JP2013002283A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device which concurrently cleans PM and NOx in exhaust emission and prevents thermal deterioration of an SCR catalyst, and with which the SCR catalyst can be made active at a low temperature.SOLUTION: During high-temperature exhaust emission, a butterfly valve 25 is opened and a part of high-temperature exhaust emission at a downstream side of a DPF 31 passes through a bypass passage 23. Before opening the butterfly valve, on the other hand, an oxidation catalyst 35 has a temperature close to an external air temperature. The oxidation catalyst 35 has heat capacity and when high-temperature exhaust emission passes through the oxidation catalyst 35, a part of the quantity of heat of the high temperature exhaust emission is absorbed by the oxidation catalyst 35, and the temperature of the high-temperature exhaust emission is lowered. Thus, high-temperature exhaust emission is prevented from flowing into an SCR catalyst 33 and the SCR catalyst is prevented from being thermally deteriorated. During low-temperature exhaust emission, the butterfly valve 25 is opened and a part of exhaust emission passes through the bypass passage 23. At such a time, NO is decreased by oxidation by the oxidation catalyst 35, NO2 is increased and a molar ratio NO2/NO is increased. By making the molar ratio closer to 1, activation of the SCR catalyst 33 is attained.

Description

  The present invention relates to an exhaust gas purification device, and more particularly to an exhaust gas purification device that simultaneously purifies PM (particulate matter) and NOx (nitrogen oxide) in exhaust gas.

  Diesel engines are widely used in commercial vehicles, generators, industrial machines, work vehicles, and the like, but in recent years, with increasing interest in global environmental conservation, regulations on exhaust emissions from engines have been strengthened. As a catalyst for purifying exhaust from a gasoline engine, a three-way catalyst that simultaneously oxidizes carbon monoxide (CO) and hydrocarbons (HC) and reduces NOx is used. On the other hand, in the case of a diesel engine, since it is an oxygen-excess atmosphere, there arises a problem that it is difficult to purify NOx with a general three-way catalyst. Therefore, many methods have been proposed to purify the exhaust from the diesel engine. Among them, a method of removing nitrogen oxides (NOx) with an SCR catalyst after removing particulate pollutants (PM) with DPF (diesel particulate filter) has become the mainstream.

  The DPF is composed of a number of elongated cells defined by, for example, a porous ceramic material partition wall, and one end of each cell is closed by a plug (plug member). Therefore, at the time of PM collection, the exhaust gas flowing in from the cell opened on the upstream side of the exhaust gas passes through the partition and passes through the filter to the cell opened on the downstream side of the exhaust gas. When passing through the partition, PM in the exhaust is captured by the partition.

  With the use of the DPF, when the amount of PM accumulated inside increases, the air permeability is gradually impaired, and the collection performance is also lowered. Therefore, regeneration processing is performed to eliminate the clogging by burning and removing PM accumulated on the DPF at an appropriate timing.

A reducing agent injection nozzle is provided upstream of the SCR catalyst. When urea water is injected from the reducing agent injection nozzle into the exhaust gas, ammonia is generated by hydrolysis. By the SCR catalyst, NOx in the exhaust gas reacts with ammonia and is purified as shown in equations (1) to (3).
4NO + 4NH3 + O2 → 4N2 + 6H2O Formula (1)
8NH3 + 6NO2 → 7N2 + 12H2O ... Formula (2)
2NH3 + NO + NO2 → 2N2 + 3H2O Formula (3)

  The SCR catalyst has the following problems at high exhaust temperatures and low exhaust temperatures.

  First, a problem related to high exhaust temperature will be described.

  During the regeneration process of DPF, PM containing carbon as a main component burns, so that high-temperature exhaust gas is generated downstream of the DPF. When the high-temperature exhaust gas flows into the SCR catalyst, the SCR catalyst may be thermally deteriorated.

  Conventional techniques for preventing this thermal degradation are disclosed in Patent Document 1 and Patent Document 2. The exhaust emission control device according to the prior art is basically configured such that a pre-stage oxidation catalyst, a DPF, an injection nozzle, and an SCR catalyst are arranged in order from the upstream in the exhaust passage of esidin. Furthermore, as a characteristic configuration, a bypass passage is provided that branches from the DPF downstream and merges downstream of the SCR catalyst.

  When the exhaust gas becomes high temperature due to the regeneration process of the DPF, the high temperature exhaust gas is guided to the bypass passage and bypasses the injection nozzle and the SCR catalyst. Thereby, high temperature exhaust is prevented from flowing into the SCR catalyst, and thermal degradation of the SCR catalyst is prevented.

  Next, the problem concerning the low temperature of the exhaust will be described.

  The engine exhaust temperature is low during idling and other low engine speeds and low engine load conditions.

  Among the above formulas (1) to (3), when the exhaust gas is at a low temperature, the reaction rate of the formula (3) is faster than the others, and NOx can be efficiently purified. However, since most of the NOx is NO in the exhaust discharged from the engine, the reaction of the formula (1) is performed and purification cannot be performed efficiently.

