Control device of exhaust sensor

A control device of an exhaust sensor comprises a heater control part configured to set a target temperature of an electrochemical cell and control a heater so that a temperature of the electrochemical cell becomes the target temperature, and a judging part configured to judge whether a water repellency of a protective layer is falling when the heater control part sets the target temperature to a temperature of a lowest temperature at which a Leidenfrost phenomenon occurs at an outer surface of the protective layer or more. The heater control part is configured to rise the target temperature when the judging part judges that the water repellency of the protective layer is falling.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Japanese Patent Application No. 2016-118389 filed on Jun. 14, 2016, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a control device of an exhaust sensor.

BACKGROUND ART

It has been known in the past to arrange an exhaust sensor in an exhaust passage of an internal combustion engine to detect a specific component in the exhaust gas (for example, see PLTs 1 to 3). The exhaust sensor described in PLT 1 is provided with an element body provided with an electrochemical cell, and a protective layer formed on the outer surface of the element body and comprised of a porous ceramic. The exhaust sensor is fastened to an exhaust pipe so as to be exposed to exhaust gas. Part of the exhaust gas passes through the protective layer and flows to the inside of the element body. Further, the exhaust sensor is provided with a heater for heating the element body so that the electrochemical cell becomes a predetermined operating temperature or more.

In this regard, when the temperature of the exhaust pipe is the dew point temperature of water or less, the water vapor in the exhaust gas condenses and condensed water is generated. If there is condensed water in the exhaust passage, drops of the condensed water strike the protective layer of the exhaust sensor together with the exhaust gas. If the protective layer does not have water repellency, the drops of water striking the protective layer will penetrate to the inside of the protective layer. If the temperature of the protective layer is high due to heating by the heater, the drops of water penetrating the protective layer will evaporate inside the protective layer. As a result, thermal shock will be given to the protective layer and element body and the element of the exhaust sensor will sometimes crack.

Therefore, PLT 1 describes preventing cracking of the element of the exhaust sensor by utilizing the Leidenfrost phenomenon to give the protective layer of the exhaust sensor water repellency. The “Leidenfrost phenomenon” is the phenomenon where when drops of water strike a high temperature protective layer, a film of water vapor is formed between the protective layer and drops of water whereby transfer of heat between the protective layer and the drops of water is suppressed. If the Leidenfrost phenomenon occurs, the drops of water are repelled from the protective layer, so water is kept from penetrating the protective layer.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

However, if soot deposits on the protective layer, the exhaust sensor becomes degraded, etc., the thermal conductivity of the protective layer will fall. As a result, the Leidenfrost phenomenon will become harder to occur, and the water repellency of the protective layer will fall. If the water repellency of the protective layer falls, a part of the drops of water striking the protective layer will penetrate through the protective layer. If the degree of fall of water repellency of the protective layer becomes larger, the amount of water penetrating through the protective layer will increase and the element of the exhaust sensor is liable to crack.

Therefore, an object of the present invention is to provide a control device of an exhaust sensor able to prevent an element of an exhaust sensor from cracking due to coverage by water even if the water repellency of the protective layer of the exhaust sensor falls.

Solution to Problem

In order to solve the above problem, in a first aspect, there is provided a control device of an exhaust sensor controlling an exhaust sensor arranged in an exhaust passage of an internal combustion engine and detecting a specific component in exhaust gas, wherein the exhaust sensor comprises an element body provided with an electrochemical cell, a protective layer formed on an outer surface of the element body and comprised of a porous ceramic, and a heater heating the element body and the protective layer, the control device comprises a heater control part configured to set a target temperature of the electrochemical cell and control the heater so that a temperature of the electrochemical cell becomes the target temperature, and a judging part configured to judge whether a water repellency of the protective layer is falling when the heater control part sets the target temperature to a temperature of a lowest temperature at which a Leidenfrost phenomenon occurs at an outer surface of the protective layer or more, and the heater control part is configured to raise the target temperature when the judging part judges that the water repellency of the protective layer is falling.

In a second aspect, the judging part is configured to judge a degree of fall of water repellency of the protective layer and the heater control part is configured to increase an amount of rise of the target temperature when the degree of fall of water repellency of the protective layer is relatively large compared with when the degree of fall of water repellency of the protective layer is relatively small, in the first aspect.

In a third aspect, the heater control part is configured to raise the target temperature so that a temperature of the outer surface of the protective layer becomes a combustion temperature of soot or more when the judging part judges the water repellency of the protective layer is falling, in the first or second aspect.

In a forth aspect, if the judging part does not judge that the water repellency of the protective layer has again fallen from when first rising the target temperature after startup of the internal combustion engine to when the internal combustion engine stops, the heater control part returns the target temperature after restart of the internal combustion engine to a value before the rise, and if the judging part judges that the water repellency of the protective layer has again fallen from when first rising the target temperature after startup of the internal combustion engine to when the internal combustion engine stops, the heater control part further raises the target temperature and maintains the target temperature after restart of the internal combustion engine at a value after the rise, in the third aspect.

In a fifth aspect, the heater control part is configured to set the target temperature to a temperature less than the lowest temperature or turn the heater off if rising the target temperature a plurality of times causes the target temperature to exceed a predetermined upper limit temperature, in any one of the first to fourth aspects.

In a sixth aspect, the control device further comprises an exhaust pipe temperature estimating part configured to estimate a temperature of an exhaust pipe around the exhaust sensor, and, after the temperature of the exhaust pipe estimated by the exhaust pipe temperature estimating part reaches a predetermined temperature of a dew point temperature or more, the judging part does not judge whether the water repellency of the protective layer is falling and the heater control part sets the target temperature to a predetermined operating temperature, in any one of the first to fifth aspects.

Advantageous Effects of Invention

According to the present invention, there is provided a control device of an exhaust sensor able to prevent an element of an exhaust sensor from cracking due to coverage by water even if the water repellency of the protective layer of the exhaust sensor falls.

DESCRIPTION OF EMBODIMENTS

Below, referring to the drawings, embodiments of the present invention will be explained in detail. Note that, in the following explanation, similar components will be assigned the same reference notations.

First Embodiment

First, referring toFIG. 1toFIG. 8, a first embodiment of the present invention will be explained.

<Explanation of Internal Combustion Engine as Whole>

FIG. 1is a view schematically showing an internal combustion engine1in which a control device of an exhaust sensor according to the first embodiment of the present invention is used. The internal combustion engine1shown inFIG. 1is a compression ignition type internal combustion engine (diesel engine). The internal combustion engine1is for example mounted in a vehicle.

Referring toFIG. 1, the internal combustion engine1is provided with an engine body100, a combustion chamber2of each cylinder, an electronically controlled fuel injector3injecting fuel into each combustion chamber2, an intake manifold4, and an exhaust manifold5. The intake manifold4is connected through an intake pipe6to an outlet of a compressor7aof a turbocharger7. The inlet of the compressor7ais connected through the intake pipe6to an air cleaner8. Inside the intake pipe6, a throttle valve9driven by a step motor is arranged. Furthermore, around the intake pipe6, a cooling device13for cooling the intake air flowing through the inside of the intake pipe6is arranged. In the internal combustion engine1shown inFIG. 1, engine cooling water is guided to the inside of the cooling device13and cools the intake air. The intake manifold4and intake pipe6form an intake passage guiding air to the inside of each combustion chamber2.

On the other hand, the exhaust manifold5is connected through an exhaust pipe27to an inlet of a turbine7bof the turbocharger7. The outlet of the turbine7bis connected through the exhaust pipe27to a casing29housing an exhaust purification catalyst28. The exhaust manifold5and exhaust pipe27form an exhaust passage discharging exhaust gas generated by combustion of the air-fuel mixture in each combustion chamber2. The exhaust purification catalyst28is, for example, a selective catalytic reduction type NOXreduction catalyst (SCR catalyst) or an NOXstorage and reduction catalyst for removing the NOXin the exhaust gas by reduction. Further, inside the exhaust passage, to reduce particulate matter (PM) in the exhaust gas, an oxidation catalyst, diesel particulate filter (DPF), etc. may be arranged.

The exhaust manifold5and the intake manifold4are connected through an exhaust gas recirculation (below, referred to as “EGR”) passage14. Inside the EGR passage14, an electronically controlled EGR control valve15is arranged. Further, around the EGR passage14, an EGR cooling device20is arranged for cooling the EGR gas flowing through the inside of the EGR passage14. In the embodiment shown inFIG. 1, the engine cooling water is guided to the inside of the EGR cooling device20and cools the EGR gas.

The fuel is supplied by an electronically controlled variable discharge fuel pump19from a fuel tank33through a fuel pipe34to the inside of a common rail18. The fuel supplied to the inside of the common rail18is supplied through the individual fuel supply pipes17to the individual fuel injectors3.

The various control routines of the internal combustion engine1are performed by the electronic control unit (ECU)80. The ECU80is comprised of a digital computer provided with components connected to each other through a bidirectional bus81such as a ROM (read only memory)82, RAM (random access memory)83, CPU (microprocessor)84, input port85, and output port86. Outputs of a load sensor101and an air-flow meter102are input through corresponding AD converters87to the input port85. On the other hand, the output port86is connected through corresponding drive circuits88to the fuel injectors3, throttle valve drive step motor, EGR control valve15, and fuel pump19.

