JP2006026582A - Exhaust gas processor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas processor which can control the fluctuation of carrier temperature by increasing the thermal capacity of a catalyst carrier to maintain the temperature of a catalyst within its active temperature region. <P>SOLUTION: In the carrier disposed in the exhaust passage of an internal combustion engine and made of a honeycomb structure having many circulation holes separated by partition walls, the catalyst carrier having the function of occluding NOx when an exhaust gas air-fuel ratio is lean, and releasing and reducing the occluded NOx when the exhaust gas air-fuel ratio is rich, is formed so as to maintain a thermal capacity of ≥0.4 J/cm<SP>3</SP>×°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an exhaust gas processing apparatus that is disposed in an exhaust passage of an internal combustion engine, and in particular, processes NOx in exhaust gas.

  In order to make the exhaust gas harmless and exhaust, the internal combustion engine is equipped with a catalyst or a filter, which is an exhaust gas treatment device corresponding to the exhaust gas characteristics exhausted by each internal combustion engine, in the exhaust path. Among them, the catalyst is installed in the exhaust passage, and supports various catalytic substances on a carrier having a large number of ventilation passages, thereby operating the harmful substances (CO, HC, NOx, etc.) in the exhaust gas to purify the harmless substances. it can.

For example, as a catalyst for treating NOx in exhaust gas, a catalyst in which a noble metal such as platinum, palladium and rhodium and a metal element having a function of occluding NOx are supported on an oxide carrier is well known. The carrier is made of various oxides having a high surface area such as alumina, silica, silica alumina, etc., and the exhaust gas purification device for automobiles is less clogged and can withstand high temperatures of about 1000 ° C. and has good formability and is a cordierite honeycomb. A carrier is used.
An example of a catalyst using such a honeycomb carrier is disclosed in Japanese Patent Application Laid-Open No. 2003-1112048 (Patent Document 1).

JP 2003-1112048 A

  By the way, the catalyst used for the exhaust system of the engine mounted on the vehicle employs a system in which cordierite or metal is used as a carrier and the catalyst is coated thereon. However, in a diesel engine having a region where the exhaust temperature is extremely low, an operating region in which the exhaust temperature is equal to or lower than the catalyst activation temperature (200 ° C. or less) is likely to occur, and in this operating region, the exhaust gas purification performance deteriorates rapidly.

This is mainly because the heat capacity of the carrier is low (small), and when exhaust gas having a low temperature flows, the exhaust gas temperature follows and the catalyst carrier is immediately cooled.
Furthermore, in the case of a NOx purification device using a reducing agent, there is a purification window that is in the optimum catalyst temperature range for NOx purification. In this case, particularly in a carrier having a low heat capacity, the catalyst temperature immediately increases due to a sudden rise in exhaust gas temperature. If the temperature rises and the catalyst temperature (support) becomes too high, the reducing agent does not react with NOx, but reacts with oxygen, resulting in insufficient purification of NOx.

  In this way, a catalyst with a low heat capacity has good followability with respect to the exhaust gas temperature and is suitable for early activation.However, depending on how it is used, the frequency of deviating from the activation temperature increases, and the catalyst performance may not be fully exploited. The present inventor has paid attention to this point and derived the present invention.

That is, when purifying exhaust gas whose exhaust temperature varies variously, it is estimated that high performance can be obtained by keeping the temperature in the catalyst as uniform as possible in the active temperature range over time. For this purpose, the heat capacity of the carrier supporting the catalyst is increased, and the activity is maintained by the heat of the carrier even when the temperature is low from the high temperature. Conversely, even if there is a sudden temperature rise, It is considered effective to absorb the catalyst so that the temperature in the catalyst stays within the activation temperature, and always keep the catalyst at the activation temperature.
It is an object of the present invention to provide an exhaust gas treatment apparatus that can suppress the fluctuation of the carrier temperature by keeping the heat capacity of the catalyst carrier relatively large and maintain the catalyst temperature within the active temperature range.

