Abstrict An air-flow meter wherein a bypass air passage for bypassing a
main air passage is provided in a throttle chamber of an internal
combustion engine, and a hot wire is arranged within the bypass
air passage, with the amount of air flow being measured on the basis
of a change in the resistance of the hot wire. A portion for diverting
the direction of air streams is disposed in an upstream part of
the bypass air passage so as to assign a dust removing function,
and a diameter of the hot wire is selected so that dust particles
flowing to the vicinity of the hot wire do not adhere to the hot
wire. A surface of the hot wire is coated with glass, and a temperature
of the surface of the hot wire is maintained at or above about 100.degree.
C. to enhance the prevention of dust particles adhering to the hot
wire.
Claims What we claim is:
1. An air-flow meter for an internal combustion engine, the air
flow meter comprising:
a main air passage means for enabling an air flow therethrough,
said main air passage means including a Venturi portion means;
a bypass air passage means for bypassing said main air passage,
said bypass air passage means including an inlet means and an outlet
means, said inlet means opening at a position upstream of the Venturi
portion, said outlet means opening at a position downstream the
Venturi portion, said bypass air passage means further including
at least one bent portion therein for removing dust particles in
the air flowing therethrough;
at least one hot wire means disposed in the bypass passage means
at a position downstream of said bent portion, said hot wire being
adapted to change resistance in dependence upon an amount of air
flow in said bypass passage means;
means for detecting the change of resistance of said hot wire means
and for measuring the amount of air flow;
means for maintaining a surface temperature of said hot wire at
100.degree. C. or above; and
wherein said bent portion has an entrance width D, said hot wire
means has a diameter d, and wherein a ratio of d/D is such that
collision of dust in the air flowing through said bypass passage
means against said hot wire means is substantially prevented.
2. An air-flow meter according to claim 1 wherein said ratio of
d/D is greater than 0.02.
3. An air-flow meter according to one of claims 1 or 2 wherein
said inlet means of said bypass passage means opens in parallel
to a direction of air flow in said main air passage means.
4. An air-flow meter according to one of claims 1 or 2 wherein
said inlet means of said bypass passage means open perpendicularly
to a direction of air flow in said main air passage means.
5. An air-flow meter according to claim 4 wherein said hot wire
means comprises a ceramic cylinder, a resistance wire would around
said ceramic cylinder, and a glass coating surrounding said resistance
wire.
6. An air flow meter for an internal combustion engine, the air
flow meter comprising a main air passage means for enabling an air
flow therethrough, said main air passage means including a venturi
portion means; a bypass air passage means for bypassing said main
air passage, said bypass air passage means including an inlet means
and an outlet means, said inlet means opening at a position upstream
of the venturi portion, said outlet means opening at a position
downstream of the venturi portion, said bypass air passage means
further including at least one bent portion therein for removing
dust particles in air flowing therethrough; at least one hot wire
means disposed in the bypass passage means at a position downstream
of the bent portion, said hot wire means being adapted to change
resistance in dependence upon an amount of air flow in said bypass
passage means, said hot wire means comprises a ceramic cylinder,
a resistance wire would around said ceramic cylinder, and a glass
coating surrounding said resistance wire; means for detecting a
change of resistance of said hot wire means and for measuring the
amount of air flow; and means for maintaining a surface temperature
of said hot wire means at 100.degree. C. or above.
7. An air-flow meter according to claim 6 wherein said inlet means
of said bypass passage means opens in parallel to a direction of
air flow in said main air passage means.
8. An air-flow meter according to claim 6 wherein said inlet means
of said bypass passage means open perpendicularly to a direction
of air flow in said main air passage means.
Description BACKGROUND OF THE INVENTION
The present invention relates to an air-flow meter for an internal
combustion engine and, more particularly, to an air-flow meter for
controlling a fuel injector of an internal combustion engine with
the air flow meter including and exposed hot wire arranged in an
air passage of the internal combustion engine so as to measure the
amount of suction air flow.
In order to maintain a proper combustion process in an internal
combustion engine, the air-fuel ratio of a mixture to be fed into
the internal combustion engine must be maintained at an appropriate
value, and the amount of fuel to be injected into a combustion chamber
by a fuel injector must be controlled in correspondence with the
amount of air flow. Consequently, it is important to accurately
sense the amount of air flow into the internal combustion engine.
