JP2015086848A - Exhaust emission control system of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology which can further surely and efficiently improve a NOx purification rate of an exhaust emission control system as a whole even if a temperature of a first selective reduction catalyst is high, in the exhaust emission control system which arranges the first selective reduction catalyst at an upstream side in an exhaust passage, arranges a second selective reduction catalyst at a downstream side, and supplies urea water as a reductant from a further upstream side of the first selective reduction catalyst.SOLUTION: When it is determined that a temperature of a second selective reduction catalyst is not lower than a threshold at a high temperature side at which the reduction purification of NOx can be efficiently performed or not (S102), and when it is determined that at least a part of a reductant supplied from an upstream side of the first selective reduction catalyst slips through the first selective reduction catalyst, and can be supplied to the second selective reduction catalyst (S202), control for cooling the second selective catalyst is performed (S103).

Description

  The present invention relates to an exhaust purification system that purifies nitrogen oxides and particulate matter in exhaust gas from an internal combustion engine.

  As an exhaust gas purification system for reducing nitrogen oxides (hereinafter referred to as “NOx”) in exhaust gas of an internal combustion engine, a selective catalytic reduction type catalyst that selectively reduces NOx in exhaust gas in an exhaust passage ( (Hereinafter also referred to as “SCR”) and a reducing agent injection valve that injects a urea aqueous solution as a reducing agent upstream of the SCR. In this type of exhaust purification system, a decrease in NOx reduction efficiency in the SCR is suppressed by controlling the injection amount of the urea aqueous solution while predicting the temperature transition of the SCR.

  In addition, the filter that collects particulate matter (hereinafter also referred to as “PM: Particulate Matter”) is provided with SCRF that is capable of purifying NOx and particulate matter in exhaust gas by coating SCR, and the SCR is disposed downstream thereof. An exhaust purification system is also known that supplies an aqueous urea solution as a reducing agent from upstream of the SCRF. In the system having such a configuration, when the temperature of the SCRF and the SCR becomes high during the regeneration process of the PM trapping ability of the filter, the oxidation reaction of the reducing agent itself occurs simultaneously with the NOx reduction reaction. The NOx purification performance of the entire purification system may be reduced.

On the other hand, it is conceivable that the downstream SCR is cooled to purify NOx that has passed through the SCRF or NOx produced by oxidation of NH ₃ with the SCRF by the downstream SCR. Since the inflow amount of the agent increases and decreases in relation to the temperature of the upstream SCRF, the supply amount of the reducing agent, etc., if the SCR cooling control is not properly performed, the reducing agent flows out from the downstream SCR or the NOx purification in the SCR. The rate may have decreased. Further, when there is no inflow of the reducing agent to the downstream SCR, there is a possibility that energy is wasted by performing cooling control of the SCR.

JP 2012-167549 A JP 2012-004786 A JP 2011-069279 A JP 2011-069231 A JP 2003-293736 A JP 2003-232216 A JP 2012-215154 A Japanese Patent Laid-Open No. 2007-247652 JP 2007-270643 A

  The present invention has been made in view of the above circumstances, and an object thereof is to further arrange the first selective reduction catalyst by disposing a first selective reduction catalyst on the upstream side and a second selective reduction catalyst on the downstream side in the exhaust passage. In an exhaust purification system that supplies urea water as a reducing agent from the upstream side, even if the temperature of the first selective reduction catalyst is high, the NOx purification rate of the entire exhaust purification system is improved more reliably or more efficiently. It is to provide the technology that can be made.

  To achieve the above object, the present invention provides a first selective reduction catalyst for reducing and purifying NOx in exhaust gas upon receipt of a reducing agent, and a second selective reduction catalyst provided downstream of the first selective reduction catalyst And a reducing agent supply means for supplying a reducing agent from the upstream side of the first selective reduction catalyst, and the exhaust gas purification system for an internal combustion engine efficiently reduces and purifies NOx when the temperature of the second selective reduction catalyst is It is determined that it is equal to or higher than the threshold value on the high temperature side, and at least a part of the reducing agent supplied from the reducing agent supply means can pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst. In this case, the greatest feature is to perform control to cool the second selective reduction catalyst.

More specifically, a first selective reduction catalyst that is provided in the exhaust passage of the internal combustion engine and receives the supply of the reducing agent to reduce and purify NOx in the exhaust,
A second selective reduction catalyst provided downstream of the first selective reduction catalyst in the exhaust passage;
Reducing agent supply means for supplying a reducing agent to the exhaust gas passing through the exhaust passage on the upstream side of the first selective reduction catalyst in the exhaust passage;
An exhaust gas purification system for an internal combustion engine comprising:
Temperature acquisition means for acquiring the temperature of the second selective reduction catalyst;
Cooling means for cooling the second selective reduction catalyst;
Reducing agent supply determining means for determining that at least a part of the reducing agent supplied from the reducing agent supply means can pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst;
The temperature of the second selective reduction catalyst acquired by the temperature acquisition means is equal to or higher than a cooling determination temperature, which is a high temperature side threshold value indicating whether the NOx reduction purification can be efficiently performed, and
When the reducing agent supply determination means determines that the reducing agent supplied from the reducing agent supply means can pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst,
An exhaust purification system for an internal combustion engine, wherein the cooling means cools the second selective reduction catalyst.

