JP2013189882A - Exhaust gas purification device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purification device that can improve NOpurification efficiency.SOLUTION: An exhaust gas purification device includes: a DOC 22 provided in an exhaust path 12 in which an exhaust gas of an engine 30 circulates; a catalyst 16a provided at the upstream side from the DOC 22 of the exhaust path 12; an EHC 16 including the catalyst 16a and a heater 16b for heating the DOC 22; an SCR catalyst 26 provided in the downstream side of the exhaust path 22 from the DOC 22; and a supply part 28 provided in the upstream side from the DOC 22 of the exhaust path 12 and supplying a secondary air to the exhaust path 12. The EHC 16 and the supply part 28 are the exhaust gas purification device which controls a temperature T3 so that the temperature T3 of the DOC 22 is within a range 11.

Description

  The present invention relates to an exhaust gas purification device.

In order to reduce air pollution, it is required to purify nitrogen oxide (NOx) contained in exhaust gas of an internal combustion engine (engine). In order to purify NOx, an SCR catalyst (Selective Catalytic Reduction) may be provided in an exhaust path through which exhaust gas flows. The SCR catalyst reacts ammonia produced from urea water with NOx, and decomposes NOx into nitrogen and water. When the molar ratio (NO: NO ₂ ) of nitric oxide (NO) to nitrogen dioxide (NO ₂ ) is 1: 1, the NOx purification efficiency in the SCR catalyst is maximized. For example, Patent Document 1 describes a technique in which an oxidation catalyst having a heater is provided upstream of the SCR catalyst in the exhaust path. The oxidation catalyst is heated by the heater, to advance the oxidation reaction of the NO with NO _2. Thus, NO: NO ₂ in the exhaust gas is adjusted to 1: 1.

JP 2010-265862 A

However, in the conventional technology, NO: NO ₂ greatly deviates from 1: 1, so that the NOx purification efficiency of the SCR catalyst may decrease. In view of the above problems, the present invention aims to provide an exhaust gas purification device capable of improving NOx purification efficiency.

  The present invention relates to an oxidation catalyst provided in an exhaust path through which exhaust gas of an internal combustion engine flows, a catalyst provided upstream of the oxidation catalyst in the exhaust path and purifying hydrocarbons, the catalyst, and the oxidation catalyst An electrically heated catalyst including a heating unit for heating the SCR catalyst, an SCR catalyst provided on the downstream side of the exhaust path from the oxidation catalyst, and provided on the upstream side of the oxidation catalyst in the exhaust path, An exhaust gas for controlling the temperature of the oxidation catalyst so that the temperature of the oxidation catalyst falls within a predetermined range. It is a purification device.

  In the above configuration, the electric heating catalyst and the supply unit are configured such that the ratio of nitrogen monoxide and nitrogen dioxide in the exhaust gas flowing into the SCR catalyst is 1: 1 or close to 1: 1. The temperature of the oxidation catalyst can be controlled.

  In the above configuration, the electrically heated catalyst heats the oxidation catalyst when the temperature of the oxidation catalyst is lower than a first temperature, and the supply unit is configured such that the temperature of the oxidation catalyst is higher than the first temperature. When the temperature is higher than a certain second temperature, the secondary air can be supplied to the exhaust path.

  In the above configuration, a filter provided upstream of the electrically heated catalyst in the exhaust path is provided, and the heating unit heats the catalyst before starting the PM regeneration process of the filter. it can.

  In the above configuration, the exhaust gas measured by the temperature measurement unit is provided between the electric heating catalyst and the oxidation catalyst, and includes a temperature measurement unit that determines the temperature of the exhaust gas flowing through the exhaust path. When the temperature of gas is higher than the temperature of the said electrically heated catalyst, the said heating part can be set as the structure which heats.

  In the above configuration, a flow rate acquisition unit that acquires a flow rate of exhaust gas flowing through the exhaust path is provided, and when the flow rate is equal to or less than a predetermined value, the supply unit supplies secondary air to the exhaust path. be able to.

