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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to fluid flowmeters in general and particularly to
such flowmeters designed specifically for use in automotive internal
combustion engines.
2. Description of the Prior Art
Fluid flowmeters of the pressure drop or differential pressure type are
well known. Orifice plate and venturi flowmeters are probably the most
common of the pressure drop type. Orifice plate flowmeters are
inexpensive, but they are inherently high energy loss devices since the
measured pressure drop across the orifice is non-recoverable, i.e., the
drop in pressure is a drop in total pressure. Venturi flowmeters are low
energy loss devices relative to orifice plate flowmeters since most of the
pressure drop in the venturi throat is recoverable at the venturi outlet,
i.e., the drop in pressure in the throat is due to an increase in kinetic
energy of the fluid. However, when either of these flowmeters are used to
measure fluid flow which varies over a wide range, such as air flow to an
automotive engine, they either overly restrict total air flow at high
engine speeds and loads if they are sized small enough to provide an
adequate differential pressure signal at low engine speeds and loads, or
they provide an inadequate differential pressure signal at low engine
speeds and loads if they are sized larger.
SUMMARY OF THE INVENTION
The invention disclosed herein represents an improvement of the fluid
flowmeter disclosed in copending application U.S. Ser. No. 845,751 filed
Oct. 26, 1977.
An object of this invention is to provide a fluid flowmeter which is low in
cost, high in accuracy, and operable to provide an easily measurable
pressure differential signal at low fluid flows and a low pressure drop at
high fluid flows. Another object of this invention is to provide a fluid
flowmeter which operates to cause the pressure differential signal to vary
substantially linearly with fluid flow over an extended range of
operation. Another object of this invention is to provide a fluid
flowmeter which has relatively few moving parts in the flow area of the
meter and is therefore durable and quick in response time. Another object
of this invention is to provide a fluid flowmeter having means to amplify
a pressure differential signal without appreciably increasing the total
pressure drop across the entire flowmeter.
Accordingly to a feature of the invention, the flowmeter includes means for
swirling a fluid flowing in a passage. The swirl means includes means
operative to increase the rate of the swirl in response to increasing
fluid flow and means operative to decrease the rate of such increase in
response to such increasing fluid flow.
According to another feature of the invention, the flowmeter includes a
main passage having a fluid flow therein, swirl means operative to swirl
the fluid about a central axis defined by the passage and thereby impart a
high tangential velocity vector to the fluid about the central axis for
forming a low static pressure region about the central axis, means for
sensing the low static pressure, and means for sensing the pressure of the
fluid before the swirl means. A geometrically reconfigurable element in
the main passage operates to vary the magnitude of the swirl such that it
increases at a decreasing rate with increasing fluid flow.
According to another feature of the invention, the flowmeter of the above
feature includes a secondary passage disposed about the central axis for
communicating a portion of the fluid in the main passage upstream of the
swirl means with the low pressure region.
According to another feature of the invention, the secondary passage of the
above feature is a venturi tube having a throat and means for sensing the
low static pressure in the throat.
According to another feature of the invention, a solenoid valve having
multiple valving members operates to selectively interconnect the static
pressure signal, the stagnation pressure signal, and a manifold vacuum
signal with an absolute pressure transducer.
In the preferred embodiment of the invention, the geometrically
reconfigurable element comprises a plurality of flexible swirl vanes which
present a pitch to the fluid in the main air flow passage which increases
with increasing fluid flow.
In an alternative embodiment of the invention, flexible swirl vanes as
described above are combined with fixed swirl vanes which are disposed
radially in the main passage at an angle oblique to the fluid flow lending
structural support to the flexible vanes.
In another alternative embodiment of the invention, an iris member is
disposed within the main passage immediately upstream of a set of fixed
swirl vanes which effectively presents swirl vanes of varying tip radius
to the fluid flowing in the main passage.
According to another feature of the invention, pressure relief means are
provided downstream of the swirl vanes to form a path of fluid
communication between the main passage and the atmosphere upon the
reversal of fluid flow direction, such as when an engine with which the
meter is associated, backfires.
Various other features and advantages of this invention will become
apparent upon reading the following specification, which, along with the
patent drawings, describes and discloses a preferred illustrative
embodiment of the invention in detail.
