Abstrict A detecting module for an air flow meter for internal combustion
engines has an arm provided with a heat wire sensor at one end thereof
and with a signal processing unit at the other end, and is provided
with a predetermined constant length. A body member assembled with
the detecting module is selected from among several varieties of
body members produced for engines with different stroke volumes,
each of which has an main air path formed by an axial bore having
a diameter commensurate with the stroke volume. A bridging member
formed across the axial bore which forms the main air path is provided
therein with a by-pass path for by-passing a part of the airflow
within the main air path and a communicating hollow portion for
accommodating the arm. The body member is further provided with
a mount for fixing the detecting module, thereon, the height of
which is different in each variety of the body members, so that
the heat wire sensor is always placed in a predetermined metering
area within the by-pass path, irrespective of the diameter of the
axial bore forming the main air path, when the arm is accommodated
in the hollow portion.
Claims We claim:
1. A method of manufacturing an air flow meter for an internal
combustion engine comprising the steps of:
producing a single type of detecting module having a metering element
to be exposed to an intake airflow of the engine, a signal processing
unit, electrically coupled to the metering element, for processing
signals produced by the metering element to generate an output signal
in response to the flow rate of the intake airflow, and an arm provided
with the metering element at one end thereof and with the signal
processing unit at the other end, through which electric connecting
means is provided for connecting said metering element to said signal
processing unit;
producing plural varieties of body members for engines with the
different stroke volumes, each of which body members comprises an
axial bore defining an air path through which the intake airflow
flows, the respective varieties of body members having axial bores
of respectively different bore diameter in accordance with different
stroke volumes of the engines, and a transverse bore extending from
a hole in the wall of said body member and communicating with said
axial bore;
selecting a body member having an axial bore with a bore diameter
commensurate with the stroke volume of engine in which the air flow
meter is to be used; and
assembling the detecting module with the selected body member,
by securing said detecting module into said transverse bore in such
a way that, considering the length of the arm of the detecting module
and the diameter of the axial bore of the selected body member,
the metering element is exposed to the air flowing in a predetermined
metering area within the air path, irrespective of the variety of
the selected body member.
2. A method according to claim 1 wherein said different varieties
of body members are produced so as to have a mount for said detecting
module formed integrally with an outer side wall of the body member,
the height of the mount formed on each variety of body member being
determined in accordance with the diameter of said axial bore forming
the air path in the body member and the length of the arm.
3. A method according to claim 1 wherein said step of assembling
includes inserting a spacer between said signal processing unit
and said body member, which spacer has a thickness which is determined
for each variety of body member in accordance with the diameter
of said axial bore forming the air path in the body member and the
length of the arm.
4. A method according to claim 1 wherein the predetermined metering
area is an area which has a diameter smaller than D/2 and is centered
in the air path, in which D is the diameter of said axial bore forming
the air path.
5. A method according to claim 1 wherein the predetermined metering
area is an area which has a diameter smaller than D/3 and is centered
in the air path, in which D is the diameter of said axial bore forming
the air path.
6. A method according to claim 1 wherein said body member has
a transverse member projecting across the air path at almost a right
angle to the direction of the airflow to be metered, which transverse
member is formed with a by-pass path for by-passing part of airflow
within the air path and a hollow portion for accommodating the arm
inserted through the hole formed in the wall of said body member;
wherein the one end of the arm is placed in a predetermined position
within the by-pass path, when the arm is accommodated in the hollow
portion; and wherein the by-pass path includes a portion lying substantially
parallel to the arm accommodated in the hollow portion, the method
further comprising the step of:
determining the length of said parallel portion of the by-pass
path for each variety of body member so that the metering element
carried on the one end of the arm is placed in said predetermined
position within the by-pass path to accommodate the diameter of
the axial bore forming the air path of the selected body member
and the fixed length of the arm.
