Abstract:
A diesel particulate filter has a thin band of washcoated filter material either on the inlet of the upstream side or the outlet of the downstream side. The washcoating is with platinum group metals (PGM), e.g., Pt and Pd added to the surface and pore structure of a DPF. The PDF should provide comparable or improved distance between active regeneration and/or should prevent HC/CO slip during active DPF regeneration.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to motor vehicles, such as trucks, that are powered by internal combustion engines, particularly diesel engines that have exhaust gas treatment devices for treating exhaust gases passing through their exhaust systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    A known system for treating exhaust gas passing through an exhaust system of a diesel engine comprises a diesel oxidation catalyst (DOC) that oxidizes hydrocarbons (HC) to CO2 and H2O and converts NO to NO2 , and a diesel particulate filter (DPF) that traps diesel particulate matter (DPM). DPM includes soot or carbon, the soluble organic fraction (SOF), and ash (i.e. lube oil additives etc.). The DPF is located downstream of the DOC in the exhaust gas flow. The combination of these two exhaust gas treatment devices prevents significant amounts of pollutants such as hydrocarbons, carbon monoxide, soot, SOF, and ash, from entering the atmosphere. The trapping of DPM by the DPF prevents black smoke from being emitted from a vehicle&#39;s exhaust pipe. 
         [0003]    The DOC oxidizes hydrocarbons (HC) and converts NO to NO2 . The organic constituents of trapped DPM within the DPF, i.e., carbon and SOF, are oxidized within the DPF, using the NO2 generated by the DOC, to form CO2 and H2O, which can then exit the exhaust pipe to atmosphere. 
         [0004]    The rate at which trapped carbon is oxidized to CO2 is controlled not only by the concentration of NO2 or O2 but also by temperature. Specifically, there are three important temperature parameters for a DPF. 
         [0005]    The first temperature parameter is the oxidation catalyst&#39;s “light off” temperature, below which catalyst activity is too low to oxidize HC. Light off temperature is typically around 180-200° C. 
         [0006]    The second temperature parameter controls the conversion of NO to NO2 . This NO conversion temperature spans a range of temperatures having both a lower bound and an upper bound, which are defined as the minimum temperature and the maximum temperature at which 40% or greater NO conversion is achieved. The conversion temperature window defined by those two bounds extends from approximately 250° C. to approximately 450° C. 
         [0007]    The third temperature parameter is related to the rate at which carbon is oxidized in the filter. Reference sources in relevant literature call that temperature the “Balance Point Temperature” (or BPT). It is the temperature at which the rate of oxidation of particulate, also sometimes referred to as the rate of DPF regeneration, is equal to the rate of accumulation of particulate. The BPT is one of the parameters that determines the ability of a DPF to enable a diesel engine to meet expected tailpipe emissions laws and/or regulations. 
         [0008]    Typically, a diesel engine runs relatively lean and relatively cool compared to a gasoline engine. That factor makes natural achievement of BPT problematic. 
         [0009]    Therefore, a DPF requires regeneration from time to time in order to maintain particulate trapping efficiency. Regeneration involves the presence of conditions that will burn off trapped particulates whose unchecked accumulation would otherwise impair DPF effectiveness. While “regeneration” refers to the general process of burning off DPM, two particular types of regeneration are recognized by those familiar with the regeneration technology as presently being applied to motor vehicle engines. 
         [0010]    “Passive regeneration” is generally understood to mean regeneration that can occur anytime that the engine is operating under conditions that burn off DPM without initiating a specific regeneration strategy embodied by algorithms in an engine control system. “Active regeneration” is generally understood to mean regeneration that is initiated intentionally, either by the engine control system on its own initiative or by the driver causing the engine control system to initiate a programmed regeneration strategy, with the goal of elevating temperature of exhaust gases entering the DPF to a range suitable for initiating and maintaining burning of trapped particulates. 
         [0011]    Active regeneration may be initiated even before a DPF becomes loaded with DPM to an extent where regeneration would be mandated by the engine control system on its own. When DPM loading beyond that extent is indicated to the engine control system, the control system forces active regeneration, and that is sometimes referred to simply as a forced regeneration. 
         [0012]    The creation of conditions for initiating and continuing active regeneration, whether forced or not, generally involves elevating the temperature of exhaust gas entering the DPF to a suitably high temperature. 
         [0013]    There are several methods for initiating a forced regeneration of a DPF such as retarding the start of main fuel injections or post-injection of diesel fuel to elevate exhaust gas temperatures entering the DPF while still leaving excess oxygen for burning the trapped particulate matter. Post-injection may be used in conjunction with other procedures and/or devices for elevating exhaust gas temperature to the relatively high temperatures needed for active DPF regeneration. 
         [0014]    These methods are able to increase the exhaust gas temperature sufficiently to elevate the catalyst&#39;s temperature above catalyst “light off” temperature and provide excess HC that can be oxidized by the catalyst. Such HC oxidation provides the necessary heat to raise the temperature in the DPF above the BPT. 
