Heating resistor type air flow meter with a measuring module inside the main air flow passage body

A highly accurate, low-cost and easy-to-handle heating resistor type flow meter has a main air flow passage body forming a main air flow passage for allowing air flowing therethrough. A measuring module has a heating resistor for measuring a flow rate of the air inserted inside the main air flow passage body: The measuring module comprises the heating resistor inside an auxiliary air flow passage body forming an L-shaped auxiliary air flow passage having an inlet opening portion opening in a direction perpendicular to a main flow line of the air and an outlet opening portion opening in a direction parallel to the main flow line. The main air flow passage body comprises an orifice on a periphery of the inner side wall positioned in an upstream side of the air flow passage body. Both of the inlet opening portion and the outlet opening portion are arranged within a flow flux zone formed by extending the air flow from a top edge of the orifice in a direction parallel to the main flow line.

BACKGROUND AND SUMMARY OF THE INVENTION 
This application claims the priority of 8-231620, the disclosure of which 
is expressly incorporated by reference herein. 
The present invention relates to an air flow meter for measuring an air 
flow rate, and more particularly to a heating resistor type air flow meter 
suitable for measuring an intake air flow rate of an internal combustion 
engine of a vehicle. 
The conventional way of improving the measuring accuracy of a heating 
resistor type air flow meter used in an internal combustion engine under a 
pulsating flow condition, as disclosed in Japanese Patent Application 
Laid-Open No. 2-1518, provides a flow passage having an L-shaped detecting 
tube. That is, the flow passage comprises a wall against backward flow so 
that the back flow does not directly impinge on the heating resistor. 
Although such a flow passage construction cannot suppress back flow, it is 
possible to moderate a so-called binary-value phenomenon, that is, 
decrease of a detected value in the heating resistor type air flow meter 
which is caused when a pulsation amplitude of air flow increases. 
Further, a flow passage construction having an orifice is disclosed in 
Japanese Patent Application Laid-Open No. 1-110220. In this construction, 
a heating resistor is arranged just downstream of an orifice inside a 
detecting tube which is a nearly straight and short tube parallel to the 
main flow direction. 
In the prior art described above, it is impossible to measure flow speed by 
identifying direction of the flow. Therefore, when averaged output signals 
of the heating resistor type air flow meter are plotted as the boost 
pressure is being varied by gradually opening the throttle valve while 
rotating speed of the engine is kept constant, the averaged output signal 
gradually increases, but shows a jump-up phenomenon at boost pressures 
above a certain point indicating a plus side measuring error to an actual 
flow speed (flow rate), as shown in FIG. 12(b). The phenomenon is caused 
by the amplitude of pulsation of the heating resistor type air flow meter 
gradually increasing as opening degree of the throttle valve is increased 
and finally back flow occurs at opening degrees of the throttle valve 
above a point B, as shown in FIG. 12(b). The heating resistor type air 
flow meter cannot identify flow direction. Therefore, when back flow 
occurs, the averaged output increases because flow speed is equally 
detected independently of forward flow and back flow. It is known that 
this phenomenon often occurs particularly in an engine having four or less 
cylinders at a comparatively low rotating speed range of 1000 to 2000 rpm, 
and hardly occurs in an engine having more than four cylinders. 
It is possible to reduce the error caused by back flow by employing one of 
the prior art teachings described above in which a wall against backward 
flow is provided in the flow passage so that the back flow does not 
directly impinge on the heating resistor. However, the error can be 
reduced by only a half. This is because when back flow occurs, forward 
flow increases by an amount of the back flow at the same time. 
Further, it is difficult to prevent the back flow in an intake flow passage 
from occurring because of structures of the engine and the intake flow 
passage. Accordingly, in order to reduce the error caused by back flow, it 
is necessary to employ a complex method such as a structure in which an 
amount of back flow rate is subtracted from an amount of forward flow rate 
or a structure in which both of a forward flow rate and a back flow rate 
are separately measured. 
An object of the present invention is to provide a low-cost and 
easy-to-handle heating resistor type flow meter by improving the measuring 
accuracy, including deviation accuracy, under pulsating flow accompanying 
back flow when the heating resistor type flow meter is mounted on a 
vehicle. 
