US20140311123A1

US20140311123A1 - Electrically heated doc using hcscr cold start nox controls

Info

Publication number: US20140311123A1
Application number: US13/866,429
Authority: US
Inventors: Eugene V. Gonze; Charles Gough
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2013-04-19
Filing date: 2013-04-19
Publication date: 2014-10-23
Also published as: DE102014105043A1

Abstract

An exhaust gas treatment system for an internal combustion engine is provided. The system includes an exhaust gas conduit extending from the engine configured to receive an exhaust gas stream from the engine, a first injector in fluid communication with the exhaust gas conduit configured to selectively inject fuel containing unburned hydrocarbon (HC) into the exhaust gas conduit and an oxidation catalyst disposed in the exhaust gas conduit downstream from the first injector. The system further includes a hydrocarbon selective catalyst reduction (HCSCR) catalyst applied on the oxidation catalyst, a heating device positioned at an upstream side of the oxidation catalyst configured to heat the oxidation catalyst and HCSCR catalyst, a selective catalyst reduction (SCR) device disposed within the exhaust gas conduit downstream from the oxidation catalyst, and a particulate filter positioned downstream from the selective catalyst reduction device.

Description

FIELD OF THE INVENTION

The subject invention relates to an exhaust gas treatment system, and in particular, an exhaust gas treatment system with improved operating characteristics at low temperatures.

BACKGROUND

Exhaust gas emitted from an internal combustion engine is a heterogeneous mixture that may contain gaseous emissions such as a carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in an engine exhaust system to convert certain, or all of the exhaust constituents into non-regulated exhaust gas components.
In an exhaust treatment technology, there are several known catalyst and filter structures used that have displayed effectiveness in reducing or removing regulated constituents from exhaust gas. For example, an oxidation catalyst may be used to oxidize CO and HC from the exhaust gas into CO2 and water. A selective catalyst reduction (SCR) device may be used in conjunction with a reductant to reduce or remove NOx from the exhaust gas. A particulate filter may be used to trap particulates remaining in the exhaust gas.
Oxidation of the HC and CO may occur at the oxidation catalyst once the oxidation catalyst reaches an adequate operating temperature. In addition, NOx reduction or removal may occur at the SCR device once the SCR device reaches an adequate operating temperature, typically around 200° C. However, an oxidation catalyst and SCR device do not efficiently oxidize HC and CO, and remove or reduce NOx, respectively, at low temperatures, for example during cold start of an internal combustion engine. Thus, from a time period between the start of the engine until a sufficient operating temperature is reached in an exhaust gas conduit, and in turn, at the oxidation catalyst and SCR device, regulated constituents such as HC, CO and NOx may not be satisfactorily reduced.
Accordingly, it is desirable to provide an apparatus and method for oxidizing and/or reducing regulated constituents in an exhaust gas stream during a cold start of an internal combustion engine.

