Patent Publication Number: US-9410467-B2

Title: Methods and systems for humidity detection via an exhaust gas sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/745,639, entitled “METHODS AND SYSTEMS FOR HUMIDITY DETECTION VIA AN EXHAUST GAS SENSOR,” filed on Jan. 18, 2013, now U.S. Pat. No. 8,857,155, the entire contents of which are hereby incorporated by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     The present application relates generally to ambient humidity detection via an exhaust gas sensor coupled in an exhaust system of an internal combustion engine. 
     BACKGROUND AND SUMMARY 
     During engine non-fueling conditions in which at least one intake valve and one exhaust valve are operating, such as deceleration fuel shut off (DFSO), ambient air may flow through engine cylinders and into the exhaust system. In some examples, an exhaust gas sensor may be utilized to determine ambient humidity during the engine non-fueling conditions. It may take a long time for the exhaust flow to be devoid of hydrocarbons during the engine non-fueling conditions, however, and, as such, an accurate indication of ambient humidity may be delayed. 
     The inventors herein have recognized the above issue and have devised an approach to at least partially address it. Thus, a method for an engine system which includes an exhaust gas sensor is disclosed. In one example, the method includes, during engine non-fueling conditions, where at least one intake valve and one exhaust valve are operating: modulating a reference voltage of the sensor; generating an ambient humidity based on a corresponding change in pumping current of the sensor; and, during selected operating conditions, adjusting an engine operating parameter based on the ambient humidity. 
     By modulating the reference voltage and determining the change in pumping current while the air fuel ratio is still changing during non-fueling conditions, such as DFSO, the effect of the changing air fuel ratio may be nullified. As such, the ambient humidity may be determined in a shorter amount of time, as the exhaust air fuel ratio does not have to be stable before an accurate indication of ambient humidity may be determined. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example embodiment of a combustion chamber in an engine system including an exhaust system and an exhaust gas recirculation system. 
         FIG. 2  shows a schematic diagram of an example exhaust gas sensor. 
         FIG. 3  is a flow chart illustrating a routine for determining a measurement mode of an exhaust gas sensor. 
         FIG. 4  is a flow chart illustrating a routine for determining ambient humidity based on an exhaust gas sensor. 
         FIG. 5  shows a graph illustrating reference voltage and pumping current of an exhaust gas sensor during deceleration fuel cut off. 
         FIG. 6  is a flow chart illustrating a routine for adjusting engine operating parameters based on an ambient humidity generated by an exhaust gas sensor. 
     
    
    
     DETAILED DESCRIPTION 
     The following description relates to methods and systems for an engine system with an exhaust gas sensor. In one example, a method comprises, during engine non-fueling conditions, where at least one intake valve and one exhaust valve are operating: modulating a reference voltage of the sensor, generating an ambient humidity based on a corresponding change in pumping current of the sensor, and adjusting an engine operating parameter based on the ambient humidity. As an example, the change in pumping current may be averaged over a duration during the non-fueling conditions. In this way, accuracy of the humidity determination based on the change in pumping current may be improved, for example. Further, the ambient humidity determination may be made in a reduced amount of time, as averaging the change in pumping current reduces the effect of a changing air fuel ratio. Once the ambient humidity is determined, one or more engine operating parameters may be adjusted during fueling conditions, for example. In one example, an amount of exhaust gas recirculation (EGR) is adjusted based on the ambient humidity. In this way, the system can nullify the effect of the changing air fuel ratio by modulating the reference voltage. 
       FIG. 1  is a schematic diagram showing one cylinder of a multi-cylinder engine  10  in an engine system  100 , which may be included in a propulsion system of an automobile. The engine  10  may be controlled at least partially by a control system including a controller  12  and by input from a vehicle operator  132  via an input device  130 . In this example, the input device  130  includes an accelerator pedal and a pedal position sensor  134  for generating a proportional pedal position signal PP. A combustion chamber (i.e., cylinder)  30  of the engine  10  may include combustion chamber walls  32  with a piston  36  positioned therein. The piston  36  may be coupled to a crankshaft  40  so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. The crankshaft  40  may be coupled to at least one drive wheel of a vehicle via an intermediate transmission system. Further, a starter motor may be coupled to the crankshaft  40  via a flywheel to enable a starting operation of the engine  10 . 
