Abstract:
A method for controlling injection of a reductant into an exhaust system of an internal combustion engine, which includes measuring temperature at a plurality of locations in the exhaust system relative to an SCR catalyst, determining an average temperature as a function of the measured temperatures, and controlling injecting of a reductant into the exhaust upstream of the catalyst as a function of the average temperature. The average temperature may be a weighted average where temperature measurements from at least some locations upstream of the SCR catalyst may be assigned greater weight than temperature measurements proximate the SCR catalyst.

Description:
BACKGROUND 
       [0001]    Selective catalytic reduction (SCR) is commonly used to remove NO x  (i.e., oxides of nitrogen) from the exhaust gas produced by internal engines, such as diesel or other lean burn (gasoline) engines. In such systems, NO x  is continuously removed from the exhaust gas by injection of a reductant into the exhaust gas prior to entering an SCR catalyst capable of achieving a high conversion of NO x . 
         [0002]    Ammonia is often used as the reductant in SCR systems. The ammonia is introduced into the exhaust gas by controlled injection either of gaseous ammonia, aqueous ammonia or indirectly as urea dissolved in water. The SCR catalyst, which is positioned in the exhaust gas stream, causes a reaction between NO x  present in the exhaust gas and a NO x  reducing agent (e.g., ammonia) to convert the NO x  into nitrogen and water. 
         [0003]    Proper operation of the SCR system involves precise control of the amount (i.e., dosing level) of ammonia (or other reductant) that is injected into the exhaust gas stream. If too little reductant is used, the catalyst will convert an insufficient amount of NOx. If too much reductant is used, a portion of the ammonia will pass unreacted through the catalyst in a condition known as “ammonia slip.” Thus, it is desirable to be able to detect the occurrence of “ammonia slip” conditions in order to regulate dosing levels. 
       SUMMARY 
       [0004]    Aspects and embodiments of the present technology described herein relate to one or more systems and methods for controlling injection of a reductant into an exhaust system of an internal combustion engine. The exhaust system includes an SCR catalyst that reacts with the reductant to reduce NOx in the engine&#39;s exhaust. The method includes measuring temperature at a plurality of locations in the exhaust system relative to the catalyst, determining an average temperature as a function of the measured temperatures, and controlling injecting of a reductant into the exhaust upstream of the catalyst as a function of the average temperature. In some embodiments, the average temperature may be a weighted average. In some embodiments, temperature measurements from at least some locations upstream of the SCR catalyst may be assigned greater weight than temperature measurements proximate the SCR catalyst. 
         [0005]    The exhaust system may include a diesel oxidation catalyst (DOC) interposed in the exhaust system between the engine and the SCR catalyst. In such configurations, the method may include measuring a temperature at an inlet of the DOC, measuring a temperature at an inlet of the SCR catalyst and measuring a temperature at an outlet of DOC. The average temperature may be a weighted average in which the temperature measurement at the inlet of the DOC is assigned a greater weighting than the measurements at the inlet and outlet of the SCR catalyst. 
         [0006]    In some embodiments, the method may modify reductant injection when the average temperature is outside of a predetermined range. In some embodiments, the method may reduce reductant injection when the average temperature is above a preselected threshold. 
         [0007]    In some embodiments, the system may include NOx particulate filter which comprises the SCR catalyst and a diesel particulate filter. 
         [0008]    Certain embodiments relate to a method of controlling the injection of a reductant into an exhaust system of an internal combustion engine, where the exhaust system includes an SCR catalyst that reacts with the reductant to reduce NOx in the engine&#39;s exhaust and a DOC located upstream of the SCR catalyst. The method measures temperature at a plurality of locations in the exhaust system, including at least an inlet of the DOC, an inlet of the SCR catalyst, and an outlet of the SCR catalyst. The method determines an average temperature as a function of the measured temperatures. In at least some embodiments, the average temperature may be a weighted average in which the temperature measurement from the DOC inlet is given a greater weight than temperature measurements from the inlet and outlet of the SCR catalyst. The method controls injection of reductant into the exhaust system as a function of the average temperature. 
         [0009]    Certain embodiments of the present technology relate to a system for controlling the injection of a reductant into an exhaust system of an internal combustion engine. The exhaust system includes an SCR catalyst that reacts with the reductant to reduce NOx in the engine&#39;s exhaust and a DOC located upstream of the SCR catalyst. The system includes a first temperature sensor which senses temperature at an inlet of the DOC and producing a first temperature signal responsive thereto. A second temperature sensor senses temperature at an inlet of the SCR catalyst and produces a second temperature signal responsive thereto. A third temperature sensor senses a temperature at an inlet of the SCR catalyst and produces a third temperature signal responsive thereto. A controller receives the temperature signals and controls injection of reductant into the exhaust system as a function of the temperature signals. In at least some embodiments, the controller regulates injection of reductant as a function of an average of the first, second and third temperature signals. In some embodiments, the average temperature is a weighted average, wherein the temperature measurement from the DOC inlet is given a greater weight than temperature measurements from the inlet and outlet of the SCR catalyst. In some embodiments, the controller reduces reductant injection when the average temperature is above a preselected threshold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic illustration of an internal combustion engine with an exhaust gas SCR system. 
