Abstrict A hot-wire air flow meter for detecting air flow intake has a main
passage (21) and a generally parallel sub-passage (31) for bypassing
said main passage, and located in the sub-passage is a hot-wire
element (2a) for detecting air flow intake. A dished member (34)
is located upstream from an inlet (31a) of the sub-passage, the
dished member having a base portion adjacent the sub-passage inlet
and an upstream projecting edge (30a) extending from said base portion.
The upstream extending edge (30a) at least partially surrounds the
inlet (31a) and is located between the sub-passage inlet and the
main passage, the effect of the dished member tending to stabilize
air flow entering the sub-passage so that the air flow is less affected
by bends in pipework upstream from the air flow meter.
Claims We claim:
1. A hot-wire air flow meter for detecting air flow intake includes
a main passage, a sub-passage bypassing said main passage, a hot-wire
element located in said sub-passage for detecting said air flow
intake, and a dished member upstream from an inlet of said sub-passage,
said dished member having a base portion adjacent said sub-passage
inlet and an upstream projecting edge extending from said base portion,
said projecting edge at least partially surrounding said inlet and
being located between said inlet said main passage, whereby said
dished member tends to stabilise air flow entering said sub-passage
by said inlet.
2. A hot-wire air flow meter as claimed in claim 1 wherein the
base portion is substantially orthogonal to the longitudinal axis
of the main passage and said base is substantially flat.
3. A hot-wire air flow meter as claimed in claim 1 wherein said
edge entirely surrounds said inlet.
4. A hot-wire air flow meter as claimed in claim 1 wherein said
sub-passage has a longitudinal axis which is substantially parallel
with but eccentric from the longitudinal axis of the main passage
and said sub-passage is positioned toward one side of the base portion.
5. A hot-wire air flow meter as claimed in claim 4 wherein said
sub-passage is positioned adjacent said edge.
6. A hot-wire air flow meter as claimed in claim 1 wherein said
edge extends toward the longitudinal axis of said main passage.
7. A hot-wire air flow meter as claimed in claim 1 wherein said
edge extends across the longitudinal axis of said main passage.
8. A hot-wire air flow meter as claimed in claim 1 wherein said
dished member base portion and a projecting edge are oblong in shape.
9. A hot-wire air flow meter as claimed in claim 8 wherein a major
part of said oblong is eccentric with respect to the longitudinal
axis of said main passage.
10. A hot-wire air flow meter as claimed in claim 1 wherein the
base portion and the projecting edge are fan-shaped with the axis
of the fan locating the inlet.
11. A hot-wire air flow meter as claimed in claim 1 wherein the
main passage has a radially inwardly directed wall, inclined inwardly
downstream, and the projecting edge extendingly projects upstream
therefrom.
12. A hot-wire air flow meter as claimed in claim 11 wherein the
distance said edge projects upstream from said inclined wall is
approximately twice the depth of the dished member.
13. A hot-wire air flow meter as claimed in claim 1 wherein the
edge adjoins an inner wall of said main passage, said inner wall
of said main passage having an inlet thereof formed in the shape
of a venturi.
14. A hot-wire air flow meter as claimed in claim 1 wherein the
sub-passage is co-axial with the longitudinal axis of the main passage.
15. A hot-wire air flow meter as claimed in claim 14 wherein the
dished member has a base and a projecting edge which are both oval
with the minor axis of said oval being orthogonal to the longitudinal
axis of said main passage.
16. A hot-wire air flow meter as claimed in claim 1 wherein the
depth of the dished member is substantially the same as the radius
of the sub-passage.
17. A hot-wire air flow meter as claimed in claim 1 wherein the
sub-passage extends from said dished member to a radially extending
passage, a radially outer end of said radial passage communicating
with a downstream end of said main passage.
18. A hot-wire air flow meter as claimed in claim 17 wherein a
baffle plate is provided to partially cover a downstream outlet
of the radial passage.
19. A hot-wire air flow meter as claimed in claim 1 wherein flexible
closure means are provided at the downstream end of said sub-passage,
said flexible closure member being arranged to open or close the
outlet of said sub-passage in dependence upon the direction of air
pressure.
20. A hot-wire air flow meter as claimed in claim 1 wherein said
sub-passage is parallel to but eccentric from the longitudinal axis
of the main passage and arcuately located partly about an entrance
of said sub-passage at the upstream end thereof is a plate, said
plate being positioned between the sub-passage and main passage,
a base part of the dished member being positioned radially outwardly
from the sub-passage with respect to the main passage longitudinal
axis, said base part being substantially orthogonal with respect
to said main passage longitudinal axis.
21. A hot-wire air flow meter as claimed in claim 20 wherein the
downstream end of said sub-passage is arcuately formed about the
main passage and enters into the main passage at a circumferential
portion thereof angularly spaced from said sub-passage.
22. A hot-wire air flow meter as claimed in claim 1 wherein said
sub-passage is formed in a bridge extending radially of the main
passage longitudinal axis, said bridge being formed integrally with
a body of said meter, and said main passage being divided into two
parts by said bridge.
