Patent Publication Number: US-11643961-B2

Title: Reductant deposit detection using a radiofrequency sensor

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 16/621,584, filed Dec. 11, 2019, which is the U.S. national stage of PCT Application No. PCT/US2018/039276, filed Jun. 25, 2018, which claims the benefit of, and priority to, U.S. Provisional Patent Application No. 62/525,415, filed Jun. 27, 2017. The contents of these applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present application relates generally to the field of aftertreatment systems for internal combustion engines. 
     BACKGROUND 
     For internal combustion engines, such as diesel engines, nitrogen oxide (NO x ) compounds may be emitted in the exhaust. To reduce NO x  emissions, a selective catalytic reduction (SCR) process may be implemented to convert the NO x  compounds into more neutral compounds, such as diatomic nitrogen, water, or carbon dioxide, with the aid of a catalyst and a reductant. The catalyst may be included in a catalyst chamber of an exhaust system, such as that of a vehicle or power generation unit. A reductant, such as anhydrous ammonia, aqueous ammonia, or urea, is typically introduced into the exhaust gas flow prior to the catalyst chamber. To introduce the reductant into the exhaust gas flow for the selective catalytic reduction process, a selective catalytic reduction system may dose or otherwise introduce the reductant through a dosing module that vaporizes or sprays the reductant into an exhaust pipe of the exhaust system up-stream of the catalyst chamber. The selective catalytic reduction system may include one or more sensors to monitor conditions within the exhaust system. 
     SUMMARY 
     Implementations described herein relate to reductant deposit detection using a radiofrequency sensor. 
     One embodiment relates to a process for detecting reductant deposits. The process includes accessing data indicative of signal output from a radiofrequency sensor positioned proximate a decomposition reactor tube; comparing the data indicative of signal output from the radiofrequency sensor to a deposit formation threshold; and activating a deposit mitigation process responsive to the data indicative of signal output from the radiofrequency sensor exceeding the deposit formation threshold. 
     Another embodiment relates to an aftertreatment system. The aftertreatment system includes a decomposition reactor tube, a doser, a first radiofrequency device, and a second radiofrequency device. The doser is coupled to the decomposition reactor tube and configured to dose exhaust gas within the decomposition reactor tube with reductant. The first radiofrequency device is coupled to the decomposition reactor tube. The first radiofrequency device includes a first radiofrequency communicator configured to receive a radiofrequency signal from within the decomposition reactor tube. The second radiofrequency device is coupled to the decomposition reactor tube. The second radiofrequency device includes a second radiofrequency communicator configured to transmit the radiofrequency signal from within the decomposition reactor tube. 
    
    
     
       BRIEF DESCRIPTION 
       The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the disclosure will become apparent from the description, the drawings, and the claims, in which: 
         FIG.  1    is a block schematic diagram of an example selective catalytic reduction system having an example reductant delivery system for an exhaust system; and 
         FIG.  2    is a block schematic depicting an exhaust system with radiofrequency sensors deployed proximate the decomposition reactor tube to detect reductant deposits. 
         FIG.  3    is a block schematic depicting a reductant deposit mitigation system. 
         FIG.  4    is a block diagram of a process for mitigating reductant deposits. 
     
    
    
     It will be recognized that some or all of the figures are schematic representations for purposes of illustration. The figures are provided for the purpose of illustrating one or more implementations with the explicit understanding that they will not be used to limit the scope or the meaning of the claims. 
     DETAILED DESCRIPTION 
     Following below are more detailed descriptions of various concepts related to, and implementations of, methods, apparatuses, and systems for reductant deposit detection using radiofrequency sensors. The various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the described concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes. 
     I. Overview 
     With increasing endeavors to reduce emissions, the desired total tailpipe NO x  (i.e., the amount of NO x  emitted from the exit of the exhaust system) for a vehicle has dropped exponentially since selective catalytic reduction systems were first introduced. 
