Abstrict A thermal air flow meter consists essentially of a hollow cylindrical
support made of an insulating material, a heat-sensitive resistor
formed on top of the support, and lead wires inserted into the hollow
part of the support from either end and bonded thereto by glass.
This flow meter measures the flow rate from changes in the resistance
of the heat-sensitive resistor with temperature. The ratio of the
inner diameter d.sub.2 of the support to the outer diameter d.sub.3
of the lead wires (d.sub.2 /d.sub.3) is at least 1.7. This arrangement
can provide a thermal air flow meter with a quick response to changes
in flow rate.
Claims What is claimed is:
1. In a thermal air flow meter which consists essentially of a
hollow cylindrical support made of an insulating material, a heat-sensitive
resistor formed on top of said support, and lead wires inserted
into the hollow part of said support from either end of said support
and bonded to said support by a heat-insulating adhesive, and which
measures the flow rate on the basis of changes in resistance due
to the temperature of said heat-sensitive resistor, the improvement
wherein the ratio of the inner diameter d.sub.2 of said support
to the outer diameter d.sub.3 of said lead wires is at least 1.7.
2. The thermal air flow meter as defined in claim 1 wherein the
ratio d.sub.2 /d.sub.3 is between 1.8 to 2.0.
3. The thermal air flow meter as defined in claim 1 wherein said
adhesive is borosilicate glass.
4. The thermal air flow meter as defined in claim 1 wherein said
adhesive is lead glass with a softening point of at least 800.degree.
C.
5. The thermal air flow meter as defined in claim 1 wherein the
length l.sub.2 of each of said lead wires from the end of said support
to the attachment position thereof is at least 1.5 times the length
l.sub.1 of said support.
Description BACKGROUND OF THE INVENTION
This invention relates to a thermal air flow meter. More particularly,
the present invention relates to a thermal air flow meter which
is suitable for measuring the quantity of air taken in by an internal-combustion
engine of an automobile.
In an internal-combustion engine of an automobile equipped with
a fuel injection system, the operational state of the internal-combustion
engine is detected by signals from various sensors, and the quantity
of fuel injected by the fuel injection system, etc., is controlled.
A thermal air flow meter is one of these sensors, it is used for
measuring the quantity of air taken in by the engine. Thermal air
flow meters of various types are known, but thermal air flow meters
using temperature-dependent, heat-sensitive resistors have gained
a wide application because they provide a high measurement accuracy.
As disclosed in U.S. Pat. No. 4264961 for example, an advantageous
thermal air flow meter has a construction in which part of the air
flowing through an intake pipe is introduced into a by-pass pipe
past a heat-sensitive resistor disposed in this by-pass pipe. This
is advantageous because such a flow meter undergoes hardly any mechanical
damage due to backfires that can occur when the engine is not running
properly.
The heat-sensitive resistor has the following construction. Platinum
wire of 20 .mu.m diameter is wound around an alumina bobbin which
is 0.5 mm in diameter and 2 mm long to form the heat-sensitive resistor
for the thermal air flow meter. Lead wires consisting principally
of platinum are bonded to both ends of the bobbin by adhesive, and
the lead wires are spot-welded to supports. The thermal air flow
meter using this heat-sensitive resistor has the problem that when
the flow rate changes suddenly, particularly when the flow rate
increases suddenly, the response of the flow meter drops.
Flow meters of this construction are used mainly in multi-point
fuel injection systems. Recently, engines with single point fuel
injection systems have been examined. It has been found that when
a flow meter of the construction described above is used in a single
point fuel injection system, its response during acceleration is
low, and this must be improved. In a single point fuel injection
system, a single fuel injection valve is provided at the point at
which the intake pipes of the engine join, and hence the distances
from the fuel injection position to the cylinder inlets are longer
than those of a multi-point fuel injection system. This means that
the time taken for the fuel to arrive at each cylinder is longer.
The distance from the fuel injection position to each cylinder varies
from cylinder to cylinder. This means that if the shape of the intake
pipes changes because of changes to the engine, delicate matching
must be carried out. Accordingly, the detection accuracy must be
improved by use of a very accurate flow meter, particularly at high
speeds, so as to accurately follow pulsations in the flow of intake
air to the engine.
As a result of examining the response of thermal air flow meters,
it has been found that when the flow rate is changed in steps, a
time constant 1.tau. (the time at which 63% of full scale is reached)
is rapid but 3.tau. (the time at which 95% of full scale is reached)
is slow. 1.tau. is mainly determined by the response of the driving
circuit of the flow meter, while 3.tau. is determined by the thermal
characteristics of the heat-sensitive resistor and its supports.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
thermal air flow meter which has a good response to changes in flow
rate.