           [0011]      FIG. 2  is flow chart of an exemplary method for detecting ammonia slip in an engine exhaust system according to certain embodiments of the present technology. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Various examples of embodiments of the present technology will be described more fully hereinafter with reference to the accompanying drawings, in which such examples of embodiments are shown. Like reference numbers refer to like elements throughout. Other embodiments of the presently described technology may, however, be in many different forms and are not limited solely to the embodiments set forth herein. Rather, these embodiments are examples representative of the present technology. Rights based on this disclosure have the full scope indicated by the claims. 
         [0013]      FIG. 1  shows an exemplary schematic depiction of an internal combustion engine  10  and an exhaust aftertreatment system  12 . The engine  10  can be used, for example, to power a vehicle such as an over-the-road vehicle (not shown). The engine  10  can be a compression ignition engine, such as a diesel engine, for example. The exhaust aftertreatment system  12  may include a diesel oxidation catalyst (DOC)  14  and a NO x  particulate filter (“NPF”)  16 . The NPF may consist of an SCR catalyst  18  and a diesel particulate filter (“DPF”)  20 . The SCR catalyst  18  is part of an SCR system  21  that also includes a reductant supply  22 , a reductant injector  24 , an electronic control unit (“ECU”)  26  and a plurality of sensors. In the illustrated embodiment, the sensors in the SCR system include an upstream NO x  detector  30 , a downstream NO x  detector  32  and a plurality of temperature sensors. In the illustrated embodiment, a first temperature sensor  36  is positioned near the inlet of the DOC  36 , a second temperature sensor  38  is positioned near the inlet of the NPF  16 , and a third temperature sensor  40  is positioned near the outlet of the NPF  16 . 
         [0014]    The ECU  26  controls delivery of a reductant, such as ammonia, from the reductant supply  22  and into an exhaust system  28  through the reductant injector  24 . The reductant supply  22  can include canisters (not shown) for storing ammonia in solid form. In most systems, a plurality of canisters will be used to provide greater travel distance between recharging. A heating jacket (not shown) is typically used around the canister to bring the solid ammonia to a sublimation temperature. Once converted to a gas, the ammonia is directed to the reductant injector  24 . The reductant injector  24  is positioned in the exhaust system  28  upstream from the catalyst  18 . As the ammonia is injected into the exhaust system  28 , it mixes with the exhaust gas and this mixture flows through the catalyst  18 . The catalyst  18  causes a reaction between NO x  present in the exhaust gas and a NO x  reducing agent (e.g., ammonia) to reduce/convert the NO x  into nitrogen and water, which then passes out of the tailpipe  34  and into the environment. While the SCR system  21  has been described in the context of solid ammonia, it will be appreciated that the SCR system could alternatively use a reductant such as pure anhydrous ammonia, aqueous ammonia or urea, for example. 
         [0015]    The upstream NO x  sensor  30  is positioned to detect the level of NO x  in the exhaust stream at a location upstream of the catalyst  18  and produce a responsive upstream NO x  signal. As shown in  FIG. 1 , the upstream NO x  sensor  30  may be positioned in the exhaust system  28  between the engine  10  and the injector  24 . The downstream NO x  sensor  32  may be positioned to detect the level of NO x  in the exhaust stream at a location downstream of the catalyst  18  and produce a responsive downstream NO x  signal. 
         [0016]    The ECU  26  is connected to receive the upstream and downstream NO x  signals from the sensors  30  and  32 , as well as the signals from the temperature sensors  36 ,  38 ,  40 . The ECU  26  may be configured to control reductant dosing from the injector  24  in response to signals from the temperature sensors  36 ,  38 ,  40  and the NO x  sensors  30 ,  32  (as well as other sensed parameters). In this regard, changes in the temperature of the NPF  16  can affect the ammonia storage capacity of the SCR catalyst  18 . For example, the catalyst  18  may be configured to operate most efficiently over an exhaust temperature range where the engine operates a majority of time or where the engine produces undesirable amounts of NO x . When the temperature in the NPF is outside of this operating range, the efficiency of the SCR catalyst  18  may be adversely impacted. For example, an increase in the temperature of the NPF  16  can reduce the storage capacity of the catalyst  18 , which can result in ammonia slip. 
