System and method to minimize the amount of NOx emissions by optimizing the amount of supplied reductant

Abstract

A method is described for controlling lean and rich operation of an internal combustion engine coupled to a lean NOx trap. In one example, the termination of the lean air-fuel mixture is based on an oxygen storage capacity of the NOx trap. In another example, the level and duration of rich air-fuel ratio purging operation is also controlled based on the oxygen storage capacity of the NOx trap.

Description

[0001] The present application incorporates by reference, in its entirety and for all purposes, the entire contents of Ser. No. ______, filed ______, assigned to the same assignee as the present application, titled, “SYSTEM AND METHOD TO MINIMIZE THE AMOUNT OF NOx RELEASED FROM A NOx TRAP”, attorney docket FGT.3A8 (202-1552).

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] In a vehicle having an internal combustion engine, it can be beneficial to have an emission control system capable of storing NOx. The control of emitted NOx is particularly important when the engine is operating lean. While lean operation improves fuel economy, the exhaust during lean operation may contain increased amounts of excess oxygen for extended periods of time, thereby affecting NOx conversion of three way catalysts.

[0003] Emission control devices capable of storing NOx during lean operating conditions (NOx traps) can be used to store the generated NOx. Periodically, as the NOx capacity of a NOx trap is approached, the air-fuel ratio (A/F ratio) can be driven to a rich condition for a period to convert at least a portion of the stored NOx and regenerate the NOx storage capacity of the trap. This can be referred to as NOx trap purging.

[0004] In addition to storing NOx during lean/rich cycling, the NOx traps may also provide HC, CO, and NOx conversion when the A/F ratio is controlled about stoichiometry. This is beneficial, for example, during high load operation. If NOx traps contain low amounts of oxygen storage capacity (OSC), the ability of these catalysts to convert CO and NOx under the oscillatory A/F conditions characteristic of closed-loop control systems can be limited. To provide three-way conversion in this oscillatory environment, NOx traps often contain oxides of cerium or mixed oxides of cerium and zirconium. In addition, the cerium may provide OSC during lean operating conditions and release oxygen during the rich operation to providing oxidants for converting the HC and CO.

[0005] The inventors herein have recognized that the inclusion of cerium in a NOx trap may also have disadvantages effects on NOx emission control. Among these effects are that cerium may require additional reductants during the rich operation purges (e.g., to reduce stored oxygen due to the OSC provided by the cerium), and that during a purge the OSC provided by cerium may cause some of the stored NOx to be released from the trap without being reduced to N2. This purge NOx release may be particularly evident at temperatures of 350° C. and above, and may increase as the amount of stored NOx is increased.

[0006] Thus, the inventors herein have recognized that NOx emission control systems based on NOx trap storage efficiency or storage capacity may be ineffective at reducing NOx emissions, and specifically, that NOx tailpipe emissions during a rich purge may be attributed to an inadequate concentration of reductants being supplied due in part to the OSC element within the NOx trap.

[0007] The above disadvantage can be overcome by a method for controlling an engine having an exhaust with an emission control device capable of storing NOx during lean operating conditions, and converting at least a portion of said NOx during stoichiometric or rich operating conditions, the method comprising:

[0008] operating the engine to produce a lean exhaust gas mixture fed to the emission control device;

[0009] after said lean operation, operating the engine to produce a rich exhaust gas mixture fed to the emission control device, said rich air-fuel ratio determined as a function of at least the oxygen storage capacity of the device.

[0010] Thus the inventors herein have recognized that the NOx emitted from a NOx trap during a rich purge may be further reduced by correlating the level of reductants in the rich air-gas mixture to the NOx trap's OSC. Note that the OSC of the NOx trap will vary over the life of the NOx trap due to various influences including thermal degradation, cerium deterioration, sulfur degradation, and desulfation operation. As trap OSC decreases with age the magnitude of the purge reductants may be decreased while still limiting the purge NOx release.

