JP2005163590A - Engine exhaust purification system - Google Patents

Abstract

An object of the present invention is to optimize the timing at which a rich spike for desorbing and reducing NOx trapped in a NOx trap catalyst is executed, and further reduce NOx emission.
The amount of NOx trapped in the NOx trap catalyst is determined based on exhaust characteristics (S1), and it is determined whether the amount of NOx trapped in the NOx trap catalyst has reached a predetermined amount (S2). When it is determined that the trap amount has reached a predetermined amount and the catalyst temperature of the NOx trap catalyst is within a predetermined temperature range, a rich spike that makes the air-fuel ratio of the engine rich is executed (S8). The predetermined temperature range allows NOx trapped in the NOx trap catalyst to be desorbed and reduced when the rich spike is executed, and the NOx trap from the engine even if the air-fuel ratio of the engine is lean after the rich spike is executed. The temperature range is set so that NOx discharged to the catalyst can be trapped by the NOx trap catalyst.
[Selection] Figure 2

Description

  The present invention relates to an exhaust emission control device for an engine, and more particularly to an exhaust gas purification apparatus that includes a NOx trap catalyst in an exhaust passage for trapping, desorbing, or reducing NOx in exhaust gas in accordance with an exhaust air / fuel ratio.

  Patent Document 1 discloses an exhaust gas in which an NOx trap catalyst that traps, desorbs, or reduces NOx in exhaust gas is provided in an exhaust passage of an engine in accordance with an exhaust air-fuel ratio in order to reduce the amount of NOx exhausted into the atmosphere. A purification device is disclosed.

There is an upper limit to the amount of NOx that can be trapped by the NOx trap catalyst, and in the exhaust purification device according to Patent Document 1, the air-fuel ratio of the engine 1 is temporarily made rich when the amount of trapped NOx reaches a certain level. A rich spike is executed to desorb and reduce NOx trapped in the NOx trap catalyst.
JP 2003-56379 A

  When the rich spike is executed, in order to efficiently reduce and purify NOx desorbed from the NOx trap catalyst, the temperature of the NOx trap catalyst needs to be within a predetermined temperature range. In other words, if the temperature of the NOx trap catalyst is too low, the catalyst is not activated, so even if rich spike is performed, NOx desorbed from the NOx trap catalyst is released into the atmosphere without being reduced. Conversely, if the temperature of the NOx trap catalyst is too high, NOx can be desorbed and reduced by the rich spike, but the NOx trap catalyst is difficult to trap NOx due to the high temperature. The NOx flowing into the NOx trap catalyst during operation cannot be sufficiently trapped.

  The exhaust purification device according to Patent Document 1 is configured to execute a rich spike when the amount of trapped NOx reaches a predetermined amount, so that the catalyst temperature of the NOx trap catalyst is outside the predetermined temperature range. In other words, the rich spike is executed, which may increase the amount of NOx emission. Further, even if the trapped amount of NOx reaches a predetermined amount, when the exhaust flow rate from the engine is small and the amount of NOx exhausted from the engine is small, the NOx is immediately executed rather than immediately executing the rich spike. It can be said that it is more advantageous in terms of fuel consumption and exhaust performance to continue the purification of NOx with the purification capacity remaining in the trap catalyst.

  The present invention has been made in view of the technical problems of the prior art, and in an exhaust purification device for an engine provided with a NOx trap catalyst in an exhaust passage, NOx trapped in the NOx trap catalyst is desorbed and reduced. It is an object to optimize the timing at which a rich spike is performed to reduce NOx emissions into the atmosphere.

  Based on the exhaust characteristics, the amount of NOx trapped in the NOx trap catalyst is obtained, and it is determined whether the amount of NOx trapped in the NOx trap catalyst has reached a predetermined amount. When the NOx trapped by the NOx trap catalyst has reached a predetermined amount and the catalyst temperature of the NOx trap catalyst is determined to be within the predetermined temperature range, a rich spike is executed to make the engine air-fuel ratio rich. To do. The predetermined temperature range allows the NOx trapped in the NOx trap catalyst to be desorbed and reduced when a rich spike that makes the engine air-fuel ratio rich is executed, and the engine air-fuel ratio is reduced after the rich spike is executed. The temperature range is set so that NOx discharged from the engine to the NOx trap catalyst when leaning can be trapped by the NOx trap catalyst.