23. An internal combustion engine including a hot-wire air flow
meter according to claim 1 a speed sensor for detecting the rotational
speed of said internal combustion engine, at least one fuel injector
for injecting fuel, and a control unit for controlling the amount
of fuel injection by receiving output signals of said hot-wire air
flow meter and said speed sensor and calculating said amount of
fuel injection corresponding to the amount of intake air.
Description BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to a hot-wire air flow meter and
to an internal combustion engine provided with such a meter. More
particularly, the invention relates to a hot-wire air flow meter
suitable as an air flow meter which constitutes the intake system
of an automobile internal combustion engine and detects the amount
of intake air thereof so as to control the amount of fuel injected.
2) Description of Related Art
In a conventional hot-wire air flow meter, a main air flow passage
has a sub-passage disposed in a central portion thereof and a hot-wire
element is provided in the sub-passage, as disclosed in Jap. Pat.
Laid-Open Nos. 50520/1975 146369/1975 and 69021/1980. In a hot-wire
flow meter having the structure such as that disclosed in Jap. Pat.
Laid-Open No. 50520/1975 however, the hot-wire element is defenseless
against blow-back due to engine back fire caused when, for example,
the timing element is mistimed. As a countermeasure, a structure
of protecting the hot-wire element from back fire is disclosed in
Jap. Pat. Laid-Open Nos. 146369/1975 and 69021/1980. However, due
to the nonlinearilty of a hot-wire element, that is the thermal
conductivity is not proportional to the voltage frequency output
thereof, a hot-wire element generally has an output characteristic
which is lowered in spite of the increased average flow rate when
the hot-wire element is placed in a large flow of pulsating air
caused by piston movement in an internal combustion engine. All
of the above-described prior art air flow meters have the disadvantage
that they cannot accurately detect the flow rate of a pulsating
air flow.
As disclosed in Jap. Utility Model Laid-Open No. 135127/1981 and
Jap. Pat. Laid-Open No. 185118/1985 in some hot-wire air flow meters,
a sub-passage with a hot-wire element provided therein is disposed
in the main passage such that the fluid resistance of the sub-passage
downstream from the hot-wire element is increased as a countermeasure
to back firing or in order to accurately detect the pulsating flow
and the entrance opening of the sub-passage is parallel to or almost
parallel to the main air current. In other words, the dynamic pressure
of the back flow which acts on the entrance opening is reduced and
the flow going toward the hot-wire element is attenuated, thereby
enhancing the resistance to back fire. Since the downstream exit
of the sub-passage is directly in line and substantially parallel
to the main air flow current, the flow in the sub-passage fluctuates
due to the static pressure which is caused by the mixture of the
flow from the sub-passage and main passage at this portion. This
appears as noise on the hot-wire element. Although high-frequency
noise is cut off to a certain degree, the noise caused by the above-described
fluctuation becomes a problem in controlling the system when the
engine is driven at a low speed. In addition, the known structure,
has a long axial length making installation in an automobile difficult
and is made of a number of parts making the cost of manufacture
high.
There are hot-wire air flow meters in which a sub-passage with
a hot-wire element provided therein is disposed outside of the main
passage as a countermeasure for back fire and to stabilise the output
of the hot-wire element with respect to the intake pulsation, as
disclosed in Jap. Pat. Laid-Open Nos. 13557/1972 109816/1983 76012/1981
and 28017/1986. The embodiments described in these specifications
have the disadvantage that the detection error in the flow rate
is increased due to thermal conditions such as the thermal conduction
from the engine, the heat of the hot-wire element itself, or the
heat of the engine and the rise in temperature in the engine compartment
caused by solar radiation, as pointed out in Jap. Pat. Laid-Open
No. 76012/1981. That is, since the sub-passage portion is provided
in the interior of the body wall which has a large heat capacity
and does not have a wide heat transfer area with respect to the
air flow, the temperature of the air flow in the sub-passage is
influenced by the temperature of the passage wall and the difference
in temperature between the air flow in the sub-passage and the air
flow in the main passage is increased. This leads to an increase
in the error in the measurement of the intake air flow.
Jap. Pat. Laid-Open No. 250260/1985 discloses a structure in which
the entrance of the sub-passage has a bell shaped mouth having a
large throat area ratio so as to reduce the error in measurement
even when the air flow upstream of the entrance of the sub-passage
is greatly deflected. This structure, however, does not effect an
improvement on the measurement accuracy (the stabilisation of the
distribution of the air flow in the main passage and the sub-passage)
if the air flow upstream of the entrance of the sub-passage has
a large speed distribution and a large pressure distribution. This
fact is prominent when the sub-passage is provided eccentrically
with the main passage. In addition, this structure has the disadvantage
that the flow rate in the sub-passage is increased when there is
a large amount of air flow, so that a large amount of dust adheres
to the hot-wire element, thereby varying the output characteristic
with time.
In the above-described prior art, some have a structure unsuitable
for practical use because they do not withstand engine back fire
and strong blow-back of the engine and they cannot accurately detect
average flow rate of a pulsating flow. Moreover some cannot accurately
measure the flow rate when the flow varies due to changes in thermal
conditions to which the air flow meter is exposed, nor when different
shapes of the constituent parts of the intake pipe passage are arranged
upstream of the air cleaner, duct, etc.; additionally since the
noise of the output of the hot wire element is large, sufficient
control of the engine when driven at the optimum ratio is not carried
out, thereby obstructing cleaning of the exhaust gas of the engine,
reduction in fuel cost, and improvement in operability, etc. Other
prior art meters increase the pressure loss in the intake pipe passage
and the weight of the system including the engine, thereby obstructing
any reduction in fuel cost, and reduce the space in the engine compartment,
etc.