The load sensor101generates an output voltage proportional to an amount of depression of an accelerator pedal120. Therefore, the load sensor101detects the engine load. The air-flow meter102is arranged inside the intake passage between the air cleaner8and compressor7aand detects the amount of air flowing through the inside of the intake pipe6. Furthermore, a crank angle sensor108generating an output pulse every time the crankshaft rotates by for example 15° is connected to the input port85. The crank angle sensor108is used to detect the engine speed.

Note that, the internal combustion engine1may be a spark ignition type internal combustion engine with spark plugs arranged in the combustion chambers. Further, specific configurations of the internal combustion engine1such as the cylinder array, configuration of the intake and exhaust systems, and presence or absence of a turbocharger may differ from the configuration shown inFIG. 1.

In the present embodiment, as the exhaust sensor controlled by the control device of an exhaust sensor, an air-fuel ratio sensor10is arranged at the exhaust passage of the internal combustion engine1. The air-fuel ratio sensor10detects a specific component in the exhaust gas flowing through the exhaust passage of the internal combustion engine1. Specifically, the air-fuel ratio sensor10detects the concentration of oxygen in the exhaust gas to thereby linearly detect the air-fuel ratio of the exhaust gas.

In the present embodiment, the air-fuel ratio sensor10is arranged in the exhaust passage at the upstream side of the exhaust purification catalyst28in the direction of flow of exhaust gas. Note that, the air-fuel ratio sensor10may be arranged at another position in the exhaust passage, for example, at the downstream side of the exhaust purification catalyst28in the direction of flow of exhaust gas.

Below, referring toFIG. 2andFIG. 3, the configuration of the air-fuel ratio sensor10will be explained.FIG. 2is an enlarged view of the air-fuel ratio sensor10. InFIG. 2, the front end side of the air-fuel ratio sensor10is shown by a cross-sectional view. The air-fuel ratio sensor10is fastened to the exhaust pipe27in the state with the front end part11inserted inside the exhaust pipe27. The air-fuel ratio sensor10is provided with a sensor element12having a plate-like shape at its inside.

FIG. 3is a cross-sectional view of a sensor element12of the air-fuel ratio sensor10along the line A-A ofFIG. 2. As shown inFIG. 3, the sensor element12of the air-fuel ratio sensor10is provided with an element body50provided with a sensor cell51and a protective layer60formed on the outer surface of the element body50.

The element body50is provided with a measured gas chamber30and a reference gas chamber31. When the air-fuel ratio sensor10is arranged in the exhaust passage of the internal combustion engine1, exhaust gas flowing through the exhaust passage is introduced into the measured gas chamber30as the measured gas. Reference gas is introduced into the reference gas chamber31. The reference gas is for example the atmosphere. In this case, the reference gas chamber31is opened to the atmosphere.

The air-fuel ratio sensor10is a laminate type air-fuel ratio sensor comprised of a plurality of layers laminated together. The element body50is provided with a solid electrolyte layer40, diffusion regulating layer16, first barrier layer21, second barrier layer22, and third barrier layer23. The solid electrolyte layer40is a thin plate member having oxide ion conductivity. The solid electrolyte layer40is, for example, a sintered body of ZrO2(zirconia), HfO2, ThO2, Bi2O3, etc. to which CaO, MgO, Y2O3, Yb2O3, etc. is added as a stabilizer. The diffusion regulating layer16is a thin plate member having gas permeability. The diffusion regulating layer16is, for example, comprised of alumina, magnesia, silica, spinel, mullite, or other porous ceramic. The barrier layers21to23are gas barrier type thin sheet members, and, for example, include alumina.

The layers of the element body50are comprised of, from the bottom ofFIG. 3, the first barrier layer21, second barrier layer22, solid electrolyte layer40, diffusion regulating layer16, and third barrier layer23stacked in that order. The measured gas chamber30is formed and defined by the solid electrolyte layer40, diffusion regulating layer16, and third barrier layer23. The exhaust gas passes through the protective layer60and diffusion regulating layer16and is introduced to the inside of the measured gas chamber30. The diffusion regulating layer16regulates the diffusion of the measured gas. Note that, the measured gas chamber30may be configured in any form so long as adjoining the solid electrolyte layer40and having the measured gas introduced into it.

The reference gas chamber31is formed and defined by the solid electrolyte layer40and the second barrier layer22. Note that, the reference gas chamber31may be configured in any form so long as adjoining the solid electrolyte layer40and having the reference gas flow into it.

The sensor cell51is an electrochemical cell having a solid electrolyte layer40, first electrode41, and second electrode42. The first electrode41is arranged on the surface of the solid electrolyte layer40on the measured gas chamber30side so that it is exposed to the measured gas of the measured gas chamber30. On the other hand, the second electrode42is arranged on the surface of the solid electrolyte layer40on the reference gas chamber31side so that it is exposed to the reference gas inside the reference gas chamber31. The first electrode41and the second electrode42are arranged so as to face each other across the solid electrolyte layer40. The first electrode41and second electrode42are comprised of platinum (Pt) or another precious metal with a high catalytic activity. For example, the first electrode41and second electrode42are porous cermet electrodes including mainly Pt.

The protective layer60is formed on the outer surface of the element body50so as to cover the entire outer surface of the element body50. The protective layer60has a gas permeability and is comprised of alumina, titania, zirconia, silicon carbide, silicon nitride, zinc oxide, and other porous ceramic.

The sensor element12is further provided with a heater55. In the present embodiment, the heater55, as shown inFIG. 3, is arranged between the first barrier layer21and the second barrier layer22. The heater55is, for example, a thin plate member of cermet including platinum (Pt) and ceramic (for example, alumina etc.) and forms a heat generating element generating heat by conduction of current. The heater55heats the element body50and protective layer60.

The first electrode41and second electrode42of the sensor cell51are connected to an electrical circuit70. The electrical circuit70is provided with a power supply71and current detector72. The power supply71applies voltage across the electrodes so that the potential of the second electrode42becomes higher than the potential of the first electrode41. The output port86of the ECU80is connected through a corresponding drive circuit88to the power supply71. Therefore, the ECU80can control the power supply71and control the voltage applied to the sensor cell51. Further, the current detector72detects the current flowing through the sensor cell51as the output of the sensor cell51. The output of the current detector72is input through the corresponding AD converter87to the input port85of the ECU80. Therefore, the ECU80can acquire the output of the sensor cell51detected by the current detector72from the current detector72.

The air-fuel ratio sensor10detects the limit current flowing through the sensor cell51when applying predetermined voltage to the sensor cell51so as to detect the air-fuel ratio of the exhaust gas. Therefore, the air-fuel ratio sensor10in the present embodiment is a so-called limit current type air-fuel ratio sensor.

In this regard, when the temperature of the exhaust pipe27is the dew point temperature of water or less, the water vapor in the exhaust gas condenses and condensed water is formed. If there is condensed water in the exhaust passage, the drops of the condensed water strike the protective layer60of the air-fuel ratio sensor10together with the exhaust gas. When the protective layer60does not have water repellency, the drops of water striking the protective layer60penetrate to the inside of the protective layer60. When due to heating by the heater55, the temperature of the protective layer60is high, the drops of water penetrating to the protective layer60evaporate inside the protective layer60. As a result, the protective layer60and element body50are given thermal shock and the element of the air-fuel ratio sensor10sometimes cracks.

The protective layer60has water repellency when the temperature is high. This property is obtained by causing the Leidenfrost phenomenon. The “Leidenfrost phenomenon” is the phenomenon where when drops of water strike a high temperature protective layer60, a film of water vapor is formed between the protective layer60and drops of water whereby transfer of heat between the protective layer60and the drops of water is suppressed. If the Leidenfrost phenomenon occurs, the drops of water are repelled from the protective layer60, so water is kept from penetrating the protective layer60.

<Drop in Water Repellency of Protective Layer>

However, if soot deposits on the protective layer60, the air-fuel ratio sensor10deteriorates, etc., the thermal conductivity of the protective layer60will fall.FIG. 4is a graph showing a region where the Leidenfrost phenomenon occurs when changing the surface temperature and thermal conductivity of the protective layer60. InFIG. 4, the region where the Leidenfrost phenomenon occurs is shown by hatching.

As shown inFIG. 4, if the thermal conductivity of the protective layer60falls, the Leidenfrost phenomenon will become harder to occur and the temperature required for causing the Leidenfrost phenomenon will rise. That is, if the thermal conductivity of the protective layer60falls, the water repellency of the protective layer60will fall. If the water repellency of the protective layer60falls, part of the drops of water striking the protective layer60will penetrate through the protective layer60. If the degree of fall of the water repellency of the protective layer60becomes larger, the amount of water penetrating through the protective layer60will increase, so the element of the air-fuel ratio sensor10is liable to crack.

<Explanation of Control Device of Exhaust Sensor>

Therefore, the control device of an exhaust sensor according to the present embodiment performs the following control at the time of the startup of the internal combustion engine1so as to prevent the element of the air-fuel ratio sensor10from cracking due to coverage by water even if the water repellency of the protective layer60of the air-fuel ratio sensor10falls.FIG. 5is a block diagram schematically showing the configuration of a control device of an exhaust sensor according to a first embodiment of the present invention. The control device of an exhaust sensor is provided with a cell temperature detecting part80a, a heater control part80b, and a judging part80c. In the present embodiment, the cell temperature detecting part80a, the heater control part80b, and the judging part80care parts of the ECU80.

