Abstract:
A fluid delivery system includes a reservoir, a pump, an injector, and a pressure regulator. The reservoir encloses a fluid and includes a reservoir body forming a reservoir volume that encloses the fluid. The reservoir includes a draw conduit and a return conduit. The pump has an inlet connected to the draw conduit and provides fluid at the operating pressure and at a desired flow to a pressure line that is fluidly connected to an outlet of the pump. The injector selectively opens to allow an injected fluid flow to pass therethrough. The return orifice fluidly connects the pressure line with the return conduit, and the pressure regulator provides a regulated fluid flow to the fluid reservoir. During operation, the desired fluid flow is equal to a sum of the injected fluid flow, the return fluid flow and the regulated fluid flow.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to fluid delivery systems used in association with vehicles and, more particularly, to a fluid delivery system for diesel exhaust fluid for use with engine emission control systems. 
       BACKGROUND 
       [0002]    One known method for abating certain diesel engine exhaust constituents is by use of an exhaust after-treatment system that utilizes Selective Catalytic Reduction (SCR) of nitrogen oxides. In a typical SCR system, diesel exhaust fluid (DEF), which may include urea or a urea-based water solution, is mixed with exhaust gas before being provided to an appropriate catalyst. In some applications, the DEF is injected directly into an exhaust passage through a specialized injector device. In the case of urea, the injected DEF mixes with exhaust gas and breaks down to provide ammonia (NH 3 ) in the exhaust stream. The ammonia then reacts with nitrogen oxides (NO x ) in the exhaust at a catalyst to provide nitrogen gas (N 2 ) and water (H 2 O). 
         [0003]    As can be appreciated, SCR systems require the presence of some form of DEF sufficiently close to the engine system such that the engine can be continuously supplied during operation. Various DEF delivery systems are known and used in engine applications. In known DEF injection systems, a reservoir is installed onto a vehicle for containing the DEF, which is drawn from the reservoir and delivered in metered amounts to the engine exhaust system. 
         [0004]    In most engine applications, a precise delivery of DEF is required to achieve a desired and sufficient abatement of undesirable exhaust constituents as well as to avoid frequent fluid replenishments. For example, a fluid flow that is below a desired rate, depending on engine operating conditions, may not sufficiently abate engine emissions. Similarly, a fluid flow that is above a desired rate may deplete fluid supply in the vehicle prematurely, which can lead to more frequent vehicle service and/or insufficient emissions abatement due to lack of fluid after the fluid has been prematurely depleted and before it can be replenished. 
       SUMMARY 
       [0005]    The disclosure describes, in one aspect, a fluid delivery system. The fluid delivery system includes a fluid reservoir adapted to enclose a fluid therewithin, the fluid reservoir comprising a reservoir body forming a reservoir volume that encloses the fluid therewithin and that includes a fluid draw conduit, which is configured to draw fluid from the reservoir volume, and a fluid return conduit, which is configured to return fluid to the reservoir volume. The fluid delivery system further includes a pump having an inlet fluidly connected to the fluid draw conduit such that the pump can draw fluid from the fluid reservoir, increase a pressure of the fluid to an operating pressure, and provide fluid at the operating pressure and at a desired fluid flow to a pressure line that is fluidly connected to an outlet of the pump. The fluid delivery system also includes a fluid injector fluidly in communication with the pressure line, the fluid injector configured to selectively open and allow pressurized fluid at a predetermined, injected fluid flow to pass therethrough when the fluid injector is open. A return orifice fluidly connecting the pressure line at a location downstream of the fluid injector with the fluid return conduit such that a return fluid flow is returned to the fluid reservoir can optionally be used, but is not required for all embodiments. A pressure regulator having a regulator inlet in fluid communication with the pressure line and a regulator outlet in fluid communication with the fluid return conduit is configured to provide a regulated fluid flow to the fluid reservoir when the operating pressure exceeds a pressure regulator opening pressure. During operation, the desired fluid flow is equal to a sum of the injected fluid flow, the return fluid flow and the regulated fluid flow. 
         [0006]    In another aspect, the disclosure describes an exhaust after-treatment system for a machine. The system includes a diesel exhaust fluid (DEF) container adapted to enclose a DEF fluid therewithin. The DEF container comprises a reservoir body forming a reservoir volume that encloses the DEF and that includes a DEF draw conduit, which is configured to draw DEF from the reservoir volume, and a DEF return conduit, which is configured to return DEF to the reservoir volume. A DEF injector is configured to inject DEF from the container into an exhaust passage of an engine. A pump has an inlet fluidly connected to the DEF draw conduit such that the pump can draw DEF from the fluid reservoir and provide it at an operating pressure to the DEF injector through a pressure line. A return line has a return orifice and configured to return unused DEF from the DEF injector to the DEF container. A pressure regulator is configured to maintain a fluid pressure of the DEF provided to the DEF injector substantially constant by continuously shunting DEF from an outlet of the DEF pump to the DEF container. During operation, a DEF flow provided by the pump is equal to a DEF flow injected by the DEF injector, a second DEF flow returned to the DEF container, and a third DEF flow shunted to the DEF container by the pressure regulator. 