  Here, the first selective reduction catalyst that receives the supply of the reducing agent to reduce and purify NOx in the exhaust, the second selective reduction catalyst provided downstream from the first selective reduction catalyst, and the first selective reduction catalyst In an exhaust gas purification system for an internal combustion engine provided with a reducing agent supply means for supplying a reducing agent from the upstream side, if the temperature of the first selective reduction catalyst is high, the second selective reduction catalyst must also purify NOx. It may not be possible. In that case, the temperature of the second selective reduction catalyst is also high, and it is often difficult to efficiently perform reduction and purification of NOx. Therefore, it is more efficient to cool the second selective reduction catalyst. In the second selective reduction catalyst, NOx can be purified.

  On the other hand, in order to purify NOx in the second selective reduction catalyst, it is necessary that at least a part of the reducing agent supplied from the reducing agent supply means can pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst. It becomes. That is, if at least a part of the reducing agent supplied from the reducing agent supply means cannot pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst, the control itself for cooling the second selective reduction catalyst is useless. become.

  On the other hand, in the present invention, the temperature of the second selective reduction catalyst is equal to or higher than the high temperature side threshold value indicating whether or not the reduction and purification of NOx can be efficiently performed, and the reduction supplied from the reducing agent supply means When it is determined that at least part of the agent can pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst, control is performed to cool the second selective reduction catalyst. According to this, only when NOx reduction and purification can be performed in the second selective reduction catalyst in the sense of the presence of the reducing agent, the second selective reduction catalyst is cooled, and the temperature of the second selective reduction catalyst is set to the first selective reduction catalyst. It becomes possible to maintain NOx discharged to the downstream side of the selective reduction catalyst at a temperature at which it can be efficiently purified.

  As a result, NOx discharged to the downstream side of the first selective reduction catalyst can be reduced and purified more reliably by the second selective reduction catalyst, and unnecessary cooling control can be prevented. Therefore, the exhaust gas purification system as a whole can more efficiently purify NOx in the exhaust gas.

  In the present invention, the temperature of the second selective reduction catalyst during cooling may be set to a target catalyst temperature at which a higher NOx purification rate can be obtained. According to this, the NOx discharged to the downstream side of the first selective reduction catalyst can be purified more efficiently by cooling the second selective reduction catalyst by the cooling means.

  The present invention further includes first temperature acquisition means for acquiring the temperature of the first selective reduction catalyst. In addition to the above-described conditions, the temperature of the first selective reduction catalyst is sufficient for NOx in the first selective reduction catalyst. The second selective reduction catalyst may be cooled when the purification rate is equal to or higher than the high temperature threshold. According to this, since the second selective reduction catalyst can be cooled only when the temperature of the first selective reduction catalyst is such a high temperature that the NOx purification rate decreases, it is possible to prevent wasteful cooling control also in this respect. Can do.

  In the present invention, when determining whether at least a part of the reducing agent supplied from the reducing agent supply means can pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst, In addition, the determination may be made based on at least one of the exhaust gas flow rate, the NOx concentration, and the reducing agent supply amount. According to this, the amount of the reducing agent that can pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst can be acquired with higher accuracy, and the cooling control of the second selective reduction catalyst can be performed with higher accuracy. It can be determined whether or not to do so.

  In the present invention, when determining whether at least a part of the reducing agent supplied from the reducing agent supply means can pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst, the above conditions are satisfied. In addition, the determination may be made based on the amount of reducing agent supplied from the reducing agent supply means. According to this, the amount of the reducing agent that can pass through the first selective reduction catalyst can be acquired with higher accuracy, and it can be determined whether or not the cooling control of the second selective reduction catalyst is to be performed with higher accuracy. it can.

  Further, in the present invention, when the amount of reducing agent that passes through the first selective reduction catalyst is larger than the amount necessary for reducing and purifying NOx discharged downstream of the first selective reduction catalyst, The temperature of the second selective reduction catalyst may be set to a target catalyst temperature capable of oxidizing the excess reducing agent. According to this, it can suppress more reliably that excess reducing agent is discharged | emitted by the downstream of a 2nd selective reduction catalyst.

  In the present invention, the optimum reducing agent supply amount for more reliably supplying the reducing agent to the second selective reduction catalyst is determined based on at least one of the temperature, exhaust flow rate, and NOx concentration of the first selective reduction catalyst. The reducing agent supply means may acquire the reducing agent according to the optimum reducing agent supply amount to the exhaust gas. According to this, a reducing agent can be more reliably supplied to a 2nd selective reduction catalyst, and the NOx purification rate as the whole exhaust gas purification system can be improved more reliably.

  The means for solving the problems in the present invention can be used in combination as much as possible.

In the present invention, the first selective reduction catalyst is arranged on the upstream side in the exhaust passage, the second selective reduction catalyst is arranged on the downstream side, and urea water as a reducing agent is supplied from the further upstream side of the first selective reduction catalyst. In the exhaust purification system, even when the temperature of the first selective reduction catalyst is high, the NOx purification rate of the entire exhaust purification system can be improved more reliably or more efficiently.

