Patent Application
20190063285
The present disclosure relates to exhaust systems for internal combustion engines, and more particularly to exhaust systems using adaptive Selective Catalytic Reduction (SCR) systems for emission control.
Exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions, typically disposed on catalyst supports or substrates, are provided in an engine exhaust system as part of an after-treatment system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
Exhaust gas treatment systems typically include Selective Catalytic Reduction (SCR) devices. An SCR device includes a substrate having an SCR catalyst disposed thereon to reduce the amount of NOx in the exhaust gas. The typical exhaust treatment system also includes a reductant delivery system that injects a reductant such as, for example, ammonia (NH3), urea (CO(NH2)2, and other reductants. The SCR device makes use of NH3 to reduce the NOx. For example, when the proper amount of NH3 is supplied to the SCR device under the proper conditions, the NH3 reacts with the NOx in the presence of the SCR catalyst to reduce the NOx emissions. If the reduction reaction rate is too slow, or if there is excess ammonia in the exhaust, ammonia can slip from the SCR. On the other hand, if there is too little ammonia in the exhaust, NOx conversion efficiency will be decreased.
An emissions control system according to one, non-limiting, embodiment of the present disclosure treats exhaust gas of a combustion engine. The system includes a Selective Catalytic Reduction (SCR) device adapted to reduce emissions, a reductant injector adapted to inject a reductant into the SCR device, a downstream NOx sensor disposed downstream of the SCR device, a controller, an iterative model, and a lookup table. The controller includes a processor and an electronic storage medium. The iterative model and the lookup table are stored in the electronic storage medium. The processor is configured to perform short term and long term adaptive control by confirming at least one short term enablement criteria is met. Once confirmed, the processor calculates a normalized chemical model error utilizing, in-part, the iterative model and a downstream NOx signal received from the downstream NOx sensor. The processor then integrates the normalized chemical model error to produce an integrated normalized chemical model error, and confirms that the integrated normalized chemical model error exceeds an error threshold. The processor may then proceed toward the long term adaptive control, and confirms at least one long term adaptation enablement criteria is met. A current long term adaptive factor and the integrated normalized chemical model error is then applied to the lookup table to determine a new long term adaptive factor. The new long term adaptive factor is multiplied against an energization time of the reductant injector.
Additionally to the foregoing embodiment, the emissions control system includes an upstream NOx sensor disposed upstream of the reductant injector and the SCR device, wherein the processor is configured to receive an upstream NOx signal from the upstream NOx sensor to calculate the normalized chemical model error.
In the alternative or additionally thereto, in the foregoing embodiment, the normalized chemical model error is associated with the difference between a model-predicted NOx level taken from the iterative model, and an actual NOx level taken form the downstream NOx signal.
In the alternative or additionally thereto, in the foregoing embodiment, the normalized chemical model error is normalized by magnitude.
In the alternative or additionally thereto, in the foregoing embodiment, the at least one short term enablement criteria includes at least one of a normalized error being greater than a first threshold, a NOx gradient being less than a second threshold, a reductant-consumed being greater than a third threshold, a temperature being greater than a fourth threshold and less than a fifth threshold, a temperature gradient being less than a sixth threshold, a reductant storage level deviation being less than a seventh threshold, and a combustion mode.
In the alternative or additionally thereto, in the foregoing embodiment, the at least one long term enablement criteria includes at least one of the normalized error being greater than an eighth threshold, the NOx gradient being less than a ninth threshold, the reductant consumed is greater than a tenth threshold, the temperature is greater than an eleventh threshold and less than a twelfth threshold, the temperature gradient is less than a thirteenth threshold, the reductant storage deviation is less than a fourteenth threshold, and the combustion mode.
In the alternative or additionally thereto, in the foregoing embodiment, the at least one short term enablement criteria is independent from the at least one long term enablement criteria.
An emissions control system for treating exhaust gas of a combustion engine according to another, non-limiting, embodiment includes a Selective Catalytic Reduction (SCR) device, a first NOx sensor, and a controller. The controller is configured to perform short term and long term adaptive control by comparing a first NOx measurement from the first NOx sensor with a predicted NOx value based at least in-part on an initial chemical model. In response to a short term enablement criteria being met, the controller calculates a normalized chemical model error, integrates the normalized chemical model error, and calculates a new long term adaptive factor if the integrated normalized chemical model error exceeds a threshold.
