High efficiency conversion of nitrogen oxides in an exhaust aftertreatment device at low temperature
A system and method for providing reductant to a lean NOx catalyst, when the temperature in the lean NOx catalyst is greater than 300° C., is disclosed. After the reductant is supplied under these conditions, the NOx conversion efficiency of the lean NOx catalyst in the 140-250° C. temperature range is increased.
The present invention relates to a system and a method for improving conversion efficiency of a lean NOx catalyst in a diesel or lean burn gasoline engine, and, more particularly, to improving conversion efficiency by controlling delivery of a NOx reductant.
Internal combustion engines commonly rely on exhaust aftertreatment devices to convert regulated components: carbon monoxide, hydrocarbons, and nitrogen oxides (NOx), into carbon dioxide, water, nitrogen, and oxygen. Exhaust catalysts have been extensively developed to obtain high conversion efficiencies on stoichiometric exhaust gases. Stoichiometric conditions are achieved when the fuel and oxidizer supplied to the engine is in a proportion which, if reaction of the fuel were complete, produce carbon dioxide, water, and nitrogen. It is known to those skilled in the art, though, that higher fuel efficiency is obtained from engines operating at air-fuel ratios lean of stoichiometric, that is, with an excess of air. These lean burning engines may be diesel engines, stratified-charge gasoline engines in which the fuel and air are only partially mixed, and homogeneous-charge, lean-burn gasoline engines in which the fuel and air are mostly premixed prior to combustion. Because of the desire for high fuel efficiency, lean burning engines are in production and continue to be developed. It is known to those skilled in the art to use a NOx catalyst and continuously supply reductant to the catalyst to convert NOx while operating lean.
The inventors of the present invention have recognized that if reductant is supplied when the catalyst is at high temperatures, the subsequent NOx conversion efficiency of the catalyst is higher than heretofore possible in the 140-250° C. temperature range.
SUMMARY OF INVENTIONThe inventors of the present invention have recognized that substantially higher NOx conversion efficiencies of a lean NOx catalyst can be achieved by supplying reductant when the temperature in the lean NOx catalyst is greater than about 300° C. The inventors recognized that a lean NOx catalyst may periodically achieve the desired temperature range and reductant may be supplied to the lean NOx catalyst in response.
Disadvantages of prior approaches are overcome by a method for controlling reductant addition to exhaust gases of an internal combustion engine. The reductant and exhaust gases flow into a catalyst coupled to the engine. An indication that temperature of the catalyst is higher than a predetermined temperature is provided. In response to the indication, reductant is added into the exhaust gases. An estimate of a stored quantity of reductant within the catalyst is provided. The reductant addition step is substantially discontinued when the stored quantity exceeds a predetermined quantity.
OLE_LINK5A primary advantage of the present invention is that a lean NOx catalyst processing lean exhaust gases operates with substantially higher conversion efficiency in a lower temperature range than heretofore possible.OLE_LINK5Another advantage of the present invention is that after storing reductant under prescribed conditions, reductant delivery can be lessened or discontinued. Furthermore, significantly less reductant is supplied to the catalyst than prior art methods.
Yet another advantage of the invention herein, over prior art, is that because less reductant is supplied to the catalyst, less reductant slips through the catalyst into the tailpipe.
The above advantages and other advantages, objects, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.
BRIEF DESCRIPTION OF DRAWINGSThe advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein:
In
Exhaust gases of engine 10 are directed into a lean NOx catalyst (LNC) 30, described in more detail later herein. Upstream of lean NOx catalyst 30 is reductant injector 20, which is supplied reductant from reductant tank 34. Reductant is injected into the exhaust gases upstream of lean NOx catalyst 30. ECU 40 controls reductant injector 20. If reductant is fuel, the reductant may be injected by injectors 12 into the combustion chamber directly. The fuel injected by injectors 12 to be used as reductant would be injected at such a time in the cycle to avoid being consumed by the combustion event. Lean NOx catalyst 30 may contain a resistive heating element, so that it may be electrically heated, by connecting it to battery 32 by electrical wires 38, which include a switch 28. An electrical voltage may be applied or discontinued by closing or opening, respectively, switch 28.
