JP5560089B2 - 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.

An oxidation catalyst, a reducing agent addition valve, and a selective reduction type NOx catalyst (hereinafter referred to as an SCR catalyst) are arranged in this order from the upstream in the exhaust passage of the internal combustion engine, and NH ₃ (generated from the reducing agent added from the reducing agent addition valve ( Ammonia) is used to purify NOx in the exhaust gas flowing into the SCR catalyst. Here, as a method for adding the reducing agent, two methods have been conventionally used (see, for example, Patent Document 1 (paragraphs 0005 to 0008)).

The first method is an equivalent addition method in which the amount of NOx discharged from the internal combustion engine and flowing into the SCR catalyst is estimated, and a reducing agent corresponding to the amount of NOx (equivalent ratio 1) is continuously added. is there. In the case of this method, when the NOx emission amount changes suddenly due to a delay in response to the addition control of the reducing agent, a delay from hydrolysis of the reducing agent to NH ₃ , etc., it is not possible to respond in time. It is difficult to make a quantity. Therefore, NH ₃ slip due to excessive addition and NOx purification rate may decrease due to insufficient addition.

As a second method, the NH ₃ adsorption function of the SCR catalyst is used, NH ₃ is adsorbed and held in advance within a range not exceeding the saturated adsorption amount, and NOx is reduced and purified and adsorbed on the SCR catalyst. This is an adsorption addition method in which as soon as NH ₃ is consumed, a reducing agent is added by an amount corresponding to the amount of NH ₃ consumed. In the case of this method, as compared with the first method, it is easy to cope with a response delay and an abrupt change in the amount of NOx. Therefore, conventionally, the second method is mainly employed.

Here, in the case of the second method, as described in paragraph 0003 of Patent Document 4, the amount of NH ₃ to be adsorbed on the SCR catalyst is as much as possible as long as it does not exceed the saturated adsorption amount of the SCR catalyst. Many were considered better. However, the amount of NH ₃ to be adsorbed on the SCR catalyst is not necessarily large, and even when NH ₃ is adsorbed, it is better to keep the state where a reducing agent is added as much as possible. It has become clear that you can get. There the other hand, when kept in a state that the addition of the reducing agent, also NH ₃ slip is liable to occur that NH ₃ will slip through the SCR catalyst.

JP 2008-261253 A JP 2009-293444 A JP 2005-226504 A JP 2005-127256 A

The present invention has been made in view of the above problems, and an object of the present invention is to achieve both of ensuring a high NOx purification rate and suppressing NH ₃ slip in a selective reduction type NOx catalyst in an exhaust purification device of an internal combustion engine. It is to provide technology to be realized.

In the present invention, the following configuration is adopted. That is, the present invention
A selective reduction type NOx catalyst disposed in an exhaust passage of the internal combustion engine;
A reducing agent addition unit that is disposed in the exhaust passage upstream of the selective reduction type NOx catalyst and adds a reducing agent for supplying NH ₃ to the selective reduction type NOx catalyst;
An exhaust purification device for an internal combustion engine comprising:
A purification rate calculation unit for calculating a NOx purification rate of the selective reduction type NOx catalyst;
An estimated adsorption amount calculation unit that calculates an estimated adsorption amount of NH ₃ adsorbed on the selective reduction type NOx catalyst based on the NOx purification rate calculated by the purification rate calculation unit;
Whether the estimated adsorption amount calculated by the estimated adsorption amount calculation unit can achieve a certain NOx purification rate in all operating states of the internal combustion engine and can suppress the slip of NH ₃ from the selective reduction type NOx catalyst. When the amount of the reducing agent is not less than a predetermined amount that is a threshold value, a reducing agent of a specified amount or more is added from the reducing agent addition unit. An addition control unit for adding the reduced reducing agent from the reducing agent addition unit;
An exhaust emission control device for an internal combustion engine, comprising:

Here, the predetermined amount means that when the estimated adsorption amount is less than that, a NOx purification rate of a certain level or more can be realized in any operating state of the internal combustion engine, and NH ₃ slip from the selective reduction type NOx catalyst can be achieved. This is an amount that can be suppressed, and is a threshold value that indicates whether or not an NOx purification rate of a certain level or more can be realized and NH ₃ slip from the selective reduction type NOx catalyst can be suppressed in all operating states of the internal combustion engine.

Selective reduction type NOx catalyst is not that it more the amount of NH ₃ adsorbed, even in a state in which adsorbs NH _3, towards keeping in a state that the addition of as much as possible reducing agent is higher NOx It has become clear that a purification rate can be obtained. There the other hand, when kept in a state that the addition of the reducing agent, also NH ₃ slip is liable to occur that NH ₃ will slip through the selective reduction NOx catalyst.

