JP3966082B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine that includes a NOx storage reduction catalyst in an exhaust passage of the internal combustion engine and performs NOx reduction by rich spike processing.
[0002]
[Prior art]
As an exhaust gas purification system for an internal combustion engine, a system using a NOx occlusion reduction type catalyst is known (Japanese Patent Laid-Open No. 2000-161105). The NOx occlusion reduction type catalyst can oxidize HC (hydrocarbon) and CO because the exhaust air-fuel ratio is lean during combustion at a fuel concentration lower than the stoichiometric air-fuel ratio such as stratified combustion or lean combustion. Since NOx cannot be reduced, NOx in the exhaust gas is occluded and is not discharged outside. When the exhaust air-fuel ratio becomes rich due to combustion at a fuel concentration higher than the stoichiometric air-fuel ratio, NOx can be reduced. Therefore, the stored NOx is released and NOx is removed from the exhaust gas by the platinum catalyst. Reduced and purified with CO.
[0003]
In such an exhaust purification system using a NOx storage reduction catalyst, when the exhaust air-fuel ratio is made lean by stratified combustion or lean combustion as described above, the NOx storage amount in the NOx storage reduction catalyst increases to a predetermined amount. Is reached, rich spike processing is executed to supply high-concentration fuel to the internal combustion engine. By this rich spike treatment, a large amount of unburned gas such as HC and CO is discharged into the exhaust gas, and these unburned gases act as a reducing agent on the NOx occlusion reduction type catalyst and are stored as described above. NOx that is present is quickly reduced and disappears. Therefore, the NOx occlusion reduction type catalyst can again occlude NOx.
[0004]
In the disclosed technology described above, during the rich spike processing in which the air-fuel ratio is set to a high fuel concentration, a large change in the air-fuel ratio is suppressed to prevent misfire and stable even at low rotation and low load such as during idling. The engine can be operated.
[0005]
[Problems to be solved by the invention]
However, in stratified combustion or lean combustion at low rotation and low load such as when idling, a state where a large amount of HC is contained in the exhaust occurs, and this causes the NOx storage reduction catalyst to be poisoned by HC. There is.
[0006]
That is, when a high concentration of HC comes into contact with the surface of the platinum-based catalyst, an HC layer starts to form, and the catalytic reaction rate for oxidizing HC by oxygen in the exhaust gas is reduced to cause further accumulation of HC. It is thought that a vicious circle occurs in which the thickness of the film increases. Thus, even if a large amount of oxygen is present in the exhaust due to combustion at a fuel concentration lower than the stoichiometric air-fuel ratio, once the thick HC layer is formed, the removal of HC by oxidation does not proceed and the thick HC layer Will be maintained.
[0007]
In this way, in the state where the surface of the platinum-based catalyst is thickly covered with the HC layer, even if the rich spike process is executed, NOx released by the rich atmosphere is prevented from contacting the platinum-based catalyst by the thick HC layer. This leads to a situation where it cannot be sufficiently reduced and purified. As a result, exhaust emission may be deteriorated, and fuel for the rich spike is wasted.
[0008]
An object of the present invention is to suppress deterioration of exhaust emission of an internal combustion engine due to inhibition of catalytic reaction of a NOx occlusion reduction type catalyst by HC and prevent waste of fuel.
[0009]
[Means for Solving the Problems]
In the following, means for achieving the above object and its effects are described.
The exhaust gas purification apparatus for an internal combustion engine according to claim 1 stores a NOx occlusion reduction type catalyst that occludes NOx in exhaust gas when the exhaust air-fuel ratio is lean and reduces NOx occluded when the exhaust air-fuel ratio is rich. An exhaust purification device for an internal combustion engine, which is provided in an exhaust passage of an internal combustion engine and performs reduction of the NOx by a rich spike process for temporarily enriching the exhaust air / fuel ratio, wherein the NOx occlusion reduction is performed when the exhaust air / fuel ratio is lean It is determined that there is a risk of poisoning by the poisoning determination means for determining whether the type catalyst is likely to be poisoned by hydrocarbons and at the start of execution of the rich spike process. In this case, the air-fuel ratio control means for executing the rich spike processing after temporarily setting the exhaust air-fuel ratio to the stoichiometric air-fuel ratio in advance is provided.
[0010]
The air-fuel ratio control means temporarily sets the exhaust air-fuel ratio to the stoichiometric air-fuel ratio prior to execution of the rich spike process when it is determined by the poisoning determination means that there is a possibility of hydrocarbon poisoning. In this way, by combustion at the stoichiometric air-fuel ratio in the internal combustion engine, the combustion of the air-fuel mixture becomes good and the amount of hydrocarbons in the exhaust gas is reduced. Reactive oxygen is supplied in a well-balanced manner. Since the exhaust gas is in a stoichiometric air-fuel ratio state, the NOx occlusion reduction catalyst functions sufficiently as a three-way catalyst, and the oxidation-reduction reaction is promoted.
[0011]
In such a state, the thick hydrocarbon layer covering the surface of the NOx storage reduction catalyst gradually disappears as the oxidation proceeds. Therefore, the catalytic activity for reducing NOx with the reducing agent can be recovered.
[0012]
As described above, the air-fuel ratio control means executes the rich spike process after recovering the catalytic activity for reducing NOx. Therefore, NOx released from the NOx occlusion reduction type catalyst in the rich atmosphere is reduced by hydrocarbons and CO as reducing agents. Thus, reduction and purification can be ensured. For this reason, hydrocarbons and CO as a reducing agent, and NOx released from the NOx storage reduction catalyst due to a rich atmosphere are not released in large quantities downstream of the NOx storage reduction catalyst.
[0013]
Note that the rich spike processing is delayed due to the existence of the period in which the exhaust air-fuel ratio is in the stoichiometric air-fuel ratio state, but during this period, the exhaust air-fuel ratio is in the stoichiometric air-fuel ratio state and the lean state is not continued. So NOx emissions will not deteriorate.