Exhaust gas sensor 22 may be a NOx sensor, placed in the exhaust line upstream of lean NOx catalyst 30 to detect concentration of NOx entering lean NOx catalyst 30. Exhaust gas sensor 44 may be a NOx sensor used to detect effectiveness of lean NOx catalyst 30. Exhaust gas sensor 46 may be an ammonia sensor to detect slippage of ammonia-containing reductant from lean NOx catalyst 30. Alternatively, exhaust gas sensor 46 may be a hydrocarbon sensor, in the event that the reductant is a hydrocarbon.
The term lean, used herein with reference to the mixture supplied to the combustion chamber of engine 10 or of the exhaust gases supplied to the lean NOx catalyst 30, refers to the chemical stoichiometry of the gases. Mixtures containing air in excess of that required to fully consume the fuel are referred to as lean. Rich mixtures contain excess fuel. The products of lean combustion produce lean exhaust gases and vice versa.
ECU 40 has a microprocessor 50, called a central processing unit (CPU), in communication with memory management unit (MMU) 60. MMU 60 controls the movement of data among the various computer readable storage media and communicates data to and from CPU 50. The computer readable storage media preferably include volatile and nonvolatile storage in read-only memory (ROM) 58, random-access memory (RAM) 56, and keep-alive memory (KAM) 54, for example. KAM 54 may be used to store various operating variables while CPU 50 is powered down. The computer-readable storage media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by CPU 50 in controlling the engine or vehicle into which the engine is mounted. The computer-readable storage media may also include floppy disks, CD-ROMs, hard disks, and the like. CPU 50 communicates with various sensors and actuators via an input/output (I/O) interface 52. Examples of items that are actuated under control by CPU 50, through I/O interface 52, are fuel injection timing, fuel injection rate, fuel injection duration, EGR valve position, throttle valve position, and reductant injection timing and duration. Sensors communicating input through I/O interface 52 may be indicating engine speed, vehicle speed, coolant temperature, manifold pressure, pedal position, throttle valve position, EGR valve position, air temperature, and exhaust temperature. Some ECU 40 architectures do not contain MMU 60. If no MMU 60 is employed, CPU 50 manages data and connects directly to ROM 58, RAM 56, and KAM 54. Of course, the present invention could utilize more than one CPU 50 to provide engine/vehicle control and ECU 40 may contain multiple ROM 58, RAM 56, and KAM 54 coupled to MMU 60 or CPU 50 depending upon the particular application.
Lean NOx catalyst 30 is an exhaust aftertreatment device which processes the products of lean combustion. Although gases within LNC 30 are overall lean, a condition which normally favors oxidation, NOx reduction can occur on catalyst surfaces in the presence of reductant. A reductant, such as hydrocarbons or ammonia, is absorbed on catalyst surfaces to promote NOx reaction to benign products, N and H2O. An example formulation for LNC 30 is one with Cu-β-zeolite and no precious metals.
Prior to explaining how the present invention allows injection of a lesser amount of reductant than prior art methods while achieving even higher NOx conversion efficiency, phenomena relevant to the present invention, which were discovered by the inventors herein, is discussed.
Referring now to
At temperatures below a threshold temperature (believed to be about 300° C. based on experimental findings), reductant absorption on active sites is negligible due to NOx inhibition. When the temperature of the LNC approaches the threshold temperature, NOx desorbs from the active sites, allowing reductant to occupy them. Curve 72 of
The significance of absorbing reductant on active sites is shown in
In the discussion above, the term NOx conversion efficiency is used in conjunction with LNC 30; another term that may applied to explain the phenomena is reaction rate. Reductant that is supplied under prescribed conditions has a faster reaction rate with NOx leading to a higher NOx conversion efficiency. The inventors of the present invention theorize that a higher reaction rate involving reductant and NOx is achieved by storing reductant on active sites within the catalyst. Conversely, reductant that is stored on inactive sites reacts with NOx at a lower reaction rate, thus leading to a lower NOx conversion efficiency.
The above discussion of NOx absorption in LNC 30 may erroneously cause the reader of this specification to assume that LNC 30 absorbs a substantial quantity of NOx. The quantity of NOx absorbed in LNC 30 is neglible in relation to exhaust levels of NOx; nevertheless, the small quantity of NOx that is absorbed in LNC 30 is sufficient to inhibit the reductant from being absorbed on active sites in LNC 30.
