Abstract:
A control system is described for optimizing the reduction of nitrogen oxides in exhaust gas produced by an internal combustion engine, especially those internal combustion engines that employ a lean air-fuel ratio. The control system employs a temperature control assembly that is capable of selectively heating the exhaust gas prior to introduction into the NO x  catalyst system, thus rapidly bringing the temperature of the NO x  catalyst system up to operating temperature.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an emissions reduction system employed in an exhaust passage of an internal combustion engine, and more particularly to a control system for optimizing the reduction of nitrogen oxides in exhaust gas produced by an internal combustion engine employing a lean air-fuel ratio. 
     2. Background and Summary of the Invention 
     Increasingly stringent government regulations for the allowable emission levels of carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NO x ) have resulted in the use of catalytic converters on most passenger vehicles sold in the United States. The task of the catalytic converter is to promote chemical reactions for the conversion of these pollutants to carbon dioxide, water, and nitrogen. 
     For automotive exhaust applications, the pollutant removal reactions are the oxidation of carbon monoxide and hydrocarbons and the reduction of nitrogen oxides. 
     Converters are of two basic catalyst types: the two-way converter (oxidation) and the three-way converter (oxidation and reduction). Both types typically employ either a pellet or monolith design. 
     The two-way catalytic converter is placed in the exhaust system between the exhaust manifold and muffler. When the hot gases are forced through the converter, they contact the catalyst-coated pellets or honeycomb, depending on the type. The resulting exothermic reaction cataylzed by the catalyst causes a rapid increase in the exhaust temperature. This, in turn, causes the carbon monoxide and hydrocarbons to change (by means of an oxidizing process) into water (H 2 O) vapor and carbon dioxide (CO 2 ) gas. The two-way oxidizing converter does not reduce the nitrogen oxides (NO x ). 
     The three-way converter uses an additional catalyst bed typically coated with platinum, palladium, rhodium, and combinations thereof. The three-way converter is capable of removing all three pollutants (i.e., carbon monoxide, hydrocarbons, and nitrogen oxides) simultaneously, provided that the catalyst is maintained in a chemically correct environment that is neither overly oxidizing or reducing. 
     Although catalytic converters work well with engines employing a stoichiometric air-fuel ratio (i.e., 14.7:1), they are not as effective with engines employing a lean air-fuel ratio (i.e., 16.0:1 or higher). Examples of engines exhibiting lean burn operation are diesel and certain newer generation gasoline engines (e.g., direct injection gasoline engines). 
     A type of catalyst for removing NO x  from the exhaust gas of internal combustion engines during lean burn operation, often called a “NO x  trap” or “NO x  absorber,” is disclosed in U.S. Pat. No. 5,404,719 issued Apr. 11, 1995. This catalyst generally comprises alkaline metals or alkaline earth materials like potassium or strontium in combination with a precious metal like platinum. Under conditions of excess oxygen, i.e., when the exhaust gas is lean, this trap is capable of storing/absorbing, nitrogen oxides. When the oxygen concentration of the exhaust gas is lowered, the NO x  is released from the NO x  trap catalyst. These traps thus operate in a different way compared to conventional lean-burn catalysts. More particularly, the widely held mechanism for NO x  trap operation is that the precious metal first oxidizes NO to NO 2  and the NO 2  subsequently forms a nitrate complex with the alkaline material. In a stoichiometric or rich environment, the nitrate is thermodynamically unstable, and the stored NO x  is released. NO x  then catalytically reacts with excess reducing species in the exhaust gas to form N 2 . 
     Another catalyst system for removing NO x  from exhaust gases of lean burn internal combustion engines is referred to as the Selective Catalytic Reduction (SCR) system. The SCR catalyst system uses an additive, such as urea, which is introduced into the exhaust stream wherein it combines with the NO x  over a suitable catalyst to eliminate the NO x  from the tailpipe emissions. Urea decomposes in the heat of the exhaust stream into ammonia which reacts with the NO x . Ammonia itself could be introduced but is much more dangerous to have onboard the vehicle. The reaction between the ammonia and the NO x  over the catalyst is highly temperature dependent. If too much urea is metered into the exhaust system and there is either insufficient NO x  in the exhaust stream or the catalyst temperature is too low to promote efficient conversion, then ammonia gas will exit the tailpipe as a dangerous and foul smelling gas. Therefore, the proper control of the quantity of urea injected is very important as well as the temperature of the NO x  catalyst system. 
