Patent Publication Number: US-7216481-B2

Title: Method and apparatus for regeneration of engine exhaust aftertreatment devices

Description:
TECHNICAL FIELD 
   The present invention relates to devices for exhaust aftertreatment for internal combustion engines; more particularly, to a mechanism for regenerating such aftertreatment devices that becomes fouled or loaded, in the case of traps, by exhaust through use; and most particularly, to method and apparatus for regeneration of an installed first aftertreatment device (AD 1 ), for example, a diesel particulate filter (DPF) and a second aftertreatment device (AD 2 ), for example, a nitrogen oxides (NOx) adsorber, using hydrogen-rich reformate generated by a catalytic hydrocarbon reformer. 
   BACKGROUND OF THE INVENTION 
   Exhaust aftertreatment devices for reducing emissions from internal combustion engines are well known. It is known in the diesel engine art to provide in series a plurality of exhaust aftertreatment devices, referred to herein for simplicity as AD 1  and AD 2 . Especially in treatment of diesel engine exhaust, such devices are designed to collect or trap undesirable exhaust constituents such as particulates or NOx, becoming full over time. They may also become contaminated by exhaust constituents which inactivate the aftertreatment device chemically, such as sulfur, or physically, such as ash, which can cause clogging or other dysfunction from prolonged exposure to the exhaust stream. Thus, it is important to be able to clean, or “regenerate,” inline exhaust amelioration devices as needed, while the engine is running. 
   It is further known in the prior art to provide a catalytic hydrocarbon reformer for generating hydrogen-rich reformate which is added to the engine exhaust stream upstream of the aftertreatment devices. The hydrogen attacks and removes deposits in the devices. In a typical cleaning duty cycle for an 8-cylinder light duty diesel vehicle, reformate is introduced into the exhaust stream for approximately 10 seconds, followed by approximately 70 seconds of little or no reformate. Typically, about 20 grams per second of reformate is needed for adequate regeneration. 
   In a prior art arrangement, the reformer takes in hydrocarbon fuel and fresh air to produce the reformate. To inject this reformate into the exhaust stream ahead of the aftertreatment devices requires that the pressure of the reformate be higher than the exhaust backpressure, P engine , at all speeds and loads, so that the reformate will flow into the exhaust stream. The apparatus must include a pump to raise the pressure of the reformate output stream to a pressure of approximately 80–100 kPa above ambient pressure, P ambient , which is the nominal inlet air pressure for the engine and the reformer, to overcome the exhaust backpressure. If the pump is 80% efficient, for example, an electric motor of about 1.5 kW input is required to run the pump. This size electric motor is large, expensive, and not practically powered by conventional 12–14 volt electrical systems provided in typical vehicles. 
   What is needed in the art is an improved method and apparatus for providing reformate into an engine exhaust stream which reduces the required size of the pump and pump motor. 
   It is a principal object of the present invention to reduce the pressure head against which a reformate pump must operate, thereby reducing the required size of the pump and pump motor. 
   SUMMARY OF THE INVENTION 
   Briefly described, a method and apparatus in accordance with the invention for providing reformate into the exhaust stream of a gasoline (spark-ignited) or diesel (compression-ignited) internal combustion engine by means which reduces the required size of the reformate pump and pump motor for pressurizing a hydrocarbon catalytic reformer and a distribution valve for dividing the reformate output of the reformer and sending it to a plurality of different points in the engine exhaust stream. The engine exhaust system includes a plurality of aftertreatment devices such as a particulate trap and an NOx filter. The chosen reformer is effective with an oxidizing input comprising oxygen-depleted engine exhaust rather than ambient air, for reaction with hydrocarbon fuel. The reformer draws its oxidizing intake from the engine exhaust at exhaust line pressure and discharges its reformate back into the engine exhaust at any of several locations. Thus, the only pressure drop that the pump must overcome is that within the reformate supply system between the reformer take-off point and the reformate entry point. In a configuration wherein the exhaust is taken off ahead of the inline particulate trap, a separate particulate filter is preferably incorporated into the reformer supply line. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic drawing of a prior art apparatus configuration for supplying reformate to an engine exhaust system including first and second aftertreatment devices AD 1  and AD 2 ; 
       FIG. 2  is a schematic drawing of a first embodiment of an apparatus configuration in accordance with the invention for supplying reformate to an engine exhaust system including first and second aftertreatment devices AD 1  and AD 2 ; 
       FIG. 3  is a schematic drawing of a second embodiment of an apparatus configuration in accordance with the invention for supplying reformate to an engine exhaust system including first and second aftertreatment devices AD 1  and AD 2 ; and 
       FIG. 4  is a schematic drawing of an apparatus configuration in accordance with the invention including a reformate storage vessel. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , an exhaust system for an internal combustion engine  01  such as, for example, a diesel engine  02 , includes a first exhaust aftertreatment device  03  (AD 1 ) comprising a diesel particulate filter (DPF) and in series a second exhaust aftertreatment device  04  (AD 2 ) comprising a nitrogen oxides adsorber. The exhaust system may further include mufflers, resonators, oxidation catalysts, and/or other exhaust components know in the art. An exhaust pipe  05 , or any other means for communicating gas flow, runs from engine  01  through devices  03 , 04  to a tailpipe  06 . 
   In a prior art arrangement  10  for providing hydrogen-rich reformate  12  to exhaust pipe  05  for regeneration of AD 1  and AD 2 , a source of reformate  14 , such as a catalytic hydrocarbon reformer, is supplied with fresh air  16  by a gas pump  18 , and with metered hydrocarbon fuel  20 , to form reformate  12  which is directed to a controllable splitter valve  22 . Valve  22  divides the flow of reformate  12  into first and second streams  24 ,  26  which are directed into exhaust pipe  05  at point  28  ahead of AD 1 , and at point  30  ahead of AD 2 , respectively. Air  32  entering pump  18  is at ambient pressure, P ambient . 
   As noted above, a serious problem with prior art arrangement  10  is that an undesirably large pump motor and high-efficiency pump  18  is required to overcome high backpressure encountered for injecting reformate into exhaust line  05 . This may be quantified as follows, where ΔP pump  is the increase in air pressure required of pump  18 :
 