[0011] In another aspect, the above disadvantage can be overcome by a computer storage medium having instructions encoded therein for controlling an engine having an exhaust with an emission control device capable of storing NOx during lean operating conditions, and converting at least a portion of said NOx during stoichiometric or rich operating conditions, said medium comprising:

[0012] code for operating the engine to produce a lean exhaust gas mixture fed to the emission control device;

[0013] after said lean operation, code for operating the engine to produce a rich exhaust gas mixture fed to the emission control device, said rich air-fuel ratio determined as a function of at least the oxygen storage capacity of the device.

[0014] Further, the inventors herein have also recognized that, several other factors besides the level of supplied reductants influence the amount of NOx released from NOx trap during a purge. These factors include the amount of NOx stored, the OSC of the trap, and the trap temperature. All these factors can be considered in order to reduce the amount of emitted NOx. Thus, the inventors have discerned that NOx trap purging can be improved when the engine operates to produce a rich air-fuel ratio selected to provide a level of reductant dependent on the trap temperature and trap OSC.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The advantages described herein will be more fully understood by reading examples of embodiments in which the invention is used to advantage, with reference to the drawings, wherein:

[0016] FIG. 1 is a block diagram of an engine in which the invention is used to advantage;

[0017] FIGS. 2 and 3 are exemplary routines for controlling an engine having an emission control device capable of storing NOx under lean operating conditions; and

[0018] FIGS. 4 through 11 are graphs of experimental results involving NOx emission control devices.

DESCRIPTION OF EXAMPLE EMBODIMENT(S)

[0019] Referring to FIG. 1, an internal combustion engine 10 may include at least one emission control devices capable of storing NOx during lean operating conditions. Engine 10 includes combustion chamber 30 and cylinder walls 32 with piston 36 positioned therein and connected to crankshaft 13. Combustion chamber 30 communicates with intake manifold 44 and exhaust manifold 48 via respective intake valve 52 and exhaust valve 54. Exhaust gas oxygen sensor 16 is coupled to exhaust manifold 48 of engine 10 upstream of catalytic converter 20. A HEGO, UEGO, or exhaust gas NOx sensor 18 is coupled to the exhaust manifold 48 upstream of NOx trap 22, and a second HEGO, UEGO, or NOx sensor 24 is coupled to manifold 48 downstream of NOx trap 22.

[0020] Intake manifold 44 communicates with throttle body 64 via throttle plate 66. Throttle plate 66 is controlled by electric motor 67, which receives a signal from ETC driver 69. ETC driver 69 receives control signal (DC) from controller 12. Intake manifold 44 is also shown having fuel injector 68 coupled thereto for delivering fuel in proportion to the pulse width of signal (fpw) from controller 12. Fuel is delivered to fuel injector 68 by a fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown).

[0021] Engine 10 further includes distributorless ignition system 88 to provide ignition spark to combustion chamber 30 via spark plug 92 in response to controller 12. In the embodiment described herein, controller 12 is a microcomputer including: microprocessor unit 102, input/output ports 104, electronic memory chip 106, which is an electronically programmable memory in this particular example, random access memory 108, and a data bus.

[0022] Controller 12 receives various signals from sensors coupled to engine 10, in addition to those signals previously discussed, including: measurements of inducted mass air flow (MAF) from mass air flow sensor 110 coupled to throttle body 64; engine coolant temperature (ECT) from temperature sensor 112 coupled to cooling jacket 114; a measurement of throttle position (TP) from throttle position sensor 117 coupled to throttle plate 66; a measurement of turbine speed (Wt) from turbine speed sensor 119, a profile ignition pickup signal (PIP) from Hall effect sensor 118 coupled to crankshaft 13 indicating and engine speed (N); and an optional sensor to detect feedgas NOx concentration (not shown).

[0023] Continuing with FIG. 1, accelerator pedal 130 is shown communicating with the driver's foot 132. Accelerator pedal position (PP) is measured by pedal position sensor 134 and sent to controller 12.

[0024] In an alternative embodiment, where an electronically controlled throttle is not used, an air bypass valve (not shown) can be installed to allow a controlled amount of air to bypass throttle plate 62. In this alternative embodiment, the air bypass valve (not shown) receives a control signal (not shown) from controller 12.

