Abstract:
This disclosure provides an exhaust flow detection and variable dosing system and method for treating exhaust flow from an engine. The system includes first and second exhaust flow legs, a cross passage connecting these legs upstream of SCRs and a sensor positioned along the cross passage to detect at least one of differential pressure between the exhaust flow legs, and exhaust flow in the cross passage. A dosing circuit connects a dosing treatment supply to each of the exhaust flow legs at or upstream of the SCRs, and at least one dosing device positioned along the dosing circuit to control the amount of the dosing agent delivered to each exhaust leg. An electronic control unit controls the amount of a dosing agent delivered to the exhaust flow legs independently based on exhaust flows determined for each leg using at least one of the differential pressure and cross passage exhaust flow.

Description:
TECHNICAL FIELD 
     The inventions relate to internal combustion engines with multi-leg after-treatment systems and, more particularly, to methods of detecting exhaust flow in each exhaust leg of a diesel engine during operation. 
     BACKGROUND 
     Environmental concerns have resulted in increasingly strict regulation of engine emissions by governmental agencies. For example, reduction of nitrogen-oxygen compounds (NOx) in exhaust emissions from internal combustion engines has become increasingly important and current indications are that this trend will continue. 
     Future emission levels of diesel engines will have to be reduced in order to meet Environmental Protection Agency (EPA) regulated levels. For example, proposed Ultra-Low Emissions Vehicle (ULEV) emission levels for light-duty vehicles up to model year 2004 are 0.2 gm/mi NOx and 0.08 gm/mi particulate matter (PM). Beginning with the 2004 model year, all light-duty Low Emission Vehicles (LEVs) and Ultra-Low Emission Vehicles (ULEVs) in California have to meet a 0.05 gm/mi NOx standard to be phased in over a three year period. In addition to the NOx standard, a full useful life PM standard of 0.01 gm/mi also have to be met. The EPA has also proposed tighter regulations for off-road diesel engines, requiring them to emit 90% less particulate matter and nitrogen oxides, by 2014 than they do today. 
     Traditional methods of in-cylinder emission reduction techniques such as exhaust gas recirculation (EGR) and injection rate shaping, by themselves will not be able to achieve these low emission levels required by the standards. After-treatment technologies will have to be used, and will have to be further developed in order to meet the future low emission requirements of the diesel engine. 
     A promising after-treatment technology designed to meet future NOx emission standards is Selective Catalytic Reduction (SCR) catalysts which have the potential to greatly reduce NOx emissions from internal combustion engines. Under some operating conditions SCR catalysts can reduce the level of NOx emitted from an internal combustion engine by as much as 60-90%. In SCR catalytic reduction systems, a reductant, such as urea, is introduced into the exhaust stream upstream of the catalyst chamber to react with the NOx on the surface of the precious metal catalyst to convert NOx into nitrogen and water vapor which is then released into the atmosphere. An accurate measurement of the exhaust flow is required in order to dose the correct amount of urea for obtaining the maximum NOx conversion efficiency with minimal NH3 slip at the tailpipe of the system. 
     SUMMARY OF THE INVENTION 
     The invention provides an exhaust flow detection and variable dosing system for treating exhaust flow from an engine, comprising a first exhaust flow leg positioned to receive the exhaust flow from the engine; a first selective catalytic reducer positioned along the first exhaust flow leg; a second exhaust flow leg positioned to receive the exhaust flow from the engine in parallel to the exhaust flow in the first exhaust flow leg; a second selective catalytic reducer positioned along the second exhaust flow leg; a cross passage connecting the first and the second exhaust flow legs upstream of the selective catalytic reducers to receive exhaust flow; and a sensor positioned along the cross passage to detect at least one of differential pressure between the first and the second exhaust flow legs, and exhaust flow in the cross passage. The sensor is adapted to generate a cross passage signal corresponding to at least one of the differential pressure and the cross passage exhaust flow. The system also includes a dosing treatment supply containing a dosing agent; a dosing circuit connecting the dosing treatment supply to each of the first and the second exhaust flow legs at or upstream of the first and the second selective catalytic reducers; at least one dosing device positioned along the dosing circuit to control the amount of the dosing agent delivered to each exhaust leg; and an electronic control unit adapted to receive the cross passage signal and independently control the amount of the dosing agent delivered to the first and the second exhaust flow legs based on the cross passage signal. 
