Patent Application
20110072798
The present invention relates generally to an exhaust gas treatment system for use with an internal combustion engine system where the treatment system is of the type using a selective catalytic reduction (SCR) catalyst, and more specifically to systems and methods for operation of the same.
There have been a variety of exhaust gas treatment systems developed in the art to minimize emission of undesirable constituent components of internal combustion engine exhaust gas. For example, it is known to reduce NOx emissions using a SCR catalyst, treatment device that includes a catalyst and a system that is operable to inject material, such as ammonia (NH3), into the exhaust gas feedstream ahead of the catalyst. The SCR catalyst is constructed so as to promote the reduction of NOx by NH3 (or other reductant, such as aqueous urea which undergoes decomposition in the exhaust to produce NH3). NH3 or urea selectively combine with NOx to form N2 and H2O in the presence of the SCR catalyst, as described generally in U.S. Patent Publication 2007/0271908 entitled “ENGINE EXHAUST EMISSION CONTROL SYSTEM PROVIDING ON-BOARD AMMONIA GENERATION”. For diesel engines, for example, selective catalytic reduction (SCR) of NOx with ammonia is perhaps the most selective and active reaction for the removal of NOx in the presence of excess oxygen. The NH3 source must be periodically replenished and the injection of NH3 into the SCR catalyst requires precise control. Over-injection may cause a release of NH3 (“slip”) out of the tailpipe into the atmosphere, while under-injection may result in inadequate emissions reduction (i.e., inadequate NOx conversion to N2 and H2O).
These systems have been amply demonstrated in the stationary catalytic applications. For mobile applications where it is generally not possible (or at least not desirable) to use ammonia directly, urea-water solutions have been proven to be suitable sources of ammonia in the exhaust gas stream. This has made SCR possible for a wide range of vehicle applications.
Increasingly stringent demands for low tail pipe emissions of NOx have been placed on heavy duty diesel powered vehicles. Liquid urea dosing systems with selective catalytic NOx reduction (SCR) technologies have been developed in the art that provide potentially viable solutions for meeting current and future diesel NOx emission standards around the world. Ammonia emissions may also be set by regulation or simply as a matter of quality. For example, European emission standards (e.g., EU 6) for NH3 slip targets specify 10 ppm average and 30 ppm peak. However, the challenge described above remains, namely, that such treatment systems achieve maximum NOx reduction (i.e., at least meeting NOx emissions criteria) while at the same time maintaining acceptable NH3 emissions, particularly over the service life of the treatment system.
However, there are situations where conventional controls are unable to regulate, albeit for relatively short periods of time, NH3 slips to within acceptable levels, even when the dosing control disables NH3 dosing entirely. These situations are undesirable.
There is therefore a need for systems and methods of operating a exhaust gas treatment system that minimize or eliminate one or more of the problems set forth above.
The invention provides an advantage for exhaust gas treatment systems that use ammonia or other reductant (e.g., aqueous urea solution) injection in combination with an SCR catalyst for NOx removal from the engine exhaust gas. Embodiments consistent with the invention involve interaction between the exhaust gas treatment system and the engine system so that the needs of the exhaust gas treatment system are satisfied. For example, when the exhaust treatment system determines that it does not have the ability to control tailpipe NH3 slip, such interaction may involve transmitting a control request to the engine system for increasing the engine-out NOx level, for the purpose of reducing NH3 slip to within acceptable levels. Through the foregoing interaction, the goals of the exhaust treatment system can be met.
