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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to an intake air flow meter for internal combustion
engines of automobiles and particularly to a heat sensing wire type air
flow meter.
There are various methods of measuring the volume of the intake air drawn
into an internal combustion engine. Among these, the heat sensing wire
type air flow meter is most widely used since it has a good response and
is capable of measuring the mass of air drawn in so that pressure
correction is not necessary. This kind of air flow meter comprises a
platinum wire of 70 to 100 .mu.m in diameter which is strained in the air
horn as proposed in the patent application of Japanese Patent Publication
No. 48893/1974 (U.S. Pat. No. 3747577), the Laid-Open No. 19227/1972 and
the Laid-Open No. 64134/1976. The flow meters in these applications,
however, have a problem of durability and especially have other problems
that the detecting portion is likely to be damaged by backfiring caused
when the engine is not running in good condition.
In order to solve these problems, an improved air flow meter has been
proposed in which the detecting portion is comprised by winding a platinum
wire on a supporting body such as hollow body made of ceramic material and
having a coating material thereon, whereby the air flow meter has improved
mechanical strength (Japanese Patent Application Nos. 53-42547 and
53-65748).
The detecting portion above, however, is likely to be affected by heat
transmitted from a member supporting the detecting portion, since the
ratio of length to diameter of the platinum wire of which the detecting
portion is comprised, is comparatively small and the detecting portion is
supported by lead wires whose portions exposed in an air flow passage are
not small enough in diameter to prevent heat from being transmitted.
Accordingly, the output signal of the air flow meter is affected by the
wall temperature of the air horn.
SUMMARY OF THE INVENION
One object of the invention is to provide an air flow meter which is
capable of measuring with high accuracy the volume of air drawn in under
any running condition of the engine.
Another object of the invention is to provide an air flow meter which is
capable of measuring with high accuracy the volume of air drawn in without
the influence of the wall temperature under any running condition of the
engine.
Still another object of the invention is to provide an air flow meter which
provides an improved proportional relationship between the main intake air
mass and the bypass air mass under any running condition of the engine.
A feature according to one embodiment of the invention is that the lead
wire supporting the temperature compensating resistor has the same length
as that supporting the air velocity measuring resistor or is longer than
that supporting the air velocity measuring resistor.
A feature according to another embodiment of the invention is that the air
flow meter is installed in the bypass passage having a flat rectangular
cross section, the internal surface of which is formed of heat insulating
material.
A feature according to still another embodiment of the invention is that
the air flow meter is controlled by a constant voltage circuit in which a
suitably adjusted constant voltage is added to the potential difference
between two intermediate points of a bridge circuit incorporating the heat
sensing resistors.
According to the present invention the volume of air drawn in is measured
with high accuracy under any running condition of the engine.
According to one embodiment of the invention the influence of the wall
temperature is reduced whereby the volume of air drawn in is measured with
high accuracy under any running condition of the engine.
According to another embodiment of the invention the proportional
relationship between the main intake air mass and the bypass air mass is
improved whereby the volume of air drawn in is measured with high accuracy
under any running condition of the engine.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front view of an air horn employing an embodiment of the air
flow meter of the invention;
FIG. 2 is a cross-sectional view taken along the line II--II' of FIG. 1;
FIG. 3 is a cross-sectional view taken along the line III--III' of FIG. 1;
FIGS. 4(A)-4(C) are cross-sectional views showing how the sensing elements
are mounted in the bypass passage;
FIGS. 5(A)-5(C) are views showing how the air flow meter of the invention
is supported by lead wires;
FIGS. 6(A), 6(B), 7(A), 7(B), 8(A) and 8(B) are diagrams showing the
relation of the output signal of the air flow meter of the invention to
the wall temperature under the constant air mass flow;
FIG. 9 is a diagram showing the relation between the length of the lead
wires supporting the resistors; FIGS. 10(A)-10(B) are enlarged
cross-sectional views showing the bypass passage in which the air flow
meter of the invention is installed;
FIG. 11 is a diagram showing the relation between the main air volume
through the intake passage and the bypass air volume through the bypass
passage;
FIG. 12 is a circuit of an embodiment of the heat sensing wire type air
flow meter constructed according to the invention;
FIGS. 13 and 14 are circuits modified from that shown in FIG. 12;
FIG. 15 is a circuit of still another embodiment of the air flow meter
constructed according to the present invention; and
FIG. 16 is a diagram showing the relation between the volume of air flowing
past the bypass passage and the output of the air flow meter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIGS. 1, 2 and 3, an air horn 10 has an intake passage 12
running therethrough, in which a venturi 14 is formed. The flange 17 at
the lower end of the air horn 10 is connected with a throttle valve
chamber (not shown). Provided downstream of the throttle valve in the
throttle valve chamber is a fuel injection valve (not shown). Air drawn in
as indicated by the arrow in FIG. 2 passes through the venturi 14, and a
part of it also passes through a bypass passage 30. The air that has
entered bypass inlet 16 located upstream of the venturi 14 is drawn
through the bypass passage 30 to a bypass outlet 18 located at the venturi
14 by the vacuum developed at the bypass outlet 18, from which the air
comes out into the intake passage 12.
