Patent Publication Number: US-10767532-B2

Title: Exhaust gas treatment system and method having improved low temperature performance

Description:
BACKGROUND 
     The present disclosure relates to exhaust systems and more particularly to diesel exhaust gas treatment systems. 
     Diesel exhaust is typically subject to emissions regulations covering a variety of emission components, including particulate matter and nitrogen oxides (NO x ). A variety of exhaust treatment devices have been developed to reduce these emission components. For example, a diesel particulate filter (DPF) can be used to trap diesel particulate matter and oxidize soot, and a selective catalytic reduction (SCR) element can be used to convert the NO x  present in exhaust gas into other compounds, such as nitrogen, water, and carbon dioxide. A selective catalytic reduction on filter (SCR+F) element combines SCR and DPF functionality such that NO x  reduction and particulate matter filtration and oxidation can occur in a single element. 
     Typically, diesel exhaust fluid (DEF)—a solution of urea and deionized water—is injected upstream of the SCR element to provide ammonia, which acts as a reducing agent and reacts with the NO x  in the presence of the SCR catalyst in accordance with Equation (1):
 
NO+NO 2 +2NH 3 →2N 2 +3H 2 O  (1)
 
The NO x  and ammonia are thus converted into nitrogen and water.
 
     SUMMARY 
     A diesel oxidation catalyst (DOC) is typically provided upstream of a SCR and DPF or a SCR+F element. The DOC includes one or more precious group metals (e.g., platinum, palladium, etc.) that act as a catalyst to reduce emission of carbon monoxide, hydrocarbons, and volatile organic compounds. The DOC also oxidizes NO to NO 2 , which promotes faster SCR reactions at exhaust temperatures above 250 degrees Celsius. However, at low temperatures (e.g., about 200 degrees Celsius or less) that occur during a cold start state of the engine or during very cold ambient operating conditions, the DOC will consume NO 2  by reacting NO 2  with carbon monoxide and hydrocarbons in the exhaust gas. This reduces the efficacy of downstream SCR or SCR+F elements, which require the presence of NO 2 . The DOC also adds thermal mass to the exhaust gas treatment system, which delays warm-up of the SCR or SCR+F elements. In addition, injecting DEF into exhaust gas at low temperatures can result in the undesirable formation of urea deposits. 
     Low temperature NO x  reduction is an increasingly important consideration as emissions regulations become more stringent. Accordingly, a need exists for an exhaust gas treatment system able to effectively reduce NO x  from exhaust gas at low temperatures. 
     The present disclosure provides, in another aspect, a method of treating exhaust gas from an internal combustion engine. The method includes sensing a temperature of the exhaust gas with a temperature sensor, comparing the sensed temperature with a threshold temperature, injecting a first reductant into the exhaust gas at a first location if the sensed temperature is less than the threshold temperature, and injecting a second reductant having a different composition than the first reductant into the exhaust gas at a second location if the sensed temperature is greater than the threshold temperature. 
     The present disclosure provides, in another aspect, a method of treating exhaust gas from an internal combustion engine. The method includes injecting an ammonium carbamate solution into the exhaust gas at a first location upstream of a first treatment element during a cold temperature operating state, injecting diesel exhaust fluid (DEF) into the exhaust gas at a second location between the first treatment element and a second treatment element located downstream of the first treatment element during a warm temperature operating state, converting DEF into additional ammonium carbamate solution in a reaction chamber during the warm temperature operating state, and storing the converted ammonium carbamate solution for use during the cold temperature operating state. 
     The present disclosure provides, in another aspect, a method of treating exhaust gas from an internal combustion engine. The method includes storing diesel exhaust fluid (DEF) in a reservoir, transferring a quantity of the DEF to a reaction chamber, heating the reaction chamber to an elevated temperature to convert at least a portion of the DEF to ammonium carbamate, storing the ammonium carbamate in the reaction chamber; and actuating a valve to select one of the stored ammonium carbamate and the stored DEF for injection into the exhaust gas. 
     Other features and aspects of the disclosure will become apparent by consideration of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a vehicle in which the disclosed system and method for regulating exhaust emissions may be implemented. 
         FIG. 2  is a schematic diagram of an exhaust gas treatment system according to one embodiment. 
         FIG. 3  is a block diagram of an electronic control unit of the exhaust gas treatment system of  FIG. 2 . 
         FIG. 4  is a schematic diagram of an exhaust gas treatment system according to another embodiment. 
     
