EXHAUST EMISSION CONTROL DEVICE
An exhaust emission control device is provided which can effectively regenerate a NOx absorption reduction catalyst even in a relatively low temperature region to improve NOx reduction ratio while minimizing deterioration of fuel economy. Incorporated in an exhaust pipe 4 through which exhaust gas 3 discharged from a diesel engine 1 via an exhaust manifold 2 flows is a reformer 8 having a fuel adding device 5, a heater 6 and a reforming catalyst 7. Arranged downstream of the reformer is a NOx absorption reduction catalyst 9.
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The present invention relates to an exhaust emission control device.
BACKGROUND ARTExhaust gas from a diesel engine has heretofore been purified by a catalyst incorporated in an exhaust pipe through which the exhaust gas flows. Known as this kind of catalyst is a NOx-absorption reduction catalyst which has a property of oxidizing NOx in the exhaust gas to temporarily absorb the same in the form of nitrate when an air/fuel ratio of the exhaust gas is lean, and conducting decomposition into NOx for reduction and purification thereof with the assistance of unburned HC and CO when the oxygen concentration in the exhaust gas is lowered.
Known as this kind of NOx-absorption reduction catalyst having the above-mentioned property is, for example, a catalyst made from alumina and carrying platinum and barium or a catalyst made from alumina and carrying platinum and potassium.
Since no further NOx can be absorbed once an absorbed NOx amount increases into saturation in the NOx-absorption reduction catalyst, it is periodically required to lower the O2 concentration in the exhaust gas flowing to the NOx-absorption reduction catalyst to decompose and discharge NOx.
For example, in application to a gasoline engine, lowering the operational air/fuel ratio in the engine (operating the engine with rich air/fuel ratio) can lower the O2 concentration and increase the reduction components such as unburned HC and CO in the exhaust gas for facilitation of decomposition and discharge of NOx. However, in use of a NOx-absorption reduction catalyst in an exhaust emission control device for a diesel engine, it is difficult to operate the engine with rich air/fuel ratio.
Thus, it has been necessary that fuel (HC) is added to the exhaust gas upstream of a NOx-absorption reduction catalyst, the added fuel being reacted as reducing agent with O2 on the reduction catalyst so as to lower the O2 concentration in the exhaust gas (see, for example, Reference 1).
[Reference 1] JP 2000-356127A
SUMMARY OF THE INVENTION Problems to be Solved by the InventionHowever, in such fuel addition upstream of a NOx-absorption reduction catalyst, part of HC produced due to evaporation of the added fuel reacts with O2 (combustion) in the exhaust gas on the NOx-absorption reduction catalyst, and decomposition and discharge of NOx are started after the O2 concentration becomes substantially zero in an ambient atmosphere around the NOx-absorption reduction catalyst. Thus, in a driving condition that a combustion temperature (about 220-250° C.) necessary for reaction of HC with O2 (combustion) cannot be obtained on the NOx-absorption reduction catalyst (for example, in driving at reduced speed on city roads often congested with traffic), NOx cannot be efficiently decomposed and discharged from the reduction catalyst and the regeneration of the reduction catalyst does not efficiently progress, disadvantageously resulting in decrease of recovery ratio of NOx-absorption sites occupied in volume of the catalyst to deteriorate absorption capacity.
Temperature raise control to the engine and/or increase in amount of fuel to be added so as to overcome the problem will, however, bring about substantial deterioration of fuel economy, resulting in difficulty in practical application.
The invention was made in view of the above and has its object to provide an exhaust emission control device which can effectively regenerate a NOx-absorption reduction catalyst even in a relatively low temperature region to improve NOx reduction ratio while minimizing deterioration of fuel economy.
Means or Measures for Solving the ProblemsThe invention is directed to an exhaust gas emission control device comprising a reformer incorporated in a gas pipe through which exhaust gas from an engine flows, said reformer having a fuel adding device, a heater and a reforming catalyst, and a NOx-absorption reduction catalyst arranged downstream of the reformer.
The above-mentioned means can obtain the following workings or operations.
With the exhaust gas from the engine being delivered via the exhaust pipe to the reformer, fuel is added as reducing agent by the fuel adding device on an entry side of the heater. Thus, the fuel is heated by the heater to produce concentrated HC gas with the exhaust gas being raised to a temperature required for action of the reforming catalyst. The HC gas is guided together with the exhaust gas to the reforming catalyst. After it reacts with O2 coexisting in the ambient atmosphere to raise the ambient temperature and O2 is consumed, the HC gas is decomposed into highly reactive H2 and CO which are guided to a posterior NOx-absorption reduction catalyst and, on a surface of the NOx-absorption reduction catalyst, NOx is effectively reduced into N2 even at a temperature lower than a conventional combustion temperature in fuel addition.
