Abstract:
A purge procedure which is part of an injector, that may be used as part of a reductant delivery unit (RDU), where the RDU is part of a selective catalytic reduction system for injecting diesel exhaust fluid into an exhaust system, to control exhaust emissions. The RDU delivers a reductant carrier to the engine exhaust system. The purge process includes a control strategy to improve the quality of the purge cycle (i.e., increase the amount of fluid evacuated). The sequence of the purge event is adjusted in order to generate a strong vacuum in the fluid supply line and the injector—this enhances the efficiency of the purge by increasing the initial flow rates through the injector. However, upon opening the injector, the pressure inside the fluid path increases to a level just below the ambient pressure outside the injector, therefore the gas flow rate is substantially reduced.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to a purge procedure for a reductant delivery unit which is part of a selective catalytic reduction system. 
       BACKGROUND OF THE INVENTION 
       [0002]    New emissions legislation in Europe and North America is driving the implementation of new exhaust aftertreatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide emissions (NOx), that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust aftertreatment technologies are currently being developed that treat NOx under these conditions. 
         [0003]    One of these technologies includes a catalyst that facilitates the reactions of ammonia (NH 3 ) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N 2 ) and water (H 2 O). This technology is referred to as Selective Catalytic Reduction (SCR). Ammonia is difficult to handle in its pure form in the automotive environment, therefore it is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea (CO(NH 2 ) 2 ). The solution is referred to as AUS-32, and is also known under its commercial name of AdBlue. The urea is delivered to the hot exhaust stream typically through the use of an injector, and is transformed into ammonia prior to entry in the catalyst. More specifically, the urea is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (CO 2 ), the ammonia resulting from the thermolysis and the hydrolysis then undergoes a catalyzed reaction with the nitrogen oxides as described previously. 
         [0004]    AUS-32, or AdBlue, has a freezing point of −11 C, and system freezing is expected to occur in cold climates. Since these fluids are aqueous, a volume expansion happens after the transition to the solid state upon freezing. This expanding ice can exert significant forces on any enclosed volumes, such as an injector, or fluid supply pipes. This expansion may cause damage to the injection unit, therefore, injection systems typically purge the injection unit when the engine shuts down to remove the fluid contained therein. 
         [0005]    In the known system configurations, injector purging is used to remove fluid from the injector when the injector is not in use. It has been found that the efficiency of this method is not 100%, i.e., a certain amount of fluid remains in the injector unit. Although the amount of remaining fluid may not always be sufficient to risk damage to the injector upon freezing (expansion volume is available for the expanding ice), a risk remains that during engine hot soaks, the remaining fluid could be exposed to high temperature. This high temperature exposure could result in the decomposition of the AUS-32 which would also cause damage to the injection unit. 
         [0006]    In other types of designs, it has been found that the remaining fluid tends to collect in the upper portion of the injector, in the volume between the filter and the inlet tube. Many types of injectors have O-rings which are used in combination with an injector cup to provide a sealing function, and prevent the remaining fluid from leaking. However, in some injectors, there is a potential leak path for the AUS-32 past the installed O-ring which cooperates with the injector cup to provide a sealing function. Although this sealing path created by the 0-ring is typically sufficient for liquids, it has been found that AUS-32 solution is prone to breaching seals of this type in the form of creeping urea crystals. At the fluid boundary layer, if there has been a minimal bypass of the sealing joint, fluid evaporates and leaves behind urea in its solid form. This provides a wicking path for more liquid urea solution, which establishes another boundary layer, evaporates, and leaves behind more solid urea. This creeping mechanism has often been observed on AUS-32 systems. 
         [0007]    Accordingly, there exists a need for a way to purge an RDU, thereby sufficiently remove fluid from the RDU, and reduce or prevent the creeping mechanism as described above. 
       SUMMARY OF THE INVENTION 
       [0008]    The purge procedure of the present invention is part of an injector, which may be used as part of a reductant delivery unit (RDU), where the RDU is part of a selective catalytic reduction (SCR) system for injecting diesel exhaust fluid (DEF) into an exhaust system, and is used to control exhaust emissions. 
