Patent Publication Number: US-7591131-B2

Title: Low pressure EGR system having full range capability

Description:
This invention was made with Government support under DOE Contract No. DE-FC05-00OR22806 awarded by the U.S. Department of Energy. Accordingly, the Government may have certain rights to this invention. 

   TECHNICAL FIELD 
   The present disclosure relates generally to an exhaust gas recirculation (EGR) system and, more particularly, to a low pressure exhaust gas recirculation system operable to recirculate exhaust gas back into an engine under a full range of conditions without throttling of intake air. 
   BACKGROUND 
   Internal combustion engines such as gasoline engines, diesel engines, and gaseous fuel-powered engines exhaust a complex mixture of air pollutants. These air pollutants are composed of solid particulate matter and gaseous compounds including nitrous oxides (NOx). Due to increased attention on the environment, exhaust emission standards have become more stringent and the amount of solid particulate matter and gaseous compounds emitted to the atmosphere from an engine is regulated depending on the type of engine, size of engine, and/or class of engine. 
   One method that has been implemented by engine manufacturers to comply with the regulation of these engine emissions has been to implement exhaust gas recirculation (EGR). EGR systems recirculate the exhaust gas by-products into the intake air supply of the internal combustion engine. The exhaust gas, which is redirected to a cylinder of the engine, reduces the concentration of oxygen therein, thereby increasing the heat capacity of the mixture and lowering the maximum combustion temperature within the cylinder. The lowered maximum combustion temperature and reduced oxygen slow the chemical reactions responsible for the formation of NOx, thereby reducing the amount of NOx emitted by the engine. In addition, the particulate matter entrained in the exhaust is burned upon reintroduction into the engine cylinder to further reduce the exhaust gas by-products. 
   One available type of EGR system is called a low pressure system. Low pressure EGR systems draw low pressure exhaust from downstream of an engine&#39;s turbine and direct the exhaust to a location upstream of the engine&#39;s compressor. An example of a low pressure EGR system was disclosed in U.S. Pat. Publication No. 2006/0156724 (the &#39;724 publication) by Dismon et al. on Jul. 20, 2006. Specifically, the &#39;724 publication disclosed an exhaust gas return system having a particulate trap located in series with and downstream of a turbine. The exhaust gas return system also has a catalyst located in series with and downstream of the particulate trap. Exhaust gas is drawn from a location between the particulate filter and the catalyst for return to an air inlet passageway upstream of a compressor. An exhaust gas return valve is disposed within an exhaust gas line between the particulate filter and the catalyst to control the flow rate of returned exhaust gases. 
   Although the low pressure exhaust gas return system of the &#39;724 publication may reduce the amount of NOx and particulate matter exhausted to the atmosphere, it may be limited. In particular, there may be some situations where the pressure differential between the exhaust and intake air is insufficient for proper operation. In other words, it is possible for the pressure of the recirculated exhaust to be substantially the same as or even lower than the pressure of the intake air. In these situations, the exhaust will flow poorly or not at all into the air inlet passageway. Without sufficient return of the exhaust, the engine&#39;s emissions may fail to be compliant with the environmental regulations. 
   Further, the disclosed placement of the exhaust gas return valve may be problematic. Specifically, because this valve is located within the exhaust gas line, the temperatures experienced by the valve may be excessive. These high temperatures may degrade the valve over time, possibly resulting in premature failure of the valve. 
   The disclosed EGR system is directed to overcoming one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   In one aspect, the present disclosure is directed to an exhaust treatment system for an engine. The exhaust treatment system may include an air induction circuit, an exhaust circuit, and an exhaust gas recirculation circuit. The air induction circuit may be configured to direct air into the engine. The exhaust circuit may be configured to direct exhaust from the engine and include a turbine driven by the exhaust, a particulate filter disposed in series with and located downstream of the turbine, and a catalytic device disposed in series with and located downstream of the particulate filter. The exhaust gas recirculation circuit may be configured to selectively redirect at least a portion of the exhaust from between the particulate filter and the catalytic device to the air induction circuit. The catalytic device is selected to create a backpressure within the exhaust circuit sufficient to ensure that, under normal engine operating conditions above low idle, exhaust can flow into the air induction circuit without throttling of the air directed into the engine. 
