Patent Publication Number: US-9897030-B2

Title: Methods and systems for an oxygen sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 14/052,637, entitled “METHODS AND SYSTEMS FOR AN OXYGEN SENSOR,” filed on Oct. 11, 2013, the entire contents of which are hereby incorporated by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     The present application relates generally to a gas constituent sensor included in an intake system of an internal combustion engine. 
     BACKGROUND AND SUMMARY 
     Engine systems may utilize recirculation of exhaust gas from an engine exhaust system to an engine intake system (intake passage), a process referred to as exhaust gas recirculation (EGR), to reduce regulated emissions. An EGR system may include various sensors to measure and/or control the EGR. As one example, the EGR system may include an intake gas constituent sensor, such as an oxygen sensor, which may be employed to measure oxygen to determine the proportion of combusted gases in an intake passage of the engine. The sensor may also be used during non-EGR conditions to determine the oxygen content of fresh intake air. The EGR system may additionally or optionally include an exhaust gas oxygen sensor coupled to the exhaust manifold for estimating a combustion air-fuel ratio. 
     As such, when the intake oxygen sensor is used for EGR control, the EGR is measured as a function of the change in oxygen due to EGR as a diluent. To determine the change in the amount of oxygen, a reference point corresponding to an oxygen reading when no EGR is present is required. Such a reference point is called the “zero point” of the oxygen sensor. Due to the sensitivity of the oxygen sensor to pressure, as well as aging, there may be large deviations in the “zero point” at different engine operating conditions. Therefore the oxygen sensor may need to be regularly calibrated and a correction factor may need to be learned. 
     One example method for calibrating an intake gas oxygen sensor is depicted by Matsubara et al. in U.S. Pat. No. 6,742,379. Therein, a calibration coefficient is calculated to calibrate the output of an intake oxygen sensor and an intake passage pressure during selected engine conditions where the intake pressure is stable, that is, within a threshold. If the calibration coefficient is outside a threshold, it may be determined that the sensor is degraded. 
     However the inventors herein have recognized that such an approach may not learn the correction calibration coefficient if an EGR valve is leaking. Specifically, due to the location of the intake oxygen sensor downstream of the EGR valve and downstream of an outlet of a low-pressure EGR passage, in the event of EGR valve leakage, exhaust gas may leak out of the EGR passage and onto the sensor. The output of the oxygen sensor may consequently be corrupted and the EGR dilution estimated may be lower than the actual value. As a result, EGR control may be degraded. 
     In one example, a method for an engine comprises: learning a reference point for an intake oxygen sensor at a reference intake pressure during engine non-fueling conditions; and adjusting EGR flow to the engine based on an intake oxygen concentration estimated by the sensor relative to the learned reference point, and further based on a change in intake pressure from the reference intake pressure. In this way, an intake oxygen sensor can be calibrated without being effected by EGR valve leakage. 
     As an example, during selected engine non-fueling conditions, such as during a deceleration fuel shut-off condition, an adaptation of the intake oxygen sensor may be performed. During the adaptation, an output of the intake oxygen sensor may be monitored for a duration of the engine non-fueling condition. A relationship between the output of the sensor at a reference intake pressure may be learned and corrected for factors such as humidity. When the adaptation is complete, the output of the intake oxygen sensor may be used to estimate an EGR concentration, and thereby adjust an EGR flow. Specifically, the output may be adjusted with a pressure correction factor based on the current intake pressure and the reference intake pressure, and the corrected oxygen sensor output may be used to more accurately estimate the change in intake oxygen concentration with EGR dilution. By correcting for pressure changes, the pressure effect on oxygen sensor readings is compensated for. As such, even if an EGR valve is leaking, during non-fueling conditions, the charge leaked over the intake oxygen sensor is air. Therefore, by performing the adaptation during non-fueling conditions, even if an EGR valve is leaking, the output of the oxygen sensor can be relied on. In addition, based on a comparison of the zero point learned during DFSO conditions relative to a zero point learned during an idle adaptation of the sensor, EGR valve leakage can be diagnosed. For example, if the idle adaptation value differs from the DFSO adaptation value by more than a threshold amount, it may be determined that the EGR valve is leaking. Accordingly, EGR control may be modified. For example, instead of relying on the oxygen sensor output for feedback control of EGR flow, in the event of an EGR valve leakage, only feed-forward adjustments to EGR flow may be performed. 
     In this way, a relationship between an intake oxygen sensor and an intake pressure sensor can be learned, independent of the accuracy of either sensor, and used to adjust EGR flow. By performing the learning during non-fueling conditions, corruption of sensor output due exhaust residuals received from a leaking EGR valve can reduced. By comparing the DFSO adaptation to an idle adaptation, EGR valve diagnostics can also be performed. Overall, the accuracy of EGR estimation is increased, allowing for improved EGR control. 
     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 
         FIGS. 1-2  are schematic diagrams of an engine system. 
         FIG. 3  is a map depicting the relationship between intake pressure and the pumping current of an intake oxygen sensor. 
         FIG. 4  depicts a flowchart for performing a zero point estimation for an intake oxygen sensor during engine idling conditions. 
         FIG. 5  depicts a flowchart for performing a zero point estimation for an intake oxygen sensor during engine non-fueling conditions. 
         FIG. 6  depicts a flowchart for identifying degradation of an EGR valve based on the zero point estimated using idle adaptation and the zero point estimated using DFSO adaptation. 
         FIG. 7  shows an example idle adaptation. 
         FIG. 8  depicts a flowchart for correcting a learned zero point based on ambient humidity. 
         FIG. 9  depicts a flowchart for EGR control using the learned intake oxygen zero point. 
     
    
    
     DETAILED DESCRIPTION 
     The present description is related to methods and system for learning a reference point, or zero point, for an intake oxygen sensor, such as the sensor coupled to the engine systems of  FIGS. 1-2 . The reference point may be determined based on a learned relationship between the output of the intake oxygen sensor and an output of an intake pressure sensor at selected conditions ( FIG. 3 ). A controller may be configured to perform a control routine, such as the routine of  FIGS. 4-5  to learn the zero point for the intake oxygen sensor during an idle adaptation or during a DFSO adaptation. The learned reference point may be corrected based on ambient humidity ( FIG. 8 ). The controller may also be configured to perform a routine ( FIG. 6 ) to identify EGR valve leakage based on discrepancies between the zero point estimated at idle conditions and the zero point estimated at DFSO conditions. In response to EGR valve leakage, EGR control may be adjusted ( FIG. 9 ) so as to vary the feedback component of EGR control from the oxygen sensor. An example idle adaptation is shown at  FIG. 7 . In this way, an intake oxygen sensor reading may be corrected for aging, part-to-part variations, and effects from fuel and reductants. 
       FIG. 1  shows a schematic depiction of an example turbocharged engine system  100  including a multi-cylinder internal combustion engine  10  and twin turbochargers  120  and  130 . As one non-limiting example, engine system  100  can be included as part of a propulsion system for a passenger vehicle. Engine system  100  can receive intake air via intake passage  140 . Intake passage  140  can include an air filter  156  and an EGR throttle valve  230 . Engine system  100  may be a split-engine system wherein intake passage  140  is branched downstream of EGR throttle valve  230  into first and second parallel intake passages, each including a turbocharger compressor. Specifically, at least a portion of intake air is directed to compressor  122  of turbocharger  120  via a first parallel intake passage  142  and at least another portion of the intake air is directed to compressor  132  of turbocharger  130  via a second parallel intake passage  144  of the intake passage  140 . 
     The first portion of the total intake air that is compressed by compressor  122  may be supplied to intake manifold  160  via first parallel branched intake passage  146 . In this way, intake passages  142  and  146  form a first parallel branch of the engine&#39;s air intake system. Similarly, a second portion of the total intake air can be compressed via compressor  132  where it may be supplied to intake manifold  160  via second parallel branched intake passage  148 . Thus, intake passages  144  and  148  form a second parallel branch of the engine&#39;s air intake system. As shown in  FIG. 1 , intake air from intake passages  146  and  148  can be recombined via a common intake passage  149  before reaching intake manifold  160 , where the intake air may be provided to the engine. 
