Abstrict A flow meter is formed with a passage for introducing and carrying
air to measure and has a sensing part disposed in the passage for
measuring a flow rate of the air. A contraction part is formed on
an inner surface of the passage so that the contraction part faces
the sensing part. The contraction part narrows the flow of the air
flowing near the sensing part. A flat surface of the contraction
part is formed in parallel with the surface of the sensing part
and at least upstream of the sensing part. Thus, collision of particles
against the sensing part and adhesion of contaminants to the sensing
pert are inhibited. Meanwhile, the flow rate of the air is measured
stably and accurately.
Claims What is claimed is:
1. A flow meter that is formed with a passage for introducing and
carrying air, which is to be measured, and has a sensing part disposed
in the passage for measuring a flow rate of the air, the flow meter
comprising: a contraction part formed on an inner surface of the
passage so that the contraction part faces the sensing part and
narrows a flow of the air flowing near the sensing part, wherein
the contraction part has a flat surface formed in parallel with
a surface of the sensing part and at least upstream of the sensing
part.
2. The flow meter set forth in claim 1 wherein the contraction
part is formed so that a ratio of a length of the flat surface in
the upstream of the sensing part in a direction of the flow to a
distance between the flat surface of the contraction part and the
sensing part is set to be equal to or larger than 1.
3. The flow meter set forth in claim 2 wherein the contraction
part is a protrusion protruding from the inner surface of the passage
toward a center of the passage for narrowing the area of the passage.
4. The flow meter set forth in claim 2 further comprising an opposite
side contraction part formed on the inner surface of the passage
opposite from the sensing part.
5. The flow meter set forth in claim 4 wherein the contraction
part facing the sensing part and the opposite side contraction part
are formed symmetrically across the sensing part.
6. The flow meter set forth in claim 4 wherein the opposite side
contraction part is smaller than the contraction part facing the
sensing part.
7. The flow meter set forth in claim 2 wherein the flat surface
of the contraction part is formed upstream and downstream of the
sensing part.
8. The flow meter set forth in claim 2 wherein an upstream end
and a downstream end of the contraction part are tapered.
9. The flow meter set forth in claim 1 wherein the passage is
a bypass passage for branching and carrying part of air flowing
through a main passage.
Description CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is based on and incorporates herein by
reference Japanese Patent Application No. 2002-70567 filed on Mar.
14 2002.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a flow meter for measuring
flow rate of air and in particular to a flow meter applied to an
intake air flow meter for an internal combustion engine.
[0004] 2. Description of Related Art
[0005] Conventionally, a flow meter having a membrane airflow sensor
is used for measuring a flow rate of intake air of an internal combustion
engine. The airflow sensor is disposed in a passage carrying the
air to measure.
[0006] An air filter is disposed in an intake system of the engine
in order to eliminate particles such as sand, which are included
in the intake air and have relatively large diameters. However,
the intake air also includes particles that have relatively small
diameters (hundreds of micrometers, for instance) and are difficult
to eliminate with the air filter. In a case where the particles
having relatively small diameters enter the passage, a sensing part
of the flow meter might be damaged if the particles collide with
the sensing part, for instance, at a speed of tens of meters per
second. Specifically, if the sensing part is a membrane airflow
sensor, the membrane of the airflow sensor is very thin. For instance,
the thickness of the membrane is approximately 1 micrometer. Therefore,
the membrane will be easily damaged if the particles collide with
it.
[0007] A flow meter disclosed in U.S. Pat. No. 5404753 (DE4219454A1)
or U.S. Pat. No. 6422070 B2 (DE4407209A1) is formed with a contraction
part in an air passage in which a flow meter is disposed. The contraction
part narrows the cross-sectional area of the passage gradually in
a direction of the flow. The contraction part straightens the flow
of the air flowing near a sensing part of the flow meter and carries
particles included in the air in parallel with the sensing part,
in order to reduce the influence of the particles to the sensing
part.
[0008] In the above flow meter, the contraction part prevents the
collision of the particles against the sensing part to some extent.
