JP4481552B2 - Heat resistance type air flow measuring device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an air flow rate measuring device that measures the flow rate of air flowing through an intake air passage of an internal combustion engine.
[0002]
[Prior art]
As a known example showing a conventional example, there is an engine control device described in JP-A-2000-169795. Japanese Patent Application Laid-Open No. 2000-169795 is an invention for preventing the adhesion of dirt on a sensor or actuator element used in an intake pipe of an automobile. The sensor or actuator element is provided with a water-repellent or oil-repellent anti-adhesion coating such as a fluorine coating. There is a statement to that effect. The effect of the invention can prevent the adhesion of sewage, petroleum, splashes, silicon oil, soot, salt, hydrocarbons, and dust particles present in the intake pipe.
[0003]
[Problems to be solved by the invention]
The present invention prevents salt from adhering to a heating resistance type air flow measuring device among sewage, petroleum, splash, silicon oil, soot, salt, hydrocarbon, dust particles present in the intake pipe. . When rain and snow containing sea salt in the coastal region or snowmelt salt in cold regions is inhaled, there is a problem that the salt accumulates and adheres to the heating resistor. The deposition and fixation of salt on the heating resistor causes deterioration in measurement accuracy of the air flow measuring device and corrosion of the heating resistor.
[0004]
Therefore, by providing a water-repellent or oil-repellent anti-adhesion coating as described in the above-mentioned prior art, it is possible to prevent the salt from depositing and sticking when water droplets containing salt come into contact with the heating resistor and evaporate. Was attempted. However, even when a water-repellent or oil-repellent anti-adhesion coating is used, when the surface temperature of the heating resistor is low, there is a problem that salt is deposited and fixed on the surface of the heating resistor.
[0005]
SUMMARY OF THE INVENTION An object of the present invention is to provide a heating resistance type air flow rate measuring device that can eliminate the problem that salt is deposited and fixed on the surface of a heating resistor.
[0006]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the temperature of the heating resistor is set to a temperature at which water droplets contacting the surface of the heating resistor evaporate or disappear due to film boiling. Also, when starting or stopping the heating resistance air flow rate measuring device, set the temperature of the heating resistor to a temperature at which droplets that contact the surface of the heating resistor evaporate or disappear due to film boiling. Also good. Further, a water-repellent and oil-repellent protective coating may be provided on the surface of the heating resistor.
[0007]
There are the following four states as a phenomenon that water droplets on the surface of the material disappear when heated.
1. Convection: Evaporates by being heated by convection below the boiling start temperature.
2. Nucleate boiling: Boiling in a dent or protrusion on the heat transfer surface. Convection in other parts.
3. Transition boiling: Transition region from nucleate boiling to film boiling.
4). Film boiling: The entire heat transfer surface is covered with steam.
[0008]
Under the temperature condition of nucleate boiling (transition boiling), the heat transfer surface has a part that is partially boiling and a part that is not boiling. The water droplets and the heat transfer surface are in contact with each other at the boiling point. As water evaporates, the salt concentration in the water drops increases. Precipitation of the salt begins when the salt exceeds the saturation concentration. Since the water droplets are in contact with the heat transfer surface, the salt is deposited on the heat transfer surface, so the adhesion between the salt and the heat transfer surface is high.
[0009]
Under the temperature condition of film boiling, the heat transfer surface is covered with steam, so that the heat transfer is smaller than that of nucleate boiling. That is, it is a quiet and stable boiling form with the film separated by heat conduction in the vapor film. When the salt concentration in the water drops increases and the salt exceeds the saturation concentration, salt precipitation begins. The water droplets are not in contact with the heat transfer surface, but only in contact with the vapor film. Therefore, since salt precipitates in a state of floating in the air, the adhesion between the salt and the heat transfer surface is extremely low.
[0010]
Here, the set temperature of the heating resistor is not set as high as possible. When set to high temperature, the life of the heating resistor of the heating resistance type air flow measuring device is remarkably shortened, or the electronic parts and connections mounted in the electronic circuit are also maintained at a high temperature. This is because of concern. For this reason, the maximum temperature is set to 460 ° C., preferably 350 ° C. or lower.