[0014]
In this way, it is possible to suppress the deterioration of exhaust emission caused by the inhibition of the catalytic reaction of the NOx occlusion reduction catalyst by hydrocarbons, and to prevent fuel waste.
The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein the poisoning determination means obtains a hydrocarbon concentration of exhaust gas flowing into the NOx occlusion reduction type catalyst, and the exhaust air-fuel ratio is lean. Sometimes, when the hydrocarbon concentration is equal to or higher than the high concentration determination value, it is determined that the NOx occlusion reduction type catalyst may be poisoned by hydrocarbons.
[0015]
When the hydrocarbon concentration of the exhaust gas flowing into the NOx storage reduction catalyst when the exhaust air-fuel ratio is lean is equal to or higher than the high concentration determination value, hydrocarbons that cannot be oxidized in the NOx storage reduction catalyst cover the NOx storage reduction catalyst. It can be determined that there is a risk of poisoning. When the poisoning determination means determines such a situation, as described above, the air-fuel ratio control means executes the rich spike process after temporarily setting the exhaust air-fuel ratio to the stoichiometric air-fuel ratio in advance. As a result, it is possible to reliably suppress the deterioration of exhaust emission caused by the inhibition of the catalytic reaction of the NOx occlusion reduction catalyst by hydrocarbons, and to prevent waste of fuel.
[0016]
According to a third aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine, wherein, in the second aspect, the air-fuel ratio control means when the poisoning judgment means judges that the hydrocarbon concentration is not less than a high concentration judgment value. Is characterized by lowering the richness in the rich spike processing.
[0017]
Thus, in the situation where the poisoning determination means determines that the hydrocarbon concentration is equal to or higher than the high concentration determination value, even if the fuel increase necessary for the rich spike process is being executed, The hydrocarbon concentration increases more than necessary. As a result, part of the hydrocarbons and CO as the reducing agent is not used for the reduction purification of NOx released from the NOx storage reduction catalyst, but is released downstream of the NOx storage reduction catalyst without being purified. May cause deterioration.
[0018]
For this reason, when it is determined by the poisoning determination means that the hydrocarbon concentration is equal to or higher than the high concentration determination value, by reducing the richness of the rich spike process, hydrocarbons and CO as a reducing agent are reduced. The exhaust gas can be purified by reliably reacting with NOx oxygen. As a result, the deterioration of exhaust emission during the rich spike process can be effectively suppressed, and fuel waste can be effectively prevented.
[0019]
According to a fourth aspect of the present invention, there is provided an exhaust gas purification apparatus for an internal combustion engine according to the third aspect, wherein the air-fuel ratio control means sets the reduction of the rich degree in the rich spike processing in accordance with the operating state of the internal combustion engine. And
[0020]
In addition, since the hydrocarbon concentration in the exhaust gas changes according to the operating state of the internal combustion engine, the rich spike processing can be performed more appropriately by setting the reduction of the rich degree in the rich spike processing according to the operating state of the internal combustion engine. It is possible to suppress deterioration of exhaust emission at the time, and to prevent fuel waste more effectively.
[0021]
6. The exhaust gas purification apparatus for an internal combustion engine according to claim 5, wherein the air-fuel ratio control means determines the end of the period in which the exhaust air-fuel ratio is set to the stoichiometric air-fuel ratio in advance. Judgment is made according to the quantity.
[0022]
The period in which the hydrocarbon covering the surface of the NOx storage reduction catalyst is oxidized and disappeared by setting the exhaust gas to the stoichiometric air-fuel ratio mainly depends on the amount of unreacted oxygen supplied by the exhaust gas. Therefore, the end of the period in which the exhaust air-fuel ratio is set to the stoichiometric air-fuel ratio performed prior to the rich spike process can be appropriately determined according to the amount of exhaust gas. As a result, hydrocarbons covering the surface of the NOx occlusion reduction catalyst can be surely lost, so that deterioration of exhaust emission during the rich spike process is effectively suppressed, and fuel waste is effectively prevented. be able to.
[0023]
The exhaust gas purification apparatus for an internal combustion engine according to claim 6, wherein the air-fuel ratio control means is configured to operate the internal combustion engine during a period in which the exhaust air-fuel ratio is set to the stoichiometric air-fuel ratio in advance. It is set according to the above.
[0024]
The period during which the hydrocarbon covering the surface of the NOx storage reduction catalyst is oxidized and disappeared by setting the exhaust gas to the stoichiometric air-fuel ratio depends on the amount of unreacted oxygen supplied by the exhaust gas. Since the amount of unreacted oxygen varies depending on the operating state of the internal combustion engine, the period during which the exhaust air-fuel ratio is the stoichiometric air-fuel ratio can be appropriately set according to the operating state of the internal combustion engine. As a result, hydrocarbons covering the surface of the NOx occlusion reduction catalyst can be surely lost, so that deterioration of exhaust emission during the rich spike process is effectively suppressed, and fuel waste is effectively prevented. be able to.
[0025]
The exhaust gas purification apparatus for an internal combustion engine according to claim 7, wherein a three-way catalyst is provided upstream of the NOx storage reduction catalyst in the exhaust passage in any one of claims 1 to 6, and the poisoning determination is performed. The means determines whether the NOx occlusion reduction type catalyst is likely to be poisoned by hydrocarbons when the exhaust air-fuel ratio is lean, based on information on the hydrocarbon purification rate of the three-way catalyst. And
[0026]
When a three-way catalyst is present upstream of the NOx storage reduction catalyst, the amount of hydrocarbons flowing into the NOx storage reduction catalyst is affected by the degree of hydrocarbon purification by the three-way catalyst.
[0027]
Therefore, the poisoning determination means can accurately determine whether or not the NOx occlusion reduction type catalyst is likely to be poisoned by hydrocarbons based on the information on the hydrocarbon purification rate of the three-way catalyst. For this reason, it is possible to appropriately determine whether or not the exhaust air-fuel ratio is the stoichiometric air-fuel ratio. Therefore, it is possible to reliably suppress the deterioration of exhaust emissions caused by the inhibition of the catalytic reaction of the NOx occlusion reduction catalyst by hydrocarbons and to ensure fuel waste. Can be prevented.
