JP4682829B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

When NOx stored in the NOx storage reduction catalyst is reduced, the NOx storage reduction
So-called rich spike control is executed in which the air-fuel ratio of the exhaust gas flowing into the catalyst is made rich in a spike manner (short time) in a relatively short cycle. If the amount of reducing agent added to the exhaust gas during the rich spike control is too large, the amount of HC adhering to the NOx storage reduction catalyst increases and so-called HC poisoning occurs. Further, if the amount of the reducing agent added to the exhaust gas is too small, NOx stored in the NOx storage reduction catalyst is not sufficiently reduced. Furthermore, since a certain amount of time is required until the reduction reaction of NOx stored in the NOx storage reduction catalyst is completed, if the time during which the rich state is continued (hereinafter referred to as rich duration time) is too short. Further, NOx stored in the NOx storage reduction catalyst is not sufficiently reduced. On the other hand, if the rich duration is too long, excess HC adheres to the NOx storage reduction catalyst, causing HC poisoning,
C may pass through the NOx storage reduction catalyst.

In contrast, when reducing NOx occluded in the NOx storage reduction catalyst, a technique is performed in which a reducing agent is added a plurality of times during one rich spike, and the start timing of each reducing agent addition is synchronized with the crank angle. It has been known. (For example, refer to Patent Document 1). According to this technique, the reducing agent can be put on the flow of exhaust gas, so that the reducing agent can quickly reach the NOx storage reduction catalyst.
JP 2002-106332 A Japanese Patent Laid-Open No. 11-62666

However, since the reducing agent is added based on the crank angle, the addition interval until the next reducing agent addition becomes shorter as the reducing agent addition amount increases. In this case, if the amount of addition of the reducing agent is large, HC poisoning may not be recovered, and the NOx purification rate may be reduced.

  The present invention has been made in view of the above-described problems, and in an exhaust gas purification apparatus for an internal combustion engine, a more appropriate addition interval when adding a reducing agent a plurality of times during one rich spike. It is an object to provide a technique capable of setting the above.

In order to achieve the above object, an exhaust gas purification apparatus for an internal combustion engine according to the present invention employs the following means. That is, the exhaust gas purification apparatus for an internal combustion engine according to the present invention is
Reducing agent addition means for adding a reducing agent into the exhaust gas discharged from the internal combustion engine;
A NOx storage reduction catalyst in which NOx is stored and NOx stored is reduced by the reducing agent added by the reducing agent adding means;
With
When reducing the NOx occluded in the NOx storage reduction catalyst to the target air-fuel ratio for reducing the exhaust air-fuel ratio, an addition period that is a period during which the reducing agent is added during the target air-fuel ratio is set. A plurality is provided, and an addition interval between the end of each addition period and the start of the next addition period is determined in accordance with the amount of reducing agent added in the immediately preceding addition period.

Here, NOx contained in the exhaust gas is stored in the NOx storage reduction catalyst, and then the reducing agent is added to the exhaust gas by the reducing agent adding means to reduce the NOx by setting the air fuel ratio of the exhaust gas to the target air fuel ratio. be able to. The “target air-fuel ratio” is an air-fuel ratio of exhaust gas that is required for reducing NOx stored in the NOx storage reduction catalyst, and may be, for example, near or below stoichiometric. By maintaining this target air-fuel ratio, NOx stored in the NOx storage reduction catalyst is reduced.

However, when a reducing agent is added, the reducing agent may adhere to the NOx storage reduction catalyst and HC poisoning may occur. This HC poisoning can be eliminated by supplying oxygen. In addition, when reducing agent is added multiple times with one rich spike, oxygen is supplied to the NOx storage reduction catalyst from when the reducing agent is added until the next reducing agent is added. Can do. As described above, since the reducing agent and oxygen are alternately supplied so that appropriate oxygen is contained in the exhaust gas, the air-fuel ratio of the exhaust gas is reduced to the target air amount while suppressing excessive richness. The fuel ratio can be set. Therefore, the NOx purification rate in the NOx storage reduction catalyst can be improved. Then, by changing the addition interval according to the immediately preceding reducing agent addition amount, it becomes possible to supply the reducing agent and oxygen to the NOx storage reduction catalyst in a well-balanced manner. That is, as the reducing agent addition amount per time increases, the HC poisoning can be recovered by supplying oxygen by increasing the reducing agent addition interval, so the NOx purification rate in the NOx storage reduction catalyst can be increased. Can be improved.

