Patent Publication Number: US-9897027-B2

Title: Methods and systems for adjusting EGR based on an impact of PCV hydrocarbons on an intake oxygen sensor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. patent application Ser. No. 14/252,693, entitled “METHODS AND SYSTEMS FOR ADJUSTING EGR BASED ON AN IMPACT OF PCV HYDROCARBONS ON AN INTAKE OXYGEN SENSOR,” filed on Apr. 14, 2014, now U.S. Pat. No. 9,441,564, the entire contents of which are incorporated herein by reference for all purposes. 
    
    
     FIELD 
     The present description relates generally to a gas constituent sensor included in an intake system of an internal combustion engine. 
     BACKGROUND/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 and improve fuel economy. 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 during non-EGR conditions to determine the oxygen content of fresh intake air. During EGR conditions, the sensor may be used to infer EGR based on a change in oxygen concentration due to addition of EGR as a diluent. One example of such an intake oxygen sensor is shown by Matsubara et al. in U.S. Pat. No. 6,742,379. 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, due to the location of the oxygen sensor downstream of a charge air cooler in the high pressure air induction system, the sensor may be sensitive to the presence of fuel vapor and other reductants and oxidants such as oil mist. For example, during boosted engine operation, purge air and/or blow-by gases may be received at a compressor inlet location. Hydrocarbons ingested from purge air, the positive crankcase ventilation (PCV), and/or rich EGR can consume oxygen on the sensor catalytic surface and reduce the oxygen concentration detected by the sensor. In some cases, the reductants may also react with the sensing element of the oxygen sensor. The reduction in oxygen at the sensor may be incorrectly interpreted as a diluent when using the change in oxygen to estimate EGR. Thus, the sensor measurements may be confounded by the various sensitivities, the accuracy of the sensor may be reduced, and measurement and/or control of EGR may be degraded. 
     In one example, the issues described above may be addressed by a method for an engine comprising: disabling EGR flow responsive to an impact of PCV flow hydrocarbons on an output of an intake oxygen sensor increasing above a threshold when purge flow is disabled, the impact of PCV flow hydrocarbons based a difference between the output of the intake oxygen sensor and an output of a DPOV sensor when EGR is flowing. In this way, EGR adjustments based on intake oxygen sensor outputs affected by PCV flow hydrocarbons may be reduced. As a result, accuracy of EGR control may be increased and engine emissions may be maintained at target levels. 
     For example, during boosted engine operation when EGR is flowing and PCV flow is enabled, hydrocarbons in the PCV flow may cause a decrease in the intake oxygen measured by the intake oxygen sensor. Therefore, when the engine is boosted and an impact of PCV flow hydrocarbons on the output of the intake oxygen sensor is above a threshold when purge is disabled, an engine controller may disable EGR until the impact of PCV flow has decreased back below the threshold. As a result, the controller may not adjust EGR based on an intake oxygen sensor output impacted by increased PCV hydrocarbons in the intake airflow. In one example, the impact of PCV flow hydrocarbons on the output of the intake oxygen sensor may be based on a difference between the intake oxygen sensor output and an output of a DPOV sensor positioned in a low-pressure EGR passage when EGR is flowing. In another example, the impact of PCV flow hydrocarbons on the output of the intake oxygen sensor may be based on a difference between the intake oxygen sensor output and expected (e.g., estimated) blow-by. In yet another example, the impact of PCV flow hydrocarbons may be based on an indication that an estimation for fuel concentration in engine oil is degraded, the indication responsive to an expected output of the intake oxygen sensor differing from an actual output of the intake oxygen sensor by a threshold amount, the expected output of the intake oxygen sensor based on an estimated fuel evaporation rate from the engine oil. Thus, the threshold amount may indicate an increased amount of hydrocarbons in the intake airflow which are impacting the output of the intake oxygen sensor. In this way, the controller may disable EGR flow when the impact of PCV flow on the intake oxygen sensor is above a threshold and may result in degraded EGR flow 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 impact of PCV hydrocarbons the oxygen concentration estimated by an intake oxygen sensor. 
         FIGS. 4A-B  show a method for disabling EGR flow when a hydrocarbon impact on an intake oxygen sensor is greater than a threshold. 
         FIG. 5  shows a method for estimating a fuel concentration in engine oil and a fuel evaporation rate from the engine oil. 
         FIG. 6  shows a graph of example adjustments to EGR flow based on estimates of an impact of PCV hydrocarbons on an output of an intake oxygen sensor. 
     
    
    
     DETAILED DESCRIPTION 
     The following description relates to systems and methods for estimating an impact of PCV hydrocarbons on an output of an intake oxygen sensor and estimating a fuel concentration in engine oil.  FIGS. 1-2  show example engines including a low-pressure exhaust gas recirculation (EGR) passage, a PCV system, and an intake oxygen sensor positioned in an intake passage downstream from the inlet of the LP-EGR passage and the inlet of the PCV system (during boosted operation) to the intake passage. During boosted engine operation, hydrocarbons (HCs) from the engine crankcase may enter the intake passage via PCV flow upstream of the intake oxygen sensor. As a result, a decrease in intake oxygen measured by the intake oxygen sensor may result from the PCV flow HCs and any additional diluents in the intake airflow (e.g., EGR or purge flow). This effect is shown at  FIG. 3 . However, the intake oxygen sensor may assume the decrease in intake oxygen is due to EGR alone and use this measurement to estimate EGR flow and adjust LP-EGR flow of the engine. As a result, EGR flow may not be adjusted to the desired level (e.g., may be reduced more than necessary).  FIGS. 4A-B  show a method for estimating the impact of PCV HCs on the intake oxygen sensor output (e.g., PCV noise at the intake oxygen sensor) when purge is disabled. If the impact of PCV HCs on the intake oxygen sensor is greater than threshold, an engine controller may disable LP-EGR for a duration until the PCV noise is reduced back below the threshold. A source of HCs in the PCV flow may result from fuel in the engine oil in the crankcase. As engine oil temperature increases, a greater amount of HCs may be released into the air and enter the intake passage via the PCV flow. Example adjustments to EGR based on PCV noise are shown at  FIG. 6 . Additionally, a method for estimating the fuel concentration in the engine oil and a fuel evaporation rate from the engine oil is shown at  FIG. 5 . The controller may adjust engine operation responsive to the fuel concentration and fuel evaporation rate. For example, the intake oxygen sensor output may be adjusted and corrected for PCV flow based on the estimated fuel concentration in the engine oil. Additionally, flags indicating a need to disable purge via the method presented at  FIG. 4  may be generated responsive to the fuel evaporation rate relative to intake oxygen sensor outputs. In this way, EGR adjustments due to inaccurate EGR flow estimates from an intake oxygen sensor impacted by PCV flow HCs may be reduced. 
       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 , which may be identical. As one non-limiting example, engine system  100  can be included as part of a propulsion system for a passenger vehicle. While not depicted herein, other engine configurations such as an engine with a single turbocharger may be used without departing from the scope of this disclosure. 
     Engine system  100  may be controlled at least partially by a controller  12  and by input from a vehicle operator  190  via an input device  192 . In this example, input device  192  includes an accelerator pedal and a pedal position sensor  194  for generating a proportional pedal position signal PP. Controller  12  may be a microcomputer including the following: a microprocessor unit, input/output ports, an electronic storage medium for executable programs and calibration values (e.g., a read only memory chip), random access memory, keep alive memory, and a data bus. The storage medium read-only memory may be programmed with computer readable data representing non-transitory instructions executable by the microprocessor for performing the routines described herein as well as other variants that are anticipated but not specifically listed. Controller  12  may be configured to receive information from a plurality of sensors  165  and to send control signals to a plurality of actuators  175  (various examples of which are described herein). 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 . 
     Engine system  100  may receive intake air via intake passage  140 . As shown at  FIG. 1 , intake passage  140  may include an air filter  156  and an air induction system (AIS) throttle  115 . The position of AIS throttle  115  may be adjusted by the control system via a throttle actuator  117  communicatively coupled to controller  12 . 
     At least a portion of the intake air may be directed to a compressor  122  of turbocharger  120  via a first branch of the intake passage  140  as indicated at  142  and at least a portion of the intake air may be directed to a compressor  132  of turbocharger  130  via a second branch of the intake passage  140  as indicated at  144 . Accordingly, engine system  100  includes a low-pressure AIS system (LP AIS)  191  upstream of compressors  122  and  132 , and a high-pressure AIS system (HP AIS)  193  downstream of compressors  122  and  132 . 
