Exhaust gas purifier

Abstract

Incorporated in an exhaust pipe are an HC-SCR NO.sub.x catalyst capable of reducing NO.sub.x at temperatures less than a set temperature T and a catalyzed particulate filter with an oxidation catalyst capable of reducing NO.sub.x at temperatures not less than the set temperature T. When an exhaust gas temperature is less than the set temperature T, fuel with a set flow rate Q is intermittently added from a fuel addition unit on an entry side of the HC-SCR NO.sub.x catalyst to the HC-SCR NO.sub.x catalyst; when the exhaust gas temperature is not less than the set temperature T, the fuel with flow rate Q′ not less than the set flow rate Q is temporarily rich-spike added from the fuel addition unit and is made arrival at a catalyzed particulate filter. With an active temperature range being expanded, exhaust emission control is performed in a wide temperature range.

Claims

1. An exhaust emission control device, comprising: an HC-SCR NO.sub.x catalyst incorporated in an exhaust pipe for discharge of exhaust gas from a diesel engine and that reduces NO.sub.x at temperatures lower than a set temperature, a catalyzed particulate filter arranged downstream of said HC-SCR NO.sub.x catalyst and integrally carrying an oxidation catalyst that reduces NO.sub.x at temperatures not lower than said set temperature, a downstream oxidation catalyst arranged downstream of said catalyzed particulate filter, a fuel addition unit for additively supplying the fuel to the exhaust gas on an entry side of said HC-SCR NO.sub.x catalyst, a temperature sensor for measuring a temperature at between said HC-SCR NO.sub.x catalyst and said catalyzed particulate filter, and a controller for outputting a control signal to said fuel addition unit so as to intermittently additively supply fuel with a set flow rate to said HC-SCR NO.sub.x catalyst when the temperature measured by said temperature sensor is lower than the set temperature and so as to make temporary rich-spike addition of the fuel with a flow rate not lower than said set flow rate to make arrival thereof at said catalyzed particulate filter when the temperature measured by said temperature sensor is not lower than the set temperature, wherein said set temperature is 300° C.

2. The exhaust emission control device as claimed in claim 1, wherein said HC-SCR NO.sub.x catalyst is a catalyst with active metal being a platinum group element, and the oxidation catalyst carried by said catalyzed particulate filter is a catalyst with active metal being copper.

3. The exhaust emission control device as claimed in claim 1, wherein said HC-SCR NO.sub.x catalyst is a catalyst with active metal being a platinum group element, and the oxidation catalyst carried by said catalyzed particulate filter is a catalyst with active metal being silver.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is an overall schematic diagram showing an example of a conventional exhaust emission control device;

(2) FIG. 2 is an overall schematic diagram showing an embodiment of an exhaust emission control device according to the invention;

(3) FIGS. 3A and 3B are diagrams each showing a temperature-based correspondence relationship between a fuel addition pattern and an air/fuel ratio in the embodiment of the exhaust emission control device according to the invention and each with an abscissa being a time axis, FIG. 3A being a diagram when the temperature is lower than a set temperature T (300° C.), FIG. 3B being a diagram when the temperature is not lower than the set temperature T (300° C.);

(4) FIG. 4A is a diagram showing inherent performances of an HC-SCR NO.sub.x catalyst and an oxidation catalyst with active metal being copper, respectively, and FIG. 4B is a diagram showing a relationship between a catalyst temperature and a NO.sub.x reduction ratio in the embodiment of the exhaust emission control device according to the invention (in a case where the oxidation catalyst carried by the catalyzed particulate filter is a catalyst with active metal being copper);

(5) FIG. 5A is a diagram showing inherent performances of the HC-SCR NO.sub.x catalyst and an oxidation catalyst with active metal being silver, respectively, and FIG. 5B is a diagram showing a relationship between the catalyst temperature and the NO.sub.x reduction ratio in the embodiment of the exhaust emission control device according to the invention (in a case where the oxidation catalyst carried by the catalyzed particulate filter is a catalyst with active metal being silver); and

(6) FIG. 6 is an overall schematic diagram showing a further embodiment of the exhaust emission control device according to the invention.