     A first EGR throttle valve  230  may be positioned in the engine intake upstream of the first and second parallel intake passages  142  and  144 , while a second air intake throttle valve  158  may be positioned in the engine intake downstream of the first and second parallel intake passages  142  and  144 , and downstream of the first and second parallel branched intake passages  146  and  148 , for example, in common intake passage  149 . 
     In some examples, intake manifold  160  may include an intake manifold pressure sensor  182  for estimating a manifold pressure (MAP) and/or an intake manifold temperature sensor  183  for estimating a manifold air temperature (MCT), each communicating with controller  12 . 
     Intake passage  149  can include a charge air cooler (CAC)  154  and/or a throttle (such as second throttle valve  158 ). The position of throttle valve  158  can be adjusted by the control system via a throttle actuator (not shown) communicatively coupled to controller  12 . An anti-surge valve  152  may be provided to selectively bypass the compressor stages of turbochargers  120  and  130  via bypass passage  150 . As one example, anti-surge valve  152  can open to enable flow through bypass passage  150  when the intake air pressure downstream of the compressors attains a threshold value. 
     Intake manifold  160  may further include an intake gas oxygen sensor  172 . In one example, the oxygen sensor is a UEGO sensor. As elaborated herein, the intake gas oxygen sensor may be configured to provide an estimate regarding the oxygen content of fresh air received in the intake manifold. In addition, when EGR is flowing, a change in oxygen concentration at the sensor may be used to infer an EGR amount and used for accurate EGR flow control. In the depicted example, oxygen sensor  162  is positioned upstream of throttle  158  and downstream of charge air cooler  154 . However, in alternate embodiments, the oxygen sensor may be positioned upstream of the CAC. 
     A pressure sensor  174  may be positioned alongside the oxygen sensor for estimating an intake pressure at which an output of the oxygen sensor is received. Since the output of the oxygen sensor is influenced by the intake pressure, a reference oxygen sensor output may be learned at a reference intake pressure. In one example, the reference intake pressure is a throttle inlet pressure (TIP) where pressure sensor  174  is a TIP sensor. In alternate examples, the reference intake pressure is a manifold pressure (MAP) as sensed by MAP sensor  182 . 
     A humidity sensor  173  may be positioned alongside the intake oxygen sensor and the intake pressure sensor. Specifically, as depicted, each of the humidity sensor  173 , intake oxygen sensor  172 , and intake pressure sensor  174  are positioned upstream of intake throttle  158  and downstream of charge air cooler  154  in the engine intake manifold. The humidity sensor may be configured to provide an estimate of the ambient humidity. As elaborated with reference to FIG.  8 , a controller may estimate an ambient humidity while learning a reference point for the intake oxygen sensor at a reference intake pressure and correct the learned reference point based on the estimated ambient humidity. This allows variations in oxygen sensor output due to variations in ambient humidity to be learned and used for accurately estimating EGR. 
     Engine  10  may include a plurality of cylinders  14 . In the depicted example, engine  10  includes six cylinders arrange in a V-configuration. Specifically, the six cylinders are arranged on two banks  13  and  15 , with each bank including three cylinders. In alternate examples, engine  10  can include two or more cylinders such as 3, 4, 5, 8, 10 or more cylinders. These various cylinders can be equally divided and arranged in alternate configurations, such as V, in-line, boxed, etc. Each cylinder  14  may be configured with a fuel injector  166 . In the depicted example, fuel injector  166  is a direct in-cylinder injector. However, in other examples, fuel injector  166  can be configured as a port based fuel injector. 
     Intake air supplied to each cylinder  14  (herein, also referred to as combustion chamber  14 ) via common intake passage  149  may be used for fuel combustion and products of combustion may then be exhausted from via bank-specific parallel exhaust passages. In the depicted example, a first bank  13  of cylinders of engine  10  can exhaust products of combustion via a first parallel exhaust passage  17  and a second bank  15  of cylinders can exhaust products of combustion via a second parallel exhaust passage  19 . Each of the first and second parallel exhaust passages  17  and  19  may further include a turbocharger turbine. Specifically, products of combustion that are exhausted via exhaust passage  17  can be directed through exhaust turbine  124  of turbocharger  120 , which in turn can provide mechanical work to compressor  122  via shaft  126  in order to provide compression to the intake air. Alternatively, some or all of the exhaust gases flowing through exhaust passage  17  can bypass turbine  124  via turbine bypass passage  123  as controlled by wastegate  128 . Similarly, products of combustion that are exhausted via exhaust passage  19  can be directed through exhaust turbine  134  of turbocharger  130 , which in turn can provide mechanical work to compressor  132  via shaft  136  in order to provide compression to intake air flowing through the second branch of the engine&#39;s intake system. Alternatively, some or all of the exhaust gas flowing through exhaust passage  19  can bypass turbine  134  via turbine bypass passage  133  as controlled by wastegate  138 . 
     In some examples, exhaust turbines  124  and  134  may be configured as variable geometry turbines, wherein controller  12  may adjust the position of the turbine impeller blades (or vanes) to vary the level of energy that is obtained from the exhaust gas flow and imparted to their respective compressor. Alternatively, exhaust turbines  124  and  134  may be configured as variable nozzle turbines, wherein controller  12  may adjust the position of the turbine nozzle to vary the level of energy that is obtained from the exhaust gas flow and imparted to their respective compressor. For example, the control system can be configured to independently vary the vane or nozzle position of the exhaust gas turbines  124  and  134  via respective actuators. 
     Exhaust gases in first parallel exhaust passage  17  may be directed to the atmosphere via branched parallel exhaust passage  170  while exhaust gases in second parallel exhaust passage  19  may be directed to the atmosphere via branched parallel exhaust passage  180 . Exhaust passages  170  and  180  may include one or more exhaust after-treatment devices, such as a catalyst, and one or more exhaust gas sensors. 
     Engine  10  may further include one or more exhaust gas recirculation (EGR) passages, or loops, for recirculating at least a portion of exhaust gas from the exhaust manifold to the intake manifold. These may include high-pressure EGR loops for proving high-pressure EGR (HP-EGR) and low-pressure EGR-loops for providing low-pressure EGR (LP-EGR). In one example, HP-EGR may be provided in the absence of boost provided by turbochargers  120 ,  130 , while LP-EGR may be provided in the presence of turbocharger boost and/or when exhaust gas temperature is above a threshold. In still other examples, both HP-EGR and LP-EGR may be provided simultaneously. 
     In the depicted example, engine  10  may include a low-pressure EGR loop  202  for recirculating at least some exhaust gas from the first branched parallel exhaust passage  170 , downstream of the turbine  124 , to the first parallel intake passage  142 , upstream of the compressor  122 . In some embodiments, a second low-pressure EGR loop (not shown) may be likewise provided for recirculating at least some exhaust gas from the second branched parallel exhaust passage  180 , downstream of the turbine  134 , to the second parallel intake passage  144 , upstream of the compressor  132 . LP-EGR loop  202  may include LP-EGR valve  204  for controlling an EGR flow (i.e., an amount of exhaust gas recirculated) through the loops, as well as an EGR cooler  206  for lowering a temperature of exhaust gas flowing through the EGR loop before recirculation into the engine intake. The LP-EGR valve  204  can be positioned upstream or downstream of the LP EGR cooler  206 . Under certain conditions, the EGR cooler  206  may also be used to heat the exhaust gas flowing through LP-EGR loop  202  before the exhaust gas enters the compressor to avoid water droplets impinging on the compressors. 