As a result, even in a driving condition that a reaction temperature necessary for reaction of HC with O2 (combustion) cannot be obtained on the NOx-absorption reduction catalyst (for example, in driving at reduced speed on city roads often congested with traffic), by operating the reformer with a minimum level of electricity and without temperature raise control to the engine nor increase in amount of fuel to be added, NOx can be efficiently decomposed and discharged from the NOx-absorption reduction catalyst, so that regeneration of the NOx-absorption reduction catalyst effectively progresses, resulting in increase of recovery ratio of NOx-absorption sites occupied in volume of the catalyst to prevent deterioration of absorption capacity and of fuel economy, leading to attainment of practical application.
In the exhaust emission control device, the heater in the reformer may be constituted by an anterior oxidation catalyst unit with oxidation catalyst having heating function being evenly arranged.
The anterior oxidation catalyst unit may be a membrane metal heater comprising a strip-like metal membrane coated with oxidation catalyst and capable of temperature raising through application of current so as to serve as oxidation catalyst with heating function and an insulating alumina tape as oxidation catalyst, the metal membrane and the insulating tape being wound together spirally in an overlapping manner.
Alternatively, the anterior oxidation catalyst unit may be a wire mesh heater comprising wire sheets coated with oxidation catalyst and capable of temperature raising through application of current so as to serve as oxidation catalyst with heating function and insulating alumina meshes as oxidation catalyst, the wire sheet and the insulating mesh being laminated alternately and plurally in a direction of flow of the exhaust gas.
In the exhaust emission control device, the heater in the reformer may comprise an anterior oxidation catalyst unit with a centrally arranged oxidation catalyst with heating function, a mixer being arranged between the anterior oxidation catalyst unit and the reforming catalyst.
Effects of the InventionAn exhaust emission control device according to the invention has excellent effects that a NOx-absorption reduction catalyst can be effectively regenerated even in a relatively low temperature region to improve NOx reduction ratio while minimizing the deterioration of fuel economy.
1 diesel engine (engine)
3 exhaust gas
4 exhaust pipe
5 fuel adding device
6 heater
7 reforming catalyst
8 reformer
9 NOx-absorption reduction catalyst
15 fuel
16 metal membrane
17 insulating alumina tape
18 membrane metal heater
19 wire sheet
19′ insulating alumina mesh
20 wire mesh heater
32 oxidation catalyst with heating function
33 anterior oxidation catalyst unit
34 mixer
BEST MODE FOR CARRYING OUT THE INVENTIONNext, embodiments of the invention will be described in conjunction with the drawings.
The fuel adding device 5 in the reformer 8 comprises an injection nozzle 10 arranged on the entry side of the heater 6 and connected via a fuel supply pipe 12 to a fuel tank 11. Fuel 15 such as light oil may be added as reducing agent to the entry side of the heater 6 via the injection nozzle 10 by driving a supply pump 13, which is incorporated in the fuel supply pipe 12, and opening an addition valve 14.
The heater 6 in the reformer 8 is constituted by an anterior oxidation catalyst unit with evenly arranged oxidation catalyst with heating function. The heater 6 may be, for example, a membrane metal heater 18 as shown in
The reforming catalyst 7 in the reformer 8 serves to decompose the HC component, which is produced due to fuel addition by the fuel adding device 5, into H2 and CO in the exhaust gas 3. This kind of reforming catalyst 7 may comprise, for example, an oxide such as alumina or silica or a complex oxide such as zeolite as a carrier carrying for example Pd, Pt or Rh as active metal.
In a controller 22 serving as engine control computer (ECU: Electronic Control Unit), current driving conditions are determined by a revolution speed signal 23a from a revolution speed sensor 23 which detects revolution speed of the diesel engine 1, a load signal 24a from an accelerator sensor 24 which detects a step-in angle of an accelerator pedal and a temperature signal 25a from a temperature sensor 25 which detects temperature of the exhaust gas 3 having passed through the reformer 8 and NOx-absorption reduction catalyst 9. On the basis of the current driving conditions thus determined, an amount of fuel 15 to be added by the fuel adding device 5, time of current application to the heater 6 and the like are determined so that the controller 22 outputs a drive command signal 13a to the supply pump 13, an opening/closing command signal 14a to the addition valve 14 and a current application command signal 6a to the heater 6. Thus, the fuel 15 is injected via the injection nozzle 10 in the fuel adding device 5 and is passed through the heater 6 to produce concentrated HC gas and raise the exhaust gas 3 to a temperature (about 300° C.) required for action of the reforming catalyst 7. Then, the HC gas is guided together with the exhaust gas 3 to the reforming catalyst 7 and reacts with O2 coexisting in the ambient atmosphere to raise the ambient temperature and consumes O2; thereafter, the HC gas is decomposed into highly reactive H2 and CO which are introduced into the posterior NOx-absorption reduction catalyst 9 for regeneration of the same.