     A positive crankcase ventilation (PCV) conduit  198  (e.g., push-side pipe) may couple a crankcase (not shown) to the second branch  144  of the intake passage such that gases in the crankcase may be vented in a controlled manner from the crankcase. Further, evaporative emissions from a fuel vapor canister (not shown) may be vented into the intake passage through a fuel vapor purge conduit  195  coupling the fuel vapor canister to the second branch  144  of the intake passage. 
     The first portion of the total intake air can be compressed via compressor  122  where it may be supplied to intake manifold  160  via intake air passage  146 . Thus, intake passages  142  and  146  form a first 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 intake air passage  148 . Thus, intake passages  144  and  148  form a second branch of the engine&#39;s air intake system. As shown at  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. 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 . In the depicted example, intake passage  149  also includes a charge air cooler (CAC)  154  and a throttle  158 . The position of throttle  158  may be adjusted by the control system via a throttle actuator  157  communicatively coupled to controller  12 . As shown, throttle  158  may be arranged in intake passage  149  downstream of CAC  154 , and may be configured to adjust the flow of an intake gas stream entering engine  10 . 
     As shown at  FIG. 1 , a compressor bypass valve (CBV)  152  may be arranged in CBV passage  150  and a CBV  155  may be arranged in CBV passage  151 . In one example, CBVs  152  and  155  may be electronic pneumatic CBVs (EPCBVs). CBVs  152  and  155  may be controlled to enable release of pressure in the intake system when the engine is boosted. An upstream end of CBV passage  150  may be coupled with intake passage  148  downstream of compressor  132 , and a downstream end of CBV passage  150  may be coupled with intake passage  144  upstream of compressor  132 . Similarly, an upstream end of a CBV passage  151  may be coupled with intake passage  146  downstream of compressor  122 , and a downstream end of CBV passage  151  may be coupled with intake passage  142  upstream of compressor  122 . Depending on a position of each CBV, air compressed by the corresponding compressor may be recirculated into the intake passage upstream of the compressor (e.g., intake passage  144  for compressor  132  and intake passage  142  for compressor  122 ). For example, CBV  152  may open to recirculate compressed air upstream of compressor  132  and/or CBV  155  may open to recirculate compressed air upstream of compressor  122  to release pressure in the intake system during selected conditions to reduce the effects of compressor surge loading. CBVs  155  and  152  may be either actively or passively controlled by the control system. 
     As shown, a compressor inlet pressure (CIP) sensor  196  is arranged in the intake passage  142  and a HP AIS pressure sensor  169  is arranged in intake passage  149 . However, in other anticipated embodiments, sensors  196  and  169  may be arranged at other locations within the LP AIS and HP AIS, respectively. Among other functions, CIP sensor  196  may be used to determine a pressure downstream of an EGR valve  121 . 
     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 via bank-specific exhaust passages. In the depicted example, a first bank  13  of cylinders of engine  10  can exhaust products of combustion via a common exhaust passage  17  and a second bank  15  of cylinders can exhaust products of combustion via a common exhaust passage  19 . 
     The position of intake and exhaust valves of each cylinder  14  may be regulated via hydraulically actuated lifters coupled to valve pushrods, or via 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 intake 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. In still another alternative embodiment, the cams may not be adjustable. 
     Products of combustion that are exhausted by engine  10  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 . The position of wastegate  128  may be controlled by an actuator (not shown) as directed by controller  12 . As one non-limiting example, controller  12  can adjust the position of the wastegate  128  via pneumatic actuator controlled by a solenoid valve. For example, the solenoid valve may receive a signal for facilitating the actuation of wastegate  128  via the pneumatic actuator based on the difference in air pressures between intake passage  142  arranged upstream of compressor  122  and intake passage  149  arranged downstream of compressor  122 . In other examples, other suitable approaches other than a solenoid valve may be used for actuating wastegate  128 . 
     Similarly, products of combustion that are exhausted by engine  10  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 gases flowing through exhaust passage  19  can bypass turbine  134  via turbine bypass passage  133  as controlled by wastegate  138 . The position of wastegate  138  may be controlled by an actuator (not shown) as directed by controller  12 . As one non-limiting example, controller  12  can adjust the position of wastegate  138  via a solenoid valve controlling a pneumatic actuator. For example, the solenoid valve may receive a signal for facilitating the actuation of wastegate  138  via the pneumatic actuator based on the difference in air pressures between intake passage  144  arranged upstream of compressor  132  and intake passage  149  arranged downstream of compressor  132 . In other examples, other suitable approaches other than a solenoid valve may be used for actuating 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. 
     Products of combustion exhausted by the cylinders via exhaust passage  19  may be directed to the atmosphere via exhaust passage  180  downstream of turbine  134 , while combustion products exhausted via exhaust passage  17  may be directed to the atmosphere via exhaust passage  170  downstream of turbine  124 . 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. For example, as shown at  FIG. 1 , exhaust passage  170  may include an emission control device  129  arranged downstream of the turbine  124 , and exhaust passage  180  may include an emission control device  127  arranged downstream of the turbine  134 . Emission control devices  127  and  129  may be selective catalytic reduction (SCR) devices, three way catalysts (TWC), NO x  traps, various other emission control devices, or combinations thereof. Further, in some embodiments, during operation of the engine  10 , emission control devices  127  and  129  may be periodically regenerated by operating at least one cylinder of the engine within a particular air/fuel ratio, for example. 
     Engine system  100  may further include one or more exhaust gas recirculation (EGR) systems for recirculating at least a portion of exhaust gas from the exhaust manifold to the intake manifold. These may include one or more high-pressure EGR systems for proving high pressure EGR (HP EGR) and one or more 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 system  100  may include a low-pressure (LP) EGR system  108 . LP EGR system  108  routes a desired portion of exhaust gas from exhaust passage  170  to intake passage  142 . In the depicted embodiment, EGR is routed in an EGR passage  197  from downstream of turbine  124  to intake passage  142  at a mixing point located upstream of compressor  122 . The amount of EGR provided to intake passage  142  may be varied by the controller  12  via EGR valve  121  coupled in the LP EGR system  108 . In the example embodiment shown at  FIG. 1 , LP EGR system  108  includes an EGR cooler  113  positioned upstream of EGR valve  121 . EGR cooler  113  may reject heat from the recirculated exhaust gas to engine coolant, for example. The LP EGR system may include a differential pressure over valve (DPOV) sensor  125 . In one example, an EGR flow rate may be estimated based on the DPOV system which includes the DPOV sensor  125  that detects a pressure difference between an upstream region of the EGR valve  121  and a downstream region of EGR valve  121 . EGR flow rate (e.g., LP EGR flow rate) determined by the DPOV system may be further based on an EGR temperature detected by an EGR temperature sensor  135  located downstream of EGR valve  121  and an area of EGR valve opening detected by an EGR valve lift sensor  131 . In another example, EGR flow rate may be determined based on outputs from an EGR measurement system that includes an intake oxygen sensor  168 , mass air flow sensor (not shown), manifold absolute pressure (MAP) sensor  182  and manifold temperature sensor  183 . In some examples, both the EGR measurement systems (that is, the DPOV system including differential pressure sensor  125  and the EGR measurement system including intake oxygen sensor  168 ) may be used to determine, monitor and adjust EGR flow rate. 
     In an alternate embodiment, the engine system may include a second LP EGR system (not shown) that routes a desired portion of exhaust gas from exhaust passage  180  to intake passage  144 . In another alternate embodiment, the engine system may include both the LP EGR systems (one routing exhaust gas from exhaust passage  180  to intake passage  144 , and another routing exhaust gas from exhaust passage  170  to intake passage  142 ) described above. 
     In the depicted example, the engine system  100  may also include a HP EGR system  206 . HP EGR system  206  routes a desired portion of exhaust gas from common exhaust passage  17 , upstream of the turbine  124 , to intake manifold  160 , downstream of intake throttle  158 . Alternatively, the HP EGR system  206  may be positioned between exhaust passage  17  and the intake passage  193 , downstream of the compressor  122  and upstream of the CAC  154 . The amount of HP EGR provided to intake manifold  160  may be varied by the controller  12  via EGR valve  210  coupled in the HP EGR passage  208 . In the example embodiment shown at  FIG. 1 , HP EGR system  206  includes an EGR cooler  212  positioned upstream of EGR valve  210 . EGR cooler  212  may reject heat from the recirculated exhaust gas to engine coolant, for example. The HP EGR system  206  includes a differential pressure over valve (DPOV) sensor  216 . In one example, an EGR flow rate (e.g., HP EGR flow rate) may be estimated based on the DPOV system which includes the DPOV sensor  216  that detects a pressure difference between an upstream region of EGR valve  210  and a downstream region of EGR valve  210 . EGR flow rate determined by the DPOV system may be further based on an EGR temperature detected by an EGR temperature sensor  220  located downstream of EGR valve  210  and an area of EGR valve opening detected by an EGR valve lift sensor  214 . In alternate embodiments, the HP EGR passage  208  may not include a DPOV system. 
