Method and system for humidity sensor diagnostics

Methods and systems are provided for an engine including a humidity sensor. Degradation of the humidity sensor may be determined based on a change in intake air relative humidity as compared to a change in intake air temperature or pressure, under selected conditions. An amount of exhaust gas recirculated to an engine intake is adjusted differently based on whether the humidity sensor is degraded or functional.

FIELD

The present application relates to diagnostics for a humidity sensor in a vehicle engine system.

BACKGROUND AND SUMMARY

Engine systems may be configured with exhaust gas recirculation (EGR) systems via which at least a portion of the exhaust gas is recirculated to the engine intake. Various sensors may be coupled in the engine system to estimate the amount of EGR being delivered to the engine. These may include, for example, various temperature, pressure, oxygen, and humidity sensors. Since the accuracy of the EGR estimation relies on the correct functioning of the various sensors, periodic sensor diagnostics are used.

One example approach for diagnosing a humidity sensor is illustrated by Xiao et al. in U.S. Pat. No. 7,715,976. Therein, humidity sensor degradation is determined based on a comparison of an intake humidity estimated by a first humidity sensor in the intake manifold with an exhaust humidity estimated by a second humidity sensor in the exhaust manifold and an ambient humidity estimated by a third humidity sensor located outside of the engine. The sensor readings are compared during conditions when all the sensor readings are expected to be substantially equal, such as during engine non-fueling conditions in which the EGR valve is closed. If the readings of the three humidity sensors differ by more than a threshold, humidity sensor degradation may be determined. For example, if the ambient humidity and the exhaust humidity are substantially equal, and the intake humidity differs by greater than a threshold amount from them, degradation of the intake humidity sensor may be determined.

However, the inventors herein have identified a potential issue with such an approach. The accuracy of determining degradation of any one humidity sensor may depend on the proper functioning of the other humidity sensors. Further, multiple humidity sensors may not be needed for engine control. For example, the inventors herein have recognized that even in a dual intake path system, it may be possible to effectively operate the engine with reduced emissions using asymmetric humidity sensing.

Thus in one example, the above issue may be at least partly addressed by a method of operating an engine including a humidity sensor positioned downstream of an EGR throttle valve. In one embodiment, the method comprises, closing the EGR throttle valve, and indicating humidity sensor degradation based on each of a change in intake air relative humidity and pressure responsive to the EGR throttle valve closing.

For example, during selected engine operating conditions, an EGR throttle valve may be temporarily closed while torque disturbances are compensated for by corresponding adjustments to a downstream air intake throttle valve. Since ambient air entering the engine intake is at atmospheric pressure, the air pressure downstream of the EGR throttle valve decreases as the EGR throttle valve closes. Thus, by changing the position of the EGR throttle valve, air pressure experienced at a humidity sensor, positioned downstream of the EGR throttle valve, may be changed. Since relative humidity is a measure of the percentage of water vapor per area at a specific pressure, the relative humidity is expected to change in accordance with the change in air pressure. An engine controller may compare a change in the relative humidity, as estimated by the humidity sensor, with the change in the intake air pressure, as estimated by a pressure sensor positioned in the intake manifold, downstream of the EGR throttle valve. If the change in relative humidity is not proportional to the change in air pressure (as determined by a difference or ratio of the change in humidity to the change in pressure), sensor degradation may be determined and a corresponding diagnostic code may be set. In other words, by utilizing the pressure effect generated on the humidity sensor, it is possible to correlate proper humidity sensor operation with the change in pressure. Further, by using the EGR throttle to generate the pressure change, it is possible to still maintain engine torque by a corresponding adjustment to the air intake throttle.

DETAILED DESCRIPTION

The following description relates to systems and methods for diagnosing a humidity sensor coupled in an engine system (FIGS. 1-2). Based on an intake air relative humidity, as determined by the humidity sensor, an EGR flow recirculated to the engine intake may be adjusted (FIG. 3). Further, the humidity sensor may be periodically diagnosed. Specifically, during selected conditions, the relative humidity output by the humidity sensor may be compared to an intake air pressure or temperature. Based on correlations between changes in the estimated relative humidity and changes in the estimated intake air temperature or pressure, humidity sensor degradation may be indicated. In one example, an engine controller may be configured to perform a diagnostic routine, such as depicted inFIG. 4, to identify humidity sensor degradation based on a change in relative humidity responsive to a change in intake air pressure generated by a temporary closing of an EGR throttle valve. In another example, the controller may perform a diagnostic routine, as depicted inFIG. 5, to identify humidity sensor degradation based on a change in humidity responsive to a change in intake air temperature over a duration since an engine cold-start. Example maps that may be used to identify humidity sensor degradation are illustrated inFIGS. 6-7. In this way, humidity sensor degradation may be diagnosed without relying on additional humidity sensors.