DESCRIPTION OF EMBODIMENTS

(7) Now, embodiments of the invention will be described in conjunction with the drawings.

(8) FIGS. 2-5B show an embodiment of an exhaust emission control device according to the invention in which parts identical with those in FIG. 1 are represented by the same reference numerals. As shown in FIGS. 2-5B, the embodiment, which has a fundamental structure similar to that of the conventional device shown in FIG. 1, resides in that an HC-SCR NO.sub.x catalyst 12 incorporated in an exhaust pipe 11 for discharge of exhaust gas 9 from a diesel engine 1 is a catalyst capable of reducing NO.sub.x at temperatures lower than a set temperature T (300° C.), an oxidation catalyst carried by a catalyzed particulate filter 13 arranged downstream of the HC-SCR NO.sub.x catalyst 12 is a catalyst capable of reducing NO.sub.x at temperatures not lower than the set temperature T (300° C.) and a temperature sensor 19 is arranged between the HC-SCR NO.sub.x catalyst 12 and the catalyzed particulate filter 13 and is configured to output a control signal 20a to a fuel addition unit 15 for intermittent addition of the fuel with a set flow rate Q (See FIG. 3A) to the HC-SCR NO.sub.x catalyst 12 when a temperature 19a measured by the temperature sensor 19 is lower than the set temperature T (300° C.) and for temporary rich-spike addition of the fuel with a flow rate Q′ not less than the set flow rate Q (see FIG. 3B) to make arrival thereof at the catalyzed particulate filter 13 when the temperature 19a measured by the temperature sensor 19 is not less than the set temperature T (300° C.).

(9) In the embodiment, arranged on a discharge side of the fuel pressure pump 17 of the fuel addition unit 15 is a control valve 21 which is controlled in opening degree by the control signal 20a from the controller 20 so as to make the intermittent addition or the temporary rich-spike addition of the fuel.

(10) Usable as the HC-SCR NO.sub.x catalyst 12 is a catalyst with an active metal being a platinum group element such as platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir) or osmium (Os) and with a carrier being zeolite or alumina. Usable as the oxidation catalyst carried by the catalyzed particulate filter 13 is a catalyst with, for example, the active metal being a transition metal such as copper (Cu) or silver (Ag). Usable as the downstream oxidation catalyst 14 is a catalyst, similar to the HC-SCR NO.sub.x catalyst 12, with the active metal being a platinum group element and with the carrier being zeolite or alumina.

(11) Next, an mode of operation of the above embodiment will be described.

(12) During an operation of the diesel engine 1, the temperature 19a at between the HC-SCR NO.sub.x catalyst 12 and the catalyzed particulate filter 13 is measured by the temperature sensor 19. When the temperature 19a measured by the temperature sensor 19 is lower than the set temperature T (300° C.), the control signal 20a outputted from the controller 20 to the control valve 21 of the fuel addition unit 15 controls the opening degree of the control valve 21 such that, as shown in FIG. 3A, the fuel with the set flow rate Q is intermittently added and supplied to the HC-SCR NO.sub.x catalyst 12. With such intermittent addition and supply of the fuel with the set flow rate Q to the HC-SCR NO.sub.x catalyst 12, HC decomposionally produced from the fuel is reacted with NO.sub.x in the exhaust gas 9 in the HC-SCR NO.sub.x catalyst 12, thereby making reduction of NO.sub.x emission.