     Likewise, the engine may include a second high-pressure EGR loop (not shown) for recirculating at least some exhaust gas from the exhaust passage  19 , upstream of the turbine  134 , to the intake passage  148 , downstream of the compressor  132 , or to the intake manifold  160 , downstream of intake throttle  158 . EGR flow through HP-EGR loops  208  may be controlled via HP-EGR valve  210 . 
     EGR valve  121  and EGR valve  210  may be configured to adjust an amount and/or rate of exhaust gas diverted through the corresponding EGR passages to achieve a desired EGR dilution percentage of the intake charge entering the engine, where an intake charge with a higher EGR dilution percentage includes a higher proportion of recirculated exhaust gas to air than an intake charge with a lower EGR dilution percentage. In addition to the position of the EGR valves, it will be appreciated that AIS throttle position of the AIS throttle  115 , and other actuators may also affect the EGR dilution percentage of the intake charge. As an example, AIS throttle position may increase the pressure drop over the LP EGR system, allowing more flow of LP EGR into the intake system. As a result, this may increase the EGR dilution percentage, whereas less LP EGR flow into the intake system may decrease the EGR dilution percentage (e.g., percentage EGR). Accordingly, EGR dilution of the intake charge may be controlled via control of one or more of EGR valve position and AIS throttle position among other parameters. Thus, adjusting one or more of the EGR valves  121  and  210  and/or the AIS throttle  115  may adjust and EGR flow amount (or rate) and subsequently a percentage EGR in the mass air flow (e.g., air charge entering the intake manifold). 
     The engine  10  may further include one or more oxygen sensors positioned in the common intake passage  149 . As such, the one or more oxygen sensors may be referred to as intake oxygen sensors. In the depicted embodiment, an intake oxygen sensor  168  is positioned upstream of throttle  158  and downstream of CAC  154 . However, in other embodiments, intake oxygen sensor  168  may be arranged at another location along intake passage  149 , such as upstream of the CAC  154 . Intake oxygen sensor (IAO2)  168  may be any suitable sensor for providing an indication of the oxygen concentration of the intake charge air (e.g., air flowing through the common intake passage  149 ), such as a linear oxygen sensor, intake UEGO (universal or wide-range exhaust gas oxygen) sensor, two-state oxygen sensor, etc. In one example, the intake oxygen sensors  168  may be an intake oxygen sensor including a heated element as the measuring element. During operation, a pumping current of the intake oxygen sensor may be indicative of an amount of oxygen in the gas flow. 
     A pressure sensor  172  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  172  is a TIP sensor. In alternate examples, the reference intake pressure is a manifold pressure (MAP) as sensed by MAP sensor  182 . 
     Engine system  100  may include various sensors  165 , in addition to those mentioned above. As shown in  FIG. 1 , common intake passage  149  may include a throttle inlet temperature sensor  173  for estimating a throttle air temperature (TCT). Further, while not depicted herein, each of intake passages  142  and  144  may include a mass air flow sensor or alternatively the mass air flow sensor can be located in common duct  140 . 
     Humidity sensor  189  may be included in only one of the parallel intake passages. As shown in  FIG. 1 , the humidity sensor  189  is positioned in the intake passage  142  (e.g., non PCV and non-purge bank of the intake passage), upstream of the CAC  154  and an outlet of the LP EGR passage  197  into the intake passage  142  (e.g., junction between the LP EGR passage  197  and the intake passage  142  where LP EGR enters the intake passage  142 ). Humidity sensor  189  may be configured to estimate a relative humidity of the intake air. In one embodiment, humidity sensor  189  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. 
     Intake oxygen sensor  168  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  121 . Specifically, a change in the output of the sensor upon opening the EGR valve  121  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. For example, upon applying a reference voltage (Vs) to the sensor, a pumping current (Ip) is output by the sensor. The change in oxygen concentration may be proportional to the change in pumping current (delta Ip) output by the sensor in the presence of EGR relative to sensor output in the absence of EGR (the zero point). Based on a deviation of the estimated EGR flow from the expected (or target) EGR flow, further EGR control may be performed. 
     A zero point estimation of the intake oxygen sensor  168  may be performed during idle conditions where intake pressure fluctuations are minimal and when no PCV or purge air is ingested into the low pressure induction system. In addition, the idle adaptation may be performed periodically, such as at every first idle following an engine start, to compensate for the effect of sensor aging and part-to-part variability on the sensor output. 
     A zero point estimation of the intake oxygen sensor may alternatively be performed during engine non-fueling conditions, such as during a deceleration fuel shut off (DFSO). 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 can be reduced. 
     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 (CPV)  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  112  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. The duty cycle may include a frequency (e.g., rate) of opening and closing the canister purge valve  112 . 
     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  168 . 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 purge air to be advantageously harnessed for vacuum generation. 
     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  168 . 
     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, during boosted engine operation, PCV gases flow in a first direction (arrow  264 ) and are received in the engine intake upstream of the intake oxygen sensor  168 . Specifically, 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  168  and therefore do affect the output of oxygen sensor  168 . The boosted conditions may include intake manifold pressure above ambient pressure. 
     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, during non-boosted engine operating, PCV gases flow in a second direction (arrow  262 ) different from the first direction and are received in the engine intake downstream of the intake oxygen sensor  168 . In the depicted example, the second direction of PCV flow during non-boosted engine operation is opposite of the first direction of PCV flow during boosted engine operation (compare arrows  262  and  264 ). Specifically, during non-boosted operation, PCV air is directed into the intake manifold  160 , directly, downstream of throttle  158  via the vacuum side PCV hose  254 . The PCV flow may be directed to the intake manifold  160  upon passage through a vacuum side oil separator  258 . Herein, the PCV air is introduced downstream of intake oxygen sensor  168 , and therefore does not affect the output of oxygen sensor  168 . Thus, due to the specific engine configuration, during boosted engine operation, PCV and purge air hydrocarbons are ingested into the engine intake manifold upstream of the intake oxygen sensor  168  and are ingested into the engine intake manifold downstream of the intake oxygen sensor during non-boosted conditions. 
     As previously discussed, the intake air oxygen sensor  168  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, the sensor may output a reading or pumping current corresponding to a lower oxygen concentration. During the estimation, a nominal reference voltage (e.g., at 450 mV), or Nernst voltage, is applied to the sensor and an output (e.g., a pumping current output by the sensor upon application of the lower reference voltage) is noted. Based on the output of the sensor relative to a zero point of the sensor (that is, sensor output at no EGR conditions), a change in oxygen concentration is learned, and an intake dilution with EGR is inferred. 
     However, if the EGR estimation is performed during conditions when purging and/or crankcase ventilation is enabled (e.g., PCV flow is enabled), an output of the sensor is corrupted. Said another way, PCV and/or fuel vapor purge flow may cause an error in the output of the intake oxygen sensor. As such, purge air and/or positive crankcase ventilation hydrocarbons (e.g., PCV flow) may be ingested during boosted engine operating conditions along boost path  92  and boost side PCV hose  252  when the purge valve  112  is open and/or the PCV valve  256  is closed. The sensor output may be corrupted primarily due to the ingested hydrocarbons reacting with ambient oxygen at the sensing element of the intake sensor. This reduces the (local) oxygen concentration read by the sensor. Since the output of the sensor and the change in oxygen concentration is used to infer an EGR dilution of intake aircharge, the reduced oxygen concentration read by the intake oxygen sensor in the presence of purge air and/or PCV may be incorrectly interpreted as additional diluent. This impacts the EGR estimation and the subsequent EGR control. Specifically, EGR may be over-estimated. 
       FIG. 3  depicts this variation in the reading of the intake sensor. Specifically, map  300  depicts an oxygen concentration estimated by an intake manifold oxygen sensor along the y-axis and a PCV hydrocarbon (HC) content along the x-axis at a given EGR level. As the amount of PCV HCs ingested into the engine intake manifold increases, such as when PCV is enabled or flowing from the push-side pipe (e.g., conduit  198 ) during boosted conditions, the hydrocarbons react with oxygen at the sensing element of the intake oxygen sensor. The oxygen is consumed and water and carbon dioxide is released. As a result, the estimated oxygen concentration is reduced, even though an amount of EGR flow may remain constant. This reduction in oxygen concentration estimated by the oxygen sensor may be inferred as an increased dilution (or replacement of oxygen with EGR). Thus, the controller may infer that there is a larger amount of EGR flow available than actually is present. If not corrected for the hydrocarbon effect, a controller may decrease EGR flow in response to an incorrect indication of higher EGR dilution, degrading EGR control. For example, during purge and/or PCV flow conditions resulting in EGR over-estimation, the controller may decrease an opening of the EGR valve in response to a higher EGR estimate (based on a lower intake oxygen measurement from the intake oxygen sensor). However, actual EGR may be lower than the estimated level. Thus, EGR flow may be incorrectly reduced instead of maintained or increased. This may, in turn, result in increased engine emissions and degraded engine performance. 
