JP2000304585A - Flow measurement device - Google Patents

Abstract

(57) [Problem] To improve the long-term reliability of a flow measurement device employing a sensor element which has high performance but is easily damaged by contaminants. A louver (11), a guide (12) forming a predetermined angle with respect to the louver, and an exhaust port (13) for removing contaminants filtered by a combination of the louver and the guide are combined. Is used to allow a clean fluid to flow to the flow rate detection unit 20.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow rate measuring device for measuring a flow rate of a gas or a liquid. Also, the present invention relates to a combustion control system that measures an intake air amount in an automobile engine and determines a fuel injection amount based on a result of the measurement.

[0002]

2. Description of the Related Art There are various conventional flow rate measuring devices. Here, the measurement of the intake air amount of an automobile will be described as an example.
As a typical air flow measuring device, there is a hot wire air flow sensor, especially a so-called wound air flow sensor. The flow rate detection part of the sensor is a platinum wire wrapped around a small alumina bobbin,
Use with heating. When air hits the heated flow detector, molecules in the air take away heat from the flow detector, the temperature of the flow detector decreases, and the electrical resistance of the platinum wire changes. The change in the electric resistance value is measured by applying, for example, a Wheatstone bridge circuit. The change in the electric resistance value correlates with the number of gas molecules that have caused a temperature change in the flow detection unit, that is, the mass flow rate of the intake air. Therefore, the use of the winding type air flow sensor makes it possible to directly measure not the volume flow rate of gas but the mass flow rate required for controlling combustion of the engine.

[0003] The above-mentioned winding type air flow sensor is configured such that a platinum element wire is wound on an alumina bobbin instead of as it is,
Even if dust or droplets hit the flow rate detection unit, the structure is not easily broken. In measuring the intake air volume of an automobile engine,
Although fine dust and liquid droplets that have passed through an air filter provided upstream of the intake pipe may hit the flow rate detection unit, highly reliable measurement can be realized over a long period of time by using the hot wire airflow sensor.

On the other hand, in recent years, an air flow sensor using a small and high-performance silicon element which has better responsiveness than the above-mentioned wound type air flow sensor has attracted attention. This will be referred to as a silicon airflow sensor for simplicity. There are several types of silicon airflow sensors, but the basic measurement principle is the same as that of the above-mentioned wound type airflow sensor. The flow rate detection unit is composed of a small diaphragm provided on a silicon substrate and a resistance wire patterned thereon. The air flow rate is measured based on the change in the resistance value of the resistance wire. The dimensions of the diaphragm are extremely small, for example, an area of several mm square or less and a thickness of about 1 micron, and are usually made using silicon micromachining technology such as etching. Since the resistance wire on the thin diaphragm is almost thermally insulated from the surroundings and has a small heat capacity, a high response of, for example, 1 millisecond can be secured as a sensor. Further, it is easy to add a function of detecting a backflow from the engine combustion chamber, for example, by changing the pattern of the resistance line even though it is a small sensor.

If a silicon air flow sensor is used, the amount of intake air of the engine can be accurately grasped without a response delay, so that in the case of a multi-cylinder engine, it is easy to determine which cylinder the air passing through the sensor flows into. . Therefore, the air amount for each cylinder can be accurately obtained. According to this, in the combustion control of the engine, the amount of intake air for each cylinder is accurately measured, and based on the result, the fuel injection amount can be optimally controlled for each cylinder.

[0006]

However, the silicon air flow sensor has a problem in securing long-term reliability. Specifically, if contaminants such as fine dust and droplets contained in the measurement fluid that has passed through the air cleaner violently hit the small diaphragm at a high engine speed, mechanical damage to the diaphragm may occur. is there. Since the contaminants after passing through the air cleaner have a very small particle size of, for example, about 10 microns, they do not affect the original function of the engine, but it is necessary to consider the influence on the silicon air flow sensor having a 1-micron-thick diaphragm. There is. As a countermeasure example, a method of providing a mesh-like filter different from a normal air cleaner in the upstream portion of the sensor of the engine intake pipe has been adopted, but the size of dust passing through the air cleaner of the intake pipe is as described above. Since these filters are extremely small, there is a problem in that if they are sufficiently removed, the pressure loss in the filter becomes large and the function as an engine is hindered.

