JP3788049B2 - Exhaust gas purification device for lean combustion internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust emission control device for an internal combustion engine, and more particularly to O.₂The present invention relates to an exhaust gas purification apparatus for an internal combustion engine provided with an exhaust gas purification catalyst having a storage function.
[0002]
[Prior art]
O in the exhaust passage of an engine operated near the stoichiometric air-fuel ratio₂HC, CO, NO in exhaust by arranging exhaust purification three-way catalyst such as three-way catalyst with storage function_XTechniques for purifying these three components are known. Three-way catalyst O₂The storage function is a function of absorbing and holding oxygen components in the exhaust gas in the catalyst when the air-fuel ratio of the inflowing exhaust gas is lean, and releasing the absorbed oxygen when the air-fuel ratio of the inflowing exhaust gas is rich. . As is well known, the three-way catalyst has HC, CO, NO in the exhaust when the inflowing exhaust air-fuel ratio is in a narrow range near the stoichiometric air-fuel ratio._XThese three components can be purified at the same time, but when the exhaust air-fuel ratio deviates from the stoichiometric air-fuel ratio, the three components cannot be purified simultaneously. On the other hand, O₂When the storage function is added, when the exhaust gas flowing into the three-way catalyst becomes leaner than the stoichiometric air-fuel ratio, excess oxygen in the exhaust gas is absorbed by the catalyst, and when it becomes rich, oxygen is released from the catalyst. Even when the air-fuel ratio of the exhaust gas flowing into the catalyst deviates from the stoichiometric air-fuel ratio, the atmosphere of the three-way catalyst can be maintained near the stoichiometric air-fuel ratio. For this reason, the exhaust of an engine operated at an air-fuel ratio near the stoichiometric air-fuel ratio is reduced to O₂By purifying using a three-way catalyst having a storage function, HC, CO, NO_XThese three components can be purified well.
[0003]
On the other hand, when the inflowing exhaust air-fuel ratio is lean, the NO in the exhaust_X(Nitrogen oxide) absorbed, NO absorbed when the oxygen concentration in the inflowing exhaust gas decreases_XNO release_XOcclusion reduction catalysts are known.
NO_XAs an example of an exhaust gas purification apparatus using an occlusion reduction catalyst, for example, there is one described in Japanese Patent Registration No. 2600492. The exhaust purification device of the above-mentioned patent has NO in an exhaust passage of an engine that performs lean air-fuel ratio operation._XAn NOx storage reduction catalyst is installed and NO during the lean air-fuel ratio operation of the engine._XNO in exhaust gas to storage reduction catalyst_XAbsorbs NO_XNO of storage reduction catalyst_XBy performing a rich spike operation in which the engine is operated at a stoichiometric air-fuel ratio or a rich air-fuel ratio for a short time when the amount of absorption increases, NO_XNO absorbed from the storage reduction catalyst_XAs well as released NO_XReduce and purify. That is, when the air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio, the oxygen concentration in the exhaust gas rapidly decreases as compared to the lean air-fuel ratio exhaust gas, and the amount of unburned HC and CO components in the exhaust gas rapidly increases. To increase. Therefore, when the engine operating air-fuel ratio is switched to the stoichiometric air-fuel ratio or the rich air-fuel ratio by the rich spike operation, NO_XThe air-fuel ratio of the exhaust gas flowing into the storage reduction catalyst changes from the lean air-fuel ratio to the stoichiometric air-fuel ratio or rich air-fuel ratio, and NO decreases due to a decrease in the oxygen concentration in the exhaust gas._XNO from the storage reduction catalyst_XIs released. In addition, since the stoichiometric or rich air-fuel ratio exhaust gas contains a relatively large amount of unburned HC and CO components as described above, NO._XNO released from the storage reduction catalyst_XReacts with unburned HC and CO components in the exhaust and is reduced.
[0004]
[Problems to be solved by the invention]
According to the exhaust purification device described in the above-mentioned Patent Registration No. 2600602, NO generated during engine lean air-fuel ratio operation_XCan be efficiently purified. However, the device of the above-mentioned Patent Registration No.₂Problems may arise when a three-way catalyst having a storage function is added.
[0005]
The main purpose of the start catalyst is to remove HC and CO components that are released in large quantities from the engine when the engine is started. The exhaust passage is designed to increase the temperature in a short time after the engine starts and to reach the catalyst activation temperature. It is necessary to place it as close to the engine as possible. For this reason, when adding to the exhaust gas purification device of the above-mentioned Patent Registration No. 2600492, the start catalyst is NO_XArranged in the exhaust passage upstream of the storage reduction catalyst.
[0006]
However, O₂NO with a catalyst having a storage function as a start catalyst_XWhen it is placed in the exhaust passage upstream of the storage reduction catalyst, NO is applied during lean air-fuel ratio operation._XNO from storage reduction catalyst_XWhen rich spike operation is performed for release and reduction purification of NO_XIs NO_XIt has been found that there is a problem of being released from the storage reduction catalyst without being purified.
[0007]
The problem is NO during rich spike operation._XThe change in the air-fuel ratio of the exhaust gas flowing into the storage reduction catalyst from lean to rich is the start catalyst's O₂This is thought to be due to a delay due to the storage function.
That is, when the rich spike operation is performed, the air-fuel ratio of the exhaust gas from the engine changes rapidly from lean to rich, but the start catalyst is O₂Due to the storage function, when this rich air-fuel ratio exhaust gas flows into the start catalyst, the absorbed oxygen is released from the start catalyst, and the air-fuel ratio of the exhaust gas flowing out from the start catalyst is maintained near the stoichiometric air-fuel ratio. Is done. For this reason, even if the rich spike operation is started, NO_XThe air-fuel ratio of the exhaust gas flowing into the storage reduction catalyst does not become a sufficiently rich air-fuel ratio until the start catalyst completely releases all the oxygen absorbed, and may be maintained at a lean air-fuel ratio close to the theoretical air-fuel ratio. .
[0008]
NO_XWhen the air-fuel ratio of the exhaust gas flowing into the storage reduction catalyst changes from the lean air-fuel ratio to the lean air-fuel ratio close to the stoichiometric air-fuel ratio, NO_XThe oxygen concentration near the surface of the storage reduction catalyst decreases rapidly. As will be described later, NO_XThe storage reduction catalyst is NO in the form of nitrate ions combined with alkaline earth (eg, Ba)._XIs retained inside the catalyst, but if the oxygen concentration near the catalyst surface decreases, NO_XNO retained near the surface of the storage reduction catalyst_XAre simultaneously released from the catalyst surface. In this case, NO_XIf the exhaust gas flowing into the storage reduction catalyst is maintained at a lean air-fuel ratio close to the stoichiometric air-fuel ratio, the released NO is released into the exhaust gas._XNO is released because it does not contain sufficient amounts of HC and CO to reduce the total amount of NO._XPart of the_XIt will flow out to the downstream side of the storage reduction catalyst. For this reason, start catalyst O₂With storage function, NO during rich spike operation_XIf the exhaust air-fuel ratio flowing into the storage reduction catalyst is delayed until it reaches a sufficiently rich air-fuel ratio, NO_XUnpurified NO from the storage reduction catalyst_XWill be leaked.
[0009]
As described above, when the entire amount of oxygen absorbed by the start catalyst is released, the air-fuel ratio of the exhaust on the downstream side of the start catalyst also becomes the same rich air-fuel ratio as that on the upstream side of the start catalyst._XA sufficiently rich air-fuel ratio exhaust gas is supplied to the storage reduction catalyst. Therefore, if a certain amount of time has elapsed since the start of the rich spike operation, NO_XNO released from the storage reduction catalyst_XIs NO_XThe total amount of the NOx storage reduction catalyst is purified, and NO_XUnpurified NO from the storage reduction catalyst_XThe outflow stops. However, as described above, NO is determined for each rich spike operation._XUnpurified NO from the storage reduction catalyst_XAs a result, NO as a whole_XThere arises a problem that the purification rate is lowered.
[0010]
Further, in an engine in which the operating air-fuel ratio of the engine is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio or rich air-fuel ratio according to the operating state of the engine, the air-fuel ratio of the exhaust from the engine does not become lean even without performing a rich spike operation. In some cases, the air-fuel ratio can be switched to the stoichiometric air-fuel ratio or the rich air-fuel ratio.₂Due to the storage function, NO when switching the operating air-fuel ratio_XWhen the exhaust air-fuel ratio flowing into the storage reduction catalyst is temporarily maintained at a lean air-fuel ratio in the vicinity of the theoretical air-fuel ratio, an unpurified NO as in the above case is generated._XIs emitted, and the exhaust property deteriorates.
[0011]
In view of the above problems, the present invention provides O₂It is intended to prevent a delay in the change of the exhaust air-fuel ratio downstream of the exhaust purification catalyst from the lean air-fuel ratio to the stoichiometric air-fuel ratio or rich air-fuel ratio when an exhaust purification catalyst having a storage function is arranged in the exhaust passage. .
[0012]
[Means for Solving the Problems]
  According to the first aspect of the present invention, there is provided an exhaust emission control device for an internal combustion engine that performs switching of an operating air-fuel ratio between a lean air-fuel ratio operation and a stoichiometric or rich air-fuel ratio operation as required.
  O placed in the engine exhaust passage₂An exhaust purification catalyst having a storage function;
  When the engine is switched from lean air-fuel ratio operation to stoichiometric air-fuel ratio operation or rich air-fuel ratio operation, fuel that does not contribute to combustion is supplied to the engine, and the exhaust air-fuel ratio flowing into the exhaust purification catalyst is made rich air-fuel ratio. Storage reduction means for reducing the amount of oxygen stored in the exhaust purification catalyst;Further, when the air-fuel ratio of the exhaust flowing into the exhaust passage downstream of the exhaust purification catalyst is a lean air-fuel ratio, the NO in the exhaust _X NO absorbed when the oxygen concentration in the exhaust gas _X NO release _X A storage reduction catalyst, and the storage reducing means further includes NO. _X NO absorbed from the storage reduction catalyst _X An exhaust purification device for an internal combustion engine is provided that reduces the amount of oxygen stored in the exhaust purification catalyst when it should be released.