  In the present invention, the addition interval can be determined as a time for recovering the HC poisoning of the NOx storage reduction catalyst caused by the immediately preceding addition of the reducing agent.

The amount of HC poisoning, which is the amount of HC adhering to the NOx storage reduction catalyst, has a correlation with the amount of reducing agent added from the reducing agent addition means. Therefore, the amount of oxygen necessary for eliminating HC poisoning is also correlated with the amount of reducing agent added. Therefore, even when HC poisoning occurs due to the addition amount in each reducing agent addition, the HC poisoning can be recovered by providing the addition interval according to the HC poisoning amount. That is, the NOx purification rate can be improved by providing an addition interval necessary to recover HC poisoning.

In order to achieve the above object, an exhaust gas purification apparatus for an internal combustion engine according to the present invention may employ the following means. That is, the exhaust gas purification apparatus for an internal combustion engine according to the present invention is
Reducing agent addition means for adding a reducing agent into the exhaust;
A NOx storage reduction catalyst in which NOx is stored and NOx stored is reduced by the reducing agent added by the reducing agent adding means;
When reducing the NOx occluded in the NOx storage reduction catalyst to the target air-fuel ratio for reducing the exhaust air-fuel ratio, an addition period that is a period during which the reducing agent is added during the target air-fuel ratio is set. A plurality of addition intervals between the end of each addition period and the start of the next addition period, when the flow rate of exhaust gas is larger than a predetermined amount and the temperature of the NOx storage reduction catalyst is higher than a predetermined temperature An addition interval switching means that is determined according to the reducing agent addition amount in the immediately preceding addition period, and that is determined according to the crank angle when the flow rate of the exhaust gas is a predetermined amount or less or the temperature of the NOx catalyst is a predetermined temperature or less;
It is good also as providing.

When the exhaust gas flow rate is less than the specified amount or the temperature of the NOx storage reduction catalyst is less than the specified temperature, the atomization of the reducing agent is not promoted. It becomes difficult to significantly improve the rate. On the other hand, by adding the reducing agent according to the exhaust stroke (that is, adding the reducing agent according to the crank angle), the reducing agent can be put on the exhaust flow, so that the NOx storage reduction catalyst can be promptly used. A reducing agent can be supplied. Moreover, since the diffusion of the reducing agent can be suppressed, occlusion reduction type N
The controllability of the exhaust air-fuel ratio in the Ox catalyst can be improved.

The addition interval switching means sets the addition interval according to the reducing agent addition amount when the NOx purification rate can be improved, and sets the addition interval according to the crank angle otherwise. The “when the flow rate of exhaust gas is larger than a predetermined amount and the temperature of the NOx storage reduction catalyst is higher than a predetermined temperature” means that the addition interval is determined according to the amount of reducing agent rather than being determined according to the crank angle. It may be a time when the NOx purification rate can be improved if it is determined. That is, the “predetermined amount” and “predetermined temperature” are the maximum of the flow rate of exhaust gas and the temperature of the NOx storage reduction catalyst when the NOx purification rate cannot be improved even if the addition interval is set according to the reducing agent addition amount. Can be a value.

  In this way, when the NOx purification rate can be improved, the NOx purification rate is improved, and when the NOx purification rate cannot be improved, the controllability of the exhaust air-fuel ratio in the NOx storage reduction catalyst is improved. Can be made.

  According to the exhaust gas purification apparatus for an internal combustion engine according to the present invention, a more appropriate addition interval can be set when a reducing agent is added a plurality of times during one rich spike.

  Hereinafter, specific embodiments of an exhaust emission control device for an internal combustion engine according to the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine 1 to which an exhaust gas purification apparatus for an internal combustion engine according to this embodiment is applied and an intake / exhaust system thereof. The internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine.

  The internal combustion engine 1 is provided with a fuel injection valve 11 that supplies fuel into the cylinder of the internal combustion engine. Further, an exhaust passage 2 leading to the combustion chamber is connected to the internal combustion engine 1. This exhaust passage 2 communicates with the atmosphere downstream.