[0007] The above-mentioned reliability problem has been pointed out also with respect to a general industrial measurement air flow sensor for measuring a flow rate by directly exposing a platinum wire to a flow. In this case, since the heat capacity of the flow rate detection unit is kept small without winding the platinum wire around the alumina bobbin, high responsiveness can be obtained, but the platinum wire is hit by the collision of contaminants such as dust and water droplets. May be disconnected and the sensor needs to be replaced frequently.

[0008]

In order to solve the above-mentioned problems without changing the structure and the detection method of the flow rate detecting unit, it is necessary to use a particle size of 1%.
It is sufficient if there is a filter means capable of removing contaminants of about 0 μm, having a small pressure loss against the flow, and being easy to mount like a normal air filter. The one closest to this is called an inertial filter, and its basic structure is shown in FIG. The flow of contaminated air containing contaminants such as dust and water droplets is rapidly reversed by the blades. At this time, the contaminants cannot follow the flow of air due to their inertia, are filtered to the exhaust duct side, and are removed outside the passage by the blower. Since the filter uses a change in the flow direction, the pressure loss is small. On the other hand, in the inertial filter shown in FIG. 11, an exhaust duct system is indispensable for sending clean air downstream. Therefore, it is difficult to mount the system on an existing system such as an engine as it is. However, in order to solve the above-mentioned problem, only the air flowing through the flow detection unit of the flow measurement device needs to be cleaned, and there is room for improvement. Looking at the example of a car engine,
Since the air in the intake pipe has already passed through a normal air cleaner, contaminants contained therein do not affect the original function of the engine even if they adversely affect the flow rate measuring device. Therefore, an inertial filter without the exhaust duct system may be configured. By combining an inertial filter structure without an exhaust duct system and a high-performance airflow sensor, it is possible to provide a flow rate measuring device capable of measuring a flow rate with high accuracy without greatly modifying an existing system such as an engine. The present invention can be widely applied when the contaminants contained in the flow do not affect the original function of the system, but affect the reliability of the high-performance flow detection unit.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and its means is to bring the movement (speed and direction) of a fluid flowing from a main flow to be subjected to flow measurement into a flow detection unit to a desired state. A flow measuring device provided with a flow control means,
The flow control unit includes a flow direction changing unit that functions as an inertial filter that removes a pollutant that hinders measurement, and arranges the flow rate detection unit in a clean flow after passing through the flow direction changing unit. And a second fluid purifying means (filter) different from the flow direction changing means is provided on the upstream side of the flow measuring apparatus, and the flow rate detecting unit is provided. A flow rate measuring device characterized in that the clean fluid that has passed and the contaminant that has been filtered by the flow direction changing means are combined again downstream of the flow rate detection unit. Here, the second fluid purifying means is a fluid purifying means for removing contaminants in the fluid that affect the original function of the system.

The flow direction changing means may include a louver in which blades having a length L are inclined at an angle θ and arranged at intervals P, a guide forming an angle δ with respect to the louver, and a combination of the louver and the guide. A flow rate measuring device characterized by having an exhaust port for removing contaminants filtered by the apparatus, and further measures an intake air amount using these flow rate measuring devices to control a fuel injection amount. Is a combustion control system for an automobile engine.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail with reference to the drawings. Hereinafter, the same numbers indicate the same or similar objects.

FIG. 1 is an explanatory view of a flow rate measuring apparatus according to a first embodiment of the present invention. The measurement target is the intake air amount of the automobile engine.

First, the configuration of the flow rate measuring device will be described. 11 is a louver in which a plurality of blades are arranged, 12 is a flow guide that forms a certain angle with respect to the louver,
Reference numeral 13 denotes an outlet for removing contaminants such as dust and water droplets filtered by the combination of the louver and the guide. The louver 11, the guide 12, and the exclusion port 13 are combined to constitute a flow direction changing unit that functions as an inertial filter. Reference numeral 20 denotes a flow detection unit, reference numeral 30 denotes a protection tube of the flow detection unit, and reference numeral 40 denotes signal processing means for correcting the detected flow signal. The flow rate detection unit 20, the protection tube 30, and the signal processing unit 40 together constitute a normal airflow sensor. In the case of a silicon airflow sensor, the flow detection unit 20 is a sensor element manufactured by processing silicon. Reference numeral 50 denotes a part of the engine intake pipe, and reference numeral 51 denotes a connection means for the pipe. Reference numeral 60 denotes an air cleaner provided upstream of the engine intake pipe. In FIG. 11, contaminants such as dust and water droplets are indicated by white circles for easy understanding, but the actual size is a small contaminant having a particle size of, for example, about 10 microns. The larger particle size contaminants have already been filtered by the air cleaner 60. The contaminants that have passed through the air cleaner 60 are:
Although it does not affect the original function of the engine, if the pollutant vigorously collides with the flow rate detection unit of the silicon airflow sensor during high-speed rotation of the engine, the flow rate detection unit may be mechanically damaged.