Δ P   pump   &gt;P   engine   −P   ambient   =ca.  80–100 kPa  (Eq. 1)
 
   Referring to  FIG. 2 , in an improved arrangement  110  for providing reformate to an engine exhaust system, the engine and exhaust aftertreatment components are as in the prior art. Also, as in the prior art, a pump supplies reforming oxygen to a reformate source from which the reformate stream is split by valve  22  into two streams  24 , 26  which enter exhaust pipe  05  at points  28  and  30 , respectively. 
   The improvement in arrangement  110  is that the reforming oxygen supply  132  is drawn from pressurized engine exhaust in exhaust pipe  05  at point  134  upstream of point  28 , rather than from ambient air as in the prior art, thus reducing the pressure differential that the pump must produce and allowing use of a much smaller motor and pump  118  than prior art pump  18 . Engine exhaust, especially diesel exhaust, contains a substantial percentage of oxygen which may be employed in reforming fuel  20 , although a different reformate source  114  may be required that is effective with an input that is oxygen-depleted engine exhaust rather than ambient air  32  in  FIG. 1 . Also, because supply  132  is taken off ahead of particulate filter  03 , a small inline particulate filter  103  is preferred to keep fouling particulates out of reformate source  114 . In use, filter  103  would be scheduled for change at regular maintenance intervals. 
   Embodiment  110  offers the lowest pressure differential possible for pump  118 , as the pump must overcome only the pressure drop across filter  103  (ΔP filter ), reformate source  114  (ΔP reformer ) and valve  22  (ΔP valve ) to inject reformate at point  28  for regeneration of aftertreatment device  03  (AD 1 ):
 
Δ P   pump   &gt;ΔP   filter   +P   reformer   +ΔP   valve   (Eq. 2)
 
   For regeneration of aftertreatment device  04  (AD 2 ), the pump pressure difference is even lower, as the back pressure against which the pump must operate in injecting reformate at point  30  is reduced by the pressure drop across AD 1   03 , (ΔP AD1 ).
 
