JP3972864B2 - Exhaust gas purification system for internal combustion engine - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purification system for an internal combustion engine.
[0002]
[Prior art]
In exhaust purification of an internal combustion engine, for example, a storage reduction type NOx catalyst (hereinafter referred to as NOx catalyst) having a function of holding (including storage, absorption, and adsorption) sulfur oxide (SOx) and nitrogen oxide (NOx), etc. May be installed in the exhaust passage of the internal combustion engine. In this case, the NOx catalyst or the like retains sulfur oxides (SOx) generated by combustion of sulfur contained in the fuel. For example, in a NOx catalyst, SOx is retained by the same mechanism as in the case of retaining NOx, but the retained SOx is less likely to desorb than NOx, and NOx is released in a reducing atmosphere with a reduced oxygen concentration. However, SOx does not leave and gradually accumulates in the NOx catalyst. This is called sulfur poisoning (SOx poisoning), and causes a decrease in the NOx purification rate of the NOx catalyst. Therefore, it is necessary to perform poisoning recovery processing for recovering the NOx catalyst from SOx poisoning at an appropriate time.
[0003]
At this time, the SOx retained in the NOx catalyst can be desorbed and released for the first time by circulating the exhaust gas with a reduced oxygen concentration while keeping the NOx catalyst at a high temperature (eg, about 600 to 650 ° C.). Become.
[0004]
However, in an internal combustion engine or the like capable of lean combustion, since the exhaust temperature during engine operation is low, it is usually necessary to raise the temperature of the NOx catalyst to a temperature at which SOx can be released. Therefore, for example, SOx release is performed by executing control to lower the oxygen concentration of the exhaust gas while increasing the bed temperature of the NOx catalyst by adding fuel to the exhaust gas.
[0005]
At this time, as SOx is released, H₂Since there is a problem that S is generated and gives off a strange odor, H₂In order to suppress the generation of S, intermittent rich in which the period during which the air-fuel ratio of the exhaust gas becomes lean and the period during which the exhaust air is rich is alternately repeated is performed (for example, see Patent Document 1).
[0006]
[Patent Document 1]
JP 2001-82137 A
[Patent Document 2]
JP 2000-161107 A
[Patent Document 3]
JP 2000-274232 A
[0007]
[Problems to be solved by the invention]
However, during the lean period in such intermittent rich, SOx release from the NOx catalyst is suppressed, and as a result, a long time is required for SOx release. In order to solve this problem, increasing the amount of fuel added to lower the air-fuel ratio so as to increase the SOx emission efficiency during the rich period, and increasing the degree of richness increases the SOx emission amount, but hydrocarbons ( (Hereinafter referred to as HC) also increases, leading to deterioration of exhaust emissions.
[0008]
The present invention has been made in view of the above circumstances, and provides an exhaust purification system that suppresses the release of HC and efficiently releases SOx when performing SOx release control of a NOx catalyst or the like. Doing this is a technical issue.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the present invention performs fuel addition densely while maintaining the air-fuel ratio of exhaust flowing into the catalyst having SOx retention ability and oxidation ability during the SOx release control at a stoichiometric or rich state. By alternately providing the period and the period to be sparse, excessive increase in the catalyst temperature and HC emission are suppressed even during the period in which the air-fuel ratio is maintained at stoichiometric or rich.
[0010]
That is, the first invention is a means for adding fuel to the exhaust provided in the exhaust system,
A catalyst having SOx retention ability and oxidation ability disposed downstream of the fuel addition means,
When the amount of SOx retained in the catalyst having SOx retention ability and oxidation ability exceeds a predetermined amount, the temperature of the catalyst having SOx retention ability and oxidation ability by the fuel addition means is equal to or higher than a predetermined temperature. SOx that releases the SOx retained in the catalyst having the SOx retaining ability and the oxidizing ability by setting the atmosphere of the exhaust gas flowing into the catalyst having the SOx retaining ability and the oxidizing ability as a stoichiometric or rich air-fuel ratio. In an exhaust gas purification system for an internal combustion engine that performs emission control,
During the SOx release control, while maintaining the SOx retention ability and the oxidation ability at a stoichiometric or rich air-fuel ratio, the fuel addition interval by the fuel addition means alternates between a dense period and a sparse period It has the addition density control means which controls to repeat.
