Measuring device for measuring the mass of a medium flowing in a line

A measuring device for measuring the mass of a medium flowing in a flow line, in particular the aspirated air mass of an internal combustion engine, including a measuring element bathed by the flowing medium and disposed in a flow conduit of the measuring device provided in the flow line. The flow conduit extends in a primary flow direction between an inlet opening that communicates with the flow line and at least one outlet opening that discharges into the flow line downstream of the inlet opening. The flow conduit branches at a first dividing point, disposed between the inlet opening and the measuring element, into a measuring conduit, in which the measuring element is disposed, and a first bypass conduit, which bypasses the measuring element in the primary flow direction.

PRIOR ART
 The invention is based on a measuring device for measuring the mass of a
 medium flowing in a line, also known as a flow rate meter. From German
 Patent DE 44 07209 C2, a measuring device is already known in which a flow
 conduit is integrated with a measuring module. The flow conduit receives
 the measuring element and tapers increasingly in the flow direction,
 beginning at an inlet opening. The tapered portion is adjoined by the
 S-shaped deflection conduit, which has a rectangular cross-sectional
 profile. The measuring module is embodied as a plug-in component. A
 substrate part of the measuring module can be inserted sealingly into the
 wall of the line to be measured and receives an electronic evaluation
 circuit.
 As the measuring element, a micromechanical component, of the kind known
 for instance from German Patent Disclosure DE 43 38 891 A1, is especially
 suitable. In the measuring element known from DE 43 38 891 A1, two
 temperature-sensitive resistors are integrated; they can for instance
 comprise silicon oxide or silicon nitride, and they have low thermal
 conductivity and a low specific thermal capacity. The two
 temperature-sensitive resistors are thermally insulated from one another
 by a silicon frame. While one of the temperature-dependent resistors acts
 as the actual measuring sensor, the second temperature-dependent resistor
 serves as the sensor for the flowing medium.
 From German Patent DE 36 27 465 C2, it is known to incline a measuring
 element for measuring the air quantity in an intake conduit by a
 predetermined angle relative to the flow direction, in order to reduce the
 adhesion of suspended particles to the measuring element. It is also known
 from this patent to provide the end faces of the measuring element both
 facing toward and away from the air stream with wedge-like protrusions,
 once again to lessen the adhesion of suspended particles in the air
 stream. From German Patent DE 39 41 330 C2, it is known to incline the
 surface of a temperature-sensitive measuring element by a predetermined
 angle relative to the flow direction of the medium to be measured. Since
 the angle dependency of the measuring characteristic is relatively great
 if the measuring element is inclined only slightly relative to the flow
 direction, or in an extreme case is oriented parallel to the flow
 direction, yet at greater angles of inclination between the measuring
 surface of the measuring element and the flow direction of the medium the
 angle dependency of the measuring characteristic is less, the teaching of
 this Patent yields a relatively reliable, replicable measurement result if
 the angle between the flow direction of the medium and the measuring
 surface of the measuring element is within a range of between 20.degree.
 and 60.degree..
 The known measuring devices have the disadvantage, however, that the
 measuring element can be destroyed by dirt particles, especially dust
 particles, and trained in the flowing medium, if the dirt particles
 collide with the measuring element. Especially when micromechanical
 components, of the kind described for instance in DE 43 38 891 A1, are
 used as the measuring elements, the dirt particles can strike the
 relatively thin diaphragm and do lasting harm. The result can be increased
 wear of the measuring element and premature failure. In addition, oily or
 greasy dirt particles can settle on the measuring element, and
 particularly on its diaphragm and act as adhesion promoters for solid
 particles, such as dust or grains of sand, and persistently soil the
 measuring element. This destroys the thermal coupling between the
 measuring element and the flowing medium, causing a shift in the
 measurement characteristic curve that necessarily leads to measurement
 errors. If the measuring device is used to detect the aspirated air in the
 intake conduit of an internal combustion engine, for instance, the result
 can be incorrect triggering of the fuel injection valves and thus a less
 than optimal setting of the fuel-air mixture, so that as the measuring
 element becomes increasingly soiled, the engine exhaust emissions become
 worse.