According to a first aspect of the present invention, there is provided an exhaust gas processing apparatus according to a first aspect of the present invention, wherein a carrier having a honeycomb structure having a large number of flow holes arranged in an exhaust passage of an internal combustion engine and partitioned by a partition wall has a lean air-fuel ratio. In the exhaust gas processing apparatus carrying a catalyst having a function of storing NOx in the exhaust gas and releasing and reducing the stored NOx when the air-fuel ratio of the inflowing exhaust gas becomes rich, the heat capacity of the carrier is 0 4 J / cm ³ · ° C. or more.

According to a second aspect of the present invention, in the exhaust gas treatment apparatus according to the first aspect, the carrier has a heat capacity of 0.6 J / cm ³ · ° C. or more.

  According to a third aspect of the present invention, in the exhaust gas processing apparatus according to the first or second aspect, an NOx storage catalyst comprising a carrier having a smaller heat capacity than the carrier is provided in the exhaust passage upstream of the carrier. And

According to the present invention, since the heat capacity of the carrier is 0.4 J / cm ³ · ° C. or more, the fluctuation in the carrier temperature with respect to the excessive fluctuation in the exhaust gas temperature is suppressed and smoothed. It is possible to suppress following the excessive fluctuation in height. For this reason, the temperature of the catalyst is maintained within the active temperature range, and the carrier thermal deterioration caused by excessively high temperature fluctuation of the carrier and the thermal dissociation of NOx occluded in the catalyst in a low temperature state, that is, the release of NOx at a high temperature. This can improve the purification efficiency.

Furthermore, according to the present invention, when the heat capacity of the carrier is 0.6 J / cm ³ · ° C. or more, the fluctuation in the carrier temperature with respect to the excessive fluctuation in the exhaust gas temperature is more reliably suppressed and becomes gentle. Can more reliably prevent the exhaust gas temperature from following the fluctuations in the exhaust gas temperature, the temperature of the catalyst is more reliably maintained within the active temperature range, and the carrier heat deterioration caused by excessively high temperature fluctuations or low temperature conditions Thus, the thermal dissociation of NOx absorbed in the catalyst can be prevented, and the purification efficiency can be improved more reliably.

  Further, according to the present invention, since the NOx occlusion catalyst having a small heat capacity is provided on the exhaust upstream side of the exhaust purification device, the front-stage NOx catalyst is activated more rapidly than the downstream carrier even at the time of starting, etc. The purification efficiency can be prevented from being lowered until the temperature of the downstream carrier reaches the activation temperature, and good purification performance can be obtained in the entire operation region.

Hereinafter, an example in which an exhaust gas treatment apparatus as an embodiment of the present invention is applied to a diesel engine (hereinafter simply referred to as an engine) 1 will be described.
The engine 1 includes four combustion chambers 2 in series, and each combustion chamber 2 is provided with a fuel injection valve 3 that directly injects fuel. Here, the fuel (light oil) in the fuel tank 4 is pressurized by the high-pressure fuel injection pump 5 and is pumped to the common rail 6 (pressure accumulating chamber), and is injected into each cylinder from the common rail 6 via the fuel injection valve 3. The fuel injection valve 3 here has a well-known configuration in which the fuel injection amount Q and the injection timing are controlled in accordance with an injection pulse output from an ECU 7 described later. The fuel injection amount Q is also load information of the engine 1 and is used in the NOx emission reduction process described later.

  An intake port (not shown) extending from one side of each combustion chamber 2 communicates with the intake manifold 8, and an intake pipe 9 that forms an intake passage I is connected to the intake manifold 8. The intake pipe 9 pressurizes the intake air drawn from the air cleaner 11 by the supercharger 12, cools the intake air from the supercharger 12 by the intercooler 13, and then introduces the intake air into the intake manifold 8. Reference numeral 14 denotes a radiator for heat radiation of a cooling water circulation system (not shown).

  An exhaust port (not shown) extending from the other side of each combustion chamber 2 communicates with the exhaust manifold 15, and an exhaust pipe 16 that forms an exhaust path Ex is connected to the exhaust manifold 15. The exhaust pipe 16 includes a turbine 121 of the supercharger 12 driven by receiving exhaust gas flow energy from upstream to downstream, a downstream NOx storage catalyst 17, a NOx catalyst 18, and a muffler (not shown). Are arranged in this order.

