Abstrict An air flow meter for detecting the air flow supplied to an internal
combustion engine includes a hot-wire disposed in a by-pass air
passage opening at its one end to a portion of an air passage upstream
from a venturi portion formed in the air passage and at its other
end in the venturi portion. The by-pass air passage is provided
to lengthen a distance between the inlet opening of the by-pass
passage and the hot-wire, to thereby settle the flow of air in the
by-pass passage at least in the area around the hot-wire. The inlet
opening of the by-pass air passage may open in a plane perpendicular
to the axis of the venturi portion and within the area of upper
projection of the venturi portion, to prevent carbon and oil particles
from coming into the by-pass passage to thereby prevent the contamination
of the hot-wire.
Claims What is claimed is:
1. An air flow meter assembly for internal combustion engines,
the air flow meter assembly comprising:
an air cleaner means for intaking air supplied to said engine;
throttle valve means for controlling the air flow to be supplied
to said engine;
an air passage extending between said air cleaner means and said
throttle valve means and having a venturi portion;
a bypass air passage means for bypassing an air flow around the
venturi portion including a first by-pass air passage, a first end
of said by-pass air passage having an inlet opening disposed at
a portion of said air passage at a position upstream of said venturi
portion, a second end of said first by-pass air passage opening
into said venturi portion, and means for forming a second by-pass
air passage and having a first end attached to the first end of
said first by-pass passage;
means disposed in said first by-pass air passage for detecting
the air flow through said by-pass air passage means; and
wherein said second by-pass air passage has a predetermined length
such that a second end of said second by-pass passage is spaced
from said means for detecting the air flow such that the air flowing
into the by-pass air passage means is settled prior to the air flow
reaching said means for detecting the air flow, and said second
end of said second by-pass passage projects into said air passage.
2. An air flow meter assembly as claimed in claim 1 wherein said
means for forming said second by-pass passage is a cylindrical member,
the distance between an inlet opening of the first end of said second
by-pass passage and said means for detecting the air flow is at
least twice as large as a diameter of said by-pass air passage means.
3. An air flow meter assembly as claimed in claim 1 wherein said
first end of said first by-pass air passage terminates in a land
provided on said air passage.
4. An air flow meter assembly as claimed in claim 1 wherein said
first by-pass air passage and said second by-pass air passage have
identical cross-sections.
5. An air flow meter assembly as claimed in claim 1 wherein an
inlet opening of said second by-pass air passage is disposed within
an area of an upper projection of said venturi portion and is disposed
in a plane substantially parallel to a longitudinal venturi portion.
6. An air flow meter assembly as claimed in claim 5 wherein said
first by-pass air passage has a substantially L-shaped form.
7. An air flow meter assembly as claimed in claim 1 wherein an
inlet opening of said second by-pass air passage opens in a plane
substantially perpendicular to a longitudinal axis of said venturi
portion.
8. An air flow meter assembly as claimed in claim 1 wherein said
means for detecting air flow inluces a hot wire.
9. An air flow meter assembly as claimed in claim 1 wherein means
for settling the flow of air are disposed in said second by-pass
air passage.
10. An air flow meter assembly as claimed in claim 1 wherein said
first by-pass air passage extends substantially in parallel with
said air passage.
11. An air flow meter assembly as claimed in claim 1 wherein the
distance between an inlet opening of said first end of said second
by-pass air passage and said means for detecting the air flow is
between 5-10 times as large as a diameter of the by-pass air passage
means.
12. An air flow meter assembly for an internal combustion engine,
the air flow meter assembly comprising intake means for supplying
air to the engine, throttle valve means for controlling the air
supplied to the engine, an air passage extending between said intake
means and said throttle valve means, a venturi means arranged in
said air passage, a by-pass means for by-passing the air flow around
the venturi means, said by-pass means including a first end having
an inlet opening disposed at a portion of the air passage at a position
upstream of said venturi means and a second end opening into the
air passage at a position downstream of said venturi means, means
disposed in the by-pass means for detecting the air flow through
the by-pass means, and means mounted on the first end of said by-pass
means and projecting into said air passage for reducing a turbulence
of the air flow at the first end of said by-pass means such that
the air flow through the by-pass means is settled prior to reaching
said means for detecting the air flow.