         [0017]    In addition to controlling the dosing or metering of ammonia, the ECU  26  can also store information such as the amount of ammonia being delivered, the canister providing the ammonia, the starting volume of deliverable ammonia in the canister, and other such data which may be relevant to determining the amount of deliverable ammonia in each canister. The information may be monitored on a periodic or continuous basis. When the ECU  26  determines that the amount of deliverable ammonia is below a predetermined level, a status indicator (not shown) electronically connected to the controller  26  can be activated. 
         [0018]      FIG. 2  is a flow chart of an exemplary method  200  according to certain aspects of the present technology. The method  200  begins in step  205 . Control is then passed to step  210  where the method determines the temperature at a plurality of preselected locations in the exhaust system. In the illustrated embodiment, the method determines the temperature T1 at the inlet of the DOC by reading the output of the first temperature sensor, the temperature T2 at the inlet of the NPF by reading the output of the second temperature sensor, and the temperature T3 at the outlet of the NPF by reading the output of the third temperature sensor. 
         [0019]    Control is then passed to step  215  where the method determines a predictive NPF temperature T NPF  based on the temperature readings taken in step  210 . In at least some embodiments described herein, the predictive NPF temperature T NPF  may be a weighted average of the temperature readings from the temperature sensors  36 ,  38 ,  40 . In some embodiments, the upstream temperature readings, e.g., at the inlet of the DOC  14 , are weighted more heavily than the downstream temperature readings, e.g., at the inlet and outlet of the NPF  16 . Using a weighted average, where the upstream temperature readings are given a higher weighting, results in a temperature value that is predictive of temperature changes that will occur in the NPF. For example, in certain embodiments, the predictive NPF temperature T NPF  is determined in accordance with the following formula: 
         [0000]        T   NPF =(( T 13)+ T 2+ T 1)/5 
         [0020]    As can be seen, in the above formula, the temperature at the inlet of the DOC is weighted more heavily than the temperatures at the inlet and outlet of the NPF. The above formula is merely exemplary of one strategy that may be used to predict temperature changes in the NPF before they occur. The number and location of the temperature sensors may be varied in accordance with the configuration of the exhaust aftertreatment system, for example. In addition, in some embodiments, the weighting factors may be adjusted (e.g., dynamically) based on other operating conditions. For example, in some embodiments, the weighting parameters may be adjusted as a function of engine operating condition. In some embodiments, a higher weighting factor may be used for the upstream temperature sensors when the engine is undergoing a transient operation versus the weighting factors that are used during steady state operation. Further, in some embodiments, it may be desirable to employ a strategy that uses simulated map-based temperature sensors. 
         [0021]    After the predictive NPF temperature T NPF  is determined in step  215 , control is passed to step  220  where the method determines an ammonia dose based on the predictive NPF temperature T NPF  and other control parameters, such as the upstream and/or downstream NO x  values. For example, where the predictive NPF temperature T NPF  increases above a temperature threshold at which ammonia slippage will occur, the method can reduce the ammonia dose to reduce/limit ammonia slippage. Using a weighted average as discussed above will cause the predictive NPF temperature T NPF  reading to increase before the temperature of the NPF actually reaches the temperature threshold. According, any corrective action, such as adjusting the ammonia dose, can be taken in advance. 
         [0022]    At least some embodiments of the present technology relate to an SCR system  21  for controlling operation of an exhaust aftertreatment system  12  and for reducing ammonia slip. Referring again to  FIG. 1 , the system  21  may generally include the injector  24 , the reductant supply  22 , the upstream NO x  sensor  30 , the downstream NO x  sensor  32 , the ECU  26  and the temperature sensors  36 ,  38 ,  40 . The ECU  26  may be configured to receive signals from the temperature sensors  36 ,  38 ,  40  and the NO x  sensors, and to responsively control operation of the injector  24 . In at least some embodiments, the ECU  26  develops a predictive NPF temperature T NPF  based on the readings from the temperature sensors  36 ,  38 ,  40 . The predictive NPF temperature T NPF  may be a weighted average, where at least some of the temperature signals are weighted differently and have different weighting factors. In some embodiments, the temperature signals from sensors positioned upstream of the NPF  16  may be given a greater weighting than sensors that are proximate to the NPF  16 . The ECU  26  may use the predictive NPF temperature T NPF  to regulate operation of the injector  24  to regulate dosing of reductant into the exhaust system. For example, when the predictive NPF temperature T NPF  falls outside of a preselected range, the ECU  26  may reduce the reductant dose to reduce ammonia slip.