Abstract:
The present invention provides a thermal flowmeter having good measurement accuracy by reducing deviation in the flow velocity distribution of a gas under measurement flowing through an auxiliary passage. An auxiliary passage  330  for taking in a portion of a gas under measurement IA flowing through a main passage  124  has a curved passage  32   a  that bends toward a flowrate measurement element  602 . The curved passage  32   a  has a resistance portion  50  formed therein that applies resistance to the flow of the gas under measurement IA flowing through the outer peripheral side CO of the curved passage  32   a  so that the pressure loss of the gas under measurement IA flowing through the outer peripheral side CO is high compared to that of the gas flowing through the inner peripheral side CI of the curved passage  32   a.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a thermal flowmeter. 
       BACKGROUND ART 
       [0002]    A thermal flowmeter is used to measure a flow amount of a gas and includes a flow measurement element which measures a flow amount. Here, the flow amount of the gas is measured in such a manner that heat is transmitted between the flow measurement element and the gas which is a measurement object. The flow amount which is measured by the thermal flowmeter is widely used as an important control parameter of various devices. The thermal flowmeter has a feature that a gas flow amount, for example, a mass flow amount can be measured with relatively high accuracy compared to other flowmeters. 
         [0003]    However, there has been a desire to further improve measurement accuracy of a gas flow amount. For example, in a vehicle equipped with an internal combustion engine, there is an extremely high demand of saving fuel or purifying an exhaust gas. In order to handle these demands, an intake air amount which is an important parameter of the internal combustion engine needs to be measured with high accuracy. 
         [0004]    A thermal flowmeter which measures an amount of intake air led to an internal combustion engine includes a sub-passage which takes a part of the intake air amount and a flow measurement element which is disposed in the sub-passage. Here, a state of a measurement object gas flowing in the sub-passage is measured by the transmission of heat between the flow measurement element and the measurement object gas and an electric signal indicating an amount of the intake air led to the internal combustion engine is output. 
         [0005]    For example, as a technology of such a thermal flowmeter, PTL 1 discloses a “flow amount measurement device in which a plate-shaped board is disposed to form a fluid passage at each of a sensor element mounting face in the plate-shaped board and a rear face opposite to the sensor element mounting face and a curve passage portion is formed at an upstream side of the plate-shaped board of a sub-passage so that a direction changes.” In this Patent Literature, a “wall face in the vicinity of the curve passage portion is provided with an inclined portion which is inclined so that an end located near a side wall face of the curve passage portion facing the sensor element mounting face is located at an inner loop of the curve passage portion in a direction following the side wall face.” 
       CITATION LIST 
     Patent Literature 
       [0006]    PTL 1: JP 2011-75359 A 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0007]    In the thermal flowmeter of PTL 1, the inclined portion is provided at the wall face (an outer peripheral wall face) in the vicinity of the curve passage portion (a curved passage) to suppress an intrusion of dust toward the sensor element. However, if the inclined portion is simply provided at the wall face (the outer peripheral wall face) in the outer periphery while any resistance is not given to a flowing measurement object gas, it is found that a separation flow (a separation vortex) is generated at the wall face (an inner peripheral wall face) of the inner loop from a fluid analysis conducted by the inventors. As a result, a flow is biased toward the sensor element because a flow rate distribution becomes fast at the vicinity side (the outer peripheral side) compared to the inner loop side (the inner peripheral side). This bias changes when an excessive flow is generated during a pulsation and thus the flow rate on the flow measurement element becomes different from that of a non-transient period. As a result, a measurement error occurs during a pulsation. 
         [0008]    The invention is made in view of such circumstances and an object of the invention is to provide a thermal flowmeter capable of improving measurement accuracy by reducing a bias of a flow rate distribution of a measurement object gas flowing in a sub-passage. 
       Solution to Problem 
       [0009]    In order to attain the above-described object, according to the invention, there is provided a thermal flowmeter which includes a sub-passage taking a part of a measurement object gas flowing in a main passage and a flow measurement element measuring a flow amount of the measurement object gas flowing in the sub-passage and measures a flow amount of the measurement object gas flowing in the main passage on the basis of a measurement value obtained by the flow measurement element, in which the sub-passage includes a curved passage which is bent toward the flow measurement element so that the measurement object gas taken from the main passage flows to the flow measurement element, and in which the curved passage is provided with a resistance portion which applies a resistance to a flow of the measurement object gas flowing at an outer peripheral side of the curved passage so that pressure loss of the measurement object gas flowing at the outer peripheral side of the curved passage becomes higher than that of an inner peripheral side of the curved passage. 
       Advantageous Effects of Invention 
       [0010]    According to the invention, it is possible to improve measurement accuracy of a measurement object gas by reducing a bias of a flow rate distribution of the measurement object gas flowing in a sub-passage. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a system diagram illustrating an embodiment in which a thermal flowmeter according to the invention is used in an internal combustion engine control system. 
           [0012]      FIG. 2A  is a front view illustrating an appearance of the thermal flowmeter according to a first embodiment of the invention. 
           [0013]      FIG. 2B  is a left side view illustrating an appearance of the thermal flowmeter according to the first embodiment of the invention. 
           [0014]      FIG. 2C  is a rear view illustrating an appearance of the thermal flowmeter according to the first embodiment of the invention. 
           [0015]      FIG. 2D  is a right side view illustrating an appearance of the thermal flowmeter according to the first embodiment of the invention. 
           [0016]      FIG. 3A  is a front view illustrating a state of a housing in which a front cover is removed from the thermal flowmeter according to the first embodiment of the invention. 