[0011]
Water droplets containing salt that have passed through the air cleaner come into contact with the heating resistor. The liquid disappears in the convection, nucleate boiling, transition boiling, or film boiling state as described above depending on the degree of heating from the surface of the heating resistor. When the temperature of the heating resistor is such that it evaporates or disappears due to film boiling, or higher, the surface of the heating resistor has the same or higher water repellency or oil repellency than water. Does not adhere as a water film. Even if water droplets remain in the recesses of the heating resistor, the surface of the heating resistor and the water droplets are completely separated by steam, so the salt in the water droplets does not accumulate and adhere to the surface of the heating resistor. Precipitate in a lump. Since this massive salt is precipitated in a state of floating from the surface of the heating resistor, if there is a flow of intake air, it is blown away. Since the effect of water repellency or oil repellency due to the film boiling phenomenon depends only on the surface temperature, the same effect can be obtained even after long-term use.
[0012]
When the surface temperature of the heating resistor is less than the temperature at which film boiling evaporates, the water film evaporates and disappears by convection, nucleate boiling, or transition boiling while water droplets get wet with the surface of the heating resistor and adhere as a water film with a predetermined contact angle. To do. In this case, since the salt and the heating resistor are in contact with each other in the process of evaporating the water film, the salt in the water droplet is deposited and fixed on the surface of the heating resistor.
[0013]
Therefore, according to the present invention, if the temperature of the heating resistor is set to a temperature at which water droplets contacting the surface of the heating resistor evaporate or disappear due to film boiling or higher, the salt-containing water droplets are removed from the heating resistor. It is blown away without adhering to the surface. Even if it stays in the recess, the surface of the heating resistor and the water droplets are completely separated by steam, so the salt in the water droplets is deposited and blown away in a lump without sticking to the surface of the heating resistor. It is. With the above mechanism, it is possible to prevent salt from being deposited and fixed on the surface of the heating resistor. In addition, even if dirt adheres to the surface of the heating resistor, there is an effect.
Further, when the heating resistance type air flow rate measuring device is stopped, salt deposition due to water droplets due to condensation can be prevented by providing a water-repellent and oil-repellent adhesion preventing surface coating on the surface of the heating resistor. In addition, even when the heating resistance air flow rate measuring device is started or stopped, the temperature of the heating resistor is set to a temperature at which water droplets that contact the surface of the heating resistor evaporate due to film boiling or higher. Can be prevented.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a configuration diagram of a heating resistance type air flow rate measuring apparatus according to an embodiment of the present invention.
The heating resistor type air flow rate measuring device 3 has a heating resistor 1 and a temperature sensitive resistor 2 arranged inside an intake pipe 11 of the internal combustion engine, and the heating resistor 1 and the temperature sensitive resistor 2 are connected to a power management circuit 4. The electronic circuit is composed of a heating resistor heating control circuit (hereinafter referred to as a control circuit) 5, an output adjustment circuit 6, and the like. The heating temperature of the heating resistor 1 is controlled by the control circuit 5 so as to have a substantially constant temperature difference from the intake air temperature detected by the temperature-sensitive resistor 2, and accordingly, the amount of heat released from the heating resistor 1 to the intake air 12. Thus, the intake air flow rate can be detected. The flow rate detection method of the heating resistance type air flow rate measuring device 3 includes other methods such as a detection method using a heater and a temperature detection resistor heated by the heater. I will omit the explanation. This heating resistance type air flow rate measuring device has a power source terminal 7a connected to a power source, a flow rate output terminal 8c that outputs a flow rate signal, a temperature output terminal 9c that outputs an intake air temperature signal, and a ground terminal 10, and Electrically connected.
