Abstract:
A method of controlling generation of nitrogen oxides in an internal combustion engine is provided with the steps of: combusting a fuel and air mixture within a combustion cylinder; determining a pressure in the combustion cylinder and a position of a piston within the combustion cylinder; calculating an amount of nitrogen oxides generated with the combusting step, dependent upon the determining step; storing a history of the calculated amount of nitrogen oxides in a memory device; and controlling an output action, dependent upon the calculated amount of nitrogen oxides, the stored history of nitrogen oxides and a threshold value of the nitrogen oxides.

Description:
TECHNICAL FIELD 
     The present invention relates to internal combustion engines, and, more particularly, to a method of controlling generation of nitrogen oxides in an internal combustion engine. 
     BACKGROUND 
     An internal combustion engine generally is of two basic types, i.e., a spark ignition engine and a compression combustion engine. A spark ignition engine uses a spark plug to ignite the fuel and air mixture which is injected into the combustion chamber. A compression combustion engine utilizes the energy resulting from compression of the fuel and air mixture as the piston travels toward a top dead center position within the combustion cylinder to ignite the fuel and air mixture. Regardless of whether the internal combustion engine is a spark ignition engine or a compression combustion engine, it is desirable to control the point in time at which combustion occurs relative to the position of the piston within the combustion cylinder. 
     Cycle-to-cycle variations in the combustion event are undesirable characteristics of operating and running a spark ignition engine. The causes of these combustion variations have been attributed to variations in the air/fuel mixture, motion or turbulence (especially in the vicinity of the spark plug), fuel and air charging, and fresh air and residual mixing. The results of these combustion variations are variations in work output or indicated mean effective pressure (IMEP), combustion efficiency, and emissions on a cycle-to-cycle basis (such as nitrogen oxides (NOx)). These combustion variations can manifest themselves in a variety of ways including randomly varying misfires, slow burns, partial burns and fast burns, including detonation or knock. These phenomena are generally more evident under high throttle, high exhaust gas recirculation (EGR), low speed, low turbulence, cold start and lean air/fuel ratio engine operation conditions. 
     The timing of spark ignition is important in obtaining maximum or desired efficiency and proper operating characteristics of the internal combustion engine. It is also generally understood that the resultant combustion event is a function of ignition and early flame development, and a poor combustion event is known to be primarily a function of those conditions that are present in that individual cycle. 
     It is known to provide a plurality of pressure sensors which sense pressures within respective combustion cylinders at discrete points in time for the purpose of analyzing a combustion event. Signals from the pressure sensors may be transmitted to an Electronic Control Module (ECM) for the purpose of controlling the timing of the combustion event within the combustion cylinder as the piston reciprocates between a bottom dead center position and a top dead center position. Sensing pressures within combustion cylinders for the purpose of controlling the timing of the engine is disclosed, e.g., in U.S. Pat. Nos. 4,063,538 (Powell et al.), 4,736,724 (Hamburg et al.), 5,276,625 (Nakaniwa), and 5,359,833 (Baldwin et al.). Examples of pressure sensors which withstand the harsh operating environment in a combustion cylinder are disclosed in U.S. Pat. Nos. 5,714,680 (Taylor et al.), 5,452,087 (Taylor et al.), and 5,168,854 (Hashimoto et al.). 
     The present invention is directed to overcoming one or more of the problems as set forth above. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a method of controlling generation of nitrogen oxides in an internal combustion engine is provided with the steps of: combusting a fuel and air mixture within a combustion cylinder; determining a pressure in the combustion cylinder and a position of a piston within the combustion cylinder; calculating an amount of nitrogen oxides generated with the combusting step, dependent upon the determining step; storing a history of the calculated amount of nitrogen oxides in a memory device; and controlling an output action, dependent upon the calculated amount of nitrogen oxides, the stored history of nitrogen oxides and a threshold value of the nitrogen oxides. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of an embodiment of an internal combustion engine in which a method of controlling generation of nitrogen oxides of the present invention may be carried out; 
     FIG. 2 is a schematic illustration of a combustion cylinder in which a combustion event occurs; 
     FIG. 3 is a graphical illustration of an occurrence of detonation with respect to a pressure profile curve of a combustion cylinder within an internal combustion engine; 
     FIG. 4 is a graphical illustration of the heat release within a combustion cylinder, relative to the position of the piston within the combustion cylinder; and 
     FIG. 5 is a block diagram of one embodiment of a method of the present invention which may be utilized with the internal combustion engine of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, and more particularly to FIG. 1, there is shown a schematic view of an embodiment of a spark ignition combustion engine  10  which may be used to carry out a method of the present invention for controlling the generation of NOx. Internal combustion engine  10  generally includes an Electronic Control Module (ECM)  12 , an Electronic Control Module (ECM)  14  and sensors  16 ,  18 ,  20  and  22 . 