  On the other hand, Patent Document 3 discloses a conventional technique related to low temperature activity of an SCR catalyst. The exhaust emission control device according to the prior art includes a main passage provided with an oxidation catalyst and a bypass passage not provided with an oxidation catalyst, and generates NO2 by the oxidation catalyst and estimates the NO2 adsorption state of the oxidation catalyst. By adjusting the exhaust flow rate passing through the main passage and the exhaust flow rate passing through the bypass passage based on the increase or decrease in the estimated NO2 adsorption amount, the NO: NO2 molar ratio of NOx flowing into the SCR catalyst can be 1: 1 (NO2 / NO = 1).

  Thereby, NOx can be efficiently purified by the reaction of the formula (3) even at a low exhaust temperature.

JP 2010-150978 A Japanese Patent No. 4290037 JP 2009-216019 A

  However, in the prior arts disclosed in Patent Documents 1 and 2, during the DPF regeneration process, the high-temperature exhaust gas bypasses the injection nozzle and the SCR catalyst, so NOx is not purified.

On the other hand, the prior art disclosed in Patent Document 3 relates to NOx purification, and does not consider DPF regeneration processing. When the DPF is provided in the above-described conventional technique disclosed in Patent Document 3, during the DPF regeneration process, NO2 in the exhaust is consumed by the reaction of the formula (4), while NO is generated. It is difficult to estimate the adsorption state. Further, NO2 in the exhaust gas is consumed by the DPF regeneration process, but NO is generated, so it is difficult to make the molar ratio close to 1.
C + 2NO2 → CO2 + 2NO ... Formula (4)

  Moreover, the said prior art currently disclosed by patent document 1, 2 is a technique which prevents the SCR catalyst thermal deterioration at the time of exhaust high temperature, and the said prior art disclosed by patent document 3 is SCR at the time of exhaust low temperature. This is a technology related to the low-temperature activity of the catalyst, and both solve the conflicting problems. Therefore, it is not easy to combine both.

  An object of the present invention is to provide an exhaust purification device capable of simultaneously purifying PM and NOx in exhaust gas, preventing thermal degradation of the SCR catalyst, and achieving low-temperature activity of the SCR catalyst.

  (1) In order to achieve the above object, the present invention is provided with a pre-stage oxidation catalyst, a DPF, a liquid reducing agent, or an injection nozzle for supplying a precursor thereof, and an SCR catalyst provided in the exhaust passage of ethidine. In the exhaust gas purification apparatus, a bypass passage branched from the downstream of the DPF and merged upstream of the injection nozzle, an oxidation catalyst disposed in the bypass passage and having a heat absorption function, and a part of the exhaust gas that has passed through the DPF And an exhaust flow rate adjusting means for adjusting an exhaust flow rate passing through the bypass passage.

  In the present invention configured as described above, when the exhaust gas flow rate is adjusted at high exhaust temperature, the high temperature exhaust gas is cooled by the heat absorption function of the oxidation catalyst disposed in the bypass passage. Thereby, the subject at the time of exhaust high temperature can be solved. On the other hand, when the exhaust gas temperature is low and the exhaust gas flow rate is adjusted and a part of the exhaust gas passes through the oxidation catalyst provided in the bypass passage, the NO2 / NO molar ratio increases. By setting the molar ratio to around 1, the problem at the time of exhaust gas low temperature can be solved.

  In this way, the conflicting problems of the problem at the time of exhaust high temperature and the problem at the time of exhaust low temperature are solved by a common configuration of the oxidation catalyst and the exhaust flow rate adjusting means disposed in the bypass passage. The configuration can be simplified by adopting a common configuration as compared to providing separate configurations corresponding to each problem. Such a simple configuration can provide effects such as cost reduction and easy maintenance.

  (2) In the above (1), preferably, exhaust gas temperature detecting means for detecting an exhaust gas temperature upstream of the SCR catalyst is further provided, and the exhaust gas flow rate adjusting means causes the exhaust gas temperature detecting means to thermally deteriorate the SCR catalyst. When a temperature equal to or higher than the first set temperature is detected, a part of the exhaust gas that has passed through the DPF is guided to the bypass passage, and the exhaust gas flow rate is adjusted so that the exhaust temperature upstream of the SCR catalyst is lower than the first set temperature. It has a high temperature suppression function.

  When the exhaust gas is hot, the high temperature exhaust gas is cooled by the endothermic function of the oxidation catalyst and guided to the SCR catalyst. Thereby, NOx can be purified while preventing thermal degradation of the SCR catalyst.