Although the louver 11 and the guide 12 only show cross sections, for example, the louver 11 draws a plate made of metal or heat-resistant resin so that air passing through the upper half of the intake pipe 50 passes between the two. The guides 12 can be configured by placing a plate in the longitudinal direction of the intake pipe, arranged in a blind so as to fit in the pipe. The louver 11 and the guide 12 may be inserted later into the intake pipe. However, if the intake pipe divided into two in the cross section in the figure is molded and the structure of the louver 11 and the guide 12 is simultaneously molded in advance, two louvers 11 and the guide 12 are formed. By simply combining the divided members, an intake pipe having a built-in flow direction changing means can be configured.
Each member can be molded using a metal material, ceramics, heat-resistant resin, or the like.

Next, the function of each component will be described.
Part of the air that has passed through the air cleaner 60 is
1 and between the guide 12. Although the flow reverses its direction when passing through the louver 11, the contaminants in the fluid are not able to follow the direction change due to its inertia and are left behind. The filtered contaminants are pushed to the outlet 13. Here, since the flow (main flow) outside the discharge port 13 is generally faster than the flow inside the discharge port 13, the contaminants are drawn out from the discharge port 13 by the differential pressure, merge with the main flow again, and Also flows downstream. On the other hand, since the flow passing through the flow detection unit 20 is a clean flow passing through the louver 11, the flow detection unit is not exposed to contaminants. The protection tube 30 is a means for protecting the flow rate detection unit 20 from a blowback from the combustion chamber side.
If the flow passing through 1 is turbulent, it also serves to reduce the turbulence. The flow rate detected by the flow rate detection unit 20 is:
It correlates with the flow rate of the main flow to be measured, but it is not. This is corrected by the signal processing means 40, and the result is used as the output of the sensor. For the correction of the signal, a correction function obtained experimentally in advance may be used, or the value of the data map adjusted for each sensor may be interpolated according to the signal and output.

FIG. 2 shows the details of the flow direction changing means. FIG. 2 shows only the cross section as in FIG. The flow direction changing means is filtered by a louver 11 in which blades having a length L are inclined at an angle θ and arranged at intervals P, a guide 12 forming an angle δ with respect to the louver 11, and a combination of the louver 11 and the guide 12. And a discharge port 13 for removing contaminants. Here, the angle δ may be changed depending on the location.
Changing each parameter changes the dust collection efficiency. The dust collection efficiency of the conventional inertial filter shown in FIG. 52 No. 10 (1970) reports a study by Okada et al. According to this, in the case of an inertial separation type dust collector for a vehicle, the length L of the blade and the interval P are equally set, specifically, 20 mm, and the inclination angle θ of the blade is 20 °.
When the angle δ is set to 4 ° and the angle δ is set to 4 °, the dust collection efficiency of about 80% has been experimentally confirmed with dust particles having a particle diameter of 10 μm. The size of the flow direction changing means of the present invention is different from that of the above-mentioned inertial separation type dust collector for vehicles. However, if the above parameters are referred to in order to reproduce the flow functioning as an inertial filter, the length L of the blade and the interval P must be equal, and the flow in the intake pipe is not necessarily parallel to the pipe. Angle θ is in the range of 10 ° to 30 °,
It is desirable to set the angle δ at about 20 ° or less, preferably at least 10 ° or less near the exhaust port 13 and preferably at about 4 ° in order to reduce the overall size while maintaining the dust collection efficiency. Two criteria. Here, the length L and the interval P of the blades used in the inertial separation type dust collector for vehicles are used.
The dimension of 20 mm is a value depending on the size of the apparatus, and is not suitable for the flow direction changing means of the present invention. Blade length L
And the interval P can be used as an adjustment parameter for each device.

The speed of flow also affects the dust collection efficiency of the inertial filter. If the flow velocity is reduced, the inertia effect is reduced, and good dust collection efficiency cannot be obtained. However, since our objective is to effectively remove contaminants that hit the flow detector at a high speed, it is safe for contaminants to remain and float in the low-speed intake airflow.