An air flow meter 50 according to one embodiment of the invention shown in
FIG. 2 is installed in the bypass passage 30 and may be installed in the
intake passage 12.
The air flow meter 50 according to the invention comprises an air velocity
measuring resistor 501, a temperature compensating resistor 503, lead
wires 302, 304 for supporting the resistors 501, 503 thereon respectively
and supplying electric power therethrough, and an electronic circuit 70
for controlling the resistors 501 and 503.
The air velocity measuring resistor 501 is arranged above the temperature
compensating resistor 503 as shown in FIG. 2, and FIG. 4(A) and 4(B),
however, the two resistors 501, 503 may be arranged side by side as shown
in FIG. 4(C).
The lead wires 302 and 304 extend through a supporting member 310 arranged
in a cover 308 and are connected to the electronic circuit 70 which is
protected by a circuit cover 72 and will be described in detail
hereinafter.
The cover 308 forming the bypass passage 30 is made of heat insulating
material such as synthetic resin or plastic to insulate the bypass passage
30 from external temperature influences and improve the electrical
insulation of the heat sensing resistors 501 and 503.
Each resistor 501 and 503 installed in the bypass passage 30 is provided
with connecting lead wires 306, 306' and 307, 307' at the opposite ends
thereof and is supported through these connecting lead wires 306, 306' and
307, 307' by the lead wires 302, 302' and 304, 304' whose portions exposed
in the bypass passage 30 have the same length as shown in FIGS. 5(A) and
(B).
The current supplied through the lead wires 302 and 304 heats the air
velocity measuring resistor 501 to a predetermined temperature but the
temperature compensating resistor 503 is heated only to a temperature
slightly higher than the atmospheric temperature.
For example, the temperature of the air velocity measuring resistor 501 is
about 100.degree. to 200.degree. C. higher than that of the air drawn in,
and the temperature of the temperature compensating resistor 503 is almost
equal to that of the air drawn in.
When the temperature of the air flowing through the bypass passage 30 is
different from that of the cover 308, heat flows through the lead wires
302 and 304 to change the temperature of the resistors 501 and 503.
That is, when the temperature of the cover 308 is higher than that of the
air flowing through, the heats are transmitted from the cover 308 through
the lead wires 302 and 304 to the resistors 501 and 503, especially to the
temperature compensating resistor 503.
To the contrary, when the temperature of the cover 308 is almost equal to
or lower than that of the air flowing through, the heat is transmitted
from the resistors 501 and 503 through the lead wires 302 and 304 to the
cover 308.
The temperature difference between the air flowing through and the cover
308 decreases in accordance with the increase of the volume of air flowing
through since an active heat exchange is carried out between the air
flowing through and the cover 308.
In general, the heat resistance of the lead wire is directly proportional
to the length and inversely proportional to the cross-sectional area
thereof.
Accordingly, the heat exchanged between the cover 308 and the resistors 501
and 503 can be made small by making the heat resistance of the lead wires
302 and 304 large, i.e., by making the lead wires long and/or small in
diameter.
The results of an experiment carried out by the inventors are shown in
FIGS. 6(A)-8(B), according to which the influence of the wall temperature
to the output signal can be understood, when the relative length of the
lead wires 302 and 304 supporting the resistors 501 and 503 changes under
the condition of the constant air mass flow of 0.1 m.sup.3 /min. and 1
m.sup.3 /min.