An occlusion reduction type NOx catalyst 3 (hereinafter referred to as NOx catalyst 3) is provided in the middle of the exhaust passage 2. The NOx catalyst 3 stores NOx in the exhaust when the oxygen concentration of the inflowing exhaust gas is high, and the NOx that has been stored when the oxygen concentration of the inflowing exhaust gas is low and a reducing agent is present.
Has the function of reducing

An air-fuel ratio sensor 4 that outputs a signal corresponding to the air-fuel ratio of the exhaust gas flowing in the exhaust passage 2 is attached to the exhaust passage 2 downstream of the NOx catalyst 3. An exhaust temperature sensor 5 that outputs a signal corresponding to the temperature of the exhaust gas flowing through the exhaust passage 2 is attached to the exhaust passage 2 upstream of the NOx catalyst 3. The exhaust temperature sensor 5 detects the temperature of the NOx catalyst 3.

The exhaust passage 2 upstream of the NOx catalyst 3 is provided with a fuel addition valve 6 for adding fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust passage 2. The fuel addition valve 6 is opened by a signal from the ECU 7 described later and injects fuel into the exhaust. The fuel injected from the fuel addition valve 6 into the exhaust passage 2 makes the air-fuel ratio of the exhaust flowing from the upstream of the exhaust passage 2 rich. At the time of NOx reduction, so-called rich spike control is performed in which the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 3 is made rich in a spike (short time) with a relatively short cycle. In this embodiment, the fuel addition valve 6 corresponds to the reducing agent addition means in the present invention.

  Furthermore, an intake passage 8 that leads to the combustion chamber is connected to the internal combustion engine 1. An air flow meter 9 for outputting a signal corresponding to the amount of air flowing through the intake passage 8 is provided in the intake passage 8. The air flow meter 9 detects the intake air amount of the internal combustion engine 1. In this embodiment, the intake air amount detected by the air flow meter 9 is assumed to be equal to the exhaust amount.

  The internal combustion engine 1 configured as described above is provided with an ECU 7 that is an electronic control unit for controlling the internal combustion engine 1. The ECU 7 is a unit that controls the operation state of the internal combustion engine 1 in accordance with the operation conditions of the internal combustion engine 1 and the request of the driver. An air-fuel ratio sensor 4, an exhaust gas temperature sensor 5, and an air flow meter 9 are connected to the ECU 7 via electric wiring, and these output signals are input to the ECU 7. On the other hand, the fuel injection valve 11 and the fuel addition valve 6 are connected to the ECU 7 via electric wiring, and the fuel injection valve 11 and the fuel addition valve 6 are controlled by the ECU 7.

  Here, FIG. 2 is a time chart showing the waveform of the command signal of the ECU 7 sent to the fuel addition valve 6 and the change of the air-fuel ratio corresponding to the waveform on the same time axis. FIG. 2A is a time chart showing the transition of the command signal of the ECU 7, and FIG. 2B is a time chart showing the transition of the air-fuel ratio.

The fuel addition valve 6 is opened when the command signal shown in FIG. 2A is on (“ON”), and fuel is injected. By performing the fuel addition, the air-fuel ratio of the exhaust gas flowing into the NOx catalyst 3 becomes low (a rich spike is formed). Here, the longer the addition period (see FIG. 2A) and the shorter the addition interval (see FIG. 2A), the more the change in the air-fuel ratio (see FIG. 2B). Will grow. Further, the longer the total addition period (see FIG. 2A), the longer the rich spike formation period (see FIG. 2B). On the other hand, the length of the fuel addition suspension period (see FIG. 2A) is the length of the period in which the lean atmosphere continues (see FIG. 2B) between the continuously formed rich spikes. Corresponding to Note that a period obtained by adding the addition period and the subsequent addition interval, that is, a period from the start of the addition period to the start of the next addition period is defined as the addition interval.