The front NOx storage catalyst 17 is made of a honeycomb cordierite carrier 171, which includes an alkali metal such as potassium K and sodium Na, an alkaline earth such as barium Ba and calcium Ca, and a rare earth such as lanthanum La and cerium Ce. And at least one component selected from the above and a noble metal such as platinum Pt. The cordierite carrier 171 has a smaller overall capacity than the carrier 19 in the NOx storage catalyst 18 described later, and the heat capacity of the cordierite carrier 171 is relatively low (small) 0.39 J / cm ³ · ° C. A structure that facilitates early activation is employed to suppress a decrease in NOx purification efficiency at low temperatures.

  Although the preceding stage NOx occlusion catalyst 17 having a cordierite carrier as the former stage catalyst has been described, a metal carrier may be used instead, and a low capacity oxidation catalyst (instead of the preceding stage NOx occlusion catalyst 17) (Not shown) may be employed, and in this case, a decrease in the HC purification efficiency at low temperatures can be suppressed, and an increase in exhaust gas temperature on the downstream side can be measured, so that the NOx occlusion catalyst 18 can be activated early and NOx It is possible to expand the operating range that can be reduced.

As shown in FIGS. 1 and 2, the NOx storage catalyst 18 accommodates a carrier 19 as a NOx storage reduction catalyst in its casing 181. The entire support 19 is made of porous silicon carbide (SiC), has a relatively large capacity, and has a relatively large heat capacity of 0.65 J / cm ³ · ° C. Has been.
The carrier 19 has a honeycomb structure having flow passages r which are a large number of straight flow holes partitioned by partition walls w.

Each flow hole r of the carrier 19 is open at both ends so that the exhaust gas can easily pass therethrough. A catalytic element that functions as a NOx occlusion reduction catalyst is uniformly supported on the entire surface area of the carrier 19, and specifically, alkaline metals such as potassium K and sodium Na, alkaline earth such as barium Ba and calcium Ca. And at least one component selected from rare earths such as lanthanum La and cerium Ce, and a noble metal such as platinum Pt. The NOx occlusion reduction catalyst 19 stores NOx (NO ₂ , NO) in the exhaust when the air-fuel ratio of the inflowing exhaust gas is lean, and absorbs NOx that releases the stored NOx when the inflowing exhaust gas becomes rich. Perform release action.

When the oxygen concentration in the exhaust gas increases (that is, when the air-fuel ratio of the exhaust gas becomes a lean air-fuel ratio), the NOx occlusion-reduction catalyst 18 is in contact with NOx (NO is the main component) in the exhaust gas on the platinum Pt. An oxidation reaction occurs to generate NO ₂ . Further, NO ₂ in the inflowing exhaust gas is further oxidized on platinum Pt and diffuses into the storage agent while being combined with barium oxide BaO as the storage agent. For this reason, NOx in the exhaust gas is stored in the NOx storage reduction catalyst 19 in a lean atmosphere.

Further, when the oxygen concentration in the inflowing exhaust gas decreases (that is, when the air-fuel ratio of the exhaust gas decreases to a rich air-fuel ratio equal to or lower than the stoichiometric air-fuel ratio), the amount of NO ₂ generated on the platinum Pt decreases, so that the reaction now proceeds in the reverse direction, NOx in the absorbent is to be released from the NOx storage reduction catalyst 19 in the form of NO _2. In this case, if components such as HC and CO are present in the exhaust gas, NO ₂ is reduced by N ₂ by these components on platinum Pt. Note that the same applies to the above-described previous NOx storage reduction catalyst 17.

  Further, as shown in FIG. 3, the NOx occlusion reduction catalyst 18 is operated in a purification window Eg in which the catalyst temperature Tc, which is the temperature inside the catalyst, is approximately 200 ° C. to 350 ° C., which is the optimum catalyst activation temperature range for NOx purification. In this case, a high purification rate can be maintained. On the other hand, as it goes to the lower side region Eld from the purification window Eg, it enters the inactive region, the purification reaction sharply decreases, and when the carrier temperature becomes too high as it goes to the higher side region Eud from the purification window Eg, it stores NOx. Occlusion reaction does not occur due to the thermal equilibrium relationship of the agent, and NOx cannot be purified. If NOx is occluded in the occlusion agent, NOx is released (thermal dissociation).

