Abstrict An air flow meter including an air flow rate measuring tube located
in a main air passageway and having an air inlet portion and an
air outlet portion of a bypass passageway formed therein. The air
inlet portion is located substantially in a central portion of the
cross-sectional surface of the main air passageway, and a hot wire
type air flow sensor is mounted in the bypass passageway. The air
flow meter is suitable for use with an internal combustion engine
of an automotive vehicle.
Claims What is claimed is:
1. An air flow meter in an internal combustion engine, the air
flow meter comprising:
a main passageway having an air inlet portion;
an air flow rate measuring tube mounted in said main passageway
and extending over a predetermined axial length thereof;
a bypass passageway formed in the air flow rate measuring tube
and including an air inlet portion located substantially in a central
portion of a cross-sectional area of said main passageway and an
outlet portion disposed downstream of the air inlet portion of said
main passageway, said bypass passageway including at least one substantially
straight portion having a constant cross-sectional area, and at
least one further portion extending substantially at a right angle
to said substantially straight portion, said bypass passageway having
an axial length greater than said predetermined axial length of
said air flow rate measuring tube;
a hot wire type air flow sensor mounted in the at least one substantially
straight portion of the bypass passageway;
a control means including a control circuit means for controlling
an operation of said hot wire type air flow sensor;
wherein said hot wire type air flow sensor and said control means
are formed as an integral unit forming a sensor assembly insertable
in said air flow rate measuring tube, said sensor assembly in said
air flow rate measuring tube and said air flow rate measuring tube
form a unit insertable in said air inlet portion, and
wherein the air flow rate measuring tube including the bypass passageway
and the hot wire type air flow sensor is exposed to the main air
flow so that a temperature of the air in the bypass passageway becomes
substantially equal to a temperature of the air in the main passageway.
2. An air flow meter as claimed in claim 1 wherein said hot wire
type air flow sensor comprises wound wire type probes.
3. An air flow meter as claimed in claim 1 or 2 wherein said air
flow rate measuring tube is located transversely of said main passageway.
4. An air flow meter as claimed in claim 3 wherein said bypass
passageway comprises a first passageway portion contiguous with
said air inlet portion and extending lengthwise of said main passageway,
and a second passageway portion located substantially at right angles
to said first passageway portion and extending lengthwise of said
air flow rate measuring tube, said second passageway portion being
contiguous at opposite ends thereof with openings of said air outlet
portion located at opposite sides of the air flow rate measuring
tube.
5. An air flow meter as claimed in claim 1 or 2 wherein said air
flow rate measuring tube is located lengthwise of said main passageway.
6. An air flow meter according to claim 1 wherein an outlet of
the air flow measuring tube is substantially perpendicular to a
flow direction of the main passageway.
7. An air flow meter according to claim 4 wherein at least for
symmetrically disposed outlet means are provided in said second
passageway for equalizing a difference in pressure.
Description BACKGROUND OF THE INVENTION
This invention relates to air flow meters of a hot wire type having
a bypass passageway, and more particularly to an air flow meter
of the hot wire type suitable for use in measuring the flow rate
of air drawn by suction into an internal combustion engine of an
automotive vehicle.
A hot wire type air flow meter for measuring the flow rate of air
drawn by suction into an internal combustion engine of an automotive
vehicle is disclosed in SAE Technical Paper Series 800468 for example.
This type of air flow meter comprises bare hot wires suspended in
a passageway for a flow of air drawn by suction into the internal
combustion engine, to obtain measurements of air flow. Although
the bare hot wires offer the advantage of being prompt in response,
they suffer the disadvantage that they are liable to be damaged
because of the backfire in the air passageway.
