Abstract:
A real time doser efficiency monitoring method is described that measures the average instant pressure difference within one duty cycle of the doser injector. The disclosed method results in improved doser efficiency monitoring. The disclosed method can be implemented in a number of areas. For example, in a diesel truck application, the doser efficiency can be monitored accurately, for example within 5% error, all the time, no matter whether the truck is in a transient or steady state.

Description:
FIELD 
     This disclosure relates to an exhaust gas aftertreatment system and a doser system used with the aftertreatment system to inject a dosing agent into exhaust gas in the aftertreatment system. 
     BACKGROUND 
     The use of an aftertreatment system to treat exhaust gas before the exhaust gas is exhausted to atmosphere is known. One known aftertreatment system uses a diesel oxidation catalyst (DOC) device that is intended to react with the exhaust gas to convert nitric oxide to nitrogen dioxide. In the case of diesel exhaust, a diesel particulate filter (DPF) can also be provided downstream of the DOC to physically remove soot or particulate matter from the exhaust flow. 
     When exhaust gas temperatures are sufficiently high, soot is continually removed from the DPF by oxidation of the soot. When the exhaust gas temperature is not sufficiently high, active regeneration is used. In the case of diesel engine exhaust, one form of active regeneration occurs by injecting fuel into the exhaust gas upstream of the DOC. The resulting chemical reaction between the fuel and the DOC raises the exhaust gas temperature high enough to oxidize the soot in the DPF. 
     A doser system that includes a doser injector is used to inject the fuel into the exhaust gas. Deterioration of the doser injector can occur over its lifetime, for example due to doser tip carboning or a reduction of doser stroke. It is currently believed by the inventors that doser deterioration is the most frequent mode of failure in aftertreatment systems. A known doser monitoring method that attempts to determine the efficiency of the doser injector senses the temperature difference across the DOC. However, the effectiveness of this method is decreased by deterioration of the DOC which cannot be independently monitored. 
     SUMMARY 
     A real time doser efficiency monitoring method is described that measures the average instant pressure difference within one duty cycle of the doser injector. The disclosed method results in improved doser efficiency monitoring. The disclosed method can be implemented in a number of areas. For example, in a diesel truck application, the doser efficiency can be monitored accurately, for example within 5% error, all the time, no matter whether the truck is in a transient or steady state. 
     In one embodiment, a method of monitoring the efficiency of a doser injector that is configured and arranged to inject a dosing agent into exhaust gas comprises determining the average instant pressure difference of the dosing agent at a dosing agent shut-off valve assembly within a duty cycle of the doser injector. The doser injector is preferably pulse-width modulation controlled. 
     In another embodiment, a method of monitoring the efficiency of a doser injector that is configured and arranged to inject a dosing agent into exhaust gas comprises, in a single duty cycle of the doser injector, determining an average pressure of the dosing agent when the doser injector is off and determining an average pressure of the dosing agent when the doser injector is on, the pressure measurements occurring at a dosing agent shut-off valve assembly. The difference between the dosing agent average pressure when the doser injector is off and the dosing agent average pressure when the doser injector is on is then determined and used to calculate the average instant pressure difference. 
     The method can be implemented by a doser system that comprises a doser injector that is configured and arranged to inject a dosing agent into exhaust gas, a dosing agent supply line connected to the doser injector, and a dosing agent shut-off valve assembly connected to the supply line that is configured and arranged to control the flow of the dosing agent in the supply line and to the doser injector. The valve assembly includes a pressure sensor for detecting dosing agent pressure in the valve assembly. A controller monitors the efficiency of the doser injector, with the controller determining the average instant pressure difference of the dosing agent at the dosing agent shut-off valve assembly within a duty cycle of the doser injector. 
     The dosing agent can be fuel, for example diesel fuel, alcohols, urea, ammonia, natural gas, and other agents suitable for use in aftertreatment of exhaust gases. 