           [0017]      FIG. 3B  is a rear view illustrating a state of a housing in which a rear cover is removed from the thermal flowmeter according to the first embodiment of the invention. 
           [0018]      FIG. 4  is a cross-sectional view taken along a line A-A of  FIG. 2A . 
           [0019]      FIG. 5  is a main enlarged view of the sub-passage illustrated in  FIG. 3B . 
           [0020]      FIG. 6  is a cross-sectional view taken along a line B-B of  FIG. 5 . 
           [0021]      FIG. 7( a )  is a diagram illustrating a relation between pressure loss and a flow rate distribution inside a curved passage of a sub-passage of a thermal flowmeter of the related art and  FIG. 7( b )  is a diagram illustrating a relation between pressure loss and a flow rate distribution inside a curved passage of a sub-passage of the thermal flowmeter illustrated in  FIG. 6 . 
           [0022]      FIG. 8  is a modified example of the curved passage of the sub-passage of the thermal flowmeter illustrated in  FIG. 6 . 
           [0023]      FIG. 9  is another modified example of the curved passage of the sub-passage of the thermal flowmeter illustrated in  FIG. 6 . 
           [0024]      FIG. 10  is still another modified example of the curved passage of the sub-passage of the thermal flowmeter illustrated in  FIG. 6 . 
           [0025]      FIG. 11  is a main enlarged view of a sub-passage according to a second embodiment corresponding to a main enlarged view of the sub-passage illustrated in  FIG. 3B . 
           [0026]      FIG. 12  is a cross-sectional view taken along a line C-C of  FIG. 11 . 
           [0027]      FIG. 13( a )  is a diagram illustrating a relation between pressure loss and a flow rate distribution inside a curved passage of a sub-passage of a thermal flowmeter of the related art and  FIG. 13( b )  is a diagram illustrating a relation between pressure loss and a flow rate distribution inside a curved passage of the sub-passage of the thermal flowmeter illustrated in  FIG. 11 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]    Hereinafter, an embodiment of the invention will be described with reference to the drawings. 
         [0029]    1. Internal Combustion Engine Control System and Thermal Flowmeter Disposed Therein 
         [0030]      FIG. 1  is a system diagram illustrating an embodiment in which a thermal flowmeter according to the embodiment is used in an electronic fuel injection type internal combustion engine control system. As illustrated in  FIG. 1 , intake air which is a measurement object gas IA is suctioned from an air cleaner  122  and is led to a combustion chamber of an engine cylinder  112  through, for example, an intake body including an intake pipe  71  provided with a main passage  124 , a throttle body  126 , and an intake manifold  128  on the basis of an operation of an internal combustion engine  110  including the engine cylinder  112  and an engine piston  114 . 
         [0031]    A flow amount of the measurement object gas IA which is the intake air led to the combustion chamber is measured by a thermal flowmeter  30  according to the embodiment. Then, on the basis of the measured flow amount, fuel is supplied from a fuel injection valve  152  and is led to the combustion chamber along with the measurement object gas IA which is the intake air in a mixed gas state. Additionally, in the embodiment, the fuel injection valve  152  is provided at an intake port of the internal combustion engine. Here, the fuel injected to the intake port forms a mixed gas along with the measurement object gas IA which is the intake air and the mixed gas is led to the combustion chamber through the intake valve  116  so that the mixed gas is burned and mechanical energy is generated. 
         [0032]    The thermal flowmeter  30  can be similarly used not only in a system that injects a fuel to the intake port of the internal combustion engine illustrated in  FIG. 1  but also in a system in which a fuel is directly injected to each combustion chamber. Both systems have substantially the same basic concept in a control parameter measurement method including a method of using the thermal flowmeter  30  and an internal combustion engine control method including a fuel supply amount or an ignition timing. Then, as a representative example of both systems, a system that injects a fuel to the intake port is illustrated in  FIG. 1 . 
         [0033]    The fuel and the air which are led to the combustion chamber are kept in a state in which the fuel and the air are mixed with each other and are burned to explode by a spark ignition of an ignition plug  154  so that mechanical energy is generated. The burned gas is led from an exhaust valve  118  to an exhaust pipe and is discharged as exhaust air EA from the exhaust pipe to the outside of a vehicle. A flow amount of the measurement object gas IA which is the intake air led to the combustion chamber is controlled by a throttle valve  132  an opening degree of which changes on the basis of an operation of an accelerator pedal. A fuel supply amount is controlled on the basis of the flow amount of the intake air led to the combustion chamber. Then, a driver can control the mechanical energy generated by the internal combustion engine by controlling the opening degree of the throttle valve  132  to control the flow amount of the intake air led to the combustion chamber. 
         [0034]    A flow amount, a humidity, and a temperature of the measurement object gas IA which is the intake air taken from the air cleaner  122  and flowing through the main passage  124  are measured by the thermal flowmeter  30  and electric signals representing the flow amount, the humidity, and the temperature of the intake air are input from the thermal flowmeter  30  to a control device  200 . Further, an output of a throttle angle sensor  144  that measures the opening degree of the throttle valve  132  is input to the control device  200 . Furthermore, an output of a rotation angle sensor  146  is input to the control device  200  in order to measure a position or a state of the engine piston  114 , the intake valve  116 , or the exhaust valve  118  of the internal combustion engine and a rotation speed of the internal combustion engine. Moreover, an output of an oxygen sensor  148  is input to the control device  200  in order to measure a mixture ratio between a fuel amount and an air amount from the state of the exhaust air EA. 