Δ P   pump &gt;(Δ P   filter   +P   reformer   +ΔP   valve )−Δ P   AD1   (Eq. 3)
 
   Referring to  FIG. 3 , in a second improved arrangement  210  for providing reformate to an engine exhaust system, the engine and exhaust aftertreatment components are as in the prior art. Also, a pump supplies reforming oxygen to a reformate source, from which the reformate stream is split by valve  22  into two streams  24 , 26  which enter exhaust pipe  05  at points  28  and  30 , respectively. 
   The improvement in arrangement  210  is that the reforming oxygen supply  232  is drawn from pressurized engine exhaust in exhaust pipe  05  at point  234  between AD 1  and AD 2 . Because supply  132  is taken off downstream of particulate filter  03 , inline particulate filter  103  is not needed in this embodiment. 
   Embodiment  210  offers the next lowest pressure differential possible for pump  118 . This embodiment has the disadvantage that the pump pressure difference is higher than in embodiment  110  by the amount equal to the pressure difference across AD 1   03 . The pump must overcome not only the pressure drop across reformate source  114  (ΔP reformer ) valve  22  (ΔP valve ), but also the pressure drop across AD 1  (ΔP AD1 ) to inject reformate at point  28  for regeneration of aftertreatment  03  (AD 1 ):
 
Δ P   pump   &gt;ΔP   reformer   +ΔP   valve   +ΔP   AD1   (Eq. 4)
 
   However, for regeneration of aftertreatment device  04  (AD 2 ), the pump pressure difference is even lower than in embodiment  110 , as the backpressure against which the pump must operate in injecting reformate at point  30  is only the pressure drop across the reformer (ΔP reformer ) and the valve (ΔP valve ).
 
Δ P   pump   &gt;ΔP   reformer   +ΔP   valve   (Eq. 5)
 
   It will be seen by one of ordinary skill in the art that a third configuration (not shown) is possible wherein the exhaust feed to the pump is taken from tail pipe  06 . However, because the exhaust backpressure in the tailpipe is very nearly P ambient , such an embodiment offers little advantage over the prior art arrangement  10  shown in  FIG. 1 . 
   Sufficient amounts of oxygen must be present in the exhaust stream to produce reformate by the reformate source. On the other hand, for successful particulate filter regeneration, no or a minimal amount of oxygen should be present in the exhaust stream during the regeneration cycle. Therefore, a means is provided, as shown in  FIG. 4 , to assure that reformate will be available for particulate filter regeneration when the exhaust composition is suitable for filter regeneration (that is, when the exhaust contains no or a minimal amount of oxygen such as during rich engine operation). 
   Referring to  FIG. 4 , reformate storage vessel  330  is shown in flow communication between reformate source  114  and stream  24  directed toward the particulate filter (shown in  FIGS. 2 and 3 ). Vessel  330  includes one-way check valve  331 . Valves  22   a  and  22   b  are also provided. In operation, reformate  12  is generated by reformate source  114  during normal diesel engine operation when sufficient oxygen is present in the engine&#39;s exhaust as feed stock to the reformate source. Reformate  12 , produced by reformate source  114  from oxygen laden engine exhaust  332 , is fed to the nitrogen oxides adsorber (shown in  FIGS. 2 and 3 ) via stream  26  through control valve  22   a  for regeneration of the adsorber as needed, as in embodiments  110  and  210 . 
   Reformate  12  produced by reformate source  114  is also fed to vessel  330  where it is stored until needed to regenerate the particulate filter via stream  24 . Reformate  12  stored in vessel  330  may then be selectively fed by control valve  22   b  to the particulate filter for regeneration only when minimal or no oxygen is present in the engine exhaust (such as during rich engine operation). 
   While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.