13. An air flow meter assembly according to claim 12 wherein said
means for reducing a turbulence includes a hollow cylindrical member
mounted to the first end of said by-pass means, said cylindrical
member having a predetermined length such that a distance between
a second end of the cylindrical member and said means for detecting
the air flow is between 5-10 times a diameter of the by-pass passage
means.
14. An air flow meter assembly according to claim 13 wherein said
means for reducing a turbulence further includes an air flow settling
means mounted in said cylindrical member.
15. An air flow meter assembly according to claim 13 wheein the
second end of said cylindrical member is disposed in a plane substantially
parallel to a longitudinal axis of said venturi means.
16. An air flow meter assembly for internal combustion engines,
the air flow meter assembly comprising:
an air cleaner means for intaking air supplied to said engine;
a throttle valve means for controlling the air flow to be supplied
to said engine;
an air passage extending between said air cleaner means and said
throttle valve means and having a venturi portion;
a by-pass air passage means for by-passing an air flow around the
venturi portion including a first by-pass air passage, a first end
of said first by-pass air passage having an inlet opening disposed
at a portion of said air passage at a position upstream of said
venturi portion, a second end of said first by-pass air passage
opening into said venturi portion, and means for forming a second
by-pass air passage and having a first end communicating with the
first end of the first by-pass passage;
means disposed in said first by-pass air passage for detecting
the air flow through said by-pass air passage means; and
wherein said second by-pass air passage has a predetermined length
so as to project into said air passage with a second end of said
second by-pass passage being spaced from said means for detecting
the air flow such that the air flowing into the by-pass air passage
means is settled prior to the air flow reaching said means for detecting
the air flow.
17. An air flow meter assembly for internal combustion engines,
the air flow meter assembly comprising:
an air cleaner means for intaking air supplied to said engine;
throttle valve means for controlling the air flow to be supplied
to said engine;
an air passage extending between said air cleaner means and said
throttle valve means and having a venturi portion;
a by-pass air passage means for by-passing an air flow around the
venturi portion, a first end of said by-pass air passage having
an inlet opening disposed at a portion of said air passage at a
position upstream from said venturi portion, a second end of said
first by-pass air passage opening into said venturi portion, means
for forming a second by-pass air passage and having a first end
attached to the first end of said first by-pass passage;
means disposed in said first by-pass air passage for detecting
the air flow through said by-pass air passage means; and
wherein said second by-pass air passage has a predetermined length
so as to project into said air passage with a second end of the
second by-pass passage being spaced from means for detecting the
air flow such that the air flowing into the by-pass air passage
means is settled prior to the air flow reaching the means for detecting
the air flow, and said second end of said second by-pass passage
extends into a projected plane of said venturi portion.
Description BACKGROUND OF THE INVENTION
The present invention relates to an air flow meter assembly and,
more particularly, to a hot-wire air flow meter assembly suitable
for use in a fuel supplying device for internal combustion engines.
Various types of air flow meters for internal combustion engines
have been proposed and, for example, in Japanese Patent Laid Open
application No. 55-37555 (Mar. 15 1980), an air flow meter is proposed
having a hot-wire disposed in a by-pass air passage provided in
parallel with an air passage leading from an air cleaner and having
a venturi portion and a throttle valve through which the air is
supplied to the engine. An output signal from the hot-wire corresponds
to the total air flow supplied to the engine through the air passage,
and the output signal is delivered, for example, to a control unit
for calculation of an optimum air-fuel ratio of the mixture. A fuel
injector or a carburetor is controlled in accordance with the optimum
air-fuel ratio calculated in the control unit, so that the mixture
is fed to the engine at the optimized air-fuel ratio.
A prior-art air flow meter of the aforementioned type has a number
of disadvantages. Namely, the air flowing into the air passage through
the air cleaner generates vortexes to form complicated stream lines.
This turbulence of the air adversely affects the air flowing through
the by-pass air passage to cause a disturbance of the output signal
from the hot-wire, resulting in an incorrect indication of the air
flow.
Additionally, it is often experienced that contaminants such as
carbon particles, oil particles and so forth come into the by-pass
passage due to the phenomenon peculiar in engines such as back-firing.