7. A method according to claim 1 wherein said body member has
a transverse member projecting across the air path at almost a right
angle to the direction of the airflow to be metered, which transverse
member is formed with a by-pass path for by-passing part of airflow
within the air path and a hollow portion for accommodating the arm
inserted through the hole formed in the wall of said body member;
wherein the one end of the arm is placed in a predetermined position
within the by-pass path, when the arm is accommodated in the hollow
portion; and wherein the by-pass path is formed at least in part
by a pipe which is connected to a part of the projecting member
and includes a portion bent so as to be substantially parallel to
the arm accommodated in the hollow portion of said projecting member,
the method further including the step of:
determining the length of said bent portion of said pipe for each
variety of body member so that the metering element carried on the
one end of the arm is placed in said predetermined position within
the by-pass path to accommodate the diameter of the axial bore forming
the air path of the selected body member and the fixed length of
the arm.
8. A method according to claim 1 wherein said body member has
a transverse member projecting across the air path at almost a right
angle to the direction of the airflow to be metered, which transverse
member is formed with a by-pass path for by-passing part of airflow
within the air path and a hollow portion for accommodating the arm
inserted through the hole formed in the wall of said body member;
wherein the one end of the arm is placed in a predetermined position
within the by-pass path, when the arm is accommodated in the hollow
portion; and wherein an inlet of the by-pass path is formed in a
shape of a race track, the length of which is included in the predetermined
metering area, the method further comprising the step of:
locating the by-pass path within said inlet at a position such
that the metering element carried on the one end of the arm is placed
in said predetermined position to accommodate the diameter of the
axial bore forming the air path of the selected body member and
the fixed length of the arm.
9. A method for manufacturing an air flow meter for an internal
combustion engine comprising the steps of:
producing plural varieties of body members, each variety having
a different size axial bore defined in the body member and through
which intake air flows;
selecting one from among the plural varieties of body members having
a bore commensurate with the stroke volume of the engine for which
the air flow meter is used;
producing a detecting module having a metering element exposed
to intake airflow, a signal processing unit electrically coupled
to the metering element for processing a signal from the metering
element to generate an output signal in response to the flow rate
of the intake airflow, and an arm having a constant length which
is longer than Dmax/4 and shorter than 3Dmax/4 wherein Dmax is
the largest one of the bores of the plural varieties of body members,
which is provided with the metering element at one end thereof and
with the signal processing unit at the other end, and through which
electric connecting means is provided; and
assembling the detecting module with the selected body member,
including interposing an appropriate attaching member between the
selected body member and the detecting module in accordance with
the length of the arm, whereby one end of the arm is positioned
in a predetermined metering area within the air path of the selected
body member, irrespective of the bore of the selected body member,
the metering area being determined by both the largest one and the
smallest one among bores of air paths of the plural varieties of
said body members.
10. A method of manufacturing an air flow meter for an internal
combustion engine comprising the steps of:
producing plural varieties of body members, each having a different
size bore defined in the body member and through which intake air
flows;
selecting one from among the plural varieties of body members having
a bore commensurate with the stroke volume of the engine for which
the air flow meter is used;
producing a detecting module having a metering element exposed
to intake airflow, a signal processing unit electrically coupled
to the metering element for processing a signal from the metering
element to generate an output signal in response to the flow rate
of the intake airflow, and an arm having a constant length which
is longer than Dmax/4 and shorter than 3Dmin/4 wherein Dmax and
Dmin are the largest one and the smallest one of the bores of the
plural varieties of body members, respectively, which is provided
with the metering element at one end thereof and with the signal
processing unit at the other end, and through which electric connecting
means is provided; and
assembling the detecting module with the selected body member,
including interposing an appropriate attaching member between the
selected body member and the detecting module in accordance with
the predetermined length of the arm, whereby one end of the arm
is positioned in a predetermined metering area within the air path
of the selected body member, irrespective of the bore of the selected
body member, the metering area being determined by both the largest
one and the smallest one among bores of air paths of the plural
varieties of body members.
Description BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to the improvement of an air flow
meter for internal combustion engines and a manufacturing method
thereof, and particularly to an improved air flow meter having a
structural configuration which is suited for manufacture with a
reduced manufacturing cost.
2. Description of the related art
As is well known, an internal combustion engine is equipped with
an air flow meter for metering the amount of intake air supplied
to the engine with a view toward attaining exhaust gas purification,
economic fuel consumption and so on. Such an air flow meter is usually
located in an appropriate portion of the suction system of the engine
between the air cleaner and the throttle valve.