A heating resistor type flow meter to attain the above object comprises a 
main air flow passage body forming a main air flow passage for allowing a 
fluid to be measured flowing therethrough; and a measuring module having a 
heating resistor for measuring a flow rate of the fluid to be measured, 
inserted inside the main air flow passage body, wherein 
the measuring module comprises the heating resistor inside an auxiliary 
shaped auxiliary air flow passage body forming an L-shaped auxiliary air 
flow passage having an inlet opening portion opening in a direction 
perpendicular to a main flow line of the fluid to be measured and an 
outlet opening an portion opening in a direction parallel to the main flow 
line; 
the main air flow passage body comprises an orifice on a periphery of the 
inner side wall positioned in an upstream side of the air flow passage 
body; and both of the inlet opening portion and the outlet opening portion 
are arranged within a flow flux zone formed by extending the fluid to be 
measured from a top edge of orifice in a direction parallel to the main 
flow line. 
According to the present invention, because increase of flow speed within 
the flow flux zone formed by the orifice reduces an effect of back flow 
flowing in the auxiliary air flow passage body having the both opening 
portions arranged within the flow flux zone, the measuring accuracy can be 
improved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The heating resistor type air flow meter (hereinafter referred to as "flow 
meter") comprises a measuring module 52 for measuring flow rate, a body 
53, that is, a main air flow passage body 20, and parts for attaching the 
body 53 to the measuring module 52 such as screws 54a, a seal 54b and so 
on which form a main air flow passage 22. 
A hole 25 is bored on a wall of the main air flow passage body 20 of the 
body 53 forming the main air flow passage 22, and the measuring module 52 
of an auxiliary air flow passage body 10 is inserted through the hole 25 
and fixed to the main air flow passage body 20 using the screws 54a so as 
to maintain mechanical strength between a mounting surface of the main air 
flow passage 20 and a mounting surface of a housing 1. The seal 54b is 
attached between the measuring module 52 and the body 53 of the main air 
flow passage body 20 to keep air-tightness. 
The measuring module 52 is mainly composed of the housing 1 containing a 
circuit board 2 for mounting a drive circuit to be described later and the 
auxiliary air flow passage body 10 made of a non-conductive material. In 
the auxiliary air flow passage body 10, a heating resistor 3 for detecting 
an air flow rate and a temperature-sensing resistor 4 for compensating 
intake air temperature are arranged so as to be electrically connected to 
the circuit board 2 through a support body 5 made of a conductive 
material. That is, the housing 1, the circuit board 2, the heating 
resistor 3, the temperature-sensing resistor 4, the auxiliary air flow 
passage 10 and so on are integrated in a unit as the measuring module 52. 
In regard to the operational principle of flow measurement in the 
above-mentioned flow meter, the construction of circuit will be described 
first. FIG. 3 is a circuit diagram showing the construction of the heating 
resistor type air flow meter of FIG. 1. The drive circuit formed on the 
circuit board 2 of the flow meter is composed of roughly a bridge circuit 
and a feedback circuit. The bridge circuit is constructed by the heating 
resistor 3 (RH) for measuring an intake air flow rate, the 
temperature-sensing resistor 4 (RC) for compensating intake air 
temperature and resistors R10 and R11, and heating current Ih is conducted 
to the heating resistor RH being controlled by feedback using an operation 
amplifier OP1 so as to keep a constant temperature difference between the 
heating resistor RH and the temperature-sensing resistor RC to output an 
output signal V2 corresponding to an air flow rate. When the air flow 
speed is fast, the heating current Ih is increased since an amount of heat 
dissipated from the heating resistor RH is large. On the other hand, when 
the air flow speed is slow, the heating current may be small since an 
amount of heat dissipated from the heating resistor RH is small. Therein, 
since the amount of heat dissipated from the heating resistor RH is 
independent of the direction of air flow, that is, forward flow or back 
flow, the heating current Ih flows even when air flows backward and as a 
result the jump-up phenomenon of the flow meter occurs. 
The auxiliary air flow passage body 10 of a "passage structure forming an 
L-shaped detecting tube" forms a nearly L-shaped auxiliary air flow 
passage 13. That is, the passage 13 comprises a longitudinal passage 13a 
and a lateral passage 13b which is composed of an auxiliary air passage 
inlet port 11 opening in a direction perpendicular to a main flow line of 
the forward direction 23 of air flow; the longitudinal passage 13a 
extending parallel to the main flow line from the auxiliary air passage 
inlet port 11; the lateral passage 13b communicating with the longitudinal 
passage 13a and bending nearly at right angle, and extending perpendicular 
to the main flow line; and an auxiliary air passage outlet port 12 
positioned at the rear end of the longitudinal passage 13a and opening 
parallel to the main flow line. In general, the heating resistors such as 
the heating resistor 3 and the temperature-sensing resistor 4 are arranged 
on inner portion of the longitudinal passage 13a. 
