JP2005002954A - Exhaust gas cleaning system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning system which uses an NOx occulsion reduction type catalyst for clean-up of NOx in exhaust gas, so as to improve the rate of clean-up of NOx while preventing a reducing agent from being discharged into the atmosphere. <P>SOLUTION: The system is arranged such that the NOx accumulation amount in the NOx occulsion reduction type catalyst 3 is computed on the basis of the NOx concentration before and behind the NOx occulsion reduction type catalyst 3 detected by an NOx concentration detection means C13 and the amount of intake air detected by an intake air amount detection means C11. In the system, the amount of discharge of NOx per unit time when rich control is carried out, is computed on the basis of the catalyst temperature index value detected by a catalyst temperature detection means C12. A period of time for the rich control is decided depending on the computed NOx accumulation amount and the computed amount of discharge of NOx per unit time. The air fuel ratio of the exhaust gas is controlled to be in a rich state by an air fuel ratio control means C31 for the period of time for the rich control. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification system for an internal combustion engine that includes a NOx storage reduction catalyst that reduces and purifies NOx (nitrogen oxides) in the exhaust gas of the internal combustion engine.
[0002]
[Prior art]
Various studies and proposals have been made on NOx catalysts for reducing and removing NOx from internal combustion engines such as diesel engines and some gasoline engines and exhaust gases from various combustion devices. One of them is a NOx occlusion reduction type catalyst as a NOx reduction catalyst for diesel engines, which can effectively purify NOx in exhaust gas.
[0003]
As shown in FIG. 7, the NOx occlusion reduction catalyst basically has a noble metal 32 such as platinum (Pt) or palladium (Pd) that promotes an oxidation / reduction reaction on a catalyst carrier 31 such as alumina. , A catalyst carrying a NOx occlusion material (NOx occlusion material) 33 having a function of occlusion / release NOx formed of alkaline earth metals such as barium (Ba).
[0004]
In this NOx occlusion reduction type catalyst, the air-fuel ratio of the inflowing exhaust gas is lean (high oxygen concentration) and oxygen (O₂) Is present, NO in the exhaust gas is oxidized by the noble metals 32 as shown in FIG.₂And this NO₂NOx occlusion material 33 with nitrate (Ba₂NO₄).
[0005]
In addition, when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or a rich (low oxygen concentration) state, and no oxygen exists in the atmosphere, as shown in FIG. 7B, a NOx storage material such as Ba or the like 33 binds to CO and from nitrates to NO₂Is decomposed and released, and this released NO₂Is reduced by unburned HC, CO, etc. contained in the exhaust gas by the three-way function of the noble metals 32 and N₂The various components in the exhaust gas are CO₂, H₂O, N₂It is released into the atmosphere as a harmless substance.
[0006]
Therefore, in an exhaust gas purification system equipped with a NOx occlusion reduction type catalyst, when the NOx occlusion capacity becomes close to saturation, the air-fuel ratio of the exhaust gas is made rich, and the oxygen concentration of the inflowing exhaust gas is reduced, for recovering the NOx occlusion capacity By performing the rich control, the absorbed NOx is released and the released NOx is reduced by the noble metal catalyst.
[0007]
In order to effectively function the NOx occlusion reduction type catalyst, it is necessary to supply a sufficient amount of reducing agent necessary for reducing the NOx occluded in the lean state in the rich state.
[0008]
However, NO from the above nitrates₂The chemical reaction of the decomposition and release of is slow at low temperatures and fast at high temperatures.₂Since the decomposition / desorption rate of NOx depends on the catalyst temperature, the storage function and the reduction function of the NOx storage reduction catalyst depend on the catalyst temperature as shown in FIG.
[0009]
That is, in the extremely low temperature region (A), the NOx storage function and the reduction function are low, and NOx is not stored and reduced. In the low temperature region (B) around 200 ° C., NO₂Since the release rate (decomposition and separation rate) is small, the occlusion function is high but the reduction function is low, and NOx is occluded but the reduction ability is insufficient. Furthermore, in the middle temperature region (C) around 450 ° C., NO₂Since the release rate of the water is large, both the occlusion function and the reduction function are high, and the occlusion ability and the reduction ability are sufficient. In the high temperature region (D) of about 600 ° C. or higher, the occlusion amount decreases due to the temperature rise, and the occlusion capacity decreases.