As a typical example, an air flow meter of this type is disclosed,
for example, in the Nishimura et al U.S. Pat. No. 4709581 which
is assigned to the same assignee as the present application. The
Nishimura et al air flow meter comprises a body member, in which
an air path having a certain bore is defined, and a metering element,
such as a known heat wire sensor, which is located in an appropriate
portion within the defined air path. The body member is, as mentioned
above, equipped between an air cleaner and a throttle valve, so
that the metering element meters the amount of intake air sucked
into the internal combustion engine.
It is also known that an air flow meter is generally designed to
provide the most suitable metering range of air flow rate from the
point of view of metering accuracy, as well as the reliability and
durability, of the air flow meter; i.e., the lower limit of the
metering range is determined based on the required metering accuracy
and the upper limit thereof depends on the expected reliability
and durability of the flow meter.
On the other hand, the required amount of intake air to be supplied
to an internal combustion engine differs to a large extent, depending
on the stroke volume of the engine. If, therefore, the bore of an
air path of an air flow meter is always constant irrespective of
the stroke volume of the engine, the flow rate of the intake air
flowing near the metering element of the flow meter will vary widely
in accordance with the stroke volume of the engine. The change in
the flow rate of the intake air may consequently exceed the lower
or the upper limit of the aforesaid suitable metering range.
To prevent this, there have been produced several varieties of
air flow meters for engines having different stroke volumes. In
such air flow meters, an air path defined in the body member has
a different bore in accordance with the stroke volume of the engine,
to which the air flow meter is to be equipped, whereby the change
in the flow rate of intake air flowing near the metering element
is established within a suitable range as mentioned above. By way
of example, the assignee of the present application has manufactured
three or four varieties of air flow meters having bores between
60 mm and 80 mm for engines having a stroke volume of from one to
five liters.
In such a case, however, a number of kinds of parts are required
for assembling several varieties of air flow meters, so that the
following inconvenience results. Firstly, the manufacturing cost
of parts for air flow meters cannot be reduced sufficiently, because
many kinds of parts to be used in relatively small amounts must
be prepared. As a result, the total cost of manufacture of such
an air flow meter could not be reduced sufficiently.
Further, there is another problem in the manufacturing process.
Namely, since may kinds of parts are required for several varieties
of air flow meters, assemblers are forced to pay careful attention
to the correct selection of parts to be fitted for each type of
meter and the proper assembly thereof. Consequently, there the good
possibility to cause manufacturing errors as a result of false selection
of parts and improper assembly of parts.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improved air
flow meter having a structural configuration suited for manufacture
with a reduced manufacturing cost and a manufacturing method thereof.
An air flow meter for an internal combustion engine has a detecting
module with a metering element exposed to an intake airflow of the
engine, a signal processing unit electrically coupled to the metering
element for processing signals produced by the metering element
to generate an output signal in response to the flow rate of the
intake airflow, and an arm provided with the metering element at
one end thereof and with the signal processing unit at the other
end, through which electric connecting means. In accordance with
the invention, the air flow meter further includes a body member,
in which an air flow path for the intake air flow is defined, and
which is assembled with the detecting module by inserting the arm
thereof in a hole formed in a wall of the body member, whereby the
one end of the arm is exposed to the airflow within the air path.
There is further provided an attaching member interposed between
the detecting module and the body member. The improvement according
to the present invention resides in the fact that the arm of the
detecting module has a predetermined constant length, and the body
member is selected from among plural varieties of body members designed
for internal combustion engines having different stroke volumes,
wherein the length of the arm is determined so that the metering
element is exposed to the air flowing in a predetermined metering
area within the air path, irrespective of the selected body member,
when the detecting module is assembled with the selected body member.