     One catalyst system that utilizes a SCR system is marketed under the tradename SINOX™ by Siemens Automotive Corporation (Auburn Hills, Mich.). The SINOX™ system&#39;s electronic control system processes temperature and emission level information fed from sensors, and then meters and injects appropriate amounts of urea into the catalyst system. A reduction of the NO x  levels of up to 70% is claimed by the manufacturer. 
     However, the aforementioned methods of removing NO x  from the exhaust gas of a lean burn internal combustion engine have failed to achieve an optimal level of NO x  reduction. In particular, previous methods have not achieved the proper control and maintenance of the temperature of the NO x  catalyst system within an optimal temperature range. Accordingly, the level of NO x  exiting the tailpipe is relatively high when the automobile is initially started and the NO x  catalyst system is relatively cool. The level of NO x  exiting the tailpipe decreases gradually as the NO x  catalyst system warms up to operating temperatures, thus becoming more efficient. However, in the interim, unnecessarily high amounts of NO x  have exited the tailpipe into the atmosphere, thus contributing to pollution concerns. 
     Therefore, there exists a need for a control system for optimizing the removal of nitrogen oxides from the exhaust gas of a lean burn internal combustion engine. 
     In accordance with one aspect of the present invention, a control system for use in an internal combustion engine automobile having a NO x  catalyst system, comprises a temperature control assembly placed upstream of the NO x  catalyst system, the temperature control assembly selectively adjusting the temperature of the exhaust gas prior to the exhaust gas being introduced into the NO x  catalyst system. 
     In accordance with another aspect of the present invention, a control system for use in an internal combustion engine automobile having a NO x  catalyst system, comprises a temperature control assembly placed upstream of the NO x  catalyst system, the temperature control assembly selectively adjusting the temperature of the exhaust gas prior to the exhaust gas being introduced into the NO x  catalyst system. A temperature sensor is placed downstream of the temperature control assembly and upstream of the NO x  catalyst system, the first temperature sensor detecting the temperature of the exhaust gas prior to introduction into the NO x  catalyst system. 
     In accordance with yet another aspect of the present invention, a control system for use in an internal combustion engine automobile having a NO x  catalyst system, comprises a temperature control assembly placed upstream of the NO x  catalyst system, the temperature control assembly selectively adjusting the temperature of the exhaust gas prior to the exhaust gas being introduced into the NO x  catalyst system. A NO x  sensor is placed downstream of the temperature control assembly and upstream of the NO x  catalyst system, the NO x  sensor sensing the level of NO x  in the exhaust gas. A first temperature sensor is placed downstream of the temperature control assembly and upstream of the NO x  catalyst system, the first temperature sensor detecting the temperature of the exhaust gas prior to introduction into the NO x  catalyst system. A second temperature sensor is placed downstream of the NO x  catalyst system, the second temperature sensor detecting the temperature of the exhaust gas after exiting the NO x  catalyst system. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a schematic view of a control system for optimizing the reduction of nitrogen oxides in exhaust gas produced by an internal combustion engine, in accordance with one embodiment of the present invention; and 
     FIG. 2 is a schematic view of a control system for optimizing the reduction of nitrogen oxides in exhaust gas produced by an internal combustion engine, in accordance with an alternative embodiment of the present invention. 
    
    
     The same reference numerals refer to the same parts throughout the various Figures. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Because the catalytic efficiency of the NO x  catalyst system is highly temperature dependent, and diesel exhaust gases are relatively cool, the addition of a control system upstream of the NO x  catalyst system is proposed to bring the NO x  catalyst system up to operating, and preferably optimal, temperature as quickly as possible. In this manner, the quantity of NO x  in the exhaust gas exiting the tailpipe will be reduced to a greater extent than with previous methods. 