         [0035]    The control device  200  calculates a fuel injection amount or an ignition timing on the basis of the flow amount, the humidity, and the temperature of the intake air output from the thermal flowmeter  30  and the rotation speed of the internal combustion engine output from the rotation angle sensor  146 . On the basis of these calculation results, the fuel supply amount of the fuel injection valve  152  and the ignition timing of the ignition plug  154  are controlled. In fact, the fuel supply amount or the ignition timing is controlled on the basis of an intake air temperature measured by the thermal flowmeter  30 , a change in throttle angle, a change in engine rotation speed, and an air fuel ratio measured by the oxygen sensor  148 . The control device  200  further controls an amount of air bypassing the throttle valve  132  through an idle air control valve  156  in an idle operation state of the internal combustion engine so that the rotation speed of the internal combustion engine in the idle operation state is controlled. 
         [0036]    The fuel supply amount or the ignition timing which is a key control amount of the internal combustion engine is calculated by using the output of the thermal flowmeter  30  as main parameters. Thus, it is important to improve the measurement accuracy of the thermal flowmeter  30 , to suppress a change in measurement accuracy with time, and to improve the reliability thereof in order to improve the vehicle control accuracy or ensure the reliability thereof. Particularly, in recent years, there has been an extremely high demand of saving fuel and purifying an exhaust gas in the vehicle. In order to handle these demands, it is important to improve the measurement accuracy of the flow amount of the measurement object gas IA which is the intake air measured by the thermal flowmeter  30 . 
         [0037]    2. Appearance of Thermal Flowmeter and Attachment State Thereof 
         [0038]      FIG. 2  illustrates an appearance of the thermal flowmeter  30 .  FIG. 2A  is a front view of the thermal flowmeter  30 ,  FIG. 2B  is a left side view,  FIG. 2C  is a rear view, and  FIG. 2D  is a right side view. 
         [0039]    The thermal flowmeter  30  includes a housing  302 , a front cover  303 , and a rear cover  304 . The housing  302  includes a flange  312  which fixes the thermal flowmeter  30  to an intake body forming a main passage, an external connection portion (a connector portion)  305  which includes an external terminal electrically connected to an external device, and a measurement unit  310  which measures a flow amount. A sub-passage groove which forms a sub-passage is provided inside the measurement unit  310 . 
         [0040]    When the front cover  303  and the rear cover  304  of the thermal flowmeter  30  are covered, a casing with a sub-passage is formed. A circuit package  400  which includes a flow measurement element  602  measuring a flow amount of the measurement object gas IA flowing in the main passage or a temperature detection unit  452  measuring a temperature of the measurement object gas IA flowing in the main passage  124  is provided inside the measurement unit  310  (see  FIGS. 3A and 3B ). 
         [0041]    In the thermal flowmeter  30 , the measurement unit  310  is supported inside the main passage in a cantilevered manner while the flange  312  is fixed to the intake body (the intake pipe)  71 .  FIGS. 2A and 3B  illustrate the intake pipe  71  by a virtual line in order to clarify a positional relation between the thermal flowmeter  30  and the intake pipe  71 . 
         [0042]    The measurement unit  310  of the thermal flowmeter  30  is formed in a shape which extends from the flange  312  toward the center of the main passage  124  in the radial direction and a front end thereof is provided with a main intake opening  350  (see  FIG. 2C ) which takes a part of the measurement object gas IA which is the intake air into the sub-passage and a discharge opening  355  (see  FIG. 2D ) which returns the measurement object gas IA from the sub-passage to the main passage  124 . 
         [0043]    Since the main intake opening  350  of the thermal flowmeter  30  is provided at the front end side of the measurement unit  310  which extends from the flange  312  toward the center of the main passage in the radial direction, a gas which is separated from an inner wall face of the main passage can be taken into the sub-passage. Accordingly, since it is possible to avoid an influence of a temperature of the inner wall face of the main passage, it is possible to suppress deterioration in measurement accuracy of the flow amount or the temperature of the gas. Further, as will be described later, in the embodiment, the center of the main intake opening  350  is offset with respect to a center line CL following a direction D in which the measurement object gas IA of the main passage  124  flows. 
         [0044]    Further, since a fluid resistance is large in the vicinity of the inner wall face of the main passage  124 , a flow rate decreases compared to an average flow rate of the main passage. In the thermal flowmeter  30  of the embodiment, since the main intake opening  350  is provided at the front end of the thin and elongated measurement unit  310  extending from the flange  312  toward the center of the main passage, it is possible to take a gas having a high flow rate at the center portion of the main passage into the sub-passage (the measurement passage). Further, since the discharge opening  355  of the sub-passage is also provided at the front end of the measurement unit  310 , it is possible to return a gas flowing through the sub-passage to the vicinity of the center portion of the main passage  124  in which the flow rate is high. 
         [0045]    The measurement unit  310  is formed in a shape which extends along an axis directed toward the center from the outer wall of the main passage  124 , but a width thereof is formed in a narrow shape as illustrated in  FIGS. 2B and 2D . That is, the measurement unit  310  of the thermal flowmeter  30  is formed in a shape in which a side face has a small width and a front face is substantially rectangular. Accordingly, the thermal flowmeter  30  can include a sub-passage which has a small fluid resistance with respect to the measurement object gas IA and has a sufficient length. 
         [0046]    The temperature detection unit  452  which measures the temperature of the measurement object gas IA is provided at a position in which an upstream outer wall inside the measurement unit  310  is recessed toward the downstream side at the center portion of the measurement unit  310  so as to have a shape which protrudes toward the upstream side from the upstream outer wall. 
         [0047]    Each of the front cover  303  and the rear cover  304  is formed in a thin plate shape with a wide cooling face. For this reason, since an air resistance in the thermal flowmeter  30  is decreased, there is an effect in which a cooling operation is easily performed by the measurement object gas flowing in the main passage  124 . 