The invention makes reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a fluid flowmeter embodying the present invention
illustrating its interconnection with a pressure transducer and a source
of engine manifold vacuum pressure;
FIG. 2 is a cross sectional view on an enlarged scale taken on line 2--2 of
FIG. 1;
FIG. 3 is a view on yet a further enlarged scale showing details of the
venturi tube-swirl vane subassembly of the flowmeter seen in FIGS. 1 and
2;
FIG. 4 is a cross sectional view on an enlarged scale of the valve section
of the flowmeter of FIGS. 1 and 2, illustrating the internal details
thereof;
FIG. 5 is a cross sectional view taken on line 5--5 of FIG. 4;
FIG. 6 is a cross sectional view of the flowmeter section of an alternative
embodiment of the invention;
FIG. 7 is a plan view of a flexible iris employed in the flowmeter section
of FIG. 6;
FIG. 8 is a perspective view of the flexible swirl vanes and straightening
vanes of another alternative embodiment of the invention;
FIG. 9 is a broken sectional view illustrating an optional pressure bypass
relief valve for use with the flowmeter of FIGS. 1 and 2; and
FIG. 10 is a pressure differential signal response graph comparing the
differential pressure and total meter pressure drop versus that in prior
art units, over a typical range of operation.
DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS
The preferred embodiment of the invention fluid flowmeter assembly 10 is
illustrated in FIGS. 1 and 2. The flowmeter assembly 10 comprises a
flowmeter section 12, a valve section 14, and a remote pressure transducer
16 which is in fluid communication with the valve section 14 through a
pressure line 18. The flowmeter assembly 10, as disclosed, is adapted for
measuring the mass air flow to an internal combustion engine. However, the
flowmeter section 12, with or without valve section 14 and/or transducer
16, may be used in other environments as a mass flowmeter or a volumetric
flowmeter. Additionally, the pressure transducer 16 can be mounted
directly upon the valve section 14 or remotely as illustrated, depending
upon the intended application. In the preferred embodiment, the line 18 is
interconnected to the valve section 14 and the transducer 16 by
conventional fittings 20. A second pressure line 22 interconnects the
valve section 14 and the intake manifold of the engine with which the
flowmeter 10 is associated. It is contemplated that line 22 could
alternatively communicate with other pressure sources such as the
atmosphere.
Flowmeter section 12 comprises a round walled tubular housing 24; a venturi
tube 26 disposed along the central axis of housing 24 and having a lower
portion 26b and a reduced diameter upper portion 26a joined to lower
portion 26b by shoulder 26c; a set of six radially disposed air
straightening vanes 28 fixed at their radially inner ends to the outer
circumferential wall of the reduced diameter portion 26a of venturi tube
26 and at their radially outer ends to the inner circumferential wall of
housing 24; and a set of six radially disposed swirl vanes 30 fixed at
their radially inner ends to the outer circumferential wall of the lower
portion 26b of venturi 26 at a 50.degree. angle with respect to the air
flowing parallel to the central axis. It is contemplated that this angle
can be changed depending upon the application and operating
characteristics of the engine with which the flowmeter assembly 10 is to
be employed. In the preferred embodiment of the invention, the swirl vanes
30 depend from the lowermost end of the venturi tube 26 and extend
radially outwardly substantially to the inside wall of the housing 24. The
venturi tube 26 and the swirl vanes 30 are illustrated as a separate
component or subassembly but it is contemplated that they could be
integerly molded with the straightening vanes 28 and the housing 24 in
zinc, aluminum, plastic or other suitable material. If necessitated by
given design considerations, the radially outwardmost ends of the swirl
vanes 30 can be fixed to the inside wall of the housing 24 for increased
structural integrity. The venturi tube 26 may be replaced by a straight
wall tube or a substantially straight wall tube. However, the venturi tube
26 has been found to provide a lower static pressure than the straight
wall tubes, particularly when the total air flow through the flowmeter is
low, and therefore provides a greater differential pressure signal.
Referring to FIG. 3, six flexible swirl vanes 32 are provided, one being
fixably associated with each fixed swirl vane 30. Each flexible swirl vane
32 is fixed to the swirl vane 30 associated therewith along the uppermost
edge thereof and extends from that edge in cantilever fashion. Each
flexible swirl vane 32 has an arcuate section resulting in its being
substantially normal to the direction of air flow through the flowmeter
section 12, when in the relaxed position as illustrated. The flexible
vanes 32 depend entirely from the fixed vanes 30 and project towards the
next adjacent fixed vane 30 to a nearly abutting relationship, whereby, in
combination, the six flexible vanes 32 substantially close the air
passageway through the housing 24 with the exception of the venturi tube
26.