     In one example, adjusting an intake oxygen measurement based on PCV flow may increase the accuracy of EGR flow estimates. Specifically, under certain engine operating conditions, an engine controller (such as controller  12  shown in  FIG. 1 ) may determine a PCV flow contribution to the intake oxygen concentration measured at an intake oxygen sensor (such as the intake oxygen sensor  168  shown in  FIGS. 1-2 ). If the PCV flow effect on intake oxygen under boost conditions is known, the controller may use this to correct the measured intake oxygen used to estimate EGR flow. As such, the EGR estimate may be corrected based on PCV flow. 
     For example, a blow-by map may be stored within a memory of the controller. The blow-by map may include an expected blow-by (e.g., expected amount of combustion chamber gases (predominantly inert) leaking through piston rings and/or compressor/turbine seals and flowing via PCV push-side pipe (e.g., conduit  198 ) to the intake and intake oxygen sensor) for current engine operating conditions. The blow-by map may be pre-determined during engine testing and may include an expected amount of blow-by for a current manifold pressure (MAP) and engine speed. In this way, the blow-by map may be in a form of a look-up table and may be used as a baseline for PCV push side flow (e.g., flow from PCV and to intake upstream of the oxygen sensor) with no hydrocarbons. Any measured hydrocarbons in excess of this amount may indicate excessive evaporation of crankcase fuel and hence high levels of noise to the IAO2 sensor reading. 
     One source of the hydrocarbons (HCs) in PCV flow may be from fuel accumulation in the engine oil in the crankcase of the engine. During engine cold start and warm-up conditions fuel may accumulate in the engine oil. Then, when the engine oil is warming up and/or after the engine oil has warmed up to a steady-state operating temperature, the accumulated fuel may be released as HCs into the air and PCV flow. The released HCs may affect fuel control and engine oil viscosity, thereby decreasing engine durability. As discussed above, when the engine is boosted, PCV flow may enter the engine intake upstream of the intake oxygen sensor. As a result, the HCs in the PCV may also affect the output of the intake oxygen sensor, thereby decreasing the accuracy of EGR flow estimation from the intake oxygen sensor output. In this way, the HCs in the intake airflow upstream of the intake oxygen sensor may result in measurement noise at the intake oxygen sensor. 
     By determining a fuel concentration in the engine oil, a hydrocarbon concentration in the intake airflow upstream of the intake oxygen sensor, and/or a fuel (e.g., hydrocarbon) evaporation rate from the engine oil, the effect of released HCs on engine fueling, the intake oxygen sensor, and consequently EGR flow estimates may be learned. This learned data may then be used to adjust engine operation including engine fueling, EGR flow rate, purge control, oil quality or viscosity monitor, etc. An instantaneous hydrocarbon concentration in the engine oil and fuel evaporation rate from the engine oil may be estimated based on one or more of engine oil temperature (EOT), an engine boosting condition, fuel composition (e.g., ethanol content of fuel used in the engine), compressor inlet pressure, crankcase pressure, and intake oxygen concentration measured from the intake oxygen sensor (e.g., such as the intake oxygen sensor  168  shown in  FIGS. 1-2 ), or a model of any of or any combination of the above measurements. Specifically, the method of determining the instantaneous hydrocarbon concentration in engine oil and/or the fuel evaporation rate may include obtaining an intake oxygen sensor reading when EGR flow and purge flow are disabled and when the engine is boosted. As a result, the decrease in oxygen concentration measured at the intake oxygen sensor may be due to HCs from PCV flow alone and not due to additional diluents such as EGR flow and purge flow HCs. Further, when the engine is boosted, HCs from the crankcase are directed to the intake passage upstream of the intake oxygen sensor. The intake oxygen sensor may then be divided by an estimated vapor pressure to determine the instantaneous concentration of HCs in the engine oil. The vapor pressure may be based on the EOT and the fuel composition (e.g., the amount of heavy vs. light ends in the fuel). The fuel evaporation rate may then be determined based on a HC concentration gradient between the liquid and gaseous phases. The HC concentration in the liquid phase is the concentration of HCs in the engine oil and the HC concentration in the gaseous phased is approximated by the intake oxygen measurement of the intake oxygen sensor. The instantaneous HC concentration in engine oil and the fuel evaporation rate may be stored in a memory of the controller and then updated as subsequent intake oxygen sensor measurements are obtained. 
     In one example, the controller may use the determined fuel evaporation rate to adjust fuel injection to the engine. For example, as the estimated fuel evaporation rate increases, the controller may decrease fueling to the engine. In this way, the controller may adjust fuel injection based on the fuel evaporation rate estimates. Additionally, the controller may use the determined instantaneous HC concentration to adjust the intake oxygen sensor output (e.g., correct the intake oxygen sensor output for PCV HCs) and then estimate EGR flow based on the adjusted intake oxygen sensor output. The controller may then adjust an EGR valve based on the estimated EGR flow, thereby resulting in EGR control with increased accuracy. Methods for determining fuel evaporation rate, instantaneous hydrocarbon concentration in the engine oil, and adjusting engine operation based these values are discussed further below with reference to  FIGS. 4-5 . 
     Additionally, when HCs exiting the crankcase through the PCV flow affect the intake oxygen sensor output, the controller may disable EGR (e.g., close an EGR valve) until the PCV impact on measured intake oxygen reduces below a threshold. In this way, EGR flow adjustments based on intake oxygen measurements reflecting a decrease in intake oxygen due to EGR and PCV HCs may be reduced. For example, if the engine system includes a LP EGR system, the controller may disable LP EGR flow when EGR flow estimates based on intake oxygen sensor output may have reduced accuracy due to PCV flow HCs. More specifically, the controller may disable LP EGR flow when the PCV HC impact on the intake oxygen sensor is greater than a threshold. The threshold may be based on a PCV HC amount that results in an EGR flow estimate different than actual EGR flow by an amount that may result in degraded EGR control. In one example, the PCV HC impact on the sensor may be determined based on a difference between the intake oxygen sensor output and estimated blow-by (determined from blow-by map) when both purge and EGR (LP EGR) are disabled (e.g., turned off). For example, the intake oxygen sensor output may be a change in intake oxygen (from a baseline or zero point value) due to HCs in the intake airflow. A difference between this intake oxygen sensor output and the expected blow-by indicate a larger amount of HCs than expected in the intake airflow. The increased amount of HCs may be from PCV and may result in degraded EGR estimates and EGR control. Thus, if the difference between the intake oxygen sensor output and the expected blow-by is greater than a threshold, the controller may disable EGR until the PCV HC impact decreases back below the threshold. 
     In another example, the PCV HC impact on the intake oxygen sensor may be determined based on a difference between a DPOV sensor reading and the intake oxygen sensor reading when purge is disabled and when LP EGR is not disabled (e.g., EGR is flowing). As described above, the DPOV sensor may be used to determine EGR flow. A first EGR flow estimate based on the DPOV sensor output may then be compared to a second EGR flow estimate based on the intake oxygen sensor output. If the difference between the DPOV sensor estimate and the intake oxygen sensor estimate for EGR is greater than a threshold, HCs from PCV may be affecting the oxygen reading and the controller may disable EGR until the PCV HC impact decreases back below the threshold. The controller may determine the PCV HC impact on the intake oxygen sensor when the engine is boosted, for example, only when the engine is boosted. Additionally, the controller may disable EGR based on the determined PCV HC impact only when the engine is boosted since the intake oxygen sensor reading is not affected by PCV HCs during non-boosted engine operation when the PCV HCs enter the intake passage downstream of the intake oxygen sensor. 