The cell temperature detecting part80adetects the temperature of the sensor cell51. Specifically, the cell temperature detecting part80acalculates the temperature of the sensor cell51based on an impedance of the sensor cell51. The cell temperature detecting part80acalculates the impedance of the sensor cell51based on the output of the sensor cell51detected by the current detector72when high frequency voltage is applied from the power supply71to the sensor cell51. Note that, the cell temperature detecting part80amay calculate the temperature of the sensor cell51based on an interelectrode resistance of the sensor cell51. Further, when the inside of the exhaust sensor (in the present embodiment, air-fuel ratio sensor10) is provided with a thermocouple, the cell temperature detecting part80amay use the thermocouple to detect the temperature of the sensor cell51.

The heater control part80bsets the target temperature of the sensor cell51and controls the heater55so that the temperature of the sensor cell51becomes the target temperature. The heater control part80bcontrols the heater55through the heater control circuit56. Specifically, the heater control part80bcontrols by feedback the power supplied to the heater55through the heater control circuit56so that the temperature of the sensor cell51detected by the cell temperature detecting part80abecomes the target temperature. When the sensor cell51is heated by the heater55, the protective layer60is also similarly heated by the heater55. For this reason, the temperature of the protective layer60is correlated with the temperature of the sensor cell51. Therefore, due to the above-mentioned feedback control, the heater control part80bcan control not only the temperature of the sensor cell51but also the temperature of the protective layer60.

The heater control part80bsets the target temperature of the sensor cell51to a temperature of the lowest temperature at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60or more so as to prevent the element of the air-fuel ratio sensor10from cracking due to coverage by water at the time of startup of the internal combustion engine1or after startup. The judging part80cjudges whether the water repellency of the protective layer60is falling when the heater control part80bsets the target temperature to a temperature of lowest temperature at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60or more. The “lowest temperature at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60” is the lower limit value of the temperature at which the Leidenfrost phenomenon occurs when an extremely small amount of drops of water strike the protective layer60and, for example, is 400° C.

The judging part80cjudges that the water repellency of the protective layer60is falling when the condition for judging abnormality is satisfied. If the water repellency of the protective layer60falls, part of the water striking the protective layer60penetrates through the protective layer60and the temperature of the protective layer60and sensor cell51falls. Further, the speed of fall of the temperature of the sensor cell51at this time is faster than the speed of fall of the temperature of the sensor cell51when turning the heater55off. For this reason, in the present embodiment, the condition for judging abnormality includes the temperature of the sensor cell51detected by the cell temperature detecting part80afalling from the target temperature and the speed of fall of the temperature of the sensor cell51at this time being faster than the speed of fall of the temperature of the sensor cell51when turning the heater55off. The speed of fall of the temperature of the sensor cell51when turning the heater55off is determined in advance by experiments or calculations. Note that, the “speed of fall of temperature” means the amount of fall in temperature per unit time.

As shown inFIG. 4, even if the thermal conductivity of the protective layer60falls and the water repellency of the protective layer60falls, it is possible to cause the Leidenfrost phenomenon by rising the surface temperature of the protective layer60. For this reason, the heater control part80braises the target temperature of the sensor cell51when the judging part80cjudges that the water repellency of the protective layer60is falling. Due to this, even if the water repellency of the protective layer60is falling, it is possible to cause the Leidenfrost phenomenon at the outer surface of the protective layer60and possible to prevent the element of the air-fuel ratio sensor10from cracking.

Further, one of the reasons why the water repellency of the protective layer60falls is the deposition of soot on the protective layer60. For this reason, preferably, the heater control part80braises the target temperature of the sensor cell51so that the outer surface of the protective layer60becomes the combustion temperature of soot or more when the judging part80cjudges that the water repellency of the protective layer60is falling. Due to this, it is possible to remove soot from the protective layer60. When deposition of soot on the protective layer60causes the water repellency of the protective layer60to fall, the water repellency of the protective layer60can be restored. The combustion temperature of soot is 750° C. or more, and thus, for example, the target temperature of the sensor cell51is risen to 750° C. or more.

<Explanation of Control Using Time Chart>

Below, referring to the time chart ofFIG. 6, the control performed by the control device of an exhaust sensor in the present embodiment will be specifically explained.FIG. 6is a schematic time chart of the engine load, the water repellency of the protective layer, and the target temperature of the sensor cell51after making the internal combustion engine1start up.

In the illustrated example, at the time to, the internal combustion engine1is started up. If the internal combustion engine1is started up, the target temperature of the sensor cell51is set to the initial temperature T0. The initial temperature T is the temperature of the lowest temperature at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60or more and, for example, is 750° C.

In the illustrated example, at the time t1, it is judged that the water repellency of the protective layer60is falling. For this reason, at the time t1, the target temperature of the sensor cell51is risen from the initial temperature T0to the first temperature T1. The first temperature T1is the temperature at which the outer surface of the protective layer60becomes the combustion temperature of soot or more and, for example, is 800° C.

<Control Routine of Processing for Judging Abnormality>

Below, referring to the flow chart ofFIG. 7, control for judging a fall in the water repellency of the protective layer60will be explained in detail.FIG. 7is a flow chart showing a control routine of processing for judging abnormality in the first embodiment of the present invention. The illustrated control routine is repeatedly performed by the ECU80at predetermined time intervals after the startup of the internal combustion engine1.

First, at step S101, the judging part80cjudges whether the target temperature TT of the sensor cell51set by the heater control part80bis a temperature of the lowest temperature TL at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60or more. The lowest temperature TL is for example 400° C.

When it is judged at step S101that the target temperature TT is less than the lowest temperature TL, the present control routine is ended. In this case, it is not judged whether the water repellency of the protective layer60is falling. On the other hand, when it is judged at step S101that the target temperature TT is the lowest temperature TL or more, the present control routine proceeds to step S102.

At step S102, the judging part80cjudges whether the temperature of the sensor cell51is falling from the target temperature TT. The temperature of the sensor cell51is detected by the cell temperature detecting part80a. If it is judged at step S102that the temperature of the sensor cell51is falling from the target temperature TT, the present control routine proceeds to step S103.

At step S103the judging part80cjudges whether the speed of fall Vdt of the temperature of the sensor cell51is faster than the speed of fall Voff of the temperature of the sensor cell51when turning the heater55off. The speed of fall Vdt is detected by the cell temperature detecting part80a. The speed of fall Voff of the temperature of the sensor cell51when turning the heater55off is determined in advance by experiments or calculations. If it is judged at step S103that the speed of fall Vdt is faster than the speed of fall Voff, the present control routine proceeds to step S104.

At step S104, the judging part80cjudges that the water repellency of the protective layer60is falling and sets the sensor abnormality flag Fsa to “1”. The initial value of the sensor abnormality flag Fsa is zero. Further, the sensor abnormality flag Fsa is made zero when the ignition switch of the vehicle carrying the internal combustion engine1is turned off or when the internal combustion engine1is stopped. After step S104, the present control routine is ended.

On the other hand, if it is judged at step S102that the temperature of the sensor cell51has not fallen from the target temperature TT or if it is judged at step S103that the speed of fall Vdt is the speed of fall Voff or less, the present control routine is ended.

<Processing for Setting Target Temperature>

Below, referring to the flow chart ofFIG. 8, control for setting the target temperature of the sensor cell51will be explained.FIG. 8is a flow chart showing the control routine of processing for setting a target temperature in the first embodiment of the present invention. The present control routine is repeatedly performed by the ECU80at predetermined time intervals after the startup of the internal combustion engine1.

First, at step S201, the heater control part80bjudges whether the sensor abnormality flag Fsa has been set to “1”. If it is judged that the sensor abnormality flag Fsa has been set to “1”, the present control routine proceeds to step S202. In this case, the judging part80cjudges that the water repellency of the protective layer60is falling.

At step S202, the heater control part80braises the target temperature TT. Specifically, the heater control part80bmakes the value of the amount of rise RT of the target temperature TT of the sensor cell51added to the current target temperature TT the new target temperature TT. The amount of rise RT is predetermined and for example is 50° C. to 100° C. The initial value of the target temperature TT is a temperature of the lowest temperature at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60or more and, for example, is 400° C. or more. Further, the target temperature TT is returned to the initial value when the ignition switch of the vehicle carrying the internal combustion engine1is turned off or the internal combustion engine1is stopped. Note that, the initial value of the target temperature TT may be set to a temperature higher than the operating temperature of the sensor cell51, for example, 700° C. or more. The operating temperature of the sensor cell51is the activation temperature of the sensor cell51or more and, for example, is 600° C. to 650° C.

Next, at step S203, the heater control part80bsets the sensor abnormality flag Fsa to zero. After step S203, the present control routine is ended.

On the other hand, if it is judged at step S201that the sensor abnormality flag Fsa has been set to zero, the present control routine is ended. In this case, the target temperature TT is maintained at the current target temperature.

Second Embodiment

The configuration and control of the control device of an exhaust sensor according to a second embodiment are basically similar to the configuration and control of the control device of an exhaust sensor according to the first embodiment except for the points explained below. For this reason, below, the second embodiment of the present invention will be explained centered on the parts different from the first embodiment.

