Patent Application
20130019589
This disclosure relates to compression-ignition internal combustion engine emissions aftertreatment.
The statements in this section merely provide background information related to the present disclosure. Accordingly, such statements are not intended to constitute an admission of prior art.
Compression-ignition internal combustion engines operate at lean air/fuel ratios to achieve desirable fuel efficiencies. Lean engine operation may produce oxides of nitrogen (NOx) when nitrogen and oxygen molecules present in engine intake air disassociate in the high temperatures of combustion. Rates of NOx production follow known relationships in the combustion process, for example with higher rates of NOx production being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures.
NOx molecules may be reduced to elemental nitrogen and oxygen in aftertreatment devices. Efficacy of known aftertreatment devices is dependent upon operating conditions including operating temperature, which is associated with exhaust gas flow temperatures and engine air/fuel ratio. Aftertreatment devices include materials prone to damage or degradation when exposed to elevated temperatures and/or contaminants in the exhaust gas feedstream.
Aftertreatment systems include catalytic devices to generate chemical reactions to treat exhaust gas constituents. Three-way catalytic devices (TWC) oxidize and reduce exhaust gas constituents. NOx adsorbers store NOx, which may be subsequently desorbed and reduced under specific engine operating conditions. One known strategy includes using a NOx adsorber to store NOx emissions during lean operations and then purging and reducing the stored NOx to nitrogen and water using a TWC during rich engine operating conditions. Particulate filters (DPF) are able to remove particulate matter in the exhaust gas feedstream, which may then be periodically purged, e.g. during high temperature regeneration events.
One known aftertreatment device is a selective catalytic reduction device (SCR). The SCR device includes catalytic material that promotes the reaction of NOx with a reductant such as ammonia (NH3) or urea to produce nitrogen and water. Reductants, e.g. urea, may be injected into an exhaust gas feedstream upstream of the SCR device, which requires an injection system, storage tank and a control scheme. Reductants, e.g. NH3, may be generated in an exhaust gas feedstream upstream of the SCR device during specific engine operating conditions.
Catalytic materials used in SCR devices include vanadium (V) and tungsten (W) on titanium (Ti) and base metals including iron (Fe) or copper (Cu) with a zeolite washcoat. Catalytic materials including copper may perform effectively at lower temperatures but have been shown to have poor thermal durability at higher temperatures. Catalytic materials including iron may perform well at higher temperatures but with decreasing reductant storage efficiency at lower temperatures.
Known SCR devices preferably operate within an operating temperature range of 150° C. to 600° C. The temperature range may vary depending on the catalyst materials. An operating temperature range may decrease during or after higher engine load operation. Temperatures greater than 600° C. may cause reductants to breakthrough and degrade an SCR catalyst, while the effectiveness of NOx processing decreases at temperatures lower than 150° C.
An internal combustion engine configured to operate in a compression-ignition combustion mode includes an exhaust aftertreatment system. The exhaust aftertreatment system includes a catalyst device fluidly coupled upstream of an ammonia-selective catalytic reduction device. The, catalyst device includes first, second, and third elements fluidly coupled in series. The first element includes a three-way catalytic element, the second element includes a NOx adsorber, and the third element includes an oxidation catalytic element.
One or more embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:
Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments only and not for the purpose of limiting the same,
The exhaust manifold 18 channels the exhaust gas feedstream of the internal combustion engine 10 to the passive NH3-SCR exhaust aftertreatment system 40. A second sensing device 45 is configured to monitor the exhaust gas feedstream downstream of the NH3-SCR catalyst 44, and may include, e.g., a NOx sensor, a NH3 sensor, or another suitable sensor. The second sensing device 45 generates a signal 81 readable by the control module 50 for purposes of control and diagnostics.
The passive NH3-SCR exhaust aftertreatment system 40 includes the first catalytic converter 42 fluidly coupled to and upstream of the aforementioned NH3-SCR catalyst 44. The first catalytic converter 42 includes first, second and third catalyst elements 51, 53, and 55, respectively. The first, second and third catalyst elements 51, 53, and 55 are arranged in series with the first catalyst element 51 fluidly coupled to the exhaust manifold 18 and configured to treat engine-out exhaust gas. Exhaust gas treated in the first catalyst element 51 passes to the second catalyst element 53 and then passes to the third catalyst element 55, as is appreciated. Each of the first, second and third catalyst elements 51, 53, and 55 includes a ceramic or metallic substrate element that is coated as described herein.
