JP2008069708A - Exhaust gas state estimation device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To estimate the state of the discharged NOx precisely. <P>SOLUTION: Inside an ECU (100) of an engine system (10), an emission estimation model (300) for estimating the state of the catalyst exhaust gas discharged from a three-way catalyst (223) of an engine (200) is configured. A catalyst model (310) in the emission estimation model (300) receives an input of an index value from a catalyst inflow model (320) and a catalyst degradation model (330), estimates the purification performance for each of substances to be cleaned according to a model formula wherein the coefficient is identified to approximate the purification rate of an actual substance to be cleaned beforehand, and outputs the emission value in the end. At this time, with respect to NOx, the influence of the oxygen concentration in the catalyst is considered, and instead of the model formula, a model formula wherein reduction reaction of NOx is set to start after the oxygen concentration in the catalyst is decreased to some extent is adopted. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technical field of an exhaust gas state estimation device for estimating the state of exhaust gas discharged from a catalyst in an internal combustion engine provided with the catalyst.

  In this type of technical field, a method for analyzing a reaction rate related to a catalytic reaction has been proposed (see, for example, Patent Document 1). According to the method for predicting a change in concentration of a predetermined component in a fluid and a reaction rate analysis method (hereinafter referred to as “conventional technology”) disclosed in Patent Document 1, the component concentration of exhaust gas changes due to contact with a catalyst. A reaction model is set, and based on the reaction model, a speed equation representing the concentration change amount of the predetermined component and a position / concentration characteristic representing the concentration change of the predetermined component are obtained to predict the concentration change of the predetermined component. For this reason, it is said that it is possible to estimate the influence on the change in the concentration of the predetermined component when conditions such as the length of the catalyst carrier and the initial concentration of the predetermined component are changed.

JP 2004-8908 A

  In the catalyst, a reduction reaction occurs in NOx (Nitrogen Oxide) and oxygen is desorbed. When oxygen coexists in the catalyst, the reduction reaction proceeds in a region where the oxygen concentration is relatively high. Is easily delayed. However, in the conventional technique, since the influence of such oxygen concentration on the reduction reaction is not taken into consideration, the prediction accuracy of the NOx concentration change is likely to decrease particularly when the oxygen concentration in the catalyst is high. That is, the conventional technique has a technical problem that it is difficult to sufficiently estimate the NOx concentration change depending on the operating conditions of the internal combustion engine.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an exhaust gas state estimation device capable of estimating the state of exhausted NOx with high accuracy.

  In order to solve the above-described problem, an exhaust gas state estimation device according to the present invention is an exhaust gas state estimation device that estimates the state of exhaust gas discharged from a catalyst in an internal combustion engine equipped with a catalyst, A means for specifying the state of the inflowing gas flowing into the catalyst, and a term corresponding to each concentration, each of which is constructed in advance as prescribing the purification characteristics of each of the purification target substances in the catalyst; In accordance with a reference model equation including a product of a reaction rate constant related to a catalytic reaction in the catalyst, an estimation means for estimating the state of the exhaust gas based on the state of the specified inflow gas, and the purification target substance A first correction unit that corrects the reference model equation so that the purification rate of the nitrogen oxide increases as the oxygen concentration decreases in the case of the nitrogen oxide. It is characterized by.

  The “internal combustion engine” according to the present invention has, for example, a plurality of cylinders, and an explosive force generated when fuel is burned in a combustion chamber in each of the plurality of cylinders, for example, a mechanical force such as a piston and a connecting rod. This is a concept that encompasses an engine that can be extracted as power via an input / output shaft such as a crankshaft through a simple transmission path, and refers to, for example, a 2-cycle or 4-cycle reciprocating engine.

  According to the exhaust gas state estimation device of the present invention, during its operation, for example, a specification configured as various processing units such as an ECU (Electronic Control Unit), various controllers or various computer systems such as a microcomputer device, etc. The state of the inflow gas flowing into the catalyst is specified by the means.

  Here, the “inflow gas state” is a concept including a state quantitatively represented by an index value that can be used for estimation of an exhaust gas state, which will be described later. In other words, it includes at least part of the component ratio, component concentration, component amount, etc., and the flow rate and temperature of the inflowing gas.

  Note that “specific” in the present invention refers to, for example, detecting directly or indirectly as a physical numerical value or an electrical signal corresponding to the physical numerical value via some detection means, or via any detection means. Select the appropriate numerical value from a map or the like stored in advance in a suitable storage means based on a physical numerical value that is directly or indirectly detected, for example, in the form of an electrical signal or the like and has a corresponding relationship with the specific object. Deriving from such a physical numerical value or a selected numerical value according to a preset algorithm or calculation formula, or the numerical value detected, selected or derived in this way, for example, in the form of an electrical signal or the like It is a broad concept encompassing simply acquiring.

