JP3820990B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification apparatus for an internal combustion engine, and more particularly, traps NOx in exhaust when the air-fuel ratio of inflowing exhaust is lean, and reduces and purifies NOx trapped when the air-fuel ratio of inflowing exhaust is rich. It is related with what is provided with the NOx trap catalyst.
[0002]
[Prior art]
As a conventional technique, Japanese Patent No. 2600492 discloses trapping NOx in exhaust when the air-fuel ratio of inflowing exhaust is lean and trapping when the air-fuel ratio of inflowing exhaust is rich in the exhaust passage of the engine. A technology is disclosed in which a NOx trap catalyst for reducing and purifying NOx is disposed, and the exhaust air-fuel ratio is made rich at the reduction and purification timing of the NOx trap catalyst to purify NOx.
[0003]
[Problems to be solved by the invention]
In the above prior art, the reduction and purification of NOx by the NOx trap catalyst enriches the exhaust air-fuel ratio, that is, supplies HC and CO as reducing agents to the NOx trap catalyst, and these and NOx are in a reducing atmosphere. By reacting with
[0004]
However, oxygen may be present in the exhaust gas even when the exhaust air-fuel ratio is rich. If oxygen is present in the exhaust, the reaction between NOx and the reducing agent (HC, CO) in the reducing atmosphere will occur unless oxygen is consumed by the oxidation reaction of HC and CO to create a reducing atmosphere near the catalyst. Absent. For this reason, even in the same rich state (the same exhaust air-fuel ratio), the greater the amount of oxygen in the exhaust, the less the reduction reaction between HC, CO and NOx occurs.
[0005]
Therefore, when the NOx trap catalyst is in a poor active state, that is, when the carrier temperature of the NOx trap catalyst is low, if NO exists in the exhaust gas, the NOx trap catalyst cannot sufficiently purify NOx.
In other words, the above conventional technique does not consider the presence of oxygen in the exhaust gas in the rich state in which NOx is reduced and purified. Therefore, when the temperature of the NOx trap catalyst is low, NOx cannot be sufficiently purified and exhaust gas is deteriorated. There was a problem.
[0006]
An object of the present invention is to provide an exhaust emission control device for an internal combustion engine that can solve such a conventional problem.
[0007]
[Means for Solving the Problems]
For this reason, according to the first aspect of the present invention, the NOx in the exhaust gas is trapped when the air-fuel ratio of the exhaust gas flowing into the engine is lean, and trapped when the air-fuel ratio of the exhaust gas flowing in is rich. A NOx trap catalyst for reducing and purifying NOx; a reduction and purification timing determining means for determining a reduction and purification timing of the NOx trap catalyst; a first air-fuel ratio enrichment method for enriching the exhaust air-fuel ratio at the reduction and purification timing; It is possible to selectively switch between the second air-fuel ratio enrichment method that makes the oxygen amount in the exhaust gas smaller than the first air-fuel ratio enrichment method and the exhaust air-fuel ratio rich, and the temperature of the NOx trap catalyst is high. Air-fuel ratio enrichment means that sometimes selects the first air-fuel ratio enrichment method and selects the second air-fuel ratio enrichment method when the temperature of the NOx trap catalyst is low. It is characterized in.
[0008]
According to a second aspect of the present invention, there is provided target rich air-fuel ratio setting means for setting a target rich air-fuel ratio that makes the exhaust air-fuel ratio rich. In any of the air-fuel ratio enrichment method and the second air-fuel ratio enrichment method, the exhaust air-fuel ratio is set to the same target rich air-fuel ratio by the target rich air-fuel ratio setting means .
According to a third aspect of the present invention, the air-fuel ratio enriching means includes means for directly detecting the temperature of the NOx trap catalyst or estimating the temperature based on at least one of an exhaust temperature, a temperature in the vicinity of the catalyst, and an operating state of the engine. It is characterized by that.