1 is a diagram illustrating a schematic configuration of an internal combustion engine, an exhaust system, and a control system according to Embodiment 1 of the present invention. In the embodiment of the present invention, the temperature of the catalyst for selective catalytic reduction method, a conceptual diagram of the relationship between the oxidation rate of NOx purification rate and NH ₃ in the catalyst. It is a graph of the relationship between the temperature of SCRF in Example 1 of this invention, and the slipping amount of a reducing agent. It is a flowchart about the SCR temperature control routine in Example 1 of this invention. It is a graph which shows the change of each parameter relevant to SCRF and SCR at the time of performing the SCR temperature control routine in Example 1 of this invention. It is a figure which shows schematic structure of the internal combustion engine which concerns on Example 2 of this invention, an exhaust system, and a control system. What is the relationship between the amount of NH ₃ supplied to the SCRF used for NOx purification, the amount oxidized by the SCRF, and the amount passing through the SCRF by changing each parameter in Example 2 of the present invention. It is a graph which shows whether it changes into. It is a flowchart about the SCR temperature control routine 2 in Example 2 of this invention. It is a flowchart about the SCR temperature control routine 3 in Example 3 of this invention. It is the schematic of the specific example of the cooling device of the exhaust gas in the Example of this invention.

  The best mode for carrying out the present invention will be exemplarily described in detail below with reference to the drawings.

<Example 1>
FIG. 1 is a diagram showing a schematic configuration of an internal combustion engine 1 according to the present embodiment and its exhaust system and control system. In FIG. 1, the inside of the internal combustion engine 1 and its intake system are omitted. In FIG. 1, the internal combustion engine 1 is connected to an exhaust pipe 5 as an exhaust passage through which exhaust gas discharged from the internal combustion engine 1 flows, and this exhaust pipe 5 is connected downstream to a muffler (not shown). In the middle of the exhaust pipe 5, an SCRF 10 is disposed as a first selective reduction catalyst that can purify NOx and PM in the exhaust by coating SCR on a filter that collects PM in the exhaust.

  Further, on the downstream side of the SCRF 10 in the exhaust pipe 5, an SCR 11 as a second selective reduction catalyst, which is a selective catalytic reduction type catalyst that selectively reduces NOx in the exhaust, is disposed. The exhaust pipe 5 in the meantime is provided with a cooling device 17 as a cooling means for cooling the exhaust gas passing through the exhaust pipe 5. Further, a reducing agent injection valve 14 as a reducing agent supply means for injecting an aqueous urea solution as a reducing agent is disposed upstream of the SCRF 10 in the exhaust pipe 5. Further, immediately upstream of the SCRF 10 and the SCR 11, a first temperature sensor 15 as a first temperature acquisition unit for detecting each temperature and a second temperature sensor 16 as a temperature acquisition unit are provided.

The internal combustion engine 1 configured as described above and its exhaust system are provided with an electronic control unit (ECU) 20 for controlling the internal combustion engine 1 and the exhaust system. The ECU 20 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 driver's request, and performs control related to the exhaust purification system including the SCRF 10 and the SCR 11.

  The ECU 20 includes a CPU, a ROM, a RAM, and the like. The ROM stores a program for performing various controls of the internal combustion engine 1 and a map storing data. The SCR temperature control routine in the present embodiment described below is also one of the programs stored in the ROM in the ECU 20. In the present embodiment, the exhaust purification system includes a configuration excluding the internal combustion engine 1 from the configuration shown in FIG.

Next, exhaust purification by the SCRF 10 and the SCR 11 in the present embodiment will be described. Exhaust gas discharged from the internal combustion engine 1 flows into the SCRF 10 through the exhaust pipe 5 and PM in the exhaust gas is collected by a filter in the SCRF 10. Further, urea water injected from the reducing agent injection valve 14 is added to the exhaust gas flowing into the SCRF 10 in accordance with the amount of NOx contained in the exhaust gas. The urea water added to the exhaust gas is first hydrolyzed by the heat of the exhaust gas and the water vapor in the exhaust gas at SCRF 10 to become NH ₃ (ammonia) and CO 2 (carbon dioxide).

Most of NH ₃ produced in SCRF 10 is adsorbed by SCRF 10 when the temperature of SCRF 10 is low. When the temperature of SCRF 10 rises above the catalyst activation temperature, NH ₃ adsorbed on the ammonia adsorption layer of SCRF 10 reacts with NOx in the exhaust gas to become nitrogen and water, and NOx in the exhaust is purified. Subsequently, the exhaust discharged from the SCRF 10 flows into the SCR 11. In the SCR 11, NH ₃ that is not consumed by the SCRF 10 and remains in the exhaust is adsorbed. In the SCR 11, NOx in the exhaust gas flowing into the SCR 11 is reduced and purified by NH ₃ that has passed through without being consumed in the SCRF 10 in this way. The exhaust gas that has passed through the SCR 11 and purified NOx is discharged into the atmosphere.

By the way, when the filter PM regeneration control is performed in the SCRF 10, the temperature of the SCRF 10 becomes a high temperature of about 600 to 650 ° C. Under such a high temperature condition, the oxidation reaction of urea added to the exhaust gas from the reducing agent injection valve 14 proceeds simultaneously with the NOx reduction reaction in SCRF 10, so that NH ₃ used for the NOx reduction reaction in SCRF 10 The amount decreases. As a result, the NOx purification rate in SCRF 10 may decrease.