In the manufacture of the air flow meter according to the present
invention, only one variety of detecting module is required for
every variety of air flow meter. Accordingly, the mass production
of detecting modules becomes possible, and therefore a detecting
module can be manufactured at the low cost, which contributes to
the reduction of the manufacturing cost of the air flow meter as
a whole. Further, even when any variety of air flow meter is assembled,
assemblers are not required to pay special attention to selection
of an appropriate detecting module, which can reduce the possibility
of mistake in assembling a detecting module and a body member.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a, 1b to 6a, 6b show sectional views of an air flow meter
according to several embodiments of the present invention, in which
the figures with suffix a show sectional views along the line A--A
in the figures with suffix b, and vice versa;
FIGS. 7a and 7b show sectional views of an air flow meter according
to still another embodiment of the present invention;
FIG. 8a is a chart showing the distribution, obtained by computer
simulation, of partial flow rates within an air path just downstream
of a curved passage in a bent element shown in front elevation and
side section in FIG. 8b;
FIG. 9a is a chart showing the distribution, obtained by computer
simulation, of partial flow rates within an air path just downstream
of another type of a curved passage in a right-angled elbow shown
in front elevation and side section in FIG. 9b;
FIGS. 10a to 10c are diagrams for explaining the assumption and
the result of the computer-simulation for simulating the state of
variation of flow rates in an air path, which is just downstream
of the bent element as shown in FIG. 8b and the right-angled elbow
as shown in FIG. 9b; and
FIG. 11 is a diagram for explaining the principle for determining
a suitable metering area within an air path.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following, an airflow meter according to a first embodiment
of the present invention will be described, referring to FIGS. 1a
and 1b showing sectional views thereof.
As shown in the figures, main air path 3 with the bore D.sub.S
is defined in body member 1 made by die-casting of aluminum. This
body member 1 is equipped in an appropriate portion of a suction
system of an internal combustion engine between an air cleaner and
a throttle valve, so that intake air flows through the main air
path 3. Bridging member 5 which extends across the bore of the
main air path 3 is also die-casted integrally with the body member
1.
In the bridging member 5 there is formed a path 7 for by-passing
part of the air flowing through the main air path 3. Air, which
enters into the by-pass path 7 from its inlet 9 is branched into
two flows on its way and is exhausted from two outlets 11a, 11b
into the main air path 3 again. The outlets 11a, 11b are provided
with a bottom plate 15 which prevents the blow-back of the engine
from undesirably influencing the airflow within the by-pass path
7. In the bridging member 5 there is also formed a hollow portion
17 which communicates with the outside of the body member 1 through
a hole provided in a wall of the body member 1. A part of a detecting
module described below is inserted into the hollow portion 17 through
the hole in the body wall.
The detecting module is composed of arm 19 metering element 21
attached on one end of the arm 19 and signal processing unit 23
attached on the other end of the arm 19. In FIG. 1a, the metering
element 21 is shown as an element of the heat wire type, but the
present invention has no limitation as to the type of metering element
to be used. A metering element of such a known type in which a thin-film
resistor is formed on a ceramic base plate can also be used. The
signal processing unit 23 includes therein a circuit for processing
signals from the metering element 21 to generate an output signal
in response to the flow rate of airflow.
The arm 19 of the detecting module has such a length La that when
the arm 19 is accommodated in the hollow portion 17 of the body
member 1 the metering element 21 is placed at a predetermined metering
point within the by-pass path 7 (in FIG. 1b, the metering element
21 is not shown for simplification of the drawing).
La in FIG. 1a is indicated as the length of the arm 19 including
the metering element 21. However, since the metering element 21
is considerably small in size, compared with the arm 19 there is
no substantial difference, whether La is represented either as the
length of the arm including the metering element or as the arm's
own length. In any event, the length of the arm itself can be determined
on the basis the required length La and a known size of the metering
element 21. More importantly, it is to be noted that, as will become
apparent from the following explanation, the most significant aspect
of the present invention is that the length La of the arm 19 is
always constant, irrespective of the diameter of the bore D.sub.S
of the main air path 3.
On the other hand, the body member 1 is provided with an attaching
mount 25 on an outer wall thereof. The height H.sub.1 of the mount
25 which is measured from the inner wall of the main air path 3
is determined in accordance with the diameter of the bore D.sub.S
of the main air path 3 whereby the distance between an attaching
surface of the amount 25 and the predetermined metering point within
the by-pass path 7 can be almost equal to the length La of the arm
19. Namely, while there are manufactured several varieties of body
members, having the different bore diameters, for engines with different
stroke volumes, each variety of body member has the height of the
attaching mount thereof adjusted to make the distance between the
attaching surface of the mount and a predetermined metering point
within by-pass path always equal to the length La of the arm of
the detecting module.