[0011]
In the SOx release control, if fuel addition into the exhaust gas is performed densely, the amount of SOx released from the oxidation catalyst increases, but the amount of HC released also increases. Therefore, by alternately repeating a dense fuel addition period and a sparse fuel addition period in the rich period, oxygen is supplied to the oxidation catalyst during the fuel addition suspension period, thereby improving the oxidation ability of the oxidation catalyst. To suppress HC emissions. In this case, SOx is released during a period when fuel addition is dense, and HC emission suppression and catalyst temperature control (overheating prevention) are realized during a sparse period.
[0012]
The SOx release control is executed by an intermittent rich that repeats a rich period in which the air-fuel ratio of the exhaust flowing through the oxidation catalyst is rich and a lean period in which the air-fuel ratio of the exhaust is lean. The control by the addition density control means is It can be performed during the rich period.
[0013]
When the catalyst temperature control is performed only during the lean period of the intermittent rich, the lean period becomes longer in order to prevent the temperature of the oxidation catalyst from rising excessively. As a result, the time required for SOx release also becomes longer. In order to avoid this, it is preferable to control the temperature of the oxidation catalyst within an appropriate range by controlling the fuel addition interval to alternately repeat the dense period and the sparse period even in the rich period. As a result, the lean period can be shortened, and the entire time required for SOx release can be avoided.
[0014]
Further, it is preferable to increase the proportion of the fuel addition interval that is closer as the load on the internal combustion engine becomes smaller.
[0015]
Because the amount of exhaust is small when the internal combustion engine is lightly loaded, the amount of fuel added to the exhaust is reduced as a whole, and even if the fuel addition interval is increased and the degree of richness is increased, HC emissions are reduced. It is. In this way, SOx can be released efficiently.
[0016]
Furthermore, it is preferable that the ratio of the fuel addition interval is smaller as the air-fuel ratio of the exhaust gas discharged from the cylinder of the internal combustion engine is lower.
[0017]
When the air-fuel ratio of the basic air-fuel mixture in the cylinder is low, the exhaust discharged from the cylinder contains a lot of HC, so the amount of fuel added to the exhaust is reduced to reduce the amount of HC and suppress HC emissions To do.
In the intermittent rich rich period, it is preferable to increase the ratio of the sparse fuel addition interval as the rich period progresses toward the end.
[0018]
In a state where the rich spike is continued, the longer the rich time is, the more the added fuel is adsorbed on the oxidation catalyst and the HC is easily discharged.
[0019]
The fuel addition amount per unit time is set constant by increasing the fuel addition amount at one time in the period in which the fuel addition interval is sparser than the fuel addition amount in a period in which the fuel addition interval is dense. It is preferable to maintain the air-fuel ratio of the exhaust gas.
[0020]
In this way, the air-fuel ratio is prevented from deviating to the lean side, thereby preventing the efficiency reduction of SOx emission.
[0021]
In the present invention, the above-described controls can be combined as much as possible.
[0022]
According to the present invention, when performing SOx release control in S0x poisoning recovery control of a NOx catalyst or the like by adding fuel to the exhaust system, HC release is suppressed and SOx is efficiently released.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, specific embodiments of the regeneration control of the exhaust purification system according to the present invention will be described with reference to the drawings. Here, the case where the regeneration control of the exhaust purification system according to the present invention is applied to a diesel engine for driving a vehicle will be described as an example.
(Embodiment 1)
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine 1 and its intake / exhaust system according to the present embodiment.
[0024]
An internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine having four cylinders 2. The internal combustion engine 1 includes a fuel injection valve 3 that injects fuel directly into the combustion chamber of each cylinder 2. Each fuel injection valve 3 is connected to a pressure accumulation chamber (common rail) 4 that accumulates fuel to a predetermined pressure. The common rail 4 communicates with a fuel pump 6 through a fuel supply pipe 5.
[0025]
The fuel discharged from the fuel pump 6 is supplied to the common rail 4 through the fuel supply pipe 5, accumulated in the common rail 4 to a predetermined pressure, and distributed to the fuel injection valves 3 of each cylinder 2.
[0026]
Next, an intake branch pipe 8 is connected to the internal combustion engine 1, and each branch pipe of the intake branch pipe 8 communicates with a combustion chamber of each cylinder 2 via an intake port (not shown). .
[0027]
The intake branch pipe 8 is connected to an intake pipe 9, and an air flow meter 11 for outputting an electric signal corresponding to the mass of the intake air flowing is attached to the intake pipe 9.
[0028]
The intake pipe 9 is provided with a compressor housing 15a of a centrifugal supercharger (turbocharger) 15 that operates using exhaust energy as a drive source.