SUMMARY OF THE INVENTION

In one exemplary embodiment of the invention, an exhaust gas treatment system for an internal combustion engine is provided. The system includes an exhaust gas conduit extending from the engine configured to receive an exhaust gas stream from the engine, a first injector in fluid communication with the exhaust gas conduit configured to selectively inject fuel containing unburned hydrocarbon (HC) into the exhaust gas conduit and an oxidation catalyst disposed in the exhaust gas conduit downstream from the first injector. The system further includes a hydrocarbon selective catalyst reduction (HCSCR) catalyst applied on the oxidation catalyst, a heating device positioned at an upstream side of the oxidation catalyst configured to heat the oxidation catalyst and HCSCR catalyst, a selective catalyst reduction (SCR) device disposed within the exhaust gas conduit downstream from the oxidation catalyst, and a particulate filter positioned downstream from the selective catalyst reduction device.
In another exemplary embodiment of the invention there is provided a method of operating an exhaust gas treatment system to reduce NOx in an exhaust gas stream during a cold start of an internal combustion engine. The exhaust gas treatment system includes an exhaust gas conduit, a first injector, a heating device, an oxidation catalyst, a hydrocarbon selective catalyst reduction (HCSCR) catalyst applied to the oxidation catalyst, a second injector, a selective catalyst reduction (SCR) device and a particulate filter. The method includes monitoring a temperature of the oxidation catalyst with at least one temperature sensor of a plurality of temperature sensors, activating the heating device in response to the temperature of the oxidation catalyst being below a first threshold temperature, monitoring a temperature of the heating device with at least one temperature sensor of the plurality of temperature sensors, operating the first injector to inject a fuel containing unburned hydrocarbon (HC) into the exhaust gas conduit in response to the temperature of the heating device exceeding a second threshold temperature, monitoring a temperature of the SCR device with at least one temperature sensor of the plurality of temperature sensors, and deactivating the heating device and ending operation of the first injector in response to the temperature of the SCR device exceeding a third threshold temperature.
The above features and advantages and other features and advantages of the invention are readily apparent from the following detailed description of the invention when taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, advantages and details appear, by way of example only, in the following detailed description of embodiments, the detailed description referring to the drawings in which:

FIG. 1 is a diagram showing an exhaust gas treatment system using HCSCR cold start NOx controls according to an exemplary embodiment of the subject invention; and

FIG. 2 is a diagram showing a method of operation of an exhaust gas treatment system using HCSCR cold start NOx controls according to an exemplary embodiment of the subject invention.