On the other hand, the main air flow passage body 20 of the body 53 
comprises an orifice 21 formed on a periphery of the inner side wall the 
main air flow passage body 20, positioned in an upstream side of the 
inserted air flow passage body 10; and the both opening portions 
(surfaces), the auxiliary air passage inlet port 11 of the inlet opening 
portion (surface) of the auxiliary air flow passage body 10 and the 
auxiliary air passage outlet port 12 of the outlet opening portion 
(surface), are arranged within a flow flux zone D formed by extending air 
flow 23 in the forward direction of the fluid to be measured from a top 
edge of the orifice 21 in a direction parallel to the main flow line. As 
shown in FIG. 1, the inside of the zone surrounded by the flow lines G1, 
G2 extends in the direction parallel to the main flow line from the top 
edge of the orifice 21. For instance, the flow flux zone corresponding to 
a cylinder having an inner diameter D, when the main air flow passage body 
20 is cylindrical as shown in FIG. 2. 
That is, as shown in FIG. 2, the shape of the main air flow passage body 20 
to be inserted with the auxiliary air flow passage body 10 is nearly 
cylindrical (circular-tube-shaped), and an effective cross-sectional area 
defined by the flow flux of the air flow of the fluid to be measured 
flowing through the main air flow passage 22 formed by the main air flow 
passage body 10 includes configurational positions of the inlet and outlet 
opening portions (the auxiliary air flow passage inlet port 11 and the 
auxiliary air flow passage outlet port 12) of the auxiliary air flow 
passage body 10. 
In other words, the orifice 21 is provided in the periphery of the inner 
side wall of the main air flow passage body 20, positioned in an upstream 
side of the inserted air flow passage body 10. The cross-sectional shape 
of the orifice 21 is a venturi-shape having its center axis nearly equal 
to that of the main air flow passage 22, an upstream side of the orifice 
21 is nearly arc-shaped and a direction of a wall surface of the orifice 
21 in the downstream side is nearly normal to the direction of the forward 
air flow 23. Further, in regard to the configuration of the orifice and 
the inlet and outlet ports of the auxiliary air flow passage inlet port 11 
and the auxiliary air flow passage outlet port 12 of the auxiliary air 
flow passage body 10, both of the auxiliary air flow passage inlet port 11 
and the auxiliary air flow passage outlet port 12 are arranged in the 
inner side of the orifice diameter D (the flow flux zone D in the figure) 
when seen from the upstream side, as shown in FIG. 2. It is preferable 
that the auxiliary air flow passage inlet port 11 is arranged at a 
position in the wall side of the passage and near the inner side of the 
flow line G1 shown in FIG. 1 and the auxiliary air flow passage outlet 
port 12 is arranged at a position in the wall side of the passage and near 
the inner side of the flow line G2 shown in FIG. 1. 
The reason why the upstream side half of the orifice 21 is formed 
arc-shaped (bell-mouth shaped) is that the air flow near the center of the 
flow passage downstream of the orifice 21 is prevented from being 
disturbed, and the reason why the direction of the wall surface in the 
downstream side half is formed nearly normal to the direction of the main 
flow line is that the forward air flow 23 downstream of the orifice 21 is 
easily flow separated. By doing so, it is possible to increase flow speed 
of forward flow under pulsating flow condition downstream and inside the 
diameter of the orifice 21 without disturbing the flow. 
The heating resistor type flow meter in accordance with the present 
invention comprises a main air flow passage body forming a main air flow 
passage for allowing a fluid to be measured flowing therethrough; and a 
measuring module having a heating resistor for measuring a flow rate of 
the fluid to be measured, inserted inside the main air flow passage body, 
wherein the measuring module comprises the heating resistor inside an 
auxiliary air flow passage body forming an L-shaped auxiliary air flow 
passage having an inlet opening portion opening in a direction 
perpendicular to a main flow line of the fluid to be measured and an 
outlet opening portion opening in a direction parallel to the main flow 
line; the main air flow passage body comprises an orifice on a periphery 
of the inner side wall positioned in an upstream side of the air flow 
passage body; and both the inlet opening portion and the outlet opening 
portion are arranged within a flow flux zone formed by extending the fluid 
to be measured from a top edge of the orifice in a direction parallel to 
the main flow line. 