It is an object of this invention to provide a hot-wire air flow
meter which achieves a reduction in fuel cost of the engine system
and occupies less space in the engine compartment, and which is
capable of detecting accurately the amount of intake air under various
conditions.
It is another object of the present invention to provide an internal
combustion engine which is capable of the optimum control of the
air fuel ratio by using the above-described hot-wire air flow meter.
SUMMARY OF THE INVENTION
According to one aspect of this invention there is provided a hot-wire
air flow meter for detecting air flow intake includes a main passage,
a sub-passage bypassing said main passage, a hot-wire element located
in said sub-passage for detecting said air flow intake, and a dished
member upstream from an inlet of said sub-passage, said dished member
having a base portion adjacent said sub-passage inlet and an upstream
projecting edge extending from said base portion, said projecting
edge at least partially surrounding said inlet and being located
between said inlet and said main passage, whereby said dished member
tends to stabilise air flow entering said sub-passage by said inlet.
Advantageously the base portion is substantially orthogonal to
the longitudinal axis of the main passage and said base is substantially
flat, and in one embodiment said edge entirely surrounds said inlet.
In an embodiment said sub-passage has a longitudinal axis which
is substantially parallel with but eccentric from the longitudinal
axis of the main passage and said sub-passage is positioned toward
one side of the base portion, and advantageously said sub-passage
is positioned adjacent said edge.
The edge may extend toward the longitudinal axis of said main passage
or the edge may extend across the longitudinal axis of said main
passage. In an embodiment of the invention said dished member base
portion and a projecting edge are oblong in shape, and in such an
embodiment a major part of said oblong is eccentric with respect
to the longitudinal axis of said main passage.
In another embodiment of the invention the base portion and the
projecting edge are fan-shaped with the axis of the fan locating
the inlet. In a further embodiment of the invention the main passage
has a radially inwardly directed wall, inclined inwardly downstream,
and the projecting edge extendingly projects upstream, and in such
further embodiment the distance said edge projects upstream from
said inclined wall is approximately twice the depth of the dished
member.
In yet another embodiment of the invention the edge adjoins an
inner wall of said main passage, said inner wall of said main passage
having an inlet thereof formed in the shape of a venturi.
In an alternative embodiment the sub-passage is co-axial with the
longitudinal axis of the main passage, and the dished member advantageously
has a base and a projecting edge which are both oval with the minor
axis of said oval being orthogonal to the longitudinal axis of said
main passage.
Preferably the depth of the dished member is substantially the
same as the radius of the sub-passage.
Where the sub-passage has a longitudinal axis which is substantially
parallel with an eccentric from the longitudinal axis of the main
passage, advantageously the sub-passage extends from said dished
member to a radially extending passage, a radially outer end of
said radial passage communicating with a downstream end of said
main passage, and preferably a baffle plate is provided to partially
cover a downstream outlet of the radial passage.
A flexible closure means may be provided at the downstream end
of said sub-passage, said flexible closure member being arranged
to open or close the outlet of said sub-passage in dependence upon
the direction of air pressure.
In a further embodiment of the invention said sub-passage is parallel
to but eccentric from the longitudinal axis of the main passage
and arcuately located partly about an entrance of said sub-passage
at the upstream end thereof is a plate, said plate being positioned
between the sub-passage and main passage, a base part of the dished
member being positioned radially outwardly from the sub-passage
with respect to the main passage longitudinal axis, said base part
being substantially orthogonal with respect to said main passage
longitudinal axis. In said still further embodiment the downstream
end of said sub-passage is arcuately formed about the main passage
and enters into the main passage at a circumferential portion thereof
angularly spaced from said sub-passage. Conveniently said sub-passage
is formed in a bridge extending radially of the main passage longitudinal
axis, said bridge being formed integrally with a body of said meter,
and said main passage being divided into two parts by said bridge.
According to another aspect of this invention there is provided
an internal combustion engine including a hot-wire air flow meter
in accordance with said one aspect, a speed sensor for detecting
the rotational speed of said internal combustion engine, at least
one fuel injector for injecting fuel, and a control unit for controlling
the amount of fuel injection by receiving output signals of said
hot-wire air flow meter and said speed sensor and calculating said
amount of fuel injection corresponding to the amount of intake air.
The edge on the periphery of the entrance opening of the sub-passage
averages the variation in the flow rate distribution and the pressure
distribution of the flow, and so stabilises the rate of the flow
rate of the air flow in the sub-passage with respect to the total
flow rate.
By providing a sub-passage for a hot-wire air flow meter with a
hot-wire element therein in parallel with a main passage, the heat
exchange of the sub-passage wall with respect to the main current
is enlarged and the temperature of the sub-passage wall is maintained
constantly at a temperature close to the temperature of the intake
air. Also, dynamic pressure of the backward flow on the sub-passage
may be prevented from being applied directly to the exit opening
thereof when the engine backfires or blows back.