[0028]
The exhaust gas purification apparatus for an internal combustion engine according to claim 8, wherein the information on the hydrocarbon purification rate of the three-way catalyst is the air-fuel ratio of the exhaust gas flowing into the three-way catalyst. To do.
[0029]
The hydrocarbon purification state of the three-way catalyst changes depending on the air-fuel ratio of the exhaust gas flowing into the three-way catalyst, and the amount of hydrocarbons in the exhaust gas supplied from the three-way catalyst to the NOx storage reduction type catalyst also changes. For this reason, the risk of hydrocarbon poisoning of the NOx storage reduction catalyst can be determined based on the air-fuel ratio of the exhaust gas flowing into the three-way catalyst. Therefore, it is possible to appropriately determine whether or not the exhaust air-fuel ratio is the stoichiometric air-fuel ratio. Therefore, it is possible to reliably suppress the deterioration of exhaust emission caused by the inhibition of the catalytic reaction of the NOx occlusion reduction catalyst by hydrocarbons and to ensure fuel waste. Can be prevented.
[0030]
The exhaust gas purification apparatus for an internal combustion engine according to claim 9 is characterized in that, in claim 7, the information relating to the hydrocarbon purification rate of the three-way catalyst is a bed temperature of the three-way catalyst.
[0031]
Since the hydrocarbon purification capacity of the three-way catalyst changes depending on the bed temperature, the amount of hydrocarbons in the exhaust gas supplied from the three-way catalyst to the NOx occlusion reduction type catalyst also changes depending on the bed temperature of the three-way catalyst. For this reason, the risk of hydrocarbon poisoning of the NOx storage reduction catalyst can be determined based on the bed temperature of the three-way catalyst. Therefore, it is possible to appropriately determine whether or not the exhaust air-fuel ratio is the stoichiometric air-fuel ratio. Therefore, it is possible to reliably suppress the deterioration of exhaust emission caused by the inhibition of the catalytic reaction of the NOx occlusion reduction catalyst by hydrocarbons and to ensure fuel waste. Can be prevented.
[0032]
DETAILED DESCRIPTION OF THE INVENTION
[Embodiment 1]
FIG. 1 shows a schematic configuration of a direct injection gasoline engine (hereinafter abbreviated as “engine”) 2 and an electronic control unit (hereinafter referred to as “ECU”) 4 mounted on a vehicle. Each cylinder of the engine 2 is provided with a fuel injection valve 6 for directly injecting fuel into the combustion chamber, and an ignition plug 8 for igniting the injected fuel. A throttle valve 12 whose opening degree is adjusted by a motor is provided in the middle of an intake passage 10 connected to the combustion chamber via an intake valve (not shown). The intake air amount GA (mg / sec) supplied to each cylinder is adjusted by the opening of the throttle valve 12 (throttle opening TA). The throttle opening degree TA is detected by the throttle opening degree sensor 14, and the intake air amount GA is detected by the intake air amount sensor 16 and read into the ECU 4.
[0033]
In the middle of an exhaust path 18 connected to the combustion chamber via an exhaust valve (not shown), a start catalyst 20 as a three-way catalyst is provided on the upstream side, and a NOx occlusion reduction type catalyst 22 is provided on the downstream side. .
[0034]
An air-fuel ratio sensor 24 that detects the air-fuel ratio from the exhaust component is provided on the upstream side of the start catalyst 20. A first oxygen sensor 26 that detects oxygen in the exhaust component is detected between the start catalyst 20 and the NOx storage reduction catalyst 22, and oxygen in the exhaust component is detected downstream of the NOx storage reduction catalyst 22. A second oxygen sensor 28 is provided.
[0035]
The ECU 4 is an engine control circuit configured mainly with a digital computer. The ECU 4 detects an amount of depression of the accelerator pedal 30 (accelerator opening ACCP) in addition to the throttle opening sensor 14, the intake air amount sensor 16, the air-fuel ratio sensor 24, and the two oxygen sensors 26 and 28. The signal is input from 32. Signals are also input from an engine speed sensor 34 that detects the engine speed NE from the rotation of the crankshaft (not shown), a coolant temperature sensor 36 that detects the coolant temperature THW of the engine 2, and the like. In addition to such sensors, sensors necessary for engine control such as a vehicle speed sensor are provided, although not shown.
[0036]
The ECU 4 appropriately controls the fuel injection timing, the fuel injection amount, the ignition timing, the throttle opening degree TA, and the like of the engine 2 based on the detection contents from the various sensors described above. Thus, for example, the combustion mode is switched between stratified combustion and homogeneous combustion in accordance with the operating state. In the first embodiment, during the normal operation excluding the cold state, the combustion mode is determined based on the map of the engine speed NE and the load factor eklq. Specifically, stratified combustion is selected in the region of the low engine speed NE including the idle operation region and the low load factor eklq, and in other cases, homogeneous combustion is selected. Here, the load factor eklq is a value obtained from a map using, for example, the accelerator opening ACCP and the engine speed NE as parameters, indicating the ratio of the current load to the maximum engine load.
[0037]
When homogeneous combustion is selected, the amount of fuel that becomes the stoichiometric air-fuel ratio corresponding to the intake air amount GA (temporarily richer than the stoichiometric air-fuel ratio may be rich) is taken in the intake stroke. The fuel is injected and homogeneous combustion is performed in the combustion chamber at the stoichiometric air-fuel ratio (rich in some cases). In this case, the exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio (rich in some cases).
[0038]
When stratified combustion is selected, the throttle valve 12 opens and the amount of fuel smaller than the stoichiometric air-fuel ratio is injected in the compression stroke according to the load factor eklq, and lean stratified combustion is performed in the combustion chamber. . In this case, the exhaust air-fuel ratio becomes lean.
[0039]
Next, in the present embodiment, a rich spike setting process among the processes for the NOx storage reduction catalyst 22 executed by the ECU 4 will be described. FIG. 2 shows the rich spike setting process. This process is a process that is repeatedly executed at regular time intervals.