     With reference to FIG. 1, there is generally shown a control system  10  for optimizing the reduction of nitrogen oxides in exhaust gas produced by an internal combustion engine, in accordance with one embodiment of the present invention. The control system  10  is intended to be employed with catalytic converters having a NO x  catalyst system, especially a lean NO x  catalyst system. The control system  10  can be placed immediately downstream of the vehicle&#39;s exhaust manifold, or alternatively, immediately downstream of the vehicle&#39;s conventional catalytic converter system. 
     The control system  10  is comprised primarily of a temperature control assembly  12 , an actuation assembly  14 , and an engine control assembly  16 . 
     The intended purpose of the temperature control assembly  12  is to selectively heat the exhaust gas E emitted or expelled from the exhaust manifold  18  in order to raise the temperature of the exhaust gas E. The heating of the exhaust gas E is preferably accomplished by disposing at least one heating element (not shown) within the temperature control assembly  12 . The heating element is preferably comprised of a heat conducting material, such as, but not limited to metal. The heating element is preferably heated by the use of electrical power. Thus, when the exhaust gas E flows past the heating element, the temperature of the exhaust gas E may be raised rapidly. 
     Under certain circumstances, it may not be necessary to raise the temperature of the exhaust gas E. In this case, it is preferable to be able to control the flow of electrical power to the heating element of the temperature control assembly  12 , so that the heating element is not constantly receiving electrical power and potentially overheating and causing a fire. Thus, an actuation assembly  14 , such as, but not limited to a switch, circuit breaker, or like device is employed. The actuation assembly  14  is preferably in electrical communication with the temperature control assembly  12 . The actuation assembly  14  is capable of selectively permitting electrical power to flow to the heating element of the temperature control assembly  12 . 
     In order to control the activity of the actuation assembly  14 , an engine control assembly  16 , such as, but not limited to an engine controller module, computer, microprocessor, or like device may be employed. The engine control assembly  16  is preferably in electrical communication with the actuation assembly  14 . By way of a nonlimiting example, the engine control assembly  16  may receive data input from various sensors or monitors indicating that the temperature of the exhaust gas E needs to be raised, for example when the automobile&#39;s engine is initially started. These sensors could detect data from various vehicle performance parameters. By way of a nonlimiting example, a vehicle speed sensor  20 , an engine load sensor  22 , and a coolant temperature sensor  24  may be employed to provide data to the engine control assembly  16 . If the data indicates that the temperature of the exhaust gas E needs to be raised, the engine control assembly  16  causes the actuation assembly  14  to permit electrical power to flow to the heating element of the temperature control assembly  12 . Conversely, if the data indicates that the temperature of the exhaust gas E does not need to be raised, the engine control assembly  16  causes the actuation assembly  14  to restrict the flow of electrical power to the heating element of the temperature control assembly  12 . In this manner, the temperature of the exhaust gas E may be carefully controlled. 
     Once the heated exhaust gas E′ has passed through the temperature control assembly  12 , it is then introduced into a passageway  26  disposed between the temperature control assembly  12  and a NO x  catalyst system  28 . Prior to the exhaust gas E′ being introduced into the NO x  catalyst system  28 , a metered amount of urea is selectively introduced into the passageway  26  by a urea injection system  30 . The urea injection system  30  is preferably in communication with the control system  10  so as to optimize the efficiency of the urea injection process. The urea injection system  30  is primarily comprised of a urea storage tank  32 , a urea pump  34 , a urea injector  36 , and a passageway  38  in fluid communication with passageway  26 . The urea and the exhaust gas E′ come into contact with one another in passageway  26 ; however, there is very little chemical interaction occurring between the two substances at this point. 
     The urea/exhaust gas E′ mixture is then introduced into the NO x  catalyst system  28 , whereupon the catalyst acts to convert the NO x  into nitrogen. As previously noted, it is important that the NO x  catalyst system  28  operate within a certain temperature range in order to efficiently convert the NO x  into nitrogen. Accordingly, the use of heated exhaust gas ensures that the temperature of the NO x  catalyst system  28  will rapidly rise to operational levels even if the vehicle&#39;s engine is relatively cold. 
