Methods and systems for operating an oxygen sensor with variable voltage

Abstract

Engine process, which includes the following: Gradual increase of a reference voltage of an oxygen sensor (126) from a first voltage to a second voltage with a rate of increase, wherein the rate of increase is based on engine operating conditions.

Description

Field of invention

[0001] The present description generally refers to methods and systems for adjusting the operation of a variable voltage oxygen sensor of an internal combustion engine in order to reduce sensor deterioration. Background / Summary

[0002] Intake and / or exhaust sensors can be used to provide measurements of various exhaust gas components. For example, US 2012 / 0037134A1 describes the detection of engine intake dilution using an intake oxygen sensor. In alternative approaches, engine dilution can be estimated by an exhaust oxygen sensor. The estimated engine dilution can be used to adjust various engine operating parameters, such as fuel delivery and the air-fuel ratio. As another example, US 5145566A describes the detection of water content in exhaust gas using an exhaust oxygen sensor. Estimating water content using an intake or exhaust oxygen sensor can be used to infer ambient humidity during engine operation and / or the alcohol content of fuel burned in the engine.

[0003] German patent application DE 36 06 045 A1 is known from the prior art. This relates to an air-fuel ratio sensor, whereby the ratio sensor can detect a performance impairment of measuring elements and correct its output signal accordingly. Furthermore, a change in a reference voltage at an oxygen sensor is described, wherein the change depends on operating conditions.

[0004] DE 10 2014 218 971 A1 relates to a method and a system for estimating a crankcase ventilation flow to a power engine based on the output of an exhaust gas oxygen sensor. Furthermore, a modulation of the reference voltage of the exhaust gas oxygen sensor is described.

[0005] In some examples, the oxygen sensor can be a variable voltage (VVs) oxygen sensor. The reference voltage of the VVs oxygen sensor can be adjustable between a lower base voltage, at which water does not dissociate, and a higher target voltage, at which water dissociates. The outputs of the oxygen sensor at these two reference voltages can then be used to determine the water content of the engine's intake air or exhaust air.

[0006] However, the inventors have identified potential problems with repeatedly changing the reference voltages of the VVs oxygen sensor directly from the base voltage to the target voltage and back again. For example, repeated transitions between reference voltages can lead to sensor blackening and / or pump current oscillation. Specifically, transitions in a single step between a lower and a higher reference voltage can result in significant overshoots of both the cell pump voltage and the pump current of the VVs sensor. This can lead to sensor degradation if repeated over thousands of cycles. Long settling times caused by oscillation can impair the oxygen sensor's ability to return to its base reference voltage, thereby increasing the time spent in fuel-idle operation.

[0007] In one example, the problems described above can be addressed by a method for gradually increasing the reference voltage of an oxygen sensor from a first voltage to a second voltage at a rate determined by the engine operating conditions. This allows the reference voltage of the oxygen sensor to be changed more gradually, thereby reducing stress on the sensor and improving its lifespan.

[0008] For example, increasing the oxygen sensor's reference voltage from the first voltage to the second voltage can involve the incremental transition from the first to the second voltage in a series of steps, rather than a single step. The rate of increase can be based on engine operating conditions, such as ambient humidity and engine fuel supply. For instance, sensor overshoot, undershoot, and settling may increase under low humidity conditions and / or when primarily ambient air flows over the sensor (such as during a fuel cut-off event upon deceleration). Thus, by decreasing the rate of increase under low humidity conditions and / or reduced engine fuel supply, sensor blackening and aftershoot due to overshoot, undershoot, and settling times can be reduced, thereby minimizing oxygen sensor degradation.

[0009] It is understood that the above summary is provided to introduce, in simplified form, a selection of concepts that are further described in the detailed description. It is not intended to identify essential or key features of the claimed subject matter, the scope of which is defined solely by the claims following the detailed description. Furthermore, the claimed subject matter is not limited to implementations that remedy any disadvantages mentioned above or in any part of this disclosure. Brief description of the drawings Fig. Figure 1 shows a schematic representation of an engine with an exhaust oxygen sensor and an intake oxygen sensor. Fig. Figure 2 shows a schematic representation of an example oxygen sensor. Fig. Figure 3 shows two curves of exemplary changes in a reference voltage (Vs) of an oxygen sensor using a single change step and the resulting pump current (Ip) at different vehicle fuel supply conditions. Fig. Figure 4 shows a curve of exemplary changes in the pump current of an oxygen sensor in response to changes in the reference voltage of the oxygen sensor at different rates. Fig. Figure 5 shows an exemplary curve of a change in the reference voltage of an oxygen sensor, including a single change step and a change with a slower increasing rate of change. Fig. Figure 6 shows a flowchart illustrating a procedure for adjusting the operation of an oxygen sensor. Fig. Figure 7 shows a curve illustrating changes in the rate of increase of a reference voltage of an oxygen sensor based on engine operating parameters. Detailed description

[0010] The following detailed description refers to systems and methods for adjusting the rate of change of a reference voltage of an oxygen sensor. A vehicle engine, such as the one in Fig. Figure 1 shows an oxygen sensor located in an intake and / or exhaust port of the engine. The oxygen sensor may be a variable voltage (VV) oxygen sensor, as shown in Figure 1. Fig. Figure 2 shows that the reference voltage of the VVs oxygen sensor is adjustable between a lower base reference voltage and a higher reference voltage. In one example, the transition between a lower and a higher reference voltage value in a single step can lead to high overshoots and / or undershoots of both the cell pump voltage and the pump current of the VVs sensor, as shown in Figure 2. Fig. 3. This can lead to degradation of the oxygen sensor if repeated over several successive cycles. For example, oxygen sensor degradation can be reduced by gradually increasing the oxygen sensor reference voltage from a lower first reference voltage to a higher second reference voltage at a steeper rate or in incremental steps (e.g., increasing). In this way, the overshoots and undershoots of the cell pump voltage and pump current can be reduced, as shown in Fig. Figure 4 shows exemplary methods for gradually changing the reference voltage of an oxygen sensor to reduce deterioration. Fig. Figure 5 shows that, for example, a method may involve increasing the reference voltage of the oxygen sensor from a lower initial voltage to an intermediate voltage in a single change step, and then increasing it from the intermediate voltage to a higher, second voltage at a reduced rate, with the intermediate voltage being between the first and second voltages. The rate of change of the reference voltage may also be set based on engine operating parameters, such as engine fuel supply and / or ambient humidity, as shown in Figure 5. Fig. Figure 7 is shown. In this way, the stress on the oxygen sensor can be reduced, and the lifespan of the oxygen sensor can be increased, thereby increasing engine control based on an oxygen sensor output.

