JP6114674B2 - Thermal flow meter - Google Patents

Description

  The present invention relates to a thermal flow meter.

  A thermal flow meter that measures the flow rate of gas includes a flow rate detection unit for measuring the flow rate, and performs heat transfer between the flow rate detection unit and the gas to be measured, thereby reducing the flow rate of the gas. It is configured to measure. The flow rate measured by the thermal flow meter is widely used as an important control parameter for various devices. A feature of the thermal flow meter is that it can measure a gas flow rate, for example, a mass flow rate, with relatively high accuracy compared to other types of flow meters.

  However, further improvement in gas flow rate measurement accuracy is desired. For example, a vehicle equipped with an internal combustion engine has a very high demand for fuel saving and exhaust gas purification. In order to meet these demands, it is required to measure the intake air amount, which is a main parameter of the internal combustion engine, with high accuracy.

  A thermal flow meter for measuring the amount of intake air led to an internal combustion engine includes a sub-passage that takes in a part of the intake air amount and a flow rate detector disposed in the sub-passage, and the flow rate detector is a gas to be measured. The state of the gas to be measured flowing through the sub-passage is measured by performing heat transfer between and the electric signal, and an electric signal representing the amount of intake air guided to the internal combustion engine is output.

  For example, as such a technique, Patent Document 1 discloses that “a bypass flow channel that takes in part of the air flowing inside the duct, and a part of the air that is branched from the bypass flow channel and flows through the bypass flow channel” And a sub-passage channel, and a sub-passage channel, and a flow rate sensor for measuring a flow rate of air flowing in the sub-bypass channel. The thermal flow meter is described that separates dust contained in the air flowing through the duct and flows the dust to the bypass flow path side.

JP 2011-1112569 A

  However, as shown in Patent Document 1, a bypass passage (first passage) and a sub-pipas passage (second passage) that branches to the bypass passage are provided as sub-passages, and a flow sensor ( In the case where the flow rate detection unit) is provided, the pollutant easily adheres to the branch portion between the first passage and the second passage. Due to the adhesion of this fouling substance, the diversion ratio of the gas to be measured flowing in the first passage and the second passage is changed as compared with the initial state, and the flow rate passing through the flow rate detector is changed. There was a possibility that the detection characteristics of the change.

  The present invention has been made in view of the above points, and the object of the present invention is that even if a pollutant flows into the sub-passage, the characteristic of detecting the gas to be measured by the pollutant is continuous. An object of the present invention is to provide a thermal type flow meter that can suppress the change to the above.

  In view of the above problems, the thermal flow meter according to the present invention includes a sub-passage that takes in part of the gas to be measured flowing through the main passage, and a flow rate detection unit that detects the flow rate of the gas to be measured flowing through the sub-passage. The sub-passage includes a first passage formed from a main intake port that takes in the measurement gas flowing through the main passage to a discharge port that discharges a part of the taken measurement gas; and And a second passage formed toward the flow rate detection unit from a sub-intake port that takes in the gas to be measured flowing into the first passage.

  Here, among the opening edges that form the sub intake port, first, the downstream opening edge located on the downstream side of the first passage is the upstream side located on the upstream side of the first passage. Of the wall surface forming the first passage, the opening wall is formed so as to be separated from the wall surface facing the sub intake port, or the downstream opening edge and the upstream opening. The edge is at the same distance as the wall facing the sub-inlet.

  Next, among the virtual planes perpendicular to the flow direction of the gas to be measured flowing through the main passage, the virtual plane passing through the first passage in the virtual plane passing through the downstream end of the downstream opening edge. The downstream side along the flow direction of the gas to be measured at which the measurement gas has the maximum flow velocity, or from the surface along the flow direction toward the side where the second passage extends from the sub intake port A wall surface of the first passage extends from the opening edge to the downstream side of the first passage in the vicinity of the downstream opening edge.

  According to the present invention, even if a fouling substance flows into the sub-passage, it is possible to suppress the characteristic of detecting the measurement target gas from being changed by the fouling substance over time.