Furthermore, by providing a member for preventing backward flow
from entering the sub-passage, the fluctuation in static pressure
due to the mixture of the flow at the respective exits of the sub-passage
and the main passage is reduced in the vicinity of the outflow portion,
thereby stabilising the difference in the pressure between the entrance
and the exit of the sub-passage. Thus, the flow within the sub-passage
is stabilised and the fluctuation of the flow is eliminated.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example with reference
to the accompanying drawings in which:
FIG. 1 shows the structure of an internal combustion engine using
a hot-wire air flow meter according to the present invention;
FIG. 2 is a sectional view of a first embodiment of a hot-wire
air flow meter in accordance with this invention;
FIG. 3 is a sectional view of the embodiment shown in FIG. 2 taken
along the line III--III;
FIG. 4 is a sectional view of the embodiment shown in FIG. 2 taken
along the line IV--IV;
FIG. 5 is a sectional view of the embodiment shown in FIG. 2 taken
along the line V--V;
FIGS. 6 and 7 show examples of distribution of the flow rate on
the upstream side of the air flow meter in the structure shown in
FIG. 1;
FIGS. 8 and 9 show examples of distribution of the pressure in
the examples shown in FIGS. 6 and 7;
FIG. 10 is a sectional view of a second embodiment of a hot-wire
air flow meter in accordance with this invention;
FIG. 11 shows the second embodiment shown in FIG. 10 viewed in
the direction XI--XI indicated by the arrows;
FIG. 12 is a sectional view of a third embodiment of a hot-wire
air flow meter in accordance with this invention;
FIG. 13 is a sectional view of the embodiment shown in FIG. 12
taken along the line XIII--XIII;
FIG. 14 is a sectional view of a fourth embodiment of a hot-wire
air flow meter in accordance with this invention;
FIG. 15 is a sectional view of the embodiment shown in FIG. 14
taken along the line XV--XV;
FIG. 16 is a sectional view of a fifth embodiment of a hot-wire
air flow meter in accordance with this invention;
FIG. 17 is a sectional view of the embodiment shown in FIG. 12
taken along the line XVII--XVII;
FIG. 18 is a sectional view of a sixth embodiment of a hot-wire
air flow meter in accordance with this invention;
FIG. 19 is a sectional view of the embodiment shown in FIG. 18
taken along the line XIX--XIX;
FIG. 20 is a sectional view of the embodiment shown in FIG. 18
taken along the line XX--XX;
FIG. 21 is a sectional view of a seventh embodiment of a hot-wire
air flow meter in accordance with this invention;
FIG. 22 is a sectional view of an eighth embodiment of a hot-wire
air flow meter in accordance with this invention;
FIG. 23 is a sectional view of a ninth embodiment of a hot-wire
air flow meter in accordance with this invention;
FIG. 24 is a sectional view of a tenth embodiment of a hot-wire
air flow meter in accordance with this invention;
FIG. 25 is a sectional view of the embodiment shown in FIG. 24
taken along the line XXV--XXV;
FIG. 26 is a sectional view of a prior art hot-wire air flow meter;
FIG. 27 is a sectional view of the prior art shown in FIG. 26
taken along the line XXVII--XXVII; and
FIG. 28 shows in graphical form results of experiments.
In the Figures like reference numerals denote like parts.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows an embodiment of an electronically controlled type
internal combustion engine to which a hot-wire air flow meter of
the present invention is applied.
In FIG. 1 the internal combustion engine has a cylinder 500 to
which is sucked intake air 502 through an air filter 503 and is
supplied thereto through an intake duct 504 the hot-wire air flow
meter 1 and an intake manifold 501. The air flow meter 1 is provided
with a sub-passage 31 projecting into a main passage 21 and in
the sub-passage 31 is provided a hot-wire element 2a and a temperature
correction element 2b which are both integral with a sensor unit
2. The air flow meter 1 detects the air flow rate in the sub-passage
and obtains an output indicative of the total amount of intake air.
In the passage of the air flow meter 1 is provided a throttle valve
3 which interlocks with the accelerator pedal of a vehicle and controls
the amount of intake air. The air flow meter 1 is further provided
with an idle speed control (ISC) valve 8 for controlling the flow
rate of the air when the throttle valve 3 is fully closed (idling).
Fuel is supplied by a pump 506 from a fuel tank 505 and injected
by injectors 507 into the intake manifold 501 so as to be supplied
to the internal combustion engine 500 together with air.
The output signal of the sensor unit 2 of the hot-wire element
2a, the rotational angle signal of the throttle valve 3 the output
signal of an oxygen concentration sensor 508 provided on an exhaust
manifold 511 and the output signal of a rotational speed sensor
509 are input to a control unit 510 which calculates the amount
of fuel to be injected, the opening angle of the idle speed control
(ISC) valve, and in accordance with the results, so the control
unit 510 controls the injectors 507 the ISC valve 8 and ignition
timing equipment (not shown).
