JP2996004B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine having an exhaust system provided with a so-called lean NOx catalyst capable of reducing NOx (nitrogen oxide) in exhaust gas produced at a lean air-fuel ratio.

[0002]

2. Description of the Related Art Japanese Patent Application Laid-Open No. 1-130735 discloses a zeolite in which a transition metal is supported by ion exchange or a noble metal is supported.
A catalyst for purifying NOx in the presence of (hydrocarbon) (hereinafter referred to as a lean NOx catalyst) is disclosed.

[0003] Since it has been found that a lean NOx catalyst exhibits a higher NOx purification rate when the temperature is in a rising state than in a steady or falling state, the present applicant has
In Japanese Patent Application No. 2-317664 (prior to publication), the same type of lean NOx catalyst is arranged in parallel in the exhaust system of an internal combustion engine,
An invention has been proposed in which the exhaust gas flow is alternately switched at certain time intervals alternately for all operating conditions to forcibly and repeatedly create a catalyst temperature increasing process, thereby improving the NOx purification rate.

[0004]

However, in the case where the exhaust gas is switched between the same type of lean NOx catalysts at fixed time intervals as in the prior application, it is difficult to cope with a change in the exhaust gas temperature, and the high NOx purification rate is high. If the temperature range deviates from the temperature range that indicates
There is a problem that a purification rate cannot be obtained.

Further, in the prior application, since the exhaust flow is switched at a constant rate with respect to all operating conditions, the amount of NOx in the exhaust gas of the engine increases and the temperature rises rapidly, as in acceleration.
Suppression of the partial oxidation of NO and the generation of active species suppresses the optimum NOx, especially when NOx countermeasures are required.
It is hard to say that it is a measure.

It is an object of the present invention to provide a method for controlling the speed of N
Even under operating conditions where Ox countermeasures are required, high NO
An object of the present invention is to provide an exhaust gas purification device for an internal combustion engine that can exhibit x purification rate.

[0007]

According to the present invention, the above object is achieved by the following apparatus for purifying exhaust gas of an internal combustion engine. That is, an internal combustion engine capable of lean burn and an exhaust passage thereof, at least one first lean NOx catalyst and at least one second lean NOx catalyst provided in parallel with each other in the exhaust passage, Lean N of 2
The second lean NOx provided for the Ox catalyst
Temperature maintaining means for maintaining the temperature of the catalyst at a predetermined temperature; valve means provided upstream of the second lean NOx catalyst for changing the flow rate of exhaust gas to the second lean NOx catalyst; operating state of the internal combustion engine When the operating state detecting means determines that the current operating state of the internal combustion engine is accelerating, the amount of exhaust flowing through the second lean NOx catalyst is increased, and the current internal combustion engine When it is determined that the operating state is at the time of non-acceleration, the second lean NOx
An exhaust gas purification device for an internal combustion engine, comprising: valve means control means for controlling the valve means so as to reduce the amount of exhaust flowing through the catalyst or maintain the reduced amount.

[0008]

In the exhaust gas purifying apparatus for an internal combustion engine according to the present invention, during non-acceleration, exhaust gas is mainly flowed to the first lean NOx catalyst, and the second lean NOx catalyst is brought into a state of being at rest or close to being at rest. It is kept warm as well. At the time of acceleration, the first lean NOx catalyst and the second lean N
Exhaust gas flows through both of the Ox catalysts and the second lean NO
The x catalyst is also activated. The second lean NOx catalyst is maintained near the lower limit temperature of the temperature range exhibiting a high NOx purification rate even during a halt, and immediately enters the temperature range exhibiting a high NOx purification rate when the operating state changes during acceleration. Therefore, NOx can be reliably purified, and the effect of improving the NOx purification rate at the time of temperature rise and the effect of increasing the NOx purification rate by the low space velocity obtained by flowing exhaust gas through the two catalysts work. A high NOx purification rate can be obtained.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Four embodiments of an exhaust gas purifying apparatus for an internal combustion engine according to the present invention will be described below with reference to the drawings. FIGS. 1 to 4 show a configuration common to each embodiment, and FIGS. 5 to 8 show configurations unique to each embodiment. First, the common configuration will be described. As shown in FIG. 1, an exhaust passage 4 of an internal combustion engine (including a diesel engine) 2 capable of lean combustion is provided with a portion in which a plurality of passages 4a and 4b are arranged in parallel with each other. Lean NOx catalyst 6a, 6
b is provided. These plural lean NOx catalysts 6
a and 6b are referred to as first, second,..., lean NOx catalysts, respectively.