         [0007]    In yet another aspect, the disclosure describes a method for operating a fluid system. The method includes drawing fluid from a reservoir with a pump, pressurizing the fluid with the pump to provide a desired fluid flow to a pressure line, circulating a return flow of fluid from the pressure line back to the reservoir through a return orifice continuously during operation, selectively injecting an injected flow of fluid from the pressure line through a fluid injector, shunting a regulated flow of fluid of fluid from the pressure line, and returning the regulated flow back to the reservoir continuously during operation, and adjusting, over a long term, the desired flow by comparing a pressure of fluid in the pressure line with a desired pressure. The desired flow is selected, based on environmental variables, and selectively set by appropriately commanding the pump. At all times during operation, the desired flow is equal to the sum of the return flow, the injected flow, and the regulated flow, not considering any leakage of fluid or other fluid loss from the system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram of an engine having a SCR system in accordance with the disclosure. 
           [0009]      FIG. 2  is a cross section of a fluid reservoir in accordance with the disclosure. 
           [0010]      FIG. 3  is a schematic representation of a fluid delivery system in accordance with the disclosure. 
           [0011]      FIG. 4  is a flowchart for a process in accordance with the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    This disclosure relates to emission control systems for engines and, more particularly, to DEF metering and delivery systems for use with SCR-based after-treatment systems for diesel engines used on stationary or mobile machines. The machines contemplated in the present disclosure can be used in a variety of applications and environments. For example, any machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, marine or any other industry known in the art is contemplated. For example, the type of machine contemplated herein may be an earth-moving machine, such as a wheel loader, excavator, dump truck, backhoe, material handler, locomotive, paver or the like. Apart from mobile machines, the machine contemplated may be a stationary or portable machine such as a generator set, an engine driving a gas compressor or pump, and the like. Moreover, the machine may include or be associated with work implements such as those utilized and employed for a variety of tasks, including, for example, loading, compacting, lifting, brushing, and include, for example, buckets, compactors, forked lifting devices, brushes, grapples, cutters, shears, blades, breakers/hammers, augers, and others. 
         [0013]      FIG. 1  is a block diagram of an exhaust after-treatment system  101  associated with an engine  102  of a machine  100 . The system  101  may be modularly packaged as shown in the illustrated embodiment for retrofit onto existing engines or, alternatively, for installation on new engines. In the illustrated embodiment, the system  101  includes a first module  104  that is fluidly connected to an exhaust conduit  106  of the engine  102 . During engine operation, the first module  104  is arranged to internally receive engine exhaust gas from the conduit  106 . The first module  104  may contain various exhaust gas treatment devices such as a diesel oxidation catalyst (DOC)  108  and a diesel particulate filter (DPF)  110 , but other devices may be used. The first module  104  and the components found therein are optional and may be omitted for various engine applications in which the exhaust-treatment function provided by the first module  104  is not required. In the illustrated embodiment, exhaust gas provided to the first module  104  by the engine  102  may first pass through the DOC  108  and then through the DPF  110  before entering a transfer conduit  112 . 
         [0014]    The transfer conduit  112  fluidly interconnects the first module  104  with a second module  114  such that exhaust gas from the engine  102  may pass through the first and second modules  104  and  114  in series before being released at a stack  120  that is connected to the second module. In the illustrated embodiment, the second module  114  encloses a SCR catalyst  116  and an Ammonia Oxidation Catalyst (AMOX)  118 . The SCR catalyst  116  and AMOX  118  operate to treat exhaust gas from the engine  102  in the presence of ammonia, which is provided after degradation of a urea-containing solution injected into the exhaust gas in the transfer conduit  112 . 
         [0015]    More specifically, a urea-containing water solution, which is commonly referred to as diesel exhaust fluid (DEF)  121 , is injected into the transfer conduit  112  by a DEF injector  122 . The DEF  121  is contained within a reservoir  128  and is provided to the DEF injector  122  by a pump  126 . As the DEF  121  is injected into the transfer conduit  112 , it mixes with exhaust gas passing therethrough and is carried to the second module  114 . To promote mixing of DEF with exhaust, a mixer  124  may be disposed along the transfer conduit  112 . The amount of DEF that may be injected into the transfer conduit  112  may be appropriately metered based on engine operating conditions. Accordingly, a desired amount of fluid at desired times may be provided to the transfer conduit  112  by the DEF delivery system. 