     The combustion chamber  30  may receive intake air from an intake manifold  44  via an intake passage  42  and may exhaust combustion gases via an exhaust passage  48 . The intake manifold  44  and the exhaust passage  48  can selectively communicate with the combustion chamber  30  via respective intake valve  52  and exhaust valve  54 . In some embodiments, the combustion chamber  30  may include two or more intake valves and/or two or more exhaust valves. 
     In this example, the intake valve  52  and exhaust valve  54  may be controlled by cam actuation via respective cam actuation systems  51  and  53 . The cam actuation systems  51  and  53  may each include one or more cams and may utilize one or more of cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT), and/or variable valve lift (VVL) systems that may be operated by the controller  12  to vary valve operation. The position of the intake valve  52  and exhaust valve  54  may be determined by position sensors  55  and  57 , respectively. In alternative embodiments, the intake valve  52  and/or exhaust valve  54  may be controlled by electric valve actuation. For example, the cylinder  30  may alternatively include an intake valve controlled via electric valve actuation and an exhaust valve controlled via cam actuation including CPS and/or VCT systems. 
     A fuel injector  66  is shown coupled directly to combustion chamber  30  for injecting fuel directly therein in proportion to the pulse width of signal FPW received from the controller  12  via an electronic driver  68 . In this manner, the fuel injector  66  provides what is known as direct injection of fuel into the combustion chamber  30 . The fuel injector may be mounted in the side of the combustion chamber or in the top of the combustion chamber (as shown), for example. Fuel may be delivered to the fuel injector  66  by a fuel system (not shown) including a fuel tank, a fuel pump, and a fuel rail. In some embodiments, the combustion chamber  30  may alternatively or additionally include a fuel injector arranged in the intake manifold  44  in a configuration that provides what is known as port injection of fuel into the intake port upstream of the combustion chamber  30 . 
     The intake passage  42  may include a throttle  62  having a throttle plate  64 . In this particular example, the position of throttle plate  64  may be varied by the controller  12  via a signal provided to an electric motor or actuator included with the throttle  62 , a configuration that is commonly referred to as electronic throttle control (ETC). In this manner, the throttle  62  may be operated to vary the intake air provided to the combustion chamber  30  among other engine cylinders. The position of the throttle plate  64  may be provided to the controller  12  by a throttle position signal TP. The intake passage  42  may include a mass air flow sensor  120  and a manifold air pressure sensor  122  for providing respective signals MAF and MAP to the controller  12 . 
     An exhaust gas sensor  126  is shown coupled to the exhaust passage  48  upstream of an emission control device  70 . The sensor  126  may be any suitable sensor for providing an indication of exhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), a two-state oxygen sensor or EGO, a HEGO (heated EGO), a NO x , HC, or CO sensor. The emission control device  70  is shown arranged along the exhaust passage  48  downstream of the exhaust gas sensor  126 . The device  70  may be a three way catalyst (TWC), NO x  trap, various other emission control devices, or combinations thereof. In some embodiments, during operation of the engine  10 , the emission control device  70  may be periodically reset by operating at least one cylinder of the engine within a particular air/fuel ratio. 
     Further, in the disclosed embodiments, an exhaust gas recirculation (EGR) system  140  may route a desired portion of exhaust gas from the exhaust passage  48  to the intake manifold  44  via an EGR passage  142 . The amount of EGR provided to the intake manifold  44  may be varied by the controller  12  via an EGR valve  144 . Further, an EGR sensor  146  may be arranged within the EGR passage  142  and may provide an indication of one or more of pressure, temperature, and constituent concentration of the exhaust gas. Under some conditions, the EGR system  140  may be used to regulate the temperature of the air and fuel mixture within the combustion chamber, thus providing a method of controlling the timing of ignition during some combustion modes. Further, during some conditions, a portion of combustion gases may be retained or trapped in the combustion chamber by controlling exhaust valve timing, such as by controlling a variable valve timing mechanism. 