Description will be made below the mechanism for reducing the jump-up error 
and the binary-value phenomenon caused by the effect of back flow by 
providing the orifice in the upstream side of the L-shaped auxiliary air 
flow passage body which is a characteristic of the present invention. 
Initially, comparison of effects of presence and absence of the orifice 
will be described, referring to FIGS. 4A, 4B, 5A, 5B. 
FIG. 4A and 4B show waveforms for cases without orifice and with orifice, 
respectively. In a case of a conventional flow meter without orifice, when 
back flow occurs in the main air flow passage as shown by the waveform of 
FIG. 4A, the waveform of an actually detected signal becomes a waveform 
folded at a line nearly zero flow speed as shown by the hatched lines 
since the flow direction cannot be detected solely by the heating 
resistor. Further, by employing the L-shaped auxiliary air flow passage 
described above, it is possible to prevent back flow from entering into 
the auxiliary air flow passage as shown by the waveform of effect of 
auxiliary air flow passage of FIG. 4A. 
Furthermore, when an amplitude of flow speed is large enough to cause back 
flow at an average flow speed of U1 in the case without orifice, it is 
possible to prevent back flow from entering into the auxiliary air flow 
passage by the effect of auxiliary air flow passage. However, an average 
value of a waveform, taking response time lag of the heating resistor into 
consideration, is increased by .DELTA.U1 since an amount corresponding to 
back flow is not subtracted from the average value and accordingly the 
forward flow is increased by the corresponding amount. The value .DELTA.U1 
is a detected error due to back flow. 
On the other hand, in a case of arranging an orifice in the upstream side 
of the L-shaped auxiliary air flow passage, since flow separation eddies 
are generated in the downstream side of the orifice, the effective 
crosssectional area of the main air flow passage is narrowed, the average 
flow speed U2 becomes faster than U1 and the pulsating amplitude is also 
increased in the portion in which the auxiliary air flow passage is 
arranged. However, since as to the back flow there is no means for 
reducing effective cross-sectional area in the portion of the auxiliary 
air flow passage, that is, the orifice in the upstream side of the 
auxiliary air flow passage is not related to the back flow, the values 
.DELTA.U1 and .DELTA.U2 as the effect of back flow (flow rate of back 
flow) become nearly equal. That is, it is possible to increase the average 
flow speed solely without changing back flow rate by arranging the orifice 
in the upstream side of the auxiliary air flow passage. 
Therefore, from the above relations, that is, U1&lt;U2, .DELTA.U1=.DELTA.U2, 
the relation (.DELTA.U1/U1)&gt;(.DELTA.U2/U2) is satisfied, and accordingly 
the measuring error (jump-up error) of the flow meter due to back flow in 
the case of providing the orifice in the upstream side of the auxiliary 
air flow passage can be reduced compared to the measuring error in the 
case without the orifice. 
On the other hand, provision of the orifice in the upstream side of the 
auxiliary air flow passage has another effect that it is possible to 
moderate a so-called binary-value phenomenon, that is, decrease of a 
detected value in the flow meter which is caused when a pulsation 
amplitude of air flow increases and even without occurrence of back flow. 
As shown in FIG. 13, the binary-value phenomenon is a decrease in output 
signal which is caused when intake back pressure is varied by gradually 
opening a throttle valve while the rotating speed of an engine is kept 
constant. The reason why this phenomenon is caused is that the output 
characteristic of the heating resistor in regard to air flow rate (flow 
speed) has a non-linear relation. 
When such a phenomenon occurs, a control system of an engine cannot perform 
an accurate fuel control because there are two different operating 
conditions to an equal indication value of flow rate. As having been 
described above in connection with the prior art, this phenomenon can be 
avoided to a certain degree by arranging a heating resistor inside an 
L-shaped auxiliary air flow passage having a bent portion without orifice. 
However, in order to moderate the binary-value phenomenon for all kinds of 
engines, it is necessary to optimize the shape of the auxiliary air flow 
passage for each kind of engine. On the other hand, the orifice in the 
upstream side of the auxiliary air flow passage provided in the heating 
resistor type air flow meter in accordance with the present invention is 
effective for moderating the binary-value phenomenon for all kinds of 
engines. The binary-value phenomenon will be described below, referring to 
FIGS. 5A and 5B showing flow velocity distributions for cases without 
orifice and with orifice, respectively. 