    
    
     Before any embodiments are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The disclosure is capable of supporting other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary vehicle  10  including a diesel-powered internal combustion engine  14  and an exhaust gas treatment system  100  according to one embodiment. The illustrated vehicle  10  is a utility tractor, but the exhaust gas treatment system  100  is not so limited in application and can be used in conjunction with any diesel-powered internal combustion engine. For example, the exhaust gas treatment system  100  can be used in other work vehicles, passenger vehicles, or other equipment powered by a diesel engine (e.g., generators, compressors, pumps, and the like). 
     With reference to  FIG. 2 , the exhaust gas treatment system  100  includes an exhaust pathway  104  (e.g., an exhaust pipe) having an inlet or upstream side  108  and an outlet or downstream side  112 . A first treatment element  116 , a second treatment element  120 , and a third treatment element  124  are located in series along the exhaust pathway  104 , between the inlet  108  and the outlet  112 . A first transition pipe  126   a  couples the engine  14  and the first treatment element  116 , a second transition pipe  126   b  couples the first treatment element  116  and the second treatment element  120 , and a third transition pipe  126   c  couples the second treatment element  120  and the third treatment element  124 . The numeric designations “first,” “second,” etc. are used herein for convenience and should not be regarded as defining order, quantity, or relative position. In the illustrated embodiment, the second treatment element  120  is an intermediate treatment element disposed between the first treatment element  116  and the third treatment element  124 ; however, in other embodiments, the relative positions of the treatment elements  116 ,  120 ,  124  may vary. 
     The first treatment element  116  in the illustrated embodiment includes a first selective catalytic reduction (SCR) element  130 . The first SCR element  130  may include, for example, a catalytic washcoat on a monolithic support material, such as ceramic. The washcoat may include one or more metal-based catalysts, such as a copper-based catalyst, and iron-based catalyst, a vanadium-based catalyst, or the like, coated and extruded. Alternatively or additionally, the washcoat may include one or more zeolites. The first SCR element  130  may be used to reduce NO x  from exhaust gas passing through the first SCR element  130 . In other embodiments, the first treatment element  116  may include a different exhaust treatment configuration. 
     The illustrated second treatment element  120  includes a diesel oxidation catalyst (DOC)  134  having, for example, a honeycomb support coated with a catalytic material containing one or more precious metals, such as a platinum group metal. The DOC  134  may be used to reduce carbon monoxide and hydrocarbons from exhaust passing through the DOC  134 . The second treatment element  120  also includes a diesel particulate filter (DPF)  138  having porous filter media configured to capture particulate matter and oxidize soot from the exhaust. The illustrated DPF  138  is coupled to the DOC  134  downstream of the DOC  134 . In other embodiments, the second treatment element  120  may include a different exhaust treatment configuration. 
     The third treatment element  124  in the illustrated embodiment includes a second SCR element  142  and an ammonia oxidation catalyst (AOC)  146 . Like the first SCR element  130 , the second SCR element  142  may include a catalytic washcoat on a monolithic support material. The first and second SCR elements  130 ,  142  may use the same or different washcoat compositions. In some embodiments, the second SCR element  142  has a larger NO x  reduction capacity than the first SCR element  130 , but the relative NO x  reduction capacities of the SCR elements  130 ,  142  may vary. The second SCR element  142  and the AOC  146  are positioned in series, with the AOC  146  located downstream of the second SCR element  142 . The AOC  146  may be used to convert excess ammonia leaving the SCR element  142  (e.g., to nitrogen and water). In some embodiments, the AOC  146  may be omitted. Alternatively, the AOC  146  may be provided as a separate treatment element positioned downstream of the third treatment element  124 . 
     With continued reference to  FIG. 2 , the exhaust gas treatment system  100  includes reaction chamber  150  that generates and stores a first reductant and a reservoir  154  that stores a second reductant having a different composition than the first reductant. The reaction chamber  150  is made of a non-reactive, corrosion-resistant material such as stainless steel and is constructed to withstand sustained high internal pressure. In the illustrated embodiment, the reaction chamber  150  is fluidly coupled to the reservoir  154  to receive a quantity of the second reductant from the reservoir and to selectively convert the second reductant into the first reductant. The second reductant is a urea-based solution, which is preferably diesel exhaust fluid (DEF), and the first reductant is a solution containing ammonium carbamate (NH 2 COONH 4 ). The reaction chamber  150  includes a heater (not shown) that can heat the second reductant in the reaction chamber  150  to an elevated temperature (e.g., about 150 degrees Celsius or more) at which hydrolysis of the urea occurs, in accordance with Equation (2):
 