   In another aspect, the present disclosure is directed to a method of producing power. The method may include mixing intake air with fuel, and combusting the mixture to generate power and a flow of exhaust. The method may also include utilizing the exhaust to compress the intake air, and removing particulate matter from the exhaust. The method may also include catalyzing the exhaust to reduce a constituent of the exhaust, and redirecting the particulate-reduced exhaust to mix with the intake air. The step of catalyzing creates a backpressure within the exhaust sufficient to ensure that, under normal combustion conditions above low idle, the exhaust can be redirected to mix with the intake air without throttling of the intake air. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic illustration of an exemplary disclosed power unit. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a power unit  100  having an exhaust treatment system  102 . For the purposes of this disclosure, power unit  100  is depicted and described as a four-stroke diesel engine. One skilled in the art will recognize, however, that power unit  100  may be any other type of internal combustion engine such as, for example, a gasoline engine or a gaseous fuel-powered engine. Further, power unit  100  may be any other type of power and exhaust producing device such as, for example, or a furnace. Generally, power unit  100  may combust a fuel/air mixture to generate power and exhaust, and direct that exhaust to exhaust treatment system  102 . Exhaust treatment system  102  may receive the exhaust, treat the exhaust, and direct the exhaust into the atmosphere. 
   Power unit  100  may include an engine block  104  that at least partially defines a plurality of combustion chambers  106  in fluid communication with both an intake manifold  108  and an exhaust manifold  110 . In the illustrated embodiment, power unit  100  includes four combustion chambers  106 . However, it is contemplated that power unit  100  may include a greater or lesser number of combustion chambers  106  and that combustion chambers  106  may be disposed in an “in-line” configuration, a “V” configuration, or any other suitable configuration. 
   Power unit  100  may compress a mixture of fuel and air, which is then controllably combusted to produce a power output and exhaust. Each combustion chamber  106  may receive fuel and air, house the combustion of the fuel and air, and direct exhaust resulting from the combustion process to exhaust manifold  110 . The exhaust may contain carbon monoxide, oxides of nitrogen, carbon dioxide, aldehydes, soot, oxygen, nitrogen, water vapor, and/or hydrocarbons such as hydrogen and methane. One skilled in the art will recognize that power unit  100  may include a plurality of other components such as a fuel tank, one or more fuel injectors, various control valves, a pre-combustion chamber, or other components consistent with the process of generating power and exhaust. 
   Intake manifold  108  may have one or more inlet ports, and direct air or a mixture of air and other gases from a passageway in fluid communication with the inlet ports to combustion chambers  106 . Similarly, exhaust manifold  110  may have one or more outlet ports, and direct exhaust from combustion chambers  106  to a passageway in fluid communication with the outlet ports. It is contemplated that power unit  100  may contain a plurality of intake and/or exhaust manifolds to direct air and exhaust to and from combustion chambers  106 , respectively. 
   Exhaust treatment system  102  may include an air induction circuit  112 , an exhaust circuit  114 , and an exhaust gas recirculation (EGR) circuit  116 . Air induction circuit  112  may draw air or a mixture of air and other gases into power unit  100  for combustion, which may produce power and exhaust. Exhaust circuit  114  may direct a portion of the exhaust from power unit  100  to the atmosphere, while EGR circuit  116  may recirculate the remaining portion of the exhaust from exhaust circuit  114  to air induction circuit  112 . 
   Air induction circuit  112  may include components that introduce charged air into combustion chambers  106  of power unit  100 . For example, air induction circuit  112  may include an air inlet port  118 , an intake passageway  120 , a compressor  122 , an intake fluid conduit  124 , and an air cooler  126 . It is contemplated that additional and/or different components may be included within air induction circuit  112  such as, for example, a wastegate, a bypass system, a control system, and other means known in the art for introducing charged air into combustion chambers  106 . 
   Air inlet port  118  may fluidly communicate with intake passageway  120 , and may be associated with an air cleaner to clean the air entering air induction circuit  112 . Intake passageway  120  may also fluidly communicate compressor  122  with air inlet port  118 . 
   Compressor  122  may be fluidly connected to the one or more inlet ports of intake manifold  108  via intake fluid conduit  124  to compress the air flowing into power unit  100 . Compressor  122  may embody a fixed geometry compressor, a variable geometry compressor, or any other type of compressor known in the art. It is contemplated that multiple compressors  122  may alternatively be included within air induction circuit  112  and disposed in a series or parallel relationship. It is further contemplated, however, that compressor  122  may be absent, if a naturally-aspirated engine is desired. 