FIGS. 2a and 2b shows an example of an air flow meter for engines
having a larger stroke volume. In those figures, corresponding parts
are indicated by the same reference numerals as in FIGS. 1a and
1b. The body member 1 of this example has a main air path 3 the
bore D.sub.L of which is larger than the bore D.sub.S of the air
flow meter shown in FIGS. 1a and 1b. Also in this case, however,
the distance between the attaching surface of the mount 25 and the
predetermined metering point within the by-pass path 7 is made equal
to the length La of the arm 19 of the detecting module. To this
end, the height H.sub.2 of the mount 25 which is measured from
the inner wall of the main air path 3 is smaller than the height
H.sub.1 in the case of FIGS. 1a and 1b. In this manner, the same
detecting module can be used with various varieties of air flow
meters for engines having different stroke volumes.
In the following, an actual example of the length La of an arm
and the bore D.sub.S or D.sub.S of a main air path of a body member
will be given. As already described, the assignee of the present
application manufactures three or four varieties of air flow meters
for engines having stroke volumes from one to five liters. Body
members of those air flow meters have a main air path, the bore
of which is from 60 mm to 80 mm. The length La of the arm of a common
detecting module is 50 mm.
If, therefore, such a detecting module is assembled with body members
having a main air path with a bore of 60 mm and the body members
are provided with an attaching mount which is 70 mm high, the end
of the arm of the detecting module, at which a metering element
is provided, will be placed in almost the center of the main air
path. Therefore, the by-pass path should be aligned with the center
of the main air path. Further, if the detecting module is assembled
with body members having a main air path with a bore of 70 mm or
80 mm, the body members should be provided with an attaching mount
of 15 mm or 10 mm high, respectively, whereby a metering element
end of the arm will be placed in almost the center of the main air
path of the respective body members.
The function of an attaching mount as mentioned above can also
be attained by a spacer interposed between an attaching mount and
a casing of a signal processing unit. FIGS. 3a and 3b show sectional
views of an air flow meter according to the present invention, in
which such a spacer is used. In those figures, the same reference
numerals denote corresponding parts in the respective figures.
In FIGS. 3a and 3b, spacer 27 is interposed between the mount 25
and the signal processing unit 23. In this case, the mount 25 is
sufficient to provide a flat surface for fixing the spacer 27 thereon,
and the distance between the attaching surface of the spacer 27
and a predetermined metering point within the by-pass path 7 can
be adjusted by the thickness Ts of the spacer 27 so as to become
equal to the length La of the arm 19. FIGS. 3a and 3b show the case
where an air flow meter has a main air path with the bore D.sub.S,
however the same affect as mentioned above is provided in the case
where an air flow meter has a main air path with a bore D.sub.L,
which is larger than D.sub.S. Namely, the thickness Ts of the spacer
27 is determined in accordance with the bore of the main air path
3.
Further, it will be easily understood that the adjustment of the
distance between an attaching surface of a detecting module and
a predetermined metering point within a by-pass path can be effected
by the combination of both the height of the attaching mount and
the thickness of the spacer.
Moreover, the following will be understood from the aforesaid explanation
concerning the length La of the arm 19. Namely, in the embodiments
as shown in FIGS. 1a, 1b to 3a, 3b, the length La is conveniently
determined on the basis of the maximum one of the bores of body
members provided for various varieties of air flow meters, because
if La is too long for a body member with a certain bore, the adjustment
may be arbitrarily effected by selecting the height of the attaching
mount or the thickness of the spacer.
FIGS. 4a and 4b are sectional views of an air flow meter according
to still another embodiment of the present invention. As apparent
from the figures, the by-pass path 7 in this embodiment is bent
in the bridging member 5 to have a horizontal portion 29 more generally,
a portion almost parallel to the arm 19 accommodated in the hollow
portion 17. With provision of this horizontal portion 29 the length
of the arm 19 of the detecting module can be reduced to La', which
is smaller than La as shown in the previous figures, while keeping
the position of the inlet 9 of the by-pass path 7 at almost the
center of the main air path 3.