If the protective layer60is covered by water when the water repellency of the protective layer60is falling and the temperature of the sensor cell51falls from the target temperature, the amount of fall of the temperature of the sensor cell51from the target temperature becomes a predetermined amount or more. For this reason, in the second embodiment, the condition for judging abnormality includes the amount of fall of the temperature of the sensor cell51from the target temperature being a predetermined amount or more. The predetermined amount is determined in advance by experiments or calculations and is for example 15° C. In the second embodiment, it is possible to suppress misjudging that the water repellency of the protective layer60is falling by detecting change of the temperature of the sensor cell51due to factors other than coverage by water, so it is possible to more precisely detect a fall in the water repellency of the protective layer60.

Further, in the second embodiment, the judging part80cjudges the degree of fall of the water repellency of the protective layer60. The judging part80cjudges the degree of fall of the water repellency of the protective layer60to be larger the larger the amount of fall of the temperature of the sensor cell51from the target temperature when the temperature of the sensor cell51falls from the target temperature at a speed faster than the speed of fall of the temperature of the sensor cell51when turning the heater55off. The judging part80cuses a map such as shown inFIG. 9to calculate the degree of fall of the water repellency of the protective layer60. In this map, the degree of fall of the water repellency of the protective layer60is shown as a function of the amount of fall in temperature ΔT of the sensor cell51.

As will be understood fromFIG. 9, the larger the degree of fall of the water repellency of the protective layer60, the higher the temperature required for causing the Leidenfrost phenomenon. For this reason, the heater control part80bincreases the amount of rise of the target temperature of the sensor cell51when the degree of fall of the water repellency of the protective layer60is relatively large compared with when the degree of fall of the water repellency of the protective layer60is relatively small. Due to this, it is possible to use the Leidenfrost phenomenon to prevent the element of the air-fuel ratio sensor10from cracking while suppressing an increase in the power consumption of the heater55due to rising the target temperature.

The amount of rise of the target temperature of the sensor cell51is, for example, calculated using a map such as shown inFIG. 10. In this map, the amount of rise of the target temperature of the sensor cell51is shown as a function of the degree of fall of the water repellency of the protective layer60. Note that, the amount of rise of the target temperature may be made larger in stages (in steps) as the degree of fall of the water repellency becomes larger, as shown inFIG. 10by the broken line.

<Control Routine of Processing for Judging Abnormality>

FIG. 11is a flow chart showing a control routine of processing for judging abnormality in the second embodiment of the present invention. The present control routine is repeatedly performed by the ECU80at predetermined time intervals after the startup of the internal combustion engine1. Step S301to step S303inFIG. 11are similar to step S101to step S103inFIG. 7, so explanations will be omitted.

The present control routine proceeds to step S304if at step S303it is judged that the speed of fall Vdt is faster than the speed of fall Voff. At step S304, the judging part80cjudges whether the amount of fall ΔT of the temperature of the sensor cell51from the target temperature TT is a predetermined amount A or more. The amount of fall ΔT is detected by the cell temperature detecting part80a. The predetermined amount A is for example 15° C. If at step S304it is judged that the amount of fall ΔT is the predetermined amount A or more, the present control routine proceeds to step S305.

At step S305, the judging part80cjudges that the water repellency of the protective layer60is falling and sets the sensor abnormality flag Fsa to “1”. The initial value of the sensor abnormality flag Fsa is zero. Further, the sensor abnormality flag Fsa is made zero when the ignition switch of the vehicle carrying the internal combustion engine1is turned off or when the internal combustion engine1is stopped.

Next, at step S306, the judging part80cjudges the degree of fall of the water repellency of the protective layer60. The judging part80cjudges the degree of fall of the water repellency of the protective layer60to be larger the larger the amount of fall of the temperature of the sensor cell51from the target temperature when the temperature falls from the target temperature at a speed faster than the speed of fall of the temperature of the sensor cell51when turning the heater55off. Specifically, the judging part80cuses a map such as shown inFIG. 9to calculate the degree of fall of the water repellency of the protective layer60based on the amount of fall ΔT of the temperature of the sensor cell51from the target temperature TT. After step S306, the present control routine is ended.

On the other hand, if at step S304the amount of fall ΔT is less than the predetermined amount A, the present control routine is ended.

<Processing for Setting Target Temperature>

FIG. 12is a flow chart showing the control routine of processing for setting a target temperature in the second embodiment of the present invention. The present control routine is repeatedly performed by the ECU80at predetermined time intervals after the startup of the internal combustion engine1. Step S401and step S404atFIG. 12are similar to step S201and step S203atFIG. 8, so explanations will be omitted.

The present control routine proceeds to step S402if at step S401it is judged that the sensor abnormality flag Fsa is set to “1”.

At step S402, the heater control part80bcalculates the amount of rise RT of the target temperature TT of the sensor cell51based on the degree of fall of the water repellency of the protective layer60. The degree of fall of the water repellency of the protective layer60is judged by the judging part80c. The heater control part80bincreases the amount of rise RT when the degree of fall of the water repellency of the protective layer60is relatively large compared to when the degree of fall of the water repellency of the protective layer60is relatively small. For example, the heater control part80buses a map such as shown inFIG. 10to calculate the amount of rise RT.

Next, at step S403, the heater control part80braises the target temperature TT. Specifically, the heater control part80bmakes the value of the amount of rise RT calculated at step S402added to the value of the current target temperature TT the new target temperature TT. The initial value of the target temperature TT is a temperature of the lowest temperature at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60or more and is for example 400° C. or more. Further, the target temperature TT is returned to the initial value when the ignition switch of the vehicle carrying the internal combustion engine1is turned off or when the internal combustion engine1is stopped. Note that, the initial value of the target temperature TT may be set to a temperature of a temperature higher than the operating temperature of the sensor cell51, for example, 700° C., or more. The operating temperature of the sensor cell51is the activation temperature of the sensor cell51or more and, for example, is 600° C. to 650° C.

Third Embodiment

The configuration and control of the control device of an exhaust sensor according to a third embodiment are basically similar to the configuration and control of the control device of an exhaust sensor according to the first embodiment except for the points explained below. For this reason, below, the third embodiment of the present invention will be explained centered on the parts different from the first embodiment.

The water penetrating the protective layer60evaporates in the protective layer60. For this reason, if the protective layer60is covered by water when the water repellency of the protective layer60is falling, the temperature of the sensor cell51will fall from the target temperature, then will again rise to the target temperature. Further, the time period from when the temperature of the sensor cell51falls from the target temperature to when it rises to the target temperature is shorter than the ignition period of the internal combustion engine1(time interval from when ignition is performed in a certain cylinder to when ignition is performed in the next cylinder). For this reason, in the third embodiment, the condition for judging abnormality includes the temperature of the sensor cell51falling from the target temperature then rising to the target temperature and the time period from when the temperature of the sensor cell51falls from the target temperature to when it rises to the target temperature being shorter than the ignition period at the internal combustion engine1. The time period from when the temperature of the sensor cell51falls from the target temperature to when it rises to the target temperature is detected by the cell temperature detecting part80a. The ignition period in the internal combustion engine1is calculated based on the number of cylinders of the internal combustion engine1and the engine speed. The engine speed is detected by the crank angle sensor108.

FIG. 13is a schematic time chart of the temperature of the sensor cell51detected by the cell temperature detecting part80awhen the water repellency of the protective layer60is falling.FIG. 13shows the time period Tdu from when the temperature of the sensor cell51falls from the target temperature TT to when it rises to the target temperature TT.

Further, when coverage by water causes the temperature of the sensor cell51to change, the speed of fall and speed of rise of the temperature of the sensor cell51become a predetermined speed or more. For this reason, in the third embodiment, the condition for judging abnormality includes the temperature of the sensor cell51falling from the target temperature, then rising to the target temperature and the speed of fall and speed of rise of the temperature of the sensor cell51being a predetermined speed or more. The speed of fall and speed of rise of the temperature are detected by the cell temperature detecting part80a. Further, the predetermined speed is determined in advance by experiments or calculations and is, for example, 1500 (° C./sec).

Further, if water penetrates the protective layer60, since time is taken for the water to evaporate, the speed of rise of the temperature of the sensor cell51becomes slower than the speed of fall of the temperature of the sensor cell51. For this reason, in the third embodiment, the condition for judging abnormality includes the temperature of the sensor cell51falling from the target temperature, then rising to the target temperature and the speed of fall of the temperature of the sensor cell51being faster than the speed of rise of the temperature of the sensor cell51.

In the third embodiment, it is possible to suppress misjudging that the water repellency of the protective layer60is falling by detecting a change in the temperature of the sensor cell51due to factors other than coverage by water, so it is possible to more precisely detect the water repellency of the protective layer60.

<Control Routine of Processing for Judging Abnormality>

FIG. 14is a flow chart showing a control routine of processing for judging abnormality in a third embodiment of the present invention. The present control routine is repeatedly performed by the ECU80at predetermined time intervals after the startup of the internal combustion engine1. Step S501to step S503inFIG. 14are similar to step S101to step S103inFIG. 7, so explanations will be omitted.

The present control routine proceeds to step S504if at step S503it is judged that the speed of fall Vdt is faster than the speed of fall Voff. At step S504, the judging part80cjudges whether the temperature of the sensor cell51fell from the target temperature TT, then rose to the target temperature TT. If at step S504it is judged that the temperature of the sensor cell51fell from the target temperature TT, then rose to the target temperature TT, the present control routine proceeds to step S505.