The flexible swirl vanes 32 are preferrably constructed from brass or
stainless steel shim stock but it is contemplated that any other suitable
material can be substituted. The uppermost end of each flexible vane 32 is
illustrated as having a U-shaped section 34 which embraces the uppermost
edge of the associated fixed vane 30. Attachment can be completed by
rivets, welding, adhesives or the like. Additionally, it is contemplated
that in certain applications, the flexible vanes 32 could be molded as an
integral part of the fixed vanes 30.
Tapered shim stock has been found to be preferrable to flat shim stock for
application in the flexible vanes 32. In fabricating the flexible vanes
32, the taper gradient is aligned substantially radially with the thickest
portion being at the radially outwardmost end of the flexible vanes 32.
The construction results in a characteristic radial spring constant
gradient which allows the radially innermost portion of each flexible
swirl vane 32 to deflect before the radially outermost portion of that
vane. Thus, at low air flow rates, the effective swirl vane tip radius is
reduced, thereby concentrating the air flow radially inwardly,
accentuating the pressure drop at the outlet of the venturi tube 26. It is
contemplated that any number of structural variations of the flexible
swirl vane 32 can be empirically derived herefrom given a set of system
perameters. The arrangement of swirl vanes 30 and 32 described herein are
therefore intended as being illustrative only.
In application, as air passes through the flowmeter section 12 a
differential pressure is developed across the flexible swirl vanes 32
causing them to deflect downwardly, ultimately approaching the pitch of
the fixed vanes 30 at full air flow rating of the flowmeter 10. Thus, at
relatively low air flow rates, the pressure drop across the swirl vanes 32
and thus, the meter 10 will be relatively high as opposed to fixed
geometry type flowmeters. At higher flow rates, the flexible vanes 32 will
be displaced downwardly, increasing the effective pitch presented by the
swirl vanes 30 and 32 to the air flowing through the flowmeter section 12
with a reduced relative pressure drop. The nature of the material selected
for the flexible swirl vanes 32 causes them to act as springs with the
released position of the "springs" seen in solid lines in FIG. 3. FIG. 3
illustrates one of the flexible vanes 32 in phantom in two intermediate
positions 32' and 32" which the vane 32 would assume with increasing air
flow.
The housing 24 defines a main air flow passage 36 and includes beaded ends
38 and 40 at its inlet and outlet respectively, for connecting the housing
24 into a duct system. A boss 42 is provided for mounting the valve
section 14 thereto by screws 44. A stagnation pressure passage 46 passes
through the wall of the housing 24 and radially inwardly within one of the
straightening vanes 28, emerging upwardly therefrom through three radially
spaced stagnation pressure ports 48. Three stagnation ports 48 are
employed to develop an average stagnation pressure signal. The signal is
transmitted through the stagnation pressure passage 46 and into the valve
section 14. A static pressure passage 50 likewise passes through the same
straightening vane 28. Passage 59 passes radially inwardly through the
wall of the housing 24 through vane 28 and emerges at the throat area of
the venturi 26 through a conical static pressure port 52 in the wall of
the venturi. The area in which the stagnation pressure is sensed is
designated P.sub.1 and the area in which the static pressure is sensed is
designated P.sub.4. The radially outer end of static pressure passage 50
is in fluid communication with the valve section 14.