In
Next, mode of working or operation of the above embodiment will be described.
With the exhaust gas 3 from the diesel engine 1 being delivered via the exhaust pipe 4 to the reformer 8, the fuel 15 is added as reducing agent via the injection nozzle 10 of the fuel adding device 5 to the entry side of the heater 6 such as the membrane metal heater 18 (see
If the heater 6 were constituted by the anterior oxidation catalyst unit solely with oxidation catalysts having heating function, the whole of the anterior oxidation catalyst unit must be raised to a temperature required for action thereof, which would require greater electricity. In view of the fact that a percentage of the fuel 15 required to be oxidized by the anterior oxidation catalyst unit is about 40% or less, in the embodiment, the heater 6 is constituted by an anterior oxidation catalyst unit with the evenly arranged oxidation catalyst or catalysts with heating function such as the membrane metal heater 18 (see
The reason why the reforming catalyst 7 is made to have no heating function is that if the reforming catalyst 7 were of heating function, then the temperature on the entry side of the reforming catalyst 7 might not be sufficiently raised and would bring abut wasteful portions as catalyst. In the embodiment, the anterior oxidation catalyst unit is arranged upstream of the reforming catalyst 7, so that the whole including the entry side of the reforming catalyst 7 can be increased in temperature and bring about no wasteful portions.
As a result, even in a driving condition at lower load region that a combustion temperature (about 220-250° C.) necessary for reaction of HC with O2 (combustion) cannot be obtained on the NOx-absorption reduction catalyst 9 (for example, in driving at reduced speed on city roads often congested with traffic), by operating the reformer 8 with a minimum level of electricity and without temperature raise control to the diesel engine 1 nor increase in amount of fuel to be added, NOx can be efficiently decomposed and discharged from the NOx-absorption reduction catalyst 9, so that regeneration of the NOx-absorption reduction catalyst 9 effectively progresses, resulting in increase of recovery ratio of NOx-absorption sites occupied in volume of the catalyst to prevent deterioration of absorption capacity and of fuel economy, leading to attainment of practical application.
Thus, while minimizing the deterioration of fuel economy, regeneration of the NOx absorption reduction catalyst can be efficiently conducted even in a relatively low temperature region to improve NOx reduction ratio.
The mixer 34 is constituted, for example, by a plurality of disks 36 arranged in a direction of flow of the exhaust gas 3, each disk being partly cut out and having a number of through holes 35 with different diameters as shown in
In the embodiment shown in
In the embodiment shown in
As a result, just like the embodiment shown in
Thus, also in the embodiment shown in
It is to be understood that an exhaust emission control device according to the invention is not limited to the above embodiments and that various changes and modifications may be made without departing from the scope of the invention.
Claims
1. An exhaust emission control device comprising a reformer incorporated in an exhaust pipe through which exhaust gas from an engine flows, said reformer having a fuel adding device, a heater and a reforming catalyst, and a NOx-absorption reduction catalyst arranged downstream of the reformer.
2. An exhaust emission control device as claimed in claim 1, wherein the heater in the reformer is constituted by an anterior oxidation catalyst unit with oxidation catalyst having heating function being evenly arranged.
3. An exhaust emission control device as claimed in claim 2, wherein the anterior oxidation catalyst unit is a membrane metal heater comprising a strip-like metal membrane coated with oxidation catalyst and capable of temperature raising through application of current so as to serve as oxidation catalyst with heating function and an insulating alumina tape as oxidation catalyst, the metal membrane and the insulating tape being wound together spirally in an overlapping manner.
4. An exhaust emission control device as claimed in claim 2, wherein the anterior oxidation catalyst unit is a wire mesh heater comprising wire sheets coated with oxidation catalyst and capable of temperature raising through application of current so as to serve as oxidation catalyst with heating function and insulating alumina meshes as oxidation catalyst, the wire sheet and the insulating mesh being laminated alternately and plurally in a direction of flow of the exhaust gas.
5. An exhaust emission control device as claimed in claim 1, wherein the heater in the reformer comprises an anterior oxidation catalyst unit with centrally arranged oxidation catalyst with heating function, a mixer being arranged between said anterior oxidation catalyst unit and the reforming catalyst.
Type: Application
Filed: Dec 26, 2006
Publication Date: Oct 29, 2009
Applicant: HINO MOTORS, LTD. (Hino-shi)
Inventor: Yoshihide Takenaka (Tokyo)
Application Number: 12/158,705
International Classification: F01N 3/023 (20060101); F01N 3/035 (20060101); F01N 3/10 (20060101);