  In this embodiment, since a diesel engine is used as the engine 1, the engine exhaust is normally at a lean air-fuel ratio, and the NOx occlusion reduction catalyst 19 occludes NOx in the exhaust. However, when the amount of NOx occluded in the NOx occlusion reduction catalyst 19 increases, the occlusion agent (BaO or the like) is saturated, and the NOx occlusion reduction catalyst 19 cannot occlude NOx in the exhaust. Therefore, as described later, the reducing agent supply nozzle 25 connected to the reducing agent supply device 24 is driven to forcibly form a rich atmosphere before the NOx amount absorbed by the NOx storage reduction catalyst 19 is saturated. Thus, NOx is released from the NOx occlusion reduction catalyst 19 for reduction and purification.

Here, the reducing agent supply device 24 uses the fuel (light oil) in the fuel tank 4 as the reducing agent.
The reducing agent supply device 24 circulates the fuel (light oil) in the fuel tank 4 to the fuel circulation path 22 with the circulation pump 21, and the flow control valve 23 in the middle of the fuel circulation path 22 in accordance with a control signal from the ECU 7 described later. By driving to open and close, pressurized fuel (light oil) is supplied from the reducing agent supply nozzle 25 to the NOx occlusion reduction catalyst 19.

By supplying fuel from the reducing agent supply nozzle 25, the NOx occlusion reduction catalyst 19 is maintained in a rich atmosphere, and NOx is released and reduced and purified. In forming the rich atmosphere, the fuel may be pump-injected in the cylinder for enrichment.
The vehicle is provided with an engine control unit (hereinafter simply referred to as ECU 7) which is an engine control means.

  A pulse signal is input to an input port (not shown) of the ECU 7 from a rotation speed sensor 26 arranged in the vicinity of the crankshaft (not shown) at every constant crankshaft rotation angle, and is used to calculate the engine speed Ne. A signal representing the driver's accelerator pedal depression amount (accelerator opening) θa is input from an accelerator opening sensor 27 arranged on an accelerator pedal (not shown). The ECU 7 calculates the engine basic fuel injection amount Q0 and the fuel injection timing based on the accelerator opening θa detected by the accelerator opening sensor 27 and the engine speed Ne, and the basic fuel injection amount Q0 is determined according to the engine operating state. Then, the engine fuel injection amount Q and the fuel injection timing are set. The temperature Tc of the NOx storage reduction catalyst 18 is detected by the temperature sensor 28 and input to the ECU 7.

  On the other hand, an output port (not shown) of the ECU 7 is connected to the fuel injection valve 3 of each cylinder via a fuel injection circuit (driver) 29 in order to control the fuel injection amount Q and the fuel injection timing to each cylinder. Moreover, it is connected to the high-pressure fuel pump 5 via a drive circuit (not shown) to control the amount of fuel pumped from the high-pressure fuel pump 5 to the common rail 6. Further, a flow rate control valve 23 of the reducing agent supply device 24 is connected to the output port of the ECU 7. This flow control valve 23 operates to supply the reducing agent from the reducing agent supply nozzle 25 to the NOx storage reduction catalyst 19 when NOx should be released from the NOx storage reduction catalyst 19.

  The ECU 7 performs engine control. In particular, when attention is paid only to the exhaust gas purification device of the engine 1, the ECU 7 has functions as NOx occlusion amount count integration means A1, rich operation determination means A2, and rich operation control means A3. Here, the NOx occlusion amount count integration means A1 sets a count amount qn corresponding to the NOx amount absorbed per unit time by the NOx occlusion reduction catalyst 18 in accordance with the operating state of the engine using a count amount map (not shown). The count amount qn is integrated during engine operation, and an integrated value (ΣCount ← ΣCount + qn) is obtained sequentially.