         [0016]    As can be appreciated, the location of the DEF injector  122  on the transfer conduit  112  can expose the injector to relatively high temperatures due to heating from exhaust gas during operation. In the illustrated exemplary embodiment, a flow of engine coolant is provided through the injector, but such coolant flow is optional. Alternatively, DEF may be used as a coolant medium. 
         [0017]    A cross section of one embodiment for the urea container or delivery reservoir  128  is shown in  FIG. 2 . In this embodiment, the reservoir is denoted by reference numeral  200 . To further facilitate the expedient filling of the reservoir  200 , in the installation shown in  FIG. 2 , an air gap or fill-vent opening  414  may be provided. In the illustrated embodiment, the fill-vent opening  414  is formed by a gap provided between inner and outer cylindrical walls  416  and  418  that are concentrically disposed within the fill opening of the reservoir  200 . As shown, the inner and outer walls  416  and  418  extend at the same distance, D, within the reservoir  200  to define a maximum reservoir fill level. In this way, when the fluid  420  reaches the fill level at the height D below the top wall of the reservoir  200 , the venting will cease and the operator will know that the reservoir has been filled to capacity. The desired fill level of the reservoir may be selected on numerous parameters such as reservoir capacity and the like, and may also provide a free space  422  at the top of the reservoir to account for fluid expansion due to heating and/or freezing without damaging the reservoir walls. A sock filter  428  surrounds the heater  426  and the pickup tube or fluid draw conduit  424 . 
         [0018]    Fluid  420  may be drawn from the reservoir  200  via a draw line  424 . The draw line  424  may dray fluid from the bottom of the reservoir  200  and be surrounded by a heater  426  that can effectively melt frozen DEF fluid under cold operating conditions such that liquid DEF can be provided at a supply outlet  427 . The liquid DEF at the supply outlet  427  may be delivered to a pump, for example, the pump  126  shown in  FIG. 1 . 
         [0019]    A schematic of one embodiment for a fluid delivery system  300  is shown in  FIG. 3 . This embodiment, and especially the DEF filtering shown are exemplary and should not be understood as limiting. The delivery system  300  includes a reservoir  302 , for example, the reservoir  200  as shown in  FIG. 2 , which contains DEF fluid for use by the system  300 . A fluid draw conduit  304  is disposed within the reservoir  302  and arranged and configured to draw DEF fluid from therewithin. A staged filter arrangement includes an outer filter  306 , such as the sock filter  428  shown in  FIG. 2 , and a secondary filter  308  disposed along the fluid draw conduit  304 . Fluid drawn from the draw conduit  304  is provided to a suction line  310  that includes a primary filtration device  312 . Filtered fluid from the suction line  310  is provided to a DEF pump  314 . The DEF pump  314  may be enclosed in a housing  316  that includes a motor  318  connected to a pump  320 . The pump  320  may be a variable or fixed displacement pump operating at a variable or fixed speed depending on system configuration, as will be discussed hereinafter. An internal filter  322  may further filter the fluid before the same enters the pump  320 . A pressure sensor  324  disposed to measure fluid pressure at the outlet of the pump  320  is configured to provide a pressure signal indicative of a fluid pressure at the pump outlet to a controller  326  associated with the system  300 . 
         [0020]    Pressurized fluid at the outlet of the pump  320  is provided to a pressure line  328 . The pressure line  328  as shown in the illustrated embodiment includes a pressure junction  330  that provides, in parallel fluid circuit arrangement, fluid at pump pressure to a pressure regulator  332  and to a DEF injector  334 , for example, the DEF injector  122  ( FIG. 1 ). During operation, a continuous flow of DEF fluid passes through the pressure line  328  and through a return orifice  336 , which is disposed downstream of the DEF injector  334 , before being provided back to the reservoir  302  via a return line  338 . In other words, in one embodiment, the pressure regulator and the return orifice are in parallel fluid connection between the pressure junction and the reservoir. Of course, the return orifice may be placed elsewhere in the system, in series with the pressure regulator, and at other locations. When the return orifice is placed along a circuit branch that includes the DEF injector, a continuous flow of DEF can also act to cool the injector. Such continuous fluid circulation can also act to maintain the fluid well mixed, can control the temperature of the fluid under certain operating conditions such as cold operation, as well as ensure that an ample supply of pressurized fluid is available to the DEF injector  334  at all times during operation. In this way, in the embodiment illustrated, when a predetermined amount of fluid is desired for injection from the injector  334 , the controller  326 , based on the pressure signal from the sensor  324 , may send a command signal such as a Pulse Width Modulated (PWM) signal to open the injector  334  for a predetermined period to allow a predetermined amount of fluid to be injected thereby. 
