Patent Publication Number: US-6662795-B2

Title: Method and apparatus configured to maintain a desired engine emissions level

Description:
TECHNICAL FIELD 
     This invention relates generally to a method and apparatus of controlling an engine, and more particularly, to an apparatus and method configured to maintain a desired emissions level of an engine. 
     BACKGROUND 
     Engine emissions, such as NOx emissions, play an important role in engine control. In some applications, emitting higher than desired NOx levels, while still within regulatory standards, may cause problems in the particular application the engine is being used. For example, in a greenhouse a low level of NOx is desirably maintained at an even level. However, current control systems are unable to do this. If the engine emissions are higher than a designated amount, then the emissions adversely effect the greenhouse. On the other hand, if the engine emissions are below the designated amount, then overall engine performance may suffer. That is, engine efficiency decreases as NOx emissions levels decrease. Therefore running the engine in an operating range where lower NOx levels are being emitted than necessary to meet site or regulatory emissions restrictions, causes a reduction in engine operating efficiency. 
     Changes in the ambient conditions may have a significant impact on the NOx emissions, and in particular the ability to maintain the NOx emissions at a desired level. For example, as the specific humidity increases in the air within the intake manifold, the higher water content in the intake air reduces the peak combustion temperature, and therefore reduces the NOx formation. In addition, the higher specific humidity means there is less oxygen in the cylinder during combustion, and therefore less oxygen exhausted from the cylinder. Both of these issues lead to a reduced oxygen content in the exhaust stream of the engine. Without accounting for the changes in the ambient conditions, the reduced oxygen content may be misinterpreted by a control algorithm which may either unnecessarily adjust the air fuel ratio, or adjust the air fuel ratio in the wrong manner, causing decreased performance in the engine. 
     Some systems calculate a specific humidity, and use the specific humidity to modify the determined lean limit of the engine. However, operating the engine at a lean limit, and modifying the lean limit to account for changes in the specific humidity does not address the problem of operating an engine in a manner to maintain a desired emissions level despite changes in the ambient conditions, such as specific humidity and/or exhaust pressure. 
     The present invention is directed to overcoming one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a method of maintaining a desired emissions level of an engine having an intake manifold and an exhaust manifold, and an exhaust stack is disclosed. The method includes the steps of establishing a desired emissions level, establishing an engine speed, establishing an engine load, establishing at least one characteristic of one of an intake air and an exhaust gas, and determining a fuel command in response to the engine speed, the engine load, and the desired emissions level, the fuel command resulting in the engine maintaining the desired emissions level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of one embodiment of a fuel system; 
     FIG. 2 is an illustration of one embodiment of a method of maintaining a desired emissions level; 
     FIG. 3 is a block diagram of one embodiment of a method of maintaining a desired emissions level; 
     FIG. 4 a  is a map illustrating the desired air/fuel ratio as a function of engine speed and engine load; 
     FIG. 4 b  is a map illustrating desired oxygen as a function of engine speed and engine load; 
     FIG. 5 is an illustration of the relationship between specific humidity and desired oxygen; 
     FIG. 6 is an illustration of the relationship between exhaust stack pressure and desired oxygen; 
     FIG. 7 a  is an illustration of the impact of changes in the specific humidity on the actual emissions level; and 
     FIG. 7 b  is an illustration of the impact of changes in the exhaust pressure on the actual emissions level. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides a method and apparatus of maintaining a desired emissions level for an engine. FIG. 1 is an illustration of one embodiment of a fuel system  100  of an engine incorporating the present invention. A fuel control valve  104 , such as a TechJet™, enables fuel to flow to an air/fuel mixer  108 . The air/fuel mixture passes through a compressor  110  and after cooler  114 . A throttle  116  controls the volume of air/fuel mixture that flows into an intake manifold  118 . The manifold  118  delivers the fuel to one or more cylinders  120 . The exhaust from the cylinders  120  passes through an exhaust manifold  122 , a turbine  112 , and an exhaust stack  124 . 
     A specific humidity sensor  130  may be located in the intake air stream. In one embodiment, the specific humidity sensor  130  is located in the inlet air before the turbo compressor  110 . Alternatively the specific humidity sensor  130  may be located in the intake manifold  118 . The specific humidity sensor  130  measures the specific humidity of the intake air within the manifold, and responsively delivers a corresponding specific humidity signal to the controller  102 . 