         [0048]    An external terminal and a correction terminal, which are not illustrated, are provided inside the external connection portion  305 . The external terminal includes a terminal which outputs a temperature and a flow amount corresponding to a measurement result and a power terminal which supplies DC power. The correction terminal is a terminal which is used to store a correction value for the thermal flowmeter  30  in a memory inside the thermal flowmeter  30 . 
         [0049]    3. Sub-Passage and Circuit Package Inside Housing 
         [0050]    Next, configurations of the sub-passage and the circuit package provided inside the housing  302  will be described with reference of  FIGS. 3A and 3B .  FIGS. 3A and 3B  illustrate a state of the housing  302  in which the front cover  303  or the rear cover  304  is removed from the thermal flowmeter  30 .  FIG. 3A  is a front view illustrating a state of a housing  302  in which a front cover  303  is removed from a thermal flowmeter according to a first embodiment of the invention and  FIG. 3B  is a rear view illustrating a state of a housing  302  in which a rear cover  304  is removed from the thermal flowmeter  30  according to the first embodiment of the invention. 
         [0051]    A sub-passage groove for molding a sub-passage at the front end side of the measurement unit  310  is provided in the housing  302 . A sub-passage  330  is a passage which is formed inside the thermal flowmeter  30  to take a part of the measurement object gas flowing in the main passage  124 . In the embodiment, sub-passage grooves  332  and  334  are provided at both front and rear faces of the housing  302 . When the front and rear faces of the housing  302  are covered by the front cover  303  and the rear cover  304 , the continuous sub-passage  330  is formed at both faces of the housing  302 . With such a structure, both a front sub-passage groove  332  and a rear sub-passage groove  334  are formed in a part of the housing  302  and a penetration hole  382  penetrating the housing  302  is formed to connect both grooves by the use of dies which are formed at both faces of the housing  302  in a step (a resin molding step) of molding the housing  302  by a second resin (a thermoplastic resin). Accordingly, the flow measurement element  602  of the circuit package  400  can be disposed at the penetration hole  382 . 
         [0052]    As illustrated in  FIG. 3B , a part of the measurement object gas IA which flows in the main passage is taken from the main intake opening  350  into the rear sub-passage groove  334  and flows in the rear sub-passage groove  334 . When the rear sub-passage groove  334  is covered by the rear cover  304 , a part of upstream sides of a first passage  31  and a second passage  32  in the sub-passage  330  are formed in the thermal flowmeter  30 . Here, the first passage  31  corresponds to a “discharge passage” of the invention and a passage (a sensor upstream passage  32   a  to be described later) which is located at the upstream side of the flow measurement element  602  in the second passage  32  corresponds to a “curved passage.” 
         [0053]    The first passage (discharge passage)  31  is a pollutant material discharge passage which is formed from the main intake opening  350  taking the measurement object gas IA flowing in the main passage  124  to the discharge opening  355  discharging a part of the taken measurement object gas IA. The second passage  32  is a flow amount measurement passage which is formed from the sub-intake opening  34  taking the measurement object gas IA flowing in the first passage  31  toward the flow measurement element  602 . The main intake opening  350  is opened to an upstream face of the main passage  124 , the discharge opening  355  is opened to a downstream face of the main passage  124 , and an opening area of the discharge opening  355  is smaller than an opening area of the main intake opening  350 . Accordingly, the measurement object gas IA can also easily flow from the main intake opening  350  to the second passage  32 . 
         [0054]    In the rear face sub-passage groove  334 , a passage groove of the second passage  32  (the passage to the flow measurement element  602 ) is formed in a shape which becomes deeper as it goes in a flow direction. Then, the measurement object gas IA gradually moves toward the front cover  303  as the measurement gas flows along the groove. The rear sub-passage groove  334  is provided with a steep inclined portion  347  which becomes steeply deeper at an upstream portion  342  of the circuit package  400 . A part of air having a small mass moves along the steep inclined portion  347  and flows toward a measurement flow passage face  430  illustrated in  FIG. 4  at the upstream portion  342  in the penetration hole  382  of the circuit package  400 . Meanwhile, since a path of a foreign material having a large mass cannot be abruptly changed due to a centrifugal force, the foreign material cannot flow along the steep inclined portion  347 , but flows toward a measurement flow passage rear face  431  illustrated in  FIG. 4 . Subsequently, the air flows in the front sub-passage groove  332  illustrated in  FIG. 3A  through a downstream portion  341  in the penetration hole  382 . 
         [0055]    As described above, a portion including the measurement flow passage face  430  of the circuit package  400  is disposed inside a cavity of the penetration hole  382  and in the penetration hole  382 , the rear sub-passage groove  334  and the front sub-passage groove  332  are connected to both left and right sides of the circuit package  400  with the measurement flow passage face  430 . 
         [0056]    As illustrated in  FIG. 3A , the air which is the measurement object gas IA flows from the upstream portion  342  along the measurement flow passage face  430  in the penetration hole  382 . At this time, the flow amount of the measurement object gas IA is measured by the transmission of heat of the flow measurement element  602  measuring the flow amount through a measurement front face (a heat transmission face)  437  which is provided at the flow measurement element  602  and is exposed into the second passage  32 . Additionally, the flow amount measurement principle may be a general measurement principle of the thermal flowmeter. Here, a measurement configuration is not particularly limited as long as the flow amount of the measurement object gas flowing in the main passage can be measured on the basis of the measurement value measured by the flow measurement element  602  of the circuit package  400  as in the embodiment. 