Additionally to the foregoing embodiment, the emissions control system includes a lookup table stored in the controller and configured to cross reference a current long term adaptive factor to the integrated normalized chemical model error to calculate the new long term adaptive factor.
In the alternative or additionally thereto, in the foregoing embodiment, the normalized chemical model error is equal to a delta between the first NOx measurement and a predicted NOx value based on the initial chemical model, and normalized based on magnitude.
In the alternative or additionally thereto, in the foregoing embodiment, the first NOx sensor is located downstream from the SCR device.
In the alternative or additionally thereto, in the foregoing embodiment, the emissions control system includes a second NOx sensor, wherein the normalized chemical model error is based on the initial chemical model, the first NOx measurement, and an upstream NOx measurement from the second NOx sensor located upstream from the SCR device, and wherein the first NOx sensor is located downstream from the SCR device.
In the alternative or additionally thereto, in the foregoing embodiment, the emissions control system includes a temperature sensor configured to send a temperature measurement to the controller, wherein the initial chemical model is generated by the controller and is at least based on the temperature measurement, the upstream NOx measurement, and the first NOx measurement.
In the alternative or additionally thereto, in the foregoing embodiment, the enablement criteria is a short term enablement criteria.
In the alternative or additionally thereto, in the foregoing embodiment, the short term enablement criteria includes at least one of a normalized error being greater than a first threshold, a NOx gradient being less than a second threshold, a reductant-consumed being greater than a third threshold, a temperature being greater than a fourth threshold and less than a fifth threshold, a temperature gradient being less than a sixth threshold, a reductant storage level deviation being less than a seventh threshold, and a combustion mode.
In the alternative or additionally thereto, in the foregoing embodiment, the long term adaptive factor determination is conducted when a long term adaptation enablement criteria is met.
In the alternative or additionally thereto, in the foregoing embodiment, the long term adaptation enablement criteria is independent from the short term enablement criteria.
In the alternative or additionally thereto, in the foregoing embodiment, the long term adaptation enablement criteria includes at least one of the normalized error being greater than an eighth threshold, the NOx gradient being less than a ninth threshold, the reductant consumed is greater than a tenth threshold, the temperature is greater than an eleventh threshold and less than a twelfth threshold, the temperature gradient is less than a thirteenth threshold, the reductant storage deviation is less than a fourteenth threshold, and the combustion mode.
In the alternative or additionally thereto, in the foregoing embodiment, the eighth threshold is greater than the first threshold.
In the alternative or additionally thereto, in the foregoing embodiment, the emission control system includes a reductant injector, wherein the new long term adaptive factor is generally multiplied by an energization time of the reductant injector.
The above features and advantages, and other features and advantages of the disclosure, are readily apparent from the following detailed description when taken in connection with the accompanying drawings.
Other features, advantages and details appear, by way of example only, in the following detailed description, the detailed description referring to the drawings in which:
The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. As used herein, the term module refers to processing circuitry that may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory module that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.
A motor vehicle, in accordance with an aspect of an exemplary embodiment, is indicated generally at 20 in
An Internal Combustion Engine (ICE) system 30 of the motor vehicle 20 may include a combustion engine 32, an exhaust system 34, and a controller 36. The engine compartment 24 may generally house the combustion engine 32. Examples of combustion engines 32 may include a diesel engine, a gasoline or heptane engine, and others.
The engine 32 of the ICE system 30 may include a plurality of reciprocating pistons attached to a crankshaft, which may be operably attached to a driveline, such as a vehicle driveline, to power a vehicle (e.g., deliver tractive torque to the driveline). For example, the ICE system 30 may be any engine configuration or application, including various vehicular applications (e.g., automotive, marine, and the like), as well as various non-vehicular applications (e.g., pumps, generators, and the like). While the ICE system may be described in a vehicular context (e.g., generating torque), other non-vehicular applications are within the scope of this disclosure. Therefore, when reference is made to a vehicle, such disclosure should be interpreted as being applicable to any application of the ICE system 30.
Moreover, the ICE system 30 may generally represent any device capable of generating an exhaust gas stream generally directed and treated by the exhaust system 34. The exhaust gas may include and chemical species or mixture of chemical species; in gaseous, liquid, or solid form, that may require treatment. In one example, the exhaust gas may generally include gaseous (e.g., NOx, O2), carbonaceous, and/or particulate matter species. An exhaust gas stream may contain a mixture of one or more NOx species, one or more liquid hydrocarbon species, and one more solid particulate species (e.g., ash). It should be further understood that the embodiments disclosed herein may be applicable to treatment of effluent streams not comprising carbonaceous and/or particulate matter species. Exhaust gas particulate matter generally includes carbonaceous soot, and other solid and/or liquid carbon-containing species which are germane to combustion engine exhaust gas, or form within the exhaust system 34.