[0009]
According to a fourth aspect of the present invention, there is provided an EGR valve disposed in an EGR passage that recirculates part of the exhaust gas from an exhaust passage of the engine to the intake passage, and an intake throttle valve disposed in the intake passage of the engine. When the first air-fuel ratio enrichment method is selected, the air-fuel ratio enrichment means enriches the exhaust air-fuel ratio at least with the EGR valve, and when the second air-fuel ratio enrichment method is selected, the intake throttle valve The exhaust air-fuel ratio is made rich.
[0010]
According to a fifth aspect of the present invention, the air-fuel ratio enrichment means includes a fuel injection device that enables post injection that injects a small amount of fuel after main injection, and an intake throttle valve that is disposed in an intake passage of the engine. When the first air-fuel ratio enrichment method is selected, the exhaust air-fuel ratio is enriched by at least post injection, and when the second air-fuel ratio enrichment method is selected, the exhaust air-fuel ratio is selected by the intake throttle valve. It is characterized by enriching.
[0011]
【The invention's effect】
According to the first aspect of the present invention, when the reduction purification by the rich spike of the NOx trap catalyst is performed, when the temperature of the NOx trap catalyst is low, the NOx purification rate is low. If there is a large amount, NOx may not be sufficiently reduced and purified. Therefore, by enriching the exhaust air / fuel ratio by an air / fuel ratio enrichment method that can reduce the amount of oxygen in the exhaust, even if the catalyst activity is low. The NOx purification performance can be maintained.
[0012]
According to the invention of claim 2, in any of the air-fuel ratio enrichment methods, by making the exhaust air-fuel ratio the same target rich air-fuel ratio by the same setting means, fluctuations in the exhaust air-fuel ratio can be prevented, and NOx, HC It is possible to suppress fluctuations in emissions such as CO.
According to the invention of claim 3, by providing the means for directly detecting or estimating the temperature of the NOx trap catalyst, the NOx purification rate depending on the temperature of the NOx trap catalyst can be accurately grasped.
[0013]
According to the invention of claim 4, when the exhaust air-fuel ratio is enriched while performing EGR when performing a normal rich spike, when reducing the oxygen amount in the exhaust during the rich spike, the EGR is stopped. By enriching the exhaust air-fuel ratio with the intake throttle valve, it is possible to burn all the oxygen that should be consumed by combustion, and to reliably reduce the amount of oxygen in the exhaust.
[0014]
According to the invention of claim 5, when the exhaust air-fuel ratio is enriched by post-injection when performing normal rich spike, post-injection is stopped or reduced when reducing the amount of oxygen in the exhaust during rich spike. Thus, the amount of oxygen in the exhaust can be reliably reduced by enriching the exhaust air-fuel ratio with the intake throttle valve.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a system diagram of an internal combustion engine (here, a diesel engine) showing an embodiment of the present invention.
The intake passage 2 of the diesel engine 1 is provided with an intake compressor of a variable nozzle type turbocharger 3. The intake air is supercharged by the intake compressor, cooled by the intercooler 4, passed through the intake throttle valve 5, and then the collector. 6 and then flows into the combustion chamber of each cylinder. The fuel is increased in pressure by the common rail type fuel injection device, that is, by the high pressure fuel pump 7, sent to the common rail 8, and directly injected from the fuel injection valve 9 of each cylinder into the combustion chamber. The air that has flowed into the combustion chamber and the injected fuel are combusted by compression ignition, and the exhaust gas flows out to the exhaust passage 10.
[0016]
Part of the exhaust gas flowing into the exhaust passage 10 is recirculated to the intake side via the EGR valve 12 through the EGR passage 11 as EGR gas. The remainder of the exhaust passes through the exhaust turbine of the variable nozzle type turbocharger 3 and drives it.
Here, downstream of the exhaust turbine in the exhaust passage 10, for exhaust purification, NOx in the exhaust is trapped when the air-fuel ratio of the inflowing exhaust is lean, and trapped when the air-fuel ratio of the inflowing exhaust is rich A NOx trap catalyst 13 for reducing and purifying NOx is disposed. Further, the NOx trap catalyst 13 carries a noble metal and has a function of oxidizing HC and CO in the exhaust gas, and serves as a NOx trap catalyst with an oxidation function.
