JP2006183525A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform NOx reduction processing in the proper timing, by accurately estimating the total quantity of NOx trapped in a NOx trap catalyst in response to a change in trap efficiency. <P>SOLUTION: While estimating an NOx quantity trapped in the NOx trap catalyst per unit time on the basis of the suction air volume, a correction factor kNOxbed for correcting a NOx trap quantity per unit time is set in response to the catalytic carrier temperature and a catalytic deterioration state. The correction factor kNOxbed is set in response to a state of reducing the trap efficiency in the NOx trap catalyst 11 in a low temperature area and a high temperature area and reducing when the catalyst deteriorates, to thereby avoid estimating the NOx trap quantity per unit time larger than an actual quantity. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine that includes a NOx trap catalyst in an exhaust passage to purify NOx in exhaust gas.

  Patent Document 1 includes a NOx trap catalyst in an exhaust passage that traps NOx in exhaust flowing when the exhaust air-fuel ratio is lean and desorbs and purifies NOx trapped when the exhaust air-fuel ratio is rich or stoichiometric. An exhaust purification device is disclosed that estimates the total amount of NOx trapped in the NOx trap catalyst and reduces the NOx trapped in the NOx trap catalyst by reducing the air-fuel ratio when the total amount increases. Yes.

The NOx trap total amount is estimated by an integration process in which a predetermined addition amount determined according to the engine operating state (engine rotation speed and fuel injection amount) is added at regular intervals.
JP 2000-130154 A

By the way, the NOx trap efficiency in the NOx trap catalyst changes depending on the carrier temperature and also changes due to the deterioration of the NOx trap catalyst. Even if the NOx emission amount from the internal combustion engine is the same, the carrier temperature and the catalyst deterioration are affected. The amount of NOx actually trapped by the NOx trap catalyst varies depending on the difference in the trapping efficiency that depends on it.
However, conventionally, the estimation of the total amount of NOx traps does not take into account the difference in trap efficiency depending on the carrier temperature and catalyst deterioration, and depending on the conditions of the carrier temperature and the deterioration state of the catalyst, the total amount of NOx traps is more than actual. However, there is a problem that the NOx reduction process for enriching the exhaust air-fuel ratio is performed at an excessively early timing to reduce the fuel efficiency of the engine.

  The present invention has been made in view of the above problems, and the total amount of NOx trapped in the NOx trap catalyst can be accurately estimated in accordance with changes in trap efficiency due to carrier temperature or catalyst deterioration, so that NOx reduction can be performed. The purpose is to enable processing to be performed at an appropriate timing.

  Therefore, in the present invention, the NOx trap amount per unit time in the NOx trap catalyst is calculated based on the engine operating state, and the exhaust air amount is calculated based on the total NOx trap amount obtained by integrating the NOx trap amount per unit time. In an exhaust gas purification apparatus for an internal combustion engine that controls the fuel ratio to the stoichiometric air-fuel ratio or richly, the carrier temperature of the NOx trap catalyst is detected, the deterioration of the NOx trap catalyst is diagnosed, and based on the carrier temperature and the result of the deterioration diagnosis Thus, the NOx trap amount per unit time is corrected.

  According to the above configuration, the total amount of traps in the NOx trap catalyst can be obtained in correspondence with the characteristics in which the trap efficiency of NOx in the NOx trap catalyst changes according to the carrier temperature of the NOx trap catalyst and the catalyst deterioration state. The determination of the air-fuel ratio control timing based on the total amount can be performed more appropriately.

Embodiments of the present invention will be described below.
FIG. 1 is a system diagram of a turbocharged diesel engine (internal combustion engine).
As shown in the figure, the engine body 1 is provided with a common rail fuel injection system including a common rail 2, a fuel injection valve 3 and a fuel pump (not shown) as components, and supplies high-pressure fuel to the engine body 1.

The fuel injection valve 3 directly injects fuel into the combustion chamber and can perform pilot injection before main injection. The fuel injection pressure can be changed by changing the fuel pressure in the common rail 2. Can be controlled.
The compressor 4 a of the supercharger 4 is interposed in the intake passage 5 and supplies compressed air to the engine body 1.

