JP2000199421A - ENGINE NOx REDUCTION DEVICE - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an NOx catalyst from being deteriorated and maintain initial performance for an extended period of time by adding a reducing agent to exhaust emissions within a predetermined exhaust emission temperature range that is either lower or higher than an exhaust emission temperature range within which NOx contained in exhaust emissions is reduced and providing an activation means that activates the NOx catalyst. SOLUTION: A NOx catalyst 3 that generates activation phenomenon is installed in midway of an exhaust pipe 2 of a diesel engine. Light oil, the fuel for an engine 1, is made possible for use as a reducing agent used for the activation process. An exhaust gas temperature sensor 7, which detects the temperature of exhaust gas at a catalyst inlet, is installed near the inlet and on the upstream side of the NOx catalyst 3. In addition, a catalyst temperature sensor 8 is installed inside the NOx catalyst 3. An ECU 4 calculates a temperature difference based on outputs from the sensors 7, 8, and evaluates the degree of deterioration of the NOx catalyst 3. Fuel (reducing agent) is injected incidentally for NOx reduction process in the temperature ranges on the lower and higher sides of a peak temperature, at which the NOx purification factor reaches a peak.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine NOx reduction apparatus, and more particularly to an engine NOx reduction apparatus suitable for a diesel engine or a lean burn gasoline engine.

[0002]

2. Description of the Related Art Generally, in a diesel engine or a lean burn (lean burn) gasoline engine, combustion is performed in an excess air state, and exhaust gas is not stoichiometric. However, recently, a zeolite-based NOx catalyst supporting a transition metal has appeared,
This technology has attracted attention because NOx can be reduced in the presence of a reducing agent such as C (Japanese Patent Laid-Open No. 4-26).
5414, JP-A-8-312336, etc.).

The characteristics of this NOx catalyst are that the active temperature range in which the NOx purification process can be performed is relatively narrow, that the NOx purification rate has a peak in this temperature range, and that the temperature range becomes high due to deterioration (aging). Shifting to the side is raised (see FIG. 9).

Further, as described in Japanese Patent Application No. 10-72335 by the present applicant, for example, a specific type of NOx catalyst containing Ir (iridium) as a component has an activation phenomenon, which is once deteriorated and degraded once in an active temperature range. Is shifted to a high temperature side, but is activated while continuing to use and shifts to a low temperature side again. For this reason, in the prior application, by switching the reducing agent addition map in accordance with the shift of the active temperature region and by switching the temperature at which the reducing agent addition is started, the optimum reducing agent addition, that is, the NOx purification process according to the shift state is performed. Had to do it.

[0005]

However, in the control of the prior application, the map switching is performed only in accordance with the deterioration and activation of the catalyst, and the deterioration itself cannot be suppressed. It was just something to be done. Normally, in order to adjust the active temperature range of the catalyst in the initial state to the exhaust gas temperature range in which purification is desired most, if the active temperature range shifts due to deterioration, desired performance cannot be obtained naturally, and the NOx emission increases from the initial schedule. Activation is also secondary or passive, so it takes a relatively long time to activate,
As a result, the use time in the deteriorated state increases, which also contributes to an increase in NOx emission.

[0006]

According to the present invention, a NOx catalyst is provided in an exhaust passage of an engine, and a reducing agent is added to the exhaust gas within a predetermined exhaust gas temperature range to reduce the NOx in the exhaust gas. In the engine NOx reduction device,
An activation treatment means for adding a reducing agent to the exhaust gas within a predetermined exhaust gas temperature range lower or higher than the exhaust gas temperature range and activating the NOx catalyst is provided.

[0007] According to this, since the NOx catalyst can be activated at a time other than during the NOx purification processing, the NOx catalyst can be actively activated and the initial performance can be maintained.

Here, it is preferable that the activation processing means executes the activation processing every predetermined engine operation time.

Further, the activation processing means cumulatively calculates the engine operating time while performing weighting according to the exhaust gas temperature during the operation of the engine, and executes the activation processing when the calculated value reaches a predetermined value. It is preferable to carry out.