[0017]
In order to control the engine 1, the control unit 20 includes a rotation speed sensor 21 for detecting the engine rotation speed Ne, an accelerator opening sensor 22 for detecting the accelerator opening APO, and an air flow meter 23 for detecting the intake air amount Qa. A signal is being input.
Further, a catalyst temperature sensor 24 for detecting the temperature (catalyst temperature) Tc of the NOx trap catalyst 13 and an air / fuel ratio sensor 25 for detecting the exhaust air / fuel ratio on the outlet side of the NOx trap catalyst 13 in the exhaust passage 10 are provided. A signal is also input to the control unit 20. However, the temperature of the NOx trap catalyst 13 may be detected (estimated) indirectly based on the signal by providing an exhaust temperature sensor on the outlet side of the NOx trap catalyst 13, or in the vicinity of other catalysts. It may be estimated from the temperature or estimated from the operating state of the engine.
[0018]
Based on these input signals, the control unit 20 controls the fuel injection amount and the injection timing of the main injection by the fuel injection valve 9 and the post injection performed after the main injection (expansion stroke or exhaust stroke) under predetermined operating conditions. The fuel injection command signal to the fuel injection valve 9, the opening command signal to the intake throttle valve 5, the opening command signal to the EGR valve 12, etc. are output.
[0019]
Here, the control unit 20 performs exhaust purification control for reducing and purifying NOx trapped and accumulated in the NOx trap catalyst 13, and the exhaust purification control will be described in detail below.
2 to 3 are flowcharts of exhaust purification control executed by the control unit 20.
[0020]
In S1-1, the engine speed Ne, the accelerator opening APO, the intake air amount Qa, and the catalyst temperature Tc are detected based on signals from the rotation speed sensor, accelerator opening sensor, air flow meter, and catalyst temperature sensor.
In S1-2, the fuel injection amount Qf for main injection is calculated by referring to a map using the engine speed Ne and the accelerator opening APO as parameters.
[0021]
In S1-3, the NOx deposition amount trapped and deposited on the NOx trap catalyst is detected. However, since it is difficult to directly detect the NOx accumulation amount, the NOx generation amount per unit time is predicted from the engine speed Ne and the fuel injection amount Qf, and the NOx accumulation amount per unit time is considered in consideration of the trap rate. It is detected indirectly by calculating and accumulating these. Alternatively, the NOx accumulation amount may be estimated from the integrated value of the engine speed.
[0022]
In S1-4, it is determined whether or not the reg1 flag indicating that the rich spike mode is active when the catalyst is active is set. When the reg1 flag = 1, the control proceeds to the rich spike mode control when the catalyst is active after S2-1 (FIG. 3).
In S1-5, it is determined whether or not the reg2 flag indicating that the rich spike mode when the catalyst activity is low is set. When the reg2 flag = 1, the control proceeds to the rich spike mode control when the catalyst activity is low after S3-1 (FIG. 3).
[0023]
In S1-6, it is determined whether or not the NOx accumulation amount detected in S1-3 is greater than a predetermined value NOx1 in order to determine whether or not it is the NOx trap catalyst regeneration timing (reduction purification timing).
If the NOx accumulation amount is equal to or less than the predetermined value NOx1, it is not the regeneration time, so the process is terminated. If the NOx accumulation amount exceeds the predetermined value NOx1, it is determined as the regeneration time, and the process proceeds to S1-7.
[0024]
In S1-7, the activity of the NOx trap catalyst is determined. Here, as shown in FIG. 4, the NOx purification performance of the catalyst starts to develop from the light-off temperature T2, but is not yet sufficient. Therefore, if the NOx purification rate becomes higher than the temperature T1 at which the NOx purification rate becomes sufficiently high, the catalyst is active. Judge. It is determined that the activity is low between T1 and T2, and it is determined that there is no activity at a temperature lower than T2.
[0025]
Therefore, in S1-7, it is determined whether or not the catalyst temperature Tc exceeds T1, and when Tc> T1, it is determined in S1-8 that the catalyst is active, In order to enter the rich spike mode, the reg1 flag is set to 1.