To obviate this disadvantage, proposals have been made, for example,
in Japanese Patent Laid-Open No. 16259/82 corresponding to DE-OS
No. 3019544 to use a hot wire type air flow meter comprising a
venturi located in a main passageway for a flow of air drawn by
suction into the internal combustion chamber, a bypass passageway
connected to the main passageway and allowing a portion of the air
flowing through the main passageway to be diverted and flow therethrough
as a bypass flow by utilizing the pressure differential between
a portion of the main passageway upstream of the venturi and a portion
of the main passageway in the vicinity of the narrowest portion
of the ventury, and a hot wire type air flow sensor located in the
bypass passageway to monitor the bypass flow of air so as to determine
the flow rate of all the air drawn by suction into the internal
combustion chamber based on the flow velocity sensed by the air
flow sensor. The use of the hot wire type air flow meter provided
with a bypass passageway has raised the problem that, when the air
flow to be measured is a pulsating flow, the hot wires are delayed
in response and cause the measurements obtained to become smaller
than the actual mean air flow rate. To solve this problem, a proposal
has been made, in Japanese Patent Laid-Open No. 135916/83 corresponding
to U.S. Ser. No. 461556 to increase the distance between an inlet
portion and an outlet portion of the bypass passageway as compared
with the distance obtained by measuring the distance in an axial
direction along the main passageway, to compensate for the error
in measurements. In this hot wire type air flow meter, a flow of
air introduced into the air flow meter is split into two air flows
at the inlet portion of the bypass passageway or a junction, so
that one air flow proceeds through the main passageway and the other
air flow through the bypass passageway until the two air flows join
at the outlet portion of the bypass passageway or a confluence and
flow downwardly out of the air flow meter. The flow velocity of
the air flow through the bypass passageway is measured by a hot
wire probe and a temperature probe, and the relation between air
velocity signals produced by the air flow sensor which senses the
air flow velocity through the bypass passageway and the flow rate
of all the air flowing through the air flow meter is set beforehand,
to allow the flow rate of all the air to be determined based on
the flow velocity signals produced by the air flow sensor located
in the bypass passageway. In order to ensure that the determination
of the flow rate of all the air flowing through the air flow meter
is achieved with a high degree of accuracy and precision, the ratio
of the air flow through the bypass passageway to the air flow through
the main passageway should remain constant at all times, even if
the flow rate of all the air flowing through the air flow meter
undergoes fluctuations.
However, this hot wire type air flow meter is not without a disadvantage.
When the wall of the air flow meter is heated, the flow rate of
air flow through the bypass passageway becomes lower than the flow
rate of air flow through the main passageway, with the result that
the air in the bypass passageway becomes higher in temperature than
the air in the main passageway. As a result, the air in the bypass
passageway is expanded and the resistance offered by the passageway
to the air flow increases, causing the flow rate of air through
the bypass passageway to become relatively lower than the flow rate
of air through the main passageway. Thus, the hot wire type air
flow meter of the aforesaid construction suffers the disadvantage
that heating or cooling the wall of the air flow meter causes a
drop in the accuracy of the value of an air flow determined by the
air flow meter.
In the hot wire type air flow meter of the aforesaid construction,
a portion of the air flow through the main passageway which is located
near the wall of the main passageway is introduced into the bypass
passageway and sensed by the air flow sensor. Generally, a portion
of an air flow through an air passageway which is located near the
wall of the air passageway tends to become more turbulent than a
portion flowing through the center of the air passageway. Thus,
signals produced by the hot wire type air flow sensor that monitors
the air flow through the bypass passageway which is constituted
by the portion of the air flow through the main passageway which
is high in turbulence would have a high noise to signal ratio (N/S
ratio).
SUMMARY OF THE INVENTION
This invention has been developed for the purpose of avoiding the
aforesaid disadvantages of the prior art. Accordingly, the invention
has as its object the provision of a hot wire type air flow meter
capable of achieving a high degree of accuracy and precision in
obtaining measurements of an air flow rate without being influenced
by the heat in the air passageway and producing signals of low N/S
ratio by an air flow sensor.
To accomplish the aforesaid object, the air flow meter according
to the invention comprises an air flow rate measuring tube located
in a main air passageway, the air flow rate measuring tube having
an air inlet and air outlet and serving as a bypass passageway,
and a hot wire type air flow sensor located in the air flow rate
measuring tube, the air flow sensor being located substantially
in a central portion of the cross-sectional surface of the main
air passageway to monitor the air velocity of an air flow through
the bypass passageway.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of the air flow meter comprising a first
embodiment of the invention;
FIG. 2 is a sectional view taken along the line II--II in FIG.
1;
FIG. 3 is a sectional view taken along the line III--III in FIG.
1;
FIG. 4 is a sectional view taken along the line IV--IV in FIG.
2;
FIG. 5 is a graphical illustration of the relationship between
the air flow rate and the error resulting in measuring the air flow
rate established when the walls of the air flow meter according
to the invention shown in FIGS. 1-4 and an air flow meter of the
prior art were heated;
FIG. 6 is a graphical illustration of the relationship between
the air flow rate and the noise to signal ratio (N/S ratio) in the
air flow meter according to the invention shown in FIGS. 1-4 and
an air flow meter of the prior art;
FIG. 7 is a vertical sectional view of the air flow meter comprising
a second embodiment of the invention;
FIG. 8 is a plan view of the air flow meter comprising a third
embodiment of the invention;
FIG. 9 is a sectional view taken along the line IX--IX in FIG.