As shown in FIGS. 5A and 5B, a flow distribution of air flow in a duct 
generally shows a parabolic distribution in a steady state condition. 
However, under a pulsating flow condition, the distribution profile 
changes from the parabolic flow velocity distribution to a flat velocity 
distribution as the amplitude of flow speed is gradually increased. With 
comparing the flow velocity distribution by presence and absence of the 
orifice, the distribution in the case without orifice is seen in FIG. 5A 
and the distribution in the case with orifice is seen in FIG. 5B. 
Referring to FIG. 5B, when the orifice 21 exists, air is difficult to flow 
in the vicinity of the wall surface of the main air flow passage 22 since 
the vicinity of the wall is shadowed by the orifice 21. Thereby, speed of 
the air flow in the other portion, that is, in the downstream portion of 
the zone D (for example, cylindrical portion having an inner diameter of 
D) of the orifice 21 is extremely increased. Further, an increased amount 
of flow speed is larger in a position in the wall side of the passage 
apart from the center the passage of the inner diameter downstream portion 
of the orifice 21 shown in the figure than in the center of the passage. 
This is the reason why the auxiliary air flow passage inlet port 11 is 
arranged at a position in the wall side of the passage and near the inner 
side of the flow line G1 and the auxiliary air flow passage outlet port 12 
is arranged at a position in the wall side of the passage and near the 
inner side of the flow line G2. 
As described above, between the increased amount of flow speed .DELTA.U1' 
at a position in the wall side of the passage shown in FIG. 5A and the 
increased amount of flow speed .DELTA.U2' at a position in the wall side 
of the passage shown in FIG. 5B there is a relation .DELTA.U1'&lt;.DELTA.U2'. 
Therefore, by appropriately arranging the inlet port and the outlet port 
of the auxiliary air flow passage in the downstream portion of the zone D, 
flow speed of air flowing in the auxiliary air flow passage is also 
increased as an amplitude of pulsation increases. Therefore, even if an 
output of the heating resistor is decreased due to the non-linearity, the 
increased amount of flow speed increasing the flow speed flowing in the 
auxiliary air flow passage compensates for the corresponding decreasing 
amount. 
However, when the dimension (inner diameter D) of the orifice is reduced 
too much, the increased amount of flow speed becomes excessively large and 
consequently there occurs a phenomenon that the output of the heating 
resistor increases regardless of absence of occurrence of back flow. 
Therefore, in taking it into consideration a decrease in the effect of 
back flow and a reduction of the binary-value phenomenon, a ratio of the 
effective cross-sectional area A1 of the main air flow passage 20 to the 
effective cross-sectional area A2 of the orifice 21 (the effective 
crosssectional area of the zone D) should be set to an optimum value to be 
described below. 
Since the effect of increasing flow speed described above is large at a 
position where flow speed is large, it is important that the inlet port 
and the outlet port of the auxiliary air flow passage are arranged in a 
downstream portion inside the zone D (for example, cylindrical portion 
having a diameter D) of the orifice 21. That is, it is necessary that the 
inlet port 11 of the auxiliary air flow passage opening nearly normal to 
the direction of the main flow line of the air flow should be arranged in 
such a configuration that kinetic pressure directly acts on the inlet port 
11, and the outlet port 12 of the auxiliary air flow passage opening 
nearly parallel to the direction of the main flow line of the air flow 
should be arranged in such a configuration that sucking effect in the 
outlet port is increased by giving kinetic pressure in the upstream side 
of the outlet port and generating flow separation eddies. 
Further, since the outlet port 12 of the auxiliary air flow passage opens 
nearly parallel to the direction of the main flow line of the air flow, it 
is required to suppress loss by collision of air flow with the wall 
surface of the main air flow passage body 20. Therefore, the outlet port 
12 of the auxiliary air flow passage should be arranged appropriately 
apart from the wall surface. 
Results of an experimental study on the above-mentioned orifice dimension 
using a actual vehicle will be described below, referring to FIG. 6 and 
FIG. 7. 