CO(NH 2 ) 2 +H 2 O→NH 2 COONH 4   (2)
 
The heater preferably includes one or more electric heating elements powered by the engine  14 , via the electrical system of the vehicle  10 . However, the heater may use any other suitable means for heating the reaction chamber  150 .
 
     The reaction chamber  150  and the reservoir  154  are each selectively fluidly coupled to an inlet  158  of a pump  162  via a valve  166 . The valve  166  is a directional control valve that is actuatable between a first position in which the valve  166  fluidly communicates the reaction chamber  150  with the pump  162  and a second position in which the valve  166  fluidly communicates the reservoir  154  with the pump  162 . The valve  166  is preferably a solenoid-actuated valve capable of remote actuation. The pump  162  can thus draw the first reductant from the reaction chamber  150  or the second reductant from the reservoir  154 , depending on the position of the valve  166 . In some embodiments, the pump  162  may be a variable speed pump. 
     With continued reference to  FIG. 2 , the exhaust gas treatment system  100  further includes a first injector  170  and a second injector  174  coupled to the exhaust pathway  104 . In the illustrated embodiment, the pump  162  includes a first outlet  178  fluidly coupled to the first injector  170  and a second outlet  182  fluidly coupled to the second injector  174 . The pump  162  may include an integrated directional control valve configured to direct the output flow of the pump to the first injector  170 , the second injector  174 , or both. Alternatively, the pump  162  may be provided with a single outlet, and flow to the first and second injectors  170 ,  174  may be controlled by one or more valves downstream of the pump  162 . The first injector  170  is positioned to introduce reductant into the first transition pipe  126   a , upstream of the first treatment element  116 . The second injector  174  is positioned to introduce reductant into the third transition pipe  126   c , downstream of the second treatment element  120  and upstream of the third treatment element  124  (i.e. between the first and third treatment elements  116 ,  124 ). 
     An electronic control unit (ECU  186 ) is provided to actively control various aspects of the operation of the exhaust gas treatment system  100 .  FIG. 3  illustrates an example of the ECU  186 . The ECU  186  includes a plurality of electrical and electronic components that provide power, operational control, and protection to the components and modules within the ECU  186 . In particular, the ECU  186  includes, among other things, an electronic processor  187  (e.g., a programmable microprocessor, microcontroller, or similar device), non-transitory, machine-readable memory  188 , and an input/output interface  189 . The electronic processor  187  is communicatively coupled to the memory  188  and configured to retrieve from memory  188  and execute, among other things, instructions related to the control processes and methods described herein. In other embodiments, the ECU  186  includes additional, fewer, or different components. 
     The ECU  186  is communicatively coupled (e.g., via a wired or wireless connection to the input/output interface  189 ) to one or more sensors  190 , which provide feedback to the ECU  186  indicative of the operating state and performance of the exhaust gas treatment system  100 . In the illustrated embodiment, the system  100  includes a first temperature sensor  190   a , a second temperature sensor  190   b , a first NO x  sensor  190   c , and a second NO x  sensor  190   d  ( FIG. 2 ). In some embodiments, additional or alternative sensors may be provided to monitor various other parameters of the exhaust gas treatment system  100 . These sensors may monitor, for example, NO x  concentrations, ammonia concentrations, temperature, exhaust flow rate, pressure drop, ash loading, etc. at one or more points along the exhaust pathway  104 . In the illustrated embodiment, the ECU  186  is also communicatively coupled to the pump  162 , the valve  166 , and the reaction chamber  150  (via the input/output interface  189 ) for controlling their operation. The ECU  186  may also be communicatively coupled to the injectors  170 ,  174  for modulating the flow of reductant through each of the injectors. In addition, the ECU  186  may be configured to communicate with external systems including, for example, engine controls and/or operator controls. 
     In operation, untreated exhaust from the internal combustion engine  14  is directed into the exhaust pathway  104  at the inlet  108  ( FIG. 2 ). The ECU  186  periodically or continuously monitors the temperature sensors  190   a ,  190   b  in order to determine the temperature of the exhaust gas as it enters the first treatment element  116  and the second treatment element  124 , respectively. The ECU  186  compares the sensed temperature from each sensor  190   a ,  190   b  with a threshold temperature stored in the memory  188 . If the sensed temperatures are greater than the threshold temperature, which is expected during ordinary operation, the ECU  186  controls the exhaust gas treatment system  100  according to a warm temperature operating routine. If one or both of the sensed temperatures is less than the threshold temperature, which may occur, for example, after cold-starting the engine  14  or when operating the vehicle  10  in cold ambient conditions, the ECU  186  controls the exhaust gas treatment system  100  according to a cold temperature operating routine. In the illustrated embodiment, the threshold temperature is about 200 degrees Celsius, which corresponds to the minimum temperature at which DEF can be introduced into the exhaust pathway  104  without significant urea deposition. 