     Engine  10  may further include a first high-pressure EGR loop  208  for recirculating at least some exhaust gas from the first parallel exhaust passage  17 , upstream of the turbine  124 , to the intake manifold  160  downstream of the engine throttle  158 . Likewise, the engine may include a second high-pressure EGR loop (not shown) for recirculating at least some exhaust gas from the second parallel exhaust passage  19 , upstream of the turbine  134 , to the intake manifold  160  downstream of the engine throttle  158 . EGR flow through HP-EGR loops  208  may be controlled via HP-EGR valve  210 . If two HP-EGR loops are present coupled to each branch of the air induction system, they may each utilize their own HP-EGR valves  210  or join together prior to and share the same HP-EGR valve before introduction into the intake manifold. It will be appreciated that as an alternate to the above described single and dual HP-EGR loop configurations, HP-EGR may be introduced into intake passages  146  and/or  148  instead of into intake manifold  160 . 
     A PCV port  102  may be configured to deliver crankcase ventilation gases (blow-by gases) to the engine intake manifold along second parallel intake passage  144 . In some embodiments, flow of PCV air through PCV port  102  may be controlled by a dedicated PCV port valve. Likewise, a purge port  104  may be configured to deliver purge gases from a fuel system canister to the engine intake manifold along passage  144 . In some embodiments, flow of purge air through purge port  104  may be controlled by a dedicated purge port valve. As elaborated with reference to  FIG. 2 , the PCV and purge ports in the pre-compressor air induction tube only flow into the induction tube during boosted conditions. In non-boosted conditions, purge and PCV air are supplied directly to the intake manifold. In other words, during boosted conditions, the purge and PCV gases are received upstream of intake oxygen sensor  172 , and therefore affect the output of the sensor during boosted conditions. In comparison, during non-boosted conditions, the purge and PCV gases are received downstream of intake oxygen sensor  172 , and therefore do not affect the output of the sensor during non-boosted conditions. 
     Humidity sensor  232  and pressure sensor  234  may be included in only one of the parallel intake passages (herein, depicted in the first parallel intake air passage  142  but not in the second parallel intake passage  144 ), downstream of EGR throttle valve  230 . Specifically, the humidity sensor and the pressure sensor may be included in the intake passage not receiving the PCV or purge air. Humidity sensor  232  may be configured to estimate a relative humidity of the intake air. In one embodiment, humidity sensor  232  is a UEGO sensor configured to estimate the relative humidity of the intake air based on the output of the sensor at one or more voltages. Since purge air and PCV air can confound the results of the humidity sensor, the purge port and PCV port are positioned in a distinct intake passage from the humidity sensor. Pressure sensor  234  may be configured to estimate a pressure of the intake air. In some embodiments, a temperature sensor may also be included in the same parallel intake passage, downstream of the EGR throttle valve  230 . 
     As such, intake oxygen sensor  172  may be used for estimating an intake oxygen concentration and inferring an amount of EGR flow through the engine based on a change in the intake oxygen concentration upon opening of the EGR valve  204 . Specifically, a change in the output of the sensor upon opening the EGR valve is compared to a reference point where the sensor is operating with no EGR (the zero point). Based on the change (e.g., decrease) in oxygen amount from the time of operating with no EGR, an EGR flow currently provided to the engine can be calculated. Then, based on a deviation of the estimated EGR flow from the expected (or target) EGR flow, further EGR control may be performed. As elaborated with reference to  FIG. 9 , a controller may feed-forward adjust the opening of the EGR valve based on engine speed-load conditions while feedback adjusting the EGR valve based on an EGR flow estimated by the oxygen sensor. However, EGR estimation and EGR control requires accurate estimation of the zero point. Since the output of the oxygen sensor is impacted by changes in intake pressure, changes in exhaust air-fuel ratio, part-to-part variations, and reductants (such as those from PCV and purge gases), accurate zero point estimation can be complicated. Without accurate zero point estimation, however, EGR flow control may be not be reliably performed. 
     To overcome these issues, a zero point estimation of the intake oxygen sensor is performed during idle conditions, herein also referred to as an idle adaptation, and discussed at  FIG. 4 . By performing the adaptation during idling conditions, where intake pressure fluctuations are minimal and when no PCV or purge air is ingested into the low pressure air induction system upstream of the compressor, sensor reading variations due to those noise factors is reduced. As such, purge and PCV air may flow into the engine during idle via the intake manifold. However, they will not affect the intake oxygen sensor output since they are ingested downstream of the sensor, directly into the intake manifold. By also performing the idle adaptation periodically, such as at every first idle following an engine start, the effect of sensor aging and part-to-part variability on the sensor output is also corrected for. Overall a more accurate zero point can be learned. 
     A zero point estimation of the intake oxygen sensor is also performed during engine non-fueling conditions, such as during a deceleration fuel shut off (DFSO), herein also referred to as a DFSO adaptation, and discussed at  FIG. 5 . By performing the adaptation during DFSO conditions, in addition to reduced noise factors such as those achieved during idle adaptation, sensor reading variations due to EGR valve leakage is also reduced. 
     Returning to  FIG. 1 , the position of intake and exhaust valves of each cylinder  14  may be regulated via hydraulically actuated lifters coupled to valve pushrods, or via direct acting mechanical buckets in which cam lobes are used. In this example, at least the intake valves of each cylinder  14  may be controlled by cam actuation using a cam actuation system. Specifically, the valve cam actuation system  25  may include one or more cams and may utilize variable cam timing or lift for intake and/or exhaust valves. In alternative embodiments, the intake valves may be controlled by electric valve actuation. Similarly, the exhaust valves may be controlled by cam actuation systems or electric valve actuation. 
     Engine system  100  may be controlled at least partially by a control system  15  including controller  12  and by input from a vehicle operator via an input device (not shown). Control system  15  is shown receiving information from a plurality of sensors  16  (various examples of which are described herein) and sending control signals to a plurality of actuators  81 . As one example, sensors  16  may include humidity sensor  232 , intake air pressure sensor  234 , MAP sensor  182 , MCT sensor  183 , TIP sensor  174 , and intake air oxygen sensor  172 . In some examples, common intake passage  149  may further include a throttle inlet temperature sensor for estimating a throttle air temperature (TCT). In other examples, one or more of the EGR passages may include pressure, temperature, and air-to-fuel ratio sensors, for determining EGR flow characteristics. As another example, actuators  81  may include fuel injector  166 , HP-EGR valve  210 , LP-EGR valve  204 , throttle valves  158  and  230 , and wastegates  128 ,  138 . Other actuators, such as a variety of additional valves and throttles, may be coupled to various locations in engine system  100 . Controller  12  may receive input data from the various sensors, process the input data, and trigger the actuators in response to the processed input data based on instruction or code programmed therein corresponding to one or more routines. Example control routines are described herein with regard to  FIGS. 4-6 and 8 . 
     Now turning to  FIG. 2 , another example embodiment  200  of the engine of  FIG. 1  is shown. As such, components previously introduced in  FIG. 1  are numbered similarly and not re-introduced here for reasons of brevity. 
     Embodiment  200  shows a fuel tank  218  configured to deliver fuel to engine fuel injectors. A fuel pump (not shown) immersed in fuel tank  218  may be configured to pressurize fuel delivered to the injectors of engine  10 , such as to injector  166 . Fuel may be pumped into the fuel tank from an external source through a refueling door (not shown). Fuel tank  218  may hold a plurality of fuel blends, including fuel with a range of alcohol concentrations, such as various gasoline-ethanol blends, including E10, E85, gasoline, etc., and combinations thereof. A fuel level sensor  219  located in fuel tank  218  may provide an indication of the fuel level to controller  12 . As depicted, fuel level sensor  219  may comprise a float connected to a variable resistor. Alternatively, other types of fuel level sensors may be used. One or more other sensors may be coupled to fuel tank  218  such as a fuel tank pressure transducer  220  for estimating a fuel tank pressure. 
     Vapors generated in fuel tank  218  may be routed to fuel vapor canister  22 , via conduit  31 , before being purged to engine intake  23 . These may include, for example, diurnal and refueling fuel tank vapors. The canister may be filled with an appropriate adsorbent, such as activated charcoal, for temporarily trapping fuel vapors (including vaporized hydrocarbons) generated in the fuel tank. Then, during a later engine operation, when purge conditions are met, such as when the canister is saturated, the fuel vapors may be purged from the canister into the engine intake by opening canister purge valve  112  and canister vent valve  114 . 