Therefore, in the present invention, when the estimated adsorption amount is equal to or less than the predetermined amount, it is possible to realize a NOx purification rate of a certain level or more and suppress slip of NH ₃ from the selective reduction type NOx catalyst in all operating states of the internal combustion engine. Add more reducing agent than specified. On the other hand, if the estimated adsorption amount exceeds a predetermined amount, NH ₃ slip from the selective reduction-type NOx catalyst is less likely to occur. Therefore, as the estimated adsorption amount increases, the reducing agent whose amount is reduced is added. Thereby, even if the estimated adsorption amount exceeds a predetermined amount, the reducing agent can be added as much as possible within a range not exceeding the saturated adsorption amount of the selective reduction type NOx catalyst. Therefore, it is possible to achieve both a high NOx purification rate and NH ₃ slip suppression in the selective reduction type NOx catalyst.

  It is preferable to further include a specified amount calculation unit that calculates a specified amount of the reducing agent based on the NOx purification rate calculated by the purification rate calculation unit.

  According to this, since the prescribed amount of reducing agent is calculated based on the NOx purification rate, the prescribed amount of reducing agent can be set to an amount suitable for NOx purification.

The purification rate calculation unit includes an assumed purification rate calculation unit that calculates a NOx purification rate from a ratio of NO to NO ₂ flowing into the selective reduction NOx catalyst, a temperature of the selective reduction NOx catalyst, and an exhaust flow rate; An actual purification rate calculation unit that calculates a NOx purification rate from the NOx concentration entering and exiting the selective reduction type NOx catalyst,
The estimated adsorption amount calculation unit calculates the estimated adsorption amount based on the NOx purification rate calculated by the actual purification rate calculation unit,
The prescribed amount calculation unit may calculate a prescribed amount of the reducing agent based on the NOx purification rate calculated by the assumed purification rate calculation unit.

Assumed purification ratio calculating section, the ratio of NO to NO ₂ flowing into the selective reduction type NOx catalyst, the temperature of the selective reduction type NOx catalyst, and therefore calculates the NOx purification ratio from the exhaust flow, now the current NOx purification rate Predictable. Since the actual purification rate calculation unit calculates the NOx purification rate from the NOx concentration entering and exiting the selective reduction type NOx catalyst, the actual actual NOx purification rate can be derived although there is a time lag. For this reason, although the estimated adsorption amount has a time lag, the latest actual amount can be derived, and the required amount of the reducing agent can be derived now.

The purification rate calculation unit is an assumed purification rate calculation unit that calculates a NOx purification rate from a ratio of NO to NO ₂ flowing into the selective reduction type NOx catalyst, a temperature of the selective reduction type NOx catalyst, and an exhaust flow rate. Good.

Assumed purification ratio calculating section, the ratio of NO to NO ₂ flowing into the selective reduction type NOx catalyst, the temperature of the selective reduction type NOx catalyst, and therefore calculates the NOx purification ratio from the exhaust flow, now the current NOx purification rate Predictable. For this reason, the present amount can be derived as the estimated amount of adsorption, and the present amount necessary as the defined amount of the reducing agent can be derived.

  The purification rate calculation unit may be an actual purification rate calculation unit that calculates a NOx purification rate from a NOx concentration entering and exiting the selective reduction type NOx catalyst.

  Since the actual purification rate calculation unit calculates the NOx purification rate from the NOx concentration entering and exiting the selective reduction type NOx catalyst, the actual actual NOx purification rate can be derived although there is a time lag. For this reason, the estimated amount of adsorption can derive the actual amount in the latest although there is a time lag, and the specified amount of the reducing agent can derive the amount actually required in the immediate vicinity of the time lag.

  Based on the estimated adsorption amount calculated by the estimated adsorption amount calculation unit and a predetermined amount for calculating the amount of reducing agent added by the addition control unit, an increase / decrease amount multiplied by a prescribed amount of the reducing agent It is preferable to further include an increase / decrease amount rate calculation unit for calculating the rate.

  According to this, the amount of the reducing agent added by the addition control unit can be calculated by multiplying the prescribed amount of the reducing agent by the increase / decrease rate.

  The addition control unit may add a specified amount of a reducing agent from the reducing agent adding unit when the estimated adsorption amount is equal to or less than a predetermined amount.

According to this, since the specified amount of reducing agent is added when the estimated adsorption amount is less than or equal to the predetermined amount, the adsorption amount of NH ₃ of the selective reduction type NOx catalyst is surely slowly increased, and the adsorption amount of NH ₃ rapidly increases. By increasing, it is possible to avoid an unexpected NH ₃ slip.

  The addition control unit may add, from the reducing agent addition unit, a reducing agent that increases a specified amount as the estimated adsorption amount is smaller when the estimated adsorption amount is equal to or less than a predetermined amount.