[0029]
In the intake system configured as described above, the intake air flowing into the air cleaner box 10 flows into the compressor housing 15a via the intake pipe 9. The intake air flowing into the compressor housing 15a is compressed by the rotation of the compressor wheel built in the compressor housing 15a and flows into the intake branch pipe 8. The intake air flowing into the intake branch pipe 8 is distributed to the combustion chamber of each cylinder 2 via each branch pipe. The fuel injected from the fuel injection valve 3 of each cylinder 2 is mixed with the intake air and burned.
[0030]
On the other hand, an exhaust branch pipe 18 is connected to the internal combustion engine 1, and each branch pipe of the exhaust branch pipe 18 communicates with a combustion chamber of each cylinder 2 via an exhaust port (not shown). Further, a crank position sensor 33 that outputs an electrical signal corresponding to the rotational position of the crankshaft is provided.
[0031]
The exhaust branch pipe 18 is connected to the turbine housing 15 b of the centrifugal supercharger 15. The turbine housing 15b is connected to an exhaust pipe 19, and the exhaust pipe 19 is connected to a muffler (not shown) downstream.
[0032]
In the middle of the exhaust pipe 19, a NOx catalyst 20 is provided.
[0033]
An exhaust gas temperature sensor 23 that outputs an electrical signal corresponding to the temperature of the exhaust gas flowing through the exhaust pipe 19 is attached to the exhaust pipe 19 downstream of the NOx catalyst 20.
[0034]
The NOx catalyst 20 is formed of a porous material such as cordierite, for example. For example, alumina is used as a carrier, and potassium (K), sodium (Na), lithium (Li), or cesium (on the carrier). At least one selected from an alkali metal such as Cs), an alkaline earth such as barium (Ba) or calcium (Ca), and a rare earth such as lanthanum (La) or yttrium (Y), platinum (Pt), etc. The noble metal is supported.
[0035]
The NOx catalyst 20 absorbs nitrogen oxide (NOx) in the exhaust when the oxygen concentration of the inflowing exhaust is high.
[0036]
On the other hand, the NOx catalyst 20 releases the absorbed nitrogen oxide (NOx) when the oxygen concentration of the inflowing exhaust gas decreases. At that time, if a reducing component such as hydrocarbon (HC) or carbon monoxide (CO) is present in the exhaust, the NOx catalyst 20 converts the released nitrogen oxide (NOx) into nitrogen (N₂).
[0037]
By the way, when the internal combustion engine 1 is in a lean combustion operation, the air-fuel ratio of the exhaust discharged from the internal combustion engine 1 becomes a lean atmosphere, and the oxygen concentration in the exhaust becomes high. Therefore, the NOx contained in the exhaust becomes the NOx catalyst 20. However, if the lean combustion operation of the internal combustion engine 1 is continued for a long time, the NOx retention capacity of the NOx catalyst 20 is saturated, and the NOx in the exhaust is not retained by the NOx catalyst 20 but is released into the atmosphere. There is a possibility of being.
[0038]
In particular, in a diesel engine such as the internal combustion engine 1, the lean air-fuel ratio mixture is burned in the most operating region, and the exhaust air-fuel ratio becomes the lean air-fuel ratio in the most operating region accordingly. The NOx retention capacity of the catalyst 20 is easily saturated.
[0039]
Note that the lean air-fuel ratio here is, for example, in the range of 20 to 50 in a diesel engine, and means a region in which NOx cannot be purified by a three-way catalyst. Therefore, when the internal combustion engine 1 is operated in lean combustion, before the NOx retention capacity of the NOx catalyst 20 is saturated, the oxygen concentration in the exhaust gas flowing into the NOx catalyst 20 is reduced and the concentration of the fuel as the reducing agent is reduced. It is necessary to increase and reduce the NOx held in the NOx catalyst 20.
[0040]
One method for reducing the oxygen concentration in this way is to add fuel to the exhaust. In order to perform this fuel addition, a fuel supply mechanism for adding fuel (light oil) as a reducing agent to the exhaust gas flowing through the exhaust pipe 19 upstream from the NOx catalyst 20 is installed, and fuel is supplied from the fuel supply mechanism into the exhaust gas. As a result, the oxygen concentration of the exhaust gas flowing into the NOx catalyst 20 can be reduced and the fuel concentration can be increased.
