Abstrict A hot wire air flow meter arrangement comprises a primary and an
auxiliary air flow path defined in the air induction passage. A
hot wire is provided in the primary air flow path for monitoring
air flow rate therethrough. A flow restricting means is provided
in the auxiliary air flow path for restricting air flow therethrough.
The flow restricting means is selectively operative so that it is
effective for restricting air flow in a low engine load condition
to force the major part of induction air to flow through the primary
air flow path and is not effective at high engine load range so
as to minimize flow resistance for induction of sufficient amount
of air to an internal combustion engine.
Claims What is claimed is:
1. A hot wire flow meter arrangement for monitoring an intake air
flow rate flowing through an air induction system of an internal
combustion engine, comprising:
means for defining a measuring portion where the intake air flow
rate is to be measured, in said air induction system;
an electrically conductive resistor body disposed within said measuring
portion in such an orientation as to subject air flow through said
measuring portion;
an electric circuit connected to said resistor body for supplying
a controlled voltage for said resistor body for heating the latter
at a predetermined temperature, said electric circuit outputting
an output signal representative of the voltage supplied to said
resistor body; and
means for controlling the path area of said measuring portion in
such a manner that the path area of said measuring portion varies
at least between a first minimum area and a second maximum area,
said path area controlling means being operated to said second position
while the engine load is heavier than a predetermined criterion
and to said first position while the engine load is smaller than
said criterion.
2. A hot wire air flow meter arrangement as set forth in claim
1 which further comprises means for deriving said intake air flow
rate on the basis of said output signal of said electric circuit
according to a first characteristic when said path area controlling
means is placed at the first position and according to a second
characteristic when said path area controlling means is placed at
the second position.
3. A hot wire flow meter arrangement as set forth in claim 1 which
further comprises means for restricting intake air flowing into
said first path for regulating air flow through said first path.
4. A hot wire flow meter arrangement as set forth in claim 1 wherein
said detecting means detects said engine load condition on the basis
of an angular position of a throttle valve disposed in said air
induction system.
5. A hot wire flow meter arrangement as set forth in claim 1 wherein
said detecting means detects said engine load condition on the basis
of an angular position of a throttle valve disposed in said air
induction system and an engine revolution speed.
6. A hot wire flow meter arrangement as set forth in claim 1 wherein
said detector means detects said engine load condition on the basis
of said output signal of said electric circuit.
7. A hot wire flow meter arrangement for monitoring an intake air
flow rate flowing through an air induction system of an internal
combustion engine, comprising:
means for separating a measuring portion where the intake air flow
rate is to be measured, into a first and a second paths;
an electrically conductive resistor body disposed within said first
path in such an orientation as to subject air flow through said
first path;
an electric circuit connected to said resistor body for supplying
a controlled voltage for said resistor body for heating the latter
at a predetermined temperature, said electric circuit outputting
an output signal representative of the voltage supplied to said
resistor body;
means for controlling path area of said second path for restricting
air flow passing therethrough;
means for detecting an engine load condition for controlling said
path area controlling means at least between a first position in
which said second path becomes minimum and a second position in
which said second path becomes maximum, said detecting means operating
said path area controlling means to said second position while the
engine load is heavier than a predetermined criterion and to said
first position while the engine load is smaller than said criterion;
and
means for deriving said intake air flow rate on the basis of said
output signal of said electric circuit according to a first characteristic
when said path area controlling means is placed at the first position
and according to a second characteristic when said path area controlling
means is placed at the second position.
8. A hot wire flow meter arrangement as set forth in claim 7 which
further comprises means for restricting intake air flowing into
said first path for regulating air flow through said first path.
9. A hot wire flow meter arrangement as set forth in claim 7 wherein
said detecting means detects said engine load condition on the basis
of an angular position of a throttle valve disposed in said air
induction system.
10. A hot wire flow meter arrangement as set forth in claim 7
wherein said detecting means detects said engine load condition
on the basis of an angular position of a throttle valve disposed
in said air induction system and an engine revolution speed.