In, for example, Japanese Laid Open Patent Application No. 56-18721
a hot-wire type air-flow meter for an internal combustion engine
is proposed wherein, in order to prevent damage of a hot wire due
to a backfire, a bypass air passage for bypassing a main air passage
is provided, and the hot wire is arranged in the bypass air passage,
with the amount of air flow being sensed on the basis of a change
in the resistance of the hot wire. A disadvantage of this arrangement
resides in the fact that dust particles flowing from the upper stream
of the bypass air passage, pollutant particles contained in a reverse
flow gas attributed to backward flow or backfire from the engine,
etc. adhere to the hot wire, and an output signal from the hot wire
changes with time, so as to result in a large error in the measured
value of the amount of air flow.
SUMMARY OF THE INVENTION
An object of the present invention is to avoid the aforementioned
disadvantages by providing an air-flow meter for internal combustion
engines which has a high reliability.
Another object of the present invention is to provide an air-flow
meter for internal combustion engines in which an ouput signal from
a hot wire does not change with time.
In accordance with advantageous features of the present invention,
a path from the inlet of a bypass passage to a passage part where
a hot wire is located has such a shape that dust particles etc.
can be removed by the inertial separation, and a diameter of the
hot wire is selected so that the dust particles etc. flowing into
the passage part where the hot wire is located may not collide against
the hot wire.
Other and further objects, features and advantages of the invention
will appear more fully from the following description when taken
in connection with the accompanying drawings which show, for the
purposes of illustration only, several embodiments in accordance
with the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial cross-sectional view of an internal combustion
engine for automobiles to which the present invention is applied;
FIG. 2 is a cross-sectional view of a square bent pipe for explaining
the principle of dust removal;
FIG. 3 is a graph illustrating variations in the passing rate of
dust particles in the construction of FIG. 2;
FIG. 4 is an explanatory diagram illustrating a collision of dust
particles against a hot wire;
FIG. 5 is a graph illustrating variations in a collision rate of
dust particles against the hot wire in the construction of FIG.
4;
FIG. 6 is a schematic diagram of a bypass air passage formed as
a square pipe having a bent portion in an upstream portion thereof;
FIG. 7 is a graph illustrating variations in the collision rate
of dust particles against a hot wire in the bypass air passage illustrated
in FIG. 6;
FIG. 8 is a schematic diagram of a bypass air passage formed as
a circular pipe having a bent portion at an upstream end thereof;
FIG. 9 is a graph illustrating variations in the rate of dust particles
passing through the bent portion of the bypass air passage illustrated
in FIG. 8;
FIG. 10 is a graph illustrating variations in the collision rate
of dust particles against a hot wire in the bypass air passage illustrated
in FIG. 8;
FIG. 11a is a schematic cross-sectional view of a bypass passage
constructed in accordance with the present invention;
FIG. 11b is a graph illustrating a relationship between a ratio
of a diameter of the hot wire to the width of the entrance of the
bending portion and the rate of collision of dust particles, in
the bypass air passage illustrated in FIG. 6;
FIG. 12a is a schematic cross-sectional view of a bypass passage
constructed in accordance with the present invention;
FIG. 12b is a graph showing the relationship between the ratio
of the diameter of the hot wire to the width of the entrance of
the bending portion and the rate of collision of dust particles,
in the bypass air passage illustrated in FIG. 8;
FIG. 13 is a cross-sectional view of a hot wire according to the
present invention;
FIG. 14a is a graph illustrating a relationship between a surface
temperature of a metal rod and adherence of dust particles;
FIG. 14b is a partially schematic partial cross-sectional view
of a hot wire construction employed to determine the relationship
illustrated in FIG. 14a;
FIG. 15 is a cross-sectional view of a throttle chamber equipped
with an embodiment of an air-flow meter according to the present
invention;
FIG. 16 is a cross-sectional view of another embodiment of the
air-flow meter according to the present invention; and
FIG. 17 is a graph illustrating variations-with-time in an output
value of the air-flow meter of FIG. 15.
DETAILED DESCRIPTION
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 electronically-controlled
fuel injection type engine includes an air-flow meter 1 for detecting
an amount of flow of air drawn from an air cleaner 2 into a combustion
chamber 3 by suction, with the detected amount of suction air being
supplied to a microcomputer 4. The microcomputer 4 computes the
amount of fuel supply corresponding to the particular amount of
air, and thus controls the amount of fuel to be injected from a
fuel injector 5. The fuel-air mixture is fed into the combustion
chamber 3 and is fired by an ignition plug 6 so that a piston 7
is pushed down. The ignition timing is controlled by the microcomputer
4 with a control signal from the microcomputer 4 being delivered
to an ignition coil 8 which generates a high voltage to thereby
spark the ignition plug 6. The resulting exhaust gas is emitted
from an exhaust valve (not shown) through an exhaust pipe 9 with
the oxygen content of the exhaust gas being detected by an O.sub.2
-sensor 10. Upon receiving a detection output signal of the O.sub.2
-sensor 10 the microcomputer 4 compensates the amount of fuel injection
so that the air-fuel ratio may become the theoretical value.