[0041]
As shown in FIG. 1, the fuel supply mechanism is attached so that its injection hole faces the exhaust branch pipe 18, and a fuel addition valve 28 that opens by a signal from an ECU 35 to be described later and injects fuel. And a fuel supply passage 29 that guides the fuel discharged from the fuel pump 6 to the fuel addition valve 28.
[0042]
In such a fuel supply mechanism, high-pressure fuel discharged from the fuel pump 6 is applied to the fuel addition valve 28 via the fuel supply path 29. Then, the fuel addition valve 28 is opened by a signal from the ECU 35 and fuel as a reducing agent is injected into the exhaust branch pipe 18.
[0043]
The fuel injected from the fuel addition valve 28 into the exhaust branch pipe 18 reduces the oxygen concentration of the exhaust gas flowing from the upstream of the exhaust branch pipe 18, reaches the NOx catalyst 20, and is held by the NOx catalyst 20. NOx will be reduced.
[0044]
Thereafter, the fuel addition valve 28 is closed by a signal from the ECU 35, and the addition of fuel into the exhaust branch pipe 18 is stopped.
[0045]
The internal combustion engine 1 configured as described above is provided with an electronic control unit (ECU) 35 for controlling the internal combustion engine 1. The ECU 35 is a unit that controls the operation state of the internal combustion engine 1 in accordance with the operation conditions of the internal combustion engine 1 and the request of the driver.
[0046]
Various sensors are connected to the ECU 35 through electric wiring, and in addition to the output signals of the various sensors described above, an output signal of the accelerator opening sensor 36 that outputs an electric signal corresponding to the amount of depression of the accelerator by the driver. It is designed to be entered.
[0047]
On the other hand, the fuel injection valve 3, the fuel addition valve 28, and the like are connected to the ECU 35 via electric wiring, and the above-described parts can be controlled by the ECU 35.
[0048]
By the way, it is possible to reduce NOx held in the NOx catalyst by making the air-fuel ratio of the exhaust gas spike like the target rich air-fuel ratio. However, as described above, in the NOx catalyst, SOx is retained by the same mechanism as in the case of retaining NOx, and the retained SOx is less likely to be released than NOx, and NOx is released in a reducing atmosphere in which the oxygen concentration is reduced. Even if it is performed, SOx does not leave and gradually accumulates in the NOx catalyst. Therefore, a poisoning recovery process for recovering the NOx catalyst from SOx poisoning at an appropriate time is performed. In the SOx poisoning recovery control, the ECU 35 performs a predetermined SOx poisoning elimination process to eliminate the NOx catalyst 20 poisoning caused by SOx.
[0049]
The fuel of the internal combustion engine 1 usually contains sulfur (S), and when such fuel burns in the internal combustion engine 1, sulfur dioxide (SO₂) And sulfur trioxide (SO_Three) And other sulfur oxides (SOx).
[0050]
As a method for eliminating SOx poisoning of the NOx catalyst 20, the temperature of the atmosphere of the NOx catalyst 20 is raised to a high temperature range of approximately 600 to 650 ° C., and the oxygen concentration of the exhaust gas flowing into the NOx catalyst 20 is lowered. , Barium sulfate (BaSO) absorbed in the NOx catalyst 20_Four) SO_Three ^-Or SO_Four ^-Pyrolyzed, and then SO_Three ^-Or SO_Four ^-Reacts with hydrocarbons (HC) and carbon monoxide (CO) in the exhaust to produce gaseous SO₂ ^-A method of releasing can be exemplified.
[0051]
Therefore, the ECU 35, for example, adds fuel into the exhaust gas from the fuel addition valve 28 to oxidize those unburned fuel components in the NOx catalyst 20, and the bed temperature of the NOx catalyst 20 by the heat generated during the oxidation. To increase. At the same time, fuel may be injected secondarily from the fuel injection valve 3 during the expansion stroke or exhaust stroke of each cylinder.
[0052]
By adding the fuel as described above, the bed temperature of the NOx catalyst 20 rises to a high temperature range of about 600 ° C. to 650 ° C. After that, the ECU 35 continues to reduce the oxygen concentration of the exhaust gas flowing into the NOx catalyst 20. Fuel is injected from the fuel addition valve 28.
[0053]
When such poisoning recovery processing is executed, the oxygen concentration of the exhaust gas flowing into the NOx catalyst 20 becomes low under a situation where the bed temperature of the NOx catalyst 20 is high, so that the barium sulfate retained in the NOx catalyst 20 is reduced. (BaSO_Four) Is SO_Three ^-Or SO_Four ^-They are pyrolyzed into SO_Three ^-Or SO_Four ^-Reacts with hydrocarbons (HC) and carbon monoxide (CO) in the exhaust, and the SOx poisoning of the NOx catalyst 20 is recovered.