     Canister  22  includes a vent  27  for routing gases out of the canister  22  to the atmosphere when storing, or trapping, fuel vapors from fuel tank  218 . Vent  27  may also allow fresh air to be drawn into fuel vapor canister  22  when purging stored fuel vapors to engine intake  23  via purge lines  90  or  92  (depending on boost level) and purge valve  112 . While this example shows vent  27  communicating with fresh, unheated air, various modifications may also be used. Vent  27  may include a canister vent valve  114  to adjust a flow of air and vapors between canister  22  and the atmosphere. The vent valve may be opened during fuel vapor storing operations (for example, during fuel tank refueling and while the engine is not running) so that air, stripped of fuel vapor after having passed through the canister, can be pushed out to the atmosphere. Likewise, during purging operations (for example, during canister regeneration and while the engine is running), the vent valve may be opened to allow a flow of fresh air to strip the fuel vapors stored in the canister. 
     Fuel vapors released from canister  22 , for example during a purging operation, may be directed into engine intake manifold  160  via purge line  28 . The flow of vapors along purge line  28  may be regulated by canister purge valve  112 , coupled between the fuel vapor canister and the engine intake. The quantity and rate of vapors released by the canister purge valve may be determined by the duty cycle of an associated canister purge valve solenoid (not shown). As such, the duty cycle of the canister purge valve solenoid may be determined by the vehicle&#39;s powertrain control module (PCM), such as controller  12 , responsive to engine operating conditions, including, for example, engine speed-load conditions, an air-fuel ratio, a canister load, etc. 
     An optional canister check valve (not shown) may be included in purge line  28  to prevent intake manifold pressure from flowing gases in the opposite direction of the purge flow. As such, the check valve may be necessary if the canister purge valve control is not accurately timed or the canister purge valve itself can be forced open by a high intake manifold pressure. An estimate of the manifold absolute pressure (MAP) may be obtained from MAP sensor  182  coupled to intake manifold  160 , and communicated with controller  12 . Alternatively, MAP may be inferred from alternate engine operating conditions, such as mass air flow (MAF), as measured by a MAF sensor coupled to the intake manifold. 
     Purge hydrocarbons may be directed to intake manifold  160  via either a boost path  92  or a vacuum path  90  based on engine operating conditions. Specifically, during conditions when turbocharger  120  is operated to provide a boosted aircharge to the intake manifold, the elevated pressure in the intake manifold causes one-way valve  94  in the vacuum path  90  to close while opening one-way valve  96  in the boost path  92 . As a result, purge air is directed into the air intake passage  140 , downstream of air filter  156  and upstream of charge air cooler  154  via the boost path  92 . Herein, the purge air is introduced upstream of intake oxygen sensor  172 . In some embodiments, as depicted, a venturi  98  may be positioned in the boost path such that the purge air is directed to the intake upon passing through the venturi and passage  99 . This allows the flow of compressor bypass air to be advantageously harnessed for enhanced purge flow. 
     During conditions when engine  10  is operated without boost, elevated vacuum in the intake manifold causes one-way valve  94  in the vacuum path to open while closing one-way valve  96  in the boost path. As a result, purge air is directed into the intake manifold  160 , downstream of throttle  158  via the vacuum path  90 . Herein, the purge air is introduced downstream of intake oxygen sensor  172 , directly into intake manifold  160 , and therefore does not affect the output of oxygen sensor  172 . In comparison, during conditions when engine  10  is operated with boost, the purge air is introduced upstream of intake oxygen sensor  172 , and therefore does affect the output of oxygen sensor  172 . 
     PCV hydrocarbons may also be directed to intake manifold  160  via either a boost side PCV hose  252  or a vacuum side PCV hose  254  based on engine operating conditions. Specifically, blow-by gases from engine cylinders  14  flow past the piston rings and enter crankcase  255 . During conditions when turbocharger  120  is operated to provide a boosted aircharge to the intake manifold, the elevated pressure in the intake manifold causes one-way valve  256  in vacuum side PCV hose  254  to close. As a result, PCV air is directed into the air intake passage  140 , downstream of air filter  156  and upstream of charge air cooler  154  via boost side PCV hose  252 . The PCV flow may be directed to the intake passage upon passage through a boost side oil separator  260 . The boost side oil separator may be integrated into the cam cover or may be an external component. Thus, during boosted conditions, the PCV gases are introduced upstream of intake oxygen sensor  172  therefore do affect the output of oxygen sensor  172 . 
     In comparison, during conditions when engine  10  is operated without boost, elevated vacuum in the intake manifold causes one-way valve  256  in the vacuum side PCV hose  254  to open. As a result, PCV air is directed into the intake manifold  160 , directly, downstream of throttle  158  via the vacuum side PCV hose  254 . Herein, the PCV air is introduced downstream of intake oxygen sensor  172 , and therefore does not affect the output of oxygen sensor  172 . 
     Thus, due to the specific engine configuration, during engine idle conditions, when no boosted aircharge is provided, a reference point (herein also referred to as the zero point) of the intake air sensor may be learned without incurring interference from PCV and purge air hydrocarbons. 
     As such, the intake air oxygen sensor can be used to measure the amount of EGR in the intake aircharge as a function of the amount of change in oxygen content due to the addition of EGR as a diluent. Thus, as more EGR is introduced, a sensor output corresponding to a lower oxygen concentration may be output. However, to accurately determine this change in the amount of oxygen, it is important to know the oxygen reading of the sensor when no EGR is present. This reference point, also known as a zero point, needs to be calibrated and learned. However, the zero point reading has a large range of values that vary based on intake pressure, sensor age, and part-to-part variation, rendering accurate EGR measurement difficult. 
       FIG. 3  depicts this variation in the reading of the intake sensor. Specifically, map  300  depicts intake pressure along the x-axis and a pumping current output by the sensor, upon application of a reference voltage, along the y-axis. Plots  301   a - d  show a first set of intake oxygen sensor outputs at a first condition with no EGR. Plots  302   a - d ,  303   a - d , and  304   a - d  show the sensor outputs at gradually increasing EGR levels, with  304   a - d  representing a nominal EGR percentage. 
     As can be seen by comparing the output at any given intake pressure (compare  301   a  to  301   b, c , and  d , and likewise for each set), a large amount of piece to piece variation occurs in the base oxygen measurement output by the sensor. As such, this piece-to-piece variation accounts for the largest amount of variation in the output of a given sensor. In addition, aging of the sensor adds to the variation. The variation makes learning of the zero point difficult, confounding the results of an EGR estimation. 
     As elaborated with reference to  FIG. 4 , the variation can be reduced by performing an idle adaptation for the sensor at each engine start. Specifically, at the first engine idle since every engine start, a zero point of the sensor may be learned and updated. This allows part-to-part variation and sensor aging to be learned and compensated for. By then using the most recently learned zero point as a reference for EGR estimation, EGR amounts can be determined more accurately and reliably. 
     Now turning to  FIG. 4 , an example routine  400  for learning a zero point of an intake oxygen sensor during selected engine idling conditions is shown. The method allows for a reference point of the sensor to be accurately learned without being confounded by PCV or purge hydrocarbons. In addition by learning the relationship between the intake pressure and the oxygen sensor output, oxygen concentrations and EGR flow can be measured accurately even if there is any inaccuracy in either sensor. 
     At  402 , the routine includes estimating and/or measuring engine operating conditions. These may include, for example, engine speed, torque demand, barometric pressure, engine temperature, etc. Next it may be determined if selected engine idling conditions are present. As elaborated below at  404  and  406 , the selected engine idling conditions may include a first engine idle since installation of one of a new intake oxygen sensor or new intake pressure sensor, or a first engine idle since an engine start. 
     Specifically, at  404 , it may be determined if a new intake air oxygen (IAO2) sensor or a new intake pressure sensor was installed in the vehicle. For example, it may be determined if a new sensor was installed since the last engine shut-down and the current engine start. In one example, following installation of a new sensor, an indication that calibration of the new sensor is required may be received at the controller. 