     The controller  12  is shown in  FIG. 1  as a microcomputer, including a microprocessor unit  102 , input/output ports  104 , an electronic storage medium for executable programs and calibration values shown as read only memory chip  106  in this particular example, random access memory  108 , keep alive memory  110 , and a data bus. The controller  12  may receive various signals from sensors coupled to the engine  10 , in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF) from the mass air flow sensor  120 ; engine coolant temperature (ECT) from a temperature sensor  112  coupled to a cooling sleeve  114 ; a profile ignition pickup signal (PIP) from a Hall effect sensor  118  (or other type) coupled to crankshaft  40 ; throttle position (TP) from a throttle position sensor; and absolute manifold pressure signal, MAP, from the sensor  122 . Engine speed signal, RPM, may be generated by the controller  12  from signal PIP. Manifold pressure signal MAP from a manifold pressure sensor may be used to provide an indication of vacuum, or pressure, in the intake manifold. Note that various combinations of the above sensors may be used, such as a MAF sensor without a MAP sensor, or vice versa. During stoichiometric operation, the MAP sensor can give an indication of engine torque. Further, this sensor, along with the detected engine speed, can provide an estimate of charge (including air) inducted into the cylinder. In one example, the sensor  118 , which is also used as an engine speed sensor, may produce a predetermined number of equally spaced pulses every revolution of the crankshaft. 
     The storage medium read-only memory  106  can be programmed with computer readable data representing non-transitory instructions executable by the processor  102  for performing the methods described below as well as other variants that are anticipated but not specifically listed. 
     As described above,  FIG. 1  shows only one cylinder of a multi-cylinder engine, and each cylinder may similarly include its own set of intake/exhaust valves, fuel injector, spark plug, etc. 
       FIG. 2  shows a schematic view of an example embodiment of an exhaust gas sensor, such as a UEGO sensor  200  configured to measure a concentration of oxygen (O 2 ) in an exhaust gas stream. The sensor  200  may operate as the exhaust gas sensor  126  described above with reference to  FIG. 1 , for example. The sensor  200  comprises a plurality of layers of one or more ceramic materials arranged in a stacked configuration. In the embodiment of  FIG. 2 , five ceramic layers are depicted as layers  201 ,  202 ,  203 ,  204 , and  205 . These layers include one or more layers of a solid electrolyte capable of conducting ionic oxygen. Examples of suitable solid electrolytes include, but are not limited to, zirconium oxide-based materials. Further, in some embodiments such as that shown in  FIG. 2 , a heater  207  may be disposed in thermal communication with the layers to increase the ionic conductivity of the layers. While the depicted UEGO sensor  200  is formed from five ceramic layers, it will be appreciated that the UEGO sensor may include other suitable numbers of ceramic layers. 
     The layer  202  includes a material or materials creating a diffusion path  210 . The diffusion path  210  is configured to introduce exhaust gases into a first internal cavity  222  via diffusion. The diffusion path  210  may be configured to allow one or more components of exhaust gases, including but not limited to a desired analyte (e.g., O 2 ), to diffuse into the internal cavity  222  at a more limiting rate than the analyte can be pumped in or out by pumping electrodes pair  212  and  214 . In this manner, a stoichiometric level of O 2  may be obtained in the first internal cavity  222 . 
     The sensor  200  further includes a second internal cavity  224  within the layer  204  separated from the first internal cavity  222  by the layer  203 . The second internal cavity  224  is configured to maintain a constant oxygen partial pressure equivalent to a stoichiometric condition, e.g., an oxygen level present in the second internal cavity  224  is equal to that which the exhaust gas would have if the air-fuel ratio was stoichiometric. The oxygen concentration in the second internal cavity  224  is held constant by pumping current I cp . Herein, the second internal cavity  224  may be referred to as a reference cell. 
     A pair of sensing electrodes  216  and  218  is disposed in communication with first internal cavity  222  and the reference cell  224 . The sensing electrodes pair  216  and  218  detects a concentration gradient that may develop between the first internal cavity  222  and the reference cell  224  due to an oxygen concentration in the exhaust gas that is higher than or lower than the stoichiometric level. A high oxygen concentration may be caused by a lean exhaust gas mixture, while a low oxygen concentration may be caused by a rich mixture, for example. 