         [0057]    Both the measurement object gas IA having passed through the measurement flow passage face  430  and the air flowing from the downstream portion  341  of the circuit package  400  to the front sub-passage groove  332  flow along the front sub-passage groove  332  and are discharged from an exit groove  353  of the second passage  32  to the main passage  124  through the discharge opening facing the downstream face of the main passage  124 . 
         [0058]    In the embodiment, the second passage  32  which is formed by the rear sub-passage groove  334  is directed toward a flange direction from a front end of the housing  302  while depicting a curve and the measurement object gas IA flowing in the sub-passage  330  at a position closest to the flange becomes a flow opposite to the flow of the main passage  124 . At the penetration hole  382  which becomes a part of the flow in the opposite direction, a sensor upstream passage (a curved passage)  32   a  which is provided at the rear face side of the second passage  32  provided at one side of the housing  302  is connected to a sensor downstream passage  32   b  which is provided at the front face side of the second passage  32  provided at the other side thereof. The sensor upstream passage (the curved passage)  32   a  is a passage which is bent in one direction toward the flow measurement element  602  so that the measurement object gas IA taken from the main passage  124  flows to the flow measurement element  602  and includes a part of the penetration hole  382  at the upstream side of the flow measurement element  602 . 
         [0059]    That is, in the embodiment, a front end side of the circuit package  400  is disposed inside a cavity of the penetration hole  382 . A space of the upstream portion  342  located at the upstream side of the circuit package  400  and a space of the downstream portion  341  located at the downstream side of the circuit package  400  are included in the penetration hole  382  and as described above, the penetration hole  382  is drilled to penetrate the front and rear face sides of the housing  302 . Accordingly, as described above, the sensor upstream passage  32   a  which is formed by the front sub-passage groove  332  at the front face side of the housing  302  communicates with the sensor downstream passage  32   b  which is formed by the rear sub-passage groove  334  at the rear face side at the penetration hole  382 . The sensor downstream passage  32   b  is a passage which is bent in one direction toward the discharge opening  355  so that the measurement object gas IA having passed through the flow measurement element  602  flows to the discharge opening  355  and includes a part of a penetration hole  382  located at the downstream side of the flow measurement element  602 . 
         [0060]    Further, as illustrated in  FIG. 4 , a space near the measurement flow passage face  430  and a space near the measurement flow passage rear face  431  are defined by the circuit package  400  inserted into the housing  302 , but is not defined by the housing  302 . One space which is formed by the space of the upstream portion  342 , the space of the downstream portion  341 , the space near the measurement flow passage face  430 , and the space near the measurement flow passage rear face  431  is continuously formed at the front and rear faces of the housing  302  and the circuit package  400  which is inserted into the housing  302  protrudes to one space in a cantilevered manner. With such a configuration, it is possible to mold the sub-passage groove at both faces of the housing  302  by one resin molding step and to perform a molding operation in accordance with a structure of connecting the sub-passage grooves at both faces. 
         [0061]    The circuit package  400  is fixed to be buried in the housing  302  by fixing portions  372 ,  373 , and  376  of the housing  302  molded by the second resin. Such a fixing structure can be mounted on the thermal flowmeter  30  in such a manner that the housing  302  is molded by the second resin and the circuit package  400  is insert-molded in the housing  302 . Additionally, in the embodiment, the first resin is a resin for molding the circuit package  400  and the second resin is a resin for molding the housing  302 . 
         [0062]    A front sub-passage inner peripheral wall (a second passage wall)  393  and a front sub-passage outer peripheral wall (a second passage wall)  394  are provided at both sides of the front sub-passage groove  332  and front ends of the front sub-passage inner peripheral wall  393  and the front sub-passage outer peripheral wall  394  in the height direction adhere to an inner face of the front cover  303  so that a part of the sensor downstream passage  32   b  of the housing  302  is formed. 
         [0063]    The measurement object gas IA which is taken from the main intake opening  350  and flows in the first passage  31  formed by the rear sub-passage groove  334  flows from the right side to the left side in  FIG. 3B . Here, a part of the taken measurement object gas IA flows to be distributed to the sub-intake opening  34  of the second passage  32  formed to be branched from the first passage  31 . The flowing measurement object gas IA flows toward a flow passage  386  which is formed by a front face of the measurement flow passage face  430  of the circuit package  400  and a protrusion  356  provided in the front cover  303  through the upstream portion  342  of the penetration hole  382  (see  FIG. 4 ). 
         [0064]    The other measurement object gas IA flows toward a flow passage  387  which is formed by the measurement flow passage rear face  431  and the rear cover  304 . Subsequently, the measurement object gas IA having passed through the flow passage  387  moves to the front sub-passage groove  332  through the downstream portion  341  of the penetration hole  382  and is merged with the measurement object gas IA flowing in the flow passage  386 . The merged measurement object gas IA flows in the front sub-passage groove  332  and is discharged from the discharge opening  355  formed in the housing to the main passage  124  through an exit  352 . 
         [0065]    The sub-passage groove is formed so that the measurement object gas IA led from the rear sub-passage groove  334  to the flow passage  386  through the upstream portion  342  of the penetration hole  382  is bent more than the flow passage led to the flow passage  387 . Accordingly, a material having a large mass such as a garbage included in the measurement object gas IA is accumulated in the flow passage  387  which is bent to a small degree. 
         [0066]    The protrusion  356  in the flow passage  386  forms a diaphragm so that the measurement object gas IA becomes a laminar flow having a small vortex. Further, the protrusion  356  increases the flow rate of the measurement object gas IA. Accordingly, the measurement accuracy is improved. The protrusion  356  is formed at the front cover  303  which is a cover facing the measurement face exposure portion  436  of the flow measurement element  602  provided at the measurement flow passage face  430 . 