[0010]
Considering the catalyst temperature dependence of the NOx storage reduction catalyst of these NOx storage reduction type catalysts, the higher the temperature (catalyst temperature, etc.) representing the temperature of the NOx absorbent (NOx storage material), the greater the degree of richness. There has been proposed an exhaust purification device for an internal combustion engine provided with NOx release control means for shortening the time to make or enrich (for example, see Patent Document 1).
[0011]
[Patent Document 1]
Japanese Patent No. 2722951 (2nd page)
[0012]
[Problems to be solved by the invention]
However, if the rich control time is determined in accordance with the NOx accumulation amount actually accumulated in the NOx occlusion reduction catalyst and an appropriate amount of reducing agent is not supplied, the NOx amount released from the NOx occlusion material and the reduction are reduced. Since the amount of the agent cannot be balanced, there arises a problem that NOx or the reducing agent is discharged downstream of the catalyst.
[0013]
That is, as shown in FIG.₂In the temperature region (C) where the decomposition and release rate of the catalyst is sufficiently activated, NO₂Since the separation / separation speed is large and the reduction function works sufficiently, the NOx purification rate is reduced, and the amount of CO used as a reducing agent in the catalytic reaction increases, so that it is discharged to the downstream side of the catalyst (slip: SLIP) is also reduced.
[0014]
On the other hand, as shown in FIG.₂In the region (B) where the decomposition and desorption rate is slow, the decomposition rate is low and only the surface of the NOx storage material reacts, so that the time required for the decomposition reaction is insufficient,₂Accumulates without being completely desorbed from the occlusion material, and the amount of NOx discharged gradually increases. Further, when a reducing agent such as CO is supplied, an excessive reducing agent is supplied with respect to the reaction rate of NOx reduction, so that an excessive reducing agent that is not used is discharged (slip: SLIP) to the downstream side of the catalyst. .
[0015]
The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a NOx occlusion reduction type catalyst in an exhaust gas purification system that uses a NOx occlusion reduction type catalyst to purify NOx in exhaust gas. The exhaust gas air-fuel ratio is richly controlled by detecting the catalyst temperature index value such as the exhaust gas temperature upstream of the engine and calculating the rich control time and the reducing agent supply amount according to the detected catalyst temperature index value. Thus, an object of the present invention is to provide an exhaust gas purification system capable of improving the NOx purification rate while preventing the reducing agent from being discharged into the atmosphere.
[0016]
[Means for Solving the Problems]
An exhaust gas purification system for an internal combustion engine for achieving the above-described object is provided for recovering the NOx occlusion reduction type catalyst disposed in the exhaust passage of the internal combustion engine and the NOx occlusion ability of the NOx occlusion reduction type catalyst. In the exhaust gas purification system provided with the catalyst regeneration control device, the catalyst regeneration control device comprises intake air amount detection means, air-fuel ratio control means, catalyst temperature detection means, and NOx concentration detection means, and the NOx concentration detection means The cumulative NOx amount in the NOx occlusion reduction catalyst is calculated from the NOx concentration before and after the NOx occlusion reduction catalyst detected in step 1 and the intake air amount detected by the intake air amount detection means, and the catalyst temperature detection The amount of NOx released per unit time when rich control is performed is calculated from the catalyst temperature index value detected by the means, and the calculated accumulated NOx amount is simply calculated. Determine the rich control time from the NOx emission amount per unit time, during the rich control time, configured to control the air-fuel ratio in the exhaust gas to a rich state by said air-fuel ratio control means.