 A further disadvantage of the known measuring device is that the
 measurement accuracy is still not optimal in the case of pulsating flows
 in the line to be measured.
 ADVANTAGES OF THE INVENTION
 The measuring device of the invention for measuring the mass of a medium
 flowing in the line, has the advantage over the prior art that dirt
 particles entrained in the flowing medium are largely prevented from
 impinging on the measuring element and at least are reduced. Particularly
 the diaphragm of a measuring element embodied as a micromechanical
 component is largely protected by the provision of the invention against
 the collision of dirt particles entrained in the flowing medium, so that
 the service life of the measuring element is prolonged substantially. By
 dividing the flow conduit into a measuring conduit, which receives the
 measuring element, and a bypass conduit that bypasses the measuring
 element, it is attained that the dirt particles are substantially carried
 away through the bypass conduit and bypass the measuring element, while
 relatively little contaminated medium flows past the measuring element
 through the measuring conduit. This reduces the risk of collision of the
 measuring element considerably, and especially a thin, vulnerable
 diaphragm of the measuring element, with the dirt particles considerably.
 Since the incidence of oily and greasy dirt particles on the measuring
 element is furthermore reduced, soiling from dust and other solid
 particles adhering to the measuring element is largely prevented. This
 counteracts any change in the characteristic curve and increases the
 reliability of the measurement result obtained. If the measuring device is
 used to detect the aspirated air mass in an internal combustion engine,
 the engine emissions are therefore not made permanently worse.
 Advantageous refinements of and improvements to the measuring device
 defined herein are possible with the provisions recited in hereinafter.
 It is especially advantageous if the flow conduit, between the inlet
 opening and the dividing point at which the flow conduit branches into the
 measuring conduit and the bypass conduit, has a curved portion, and the
 measuring conduit adjoins an inner region with a relatively small radius
 of curvature while the bypass conduit adjoins a peripheral region with a
 relatively large radius of curvature of the curved portion. As a result of
 the centrifugal forces acting on the dirt particles in the curved portion,
 the dirt particles are positively displaced outward into the peripheral
 region, so that the peripheral region of the curved portion is
 contaminated with relatively many dirt particles, while the inner region
 of the curved portion is contaminated with relatively few dirt particles.
 Most of the dirt particles therefore enter the bypass conduit bypassing
 the measuring element, and do not enter the measuring conduit, and the
 contamination of the medium bathing the measuring element is reduced
 markedly.
 As an alternative to this, it is also possible to offset the measuring
 conduit radially from the inlet opening relative to a longitudinal axis of
 the line to be measured. As a result, the measuring conduit is located
 largely outside the flight path of the dirt particles, which extends
 substantially parallel to the longitudinal axis of the line and is thus
 predetermined by the projection of the inlet opening parallel to the
 longitudinal axis of the line.
 Between the measuring conduit and the bypass conduit, a partition can be
 provided; the bypass conduit and the measuring conduit can either reunite
 downstream of the measuring element and emerge at a common outlet opening,
 or the measuring conduit and the bypass conduit can be extended onward in
 the measuring device in the form of separate conduits with separate
 conduits with separate outlet openings. Especially when the measuring
 conduit and the bypass conduit unite again downstream of the measuring
 element to form a common flow conduit, such as an S-shaped deflection
 conduit, it is advantageous to make the partition streamlined in
 cross-sectional profile, in order to prevent flow separations and to
 present the least possible flow resistance to the flowing medium.
 The outlet openings of the measuring conduit and of the bypass conduit are
 preferably disposed on a trailing end of the measuring device, which is
 located opposite the inlet opening disposed on a leading end.