In one aspect of the invention, a method is provided for reductant slip control. The method is applicable for use in internal combustion engine systems producing an exhaust gas stream destined for an exhaust treatment system. The method involves the step of determining an operating characteristic associated with the exhaust gas treatment system. In one embodiment, this characteristic may be a reductant slip level (e.g., NH3 slip level). The next step may involve forming a control request (e.g., in the dosing control portion of the overall exhaust treatment system) based on the determined operating characteristic. In an embodiment where the exhaust treatment characteristic is NH3 slip, this step may involve generating a message operative to alter the operation of the engine system so as to increase the engine-out NOx level. Finally, transmitting the control request (i.e., message) to the engine system (e.g., an engine control unit (ECU)). In a preferred embodiment, the message may communicate the request to the ECU to decrease the EGR rate, which in turn results in an increase in the engine-out NOx level. The increased amount of NOx provided to the selective catalyst reduction (SCR) catalyst can react with the excess stored NH3, resulting in a reduction in the NH3 slip level to within acceptable limits. In one embodiment, the control request is transmitted only when the exhaust gas treatment system has run out of authority to further decrease NH3 injection (e.g., has already disabled NH3 dosing) but is unable to maintain control of the NH3 slip within acceptable thresholds. It should be appreciated that other requests can be made by the dosing control directed to other aspects of engine operation, all in furtherance of and to meet the needs/goals of the treatment system.
A corresponding system is also presented.
The present invention will now be described by way of example, with reference to the accompanying drawings:
Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,
In the illustrative embodiment, the engine 10 may be a turbocharged diesel engine. In a constructed embodiment, the engine 10 comprised a conventional 6.6-liter, 8-cylinder turbocharged diesel engine commercially available under the DuraMax trade designation. As also shown, the engine 10 may be equipped with an exhaust gas recirculation (EGR) valve 11 and optionally an EGR cooler 13, both elements of which may comprise conventional components. As known, EGR involves recirculating a portion of the engine exhaust (engine-out) to the engine intake. The recirculated gas is generally inert and serves to dilute the intake charge, among other things. One result of EGR is a reduction in the NOx concentration level in the engine-out exhaust gas stream. It should be understood that while the illustrated approach for implementing EGR is common, particularly with contemporary diesel engines, it is exemplary and not limiting in nature. Alternate approaches may be employed. For example, it is known to provide cam phasing control, where the overlap of intake and exhaust valves is controlled to achieve the same effect. While the cam phasing approach can be implemented without separate EGR valve/plumbing, it adds other requirements, as known. Accordingly, EGR as used herein refers to redirecting and/or controlling the amount of exhaust gas redirected and/or retained in the cylinder as an inert gas, which does not combust. Since the inert gas does not combust, the higher the EGR rate, typically the lower the combustion temperature (and vice-versa), which avoids the temperature range where NOx is generated.
The software algorithms and calibrations which are executed in the ECU 16 may generally comprise conventional strategies known to those of ordinary skill in the art. Overall, in response to the various inputs, the ECU 16 develops the necessary outputs to control the throttle valve position (for engine load control), fueling (fuel injector opening, duration and closing), spark (ignition timing—if so equipped) and other aspects, all as known in the art.
In addition to the control of the engine 10, the ECU 16 is also typically configured to perform various diagnostics. For this purpose, the ECU 16 may be configured to include a diagnostic data manager or the like, a higher level service arranged to manage the reports received from various lower level diagnostic routines/circuits, and set or reset diagnostic trouble code(s)/service codes, as well as activate or extinguish various alerts, all as known generally in the art. For example only, such a diagnostic data manager may be pre-configured such that certain non-continuous monitoring diagnostics require that such diagnostic fail twice before a diagnostic trouble code (DTC) is set and a malfunction indicator lamp (MIL) is illuminated. As shown in
The exhaust gas treatment system 14 may be a selective catalytic reduction (SCR) catalyst based system. As shown in
NH3 Slip and EGR Control. As described in the Background, the SCR-based exhaust treatment system 14 includes a precision dosing control configured to inject a measured amount of reductant (NH3 or aqueous urea) to achieve the dual goals of reducing tailpipe NOx emissions while also maintaining reductant (NH3) slip within acceptable concentration levels. Over-injection may cause a release of NH3 (“slip”) out of the tailpipe into the atmosphere, while under-injection may result in inadequate emissions reduction (i.e., inadequate NOx conversion to N2 and H2O). However, under certain circumstances, tailpipe NH3 concentration levels exceed desired thresholds. Within the SCR system 14, there exists predominantly two ways to deal with (reduce) NH3 slip: (1) reduce or discontinue the introduction of ammonia into the SCR catalyst by adjusting the dosing control method; or (2) ensure sufficient NOx for conversion with the ammonia stored in the SCR catalyst. To solve the problem of uncontrolled NH3 slip, the invention provides a mechanism to request an increase in the amount of NOx available to the SCR control system. The request may be by way of an internal data control message or by an external serial data message, in any case both directed to the engine control unit (ECU) 16 and requesting that the ECU increase engine-out NOx production.