[0013]
  According to the invention of claim 2, the NO during the lean air-fuel ratio operation of the engine. _X NO absorbed from the storage reduction catalyst _X For reducing the amount of oxygen stored in the exhaust purification catalyst by the storage reduction means during the rich spike operation. An exhaust emission control device for an internal combustion engine according to claim 1 is provided.
[0014]
  That is, in the first and second aspects of the invention, when the operating air-fuel ratio of the engine is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio or rich air-fuel ratio, fuel that does not contribute to combustion is supplied to the engine. This fuel becomes an unburned HC component without burning, and is discharged from the engine together with the exhaust. For this reason, exhaust gas having a rich air-fuel ratio and containing a large amount of unburned HC flows into the exhaust purification catalyst. In this case, the exhaust purification catalyst O ₂ Oxygen is released from the exhaust purification catalyst by the storage function. But O ₂ Since there is a limit to the rate of oxygen released by storage, if a large amount of unburned HC components are contained in the inflowing exhaust, the released oxygen cannot oxidize the entire amount of unburned HC components in the exhaust, The air-fuel ratio of the exhaust downstream of the exhaust purification catalyst becomes an air-fuel ratio richer than the stoichiometric air-fuel ratio. That is, since oxygen stored in the exhaust purification catalyst is released and immediately consumed, the exhaust air-fuel ratio downstream of the exhaust purification catalyst immediately changes to the rich air-fuel ratio. For this reason, the exhaust purification catalyst O ₂ The delay of the air-fuel ratio change due to the storage function is prevented. The supply of fuel that does not contribute to combustion is stopped when the amount of oxygen stored in the exhaust purification catalyst is sufficiently reduced (that is, when the release of oxygen from the exhaust purification catalyst does not cause a practical problem). The Further, as the storage reducing means, in an engine having an in-cylinder fuel injection valve that directly injects fuel into the cylinder, the fuel may be injected into the cylinder in the expansion stroke or the exhaust stroke of each cylinder. Alternatively, an engine having an exhaust port fuel injection valve that injects fuel into each cylinder exhaust port may inject fuel into the exhaust port. O ₂ Supply of fuel that does not contribute to combustion by the storage reduction means may be performed during lean air-fuel ratio operation immediately before switching of the engine operating air-fuel ratio, or during operation of the stoichiometric air-fuel ratio or rich air-fuel ratio immediately after switching. good.
[0016]
  Furthermore, Claim 1 and Claim 2In the invention of₂NO on the downstream side of the exhaust purification catalyst with storage function_XThe NOx storage reduction catalyst is arranged, NO_XNO absorbed from the storage reduction catalyst_XExhaust gas purification by storage reduction means when it should be released₂Storage function is reduced. Therefore, not only when the engine operating air-fuel ratio is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio or rich air-fuel ratio due to changes in the operating state,_XNO from the storage reduction catalyst_XNO when releasing_XNO is also used when the air-fuel ratio of the exhaust gas flowing into the storage reduction catalyst is changed from the lean air-fuel ratio to the stoichiometric or rich air-fuel ratio._XThe air-fuel ratio of the exhaust gas flowing into the storage reduction catalyst immediately changes from a lean air-fuel ratio to a sufficiently rich air-fuel ratio, and NO._XUnpurified NO from the storage reduction catalyst_XIs prevented from flowing out.
[0017]
  Claim 3According to the invention described in the above, it is an exhaust purification device for an internal combustion engine that performs a lean air-fuel ratio operation as necessary, and includes an O disposed in the engine exhaust passage.₂An exhaust purification catalyst having a storage function and NO in the exhaust when the air-fuel ratio of the inflowing exhaust disposed in the exhaust passage downstream of the exhaust purification catalyst is a lean air-fuel ratio_XNO is absorbed when the air-fuel ratio of the exhaust that absorbs and flows in becomes rich._XNO release_XThe storage reduction catalyst and the NO during the lean air-fuel ratio operation of the engine_XNO absorbed from the storage reduction catalyst_XA rich spike operation for switching the operating air-fuel ratio of the engine to a rich air-fuel ratio for a short time when the engine should be released, and a rich spike operation for the exhaust air-fuel ratio flowing into the exhaust purification catalyst for a predetermined period immediately after the start of the rich spike operation There is provided an exhaust gas purification apparatus for an internal combustion engine, comprising a storage reduction means for reducing the amount of oxygen stored in the exhaust gas purification catalyst by making it richer than the air-fuel ratio therein.
[0018]
  That is,Claim 3In the invention of NO_XNO from storage reduction catalyst_XWhen the rich spike operation is performed for release and reduction purification, the air-fuel ratio of the exhaust flowing into the exhaust purification catalyst for a predetermined period immediately after the start of the rich spike is kept richer than the exhaust air-fuel ratio during the subsequent rich spike operation Is done. As a result, the exhaust purification catalyst removes O₂While oxygen is released by the storage function, the exhaust gas contains sufficient amounts of unburned HC and CO components to consume the entire amount of released oxygen, and oxygen is released from the exhaust purification catalyst. Even during this time, the exhaust air-fuel ratio downstream of the exhaust purification catalyst becomes a sufficiently rich air-fuel ratio. Therefore, the NO on the downstream side of the exhaust purification catalyst_XThe storage reduction catalyst is supplied with exhaust gas with a sufficiently rich air-fuel ratio from the start of the rich spike operation._XUnpurified NO from the storage reduction catalyst_XIs prevented from flowing out. Note that the air-fuel ratio of the exhaust flowing into the exhaust purification catalyst immediately after the start of the rich spike is sufficient to consume the entire amount of oxygen released from the exhaust purification catalyst, and downstream NO._XNO released from the storage reduction catalyst_XThe amount of unburned HC and CO components is set so as to include the amount sufficient to purify the total amount. In addition, as storage reduction means,Claims 1 to 2In addition to supplying fuel that does not contribute to combustion, such as those that inject fuel into the cylinder during cylinder expansion or exhaust stroke, or those that inject fuel into the exhaust port, The operating air-fuel ratio of the engine may be made richer than during the period and the subsequent rich spike operation. The predetermined period is set to a time sufficient for releasing the entire amount of oxygen absorbed from the exhaust purification catalyst.
[0019]
  Claim 4According to the invention described above, the storage reduction means includes storage estimation means for estimating the amount of oxygen stored in the exhaust purification catalyst based on the operating state of the engine, and the storage reduction means includes the storage reduction means according to the estimated stored oxygen amount. Claims for performing oxygen reduction operation1 to 3An exhaust emission control device for an internal combustion engine according to any one of the above is provided.
  That is,Claim 4In this invention, the storage reduction means estimates the amount of oxygen stored in the exhaust purification catalyst, and performs an oxygen amount reduction operation according to the estimated amount of oxygen. For example, the storage reduction means decreases the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst as the amount of stored oxygen increases (that is, deepens the degree of richness), or lengthens the time for continuing the oxygen amount reduction operation. As a result, the oxygen amount reduction operation can be performed accurately, and a delay in the change of the exhaust air-fuel ratio downstream of the exhaust purification catalyst can be surely prevented. It should be noted that the storage oxygen amount estimation of the exhaust purification catalyst by the storage reducing means is based on the engine operating state such as the catalyst temperature, the exhaust flow rate, the history of engine air-fuel ratio change (duration of lean air-fuel ratio operation and rich air-fuel ratio operation), Based on.
[0020]
  Claim 5According to the present invention, the storage estimation means estimates the amount of oxygen stored in the exhaust purification catalyst based on the deterioration state of the exhaust purification catalyst in addition to the operating state of the engine.Claim 4An exhaust emission control device for an internal combustion engine as described in 1) is provided.
  Claim 5In this invention, the storage reduction means estimates the stored oxygen amount based on the deterioration state of the exhaust purification catalyst in addition to the engine operating state. O₂The storage function decreases with the deterioration of the exhaust purification catalyst, and the oxygen amount (saturated oxygen amount) that can be stored in the exhaust purification catalyst decreases with the deterioration of the exhaust purification catalyst. That is, the exhaust purification catalyst does not store oxygen in an amount greater than the saturated oxygen amount. Therefore, by considering the deterioration state of the exhaust purification catalyst, it is possible to estimate the stored oxygen amount of the exhaust purification catalyst more accurately, and it is possible to perform the oxygen amount reduction operation more accurately.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a diagram showing a schematic configuration of an embodiment in which the present invention is applied to an automobile internal combustion engine.
In FIG. 1, reference numeral 1 denotes an automobile internal combustion engine. In the present embodiment, the engine 1 is a four-cylinder gasoline engine having four cylinders # 1 to # 4, and the fuel injection valves 111 to 114 for injecting fuel directly into the cylinders are provided for the # 1 to # 4 cylinders. Is provided. As will be described later, the internal combustion engine 1 of the present embodiment is a lean burn engine that can be operated at an air fuel ratio higher (lean) than the stoichiometric air fuel ratio.
[0022]
Further, in the present embodiment, the cylinders # 1 to # 4 are grouped into two cylinder groups including two cylinders whose ignition timings are not continuous with each other. (For example, in the embodiment of FIG. 1, the cylinder firing order is 1-3-4-2, and the cylinders # 1 and # 4 and the cylinders # 2 and # 3 each constitute a cylinder group. In addition, the exhaust port of each cylinder is connected to an exhaust manifold for each cylinder group, and is connected to an exhaust passage for each cylinder group. In FIG. 1, reference numeral 21a denotes an exhaust manifold for connecting the exhaust ports of the cylinder group consisting of # 1 and # 4 cylinders to the individual exhaust passage 2a, and 21b denotes the exhaust port of the cylinder group consisting of # 2 and # 4 cylinders to the individual exhaust passage 2b. Is an exhaust manifold connected to In the present embodiment, start catalysts (hereinafter referred to as “SC”) 5a and 5b made of a three-way catalyst are disposed on the individual exhaust passages 2a and 2b, respectively. Further, the individual exhaust passages 2a and 2b merge with the common exhaust passage 2 on the downstream side of the SC.