     While reducing the NO x  produced by an engine is one way of addressing the reduction of total tailpipe NO x , that approach may result in a reduced fuel economy. With some engine systems moving in the direction of high engine-out NO x , reducing tailpipe NO x  may be shifted to the exhaust gas treatment system (EGTS). At the high levels of reductant dosing to maintain the selective catalytic reduction NO x  conversion efficiency targets, occasionally reductant may form deposits on a decomposition reactor tube (DRT), which can result in one or more failure modes. The presence of reductant deposits can reduce the selective catalytic reduction NO x  conversion capability since the NH 3  is not being stored on the catalyst to help with NO x  reduction, but is instead forming deposits/puddles at the front face of the selective catalytic reduction catalyst. Reductant deposits can also increase the backpressure on the engine, forcing it to operate less efficiently. 
     Some current methods of detecting a reductant deposit rely on either an increase in the backpressure or a decrease in the selective catalytic reduction NO x  conversion efficiency as measured from an selective catalytic reduction inlet NO x  sensor and selective catalytic reduction outlet NO x  sensor. Both methods detect the presence of reductant deposits, but are reactive in nature. That is, each of the foregoing methods detects the reductant deposits after they have formed. Additionally, neither method determines a quantity of reductant deposits. Furthermore, the use of NO x  sensors can result in additional failure modes, such as cross-sensitivity to NH 3  resulting in a false-positive when an increased NH 3  slip from the selective catalytic reduction is misconstrued as reduced NO x  conversion capability. 
     II. Overview of Aftertreatment System 
       FIG.  1    depicts an aftertreatment system  100  having an example reductant delivery system  110  for an exhaust system  190 . The aftertreatment system  100  includes a particulate filter, for example a diesel particulate filter (DPF)  102 , the reductant delivery system  110 , a decomposition chamber or reactor tube  104 , a selective catalytic reduction catalyst  106 , and a sensor  150 . 
     The diesel particulate filter  102  is configured to remove particulate matter, such as soot, from exhaust gas flowing in the exhaust system  190 . The diesel particulate filter  102  includes an inlet, where the exhaust gas is received, and an outlet, where the exhaust gas exits after having particulate matter substantially filtered from the exhaust gas and/or converting the particulate matter into carbon dioxide. 
     The decomposition chamber  104  is configured to convert a reductant, such as urea, aqueous ammonia, or diesel exhaust fluid (DEF), into ammonia. The decomposition chamber  104  includes a reductant delivery system  110  having a dosing module  112  configured to dose the reductant into the decomposition chamber  104 . In some implementations, the reductant is injected upstream of the selective catalytic reduction catalyst  106 . The reductant droplets then undergo the processes of evaporation, thermolysis, and hydrolysis to form gaseous ammonia within the exhaust system  190 . The decomposition chamber  104  includes an inlet in fluid communication with the diesel particulate filter  102  to receive the exhaust gas containing NO x  emissions and an outlet for the exhaust gas, NO x  emissions, ammonia, and/or remaining reductant to flow to the selective catalytic reduction catalyst  106 . 
     The decomposition chamber  104  includes the dosing module  112  mounted to the decomposition chamber  104  such that the dosing module  112  may dose the reductant into the exhaust gases flowing in the exhaust system  190 . The dosing module  112  may include an insulator  114  interposed between a portion of the dosing module  112  and the portion of the decomposition chamber  104  to which the dosing module  112  is mounted. The dosing module  112  is fluidly coupled to one or more reductant sources  116 . In some implementations, a pump  118  may be used to pressurize the reductant from the reductant source  116  for delivery to the dosing module  112 . 
     The dosing module  112  and pump  118  are also electrically or communicatively coupled to a controller  120 . The controller  120  is configured to control the dosing module  112  to dose reductant into the decomposition chamber  104 . The controller  120  may also be configured to control the pump  118 . The controller  120  may include a microprocessor, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), etc., or combinations thereof. The controller  120  may include memory which may include, but is not limited to, electronic, optical, magnetic, or any other storage or transmission device capable of providing a processor, application-specific integrated circuit, field-programmable gate array, etc. with program instructions. The memory may include a memory chip, electrically erasable programmable read-only memory (EEPROM), erasable programmable read only memory (EPROM), flash memory, or any other suitable memory from which the controller  120  can read instructions. The instructions may include code from any suitable programming language. 