1 is a system diagram showing an embodiment in which a thermal flow meter according to the present invention is used in an internal combustion engine control system. The front view which shows the external appearance of the thermal type flow meter which concerns on this invention. The left view which shows the external appearance of the thermal type flow meter which concerns on this invention. The rear view which shows the external appearance of the thermal type flow meter which concerns on this invention. The right view which shows the external appearance of the thermal type flow meter which concerns on this invention. The front view which shows the state of the housing which removed the front cover from the thermal type flow meter which concerns on this invention. The rear view which shows the state of the housing which removed the back cover from the thermal type flow meter which concerns on this invention. AA sectional drawing of FIG. 2A. The schematic diagram of the subchannel | path used as the comparative example of a present Example. The principal part enlarged view of the subchannel | path shown to FIG. 3B. FIG. 7 is a schematic conceptual diagram of the auxiliary passage shown in FIG. 6. The schematic diagram which showed the modification of FIG. The schematic diagram which showed another modification of FIG. The figure which showed another modification of FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a system diagram showing an embodiment in which a thermal flow meter according to the present embodiment is used in an electronic fuel injection type internal combustion engine control system. As shown in FIG. 1, based on the operation of an internal combustion engine 110 including an engine cylinder 112 and an engine piston 114, intake air is sucked from an air cleaner 122 as a measured gas IA and includes an intake pipe 71 in which a main passage 124 is formed. For example, the air is guided to the combustion chamber of the engine cylinder 112 through the intake body, the throttle body 126, and the intake manifold 128.

  The flow rate of the gas to be measured IA that is the intake air led to the combustion chamber is measured by the thermal flow meter 30 according to the present embodiment, and fuel is supplied from the fuel injection valve 152 based on the measured flow rate. Together with the gas to be measured IA, which is introduced into the combustion chamber in the state of the air-fuel mixture. In the present embodiment, the fuel injection valve 152 is provided at the intake port of the internal combustion engine, and the fuel injected into the intake port forms an air-fuel mixture together with the measured gas IA that is intake air, and is connected via the intake valve 116. It is guided to the combustion chamber and burns to generate mechanical energy.

  The thermal flow meter 30 can be used not only for the method of injecting fuel into the intake port of the internal combustion engine shown in FIG. 1 but also for the method of directly injecting fuel into each combustion chamber. In both types, the basic concept of the control parameter measurement method including the method of using the thermal flow meter 30 and the control method of the internal combustion engine including the fuel supply amount and the ignition timing is substantially the same. A method of injecting fuel into the port is shown in FIG.

  The fuel and air guided to the combustion chamber are in a mixed state of fuel and air, and are burned explosively by spark ignition of the spark plug 154 to generate mechanical energy. The combusted gas is guided from the exhaust valve 118 to the exhaust pipe, and is exhausted from the exhaust pipe to the outside as exhaust EA. The flow rate of the gas to be measured IA that is the intake air led to the combustion chamber is controlled by a throttle valve 132 whose opening degree changes based on the operation of the accelerator pedal. The fuel supply amount is controlled based on the flow rate of the intake air guided to the combustion chamber, and the driver controls the flow rate of the intake air guided to the combustion chamber by controlling the opening degree of the throttle valve 132, thereby The mechanical energy generated by the engine can be controlled.

  The flow rate, humidity, and temperature of the measurement target gas IA that is the intake air that is taken in from the air cleaner 122 and flows through the main passage 124 are measured by the thermal flow meter 30, and the flow rate, humidity, and temperature of the intake air are measured from the thermal flow meter 30. The electric signal to be represented is input to the control device 200. Further, the output of the throttle angle sensor 144 that measures the opening degree of the throttle valve 132 is input to the control device 200, and the positions and states of the engine piston 114, the intake valve 116, and the exhaust valve 118 of the internal combustion engine, and the rotation of the internal combustion engine. In order to measure the speed, the output of the rotation angle sensor 146 is input to the control device 200. In order to measure the state of the mixture ratio between the fuel amount and the air amount from the state of the exhaust EA, the output of the oxygen sensor 148 is input to the control device 200.

  The control device 200 calculates the fuel injection amount and the ignition timing based on the flow rate, humidity, and temperature of the intake air that is the output of the thermal flow meter 30 and the rotational speed of the internal combustion engine from the rotation angle sensor 146. Based on these calculation results, the amount of fuel supplied from the fuel injection valve 152 and the ignition timing ignited by the spark plug 154 are controlled. The fuel supply amount and ignition timing are actually based on the intake air temperature and throttle angle change state measured by the thermal flow meter 30, the engine speed change state, and the air-fuel ratio state measured by the oxygen sensor 148. Are controlled. The control device 200 further controls the amount of air that bypasses the throttle valve 132 by the idle air control valve 156 in the idle operation state of the internal combustion engine, thereby controlling the rotational speed of the internal combustion engine in the idle operation state.

  Both the fuel supply amount and the ignition timing, which are the main control amounts of the internal combustion engine, are calculated using the output of the thermal flow meter 30 as a main parameter. Therefore, improvement in measurement accuracy of the thermal flow meter 30, suppression of changes over time, and improvement in reliability are important in terms of improving the control accuracy of the vehicle and ensuring reliability. In particular, in recent years, there has been a very high demand for fuel efficiency of vehicles and a very high demand for exhaust gas purification. In order to meet these demands, it is extremely important to improve the measurement accuracy of the flow rate of the measurement target gas IA that is the intake air measured by the thermal flow meter 30.