DESCRIPTION OF THE EMBODIMENTS

The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.
In accordance with an exemplary embodiment of the subject invention, and with reference to FIG. 1, an exhaust gas treatment system 20 is provided for the reduction of regulated exhaust gas constituents emitted by an internal combustion engine 22. It is understood that the exhaust treatment system 20 described herein may be used in various engine systems utilizing an exhaust gas particulate filter. Such internal combustion engine systems may include, but are not limited to, diesel systems, gasoline systems and various homogeneous charge compression ignition engine systems.
The exhaust gas treat system 20 includes at least one exhaust gas conduit 30 extending from the engine 22. An exhaust gas stream 25 exits the engine 22 and flows into the exhaust gas conduit 30. The exhaust gas treatment system 20 includes an oxidation catalyst (OC) 32 positioned within the exhaust gas conduit 30 in a flow path of the exhaust gas stream 25. The oxidation catalyst 32 may include a flow-through metal or ceramic monolith substrate that is packaged in a rigid shell or canister having an inlet and an outlet in fluid communication with the exhaust gas conduit. The substrate may include an oxidation catalyst compound (not shown) disposed thereon which may be applied as a wash coat and may contain platinum group metals such as platinum (Pt), palladium (Pd) rhodium (Rh) or other suitable oxidizing catalysts, or combination thereof. The oxidation catalyst 32 is useful in treating unburned gaseous and non-volatile HC and CO in the exhaust gas stream 25, which are oxidized to form carbon dioxide and water.
The exhaust gas treatment system 20 further includes a selective catalytic reduction (SCR) device 34 disposed within the exhaust gas conduit 30 downstream from the oxidation catalyst 32. The SCR device 34 is positioned in fluid communication with the exhaust gas stream 25. Similar to the oxidation catalyst 32, the SCR device 34 may also include a flow-through ceramic or metal monolith substrate which is packaged in a rigid shell or canister having an inlet and an outlet in fluid communication with the exhaust gas conduit. The substrate has an SCR catalyst composition (not shown) applied thereto. The SCR catalyst composition preferably contains a zeolite and one or more base metal components such as iron (“Fe”), cobalt (Co”), copper (“Cu”) or vanadium (“V”) which can operate efficiently to convert NOx constituents in the exhaust gas in the presence of a reductant.
The exhaust gas treatment system 20 further includes a particulate filter assembly 36. The particulate filter assembly 36 is in fluid communication with the exhaust gas stream 25 in the exhaust gas conduit 30 and is configured to receive the exhaust gas stream 25. The particulate filter assembly 36 may be positioned downstream from the SCR device 34 and operates to filter the exhaust gas stream 25 of carbon and other particulates.
The particulate filter assembly 36 includes a filter 38. In an exemplary embodiment, the filter 38 may be formed using a ceramic wall flow monolith filter that is packaged in a rigid, heat resistant shell or canister having an inlet end and an outlet end in fluid communication with the exhaust gas conduit 30. The ceramic wall flow monolith filter 38 may be a monolith particulate trap, and include a plurality of longitudinally extending passages that are defined by longitudinally extending walls. The passages include a subset of inlet passages that have an open inlet end and a closed outlet end, and a subset of outlet passages having a closed inlet end and an open outlet end. Exhaust gas entering the filter 38 through the inlet ends of the inlet passages is forced to migrate through adjacent longitudinally extending walls to the outlet passages due to adjacent inlet and outlet passages being plugged or closed at opposite ends. The exhaust gas stream 25 is filtered of carbon and other particulates through this wall flow mechanism. The filtered particulates are deposited on the longitudinally extending walls of the inlet passages and, over time, will have the effect of increasing the exhaust gas backpressure experienced by the engine 22. The walls of the wall flow monolith filter 38 may comprise a porous ceramic honeycomb wall of cordierite material. Any type of ceramic material suitable for the purpose set forth herein may be utilized. It is understood that the ceramic wall flow filter 38 described above is merely exemplary in nature, and other suitable filters are envisioned. For example, particulate filter assembly 36 may include other filter devices such as wound or packed fiber filters, open cell foams, sintered metal fibers, etc., in addition to, or in place of the filter 38 described above.
The exhaust gas treatment system 20 further includes a heating device 40 positioned proximate to an upstream side of the oxidation catalyst 32. The heating device 40 may be an electric heating device (“EHC”) configured to provide heat to the oxidation catalyst 32. The heating device 40 is configured to operate during cold-start scenarios to more quickly activate the oxidation catalyst 32 so that the oxidation catalyst 32 may successfully oxidize HC and CO into CO2 and water, thereby removing the regulated constituents from the exhaust gas.