[0010] The lean NOx catalyst is a catalyst capable of purifying NOx in lean air-fuel ratio exhaust gas, and the ion exchange of a transition metal such as Cu (copper) with a conventionally known catalyst is performed. A transition metal / zeolite catalyst that reduces NOx in the presence of HC (hydrocarbon) supported on zeolite, or a Pt / platinum that supports Pt (platinum) on alumina
Alumina catalysts are known.

[0011] The lean NOx catalyst has a temperature range (temperature window) showing a high NOx purification rate, and when the temperature deviates from this temperature window, the NOx purification rate decreases. Transition metal / zeolite catalysts have a temperature window in the range of about 350-550 ° C, and Pt / alumina catalysts have a temperature window in the range of about 250-450 ° C. In addition, as shown in FIG. 4, the lean NOx catalyst exhibits a higher NOx purification rate even within the temperature window, particularly when the temperature is raised, than when the temperature is constant or when the temperature is lowered.

In the passage 4b in which the second lean NOx catalyst 6b is provided, a valve means 8 for changing the flow rate of exhaust gas flowing through the second lean NOx catalyst 6b is provided upstream of the second lean NOx catalyst 6b. . When the valve means 8 is open, the exhaust gas also flows to the second lean NOx catalyst 6b. Therefore, the exhaust gas flows to both the lean NOx catalysts 6a and 6b, and when the valve means 8 is closed or close to closed, the second lean NOx catalyst 6b is closed. Even if the flow of exhaust gas to the NOx catalyst 6b stops or does not flow, the flow is reduced to a small amount. The first lean NOx catalyst 6a is mainly used, and the second lean NOx catalyst 6b is brought into a state of being at or almost stopped. Valve means 8
The opening and closing of are controlled by a command from an electronic control unit (ECU) 10 described later.

When the second lean NOx catalyst 6b is at rest or in a state close to rest, the catalyst temperature drops due to natural heat radiation, and eventually falls out of the temperature window to a lower temperature side. 8 is opened and the exhaust gas flows toward the second lean NOx catalyst 6b, the second lean N
Because the temperature of the second lean NOx catalyst 6b does not immediately return to the temperature window due to the heat capacity of the Ox catalyst 6b, the NOx of the second lean NOx catalyst 6b is temporarily stopped.
The state where the x purification rate is low continues.

To prevent the occurrence of such a state,
Even when the second lean NOx catalyst 6b is at rest or near to rest, the second lean NOx catalyst 6b is maintained near the lower limit of the temperature window where the second lean NOx catalyst exhibits a high NOx purification rate. A temperature maintaining means 12 is provided for the second lean NOx catalyst 6b so as to immediately bring the second lean NOx catalyst bed temperature into the temperature window when the valve means 8 is opened. ing. Such a temperature maintaining means 12 comprises a combination of at least one of a heat insulating material, a heater and a cooler.

An operating state detecting means for detecting the operating state of the internal combustion engine 2 is provided. The operating state detecting means includes, for example, a crank angle sensor 14 built in a distributor for detecting the engine speed NE,
It includes an intake pipe negative pressure center 16 provided downstream of the throttle valve for detecting the engine load. These sensors 1
The outputs of 4 and 16 are input to the ECU 10.

The ECU 10 comprises a microcomputer, and has an input interface, an output interface, a read-only memory (ROM) as a read-only storage element, a random access memory (RAM) for temporary storage, and a central processor unit (CPU) for executing operations. ). If the input is an analog signal,
The signal is converted into a digital signal by an analog / digital converter and input to the input interface. The command of the calculation result in the CPU is sent to the actuator of the valve means 8 via the output interface, and controls the opening and closing of the valve means 8.

The ROM stores a control routine of the valve means control means for controlling the opening and closing of the valve means 8 as shown in FIG. 2, and this routine is read out by the CPU and executed. The control routine of FIG. 2 is interrupted at regular intervals, for example, every second.

First, at step 102, the engine operating state,
For example, the engine speed NE and the engine load PM are read and calculated. Subsequently, at step 104, the engine speed NE
Change rate (time derivative) dNE / dt and engine load PM
Is calculated as dPM / dt.

Subsequently, at step 106, it is determined whether or not the current operation state is during acceleration. For example, when the rate of change of the engine speed exceeds a certain value and the rate of change of the engine load exceeds a certain value, it is determined that the vehicle is accelerating. Otherwise, it is determined that the vehicle is not accelerating.