     When executing the warm temperature operating routine, the ECU  186  actuates the valve  166  to the second position. The ECU  186  then activates the pump  162 , which draws DEF directly from the reservoir  154 . In some embodiments, the ECU  186  controls the pump  162  to only supply DEF to the second injector  174 , such that substantially no DEF flows to the first injector  170 . Alternatively, the ECU  186  may control the pump  162  to supply DEF to both the first and second injectors  170 ,  174 . 
     Untreated exhaust flows from the engine  14  into the first treatment element  116 , which reduces NO x  in the presence of the catalyst in the first SCR element  130  to form nitrogen and water. The exhaust then flows into the second treatment element  120 . The DOC  134  reduces carbon monoxide and hydrocarbons from the exhaust, and the DPF  138  captures particulate matter from the exhaust. At warm temperatures, the DOC  134  also increases the fraction of NO 2  in the exhaust, which promotes oxidation of soot on the filter media of the DPF  138 . 
     The DEF is injected into the partially-treated exhaust downstream of the second treatment element  120  via the second injector  174 . The DEF and exhaust mixture then enters the third treatment element  124 . The DEF decomposes to produce ammonia, which reacts with NO x  in the presence of the catalyst in the second SCR element  142  to form nitrogen and water. Any unreacted ammonia is subsequently oxidized in the AOC  146 . The treated exhaust then exits the exhaust gas treatment system  100  through the outlet  112 . 
     The ECU  186  may vary the flow rate of DEF through one or both of the injectors  170 ,  174  based on NO x  concentration data received by the ECU  186  from the NO x  sensors  190   c ,  190   d  and on predictive modeling of the combustion process in the engine  14 . For example, if the NO x  sensor  190   d  detects an elevated level of NO x  in the treated exhaust downstream of the third treatment element  124 , the ECU  186  may command the pump  162  to increase the flow rate of DEF injection through either or both the first and second injectors  170 ,  174 . Because proper soot oxidation in the DPF  138  requires the presence of NO 2 , the amount of reductant flowing through the first injector  170  is limited so that some of the NO 2  in the exhaust remains unreacted through the first treatment element  116 . 
     During the warm temperature operating routine, the ECU  186  may command the reaction chamber  150  to generate ammonium carbamate. The ECU  186  may initiate generation of ammonium carbamate based on feedback indicating that the available supply of ammonium carbamate must be replenished. For example, the ECU  186  may receive feedback from a fluid level sensor in the reaction chamber  150 , a flow meter downstream of the reaction chamber  150 , a counter, a timer, a command received from a user of the vehicle  10 , or the like. Once the ECU  186  initiates ammonium carbamate generation, the reaction chamber  150  receives a quantity of DEF from the reservoir  154  (e.g., by opening a valve or operating a pump (not shown) to transfer DEF from the reservoir  154  into the reaction chamber  150 . The ECU  186  then seals the reaction chamber  150  and initiates a heating cycle to heat the contents of the reaction chamber  150  to an elevated temperature (e.g., at least 150 degrees Celsius). The DEF undergoes hydrolysis in accordance with Equation (2) and produces an ammonium carbamate solution, which then remains in the reaction chamber  150  for future use. 
     When executing the cold temperature operating routine, the ECU  186  actuates the valve  166  to the first position. The ECU  186  then activates the pump  162 , which draws the ammonium carbamate solution from the reaction chamber  150 . Ammonium carbamate solution advantageously has a lower freezing point than DEF and will remain liquid in cold weather. In extremely cold weather, however, should the ammonium carbamate solution freeze, the ECU  186  may active the heater of the reaction chamber  150  in order to thaw the ammonium carbamate solution. In some embodiments, the ECU  186  controls the pump  162  to only supply ammonium carbamate to the first injector  170 , such that substantially no ammonium carbamate flows to the second injector  174 . Alternatively, the ECU  186  may control the pump  162  to supply the ammonium carbamate to both the first and second injectors  170 ,  174 . 
     Untreated exhaust flows from the engine  14 , mixes with the ammonium carbamate, and then enters the first treatment element  116 . Ammonium carbamate decomposes at relatively low exhaust temperatures (i.e. above about 50 degrees Celsius) to form ammonia and carbon dioxide in accordance with Equation (3):
 