     The system may also include an oxidation catalyst and a particulate filter in each of the first and the second exhaust flow legs upstream of a connection of the dosing circuit to the first and the second exhaust flow legs. The cross passage may be connected to the first and the second exhaust legs downstream of the particulate filter. The sensor may be differential pressure sensor connected to the cross passage at two locations, and the cross passage may include a venturi positioned between the two locations. The sensor may comprise at least one mass flow sensor. The electronic control unit may be adapted to generate a dosing control signal based on the cross passage signal to control the at least one dosing device to vary the amount of the dosing agent delivered to at least one of the first and the second exhaust flow legs. The electronic control unit may be adapted to determine an exhaust mass flow for each of the first and the second exhaust flow legs based on the cross passage signal, determine a NOx mass flow in each of the first and the second exhaust flow legs based on the respective exhaust mass flow, and control the amount of the dosing agent delivered to at least one of the first and the second exhaust legs based on the respective NOx mass flow. 
     A method of detecting exhaust flow and treating exhaust flow in an engine exhaust system having multiple exhaust legs is also provided that includes flowing exhaust gas through a first exhaust flow leg containing a first selective catalytic reducer; flowing exhaust gas through a second exhaust flow leg containing a second selective catalytic reducer and positioned in parallel to the exhaust flow in the first exhaust flow leg; detecting a differential pressure between the first and the second exhaust flow legs and generating a pressure signal corresponding to the differential pressure. The method also includes delivering an amount of a dosing agent to each of the first and the second exhaust flow legs based on the pressure signal. 
     The method may further include generating a dosing control signal based on the pressure signal to control the amount of the dosing agent delivered to at least one of the first and the second exhaust flow legs, and determining an exhaust mass flow for each of the first and the second exhaust flow legs based on the pressure signal. The method may further include determining a NOx mass flow in each of the first and the second exhaust flow legs based on the respective exhaust mass flow, and controlling the amount of the dosing agent delivered to at least one of the first and the second exhaust legs based on the respective NOx mass flow. The method may further include a venturi connected to both the first exhaust flow leg and the second exhaust flow leg, and a pressure sensor to detect the differential pressure across the venturi. 
     The invention also provides a method of detecting exhaust flow and treating exhaust flow in an engine exhaust system having multiple exhaust legs, comprising flowing exhaust gas through a first exhaust flow leg containing a first selective catalytic reducer; flowing exhaust gas through a second exhaust flow leg containing a second selective catalytic reducer and positioned in parallel to the exhaust flow in the first exhaust flow leg; providing a cross passage connecting the first and the second exhaust flow legs upstream of the selective catalytic reducers to receive exhaust flow; detecting a mass flow of the exhaust gas flowing in the cross passage and generating a mass flow; and delivering an amount of a dosing agent to each of the first and the second exhaust flow legs based on the mass flow signal. The method may also include generating a dosing control signal based on the mass flow signal to control the amount of the dosing agent delivered to at least one of the first and the second exhaust flow legs. The method may further include determining an exhaust mass flow for each of the first and the second exhaust flow legs based on the mass flow signal, determining a NOx mass flow in each of the first and the second exhaust flow legs based on the respective exhaust mass flow, and controlling the amount of the dosing agent delivered to at least one of the first and the second exhaust legs based on the respective NOx mass flow. 
     Advantages and features of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention when viewed in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a multi leg urea dosed after-treatment system according to an exemplary embodiment with DOC, (Diesel Oxidation Catalyst), DPF (Diesel Particulate Filter), and SCR (Selective Catalytic Reduction chamber) used to reduce NOx emission produced by an internal combustion engine; and 
         FIG. 2  is a schematic block diagram of another exemplary embodiment similar to the embodiment of  FIG. 1  except one or more mass air flow sensors are used to detect the exhaust flow in the cross passage. 