[0023]
On the common exhaust passage 2, NO, which will be described later, is provided._XAn occlusion reduction catalyst 7 is arranged. In FIG. 1, 29a and 29b indicate air-fuel ratio sensors disposed upstream of the start catalyst 5a and 5b of the individual exhaust passages 2a and 2b, and 31 indicates the NO in the exhaust passage 2._XAn air-fuel ratio sensor disposed at the outlet of the storage reduction catalyst 7. The air-fuel ratio sensors 29a, 29b, and 31 are so-called linear air-fuel ratio sensors that output a voltage signal corresponding to the exhaust air-fuel ratio in a wide air-fuel ratio range.
Further, an electronic control unit (ECU) of the engine 1 is indicated by 30 in FIG. In this embodiment, the ECU 30 is a microcomputer having a known configuration including a RAM, a ROM, and a CPU, and performs basic control such as ignition timing control and fuel injection control of the engine 1. In the present embodiment, the ECU 30 performs the basic control described above, and changes the fuel injection mode of the in-cylinder injection valves 111 to 114 in accordance with the engine operating state to change the operating air-fuel ratio of the engine, as will be described later. In addition to controlling, NO_XNO absorbed from the storage reduction catalyst 7_XIn order to release the engine, a rich spike operation is performed to switch the air-fuel ratio for a short time to the rich air-fuel ratio during the lean air-fuel ratio operation of the engine. Further, the ECU 30 performs a stored oxygen amount reduction operation for reducing the oxygen amount stored in the SCs 5a and 5b when the engine operating air-fuel ratio is changed from lean to rich or during the rich spike operation.
[0024]
The input port of the ECU 30 includes a signal indicating the exhaust air / fuel ratio at the inlets of the start catalyst 5a and 5b from the air / fuel ratio sensors 29a and 29b, and the NO / NO from the air / fuel ratio sensor 31._XIn addition to a signal representing the exhaust air / fuel ratio at the outlet of the storage reduction catalyst 7 and a signal corresponding to the intake pressure of the engine from an intake pressure sensor 33 provided in an unillustrated engine intake manifold, an engine crankshaft ( A signal corresponding to the engine speed is input from a speed sensor 35 arranged in the vicinity. Furthermore, in the present embodiment, a signal representing the accelerator pedal depression amount (accelerator opening) of the driver is input to an input port of the ECU 30 from an accelerator opening sensor 37 disposed in the vicinity of an accelerator pedal (not shown) of the engine 1. Has been. Further, the output port of the ECU 30 is connected to the fuel injection valves 111 to 114 of each cylinder through a fuel injection circuit (not shown) in order to control the fuel injection amount and fuel injection timing to each cylinder.
[0025]
In the present embodiment, the ECU 30 operates the engine 1 in the following five combustion modes according to the operating state of the engine.
(1) Lean air-fuel ratio stratified combustion (injection once in the compression stroke)
(2) Lean air-fuel ratio homogeneous mixture / stratified combustion (intake stroke / compression stroke twice injection)
(3) Lean air-fuel ratio homogeneous mixture combustion (intake stroke one injection)
(4) Theoretical air-fuel ratio homogeneous mixture combustion (intake stroke one injection)
(5) Rich air-fuel ratio homogeneous mixture combustion (intake stroke one injection)
That is, in the light load operation region of the engine 1, the lean air-fuel ratio stratified combustion (1) is performed. In this state, in-cylinder fuel injection is performed only once in the latter half of the compression stroke of each cylinder, and the injected fuel forms a combustible mixture layer near the cylinder spark plug. Further, the fuel injection amount in this operation state is extremely small, and the air-fuel ratio as a whole in the cylinder is about 25 to 30.
[0026]
Further, when the load increases from the state (1) to the low load operation region, (2) the lean air-fuel ratio homogeneous mixture / stratified combustion is performed. As the engine load increases, the amount of fuel injected into the cylinder increases. However, in the stratified combustion of (1), the fuel injection is performed in the latter half of the compression stroke, so the injection time is limited and the amount of fuel that can be stratified Has its limits. Therefore, in this load region, a target amount of fuel is supplied to the cylinders by injecting in advance into the first half of the intake stroke an amount of fuel that is insufficient only by fuel injection in the latter half of the compression stroke. The fuel injected into the cylinder in the first half of the intake stroke generates a very lean homogeneous mixture by the time of ignition. In the latter half of the compression stroke, further fuel is injected into this extremely lean homogeneous mixture to generate a combustible mixture layer that can be ignited in the vicinity of the spark plug. At the time of ignition, the combustible air-fuel mixture layer starts to burn, and the flame propagates to the surrounding lean air-fuel mixture layer, so that stable combustion is performed. In this state, the amount of fuel supplied by the injection in the intake stroke and the compression stroke is increased from (1), but the overall air-fuel ratio becomes slightly low (for example, about 20 to 30 in the air-fuel ratio).
[0027]
When the engine load further increases, the engine 1 performs the lean air-fuel ratio homogeneous mixture combustion of (3) above. In this state, fuel injection is executed only once in the first half of the intake stroke, and the fuel injection amount is further increased from the above (2). In this state, the homogeneous air-fuel mixture generated in the cylinder has a lean air-fuel ratio that is relatively close to the stoichiometric air-fuel ratio (for example, about 15 to 25 as the air-fuel ratio).
[0028]
When the engine load further increases and the engine high load operation region is reached, the fuel is further increased from the state (3), and the stoichiometric air-fuel ratio homogeneous mixture operation (4) is performed. In this state, a homogeneous air-fuel mixture having a stoichiometric air-fuel ratio is generated in the cylinder, and the engine output increases. When the engine load is further increased and the engine is fully loaded, the fuel injection amount is further increased from the state (4), and the rich air-fuel ratio homogeneous mixture operation (5) is performed. In this state, the air-fuel ratio of the homogeneous mixture generated in the cylinder becomes rich (for example, about 12 to 14 as the air-fuel ratio).
[0029]
In the present embodiment, the optimum operation mode (above (1) to (5)) is set in advance based on experiments or the like according to the accelerator opening (the amount by which the driver depresses the accelerator pedal) and the engine speed. , Stored in the ROM of the ECU 30 as a map using the accelerator opening and the engine speed. During the engine 1 operation, the ECU 30 determines which one of the above operation modes (1) to (5) should be selected based on the accelerator opening detected by the accelerator opening sensor 37 and the engine speed, The fuel injection amount, the fuel injection timing and the number of times are determined according to the mode.
[0030]
That is, when the mode (1) to (3) (lean air-fuel ratio combustion) is selected, the ECU 30 determines the accelerator based on the map prepared in advance for each mode (1) to (3). The fuel injection amount is determined from the opening degree and the engine speed. When the modes (4) and (5) (theoretical air-fuel ratio or rich air-fuel ratio homogeneous mixture combustion) are selected, the ECU 30 is prepared in advance for each of the modes (4) and (5). Based on the map, the fuel injection amount is set based on the intake pressure detected by the intake pressure sensor 33 and the engine speed.
[0031]
When mode (4) (stoichiometric air-fuel ratio homogeneous mixture combustion) is selected, the ECU 30 further calculates the fuel injection amount calculated as described above from the air-fuel ratio sensor so that the engine exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio. Feedback correction is performed based on the outputs of 29a and 29b.
As described above, in the engine 1 of the present embodiment, the fuel injection amount is increased as the engine load increases, and the operation mode is changed according to the fuel injection amount.
[0032]
Next, the start catalyst 5a, 5b and NO of this embodiment_XThe storage reduction catalyst will be described.
The start catalyst (SC) 5a, 5b uses a carrier such as cordierite formed in a honeycomb shape, and a thin coating of alumina is formed on the surface of the carrier, and platinum Pt, palladium Pd, rhodium Rh, etc. are formed on the alumina layer. It is comprised as a three-way catalyst which supported the noble metal catalyst component. The three-way catalyst is near HC, CO, NO near the stoichiometric air-fuel ratio._XThese three components are purified with high efficiency. The three way catalyst is NO when the air-fuel ratio of the inflowing exhaust gas becomes higher than the stoichiometric air-fuel ratio._XBecause the reduction capacity of the engine 1 is reduced, the NO in the exhaust when the engine 1 is operated with a lean air-fuel ratio is reduced._XCannot be sufficiently purified.
[0033]
Further, the SCs 5a and 5b are arranged in portions close to the engine 1 in the exhaust passages 2a and 2b so that the activation temperature of the catalyst can be reached in a short time after the engine is started and the catalytic action can be started, thereby reducing the heat capacity. Therefore, it has a relatively small capacity.
Next, SC5a, 5b O₂The storage function will be described.
In general, when an exhaust purification catalyst such as a three-way catalyst is loaded with a metal component such as cerium (Ce) in addition to the catalyst component, the exhaust purification catalyst functions as an oxygen storage function (O₂It is known that the storage function will be exhibited. That is, cerium supported on the catalyst as an additive is combined with oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the catalyst is higher than the stoichiometric air-fuel ratio (when the exhaust air-fuel ratio is lean). Cerium) to store oxygen. When the air-fuel ratio of the inflowing exhaust gas is equal to or lower than the stoichiometric air-fuel ratio (when the exhaust air-fuel ratio is rich), oxygen is released because ceria releases oxygen and returns to metallic cerium. O₂In the exhaust purification catalyst having a storage function, even when the exhaust air-fuel ratio flowing into the catalyst changes from the rich air-fuel ratio to the lean air-fuel ratio, oxygen in the exhaust is absorbed by cerium, so that the oxygen concentration in the inflowing exhaust gas decreases. For this reason, the exhaust air-fuel ratio at the catalyst outlet is close to the stoichiometric air-fuel ratio while oxygen is absorbed by cerium. Further, when the total amount of cerium supported by the catalyst is combined with oxygen (that is, when the catalyst is saturated with oxygen) and can no longer absorb oxygen, the exhaust air-fuel ratio at the exhaust purification outlet becomes the exhaust gas at the catalyst inlet. It changes to the same lean air-fuel ratio as the air-fuel ratio. Similarly, in a state where cerium has sufficiently absorbed oxygen, when the air-fuel ratio of the exhaust flowing into the catalyst changes from the lean air-fuel ratio to the rich air-fuel ratio, oxygen is released from cerium, and the oxygen concentration in the exhaust increases. Thus, the air-fuel ratio at the catalyst outlet is close to the theoretical air-fuel ratio. Also in this case, after the total amount of oxygen combined with cerium is released, oxygen is not released from the catalyst any more, so the exhaust air-fuel ratio at the catalyst outlet is the same as the rich air-fuel ratio at the catalyst inlet. Become. That is, the exhaust purification catalyst is O₂If the storage function is provided, the change from lean to rich or rich to lean of the exhaust air-fuel ratio on the downstream side of the catalyst will be delayed as compared to the upstream side of the catalyst.