         [0067]    Here, as illustrated in  FIG. 3B , the rear sub-passage groove  334  is formed by a first passage wall  395 , a rear sub-passage inner peripheral wall (a second passage wall)  392 , and a rear sub-passage outer peripheral wall (a second passage wall)  391  which are provided to face one another. When front ends of the rear sub-passage inner peripheral wall  392  and the rear sub-passage outer peripheral wall  391  in the height direction adhere to the inner face of the rear cover  304 , a part of the sensor upstream passage (the curved passage)  32   a  of the second passage  32  and the first passage  31  of the housing  302  are formed. 
         [0068]    As illustrated in  FIGS. 3A and 3B , a cavity portion  336  is formed between a portion provided with the sub-passage groove and the flange  312  in the housing  302 . A terminal connection portion  320  which connects a lead terminal  412  of the circuit package  400  and the connection terminal  306  of the external connection portion  305  is provided inside the cavity portion  336 . A lead terminal  412  and a connection terminal  306  (an inner end  361 ) are electrically connected to each other by spot-welding or laser-welding. 
         [0069]    4. Resistance Portion of Sub-Passage  220   
         [0070]      FIG. 5  is a main enlarged view of the sub-passage illustrated in  FIG. 3B . Further,  FIG. 6  is a cross-sectional view taken along a line B-B of  FIG. 5 ,  FIG. 7( a )  is a diagram illustrating a relation between pressure loss and a flow rate distribution inside a curved passage of a sub-passage of a thermal flowmeter of the related art, and  FIG. 7( b )  is a diagram illustrating a relation between pressure loss and a flow rate distribution inside a curved passage of the sub-passage of the thermal flowmeter illustrated in  FIG. 6 . 
         [0071]    As illustrated in  FIGS. 5 and 6 , as described above, the sub-passage  330  of the thermal flowmeter  30  according to the embodiment includes the curved passage (the sensor upstream passage)  32   a  which is bent toward the flow measurement element  602  so that the measurement object gas IA taken from the main passage  124  flows to the flow measurement element  602 . 
         [0072]    The curved passage  32   a  is provided with a resistance portion  50  which applies a resistance to the flow of the measurement object gas IA flowing at the outer peripheral side CO so that the pressure loss of the measurement object gas IA flowing at the outer peripheral side CO of the curved passage  32   a  becomes higher than that of the inner peripheral side CI of the curved passage  32   a  (which will be described in detail by referring to  FIG. 7( b ) ). The resistance portion  50  is formed along the outer peripheral wall face  42  of the outer peripheral side CO of the curved passage  32   a  illustrated in  FIG. 5 . As illustrated in  FIG. 5 , an upstream end  50   a  of the resistance portion  50  is formed at the outer peripheral wall face  42  near the flow measurement element  602  in relation to the sub-intake opening  34  and a downstream end  50   b  of the resistance portion  50  is formed to the upstream outer peripheral wall face  42  of the flow measurement element  602  forming the penetration hole  382  (see a bold line of  FIG. 5 ). 
         [0073]    As illustrated in  FIG. 6 , in the embodiment, the resistance portion  50  corresponds to a pair of opposite inclined faces  52   a  and  52   b  which are formed at the outer peripheral wall face  42  of the outer peripheral side CO of the curved passage  32   a  in the flow direction of the measurement object gas IA so that a resistance is applied to the flow of the measurement object gas IA flowing at the outer peripheral side CO. Each of the inclined faces  52   a  and  52   b  is inclined with respect to a virtual plane F following the measurement front face  437  of the flow measurement element  602 . 
         [0074]    Here, one inclined face  52   a  is formed at the front face of the protrusion portion  57  provided at the inner face of the rear cover  304  and the other inclined face  52   b  is formed at a wall face of the rear sub-passage outer peripheral wall  391  (see  FIG. 3B ) of the housing  302  and a wall face forming the penetration hole  382  continuous thereto. A gap  58  is formed between one inclined face  52   a  and the other inclined face  52   b . Specifically, the gap  58  is formed in the same direction as the extension direction of the virtual plane F. Since the gap  58  is provided, it is possible to further increase the pressure loss of the measurement object gas IA flowing at the outer peripheral side CO of the curved passage  32   a  compared to the inner peripheral side CI of the curved passage  32   a.    
         [0075]    Further, in the embodiment, the pair of inclined faces  52   a  and  52   b  is formed so that the vicinity of the bottom portion  55   a  of a groove portion  55  formed by the pair of opposite inclined faces  52   a  and  52   b  is located on the virtual plane F. 
         [0076]    Incidentally, in the related art, since an outer peripheral wall face  91  which is orthogonal to the virtual plane F or a dust prevention inclined face  92  which is inclined in one direction is provided as indicated by a dashed line of  FIG. 6 , a separation flow (a separation vortex) is generated in the vicinity of the inner peripheral wall face  43 . Accordingly, as illustrated in  FIG. 7( a ) , the pressure loss of the measurement object gas IA flowing at the inner peripheral side CI increases compared to the outer peripheral side CO. As a result, the measurement object gas IA flows to the outer peripheral side CO compared to the inner peripheral side CI and thus the flow rate of the measurement object gas IA flowing at the outer peripheral side CO becomes fast. Accordingly, the flow of the measurement object gas IA directed toward the flow measurement element  602  is biased, a bias of a flow in the event of an excessive flow during a pulsation changes, and the flow rate of the measurement object gas IA on the flow measurement element  602  becomes different compared to the non-transient period. As a result, a measurement error occurs in the flow measurement element  602  in the event of a pulsation. 