  Even if the rich spike is executed when the catalyst temperature of the NOx trap catalyst is too low, NOx desorbed from the NOx trap catalyst cannot be reduced sufficiently, and the rich spike is executed when the catalyst temperature is too high. However, NOx cannot be sufficiently trapped in the subsequent lean operation, and in any case, the amount of NOx discharged into the atmosphere is increased. However, according to the present invention, since the rich spike is executed only when the catalyst temperature of the NOx trap catalyst is within the predetermined temperature range, the NOx emission amount can be suppressed.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a schematic configuration diagram of an exhaust emission control device for an engine according to the present invention. The engine 1 is a diesel engine, and fuel is supplied to the engine 1 from a fuel injection valve 6. Although the case where the engine 1 is a diesel engine will be described here, the engine 1 may of course be a gasoline engine.

  An intake passage 2 for introducing air into the engine 1 includes an air flow meter 3 for detecting the intake air amount of the engine 1, an electronically controlled throttle valve 4 for adjusting the intake air amount, and a throttle valve for detecting the opening degree of the throttle valve 4. An opening sensor 5 is provided.

  The exhaust passage 7 for exhausting exhaust from the engine 1 is provided with a NOx trap catalyst 8. The air-fuel ratio at the inlet of the NOx trap catalyst 8 and the air-fuel ratio at the outlet are the inlet and outlet of the NOx trap catalyst 8. Are detected by air-fuel ratio sensors 10 and 11, respectively. Further, the NOx trap catalyst 8 is provided with a catalyst temperature sensor 9 for detecting its internal temperature (catalyst temperature), and its active state can be determined based on the catalyst temperature. The NOx trap catalyst 8 traps NOx in the exhaust at the lean air-fuel ratio and traps the NOx at the rich air-fuel ratio, and a three-way catalyst function that performs oxidation and reduction of HC, CO, NOx in the exhaust at the stoichiometric air-fuel ratio (stoichiometric). The catalyst has a NOx trap function for desorption and reduction. For example, the catalyst support is configured by coating a NOx trap layer and a three-way catalyst layer on a catalyst carrier.

  In order for the NOx trap catalyst 8 to exhibit its catalytic function, the catalyst temperature must be in a predetermined temperature range and be in an active state. For this reason, the NOx trap catalyst 8 is disposed at a predetermined position in the exhaust passage 7 so as to be maintained in a predetermined temperature range by the exhaust gas flowing in. Here, only the NOx trap catalyst 8 is disposed in the exhaust passage 7, but other catalysts are provided upstream, downstream, or both of the NOx trap catalyst 8 depending on the exhaust performance required for the engine 1. It may be additionally provided. The NOx trap catalyst 8 is also a catalyst having a three-way catalyst function. However, the NOx trap catalyst 8 is provided with only the NOx trap function, and the three-way catalyst function is provided by separately providing a three-way catalyst. Also good.

  Further, the EGR passage 15 connects the middle of the exhaust passage 7 and the downstream side of the throttle valve 4 in the intake passage 2. An EGR valve 16 is provided in the middle of the EGR passage 15, and the amount of exhaust gas recirculated can be adjusted by adjusting the opening degree of the EGR valve 16. The amount of exhaust gas that is recirculated is adjusted according to the operating state (rotational speed, load, combustion state) of the engine 1.

  Various detection signals output from the air flow meter 3, throttle valve opening sensor 5, air-fuel ratio sensors 10 and 11, catalyst temperature sensor 9, engine rotation speed sensor 17, accelerator operation amount sensor (not shown), vehicle speed sensor, etc. are sent to the controller 20. Entered. The controller 20 includes one or more arithmetic units, a memory, an input / output interface, and the like. Based on the input signal and the internal parameters, the intake air amount and the fuel injection valve 6 are supplied via the throttle valve 4. The fuel injection amount, the fuel injection timing, and the exhaust gas recirculation amount are controlled via the EGR valve 16, respectively.