FIG. 1shows a schematic depiction of an example turbocharged engine system100including a multi-cylinder internal combustion engine10and twin turbochargers120and130. As one non-limiting example, engine system100can be included as part of a propulsion system for a passenger vehicle. Engine system100can receive intake air via intake passage140. Intake passage140can include an air filter156and an EGR throttle valve230. Engine system100may be a split-engine system wherein intake passage140is branched downstream of EGR throttle valve230into first and second parallel intake passages, each including a turbocharger compressor. Specifically, at least a portion of intake air is directed to compressor122of turbocharger120via a first parallel intake passage142and at least another portion of the intake air is directed to compressor132of turbocharger130via a second parallel intake passage144of the intake passage140.

The first portion of the total intake air that is compressed by compressor122may be supplied to intake manifold160via first parallel branched intake passage146. In this way, intake passages142and146form a first parallel branch of the engine's air intake system. Similarly, a second portion of the total intake air can be compressed via compressor132where it may be supplied to intake manifold160via second parallel branched intake passage148. Thus, intake passages144and148form a second parallel branch of the engine's air intake system. As shown inFIG. 1, intake air from intake passages146and148can be recombined via a common intake passage149before reaching intake manifold160, where the intake air may be provided to the engine.

A first EGR throttle valve230may be positioned in the engine intake upstream of the first and second parallel intake passages142and144, while a second air intake throttle valve158may be positioned in the engine intake downstream of the first and second parallel intake passages142and144, and downstream of the first and second parallel branched intake passages146and148, for example, in common intake passage149.

In some examples, intake manifold160may include an intake manifold pressure sensor182for estimating a manifold pressure (MAP) and/or an intake manifold temperature sensor183for estimating a manifold air temperature (MCT), each communicating with controller12. Intake passage149can include an air cooler154and/or a throttle (such as second throttle valve158). The position of throttle valve158can be adjusted by the control system via a throttle actuator (not shown) communicatively coupled to controller12. An anti-surge valve152may be provided to selectively bypass the compressor stages of turbochargers120and130via bypass passage150. As one example, anti-surge valve152can open to enable flow through bypass passage150when the intake air pressure upstream of the compressors attains a threshold value.

Engine10may include a plurality of cylinders14. In the depicted example, engine10includes six cylinders arrange in a V-configuration. Specifically, the six cylinders are arranged on two banks13and15, with each bank including three cylinders. In alternate examples, engine10can include two or more cylinders such as 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 cylinder14may be configured with a fuel injector166. In the depicted example, fuel injector166is a direct in-cylinder injector. However, in other examples, fuel injector166can be configured as a port based fuel injector. Further details of a single cylinder14are described below inFIG. 2.

Intake air supplied to each cylinder14(herein, also referred to as combustion chamber14) via common intake passage149may be used for fuel combustion and products of combustion may then be exhausted from via bank-specific parallel exhaust passages. In the depicted example, a first bank13of cylinders of engine10can exhaust products of combustion via a first parallel exhaust passage17and a second bank15of cylinders can exhaust products of combustion via a second parallel exhaust passage19. Each of the first and second parallel exhaust passages17and19may further include a turbocharger turbine. Specifically, products of combustion that are exhausted via exhaust passage17can be directed through exhaust turbine124of turbocharger120, which in turn can provide mechanical work to compressor122via shaft126in order to provide compression to the intake air. Alternatively, some or all of the exhaust gases flowing through exhaust passage17can bypass turbine124via turbine bypass passage123as controlled by wastegate128. Similarly, products of combustion that are exhausted via exhaust passage19can be directed through exhaust turbine134of turbocharger130, which in turn can provide mechanical work to compressor132via shaft136in order to provide compression to intake air flowing through the second branch of the engine's intake system. Alternatively, some or all of the exhaust gas flowing through exhaust passage19can bypass turbine134via turbine bypass passage133as controlled by wastegate138.