Referring to FIGS. 6-9 in FIGS. 6 and 7 examples are shown of
the distribution of the axial flow rate immediately before the air
flow meter 1 in the embodiment of the structure shown in FIG. 1
and FIGS. 8 and 9 show examples of distribution of the static pressure,
especially in the case in which there are some bent portions in
the intake duct 504 and the air flow meter 1 is disposed immediately
after a bent portion, as in the embodiment of the system shown in
FIG. 1. As seen from the examples of FIGS. 6 and 7 in the flow
immediately before the air flow meter 1 the flow rate is high on
the outside of the bend (on the lefthand side of each drawing) and
low on the inside thereof (on the righthand side of each drawing),
while the static pressure (shown in the examples of FIGS. 8 and
9) is high on the outside and low on the inside. In other words,
there is a deflection in the flow immediately before the air flow
meter 1. Examples 1 and 2 show the difference in the flow which
may be caused by a combination of variation in the shape and the
fixing positions of the filter 503 and the intake duct 504. In each
example, amount of air flow is about 20 g/s.
Such a deflection of flow is well known as the flow immediately
after a bent pipe (bend) in hydromechanics (e.g. Hydraulics by Matsuki
Itaya; Lectures on Mechanical Engineering published by Japan Society
of Mechanical Engineers and Hydromechanics by Yoshimasa Furuya and
two others, published by Asakura Shoten). It is also known that
two vortices (not shown in FIGS. 6 to 9) are produced following
a right angle bend which when viewed transversely of the pipe are
contra-rotating about respective halves of the pipe and from a secondary
flow.
FIGS. 2 to 5 which show a first embodiment of a hot-wire air flow
meter according to the present invention will now be described.
The air flow meter 20 has a body 20a, a throttle valve body 20b
and an ISC valve body 20c which are die cast integrally with each
other. At the entrance of the air flow meter body 20a, a rectifier
lattice body (honeycomb) 40 is provided. In the downstream of the
lattice body 40 a bridge 30 is formed by a transverse web 30c and
a part circular member 30b and defines the sub-passage 31 the bridge
30 being die cast integrally with the body 20a across the main passage
21.
The sub-passage 31 has a smaller diameter than the main passage
21 and is composed of an axial sub-passage 31b having a circular
cross-section which is substantially parallel with the main passage
axis but eccentric therewith. The axial sub-passage joins one end
of the radial sub-passage 31c having a rectangular cross-section,
the radial sub-passage being substantially orthogonal to the sub-passage
31b and the other end of the sub-passage 31c joins the main passage
21. The sub-passage 31c is composed of a groove provided at the
end of the downstream of the bridge 30 and a cover 32 is fixed
to the downstream end of the bridge 30 by a screw 33. The lower
(as shown in FIGS. 2 and 5) end portion 32a of the cover 32 has
a smaller width than the groove of the sub-passage 31c, but has
such a configuration as to overlap with the exit opening (outflow
portion) 31d of the sub-passage. The resistance to air flow of the
sub-passage 31 is larger than that of the main passage 21 due to
the fluid resistance of the rectangular cross-section passage being
bent into an L-shape. By virtue of the above-described structure,
most parts of the outer wall of the bridge 30 are in contact with
the main current of the intake air, so that the temperature of the
passage wall of the sub-passage 31b is maintained at a substantially
equal temperature to the temperature of the intake air and the heat
entering from the outside, such as from the engine, is cooled by
the intake air, thereby enabling measurement of the air flow rate
to be produced with only a small error. The force of backward flow
of air such as caused by back fire of the internal combustion engine
entering the sub-passage 31 is lessened by the lower (as viewed
in FIGS. 2 and 5) end portion 32a of the cover which covers the
exit opening 31d so as to protect the hot-wire element 2a, and element
2b. Since the passage 31c produces a resistance having an attenuating
action to pulses, the hot-wire element 2a in the sub-passage is
prevented from abnormal output due to pulsation.
The sensor circuit unit 2 has a hole having substantially the same
diameter as the axial sub-passage 31b so that the hot-wire element
2a and the temperature correction element 2b are situated in the
sub-passage 31b. A mold portion 2c which constitutes a part of the
sub-passage 31b is screwed to the body 20a by screw members 41a,
41b in such a manner as to be inserted from the outside of the body
20a.
The upstream end of the axial sub-passage 31b has a circular entrance
opening 31a located in a substantially flat base, i.e. downstream
part of an oblong, dished, recessed, member 34 having a peripheral
edge 30a which projects upstream from the entrance opening 31a and
which forms an upstream end projection of the bridge 30. The edge
30a is disposed in the main passage 21 at position located inwardly
from the main passage wall 20a. The edge 30a of the bridge 30 is
so formed as to project in an upstream direction from an adjoining
inclined wall surface 28 of the main passage 21 by a length equivalent
to two times the depth of the recessed member 34.
In this embodiment, the entrance opening 31a is provided eccentrically
at a position toward the upper portion in FIG. 2 of the base of
the dished member 34 namely, closer to the sensor unit 2. The opposite
portion of the dished member 34 with respect to the entrance opening
31a is extended substantially toward the center of the main passage
21 and the axial depth of the dished member 34 is about the same
as the radius of the axial sub-passage 31b. Provision of the recessed
member 34 makes the air flow to the sub-passage 31 insensitive to
differences in the upstream air flow caused by the variations of
the shapes and the fixing states of the air cleaner and the intake
duct having bent portions, as shown in FIGS. 6 to 9. In other words,
the distribution of the flow rate to the sub-passage 31 is stabilised
by the member 34 which may, therefore, be considered as a stabilising
member. This embodiment is especially effective in the case in which
there is no alternative but to dispose the entrance opening 31a
immediately after an intake duct having a bent portion.