  For example, the specifying means has a direct or indirect correspondence relationship with the above-described inflow gas state such as the engine speed, load factor, air-fuel ratio, ignition timing, and valve timing of the internal combustion engine, for example, experimentally in advance. In particular, it is set in advance, for example, based on various index values that define the operating state of the internal combustion engine, which is determined with reliability to the extent that no practical disadvantage is manifested theoretically or based on simulation or the like. The state of the inflowing gas is specified by calculating the state of the inflowing gas as an appropriate index value according to the algorithm or the calculation formula, or by selecting an appropriate value from a map or the like based on the various index values.

  When the state of the inflowing gas is specified by the specifying means as described above, the estimation means configured as various processing units such as an ECU, various controllers or various computer systems such as a microcomputer device, etc., according to a reference model formula constructed in advance. The state of the exhaust gas is estimated based on the state of the inflowing gas specified by the specifying means.

  Here, the “reference model equation” is an equation constructed for each of the purification target substances in the catalyst in advance in order to prescribe the purification characteristics of each of the purification target substances. It is a formula including the product with the reaction rate constant concerning a catalytic reaction. This reference model equation is a function of the estimated inflow gas state, and the estimation means is obtained by substituting, for example, the specified inflow gas state (that is, various index values as forms) into this reference model equation. For example, the state of the exhaust gas is estimated based on the purification rate, the purification amount, or the purification rate (reaction rate of the catalytic reaction) for each of the substances to be purified.

  Here, the “exhaust gas state” is a concept including a state represented by an index value that can be used for evaluation of emission characteristics as a concept including the type and amount of emission in an internal combustion engine. The state relating to the gas may be at least partially equivalent in terms of meaning, and includes, for example, the components of the substance to be purified in the exhaust gas, the component ratio, the component amount and the component concentration, the temperature of the exhaust gas, etc. .

  The “reaction rate constant” is, for example, various experimentally, experimentally, empirically, theoretically, or based on simulations so that the purification characteristics of the actual catalyst can be expressed approximately or alternatively. In consideration of the fact that the coefficient or the value of the characteristic term is determined (identified) and multiplied by the concentration of the purification target substance, the purification characteristic of the purification target substance (for example, the purification amount, the purification rate or the It is a constant that determines the purification rate or the reaction rate in the catalyst.

  The “term corresponding to the concentration of the substance to be purified” is a concept that includes the concentration and includes a term that can approximate or substitute the reference model equation for the purification characteristics in the actual catalyst. For example, the power term of the concentration may be used.

  Here, in particular, when the substance to be purified is nitrogen oxide (hereinafter referred to as NOx) containing NO (Nitric Oxide) or the like, the desorption of oxygen occurs in a region where the oxygen concentration in the catalyst is relatively high. It is obstructed and the progress of the reduction reaction tends to be delayed. Therefore, when trying to express the NOx purification characteristics by the above-mentioned reference model equation, the oxygen concentration in the catalyst tends to be relatively high, for example, in a high load region (high flow rate region) of the internal combustion engine, etc. The estimation accuracy related to the state is likely to decrease.

  Therefore, in the exhaust gas state estimation device according to the present invention, the purification target is made to be NOx by the action of the first correction means configured as various processing units such as an ECU, various controllers or various computer systems such as a microcomputer device. If so, the reference model formula is corrected. More specifically, the first correction unit corrects the reference model equation so that the NOx purification rate increases as the oxygen concentration decreases.

  Note that “so that the purification rate is increased” means that the reference model formula itself directly defines the NOx purification rate as long as it is known that the purification rate increases or the purification rate is estimated to increase. This is to say that it may be irrelevant to whether or not the expression is. Further, the correction mode according to the first correction means increases the NOx purification rate in accordance with the decrease in oxygen concentration, and this type of correction is not performed for the NOx purification characteristic defined by the reference model equation. The purpose is not limited as long as it can be approximated to the actual NOx purification characteristics considerably more than the case. For example, such correction may be made by adding, subtracting, multiplying, or dividing some correction term to the reference model formula. Further, “in response to a decrease in the oxygen concentration” means that the change in the oxygen concentration does not necessarily correspond to the change in the purification rate on a one-to-one basis. May be in a one-to-many relationship with each other.

  In one aspect of the exhaust gas state estimating apparatus according to the present invention, the exhaust gas state estimating apparatus further includes second correction means for correcting the reference model equation based on a thermal load applied to the catalyst.

  According to this aspect, for example, the reference model formula is applied to the heat load applied to the catalyst by the action of the second correction unit configured as various processing units such as an ECU, various controllers or various computer systems such as a microcomputer device. Based on the correction.