[0034]
SC5a and 5b of this embodiment are O₂Since the storage function is added, when the operating air-fuel ratio of the engine changes from lean to rich, the change in the exhaust air-fuel ratio downstream of the SCs 5a and 5b is delayed and temporarily maintained at the air-fuel ratio in the vicinity of the theoretical air-fuel ratio. A period will occur.
Next, NO of this embodiment_XThe storage reduction catalyst 7 will be described. NO of this embodiment_XThe occlusion reduction catalyst 7 uses, for example, alumina as a carrier, and an alkali metal such as potassium K, sodium Na, lithium Li, and cesium Cs, alkaline earth such as barium Ba and calcium Ca, lanthanum La, cerium, and the like. It carries at least one component selected from rare earths such as Ce and yttrium Y and a noble metal such as platinum Pt. NO_XThe NOx storage reduction catalyst is used when the air-fuel ratio of the inflowing exhaust gas is lean._X(NO₂, NO) to nitrate ion NO_Three ^-NO is absorbed when the inflowing exhaust gas becomes rich_XNO release_XPerforms absorption and release action.
[0035]
This absorption / release mechanism will be described below using platinum Pt and barium Ba as an example, but the same mechanism can be obtained by using other noble metals, alkali metals, alkaline earths, and rare earths.
When the oxygen concentration in the inflowing exhaust gas increases (that is, when the air-fuel ratio of the exhaust gas becomes a lean air-fuel ratio), these oxygens become O on the platinum Pt.₂ ^-Or O^2-NO in the exhaust_XIs O on platinum Pt₂ ^-Or O^2-To react with NO₂Is generated. In addition, NO in inflow exhaust₂And NO produced by the above₂Is absorbed in the absorbent while being further oxidized on platinum Pt and combined with barium oxide BaO and nitrate ions NO._Three ^-Diffuses into the absorbent in the form of For this reason, NO in the exhaust gas in a lean atmosphere_XIs NO_XIt becomes absorbed in the form of nitrate in the absorbent.
[0036]
Further, when the oxygen concentration in the inflowing exhaust gas is greatly reduced (that is, when the air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio or the rich air-fuel ratio), NO on the platinum Pt is increased.₂Since the production amount decreases, the reaction proceeds in the reverse direction, and nitrate ion NO in the absorbent_Three ^-Is NO₂From the absorbent in the form of In this case, reducing components such as CO, HC, CO₂If there are components such as NO on the platinum Pt,₂Is reduced.
[0037]
In the present embodiment, the engine 1 capable of lean air-fuel ratio operation is used, and when the engine 1 is operated at the lean air-fuel ratio, NO_XThe storage reduction catalyst is NO in the exhaust gas flowing in._XAbsorbs. Further, when the engine 1 is operated at a rich air-fuel ratio, NO_XThe NOx storage reduction catalyst 7 has absorbed NO._XRelease, reduce and purify. In this embodiment, NO during lean air-fuel ratio operation._XNO absorbed by the storage reduction catalyst 7_XWhen the amount increases, a rich spike operation is performed in which the engine air-fuel ratio is switched from a lean air-fuel ratio to a rich air-fuel ratio for a short time._XNO from storage reduction catalyst_XRelease and reduction purification (NO_XThe regeneration of the storage reduction catalyst is performed.
In this embodiment, the ECU 30 is NO_XNO by increasing or decreasing the counter value_XNO absorbed and stored by the storage reduction catalyst 7_XEstimate the amount. NO_XNO absorbed by the storage reduction catalyst 7 per unit time_XAmount of NO_XNO in exhaust flowing into the storage reduction catalyst per unit time_XQuantity, ie NO generated per unit time in engine 1_XIt is proportional to the amount. On the other hand, NO generated per unit time in the engine_XIs determined by the amount of fuel supplied to the engine, air-fuel ratio, exhaust flow rate, etc., so if the engine operating conditions are determined, NO_XNO absorbed by the storage reduction catalyst_XYou can know the amount. In the present embodiment, the engine operation conditions (accelerator opening, engine speed, intake air amount, intake air pressure, air fuel ratio, fuel supply amount, etc.) are changed in advance, and the NO generated by the engine per unit time._XMeasure the amount, NO_XNO absorbed by the storage reduction catalyst 7 per unit time_XThe amount is stored in the ROM of the ECU 30 in the form of a numerical map using, for example, the engine load (fuel injection amount) and the engine speed. The ECU 30 uses this map to determine NO per unit time from the engine load (fuel injection amount) and the engine speed at regular intervals (every unit time)._XNO absorbed by the storage reduction catalyst_XCalculate the amount, NO_XSet this counter to NO_XIncrease the amount absorbed. This makes NO_XThe counter value is always NO_XNO absorbed by the storage reduction catalyst 7_XTo represent the amount of. The ECU 30 performs the above NO during the lean air-fuel ratio operation of the engine._XWhen the counter value increases to a predetermined value or more, the engine is operated for a short time (for example, about 0.5 to 1 second) in the above-described mode (4) or (5) (theoretical air-fuel ratio or rich air-fuel ratio homogeneous mixture combustion) ) Perform a rich spike operation. As a result, NO_XNO absorbed from the storage reduction catalyst_XIs released and reduced and purified. Note that the time required to keep the exhaust air-fuel ratio rich with a rich spike is NO in detail._XIt is determined by experiments or the like based on the type and capacity of the storage reduction catalyst. Also, execute rich spike and NO_XNO from the storage reduction catalyst_XNO is released and reduced and purified_XThe counter value is reset to zero. Like this, NO_XNO of storage reduction catalyst 7_XBy performing rich spike according to the amount of absorption, NO_XThe storage reduction catalyst 7 is properly regenerated, and NO_XNO absorbed by the storage reduction catalyst_XSaturation is prevented.
[0038]
However, as described above, in this embodiment, NO is used._XO in the exhaust passage upstream of the storage reduction catalyst 7₂SCs 5a and 5b having a storage function are provided. For this reason, even if rich air-fuel ratio exhaust gas flows into the SC 5a, 5b from the engine during a rich spike, the NO on the downstream of the SC 5a, 5b_XOccurrence of lean air-fuel ratio exhaust near the stoichiometric air-fuel ratio occurs when the storage reduction catalyst 7 releases oxygen at SCs 5a and 5b._XUnpurified NO from the storage reduction catalyst 7_XMay leak. Similarly, the operating air-fuel ratio of the engine changes from the lean air-fuel ratio (the operating modes from (1) to (3) described above) to the stoichiometric air-fuel ratio or rich air-fuel ratio (the above-mentioned (4) or (5) Operation mode is switched to NO immediately after switching._XUnpurified NO from the storage reduction catalyst 7_XMay occur.
[0039]
Therefore, in the embodiment described below, when the engine air-fuel ratio is switched from the lean air-fuel ratio to the stoichiometric air-fuel ratio or the rich air-fuel ratio for rich spike operation, operation mode switching, etc., the air-fuel ratio of the exhaust gas that flows into the SCs 5a, 5b in advance. By making the SC 5a, 5b O₂Resolves problems caused by storage.
By making the exhaust air-fuel ratio flowing into the SCs 5a and 5b rich, the exhaust gas containing a large amount of HC and CO components flows into the SCs 5a and 5b. For this reason, O₂Oxygen stored in the catalyst by the storage is consumed to oxidize HC and CO components in the exhaust, and the release of oxygen from the catalyst is completed in a short time. The amount of the HC and CO components is set to be larger than the amount that consumes the total amount of oxygen released from the catalyst.₂Even when oxygen is released from the catalyst by the storage function, the exhaust on the downstream side of SC5a and 5b is maintained at a rich air-fuel ratio. As a result, NO_XUnpurified NO from the storage reduction catalyst 7_XIs prevented from flowing out.
[0040]
The operation for reducing the stored oxygen that makes the exhaust air-fuel ratio flowing into the SCs 5a, 5b rich when the engine air-fuel ratio is switched is, for example, (A) From the in-cylinder injection valve of each cylinder into the cylinder during the cylinder expansion stroke or exhaust stroke (B) An exhaust port fuel injection valve that injects fuel into the exhaust port of each cylinder is provided to inject fuel into the engine exhaust port (hereinafter referred to as “exhaust port injection”) (C) There is a method of temporarily enriching the engine combustion air-fuel ratio temporarily when switching the engine air-fuel ratio. In the methods (A) and (B) described above, the fuel injected into the cylinder expansion, exhaust stroke, and exhaust port is vaporized without burning, and a large amount of HC and CO components are generated in the exhaust. That is, since these fuels do not contribute to combustion, even when a relatively large amount of fuel is supplied, there is an advantage that the engine output does not fluctuate. On the other hand, since these fuels do not contribute to combustion, a relatively large amount of oxygen remains in the exhaust when the engine is operated at a lean air-fuel ratio. That is, when fuel that does not contribute to combustion is supplied to the engine as described above, the exhaust air-fuel ratio becomes a rich air-fuel ratio as a whole, but unreacted oxygen, HC, and CO components are separated in the exhaust. Will exist. For this reason, these oxygen, HC, and CO components react on SC5a and 5b, and the temperature of SC5a and 5b may rise excessively depending on the operating conditions.