The first catalyst element 51 is preferably a catalytic element that includes a substrate element coated with a washcoat that is capable of oxidizing HC and CO molecules and reducing NOx molecules in response to engine operating conditions including air/fuel ratio. The catalytically active materials include Pd/Al2O3 in one embodiment. Alternatively, the first catalyst element 51 may be another suitable three-way catalytic element including a substrate element coated with a catalytically active washcoat that oxidizes HC and CO molecules and reduces NOx molecules in response to engine operating conditions.
The second catalyst element 53 is preferably a NOx adsorber that includes a substrate element coated with a washcoat that is capable of adsorbing and desorbing NOx molecules. The substrate element is coated with a washcoat including LaMnO2 and BaO in one embodiment. As such, there are preferably no platinum-group metals (e.g., platinum, palladium, and rhodium) used in the NOx adsorber. Alternatively, the second catalyst element 53 may be any other NOx adsorber element including a substrate element coated with a suitable washcoat that is capable of adsorbing and desorbing NOx molecules.
The third catalyst element 55 is preferably a catalytic element including a substrate element coated with a washcoat containing one or more catalytically active materials for oxidizing hydrocarbons in the exhaust gas feedstream. The catalytically active materials include Rh/CeO2 and Al2O3 in one embodiment. Alternatively, the third catalyst element 55 may be another suitable oxidation catalytic element or three-way catalytic electric including a substrate element coated with a catalytically active washcoat. This may include a three-way catalyst device that is capable of oxidizing HC and CO molecules and reducing NOx molecules in response to engine operating conditions including air/fuel ratio.
Ammonia (NH3) may be passively generated in the first catalytic converter 42 using a system wherein engine operation is periodically modulated to generate an exhaust gas feedstream that includes nitric oxide (NO), carbon monoxide (CO), and hydrogen (H2). The exhaust gas feedstream produces NH3 in the first catalytic converter 42 under specific operating conditions. It is appreciated that the amount of NH3 that is produced in either or both the first, three-way catalytic element 51 and the third oxidation catalytic element 55 is limited by the engine-out NOx level. Additional H2 is available, which may be used to produce NH3. The chemical equation expressing this relationship as follows.
NOx+H2/CONH3+CO2 [1]
The NH3-SCR catalyst 44 includes one or more substrate elements preferably fabricated from cordierite material with a multiplicity of flowthrough passageways that are preferably coated with a zeolite washcoat and catalytic material including, e.g., a catalytically active base metal. The catalytically active materials store NH3, and release stored NH3 for reacting with NOx molecules in the exhaust gas feedstream. It is appreciated that the storage capacity of a NH3-SCR catalyst, i.e., the mass amount of NH3 that may be stored on a NH3-SCR catalyst, correlates to an inlet temperature of the NH3-SCR catalyst. When the inlet temperature increases above a threshold temperature, the storage capacity decreases.
The particulate filter 46 is fluidly coupled downstream of the NH3-SCR catalyst 44, and includes a ceramic filter element configured to trap particulate matter. In one embodiment, the ceramic filter element is a wall-flow filtering element. In one embodiment, the ceramic filter element is coated with a washcoat including suitable catalytically active materials. Particulate filters 46 may include other suitable features for trapping and oxidizing particulate matter produced during combustion.
A control module 50 is signally connected to engine sensors and operatively connected to engine actuators to execute control schemes to control operation of the engine 10 to form cylinder charges in response to an operator command. The sensors include, e.g., the air/fuel ratio sensing device 41, the mass airflow sensing device 24, and the pressure sensor(s) 30. The actuators include, e.g., the fuel injector 28, the throttle device 23, and the EGR valve 22. The control module 50 operates the fuel injector 28 by commanding a pulsewidth 77 to deliver a fuel pulse to the combustion chamber 16. The pulsewidth 77 is an elapsed time period during which the fuel injector 28 is opened and delivering the fuel pulse. The delivered fuel pulse interacts with intake air and any internally retained and externally recirculated exhaust gases to form a cylinder charge in the combustion chamber 16 in response an operator torque request. It is appreciated that the control module 50 may command multiple fuel injection events using multiple pulsewidths 77 to cause the fuel injector 28 to deliver the fuel pulse to the combustion chamber 16 during each cylinder event.