[0011] Now with reference to Fig. Figure 1: A schematic diagram illustrates a cylinder of a multi-cylinder engine 10, which may be included in a propulsion system of a motor vehicle. The engine 10 may be controlled, at least partially, by a control system comprising a controller 12 and by input from a vehicle user 132 via an input device 130. In this example, the input device 130 includes an accelerator pedal and a pedal position sensor 134 for generating a proportional pedal position signal PP. A combustion chamber (i.e., a cylinder) 30 of the engine 10 may contain combustion chamber walls 32 with a piston 36 positioned therein. The piston 36 may be coupled to a crankshaft 40 such that a reciprocating motion of the piston is translated into a rotary motion of the crankshaft. The crankshaft 40 may be coupled to at least one drive wheel of a vehicle via an intervening transmission system.Furthermore, a starter motor can be coupled to the crankshaft 40 via a flywheel to enable starting operation of the engine 10.

[0012] The combustion chamber 30 can draw in intake air from an intake manifold 44 via an intake port 42 and can expel combustion gases via an exhaust port 48. The intake manifold 44 and the exhaust port 48 can be selectively connected to the combustion chamber 30 via the respective intake valve 52 and exhaust valve 54. In some embodiments, the combustion chamber 30 can contain two or more intake valves and / or two or more exhaust valves.

[0013] In this example, the inlet valve 52 and the exhaust valve 54 can be controlled by cam actuation using the corresponding cam actuation systems 51 and 53. The cam actuation systems 51 and 53 can each contain one or more cams and utilize one or more of the following systems: cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT), and / or variable valve lift (VVL), which can be operated by a controller 12 to modify the valve operation. The position of the inlet valve 52 and the position of the exhaust valve 54 can be determined by the position sensors 55 and 57, respectively. In alternative embodiments, the inlet valve 52 and / or the exhaust valve 54 can be controlled by electric valve actuation.For example, cylinder 30 can alternatively include an inlet valve controlled by electric valve actuation and an exhaust valve controlled by cam actuation including CPS and / or VCT systems.

[0014] In some embodiments, each cylinder of the engine 10 can be configured with one or more injectors to supply fuel to it. As a non-limiting example, cylinder 30 is shown with a fuel injector 66. The fuel injector 66 is shown directly coupled to cylinder 30 to inject fuel therein in proportion to the pulse width of the FPW signal received from the controller 12 via the electronic driver stage 68. In this way, the fuel injector 66 provides what is referred to as direct injection of fuel into the combustion cylinder 30 (hereinafter also referred to as "DI").

[0015] It is understood that in an alternative embodiment, the injector 66 can be a channel injector that supplies fuel to the intake port upstream of cylinder 30. It is also understood that cylinder 30 can receive fuel from multiple injectors, such as multiple channel injectors, multiple direct injectors, or a combination thereof.

[0016] A fuel tank in a fuel system 172 can hold fuel of varying qualities, such as different fuel compositions. These differences can include varying alcohol content, octane rating, heat of vaporization, fuel blends, and / or combinations thereof. The engine can use a fuel blend containing alcohol, such as E85 (which consists of approximately 85% ethanol and 15% gasoline) or M85 (which consists of approximately 85% methanol and 15% gasoline). Alternatively, the engine can operate on other ratios of gasoline and ethanol stored in the tank, including 100% gasoline and 100% ethanol, and varying ratios between them, depending on the alcohol content of the fuel added to the tank by the user. Furthermore, the fuel characteristics of the fuel in the tank can change frequently.For example, the driver might fill the fuel tank with E85 one day, E10 the next, and E50 the next. Therefore, the fuel composition in the tank can change dynamically based on the level and composition of the fuel remaining at the time of filling.

[0017] The fluctuations from day to day during refueling can thus lead to frequently changing fuel compositions in the fuel system 172, which affects the fuel composition supplied by the injector 66 and / or the fuel quality. The different fuel compositions injected by the injector 66 can be referred to here as a fuel type. For example, the different fuel compositions can be qualitatively described by their Research Octane Number (RON), alcohol percentage, ethanol percentage, etc.

[0018] While in one embodiment the engine can be operated by injecting the variable fuel mixture via a direct injection nozzle, it is understood that in alternative embodiments the engine can be operated using two injection nozzles and varying the relative injection quantity from each nozzle. If the engine is operated with supercharging from a charging device, such as a turbocharger or a pre-compressor (not shown), it is further understood that the boost limit can be increased by increasing the alcohol content of the variable fuel mixture.

[0019] Continue with Fig. 1: The intake port 42 can contain a throttle 62 with a throttle valve 64. In this particular example, the position of the throttle valve 64 can be modified by the control unit 12 via a signal provided to an electric motor or actuator contained within the throttle 62, a configuration commonly referred to as electronic throttle control (ETC). In this way, the throttle 62 can be operated to modify the intake air supplied to the combustion chamber 30, among other engine cylinders. The position of the throttle valve 64 can be provided to the control unit 12 by a throttle position signal TP. The intake port 42 can contain a mass airflow sensor 120 and a manifold pressure sensor 122 to provide the respective MAF (mass airflow) and MAP (manifold air pressure) signals to the control unit 12.

[0020] An ignition system 88 can provide a spark to the combustion chamber 30 via a spark plug 92 in selected operating modes in response to the ignition advance signal SA (Spark Advance) from the control unit 12. Although spark ignition components are shown, in some embodiments the combustion chamber 30 or one or more other combustion chambers of the engine 10 can be operated in a compression ignition mode with or without a spark.

[0021] An oxygen sensor 126 with variable voltage (VVs) is shown coupled to the exhaust channel 48 and located upstream of an exhaust gas purification device 70. The exhaust gas purification device 70 is shown arranged along the exhaust channel 48 and downstream of the VVs oxygen sensor 126. The device 70 can be a three-way catalyst (TWC), a NOₓ catalyst, or a catalytic converter. x-trap, various other exhaust gas purification devices, or combinations thereof. In some embodiments, the exhaust gas purification device 70 can be periodically reset during the operation of the engine 10 by operating at least one cylinder of the engine in a special air-fuel ratio.

[0022] As in the example in Fig. As shown in Figure 1, the system further includes an intake air sensor 127 coupled to the intake channel 44. The sensor 127 can be a VVs oxygen sensor, but it can also be any sensor suitable for providing an indication of the air-fuel ratio in the exhaust gas, such as a linear oxygen sensor or wideband lambda sensor (UEGO), a two-point oxygen sensor or lambda sensor (EGO), a heated lambda sensor (Heated EGO), or an NO sensor. x -, HC or CO sensor.