In some examples, exhaust turbines124and134may be configured as variable geometry turbines, wherein controller12may 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 turbines124and134may be configured as variable nozzle turbines, wherein controller12may 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 turbines124and134via respective actuators.

Exhaust gases in first parallel exhaust passage17may be directed to the atmosphere via branched parallel exhaust passage170while exhaust gases in second parallel exhaust passage19may be directed to the atmosphere via branched parallel exhaust passage180. Exhaust passages170and180may include one or more exhaust after-treatment devices, such as a catalyst, and one or more exhaust gas sensors, as further elaborated inFIG. 2.

Engine10may further include one or more exhaust gas recirculation (EGR) passages, or loops, for recirculating at least a portion of exhaust gas from first and second parallel exhaust passages17and19and/or first and second parallel branched exhaust passages170and180, to first and second parallel intake passages142and144, and/or parallel branched intake passages146and148. These may include high-pressure EGR loops for proving high-pressure EGR (HP-EGR) and low-pressure EGR-loops for providing low-pressure EGR (LP-EGR). In one example, HP-EGR may be provided in the absence of boost provided by turbochargers120,130, 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, engine10may include a first low-pressure EGR loop202for recirculating at least some exhaust gas from the first branched parallel exhaust passage170, downstream of the turbine124, to the first parallel intake passage142, upstream of the compressor122. Likewise, the engine may include a second low-pressure EGR loop212for recirculating at least some exhaust gas from the second branched parallel exhaust passage180, downstream of the turbine134, to the second parallel intake passage144, upstream of the compressor132. First and second LP-EGR loops202and212may include respective LP-EGR valves204and214for controlling an EGR flow (i.e., an amount of exhaust gas recirculated) through the loops, as well as respective charge air coolers206and216for lowering a temperature of exhaust gas flowing through the respective EGR loops before recirculation into the engine intake. Under certain conditions, the charge air coolers206,216may also be used to heat the exhaust gas flowing through LP-EGR loops202,212before the exhaust gas enters the compressor to avoid water droplets impinging on the compressors.

Engine10may further include a first high-pressure EGR loop208for recirculating at least some exhaust gas from the first parallel exhaust passage17, upstream of the turbine124, to the first branched parallel intake passage146, downstream of the compressor122. Likewise, the engine may include a second high-pressure EGR loop218for recirculating at least some exhaust gas from the second parallel exhaust passage18, upstream of the turbine134, to the second branched parallel intake passage148, downstream of the compressor132. EGR flow through HP-EGR loops208and218may be controlled via respective HP-EGR valves210and220.

Humidity sensor232and pressure sensor234may be included in only one of the parallel intake passages (herein, depicted in the first parallel intake air passage142but not in the second parallel intake passage144), downstream of EGR throttle valve230. Humidity sensor232may be configured to estimate a relative humidity of the intake air. Pressure sensor234may be configured to estimate a pressure of the intake air. In some embodiments, a temperature sensor may also be included in the same parallel intake passage, downstream of the EGR throttle valve230.

As elaborated inFIGS. 3-5, an engine controller may determine whether the humidity sensor is functional or degraded based on correlations between a relative humidity output by the humidity sensor as an intake pressure or intake temperature changes. If the humidity sensor is functioning, an amount of exhaust gas recirculated to the engine intake through the HP-EGR and/or LP-EGR loops may be adjusted based the output of the humidity sensor. For example, an EGR equivalent of the relative humidity sensed by the humidity sensor may be determined, and a position of LP-EGR valves204and214and/or HP-EGR valves210and220may be accordingly adjusted to provide the desired LP-EGR and/or HP-EGR, respectively. In comparison, if the humidity sensor is degraded, the engine controller may assume a maximum humidity condition (based on engine operating conditions), calculate the equivalent EGR, and accordingly adjust the LP-EGR and/or HP-EGR valves. By adjusting an EGR flow to both intake branches of the split engine system based on the output of a single humidity sensor in only one of the intake branches, the number of sensors required for engine EGR control can be reduced without compromising the accuracy of EGR control. By not requiring dedicated humidity sensors in each intake passage branch, (although additional humidity sensors may be provided in alternate embodiments, if desired) components reduction benefits are achieved.