  Here, the heat load applied to the catalyst is preferably a function of the temperature of the inflowing gas flowing into the catalyst, the temperature of the catalyst, the flow rate of the inflowing gas, the oxygen concentration in the inflowing gas, or the air-fuel ratio, For example, the second correction unit may include a correction coefficient, a correction equation, or a correction function that is variable according to an index value representing these (preferably a part of the state of the inflowing gas specified by the specifying unit). The reference model formula is corrected by selecting from a map stored in the appropriate storage means in advance, or by obtaining or calculating in accordance with a preset algorithm or calculation formula.

  The purification characteristics of the purification target substance in the catalyst are correspondingly deteriorated according to the thermal load applied to the catalyst. According to this aspect, the reference model equation, that is, the purification characteristic in the catalyst is corrected according to the thermal load. Therefore, the state of the exhaust gas finally estimated by the estimation means takes into account the thermal load, and it becomes possible to estimate the state of the exhaust gas more accurately.

  In another aspect of the exhaust gas state estimating apparatus according to the present invention, the reaction rate constant represents a term corresponding to a frequency factor representing the number of active sites in the catalyst and energy when the catalytic reaction occurs. And a term corresponding to the activation energy.

  According to this aspect, the reaction rate constant constituting the reference model equation includes the frequency factor representing the number of active points and the activation energy term representing the energy required when the catalytic reaction occurs. The purification characteristics of the substance to be purified can be brought closer to the actual purification characteristics, and the state of the exhaust gas can be estimated with high accuracy.

  According to this aspect, the deterioration of the catalyst can be suitably approximated by correcting the terms of the frequency factor and the activation energy according to the thermal load in the correction according to the second correction unit described above. .

  In another aspect of the exhaust gas state estimating apparatus according to the present invention, the reference model equation adds the product of the product and the concentration of the reducing agent corresponding to the substance to be purified for each type of the reducing agent. Is included.

  According to this aspect, the reference model equation further includes a reducing agent (for example, CO (carbon monoxide) or HC (carbonization if the purification target substance is NOx) in addition to the product of the reaction rate constant and the concentration of the purification target substance. The addition term formed by adding the term multiplied by the concentration of hydrogen) etc. for each type of the reducing agent is included. That is, in this aspect, the reference model equation defines the purification characteristics of the purification target substance in consideration of not only the concentration of the purification target substance but also the influence of the reducing agent. Therefore, it becomes possible to estimate the state of the exhaust gas more precisely.

  When the influence of the reducing agent is taken into account, for each of the products of the term corresponding to the concentration of the substance to be purified and the term corresponding to the concentration of the reducing agent and the reaction rate constant, which constitute the addition term described above, for example, It is necessary to identify coefficients and characteristic terms related to the reaction rate constant (that is, for example, the above-described frequency factor, activation energy, or further suppression term), and the time load related to identification tends to increase. Therefore, whether or not the influence of the reducing agent is taken into account may be determined individually and concretely each time, for example, in accordance with vehicle specifications, destinations, required performance of the internal combustion engine, and the like.

  Such an operation and other advantages of the present invention will become apparent from the embodiments described below.

<Embodiment of the Invention>
Various preferred embodiments of the present invention will be described below with reference to the drawings.

<1: First Embodiment>
<1-1: Configuration of Embodiment>
<1-1-1: Configuration of engine system>
First, the configuration of the engine system 10 according to the first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram of the engine system 10.

  In FIG. 1, the engine system 10 includes an ECU 100 and an engine 200.

  The ECU 100 is an electronic control unit that includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like, and controls the entire operation of the engine system 10. In addition, an emission estimation model 300 (described later) for estimating the emission state of the engine 200 (that is, an example of the “exhaust gas state” according to the present invention) is built in the ECU 100. The ECU 100 is also configured to function as an example of the “exhaust gas state estimation device” according to the present invention.

  The engine 200 is an example of the “internal combustion engine” according to the present invention. The engine 200 causes the air-fuel mixture to explode by an ignition operation of an ignition device 202 in which a part of a spark plug that is a part of the cylinder 201 is exposed, and the reciprocating motion of the piston 203 generated according to the explosive force is connected. It can be converted into a rotational motion of the crankshaft 205 via the rod 204. A crank position sensor 206 that detects the rotational position (ie, crank angle) of the crankshaft 205 is installed in the vicinity of the crankshaft 205. The crank position sensor 206 is electrically connected to the ECU 100, and the ECU 100 is configured to be able to control the ignition timing and the like of the ignition device 202 based on the crank angle detected by the crank position sensor 206. Yes. Further, the ECU 100 is configured to be able to calculate the engine speed Ne of the engine 200 based on the rotational position of the crankshaft 205. Below, the principal part structure of the engine 200 is demonstrated with a part of the operation | movement.