11. A hot wire flow meter arrangement as set forth in claim 7
wherein said detector means detects said engine load condition on
the basis of said output signal of said electric circuit.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an arrangement for monitoring
a flow rate of an intake air flowing through an induction system
of an internal combustion engine. More specifically, the invention
relates to a hot wire flow meter arrangement which can achieve high
accuracy in measurement of the intake air flow rate in all engine
driving range.
2. Description of Background Art
Hot wire air flow meters, to which the present invention concerns,
have already been widely known in the automotive technologies. The
hot wire air flow meters are known as advantageous in comparison
with the conventionally known flap type air flow meter because of
lesser flow resistance against the flow of intake air flowing through
an air induction passage.
In general, the hot wire air flow meter comprises an electrically
conductive resistor body in a form of a wire, a strip, a tape or
so forth. The resistor body is heated at a predetermined reference
temperature so as to set its resistance at a reference value. The
heated resistor body is placed in the air induction passage to subject
air flow. Subjecting air flow, the temperature of the resistor body
is lowered in a magnitude corresponding to the air flow rate or
air flow velocity through the air induction passage. According to
temperature variation, resistance of the resistor body varies holding
a known relationship with variation of the temperature of the resistor
body. The resistor body is connected to a detector circuit for monitoring
variation of the resistance in the resistor body and whereby derives
the air flow rate in the air induction passage. The intake air flow
rate is used in an engine operation control, such as a fuel injection
control, air/fuel ratio control, spark ignition timing control and
so forth, as an engine load condition representative parameter.
In such hot wire flow meter, a difficulty is encountered in maintaining
satisfactorily high accuracy in substantially low engine load condition.
Namely, at substantially low engine load condition, such as that
in the engine idling condition, flow velocity of the intake air
is substantially low. In the low engine load range, substantially
low air flow velocity results in substantially small resistance
variation in the extent that the noise component in various source
and introduced in the detector circuit may cause substantial influence
in deriving the air flow rate. Particularly, in case of high performance
engine which has relatively large path area induction system, air
flow velocity in the induction system at the low engine load condition
becomes smaller to increase difficulty in accurately measuring the
intake air flow rate.
In addition, the hot wire flow meter of the conventional construction
is also defective at engine full load condition. Namely, when a
throttle valve is fully open to provide a maximum intake air flow
area for maximum amount of intake air, partial concentration of
air flow tends to be caused for lowering the measured intake air
flow rate across a value which should be obtained in relation to
the actual intake air flow rate. Influence of partial concentration
of intake air can be avoided by providing flow restrictive flow
regulating member, such as a flow restriction ring, in an orientation
upstream of the position of the hot wire. Such arrangement may improve
accuracy of measurement of the intake air flow rate. However, the
flow restriction may cause degradation of performance of air induction.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide
a hot wire air flow meter arrangement which can hold high accuracy
in measurement of intake air flow rate in all engine driving range.
Another object of the invention is to provide a hot wire air flow
meter arrangement which can achieve high accuracy without causing
degradation of air induction efficiency.
In order to accomplish aforementioned and other objects, a hot
wire air flow meter arrangement, according to the present invention,
comprises a primary and an auxiliary air flow path defined in the
air induction passage. A hot wire is provided in the primary air
flow path for monitoring air flow rate therethrough. A flow restricting
means is provided in the auxiliary air flow path for restricting
air flow therethrough. The flow restricting means is selectively
operative so that it is effective for restricting air flow in a
low engine load condition so that major part of induction air flows
through the primary air flow path and is not effective at high engine
load range so as to minimize flow resistance for induction of sufficient
amount of air to an internal combustion engine.