These contaminants undesirably attach to the surface of the hot-wire
to deteriorate the sensitivity thereof.
SUMMARY OF THE INVENTION
Accordingly, the aim underlying the present invention essentially
resides in providing an air flow meter assembly capable of precisely
detecting the air flow, while eliminating the above-described disadvantages
of the prior art.
Namely, an object of the invention is to provide an air flow meter
assembly capable of correctly detecting the air flow without being
disturbed by the turbulence of the air flowing into the air passage.
Another object of the invention is to provide an air flow meter
assembly in which the contamination of the detecting means by the
carbon and oil particles is avoided to ensure a correct measurement
of the air flow.
To these ends, according to the invention, there is provided an
air flow meter assembly comprising: an air passage having a venturi;
a first by-pass air passage disposed in parallel with the air passage;
and a detecting means disposed in the first by-pass air passage
and adapted to generate an output signal corresponding to the air
flow passing through the first by-pass air passage, with a cylindrical
second air by-pass air passage being disposed at the upstream end
opening of the first by-pass air passage and adapted to settle the
flow of air in the first by-pass air passage.
According to the invention, a length of a portion of the by-pass
air passage upstream from the hot wire is selected to be at least
twice as large as a diameter of the first by-pass air passage.
According to further features of the invention, a cylindrical second
by-pass air passage is provided at the upstream end opening of the
first by-pass air passage within the area of upper projection of
the venturi portion of the air passage, the second by-pass air passage
opening along a plane normal to an axis of the venturi portion.
The above and other objects, as well as advantageous features of
the invention will become clear from the following description of
the preferred embodiments taken in conjunction with the acconpanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a hot-wire air flow meter assembly
constructed in accordance with the present invention;
FIG. 2 is a cross-sectional view of a modification of a pipe incorporated
in the hot-wire air flow meter assembly as shown in FIG. 1; and
FIG. 3A is a cross-sectional view a hot-wire air flow meter assembly
constructed in accordance with another embodiment of the present
invention; and
FIG. 3B is a top plan view of the hot-wire air flow meter assembly
of FIG. 3A.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings wherein like reference numerals are
used throughout the various views to designate like parts and, more
particularly, to FIG. 1 according to this figure, an air cleaner
means 1 is disposed on an air passage 2 provided with a venturi
portion 3 formed on the inner peripheral surface at a middle portion
thereof, with valves 4 5 and a fuel injector 6 being secured to
the lower portion of the air passage 2. Air drawn in through the
air cleaner means 1 flows through the venturi portion 3 of the air
passage 2 and is mixed with the fuel injected by the injector 6
to form an air-fuel mixture which is fed into the cylinders of an
internal combustion engine (not shown).
As is well known, the flow Q.sub.A of the air supplied to the engine
is detected and the value of which is delivered to a control unit
7 which determines and sets the optimum air-fuel ratio (A/F) of
the mixture supplied to the engine. The length of time of each injection
from the injector 6 is determined in accordance with the determined
optimum air-fuel ratio (A/F).
The throttle body including the air passage 2 having the venturi
portion 3 is formed by, for example, a die-casting from aluminum,
to have a first by-pass air passage 8 formed at a portion thereof
near the venturi portion 3. The first by-pass air passage 8 has
an inlet opening which opens to a land 10 formed at one portion
of the air horn 9 on the top of the air passage 2 and an outlet
opening communicated with an air passage 2 through a slit 11 formed
in the throat portion of the venturi portion 3. A pipe or second
by-pass air passage 12 made of, for example, a metallic material,
is secured to the upper portion of the first by-pass air passage
8. A heat-sensitive resistor body, i.e. a hot wire 13 is disposed
in the first by-pass air passage 8. The hot-wire 13 is adapted to
change its electric resistance in accordance with the change in
the flow of air flowing therearound.
Although not shown, the hot-wire 13 disposed in the first by-pass
air passage, is usually composed of two hot-wire elements which
constitute two of four sides of a Wheatstone bridge circuit.