     
    
    
     DETAILED DESCRIPTION 
     Applicant has recognized the need to more precisely control the amount of reductant, e.g. urea or ammonia, injected into each leg of a multi-leg exhaust after-treatment system. If too much reductant is injected, the reductant may undesirably pass through the system to the atmosphere thereby increasing operation costs while also possibly contaminating the environment with increased slip of NH3 from the tailpipe of the system into the atmosphere. In addition, excess reductant or dosing agent may accumulate in the catalyst in a manner which deters the conversion effectiveness of the catalyst. On the other hand, an inadequate amount of reductant supplied to the SCR chamber may reduce emissions conversion by the SCR device possibly resulting in emissions noncompliance. The exemplary embodiments consistent with the claimed inventions provide an engine having an exhaust after-treatment system that maintains minimal variability in the NOx emissions exiting the exhaust system thereby maintaining desirable emissions conversion efficiency for emissions compliance and minimizing associated fuel economy penalties, while minimizing the release of un-reacted reductant into the atmosphere. 
     Referring now to  FIG. 1 , illustrated is a schematic block diagram of an exemplary embodiment of an internal combustion engine  100  with a multi-leg exhaust gas after-treatment system  200  for implementing the method consistent with the claimed inventions. System  200  may be used to remove particulates and various chemical compounds from exhaust gas created by internal combustion engine  100 . System  200  includes exhaust legs  600  and  700 , connected to the engine to receive exhaust gas from the engine  100 . Exhaust legs  600 ,  700  may be connected downstream of a common exhaust manifold. Also engine  100  may include one or more turbochargers. Each leg  600 ,  700  includes a diesel oxidation catalyst (DOC)  630 ,  730  and a diesel particulate filter (DPF)  632 ,  732  positioned downstream of the respective DOC  630 ,  730 . DOC  630 ,  730  and DPF  632 ,  732  may be any conventional DOC and DPF devices well known in the art. Each leg  600 ,  700  also includes a selective catalytic reduction (SCR) device  606 ,  706  positioned downstream of the respective DPF  632 ,  732  for removing NOx emissions from the exhaust gas. Each SCR device  606 ,  706  includes an absorber or catalyst, such as a manufactured from various ceramic materials used as a carrier, such as titanium oxide, and active catalytic components such as either oxides of base metals (such as vanadium and tungsten), zeolites, or various precious metals. 
     In addition, a dosing system is provided in the form of a dosing treatment supply  602  and dosing devices, i.e. injectors,  604 ,  704 , fluidly connected to dosing treatment supply  602  by respective dosing circuits  610 ,  710 . The dosing system may also include a pump (not shown). A reductant, such as ammonia or urea, stored in dosing treatment supply  602 , is selectively and controllably added to the exhaust flow upstream of the SCR devices  606 ,  706  and absorbed onto the SCR catalyst where it is used to convert the NOx emissions in the exhaust gas flow to nitrogen and water, and in the case of urea, also into carbon dioxide. The predetermined amount of reductant to be injected into one leg of the system  200  may be delivered in a particular rate shape, such as disclosed in U.S. Pat. No. 7,587,890, the entire contents of which is hereby incorporated by reference. 
     An electronic control module or unit  800  is used to process data received from various sensors, detectors, and components relating to engine and exhaust system conditions, and to generate control signals based on this information and perhaps other information, for example, stored in a database or memory integral to or separate from ECU  800 . ECU  800  may include a processor and software modules or routines that are executable by the ECU  800 . In alternative embodiments, ECU  800  may include electronic circuits for performing some or all of the processing, including analog and/or digital circuitry. The modules may comprise a combination of electronic circuits and microprocessor based components. Electronic control unit  800  may be integral with the engine control unit, or exist as a separate component. For example, ECU  800  may receive data indicative of engine performance, and exhaust gas composition including but not limited to engine sensor data, such as engine position sensor data, speed sensor data, exhaust mass flow sensor data, fuel rate data, pressure sensor data, temperature sensor data from locations throughout the engine and the exhaust system, NOx sensor data, and other data, all indicated generally as sensor data inputs  804 . ECU  800  may then generate control signals or outputs  802  to control various components in the engine and exhaust system. In particular, in the exemplary embodiments consistent with the claimed inventions, ECU  800  is operably connected to dosing devices  604 ,  704  by wired or wireless connections  640 ,  740  so that a control signal generated by ECU  800  is transmitted to devices  604 ,  704  to control the level or amount of reductant dosing. 