Operation of flowmeter section 12 is as follows: Air enters the inlet of
the main passage 36 with a given axial or transport velocity. The
stagnation pressure P.sub.1 of the entering air is sensed by stagnation
ports 48. These ports or alternatively a single port may be disposed
further upstream or external of the main passage. A portion of the air
flows through the inlet of the venturi 26 and the remainder of the air
flows through the straightening vanes 28; vane 28 function to remove
substantially all pre-swirl in the air due to upstream conditions. When
the air leaves the straightening vanes 28, it has a substantially uniform
and homogeneous axial flow pattern parallel to the central axis as it
enters the swirl vanes 30 and 32. The flexible swirl vanes 32 impart a
tangential swirl velocity vector to the air passing thereby which varies
in magnitude as a function of the air velocity. If the flexible swirl
vanes 32 were absent, the fixed vanes 30 would impart a substantially
50.degree. swirl velocity vector to the air. The magnitude of the vector
varies substantially proportionally with the incoming air velocity. This
is the characteristic which limits the effective operating range of a
flowmeter that does not have a geometrically reconfigurable element such
as the swirl vanes 32. In the preferred embodiment, as the velocity of the
incoming air increases, the effective pitch, presented by the flexible
vane 32 will increase and the tangential velocity vector imparted into the
swirl will vary with incoming air velocity but at a decreasing rate. The
axial and tangential velocity vector components of the swirl velocity form
an air flow pattern (known as a forced-vortex flow) similar to a tornado
which has a high velocity at the center axis and a diminishing velocity
gradient radially outward from the central axis. This resulting radial
velocity gradient forms an associated radial pressure gradient having a
static pressure P.sub.2 at the radius of mean mass flow through the main
passage, and a low static pressure region P.sub.3 at the central axis. The
radius of mean mass flow is, understood to mean the radius at which one
half of the fluid mass passes through the annular sector disposed radially
outwardly therefrom and the other half passes through the annular sector
disposed radially inwardly therefrom. This low pressure accelerates the
air leaving the outlet of the venturi 26 and therefore amplifies the
velocity of the air flowing through the venturi 26. As a result the static
pressure P.sub.4 of the air of the venturi throat 52 is proportionally
decreased. The total volumetric or mass air flow through the flowmeter 10
may then be calculated in a known manner by known fluid flow principles in
conjunction with the expression
P.sub.1 - P.sub.4 = K(P.sub.1 - P.sub.2)
wherein K is a proportionally constant. K may be empirically determined.
The static pressure P.sub.4 in the venturi throat 52 is directly related to
the total air flow through the meter 10 since the amount of air flowing
through the venturi 26 is directly related to the low static pressure
P.sub.3 produced by the swirl vanes 32. Hence, the disclosed flowmeter 10
does not depend upon or use localized air flow techniques to measure air
flow, which localized air flow in some modes of operation may be far from
representative of the total air flow through a meter.
Valve section 14, which is shown in greater detail in FIG. 4, includes a
gasket 56 disposed intermediate a puck-shaped housing 58 and the wall of
housing 24. A second gasket 60 is disposed intermediate the puck-shaped
housing 58 and cover plate 62. The gaskets 56 and 60, housing 58 and cover
plate 62 are held together by the screws 44 which pass therethrough and
threadably engage the boss 42. The housing 58 has two axially disposed
bores 64 and 66 therein. The bores 64 and 66 are substantially closed at
one end by gasket 56 and at the other end by gasket 60. Solenoids 68 and
70 are disposed in the bores 64 and 66 respectively. Threaded ports 72 and
74 are provided in the cover plate 62 coaxially with bores 64 and 66
respectively to provide fluid communication with the pressure transducer
16 and an external pressure source (such as manifold vacuum) by threadable
engagement with standard fittings 20. Since solenoids 68 and 70 are
identical, a detailed description of the internal construction of only one
(68) will be given. It is also contemplated that other commercially
available designs could be substituted.
The stagnation pressure passage 46 passes radially outwardly through the
wall of the housing 24 and the gasket 56, opening into a stagnation
pressure passage port 76 within the solenoid 68. The stagnation pressure
port 76 opens into a central bore 78 within the solenoid 68, within which
is slidably disposed a plunger 80. With the plunger 80 in the position
illustrated, the stagnation pressure passage 46 is in fluid communication
with an outlet passage 82 through axially aligned grooves 84 (FIG. 5)
within the plunger 80. A chamber vertically elongated slot 83 within the
gasket 60 provides fluid communication between solenoid 68 and an axial
inlet passage 86 of the other solenoid 70. Solenoid 68 also has an axially
disposed inlet passage 88 which is in fluid communication with the
threaded port 72 through an aperture 89 within the gasket 60. The plunger
80 is biased to the right by means of a spring 90.
A coil 92 disposed coaxially with the plunger 80 is arranged to counteract
the spring 90 when energized. The solenoid 68 has a cylindrical housing 94
and endplugs 96 and 98, all of which are constructed of ferrous material
to define a magnetic path in combination with the plunger 80. The magnetic
operating principles of the solenoid 68 and 80 are well known in the art
and will not be elaborated upon. The coil 92 has provisions for
interconnection with a source of electrical excitation (not illustrated).