According to this, since the amount of reducing agent is increased as the estimated adsorption amount is smaller, the adsorption amount of NH ₃ of the selective reduction type NOx catalyst can be increased more quickly, and the NOx purification rate of the selective reduction type NOx catalyst can be increased more quickly. it can.

The present invention also provides
A selective reduction type NOx catalyst disposed in an exhaust passage of the internal combustion engine;
A reducing agent addition unit that is disposed in the exhaust passage upstream of the selective reduction type NOx catalyst and adds a reducing agent for supplying NH ₃ to the selective reduction type NOx catalyst;
A reducing agent addition method for an exhaust gas purification apparatus for an internal combustion engine comprising:
Calculating the NOx purification rate of the selective reduction type NOx catalyst;
Based on the calculated NOx purification rate, an estimated adsorption amount of NH ₃ adsorbed on the selective reduction type NOx catalyst is calculated,
The calculated estimated adsorption amount is a threshold value as to whether or not a NOx purification rate of a certain level or more can be realized and NH ₃ slip from the selective reduction type NOx catalyst can be suppressed in all operating states of the internal combustion engine. When the amount is below the fixed amount, a specified amount or more of the reducing agent is added from the reducing agent addition unit, and when the estimated adsorption amount exceeds a predetermined amount, the reducing agent is reduced by reducing the specified amount as the estimated adsorption amount increases. A reducing agent addition method for an exhaust gas purification apparatus for an internal combustion engine, wherein the addition is performed from an agent addition unit.

According to the present invention as well, in the exhaust gas purification apparatus for an internal combustion engine, it is possible to achieve both a high NOx purification rate and NH ₃ slip suppression in the selective reduction type NOx catalyst.

According to the present invention, in the exhaust gas purification apparatus for an internal combustion engine, it is possible to achieve both a high NOx purification rate and NH ₃ slip suppression in the selective reduction type NOx catalyst.

1 is a diagram illustrating a schematic configuration of an internal combustion engine according to Embodiment 1 of the present invention. NH of the SCR catalyst ₃ is a diagram showing the relationship between adsorption amount and NOx purification rate. FIG. 3 is a control block diagram in the ECU according to the first embodiment. It is a figure which shows the relationship between the estimated amount of adsorption | suction which concerns on Example 1, and the increase / decrease rate. It is a figure which shows the relationship between the SCR catalyst bed temperature which concerns on Example 1, and predetermined amount and limit adsorption amount. FIG. 4 is a diagram showing a relationship between an SCR catalyst bed temperature, a predetermined amount, and a limit adsorption amount according to another example of Example 1. 3 is a flowchart illustrating a urea water addition control routine according to the first embodiment. It is a figure which shows the relationship between the estimated amount of adsorption | suction which concerns on the modification 1, and the increase / decrease amount rate. FIG. 10 is a control block diagram in an ECU according to Modification 2. 10 is a flowchart showing a urea water addition control routine according to Modification 2.

  Specific examples of the present invention will be described below.

<Example 1>
(Internal combustion engine)
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine according to Embodiment 1 of the present invention. The internal combustion engine 1 shown in FIG. 1 is a four-stroke cycle diesel engine for driving a vehicle having four cylinders. Connected to the internal combustion engine 1 is an exhaust passage 2 through which the exhaust discharged from the internal combustion engine 1 flows.

A selective reduction type NOx catalyst (hereinafter referred to as SCR catalyst) 3 is disposed in the middle of the exhaust passage 2. The SCR catalyst 3 reduces and purifies NOx in the exhaust gas using NH ₃ (ammonia). For example, NO is reduced to N ₂ by a reaction such as 4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O. NO ₂ is reduced to N ₂ by a reaction such as 6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O. NO and NO ₂ are reduced to N ₂ by a reaction such as NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O. The SCR catalyst 3 has a function of adsorbing NH ₃ . The SCR catalyst 3 is formed of zeolite or the like.

In the exhaust passage 2 upstream of the SCR catalyst 3, a urea water addition valve 4 for adding a urea aqueous solution (hereinafter referred to as urea water) as a reducing agent hydrolyzed to NH ₃ supplied to the SCR catalyst 3 is arranged. Yes. From the urea water addition valve 4, urea water stored in the urea water tank 5 is injected into the exhaust passage 2 based on a command. The injected urea water is hydrolyzed using exhaust heat in a reaction such as (NH ₂ ) 2 CO + H ₂ O → 2NH ₃ + CO ₂ to generate NH ₃ . The urea water addition valve 4 corresponds to the reducing agent addition part of the present invention. As the reducing agent, an ammonia-based solution such as an aqueous ammonia solution can be used in addition to the urea water.