A test was conducted using an engine on a bench in the same procedure as in 
FIGS. 12A and 12B by gradually opening the throttle valve while keeping 
constant rotating speed of the engine, and an detected error indicating 
the heating resistor at full open state of the throttle valve was plotted 
with varying dimension (inner diameter D) of the orifice. From the test, 
in regard to dimension of the orifice as shown in FIG. 5B and FIG. 6, an 
effect of reducing the jump-up error due to back flow could be obtained in 
a range of the contraction ratio R=(A2/A1).gtoreq.7%, where A1 is the 
effective cross-sectional area of the main air flow passage in which the 
auxiliary air flow passage was placed, and A2 is the effective 
cross-sectional area of the orifice having an inner diameter of D. 
On the other hand, when the contraction ratio R was smaller than 70%, it 
was found that the output was caused to increase. The reason is that the 
detected flow speed itself increases downstream of the orifice when the 
amplitude of the pulsating flow increases, as described above. A test 
result at a rotating speed with back flow not occurring is also shown in 
the figure for purpose of reference. It was confirmed that the output 
rapidly increases with a contraction ratio R&lt;70%. 
Therefore, it may be preferable that the ratio of the cross-sectional area 
A2 of the orifice to the cross-sectional area A1 of the main air flow 
passage in which the auxiliary air flow passage was placed satisfies the 
relation R=(A2/A1).gtoreq.70%. However, in taking the case of the 
contraction ratio R of 100% (corresponding to the conventional technology) 
into consideration, it can be said that the range 90%.gtoreq.R.gtoreq.70% 
is preferable. Particularly, in order to reduce the error to one-half, the 
range 80%.gtoreq.R.gtoreq.70% is preferable. Further, it has been 
confirmed from the test result that the effect of reducing the jump-up 
error is good when 90%.gtoreq.R.gtoreq.70% and a distance L from the 
orifice 21 to the inlet opening portion 11, shown in FIG. 1, is near a 
value satisfying the relation L=0.7D. 
Description will be made below on the relationship between positional 
relationship of the orifice and the inlet and outlet ports of the 
auxiliary air flow passage and output noise of the flow meter under a 
steady state condition, referring to FIG. 7 in which the ordinate 
indicates value of output noise and the abscissa indicates contraction 
ratio R as in FIG. 6. 
Dimension of a sample orifice used in this test had a contraction ratio R 
of nearly 60%. Therefore, a contraction ratio R smaller than 60% means 
that the both positions of the inlet port and outlet port of the auxiliary 
air flow passage are within a zone shadowed by the orifice 21 (a wall side 
zone outside a zone surrounded by the main flow lines G1 and G2 shown in 
FIG. 1). 
As shown in FIG. 7, within the range of the contraction ratio R of 
100.about.60%. the output noise decreases as the contraction ratio R is 
decreased. However, in the range of the contraction ratio smaller than 
60%, the output noise is clearly increased. That is, when the both 
positions of the inlet port and outlet port of the auxiliary air flow 
passage are within the zone D (the zone within the zone surrounded by the 
main flow lines G1 and G2 shown in FIG. 1), the output noise is small. It 
has been found that the output noise can be reduced basically by 
increasing the contraction ratio R, that is, by decreasing the dimension 
of the orifice to increase flow speed of the air. Further, it has been 
found that flow in the main air flow passage is disturbed and accordingly 
output noise is increased when then orifice exists upstream and flow 
separation occurs downstream, that is, the both positions of the inlet 
port and the outlet port of the auxiliary air flow passage are within the 
zone shadowed by the orifice 21. 
From the above result, both the inlet port and the outlet port require that 
upstream flow is not disturbed since a value of flow speed in the 
auxiliary air flow passage is determined by a pressure difference between 
the inlet port and the outlet port. Therefore, in a case of an orifice or 
the like arranged in the upstream side of the auxiliary air flow passage, 
it is necessary to take the relative positions of the inlet port and the 
outlet port of the auxiliary air flow passage and the dimension of the 
orifice into consideration from viewpoint of output noise. 
From the results of FIG. 6 and FIG. 7, in order to reduce the jump-up error 
and the output noise, it can be said that the range 
90%.gtoreq.R.gtoreq.70% is preferable. 
FIG. 8 is a cross-sectional view showing another embodiment of a heating 
resistor type air flow meter in accordance with the present invention. The 
figure shows a cross-sectional side view of the flow meter in which a 
straight pipe 41a having an orifice forming a main air flow passage is 
integrated as a part of an air cleaner clean side 41 of an air cleaner 68. 