     The pair of pumping electrodes  212  and  214  is disposed in communication with the internal cavity  222 , and is configured to electrochemically pump a selected gas constituent (e.g., O 2 ) from the internal cavity  222  through the layer  201  and out of the sensor  200 . Alternatively, the pair of pumping electrodes  212  and  214  may be configured to electrochemically pump a selected gas through the layer  201  and into the internal cavity  222 . Herein, the pumping electrodes pair  212  and  214  may be referred to as an O 2  pumping cell. 
     The electrodes  212 ,  214 ,  216 , and  218  may be made of various suitable materials. In some embodiments, the electrodes  212 ,  214 ,  216 , and  218  may be at least partially made of a material that catalyzes the dissociation of molecular oxygen. Examples of such materials include, but are not limited to, electrodes containing platinum and/or gold. 
     The process of electrochemically pumping the oxygen out of or into the internal cavity  222  includes applying an electric current I p  across the pumping electrodes pair  212  and  214 . The pumping current I p  applied to the O 2  pumping cell pumps oxygen into or out of the first internal cavity  222  in order to maintain a stoichiometric level of oxygen in the cavity pumping cell. The pumping current I p  is proportional to the concentration of oxygen in the exhaust gas. Thus, a lean mixture will cause oxygen to be pumped out of the internal cavity  222  and a rich mixture will cause oxygen to be pumped into the internal cavity  222 . 
     A control system (not shown in  FIG. 2 ) generates the pumping voltage signal V p  as a function of the intensity of the pumping current I p  required to maintain a stoichiometric level within the first internal cavity  222 . 
     It should be appreciated that the UEGO sensor described herein is merely an example embodiment of a UEGO sensor, and that other embodiments of UEGO sensors may have additional and/or alternative features and/or designs. 
       FIGS. 3, 4, and 6  show flow charts illustrating routines for an exhaust gas sensor and an engine system, respectively. For example, the routine shown in  FIG. 3  determines whether the sensor should be operated to measure exhaust gas oxygen concentration or ambient humidity based on fueling conditions of the engine. The routine shown in  FIG. 4  determines the ambient humidity based on an exhaust gas sensor, such as the exhaust gas sensor  200  described above with reference to  FIG. 2 .  FIG. 6  shows a routine for adjusting an engine operating parameter based on the ambient humidity determined via the routine shown in  FIG. 3 . 
       FIG. 3  shows a flow chart illustrating a routine  300  for controlling an exhaust gas sensor, such as the exhaust gas sensor described above with reference to  FIG. 2  and positioned as shown in  FIG. 1 , based on engine fueling conditions. Specifically, the routine determines if the engine system is operating under non-fueling conditions and adjusts a measurement mode of the sensor accordingly. For example, during non-fueling conditions, the sensor is operated in a mode to determine ambient humidity and during fueling conditions, the sensor is operated in a mode to measure exhaust gas oxygen concentration to determine air fuel ratio. 
     At  302  of routine  300  in  FIG. 3 , engine operating conditions are determined. As non-limiting examples, the engine operating conditions may include actual/desired amount of EGR, spark timing, air-fuel ratio, etc. 
     Once the operating conditions are determined, it is determined if the engine is under non-fueling conditions at  304  of routine  300 . Non-fueling conditions include engine operating conditions in which the fuel supply is interrupted but the engine continues spinning and at least one intake valve and one exhaust valve are operating; thus, air is flowing through one or more of the cylinders, but fuel is not injected in the cylinders. Under non-fueling conditions, combustion is not carried out and ambient air may move through the cylinder from the intake passage to the exhaust passage. In this way, a sensor, such as an exhaust gas oxygen sensor, may receive ambient air on which measurements, such as ambient humidity detection, may be performed. 
     Non-fueling conditions may include, for example, deceleration fuel shut off (DFSO). DFSO is responsive to the operator pedal (e.g., in response to a driver tip-out and where the vehicle accelerates greater than a threshold amount). DSFO conditions may occur repeatedly during a drive cycle, and, thus, numerous indications of the ambient humidity may be generated throughout the drive cycle, such as during each DFSO event. As such, the overall efficiency of the engine may be maintained during driving cycles in which the ambient humidity fluctuates. The ambient humidity may fluctuate due to a change in altitude or temperature or when the vehicle enters/exits fog or rain, for example. 