The control module 50 operates the EGR valve 22 by commanding an EGR valve opening command 78 to cause the EGR valve 22 to effect a preferred EGR flowrate to achieve a preferred EGR fraction in the cylinder charge. It is appreciated that age, calibration, contamination and other factors may affect operation of the EGR system 20, thus causing variations in in-cylinder air/fuel ratio of the cylinder charge.
The control module 50 may operate the throttle device 23 by commanding a throttle valve opening command 76 to command a preferred fresh air mass flowrate for the cylinder charge. In one embodiment, the control module 50 operates a turbocharger device to command a preferred boost pressure associated with the cylinder charge.
Control module, module, control, controller, control unit, processor and similar terms mean any one or various combinations of one or more of Application Specific Integrated Circuit(s) (ASIC), electronic circuit(s), central processing unit(s) (preferably microprocessor(s)) and associated memory and storage (read only, programmable read only, random access, hard drive, etc.) executing one or more software or firmware programs or routines, combinational logic circuit(s), input/output circuit(s) and devices, appropriate signal conditioning and buffer circuitry, and other components to provide the described functionality. Software, firmware, programs, instructions, routines, code, algorithms and similar terms mean any controller executable instruction sets including calibrations and look-up tables. The control module has a set of control routines executed to provide the desired functions. Routines are executed, such as by a central processing unit, and are operable to monitor inputs from sensing devices and other networked control modules, and execute control and diagnostic routines to control operation of actuators. Routines may be executed at regular intervals, for example each 3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing engine and vehicle operation. Alternatively, routines may be executed in response to occurrence of an event.
It is appreciated that the engine-out NOx in the exhaust gas feedstream of a compression-ignition engine is substantially lower than that for a spark-ignition engine. During ongoing engine operation, NOx emissions generated during cold-start and lean air/fuel engine operation is stored on the second NOx adsorber catalyst element 53, represented as follows.
NOx+MOM(NO3)2 [2]
During engine operation under other conditions, the stored NOx is used to produce additional NH3 that passes through to the NH3-SCR catalyst 44 and stored for NOx reduction, represented as follows.
NOx+H2/CONH3+CO2 [3]
The configuration of the passive NH3-SCR exhaust aftertreatment system 40 including the first catalytic converter 42 as described herein enables use of a passive NH3-SCR exhaust aftertreatment system in a compression-ignition engine system. The first catalytic converter 42 produces NH3, thus minimizing any fuel penalty associated with rich events required to produce NH3. Absorbing NOx in the first catalytic converter 42 reduces reliance upon the NH3-SCR catalyst 44 to achieve NOx reduction during lean operation.
An exemplary control scheme for managing an exhaust gas feedstream from the engine 10 coupled to the passive NH3-SCR exhaust aftertreatment system 40 includes a process for repetitively cycling between a lean air/fuel ratio and a stoichiometric or rich air/fuel ratio, depending upon operating conditions. Preferably the exhaust gas feedstream and/or selected elements of the passive NH3-SCR exhaust aftertreatment system 40 are monitored to detect or otherwise determine NOx breakthrough and NH3 breakthrough downstream of the NH3-SCR catalyst 44, e.g., using the second sensing device 45. Monitoring the NH3-SCR catalyst 44 preferably includes monitoring temperature of the exhaust gas feedstream proximal to the NH3-SCR catalyst 44 to ensure the temperature of the NH3-SCR catalyst 44 is within a preferred temperature range. In one embodiment, the preferred temperature range is 150° C. to 450° C. When the temperature of the NH3-SCR catalyst 44 is maintained within the preferred temperature range, engine operation may be modulated to produce nitric oxide (NO), carbon monoxide (CO), and hydrogen (H2) to produce NH3 in the first catalytic converter 42 that may be transferred downstream to the NH3-SCR catalyst 44 for storage and subsequent NOx reduction.