[0017]
The catalyst temperature index value used here is a physical quantity or numerical value correlated with the catalyst temperature. The measured catalyst temperature itself or a temperature at which the catalyst temperature can be estimated, for example, the measured exhaust temperature, engine The catalyst temperature estimated from the map data or the like input in advance from the load and the rotational speed of the catalyst. In general, since it is difficult to directly measure the catalyst temperature, the exhaust temperature and the like are substituted for the catalyst temperature. Here, an index value that substitutes for the catalyst temperature is referred to as a catalyst temperature index value.
[0018]
In the exhaust gas purification system, the catalyst regeneration control device includes a reducing agent supply unit, and calculates a reducing agent supply amount per unit time from a catalyst temperature index value detected by the catalyst temperature detection unit, The reducing agent is supplied by the reducing agent supply means so that the reducing agent supply amount per unit time is obtained.
[0019]
Alternatively, in the above exhaust gas purification system, the catalyst regeneration control device includes a reducing agent supply means, calculates a reducing agent supply amount per unit time from the NOx release amount per unit time, and supplies the reducing agent supply It is comprised so that a reducing agent may be supplied so that it may become the reducing agent supply amount per said unit time by a means.
[0020]
Further, in the exhaust gas purification system, the air-fuel ratio control unit is configured to replace the air-fuel ratio in the exhaust gas in the rich state by the air-fuel ratio control unit during the rich control time. To control the air-fuel ratio in the exhaust gas to a rich state for a predetermined time, calculate the NOx occlusion amount calculated from the NOx release amount per unit time and the predetermined time, and calculate The remaining amount of NOx is calculated by subtracting the reduced amount from the accumulated NOx amount, and the air-fuel ratio in the exhaust gas is controlled to be rich until the remaining amount of NOx becomes zero or less. .
[0021]
According to these configurations, since the NOx accumulated amount is calculated from the NOx concentration before and after the NOx storage reduction catalyst and the intake air amount, not only the engine operating state change but also the environmental change and the transient mode are calculated. Therefore, it is possible to cope with a change in the NOx emission amount due to the above, and the estimation accuracy becomes higher.
[0022]
Further, since the NOx release amount at the time of rich control is calculated based on the catalyst temperature detected by the catalyst temperature detecting means, it is possible to perform estimation in consideration of the temperature characteristics of the NOx occlusion reduction type catalyst. Increases accuracy.
[0023]
Since the rich control time is determined from the NOx cumulative amount estimated with high accuracy and the NOx release amount per unit time, useless rich control that is not related to the recovery of the NOx storage capacity becomes unnecessary, and fuel consumption is reduced. Savings.
[0024]
Furthermore, the reducing agent supply amount per unit time is also calculated based on the catalyst temperature, or calculated based on the NOx release amount per unit time, and supplied in an amount corresponding to the NOx release amount per unit time. Therefore, the amount becomes necessary and sufficient for the reduction of NOx, and the emission of NOx and the reducing agent to the atmosphere can be prevented. Further, since the rich control time during which NOx is released is also a necessary and sufficient time, the reducing agent is not wastedly released and the reducing agent can be saved.
[0025]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an exhaust gas purification system according to an embodiment of the present invention will be described with reference to the drawings.
[0026]
As shown in FIG. 1, in an automobile provided with this exhaust gas purification system 1, a mass air flow sensor 11, an intercooler 12, and a throttle valve (intake throttle valve) 13 from an upstream side to an intake passage 10 of an engine (internal combustion engine) E. And an λ (excess air ratio) sensor 21, an exhaust temperature sensor 22, an inlet-side NOx concentration sensor 23, an NOx storage reduction catalyst 3, an outlet-side NOx concentration sensor 24, and a muffler from upstream to the exhaust passage 20. A vessel 8 is provided. Further, an EGR passage 40 for recirculating exhaust gas to the intake side is provided.
[0027]
A common rail injection system 51 that performs fuel injection of the engine E and an electronic control device (electronic control box) 5 called an ECU (engine control unit) that controls the entire engine are provided. The common rail injection system 51 is supplied with fuel from the fuel tank 9, and the electronic control unit 5 is supplied with power from the battery 7.