     In yet another example, the instantaneous HC concentration and/or the fuel evaporation rate determined by the method described above and presented at  FIG. 5  may be used in determining the PCV HC impact on the intake oxygen sensor. If the PCV impact on the intake oxygen sensor cannot be determined or compensated for (e.g., cannot determine the instantaneous HC concentration in engine oil and/or the evaporation rate), the controller may set a diagnostic code (e.g., flag). This diagnostic code may then be used by the controller to trigger disabling the EGR flow. The PCV flow impact may not be able to be determined or compensated for during conditions when the fuel evaporation model is degraded. For example, the fuel evaporation rate may be used to predict subsequent intake oxygen measurements from the intake oxygen sensor. If a predicted intake oxygen sensor output based on the estimated fuel evaporation rate differs from an actual intake oxygen sensor output, the estimated evaporation rate may not be accurate. As a result, the controller may set a flag or diagnostic code indicating that the estimated HC concentration values used for intake oxygen sensor output compensation are degraded. As a result, the controller may set a flag and/or command that EGR be disabled for a duration until the accuracy of the fuel evaporation rate model increases back above a threshold. 
     In this way, when HCs are impacting intake oxygen sensor measurements, thereby resulting in inaccurate EGR flow estimates, the controller may disable EGR flow until the PCV HC impact decreases below a set threshold. As a result, EGR flow may only be enabled and adjusted when the PCV flow effect on the intake oxygen sensor may be compensated for, thereby resulting in EGR flow estimates of increased accuracy. Further, by estimating the evaporation rate and/or the instantaneous HC concentration in the intake air upstream from the intake oxygen sensor, the controller may improve the accuracy of fuel injection and EGR flow adjustments, thereby increasing engine efficiency. 
     The systems of  FIGS. 1-2  described above provide for an engine system, comprising: an intake manifold, a crankcase coupled to the intake manifold via a PCV valve, a turbocharger with an intake compressor, an exhaust turbine, and a charge air cooler, an intake throttle coupled to the intake manifold downstream of the charge air cooler, and a canister configured to receive fuel vapors from a fuel tank, the canister coupled to the intake manifold via a purge valve. The system further comprises a low-pressure exhaust gas recirculation (EGR) passage coupled between an exhaust passage downstream of the exhaust turbine and an intake passage upstream of the intake compressor, the low-pressure EGR passage including a low-pressure EGR valve and low-pressure DPOV sensor for measuring low-pressure EGR flow. The system further comprises an intake oxygen sensor coupled to the intake manifold downstream of the charge air cooler and upstream of the intake throttle and a controller with computer readable instructions for disabling EGR flow responsive to a difference between an output of the intake oxygen sensor and an output of the DPOV sensor increasing above a first threshold when EGR is flowing and purge flow is disabled. The computer readable instructions further include instructions for maintaining EGR flow disabled responsive to a difference between the output of the intake oxygen sensor and expected blow-by increasing above a second threshold when EGR is not flowing and purge flow is disabled. The instructions also include instructions for maintaining the EGR flow disabled until the difference between the output of the intake oxygen sensor and expected blow-by decreases back below the second threshold. 
     As another embodiment, the computer readable instructions include instructions for: adjusting the low-pressure EGR valve based on an estimated fuel concentration in engine oil and an output of the intake oxygen sensor, the estimated fuel concentration in engine oil based on the output of the intake oxygen sensor when purge and EGR flow are disabled, engine oil temperature, and fuel composition. The computer readable instructions further include instructions for adjusting fuel injection to the injection based on an evaporation rate of fuel from the crankcase, the evaporation rate based on a concentration gradient between the estimated fuel concentration in engine oil and the output of the intake oxygen sensor. In another example, the computer readable instructions further include closing the low-pressure EGR valve in order to disable EGR flow responsive to a difference between a predicted output of the intake oxygen sensor and an actual output of the intake oxygen sensor being greater than a threshold amount, the predicted output of the intake oxygen sensor based on the evaporation rate. The threshold amount may be an amount indicative of increased hydrocarbons in the intake airflow upstream of the intake airflow. As a result, compensation for the PCV hydrocarbons may not be possible for EGR flow estimation. 
     Turning now to  FIGS. 4A-B , a method  400  is shown for disabling EGR flow when a hydrocarbon impact on an intake oxygen sensor is greater than a threshold. As described above, an increase in HC impact on the intake oxygen sensor (IAO2) may be due to PCV flow HCs during boosted engine operation. As shown in  FIGS. 1-2 , the IAO2 may be positioned in an intake passage, downstream of a compressor, an inlet of a LP EGR passage into the intake passage, and a push-side PCV passage (e.g., boost path  92  shown in  FIG. 2 ). The LP EGR passage may include a DPOV sensor coupled to the LP EGR valve. Instructions for carrying out method  400  may be stored in a memory of an engine controller such as controller  12  shown in  FIGS. 1-2 . Further, method  400  may be executed by the controller. 
     Method  400  begins by estimating and/or measuring engine operating conditions at  402 . Engine operating conditions may include an engine boost condition (e.g., boost level and boost on/off), EGR flow, MAP, engine speed, engine load, engine oil temperature (EOT), barometric pressure, humidity, crankcase pressure, etc. At  404 , the method includes determining if the engine is boosted. If the engine is not boosted, the method continues to  406  to adjust EGR flow based on the IAO2 output and not estimate PCV (or HC) noise at the IAO2. The impact of PCV HCs on the IAO2 output may be referred to herein as PCV noise at the IAO2. As discussed above, when the engine is unboosted, PCV flow enters the engine intake downstream of the IAO2, thereby having no effect on the intake oxygen measured by the IAO2. However, if the engine is boosted, PCV HCs may enter the intake airflow upstream of the IAO2, thereby impacting the IAO2. 
     If the engine is boosted, the method continues on to  408  to determine if purge flow is off. For example, if a purge valve is closed and no purge is flowing to the intake passage, purge flow is disabled (e.g., off). As discussed above, in order to determine the impact of PCV HCs on the IAO2, purge must be disabled. Thus, if purge is not disabled, the method continues to  410  to determine if it is time to disable purge. The routine for determining the HC impact on the IAO2 may be run at a set frequency. For example, the IAO2 output relative to estimated blow-by or a DPOV sensor output (based on whether EGR is flowing or not, as described further below) may be checked at a set frequency in order to determine the PCV noise at the IAO2. If it is not time to disable purge for determining the PCV HC impact on the IAO2, the controller does not disable purge at  411 . The method may return and wait until it is time to disable purge, as defined by the set checking frequency. In one example, the set checking frequency may be based on EOT. Specifically, if the engine is warming up (e.g., EOT is below a steady-state operating temperature), the checking frequency may be set to a first level based on the increasing EOT. For example, for each threshold increase in EOT (e.g., 5° C.) the controller may disable purge and determine the PCV HC impact on the IAO2. Once the EOT reaches steady-state such that the EOT is relatively constant, the controller may disable purge and determine the PCV noise less frequently. For example, purge may only be disabled once for determining PCV noise when the EOT is at steady-state. Then, following steady-state when the EOT begins to increase or decrease, the checking frequency may return to the first level based on the change in EOT. 
     Conversely at  410 , if it is time to estimate PCV noise at the IAO2, the controller may disable purge at  412 . Disabling purge may include closing a purge valve (e.g., CPV valve  112  shown in  FIG. 2 ). At  414 , the method includes determining if EGR is disabled (e.g., turned off). EGR may be disabled if the EGR valve is closed. As discussed above, the EGR may be LP EGR including a LP EGR passage with an inlet positioned upstream of the compressor and IAO2 in the intake passage. If EGR is disabled, the method continues on to  417  to obtain a measurement from the IAO2. The method may then continue on to  418  to determine the HC concentration in engine oil and/or a fuel evaporation rate from the engine oil based on the IAO2 output. The method for determining the HC concentration and fuel evaporation rate is shown at  FIG. 5 , described further below. In alternate embodiments, method  400  may not include determining the HC concentration in the oil and the fuel evaporation rate. 