  The rich operation determination means A2 issues a rich operation command (rich spike) when the count integrated value ΣCount exceeds a threshold Limit which is a preset NOx carrying amount. Here, the threshold Limit is set immediately before the NOx amount is saturated or an occlusion amount with a predetermined margin. The rich operation control means A3 drives the engine 1 for a predetermined time in response to the rich operation command, and during this time, pressurized fuel (light oil) is supplied from the reducing agent supply nozzle 25 to the NOx occlusion reduction catalyst 18, and oxygen in the exhaust gas is exhausted. Reduce the concentration to reduce NOx.

The operation of such an exhaust gas purification apparatus for an internal combustion engine will be described.
When the engine starts operation, the ECU 7 executes fuel injection control based on the operation information. Meanwhile, the engine speed Ne and the count quantity qn corresponding to the fuel injection quantity Q are obtained by a predetermined count quantity map (not shown), and the current count quantity qn is added to the previous value ΣCount to obtain the current integrated value. ΣCount (← ΣCount + qn) is updated and obtained. Next, when the latest integrated value ΣCount exceeds a predetermined threshold Limit, a rich operation command is issued, and the engine 1 is richly operated for a predetermined time accordingly.

Specifically, the flow control valve 23 of the reducing agent supply device 24 is driven, fuel (light oil) is supplied from the reducing agent supply nozzle 25 to the NOx occlusion reduction catalyst 18, the oxygen concentration in the exhaust gas is lowered, and the platinum Pt The amount of NO ₂ produced at Accordingly, NOx in the absorbent is released from the NOx storage reduction catalyst 18 in the form of NO _2, moreover, NO ₂ is reduced processed by these components on HC, platinum when components such as CO are present Pt in the exhaust Is done.

It is assumed that the exhaust gas purification device for the internal combustion engine using the NOx occlusion reduction catalyst 19 is driven. In this case, the NOx purification rate of the NOx occlusion reduction catalyst 18 is maintained at a relatively high level in the purification window Eg where the catalyst temperature Tc is 200 ° C. to 350 ° C., as shown in FIG. Here, the carrier 19 of the NOx occlusion reduction catalyst 18 is made of porous silicon carbide, and its heat capacity is a relatively high value of 0.65 J / cm ³ · ° C. Such a NOx occlusion reduction catalyst 18 has a test result that the average NOx purification rate within the purification window Eg is 73%. This is a result of 10% improvement in the purification rate compared with the average NOx purification rate test result in the purification window when the heat capacity of the conventional cordierite carrier is 0.39 J / cm ³ · ° C. .

Further, FIG. 4 shows the temporal change characteristics of the exhaust gas temperature Tg and the catalyst temperature Tc when the vehicle equipped with such an exhaust gas purification device for an internal combustion engine is running.
In FIG. 4, the catalyst temperature Tc (shown by a solid line in FIG. 4) is present in the steady operation region E <b> 1, even though the exhaust gas temperature Tg (shown by a broken line in FIG. 4) is also an operating region in which the exhaust gas temperature frequently decreases to around 200 ° C. However, it is maintained at a temperature around 250 ° C., which can ensure a relatively high NOx purification rate. Incidentally, the fluctuation of the catalyst temperature Tc 'in the case of using the conventional use of heat capacity 0.39J / cm 3 ^· ℃ cordierite carrier, shown in comparison with one-dot chain line in FIG. 4. Here, the catalyst temperature Tc ′ fluctuation of the cordierite carrier shows a movement that relatively follows the fluctuation of the exhaust gas temperature Tg, and it is difficult to maintain a temperature state where a high purification rate can be obtained.

Further, even when the operating state changes to the high-load side operation state E2, the exhaust gas temperature Tg fluctuates close to 400 ° C., whereas the catalyst temperature Tc is fluctuated so as to ensure a relatively high NOx purification rate. It is maintained at a temperature close to 350 ° C.
Thus, when the time-dependent transition of the catalyst temperature Tc of the NOx storage reduction catalyst 18 is compared with the time-dependent transition of the exhaust gas temperature Tg, the carrier 19 is formed with a relatively large heat capacity of 0.65 J / cm ³ · ° C. As a result, the fluctuation of the catalyst temperature Tc is moderated compared to the fluctuation of the exhaust gas temperature Tg, and the fluctuation of the catalyst temperature Tc is suppressed to follow the exhaust gas temperature Tg. Can be maintained in a temperature state. Moreover, thermal degradation of the catalyst caused by excessively high temperature fluctuation of the catalyst temperature Tc, which is the carrier temperature, thermal dissociation of NOx occurring in the case of the NOx occlusion reduction catalyst 18, that is, the high temperature of NOx occluded in the carrier 191. The time emission can be prevented, and the exhaust gas purification efficiency can be improved without being influenced by the running state.