In a thermal air flow meter for measuring flow rate of the type
which consists of a hollow cylindrical support made of an insulating
material, a heat-sensitive resistor formed on this support, and
lead wires inserted into the hollow part of the support from either
side and bonded thereto by a heat-insulating adhesive, and which
measures the flow rate on the basis of resistance changes in the
heat-sensitive resistor due to temperature, the object of the present
invention stated above can be accomplished by an thermal air flow
meter which satisfies the relationship d.sub.2 /d.sub.3 .gtoreq.1.7
where d.sub.2 is the inner diameter of the support and d.sub.3 is
the outer diameter of the lead wire.
The effect described above can be achieved because the temperature
of the heat-sensitive resistor can sufficiently follow the flow
rate even when the heat transfer from the lead wires to the heat-sensitive
resistor drops and the flow rate changes rapidly.
As the heat-insulating adhesive, borosilicate glass or lead glass
with a softening point of at least 800.degree. C. is preferred,
because of their heat-insulating properties and bond strength.
If the diameter of the lead wires is reduced, the problem will
occur that the mechanical strength of the lead wires will drop,
and the measurement accuracy will also drop because of oscillations
of the heat-sensitive resistor, etc. For this reason, the value
of d.sub.2 /d.sub.3 can not be increased very far: its maximum is
approximately 2.3. The optimal range is between about 1.8 to about
2.0.
If the length of each lead wire is at least 1.5 times that of the
supports, the heat transferred from the supports through the lead
wires can be reduced, and hence the response can be further improved.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of a thermal air flow meter;
FIG. 2 is a sectional view of a thermal air flow meter used for
measuring the quantity of intake air in an internal-combustion engine;
FIG. 3 is a partial sectioned view of the heat-sensitive resistor
of the thermal air flow meter in accordance one embodiment of the
present invention;
FIG. 3A is an enlarged section through the heat-sensitive resistor
of FIG. 3;
FIGS. 4 and 5 are graphs of measured values of air flow rate;
FIG. 6 is a graph of the relationship between the ratio of the
inner diameter of the support of the heat-sensitive resistor to
the outer diameter of the lead wire, and step response;
FIG. 7 is a partial sectioned view of the heat-sensitive resistor
of the thermal air flow meter in accordance with another embodiment
of the present invention; and
FIG. 8 is a graph comparing the power consumption of a prior-art
flow meter with that of one embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a schematic diagram of the thermal air flow meter. In
the drawing, a heat-generating resistor element 1 forms a bridge
together with other resistors 11 12 and 13. The voltage differences
of these bridge resistors are differentially amplified by an amplifier
14 to form a feedback circuit for driving a transistor 15. The
heat-generating resistor element 1 is positioned in a fluid passage
together with a temperature-compensating resistor 13. The temperature
of the heat-generating resistor element 1 is controlled by the feedback
circuit so that it is always kept at a predetermined temperature
difference (between about 100.degree. to about 200.degree. C.) above
ambient temperature. The heat-generating resistor element 1 and
the temperature-compensating resistor 13 are heat-sensitive resistors.
FIG. 2 illustrates a thermal air flow meter for measuring the quantity
of intake air in an internal-combustion engine of an automobile.
The heat-generating resistor element 1 and the temperature-compensating
resistor 13 are supported by support pins 21 22 and 23 24 respectively,
and are mounted in a chamber forming a fluid passage and integrated
into a 4-terminal holder 25 into which the support pins 21 through
24 are inserted. The driving circuit portion which is connected
electrically to the heat-generating resistor element 1 and to the
temperature-compensating resistor 13 is held in a housing 20.
A by-pass passage 33 is formed within the chamber so as to extend
from an inlet portion 30 through a static pressure intake port 32
which opens off a main passage 31. The heat-generating resistor
element 1 and the temperature-compensating resistor 13 are positioned
in a central straight pipe portion 34 of the by-pass passage 33.
The downstream portion of the by-pass passage 33 extends for a predetermined
distance in an annular form around the circumferential direction
of the main passage 31 and joins a venturi portion 35 of the main
passage 31. A discharge port 36 is connected to the intake pipes
of the engine by a duct interrupted by a throttle chamber. The chamber
inlet portion 30 is connected to an air cleaner. The discharge port
36 is connected to the intake pipes by the throttle chamber. A typical
construction of a single point fuel injection flow meter is such
that the flow meter is integrated with a throttle chamber which
contains an injector, and is mounted directly in the place at which
the intake pipes join.