In FIGS. 6(A)-8(B), the abscissa represents wall temperature (.degree. C.)
and the ordinate represents output signal (in volts) of the air flow meter
of the invention.
In the case where the lead wires 302 and 304 whose portions exposed in the
bypass passage 30 have the same length as shown in FIG. 5(A) and 5(D)
hardly any influence of the wall temperature to the output signal is
expected at the constant air mass flow of 1 m.sup.3 /min. as shown in FIG.
6(B). For a constant flow mass rate of 0.1 m.sup.3 /min., there is a
slight increase with temperature, as shown in FIG. 6(A).
In the case where the lead wire 304 supporting the temperature compensating
resistor 503 is longer than that supporting the air velocity measuring
resistor 501 as shown in FIGS. 5(B) and 5(E), hardly any influence of the
wall temperature to the output signal is expected at the constant air mass
flow of 0.1 m.sup.3 /min. and 1 m.sup.3 /min., as shown in FIGS. 7(A) and
7(B), respectively, and especially the output signal slightly decreases at
the constant air mass flow of 1 m.sup.3 /min. in accordance with the
increase of the wall temperature, as shown in FIG. 7(B).
In the case where the lead wire 302 supporting the air velocity measuring
resistor 501 is longer than that supporting the temperature compensating
resistor 503 as shown in FIGS. 5(C) and 5(F), the output signal increases
in accordance with the increase of the wall temperature at the respective
constant air mass flows of 0.1 m.sup.3 /min and 1 m.sup.3 /min, as shown
in FIG. 8(A) and 8(B).
As is apparent from the above, the temperature compensating resistor 503 is
likely to be affected by heat transmitted thereto from the cover 308 or by
heat transmitted therefrom to the cover 308.
In the case of a large amount of air flowing through passage 30, it is
preferable to make the lead wire 304 supporting the temperature
compensating resistor 503 longer than or equal to that supporting the air
velocity measuring resistor 501 as shown in FIGS. 5(A), 5(D) and 5(B),
5(E), the reason for which can be understood by referring to FIGS. 6(B)
and 7(B).
In the case of relatively small amount of air flowing through, it is
preferable to make the lead wire 304 supporting the temperature
compensating resistor 503 longer than that supporting the air velocity
measuring resistor 501 as shown in FIGS. 5(B) and 5(E), the reason for
which is understood by FIG. 7(A).
For this reason, the temperature compensating resistor 503 is preferably
supported by the lead wire 304 having no less heat resistance than that of
the lead wire 302 supporting the air velocity measuring resistor 501 in
order to measure with high accuracy the volume of air drawn in under any
running condition of the engine.
Accordingly, the lead wire 304 of the temperature compensating resistor 503
may be made longer and/or smaller in diameter than that of the air
velocity measuring resistor 501.
FIG. 9 is a diagram showing the relation between the length of the lead
wires 302 and 304 for which there is headly any influence due to the wall
temperature on the output signal, when the wall temperature and the
temperature of the air flowing through at a constant velocity of 10 m/sec
are different, representing the diameter of the lead wires as a parameter.
The lead wires are made of the same material, such as stainless steel, and
have the same diameter.
The absissa represents the length of the lead wires supporting the air
velocity measuring resistor 501, and the ordinate represents the ratio of
the length of the lead wires supporting the air velocity measuring
resistor 501 to that of the lead wires supporting the temperature
compensating resistor 503.
As is apparent from FIG. 9, in the case where both of the lead wires 302
supporting the air velocity measuring resistor 501 and the lead wires 304
supporting the temperature compensating resistor 503 are long, both of
them are required to have the same length. To the contrary, in the case
where both are short, the lead wires 304 supporting the temperature
compensating resistor 503 are required to be made longer than those
supporting the air velocity measuring resistor 501.
As the diameter of the lead wires 302 is made larger, the lead wires 304
supporting the temperature compensating resistor are required to be made
longer than that supporting the air velocity measuring resistor, as shown
in FIG. 9.