[0040]
When this process is started, it is first determined whether or not a stratified combustion request is made because the engine operating state corresponds to the stratified combustion region described above (S100). If the stratified combustion request is not requested (“NO” in S100), that is, if the homogeneous combustion request is requested, the present process is temporarily terminated as it is.
[0041]
On the other hand, if it is at the time of the stratified combustion request (“YES” in S100), it is next determined whether or not the rich spike execution flag Frs is “OFF” (S102). The rich spike execution flag Frs is a flag for requesting the start of the rich spike, and Frs = “OFF” is set in the initial setting when the ECU 4 is turned on. If Frs = “OFF” (“YES” in S102), it is next determined whether or not the NOx occlusion amount eqnoxcnt of the NOx occlusion reduction type catalyst 22 is equal to or greater than a predetermined value N (reference amount for executing rich spike). (S104). This NOx occlusion amount eqnoxcnt is obtained by repeatedly obtaining the NOx amount and the reducing agent amount contained in the exhaust gas flowing into the NOx occlusion reduction type catalyst 22 according to the operating state of the engine 2 in a separate process (not shown). , The NOx amount is integrated and continuously calculated by subtracting the NOx amount reduced by the reducing agent.
[0042]
If eqnoxcnt <N (“NO” in S104), it is not the timing to reduce with a rich spike, so this process is temporarily terminated as it is.
In particular, when eqnoxcnt ≧ N is satisfied when NOx is continuously supplied to the NOx occlusion reduction type catalyst 22 by continuing the stratified combustion (“YES” in S104), the rich spike execution flag Frs is set to “ON”. (S106). Next, it is determined whether or not the inflow HC concentration thccl to the NOx storage reduction type catalyst 22 is equal to or higher than the high concentration determination value Dhc (S108).
[0043]
The inflow HC concentration thccl represents the concentration of HC flowing into the NOx storage reduction catalyst 22, and is a value estimated as described later. The high density determination value Dhc is set as follows. That is, as shown in the graph of FIG. 3, when the inflow HC concentration thccl to the NOx occlusion reduction type catalyst 22 is 5000 ppm or less, the NOx reduction rate at the time of rich is 100% and NOx is completely purified. However, when the inflow HC concentration thccl exceeds 5000 ppm, HC begins to cover the NOx occlusion reduction type catalyst 22, so that the NOx reduction rate decreases, and at about 6000 ppm, the NOx reduction rate becomes 0%. Further, when the inflow HC concentration thccl further increases, the stored NOx release amount is added, and the NOx amount discharged downstream from the NOx storage reduction catalyst 22 increases. Therefore, in the present embodiment, the high concentration determination value Dhc is set to 5000 ppm in order to determine the timing at which HC begins to cover the NOx storage reduction catalyst 22.
[0044]
As described above, when thcc1 ≧ Dhc (“YES” in S108), it is estimated that the NOx storage reduction catalyst 22 is covered or started to be covered with HC. “ON” is set in Fst (S110). Next, an initial value is set to the HC consumption total gas amount Gst (S112). This initial value is the amount of exhaust gas at the stoichiometric air-fuel ratio necessary for consuming the HC covering the NOx storage reduction catalyst 22, and is a value obtained in advance by experiments and stored in the ROM of the ECU 4. It is. The initial value may be a constant value, but based on the value of the inflow HC concentration thccl determined in the immediately preceding step S108, the larger the inflow HC concentration thccl, the larger the initial value may be set. good.
[0045]
Next, the rich spike correction flag Fkrs is set to “ON” (S114). Thus, the rich spike setting process is temporarily terminated.
In the next control cycle, since the rich spike execution flag Frs is set to “ON” (“NO” in S102), no substantial processing is performed in the rich spike setting processing.
[0046]
On the other hand, when thcc1 <Dhc (“NO” in S108), it is presumed that the NOx occlusion reduction type catalyst 22 is not covered with HC, and therefore, the theoretical air-fuel ratio combustion execution flag Fst is set to “OFF” ( S116). Next, the rich spike correction flag Fkrs is set to “OFF” (S118). Thus, the rich spike setting process is temporarily terminated.
[0047]
Here, the calculation process of the inflow HC concentration thccl will be described with reference to FIG. This process is repeatedly executed at regular time intervals. When this processing is started, first, the inflow-side HC concentration thcin of the start catalyst 20 is obtained from the map Mthcin based on the load factor eklq and the engine speed NE (S210). This map Mthcin is set in advance by experimentally obtaining the HC concentration in exhaust discharged from the combustion chamber of the engine 2 using the load factor eklq and the engine speed NE as parameters. Next, the air-fuel ratio AFsc on the inflow side of the start catalyst 20 is detected from the output of the air-fuel ratio sensor 24 (S212).
[0048]
Then, the bed temperature esctempave of the start catalyst 20 is detected (S214). The bed temperature esctempave of the start catalyst 20 is estimated from the engine speed NE and the intake air amount GA by a bed temperature estimation process separately performed by the ECU 4. As the bed temperature estimation process, for example, the catalyst bed temperature can be estimated as the exhaust temperature obtained from the engine speed NE and the intake air amount GA during stable operation of the engine 2. When the engine 2 is in transition, the catalyst bed temperature is repeated and calculated so as to follow the exhaust temperature based on the time constant based on the intake air amount GA, thereby obtaining the bed temperature esctempave. Instead of estimating in this way, a temperature sensor may be provided in the start catalyst 20 to directly measure the bed temperature.
[0049]
Next, the HC purification rate redhc (%) of the start catalyst 20 is obtained from the map Mredhc based on the air-fuel ratio AFsc on the inflow side of the start catalyst 20 and the bed temperature esctempave of the start catalyst 20 (S216). This map Mredhc is previously set by experimentally measuring the purification state by oxidation of HC in the start catalyst 20 using the air-fuel ratio AFsc and the bed temperature esctempave as parameters.
[0050]
Next, as shown in the following equation 1, the HC concentration thccl on the inflow side of the NOx storage reduction type catalyst 22 is calculated (S218).