At step S505, the judging part80cjudges whether the time period Tdu from when the temperature of the sensor cell51falls from the target temperature TT to when it rises to the target temperature TT is shorter than the ignition period Ti in the internal combustion engine1. The time period Tdu is detected by the cell temperature detecting part80a. The ignition period Ti in the internal combustion engine1is calculated based on the number of cylinders of the internal combustion engine1and the engine speed. If at step S505it is judged that the time period Tdu is shorter than the ignition period Ti, the present control routine proceeds to step S506.

At step S506, the judging part80cjudges whether the speed of fall Vdt and the speed of rise Vut of the temperature of the sensor cell51Vdt are a predetermined speed Vref or more. The speed of fall Vdt and the speed of rise Vut of the temperature of the sensor cell51are detected by the cell temperature detecting part80a. The predetermined speed Vref is for example 1500 (° C./sec). If at step S506it is judged that the speed of fall Vdt and the speed of rise Vut are the predetermined speed Vref or more, the present control routine proceeds to step S507.

At step S507, the judging part80cjudges whether the speed of fall Vdt of the temperature of the sensor cell51is faster than the speed of rise Vut of the temperature of the sensor cell51. If at step S507it is judged that the speed of fall Vdt is faster than the speed of rise Vut of the temperature, the present control routine proceeds to step S508.

At step S508, the judging part80cjudges that the water repellency of the protective layer60is falling and sets the sensor abnormality flag Fsa to “1”. The initial value of the sensor abnormality flag Fsa is zero. Further, the sensor abnormality flag Fsa is made zero when the ignition switch of the vehicle carrying the internal combustion engine1is turned off or when the internal combustion engine1is stopped. After step S508, the present control routine is ended.

On the other hand, if at step S504it is judged that the temperature of the sensor cell51did not fall from the target temperature TT, then rise to the target temperature TT, if at step S505it is judged that the time period Tdu is the ignition period Ti or more, if at step S506it is judged that the speed of fall Vdt and the speed of rise Vut are less than the predetermined speed Vref, or if at step S507it is judged that the speed of fall Vdt is the speed of rise Vut of the temperature or less, the present control routine is ended.

Note that, in the present control routine, any one or two of step S505to step S507may be omitted.

Fourth Embodiment

The configuration and control of the control device of an exhaust sensor according to a fourth embodiment are basically similar to the configuration and control of the control device of an exhaust sensor according to the first embodiment except for the points explained below. For this reason, below, the fourth embodiment of the present invention will be explained centered on the parts different from the first embodiment.

FIG. 15is a block diagram schematically showing the configuration of a control device of an exhaust sensor according to the fourth embodiment of the present invention. The control device of an exhaust sensor is further provided with an output detecting part80d. The output detecting part80ddetects the output of the air-fuel ratio sensor10detected by the current detector72. In the present embodiment, the output detecting part80dis part of the ECU80.

If the water repellency of the protective layer60falls, part of the water striking the protective layer60penetrates the protective layer60. As a result, exhaust gas is obstructed from passing through the diffusion regulating layer16and flowing into the measured gas chamber30. For this reason, if the protective layer60is covered by water when the absolute value of the output of the air-fuel ratio sensor10is the value of a reference value or more and the water repellency of the protective layer60is falling, the absolute value of the output of the air-fuel ratio sensor10will fall toward zero.

For this reason, in the fourth embodiment, the condition for judging abnormality includes the absolute value of the output of the air-fuel ratio sensor10detected by the output detecting part80dfalling from a value of the reference value or more when the temperature of the sensor cell51falls from the target temperature. The reference value is determined in advance and is, for example, the value of the output corresponding to an air-fuel ratio of 14.65 or the absolute value of the output corresponding to an air-fuel ratio of 14.55.

Further, if the protective layer60is covered by water and the absolute value of the output of the air-fuel ratio sensor10falls from a value of the reference value or more when the water repellency of the protective layer60is falling, the amount of fall from a value of a reference value of the absolute value of the output of the air-fuel ratio sensor10or from more becomes a predetermined amount or more. For this reason, in the fourth embodiment, the condition for judging abnormality includes the amount of fall from a reference value of an absolute value of the output of the air-fuel ratio sensor10or more being a predetermined amount or more. The predetermined amount is determined in advance by experiments or calculations and for example is 10% of the absolute value of the output before the fall.

Further, if the protective layer60is covered by water when the water repellency of the protective layer60falls, the absolute value of the output of the air-fuel ratio sensor10falls from a value of the reference value or more, then again rises to a value of the reference value or more. Further, the time period from when the absolute value of the output of the air-fuel ratio sensor10falls from a value of a reference value or more to when it rises to a value of the reference value or more is shorter than the ignition period in the internal combustion engine1. For this reason, in the fourth embodiment, the condition for judging abnormality includes the absolute value of the output of the air-fuel ratio sensor10falling from a value of the reference value or more, then rising to a value of the reference value or more and the time period from when the absolute value of the output of the air-fuel ratio sensor10falls from a value of the reference value or more to when it rises to a value of the reference value or more being shorter than the ignition period in the internal combustion engine1. The time period from when the absolute value of the output of the air-fuel ratio sensor10falls from a value of the reference value or more to when it rises to a value of the reference value or more is detected by the output detecting part80d. The ignition period at the internal combustion engine1is calculated based on the number of cylinders of the internal combustion engine1and the engine speed. The engine speed is detected by the crank angle sensor108.

Further, if the absolute value of the output of the air-fuel ratio sensor10changes due to coverage by water, the speed of fall and the speed of rise of the absolute value of the output of the air-fuel ratio sensor10becomes a predetermined speed or more. For this reason, in the fourth embodiment, the condition for judging abnormality includes the absolute value of the output of the air-fuel ratio sensor10falling from a value of a reference value or more, then rising to a value of the reference value or more and the speed of fall and the speed of rise of the absolute value of the output being a predetermined speed or more. The speed of fall and the speed of rise of the absolute value of the output are detected by the output detecting part80d. Further, the predetermined speed is determined in advance by experiments or calculations and, for example, is the rate of change of output corresponding to a rate of change of air-fuel ratio of 100/sec. Note that, this value is larger than the amount of change of output occurring due to normal combustion.

Further, if water penetrates the protective layer60, since time is taken for the water to evaporate, the speed of rise of the absolute value of the output of the air-fuel ratio sensor10becomes slower than the speed of fall of the absolute value of the output. For this reason, in the fourth embodiment, the condition for judging abnormality includes the absolute value of the output of the air-fuel ratio sensor10falling from a value of the reference value or more, then rising to a value of the reference value or more and the speed of fall of the absolute value of the output being faster than the speed of rise of the absolute value of the output.

Further, when the air-fuel ratio of the exhaust gas introduced into the measured gas chamber30is at the stoichiometric air-fuel ratio (14.60), the output of the air-fuel ratio sensor10becomes substantially zero. However, if the water repellency of the protective layer60falls, the concentration of water in the exhaust gas introduced into the measured gas chamber30becomes higher and part of the oxygen atoms in the water molecules are broken down at the sensor cell51. As a result, if the protective layer60is covered by water when the output of the air-fuel ratio sensor10is a value in a near zero region and the water repellency of the protective layer60is falling, the output of the air-fuel ratio sensor10temporarily rises.

For this reason, in the fourth embodiment, the condition for judging abnormality includes the output of the air-fuel ratio sensor10detected by the output detecting part80drising from a value in the near zero region when the temperature of the sensor cell51falls. The near zero region is determined in advance and is a range of output corresponding to a range of air-fuel ratio of, for example, 14.55 to 14.65.

Further, the time period from when the output of the air-fuel ratio sensor10rises from a value in the near zero region to when it falls to a value in the near zero region is shorter than the ignition period of the internal combustion engine1. For this reason, in the fourth embodiment, the condition for judging abnormality includes the output of the air-fuel ratio sensor10rising from a value in the near zero region, then falling to a value in the near zero region and the time period from when the output of the air-fuel ratio sensor10rises from a value in the near zero region to when it falls to a value in the near zero region being shorter than the ignition period in the internal combustion engine1. The time period from when the output of the air-fuel ratio sensor10rises from a value in the near zero region, then falls to a value in the near zero region is detected by the output detecting part80d. The ignition period in the internal combustion engine1is calculated based on the number of cylinders of the internal combustion engine1and the engine speed. The engine speed is detected by the crank angle sensor108.

In the fourth embodiment, it is possible to suppress misjudging that the water repellency of the protective layer60is falling by judging a fall in the water repellency of the protective layer60based on not only the change of the temperature of the sensor cell51but also the change of the output of the air-fuel ratio sensor10, so it is possible to more precisely detect a fall in the water repellency of the protective layer60.

<Control Routine of Processing for Judging Abnormality>

FIG. 16toFIG. 18are flow charts showing a control routine of processing for judging abnormality in a fourth embodiment of the present invention. The present control routine is repeatedly performed by the ECU80at predetermined time intervals after the startup of the internal combustion engine1. Step S601to step S603inFIG. 16are similar to step S101to step S103inFIG. 7, so explanations will be omitted.