  At the time of fuel combustion in the cylinder 201, the air sucked from the outside passes through the intake pipe 207 and is mixed with the fuel injected from the injector 214 at the intake port 213 to become the above-mentioned air-fuel mixture. The fuel is stored in the fuel tank 215 and is pumped and supplied to the injector 214 via the delivery pipe 216 by the action of the low pressure pump 217. The injector 214 is electrically connected to the ECU 100, and is configured to be able to inject the supplied fuel into the intake port 213 according to the control of the ECU 100.

  The communication state between the inside of the cylinder 201 and the intake pipe 207 is controlled by opening and closing the intake valve 218. The air-fuel mixture combusted inside the cylinder 201 becomes exhaust and is led to the exhaust pipe 221 via the exhaust port 220 when the exhaust valve 219 that opens and closes in conjunction with the opening and closing of the intake valve 218 is opened.

  A cleaner 208 is disposed on the intake pipe 207 to purify air sucked from the outside. An air flow meter 209 is further disposed on the downstream side (cylinder side) of the cleaner 208. The air flow meter 209 has a form called a hot wire type, and is configured to be able to directly detect the mass flow rate of the sucked air. The air flow meter 209 is electrically connected to the ECU 100, and the detected mass flow rate of the intake air is constantly grasped by the ECU 100.

  A throttle valve 210 that adjusts the amount of intake air related to the air sucked into the cylinder 201 is disposed downstream of the air flow meter 209 in the intake pipe 207. A throttle position sensor 212 is electrically connected to the throttle valve 210, and is configured to be able to detect the throttle opening that is the opening.

  The slot valve motor 211 is a motor that is electrically connected to the ECU 100 and configured to drive the throttle valve 210. The ECU 100 is configured to be able to control the driving state of the throttle valve motor 211 based on an accelerator opening detected by an unillustrated accelerator position sensor, whereby the opening / closing state of the throttle valve 210 (that is, the throttle valve) Opening degree) is controlled.

  The throttle valve 210 is a kind of electronically controlled throttle valve as described above, and the throttle opening degree can be controlled by the ECU 100 regardless of the driver's intention (that is, the accelerator opening degree).

  A three-way catalyst 223 is installed in the exhaust pipe 221. The three-way catalyst 223 is a catalyst that can be purified by oxidizing CO and HC in exhaust gas discharged from the engine 200 and reducing NOx, respectively. Is an example.

  An air-fuel ratio sensor 222 is disposed upstream of the three-way catalyst 223 in the exhaust pipe 221. The air-fuel ratio sensor 222 is configured to be able to detect the air-fuel ratio of the engine 200 from the exhaust gas discharged through the exhaust port 220. The air-fuel ratio sensor 222 is electrically connected to the ECU 100, and the detected air-fuel ratio is constantly grasped by the ECU 100.

  In addition, a temperature sensor 224 for detecting the temperature of the cooling water for cooling the engine 200 is disposed in the water jacket installed in the cylinder block that houses the cylinder 201. The temperature sensor 224 is electrically connected to the ECU 100, and the detected temperature of the cooling water is constantly grasped by the ECU 100.

<1-1-2: Configuration of Emission Estimation Model>
Next, the configuration of the emission estimation model 300 built in the ECU 100 will be described with reference to FIG. FIG. 2 is a schematic diagram conceptually showing the configuration of the emission estimation model 300.

  In FIG. 2, the emission estimation model 300 includes a catalyst model 310, a catalyst inflow gas model 320, and a catalyst deterioration model 330.

  The catalyst model 310 may output an emission value representing the emission state of the catalyst exhaust gas discharged from the three-way catalyst 223 (for example, the component, component ratio, component concentration, or component amount of the catalyst exhaust gas) as an estimation result. It is a model that can be configured. The catalyst model 310 is a model of a catalytic reaction that occurs in an actual catalyst (for example, a three-way catalyst 223 or a catalyst having a configuration equivalent thereto). The “estimating means” and “first correction” according to the present invention are modeled. Means "and" second correction means "are configured to function as examples.

  The catalyst inflow gas model 320 is input with index values (corresponding to “engine condition index value” in the figure) such as the engine speed Ne, the load factor KL, the air-fuel ratio A / F, the ignition timing SA, and the valve timing VT in the engine 200. The state of the catalyst inflow gas flowing into the three-way catalyst 223 in accordance with a preset algorithm and calculation formula (for example, the catalyst inflow gas component, component ratio, component concentration or component amount, catalyst inflow gas temperature and flow rate, etc. ) Can be calculated. The catalyst inflow gas model 320 is electrically connected to the catalyst model 310, and the calculated index value indicating the state of the catalyst inflow gas is input to the catalyst model 310. The catalyst inflow gas model 320 is an example of the “specifying means” according to the present invention.