FIG. 3 is an explanatory view of a flow rate measuring apparatus according to a second embodiment of the present invention. This embodiment shows a case where the flow direction changing means described in the first embodiment of the present invention is applied to the entire intake pipe. In this case, since the flow always passes through the louver 11, the change in the ratio of the flow passing through the flow rate detection unit 20 to the main flow, that is, the change in the split flow ratio, can be suppressed to be small over the flow velocity range to be measured. For this reason, the correction processing of the signal processing means 40 becomes easy.

FIG. 4 is an explanatory view of a flow rate measuring device according to a third embodiment of the present invention. The present embodiment shows a case where the flow direction changing means described in the first embodiment of the present invention is symmetrical with respect to the forward flow direction of the main flow to be measured and the reverse flow direction thereof. According to the present embodiment, the forward flow and the backward flow passing through the flow detection unit can be made the same. Therefore, if a sensor element capable of detecting not only the forward flow but also the backward flow is employed in the flow detection unit 20, any of the forward flow and the backward flow can be used. It can be detected accurately. Therefore, even when the flow of the intake air of the engine has a pulsation, the intake air amount to each cylinder can be accurately measured. In addition, reference numeral 70 in FIG. 4 constitutes an intake pipe integrated structure module including a flow rate detecting unit and a flow direction changing unit, and detachable from the suction pipe unit. When the module is adopted, mounting on the engine becomes easy.

FIG. 5 is an explanatory view of a flow rate measuring apparatus according to a fourth embodiment of the present invention. In the present embodiment, the flow direction changing means described in the first embodiment of the present invention is replaced with the flow detecting unit 20.
5 shows a case where it is mounted in the sub-passage 90 attached to the sub-passage. Since the flow direction is changed by a short pipe, the flow rectifying blades 14 are provided on the clean fluid side near the louver 11 to regulate the flow so that the flow passing through the flow rate detection unit 20 is not disturbed. In the pollutant discharge port 13, the pressure change caused by the external flow is used to promote the removal of the pollutant. The structure of the discharge port 13 is a passage structure that is wide on the inside and narrow on the outside. This can suppress the inflow of air from the outside. Further, a projection 15 is provided on the outlet side of the outlet 13 so that a portion having a low pressure is effectively generated on the outlet side of the outlet 13 together with the external flow. Reference numeral 80 denotes an external electrical connection terminal.

FIG. 6 is a sectional view taken along the line AA of the flow rate measuring apparatus according to the fourth embodiment of the present invention. The air flowing through the sub passage 90 is effectively collected by the flow detecting unit 20 by the constricted structure 91 of the passage. As a result, the turbulence of the flow can be suppressed, so that the flow rate detector 20 can detect the flow rate with a good S / N ratio. The sub-passage 90 shown in FIG. 5 and FIG. 6 can be constructed by being divided into two members as shown in, for example, B and C in FIG. For each of the two parts, louver 11 and rectifying blade 1
If the part 4 is integrally molded, a desired flow direction changing means can be obtained only by combining these parts.
Of course, the method of dividing into each member is not limited to one, and for example, various structures may be formed on one member and the other may be like a lid. Sub-passage 9 including flow direction changing means
0 can be molded using a metal material, ceramics, a resin having heat resistance, or the like.

FIG. 7 is an explanatory view of a flow rate measuring apparatus according to a fifth embodiment of the present invention. In the present embodiment, the flow direction changing means described in the first embodiment of the present invention is replaced with the flow detecting unit 20.
14 shows another structure mounted in the sub-passage 90 attached to the sub-passage 90. The air that has passed through the flow direction changing means composed of the louver 11, the guide 12, and the exhaust port 13 flows as it is in the direction of the louver, passes through the flow rate detection unit 20, and then passes through the sub-passage 90.
Of the louver 16 provided on the side surface of the container. Compared to the fourth embodiment of the present invention, the structure shown in FIG. 7 allows the flow to escape to a plurality of paths when excessive air flows in, so that the sensor has a non-linear characteristic in a high flow rate region, for example. When the flow rate detection unit 20 is used as the element, the flow rate detection range can be expanded. Also in this case, if there is a one-to-one correspondence between the output of the flow detection unit 20 and the main flow, the signal processing unit 40 can correct the flow signal.

FIG. 8 is a sectional view taken along the line AA of the flow rate measuring apparatus according to the fifth embodiment of the present invention. In order to effectively realize the flow in the sub-passage indicated by the arrow in FIG. 7, the louver provided on the side surface of the sub-passage 90 has a blade shape. A projection 15 is provided on the outer wall of the sub passage. These have a symmetrical structure in each of the forward flow and the backward flow, so that the flow detection unit 20 can detect both flows with the same sensitivity.