         [0009]    The RDU delivers a reductant carrier (e.g. aqueous urea solution) to the engine exhaust system. The solution is transformed into ammonia which then reacts with the exhaust oxides of nitrogen in a catalytic environment to produce nitrogen and H 2 O. One type of urea, commercially known as AdBlue, has a freezing point of −11° C. In order to prevent component damage during freezing conditions, AdBlue injection systems remove fluid from the injector by purging. This invention improves the purging efficiency of the RDU. 
         [0010]    In one embodiment, the present invention is a system for purging an injector, including a pumping mechanism having multiple modes of operation, an injector in fluid communication with the pumping mechanism, and a valve portion being part of the injector, where the valve portion has an open position and a closed position. The pumping mechanism is placed in a first mode of operation such that the pumping mechanism directs pressurized fluid to the injector, and the valve portion is changed between the open and closed positions to selectively dispense fluid into an exhaust flow path. The pumping mechanism may also be placed in a second mode of operation such that the pumping mechanism generates a vacuum when the valve portion is in the closed position, and the pumping mechanism directs fluid away from the injector when the valve portion is in the open position. 
         [0011]    In another embodiment, the system of the present invention includes a pumping mechanism for transferring fluid, an injector, a valve portion which is part of the injector, where the valve portion is moveable between and open position, a closed position, and anywhere therebetween, and a purge valve in fluid communication with the pumping mechanism and the injector. The purge valve is placed in a first configuration such that the purge valve directs pressurized fluid from the pumping mechanism to the injector. The purge valve is placed in a second configuration such that the pumping mechanism generates a vacuum when the valve portion is in the closed position, and the purge valve directs fluid from the injector to the pumping mechanism when the valve portion is in the open position. 
         [0012]    The purge valve includes a first portion and a second portion connected to the first portion. The pumping mechanism directs fluid from the first portion through the pumping mechanism, through the second portion and to the injector when the purge valve is in the first configuration. The pumping mechanism directs fluid away from the injector, through the first portion and through the pumping mechanism, when the purge valve is in the second configuration. 
         [0013]    The purge process of the present invention includes a control strategy to improve the quality of the purge cycle (i.e., increase the amount of fluid evacuated from the RDU). The sequence of the purge event is adjusted in order to generate a strong vacuum in the fluid supply line and the injector—this enhances the efficiency of the purge by increasing the initial flow rates through the injector. However, upon opening the injector, the pressure inside the fluid path increases to a level just below the ambient pressure outside the injector, therefore the gas flow rate is substantially reduced. 
         [0014]    The purge process of the present invention includes multiple vacuum generating sequences during the purge event. Initially, the pumping mechanism and the purge control valve are activated (or, in alternate embodiments, the pumping mechanism is activated in “reverse” mode), and the valve portion of the injector remains closed. After a predetermined time is reached, or if a predetermined vacuum level is attained, the valve portion of the injector is opened. The valve portion of the injector remains open for a predetermined time, or until the pressure rises to a predetermined level. 
         [0015]    This cycle may then be repeated as many times as necessary. It is therefore an object of this invention to maximize the fluid volume evacuated from the injector. It is another object of this invention to provide an additional sealing barrier to reduce the sealing load of the existing sealing elements. It is another object of the invention to increase the amount of time that the gas flow rate is high and efficient for scavenging the injector, due to the high level of generated vacuum. It is yet another object of this invention to allow for the trapped fluid in the off-axis volumes (such as the volume bounded by the injector cup and the main gas stream) to flow back into the main gas stream—this permits the next wave of high intensity gas flow to remove this fluid which was previously trapped in the inaccessible volumes of the injector. 
         [0016]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
           [0018]      FIG. 1  is a sectional side view of a reductant delivery unit used as part of a purge procedure, according to embodiments of the present invention; 
           [0019]      FIG. 2  is a sectional side view of a reductant delivery unit connected to an exhaust boss used as part of a purge procedure, according to embodiments of the present invention; 
           [0020]      FIG. 3  is a diagram of system incorporating a purge procedure, according to embodiments of the present invention; and 
           [0021]      FIG. 4  is a diagram depicting the purge procedure, according to embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
         [0023]    A reductant delivery unit (RDU) having a valve portion which incorporates multiple vacuum generating sequences during a purge event is shown in the Figures generally at  10 . The RDU  10  includes a solenoid fluid injector, generally indicated at  12 , that provides a metering function of fluid and provides the spray preparation of the fluid into the exhaust path of a vehicle in a dosing application. Thus, the fluid injector  12  is constructed and arranged to be associated with an exhaust gas flow path upstream of a selective catalytic reduction (SCR) catalytic converter. The fluid injector  12  is preferably an electrically operated, solenoid fuel injector. Thus, the injector  12  has a coil  14  and a movable armature  16 . 