     ECM  12  is a conventional ECM found onboard a vehicle, such as an on-road vehicle, off-road vehicle, etc. ECM  12  includes suitable input/output (IO) circuitry allowing ECM  12  to communicate either unidirectionally and/or bi-directionally with sensors  16 ,  18  and  20 , and ECM  14 , as indicated by lines  24 ,  26 ,  28  and  30 , respectively. In the embodiment shown, lines  24 ,  26  and  28  transmit data in a unidirectional manner from sensors  16 ,  18  and  20  to ECM I 2 . Line  30  communicates data in a bidirectional manner with ECM  14 . Output lines  32 A,  32 B and  32  C are used to effect an action from ECM  12 , depending upon the value of the sensed signals. Output line  32 A is used to adjust a timing of the combustion within a combustion cylinder  34  (FIG.  2 ), output line  32 B is used to adjust an air flow ratio and output line  32 C is used for diagnostics/prognostics. 
     Sensor  16  is used to sense a manifold air pressure within spark ignition combustion engine  10  and provides a plurality of discrete signals to ECM  12  corresponding to the sensed manifold air pressures. Sensor  18  is used to sense a manifold air temperature and provides a plurality of signals to ECM  12  via line  26 . Sensing manifold air pressure and manifold air temperature is optional in the embodiment shown, as indicated by the dashed lines. Sensor  20  is used to sense an engine speed and/or engine coolant temperature and provides a plurality of signals via line  28  to ECM  12 . ECM  12  may analyze the values of the signals sensed by sensors  16 ,  18  and  20  or may pass the data to ECM  14  via line  30 . 
     ECM  14  is used to control the generation of NOx within internal combustion engine  10 , and communicates in a bidirectional manner with ECM  12  via line  30 . In the embodiment shown, ECM  14  is a separate ECM which is coupled with ECM  12  via line  30 . However, it is also to be understood that ECM  14  and ECM  12  may be combined into a common ECM, depending upon the particular application. 
     Pressure sensors  22 - 22   n  sense pressures within respective combustion cylinders  34  of internal combustion engine  10 . The number “n” of pressure sensors  22  corresponds to the number of combustion cylinders within internal combustion engine  10 . Sensors  22   l - 22   n  sense a plurality of pressures at discrete points in time within corresponding combustion cylinders  34  and provide a plurality of pressure signals to ECM  14  via lines  36 . In the embodiment shown, lines  36  are assumed to be bus lines such that a common bus is used to communicate with ECM  14 . However, it is to be understood that each pressure sensor  22   l - 22   n  may include a direct connection with ECM  14 , depending upon the IO configuration of ECM  14 . 
     As shown in FIG. 2, each of the plurality of combustion cylinders  34  includes a piston  38  which is slidably disposed therein. Piston  38  may include a contoured crown, as shown, which affects the fluid dynamics of the fuel and air mixture in combustion chamber  40  within combustion cylinder  34 . A spark plug  42  ignites the fuel and air mixture in combustion chamber  40  at selected points in time as piston  38  moves between a top dead center position and a bottom dead center position. The combustion propagation proceeds in multiple directions, as indicated by direction arrows  44 . Pressure sensor  22  is in fluid communication with combustion chamber  40  and senses a plurality of pressures at discrete points in time. Pressure sensor  22  may be positioned at the axial end of combustion cylinder  34  as shown, or may be positioned at some other desired location (such as a sidewall of combustion cylinder  34 ), depending upon the particular application. 
     As shown in FIG. 2, it is possible that not all of the fuel and air mixture combusts during the primary exothermic chemical reaction within combustion chamber  40 . Some of the non-combusted fuel which remains within combustion chamber  40  typically may be located in areas within combustion chamber  40  away from spark plug  42 , as illustrated by fuel and air mixture pocket  46 . It is possible for this fuel and air pocket to combust separately from the primary charge of fuel and air which is injected into combustion chamber  40 , thereby causing detonation with an additional shock wave to occur within combustion chamber  40 . The uncombusted fuel and air mixture and/or possible detonation (as well as other parameters) affects the combustion event within combustion cylinder  34 , which in turn may affect the generation of NOx. 
     Referring to FIG. 3, a pressure profile curve is shown with the piston position being represented on the horizontal axis and the pressure within the combustion chamber being represented on the vertical axis. During normal operation (indicated by the dashed line), the pressure within combustion cylinder  34  reaches a maximum near or shortly after a top dead center position of the piston  38  within combustion cylinder  34 . Typically, detonation does not occur during normal operation. 