The inflowing air from the air cleaner 2 passes through a main
air passage 26 and is drawn into the combustion chamber 3 by suction
with a part of the suction air being introduced into a bypass air
passage 24 so as to be sensed by a hot wire 22 to determine the
amount of air flow through the bypass passsage 24 and thereby measure,
the amount of flow of the suction air into the combustion chamber
3. When dust is contained in the air passing through the bypass
24 it adheres to the hot wire 22 and increases the resistance of
this hot wire 22 so that the amount of air flow cannot be accurately
measured. In the present invention the amount of dust to be contained
in the air passing near the hot wire 22 is reduced, and, a diameter
of the hot wire 22 is selected so that the dust may fail to collide
against the hot wire 22. Since dust particles having diameters of
about 10 .mu.m and above are removed by the air cleaner 2 smaller
dust particles may be prevented from adhering to the hot wire 22.
As shown in FIG. 2 the section of the entrance of the bending
portion is a square of (b.times.b), and that of the exit is a rectangle
of (2b.times.b). The solid line arrows indicate the streams of air,
with the broken line arrows indicating the streams of dust. Since
the flow passage curves abruptly, although the air flows while turning
along the flow passage, most of the dust particles cannot turn and
they are separated from the air streams and collide against the
wall of the flow passage. Assuming that the dust particles floating
on the air streams flowing within a distance C.sub.s from the surface
of the wall located on the outer side of the bending collide against
the wall of the flow passage, the factor of collision of the dust
particles against the bent wall is expressed by C.sub.s /b, and
hence, the factor X.sub.s of dust particles passing through the
bending portion becomes:
When the flow passage is constructed as shown in FIG. 2 the calculated
results of the factor X.sub.s of dust particles passing through
the bending portion are shown in FIG. 3 where the air flow speed
is graduated on a logarithmic scale. In obtaining the results, the
streams in FIG. 2 were solved with the potential flow, and numerical
analyses were performed by applying the equation of motion of the
dust particle to the solution. The diameter of the dust particle
at this time was assumed to be 5 .mu.m taking into consideration
the fact that the dust particles arriving through the air cleaner
are less than 10 .mu.m. Additionally, the density of the dust particles
was assumed to be 2000 kg/m.sup.3 taking into consideration the
densities of soil, sand, silicon, sulfur, etc. As shown in FIG.
3 the factor X.sub.s of the dust particles passing through the
bending portion varies in dependence upon a slit width b and the
air flow speed. It is more difficult for the dust particles to pass
through the bending portion as the slit width b is smaller and as
the air flow speed is higher.
As shown in FIG. 4 the hot wire 22 is a circular cylinder having
a cross-sectional diameter d, with the solid line arrows indicating
the air streams and the broken line arrows indicating the dust streams.
As can be seen in FIG. 4 the air streams avoid the hot wire 22
while the flow of dust particles separate from the air streams near
the circular cylinder due to inertia, and those of the dust particles
which flow in a vicinity of the axis of the circular cylinder collide
against the circular cylinder. Assuming C.sub.B denotes a critical
value of the thickness of the air streams for permitting the dust
particles to flow without colliding against the circular cylinder,
a factor X.sub.B of collision of the dust particles against the
circular cylinder is expressed by:
FIG. 5 illustrates calculated results of the collision factor X.sub.B
of the dust particles against the circular cylinder, with the air
flow speed being graduated on a logarithmic scale. The conditions
under which the calculation of the collision factor X.sub.B is made
are the same as those of FIG. 3. As shown in FIG. 5 the collision
factor X.sub.B of the dust particles against the hot wire 22 varies
in dependence upon the diameter d of the hot wire 22 and the air
flow speed. As the diameter d of the hot wire 22 increases and as
the air flow speed is reduced, the collision factor X.sub.B becomes
smaller, so that the precision of the air flow value sensed by the
hot wire 22 is more enhanced.