[0015]
The heating resistor 1 of the heating resistance type air flow rate measuring device 3 is heated and controlled during flow rate measurement, that is, during engine driving. Since the heating temperature of the heat generating resistor and the intake air temperature are a constant temperature difference, the surface temperature of the heat generating resistor is relatively low when the intake air temperature (outside air temperature) is low (for example, in a cold region). Although the heating resistance type air flow rate measuring device 3 for measuring the engine flow rate varies depending on the model, the heating temperature of the heating resistor 1 is usually 200 ° C. higher than the intake air temperature detected by the temperature sensing resistor 2 by the control circuit 5. Controlled. Since the intake air temperature is targeted at -40 to 120 ° C, the surface temperature of the heating resistor of the conventional product is set to 160 to 320 ° C.
[0016]
In the present embodiment, the heating resistor type air in which the control circuit 5 is set so that the surface temperature of the heating resistor 1 is maintained at a temperature at which water contacting the surface of the heating resistor evaporates by film boiling or higher is maintained. This is a flow rate measuring device 3. In other words, even when the intake air temperature (outside air temperature) is low, the temperature is maintained at a temperature at which water droplets contacting the surface of the heat generating resistor 1 disappear due to film boiling or higher. If the temperature of the heating resistor is maintained at 300 ° C. or higher in all use environments, the salt contained in the droplets does not deposit and adhere to the heating resistor.
A means for selecting and setting one of the surface temperatures described above by storing a plurality of surface temperatures of the heating resistor 1 in the electronic circuit may be added.
[0017]
FIG. 2 shows the relationship between the lifetime until the water droplets disappear and the surface temperature when 10 μl of 3% NaCl water droplets are dropped on the high temperature surface. As the surface temperature increases, the droplets evaporate and disappear due to convection, nucleate boiling, and transition boiling, and evaporate and disappear due to film boiling above 300 ° C. Above 300 ° C., the surface and water droplets were completely separated from each other by steam, so that the water droplets remained stationary with a spheroid shape. When dirt adheres to the surface of the heating resistor, these deposits become foaming points, and nucleate boiling occurs at a lower temperature than a smooth surface. However, even if the surface is prone to nucleate boiling, water droplets become film boiling if the surface temperature is as high as 300 ° C. or higher.
[0018]
In this way, when the temperature of the heating resistor is equal to or higher than the temperature of film boiling, water droplets do not adhere and a water film is not formed as much as or more than when the surface of the heating resistor is water-repellent or oil-repellent. Even if the water droplet stays in the recess of the heating resistor, the surface of the heating resistor and the water droplet are completely separated from each other by vapor, so even if the water droplet 18 evaporates and disappears as shown in FIG. The salt 20 is deposited in a lump without being deposited and fixed on the surface of the heating resistor 1. Since the massive salt 20 is precipitated in a state of floating from the surface of the heating resistor 1, it is blown away by the flow of intake air. Although the performance of the water-repellent or oil-repellent anti-adhesion surface coating 15 may deteriorate due to long-term use, the effect of water-repellency or oil-repellency due to the transition boiling or film boiling phenomenon depends only on the surface temperature, so that it can be used for a long time. The same effect can be obtained later.
[0019]
On the other hand, when the temperature of the heating resistor 1 is equal to or lower than the film boiling, water droplets are wetted on the surface of the heating resistor and attached with a predetermined contact angle, and convection, nucleate boiling, or transition boiling in the water film 19 state. As a result, water evaporates and disappears. In this case, as shown in FIG. 3b, since the salt 20 and the heating resistor are in contact with each other in the process of evaporating the water film 19, the salt 20 is deposited and fixed on the heating resistor 1. In this case, it is difficult to blow off with the flow of intake air.
