JP2006322389A - Catalyst degrading prevention device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively prevent a catalyst layer in an air-fuel ratio sensor from being degraded. <P>SOLUTION: The air-fuel ratio sensor 221 comprises a sensor element 221a, the catalyst layer 221f, and a heater 221g. The heater 221g is so formed that an energization amount is duty-controlled by an ECU 100. When requirements for performing fuel cut are satisfied, the ECU 100 determines whether or not the temperature of the sensor element 221a is higher than the degrading start temperature of the catalyst layer 221f. When the temperature of the sensor element is higher than the degrading start temperature, it controls the energization amount to the heater 221g so that the temperature of the sensor element 221a can be lowered to a predetermined lower limit temperature. When the temperature of the sensor element 221a is lowered to less than the degrading start temperature by the control, the ECU 100 performs the fuel cut. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technical field of a catalyst deterioration preventing device for preventing deterioration of a catalyst provided in an air-fuel ratio sensor.

  As an air-fuel ratio sensor including a catalyst, a sensor provided with a heater has been proposed (for example, see Patent Document 1). According to the air-fuel ratio control device for a gas engine disclosed in Patent Document 1 (hereinafter referred to as “conventional technology”), the surface of the exhaust gas side electrode in the sensor is kept at 600 ° C. or higher by a heater, thereby It is said that it is removed by spontaneous combustion, prevents output deviation of the sensor, and enables proper air-fuel ratio control.

  In addition, as a technique for preventing the deterioration of the catalyst in the catalytic converter for exhaust gas purification, a technique for prohibiting fuel cut at a high temperature has been proposed (for example, see Patent Document 2).

  In addition, the technique which controls heater energization in consideration of an operation history at the time of fuel cut operation execution is also proposed (for example, refer to patent documents 3).

  In addition, a technique for controlling the heater of the lean sensor in accordance with the operating state in consideration of the fact that the lean sensor is heated by the exhaust gas has been proposed (for example, see Patent Document 4).

  In addition, a technique for controlling the heater energization according to the exhaust temperature determined by the operation parameter has been proposed (see, for example, Patent Document 5).

Japanese Patent Laid-Open No. 11-30141 JP-A-8-144814 Japanese Patent No. 2816553 Japanese Patent No. 2887351 JP-A-6-194338

  In an internal combustion engine, fuel supply is sometimes stopped (fuel cut) from the viewpoint of efficiently using fuel. When the fuel cut is performed, the exhaust pipe has a lean atmosphere. Therefore, when the surface of the exhaust gas side electrode is maintained at 600 ° C. or higher as in the conventional technique, the catalyst provided in the air-fuel ratio sensor is accompanied by the fuel cut. Will be exposed to a high temperature and lean atmosphere. On the other hand, since the deterioration of such a catalyst is accelerated in a high temperature and lean atmosphere, as a result, the deterioration of the catalyst is promoted with a high probability when the fuel supply is cut. On the other hand, if fuel cut at high temperatures is prohibited, even if the catalyst can be prevented from degrading to some extent, the economy may be sacrificed. That is, the conventional technique has a technical problem that it is difficult to effectively prevent catalyst deterioration in the air-fuel ratio sensor.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a catalyst deterioration prevention device that can effectively prevent catalyst deterioration in an air-fuel ratio sensor including a catalyst.

In order to solve the above-described problems, a catalyst deterioration prevention device according to the present invention is installed in an exhaust system of an internal combustion engine, (i) a catalyst for removing hydrogen contained in the exhaust system, and (ii) the internal combustion engine. Deterioration of the catalyst is prevented in a vehicle having a sensor element for detecting the air-fuel ratio of the engine, and (iii) an air-fuel ratio sensor having a heater for heating the sensor element, and supply means for supplying fuel to the internal combustion engine A catalyst deterioration prevention device, which is predetermined as a condition for specifying the temperature of the sensor element, and controlling the supply means so that the fuel is supplied, and stopping the supply of the fuel. and supply control means for supply of the fuel to control the supply means so as to be stopped when a predetermined supply stop condition is satisfied, it the active temperature T ₁ of the temperature is given of the sensor element And when the supply stop condition is satisfied and the temperature of the sensor element is equal to or higher than the deterioration start temperature T ₂ (T ₂ <T ₁ ) of the catalyst. And a heater control means for controlling the heater so that the temperature is lowered.

  In the present invention, the “internal combustion engine” is a concept that encompasses an engine that operates by directly using a gas generated by fuel combustion, and preferably refers to an engine that uses gasoline, light oil, LPG, or the like as fuel. Further, the “exhaust system” in the present invention is a concept including a mechanism for exhausting gas from the inside of a cylinder (cylinder) via an exhaust valve. Typically, on the exhaust pipe (exhaust manifold), In particular, it refers to the front stage (cylinder side) of a catalytic device such as a catalytic converter that purifies HC (hydrocarbon), CO (carbon monoxide) and NOx (nitrogen oxide) in the exhausted gas. It is not limited to this as long as it is done.