As apparent from FIGS. 3 and 5 upon sensing the air flow by providing
the bent portion near the inlet of the bypass air passage and arranging
the hot wire 22 in the bypass passage, the slit width b of the entrance
of the bent portion may be small and the diameter d of the hot wire
22 may be large, in order to prevent the dust particles from adhering
to the hot wire 22 these dust particles arriving from the inlet
of the bypass passage in a manner so as to coexist with the air.
Also, when the air flow speed in the slit portion is high and the
air flow speed near the hot wire 22 is low, the adherence of the
dust particles to the hot wire 22 is reduced. This measure is achieved
by properly selecting a ratio between a cross-sectional area of
the bypass passage in a vicinity of the hot wire 22 and that of
the passage in the upper stream.
In FIG. 6 the bypass air passage 24 is fashioned as a crank-like
rectangular pipe, in which b denotes the slit width of the upstream
part of the bent portion, A.sub.S a cross-sectional area of the
slit portion, c a width of a portion of air passage 24 where the
hot wire 22 is arranged, A.sub.B a cross-sectional area of a further
portion of the air passage 24 and d a diameter of the hot wire
22. The factor X.sub.B of collision of dust particles against the
hot wire 22 in the case where air containing the dust particles
is caused to flow through the passage 24 was evaluated, with the
passage 24 having a slit width b=1 mm and hot wire diameter d=0.5
mm were chosen as examples of practical dimensions. Considering
the fact that dust particles having diameters of 5 .mu.m-10 .mu.m
enter the bypass air passage most frequently after passing through
the air cleaner 2 the calculation was conducted for dust particles
respectively having diameters of 5 .mu.m and 10 .mu.m.
The results of the above mentioned calculations are illustrated
in FIG. 7 wherein curve I represents the collision factor X.sub.B
of the dust particles against the hot wire 22 with A.sub.S being
equal to A.sub.B so as to equalize an air flow speed V.sub.S in
the slit portion and an air flow speed V.sub.B in the passage surrounding
the hot wire 22. The curve II represents the collision factor X.sub.B,
with A.sub.S :A.sub.B =1:2 and V.sub.S :V.sub.B =2:1. The air flow
speed V.sub.S corresponds to the speed at the entrance of the slit
portion, and a collision factor Y of the dust particles against
the hot wire 22 is obtained by obtaining the product between equations
(1) and (2). More particularly, the collision factor Y is expressed
by the following relationship:
As shown in FIG. 7 the collision factor Y of the dust particles
against the hot wire 22 varies in dependence upon the air flow speed
V.sub.S, but it is at most about 0.2 which signifies that most
of the dust particles do not adhere to the hot wire 22. Further,
it has been confirmed that the dust removing effect increases when
the passage around the hot wire 22 is increased. It is also noted
that the maximum value of the collision factor Y of the dust particles
is substantially constant irrespective of the air flow speed V.sub.S,
and that the difference of the diameters of the dust particles brings
forth no considerable difference in the collision factor Y.
As shown in FIG. 8 the bypass air passage is cylindrical having
a bore diameter D.sub.n, and a hot wire diameter d. The hot wire
22 is arranged near an axis of the pipe and FIG. 9 provides an illustration
of calculated results of a rate of dust particles passing through
the bending portion 26 when air containing the dust particles flows
through the passage 24. As apparent from FIG. 9 the results illustrated
therein with the bypass air passage of FIG. 8 has the same tendency
as illustrated in FIG. 3.
As apparent from FIG. 10 the calculated results of the collision
factor of the dust particles against the hot wire 22 when air containing
the dust particles flows through the bypass passage 24 shown in
FIG. 8 has the same tendency as shown in FIG. 7. Thus, from FIGS.
9 and 10 it becomes apparent that the bent pipe of a circular cross-section
attains the same effects as when the bent pipe is of a square cross-section.
In the instant specification, the width b of the entrance of the
bent portion of square pipe section of FIG. 2 or 6 and the diameter
D of the entrance of the circular bent portion in FIG. 8 etc. shall
be generally termed the "width of the entrance of the bent
portion".
As noted hereinabove in connection with FIG. 5 when the diameter
d of the hot wire 22 is increased, the collision factor of dust
particles against the hot wire 22 is reduced.
