Gas flow measuring apparatus

An intake barrel connected to an intake pipe of an internal combustion engine is provided therein with a venturi member. Openings are formed in the wall of the intake barrel at the upstream and downstream sides of the venturi member. A bypass passage forming member provided with a groove of a length greater than the distance between said openings is fixed to the outside of the intake barrel. A bypass passage is formed by these openings and the groove. A spacer is interposed between the intake barrel and the bypass passage forming member. The cross-sectional area of the bypass passage can be varied by varying the thickness of the spacer.

BACKGROUND OF THE INVENTION 
The present invention relates to a flow measuring device and, more 
particularly, to an apparatus for measuring the flow rate of a gas. 
An air flow measuring apparatus having a heat generating resistance body 
such as a platinum wire mounted in a path of air flow by means of an 
attachment is known from the specification of U.S. Pat. No. 3,824,966 
issued to Schneider et al. on July 23, 1974. This air flow measuring 
apparatus suffers, when used as an air flow measuring apparatus for 
measuring the intake air flow rate in an internal combustion engine, that 
the wire is burnt out or deteriorated due to the reverse flow of gas in 
the event of a back fire and, accordingly, has a poor durability. In 
addition, the adjustment of the tension of the wire is extremely 
difficult. Namely, if the tension is excessively large, the wire becomes 
more liable to be broken. To the contrary, a too low tension 
inconveniently permits the wire to be vibrated to cause a degradation of 
the precision of measurement. 
The U.S. Pat. No. 3,559,482 granted on Baker et al on Feb. 2, 1971 proposes 
to dispose an air flow measuring apparatus in a bypass passage between an 
opening disposed at a portion of the air intake passage downstream from a 
venturi and an opening formed at a portion of the same upstream from the 
venturi. This air flow measuring apparatus is superior in that it can 
shelter from the reversing flow of gas in the event of back fire and that 
the installation of the air flow measuring apparatus is comparatively 
easy. In addition, the size of the apparatus can be made considerably 
small. 
These known air flow measuring apparatus, however, involve a common 
disadvantage that the detection or measuring characteristic is determined 
by the construction of the air passage or the bypass passage and the 
calibration characteristic cannot be altered easily. 
For instance, when these known air flow measuring apparatus are applied to 
engines having different capacities, it is necessary to modify the shape 
and construction of the air passage or the bypass passage and, in 
addition, to partially modify the arrangement of the platinum wire and the 
circuit arrangement. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to provide an air flow 
measuring apparatus easily adjustable for different measuring 
characteristics and, hence, suitable for a mass-production. 
To this end, according to the invention, there is provided an air flow 
measuring apparatus comprising a bypass passage forming member fixed to 
the outer periphery of an intake barrel which forms a main intake air 
passage with a spacer interposed therebetween, said bypass passage forming 
member being provided with an elongated groove so that bypass passage is 
formed between said bypass passage forming member and said intake barrel, 
and a flow meter interposed in said bypass passage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, an intake barrel 10 is a stepped cylindrical 
member the inside diameter of which is greater at the upstream side than 
at the downstream side thereof. A venturi member 12 is inserted into the 
intake barrel from the upstream side end of the latter, and is fixed to 
the inside of the intake barrel 10 by means of a pipe 18 which is driven 
into an opening 14 formed at the outside of the central portion of the 
venturi member 12 and an opening 16 formed in the wall of the intake 
barrel 10 at a portion of the latter confronting the opening 14. An 
opening 20 is formed at the narrowest throat portion 19 of the venturi 
member 12 so as to communicate with the pipe 18. An opening 22 is formed 
in the wall of the intake barrel 10 at a portion of the latter upstream 
from the venturi member 12. The opening 22 and the opening 20 in the 
venturi member 12 are located on a line parallel to the central axis of 
the intake barrel 10. The opening 22 is provided with a filter 24. 