FIG. 3 shows in detail the heat-generating resistor element 1 in
accordance with one embodiment of the present invention, and FIG.
3A is an enlarged section through the heat-generating resistor element.
The heat-generating resistor element 1 will now be described in
detail with reference to these drawings. An alumina pipe or bobbin
2 used as an insulating material is provided, and lead wires 3
4 are concentrically inserted into either end of the pipe 2. The
length l.sub.1 of the pipe is 2.0 mm, the length l.sub.2 of the
lead wires is 2.5 mm, and the length l.sub.3 by which each lead
wire is inserted into the pipe 2 is about 0.5 mm. Adhesive 66'
is provided between the lead wires 3 4 and the pipe 2 to mechanically
connect them. They are bonded at least 1000.degree. C. using borosilicate
glass powder as the adhesive 66'. A thin platinum wire 7 of 20
.mu.m diameter is wound around the outer circumference of the pipe
2. The ends of this thin platinum wire 7 are spot-welded at spot-weld
portions 99' to the lead wires 34 respectively, to provide electric
connection. Lead glass powder blended with a binder is applied so
as to cover the thin platinum wire 7 and the spot-weld portions
99', and this is then baked at about 600.degree. C. to form a protective
layer 8. The lead wires 34 are spot-welded to pins 21 22 respectively.
In resistors of the construction described above, measurements
were made of changes in response characteristics due to changes
in the outer diameter d.sub.1 of the pipe 2 its inner diameter
d.sub.2 and the outer diameter d.sub.3 of the lead wires 3 4 and
also due to stepped changes in the air flow rate. The results are
shown in FIG. 4. This graph illustrates a test in which the outer
diameter d.sub.1 of the pipe was 0.6 mm, its inner diameter d.sub.2
was 0.3 mm, and the outer diameter d.sub.3 of the lead wires 3
4 was 0.225 mm. In this test, the response time to 3.tau. (the time
at which 95% of full scale was reached) for a stepped rise was 1700
ms, and the response time to 3.tau. for a stepped fall was 107 ms.
It can be seen that the response time at the time of rise was particularly
long.
FIG. 5 illustrates a test in which the outer diameter d.sub.1 of
the pipe 2 was 0.6 mm, its inner diameter d.sub.2 was 0.3 mm, and
the outer diameter d.sub.3 of the lead wires 3 4 was 0.16 mm. The
response time to 3.tau. for a stepped rise was much better at 142
ms, with the response time to 3.tau. for a stepped fall also improved
at 65 ms.
FIG. 6 illustrates tests in which the outer diameter d.sub.1 of
the pipe 2 was 0.6 mm, its inner diameter d.sub.2 was 0.3 mm, and
the outer diameter d.sub.3 of the lead wires 3 4 were 0.225 mm,
0.2 mm, 0.18 mm, 0.175 mm, 0.16 mm, or 0.15 mm. The graph shows
the response time to 3.tau. in relation to the ratio d.sub.2 /d.sub.3
for a stepped rise. It can be seen that when the ratio d.sub.2 /d.sub.3
was at least 1.7 the response time was much better. When the d.sub.2
/d.sub.3 was close to 1 the thickness of the adhesive layers 66'
was small so that hardly any heat insulating effect could be obtained.
If the ratio d.sub.2 /d.sub.3 is at least 1.7 the difference between
the inner diameter of the pipe 2 and the diameter of the lead wires
3 4 is so large that the thickness of the adhesive layers 6 6'
also becomes large so that the heat insulating effect provided by
these adhesive layers can be obtained. Accordingly, thermal equilibrium
of the pipe 2 is reached almost as soon as the adhesive layers 6
6' receive the heat transferred from the pipe 2 and start to transfer
the heat to the lead wires 3 4 and the temperature distribution
of the adhesive layers 6 6' also reaches substantially its final
value. Thus, a thermal air flow meter with a good response can be
obtained. If d.sub.2 /d.sub.3 is greater than 1.7 the response
time is hardly affected by the increase in the thickness of the
adhesive layers 6 6', but the diameter of the lead wires 3 4 is
small and their mechanical tensile strength (which must be at least
800 gr) drops. Hence, d.sub.3 must be 0.13 mm. In other words, the
limit for d.sub.2 /d.sub.3 is 2.3 when d.sub.2 is 0.3 mm. Because
of machining tolerances, the optimal ratio is between 1.8 to 2.0.