When the air flow speed changes into arbitrary values, the integrated
value of a collison factor curve, e.g., the area S of a part enclosed
with the curve I and the axis of abscissas in FIG. 7 denotes the
summation of probabilities at which the dust particles collide against
the hot wire 22 and it serves as a criterion for judging whether
or not the dust particles easily adhere to the hot wire 22. Therefore,
considering what proportion the integral value S of the actual collision
factor curve assumes in the integral value S.sub.o of a collision
factor curve, assuming that the collision factor is equal to one
at the flow speed at which the collision factor curve assumes the
maximum value, the quantity of adherence of the dust particles to
the hot wire 22 may be determined in accordance with the following
relationship:
FIG. 11a provides an illustration of the calculated results of
variations of the ratio S/S.sub.o dependent upon the ratio (d/D)
of the diameter of the hot wire 22 to the width of the entrance
of the bent portion, where the hot wire 22 is arranged within the
bypass passage having the bent portion in an upstream part thereof,
with a bypass passage dimensioned as shown in FIG. 11b. More particularly,
in FIG. 11b, the bypass passage has a bent portion 30 with an entrance
width D and an exit width H, and a hot wire having a diameter d.
As shown in FIG. 11b, the ratio S/S.sub.o increases with a decrease
in the ratio d/D irrespective of the dimensions of the bent portion
30. Particularly, when the ratio d/D becomes less than about 0.02
the value S/S.sub.o increases abruptly. In other words, when the
ratio d/D is set at about 0.02 or greater, the value S/S.sub.o becomes
less than 0.1 and the quantity of adherence of the dust particles
to the hot wire 22 can be made very small in practical use. While,
in FIG. 11b, the value S/S.sub.o has been calculated by giving the
ratio of the diameter d of the hot wire 22 to the diameter D of
the entrance of the bent portion 30 results of similar tendency
are obtained even when the value S/S.sub.o is calculated by giving
the ratio of the diameter d of the hot wire to the exit width H
of the exit of the bent portion 32.
The calculated results of variations in the ratio S/S.sub.o in
dependence upon the ratio of the diameter d of the hot wire to the
diameter D of the entrance of the bent portion, where the hot wire
is arranged in the bypass passage of circular cross section (FIG.
12a) having the bent portion at an upstream part thereof is illustrated
in FIG. 12b. As with the bypass passage of FIG. 11a, when d/D is
smaller than about 0.02 S/S.sub.o increases abruptly, and when
d/D is greater than about 0.02 S/S.sub.o can be maintained below
0.1 so that the quantity of adherence of the dust particles to
the hot wire becomes very small.
In this manner, the bent portion for diverting the air streams
is disposed in the upstream part of the bypass passage, and the
ratio of the diameter of the hot wire to the diameter of the bending
portion is properly selected, whereby the quantity of dust particles
adhering to the hot wire can be extremely reduced to prevent the
lowering of an output signal from the hot wire.
Further, when the hot wire is constructed in the manner illustrated
in FIG. 13 wherein a fine wire of platinum or the like is wound
on the surface of a ceramic tube and then coated with glass is used
as the hot wire, the adherence of dust particles to the hot wire
can be further prevented. Referring to FIG. 13 a resistance wire
such as platinum wire 36 is wound on the surface of a ceramic tube
34 and is covered with a glass coating 38 with leads 40 being inserted
into both the ends of the ceramic tube 34. In FIG. 13 the diameter
of the hot wire is 0.5 mm in order to reduce the collision of dust
particles to the utmost. Coating the surface of the hot wire with
glass in this manner contributes to the prevention of the lowering
of the output from the hot wire for the reason that it is difficult
for dust to adhere to the glass even when it has collided against
the hot wire. Moreover, the heat capacity of the glass film brings
forth the effect that, even when air streams have pulsated, the
output signal is averaged to fluctuate little.
Since moisture in the air passing through the bypass passage increases
the adherence between the dust particles and the hot wire, removing
moisture from the air flow is also effective for preventing the
colliding dust particles from adhering to the hot wire.
In this connection, FIGS. 14a, 14b illustrate the result of an
experiment for clarifying a relationship between the set temperature
of the hot wire and the state of adherence of dust particles. As
shown in FIG. 14b, a metal rod 42 is provided with recesses at intervals
of 10 mm, and thermocouples 44 are inserted therein. While the metal
rod 42 was heated in each of three heating modes by a heater 46
located at the left end thereof, test dust particles principally
containing SiO.sub.2 and mixed in the air were caused to flow from
above. When the surface temperature distribution of the metal rod
42 was investigated under such conditions, curves T.sub.1 -T.sub.3
shown in FIG. 14a were obtained. The test dust particles adhered
to the metal rod 42 in a temperature range indicated in solid line
in FIG. 14a, whereas, the particles did not adhere in a temperature
range indicated in broken line. Thus, it has been determined that,
when the surface temperature of the metal rod 42 is held at 100.degree.