The oxidation catalyst 32 is coated with a hydrocarbon selective catalyst reduction (HCSCR) catalyst 42. The HCSCR catalyst may be applied to an upstream end of the oxidation catalyst 32, downstream from the heating device 40. The HCSCR catalyst 42 may reduce NOx levels in the exhaust gas stream 25 as the exhaust gas stream 25 passes through the oxidation catalyst 32. NOx reduction may begin when the oxidation catalyst 32 is heated to approximately 300° C. Thus, during a cold start, while the SCR device 34 is warming up and has not reached a temperature sufficient for NOx reduction, the oxidation catalyst 32 may be heated and operate to reduce NOx from the exhaust gas stream 25 as further described below.
In an exemplary embodiment, a first injector 44 is disposed upstream from the heating device 40 and oxidation catalyst device 32 that is coated with the HCSCR catalyst 42. The first injector 44 is disposed in fluid communication with the exhaust gas stream 25 in the exhaust gas conduit 30. In an exemplary embodiment, the first injector 44 is a fuel injector. The first injector 44 is in fluid communication with a fuel containing unburned HC (not shown) in a fuel supply tank (not shown) through a fuel conduit (not shown). The first injector 44 is configured to selectively inject the fuel containing unburned HC into the exhaust gas conduit 30 to flow with the exhaust gas stream 25 through the HCSCR coated oxidation catalyst 32.
The exhaust gas treatment system 20 may further include a second injector 46 disposed downstream from the oxidation catalyst 32 and upstream from the SCR device 34. The second injector 46 is in fluid communication with the exhaust gas conduit 30 and is configured to periodically and selectively inject a reductant such as urea or ammonia, or a combination thereof, into the exhaust gas stream 25. Other suitable methods of delivery of the reductant to the exhaust gas stream 25 may be used. The reductant is supplied from a reductant supply tank (not shown) through a supply conduit (not shown). The reductant may be in the form of a gas, a liquid or an aqueous urea solution and may be mixed with air in the second injector 46 to aid in the dispersion of the injected spray in the exhaust gas. A mixing arrangement 48 may be positioned in the exhaust gas conduit 30 downstream from the second injector 46 to assist in mixing the injected reductant with the exhaust gas stream 25.
The exhaust gas treatment system 20 may further include a plurality of sensors positioned within and along the exhaust gas conduit 30. In an exemplary embodiment, the plurality of sensors may include a plurality of temperature sensors 50, 52, 54, 56. For example, a first temperature sensor 50 may be positioned upstream from the heating device 40 and oxidation catalyst 32, a second temperature sensor 52 may be positioned downstream from the oxidation catalyst 32 and upstream from the SCR device 34, a third temperature sensor 54 may be positioned between the SCR device 34 and the filter 38 and a fourth temperature sensor 56 may be positioned downstream from the filter 38.
The plurality of sensors may further include at least one NOx sensor 58 and a plurality of pressure sensors 60, 62. In an exemplary embodiment, the at least one NOx sensor 58 is positioned downstream from the oxidation catalyst 32 and upstream of the SCR device 34 and is configured to measure NOx in the exhaust gas stream 25. The plurality of pressure sensors 60, 62 may include a first pressure sensor 60 positioned between the SCR device 34 and filter 38 and a second pressure sensor 62 positioned downstream from the filter 38. The first and second pressure sensors 60, 62 may measure the flow of the exhaust gas stream 25 through the exhaust gas conduit 30. It is understood that the number and positioning of the temperature sensors 50, 52, 54, 56, NOx sensor 58 and pressure sensors 60, 62 may vary from the examples described above.
A controller 64 such as a vehicle or engine controller is operably connected to, and monitors, the engine 22 and exhaust gas treatment system 20 through signal communication with the plurality of sensors 50-62, including the temperature sensors 50, 52, 54, 56, the at least one NOx sensor 58 and the pressure sensors 60, 62. The controller 64 may include, for example, an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. In addition, the controller 64 may be communicatively connected to the first injector 44, the second injector 46 and the heating device 40. Accordingly, the controller 64 may selectively operate the first and second injectors 44, 46 and heating device 40 for NOx emission control in response to different temperature profiles in the exhaust gas stream 25 as described below. Controlling of the NOx emissions may be in response to, at least partially, signals received from the various sensors, including the temperature sensors 50, 52, 54, 56, the at least one NOx sensor 58 and the pressure sensors 60, 62.
In use, the SCR device 34 operates to reduce NOx levels in the exhaust gas stream 25. However, the SCR device 34 must first reach a sufficient temperature to activate, or light-off, catalyst compounds thereon to reduce the NOx levels. Typically, this temperature is around 200° C. A delay may occur in reaching this temperature because a thermal mass of the upstream exhaust system delays thermal energy to the SCR device 34.