     The selective catalytic reduction catalyst  106  is configured to assist in the reduction of NO x  emissions by accelerating a NO x  reduction process between the ammonia and the NO x  of the exhaust gas into diatomic nitrogen, water, and/or carbon dioxide. The selective catalytic reduction catalyst  106  includes inlet in fluid communication with the decomposition chamber  104  from which exhaust gas and reductant is received and an outlet in fluid communication with an end of the exhaust system  190 . 
     The exhaust system  190  may further include an oxidation catalyst, for example a diesel oxidation catalyst (DOC), in fluid communication with the exhaust system  190  (e.g., downstream of the selective catalytic reduction catalyst  106  or upstream of the diesel particulate filter  102 ) to oxidize hydrocarbons and carbon monoxide in the exhaust gas. 
     In some implementations, the diesel particulate filter  102  may be positioned downstream of the decomposition chamber or reactor tube  104 . For instance, the diesel particulate filter  102  and the selective catalytic reduction catalyst  106  may be combined into a single unit, such as a diesel particulate filter with selective catalytic reduction-coating (SDPF). In some implementations, the dosing module  112  may instead be positioned downstream of a turbocharger or upstream of a turbocharger. 
     The sensor  150  may be coupled to the exhaust system  190  to detect a condition of the exhaust gas flowing through the exhaust system  190 . In some implementations, the sensor  150  may have a portion disposed within the exhaust system  190 , such as a tip of the sensor  150  may extend into a portion of the exhaust system  190 . In other implementations, the sensor  150  may receive exhaust gas through another conduit, such as a sample pipe extending from the exhaust system  190 . While the sensor  150  is depicted as positioned downstream of the selective catalytic reduction catalyst  106 , it should be understood that the sensor  150  may be positioned at any other position of the exhaust system  190 , including upstream of the diesel particulate filter  102 , within the diesel particulate filter  102 , between the diesel particulate filter  102  and the decomposition chamber  104 , within the decomposition chamber  104 , between the decomposition chamber  104  and the selective catalytic reduction catalyst  106 , within the selective catalytic reduction catalyst  106 , or downstream of the selective catalytic reduction catalyst  106 . In addition, two or more sensors  150  may be utilized for detecting a condition of the exhaust gas, such as two, three, four, five, or six sensors  150  with each sensor  150  located at one of the foregoing positions of the exhaust system  190 . 
     III. Example Deposit Detection Using a Radiofrequency Sensor 
     In some implementations, radiofrequency (RF) sensing technology can be used to detect the presence of reductant or NH 3  deposits in the decomposition reactor tube (DRT)  104 . In some implementations, the radiofrequency sensor may be a radiometer. The radiometer may have an operational range between 0.4 GHz and 2.5 GHz. In addition, the radiofrequency sensor can be used to determine the amount of NH 3  storage on the selective catalytic reduction catalyst  106  based on calibrations of the radiofrequency sensor output. As the selective catalytic reduction catalyst  106  acts as a resonant cavity, the radiofrequency sensor can detect a base level radiofrequency measurement for a selective catalytic reduction catalyst  106  with no NH 3  storage. The base level radiofrequency measurement can be measured and stored in a machine readable medium. In some implementations, the base level radiofrequency measurement for the selective catalytic reduction catalyst  106  may be initially stored when the aftertreatment system is first constructed. In other implementations, the base level radiofrequency measurement for the selective catalytic reduction catalyst  106  may be stored upon each key-on event for a vehicle. In still further implementations, the base level radiofrequency measurement for the selective catalytic reduction catalyst  106  may be stored when the selective catalytic reduction catalyst  106  has no NH 3  storage for a predetermined period of time (e.g., for one minute), such that the base level radiofrequency measurement can be reset each time the stored NH 3  on the selective catalytic reduction catalyst  106  is known to be exhausted. To detect the amount of NH 3  storage on the selective catalytic reduction catalyst  106 , a measured radiofrequency signal can be compared to the stored base radiofrequency level measurement to compare the variations in signal peaks at resonant modes. That is, when NH 3  is stored on the selective catalytic reduction catalyst  106 , the dielectric properties of the cavity of the selective catalytic reduction catalyst  106  changes and affects the signal peaks at resonant modes. A comparison of the change in measured signal peak at resonant modes can be compared to a stored table of known NH 3  storage amounts and/or a NH 3  storage transfer function can be empirically determined based on the stored base radiofrequency level measurement and used to calculate an amount of NH 3  storage on the selective catalytic reduction catalyst  106 . In some implementations, the foregoing radiofrequency sensor measurement to detect NH 3  storage can be used to detect other contaminants on other aftertreatment components, such as the diesel particulate filter  102 , diesel oxidation catalyst, etc. In still other implementations, the radiofrequency sensor may be used to detect contamination deposits in other cavities of a vehicle. 