[0028]
In this exhaust gas purification system 1, the air A passes through the mass air flow sensor 11 and the intercooler 12, and the intake air flow rate is adjusted by the throttle valve 13 controlled by the electronic control unit 5. Supplied into the cylinder.
[0029]
Further, the exhaust gas G exits the exhaust manifold of the engine E, passes through the NOx occlusion reduction type catalyst 3 in the exhaust passage 20, becomes purified exhaust gas Gc, passes through the silencer 8, and is discharged into the atmosphere. Then, the EGR gas that is a part of the exhaust gas G enters the intake manifold through the EGR passage 40 and recirculates.
[0030]
As shown in FIG. 7, the NOx occlusion reduction catalyst 3 uses a monolith honeycomb cell formed of γ alumina or the like as a carrier 31, and a catalyst metal 32 and a NOx occlusion material (NOx occlusion material) are formed on the surface of the carrier 31. (Material) 33 is supported.
[0031]
The catalytic metal 32 can be formed of platinum (Pt), palladium (Pd), or the like having oxidation activity in a temperature range higher than the activation start temperature. The NOx occlusion material 33 is composed of alkali metals such as potassium (K), sodium (Na), lithium (Li) and cesium (Cs), alkaline earth metals such as barium (Ba) and calcium (Ca), lanthanum ( La), rare earth such as yttrium (Y), etc. can be formed by any one or a combination thereof, storing NOx when the oxygen concentration in the gas is high, and releasing NOx when the oxygen concentration in the gas is low To do.
[0032]
In the NOx occlusion reduction type catalyst 3, as shown in FIG. 7A, NO in the exhaust gas is catalyzed by the catalyst metal 32 in a high oxygen concentration atmosphere in which the exhaust gas is lean (lean combustion). Oxidized by NO₂And NO₃ ⁻The NOx storage material 33 diffuses into the catalyst in the form of nitrate (Ba (NO₃)₂) Is absorbed. That is, barium carbonate (BaCO₃) To barium nitrate (Ba (NO₃)₂) To selectively NO₂Occlude.
[0033]
Then, as shown in FIG. 7B, when the exhaust gas becomes rich and the oxygen concentration decreases, NO₃ ⁻Is NO₂In the form of NOx occlusion material 33. That is, barium nitrate (Ba (NO₃)₂) To barium carbonate (BaCO)₃) To change to NO₂Release. This released NO₂Is unburned HC, CO or H contained in the exhaust gas₂Catalytic action of the catalytic metal 32 by a reducing agent such as N₂Reduced to This reduction action can prevent NOx from being released into the atmosphere.
[0034]
The exhaust gas rich state here does not necessarily need to be richly burned in the cylinder bore, but the amount of air and fuel supplied into the exhaust gas flowing into the NOx storage reduction catalyst 3 (burned in the cylinder bore). (Including the minute) is close to the stoichiometric air-fuel ratio, or the fuel is richer than the stoichiometric air-fuel ratio.
[0035]
In the exhaust gas purification system 1 of the present invention, a catalyst regeneration control device 5 a for recovering the NOx storage capacity of the NOx storage reduction catalyst 3 is provided in the electronic control device 5.
[0036]
The catalyst regeneration control device 5a includes a catalyst regeneration means C1 as shown in FIG. 2, and the catalyst generation means C1 includes an intake air amount detection means C11, a catalyst temperature detection means C12, and a NOx concentration detection means C13. NOx cumulative amount calculation means C21, NOx release amount calculation means C22, rich control time calculation means C23, reducing agent supply amount calculation means C24, air-fuel ratio control means C31, and reducing agent supply means C32.
[0037]
The intake air amount detection means C11 detects the intake air amount by the mass air flow sensor 11, and the catalyst temperature detection means C12 detects the catalyst temperature index value, but it is detected by the exhaust temperature sensor 22 instead of the catalyst temperature. The exhaust temperature Tg thus used is used as the catalyst temperature index value. Further, the NOx concentration detection means C13 detects the NOx concentration before and after the NOx storage reduction catalyst 3 by the inlet side NOx concentration sensor 23 and the outlet side NOx concentration sensor 24.