On the other hand, in the present embodiment, the cooling device 17 cools the exhaust discharged from the SCRF 10 so that the SCR 11 is not exposed to a high temperature, and further supplies NH ₃ that has passed through the SCRF 10 to the SCR 11. As a result, NOx that could not be purified by the SCRF 10 is more reliably reduced and purified by the SCR 11.

Here, FIG. 2 shows a conceptual diagram of the relationship between the temperature of the selective catalytic reduction type catalyst (SCRF10 or SCR11), the NOx purification rate and the NH ₃ oxidation ratio in the catalyst. FIG. 2 (a) shows the relationship between the catalyst temperature and the NOx purification rate, and FIG. 2 (b) shows the relationship between the catalyst temperature and the NH ₃ oxidation rate. As can be seen from FIG. 2 (b), as the temperature of the catalyst rises, the oxidation rate of NH _{3 in} the catalyst rises rapidly. Then, since the proportion of NH ₃ used for NOx purification is relatively reduced, the NOx purification rate in the catalyst is reduced as compared with the case where there is no increase in the oxidation rate of NH ₃ .

Thus, in both the SCRF 10 and the SCR 11, the NOx purification rate and the NH ₃ oxidation ratio change depending on the catalyst temperature. Therefore, when cooling the exhaust discharged from the SCRF 10 as described above, Characteristics should be considered.

Next, FIG. 3 shows a graph of the relationship between the temperature of SCRF 11 and the amount of slipping through the reducing agent. A black triangle line is a line when the supply amount of the reducing agent is further increased with respect to the black circle line. From FIG. 3, it can be understood that the amount of NH ₃ flowing into the SCR 11 varies depending on the temperature of the SCRF 10 and the supply amount of the reducing agent. Thus, when cooling the exhaust discharged from the SCRF 10, it should be considered that the amount of the reducing agent flowing into the SCR 11 varies depending on the temperature of the SCRF 10 and the amount of reducing agent supplied. For example, NOx reduction cannot be performed in the SCR 11 even if the exhaust gas is cooled in a state where no reducing agent flows into the SCR 11.

  On the other hand, in the present embodiment, the temperatures of the SCRF 10 and the SCR 11 are acquired, the amount of reducing agent that passes through the SCRF 10 and flows into the SCR 11 is predicted from the acquired temperature, and the cooling of the SCR 11 is based on the predicted value. The presence or absence was determined and control was executed. In FIG. 4, the flowchart about the SCR temperature control routine in a present Example is shown. The SCR temperature control routine is a program stored in the ROM of the ECU 20 and executed by the CPU. The SCR temperature control routine is a routine that is periodically executed every predetermined time while the internal combustion engine 1 is in operation.

  When the SCR temperature control routine is executed, first, in S101, it is determined whether the temperature Tscrf of the SCRF 10 is equal to or higher than the cooling determination threshold T1. More specifically, the temperature Tscrf of the SCRF 10 is calculated based on the output of the first temperature sensor 15. In addition, when the temperature of the SCRF 10 is higher than this, the cooling determination threshold T1 is such that the temperature of the exhaust exhausted from the SCRF 10 is also high, and the cooling control of the SCR 11 is performed to maintain the temperature of the SCR 11 at a high NOx purification rate. Otherwise, the temperature is judged to be difficult to maintain a sufficient NOx purification rate for the entire exhaust purification system. The cooling determination threshold value T1 may be determined experimentally or theoretically in advance. In S101, when it is determined that the temperature Tscrf of the SCRF 10 is lower than the cooling determination threshold T1, it is determined that the cooling control of the SCR 11 is unnecessary, and thus this routine is temporarily ended. On the other hand, when it is determined that the temperature Tscrf of the SCRF 10 is equal to or higher than the cooling determination threshold value T1, the process proceeds to S102.

In S102, it is determined whether or not the temperature Tscr of the SCR 11 is equal to or higher than the cooling determination threshold T2. More specifically, the temperature Tscr of the SCR 11 is calculated based on the output of the second temperature sensor 16. Further, the cooling determination threshold T2 is a temperature at which when the temperature of the SCR 11 is higher than this, it is determined that a sufficient NOx purification rate cannot be obtained due to the oxidation of the reducing agent NH _{3 in} the SCR 11. The cooling determination threshold value T2 may be determined experimentally or theoretically in advance. If it is determined in S102 that the temperature Tscr of the SCR 11 is lower than the cooling determination threshold value T2, it is determined that the cooling control of the SCR 11 is unnecessary, and thus this routine is temporarily ended. On the other hand, when it is determined that the temperature Tscr of the SCR 11 is equal to or higher than the cooling determination threshold T2, the process proceeds to S103.

  In S103, cooling control of the SCR 11 is performed. Here, the target catalyst temperature Ttrg is calculated in the ECU 20. This target catalyst temperature Ttrg may be determined as a temperature at which an optimal NOx purification rate is obtained in the SCR 11 using an estimation model and map of the relationship between the temperature of the SCR 11 and the purification rate, or experimentally and theoretically in advance. It may be set to the obtained value. Then, the SCR 11 is cooled so that the temperature of the SCR 11 becomes the target catalyst temperature Ttrg. More specifically, in this embodiment, the cooling water of the internal combustion engine 1 is passed around the exhaust pipe 5 between the SCRF 10 and the SCR 11, and the flow rate of this cooling water is increased to cool the SCR 11. . When the process of S103 ends, the process proceeds to S104.