  FIG. 2 shows the appearance of the thermal flow meter 30. 2A is a front view of the thermal flow meter 30, FIG. 2B is a left side view, FIG. 2C is a rear view, and FIG. 2D is a right side view. The thermal flow meter 30 includes a housing 302, a front cover 303, and a back cover 304. The housing 302 has an external connection portion (connector portion) having a flange 312 for fixing the thermal flow meter 30 to the intake body constituting the main passage and an external terminal for electrical connection with an external device. 305 and a measurement unit 310 for measuring a flow rate and the like. A sub-passage groove for making a sub-passage is provided inside the measurement unit 310.

  By covering the front cover 303 and the back cover 304 described above, a casing having a sub-passage is formed. Inside the measurement unit 310, a circuit including a flow rate detection unit 602 for measuring the flow rate of the measurement target gas IA flowing through the main passage and a temperature detection unit 452 for measuring the temperature of the measurement target gas IA flowing through the main passage. A package 400 is provided (see FIGS. 3A and 3B).

  In the thermal flow meter 30, the measurement unit 310 is supported in a cantilever manner in the main passage by fixing the flange 312 to the intake body (intake pipe) 71. In FIG. 2A and FIG. 3B, in order to clarify the positional relationship between the thermal flow meter 30 and the intake pipe 71, the intake pipe 71 is indicated by a virtual line.

  The measurement unit 310 of the thermal flow meter 30 has a shape extending long from the flange 312 toward the center of the main passage 124 in the radial direction, and a part of the measurement target gas IA such as intake air is provided at the tip thereof. A main intake port 350 (see FIG. 2C) for taking in the sub-passage and an exhaust port 355 (see FIG. 2D) for returning the measured gas IA from the sub-passage to the main passage 124 are provided.

  The main intake port 350 of the thermal flow meter 30 is provided on the distal end side of the measuring unit 310 extending from the flange 312 toward the central direction in the radial direction of the main passage, so that the portion away from the inner wall surface of the main passage is provided. Gas can be taken into the secondary passage. Thereby, it becomes difficult to be influenced by the temperature of the inner wall surface of the main passage, and a decrease in measurement accuracy of the gas flow rate and temperature can be suppressed. As will be described later, in the present embodiment, the center of the main intake port 350 is offset with respect to the center line CL along the direction D in which the measured gas IA in the main passage 124 flows.

  Further, the fluid resistance is large near the inner wall surface of the main passage 124, and the flow velocity is lower than the average flow velocity of the main passage. In the thermal type flow meter 30 of the present embodiment, the main intake port 350 is provided at the distal end portion of the thin and long measuring unit 310 extending from the flange 312 toward the center of the main passage. Gas can be taken into the sub-passage (measurement passage). Further, since the discharge port 355 of the sub passage is also provided at the tip of the measuring unit 310, the gas flowing in the sub passage can be returned to the vicinity of the central portion of the main passage 124 having a high flow velocity.

  The measuring unit 310 has a shape that extends long along the axis from the outer wall of the main passage 124 toward the center, but has a narrow shape as shown in FIGS. 2B and 2D. That is, the measurement unit 310 of the thermal flow meter 30 has a side surface that is thin and has a substantially rectangular front surface. Thus, the thermal flow meter 30 can be provided with a sufficiently long sub-passage with a reduced fluid resistance with respect to the gas to be measured IA.

  The temperature detection unit 452 for measuring the temperature of the measurement target gas IA is located at the center of the measurement unit 310 at a position where the upstream outer wall in the measurement unit 310 is recessed toward the downstream side, upstream from the upstream outer wall. It is provided with a shape that protrudes toward the surface.

  The front cover 303 and the back cover 304 are formed in a thin plate shape and have a shape with a wide cooling surface. For this reason, the thermal flow meter 30 has an effect that air resistance is reduced, and further, the thermal flow meter 30 is easily cooled by the gas to be measured flowing through the main passage 124.

  An external terminal and a correction terminal (not shown) are provided inside the external connection unit 305. The external terminal includes a terminal for outputting a flow rate and temperature as measurement results, and a power supply terminal for supplying DC power. The correction terminal is a terminal used to store a correction value related to the thermal flow meter 30 in a memory inside the thermal flow meter 30.

  Next, with reference to FIG. 3A and FIG. 3B, the configuration of the sub-path and the circuit package configured in the housing 302 will be described. 3A and 3B show a state of the housing 302 with the front cover 303 or the back cover 304 removed from the thermal flow meter 30. FIG. FIG. 3A is a front view showing the state of the housing with the front cover removed from the thermal flow meter according to the present invention, and FIG. 3B shows the state of the housing with the back cover removed from the thermal flow meter according to the present invention. FIG.