The flow Q.sub.A of the air flowing in the air passage 2 of the
throttle body is usually determined by the venturi portion 3 and
the flow of the air in the first by-pass air passage 8 is changed
in proportion to the change in the flow Q.sub.A in the air passage
2. The hot wire 13 which changes its resistance in response to the
change in the flow Q.sub.A of air in the first by-pass passage 8
therefore, can generate an output signal proportional to the flow
of air supplied to the engine.
In general, in the air flow meter assembly having a by-pass air
passage 8 formed in parallell with an air passage 2 the air stream
at the upstream portion of the by-pass air passage 8 tends to be
disturbed by, for example, vortexes.
Various experiments having conducted to find out the following
fact to achieve the present invention. Namely, the unfavorable influence
of the turbulence of the air flow appearing in the upstream end
portion of the by-pass air passage can be reduced to a negligibly
small level by selecting the length l of the portion of the by-pass
air passage upstream from the hot-wire 13 to be at least twice,
preferably 5 to 10 times, as large as the diameter D of the by-pass
air passage. On the other hand, the length of the by-pass air passage,
particularly the length of the portion thereof upstream from the
hot-wire, is limited by the size of the throttle body or the construction
of the same. Therefore, it is extremely difficult to obtain the
desired length of that portion of the by-pass air passage.
In the embodiment described above a metallic cylindrical pipe or
second by-pass air passage 12 is attached to the upstream inlet
portion of the by-pass air passage 8 in order to attain the desired
length of the portion of the by-pass air passage upstream from the
hot-wire. The pipe 12 however, may be formed from a material other
than a metal and can have a cross-sectional shape other than circular.
It is, however, desirable that the pipe 12 has an inner bore of
a cross-section coinciding with the cross-section of the first by-pass
air passage 8 and, preferably, that the cross-sectional area of
the inner bore of the pipe 12 coincides with that of the first by-pass
air passage 8 in order that the air flow in the by-pass air passage
may not be disturbed.
Referring to FIG. 2 a honey-comb-like flow settling grid 14 is
provided at the upstream side end of the pipe 12 secured to the
upstream inlet opening of the first by-pass air passage 8. This
flow settling grid 14 effectively further settles the stream of
air flowing in the by-pass air passage.
As shown in FIGS. 3A and 3B, a hot wire air flow meter assembly
includes a throttle valve 15 mounted on a rotary shaft 16 disposed
at the downstream side of the air passage 2 and acts to control
the air flow supply to the engine (not shown). A fuel injector or
a carburetor of the same type as that shown in FIG. 1 secured to
the lower portion of the throttle body having the throttle valve
15.
As will be seen from FIGS. 3A, 3B, a pipe or second by-pass air
passage 20 bent at a predetermined curvature, is attached to the
upstream inlet portion of the first by-pass air passage 8. The pipe
20 is made of, for example, a metal, and has a flange member 21
attached thereto by means of welding or the like. The pipe 20 is
firmly fixed to the land 10 of the throttle body by means of screws
22. Furthermore, a supporting member 23 of, for example, a metal,
is secured to the same land portion by means of screws 24 to support
of the bent pipe 20.
As will be seen from FIGS. 3A, 3B, the bent pipe 20 has an inlet
opening in an area of the upper projection of the venturi portion
3 and is disposed in a plane substantially perpendicular to the
axis of the venturi portion 3.
In still another embodiment, the inlet opening of the pipe 20 opens
on a straight line substantially perpendicular to the axis of the
venturi portion 3.
Due to the bending of the pipe 20 carbon and oil particles having
comparatively large inertia can hardly be introduced into the pipe
20 while the air having comparatively small inertia is allowed
to easily come into the pipe 20. Consequently, the attaching of
carbon and oil particles to the hot-wire 13 attributable to the
back-fire and other phenomenon, can fairly be avoided to ensure
a correct measuring of the air flow. The change in the resistance
of the hot-wire 13 is detected by a detecting means 25 and is sent
to the control unit (not shown).
According to the invention, it is possible to stabilize the flow
of air in the by-pass air passage, particularly in the region around
the hot-wire to achieve a stable heat transfer between the hot-wire
and the air flowing therearound, so that the air flow can be detected
at a high precision. Additionally, it is possible to eliminate unfavorable
effects of carbon and oil particles on the measurement of the air
flow. |