  (3) In the above (1), preferably, the exhaust gas temperature detecting means for detecting the exhaust gas temperature upstream of the SCR catalyst, and the molar ratio (NO2 / NO) of nitrogen dioxide to nitrogen monoxide upstream of the SCR catalyst are estimated. The exhaust flow rate adjusting means further comprises a part of the exhaust gas that has passed through the DPF when the exhaust gas temperature detecting means detects a temperature lower than a second set temperature at which the SCR catalyst is difficult to activate. A low temperature activation function unit is provided for adjusting the exhaust gas flow rate so as to lead to the bypass passage and the estimated molar ratio is 1.

  When the exhaust gas is at a low temperature, NO is oxidized by the oxidation catalyst so that the NO2 / NO molar ratio is around 1. Thereby, activation of an SCR catalyst can be aimed at.

  (4) In the above (3), preferably further comprising NOx concentration detecting means for detecting the NOx concentration upstream and downstream of the SCR catalyst, wherein the molar ratio estimating means is the SCR catalyst detected by the NOx concentration detecting means. The molar ratio is estimated based on the NOx purification rate obtained from the upstream and downstream NOx concentrations and the exhaust temperature upstream of the SCR catalyst detected by the exhaust temperature detecting means.

  Thereby, the molar ratio of NO2 / NO can be estimated accurately and the reducing agent can be supplied without excess or deficiency. As a result, NOx can be efficiently purified and ammonia slip can be prevented.

  (5) In the above (1), preferably, the exhaust flow rate adjusting means has a butterfly valve.

  Thereby, simplification of a structure can be achieved.

  According to the present invention, PM and NOx in exhaust gas can be purified simultaneously. In particular, even when the exhaust gas becomes high temperature due to the DPF regeneration process, NOx can be purified while preventing thermal degradation of the SCR catalyst. In addition, the SCR catalyst can be activated at a low temperature even when the exhaust gas is at a low temperature.

It is a figure which shows the whole structure of an exhaust-gas purification apparatus. It is a figure which shows the exhaust flow volume adjustment control flow at the time of exhaust high temperature in addition to the regeneration control flow. It is a figure which shows the exhaust gas flow rate adjustment control flow at the time of exhaust gas low temperature in addition to a reducing agent supply control flow. It is a figure which shows the purification performance of the SCR catalyst at the time of exhaust gas low temperature. It is a figure which shows an example of a molar ratio estimation map.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

~Constitution~
FIG. 1 is a diagram showing an overall configuration of an exhaust emission control apparatus according to this embodiment of the present invention.

  A work vehicle (for example, a hydraulic excavator) is equipped with a diesel engine 10. The exhaust purification device 20 is provided in the exhaust pipe 21 of the engine 10. As a basic configuration, a front-stage oxidation catalyst 30, a DPF 31, an injection nozzle 32, an SCR catalyst 33, and a rear-stage oxidation catalyst 34 are arranged in the exhaust pipe 21 in order from the upstream. Further, as a characteristic configuration, a bypass passage 23 branched from the exhaust pipe 21 downstream of the DPF 31 and merged with the exhaust pipe 21 upstream of the injection nozzle 32, an oxidation catalyst 35 disposed in the bypass passage 23, and an inlet of the bypass passage 23 A provided butterfly valve 25 is provided.

  The pre-stage oxidation catalyst 30 is not particularly limited as long as it is a catalyst that can oxidize NO to NO2. For example, at least one of noble metals such as platinum, palladium, iridium, and rhodium is titania, zirconia, alumina, or the like. A catalyst in which the loaded catalyst component is loaded on a cordierite honeycomb structure or the like is suitable.

  By arranging the pre-stage oxidation catalyst 30 upstream of the DPF 31, the particulates collected and deposited in the DPF 31 react with NO 2 supplied from the oxidation catalyst 30 and are oxidized even at a relatively low temperature (eg, 300 ° C.). The Further, the pre-oxidation catalyst 30 oxidizes an unburned fuel (hydrocarbon: HC), carbon monoxide (CO), or the like in the exhaust, and raises the temperature of the exhaust by heat generated by the oxidation. Then, the temperature of the DPF 31 on the downstream side is raised by the heated exhaust gas. When the DPF 31 is clogged, it can be oxidized and removed by oxygen.

  The DPF 31 collects PM with a honeycomb structure. The PM deposited on the DPF 31 is burned and removed by the regeneration process.

  The differential pressure sensor 40 is provided before and after the DPF 31 and detects an exhaust differential pressure. Based on the detected value, the PM accumulation amount is estimated, and regeneration control such as regeneration start / regeneration end is performed.

  The exhaust temperature sensors 42 and 43 are provided on the upstream side and the downstream side of the pre-stage oxidation catalyst 30. During regeneration control, the exhaust gas temperature is monitored to prevent melting of the DPF 31 due to the accumulated PM burning at a stroke and abnormally rising in temperature.

  The injection nozzle 32 injects urea water, which is a precursor of ammonia, which is a reducing agent, into the exhaust gas. Ammonia is generated by hydrolyzing the urea water. The urea water is stored in the urea water tank 50. A pipe connecting the urea water tank 50 and the injection nozzle 32 is provided with a reducing agent supply pump 51, a reducing agent control valve 52, and a reducing agent injection device 54. The urea water tank 50 is provided with a reducing agent concentration sensor 55 that detects the concentration of the reducing agent.