[0023] Furthermore, in the disclosed embodiments, an exhaust gas recirculation (EGR) system can direct a desired proportion of exhaust gas from the exhaust port 48 into the intake port 44 via an EGR channel 140. The amount of EGR supplied to the intake port 44 can be varied by the control unit 12 via an EGR valve 142. An EGR sensor 144 can also be arranged in the EGR channel 140 and provide information on one or more of the following values: exhaust gas pressure, temperature, and concentration. Under certain conditions, the EGR system can be used to control the temperature of the air-fuel mixture in the combustion chamber, thereby providing a method for controlling the ignition timing in some combustion modes. Furthermore, under certain conditions, a portion of the combustion gases can be retained or captured in the combustion chamber by controlling the timing of the exhaust valve, for example, by controlling a variable valve timing mechanism.

[0024] Control unit 12 is in Fig. 1 is shown as a microcomputer comprising a microprocessor unit (CPU) 102, input / output ports 104 (I / O), an electronic storage medium for executable programs and calibration values, shown in this particular example as a read-only memory (ROM) chip 106, a random access memory (RAM) 108, a keep-alive memory (KAM) 110 and a data bus.The control unit 12 can receive various signals from sensors coupled to the engine 10, in addition to the signals discussed previously, including induced mass airflow (MAF) measurement from the mass airflow sensor 120; engine coolant temperature (ECT) from a temperature sensor 112 coupled to a cooling jacket 114; a profile ignition pickup (PIP) signal from a Hall sensor 118 (or other type) coupled to the crankshaft 40; throttle position (TP) from a throttle position sensor; and a manifold air pressure (MAP) signal from sensor 122. The engine speed (RPM) signal can be generated by the control unit 12 from the PIP signal.

[0025] The read-only storage medium 106 can be programmed with computer-readable data representing instructions that can be executed by the processor 102 to perform the procedures described below, as well as other variations that are expected but not specifically listed.

[0026] As described above, shows Fig. 1 only one cylinder of a multi-cylinder engine, and each cylinder can equally contain its own set of inlet / exhaust valves, fuel injector, spark plug, etc.

[0027] Next, we will show Fig. Figure 2 shows a schematic view of an embodiment of an oxygen sensor 200 designed to measure the concentration of oxygen (O2) in an inlet air stream in an inlet channel or in an exhaust gas stream in an outlet channel. The sensor 200 can, for example, be a VVs oxygen sensor 126 or 127 made of Fig. 1. The sensor 200 comprises several layers of one or more ceramic materials arranged in a stacked configuration. In the embodiment made of Fig. Figure 2 shows five ceramic layers, designated as layers 201, 202, 203, 204, and 205. These layers contain one or more layers of a solid electrolyte capable of conducting oxygen in ionic form. Examples of suitable solid electrolytes include, but are not limited to, zirconium oxide-based materials. Furthermore, in some embodiments, a heating element 207 may be arranged in thermal contact with the layers to increase their ionic conductivity. Although the oxygen sensor shown is constructed from five ceramic layers, it is understood that the oxygen sensor may contain other suitable numbers of ceramic layers.

[0028] Layer 202 contains a material or materials that create a diffusion path 210. The diffusion path 210 is designed to introduce exhaust gases into a first, inner cavity 222 by diffusion. The diffusion path 210 can be designed to allow one or more components of the intake air or the exhaust gases, including but not limited to a desired analyte (e.g., O2), to diffuse into the inner cavity 222 at a more limited rate than the rate at which the analyte can be pumped in or out by a pump electrode pair 212 and 214. In this way, a stoichiometric O2 level can be achieved in the first, inner cavity 222.

[0029] The sensor 200 further includes a second, inner cavity 224 within layer 204, which is separated from the first, inner cavity 222 by layer 203. The second, inner cavity 224 is designed to maintain a constant oxygen partial pressure equivalent to a stoichiometric state; for example, the oxygen level present in the second, inner cavity 224 is equal to that which the intake air or exhaust gas would have if the air-fuel ratio were stoichiometric. The oxygen concentration in the second, inner cavity 224 is controlled by the pump voltage V. cp kept constant. Here, the second, inner cavity 224 can be referred to as a reference cell.

[0030] A pair of scanning electrodes 216 and 218 is arranged in conjunction with the first, inner cavity 222 and the reference cell 224. The scanning electrode pair 216 and 218 detects a concentration gradient that can develop between the first inner cavity 222 and the reference cell 224 due to an oxygen concentration in the intake air or exhaust gas being higher than or lower than the stoichiometric level. A high oxygen concentration can be caused by a lean intake air or exhaust gas mixture, while a low oxygen concentration can be caused by a rich mixture.

[0031] A pair of pump electrodes 212 and 214 is arranged in conjunction with the inner cavity 222 and is designed to electrochemically pump a selected gas component (e.g., O2) from the inner cavity 222 through the layer 201 and out of the sensor 200. Alternatively, the pair of pump electrodes 212 and 214 can be designed to electrochemically pump a selected gas through the layer 201 and into the inner cavity 222. In this case, the pump electrode pair 212 and 214 can be referred to as an O2 pump cell.

[0032] The electrodes 212, 214, 216, and 218 can be made of various suitable materials. In some embodiments, the electrodes 212, 214, 216, and 218 can consist at least partially of a material that catalyzes the dissociation of molecular oxygen. Examples of such materials include, but are not limited to, electrodes comprising platinum and / or silver.

[0033] The process of electrochemically pumping oxygen from or into the inner cavity 222 includes applying a voltage V p above the pump electrode pair 212 and 214. The pump voltage V applied to the O2 pump cell p It pumps oxygen into or out of the first, inner cavity 222 to maintain a stoichiometric oxygen level in the pump cell cavity. The resulting pump flow I p is proportional to the oxygen concentration in the exhaust gas. A control system (in Fig. 2 (not shown) generates the pump current signal I p as a function of the strength of the applied pump voltage V p, which is required to maintain a stoichiometric level in the first, inner cavity 222. Thus, a lean mixture will cause oxygen to be pumped out of the inner cavity 222, and a rich mixture will cause oxygen to be pumped into the inner cavity 222.

[0034] It is understood that the oxygen sensor described here is merely an exemplary embodiment of an oxygen sensor and that other embodiments of oxygen sensors may have additional and / or alternative features and / or designs.

[0035] In one example, the oxygen sensor can measure 200 (and the one in Fig. The oxygen sensors shown in Figure 1 (126 and 127) are variable-voltage (Vs or VVs) oxygen sensors, in which the sensor's reference voltage is modulated between a lower or base voltage, at which oxygen is detected (and water is not dissociated), and a higher voltage, at which water molecules in the gas stream can dissociate. For example, during base operation, the oxygen sensor can operate at the base reference voltage. At the base reference voltage, if water comes into contact with the sensor, the sensor's heated element can vaporize the water and measure it as local vapor or diluent. The oxygen sensor can also operate in a second mode, in which the reference voltage is increased to a second reference voltage. The second reference voltage can be higher than the base reference voltage.When the inlet oxygen sensor is operated at the second reference voltage, the sensor's heated element dissociates water in the air and subsequently measures the water concentration. Thus, the resulting pump current of the sensor at the second reference voltage can indicate the amount of oxygen in the gas stream plus the amount of oxygen from dissociated water molecules. A change in the pump current between the first and second voltages can then indicate the amount of water in a gas stream in which the oxygen sensor is positioned.