  The housing 302 is provided with a sub-passage groove for forming a sub-passage on the distal end side of the measuring unit 310. The sub passage 330 is a passage formed in the thermal flow meter 30 in order to take in a part of the gas to be measured flowing through the main passage 124. In this embodiment, auxiliary passage grooves 332 and 334 are provided on both the front and back surfaces of the housing 302. By covering the front cover 303 and the back cover 304 on the front surface and the back surface of the housing 302, continuous sub-passages 330 are formed on both surfaces of the housing 302. With such a structure, when the housing 302 is molded (resin molding process), molds provided on both surfaces of the housing 302 are used, and both the front side sub passage groove 332 and the back side sub passage groove 334 are formed in the housing 302. A through portion 382 that penetrates the housing 302 is formed so as to connect them, and the flow rate detecting element (flow rate detecting portion) 602 of the circuit package 400 can be disposed in the through portion 382.

  As shown in FIG. 3B, a part of the measurement target gas IA flowing through the main passage is taken into the back side sub passage groove 334 from the main intake port 350 via the inlet groove 531 and flows through the back side sub passage groove 334. . By covering the back cover 304 with the back side sub-passage groove 334, a part of the sub-passage 330 upstream of the first passage 31 and the second passage 32 is formed in the thermal flow meter 30.

  The first passage 31 is a passage for discharging a pollutant formed from a main intake port 350 for taking in the measurement target gas IA flowing through the main passage 124 to a discharge port 355 for discharging a part of the taken measurement target gas IA. It is. The second passage 32 is a flow rate measurement passage formed toward the flow rate detection unit 602 from the sub intake port 34 for taking in the measurement target gas IA flowing in the first passage 31. The main intake port 350 is opened facing the upstream side of the main passage 124, the discharge port 355 is opened facing the downstream side of the main passage 124, and the opening area of the discharge port 355 is: It is smaller than the opening area of the main intake port 350. As a result, the gas IA to be measured from the main intake port 350 can easily flow into the second passage 32.

  Of the back side auxiliary passage groove 334, the passage groove of the second passage 32 (passage to the flow rate detection unit 602) has a shape that becomes deeper as it advances in the flow direction, and in the front side direction as it flows along the groove. The gas to be measured IA moves gradually. The back side sub-passage groove 334 is provided with a steeply inclined portion 347 that suddenly deepens upstream of the circuit package 400. A part of the air having a small mass moves along the steeply inclined portion 347 and flows in the upstream portion 342 of the through portion 382 of the circuit package 400 toward the measurement channel surface 430 shown in FIG. On the other hand, a foreign substance having a large mass cannot easily flow along the steeply inclined portion 347 because it is difficult to change the course due to centrifugal force, and flows along the measurement channel back surface 431 shown in FIG. After that, it passes through the downstream portion 341 of the penetrating portion 382 and flows through the front side auxiliary passage groove 332 shown in FIG. 3A.

  As described above, the portion of the circuit package 400 that includes the measurement flow path surface 430 is disposed in the cavity of the through portion 382, and the through portion 382 is provided on the left and right sides of the circuit package 400 having the measurement flow passage surface 430. The groove 334 and the front side auxiliary passage groove 332 are connected.

  As shown in FIG. 3A, in the penetrating portion 382, air that is the measurement target gas IA flows from the upstream portion 342 along the measurement channel surface 430. At this time, heat transfer is performed with the flow rate detection unit 602 for measuring the flow rate through the heat transfer surface provided in the flow rate detection unit 602, and the flow rate is measured. Note that this flow rate measurement principle may be a general detection principle for a thermal flow meter, and flows through the main passage based on the measurement value measured by the flow rate detection unit 602 of the circuit package 400 as in this embodiment. As long as the flow rate of the gas to be measured can be detected, the configuration for detection is not particularly limited.

  Both the gas IA to be measured that has passed through the measurement flow path surface 430 and the air that has flowed from the downstream portion 341 of the circuit package 400 to the front side sub-passage groove 332 flow along the front side sub-passage groove 332, and exit from the second passage 32. An outlet groove 353 forming 352 is discharged into the main passage 124.

  In this embodiment, the second passage formed by the back side sub-passage groove 334 is curved toward the flange direction from the tip of the housing 302 while drawing a curve, and the measured gas IA flowing through the sub-passage is closest to the flange side. The flow is in the opposite direction to the flow of the main passage 124. The sensor upstream side passage 32a provided on the back surface side of the second passage 32 provided on one side of the housing 302 is a second portion provided on the other side of the second passage 32 provided on the one side of the housing 302. The sensor downstream side passage 32b provided on the surface side of the second passage 32 is connected.