     Exhaust after-treatment system  200  may include various sensors and detectors for sensing and generating data relating to aspects of the exhaust system. For example, each leg may be equipped with an unspent urea sensor  624 ,  724  detecting the amount of urea present in the exhaust flow downstream of SCR device  606 ,  706  and reporting this information to ECU  800 . Sensors  608 ,  708  are NOx emissions sensors to detect the amount of NOx emissions in the exhaust stream. Other sensors  612 ,  614 ,  616 ,  618 ,  620 , and  622  may be provided at various locations along the exhaust system. However, these and other sensors may include pressure sensors, lambda sensors and mass air flow sensors. Each sensor position can be equipped with more than one sensor so as to provide redundant data affirming a more accurate operation, or redundancy in its basic form, to assure a lower failure rate. Each sensor connects to ECU  800  to report its data via a data signal as an input shown generally at  804 . 
     Most importantly, in the exemplary embodiment of  FIG. 1 , a feature is incorporated to detect a differential pressure between exhaust legs  600  and  700 . Specifically, the feature includes a cross passage  500  having one end communicating with exhaust leg  600  upstream of SCR device  606  and an opposite end communicating with exhaust leg  700  upstream of SCR device  706 , and a pressure differential sensor  502  positioned to measure the pressure drop/difference across cross passage  500 . Cross passage  500  is preferably sized and dimensioned with a cross-sectional flow area smaller than the cross-sectional flow area of each of exhaust legs  600 ,  700 . For example, the cross-sectional flow area of cross passage  500  may be approximately 0.01-0.04 m 2 . As a result, cross passage  500  permits only a small amount of exhaust gas to flow through cross passage  500 . An orifice or venturi  501  may be provided along cross passage  500  to create a measurable pressure drop based on the pressure difference between exhaust legs  600  and  700 . Pressure differential sensor  502  may be fluidly connected to cross passage  500  on either side of orifice or venturi  501  to enable differential pressure sensing. In another exemplary embodiment, cross passage  500  may be sized small enough to create a pressure drop without the need for a specific orifice or venturi. Pressure differential sensor  502  may be any sensor or detector capable of sensing the pressure difference across cross passage  500 , i.e. across orifice or venturi  501 . For example, in another exemplary embodiment, a pressure differential sensor, such as a diaphragm type sensor, may be positioned within cross passage  500 . Regardless of the type of sensor, a data signal representative of the pressure differential is delivered to ECU  800 . 
       FIG. 2  illustrates another exemplary embodiment of the exhaust after-treatment system and method which is similar to the embodiment of  FIG. 1  except a mass air flow sensor  504  is used in cross passage  500  instead of pressure differential sensor  502 . The mass flow sensor  504  is positioned in cross passage  500  to measure or detect the exhaust gas flow between legs  600  and  700 . Mass flow sensor  504  may be of the type that works more effectively at lower temperatures. In this case, a cooling mechanism  503 , such as a jacket of engine coolant or engine oil, may be provided along cross passage  500  to reduce the exhaust gas temperature in cross passage  500 . Mass flow sensor  504  may be either made using the hot-wire anemometer principle of operation or could utilize any other mechanism such as the use of ultrasonic or surface acoustic-wave type to measure flow. 
     During operation, exhaust gas is generated by engine  100  and flows into respective legs  600 ,  700  of exhaust after-treatment system  200 . The exhaust gas continues in each leg flowing through DOC  630 ,  730 , DPF  632 ,  732 , and then through SCR devices  606 ,  706  before exiting the system through the exhaust outlet, or common tail pipe, depicted as “exhaust out”. In order to dose the accurate amount of urea needed to convert NOx emissions and maintain SCR conversion efficiency at an optimum level, a volumetric estimate of the amount of NOx in the exhaust is needed for an accurate dosing calculation. Therefore, an accurate measurement of the exhaust flow, from which to calculate the amount of NOx, is needed. In addition, the exhaust flow in each leg  600 ,  700  will be different, and will vary throughout operation due to the presence of the DPFs  632 ,  732  and other components that likely cause a difference in back pressure in the legs and thus a difference in the amount of exhaust flow through the legs. For example, the exhaust flow through DPFs  632 ,  732  will vary as the filter element becomes loaded with particulate matter creating more back pressure and then are regenerated resulting in less back pressure. Dosing the same amount of reductant, i.e. urea, to each leg with the assumption that the exhaust flow is the same in each leg throughout operation would necessarily result in less than optimum NOx conversion. System  200  provides an accurate estimate of the flow difference between the legs, thus permitting determination of the exhaust flow, and consequently the NOx flow, in each leg thereby ensuring accurate reductant dosing into both legs. 