In the case of Tc ≦ T1, it is determined in S1-9 whether or not the catalyst temperature Tc exceeds T2, that is, whether or not the catalyst temperature Tc is between T1 and T2, and if Tc> T2, In S1-10, it is determined that the activity is low, and the reg2 flag is set to 1 to enter the rich spike mode when the catalyst activity is low.
[0026]
In the case of Tc ≦ T2, since there is no activity and no purification performance can be expected, the process is terminated by waiting for the regeneration process until it is warmed up.
Next, the rich spike mode at the time of catalyst activation after S2-1 when the reg1 flag = 1 is described.
In S2-1, the first air-fuel ratio enrichment method for enriching the exhaust air-fuel ratio is selected to enrich the exhaust air-fuel ratio.
[0027]
Specifically, the first air-fuel ratio enrichment method is one of the following (1) to (3).
(1) If the operating condition is to perform EGR, the target λ at the time of rich spike is set to a predetermined value λ1 while the EGR is continued, and the intake throttle valve control (control to the opening decrease side) 5 to achieve the target λ = λ1 (described in the flow). As for the error, feedback control is performed based on the signal from the air-fuel ratio sensor on the catalyst outlet side.
[0028]
(2) Enrichment is achieved by supplying fuel that does not contribute to combustion by post injection. In this case, the target λ = λ1 is achieved by controlling the post injection amount for the rich spike shown in FIG.
(3) The target λ = λ1 is achieved by enriching with EGR, intake throttle and post-injection.
[0029]
In S2-2, it is determined whether or not the elapsed time (rich spike time) t from the start of the rich spike has exceeded a predetermined time t1, and if so, it is considered that the regeneration of the catalyst has been completed. The accumulated value of the NOx accumulation amount is cleared at 3 and the reg1 flag is set to 0 at S2-4.
Next, the rich spike mode when the catalyst activity after S3-1 when the reg2 flag is 1 is low will be described.
[0030]
In S3-1, the exhaust air / fuel ratio is enriched by selecting the second air / fuel ratio enrichment method that makes the amount of oxygen in the exhaust gas smaller than the first air / fuel ratio enrichment method and also makes the exhaust air / fuel ratio rich.
The second air-fuel ratio enrichment method is based on the following (1) to (3) corresponding to (1) to (3) in the first air-fuel ratio enrichment method.
[0031]
(1) When the normal rich spike is caused by EGR and the intake throttle, EGR is stopped, the target λ at the time of the rich spike is set to a predetermined value λ1, and the EGR shown in FIG. 6 is performed by controlling the intake throttle valve The target λ = λ1 is achieved by controlling the target intake air amount when there is not (described in the flow). As for the error, feedback control is performed based on the signal from the air-fuel ratio sensor on the catalyst outlet side.
[0032]
(2) When the normal rich spike is caused by post injection, the opening of the intake throttle valve is decreased, and the post injection amount is decreased by that amount, thereby achieving the target λ = λ1.
(3) When the normal rich spike is caused by EGR, intake throttle and post-injection, control is performed by intake throttle only (no EGR, no post-injection) or intake throttle and post-injection amount reduction (no EGR), and the target λ = Λ1 is achieved.
[0033]
In any case, by setting the target λ at the time of rich spike to the same value as λ1 set in S2-1 even if the residual oxygen amount is reduced, the emission deteriorates due to excessive spikes, and the spike becomes shallower. It is possible to suppress a decrease in NOx purification performance.
In S3-2, it is determined whether or not the elapsed time (rich spike time) t from the start of the rich spike has exceeded a predetermined time t2, and if so, it is considered that the regeneration of the catalyst has been completed. The accumulated value of the NOx accumulation amount is cleared at 3 and the reg2 flag is set to 0 at S3-4. In addition, in the air-fuel ratio enrichment after stopping the EGR, it takes time for the EGR gas to escape immediately after the EGR is stopped. Therefore, by setting t2> t1, a rich gas with less oxygen is sufficiently supplied to the catalyst. The reproduction effect by the rich spike can be sufficiently obtained.