     In addition to detecting the NH 3  storage on a catalyst, the radiofrequency sensors can be used to detect reductant deposits within the decomposition reactor tube  104 . As the decomposition reactor tube  104  acts as a resonant cavity, similar to the selective catalytic reduction catalyst, the radiofrequency sensor can detect a base level radiofrequency measurement for a decomposition reactor tube  104  with no reductant deposits. The base level radiofrequency measurement can be measured and stored in a machine readable medium. In some implementations, the base level radiofrequency measurement for the decomposition reactor tube  104  may be initially stored when the aftertreatment system is first constructed. In other implementations, the base level radiofrequency measurement for the decomposition reactor tube  104  may be stored upon each key-on event for a vehicle. In still further implementations, the base level radiofrequency measurement for the decomposition reactor tube  104  may be stored after a regeneration event to clear reductant deposits in the decomposition reactor tube  104 . To detect the amount of reductant deposits in the decomposition reactor tube  104 , a measured radiofrequency signal can be compared to the stored base radiofrequency level measurement to compare the variations in signal peaks at resonant modes. That is, when reductant deposits form in the decomposition reactor tube  104 , the dielectric properties of the cavity of the decomposition reactor tube  104  changes and affects the signal peaks at resonant modes. A comparison of the change in measured signal peak at resonant modes can be compared to a stored table of known reductant deposit amounts and/or a reductant deposit transfer function can be empirically determined based on the stored base radiofrequency level measurement and used to calculate an amount of reductant deposits in the decomposition reactor tube  104 . 
     If the amount of NH 3  storage and/or reductant deposit is known, then control strategies can be implemented to modulate the engine-out NO x , exhaust gas temperature, exhaust gas mass-flow, and/or reductant dosing to control the amount of NH 3  storage and/or reduce the amount of reductant deposit and/or eliminate it entirely. 
     In the implementation shown in  FIG.  2   , radiofrequency sensor probes can be placed at an upstream position and/or a downstream position of a decomposition reactor tube and/or selective catalytic reduction catalyst. The position of the radiofrequency sensor probes define the boundaries of the Faraday cage/shield formed by the aftertreatment system components. Accordingly, the base level radiofrequency measurement is taken for a particular location of the radiofrequency sensor probe for calibration purposes and any changes to the radiofrequency sensor probe location will need recalibration. The exhaust tube housing acts as a Faraday shield and helps exclude electrostatic and electromagnetic interference from other components of the exhaust system and/or vehicle. The radiofrequency signal is affected in a repeatable way in the presence of reductant deposits (amplitude, phase shift, etc.) and these calibrated radiofrequency signal responses are used to detect the presence and/or quantity of NH 3  storage and/or reductant deposits using an NH 3  storage and/or reductant deposit transfer function. 
     In some implementations, two radiofrequency sensor probes can be used to calculate a reflected parameter, S 11 , and a transmission parameter, S 12 , based on radiofrequency scattering, which can be used to detect NH 3  storage and/or reductant deposit amounts. In some implementations, radiofrequency noise, such as from temperature variations within the aftertreatment system, can be compensated based on measuring the temperature with a temperature sensor. 
     Based on the radiofrequency signal response, the engine-out NO x , engine temperature and/or exhaust mass flow can be modified to gradually decompose the reductant deposits. In some implementations, the radiofrequency signal response can be integrated into a reductant dosing strategy to improve robustness and reduce the likelihood of failure modes due to reductant deposits. 
       FIG.  3    illustrates a reductant deposit mitigation system  300 , according to an example embodiment. The reductant deposit mitigation system  300  is implemented in an aftertreatment system  302 . The aftertreatment system  302  includes an upstream exhaust component  304 . The upstream exhaust component  304  receives exhaust gases from an internal combustion engine (e.g., a diesel internal combustion engine, etc.). In various embodiments, the upstream exhaust component  304  is a manifold of the internal combustion engine. In other embodiments, the upstream exhaust component  304  is a turbocharger of the internal combustion engine. In still other embodiments, the upstream exhaust component  304  is a component of a waste heat recovery system. 
