Process and system for removing soot from particulate filters of vehicle exhaust systems

A motor vehicle exhaust aftertreatment system includes an exhaust conduit that includes a NOx adsorber, a particulate filter, a reformer for generating reformate containing hydrogen and carbon monoxide from a fuel source, a reformate conduit, and an oxygen sensor. In response to a pressure drop through the particulate filter attaining a threshold value, reformate is introduced under selected controlled flow conditions into the exhaust conduit and caused to undergo combustion. The resulting exotherm maintains the temperature within the particulate filter within a selected range that is effective to cause oxidation and removal of soot from the particulate filter. Introduction of reformate into the exhaust conduit is discontinued when the pressure drop through the particulate filter is decreased to a selected value.

TECHNICAL FIELD

The present invention relates to the aftertreatment of motor vehicle exhaust and, more particularly, to a process and system for removing soot from particulate filters of vehicle exhaust aftertreatment systems.

BACKGROUND OF THE INVENTION

Internal combustion engines, including diesel engines, operate by the controlled combustion of hydrocarbon fuels and produce exhaust gases containing complete combustion products such as carbon dioxide (CO2) and water (H2O), and incomplete combustion products such as carbon monoxide (CO) and unburned hydrocarbons (HC). Further, the very high temperatures produced by the burning of the hydrocarbon fuels with air results in the detrimental formation of nitrogen oxide compounds (NOx). Certain undesirable components of the exhaust, including CO, HC, NOx, and soot particulates must be controlled to meet government emissions regulations.

Diesel engines are characterized by higher thermal efficiency than gasoline engines because of their high compression ratios, but they typically generate higher levels of NOxand particulate emissions than gasoline engines. To reduce these emissions to required low levels, premixed diesel combustion technology is being developed that provides for the fuel-air charge to be well mixed and diluted, thereby enabling combustion to occur at low temperatures without local rich zones. One approach to premixed combustion is to reduce engine compression ratio, increase charge dilution with exhaust gas, and inject fuel incrementally into the cylinder during the compression stroke. Generally, this lengthens the ignition delay period to provide more time for fuel-air mixing. This approach works best at medium engine loads but not very well at high loads or very low loads. Engine load refers to relative torque, i.e., the ratio of actual torque to maximum torque at a given engine speed. Medium loads may be defined as lying between about one-third and about two-thirds of maximum torque. Accordingly, low loads are below about one-third maximum torque, and high loads are above about two-thirds maximum torque.

For high loads, detonation of the fuel-air mixture may produce high combustion rates and noise. For very low loads, the mixture is very lean and ignition may become unstable, with increased occurrence of misfire cycles. The main technical challenges are control of combustion initiation, timing and rate to achieve effective premixed combustion over an extended range of engine load. Another goal is to improve emission aftertreatment performance without compromising overall engine efficiency under operating conditions for which premixed combustion cannot be achieved.

The quantities of pollutants generated by incomplete combustion varies with operating conditions of the engine but are influenced predominantly by the air-to-fuel ratio in the combustion cylinder. Conditions conducive to reducing carbon monoxide and unburned hydrocarbons, i.e., a fuel mixture just lean of stoichiometric and high combustion temperatures, cause an increased formation of NOx, and conditions conducive to reducing the formation of NOx, i.e., rich fuel mixture and low combustion temperatures, cause an increase in carbon monoxide and unburned hydrocarbons in the exhaust gases. As a result, significant amounts of CO, HC and NOxare emitted within the region of stable operation of an internal combustion engine.

One approach for treating nitrogen oxides in exhaust gases is to incorporate a NOxadsorber, also referred to as a “lean-NOxtrap,” in the exhaust lines. The NOxadsorber promotes the catalytic oxidation of nitrogen oxides by catalytic metal components effective for such oxidation, such as precious metals. The formation of NO2is generally followed by the formation of a nitrate when the NO2is adsorbed onto the catalyst surface. The NO2is thus “trapped”, i.e., stored, on the catalyst surface in the nitrate form. The system can be periodically operated under fuel-rich combustion to regenerate the NOxadsorber. During this period of fuel-rich combustion, the absence of oxygen and the presence of a reducing agent promote the release and subsequent reduction of the stored nitrogen oxides. However, this period of fuel-rich combustion may also result in a significant fuel penalty.

As already noted, exhaust gas streams can further comprise particulate matter such as carbon-containing particles or soot. A particulate filter is commonly used with a compression-ignition engine to prevent the carbon particles or the soot from exiting a tailpipe. The particulate filter may be a stand-alone device separate and distinct from devices employing catalytic elements for removing undesirable NOxgaseous components. Carbon particles can be trapped in the particulate filter and then periodically burned to regenerate the filter.

Reformates are hydrogen-enriched fuels that can be produced from a variety of sources, including gasoline, diesel, and other liquid or gaseous fuels. On-board reformers for producing hydrogen-enriched reformate fuels are described in, for example, U.S. Pat. Nos. 6,655,130 and 6,832,473 and U.S. Patent Application Publication Nos. 2004/0146458 and 2005/0022450, the disclosures of which are incorporated herein by reference.