However, the particles have some weight and inertia. Therefore,
there is a possibility that the flow of the particles might not
be straightened sufficiently compared with the airflow, depending
on the shape of the contraction part. Accordingly, the particles
whose flow is not straightened sufficiently might collide with the
sensing part and might damage the sensing part.
[0009] A flow meter disclosed in U.S. Pat. No. 6336360 B1 (JP-A-11-248505)
has a sensing part disposed in an air passage formed generally in
a sigmoid shape. The air passage is formed with folded portions
upstream and downstream of the sensing part in order to deflect
the airflow and to inhibit the collision of the particles against
the sensing part.
[0010] In the above flow meter, the particles bypass the sensing
part to some extent, since the air passage has the folded portions.
However, the effect of the folded parts is not enough to fully prevent
the particles from colliding with the sensing part. As a result,
the sensing part might be damaged by the particles, or the detection
accuracy might be degraded because of adhesion of contaminants to
the sensing part.
SUMMARY OF THE INVENTION
[0011] It is therefore an object of the present invention to provide
a flow meter that prevents particles from colliding with a sensing
part and prevents contaminants from adhering to the sensing part,
whereby measuring air flow rate stably and accurately.
[0012] According to an aspect of the present invention, a flow
meter is formed with a passage for introducing and carrying air
to measure, and has a sensing part disposed in the passage. The
flow meter is formed with a contraction part for narrowing the flow
of the air flowing near the sensing part. The contraction part is
formed on the inner surface of the passage so that the contraction
part faces the sensing part. The contraction part is formed with
a flat surface in parallel with the sensing part and at least upstream
of the sensing part.
[0013] Since the flat surface of the contraction part is formed
at least upstream of the sensing part, the air to measure flows
along the flat surface of the contraction part and in parallel with
the surface of the sensing part when the airflow enters the passage
and flows near the sensing part. Meanwhile, the airflow is favorably
straightened. Accordingly, the collision of the particles in acute
angles against the sensing part is inhibited. Therefore, impulsive
force generated when the particles collide with the sensing part
is reduced, and the damage in the sensing part or adhesion of contaminants
to the sensing part is inhibited. In addition, the airflow is favorably
narrowed near the contraction part. Accordingly, the speed of the
airflow is increased and the airflow is stabilized. As a result,
the flow rate of the air is measured accurately.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Features and advantages of embodiments will be appreciated,
as well as methods of operation and the function of the related
parts, from a study of the following detailed description, the appended
claims, and the drawings, all of which form a part of this application.
In the drawings:
[0015] FIG. 1 is a cross-sectional view showing a flow meter according
to a first embodiment of the present invention;
[0016] FIG. 2 is an elevation view showing the flow meter according
to the first embodiment;
[0017] FIG. 3 is a cross-sectional view showing the flow meter
attached to an intake pipe according to the first embodiment;
[0018] FIG. 4 is a cross-sectional view showing the flow meter
along line IV-IV in FIG. 1 according to the first embodiment;
[0019] FIG. 5 is a cross-sectional view showing the flow meter
along line V-V in FIG. 1 according to the first embodiment;
[0020] FIG. 6 is a graph showing a relation between a ratio of
length L of a flat surface to a distance S and influence of particles
to a sensing part according to the first embodiment;
[0021] FIG. 7 is a cross-sectional view showing a flow meter according
to a second embodiment of the present invention;
[0022] FIG. 8 is a cross-sectional view showing a flow meter according
to a third embodiment of the present invention; and
[0023] FIG. 9 is a cross-sectional view showing a flow meter according
to a fourth embodiment of the present invention.
DETAILED DESCRIPTION OF THE REFERRED EMBODIMENT
[0024] (First Embodiment)
[0025] Referring first to FIG. 1 a flow meter is illustrated.
A passage 3 for introducing and carrying air to measure is formed
in a main body 1 of the flow meter. The flow meter has a sensing
part 4 on a base 14 formed in the passage 3. The sensing part 4
is exposed to the airflow, and measures the flow rate of the air.