[0025] As will be appreciated by one of ordinary skill in the art, the specific routines described below in the flowcharts may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the invention, but is provided for ease of illustration and description. Although not explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending on the particular strategy being used. Further, these Figures graphically represent code to be programmed into the computer readable storage medium in controller 12.

[0026] Referring now to FIG. 2, a routine is described for controlling an engine having an emission control device capable of storing NOx under lean operating conditions such as a NOx trap. First, in step 210, the routine determines whether the engine is operating under lean conditions. For example, the routine determines whether the engine is operating at an A/F ratio of greater than stoichiometric mixture of 14.6 to 1.

[0027] Next, in step 212, the routine determines the OSC of the NOx trap. This can be determined in a variety of ways. For example, the amount of OSC in the trap can be estimated from the difference in switch times during the previous rich-to-lean transition. Alternatively, an average of several previous transitions can be used. In step 214 the routine then determines the A/F ratio to be supplied during a rich purge based on the OSC. Then, following a duration of time determined by operating conditions, in step 216, the routine ends lean operation and operates the engine to produce a rich A/F exhaust gas mixture according to the OSC-determined A/F ratio.

[0028] FIG. 3 describes an alternative routine for controlling emissions from an engine having a NOx trap. In step 310, the routine determines if the engine is operating under lean conditions, in step 312, the routine determines the OSC of the NOx trap, and in step 314 the routine determines the temperature of the NOx trap. NOx trap temperature can be determined in a variety of ways including measured directly using a thermocouple (or other temperature measurement device) or estimated from a temperature model.

[0029] In step 316, the routine determines a rich A/F ratio to be supplied during an upcoming purge based on NOx trap OSC and temperature. In step 318 the routine ends lean operation and operates the engine to produce a rich A/F exhaust gas mixture according to the OSC and temperature determined A/F ratio. Thus, in the routine of FIG. 3 the A/F ratio supplied in a rich purge is based on both the OSC and temperature of a NOx trap.

[0030] The inventors performed a number of experiments on NOx traps. These experimental results revealed the complex relationships between NOx trap OSC, the concentration of reductant supplied in a rich purge, the NOx trap temperature, and the amount of NOx emitted during a purge. Such data can be used in the determinations and calculations described above with respect to FIGS. 2 and 3. FIGS. 4-12 summarize these experimental findings.

[0031] FIG. 4 is a graph of the NOx Efficiency versus lambda value at 500° C. for NOx traps with and without cerium after high temperature aging. This shows that a NOx trap containing cerium had much better NOx conversion at stoichiometry (lambda=1.0) than a trap without cerium after the traps were aged on a high temperature schedule with maximum temperatures near 1000° C. Therefore, the NOx trap itself may contain some OSC in order to provide high CO and NOx conversion under these conditions.

[0032] FIGS. 5 and 6 further show example effects of including OSC in a NOx trap. These figures depict the results from experiments comparing the NOx release for a stabilized trap with no oxygen storage capacity and a stabilized trap containing oxygen storage capacity. FIG. 5 shows that when these traps were evaluated at 350° C. using a 5 minute lean/3 minute rich cycle the trap without OSC was less effective at storing NOx during the 5 minute lean period, but it exhibited much less NOx release during the purge than the trap containing OSC.

[0033] FIG. 6 shows the percent NOx release at different temperatures for the NOx traps with and without OSC. In this experiment the amount of NOx stored during the lean period and the amount of NOx released during the rich period were calculated, and the amount of NOx release was then determined as a percentage of the amount of NOx stored. For both NOx traps, the percent NOx release generally increased as the temperature increased beyond 350° C. As can be seen the trap with OSC had a significantly higher percentage of NOx release than the trap without OSC at these higher temperatures. FIG. 6 clearly indicates that the percentage of NOx release depends on the temperature and the oxygen storage capacity of the trap.