[0034]
The above control will be described in more detail.
As shown in FIG. 7, when the rich operation is realized in the diesel engine, the combustion becomes slightly unstable when the EGR is performed, and the residual oxygen in the exhaust gas with the same exhaust λ as compared with the condition where the EGR is not performed. The amount increases (the more oxygen, the more CO and HC emissions). Further, when the exhaust λ is made rich by adding fuel that does not contribute to combustion by controlling post injection, the amount of residual oxygen increases as in the case of performing the EGR.
[0035]
Here, as shown in FIG. 8, in the state where the exhaust λ <1 (rich) and no oxygen, NOx can be purified by reacting with the reducing components (HC, CO) in the exhaust in the NOx trap catalyst. However, as shown in FIG. 9, when oxygen is present even if the exhaust λ <1 (rich), first, oxygen is consumed to create a reducing atmosphere, and NOx is purified in the reducing atmosphere. In particular, when the exhaust gas λ <1 and oxygen remains and the activity is low because the catalyst temperature is low, the reaction time is sufficient, but since the activity of the catalyst is low, most of the time is spent only on the oxidation reaction. As a result, NOx purification in a reducing atmosphere cannot be expected.
[0036]
Therefore, when the temperature of the catalyst is low, since the oxidation activity is low when the oxygen remaining amount is large, most of the usable reaction time must be consumed for oxygen consumption. Therefore, NOx can be reduced by reducing the amount of oxygen remaining in the exhaust gas. To prevent it from being purified.
For this reason, when it is determined from the catalyst temperature that the catalyst activity (NOx purification rate) is low, the air-fuel ratio is enriched by a method capable of reducing the amount of oxygen in the exhaust gas.
[0037]
Supplementing the air-fuel ratio enrichment method, the normal rich spike, that is, the rich spike when there is no need to reduce the oxygen amount (first air-fuel ratio enrichment method) is at least one of EGR, intake throttle valve, and post injection. Use one.
When using EGR, when performing a normal rich spike, the EGR valve opening is increased so as to increase the EGR ratio, and the intake throttle valve opening is decreased to reduce the excess air ratio.
[0038]
By the way, since the distribution of the EGR gas sucked into the cylinder (oxygen concentration in the cylinder) is not uniform, when the injected fuel burns, the fuel that exists in the region where the EGR gas concentration is high (low oxygen concentration) Is difficult to burn and is discharged as HC. That is, since the fuel to be combusted does not combust, the oxygen to be consumed by the combustion is discharged as it is without being consumed.
[0039]
Therefore, when it is desired to reduce the amount of oxygen during the rich spike (in the second air-fuel ratio enrichment method), by realizing the target excess air ratio without EGR, all the oxygen that should be consumed by combustion is burned. As a result, the amount of oxygen in the exhaust gas can be reduced.
That is, when the rich spike when there is no need to reduce the oxygen amount is performed by the EGR and the intake throttle, the rich spike when reducing the oxygen amount is performed only by the intake throttle (no EGR).
[0040]
Further, when the rich spike when there is no need to reduce the oxygen amount is performed only by the post injection (FIG. 10 shows a characteristic diagram of the post injection amount for the rich spike), the rich spike when the oxygen amount is reduced. This can be realized by reducing the intake air amount by reducing the intake throttle valve opening to reduce the amount of brew in the exhaust, and thereby reducing the post injection amount by the amount that the excess air ratio decreases.
[0041]
That is, when controlling the exhaust λ by controlling the injection amount such as post injection (by increasing the unburned fuel), the exhaust λ control by controlling the air amount is changed from the exhaust λ control by controlling the unburned fuel. By switching to, it becomes possible to reduce the amount of residual oxygen in the exhaust.
Furthermore, when the rich spike when there is no need to reduce the oxygen amount is performed by EGR, intake throttle and post injection, is the rich spike when reducing the oxygen amount only the intake throttle (no EGR, no post injection)? Alternatively, it can be performed by reducing the intake throttle and the post injection amount (the reduction amount depends on the intake throttle amount).