         [0077]    Here, in the embodiment, since the resistance portion  50  is provided, an effective cross-sectional area of the measurement object gas IA flowing to the outer peripheral side CO is decreased so that a resistance is applied to the flow of the measurement object gas IA flowing at the outer peripheral side CO. Accordingly, as illustrated in  FIG. 7( b ) , the pressure loss of the measurement object gas IA flowing at the outer peripheral side CO of the curved passage  32   a  can be increased compared to the inner peripheral side CO. Accordingly, since a separation flow (a separation vortex) of the measurement object gas IA generated in the vicinity of the inner peripheral wall face  43  is reduced, it is possible to suppress the flow of the measurement object gas IA flowing in the curved passage  32   a  from being biased to the outer peripheral side CO. 
         [0078]    Further, in the embodiment, since the pair of opposite inclined faces  52   a  and  52   b  is provided, it is possible to increase the pressure loss of the measurement object gas IA flowing in the vicinity of the groove portion  55  formed by the inclined faces  52   a  and  52   b . Accordingly, it is also possible to reduce a bias of the flow of the measurement object gas IA in a direction perpendicular to the virtual plane F, that is, the thickness direction of the flowmeter. Particularly, since the pair of inclined faces  52   a  and  52   b  is formed so that the vicinity of the bottom portion  55   a  of the groove portion  55  is located on the virtual plane F, it is possible to uniformly adjust the flow rate distribution of the measurement object gas IA flowing in the vicinity of the flow measurement element  602 . 
         [0079]      FIGS. 8 to 10  are modified examples of the curved passage of the sub-passage of the thermal flowmeter illustrated in  FIG. 6 . For example, as illustrated in  FIG. 8 , even in the modified example, as in the above-described embodiment, the pair of opposite inclined faces  52   a  and  52   b  is formed at the outer peripheral wall face  42  of the outer peripheral side CO of the curved passage  32   a  following the flow direction of the measurement object gas IA so that a resistance is applied to the flow of the measurement object gas IA flowing at the outer peripheral side CO and the gap  58  is formed between the inclined faces  52   a  and  52   b . Here, a bottom face  52   c  of the groove portion  55  is formed between the inclined faces  52   a  and  52   b  and the bottom face  52   c  is a face which is orthogonal to the virtual plane F. In the modified example, the pair of opposite inclined faces  52   a  and  52   b  is inclined with respect to the virtual plane F so that the virtual plane F is interposed therebetween. Further, the bottom face  52   c  extends toward the upside of the measurement front face  437  of the flow measurement element  602  (that is, the side of the front cover  303  in relation to the measurement front face  435 ) from the virtual plane F. 
         [0080]    According to the modified example, since the pair of opposite inclined faces  52   a  and  52   b  is inclined with respect to the virtual plane F so that the virtual plane F is interposed therebetween, it is possible to increase the pressure loss of the measurement object gas IA flowing in the groove portion  55 . Particularly, since the bottom face  52   c  of the groove portion  55  is located on the virtual plane F, it is possible to further uniformly adjust the flow rate distribution of the measurement object gas IA flowing in the measurement front face  437  of the flow measurement element  602 . Further, since the bottom face  52   c  of the groove portion  55  extends toward the upside of the measurement front face  437  of the flow measurement element  602 , it is possible to further uniformly adjust the flow rate distribution of the measurement object gas IA flowing in the measurement front face  437  of the flow measurement element  602 . 
         [0081]    In the embodiment, the pair of inclined faces  52   a  and  52   b  corresponds to planar inclined faces (see  FIGS. 6 and 8 ). For example, as illustrated in  FIG. 9 , even when the pair of inclined faces corresponds to inclined faces  52   a ′ and  52   b ′ serving as convex faces, it is possible to improve the pressure loss of the outer peripheral side CO compared to the inner peripheral side CI by applying a resistance to the flow of the measurement object gas IA flowing at the outer peripheral side CO of the curved passage  32   a . Accordingly, it is possible to suppress the flow of the measurement object gas IA flowing in the curved passage  32   a  from being biased to the outer peripheral side CO. 
         [0082]    Further, if a resistance can be applied to the flow of the measurement object gas IA flowing at the outer peripheral side CO so that the pressure loss of the measurement object gas IA flowing at the outer peripheral side CO of the curved passage  32   a  becomes higher than that of the inner peripheral side CI of the curved passage  32   a , for example, as illustrated in  FIG. 10 , a plurality of convex claw portions  56  may be provided at the outer peripheral wall face  42  of the outer peripheral side CO of the curved passage  32   a  along the flow direction of the measurement object gas IA. 
         [0083]      FIG. 11  is a main enlarged view of a sub-passage according to a second embodiment corresponding to a main enlarged view of the sub-passage illustrated in  FIG. 3B .  FIG. 12  is a cross-sectional view taken along a line C-C of  FIG. 11 ,  FIG. 13( a )  is a diagram illustrating a relation between pressure loss and a flow rate distribution inside a curved passage of a sub-passage of a thermal flowmeter of the related art, and  FIG. 13( b )  is a diagram illustrating a relation between pressure loss and a flow rate distribution inside the bend passage of the sub-passage of the thermal flowmeter illustrated in  FIG. 11 . Additionally,  FIG. 13( a )  is a diagram similar to that of  FIG. 5( a )  and is provided for the comparison with  FIG. 13( b ) . 
         [0084]    The thermal flowmeter according to the second embodiment is different from that of the first embodiment as below. In the first embodiment, the resistance portion  50  is provided at the outer peripheral wall face  42  of the curved passage  32   a , but in the second embodiment, the resistance portion  50  is provided inside the curved passage  32   a.    