Discussion of storage of NOx may also cause the reader of the specification to confuse a LNC with a lean NOx trap (LNT). Distinctions between a LNT and a LNC are demonstrated in
Referring now to curves 84, 86, and 88 of
Referring now to
Curve 86 of
Techniques by which the temperature can be raised in the catalyst are relevant to the present invention. Thus, methods known by those skilled in the art, which cause exhaust gas temperature increase are provided in Table 1.
Any of the methods in Table 1 may be used to achieve a temperature increase.
Storing reductant on active sites in LNC 30 may be accomplished in a passive manner or actively pursued. ECU 40 may determine that engine 10 is operating at a condition conducive to storing reductant on active sites and command reductant injector 20 to supply reductant during such a condition, an example of passively exploiting the phenomenon. Or, ECU 40 may actively cause engine 10 to operate at a condition which provides the necessary conditions within LNC 30 to absorb reductant on active sites.
In block 106 of
A passive scheme, by which the present invention may be practiced, is shown in
Prior art methods and the present invention are compared in Table 2.
Prior art methods X and Y demonstrate NOx and fuel efficiency tradeoffs: method Y suffers in NOx conversion efficiency and method X suffers in fuel efficiency. Fuel efficiency suffers with method X because methods by which exhaust temperature is raised lead to a fuel economy penalty. The present invention (shown as curve 76 in
The inventors of the present invention have found that reductant supplied to the catalyst during conditions of NOx inhibition is stored on inactive sites. They have also discovered that if NOx inhibiting effects are subsequently removed, reductant stored on inactive sites diffuses to active sites. This phenomenon may also be exploited by supplying reductant at any operating condition and subsequently causing a condition in the engine at which the NOx inhibition is no longer present to achieve the desired effect, i.e., reductant absorbed on active sites.
The embodiments discussed above relate to supplying reductant when prescribed operating conditions prevail in the LNC 30. Although LNC 30 provides higher NOx conversion efficiency by supplying the reductant accordingly, it may be found preferable to utilize a strategy combining both prior art reductant supply method Y and the invention herein to achieve a desired NOx reduction with a minimum penalty on fuel economy.
The embodiments discussed above relate most closely to a diesel engine. However, the invention may apply to any lean-burning combustion system for which reduction of exhaust NOx is desired.
While several modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. The above-described embodiments are intended to be illustrative of the invention, which may be modified within the scope of the following claims.
Claims
1. A method for controlling reductant added to exhaust gases of an internal combustion engine, the reductant and exhaust gases flowing into a catalyst coupled to the engine, comprising the steps of:
- providing an indication that temperature of the catalyst is higher than a predetermined temperature;
- in response to said indication, adding reductant into the exhaust gases;
- providing an estimate of a stored quantity of reductant within the catalyst; and
- substantially discontinuing said step of adding reductant when said stored quantity exceeds a predetermined quantity.
2. The method of claim 1 wherein said predetermined quantity is based on a reductant storage capacity of the catalyst.
3. The method of claim 1 wherein said predetermined temperature is 300 degrees Celsius.
4. The method of claim 2 wherein said reductant storage capacity of the catalyst depends on a temperature of the catalyst.
5. The method of claim 1 wherein the reductant is added by an injector disposed upstream of the catalyst and downstream of the engine.
6. The method of claim 1 wherein the reductant contains ammonia.
7. The method of claim 1 wherein the reductant is added by an injector disposed in a combustion chamber of the engine.
8. A system for controlling reductant addition into lean exhaust gases discharged from an internal combustion engine, the reductant being added upstream of a catalyst coupled to the engine, comprising:
- an injector for adding the reductant into the exhaust gases; and
- an engine controller operably connected to the engine and said injector which actuates said injector to supply the reductant in response to an indication that temperature of the catalyst is greater than a predetermined temperature, provides an estimate of a quantity of reductant stored in the catalyst, and when said quantity exceeds a predetermined quantity, substantially discontinues said addition of reductant.
9. The system of claim 8, further comprising a temperature sensor disposed upstream of the catalyst providing said indication of temperature.
10. The system of claim 8, further comprising an exhaust gas component sensor disposed in an exhaust line downstream of the catalyst and coupled to said engine control unit.
11. The system of claim 10, wherein said exhaust gas component sensor is a reductant sensor and said engine control unit discontinues said addition of reductant based on a signal from said reductant sensor.