  By the way, the NOx trap catalyst 8 functions to trap NOx in the exhaust gas when the inflowing exhaust air-fuel ratio is lean and reduce the NOx concentration in the exhaust gas, but the total amount of NOx that can be trapped by the NOx trap catalyst 8. Has an upper limit, and when the amount of trapped NOx reaches the upper limit, NOx cannot be trapped.

  Therefore, the amount of NOx trapped (NOx trap amount) is monitored, and when the NOx trap amount reaches a predetermined amount close to the upper limit, the air-fuel ratio of the engine 1 is temporarily enriched (rich spike). Therefore, it is necessary to desorb and reduce the NOx trapped in the NOx trap catalyst 8 and restore the NOx trapping ability of the NOx trap catalyst 8. However, when the temperature of the NOx trap catalyst 8 is not within the predetermined temperature range, the desorbed NOx may be released into the atmosphere without being reduced even when the rich spike is executed (when the catalyst temperature is low), or Even if NOx can be desorbed and reduced, NOx discharged from the engine 1 during subsequent lean operation may not be trapped (when the catalyst temperature is high), leading to an increase in NOx emission. There is also.

  Even if the amount of NOx trapped in the NOx trap catalyst 8 reaches a predetermined amount, when the exhaust gas flow rate from the engine 1 is small and the amount of NOx discharged is small, it remains in the NOx trap catalyst 8. In some cases, the NOx trapping capacity that has been provided can be sufficiently processed. In such a case, if the NOx trap catalyst 8 continues to process NOx without performing the rich spike, the chance of the rich spike being performed can be reduced. This is advantageous in terms of fuel consumption and exhaust performance.

  Therefore, in the exhaust gas purification apparatus according to the present invention, even when the amount NOX of NOx trapped in the NOx trap catalyst 8 reaches the predetermined amount SNOX, the exhaust flow rate from the engine 1 is small and the exhausted NOx. When the amount of NO is also small, the NOx processing by the NOx trap catalyst 8 is continued without performing rich spike. Further, when the amount of NOx trapped in the NOx trap catalyst 8 reaches the predetermined amount SNOX, the exhaust flow rate from the engine 1 is large, and the amount of NOx discharged is also large, the catalyst temperature of the NOx trap catalyst 8 is increased. Accordingly, any one of rich spike, NOx reduction processing for reducing the amount of NOx discharged from the engine 1, and stoichiometric operation for operating the engine 1 at the stoichiometric air-fuel ratio is performed, and high rotation with a large exhaust flow rate from the engine 1 is performed. The amount of NOx discharged into the atmosphere is sufficiently suppressed even during operation and high load operation. Hereinafter, the content of the NOx process performed by the controller 20 will be described with reference to the flowchart.

  FIG. 2 is a flowchart showing the contents of the NOx process performed by the controller 20.

  According to this, first, in step S1, the amount of NOx trapped in the NOx trap catalyst 8 (hereinafter referred to as NOx trap amount) NOX is calculated based on the exhaust characteristics. As a method for calculating the NOx trap amount, for example, there is a method of obtaining by subtracting the integrated value of the NOx amount released from the NOx trap catalyst 8 from the integrated value of the NOx amount discharged from the engine 1. For the amount of NOx discharged from the engine 1, a map that defines the relationship among the rotational speed of the engine 1, the amount of intake air, and the amount of NOx discharged from the engine 1 is created in advance by experiment, and this map is referred to. Can be obtained. Further, the amount of NOx released from the NOx trap catalyst 8 includes the flow rate of the exhaust gas flowing into the NOx trap catalyst 8 (≈the intake air amount of the engine 1), the air-fuel ratio of the exhaust gas flowing into the NOx trap catalyst 8, and the NOx. A map defining the relationship of the amount of NOx discharged from the trap catalyst 8 can be created in advance by experiments and can be obtained with reference to this map. The method for calculating the NOx trap amount mentioned here is only an example, and various already proposed methods for calculating and estimating various NOx trap amounts, such as a method for estimating from the NOx concentration of the exhaust gas flowing out from the NOx trap catalyst 8. Can be adopted.

