Internal Combustion Engine Exhaust Aftertreatment System

Abstract

An engine exhaust aftertreatment system having an organization and arrangement of certain selected components which achieve significant catalytic reduction of the known NOx pollutants (NO and NO.sub.2) in tailpipe-out exhaust, while also achieving significant catalytic reduction of sulfate pollutants in tailpipe-out exhaust.

Claims

1. A motor vehicle comprising; a diesel engine having an engine exhaust aftertreatment system which has an entrance through which diesel exhaust coining from the engine enters the aftertreatment system, an exhaust flow path for treating diesel exhaust which has entered the aftertreatment system as diesel exhaust flows along the exhaust flow path, and an exit through which treated diesel exhaust exits the aftertreatment system, the exhaust flow path containing, seriatim from the entrance: 1) a first DOC for catalytic treatment of exhaust coining from the engine, the first DOC having catalytic material in a non-catalytic washcoat which has been applied to a non-catalytic substrate, the catalytic material consisting essentially of elemental palladium and being essentially free of constituents for catalytically converting NO to NO.sub.2 and SO.sub.2 to SO.sub.3, 2) a first mixing chamber space and a first DEF injector for injecting DEF into exhaust flow passing through the first mixing chamber space from the first DOC, 3) a first SCR catalyst comprising a metal/zeolite for treating exhaust flow coining from the first mixing chamber space, 4) a DPF, 5) a second mixing chamber space and a second DEF injector for injecting DEF into exhaust flow coining from the DPF, and 6) a second SCR catalyst comprising a metal/zeolite for treating exhaust coining from the second mixing chamber space; at least one tank for storing DEF which is to be injected by the first and second DEF injectors; and a controller for controlling quantities of DEF injected by the first DEF injector and the second DEF injector.

2. A motor vehicle as set forth in claim 1 further comprising a first ASC catalyst through which exhaust coining from the first SCR catalyst passes before reaching the DPF, and a second ASC catalyst through which exhaust coining from the second SCR catalyst passes before reaching the exit.

3. A motor vehicle as set forth in claim 2 in which the first mixing chamber space comprises a first static mixer, and the second mixing chamber space comprises a second static mixer.

4. A motor vehicle as set forth in claim 1 in which the engine comprises a turbocharger having a turbine through which exhaust coining from the engine passes before entering the entrance of the exhaust aftertreatment system.

5. A method for aftertreatment of diesel exhaust coining from a diesel engine in a motor vehicle, the method comprising; flowing diesel exhaust through an exhaust flow path extending between an entrance and an exit of an aftertreatment system containing, seriatim from the entrance: 1) a first DOC for catalytic treatment of exhaust coining from the engine, the first DOC having catalytic material in a non-catalytic washcoat which has been applied to a non-catalytic substrate, the catalytic material consisting essentially of elemental palladium and being essentially free of constituents for catalytically converting NO to NO.sub.2 and SO.sub.2 to SO.sub.3, 2) a first mixing chamber space, while injecting DEF into exhaust flow passing through the first mixing chamber space from the first DOC, 3) a first SCR catalyst comprising a metal/zeolite for treating exhaust flow coining from the first mixing chamber space, 4) a DPF, 5) a second mixing chamber space, while injecting DEF into exhaust flow coining from the DPF, and 6) a second SCR catalyst comprising a metal/zeolite for treating exhaust coining from the second mixing chamber space before the exhaust reaches the exit; and controlling quantities of DEF injected into the first mixing chamber space and the second mixing chamber space.

6. A method as set forth in claim 5 further comprising flowing exhaust coming from the first SCR catalyst through a first ASC catalyst before the exhaust reaches the DPF, and flowing exhaust coining from the second SCR catalyst through a second ASC catalyst passes before the exhaust reaches the exit.

7. A method as set forth in claim 6 further comprising using a first static mixer in the first mixing chamber space to mix DEF with exhaust, and using a second static mixer in the second mixing chamber space to mix DEF with exhaust.