According to one aspect of the invention, a hot wire flow meter
arrangement for monitoring an intake air flow rate flowing through
an air induction system of an internal combustion engine, comprises:
means for defining a measuring portion where the intake air flow
rate is to be measured, in the air induction system;
an electrically conductive resistor body disposed within the measuring
portion in such an orientation as to subject air flow through the
measuring portion;
an electric circuit connected to the resistor body for supplying
a controlled voltage for the resistor body for heating the latter
at a predetermined temperture, the electric circuit outputting an
output signal representative of the voltage supplied to the resistor
body; and
means for controlling path area of the measuring portion in such
a manner that the path area ofthe measuring portion varies least
between a first minimum area and a second maximum area, the path
area controlling means being operated to the second position while
the engine load is heavier than a predetermined criterion and to
the first position while the engine load is smaller than the criterion.
A hot wire air flow meter arrangement may further comprises means
for deriving the intake air flow rate on the basis of the output
signal of the electric circuit according to a first characteristics
when the path area controlling means is placed at the first position
and according to a second characteristics when the path area controlling
means is placed at the second position.
According to another aspect of the invention, a hot wire flow meter
arrangement for monitoring an intake air flow rate flowing through
an air induction system of an internal combustion engine, comprises:
means for separating a measuring portion where the intake air flow
rate is to be measured, into a first and a second paths;
an electrically conductive resistor body disposed within the first
path in such an orientation as to subject air flow through the first
path;
an electric circuit connected to the resistor body for supplying
a controlled voltage for the resistor body for heating the latter
at a predetermined temperature, the electric circuit outputting
an output signal representative of the voltage supplied to the resistor
body;
means for controlling path area of the second path for restricting
air flow passing therethrough;
means for detecting an engine load condition for controlling the
path area controlling means at least between a first position in
which the second path becomes minimum and a second position in which
the second path becomes maximum, the detecting means operating the
path area controlling means to the second position while the engine
load is heavier than a predetermined criterion and to the first
position while the engine load is smaller than the criterion; and
means for deriving the intake air flow rate on the basis of the
output signal of the electric circuit according to a first characteristics
when the path area controlling means is placed at the first position
and according to a second characteristics when the path area controlling
means is placed at the second position.
The hot wire flow meter arrangement may further comprise means
for restricting intake air flowing into the first path for regulating
air flow through the first path.
The detecting means detects the engine load condition on the basis
of an angular position of a throttle valve disposed in the air induction
system; on the basis of an angular position of a throttle valve
disposed in the air induction system and an engine revolution speed,
or detects the engine load condition on the basis of the output
signal of the electric circuit.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood more fully from the detailed
description given herebelow and from the accompanying drawings of
the preferred embodiment of the invention, which, however, should
not be taken to limit the invention to the specific embodiment,
but are for explanation and understanding only.
In the drawings:
FIG. 1 is a fragmentary diagram of an air induction system for
an internal combustion engine, in which the preferred embodiment
of a hot wire flow meter, according to the present invention, is
employed;
FIG. 2 is a sectional view of the portion of the air induction
system, where the preferred embodiment of the hot wire air flow
meter is provided;
FIG. 3 is a graph showing relationship between an output voltage
of the preferred embodiment of the hot wire air flow meter and an
intake air flow rate represented by the output voltage;
FIG. 4 is a graph showing relationship between a pressure loss
and intake air flow rate; and
FIG. 5 is a graph showing variation of output voltage of the conventional
hot air flow meter and an intake air flow rate at a constant engine
speed.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, particularly to FIG. 1 the preferred
embodiment of a hot wire flow meter arrangement, according to the
present invention, is applied to an air induction system of an internal
combustion engine 1. The air induction system includes an air cleaner
2 and an intake duct 3 defining an air intake passge. A throttle
valve 4 is disposed within the intake duct 3 for adjusting an intake
air amount to introduce the engine 1.