[0042]
In the present embodiment, the portion S1-6 in FIG. 2 corresponds to the reduction purification timing determination means, and the portions S1-7 to S1-10 in FIG. 2 and S2-1 and S3-1 in FIG. This corresponds to air-fuel ratio enrichment means including target rich air-fuel ratio setting means.
[Brief description of the drawings]
FIG. 1 is a system diagram of an engine showing an embodiment of the present invention. FIG. 2 is a flowchart of exhaust purification control (part 1).
FIG. 3 is a flowchart of exhaust purification control (part 2).
FIG. 4 is a diagram showing the relationship between temperature and catalyst activity. FIG. 5 is a diagram showing a target intake air amount during rich spike. FIG. 6 is a diagram showing a target intake air amount during rich spike without EGR. 7] Fig. 8 is a graph showing changes in components in exhaust with and without EGR. Fig. 8 is a diagram showing reactions in the absence of oxygen at exhaust λ <1. Fig. 9 is a reaction in the presence of oxygen at exhaust λ <1. Figure [Figure 10] Figure showing the post injection amount during rich spike [Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Engine 2 Intake passage 5 Intake throttle valve 8 Common rail 9 Fuel injection valve 10 Exhaust passage 11 EGR passage 12 EGR valve 13 NOx trap catalyst 20 Control unit 21 Rotational speed sensor 22 Acceleration opening degree sensor 23 Air flow meter 24 Catalyst temperature sensor 25 Air-fuel ratio Sensor

Claims (5)

An NOx trap catalyst that is disposed in the exhaust passage of the engine and traps NOx in the exhaust when the air-fuel ratio of the inflowing exhaust is lean and reduces and purifies NOx trapped when the air-fuel ratio of the inflowing exhaust is rich;
Reduction purification timing determination means for determining the reduction purification timing of the NOx trap catalyst;
A first air-fuel ratio enrichment method for enriching the exhaust air-fuel ratio at the reduction purification time, and a second air-fuel ratio enrichment for reducing the amount of oxygen in the exhaust gas as compared to the first air-fuel ratio enrichment method and enriching the exhaust air-fuel ratio. The first air-fuel ratio enrichment method is selected when the temperature of the NOx trap catalyst is high, and the second air-fuel ratio enrichment method is selected when the temperature of the NOx trap catalyst is low. An air-fuel ratio enrichment means for selecting a fuel ratio enrichment method;
An exhaust emission control device for an internal combustion engine, comprising: A target rich air-fuel ratio setting means for setting a target rich air-fuel ratio that makes the exhaust air-fuel ratio rich;
When the exhaust air-fuel ratio is made rich, the air-fuel ratio enrichment means sets the exhaust air-fuel ratio to the target rich air-fuel ratio setting means in both the first air-fuel ratio enrichment method and the second air-fuel ratio enrichment method. 2. The exhaust emission control device for an internal combustion engine according to claim 1, wherein the same target rich air-fuel ratio is set. The air-fuel ratio enriching means includes means for directly detecting the temperature of the NOx trap catalyst or estimating the temperature based on at least one of an exhaust temperature, a temperature in the vicinity of the catalyst, and an operating state of the engine. The exhaust emission control device for an internal combustion engine according to claim 1 or 2. An EGR valve disposed in an EGR passage that recirculates part of the exhaust from the exhaust passage of the engine to the intake passage, and an intake throttle valve disposed in the intake passage of the engine,
The air-fuel ratio enrichment means enriches the exhaust air-fuel ratio with at least an EGR valve when the first air-fuel ratio enrichment method is selected, and selects the intake air when the second air-fuel ratio enrichment method is selected. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the exhaust air-fuel ratio is enriched by a throttle valve. A fuel injection device that enables post injection that injects a small amount of fuel after the main injection, and an intake throttle valve that is arranged in the intake passage of the engine,
The air-fuel ratio enrichment means enriches the exhaust air-fuel ratio by at least post injection when the first air-fuel ratio enrichment method is selected, and selects the intake air when the second air-fuel ratio enrichment method is selected. The exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the exhaust air-fuel ratio is enriched by a throttle valve.

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