8;
FIG. 10 is a sectional view taken along the line X--X in FIG. 9;
FIG. 11 is a sectional view taken along the line XI--XI in FIG.
9;
FIG. 12 is a plan view of the air flow meter comprising a fourth
embodiment of the invention;
FIG. 13 is a sectional view taken along the line XIII--XIII in
FIG. 12;
FIG. 14 is a sectional view taken along the line XIV--XIV in FIG.
13;
FIG. 15 is a plan view of the air flow meter according to the invention
shown in FIGS. 1-4 shown in combination with a fuel injection valve;
FIG. 16 is a sectional view taken along the line XVI--XVI in FIG.
15;
FIG. 17 is a view similar to that of FIG. 16 but showing the ring
in a position farther remote from the flow rate measuring tube than
in FIG. 16;
FIG. 18 is a plan view of the air flow meter comprising a fifth
embodiment of the invention;
FIG. 19 is a sectional view taken along the line XIX--XIX in FIG.
18;
FIG. 20 is a sectional view taken along the line XX--XX in FIG.
18;
FIG. 21 is a plan view of the air flow meter comprising a sixth
embodiment of the invention;
FIG. 22 is a sectional view taken along the line XXII'XXII in FIG.
21;
FIG. 23 is a sectional view taken along the line XXIII--XXIII in
FIG. 22;
FIG. 24 is a plan view of the air flow meter comprising a seventh
embodiment of the invention;
FIG. 25 is a sectional view taken along the line XXV--XXV in FIG.
24;
FIG. 26 is a sectional view taken along the line XXVI--XXVI in
FIG. 24;
FIG. 27 is a vertical sectional view of the air flow meter comprising
an eighth embodiment of the invention;
FIG. 28 is a sectional view taken along the line XXVIII--XXVIII
in FIG. 27;
FIG. 29 is a vertical sectional view of the air flow meter comprising
a modification of the embodiment shown in FIGS. 27 and 28;
FIG. 30 is a sectional view taken along the line XXX--XXX in FIG.
29;
FIG. 31 is a vertical sectional view of a modification of the third
embodiment shown in FIGS. 8-11;
FIG. 32 is a sectional view taken along the line XXXII--XXXII in
FIG. 31; and
FIG. 33 is a bottom plan view of the modification of the embodiment
shown in FIGS. 12-14.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A first embodiment of the invention will be described by referring
to FIGS. 1-6.
Referring to FIGS. 1 and 2 an air flow meter 1 comprises a suction
conduit 8 defining a main passageway 4 therein, a venturi 9 formed
on an inner periphery of the suction conduit 8 and an air flow
rate measuring tube 10 located at the throat of the venturi 9 substantially
at right angles to the direction in which air flows in the main
passageway 4. The air flow rate measuring tube 10 is fitted at one
end thereof located in the suction conduit 8 in a recess 11 formed
at one portion of the throat of the venturi 9 and secured at an
opposite end thereof to an outer periphery of the suction conduit
8 by screws 13 through a flange 12A of a control section 12 including
a control circuit. A junction 2 which serves as an air inlet of
a bypass passageway 3 is formed in the flow rate measuring tube
10 in a position located substantially on the center axis of the
main passageway 4 or substantially in a central portion of the cross-sectional
surface of the main passageway 4 and maintained in communication
with a confluence 5 which serves as an air outlet of the bypass
passageway formed in the flowrate measuring tube 10. The bypass
passageway 3 formed in the air flow rate measuring tube 10 comprises
a first or sensor mounting passageway portion 14 contiguous with
the junction 2 and extending substantially along the center axis
of the main passageway 4 a second passageway portion 15 extending
substantially at right angles to the sensor mounting passageway
section 14 a third passageway portion 16 extending substantially
at right angles to the second passageway portion 15 and a fourth
passageway portion 17 extending substantially at right angles to
the third passageway portion 16 and contiguous with the confluence
5 which, as shown in FIGS. 2 and 3 is located at one side of the
flow rate measuring tube 10. A hot wire probe 6 and a temperature
probe 7 both of wound wire type, are mounted in the sensor mounting
passageway portion 14 with the hot wire probe 6 temperature probe
7 and control section 12 forming an integral unit enabling a sensor
assembly to be inserted into the air flow rate measuring tube 10
which, in turn, is inserted as a unit into the main passageway 4.