12. The system of claim 10, wherein said exhaust gas component sensor is a NOx sensor.
13. The system of claim 12, wherein a signal from said NOx sensor is used in said estimation of a quantity of reductant stored in the catalyst.
14. The system of claim 8, wherein said reductant contains ammonia.
15-16. (canceled)
17. A computer readable storage medium having stored data representing instructions executable by a computer to control an injector which supplies reductant into exhaust gases discharged from an internal combustion engine, wherein the exhaust gases contain excess oxygen, the reductant being added to the exhaust gases upstream of a catalyst coupled to the engine, comprising:
- instructions for providing an indication of a temperature of the catalyst; and
- instructions for commanding the injector to supply reductant when said indicated temperature is greater than a threshold temperature; and
- instructions to substantially reduce said injection of reductant when a quantity of reductant stored in the catalyst is substantially equal to a predetermined quantity.
18. The storage medium of claim 17 wherein a reductant sensor is disposed in an exhaust duct downstream of the catalyst, further comprising instructions to substantially discontinue said injection of reductant based on a signal from said reductant sensor.
19. A method for controlling reductant added to exhaust gases of an internal combustion engine, the reductant and exhaust gases flowing into a catalyst coupled to the engine, comprising the steps of:
- determining a temperature in the catalyst; estimating a stored quantity of reductant in the catalyst; adding reductant to the exhaust gases when said catalyst temperature is greater than a predetermined temperature and said stored quantity is less than a first predetermined quantity; and discontinuing said step of adding reductant when said stored quantity is greater than a second predetermined quantity, said second predetermined quantity being greater than or equal to said first predetermined quantity.
20. A system for controlling an amount of reductant added to exhaust gases from an internal combustion engine, comprising the steps of:
- an injector for adding the reductant into the engine exhaust gases upstream of a catalyst coupled to said engine; and
- an engine controller operably connected to said engine and said injector, said controller configured to actuate said injector to supply said reductant at a rate greater than a consumption rate of reductant in said catalyst when a temperature of said catalyst is greater than a predetermined temperature, said controller further configured to estimate a quantity of reductant stored in said catalyst, said controller further configured to substantially discontinue said addition of reductant when said quantity of stored reductant exceeds a predetermined quantity.
21. The system of claim 20 wherein said supplying rate of said reductant is substantially greater than a consumption rate of said reductant in said catalyst.
22. A method for controlling reductant added to exhaust gases from an internal combustion engine, the reductant and exhaust gases flowing into a catalyst coupled to the engine, comprising the steps of:
- adding reductant into the exhaust gases at a first delivery rate;
- indicating when a temperature of the catalyst is greater than a predetermined temperature; adding reductant at a second delivery rate into the exhaust gases in response to said indication, estimating of a stored quantity of reductant within the catalyst; and
- adding reductant at said first delivery rate when said stored quantity exceeds a predetermined quantity, said first delivery rate being less than said second delivery rate.
23. The method of claim 22 wherein said first delivery rate equals zero.
24. The method of claim 22 wherein said first delivery rate is substantially less than said second delivery rate.
25. A method for reducing NOx in exhaust gases from an engine, said engine communicating exhaust gases to a catalyst, comprising the steps of:
- storing reductant on active sites of said catalyst when a temperature of the catalyst is greater than a predetermined temperature; and,
- utilizing only said stored reductant in said catalyst for reducing NOx in said exhaust gases for a first time period.
26. The method of claim 25 further comprising:
- supplying additional reductant to said catalyst after said first time period to reduce NOx in said exhaust gases and to store additional reductant on said active sites.
27. A method for reducing NOx in exhaust gases from an engine, said engine communicating exhaust gases to a lean NOx catalyst, comprising the steps of:
- iteratively supplying pulses of reductant to a lean NOx catalyst during engine operation, wherein between said pulses of reductant a portion of reductant stored in said lean NOx catalyst is utilized to continue reducing NOx, said pulsing commences when temperature in said lean NOx catalyst exceeds a predetermined temperature, and said pulsing terminates when an estimate of a quantity of reductant stored in the catalyst exceeds a predetermined quantity.
Type: Application
Filed: Jan 20, 2004
Publication Date: Jan 13, 2005
Inventors: Lifeng Xu (Farmington Hills, MI), Robert McCabe (Lathrup Village, MI)
Application Number: 10/760,761