(13) During the operation of the diesel engine 1, when the temperature 19a measured by the temperature sensor 19 is not less than the set temperature T (300° C.), the control signal 20a outputted from the controller 20 to the control valve 21 of the fuel addition unit 15 controls the opening degree of the control valve 21 such that, as shown in FIG. 3B, the fuel with the flow rate Q′ not less than the set flow rate Q is temporarily rich-spike added. With such temporary rich-spike addition of the fuel with the flow rate Q′ not less than the set flow rate Q, reliably the fuel is passed through the HC-SCR NO.sub.x catalyst 12 and is made arrival at the catalyzed particulate filter 13 where HC decomposionally produced from the fuel is reacted with NO.sub.x in the exhaust gas 9 to make reduction of NO.sub.x emission.

(14) The HC-SCR NO.sub.x catalyst 12 and the oxidation catalyst carried by the catalyzed particulate filter 13 with active metal being copper have inherent performances respectively as shown in FIG. 4A. As in the embodiment, with temperature-based changeover control between intermittent addition and temporary rich-spike addition of fuel, an active temperature range is expanded as shown in FIG. 4B.

(15) The HC-SCR NO.sub.x catalyst 12 and the oxidation catalyst carried by the catalyzed particulate filter 13 with active metal being silver have inherent performances respectively as shown in FIG. 5A. As in the embodiment, with temperature-based changeover control between intermittent addition and temporary rich-spike addition of fuel, an active temperature range is expanded as shown in FIG. 5B.

(16) As a result, in comparison with conventional exhaust emission control devices, an active temperature range is expanded; exhaust emission control can be made with a wide temperature range; a NO.sub.x reduction ratio can be enhanced not only in a low and medium temperature range of about 150-300° C. but also at high temperatures not less than 300° C.

(17) Thus, the active temperature range can be expanded; exhaust emission control can be made with a wide temperature range; and a NO.sub.x reduction ratio can be enhanced.

(18) FIG. 6 shows a further embodiment of an exhaust emission control device according to the invention in which parts identical with those in FIG. 2 are represented by the same reference numeral. Additionally provided between the catalyzed particulate filter 13 and the downstream oxidation catalyst 14 are a fuel addition unit 15′, a platinum-group-element HC-SCR NO.sub.x catalyst 12′ having performance of oxidizing NO.sub.x and a platinum-group-element HC-SCR NO.sub.x catalyst 12″.

(19) The fuel addition unit 15′ comprises, like the fuel addition unit 15, a fuel tank 16′ for storage of the fuel, a fuel pressure pump 17′ for pumping of the fuel stored in the fuel tank 16′, a fuel addition nozzle 18′ for spray of the fuel pumped by the fuel pressure pump 17′ to an entry side of the HC-SCR NO.sub.x catalyst 12′ and a control valve 21′ arranged on a discharge side of the fuel pressure pump 17′ and controlled in opening degree by a control signal 20a′ outputted from the controller 20. The fuel stored in the fuel tank 16′ is pumped, with an opening degree of the control valve 21′ being controlled, by the fuel pressure pump 17′ to the fuel addition nozzle 18′ from which the fuel is sprayed to the exhaust gas 9 having passed through the catalyzed particulate filter 13; in the HC-SCR NO.sub.x catalysts 12′ and 12″, HC decomposionally produced from the fuel is reacted with NO.sub.x in the exhaust gas 9 to further make reduction of NO.sub.x emission, and HC, CO and the like in the exhaust gas 9 are oxidatively removed by the downstream oxidation catalyst 14.

(20) With the structure shown in FIG. 6, further reduction of NO.sub.x emission can be attained.

(21) It is to be understood that an exhaust emission control device according to the invention is not limited to the above embodiments and various changes and modifications may be made without departing from the scope of the invention.

INDUSTRIAL APPLICABILITY

(22) An exhaust emission control device according to the invention can be utilized for a vehicle with a diesel engine.

REFERENCE SIGNS LIST

(23) 1 diesel engine 9 exhaust gas 11 exhaust pipe 12 HC-SCR NO.sub.x catalyst 13 catalyzed particulate filter 14 downstream oxidation catalyst 15 fuel addition unit 19 temperature sensor 19a temperature 20 controller 20a control signal 21 control valve Q set flow rate Q′ flow rate T set temperature