[0054]
In the above SOx poisoning recovery process, the rich spike is executed to reduce the oxygen concentration in the exhaust gas. At this time, the rich spike is executed after a predetermined pause period so that the air-fuel ratio of the exhaust gas has a rich atmosphere period and a lean atmosphere period. Furthermore, if one rich spike is formed by multiple fuel injections, the air-fuel ratio can be adjusted so as not to become excessively rich. The temperature of the NOx catalyst 20 is increased by gradually increasing the HC concentration in the exhaust gas by intermittent fuel supply from the fuel addition valve 28 as described above.
[0055]
In this case, if a large amount of fuel is injected at a time, the air-fuel ratio of the exhaust gas may become excessively rich, and part of unburned fuel that cannot be reacted by the NOx catalyst 20 may flow downstream. Therefore, in the present embodiment, a small atmosphere is divided into a plurality of times, that is, injected in pulses, thereby forming a rich atmosphere while suppressing over-richness.
[0056]
The actual air-fuel ratio of the exhaust gas is detected by the air-fuel ratio sensor 24 provided upstream of the NOx catalyst 20, and the amount of fuel added to the exhaust gas from the fuel addition valve 28 by the command of the ECU 35 becomes the target air-fuel ratio. Feedback control may be performed.
[0057]
Such SOx poisoning recovery control can be determined to be executed in a region where the engine speed NE and the engine output torque T are predetermined values.
[0058]
The control will be described in more detail. For example, as shown in FIG. 3, the HC concentration and the temperature of the NOx catalyst 20 can be converged to desired values by adjusting the waveform of the command signal of the ECU 35 sent to the fuel addition valve 28. it can.
[0059]
First, the fuel injection valve 28 opens when the command signal is “ON”, and supplies fuel into the exhaust branch pipe 18 at a predetermined pressure. By such fuel addition, a rich spike having a high HC concentration is formed.
[0060]
At this time, as shown in FIG. 3, the longer the addition period Csec, the greater the amount of change in HC concentration, and the longer the total addition period TCsec (the more the number of additions), the longer the rich spike formation period. .
[0061]
Further, the shorter the addition interval Bms, the higher the HC concentration in the exhaust gas and the greater the increase in the filter temperature.
[0062]
On the other hand, the length of the fuel addition suspension period Dsec corresponds to the length of the period in which the HC concentration is low (the period in which the lean atmosphere continues between the continuously formed rich spikes). The bed temperature of the NOx catalyst 20 can be adjusted by the length of the pause period Dsec. That is, the temperature of the NOx catalyst 20 becomes lower as the pause period Dsec is longer.
(Embodiment 1)
In the present embodiment, in addition to the above basic control, as shown in FIGS. 4A and 4B, the fuel addition density is further changed during the rich spike so that the pulsed fuel addition is dense. A period and a sparse period are provided. In this way, the release of HC from the NOx catalyst 20 can be suppressed, and the predetermined air-fuel ratio can be maintained, so that SOx can be effectively released. Here, a period in which the fuel addition is sparse is provided, and oxygen is supplied to the NOx catalyst 20 during the fuel addition stop period to maintain the oxidizing ability of the NOx catalyst 20, thereby suppressing HC emission.
[0063]
If the fuel addition is made dense as shown in FIGS. 4A and 4B, the output of the air-fuel ratio sensor 24 stably changes on the rich side lower than the stoichiometric air-fuel ratio correspondingly.
[0064]
On the other hand, when the fuel addition is sparse, the air-fuel ratio changes to the lean side at the interval of pulse addition, so that the air-fuel ratio fluctuates as a whole. Therefore, it is avoided that the air-fuel ratio of the exhaust gas becomes excessively rich during fuel addition. At this time, if the whole area during fuel addition is made dense, a constant amount of HC is always released during the rich spike as shown by a broken line 201 in FIG. On the other hand, by repeating dense and sparse, the amount of HC released changes as shown by the solid line 202, so the amount released during the rich spike is less than that during the whole period. Obviously decrease.
[0065]
On the other hand, SOx is released as indicated by a broken line 203 in FIG. On the other hand, when densely and sparsely repeats, it is released as shown by the solid line 204, and the amount of extraction is reduced as compared with the case where addition is densely performed, but the overall amount of release is not greatly reduced. .
