Abstrict The invention relates to an air flow rate measuring device making
use of a hot wire type air flow meter and adapted to measure the
true intake air flow rate while compensating for the error attributable
to the reversing of the air. The device includes an operation unit
adapted to accumulate the output from the hot wire type air flow
meter to calculate an apparent intake air flow rate Q.sub.a =Q.sub.1
+Q.sub.2 where Q.sub.1 is the flow rate during reversing of the
air while Q.sub.2 represents the flow rate during forward flowing
of the air. The apparent flow rate Q.sub.a is discriminated by a
signal derived from means for detecting the period in which the
reversing of intake air takes place into the flow rate Q.sub.1 in
the reversing or backward direction and the flow rate Q.sub.1 in
the forward direction, by means of the operation unit. Another operation
unit performs the calculation of difference Q.sub.2 -Q.sub.1 to
determine the true air flow rate Q.sub.o.
Claims What is claimed is:
1. An air flow rate measuring device making use of a hot wire type
air flow meter comprising: a hot wire type air flow meter including
a hot wire type flow rate measuring sensor disposed in the intake
air passage of an internal combustion engine and a control drive
circuit connected to said sensor, said hot wire type air flow meter
being adapted to produce a first signal corresponding to the flow
rate of air in said intake air passage; a reversing flow signal
generating means adapted to generate a second signal representing
the period of occurence of reversing or backward flow of air in
said intake air passage; a first operation unit connected to said
control drive circuit and said reversing flow signal generating
means and adapted to generate, upon accumulation of the first signal
representing said flow rate of air, a third signal representing
the amount of air in said period of occurence of reversing flow
and a fourth signal representing the amount of air in the period
other than said period of occurence of reversing flow; and a second
operation unit adapted for subtracting said third signal from said
fourth signal derived from said first operation unit.
2. An air flow rate measuring device making use of a hot wire type
air flow meter as claimed in claim 1 wherein said reversing flow
signal generating means is a memory device adapted to memorize the
crank angle at which the reversing flow is commenced and the crank
angle at which the reversing flow is ceased in relation to the engine
speed, and to deliver the crank angles in accordance with the actual
engine speed.
3. An air flow rate measuring device making use of a hot wire type
air flow meter as claimed in claim 1 wherein said reversing flow
signal generating means include a pair of hot wire type sensors
disposed at an upstream portion and a downstream portion of said
air passage, and a drive control circuit adapted for discriminating
the phase relation between the outputs from said sensors.
4. An air flow rate measuring device making use of a hot wire type
air flow meter as claimed in claim 1 wherein said reversing flow
signal generating means include a temperature sensor disposed in
the air passage, a drive control circuit connected to said temperature
sensor, and a reversing flow discrimination circuit adapted to produce
an output over a period in which the output from said drive control
circuit representing the temperature exceeds a predetermined level.
Description TECHNICAL FIELD
The present invention relates to an air flow rate measuring device
and, more particularly, to an air flow rate measuring device incorporating
a hot wire type air flow meter suitable for use in automobiles.
BACKGROUND ART
A device for measuring the flow rate of intake air in an automobile
engine, of the type having a resistance element having a resistance
temperature coefficient and disposed in the intake air passage and
adapted to measure the intake air flow rate using the change in
the resistance, is known by, for example, U.S. Pat. No. 3747577.
This resistance element is heated within the intake air passage
of the engine by electric current supplied thereto, and the resistance
value thereof is changed in accordance with the flow rate of the
intake air in the intake passage.
In automobile engines, the air-fuel ratio is controlled upon detection
of the intake air flow rate per one suction stroke of the engine
by accumulating the intake air flow rate. It is, therefore, very
important to precisely detect the flow rate of the intake air. An
inaccurate measurement leads to an incorrect control of the air-fuel
ratio.
The size of the hot wire type air flow meter is advantageously
small but this type of flow meter has a drawback that it cannot
detect the direction of flow of the air. Therefore, it is not possible
to measure the correct or true intake air flow rate by a mere accumulation
of the output of the hot wire type air flow meter, particularly
in the low speed region of operation of a 4-cylinder internal combustion
engine.
DISCLOSURE OF INVENTION
Accordingly, an object of the invention is to provide an air flow
rate measuring device having a hot wire type air flow meter improved
for use in automobile engines.
Another object of the invention is to provide an air flow rate
measuring device incorporating a hot wire type air flow meter capable
of measuring the true intake air flow rate in automobile engines.