     At  420 , the method includes determining if a difference between the IAO2 output and an estimated blow-by is greater than a first threshold and/or if a flag has been set based on the fuel evaporation rate (as determined by the method presented at  FIG. 5 ). For example, the IAO2 output may be a change in intake oxygen from a reference point due to diluents in the airflow. Since EGR and purge are both disabled, the decrease in intake oxygen measured by the IAO2 may be due to PCV HCs alone and not due to EGR and purge flow. As discussed above, the expected blow-by may be an expected amount of HCs in the intake airflow from PCV flow at the current engine operating conditions. Determining the expected blow-by may include looking up the expected blow-by in a look-up table or map stored in the memory of the controller. The expected blow-by may be a function of the current MAP and engine speed. Alternatively, the blow-by map may be determined by using the intake oxygen sensor measurement after an oil change when the fuel in the oil is negligible. The first threshold may be based on an amount of HCs indicating increased PCV HCs at the IAO2. The increased PCV HCs may be indicative of increased PCV noise resulting in EGR flow estimates of reduced accuracy. Additionally, the flag based on the fuel evaporation rate may be indicative of more PCV HCs than predicted by the fuel evaporation rate in the intake airflow. As a result, IAO2 compensation based on the estimated HC concentration in the engine oil (and the fuel evaporation rate) may not be accurate and may lead to EGR flow estimates of reduce accuracy. Thus, if the diagnostic flag indicating degraded fuel evaporation rate estimation is set and/or the difference between the IAO2 output and the expected blow-by is greater than the first threshold, the controller may disable EGR at  430  (shown in  FIG. 4B ). The controller may disable EGR until the PCV HC impact on the IAO2 decreases back below the threshold. Thus, the method at  430  may also include re-checking the PCV noise at a second frequency. The second frequency may be different than the set checking frequency described above at  410 . For example, the second frequency may be a set amount of time or duration between determining the PCV noise at the IAO2. In some examples, the second frequency may be greater than the set checking frequency (and the first frequency as described further below with reference to  424 ) such that the PCV noise is checked more frequently when EGR has been disabled due to the PCV HC impact greater than the threshold. If the impact of PCV flow HCs on the IAO2 output is less than or equal to the threshold (e.g., the first threshold) during the re-checking at  432 , the method continues to  434  to re-enable EGR. The method at  434  then returns to checking for PCV noise at a first frequency based on EOT. 
     Returning to  420 , if neither a flag is set based on the fuel evaporation rate nor the difference between the IAO2 output and the expected blow-by is greater than the first threshold, the method continues on to  424  to not disable EGR. The method at  424  also includes re-checking PCV noise at the first frequency based on EOT, the first frequency lower than the second frequency such that the PCV noise is checked less often at the first frequency (e.g., a duration between subsequent PCV noise checks is longer at the first frequency when EGR is not disabled than at the second frequency when EGR is disabled). 
     Returning to  414 , if EGR is not disabled (e.g., LP EGR is flowing and a LP EGR valve is at least partially open), the method continues on to  415  to determine if the engine includes a DPOV sensor. In some embodiments, the method at  415  may also include determining if the DPOV sensor is currently functioning properly. If a DPOV sensor is not present in the engine system (e.g., a DPOV sensor is not present for measuring LP EGR and/or HP EGR) or if the DPOV sensor is degraded, the method proceeds to  416  to disable EGR and continues on to  417  to obtain a measurement from the IAO2. The method then proceeds to  418  to determine the HC concentration in the oil and the evaporation rate from the IAO2 output and expected blow-by and not the DPOV sensor output. In this way, if no DPOV sensor is present, the method may turn off EGR in order to determine the impact of HCs on the IAO2 (e.g., determine the noise at the IAO2). 
     If, at  415 , a DPOV sensor is included in the engine, the method may continue to  426  to obtain a measurement from the IAO2 and a measurement from the DPOV sensor in the LP EGR passage. As a result of the engine system including a DPOV sensor, EGR does not need to be turned off in order to determine the impact of HCs on the IAO2. Thus, determining PCV noise at the IAO2 may be performed less intrusively when a DPOV sensor is present and may be used to estimate EGR flow for comparison with the IAO2 output. 
     At  428 , the method includes determining if an absolute value of a difference between the IAO2 output and the DPOV sensor output is greater than a second threshold. The second threshold may be indicative of an increased amount of HCs from PCV flow in the intake airflow which may result in a lower intake oxygen sensor measurement, thereby resulting in an EGR flow measurement of reduced accuracy. This may result in incorrect EGR flow adjustment, thereby degrading engine control. If the difference between the IAO2 output and the DPOV sensor output is not greater than the second threshold, the method continues on to  424  to not disable EGR and continue checking for PCV noise at the IAO2 at the first frequency based on EOT. However, if the difference between the IAO2 output and the DPOV sensor output is greater than the second threshold, the method continues on to  420  (shown in  FIG. 4B ) to disable EGR and re-check for PCV noise at the second frequency based on a set time duration. The method than continues on to  432  and  434  as described above. For example, after disabling EGR the method at  432  includes determining if the impact of PCV flow HCs on the IAO2 output is less than or equal to the threshold. The impact of PCV flow HCs may be determined from the difference between the IAO2 output and estimated blow-by and not the difference between the IAO2 output and the DPOV sensor output since EGR is disabled at  432 . 
     In this way, a method for an engine comprises disabling EGR flow responsive to an impact of PCV flow hydrocarbons on an output of an intake oxygen sensor increasing above a threshold when purge flow is disabled, the impact of PCV flow hydrocarbons based a difference between the output of the intake oxygen sensor and an output of a DPOV sensor when EGR is flowing. In one example, the impact of PCV flow hydrocarbons is based on a difference between the output of the intake oxygen sensor and expected blow-by when EGR is not flowing. The expected blow-by is based on a pre-determined blow-by amount for a current manifold pressure and engine speed. For example, the expected blow-by may be stored within a look-up table in a memory of a controller of the engine. Inputs to the look-up table may include the current manifold pressure and engine speed. In alternate embodiments, the inputs to the look-up table may be alternative or additional engine operating conditions such as boost level and/or engine oil temperature. 
     The method further comprises when EGR flow is not disabled due to the impact of PCV flow hydrocarbons being below the threshold, disabling purge and determining the impact of PCV flow hydrocarbons while purge is disabled at a first frequency, the first frequency based on an engine oil temperature. Additionally, the method comprises after disabling EGR flow responsive to the impact of PCV flow hydrocarbons, disabling purge and determining a subsequent impact of PCV flow hydrocarbons while purge is disabled at a second frequency, the second frequency different than the first frequency. For example, the second frequency is a set time-based frequency and the first frequency is based on a set change in the engine oil temperature, the second frequency higher than the first frequency such that purge is disabled more often at the second frequency. 
     The method further comprises re-enabling EGR flow responsive to the impact of PCV flow hydrocarbons on the output of the intake oxygen sensor decreasing below the threshold. In another example, the disabling EGR flow responsive to the impact of PCV hydrocarbons on the output of the intake oxygen sensor increasing above a threshold includes disabling EGR responsive to a diagnostic flag indicating degradation of an estimated fuel concentration in engine oil, the diagnostic flag set in response to an expected output of the intake oxygen sensor differing from an actual output of the intake oxygen sensor by a threshold amount, the expected output of the intake oxygen sensor based on an estimated fuel evaporation rate from the engine oil. The estimated fuel evaporation rate is based on the output of the intake oxygen sensor, the estimated fuel concentration in engine oil, fuel composition, and engine oil temperature. 
     Disabling EGR flow includes disabling EGR during boosted engine operation. Additionally, disabling EGR flow includes closing an EGR valve positioned in a low-pressure EGR passage, the low-pressure EGR passage positioned between an exhaust passage downstream of a turbine and an intake passage upstream of a compressor. The intake oxygen sensor is positioned downstream of an inlet of the low-pressure EGR passage into the intake passage and the DPOV sensor is positioned in the low-pressure EGR passage. 
     In another example, the disabling EGR flow described above may be responsive to a degree of an impact of PCV flow hydrocarbons on the output of the intake oxygen sensor. The degree of impact of the PCV flow hydrocarbons may be based on the magnitude of the difference between the intake oxygen sensor output and the DPOV sensor output (if EGR is flowing) or the magnitude of difference between the intake oxygen sensor output and the expected blow-by (if EGR is not flowing). As the magnitude of one or more of these differences increases, the degree of impact of PCV flow hydrocarbons on the intake oxygen sensor may increase. The controller may then disable or not disable EGR based on the degree of the impact of PCV flow hydrocarbons. For example, if the degree of impact is greater than a threshold the controller may disable EGR. In another example, the controller may disable EGR for a longer duration and/or increase the impact of PCV flow checking frequency (e.g., second frequency described above) as the degree of the impact increases. 
     As described above, HCs from PCV flow may impact the output of the IAO2 and consequently influence the EGR flow estimate based on the IAO2 output. HCs released into the intake passage via the PCV flow may result from evaporation of fuel in engine oil in the engine crankcase. As the engine warms up, fuel may evaporate from the engine oil and be released as HCs into the PCV flow. These HCs may impact the IAO2 output during boosted engine operation. Fuel in the engine oil may also affect additional engine controls such as engine fueling.  FIG. 5  shows a method  500  for estimating a fuel concentration in engine oil and a fuel evaporation rate from the engine oil. Instructions for executing method  500  may be stored in a controller (such as controller  12  shown in  FIGS. 1-2 ). Further, the controller may execute the method  500  as described below. 