  The SCR catalyst 33 is not particularly limited as long as it is a catalyst normally used for denitration. For example, a catalyst in which titanium oxide is loaded with a denitration active component such as vanadium or tungsten, or a transition metal such as copper, iron, or cerium is used. A catalyst in which the ion-exchanged zeolite is supported on a cordierite honeycomb structure or the like is suitable.

  The exhaust temperature sensor 45 is provided on the upstream side of the SCR catalyst 33 and detects the exhaust temperature. A catalyst temperature sensor that directly detects the temperature of the SCR catalyst 33 may be used.

  The NOx concentration sensors 46 and 47 are provided on the upstream side and the downstream side of the SCR catalyst 33.

  The post-stage oxidation catalyst 34 is provided on the downstream side of the SCR catalyst 33, and decomposes excess ammonia (generation of ammonia slip) due to supply of excess urea water. Note that the downstream oxidation catalyst 34 may be omitted as long as the reducing agent supply control that does not generate ammonia slip can be performed.

  The oxidation catalyst 35 may be the same as the pre-stage oxidation catalyst 30, but has an endothermic function by heat capacity (detailed in ~ operation ~).

  The butterfly valve 25 is adjustable in its opening degree, and adjusts the exhaust gas flow rate passing through the bypass passage 23 by adjusting the opening degree (detailed in ~ control ~ and ~ operation ~).

~control~
The ECU 60 controls the rotational speed and torque of the engine 10 and performs various controls. For example, the ECU 60 inputs detection signals from the differential pressure sensor 40 and the exhaust temperature sensors 42 and 43 and performs regeneration control of the DPF 31. In addition, a signal from a rotational speed sensor or various sensors is input, various calculations are performed, and the calculation results are output to various devices such as an injector, an intake control valve, and an EGR valve of each cylinder.

  The ECU 60 is connected to the DCU 70 via CAN communication. For example, the DCU 70 receives detection signals from the exhaust temperature sensor 45 and the NOx concentration sensors 46 and 47 that are input to the ECU 60.

  The DCU 70 calculates the necessary reducing agent supply amount based on the NOx concentration by the NOx concentration sensor 46, the NO2 / NO molar ratio, and the reducing agent concentration by the reducing agent concentration sensor 55, and controls the reducing agent supply pump 51 and the reducing agent. The valve 52 is controlled (reducing agent supply control).

  The estimation of the molar ratio will be described in detail in “Operation”.

  In the present embodiment, the DCU 70 controls the opening degree of the butterfly valve 25 (exhaust flow rate adjustment control). Note that the ECU 60 may control the butterfly valve 25.

  The DCU 70 includes a high temperature suppression function unit 70a, a low temperature activation function unit 70b, and a molar ratio estimation function unit 70c that perform characteristic control of the present embodiment.

  FIG. 2 is a diagram showing an exhaust flow rate adjustment control flow by the DCU 70 at a high exhaust temperature in addition to the regeneration control flow by the ECU 60.

  First, reproduction control will be described.

  The ECU 60 estimates the PM accumulation amount based on the detection signal of the differential pressure sensor 40 (step S11). Then, it is determined whether or not the PM accumulation amount is not less than a set value (regeneration start reference value) (step S12). If it is determined that the PM accumulation amount is not equal to or greater than the set value, the PM accumulation amount estimation is continued. When it is determined that the PM accumulation amount is equal to or greater than the set value, the regeneration process is started (step S13). In addition, you may start a reproduction | regeneration process based on the working time of a working machine with PM deposition amount.

  During the regeneration process, it is determined whether or not the detected temperature T of the exhaust temperature sensor 45 is equal to or higher than the first set temperature T1 (step S21) (details will be described later).

  Further, it is determined whether or not the PM accumulation amount is equal to or less than a set value (regeneration end reference value) (step S14). If it is determined that the PM accumulation amount is not less than the set value, the regeneration process is continued. If it is determined that the PM accumulation amount is less than or equal to the set value, the regeneration process is terminated (step S15). Note that the regeneration process may be terminated based on the regeneration processing time together with the PM deposition amount.

  Next, exhaust flow rate adjustment control at high exhaust temperature, which is characteristic control of the present embodiment, will be described.

  During the regeneration process, it is determined whether or not the detected temperature T of the exhaust temperature sensor 45 is equal to or higher than the first set temperature T1 (step S21). If it is determined that the detected temperature T is equal to or higher than the first set temperature T1, the butterfly valve 25 is opened, and a part of the exhaust is guided to the bypass passage 23 (step S22).