[0020]
Another embodiment of the present invention is shown below. In the heating resistance type air flow measuring device of the present invention, in order to make the temperature of the heating resistor 300 ° C. or higher in all use environments, a temperature difference of 340 ° C. with respect to the intake air temperature is set and the temperature of the heating resistor is 300 to 460. It becomes ℃. Since the conventional product sets an intake air temperature of −40 to 120 ° C. and a temperature difference of 200 ° C. with respect to the intake air temperature, the temperature of the heating resistor is 160 to 320 ° C. Thus, when the heating resistor is used at a maximum of 460 ° C., the life of the resistor is reduced, so that it may be difficult to always use the sensor at 460 ° C. Therefore, at the start-up of the heating resistance type air flow measuring device, the surface temperature of the heating resistor 1 is maintained at a temperature at which the water droplets contacting the surface of the heating resistor evaporate by film boiling is 300 ° C. or higher. The control circuit 5 is set, and after a predetermined time, the temperature of the heating resistor is set to 160 to 320 ° C. as in the conventional case. When the intake air temperature is as high as 120 ° C., the salt does not deposit and adhere because it is always above 300 ° C. However, when the intake air temperature is as low as −40 ° C., the temperature of the heating resistor is 160 ° C., and salt may remain during operation of the heating resistance type air flow measuring device. If dew condensation occurs when the heating resistance type air flow measuring device stops, salt will deliquesce and form a water film. This water film can be blown off in a film boiling state by heating at 300 ° C. or higher when the heating resistance type air flow measuring device is activated. Therefore, salt does not deposit and adhere to the surface of the heating resistor.
In the above example, the temperature of the heating resistor is set to 300 to 460 ° C., but it is desirable to set the temperature within 300 to 350 ° C. from the viewpoint of extending the life of the heating resistor.
[0021]
Further, when the heating resistance type air flow measuring device is stopped, the surface temperature of the heating resistor 1 is maintained at a temperature at which the water droplets contacting the surface of the heating resistor evaporate due to film boiling is 300 ° C. or higher. By setting the control circuit 5 and setting the temperature of the heating resistor to 160 to 320 ° C. as in the conventional case after a certain time, the temperature evaporating by film boiling at the time of startup is maintained at a temperature of 300 ° C. or higher. There is a similar effect.
[0022]
Since the temperature of the heating resistor decreases when the engine is stopped, dirt or condensed water may adhere to the surface of the heating resistor during this period. In addition, when water droplets containing rock salt adhere to the sensor, some of them change to a deliquescent basic substance, and this substance may absorb water when the sensor is stopped. Therefore, another embodiment of the present invention will be shown. When the sensor is stopped, water and oil repellent protective coatings prevent dirt and condensed water from adhering, and when the sensor is in operation, heat is generated at the temperature at which water droplets that contact the surface of the heating resistor evaporate due to film boiling or above. The temperature of the resistor can be set to prevent salt adhesion. FIG. 4 shows the heating resistor 1 of the present invention. The heating resistor is structured in the form of a small piece made of structured silicon (FIG. 4a), but is composed of a metal wire wound around an electrically insulating substrate (FIG. 4b). But you can. In FIG. 4a, a heating resistor 1 and a temperature sensitive resistor 2 such as platinum or polysilicon are formed on a diaphragm 14 made of SiO2 or SiN manufactured in the semiconductor manufacturing process based on the silicon substrate 13. In FIG. 4 b, platinum thin wires 17 are formed on a cylindrical alumina insulating substrate 16. A water-repellent and oil-repellent protective coating 15 is provided on the surface of these heat generating resistors. The protective coating needs to cover the entire heating resistor. That is, if the protective coating is partially missing, salt deposition and sticking occurs starting from the missing portion.
[0023]
FIG. 5A is a schematic plan view of the measuring element 21 of the thermal air flow meter. A thin film diaphragm having a cavity 23 on the lower surface is formed at the center of a rectangular polycrystalline silicon substrate 22 that is a substrate. The thin film diaphragm is provided with a heating resistor 226, side temperature resistors 24 and 25 for measuring the temperature of the heating resistor 26, and an air temperature side temperature resistor 27 for measuring the air temperature. The heating resistor 26 and the temperature measuring resistors 24, 25, and 27 functioning as a micro heater are formed by adopting the same film structure.
FIG. 5B shows a cross section of the thin film diaphragm shown in FIG. 5A cut along the line AA. The heating resistor 26 and the resistance temperature detectors 24, 25, 27 are shown in FIG. The lower thin film 28 and the upper thin film 29 are sandwiched.