As described above, since the entrance opening 31a is eccentric
toward the sensor unit 2 the effective length of the radial sub-passage
31c can be increased to more than the radius of the main passage
21. A lower end portion 30b of the bridge 30 has a part circular
side wall 30d having a height in the meter axial direction about
1/2 the depth of the radial sub-passage 31c measured in the meter
axial direction and the lower end portion 30b in conjunction with
wall 30d serve as an effective air breaking wall with respect to
the exit opening 31d, which is situated slightly below (as shown
in FIG. 2) the downstream throttle valve shaft 4 to thereby prevent
the flow in the exit opening 31d from disturbing the main air flow
current. A rib 30c is provided chiefly for the purpose of improving
the melt flow at the time of die casting, but it also has an effect
to prevent transverse circulation of the main air flow current.
These portions of the bridge 30 stabilise the flow and reduce the
noise of the output of the hot-wire element 2a.
A necked portion 22 of the main passage wall is provided slightly
downstream from the exit opening 31d of the sub-passage 31c to stabilise
the flow in the sub-passage 31 with respect to the movement of the
throttle valve 3; in other words, to stabilise the distribution
of the air flow of the main passage 21 and the sub-passage 31 and,
as a result, enable the throttle valve 3 to be provided in proximity
to the air flow meter. In this respect, the throttle valve 3 for
controlling the amount of air is located downstream of the hot-wire
meter and the valve shaft 4 for driving the throttle valve 3 penetrates
the body 20b. On the exterior of the body 20b are provided a lever
mechanism 5 for driving the shaft 4 a spring 6 and a throttle position
sensor 7 for detecting the rotational angle of the shaft. At a portion
of the ISC valve body 20c are provided the ISC valve 8 for controlling
the amount of air flow rate during the idling of the internal combustion
engine and air passages 23 24 and 25 to the ISC valve 8. Since
the passages 23 and 25 are formed from the outside of the body 20c,
plugs 26 and 27 are used to fill the unnecessary hole portions.
In this embodiment, it is possible to realise a hot-wire air flow
meter at a low cost which is capable of measuring the amount of
intake air flow of an internal combustion engine with accuracy and
high reliability even where there is a very complicated intake passage
upstream of the air flow meter and there is a wide variation in
the shape and the connection of the intake passage, the meter having
a short axial dimension and being light in weight. It is therefore
possible to realise an engine system which is capable of achieving
a cleaner exhaust gas and reduction in fuel used.
FIGS. 10 and 11 show a second embodiment of a hot-wire air flow
meter according to the present invention. The entrance opening 91a
of a sub-passage 91 is provided at the base portion of a recessed
member 94 which is fundamentally the same as the recessed member
34 in the first embodiment. In place of the rectifying member honeycomb
40 in the first embodiment, a throat portion 81a is provided at
the entrance portion of a main passage 81. Provision of the throat
portion 81a reduces the diameter of the main passage, thereby reducing
the pressure loss and the maximum flow rate of the main passage
81 which exerts influence on the distribution of the air flow in
the sub-passage 91. Therefore, the lower end (as viewed in FIG.
10), portion 90c, of a bridge 90 is only composed of a rib member,
so that the minimum cross-section of the main passage is greatly
reduced in comparison with that in the first embodiment.
A cover 92 constituting a radial sub-passage 91c is fixed by the
screw 33 to the downstream end of the bridge 90 which is integral
with a body 80. The width of a lower end portion 92a of the cover
92 is slightly smaller than the width of the groove of the sub-passage
91c, as in the first embodiment. The lower end portion 92a is bent
toward the upstream in the axial direction so as to cover the exit
opening 91d of the sub-passage 91c. The end of the lower end portion
92a of the cover 92 is substantially in contact with the downstream
end of the bridge over the rib 90c. In this way, any disturbance
of the main air flow current by the lower end portion 92a of the
cover 92 is prevented and it is possible to make the power of the
backward flow entering the sub-passage 91 less than that in the
first embodiment.
The throat portion 81a at the entrance of the air flow meter is
not as satisfactory as a rectifying member such as a honeycomb with
respect to a strong circulating current, but it has the action of
reducing the boundary layer produced in any upstream bent portion
and of suppressing the disturbance produced within the boundary
layer by the virtue of the venturi effect. Thus, this embodiment
is used where a lower cost is required than the cost of the first
embodiment. The pressure loss can be kept equal to or smaller than
that in the first embodiment because there is no honeycomb.
FIGS. 12 and 13 show a third embodiment of a hot-wire air flow
meter according to the present invention. A recessed member 114
is provided at the upstream end of a bridge 110 which is integral
with a body 100 and the entrance opening 111a of a sub-passage is
provided at the base portion of the dished, recessed, member 114.