  The catalyst deterioration model 330 is electrically connected to the catalyst model 310 and models the deterioration of the purification characteristics in the three-way catalyst 223 caused by a heat load. The catalyst deterioration model 330 receives a thermal load index value including the catalyst temperature and oxygen concentration from the catalyst model 310 as an input, calculates a correction value according to the thermal load based on the received thermal load index value, and generates a catalyst model. It is configured to be able to output to 310. The catalyst deterioration model 330 is an example of the “second correction unit” according to the present invention together with the catalyst model 310.

<1-2: Operation of Embodiment>
Hereinafter, the operation of the emission estimation model 300 will be described as the operation of the present embodiment. The three-way catalyst 223 mainly undergoes various catalytic reactions such as NOx reduction reaction (desorption reaction of oxygen from NOx), CO oxidation reaction, and HC oxidation reaction. This reduction reaction will be described as an example.

<1-2-1: Basic operation of emission state estimation>
In the catalyst model 310, basically, the purification rate in the three-way catalyst 223 is defined for each of the substances to be purified by the following formula (1) (an example of the “reference model formula” according to the present invention). In addition, [NOx] in the equation (1) represents the concentration of NOx as one of the purification target substances, and an initial value is an index that defines the state of the catalyst inflow gas described above from the catalyst inflow gas model 320. Obtained as part of the value. Further, k is a power exponent that is basically set to “1” and is appropriately adjusted in the process of identifying the purification rate.

d [NOx] / dt = K · [NOx] ^ k (1)
Here, K in the above equation (1) is a reaction rate constant, and is defined by the following equation (2).

K = A / G · exp (-E / RT) (2)
Here, A represents a frequency factor, G represents a suppression term, and E represents activation energy. R is a known gas constant, and T is the absolute temperature of the catalyst inflow gas. Here, the frequency factor A is a parameter indicating the number of active points in the three-way catalyst 223, and indicates that the larger the value, the more the number of active points. Therefore, the larger the value of the frequency factor A, the more the catalytic reaction in the three-way catalyst 223 is promoted.

  The activation energy E represents the energy required when the catalytic reaction occurs, and the smaller the activation energy E, the easier the catalytic reaction occurs. That is, the smaller the activation energy E, the larger the value of the reaction rate constant K. Further, the suppression term G represents a factor that suppresses the catalytic reaction, and the larger the value, the more the catalytic reaction is suppressed. That is, the value of the reaction rate constant K becomes small.

  When the purification rate in the three-way catalyst 223 is calculated for each of the substances to be purified according to the above equation (1), the catalyst model 310 first relates to the shape of the three-way catalyst 223 stored in advance in a ROM or the like. Physical property values, for example, index values such as the cell opening area, the number of cells, and the heat transfer coefficient are input. Then, from the catalyst inflow gas model 320, the above-described index value that defines the state of the catalyst inflow gas is input.

  When these physical property values and index values are input, the catalyst model 310 repeats the numerical calculation processing according to the equation (1) at a frequency corresponding to the set number of cells. More specifically, the catalyst model 310, for example, for the first cell, the NOx concentration that is the initial value (that is, the NOx concentration of the catalyst inflow gas) and the numerical calculation based on the equation (1) NOx purification rate is obtained, and based on the obtained purification rate, the NOx concentration to be subjected to the numerical calculation processing related to the purification rate estimation in the second cell is obtained.

  The catalyst model 310 further repeats the same processing for the set number of cells, and finally calculates the NOx concentration at the outlet of the three-way catalyst 223, that is, in the catalyst exhaust gas. When the NOx concentration in the catalyst exhaust gas is calculated, the catalyst model 310 calculates an emission value based on the NOx concentration and outputs it as an emission state estimation result. In the catalyst model 310, such a process is performed in parallel for each of the substances to be purified.

  On the other hand, the three-way catalyst 223 becomes a high temperature due to exhaust gas (substantially catalyst inflow gas) exhausted from the cylinder 201, and the air-fuel ratio or intake as a catalyst temperature or oxygen concentration (or oxygen concentration is defined). Deteriorated by heat load factors such as air volume. The catalyst deterioration model 330 calculates a heat load correction coefficient for correcting the above equation (1) based on the heat load index values such as the catalyst temperature and the oxygen concentration input from the catalyst model 310. The calculated thermal load correction coefficient is input to the catalyst model 310.