With reference to
The emissions control system 40 may include an Oxidation Catalyst (OC) device 46, a Selective Catalytic Reduction (SCR) assembly 48, particulate filter devices (not shown), and other exhaust treatment devices. The SCR assembly 48 may be located downstream of the OC device 46 with respect to the exhaust conduit 38.
The OC device 46 of the emissions control system 40 may be one of various flow-through, oxidation catalyst devices known in the art. The OC device 46 may include a flow-through metal or ceramic monolith substrate 50. The substrate 50 may be packaged in a stainless steel shell or canister having an inlet and an outlet in fluid communication with the exhaust gas conduit 38. The substrate 50 may include an oxidation catalyst compound disposed thereon. The oxidation catalyst compound may be applied as a washcoat and may contain platinum group metals such as platinum (Pt), palladium (Pd), rhodium (Rh) or other suitable oxidizing catalysts, or combination thereof. The OC device 46 is useful in treating unburned gaseous and non-volatile HC and CO, that are oxidized to form carbon dioxide and water. A washcoat layer includes a compositionally distinct layer of material disposed on the surface of the monolithic substrate 50 or an underlying washcoat layer. A catalyst may contain one or more washcoat layers, and each washcoat layer may have unique chemical catalytic functions.
The SCR assembly 48 of the emissions control system 40 may be adapted to receive exhaust gas 44 treated by the OC device 46, and/or originating from the combustion engine 32, and reduce Nitrogen Oxide (NOx) constituents in the exhaust gas 44. NOx constituents may include NyOx species, wherein y>0 and x>0. Non-limiting examples of NOx may include NO, NO2, N2O, N2O2, N2O3, N2O4, and N2O5. More particularly, the SCR assembly 48 may convert NOx to diatomic nitrogen (N2) and water.
The SCR assembly 48 may include at least a portion of the controller 36, an SCR device or canister 52, an upstream NOx sensor 54, a downstream NOx sensor 56, at least one temperature sensor 58, at least one pressure sensor 60, a reductant injector 62, and a reductant supply source 64. The SCR device 52 is in fluid communication with the exhaust gas conduit 44 for treatment of the exhaust gas 44. The NOx sensor 54 may be located upstream of the SCR device 52 and downstream of the OC device 46 for measuring NOx constituents in the exhaust gas 44 before the exhaust gas enters the SCR device 52. The NOx sensor 56 may be located downstream of the SCR device 52 for measuring NOx constituents in the exhaust gas 44 after the exhaust gas exits the SCR device 52. The temperature sensor 58 may be located upstream of the SCR device 52 and downstream of the reductant injector 62 for measuring exhaust gas temperature. Although the SCR device 52 is illustrated downstream from the OC device 46, it is contemplated and understood that the SCR device 52 may be located upstream from the OC device 46.
The at least one pressure sensor 60 (e.g., differential pressure sensor) may be adapted to determine the pressure differential across the SCR device 52. Although a single differential pressure sensor 60 is illustrated, it is appreciated that a plurality of pressure sensors may be used to determine the pressure differential of the SCR device 52. For example, a first pressure sensor may be disposed at an inlet of the SCR device 52 and a second pressure sensor may be disposed at an outlet of the SCR device 52. Accordingly, the difference between the pressure detected by the second pressure sensor and the pressure detected by the first pressure sensor may indicate the pressure differential across the SCR device 52. It should be noted that in other examples, the sensors may include different, additional, or fewer sensors than the sensors 54, 56, 58, 60 described.
The reductant injector 62 of the SCR assembly 48 may be generally mounted to the exhaust gas conduit 38 upstream of the SCR device 52 (i.e., between the upstream NOx sensor 54 and the temperature sensor 58), and is configured to disperse a controlled amount of a reductant 66 into the flow of exhaust gas 44. The reductant 66 is stored and supplied to the injector 62 by the reductant supply source 64, and may be in the form of a gas, a liquid, or an aqueous solution (e.g., aqueous urea solution). The reductant 66 may be mixed with air in the injector 62 to aid in the dispersion of the injected spray. The SCR device 52 utilizes the reductant 66, such as ammonia (NH3), to reduce the NOx.