  The said structure WHEREIN: The said supply part can be set as the structure provided between the said electrically heated catalyst and the said oxidation catalyst.

  ADVANTAGE OF THE INVENTION According to this invention, the exhaust gas purification apparatus which can improve NOx purification efficiency can be provided.

FIG. 1 is a schematic view illustrating an engine system including the exhaust gas purifying apparatus according to the first embodiment. FIG. 2A is a graph illustrating the NOx purification efficiency of the SCR catalyst. FIG. 2B is a graph illustrating the relationship between the DOC temperature T3 and NO ₂ / NOx. FIG. 3 is a functional block diagram illustrating the configuration of the ECU. FIG. 4A and FIG. 4B are flowcharts illustrating the control of the exhaust gas purifying apparatus according to the first embodiment. FIG. 5 is a flowchart illustrating the control of the exhaust gas purification device. FIG. 6 is a flowchart illustrating the control of the exhaust gas purifying apparatus according to the second embodiment. FIG. 7 is a graph for explaining a change in the temperature T3 in the second embodiment.

  Embodiments of the present invention will be described with reference to the drawings.

  Example 1 is an example in which a catalyst for purifying HC (hydrocarbon) is used and the temperature of the oxidation catalyst is adjusted. FIG. 1 is a schematic view illustrating an engine system 100 including an exhaust gas purification apparatus according to the first embodiment.

  As shown in FIG. 1, engine system 100 includes an ECU (Engine Control Unit) 10, an exhaust path 12, and an engine 30. The engine 30 is, for example, a diesel engine. Exhaust gas discharged from the engine 30 flows through the exhaust path 12 and is discharged outside the vehicle on which the engine 30 is mounted. From the upstream side (engine 30 side), a differential pressure sensor 13, a DPF (Diesel Particulate Filter) 14, an EHC (Electric Heated Catalyst) 16, and a temperature sensor 18 are connected to the exhaust path 12. And 20, a DOC (Diesel Oxidation Catalyst) 22, a urea water injector 24, and an SCR catalyst 26 are provided. A secondary air path 28 a is connected between the temperature sensor 18 and the temperature sensor 20 in the exhaust path 12. A valve 28b is provided in the secondary air path 28a. The secondary air path 28 a and the valve 28 b function as a supply unit 28 that supplies secondary air to the exhaust path 12.

  The differential pressure sensor 13 measures the difference (differential pressure) between the upstream pressure and the downstream pressure of the DPF 14. The DPF 14 collects PM (Particulate Matter) in the exhaust gas. The EHC 16 includes a catalyst 16a, a heater 16b, and a temperature sensor (not shown). The heater 16b generates heat when energized. The catalyst 16a is activated by being heated by the heater 16b, and is purified by burning HC. The temperature sensor included in the EHC 16 measures the temperature T1 of the EHC 16. The temperature sensor 18 measures the temperature T2 of the exhaust gas between the EHC 16 and the secondary air path 28a. The temperature sensor 20 measures the temperature T3 of the DOC 22. The DOC 22 is heated by the exhaust gas heated by the heater 16b reaching the DOC 22.

浄化された排気ガスは車両の外部に排出される。 In DOC22, the oxidation reaction is performed NO contained in the exhaust gas is NO _2. The urea water injector 24 injects urea water toward the SCR catalyst 26. The SCR catalyst 26 purifies NOx by a chemical reaction between ammonia (NH ₃ ) generated by hydrolysis of urea water and NOx. In the SCR catalyst 26, NOx is purified by a reaction represented by the following chemical reaction formula.

The purified exhaust gas is discharged outside the vehicle.