   Air cooler  126  may facilitate the transfer of heat to or from the air compressed by compressor  122 , prior to the compressed air entering intake manifold  108 . For example, air cooler  126  may embody an air-to-air heat exchanger or a liquid-to-air heat exchanger. Air cooler  126  may include a tube and shell type heat exchanger, a plate type heat exchanger, or any other type of heat exchanger known in the art. In the embodiment exemplified by  FIG. 1 , air cooler  126  is disposed downstream of compressor  122  and upstream of intake manifold  108 . However air cooler  126  may alternatively be located upstream of compressor  122 , if desired. 
   Exhaust circuit  114  may include components that treat and fluidly direct the exhaust from combustion chambers  106 . For example, exhaust circuit  114  may include a turbine  128 , an exhaust fluid conduit  130 , an exhaust passageway  132 , a particulate filter  134 , a catalytic device  136 , and an exhaust port  138 . It is contemplated that exhaust circuit  114  may include additional and/or different components than those recited above such as, for example, one or more additional catalytic devices  150  disposed in a series or parallel relationship with catalytic device  136 , or any other exhaust circuit component known in the art. 
   Turbine  128  may receive the exhaust from combustion chambers  106  via exhaust fluid conduit  130 , which may be in fluid communication with the one or more outlets of exhaust manifold  110 . Turbine  128  may be connected to drive compressor  122 , with turbine  128  and compressor  122 , together, embodying a turbocharger. In particular, as the hot exhaust gases exiting power unit  100  expand against the blades (not shown) of turbine  128 , turbine  128  may rotate and drive compressor  122 . It is contemplated that more than one turbine  128  may alternatively be included within exhaust circuit  114  and disposed in a parallel or series relationship, if desired. The one or more turbines  128  may further be arranged in a turbocompounding configuration wherein at least one turbine is coupled with power unit  100  such that power produced by the turbine is returned to power unit  100 . For example, a turbine may be disposed in a series relationship with turbine  128  and mechanically, hydraulically, or electrically linked to the crankshaft (not shown) of power unit  100 . It is also contemplated that turbine  128  may be omitted and compressor  122  driven by power unit  100  mechanically, hydraulically, electrically, or in any other manner known in the art, if desired. 
   After exiting turbine  128 , the exhaust may be fluidly directed through exhaust passageway  132 . Particulate filter  134  may be disposed within exhaust passageway  132  downstream of turbine  128 . As exhaust from power unit  100  flows through exhaust passageway  132 , particulate filter  134  may remove particulate matter from the exhaust flow. Particulate filter  134  may include, among other things, a wire mesh or ceramic honeycomb filtration medium, or a wall-flow style filter. 
   Catalytic device  136  may also be disposed within exhaust passageway  132 , downstream of particulate filter  134 . Catalytic device  136  may include one or more substrates coated with or otherwise containing a liquid or gaseous catalyst such as, for example, a precious metal-containing washcoat. The catalyst may be utilized to reduce the by-products of combustion in the exhaust flow by means of, for example, selective catalytic reduction or NOx trapping. In one example, a reagent urea may be injected into the exhaust flow upstream of catalytic device  136 . The reagent may decompose to ammonia, which may react with the NOx in the exhaust gas across the catalyst to form H2O and N2. In another example, NOx in the exhaust gas may be trapped by a NOx trap, such as a barium salt NOx trap, and periodically be released and reduced across the catalyst to form CO2 and N2. Catalytic device  136  may also oxidize particulate matter that remains in the exhaust flow after passing through particulate filter  134 . 
   The size, thickness, and/or other parameters of catalytic device  136  may be chosen such that the backpressure produced from running the exhaust gas through it during operation of power unit  100  is sufficient to always drive some amount of the exhaust gas into EGR circuit  116 . For example, the minimum backpressure created by catalytic device  136  may be at least 1 kPa during normal operating conditions of power unit  100  above low idle. However, the size of catalytic device  136  may be preferably chosen such that the backpressure ranges from 10-30 kPa during rated power unit  100  operation. In a most-preferred embodiment, the size of catalytic device  136  may be chosen such that the backpressure ranges from 10-15 kPa during rated operation of power unit  100 . Normal operating conditions above low idle may include engine speeds ranging from above 700 rpm to about 2300 rpm. Rated operation of power unit  100  may be one or more conditions at which the manufacturer of power unit  100  guarantees a particular performance, and at which power unit  100  is designed to run most of the time and run optimally. This may correspond with one or more speeds and/or one or more torque outputs. For example, power unit  100  may have a rated operating speed of about 1800 rpm. Thus, the size of catalytic device  136  may be chosen such that the backpressure is at least 1 kPa when power unit  100  operates at greater than 700 rpm, and ranges from 10-15 kPa when power unit  100  operates at 1800 rpm. 