The bypass passageway 3 of the aforesaid construction has a length
which is greater than the length of the main passageway 4 corresponding
to the distance between the inlet or junction 2 of the bypass passageway
3 and the outlet or confluence 5 thereof as measured along the center
axis of the main passageway 4. FIGS. 3 and 4 show the relative positions
of the junction 2 bypass passageway 3 and confluence 5 in sectional
views along the lines III--III and IV--IV, respectively, in FIG.
2.
Operation of the first embodiment of the invention of the aforesaid
construction will be described. A portion of an air flow introduced
into the air flow meter 1 is led through the junction 2 into the
bypass passageway 3 as indicated by an arrow and released through
the confluence 5 into the main passageway 4 where it joins the air
flow therein. The air flow led into the bypass passageway 3 has
its flow velocity measured by the hot wire probe 6. Since the ratio
of the air flow rate through the bypass passage 3 to the air flow
rate through the main passageway 4 is substantially constant, it
is possible to obtain the flow rate of air through the main passageway
4 based on a signal produced by the hot wire probe 6 by establishing
beforehand the relationship between the air flow rate through the
main passageway 4 and the signal produced by the hot wire probe
6.
As shown in FIGS. 1 and 2 the flow rate measuring tube 10 defining
the bypass passageway 3 therein is exposed to the air flow through
the main passageway 4. Thus, even if the suction conduit 8 is heated
or cooled from outside, the wall of the air flow rate measuring
tube 10 has a temperature which is kept substantially at the same
level as the temperature of the air flowing through the main passageway
4. This makes it possible to keep the ratio of the flow rate of
air flowing through the bypass passageway 3 to the flow rate of
air flowing through the main passageway 4 substantially constant
regardless of whether or not the suction conduit 8 is heated, thereby
eliminating the risk that the measurements might have errors.
FIG. 5 shows the results of tests conducted on errors in measurements
which might result due to variations in air flow rate when air temperature
is 20.degree. C. and the temperature of the wall of the suction
conduit 8 is 70.degree. C. In an air flow meter of the prior art,
the temperature of air flowing through the bypass passageway 3 rises
and the air expands, particularly in the low air flow rate range,
when the wall of the suction conduit 8 is heated. The expansion
of air causes a relative reduction to occur in the flow rate of
air through the bypass passageway 3 with the result that the signal
produced by the hot wire probe 6 located in the bypass passage 3
has a value which is 5-10% lower than the actual value of the air
flow rate. Meanwhile, in the air flow meter 1 according to the invention,
the error described hereinabove can be eliminated because the air
flow through the bypass passageway 3 shows substantially no rise
in temperature.
FIG. 6 is a graph showing the results of tests conducted on the
relation between the N/X ratio of the signal produced by the hot
wire probe 6 and the air flow rate through the bypass passage 3
in the air flow meter according to the invention and an air flow
meter of the prior art. In the air flow meter 1 according to the
invention, the air flowing into the bypass passageway 3 comes from
a central portion of the air flow through the main passageway 4
which is relatively free from turbulence. Meanwhile, in an air flow
meter of the prior art, the junction at the main passageway serving
as an inlet of the bypass passageway 3 is located at the outer peripheral
portion of the main passageway 4. Therefore, the air flowing into
the bypass passageway 3 comes from a portion of the air flow through
the main passageway 4 located mere the wall thereof which is relatively
high in turbulence. The results of tests shown in FIG. 6 indicate
that the N/S ratio of the signal produced by the hot wire probe
of the air flow meter of the prior art is substantially three times
as high as that of the signal produced by the hot wire probe 6 of
the air flow meter 1 according to the invention. This is one of
the outstanding characteristics of the invention.
The air flow meter 1 according to the invention has a tolerance
which is greatly reduced to enable a mass production of the air
flow meter to be achieved. More specifically, the suction conduit
8 is formed at one portion of its inner peripheral surface with
the venturi 9 which has its throat machined with a high degree of
precision, and the flow rate measuring tube 10 has its outer peripheral
surface machined with a high degree of precision, and fitted to
the throat of the venturi 9. By this structural arrangement, variations
in pressure differential between the junction 2 and confluence 5
of the bypass passageway 3 caused by the air flow through the main
passageway 4 can be minimized. To reduce variations in the flow
rate of air through the bypass passageway 3 which would be caused
by this pressure differential, a portion of the bypass passageway
3 has only to be narrowed and machined with a high degree of precision,
so that the resistance offered to the flow of air through the bypass
passage 3 would be concentrated in this narrowed portion.