[0038]
Further, the NOx accumulation amount calculating means C21 calculates the NOx occlusion reduction type catalyst 3 from the intake air quantity detected by the intake air quantity detection means C11 and the NOx concentration before and after the NOx occlusion reduction type catalyst 3 detected by the NOx concentration detection means C13. The cumulative amount of NOx at is calculated.
[0039]
The NOx release amount calculating means C22 calculates the NOx release amount per unit time from the exhaust temperature (catalyst temperature index value) detected by the catalyst temperature detecting means C12, and the rich control time calculating means C23 is the NOx accumulated amount calculating means C21. The rich control time is calculated from the NOx accumulated amount calculated in step S1 and the NOx release amount per unit time calculated by the NOx release amount calculation means C22. The reducing agent supply amount calculating means C24 calculates the reducing agent supply amount per unit time from the exhaust temperature (catalyst temperature index value) detected by the catalyst temperature detecting means C12.
[0040]
The air-fuel ratio control means C31 is a means for bringing the exhaust gas into a rich state and bringing the atmosphere around the catalyst into a low oxygen or oxygen-free state in order to recover the NOx storage capacity of the NOx storage reduction catalyst 3. In the air-fuel ratio control to make this rich state, the oxygen concentration is reduced to zero by adjusting the fuel injection amount and injection timing of the multistage injection by the fuel injection control into the cylinder, adjusting the EGR, adjusting the intake throttle, etc. Make it near rich.
[0041]
The reducing agent supply means C32 is a means for supplying a reducing agent such as HC or CO for reducing NOx released from the NOx occlusion reduction type catalyst 3 and is reduced by post injection or the like in fuel injection into the cylinder. The reducing agent supply amount is adjusted by adjusting the amount and timing of the post injection and adjusting the EGR amount.
[0042]
In this exhaust gas purification system 1, regeneration control of the NOx occlusion reduction type catalyst 3 is performed by the catalyst regeneration means C1 of the catalyst regeneration control device 5a according to the control flow illustrated in FIG. 3 and FIG. 3 and 4 are shown to be executed in parallel with other control flows of the engine when the engine 10 is operated.
[0043]
When the control flow in FIG. 3 starts, data such as the NOx accumulation amount Nt at the end of the previous control, the rich control flag F, the actual rich control elapsed time tr are input in preparation for control in step S10, and the next step In S20, whether or not the rich control is being performed is determined by whether or not the rich control flag F is set (F = 1) or not (F = 0).
[0044]
If it is determined in step S20 that the rich control is not being performed, the process proceeds to step S21. If it is determined that the rich control is being performed, the rich control operation in step S40 is performed.
[0045]
In step S21, it is determined whether or not rich control for recovery of the NOx storage capacity is necessary, for example, based on whether or not the NOx accumulated amount Nt exceeds a predetermined determination value Nt0. If it is determined in this determination that the rich control operation is not necessary, the normal lean mode operation that is the normal lean combustion (lean combustion) operation in the lean mode operation of step S30 is performed for a predetermined time (rich control determination interval). (Time relating to) for Δtl, and the process returns to step S20. Further, when it is determined that the rich control operation is necessary, the rich control operation of step S40 is performed.
[0046]
In the lean mode operation in step S30, the inlet NOx concentration sensor 23 inputs the inlet NOx concentration Na, the outlet NOx concentration sensor 23 inputs the outlet NOx concentration Nb, and the mass airflow sensor 11 inputs the intake air amount Va. From the concentration value difference ΔN (= Na−Nb) and the intake air amount Va, a NOx cumulative amount Nc (= ΔN × Va) per unit time is calculated, this is cumulatively calculated, and NOx cumulative in this lean mode operation is calculated. The amount Nt (= Nt + ΔN × Va × Δtl) is calculated.