In S104, it is determined whether the temperature Tscrf of the SCRF 10 is lower than the cooling determination threshold value T1. Here, when it is determined that the temperature Tscrf of the SCRF 10 is lower than the cooling determination threshold value T1, it is determined that it is not necessary to continue the cooling control of the SCR 11 any more, so the cooling control of the SCR 11 is stopped and this routine is performed. Is temporarily terminated. On the other hand, when it is determined that the temperature Tscrf of the SCRF 10 is equal to or higher than the cooling determination threshold value T1, the process proceeds to S105.

  In S105, it is determined whether or not the temperature Tscr of the SCR 11 is lower than the target catalyst temperature Ttrg. Here, when it is determined that the temperature Tscr of the SCR 11 is equal to or higher than the target catalyst temperature Ttrg, it is determined that further cooling control needs to be continued, so the process returns to before the process of S103. On the other hand, if it is determined that the temperature Tscr of the SCR 11 is lower than the target catalyst temperature Ttrg, it is determined that it is not necessary to continue the cooling control of the SCR 11 any more, so the cooling control of the SCR 11 is stopped and this routine is executed. Exit once.

Note that the execution of this routine is based on the premise that the NH ₃ has slipped through the SCR 11 to the extent that NOx purification is possible unless the temperature of the SCRF 10 is extremely high. Therefore, in this embodiment, the ECU 20 that executes the process of S101 corresponds to a reducing agent supply determination unit. Of course, if the temperature of the SCRF 10 is equal to or higher than a predetermined threshold value higher than the cooling determination threshold value T1 in S101, the SCR 11 determines that NH ₃ is not slipped through to the extent that NOx purification is possible, and the SCR temperature control routine It is good also as control which once complete | finishes.

FIG. 5 shows changes in parameters related to the SCRF 10 and the SCR 11 when the above SCR temperature control routine is executed. FIG. 5A shows the temperature change of SCRF 10 and SCR 11. FIG. 5B shows changes in NOx concentration in the exhaust gas flowing into the SCRF 10 and the exhaust gas exhausted from the SCR 11. FIG. 5C shows changes in the amount of oxidation of NH ₃ on SCRF10 and SCR11.

  As shown in FIG. 5A, when the PM regeneration process of the filter of the SCRF 10 of the internal combustion engine 1 is started, the temperatures of the SCRF 10 and the SCR 11 continue to rise. Then, the cooling control of the SCR 11 is started at the time t1 when the temperature of the SCRF 10 rises to become the cooling determination temperature T1 or higher, and further, the temperature of the SCR 11 rises to become the cooling determination temperature T2 or higher. Thereby, the temperature of SCR11 begins to fall. Then, at time t2 when the SCR temperature becomes lower than the target catalyst temperature Ttrg, the cooling control is stopped.

Further, the NOx concentration in the exhaust gas flowing into the SCRF 10 is constant as shown in FIG. As the temperature of the SCRF 10 and the SCR 11 rises, the NOx purification rate in the SCRF 10 and the SCR 11 once rises, so the NOx concentration in the exhaust discharged from the SCR 11 decreases. If the temperature of the SCRF 10 and the SCR 11 continues to rise further, the amount of oxidation of NH ₃ on the SCRF 10 and the SCR 11 starts to increase as shown in FIG. 5 (c), so that from the SCR 11 as shown in FIG. 5 (b). The NOx concentration in the discharged exhaust gas increases again.

When the cooling control of the SCR 11 is started at the time t1, as the temperature of the SCR 11 decreases, the amount of oxidation of NH ₃ in the SCR 11 decreases and the NOx purification rate increases. As a result, the exhaust discharged from the SCR 11 The NOx concentration in the case also decreases.

As described above, according to the present embodiment, the temperature of the SCRF 10 is a high temperature at which it is determined that the cooling control of the SCR 11 is necessary due to the PM regeneration process, and the NOx purification rate is sufficient even when the temperature of the SCR 11 is the SCR 11. When the temperature is determined not to be obtained, the cooling control of the SCR 11 is performed to maintain the SCR 11 in a high NOx purification rate. Further, the NOx increasing due to the decrease in the NOx purification rate due to the temperature rise of the SCRF 10 and the oxidation of NH ₃ itself can be sufficiently reduced and purified in the SCR 11.

In the present embodiment, a sufficient NOx purification rate cannot be expected in SCRF10, and SCR1
Since the cooling control of the SCR 11 is performed only when the temperature drop of 1 works effectively, the SCRF 10 and the SCR 11 can be used up effectively and the exhaust gas can be purified efficiently.

<Example 2>
Next, a second embodiment of the present invention will be described. In the second embodiment, the NOx concentration, the exhaust flow rate, the catalyst temperature, and the reducing agent supply amount in the exhaust gas flowing into the SCRF 10 are acquired, and the optimum cooling amount of the SCR 11 is calculated from these values, and the target temperature for optimum cooling control is calculated. An example in which the cooling control of the SCR 11 is performed based on this value will be described.