The air cleaner 68 of one of intake pipe structural members composing the 
intake system is composed of the air cleaner clean side 41 integrating the 
straight pipe 41a having the orifice of the main air flow passage body 20 
as a unit and an air cleaner dirty side 42 and an air filter element 43. 
In this embodiment, the orifice 21 is provided in an intake air outlet 
portion of the air cleaner clean side 41 (a duct placed in the downstream 
side of the air filter 43), and the straight pipe 41a as the flow meter is 
integrally connected downstream of the orifice, and the measuring module 
52 of the auxiliary air flow passage 10 shown in FIG. 1 is inserted into a 
hole 25 provided on a wall surface of the straight pipe 41a having the 
orifice. In this embodiment, since the air cleaner 68 of an existing 
intake pipe structural member serves as the main air flow passage body 20 
having the orifice 21 and the hole 25, a dedicated air flow passage is not 
required and accordingly the system cost can be reduced. 
FIG. 9 is a cross-sectional view showing a further embodiment of a heating 
resistor type air flow meter in accordance with the present invention. The 
figure shows the flow meter in which an orifice 21 is provided in a part 
of an air cleaner clean side 41, and further a main air flow passage body 
20 is connected to the air cleaner clean side 41. FIG. 10 is an enlarged 
view showing the joint portion of FIG. 9. The figure shows the details of 
the joint portion of a soldering portion 47 and the joint portion 48. 
This embodiment is basically the same as the embodiment of FIG. 8, but the 
outlet opening portion of the intake air outlet portion of the air cleaner 
68 is formed in a bell-mouth shaped orifice 21, and a body 53 of the main 
air flow passage body 20 of the flow meter is mechanically connected to 
the downstream side by bonding or screw fastening in the soldering portion 
47 and the joint portion 48. That is, the orifice of the main air flow 
passage body composing the heating resistor type air flow meter is 
separated from the main air flow passage body, and the orifice is provided 
in the intake air outlet portion of the intake pipe structural member to 
be connected with the main air flow passage. In this embodiment, since the 
existing intake pipe structural member serves as the orifice 21, a 
dedicated air flow passage is not required. Further, an exiting heating 
resistor type air flow meter without orifice can be used. Accordingly, the 
system cost can be reduced. 
FIG. 11 is a view showing an embodiment of an internal combustion engine of 
an electronic fuel injection control type mounting a flow meter in 
accordance with the present invention. The figure shows an embodiment of a 
fuel control engine for controlling a fuel supplying rate using an air 
flow rate signal obtained from a heating resistor type air flow meter of 
the present embodiment. 
Referring to the figure, intake air 67 to be sucked is taken in an engine 
cylinder 62 through an intake system composed of an air cleaner 67, a body 
53 of the flow meter, a duct 55, a throttle body 58, an intake manifold 59 
having an injector 60 to which fuel is supplied and so on. On the other 
hand, exhaust gas generated in the engine cylinder 62 is exhausted through 
an exhaust manifold 64. 
A control unit 66 receives an airflow rate signal output from a measuring 
module 52 of the flow meter, a throttle valve angle signal output from a 
throttle valve angle sensor 57, an oxygen concentration signal output from 
an oxygen concentration meter 65 provided in the exhaust manifold 64, an 
engine rotating speed signal output from an engine rotating speed meter 61 
and so on. The control unit 66 sequentially calculates these signals to 
obtain an optimum fuel injecting rate and an optimum idling air control 
valve opening degree, and controls the injector 60 and the idle air 
control valve 56 using the obtained values so as to supply a flow rate of 
fuel matching with an intake air flow rate. 
Since an intake air flow rate can be appropriately measured in the internal 
combustion engine of an electronic fuel injection control type mounting 
the flow meter in accordance with the present invention, the electronic 
fuel injection control can be properly performed and consequently an 
amount of unburned fuel gas in the exhaust gas can be reduced. 
According to the present invention, increase of flow speed in the flow flux 
zone formed by the orifice of a simple structure reduces the jump-up error 
under pulsating flow accompanying back flow when a heating resistor type 
flow meter is mounted on a vehicle and the bad effect of the binary-value 
phenomena, and thereby a low-cost and highly accurate heating resistor 
type flow meter can be provided. 
Further, appropriate fuel control can be performed when a driver steps on 
the accelerator, and therefore there is an effect in cleaning of exhaust 
gas of an internal combustion engine of an electronic fuel injection type. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.