  The oxidation catalyst 35 has an endothermic function, and the high-temperature exhaust gas is cooled by passing through the bypass passage 23. As the opening degree of the butterfly valve 25 increases, the passage flow rate of the bypass passage 23 increases, and the temperature of the high-temperature exhaust gas further decreases. The DCU 70 adjusts the opening degree of the butterfly valve 25 based on the detected temperature T of the exhaust temperature sensor 45 (step S23).

  The DCU 70 verifies whether the flow rate adjustment is optimal (step S24). Specifically, it is determined whether or not the detected temperature T of the exhaust temperature sensor 45 has dropped from the first set temperature T1 by a predetermined temperature (for example, 100 ° C.) or more. If it is determined that the flow rate adjustment is not optimal, the flow rate adjustment is continued. When it is determined that the flow rate adjustment is optimal, the process returns to the regeneration control.

  After completion of the regeneration process, the butterfly valve 25 is closed (step S25).

  Here, the first set temperature T1 is the lower limit (for example, 700 ° C.) of the exhaust gas temperature range that causes the SCR catalyst 33 to thermally deteriorate.

  FIG. 3 is a diagram showing an exhaust flow rate adjustment control flow by the DCU 70 at a low exhaust temperature in addition to the reducing agent supply control flow by the DCU 70.

  First, the reducing agent supply control will be described.

  The DCU 70 determines whether or not the detected temperature T of the exhaust temperature sensor 45 is lower than the second set temperature T2 (step S41) (details will be described later). When it is determined that the detected temperature T is not lower than the second set temperature T2, reducing agent supply control is started.

  The DCU 70 inputs the detection signal of the NOx concentration sensor 46 via the ECU 60 and CAN communication, and measures the NOx concentration upstream of the SCR catalyst 33 (step S31). At the same time, the NO2 / NO molar ratio is estimated. On the other hand, the detection signal of the reducing agent concentration sensor 55 is input, and the reducing agent concentration is measured (step S32). Based on the NOx concentration, the NO2 / NO molar ratio, and the reducing agent concentration, a necessary reducing agent supply amount is calculated (step S33). By controlling the pressure / flow rate of the reducing agent supply pump 51 and the opening of the reducing agent control valve 52, injection is started so as to supply a predetermined amount of reducing agent into the exhaust gas (step S34).

  The DCU 70 measures the NOx concentration downstream of the SCR catalyst 33 by the NOx concentration sensor 47, and determines whether or not the NOx concentration is equal to or less than the regulation value (step S35). If it is determined that the NOx concentration is not lower than the regulation value, the injection is continued. If it is determined that the NOx concentration is less than or equal to the regulation value, the injection is terminated (step S36). Note that, during the reducing agent supply control, the necessary reducing agent supply amount is appropriately corrected based on the latest information.

  Next, exhaust flow rate adjustment control at low exhaust temperature, which is characteristic control of the present embodiment, will be described.

  Prior to the reducing agent supply control, the DCU 70 determines whether or not the detected temperature T of the exhaust temperature sensor 45 is lower than the second set temperature T2 (step S41).

  If it is determined that the detected temperature T is lower than the second set temperature T2, the butterfly valve 25 is opened, and a part of the exhaust is guided to the bypass passage 23 (step S42).

  The oxidation catalyst 35 oxidizes NO and generates NO2. As the opening degree of the butterfly valve 25 increases, the flow rate through the bypass passage 23 increases, NO decreases, NO2 increases, and as a result, the NO2 / NO molar ratio increases. The DCU 70 adjusts the opening of the butterfly valve 25 based on the NO2 / NO molar ratio (step S43).

  The DCU 70 verifies whether the flow rate adjustment is optimal (step S44). Specifically, it is determined whether or not the NO2 / NO molar ratio is around 1. If it is determined that the flow rate adjustment is not optimal, the flow rate adjustment is continued. When it is determined that the flow rate adjustment is optimal, reducing agent supply control is performed.

  After completion of the reducing agent supply control, the butterfly valve 25 is closed (step S45).

  Here, the second set temperature T2 is the upper limit (for example, 330 ° C.) of the exhaust temperature range where the SCR catalyst 33 is difficult to activate.

-Correspondence with claims-
In the present embodiment, the bypass passage 23 is a bypass passage that branches from the downstream of the DPF 31 and joins upstream of the injection nozzle 32, and the oxidation catalyst 35 is an oxidation catalyst that is disposed in the bypass passage 23 and has a heat absorption function. Part of the functions of the butterfly valve 25 and the DCU 70 constitute exhaust flow rate adjusting means for guiding a part of the exhaust gas that has passed through the DPF 31 to the bypass passage 23 and adjusting the exhaust flow rate that passes through the bypass passage 23.

  When the exhaust temperature sensor 45 detects the temperature equal to or higher than the first set temperature T1, the high temperature suppression function unit 70a and the processing of step S21 to step S25 by the DCU 70 guide part of the exhaust that has passed through the DPF 31 to the bypass passage 23. A high temperature suppression function unit that adjusts the exhaust gas flow rate is configured such that the exhaust gas temperature upstream of the SCR catalyst 33 is lower than the first set temperature T1.