The passage 3 is a bypass passage for branching and carrying part
of the air flowing through an intake pipe 10 of an internal combustion
engine and the like. As shown in FIG. 1 the passage 3 is formed
generally in the shape of a reversed letter `U` (or in a sigmoid
shape). The cross-section of the passage 3 is formed in a rectangular
shape as shown in FIG. 5. An upstream end of the passage 3 in the
right side in FIG. 1 provides an inlet 15 and a downstream end thereof
in the left side in FIG. 1 provides an outlet 16.
[0026] An inlet side bent part 5 is formed in the passage 3 near
the inlet 15. On the other hand, an outlet side bent part 7 is formed
in the passage 3 near the outlet 16. A middle bent part 6 is formed
in the passage 3 at the middle of the passage 3. The base 14 is
disposed between the inlet side bent part 5 and middle bent part
6 in the passage 3. The base 14 is formed in the shape of a plate
and is disposed so that the base 14 crosses approximately the center
of the cross-section of the passage 3. The sensing part 4 and the
base 14 are disposed so that the surfaces thereof are parallel to
the airflow as shown in FIG. 4. The sensing part 4 is disposed on
the base 14 so that the sensing part 4 is exposed to the airflow.
[0027] As shown in FIG. 4 a contraction part 20 is formed on the
inner surface of the passage 3 so that the contraction part 20 faces
the sensing part 4. The contraction part 20 narrows the flow of
the air to measure, which flows near the sensing part 4. A flat
surface 20a is formed on the contraction part 20 in parallel with
the surface of the sensing part 4. The flat surface 20a is formed
at least upstream of the sensing part 4. Another contraction part
21 is formed on the inner surface of the passage 3 opposite from
the sensing part 4. The contraction part 21 is formed in the same
shape and in the same orientation as the contraction part 20. Thus,
the contraction parts 20 21 are disposed symmetrically across the
base 14 and the sensing part 4. The contraction parts 20 21 are
protrusions protruding from the inner surface of the passage 3 toward
the center of the passage 3 for narrowing the passage area.
[0028] A relation between a length L of the flat surface 20a in
the upstream of the sensing part 4 in the flow direction and a distance
S from the contraction part 20 to the sensing part 4 shown in FIG.
4 is a significant factor for reducing influence of particles to
the sensing part 4. Therefore, the relation between the length L
and the distance S is specified based on a graph shown in FIG. 6.
[0029] The graph shown in FIG. 6 is based on results of experiment.
In the experiment, the degree of the influence, the damage, to the
sensing part 4 caused by the particles was measured while changing
the ratio (L/S) of the length L to the distance S. Then the results
of the experiment were plotted as shown in FIG. 6.
[0030] As shown in FIG. 6 the damage to the sensing part 4 caused
by the particles is effectively reduced in a range in which the
ratio L/S is equal to or more than 1. On the other hand, the influence
of the particles increases in a range in which the ratio L/S is
less than 1.
[0031] Therefore, the relation between the length L and the distance
S is set so that the ratio L/S is equal to or more than 1 in order
to reduce the influence of the particles to the sensing part 4.
[0032] A circuit module 2 is attached to the top of the main body
1 through a sealing member 12 such as an O-ring as shown in FIGS.
1 and 2. A circuit for handling signals is disposed in the circuit
module 2. A connector 11 for power source line and signal lines
is disposed on the side surface of the circuit module 2.
[0033] The main body 1 is made of synthetic resin, such as polybutylene
terephthalate (PBT) or poly phenylene sulfide (PPS) including glass
fibers.
[0034] The sensing part 4 is constructed with a membrane airflow
sensor, for instance. The sensing part 4 has a membrane formed on
a semiconductor substrate 17 and an intake air temperature detection
resistor formed near the membrane. The membrane includes a flow
rate detection resistor and an exothermic resistor. The sensing
part 4 measures the air flow rate by the use of temperature-resistance
characteristics of the resistor. The semiconductor substrate 17
is fixed on a surface of the base 14 and is exposed to the airflow.
Terminals on the substrate 17 are connected with the circuit in
the circuit module 2 through wire bonding and the like.
[0035] As shown in FIG. 3 the flow meter is mounted by inserting
the main body 1 into a hole formed on the intake pipe 10. The flow
meter is mounted so that the inlet 15 faces upstream of the airflow
and the outlet 16 faces downstream of the airflow. The sealing member
12 seals a gap between the flow meter and the intake pipe.