[0010] It is preferable that the activation processing means executes the activation processing every predetermined traveling distance of the vehicle.

The activation processing means executes the addition of the reducing agent by sub-injection of fuel into the engine cylinder,
Preferably, the sub-injection amount is determined for each engine speed based on a predetermined map.

[0012]

Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 8 shows an engine NOx reduction device according to the present invention. The engine 1 is a diesel engine, and an exhaust path of the engine 1 is formed by an exhaust pipe 2, and a NOx catalyst 3 is provided in the exhaust pipe 2.

As the NOx catalyst 3, one that causes an activation phenomenon is used, and here, Ir is contained as an active metal species. However, the NOx catalyst 3 may be any one that can be activated. Activation requires the presence of a reducing agent. Therefore, the fuel of the engine 1, that is, light oil, is used as the reducing agent. HC contained in light oil plays a substantial role as a reducing agent.

The engine 1 has an electronic control unit (hereinafter referred to as EC).
U) (electronic control). That is, the ECU 4
Is to calculate the current crank angle and the engine rotational speed N _E based on the output of the crank angle sensor 5, and calculates the current engine load Q based on the output of the engine load sensor 6, a predetermined injection amount from these values The target fuel injection amount and the target injection timing are determined according to the map and the injection timing map, and the injection amount control and the injection timing control are executed based on these.

In particular, as shown in the frame of the figure, apart from the main injection for the combustion stroke performed near the compression top dead center, a small amount of sub-injection is also performed near the exhaust top dead center. Light oil as a reducing agent is added and mixed therein. In this case, it is not necessary to separately provide a nozzle device dedicated for adding the reducing agent, and simplification and cost reduction can be achieved.
The sub-injection will be described later in detail.

Incidentally, the method of adding the reducing agent is not limited to such a sub-injection near the exhaust top dead center. The addition timing may be the expansion stroke or the exhaust stroke, or the addition may be performed directly in the exhaust passage upstream of the NOx catalyst 3 using a dedicated nozzle.

An exhaust gas temperature sensor 7 for detecting the exhaust gas temperature T _IN at the catalyst inlet (hereinafter referred to as catalyst inlet temperature) is provided near and upstream of the inlet of the NOx catalyst 3. EC
U4 determines whether or not sub-injection can be executed based on the output of the sensor 7, as described later. A catalyst temperature sensor 8 is embedded inside the NOx catalyst 3. The ECU 4 calculates a temperature difference based on the output of the sensor 8 and the output of the exhaust gas temperature sensor 7 to determine the deterioration state of the NOx catalyst 3 (Japanese Patent Laid-Open No. 8-31).
No. 2336). On the downstream side of the NOx catalyst 3, NOx
A sensor 9 is provided, and the ECU 4 detects the NOx concentration in the exhaust gas based on the output of the sensor 9.

The NOx catalyst 3 has a purifying characteristic as shown in FIG. N That is, in the initial state shown by the solid line, the catalyst inlet temperature T _IN activity start temperature T _ST (here 350 ° C.) was activated at or over, as the rise of the subsequent catalyst inlet temperature T _IN
Gradually increasing the Ox purification rate, it peaked in the NOx purification rate at the temperature T _P, gradually reducing the NOx purification rate at higher temperatures. Therefore, in this apparatus, the temperature T _ON or more than this lower temperature side the boundary of the peak temperature T _P, the temperature T _OFF of the high-temperature side
Within the following temperature range T _Z, running sub injection, performs addition of the reducing agent into the exhaust gas, so as to perform the reduction treatment or purification treatment of NOx. T _ON purification starting temperature, T _OFF purification stop temperature, the T _ON or T _OFF following temperature range T _Z that purification temperature region. Here, T _ON = 400
℃, T _OFF = 500 ℃.

As shown by the broken line, in the NOx catalyst 3, when the catalyst inlet temperature T _IN is higher than the activation start temperature T _ST,
And CO can also be purified. That is, a reduction / oxidation reaction is caused between NOx and HC and CO, so that HC and CO can also be purified. As the purification characteristics of HC and CO, when the catalyst inlet temperature T _IN becomes a value slightly higher than the activation start temperature T _ST , the purification rate starts to rise relatively sharply and reaches a maximum at a predetermined temperature slightly lower than T _ON. Value reached,
Thereafter, the maximum value is maintained.