Also in this embodiment, if the length Lh of the horizontal portion
29 in the by-pass path 7 is determined in accordance with the bore
of the main air path 3 a detecting module having an arm 19 the
length La' of which is always constant irrespective of the bore
of the main air path 3 can be assembled with every variety of the
body member 1.
A modification of the embodiment of FIGS. 4a, 4b is shown in FIGS.
5a and 5b, in which the inlet side of the by-pass path 7 is formed
by a curved pipe 31. This curved pipe 31 is inserted into a predetermined
hole provided in the bridging member 5. This curved pipe 31 has
a horizontal offset of a length Lh, so that the arm 19 in this modification
can also be reduced to a length La', which is smaller than La. According
to this modification, the manufacture of the body member 1 becomes
simple, because the by-pass path does not have such a complicated
configuration as in FIGS. 4a and 4b.
FIGS. 6a and 6b show sectional views of an air flow meter according
to a further embodiment of the present invention, in which the inlet
9 of the by-pass path 7 has an opening 32 formed in an elliptical
shape or in the shape of race track. The ellipse-shaped opening
32 has a width W in the direction of its long axis. The width W
is determined to be within an area as described later in detail.
In this embodiment, the inlet 9 of the by-pass path 7 is put within
the width W of the ellipse-shaped opening 32 whereby stable airflow
suited for metering can be taken into the by-pass path 7. In addition,
the distance between an attaching surface of the mount 25 and the
metering point within the by-pass path 7 can be made equal to the
length La of the arm 19 of the detecting module by locating the
inlet 9 of the by-pass path 7 at an appropriate position within
the width W of the ellipse-shaped opening 32 in accordance with
the bore of the main air path 3 of the body member 1. With this
expedient, the same effect as in the structure shown in FIGS. 4a,
4b and 5a, 5b is attainable.
In the embodiments as shown in FIGS. 4a, 4b to 6a, 6b, the length
La' (or La) of the arm 19 is conveniently determined on the basis
of the minimum one of the bores of body members designed for various
varieties of air flow meters, contrary to those as shown in FIGS.
1a, 1b, 2a, 2b, 3a and 3b. In this case, the adaptive positioning
of the metering element 21 can be effected by the length Lh of the
horizontal portion of the by-pass path 7.
In all the embodiments as described above, a by-pass path, in which
there exists a metering point, is formed within a main air path
in order to obtain a stable airflow suitable for metering. If, however,
airflow within a main air path is very stable or there is found
an area of stable airflow at a particular location within a main
air path, a by-pass path is not always necessary. FIGS. 7a and 7b
show sectional views of an air flow meter in such a case.
As apparent from those figures, in this embodiment, the arm 19
of the detecting module has a small hole 33 at one end thereof,
which opens toward both the upstream and the downstream directions
of airflow within the main air path 3. In the hole 33 there is
provided the metering element 21. Since the length of the hole 33
is considerably short in the direction of airflow, the hole 33 does
not have the effect of rectifying unstable airflow to the extent
that the by-pass path 7 has. Therefore, it can be said that the
hole 33 is provided simply to protect the metering element 21 from
mechanical damage. Since the arm 19 of the detecting module is inserted
within the main air path 3 through a hole provided in a wall of
the body member 1 the metering element 21 attached at one end of
the arm 19 is directly exposed to the airflow within the main air
path 3.
In the following, the behavior of the air flowing through the main
air path will be discussed to find an area of stable airflow as
mentioned above.
As is well known, there are usually included several curved portions
in a suction pipe communicating with the main air path of an air
flow meter from an air cleaner, because the suction pipe must be
arranged in the engine compartment of an automobile, which compartment
is not always wide sufficiently. It is also known that there occurs
a disturbance in the airflow which passes through a curved portion,
i.e. the flow velocity is varies over the section of a pipe just
downstream of a curved portion.
FIGS. 8a and 9a are charts representing the distributions of partial
flow velocities in an airflow just downstream of a curved portion,
which were obtained by computer simulation conducted by the inventors.
