JP5085393B2 - Exhaust purification device - Google Patents

Description

  The present invention relates to an exhaust emission control device.

  Conventionally, in a diesel engine of an automobile, a part of the exhaust gas is extracted from the exhaust side and returned to the intake side, and the exhaust gas returned to the intake side suppresses the combustion of fuel in the engine to reduce the combustion temperature. Some of them adopt so-called exhaust gas recirculation (EGR), which reduces the generation of NOx by lowering.

  However, reducing NOx by exhaust gas recirculation is in a trade-off relationship with the generation of black smoke due to poor combustion in the cylinder, so from the viewpoint of suppressing the generation of black smoke. There is a problem in that the amount of exhaust gas recirculation is limited, and it is difficult to significantly reduce NOx simply by recirculating exhaust gas.

  For this reason, in recent years, fuel injection that should normally be performed near the compression top dead center is performed at a timing earlier than the compression top dead center, and fuel premixing is promoted by prior injection of fuel into the cylinder. Adoption of premixed compression ignition that suppresses the generation of black smoke by igniting and burning after that has been studied (for example, see Patent Document 1).

  That is, if combustion is performed using such premixed compression ignition, combustion is performed in a state where the fuel is well dispersed and mixed and evenly diluted, so the combustion temperature is suppressed to a relatively low level. The generation of NOx is reduced, and moreover, it is difficult to produce a fuel-rich portion locally, which is effective in suppressing the generation of black smoke.

When adopting premixed compression ignition, it is desirable that the ignition be performed near the compression top dead center even if the fuel injection timing is advanced from the compression top dead center. There is a problem that pre-ignition occurs at a timing earlier than the compression top dead center, resulting in deterioration of combustion noise and loss of heat efficiency, especially in high-load operating conditions where the engine temperature is high, mixing with compression As the temperature of the air reaches the ignition temperature early, pre-ignition occurs at a timing much earlier than the compression top dead center, resulting in a significant deterioration in combustion noise. However, there is no prospect of adopting premixed compression ignition.
Japanese Patent Application Laid-Open No. 11-107772

  However, in the operation of the premixed compression ignition as described above, there is a problem that a large amount of HC and CO is generated as a contradiction that can reduce NOx, and the exhaust is performed so that these HC and CO are not discharged outside the vehicle. Although it is necessary to install an oxidation catalyst in the middle of the pipe, the exhaust temperature is generally low in the light load operation region where premixed compression ignition is applied, and the oxidation catalyst cannot exhibit good catalytic activity. There was a problem that sufficient exhaust purification performance could not be obtained.

  The present invention has been made in view of the above-described circumstances, and an exhaust purification device that can efficiently and reliably purify HC and CO generated by the application of premixed compression ignition and at the same time efficiently reduce NOx. It is intended to provide.

The present invention generates black smoke by performing fuel injection at a timing earlier than the compression top dead center at the time of light load, promoting premixing of fuel, and performing premixed compression ignition in which combustion is performed in a combustion chamber. An exhaust emission control device for a diesel engine, which is capable of suppressing the pre-oxidation catalyst that preferentially oxidizes CO over HC at the upstream portion of the exhaust pipe of the diesel engine, and is disposed downstream of the pre-oxidation catalyst. A selective reduction type catalyst having a catalytic function for selectively reacting NOx with HC even in the presence of oxygen and an NOx adsorption capability is provided, and fuel is introduced into the exhaust gas between the selective reduction type catalyst and the pre-oxidation catalyst. The fuel adding means to be added is provided , and the fuel adding means is configured to cause the fuel to be added in an operation region where premixed compression ignition is not applied .

  In this way, even if premixed compression ignition is applied at a light load with a low exhaust temperature and a large amount of HC and CO is generated, the pre-oxidation catalyst provided in the upstream portion of the exhaust pipe is a diesel engine. Since it has been exposed to high-temperature exhaust gas immediately after being discharged from the catalyst and has already obtained sufficient catalytic activity to oxidize CO and HC, CO and HC are oxidized by this pre-oxidation catalyst, and the oxidation reaction The temperature of the exhaust gas is increased by heat.

At this time, CO is more reactive than HC, and the reaction rate of the oxidation treatment from CO to CO ₂ is fast. Therefore, the catalytic activity obtained only by providing a pre-oxidation catalyst upstream of the exhaust pipe is sufficient for CO. Although most of the HC can be oxidized, it is difficult to oxidize most of the slow HC with the pre-oxidation catalyst, and most of the HC passes through the pre-oxidation catalyst and goes downstream. .