  A first NOx sensor 6 for detecting the NOx concentration in the exhaust gas flowing into the SCR catalyst 3 is disposed in the exhaust passage 2 immediately upstream of the urea water addition valve 4. The first NOx sensor 6 corresponds to a first NOx concentration acquisition unit. A second NOx sensor 7 that detects the NOx concentration in the exhaust gas flowing out from the SCR catalyst 3 is disposed in the exhaust passage 2 immediately downstream of the SCR catalyst 3. The second NOx sensor 7 corresponds to a second NOx concentration acquisition unit. The first NOx concentration acquisition unit and the second NOx concentration acquisition unit may estimate the NOx concentration from the operating state of the internal combustion engine 1 and a NOx concentration estimation map calculated in advance. The SCR catalyst 3 is provided with a temperature sensor 8 for detecting the SCR catalyst bed temperature. The temperature sensor 8 corresponds to a catalyst temperature acquisition unit. The catalyst temperature acquisition unit may estimate the SCR catalyst bed temperature from the exhaust temperature arranged in the exhaust passage 2 and a previously calculated catalyst bed temperature estimation map.

  The internal combustion engine 1 configured as described above is provided with an electronic control unit (hereinafter referred to as ECU) 9. The ECU 9 is electrically connected to a first NOx sensor 6, a second NOx sensor 7, a temperature sensor 8, and a crank position sensor and an accelerator opening sensor (not shown). These output signals are input to the ECU 9. Further, the urea water addition valve 4 is electrically connected to the ECU 9 and is controlled by the ECU 9.

(Urea water addition control)
Conventionally, as a case of adding urea water to the SCR catalyst 3, mainly by using the function of adsorbing NH ₃ in the SCR catalyst 3, the NH ₃ SCR catalyst 3 at the target adsorption amount of not exceeding a pre-saturation adsorption amount Adsorption and addition of supplying urea water in an amount corresponding to the amount of NH ₃ consumed so as to reach the target adsorption amount as soon as NH ₃ held by reduction of NOx is consumed. The method was used.

Here, in the case of this method, it was considered that the NH ₃ amount to be adsorbed on the SCR catalyst 3 as the target adsorption amount should be as large as possible as long as the saturated adsorption amount of the SCR catalyst 3 is not exceeded. . However, SCR catalyst 3 NH ₃ amount adsorbed to and not that it The more, even in a state where the adsorbed NH _3, and as far as possible the urea water added to the state is to keep it in the higher NOx purifying the It has become clear that rates can be obtained. FIG. 2 is a graph showing the relationship between the NH ₃ adsorption amount of the SCR catalyst and the NOx purification rate. As shown in FIG. 2, even with the same NH ₃ adsorption amount, the NOx purification rate is higher when urea water is added than when it is not added. There the other hand, when kept in a state that the addition of urea water, even NH ₃ slip is liable to occur that NH ₃ will slip through the SCR catalyst 3.

Therefore, in this embodiment, the estimated adsorption amount of NH ₃ adsorbed on the SCR catalyst 3 can realize a NOx purification rate higher than a certain level in all operating states of the internal combustion engine 1 and can prevent slip of NH ₃ from the SCR catalyst 3. When a predetermined amount of urea water is added from the urea water addition valve 4 when the amount is equal to or less than a predetermined amount which is a threshold value indicating whether or not the amount can be suppressed, and the estimated amount of adsorption exceeds the predetermined amount, the larger the estimated amount of adsorption The urea water whose amount was reduced was added from the urea water addition valve 4.

Here, the predetermined amount means that when the estimated adsorption amount of NH ₃ adsorbed on the SCR catalyst 3 is an amount smaller than that, an NOx purification rate of a certain level or more can be realized in any operating state of the internal combustion engine 1 and the SCR catalyst. This is an amount that can suppress the slip of NH 3 from ₃ , a threshold that determines whether or not a NOx purification rate of a certain level or more can be realized and the slip of NH ₃ from the SCR catalyst 3 can be suppressed in any operating state of the internal combustion engine 1. It is. As the predetermined amount in this embodiment, a purification rate saturation adsorption amount at which the NOx purification rate is saturated with respect to the adsorption amount of the SCR catalyst 3 is used. However, the predetermined amount is not limited to the purification rate saturated adsorption amount.

According to the present embodiment, when the estimated adsorption amount of NH ₃ adsorbed on the SCR catalyst 3 is equal to or less than a predetermined amount, a NOx purification rate higher than a certain level can be realized in all operating states of the internal combustion engine 1 and Since a slip of NH ₃ can be suppressed, a specified amount of urea water is added. On the other hand, if the estimated amount of adsorption exceeds a predetermined amount, NH ₃ slip from the SCR catalyst 3 is not likely to occur, so urea water whose amount is reduced by a specified amount is added as the estimated amount of adsorption increases. Thereby, even if the estimated adsorption amount exceeds a predetermined amount, urea water can be added as much as possible within a range not exceeding the saturated adsorption amount of the SCR catalyst 3. Therefore, it is possible to achieve both a high NOx purification rate in the SCR catalyst 3 and NH ₃ slip suppression.