[0034] FIG. 7 reveals that the amount of NOx release is also a function of the amount of reductant supplied to the trap during the purge. A NOx trap pre-conditioned at 600° C. was evaluated for NOx storage and NOx release at 500° C. using a test cycle with a lean period of two minutes and a rich period of one minute. The CO level during the rich purges was varied from 0.5% to 13%. FIG. 7 shows the tailpipe NOx for these tests with the different purge CO levels. The amount of NOx release decreased significantly as the purge CO level increased. This shows that the amount of NOx release can be decreased by providing higher concentrations of reductant during the purges (i.e., by using richer A/F ratios during the purges). As shown by FIG. 6 above, the amount of reductant needed to minimize the NOx release can be advantageously determined based on the temperature and the level of OSC in the trap.

[0035] Studies were performed on NOx traps that contained different amounts of a ceria-containing mixed oxide. After stabilization at 600° C., samples of these traps were evaluated for oxygen storage capacity at different temperatures. In this study the sample was reduced for 30 seconds in 1% CO and then oxidized for 30 seconds in 0.5% O2. The total OSC was determined from the amount of oxygen taken up during the 30 second lean period and normalized by the weight of the sample. The “fast” OSC was defined as the amount of oxygen taken up by the trap before any breakthrough of oxygen occurred. Oxygen storage tests were also performed on samples of these traps that had been aged for 50 hours on a high temperature aging cycle with maximum temperatures of 1000° C. In FIG. 8 the fast OSC results were taken for the fresh and aged samples at 425° C. Either type of catalyst can be used herein. Note that this also illustrates optional advantages that can be obtained by tracking OSC over the life of the catalyst as the OSC changes with aging.

[0036] FIG. 9 is a graph of NOx release versus purge CO level on a 2/1 cycle at 425° C. for stabilized NOx traps with different levels of mixed oxide. During this experiment samples of traps stabilized at 600° C. were evaluated for NOx storage and NOx release at 425° C. by utilizing the 2/1 cycle. The CO level during the rich purges was varied from as low as 0.5% to as high as 16%. During the purges, the feedgas also contained H2 at ⅓ of the CO level to reflect the approximate ratio of these species in actual engine exhaust. The amount of NOx storage was relatively constant at around 12-13 mg/in3 for these tests, and the amount of NOx release was calculated for the different levels of CO and normalized by the volume of the sample. FIG. 9 shows the amount of NOx release as a function of the purge CO level for samples with 0% and 75% mixed oxide (MO), which had oxygen storage levels of 8 and 103 micromoles 0/gram of trap. For a given level of CO, the sample with 0% MO had much lower NOx release than the sample with 75% MO. Thus, a much higher level of CO may be used for the high OSC trap to achieve the same NOx release as the low OSC trap.

[0037] FIG. 10 is a graph of data from the same study on samples with 0, 12, 37, and 75% MO. The data for these four traps were analyzed to determine the amount of CO supplied to achieve specified levels of NOx release. FIG. 10 shows the CO required to achieve NOx release levels of 0.2, 0.4, 0.6, 0.8, and 1.0 mg/in3 as a function of the measured OSC of the sample. As such, to achieve a relatively constant level of purge NOx release, less reductant is required as the OSC of the trap decreases.

[0038] FIG. 11 demonstrates that similar OSC and NOx release results were obtained at a temperature of 500° C. A permissible level of NOx release may be specified so that the required level of CO can be determined for the different levels of OSC. As an example, in FIG. 11 a level of 0.6 mg/in3 was specified. FIG. 11 shows the CO required to achieve a NOx release of 0.6 mg/in3 at both 425° C. and 500° C. For a given level of OSC, much more CO is required at 500° C. than at 425° C. to achieve the 0.6 mg/in3 of NOx release.