[0066]
Therefore, the effect of suppressing the release of HC is great by repeating dense and sparse, and the release action of SOx is not greatly impaired.
[0067]
In addition, it is preferable that the ratio of the fuel addition is small and the ratio of the sparseness is increased as the amount of SOx held in the NOx catalyst 20 is larger. When the amount of SOx retained is large, the activity (oxidizing ability) of the NOx catalyst 20 is lowered, and therefore HC is easily discharged. Therefore, HC emissions are suppressed by increasing the sparse ratio.
[0068]
The amount of SOx retained in the NOx catalyst 20 can be calculated based on a two-dimensional map of engine speed NE × fuel injection amount Q. In addition, it can be obtained from the fuel consumption, the output signal from the NOx sensor 22, the vehicle travel distance, and the like. Alternatively, since the NOx catalyst 20 is poisoned by the sulfur component in the fuel, the fuel consumption may be integrated and stored in the ECU 35, and the SOx retention amount may be obtained from this fuel consumption. Further, as SOx poisoning proceeds, the amount of NOx retained in the NOx catalyst decreases, and the amount of NOx flowing downstream of the NOx catalyst 20 increases. Therefore, the NOx sensor 22 may be provided downstream of the NOx catalyst 20, and the SOx retention amount may be obtained based on this output signal. Furthermore, assuming that the SOx retention amount increases according to the vehicle travel distance, it is also possible to obtain the SOx retention amount based on this vehicle travel distance.
[0069]
Next, it is desirable to detect thermal degradation of the NOx catalyst 20 and change the density of fuel addition according to the degree of thermal degradation. Since the activity of the catalyst also decreases due to thermal deterioration, HC is easily discharged when the degree of deterioration is high.
[0070]
Therefore, by increasing the ratio of sparse fuel addition, HC emissions are suppressed.
[0071]
In order to determine the degree of thermal deterioration of the NOx catalyst, for example, the NOx sensor 22 provided in the exhaust passage downstream of the NOx catalyst 20 measures and compares the NOx concentration passing through the NOx catalyst 20 before and after the SOx poisoning recovery process. And a method of determining from the operation history such as the number of times of performing the SOx poisoning recovery process.
[0072]
In this way, as the degree of deterioration of the NOx catalyst 20 increases, the ratio of fuel addition is reduced and the ratio of sparseness is increased.
[0073]
Next, the SOx poisoning recovery control processed by the ECU 35 will be described based on the flowchart shown in FIG.
[0074]
First, in step S101, the ECU 35 stores output signals of various sensors on the RAM 353 in order to collect an operation history from the start of engine operation. For example, the elapsed time from the start of engine operation, the amount of fuel supplied to each cylinder 2 to satisfy the target required torque, the amount of air drawn into each cylinder 2, the elapsed time since the previous fuel supply, the vehicle For example, the integrated value of the number of mileage and the exhaust temperature. The collected operation history is read out to the CPU 351, and it is determined in the CPU 351 whether or not the SOx poisoning recovery control condition, that is, the fuel supply execution condition is satisfied. The SOx poisoning recovery control condition is whether or not the amount of SOx absorbed in the NOx catalyst 20 is a predetermined amount or more. When this condition is not satisfied, the execution of this processing routine is temporarily terminated.
[0075]
When the above condition is satisfied, the process proceeds to step S102, and the supply amount of the added fuel is calculated. Here, the base addition amount of fuel that is used for recovery of SOx poisoning is determined using the air-fuel ratio of the air-fuel mixture that is currently used for engine operation and the SOx absorption amount calculated in step S101 as parameters. Calculation is performed based on a base addition amount calculation map prepared in advance on the ROM 352. The fuel supply amount calculation map is created based on various preliminary experiments.
[0076]
Next, proceeding to step S103, the degree of thermal deterioration of the NOx catalyst 20 is detected.
[0077]
Then, it progresses to step S104 and the conditions of material addition are determined based on the information obtained by said step S101 to S103. That is, the total addition time TCsec, the addition time Csec, the fuel addition interval Bms, and the addition stop time Dsec are determined from the added fuel amount, the S0x absorption amount, and the degree of thermal deterioration of the NOx catalyst 20.