         [0085]    Specifically, as illustrated in  FIGS. 11 and 12 , a partition wall (a partition plate)  60  which defines the curved passage  32   a  and serves as the resistance portion  50  is formed at the inner peripheral passage  32   c  through which the measurement object gas IA flows to the inner peripheral side CI of the curved passage  32   a  and the outer peripheral passage  32   d  through which the measurement object gas IA flows to the outer peripheral side CO of the curved passage  32   a.    
         [0086]    The partition wall  60  corresponds to a “resistance portion” of the invention. The partition wall  60  is formed along the flow direction of the measurement object gas IA of the curved passage  32   a  and is formed near the outer peripheral wall face  42  of the curved passage  32   a . The upstream end  60   a  of the partition wall  60  is formed near the flow measurement element  602  in relation to the sub-intake opening  34  and the downstream end  60   b  of the partition wall  50  is formed to the upstream outer peripheral wall face  42  of the flow measurement element  602  forming the penetration hole  382 . 
         [0087]    Here, as described in the first embodiment, the sub-intake opening  34  is an intake opening which takes the measurement object gas IA flowing in the discharge passage (the first passage)  31  into the curved passage  32   a  of the sub-passage  330  and the discharge passage  31  is a passage which is formed to the discharge opening  355  discharging a part of the taken measurement object gas IA from the main intake opening  350  taking the measurement object gas IA flowing in the main passage  124 . 
         [0088]    In the embodiment, the partition wall  60  is formed at the housing  302 , but if the curved passage  32   a  can be divided into the inner peripheral passage  32   c  and the outer peripheral passage  32   d , the partition wall  60  may be formed at the rear cover  304 . Further, the partition wall  60  may be formed by a part of the housing  302  and the rear cover  304 . 
         [0089]    Since such a partition wall  60  is provided, the measurement object gas IA which flows into the curved passage  32   a  separately flows into the inner peripheral passage  32   c  and the outer peripheral passage  32   d  at a halfway position. Since the partition wall  60  is formed near the outer peripheral wall face  42  of the curved passage  32   a  and the flow passage length of the outer peripheral passage  32   d  is longer than the flow passage length of the inner peripheral passage  32   c , the measurement object gas IA hardly flows in the outer peripheral passage  32   d  compared to the inner peripheral passage  32   c . That is, since the partition wall  60  is provided, a resistance is applied to the flow of the measurement object gas IA flowing at the outer peripheral side CO. Accordingly, as illustrated in  FIG. 13( b ) , it is possible to increase the pressure loss of the measurement object gas IA which flows at the outer peripheral side CO of the curved passage  32   a . Accordingly, since a separation flow (a separation vortex) of the measurement object gas IA generated in the vicinity of the inner peripheral wall face  43  is reduced, it is possible to suppress the flow of the measurement object gas IA flowing in the curved passage  32   a  from being biased to the outer peripheral side CO as illustrated in  FIG. 13( a ) . 
         [0090]    Particularly, since the upstream end  60   a  of the partition wall  60  is formed near the flow measurement element  602  in relation to the sub-intake opening  340 , the measurement object gas IA is not divided by the partition wall  60  in the sub-intake opening  340 . Accordingly, regarding the measurement object gas IA taken from the sub-intake opening  340 , the pressure loss of the measurement object gas IA flowing at the outer peripheral side CO of the curved passage  32   a  increases. Thus, it is possible to further reduce a separation flow (a separation vortex) of the measurement object gas IA generated in the vicinity of the inner peripheral wall face  43 . 
         [0091]    While the embodiments of the invention have been described, the invention is not limited to the aforementioned embodiments and various modifications in design can be made without departing from the spirit of the invention of claims. For example, the aforementioned embodiments have been described in detail in order to easily describe the invention and all configurations are not essentially necessary in the invention. Further, a part of the configuration of a certain embodiment can be replaced by the configurations of the other embodiments and the configuration of the other embodiment can be added to the configuration of a certain embodiment. Furthermore, a part of the configurations of the embodiments can be added, deleted, and replaced. 
         [0092]    For example, in the first and second embodiments, the first passage (the discharge passage) is provided as a part of the sub-passage, but if the above-described effect can be anticipated, the sub-passage may include only the second passage which is the flow amount measurement passage. 
         [0093]    In the first and second embodiments, the resistance portion is provided at the curved passage which is the sensor upstream passage, but the same configuration may be provided at the sensor downstream passage in consideration of a reverse flow generated during a pulsation. Further, the curved passage which is the sensor upstream passage of the first embodiment may be further provided with the partition wall illustrated in the second embodiment. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           30  thermal flowmeter 
           31  first passage (discharge passage) 
           31 A upstream passage 
           32  second passage 
           32   a  sensor upstream passage (curved passage) 
           32   b  sensor downstream passage 
           32   c  inner peripheral passage 
           32   d  outer peripheral passage 
           34  sub-intake opening 
           50  resistance portion 
           52   a ,  52   b  inclined face 
           52   a ′,  52   b ′ inclined face 
           52   c  bottom face 
           42  outer peripheral wall face 
           43  inner peripheral wall face 
           55  groove portion 
           55   a  bottom portion of groove portion 
           58  gap 
           60  partition wall 
           124  main passage 
           302  housing 
           303  front cover 
           304  rear cover 
           330  sub-passage 
           350  main intake opening 
           355  discharge opening 
           437  measurement front face (heat transmission face) 
           602  flow measurement element 
         CI inner peripheral side 
         CO outer peripheral side 
         IA measurement object gas 
         F virtual plane