     The aftertreatment system  302  also includes an upstream exhaust conduit  306  (e.g., exhaust pipe, etc.). The upstream exhaust conduit  306  receives the exhaust gases from the upstream exhaust component  304 . The aftertreatment system  302  also includes a diesel oxidation catalyst  308 . The diesel oxidation catalyst  308  oxidizes hydrocarbons and carbon monoxide in the exhaust gases received from the upstream exhaust component  304 . As a result, the diesel oxidation catalyst  308  may provide a mixture of carbon dioxide and water, among other components. 
     The aftertreatment system  302  also includes a diesel particulate filter  310 . The diesel particulate filter  310  receives the exhaust gases from the diesel oxidation catalyst  308 . In various embodiments, the diesel oxidation catalyst  308  is positioned immediately upstream of the diesel particulate filter  310  such that the diesel oxidation catalyst  308  and the diesel particulate filter  310  are contained in some same housing and are not separated by an exhaust conduit. In other embodiments, the diesel oxidation catalyst  308  and the diesel particulate filter  310  are separated by an exhaust conduit similar to the upstream exhaust conduit  306 . The diesel particulate filter  310  removes particulate matter, such as soot, from the exhaust gases provided by the diesel oxidation catalyst  308 . The diesel particulate filter  310  includes an inlet, where the exhaust gases are received, and an outlet, where the exhaust gases exit after having particulate matter substantially filtered from the exhaust gases and/or converted into carbon dioxide. 
     The aftertreatment system  302  also includes a decomposition reactor tube  312 . The decomposition reactor tube  312  receives the exhaust gases from the diesel particulate filter  310 . In various embodiments, the diesel particulate filter  310  is positioned immediately upstream of the decomposition reactor tube  312  such that the diesel particulate filter  310  and the decomposition reactor tube  312  are contained in some same housing and are not separated by an exhaust conduit. In other embodiments, the diesel particulate filter  310  and the decomposition reactor tube  312  are separated by an exhaust conduit similar to the upstream exhaust conduit  306 . The decomposition reactor tube  312  converts reductant provided by a doser  314  into ammonia, NH 3 , through hydrolysis. In various embodiments, the doser  314  is coupled to the decomposition reactor tube  312  such that the reductant is provided directly into the decomposition reactor tube  312 . In other embodiments, the doser  314  is coupled to the aftertreatment system  302  upstream of the decomposition reactor tube  312 . For example, the doser  314  may be coupled to the diesel particulate filter  310  or the diesel oxidation catalyst  308 . In various embodiments, the decomposition reactor tube  312  may include various mixers (e.g., baffles, vanes, etc.) and other flow devices configured to facilitate mixing of the exhaust gases and reductant. For example, the decomposition reactor tube  312  may include a mixing device configured to impart a swirl flow. The decomposition reactor tube  312  includes an inlet, where the exhaust gases are received, and an outlet, where the exhaust gases exit (e.g., after being mixed with reductant, etc.). 
     The aftertreatment system  302  also includes a selective catalytic reduction catalyst  316 . The selective catalytic reduction catalyst  316  receives the exhaust gases (e.g., a mixture of the exhaust gases and reductant, etc.) from the decomposition reactor tube  312 . In various embodiments, the decomposition reactor tube  312  is positioned immediately upstream of the selective catalytic reduction catalyst  316  such that the decomposition reactor tube  312  and the selective catalytic reduction catalyst  316  are contained in some same housing and are not separated by an exhaust conduit. In other embodiments, the decomposition reactor tube  312  and the selective catalytic reduction catalyst  316  are separated by an exhaust conduit similar to the upstream exhaust conduit  306 . The selective catalytic reduction catalyst  316  converts NO x  into nitrogen gas and water vapor. The selective catalytic reduction catalyst  316  may include various catalysts such as, for example, ceramic catalysts, titanium oxide catalysts, vanadium catalysts, molybdenum catalysts, tungsten catalysts, zeolite catalysts, activated carbon catalysts, and other similar catalysts. The selective catalytic reduction catalyst  316  includes an inlet, where the exhaust gases are received, and an outlet, where the exhaust gases exit. 