  In this embodiment, the front end side of the circuit package 400 is disposed in the cavity of the through portion 382. The space of the upstream portion 342 located on the upstream side of the circuit package 400 and the space of the downstream portion 341 located on the downstream side of the circuit package 400 are included in the penetration portion 382, and the penetration portion 382 is as described above. The housing 302 is pierced so as to penetrate the front surface side and the back surface side. As a result, as described above, at the through portion 382, the sensor upstream side passage 32a formed by the front side secondary passage groove 332 on the front surface side of the housing 302 and the sensor downstream side secondary passage groove 334 formed by the back side secondary passage groove 334 are formed. The passage 32b communicates.

  As shown in FIG. 4, the space on the measurement flow path surface 430 side and the space on the measurement flow path back surface 431 side are separated by being inserted into the housing 302, and are not separated by the housing 302. . One space formed by the space of the upstream portion 342, the space of the downstream portion 341, the space on the measurement flow channel surface 430 side, and the space on the measurement flow channel back surface 431 side is the front and back surfaces of the housing 302. The circuit package 400 inserted into the housing 302 protrudes in a cantilever manner in this one space. With such a configuration, the sub-passage grooves can be formed on both surfaces of the housing 302 in a single resin molding step, and the structure connecting the sub-passage grooves on both surfaces can be formed together.

  The circuit package 400 is fixed by being embedded in a fixing portion 372, 373, 376 of the housing 302 by a resin mold. Such a fixing structure can be mounted on the thermal flow meter 30 by insert molding the circuit package 400 into the housing 302 simultaneously with resin molding of the housing 302.

  As shown in FIG. 3B, the back side sub-passage groove 334 includes a first passage wall 395, a back side sub-passage inner peripheral wall (second passage wall) 392, and a back side sub-passage outer peripheral wall ( Second wall 391). The front end portions of the back side sub-passage inner peripheral wall 392 and the back side sub-passage outer peripheral wall 391 and the inner side surface of the back cover 304 are in close contact with each other, whereby the first passage 31 and the second passage of the housing 302 are in close contact with each other. A sensor upstream side passage 32a of the passage 32 is formed.

  On the other hand, a front side sub-passage inner peripheral wall (second passage wall) 393 and a front side sub-passage outer peripheral wall (second passage wall) 394 are provided on both sides of the front side sub-passage groove 332, and these front side sub-passage inner peripheral walls 393. And the front end portion in the height direction of the front side sub-passage outer peripheral wall 394 and the inner side surface of the front cover 303 are in close contact with each other, so that the downstream side sub-passage of the housing 302 is formed.

  The measured gas IA that has been taken in from the main inlet 350 and has flowed through the first passage 31 constituted by the back side sub-passage groove 334 flows from the right side to the left side in FIG. 3B. Here, a part of the taken measurement gas IA flows in a diverted flow into the sub intake port 34 of the second passage 32 formed to branch from the first passage 31. The gas to be measured IA flows through the upstream portion 342 of the penetrating portion 382 through the channel 386 formed by the surface of the measurement channel surface 430 of the circuit package 400 and the protrusion 356 provided on the front cover 303. Flows (see FIG. 4).

  The other measurement target gas IA flows through the flow path 387 formed by the measurement flow path back surface 431 and the back cover 304. Thereafter, the measured gas IA that has flowed through the flow path 387 moves toward the front side sub-passage groove 332 via the downstream portion 341 of the penetrating portion 382, and merges with the measured gas IA that flows through the flow path 386. The gas to be measured IA that has joined flows through the front side sub-passage groove 332 and is discharged from the outlet 352 to the main passage.

  The sub-passage groove is such that the measured gas IA guided from the back side sub-passage groove 334 to the flow path 386 via the upstream portion 342 of the penetrating part 382 is bent more than the flow path guided to the flow path 387. Is formed. Thereby, a substance having a large mass such as dust contained in the measurement target gas IA collects in the flow path 387 with less bending.

  In the flow path 386, the protruding portion 356 forms a throttle, and the measured gas IA is made into a laminar flow with little vortex. Further, the protrusion 356 increases the flow velocity of the measurement target gas IA. Thereby, measurement accuracy improves. The protrusion 356 is formed on the front cover 303, which is a cover that faces the heat transfer surface exposed portion 436 of the flow rate detector 602 provided on the measurement flow path surface 430.