Combustion of a hydrogen-enriched reformate fuel produced by an on-board reformer can be employed to burn accumulated soot from a particulate filter, but the combustion needs to be carefully controlled to prevent overheating and consequent damage to the filter, in particular, the porous filter support. Controlling the soot-burning exotherm would, for example, permit the use of cordierite as a support material in place of the more expensive silicon carbide.

The motor vehicle exhaust system and process for removing soot from a particulate filter in accordance with the present invention provides for the controlled combustion of reformate with oxygen in the exhaust conduit, resulting in the effective removal of soot from a particulate filter, without attendant damage to the filter.

SUMMARY OF THE INVENTION

The present invention is directed to a motor vehicle exhaust aftertreatment system that comprises: an exhaust conduit for conveying exhaust fluid from an engine comprising a NOxadsorber that includes a NOxadsorber inlet and outlet, and further comprising a particulate filter that includes a particulate filter inlet and outlet; a reformer for generating reformate containing hydrogen and carbon monoxide from fuel supplied from a fuel source; a reformate conduit for conveying reformate from the reformer into the exhaust conduit upstream of the particulate filter; an oxygen sensor disposed in the exhaust conduit downstream of the reformate conduit and upstream of the particulate filter for determining the oxygen concentration of fluid in the exhaust conduit, the oxygen concentration being maintained within a selected concentration range; a pressure sensor disposed at each of the particulate filter inlet and outlet for determining pressure drop through the particulate filter; and a temperature sensor disposed at each of the particulate filter inlet and outlet.

In response to a pressure drop through the particulate filter attaining a threshold value, as determined by the pressure sensors disposed at the particulate filter inlet and outlet, reformate is introduced under selected controlled flow conditions from the reformer through the reformate conduit into the exhaust conduit and caused to undergo combustion, resulting in an exotherm. The selected controlled flow and subsequent combustion of the reformate in the exhaust conduit is carried out under conditions effective to maintain the temperature within the particulate filter within a selected temperature range, as measured by the temperature sensors respectively disposed at the inlet and outlet.

The exotherm resulting from combustion of the reformate heats fluid passing through the particulate filter to a temperature effective to oxidize and thereby remove soot from the particulate filter. Introduction of reformate into the exhaust conduit is discontinued when the pressure drop through the particulate filter is decreased to a selected value.

The present invention is further directed to a process for removing soot from a particulate filter using the motor vehicle exhaust aftertreatment system just described.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1schematically depicts a motor vehicle exhaust aftertreatment system100in accordance with the present invention. An exhaust conduit101that conveys exhaust fluid from a diesel engine E includes a NOxadsorber102having an inlet103and an outlet104and a particulate filter105having an inlet106and an outlet107. NOxadsorber102further includes an oxidation catalyst102a.

System100also includes a reformer108that generates a hydrogen-containing reformate fuel from a fuel source109. The reformate is introduced, preferably by intermittent pulsed flow, into exhaust conduit101via reformate conduit110. An oxygen sensor111, which is disposed in exhaust conduit101downstream of reformate conduit110and upstream of particulate filter105, measures the concentration of oxygen in the fluid flowing through exhaust conduit101. To ensure efficient soot removal from particulate filter105, an oxygen concentration of at least about 10 vol. %, preferably about 11 vol. % to about 19. vol. %, is desired.

Pressure sensors112and113, disposed at, respectively, particulate filter inlet106and outlet107, enable the measurement of pressure drop through particulate filter105. In response to a pressure drop through particulate filter105attaining a threshold value, as determined by pressure sensors112and113, an exotherm is generated in exhaust conduit101by combustion of reformate promoted by catalyst102aof NOxadsorber102. Introduction of reformate into exhaust conduit101is discontinued when the pressure drop through particulate filter105is decreased to a selected value.

Temperature sensors114and115, disposed at, respectively, inlet106and outlet107of particulate filter105, enable the temperature range within particulate filter105to be measured and controlled. For effective soot removal, a selected temperature range within particulate filter105of about 600° C. to about 750° C. is preferred.

FIG. 2schematically depicts a second embodiment200of the motor vehicle exhaust aftertreatment system of the present invention. An exhaust conduit201that conveys exhaust fluid from a diesel engine E includes a NOxadsorber202having an inlet203and an outlet204and a particulate filter205having an inlet206and an outlet207. NOxadsorber202further includes an oxidation catalyst202a.

System200also includes a reformer208that generates a hydrogen-containing reformate fuel from a fuel source209. The reformate is conveyed to exhaust conduit201via reformate conduit210. An oxygen sensor211, which is disposed in exhaust conduit201between engine E and a burner216, measures the concentration of oxygen in the fluid flowing through exhaust conduit201.

Pressure sensors212and213, disposed at, respectively, particulate filter inlet206and outlet207, enable the measurement of pressure drop through particulate filter205. Temperature sensors214and215, similarly disposed at, respectively, inlet206and outlet207, enable the temperature range within particulate filter205to be measured and controlled.