[0041]
In the method (C), the combustion air-fuel ratio of the engine itself temporarily becomes a rich rich air-fuel ratio, so that there is almost no unreacted oxygen in the exhaust, and the problem of overheating of the SCs 5a and 5b does not occur. The engine-generated torque increases due to combustion of a large amount of fuel, and output torque fluctuations may occur depending on the operating state of the engine.
Accordingly, it is preferable to select which of the methods (A) to (C) is selected according to the engine characteristics and the operating state.
[0042]
In addition, since the case of applying the method (B) (exhaust port fuel injection) is almost the same as the case of the above (A) (secondary fuel injection), the following embodiments (A) and ( The case where the method of C) is applied will be described as an example.
(1) First embodiment
FIG. 2 is a flowchart for explaining the stored oxygen amount reducing operation of the SCs 5a and 5b in the first embodiment of the present invention. This operation is executed by the ECU 30 at predetermined intervals (for example, every constant crankshaft rotation angle).
[0043]
In the operation of FIG. 2, when the operating air-fuel ratio is switched from the lean air-fuel ratio operation to the stoichiometric air-fuel ratio operation or the rich air-fuel ratio operation due to a change in the engine operating conditions, and NO_XNO from the storage reduction catalyst 7_XDuring the rich spike operation for releasing, immediately before switching the operating air-fuel ratio of the engine, the amount of stored oxygen in the SCs 5a and 5b is reduced by injecting fuel from the in-cylinder fuel injection valve during the expansion or exhaust stroke of each cylinder. Yes. That is, in this embodiment, O from SC5a, 5b.₂The engine operating air-fuel ratio is switched after the oxygen release by the storage function is completed.
[0044]
As a result, when the engine operating air-fuel ratio is switched from lean to rich (or stoichiometric air-fuel ratio), oxygen release from the SCs 5a and 5b does not occur._XThe air-fuel ratio of the exhaust gas flowing into the storage reduction catalyst 7 immediately changes from the lean air-fuel ratio to the rich air-fuel ratio (or stoichiometric air-fuel ratio)._XUnpurified NO from the storage reduction catalyst 7_XIs prevented from being released.
[0045]
When the operation starts in FIG. 2, in step 201, the accelerator opening (the driver's accelerator pedal depression amount) ACCP from the accelerator opening sensor 37 is calculated based on the output of the rotation speed sensor 35. The number NE and the stored oxygen amount OSC of the SCs 5a and 5b are read. The calculation of the stored oxygen amount OSC of SC5a and 5b will be described in detail later.
[0046]
Next, at step 203, based on the accelerator opening ACCP and the engine speed NE read in the above, the optimum operation mode M among the operation modes {circle around (1)} to {circle around (5)} above.₁Is selected. In this embodiment, the optimum operation mode for each accelerator opening and engine speed is stored in the ROM of the ECU 30 as a numerical table using the accelerator opening ACCP and the rotational speed NE as parameters. The optimum operation mode (1) to (5) is selected from this numerical table based on the ACCP and NE read in. M in step 203₁(M₁= (1) to (5)) represents an optimum operation mode in view of the current engine operation conditions (that is, an operation mode that is a target of the switching operation in step 223 described later).
[0047]
Next, at step 205, the current operation mode M₀Is a lean air-fuel ratio operation (any one of the above-mentioned modes (1) to (3)). M₀Is a parameter indicating in which operation mode (1) to (5) the engine is currently operated (M₀= (1) to (5)). If the lean air-fuel ratio operation is not currently performed in step 205, that is, if the current stoichiometric air-fuel ratio or rich air-fuel ratio operation is currently performed, the target operation mode M₁No change of the air-fuel ratio from the lean air-fuel ratio to the rich air-fuel ratio (or stoichiometric air-fuel ratio) occurs even if is any of (1) to (5), NO_XUnpurified NO from the storage reduction catalyst 7_XTherefore, step 223 is executed immediately and the engine operation mode is set to the target operation mode M.₁(If the vehicle is currently operating in the target operation mode, the current mode is continued). Then, after the switching is completed, in step 225, the current operation mode M₀Is the operation mode after switching (M₁) Is updated to a value according to.
[0048]
On the other hand, if the engine is currently operated in the operation mode from (1) to (3) in step 205, the current NO is determined in step 207._XNO from the storage reduction catalyst 7_XIt is determined based on the value of the rich spike flag FR whether or not execution of a rich spike operation for releasing the battery is required. As described above, in the present embodiment, the ECU 30 performs NO based on the engine operating state by a routine (not shown) that is separately executed._XNO absorbed by the storage reduction catalyst 7_XNO representing quantity_XThe value of the counter CNOX is integrated, and when the value of the counter CNOX exceeds a predetermined value, the value of the rich spike flag FR is set to 1. If the rich spike operation is currently requested in step 207, the stored oxygen amount reducing operation (steps 213 to 217) to be described later needs to be executed, so the operation directly proceeds to step 211. On the other hand, if the rich spike operation is not currently requested, in step 209, the target operation mode M₁Is rich air fuel ratio or theoretical air fuel ratio operation (mode (4) or (5)). Target operation mode M₁If the mode is neither mode {circle over (4)} nor {circle over (5)}, the switching from the lean air-fuel ratio operation to the rich air-fuel ratio operation does not occur in this case as well, so the routine proceeds to step 223 and the operation mode is switched to the target mode. Is done.
[0049]
On the other hand, if FR = 1 (rich spike request) in step 207 and if the target operation mode is (4) or (5) in step 209, that is, the engine operating air-fuel ratio is rich from the lean air-fuel ratio. Since it is necessary to switch to the air-fuel ratio (or the stoichiometric air-fuel ratio), the process proceeds to step 211 to determine whether or not the current oxygen amount reduction operation has been completed. If the reduction operation is not completed, secondary fuel injection (in-cylinder fuel injection in the expansion or exhaust stroke) for reducing the amount of oxygen in steps 213 to 217 is performed, and fuel that does not contribute to combustion is supplied to the cylinder. Supply.
[0050]
That is, in step 213, the entire amount of oxygen stored in SC5a, 5b is consumed from the current stored oxygen amount OSC of SC5a, 5b read in step 201, and the exhaust on the downstream side of SC5a, 5b is richer than the stoichiometric air-fuel ratio. The total amount of fuel (HC amount) required to maintain the fuel amount is calculated, and the total amount of fuel is divided by a predetermined number of secondary fuel injections (described later) to obtain two The next fuel injection amount is calculated. Then, in step 215, it is determined whether or not it is time to set the secondary fuel injection amount for any of the cylinders. If it is set timing, the calculated secondary fuel injection amount is sent to the fuel injection circuit in step 217. Set. Thus, when the secondary fuel injection timing (expansion or exhaust stroke) comes, secondary fuel injection is executed in each cylinder. When secondary fuel injection is executed a predetermined number of times (the number of cylinders), it is determined in step 211 that the oxygen amount reduction operation has been completed, and step 219 and subsequent steps are executed.
[0051]
That is, when the stored oxygen amount reduction operation is finished and the oxygen release from the SCs 5a and 5b is finished, it is determined in step 219 whether the rich spike operation is currently requested (FR = 1), and the rich spike operation is requested. If so, the rich spike operation is executed in step 221. If not, the target operation mode M is executed in step 223.₁Switching to the rich or stoichiometric air-fuel ratio operation mode in this case is executed.
[0052]
Note that in the rich spike operation of step 221, the engine is NO._XUntil the value of the counter CNOX becomes 0, it is operated by the rich air-fuel ratio homogeneous mixture combustion in the mode (5), and NO_XNO absorbed by the storage reduction catalyst 7_XIs released and reduced and purified.
Next, the number of secondary fuel injections in this embodiment will be described. In the present embodiment, the secondary fuel injection is performed once for each of the cylinder groups # 1, 4 and # 2, 3 or once for all the cylinders # 1 to # 4. That is, since the capacity of the SCs 5a and 5b is relatively small, when the amount of oxygen stored in the SC can be reduced by one secondary fuel injection for each SC 5a and 5b, the secondary fuel is only once for each cylinder group. When the injection is performed and the capacity of the SCs 5a and 5b is relatively large, and the amount of stored oxygen cannot be sufficiently reduced by one secondary fuel injection, the SCs 5a and 5b are twice (ie, # 1 to # 4). Secondary fuel injection is performed once for each cylinder). Which secondary fuel injection is to be executed is determined in advance according to the capacity of the SCs 5a and 5b. Since each cylinder group is composed of cylinders whose firing order is not continuous, when the secondary fuel injection is executed once for each cylinder group, the two cylinders having the continuous firing order are performed in step 211 described above. (For example, # 1 and # 3, or # 3 and # 2 etc.), when the secondary fuel injection is executed once, it is determined that the stored oxygen amount reduction operation is completed.
[0053]
Further, as described above, in the present embodiment, the lean air-fuel ratio operation mode (1 to 3) is continued in the engine while the stored oxygen amount reduction operation is being executed.
(2) Second embodiment
Next, a second embodiment of the present invention will be described. In the first embodiment, the operation of reducing the stored oxygen amount of the SCs 5a and 5b is performed only by the secondary fuel injection, and the operation mode of the rich air-fuel ratio operation is changed until the stored oxygen amount reducing operation (secondary fuel injection) is completed. No switching was performed. In the present embodiment, in an engine that requires secondary fuel injection once for all cylinders, the operation mode switching (intake stroke fuel injection) is performed at the timing when the switching command from the lean air-fuel ratio operation mode to the rich air-fuel ratio operation mode is issued. For cylinders that can be switched to intake stroke fuel injection, the intake stroke fuel injection amount is increased by the amount corresponding to the secondary fuel injection amount, and for cylinders that do not make the transition to intake stroke fuel injection in time Perform fuel injection.