This embodiment is different from the first embodiment in that the
recessed member is also extended upwardly (as shown in FIG. 12)
of the entrance opening portion 111a, and in that the portion of
the recessed member below the entrance opening 111a is longer. This
structure enables the pressure to be averaged over a wide range.
In this case, however, the function is not effective unless the
depth of the recessed member 114 is set to be larger than in the
first and second embodiments.
FIGS. 14 and 15 show a fourth embodiment of a hot-wire air flow
meter according to the present invention. A recessed member 134
is composed of two parallel edges 130a projecting upstream at the
end of a bridge 130 which is integral with a body 120 and the inner
walls 120a, 120b of the entrance of the air flow meter and the base
surfaces 134a, 134b of the dished, recessed, member 134 are formed
as a continuous smooth wall surface. The base surfaces 134a and
134b of the recessed member 134 have a gentle inclination toward
the entrance opening 131a of a sub-passage 131.
The entrance inner walls 120a, 120b , which are shown vertically
in the drawings, reduce the air current and stabilise the flow along
the wall surfaces. The inclination of the base surfaces 134a, 134b
makes the air stagnating in the recessed member 134 readily flow
toward the entrance opening 131a . By virtue of this structure,
stable distribution of air flow and a lower noise of the output
of the hot-wire element, such as is shown in FIGS. 6 to 9 can be
realised without a honeycomb.
FIGS. 16 and 17 show a fifth embodiment of a hot-wire air flow
meter according to the present invention. A dished, recessed, member
154 is composed of an edge 150a projecting upstream from an end
of a bridge 150 which is integral with a body 140. This embodiment
is different from the first embodiment in that the recessed member
154 is provided in the shape of a sectorially shaped fan in the
upper portion of FIGS. 16 and 17 such that the entrance opening
151a of a sub-passage 151 constitutes the pivot of the fan. The
upper portion (shown in FIG. 16) of the base of the recessed member
154 is substantially flat. This embodiment is especially effective
in the case where the meter is to be located close to a bend and
in the entrance opening 151a of the sub-passage is arranged to be
orthogonal to a line connecting the inside and the outside of the
bend. Since the variation of the air speed is large in the vicinity
of the inside wall, as shown in the distribution of the speed in
FIGS. 6 and 7 averaging this portion is an effective countermeasure.
FIGS. 18 to 20 show a sixth embodiment of a hot-wire air flow meter
according to the present invention. A recessed member 174 is surrounded
by an oval edge 170a projecting upstream at the upstream end of
a bridge 170 which bridge is integral with a body 160 and the entrance
opening 171a of the sub-passage 171 is provided at the base portion
of the dished, recessed member 174 the base thereof being substantially
flat. This embodiment is different from the first to fifth embodiments
in that the longitudinal axis of an axial sub-passage 171b is coaxial
with the longitudinal axis of the main passage 161 and upper and
lower (as shown in FIG. 18) radial sub-passages 171c are provided.
Therefore, a cover 172 for the radial sub-passages 171c has the
form of a plate with both upper and lower end portions 172a thereof
having a smaller width. Two exit openings 171d are naturally provided
on the sub-passage 171c at the upper portion and the lower portion,
respectively, as shown in FIG. 18. A long molded portion 162c of
a sensor unit 162 is provided so that sensors 162a and 162b are
situated within the axial sub-passage 171b.
The entrance wall 160a of the air flow meter body 160 adopts the
configuration of a venturi as in the second embodiment.
In this embodiment, since the flow at the central portion is fundamentally
stable, a more stable property is obtained than the experiments
which will be described later, but increase in the length of the
molded portion 162c of the sensor unit 162 disadvantageously leads
to a rise in cost.
FIG. 21 shows a seventh embodiment of the present invention. A
sensor circuit unit 182 is fixed to the block for holding the hot-wire
elements which block is provided separately from a body 180. An
entrance member 195 of a sub-passage 191 has a dished, recessed,
portion 194 is fixed at the upstream end of a bridge 190 by a screw
196 as shown in FIG. 21. The entrance opening 191a of the sub-passage
191 is provided in the flat base portion of the recessed portion
194 of the member 195 and the upstream projecting edge of member
195 surrounds opening 191a to separate it from the main passage
181. The lower end portion 192a of a cover member 192 which is fixed
to the downstream end of the bridge 190 is bent toward the upstream
in the same way as in the second embodiment shown in FIG. 10. This
is because while the hot-wire element holder block is integral with
the body in each of the first to sixth embodiments, these elements
are separate from each other in the structure of this embodiment
and the main passage 181 is also situated at the lower (as shown
in FIG. 21) end portion of the block 190.
This structure has the disadvantage that the number of parts increases,
but has the advantage of easy maintenance since parts may be replaced.
In addition, since it is possible to position the member 195 which
constitutes the recessed portion 194 at a position slightly rotated
around the entrance opening 191a by changing the fixing position
of the screw 196 this embodiment can cope with a wider range in
the shapes of the elements of the upstream intake pipe and the positions
for mounting the air flow meter.