The position of intake and exhaust valves of each cylinder14may be regulated via hydraulically actuated lifters coupled to valve pushrods, or via a cam profile switching mechanism in which cam lobes are used. In this example, at least the intake valves of each cylinder14may be controlled by cam actuation using a cam actuation system. Specifically, the intake valve cam actuation system25may include one or more cams and may utilize variable cam timing or lift for intake and/or exhaust valves. In alternative embodiments, the intake valves may be controlled by electric valve actuation. Similarly, the exhaust valves may be controlled by cam actuation systems or electric valve actuation.

Engine system100may be controlled at least partially by a control system15including controller12and by input from a vehicle operator via an input device (not shown). Control system15is shown receiving information from a plurality of sensors16(various examples of which are described herein) and sending control signals to a plurality of actuators81. As one example, sensors16may include humidity sensor232, intake air pressure sensor234, MAP sensor182, and MAT sensor183. In some examples, common intake passage149may include a throttle inlet pressure (TIP) sensor for estimating a throttle inlet pressure (TIP) and/or a throttle inlet temperature sensor for estimating a throttle air temperature (TCT). In other examples, one or more of the EGR passages may include pressure, temperature, and air-to-fuel ratio sensors, for determining EGR flow characteristics. Additional system sensors and actuators are elaborated below with reference toFIG. 2. As another example, actuators81may include fuel injector166, HP-EGR valves210and220, LP-EGR valves204and214, throttle valves158and230, and wastegates128,138. Other actuators, such as a variety of additional valves and throttles, may be coupled to various locations in engine system100. Controller12may 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 toFIGS. 3-5.

FIG. 2depicts an example embodiment of a cylinder or combustion chamber of internal combustion engine10. Engine10may receive control parameters from controller12and input from vehicle operator190via an input device192, such as an accelerator pedal and a pedal position sensor194for generating a proportional pedal position signal PP. Cylinder (herein also “combustion chamber”)14of engine10may include combustion chamber walls236with piston238positioned therein. Piston238may be coupled to crankshaft240so that reciprocating motion of the piston is translated into rotational motion of the crankshaft. Crankshaft240may be coupled to at least one drive wheel of the passenger vehicle via a transmission system. Further, a starter motor may be coupled to crankshaft240via a flywheel to enable a starting operation of engine10.

Cylinder14can receive intake air via a series of intake air passages242,244, and246. Intake air passage246can communicate with other cylinders of engine10in addition to cylinder14. In some embodiments, one or more of the intake passages may include a boosting device such as a turbocharger280. For example,FIG. 2shows engine10configured with a turbocharger including a compressor282arranged between intake passages242and244, and an exhaust turbine284arranged along exhaust passage248. Compressor282may be at least partially powered by exhaust turbine284via a shaft286where the boosting device is configured as a turbocharger. A throttle valve158including a throttle plate164may be provided along an intake passage of the engine for varying the flow rate and/or pressure of intake air provided to the engine cylinders. For example, throttle valve158may be disposed downstream of compressor282as shown inFIG. 2, or alternatively may be provided upstream of the compressor.

Exhaust passage248can receive exhaust gases from other cylinders of engine10in addition to cylinder14. Exhaust gas sensor228is shown coupled to exhaust passage248upstream of emission control device278. Sensor228may be selected from among various suitable sensors for providing an indication of exhaust gas air/fuel ratio such as a linear oxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), a two-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), a NOx, HC, or CO sensor, for example. Emission control device278may be a three way catalyst (TWC), NOx trap, various other emission control devices, or combinations thereof.

Exhaust temperature may be estimated by one or more temperature sensors (not shown) located in exhaust passage248. Alternatively, exhaust temperature may be inferred based on engine operating conditions such as speed, load, air-fuel ratio (AFR), spark retard, etc. Further, exhaust temperature may be computed by one or more exhaust gas sensors228. It may be appreciated that the exhaust gas temperature may alternatively be estimated by any combination of temperature estimation methods listed herein.