  When the NOx trap amount NOX is calculated, the process proceeds to step S2 to determine whether the NOx trap amount NOX exceeds a predetermined amount SNOX. The predetermined amount SNOX is set to a value smaller than the maximum amount of NOx that can be trapped by the NOx trap catalyst 8, and when the NOx trap amount NOX does not exceed the predetermined amount SNOX, the NOx trap catalyst 8 It is determined that sufficient NOx trap capability remains. When it is determined that the NOx trap amount NOX does not exceed the predetermined amount SNOX and sufficient NOx trap capability remains in the NOx trap catalyst 8, the process proceeds to step S3, where the rich spike is not performed, and the NOx trap catalyst. The NOx trap in the exhaust by 8 is continued. On the other hand, if the NOx trap amount NOX exceeds the predetermined amount SNOX, the process proceeds to step S4. The processing in steps S1 and S2 corresponds to NOx trap amount determination means.

  In step S4, it is determined whether the throttle valve opening TVO is larger than a predetermined small throttle opening STVO. When it is not larger than the predetermined small throttle opening STVO, the exhaust flow rate discharged from the engine 1 is small and the amount of NOx discharged is small, so even if the NOx trap amount NOX reaches the predetermined amount SNOX, The NOx trap ability remaining in the NOx trap catalyst 8 can sufficiently trap NOx in the exhaust gas. Therefore, in this case, the process proceeds to step S3, and the NOx trap by the NOx trap catalyst 8 is continued without executing the rich spike. The larger the predetermined amount SNOX is set, the lower the NOx trap ability of the NOx trap catalyst 8 when the routine proceeds to step S4. Therefore, the larger the predetermined amount SNOX is set, the smaller the STVO is set. Is done. The processing in step S4 corresponds to NOx trap capability determination means.

  In step S4, the exhaust flow rate from the engine 1 and the exhausted NOx amount are determined based on the throttle valve opening TVO. However, the accelerator pedal operation amount may be used instead of the throttle valve opening TVO. Absent. Further, since the exhaust flow rate of the engine 1 becomes substantially equal to the intake air amount detected by the air flow meter 3, the intake air amount detected by the air flow meter 3 may be used instead of the throttle valve opening TVO.

  If it is determined in step S4 that the throttle valve opening TVO is larger than the predetermined small throttle opening STVO, the catalyst temperature Tnox of the NOx trap catalyst 8 is set to a predetermined temperature range (Tmin to Tmax) in steps S5 and S6. It is judged whether there is. The catalyst temperature is detected by the catalyst temperature sensor 9, but can be estimated from the operating state of the engine 1. In the predetermined temperature range, when the catalyst of the NOx trap catalyst 8 becomes active and the rich spike is executed, the NOx trapped in the NOx trap catalyst 8 can be sufficiently desorbed and reduced, and the engine after the rich spike is executed. Even if 1 is operated at a lean air-fuel ratio, the temperature range is set so that the NOx trap catalyst 8 can sufficiently trap NOx in the exhaust gas. The processing in steps S5 and S6 corresponds to the catalyst temperature determination means.

  When the catalyst temperature Tnox is lower than the lower limit Tmin of the predetermined temperature range, the process proceeds to step S7, and NOx reduction processing for reducing the amount of NOx discharged from the engine 1 itself is executed. This is because if the rich spike is executed in a state where the catalyst temperature is low, NOx can be desorbed from the NOx trap catalyst 8, but the NOx reducing ability of the catalyst is not sufficient and the desorbed NOx cannot be sufficiently purified. This is because, in order to reduce the NOx emission amount into the atmosphere, it is necessary to cope with a reduction in the NOx amount itself discharged from the engine 1.