FIG. 2A is a graph illustrating the NOx purification efficiency of the SCR catalyst. The horizontal axis represents the molar ratio NO ₂ / NOx, and the vertical axis represents the NOx purification efficiency. As shown in FIG. 2A, when NO ₂ /NOx≈0.5, the NOx purification efficiency is maximized. Note that when NO: NO ₂ = 1, NO ₂ /NOx≈0.5.

Since NO is oxidized to NO ₂ in the DOC 22, NO: NO ₂ becomes 1: 1 or approaches 1: 1. _NO: NO 2 is dependent on the temperature T3 of DOC22. FIG. 2B is a graph illustrating the relationship between the temperature T3 of the DOC 22 and NO ₂ / NOx. The horizontal axis represents the temperature T3 of the DOC 22, and the vertical axis represents NO ₂ / NOx after passing through the DOC 22. As shown in FIG. 2B, when T3 = T0, NO ₂ / NOx is 0.5. When the temperature T3 is in the range 11 of T0-t or more and T0 + t or less, NO ₂ / NOx is close to 0.5, for example, R1 or more and 0.5 or less. R1 is, for example, 0.45 and 0.48. Since NO ₂ / NOx is 0.5 or a value close to 0.5, the NOx purification efficiency is increased. When the temperature T3 is lower than T0-t or higher than T0 + t, NO ₂ / NOx greatly deviates from 0.5. For this reason, NOx purification efficiency falls. The arrows A1 and A2 will be described later.

When the exhaust gas contains HC functioning as a reducing agent, a reduction reaction in which NO ₂ becomes NO proceeds. As a result, NO: NO ₂ deviates significantly from 1: 1. That is, NO ₂ / NOx falls below 0.5. The NOx purification efficiency in the SCR catalyst 26 decreases to a region on the left side of the peak in FIG. Even the temperature T3 of DOC22 is T0-t or more, even within the range of T0 + t, NO by the generation of HC: NO ₂ is 1: deviates greatly from 1. HC occurs, for example, immediately after the start of the PM regeneration process. In the PM regeneration process, fuel is injected from an injector (not shown) included in the engine 30 toward the DPF 14, and the PM and fuel collected by the DPF 14 are combusted. For example, when the temperature of the DPF 14 is not sufficiently high, such as immediately after the start of the PM regeneration process, incomplete combustion occurs in the DPF 14. Thereby, HC is generated.

  FIG. 3 is a functional block diagram illustrating the configuration of the ECU. The ECU 10 functions as a PM regeneration processing control unit 40, a determination unit 42, an energization control unit 44, a valve control unit 46, a gas flow rate acquisition unit 48, and a heat quantity setting unit 50. The PM regeneration process control unit 40 acquires the differential pressure measured by the differential pressure sensor 13, and controls the start and end of the PM regeneration process based on the differential pressure. The PM regeneration processing control unit 40 estimates the PM accumulation amount in the DPF 14 based on the differential pressure, and causes the injector included in the engine 30 to inject fuel toward the DPF 14 if the PM accumulation amount is equal to or greater than a predetermined value. The determination unit 42 acquires the temperature measured by the temperature sensors 18 and 20 and the temperature of the EHC 16, compares the temperatures, and determines whether the temperature is high or low. Moreover, the determination part 42 determines whether HC has generate | occur | produced based on the acquired temperature. The energization control unit 44 controls energization to the heater 16b. The valve control unit 46 controls opening and closing of the valve 28b. The ECU 10 controls the urea water injection by the urea water injector 24. The gas flow rate acquisition unit 48 and the heat quantity setting unit 50 will be described in the second embodiment.

  FIG. 4A to FIG. 5 are flowcharts illustrating the control of the exhaust gas purification device. As shown in FIG. 4A, the PM regeneration process control unit 40 determines whether to start the PM regeneration process (step S10). In the case of No, the control ends (A in FIGS. 4A and 4B). In the case of Yes, the energization control unit 44 starts energizing the heater 16b of the EHC 16 (heater ON, step S11). After step S11, the PM regeneration process control unit 40 starts the PM regeneration process (step S12). The determination unit 42 determines whether the temperature T1 of the EHC 16 is lower than the temperature T2 measured by the temperature sensor 18 (step S13).