     An oxygen sensing device  152 , may be located in the exhaust stream of the engine. The oxygen sensing device  152  senses the gases being exhausted from the engine, i.e., one or more cylinders of the engine, and responsively generates a signal indicative of the oxygen content of the exhaust gases, to the controller  102 . In one embodiment, the oxygen sensing device  152  is located in the exhaust manifold  122 . Alternatively the oxygen sensing device  152  is located in the exhaust stack  124 . In one embodiment, the oxygen sensing device  152  may be an automotive-type, heated sensor such as NTK TL6312. Some oxygen sensing devices such as NTK TL6312 may be sensitive to the pressure they are exposed to. Other types of oxygen sensing devices  152 , such as an electrochemical cell type oxygen sensor are less sensitive to pressure, or not sensitive to pressure at all. 
     A pressure sensing device  154 , may be used to sense the pressure located in the exhaust stream of the engine. In one embodiment, the pressure sensing device  154  is located adjacent to the oxygen sensing device  152  and delivers a pressure signal to the controller  102  indicative of the pressure experienced by the oxygen sensing device  152 . The pressure sensing device  154  and the oxygen sensing device  152  may be located in the exhaust stack  124  of the engine. In one embodiment, the pressure sensing device  154  is an exhaust pressure sensor. Alternatively, the pressure sensing device may be located in a pipe  170 . The pipe  170  is connected to the exhaust stream such that one end is open to the exhaust stream of the engine, and the other end of the pipe  170  is open to the ambient air. In this manner, the sensing device  154  (illustrated as  154 ′ in this alternative location of FIG. 1) may sense the pressure of the exhaust stream without being directly exposed to the extreme temperature of the exhaust gases. In an alternative embodiment, the sensing device  154  may be configured to sense the atmospheric pressure as opposed to the exhaust pressure. In this embodiment, the sensing device  154  may be an ambient air pressure sensor. As will be discussed, the exhaust pressure or ambient air pressure may then be used to account for changes in the ambient conditions with respect to fuel calculations. 
     In one embodiment, a pressure sensing device  156  may be configured to sense the pressure in the intake manifold  118 , an deliver a signal indicative of the intake air pressure to the controller  102 . 
     In one embodiment, a temperature sensing device  132  is located in the intake manifold  118 . The temperature sensing device  132  is configured to deliver a temperature signal to the controller  102  indicative of the temperature of the air in the intake manifold  112 . 
     An engine speed sensing device  134  is electrically connected to the controller  102 . The speed sensing device  132  can be any type of sensor that produces an electrical signal indicative of engine speed. For example, in one embodiment, the speed sensor  132  is mounted on an engine flywheel housing (not shown) and produces a digital speed signal in response to the speed of the flywheel mounted on an engine crankshaft (not shown). Alternatively, the speed sensing device  132  may be an in-cylinder sensing device configured to deliver a signal to the controller  102  indicative of the speed of the engine. 
     The controller  102  receives inputs from the oxygen sensing device  152 , speed sensing device  134 , and one or more of the pressure sensing device  154 , a temperature sensing device  132 , and a humidity sensor  130 . The controller  102  may receive continuous updates from the sensors. The controller  102  determines a throttle position and a fuel control valve position in response to the input signals, and sends the appropriate commands to a throttle actuator  124 , and a fuel actuator  126  respectively. That is, one or more software algorithms executing on the controller  102  receive the input signals, and responsively determine the appropriate throttle and fuel commands in order to maintain the desired emissions level, and generate the corresponding command signals. 
     The controller  102  delivers the throttle command to a throttle actuator  128 . The throttle actuator  128  will control the position of the throttle  116  in response to the throttle command. 
     The controller  102  also delivers a fuel command to a fuel valve actuator  126 . The fuel valve actuator  126  will control the position of the fuel control valve  104  in response to the fuel command. 
     FIG. 2 illustrates the one embodiment of the method of the present invention. The present invention includes a method of maintaining a desired emissions level of an engine having an intake manifold  118  and an exhaust manifold  122  and an exhaust stack  124 . The method includes the steps of establishing a desired emissions level, establishing an engine speed, establishing an engine load, establishing at least one characteristic of one of an intake air and an exhaust gas, determining a fuel command in response to the engine speed, the engine load, the desired emissions level, and the at least one established characteristic. 