  FIG. 3 is a flowchart showing details of the NOx reduction process in step S7.

  According to this, the target excess air ratio tλ is corrected by multiplying the target excess air ratio tλ of the engine 1 set in accordance with the operating state of the engine 1 in step S10 by the coefficient A. The coefficient A is set to a value less than 1 so that the corrected target excess air ratio tλ is smaller than the value before correction, thereby correcting the throttle valve opening to the small side and the intake of the engine 1 Try to reduce the amount of air.

  In step S11, the target exhaust gas recirculation amount (or target exhaust gas recirculation rate) tEGR set according to the operating state of the engine 1 is divided by the coefficient A to correct the target exhaust gas recirculation amount. Since the coefficient is set to a value less than 1, the corrected target exhaust gas recirculation amount tEGR is larger than the value before correction, and the exhaust gas recirculation amount of the engine 1 is increased.

  Here, the target excess air ratio tλ and the target exhaust gas recirculation amount tEGR are corrected by the same coefficient A, but the correction coefficient may be different for the target excess air ratio tλ and the target exhaust gas recirculation amount tEGR. The coefficient A may be a fixed value or may be variable according to the operating state of the engine 1 and the state of the NOx trap catalyst 8 (catalyst temperature, NOx trap amount, etc.). Alternatively, the target excess air ratio map and the target exhaust gas recirculation amount map for NOx reduction processing are prepared separately from the map used during normal operation, and the target excess air ratio tλ is referred to by referring to this separately prepared map during NOx reduction processing. The target exhaust gas recirculation rate tEGR may be calculated. In this case, under the same operating conditions, the value obtained by referring to the target excess air ratio map for NOx reduction processing is smaller than the value obtained by referring to the normal time map, and conversely, for NOx reduction processing. The value obtained by referring to the target exhaust gas recirculation amount map is larger than the value obtained by referring to the normal time map.

  Further, here, both the target excess air ratio tλ and the target exhaust gas recirculation amount tEGR amount are corrected. However, if at least one of them is corrected, the NOx amount exhausted from the engine 1 can be reduced. Only one of tλ and target exhaust gas recirculation amount tEGR may be corrected. Note that if both the target excess air ratio tλ and the target exhaust gas recirculation amount tEGR are corrected as in this embodiment, a high NOx reduction effect can be obtained.

  Returning to FIG. 3, the description will be continued. When the catalyst temperature Tnox is within a predetermined temperature range (Tmin <Tnox <Tmax), the process proceeds to step S8 where a rich spike is executed and trapped on the NOx trap catalyst 8. NOx is removed and reduced. When the catalyst temperature Tnox is in the predetermined temperature range, the NOx purification capacity of the NOx trap catalyst 8 is high. Therefore, by executing rich spike, NOx is desorbed from the NOx trap catalyst 8 and the desorbed NOx is removed. It can be fully reduced. Moreover, the NOx trap ability after the rich spike is sufficiently secured.

  On the other hand, when the catalyst temperature Tnox exceeds the upper limit temperature Tmax of the predetermined temperature range, the process proceeds to step S9. In step S9, the rich spike is not executed, the target excess air ratio tλ is set to 1, and the engine 1 is operated at the stoichiometric air-fuel ratio (stoichiometric operation). In this state, if rich spike is executed, NOx can be efficiently desorbed and reduced from the NOx trap catalyst, but the ability of the NOx trap catalyst to trap NOx is reduced. This is because NOx discharged from the engine 1 cannot be sufficiently purified. Therefore, in step S9, the engine 1 is operated at the stoichiometric air-fuel ratio so that the amount of NOx released to the atmosphere can be reduced without using the NOx trapping capability of the NOx trap catalyst 8. Since the NOx trap catalyst 8 also has a three-way catalyst function, the NOx discharged from the engine 1 is purified together with HC and CO by operating the engine 1 at the stoichiometric air-fuel ratio, and the NOx discharged into the atmosphere is reduced. can do. Steps S3 and S7 to S9 correspond to NOx processing means.