In the exemplary embodiments above, the electrically heated, HCSCR coated oxidation catalyst 32 aims to reduce NOx levels in the exhaust gas stream 25 during a cold start scenario, in a time period before the SCR device 34 reaches a sufficient temperature to reduce NOx levels. For example, in the cold start scenario, the first injector 44 injects a fuel containing unburned HC into the exhaust gas conduit 30 upstream of the heating device 40, the HCSCR catalyst 42 and the oxidation catalyst 32. The heating device 40 and injected fuel cause an exothermic reaction which brings the HCSCR catalyst 42 on the oxidation catalyst to a temperature (typically around 300° C.) sufficient to reduce NOx levels in the exhaust gas stream 25 flowing through the oxidation catalyst 32. During this time, heat from the exhaust gas stream 25 along with heat generated by the heating device 40 and oxidizing injected fuel flows downstream and acts to increase the temperature of the SCR device 34. After the SCR device 34 reaches a temperature (typically around 200° C.) where it is operable to reduce NOx in the exhaust gas stream 25, the heating device 40 and injected fuel are turned off and the HCSCR function is no longer used.
Referring to FIG. 2, the controller 64 controls the operation of the exhaust gas treatment system 20 using cold start NOx controls. A method of operating the exhaust gas treatment system 20 begins at 110 and may run continuously following a cold start of the engine 22. At 120, the controller 64 monitors a temperature of the oxidation catalyst 32 via the temperature sensors, for example, the first and second temperature sensors 50, 52 to determine if the oxidation catalyst 32 needs to be heated. At 130, if the controller 64 determines that the temperature of the oxidation catalyst 32 is below a first threshold temperature, i.e., the temperature of the oxidation catalyst 32 is insufficient for the HCSCR catalyst 42 to reduce NOx levels in the exhaust gas stream 25, the controller 64 activates the heating device 40. At 140, the controller 64 monitors the temperature of the heating device 40 via the temperature sensors, for example, first and second temperature sensors 50, 52. At 150, if the controller 64 determines that the heating device 40 is at or above a second threshold temperature, i.e., a temperature sufficient to cause an exothermic reaction, the controller 64 operates the first injector 44 to inject the fuel containing unburned HC into the exhaust gas stream 25 in the exhaust gas conduit 30 upstream from the heating device 40, HCSCR catalyst 42 and oxidation catalyst 32. In an exemplary embodiment, injection of the fuel through the first injector 44 is a function of the temperature of heating device 40, the temperature of the oxidation catalyst 32 and the exhaust flow in the exhaust gas conduit as measured by the pressure sensors 60, 62. The injection of the fuel is also a function of the HCSCR catalyst and the NOx levels in the exhaust gas stream 25, as measured by the at least one NOx sensor 58.
At 160, the controller 64 monitors a temperature of the SCR device 34 via the temperature sensors, for example, the third and fourth temperature sensors 54, 56. At 170, if the controller 64 determines the temperature of the SCR device 34 is greater than a third threshold temperature, i.e., a temperature where the SCR device 34 is lit off to reduce NOx levels, the controller 64 deactivates the heating device 40 and the first injector 44. At this point, the heating device is turned off and the fuel containing unburned HC is no longer injected into the exhaust gas stream 25. At 180, the controller 64 ends the method.
Referring still to FIG. 2, at 125, if the controller 64 determines that oxidation catalyst 32 temperature (monitored at 120) is greater than the first threshold temperature, the controller 64 operates the first injector 44 to inject fuel containing unburned HC into the exhaust gas stream 25. Here, the fuel injected is a function of the HCSCR catalyst 42 and NOx as measured by the at least one NOx sensor 58. The controller 64 proceeds to monitor the SCR device 34 temperature at 160.
Further, if the controller 64 determines that the heating device 40 temperature (monitored at 140) is less than the second threshold temperature, then the controller 64 ends the method at 180. If the controller 64 determines that the SCR device 32 temperature (monitored at 160) is less than the third threshold temperature, the controller 64 ends the method at 180.
In the exemplary embodiments above, the heating device 40 operates to heat the HCSCR catalyst 42 on the oxidation catalyst 32 to a temperature sufficient to reduce NOx within the exhaust gas stream 25 prior to the SCR device 34 lighting off That is, during a cold start scenario of the engine, prior to the SCR device 34 reaching a temperature sufficient to reduce NOx levels in the exhaust gas stream 25, the oxidation catalyst 32 and HCSCR catalyst 42 may be heated by the heating device 40. The HCSCR catalyst may be activated around 300° C. and reduce NOx levels in the exhaust gas stream 25 upon injection of a fuel containing unburned HC before the SCR device 34 reaches a temperature sufficient for NOx reduction. Accordingly, the exhaust treatment system 20 may reduce NOx levels in the exhaust gas stream 25 during a cold start scenario, thereby reducing the release of regulated exhaust constituents to the atmosphere.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the application.