Owing to the influence exerted by free convection, the performance
of a hot wire air flow sensor is reduced in a low speed range below
0.5 m/s. The resistance offered by the main passageway 4 to the
air flow in the air flow meter 1 should be reduced to obtain a high
performance of the engine at full-open throttle, making it necessary
to increase the cross-sectional area of the main passageway 4. As
a result, the air flow velocity at the junction 2 or inlet of the
bypass passageway 3 drops and the air flow velocity in the vicinity
of the hot wire probe 6 tends to drop. Thus, to reduce variations
in the air flow velocity through the bypass passageway 3 and raise
the air flow velocity in the vicinity of the hot wire probe 6 the
cross-sectional area of the sensor mounting passageway portion 14
of the bypass passageway 3 has only to be reduced to less than 80%
of the cross-sectional area of the passageway portions 15 16 and
17 of the bypass passageway 3. This allows the air flow rate through
the bypass passageway 3 to be determined by the resistance offered
to the air flow by the sensor mounting passageway portion 14 which
can be readily machined with a high degree of precision. In addition,
since the sensor mounting passageway portion 14 is narrower than
the passageway portions 15 16 and 17 it is possible to give a
highest air flow velocity to the air flowing through the first or
sensor mounting passageway portion 14. In the embodiment of FIG.
2 air flow meter 101 comprises a suction conduit 108 of substantially
cylindrical configuration, and an air flow rate measuring tube 10
substantially similar in construction to the air flow rate measuring
tube shown in FIG. 2 which is inserted in the suction conduit 108.
In FIG. 7 the main passageway 104 defined in the suction tube
108 has its transverse cross-sectional area reduced by fitting
the air flow rate measuring tube 10 in the suction conduit 108
to thereby accelerate the air flow. Thus, a negative or subatmospheric
pressure is produced at the confluence 5 or the outlet of the bypass
passageway 3 and the dynamic pressure of an air flow led into the
air flow meter 101 acts on the junction 2 or inlet of the bypass
passageway 3. As a result, the flow velocity of air flowing in the
bypass passageway 3 is substantially at the same level as the flow
rate of air flowing through the bypass passageway 3 of the embodiment
shown in FIGS. 1 and 2.
In the embodiment of FIGS. 8-11 the invention of this embodiment
which air flow meter 201 of this embodiment which has the same suction
conduit 8 as shown in FIG. 2 and comprises an air flow rate measuring
tube 210 formed therein with a bypass passageway 203 extending from
a junction 202 serving as an air inlet through a first passageway
portion 214 located substantially lengthwise of the suction conduit
8 a second passageway portion 215 located substantially at right
angles to the first passageway portion 214 and a third passageway
portion 216 located substantially at right angles to the second
passageway portion 215 to a confluence 205 serving as an air outlet
whcih opens at one side of the flow rate measuring tube 210. A hot
wire probe 206 and a temperature probe 207 are mounted in the second
passageway portion 215 and third passageway portion 216 respectively.
The first passageway portion 214 is formed with a recess 218 at
its bottom, so that an air flow led into the bypass passageway 203
through the junction 202 alters its direction substantially 90 degrees
at the end of the first passageway portion 214 and enters the second
and third passageway portions 215 and 216 successively where the
hot wire and temperature probes 206 and 207 are located, before
being released into the main passageway 204 through the confluence
205. In the air flow meter of FIGS. 8-11 the air flow alters its
direction upstream of the hot wire probe 206 so that dust in the
air flow is separated by inertia and collects in the recess 218
at the bottom of the first passageway portion 214. Since the dust
is prevented from impinging on the hot wire probe 206 the dust
is prevented from depositing on the hot wire probe 206 thereby
reducing a signal drift of the air flow sensor which might otherwise
be caused to occur by the dust deposited on the surface of the hot
wire probe 206.
In FIGS. 12-14 an air flow meter 301 is provided which has the
same suction conduit 8 as shown in FIG. 2 comprises an air flow
measuring tube 301 formed therein with a bypass passageway 303 extending
from a junction 302 serving as an air inlet through a first passageway
portion 314 located lengthwise of the suction conduit 8 and a second
passageway portion 315 located substantially at right angles to
the first passageway portion 314 and lengthwise of the measuring
tube 310 to a confluence 305 serving as an air outlet which opens
at either end portion of the second passageway portion 315 at either
side of the flow rate measuring tube 310. A hot wire probe 316 and
a temperature probe 317 are mounted in the first passageway portion
314. In FIGS. 12-14 an air flow led into the bypass passageway
303 at the junction 302 proceeds through the sensor mounting first
passageway portion 314 and alters its direction 90 degrees when
it enters the second passageway portion 315 where the air flow splits
into two branches flowing in opposite directions and released into
a main passageway defined by the suction conduit 8 through the confluence
5 having four openings. Thus, the bypass passageway 314 is relatively
elongated and can be readily formed in the air flow rate measuring
tube 301.