  FIG. 4 shows a state in which the driver depresses the accelerator pedal and accelerates when the NOx trap amount NOX reaches the predetermined amount SNOX and the temperature of the NOx trap catalyst is low. In the exhaust purification apparatus according to the present invention, the rich spike is not performed in such a situation, and the NOx reduction process (step S7, FIG. 3) for reducing the amount of NOx discharged from the engine 1 is performed. By this NOx reduction processing, the excess air ratio of the engine 1 is corrected to the small side, the throttle valve opening is reduced, the intake air amount is reduced, and the NOx concentration in the exhaust is reduced. Therefore, although the acceleration is in a state where the NOx trapping capacity of the NOx trap catalyst 8 is lowered, the amount of NOx discharged into the atmosphere can be reduced by reducing the NOx concentration in the exhaust gas. .

  Next, the effect by this invention is demonstrated.

  According to the present invention, in an exhaust purification device for an engine 1 provided with an NOx trap catalyst 8 in the exhaust passage 2 for trapping or desorbing NOx according to the air-fuel ratio of the inflowing exhaust, the NOx trap is based on the exhaust characteristics of the engine 1. Determining the amount of NOx trapped in the catalyst 8, determining whether the NOx trap amount of the NOx trap catalyst 8 has reached a predetermined amount, determining whether the catalyst temperature of the NOx trap catalyst 8 is within a predetermined temperature range, A rich spike is executed when it is determined that the NOx trap amount has reached a predetermined amount and the catalyst temperature is within a predetermined temperature range. The predetermined temperature range can sufficiently desorb and reduce NOx trapped in the NOx trap catalyst 8 when a rich spike that makes the air-fuel ratio of the engine 1 rich is executed, and the engine after the rich spike is executed. When the air-fuel ratio of the engine is made lean, NOx discharged from the engine 1 to the NOx trap catalyst 8 is set to a temperature range in which the NOx trap catalyst 8 can be sufficiently trapped.

  When the NOx trap amount reaches a predetermined amount, it is necessary to desorb and reduce the trapped NOx by executing a rich spike, but the rich spike is performed when the catalyst temperature of the NOx trap catalyst 8 is too low. Even so, the NOx desorbed from the NOx trap catalyst 8 cannot be sufficiently reduced, and instead the amount of NOx discharged into the atmosphere is increased. On the other hand, if the catalyst temperature is too high, NOx trapped in the NOx trap catalyst 8 can be sufficiently desorbed and reduced by executing rich spike, but the NOx trap catalyst 8 has a high ability to desorb NOx. Since the NOx trapping capability is too low, NOx cannot be sufficiently trapped in the subsequent lean operation, and the amount of NOx discharged into the atmosphere is also increased. However, according to the present invention, since the rich spike is executed only when the catalyst temperature of the NOx trap catalyst 8 is within a predetermined temperature range, the amount of NOx discharged into the atmosphere can be suppressed.

  Further, when it is determined that the catalyst temperature is lower than the lower limit of the predetermined temperature range, a NOx reduction process for reducing the amount of NOx discharged from the engine 1 to the NOx trap catalyst 8 is executed instead of executing the rich spike. By doing so, even if the catalyst temperature is not in the predetermined temperature range and the rich spike is not executed, the amount of NOx discharged into the atmosphere can be suppressed. As the NOx reduction processing, processing for reducing the intake air amount of the engine 1 (correction for reducing the target excess air ratio in this embodiment), processing for increasing the exhaust gas recirculation amount of the engine 1 (target exhaust gas recirculation amount in this embodiment) Correction that increases the amount of NOx) can be considered, and by executing both processes together, a high NOx reduction effect can be obtained.

  When it is determined that the catalyst temperature is higher than the upper limit of the predetermined temperature range, the NOx discharged from the engine 1 is reduced to HC, by operating the engine 1 at the stoichiometric air-fuel ratio instead of executing the rich spike. NOx that is purified together with CO and discharged into the atmosphere can be reduced.