     If a new oxygen sensor or pressure sensor was installed, then at  405 , the routine includes resetting the previously learned adaptive values of the intake air oxygen sensor. That is, the previously learned zero point and pressure correction factors saved in a look-up table of the controller&#39;s memory (e.g., in the KAM) may be reset. Then, the table may be repopulated with data from the current zero point learning, and subsequent iterations of the routine. 
     If a new oxygen or pressure sensor was not installed, or after resetting the table if a new sensor was installed, the routine proceeds to  406  to confirm a first engine idle condition since the current engine start. If a first engine idle condition is not confirmed, at  407 , the look-up table in the controller&#39;s memory may not be further updated and the current zero point readings may be used. As such, by re-learning the reference point each time a new sensor is installed, differences in oxygen sensor readings due to part-to-part variations can be better accounted for. By updating and re-learning the reference point on each engine start, differences in oxygen sensor readings due to sensor aging can be better accounted for. 
     Upon confirmation of a first engine idle condition since the current engine start, at  408 , the routine includes learning a reference point for the intake oxygen sensor at a reference intake pressure during the selected engine idling condition. Specifically, the controller may learn the oxygen sensor output at the first engine idle condition and may also note the reference intake pressure at which the oxygen sensor output was learned. The controller may update the look-up table saved in the controller&#39;s KAM with the oxygen sensor output learned at the given pressure. In one example, the reference intake pressure is a throttle inlet pressure estimated by a TIP sensor coupled to the intake manifold at a location similar to the oxygen sensor (e.g., downstream of the charge air cooler and upstream of the intake throttle). In another example, the reference intake pressure is a manifold pressure estimated by a MAP sensor coupled to the intake manifold at a location similar to the oxygen sensor. 
     As such, learning the reference point includes learning a relationship between a first output of the intake oxygen sensor at a first intake pressure during the first engine idle since start, and then using the learned relationship, the idle reference oxygen (iao2_ref) at the reference pressure (iao2_ref_press) is calculated. It is calculated by determining a correction factor (iao2_press_corr) as:
 
 iao 2_press_corr= a 0+ a 1*( iao 2_ref_press− iao 2_press)+ a 2*( iao 2_ref_press− iao 2_press) 2  
 
     The idle reference oxygen is then calculated as: iao2_ref=iao2_o2*iao2_press_corr 
     By performing this learning during idle conditions, various advantages are achieved. First, any error caused due to noise factors from purge or PCV hydrocarbons is reduced. Second, since pressure changes at the intake oxygen sensor location are minimal during idle conditions, changes in the oxygen sensor output due to the pressure effect (as described at  FIG. 3 ) are also minimized. Overall, a more accurate zero point learning can be achieved. 
     At  410 , the intake oxygen sensor output is corrected for humidity. As elaborated with reference to  FIG. 8 , the output of the intake oxygen sensor estimated at the reference pressure is corrected with a correction factor based on ambient humidity. This may include correcting for no humidity (that is, zero % humidity or dry conditions) wherein the output of the oxygen sensor is corrected by removing all the contribution of humidity. Alternatively, this may include correcting to a known standard or reference humidity level. For example, the oxygen sensor output may be corrected to a reference 1.2% of humidity. 
     At  412 , it may be determined if the idle adaptation is complete. As such, the intake oxygen sensor readings at the given reference intake pressure may be monitored for a duration of the first engine idle since the engine start and the look-up table may be continue to be populated over the duration with readings from the intake oxygen sensor. In one example, when the idle adaptation is initiated at  408 , a timer may be started and at  412 , it may be determined if a threshold duration has elapsed on the timer. In one example, the idle adaptation may be complete if 15 seconds has elapsed. 
     Upon confirming that the idle adaptation has been completed, at  414 , the routine includes calculating a pressure correction factor. The pressure correction factor is a factor that compensates for the effect of intake pressure on the output of the intake oxygen sensor. The pressure correction factor is determined by taking the ratio of the measured oxygen and the reference oxygen reading (iao2_ref), the reference oxygen reading being the reference oxygen reading of the intake oxygen sensor at the reference pressure. Nominally, the reference pressure may be 100 kPa. The pressure correction adaptation may be performed by calculating a pressure correction factor based on the output of the intake oxygen sensor (iao2_o2) relative to the zero point (iao2_ref) of the sensor (that is, iao2_o2/iao2_ref). In addition, a delta pressure may also be determined based on the reference pressure, wherein the delta pressure is calculated as TIP−iao2_ref_press. Herein, TIP may be the same as boost pressure. The delta pressure is calculated as the difference between the measured boost pressure TIP and the reference pressure. The delta pressure from the reference pressure provides information about the change in oxygen reading from iao2_ref versus the change in the pressure from the reference pressure. The reference pressure corresponds with the pressure at which iao2_ref was determined. 
     At  418 , the routine includes calculating and learning the zero point of the intake oxygen sensor. This may include, for example, performing a recursive least squares adaptation for pressure correction. The correction may be denoted as: 
     Iao2_press_corr_new=a2*dp_corr 2 +a1*dp_corr+a0, wherein a0, a1, and a2 are pressure correction coefficients, and dp_corr is the delta pressure correction (that is, delta pressure from reference pressure). 
     Once the zero point is learned, an EGR flow to the engine can be adjusted based on an output of the intake oxygen sensor during EGR conditions, as elaborated at  FIG. 9 . Therein, an EGR flow to the engine is adjusted based on an intake oxygen concentration estimated by the intake oxygen sensor relative to the learned reference point, and further based on a change in intake pressure relative to the reference intake pressure (where the reference point was learned). 
     At  420 , the routine includes diagnosing an EGR valve based on the zero point estimated during the idle adaptation relative to a zero point estimated during selected engine non-fueling conditions, such as during a deceleration fuel shut off (DFSO) adaptation. An example DFSO adaptation is described in  FIG. 5 . As such, the zero point learned during the idle adaptation may be a first learned reference point, while the zero point learned during the DFSO adaptation may be a second learned reference point (both learned at a given reference intake pressure). As elaborated at  FIG. 6 , the controller may indicate EGR valve degradation based on a difference between the first learned reference point and the second learned reference point being larger than a threshold amount. 
     As such, while the idle adaptation performed during idle conditions removes the effect of purge and PCV hydrocarbons on the intake oxygen sensor output, as well as the effect of pressure variations, the idle adaptation is susceptible to EGR leakage. Thus, if the EGR valve is leaking, assuming there is no EGR backflow, EGR may flow over the intake oxygen sensor even during the idle conditions. As a result, the output from the oxygen sensor output may be lower than the actual value. In comparison, an adaptation performed during DFSO is insensitive to the effect of a leaking EGR valve. This is because even if the valve were leaking, the leaking “EGR” would be air since no fuel is being injected during these conditions. As a result, the exhaust leak does not affect the output of the oxygen sensor. Thus, by comparing the zero point learned during the idle adaptation with the zero point learned during the DFSO adaptation, EGR valve leakage can be identified. 
     An example idle adaptation is shown with reference to  FIG. 7 . Map  700  depicts an idle adaptation timer at plot  702 , and a change in oxygen concentration sensed by the intake oxygen sensor at plot  704 . 
     Prior to t 1 , idle adaptation conditions may not be present. At t 1 , a first engine idle since an engine start may be confirmed and an idle adaptation timer may be started. Plot  704  (solid line) shows a zero point of the intake oxygen sensor relative to an expected value  708 . Plot  706  (dashed line) shows the intake sensor output. As such, prior to the idle adaptation, the deviation of the estimated zero point from the expected zero point is larger. Then, during the adaptation, based on the sensor reading (plot  706 ), the zero point is corrected and the learned zero point gradually merges with the expected value. At t 2 , the idle adaptation is completed and the learned zero point is used for accurate EGR control. 