     In a first control block  202 , a desired emissions level is established. In one embodiment, the desired emissions level is a desired NOx level emitted by the engine. The desired NOx level may be established based upon local emissions regulations or site specific emissions regulations. For example, there may be applications, such as operation within a greenhouse, which require the emissions to be lower than specified in local emissions regulations. The desired emissions level may include a range. For example, the desired emissions level may include a designated value, plus or minus five percent of the designated value. Therefore, in one embodiment, maintaining the desired emissions level includes maintaining the actual emissions level within a desired emissions range. Alternatively the desired emissions level may include the designated value plus or minus a second designated value. In one embodiment, the operator may deliver a parameter indicative of the desired emissions level into the controller  102 , as will be described. 
     In one embodiment, once a desired emissions level has been established, an operator may determine a desired rated oxygen to be exhausted by the engine in order to achieve the desired emissions level. The desired rated oxygen is a parameter used to determine the fuel command, as will be explained. The desired rated oxygen may be determined in response to an actual and the desired NOx level. For example, during initial configuration, a desired rated oxygen level may be established based upon a look up table or map which correlates desired rated oxygen as a function of desired NOx level, and actual NOx level, in order to achieve the desired emissions level. The maps or look up tables may be empirically determined. In an alternative embodiment, the desired rated oxygen may be established based upon calculations including the desired and actual NOx levels. Then, if the actual NOx emitted by the engine is greater than the desired NOx, the desired rated oxygen parameter may be adjusted in a manner to effect a change in the fuel command such that the actual NOx emissions change until within a threshold of the desired NOx emissions. For example, an operator may determine an actual NOx emissions through the use of a sensing device, such as a NOx analyzer. The actual NOx emissions may be compared to the desired NOx emissions. A desired rated oxygen to be exhausted by the engine may be determined in response to the comparison. For example, the NOx emissions error may be used to modify the previous value of desired rated oxygen to determine upcoming fuel command. 
     Therefore, in one embodiment, the operator may input a parameter indicative of the desired rated oxygen to the controller  102 , in response to the actual and desired NOx levels. That is, the operator may input a parameter indicative of the desired emissions level into the controller  102 , such as the desired rated oxygen level. The desired rated oxygen may be established in response to an operator input into the controller  102 . The operator input may be used to modify the desired rated oxygen exhausted by the engine until the actual NOx emissions is equivalent, or within a threshold, or range, of the desired NOx emissions. The desired rated oxygen is then used to determine a fuel command, as is described below. The fuel command is delivered to the system, in one embodiment, and the actual NOx emissions are again compared to the desired NOx emissions. A modification to the desired rated oxygen is made in response to the comparison if necessary, and the process is repeated. Otherwise, if the actual NOx emissions is equal to, or within a threshold of the desired NOx emissions, then the desired rated oxygen is left unmodified. In one embodiment, the desire rated oxygen is determined while the engine is operating at rated load, e.g., full load. In one embodiment, the establishment of the desired emissions level, and corresponding desired rated oxygen level may be considered an initialization step for the engine. The initialization step may be performed periodically, every time the engine is started, or at some other desired interval. 
     The desired rated oxygen level, as discussed, is a value which may be dynamically established based upon an operator input. For example, when an operator starts an engine, the operator may input a value indicative of the desired rated oxygen to be emitted by the engine, which will be received and stored by the controller  102 . The desired rated oxygen level, or value indicative thereof, may then be used by the controller  102  for future operations until the value is changed by an operator. In one embodiment, the desired rated oxygen level may be input by the operator via an operator input device, such as a keypad (not shown), touch screen display (not shown), or other analogous input device. In one embodiment, the desired rated oxygen level may be input by a service technician using a service tool (not shown) which may access the controller  102 . In another embodiment, the operator input device may include a receiving device (not shown). For example, the desired rated oxygen level may be received by a receiving device (not shown), from a remote location. A central office may be in communication with a remotely located engine, via satellite or wireless communication techniques, and send the desired oxygen level to the controller  102 . 
     In an alternative embodiment, the desired emissions level may be considered to include, or be the desired oxygen level. In this embodiment, the desired rated oxygen level may be input to the controller associated with the engine as indicated above. 
     In an alternative embodiment, the desired emissions level, e.g., desired NOx level, may be delivered to the controller  102 , and the desired rated oxygen exhausted by the engine may be determined in response to the desired emissions level. The desired emissions level may be delivered to the controller via an operator input device as described above. The desired rated oxygen exhausted may be determined from a map which has been empirically established which indicates desired rated oxygen as a function of desired emissions levels. Alternatively, the desired rated oxygen may be determined based on a calculation involving the desired emissions levels. In yet another embodiment, a desired emissions level may be established and delivered to the controller  102  prior to delivery of the engine to the location where the engine is to be used. 