8. A method as set forth in claim 5 further comprising flowing exhaust through a turbine of a turbocharger before exhaust enters the entrance of the exhaust aftertreatment system.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 shows a motor vehicle which is propelled by an internal combustion engine.

[0021] FIG. 2 is a general schematic diagram of the engine and its exhaust aftertreatment system.

DETAILED DESCRIPTION

[0022] FIG. 1 shows a truck vehicle 10, such as a highway tractor for example, having a chassis 12 and a cab 14 supported on a frame of chassis 12 which also supports a fuel-consuming engine 16 of a powertrain 18. Engine 16 operates through a drivetrain of powertrain 18 to tandem axle drive wheels 20 which propel the truck vehicle on an underlying surface such as a roadway.

[0023] FIG. 2 shows a portion of an engine intake system 22 for conveying air to cylinders 24 of engine 16 into which fuel is injected and within which injected fuel is combusted to operate the engine. It also shows a portion of an engine exhaust system 26 which includes an exhaust aftertreatment system 28 which treats exhaust resulting from combustion of fuel in cylinders 24, i.e. “engine-out” exhaust, as the exhaust flows through the aftertreatment system before treated exhaust exits the aftertreatment system to pass through and out of a tailpipe 30 into surrounding atmosphere.

[0024] Engine 16 is representative of a turbocharged diesel engine which comprises a turbocharger 32 having a turbine 34 operated by engine-out exhaust before exhaust enters aftertreatment system 28. Turbine 34 operates a compressor 36 to create charge air which enters cylinders 24 from intake system 22, and in doing so decreases. Other components associated with this type of engine, such as a charge air cooler, an air filter, etc. for example, are not shown in the drawing.

[0025] An engine controller comprises a processor-based engine control unit (ECU) 38 which controls various aspects of engine operation, such as injection of fuel into engine cylinders 24. Control of fuel injection and other functions is accomplished by processing various input data to develop control data for controlling those functions.

[0026] Exhaust aftertreatment system 28 is shown in FIG. 2 to comprise structure through which exhaust is constrained to pass before exiting exhaust system 26. It should be understood that various components of the illustrated structure are shown schematically rather than as actual components, many of which are well-known in aftertreatment systems. Aftertreatment system 28 comprises an enclosure 40 providing an exhaust flow path between an exhaust entrance 42 at an upstream end, and an exhaust exit 44 at a downstream end. Arrows 46 indicate a direction of exhaust flow into, through, and out of the interior of enclosure 40. After passing through entrance 42, the exhaust flow is constrained to pass in succession across surfaces of a first DOC 48 (preferably a close-coupled DOC), then through a first mixing chamber space 50, and then across surfaces of a first SCR catalyst 52, of a first ASC 54, and of a second DOC 56 before arriving at a DPF 58. Downstream of DPF 58, the exhaust flow passes first through a second mixing chamber space 60 and then in succession across surfaces of a second SCR catalyst 62 and of a second ASC 64.

[0027] Enclosure 40 may be mounted on a frame rail of chassis 12, or alternately, certain components of aftertreatment system 28 may be housed within individual enclosures which are connected into the system by pipes.

[0028] DOC 48 treats engine exhaust by removing certain entrained matter and promoting chemical reaction of certain exhaust constituents, as mentioned earlier. First SCR catalyst 52 promotes further chemical reactions of certain constituents, primarily NOx, and first ASC 54 promotes reactions which convert ammonia into non-pollutant gases.

[0029] DPF 58 traps entrained soot to remove the trapped soot from the exhaust flow. Second SCR catalyst 62 promotes chemical reactions of certain exhaust constituents, and second ASC 64 promotes reactions which convert ammonia into non-pollutant gases, specifically nitrogen and water (vapor).

[0030] First SCR catalyst 52 is separated from DOC 48 by first mixing chamber space 50, and similarly, second SCR catalyst 62 is separated from DPF 58 by second mixing chamber space 60. DEF is introduced into each mixing chamber space via a respective DEF injector 70, 72 for entrainment and mixing with exhaust flow through the respective mixing chamber space. A mixing chamber space may, or may not, contain a physical element, such as a static mixer, which promotes wide distribution of mixing. Such physical elements are shown in phantom and marked by reference numerals 74, 76 in FIG. 2. Each DEF injector is also designed to promote wide distribution of DEF into the exhaust flow.