NH 2 COONH 4 →2NH 3 +CO 2   (3)
 
     The ammonia and NO x  react in the presence of the catalyst in the first SCR element  130  to form nitrogen and water. The exhaust then flows into the second treatment element  120 . The DOC  134  reduces carbon monoxide and hydrocarbons from the exhaust, and the DPF  138  captures particulate matter from the exhaust. Any remaining NO x  is reacted in the second SCR element  142 , while any unreacted ammonia is subsequently oxidized in the AOC  146 . The treated exhaust then exits the exhaust gas treatment system  100  through the outlet  112 . The ECU  186  may vary the flow rate of ammonium carbamate through one or both of the injectors  170 ,  174  based on NO x  concentration data received by the ECU  186  from the NO x  sensors  190   c ,  190   d . Because the ammonium carbamate solution that is transformed from DEF in the reaction chamber  150  does not change the solution density (i.e. there is no material loss), the ECU  186  need not make any control compensation to deliver the desired amount of ammonia for the SCR reactions when switching between DEF and the ammonium carbamate solution. 
     By selectively introducing ammonium carbamate into the exhaust pathway  104  instead of DEF at low temperatures, the exhaust gas treatment system  100  can achieve effective low temperature NO x  reduction. In addition, because the exhaust treatment system  100  can produce the ammonium carbamate on demand, the user need only supply the exhaust treatment system  100  with DEF, which is inexpensive and widely available. 
       FIG. 4  illustrates an exhaust gas treatment system  100 ′ according to another embodiment. The exhaust gas treatment system  100 ′ is similar to the exhaust gas treatment system  100  described above with reference to  FIG. 2 , except that the first treatment element  116  further includes an AOC  131  downstream of the first SCR element  130 . The AOC  131  may advantageously avoid conversion of excess ammonia into NO x  by the precious metal containing DOC  134  or any precious metals that have migrated from the DOC  134  and become entrained on the filter substrate of the DPF  138 . 
     Various features of the disclosure are set forth in the following claims.