Each cylinder of engine10may include one or more intake valves and one or more exhaust valves. For example, cylinder14is shown including at least one intake poppet valve250and at least one exhaust poppet valve256located at an upper region of cylinder14. In some embodiments, each cylinder of engine10, including cylinder14, may include at least two intake poppet valves and at least two exhaust poppet valves located at an upper region of the cylinder. The valves of cylinder14may be deactivated via hydraulically actuated lifters coupled to valve pushrods, or via a cam profile switching mechanism in which a cam lobe with no lift is used for deactivated valves. In this example, deactivation of intake valve250and exhaust valve256may be controlled by cam actuation via respective cam actuation systems251and253. Cam actuation systems251and253may each include one or more cams and may utilize one or more of cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT) and/or variable valve lift (VVL) systems that may be operated by controller12to vary valve operation. In alternative embodiments, the intake and/or exhaust valve may be controlled by electric valve actuation. In one example, cylinder14may include an intake valve controlled via cam actuation including VCT systems and an exhaust valve controlled via electric valve actuation.

In some embodiments, each cylinder of engine10may include a spark plug292for initiating combustion. Ignition system290can provide an ignition spark to combustion chamber14via spark plug292in response to spark advance signal SA from controller12, under select operating modes. However, in some embodiments, spark plug292may be omitted, such as where engine10may initiate combustion by auto-ignition or by injection of fuel, as may be the case with some diesel engines.

In some embodiments, each cylinder of engine10may be configured with one or more fuel injectors for providing fuel thereto. As a non-limiting example, cylinder14is shown including one fuel injector166. Fuel injector166is shown coupled directly to cylinder14for injecting fuel directly therein in proportion to the pulse width of signal FPW-1received from controller12via electronic driver168. In this manner, fuel injector166provides what is known as direct injection (hereafter also referred to as “DI”) of fuel into combustion cylinder14. Alternatively, the injector may be located overhead and near the intake valve to improve mixing. Fuel may be delivered to fuel injector166from a high pressure fuel system8including fuel tanks, fuel pumps, and a fuel rail. Alternatively, fuel may be delivered by a single stage fuel pump at lower pressure, in which case the timing of the direct fuel injection may be more limited during the compression stroke than if a high pressure fuel system is used. Further, while not shown, the fuel tanks may have a pressure transducer providing a signal to controller12. It will be appreciated that, in an alternate embodiment, injector166may be a port injector providing fuel into the intake port upstream of cylinder14.

Controller12is shown inFIG. 2as a microcomputer, including microprocessor unit106, input/output ports108, an electronic storage medium for executable programs and calibration values shown as read only memory (ROM) chip110in this particular example, random access memory (RAM)112, keep alive memory (KAM)114, and a data bus. Storage medium read-only memory110can be programmed with computer readable data representing instructions executable by processor102for performing the methods described below as well as other variants that are anticipated but not specifically listed. Controller12may receive various signals from sensors coupled to engine10, in addition to those signals previously discussed, including measurement of inducted mass air flow (MAF) from mass air flow sensor231; engine coolant temperature (ECT) from temperature sensor116coupled to cooling sleeve118; a profile ignition pickup signal (PIP) from Hall effect sensor260(or other type) coupled to crankshaft240; throttle position (TP) from a throttle position sensor; and absolute manifold air pressure signal (MAP) from sensor182. Engine speed signal, RPM, may be generated by controller12from signal PIP. Further, crankshaft position, as well as crankshaft acceleration, and crankshaft oscillations may also be identified based on the signal PIP. Manifold air pressure signal MAP from manifold pressure sensor182may be used to provide an indication of vacuum, or pressure, in the intake manifold. Further, as noted herein, manifold pressure may be estimated based on other operating parameters, such as based on MAF and RPM, for example.

Engine10further includes a humidity sensor232. The humidity sensor may detect a water vapor concentration of air entering the intake manifold via intake passage242. As previously elaborated, humidity sensor232may be positioned downstream of an EGR throttle valve (230,FIG. 1) but upstream of the intake throttle valve158. A relative humidity reading generated by the humidity sensor is indicative of the humidity of fresh air or a combination of fresh air and recirculated exhaust air, based on the position of EGR throttle valve230and the LP-EGR and HP-EGR valves (ofFIG. 1).

As described above,FIG. 2shows only one cylinder of a multi-cylinder engine, and that each cylinder may similarly include its own set of intake/exhaust valves, fuel injector, spark plug, etc.