In the illustrative diesel engine embodiment, it is known to provide aggressive EGR schedules (i.e., EGR rates), which among other things tends to minimize engine-out NOx levels (i.e., contained in engine-out exhaust gas stream 12E-O) ostensibly to meet emissions compliance criteria. EGR schedules are typically determined by engine speed and engine load. “EGR rate” is the amount of exhaust gas returned to the cylinder to be mixed with intake air. There are a variety of measurements for this amount of EGR. In one embodiment, an EGR rate may be expressed as an EGR flow in grams per second, but a ratio (e.g., percentage) of cylinder volume relative to intake versus exhaust gases is also common. Accordingly, in a conventional configuration, existing diesel engine control schemes would have room for a reduction in the normal EGR rate so as to increase the engine-out NOx concentration levels. SCR chemistry demonstrates that the reduction of NH3 available in the SCR catalyst (i.e., SCR catalyst 38—
When the SCR dosing control 80 (best shown in
In step 110, the dosing control 80 executes in accordance with a predetermined control strategy configured to optimize the injection amount (rate) of the NOx reductant being used (e.g., aqueous urea). This step involves monitoring the reductant storage capacity (i.e., the theta parameter θNH3) as well as the NH3 concentration level being emitted from the tailpipe (i.e., the NH3 slip). The method then proceeds to step 112.
In step 112, the dosing control 80 determines whether it can control (i.e., determine a value for) a target theta (target θNH3) such that the NH3 slip can be maintained within acceptable limits. In this regard, the dosing control 80 may rely on, among other things, the various outputs of the SCR model 82, the characteristics of the various control blocks (e.g., see
In step 114, the dosing control 80 disables or otherwise discontinues reductant dosing entirely. This is the control's first response to excessive NH3 slip. This step is adapted to reduce the amount of stored NH3 in the SCR catalyst 38, with the end goal of reducing the NH3 slip to within acceptable limits. The method then proceeds to step 116.
In step 116, the dosing control 80 again determines whether the exhaust treatment system 14 has the ability to control the NH3 slip to within acceptable limits. In other words, has the previous step of disabling reductant (urea) dosing reduced the stored NH3 to levels such that the dosing control 80 can now regulate the operation of the exhaust treatment system so that tailpipe NH3 emissions are within acceptable limits? If the answer in this decision block is YES, then the method branches to step 110 (“monitoring”). Otherwise, if the answer is NO (i.e., if the previous step of disabling the reductant dosing is inadequate to allow regulation of NH3 slip to within acceptable limits), then the method branches to step 118.
In step 118, the dosing control 80 is configured to transmit a control message to the engine control unit (ECU) 16 requesting an increase in the engine-out NOx level. More specifically, in an embodiment, the dosing control 80 may form the control message so as to request a decrease in the EGR rate (as a means of increasing the engine out NOx) by a predetermined amount that is to be used by the ECU 16. The predetermined amount of EGR rate decrease may be fixed or may alternately be based on the determined NH3 slip concentration level. The increase in the engine-out NOx level is adapted to reduce in a corresponding fashion the NH3 slip level due the increased availability of NOx in the SCR catalyst 38.