[0036] In this way, the VVs oxygen sensor can be used from Fig. 2. It can be used to estimate the oxygen content of the fresh air drawn in at the intake manifold (if the sensor is positioned in an intake port of the engine) or the oxygen content of the exhaust gas expelled from the engine (if the sensor is positioned in an exhaust port downstream of an engine cylinder). The VVs oxygen sensor can also be used to estimate the amount of alcohol in the fuel burned in the engine and the ambient humidity.

[0037] With reference to Fig. 3: Two curves, 300 and 308, show a changing reference voltage and the resulting pump current over time for a variable-voltage oxygen sensor (e.g., the one in Fig. Figure 1 shows the VVs oxygen sensor 126 or 127). Curve 300 consists of two waveforms 302 and 304, which show the reference voltage and pump current of an oxygen sensor (e.g., the VVs oxygen sensor) over time under typical outlet conditions. Both the reference voltage and the pump current of the oxygen sensor change periodically between a lower first value and a higher second value. The first reference voltage V1 can be such that oxygen is pumped out of the cell, but low enough so that oxygen compounds, such as water, are not dissociated at the sensor (e.g., V1 in one example can be approximately 450 mV). Applying the first reference voltage V1 can generate an output from the sensor in the form of a first pump current I1, which indicates the amount of oxygen in the sample gas.Once the oxygen quantity has been determined, a second pump voltage V2 can be applied to the oxygen pump cell of the oxygen sensor. The second voltage V2 can be higher than the first voltage V1 applied to the sensor. In particular, the second voltage V2 can have a value high enough to dissociate a desired oxygen compound. For example, the second voltage V2 can be high enough to dissociate water molecules into hydrogen and oxygen (e.g., V2 can be approximately 1.1 V in one example). Applying the second voltage V2 can generate a second pump current I2, which indicates the quantity of oxygen and water in the sample gas. It is understood that the term "water" in "quantity of oxygen and water," as used here, refers to the quantity of oxygen from the dissociated water molecules in the sample gas.

[0038] The ambient humidity (e.g., the absolute humidity of the fresh air surrounding the vehicle) can be determined based on the first and second pump streams during a no-fuel-supply condition (e.g., a DFSO event). In this case, the first pump stream can be subtracted from the second to obtain a value indicating the amount of oxygen from dissociated water molecules (e.g., the amount of water) in the sample gas. This value can be proportional to the ambient humidity. Additionally, the amount of alcohol in the fuel, and thus the fuel type, can be identified under normal fuel supply conditions. A normal fuel supply condition can refer to any condition in which fuel is injected into the engine cylinders. In this case, the amount of water in the exhaust gas can be proportional to the amount of alcohol (e.g., a percentage of ethanol) in the fuel injected into the engine.Because ambient humidity also contributes to the amount of water in the exhaust gas, the ambient humidity determined under a condition without fuel supply (e.g. a DFSO event) can be subtracted from the amount of water to obtain a more accurate measurement of the amount of alcohol in the fuel.

[0039] Returning to curve 300: The reference voltage of the oxygen sensor transitions between the first voltage V1 and the second voltage V2 in a single step. In other words, the reference voltage changes directly from the first voltage V1 to the second voltage V2 and back again, without any intermediate voltages between the first voltage V1 and the second voltage V2. Similarly, the pump current also transitions from the first pump current I1 to the second pump current I2 and back again. However, the pump current undershoots its lower value (I1) as a result of the reference voltage change, which occurs in a single step from its upper to its lower value, as shown in curve 300. This Ip undershoot 306 can lead to sensor degradation if it persists over several repeated cycles (e.g., thousands of cycles).

[0040] Curve 308 consists of two curves, 310 and 312, which show the reference voltage and pump current of the oxygen sensor over time, respectively, during engine conditions without fuel supply. In one example, a fuel cut-off during deceleration (DFSO) can be considered a condition without fuel supply. During a DFSO event, only ambient air flows past the oxygen sensor, and thus the sensor is exposed to a condition with lower humidity than during normal fuel supply conditions (e.g., when the engine is injecting fuel into the cylinders). As can be seen from curve 312, Ip undershoots can worsen under conditions with lower humidity. The Ip undershoot 316 during DFSO conditions is larger than the Ip undershoot 306 under normal fuel supply conditions.Additionally, during DFSO states, an Ip overshoot (Ip 314) is observed when the reference voltage is increased from a first, lower voltage to a second, higher voltage. In this case, the Ip exceeds its upper target value (I3) and then requires time to settle back to the upper Ip value (I3). Both the Ip overshoot (Ip 314) and the lack of settling time can cause sensor degradation. Conditions with lower humidity, exemplified by DFSO events, can lead to both Ip overshoots and Ip undershoots, and therefore can result in increased sensor damage and reduced sensor function. Thus, both changes in the reference voltage of an oxygen sensor and conditions with lower humidity can contribute to sensor degradation.

[0041] With reference to Fig. 4: A curve 400 shows two curves of an oxygen sensor Ip resulting from two different rise rates used to adjust the oxygen sensor Vs between a first lower value and a second higher value. Vs switches to determine various engine operating parameters (e.g., water content of the exhaust gas, water content of the intake air, ambient humidity, amount of alcohol in the fuel injected into the engine, etc.) using the previously described Fig. The procedures described in section 3 relate to the first and second values. Vs curves 402 and 404 show a first, lower voltage (V1) that is ramped up to a second, higher voltage (V2), but at different rates. Vs curve 402 shows the rate of increase from V1 to V2 and the rate of decrease from V2 to V1 as being smaller than the value in Vs curve 404. Curve 400 also includes the Ip curve 406, which results from the voltage applied in Vs curve 402, and the Ip curve 408, which results from the voltage applied in Vs curve 404. In Ip curve 406, the magnitude of the Ip overshoot 412 and the magnitude of the Ip undershoot 416 are smaller than the magnitude of the Ip overshoot 410 and the magnitude of the Ip undershoot 418 in curve 408. Thus, ramping up the reference voltage of the oxygen sensor from V1 to V2 at a more gradual rate, as in curve 402, compared to the faster rate in curve 404, can reduce the magnitude of the Ip overshoot.Similarly, reducing the voltage from V2 to V1 at a more gradual rate, as in curve 402, compared to the faster rate in curve 404, can reduce the size of an Ip undershoot.