The above is the initial characteristics of the NOx catalyst 3;
When the catalyst 3 deteriorates (changes with time), the entire NOx purification curve and the entire HC and CO purification curves shift to higher temperatures as indicated by the dashed line. Accordingly, in the prior application, T _ON , T _OFF , and T _Z are shifted by map switching. Although the same control can be performed by the present apparatus, the description is not given here, and the following description will be made on the assumption that the catalyst is in the initial state.

The auxiliary injection is executed in accordance with the particular portion Z of the secondary injection amount map shown in FIG. 6 in the purification temperature region T _Z. That is, the sub-injection amount map is created in advance by experiments or the like and stored in the ECU 4, and is used for uniquely determining the sub-injection amount based on the engine speed _NE and the engine load Q. On this map, when the engine rotation speed _NE and the engine load Q are on the line on the lower right, which is written as T _ON and T _OFF , the catalyst inlet temperature T _IN
Become T _ON and T _OFF . Therefore, the portion Z sandwiched between them
The purification corresponding to the temperature region T _Z, each moiety Z in each engine rotational speed N _E, for each engine load Q, i.e. sub-injection amount for each mesh is defined.

FIG. 5 shows a control flow of the NOx purification process. This control takes a predetermined control time (several tens of msec)
This is an interrupt process executed by the ECU 4 every time.
First, the ECU 4 determines in step 31 the current catalyst inlet temperature T
_IN , the engine speed _NE and the engine load Q are read. Then, in step 32, whether or not T _ON ≦ T _IN ≦ T _{OFF is} satisfied,
That it is determined whether the catalyst inlet temperature T _IN of the current is in purification temperature region T _Z. When it is determined that T _ON ≦ T _IN ≦ T _OFF , the routine proceeds to step 33, where the target sub-injection amount is calculated from the sub-injection amount map, and in step 34, the sub-injection corresponding to the target sub-injection amount is executed. To end. On the other hand, if it is determined in step 32 that T _ON ≤T _IN ≤T _{OFF, the} control is immediately terminated. At this time, the NOx purification processing is not performed.

In the above-described NOx purification process, the NOx catalyst 3 may be activated by the addition of a reducing agent while the catalyst is active. However, this is a secondary or passive activity, not a positive one. Therefore, the present apparatus is provided with activation processing means for activating the NOx catalyst 3, so that the NOx catalyst 3 is actively activated. The contents will be described below.

As shown in FIG. 6, in the sub-injection amount map, a portion FK is defined in addition to the portion Z. This portion FK in a portion used for the activation treatment, and the sub injection amount is defined for each engine rotational speed N _E. That mesh cutting maps made only with respect to the engine rotational speed N _E, not been made with respect to the engine load Q. The portion FK is a lower temperature region and a higher temperature region adjacent to the portion Z. That is, on the map, the engine rotation speed N _E
When the engine load Q and the engine load Q are on a line written as _TL , the catalyst inlet temperature T _IN becomes _TL = T _ST (here, 350 ° C.)
Becomes Therefore, on the low temperature side, the activation process is performed within the temperature range of T _L ≦ T _IN ≦ T _ON . This temperature region is called a low-temperature activation temperature region. The _TL line on the map is the _TON line translated in parallel to the low load side. In the low activation temperature region, the NOx catalyst 3 is in an active state.

On the other hand, on the high temperature side, the activation process is performed within the temperature range of T _IN ≧ T _OFF . This temperature region is called a high-temperature activation temperature region.

FIG. 4 shows a main flow of the activation processing control. This flow is an interrupt process that is repeatedly executed at predetermined control times. First, the ECU 4 executes step 4
It is determined whether the predetermined activation processing execution time has come in 1 or not,
If it is determined that the time is reached, the activation process is executed in step 42, and the present flow ends. If it is determined in step 41 that it is not the time, this flow is immediately ended. That is, the activation process here is executed at predetermined intervals. The details will be described below.