     The disclosed method can complete monitoring within fraction of seconds, which works well even during transient engine operations and dosing. The disclosed method also has increased accuracy. The average instant pressure difference is the maximum pressure drop so it has a better signal-to-noise ratio. The disclosed method is also independent of the performance, e.g. degradation, of individual aftertreatment components as is the current temperature based efficiency monitoring method. Further, the disclosed method is independent of the dosing command. 
     The disclosed method permits compliance with the on-board diagnostics requirement for the year 2010, which requires independent monitoring for each aftertreatment component. In addition, the higher efficiency achieved by the disclosed method reduces the injection of excess fuel, called hydrocarbon slip, thereby avoiding violation of hydrocarbon emission regulations. Further, the occurrence of false detected “bad” dosers is reduced, thereby reducing warranty costs of doser replacement. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary doser system that can implement the real time doser efficiency monitoring method. 
         FIG. 2  illustrates the shut-off valve assembly. 
         FIG. 3  is detailed view of the portion in box  3  of  FIG. 2  illustrating the trim orifice in the shut-off valve assembly. 
         FIG. 4  depicts a pressure reading over one cycle period of the doser injector. 
         FIG. 5  is a graph of the dosing agent pressure versus time at different dosing rates. 
         FIG. 6  is a graph of the doser efficiency versus instant pressure difference for 6 doser injectors with differing deterioration levels. 
         FIG. 7  is a graph of dosing agent pressure and dosing rate versus time. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a doser system  10  for an exhaust gas aftertreatment system is illustrated. For sake of convenience in describing the unique concepts, this description will describe the doser system  10  as being a hydrocarbon doser system for a diesel fuel engine that injects diesel fuel into exhaust gas from the engine. However, it is to be realized that the unique concepts described herein can be applied to other doser systems that inject other types of dosing agents. 
     The basic configuration and operation of the doser system  10  and aftertreatment system are well known to persons of ordinary skill in the art. The doser system  10  includes a doser injector  12  that is connected to an exhaust gas connection tube  14  connected to the exhaust from an engine (not illustrated). As part of the aftertreatment system, exhaust gases in the connection tube  14  flow to a diesel oxidation catalyst (DOC) device that is intended to react with the exhaust gas to convert nitric oxide to nitrogen dioxide. A diesel particulate filter (DPF) is provided downstream of the DOC to remove soot or particulate matter from the exhaust flow. 
     The doser injector  12  is configured and arranged to inject a dosing agent, which in this exemplary embodiment is diesel fuel, into the exhaust gas in the tube  14  to increase the temperature of the DOC. The fuel is supplied via a fuel supply line  16 . A shut-off valve assembly  18  is connected to the supply line  16  and is configured and arranged to control the flow of fuel in the supply line  16  and to the doser injector  12 . 
     Details of the shut-off valve assembly  18  are illustrated in  FIGS. 2 and 3 . The assembly  18  includes a fuel inlet port  20 , a fuel outlet port  22  connected to the supply line  16 , and a drain port  24 . A pressure sensor  26  connected to the valve assembly  18  senses fuel pressure in the assembly  18 . A trim orifice  28  is provided to keep the fuel pressure in the assembly  18  more stable. The construction and operation of the valve assembly  18  illustrated in  FIGS. 2 and 3  are conventional. 
     Returning to  FIG. 1 , a controller  30  is connected to the pressure sensor  26  and receives pressure readings therefrom. The controller  30  monitors the efficiency of the doser injector  12  by determining the average instant pressure difference of the fuel at the shut-off valve assembly  18  within one duty cycle of the doser injector which is pulse-width modulation (PWM) controlled. The controller  30 , which can be an electronic control module (ECM), can also control the aftertreatment system. The doser injector  12  is controlled by a separate PWM controller  32 . 
     The fuel dosing rate is controlled by the duty cycle of the PWM controller.  FIG. 4  shows one cycle period T of doser pressure, with P off  and P on  being the fuel pressure measured by the pressure sensor  26  when the doser injector is turned off and on, respectively. All references to pressure herein and the pressures shown in  FIGS. 5-7  are the fuel pressure measured by the pressure sensor  26  in the valve assembly  18 . P avg  is the average pressure when the doser injects fuel at that duty cycle, calculated as follows: 
     