During acceleration, the amount of NOx in the exhaust gas from the internal combustion engine increases, and the exhaust gas temperature also rises sharply. Therefore, when it is determined that there is acceleration, NOx is effectively purified by both the first lean NOx catalyst 6a and the second lean NOx catalyst 6b.
It is desired to purify NOx only with the lean NOx catalyst 6a and leave the second lean NOx catalyst 6b at rest or in a state close to rest.

In order to perform such control, step 1
If it is determined in step 06 that the current engine operating state is during acceleration, the routine proceeds to step 110, where the valve means 8 is opened to increase the amount of exhaust flowing through the second lean NOx catalyst 6b.
When it is determined in step 106 that the current engine operating state is during non-acceleration, the routine proceeds to step 108, in which the valve means 8 is closed or close to closed or is maintained in that state. The exhaust gas flowing to the lean NOx catalyst 6b is reduced (including 0) or the reduced state is maintained. From steps 108 and 110, the process proceeds to the return step, and the calculation of the interrupt cycle ends. FIG. 3 shows the second lean NOx catalyst 6b under the control of the routine of FIG.
Is shown in a time chart.

Next, the operation common to the embodiments will be described. At the time of non-acceleration, exhaust gas is mainly flown to the first lean NOx catalyst 6a, and NO is mainly emitted to the first lean NOx catalyst 6a.
x is purified. At this time, the second lean NOx catalyst 6
b is in a resting or near resting state, and is maintained at a temperature near the lower limit of the temperature window by the temperature maintaining means 12 (portion A in FIG. 3).

When the operating state of the internal combustion engine changes from non-acceleration to acceleration, the valve means 8 is opened and the second lean NOx
Exhaust also flows through the catalyst 6b, and the first lean NOx catalyst 6a
And the second lean NOx catalyst 6b purifies NOx. At this time, the temperature of the second lean NOx catalyst 6b rises (part B in FIG. 3), so that a good NOx purification rate in the temperature rise process of FIG. 4 is obtained. Also, the second lean N
Since the Ox catalyst 6b is kept near the lower limit of the temperature window during non-acceleration, it can immediately enter the temperature window when the temperature rises, and quickly starts showing a high NOx purification rate. Further, the first and second lean NOx catalysts 6a, 6
Since the exhaust gas flows through both of the catalysts b, the space velocity of the exhaust gas at both the catalysts 6a and 6b is also low, and a high NOx purification rate can be exhibited.

Next, the configuration and operation unique to each embodiment will be described. FIG. 5 shows a partial system of the first embodiment. In the first embodiment, the first lean NOx catalyst 6a comprises a transition metal / zeolite catalyst, and the second lean NOx catalyst 6a
b consists of a Pt / alumina catalyst. Second lean NOx
During non-acceleration, the catalyst 6b is maintained at about 250 ° C. by the temperature maintaining means 12.

At the time of acceleration, the exhaust gas temperature rises and the HC in the exhaust gas is directly oxidized, and the HC becomes slightly insufficient.
Since the alumina catalyst can purify NOx even with a small amount of HC, it exhibits a high NOx purification rate. In addition, during acceleration, the space velocity of the transition metal / zeolite catalyst decreases, and the zeolite-based catalyst exhibits a good NOx purification ability because the NOx purification rate increases as the space velocity decreases.

FIG. 6 shows a partial system of the second embodiment. In the second embodiment, the first lean NOx catalyst 6a
t / alumina catalyst, the second lean NOx catalyst 6
b consists of a transition metal / zeolite catalyst. The second lean NOx catalyst 6b is maintained at about 350 ° C. by the temperature maintaining means 12 during non-acceleration.

At the time of acceleration, the gas flows through both catalysts 6a and 6b, so that the flow rate of exhaust gas flowing through the Pt / alumina catalyst decreases and the rise in the Pt / alumina catalyst bed temperature is suppressed.
/ Alumina catalyst shows good NOx purification rate. Also, since the transition metal / zeolite catalyst is kept at a relatively low temperature during non-acceleration and adsorbs HC by leaking a small amount of exhaust gas, use the HC that was adsorbed when changed during acceleration. And exhibit a high NOx purification rate.

FIG. 7 shows a partial system of the third embodiment. In the third embodiment, both the first lean NOx catalyst 6a and the second lean NOx catalyst 6b are made of a transition metal / zeolite catalyst. The second lean NOx catalyst 6b is maintained at about 350 ° C. by the temperature maintaining means 12 during non-acceleration.