     The aftertreatment system  302  also includes an ammonia slip catalyst (ASC)  318 . The ammonia slip catalyst  318  receives the exhaust gases from the selective catalytic reduction catalyst  316 . In various embodiments, the selective catalytic reduction catalyst  316  is positioned immediately upstream of the ammonia slip catalyst  318  such that the selective catalytic reduction catalyst  316  and the ammonia slip catalyst  318  are contained in some same housing and are not separated by an exhaust conduit. In other embodiments, the selective catalytic reduction catalyst  316  and the ammonia slip catalyst  318  are separated by an exhaust conduit similar to the upstream exhaust conduit  306 . The ammonia slip catalyst  318  mitigates emission of NH 3  and/or converts NO x  to nitrogen gas. The ammonia slip catalyst  318  includes an inlet, where the exhaust gases are received, and an outlet, where the exhaust gases exit. 
     The aftertreatment system  302  also includes a downstream exhaust conduit  320  (e.g., exhaust pipe, etc.). The downstream exhaust conduit  320  receives the exhaust gases from the ammonia slip catalyst  318 . The aftertreatment system  302  includes a downstream exhaust component  322 . The downstream exhaust component  322  receives exhaust gases from the downstream exhaust conduit  320 . In various embodiments, the downstream exhaust component  322  is a tailpipe (e.g., muffler, etc.). 
     The reductant deposit mitigation system  300  includes a first radiofrequency device  324  and a second radiofrequency device  326 . The first radiofrequency device  324  is positioned between the diesel particulate filter  310  and the decomposition reactor tube  312 . The second radiofrequency device  326  is positioned between the decomposition reactor tube  312  and the selective catalytic reduction catalyst  316 . The first radiofrequency device  324  includes a first radiofrequency communicator  328  positioned within the diesel particulate filter  310  and/or the decomposition reactor tube  312 . The second radiofrequency device  326  includes a second radiofrequency communicator  330  positioned within the decomposition reactor tube  312  and/or the selective catalytic reduction catalyst  316 . The decomposition reactor tube  312 , as well as the diesel particulate filter  310  and/or the selective catalytic reduction catalyst  316  in some embodiments, creates a Faraday cage around the first radiofrequency communicator  328  and the second radiofrequency communicator  330 . The Faraday cage facilitates calibration of the first radiofrequency communicator  328  and the second radiofrequency communicator  330  because outside radiofrequency signals are substantially blocked from entering the Faraday cage. Additionally, the Faraday cage substantially excludes electrostatic and electromagnetic interference from other components of the exhaust system and/or vehicle. 
     The first radiofrequency communicator  328  is a radiofrequency transmitter, a radiofrequency receiver, or a radiofrequency transceiver. The second radiofrequency communicator  330  is a radiofrequency transmitter, a radiofrequency receiver, or a radiofrequency transceiver. In various embodiments, the first radiofrequency communicator  328  is one of a radiofrequency transmitter and a radiofrequency receiver and the second radiofrequency communicator  330  is the other of the radiofrequency transmitter and the radiofrequency receiver. For example, in some embodiments, the first radiofrequency communicator  328  is a radiofrequency transmitter and the second radiofrequency communicator  330  is a radiofrequency receiver. In other embodiments, the first radiofrequency communicator  328  is a radiofrequency receiver and the second radiofrequency communicator  330  is a radiofrequency transmitter. In still other embodiments, both the first radiofrequency communicator  328  and the second radiofrequency communicator  330  are radiofrequency transceivers. A radiofrequency transceiver includes a radiofrequency transmitter and a radiofrequency receiver. 
     At least one of the first radiofrequency communicator  328  and the second radiofrequency communicator  330  is configured to transmit a radiofrequency signal to the other of the first radiofrequency communicator  328  and the second radiofrequency communicator  330 . The radiofrequency signal is affected in a repeatable way in the presence of reductant deposits (amplitude, phase shift, etc.). In embodiments where the first radiofrequency communicator  328  and the second radiofrequency communicator  330  are each radiofrequency transceivers, a first radiofrequency signal may be transmitted from the first radiofrequency communicator  328  to the second radiofrequency communicator  330  and a second radiofrequency signal may be transmitted from the second radiofrequency communicator  330  to the first radiofrequency communicator  328  (e.g., simultaneously, in alternating fashion, etc.). 