[0051]
[Expression 1]
thccl ← thcin × redhc / 100 [Equation 1]
In this way, the inflow HC concentration thccl with respect to the NOx occlusion reduction type catalyst 22 is estimated, and this process is temporarily terminated.
[0052]
Next, a fuel injection control process performed using the rich spike execution flag Frs, the stoichiometric air-fuel ratio combustion execution flag Fst, the HC consumption total gas amount Gst, and the rich spike correction flag Fkrs set in the rich spike setting process (FIG. 2). (FIG. 5) and the HC removal completion determination process (FIG. 6) will be described.
[0053]
The fuel injection control process (FIG. 5) is a process that is executed at every fixed crank angle (180 ° for a 4-cylinder engine and 120 ° for a 6-cylinder engine). When this process is started, it is first determined whether or not stratified combustion is requested (S300). This determination is as described in step S100 of the rich spike setting process (FIG. 2).
[0054]
If not at the time of stratified combustion request (“NO” in S300), homogeneous combustion is set (S302). With this setting, in the intake stroke, a fuel amount that becomes the stoichiometric air-fuel ratio (sometimes rich) corresponding to the intake air amount GA is injected into the combustion chamber. As a result, homogeneous combustion at the stoichiometric air fuel ratio (rich in some cases) is performed.
[0055]
If the stratified charge combustion is requested (“YES” in S300), it is determined whether the rich spike execution flag Frs is “ON” (S304). If Frs = “OFF” (“NO” in S304), the rich spike is not executed, so stratified combustion is set (S306). With this setting, in the compression stroke, a fuel amount smaller than the stoichiometric air-fuel ratio corresponding to the load factor eklq is injected into the combustion chamber. This makes stratified combustion in lean.
[0056]
On the other hand, if Frs = “ON” (“YES” in S304), it is next determined whether or not the theoretical air-fuel ratio combustion execution flag Fst is “ON” (S308). If the inflow HC concentration thcc1 ≧ Dhc is determined in step S108 of the rich spike setting process (FIG. 2) and Fst = “ON” is set (“YES” in S308), the target air-fuel ratio is set. The theoretical air-fuel ratio is set to AFt (S310). Then, the fuel amount Q is calculated based on the intake air amount GA detected by the intake air amount sensor 16 and the correction value by the air-fuel ratio feedback control by the air-fuel ratio sensor 24 so that the target air-fuel ratio AFt is achieved. The fuel amount Q is set so as to be injected into the combustion chamber in the intake stroke (S312). As a result, homogeneous combustion at the stoichiometric air-fuel ratio is performed.
[0057]
This homogeneous combustion at the stoichiometric air-fuel ratio continues until the stoichiometric air-fuel ratio combustion execution flag Fst is set to “OFF” by the HC removal completion determination process (FIG. 6).
The HC removal completion determination process (FIG. 6) will be described. This process is repeatedly executed in the same cycle as the fuel injection control process (FIG. 5). When this process is started, it is first determined whether or not Fst = "ON" (S400). If Fst = “ON” by executing step S110 of the rich spike setting process (FIG. 2) (“YES” in S400), then NOx for each control cycle based on the operating state of the engine 2 A gas amount dgas flowing into the storage reduction catalyst 22 is calculated (S402). That is, the gas amount dgas of the exhaust gas homogeneously burned at the stoichiometric air-fuel ratio is set based on the engine speed NE and the load factor eklq.
[0058]
Next, this gas amount dgas is added to the total gas amount Tgas (S404). As a result, the total gas amount Tgas represents the amount of exhaust gas that has passed through the NOx storage reduction catalyst 22 when Fst = “ON” continues.
[0059]
Then, it is determined whether or not the total gas amount Tgas is greater than or equal to the HC consumption total gas amount Gst set in step S112 of the rich spike setting process (FIG. 2) (S406). If Tgas <Gst (“NO” in S406), it is presumed that the HC covering the NOx storage reduction catalyst 22 has not been completely removed, so the HC removal completion determination process is temporarily performed as it is (FIG. 6). Exit.
[0060]
If the total gas amount Tgas gradually increases and Tgas ≧ Gst (“YES” in S406), the HC covering the NOx storage reduction catalyst 22 is oxidized and completely removed under the atmosphere of the theoretical air-fuel ratio. Can be estimated. For this reason, the theoretical air-fuel ratio combustion execution flag Fst is set to “OFF” (S408). Thus, the HC removal completion determination process (FIG. 6) is temporarily terminated.
[0061]
In the next control cycle, since Fst = “OFF” (“NO” in S400), “0” is set to the total gas amount Tgas (S410), and the process is temporarily ended. Thereafter, as long as Fst = “OFF” (“NO” in S400), this state continues.
[0062]
Returning to the description of the fuel injection control process (FIG. 5), when Fst = “OFF” is set in the HC removal completion determination process (FIG. 6) (“NO” in S308), the NOx occlusion reduction type catalyst is then displayed. It is determined whether or not the output of the second oxygen sensor 28 provided on the outlet side of 22 is in a lean state (S314). Since the rich spike process has not actually started yet, the output of the second oxygen sensor 28 indicates a lean state (“YES” in S314). Therefore, the rich spike air-fuel ratio AFrs is set according to the operating state of the engine 2 (S316).
[0063]
Next, it is determined whether or not the rich spike correction flag Fkrs is “ON” (S318). That is, when starting this rich spike process, it is determined whether or not thcc1 ≧ Dhc (FIG. 2: “YES” in S108). If Fkrs = “ON” (“YES” in S318), then the rich spike correction coefficient Krs (> 1.0) is set according to the operating state of the engine 2 (S320). Here, the rich spike correction coefficient Krs is increased as the engine speed NE is lower, and the rich spike correction coefficient Krs is increased as the load factor eklq is lower.
[0064]
Then, the target air-fuel ratio AFt is set as in the following expression 2 (S322).
