH is set in advance, for example, to a value less than 1/2 of the maximum oxygen storage amount downstream of the catalyst. If it is determined in step S202 that the oxygen storage amount OSAd is equal to or less than the threshold value TH, this control routine proceeds to step S203.

In step S203, the air-fuel ratio control unit 62 determines the amount of oxygen to be supplied to the downstream of the catalyst based on the oxygen storage amount OSAd of the downstream of the catalyst calculated by the oxygen storage amount calculation unit 61. At this time, when the oxygen storage amount OSAd is relatively small, the air-fuel ratio control unit 62 increases the amount of oxygen to be supplied to the downstream of the catalyst compared to when the oxygen storage amount OSAd is relatively large. For example, the air-fuel ratio control unit 62 determines the amount of oxygen to be supplied based on the oxygen storage amount OSAd using a map such as that shown in FIG. 8. Specifically, as shown by the solid line in FIG. 8, the air-fuel ratio control unit 62 linearly increases the amount of oxygen to be supplied as the oxygen storage amount OSAd decreases. Note that, as shown by the dashed line in FIG. 8, the air-fuel ratio control unit 62 may increase the amount of oxygen to be supplied in stages as the oxygen storage amount OSAd decreases.

Next, in step S204, the air-fuel ratio control unit 62 controls the air-fuel ratio of the inflowing exhaust gas so that the supply amount of oxygen determined in step S203 is supplied to the downstream portion of the catalyst. For example, the air-fuel ratio control unit 62 executes lean control to make the air-fuel ratio of the inflowing exhaust gas leaner than the theoretical air-fuel ratio. At this time, the air-fuel ratio control unit 62 increases the lean degree of the air-fuel ratio of the inflowing exhaust gas in the lean control as the supply amount of oxygen increases. That is, when the oxygen storage amount OSAd is relatively small, the air-fuel ratio control unit 62 increases the lean degree of the air-fuel ratio of the inflowing exhaust gas in the lean control compared to when the oxygen storage amount OSAd is relatively large. Note that the lean degree means the difference between the air-fuel ratio leaner than the theoretical air-fuel ratio and the theoretical air-fuel ratio. In addition, the air-fuel ratio control unit 62 may extend the execution time (continuation time) of the lean control as the supply amount of oxygen increases. That is, when the oxygen storage amount OSAd is relatively small, the air-fuel ratio control unit 62 may extend the execution time of the lean control compared to when the oxygen storage amount OSAd is relatively large.

Also, the air-fuel ratio control unit 62 may supply oxygen downstream of the catalyst by executing fuel cut control. In this case, for example, the air-fuel ratio control unit 62 may extend the execution time (duration) of the fuel cut control as the amount of oxygen supplied increases, or may increase the opening of the throttle valve 18 during the fuel cut control as the amount of oxygen supplied increases. After step S204, this control routine ends.

On the other hand, if it is determined in step S202 that the oxygen storage amount OSAd is greater than the threshold value TH, the control routine proceeds to step S205. In step S205, similar to step S102 in FIG. 5, the air-fuel ratio control unit 62 determines the amount of reducing agent to be supplied to the catalyst 20 based on the oxygen storage amount OSAd downstream of the catalyst calculated by the oxygen storage amount calculation unit 61.

Next, in step S206, similar to step S103 in FIG. 5, the air-fuel ratio control unit 62 executes rich control so that the amount of reducing agent determined in step S205 is supplied to the catalyst 20. After step S206, this control routine ends.

Third Embodiment
The configuration and control of the exhaust purification device in the third embodiment are basically the same as those of the exhaust purification device in the first embodiment, except for the points described below. Therefore, the third embodiment of the present invention will be described below, focusing on the differences from the first embodiment.

Figure 9 is a functional block diagram of the ECU 31 in the third embodiment. In the third embodiment, the ECU 31 has an output control unit 63 in addition to an oxygen storage amount calculation unit 61 and an air-fuel ratio control unit 62. The oxygen storage amount calculation unit 61, the air-fuel ratio control unit 62, and the output control unit 63 are functional modules that are realized by the CPU 35 of the ECU 31 executing a program stored in the ROM 34 of the ECU 31.

In the third embodiment, a vehicle equipped with an internal combustion engine includes a motor (electric motor) in addition to the internal combustion engine as a power source for the vehicle. In other words, the vehicle to which the exhaust purification device for an internal combustion engine according to the third embodiment is applied is a so-called hybrid vehicle (HEV) or plug-in hybrid vehicle (PHEV). The output control unit 63 controls the output of the internal combustion engine and the output of the motor according to the required output, etc.