 Especially preferably, the flow conduit has a second dividing point, where
 the flow conduit branches off counter to the primary flow direction into
 the measuring conduit and a second bypass conduit. Especially in the event
 of pulsating flows, in which a reverse flow component counter to the
 primary flow direction occurs, this provision is advantageous, since then
 the medium flowing past the measuring element is freed of dirt particles
 in the reverse direction as well. To that end, a second curved portion is
 advantageously provided between the outlet opening and the second dividing
 point. The first curved portion and the second curved portion are
 preferably embodied symmetrically to one another, so that even in the
 event of a strongly reverse-pulsating flow, only a relatively slight
 measurement error occurs. The two curved portions and the likewise curved
 measuring conduit advantageously combine to form a loop that encloses an
 angle of approximately 360.degree..

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
 FIG. 1, in a sectional view, shows a side view of a measuring device 1
 according to the invention, which is used to measure the mass of a flowing
 medium, in particular the aspirated air mass of internal combustion
 engines.
 The measuring device 1 detects the mass of a medium flowing in a flow line
 2. The flow line 2 is shown merely schematically and extends along a
 longitudinal axis 3, at least in the region of the measuring device 1. The
 flow line 2 may for instance be an intake line of an internal combustion
 engine, by way of which the engine can aspirate air from the environment.
 In the exemplary embodiments shown, the medium, such as the aspirated air,
 flows from right to left through the flow line 2. The flow direction in
 the flow line 2 is indicated by an arrow 4.
 The measuring device 1 preferably has a slender shape extending radially in
 the flow line 2, and it can preferably be inserted, for instance in
 plug-in fashion, into an opening made in the wall 5 of the flow line 2.
 Embodying the measuring device 1 as a plug-in module that can be plugged
 into the wall 5 of the flow line 2 makes especially simple installation
 and maintenance possible. In a preferred embodiment, an electronic
 evaluation circuit 6 can be integrated with the measuring device 1, for
 instance being cast integrally with the measuring device. It is equally
 conceivable to accommodate an electronic evaluation circuit outside the
 wall 5. Suitable contacts 8 for supplying current to the measuring device
 1 and for picking up the measurement signal obtained by the measuring
 device 1 are provided on a plug portion 7 that protrudes from the wall 5
 of the flow line 2 and are connected to the evaluation circuit 6 via
 connecting lines 9.
 The measuring device 1 can for instance be made in one piece of plastic as
 a plastic injection molded part. The measuring device 1 has a flow conduit
 10, which is disposed in the manner of a bypass line, parallel to the
 primary flow cross section 11 of the flow line 2. The flow conduit 10
 extends from an inlet opening 12 to one or more outlet openings. In the
 exemplary embodiment shown in FIG. 1, a first outlet opening 13 and a
 second outlet opening 14 are provided. The primary flow direction at the
 inlet opening 12 is indicated by an arrow 16, and the primary flow
 direction at the outlet openings 13 and 14 is indicated by a respective
 arrow 17 and 18. The primary flow direction inside the flow conduit 10 is
 indicated by an arrow 19.
 According to the invention, the flow conduit 10 branches at a dividing
 point 15 into a measuring conduit 20, in which a measuring element 21 is
 disposed, and a bypass conduit 22 that bypasses the measuring element. The
 measuring element 21 is connected to the evaluation circuit 6 via
 connecting lines 23 and is preferably embodied as a micromechanical
 component, of the kind proposed for instance in DE 43 38 891 A1. The
 measuring element 21 in a manner known per se has at least one but
 preferably two temperature-sensitive resistor elements, which are embodied
 on a dielectric diaphragm, for instance of silicon oxide or silicon
 nitride. The dielectric diaphragm has the advantage of an only slight
 thermal capacity and a relatively slight thermal conductivity, so that the
 response performance of the measuring element is relatively fast.