[0047]
In the rich control at step S40, the unit time when the exhaust temperature (catalyst temperature index value) Tg is input from the exhaust temperature sensor 22 by inputting the measurement data at step S41, and the rich control is performed at the next step S42. The NOx emission amount (decomposition and separation amount) Nd per unit is calculated. This NOx release amount Nd is calculated in advance by storing the relationship between the exhaust temperature Tg and the NOx release amount Nd obtained in advance by experiments or the like using map data or a function, and from these detected exhaust temperature Tg, This is performed by a method of obtaining the NOx emission amount Nd per unit time when rich control is performed using the map data of FIG.
[0048]
In the next step S43, a reducing agent supply amount Md per unit time is calculated. The calculation of the reducing agent supply amount Ma is performed by storing the relationship between the exhaust gas temperature Tg and the reducing agent supply amount Ma previously obtained through experiments or the like using map data or a function, and using the detected exhaust gas temperature Tg. These map data may be used to calculate the reducing agent supply amount Ma per unit time, or refer to the NOx release amount Nd per unit time calculated by the NOx release amount calculation means C22. And you may carry out by the method of correct | amending this reducing agent supply amount Ma.
[0049]
In the next step S44, whether or not the rich timer is on, whether or not the actual rich control elapsed time tr measured by the rich timer is a positive value (tr> 0), or rich control It is determined whether the flag F is set (F = 1) or not (F = 0). If it is determined in this determination that the rich timer is not on (tr = 0), that is, the rich control operation has not yet started (F = 0), the rich control start work is performed in steps S45 to S47. .
[0050]
In step S45, the rich control time tr0 is calculated from the NOx accumulated amount Nt and the NOx release amount Nd per unit time when rich control is performed. When the catalyst temperature in the rich control time tr0 is approximated to be constant, the rich control time tr0 is calculated by dividing the NOx accumulated amount Nt by the NOx release amount Nd per unit time to obtain the rich control time tr0 ( = Nt / Nd) can be calculated.
[0051]
In step S46, the rich control operation is started and the rich control flag F is set (F = 1). At the same time, in step S47, the rich timer is activated and measurement of the actual rich control elapsed time tr is started.
[0052]
In this rich control operation, the exhaust gas is brought into a rich state in which the oxygen concentration is close to zero by adjusting the fuel injection amount and injection timing of the multistage injection by the fuel injection control into the cylinder, adjusting the EGR, adjusting the intake throttle, and the like. .
[0053]
That is, in the fuel injection control, multistage injection is performed, λ (excess air ratio) detected by the λ sensor 21 is monitored, and λ is feedback-controlled so as to become the target λt. That is, the oxygen concentration at the catalyst inlet is set to NO from the NOx storage material.₂The oxygen concentration is controlled to be lower than the possible oxygen concentration (for example, 1%). At this time, the EGR amount and the intake throttle amount are feedback-controlled while monitoring the output of the mass air flow sensor 11 that measures the intake amount.
[0054]
In step S48, it is determined whether or not the rich control operation is ended based on whether or not the actual rich control elapsed time tr exceeds the rich control time tr0. If the determination is not finished (tr <tr0), the actual rich control elapsed time tr is stored in the memory in step S50, and the process returns to step S11.
[0055]
If it is determined in step S48 that the actual rich control elapsed time tr exceeds the rich control time tr0 and the rich control operation is terminated, the NOx accumulated amount Nt and the rich control flag F are reset in step S49. Further, the rich control operation end work such as stop of the rich timer (Nt = 0, F = 0, tr = 0) is performed, the actual rich control elapsed time tr is stored in the memory in step S50, and the process returns to step S20.
[0056]
Then, steps S20 to S30 or steps S20 to S40 are repeatedly executed until the engine key is turned off.
[0057]
If the engine key is turned off during the execution of this control flow, an interrupt in step S60 occurs, and in step S61, an end work such as storing the NOx accumulation amount Nt or the like in the memory is performed and stopped. finish.