As described above, when the amount of oxygen stored downstream of the catalyst is small, the air-fuel ratio control unit 62 limits the amount of reducing agent supplied to the catalyst 20 in rich control. As a result, the output of the internal combustion engine decreases, and there is a risk that the output of the internal combustion engine will be insufficient for the required output according to the accelerator opening (amount of depression of the accelerator pedal 50) etc.

Therefore, in the third embodiment, when the output of the internal combustion engine is insufficient for the required output due to a decrease in the amount of reducing agent supplied to the catalyst 20, the output control unit 63 causes the motor to output the insufficient output for the required output. This makes it possible to suppress ammonia from leaking out of the catalyst 20 while suppressing a decrease in the vehicle's acceleration performance due to a decrease in the output of the internal combustion engine.

Figure 10 is a flowchart showing a control routine for the reducing agent supply process in the third embodiment. This control routine is repeatedly executed by the ECU 31 at predetermined time intervals.

First, in step S301, the oxygen storage amount calculation unit 61 calculates the oxygen storage amount OSAd downstream of the catalyst, similar to step S101 in FIG. 5. Next, in step S302, the air-fuel ratio control unit 62 determines the amount of reducing agent to be supplied to the catalyst 20 based on the oxygen storage amount OSAd downstream of the catalyst calculated by the oxygen storage amount calculation unit 61, similar to step S102 in FIG. 5.

Next, in step S303, the air-fuel ratio control unit 62 determines whether the output of the internal combustion engine will be insufficient for the required output when the amount of reducing agent determined in step S102 is supplied to the catalyst 20. If it is determined that the output of the internal combustion engine will be insufficient for the required output, the control routine proceeds to step S304.

In step S304, the output control unit 63 increases the output of the motor by, for example, increasing the amount of power supplied from the battery to the motor so as to offset the decrease in output of the internal combustion engine. That is, the output control unit 63 causes the motor to output an output that is insufficient relative to the required output. Next, in step S305, similar to step S103 in FIG. 5, the air-fuel ratio control unit 62 executes rich control so that the amount of reducing agent determined in step S102 is supplied to the catalyst 20. After step S305, this control routine ends.

On the other hand, if it is determined in step S303 that the output of the internal combustion engine is not insufficient for the required output, this control routine skips step S304 and proceeds to step S305. In step S305, similar to step S103 in FIG. 5, the air-fuel ratio control unit 62 executes rich control so that the amount of reducing agent determined in step S102 is supplied to the catalyst 20. After step S305, this control routine ends.

<Other embodiments>
Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the claims. For example, in the above-mentioned embodiment, the upstream portion and the downstream portion of the catalyst are integrally formed as the catalyst 20, but the upstream portion and the downstream portion of the catalyst may be provided separately in the exhaust passage as separate catalysts.

The above-described embodiments can be implemented in any combination. For example, when the second and third embodiments are combined, steps S303 and S304 in FIG. 10 are executed between steps S205 and S206 in the control routine for the reducing agent supply process in FIG. 7.

20 Catalyst 31 Electronic control unit (ECU)
61 Oxygen storage amount calculation unit 62 Air-fuel ratio control unit

Claims (5)

a catalyst disposed in an exhaust passage of the internal combustion engine and capable of storing oxygen;
an oxygen storage amount calculation unit that calculates an oxygen storage amount in a downstream portion of the catalyst;
an air-fuel ratio control unit for controlling an air-fuel ratio of an inflow exhaust gas flowing into the catalyst,
The air-fuel ratio control unit executes rich control to make the air-fuel ratio of the inflowing exhaust gas richer than the stoichiometric air-fuel ratio, and when the oxygen storage amount is relatively large, controls the air-fuel ratio of the inflowing exhaust gas so that the amount of reducing agent supplied to the catalyst in the rich control is greater than when the oxygen storage amount is relatively small. The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the air-fuel ratio control unit increases the richness of the air-fuel ratio of the inflowing exhaust gas in the rich control when the oxygen storage amount is relatively large, compared to when the oxygen storage amount is relatively small. The exhaust gas purification device for an internal combustion engine according to claim 1 or 2, wherein the air-fuel ratio control section extends the execution time of the rich control when the amount of oxygen stored is relatively large, compared to when the amount of oxygen stored is relatively small. The exhaust gas purification device for an internal combustion engine according to any one of claims 1 to 3, wherein the air-fuel ratio control unit controls the air-fuel ratio of the inflowing exhaust gas so as to increase the amount of oxygen stored when the amount of oxygen stored is equal to or less than a predetermined threshold value. an output control unit that controls an output of the internal combustion engine and an output of a motor provided in a vehicle equipped with the internal combustion engine as a power source of the vehicle;
5. The exhaust gas purification device for an internal combustion engine according to claim 1, wherein, when an output of the internal combustion engine is insufficient for a required output due to a decrease in the amount of the reducing agent supplied, the output control unit causes the motor to output an output that is insufficient for the required output.

Images