[0054]
FIG. 3 is a diagram for explaining the timing of secondary fuel injection (in FIG. 3, an example in which secondary fuel injection is performed from the end of the expansion stroke to the beginning of the exhaust stroke) and the intake stroke fuel injection increase amount according to the present embodiment. FIG. 3 shows the timing of switching from mode (1) (lean stratified charge combustion (single compression stroke injection) to mode (4) (stoichiometric air-fuel ratio homogeneous mixture combustion (intake stroke injection)). Switching between modes is the same as in FIG.
[0055]
FIG. 3 shows the fuel injection timing of each of the # 1 to # 4 cylinders and the fuel injection amount setting timing for performing the fuel injection. In FIG. 3, CSET is the compression stroke fuel injection amount setting timing, CINJ is the compression stroke fuel injection execution timing, EXSET is the secondary fuel injection amount setting timing, EXINJ is the secondary fuel injection execution timing, and ISET is the intake stroke fuel injection amount setting timing. , IINJ indicate the intake stroke fuel injection execution timing, respectively. Further, CH in FIG. 3 indicates the start timing of the stored oxygen amount reduction operation for switching the operation mode. In FIG. 3, “suction”, “pressure”, “expansion”, and “exhaust” represent the intake stroke, compression stroke, expansion stroke, and exhaust stroke of each cylinder, respectively. As shown in FIG. 3, in the present embodiment, the secondary fuel injection amount is set at the end of the compression stroke (EXSET), and the intake stroke fuel injection amount is set at the beginning of the exhaust stroke (ISET).
[0056]
Now, assuming that an operation for reducing the amount of oxygen stored in the catalyst for switching the operation mode is started at the timing of FIG. 3CH, CH corresponds to the middle of the compression stroke in the # 1 cylinder, so the intake stroke fuel injection timing (IINJ) has already been set. The compression stroke fuel injection amount has been set at the timing CSET. Accordingly, since the operation mode cannot be switched immediately for the # 1 cylinder, the compression stroke fuel injection IINJ is executed as it is, and the secondary fuel injection amount is set at the timing EXSET and the secondary fuel injection (EXINJ) is executed.
[0057]
On the other hand, in the # 3 cylinder, the CH timing corresponds to the middle stage of the intake stroke, but at this timing, the intake stroke fuel injection amount set timing has already passed, so that it is not possible to immediately shift to the intake stroke fuel injection. Therefore, in the # 3 cylinder, the compression stroke fuel injection is executed as it is, the compression stroke fuel injection amount is set by CSET, and the secondary fuel injection amount set timing (EXSET) at the end of the subsequent compression stroke is set. The secondary fuel injection is executed with the amount set.
[0058]
Similarly, in the # 4 cylinder, the CH timing corresponds to the middle of the exhaust stroke, but in this case as well, since the intake stroke fuel injection amount set timing (IINJ) has passed, the process immediately shifts to intake stroke fuel injection. I can't. Accordingly, the secondary fuel injection (EXINJ) is executed while continuing the compression stroke fuel injection (CINJ) as in the case of the # 3 cylinder.
[0059]
On the other hand, in the # 2 cylinder, since the CH timing corresponds to the middle stage of the expansion stroke, the intake stroke fuel injection amount set timing (ISET) has not yet been reached, and it is possible to shift to the intake stroke fuel injection. Therefore, in the # 2 cylinder, the operation mode is switched to perform the intake stroke fuel injection, and the fuel injection amount set by ISET is increased by the amount corresponding to the secondary fuel injection amount. That is, for the # 2 cylinder, the operation mode is switched without performing the secondary fuel injection, and instead, the fuel injection is performed by adding the equivalent amount of the secondary fuel injection to the intake stroke fuel injection amount immediately after the operation mode switching. Set the amount.
[0060]
As can be seen from the timing chart of FIG. 3, in this embodiment, after the start of the catalyst storage oxygen amount reduction operation for switching the operation mode, the secondary fuel injection amount set timing precedes the intake stroke fuel injection amount set timing. For the cylinders (# 1, # 3, and # 4 in the case of FIG. 3), the secondary fuel injection is executed while continuing the compression stroke fuel injection (that is, without switching the operation mode). For the cylinder (# 2 cylinder) whose amount set timing is ahead of the secondary fuel injection amount set timing, the operation mode is switched to perform the intake stroke fuel injection, and during the intake stroke fuel injection, it corresponds to the secondary fuel injection amount of the other cylinders The amount of fuel for the minute is increased. In this case as well, the operation for reducing the amount of oxygen stored in the catalyst is completed when the secondary fuel injection or the intake stroke fuel injection amount is increased once for all cylinders, and thereafter the operation mode is switched for all cylinders. .
[0061]
That is, in this embodiment, the operation for reducing the storage amount of the catalyst is started before the operation mode is switched (# 1, # 3, # 4 cylinder) and is ended after the operation mode is switched (# 2 cylinder). Thereby, the operation mode switching time can be shortened. FIG. 4 is a flowchart for explaining the stored oxygen amount reducing operation of the present embodiment. 4 is performed as a routine executed by the ECU 30 at predetermined intervals. The flowchart of FIG. 4 includes steps 213 to 217 of the flowchart of FIG.414, 415, 416 and 4172 is different from the flowchart of FIG. Therefore, only the differences will be described here.
[0062]
In step 413, the amount of one secondary fuel injection is set from the stored oxygen amount OSC of SC5a, 5b as in step 213 in FIG.4If the current intake stroke fuel injection amount set timing (FIG. 3 ISET) is reached, step 41 is performed.6Then, an amount obtained by adding the secondary fuel injection amount calculated in step 413 to the intake stroke fuel injection amount after the operation mode switching is increased and set as the intake stroke fuel injection amount. If the intake stroke fuel injection amount setting timing is not reached, step 41 is performed.5, Step 417Then, the secondary fuel injection amount is set. As a result, in the cylinder in time for the intake stroke fuel injection amount set timing (ISET), the operation mode is switched and the fuel injection amount is increased instead of the secondary fuel injection.
[0063]
In the present embodiment, the operation for reducing the stored oxygen amount is performed by executing the secondary fuel injection in the cylinders other than the cylinders in time for the set timing of the intake stroke fuel injection, but the secondary fuel injection is not performed. In each cylinder, the intake stroke fuel injection amount may be increased from the next intake stroke fuel injection amount setting timing by the amount corresponding to the secondary fuel injection amount as in the above-described # 2 cylinder. In this case, the operation for reducing the amount of oxygen stored in the catalyst is performed immediately after switching the operation mode in each cylinder.
[0064]
(3) Third embodiment
Next, a third embodiment of the present invention will be described. In the first embodiment described above, when the engine operation mode is switched, the operation mode is switched after the operation for reducing the amount of oxygen stored in the catalyst is completed. In the second embodiment, the operation mode is switched for some or all cylinders. Immediately after that, a reduction operation is performed. On the other hand, in this embodiment, the stored oxygen amount reducing operation is performed independently of the switching of the operation mode. That is, the operation mode is switched as usual in each cylinder, and the secondary fuel injection is executed regardless of the operation mode until the switching of the operation mode of each cylinder is completed. In actual operation, for example, from the very lean combustion state of mode (1) (lean air-fuel ratio stratified combustion (injection once in the compression stroke)), sudden acceleration or the like (5) (rich air-fuel ratio homogeneous mixture combustion (intake stroke 1) It may be necessary to shift to the rich air-fuel ratio combustion)), but in such a case, switching the operation mode directly from (1) to (5) causes a sudden change in the combustion air-fuel ratio. There may be sudden fluctuations in the output torque. In such a case, the mode (1) to (2) (lean air-fuel ratio homogeneous mixture / stratified combustion (intake stroke / compression stroke 2) is temporarily performed without switching the operation mode directly from mode (1) to (4). In some cases, the operation mode is switched to (5) via (3) (3) (lean air-fuel ratio homogeneous mixture combustion (intake of one intake stroke))).
[0065]
In this embodiment, when the operation mode is switched, the switching operations such as (1) → (2) → (3) → (5) are executed independently, and at the same time, the secondary fuel injection is executed until the mode switching is completed. ing. That is, in the present embodiment, the operation mode switching and the catalyst storage oxygen amount reduction operation are performed in parallel. This prevents the operation mode switching time from being affected by the execution of the stored oxygen amount reduction operation.
[0066]
FIG. 5 is a flowchart illustrating an operation for reducing the amount of stored oxygen in the catalyst according to the present embodiment. This operation is performed as a routine executed by the ECU 30 at predetermined intervals.
In FIG. 5, steps 501 to 509 indicate the same operations as steps 201 to 209 in FIG. Also in this embodiment, if the operation mode is not switched from the lean air-fuel ratio to the rich air-fuel ratio in steps 501 to 509, the process immediately proceeds to step 523 and the target operation mode M₁And current operation mode M₀A mode switching operation corresponding to is performed.
[0067]
On the other hand, when it is necessary to switch the operation mode from the lean air-fuel ratio operation to the rich air-fuel ratio operation in steps 501 to 509, the process proceeds to step 511 and the target operation mode M₁And current operation mode M₀And M₀→ M₁The number of secondary fuel injections that can be executed is calculated on the basis of the number of cycles required for the transition to, and the amount of secondary fuel injection per one time is calculated from the number of secondary fuel injections executed and the current catalyst storage oxygen amount OSC Is calculated. The secondary fuel injection amount is calculated as an amount capable of generating HC that consumes the entire amount of oxygen released from the SCs 5a and 5b and maintains the exhaust gas downstream of the SCs 5a and 5b at a rich air-fuel ratio.
[0068]
Then, after calculating the secondary fuel injection amount, in steps 513 to 521, the secondary fuel injection is executed until the switching of the operation mode is completed. At this time, in steps 521 and 523, the shift to the rich spike operation (step 521) and the current operation mode M are performed in parallel with the secondary fuel injection.₀And target operation mode M₁A transition operation is performed according to. When the transition operation of step 513 or step 521 is completed, it is determined in step 513 that the mode switching has been completed, and the secondary fuel injection is stopped.