Here, in the N0x storage catalyst 18 of FIG. 1, silicon carbide sic was used as a carrier in order to increase the heat capacity. The reason for this is to increase the wall pressure in existing cordierite or to make the wall unporous (dense) as a method of increasing the heat capacity. However, increasing the wall pressure may lead to an increase in exhaust pressure. Clogging due to wrinkles is likely to occur, and when the wall is made unporous (impact), the impact strength is reduced, which is not practical as a method for increasing the heat capacity. In addition, when the carrier is made of metal, it is possible to increase the mature capacity by increasing the thickness of the foil film of the carrier. However, in this case, the moldability deteriorates and is not practical. . On the other hand, in the case of silicon carbide SiC, the specific gravity per unit is about 1.24 times that of cordierite, and it can be easily formed by extrusion, so it can be said that it is a material suitable for increasing the heat capacity. In addition, by using silicon carbide SiC having a relatively large apparent specific gravity as a carrier, it becomes easy to form a silicon substrate with a heat capacity of 0.6 J / cm ³ · ° C. or more.

Further, since the heat capacity of the NOx occlusion reduction catalyst 18 is set to be large, it is difficult to increase the activation temperature, but here, the heat capacity of the cordierite carrier 171 is 0. 0 between the engine 1 and the NOx occlusion reduction catalyst 18. A pre-stage NOx storage catalyst 17 having a relatively low (small) 39 J / cm ³ · ° C. was provided. For this reason, the front-stage NOx storage catalyst 17 set to a relatively small heat capacity is activated during a low temperature operation at a relatively low exhaust gas temperature Tg even if the NOx storage reduction catalyst 18 having a large heat capacity is in the inactive region. Therefore, a decrease in NOx purification efficiency when the exhaust gas temperature Tg is low can be suppressed, and good purification efficiency can be obtained in the entire operation region.

  In the above, the exhaust gas treatment device is a through-type NOx occlusion catalyst or carrier applied in the exhaust system of a diesel engine, but the exhaust side is closed by alternately closing the inlet side and the outlet side of the carrier 19. A NOx occlusion catalyst carrier having a fine particle filter function may be used. Further, it can be applied to a NOx occlusion catalyst in a gasoline engine.

1 is an overall schematic configuration diagram of a NOx occlusion reduction catalyst as an exhaust gas treatment device according to an embodiment of the present invention and an internal combustion engine equipped with the catalyst. FIG. 2 is an enlarged cutaway cross-sectional view of the NOx storage reduction catalyst of FIG. 1. FIG. 2 is a characteristic diagram of NOx purification rate-catalyst temperature when the NOx storage reduction catalyst of FIG. 1 is in operation. FIG. 2 is a characteristic diagram of catalyst temperature-operation time when the NOx storage reduction catalyst of FIG. 1 is operated.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Engine 18 NOx occlusion reduction catalyst 19 Carrier r Flow hole w Bulkhead Ex Exhaust path L1 Axis of partition

Claims (3)

NOx in the exhaust gas is occluded in the carrier having a honeycomb structure arranged in the exhaust passage of the internal combustion engine and having a number of flow holes partitioned by the partition walls when the air-fuel ratio of the exhaust gas is lean. In an exhaust gas treatment apparatus carrying a catalyst having a function of releasing and reducing NOx occluded when the fuel ratio becomes rich, the heat capacity of the carrier is 0.4 J / cm ³ · ° C. or more. Exhaust gas treatment device. 2. The exhaust gas treatment apparatus according to claim 1, wherein the carrier has a heat capacity of 0.6 J / cm ³ · ° C. or more. 3. The exhaust gas treatment apparatus according to claim 1, wherein an NOx occlusion catalyst made of a carrier having a smaller heat capacity than the carrier is provided in the exhaust passage upstream of the carrier.

Images