FIGS. 2 and 3 show a sub-flow for judging the activation processing execution timing. Here, for the determination, both a method of making a determination based on the engine operating time (FIG. 2) and a method of making a determination based on the traveling distance of the vehicle equipped with the present apparatus (FIG. 3) are used in combination. 2 and 3
Are performed in parallel, and when it is determined that the execution time has arrived in any of the flows, the activation process is executed. These flows are also interrupt processes repeatedly executed at every predetermined control time t.

The engine operating time is cumulatively calculated by a built-in timer of the ECU 4. Here, a weighting of the time corresponding to the catalyst inlet temperature T _IN , that is, a weight calculation is performed during the calculation, and the degree of deterioration of the catalyst is taken into consideration. Included in.

FIG. 7 is a diagram showing a conversion method when converting the engine operation time into a timer count value. It is divided into four regions A, B, C and D according to the catalyst inlet temperature T _IN , and T _IN <
A region where T _H1 is satisfied is A, a region where T _H1 ≦ T _IN <T _H2 is B, a region where T _H2 ≦ T _IN <T _H3 is C, and a region where T _H3 ≦ T _IN is D. The values of _TH1 , _TH2 , and _TH3 here are 500 ° C, 600 ° C, and 700 ° C, respectively, and in particular, _TH1 =
T _OFF . In each of the areas A, B, C, and D, the actual engine operation time obtained by multiplying the actual engine operation time by 0 times, 1 time, 5 times, and 10 times is defined as a timer count value. For example, when the catalyst inlet temperature T _IN is 650 ° C., the engine enters the region C, so that the actual engine operation time of 1 second is set to 1 × 5 = 5 seconds, and this is set as the timer count value.

The A region, that is, 0 times (no count) when T _IN <500 ° C., corresponds to the previous purification temperature region T _Z if T _IN ≧ 400 ° C., and the reducing agent is added. Therefore, it is based on the reason that the substantial activation treatment has been performed and the catalyst is considered not to be activated or deteriorated, and that if T _IN <400 ° C., there is no deterioration because the catalyst temperature is low.

In this way, the sum of the timer count values calculated in each of the areas B, C, and D is calculated as _TΣB , _TΣC , T
_{と す る D.} And the sum of these _T Σ ₌ T ΣB + T ΣC
+ _TΣD .

Now, a method for determining the activation processing execution timing shown in FIG. 2 will be described. At the start of this flow _{T Σ, T ΣB, T ΣC} , the initial value of T _.SIGMA.D is 0. EC
U4 reads the catalyst inlet temperature T _IN in the first step 51, and determines whether or not T _IN ≧ T _H1 in the next step 52. If T _IN ≧ T _H1 (Y), the _routine proceeds to step 53, where T _IN
If <T _H1 (N), this flow ends. Step 53
Then, it is determined whether or not T _IN ≧ T _H2 . When T _IN ≧ TH ₂ (Y), the _routine proceeds to step 54, and when T _IN <TH ₂ (N), the _routine proceeds to step 55. In step 54, it is determined whether or not T _IN ≧ T _H3 . When T _IN ≧ _{TH 3} (Y), the _routine proceeds to step 57, and when T _IN < _{TH 3} (N), the _routine proceeds to step 56.

In step 55, _TΣB is calculated. That is, a value obtained by adding the control time t from the previous time to the present time as it is to the value of _TΣB at the time of the previous control is set as a new _TΣB . Similarly, in steps 56 and 57, _TΣC and _TΣD are calculated, respectively. In other words, T _ｔC and T _ΣD at the time of the previous control are _added to the control time t
The value obtained by adding the values obtained by multiplying 5 times and 10 times respectively to a new T
_{Let ΣC} and T _ΣD .