[0069]
Next, a method for estimating the stored oxygen amount OSC of the SCs 5a and 5b used for calculating the secondary fuel injection amount in each of the above embodiments will be described. In this embodiment, SC5a and 5b are stored from the exhaust air-fuel ratio AF at the SC5a and 5b inlets detected by the air-fuel ratio sensors 29a and 29b disposed at the SC5a and 5b inlets, and the engine intake air weight flow rate (gram / second) GA. An oxygen amount OSC is calculated.
[0070]
As mentioned above, the catalyst O₂Due to the storage function, when the exhaust air-fuel ratio flowing into SC5a, 5b is leaner than the stoichiometric air-fuel ratio, surplus oxygen in the exhaust is absorbed by SC5a, 5b, and when it is richer than the stoichiometric air-fuel ratio, oxygen absorbed from SC5a, 5b In both cases, the exhaust air-fuel ratio at the SC5a and 5b outlets is close to the stoichiometric air-fuel ratio. Accordingly, the amount of oxygen absorbed in or released from the SCs 5a and 5b corresponds to the amount of oxygen required to make the exhaust of the air-fuel ratio AF the stoichiometric air-fuel ratio.
[0071]
Now, assuming that the weight of air necessary for burning a certain amount of fuel to generate exhaust with an air-fuel ratio AF is GA, GA = AF × F. Also, GA ′ = ST × F, where GA ′ is the air weight necessary for burning the same amount of fuel to generate exhaust with the stoichiometric air-fuel ratio ST. On the other hand, the oxygen concentration in the air₂Then, in the air of weight GA and GA ′, AO respectively₂× GA and AO₂× GA '. That is, the amount of oxygen required to burn a certain amount of fuel to produce exhaust with the stoichiometric air-fuel ratio ST is AO₂× GA '= AO₂× ST × F. On the other hand, the amount of oxygen when the same fuel is burned to produce the exhaust of air-fuel ratio AF is AO₂× GA = AO₂× AF × F. Therefore, the amount of oxygen required to make the exhaust of the air-fuel ratio AF the stoichiometric air-fuel ratio, that is, the amount of oxygen absorbed by the SCs 5a and 5b when AF> ST is (AO₂× GA)-(AO₂× GA ′) = AO₂× F × (AF-ST). Also, since F = GA / AF, the oxygen release / absorption amount is
AO₂× GA × (AF-ST) / AF = AO₂× GA × (ΔAF / AF). Here, ΔAF = (AF−ST). Further, since GA is an air flow rate per unit time (second), if AF> ST, the catalyst has an AO per unit time during engine operation.₂Oxygen of × GA × (ΔAF / AF) is absorbed, and the stored oxygen amount OSC of the catalyst is AO₂It will increase by × GA × (ΔAF / AF). (If AF <ST, ΔAF becomes negative and the stored oxygen amount OSC of the catalyst decreases).
[0072]
Therefore, when the exhaust air / fuel ratio is AF and the intake air weight flow rate is GA, the change amount of the stored oxygen amount OSC per time Δt of SC5a and 5b is AO.₂× GA × (ΔAF / AF) × Δt, but since the amount of change in OSC is actually affected by the oxygen absorption / release rate of the catalyst, the actual amount of change in OSC is AO₂It is expressed as × GA × (ΔAF / AF) × Δt × K (K is a correction coefficient based on the oxygen absorption / release rate). In practice, the oxygen absorption / release rate is affected by the catalyst temperature, and increases as the catalyst temperature increases. Furthermore, the rate of oxygen absorption and release is different, and the rate of oxygen absorption is higher than the rate of release. Therefore, in the present embodiment, the amount of change in the OSC per time Δt is represented by the following equation for absorption (AF ≧ ST) and emission (AF <ST).
[0073]
Absorption (AF ≧ ST): AO₂× GA × (ΔAF / AF) × Δt × A
Release (AF <ST): AO₂× GA × (ΔAF / AF) × Δt × B
Here, A and B are correction coefficients determined by the oxygen absorption / release rate and the catalyst temperature.
FIG. 6 is a flowchart for explaining the stored oxygen amount calculation operation of the SCs 5a and 5b in the present embodiment. This operation is performed by a routine executed by the ECU 30 at regular time intervals corresponding to the above Δt. In this operation, the change amount of the stored oxygen amount OSC per time Δt of the SCs 5a and 5b is calculated using the above formula, and this change amount is integrated from the start of the engine, whereby the current stored oxygen amount of the SCs 5a and 5b. OSC is estimated.
[0074]
In the operation of FIG. 6, first, in step 601, the exhaust air-fuel ratio AF at the inlets of SC5a and 5b, the intake air weight flow rate GA of the engine, and the SC5a and 5b temperature TCAT are read. In the present embodiment, the exhaust air-fuel ratio AF is obtained as an average value of the exhaust air-fuel ratios detected by the air-fuel ratio sensors 29a and 29b at the SC5a and 5b inlets. The intake air weight flow rate GA is calculated as the product of the amount of fuel supplied to the engine per unit time (fuel injection amount) and the exhaust air-fuel ratio AF. Further, the temperature TCAT of the SCs 5a and 5b may be measured by arranging a temperature sensor on the catalyst bed, or the relationship between the engine load (fuel injection amount), the rotational speed and the exhaust temperature is obtained in advance, and the engine fuel injection is performed. The exhaust temperature may be calculated based on the amount (engine load) and the rotational speed, and this exhaust temperature may be approximately used as TCAT.
[0075]
After reading AF, GA, and TCAT as described above, it is calculated in step 603 whether AF ≧ ST (ST is the stoichiometric air-fuel ratio). If AF ≧ ST, the exhaust purification catalyst currently absorbs oxygen. Since the stored oxygen amount OSC has increased, the correction coefficient A is calculated from the oxygen absorption rate of the SCs 5a and 5b and the catalyst temperature TCAT in step 605. In step 607, the value of the stored oxygen amount OSC is (AO₂X GA x (ΔAF / AF) x Δt x A). In step 609, the increased OSC value is changed to the maximum value OSC._MAXIf the value exceeds OSC, the value of OSC_MAXSet to OSC_MAXIs the maximum oxygen amount (saturation amount) that SC5a and 5b can store.
[0076]
On the other hand, if AF <ST in step 603, the SCs 5a and 5b are currently releasing oxygen. Therefore, in step 613, the correction coefficient B is calculated based on the oxygen release rate and the catalyst temperature TCAT. In 615, the value of OSC is (AO₂* GA * (ΔAF / AF) * Δt * B) (in this case, since ΔAF <0, OSC decreases). In steps 617 and 619, the OSC value is limited to the minimum value 0, and the current operation is terminated. When the engine is started, the initial value of OSC in steps 607 and 615 is OSC._MAXSet to This is because when the engine is stopped, SC5a and 5b are in an atmospheric atmosphere (lean air-fuel ratio), and SC5a and 5b are saturated with oxygen.
[0077]
By calculating the amount of fuel required for the stored oxygen amount reduction operation of the SCs 5a and 5b using the catalyst stored oxygen amount OSC estimated by the operation of FIG. 6, in each of the foregoing embodiments, an accurate stored oxygen amount reduction operation is performed. NO when the engine operation mode is switched from the lean air-fuel ratio to the rich air-fuel ratio._XUnpurified NO from the storage reduction catalyst 7_XIs prevented from flowing out.
[0078]
Next, the saturated oxygen amount OSC of SC5a and 5b used for the operation of FIG. 6 using FIG. 7 to FIG._MAXNext, the correction will be described. In the operation of FIG. 6, the saturated oxygen amount OSC_MAXThe amount of stored oxygen OSC may be calculated with an appropriate constant value, but more precisely the OSC according to the deterioration of the catalyst._MAXIt is preferable to correct the value of. O of catalyst₂The storage function decreases as the catalyst deteriorates, and the maximum oxygen (saturation) OSC that the catalyst can store_MAXAlso go down. Therefore, in this embodiment, the deterioration state of the catalyst is determined, and the OSC is determined according to the deterioration state._MAXCorrect the value of.
[0079]
First, a method for determining the deterioration state of the catalyst will be described. In the present embodiment, the locus lengths of the output signal curves of the air-fuel ratio sensors 29a and 29b upstream of the SCs 5a and 5b and the NO_XThe deterioration state of the catalyst is determined based on the locus length of the output signal curve of the air-fuel ratio sensor 31 downstream of the storage reduction catalyst 7.
FIG. 7 shows general waveforms of the air-fuel ratio sensor output VOM provided upstream of the exhaust purification catalyst and the air-fuel ratio sensor output VOS provided downstream of the catalyst when the engine air-fuel ratio is feedback controlled to the stoichiometric air-fuel ratio. ing. In FIG. 7, (A) is O of the exhaust purification catalyst.₂Figure 7 (B) shows the waveform when the storage function is high.₂The waveforms when the storage function deteriorates are shown.