  The HC passing through the pre-oxidation catalyst reaches the downstream selective reduction catalyst together with the exhaust gas heated by the pre-oxidation catalyst, and reduces and purifies NOx adsorbed on the selective reduction catalyst so far. It is used as a reducing agent.

  That is, in the selective catalytic reduction catalyst on the downstream side, NOx adsorption has progressed before the reduction of NOx by premixed compression ignition, so the catalyst is exposed to the exhaust gas heated by the pre-oxidation catalyst. When HC is led to the selective catalytic reduction catalyst whose bed temperature has been raised, the reduced purification of adsorbed NOx is achieved using this HC as a reducing agent.

  Although the present inventors have obtained that NOx adsorption on the selective catalytic reduction catalyst is inhibited by the presence of CO in the atmosphere (the release of adsorbed NOx is promoted), the pre-oxidation catalyst If CO is removed in advance, it is possible to eliminate such a concern and make maximum use of the NOx adsorption ability of the selective catalytic reduction catalyst.

  In addition, generally at the time of medium and high loads where premixed compression ignition is not applied, fuel is added by the fuel addition means, and NOx is reduced and purified by the selective reduction catalyst using HC generated from the added fuel as a reducing agent. You can make it.

Furthermore, in the present invention, a particulate filter is provided in the preceding stage of the selective reduction catalyst, and a main oxidation catalyst is provided in the further preceding stage of the particulate filter, and fuel adding means is provided between the main oxidation catalyst and the pre-oxidation catalyst. It is preferable to arrange so that fuel is added even during forced regeneration of the particulate filter .

  In this way, it is possible to simultaneously reduce NOx and particulates. Moreover, when it is necessary to perform forced regeneration of the particulate filter, if fuel is added by the fuel addition means, the added fuel is The generated high-concentration HC undergoes an oxidation reaction while passing through the main oxidation catalyst, and the inflow of exhaust gas heated by the reaction heat raises the catalyst bed temperature of the particulate filter immediately after that to burn out the particulates. Thus, regeneration of the particulate filter is achieved.

  According to the exhaust emission control device of the present invention described above, various excellent effects as described below can be obtained.

  (I) According to the invention described in claim 1 of the present invention, HC and CO generated by applying premixed compression ignition at the time of a light load with a low exhaust temperature can be efficiently and reliably purified. Thus, it is possible to achieve a good NOx reduction by the premixed compression ignition and the selective reduction catalyst while suppressing the discharge of HC and CO to the outside of the vehicle.

  (II) According to the invention described in claim 2 of the present invention, NOx and particulates can be simultaneously reduced, and as a reducing agent for the selective catalytic reduction catalyst when premixed compression ignition is not applied. The forced regeneration of the particulate filter can be performed by diverting the fuel addition means for adding the fuel.

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

  FIG. 1 shows an example of an embodiment of the present invention. In FIG. 1, reference numeral 1 shows a diesel engine equipped with a turbocharger 2, and intake air 4 guided from an air cleaner 3 passes through an intake pipe 5 and the turbocharger. 2, the intake air 4 pressurized by the compressor 2 a is sent to the intercooler 6 to be cooled, and the intake air 4 is further guided from the intercooler 6 to the intake manifold 7. 1 is distributed to each cylinder 8 (FIG. 1 illustrates the case of inline 6 cylinders).

  Further, the exhaust gas 9 discharged from each cylinder 8 of the diesel engine 1 is sent to the turbine 2b of the turbocharger 2 through the exhaust manifold 10, and the exhaust gas 9 that has driven the turbine 2b passes through the exhaust pipe 11. It is designed to be discharged outside the vehicle.

  A pre-oxidation catalyst 12 that preferentially oxidizes CO over HC is provided at the outlet of the turbine 2b in the exhaust pipe 11 (upstream portion of the exhaust pipe 11). For example, an oxidation catalyst in which Pt / Pd / alumina is supported as a raw material may be used.

  Further, the exhaust pipe 11 downstream of the pre-oxidation catalyst 12 is provided with a selective reduction catalyst 13 having a catalytic function for selectively reacting NOx with HC and coexisting with NOx even in the presence of oxygen. The selective reduction catalyst 13 is composed of, for example, a Pt / zeolite catalyst.