[0058]
In the control flow of FIGS. 3 and 4, the exhaust temperature (catalyst temperature index value) Tg is approximated to be constant within the rich control time (between tr = 0 to tr0) within the rich control time, and at this exhaust temperature Tg. The rich control time tr0 (= Nt / Nd) is calculated from the NOx accumulated amount Nt using the NOx release amount Nd per unit time when the rich control is performed. The catalyst within the rich control time tr0 When considering the change in temperature, the determination at the end of the rich control operation can be performed by the following control.
[0059]
In this control, the influence of the catalyst temperature Tg is included for each control cycle time (Δtr) of the rich control, not within the rich control time (between 0 and tr0), and the NOx release amount ΔNd in the control cycle is expressed in unit time. The NOx emission amount Nd is calculated by multiplying the NOx emission amount Nd by the control cycle time Δtr, and subtracting the NOx emission amount ΔNd (= Nd × Δtr) from the NOx accumulation amount Nt (Nt = Nt−ΔNd), Is the time when rich control ends when the value becomes zero or negative (Nt ≦ 0). In this case, the rich control time is from the start of the rich control to the end time of the rich control.
[0060]
According to the exhaust gas purification system 1 having the above configuration, the NOx accumulated amount Nt is calculated from the NOx concentrations Na and Nb before and after the NOx storage reduction catalyst 3 and the intake air amount Va. It is possible to estimate with higher accuracy corresponding to not only the change in the environment but also the change in the environment and the change in the NOx emission amount due to the transient mode.
[0061]
Further, since the NOx release amount Nd per unit time during the rich control is calculated based on the exhaust temperature (catalyst temperature index value) Tg, the temperature characteristics of the storage and reduction ability of the NOx storage reduction catalyst 3 are taken into consideration. can do.
[0062]
Since the rich control time tr0 is determined from the NOx accumulated amount Nt estimated with high accuracy and the NOx release amount Nd per unit time, useless rich control operation not related to the recovery of the NOx storage capability is unnecessary. And saves fuel consumption.
[0063]
Further, the reducing agent supply amount Ma per unit time is also calculated based on the catalyst temperature index value Tg or the NOx release amount Nd per unit time, and the reducing agent is supplied in an amount corresponding to the NOx release amount. Therefore, NOx and reducing agent can be prevented from being discharged into the atmosphere.
[0064]
Examples of exhaust gas purification by this exhaust gas purification system 1 and comparative examples are shown in FIGS. FIGS. 5 and 6 are graphs showing NOx reduction performance when the catalyst temperature is 200 ° C., and FIG.₂NO considering decomposition reaction rate₂FIG. 6 shows an example in which the rich control time is set according to the decomposition reaction rate and the amount of reducing agent is supplied. FIG. 6 shows the NOx storage reduction catalyst NO.₂This is a comparative example that does not consider the decomposition reaction rate.
[0065]
In the comparative example, the catalyst outlet NOx gradually increases as shown by the two-dot chain line, whereas in the example, the catalyst outlet NOx hardly changes, and the effect of the present invention can be understood.
[0066]
【The invention's effect】
As described above, according to the exhaust gas purification system of the present invention, the accumulated NOx amount is calculated from the NOx concentration and the intake air amount before and after the NOx occlusion reduction type catalyst. Not only the change but also the change of environment and the change of NOx emission due to the transient mode can be dealt with, and the accumulated amount of NOx can be estimated with higher accuracy.
[0067]
In addition, taking into account the temperature characteristics of the NOx storage reduction catalyst, the amount of NOx released per unit time during rich control is calculated based on the catalyst temperature detected by the catalyst temperature detection means, and is estimated with high accuracy. Since the rich control time is determined from the accumulated amount of NOx and the amount of NOx released per unit time, useless rich control that is not related to the recovery of the NOx storage capacity becomes unnecessary, and fuel for rich control is saved. This improves fuel economy.
[0068]
Furthermore, since the reducing agent supply amount per unit time is calculated based on the catalyst temperature and is supplied in an amount corresponding to the NOx release amount per unit time, it becomes a necessary and sufficient amount for the reduction of NOx. Emissions to the atmosphere can be prevented. In addition, since the rich control time during which NOx is released is also a necessary and sufficient time, the reducing agent is not wastedly released and the reducing agent can be saved.