[0080]
As shown in FIGS. 7A and 7B, the engine air-fuel ratio (exhaust air-fuel ratio) becomes rich and lean within a relatively small range centering on the stoichiometric air-fuel ratio in a state where feedback control is performed to the stoichiometric air-fuel ratio. fluctuate. For this reason, the upstream air-fuel ratio sensor output VOM also exhibits periodic fluctuations centering on the theoretical air-fuel ratio. In this case, the catalyst O₂If the storage function is sufficiently high, the exhaust air / fuel ratio at the catalyst outlet is maintained in the vicinity of the stoichiometric air / fuel ratio even if the exhaust air / fuel ratio flowing into the catalyst slightly fluctuates around the stoichiometric air / fuel ratio. For this reason, O₂In the case of a catalyst having a sufficiently high storage function, the downstream air-fuel ratio sensor output VOS does not vary much as shown in FIG. Therefore, LOVS is relatively small in length along the locus of the output VOS. However, the catalyst deteriorates and O₂When the storage function is lowered, the amount of oxygen absorbed and released by the catalyst is lowered, so that the air-fuel ratio on the downstream side also varies in accordance with the fluctuation of the air-fuel ratio on the upstream side. Therefore, the locus length LVOS of the downstream air-fuel ratio sensor output VOS is O₂As the storage function declines, it becomes larger as shown in FIG.₂When the storage function is completely lost, the locus length LVOM of the upstream air-fuel ratio sensor output VOM becomes equal. That is, if the ratio LR (LR = LVOS / LVOM) between the locus length LVOS of the downstream air-fuel ratio sensor output VOS and the locus length LVOM of the upstream air-fuel ratio sensor output VOM during the air-fuel ratio feedback control is taken,₂When the storage function is sufficiently high, LR is much smaller than 1, and O₂As the storage function decreases, it increases and approaches 1. In the present embodiment, based on the above, the ratio LR of the trajectory lengths of the upstream air-fuel ratio sensors 29a, 29b output and the downstream air-fuel ratio sensor 31 output is set to O of SC5a, 5b.₂It is used as a parameter indicating storage function degradation. In the case of an engine having two exhaust purification catalysts 5a, 5b and two upstream air-fuel ratio sensors 29a, 29b as in the present embodiment, the average value of the outputs of the two upstream air-fuel ratio sensors 29a, 29b is obtained. The trajectory length LVOM may be calculated using the upstream air-fuel ratio sensor output VOM, or the output trajectory length is calculated for each of the air-fuel ratio sensors 29a and 29b, and the average of both trajectory lengths is used as the upstream air-fuel ratio. The sensor output trajectory length LVOM may be used.
FIG. 8 shows the maximum stored oxygen amount OSC in consideration of the deterioration of the SCs 5a and 5b of the present embodiment._MAXIt is a flowchart explaining the calculation operation of. This operation is performed as a routine executed by the ECU 30 at regular intervals.
[0081]
When the operation starts in FIG. 8, it is determined in step 801 whether or not the deterioration parameter calculation execution condition is satisfied. In this embodiment, the condition of step 801 is that the engine is operated in mode (4) (theoretical air-fuel ratio homogeneous mixture combustion (intake stroke one injection)) and the air-fuel ratio based on the air-fuel ratio sensors 29a and 29b. It is assumed that feedback control is performed. As described with reference to FIG. 7, the trajectory length ratio LR is set to 0% of the catalyst.₂This is because the trajectory length ratio LR needs to be calculated in a state where the engine air-fuel ratio is feedback controlled to the stoichiometric air-fuel ratio in order to use it as a parameter representing the storage function.
[0082]
If the condition is satisfied in step 801, the output voltage VOM of the upstream air-fuel ratio sensors 29a and 29b and the output voltage VOS of the downstream air-fuel ratio sensor 31 are read in step 803. In this embodiment, the average value of the output voltages of the sensors 29a and 29b is used as the VOM. Next, at step 805, the locus length LVOM of the upstream air-fuel ratio sensor output VOM and the locus length LVOS of the downstream air-fuel ratio sensor output VOS are calculated as follows:
LVOM = LVOM + | VOM-VOM_i-1｜
LVOS = LVOS + | VOS-VOS_i-1｜
Is calculated as Where VOM_i-1, VOS_i-1Are values of VOM and VOS at the time of the previous execution of this operation, and are updated at step 807 every time LVOM and LVOS are calculated. That is, in this embodiment, as shown in FIG. 9, | VOM-VOM_i-1| And | VOS-VOS_i-1Approximate calculation is performed using the integrated values of | as LVOM and LVOS, respectively.
[0083]
Steps 809 and 811 are operations for determining the locus length calculation period. In the present embodiment, the integration of the LVOM and LVOS is performed until the value of the counter CT that is incremented by 1 every time the operation is performed reaches a predetermined value T. The predetermined value T is set so that the total of the integration periods is about several tens of seconds.
If the period T has elapsed in step 811, the trajectory length ratio LR is calculated as LR = LVOS / LVOM from the values of LVOM and LVOS accumulated in the period in step 813. In step 815, the locus length ratio LR (O₂OSC based on the relationship set in advance from the value of the storage function parameter)_MAXCorrection coefficient RD is obtained. In step 819, the stored oxygen amount maximum value OSC of the current SC 5a, 5b._MAXBut OSC_MAX= OSC_MAX0* Calculated as RD. Where OSC_MAX0Is the maximum stored oxygen amount in a new state in which SC5a and 5b are not deteriorated at all.
[0084]
FIG. 10 is a graph showing the relationship between the locus length ratio LR and the correction coefficient RD, which is used to obtain the correction coefficient RD in step 817 of FIG. As shown in FIG. 10, the value of the correction coefficient RD is set to 1.0 when the catalyst is not deteriorated at all (LR << 1.0), and the value of LR approaches 1 as the deterioration of the catalyst proceeds. To be smaller).
[0085]
According to FIG. 10, the maximum stored oxygen amount OSC of SC5a, 5b_MAXIs set according to the degree of deterioration of the catalyst, so that the estimation accuracy of the stored oxygen amount OSC of the SCs 5a and 5b in each of the above-described embodiments is improved. Therefore, the operation for reducing the stored oxygen amount more accurately in each of the above-described embodiments. Can be performed.
[0086]
【The invention's effect】
According to the invention described in each claim, O₂When an exhaust purification catalyst having a storage function is arranged in the exhaust passage, it is possible to prevent a delay in the change of the exhaust air / fuel ratio downstream of the catalyst from the lean air / fuel ratio to the stoichiometric or rich air / fuel ratio. There is an effect.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of an embodiment in which the present invention is applied to an automobile internal combustion engine.
FIG. 2 is a flowchart for explaining a first embodiment of an operation for reducing the amount of stored oxygen of the exhaust purification catalyst of the present invention.
FIG. 3 is a timing chart for explaining a second embodiment of the stored oxygen amount reducing operation of the exhaust purification catalyst of the present invention.
FIG. 4 is a flowchart for explaining a second embodiment of the stored oxygen amount reducing operation of the exhaust purification catalyst of the present invention.
FIG. 5 is a flowchart for explaining a third embodiment of the operation for reducing the amount of stored oxygen in the exhaust purification catalyst of the present invention.
FIG. 6 is a flowchart illustrating an operation for estimating the stored oxygen amount of the exhaust purification catalyst used in the first to third embodiments.
FIG. 7 is a diagram for explaining a change between an upstream air-fuel ratio sensor output and a downstream air-fuel ratio sensor output due to deterioration of an exhaust purification catalyst.
FIG. 8 is a flowchart for explaining a stored oxygen amount estimation operation in consideration of deterioration of an exhaust purification catalyst.
9 is a diagram illustrating a method for calculating an air-fuel ratio sensor output trajectory length used in the operation of FIG.
FIG. 10 shows exhaust purification catalyst O.₂It is a figure explaining the relationship between the correction coefficient of a storage function, and locus | trajectory length ratio.
[Explanation of symbols]
1. Internal combustion engine
2 ... Exhaust passage
5a, 5b ... Start Catalyst (SC)
7 ... NO_XOcclusion reduction catalyst
29a, 29b, 31 ... air-fuel ratio sensor
30 ... Electronic control unit (ECU)

Claims (5)

An exhaust purification device for an internal combustion engine that switches the operation air-fuel ratio between a lean air-fuel ratio operation and a stoichiometric or rich air-fuel ratio operation as required,
An exhaust purification catalyst having an O ₂ storage function disposed in the engine exhaust passage;
When the engine is switched from lean air-fuel ratio operation to stoichiometric air-fuel ratio operation or rich air-fuel ratio operation, fuel that does not contribute to combustion is supplied to the engine, and the exhaust air-fuel ratio flowing into the exhaust purification catalyst is made rich air-fuel ratio. Storage reduction means for reducing the amount of oxygen stored in the exhaust purification catalyst;
Further, when the air-fuel ratio of the exhaust flowing into the exhaust passage downstream of the exhaust purification catalyst is a lean air-fuel ratio, the NO _X in the exhaust is absorbed, and the absorbed NO _X is released when the oxygen concentration in the exhaust flowing in decreases. comprising a _the NO _X storage reduction catalyst that, the,
The storage reduction means further includes an exhaust purification device for an internal combustion engine that reduces the amount of oxygen stored in the exhaust purification catalyst when NO _x absorbed from the NO _x storage reduction catalyst is to be released. The NO during the lean air-fuel ratio operation of the engine _X NO absorbed from the storage reduction catalyst _X For reducing the amount of oxygen stored in the exhaust purification catalyst by the storage reduction means during the rich spike operation. The exhaust emission control device for an internal combustion engine according to claim 1. An exhaust purification device for an internal combustion engine that performs lean air-fuel ratio operation as required,
O placed in the engine exhaust passage ₂ An exhaust purification catalyst having a storage function;
NO in the exhaust when the air-fuel ratio of the inflowing exhaust disposed in the exhaust passage downstream of the exhaust purification catalyst is a lean air-fuel ratio _X NO absorbed when the oxygen concentration in the exhaust gas _X NO release _X An occlusion reduction catalyst;
NO during engine lean air-fuel ratio operation _X NO absorbed from the storage reduction catalyst _X Means for performing a rich spike operation for switching the operating air-fuel ratio of the engine to a rich air-fuel ratio for a short time when
Storage reduction means for reducing the amount of oxygen stored in the exhaust purification catalyst by making the exhaust air / fuel ratio flowing into the exhaust purification catalyst for a predetermined period immediately after the start of the rich spike operation further richer than the air / fuel ratio during the rich spike operation When,
An exhaust purification device for an internal combustion engine, comprising: The storage reduction means includes storage estimation means for estimating an oxygen amount stored in the exhaust purification catalyst based on an operating state of the engine, and performs the oxygen amount reduction operation according to the estimated stored oxygen amount. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 3. The exhaust gas purification device for an internal combustion engine according to claim 4, wherein the storage estimation means estimates the amount of oxygen stored in the exhaust gas purification catalyst based on a deterioration state of the exhaust gas purification catalyst in addition to an operating state of the engine.

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