FIG. 8a shows the distribution of partial flow velocities in airflow
which has passed a bent element, as shown in FIG. 8b, and FIG. 9a
shows the same in airflow which has passed through a right-angled
elbow, as shown in FIG. 9b. In both the cases, the flow rate of
air flowing through the pipe is 20 g/sec. Numerals surrounded by
an iso-velocity line or lines represent the following flow velocity
ranges in terms of m/sec.
In FIG. 8a:
______________________________________ 1; 4.4-4.2 2; 4.2-4.0
3; 4.0-3.8 4; 3.8-3.6 5; 3.6-3.4 6; 3.4-3.2 7; 3.2-3.0 8; 3.0-2.8
9; 2.8-2.6 10; 2.6-2.4. ______________________________________
In FIG.9a:
______________________________________ 1; 5.0-4.8 2; 4.8-4.4
3; 4.4-4.0 4; 4.0-3.6 5; 3.6-3.2 6; 3.2-2.8 7; 2.8-2.4 8; 2.4-2.0
9; 2.0-1.6 10; 1.6-1.2 11; 1.2-0.8 12; 0.8-0.4 13; 0.4-0.2
14; 0.2-0. ______________________________________
As apparent from those figures, the iso-velocity lines are dense
in the inside of a curved portion (the upper side of the figures)
and they are sparse at the outside thereof (the lower side of the
figures). This means that the steep slope in the flow velocity change
occurs in the inside of the curved portion, and on the other hand,
the gentle slope occurs in the outside. Further, it can be understood
that the neighborhood of the center axis of the airflow has a relatively
stable flow velocity distribution.
A pipe of a suction system in an internal combustion engine usually
includes plural curved pipe elements, such as bent elements or right-angled
elbows as shown in FIGS. 8b and 9b, however a pipe element provided
just upstream an air flow meter has the most significant influence
on the metering of the airflow by the air flow meter. In the case,
therefore, where the layout of pipes of a suction system is not
known in advance or an air flow meter must be used for engines having
wide varieties of pipe layouts in a suction system, the air flow
meter is preferably designed to be capable of metering the flow
rate in the neighborhood of the center axis of the airflow within
a main air path thereof. In the following, the determination of
the range of the metering area will be discussed on the basis of
the computer simulation conducted by the inventors.
The simulation was conducted, taking account the following assumption.
As shown in FIG. 10a, two kinds of inlets of a by-pass path, which
are formed within a main air path denoted by solid circle 91 were
assumed. Namely, one (M) of the inlets has an opening formed in
a circular shape, as shown in FIGS. 1a, 1b to 5a, 5b, and the other
inlet (N) has an opening formed in a shape of race track and includes
the center C of the main air path 91 as shown in FIGS. 6a, 6b.
However, in the inlets M, N of different shapes, the centers thereof
were assumed to be both located at the point O, which is the center
of the inlet M and the center of the upper half circle of the inlet
N. Further, the inlets M, N are rotated with the center C of the
main air path centered, as shown by circle 93 of broken line. The
distance r of the center O of the inlets M, N from the center C
of the main air path, i.e. the radius r of the rotation circle 93
is also changed.
Under the assumption as mentioned above, the flow velocities u
of the airflow, which will be taken into the by-pass path through
the respective inlets M, N, are simulated. It will be understood
from the charts of FIGS. 8a and 9a that the flow velocities u vary,
as the center O of the inlets M, N and the rotation radius r change.
Then, there were calculated the rate of variation of the flow velocities
at every location of the center O of the inlets M, N, which was
compared with the flow velocity at a reference position of the center
O of the inlets M, N. In this case, the reference point was set
on the line connecting the center C of the main air path and the
point P, which is the top of the inside of the curved portion. With
the thus obtained variation rates with respect to every point on
the rotation circle 93 with the radius r, there was calculated the
deviation .lambda.r between the maximum variation rate and the minimum
one. The foregoing is represented by the following formula; ##EQU1##
wherein u.sub..theta.r represents an average flow velocity in the
inlet of the by-pass path, when the inlet is rotated with the radius
r and displaced by the angle a from the reference position (0<.theta..ltoreq.360.degree.),
and u.sub.Or represents the same, when the inlet is located at the
reference position.