FIG. 3illustrates an example routine300for adjusting one or more EGR valves (such as, an EGR throttle valve, LP-EGR valves and/or HP-EGR valves) to provide a desired amount of exhaust gas recirculation (EGR) based on a relative humidity of the intake air. The routine determines an EGR amount that is equivalent to the relative humidity, as estimated by a humidity sensor positioned in the intake passage, downstream of the EGR throttle valve. Specifically, using the water vapor concentration of the intake air, an accurate indication of the equivalent EGR amount can be generated by utilizing a mass balance formula. Based on the determined humidity equivalent EGR amount, a position of one or more EGR valve is adjusted to provide the desired EGR flow.

At302, the routine includes estimating and/or measuring engine operating conditions. These may include, for example, ignition spark timing, air-fuel ratio, engine speed, torque demand, catalyst temperature, fuel type, etc. At304, a desired EGR amount may be determined based on the estimated engine operating conditions. This may include determining an amount, flow, and temperature of exhaust gas to be recirculated to an engine intake (for example, from parallel exhaust passages to respective parallel intake passages in a split engine system). This may further include determining whether the desired amount of EGR is to be provided as LP-EGR flow, HP-EGR flow, or a combination thereof.

At306, it may be determined whether the humidity sensor is functional. As such, the humidity sensor may be periodically diagnosed using diagnostic routines, such as those elaborated with reference toFIGS. 4-5. If the humidity sensor is functional, then at308, the humidity sensor output may be received. As such, since the humidity sensor is positioned upstream of the point where exhaust gas enters the EGR system, the humidity reading of the humidity sensor is indicative of the water vapor concentration of the (fresh) intake air. At310, a mass balance formula (e.g., conservation of mass) may be applied to the received humidity data to determine the humidity equivalent EGR amount of the intake air, and accordingly determine an exhaust gas fraction to be delivered. In one example, based on the mass balance formula, and further based on the ratio of the specific heats of water and EGR, 1% water by mass may be determined to be equivalent to 1.7% EGR.

At312, upon confirming that the humidity sensor is not degraded, an amount of exhaust gas recirculated from the engine exhaust to the engine intake may be adjusted based on the output of the humidity sensor. Specifically, the position of one or more EGR valves may be adjusted to provide the desired EGR amount based on the humidity equivalent EGR amount calculated from the output of the humidity sensor. The one or more EGR valves that are adjusted may include one or more of the EGR throttle valve, the LP-EGR valves (for adjusting an amount of LP-EGR provided), and the HP-EGR valves (for adjusting an amount of HP-EGR provided). Specifically, the position of the one or more EGR valves may be adjusted to provide the difference in EGR amount (e.g., using exhaust gas and/or intake air). In one example, the relative humidity may be 40%. The engine may be calibrated at the specified humidity and the amount of scheduled EGR may be increased or decreased based on the amount of water over or under the base water concentration at the specified humidity level (i.e. 40%).

In comparison, at320, in response to the indication of humidity sensor degradation (received at306), an EGR flow to the engine may be adjusted based on a maximum humidity assumption. That is, a maximum relative humidity may be determined based on the engine operating conditions (e.g., based on ambient temperature and pressure conditions) and the EGR equivalent of the maximum assumed humidity may be determined. Accordingly, at322, the position of the one or more EGR valve may be adjusted to provide the difference of in EGR amount.

As such, the estimated intake air relative humidity is also indicative of the likelihood of condensation at the inlet and outlet of the turbocharger compressor, as well as the charge air cooler outlet and manifold. Thus, if the humidity sensor degrades, the desired EGR may set to a value such that condensation does not occur. By adjusting the delivered EGR based on the assumption of maximum (e.g., 100%) relative humidity, condensation in the engine system (in particular, at the compressor and in the EGR loops) can be reduced.

In one example, the humidity sensor may be included in a split engine system having first and second parallel intake passages, each intake passage coupled to a distinct group of cylinders. The humidity sensor may be positioned in either the first or the second intake passage. Herein, EGR flow to both intake passages (and consequently to different groups of cylinders) may be adjusted based on the output of a single humidity sensor. By reducing the number of humidity sensors required to control the engine, without compromising the accuracy of EGR determination and flow control, component reduction benefits can be achieved in the engine system.