[0042] In one example, the reference voltage change rate shown in graph 404 can be a single step change, where the reference voltage transitions directly from V1 to V2 and not back. In another example, the reference voltage change rate shown in graph 404 can be a faster rate (and thus graph 404 has a steeper slope) than the reference voltage change rate shown in graph 402 (which can have a shallower slope). Transitioning between V1 and V2 at a rate lower than a single step change and / or reducing the change rate between V1 and V2 can reduce the magnitude of Ip overshoots and Ip undershoots, thereby reducing oxygen sensor degradation due to sensor blackening and / or pump current oscillations. As a result, the oxygen sensor's lifetime can be increased.Furthermore, reducing the rate of change between V1 and V2 can decrease settling times (e.g., the time it takes for the sensor to return to its base voltage V1), thereby reducing the time spent in fuel idling operation.

[0043] Fig. Figure 5 shows a curve 500 of an exemplary slope function for the transition of a reference voltage (Vs) of an oxygen sensor between a lower first voltage V1 and a higher second voltage V2. The purpose of the slope function is to provide a more gradual transition between the first and second voltages, thus reducing sensor degradation. During the time interval 504, a first reference voltage (e.g., the first voltage) is applied across the pump cell (e.g., the pump electrode pair 212 and 214) that is low enough so that water molecules do not dissociate. In one example, the first voltage is approximately 450 mV. In other embodiments, the first voltage can be a different reference voltage at which water does not dissociate. The first voltage can generate a pump current that can be used to measure the oxygen content of the sample gas, as described in the procedures above.

[0044] The reference voltage is then increased during time interval 506 from the first voltage, at which water molecules do not dissociate, to a second voltage, at which water molecules do dissociate. In one example, the second voltage can be approximately 1.2 V (e.g., 1200 mV). In alternative embodiments, the second voltage can be a different voltage at which water molecules dissociate at the sensor. Applying the second voltage during time interval 508 can generate a second pump current, which can be used to estimate the amount of oxygen and water in the sample gas, which can then be used to estimate the ambient humidity and the alcohol content in the injected fuel, as described in more detail above. Subsequently, during time interval 510, the reference voltage is reduced from the second voltage to the first voltage, and the first voltage is maintained during time interval 512.The following is a description of several embodiments of methods for increasing or decreasing the voltages between the first, lower, and the second, higher voltages. It is important to note that the rates at which the voltages are increased or decreased can be either preset or variable in all the following methods, depending on the operating parameters of the vehicle engine. For example, if lower humidity conditions are detected, the rate at which the oxygen sensor voltage increases between the first and second reference voltages can be reduced because lower humidity conditions can lead to greater sensor degradation (as described in detail in [reference]). Fig. 7 is described).

[0045] During time intervals 506 and 510, the reference voltage can transition between the first and second voltages at an adjustable rate of increase. The rate of increase can be a linear function or another smooth function, the rate of increase of which can gradually decrease as the reference voltage approaches the second, higher voltage. In another embodiment, the voltage can be increased in a series of small, incremental steps, which can be of equal size or whose size can decrease as the reference voltage approaches the second voltage.

[0046] As shown in curve 502 during time interval 506, in another embodiment the reference voltage can be increased in a single step from the first voltage V1 to an intermediate voltage IV at which water molecules just begin to dissociate. In other words, the reference voltage can increase directly from the first voltage to the intermediate voltage IV in a single step without any additional, activated steps. In one example, as shown in curve 500, the intermediate voltage is approximately 900 mV. In another example, the intermediate voltage can be a different voltage between the first and second voltages at which water molecules transition from not dissociating to dissociating at the oxygen sensor. In yet another example, the intermediate voltage can be a reference voltage before which the dissociation of water molecules begins at the oxygen sensor.

[0047] The reference voltage then gradually increases from the intermediate voltage to the second voltage. Because there may be no sensor degradation at steps between voltage levels where water molecules do not dissociate (e.g., between 450 and 900 mV), a slope function may not be implemented to increase Vs from the first voltage to the intermediate voltage in the time interval 506. Thus, the transition from the first to the intermediate voltage in one step reduces the time spent at initial voltage levels and increases the sampling rate the sensor can perform, thereby improving its accuracy. The reduced rate at which the reference voltage can increase from the intermediate voltage to the second voltage can be determined by a linear function or other smooth function whose rate of increase can gradually decrease as the reference voltage approaches the second, higher voltage.The voltage can also be increased from the intermediate to the second voltage in a series of small, incremental steps, which may be of equal magnitude or whose magnitude may decrease as the reference voltage approaches the second voltage. This same process can be applied in reverse, as can be seen in time interval 510 of graph 502. The reference voltage can be decreased from a second voltage to an intermediate voltage at which water molecules cease to dissociate (e.g., 900 mV) in a gradually decreasing function (e.g., in incremental steps or by a linear or other smooth function). Finally, the reference voltage can be reduced from the intermediate voltage to the first voltage in a single step (or at a higher rate than the rate of increase from the second voltage to the intermediate voltage).

[0048] Therefore, in one embodiment, methods can involve increasing (or transitioning) the reference voltage of an oxygen sensor between a first and second voltage at a more gradual rate than that of a single change step. In another embodiment, methods can involve reducing the rate at which the reference voltage transitions between the first and second voltages. As a result of reducing the transition rate between the first and second oxygen sensor reference voltages, oxygen sensor degradation can be reduced.

[0049] Fig. Figure 6 shows a method 600 for adjusting the operation of an oxygen sensor (such as the one in Fig. 1 oxygen sensor 126 or 127 shown). Commands to execute the procedure 600 can be stored in a memory of the controller (e.g., in the read-only memory chip 106 of the in Fig. 1 shown in control unit 12). Therefore, the control unit can execute procedure 600 based on signals recorded from various motor sensors, as above in Fig. 1 has been described.

[0050] Method 600 begins in 602 with the estimation and / or measurement of engine operating parameters. Engine operating parameters include ambient humidity, engine speed and load, air-fuel ratio, air mass, engine temperatures, fuel injection quantity, etc. Method 600 continues from 602 to 604, where the control determines a desired rate of rise of the oxygen sensor based on the engine operating conditions. As described above, the desired rate of rise can be a desired rate at which the reference voltage of the oxygen sensor is adjusted between a first voltage and a second voltage. The first voltage can be a lower base voltage at which water molecules do not dissociate, and the second voltage can be a higher target voltage at which water molecules dissociate. The desired rate of rise can be based on engine operating conditions, including ambient humidity and engine fuel supply conditions.For example, if the ambient humidity decreases, the desired rate of increase may decrease. In another example, if the engine fuel supply decreases, the desired rate of increase may decrease. In yet another example, the desired rate of increase may be slower under conditions without fuel supply, such as during fuel cut-off during deceleration (DFSO), than when the engine is injecting fuel into the cylinders. In still other examples, the desired rate of increase may decrease if the amount of oxygen in the air surrounding the oxygen sensor increases.