  When the exhaust temperature sensor 45 detects a temperature lower than the second set temperature T2, the processing at steps S41 to S45 by the low temperature activation function unit 70b and the DCU 70 causes a part of the exhaust gas that has passed through the DPF 31 to enter the bypass passage 23. Thus, the low temperature activation function unit that adjusts the exhaust gas flow rate is configured so that the estimated molar ratio is 1.

  The exhaust temperature sensor 45, the NOx concentration sensors 46, 47, the molar ratio estimation map (see FIG. 5 to be described later), and the molar ratio estimation function unit 70c are the molar ratio of nitrogen dioxide with respect to nitrogen monoxide upstream of the SCR catalyst 33 (NO2 / The molar ratio estimating means for estimating (NO) is configured.

~ Operation ~
(Operation 1) The operation of the exhaust emission control device 20 at the normal time (T2 ≦ T <T1) will be described.

  The exhaust purification device 20 performs a regeneration process on the DPF 31 (S11 → S12 → S13 → S21 → S14 → S15 → S25). Thereby, PM is purified. During the regeneration process, the exhaust temperature sensors 42 and 43 monitor the exhaust temperature to prevent abnormal temperature rise. Therefore, there is almost no fear of thermal degradation of the SCR catalyst 33.

  On the other hand, the exhaust emission control device 20 supplies the reducing agent to the exhaust gas that has passed through the DPF 31 from the injection nozzle 32 (S41 → S12 → S13 → S21 → S14 → S15 → S25). Further, NOx is purified by the SCR catalyst 33.

  At this time, the butterfly valve 25 is closed and the exhaust flow rate passing through the bypass passage 23 is zero. In other words, all the exhaust gas passes through the exhaust pipe 21.

  Therefore, the high temperature exhaust is not led to the bypass passage 23. The oxidation catalyst 35 disposed in the bypass passage 23 has a heat capacity. As a result, the oxidation catalyst 35 becomes a temperature close to the outside air temperature (for example, 40 ° C.) due to the outside air.

  By the way, most of NOx contained in the exhaust discharged from the engine is NO. Under normal conditions, the reaction rate of the formula (1) and the reaction rate of the formula (3) are comparable (see FIG. 4). Therefore, NO can be efficiently purified by the reaction of the formula (1).

  (Operation 2) The operation of the exhaust emission control device 20 at a high temperature (T ≧ T1) will be described.

  The exhaust gas purification device 20 monitors the exhaust gas temperature during the regeneration process to prevent abnormal temperature rise. However, if the PM accumulation amount increases, the exhaust gas after passing through the DPF 31 may become hot. When the high-temperature exhaust gas flows into the SCR catalyst 33, the SCR catalyst 33 may be thermally deteriorated, and it is necessary to prevent this.

  The exhaust gas purification device 20 performs exhaust gas flow rate adjustment control when the exhaust gas temperature is high (S21 → S22 → S23 → S24 → S21). The butterfly valve 25 is opened, and a part of the high-temperature exhaust (for example, 700 ° C.) passes through the bypass passage 23. On the other hand, before the butterfly valve is opened, the oxidation catalyst 35 is at a temperature close to the outside air temperature (for example, 40 ° C.). The oxidation catalyst 35 has a heat capacity. When the high-temperature exhaust gas passes through the oxidation catalyst 35, a part of the heat quantity of the high-temperature exhaust gas is absorbed by the oxidation catalyst 35, and the high-temperature exhaust gas cools down. As the opening degree of the butterfly valve 25 increases, the passage flow rate of the bypass passage 23 increases, and the temperature of the high-temperature exhaust gas further decreases. The exhaust gas whose temperature has been lowered in the bypass passage 23 merges with the high-temperature exhaust gas in the exhaust pipe 21.

  By performing appropriate exhaust flow rate adjustment control, the detected temperature T of the exhaust temperature sensor 45 is lowered from the first set temperature T1 by a predetermined temperature (for example, 100 ° C.) or more. Thereby, it is possible to prevent the high-temperature exhaust gas from flowing into the SCR catalyst 33, and it is possible to prevent SCR catalyst thermal deterioration.

  The exhaust purification device 20 performs reducing agent supply control, and purifies NOx contained in the exhaust after passing through the DPF 31. At this time, NO contained in the exhaust gas passing through the bypass passage 23 becomes NO 2 by the oxidation catalyst 35. The molar ratio of NO2 / NO is estimated, and NOx is efficiently purified by the reactions of formulas (1) and (3).

  By the way, the DPF regeneration process is not always performed, and even if the DPF regeneration process is performed, the exhaust high temperature (T ≧ T1) is not always achieved. When the exhaust flow rate adjustment control is not necessary, the butterfly valve 25 is closed and the exhaust flow rate passing through the bypass passage 23 becomes zero. The oxidation catalyst 35 gradually returns to a temperature close to the outside air temperature (for example, 40 ° C.) by the outside air. Thereby, the oxidation catalyst 35 recovers the endothermic function.