The bypass passage 30 of the invention has a rectangular cross section with
a longer lateral side and has a curvature at the bends, as shown in FIGS.
10(A) and 10(B). In general, as the fluid flow through the main passage
decreases, the amount of fluid passing through the bypass passage
decreases at a rate greater than that of the main passage. Since the heat
sensing wire type air flow meter detects the overall intake air volume
from the amount of air passing through the bypass passage 30, the above
configuration of the bypass passage 30 is effective in improving the
proportional relationship between them. That is, since the cross section
of the bypass passage 30 is rectangular, the air flow easily becomes
laminar and its direction is smoothly changed by the curvature at the
bends, so that the turbulence of air in the bypass passage is limited to
the minimum. Therefore, a proportional relationship between the main
intake air volume and the bypass air volume can be maintained even when
the intake air flow decreases. The bypass passage 30 may have a circular
cross section.
The bypass inlet 16 and outlet 18 of the bypass passage 30 are formed
facing the intake passage 12 at right angles so that any dust carried in
the main air flow can be prevented from entering the bypass passage 30.
This keeps the resistor 501, 503 from being fouled and ensures long life.
If the engine backfires and the air flow is reversed, the air will not
enter the bypass outlet 18 and flow back through the bypass passage 30,
since the bypass outlet 18 is formed at the narrowest portion of the
venturi 14. In other words, the reverse air flow is separated from the air
horn wall by the venturi 14, so that the pressure at the bypass inlet 16
becomes almost equal to that at the bypass outlet 18 and the air does not
flow back through the bypass passage 30. Thus, no error will arise in the
measurement of the intake air volume when the engine backfires and the air
flows back. For the same reason, no measurement error will be generated by
the pulsation of the intake air flow caused by the engine, so that the
heat sensing wire type flow meter can provide signal which represents the
correct air volume taken into the combustion chambers of the engine.
As can be seen from the foregoing, the air flow meter of this embodiment
has a construction such that the resistors 501, 503 are supported on the
lead wires 302, 304 of the same length in the bypass passage 30 which is
enclosed by the cover 308 with smooth wall surfaces made of synthetic
resin material of low thermal conductivity, the bypass passage 30 having a
flat rectangular cross section and opening into the intake passage 12 at
the venturi 14 as well as at a point upstream of the venturi 14; hence
this flow meter can detect the mass of air flow passing through the bypass
passage 30 with high accuracy even when the engine backfires and the air
flows back. This construction also has an advantage of being able to
provide an improved proportional relationship between the main intake air
mass and the bypass air mass.
FIG. 11 is a diagram showing the relationship between the volume of air
flowing through the intake passage 12 and the volume of air passing
through the bypass passage 30, with the abscissa representing the main
intake air volume Q and the ordinate representing the ratio q/Q of the
bypass air volume q to the main intake air volume Q. The horizontal dashed
line 103 represents the ideal relation in which the ratio q/Q is constant
for any value of the main intake air volume Q. As a matter of fact, in the
conventional bypass type flow meter the bypass air volume q rapidly
decreases as the take intake volume Q becomes small, as indicated by the
solid line 104. However, the reduction of the bypass air volume q is
smaller for the flow meter of this invention than for the conventional
one, as shown by the one-dot line 105, indicating that the present
invention gives better proportionality between the two quantities.
FIG. 12 shows a circuit 70 for the heat sensing wire type air flow meter of
this invention. The potential Va at a point between the resistors 704, 706
is added to a constant voltage of an adder 710. This constant voltage is
set by a voltage divider 708. The potential Vb is the signal voltage of
the air flow meter and is added to the negative terminal of a differential
amplifier. Va and Vb are expressed as follows
##EQU1##
where R.sub.1 =resistance of the resistor 702,
R.sub.2 =resistance of the resistor 704,
R.sub.3 =resistance of the resistor 706,
Rt=resistance of the temperature compensating resistor 503, and V is a
potential of the emitter of the transistor 716. Assuming Vc stands for a
predetermined potential given by the variable resistor 708, and when the
potential of the adder 710 is made equal to the potential V.sub.b the
following equation holds in the circuit of FIG. 13.