  As shown in FIGS. 3A and 3B, the housing 302 has a cavity 336 formed between the flange 312 and the portion where the sub-passage groove is formed. In the hollow portion 336, a terminal connection portion 320 that connects the connection terminal 412 of the circuit package 400 and the inner end 361 of the external terminal of the external connection portion 305 is provided. The connection terminal 412 and the inner end 361 are electrically connected by spot welding or laser welding.

  Here, for example, in the case where the first passage 31 as shown in FIG. 5 is provided, an opening that forms the sub intake port 34 in the branching portion between the first passage 31 and the second passage 32. If the position of the downstream opening edge 34b located on the downstream side of the first passage 31 in the edge protrudes so as to restrict the flow path of the first passage 31, the second passage 32 is contaminated. It was found from the analysis results of the inventors that the substance easily flows. Further, in this structure, it is assumed that the pollutant easily adheres to the downstream opening edge 34b.

  Further, since the pollutant substance flowing in from the main intake port 350 easily collides with the downstream opening edge 34b, the pollutant substance is present on the downstream side wall surface 35 formed on the downstream side from the downstream opening edge 34b. Easy to deposit. Due to the accumulation of the fouling substance, the diversion ratio of the gas to be measured IA flowing through the first passage 31 and the second passage 32 changes, and the flow rate of the gas to be measured IA flowing through the second passage 32 is changed to the fouling. It has been clarified by experiments by the inventors that the material changes due to the deposition of the material.

  From such a result, the sub-passage 330 described above was formed from the main intake port 350 for taking in the measurement gas IA flowing through the main passage 124 to the discharge port 355 for discharging a part of the taken measurement gas IA. A passage structure that includes a first passage 31 and a second passage 32 that is formed toward the flow rate detection unit 602 from the sub intake port 34 that takes in the gas to be measured IA flowing in the first passage 31 is employed. In that case, it was considered important to focus on the following two points.

  That is, in the present embodiment, as shown in FIGS. 6 and 7, among the opening edges forming the sub intake port 34, the downstream opening edge 34 b positioned on the downstream side of the first passage 31 is It is formed so as to be separated from the wall surface 31b facing the sub intake port 34 among the wall surfaces forming the first passage 31 rather than the upstream opening edge 34a located on the upstream side of the first passage 31. 7 (distance L1 from the upstream opening edge 34a to the wall surface 31b> distance L2 from the downstream opening edge 34b to the wall surface 31b). The downstream opening edge 34b and the upstream opening edge 34a may be at the same distance as the wall surface 31b facing the sub intake port 34.

  By having such a positional relationship, the flow path of the first passage 31 is restricted in the downstream opening edge 34b located on the downstream side of the first passage 31 forming the sub intake port 34. Therefore, the pollutant does not easily collide with the downstream opening edge 34b, and the pollutant from the main intake port 350 easily flows toward the discharge port 355 of the first passage 31.

  Here, of the wall surfaces forming the first passage 31, the wall surface 31 b facing the sub-intake port 34 is planar, and the discharge port 355 along the flow direction D of the gas to be measured IA flowing through the main passage 124. It is formed up to. By adopting such a structure, it is possible to suppress the accumulation of fouling substances on the wall surface 31b, and it is possible to suitably discharge the fouling substances taken in from the upstream passage 31A toward the discharge port 335. .

  In the present embodiment, the upstream and downstream opening edges 34a and 34b are formed along the direction perpendicular to the paper surface of FIG. 8, and therefore, the upstream edge 34a and its upstream end coincide with each other. The downstream edge 34b is coincident with the downstream end 34c.

  Next, as shown in FIG. 7, the virtual plane P that passes through the downstream end 34 c of the downstream opening edge 34 b among the virtual planes perpendicular to the flow direction D of the gas IA to be measured flowing through the main passage 124. , The downstream side of the virtual plane S along the flow direction Fmax toward the side Q where the second passage extends from the sub intake port 34 (specifically, the flow rate detection unit 602 side in the present embodiment). A downstream side wall surface 35 of the first passage 31 extends from the opening edge 34b to the downstream side in the vicinity of the downstream opening edge 34b.

  Thereby, the pollutant does not easily collide with the downstream side wall surface 35 formed on the downstream side in the vicinity of the downstream opening edge 34b from the downstream opening edge 34b. As a result, the diversion ratio of the gas to be measured IA flowing in the first passage 31 and the second passage 32 is stabilized, the flow rate characteristic of the gas to be measured IA flowing in the second passage 32 can be stabilized, and the thermal type The detection characteristics of the flow meter 30 can be stabilized over time. In addition, as shown in FIG. 7, along the flow direction Fmax of the gas to be measured having the maximum flow velocity (specifically, along the virtual plane of FIG. 7), the first passage from the downstream opening edge 34b. 31 so that the downstream side wall surface 35 of 31 extends. In this case, the fouling substance is more likely to adhere to the downstream side wall surface 35 than in the present embodiment, but the fouling substance is less likely to adhere to the downstream side wall surface 35 than the conventional ones.