  Although the DPF regeneration process is exemplified as the reason for the high exhaust temperature, the exhaust flow rate adjustment control is also performed when the exhaust temperature becomes high for other reasons.

  (Operation 3) The operation of the exhaust emission control device 20 at a low temperature (T <T2) will be described.

  The engine exhaust temperature is low during idling and other low engine speeds and low engine load conditions.

  FIG. 4 is a view showing the purification performance of the SCR catalyst at the time of low exhaust temperature. The thin line in the figure shows the purification performance when the NO2 / NO molar ratio is 0 (that is, the reaction of the formula (1)), and the thick line in the figure shows the NO2 / NO molar ratio of 1 (that is, the reaction of the formula (3)). The purification performance is shown. The horizontal axis is the exhaust temperature, and the vertical axis is the NOx purification rate. The NOx purification rate is a value obtained by dividing the difference between the upstream and downstream NOx concentrations of the SCR catalyst 33 detected by the NOx concentration sensors 46 and 47 by the upstream NOx concentration.

  The second set temperature T2 is the upper limit (for example, 330 ° C.) of the exhaust temperature range in which the SCR catalyst 33 is difficult to activate. In other words, the equivalent purification rate is obtained in the reactions of the expressions (1) and (3). Temperature.

  At a low exhaust temperature (T <T2), a high purification rate cannot be obtained by the reaction of the formula (1). That is, NOx cannot be purified efficiently. On the other hand, in the reaction of the formula (3), a higher purification rate is obtained compared to the reaction of the formula (1).

  The exhaust gas purification device 20 performs exhaust gas flow rate adjustment control when the exhaust gas temperature is low (S41 → S42 → S43 → S44 → S31). The butterfly valve 25 is opened, and a part of the exhaust passes through the bypass passage 23. At this time, most of the NOx contained in the exhaust gas is NO. The NO contained in the exhaust gas passing through the bypass passage 23 becomes NO 2 by the oxidation catalyst 35. The exhaust gas containing NO 2 joins the exhaust gas containing NO in the exhaust pipe 21. As the opening degree of the butterfly valve 25 increases, the flow rate through the bypass passage 23 increases, NO decreases, NO2 increases, and as a result, the NO2 / NO molar ratio increases.

  Here, the molar ratio estimation will be described.

  The molar ratio estimation function unit 70c receives detection signals from the NOx concentration sensors 46 and 47, calculates the Nox purification rate, reads the molar ratio estimation map stored in the DCU 70, and reads the NOx purification rate and the exhaust temperature sensor 45. The molar ratio (NO2 / NO) of nitrogen dioxide to nitrogen monoxide upstream of the SCR catalyst 33 is estimated based on the exhaust temperature and the molar ratio estimation map.

  FIG. 5 is a diagram illustrating an example of the molar ratio estimation map. The horizontal axis is the molar ratio, and the vertical axis is the NOx purification rate. For each exhaust temperature, the relationship between the molar ratio and the NOx purification rate is recorded in advance based on experiments.

  As a general tendency, the NOx purification rate increases as the exhaust gas temperature increases. Further, at the same exhaust temperature, when the molar ratio is small, the reaction of the formula (1) has priority, so the NOx purification rate is low. As the molar ratio increases, the reaction of the formula (3) also occurs in addition to the reaction of the formula (1), the NOx purification rate increases, and when the molar ratio is close to 1, the reaction of the formula (3) takes precedence. NOx purification rate becomes the highest.

  As the molar ratio increases beyond 1, the NO2 supply becomes excessive, the reaction of formula (2) occurs in addition to the reaction of formula (3), and the NOx purification rate decreases.

  In FIG. 5, when one exhaust temperature and one NOx purification rate are determined, two solutions for the molar ratio are obtained. The molar ratio estimation function unit 70c stores the immediately preceding molar ratio estimation result and selects one of the two solutions related to the molar ratio based on whether the NOx purification rate is increasing or the NOx purification rate is decreasing. Select. Thereby, the exhaust emission control device 20 estimates the molar ratio.

  Return to the explanation of the operation at low temperature. By performing appropriate exhaust gas flow rate adjustment control, the molar ratio of NO2 / NO becomes around 1. By the reaction of formula (3), NOx can be efficiently purified even at a low exhaust temperature.

  On the other hand, there is no risk of thermal degradation of the SCR catalyst 33 at low exhaust temperatures.

  In addition, the numerical values such as the temperature shown above are illustrated to assist the understanding of the invention, and the present invention is not limited to these numerical values.

~effect~
(Effect 1) The exhaust gas purification device 20 can purify PM in exhaust gas by PM collection of the DPF 31 and DPF regeneration processing. On the other hand, NOx can be purified by the reducing agent supply from the injection nozzle 32 and the SCR catalyst 33.