  In the equation (1), the influence of the thermal load mainly appears in the frequency factor A and the activation energy E. More specifically, the greater the heat load, the smaller the frequency factor A and the greater the activation energy E. That is, the greater the heat load, the lower the NOx purification rate defined by equation (1). The catalyst deterioration model 330 calculates a thermal load correction coefficient as a comprehensive representation of the change characteristics of the frequency factor A and the activation energy E according to the thermal load, and outputs the thermal load correction coefficient to the catalyst model 310.

  The catalyst model 310 corrects the equation (1) by multiplying the equation (1) by the thermal load correction coefficient input from the catalyst deterioration model 330 when estimating the emission characteristics of the catalyst exhaust gas described above. Therefore, the catalyst model 310 can output an accurate emission value in consideration of the heat load applied to the three-way catalyst 223.

  As described above, if the values of the frequency factor A, the suppression term G, the activation energy E, and the power exponent k that constitute the equation (1) can be set to values suitable for the actual situation, the equation (1) It is possible to accurately approximate the purification rate of the substance to be purified in the actual three-way catalyst 223 by the numerical calculation processing executed by the catalyst model 310 based on the catalyst exhaust gas that is finally output from the catalyst model 310 The emission value can be accurately estimated without actually measuring the emission of the catalyst exhaust gas.

  In view of such circumstances, the estimation accuracy of the emission state in the catalyst model 310 is greatly influenced by the identification accuracy of the reaction rate constant K. Therefore, the reaction rate constant K can be identified by purifying according to the operating conditions such as the air-fuel ratio obtained from the actual catalyst (for example, the three-way catalyst 223 before being mounted on the engine 200 or a catalyst equivalent thereto). This is performed before the emission estimation model 300 is built in the ECU 100 in advance so as to conform to the actually measured value of the rate.

<1-2-2: Correction in consideration of oxygen concentration>
On the other hand, when oxygen is sufficiently present inside the three-way catalyst 223, the desorption of oxygen from NOx is hindered by such abundant oxygen, and thus the catalytic reaction of NOx tends to be delayed. That is, the purification rate tends to decrease. On the other hand, in the high load region (high flow rate region) of the engine 200 where the flow rate of the catalyst inflow gas is relatively high, in addition to the large amount of absolute oxygen in the catalyst inflow gas, the flow rate of the catalyst inflow gas is high. The oxygen concentration inside the three-way catalyst 223 tends to increase due to a decrease in the reaction rate of other oxidation reactions caused by the above.

  Therefore, for example, in the high load region of the engine 200, there is a situation in which the purification of NOx hardly progresses as it gets closer to the catalyst inlet. For this reason, in the high load region, depending on the oxygen concentration, a situation called “NOx blow-through” occurs in which NOx passes through the three-way catalyst 223 without being purified.

  However, when applying the purification rate defined by the above-described equation (1) to NOx, it is difficult to express the NOx blow-through phenomenon because the effect of such oxygen concentration is not described in equation (1). . That is, when the NOx purification rate is defined in accordance with the equation (1), it is likely to deviate significantly from the actual purification rate in the high load region, and the estimation accuracy of the emission characteristics of the catalyst exhaust gas is likely to be extremely lowered.

Here, the NOx purification characteristics of the three-way catalyst 223 will be described with reference to FIG. FIG. 3 is a schematic diagram conceptually showing changes in the concentrations of NOx and O ₂ in the three-way catalyst 223.

  In FIG. 3, the vertical axis represents the gas concentration in the three-way catalyst 223, and the horizontal axis represents the inlet of the three-way catalyst 223 (that is, the front side end of the three-way catalyst 223. The length L from (referred to as “)” is represented.

  In FIG. 3, for each of the oxygen concentration and the NOx concentration, the concentration change characteristics corresponding to two different load regions of the engine 200, that is, a low load (low flow rate) region and a high load (high flow rate) region ( That is, an example of the purification characteristic) is shown.

The change characteristic of the oxygen concentration in the low load region is shown as PRFO ₂ 1 (thin solid line) in the figure. That is, at the catalyst inlet (the position where the illustrated length L is 0), the oxygen concentration takes the initial value B and is the outlet of the three-way catalyst 223 (that is, the rear side end portion, hereinafter referred to as “catalyst outlet” as appropriate). ), That is, the density is substantially zero at the position of the illustrated length Lmax.

  The change characteristic of the NOx concentration in the low load region is represented as illustrated PRFNOx1 (thick solid line). Here, as described above, NOx purification is started inside the catalyst after the oxygen concentration has decreased to some extent. When the oxygen concentration at which the NOx purification is started is D (D <B), the position in the catalyst at which the NOx purification is started is a position corresponding to the illustrated length L1. That is, in the region from the catalyst entrance to the length L1, NOx passes through the catalyst without being purified (not purified to such an extent that it is considered practically effective). Further, when NOx purification starts from a position where the length from the catalyst inlet becomes L1, the NOx concentration gradually decreases and becomes substantially zero at the catalyst outlet.