The modulated engine operation includes operating the engine 10 rich or at stoichiometry while meeting the operator torque request and without changing engine output power. One exemplary method for operating the exemplary engine 10 rich of stoichiometry may include executing multiple fuel injection pulses during a combustion cycle including injecting a first fuel pulse into the combustion chamber 16 during each compression stroke. The mass of fuel injected during the first fuel pulse is determined based upon an amount sufficient to operate the engine 10 to meet the operator torque request and other load demands. Subsequent fuel pulses may be injected into the combustion chamber 16 during other strokes of the combustion cycle thereby generating an exhaust gas feedstream including nitric oxide (NO), carbon monoxide (CO), and hydrogen (H2) to produce NH3 in the first catalytic converter 42. In one embodiment, the subsequent fuel pulses are executed late in a power stroke or early in an exhaust stroke of the combustion cycle thereby minimizing likelihood of combustion in the combustion chamber 16.
The modulated engine operation is discontinued and the engine 10 is controlled to operate lean of stoichiometry when the NH3-SCR catalyst 44 has stored a sufficient amount of NH3, preferably before reaching a saturation point. NH3 production may instead be discontinued after a predetermined threshold of NH3 molecules are generated or when engine operating conditions are not conducive to NH3 production, e.g., during vehicle decelerations, engine idling, or engine stops. NH3 saturation may be estimated based upon a predetermined elapsed time period of operation in the modulated mode, or by monitoring the exhaust gas feedstream downstream of the NH3-SCR catalyst 44 to detect NH3 breakthrough, or determined after executing a predetermined number of cylinder events. NH3 breakthrough may be detected by monitoring signal output of an NH3 sensor configured to monitor the exhaust gas feedstream downstream of the NH3-SCR catalyst 44. In one embodiment, NH3 saturation may be estimated using a model according to methods sufficient to accurately estimate operation of the combustion cycle, aftertreatment processes, conversions, and monitored operating conditions including intake mass airflow, air/fuel ratio, engine speed, and temperatures and aging states of the first catalytic converter 42 and the NH3-SCR catalyst 44. Such a model may be calibrated according to test results corresponding to a particular hardware application.
After determining the NH3-SCR catalyst 44 is saturated with NH3, or deciding based upon other considerations including operating conditions, the modulated engine operation is discontinued and the engine operation transitions to lean engine operation, which may result in increased NOx emissions into the exhaust gas flow. The first catalytic converter 42 may reduce a portion of the NOx emissions. NH3 stored on the NH3-SCR catalyst 44 reacts with NOx thereby reducing NOx emissions and producing nitrogen and water. The NH3 stored in the NH3-SCR catalyst 44 is depleted as NH3 molecules react with NOx molecules. When the NH3 on the NH3-SCR catalyst 44 is depleted, NOx emissions may pass through the NH3-SCR catalyst 44. Therefore, the lean engine operation may be discontinued and the control system may revert to the modulated engine operation after detecting NOx breakthrough downstream from the NH3-SCR catalyst 44. NH3 depletion and any associated NOx breakthrough may be detected using the second sensing device 45. Alternatively, NH3 depletion on the NH3-SCR catalyst 44 may be estimated using an executable model.
The engine operating conditions of air/fuel ratio (207) and catalyst temperature (209) include the periodic air/fuel ratio excursions, with specific magnitudes of the air/fuel ratio excursions (208) including 15.0:1, 14.9:1, 14.8:1, 14.7:1, 14.6:1, 14.5:1, 14.4:1, 14.3:1, 14.2:1, 14.1:1, as shown.
As indicated, the concentration of engine-out H2 (213) increases as the air/fuel ratio becomes increasingly rich, whereas the H2 concentration downstream of the first catalyst element 51 (215) is negligible. However, the corresponding H2 concentration downstream of the third catalyst element 55 (217) increases when the air/fuel ratio becomes rich of stoichiometry, and reaches a peak concentration of about 1400 ppm when the air/fuel ratio excursion is at 14.4:1.
The increased H2 in the exhaust gas feedstream upstream of the upstream of the NH3-SCR catalyst 44 reacts with NOx gases to form NH3, which may be stored on the NH3-SCR catalyst 44 and used during lean operation for NOx reduction.
The disclosure has described certain preferred embodiments and modifications thereto. Further modifications and alterations may occur to others upon reading and understanding the specification. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.