Next, FIG. 3 is a diagram showing an addition interval when fuel is added from the fuel addition valve 6 in accordance with the start of the exhaust stroke of the fourth cylinder. That is, the addition interval in the case of adding fuel in synchronization with the crank angle is shown. FIG. 3A shows a case where the fuel addition amount is relatively small, and FIG. 3B shows a case where the fuel addition amount is relatively large. In either case of FIG. 3 (A) or (B), the addition interval is the same because the fuel addition is performed at the start of the exhaust stroke. However, when the amount of fuel added is relatively small, the addition interval becomes longer than the amount of fuel added, so there is a risk that the NOx catalyst 3 will have oxygen excess. Further, when the fuel addition amount is relatively large, the addition interval becomes shorter than the fuel addition amount. If HC is excessively supplied to the NOx catalyst 3, HC poisoning occurs. Therefore, in any case, the NOx purification rate may not be improved.

On the other hand, FIG. 4 is a diagram showing the addition interval when the addition interval is set according to the fuel addition amount from the fuel addition valve 6. 4A shows a case where the same amount of fuel as in FIG. 3A is added, and FIG. 4B shows a case where the same amount of fuel as in FIG. 3B is added. In FIG. 4, the addition interval is changed according to the fuel addition amount (may be a fuel addition period). That is, the longer the addition period, the longer the addition interval. Thereby, oxygen and fuel can be supplied appropriately, so that the NOx purification rate can be improved.
Therefore, in this embodiment, the addition interval is changed according to the fuel addition amount (may be a fuel addition period).

The relationship between the addition period and the addition interval or the addition interval is obtained in advance through experiments or the like and is mapped and stored in the ECU 7. For example, the ratio between the addition period and the addition interval or the addition interval is set to a constant value. This relationship may be determined such that the amount of HC poisoning of the NOx catalyst 3 is estimated and the amount is less than the allowable value. Further, since the HC poisoning amount is affected by the fuel addition amount, the exhaust gas flow rate, and the bed temperature of the NOx catalyst 3, a map using these as parameters may be used. That is, the relationship between the fuel addition amount, the exhaust gas flow rate, the bed temperature of the NOx catalyst 3 and the addition interval or addition interval may be obtained in advance and mapped. The flow rate of the exhaust gas may be equal to the intake air amount, and the bed temperature of the NOx catalyst 3 may be estimated from the temperature of the exhaust gas flowing into the NOx catalyst 3.

In addition, when the addition interval is determined so that the ratio between the addition amount and the addition interval becomes a predetermined value, the HC poisoning of the NOx catalyst 3 may increase. For this reason, a lower limit value of the addition interval may be provided so that the addition interval does not become shorter than this lower limit value.

FIG. 5 is a graph showing the relationship between the addition interval, the NOx purification rate, and the outgas average HC. The engine speed is 1800 r. p. m. The respective values when the engine generated torque is 80 Nm are shown. A solid line indicates a case where fuel is added four times per rich spike, and a broken line indicates a case where fuel is added three times per rich spike. In the solid line and the broken line, the amount of fuel added per rich spike is the same, but differs by the addition interval. The output gas average HC is the average concentration (ppm) of HC flowing out from the NOx catalyst 3.

If the addition interval is lengthened, the amount of oxygen supplied to the NOx catalyst 3 increases, so the average output gas H
C becomes lower. However, there is a peak in the NOx purification rate, and if the addition interval is extended beyond the peak, the NOx purification rate decreases. That is, if the addition interval is shorter than the addition interval at this peak, the HC poisoning does not recover, and if it is longer, the time during which the NOx catalyst 3 is in the oxidizing atmosphere becomes longer, so the NOx purification rate decreases in any case. Therefore, for example, in order to give the highest priority to the NOx purification rate, the addition interval at which the NOx purification rate is maximized is set. Here, if the addition interval is determined so that the air-fuel ratio of the atmosphere of the NOx catalyst 3 (or the air-fuel ratio of the exhaust gas downstream of the NOx catalyst 3) can maintain the stoichiometry, the NOx purification rate can be maximized. .
This is because if an addition interval for recovering the HC poisoning of the NOx catalyst 3 is provided, such an air-fuel ratio is obtained.