FIG. 3 is a control block diagram in the ECU 9 according to the present embodiment. A specific configuration for performing urea water addition control from the urea water addition valve 4 in accordance with the relationship between the estimated amount of NH ₃ adsorbed on the SCR catalyst 3 and a predetermined amount will be described with reference to FIG.

As shown in FIG. 3, the ECU 9 determines the ratio of NO to NO ₂ flowing into the SCR catalyst 3 estimated from the operating state of the internal combustion engine 1 based on the map, the temperature of the SCR catalyst 3 detected by the temperature sensor 8, and the air flow. An assumed purification rate calculation unit 9a that calculates the NOx purification rate from the exhaust gas flow rate detected by the meter 10 is provided. For the ratio of NO to NO ₂ flowing into the SCR catalyst 3, the rate of change from NO to NO ₂ in the oxidation catalyst disposed in the exhaust passage 2 upstream of the SCR catalyst 3 can also be considered. Thereby, in the assumed purification rate calculation part 9a, the present NOx purification rate of the SCR catalyst 3 can be predicted. The assumed purification rate calculation unit 9a is a part of the purification rate calculation unit.

  The ECU 9 has an actual purification rate calculation unit 9 b that calculates the NOx purification rate of the SCR catalyst 3 from the NOx concentration entering and exiting the SCR catalyst 3 detected by the first NOx sensor 6 and the second NOx sensor 7. The NOx purification rate in the actual purification rate calculation unit 9b is determined from the NOx concentration detected by the first NOx sensor 6 (hereinafter referred to as input NOx concentration) to the NOx concentration detected by the second NOx sensor 7 (hereinafter referred to as output NOx concentration). It can be obtained by dividing the subtracted value by the input NOx concentration. As a result, the actual purification rate calculation unit 9b can derive the actual NOx purification rate of the SCR catalyst 3 at the latest time, although there is a time lag. The actual purification rate calculation unit 9b is a part of the purification rate calculation unit.

  The ECU 9 includes a NOx amount calculation unit 9c that calculates the NOx amount flowing into the SCR catalyst 3 from the NOx concentration flowing into the SCR catalyst 3 detected by the first NOx sensor 7 and the exhaust flow rate detected by the air flow meter 10. . The amount of NOx can be obtained by multiplying the input NOx concentration by the exhaust gas flow rate.

The ECU 9 has a specified amount calculation unit 9d that calculates a specified amount of urea water based on the NOx purification rate calculated by the assumed purification rate calculation unit 9a. The specified amount calculation unit 9d calculates the purification NOx amount to be purified by the SCR catalyst 3 by multiplying the NOx purification rate calculated by the assumed purification rate calculation unit 9a by the NOx amount calculated by the NOx amount calculation unit 9c, The amount of urea water that generates NH ₃ that reacts with the purified NOx amount and is consumed is calculated as a specified amount. Since the assumed purification rate calculation unit 9a predicts the current NOx purification rate, the currently required amount of urea water can be derived.

The ECU 9 includes an estimated adsorption amount calculation unit 9e that calculates an estimated adsorption amount of NH ₃ adsorbed on the SCR catalyst 3 based on the NOx purification rate calculated by the actual purification rate calculation unit 9b. The estimated adsorption amount calculation unit 9e multiplies the NOx purification rate calculated by the actual purification rate calculation unit 9b by the NOx amount calculated by the NOx amount calculation unit 9c, thereby reducing the reduced NOx amount reduced and purified by the SCR catalyst 3. calculate. The amount of NH ₃ reduced to the amount of reduced NOx is the amount of consumed NH ₃ . And a quantity of urea water in the addition control unit and the previous estimated amount of adsorption and consumption NH ₃ amount, calculates an estimated amount of adsorption of NH ₃ adsorbed in the SCR catalyst 3. Although the actual purification rate calculation unit 9b derives the latest actual NOx purification rate with a time lag, the estimated adsorption amount can derive the latest actual amount with a time lag.