In response to a pressure drop through particulate filter205attaining a threshold value, as determined by pressure sensors212and213, an exotherm is generated in exhaust conduit201by combustion of reformate effected by burner216. Introduction of reformate into exhaust conduit201through burner216, which preferably is a tube burner that includes a glow plug (not shown), is discontinued when the pressure drop through particulate filter205is decreased to a selected value.

The accumulation of soot in the particulate filter of an exhaust aftertreatment system results in a pressure drop through the filter. In accordance with the present invention, reformate is introduced in a controlled flow into the exhaust conduit and caused to undergo combustion, resulting in an exotherm. Through this selected controlled reformate flow and subsequent combustion, the temperature within the particulate filter is maintained within a selected temperature range that enables the oxidation and removal of soot without attendant damage to the particulate filter. Introduction of reformate into the exhaust conduit is discontinued when the pressure drop through the particulate filter is decreased to a selected value.

The soot removal process of the present invention thus constitutes a feedback loop that is further described by the flow chart depicted inFIGS. 3A,3B,3C. Referring first toFIG. 3A, measure the inlet pressure and outlet pressure of the particulate filter at step300, and determine the difference between inlet and outlet pressures at step301. Ask whether this pressure difference exceeds the threshold value for soot burnout at step302. If the answer is No, return to step300; if Yes, turn on the reformer at step303.

At step304, measure the inlet and outlet pressures and the inlet and outlet temperatures of the particulate filter and the oxygen concentration in the exhaust conduit. Determine the difference between inlet and outlet pressures at step305, and ask whether this pressure difference exceeds the value to end burnout at step306. If the answer at step306is No, turn off the reformer at step307. If the answer at step306is Yes, ask at step308if the reformer timer on-cycle is less than 5 seconds. If the answer at step308is Yes, increase the reformer timer on-cycle at step309(FIG. 3B).

At step310, ask whether the hydrogen concentration of 5 vol. % in the exhaust conduit flow is greater than 5 vol. %. If the answer at step310is No, increase the reformate flow at step311, and return to step310. If the answer at step310is Yes, ask at step312whether the oxygen concentration in the exhaust conduit flow is 10 vol. % or greater.

If the answer at step312is No, increase the concentration of oxygen in the exhaust flow at step313, and return to step312. If the answer at step312is Yes, ask at step314(FIG. 3B) whether the exhaust temperature is greater than 700° C. If the answer at step314is No, ask at step315if the inlet temperature of the particulate filter is less than 650° C. If the answer at step315is No, go to step317. If the answer at step315is Yes, increase the reformer flow at step316, and return to step314.

If the answer at step314is Yes, ask at step317whether the exhaust temperature is less than 750° C. If the answer at step317is No, decrease the reformer flow at step318, and ask at step319if the outlet temperature of the particulate filter is greater than 700° C. If the answer at step319is No, go to step321. If the answer at step318is Yes, decrease the oxygen level at step320, and return to step317.

If the answer at step317is Yes, ask at step321whether the inlet temperature of the particulate filter is greater than 600° C. If the answer at step321is No, ask at step322whether the outlet temperature of the particulate filter is less than 600° C. If the answer at step322is No, go to step324. If the answer at step322is Yes, increase the reformer flow at step323, and return to step321.

If the answer at step321is Yes, ask at step324whether the inlet temperature of the particulate filter is less than 750° C. If the answer at step324is No, decrease the reformer flow at step325, and ask at step326if the outlet temperature of the particulate filter is greater than 700° C. If the answer at step326is Yes, decrease the concentration of oxygen in the exhaust conduit flow at step327, and return to step324. If the answer at step326is No, go to step324.

If the answer at step324is Yes, ask at step328if the outlet temperature of the particulate filter is greater than 600° C. If the answer at step328is No, ask at step329if the inlet temperature of the particulate filter is less than 750° C. If the answer at step329is Yes, increase the reformer flow at step330, and return to step328.

If the answer at step328is Yes, or if the answer at step329is No, ask at step331(FIG. 3C) whether the outlet temperature of the particulate filter is less than 700° C. If the answer at step331is No, decrease the reformer flow at step332, decrease the concentration of oxygen in the exhaust flow at step333, and return to step331.

If the answer at step331is Yes, ask at step334whether the pressure difference between the inlet and outlet pressures of the particulate filter is decreased to the value selected to end burnout. If the answer at step334is Yes, return to steps307and300(FIG. 3A). If the answer at step334is No, return to step304(FIG. 3A).

If the answer at step308(FIG. 3A), is No, turn off the reformer at step335(FIG. 3C), set the reformer timer-off equal to zero at step336, and increase the reformer timer off-cycle at step337.

Following step337, ask at step338if the inlet temperature of the particulate filter is less than 750° C. If the answer at step338is No, decrease the concentration of oxygen in the exhaust flow at step339, and return to step338.

If the answer at step338is Yes, ask at step340if the outlet temperature of the particulate filter is less than 700° C. If the answer at step340is No, decrease the concentration of oxygen in the exhaust flow at step341, and return to step340.

If the answer at step340is Yes, ask at step342if the reformer timer on-cycle is less than 5 seconds. If the answer at step342is Yes, return to step337(FIG. 3C). If the answer at step342is No, return to step303(FIG. 3A).