[0024]
As shown in FIG. 6, the thermal air flow meter having the above configuration includes a support body 40 that supports the measuring element 21, an external circuit 41, and the like. The measuring element 21 and the external circuit 41 are electrically connected by a wiring (not shown) protected by the support 40 between each terminal electrode 36 (see FIG. 5) of the measuring element 21 and the external circuit 41. ing. The measuring element 21 is disposed in a sub-passage 43 inside the intake passage 42 of the electronically controlled fuel injection device, and the external circuit 41 is installed on the outer wall surface of the intake passage 42.
[0025]
As a principle of detecting the intake flow rate, when air flows from the direction indicated by the white arrow 44, the temperature T of the air is detected by the air temperature measuring resistor, and the air detected by the heating resistor is detected. Energization is performed so as to maintain a state higher than the temperature T by a constant temperature ΔT. At this time, since the amount of heat taken from the surface varies depending on the flow rate of air flowing through the surfaces of the heating resistors 24 and 25, the current supplied to the heating resistor also varies depending on the flow rate of air. Therefore, at this time, the intake flow rate can be detected from the current supplied to the heating resistor 26. Further, the direction of the air flow can be detected by utilizing the characteristic that the temperature of the upstream temperature resistor is lowered.
[0026]
Here, a case where a water-repellent and oil-repellent protective coating is provided on the heat generating resistor will be described with respect to a conventional heat generating resistance air flow measuring device in which the temperature of the heat generating resistor is set to 160 to 320 ° C. . In this case, the water droplet contacting the surface of the heating resistor evaporates and disappears by convection, nucleate boiling, and transition boiling. The water and oil repellent protective coating has a large water and oil contact angle of about 90 to 170 degrees. When water droplets or oil droplets are dropped on the water-repellent and oil-repellent anti-adhesion surface coating, the spherical shape is maintained. However, as long as the contact angle is not 180 degrees, a part of the water droplet is in contact with the water-repellent and oil-repellent protective coating as a water film in a sense. When the surface is heated to 160 ° C. in this state, the water film 19 undergoes nucleate boiling at the contact surface as shown in FIG. It was found that 20 deposited and adhered to the surface. Accordingly, when the water-repellent and oil-repellent protective coating 15 is used alone, it is insufficient to prevent the salt 20 from being deposited and fixed. From the above, when providing a water-repellent and oil-repellent protective coating, if the temperature of the heating resistor is set to a temperature at which water droplets contacting the surface of the heating resistor evaporate due to film boiling or higher, salt deposition will occur. Sticking can be prevented.
[0027]
As described above, the following heating resistance type flow rate measuring device is configured.
A heating resistor whose surface temperature is set to detect the flow rate of the air sucked into the internal combustion engine, and the heating resistor are electrically connected, and the amount of the intake air is determined by the amount of heat released from the heating resistor or the detected temperature. A heating resistance type flow rate measuring device having an electronic circuit that outputs a signal corresponding to a flow rate, wherein the surface temperature of the heating resistor is set to any temperature within 300 ° C to 350 ° C.
[0028]
In order to detect the flow rate of air sucked into the internal combustion engine, a water-repellent and oil-repellent protective coating is provided on the surface, the heating resistor whose surface temperature is set, and the heating resistor are electrically connected, In a heating resistance type flow measuring device having an electronic circuit that outputs a signal corresponding to the flow rate of the intake air depending on the amount of heat released from the heating resistor or the detected temperature, the surface temperature of the heating resistor is any of 300 ° C. to 350 ° C. A heating resistance type flow rate measuring device characterized in that it is set to a temperature of.
[0029]
A heating resistor whose surface temperature is set in order to detect the flow rate of air sucked into the internal combustion engine, and the heating resistor are electrically connected to each other. In the heating resistance type flow rate measuring apparatus having an electronic circuit for outputting a signal corresponding to the flow rate, the electronic circuit has means for storing a plurality of surface temperatures of the heating resistor, and any one of 300 ° C. to 350 ° C. A heating resistance type flow rate measuring device characterized by selecting and setting the surface temperature.