  FIG. 6 shows the internal combustion engine 1 and its exhaust system and control system in this embodiment. In the system shown in FIG. 6, information acquired from the NOx sensor 18 and the intake air amount sensor 19 is used for cooling control of the SCR 11. In the present embodiment, the exhaust purification system includes a configuration excluding the internal combustion engine 1 among the configurations shown in FIG.

Next, FIG. 7, by changing the parameters affecting NOx reduction treatment with NH _3, of the NH ₃ supplied to SCRF10, the amount of NH ₃ used in the NOx purification, NH ₃ which is oxidized at SCRF10 the amount of a graph showing how changes to how the relationship between the amount of NH ₃ slip through SCRF10. 7A shows the relationship with the supply amount of NH ₃ as a reducing agent, FIG. 7B shows the relationship with the exhaust flow rate, and FIG. 7C shows the relationship with the filter temperature, that is, the temperature of the SCRF 10. .

As can be seen from FIG. 7 (a), when increasing the supply amount of the NH _3, in proportion to slip through the amount of NH ₃ used in the NOx purification, and the amount of NH ₃ which is oxidized at SCRF10, a SCRF10 The total amount of NH ₃ increases. Then, by increasing the supply amount of NH _3, the amount of NH ₃ used in the NOx purification, it is possible to increase the amount of NH ₃ slip through SCRF10.

As can be seen from FIG. 7 (b), the exhaust flow rate increases, among NH ₃ supplied, the amount of NH ₃ which is oxidized by the amount and SCRF10 of NH ₃ used for NOx purification is reduced, slip through SCRF10 The amount of NH ₃ increases. Moreover, as can be seen from FIG. 7 (c), the the temperature of the filter temperature ie SCRF10 rises, among NH ₃ supplied, the amount of NH ₃ used for NOx purification is reduced, NH ₃ which is oxidized at SCRF10 The amount of NH ₃ increases and the amount of NH ₃ that slips through SCRF 10 decreases. And when the temperature of SCRF10 is very high, the case where a reducing agent cannot be supplied to SCR11 may arise.

Thus, the supply amount of NH _3, the exhaust flow rate, the temperature of the SCRF11, NOx purification amount in SCRF11, oxidation of NH ₃ and, since the slipping amount of NH ₃ is determined, in response to this, the cooling temperature of SCR11 By controlling, the NOx purification rate in the SCR 11 can be improved more accurately, and the NOx purification rate of the entire exhaust purification system can be improved.

  In FIG. 8, the flowchart about the SCR temperature control routine 2 in a present Example is shown. The SCR temperature control routine 2 is a program stored in the ROM of the ECU 20 and executed by the CPU. The SCR temperature control routine 2 is a routine that is periodically executed every predetermined time while the internal combustion engine 1 is in operation.

When the SCR temperature control routine 2 is executed, first, in S101, it is determined whether the temperature Tscrf of the SCRF 10 is equal to or higher than the cooling determination threshold T1, and in S102, it is determined whether the temperature Tscr of the SCR 11 is equal to or higher than the cooling determination threshold T2. Although it is determined, since this is the same processing as the processing in the SCR temperature control routine described in the first embodiment, detailed description thereof is omitted here.

Next, in S201, the amount Qnox of unpurified NOx discharged from the SCRF 10 and the amount of slipping Qred of NH ₃ are calculated. More specifically, based on the NOx concentration of the exhaust gas flowing into the SCRF 10, the exhaust gas flow rate, the temperature of the SCRF 10, and the NH ₃ supply amount, the calculation is performed using the estimation model and the estimation map taking into account the characteristics as shown in FIG. Is done. When the process of S201 ends, the process proceeds to S202.

In S202, it is determined whether or not there is NH ₃ slip-through, that is, whether or not Qred calculated in S201 is greater than zero. Here, when Qred = 0, since there is no NH ₃ reaching the SCR 11 and NOx purification is impossible due to a shortage of the reducing agent, it is determined that the cooling control of the SCR 11 is wasteful. The routine is temporarily terminated. On the other hand, when Qred> 0, NH ₃ that can be supplied to the SCR 11 is present and NOx purification is possible. Therefore, it is determined that the effect of cooling control of the SCR 11 can be expected, and the process proceeds to S203.

In S203, it is determined whether or not the reducing agent slip-through amount Qred is larger than an amount necessary for purification of the unpurified NOx amount Qnox (hereinafter also referred to as Qnox purification usage amount). Here, when it is determined that Qred> Qnox purification usage amount, it is required to cause an oxidation reaction of NH _{3 in} the SCR 11 to consume all the excess NH ₃ with respect to the Qnox purification usage amount, and thus the process proceeds to S204. . This is because if excess NH ₃ leaks further downstream from the SCR 11, NH ₃ may be discharged to the outside or NOx generated by oxidation of NH ₃ may be discharged to the outside. On the other hand, if it is determined that Qred ≦ Qnox purification usage, the cooling control similar to the control in the SCR temperature control routine described in the first embodiment is considered sufficient, and the process proceeds to S103.