The SCR device 52 of the SCR assembly 48 may include a substrate 68. The substrate 68 may generally be a particulate filter (PF), such as a diesel particulate filter (DPF) coated with a SCR catalyst and adapted to filter or trap carbon and other particulate matter from the exhaust gas 44. The substrate 68 generally includes an inlet and an outlet in fluid communication with exhaust gas conduit 38. In another example, the substrate may be a flow-through monolith type of substrate that may generally be made of ceramic. Further examples of substrates 68 may include wound or packed fiber filters, open cell foams, sintered metal fibers, and others. The emissions control system 40 may also perform a regeneration process that regenerates the substrate 68 by burning-off the particulate matter trapped in the filter substrate.
A catalyst containing washcoat disposed on the substrate 68, (i.e., a flow through catalyst or a wall flow filter) may reduce NOx constituents in the exhaust gas 44. The catalyst containing washcoat may contain a zeolite and one or more base metal components such as iron (Fe), cobalt (Co), copper (Cu), or vanadium (V), which can operate efficiently to convert NOx constituents of the exhaust gas 44 in the presence of NH3. In one or more examples, a turbulator (i.e., mixer, not shown) may also be disposed within the exhaust conduit 38 in close proximity to the injector 62 and/or the SCR device 52 to further assist in thorough mixing of the reductant 66 with the exhaust gas 44, and/or even distribution throughout the SCR device 52. It is understood that catalyst compositions for the SCR function, and NH3 oxidation function, may reside in discrete washcoat layers on the substrate 68 or, alternatively, the compositions for the SCR and NH3 oxidation functions may reside in discrete longitudinal zones on the substrate 68.
The body of the substrate 68 may, for example, be a ceramic brick, a plate structure, or any other suitable structure such as a monolithic honeycomb structure that includes several hundred to several thousand parallel flow-through cells per square inch, although other configurations are suitable. Each of the flow-through cells may be defined by a wall surface on which the SCR catalyst composition can be washcoated. The body of the substrate 68 may be formed from a material capable of withstanding the temperatures and chemical environment associated with the exhaust gas 44. Some specific examples of materials that may be used include ceramics such as extruded cordierite, α-alumina, silicon carbide, silicon nitride, zirconia, mullite, spodumene, alumina-silica-magnesia, zirconium silicate, sillimanite, petalite, or a heat and corrosion resistant metal such as titanium or stainless steel. For example, the substrate 68 may comprise a non-sulfating TiO2 material. The body of the substrate 68 may be a PF device, as will be discussed below.
The SCR catalyst composition is generally a porous and high surface area material which can operate efficiently to convert NOx constituents in the exhaust gas 44 in the presence of the reductant 66 (e.g., ammonia). In some embodiments the zeolite may be a β-type zeolite, a Y-type zeolite, a ZM5 zeolite, or any other crystalline zeolite structure such as a Chabazite or a USY (ultra-stable Y-type) zeolite. The zeolite may comprise Chabazite or SSZ. Suitable SCR catalyst compositions may have high thermal structural stability, particularly when used in tandem with the substrate 68 as a particulate filter (PF) device or when incorporated into a SCRF device, which are regenerated via high temperature exhaust soot burning techniques.
The SCR catalyst composition may optionally further comprise one or more base metal oxides as promoters to further decrease the SO3 formation and to extend catalyst life. The one or more base metal oxides may include WO3, Al2O3, and MoO3. In one embodiment, WO3, Al2O3, and MoO3 may be used in combination with V2O5.
The SCR catalyst (i.e., substrate 68) generally uses the reductant 66 to reduce NOx species (e.g., NO and NO2) to unregulated components. Such components include one or more of species which are not NOx species, such as diatomic nitrogen (N2), nitrogen-containing inert species, or species which are considered acceptable emissions. The reductant 66 may be ammonia (NH3), such as anhydrous ammonia or aqueous ammonia, or generated from a nitrogen and hydrogen rich substance such as urea (CO(NH2)2). Additionally or alternatively, the reductant 66 may be any compound capable of decomposing or reacting in the presence of the exhaust gas 44 and/or heat to form ammonia. Equations (1)-(5) provide exemplary chemical reactions for NOx reduction involving ammonia:
6NO+4NH3→5N2+6H2O (1)
4NO+4NH3+O2→4N2+6H2O (2)
6NO2+8NH3→7N2+12H2O (3)
2NO2+4NH3+O2→3N2+6H2O (4)
NO+NO2+2NH3→2N2+3H2O (5)
It should be appreciated that Equations (1)-(5) are merely illustrative, and are not meant to confine the SCR device 52 to a particular NOx reduction mechanism or mechanisms, nor preclude the operation of other mechanisms. The SCR device 52 may be configured to perform any one of the above NOx reduction reactions, combinations of equations (1) through (5) NOx reduction reactions, and other NOx reduction reactions.