FIG. 9 is an enlarged view of the flow direction means of the flow rate measuring device according to the fifth embodiment of the present invention. In FIG.
The case where the rectifying blades 14 are provided as in the fourth embodiment of the present invention is shown. Similarly, the contaminant outlet 13 also has a passage structure that is wide on the inside and narrow on the outside.

In the above embodiment, the figure mainly shows an example in which the flow rate detecting section is a sensor element manufactured by processing silicon, but the flow rate detecting section uses a platinum resistance element wire as it is. In addition, the reliability of the flow detection device can be similarly improved for a flow detection unit that is easily damaged by contaminants in a fluid.

FIG. 10 is an explanatory diagram of an engine combustion control system according to a sixth embodiment of the present invention. 100 is a flow rate measuring device according to the present invention, 101 is a fuel injection means, 1
02 denotes an oxygen concentration measuring means for measuring the oxygen concentration in the exhaust gas, 103 denotes an electronic control unit for controlling the engine, 104 denotes a throttle of an intake pipe, and 105 denotes a combustion chamber. The signal Sa indicates an input from another sensor or an external device, and the signal Sb indicates an output to another sensor or an external device. In the above combustion control system, the flow rate measuring device 10
The intake air amount is measured based on 0, and a signal for feedforward control is output to the electronic control unit 103. Further, the oxygen concentration measuring means 102 outputs a signal for feedback control to the electronic control unit 103. The electronic control unit 103 calculates an appropriate fuel injection amount based on both measurement results, and outputs a pulse signal that gives the fuel injection means 101 a necessary fuel injection time and timing. When calculating the fuel injection amount and timing, the output of the crank angle sensor necessary for determining the engine speed and the combustion cylinder is referred to as the signal Sa. The flow rate measuring device 100 according to the present invention can secure long-term reliability while using a sensor element having high detection accuracy for the flow rate detection unit 20, so that the accuracy and reliability of the system can be improved. The flow rate measuring device 100 can be easily mounted on an engine by adopting, for example, the intake pipe integrated structure module shown in FIG. 4 or the sub-passage structure shown in FIGS. In the case of the intake pipe integrated structure module, the throttle
104 may be included in a part thereof. In this case, if a throttle opening signal is input so that the signal processing unit 40 can determine the throttle opening, accurate flow correction according to each throttle opening can also be performed. Conversely, opening control can be performed on the electronically controlled throttle. In this case, a separate electronic control unit may be provided in the intake pipe integrated structure module to correct the flow signal therein.

[0027]

According to the flow rate measuring apparatus of the present invention, the flow direction changing means having the inertial filter function can efficiently filter contaminants such as fine dust and water droplets which hinder measurement with a small pressure loss. .

Also, a second fluid purifying means different from the flow direction changing means is provided on the upstream side of the flow rate measuring device, and the clean fluid passing through the flow rate detecting unit and the flow direction changing means are filtered. By causing the contaminant to join again downstream of the flow rate detection unit, the flow rate measurement device can be easily mounted on the measured system. Here, the second fluid purifying means is a fluid purifying means for removing contaminants in the fluid that affect the original function of the system. In the case of measuring the amount of intake air of the engine, if a normal air cleaner is used as the second fluid purifying means, a simple method of replacing a part of the intake pipe with the intake pipe integrated module according to the present invention can be used for the flow rate measurement device. Engine can be implemented.

As the flow direction changing means, a louver in which blades of length L are inclined at an angle θ and arranged at an interval P, a guide forming an angle δ with respect to the louver, and a combination of the louver and the guide By using the elimination port for excluding the contaminants filtered by the filter, fine contaminants having a particle diameter of about 10 microns can be filtered while having a simple structure advantageous for mounting.

Further, in a combustion control system for an automobile engine which measures the amount of intake air using these flow rate measuring devices and controls the amount of fuel injection, accurate measurement of the amount of air can be reliably performed over a long period of time. Accuracy and reliability can be improved.

Generally, the flow control means according to the present invention;
When combined with a sensor element made of silicon and a sensor element with a high-performance but fragile structure, such as a flow rate detection unit that obtains high response by stretching a platinum resistance wire directly into the fluid, contaminants Therefore, the mechanical damage of the flow rate detecting unit due to the above can be reduced, and the accuracy and reliability of the flow rate measuring device and the system including the same can be ensured for a long time.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a flow rate measuring device according to a first embodiment of the present invention.