The ECU 9 multiplies the prescribed amount of urea water based on the estimated adsorption amount calculated by the estimated adsorption amount calculation unit 9e and a predetermined amount for calculating the amount of urea water added by the addition control unit 9g. It has an increase / decrease rate calculation unit 9f that calculates an increase / decrease rate. FIG. 4 is a diagram illustrating a relationship between the estimated adsorption amount and the increase / decrease amount rate set in the increase / decrease amount rate calculation unit 9f. A map as shown in FIG. 4 is set in the increase / decrease rate calculation unit 9f. The map shown in FIG. 4 is a linear function so that the increase / decrease amount rate is 1 when the estimated adsorption amount is equal to or less than the predetermined amount, and becomes 0 when the estimated adsorption amount exceeds the predetermined amount and the increase / decrease amount rate is the limit adsorption amount. Decrease. By taking the estimated adsorption amount into this map, the increase / decrease amount rate is calculated. Here, the map shown in FIG. 4 differs according to the SCR catalyst bed temperature. FIG. 5 is a diagram showing the relationship between the SCR catalyst bed temperature, the predetermined amount, and the limit adsorption amount. The temperature characteristics of the predetermined amount and the limit adsorption amount shown in FIG. 5 are known in advance. Therefore, the increase / decrease amount rate calculation unit 9f derives the predetermined amount and the limit adsorption amount from the temperature characteristics shown in FIG. 5 according to the SCR catalyst bed temperature detected by the temperature sensor 8. Then, a map shown in FIG. 4 is set from the derived predetermined amount and the limit adsorption amount. In addition to the characteristic shown in FIG. 5, the limit adsorption amount may be a temperature characteristic considering NH ₃ slip during transient operation. FIG. 6 is a diagram illustrating a relationship between the SCR catalyst bed temperature, a predetermined amount, and a limit adsorption amount according to another example. As shown in FIG. 6, the predetermined amount and the limit adsorption amount may be derived from those different from the temperature characteristics of FIG. The temperature characteristic of the predetermined amount shown in FIG. 6 sets the adsorption amount larger than the purification rate saturated adsorption amount, and the temperature characteristic of the limit adsorption amount considers NH ₃ slip at the time of transient operation. The adsorption amount is set to be smaller than the limit adsorption amount.

The ECU 9 has an addition control unit 9 g that performs addition control of urea water added from the urea water addition valve 4. The addition control unit 9g multiplies the specified amount calculated by the specified amount calculation unit 9d by the increase / decrease rate rate calculated by the increase / decrease rate rate calculation unit 9f to derive the addition amount of urea water. Then, the derived addition amount is added from the urea water addition valve 4. That is, since the increase / decrease amount rate is determined by the map shown in FIG. 4, if the estimated adsorption amount is equal to or less than the predetermined amount, a prescribed amount of urea water is added from the urea water addition valve 4, and the estimated adsorption amount exceeds the predetermined amount. Then, as the estimated adsorption amount increases, the urea water whose amount is reduced by the specified amount is added from the urea water addition valve 4. In this case, since the specified amount of urea water is added when the estimated adsorption amount is less than the predetermined amount, the adsorption amount of NH ₃ of the SCR catalyst 3 is surely slowly increased, and the adsorption amount of NH ₃ increases rapidly. By doing so, it is possible to avoid an unexpected NH ₃ slip.

A urea water addition control routine performed by the ECU 9 will be described based on a flowchart shown in FIG. FIG. 7 is a flowchart showing a urea water addition control routine. This routine is executed by the ECU 9 every predetermined time. When the routine shown in FIG. 7 is started, in S101, an assumed purification rate calculation unit 9a calculates an assumed purification rate. In S102, the specified amount of urea water is calculated by the specified amount calculation unit 9d from the assumed purification rate calculated by the assumed purification rate calculation unit 9a and the NOx amount calculated by the NOx amount calculation unit 9c. In S103, the increase / decrease rate calculation unit 9f calculates the increase / decrease rate. In S104, the addition amount of urea water is calculated by the addition control unit 9g from the specified amount of urea water calculated by the specified amount calculation unit 9d and the increase / decrease rate rate calculated by the increase / decrease rate rate calculation unit 9f. Run. In S105, the actual purification rate is calculated by the actual purification rate calculation unit 9b. In S106, the estimated adsorption amount calculation unit 9e calculates the SCR catalyst 3 from the previous estimated adsorption amount, the urea water addition amount added by the addition control unit 9g, and the actual purification rate calculated by the actual purification rate calculation unit 9b. The amount of NH ₃ adsorbed on the surface is estimated. After the processing of this step, this routine is once ended. According to this routine described above, it is possible to achieve both a high NOx purification rate in the SCR catalyst 3 and NH ₃ slip suppression.

<Others>
The exhaust gas purification apparatus for an internal combustion engine according to the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the gist of the present invention.