An engine speed sensor 5 is provided for the engine 1 is provided
for monitoring a revolution speed N of the engine. The engine speed
sensor 5 may comprise a crank angle sensor for monitoring a crankshaft
angular position. As is well known, the crank angle sensor produces
a crank reference signal produced at every predetermined crankshaft
angular position and a crank position signal produced at every given
angular displacement. The engine speed can be derived on the basis
of an interval of occurrence of the crank reference signal, or,
in the alternative, by counting the crank position signal. Manner
of deriving the engine speed N based on the crank reference signal
or crank position signal is known in the art and thus do not need
to discuss in detail. A throttle angle sensor 6 such as a potentiometer,
is provided for monitoring angular position the throttle valve 4
to produce a throttle angle indicative signal TVO.
A hot wire 7 is provided in the intake duct 3 at an orientation
downstream of the air cleaner 2 and upstream of the throttle valve
4. The hot wire 7 is made of white gold wire, a nickel strip or
tape or so forth. The portion of the intake duct 3 where the hot
wire 7 is disposed, is partitioned and separated into a primary
passage 3a and an auxiliary passage 3b. The hot wire 7 is disposed
in the primary air passage 3a for monitoring intake air flow rate
flowing through the primary passage. The hot wire 7 is connected
to a control circuit 8. The control circuit 8 supplies heating power
to the hot wire 7 for maintaining the temperature of the wire at
a predetermined temperature. Therefore, the output voltage VQ supplied
to the hot wire 7 for maintaining the wire temperature constant
is variable depending upon the intake air flow rate of the induction
air flowing through the primary passage 3a. The control circuit
8 further outputs the output voltage VQ.
As seen from FIG. 1 a flow restricting annular wall 11 is provided
at the upstream end of the primary passage 3a. The flow restricting
annular wall 11 is designed so as not to serve for restricting intake
air flow in the substantial magnitude and so as to regulate the
air flow through the primary passage 3a. This flow restricting annular
wall 11 is effective for minimizing pulsation of the intake air
flow, which pulsation is caused by variation of intake vacuum in
synchronism with the engine revolution. The flow restricting annular
wall 11 also serves for preventing error in measurement of the intake
air flow rate at the engine full load condition. This is advantageously
introduced technology in view of performance of measurement of the
intake flow rate near or at the engine full load range.
Namely, as shown in FIG. 5 unless the flow restriction is provided
at the orientation upstream of the hot wire, the measured intake
air flow amount at the engine full load range tends to lower than
that actually flowing through the air intake duct. For instance,
in the example of FIG. 5 when the engine speed N is maintained
constant at 1200 rpm and the pressure in the air intake duct is
varied from 500 mmHg to the pressure WOT at the throttle valve fully
open position, actually flowing intake air flow rate varies as shown
by solid line. Whereas, the air flow rate derived on the basis of
the output of the hot wire air flow meter arrangement varies as
shown by broken line. According to the shown embodiment, since the
flow restricting annular wall 11 serves for providing flow resistance,
whereby for compressing the intake at an orientation upstream of
the hot wire 7 and subsequently allow expansion to the initial volume,
air flow in the primary passage can be regulated. As a result, the
measured air flow rate becomes substantially coincide with the actual
intake air flow rate.
A butterfly valve 9 is pivotally provided in the partition for
pivotal movement at a valve shaft 9a. As seen from FIG. 2 the butterfly
valve 9 is fixed to the valve shaft 9a for rotation therewith. The
valve shaft 9a extends transversely to the axis of the intake duct
3. One end of the valve shaft 9a is connected to an actuator 10
such as an electric motor, for driving the butterfly valve 9 together
with the valve shaft 9a between an open position and a closed position.
The butterfly valve 9 employed in the preferred embodiment of the
hot wire air flow meter, according to the invention, will not subject
substantial pressure difference between upstream and downstream
thereof. Therefore, in the shown embodiment, the butterfly valve
9 may be made of synthetic resin, for example. The resin valve may
be advantageous for easy production.