         [0015]    However, during such short rich operation, the exhaust gas in enriched with hydrocarbons (HC) and carbon monoxide (CO) while the oxygen concentration in the exhaust gas is drastically depleted. 
         [0016]    The amount of HC and CO generated by the engine during the rich operation typically exceeds the stoichiometric quantity of NOx that is to be reduced over the catalyst. This excess of reductant, while necessary for high NOx reduction efficiencies, leads to HC and CO breakthroughs at the DOC outlet (“HC/Co slip”), wherein the HC/CO slip cannot be oxidized to CO2 and H2O. 
         [0017]    Traditional coated DPF have a washcoat throughout the filter to prevent HC/CO slip and increased emissions. However, a coated DPF has a lower soot-mass-limit to prevent deactivation of the catalyst from high bed temperatures generated during active regeneration. 
         [0018]    Uncoated DPF are used in conjunction with a DOC and operated in a passive manner (no active filter regeneration) or if used with an active regeneration then have a reduced rate of passive regeneration compared to a coated filter or have an increased risk of HC/CO slip. The time elapsed between active filter regeneration is decreased as a result of the lower rate of passive soot oxidation. 
       SUMMARY 
       [0019]    The disclosed embodiments of the invention provide a DPF which has a thin band of washcoated filter material either on the inlet of the upstream side or the outlet of the downstream side. The washcoating is with platinum group metals (PGM), e.g., Pt and Pd added to the surface and pore structure of a DPF. The PDF should provide comparable or improved distance between active regeneration and/or should prevent HC/CO slip during active DPF regeneration. The embodiments of the invention should also reduce the cost of the after treatment system by minimizing the PGM applied to the filter. The after treatment system should operate and function in the same manner as a fully coated DPF although the interval between active regeneration could be increased or alternatively the size of the filter can be reduced. 
         [0020]    According to a first embodiment, a coating of platinum group metals (PGM), e.g., Pt and Pd is applied to the surface and pore structure of a relatively thin band of filter media within an inlet portion of the DPF media on the upstream side. Due to the presence of the coating, additional NO2 can thus be formed and therefore increase the rate of passive regeneration. In addition, since the coating is only on a front portion of the filter, it will be able to burn any HC/CO slip from the DOC during active regeneration while not being exposed to the exotherm from burning the HC/CO slip coupled with the burning of the storage soot on the filter. It is known that the temperature rise within the DPF is greater towards the rear of the DPF as opposed to the front. This exotherm becomes pronounced in situations where regeneration has been initiated but then is quickly interrupted (e.g. drop-to-idle). By having the coating only at the front of the filter, it will be possible to minimize HC/CO slip, maintain passive regeneration and increase the soot-mass-limit of the filter. 
         [0021]    According to a second embodiment, a coating of platinum group metals (PGM), e.g., Pt and Pd is applied to the surface and pore structure of a relatively thin band of filter media within an outlet portion of the DPF media on the downstream side. Although this will not increase the rate of passive regeneration other than that from the DOC, it will allow for HC/CO slip mitigation during active regeneration. Since the coating is on the downstream side, HC/CO will be in contact with a catalyst even if it traverses the filter wall toward the inlet. Also, since the coating will not be in direct contact with soot, there will not be any accelerated soot burn during conditions such as drop-to-idle. This in turn will minimize the peak bed temperature and prevent ring-cracking, pitting or melting which could occur if the coating was on the upstream side and in contact with the soot. Though this configuration will be exposed to higher peak temperatures than if the coating was placed on the inlet of the DPF on the upstream side, the impact will not be as pronounced since this configuration is not relying on the coating to perform, enhance or promote passive regeneration. The function of the washcoat in this configuration is to burn any HC/CO slip during active regeneration. 
         [0022]    Numerous other advantages and features of the present invention will be become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a schematic illustration of a representative diesel engine and control with an exhaust after-treatment device; 
           [0024]      FIG. 2  is a schematic sectional view of a first embodiment DPF of the invention; and 
           [0025]      FIG. 3  is a schematic sectional view of a second embodiment DPF of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    While this invention is susceptible of embodiment in many different forms, there are shown in the drawings, and will be described herein in detail, specific embodiments thereof with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated. 
         [0027]      FIG. 1  shows a schematic diagram of an exemplary diesel engine  20  for powering a motor vehicle. Engine  20  has a processor-based engine control system  22  that processes data from various sources to develop various control data for controlling various aspects of engine operation. The data processed by control system  22  may originate at external sources, such as sensors, and/or be generated internally. 
         [0028]    Control system  22  includes an injector driver module  24  for controlling the operation of electric-actuated fuel injectors  26  that inject fuel into combustion chambers in the engine cylinder block  28 . A respective fuel injector  26  is associated with each cylinder and comprises a body that is mounted on the engine and has a nozzle through which fuel is injected into the corresponding engine cylinder. A processor of engine control system  22  can process data sufficiently fast to calculate, in real time, the timing and duration of injector actuation to set both the timing and the amount of fueling. 