<Modification 1>
For example, the increase / decrease amount calculation unit 9f may set a map as shown in FIG. FIG. 8 is a diagram illustrating the relationship between the estimated adsorption amount and the increase / decrease amount rate set in the increase / decrease amount rate calculation unit 9f. The map shown in FIG. 8 shows that when the estimated adsorption amount is equal to or less than the predetermined amount, the smaller the estimated adsorption amount, the larger the increase / decrease rate rate is greater than 1, and when the estimated adsorption amount is the predetermined amount, the increase / decrease rate rate becomes 1. When the amount exceeds the predetermined amount, the rate of increase / decrease decreases linearly so that the rate of increase / decrease is 0. According to this, in the addition control unit 9g, the increase / decrease amount rate is determined by the map shown in FIG. 8, and if the estimated adsorption amount is equal to or less than the predetermined amount, the urea water with the specified amount increased as the estimated adsorption amount decreases. When added from the water addition valve 4 and the estimated adsorption amount exceeds a predetermined amount, the urea water whose amount is reduced by a specified amount is added from the urea water addition valve 4 as the estimated adsorption amount increases. In this case, since the urea water is increased as the estimated adsorption amount is smaller, the NH ₃ adsorption amount of the SCR catalyst 3 can be increased more quickly, and the NOx purification rate of the SCR catalyst 3 can be increased more quickly.

<Modification 2>
A control block diagram in the ECU 9 may be as shown in FIG. In addition, description is abbreviate | omitted here about what was demonstrated with the control block diagram of FIG.

  As shown in FIG. 9, the ECU 9 multiplies the specified amount calculated by the specified amount calculation unit 9d by the increase / decrease rate rate calculated by the increase / decrease rate rate calculation unit 9f to derive the addition amount 1 of urea water. It has the quantity 1 calculation part 9h.

  In addition, the ECU 9 includes an addition amount 2 calculation unit 9i that calculates an addition amount 2 that is feedback-controlled by PID control or the like so that the predetermined amount matches the estimated adsorption amount calculated by the estimated adsorption amount calculation unit 9e. The addition amount 2 calculation unit 9i calculates the addition amount 2 as a function of a value obtained by subtracting the estimated adsorption amount from the predetermined amount. This addition amount 2 is calculated as 0 when the value obtained by subtracting the estimated adsorption amount from the predetermined amount is negative (0 or less).

  The ECU 9 has an addition control unit 9j that adds the addition amount 1 and the addition amount 2 to calculate the final addition amount and executes the addition. The addition control unit 9j adds the addition amount 1 calculated by the addition amount 1 calculation unit 9h and the addition amount 2 calculated by the addition amount 2 calculation unit 9i to derive the final addition amount of urea water. . Then, the derived final addition amount is added from the urea water addition valve 4. That is, when the value obtained by subtracting the estimated adsorption amount from the predetermined amount is negative (0 or less), it is calculated as 0. Therefore, if the estimated adsorption amount is equal to or less than the predetermined amount, When urea water is added from the urea water addition valve 4 and the estimated adsorption amount exceeds a predetermined amount, the urea water whose amount is reduced by a specified amount is added from the urea water addition valve 4 as the estimated adsorption amount increases.

The urea water addition control routine performed by the ECU 9 will be described based on the flowchart shown in FIG. FIG. 10 is a flowchart showing a urea water addition control routine. This routine is executed by the ECU 9 every predetermined time. The description of the same steps as those in the routine shown in FIG. When the routine shown in FIG. 10 is started, S101 to S103 are executed. In S201, the addition amount 1 of urea water is calculated by the addition amount 1 calculation unit 9h from the specified amount of urea water calculated by the specified amount calculation unit 9d and the increase / decrease rate rate calculated by the increase / decrease amount rate calculation unit 9f. . In S202, the addition amount 2 calculation unit 9i calculates the addition amount 2 as a function of a value obtained by subtracting the estimated adsorption amount calculated by the estimated adsorption amount calculation unit 9e from the predetermined amount. In S203, the addition control unit 9j adds the addition amount 1 and the addition amount 2 to calculate the final addition amount, and executes the addition. Steps S105 to S106 are executed, and after the processing of this step, this routine is once ended. According to this routine described above, it is possible to achieve both a high NOx purification rate in the SCR catalyst 3 and NH ₃ slip suppression.

<Modification 3>
In the first embodiment, the purification rate calculation unit is the assumed purification rate calculation unit 9a and the actual purification rate calculation unit 9b. However, only one of them may be used. If the purification rate calculation unit is only the assumed purification rate calculation unit 9a, the current NOx purification rate can be predicted, the estimated adsorption amount can be derived from the current amount, and the specified amount of urea water is the amount currently required. Can be derived. When the purification rate calculation unit is only the actual purification rate calculation unit 9b, it is possible to derive the latest actual NOx purification rate although there is a time lag, and the estimated adsorption amount can derive the latest actual amount although there is a time lag. Although there is a time lag, the required amount can be derived from the amount actually required in the immediate future.

