ELECTRONIC CONTROL SYSTEM
- Air Flow Mete
r
2
. OPTICAL KARMAN VORTEX TYP
E
This type of air flow meter directly senses the
intake air volume optically
. Compared to the
vane type air flow meter, it can be made smaller
and lighter in weight
. The simplified construction
of the air passage also reduces inlet resistance
.
This air flow meter is constructed as shown in
the following illustration
:
Mirror
LED
Leaf sprin
g
From
W~'M
I To air
air
moo,
Karman
♦
intake
creaner ~9
v
n
.toYO~ chambe
r
Pressure-
directin
Mirro
r
~ Pressur
e
~ directing
ape
rt
ur
e
OHP 1
3
Vo
rt
ex
/
___
.
. 9 Phntntrancictn
r
generator OHP 1
3
OPERATION AND FUNCTIO
N
A pillar (called the "vo
rt
ex generator") placed in
the middle of a uniform flow of air generates a
vo
rt
ex called
a "Karman
vo
rt
ex" down-stream
of the pillar
.
The frequency "f" of the Karman vortex thus
generated, the velocity of the air "V" and the
diameter of the pillar "d" have the following
relationship
:
f - K x d
®
of a piece of thin metal foil (called a "mirror") to
the pressure of the vortexes and optically
detecting the vibrations of the mirror by means
of a photocoupler (an LED combined with a
phototransistor)
.
LE
D
r=^
Phototransisto
r
Vo
rt
ex -
generato
r
The intake air volume (KS) signal is a pulse
signal like that shown below
. When the intake
air volume is low, this signal has a low
frequency
. When the intake air volume is high,
this signal has a high frequency
.
High
Voltage
signa
l
Low
Low
Intake air volume
Hig
h
OHP 1
3
ELECTRICAL CIRCUITR
Y
Air flow mete
r
Pillar (vo
rt
ex generator
)
KARMAN VORTEX
OHP 1
3
Utilizing this principle, the frequency of the
vortexes generated by the vortex generator is
measured, making it possible to determine the
air flow volume
.
Vo
rt
exes are detected by subjecting the su
rf
ace
Phototransistor
Engine EC
U
OHP 1
3
21
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