FIG. 2 is an explanatory view of a flow direction changing means according to the present invention.

FIG. 3 is an explanatory view of a flow rate measuring device according to a second embodiment of the present invention.

FIG. 4 is an explanatory diagram of a flow rate measuring device according to a third embodiment of the present invention.

FIG. 5 is an explanatory diagram of a flow rate measuring device according to a fourth embodiment of the present invention.

FIG. 6 shows a flow rate measuring device A according to a fourth embodiment of the present invention.
A sectional drawing.

FIG. 7 is an explanatory diagram of a flow rate measuring device according to a fifth embodiment of the present invention.

FIG. 8 shows a flow rate measuring device A according to a fifth embodiment of the present invention.
A sectional drawing.

FIG. 9 is an enlarged view of a flow direction changing means of a flow measuring device according to a fifth embodiment of the present invention.

FIG. 10 is an explanatory diagram of a flow rate measuring device according to a sixth embodiment of the present invention.

FIG. 11 is an explanatory diagram of an inertial filter.

[Explanation of symbols]

11: louver of the flow direction changing means, 12: guide of the flow direction changing means, 13: pollutant discharge port of the flow direction changing means, 14: rectifying vane of the flow direction changing means, 15: low pressure portion in the flow Projecting structures for making
... Louvers provided on the side of the sub passage, 20 ... Flow detector, 3
0 ... Protection pipe, 40 ... Signal processing means, 50 ... Intake pipe (pipe for flowing fluid to be measured), 51 ... Intake pipe (pipe) connecting means, 60 ... Air cleaner, 70 ... Intake pipe integrated structure module, 80 ... Electricity 90: auxiliary passage, 91: constricted structure of passage, 100: flow rate measuring device according to the present invention, 101: fuel injection means, 102: oxygen concentration measuring means, 103: electronic control unit of engine, 104: throttle , 105 ... combustion chamber.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hisao Sonobe 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Inside Hitachi, Ltd.Hitachi Research Laboratory Co., Ltd. Within Hitachi Car Engineering Co., Ltd. (72) Inventor Izumi Watanabe 2477 Takaba, Hitachinaka-shi, Ibaraki F-term within Hitachi Car Engineering Co., Ltd. 2F030 CB07 CC14 CF02 2F035 AA02 EA03 EA04 EA07 EA08

Claims (14)

[Claims] 1. A flow rate measuring apparatus comprising: a detection unit for detecting a flow rate; and a flow control unit for controlling a flow of a fluid flowing into the detection unit, wherein the flow control unit is configured to control contamination in the fluid. A flow rate measuring device, which functions as an inertial filter for removing a substance and is provided near the detection unit. 2. The apparatus according to claim 1, further comprising a second fluid purifying means provided on an upstream side, the second fluid purifying means being different from the flow direction changing means. A flow contaminant that rejoins downstream of the flow detector. 3. The louver according to claim 1, wherein the flow direction changing means includes a louver in which blades having a length L are inclined at an angle θ and arranged at intervals P, a guide forming an angle δ with respect to the louver, A flow measuring device, comprising: a discharge port for removing contaminants filtered by a combination of a louver and the guide. 4. The flow rate measuring device according to claim 3, wherein the length L of the blade and the interval P are the same. 5. The flow rate measuring device according to claim 3, wherein the angle θ is in a range of 10 ° to 30 °. 6. The flow measuring device according to claim 3, wherein the angle δ is set to 10 ° or less at least near the outlet. 7. The flow rate measuring device according to claim 3, wherein a flow regulating blade is provided on a clean fluid side near the louver. 8. The flow rate measuring device according to claim 3, wherein the exhaust port has a passage structure having a wide inside and a narrow outside. 9. The flow rate measuring device according to claim 1, wherein the flow direction changing means has a symmetrical structure with respect to a forward flow direction and a reverse flow direction of the main flow. 10. The flow rate measuring device according to claim 1, wherein the flow rate detection unit is a sensor element manufactured by processing silicon. 11. The flow measuring device according to claim 1, wherein the flow detecting unit is a platinum resistance element wire. 12. The flow measuring device according to claim 1, wherein the flow direction changing means is provided in a sub-passage integrally formed with the flow detecting unit. 13. The flow measuring device according to claim 12, wherein the sub-passage provided with the flow direction changing means is configured by combining at least two members molded in advance. 14. A combustion control system for an automobile engine which measures an intake air amount by using the flow rate measuring device according to claim 1 and controls a fuel injection amount.