[0030]
【The invention's effect】
By setting the temperature of the heating resistor to a temperature at which the water droplets contacting the surface of the heating resistor evaporate or disappear due to film boiling or higher, the deposition of the salt contained in the water droplets is prevented, thereby preventing the heating resistor. It is possible to reduce fouling and corrosion, and to reduce the measurement accuracy of the heating resistance type air flow measuring device.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a heating resistance type air flow rate measuring apparatus according to an embodiment of the present invention.
FIG. 2 is a diagram showing the relationship between the life until the water droplets on the high temperature surface evaporate and the surface temperature.
FIG. 3 is a diagram showing the form of salt on the surface of a heating resistor.
FIG. 4 is a cross-sectional view of a heating resistor according to an embodiment of the present invention.
5A and 5B are views showing an embodiment of a measuring element provided in a thermal air flow meter to which the present invention is applied, in which FIG. 5A is a schematic plan view, and FIG. The expanded sectional view in AA of 5 (a).
FIG. 6 is a diagram showing a schematic configuration of an embodiment of a thermal air flow meter to which the present invention is applied.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Heat generating resistor, 2 ... Temperature sensitive resistor, 3 ... Heat generating resistance type air flow measuring device, 4 ... Power supply management circuit, 5 ... Control circuit, 6 ... Output adjustment circuit, 7 ... Power supply line, 8 ... Flow rate signal, DESCRIPTION OF SYMBOLS 9 ... Temperature signal, 10 ... Ground line, 11 ... Intake pipe, 12 ... Intake air, 13 ... Silicon substrate, 14 ... Diaphragm, 15 ... Anti-adhesion surface coating, 16 ... Electrical insulation board, 17 ... Platinum wire, 18 ... Water drops, 19 ... Water film, 20 ... Salt

Claims (4)

A heating resistor for detecting the flow rate of air sucked into the internal combustion engine, and a signal corresponding to the flow rate of the intake air according to the amount of heat released from the heating resistor or the detected temperature are electrically connected to the heating resistor. In the heating resistance type flow measuring device having an electronic circuit to output,
A heating resistor whose entire surface is covered with a coating having water repellency and oil repellency, and when the heating resistor starts up from the time when the heating resistance type air flow measuring device is stopped, a water droplet contacting the surface is formed into a film. so that the temperature of 300 ℃ ~460 ℃ boiling evaporation vanishing temperature or higher (including the temperature), when the temperature is set, launched from the stop of the heating resistor type air flow rate measuring device, the temperature A heating resistance type air flow rate measuring device characterized by evaporating and extinguishing a water droplet used in a range and contacting the surface by film boiling.   2. The temperature of the heating resistor according to claim 1, wherein the temperature of the heating resistor exceeds the temperature (including the temperature) at which water droplets contacting the surface of the heating resistor evaporate and disappear due to film boiling only when the heating resistance type air flow rate measuring device is activated. A heating resistance type air flow rate measuring device characterized in that is set.   The temperature of the heating resistor according to claim 1, wherein the temperature of the heating resistor is higher than the temperature (including the temperature) at which water droplets contacting the surface of the heating resistor evaporate and disappear due to film boiling only when the heating resistance type air flow measuring device is stopped. A heating resistance type air flow rate measuring device characterized in that is set. A heating resistor whose surface temperature is set to detect the flow rate of the air sucked into the internal combustion engine, and the heating resistor are electrically connected, and the amount of the intake air is determined by the amount of heat released from the heating resistor or the detected temperature. In the heating resistance type flow rate measuring device having an electronic circuit that outputs a signal corresponding to the flow rate,
A heating resistor whose surface is entirely covered with a coating having water repellency and oil repellency, and the heating resistor has a surface temperature of 300 ° C. to 350 ° C. when the heating resistance air flow measuring device is started from a stop. When the temperature of the heating resistance type air flow measuring device is started from the time when the heating resistance type air flow measuring device is stopped , water droplets used in the temperature range and contacting the surface are evaporated and extinguished by film boiling. Heating resistance type flow measuring device.

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