In particular, in the present embodiment, a catalyst regeneration type particulate filter 14 that integrally supports an oxidation catalyst is accommodated in the preceding stage of the selective reduction catalyst 13, and the particulate filter 14 is made of ceramic. So that the inlets of the respective flow paths partitioned in a lattice pattern are alternately sealed, and the outlets of the flow paths that are not sealed are sealed. Thus, only the exhaust gas 9 that has permeated through the porous thin wall partitioning each flow path is discharged to the downstream side. The particulate filter 14 may be integrally supported with, for example, an oxidation catalyst obtained by adding ceria (CeO ₂ ) to Pt / Pd / alumina.

Further, a flow-through type main oxidation catalyst 15 is provided in the preceding stage of the particulate filter 14, and the main oxidation catalyst 15 is made of, for example, a raw material obtained by adding ceria (CeO ₂ ) to Pt / alumina. It only has to be supported.

  Further, a fuel addition valve 16 is provided between the main oxidation catalyst 15 and the pre-oxidation catalyst 12 as a fuel addition means for adding fuel to the exhaust gas 9. A fuel addition line 18 led from a fuel tank 17 disposed in the fuel addition line 18 is connected, and fuel in the fuel tank 17 is extracted by driving a pump 19 provided in the middle of the fuel addition line 18 so that the fuel addition valve is connected. 16 and is injected into the exhaust pipe 11 from the nozzle tip of the fuel addition valve 16.

  The diesel engine 1 is equipped with a fuel injection device 21 composed of an injector 20 provided for each cylinder 8, and the solenoid valve of each injector 20 in the fuel injection device 21 is an engine control computer (not shown). (ECU: Electronic Control Unit) properly controls the injection timing and injection amount (valve opening time) based on the load and rotation speed, but especially at light loads, near the compression top dead center. Premixed compression ignition in which main injection to be performed is performed at a timing earlier than the compression top dead center is applied.

  Further, an end portion of the exhaust manifold 10 in the direction in which the cylinders 8 are arranged and an end portion of the intake pipe 5 connected to the intake manifold 7 are connected by an EGR line 22, and the exhaust manifold 10 is extracted from the exhaust manifold 10. A part of the exhaust gas 9 is passed through an oxidation catalyst 25 for EGR to oxidize and purify HC and SOF (Soluble Organic Fraction) of the exhaust gas 9, and then cooled by a water-cooled EGR cooler 23 to EGR The air is recirculated to the intake pipe 5 through the valve 24.

  Thus, if the exhaust gas purification apparatus is configured in this way, premixed compression ignition is applied while adjusting the ignition timing by using the recirculation of the exhaust gas 9 by the EGR line 22 at the time of a light load with a low exhaust temperature. Thus, even if a large amount of HC and CO is generated in the exhaust gas 9 from the diesel engine 1, the pre-oxidation catalyst 12 provided in the upstream portion of the exhaust pipe 11 is immediately after being discharged from the diesel engine 1. Since catalytic activity sufficient to oxidize CO and HC has already been obtained by exposure to the high-temperature exhaust gas 9, CO and HC are oxidized by the pre-oxidation catalyst 12, and the exhaust gas is generated by the oxidation reaction heat. The temperature of 9 will be increased.

At this time, CO is more reactive than HC, and the reaction rate of the oxidation treatment from CO to CO ₂ is also fast. Although it is possible to oxidize most of CO, it is difficult to oxidize most of the slow HC with the pre-oxidation catalyst 12, and most of the HC passes through the pre-oxidation catalyst 12 and goes downstream. Will head.

  The HC passing through the pre-oxidation catalyst 12 reaches the selective reduction catalyst 13 on the downstream side together with the exhaust gas 9 heated by the pre-oxidation catalyst 12, and has been adsorbed by the selective reduction catalyst 13 so far. It is used as a reducing agent for reducing and purifying NOx.

  That is, in the selective catalytic reduction catalyst 13 on the downstream side, NOx adsorption has progressed before NOx reduction is achieved by premixed compression ignition, so that it is exposed to the exhaust gas 9 heated by the pre-oxidation catalyst 12. When HC is introduced to the selective catalytic reduction catalyst 13 whose catalyst bed temperature has been raised, the reduced purification of adsorbed NOx is achieved using this HC as a reducing agent.

  It has been obtained by the present inventors that NOx adsorption to the selective catalytic reduction catalyst 13 is inhibited by the presence of CO in the atmosphere (release of adsorbed NOx is promoted). If CO is removed in advance by the catalyst 12, such a concern can be eliminated and the NOx adsorption ability of the selective catalytic reduction catalyst 13 can be utilized to the maximum extent.