Again, however, the electrical theory of operation is well known in the
art and will not be expanded upon here. In the relaxed position
illustrated, the stagnation pressure passage port 76 is in fluid
communication with the outlet passage 82 and the axial inlet passage 88 is
blocked by one of two rubber seals 100 disposed within the plunger 80
coaxially with the port 76 and passage 88. When the coil 92 is energized,
the plunger 80 will be displaced to the left, whereby the stagnation
pressure passage port 76 is blocked by the seal 100 and the axial inlet
passage 88 is placed in fluid communication with the outlet passage 82.
Thus, in application, axial inlet passage 88 is in fluid communication
with a source of manifold vacuum from the engine with which the flowmeter
10 is associated and, depending upon the position of the plunger 80, the
stagnation pressure or alternatively the manifold vacuum pressure can be
selectively monitored within the chamber 83 of the valve section 14.
The static pressure passage 50 passes through the wall of housing 24 and
enters one end of a vertically elongated static pressure chamber or slot
102 within the gasket 56. The other end of the chamber 102 opens into a
static pressure passage port 104 in the left hand end of the solenoid 70.
Within the solenoid 70 the static pressure passage port 104 opens into a
central bore 78' in the same manner as the stagnation pressure passage
port 76 opens into central bore 78 within solenoid 68. With the plunger
80' in the position indicated the static pressure passage port 104 and
thus, the static pressure passage 50 is in fluid communication with the
ports 74 through an aperture 91 in the gasket 60. In application, the port
74 is in fluid communication with the pressure transducer 16 through the
conventional fittings 20 and pressure line 22 illustrated in FIGS. 1 and
2. With the plunger 80' in the position indicated, one of the rubber seals
100' closes the axial inlet passage 86. As in the case with the solenoid
68, the coil 92' of the solenoid 70 is connected with a source of
electrical excitation (not shown) whereby the plunger 80' can be displaced
to the left to close the static pressure passage port 104 with the other
rubber seal 100' and interconnect the axial inlet passage 86 with an
outlet passage 82' which in turn is in fluid communication with the port
74 through the aperture 91 in the gasket 60.
In the present referred application, by selectively energizing or releasing
the plungers 80 and 80' of the solenoids 68 and 70 respectively the
pressure transducer 16 can selectively monitor the manifold vacuum
pressure, the static pressure and the stagnation pressure. When both
solenoids 68 and 70 have their respective plungers 80 and 80' in the
relaxed or deenergized position as illustrated, the pressure transducer 16
is in fluid communication with the throat 54 of the venturi 26 through
static pressure port 52, static pressure passage 50, static pressure
chamber 102, static pressure passage port 104, central bore 78', of the
solenoid 70, the grooves 84' of the plunger 80', the outlet passage 82' of
the solenoid 70, the aperture 91, port 74, and the pressure line 18. The
manifold vacuum pressure is blocked by the rubber seal 100 in the right
hand end of the plunger 80 of the solenoid 68 and the stagnation pressure
signal is blocked by the right hand end seal 100' in the plunger 80' of
the solenoid 70.
To interconnect the stagnation pressure passage 46 with the pressure
transducer 16, the coil 92' of the solenoid 70 must be energized so that
the static pressure passage port 104 is blocked by the rubber seal 100' in
the left hand end of the plunger 80' of the solenoid 70 and the axial
inlet passage 86 is placed in fluid communication with the outlet passage
82' of the solenoid 70. To place the pressure transducer 16 in fluid
communication with the manifold of the engine with which the flowmeter 10
is associated, both solenoids 68 and 70 must be energized whereby the
seals 100 and 100' in the left hand ends of the plungers 80 and 80'
respectively block the stagnation pressure passage 56 and static pressure
passage 50 respectively, permitting fluid communication between the ports
72 and 74 within the valve section 14. It is contemplated that other
pressure sources can be monitored in a similar fashion by extending the
operation of the valve section 14 in ways which would be obvious to one
skilled in the art in light of this specification. Fewer or more solenoids
could be employed in such modifications. The illustrated configuration is
intended for example only and it is contemplated that any number of
variations could be made therefrom without offending the spirit of the
present invention.
A non-conducting coil spool 106 and 106' is employed in each solenoid 68
and 70 respectively to support the coil 92 and 92' and guide plunger 80
and 80'. FIG. 5 is a cross sectional view of the plunger 80 and coil spool
106 illustrating the interfit between the two and the configuration of the
grooves 84 within the plunger 80.