Now turning toFIG. 4, an example diagnostics routine400is described for diagnosing the humidity sensor based on an intake air pressure.

At402, engine operating conditions may be estimated and/or measured. These may include, for example, intake air pressure, temperature, humidity, engine speed, desired torque, etc. At404, a first EGR throttle valve may be closed while a second air intake throttle valve is concurrently opened. That is, the EGR throttle valve is temporarily closed while torque disturbances are transiently compensated for by opening the air intake throttle valve. As such, the EGR throttle valve may be fully closed or partially closed. In one example, the EGR throttle valve may be closed for a duration based on the engine operating conditions. In another example, the EGR throttle valve may be closed by adjusting a duty cycle of the valve, the duty cycle adjusted based on the engine operating conditions.

At406, each of a change in the intake air pressure and a change in the intake air relative humidity, resulting from the closing of the EGR throttle valve, may be determined. The change in intake air relative humidity may be based on the output of the humidity sensor positioned downstream of the EGR throttle valve, while the change in intake air pressure may be based on the output of a pressure sensor also coupled downstream of the EGR throttle valve in the intake passage. In one example, an initial humidity and pressure may be estimated when the throttle valve is closed and a final humidity and pressure may be estimated when the throttle valve is opened again, and a change in humidity and pressure accordingly calculated.

At408, a comparison of the resulting change in relative humidity (ΔH) and the resulting change in intake air pressure (ΔP), responsive to the EGR throttle valve closing, may be performed. In one example, the comparison may include determining a ratio of the change in humidity to the change in pressure. In another example, the comparison may include determining a difference (e.g., absolute difference) in the change in humidity and the change in pressure.

If a ratio of the change in pressure to the change in humidity is higher than a threshold, then at410, it may be determined that the humidity sensor is functional and that the output of the humidity sensor is reliable. In comparison, if the ratio is lower than a threshold, then at412, humidity sensor degradation may be determined. Accordingly, at414, a diagnostic code may be set. Further, as previously elaborated inFIG. 3(at320-322), in the absence of a reliable humidity sensor output, an EGR flow may be adjusted based on the assumption of a maximum humidity condition, to reduce condensation in the engine system. In an alternate example, humidity sensor degradation may be indicated in response to a difference (e.g., absolute difference) between the change in pressure and the change in humidity being higher than a threshold. In this way, by correlating expected changes in humidity with changes in pressure, humidity sensor degradation may be accurately determined without requiring additional humidity sensors.

An example of a pressure-based humidity sensor diagnostics is shown in the example map ofFIG. 6. Map600depicts an EGR throttle valve position at606, changes in intake air pressure at602, and corresponding changes in intake air humidity at604.

At t1, the EGR throttle valve may be closed for a duration lasting up to t2. As such, both the pressure sensor and the humidity sensor are positioned downstream of an EGR throttle valve in an intake air passage. Thus, in response to EGR throttle valve closing, the pressure output from the pressure sensor may start to decrease. Since a relative humidity of the intake air is based on the pressure of the intake air, the drop in pressure is expected to cause a proportional decrease in the relative humidity output by the humidity sensor. As depicted, between t1and t2, the change in relative humidity (ΔH) resulting from the valve closure, as estimated by the humidity sensor, may be proportional to the change in pressure (ΔP) resulting from the valve closure, as estimated by the pressure sensor, indicating that the humidity sensor is functional.

In response to the throttle valve being subsequently opened at t2, the intake air pressure may start to increase, and correspondingly the humidity may also increase, as expected. At an alternate time t3, the EGR throttle valve may be closed for a duration lasting up to t4. Herein, in response to the EGR throttle valve closing, the intake pressure starts to decrease, however, there is no substantial change in the estimated humidity. Thus, in response to the humidity change estimated by the humidity sensor being disproportionate to the pressure change estimated by the pressure sensor, upon throttle valve closing, an engine controller may determine that the humidity sensor is degraded and set a diagnostic code at t4.

Now turning toFIG. 5, an example diagnostics routine500is described for diagnosing the humidity sensor based on intake air temperature.