[0031] DEF is stored in a DEF storage tank 78 which is typically mounted on truck vehicle 10 at a location exposed to ambient temperatures which if low enough will freeze DEF in the DEF storage tank. When not frozen, DEF is drawn from DEF storage tank 78 by a pump 80 and delivered through a supply conduit 82 to a DEF supply module 84 at a pressure which is under control of ECU 38. Any excess of DEF delivered to DEF supply module 84 returns from the DEF supply module to the DEF storage tank through a return conduit 86.

[0032] ECU 38 monitors various relevant operating parameters of engine and measurements from various sensors associated with the aftertreatment system 28 to control operation of each DEF injector 70, 72 in coordination with control of DEF pressure so that proper quantities of DEF are injected into the respective mixing chamber spaces.

[0033] For complying with stricter tailpipe emission standards, Applicant had initially considered various modifications to the aftertreatment system of US Patent Publication No. 2019/0234283 before recognizing that the modifications failed to take the sulfur content of fuel into account.

[0034] For example, if a metal zeolite catalyst were placed upstream of the DPF, sulfur-based constituents present in untreated exhaust coining from the engine, sulfates would accumulate on surfaces of the catalyst, and over time, would lead to early catalyst failure with increasing accumulations on those surfaces because regeneration temperatures for removing them would not by sufficiently high, as mentioned earlier.

[0035] While it would be possible to de-sulfurize the upstream SCR catalyst by placing a close-coupled DOC (cc-DOC) upstream of the upstream SCR catalyst, and introducing hydrocarbons (HC) upstream of the cc-DOC, by post-injection of fuel into the engine cylinders after main combustion events and/or by injection of fuel into the engine-out exhaust, to create sufficiently high temperatures to de-sulfurize the SCR catalyst, the applicant has discovered that doing so would lead to one or more new and different problems because the cc-DOC contains platinum and/or a platinum/palladium formulation.

[0036] The presence of platinum alone or as an element of a catalytic formulation, such as platinum/palladium, will cause NO in engine-out exhaust to be oxidized to NO.sub.2 and SO.sub.2 to be oxidized to SO.sub.3. If higher levels of NO.sub.2 are introduced into the upstream SCR catalyst, it is possible that levels of N.sub.2O will increase and in turn increase greenhouse gas (GHG) content of tailpipe-out exhaust. Increased levels of SO.sub.3 entering the upstream SCR catalyst may increase the rate of the sulfur deposition on catalytic surfaces and create need for more frequent de-sulfurization events with consequent earlier failure of the upstream SCR catalyst.

[0037] The disclosed aftertreatment system avoids the potential problems with the modification just described because the catalytic material of the first DOC 48 consists essentially of elemental palladium and is essentially free of constituents for catalytically converting NO to NO.sub.2 and SO.sub.2 to SO.sub.3. Palladium can oxidize hydrocarbons in exhaust and during active regeneration generates an exotherm sufficient to remove accumulated sulfur and nitrogen compounds from downstream components without oxidizing NO to NO.sub.2 and without oxidizing SO.sub.2 to SO.sub.3.

[0038] DOC 48 comprises a substrate to which a washcoat containing elemental palladium has been applied. The substrate, commonly a ceramic, is constructed to provide an extensive surface area for supporting the washcoat which itself has dried to highly irregular shape further increasing the surface area of washcoat across which engine exhaust flows when passing through the DOC. Palladium is distributed throughout the washcoat for catalytically enabling chemical reactions between certain constituents of engine-out exhaust, such as reducing CO to CO.sub.2 and oxidizing hydrocarbons, to occur. The washcoat material is a porous refractory oxide, such as aluminum oxide. Both the washcoat material and the substrate material are non-catalytic.

[0039] While the diesel engine is one example of an internal combustion engine, the disclosed aftertreatment system may be used on any internal combustion engine which runs lean of stoichiometric (i.e. any lean burn engine).