At the time of acceleration, the gas flows into both the catalysts 6a and 6b. Therefore, even if the exhaust gas temperature rises at the engine outlet, the increase in the catalyst bed temperature of the first lean NOx catalyst 6a is suppressed, and the first
The lean NOx catalyst 6a is maintained in the temperature window and exhibits a high NOx purification rate. Also, the second lean NO
By causing a small amount of exhaust gas to leak to the x catalyst 6b during non-acceleration and adsorbing HC, it is possible to cause the second lean NOx catalyst 6b to exhibit a high NOx purification rate when changing during acceleration. it can.

FIG. 8 shows a partial system of the fourth embodiment. In the fourth embodiment, both the first lean NOx catalyst 6a and the second lean NOx catalyst 6b are composed of a Pt / alumina catalyst. The second lean NOx catalyst 6b is maintained at about 250 ° C. by the temperature maintaining means 12 during non-acceleration.

At the time of acceleration, the fuel flows to both the catalysts 6a and 6b. Therefore, even if the exhaust gas temperature rises at the engine outlet, the increase in the catalyst bed temperature of the first lean NOx catalyst 6a is suppressed, and the first lean NOx catalyst 6a Shows a high NOx purification rate maintained in the temperature window. Further, even if the exhaust gas temperature rises at the engine outlet during acceleration and the HC is directly oxidized and the HC decreases, the Pt / alumina catalyst can show NOx purification with a small amount of HC. Also high N
It shows the Ox purification rate.

[0032]

The first and second lean NOx catalysts are arranged in parallel in the exhaust passage of the internal combustion engine, the second lean NOx catalyst is provided with temperature maintaining means, and the valve means is provided upstream of the second lean NOx catalyst. And the exhaust to the second lean NOx catalyst is increased by the valve means control means during acceleration and decreased during non-acceleration.
In addition to using the second lean NOx catalyst to immediately raise the temperature to a high purification rate temperature range under low space velocity conditions,
Since a high purification rate in the temperature raising process can be used, a high NOx purification rate can be obtained.

[Brief description of the drawings]

FIG. 1 is a system diagram of an exhaust gas purification device for an internal combustion engine according to the present invention.

FIG. 2 is a flowchart of a control routine of valve means control means stored in an ECU of FIG. 1;

FIG. 3 is a time chart showing a change with time of a catalyst bed temperature of a second lean NOx catalyst when controlled by the control routine of FIG. 2;

FIG. 4 is a characteristic diagram showing a tendency of a NOx purification rate during a temperature rise process, a constant temperature, and a temperature decrease process of a lean NOx catalyst.

FIG. 5 is a partial system diagram according to the first embodiment of the present invention.

FIG. 6 is a partial system diagram according to a second embodiment of the present invention.

FIG. 7 is a partial system diagram according to a third embodiment of the present invention.

FIG. 8 is a partial system diagram according to a fourth embodiment of the present invention.

[Explanation of symbols]

 2 internal combustion engine 4 exhaust passage 4a passage 4b passage 6a first lean NOx catalyst 6b second lean NOx catalyst 8 valve means 10 ECU 12 temperature maintaining means 14 crank angle sensor 16 intake pipe negative pressure sensor

──────────────────────────────────────────────────の Continued on front page (51) Int.Cl. ⁶ Identification code FI F01N 3/24 F01N 3/24 N R

Claims (1)

(57) [Claims] 1. An internal combustion engine capable of lean-burn and an exhaust passage thereof, and at least one exhaust passage provided in the exhaust passage in parallel with each other.
The first lean NOx catalyst and the at least one second lean NOx catalyst; and maintaining the temperature of the second lean NOx catalyst provided for the second lean NOx catalyst at a predetermined temperature. A temperature maintaining means, a second lean NOx catalyst provided upstream of the second lean NOx catalyst.
Valve means for changing the exhaust flow rate to the lean NOx catalyst, operating state detecting means for detecting an operating state of the internal combustion engine, and when the operating state detecting means determines that the current internal combustion engine operating state is during acceleration. The second lean NOx
When the amount of exhaust flowing through the catalyst is increased and it is determined that the current operation state of the internal combustion engine is at the time of non-acceleration, the second lean N
An exhaust gas purification apparatus for an internal combustion engine, comprising: valve means control means for controlling the valve means so as to reduce the amount of exhaust flowing through the Ox catalyst or maintain the reduced amount.