At502, it may be confirmed that the engine is in a cold-start condition. As such, a cold-start condition may be confirmed if an exhaust catalyst temperature is below a light-off temperature and/or if the engine has not been started for a threshold duration. If an engine cold-start is not confirmed, the routine may end. At504, engine operating conditions may be estimated and/or measured. These may include, for example, intake air pressure, temperature, humidity, engine speed, desired torque, etc.

At506, an intake aircharge temperature may be monitored over a specified duration since the engine cold-start. The intake aircharge temperature may be estimated by a temperature sensor positioned in the engine intake, downstream of the first EGR throttle valve. At508, the intake air humidity may be monitored over the same duration. The intake air relative humidity may be estimated by the humidity sensor positioned in the engine intake, downstream of the first EGR throttle valve. As such, the duration over which the temperature and humidity are monitored may be adjusted based on engine operating conditions. For example, intake air temperature and humidity may continue to be monitored until an exhaust catalyst temperature stabilizes and/or reaches a light-off temperature (e.g., 180° C.).

At510, a change in the intake air temperature (ΔT) and a change in the intake air humidity (ΔH), over the specified duration, may be determined. For example, an initial humidity and temperature may be estimated at the start of the duration, and a final humidity and temperature may be estimated at the end of the duration. At512, the change in temperature (ΔT) may be compared to the change in humidity (ΔH). In one example, the comparison may include determining a ratio of the change in humidity to the change in temperature. In another example, the comparison may include determining a difference (e.g., absolute difference) in the change in humidity and the change in temperature.

If a ratio of the change in temperature to the change in humidity is higher than a threshold, then at514, it may be determined that the humidity sensor is functional and that the output of the humidity sensor is reliable. In comparison, if the ratio is lower than a threshold, then at516, humidity sensor degradation may be determined. Accordingly, at518, a diagnostic code may be set. Further, as previously elaborated inFIG. 3(at320-322), in the absence of a reliable humidity sensor output, an EGR flow may be adjusted based on the assumption of a maximum humidity condition, to reduce condensation in the engine system. In an alternate example, humidity sensor degradation may be indicated in response to a difference (e.g., absolute difference) between the change in temperature and the change in humidity being higher than a threshold. In this way, by correlating expected changes in humidity with changes in temperature, humidity sensor degradation may be accurately determined without requiring additional humidity sensors.

An example of a temperature-based humidity sensor diagnostics is shown in the example map ofFIG. 7. Map700depicts an exhaust catalyst temperature at706, changes in intake air temperature at702, and corresponding changes in intake air humidity at704.

At t0, an engine cold-start may be confirmed. For a duration since the cold-start, specifically between t0and t5, an intake air temperature (ACT) and humidity may be monitored and compared. As such, the duration may be a duration over which the exhaust catalyst temperature increases to stabilize at, or beyond, a light-off temperature. Both the intake air temperature sensor and humidity sensor may be positioned in an intake air passage, downstream of an EGR throttle valve. Thus, as the engine starts to warm over the duration since the cold-start, the intake air temperature increases. Since relative humidity is based on temperature, the rise in temperature is expected to cause the relative humidity output by the humidity sensor to decrease. Between t0and t5, a change in the air temperature (ΔT) relative to the change in humidity (ΔH) may be determined to be proportional, indicating that the humidity sensor is functional. In an alternate example, where the change in humidity is not proportional to the change temperature, humidity sensor degradation may be indicated responsive to the humidity change estimated by the humidity sensor being disproportionate to the temperature change estimated by the temperature sensor over the duration since the engine cold-start.

While the depicted diagnostic routines illustrate indicating humidity sensor degradation based on either a pressure or a temperature effect on relative humidity, it will be appreciated that in still other examples, humidity sensor degradation may be indicated based on each of a pressure and a temperature effect on relative humidity. For example, a change in intake air relative humidity output by the humidity sensor may be compared to each of an intake air temperature change output by the temperature sensor and an intake air pressure change output by the pressure sensor. In one approach, if the change in humidity does not correspond to both the change in temperature and the change in pressure, humidity sensor degradation may be indicated.

In this way, by correlating changes in intake air temperature and/or pressure with changes in intake air humidity, degradation of a humidity sensor may be identified without relying on additional humidity sensors and while using existing temperature and pressure sensors. By adjusting EGR flow to an engine based on the output of a single humidity sensor, EGR adjustments based on humidity may be accurately provided while achieving component reduction benefits.