FIGS. 15-17 show single-point fuel-injection chambers 19 and 19A
wherein the air flow meter 1 according to the invention shown in
FIGS. 1 and 2 is combined with a fuel-injection valve 20. The chambers
19 and 29A each have the fuel-injection valve 20 and a throttle
valve 21 located downstream of the air flow rate measuring tube
10. To promote atomization of fuel fed through the fuel-injection
valve 20 a piezoelectric element 22 a horn 23 and a ring 24 as
disclosed in Japanese Patent Laid-Open No. 162972/84 for example,
are provided. The single-point fuel-injection chamber 19 shown in
FIG. 16 has a restricted height H, so that the fuel-injection valve
20 is directed horizontally. Generally, when a fuel-injection valve
is directed horizontally, fuel fed through the fuel-injection valve
tends to impinge on an opposing wall surface to be deposited thereon.
To avoid this disadvantage, means for atomizing fuel by vibration
comprising the piezoelectric element 22 is provided to prevent the
fuel fed through the fuel-injection valve 20 from inpinging on a
wall surface of the chamber 19. When the clearance between the ring
24 for atomizing the deposited fuel by vibration and the flow rate
measuring tube 10 is small as indicated by T.sub.1 in FIG. 16 the
air flow is blocked by the air flow rate measuring tube 10 so that
the fuel fed through the fuel-injection valve 20 impinges on the
ring 24 and is atomized by vibration, when the clearance between
the ring 24 and air flow rate measuring tube 10 is great as indicated
by T.sub.2 in FIG. 17 the atomized particles of fuel fed through
the fuel-injection valve 20 are forced to flow downwardly by the
action of an air flow as indicated by broken lines without impinging
on the ring 24. Thus, it is necessary that the clearance T between
the ring 24 for atomizing the fuel fed through the fuel-injection
valve 20 and the air flow rate measuring tube 10 be small.
FIGS. 18-20 show a fifth embodiment of the invention in which an
air flow rate measuring tube 410 is located lengthwise of the main
passageway 4 substantially in a central portion of the bore of the
suction conduit 8 and formed with a bypass passageway 403 extending
axially of the tube 410 substantially in a central portion thereof.
A hot wire probe 406 and a temperature probe 407 are mounted in
the bypass passageway 403.
The air flow rate measuring tube 410 has a junction 402 serving
as an air inlet located subatantially on the center axis of the
main passageway 4 and the bypass passageway 403 starting at the
junction 402 has connected to its downstream end a roundabout passageway
408 which extends lengthwise of the tube 410 along the bypass passageway
403 so that an air flow led into the bypass passageway 403 from
the main passageway 4 through the junction 402 alters its direction
180 degrees at the end of the bypass passageway 403 to flow along
the roundabout passageway 408 before being released into the main
passageway 4 through a confluence 405 serving as an air outlet formed
at either side of the tube 410. A hot wire probe 406 and a temperature
probe 407 are mounted in the bypass passageway 403.
The air flow rate measuring tube 410 is secured at one side thereof
through a holder 420 to a control section 412 secured to the wall
of the suction conduit 8 and has, at an opposite side, a stay 421
threadably connected to the wall of the suction conduit 8 as indicated
at 422. Air flowing into the air flow meter 401 from above splits
into two air flows at the junction 402 serving as the air inlet
of the bypass passageway 403. One air flow proceeds through the
main passageway 4 and the other air flow is led into the bypass
passageway 403. The air flow led into the bypass passageway 403
is led into the roundabout passageway 408 after passing by the hot
wire probe 406 and temperature probe 407 and released through the
confluence 405 serving as the air outlet which opens at the throat
of the venturi 9 into the main passageway 4 so that the two air
flows join at the confluence 405. In this constructional form, the
ratio of the length of the bypass passageway 403 to the length of
a portion of the main passageway 4 located between the junction
402 and confluence 405 of the air flow rate measuring tube 410 (such
length is indicated by L in FIG. 19) can be set at any value as
desired by selecting a suitable value for the length of the roundabout
passageway 408. Thus, the ratio can be increased three to five times,
thereby minimizing errors which might occur due to a delay in the
response of the hot wire when the air flow through the bypass passageway
403 is a pulsating flow. The arrangement whereby the air flow rate
measuring tube 410 is supported in the air flow in the main passageway
4 by the stay 421 enables the temperature of the air flow rate measuring
tube 410 to remain substantially at the same level as the temperature
of the air flowing through the main passageway 4 even if the wall
of the suction conduit 8 is heated or cooled. Thus, the air flowing
through the bypass passageway 403 has the same temperature as the
air flowing through the main passageway 4 at all times, thereby
eliminating the risk that errors might occur in the measurements
made by the hot wire probe 406. The air flow led into the bypass
passageway 403 through the junction 402 comes from a central portion
of the air flow introduced into the suction conduit 8 so that the
air flowing through the bypass passageway 403 is substantially free
from turbulence and signals produced by the hot wire probe 406 are
low in N/S ratio.