  By the way, the exhaust gas purification apparatus according to the prior art has a problem that during the DPF regeneration process, high temperature exhaust gas bypasses the injection nozzle and the SCR catalyst so as to prevent thermal degradation of the SCR catalyst, so that NOx purification is not performed. It was.

  The exhaust purification device 20 lowers the temperature of the high-temperature exhaust gas by the endothermic function of the oxidation catalyst 35 and guides it to the SCR catalyst 33. Thereby, NOx can be purified while preventing thermal degradation of the SCR catalyst.

  (Effect 2) The exhaust gas purification device 20 can activate the SCR catalyst 33 by oxidizing NO with the oxidation catalyst 35 and setting the NO2 / NO molar ratio to around 1 when the exhaust gas temperature is low.

  (Effect 3) The exhaust emission control device 20 solves the conflicting problems of the problem related to the high temperature of the exhaust gas and the problem related to the low temperature of the exhaust gas by a common configuration of the oxidation catalyst 35 disposed in the butterfly valve 25 and the bypass passage 23. To do. The configuration can be simplified by adopting a common configuration as compared to providing separate configurations corresponding to each problem. Such a simple configuration can provide effects such as cost reduction and easy maintenance.

  (Effect 4) The exhaust gas purification device 20 can supply the reducing agent without excess or deficiency by accurately estimating the NO2 / NO molar ratio. As a result, NOx can be efficiently purified, and ammonia slip can be prevented.

10 Engine 20 Exhaust gas purification device 21 Exhaust pipe 23 Bypass passage 25 Butterfly valve 30 Pre-stage oxidation catalyst 31 DPF
32 injection nozzle 33 SCR catalyst 34 latter stage oxidation catalyst 35 oxidation catalyst (bypass passage arrangement)
40 Differential pressure sensor 42 Exhaust temperature sensor (front stage oxidation catalyst upstream)
43 Exhaust temperature sensor (downstream of upstream oxidation catalyst)
45 Exhaust temperature sensor (upstream of SCR catalyst)
46 NOx concentration sensor (upstream of SCR catalyst)
47 NOx concentration sensor (downstream of SCR catalyst)
50 urea water tank 51 reducing agent supply pump 52 reducing agent control valve 54 reducing agent injection device 55 reducing agent concentration sensor 60 engine control unit (ECU)
70 Reducing agent injection control unit (DCU)
70a High temperature suppression function part 70b Low temperature activation function part 70c Molar ratio estimation function part

Claims (5)

In an exhaust gas purification apparatus provided with an SCR catalyst provided in an exhaust passage of ethidine, a pre-stage oxidation catalyst, a DPF, a liquid reducing agent or a precursor for supplying the precursor, and an SCR catalyst in order from upstream
A bypass passage branched from the DPF downstream and joining upstream of the injection nozzle;
An oxidation catalyst disposed in the bypass passage and having an endothermic function;
An exhaust gas purification apparatus comprising: an exhaust gas flow rate adjusting unit that guides a part of the exhaust gas that has passed through the DPF to the bypass passage and adjusts an exhaust gas flow rate that passes through the bypass passage. The exhaust emission control device according to claim 1,
Exhaust temperature detecting means for detecting the exhaust temperature upstream of the SCR catalyst,
The exhaust flow rate adjusting means is
When the exhaust temperature detecting means detects a temperature equal to or higher than a first set temperature that causes the SCR catalyst to thermally deteriorate, a part of the exhaust gas that has passed through the DPF is guided to the bypass passage, and the exhaust temperature upstream of the SCR catalyst is the first An exhaust emission control device having a high-temperature suppression function unit that adjusts an exhaust gas flow rate so as to be lower than a set temperature. The exhaust emission control device according to claim 1,
Exhaust temperature detecting means for detecting the exhaust temperature upstream of the SCR catalyst;
A molar ratio estimating means for estimating a molar ratio of nitrogen dioxide to nitrogen monoxide (NO2 / NO) upstream of the SCR catalyst;
The exhaust flow rate adjusting means is
When the exhaust temperature detecting means detects a temperature lower than a second set temperature at which the SCR catalyst is difficult to activate, a part of the exhaust gas that has passed through the DPF is led to the bypass passage so that the estimated molar ratio becomes 1. An exhaust purification apparatus comprising a low temperature activation function unit for adjusting an exhaust flow rate. The exhaust emission control device according to claim 3,
NOx concentration detection means for detecting NOx concentration upstream and downstream of the SCR catalyst,
The molar ratio estimating means includes
Estimating the molar ratio based on the NOx purification rate obtained from the NOx concentration upstream and downstream of the SCR catalyst detected by the NOx concentration detection means and the exhaust temperature upstream of the SCR catalyst detected by the exhaust temperature detection means. A featured exhaust purification device. The exhaust emission control device according to claim 1,
The exhaust gas purification device, wherein the exhaust flow rate adjusting means has a butterfly valve.

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