[0051] In 606, the procedure involves determining the desired incremental step size based on the specified, desired rate of increase and the time interval over which the steps for the increase occur. The desired incremental step size can be based on the time interval over which the increase (or rise from the first voltage to the second voltage) occurs, the desired rate of increase, and a desired step size. The desired step size can be a desired incremental increase of the reference voltage. The desired step size can also be based on the total number of steps in the increase. Alternatively, the total number of steps in the increase can be based on the rate of increase, the step size, and the time interval for the increase.

[0052] Once the controller determines the incremental step size, it sends a signal to the oxygen sensor to adjust the reference voltage accordingly. It is important to note that during each step, as the voltage is increased from a first to a second voltage, the incremental step size may not be constant. For example, it may be desirable for the voltage increase rate to decrease as the applied voltage approaches the second, higher voltage, where greater sensor damage can occur. In another embodiment of step 606, a smooth slope function (e.g., a linear, logarithmic, or other smooth function) can be implemented. In this embodiment, the slope or increase rate of the reference voltage is determined based on step 604. The increase rate can be constant (e.g.,The rate of increase (linear) can also change during the time interval in which the voltage rises from the first to the second voltage. For example, the rate of increase may decrease as the applied voltage approaches the second voltage, at which water dissociates and can potentially cause greater damage to the sensor.

[0053] After determining the desired rate of increase, the incremental step size (if the rate of increase is not a smooth function), and the time interval for the voltage increase, procedure 600 can optionally proceed to 608, where the controller sends a signal to the oxygen sensor to increase the reference voltage from the first voltage to an intermediate voltage in one step (e.g., not at a reduced rate of increase). The intermediate voltage can be a voltage that is greater than the first voltage and less than the second voltage. In one example, the intermediate voltage can be a reference voltage at which water begins to dissociate. In another example, the intermediate reference voltage can be a reference voltage above which water dissociates. In yet another example, the intermediate voltage can be a reference voltage at which water does not dissociate.Therefore, the reference voltage of the oxygen sensor is increased via a step function from the first voltage to the intermediate voltage before the reduced rate of increase determined in step 604 is initiated.

[0054] Procedure 600 can proceed to step 610 either from step 606 or, optionally, from step 608. If procedure 600 arrives at step 610 from step 606, the controller sends a signal to the oxygen sensor to adjust the reference voltage from the first to the second voltage at a rate or step size determined in steps 604 and / or 606. Alternatively, if procedure 600 arrives at step 610 from step 608, the controller sends a signal to the oxygen sensor to adjust the reference voltage from the intermediate voltage to the second voltage at a rate or step size determined in steps 604 and / or 606.By including step 608 in procedure 600, the reference voltage can be increased from a first to a second voltage in less time than by proceeding directly from step 606 to step 610, because the gradual increase starts with a higher voltage (e.g., the intermediate voltage instead of the initial base voltage), resulting in a higher cycle rate for the sensor. Therefore, the accuracy of the oxygen sensor measurements can be improved. Furthermore, if procedure 600 additionally employs an optional step 608, the sensor's health may not be reduced, because sensor degradation may only occur during voltage step intervals where water dissociates at the sensor, as explained in more detail above. Fig. 5 is described.

[0055] Once the applied voltage in step 610 reaches the desired second upper voltage, procedure 600 continues with step 612, in which the controller can estimate the engine operating parameters based on the feedback from the sensor at the first and second voltages. The controller can determine the ambient humidity, the engine air-fuel ratio, the water content of the gas near the oxygen sensor, the alcohol content of the fuel, and the EGR rate, or similar parameters, based on the difference in the oxygen sensor's pumping current at the first and second voltages. Procedure 600 then continues with step 614, in which the controller adjusts the engine operation based on the estimated operating parameters (e.g., the operating parameters estimated based on the oxygen sensor outputs).For example, the engine can increase the amount of exhaust gas recirculated into the intake manifold if the amount of unburned fuel in the exhaust manifold reaches a sufficiently high initial threshold. In another example, the control unit can adjust the fuel injection based on the estimated air-fuel ratio.

[0056] In this way, the control unit can adjust engine operation based on outputs from the oxygen sensor. Furthermore, based on engine operating parameters, including ambient humidity and fuel supply, the control unit can determine the rate at which the oxygen sensor's reference voltage is increased from a lower initial voltage to a higher second voltage (and decreased from the second voltage back to the first). As a result, oxygen sensor degradation due to sensor blackening and oscillation can be reduced.

[0057] As in Fig. Figure 6 shows a method in one embodiment that involves increasing the reference voltage of an oxygen sensor from a first voltage to an intermediate voltage in a single step; and gradually increasing the reference voltage from the second, intermediate voltage, to a third voltage in multiple steps. In one example, the step size of each of the multiple steps is based on a desired rate of increase, a total rise time for increasing from the second, intermediate voltage, to the third voltage, and a desired step time length. Furthermore, the desired rate of increase can be based on the ambient humidity and / or the engine fuel supply conditions. The method can further include decreasing the desired rate of increase and increasing the total rise time during a fuel cut-off upon deceleration.In one example, the procedure could further involve decreasing the desired rate of rise and increasing the overall rise time as the ambient humidity decreases. The first voltage could be a base voltage at which water does not dissociate, and the third voltage could be a target voltage at which water dissociates. Furthermore, the intermediate voltage lies between the first and third voltages.

[0058] Fig. Figure 7 shows a curve 700, which demonstrates how the control (e.g., the one in Fig.The controller shown (12) can adjust the rate of rise of the reference voltage of an oxygen sensor between a lower, first voltage and a higher, second voltage based on engine operating parameters, such as fuel supply and / or ambient humidity. Graph 706 shows how the fuel injected into the engine can change over time, and similarly, graph 708 shows how the ambient humidity of the air flowing through the engine can change over time. Graph 702 shows settings of the oxygen sensor's reference voltage rate in response to changes in fuel supply and / or ambient humidity from graphs 706 and 708. Graph 704 shows the changes in the oxygen sensor's pump current resulting from graph 702.