         [0024]    The fluid injector  12  is disposed in an interior carrier  18 . An injector shield  20  is coupled to the carrier  18  by folding down tangs of a flange  22  of body  24  over shelf features of the carrier  18  and the shield  20 . Thus, the shield  20  is fixed with respect to the injector  12 . An inlet cup structure, generally indicated at  26 , includes a cup  28  and an inlet tube  32  integral with the cup  28 . The cup structure  26  is in fluid communication with an inlet  30  of the injector  12 . The inlet tube  32  is in communication with a source of urea solution that is fed to the injector  12  to be injected from an injector outlet  34  of the injector  12 . The injector outlet  34  is fluidly connected with a flange outlet  36  of an injector flange  38  that is coupled directly with an end  40  of the body  24 . The injector  12  also includes a valve portion having a seal member  42  and a seat  44 . When the coil  14  is energized, the seal member  42  of the armature  16  is lifted off the seat  44 , moving the valve portion to an open position, permitting urea solution to pass through the injector outlet  34  to flange outlet  36 . When the coil  14  is de-energized, a spring  46  biases the seal member  42  of the armature  16  into sealing engagement with the seat  44 , changing the valve portion back to a closed position. 
         [0025]    The injector flange  38  includes internal surface structure, generally indicated at  48 , that defines the flange outlet  36  that delivers fluid into an exhaust boss  50  of an exhaust flow path. Thus, as shown in  FIG. 2 , the flange  38  is coupled to an end  52  of the exhaust boss  50  with the flange outlet  36  communicating with a bore  54  of the boss  50 . The bore  54  communicates with the exhaust flow path  56 . The flange  38  provides a mechanical support that mounts the injector  12  so that the tip is placed in a remote position with respect to the hot exhaust gases. 
         [0026]    The internal surface structure  48  also includes a conical surface  58  that is joined with at least one radius surface  60 . In the embodiment, the conical surface  58  defines the open end of the flange  38  and is joined with the radius surface  60 , with the radius surface  60  being joined directly with a gasket shelf surface  62  of the flange  38 . Thus, the conical surface  58  is downstream of the radius surface  60 . The gasket shelf surface  62  is disposed generally perpendicular with respect to a longitudinal axis C of the injector  12 . A gasket  64  rests on the gasket shelf surface  62  to seal the flange  38  with respect to the carrier  18 . 
         [0027]    A diagram of an RDU  10  incorporated into an SCR system is shown in  FIG. 3 . The system includes a pump  70  having a pumping mechanism  72  in fluid communication with a solenoid control valve, shown generally at  74 , which in this embodiment is a purge valve. The purge valve  74  has two portions, a first portion  74 A, and a second portion  74 B. The valve  74  is in fluid communication with a urea tank  76  through the use of a first conduit  78 , and a second conduit  80 . A third conduit  82  also provides fluid communication between the purge valve  74 , a fourth conduit  84 , and a pressure sensor  86 . The fourth conduit  84  is also in fluid communication with the purge valve  74  and the RDU  10 . The RDU  10  is in fluid communication with the exhaust flow path  88 , and the operation of the RDU  10  is controlled by an injector driver  90 . 
         [0028]    Referring now to  FIGS. 3 and 4 , the operation of the cycle of the purge procedure of the SCR system involves several steps. The purge valve  74  is configurable is several ways. A first configuration of the purge valve  74  is shown in  FIG. 3 , where the first portion  74 A provides fluid communication between the first conduit  78  and the pumping mechanism  72 , and the second portion  74 B provides fluid communication between the fourth conduit  84  and the pumping mechanism  72 . When the purge valve  74  is in the first configuration, the pumping mechanism  72  pumps fluid from the urea tank  76  through the first conduit  78  and through the first portion  74 A of the purge valve  74  such that the fluid flows through the pumping mechanism  72 , through second portion  74 B of the purge valve  74 , through the fourth conduit  84 , and to the RDU  10 , where the injector  12  controls the amount of fluid dispensed into the exhaust flow path  88 . 