     If it is determined that the engine is not operating under non-fueling conditions, for example, fuel is injected in one or more cylinders of the engine, routine  300  moves to  308 . At  308 , the exhaust gas sensor is operated as an air-fuel ratio sensor. In this mode of operation, the sensor may be operated as a lambda sensor, for example. As a lambda sensor, the output voltage may determine whether the exhaust gas air-fuel ratio is lean or rich. Alternatively, the sensor may operate as a universal exhaust gas oxygen sensor (UEGO) and an air-fuel ratio (e.g., a degree of deviation from a stoichiometric ratio) may be obtained from the pumping current of the pumping cell of the sensor. 
     At  310  of routine  300 , the air-fuel ratio (AFR) is controlled responsive to the exhaust gas oxygen sensor. Thus, a desired exhaust gas AFR may be maintained based on feedback from the sensor during engine fueling conditions. For example, if a desired air-fuel ratio is the stoichiometric ratio and the sensor determines the exhaust gas is lean (i.e., the exhaust gas comprises excess oxygen and the AFR is less than stoichiometric), additional fuel may be injected during subsequent engine fueling operation. 
     On the other hand, if it is determined that the engine is under non-fueling conditions, the routine proceeds to  306 , and the sensor is operated to determine ambient humidity. The ambient humidity may be determined based on the sensor output, as described in greater detail below with reference to  FIG. 4 . For example, a reference voltage of the sensor may be modulated between a minimum voltage at which oxygen is detected and a voltage at which water molecules may be dissociated such that the ambient humidity may be determined. It should be understood, the ambient humidity as determined (described below with reference to  FIG. 4 ) is the absolute ambient humidity. Additionally, relative humidity may be obtained by further employing a temperature detecting device, such as a temperature sensor. 
       FIG. 4  shows a flow chart illustrating a routine  400  for determining ambient humidity via an exhaust gas sensor, such as the oxygen sensor described above with reference to  FIG. 2 , and positioned as shown in  FIG. 1 , for example. Specifically, the routine determines a duration since fuel shut off and determines an ambient humidity via the exhaust gas sensor in a manner based on the duration since fuel shut off. For example, when the duration since fuel shut off is less than a threshold duration, a reference voltage of the sensor is modulated between a first voltage and a second voltage in order to determine the ambient humidity. When the duration since fuel shut off is greater than the threshold duration, the reference voltage is not modulated. 
     At  402 , the duration since fuel shut off is determined. In some examples, the duration since fuel shut off may be a time since fuel shut off. In other examples, the duration since fuel shut off may be a number of engine cycles since fuel shut off, for example. At  404 , it is determined if the duration since fuel shut off is greater than a threshold duration. The threshold duration may be an amount of time until the exhaust is substantially free of hydrocarbons from combustion in the engine. For example, residual gases from one or more previous combustion cycles may remain in the exhaust for several cycles after fuel is shut off and the gas that is exhausted from the chamber may contain more than ambient air for a duration after fuel injection is stopped. Further, the period in which fuel is shut off may vary. For example, a vehicle operator may release the accelerator pedal and coast to a stop, resulting in a long DFSO period. In some situations, the fuel shut off period (the time from interruption of the fuel supply to restart of the fuel supply, for example) may not be long enough for the ambient air to establish an equilibrium state in the exhaust system. For example, a vehicle operator may tip-in shortly after releasing the accelerator pedal, causing DFSO to stop soon after beginning. In such a situation, routine  400  proceeds to  406 . 
     If it is determined that the duration is less than the threshold duration, the routine continues to  406  and the sensor is operated in a first mode in which the reference voltage is modulated between a first voltage and a second voltage. As one non-limiting example, the first voltage may be 450 mV and the second voltage may be 950 mV. At 450 mV, for example, the pumping current may be indicative of an amount of oxygen in the exhaust gas. At 950 mV, water molecules may be dissociated such that the pumping current is indicative of the amount of oxygen in the exhaust gas plus an amount of oxygen from dissociated water molecules. The first voltage may be a voltage at which a concentration of oxygen in the exhaust gas may be determined, for example, while the second voltage may be a voltage at which water molecules may be dissociated. In this way, a humidity of the exhaust gas may be determined based on the water concentration. 