C. or above, the moisture in the air does not adhere to the surface
of the metal rod 42 so any adherence of the test dust particles
to the metal rod 42 does not exist. accordingly, maintaining the
surface temperature of the hot wire placed in the bypass air passage
at a temperature of at least 100.degree. C. greatly contributes
to the prevention of adherence of dust particles to the hot wire.
As shown in FIG. 15 with an air flow meter of the present invention
disposed in a throttle chamber, as indicated by the air stream arrows,
a portion of the air enters an annular inlet slit 54 formed between
the body 50 of the throttle chamber and a bell-mouthed member 52
with the air having the flow direction thereof diverted by a bent
portion 56 so as to pass around a hot wire 220 and arrive at a
main air passage 60 through an annular outlet slit 58. The amount
of air flow through the bypass passage 24 is sensed by the hot wire
220 and a compensation for the sensed amount is provided by an
output signal from a temperature-compensating resistance wire 222
whereupon the total amount of the air flowing through the main air
passage 60 and the bypass air passage 24 is measured by the air-flow
meter 1. A flow signal provided from the air-flow meter 1 is applied
to the microcomputer 4 which delivers a control signal to the fuel
injector 5 so as to inject a suitable quantity of fuel. In FIG.
15 the upstream portion of the bent portion 56 i.e., the entrance
slit 54 has a width D=1 mm and the hot wire 220 has a diameter d=0.5
mm, so that d/D=0.5 which satisfies d/D.gtoreq.0.02. Therefore,
the dust particles mixed in the air are prevented from colliding
against the hot wire 220. Furthermore, even when a slight amount
of dust has collided against the hot wire 220 the dust particles
do not adhere to the hot wire 220 because the hot wire 220 is constructed
as glass-coated hot wire of the type illustrated in FIG. 13 and
the surface temperature thereof is maintained at 100.degree. C.
or above. Accordingly, an output signal from the hot wire 220 does
not change, and an accurate air-flow measurement is realized. In
addition, in the embodiment of FIG. 15 the inlet slit 54 of the
bypass air passage 24 is open in the flowing direction of the main
air streams. Therefore, an occurence of turbulent flow is difficult
in the air flowing through the bypass passage 24 and the fluctuation
of the output signal of the hot wire 220 is suppressed.
In FIG. 16 the width D of the annular entrance slit 54 is 1 mm
and the diameter d of the hot wire 220 is 0.5 mm, so that d/D.gtoreq.0.02.
Accordingly, the collision of dust particles against the hot wire
220 is prevented. Moreover, the opening direction of the entrance
slit 54 of the bypass air passage 24 is perpendicular to the flowing
direction of the main air streams, and when the air flows from the
entrance slit 54 into the bypass air passage 24 the direction of
the air streams change. Therefore, the quantity of dust particles
flowing into the bypass air passage 24 is smaller than in the type
illustrated in FIG. 15 and the function of removing dust is more
enhanced.
To determine the effectiveness of the air flow meter in the embodiment
of FIG. 15 experiments were conducted wherein air containing large
quantities of dust were directed through the air flow meter to simulate
actual repeated operation of an internal combustion engine installed
in a motor vehicle. In the experiments, hot wires having diameters
of 0.5 mm were arranged in both the main air passage and the bypass
air passage, with negative pressures before and behind the air-flow
meter were kept unchanged and the ambient air temperature was kept
substantially constant, while test dust particles were caused to
flow through both the passages at a rate of 3.0 gr/hour. In FIG.
17 E.sub.B represents the output of the hot wire placed in the
bypass air passage, E.sub.M the output of the hot wire placed in
the main air passage, P the negative pressure before and behind
the air-flow meter, and T the ambient air temperature. The test
dust particles began to flow at time=0 min., and a time interval
until the lapse of 30 min. corresponded to an actual running distance
of 100000 km. As apparent from FIG. 17 the output E.sub.M of the
hot wire placed in the main air passage provided with no means for
removing dust lowers with the lapse of time and exhibits values
smaller than actual amounts of air flow, whereas the output E.sub.B
of the air-flow meter according to the present invention does not
change with time, which signifies that an accurate air-flow measurement
is achieved.
In accordance with the present invention, an air-flow meter for
internal combustion engines is provided which produces output values
free from changes over time, and which exhibits a high reliability
as well as a high precision. |