A bypass passage forming member 28 made of a synthetic resin and provided 
with a rectangular groove 26 of a length greater than the distance between 
the openings 20,22 is fixed to the outside of the intake barrel 10. A 
spacer 32 made of a synthetic resin is placed between the bypass passage 
forming member 28 and the intake barrel 10. The spacer 32 is provided with 
a rectangular bore of a size equal to that of the groove 26. The openings 
22 and 20 are communicated with each other to form a bypass passage 34. 
A circuit board 38 is accommodated by a recess 36 formed in the bypass 
passage forming member 28 so as to oppose to the groove 26. 
Two sets of support pins 40,42 extending through the central portion of the 
bypass passage forming member 28 are connected at their one ends to the 
electronic parts of the circuit board 38, while the other ends of the same 
are connected to a flow rate sensor 44 and a temperature compensation 
sensor 46. The flow rate sensor 44 is a resistance member whose resistance 
value is varied at a high sensitivity in response to a change in the flow 
rate, while the temperature compensation sensor 46 is a resistance member 
whose resistance value depends on the air temperature irrespective of the 
air flow rate. Although not shown, the flow rate sensor 44 and the 
temperature compensation sensor 46 are connected to two sides of a bridge 
circuit formed on the circuit board 38. 
The circuit board 38 is coated with an anti-humidity insulating material 52 
so that the electric insulating performance thereof is much improved. The 
recess 36 is filled with an insulating filler 54 and is then sealed by 
means of a cover 55. 
A connector 56 provided at the outside of the bypass forming passage 28 is 
connected to a terminal 58 so as to supply the bridge circuit on the 
circuit board 38 with the required electric power. Thus, an air flow meter 
is formed on the bypass passage forming member 28 by means of the flow 
rate sensor 44, temperature compensation sensor 46 and the circuit board 
including the bridge circuit. 
The level of vacuum generated around the opening 20 of the venturi member 
12 is changed in accordance with the change of flow rate of air passing 
the intake barrel 10. Accordingly, the velocity of air flowing through the 
bypass air passage 34 is changed. The change of the velocity of air is 
detected by the flow rate sensor 44 and the temperature compensation 
sensor 46 which are connected in a bridge, and an electric output 
corresponding to the flow rate of air flowing through the intake barrel 10 
is derived from the output terminal 60. This principle of operation is 
disclosed in, for example, the specification of U.S. Pat. No. 3,824,966. 
The intake barrel 10 is communicated at its downstream side end with the 
combustion chamber of the engine through the throttle valve chamber and 
the intake pipe. A fuel injection valve is disposed at a portion of the 
throttle valve cahmber downstream from the throttle valve. The operation 
of the fuel injection valve is controlled in relation to the output signal 
derived from the output terminal 60. 
In the described embodiment, the flow meter is less liable to be affected 
by the reversing flow of gas, even when a back fire happens to take place, 
partly because the openings 20,22 of the bypass air passage 34 are 
sufficiently small and partly because these openings are arranged at a 
right angle to the intake passage, so that the erroneous measurement of 
the air flow rate is avoided. Since the bypass passage forming member 28 
is made of a synthetic resin having a low thermal conductivity, the 
temperature of air flowing through the bypass passage 34 is never changed. 
The stabilization of the air temperature in turn suppresses the density of 
air to ensure a precise measurement of the air flow rate. 
In addition, since the venturi member 12 and the spacer 32 are replaceable, 
the air flow measuring apparatus of the invention can apply to engines of 
different capacities. Namely, when the air flow measuring apparatus is 
applied to an engine having a larger capacity, the venturi member 12 is 
replaced with another venturi member having a larger cross-sectional area 
of passage and the spacer 32 is replaced with another spacer having a 
greater thickness. By so doing, the cross-sectional area of the bypass air 
passage 34 is increased to lower the flowing velocity of air flowing 
therethrough to magnify the measureable range of the intake air flow rate. 
In consequence, the intake barrel 10 and the bypass passage forming member 
28 are commonly used for various capacities of engine, so that the 
measuring apparatus can suitably be mass-produced and the efficiency of 
assembling of the air flow meter is improved.