[0039] Thus, this data can be used to generate tables, in one example, to be used during engine control to select (during real-time operation) a level of reductant (or a level of a rich air-fuel ratio) to be provided during a NOx purge. Further, the rich level can be varied during the rich purge to provide a desired profile suited to expected NOx release. Also, the rich air-fuel ratio can be selected based on a change in OSC determined from diagnostic routines detecting catalyst degradation based on changes in switching times (or switch ratios, for example) of exhaust gas oxygen sensors. Further, these determinations of degradation can be based on an amount of sulfur retained in the device. Note also that when determining OSC, OSC is somewhat sensitive to sulfur, but thermal degradation may be a larger cause of reducing OSC.

[0040] As described in the examples above, the amount of OSC in the trap can be estimated from the difference in switch times during the rich-to-lean transition. The bed temperature of the LNT can be measured using a thermocouple or estimated from a temperature model. The bed temperature and the measured amount of OSC can be used with FIG. 11 to determine the amount of CO necessary to limit the NOX release to a specified level (which can be selected to vary depending on operating conditions), which is 0.6 mg/in3 in this example. For fresh or stabilized traps that have high levels of OSC, significantly richer purges can be necessary to limit the purge NOX release to the desired level. However, as the trap ages and the OSC decreases, the magnitude of the purge can be decreased while still limiting the purge NOX release. Also, the system can operate in both a stoichiometric mode and a lean burn mode, where the NOx trap 22 contains some OSC.

[0041] This concludes the description of the invention. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the invention. Accordingly, it is intended that the scope of the invention be defined by the following claims:

Claims

1. A method for controlling an engine having an exhaust with an emission control device capable of storing NOx during lean operating conditions, and converting at least a portion of said NOx during stoichiometric or rich operating conditions, the method comprising:

operating the engine to produce a lean exhaust gas mixture fed to the emission control device;

after said lean operation, operating the engine to produce a rich exhaust gas mixture fed to the emission control device, said rich air-fuel ratio determined as a function of at least the oxygen storage capacity of the device.

2. The method of claim 1 wherein said rich air-fuel ratio is further based on temperature of the device.

3. The method of claim 1 wherein as oxygen storage capacity of the device decreases, the rich air-fuel ratio becomes less rich.

4. The method of claim 1 wherein the rich air-fuel ratio is selected to provide a select amount of CO and H2.

5. The method of claim 1 wherein the oxygen storage capacity of the device is determined based on device degradation.

6. The method of claim 5 wherein device degradation is based on at least one of an amount of sulfur contaminating the device and thermal degradation of the device.

7. The method of claim 1 wherein the oxygen storage capacity of the device is determined from rich to lean transition time.

8. A method for controlling an engine having an

exhaust with an emission control device capable of storing NOx during lean operating conditions, and converting at least a portion said NOx during stoichiometric or rich operating conditions, the method comprising:

operating the engine to produce a lean exhaust gas mixture fed to the emission control device;

estimating the amount of NOx release based on the oxygen storage capacity of the device;

after said lean operation, operating the engine to produce a rich exhaust gas mixture fed to the emission control device, said rich air-fuel ratio is determined based at least on the amount of NOx released.

9. The method of claim 8 wherein said rich air-fuel ratio is further based on temperature of the device.

10. The method of claim 8 wherein said amount of NOx release is further based on operating conditions.

11. The method of claim 8 wherein as oxygen storage capacity of the device decreases the rich air-fuel ratio becomes less rich.

12. The method of claim 8 wherein the rich air-fuel ratio is selected to provide a selected amount of CO and H2 to the device.

13. The method of claim 8 wherein the oxygen storage capacity of the device is determined based on device degradation.

14. The method of claim 13 wherein device degradation is based on an amount of sulfur contaminating the device.

15. The method of claim 8 wherein the oxygen storage capacity of the device is determined from rich to lean transition time.

16. A computer storage medium having instructions encoded therein for controlling an engine having an

exhaust with an emission control device capable of storing NOx during lean operating conditions, and converting at least a portion of said NOx during stoichiometric or rich operating conditions, said medium comprising:

code for operating the engine to produce a lean exhaust gas mixture fed to the emission control device;

after said lean operation, code for operating the engine to produce a rich exhaust gas mixture fed to the emission control device, said rich air-fuel ratio determined as a function of at least the oxygen storage capacity of the device.