  The “air-fuel ratio sensor” in the present invention outputs a physical quantity corresponding to an oxygen concentration in at least a gas flowing through an exhaust system (hereinafter referred to as “exhaust gas” as appropriate) in order to detect an air-fuel ratio in an internal combustion engine. For example, a linear air-fuel ratio sensor or an O2 sensor may be used. The physical quantity corresponding to the oxygen concentration may be, for example, a current value or a voltage value. In this case, a correspondence relationship between a physical quantity such as a current value or a voltage value as a sensor output and an air-fuel ratio may be determined in advance, or a correspondence relationship between the physical quantity and the oxygen concentration may be determined in advance. Further, the air-fuel ratio may be specified by an ECU (Electronic Controlling Unit) or the like based on such a predetermined correspondence.

  The air-fuel ratio sensor according to the present invention includes a sensor element, a heater, and a catalyst.

Here, the “sensor element” is taken out of the sensor element main body or from the sensor element through an electrode or the like according to a change in oxygen concentration in a gas flowing through the exhaust system (hereinafter referred to as “exhaust gas” as appropriate). This is a concept encompassing an element capable of changing some physical quantity, for example, zirconia (ZrO ₂ ) or an oxygen ion conductive oxide sintered body in which CaO or the like is dissolved as a stabilizer. In this case, since movement of oxygen ions occurs according to the difference in oxygen concentration at both ends of the sensor element, a physical quantity such as voltage or current generated due to the movement of oxygen ions is output as a physical quantity corresponding to the oxygen concentration. This makes it possible to detect the oxygen concentration in the exhaust gas. The shape of the sensor element is not limited as long as the oxygen concentration can be detected. For example, it may have a bottomed cylindrical shape or a flat plate shape.

  An activation temperature is defined for the sensor element based on the physical characteristics of the sensor element. The sensor element exhibits oxygen ion conduction characteristics relatively well within this active temperature range. In actual use of the air-fuel ratio sensor, it is necessary to heat the sensor element to a temperature that belongs to a temperature range corresponding to the activation temperature. This activation temperature belongs to, for example, a temperature range of 700 ° C. to 800 ° C., and is about 750 ° C. when the sensor element is zirconia, for example.

  The “heater” according to the present invention is a concept encompassing means for heating the sensor element as described above, and refers to, for example, a ceramic rod-shaped heating element disposed in proximity to the sensor element. However, the mode of the heater is not limited as long as the sensor element can be heated.

  On the other hand, it is known that the presence of hydrogen in the exhaust gas affects the detection accuracy of the oxygen concentration by the sensor element. For this reason, the air-fuel ratio sensor according to the present invention includes a catalyst for removing hydrogen. Here, the “catalyst for removing hydrogen” is a concept encompassing means capable of considerably reducing the influence of hydrogen on the sensor element as compared with the case where no such catalyst is provided. Yes, as long as such a concept is secured, it may be possible to reduce the influence of substances other than hydrogen. For example, such a catalyst may be one in which a catalyst component such as platinum or rhodium is supported on a porous layer such as alumina as a catalyst carrier. The catalyst may be installed in any position as long as the influence of hydrogen on the sensor element can be reduced, but preferably takes the form of a catalyst layer formed integrally with the sensor element.

  The air-fuel ratio sensor has various means for outputting a physical quantity corresponding to the oxygen concentration in addition to the sensor element, the heater and the catalyst. For example, the mechanical strength of the atmosphere side electrode and the exhaust side electrode formed so as to sandwich the sensor element, the diffusion resistance layer covering the exhaust side electrode, the trap layer for protecting the sensor element from poisonous substances, or the mechanical strength of the air-fuel ratio sensor A housing member or the like for maintaining may be provided as appropriate according to the aspect of the air-fuel ratio sensor.

  On the other hand, the “supply means” according to the present invention is a concept including a mechanical, physical, or electrical mechanism for supplying fuel to an internal combustion engine. For example, a fuel injection device (injector). And so on.

  According to the catalyst deterioration preventing apparatus of the present invention, the temperature of the sensor element is specified by the specifying means during the operation.

  Here, “identified” is not limited to directly measuring the temperature of an object via a temperature sensor or the like, but detects a physical quantity that changes in accordance with the temperature of the object and responds to it. It is a concept that includes indirectly, for example, estimating or estimating the temperature of an object based on the relationship. Such a physical quantity may be, for example, a value that correlates with an electrical resistance of an object such as impedance or admittance.

  The specifying means for performing such specification may include a physical, electrical, or chemical mechanism for detecting the temperature of the sensor element or a physical quantity that can be replaced with the sensor element as described above. It may be a controller that simply receives a physical quantity output via a simple mechanism as data and specifies the temperature. In such a case, a mechanism for detecting the temperature or a physical quantity that can be replaced with the temperature may be provided in the air-fuel ratio sensor.

Heater control means, based on the temperature of the specified sensor element, controls the heater so that the temperature of the sensor element becomes a predetermined activation temperature T _1.

Here, the activation temperature T ₁ is a concept that encompasses a temperature at which the sensor element can conduct oxygen ions according to the difference in oxygen concentration at both ends of the sensor element, that is, a temperature that exhibits linear characteristics according to the oxygen concentration. . As long as the concept of is ensured, the value of the active temperature T ₁ of is meant to be set freely. The heater control means is controlled so as the temperature of the sensor element becomes an active temperature T _1, necessarily, the temperature itself of the sensor element need not be maintained at the activation temperature T _1, it is appropriately increased or decreased Good. Further, the activation temperature as such a control target value may be defined as an appropriate temperature range. In that case, the activation temperature as the control target value may be changed relatively freely within the temperature range as necessary.