     In one example, a method for an engine comprises: learning a relationship between a first intake oxygen sensor output estimated at a first intake pressure during a first engine idle since engine start, and adjusting EGR flow to the engine at a second intake pressure based on a second intake oxygen sensor output estimated at the second intake pressure and the learned relationship. The adjusting includes calculating a pressure correction factor based on a difference between the first intake pressure and the second intake pressure, calculating a humidity correction factor based on a difference between ambient humidity at the second intake pressure and a reference humidity, modifying the second intake oxygen sensor output based on each of the calculated pressure correction factor, humidity correction factor, and the learned relationship, and adjusting a position of an EGR valve based on the modified second intake oxygen sensor output. The EGR valve may be coupled in a low pressure EGR passage and wherein the learning is performed at a first engine idle following each engine restart. Herein, each of the first and second intake oxygen sensor output is generated by an intake oxygen sensor coupled upstream of an intake throttle and downstream of a charge air cooler, and each of the first and second intake pressure is estimated by an intake pressure sensor coupled upstream of the intake throttle and downstream of the charge air cooler. The learning is performed at a first engine idle following installation of one or more of the intake oxygen sensor and the intake pressure sensor in the engine, so as to correct for part-to-part variations as well as sensor aging. In addition, degradation of the EGR valve can be indicated based on the first intake oxygen sensor output estimated at the first intake pressure during the first engine idle since engine start relative to a second intake oxygen sensor output estimated at the first intake pressure during an engine deceleration fuel shut-off condition. 
     Now turning to  FIG. 5 , an example routine  500  for learning a zero point of an intake oxygen sensor during selected engine non-fueling conditions is shown. The method allows for a reference point of the sensor to be accurately learned without being confounded by EGR valve leakage. 
     At  502 , as at  402 , the routine includes estimating and/or measuring engine operating conditions. These may include, for example, engine speed, torque demand, barometric pressure, engine temperature, etc. Next it may be determined if selected engine non-fueling conditions are present. As elaborated below, the selected engine non-fueling conditions may include a deceleration fuel shut-off condition. The routine may be repeated at the first DFSO event after every engine start and/or the first DFSO event after a new oxygen or pressure sensor is installed. 
     At  504 , it may be determined if a new intake air oxygen (IAO2) sensor or a new intake pressure sensor was installed in the vehicle. For example, it may be determined if a new sensor was installed since the last engine shut-down and the current engine start. In one example, following installation of a new sensor, an indication that calibration of the new sensor is required may be received at the controller. 
     If a new oxygen sensor or pressure sensor was installed, then at  505 , the routine includes resetting the previously learned adaptive values of the intake air oxygen sensor. That is, the previously learned zero point and pressure correction factors saved in a look-up table of the controller&#39;s memory (e.g., in the KAM) may be reset. Then, the table may be repopulated with data from the current zero point learning, and subsequent iterations of the DFSO adaptation routine. 
     If a new oxygen or pressure sensor was not installed, or after resetting the table if a new sensor was installed, the routine proceeds to  506  to confirm if engine non-fueling conditions are present. Specifically, a deceleration fuel shut off (DFSO) condition may be confirmed. If a DFSO condition is not confirmed, at  507 , the look-up table in the controller&#39;s memory may not be further updated and the current zero point readings may be used. As such, by re-learning the reference point each time a new sensor is installed, differences in oxygen sensor readings due to part-to-part variations can be better accounted for. By updating and re-learning the reference point on each engine start, differences in oxygen sensor readings due to sensor aging can be better accounted for. 
     Upon confirmation of the DFSO condition, at  508 , the routine includes learning a reference point for the intake oxygen sensor at a reference intake pressure during the non-fueling condition. Specifically, the controller may learn the oxygen sensor output at the first engine idle condition and may also note the reference intake pressure at which the oxygen sensor output was learned. The controller may update the look-up table saved in the controller&#39;s KAM with the oxygen sensor output learned at the given pressure. In one example, the reference intake pressure is a throttle inlet pressure estimated by a TIP sensor coupled to the intake manifold at a location similar to the oxygen sensor (e.g., downstream of the charge air cooler and upstream of the intake throttle). In another example, the reference intake pressure is a manifold pressure estimated by a MAP sensor coupled to the intake manifold at a location similar to the oxygen sensor. 
     As such, learning the reference point includes learning a relationship between a first output of the intake oxygen sensor at a first intake pressure during the first DFSO event since engine start, and then using the learned relationship to determine the zero point. The learned relationship is used to determine the zero point by calculating the oxygen reading at the reference pressure, by substituting the delta pressure from the reference pressure. By performing this learning during DFSO conditions, various advantages are achieved. First, any error caused due to noise factors from purge or PCV hydrocarbons is reduced. Second, errors due to EGR valve leakage are reduced. This is because during the non-fueling conditions, any leaked “EGR” is the same as the intake air. Overall, a more accurate zero point learning can be achieved. 
     At  510 , the intake oxygen sensor output is corrected for humidity. As elaborated with reference to  FIG. 8 , the output of the intake oxygen sensor estimated at the reference pressure is corrected with a correction factor based on ambient humidity. As such, this may include correcting for no humidity (that is, zero % humidity or dry conditions) wherein the output of the oxygen sensor is corrected by removing all the contribution of humidity. Alternatively, this may include correcting to a known standard or reference humidity level. For example, the oxygen sensor output may be corrected to a reference 1.2% of humidity. 
     At  512 , it may be determined if the DFSO adaptation is complete. As such, the intake oxygen sensor readings at the given reference intake pressure may be monitored for a duration of the DFSO and the look-up table may be continue to be populated over the duration with readings from the intake oxygen sensor. In one example, when the DFSO is initiated at  508 , a timer may be started and at  512 , it may be determined if a threshold duration has elapsed on the timer. In one example, the DFSO adaptation may be complete if 4 seconds has elapsed. 
     Upon confirming that the DFSO adaptation has been completed, at  514 , the routine includes calculating a pressure correction factor. The pressure correction factor is a factor that compensates for the effect of intake pressure on the output of the intake oxygen sensor. The pressure correction adaptation may be performed by calculating a pressure correction factor based on the output of the intake oxygen sensor (iao2_o2) relative to the zero point (iao2_ref) of the sensor (that is, iao2_o2/iao2_ref). In addition, a delta pressure may also be determined based on the reference pressure, wherein the delta pressure is calculated as TIP−iao2_ref_press. Herein, TIP may be the same as boost pressure. At idle condition, the reference intake oxygen and preference intake pressure are determined. The pressure correction factor at a given pressure condition is calculated as the ratio of intake oxygen sensor reading and the reference oxygen concentration (that is, iao2_o2/iao2_ref). This corrected factor is learned as a relation between the delta pressure and the reference pressure. By doing this, the pressure input into the relationship is normalized to the reference pressure. 
     At  518 , the routine includes calculating and learning the zero point of the intake oxygen sensor. This may include, for example, performing a recursive least squares adaptation for pressure correction. The correction may be denoted as: 
     Iao2_press_corr_new=a2*dp_corr 2 +a1*dp_corr+a0, wherein a0, a1, and a2 are pressure correction coefficients, and dp_corr is the delta pressure correction. 
     Once the zero point is learned, an EGR flow to the engine can be adjusted based on an output of the intake oxygen sensor during EGR conditions, as elaborated at  FIG. 9 . Therein, an EGR flow to the engine is adjusted based on an intake oxygen concentration estimated by the intake oxygen sensor relative to the learned reference point, and further based on a change in intake pressure relative to the reference point 
     At  520 , the routine includes diagnosing an EGR valve based on the zero point estimated during the DFSO adaptation relative to a zero point estimated during an idle adaptation. An example idle adaptation is described in  FIG. 4 . As such, the zero point learned during the idle adaptation may be a first learned reference point, while the zero point learned during the DFSO adaptation may be a second learned reference point (both learned at a given reference intake pressure). As elaborated at  FIG. 6 , the controller may indicate EGR valve degradation based on a difference between the first learned reference point and the second learned reference point being larger than a threshold amount. 
     Turning now to  FIG. 8 . An example routine  800  is shown for correcting a nominal output of an intake oxygen sensor during zero point learning based on an ambient humidity estimate. The routine allows for oxygen displaced by the humidity to be accounted for. 