     At  502 , the method includes determining an engine oil temperature (EOT). In one example, the EOT may be measured by a temperature sensor positioned in the engine oil in the crankcase or estimated through a model. At  504 , the method includes determining a vapor pressure of the fuel based on the EOT and the type of fuel being used in the engine. For example, each type of fuel may have a constituent content with different amount of heavy and light ends. In one example, the method at  504  may utilize a vapor pressure model. The vapor pressure model may use the fuel constituent content (pre-determined based on fuel type and prior knowledge of fuel species that get accumulated in the oil) and the measured EOT to determine the current fuel vapor pressure of the dominant constituents. At  506 , the method includes obtaining an IAO2 reading when both purge and EGR are disabled. The methods at  502  and  504  may be performed concurrently with the method at  506 . For example, the controller may obtain the IAO2 reading for estimating the fuel concentration in engine oil only when purge and EGR flow are disabled. In another example, the controller may obtain the IAO2 reading for estimating the fuel concentration in engine oil (and subsequently adjusting engine operation based on the estimated fuel concentration in engine oil) in response to both purge and EGR being disabled. As described above, EGR flow being disabled may include when the EGR valve (e.g., LP EGR valve) is fully closed and no EGR is flowing into the intake passage upstream of the IAO2. Further, in another example, if EGR and/or purge are disabled for the fuel concentration and/or the PCV noise estimation (described at  FIG. 4 ), EGR and/or purge may not be enabled even if requested by another engine system during the estimation period. 
     At  508  the method includes determining the instantaneous concentration of HCs in the engine oil by dividing the IAO2 output by the estimated vapor pressure. The IAO2 output may be proportional to the concentration of HCs in the gaseous phase while the determined concentration of HCs in the engine oil is the concentration of HCs in the liquid phase. At  510  the method includes determining the fuel evaporation rate from the engine oil based on a concentration gradient between the concentration of HCs in the liquid phase (e.g., oil) and the gaseous phase (e.g., air). In another embodiment, the fuel evaporation rate may be based on consecutive estimates of the fuel concentration in engine oil. 
     At  512  the method includes storing the instantaneous concentration of HCs in the oil (e.g., fuel concentration in engine oil) and the fuel evaporation rate in a memory of the controller. In one example, the fuel concentration in engine oil and the fuel evaporation may be stored as a function of EOT in a look-up table or chart. The engine controller may then reference the stored look-up table or chart during subsequent control routines wherein a fuel evaporation rate and/or fuel concentration in the engine oil is required. At  514  the method includes obtaining IAO2 measurements at a set interval or frequency and then updating the stored concentration of HCs in the oil and fuel evaporation rate based on the new IAO2 measurements, as described at steps  502 - 510 . The set interval for estimating and updating the fuel concentration in the engine oil and fuel evaporation rate data may be based on the EOT and operational state of the engine. For example, if the engine is warming up and the EOT is not at steady-state the interval for estimating may be shorter than if the EOT is at steady-state (e.g., not changing substantially). 
     At  516  the method includes adjusting engine operation based on the fuel evaporation rate and the fuel concentration in the engine oil. In one example, adjusting engine operation may include adjusting fuel injection based on the fuel evaporation rate. For example, the controller may reduce a fuel injection amount or pressure as the fuel evaporation rate increases. In another example, the controller may adjust subsequent IAO2 outputs based on the HC concentration in the oil. For example, the IAO2 output may be corrected by the HC concentration in the oil such that the corrected IAO2 output reflects a decrease in intake oxygen due to EGR only and not due to PCV HCs. As a result, the controller may estimate EGR flow (e.g., LP EGR flow) based the adjusted IAO2 output. The controller may then adjust EGR flow (e.g., adjust a LP EGR valve) based on the estimated EGR flow. In alternate examples, the controller may also adjust HP EGR flow based on the adjusted IAO2 output in order to adjust a total amount of EGR provided to the engine. In some examples, the fuel evaporation rate may be used to estimate and/or predict subsequent IAO2 outputs. If an actual IAO2 differs by a threshold amount from a predicted IAO2 output, degradation of the fuel evaporation rate may be indicated. If the accuracy of the fuel evaporation rate estimation is degraded (e.g., reduced), EGR flow estimates based on the fuel concentration in the oil may be inaccurate. As a result, the controller may set a flag indicating that the change in intake oxygen measured at the IAO2 due to PCV HCs may not be compensated for with the method outlined at  FIG. 5 . As a result, the controller may disable EGR flow for a duration until the impact of HCs at the IAO2 is reduced, as described above with reference to  FIGS. 4A-B  (at step  420 ). In yet another example, if the fuel evaporation rate exceeds a threshold rate, the controller may set a flag as an indicator to the EGR arbitration strategy shown at step  420  in  FIG. 4A . Thus, the method shown at  FIGS. 4A-4B  may include disabling EGR flow for a duration based on the estimated evaporation rate increasing above the threshold rate. In alternate examples, the fuel evaporation rate may be used to adjust additional engine controls such as fuel injection routines or adjusting estimates of engine oil viscosity for additional control routines such as oil minder. 
     As one embodiment, a method for an engine comprises adjusting engine operation based on a fuel concentration in engine oil, the fuel concentration based on an output of an intake oxygen sensor when purge and EGR flow are disabled, engine oil temperature, and fuel composition. The method further comprises estimating a fuel evaporation rate from the engine oil based on a concentration gradient between the fuel concentration in engine oil and the output of the intake oxygen sensor, the output of the intake oxygen sensor indicative of a fuel concentration in intake air. In one example, adjusting engine operation includes adjusting fuel injection to the engine based on estimated fuel evaporation rate, an amount of fuel injected decreasing with increasing estimated fuel evaporation rate. In another example, adjusting engine operation includes disabling EGR flow for a duration when an actual output of the intake oxygen sensor differs from an expected output of the intake oxygen sensor by a threshold amount, the expected output based on the estimated fuel evaporation rate. In yet another example, adjusting engine operation includes adjusting a position of an EGR valve based on the output of the intake oxygen sensor relative to the fuel concentration in engine oil. The EGR valve may be a low-pressure EGR valve in a low-pressure EGR system. In another example, the EGR valve may be a high-pressure EGR valve in a high-pressure EGR system. 
     The fuel concentration may be further based on crankcase pressure and boost conditions. For example, the fuel concentration may only be determined when the engine is boosted. The intake oxygen sensor is positioned in an intake passage downstream of an inlet of a low-pressure EGR passage into the intake passage, the low-pressure EGR passage positioned between an exhaust passage downstream of a turbine and an intake passage upstream of a compressor. 
     As another embodiment, a method for an engine comprises during boosted engine operation, flowing PCV gases to an engine intake upstream of an intake oxygen sensor; estimating a vapor pressure based on an engine oil temperature and a composition of fuel; estimating a fuel concentration in engine oil based on the estimated vapor pressure and an output of the intake oxygen sensor when purge flow and EGR are disabled; and adjusting an EGR valve based on the estimated fuel concentration in engine oil and the output of the intake oxygen sensor. The method further comprises estimating a fuel evaporation rate from the engine oil based on a concentration gradient between the output of the intake oxygen sensor and the estimated fuel concentration in engine oil. Additionally, the method comprises adjusting engine fueling based on the estimated fuel evaporation rate. 
     Further still, the method comprises setting a diagnostic flag to disable EGR and indicating degradation of the estimated fuel concentration in engine oil due to an expected output of the intake oxygen sensor differing from an actual output of the intake oxygen sensor by a threshold amount, the expected output of the intake oxygen sensor based on the estimated fuel evaporation rate. After setting the diagnostic flag to disable EGR, the method may include removing the diagnostic flag to re-enable EGR when the expected output of the intake oxygen sensor based on the estimated fuel evaporation rate is within the threshold amount of the actual output of the intake oxygen sensor. In one example, the method includes disabling purge at a first frequency in order to determine if degradation of the estimated fuel concentration in engine oil is indicated, the first frequency based on engine oil temperature when EGR is not disabled due to an impact of hydrocarbons on the output of the intake oxygen sensor. In another example, the method includes disabling purge at a second frequency, higher than the first frequency, in order to determine if degradation of the estimated fuel concentration in engine oil is indicated, the second frequency based on a set time duration when EGR has been disabled due to the impact of hydrocarbons on the output of the intake oxygen sensor. 
     Additionally, the method comprises storing the estimated fuel evaporation rate and the estimated fuel concentration in engine oil as a function of engine oil temperature in a memory of a controller of the engine. A controller of the engine may obtain an output of the intake oxygen sensor at a set interval when purge and EGR are disabled and then update the stored fuel evaporation rate and fuel concentration in engine oil, the set interval based on engine oil temperature. During non-boosted engine operation, the method includes flowing PCV gases to the engine intake downstream of the intake oxygen sensor and adjusting the EGR valve based on the output of the intake oxygen sensor on not based on the estimated fuel concentration in engine oil. 