[0078]
Next, it progresses to step S105 and it is determined whether the fuel addition determination conditions in exhaust_gas | exhaustion are satisfied. Whether the internal combustion engine 1 is in an operating state suitable for SOx poisoning recovery or whether the temperature of the NOx catalyst 20 is in a high temperature range (for example, a range of 550 to 700 ° C.) capable of thermally decomposing SOx. Further, conditions such as whether the air-fuel ratio of the exhaust gas is smaller than stoichiometric, whether fuel supply that promotes the NOx absorption / release action is in an execution state, and the like can be exemplified. When these conditions are not satisfied, the execution of this processing routine is temporarily terminated.
[0079]
If an affirmative determination is made in step S105, the process proceeds to step S106, and fuel addition is executed.
[0080]
According to this embodiment, since the fuel addition is controlled in consideration of the state of the NOx catalyst 20 (SOx absorption amount and thermal deterioration degree), it is possible to achieve both prevention of slipping of HC and SOx release at a high level.
(Embodiment 2)
As in the first embodiment, the fuel addition density is changed during the rich spike to provide a period during which the pulsed fuel addition is dense and a period during which the fuel addition is sparse, and the SOx poisoning recovery process of the NOx catalyst 20 is performed. carry out.
[0081]
However, in this embodiment, in addition to the state of the NOx catalyst 20, control is performed in consideration of the operation state of the internal combustion engine 1.
[0082]
First, according to the load on the internal combustion engine 1, the ECU 35 controls the ratio of the period during which fuel addition is dense and the period during which fuel addition is sparse to be different. Here, when the load is light, since the amount of exhaust is small, the amount of added fuel can be reduced as a whole. Therefore, the ratio of dense addition is reduced and the ratio of sparseness is increased. And as the load increases, the ratio of dense addition increases.
[0083]
Secondly, the richer the air-fuel ratio of the exhaust discharged from the cylinder of the internal combustion engine 1, the smaller the density ratio. The air-fuel ratio of the air-fuel mixture serving as the base of the air-fuel ratio of the exhaust gas is obtained from the intake air amount and the fuel injection amount, and is determined by the ECU 35. The richer the air-fuel ratio of the basic air-fuel mixture, the greater the amount of HC that is discharged into the exhaust gas. Therefore, the ECU 35 reduces the ratio of dense addition so that excessive HC is not included in the exhaust gas. , Increase the sparse ratio. Then, the control is performed so that the fuel addition increases in a dense ratio as the air-fuel ratio in the cylinder increases (lean).
[0084]
In this way, it is possible to avoid a situation in which the HC concentration in the exhaust gas becomes excessively high, so that the release of HC is suppressed.
(Embodiment 3)
Similar to the first embodiment, the fuel addition density is changed during execution of the rich spike to provide a period during which the pulsed fuel addition is dense and a period during which the pulse fuel addition is sparse. Carry out poison recovery treatment.
[0085]
In this embodiment, the fuel addition amount per time is different between the period when the fuel addition is dense and the period when the fuel addition is sparse, and the amount of fuel addition per time is the dense period during the sparse period. The amount of fuel added per unit time in the dense and sparse cases is adjusted to be the same.
[0086]
In this way, the air-fuel ratio per unit time becomes constant, and efficient SOx release can be performed.
(Other embodiments)
In the rich period when the SOx release control is performed by intermittent rich in which the rich period and the lean period are alternately repeated, the ECU 35 increases the rate at which the fuel addition interval is sparse as the rich period progresses toward the end. To do. For example, when the rich spike is continued for 10 seconds, the control is performed so that the ratio of sparseness in the latter is increased by comparing when the rich spike has elapsed 5 seconds and when 7 seconds have elapsed.
[0087]
In a state where the rich spike continues, the added fuel is adsorbed to the NOx contact 20 more as the rich time becomes longer. When such a state progresses, the NOx catalyst 20 easily discharges HC. Therefore, in order to avoid such a situation, the interval of fuel addition is made sparse and the interval period is increased. Then, since oxygen is supplied to the NOx catalyst 20 during the interval period, its oxidizing ability is recovered. Therefore, the amount of HC emission decreases.
[0088]
Further, control is performed so as to change the density of fuel addition based on the temperature of the NOx catalyst 20.
[0089]
In the SOx poisoning recovery control, as described above, the catalyst temperature is in the range of about 600 to 650 ° C., but when the catalyst temperature is low within the range, the ratio of fuel addition is increased. Since the activity (oxidizing ability) of the NOx catalyst 20 is lower than that when the temperature is further increased, it is easy to discharge HC. Therefore, HC emissions are suppressed by increasing the sparse ratio.