  As shown in B of FIG. 4B, in the case of Yes in step S13, that is, when the temperature T1 of the EHC 16 is lower than the temperature T2 of the exhaust gas downstream from the EHC 16, the determination unit 42 determines whether the HC is from the DPF 14 to the HC. It determines with having flowed out (step S14). Step S14 will be described in detail. T1 <T2 means that the exhaust gas warmed by the combustion of HC in the EHC 16 flows downstream from the EHC 16. That is, the HC flowing out from the DPF 14 flows into the EHC 16 and is burned in the EHC 16.

  After step S14, the determination unit 42 determines whether the temperature T3 of the DOC 22 measured by the temperature sensor 20 is higher than the temperature T0 (see FIG. 2B) (step S15). In the case of No, the temperature T3 may be lower than T0-t (first temperature) in FIG. Therefore, the valve controller 46 closes the valve 28b to stop the supply of the secondary air or reduce the supply amount of the secondary air (step S16). The heater 16b is turned on in step S11. Accordingly, as indicated by the arrow A1 in FIG. 2B, the DOC 22 is heated by the heater 16b. In the case of Yes, the temperature T3 may be higher than T0 + t (second temperature) in FIG. Therefore, the valve controller 46 opens the valve 28b and supplies the secondary air to the exhaust path 12 through the secondary air path 28a (step S17). As shown by an arrow A2 in FIG. 2B, the DOC 22 is cooled by secondary air. After step S16 or S17, the control ends.

  As shown in FIG. 5C, if Yes in step S13, that is, if the temperature T1 of the EHC 16 is equal to or higher than the temperature T2, the determination unit 42 determines that the outflow of HC from the DPF 14 has ended (step S18). T1 ≧ T2 means that HC is not burned in the EHC 16. That is, HC does not flow out from the DPF 14 and HC does not flow into the EHC 16.

  After step S18, the determination unit 42 determines whether the temperature T3 is within a range of T0-t or higher and T0 + t or lower (range 11 in FIG. 2B). In No, the determination part 42 determines whether temperature T3 is less than T0-t (step S20). In the case of Yes in step S20, the temperature T3 is located on the lower temperature side than the range 11 in FIG. Therefore, the energization control unit 44 energizes the heater 16b, and the valve control unit 46 closes the valve 28b (step S21). Thereby, DOC22 is heated. In the case of No in step S20, the temperature T3 is higher than T0 + t in FIG. Therefore, the energization control unit 44 stops energization to the heater 16b, and the valve control unit 46 opens the valve 28b (step S22). Thereby, the DOC 22 is cooled. After steps S21 and S22, control returns to step S19. If Yes in step S19, the energization control unit 44 stops energization of the heater 16b (heater OFF), and the valve control unit 46 closes the valve 28b (step S23). After step S23, the control ends.

According to the first embodiment, as shown in steps S16 and S17 and S21 to S23, the EHC 16 and the supply unit 28 have the temperature T3 of the DOC 22 within a predetermined range 11 (T0-t or more and T0 + t or less). Thus, the temperature T3 is controlled. For this reason, as shown in FIG. 2A, NO ₂ / NOx is close to 0.5. Further, since the EHC 16 purifies HC, the inflow of HC into the DOC 22 is suppressed. As a result, NO: NO ₂ becomes 1: 1 or becomes close to 1: 1. Therefore, the NOx purification efficiency in the SCR catalyst 26 is improved. NO: NO ₂ is in the vicinity of 1: 1, for example, 1: 0.98, 1: 0.95, 1: 0.9, 0.98: 1, 0.95: 1, or 0.9; 1 Etc. NO: NO ₂ may be in a range where the NOx purification efficiency of the SCR catalyst 26 is improved and is, for example, in the vicinity of the optimum value. The range 11 can be set by arbitrarily determining the value of t. In order to make NO: NO ₂ close to 1: 1, it is preferable to narrow the range 11 and bring the temperature T3 closer to the temperature T0.