The reductant 66 may be diluted with water, where heat (e.g., from the exhaust) evaporates the water, and ammonia is supplied to the SCR device 52. Non-ammonia reductants may be used as a full or partial alternative to ammonia as desired. In embodiments where the reductant 66 includes urea, the urea reacts with the exhaust gas 44 to produce ammonia that is supplied to the SCR device 52. Reaction (6) below provides an exemplary chemical reaction of ammonia production via urea decomposition:
CO(NH2)2+H2O→2NH3+CO2 (6)
It should be appreciated that Equation (6) is merely illustrative, and is not meant to confine the urea or other reductant 66 decomposition to a particular single mechanism, nor preclude the operation of other mechanisms.
The substrate 68 (i.e., SCR catalyst) may store the reductant 66 for interaction with the exhaust gas 44. The SCR device 52 has a reductant capacity, or an amount of reductant or reductant derivative it is capable of storing. The amount of reductant 66 stored within the SCR device 52 relative to the SCR catalyst capacity of the substrate 68 may be referred to as the SCR “reductant loading”, and may be indicated as a percent (%) loading (e.g., 90% reductant loading). During operation of SCR device 52, injected reductant 66 is stored in the SCR catalyst of the substrate 68 and consumed during reduction reactions with undesired NOx species and must be continually replenished. Determining the precise amount of reductant 66 to inject is critical to maintaining exhaust gas emissions at acceptable levels. Insufficient levels of reductant 66 within the SCR device 52 may result in undesirable NOx species emissions, referred as NOx breakthrough, that may exit the exhaust outlet pipe 42. Excessive levels of reductant 66 injected into the SCR device 52 may result in undesirable amounts of reductant 66 passing through the SCR device 52 unreacted, or exiting the SCR device 52 as an undesired reaction product, also referred to as reductant slip. Reductant slip and NOx breakthrough may also occur when the SCR catalyst of the substrate 68 is below a “light-off” temperature. SCR dosing logic may be utilized by the controller 36 to command reductant dosing.
The controller 36 may be adapted to be in electronic communication with aspects of the combustion engine 32, the reductant supply source 64, the injector 62, the sensors 54, 56, 58, 60, and other components of the ICE system 30. The controller 36 may include a processor 70 (e.g., microprocessor) and an electronic storage medium 72 that may be computer writeable and readable. In one embodiment, the controller 36 may be an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group), and an electronic memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.
In operation, the processor 70 of the controller 36 may execute the SCR dosing logic stored in the storage medium 72, and may further receive and process NOx signals (see arrows 74, 76 in
A reductant injection dosing rate (e.g., grams per second) may be determined by the processor 70 by applying a SCR chemical model 82 and processing a feedback signal (see arrow 84) from the reductant supply source 64 of the injector 62. Generally, the SCR chemical model 82, combined with the feedback signal 84 and the upstream NOx signal 74, assists in generating a prediction of the amount of reductant 66 stored in the SCR device 52. The SCR chemical model 82 may further predict NOx levels of the exhaust gas 44 discharged from the SCR device 52. Over time, the SCR chemical model 82 may be updatable by one or more process values. For example, the SCR chemical model 82 may be updatable by a short term correction factor.
Referring to
Referring again to
In one or more examples, the percentage of NOx that is removed from the exhaust gas 44 entering the SCR device 52 may be referred to as a conversion efficiency of the SCR device 52. The controller 36 may determine the conversion efficiency of the SCR device 52 based on the NOxin and NOxout signals 74, 76 generated by the respective NOx sensors 54, 56. For example, the controller 36 may determine the conversion efficiency of the SCR device 52 based on the following equation:
SCReff=(NOxin−NOxout)/NOxin (7)
Reductant (e.g., NH3) slip may also be caused because of an increase in the temperature of the SCR catalyst. For example, NH3 may desorb from the SCR catalyst of the substrate 68 when the temperature increases at times when the NH3 storage level is near to the maximum NH3 storage level. NH3 slip may also occur due to an error (e.g., storage level estimation error) or faulty component (e.g., faulty injector) of the emissions control system 34.