In S204, the optimum cooling amount is calculated. That is, the target temperature Ttrg2 for optimal cooling control is calculated based on the estimation model and map based on the unpurified NOx amount Qnox, the exhaust gas flow rate, and the NH ₃ slip-through amount Qred. This target catalyst temperature Ttrg2 for optimal cooling control is the temperature of the SCR 11 at which the unpurified NOx is reduced and purified in the SCR 11, and the surplus NH ₃ is oxidized and consumed. When the process of S204 ends, the process proceeds to S103, where catalyst cooling control is performed. The target temperature when proceeding from S204 to S103 is Ttrg2, and the target catalyst temperature when proceeding from S203 to S103 is Ttrg. When the process of S103 ends, the process proceeds to S104. Since the process of S104 is equivalent to the process in the SCR temperature control routine described in the first embodiment, the description thereof is omitted here.

  In S205, it is determined whether the temperature Tscr of the SCR 11 is lower than the target catalyst temperature. This target catalyst temperature is Ttrg2 when an affirmative determination is made in S203, and is a Ttrg when a negative determination is made in S203. Here, when it is determined that the temperature Tscr of the SCR 11 is equal to or higher than the target catalyst temperature, it is determined that it is necessary to continue the cooling control, so the process returns to S103. On the other hand, if it is determined that the temperature Tscr of the SCR 11 is lower than the target catalyst temperature, it is determined that it is not necessary to continue the cooling control of the SCR 11 any more, so the cooling control of the SCR 11 is stopped and this routine is temporarily performed. finish.

As described above, in this embodiment, based on the NOx concentration of the exhaust gas flowing into the SCRF 10, the exhaust gas flow rate, the temperature of the SCRF 10, and the NH ₃ supply amount, the unpurified NOx amount Qnox and the NH ₃ slip-through amount Qred is calculated. When Qred is larger than the amount of Qnox purification used, which is the amount of NH ₃ necessary for purification of Qnox, a target for causing a partial oxidation reaction of NH ₃ in SCR 11 and consuming all excess NH ₃ The temperature Ttrg2 is calculated, and cooling control of the SCR 11 is performed so that the temperature is cooled to the temperature. Therefore,
The outflow of NH ₃ from the SCR 11 can be suppressed more reliably, and NH ₃ and NOx generated by oxidation of NH ₃ can be suppressed from being discharged to the outside.

In the present embodiment, to determine if there is slipping amount Qred of NH _3, when there is no slipping amount Qred of NH ₃ is, NOx purification determines impossible in SCR11, cooling control of SCR11 It was decided not to do. Thereby, the cooling control of the SCR 11 can be performed only when the effect of improving the NOx purification rate can be actually expected, and waste of energy can be suppressed. In the present embodiment, the ECU 20 that executes the process of S202 corresponds to a reducing agent supply determination unit.

<Example 3>
Next, Embodiment 3 of the present invention will be described. In this embodiment, an example will be described in which the temperature of the SCRF 10, the exhaust gas flow rate, and the NOx concentration are detected or estimated, the reducing agent supply amount that maximizes the NOx purification efficiency and the temperature of the SCR 10 are calculated, and then the SCR 11 is controlled to be cooled. . The schematic configuration of the internal combustion engine 1 and its exhaust system and control system according to this embodiment is the same as that shown in FIG.

  In FIG. 9, the flowchart about the SCR temperature control routine 3 in a present Example is shown. The SCR temperature control routine 3 is a program that is stored in the ROM of the ECU 20 and executed by the CPU of the ECU 20, and is a routine that is periodically executed every predetermined time while the internal combustion engine 1 is operating.

When the SCR temperature control routine 3 is executed, first, in S101, it is determined whether the temperature Tscrf of the SCRF 10 is equal to or higher than the cooling determination threshold T1, and then in S201, the amount of unpurified NOx Qnox and NH ₃ A slip-through amount Qred is calculated. The details of the processing in S101 and S201 are the same as the details of the processing in S101 and S201 in the SCR temperature control routine 2 of the second embodiment, and thus the details are omitted here. When the process of S201 ends, the process proceeds to S301.

In S301, the optimum reducing agent supply amount Qsupply is calculated. This optimum reductant supply amount Qsupply is a reductant supply amount that reliably causes NH ₃ to slip through the SCRF 10 and reliably supplies NH ₃ to the SCR 11, resulting in a maximum NOx purification rate of the entire exhaust purification system. Based on the NOx concentration of the exhaust gas flowing into the SCRF 10, the exhaust flow rate, the temperature of the SCRF 10, the reducing agent supply amount at this time, etc., the calculation is performed using the estimation model and the estimation map taking into account the characteristics as shown in FIG. It is what is done. When the process of S301 ends, the process proceeds to S302.

In S302, the NH ₃ supply amount from the reducing agent injection valve 14 is changed to the optimum reducing agent supply amount Qsupply calculated in S301. When the process of S302 ends, the process proceeds to S102. In S102, it is determined whether or not the temperature Tscr of the SCR 11 is equal to or higher than the cooling determination threshold T2. Since the content of this process is equivalent to the process of S102 in the SCR temperature control routine of the first embodiment, detailed description thereof is omitted. If it is determined in S102 that the temperature Tscr of the SCR 11 is equal to or higher than the cooling determination threshold T2, the process proceeds to S303.