[0069]
In particular, NOx reduction and purification can be effectively performed even at a low temperature of 300 ° C. or lower where a reduction in NOx storage and reduction capability is observed.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an exhaust gas purification system according to an embodiment of the present invention.
FIG. 2 is a diagram showing a configuration of regeneration control means of the exhaust gas purification system according to the embodiment of the present invention.
FIG. 3 is a diagram showing an example of a control flow of the exhaust gas system according to the embodiment of the present invention.
4 is a control flow diagram of the rich control operation of FIG. 3. FIG.
FIG. 5 is a diagram showing NOx reduction performance in an example.
FIG. 6 is a diagram showing NOx reduction performance in a comparative example.
FIG. 7 is a diagram schematically showing the configuration and purification mechanism of a NOx storage reduction catalyst according to the present invention, where (a) is a state during lean control (NO₂(B) is a state of rich control (NO₂FIG.
FIG. 8 is a diagram showing the relationship between the storage and reduction ability of a NOx storage reduction catalyst and the catalyst temperature.
[Explanation of symbols]
1 Exhaust gas purification system
3 NOx storage reduction catalyst
5 Electronic control device (electronic control box)
5a Catalyst regeneration control device
11 Mass Air Flow Sensor
22 Exhaust temperature sensor
23 Inlet NOx concentration sensor
24 NOx concentration sensor on outlet side
E engine (internal combustion engine)
C1 Catalyst regeneration means
C11 Intake air amount detection means
C12 catalyst temperature detection means
C13 NOx concentration detection means
C21 NOx cumulative amount calculation means
C22 NOx emission amount calculation means
C23 rich control time calculation means
C24 Reducing agent supply amount calculation means
C31 air-fuel ratio control means
C32 Reducing agent supply means

Claims (4)

In an exhaust gas purification system comprising a NOx occlusion reduction type catalyst disposed in an exhaust passage of an internal combustion engine and a catalyst regeneration control device for recovering the NOx occlusion capacity of the NOx occlusion reduction type catalyst,
The catalyst regeneration control device comprises intake air amount detection means, air-fuel ratio control means, catalyst temperature detection means, and NOx concentration detection means,
A cumulative NOx amount in the NOx storage reduction catalyst is calculated from the NOx concentration before and after the NOx storage reduction catalyst detected by the NOx concentration detection means and the intake air amount detected by the intake air amount detection means. The amount of NOx released per unit time when rich control is performed from the catalyst temperature index value detected by the catalyst temperature detection means,
The rich control time is determined from the calculated NOx accumulated amount and the NOx release amount per unit time, and the air-fuel ratio in the exhaust gas is controlled to be rich by the air-fuel ratio control means during the rich control time. Exhaust gas purification system. The catalyst regeneration control device includes a reducing agent supply means, calculates a reducing agent supply amount per unit time from a catalyst temperature index value detected by the catalyst temperature detection means, and the reducing agent supply means calculates the per unit time. 2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the reducing agent is supplied so that the amount of the reducing agent is reduced. The catalyst regeneration control device includes a reducing agent supply unit, calculates a reducing agent supply amount per unit time from the NOx release amount per unit time, and supplies the reducing agent per unit time by the reducing agent supply unit. 2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the reducing agent is supplied so as to be an amount. The catalyst regeneration control device comprises:
Instead of controlling the air-fuel ratio in the exhaust gas to the rich state by the air-fuel ratio control means during the rich control time,
The air-fuel ratio control means controls the air-fuel ratio in the exhaust gas to a rich state for a predetermined time, and reduces the NOx occlusion amount calculated from the NOx release amount per unit time and the predetermined time. Calculating, subtracting the decrease from the calculated NOx cumulative amount to calculate the NOx remaining amount,
The exhaust gas purification system according to any one of claims 1 to 3, wherein the air-fuel ratio in the exhaust gas is controlled to a rich state until the remaining amount of NOx becomes zero or less.

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