Typically, the controller 36 estimates an NH3 storage level of the SCR device 52 based on the chemical model 82. In one or more examples, the NH3 storage set-point (“set-point”) is capable of being calibrated. That is, the NH3 storage set-point may be a function of exhaust flow rate and temperature. Based on the current exhaust flow and temperature, the set-point may be defined.
The controller 36 uses the chemical model 82 to estimate the current storage level of NH3 in the SCR device 52, and the storage level governor provides feedback to the injection controls to determine the injection rate of reductant to provide NH3 for reactions according to the chemical model 82, and to maintain a target storage level. The set-point may indicate a target storage level for given operating conditions (e.g., a temperature of the SCR catalyst of the substrate 68). Accordingly, the set-point may indicate a storage level (S) and a temperature (T) of the SCR device 52, see
The controller 36 may use the chemical model 82 of the SCR catalyst of the substrate 68 to predict the NOx concentration in the exhaust gases 44 entering the SCR device 52. Further, based on the predicted NOx concentration, the controller 36 may determine an amount of NH3 needed to dose the exhaust gases 44 to satisfy the emissions threshold. The controller 36 may implement an adaptive semi-closed loop control strategy to maintain the performance of the SCR device 52 according to the chemical model 82, where the controller continuously learns one or more parameters associated with the chemical model 82 according to the ongoing performance of the motor vehicle 20.
In one or more examples, the predetermined value may be determined based on a specified statistic such as a standard deviation, for example 1.5 standard deviations. Further, the predetermined value may be calibrated to a modeled downstream NOx value. The measured downstream NOx is thus normalized against the expected error of the downstream NOx sensor 56. The normalized error, 1.5 in this example, may then be compared to the threshold for entry into steady state slip detection logic. The predetermined value of the concentration of the NOx that is used as the threshold for comparison, in such cases, is computed based on the earlier values of the NOx measured by the NOx sensor 56. In other words, in the above scenario, the 37.5 ppm is used as the threshold value because 37.5 is the 1.5 standard deviation value of earlier NOx measurements. It should be noted that in one or more examples, the NOx measurement and predicted value used may be a NOx flow rate, or any other NOx attribute (i.e., instead of the NOx concentration).
A dosing governor (not shown) may be controlled by the controller 36, and is configured to monitor the reductant storage level (i.e., in the substrate 68 of the SCR device 52) generally predicted by the SCR chemical model 82, and compares the predicted reductant storage level to a preprogrammed, desired, reductant storage level. Deviations between the predicted reductant storage level and the desired reductant storage level may be continuously monitored, and a dosing adaptation (i.e., both the short term correction factor and the long term factor) may be triggered to increase or decrease reductant dosing in order to eliminate or reduce the deviation.
For example, the reductant dosing rate may be adapted to achieve a desired NOx concentration or flow rate in the exhaust gas 44 downstream of the SCR device 52, or achieve a desired NOx conversion rate. A desired conversion rate may be determined by many factors, such as the characteristics of SCR catalyst type and/or operating conditions of the ICE system 30 (e.g., engine 32 operating parameters). To achieve an optimal reductant dosing rate, the short term correction factor may be applied to the SCR chemical model 82 that generally represents the modeled NH3 storage. If the modeled and requested storage differ, then dosing is modified to achieve the desired storage. That is, the long term factor may be applied directly to the injector energizing time, and may increase or decrease dosing accordingly. The short term correction may be applied instantaneously, but the long term correction is applied only after a period of time.
Over time, inaccuracies of the SCR chemical model 82 may compound to appreciative errors between modeled SCR reductant storage level and actual storage level. Accordingly, the SCR chemical model 82 may be continuously corrected to minimize or eliminate errors. One method for correcting the SCR chemical model 82 includes comparing the modeled SCR discharge exhaust gas NOx levels to the actual NOx levels, measured by the downstream NOx sensor 56, to determine a discrepancy, and subsequently correcting the SCR chemical model 82 to eliminate or reduce the discrepancy. Because the downstream NOx sensor 56 may be cross-sensitive to the reductant 66 and the exhaust gas NOx, it is critical to distinguish between reductant measurements and NOx measurements as reductant slip may otherwise be confused with insufficient NOx conversion.