     In one embodiment, the desired rated oxygen may be a default value that is modified based upon the current operating conditions, e.g., the difference between the desired and actual NOx level. 
     In a second control block  204  an engine speed is established. In the preferred embodiment, the engine speed is established in response to the speed signal received from the engine speed sensing device  134 . 
     In a third control block  206  an engine load may be established. Engine load is generally the amount of work being performed by the engine at a particular point in time and is generally defined in terms of rated engine load or work capacity. Engine load can be measured by a wide variety of different methods known in the art such as by using the total quantity of fuel delivered, e.g., fuel rate, to the engine for a particular task or work operation as an indicator of engine load. In addition, engine load may be determined in response to a throttle input, manifold boost pressure, exhaust temperature, and or load sensor. Alternatively, or in addition to, a load signal from the generator could be used to determine load 
     In a fourth control block  208  at least one characteristic of one of an intake air and an exhaust gas is established. In one embodiment, the intake air characteristic includes a specific humidity of the air within the intake air stream. For example the characteristic may be the specific humidity of the air in the inlet air before the turbo  110 . Alternatively, or in addition to, the characteristic may include a pressure of the air within the exhaust stream of the engine. In one embodiment, the pressure is established in a manner such that the established pressure is indicative of the pressure that the oxygen sensing device  152  is exposed to. In one embodiment, the established characteristics include the air temperature within the intake manifold, the specific humidity as measured within the intake air stream, and the pressure indicative of the pressure the oxygen sensing device  152  is exposed to in the exhaust stack  124 , and the oxygen in the exhaust stream sensed by the oxygen sensing device  152 . In an alternative embodiment, the ambient air pressure, or the atmospheric air pressure, may be sensed instead of, or in addition to the exhaust pressure. 
     In a fifth control block  210  a fuel command is determined in response to the engine speed, engine load, at least one of the established characteristics, and the desired emissions level. In one embodiment, the engine speed, engine load and the desired emissions level are used to determine an air flow, desired air/fuel ratio, and a fuel correction factor. The desired fuel flow is then determined in response to the air flow, desired air/fuel ratio, and the fuel correction factor, as illustrated in FIG.  3 . For example, the desired air/fuel ratio is determined in response to the current engine speed and the current engine load. In one embodiment, a three dimensional map, or look up table, may be established through empirical analysis, which maps the desired air/fuel ratio as a function of engine speed and engine load, as illustrated in FIG. 4 a.  The desired air/fuel ratio is then determined through the use of the desired air/fuel map. 
     The air flow may be determined in response to a sensed inlet manifold pressure, a sensed inlet manifold temperature, the engine speed, and the engine load. For example, in one embodiment, a volumetric efficiency may be determined in response to the engine load and engine speed. The air flow may be calculated based on the inlet manifold air pressure, engine speed, volumetric efficiency and inlet manifold air temperature. One or more of these determinations may be based upon a map or look-up table. For example, a map may be used to determine the volumetric efficiency as a function of engine speed and engine load. The map may be empirically determined and stored in the controller. 
     A fuel correction factor may be determined in response to the desired rated oxygen exhausted by the engine, a desired oxygen to be exhausted by the engine, and the actual oxygen exhausted by the engine. As illustrated in FIG. 3, a rated oxygen offset may be determined based upon the desired rated oxygen exhausted from the engine. For example, the desired rated oxygen exhausted by the engine may be compared to a map rated oxygen exhausted by the engine. The map rated oxygen exhausted by the system may be established at particular ambient conditions, and while the engine is operating at rated load. The desired rated oxygen may be established at rated load. In one embodiment, rated load is equivalent to maximum load. Therefore, the desired rated oxygen indicates the desired oxygen at rated load, e.g., full load. The difference between the desired rated oxygen and the map rated oxygen levels is that the ambient conditions may have changed. Therefore, in one embodiment, an offset is determined by subtracting the map rated oxygen from the desired rated oxygen to reflect the potential difference in ambient conditions. 