When a 4-cylinder engine is operating at a low speed, the intake
air flow rate (instantaneous value) of the engine fluctuates largely.
In addition, in the region near the top dead center, the intake
valve and the exhaust valve are opened simultaneously so that the
intake air is forced back by the pressure which acts to reverse
the flow of gas from the exhaust valve toward the intake valve.
This reversing of the gas does not take place when the intake valve
solely is kept opened.
The reversing of the gas takes place in the low speed region of
600 to 3000 R.P.M. and in the region of low intake vacuum of below
-100 mmHg. The hot air type flow meter produces the detection signal
irrespective of the flowing direction of the air.
Representing the apparent intake air amount obtained by an integration
or accumulation of the hot wire type flow meter by Q.sub.a (m.sup.3),
the air amount flowing in the reverse direction by Q.sub.1 (m.sup.3),
the air amount in the forward direction by Q.sub.2 (m.sup.3) and
the true air amount by Q.sub.0 (m.sup.3), the following equations
(1) and (2) are established.
Thus, the true air flow rate Q.sub.0 is given as the difference
between the flow rate Q.sub.2 in the forward direction and the flow
rate Q.sub.1 in the backward direction.
According to the invention, the apparent air flow rate Q.sub.a
is measured by integrating or accumulating the output of the hot
wire type air flow meter disposed in the intake air passage, and
a signal representing the period of reversing flow is generated
to divide the apparent air flow rate Q.sub.a into the flow rate
Q.sub.1 in the backward direction and the flow rate Q.sub.2 in the
forward direction. Then, the difference Q.sub.2 -Q.sub.1 is calculated
by an operation unit to determine the true air flow rate Q.sub.0.
According to an embodiment of the invention, there is provided
a memory device adapted to memorize the crank angle corresponding
to engine speed, and a crank angle sensor adapted to produce a signal
corresponding to the crank angle of the engine. The period of the
backward flow of intake air is read out from the memory device in
accordance with the output signal from the crank angle sensor, so
that the apparent air flow rate Q.sub.a is divided into the flow
rate Q.sub.1 in the backward direction and the flow rate Q.sub.2
in the forward direction, and the difference Q.sub.2 -Q.sub.1 is
calculated by the operation unit Q.sub.2 -Q.sub.1 to determine the
true air flow rate Q.sub.0.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of an embodiment of the invention, showing
particularly the mounting structure for a hot wire type sensor;
FIG. 2 is a block diagram of a signal processing unit in an embodiment
of the invention;
FIG. 3 is a graphical representation of an analog output waveform
of a hot wire sensor;
FIG. 4 is a graphical representation of the period in which the
reversing flow of the intake air takes place;
FIG. 5 is a block diagram of another embodiment;
FIG. 6 is a graphical representation of the content of a memory
device;
FIG. 7 is a sectional view of still another embodiment;
FIG. 8 is a graphical representation of operation of the device
shown in FIG. 7;
FIG. 9 is a sectional view of a further embodiment; and
FIG. 10 is a graphical representation of operation of the device
shown in FIG. 9.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows how an air flow rate sensor 10 is mounted. At the
lower side of the main venturi 12 disposed is a throttle chamber
14 provided with a primary and a secondary throttle valves 16 and
18. Also, a fuel injector 20 for injecting the fuel at the downstream
side of the primary throttle valve is secured to the throttle chamber
14. The air flows through an air filter (not shown) provided at
the upstream side of the main venturi 12 and flows through the latter
so as to be mixed with the fuel injected from the injector to form
an air-fuel mixture which is supplied into the cylinder of the internal
combustion engine.
A part of the intake air flows into a bypass passage 24 due to
a pressure difference between the upstream side of the main venturi
12 and the slit 22 as indicated by an arrow. The bypass passage
24 is provided at a portion of a venturi chamber body 26 which forms
a main venturi 7. Namely, the air which has passed through the bypass
passage 24 merges in the flow in main venturi 12 through a slit
22 formed around the inner wall of the main venturi 12.
The flow of intake air is uniformalized by an air guiding plate
28 made of a metal or an insulating material disposed at the upstream
side of the sensor 10. The guide plate 28 has an aperture of preferably
a circular form. Accordingly, the bypass passage 24 preferably has
a circular form. A reference numeral 30 designates an orifice for
maintaining a constant value of the ratio between the flow rate
in the main venturi 12 and the flow rate in the bypass passage 24.