A passive analysis technique may be used to distinguish between reductant measurements and NOx measurements, is a correlation method that includes comparing the upstream NOx concentration, measured by the upstream NOx sensor 54, to the downstream NOx concentration, measured by the downstream NOx sensor 56. If the difference in concentration shows a diverging trend (i.e., an increasing difference), this may indicate an increase or decrease in reductant slip. The correlation analysis identifies when the measurements from the downstream NOx sensor 56 is following the pattern of measurements from the upstream NOx sensor 54 (i.e., the two sensor measurements are moving alike). The correlation is a statistical measure of the strength and direction of a linear relationship between the two NOx sensors 54, 56.
For example, the comparison includes a correlation method which includes comparing the downstream NOx concentration with the upstream NOx measurements, or the predicted NOx measurements, wherein diverging concentration directions can indicate an increase or decrease in reductant slip. For example, if the upstream NOx concentration decreases and downstream NOx concentration increases, reductant slip can be identified as increasing. Similarly, if the upstream NOx concentration increases and downstream NOx concentration decreases, reductant slip can be identified as decreasing. Thus, the divergence between the two sequences of NOx measurements can be used to determine a dosing status of the SCR device 52.
Alternatively, or in addition, the comparison may include a frequency analysis. NOx signals 74, 76 generated by the NOx sensors 54, 56 may include multiple frequency components (e.g., high frequency and low frequency) due to the variation of the NOx and reductant concentrations during the modulation/demodulation. High frequency signals generally relate only to NOx concentration, while low frequency signals generally relate to both NOx concentration and reductant concentration. High frequency signals for upstream NOx and downstream NOx are isolated and used to calculate a SCR NOx conversion ratio, which is then applied to the isolated, low-pass, upstream NOx measurement to determine a low frequency downstream NOx measurement. The calculated low frequency downstream NOx measurement is then compared to the actual isolated low frequency downstream NOx measurement, wherein a deviation between the two values may indicate reductant slip.
A drawback of passive analysis techniques (i.e., short term techniques), such as the correlation method and frequency method described above, is the reliance on proper operation of two NOx sensors 54, 56. For example, a faulty upstream NOx sensor 54 may generate a NOx signal 74 that is lower than the actual NOx level proximate the upstream NOx sensor causing the SCR chemical model 82 to predict higher reductant storage levels than the actual storage level. Accordingly, NOx breakthrough would be incorrectly identified as reductant slip, and reductant dosing would be commanded such that NOx breakthrough would be exacerbated (i.e., reductant dosing would be decreased). Further, the SCR chemical model 82 would be updated using the inaccurate upstream NOx measurement, and the exacerbated NOx breakthrough would endure. Additionally or alternatively, in a similar manner, a reductant slip may be incorrectly interpreted as NOx breakthrough.
In general, the passive analysis, or short term, techniques may be used to partially predict the presence of NH3 slip and/or NOx breakthrough. However, merely applying the short term techniques will not compensate for system drift, part-to-part variations, and other factors, thus causing the wrong slip decisions to be made, leading to wrong short term storage level corrections. Wrong short term storage level corrections may lead to the wrong long term adaptation decisions. That is, if a system drift issue is present, merely applying any short term technique may cause saturation of the NH3 slip and/or NOx breakthrough predictions. Therefore, the emissions control system 40 applies both a short term correction and a long term correction. More specifically, a long term adaptation that is only dependent on accumulated error is applied.
Referring to
Referring to
The normalized chemical model error being greater than the first threshold, and as part of the short term enablement, may be generally the difference between the model predicted NOx from SCR chemical model 82 and the actual NOx normalized by magnitude. The NOx gradient being less than a second threshold, and as part of the short term enablement, may be a rate of change (e.g., ppm/s) of NOx entering the SCR device 52. A large gradient may be an indicator of a highly transient vehicle maneuver where corrections are desirable. The reductant (e.g., NH3) consumed being greater than a pre-established threshold is generally a stability criteria for the SCR device 52.
The temperature window, as part of the short term enablement, is generally indicative of a temperature being higher than a low temperature threshold and less than a high temperature threshold. The temperature window criteria may allow alignment of short term correction with the operating range where the SCR chemical model 82 is most accurate. That is, the SCR chemical model 82 may not be as accurate at very low temperatures where performance is reduced, or at very high temperatures when there is a propensity of NH3 slip.