     In the preferred embodiment, a desired, or predicted, oxygen level exhausted may be determined. That is, for the combustion that is about to occur in response to the initial fuel command, the desired oxygen output may be determined. In the preferred embodiment, a three dimensional map, or look up table, may be established through empirical analysis, which maps desired oxygen output as a function of current engine speed and current engine load, as illustrated in FIG. 4 b.  The oxygen being exhausted by the engine, is indicative of the amount of NOx being exhausted by the engine. The desired rated oxygen, as mentioned, indicates the desired rated oxygen at rated load, e.g., full load. The desired oxygen determined based on the map illustrated in FIG. 4 b  is based upon the current engine speed and engine load, which may be different from the rated load at which the desired rated oxygen was determined. By compensating the fuel calculations in response to the desired oxygen, and the desired rated oxygen, the desired NOx emissions level may be maintained. The desired oxygen exhausted may be determined as a function of the current engine speed and current engine load. In one embodiment, the desired, or predicted oxygen level, is compensated in response to one or both of the specific humidity measurement and the established exhaust pressure. The combustion process that occurs within a cylinder is affected by the specific humidity of the air that flows into the cylinder. For example, the higher the specific humidity of the intake air, the lower the amount of oxygen that is available in the intake cylinder during combustion, and therefore, the lower the exhausted oxygen amount will be. In addition, the higher the specific humidity of the intake air, the lower the temperature of the combustion process, due in part to the fact that more energy will be expended heating the additional water in the air. As a result, the combustion temperature is lower, and therefore the NOx emissions are lower. The lower NOx emissions further reduces the amount of oxygen exhausted from the cylinder. Therefore, as the specific humidity increases, the predicted, or desired, level of oxygen in the exhaust gases is reduced, as illustrated in FIG.  5 . Therefore, to account for the particular specific humidity, a compensation factor may be determined which accounts for both the change in the temperature of the combustion process due to the specific humidity, which leads to a change in the amount of oxygen exhausted, and the change in the amount of oxygen that entered the cylinder based upon the specific humidity, which also changed the amount of oxygen exhausted. For example, the higher the specific humidity of the intake air, the cooler the combustion, and the lower the NOx emissions will be. Therefore, as the specific humidity increases, the desired level of oxygen in the exhaust gases may be reduced to maintain a NOx emissions level, as illustrated in FIG.  5 . Therefore, to account for the particular specific humidity, a compensation factor may be determined which accounts for the change in the oxygen required to maintain the desired NOx. Again, maintaining the desired NOx level may include maintaining the actual NOx level within a desired range, or threshold, of the desired NOx level. Additionally, the oxygen sensor  152  may be sensitive to pressure changes in the exhaust. The effects of this sensitivity may also included in the compensation factor. 
     In one embodiment, the use of pressure compensation may be dependent on the type of oxygen sensing device  152  used. Some types of oxygen sensors  152  are sensitive to changes in the pressure of the exhaust stack. Therefore a pressure sensing device  154  may be located adjacent to the oxygen sensing device  152  to establish the pressure experienced by the oxygen sensing device  152 . In one embodiment, depending upon the pressure sensing device used, pressure within the exhaust stack affects the desired oxygen output level in a manner as illustrated in FIG.  6 . Therefore, the predicted oxygen output may be modified to account for changes in the intake air temperature, the specific humidity of the intake air, and the exhaust pressure the oxygen sensing device is exposed to. In one embodiment, the ambient air pressure may be compensated for instead of, or in addition to the exhaust pressure. Each of these compensation factors (associated with the intake air temperature, the specific humidity of the intake air, and the exhaust pressure the oxygen sensing device) may be empirically determined and stored in a map or look up table. For example, a pressure compensation factor may be empirically determined as a function of the pressure which the oxygen sensing device is exposed to, and stored in a map or look up table, and used to compensate the desired or predicted oxygen level emitted. Alternatively, a pressure compensation factor may be determined dynamically using a formula. For example, a pressure compensation factor may be set equal to (X*Absolute Stack Pressure(KPa)). Where X is a constant. Additional variables or offsets may be used to determine the pressure compensation factor. The value for X or any other variables or offsets are implementation dependent and may vary from one engine type to another. A specific humidity compensation factor may be empirically determined as a function of the specific humidity of the air within the intake manifold, and stored in a map or look up table. In an alternative embodiment, a specific humidity compensation may be dynamically determined using a formula. For example, the specific humidity compensation factor=(Y*specific humidity (gr/lbm dry air)). In one embodiment, Y is an empirically established constant. Alternatively Y could vary as a function of the specific humidity. The value Y or any other variables or offsets is implementation dependent and may vary from one engine type to another. 
     In addition, a temperature compensation factor may be used to modify the desired oxygen exhausted by the engine in order to account for the temperature of the intake air flowing into the manifold. A temperature compensation factor may be empirically determined as a function of the temperature of the air within the intake manifold. 