         [0021]    When fluid is injected from the injector  334 , fluid pressure in the pressure line may decrease, especially if an appreciable amount of fluid is injected. Such pressure drop within the pressure line  328  will be indicated to the controller  326  by the sensor  324 . In response, the controller  326  will command the motor  318  to activate the pump  320  to supply fluid into the pressure line  328  until the desired pressure is once again established in the pressure line  328 . The initiation of the pump, however, as well as the activation and deactivation of the injector  334 , typically causes pressure pulsations, for example, standing waves or a hydraulic pressure spike of fluid pressure within the pressure line  328 . Such pressure fluctuations can interfere, at least temporarily, with the pressure signal readings from the sensor  324 . Moreover, such pressure spikes may interfere with the calculations in the controller  326  of the amount of fluid injected through the injector  334  because such fluid pressure may be above or below the predetermined system pressure that exists under stable conditions within the pressure line  328 . These and other effects in the system, which can cause instability and large fluctuations in system pressure, especially under conditions when high fluid amounts are being delivered therethrough in relatively quick succession during machine operation, which can ultimately lead to a greater or lesser fluid being provided through the injector  334  than what is desired. 
         [0022]    To address such and other related fluid pressure issues, at least in part, the pressure regulator  332  is configured to, at least in part, mitigate high pressure spikes in the pressure line  328 . As shown, the pressure regulator includes a valve element  340  that is biased in a closed position via a spring  342  and that, when open, fluidly bypasses the injector  334  by fluidly and directly connecting the pressure line  328  with the return line  338 . Although a mechanical pressure regulator is shown, an electronic pressure regulator valve may alternatively be used, or a mechanical arrangement having a different configuration than the one shown in  FIG. 3 . In the illustrated embodiment, the spring constant of the spring  342  is selected to yield an opening pressure for the valve element  340  that is about the same or just above the normal operating pressure in the pressure line  328 . Thus, pressure spikes may cause the automatic opening of the pressure regulator  332  and dispose of high pressure fluid by returning it to the reservoir  302 . However, the pressure regulator  332  alone is not sufficient to maintain a stable, reliable pressure within the pressure line  328 , and may further lead to system instability when large and relatively frequent pressure fluctuations are present. Moreover, the pressure regulator, by its operation principles, cannot address low pressure conditions, especially when the transient response of the pump  314 , for example, owing to the transient response of the pumping element  320  and/or motor  318 , is slow to respond to pressure drops in the system, for example, when large amounts of DEF are being provided to the machine through the injector  334 . 
         [0023]    These and other issues may be avoided by appropriately controlling the motor  318  with the controller  326  to drive the pump  320  such that an excess amount of fluid is provided to the pressure line  328 . In one embodiment, the pump  320  is driven by the motor  318  at a predetermined speed and/or displacement, in general, at predetermined fluid flow rate, which exceeds the return flow into the reservoir  302  through the return orifice  336  and also causes the pressure regulator  332  to be in an open position even when the injector  334  is in a fully open condition. Stated differently, the pump  320  is driven to provide an excess fluid supply to the pressure line  328  that exceeds the maximum fluid flow demand of the system  300  by a predetermined about, for example, 10 or 15% above the maximum expected flow through the DEF injector when the fluid pressure in the system is at its maximum allowable value and the injector is fully open, i.e., when the injector duty cycle is at 100%. The fluid supply from the pump, therefore, is equal to the sum of fluid injected through the injector, fluid returned to the reservoir through the return orifice, and fluid shunted from the pressure regulator at all times during operation. Of course, this equality of fluid flows does not account for other fluid losses from the system such as leaks, evaporation and the like, or fluid stored in system components such as in the various conduits or within the fluid injector, which fluid storage may occur transiently and/or occur at system startup or shutdown but is otherwise stabilized during system operation. 
         [0024]    The excessive fluid supply described above during stable system operation will not cause a concomitant fluid pressure increase in the pressure line  328  because of the action of the pressure regulator  332 . In short, when the opening pressure of the pressure regulator  332  is selected to be about equal and, preferably, just below the desired fluid pressure under steady conditions within the pressure line  328 , the excess fluid provided to the pressure line  328  will be shunted back to the reservoir  302  through the pressure regulator continuously during operation. In conditions when the injector  334  is open, the excess fluid flow provided by the pump  320  will account for the flow through the injector  334 , the flow through the return orifice  336 , and will also still cause the pressure regulator  332  to open, at least partially, to shunt fluid back to the reservoir  302 . In this way, a stable pressure can be maintained at all times within the pressure line  328 , and dampening that will reduce or eliminate pressure fluctuations within the pressure line  328  can be provided by a combination of the return orifice  336  and the flow through the pressure regulator  332 . 