On the other hand, variation characteristics of the oxygen concentration in the high load range is indicated as shown PRFO 2 2 _(shown thin dashed line). As shown in the figure, the flow rate of the catalyst inflow gas is relatively high in the high load region, so that the oxygen purification rate becomes relatively slow, and the oxygen concentration is reduced to the above-mentioned concentration D by the length L2 from the catalyst inlet. Is the position.

  The characteristic of the NOx concentration in the high load region is illustrated as PRFNOx2 (shown with a thick broken line). That is, according to the same concept as in the low load region, the NOx concentration starts to decrease at the position of the length L2 from the catalyst inlet, and the NOx concentration becomes C (B <C <D) at the catalyst outlet. In this way, in the high load region, the rate of decrease in oxygen concentration becomes slow, so NOx is not sufficiently purified inside the catalyst, and a considerable amount of NOx is exhausted without being purified, and the emission of catalyst exhaust gas deteriorates. A situation occurs.

  Such a blow-through phenomenon of NOx cannot be sufficiently expressed by the above-described equation (1). Therefore, the emission estimation result related to the catalyst model 310 and the actual emission characteristics are noticeably in a high load region. It will be a big difference.

  Therefore, in the present embodiment, an expression that defines the NOx purification rate is expressed by the following expression (3). That is, when estimating the NOx purification rate, the catalyst model 310 corrects the above equation (1) to the equation (3) in order to consider the influence of the oxygen concentration. In the equation (3), n is a power exponent to be identified so as to match the actual purification rate similarly to k, and is basically set to 1.

d [NOx] / dt = K · [NOx] ^ k / (1+ [O ₂ ] ^ n) (3)
According to the equation (3), the NOx purification rate is low in a region where the oxygen concentration is high, and NOx purification is substantially started when the oxygen concentration is sufficiently low. That is, the NOx concentration change characteristic in the catalyst conceptually shown in FIG. 3 can be approximated with high accuracy.

Of course, the correction mode when considering the influence of the oxygen concentration is not limited to this. For example, in the equation (3), the term of oxygen concentration is set to the oxygen concentration, for example, “A ₁ exp (−E ₁ / RT) May be replaced by a term that is multiplied by a constant. Here, A ₁ and E ₁ represent the frequency factor and the activation energy in the same manner as the reaction rate constant described above, and are identified in advance so as to match the actual NOx purification rate.

  Here, with reference to FIG. 4, the estimation result of the NOx concentration obtained based on the equation (3) will be described. FIG. 4 is a schematic diagram conceptually showing the characteristic of the NOx concentration in the catalyst exhaust gas with respect to the intake air amount Ga.

  In FIG. 4, a characteristic corresponding to the actual change characteristic of the NOx concentration shown in FIG. 3 is shown as PRFNOx3 (solid line) in the figure. That is, as already described, in the region where the intake air amount Ga is high (high load region), the NOx concentration in the catalyst exhaust gas rapidly increases.

  On the other hand, the NOx concentration estimated by the catalyst model 310 according to the equation (3) is expressed as illustrated PRFNOx4 (illustrated two-dot chain line). That is, since the correction term for the oxygen concentration is further applied to the equation (1), PRFNOx4 is asymptotic to PRFNOx3 and rapidly increases in a region where the intake air amount Ga is high.

  Here, FIG. 4 further shows illustrated PRFNOx 5 (illustrated broken line) and PRFNOx 6 (illustrated dashed line) as comparative examples according to the present embodiment. Both of these characteristics are changes in the NOx concentration estimated by the catalyst model 310 according to the equation (1), and since both do not consider the influence of the oxygen concentration, the NOx concentration changes with respect to the change in the intake air amount Ga. Change is linear.

  Therefore, if the estimation accuracy of the NOx concentration in the high load region is increased using PRFNOx5, the estimation accuracy of the NOx concentration in the low load region (region where the intake air amount Ga is low) decreases, and the load is reduced using PRFNOx6. If an attempt is made to increase the NOx concentration estimation accuracy in the region, the NOx concentration estimation accuracy in the high load region (region where the intake air amount Ga is high) decreases. In other words, it is clear that it is difficult to sufficiently estimate the change in the NOx concentration with respect to the oxygen concentration when the equation (1) is used.