A turbine 4b of the supercharger 4 is interposed in the exhaust passage 6, and is rotated by exhaust from the engine body 1 to drive the compressor 4a.
An air flow meter 7 is provided in the intake passage 5 upstream of the compressor 4a, and an intake throttle valve 8 is provided in the intake passage 6 downstream of the compressor 4a.
The intake throttle valve 8 is, for example, an electronically controlled intake throttle valve whose opening degree can be changed by using a step motor, and the intake air sucked into the engine body 1 according to the opening degree of the intake throttle valve 8 The amount is controlled.

An EGR passage 9 branched from the exhaust passage 6 between the engine body 1 and the turbocharger turbine 4b and connected to the intake passage 5 is provided, and an EGR valve 10 is interposed in the EGR passage 9. Yes.
A NOx trap catalyst 11 is provided in the exhaust passage 6 on the downstream side of the turbine 4b.

The EGR valve 10 is, for example, an electronically controlled throttle valve that uses a step motor, and controls the amount of exhaust gas recirculated to the intake side in accordance with the opening of the EGR valve 10.
The NOx trap catalyst 11 traps NOx in the exhaust flowing in when the exhaust air-fuel ratio is lean, and desorbs the NOx trapped when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio or rich. The original catalyst.

The NOx trap catalyst 11 is configured to trap NOx in, for example, a vanadium-based substance. When the exhaust air-fuel ratio is lean, the NOx trap catalyst 11 is oxidized to easily trap NOx on a noble metal such as platinum, and then the vanadium. Occludes in the trap material of the system.
When the exhaust air-fuel ratio becomes the stoichiometric air-fuel ratio or rich, the NOx stored in the trap material moves again onto the noble metal, where it reacts with a reducing agent such as HC or CO and is reduced to nitrogen. .

  As sensors for detecting various states, the air flow meter 7 for detecting the intake air amount Qa of the engine body 1, the rotation speed sensor 12 for detecting the engine rotation speed Ne, the accelerator opening sensor 13 for detecting the accelerator opening, the engine body. 1, a water temperature sensor 14 for detecting the cooling water temperature Tw, a thermocouple 15 (temperature sensor) for detecting the temperature of the catalyst carrier of the NOx trap catalyst 11, and an oxygen concentration in the exhaust gas provided before and after the NOx trap catalyst 11. First and second oxygen sensors 16a and 16b are provided.

The control unit 20 performs drive control of the fuel injection valve 3 and opening control of the intake throttle valve 8 and the EGR valve 10 based on detection signals from the various sensors.
In addition, when the control unit 20 estimates that the total amount of NOx trapped in the NOx trap catalyst 11 has reached the saturation amount by continuing the operation in the lean region, the control unit 20 forcibly makes the air-fuel ratio rich and the NOx. Is controlled to reduce.

The flowchart of FIG. 2 shows the NOx trap total amount estimation process.
In step S1, the fuel injection amount Qf and the air flow set based on the engine rotational speed Ne detected by the rotational speed sensor 12, the accelerator opening Acc detected by the accelerator opening sensor 13 and the engine rotational speed Ne. The intake air flow rate Qac (mg / s) per unit time calculated as described later based on the intake air amount Qa detected by the meter 7 and the NOx trap catalyst 11 detected by the thermocouple (temperature sensor) 15. The catalyst carrier temperature Tbed is read.

In step 2, from a table (see FIG. 3) showing the correlation between the intake air amount Qac per unit time and the NOx amount (see FIG. 3), the unit output discharged from the engine 1 and the NOx emission amount NOxht per unit time (20 ms) ( g / kW / 20ms).
The unit time of the NOx emission amount NOxht is 20 ms as described above, and this corresponds to the execution cycle of this routine.

The NOx emission amount NOxht is set to a larger value as the intake air amount Qac per unit time increases.
In step 3, NOx per unit time discharged from the engine 1 by the product of the unit output and NOx emission amount NOxht (g / kW / 20 ms) per unit time calculated in step 2 and the engine output Pe at that time. The emission amount (g / 20 ms) is calculated, and this is set as the NOx trap amount NOxt (g / 20 ms) per unit time trapped by the NOx trap catalyst 11.

Here, the fuel injection amount Qf read in step 1 may be regarded as the engine torque, and the engine output equivalent value Pe may be calculated by the following equation.
Pe = Ne × Qf
In step 4, a correction coefficient kNOxeoe based on the NOx emission amount per unit time is calculated using the table shown in FIG.