1: internal combustion engine, 2: exhaust passage, 3: SCR catalyst, 4: urea water addition valve, 5: urea water tank, 6: first NOx sensor, 7: second NOx sensor, 8: temperature sensor, 9: ECU, 9a : Assumed purification rate calculation unit, 9b: actual purification rate calculation unit, 9c: NOx amount calculation unit, 9d: specified amount calculation unit, 9e: estimated adsorption amount calculation unit, 9f: increase / decrease amount rate calculation unit, 9g: addition control unit , 9h: addition amount 1 calculation unit, 9i: addition amount 2 calculation unit, 9j: addition control unit, 10: air flow meter

Claims (5)

A selective reduction type NOx catalyst disposed in an exhaust passage of the internal combustion engine;
A reducing agent addition unit that is disposed in the exhaust passage upstream of the selective reduction type NOx catalyst and adds a reducing agent for supplying NH ₃ to the selective reduction type NOx catalyst;
An exhaust purification device for an internal combustion engine comprising:
A purification rate calculation unit for calculating a NOx purification rate of the selective reduction type NOx catalyst;
An estimated adsorption amount calculation unit that calculates an estimated adsorption amount of NH ₃ adsorbed on the selective reduction type NOx catalyst based on the NOx purification rate calculated by the purification rate calculation unit;
Whether the estimated adsorption amount calculated by the estimated adsorption amount calculation unit can achieve a certain NOx purification rate in all operating states of the internal combustion engine and can suppress the slip of NH ₃ from the selective reduction type NOx catalyst. When the amount of the reducing agent is not less than a predetermined amount that is a threshold value, a reducing agent of a specified amount or more is added from the reducing agent addition unit. An addition control unit for adding the reduced reducing agent from the reducing agent addition unit ;
A prescribed amount calculation unit for calculating a prescribed amount of the reducing agent based on the NOx purification rate calculated by the purification rate calculation unit,
The purification rate calculation unit includes an assumed purification rate calculation unit that calculates a NOx purification rate from a ratio of NO to NO ₂ flowing into the selective reduction NOx catalyst, a temperature of the selective reduction NOx catalyst, and an exhaust flow rate; An actual purification rate calculation unit that calculates a NOx purification rate from the NOx concentration entering and exiting the selective reduction type NOx catalyst,
The estimated adsorption amount calculation unit calculates the estimated adsorption amount based on the NOx purification rate calculated by the actual purification rate calculation unit,
The exhaust gas purification apparatus for an internal combustion engine, wherein the specified amount calculation unit calculates a specified amount of reducing agent based on the NOx purification rate calculated by the assumed purification rate calculation unit . Based on the estimated adsorption amount calculated by the estimated adsorption amount calculation unit and a predetermined amount for calculating the amount of reducing agent added by the addition control unit, an increase / decrease amount multiplied by a prescribed amount of the reducing agent The exhaust gas purification apparatus for an internal combustion engine according to claim 1, further comprising an increase / decrease amount rate calculation unit for calculating the rate. 3. The internal combustion engine according to claim 1 , wherein the addition control unit adds a specified amount of a reducing agent from the reducing agent adding unit when the estimated adsorption amount is equal to or less than a predetermined amount. Exhaust purification device. The addition control unit, said the estimated amount of adsorption is equal to or less than a predetermined amount, according to claim 1, characterized in that the addition of the reducing agent was increased a predetermined amount the estimated smaller the adsorption amount of the reducing agent addition unit or The exhaust emission control device for an internal combustion engine according to claim 2 . A selective reduction type NOx catalyst disposed in an exhaust passage of the internal combustion engine;
A reducing agent addition unit that is disposed in the exhaust passage upstream of the selective reduction type NOx catalyst and adds a reducing agent for supplying NH ₃ to the selective reduction type NOx catalyst;
A reducing agent addition method for an exhaust gas purification apparatus for an internal combustion engine comprising:
The ratio of NO to NO ₂ which flows into the selective reduction type NOx catalyst calculates the assumed NOx purification rate the temperature of the selective reduction type NOx catalyst, and the exhaust flow rate,
An actual NOx purification rate is calculated from the NOx concentration entering and exiting the selective reduction type NOx catalyst,
Based on the calculated actual NOx purification rate, an estimated adsorption amount of NH ₃ adsorbed on the selective reduction type NOx catalyst is calculated,
Based on the calculated assumed NOx purification rate, calculate a prescribed amount of reducing agent added from the reducing agent addition unit,
The calculated estimated adsorption amount is a threshold value as to whether or not a NOx purification rate of a certain level or more can be realized and NH ₃ slip from the selective reduction type NOx catalyst can be suppressed in all operating states of the internal combustion engine. When the amount is below the fixed amount, a specified amount or more of the reducing agent is added from the reducing agent addition unit, and when the estimated adsorption amount exceeds a predetermined amount, the reducing agent is reduced by reducing the specified amount as the estimated adsorption amount increases. A reducing agent addition method for an exhaust gas purification apparatus for an internal combustion engine, wherein the reducing agent addition unit is added from an agent addition unit.

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