  FIG. 6 is a diagram showing an addition interval when a plurality of fuel additions with different addition amounts are performed during one rich spike. For example, during NOx reduction, in order to quickly remove oxygen stored in the NOx catalyst 3, a large amount of fuel may be added at the initial stage of fuel addition. Even in such a case, the addition intervals (1b), (2b), and (3b) are determined according to the fuel addition amounts (1a), (2a), and (3a), respectively. The addition interval at this time can also be obtained in the same manner as in the case where all the addition amounts are the same. As described above, even when the fuel addition amount is different every time, the oxygen amount and the fuel amount can be optimized by setting the addition interval or the addition interval according to the addition amount each time. Become.

As described above, according to this embodiment, the addition interval or addition interval for recovering the HC poisoning of the NOx catalyst 3 is determined according to the fuel addition amount or the addition period, so that the NOx purification rate is improved. Can be made.

When the flow rate of the exhaust is too small or the temperature of the NOx catalyst 3 is too low, a large amount of fuel added from the fuel addition valve 6 adheres to the exhaust passage 2 and the NOx catalyst 3. Therefore, it is difficult to significantly improve the NOx purification rate even if the addition interval is changed in accordance with the fuel addition amount. On the other hand, for example, by adding fuel in accordance with the exhaust stroke of the fourth cylinder (that is, adding fuel according to the crank angle), the fuel can be put on the flow of exhaust, so that the NOx catalyst 3 can be promptly applied. Fuel can be supplied. Since the diffusion of the fuel in the exhaust gas can be suppressed, the controllability of the air-fuel ratio of the exhaust gas in the NOx catalyst 3 can be improved.

Therefore, in this embodiment, when the NOx purification rate can be improved, the addition interval corresponding to the fuel addition amount is set for the purpose of improving the NOx purification rate. On the other hand, if the NOx purification rate cannot be improved, fuel addition is performed in synchronization with the crank angle in order to improve the controllability of the air-fuel ratio of the exhaust gas in the NOx catalyst 3.

  FIG. 7 is a flowchart showing a flow of fuel addition control in the present embodiment. This routine is repeatedly executed when the NOx stored in the NOx catalyst 3 is reduced.

In step S101, the exhaust temperature T and the intake air amount Ga immediately before fuel addition are read. The exhaust temperature T is used to simply indicate the temperature of the NOx catalyst 3, and the intake air amount Ga is used to indicate the flow rate of the exhaust. The exhaust temperature T can be obtained from the exhaust temperature sensor 5. The intake air amount Ga can be obtained from the air flow meter 9.

In step S102, it is determined whether the exhaust temperature T is higher than a predetermined temperature and the intake air amount Ga is higher than a predetermined amount. When the fuel added from the fuel addition valve 6 adheres to the exhaust passage 2 or the NOx catalyst 3 and further it takes time to evaporate the fuel, the NOx purification rate is hardly improved even if the addition interval is changed. In such a case, it is better to add the fuel in accordance with the exhaust stroke in order to put the added fuel on the exhaust flow. Such a state is determined based on the exhaust temperature T and the intake air amount Ga. The predetermined temperature and the predetermined amount are obtained in advance by experiments or the like as values that can be expected to greatly improve the NOx purification rate when the addition interval is changed. If an affirmative determination is made in step S102, the process proceeds to step S103, whereas if a negative determination is made, the process proceeds to step S106. In this embodiment, the ECU 102 that executes step S102 corresponds to the addition interval switching means in the present invention.

In step S103, the fuel addition amount is calculated. Since the NOx purification rate can be improved by changing the addition interval, the fuel addition amount calculated in step S106, which will be described later, is reduced. For example, the fuel addition amount may be calculated in the same manner as in step S106, and the fuel addition amount may be reduced by an amount corresponding to the improvement in the NOx purification rate. Further, for example, the relationship between the engine speed and the engine load and the fuel addition amount may be obtained in advance and mapped, and the fuel addition amount may be obtained based on the map.

  In step S104, the addition interval is determined. The addition interval is determined in the same manner as in Example 1. For example, the relationship between the fuel addition amount calculated in step S103 and the addition interval may be mapped in advance, and the addition interval may be obtained by substituting the fuel addition amount into the map.

  In step S105, fuel addition is performed while changing the addition interval according to the fuel addition amount.