     The reductant deposit mitigation system  300  includes a controller  332 , such as a reductant deposit mitigation controller. The controller  332  is electronically communicable with the first radiofrequency device  324 , and therefore with the first radiofrequency communicator  328 , and the second radiofrequency device  326 , and therefore with the second radiofrequency communicator  330 . The controller  332  is configured to control the first radiofrequency communicator  328  and/or the second radiofrequency communicator  330  to transmit a radiofrequency signal. 
     The controller  332  includes an input/output (I/O) interface  334  and a processing circuit  336 . The input/output interface  334  facilitates interaction between the processing circuit  336  and the first radiofrequency device  324  and the second radiofrequency device  326 . The processing circuit  336  includes a processor  338  and a memory  340 . The memory  340  may include, but is not limited to, electronic, optical, magnetic, or any other storage or transmission device capable of providing the processor  338  with program instructions. The memory  340  may include a memory chip, electrically erasable programmable read-only memory, erasable programmable read only memory, flash memory, or any other suitable memory from which the modules can read instructions. The instructions may include code from any suitable programming language. 
     The memory  340  includes a number of modules (e.g., microprocessors, application-specific integrated circuit, field-programmable gate arrays, etc.). As shown in  FIG.  3   , the memory  340  includes a first radiofrequency device module  342  and a second radiofrequency device module  344 . The first radiofrequency device module  342  is configured to control interactions between the controller  332  and the first radiofrequency device  324 . The second radiofrequency device module  344  is configured to control interactions between the controller  332  and the second radiofrequency device  326 . The memory  340  may also include additional modules, such as a module for facilitating communication between the controller  332  and an engine control unit (ECU) of an internal combustion engine associated with the reductant deposit mitigation system  300 . 
     The controller  332  is configured to compare a transmitted radiofrequency signal, such as a radiofrequency signal transmitted by the first radiofrequency communicator  328 , with a received radiofrequency signal, such as an radiofrequency signal received by the second radiofrequency communicator  330 . This comparison is used by the controller  332  to detect the presence and/or quantity of NH 3  storage and/or reductant deposits within the decomposition reactor tube  312  using an NH 3  storage and/or reductant deposit transfer function. 
     The controller  332  may analyze a received radiofrequency signal to calculate a reflected parameter, S 11 , and a transmission parameter, S 12 , based on radiofrequency scattering, which can be used to detect NH 3  storage and/or reductant deposit amounts. In some implementations, radiofrequency noise, such as from temperature variations within the aftertreatment system, can be compensated by the controller  332  based on measuring the temperature with a temperature sensor. 
     Based on the comparison, the controller  332  can send a signal to an ECU of the internal combustion engine to modify the engine-out NO x , engine temperature, and/or exhaust mass flow to gradually decompose the reductant deposits. In some implementations, the comparison can be integrated into a reductant dosing strategy implemented by the ECU to improve robustness and reduce the likelihood of failure modes due to reductant deposits. 
     Rather than merely detecting the presence of the controller  332  utilizes the comparison to determine an exact quantity of reductant deposits within the decomposition reactor tube  312 . Other detection systems are not able to determine the amount of reductant deposits and rely on cross-sensitive sensors which can be inaccurate in the presence of NH 3  slip. 
     While the aftertreatment system  302  is shown as including the diesel oxidation catalyst  308 , the diesel particulate filter  310 , the selective catalytic reduction catalyst  316 , and the ammonia slip catalyst  318 , it is understood that the aftertreatment system  302  may not include any of the diesel oxidation catalyst  308 , the diesel particulate filter  310 , the selective catalytic reduction catalyst  316 , or the ammonia slip catalyst  318  such that the aftertreatment system  302  is tailored for a target application. Furthermore, the position of any of the diesel oxidation catalyst  308 , the diesel particulate filter  310 , the selective catalytic reduction catalyst  316 , and the ammonia slip catalyst  318  may be varied such that the aftertreatment system  302  is tailored for a target application. 