Instead of the correction coefficient kNOxeoe, the NOx trap amount is determined by the correction coefficient kNOxqexh calculated from the table shown in FIG. 5 based on the relationship between the engine exhaust flow rate and the NOx trap amount, and the engine operating conditions (load, rotation). In consideration of the change, the correction coefficient kNOxneqf calculated from the map shown in FIG. 6 can be used.
In step 5, a correction coefficient kNOxbed for making the trap efficiency in the NOx trap catalyst 11 change depending on the support temperature of the catalyst and the deterioration state of the catalyst is set to the support temperature detected by the thermocouple 15 and the correction coefficient kNOxbed. Based on the deterioration state of the catalyst 11 diagnosed as will be described later, calculation is performed using the table shown in FIG.

The trap efficiency, which is the efficiency with which the NOx trap catalyst 11 can actually trap from the amount of NOx in the exhaust, is low in the low temperature range (inactive temperature range) where the catalyst carrier temperature does not reach the activation temperature, as shown in FIG. In addition, the catalyst carrier temperature tends to decrease even in a high temperature range exceeding a predetermined temperature (predetermined temperature> active temperature, for example, 450 ° C.).
Furthermore, as the NOx trap catalyst 11 (trap function) deteriorates, as shown by the dotted line in FIG. 8, the tendency of the trap efficiency to decrease due to the carrier temperature becomes larger compared to the non-deteriorated state. Only a small amount of NOx can be trapped.

Therefore, as shown in FIG. 7, when the carrier temperature detected by the thermocouple 15 is in a low temperature range (inactive temperature range), the correction coefficient kNOxbed is set to a smaller value as the temperature decreases, and the predetermined coefficient In a high temperature region exceeding the temperature, the value is set to a smaller value as the temperature becomes higher, and further set to a smaller value when the catalyst 11 is deteriorated.
When the correction coefficient kNOxbed is decreased, the NOx trap amount per unit time is corrected to be smaller.

Note that the temperature of the NOx trap catalyst 11 may be estimated from the exhaust temperature or the like.
In step 6, in order to cope with the influence of the NOx trap amount per unit time in the NOx trap catalyst 11 on the total NOx trap amount NOxtrap trapped in the NOx trap catalyst 11 so far, a table as shown in FIG. A correction coefficient kNOxtrap is calculated.

In step S7, the final correction coefficient kNOx is calculated by multiplying the correction coefficients calculated in steps 4-6.
kNOx = kNOxeoe x kNOxbed x kNOxtrap
Note that kNOxqexh or kNOxneqf can be used instead of the kNOxeoe.

In Step 8, the total NOx trap amount NOxtrap (n-1) up to the previous time is added to the current NOx trap amount NOxt per unit time calculated in Step 3 and the correction coefficient kNOx calculated in Step 7 as shown in the following equation. The product is added, and the current NOx trap total amount NOxtrap is calculated.
NOxtrap = NOxtrap (n−1) + NOxt × kNOx
When the estimated NOx trap total amount NOxtrap reaches a predetermined saturation amount, it is determined that the regeneration timing of the NOx trap catalyst 11 is reached, the air-fuel ratio is enriched, and NOx trapped in the NOx trap catalyst 11 is desorbed / Reduce.

  Here, when the NOx trap catalyst 11 is in the carrier temperature range where the trap efficiency decreases, the NOx trap amount NOxt per unit time, which is the basis of the total NOx trap amount NOxtrap, is corrected to decrease by the correction coefficient kNOxbed. The total NOx trap amount NOxtrap is not overestimated by the change in trap efficiency due to the carrier temperature.

Further, the NOx trap amount NOxt per unit time is corrected to decrease by the correction coefficient kNOxbed in response to the trap efficiency decreasing due to the catalyst deterioration together with the carrier temperature. Therefore, the NOx trap total amount NOxtrap is excessively increased when the catalyst deteriorates. It will never be estimated.
Therefore, when the NOx trap total amount does not actually reach the saturation amount, the regeneration process for enriching the air-fuel ratio is performed, so that it is possible to prevent the fuel efficiency performance from being lowered.

Next, the calculation of the intake air amount Qac per unit time will be described with reference to the flowchart of FIG.
In step 11, the engine speed Ne and the intake air amount Qa detected by the air flow meter 7 are read.
In step 12, based on these values, the intake air amount Qac0 per cylinder is calculated by the following equation.