[0036] If the air passes through the intake pipe 10 due to an intake
operation of the engine, part of airflow enters the passage 3 through
the inlet 15 and passes through the passage 3. At this time, the
airflow is narrowed by the contraction parts 20 21 on both sides
near the sensing part 4 as shown in FIG. 4 and goes upward.
[0037] The air flowing near the sensing part 4 flows along the
contraction parts 20 21 and meanwhile, the airflow is favorably
straightened. Therefore, the particles included in the air flow
generally in parallel with the surface of the sensing part 4 along
the contraction parts 20 21 even if the particles have some weight
and inertia. Thus, the flow of the particles is straightened and
the collision of the particles against the surface of the sensing
part 4 in acute angles is greatly inhibited.
[0038] As a result, the impulsive force applied to the sensing
part 4 when the particles collide with the sensing part 4 is largely
reduced, and the damage to the sensing part 4 and the adhesion of
the contaminants to the sensing part 4 are inhibited. Furthermore,
since the airflow is narrowed near the sensing part 4 the speed
of the airflow increases there. As a result, the flow of the air
flowing near the sensing part 4 is stabilized and is measured accurately.
[0039] (Second Embodiment)
[0040] The second embodiment of the present invention is illustrated
in FIG. 7. As shown in FIG. 7 a contraction part 30 is formed on
the inner surface of the passage 3 near the sensing part 4. The
contraction part 30 has a flat surface 30a parallel to the surface
of the sensing part 4. The flat surface 30a extends longer than
the sensing part 4 upstream and downstream of the airflow. Another
contraction part 31 is formed on the inner surface of the passage
3 opposite from the sensing part 4. The contraction parts 30 31
are formed symmetrically with each other.
[0041] Since the flat surface 30a extends longer than the sensing
part 4 upstream and downstream of the airflow, the airflow is favorably
narrowed even if a backflow of the air is generated in the passage
3. As a result, the collision of the particles against the surface
of the sensing part 4 in acute angles is greatly inhibited, and
the damage to the sensing part 4 is reduced.
[0042] (Third Embodiment)
[0043] The third embodiment of the present invention is illustrated
in FIG. 8. As shown in FIG. 8 a contraction part 41 formed opposite
from the sensing part 4 is smaller than a contraction part 40 formed
in the sensing part 4 side. The airflow and the flow of the particles
are suitably straightened near the sensing part 4 even in the case
in which the contraction part 41 is formed relatively small as shown
in FIG. 8.
[0044] (Fourth Embodiment)
[0045] The fourth embodiment of the present invention is illustrated
in FIG. 9. As shown in FIG. 9 contraction parts 50 51 are formed
on the inner surface of the passage 3 respectively, across the sensing
part 4. The upstream end and the downstream end of the contraction
part 50 are tapered. Likewise, the upstream end and the downstream
end of the contraction part 51 are tapered. Thus, increase in air
resistance caused by the contraction parts 50 51 is minimized.
As a result, the speed of the air flowing along the contraction
parts 50 51 is maintained high, so that the measuring accuracy
of the airflow is improved.
[0046] (Effect of the Embodiments)
[0047] As explained above, in the flow meter according to the embodiments,
when the air to measure enters the passage and passes through the
sensing part, the airflow is favorably straightened by the flat
surface of the contraction part upstream of the sensing part and
flows in parallel with the surface of the sensing part. Therefore,
the collision of the particles against the surface of the sensing
part in acute angles is inhibited, even if the particles have some
weight and inertia. Accordingly, the angle in which the particles
collide with the surface of the sensing part becomes obtuse, and
the impulsive force applied by the particles to the sensing part
decreases. As a result, the damage to the sensing part and the adhesion
of the contaminants to the sensing part are inhibited. In addition,
the airflow is favorably narrowed at the contraction part near the
sensing part and the speed of the airflow is increased. As a result,
the airflow near the sensing part is stabilized, and the flow rate
of the air is measured accurately.
[0048] The present invention should not be limited to the disclosed
embodiments, but may be implemented in many other ways without departing
from the spirit of the invention.
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