Referring to FIGS. 6 and 7 an alternative embodiment of the flowmeter
section 12 is illustrated. The swirl vanes 30 are fixed. A flexible iris
108 is disposed within the air flow passage 36 within the flowmeter
section 12 substantially normal to the central axis thereof. The iris 108
is constructed of stainless steel shim stock or the like and comprises a
peripheral supportive portion 110 and a plurality of radially inwardly
projecting compliant members 112 depending from support portion 110. The
compliant members 112 are illustrated as being integral with the support
portion 110. However, it is contemplated that separate members could be
employed. The flexible iris 108 is supportively affixed to the bottom most
surface of the straightening vanes 28. In operation, at relatively low air
flow rates, the air passing through the air flow passage 36 passes either
through the venturi tube 26 or the straightening vanes 28 and the swirl
vanes 30. The air passing through the straightening vanes 28 initially is
obstructed by the compliant members 112 of the flexible iris 108.
Compliant members 112 are disposed substantially normal to the air flow.
The air passing through the straightening vanes 28 is concentrated
radially inwardly, thereby substantially increasing the velocity of the
air near the exit of the venturi 26. As was described in the detailed
description of the preferred embodiment illustrated in FIGS. 1, 2, and 3,
this increased velocity results in a decreased pressure P.sub.3 which, at
low air flow rates, amplifies the pressure differential sensed in the
venturi 26. As the air flow rate through the air flow passage 36
increases, the radially innermost ends of the of the compliant members 112
are deflected downwardly thereby exposing a larger radius of the swirl
vanes 30 to the air stream in the main passage 36. The fixed angle of the
swirl vanes 30 will impart a tangential velocity vector to the air stream
thereby causing the swirl as was described in the discussion of the
preferred embodiment. As the air flow rate increases the iris 108 will
effectively open radially outwardly to expose a larger portion of the
swirl vanes 30 to that portion of the air flow through the straightening
vanes 28. This increase of effective tip diameter of the swirl vanes 30,
which is proportional with air flow rate, results in a tengential flow
vector which increases in magnitude with increasing air flow rate but at a
decreasing rate. The compliant members 112 of the iris 108 are illustrated
in phantom in several intermediate positions 112' and 112".
Referring to FIG. 8 a second alternative embodiment of the invention is
illustrated, in which the plurality of flexible swirl vanes 114 depend
from the lowermost edge of the straightening vanes 28, and in the relaxed
position or zero air flow position, are disposed normally thereto to
present a surface substantially normal to the air flowing through the
straightening vanes 28. In the rest position, the swirl vanes 114
substantially close the arcuate sectors 115 defined by each pair of
adjacent straightening vanes 28. At low air flow rates, the swirl vanes
114 assume a relatively small pitch with respect thereto, thereby
imparting a large tangential vector to the air flow. As air flow
increases, the swirl vanes 114 deflect downwardly to effectively increase
the pitch presented to the air flowing by them whereby the tangential
vector imparted to the air increases with increasing air velocity but at a
decreasing rate. Vane 114 has an upturned foot portion 116 which is
attached to the straightening vane 128 from which the swirl vane 114
depends. The swirl vanes 114 are constructed of shim stock or the like and
are affixed to the straightening vanes 28 by riveting, welding, adhesives,
or the like. It is contemplated however, that they could also
alternatively be integrally molded with the straightening vanes 28.
Pressure transducer 16 may be any of several well known types. Herein,
transducer 16 is an absolute pressure transducer of the type described in
Society of Automotive Engineer (SAE) paper 770397 and manufactured by the
Instruments Division of Bunker Remo Corporation. The trnnsducer measures
the absolute stagnation and static pressures within the flowmeter as well
as the manifold vacuum of the engine with which the flowmeter is
associated, and produces electrical output signals representative of each
pressure. These signals may be processed by an electronic logic system to
produce a signal representative of the volumetric air flow through the
flowmeter or, since the stagnation pressure is compared with absolute
pressure, the signals may be processed with an absolute air temperature
signal provided by a tmmperature sensor 118 in the inlet of the main air
passage 36 to produce a signal representative of mass air flow through the
flowmeter 10.