When pipes of an outer diameter of 0.35 mm were examined, substantially
the same results could be obtained. If the outer diameter d.sub.1
of the pipe is too great, however, the heat capacity of the pipe
becomes too much so that response drops.
The adhesive may be lead glass with a softening point of at least
800.degree. C., besides borosilicate glass.
The glass applied as a coating over the thin platinum wire is lead
glass having a softening point of between 500.degree. to 700.degree.
C.
Since the rise response time can be shortened, the difference in
response time between rise and fall can be reduced, so that when
detecting a pulsating flow within an engine, the flow rate can be
detected with a level of accuracy close to the mean value, and the
efficiency with which the flow rate is detected can be therefore
improved.
The resistor can be formed of a thick or thin platinum film. In
other words, a thin platinum film is formed by sputtering, ion plating,
vacuum deposition, etc., or a thick platinum film can be formed
by printing or coating platinum paste, and the film is heat-treated
at a high temperature. The treatment temperature is between 700.degree.
to 800.degree. C. for a thin film, and between 900.degree. to 1000.degree.
C. for a thick film. The film resistor thus formed is trimmed by
a laser so as to obtain a predetermined resistance value.
FIG. 7 illustrates another embodiment of the present invention.
When the heat-generating heat-sensitive resistor 1 is mounted onto
the 4-terminal holder 25 the leads 3 4 can be shaped by bending
them at right angles into L-shapes at a length of 1.5 mm, for example,
from the bobbin ends. The distance l.sub.2 of each lead from the
bobbin end to the stainless steel supports 21 22 is set to satisfy
the relationship l.sub.1 /l.sub.2 <1/1.5 with respect to the
bobbin length l.sub.1. If the bobbin length l.sub.1 is 2.0 for
example, the lead length l.sub.2 is 3.5 for example. The 4-terminal
holder assembly thus assembled is electrically connected to the
driving circuit 20 for the heat-generating resistor 1 and is integrated
with the throttle chamber containing an injector. When the assembly
was incorporated in a single point fuel injection system and matching
was effected for a 4-cycle 4-cylinder engine, the operation was
found to be normal, and the operational performance during acceleration
was found to be superior to that of an automobile provided with
a carburetor.
The construction illustrated in this embodiment was found to be
sufficiently resistant to vibrations of 30 G and between 20 to 1
kHz in each of the X, Y and Z axes for 3 hours. It could also withstand
sufficiently 60 G vibrations for 1 hour in the direction of flow,
which is the most critical condition for vibration resistance. It
was also confirmed that the flow meter had no resonance points in
the range of between 20 to 1 kHz, and had a sufficiently high mechanical
durability. When an interrupted power feed test was carried out
by turning on and off the power for four seconds each at a set temperature
of 300.degree. C., it was confermed that the heat-generating resistor
could withstand 1000 cycles.
The heat-insulating structure provided by the uses of a glass adhesive
has the effect of reducing the power consumption of the heat-generating
resistor heated to a predetermined temperature. FIG. 8 is a graph
of the thermal characteristics A of a prior-art flow meter in comparison
with those B of a flow meter in accordance with one embodiment of
the present invention. The ordinate represents the power I.sup.2.sub.H
R.sub.H applied to the heat-generating resistor, and the abscissa
represents the square root of the flow rate Q. The power consumption
of the embodiment of the present invention was reduced to 1/1.5
on the side of low flow rate. The temperature difference in the
axial direction was also relatively small, and the sensitivity to
the flow was improved by about 10%.
The present invention provides the following effects.
Because heat insulation is provided by the glass bonding between
the bobbin and the leads, and the cross-sectional area of each lead
is reduced, heat transfer to the supports can be reduced and the
rise response time shortened.
The power applied to the heat-generating resistor can be reduced
and the sensitivity of the heat dissipation characteristics to the
air flow can be improved.
The difference in response time between rise and fall can be reduced
so that a value approximately equal to the true mean value of a
pulsating flow can be detected, and the accuracy with which the
flow rate is measured can be improved.
Since the heat-generating, heat-sensitive resistor and the temperature-compensating
resistor are positioned within the bypass passage, adverse influences
due to backfiring and dust can be reduced.
The present invention can provide a thermal air flow meter with
a quick response time to the changes in flow rate. |