  In this respect, as shown as PRFNOx4, when the influence of the oxygen concentration is taken into account by the correction term of the oxygen concentration, the concentration change of NOx according to the oxygen concentration (approximately, the intake air amount Ga) is accurately determined. Thus, the catalyst model 310 can estimate the emission characteristics in the catalyst exhaust gas with high accuracy.

<2: Second Embodiment>
In the first embodiment, the NOx purification rate in the three-way catalyst 223 is defined in a form including the product of the NOx concentration and the reaction rate constant K by the equation (3). However, since the NOx purification rate can also be related to the concentration of CO, HC, or the like that functions as a reducing agent, when estimating the NOx concentration change, the estimation accuracy is further improved by considering the influence of these reducing agents. there is a possibility.

  Therefore, as a second embodiment, the catalyst model 310 changes the NOx concentration in the catalyst exhaust gas based on the NOx purification rate expressed by the following equation (4) instead of the equation (3) according to the first embodiment. presume.

Purification rate = d [NOx] / dt = K _1. [NOx] ^ k _1. [First reducing agent] ^ m ₁ / (1+ [O ₂ ] ^ n ₁ ) + K _2. [NOx] ^ k _2. [Second reducing agent] ^ m ₂ / (1+ [O ₂ ] ^ n ₂ ) (4)
Here, [first reducing agent] and [second reducing agent] represent the concentration of the first reducing agent (for example, CO or HC) and the concentration of the second reducing agent (for example, HC or CO), respectively. K ₁ and K ₂ are reaction rate constants, respectively, k ₁ , k ₂ , m ₁ , and m ₂ are n ₁ and n ₂ are power exponents, respectively.

  According to the equation (4), since the concentration of the reducing agent is further considered for each reducing agent while considering the influence of the oxygen concentration as in the first embodiment, the actual NOx purification rate is more accurately determined. Approximated. However, at this time, since the load related to coefficient identification, which is the previous stage built in the ECU 100, increases, which equation the catalyst model 310 should use in estimating the emission characteristics depends on the specifications and destinations of the engine 200. Further, it may be specifically determined for each engine 200 according to the required performance and the like. In any case, the NOx purification rate can be accurately expressed as compared with the equation (1). Therefore, the NOx purification rate in the three-way catalyst 223 can be estimated with high accuracy. Finally, the emission characteristics of the catalyst exhaust gas, and particularly the accuracy related to the estimation of the NOx emission amount, are dramatically improved.

  The present invention is not limited to the above-described embodiment, and can be appropriately changed without departing from the gist or concept of the invention that can be read from the claims and the entire specification, and the state of exhaust gas accompanying such a change. The estimation device is also included in the technical scope of the present invention.

It is a mimetic diagram showing the engine system concerning a 1st embodiment of the present invention. FIG. 2 is a schematic diagram conceptually showing a configuration of an emission estimation model built in an ECU in the engine system of FIG. 1. It is a schematic diagram conceptually showing the concentration characteristics of NOx and O ₂ in a three-way catalyst. It is a schematic diagram conceptually showing the characteristic of NOx concentration in catalyst exhaust gas with respect to the amount of intake air.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Engine system, 100 ... ECU, 200 ... Engine, 201 ... Cylinder, 203 ... Piston, 205 ... Crankshaft, 222 ... Air-fuel ratio sensor, 223 ... Three-way catalyst, 300 ... Emission estimation model, 310 ... Catalyst model, 320 ... catalyst inflow gas model, 330 ... catalyst deterioration model.

Claims (4)

An exhaust gas state estimation device for estimating a state of exhaust gas discharged from a catalyst in an internal combustion engine equipped with a catalyst,
Specifying means for specifying the state of the inflowing gas flowing into the catalyst;
Including a product of a term corresponding to each concentration and a reaction rate constant relating to a catalytic reaction in the catalyst, which is established for each of the substances to be purified in advance to define the purification characteristics of the catalyst in the catalyst. Estimating means for estimating the state of the exhaust gas based on the specified state of the inflowing gas according to a reference model equation
First correction means for correcting the reference model equation so that the purification rate of the nitrogen oxides increases as the oxygen concentration decreases when the substance to be purified is nitrogen oxides. An exhaust gas state estimation device characterized by the above. The exhaust gas state estimation device according to claim 1, further comprising second correction means for correcting the reference model formula based on a thermal load applied to the catalyst. The reaction rate constant includes a term corresponding to a frequency factor representing the number of active sites in the catalyst and a term corresponding to activation energy representing energy when the catalytic reaction occurs. Item 3. The exhaust gas state estimation apparatus according to Item 1 or 2. The said reference model formula includes the addition term formed by adding the product of the said product and the density | concentration of the reducing agent corresponding to the said purification target substance for every kind of this reducing agent. The exhaust gas state estimation device according to any one of the above.

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