     Finally, the treated exhaust gas E″ exits the tailpipe  40  having had its NO x  levels reduced on the order of 90-95% compared to the levels present in the exhaust gas E emitted from the exhaust manifold  18 . Thus, the present invention achieves a significant reduction in the levels of NO x  when compared to previous methods. 
     With reference to FIG. 2, there is generally shown a control system  10  for optimizing the reduction of nitrogen oxides in exhaust gas produced by an internal combustion engine, in accordance with an alternative embodiment of the present invention. The control system  10  is generally similar to the one illustrated in FIG. 1; however, a NO x  sensor  42 , a front temperature sensor  44 , and a rear temperature sensor  46  have been added to enhance the efficiency of the control system  10 . It should be noted that all three of the aforementioned components are optional and may be employed alone or in any number of combinations with one another. Additionally, it should be noted that the positions of the NO x  sensor  42  and the front temperature sensor  44  with respect to one another is for illustrative purposes only. 
     The amount of urea injected into passageway  26  is a function of the level of NO x  in the exhaust gas emitted from the exhaust manifold, specifically exhaust gas E′. Because the control of the quantity of urea injected is important to prevent ammonia from exiting the tailpipe  40 , a NO x  sensor  42  is employed for determining the quantity of NO x  in the exhaust gas E′. 
     Preferably, the NO x  sensor  42  is in communication with passageway  26  via passageway  48 . Additionally, the NO x  sensor  42  is preferably in communication with the control system  10  so as to be able to provide real-time feedback to the engine control assembly  16  as to the amount of NO x  in the exhaust gas E′. This may be accomplished through mapping of the quantity of NO x  generated by the engine on a dynamometer. 
     Preferably, the front temperature sensor  44  is in communication with passageway  26  via passageway  50 . Additionally, the front temperature sensor  44  is preferably in communication with the control system  10  so as to be able to provide real-time feedback to the engine control assembly  16  as to the temperature of the exhaust gas E. This is accomplished by measuring the temperature drop or gain that occurs when the exhaust gas E passes through the temperature control assembly  12 . In this manner, the temperature of the exhaust gas E can be controlled to a much greater degree. 
     By way of a non-limiting example, the engine control assembly  16  may receive data input from the front temperature sensor  44  indicating that the temperature of the exhaust gas E needs to be raised. Accordingly, the engine control assembly  16  causes the actuation assembly  14  to permit the flow of electrical power to the heating element of the temperature control assembly  12 . Conversely, if the front temperature sensor  44  indicates that the temperature of the exhaust gas E does not need to be raised, or in fact lowered, the engine control assembly  16  causes the actuation assembly  14  to restrict the flow of electrical power to the heating element of the temperature control assembly  12 . 
     Preferably, the rear temperature sensor  46  is in communication with passageway  26  via passageway  52 . Additionally, the rear temperature sensor  46  is preferably in communication with the control system  10  so as to be able to provide real-time feedback to the engine control assembly  16  as to the temperature of the NO x  catalyst system  28 . This is accomplished by measuring the temperature drop or gain that occurs when the exhaust gas E′ passes through the NO x  catalyst system  28 . Accordingly, if the temperature of the NO x  catalyst system  28  needs to be raised, the engine control assembly  16  causes the actuation assembly  14  to permit the flow of electrical power to the heating element of the temperature control assembly  12 . Conversely, if the rear temperature sensor  44  indicates that the temperature of the NO x  catalyst system  28  does not need to be raised, or in fact lowered, the engine control assembly  16  causes the actuation assembly  14  to restrict the flow of electrical power to the heating element of the temperature control assembly  12 . In this manner, the temperature of the NO x  catalyst system  28  can be controlled to a great degree. 
     Preferably the control system  10 , the NO x  sensor  42 , the front temperature sensor  44 , the rear temperature sensor  46 , the urea injection system  30 , and the NO x  catalyst system  28  are in simultaneous communication with each other to enhance the efficient removal of NO x  from the exhaust gas produced by the engine. 
     In practice, the NO x  level in the exhaust gas E (detected by the NO x  sensor  42 ) is then used with the temperature of the NO x  catalyst system  28  (detected by the rear temperature sensor  46 ) to determine the precise amount of urea required to be introduced into passageway  26 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.