FIGS. 21-23 show a sixth embodiment of the invention, wherein the
air flow meter 501 comprises an air flow rate measuring tube 510
supported in the suction conduit 8 by a holder 520 and a stay 521
similar to the corresponding parts shown in FIG. 19. The air flow
rate measuring tube 510 is formed therein with a bypass passageway
503 and a roundabout passageway 509 which are constructed such that
a passageway portion 504 of the bypass passageway 503 is surrounded
by the roundabout passageway 509 substantially through a circumferential
extent of 360 degrees. Thus, air led through a junction 502 serving
as an air inlet of the bypass passageway 503 flows through the passageway
portion 504 extending lengthwise of the air flow rate measuring
tube 510 and enters the roundabout passageway 509 through an inlet
509A thereof. The air flows through the roundabout passageway 509
substantially through a circumferential extent of 360 degrees about
the bypass passageway 503 before being released into the main passageway
4 through a confluence 505 serving as an air outlet of the bypass
passageway 503 which opens in the throat of the venturi 9. By forming
the roundabout passageway 509 in annular form, it is possible to
reduce the length L.sub.1 of the air flow rate measuring tube 510
as compared with the air flow rate measuring tube 410 shown in FIG.
19. Other parts of the embodiment shown in FIG. 22 are similar to
the corresponding parts of the embodiment shown in FIG. 2. The reference
numerals 506 and 507 designate a hot wire probe and a temperature
probe, respectively, and the reference numeral 512 a control section
of the air flow rate measuring tube 510.
FIGS. 24-26 show a modification of the embodiment shown in FIGS.
18-20 which is provided with a deflector 609 at an inlet of a bypass
passageway 603 for deflecting an air flow to avoid deposition of
dust on a hot wire probe 606 which might cause a reduction in the
accuracy of the air velocity determined by the probe 606.
The air flow meter 601 shown comprises an air flow rate measuring
tube 610 which, like the corresponding part of the embodiment shown
in FIG. 19 is secured at one side thereof to a control section
612 by a holder 620 and has at an opposite side a stay 621 threadably
connected to the wall of the suction conduit 8 as indicated at 622.
A bypass passageway 603 formed in the air flow rate measuring tube
610 of the air flow meter 601 of this embodiment which is substantially
similar to the corresponding part shown in FIG. 19 except for its
inlet portion 602 is contiguous with a roundabout passageway 608
and has a confluence 605 serving as an air outlet. A hot wire probe
606 and a temperature probe 607 are mounted in the bypass passageway
603.
The embodiment of FIGS. 24-26 is characterized by the deflector
609 located at the junction 602 serving as an air inlet of the bypass
passageway 603 for deflecting an air flow to avoid deposition of
dust on the hot wire probe 606. The provision of the deflector 609
causes air to flow into the bypass passageway 603 from a direction
substantially at right angles to the direction of an air flow through
the main passageway, so that the major portion of dust in the air
introduced into the air flow meter 601 flows by inertia through
the main passageway and is prevented from entering the bypass passageway
603. Thus, deposition of dust on the probes 606 and 607 for determining
the air flow velocity in the bypass passageway 603 is prevented.
FIGS. 27 and 28 show an eighth embodiment of the invention, which
comprises the air flow rate measuring tube 510 shown in FIGS. 21-23
and a single-point fuel-injection valve 720 having a microcomputer.