  Further, at the time of medium and high loads where premixed compression ignition is not applied, fuel is added by the fuel addition valve 16, and NOx is reduced and purified by the selective reduction catalyst 13 using HC generated from the added fuel as a reducing agent. You can do it.

  Further, in the present embodiment, a particulate filter 14 is provided in front of the selective catalytic reduction catalyst 13, and a main oxidation catalyst 15 is provided further in front of the particulate filter 14, and the main oxidation catalyst 15 and the pre-oxidation catalyst 12 are provided. Since the fuel addition valve 16 is arranged between the NOx and the particulates, it is possible to simultaneously reduce NOx and the particulates, and when the particulate filter 14 needs to be forcibly regenerated. In addition, when fuel is added by the fuel addition valve 16, high-concentration HC generated from the added fuel undergoes an oxidation reaction while passing through the main oxidation catalyst 15, and immediately after the exhaust gas 9 heated by the reaction heat flows in. The catalyst bed temperature of the particulate filter 14 is raised, the particulates are burned out, and the particulate filter 14 Will be regenerated.

  Therefore, according to the above embodiment, HC and CO generated by the application of premixed compression ignition at the time of light load with low exhaust temperature can be purified efficiently and reliably. The NOx reduction by the premixed compression ignition and the selective catalytic reduction catalyst 13 can be achieved while suppressing the above.

  In fact, when the present inventors conducted a verification experiment, if the pre-oxidation catalyst 12 is eliminated from the configuration of the present embodiment, the premix compression is performed at a light load with a low exhaust temperature as shown in the graph of FIG. As soon as the required time has elapsed since the ignition was started and the catalyst bed temperatures of the main oxidation catalyst 15, the particulate filter 14, and the selective catalytic reduction catalyst 13 were lowered and the catalytic activity was lost, the HC leak gas shown by the curve A It was confirmed that the concentration and the CO leak gas concentration indicated by the curve B increased rapidly.

  On the other hand, when the pre-oxidation catalyst 12 is installed, as shown by the same graph in FIG. 3, even if the premixed compression ignition is started at a light load with a low exhaust temperature and the required time has elapsed, the main oxidation catalyst 15 , The particulate filter 14 and the catalyst bed temperature of the selective catalytic reduction catalyst 13 are maintained in the active temperature range, and the HC leak gas concentration indicated by the curve A and the CO leak gas concentration indicated by the curve B are maintained in a reduced state. It was confirmed.

  Furthermore, in this embodiment, NOx and particulates can be reduced simultaneously, and a fuel addition valve for adding fuel as a reducing agent to the selective catalytic reduction catalyst 13 when premixed compression ignition is not applied. The forced regeneration of the particulate filter 14 can be performed by diverting 16.

  Note that the exhaust emission control device of the present invention is not limited to the above-described embodiment, and it is needless to say that various modifications can be made without departing from the gist of the present invention.

It is the schematic which shows an example of the form which implements this invention. It is a graph which shows the leak gas density | concentration of HC and CO when there is no pre-oxidation catalyst. It is a graph which shows the leak gas density | concentration of HC and CO in the case of having a pre-oxidation catalyst.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Diesel engine 9 Exhaust gas 11 Exhaust pipe 12 Pre oxidation catalyst 13 Selective reduction type catalyst 14 Particulate filter 15 Main oxidation catalyst 16 Fuel addition valve (fuel addition means)

Claims (2)

The fuel injection is executed at a timing earlier than the compression top dead center at the time of light load, and the premixed compression ignition in which the fuel is premixed and then ignited and combusted in the combustion chamber can suppress the generation of black smoke. An exhaust emission control device for a diesel engine, wherein a pre-oxidation catalyst that preferentially oxidizes CO over HC is provided at an upstream portion of an exhaust pipe of the diesel engine, and the downstream side of the pre-oxidation catalyst is also in the presence of oxygen. A fuel addition for providing a selective reduction type catalyst having a catalytic function for selectively reacting NOx with HC and an NOx adsorption capability, and adding fuel into the exhaust gas between the selective reduction type catalyst and the pre-oxidation catalyst An exhaust emission control device characterized by comprising means for adding fuel in an operating region where premixed compression ignition is not applied by the fuel addition means. A particulate filter is provided in front of the selective reduction catalyst, a main oxidation catalyst is further provided in front of the particulate filter, and fuel addition means is disposed between the main oxidation catalyst and the pre-oxidation catalyst. The exhaust emission control device according to claim 1, wherein fuel is added even during forced regeneration of the filter .

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