The present control routine proceeds to step S604if at step S603it is judged that the speed of fall Vdt is faster than the speed of fall Voff. At step S604, the judging part80cjudges whether the absolute value of the output of the air-fuel ratio sensor10is falling from a value of the reference value or more. The absolute value of the output of the air-fuel ratio sensor10is detected by the output detecting part80d. The reference value is, for example, the value of the output corresponding to an air-fuel ratio of 14.65 or the absolute value of the output corresponding to an air-fuel ratio of 14.55. If at step S604it is judged that the absolute value of the output of the air-fuel ratio sensor10is falling from a value of the reference value or more, the present control routine proceeds to step S606.

At step S606, the judging part80cjudges whether the amount of fall ΔO from a value of the reference value of the absolute value of the output of the air-fuel ratio sensor10or more is a predetermined amount B or more. The predetermined amount B is, for example, 10% of the absolute value of the output before the fall. If at step S606it is judged that the amount of fall ΔO is the predetermined amount B or more, the present control routine proceeds to step S607.

At step S607, the judging part80cjudges whether the absolute value of the output of the air-fuel ratio sensor10fell from a value of the reference value or more, then rose to a value of the reference value or more. If at step S607it is judged that the absolute value of the output of the air-fuel ratio sensor10fell from a value of the reference value or more, then rose to a value of the reference value or more, the present control routine proceeds to step S608.

At step S608, it is judged whether the time period OTdu from when the absolute value of the output of the air-fuel ratio sensor10fell from a value of the reference value or more, then rose to a value of the reference value or more is shorter than the ignition period Ti in the internal combustion engine1. The time period OTdu is detected by the output detecting part80d. The ignition period Ti in the internal combustion engine1is calculated based on the number of cylinders of the internal combustion engine1and the engine speed. If at step S608it is judged that the time period OTdu is shorter than the ignition period Ti, the present control routine proceeds to step S609.

At step S609, the judging part80cjudges whether the speed of fall Vdo and the speed of rise Vuo of the absolute value of the output of the air-fuel ratio sensor10are a predetermined speed Vrefo or more. The speed of fall Vdo and the speed of rise Vuo are detected by the output detecting part80d. The predetermined speed Vrefo, for example, is the rate of change of output corresponding to a rate of change of air-fuel ratio of 100/sec. If at step S609it is judged that the speed of fall Vdo and the speed of rise Vuo are the predetermined speed Vrefo or more, the present control routine proceeds to step S610.

At step S610, the judging part80cjudges whether the speed of fall Vdo is faster than the speed of rise Vuo. If at step S610it is judged that the speed of fall Vdo is faster than the speed of rise Vuo, the present control routine proceeds to step S611.

At step S611, the judging part80cjudges that the water repellency of the protective layer60is falling and sets the sensor abnormality flag Fsa to “1”. The initial value of the sensor abnormality flag Fsa is zero. Further, the sensor abnormality flag Fsa is made zero when the ignition switch of the vehicle carrying the internal combustion engine1is turned off or when the internal combustion engine1is stopped. After step S611, the present control routine is ended.

On the other hand, if at step S606it is judged that the amount of fall ΔO is less than a predetermined amount B, if at step S607it is judged that the absolute value of the output of the air-fuel ratio sensor10did not fall from a value of the reference value or more, then rise to a value of the reference value or more, if at step S608it is judged that the time period OTdu is the ignition period Ti or more, if at step S609it is judged that speed of fall Vdo and the speed of rise Vuo are less than a predetermined speed Vrefo, or if at step S610it is judged that the speed of fall Vdo is the speed of rise Vuo or less, the present control routine is ended.

Further, if at step S604it is judged that the absolute value of the output of the air-fuel ratio sensor10has not fallen from a value of the reference value or more, the present control routine proceeds to step S605. At step S605, the judging part80cjudges whether the output of the air-fuel ratio sensor10has risen from a value in the near zero region. The near zero region is for example a range of output corresponding to a range of air-fuel ratio of 14.55 to 14.65. If at step S605it is judged that the output of the air-fuel ratio sensor10has risen from a value in the near zero region, the present control routine proceeds to step S612.

At step S612, the judging part80cjudges whether the output of the air-fuel ratio sensor10rose from a value in the near zero region, then fell to a value in the near zero region. If at step S612it is judged that the output of the air-fuel ratio sensor10rose from a value in the near zero region, then fell to a value in the near zero region, the present control routine proceeds to step S613.

At step S613, it is judged whether the time period OTud from when the output of the air-fuel ratio sensor10rose from a value in the near zero region to when it fell to a value in the near zero region is shorter than the ignition period Ti in the internal combustion engine1. The time period OTud is detected by the output detecting part80d. The ignition period Ti in the internal combustion engine1is calculated based on the number of cylinders of the internal combustion engine1and the engine speed. If at step S613it is judged that the time period OTud is shorter than the ignition period Ti, the present control routine proceeds to step S614.

At step S614, the judging part80cjudges that the water repellency of the protective layer60is falling and sets the sensor abnormality flag Fsa to “1”. After step S614, the present control routine is ended.

On the other hand, if at step S605it is judged that the output of the air-fuel ratio sensor10did not rise from a value in the near zero region, if at step S612it is judged that the output of the air-fuel ratio sensor10did not rise from a value in the near zero region, then fall to a value in the near zero region, or if at step S613it is judged that the time period OTud is the ignition period Ti or more, the present control routine is ended.

Note that, in the present control routine, step S604and step S606to step S611may be omitted. In this case, if at step S603it is judged that the speed of fall Vdt is faster than the speed of fall Voff, the present control routine proceeds to step S605. Further, in the present control routine, step S605and step S612to step S614may be omitted. In this case, if at step S604it is judged that the absolute value of the output of the air-fuel ratio sensor10has not fallen from a value of the reference value or more, the present control routine is ended. Further, any six or fewer steps in step S606to step S610and step S612to step S613may be omitted.

Fifth Embodiment

The configuration and control of the control device of an exhaust sensor according to a fifth embodiment are basically similar to the configuration and control of the control device of an exhaust sensor according to the first embodiment except for the points explained below. For this reason, below, the fifth embodiment of the present invention will be explained centered on the parts different from the first embodiment.

FIG. 19is a block diagram schematically showing the configuration of a control device of an exhaust sensor according to the fifth embodiment of the present invention. The control device of an exhaust sensor is further provided with an exhaust pipe temperature estimating part80e. In the present embodiment, the exhaust pipe temperature estimating part80eis part of the ECU80.

The exhaust pipe temperature estimating part80eestimates the temperature of the exhaust pipe27around the air-fuel ratio sensor10(below, simply referred to as the “temperature of the exhaust pipe27”).FIG. 20is a view schematically showing an internal combustion engine1in which a control device of an exhaust sensor according to the fifth embodiment of the present invention is used. For example, the exhaust pipe temperature estimating part80eestimates the temperature of the exhaust pipe27from the output of the exhaust temperature sensor105arranged in the exhaust passage near the air-fuel ratio sensor10. The exhaust temperature sensor105is arranged near the air-fuel ratio sensor10and detects the temperature of the exhaust pipe27. The output of the exhaust temperature sensor105is input through the corresponding AD converter87to the input port85of the ECU80.

Note that, the exhaust pipe temperature estimating part80emay estimate the temperature of the exhaust pipe27without using the exhaust temperature sensor105. In this case, the internal combustion engine1need not be provided with the exhaust temperature sensor105near the air-fuel ratio sensor10. For example, the exhaust pipe temperature estimating part80emay estimate the temperature of the exhaust pipe27based on the elapsed time from when the internal combustion engine1is started up. In this case, the longer the elapsed time from when the internal combustion engine1is started up, the higher the temperature of the exhaust pipe27estimated by the exhaust pipe temperature estimating part80e.

Further, the exhaust pipe temperature estimating part80emay estimate the temperature of the exhaust pipe27based on the cumulative value of the amount of intake air supplied to a combustion chamber2from when the internal combustion engine1is started up (below, referred to as “cumulative amount of air”). The cumulative amount of air is, for example, calculated based on the output of the air flow meter102. In this case, the greater the cumulative amount of air, the higher the temperature of the exhaust pipe27estimated by the exhaust pipe temperature estimating part80e. Further, the exhaust pipe temperature estimating part80emay estimate the temperature of the exhaust pipe27based on the elapsed time and the cumulative amount of air from when the internal combustion engine1is started up. In this case, the exhaust pipe temperature estimating part80e, for example, estimates the temperature of the exhaust pipe27using a map such as shown inFIG. 21. In this map, the temperature PT of the exhaust pipe27is shown as a function of the elapsed time ET and the cumulative amount of air ΣMc.

Further, the exhaust pipe temperature estimating part80emay estimate the temperature of the exhaust pipe27based on the temperature of the cooling water of the internal combustion engine1. The temperature of the cooling water is, for example, detected by a water temperature sensor (not shown) arranged in the cooling water path of the internal combustion engine1.

As explained above, when the temperature of the exhaust pipe27is the dew point temperature of water or less, the water vapor in the exhaust gas condenses and condensed water is generated. For this reason, until the temperature of the exhaust pipe27reaches the dew point temperature of water, a lot of condensed water strikes the protective layer60of the air-fuel ratio sensor10together with the exhaust gas. On the other hand, if the temperature of the exhaust pipe27reaches the dew point temperature of water, new condensed water will not be generated inside the exhaust passage. For this reason, after the temperature of the exhaust pipe27reaches the dew point temperature of water, there is little possibility of the protective layer60being covered by water.