Since the EHC 16 exerts both functions of purifying the HC and heating the DOC 22, it is not necessary to provide another heater for heating the DOC 22. For this reason, the number of parts can be reduced, and the cost of the exhaust gas purification device can be reduced. Immediately after the start of the PM regeneration process, HC is likely to be generated because the temperature of the DPF 14 is low. Therefore, as shown in step S11 of FIG. 4A, it is preferable to turn on the heater 16b of the EHC 16 before the start of the PM regeneration process. Thereby, the EHC 16 can effectively purify HC from the start of the PM regeneration process. Further, by controlling the temperature T3 when the PM regeneration process is being performed, NO ₂ / NOx approaches 0.5. Therefore, the NOx purification efficiency in the SCR catalyst 26 can be increased more effectively during the PM regeneration process. Note that the exhaust gas purification device may control the temperature T3 even when the PM regeneration process is not performed. By controlling the temperature T3 to be within the range 11, that is, NO: NO ₂ = 1: 1, the NOx purification efficiency can be increased.

As shown in step S11 of FIG. 4A and FIG. 4B, the heater 16b is turned on while HC is flowing out. For this reason, the EHC 16 can effectively purify HC. The NO ₂ / NOx can be brought closer to 0.5 more effectively, and the NOx purification efficiency in the SCR catalyst 26 can be increased.

  Example 2 is an example in which heating of the DOC 22 is performed quickly. The configuration shown in FIGS. 1 and 3 is common to the second embodiment.

  For example, when the flow rate of the gas flowing through the exhaust path 12 is small, heat is not easily transmitted to the DOC 22 even if the heater 16b is turned on. For this reason, the temperature T3 of the DOC 22 hardly increases. For example, the gas flow rate decreases when the engine 30 is idling or operating in a light load operation region. In the second embodiment, the heating of the DOC 22 is accelerated by increasing the gas flowing through the exhaust path 12 to facilitate heat transfer. The control of the exhaust gas purification device will be described.

  FIG. 6 is a flowchart illustrating the control of the exhaust gas purifying apparatus according to the second embodiment. FIG. 7 is a graph for explaining a change in the temperature T3 in the second embodiment.

  As shown in FIG. 6, the gas flow rate acquisition unit 48 (see FIG. 3) acquires the gas flow rate F flowing through the exhaust path 12 based on, for example, the load and the rotational speed of the engine 30. The gas flow rate acquisition unit 48 determines whether the gas flow rate F is equal to or less than a predetermined value F0 (step S30). If No, the control ends. In the case of Yes, the calorie | heat amount setting part 50 acquires temperature T3 of DOC22, and calculates difference (DELTA) T = T0-T3 of predetermined value T0 (refer FIG.2 (b)) and temperature T3 (step S31). The calorie | heat amount setting part 50 determines whether (DELTA) T is more than predetermined value (DELTA) T0 (step S32). If No, the control ends. This is the case, for example, when the temperature T3 is T4 shown in FIG. At this time, since the temperature T3 is rising rapidly, heating need not be accelerated.

  In the case of No in step S <b> 32, the heat quantity setting unit 50 sets the heat quantity used to accelerate the heating of the DOC 22. This is the case, for example, when the temperature T3 is T5 shown in FIG. In order to obtain the determined heat amount, the energization amount to the heater 16b and the supply amount of the secondary air are determined (steps S33 and S34). The energization control unit 44 energizes the heater 16b based on the determined energization amount. The valve control unit 46 opens the valve 28b so that the determined secondary air is supplied (step S35). After step S35, the control ends.