         [0029]    The purge valve  74  also includes a second configuration which is used during the cycle of the purge process. The first step of the cycle is shown generally at  96  in  FIG. 4 , where the pumping mechanism  72  is operating, and the purge valve  74  is changed to the second configuration. When the purge valve  74  is in the second configuration, the first portion  74 A provides fluid communication between the third conduit  82  and the pumping mechanism  72 , and the second portion  74 B provides fluid communication between the second conduit  80  and the pumping mechanism  72 . When the pumping mechanism  72  is operating, fluid is drawn to the pumping mechanism  72  from the third conduit  82  and the portion  84 A of the fourth conduit  84  downstream of the third conduit  82  and upstream of the injector  12 , creating a vacuum in the third conduit  82  and the portion  84 A of the fourth conduit  84  when the valve portion is in the closed position. In the second configuration of the purge valve  74 , the pumping mechanism  72  pumps any fluid drawn from the third conduit  82  and the portion  84 A of the fourth conduit  84 , through the first portion  74 A of the purge valve  74 , through the pumping mechanism  72 , through the second portion  74 B of the purge valve  74 , and into the second conduit  80 . When the valve portion is changed to the open position during the second step of the cycle, shown generally at  98  in  FIG. 4 , the vacuum generated in the third conduit  82  and fourth conduit  84  creates suction, and causes fluid to be drawn out of the injector  12 . 
         [0030]    During both the first step  96  and the second step  98 , the pumping mechanism  72  is operating, and the purge valve  74  is in the second configuration. The valve portion of the injector  12  remains in the closed position when the purge valve  74  is changed to the second configuration to generate the vacuum. If the valve portion of the injector  12  is opened simultaneously as the purge valve  74  is changed to the second configuration, the vacuum is not generated. 
         [0031]    Referring again to  FIGS. 1-2 , during the operation of the RDU  10 , the fluid primarily collects in an upper cavity, shown generally at  92 , and around an upper seal  94 . Once the valve portion is opened after the vacuum is generated, the air flow through the injector  12  into the fourth conduit  84  draws at least a portion of the fluid into the fourth conduit  84  towards the purge valve  74 . After the valve portion is moved to the open position, and, the air flow passes from the injector  12  into the fourth conduit  84 , the vacuum pressure decreases until eventually the air flow stabilizes, and the vacuum is minimized or non-existent. 
         [0032]    If it is desired to repeat the cycle, the valve portion is changed back to the closed position to generate the vacuum, and the valve portion is then changed to the open position to draw more fluid out of the injector  12 . While two cycles are shown in  FIG. 4 , the steps  96 , 98  of the cycle may be repeated as many times as necessary to continue to remove fluid from the injector  12 . 
         [0033]    Alternate embodiments of the present invention are also possible. In one alternate embodiment, the solenoid purge valve  74  is not used, and the pumping mechanism  72  is directly in fluid communication with the first conduit  78  and the fourth conduit  84 . In this embodiment, there is no second conduit  80  or third conduit  82 , and the pressure sensor  86  is only in fluid communication with the fourth conduit  84 . 
         [0034]    In this embodiment, the pumping mechanism  72  has multiple modes of operation. In one mode of operation, the pumping mechanism  72  is operating in a forward mode, and the fluid is drawn from the urea tank  76  through the first conduit  78 , and pumped through the pumping mechanism  72  such that the fluid flowing into the fourth conduit  84  is pressurized. The pressure of the fluid in the fourth conduit  84  is indicated by the pressure sensor  86 . The fluid in the fourth conduit  84  flows into the RDU  10 , and the injector  12  controls the amount of pressurized fluid dispensed into the exhaust flow path  88 . 
         [0035]    The pumping mechanism  72  also has another mode of operation used during the purge process, where the pumping mechanism  72  operates in a reverse mode, and fluid is drawn out of the fourth conduit  84 , and forced into the first conduit  78  by the pumping mechanism  72 . When the valve portion of the injector  12  is closed, the pumping mechanism  72  is operating in a reverse mode, a vacuum is generated in the fourth conduit  84  and the RDU  10 , such that when the valve portion of the injector  12  is open, fluid remaining the upper cavity  92  is drawn out by the air flow from the injector  12  into the fourth conduit  84 . 
         [0036]    The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.