Claims

What is claimed is:

1. An exhaust gas treatment system for an internal combustion engine, the system comprising:

an exhaust gas conduit extending from the engine configured to receive an exhaust gas stream from the engine;

a first injector in fluid communication with the exhaust gas conduit configured to selectively inject fuel containing unburned hydrocarbon (HC) into the exhaust gas conduit;

an oxidation catalyst disposed in the exhaust gas conduit downstream from the first injector;

a hydrocarbon selective catalyst reduction (HCSCR) catalyst applied on the oxidation catalyst;

a heating device positioned at an upstream side of the oxidation catalyst configured to heat the oxidation catalyst and HCSCR catalyst;

a selective catalyst reduction (SCR) device disposed in the exhaust gas conduit downstream from the oxidation catalyst; and

a particulate filter positioned downstream from the selective catalyst reduction device.

2. The exhaust gas treatment system of claim 1, further comprising a second injector disposed in fluid communication with the exhaust gas conduit and positioned between the oxidation catalyst and SCR device, the second injector configured to selectively inject a reductant into the exhaust gas conduit.

3. The exhaust gas treatment system of claim 2, wherein the heating device is an electric heating device.

4. The exhaust gas treatment system of claim 1, further comprising a plurality of temperature sensors disposed within the exhaust gas conduit.

5. The exhaust gas treatment system of claim 4, wherein the plurality of temperature sensors includes a first temperature sensor positioned upstream from the heating device, a second temperature sensor positioned downstream from the oxidation catalyst and upstream from the SCR device, a third temperature sensor positioned downstream from the SCR device and upstream from the particulate filter, and a fourth temperature sensor position downstream from the particulate filter.

6. The exhaust gas treatment system of claim 4, further comprising at least one NOx sensor disposed within the exhaust gas conduit.

7. The exhaust gas treatment system of claim 6, further comprising at least one pressure sensor disposed within the exhaust gas conduit.

8. The exhaust gas treatment system of claim 7, further comprising a controller configured to selectively operate the heating device and first injector in response to information received from at least one of the plurality of temperature sensors, at least one NOx sensor and at least one pressure sensor.

9. A method of operating an exhaust gas treatment system to reduce NOx in an exhaust gas stream during a cold start scenario of an internal combustion engine, the exhaust gas treatment system comprising an exhaust gas conduit, a first injector, a heating device, an oxidation catalyst, a hydrocarbon selective catalyst reduction (HCSCR) catalyst applied to the oxidation catalyst, a second injector, a selective catalyst reduction (SCR) device and a particulate filter, the method comprising:

monitoring a temperature of the oxidation catalyst with at least one temperature sensor of a plurality of temperature sensors;

activating the heating device in response to the temperature of the oxidation catalyst being below a first threshold temperature;

monitoring a temperature of the heating device with at least one temperature sensor of the plurality of temperature sensors;

operating the first injector to inject a fuel containing unburned hydrocarbon (HC) into the exhaust gas conduit in response to the temperature of the heating device exceeding a second threshold temperature;

monitoring a temperature of the SCR device with at least one temperature sensor of the plurality of temperature sensors; and

deactivating the heating device and ending operation of the first injector in response to the temperature of the SCR device exceeding a third threshold temperature.

10. The method of claim 9, further comprising operating the first injector to inject the fuel containing unburned HC in response to the temperature of the oxidation catalyst exceed the first threshold temperature.

11. The method of claim 10, further comprising measuring NOx levels in the exhaust gas stream with a NOx sensor and operating the first injector based on the measured NOx levels.