     At  802 , the routine includes confirming that zero point learning is enabled. Specifically, it may be confirmed that either idle adaptation or DFSO adaptation of the intake oxygen sensor is being performed, as previously discussed with reference to  FIGS. 4-5 . 
     Upon confirmation, at  804 , the routine includes learning a reference point for the intake oxygen sensor at a reference intake pressure. This includes learning a nominal amount of oxygen based on an output of the intake oxygen sensor at the reference intake pressure during selected engine idling conditions or selected engine non-fueling conditions. As such, the reference intake pressure is one of a throttle inlet pressure and an intake manifold pressure. The selected engine idling conditions include a first engine idle since an engine start, a first engine idle since installation of the intake oxygen sensor or installation of an intake pressure sensor configured to estimate the reference intake pressure. The selected non-fueling conditions include a deceleration fuel shut-off condition. 
     At  806 , an intake oxygen concentration is estimated based on the sensor output. At  808 , ambient humidity is estimated, for example, via an intake manifold humidity sensor (such as sensor  173  of  FIG. 1 ). At  810 , the routine includes calculating an amount of oxygen displaced by the estimated ambient humidity. As such, the change in oxygen concentration due to humidity may be defined as per the equation:
 
O 2 MeasuredConcentration=21%/1+volume % gwater,
 
     wherein O 2 MeasuredConcentration is the measured oxygen concentration with volume % water (fraction) amount of water in air (that is, humidity). 
     At  812 , it may be determined if the nominal oxygen concentration is to be corrected based on the ambient humidity to reflect dry conditions or standard humidity conditions. In one example, during a first condition (at  814 ), the reference point may be calibrated to dry conditions (zero humidity) where the effect of all humidity is removed from the oxygen sensor output. In another example, during a second condition (at  816 ), the reference point may be calibrated to standard humidity conditions where the effect of humidity on the oxygen sensor output is corrected to pre-defined humidity conditions. An example of a standard humidity condition may be a humidity of 8 g/kg or 1.28%. 
     If dry condition calibration is selected, then at  814 , the routine includes correcting the learned reference point by adding the calculated amount of oxygen to the learned nominal amount of oxygen. This corrects the reference point to dry air conditions (that is, zero humidity) and the effect of all humidity on the oxygen sensor output is removed. The routine then moves to  820  to update the zero point value in the adaptive values table. Specifically, the corrected zero point is learned with relation to the reference intake pressure and stored in the controller&#39;s memory. 
     If the standard humidity condition calibration is selected, then at  816 , the routine includes adding the calculated amount of oxygen to the learned nominal amount of oxygen, as at  814 . Then, at  818 , after correcting the reference point to dry air, the routine includes further calibrating the reference point to a standard humidity level. In one example, the standard humidity level is 1.2% humidity. The routine then proceeds to  520  to update the zero point value in the adaptive values table. 
     As such, the humidity corrected zero point is then used to estimate EGR and adjust EGR flow. For example, the controller may subsequently (that is, after learning and during engine non-idling conditions) adjust EGR flow to the engine based on an intake oxygen concentration estimated by the sensor relative to the corrected reference point, and further based on a change in intake pressure from the reference intake pressure. 
     In one example, the intake oxygen sensor reading may correspond to 19.5% oxygen and the estimated ambient humidity read by the humidity sensor may be 30 grams/KG of air. The humidity reading may be converted to molar percent of water as per the calculation 100*(30/1000)*29/18=4.83%, where 29 is the molecular weight of air and 18 is the molecular weight of water. The 4.83% water displaces an amount of oxygen corresponding to 4.83*21/100=1.01% oxygen, where 21 is the atmospheric dry oxygen reading. The corrected dry air reading of the intake oxygen sensor is then calculated as 19.5% (intake air sensor reading)+1.01% (humidity correction)=20.5%. 
     Alternatively, the dry air oxygen reading learned above is further adjusted to a standard humidity level oxygen reading. Therein, the humidity sensor information is used to calculate the dry air oxygen reading which is then adjusted with the amount of oxygen that would be displaceable by a calibratable amount of humidity. With reference to the above example, if the calibratable amount of humidity is 10 g/KG of air, the displaced oxygen corresponding to this amount of humidity would be 0.34%. The nominal intake oxygen sensor reading would then be adjusted to 20.5% (dry air reading)−0.34% (displaced oxygen for the calibrated humidity level)=20.16%. 
     As another example, an engine system may comprise an engine including an intake manifold, a turbocharger including an exhaust turbine and an intake compressor, a charge air cooler coupled downstream of the compressor, and an intake oxygen sensor coupled to the intake manifold downstream of the charge air cooler and upstream of an intake throttle. Alternatively, the intake oxygen sensor can be located upstream of the CAC if the total LP-EGR concentration going to the engine is well mixed. The engine system may further include a pressure sensor coupled to the intake manifold downstream of the charge air cooler and upstream of the intake throttle, as well as a humidity sensor coupled to the intake manifold downstream of the charge air cooler and upstream of an intake throttle. An EGR system may be included in the engine including an EGR passage and EGR valve for recirculating exhaust residuals from downstream of the turbine to upstream of the compressor. An engine controller may be configured with computer readable instructions for: during a first engine idle since an engine start, learning an oxygen sensor output and a humidity sensor output at a reference intake pressure and adjusting the oxygen sensor output based on the humidity sensor output. Then, during subsequent engine non-idle conditions, the controller may be configured to adjust an opening of the EGR valve based on an intake oxygen concentration estimated by the intake oxygen sensor relative to the reference oxygen sensor output, and further based on an intake pressure relative to the reference intake pressure. Herein, adjusting the oxygen sensor output based on the humidity sensor output includes, during a first condition at idle, estimating a first amount of oxygen displaced by total humidity based on the humidity sensor output and adjusting (e.g., increasing) the reference oxygen sensor output for either dry or standard humidity conditions. In comparison, during a second condition, such as non-idle conditions, the oxygen sensor can reliably predict the oxygen concentration and adjust the EGR valve accordingly having been previously corrected for part to part variations, change over time and variable humidity levels. 
     In this way, a controller may correct a first, nominal output of an intake oxygen sensor, learned during selected engine idling conditions at a reference intake pressure, based on an measured ambient humidity. The selected engine idling conditions include one of a first engine idle from engine start, a first engine idle following installation of the intake oxygen sensor, and a first engine idle following installation of an intake pressure sensor. The controller may then adjust EGR flow to the engine based on a second output of the sensor, estimated at a second intake pressure, relative to the corrected first output. The EGR flow may be further adjusted based on the second intake pressure relative to the reference intake pressure. 
     The correcting performed by the controller may include calculating an amount of oxygen displaced by the estimated ambient humidity, and increasing the first output to include the amount of displaced oxygen, wherein the increased first output is indicative of a dry air oxygen content. In this way, the effect of all humidity is removed from the oxygen sensor output. The correcting may alternatively further include adjusting the increased first output based on an amount of oxygen displaceable by a calibrated humidity level, the adjusted output indicative of a calibrated humidity air oxygen content. In this way, the oxygen sensor output is calibrated to a standard humidity level. 
     The controller may adjust the EGR flow by estimating a delivered EGR flow based on a difference between the second output and the corrected first output, and adjusting a position of an EGR valve based on a difference between the delivered EGR flow and a target EGR flow, wherein the target EGR flow based on engine speed-load conditions. 
     Now turning to  FIG. 6 , an example routine  600  for diagnosing an EGR valve coupled to a low pressure EGR system based on intake oxygen sensor reference points learned during an idle adaptation and a DFSO adaptation is shown. The method allows an EGR valve leak to be identified and compensated for. 