[0090]
Incidentally, in the above embodiment, the case where the fuel is added by the fuel addition valve 28 in the exhaust passage upstream of the catalyst in order to increase the catalyst temperature has been described. However, post injection in the cylinder in conjunction with such fuel addition. In this case, a method of supplying unburned components into the exhaust gas by sub-injection or the like and a method of increasing the unburned components in the exhaust gas by low temperature combustion are included.
[0091]
Further, without adding fuel by the fuel addition valve 28, the unburned components are supplied into the exhaust by sub-injection such as post injection, or the catalyst bed temperature is increased by increasing the unburned components in the exhaust by low temperature combustion. It is possible to apply the control of the present invention also when raising.
[0092]
In the above-described embodiment, it is preferable to provide an oxidation catalyst (not shown) downstream of the NOx catalyst 20. HC released from the NOx catalyst 20 and passing through the NOx catalyst 20 is prevented from being oxidized and released to the outside by the oxidation catalyst.
[0093]
【The invention's effect】
According to the present invention, when performing SOx release control of a NOx catalyst or the like, SOx can be efficiently released while suppressing the release of HC.
[Brief description of the drawings]
FIG. 1 is a diagram showing an entire internal combustion engine according to an embodiment of the present invention.
FIG. 2 is a diagram showing a schematic configuration of an ECU.
FIG. 3 is a diagram showing a state in which a rich spike is formed by fuel addition in SOx poisoning recovery control.
FIG. 4 is a diagram showing a state in which a rich spike is formed by the control of the present invention. (A) is a figure which shows the density of fuel addition, (b) shows the change of the air fuel ratio of exhaust gas. (C) shows the change in the amount of HC released with fuel addition, and (d) shows the change in the amount of SOx released.
FIG. 5 is a flowchart showing an execution flow of SOx poisoning recovery control in the embodiment of the present invention.
[Explanation of symbols]
1 ... Internal combustion engine
2. Cylinder
3. Fuel injection valve
4 ... Common rail
5. Fuel supply pipe
6. Fuel pump
8 ... Intake branch pipe
9. Intake pipe
18 ... Exhaust branch pipe
19 ... Exhaust pipe
20 ... NOx catalyst
22 ... NOx sensor
23 ... Exhaust temperature sensor
24 ... Air-fuel ratio sensor
28 ... Fuel addition valve
29 ... Fuel supply path
33 ... Crank position sensor
35 ... ECU
36 Accelerator opening sensor

Claims (6)

Means for adding fuel to the exhaust provided in the exhaust system;
A catalyst having SOx retention ability and oxidation ability disposed downstream of the fuel addition means,
When the amount of SOx held in the catalyst exceeds a predetermined amount, the temperature of the catalyst is set to a predetermined temperature or more by the fuel addition means, and the air-fuel ratio of the exhaust gas flowing through the oxidation catalyst is stoichiometric or rich to oxidize In an exhaust gas purification system for an internal combustion engine that performs SOx release control for releasing SOx held in a catalyst,
An addition density control means for controlling the fuel addition means to repeat a dense period and a sparse period alternately while maintaining the oxidation catalyst in a stoichiometric or rich atmosphere during the SOx release control. An exhaust purification system for an internal combustion engine, comprising: The SOx release control is executed by an intermittent rich that repeats a rich period in which the air-fuel ratio of the exhaust flowing through the oxidation catalyst is rich and a lean period in which the air-fuel ratio of the exhaust is lean. The control by the addition density control means is The exhaust purification system for an internal combustion engine according to claim 1, wherein the exhaust purification system is implemented during the rich period. 3. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the ratio of the fuel addition interval being closer is increased as the load of the internal combustion engine is reduced. The exhaust gas purification system for an internal combustion engine according to claim 1 or 2, wherein the ratio of the fuel addition interval being closer is reduced as the air-fuel ratio of the exhaust gas discharged from the cylinder of the internal combustion engine is lower. The exhaust purification system for an internal combustion engine according to claim 2, wherein in the intermittent rich rich period, as the rich period progresses toward the end, the ratio at which the fuel addition interval is sparse is increased. The fuel addition amount per unit time is set constant by increasing the fuel addition amount at one time in the period in which the fuel addition interval is sparser than the fuel addition amount in a period in which the fuel addition interval is dense. The exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 5, wherein the air-fuel ratio of the exhaust gas is maintained.

Images