In steps 55, 56 or 57, _TΣB , T
_{When ΣC} , _TΣD is calculated, the _routine proceeds to step 58, where T
Calculate _Σ = _TΣB + _TΣC + _TΣD . Then it is determined whether _T Σ ≧ D ₁ in step 59. D ₁ is at a predetermined fixed time (index time), here are set 20 hr. And _T Σ ≧ D ₁ The activation process flag FLG _FK proceeds to step 60 when the (Y) 1 (ON), and starts the activation process to be described later, T _sigma in step _61, T ΣB, T
_ΣC , T _ΣD are set to the initial value 0, and this flow ends. When _TΣ <D ₁ (N) in step 59, this flow is immediately terminated.

By repeating this flow, the engine operation time (T _Σ ) is cumulatively calculated while weighting according to the exhaust gas temperature (T _IN ). Then, when this becomes a predetermined value (D ₁ ), the activation process is executed. That is the activation process for each predetermined engine operating time D ₁ is performed.

Next, a description will be given of a method of determining the activation processing execution time based on the vehicle traveling distance shown in FIG. Here EC
A trip meter of the vehicle is connected to U4, and a distance counter built in the ECU 4 is counted up every time the number of traveling distances of the trip meter increases. Therefore, first ECU4 performs count-up of the distance count value L _sigma in the first step 71, a predetermined fixed distance and the distance count value L _sigma in the next step 72 (index distance) is compared with D _2. Here D ₂
= 1000 km. If _LΣ <D ₂ (N), the flow is terminated, and if _LΣ ≧ D ₂ (Y), the process proceeds to step 73, where the activation process flag FLG _{FK is set} to 1 (ON), and the activation process described later is started. Thereafter, in step 74, _LΣ = 0, the distance count value _LΣ is returned to the initial value, and the present flow ends. Thus it becomes activation treatment is performed for each predetermined vehicle travel distance D _2.

Next, the specific contents of the activation process will be described with reference to the flowchart shown in FIG.

The start condition of this flow is the activation processing flag FL
G _FK is 1 (ON). This flow is also an interrupt process that is repeatedly executed at every predetermined control time t _FK . In a first step 81, the ECU 4 reads the catalyst inlet temperature T _IN , and in the next step 82, the T _IN becomes T _IN
L ≦ T _IN ≦ T _ON , or T _IN ≧ T _H1 (=
T _OFF ) is determined. When the temperature is not within the temperature range (N), the present flow is terminated. When the temperature is within the temperature range (Y), the routine proceeds to step 83, where the engine speed _NE is read. Thereafter, in step 84, the target sub-injection amount for the activation process is calculated from the partial FK in the sub-injection amount map shown in FIG. 6, and in step 85, the sub-injection corresponding to the amount is executed. Next, at step 86, the ECU
The timer count value T _{@ FK} by the built-in timer _No. 4 is counted up by the control time t _FK . In the next step 87, the timer count value T _{@ FK} is compared with a predetermined value (activation processing end time) T _{@ FKEND} . Where T
_ΣFKEND is set to about ₁₅ to 20 minutes. T _ΣFK <T
_{If ΣFKEND} (N), the flow ends, and T _ΣFK ≧ T
_{If ΣFKEND} (Y), the process proceeds to step 88, where T _ΣFK is reset to the initial value 0, and the activation processing flag FLG _{FK is set} to 0.
(OFF) and ends this flow.

Thus, T _L ≤T _IN ≤T _ON or T _IN
In the exhaust gas temperature range of ≧ T _H1 (= T _OFF ), the activation process can be performed for a fixed time ( _TΣFKEND ).

As described above, according to the present apparatus, the NOx catalyst can be actively activated at times other than during the NOx purification processing, so that the deterioration of the NOx catalyst is suppressed as much as possible, and the initial performance is reduced as much as possible. While maintaining for a long time,
An increase in NOx emission can be prevented. In particular, this device uses a once-degraded N as in the prior application (Japanese Patent Application No. 10-72335).
Instead of waiting for the activation of the Ox catalyst, measures are taken to prevent deterioration as much as possible before deterioration. Therefore, the initial performance can be maintained for a longer time than before, the active temperature region of the catalyst can be kept at the exhaust gas temperature region where purification is most desired for a long time, and the most effective usage can be performed. In addition, the progress of deterioration can be minimized.