     In another example, the first voltage is 450 mV and the second voltage is 1080 mV. At 1080 mV, carbon dioxide (CO 2 ) molecules may be dissociated in addition to water molecules. In such an example, an amount of alcohol (e.g, ethanol) in the fuel may be determined based on the average change in pumping current while the voltage is modulated. 
     Continuing with  FIG. 4 , at  408 , a change in pumping current during the modulation is determined. For example, the difference in pumping current at the first reference voltage and the pumping current at the second reference voltage is determined.  FIG. 5  shows a graph illustrating an example of a modulated reference voltage  502  and corresponding change in pumping current  504  during a non-fueling condition such as DFSO. In the example depicted in  FIG. 5 , DFSO begins at a time t 1  and ends at a time t 2 . As shown, the reference voltage  502  is modulated between a first voltage V 1  and a second voltage V 2 , which is higher than the first voltage V 1 . Responsive to the changing reference voltage  502 , the pumping current  504  also changes. Thus, a change in pumping current (e.g., a delta pumping current) may be determined. The delta pumping current may be averaged over the duration of the DFSO condition such that an ambient humidity may be determined. 
     Continuing with  FIG. 4 , at  410  of routine  400 , the average change in pumping current is determined. Once the average change in pumping current is determined, a first indication of ambient humidity is determined based on the average change in pumping current at  412 . By modulating the reference voltage and determining an average change in pumping current, the effect of a changing air fuel ratio at the beginning of a fuel shut off duration when residual combustion gases may be present in the exhaust may be nullified, for example. As such, an indication of ambient humidity may be generated relatively quickly after fuel injection is suspended, even if the exhaust gas is not free of residual combustion gases. 
     Referring back to  404 , if it is determined that the duration since fuel shut off is greater than the threshold duration, the routine moves to  414  and the sensor is operated in a second mode in which the reference voltage is increased to a threshold voltage, but not modulated. The threshold voltage may be a voltage at which a desired molecule is dissociated. As an example, the reference voltage may be increased to 950 mV or another voltage at which water molecules may be dissociated. At  416 , the change in pumping current due to the increased reference voltage is determined. At  418 , a second indication of ambient humidity is determined based on the change in pumping current determined at  416 . After the threshold duration, the exhaust gas may be free from residual combustion gases. As such, an indication of ambient humidity may be generated without modulating the reference voltage at a rapid rate. 
     As described in detail above, an exhaust gas sensor may be operated in at least two modes in which the pumping voltage or pumping current of the pumping cell is monitored. As such, the sensor may be employed to determine the absolute ambient humidity of the air surrounding the vehicle as well as the air-fuel ratio of the exhaust gas. Subsequent to detection of the ambient humidity, a plurality of engine operating parameters may be adjusted for optimal engine performance, which will be explained in detail below. These parameters include, but are not limited to, an amount of exhaust gas recirculation (EGR), spark timing, air-fuel ratio, fuel injection, and valve timing. In one embodiment, one or more of these operating parameters (e.g., EGR, spark timing, air-fuel ratio, fuel injection, valve timing, etc.) are not adjusted during the modulating of the reference voltage 
       FIG. 6  shows a flow chart illustrating a routine  600  for adjusting engine operating parameters based on an ambient humidity generated by an exhaust gas sensor such as the ambient humidity generated as described with reference to  FIG. 4 , for example. Specifically, the routine determines the humidity and adjusts one or more operating parameters based on the humidity. For example, an increase in water concentration of the air surrounding the vehicle may dilute a charge mixture delivered to a combustion chamber of the engine. If one or more operating parameters are not adjusted in response to the increase in humidity, engine performance and fuel economy may decrease and emissions may increase; thus, the overall efficiency of the engine may be reduced. 
     At  602 , engine operating conditions are determined. The engine operating conditions may include EGR, spark timing, and air fuel ratio, among others, which may be affected by fluctuations of the water concentration in ambient air. 
     Once the operating conditions are determined, the routine proceeds to  604  where the ambient humidity is determined. The ambient humidity may be determined based on an exhaust gas sensor, such as the exhaust gas sensor described above with reference to  FIG. 2 . For example, the ambient humidity may be determined based on  412  or  418  of routine  400  described with reference to  FIG. 4 . 