  As shown in FIGS. 6 and 7, in this embodiment, as a more preferable aspect of the present invention, the downstream side wall surface 35 has a side Q (second passage 32 extends) on which the second passage 32 extends. A concave wall surface recessed in the existing direction). By having such a concave wall surface, as shown in FIG. 7, the concave wall surface serves as a pocket for capturing the fouling substance, and even if the fouling substance accumulates on the concave wall surface, The diversion ratio of the measurement target gas IA flowing in the second passage 32 is difficult to change.

  Further, in the present embodiment, the wall surface near the discharge port 355 downstream of the concave wall surface has a convex wall surface 36 protruding to the side opposite to the side Q on which the second passage extends. By forming the convex wall surface 36 in this manner, the flow velocity of the convex wall surface 36 increases on the wall surface from the downstream opening edge 34b to the discharge port 355, so that it is difficult for the pollutant to deposit on the upstream side in the vicinity of the discharge port 355. . In particular, by forming the upstream wall surface of the convex wall surface 36 near the discharge port 355 from the discharge port 355 so as to be parallel to the wall surface 31b facing this, Collisions can be reduced. Thereby, it can suppress that the characteristic which detects to-be-measured gas changes with time.

  Here, the side Q on which the second passage 32 extends is a side on which the second passage 32 extends from the virtual plane S along the flow direction Fmax shown in FIG. 7 in more detail. (See the arrow F drawn from the virtual plane S in FIG. 7). On the other hand, the side opposite to the side Q on which the second passage extends, which will be described later, is the side opposite to the side on which the second passage 32 extends from the virtual plane S along the flow direction Fmax. is there.

  Furthermore, in this embodiment, as a more preferable aspect of the present invention, as shown in FIG. 6, the center of the main intake port 350 of the first passage 31 is in the direction D in which the measured gas IA flows in the main passage 124. It is offset with respect to the center line CL along. In the present embodiment, since the pollutant is easily contained in the vicinity of the center line CL of the main passage 124 as compared with other portions, by disposing the center of the main intake port 350 at a position offset from this position, The amount of fouling substances flowing into the sub passage 330 can be reduced. Thereby, it can suppress that the characteristic which detects to-be-measured gas changes with time.

  In particular, in the present embodiment, the upstream passage 31A from the main intake port 350 to the sub intake port 34 (specifically, the upstream end portion) of the first passage 31 in such an offset state. Is inclined with respect to the center line CL so as to move away from the center line CL as it proceeds from the main inlet 350 to the sub inlet 34. Thus, even if the main intake port is not disposed near the center line CL, the thermal flow meter 30 is disposed at such an offset position in this way with respect to the center line CL. Since the gas to be measured IA (arrow ia in the figure) flows along the inclined direction, the main intake port 350 follows the flow (arrow ia in the figure) without obstructing the flow of the gas to be measured IA. It can be taken into the secondary passage 330.

  Furthermore, in the present embodiment, as a preferred aspect of the present invention, the upstream-side passage 31 </ b> A is inclined so as to be away from the flow rate detection unit 602 with respect to the flow direction D of the measurement target gas IA flowing through the main passage 124. This further suppresses the attachment of the fouling substance to the downstream side wall surface 35 in the vicinity of the downstream opening edge and makes it difficult for the fouling substance flowing in the upstream passage 31A to flow into the second passage 32. It is possible to facilitate the flow toward the discharge port 355 of the passage 31.

In order to further enhance such effects, it is preferable that the upstream passage 31 </ b> A is formed linearly from the main intake port 350 along the sub intake port 34. That is, in the present embodiment, the opposing wall surfaces 31a and 31a forming the upstream passage 31A are planar. As a result, the fouling substance that has flowed in from the main intake port 350 flows to the upstream passage 31A with linearity, so that the fouling substance is easily discharged toward the discharge port 355. In particular, in the present embodiment, since the discharge port 355 is formed in the flow direction of the measurement target gas IA flowing through the upstream side passage 31A, the pollutant substance flowing through the upstream side passage 31A can be efficiently flowed to the discharge port 355. it can.
In the above-described embodiment, the wall surface in the vicinity of the discharge port 355 downstream of the concave wall surface is formed with the convex wall surface 36 protruding to the side opposite to the side D on which the second passage extends. As shown in FIG. 8, the downstream side wall surface 35 from the downstream opening edge 34b to the discharge port 355 may be a concave wall surface that is recessed in the direction D in which the second passage extends. By having such a concave wall surface, the concave wall surface serves as a pocket for capturing the fouling substance. Even if the fouling substance accumulates on the concave wall surface, the first passage 31 and the second passage 32 are provided. It is difficult for the diversion ratio of the flowing measurement target gas IA to change, and it is possible to suppress changes in the characteristics of detecting the measurement target gas over time.