FIGS. 10b and 10c show the results of the simulation as mentioned
above. FIG. 10b is the graph showing the result of the simulation
conducted for the bent element as shown in FIG. 8b, and FIG. 10c
is the same for the right-angled elbow as shown in FIG. 9b. Further,
in both the figures, the broken line M indicates the simulation
results for the inlet M with the circular shaped opening and the
solid line N the simulation results for the inlet N with the race-track
shaped opening. Abscissas in those figures are marked out by both
the scale r measured from the center C of the main air path and
the scale 1 measured from the top point P of the inside of the curved
portion, both being represented in terms of the ratio to the bore
D of the main air path.
In a conventional air flow meter, the upper limit of the allowable
variation rate is about 20%. If the same allowable range of the
variation rate is accepted in this case, it can be said from the
graphs of FIGS. 10b and 10c that the distance r between the center
O of the inlet of the by-pass path and the center C of the main
air path can be D/4 at maximum, although there is an exception (cf.
the broken line M in FIG. 10c). This means that the suitable metering
area becomes a circular area of the radius D/4 (D/2 in its diameter)
with the center C centered.
If a more stable flow velocity is required, for example, the variation
rate is required to be smaller than 15%, the suitable metering area
becomes a circular area of radius D/6 (D/3 in its diameter) with
the center C centered. In that case, if the opening of the inlet
is formed in a shape of race track, the variation rate is reduced
to as small as about 8%. This means that a very stable airflow can
be obtained.
From the foregoing, the determination of the metering area will
be concluded as follows, which will be explained with reference
to FIG. 11. In the figure, the same reference numerals denote corresponding
parts as in the previous figures, and also the same symbols have
the same meaning.
In FIG. 11 broken lines E, F and G indicate the inner surfaces
of the main air paths with three varieties of bores, respectively,
wherein the broken line E indicates the inner surface of the main
air path of the maximum bore Dmax, the broken line G indicates the
same for the main air path of the minimum bore Dmin, and the broken
line F indicates the same for the main air path of a bore between
the two above. These bores Dmax and Dmin depend on the varieties
of air flow meters manufactured, which correspond to the bores of
80 mm and 60 mm in the air flow meters manufactured by the assignee
of the present application, as already described.
Further, chain lines e, f and g in the figure indicate areas, each
of which has the diameter of one third of the bore of the respective
main air paths and in which the centers of the respective main air
paths are centered for the corresponding areas. As already described.,
it is of course possible to determine these areas e, f, and g so
as to make the diameter thereof equal to one half of the bore of
the respective main air paths. When the bore of the main air path
is E, F or G, a stable airflow can be obtained in the thus determined
areas e, f and g, respectively. Therefore, the areas e,f and g are
called suitable metering areas.
In FIG. 11 therefore, the area U, which is determined by the area
e (its upper side in the figure) for the maximum bore Dmax and the
area g (its lower side in the figure) for the minimum bore Dmin,
can provide a stable airflow for every size of bore of the main
air path. Inversely, if the end of the arm 19 at which the metering
element is provided, is located in the aforesaid area U, a stable
metering of the flow rate can be always realized. Based on this
fact, the length La of the arm 19 which is available for every
variety of the body member 1 can be determined.
As apparent from FIG. 11 in which suitable metering areas are
assumed to be determined in accordance with the aforesaid one-third
rule, since the upper side of the area e measured from the point
P is represented by Dmax/3 and the lower side of the area g measured
from the point P by 2Dmin/3 the appropriate length La of the arm
19 is represented as follows:
Further, if suitable metering areas are determined in accordance
with the aforesaid one-half rule, the same is represented as follows:
The length La of the arm 19 in the embodiment shown in FIGS. 7a
and 7b can be determined as above. However, it is to be noted that
this principle can also be applied to the embodiments as shown in
FIGS. 1a, 1b to 5a, 5b. In the cases of those embodiments, the position
of the inlet 9 of the by-pass path 7 should be determined in accordance
with the above principle. Also in the embodiment of FIGS. 6a and
6b, the width W of the opening 32 of the by-pass path 7 can be determined
using the same principle.
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