FIG. 22 shows an eighth embodiment of the present invention. In
the interior of a bridge 203 which is integral with a body 200
a sub-passage 202 which consists only of an axial sub-passage is
provided coaxially with a main passage 201. The upstream end of
the bridge 203 constitutes an edge 203a surrounding a flat portion
204a formed around the entrance opening 201a of the sub-passage
201 thereby constituting a dished, recessed, member 204. To the
outflow portion of the sub-passage 202 is provided a check valve
(stabilising means) 205 made of a thin steel sheet having a retainer
206 which serves as the stopper of the check valve 205 both being
fixed by a screw member 207. The check valve 205 is deformed toward
the downstream side when the flow is normal, as shown in FIG. 22
and when the flow is reversed, the check valve 205 closes the exit
(i.e. right hand end as viewed in FIG. 22) of the sub-passage 202.
FIG. 23 shows a ninth embodiment of the present invention. In the
interior of a bridge 213 which is integral with a body 211 is
a sub-passage 212 which consists only of an axial extending sub-passage
that is eccentric with respect to a main passage 211. A dished,
recessed, member 214 is formed at the upstream end of the bridge
213 and member 214 has an upstream projecting edge 213a which peripherally
surrounds an entrance opening 212a of the sub-passage 212 provided
at the flat base portion of the recessed member 214. The edge 213a
thus separates the opening 212a from the main passage 211. To the
outflow portion, i.e. downstream end, of the sub-passage 212 is
provided a backward flow preventive valve (stabilising means) 215
made of a thin steel sheet which is secured with a retainer 216
by a screw 217.
FIGS. 24 and 25 show a tenth embodiment of the present invention.
In the interior of a thick-walled portion 230 of a body 220 are
formed a sub-passage 231 consisting of an axial sub-passage 231b
which is parallel to a main passage 221 and an arcuate circularly
cross-sectioned passage 231c which traverses around the outer periphery
of the main passage 221 and the exit opening 231d of the sub-passage
231 is disposed at the inner wall of the main passage 221. A pipe
body 225 on the downstream side and the body 220 are connected with
each other through a packing piece 224. The upstream end surface
of the thickwalled portion 230 of the sub-passage 231 constitutes
a flat surface 230a perpendicular to the air flow, and on this surface
the entrance opening 231a of the sub-passage 231 is disposed. A
curved member 232 extends upstream from and partially around entrance
231a to prevent the air flow at the portion of the flat surface
230a from flowing out to the main passage 221. Provision of the
member 232 stabilises the static pressure in the vicinity of the
entrance opening 231a and serves to separate the sub-passage entrance
from the main passage. As a result, the distribution of the air
flow in the sub-passage 231 and the main passage 221 is stabilised
with respect to a change in the velocity distribution in the air
flow on the upstream side caused by, for example, a change in the
upstream conditions.
FIGS. 26 and 27 show the structure of a conventional air flow meter,
which is an object for comparison for showing the advantages of
the present invention. An edge 250a at the upstream end of a bridge
250 which is integral with a body 240 is formed by merely projecting
the edge from the upstream end in the form of a cylinder. The uppermost
stream portion of the edge 250a therefore constitutes an entrance
opening 251a. A sub-passage 251b which is parallel to a main passage
241 is eccentrically positioned with respect to the main passage
241 being offset toward a sensor circuit unit 242.
FIG. 28 shows the results of experiments with an air flow meter
which is disposed as in the embodiment of the system shown in FIG.
1 that is, on the downstream side of air flows such as those shown
in FIGS. 6 to 9. Experiments were carried out with the conventional
air flow meter shown in FIGS. 26 and 27 and the first embodiment
of the present invention shown in FIGS. 2 to 5. The abscissa in
FIG. 28 represents the mass flow rate (q/s) of the air which flows
in the air flow meter. Since the mass flow rate covers a wide range,
the abscissa is graduated in a logarithmic scale. The ordinate represents
the rate of output change (%) of the hot-wire meter. The output
varies in accordance with the differing shapes of air filter, the
intake duct and the error in the fixing state of the intake duct.
A combination of air filter, intake duct and fixing thereof in which
the variation of output is minimum was selected as the reference
(output variation is zero), and the change of the measured outputs
is represented as the rate of output change (%). As is clear from
the results of the experiments, the conventional air flow meter
represented by the broken line shows a flow rate variation of up
to 8%, while the structure of the present invention represented
by the solid line shows a flow rate variation within .+-.2%.
Thus, in the present invention, since the change in the flow on
the upstream side due to the variations of the intake pipe elements
is substantially cancelled and the backward flow to the sub-passage
due to back fire or blow back and the disturbance of the flow due
to the mixture at the exit portion of the sub-passage are prevented,
it is possible to measure the air flow accurately under various
conditions.
Having fully described the present invention, it will now be understood
that by providing a flow stabilising means to the entrance a sub-passage
in a hot-wire air flow meter, the measurement accuracy of amount
of intake air by a hot-wire element is enhanced, and by providing
means such as a radial sub-passage or a flexing member at the output
of the sub-passage the backward flow to the sub-passage due to back
fire or blow back and the disturbance of the flow due to the mixture
at the exit portion of the sub-passage is prevented, so that it
is possible to measure the air flow accurately under various conditions.
It is to be understood that various modifications may be made and
that all such modifications falling within the spirit and scope
of the appended claims are intended to be included in the present
invention. |