In S303, the unpurified NOx amount Qnox and the reducing agent slip-through amount Qred are calculated again. More specifically, based on the NOx concentration of the exhaust gas flowing into the SCRF 10, the exhaust flow rate, the temperature of the SCRF 10, and the new NH ₃ supply amount changed in S302, an estimation model that takes into account the characteristics as shown in FIG. , Calculated using the estimated map. When the process of S303 ends, the process proceeds to S203. Since the contents of the processes from S203 to S205 are the same as the contents of the processes from S203 to S205 in the SCR temperature control routine 2 of the second embodiment, the details are omitted here.

As described above, in this embodiment, as in the second embodiment, the temperature of the SCR 11 is such that the unpurified NOx is reduced and purified in the SCR 11 and the surplus NH ₃ is oxidized and consumed. The target catalyst temperature Ttrg2 is calculated, and cooling control is performed toward the target catalyst temperature Ttrg2. In addition, an optimum reducing agent supply amount Qsuppy that maximizes the NOx purification rate of the entire exhaust purification system is calculated and supplied to the SCRF 10. Thereby, it is possible to improve the NOx purification rate of the exhaust gas purification system as a whole more reliably.

Note that, in the SCR temperature control routine 3 in the present embodiment, the optimum reducing agent supply amount Qsupply is used so that the SCR 11 always passes through NH _{3 to} the extent that NOx purification is possible. Therefore, when calculating the optimum reducing agent supply amount Qsupply, it is considered that it is determined whether or not NH ₃ slips through the SCR 11 to the extent that NOx purification is possible. Therefore, in this embodiment, it can be said that the ECU 20 that executes S301 is the reducing agent supply determination unit.

  Next, an example of a mechanism for reducing the temperature of the exhaust gas flowing into the SCR 11 in the catalyst cooling control described in the above embodiment will be described. FIG. 10 shows a specific example of the exhaust gas cooling device 17. FIG. 10A shows an example in which the exhaust gas is cooled using the cooling water of the internal combustion engine 1. In this example, the cooling water passage 17b from the radiator 17a is wound around the exhaust pipe 5. When cooling is not required, the cooling water from the radiator 17 is circulated at an extremely low flow rate, and in the cooling control of the SCR 11, the flow rate is increased according to the value of Ttrg or Ttrg2.

  In FIG. 10B, the exhaust pipe 5 is branched between the SCRF 10 and the SCR 11, and the branch pipe 5a is provided with a cooling element 17c for cooling the exhaust gas passing through the branch pipe 5a by air cooling or water cooling. ing. When cooling is unnecessary, the exhaust control valve 5b allows all the exhaust from the SCRF 10 to pass through only the exhaust pipe 5 and flow into the SCR 11. In the cooling control of the SCR 11, the amount of exhaust gas passing through the branch pipe 5 a and the flow rate of water or air flowing through the cooling element 17 c as necessary are increased according to the value of Ttrg or Ttrg 2 and flow into the SCR 11. Control the exhaust temperature.

  FIG. 10C shows an example in which a heat storage system having the heat storage material 17d is provided, and the amount of heat stored and the amount of heat released in the heat storage material 17d are controlled. That is, when cooling is unnecessary, the heat storage system is not operated. Further, in the cooling control of the SCR 11, the heat storage system is operated, and the exhaust gas flowing into the SCR 11 is cooled by moving the heat of the exhaust gas to the heat storage material 17d. When the temperature of the SCR 11 becomes too low, the heat stored in the heat storage material 17d is returned to the exhaust to raise the temperature of the exhaust flowing into the SCR 11.

DESCRIPTION OF SYMBOLS 1 ... Internal combustion engine 5 ... Exhaust pipe 10 ... SCRF
11 ... SCR
DESCRIPTION OF SYMBOLS 14 ... Reducing agent injection valve 15 ... 1st temperature sensor 16 ... 2nd temperature sensor 17 ... Cooling device 20 ... ECU

Claims (1)

A first selective reduction catalyst that is provided in an exhaust passage of the internal combustion engine and receives a supply of a reducing agent to reduce and purify NOx in the exhaust;
A second selective reduction catalyst provided downstream of the first selective reduction catalyst in the exhaust passage;
Reducing agent supply means for supplying a reducing agent to the exhaust gas passing through the exhaust passage on the upstream side of the first selective reduction catalyst in the exhaust passage;
An exhaust gas purification system for an internal combustion engine comprising:
Temperature acquisition means for acquiring the temperature of the second selective reduction catalyst;
Cooling means for cooling the second selective reduction catalyst;
Reducing agent supply determining means for determining that at least a part of the reducing agent supplied from the reducing agent supply means can pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst;
The temperature of the second selective reduction catalyst acquired by the temperature acquisition means is equal to or higher than a cooling determination temperature, which is a high temperature side threshold value indicating whether the NOx reduction purification can be efficiently performed, and
When the reducing agent supply determination means determines that the reducing agent supplied from the reducing agent supply means can pass through the first selective reduction catalyst and be supplied to the second selective reduction catalyst,
An exhaust gas purification system for an internal combustion engine, wherein the cooling means cools the second selective reduction catalyst.

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