[0065]
[Expression 2]
AFt ← AFrs x Krs ... [Formula 2]
That is, the lower the engine speed or the lower the load factor eklq, the more the target air-fuel ratio AFt is corrected to the lean side than the rich spike air-fuel ratio AFrs, thereby reducing the richness of the air-fuel ratio. This is because, in the operating state determined as thcc1 ≧ Dhc, the HC concentration in the exhaust gas is increased and the ability as a reducing agent is increased even during the rich spike, so that the fuel concentration during the rich spike is suppressed in consideration of this. It is.
[0066]
The fuel amount Q is calculated as described above so that the target air-fuel ratio AFt set in this way is achieved, and the fuel amount is set to be injected into the combustion chamber in the intake stroke (S312). As a result, rich spike processing is executed.
[0067]
When the actual NOx occlusion amount in the NOx occlusion reduction type catalyst 22 becomes “0” by the rich spike process, the output of the second oxygen sensor 28 starts to change from lean to rich (“NO” in S314). For this reason, the rich spike execution flag Frs is set to “OFF” (S324), the rich spike correction flag Fkrs is set to “OFF” (S326), stratified combustion is set (S306), and the process is temporarily terminated. . Note that eqnoxcnt = “0” is set in the above-described calculation process of the NOx occlusion amount eqnoxcnt at the timing when the output change of the second oxygen sensor 28 occurs on the rich side at this time.
[0068]
In the next control cycle, after “YES” is determined in step S300, since Frs = “OFF” (“NO” in S304), stratified combustion is set (S306).
[0069]
When thcc1 <Dhc (FIG. 2: “NO” in S108) when starting the rich spike processing this time, Fst = “OFF” (FIG. 2: S116), so from the beginning. Since “NO” is determined in the step S308, the homogeneous combustion (S310) at the stoichiometric air-fuel ratio immediately before the rich spike process is not executed. Further, since Fkrs = “OFF” (FIG. 2: S118), it is determined as “NO” in step S318, and rich spike processing is executed with the normal rich spike air-fuel ratio AFrs (S328). The fuel concentration at the time is not reduced.
[0070]
An example of processing according to the present embodiment is shown in FIG. FIG. 7A shows a case where thcc1 ≧ Dhc at the start of rich spike (t0). In this case, immediately before the rich spike processing (t1 to t2), homogeneous combustion at the stoichiometric air-fuel ratio is executed (t0 to t1). In the rich spike processing (t1 to t2), the air-fuel ratio is made larger than the normal rich spike air-fuel ratio AFrs.
[0071]
FIG. 7B shows a case where thcc1 <Dhc at the start of rich spike (t10). In this case, the homogeneous combustion at the stoichiometric air-fuel ratio is not performed immediately before the rich spike processing (t10 to t11), and the rich spike processing (t10 to t11) is immediately executed at the normal rich spike air-fuel ratio AFrs. .
[0072]
In the above-described configuration, step S108 of the rich spike setting process (FIG. 2) and the inflow HC concentration thcc1 calculation process (FIG. 4) of the NOx storage reduction catalyst correspond to the process as the poisoning determination means. Steps S110 to S114 of the rich spike setting process (FIG. 2), the HC removal completion determination process (FIG. 6), and the steps S308, S310, and S316 to S322 of the fuel injection control process (FIG. 5) serve as the air-fuel ratio control means. Equivalent to.
[0073]
According to the first embodiment described above, the following effects can be obtained.
(I). If it is determined that there is a possibility of HC poisoning by the inflow HC concentration thcc1 ≧ high concentration determination value Dhc of the NOx occlusion reduction type catalyst (FIG. 2: “YES” in S108), rich spike processing Prior to the execution of (FIG. 5: S316 to S322), the exhaust air-fuel ratio is made the stoichiometric air-fuel ratio (FIG. 5: S310). In this way, combustion at the stoichiometric air-fuel ratio is performed before the rich spike processing, so that the combustion of the air-fuel mixture is good and the amount of HC in the exhaust gas is reduced, and HC, CO, NOx and Unreacted oxygen is supplied in a well-balanced manner. Since the exhaust gas is in the stoichiometric air-fuel ratio state, the NOx occlusion reduction catalyst 22 sufficiently functions as a three-way catalyst, and the oxidation-reduction reaction is promoted.
[0074]
In such an exhaust state, the thick HC layer covering the surface of the NOx storage reduction type catalyst 22 gradually disappears as the oxidation proceeds. Therefore, since the catalytic activity for reducing NOx with the reducing agent can be recovered, when the rich spike process is performed thereafter, NOx released from the NOx storage reduction type catalyst 22 by the rich atmosphere is used as the reducing agent. Reduced and purified reliably by HC and CO. For this reason, HC and CO as the reducing agent and NOx released from the NOx storage reduction catalyst 22 by a rich atmosphere are not released in large quantities downstream of the NOx storage reduction catalyst.
[0075]
Note that the rich spike processing is delayed due to the existence of the period in which the exhaust air-fuel ratio is in the stoichiometric air-fuel ratio state, but during this period, the exhaust air-fuel ratio is in the stoichiometric air-fuel ratio state and the lean state is not continued. So NOx emissions will not deteriorate.
[0076]
In this way, it is possible to suppress the deterioration of exhaust emission due to the inhibition of the catalytic reaction of the NOx occlusion reduction type catalyst 22 by HC, and it is possible to prevent fuel waste.
(B). In a situation where it is determined that thcc1 ≧ Dhc, even if the fuel increase necessary for the rich spike process is executed, the HC concentration in the exhaust actually becomes higher than necessary. As a result, part of the hydrocarbons and CO as the reducing agent is not used for the reduction purification of NOx released from the NOx storage reduction catalyst 22, but is released downstream of the NOx storage reduction catalyst 22 without being purified. , Exhaust emissions may be deteriorated. Therefore, when it is determined that thcc1 ≧ Dhc (FIG. 5: “YES” in S318), the richness of the rich spike processing is lowered according to the engine operating state (FIG. 5: S320, S322). This makes it possible to clean the exhaust by reliably reacting HC or CO as the reducing agent with NOx oxygen, effectively suppressing the deterioration of exhaust emissions during the rich spike process, and effectively preventing fuel waste. can do.