  Furthermore, in the embodiment shown in FIG. 6 described above, the downstream opening edge 34b is downstream of the first passage 31 from the sub intake port 34 toward the side where the second passage 32 extends from the flow direction Fmax. Although the side wall surface 35 is configured to extend, for example, as shown in FIG. 9, a portion including the downstream opening edge 34 b is formed so as to protrude with respect to the first passage, and the protruding portion The distal end surface (the downstream side wall surface 35B in the vicinity of the downstream opening edge 34b from the downstream opening edge 34b) may be configured to extend toward the side on which the second passage extends.

  Furthermore, in the embodiment shown in FIG. 6 described above, the downstream side wall surface 35 is provided with a concave wall surface that is recessed on the side Q (the direction in which the second passage extends) on which the second passage 32 extends. Furthermore, as shown in FIG. 10, a through hole 35 a penetrating the concave wall surface may be formed in the concave wall surface, and the through hole 35 a communicates with the first passage 31 and the main passage 124. Is more preferable. As a result, by providing the through hole 35a, the fouling substance captured on the concave wall surface acts as an outlet for discharging the first passage 31 to the outside through the through hole 35a. Thereby, it can suppress that the shape of the 1st channel | path 31 changes with time by adhesion of a pollutant.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various designs can be made without departing from the spirit of the present invention described in the claims. It can be changed. For example, the above-described embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to one having all the configurations described. Further, a part of the configuration of an embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of an embodiment. Furthermore, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

DESCRIPTION OF SYMBOLS 30 ... Thermal type flow meter 31 ... 1st channel | path 31A ... Upstream side channel | path 32 ... 2nd channel | path 34 ... Sub intake 34a ... Upstream side opening edge part 34b ... Downstream side opening edge part 34c ... Downstream end part 35 ... Downstream side wall surface 35a ... through hole 36 ... convex wall surface 302 ... housing 303 ... front cover 304 ... back cover 350 ... main intake port 355 ... discharge port 602 ... flow rate detector

Claims (5)

A sub-passage that takes in a part of the gas to be measured flowing through the main passage; and a flow rate detection unit that detects a flow rate of the gas to be measured that flows through the sub-passage, and the main flow is measured based on a measurement value measured by the flow rate detection unit. In a thermal flow meter that detects the flow rate of the gas to be measured flowing through the passage,
The sub-passage is a first passage formed from a main intake port for taking in the gas to be measured flowing through the main passage to an exhaust port for discharging a part of the taken in gas to be measured;
A second passage formed from the sub-inlet for taking in the gas to be measured flowing into the first passage toward the flow rate detection unit,
Of the opening edge that forms the sub intake port, the downstream opening edge located on the downstream side of the first passage is more than the upstream opening edge located on the upstream side of the first passage. , Of the wall surfaces forming the first passage, are formed apart from the wall surface facing the sub intake port, or the downstream opening edge and the upstream opening edge are At the same distance as the wall facing the sub-inlet,
Of the virtual plane perpendicular to the flow direction of the gas to be measured flowing through the main passage, the gas to be measured passing through the first passage is in a virtual plane that passes through the downstream end of the downstream opening edge. along the flow direction of the measurement gas having the largest flow rate, or flow Re than said plane along the direction toward the side where the from the secondary inlet the second passage extends, said downstream opening edge from extends downstream side wall of the first passage downstream of the first passage in the vicinity of the downstream-side opening edge portion,
The downstream side wall surface has a concave wall surface recessed on the side on which the second passage extends,
The thermal flow meter according to claim 1, wherein a wall surface in the vicinity of the discharge port downstream of the concave wall surface has a convex wall surface protruding to the side opposite to the side on which the second passage extends . The thermal flow meter according to claim 1 , wherein a through-hole penetrating the concave wall surface is formed in the concave wall surface. Said center of said main inlet of the first passage, the thermal type according to claim 1, characterized in that it is offset with respect to the center line along the direction in which the measurement gas in the main passage flows Flowmeter. The upstream path from the main inlet to the secondary inlet of the first passage, away from the center line in accordance with the process proceeds to the sub-inlet from said main inlet, the center The thermal flow meter according to claim 3, wherein the thermal flow meter is inclined with respect to the line. Upstream passage from the main inlet of the first passage to said auxiliary inlet, relative to the flow direction of the measurement gas flowing through said main passage, and inclined away from the flow detector The thermal flow meter according to claim 1, wherein

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