  According to the second embodiment, the valve control unit 46 opens the valve 28 b and supplies secondary air to the exhaust path 12. As the gas flowing through the exhaust path 12 increases, the heat of the heater 16b is easily transmitted to the DOC 22. Accordingly, the heating of the DOC 22 is accelerated, and the temperature T3 quickly reaches the range 11 as indicated by an arrow A3 in FIG. For this reason, NOx purification efficiency improves. In particular, when idling or operation in a light load region is performed, the NOx purification efficiency can be effectively increased.

  After the temperature T3 reaches the range 11 by the control shown in FIG. 6, for example, the control shown in FIGS. 4A to 5 is performed. Thereby, since the temperature T3 is maintained within the range 11, high NOx purification efficiency can be obtained. The gas flow rate acquisition unit 48 may acquire the gas flow rate from an air flow meter provided in the exhaust path 12, for example. Step S34 may be performed before step S33.

  In order to control the temperature T3 using the secondary air, the secondary air path 28a is preferably provided on the upstream side of the DOC22. The secondary air path 28a may be provided on the upstream side of the EHC 16, for example. However, the EHC 16 is cooled by the secondary air, and the HC purification efficiency may be reduced. Therefore, the secondary air path 28a is preferably located between the EHC 16 and the DOC 22.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

10 ECU
11 Range 12 Exhaust path 14 DPF
16 EHC
16a catalyst 16b heater 18, 20 temperature sensor 22 DOC
26 SCR catalyst 28 Supply unit 28a Secondary air path 28b Valve 30 Engine 40 PM regeneration processing control unit 42 Determination unit 44 Energization control unit 46 Valve control unit 48 Gas flow rate acquisition unit 50 Heat quantity setting unit 100 Engine system

Claims (7)

An oxidation catalyst provided in an exhaust path through which the exhaust gas of the internal combustion engine flows;
An electrically heated catalyst provided upstream of the oxidation catalyst in the exhaust path and including a catalyst for purifying hydrocarbons, and a heating unit for heating the catalyst and the oxidation catalyst;
An SCR catalyst provided downstream of the exhaust path from the oxidation catalyst;
A supply unit provided upstream of the oxidation catalyst in the exhaust path and supplying secondary air to the exhaust path;
The exhaust gas purification apparatus, wherein the electrically heated catalyst and the supply unit control the temperature of the oxidation catalyst such that the temperature of the oxidation catalyst falls within a predetermined range.   The electric heating catalyst and the supply section are configured such that the ratio of nitrogen monoxide and nitrogen dioxide in the exhaust gas flowing into the SCR catalyst is 1: 1 or close to 1: 1. The exhaust gas purification device according to claim 1, wherein the temperature of the exhaust gas is controlled. The electrically heated catalyst heats the oxidation catalyst when the temperature of the oxidation catalyst is lower than a first temperature,
3. The exhaust according to claim 1, wherein the supply unit supplies secondary air to the exhaust path when a temperature of the oxidation catalyst is higher than a second temperature that is higher than the first temperature. Gas purification device. A filter provided upstream of the electrically heated catalyst in the exhaust path;
The exhaust gas purification device according to any one of claims 1 to 3, wherein the heating unit heats the catalyst before starting the PM regeneration process of the filter. A temperature measuring unit that is provided between the electric heating catalyst and the oxidation catalyst and determines the temperature of the exhaust gas flowing through the exhaust path;
The exhaust gas purification device according to claim 4, wherein when the temperature of the exhaust gas measured by the temperature measurement unit is higher than the temperature of the electric heating catalyst, the heating unit performs heating. A flow rate acquisition unit for acquiring a flow rate of exhaust gas flowing through the exhaust path;
6. The exhaust gas purification device according to claim 1, wherein when the flow rate is equal to or less than a predetermined value, the supply unit supplies secondary air to the exhaust path. The exhaust gas purification device according to any one of claims 1 to 6, wherein the supply unit is provided between the electric heating catalyst and the oxidation catalyst.

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