Qac0 = (Qa / Ne) × KCON #
In the above equation, KCON # is a constant determined according to the number of cylinders.
As shown in FIG. 1, the air flow meter 7 is provided in the intake passage 5 upstream of the compressor 4a, and there is a transport delay from the air flow meter 7 to the collector portion downstream of the compressor 4a.

Therefore, in step 13, the value of Qac0 before L times (L = integer constant) is obtained as the intake air amount Qacn per cylinder at the collector inlet.
In step 14, the intake air amount Qaci per cylinder at the intake valve position is calculated with respect to Qacn by the following equation (primary delay equation).
Qaci = Qaci (n-1) * (1-KIN * KVOL) + Qacn * KIN * KVOL
The Qac i (n-1) is the previous value of the intake air amount Q aci per cylinder, the KIN is a volume efficiency equivalent value for correcting the change in volume efficiency due to exhaust gas recirculation, and the KVOL is KVOL = displacement amount This is a value obtained as VE / number of cylinders NC / intake system volume VM.

Further, in step 15, the intake air amount Qaci per cylinder is converted into an intake air amount Qac (mg / s) per unit time.
Qac = Qaci × Kn # × {Ne / (60 × 2)}
However, Kn # is a constant and is 4 for 4 cylinders and 6 for 6 cylinders.
In this embodiment, when calculating the NOx emission amount NOxht per unit output and unit time, the unit time calculated based on the detection value Qa of the air flow meter in order to accurately grasp the delay of the intake air amount. Although the per intake air amount Qac is used, the detection value Qa of the air flow meter 7 may be used as it is.

As shown in FIG. 11, it is desirable that the intake air amount Q changes as tQa indicated by a one-dot chain line in accordance with changes in the fuel injection amount Qf and the engine rotational speed Ne. It changes as follows.
Here, since the detected value Qa of the air flow meter 7 also shows a change that substantially corresponds to the actual delay, it can be configured to calculate NOxht using the detected value Qa of the air flow meter 7, thereby using Qac. Thus, although the estimation accuracy of the NOx emission amount NOxht is lower than when calculating the NOx emission amount NOxht, the control logic can be simplified.

Next, the deterioration diagnosis of the NOx trap catalyst 11 will be described below.
The NOx trap catalyst 11 in the present embodiment has a NOx trap function and an oxygen storage function, and the NOx trap function and the oxygen storage function are deteriorated due to deterioration of a noble metal (such as platinum or ceria). That is, the deterioration of the NOx trap function can be estimated from the deterioration of the oxygen storage function.

The oxygen storage function is a function of storing oxygen in the catalyst when the exhaust air-fuel ratio is lean and releasing the oxygen stored in the catalyst when the exhaust air-fuel ratio becomes rich.
Therefore, when the NOx desorption / reduction process of the NOx trap catalyst 11 or when the sulfur poisoning of the NOx trap catalyst 11 is released, the engine is operated in the vicinity of the theoretical air fuel ratio, and the exhaust air fuel ratio on the upstream side of the NOx trap catalyst 11 is calculated theoretically. When fluctuating around the air / fuel ratio (excess air ratio λ = 1), the NOx trap catalyst 11 acts to suppress fluctuations in oxygen concentration due to fluctuations in the exhaust air / fuel ratio by the oxygen storage function.

In other words, the NOx trap catalyst 11 causes the upstream exhaust air-fuel ratio to lean and the oxygen in the exhaust gas increases, so that oxygen is stored inside, and conversely, the upstream exhaust air-fuel ratio swings richly, When the amount of oxygen decreases, the oxygen stored so far is released.
Therefore, the fluctuation of the exhaust air-fuel ratio (oxygen concentration) on the upstream side of the catalyst is greatly attenuated on the downstream side of the catalyst by the oxygen storage function.

Here, when the oxygen storage function in the NOx trap catalyst 11 deteriorates, the function of storing oxygen when the exhaust air-fuel ratio swings lean and the function of releasing oxygen when the exhaust air-fuel ratio swings rich decreases. Changes in the oxygen concentration upstream of the catalyst appear on the downstream side of the catalyst (see FIG. 12).
Therefore, when the engine is operated near the theoretical air-fuel ratio during the NOx desorption / reduction process of the NOx trap catalyst 11 or when the sulfur poisoning of the NOx trap catalyst 11 is released, the oxygen sensor 16a on the upstream side of the catalyst detects this. NOx trap when the fluctuation frequency on the downstream side approaches the fluctuation frequency on the upstream side by comparing the fluctuation frequency of the oxygen concentration and the fluctuation frequency of the oxygen concentration detected by the oxygen sensor 16b on the downstream side of the catalyst. The deterioration of the catalyst 11 is determined.