       
         
           
             
               
                 
                   
                     
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     The average pressure difference, ΔP avg , can be calculated as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     The average instant pressure difference, ΔP ins , is the average pressure difference by a factor of duty cycle. The average instant pressure difference is substantially independent of dosing rate. This is evident from  FIG. 5  which depicts a graph of dosing agent pressure versus time at different dosing rates. From  FIG. 5 , it can be seen that the instant pressure difference (i.e. the difference between the maximum pressure P off  and the minimum pressure P on ) remains substantially constant even with dosing rate changes. 
       FIG. 6  is a graph of the doser efficiency versus average instant pressure difference for 6 doser injectors with differing deterioration levels. From this graph, it can be determined that under the conditions set forth (e.g. at a supply pressure of about 1200 kPa) in the graph, a 10 kPa variation in instant pressure difference means approximately a 3.1% doser efficiency error. It is believed by the inventors that this level of accuracy is not achievable by doser efficiency monitoring methods in existence at the time of filing this application. 
       FIG. 7  is a graph depicting pressure measurements when the fuel dose rate changes from about 1.4 g/s to about 0.8 g/s within 2.2 seconds at a supply pressure of about 1950 kPa. The graph plots the individual instant pressure readings  40  versus time, the average pressure  42  versus time, the average instant pressure  44  versus time, and the dose rate  46  versus time. 
     First, looking at the average instant pressure difference method described herein, relying upon the average instant pressure difference within a single duty cycle eliminates duty cycle error. In addition, the average instant pressure difference method relies upon a relatively large range of instant pressure difference, shown in  FIG. 7  as about 256 kPa, over the single duty cycle. This helps to minimize the impact of pressure variations on the doser efficiency. From  FIG. 7 , the average instant pressure  44  while the doser is off holds relatively steady at about 1950 kPa, which is the assumed supply pressure. The variation in instant pressure difference while the doser injector is on varies by about 10 kPa. Assuming that the doser used in  FIG. 7  is a 100% efficient doser, and assuming that a 100% efficiency doser at 1950 kPa supply pressure has an instant pressure difference of 256 kPa, then the doser efficiency error can be determined by taking the variation in instant pressure difference, 10 kPa, and dividing it by the pressure difference range of 256 kPa. The doser efficiency error for the average instant pressure difference method is thus about 3.9%. 
     In contrast, looking at the instant pressure  40  and the average pressure  42 , one doser efficiency monitoring method in existence at the time of filing this application relies upon the average pressure  42  to determine doser efficiency. In the average pressure difference method, the dynamic range of the average pressure difference is the dynamic range of the pressure difference multiplied by a factor of duty cycle. In  FIG. 7 , the duty cycle is about 0.15 seconds. The dynamic range of the average pressure difference (i.e. the maximum average pressure minus the minimum average pressure) is about 38.5 kPa. This is a much smaller range than the average instant pressure difference method which means that pressure variations have a much greater impact on the doser efficiency. Relying on the same assumptions in the preceding paragraph, and assuming that the variation in instant pressure difference while the doser injector is on varies by about 10 kPa as above, the doser efficiency error of the average pressure difference method is 10 kPa divided by 38.5 kPa, or about 27.5%. If one factors in duty cycle error, that error becomes even larger. 
     Although the average instant pressure difference method has been described with respect to diesel fuel as the dosing agent, the concepts described herein can be applied to other dosing agents. For example, the dosing agent can be one or more of other types of fuels including hydrocarbon fuels, or other dosing agents such as alcohols, urea, ammonia, and natural gas. 
     The monitoring method described herein can be implemented in a number of different ways. For example, the monitoring method can be implemented by software residing in an aftertreatment system controller, for example in the controller  30 . Alternatively, the monitoring method can be implemented by hardware such as electronic circuitry at or near the pressure sensor  26 . 
     The concepts described herein may be embodied in other forms without departing from its spirit or characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.