Therefore, in the fifth embodiment, after the temperature of the exhaust pipe27estimated by the exhaust pipe temperature estimating part80ereaches a predetermined temperature of the dew point temperature or more, the judging part80cdoes not judge if the water repellency of the protective layer60is falling. Due to this, it is possible to suppress misjudging that the water repellency of the protective layer60is falling by detecting a change of the temperature of the sensor cell51due to factors other than coverage by water, so it is possible to more precisely detect a drop in the water repellency of the protective layer60.

Further, after the temperature of the exhaust pipe27reaches the dew point temperature of water, there is little possibility that the element of the air-fuel ratio sensor10cracks due to coverage by water. For this reason, in the fifth embodiment, after the temperature of the exhaust pipe27estimated by the exhaust pipe temperature estimating part80ereaches a predetermined temperature of the dew point temperature or more, the heater control part80bsets the target temperature of the sensor cell51to a predetermined operating temperature. The operating temperature of the sensor cell51is the activation temperature of the sensor cell51or more and, for example, is 600° C. to 650° C. Due to this, it is possible to suppress an increase in the power consumption of the heater55due to maintaining the target temperature of the sensor cell51at a temperature higher than the operating temperature.

Further, if the temperature of the exhaust pipe27reaches the boiling point of water, the condensed water which remained in the exhaust passage evaporates and condensed water no longer strikes the protective layer60. For this reason, the above predetermined temperature may be the boiling point of water. Note that, the dew point is 54° C. at atmospheric pressure (1 atm), while the boiling point of water is 100° C. at atmospheric pressure.

<Explanation of Control Using Time Chart>

Below, referring to the time chart ofFIG. 22, control performed by the control device of an exhaust sensor in the fifth embodiment will be specifically explained.FIG. 22is a schematic time chart of the engine load, the temperature of the exhaust pipe27(exhaust pipe temperature around sensor), water repellency of the protective layer, and target temperature of the sensor cell51after starting up the internal combustion engine. In the illustrated example, the temperature of the exhaust pipe27is calculated from the output of the exhaust temperature sensor105.

In the illustrated example, at the time t0, the internal combustion engine1is started up. When the internal combustion engine1is started up, the target temperature of the sensor cell51is set to the initial temperature T0. The initial temperature T0is a temperature of the lowest temperature at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60or more and, for example, is 750° C.

In the illustrated example, at the time t1, it is judged that the water repellency of the protective layer60is falling. For this reason, at the time t1, the target temperature of the sensor cell51is risen from the initial temperature T0to the first temperature T1. The first temperature T1is a temperature at which the temperature of the outer surface of the protective layer60will become the combustion temperature of soot or more and is for example 800° C.

Further, at the time t2, it is judged that the water repellency of the protective layer60has again fallen. For this reason, at the time t2, the target temperature of the sensor cell51is risen from the first temperature T1to the second temperature T2. The second temperature T2is for example 850° C.

After the time t2, at the time t3, the temperature of the exhaust pipe27reaches a predetermined temperature PTref of the dew point temperature or more. For this reason, at the time t3, the target temperature of the sensor cell51is set to the operating temperature OT. The predetermined temperature PTref is for example the dew point temperature (54° C.) while the operating temperature OT is for example 650° C.

<Control Routine of Processing for Judging Abnormality>

FIG. 23is a flow chart showing a control routine of processing for judging abnormality in the fifth embodiment of the present invention. The present control routine is repeatedly performed by the ECU80at predetermined time intervals after the startup of the internal combustion engine1.

First, at step S701, the judging part80cjudges whether the temperature PT of the exhaust pipe27is a predetermined temperature Tref, which is the dew point temperature or more, or more. The temperature PT of the exhaust pipe27is estimated by the exhaust pipe temperature estimating part80eusing any of the above-mentioned methods. The predetermined temperature Tref is for example the dew point or boiling point of water.

If at step S701it is judged that the temperature PT of the exhaust pipe27is a predetermined temperature Tref or more, the present control routine is ended. In this case, it is not judged whether the water repellency of the protective layer60is falling. On the other hand, if at step S701it is judged that the temperature PT of the exhaust pipe27is less than a predetermined temperature Tref, the present control routine proceeds to step S702.

At step S702, the judging part80cjudges whether the target temperature TT of the sensor cell51set by the heater control part80bis a temperature of the lowest temperature TL at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60or more. The lowest temperature TL is for example 400° C.

If at step S702it is judged that the target temperature TT is less than the lowest temperature TL, the present control routine is ended. In this case, it is not judged whether the water repellency of the protective layer60is falling. On the other hand, if at step S702it is judged that the target temperature TT is the lowest temperature TL or more, the present control routine proceeds to step S703.

At step S703, the judging part80cjudges whether the condition for judging abnormality is satisfied. The condition for judging abnormality is at least one of the above-mentioned conditions for judging abnormality in the explanations of the first embodiment to the fourth embodiment.

If at step S703it is judged that the condition for judging abnormality is satisfied, the present control routine proceeds to step S704. At step S704, the judging part80cjudges that the water repellency of the protective layer60is falling and sets the sensor abnormality flag Fsa to “1”. The initial value of the sensor abnormality flag Fsa is zero. Further, the sensor abnormality flag Fsa is made zero when the ignition switch of the vehicle carrying the internal combustion engine1is turned off or when the internal combustion engine1is stopped.

Next, at step S705, in the same way as step S306ofFIG. 11, the judging part80cjudges the degree of fall of the water repellency of the protective layer60. After step S705, the present control routine is ended. On the other hand, if at step S703it is judged that the condition for judging abnormality is not satisfied, the present control routine is ended.

<Processing for Setting Target Temperature>

FIG. 24is a flow chart showing the control routine of processing for setting a target temperature in the fifth embodiment of the present invention. The present control routine is repeatedly performed by the ECU80at predetermined time intervals after the startup of the internal combustion engine1.

First, at step S801, the heater control part80bjudges whether the temperature PT of the exhaust pipe27estimated by the exhaust pipe temperature estimating part80eis a predetermined temperature PTref, which is the dew point temperature or more, or more. The predetermined temperature PTref is for example the dew point (54° C.) or boiling point of water (100° C.). If at step S801it is judged that the temperature PT of the exhaust pipe27is a predetermined temperature PTref or more, the present control routine proceeds to step S802.

At step S802, the heater control part80bsets the target temperature TT of the sensor cell51to the operating temperature OT. The operating temperature OT is the activation temperature of the sensor cell51or more and, for example, is 600° C. to 650° C. After step S802, the present control routine is ended.

On the other hand, if at step S801it is judged that the temperature PT of the exhaust pipe27is less than a predetermined temperature PTref, the present control routine proceeds to step S803. Step S803to step S806are similar to step S401to step S404inFIG. 12, so explanations will be omitted.

Note that, step S705ofFIG. 23and step S804ofFIG. 24may be omitted. In this case, the amount of rise RT used at step S805is made a predetermined value, for example, 50° C. to 100° C.

Sixth Embodiment

The configuration and control of the control device of an exhaust sensor according to a sixth embodiment are basically similar to the configuration and control of the control device of an exhaust sensor according to the first embodiment except for the points explained below. For this reason, below, the sixth embodiment of the present invention will be explained centered on the parts different from the first embodiment.

In the sixth embodiment, if the judging part80cjudges that the water repellency of the protective layer60is falling, the heater control part80braises the target temperature of the sensor cell51so that the temperature of the outer surface of the protective layer60becomes the combustion temperature of soot or more. If, in the period from when rising the target temperature to when the internal combustion engine1stops, a fall in the water repellency of the protective layer60is not again detected, there is a possibility that removal of the soot enabled the water repellency of the protective layer60to recover. For this reason, in the sixth embodiment, if the judging part80cdoes not judge the water repellency of the protective layer60has again fallen in the period from when first rising the target temperature after the startup of the internal combustion engine1to when the internal combustion engine1stops, the heater control part80breturns the target temperature after restart of the internal combustion engine1to the value before the rise. Due to this, it is possible to suppress an increase in the power consumption of the heater55due to rising the target temperature over a long period of time.

On the other hand, if a fall in the water repellency of the protective layer60is again detected in the period from when rising the target temperature to when the internal combustion engine1stops, the water repellency of the protective layer may permanently fall. For this reason, in the sixth embodiment, if the judging part80cjudges that the water repellency of the protective layer60has again fallen in the period from when first rising the target temperature after the startup of the internal combustion engine1to when the internal combustion engine1stops, the heater control part80bfurther raises the target temperature and maintains the target temperature after restart of the internal combustion engine1at the value after the rise. Due to this, even after restart of the internal combustion engine1, due to the Leidenfrost phenomenon, it is possible to effectively prevent the element of the air-fuel ratio sensor10from cracking.

In this regard, there is an upper limit to the temperature of the sensor cell51able to be controlled by the heater55. However, if a fall in the water repellency of the protective layer60causes the target temperature of the sensor cell51to be made to rise a plurality of times, the target temperature will sometimes exceed the upper limit temperature. Further, if the temperature of the protective layer60is a high temperature, when water penetrates the protective layer60, the thermal shock given to the protective layer60and element body50becomes larger. For this reason, if a fall in the water repellency of the protective layer60makes it difficult for the Leidenfrost phenomenon to occur at the outer surface of the protective layer60, it is preferable to maintain the temperature of the outer surface of the protective layer60at a low temperature to prevent the element of the air-fuel ratio sensor10from cracking due to coverage by water.