The deterioration of the NOx trap catalyst 11 can be estimated from the travel distance of the vehicle, the accumulated exhaust flow rate, and the like.
Further, in the NOx desorption / reduction process of the NOx trap catalyst 11, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio or rich, and then the exhaust air-fuel ratio downstream of the NOx trap catalyst 11 is switched to the stoichiometric air-fuel ratio or rich. By measuring the time lag, the total amount of NOx actually trapped can be estimated, and the deterioration of the NOx trap catalyst 11 can be determined from the estimated value.

  Further, the degree of deterioration in the NOx trap catalyst 11 can be determined, and the correction coefficient kNOxbed can be changed in multiple stages according to the degree of deterioration.

The figure which shows the system configuration | structure of the internal combustion engine in embodiment. The flowchart which shows the estimation process of the NOx trap total amount in embodiment. The figure which shows the relationship between the amount of intake air Qac per unit time and NOx discharge | emission amount per unit time and unit time in embodiment. The figure which shows the relationship between NOx discharge | emission amount per unit time and correction coefficient kNOxeoe in embodiment. The figure which shows the relationship between the exhaust flow volume in embodiment, and the correction coefficient kNOxqexh. The figure which shows the relationship between the engine speed Ne and fuel injection quantity Qf, and correction coefficient kNOxneqf in embodiment. The figure which shows the relationship between the catalyst carrier temperature and catalyst deterioration state, and correction coefficient kNOxbed in embodiment. The figure which shows the relationship between the catalyst carrier temperature and catalyst deterioration state, and trap efficiency in embodiment. The figure which shows the relationship between NOx trap total amount and correction coefficient kNOxtrap in embodiment. The flowchart which shows the process which calculates | requires the intake air amount Qac per unit time. To. The figure which shows the change of the fuel injection quantity Qf at the time of transient driving | operation, the engine speed Ne, the intake air quantity Q, and NOx discharge | emission amount. The time chart which shows the fluctuation | variation of the oxygen concentration before and behind a NOx trap catalyst.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Engine main body, 3 ... Fuel injection valve, 4 ... Supercharger, 5 ... Intake passage, 6 ... Exhaust passage, 7 ... Air flow meter, 8 ... Intake throttle valve, 9 ... EGR passage, 10 ... EGR valve, 11 ... NOx trap catalyst, 12 ... rotational speed sensor, 13 ... accelerator sensor, 14 ... water temperature sensor, 15 ... thermocouple, 16a, 16b ... oxygen sensor, 20 control unit

Claims (4)

An NOx trap catalyst that is disposed in the exhaust passage of the internal combustion engine and traps NOx in exhaust flowing when the exhaust air-fuel ratio is lean, and desorbs and purifies trapped NOx when the exhaust air-fuel ratio is rich or stoichiometric. ,
The NOx trap amount per unit time in the NOx trap catalyst is calculated based on the engine operating state, and the exhaust air / fuel ratio is calculated based on the total NOx trap amount obtained by integrating the NOx trap amount per unit time. In an exhaust purification device for an internal combustion engine that is richly controlled,
Detecting the carrier temperature of the NOx trap catalyst, diagnosing deterioration of the NOx trap catalyst, and correcting the NOx trap amount per unit time based on the carrier temperature and the result of the deterioration diagnosis. An exhaust purification device for an internal combustion engine.   When the carrier temperature is in a temperature range where the trap efficiency is lowered, the NOx trap amount per unit time is corrected to be decreased, and further, the decrease correction is increased when the deterioration of the NOx trap catalyst is determined. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the correction is performed.   The exhaust gas purification apparatus for an internal combustion engine according to claim 2, wherein when the carrier temperature is a predetermined low temperature range and a predetermined high temperature range, the NOx trap amount per unit time is corrected to decrease.   The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the deterioration of the NOx trap catalyst is determined based on a change in oxygen concentration in the exhaust gas before and after the NOx trap catalyst. .

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