   Several environmental or contextual factors may affect the exact parameters of catalytic device  136  necessary to create the desired backpressure. These factors may include, without limitation, the operating temperature of power unit  100  and/or the ambient, the elevation of power unit  100  above sea level, the size of power unit  100 , the rated operation of power unit  100 , and the application of power unit  100 . The parameters of catalytic device  136  may further be dependent upon the components of EGR circuit  116 . For example, the length of the circuit and the size of the components included in the circuit may define a pressure drop in fluids that pass through the circuit. The value of the pressure drop may affect the desired backpressure created by catalytic device  136 , and thus the parameters of catalytic device  136  necessary to create the desired backpressure. In some situations, it may be necessary to place multiple catalytic devices  136 ,  150  in series to create this desired backpressure. The treated exhaust may then be fluidly directed through exhaust port  138  into the atmosphere. 
   EGR circuit  116  may redirect a portion of the exhaust flow of power unit  100  from exhaust circuit  114  into air induction circuit  112 . For example, EGR circuit  116  may include an EGR inlet port  140 , an EGR passageway  142 , an exhaust cooler  144 , an EGR outlet port  146 , and a mixing valve  148 . It is contemplated that EGR circuit  116  may include additional and/or different components such as a catalyst, an electrostatic precipitation device, a shield gas system, a particulate trap, and other means known in the art for redirecting exhaust from exhaust circuit  114  into air induction circuit  112 . 
   EGR inlet port  140  may be connected to exhaust circuit  114  to receive at least a portion of the exhaust flow from power unit  100 . Specifically, EGR inlet port  140  may be disposed downstream of turbine  128  to receive low pressure exhaust gas from turbine  128 . In the embodiment of  FIG. 1 , EGR inlet port  140  may also be located downstream of particulate filter  134 , but upstream of catalytic device  136 . It is contemplated that EGR inlet port  140  may alternatively be located upstream of particulate filter  134  to receive higher pressure exhaust if desired. However, in this configuration, a separate particulate trap within EGR passageway  142  may be required to reduce particulate matter in the recirculated exhaust. 
   EGR passageway  142  may fluidly connect EGR inlet port  140  to EGR outlet port  146 . Exhaust cooler  144  may be disposed within EGR passageway  142  to cool the portion of the exhaust flowing through EGR inlet port  140 . Exhaust cooler  144  may include, for example, a liquid-to-air heat exchanger, an air-to-air heat exchanger, or any other type of heat exchanger known in the art for cooling an exhaust flow. It is contemplated that exhaust cooler  144  may be omitted, if desired. 
   EGR outlet port  146  may be fluidly connected to mixing valve  148  to direct the exhaust flow from EGR passageway  142  through mixing valve  148  into intake passageway  120 . Mixing valve  148  may be fluidly connected to both EGR outlet port  146  and air induction circuit  112  to regulate the flow of exhaust from EGR circuit  116  and air from air inlet port  118 , respectively. Mixing valve  148  may include, for example, a butterfly valve element, a spool valve element, a check valve element, a gate valve element, a ball valve element, a globe valve element, or any other valve element known in the art. The valve element of mixing valve  148  may be movable between a flow-passing position and a flow-restricting position. The position of the valve element of mixing valve  148  between the flow-passing and flow-restricting positions may, at least in part, affect the amount of exhaust gas recirculated back into power unit  100 . More specifically, mixing valve  148  may selectively allow, block, or partially block the flow of exhaust from EGR passageway  142  into intake passageway  120 , thereby adjusting the air-to-exhaust ratio of gases passed into intake manifold  108 . Mixing valve  148  may be disposed within intake passageway  120  upstream of compressor  122 , so that the exhaust from EGR circuit  116  may be mixed with the air before the flow passes through compressor  122  and air cooler  126 . 
   INDUSTRIAL APPLICABILITY 
   The disclosed EGR system may be applicable to any engine where emission control is desired. The disclosed EGR system may embody a low pressure system that recirculates a portion of the exhaust from an engine back into the combustion chambers of the engine under normal engine operating conditions above low idle without throttling the intake air. The recirculated portion of the exhaust may create a lean burn condition that reduces NOx and particulate matter. The operation of power unit  100  will now be explained. 
   Atmospheric air may be drawn into air induction circuit  112  through air inlet port  118 , further passing through mixing valve  148 , and intake passageway  120 . The air may be mixed with recirculated exhaust at mixing valve  148  and may be directed through compressor  122  where it may be pressurized before entering intake manifold  108  of power unit  100 . The mixture may further pass through air cooler  126  prior to entering intake fluid conduit  124 , lowering the temperature of the air/exhaust mixture before it is combusted. 