         [0025]    To improve system control accuracy and avoid unnecessary wear on the pumping and other fluid elements of the system, the control scheme for the pump  314  operating within the controller  326  in the system  300  may account for various environmental and aging effects in the system. In one embodiment, the control algorithm, which provides a command to the motor  318  as an output, can include a closed-loop controller that is used to set the fluid flow rate of fluid provided through the pump  320  at a point that is just above the corresponding setting on the pressure regulator  332 . In one contemplated embodiment, the closed-loop controller uses a feed-forward control term to set the initial pump speed to a predetermined pump speed that yields the desired fluid flow. The predetermined pump speed can be selected or set based on pump performance mapping and environmental conditions such as ambient temp, fluid temp, altitude and pressure setting of the pressure regulator. If such predetermined pump speed setting is considered as a base or normal operating condition, the control algorithm can also monitor system pressure and use a relatively long term feedback, for example, via an integral control term having a relatively large time constant, that is based on system pressure to slowly adapt and adjust the fluid flow rate through the pump and maintain predetermined and/or desired flow margin above a maximum flow consumption of the system. In this way, the pressure can automatically control overall system pressure. 
         [0026]    A block diagram for a control  500  that controls the operation of the motor  318  and/or a displacement of the pump  320 , as applicable to the system  300  as shown in  FIG. 3 , is illustrated in  FIG. 4 . As can be appreciated, the control  500  may adjust motor speed when the motor is associated with a fixed-displacement pump to control fluid flow, or may alternatively control a pump displacement in a variable displacement pump when the same is associated with a fixed-speed motor to control fluid flow. The control  500  may operate within the controller  326 . The controller  326  may be a single controller or may include more than one controller disposed to control various functions and/or features of a machine. For example, a master controller, used to control the overall operation and function of the machine, may be cooperatively implemented with a motor or engine controller, used to control other machine systems, for example the engine  102 . In this embodiment, the term “controller” is meant to include one, two, or more controllers that may be associated with the machine  100  ( FIG. 1 ) and that may cooperate in controlling various functions and operations of the machine  100 . The functionality of the controller  326 , while shown conceptually in  FIG. 4  to include various discrete functions for illustrative purposes only, may be implemented in hardware and/or software without regard to the discrete functionality shown. Accordingly, various interfaces of the controller are described relative to components of the system  300  ( FIG. 3 ) shown in the block diagram of  FIG. 4 . Such interfaces are not intended to limit the type and number of components that are connected, nor the number of controllers that are described. 
         [0027]    During operation, the controller  500  provides a motor/pump command signal  502 , which as previously described may control the speed of a motor operating a pump and/or a displacement of a pump. In any case, the motor/pump command signal  502  is a signal that causes a change in a fluid flow provided to a pressure line of a fluid system such as the pressure line  328  in the system  300  ( FIG. 3 ). Various signals are provided as inputs to the control  500 , on the basis of which the command signal  502  is determined. In the illustrated embodiment, inputs to the control  500  include ambient temperature  504 , fluid temperature  506  and altitude  508 . These inputs, which are collectively considered environmental inputs, may include more or fewer parameters. An addition input to the control  500  is system pressure  510 . The system pressure  510  is a signal indicative of the pressure of fluid at the outlet of the pump. One example of system pressure may be the pressure signal provided by sensor  324  ( FIG. 3 ) to the controller  326  that is indicative of the real-time pressure of fluid within the pressure line  328 . 
         [0028]    The various environmental inputs, i.e., the ambient temperature  504 , fluid temperature  506  and altitude  508  in the embodiment shown in  FIG. 4 , along with a constant  512  are provided to a desired system pressure determinator function  514 . The constant  512  represents the default or designed-for opening pressure of the pressure regulator. The desired system pressure determinator function  514  in the illustrated embodiment comprises various lookup tables and compensation functions that provide, based on the then-present operating conditions of the system, an indication as to the desired pressure setting for the system. As can be appreciated, static pressure conditions such as altitude, and fluid temperature, may affect the reading of the sensor or other means used to monitor and provide an indication of the system pressure  510 . To offset or compensate for such effects, as well as mechanical pump and motor effects due to temperature, the system pressure determinator function is pre-populated or pre-programmed to provide an indication of a desired system pressure  516  that is sufficient in the system to reflect an ample supply of fluid for the fluid injector and for the pressure regulator to bypass and achieve stable system performance as described above. 
         [0029]    The desired system pressure  516  is provided to a summing junction  518 , where it is compared to the system pressure  510 . A pressure difference or error  520 , which is indicative of a difference between the desired system pressure  516  and the actual, measured or estimated system pressure  510  that is present in the system, is provided to an integral function  522 . The integral function, may be of the form shown in Equation 1 below: 
         [0000]    
       