In general terms, the method of the invention contemplates determining an operating characteristic associated with the exhaust gas treatment system, which in the example was the NH3 slip level. Next, forming a control request based on the determined operating characteristic. Here, the control request is configured to request a reduction in the prevailing EGR rate by a predetermined amount. Finally, transmitting the control request to the ECU where the control request is configured to alter the operation of the engine system in such a way as to adjust the determined operating characteristic. Here, the reduction of the EGR rate alters the operation of the engine, which results in an increase in engine-out NOx availability. This increased NOx availability, in turn, has the result of adjusting the tailpipe NH3 slip (i.e., the determined operating characteristic).
While the present invention may be used to provide the capability for a wide range of exhaust gas treatment systems to interact with engine controls so as to satisfy the needs of the exhaust treatment system, one exemplary exhaust treatment system, as shown in
Referring again to
The dosing subsystem 32 is responsive to an NH3 Request signal produced by a dosing control 80 and configured to deliver a NOx reducing agent at an injection node 68, which is introduced in the exhaust gas stream in accurate, controlled doses 70 (e.g., mass per unit time). The reducing agent (“reductant”) may be, in general, (1) NH3 gas or (2) a urea-water solution containing a predetermined known concentration of urea. The dosing unit 32 is shown in block form for clarity and may comprise a number of sub-parts, including but not limited to a fluid delivery mechanism, which may include an integral pump or other source of pressurized transport of the urea-water solution from the storage tank, a fluid regulation mechanism, such as an electronically controlled injector, nozzle or the like (at node 68), and a programmed dosing control unit. The dosing subsystem 32 may take various forms known in the art and may comprise commercially available components.
The SCR catalyst 38 is configured to provide a mechanism to promote a selective reduction reaction between NOx, on the one hand, and a reductant such as ammonia gas NH3 (or aqueous urea, which decomposes into ammonia, NH3) on the other hand. The result of such a selective reduction is, as described above in the Background, N2 and H2O. In general, the chemistry involved is well documented in the literature, well understood to those of ordinary skill in the art, and thus will not be elaborated upon in any greater detail. In one embodiment, the SCR catalyst 38 may comprise copper zeolite (Cu-zeolite) material, although other materials are known. See, for example, U.S. Pat. No. 6,576,587 entitled “HIGH SURFACE AREA LEAN NOx CATALYST” issued to Labarge et al., and U.S. Pat. No. 7,240,484 entitled “EXHAUST TREATMENT SYSTEMS AND METHODS FOR USING THE SAME” issued to Li et al., both owned by the common assignee of the present invention, and both hereby incorporated by reference in their entirety. In addition, as shown, the SCR catalyst 38 may be of multi-brick construction, including a plurality of individual bricks 381, 382 wherein each “brick” may be substantially disc-shaped. The “bricks” may be housed in a suitable enclosure, as known.
The NOx concentration sensor 40 is located upstream of the injection node 68. The NOx sensor 40 is so located so as to avoid possible interference in the NOx sensing function due to the presence of NH3 gas. The NOx sensor 40, however, may alternatively be located further upstream, between the DOC 28 and the DPF 30, or upstream of the DOC 28. In addition, the exhaust temperature is often referred to herein, and for such purpose, the temperature reading from the SCR inlet temperature sensor 44 (TIN) may be used.