In step S106, the fuel addition amount is calculated. The fuel addition amount is calculated based on the engine speed and the engine load so that the air-fuel ratio of the exhaust gas becomes the target air-fuel ratio at the time of NOx reduction. Further, the fuel addition amount may be determined so that the ratio of the intake air amount obtained by the air flow meter 9 and the fuel amount injected from the fuel injection valve 11 and the fuel addition valve 6 becomes the target air-fuel ratio. Further, the fuel addition amount may be calculated by a conventional technique.

In step S107, fuel is added according to the crank angle. Specifically, fuel addition is performed in accordance with the exhaust stroke of the fourth cylinder. By adding fuel in accordance with the exhaust stroke of a predetermined cylinder in this way, NOx can be quickly achieved even when the exhaust flow rate is small.
Since the fuel can reach the catalyst 3, the controllability of the air-fuel ratio in the NOx catalyst 3 can be improved.

Thus, the NOx purification rate in the NOx catalyst 3 can be improved by determining the addition interval. In addition, since the amount of fuel added during NOx reduction can be reduced, fuel efficiency can be improved.

It is a figure which shows schematic structure of the internal combustion engine which applies the exhaust gas purification apparatus of the internal combustion engine which concerns on an Example, and its intake / exhaust system. It is a time chart which shows the waveform of the command signal of ECU sent to a fuel addition valve, and the change of the air fuel ratio corresponding to the waveform on the same time axis. It is the figure which showed the addition space | interval in the case of adding fuel from a fuel addition valve according to the exhaust stroke start of the 4th cylinder. FIG. 3A shows a case where the fuel addition amount is relatively small, and FIG. 3B shows a case where the fuel addition amount is relatively large. It is the figure which showed the addition space | interval when the addition space | interval is set according to the fuel addition amount from a fuel addition valve. 4A shows a case where the same amount of fuel as in FIG. 3A is added, and FIG. 4B shows a case where the same amount of fuel as in FIG. 3B is added. It is the figure which showed the relationship between an addition space | interval, a NOx purification rate, and an outgas average HC. It is the figure which showed the addition interval when the fuel addition of different addition amount was performed in one rich spike. 6 is a flowchart showing a flow of fuel addition control in Embodiment 2.

Explanation of symbols

1 Internal combustion engine 2 Exhaust passage 3 NOx storage reduction catalyst 4 Air-fuel ratio sensor 5 Exhaust temperature sensor 6 Fuel addition valve 7 ECU
8 Intake passage 9 Air flow meter 11 Fuel injection valve

Claims (3)

Reducing agent addition means for adding a reducing agent into the exhaust gas discharged from the internal combustion engine;
A NOx storage reduction catalyst in which NOx is stored and NOx stored is reduced by the reducing agent added by the reducing agent adding means;
With
When reducing the NOx occluded in the NOx storage reduction catalyst to the target air-fuel ratio for reducing the exhaust air-fuel ratio, an addition period that is a period during which the reducing agent is added during the target air-fuel ratio is set. An exhaust purification apparatus for an internal combustion engine, wherein a plurality of addition intervals between the end of each addition period and the start of the next addition period are determined according to the amount of reducing agent added in the immediately preceding addition period. 2. The exhaust gas purification apparatus for an internal combustion engine according to claim 1, wherein the addition interval is determined as a time for recovering HC poisoning of the NOx storage reduction catalyst caused by the immediately preceding addition of the reducing agent. Reducing agent addition means for adding a reducing agent into the exhaust;
A NOx storage reduction catalyst in which NOx is stored and NOx stored is reduced by the reducing agent added by the reducing agent adding means;
When reducing the NOx occluded in the NOx storage reduction catalyst to the target air-fuel ratio for reducing the exhaust air-fuel ratio, an addition period that is a period during which the reducing agent is added during the target air-fuel ratio is set. A plurality of addition intervals between the end of each addition period and the start of the next addition period, when the flow rate of exhaust gas is larger than a predetermined amount and the temperature of the NOx storage reduction catalyst is higher than a predetermined temperature An addition interval switching means that is determined according to the reducing agent addition amount in the immediately preceding addition period, and that is determined according to the crank angle when the flow rate of the exhaust gas is a predetermined amount or less or the temperature of the NOx catalyst is a predetermined temperature or less;
An exhaust emission control device for an internal combustion engine, comprising:

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