[0077]
(C). The period during which the HC covering the surface of the NOx storage reduction catalyst 22 is oxidized and disappeared by setting the exhaust gas to the stoichiometric air-fuel ratio mainly depends on the amount of unreacted oxygen supplied by the exhaust gas. Therefore, in the HC removal completion determination process (FIG. 6), the end of the period in which the exhaust air-fuel ratio is the stoichiometric air-fuel ratio is determined according to the amount of exhaust gas. As a result, the HC covering the surface of the NOx occlusion reduction catalyst 22 can be surely eliminated, so that deterioration of exhaust emission during the rich spike process is effectively suppressed and fuel waste is effectively prevented. be able to.
[0078]
(D). In the present embodiment, the start catalyst 20 composed of a three-way catalyst exists on the upstream side of the NOx storage reduction catalyst 22. For this reason, the amount of HC flowing into the NOx storage reduction type catalyst 22 is influenced by the degree of purification of HC by the start catalyst 20. Accordingly, the inflow HC concentration thccl is calculated based on the HC purification rate redhc of the start catalyst 20 in the inflow HC concentration thccl calculation processing (FIG. 4) of the NOx occlusion reduction type catalyst 22, whereby the NOx storage reduction catalyst 22 It becomes possible to accurately determine whether there is a risk of poisoning by HC. The HC purification rate redhc reflects the air-fuel ratio AFsc of the exhaust gas flowing into the start catalyst and the bed temperature esctempave of the start catalyst 20.
[0079]
Therefore, it is possible to appropriately determine whether or not the exhaust air-fuel ratio is the stoichiometric air-fuel ratio. Therefore, it is possible to reliably suppress the deterioration of exhaust emission caused by the inhibition of the catalytic reaction of the NOx occlusion reduction type catalyst 22 by HC and to ensure fuel waste. Can be prevented.
[0080]
[Embodiment 2]
In the present embodiment, the process of step S113 as shown in FIG. 8 is performed instead of step S112 of FIG. 2, and the HC removal completion determination process shown in FIG. 9 is executed instead of FIG. Other configurations are the same as those of the first embodiment.
[0081]
In step S113, the homogeneous combustion time Tst at the stoichiometric air-fuel ratio is set based on the operating state of the engine 2. This homogeneous combustion time Tst represents the time until the HC covering the NOx storage reduction catalyst 22 is completely removed by the exhaust of the stoichiometric air-fuel ratio. This homogeneous combustion time Tst is set by a map formed so as to increase as the engine speed NE decreases and to increase as the load factor eklq decreases.
[0082]
The HC removal completion determination process (FIG. 9) will be described. This process is repeatedly executed at regular time intervals. When this process is started, it is first determined whether or not Fst = “ON” (S500). If it is in the state of Fst = “OFF” (“NO” in S500), “0” is set to the timer counter Tc (S502), and this process is temporarily terminated as it is.
[0083]
On the other hand, when Fst = “ON” is set by execution of step S110 of the rich spike setting process (FIG. 2) (“YES” in S500), the homogeneous combustion time Tst calculated by the timer counter Tc in step S113 next. It is determined whether or not the value is smaller (S504). Since Tc <Tst in the initial stage (“YES” in S504), the timer counter Tc is incremented (S506), and this process is temporarily terminated. Thereafter, as long as Tc <Tst (“YES” in S504), the increment of the timer counter Tc (S506) is only repeated, and Fst = “ON” remains. Therefore, in the fuel injection control process (FIG. 5), the state determined as “YES” in step S308 continues, and the homogeneous combustion state at the stoichiometric air-fuel ratio continues (S310, S312).
[0084]
When Tc ≧ Tst is satisfied by the increment of the timer counter Tc (S506) (“NO” in S504), Fst = “OFF” is set (S508). In this way, this process is once completed. In the next control cycle, “NO” is determined in step S500, and therefore “0” is set to the timer counter Tc (S502).
[0085]
Since the stoichiometric air-fuel ratio combustion execution flag Fst is switched from “ON” to “OFF” after the homogeneous combustion time Tst set in step S113 has elapsed in this way, in the fuel injection control process (FIG. 5), step S308 is performed. Is judged as “NO”, and the homogeneous combustion at the stoichiometric air-fuel ratio is completed. Then, the rich spike processing (S316 to 322) as described above is executed.
[0086]
In the above-described configuration, steps S110, S113, and S114 of the rich spike setting process (FIG. 2), the HC removal completion determination process (FIG. 9), and the steps S308, S310, and S316 to S322 of the fuel injection control process (FIG. 5) are empty. This corresponds to processing as a fuel ratio control means.
[0087]
According to the second embodiment described above, the following effects can be obtained.
(I). The effects (a), (b) and (d) of the first embodiment are produced.
(B). The period during which the HC covering the surface of the NOx storage reduction catalyst 22 is oxidized and disappeared by setting the exhaust gas to the stoichiometric air-fuel ratio depends on the amount of unreacted oxygen supplied by the exhaust gas. Depends on the operating state of the engine 2. For this reason, by setting the homogeneous combustion time Tst at the stoichiometric air-fuel ratio according to the operating state of the engine 2, the HC covering the surface of the NOx storage reduction catalyst 22 can be surely lost. For this reason, it is possible to effectively suppress the deterioration of exhaust emission during the rich spike processing and to effectively prevent fuel waste.
[0088]
[Other embodiments]
(A). In FIG. 1, a configuration in which the start catalyst 20 and the first oxygen sensor 26 are not provided in the exhaust path 18 may be employed. In this case, in the inflow HC concentration thcc1 calculation process (FIG. 4) of the NOx occlusion reduction type catalyst (FIG. 4), the load ratio eklq and the engine speed are calculated using the same map Mthcin as the start catalyst inflow HC concentration thcin is calculated in step S210. The inflow HC concentration thccl is calculated based on the number NE. Steps S212 to S218 are not executed.