[0059] Both the fuel supply and the ambient humidity are shown to be at a first level between times t0 and t1. This time interval can thus represent normal engine operating conditions, where fuel is being supplied to the engine (e.g., fuel is being injected into the engine cylinders) and the ambient humidity is above a lower threshold level. At time t1, the control unit can detect a reduction in the amount of fuel injected into the engine below a threshold level. In one example, the reduction in engine fuel supply below the threshold level could be the result of a fuel cut-off during deceleration or another condition without fuel supply. This can reduce the humidity of the gas mixture because water from the fuel contributes to the overall humidity of the gas mixture in the combustion chamber.Additionally, during periods without fuel supply, the oxygen content in the exhaust gas flowing to an exhaust oxygen sensor (located in an exhaust port downstream of the engine cylinders) can increase. This reduction in humidity and increase in oxygen content can lead to increased pump current overshoot, undershoot, and / or settling times of the oxygen sensor, contributing to sensor degradation. In response to the reduction in engine fuel supply, the control unit can thus decrease the rate at which the reference voltage of the oxygen sensor (e.g., the exhaust oxygen sensor) increases from a first voltage to a second voltage and decreases from the second voltage back to the first voltage. This can be observed in waveform 702.Between times t1 and t2, where the fuel injected into the engine decreases from the first level F1, the rate at which the reference voltage increases from the first, lower voltage to the second, higher voltage (e.g., the second slope 712 of the line of the curve 702) is smaller than the rate between times t0 and t1 (e.g., smaller than the first slope 710 of the line of the curve 702).

[0060] As soon as the fuel supply conditions return to the first level F1 at time t2, the rate of change of the reference voltage (e.g., the rate of change between the first and second voltages) is reduced to a rate similar to that observed during the time interval from t0 to t1. Thus, the rate of change of the reference voltage is increased again after time t2. At time t3, the control unit detects a decrease in ambient humidity from a first level A1 below a lower threshold level. This can occur when the vehicle enters a particularly dry environment where the humidity in the ambient air is very low. Because low humidity conditions can lead to increased oxygen sensor degradation, the control unit may reduce the rate at which the reference voltage transitions between the lower, first voltage and the higher, second voltage.As can be seen in the curve 702 after time t3, the rate of change between the lower, first voltage and the higher, second voltage (e.g. the third slope 714 of the line in curve 702) is smaller than that between times t0 and t1 and t2 and t3, when the ambient humidity is at the first level A1 and the engine fuel supply is at the first level F1.

[0061] As seen in diagram 702, the rate of rise of the voltage increase or decrease between the lower, first voltage and the higher, second voltage can be adjusted based on engine operating conditions. The rate of rise (also referred to here as the rate of change of the reference voltage) can decrease if the amount of fuel injected into the engine cylinders and / or the ambient humidity decreases. In another example, the rate of rise can decrease only if the amount of fuel injected and / or the ambient humidity falls below their respective threshold levels, where the threshold levels are based on levels at which oxygen sensor overshoot, undershoot, and / or settling times in the pump current output can lead to sensor degradation. In some examples, a DFSO event can coincide with lower humidity conditions.In this example, the rate of rise of the oxygen sensor's reference voltage can decrease in response to both the DFSO event and the reduced humidity states. For instance, the rate of rise can be reduced to a greater extent (the transition between the first and second voltages may even be more gradual) than observed between times t1 and t2 or after t3 if the controller detects both a reduced humidity state and a DFSO event. This means that a DFSO event (or a no-fuel event) and a reduced humidity event may not be mutually exclusive, and that both occurring simultaneously can result in a more gradual rate of rise than observed in curve 700.

[0062] The rate of rise, or the more gradual transition, between the first voltage (e.g., the base voltage at which water does not dissociate) and the second voltage (e.g., the target voltage at which water dissociates) is smaller than the rate of a single step (also referred to as a single change step between the first and second voltages). As described here, during motor operation, the oxygen sensor's reference voltage alternates between the lower, first voltage and the higher, second voltage over a time interval at a rate set by the controller based on motor operating conditions. The rate of change between the reference voltages can then be adjusted based on the motor operating parameters. However, the rate of change of the reference voltage (e.g.,The rate of increase (the rise rate) in one embodiment will always be smaller than the rate of change of a single, direct change step between the first and second voltages. Thus, there can be a threshold rate of increase, based on the rate of a single change step between voltages, below which the oxygen sensor rate of increase can remain.

[0063] In this way, a method can involve gradually changing the reference voltage of an oxygen sensor between a first voltage and a second voltage at a rate of increase, the rate of increase being based on engine operating conditions. As described above, the oxygen sensor can be a variable-voltage oxygen sensor positioned in an exhaust or intake port of an engine. The reference voltage of the variable-voltage oxygen sensor is adjustable between the first voltage and the second voltage, where the first voltage is a lower voltage at which water does not dissociate at the sensor, and the second voltage is a higher voltage at which water dissociates at the sensor.A technical effect is achieved by gradually adjusting the reference voltage of the oxygen sensor with a rise rate based on engine operating conditions, thereby reducing oxygen sensor degradation and increasing the accuracy of engine control based on oxygen sensor outputs.

[0064] In particular, a slower transition rate between the first and second voltages can reduce sensor overshoot, undershoot, and settling times, which can lead to sensor oscillations and blackening effects that can degrade sensor performance. Furthermore, these effects can become more pronounced when the oxygen content in the gas surrounding the oxygen sensor increases. Therefore, further reducing the slew rate of the oxygen sensor's reference voltage when the oxygen content of the exhaust and / or intake gas increases (e.g., for an exhaust oxygen sensor during reduced engine fuel supply, and for both intake and exhaust oxygen sensors during reduced ambient humidity) can further reduce oxygen sensor degradation.

[0065] In one embodiment, the method involves gradually increasing a reference voltage of an oxygen sensor from a first voltage to a second voltage at a rate of increase, the rate of increase being based on engine operating conditions. In one example, the rate of increase is based on ambient humidity, decreasing as the ambient humidity decreases. In another example, the rate of increase is based on engine fuel supply, decreasing as the engine fuel supply decreases. The method further involves decreasing the rate of increase from a first level to a lower, second level during an engine condition with no fuel supply.

[0066] In one embodiment, the method further comprises increasing the reference voltage from the first voltage to an intermediate voltage in a single step and then increasing the reference voltage from the intermediate voltage to the second voltage at a rate of increase, wherein the rate of increase is lower than that of the single step. In one example, the intermediate voltage is a voltage below which water does not dissociate and above which water dissociates. Additionally, the first voltage is a base reference voltage, and the second voltage is a target voltage at which water dissociates. The method further comprises gradually decreasing the reference voltage from the second voltage to the first voltage at a rate of decrease.In one example, the oxygen sensor is an exhaust oxygen sensor positioned in an engine's exhaust port, and the method further involves determining the water content of the exhaust air based on the oxygen sensor's outputs at the first and second voltages. The method may also include estimating the fuel alcohol content based on the determined water content. In another example, the oxygen sensor is an intake oxygen sensor positioned in an engine's intake manifold, and the method further involves determining the water content of the intake air based on the oxygen sensor's outputs at the first and second voltages.