Parts shown in FIGS. 27 and 28 which are similar to those shown
in FIG. 22 are designated by like reference characters. The fuel-injection
valve 720 is supported in the suction conduit 8 by a stay 721 and
has an upper portion connected to a lower end of the air flow rate
measuring tube 510. The stay 521 for supporting the air flow rate
measuring tube 510 and the stay 721 for supporting the fuel-injection
valve 510 are parallel to each other and extend in the same direction.
The two stays 512 and 712 are secured to a wall portion 722 of the
suction conduit 8 defining a throttle chamber. By this structural
arrangement, resistance offered to an air flow through the air flow
rate measuring tube 510 and fuel-injection valve 720 can be reduced
and the height of the throttle chamber can be reduced. In FIGS.
27 and 28 the reference numerals 723 724 and 725 designate a throttle
valve, a warm water heating passageway for vaporizing the fuel released
through the fuel-injection valve 720 and a heat-insulating material
layer 725 respectively. The heat-insulating material layer 725
performs the function of preventing the fuel-injection valve 720
and throttle chamber wall 722 from being heated by the warm water.
FIGS. 29 and 30 show a modification of the embodiment shown in
FIGS. 27 and 28 in which a roundabout passageway 709 contiguous
with a bypass passageway 703 is located along an outer periphery
of the fuel-injection valve 720 to decrease the height of a wall
portion 720 defining a throttle chamber. The arrangement of FIGS.
29 30 offers the advantage that, besides being able to decrease
the height of the throttle chamber, the outer diameter D.sub.2 of
the portion of the fuel-injection valve 720 whose outer peripheral
defines the roundabout passageway 709 is greater than the outer
diameter D.sub.1 of the air flow rate measuring tube 710 thereby
eliminating the need to provide the venturi 9 located in the main
passageway shown in FIG. 27.
As described hereinabove, the air flow rate measuring tube 710
can be kept substantially at the same temperature as air flowing
therethrough, even if the suction conduit 8 is heated. This enables
the air flow rate measuring tube 710 to be formed of synthetic resinous
material which has high formability. When the air flow rate measuring
tube 710 is formed of synthetic resinous material, the provision
of a sheath formed of conductive material to the periphery of probes
706 and 707 in the bypass passageway 703 can achieve the effect
of reducing the noise level.
The reference numerals 711 and 713 designate an outlet and a stay,
respectively. Parts shown in FIGS. 29 and 30 which are similar to
those shown in FIGS. 27 and 28 are designated by like reference
characters.
FIGS. 31-33 show a modification of the embodiment shown in FIG.
7 wherein parts similar to those shown in FIG. 7 are designated
by like reference characters. As shown, an air flow meter 101A comprises
an air flow rate measuring tube 10A formed with a bypass passageway
3A comprising a longitudinal first passageway portion 14A located
substantially in a central portion of a main passageway 104 and
having a junction 2 serving as an air inlet of the bypass passageway
3A, and a radial second passageway portion 15A located substantially
at right angles to the longitudinal first passageway portion 14A.
The radial second passageway portion 15A has in the vicinity of
either end thereof a confluence 5A, 5A serving as an air outlet
of the bypass passageway 3A which faces down stream or lengthwise
of the main passageway 104.
In the embodiment shown in FIG. 7 the confluence 5 serving as
an outlet of the bypass passageway 3 has openings located at opposite
sides of the air flow rate measuring tube 10 and each facing radially
of the main passageway 104. Thus, the air flowed through the bypass
passageway 3 and released through the confluence 5 into the main
passageway 104 impinges substantially at right angle or obliquely
to air flowing through the main passageway 104 thereby forcing
the air flow in the main passageway 104 toward the central portion
of the suction conduit 8. This means that the air flow released
from the bypass passageway 3 into the main passageway 104 is an
obstacle to the air flow through the main passageway 4 and reduces
the flow area of the main passageway 4.
In the modification shown in FIGS. 31-33 air flowing into the
bypass passageway 3A through the junction 2 first enters the longitudinal
first passageway portion 14A where air flow velocity is measured
by the hot wire probe 6 and temperature probe 7. The air flow is
divided at a junction 16A into two air flows flowing in opposite
directions through the radial second passageway portions 15A, 15A,
before being released through the confluence 5A into the main passageway
104. The confluence 5A has openings which face downstream of the
air flow through the main passageway 104 so that the air released
into the main passageway 104 through the confluence 5A flow in currents
parallel to the air flow through the main passageway 104 thereby
offering no great resistance to the flow of air through the main
passageway. This offers the advantage of minimizing the resistance
offered to the flow of air through the main passageway. |