This ratio is referred to as "shunting ratio". A dust
cover 32 covers an area greater than the area of the aperture of
the guiding plate 28 thereby to prevent dusts or other foreign
matters from coming directly into the bypass passage 24.
The sensor 10 which is disposed in the passage 24 of air and having
an electric characteristics changeable in accordance with the air
flow rate, is grounded through a resistance 32. The sensor 10 and
the resistance 32 constitute two sides of a bridge circuit.
The voltage appearing at the terminals of the resistance 32 is
transmitted to the control circuit 34. The output from the circuit
34 is transmitted to an amplifier 36 the output of which is used
for the control of the power transistor 38. The measurement sensor
10 is connected at its one end to the collector of a power transistor
38. The electric current I flowing in the measurement sensor 10
is detected by the resistance 32.
Also, the current I is controlled by the control circuit 34 and
the transistor 38 in such a manner as to maintain a constant temperature
of the sensor 10 i.e. a constant resistance value of the same.
Accordingly, the relationship between the current I and the air
flow rate Q is represented by the following equation.
It is, therefore, possible to detect the flow rate Q by measuring
the current I. In above equation, symbols K.sub.1K.sub.2 and S
represent constants, t.sub.w represents the temperature of the sensor
10 and t.sub.a represents the temperature of the atmosphere.
The analog signal Q.sub.an (m.sup.3 /min) representing the flow
rate of air in the air passage 24 is supplied to an analog-to-digital
converter (referred to as A/D converter, hereinunder) 40 and is
converted into a digital signal Q.sub.d (m.sup.3 /min). A data processing
unit 42 picks up the digital signal Q.sub.d at each predetermined
rotation angle of the crank shaft of the engine, e.g. at every 1.degree..,
and calculates the mean value of the digital signal at each predetermined
angular movement of the crank shaft, e.g. at every 5.degree., to
produce a signal Q.sub.f of a pulse frequency corresponding to the
mean air flow rate.
The crank angle sensor 44 is provided with a movable core 46 connected
to the crank shaft (not shown) of the internal combustion engine
and a pick-up 48 opposing to the movable core 46 and produces a
pulse signal at each predetermined angular movement of the crank
shaft, e.g. at every 1.degree.. A reference crank angle sensor 49
has a movable core 50 connected to the crank shaft and a pickup
51 opposing to the movable core 50 and produces an output pulse
at each time the crank shaft passes the top dead center. The pulse
of the sensor 44 is in synchronization with the sensor 49.
An operation unit 52 is adapted to accumulate, in accordance with
the outputs from the data processing unit 42 crank angle sensors
4449 and memory devices 535456 and 58 the air flow rate signal
presented by the data processing unit 42 to divide the calculated
value into the flow rate Q.sub.1 in the backward direction and the
flow rate Q.sub.2 in the forward direction. The operation unit 60
serves to determine the difference between the flow rates Q.sub.1
and Q.sub.2.
The intake air flow rate in 4-cylinder engine, particularly when
the engine speed is low, pulsates in relation to time as shown in
FIG. 3. A reversing of air from the exhaust valve side to the intake
valve side is observed when the piston takes a position around the
top dead center where both of the exhaust valve and the intake valve
are opened simultaneously. The hot wire type air flow rate sensor
10 produces a detection signal irrespective of the flowing direction
of the air, so that the output of the sensor has a pulsating waveform
as shown in FIG. 3. In this Figure, T.sub.R represents the period
over which both of the intake and exhaust valves are kept opened
to permit the reversing of the intake air, while T.sub.R represents
the period in which the exhaust valve T.sub.F is opened solely to
cause the air to flow in the forward direction. Thus, the flow rates
in one suction stroke in the backward and forward directions are
Q.sub.1 (m.sup.3) and Q.sub.2 (m.sup.3), respectively, and the total
air flow rate Q.sub.0 actually charged in one suction stroke in
the cylinder is represented as the difference Q.sub.2 -Q.sub.1.
The period T.sub.R over which the backward flow takes place has
a functional relation to the revolution speed N of the engine as
will be seen from FIG. 4. More specifically, this period corresponds
to about .+-.20.degree. around the top dead center by the crank
angle. The period is shortened as the engine speed N is increased,
and almost no reversing takes place at the high-speed region in
excess of 3000 R.P.M.