   The cooled, pressurized, air/exhaust mixture may then be directed through intake manifold  108  to combustion chambers  106 . Fuel may be mixed with the cooled, pressurized, air before or after entering combustion chambers  106 , and combusted by power unit  100  to produce mechanical work output and a hot high-pressure exhaust flow containing gaseous compounds and solid particulate matter. The hot high-pressure exhaust flow may then be directed to turbine  128  via exhaust manifold  110  and exhaust fluid conduit  130 . As the exhaust enters turbine  128 , the expansion of hot exhaust gases may cause turbine  128  to rotate, thereby rotating connected compressor  122 . The rotation of turbine  128  may cause compressor  122  to rotate and compress the air/exhaust mixture in air induction circuit  112 , thereby facilitating movement of the mixture towards power unit  100  for subsequent combustion. 
   The work performed by the expansion of the exhaust gases on turbine  128  may reduce the pressure of the exhaust. More specifically, the exhaust downstream of turbine  128  may have a lower pressure than the exhaust upstream of turbine  128 . This lower-pressure exhaust flow may then be directed along exhaust passageway  132  to particulate filter  134 . Particulate filter  134  may remove some amount of the solid particulate matter from the exhaust flow. Substantially immediately after exiting particulate filter  134 , the exhaust gas flow may be divided into two flows, including a first flow directed to EGR circuit  116  and a second flow directed through catalytic device  136  to the atmosphere, catalytic device  136  serving to reduce the amount of NOx and/or further reduce particulate matter exhausted to the atmosphere. It is contemplated that the two flows of exhaust gas may alternatively be divided upstream of particulate filter  134 , if desired. 
   The exhaust gas may be driven through EGR inlet port  140 , at least in part, by the backpressure created by catalytic device  136 . Specifically, by choosing the parameters of catalytic device  136  appropriately with respect to the operational conditions of power unit  100 , a minimum backpressure of 1 kPa may be created by catalytic device  136  under normal power unit  100  operating conditions above low idle. In preferred embodiments of the present disclosure, the size of catalytic device  136  may be chosen to create a backpressure of 10-15 kPa during rated operation of power unit  100 . The backpressure may be sufficient to ensure that the exhaust gas is driven through EGR inlet port  140  without throttling the intake air. 
   As the first exhaust flow moves through EGR inlet port  140 , it may be directed to exhaust cooler  144 . The first exhaust flow may be cooled by exhaust cooler  144  to a predetermined temperature, which may further reduce the pressure of the exhaust gases in the first exhaust flow. The first exhaust flow may then be drawn through EGR outlet port  146  and mixing valve  148  back into air induction circuit  112  by compressor  122 . The recirculated exhaust flow may then be mixed with the air entering combustion chambers  106  for subsequent combustion. 
   The exhaust gas that is mixed with air and directed to combustion chambers  106  may reduce the concentration of oxygen therein, which in turn may increase the heat capacity of the mixture and lower the maximum combustion temperature within power unit  100 . The lowered maximum combustion temperature and reduced oxygen may slow the chemical reactions responsible for the formation of nitrous oxides, thereby reducing the amount of NOx emitted by power unit  100 . 
   The present disclosure may provide an EGR system and method of recirculating exhaust gas that, by directing the exhaust gas into EGR circuit  116  from upstream of a specifically sized catalytic device, guarantees the exhaust gas will be driven by pressure sufficient to ensure proper mixing of the recirculated exhaust gas and air under normal operating conditions above low idle. This guaranteed exhaust gas recirculation may eliminate the need for air throttling, thus increasing fuel efficiency and/or leading to a lean burn condition. In addition, by choosing to direct exhaust gas into EGR circuit  116  downstream of particulate filter  134 , particulate matter in the recirculated exhaust gas may be reduced or eliminated, which may improve power unit  100  performance, prolong the life of power unit  100 , and/or improve the quality of emissions from power unit  100 . 
   The present disclosure may also provide an EGR system and method of recirculating exhaust gas that prolongs the life of mixing valve  148 . More specifically, by placing mixing valve  148  within air induction circuit  112  downstream of exhaust cooler  144 , the temperature of exhaust gases entering mixing valve  148  may be controlled to minimize or eliminate the degrading effects of high temperatures on mixing valve  148 . 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed EGR system. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed EGR system. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.