The temperature gradient being less than the sixth threshold criteria, and as part of the short term enablement, is indicative of a rate of change of temperature at the inlet of the SCR device 52. A large gradient may be an indicator of a highly transient vehicle maneuver where exhaust correction may be undesirable.
The reductant storage level deviation being less than a seventh threshold criteria, and as part of the short term enablement, allows alignment of short term correction with the operating range where the SCR chemical model 82 is most accurate. The SCR chemical model 82 may not be as accurate when the actual reductant storage level on the SCR catalyst is much higher or lower than the setpoint (i.e., seventh threshold).
The combustion mode criteria allows blocking of short term correction on combustion mode (e.g., DPF regeneration, SCR warmup, and others). Certain combustion modes may have a higher propensity for increased temperatures, decreased storage levels, and increased NH3 slip. Under such modes, or conditions, short term corrections should be avoided.
At block 204, and if the short term enablement criteria is met, the controller may calculate the normalized chemical model error. Inputs for the normalized chemical model error include the signals 74, 76 from the respective upstream and downstream NOx sensors 54, 56, and the SCR chemical model 82. The SCR chemical model 82 may be formed and developed from inputs associated with the temperature sensor 58, the NOx sensors 54, 56, and the previous SCR chemical model 82 contained within the controller 36. The normalized chemical model error may equal the delta between the measured NOx associated with the downstream NOx signal 76 and the modeled, or predicted, downstream NOx that is normalized based on magnitude of values.
At block 206, the normalized chemical model error, associated with short term control, is integrated. In one example, the task rate for this integration may be about fifty milliseconds. At block 208, if the integrated normalized chemical model error exceeds a threshold, the method 200 may proceed toward a long term adaptive factor determination. At block 210 and proceeding with the long term adaptive factor determination, a determination of whether long term adaptation enablement criteria is met. The long term adaptation enablement criteria(s) may be similar to the short term adaptation enablement criteria(s) except that the respective thresholds may be different. That is, the thresholds between the long term adaptation enablement criteria(s) may be independent from the short term adaptation enablement criteria(s). Generally, all thresholds may be SCR strategy and hardware dependent.
Examples of long term enablement criteria may include: the normalized error is greater than an eighth threshold, the NOx gradient is less than a ninth threshold, the reductant (e.g., NH3) consumed is greater than a tenth threshold (i.e., SCR device stability), the temperature is greater than an eleventh threshold and less than a twelfth threshold, a temperature gradient is less than a thirteenth threshold, a reductant storage deviation is less than a setpoint (i.e., fourteenth threshold), and the combustion mode.
In one embodiment, the normalized error threshold for the long term adaptation (i.e., eighth threshold) may be substantially larger than the normalized error threshold for the short term correction (i.e., first threshold). Furthermore, the various thresholds of the long term adaptation enablement criteria may be about twice as large as the respective thresholds of the short term adaptation criteria. In other embodiments, the thresholds may be about equivalent.
At block 212, and if the long term adaptation enablement criteria is met, the integrated normalized chemical model error 102 and the current long term adaptive factor 104 are utilized as inputs for the map or lookup table 100 to determine a new (i.e., subsequent) long term adaptive factor 106. For example, if the integrated short term normalized error 102 is 0.3 (i.e., an integrated short term normalized error associated with NH3 slip) and the current long term adaptation factor is about 0.8, the new long term adaptation factor 106 would be about 0.77 (i.e., as an example portrayal). The new long term adaptation factor 106 may then be used to compensate for system drift and/or part-to-part variation. In, one example, the new long term adaptation factor 106 may generally multiply the DEF injector energizing time (i.e., the amount of time the injector remains open).
The technical solutions described herein facilitate improvements to emissions control systems used in combustion engines, such as those used in vehicles. For example, the technical solutions determine storage correction and adaptation based on integration of a smaller error than what is used to enter a steady state reductant slip detection logic, the error indicative of a difference between downstream NOx sensor measurement and downstream NOx model. Such improvements facilitate prevention of cycling of steady state reductant slip detection when the NOx error is just high enough to cause a steady state reductant slip detection event, but the error is low enough to cause the system to cycle in and out of the steady state reductant slip detection without any adapts.
Further advantages and benefits include a system 40 configured to treat short term and long term adaptation independently. This independence contributes toward improved adaptation robustness, decreased false failures, and a reduction in the potential for DEF crystallization.
While the above disclosure has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from its scope. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiments disclosed, but will include all embodiments falling within the scope thereof.