     Once the ambient humidity is determined, the routine continues to  606  where one or more operating parameters are adjusted based on the ambient humidity. Such operating parameters may include an amount of EGR, spark timing, and air-fuel ratio, among others. As described above, in internal combustion engines, it is desirable to schedule engine operating parameters, such as spark timing, in order to optimize engine performance. In some embodiments, only one parameter may be adjusted responsive to the humidity. In other embodiments, any combination or subcombination of these operating parameters may be adjusted in response to measured fluctuations in ambient humidity. 
     In one example embodiment, an amount of EGR may be adjusted based on the measured ambient humidity. For example, in one condition, the water concentration in the air surrounding the vehicle may have increased due to a weather condition such as fog; thus, a higher humidity is detected by the exhaust gas sensor during engine non-fueling conditions. In response to the increased humidity measurement, during subsequent engine fueling operation, the EGR flow into at least one combustion chamber may be reduced. As a result, engine efficiency may be maintained. 
     Responsive to a fluctuation in absolute ambient humidity, EGR flow may be increased or decreased in at least one combustion chamber. As such, the EGR flow may be increased or decreased in only one combustion chamber, in some combustion chambers, or in all combustion chambers. Furthermore, the magnitude of change of the EGR flow may be the same for all cylinders or the magnitude of change of the EGR flow may vary by cylinder based on the specific operating conditions of each cylinder. 
     In another embodiment, spark timing may be adjusted responsive to the ambient humidity. In at least one condition, for example, spark timing may be advanced in one or more cylinders during subsequent engine fueling operation responsive to a higher humidity reading. Spark timing may be scheduled so as to reduce knock in low humidity conditions (e.g., retarded from a peak torque timing), for example. When an increase in humidity is detected by the exhaust gas sensor, spark timing may be advanced in order to maintain engine performance and operate closer to or at a peak torque spark timing. 
     Additionally, spark timing may be retarded in response to a decrease in ambient humidity. For example, a decrease in ambient humidity from a higher humidity may cause knock. If the decrease in humidity is detected by the exhaust gas sensor during non-fueling conditions, such as DFSO, spark timing may be retarded during subsequent engine fueling operation and knock may be reduced. 
     It should be noted that spark may be advanced or retarded in one or more cylinders during subsequent engine fueling operation. Further, the magnitude of change of spark timing may be the same for all cylinders or one or more cylinders may have varying magnitudes of spark advance or retard. 
     In still another example embodiment, exhaust gas air fuel ratio may be adjusted responsive to the measured ambient humidity during subsequent engine fueling operation. For example, an engine may be operating with a lean air fuel ratio optimized for low humidity. In the event of an increase in humidity, the mixture may become diluted, resulting in engine misfire. If the increase in humidity is detected by the exhaust gas sensor during non-fueling conditions, however, the air fuel ration may be adjusted so that the engine will operate with a less lean, lean air fuel ratio during subsequent fueling operation. Likewise, an air fuel ratio may be adjusted to be a more lean, lean air fuel ratio during subsequent engine fueling operation in response to a measured decrease in ambient humidity. In this way, conditions such as engine misfire due to humidity fluctuations may be reduced. 
     In some examples, an engine may be operating with a stoichiometric air fuel ratio or a rich air fuel ratio. As such, the air fuel ratio may be independent of ambient humidity and measured fluctuations in humidity may not result in an adjustment of air fuel ratio. 
     In this way, engine operating parameters may be adjusted responsive to an ambient humidity generated by an exhaust gas sensor coupled to an engine exhaust system. As DFSO may occur numerous times during a drive cycle, an ambient humidity measurement may be generated several times throughout the drive cycle and one or more engine operating parameters may be adjusted accordingly, resulting in an optimized overall engine performance despite fluctuations in ambient humidity. Furthermore, the engine operating parameters may be adjusted responsive to the ambient humidity regardless of a duration the engine non-fueling conditions, as an indication of ambient humidity may be generated in a short amount of time even if the exhaust gas is not devoid of residual combustion gases by modulating the reference voltage. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various acts, operations, or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts or functions may be repeatedly performed depending on the particular strategy being used. Further, the described acts may graphically represent code to be programmed into the computer readable storage medium in the engine control system. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and nonobvious combinations and subcombinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. 
     Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.