The engine speed sensor 5 the throttle angle sensor 6 and the
control circuit 8 are connected to a control unit 12 which may comprise
a microprocessor. The control unit 12 processes the output voltage
of the control circuit 8 for deriving the intake air flow rate indicative
signal. On the other hand, the control unit 8 monitors the throttle
valve angular position and the engine speed for deriving the valve
position of the butterfly valve 9 so as to selectively close the
valve at a predetermined engine load condition as derived based
on the throttle valve angular position.
It should be appreciated that the control unit 12 may perform not
only derivation of the intake air flow rate and control of the butterfly
valve position, but also perform the fuel injection control, spark
ignition timing control and so forth.
Operation of the above-mentioned preferred embodiment of the hot
wire air flow meter arrangement, according to the invention will
be discussed herebelow.
At first, it is assumed that the engine is driven at a relative
heavy engine load. At this condition, the control unit 12 controls
the actuator 10 so as to maintain the butterfly valve position at
a fully open position. At this condition, the intake air past the
air cleaner 2 passes both of the primary and auxiliary air passages
3a and 3b for minimizing flow resistance against the intake air.
According to the shown embodiment of the hot wire air flow meter
arrangement, the pressure loss caused by flow restricting in the
hot wire air flow meter arrangement can be maintained within an
acceptable range even at the maximum flow rate, as shown in FIG.
4. At this time, the relationship between the intake air flow rate
and the output voltage VQ of the control circuit 8 becomes as illustrated
by line A in FIG. 3. By setting the output voltage VQ variation
in relation to variation of the intake air flow rate in a form of
data map or look-up table in a memory (not shown) in the control
unit 12 the air flow rate can be easily derived in terms of the
output voltage VQ of the control circuit 8.
The control unit 12 checks the engine driving range on the basis
of the engine speed N and the throttle valve angular position TVO.
The control unit 12 detects the engine driving range to discriminate
low engine load range and high engine load range. Namely, when the
engine speed is lower than or equal to a predetermined engine speed
threshold N.sub.ref and the throttle valve open angle is smaller
than or equal to a predetermined angle TVO.sub.ref, the control
unit outputs an actuator control signal to operate the latter to
place the butterfly valve 9 at the fully closed position. In the
alternative, the control unit 12 compares the output voltage VQ
of the control circuit 8 with a predetermined threshold voltage
V.sub.0 which represents a criteria of low engine load range where
accuracy in measurement of intake air flow rate can be lowered due
to substantially low intake air flow velocity. When the output voltage
VQ is lower than or equal to the threshold voltage V.sub.0 then
the control unit 12 operates the actuator 10 to shift the butterfly
valve 10 to the fully closed position.
By positioning the butterfly valve 9 at the fully closed position,
the intake air flows only through the primary passage 3a. Therefore,
intake air flow velocity is increased. The magnitude of variation
of the output voltage VQ of the control circuit 8 in relation to
variation of the intake air flow rate in the low engine load range,
becomes greater. While the butterfly valve 9 is held at fully closed
position, the control unit 12 utilizes a characteristic as illustrated
by broken line B in FIG. 3 for deriving the intake air flow rate
based on the output voltage VQ. By this, noise which can superimpose
on the output voltage will not cause significant influence for the
resultant intake air flow rate obtained as a result of measurement.
While the present invention has been disclosed in terms of the
preferred embodiment in order to facilitate better understanding
of the invention, it should be appreciated that the invention can
be embodied in various ways without departing from the principle
of the invention. Therefore, the invention should be understood
to include all possible embodiments and modifications to the shown
embodiments which can be embodied without departing from the principle
of the invention set out in the appended claims.
Though the shown embodiment switches the position of the butterfly
valve between the fully open position and closed position, it is
possible to vary the butterfly valve position in stepwise fashion
or in linear fashion. On the other hand, though the shown embodiment
detects the low engine load condition by detecting the engine condition
based on the engine speed and the throttle valve angular position
or in the alternative based on the output voltage of the control
circuit, it may be possible to detect the low engine load condition
by means of a mechanical switch associated with the throttle valve
to detect the throttle valve open angle smaller than a predetermined
angle. |