       FIG.  4    illustrates a process  400  for mitigating reductant deposits using the reductant deposit mitigation system  300  in the aftertreatment system  302 . The process  400  begins, in block  402 , with accessing, by the controller  332 , data indicative of a signal output from a radiofrequency device (e.g., a radiofrequency sensor, the first radiofrequency device  324 , the second radiofrequency device  326 , etc.). The radiofrequency device is positioned proximate a decomposition reactor tube, such as the decomposition reactor tube  312 . Then, in block  404 , the controller  332  compares the data indicative of a signal output from the radiofrequency device to a deposit formation threshold. The deposit formation threshold may be programmed into the controller  332  or may be determined by the controller  332  (e.g., by machine learning, etc.). The deposit formation threshold may be associated with an amount of reductant deposits within the decomposition reactor tube  312  which is associated with undesirable performance of the aftertreatment system  302 . 
     The process  400  continues, in block  406 , with activating, by the controller  332 , a deposit mitigation process in response to the data indicative of a signal output from the radiofrequency device exceeding the deposit formation threshold. For example, if the controller  332  determines that 0.5 mm of reductant deposit is present on the wall of the decomposition reactor tube  312  and the deposit formation threshold is 0.45 mm, the controller  332  may activate a deposit mitigation process. The deposit mitigation process may be, for example, transmitting a signal to the ECU to modify the engine-out NO x , engine temperature, and/or exhaust mass flow to gradually decompose the reductant deposits within the decomposition reactor tube  312 . 
     In some embodiments, the process  400  includes, in block  408  which occurs after block  402  and before block  404 , with calculating, by the controller  332 , an amount of a reductant deposit (e.g., in the decomposition reactor tube  312 , etc.) based on the data indicative of a signal output from the radiofrequency device. In these embodiments, block  404  is implemented by comparing the amount of the reductant deposit to a deposit formation threshold and block  406  is implemented based on that comparison. 
     IV. Configuration of Exemplary Embodiments 
     The term “controller” encompasses all kinds of apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, a system on a chip, or multiple ones, a portion of a programmed processor, or combinations of the foregoing. The apparatus can include special purpose logic circuitry, e.g., a field-programmable gate array or an application-specific integrated circuit. The apparatus can also include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a cross-platform runtime environment, a virtual machine, or a combination of one or more of them. The apparatus and execution environment can realize various different computing model infrastructures, such as distributed computing and grid computing infrastructures. 
     A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, declarative or procedural languages, and it can be deployed in any form, including as a standalone program or as a module, component, subroutine, object, or other unit suitable for use in a computing environment. A computer program may, but need not, correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of what may be claimed, but rather as descriptions of features specific to particular implementations. Certain features described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described components and systems can generally be integrated in a single product or packaged into multiple products embodied on tangible media. 
     As utilized herein, the terms “approximately,” “about,” “substantially”, and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numerical ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described and claimed are considered to be within the scope of the invention as recited in the appended claims. Additionally, it is noted that limitations in the claims should not be interpreted as constituting “means plus function” limitations under the United States patent laws in the event that the term “means” is not used therein. 
     The terms “coupled,” “connected,” and the like as used herein mean the joining of two components directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two components or the two components and any additional intermediate components being integrally formed as a single unitary body with one another or with the two components or the two components and any additional intermediate components being attached to one another. 
     The terms “fluidly coupled,” “in fluid communication,” and the like as used herein mean the two components or objects have a pathway formed between the two components or objects in which a fluid, such as water, air, gaseous reductant, gaseous ammonia, etc., may flow, either with or without intervening components or objects. Examples of fluid couplings or configurations for enabling fluid communication may include piping, channels, or any other suitable components for enabling the flow of a fluid from one component or object to another. 
     It is important to note that the construction and arrangement of the system shown in the various exemplary implementations is illustrative only and not restrictive in character. All changes and modifications that come within the spirit and/or scope of the described implementations are desired to be protected. It should be understood that some features may not be necessary and implementations lacking the various features may be contemplated as within the scope of the application, the scope being defined by the claims that follow. In reading the claims, it is intended that when words such as “a,” “an,” “at least one,” or “at least one portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. When the language “at least a portion” and/or “a portion” is used the item can include a portion and/or the entire item unless specifically stated to the contrary.