V.sub.b =V.sub.a +V.sub.c (3)
From the equations (1), (2) and (3), the resistance Rw of the air velocity
measuring resistor 501 can be expressed as
##EQU2##
Since the emitter current I of the transistor 716 increases with
increasing air volume, Rw, the amount of which corresponds to a
temperature of the air velocity measuring resistor 501 becomes large in
the low air flow range and small in the high air flow range as will be
understood from equation (4'). Therefore, the amplitude of the signal from
the heat sensing wire type air flow meter becomes comparatively large in
the low air flow range and small in the high air flow range. As a result,
the one-dot line 105 of FIG. 11 approaches the target dashed line 103 for
which the ration q/Q is constant for all values of the main intake volume
Q. In this way, the outputs in the low air flow range can be corrected.
FIG. 13 shows a circuit of the heat sensing wire type air flow meter
modified from that shown in FIG. 12. Parts that are identical with those
in FIG. 12 are given the same reference numerals. In this circuit, the
temperature compensating resistor 503 is connected across an operational
amplifier 730 so as to control the gain thereof and further to limit the
amount of current flowing therethrough which causes self-heating thereof
thereby improving the accuracy of air temperature compensation. Reference
numerals 718, 720, 722, 724 and 726 represent fixed resistors which
constitute elements of a modified bridge circuit, 730 an operational
amplifier, 714 a differential amplifier, and 716 a transistor. The
constant voltage Vc as set by the variable resistor 708 and the output
voltage from the operational amplifiers 730 are applied to the positive
input terminal of the adder 710 and the output voltage of the adder 710 is
applied to the positive input terminal of the differential amplifier 714
to obtain the same effect as that given by the circuit of FIG. 12. A
condenser 728 is provided to prevent noise.
FIG. 14 shows a further modification of the circuit of FIG. 12, in which a
subtracter 740 subtracts the constant voltage set by the variable resistor
708 from the potential between the resistor 702 and the air velocity
measuring resistor 501 to make this voltage difference equal to the
potential at a point between the resistors 704, 706.
It is then true that
V.sub.a =V.sub.b -V.sub.c (5)
The above equation is equal to (3) and this means that the circuit shown in
FIG. 14 provides the same effect as does the circuit of FIG. 12.
FIG. 15 is still another modification of the circuit 70 of the heat sensing
wire type air flow meter constructed according to this invention. If we
let V stand for the signal from the heat sensing wire type air flow meter,
the following relation holds between the amplitude of the signal V and the
amount of air q flowing past the bypass passage 30.
##EQU3##
Therefore
##EQU4##
where c1 and c2 are constants relating to the heat sensing resistors. The
circuit of this embodiment performs the arithmetic operation represented
by the equation (7).
According to the present invention, a predetermined voltage potential is
applied to one of the input terminals of the amplifier or the differential
amplifier which is included in the closed-loop circuit which effects to
balance the potentials at two intermediate points of the Wheatstone bridge
so as to adjust or correct the output signal of the airflow meter when the
intake air volume is small thereby enabling accurate measurement of the
air flow.
FIG. 16 is a diagram showing the relation between the volume of air passing
through the bypass passage and the output of the circuit of FIG. 15. The
solid line 142 represents the output of the square-law detector 752 and
the dashed line 144 the output of the adder 760. In this way, a certain
value is added to the output of the square-law detector 752 so that the
increase in output is comparatively small when a large amount of air is
flowing through, and is large when only a small amount of air is passing
through. As a result, the measurement errors caused in the low air volume
area can be corrected. The amount of correction can be adjusted by the
variable resistor 754.
As can be seen from the foregoing, the circuit of the heat sensing wire
type air flow meter of this embodiment has an advantage that by adding a
suitably adjusted voltage to the signal representing the volume of air
flowing past the bypass passage, it is possible to correct the signal
value when the volume of intake air is small.
According to the embodiment of the invention the ouput of the closed-loop
control circuit which controls at the same value the potentials at two
points between resistors on the parallel circuits of the Wheatstone bridge
is added to a suitably adjusted voltage so as to adjust or correct the
output when the intake air volume is small. This enables accurate
measurement of the air flow.
With the air flow meter of this invention, it is possible to measure highly
accurately the intake air volume over a wide range of engine running
conditions for a long period of operation.
* * * * *
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