     It is also possible for the peak pressure to be magnified at a point in time which is delayed relative to the top dead center position of piston  38 . Detonation of fuel and air pocket  46  within combustion chamber  40  may occur along the pressure profile curve at a point in time after the peak pressure, which is referred to as “auto ignition of detonation” in FIG.  3 . This type of detonation is evidenced by higher frequency vibrations of the pressure changing from a plus to a minus value as the pressure fluctuates. This type of detonation occurring after the peak pressure has been found not to be particularly deleterious to operation of spark ignition combustion engine  10 . 
     On the other hand, detonation of fuel and air pocket  46  which occurs before the peak pressure, referred to as “hard detonation”, has been found to be deleterious to operation of compression combustion engine  10 . If hard detonation is sensed, it is possible to take various actions which either eliminate the detonation or move the detonation to a point in time after occurrence of the peak pressure such that the detonation is not harmful. For example, it is possible to adjust the timing of the ignition event; reduce an amount of fuel which is injected into combustion engine  10 ; and/or reduce a load on spark ignition combustion engine  10  to affect the location of the detonation on the pressure profile curve shown in FIG.  3 . 
     Combustion events as described above in combustion chamber  40  within combustion cylinder  34  affects the combustion efficiency and operation of internal combustion engine  10 . The combustion efficiency in turn affects the generation of NOx emitted from internal combustion engine  10 . It is possible to calculate the amount of NOx which is emitted from internal combustion engine  10  using various input parameters. The input parameters are then used to calculate the heat release during the combustion event as well as the burn temperature of the fuel and air mixture during the combustion event. 
     Referring now to FIG. 4, it may be observed that a large percentage of the heat release associated with a combustion event occurs while the piston is near a top dead center position. More particularly, a large percentage of the heat release for a given combustion event occurs when the piston moves through a position approximately 10° before top dead center to a position approximately 10° after top dead center. 
     Industrial Applicability 
     Referring now to FIG. 5, there is shown a block diagram of an embodiment of the method of the present invention for controlling the generation of NOx in internal combustion engine  10 . At block  50 , various input parameters are received and analyzed. To wit, the pressure (Pcyl) corresponding to one or more combustion cylinders sensed by a pressure sensor  22  is received. In addition, the crank angle θ of a crank shaft carrying the plurality of pistons  38  is received. The crank angle θ in turn is used to determine the position of the piston  38  within the combustion cylinder  34  for which the sensed pressure corresponds. Other operating parameters (block  52 ) such as manifold air pressure, manifold air temperature, etc. may also be sensed within internal combustion engine  10  and used at block  50 . Additionally, engine platform parameters (block  54 ) specific to a particular engine may be sensed within internal combustion engine  10  and provided for analysis at block  50 . 
     At block  56 , the various signals analyzed at block  50  are used to extract the heat release corresponding to the combustion event within combustion chamber  40 . The input parameters may be used in an individual or combined manner to calculate the heat release for the combustion event. Extracting the heat release of the combustion event using mathematical techniques is known in the art, and thus will not be described in further detail herein (see, e.g., U.S. Pat. No. 5,219,227, column  7 ). Based upon the calculated heat release, the burned temperature of the fuel and air mixture for the combustion event is then calculated (block  58 ). Again, calculating the burned temperature of a fuel and air mixture for a combustion event is known and thus not described in further detail herein. 
     At block  60 , the amount of NOx which is generated for the combustion event is calculated using the burned temperature from block  58  and (optionally) platform parameters from block  54 . The calculated NOx is then utilized within logic circuit  62  and a memory device. The calculated NOx is stored individually within the memory device (block  64 ) and/or mathematically combined with the calculated NOx for other cylinders from previous cycles (block  66 ). The individually stored NOx amounts and/or the combined NOx amounts from previous cycles are utilized by logic circuit  62 . Additionally, logic circuit  62  receives a threshold value corresponding to allowable NOx which may be generated by internal combustion engine  10 . The calculated amount of NOx from block  60 , stored history of NOx from block  66  and threshold value of allowable NOx (block  68 ) are analyzed with logic circuit  62  to determine whether an output action  70  should occur. More particularly, the calculated NOx, stored history of NOx and threshold value of NOx are mathematically combined within logic circuit  62  to determine whether an output action  70  should occur. Output actions  70  may include, e.g., adjusting the timing (block  72 ), waste gate (block  74 ), throttle (block  76 ), fuel rate (block  78 ) and/or other appropriate actions (block  80 ). 
     From the foregoing description of an embodiment of the method of the present invention, it is apparent that logic circuit  62  receives multiple inputs corresponding to the NOx generated by internal combustion engine. By basing an output action  70  upon multiple inputs, including the calculated NOx, stored history of NOx and allowable threshold value of NOx, a more accurate determination of whether to take an output action  70  is effected. The output action may include any number of output actions as shown, or may include no action. The method of the present invention therefore provides improved control over the generation of NOx within internal combustion engine  10 . 
     Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.