Referring to FIG. 9, inasmuch as each of the embodiments of the present
invention described herein substantially obstruct the air flow passage 36
at low air flow rates, and the materials used in the geometrically
reconfigurable elements 32, 112, and 114 is relatively thin and sensitive
to rapid pressure changes, it is desirable to provide a pressure release
feature to the flowmeter 10 to provide for the contingency where the
engine with which the flowmeter 10 is associated backfires or otherwise
causes the flow of air to severely reverse direction. The preferred
embodiment of the invention is the most sensitive to such a condition
inasmuch as the flexible swirl vanes 32 are nearly aligned with the next
adjacent fixed swirl vane 30. Referring to FIG. 3, of the air flow
direction were reversed, the flexible swirl vane 32 would tend to be
displaced upwardly. However, the free ends of the flexible vanes 32 would
immediately abut the overlapping surface of the next adjoining fixed swirl
vane 30 directly thereabove. This would substantially seal the main air
flow passage 36 and if the pressure generated in the backfire were severe
enough it could potentially miscalibrate the meter.
FIG. 9 illustrates a check valve 130 which threadably engages the wall of
housing 24 at a point downstream of the geometrically reconfigurable
element 32, 112, or 114. The check valve 120 has an axial bore 122
therethrough having an area of increased diameter 124 therealong. A
substantially spherical plug member 126 is slidably disposed within the
area of increased diameter 124 and is biased to the left by a spring 128.
A land 130 within the check valve 120 abuts plug member 126 to define a
left hand limit of travel. As illustrated, the right hand end of the axial
bore 122 communicates with the atmosphere and the left hand end
communicates with the air flow passage 36. Under normal conditions the
passage between the two is blocked by the plug member 126. The spring 128
has a rate which is calculated so as to hold the plug 126 in the position
illustrated during normal operating conditions with air flowing toward the
engine in the illustrated application. If the pressure in the air flow
passage 36 increases substantially, above a predetermined point, the plug
member 126 will be displaced to the right thereby opening a path of
communication between the air flow passage 36 and the atmosphere. When the
pressure drain drops below the predetermined point, the plug member 126
will again close the line of communication between the air flow passage 36
and the atmosphere. This overpressure condition is most likely to arise in
the event of engine backfire but it is contemplated that it could also
occur in various other applications. It is to be understood that the check
valve 120 is for illustrative purposes only and that any number of
variations therefrom could be made which would serve to relieve the
overpressure condition.
Referring to FIG. 10, a differential pressure versus flow rate curve and
meter pressure drop versus flow rate curve for both the preferred
embodiment of the present invention (new) and the prior art devices (old)
which employed fixed geometry swirl means are illustrated over a typical
flow rate operating range of a V-8 autombile engine. In the fixed geometry
devices, the signal pressure which is the differential (P) between the
stagnation pressure and the static pressure tends to have very little
amplification at low flow rates and has an extremely non-linear response
over a typical automobile engine flow range. By employing the variable
geometry elements described in the various embodiments of the invention,
the tangential vector imparted to the air flow or swirl rate varies at a
rate which increases with increasing flow rate at a decreasing rate. In
the prior art devices the swirl rate varied substantially linearly with
increasing flow rate. Swirl vanes impart a tangential velocity vector
(V.sub.T) component and an axial velocity vector component (V.sub.A) to
the fluid in creating the swirl. The ratio V.sub.T /V.sub.A remains
constant for fixed swirl vanes but decreases with increasing flow rate
when a geometrically reconfigurable element is introduced into the
flowmeter. By tailoring the effective spring rate of the variable geometry
elements, a substantially linear pressure signal response curve over an
extended flow range can be achieved. It is contemplated that this
tailoring can be extended to result in a non-linear response over an
extended range to amplify the low end signal and/or attenuate the high end
signal.
Similarly, the total pressure drop (TDP) of the prior art devices would
tend to increase dramatically near the high end of the flow rate range for
a given application. This is extremely undesirable. With a geometrically
reconfiguraable device, although at low flow rates the total pressure drop
is slightly higher than that of the fixed swirl vane type units, the
operating characteristic of the flowmeter provides a substantially linear
total pressure drop characteristic over the entire flow range of a typical
automotive application resulting in a total pressure drop at maximum flow
rates which is substantially smaller than that of the prior art devices.
It is to be understood that the invention has been described with reference
to specific embodiments which provide the features and advantages
previously described, and that such specific embodiments are susceptible
of modification, as will be apparent to those skilled in the art.
Accordingly, the foregoing description is not to be construed in a
limiting sense.
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