Memory devices 53 and 54 are adapted to memorize a curve C.sub.1
representing the crank angle D.sub.s of commencement of the backward
or reverse flow of air and the engine speed N, and a curve C.sub.2
representing the relationship between the crank angle D.sub.T at
which the reversing flow is ceased and the engine speed N, and deliver
respective outputs in accordance with the engine speed N derived
from the operation unit 52.
Representing the engine speed by N.sub.1 (e.g. 500 R.P.M.), the
pickup 48 of the crank angle sensor 44 delivers an output pulse
corresponding to the speed N.sub.1 to the input terminal of the
operation unit 52 which is reading the crank angle corresponding
to the engine speed N.sub.1 e.g. 20.degree. before the top dead
center. As the crank shaft angle reaches D.sub.1 shown in FIG. 3
(D.sub.1 : 20.degree. before top dead center), the operation unit
52 decides upon receipt of the signal from the memory device 53
that the period of the reversing flow has just started, and the
value accumulated by the data processing unit 42 from this moment
is stored in the memory device 56. Then, as the crank shaft angle
D.sub.2 (20.degree. after top dead center) is reached, the operation
unit 52 judges that the period T.sub.R of reversing is over, and
stops to store the accumulated value in the memory device 56. Thereafter,
the value accumulated by the data processing unit 42 is stored in
the memory device 58. The value accumulated by the data processing
unit 42 later than this moment is stored in the memory device 58
until the reversing start signal is generated. In consequence, the
memory device 56 memorizes the air flow rate Q.sub.1 in the backward
direction while the memory device 58 memorizes the air flow rate
Q.sub.2 in the forward direction. The operation unit 52 periodically
reads out the content of memory of the memory devices 56 and 58
and delivers the read-out content to the operation unit 60 to conduct
the subtraction to calculate the true air flow rate Q.sub.0 m.sup.3
i.e. Q.sub.2 -Q.sub.1.
Referring now to FIG. 5 which is a block diagram of another embodiment,
there are shown only an operation unit 70 for accumulating the signal
of the data processing unit 42 and a memory device 72 for memorizing
a predetermined data. FIG. 6 shows the result of measurement of
the ratio between the difference between the apparent total air
flow rate Q.sub.a (=Q.sub.1 +Q.sub.2) and the true air flow rate
Q.sub.0 (=Q.sub.2 -Q.sub.1), i.e. 2Q.sub.1 and the apparent total
intake air flow rate Q.sub.0. Since the true air flow rate Q.sub.0
is expressed by the following equation (3), the true air flow rate
is determined by executing this calculation.
As shown in FIG. 6 the value 2Q.sub.1 /Q.sub.1 +Q.sub.2 is determined
by the speed N of the engine, so that the value 1-2Q.sub.1 /Q.sub.1
+Q.sub.2 is beforehand memorized by the memory device 72. On the
other hand, the operation unit 70 calculates the apparent total
air flow rate Q.sub.a =Q.sub.3 -Q.sub.1 and reads the content of
the memory device 72 in accordance with the engine speed and performs
the calculation of the equation (3) to determine the true air flow
rate Q.sub.0.
FIG. 7 shows still another embodiment of the invention. More specifically,
this Figure shows only an operation unit 80 two hot wire type sensors
8284 and a drive control circuit 86. The waveforms of signals sensed
by the sensors 8284 have phase difference from each other in relation
to the crank angle, as will be seen from FIG. 8A. During the sucking
of the sucking of the air by the internal combustion engine, the
output from the donwstream side sensor 84 lags behind that from
the upstream side sensor 82. This phase relation is reversed during
the reversing or backward flow of the intake air, as shown in FIG.
8B. Therefore, the drive control circuit 86 detects the phases and
makes a judgement and supplies a signal representing the reversing
period to the operation unit 80 which in turn accumulates the output
from the hot wire type air flow meter and determines the intake
air flow rates Q.sub.1 and Q.sub.2 in the backward and forward directions.
Thereafter, the true air flow rate Q.sub.0 is determined in the
manner identical to that explained before in connection with FIG.
2.
FIG. 9 shows a further embodiment. In this Figure, there are shown
an operation unit 90 temperature sensor 92 drive control circuit
94 and a reverse flow discrimination circuit 96. As will be understood
from FIG. 10 the temperature of the air during the reversing period
T.sub.R is higher than that during the period of forward flow of
the air. It is, therefore, possible to obtain a signal representing
the period of reversing flow by setting the temperature by the reverse
flow discriminating circuit 96. Then, the true air flow rate Q.sub.0
is determined in the same manner as that explained before in connection
with FIG. 2. |