     The fuel correction factor may then be determined in response to the rated oxygen offset, the modified, or compensated, desired oxygen exhausted by the engine, and the actual oxygen as measured by the oxygen sensing device. In one embodiment, the rated oxygen offset and the modified desired oxygen level may be compared to the actual measured oxygen level. For example, the rated oxygen offset, and the modified desired oxygen level may be subtracted from the actual oxygen measurement. In the preferred embodiment, the result is then delivered to a PID controller to determine the fuel correction factor. One example, of such a PID controller is: 
     FCF=(Kp*ei)+(Ki*iei)+(KD*deltaei) 
     Where 
     FCF=the fuel correction factor 
     ei=error(desired oxygen−actual oxygen) 
     KP=Proportional gain of the governor 
     KI=Integral gain of the governor 
     KD=Derivative gain of the governor 
     deltaei=the rate of change of the error 
     iei=an integral factor. 
     In the preferred embodiment, the air flow, is then multiplied by the fuel correction factor and a map BTU, and the result divided by the desired air/fuel ratio multiplied by a customer selected BTU to determine the desired fuel flow to the cylinder, as illustrated below:          Fuel                 Command     =       (     Air                 Flow   *   Map                 BTU   *   Fuel                 Correction                 Factor     )     /     (       Desired                 Air                 Fuel                 Ratio   *   Operator     -     selected                 BTU                 value       )                       
     Where: 
     Map BTU is the heating value of the fuel that was used when the Air Fuel ratio map was created and Operator-selected BTU value is a heating value of the fuel that is currently being used. For example, an operator may input a BTU value indicative of the fuel being used, via an operator input device as described earlier. 
     A fuel command is then determined in response to the desired fuel flow, in order to deliver the desired fuel to the cylinder. 
     INDUSTRIAL APPLICABILITY 
     The present invention includes a method and apparatus of maintaining a desired emissions level of an engine having an intake manifold and an exhaust manifold, and an exhaust stack. The method includes the steps of establishing a desired emissions level, establishing an engine speed, establishing an engine load, establishing at least one characteristic of one of an intake air and an exhaust gas, and determining a fuel command in response to the engine speed, the engine load, the desired emissions level, and the established characteristics. 
     In one embodiment, an operator establishes a desired emissions level, such as a desired NOx level, based on either a local regulation or a site specific requirement. An initialization procedure may be performed whereby the operator runs the engine at a rated load, such as full load. The operator then monitors the actual engine emissions level and compares it to the desired emissions level. The difference between the actual and desired emissions level is used to determine a parameter, such as a desired rated oxygen level, which is then input to the controller associated with the engine. The desired rated oxygen level is used by a software algorithm running on the controller to determine a fuel command. The algorithm is configured to determine a fuel command in a manner such that the desired emissions level is maintained. Therefore, during the initialization procedure, the resulting actual emissions may be compared to the desired emissions level, and the desired rated oxygen level is modified accordingly until the actual emissions level is equal to, or within an acceptable range of the desired emissions levels. 
     The software algorithm executing on the controller is configured to account for changes in ambient conditions. Therefore, the specific humidity, exhaust pressure, and/or ambient air pressure may be measured and used to compensate, or modify, a desired oxygen output level. The modified, desired oxygen output level may be compared with the actual oxygen output level in the exhaust gases, and the result used to determine a fuel correction factor. The fuel correction factor is used to determine the fuel command. In this manner, algorithm compensates the fuel command based on changes in the ambient conditions, such as the specific humidity, the exhaust pressure, or the ambient air pressure. Therefore, the desired emissions levels may be maintained despite variations in ambient conditions. For example, the emissions levels may be maintained within a range or threshold of the desired emissions level, i.e., the desired emissions level includes a range or threshold value within which the actual emissions are desirably maintained. FIG. 7A illustrates test results using this invention during changes in the specific humidity. The plot  702  illustrates the actual emissions level without using the present invention, as compared to plot  704  where one embodiment of the present invention was utilized. Using one embodiment of the present invention, as the specific humidity varies, the desired emissions level was maintained. In particular, the actual emissions level was maintained within an acceptable threshold of the desired emissions level. FIG. 7B illustrates test results using this invention during changes in the exhaust pressure. The plot  706  illustrates the actual emissions level without using the present invention, as compared to plot  708  where one embodiment of the present invention was utilized. Using one embodiment of the present invention, as the exhaust pressure varies, the desired emissions level was maintained. In particular, the actual emissions level was maintained within an acceptable threshold, or range, of the desired emissions level. 
     Other aspects, objects, and advantages of the present invention can be obtained from a study of the drawings, the disclosure, and the claims.