The NH3 sensor 60 may be located at a mid-brick position, as shown in solid line (i.e., located anywhere downstream of the inlet of the SCR catalyst 38 and upstream of the outlet of the SCR catalyst 38). As illustrated, the NH3 sensor 60 may be located at approximately the center position. The sensed ammonia concentration level in this arrangement, even during nominal operation, is at a small yet detectable level of mid-brick NH3 slip, where the downstream NOx conversion with this detectable NH3 can be assumed in the presence of the rear brick, even further reducing NH3 concentration levels at the tail pipe to within acceptable levels. Alternatively, in certain embodiments, the NH3 sensor 60 may be located at the outlet of the SCR catalyst 38. The remainder of the sensors shown in
The dosing control 80 is configured to generate the NH3 Request signal that is sent to the dosing unit 36, which represents the command for a specified amount (e.g., mass rate) of reductant to be delivered to the exhaust gas stream. The dosing control 80 includes a plurality of inputs and outputs, designated 18, for interface with various sensors, other control units, etc., as described herein. Although the dosing control 80 is shown as a separate block, it should be understood that depending on the particular arrangement, the functionality of the dosing control 80 may be implemented in a separate controller, incorporated into the ECU 16, or incorporated, in whole or in part, in other control units already existing in the system (e.g., the dosing unit). Further, the dosing control 80 may be configured to perform not only control functions described herein but perform the various diagnostics also described herein as well. For such purpose, the dosing control 80 may include conventional processing apparatus known in the art, capable of executing pre-programmed instructions stored in an associated memory, all performing in accordance with the functionality described herein. That is, it is contemplated that the control processes described herein will be programmed in a preferred embodiment, with the resulting software code being stored in the associated memory. Implementation of the invention, in software, in view of the foregoing enabling description, would require no more than routine application of programming skills by one of ordinary skill in the art. Such a control may further be of the type having both ROM, RAM, a combination of non-volatile and volatile (modifiable) memory so that the software can be stored and yet allow storage and processing of dynamically produced data and/or signals.
The SCR model 82 may be configured to have access to a plurality of signals/parameters as needed to execute the predetermined calculations needed to model the catalyst 38. In the illustrative embodiment, this access to sensor outputs and other data sources may be implemented over a vehicle network (not shown), but which may be a controller area network (CAN) for certain vehicle embodiments. Alternatively, access to certain information may be direct to the extent that the dosing control 80 is integrated with the engine control function in the ECU 16. It should be understood that other variations are possible.
The SCR model 82 may comprise conventional models known in the art for modeling an SCR catalyst. In one embodiment, the SCR model 82 is responsive to a number of inputs, including: (i) predicted NO and NO2 levels 88; (ii) an inlet NOx amount, which may be derived from the NOx indicative signal 42 (best shown in
Referring again to
As shown in
The theta perturbation diagnostic block 100 is configured to perturb the target theta parameter in accordance with a small diagnostic function and to measure the resulting response to determine the state of health of one or more components of the exhaust treatment system 14. The adaptive learning diagnostic block 102 includes a diagnostic feature that monitors how much adaptation has been applied in adjusting the target theta parameter and generates an error when the level of adaptation exceeds predetermined upper and lower limits. The logic in operation is that at some level, the ability to adapt target theta values to overcome errors (e.g., reagent misdosing, reagent quality problems, SCR catalyst degradation) will reach its control limit for maintaining emissions. When this control limit is exceeded, the diagnostic generates an error. These features are described in greater detail in the co-pending patent application Ser. No. 12/327,945 entitled “DIAGNOSTIC METHODS FOR SELECTIVE CATALYTIC REDUCTION(SCR) EXHAUST TREATMENT SYSTEM”, (Attorney Docket No. DP-318283), filed 4 Dec. 2008, owned by the common assignee of the present invention, the disclosure of which is hereby incorporated by reference in its entirety.
The transient compensation block 104 involves implementing dosing reductions upon detection of certain exhaust transient conditions (“Transient Compensation”). One transient condition includes a sudden increase in the exhaust gas mass air flow, which portends a like increase in the exhaust gas temperature, which allows extra time for the dosing control to adjust NH3 dosing before possible NH3 slips can occur. Another transient condition includes an increasing exhaust temperature gradient. The NH3 slip control block 106 involves shutting-off dosing altogether when certain exhaust conditions are recognized by the dosing control (“NH3 slip control”). These features are described in greater detail in the co-pending patent application entitled EXHAUST GAS TREATMENT SYSTEM AND METHODS FOR OPERATING THE SAME (docket No. DP-318318) referred to above.
While particular embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.