[0089]
By this, the same effect can effectively suppress the deterioration of exhaust emission during the rich spike process, and can effectively prevent fuel waste.
(B). In step S314 of the fuel injection control process (FIG. 5), the end of the rich spike process is determined based on the output change of the second oxygen sensor. In addition to this, the NOx occlusion amount eqnoxcnt is always calculated from the component state of the exhaust, and at the timing when eqnoxcnt = 0 is returned by the rich spike process, it is determined as “NO” in step S314, and the rich spike process is terminated. Also good.
[0090]
At the start of the rich spike process, the time required to reduce the NOx occlusion amount eqnoxcnt is calculated based on the operating state of the engine 2 and the air-fuel ratio at the time of the rich spike. The rich spike process may be terminated by determining “NO” in S314.
[0091]
(C). In the first and second embodiments, the concentration thcc of the HC flowing into the NOx storage reduction catalyst 22 is obtained and compared with the high concentration determination value Dhc, and whether or not combustion at the stoichiometric air-fuel ratio is executed before the rich spike processing. Judged. Instead of this, the flow rate of HC flowing into the NOx storage reduction catalyst 22 may be obtained and compared with a determination value to determine whether or not to perform combustion at the stoichiometric air-fuel ratio before the rich spike process.
[0092]
(D). In each of the above-described embodiments, an example of stratified combustion of an in-cylinder injection engine has been described. However, lean homogeneous combustion is executed instead of stratified combustion in a port injection engine that injects fuel into an intake port. It can also be applied to the system. Further, the present invention can also be applied to a case where a lean homogeneous combustion is performed in a cylinder injection type engine.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an engine and an ECU as a first embodiment.
FIG. 2 is a flowchart of rich spike setting processing executed by the ECU according to the first embodiment;
FIG. 3 is a graph showing a relationship between a rich NOx reduction rate and an inflow HC concentration in a NOx occlusion reduction type catalyst.
FIG. 4 is a flowchart of a process for calculating an inflow HC concentration thccl of the NOx storage reduction catalyst executed by the ECU according to the first embodiment.
FIG. 5 is a flowchart of the same fuel injection control process.
FIG. 6 is a flowchart of HC removal completion determination processing.
7 is a timing chart illustrating an example of processing according to Embodiment 1. FIG.
FIG. 8 is a partial flowchart of rich spike setting processing according to the second embodiment;
FIG. 9 is a flowchart of HC removal completion determination processing.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 2 ... Engine, 4 ... ECU, 6 ... Fuel injection valve, 8 ... Spark plug, 10 ... Intake path, 12 ... Throttle valve, 14 ... Throttle opening sensor, 16 ... Intake air amount sensor, 18 ... Exhaust path, 20 ... Start catalyst, 22 ... NOx occlusion reduction type catalyst, 24 ... Air-fuel ratio sensor, 26 ... First oxygen sensor, 28 ... Second oxygen sensor, 30 ... Accelerator pedal, 32 ... Accelerator opening sensor, 34 ... Engine speed sensor 36: Cooling water temperature sensor.

Claims (9)

The exhaust passage of the internal combustion engine is provided with a NOx occlusion reduction type catalyst that stores NOx in the exhaust when the exhaust air-fuel ratio is lean and reduces NOx that is stored when the exhaust air-fuel ratio is rich. An exhaust purification device for an internal combustion engine that performs NOx reduction by rich spike processing that makes the exhaust gas rich, and whether or not the NOx storage reduction catalyst may be poisoned by hydrocarbons when the exhaust air-fuel ratio is lean Poison determination means for determining whether or not
If it is determined at the start of execution of the rich spike process that there is a possibility of poisoning, the rich spike process is executed after temporarily setting the exhaust air / fuel ratio to the theoretical air / fuel ratio temporarily. Air-fuel ratio control means for
An exhaust emission control device for an internal combustion engine, comprising: 2. The poison determination unit according to claim 1, wherein the poisoning determination means obtains the hydrocarbon concentration of the exhaust gas flowing into the NOx occlusion reduction type catalyst, and the hydrocarbon concentration is equal to or higher than a high concentration determination value when the exhaust air-fuel ratio is lean. An exhaust gas purification apparatus for an internal combustion engine, wherein the NOx occlusion reduction type catalyst is determined to be poisoned by hydrocarbons. 3. The air-fuel ratio control unit according to claim 2, wherein when the poisoning determination unit determines that the hydrocarbon concentration is equal to or higher than the high concentration determination value, the air-fuel ratio control unit reduces the rich degree in the rich spike processing. An exhaust gas purification apparatus for an internal combustion engine characterized by the above. 4. The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the air-fuel ratio control means sets the decrease in the rich degree in the rich spike processing according to the operating state of the internal combustion engine. 5. The internal combustion engine according to claim 1, wherein the air-fuel ratio control unit determines the end of a period in which the exhaust air-fuel ratio to be performed in advance is the stoichiometric air-fuel ratio according to the amount of exhaust gas. Exhaust purification device. 5. The exhaust gas of an internal combustion engine according to claim 1, wherein the air-fuel ratio control means sets a period in which the exhaust air-fuel ratio to be performed in advance is a stoichiometric air-fuel ratio in accordance with an operating state of the internal combustion engine. Purification equipment. 7. The information on the hydrocarbon purification rate of the three-way catalyst according to claim 1, wherein a three-way catalyst is provided on the upstream side of the NOx storage reduction catalyst in the exhaust passage, and the poisoning determination means is the hydrocarbon purification rate of the three-way catalyst. And determining whether the NOx occlusion reduction catalyst is likely to be poisoned by hydrocarbons when the exhaust air-fuel ratio is lean. 8. The exhaust gas purification apparatus for an internal combustion engine according to claim 7, wherein the information regarding the hydrocarbon purification rate of the three-way catalyst is an air-fuel ratio of exhaust gas flowing into the three-way catalyst. 8. The exhaust gas purification apparatus for an internal combustion engine according to claim 7, wherein the information relating to the hydrocarbon purification rate of the three-way catalyst is a bed temperature of the three-way catalyst.

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