  The operation of the supply means is controlled by the supply control means. The supply means is typically a fuel injection device such as an injector. Therefore, the supply control means may be a control unit such as an ECU, for example. Normally, the supply control means controls the supply means so that a predetermined amount of fuel corresponding to the engine speed, throttle valve opening, etc. is injected into the intake pipe (intake port) or cylinder. At this time, the fuel is preferably injected based on the air-fuel ratio of the internal combustion engine specified from the output value of the air-fuel ratio sensor, for example, so as to keep the air-fuel ratio in the vicinity of the stoichiometry.

  On the other hand, the supply means controls the supply means so that the supply of fuel is stopped when a predetermined supply stop condition predetermined as a condition for stopping the supply of fuel is satisfied. Here, the “supply stop condition” may be any condition as long as it is more reasonable to stop the fuel supply without hindering the operation of the vehicle. For example, the accelerator pedal may not be operated (stepped down) in a driving region that is equal to or greater than a predetermined value.

  Here, in particular, when the fuel supply is stopped (hereinafter referred to as “fuel cut is performed” as appropriate), the fuel component in the exhaust gas decreases, so the air-fuel ratio inevitably increases, that is, the combustion state. Becomes lean. However, when the catalyst provided in the air-fuel ratio sensor is exposed to a high-temperature and lean atmosphere, the deterioration is promoted, so that a situation in which the deterioration of the air-fuel ratio sensor is promoted as the fuel cut is performed may occur. .

  Therefore, in the catalyst deterioration preventing apparatus according to the present invention, the heater control means controls the heater as follows to solve the problem.

That is, the heater control means is configured so that the temperature of the sensor element is lowered when the supply stop condition is satisfied and the temperature of the sensor element is equal to or higher than the catalyst deterioration start temperature T ₂ (T ₂ <T ₁ ). To control.

  Here, the “deterioration start temperature” is a concept including a lower limit value of a temperature range in which deterioration of the catalyst is easily promoted in a lean atmosphere. At this time, the catalyst deterioration may be defined qualitatively or quantitatively. Therefore, the deterioration start temperature may be set experimentally, empirically, or based on simulation for each catalyst or for each type of catalyst.

  Thus, as a result of the heater being controlled so that the temperature of the sensor element is lowered by the heater control means, the temperature of the catalyst provided in the air-fuel ratio sensor is inevitably lowered. Therefore, deterioration of the catalyst is prevented. As the temperature of the sensor element decreases, the accuracy of oxygen concentration detection by the air-fuel ratio sensor decreases.However, since the fuel cut is performed when the supply stop condition is satisfied in this way, the oxygen cut is performed compared to the normal time. Concentration detection accuracy is not required. That is, according to the catalyst deterioration preventing apparatus according to the present invention, it is possible to effectively prevent catalyst deterioration.

  Note that there is no limitation on how the heater control means controls the heater. Preferably, the heater is controlled by so-called “duty control” in which the supplied power is controlled by controlling the energization time. In this case, the temperature of the sensor element is controlled relatively simply by reducing the duty ratio.

The specifying means is means for specifying the temperature of the sensor element, and the temperature specified by the specifying means does not necessarily match the catalyst temperature. However, by controlling the heater so that the temperature of the sensor element is lowered, at least the temperature of the catalyst is also lowered, and the effect according to the present invention is not hindered at all. When the sensor element has a catalyst, for example, in the form of a catalyst layer, the temperature of the sensor element may be treated equivalently as the temperature of the catalyst. In addition, when the catalyst is secondarily heated by the heater that warms the sensor element, the temperature of the sensor element is higher than the catalyst temperature with a high probability, and even if the temperature of the sensor element is treated as the catalyst temperature, there is a problem. Will not be affected. In the case where the deterioration starting temperature T ₂ of the catalyst, by any corrected for true deterioration start temperature is performed, deterioration of the catalyst is preset as the temperature of the sensor element which may considered to be promoted, It is possible to define the catalyst deterioration start temperature relatively accurately.

  “Supply stop condition is satisfied” means that the timing on the time axis between the timing at which the heater control unit executes control of the heater and the execution timing of fuel cut that is performed when the supply stop condition is also satisfied. This is a concept representing that there is no limitation as long as the deterioration of the catalyst can be prevented to some extent as compared with the case where such heater control is not executed. Therefore, the heater control may be executed simultaneously with the fuel cut, or such a heater control may be executed prior to the fuel cut.

In one aspect of the deterioration prevention device of the catalyst according to the present invention, the supply control means, the supply of the fuel when the temperature of the supply stop condition is satisfied and said sensor element is lower than the degradation start temperature T ₂ The supply means is controlled so as to be stopped.

  According to this aspect, the supply control means performs the fuel cut when the temperature of the sensor element becomes lower than the catalyst deterioration start temperature. Therefore, the catalyst can be further prevented from being deteriorated as compared with the case where the fuel cut is performed when the supply stop condition is satisfied. Such a control mode is particularly effective when the temperature of the sensor element is sufficiently high with respect to the deterioration start temperature of the catalyst and the time required for the temperature to decrease to the deterioration start temperature cannot be ignored. Is.

When in another aspect of the deterioration prevention device of the catalyst according to the present invention, the heater control means, the temperature of the supply stop condition is satisfied and the sensor element is the degradation start temperature T ₂ above, the sensor element The heater is controlled so that the temperature decreases to a predetermined lower limit temperature T ₃ (T ₃ <T ₂ ).

  Here, the “lower limit temperature” is a target temperature of the sensor element and a control target value of the heater control means. By setting the lower limit temperature below the deterioration start temperature, the temperature of the sensor element can be lowered below the deterioration start temperature in a relatively short time. Such a lower limit temperature may be determined in advance experimentally, empirically, or based on simulation.

In another aspect of the catalyst deterioration preventing apparatus according to the present invention, the supply control means may supply the fuel when a predetermined supply restart condition that is predetermined as a condition for restarting the fuel supply is satisfied. The supply means is controlled to resume, and the heater control means controls the heater so that the temperature of the sensor element becomes the activation temperature T ₁ when the supply restart condition is satisfied.

  While the fuel cut is performed as described above, when a predetermined supply restart condition is satisfied, the supply control unit restarts the fuel supply. The resumption of fuel supply is a process that requires immediateness as compared with the case where the supply is stopped, and is a process that should be performed regardless of the temperature of the sensor element and the catalyst. Therefore, when the temperature of the sensor element is lower than the activation temperature, it is necessary to quickly return to the activation temperature. According to this aspect, when the supply resumption condition is satisfied, the heater control means controls the heater so that the temperature of the sensor element becomes the activation temperature, so that the fuel efficiency is reduced as the catalyst is prevented from deteriorating. Is prevented. That is, deterioration of the catalyst is effectively prevented.

In this aspect, the supply control means, when the supply restart condition is satisfied and the temperature of the sensor element is lower than the semi-active temperature T ₄ (T ₄ <T ₁ ) of the sensor element, heat of the heater, the temperature may control the heater such relatively larger than the heat quantity when the is semi activation temperature T ₄ or more of the sensor element.

  Here, the semi-active temperature of the sensor element is a concept that encompasses a temperature as a threshold value where the detection accuracy of the oxygen concentration exceeds a certain level, although the reliability is lower than that in the active state. It may be defined based on an index, or may be determined experimentally, empirically, or based on simulation according to the performance required for the catalyst deterioration prevention device.

  When the sensor element is below the half-active temperature, the detection accuracy of the oxygen concentration by the air-fuel ratio sensor is low, which may affect the operation of the internal combustion engine. In this case, the reliability of the detected oxygen concentration is not greatly impaired by relatively increasing the amount of heat of the heater until the temperature of the sensor element reaches the semi-active temperature. That is, it is possible to prevent the operation of the internal combustion engine from being hindered along with prevention of catalyst deterioration. The amount of heat of the heater may be treated as equivalent to the duty ratio when the heater is duty controlled as described above.

  Such an operation and other advantages of the present invention will become apparent from the embodiments described below.

<Embodiment of the Invention>
Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings as appropriate.

<1. Configuration of Embodiment>
<1-1. Configuration of Engine System 10>
First, the configuration of the engine system according to the embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram of the engine system 10.

  In FIG. 1, an engine system 10 is mounted on a vehicle (not shown) and includes an ECU 100 and an engine 200.

  The ECU 100 is an electronic control unit that controls the operation of the engine 200, and is configured to function as an example of a catalyst deterioration prevention device according to the present invention.

  The engine 200 can cause the air-fuel mixture to explode in the cylinder 201 by the spark plug 202, and can convert the reciprocating motion of the piston 203 generated according to the explosive force into the rotational motion of the crankshaft 205 via the connection rod 204. 1 is an example of an “internal combustion engine” according to the present invention. Below, the principal part structure of the engine 200 is demonstrated with the operation | movement.

<1-2. Configuration of Engine 200>
When the fuel is burned in the cylinder 201, the air sucked from the outside passes through the intake pipe 206 and is mixed with the fuel injected from the injector 207 (an example of the “supplying means” according to the present invention) to be mixed with the air-fuel mixture described above. It becomes. The injector 207 is supplied with fuel (gasoline) from a fuel tank (not shown), and the injector 207 is configured to be able to inject the supplied fuel into the intake pipe 206 in accordance with the control of the ECU 100. Yes.

  The communication state between the inside of the cylinder 201 and the intake pipe 206 is controlled by opening and closing the intake valve 208. The air-fuel mixture burned in the cylinder 201 becomes exhaust gas, passes through an exhaust valve 209 that opens and closes in conjunction with opening and closing of the intake valve 208, and passes through an exhaust pipe 210 (that is, an example of the “exhaust system” according to the present invention). Exhausted.

  A cleaner 211 is disposed on the intake pipe 206 to purify air sucked from the outside. An air flow meter 212 is disposed on the downstream side (cylinder side) of the cleaner 211. The air flow meter 212 has a form called a hot wire type, and is configured to be able to directly measure the mass flow rate of the inhaled air. The intake pipe 206 is further provided with an intake air temperature sensor 213 for detecting the temperature of the intake air.

  A throttle valve 214 that adjusts the amount of intake air into the cylinder 201 is disposed downstream of the air flow meter 212 in the intake pipe 206. The throttle valve 214 is provided with a throttle valve motor 217 and a throttle position sensor 215. On the other hand, the depression amount of the accelerator pedal 223 is input to the ECU 100 via the accelerator position sensor 216, and a signal indicating the throttle valve opening corresponding to the output of the accelerator position sensor 216 is output from the ECU 100 to the throttle valve motor 217. The intake air volume is controlled.

  A crank position sensor 218 that detects the rotational position of the crankshaft 205 is provided in the vicinity of the crankshaft 205. The crank position sensor 218 is a sensor configured to be able to detect the position of the crankshaft 205, and the ECU 100 determines the position of the piston 203, the rotational speed of the engine 200, and the like based on the output signal of the crank position sensor 218. It is configured to be able to obtain. The position of the piston 203 is used for controlling the ignition timing in the spark plug 202 described above. The ignition timing in the spark plug 202 is, for example, retarded or advanced with respect to a preset basic value associated with the position of the piston 203.

  Further, a knock sensor 219 capable of measuring the knock strength of the engine 200 is disposed in the cylinder block that accommodates the cylinder 201, and the cooling water of the engine 200 is placed in the water jacket in the cylinder block. A water temperature sensor 220 for detecting the temperature is provided.

  A three-way catalyst 222 is installed in the exhaust pipe 210. The three-way catalyst 222 is configured to be able to purify exhaust gas by oxidizing CO (carbon monoxide) and HC (hydrocarbon) discharged from the engine 200 and reducing NOx (nitrogen oxide). Has been. An air-fuel ratio sensor 221 is disposed upstream of the three-way catalyst 222 in the exhaust pipe 210. The air-fuel ratio sensor 221 is configured to detect the oxygen concentration of the exhaust gas exhausted from the exhaust pipe 210 and output a current corresponding to the oxygen concentration to the ECU 100 as a sensor signal according to the present invention. It is an example of an “air-fuel ratio sensor”. The ECU 100 is configured to be able to acquire the air / fuel ratio of the engine 200 based on the current value input from the air / fuel ratio sensor 221.

<1-3. Configuration of Air-Fuel Ratio Sensor 221>
Next, a detailed configuration of the air-fuel ratio sensor 221 will be described with reference to FIG. FIG. 2 is a schematic enlarged sectional view of the vicinity of the air-fuel ratio sensor 221 in FIG. In the figure, the same reference numerals are assigned to the same parts as in FIG.

  In FIG. 2, a part of the air-fuel ratio sensor 221 is exposed inside the exhaust pipe 210, and the oxygen concentration in the exhaust gas flowing in the exhaust direction in the exhaust pipe 210 in the exposed part is detected at the exposed part. A signal that defines the concentration is configured to be output to the ECU 100 as a sensor output.

  Now, further details of the air-fuel ratio sensor 221 will be described with reference to FIG. FIG. 3 is a cross-sectional view taken along line A-A ′ in FIG. 2. In the figure, the same reference numerals are assigned to the same parts as in FIG.

  In FIG. 3, a sensor element 221a is a solid electrolyte made of zirconium oxide (zirconia), and constitutes an example of a “sensor element” according to the present invention. An atmosphere side electrode 221b and an exhaust side electrode 221c made of a metal material having high catalytic activity, such as platinum, are formed on part of both end faces of the sensor element 221a, and can be in contact with the atmosphere and exhaust gas, respectively. It is configured to be possible. Further, porous chemical plating is applied to the surfaces of both electrodes. Further, a predetermined bias voltage is applied between both electrodes via a power supply line (not shown). A detection terminal (not shown) is taken out from a part of the power supply line, and a sensor current resulting from the movement of oxygen ions generated according to the oxygen concentration difference between the electrodes is detected. . The detected current is output to the ECU 100 as the sensor output described above. Note that the ECU 100 controls the power supply line to the upper level, and is configured to be able to specify the impedance of the sensor element 221a in the process of controlling the power supply line. The impedance of the sensor element 221a is used to specify the temperature of the sensor element 221a described later.

  A diffusion resistance layer 221d made of a heat-resistant inorganic material such as alumina is formed at the end of the exhaust side electrode 221c opposite to the sensor element 221a so as to cover the end, and the exhaust side electrode 221c is not formed. The portion also covers the sensor element 221a. Further, on the surface of the diffusion resistance layer 221d, a trap layer 221e for protecting the sensor element 221a from poisonous substances such as lead in the exhaust gas is formed adjacent to the porous ceramic. .

  On the surface of the trap layer 221e, a catalyst layer 221f (that is, an example of the “catalyst” according to the present invention) for reducing the influence of hydrogen present in the exhaust gas is formed. The catalyst layer 221f has a configuration in which a catalyst component such as platinum or rhodium is supported on a porous layer such as alumina as a catalyst carrier.

  On the other hand, the end of the atmosphere side electrode 221b opposite to the sensor element 221a is covered with a heat transfer body 221h through a space that maintains the atmosphere. The heat transfer body 221h is configured to also cover the sensor element 221a except for the portion facing the atmosphere-side electrode 221b.

  A heater 221g is formed inside the heat transfer body 221b, and power is supplied via a power supply line (not shown). The heater 221g is configured to generate heat according to the supplied electric power and to warm the sensor element 221a via the heat transfer body 221h. The electric power supplied to the heater 221g is controlled by the ECU 100 through a control line (not shown).

  The protective cover 221i is a heat-resistant housing member, and ensures the mechanical strength of a pair of sensor parts including the sensor element 221a. Further, a vent hole 221j is appropriately formed on the surface of the protective cover 221i, and the inside and outside of the protective cover 221i are communicated with each other. The air-fuel ratio sensor 221 is configured such that exhaust gas flowing through the exhaust pipe 210 passes through the vent hole 221i and finally reaches the sensor element 221a.

  Here, the principle of air-fuel ratio detection by the air-fuel ratio sensor 221 will be described with reference to FIG. FIG. 4 is a correlation diagram between the sensor current IL that is the output of the air-fuel ratio sensor 221 and the air-fuel ratio of the engine 200.

  In FIG. 4, the horizontal axis and the vertical axis represent the air-fuel ratio of the engine 200 and the value of the sensor current IL, respectively. The value of the sensor current IL is smaller as the air-fuel ratio of the engine 200 is richer (that is, the oxygen concentration in the exhaust pipe is lower) and the air-fuel ratio is leaner (that is, the oxygen concentration in the exhaust pipe is higher). It changes to become larger. Such a correspondence relationship between the sensor current and the air-fuel ratio is stored in advance in a ROM or the like in the ECU 100 as a map for air-fuel ratio control, and the ECU 100 matches the value of the detected sensor current IL with the map. And the current air-fuel ratio of the engine 200 is specified.

<2. Operation of Embodiment>
In the engine system 10 according to the present embodiment, a fuel supply cut (fuel cut) is performed during the operation of the engine 200 for the purpose of improving economy. On the other hand, the air-fuel ratio sensor 221 generates oxygen ion movement linearly with respect to the oxygen concentration in the sensor element 221a, in the vicinity of the activation temperature (750 ° C.) of the sensor element 221a (approximately 700 ° C. to 800 ° C. temperature range). It is driven by.

  When the fuel cut is performed, the atmosphere in the exhaust pipe 210 becomes lean. However, when the exhaust gas enters a lean state in a high temperature atmosphere near the activation temperature, the catalyst layer 221f for protecting the sensor element 221a from the influence of hydrogen. Will deteriorate. Therefore, the ECU 100 is configured to perform fuel cut while preventing deterioration of the catalyst layer by the fuel cut process described below, and to effectively restart the fuel supply by the fuel cut return process.

<2-1. Details of fuel cut processing ＞
Here, the details of the fuel cut process will be described with reference to FIG. FIG. 5 is a flowchart of the fuel cut process.

  In FIG. 5, ECU 100 determines whether or not the operating condition of engine 200 satisfies a predetermined fuel cut condition (step A10).

  Here, in the present embodiment, the fuel cut condition is defined as a state where the accelerator pedal 223 is not operated in an operation region where the engine speed of the engine 200 is 2000 rpm or more. That is, the ECU 100 performs such determination based on the engine speed calculated based on the output of the crank position sensor 218 and the position of the accelerator pedal 223 detected by the accelerator position sensor 216.

  When the fuel cut condition is not satisfied (step A10: NO), the ECU 100 repeats step A10 and controls the injector 207 so that normal fuel injection is executed until the fuel cut condition is satisfied. Normally, the ECU 100 continually specifies the air-fuel ratio of the engine 200 based on the sensor current output from the air-fuel ratio sensor 221, and the fuel is set so that the air-fuel ratio becomes a value near the stoichiometric (theoretical air-fuel ratio). So-called air-fuel ratio feedback control is executed to control the injection amount of the fuel.

  On the other hand, when the fuel cut condition is satisfied (step A10: YES), the ECU 100 determines whether or not the temperature of the sensor element 221a (hereinafter referred to as “element temperature” as appropriate) is equal to or higher than the deterioration start temperature of the catalyst layer 221f. Is discriminated (step A11).

  Here, with reference to FIG. 6, the detail of detection of element temperature is demonstrated. FIG. 6 is a correlation diagram between the resistance value of the sensor element 221a and the element temperature.

  In FIG. 6, the horizontal axis and the vertical axis represent the element temperature and the impedance and admittance as the resistance value, respectively. The impedance of the sensor element 221a varies as illustrated depending on the element temperature. A storage unit such as a ROM provided in the ECU 100 stores a map corresponding to FIG. 6 in advance as a map for specifying the element temperature, and the ECU 100 detects the impedance of the sensor element 221a acquired through the air-fuel ratio sensor 221. The admittance of the sensor element 221a is calculated, and the temperature value corresponding to the admittance value is specified as the element temperature. The admittance is the reciprocal of the impedance, and the element temperature may be specified using any of them. However, when considering the change range in the element temperature control region (shown by the dotted frame), at least the element temperature control region (illustrated) Admittance is preferably used in the dotted frame).

  Returning to FIG. 5, in the present embodiment, the catalyst layer deterioration start temperature defines the lower limit of the temperature range in which the deterioration of the catalyst layer 221f progresses relatively severely in a lean atmosphere. The temperature is determined in advance experimentally, empirically, or based on simulation. The deterioration start temperature in this embodiment is set to 600 ° C.

  On the other hand, the ECU 100 normally controls the heater 221g so that the element temperature becomes the activation temperature (750 ° C.) described above, and therefore the determination in step A11 is normally “YES”. Note that the ECU 100 performs duty control on the power supplied to the heater 221g, and normally energizes the heater with a duty ratio of 0.7 (70% duty) as an upper limit. Hereinafter, the energization control with the upper limit of the duty ratio 0.7 is referred to as “normal duty”.

  When the element temperature is lower than the deterioration start temperature (step A11: NO), for example, immediately after the start of the engine 200, the possibility that the catalyst layer 221f is deteriorated due to the fuel cut is low. A fuel cut is executed (step A14).

  On the other hand, when the element temperature is equal to or higher than the deterioration start temperature (step A11: YES), the ECU 100 controls the heater 221g so that the element temperature becomes a preset lower limit temperature (step A12). Here, the lower limit temperature may be set relatively freely in a temperature range lower than the deterioration start temperature. However, it is desirable that the temperature be such that switching to the activation temperature can be performed relatively quickly. Further, considering that the element temperature does not necessarily coincide with the temperature of the catalyst layer 221f, it is more preferable that the temperature has a certain margin with respect to the deterioration start temperature. As a temperature satisfying such conditions, the lower limit temperature according to the present embodiment is set to 500 ° C. In the process related to step A12, the ECU 100 cuts off the power supply to the heater 221g so that the element temperature quickly decreases to the lower limit temperature.

  When the energization of the heater 221g is interrupted, the ECU 100 determines whether or not the element temperature has become lower than the deterioration start temperature at every fixed timing (step A13). When the element temperature is still equal to or higher than the deterioration start temperature (step A13: NO), the ECU 100 repeats step A13, and when the element temperature becomes lower than the deterioration start temperature (step A13: YES), the fuel cut is executed. (Step A14).

  As described above, according to the fuel cut processing according to the present embodiment, when the fuel cut is executed, the catalyst layer 221f of the air-fuel ratio sensor 221 is controlled below the temperature at which the catalyst layer starts to deteriorate. , Not exposed to high temperature and lean atmosphere. That is, it is possible to effectively prevent deterioration of the catalyst.

<2-2. Details of fuel cut recovery process>
Next, details of the fuel cut return process for returning from the fuel cut process of FIG. 5 and restarting the fuel supply will be described with reference to FIG. FIG. 7 is a flowchart of the fuel cut return process.

  In FIG. 7, the ECU 100 determines whether or not the operating state of the engine 200 satisfies the forced return condition (step B10). The fuel cut is performed for reasons of pursuing economic efficiency, and is a process that does not require relatively immediateness. However, returning from the fuel cut requires immediateness from the viewpoint of preventing misfire of the engine 200. It is processing.

  As a return condition from the fuel cut process, there is a condition that accompanies a natural decrease in the engine speed, for example, an operation for making an output request to the engine 200 (for example, depression of the accelerator pedal 223). There is. The forced return condition in step B10 is a condition belonging to the latter.

  When the forced return condition is satisfied (step B10: YES), the ECU 100 determines whether or not the element temperature is lower than the half-active temperature of the sensor element 221a (step B15). Here, the semi-active temperature of the sensor element 221a represents a temperature at which the air-fuel ratio sensor 221 can detect the oxygen concentration in the exhaust gas at the minimum according to the difference in oxygen concentration between the two electrodes. Therefore, the detection accuracy is degraded as compared with the sensor element 221a at the activation temperature. However, it is possible to take out an output signal corresponding to the oxygen concentration, and in that sense, the above-mentioned air-fuel ratio feedback when supplying fuel is a temperature that is possible to some extent.

  When the element temperature is equal to or higher than the half-active temperature (step B15: NO), the ECU 100 starts fuel supply with air-fuel ratio feedback (step B19), and normally sets the target value of the element temperature to the active temperature of 750 ° C. Duty heater energization is executed (step B20).

  On the other hand, when the element temperature is lower than the activation temperature (step B15: YES), the ECU 100 does not perform air-fuel ratio feedback considering that the reliability of the air-fuel ratio sensor 221 is not ensured, and simply performs the engine speed and throttle valve. Supply of fuel according to the opening is executed (step B16). At this time, in order to activate the element temperature at an early stage, energization of the heater with an enlarged duty is executed (step B17). Here, the “extended duty” according to the present embodiment refers to energization with an upper limit of a duty ratio of 1.0 (that is, 100% duty), unlike the above-described normal duty. Therefore, the amount of heat generated by the heater is larger than that in the case of normal duty, and the rate of increase in element temperature is relatively high. During the period in which the heater energization is performed with the enlarged duty, the ECU 100 determines whether or not the element temperature is equal to or higher than the semi-active temperature at every predetermined timing (step B18). When the element temperature is still lower than the half-active temperature (step B18: NO), the determination process is repeated. When the element temperature becomes equal to or higher than the half-active temperature, fuel supply based on the air-fuel ratio feedback is started in step B19. Is done. In step B20, the heater control mode is changed from the enlarged duty to the normal duty.

  In step B10, when the operating condition of engine 200 does not satisfy the forced return condition (step B10: NO), ECU 100 determines whether the engine speed is less than the speed obtained by adding the margin speed Nm to the return speed. Is discriminated (step B11).

  Here, the return rotation speed is a rotation speed that defines a natural return from the fuel cut, and in this embodiment, 2000 rotations per minute that define the fuel cut condition is set as the return rotation speed as it is. Note that the return rotational speed may be different from the rotational speed that defines the fuel cut condition. On the other hand, the margin rotation speed Nm is a value for raising the return rotation speed by a certain percentage or a predetermined amount, and is set in advance to an appropriate value experimentally, empirically, or based on simulation. Yes. In this embodiment, it is set as 500 revolutions per minute. Therefore, in step B13, it is determined whether or not the engine speed is less than 2500 revolutions per minute.