[0067] In yet another embodiment, the engine system can include an exhaust oxygen sensor positioned downstream of an engine cylinder in an exhaust channel and a control system with computer-readable commands for the following: in a first state, when fuel is supplied to the engine cylinder, gradually increasing a reference voltage of the exhaust oxygen sensor from a first voltage to a second voltage with a first rate of increase; and during a second state, when no fuel is supplied to the engine cylinder, gradually increasing the reference voltage of the exhaust oxygen sensor from the first voltage to the second voltage with a second rate of increase, the second rate of increase being more gradual than the first rate of increase.Increasing the exhaust oxygen sensor's reference voltage from the first voltage to the second voltage in the first and second states involves increasing the reference voltage from the first voltage to the second voltage in a series of incremental steps. The number of incremental steps and the magnitude of each step in the series are based on both the first and second rate of increase. Furthermore, the exhaust oxygen sensor is a variable-voltage oxygen sensor, where the first voltage is a base voltage and the second voltage is a target voltage, the target voltage being based on a desired gas component to be measured by the oxygen sensor. The engine system may also include an intake oxygen sensor positioned in an intake port upstream of the engine cylinder.

[0068] It should be noted that the exemplary control and estimation routines contained herein can be used with various engine and / or vehicle system configurations. The control procedures and routines disclosed herein can be stored as executable instructions in non-volatile memory and can be executed by the control system, which includes the controller in combination with various sensors, actuators, or other engine hardware. The specific routines described herein can represent one or more of any number of processing strategies, such as event-driven, interrupt-driven, multitasking, multithreading, and the like. Therefore, various illustrated actions, operations, and / or functions can be executed in the illustrated order or in parallel, or in some cases, omitted.Similarly, the processing sequence is not necessarily required to achieve the features and benefits of the embodiments described here, but is provided for the sake of clarity and explanation. One or more of the illustrated actions, operations, and / or functions can be executed repeatedly, depending on the specific strategy employed. Furthermore, the described actions, operations, and / or functions can graphically represent code to be programmed into non-volatile memory of the computer-readable storage medium in the engine control system, with the described actions being executed by carrying out the instructions in a system that includes the various engine hardware components in combination with the electronic control unit.

[0069] It is understood that the configurations and routines disclosed herein are by their very nature exemplary and that these specific embodiments should not be considered limiting, as numerous variations are possible. For example, the above technology can be applied to six-cylinder V-engines (V-6), four-cylinder in-line engines (I-4), six-cylinder in-line engines (I-6), twelve-cylinder V-engines (V-12), four-cylinder boxer engines (Opposed 4), and other engine types. The subject matter of this 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.

[0070] The following claims, in particular, disclose certain combinations and subcombinations that are considered novel and not obvious. These claims may refer to "one" element, "a first" element, or the equivalent thereof. Such claims should be understood as including one or more such elements, without requiring or excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and / or properties may be claimed by amending the present claims or by filing new claims in this or a related application. Such claims, whether their scope of protection is broader, narrower, or different from that of the original claims, are also considered to be included in the subject matter of the present disclosure.

Claims

[1] Engine process comprising the following: Gradual increase of a reference voltage of an oxygen sensor (126) from a first voltage to a second voltage with a rate of increase, wherein the rate of increase is based on engine operating conditions. [2] Method according to claim 1, wherein the rate of increase is based on ambient humidity, wherein the rate of increase decreases with a decrease in ambient humidity. [3] Method according to claim 1, wherein the rate of increase is based on engine fuel supply, wherein the rate of increase decreases with a reduction in engine fuel supply. [4] The method according to claim 3, which further comprises reducing the rate of increase from a first level to a lower, second level when the engine is in a no-fuel-supply condition. [5] The method of claim 1, further comprising increasing the reference voltage from the first voltage to an intermediate voltage with a single change step and then increasing the reference voltage from the intermediate voltage to the second voltage with the rate of change, wherein the rate of change of the rate of change is smaller than that of the single change step. [6] Method according to claim 5, wherein the intermediate voltage is a voltage below which water is not dissociated and above which water is dissociated. [7] Method according to claim 1, wherein the first voltage is a basic reference voltage and the second voltage is a target voltage at which water is dissociated. [8] The method of claim 1, further comprising gradually reducing the reference voltage from the second voltage to the first voltage at the decay rate. [9] Method according to claim 1, wherein the oxygen sensor (126) is an exhaust oxygen sensor positioned in an exhaust channel (48) of a motor (10), and further comprising determining a water content of the exhaust air based on outputs of the oxygen sensor (126) at the first voltage and the second voltage. [10] The method of claim 9, further comprising estimating a fuel alcohol content based on the determined water content. [11] Method according to claim 1, wherein the oxygen sensor (126) is an inlet oxygen sensor positioned in an inlet manifold (44) of an engine (10), and further comprising determining a water content of the inlet air based on outputs of the oxygen sensor (126) at the first voltage and the second voltage. [12] Engine method comprising the following: Increasing a reference voltage of an oxygen sensor (126) from a first voltage to a second voltage, an intermediate voltage, in a single step; and Incremental increase of the reference voltage from the second, intermediate voltage, to a third voltage in several steps. [13] Method according to claim 12, wherein a step size of each of the multiple steps is based on a desired rate of increase, a total rise time for increasing from the second, intermediate voltage, to the third voltage and a desired step time length. [14] Method according to claim 13, wherein the desired rate of increase is based on the ambient humidity and / or the engine fuel supply conditions. [15] The method of claim 14, further comprising reducing the desired rate of increase and increasing the total rate of increase during a fuel cut-off at deceleration. [16] The method of claim 14, further comprising reducing the desired rate of increase and increasing the total rise time when the ambient humidity decreases. [17] Method according to claim 12, wherein the first voltage is a base voltage at which water is not dissociated, and the third voltage is a target voltage at which water is dissociated, and wherein the second, the intermediate voltage, lies between the first voltage and the third voltage. [18] Engine system comprising the following: an exhaust oxygen sensor positioned downstream of an engine cylinder (30) in an exhaust port (48); and a controller with computer-readable instructions for: In a first state, when fuel is supplied to the engine cylinder (30), a reference voltage of the exhaust oxygen sensor is gradually increased from a first voltage to a second voltage with a first rate of increase; and in a second state, when no fuel is supplied to the engine cylinder (30), the reference voltage of the exhaust oxygen sensor is gradually increased from the first voltage to the second voltage with a second rate of increase, the second rate of increase being more gradual than the first rate of increase. [19] System according to claim 18, wherein increasing the reference voltage of the outlet oxygen sensor from the first voltage to the second voltage in the first and second states involves increasing the reference voltage from the first voltage to the second voltage in a series of incremental steps, wherein the number of incremental steps and the size of each incremental step are based on both the first rate of increase and the second rate of increase. [20] System according to claim 18, wherein the outlet oxygen sensor is a variable voltage oxygen sensor, wherein the first voltage is a base voltage and wherein the second voltage is a target voltage, the target voltage being based on a desired gas component to be measured by the oxygen sensor (126).

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