         
           
             
               I 
                
               
                 ( 
                 t 
                 ) 
               
             
             = 
             
               
                 K 
                 i 
               
                
               
                 
                   ∫ 
                   0 
                   t 
                 
                  
                 
                   
                     e 
                      
                     
                       ( 
                       τ 
                       ) 
                     
                   
                    
                   
                     ∂ 
                     t 
                   
                 
               
             
           
         
       
     
         [0000]    where I(t) is the integral term over time (t), K i  is a constant, and e(τ) is a function that is integrated over a period (τ). As is known, integral terms can address residual steady-state error that can occur in systems. In this case, such errors may result from various sources such as sensor error, sensor creep, system aging, filter clogging, and other effects. In the illustrated embodiment, the pressure difference or error  520  may also be provided to a sentry function  524  that can provide a system fault signal  526  indicating that system service is required or that notifies the operator of a fault when the error  520  exceeds a maximum allowable error for a predetermined period. 
         [0030]    The integral function  522  provides a correction signal  528  which passes through a delimiter  530 . A delimited correction signal  532  and the desired system pressure  516  are provided to a summing junction  534  and are compounded to provide a corrected, desired system pressure in the form of the command signal  502 . As can be appreciated, the desired system pressure  516 , which can also be expressed as a desired system flow rate setpoint, is independent of fluid use by the system and only depends on fixed system parameters such as pump and motor operation and, optionally, on environmental parameters within which the system is operating. The flow and/or pressure setpoint provided by the determinator function  514  is independent of fluid use, which leads to an inherently stable control scheme. As previously discussed, flow changes within the system are addressed by the pressure regulator such that there is always a flow excess provided to the system. The steady-state error compensation provided by the integral function  522  addresses effects that may appear in the system over time and also helps diagnose system faults. 
       INDUSTRIAL APPLICABILITY 
       [0031]    The present disclosure is applicable to emission control systems for engines and, more particularly, to emission control systems using SCR processes requiring the injection of urea-based water solutions into engine exhaust streams. In the disclosed embodiments, a feed forward controller having a long-term feedback is used to create a control arrangement in which pressure fluctuations in the high-pressure DEF fluid delivery system are avoided. In one embodiment, the system sets a predetermined DEF flow which exceeds the maximum use of DEF by the injector such that an excess flow causes a pressure regulator to open, at all times, thus controlling the pressure continuously within the system. 
         [0032]    It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
         [0033]    Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.