As shown by the dashed line in FIG.
As the x catalyst 3 deteriorates, the purification rate also decreases. Therefore, the use in the deteriorated state increases the emission amount of NOx, HC and CO. However, in the present apparatus, the deterioration hardly proceeds, and such an increase is minimized.

When the NOx catalyst 3 deteriorates, its substantial active temperature range shifts from the initial T _Z to T _ZX . However, since the sub-injection (addition of the reducing agent) is performed at T _Z , a situation in which the sub-injection is performed in the exhaust gas temperature region ΔT on the low temperature side deviating from the active temperature region due to the deviation even though purification is not performed. As a result, the useless injection causes deterioration of HC and fuel efficiency. In the present device, such a shift in the active temperature range can be suppressed for a long time, and as a result, HC and fuel efficiency can be improved.

As described above, the embodiments of the present invention are not limited to those described above. For example, in the present embodiment, the activation process is performed in both the temperature range lower or higher than the purification temperature range T _Z , but may be performed in one of the low temperature range or the high temperature range. . Further, the present invention can be applied not only to a diesel engine but also to a lean burn gasoline engine. The sub-injection amount map may be switched following the deterioration and activation of the catalyst by combining the contents of the prior application with the present invention. In the present embodiment, regarding the determination of the activation processing execution timing, the engine operation time and the vehicle traveling distance are set in a parallel relationship, and the activation processing is executed when either one of the timings comes. Since the other may arrive immediately afterward, a predetermined time delay may be performed or the later one may be canceled in order to prevent unnecessary continuous activation processing due to this.

[0045]

The present invention exhibits the following excellent effects.

(1) The deterioration of the NOx catalyst can be suppressed and its initial performance can be maintained for a long time.

(2) An increase in NOx emission can be prevented.

[Brief description of the drawings]

FIG. 1 is a flowchart showing execution contents of an activation process according to an embodiment of the present invention.

FIG. 2 is a flowchart illustrating a method of determining activation processing execution time according to an embodiment of the present invention.

FIG. 3 is a flowchart illustrating a method of determining an activation processing execution time according to an embodiment of the present invention.

FIG. 4 is a flowchart showing a main flow of activation processing control according to the embodiment of the present invention.

FIG. 5 is a flowchart showing a NOx purification processing method according to the embodiment of the present invention.

FIG. 6 shows a sub-injection map used for NOx purification processing and activation processing.

FIG. 7 is a graph showing a relationship between a catalyst inlet temperature and a timer count value in activation processing execution timing determination.

FIG. 8 is a configuration diagram showing a NOx reduction device for an engine according to an embodiment of the present invention.

FIG. 9 is a graph showing the purification characteristics of a NOx catalyst.

[Explanation of symbols]

1 engine 3 NOx catalyst _TZ purification temperature range _NE engine speed

Claims (5)

[Claims] An NOx reduction device for an engine, wherein a NOx catalyst is provided in an exhaust passage of the engine, and a reducing agent is added to the exhaust gas within a predetermined exhaust gas temperature range to reduce the NOx in the exhaust gas. An engine NOx reduction device comprising an activation processing means for adding a reducing agent to exhaust gas and activating the NOx catalyst in a predetermined exhaust gas temperature range lower or higher than the exhaust gas temperature range. 2. The NOx reduction device for an engine according to claim 1, wherein said activation processing means executes said activation processing every predetermined engine operation time. 3. The activation processing means accumulates and calculates the engine operation time while performing weighting according to the exhaust gas temperature during the operation of the engine. When the calculated value reaches a predetermined value, the activation processing means performs the activation processing. The NOx reduction device for an engine according to claim 2, which is executed. 4. The NOx reduction device for an engine according to claim 1, wherein said activation processing means executes said activation processing for each predetermined vehicle traveling distance. 5. The activation processing means executes the addition of the reducing agent by fuel sub-injection into the engine cylinder, and determines the sub-injection amount for each engine speed based on a predetermined map. The NOx reduction device for an engine according to any one of claims 1 to 4.