     Turning now to  FIG. 6 , a graphical example of adjustments to EGR flow based on estimates of the impact of PCV HCs on an IAO2 output is shown. Specifically, graph  600  shows changes in engine oil temperature (EOT) at plot  602 , changes in a purge off command (e.g., command to disable purge) at plot  604 , changes in EGR flow (e.g., LP EGR) at plot  606 , changes in boost at plot  608 , changes in a difference between an IAO2 output and DPOV sensor output at plot  610 , changes in a difference between an IAO2 output and expected blow-by at plot  612 , and changes to a set diagnostic flag based on the fuel evaporation rate at plot  614 . As discussed above, the flag based on the fuel evaporation rate may indicate the impact of PCV HCs on the IAO2 output is over a threshold. 
     Prior to time t 1 , the engine is boosted (plot  608 ) and the EOT may be increasing from a lower threshold temperature (plot  602 ), thereby indicating the engine oil is warming up. As a result, the controller may disable purge flow (or command purge flow off) at a first frequency, ΔF 1  (plot  604 ). Disabling purge flow may include closing a canister purge valve to stop the flow of purge gases to the engine intake. If the purge valve is already closed, the controller may maintain the valve in the closed position during the command to disable purge. The first frequency ΔF 1  may be based on the EOT such that purge is commanded off to determine the PCV noise on the IAO2 for every set increase in EOT. For example, the set increase may be 5° C. such that purge is disabled to perform the PCV noise check (e.g., impact of PCV HCs on the IAO2 output) every increase in EOT by 5° C. In alternate examples, the set increase in EOT may be more or less than 5° C. Also prior to time t 1 , EGR may be enabled (e.g., LP-EGR valve at least partially open and LP-EGR is flowing). After disabling purge, the controller may re-enable purge. However, if purge is commanded closed based on additional engine operating conditions, the purge valve may remain closed even if purge flow is not disabled for the PCV noise estimating routine. 
     At time t 1 , the difference between the output of the IAO2 sensor and the output of the DPOV sensor may be greater than a first threshold, T 1  (plot  610 ). Since EGR is flowing, both the IAO2 output and the DPOV sensor output may provide estimates of EGR flow. If these estimates differ by an amount greater than the first threshold T 1 , blow-by hydrocarbons from PCV flow may be affecting the IAO2 sensor output. In response to the difference between the output of the IAO2 and the output of the DPOV sensor being greater than the first threshold T 1 , the controller may disable EGR (plot  606 ). For example, the controller may close a LP EGR valve positioned in a LP EGR passage in order to stop LP EGR flow from flowing into the intake passage upstream of the IAO2 sensor. 
     After disabling EGR flow at time t 1 , the controller may re-check the PCV noise at the IAO2 by disabling purge and re-checking the difference between the IAO2 output and predicted blow-by at a second frequency ΔF 2 . The second frequency ΔF 2  may be based on set time intervals rather than based on EOT. In some examples, as shown in  FIG. 6 , the second frequency ΔF 2  may be higher than the first frequency ΔF 1  such that the PCV noise impact on the IAO2 is checked more frequently after purge has been disabled due to PCV noise being above a threshold. In alternate embodiments, the first frequency and the second frequency may be substantially the same. 
     Between time t 1  and time t 2 , a flag may be set based on the estimated fuel evaporation rate (plot  614 ). As discussed above, if the impact of PC HCs on the IAO2 output may not be compensated for using an estimated fuel concentration in the engine oil, the controller may disabled purge. For example, if the IAO2 is different than predicted by the fuel evaporation rate by a threshold amount, the controller may set the flag resulting in disabling EGR. Since EGR is already disabled between time t 1  and time t 2 , the EGR remains off responsive to the flag. 
     At time t 2 , the difference between the IAO2 output and the expected blow-by decreases back below a second threshold T 2  (plot  612 ), thereby indicating the impact of PCV noise on the IAO2 has decreased back below a set threshold. As a result, the controller may re-enable EGR responsive to the difference between the IAO2 output and the expected blow-by being below the second threshold T 2 . Re-enabling EGR flow may include opening the LP EGR valve and adjusting LP EGR flow to a requested level. Additionally, the second threshold T 2  may be different than the first threshold T 1 . 
     At time t 3  the EOT reaches steady-state such that the EOT is substantially steady and no longer increasing (plot  602 ). As a result, since EGR is enabled, the controller may only disable purge and check the impact of PCV hydrocarbons on the IAO2 once while the EOT remains at steady-state conditions. In alternate embodiments, the controller may check PCV noise and disable purge more than once, but at a frequency lower than the first frequency ΔF 1  and the second frequency ΔF 2 . 
     At time t 4  the EOT begins increasing again above the steady-state level (plot  602 ). As a result, the controller begins disabling purge and checking PCV noise at the IAO2 at the first frequency ΔF 1 . At time t 5 , the controller determines that the difference between the IAO2 output and the expected blow-by (BB) is greater than the second threshold T 2 . In response to the difference between the IAO2 output and the expected BB being greater than the second threshold T 2 , the controller commands EGR flow off (e.g., closes the LP EGR valve). However, since EGR flow is already disabled (plot  606 ), the controller maintains the disabled EGR flow at time t 5 . After time t 5 , the controller begins disabling purge and checking the PCV noise at the IAO2 at the second frequency ΔF 2 . 
     As shown at time t 1  in  FIG. 6 , during a first condition when EGR is flowing and purge is disabled, an engine controller may disable EGR when a difference between an output of an intake oxygen sensor and an output of a DPOV sensor is greater than a first threshold T 1 . As shown at time t 5 , during a second condition when EGR is not flowing and purge is disabled, the engine controller may disable EGR flow when a difference between the output of the intake oxygen sensor and an expected blow-by flow is greater than a second threshold T 2 . Disabling EGR flow during the second condition may include maintaining EGR flow off (e.g., maintain the EGR valve closed) until the difference between the intake oxygen sensor output and the expected blow-by decrease back below the second threshold T 2 . The controller may then turn on EGR if EGR is requested based on additional engine operating conditions. 
     As shown at time t 2 , the controller may re-enable EGR flow after the disabling EGR when the difference between the output of the intake oxygen sensor and the expected blow-by flow is not greater than the second threshold T 2  when purge is disabled. As discussed above the expected blow-by may be stored in a memory of a controller in a look-up table as a function of current engine speed and manifold pressure. 
     As shown prior to time t 1  and between time t 4  and time t 5 , when engine oil temperature is not at steady-state, the controller may disable purge and determine the difference between the output of the intake oxygen sensor and the output of the DPOV sensor or the difference between the output of the intake oxygen sensor and the expected blow-by flow at a first frequency. Then, as shown between time t 3  and time t 4  when the engine oil temperature is at steady-state, the controller may disable purge and determine the difference between the output of the intake oxygen sensor and the output of the DPOV sensor or the difference between the output of the intake oxygen sensor and the expected blow-by flow only once. 
     As shown between time t 1  and time t 2  and after time t 5 , after disabling EGR, the controller may disable purge and determine the difference between the output of the intake oxygen sensor and expected blow-by flow at a second frequency, the second frequency higher than the first frequency. As discussed above, the DPOV sensor is positioned in a low-pressure EGR passage and the intake oxygen sensor is positioned in an intake passage downstream from a PCV passage inlet during boosted conditions and downstream from an inlet of the low-pressure EGR passage. 
     In this way, when hydrocarbons from PCV flow are impacting the output of an intake oxygen sensor, an engine controller may temporarily disable EGR flow. Then, when the impact of PCV flow hydrocarbons reduces below a threshold, the controller may re-enable EGR flow. The controller may then estimate EGR flow based on the output of the intake oxygen sensor. In one example, the output of the intake oxygen sensor may be adjusted based on an estimated fuel concentration in engine oil. As such, a technical effect is achieved by either adjusting an intake oxygen sensor output to compensate for PCV hydrocarbons or temporarily disabling EGR flow when the effect of PCV hydrocarbons on the intake oxygen sensor is above a threshold. In this way, EGR flow adjustments may only be made when the EGR flow is estimated based on an intake oxygen sensor output reflective of a decrease in intake oxygen due to EGR only and not due to PCV flow. As a result, EGR system control may increase and engine emissions may be maintained at desired levels. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The control methods and routines disclosed herein may be stored as executable instructions in non-transitory memory. 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-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.