  Further, it is determined from the exhaust flow rate of the engine 1 whether NOx discharged from the engine 1 can be trapped by the NOx trap catalyst 8, and when NOx discharged from the engine 1 is determined to be trapped by the NOx trap catalyst 8, NOx Even if the amount of NOx trapped in the trap catalyst 8 reaches a predetermined amount, the NOx trap in the exhaust by the NOx trap catalyst 8 is continued. Thus, in a situation where the NOx trap ability remaining in the NOx trap catalyst 8 can sufficiently trap NOx in the exhaust gas, the rich spike is not performed, and the NOx trap by the NOx trap catalyst 8 is continued. Can improve the fuel efficiency and exhaust performance.

  INDUSTRIAL APPLICABILITY The present invention can be widely applied to exhaust purification devices for engines including NOx trap catalysts in the exhaust passage, including those for vehicles, and can suppress the amount of NOx discharged into the atmosphere and improve the performance of the exhaust purification device. Useful to enhance.

1 is a schematic configuration diagram of an exhaust emission control device for an engine according to the present invention. It is the flowchart which showed the content of the NOx process which a controller performs. It is the flowchart which showed the content of the NOx reduction process. It is a figure for demonstrating the effect | action of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine 2 Intake passage 3 Air flow meter 4 Throttle valve 5 Throttle valve opening sensor 6 Fuel injection valve 7 Exhaust passage 8 NOx trap catalyst 9 Catalyst temperature sensor 10, 11 Air-fuel ratio sensor 15 EGR passage 16 EGR valve 17 Rotational speed sensor 20 Controller

Claims (7)

In an engine exhaust purification device provided with an NOx trap catalyst in an exhaust passage for trapping or desorbing NOx in accordance with an air-fuel ratio of inflowing exhaust gas,
NOx trap amount determination means for determining the amount of NOx trapped in the NOx trap catalyst based on exhaust characteristics and determining whether the amount of NOx trapped in the NOx trap catalyst has reached a predetermined amount;
The NOx trap catalyst can desorb and reduce NOx when a rich spike is executed to enrich the air-fuel ratio of the engine, and after the rich spike is executed. Catalyst temperature determination means for determining whether the NOx exhausted from the engine to the NOx trap catalyst is trapped by the NOx trap catalyst even if the air-fuel ratio of the engine is lean;
NOx processing means for executing a rich spike when it is determined that the amount of NOx trapped in the NOx trap catalyst has reached a predetermined amount and the catalyst temperature is in the predetermined temperature range;
An exhaust emission control device for an engine characterized by comprising:   The NOx processing means performs a NOx reduction process for reducing the amount of NOx discharged from the engine to the NOx trap catalyst when it is determined that the catalyst temperature is lower than a lower limit of the predetermined temperature range. The exhaust emission control device for an engine according to claim 1.   The engine exhaust gas purification apparatus according to claim 2, wherein the NOx reduction process is a process of reducing an intake air amount of the engine.   The engine exhaust gas purification apparatus according to claim 2, wherein the NOx reduction process is a process of increasing an exhaust gas recirculation amount of the engine.   The engine exhaust gas purification apparatus according to claim 2, wherein the NOx reduction process is a process of decreasing an intake air amount of the engine and increasing an exhaust gas recirculation amount of the engine.   6. The NOx processing means, when determining that the catalyst temperature is higher than an upper limit of the predetermined temperature range, causes the engine to operate at a stoichiometric air-fuel ratio. The engine exhaust gas purification apparatus as described. NOx trap capability judging means for judging from the exhaust flow rate of the engine whether NOx discharged from the engine to the NOx trap catalyst can be trapped by the NOx trap catalyst;
When it is determined that the NOx exhausted from the engine can be trapped by the NOx trap catalyst, the NOx treatment means does not stop the NOx trap even if the amount of NOx trapped in the NOx trap catalyst reaches a predetermined amount. Continue NOx trap in exhaust by catalyst,
The exhaust emission control device for an engine according to any one of claims 1 to 6.

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