     At  602 , the routine includes retrieving a first reference point learned during an idle adaptation (ref_idle) such as the idle adaptation of  FIG. 4 . At  604 , the routine includes retrieving a second reference point learned during a DFSO adaptation (ref_DFSO) such as the DFSO adaptation of  FIG. 5 . At  606 , the two reference points may be compared and it may be determined if there are any discrepancies between them. Specifically, it may be determined if the first reference point is within a threshold range of the second reference point, or if they differ by more than a threshold amount. The controller may then indicate EGR valve leakage based on the first reference point of the intake oxygen sensor learned during engine idling conditions relative to the second reference point of the oxygen sensor learned during engine non-fueling conditions. Specifically, at  610 , EGR valve leakage is indicated based on a difference between the first reference point and the second reference point being larger than a threshold. The controller may indicate the EGR valve degradation by setting a diagnostic code. In comparison, at  608 , no EGR valve leakage is indicated when the difference is smaller than the threshold. 
     As discussed at  FIG. 9 , based on the indication of EGR valve leakage, EGR control responsive to an output of the intake oxygen sensor may be adjusted. Specifically, in response to the indication of no EGR valve leakage, the EGR valve may be feed-forward adjusted based on engine speed-load conditions and feedback adjusted based on an output of the intake manifold sensor relative to one of the first and second reference point. In comparison, in response to an indication of EGR valve leakage, the controller may continue feed-forward adjusting the EGR valve based on engine speed-load conditions but may terminate feedback adjusting of the EGR valve based on the output of the intake manifold sensor relative to one of the first and second reference point. 
     As used herein, indication EGR valve degradation includes indicating leakage of an EGR valve coupled to a low pressure EGR passage configured to recirculate exhaust residuals from an exhaust manifold, downstream of a turbine to an intake manifold, upstream of a compressor. The intake oxygen sensor may be coupled to the engine intake manifold, upstream of an intake throttle and either upstream or downstream of a charge air cooler, the cooler coupled downstream of the compressor. Herein, each of the first and second reference points are learned at a reference intake pressure, the reference intake pressure estimated by an intake pressure sensor coupled to the engine intake manifold, upstream of the intake throttle and downstream of the charge air cooler. 
     Now turning to  FIG. 9 , routine  900  depicts an example method for performing EGR control using the output of an intake manifold oxygen sensor relative to a zero point of the sensor learned during an idle adaptation and/or a DFSO adaptation. The method further adjusts the feed-forward feedback components of EGR control based on any indication of EGR valve degradation. 
     At  902 , the output of an intake manifold oxygen sensor is received. An intake pressure at which the output was received is also noted since the output is affected by intake pressure. At  904 , a pressure correction of the output is performed based on the intake pressure at which the sensor output was taken relative to a reference intake pressure. Also at  904 , a difference between the pressure-corrected oxygen sensor output and the zero point of the oxygen sensor is learned. As such, as an amount of EGR flow increases, exhaust dilution of intake air increases, reducing the amount of oxygen available in the intake air, and thereby reducing the output of the intake sensor. The EGR dilution may be reflected as a drop in oxygen concentration sensed by the intake oxygen sensor. 
     Thus at  906 , a change in oxygen concentration may be determined based on the determined difference between the oxygen sensor output relative to the zero point. At  908 , an amount of EGR dilution of intake air is determined based on the change in oxygen concentration. At  910 , an EGR flow is controlled based on the EGR dilution determined and the desired EGR. As used herein, the EGR flow may be a low pressure EGR flow along an EGR passage from an exhaust manifold, downstream of an exhaust turbine, to an intake manifold, upstream of an intake compressor, via an EGR valve. In addition, the EGR may be provided at a fixed rate or variable rate relative to intake air flow based on engine operating conditions. For example, at all engine speed-load conditions from a medium load down to a minimum load, low pressure EGR may be delivered at a fixed rate relative to the intake air flow (that is, at a fixed EGR percentage). In comparison, at engine speed-load conditions above a medium load, low pressure EGR may be delivered at a variable rate relative to the intake air flow (that is, at a variable EGR percentage). 
     Controlling the EGR flow includes, at  911 , feed-forward adjusting the EGR valve based on engine operating conditions, such as speed-load conditions. For example, at higher engine speed-load conditions, an opening of the EGR valve may be increased while at lower engine speed-load conditions, the EGR valve opening may be decreased. In addition, at  912 , the controlling includes feedback adjusting the EGR valve based on the calculated EGR flow relative to a desired EGR flow. For example, if the actual flow estimated by the intake oxygen sensor exceeds the desired or expected flow, the EGR valve opening may be decreased. As another example, if the actual flow estimated by the intake oxygen sensor is below the desired or expected flow, the EGR valve opening may be increased. 
     At  914 , it may be determined if there is an indication of EGR valve leakage. As elaborated at  FIG. 6 , EGR valve leakage may be identified based on deviations between an oxygen sensor zero point learned using the idle adaptation and a zero point learned using the DFSO adaptation. If no EGR valve leakage is identified, the routine may end. Else at  816 , in response to the indication of EGR valve leakage, the controller may terminate the feedback adjustment of the EGR valve based on the output of the intake oxygen sensor and temporarily shift to using only feed-forward control of the EGR valve. In alternate embodiments, in response to the indication of EGR valve leakage, EGR may be transiently disabled or a diagnostic flag may be set. 
     In other words, in response to an indication of no EGR valve leakage, the EGR valve is feed-forward adjusted based on engine speed-load conditions and feedback adjusted based on an output of the intake manifold sensor relative to at least one of the first and second reference points learned during idle and DFSO adaptations, respectively. In comparison, in response to an indication of EGR valve leakage, the EGR valve is only feed-forward adjusted based on engine speed-load conditions while feedback adjusting of the EGR valve based on the output of the intake manifold sensor relative to at least one of the first and second reference points is terminated. This allows EGR control to be improved when EGR valve leakage is known. 
     In one example, an engine system comprises an engine including an intake manifold, a turbocharger including an exhaust turbine and an intake compressor, a charge air cooler coupled downstream of the compressor, and an intake oxygen sensor coupled to the intake manifold downstream of the charge air cooler and upstream of an intake throttle. A pressure sensor may be coupled to the intake manifold downstream of the charge air cooler and upstream of the intake throttle. The engine system may further comprise an EGR system including an EGR passage and EGR valve for recirculating exhaust residuals from downstream of the turbine to upstream of the compressor. A controller of the engine system may be configured with computer readable instructions for: during a first engine idle since an engine start, learning a reference point for the oxygen sensor at a reference intake pressure; and adjust an opening of the EGR valve based on an intake oxygen concentration estimated by the sensor relative to the learned reference point, and further based on an intake pressure relative to the reference intake pressure. The controller may additionally, or optionally, during an engine deceleration fuel shut-off condition, learn a reference point for the oxygen sensor at the reference intake pressure; and adjust an opening of the EGR valve based on an intake oxygen concentration estimated by the sensor relative to the learned reference point, and further based on an intake pressure relative to the reference intake pressure. The engine system may further comprise a humidity sensor for estimating an ambient humidity, the controller then further adjusting the opening of the EGR valve based on an ambient humidity relative to a reference humidity. The controller may further determine degradation of the EGR valve based on differences between the reference points learned during the idle condition relative to the DFSO condition. 
     In this way, a relationship between an intake oxygen sensor and an intake pressure sensor can be learned at varying humidity conditions, and an EGR flow can be learned based on a change in the output of the oxygen sensor, independent of the accuracy of either the oxygen sensor or the pressure sensor. By adjusting the output of an intake oxygen sensor based on an ambient humidity estimated by an intake humidity sensor, the displacement of intake oxygen by humidity can be accurately estimated and accounted for, improving the reliability of the oxygen sensor&#39;s zero point reading. By performing the learning during idling conditions, noise factors due to ingestion of PCV and purge HCs, intake pressure variations, sensor aging, as well as part-to-part variations is reduced. By also performing the learning during engine non-fueling conditions, such as a DFSO, noise factors due to EGR valve leakage are reduced. By increasing the accuracy of the intake oxygen sensor&#39;s zero point reading, EGR can be estimated more reliably, improving EGR control. 
     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 actions, operations, and/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 actions, operations and/or functions may be repeatedly performed depending on the particular strategy being used. Further, the described actions, operations and/or functions may graphically represent code to be programmed into non-transitory memory of 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-3, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. 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 sub-combinations 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.