For this reason, in the sixth embodiment, the heater control part80bturns the heater55off if rising the target temperature of the sensor cell51a plurality of times causes the target temperature to exceed a predetermined upper limit temperature. The upper limit temperature is predetermined from the configuration of the sensor element12etc. and is for example 900° C. Note that, if turning the heater55off, the temperature of the sensor cell51becomes less than the activation temperature and accurate detection of the air-fuel ratio becomes difficult. For this reason, while the heater55is turned off, the air-fuel ratio sensor10does not detect the air-fuel ratio.

Further, in the sixth embodiment, in the same way as the fifth embodiment, the heater control part80bsets the target temperature of the sensor cell51to a predetermined operating temperature after the temperature of the exhaust pipe27estimated by the exhaust pipe temperature estimating part80ereaches a predetermined temperature of the dew point temperature or more. Due to this, the air-fuel ratio sensor10can be used to detect the air-fuel ratio after the amount of condensed water inside the exhaust passage becomes smaller.

Note that, the heater control part80bmay set the target temperature of the sensor cell51to a temperature of less than the lowest temperature at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60if rising target temperature of the sensor cell51a plurality of times causes the target temperature to exceed a predetermined upper limit temperature. For example, the heater control part80bsets the target temperature of the sensor cell51to 300° C. Due to this, compared with when turning the heater55off, after the temperature of the exhaust pipe27reaches a predetermined temperature of the dew point temperature or more, the target temperature can be made to quickly rise to the operating temperature, so it is possible to detect the air-fuel ratio using the air-fuel ratio sensor10early.

<Control Routine of Processing for Judging Abnormality>

FIG. 25is a flow chart showing a control routine of processing for judging abnormality in the sixth embodiment of the present invention. The illustrated control routine is repeatedly performed by the ECU80at predetermined time intervals after the startup of the internal combustion engine1. Step S901to step S905inFIG. 25are similar to step S701to step S705inFIG. 23, so explanations will be omitted.

The present control routine proceeds to step S906after step S905. At step S906, the judging part80cupdates the number of times of judgment COUNT.

Specifically, the judging part80cmakes the value of the current number of times of judgment COUNT plus1the new number of times of judgment COUNT. The number of times of judgment COUNT shows the number of times it is judged that the water repellency of the protective layer60is falling from when the internal combustion engine1is started to when it is stopped. The initial value of the number of times of judgment COUNT is zero. Further, the number of times of judgment COUNT is made zero when the ignition switch of the vehicle carrying the internal combustion engine1is turned off or when the internal combustion engine1is stopped. After step S906, the present control routine is ended.

<Processing for Setting Target Temperature>

FIG. 26is a flow chart showing the control routine of processing for setting a target temperature in the sixth embodiment of the present invention. The present control routine is performed repeatedly by the ECU80at predetermined time intervals after the startup of the internal combustion engine1.

First, at step S1001, the heater control part80bjudges whether the temperature PT of the exhaust pipe27estimated by the exhaust pipe temperature estimating part80eis a predetermined temperature PTref, which is the dew point temperature or more, or more. The predetermined temperature PTref is for example the dew point (54° C.) or boiling point of water (100° C.). If at step S1001it is judged that the temperature PT of the exhaust pipe27is less than a predetermined temperature PTref, the present control routine proceeds to step S1002.

At step S1002, the heater control part80bjudges whether the sensor abnormality flag Fsa has been set to “1”. If it is judged that the sensor abnormality flag Fsa has been set to zero, the present control routine proceeds to step S1003.

At step S1003, the heater control part80bsets the target temperature TT to the base temperature Tb. The initial value of the base temperature Tb is the lowest temperature at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60or more and, for example, is 400° C. or more. Note that, the initial value of the base temperature Tb may be set to a temperature higher than the operating temperature of the sensor cell51, for example, a temperature of 700° C. or more.

Next, at step S1004, the heater control part80bjudges whether the target temperature TT set at step S1003is the upper limit temperature Tut or less. The upper limit temperature Tut is for example 900° C. If it is judged at step S1004that the target temperature TT is the upper limit temperature Tut or less, the present control routine is ended.

On the other hand, if at step S1002it is judged that the sensor abnormality flag Fsa is set to “1”, the present control routine proceeds to step S1005. At step S1005, the heater control part80bjudges whether the number of times of judgment COUNT is 2 or more. The number of times of judgment COUNT is updated at step S906ofFIG. 25. If at step S1005it is judged that the number of times of judgment COUNT is “1”, the present control routine proceeds to step S1006.

At step S1006, in the same way as step S402ofFIG. 12, the target temperature TT of the heater control part80bcalculates the amount of rise RT of the sensor cell51based on the degree of fall of the water repellency of the protective layer60. Next, at step S1007, the heater control part80braises the target temperature TT. Specifically, the heater control part80bmakes the value of the amount of rise RT calculated at step S1006added to the current target temperature TT the new target temperature TT.

Next, at step S1004, the heater control part80bjudges whether the target temperature TT set at step S1007is the upper limit temperature Tut or less. If it is judged that the target temperature TT is the upper limit temperature Tut or less, the present control routine is ended.

On the other hand, if at step S1005it is judged that the number of times of judgment COUNT is 2 or more, the present control routine proceeds to step S1008. In this case, it is judged that the water repellency of the protective layer60has again fallen in the period from when rising the target temperature TT at step S1007, to when the internal combustion engine1stops.

At step S1008, in the same way as step S402ofFIG. 12, the heater control part80bcalculates the amount of rise RT of the base temperature Tb based on the degree of fall of the water repellency of the protective layer60. Next, at step S1009, the heater control part80braises the base temperature Tb. Specifically, the heater control part80bmakes the value of the amount of rise RT calculated at step S1008added to the current base temperature Tb the new base temperature Tb. The base temperature Tb is stored in the RAM83of the ECU80. The updated value is held even after the ignition switch is turned off.

Next, at step S1010, the heater control part80braises the target temperature TT. Specifically, the heater control part80bsets the target temperature TT to the base temperature Tb updated at step S1009. Next, at step S1011, the heater control part80bsets the sensor abnormality flag Fsa to zero.

Next, at step S1004, the heater control part80bjudges at step S1007, the target temperature TT set at step S1010is the upper limit temperature Tut or less. If it is judged that the target temperature TT is the upper limit temperature Tut or less, the present control routine is ended.

On the other hand, if at step S1004it is judged that the target temperature TT set at step S1003, step S1007, or step S1010is higher than the upper limit temperature Tut, the present control routine proceeds to step S1012. In this case, it is difficult to cause the Leidenfrost phenomenon to occur at the outer surface of the protective layer60and thereby prevent the element of the air-fuel ratio sensor10from cracking, so at step S1012, the heater control part80bturns the heater55off. After step S1012, the present control routine is ended.

Further, if it is judged at step S1001that the temperature PT of the exhaust pipe27is a predetermined temperature PTref or more, the present control routine proceeds to step S1013. At step S1013, the heater control part80bsets the target temperature TT of the sensor cell51to the operating temperature OT. The operating temperature OT is the activation temperature of the sensor cell51or more and, for example 600° C. to 650° C. After step S1013, the present control routine is ended.

Note that, at step S1012, the heater control part80bmay set the target temperature of the sensor cell51to a temperature (for example 300° C.) less than the lowest temperature at which the Leidenfrost phenomenon occurs at the outer surface of the protective layer60. Further, step S905ofFIG. 25and step S1006and step S1008ofFIG. 26may be omitted. In this case, the amount of rise RT used at step S1007and step S1009is made a predetermined value, for example, 50° C. to 100° C. Further, the amount of rise RT used at step S1007and the amount of rise RT used at step S1009may be different values.

Above, preferred embodiments according to the present invention were explained, but the present invention is not limited to these embodiments. Various corrections and changes may be made within the language of the claims. For example, the exhaust sensor controlled by the control device of the exhaust sensor may be an oxygen sensor detecting if the air-fuel ratio of the exhaust gas is rich or lean by detecting the oxygen in the exhaust gas. Further, the exhaust sensor may be a nitrogen oxide sensor (NOXsensor) detecting the concentration of nitrogen oxides (NOX) in the exhaust gas, a sulfur oxide sensor (SOXsensor) detecting the concentration of sulfur oxides (SOX) in the exhaust gas, etc.

Further, the element body of the exhaust sensor may be provided with another electrochemical cell in addition to the sensor cell. The other electrochemical cell is, for example, a pump cell discharging the oxygen in the measured gas from the measured gas chamber, a monitor cell detecting the concentration of a specific component in the measured gas, etc. In this case, the heater control part may set the target temperature of the pump cell or monitor cell and control the heater so that the temperature of the pump cell or monitor cell becomes the target temperature. The temperature of the pump cell or monitor cell is calculated by its impedance, etc.

Further, the above-mentioned embodiments can be freely combined and carried out. For example, after step S508ofFIG. 14, step S611ofFIG. 17or step S614ofFIG. 18, step S306ofFIG. 11may be performed.

REFERENCE SIGNS LIST