     Specifically, system  200  and the associated method permits a small amount of exhaust gas to enter cross passage  500  for sensing by differential pressure sensor  502  or mass air flow sensor  504 . In other embodiments, the sensors  502  and  504  could be used in combination and/or multiple sensors used for redundancy or to provide numerous pressure and/or flow data for ECU  800  to process. The pressure differential signal generated by differential pressure sensor  502 , or the mass flow signal generated by mass flow sensor  504 , is sent to ECU  800  and processed using standard equations and known engine data to determine the NOx mass flow which is then used to control and adjust the reductant dose for each leg independently thereby achieving optimum NOx conversion without excess reductant use. More specifically, a standard volumetric efficiency calculation for the particular engine provides the total exhaust mass flow, e.g., 10 kg/min, out of engine  100  upstream of both legs  600 ,  700 . The difference in back pressure in the legs causes the exhaust flow to be different in the legs so that the exhaust flow is not necessarily the same, e.g. 5 kg/min, but some other split, such as 7 kg/min in leg  600  and 3 kg/min in leg  700 . Sensors  502  and  504  provide a measurement of the amount of the respective measured characteristic, i.e. differential pressure, such as 0.5 kPa, or mass flow, such as 4 kg/min, and also provide an indication of the leg having the higher pressure or flow by sensing the direction of the flow within cross passage  500 . In order to determine the exhaust flow in each leg, ECU  800  uses the pressure differential data from sensor  502 , or mass flow data from sensor  504 , the indication of the leg having the higher pressure or flow and/or the leg having the lower pressure or flow, and standard flow orifice equations, to determine the exhaust mass flow in each leg. The exhaust flow value for each leg can then be used by ECU  800  to determine the NOx mass flow, for example, using the following equation: 
                 m   .       NO   x       =       NOx   ppm     *       m   .     Exhaust     ⁢       MW   NOx       MW   Exhaust               
where, {dot over (m)} NOx  is the NOx mass flow in g/sec; {dot over (m)} Exhaust  is the Exhaust flow in g/sec; MW NOx  is the molecular weight of NO 2 ; MW Exhaust  is the molecular weight of Exhaust gas; NOx ppm  is the NOx concentration in the exhaust gas as measured by the NOx sensor  608 ,  708  in ppm
 
     ECU  800  then uses the NOx mass flow value for each leg to determine the dosing requirement for each leg using a control system design that adjusts the dosing based upon the measurements of NOx, NH3, urea and the exhaust flow in accordance with known control concepts, such as described in US Patent Application Publication 2010/0229531. ECU  800  then generates control signals and sends the signals via circuits  640 ,  740  to injectors  604 ,  704  to control the injectors in such a manner to inject the desired target dosing quantity/amount of reductant, i.e. urea. The system may adjust the dosing in small increments or decrements to achieve a more balanced application of dosing agent in real time. There may be governing limits as to the amount of each increase (increment) and the amount of each decrease (decrement) in dosing. The process loops or continually operates to recalculate exhaust flow, NOx flow, and desired dosing amounts or adjustments for each leg and then controls the dosing amounts for each leg independently, all based on the latest pressure differential from sensor  502  and/or mass flow signal from sensor  504 , and other real time data, such as NOx emissions signals from sensors  608 ,  708 , thereby providing real time reductant dosing control and NOx conversion management to maintain NOx emissions within acceptable limits. 
     While various embodiments in accordance with the present invention have been shown and described, it is understood that the invention is not limited thereto. The present invention may be changed, modified and further applied by those skilled in the art. Therefore, this invention is not limited to the detail shown and described previously, but also includes all such changes and modifications.