         [0029]    Engine  20  further comprises an intake system having an intake manifold  30  mounted on block  28 . An intercooler  32  and a compressor  34  of a turbocharger  36  are upstream of manifold  30 . Compressor  34  draws air through intercooler  32  to create charge air that enters each engine cylinder from manifold  30  via a corresponding intake valve that opens and closes at proper times during engine cycles. 
         [0030]    Engine  20  also comprises an exhaust system through which exhaust gases created by combustion within the engine cylinders can pass from the engine to atmosphere. The exhaust system comprises an exhaust manifold  38  mounted on block  28 . Exhaust gases pass from each cylinder into manifold  38  via a respective exhaust valve that opens and closes at proper times during engine cycles. 
         [0031]    Turbocharging of engine  20  is accomplished by turbocharger  36  that further comprises a turbine  40  associated with the exhaust system and coupled via a shaft to compressor  34 . Hot exhaust gases acting on turbine  40  cause the turbine to operate compressor  34  to develop the charge air that provides boost for engine  20 . 
         [0032]    The exhaust system further comprises a DOC  41  and DPF  42  downstream of turbine  40  for treating exhaust gas before it passes into the atmosphere through an exhaust pipe  44 . Although the DOC  41  and the DPF  42  are shown as separate components, it is also possible that the DOC  41  and the DPF  42  share a common housing. 
         [0033]    DPF  42  physically traps a high percentage of DPM in exhaust gas passing through it, preventing the trapped DPM from passing into the atmosphere. Oxidation catalyst  46  within the DOC  41  oxidizes hydrocarbons (HC) in the incoming exhaust gas to CO2 and H2O and converts NO to NO2 . The NO2 is then used to reduce the carbon particulates trapped in DPF  42 . 
         [0034]    With regard to passive and active regeneration as mentioned above, U.S. Pat. No. 6,829,890; and U.S. Published Patent Applications 2008/0184696 and 2008/0093153 describe systems and methods for undertaking regeneration. These patents and publications are herein incorporated by reference. 
         [0035]    A first embodiment DPF  42  is shown in  FIG. 2 . The DPF includes a housing  47  having an inlet  48  and an outlet  49  and containing a filter media or filter material throughout. The filter media is composed of (but not limited to) cordierite, silicon carbide, aluminum titanate, mullite or other porous ceramic material, or woven metal or ceramic fibers. 
         [0036]    Ceramic or refractory materials for diesel particulate filters are described in U.S. Pat. Nos. 6,942,708; 4,510,265; and 4,758,272, herein incorporated by reference. 
         [0037]    A washcoat  54  of platinum group metals (PGM), e.g., Pt and Pd is applied to the surface and pore structure of a relatively thin band  55  of filter media  50  within an inlet portion of the DPF media on the upstream side of the DPF  42 . The relatively thin band  55  can be up to about 25% of the length of the media  50 . 
         [0038]    Due to the presence of the PGM, additional NO2 can thus be formed and therefore increase the rate of passive regeneration within the DPF  42 . In addition, since the coating  54  is only on a front portion of the DPF  42 , it will be able to burn any HC/CO slip from the DOC during active regeneration while not being exposed to the exotherm from burning the HC/CO slip coupled with the burning of the storage soot on the filter. It is known that the temperature rise within the DPF is greater towards the rear of the DPF as opposed to the front. This exotherm becomes pronounced in situations where regeneration has been initiated but then is quickly interrupted (e.g. drop-to-idle). By having the coating only at the front of the filter, it will be possible to minimize HC/CO slip, maintain passive regeneration and increase the soot-mass-limit of the filter. 
         [0039]    The maximum bed temperature should be limited so that the PGM does not sinter excessively. Also, the interaction between the washcoat and filter material may lead to lower tolerance to thermal events (peak bed temperature, axial/radial thermal gradient). In order to prevent excessive PGM sintering or filter failure induced from the washcoat-filter material interaction, the soot mass limit (SML) can be lowered so that an active DPF regeneration event is commanded more frequently for equivalent volume of filter. 
         [0040]    A second embodiment DPF  42   a  is shown in  FIG. 3 . A washcoat  64  of platinum group metals (PGM), e.g., Pt and Pd is applied to the surface and pore structure of a relatively thin band  65  of filter media  50  within an outlet portion of the DPF media on the downstream side of the DPF  42 . The relatively thin band  55  can be up to about 25% of the length of the media  50 . Although this will not increase the rate of passive regeneration over that generated by the DOC, it will allow for HC/CO slip mitigation during active regeneration. Since the coating is on the downstream side, HC/CO will be in contact with a catalyst even if it traverses the filter. 
         [0041]    Also, since the coating  64  will not be in direct contact with soot, there will not be any accelerated soot burn during conditions such as drop-to-idle. This in turn should minimize the peak bed temperature and prevent ring-cracking, pitting or melting. Though the coating of this embodiment will be exposed to higher peak temperatures than if the coating was placed on the inlet of the DPF on the upstream side, this should not be detrimental since this coating is not intended to perform, enhance or promote passive regeneration. The function of the washcoat in this configuration is to burn any HC/CO slip during active regeneration. 
         [0042]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred.