JP2017078674A - Sensor for exhaust gas and control method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sensor for exhaust gas that can avoid poisoning due to water vapor in an exhaust gas while preventing damage to a sensor element due to a sudden change in temperature, and a control method thereof.SOLUTION: A sensor for exhaust gas 30 comprises a sensor element 31, electrodes 32a and 32b, a heater 33, and a control device 38. The control device 38 is configured to perform control of, before energizing the electrodes 32a and 32b, energizing the heater 33 to raise the temperature of the sensor element 31 from an ambient temperature θ0 to a poisoning avoiding temperature θα set at a temperature near a temperature at which water content around the sensor element 31 evaporates before raising to an element activation temperature θβ.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an exhaust gas sensor and a control method therefor, and more particularly, damage to a sensor element due to a sudden change in temperature when the temperature of the sensor element of the exhaust gas sensor is increased at the time of starting an internal combustion engine or the like. The present invention relates to an exhaust gas sensor that prevents poisoning due to water vapor and a control method thereof.

  An internal combustion engine controls a fuel injection amount by sensing components such as oxygen concentration and nitrogen oxide (NOx) concentration in exhaust gas using an exhaust gas sensor disposed in an exhaust passage. . Examples of the exhaust gas sensor include a lambda sensor (oxygen sensor) and a NOx sensor.

  The exhaust gas sensor includes a sensor element made of an ion conductor, an electrode joined to the sensor element, and a heater for heating the sensor element. In this exhaust gas sensor, before energizing the electrodes, it is necessary to raise the temperature of the sensor element to the element activation temperature at which the sensor element is activated by energizing the heater.

  In this regard, there has been proposed an apparatus for gradually increasing the temperature of the sensor element when the temperature of the sensor element is increased to the element activation temperature by energizing the heater when starting the engine (see, for example, Patent Documents 1 and 2). . These devices increase the temperature of the sensor element stepwise from the ambient temperature to the element activation temperature by energizing the heater, thereby damaging the sensor element due to burning of water or soot due to a sudden change in temperature. Is preventing.

  However, when these devices are used repeatedly, there is a problem that the initial value (initial value) of the exhaust gas sensor shifts and components in the exhaust gas cannot be detected with high accuracy.

  Therefore, the inventors of the present invention focused on the initial target temperature when starting the temperature rise of the sensor element by energizing the heater in these devices. In these apparatuses, when the temperature of the sensor element is raised by energizing the heater, the initial target temperature is set to a temperature of 200 ° C. or higher.

  When the sensor element is heated from the ambient temperature to a temperature of 200 ° C. or more by energizing the heater, the water around the sensor element is rapidly evaporated to become water vapor. It has been found that the initial value of the exhaust gas sensor is shifted.

Japanese Patent Laid-Open No. 2003-083152 Special table 2008-530542 gazette

  An object of the present invention is to avoid poisoning due to water vapor while preventing the sensor element from being damaged due to a rapid change in temperature when the temperature of the sensor element of the exhaust gas sensor is increased at the time of starting the internal combustion engine. It is an object to provide an exhaust gas sensor and a control method thereof.

  The exhaust gas sensor of the present invention that achieves the above-described object includes a sensor element made of an ion conductor, an electrode joined to the sensor element, and the temperature of the sensor element when the electrode is energized in advance. A heater for raising the temperature to a set element activation temperature, and disposed in an exhaust passage of the internal combustion engine to detect an exhaust gas component. The temperature of the sensor element is raised from the ambient temperature to a poisoning avoidance temperature set to a temperature in the vicinity of the temperature at which the moisture around the sensor element evaporates by energizing the element, and then raised to the element activation temperature. The present invention is characterized in that a control device that performs control is provided.

  In addition, the exhaust gas sensor control method of the present invention that achieves the above object provides an exhaust gas sensor that raises the sensor element to a preset element activation temperature by energizing the heater when the electrode is energized. In the sensor control method, the temperature of the sensor element is increased from the ambient temperature to a poisoning avoidance temperature set to a temperature in the vicinity of a temperature at which moisture around the sensor element evaporates by energizing the heater. And a step of raising the temperature from the poisoning avoidance temperature to the element activation temperature, and a step of energizing the electrode after the temperature of the sensor element is raised to the element activation temperature. It is a method.

  The exhaust gas sensor here includes a lambda sensor (oxygen sensor) that detects the excess air ratio, air-fuel ratio, or oxygen concentration of the exhaust gas, and a NOx sensor that detects the NOx concentration contained in the exhaust gas. It can be illustrated.

  Further, the moisture around the sensor element referred to here is contained in droplet-like moisture present on the inner wall of the exhaust gas sensor or the inner wall of the exhaust passage, and in the exhaust gas flowing into the exhaust gas sensor. It refers to water droplets.

  In addition, the atmospheric temperature referred to here is the temperature around the exhaust gas sensor. For example, the ambient temperature indicates the temperature immediately after the start, and the exhaust gas temperature if the exhaust gas is exhausted. .

  According to the exhaust gas sensor and its control method, when the temperature of the sensor element of the exhaust gas sensor is increased at the time of starting the internal combustion engine, the target value of the first stage of the temperature increase of the sensor element by the heater When the temperature of the sensor element is increased from the ambient temperature to the poisoning avoidance temperature by energizing the heater, the poisoning avoidance temperature is set to a temperature in the vicinity of the temperature at which the water around the sensor element evaporates. In addition, the water around the sensor element can be prevented from suddenly becoming water vapor. Then, after the moisture in the exhaust gas gradually becomes water vapor, the sensor element is heated from the poisoning avoidance temperature to the element activation temperature, thereby preventing damage to the sensor element due to a rapid change in temperature, Poisoning by water vapor can be avoided.

  As a result, even if the exhaust gas sensor is used repeatedly, the initial value does not deviate and the components in the exhaust gas can be detected with high precision. By controlling the internal combustion engine based on the detected value, the exhaust gas can be detected. The purification rate and fuel consumption of harmful components in gas can be improved.

  The poisoning avoidance temperature is preferably set to a temperature of 100 ° C. or higher and 150 ° C. or lower. By setting the poisoning avoidance temperature to a temperature of 100 ° C. or higher and 150 ° C. or lower, it is possible to more reliably suppress the moisture around the sensor element from rapidly becoming water vapor.

It is a block diagram which illustrates the internal combustion engine carrying the exhaust gas sensor of embodiment of this invention. It is a block diagram which illustrates the sensor for exhaust gas of FIG. It is a flowchart which illustrates the control method of the sensor for exhaust gases of this invention. It is a time chart which illustrates transition of the temperature of a sensor element until electricity supply is started to the electrode of the sensor for exhaust gas.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 illustrates an engine 10 equipped with an exhaust gas sensor 30 according to an embodiment of the present invention. The exhaust gas sensor 30 is a lambda sensor (oxygen sensor) and detects an excess air ratio in the exhaust gas G1 discharged to the exhaust passage 20. The exhaust gas sensor 30 can be exemplified by a NOx sensor that detects the NOx concentration in the exhaust gas G1.

  As shown in FIG. 1, in the engine 10, the intake air A <b> 1 drawn into the intake passage 11 is compressed by the compressor 12 a of the turbocharger 12 and becomes high temperature, and is cooled by the intercooler 13. Thereafter, the intake air A1 is supplied from the intake manifold 14 through the intake valve 15 to the cylinder 16. The intake air A1 supplied to the cylinder 16 is mixed with the fuel injected from the fuel injection valve 17 and burned to generate heat energy, and then becomes exhaust gas G1.

  The exhaust gas G1 is exhausted from the exhaust valve 18 to the exhaust passage 20 via the exhaust manifold 19. The exhaust gas G1 thus exhausted drives the turbine 12b of the turbocharger 12, and then selectively reduces the oxidation catalyst 21, the collection filter 22, and the urea water injected from the urea water injection valve 23 as a reducing agent. After passing through the order of 24 and being purified, it is released into the atmosphere.

  A part of the exhaust gas G1 branches from the exhaust passage 20 to the EGR passage 25, is cooled by the EGR cooler 26, and then recirculates from the EGR valve 27 to the intake passage 11 as EGR gas G2.

  The exhaust gas sensor 30 is disposed in the exhaust passage 20 upstream of the oxidation catalyst 21. The position of the exhaust gas sensor 30 is not particularly limited. For example, when a NOx sensor is used as the exhaust gas sensor 30, the NOx sensor may be disposed downstream of the selective reduction catalyst 24.

FIG. 2 illustrates the configuration of the exhaust gas sensor 30. The exhaust gas sensor 30 includes a sensor element 31, electrodes 32 a and 32 b, and a heater 33. The sensor element 31 is composed of an oxygen ion conductor such as zirconia (ZrO ₂ ). The pair of electrodes 32a and 32b are joined to the sensor element 31, and the electrode 32a is disposed in the air atmosphere and the electrode 32b is disposed in the exhaust gas atmosphere. The heater 33 may be a ceramic heater that generates heat when electric power is supplied. The heater 33 is a heater that raises the temperature of the sensor element 31 to a preset element activation temperature θβ when the electrodes 32a and 32b are energized.

  The sensor element 31, the electrodes 32 a and 32 b, and the heater 33 are housed inside the housing 34. The housing 34 is formed with an exhaust gas hole 35 into which the exhaust gas G1 flows and an atmospheric hole 36 into which the atmosphere (reference air) flows.

The above configuration is a configuration in the case where the exhaust gas sensor 30 is a lambda sensor, and is not limited to this configuration. For example, as an electrode, a pair of electrodes 32a and 32b is used in a lambda sensor.
In the case of a NOx sensor, a plurality of sets of electrodes are provided.

  In the exhaust gas sensor 30, electric power is supplied to the heater 33 and the sensor element 31 is heated. The heater 33 is heated by energization of the heater 33, and the element activation in which the temperature of the sensor element 31 is set to a temperature at which the conductivity of oxygen ions of the sensor element 31 is increased by the heating of the sensor element 31 by the heater 33. The temperature is raised to the temperature θβ. When the electrodes 32a and 32b are energized, oxygen ions are detected in accordance with the difference between the oxygen partial pressure in the air atmosphere where the electrode 32a is arranged and the oxygen partial pressure in the exhaust gas atmosphere where the electrode 32b is arranged. It moves through 31. From the amount of change in electromotive force due to the movement of oxygen ions based on the difference in oxygen partial pressure, the exhaust gas sensor 30 detects the excess air ratio.

  In such an exhaust gas sensor 30, before the electrodes 32a and 32b are energized, the temperature of the sensor element 31 is raised from the ambient temperature θ0 to the poisoning avoidance temperature θα by energizing the heater 33, and then the element activity is increased. And a control device 38 that performs control to raise the temperature to the control temperature θβ.

  That is, in this exhaust gas sensor 30, the control device 38 raises the temperature of the sensor element 31 from the ambient temperature θ0 to the poisoning avoidance temperature θα by preheating before energization, and after the preheating is completed, Control is performed to raise the temperature of 31 to the element activation temperature θβ.

  The control device 38 includes a CPU that performs various processes, a ROM that temporarily stores programs used to perform the various processes, a RAM that can read and write the processing results, and various interfaces. The control device 38 is connected to a terminal 37 of the exhaust gas sensor 30 via a signal line. The control device 38 is also connected to the fuel injection valve 17, the urea water injection valve 23, the EGR valve 27, and the like.

  The control device 38 performs control such as adjustment of the fuel injection amount, adjustment of the urea water injection amount, adjustment of the recirculation amount of the EGR gas G2, and control for adjusting the temperature of the heater 33 of the exhaust gas sensor 30. Configured to do. In this embodiment, the control device that controls the temperature of the heater 33 of the exhaust gas sensor 30 may be a separate control device that can communicate with the control device that controls the engine 10.

  The poisoning avoidance temperature θα is set near the temperature at which water around the sensor element 31 evaporates. The moisture around the sensor element 31 is contained in droplet-shaped moisture present on the inner wall of the exhaust gas sensor 30 and the inner wall of the exhaust passage 20 and in the exhaust gas G1 flowing into the exhaust gas sensor 30. It refers to water droplets.

  If the poisoning avoidance temperature θα is set to a temperature lower than 100 ° C., the temperature as preheating before energization is low, and the sensor element 31 may be damaged when the temperature is raised to the element activation temperature θβ. On the other hand, when the poisoning avoidance temperature θα is set to a temperature higher than 150 ° C., the moisture around the sensor element 31 is suddenly changed to water vapor, and the exhaust gas sensor 30 is affected by water vapor poisoning.

  Therefore, the poisoning avoidance temperature θα is desirably set to a temperature of 100 ° C. or higher and 150 ° C. or lower. When the poisoning avoidance temperature θα is set to such a temperature, when the sensor element 31 is heated up to the element activation temperature θβ by the heater 33, a sudden temperature change can be avoided and the surroundings of the sensor element 31 can be avoided. It is possible to avoid that the moisture of the water suddenly changes to water vapor.

In the exhaust gas sensor 30 described above, the element activation temperature θβ is determined by the sensor element 31.
It is desirable to set the temperature at which the aromatic compound existing around An example of the aromatic compound is xylene.

  When the element activation temperature θβ is set to a temperature lower than 780 ° C., it is impossible to avoid a state in which a part of the aromatic compound does not burn and is poisoned by the aromatic compound. On the other hand, if the element activation temperature θβ is set to a temperature exceeding 900 ° C., the sensor element 31 may be damaged. Therefore, the element activation temperature θβ is preferably a temperature of 780 ° C. or higher and 900 ° C. or lower.

  As described above, when the element activation temperature θβ is set to a temperature at which the aromatic compound existing around the sensor element 31 burns, the sensor element 31 can be prevented from being poisoned by the aromatic compound. Therefore, the component of the exhaust gas G1 can be detected with higher accuracy.

  Hereinafter, the control method of the exhaust gas sensor 30 will be described as a function of the control device 38 with reference to the flowchart of FIG. In addition, this control method shall be started after the engine 10 starts. In addition, at the start, it is assumed that α is input as the variable x and β is input as the end determination value y. The change of the variable x is assumed to change in the order of α and β every time n is added.

  First, in step S10, the control device 38 inputs θ (x−n) as the current value θa. Next, in step S20, the control device 38 inputs θ (x) as the target value θb.

  For example, in the first steps S10 and S20, the ambient temperature θ0 is input as the current value θa, and the poisoning avoidance temperature θα is input as the target value θb. In the second step S10, S20, the poisoning avoidance temperature θα is input as the current value θa, and the element activation temperature θβ is input as the target value θb. The ambient temperature θ0 is the temperature around the exhaust gas sensor 30. That is, for example, the ambient temperature θ0 is set to the atmospheric temperature immediately after the start, and to the temperature of the exhaust gas G1 if the exhaust gas is exhausted.

  Next, in step S30, the control device 38 sets the power supplied to the heater 33 to a value that changes the temperature of the sensor element 31 from the current value θa to the target value θb. This step S30 refers to, for example, map data stored in the control device 38 in which the power supplied to the heater 33 based on the current value θa and the target value θb is set, and corresponds to the map data. The step of setting power can be exemplified. Next, in step S <b> 40, the control device 38 supplies the set power to the heater 33.

  Next, in step S <b> 50, the control device 38 counts the time Δt when power is supplied to the heater 33. Next, in step S60, the control device 38 determines whether or not the elapsed time Δtx set in advance for the counted time Δt has elapsed.

  The elapsed time Δtx is set to a time during which it can be determined that the temperature of the sensor element 31 has been reliably raised to the target value θb by the heater 33 supplied with electric power. This elapsed time Δtx may be calculated using map data stored in the control device 38 and created in advance through experiments or tests. As the map data, the current value θa, the target value θb, and the power supplied to the heater 33 can be used as parameters and the elapsed time Δtx can be exemplified.

  If it is determined in step S60 that the counted time Δt has passed the elapsed time Δtx, the process proceeds to step S70. On the other hand, if it is determined that the counted time Δt has not passed the elapsed time Δtx, the process returns to step S40.

  Next, in step S70, the control device 38 counts the variable x. Next, in step S80, the control device 38 determines whether or not the variable x is greater than or equal to the end determination value y. If it is determined in step S80 that the variable x is less than the end determination value y, the process returns to step S10. On the other hand, if it is determined that the variable x is equal to or greater than the end determination value y, the process proceeds to step S90.

  Next, in step S90, the control device 38 starts energizing the electrodes 32a and 32b, and this control method is completed.

  Thus, the poisoning avoidance temperature set in the vicinity of the temperature at which the moisture around the sensor element 31 evaporates as the target value θb when the temperature of the sensor element 31 is raised from the atmospheric temperature θ0 by the heating of the heater 33. By using θα, it is possible to suppress the moisture from rapidly changing to water vapor.

  Then, after the water gradually becomes water vapor, the sensor element 31 is heated from the poisoning avoidance temperature θα to the element activation temperature θβ, thereby preventing damage due to a rapid change in the temperature of the sensor element 31. In addition, poisoning due to water vapor can be avoided.

  As a result, even if the exhaust gas sensor 30 is used repeatedly, the initial value does not deviate and components in the exhaust gas G1 can be detected with high accuracy. Therefore, the engine 10 is controlled based on the detected value. The purification rate and fuel consumption of harmful components in the exhaust gas G1 can be improved.

  As an example of the control method of the exhaust gas sensor 30, open loop control has been described in which the current value θa and the target value θb are set, and the power is set so that the current value θa becomes the target value θb. On the other hand, as this control method, when the exhaust gas sensor 30 is additionally provided with a sensor for detecting the temperature of the sensor element 31, the detection value detected by the sensor is compared with the target value θb, and they match. The closed loop control which sets electric power to do can also be illustrated.

  FIG. 4 illustrates the transition of the temperature of the sensor element 31 until the energization of the electrodes 32a and 32b of the exhaust gas sensor 30 is started. In FIG. 4, the first example of the temperature change of the sensor element 31 of this embodiment is indicated by a solid line, the second example is indicated by a one-dot chain line, and the temperature change of the conventional sensor element is indicated by a dotted line. .

  In the exhaust gas sensor 30 described above, when the temperature of the sensor element 31 by the heater 33 is raised from the poisoning avoidance temperature θα to the element activation temperature θβ, the temperature is raised gradually or stepwise. It is desirable to do.

  Therefore, as a first example, the control device 38 is configured to perform control to gradually increase the temperature of the sensor element 31 from the poisoning avoidance temperature θα to the element activation temperature θβ.

  Further, as a second example, the control device 38 repeats the temperature rise by the heater 33 and the temperature rise stop, and the temperature of the sensor element 31 is changed from the poisoning avoidance temperature θα to at least one intermediate temperature θγ, Control is performed to raise the temperature stepwise up to the element activation temperature θβ. In the case of the second example, it is assumed that the change of the variable x in the above control method changes in the order of α, γ, and β every time n is added.

  The intermediate temperature θγ is set to a temperature between the poisoning avoidance temperature θα and the element activation temperature θβ. Examples of the intermediate temperature θγ include a temperature of 200 ° C. or more and 250 ° C. or less, which is a temperature at which a part of SOF (availability organic component) attached to the sensor element 31 burns. Further, examples of the intermediate temperature θγ include temperatures of 500 ° C. or more and 600 ° C. or less, which is a temperature at which soot attached to the sensor element 31 burns. Furthermore, a plurality of intermediate temperatures θγ can be set.

  Therefore, for example, after the temperature of the sensor element 31 is raised from the poisoning avoidance temperature θα to the first intermediate temperature θγ set to 200 ° C. or more and 250 ° C. or less, the first intermediate temperature θγ to 500 ° C. As described above, the temperature may be raised to the second intermediate temperature θγ set to a temperature of 600 ° C. or lower, and then the temperature may be raised from the second intermediate temperature θγ to the element activation temperature θβ.

  As shown in FIG. 4, in the first example, the temperature of the sensor element 31 rises to the poisoning avoidance temperature θα at time t1. Next, the temperature of the sensor element 31 is maintained at the poisoning avoidance temperature θα until time t2. Next, at time t3, the temperature of the sensor element 31 rises to the element activation temperature θβ, and energization of the electrodes 32a and 32b is started from time t3.

  As for the rate of increase in temperature of the sensor element 31 from time t2 to time t3, it is preferable that the rate of increase in the latter period be larger than the rate of increase in the initial period. As control for changing the rate of increase, for example, control using map data in which the rate of increase in power is set using the ambient temperature θ0, the poisoning avoidance temperature θα, and the element activation temperature θβ as parameters can be exemplified. As described above, the temperature of the sensor element 31 is gradually raised to a certain temperature and then raised to the element activation temperature θβ, whereby damage to the sensor element 31 due to a sudden temperature change can be prevented. .

  In the second example, the temperature of the sensor element 31 is raised to the intermediate temperature θγ at time t4. Next, the temperature of the sensor element 31 is maintained at the intermediate temperature θγ until time t5. Next, at time t6, the temperature of the sensor element 31 is raised to the element activation temperature θβ, and energization of the electrodes 32a and 32b is started from time t6.

  Comparing the first example and the second example, the time until the energization of the electrodes 32a and 32b is started is maintained at the intermediate temperature θγ when the temperature of the sensor element 31 is raised in the first example. The first example is shortened by the amount of time Δtγ to be used. Therefore, in the first example, since the time until the electrodes 32a and 32b are energized is shortened, it becomes possible to detect the component in the exhaust gas G1 earlier, and the purification of the harmful component in the exhaust gas G1. It is advantageous for improving the rate and fuel consumption.

  On the other hand, in the first example, as described above, when the temperature of the sensor element 31 is increased from the poisoning avoidance temperature θα to the element activation temperature θβ, it is necessary to increase the late increase rate from the initial increase rate. Yes, the control is complicated accordingly. Therefore, in the second example, the control is simplified.

  Further, when comparing the first example and the second example with the prior art, in the prior art, first, the temperature of the sensor element 31 is raised from the ambient temperature θ0 to a temperature exceeding the poisoning avoidance temperature θα. At this time, since the water around the sensor element 31 is suddenly changed to water vapor, the exhaust gas sensor 30 is affected by water vapor poisoning by the water vapor. Due to the water vapor poisoning, the initial value of the exhaust gas sensor 30 is deviated, so that highly accurate detection cannot be performed.

  As is apparent from FIG. 4, this exhaust gas sensor 30 avoids a sudden change in the moisture present around the sensor element 31 to water vapor by lowering the preheat temperature as compared with the prior art. Water vapor poisoning can be prevented.

10 Engine 20 Exhaust passage 30 Exhaust gas sensor 31 Sensor elements 32a, 32b Electrode 33 Heater 38 Control device G1 Exhaust gas θ0 Atmospheric temperature θα Poisoning avoidance temperature θβ Element activation temperature

Claims (6)

A sensor element composed of an ion conductor; an electrode joined to the sensor element; and a heater that raises the temperature of the sensor element to a preset element activation temperature when the electrode is energized. In the exhaust gas sensor that is disposed in the exhaust passage of the internal combustion engine and detects a component of the exhaust gas,
Before energizing the electrodes, energizing the heater raises the temperature of the sensor element from the ambient temperature to a poisoning avoidance temperature set to a temperature in the vicinity of the temperature at which moisture around the sensor element evaporates. An exhaust gas sensor comprising a control device that performs control to raise the temperature to the element activation temperature.   The exhaust gas sensor according to claim 1, wherein the poisoning avoidance temperature is set to a temperature of 100 ° C. or higher and 150 ° C. or lower.   The control device repeats energization to the heater and temperature rise and temperature rise stop by energization stop to change the temperature of the sensor element from the poison avoidance temperature to the poison avoidance temperature and the 3. The exhaust gas sensor according to claim 1, wherein the exhaust gas sensor is configured to perform stepwise temperature rise to the element activation temperature through at least one intermediate temperature set between the element activation temperatures.   3. The exhaust gas sensor according to claim 1, wherein the control device performs control to gradually and continuously raise the temperature of the sensor element from the poisoning avoidance temperature to the element activation temperature. 4.   The exhaust gas sensor according to any one of claims 1 to 4, wherein the element activation temperature is set to a temperature at which an aromatic compound existing around the sensor element burns. In the control method of the exhaust gas sensor, when energizing the electrode, the sensor element is heated to a preset element activation temperature by energizing the heater.
Increasing the temperature of the sensor element by energizing the heater to a poisoning avoidance temperature set to a temperature in the vicinity of the temperature at which moisture around the sensor element evaporates from the ambient temperature;
Raising the temperature from the poisoning avoidance temperature to the element activation temperature;
And a step of energizing the electrode after the temperature of the sensor element is raised to the element activation temperature.