  If the engine speed is still equal to or higher than the rotational speed obtained by raising the margin rotational speed to the return speed (step B11: NO), the ECU 100 returns the process to step B10 on the assumption that no return condition is satisfied. The engine 200 continues to monitor whether the return condition is satisfied.

  When the engine speed becomes less than the rotational speed obtained by adding the margin rotational speed to the return speed (step B11: YES), the ECU 100 presumes that the natural return condition will be satisfied soon, and the heater 221g Then, energization based on the enlarged duty is performed, and the sensor element 221a is rapidly heated (step B12). The target value of the element temperature at this time is 750 ° C., which is the activation temperature.

  While energizing the heater with the enlarged duty, the ECU 100 determines whether or not the engine speed is less than the return speed (step B13). If it is equal to or higher than the return rotational speed (step B13: NO), the ECU 100 repeats step B13 and executes only the heater energization based on the enlarged duty, assuming that the return condition is not yet satisfied. On the other hand, when the engine speed is less than the return speed (step B13: YES), the ECU 100 determines whether the element temperature is equal to or higher than the half-active temperature (step B14).

  When the element temperature is lower than the semi-active temperature of the sensor element 221a (step B14: NO), the ECU 100 starts supplying fuel without performing air-fuel ratio feedback as already described (step B16). When the temperature is equal to or higher than the half-active temperature (step B14: YES), fuel supply based on air-fuel ratio feedback is executed.

  As described above, in the engine system 10, even when the fuel supply is resumed after the fuel cut, the element temperature is reduced by the energization control of the heater 221g (for example, below the half-active temperature). Adverse effects (such as inability to feed back air-fuel ratio) can be kept small. That is, the deterioration of the catalyst layer 221f can be effectively prevented.

  The present invention is not limited to the above-described embodiments, and can be appropriately changed without departing from the gist or concept of the invention that can be read from the claims and the entire specification. Is also included in the technical scope of the present invention.

It is a mimetic diagram of an engine system concerning an embodiment of the present invention. It is a typical expanded sectional view of the air fuel ratio sensor in the engine system of FIG. FIG. 3 is a cross-sectional view taken along line A-A ′ in FIG. 2. FIG. 2 is a correlation diagram between a sensor current of an air-fuel ratio sensor in the engine system of FIG. 1 and an air-fuel ratio of the engine. It is a flowchart of the fuel cut process which ECU performs in the engine system of FIG. FIG. 2 is a correlation diagram between the resistance value of an air-fuel ratio sensor and the temperature of a sensor element in the engine system of FIG. 1. 2 is a flowchart of a fuel cut return process executed by an ECU in the engine system of FIG. 1.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Engine system, 100 ... ECU, 200 ... Engine, 210 ... Exhaust pipe, 221 ... Air-fuel ratio sensor, 221a ... Sensor element, 221f ... Catalyst layer, 221g ... Heater.

Claims (5)

(I) a catalyst for removing hydrogen contained in the exhaust system; (ii) a sensor element for detecting the air-fuel ratio of the internal combustion engine; and (iii) the sensor element. A catalyst deterioration prevention device for preventing deterioration of the catalyst in a vehicle having an air-fuel ratio sensor provided with a heater for heating and a supply means for supplying fuel to the internal combustion engine,
A specifying means for specifying the temperature of the sensor element;
The supply means is controlled so that the fuel is supplied, and the supply of the fuel is stopped when a predetermined supply stop condition predetermined as a condition for stopping the supply of the fuel is satisfied. Supply control means for controlling the supply means;
The heater is controlled so that the temperature of the sensor element becomes a predetermined activation temperature T ₁ , the supply stop condition is satisfied, and the temperature of the sensor element is the deterioration start temperature T ₂ (T ₂ <T And a heater control means for controlling the heater so that the temperature of the sensor element decreases when the temperature is equal to or higher than T ₁ ). Wherein the supply control means controls said supply means so that the supply of the fuel is stopped when the temperature of the supply stop condition is satisfied and said sensor element is lower than the degradation start temperature T ₂ The catalyst deterioration preventing apparatus according to claim 1. The heater control means, wherein when the temperature of the supply stop condition is satisfied and the sensor element is the degradation start temperature T ₂ above the lower limit temperature T ₃ temperature is given of the sensor element _(T 3 _{<T 2} The catalyst deterioration prevention device according to any one of claims 1 to 3, wherein the heater is controlled so as to decrease to a lower value. The supply control means controls the supply means so that the supply of the fuel is resumed when a predetermined supply resumption condition predetermined as a condition for resuming the supply of the fuel is satisfied,
The heater control means, when the supply restart condition is satisfied, said any one of claims 1 to 3 in which the temperature of the sensor element and the controller controls the heater so that the active temperatures T ₁ The catalyst deterioration preventing device according to item. When the supply resumption condition is satisfied and the temperature of the sensor element is lower than the semi-active temperature T ₄ (T ₄ <T ₁ ) of the sensor element, catalyst degradation preventing apparatus according to claim 4 in which the temperature of the sensor element and the controller controls the heater so relatively larger than the heat quantity when the is semi activation temperature T ₄ or more.

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