 The measuring element 21, in the preferred exemplary embodiment shown, has
 a plate-like silicon-based substrate body, with a diaphragm-like sensor
 region created by etching and having an extremely slight thickness and
 having a plurality of resistor layers, also created by etching. These
 resistor layers form at least one temperature-dependent measuring resistor
 and for instance a hot resistor. The hot resistor is preferably located in
 the middle of the diaphragm and is regulated to an over temperature with
 the aid of a temperature sensor. Upstream and downstream of the hot region
 formed by the hot resistor, there are two measuring resistors disposed
 symmetrically to the hot region. The substrate body of the measuring
 element 21 is accommodated flush in a recess of a receptacle, for instance
 of metal, and is retained there, for instance by adhesive bonding. The
 receptacle protrudes into the measuring conduit 20, so that the measuring
 element 21 is bathed by the medium flowing through the measuring conduit
 20 of the measuring device 1.
 In the exemplary embodiment shown in FIG. 1, a first curved portion 24 is
 located between the inlet opening 12 and the dividing point 15; in the
 exemplary embodiment shown, it is curved to the right, in the primary flow
 direction 19. As a result, dirt particles located in the line, which
 invade the flow conduit 10 through the inlet opening 12, are positively
 displaced outward into a peripheral region 25 of the curved portion 24 by
 centrifugal force, because of their inertial mass. These dirt particles
 may be either liquid droplets, such as water droplets or oil droplets, or
 solid particles, such as dust. The inner region 26 of the curved portion
 24, conversely, is contaminated relatively little with dirt particles,
 because of the positive displacement dictated by the centrifugal force.
 Because the measuring conduit 20 adjoins the inner region 26 of the curved
 portion 24, and conversely the bypass conduit 22 that bypasses the
 measuring element 21 adjoins the peripheral region 25 of the curved
 portion 24 in the primary flow direction 19, it is attained that the
 medium especially contaminated with dirt particles and located in the
 outer region 25 of the curved portion 24 is returned into the flow line 2
 via the second outlet opening 14, without the risk that the dirt particles
 can strike the measuring element 21 and damage it. This is especially
 important if the measuring element 21 is embodied as a micromechanical
 component as described above, with a diaphragm-like sensor region that is
 especially sensitive with respect to the impact of dirt particles. By the
 provision according to the invention, in a sense the solid or liquid phase
 of the flowing medium, which contains the dirt particles, is separated
 from the gaseous phase of the medium that is actually to be measured.
 Because the contaminated medium is carried away via the second outlet
 opening 14, it is assured that the dirt particles cannot become deposited
 in the bypass conduit 22 or in the peripheral region 25 of the curved
 portion 24, so that a self-cleaning effect is attained. In the exemplary
 embodiment shown in FIG. 1, the measuring conduit 20 and the bypass
 conduit 22 are separated by a lip-like thin partition 27. The measuring
 conduit 20 and the bypass conduit 22 extend largely parallel to one
 another, and the medium flowing through the measuring conduit 20 and the
 medium flowing through the bypass conduit 22 emerge from separate but
 adjacent outlet openings 13 and 14. The outlet openings 13 and 14 are
 located on a trailing end 28, facing away from the primary flow direction
 4 of the flow line 2, and this end is opposite a leading end 29 facing
 toward the primary flow direction 4 of the flow line 2, and this is where
 the inlet opening 12 is located. In the exemplary embodiment shown in FIG.
 1, the trailing end 28 and the lower region, in terms of FIG. 1, of the
 outflow end 29 have a curved cross-sectional constant, which is adapted to
 the flow conditions. The lip-like partition 27 is relatively simple to
 make from a production technology standpoint and can optionally also be
 inserted later into the flow conduit 10, after the forming of the flow
 conduit.
 FIG. 2 shows a second exemplary embodiment of the measuring device 1 of the
 invention. Elements already described or corresponding to them are
 provided with the same reference numerals.
 The exemplary embodiment shown in FIG. 2 differs from the exemplary
 embodiment described in conjunction with FIG. 1 on the one hand in that
 the measuring conduit 20 that receives the measuring element 21 and the
 bypass conduit 22 that bypasses the measuring element 21 are reunited,
 downstream of the partition 27 and downstream of the measuring element 21,
 respectively, to form a common flow conduit in the form of an S-shaped
 deflection conduit 40. The outlet opening 30 of the deflection conduit 40
 is located in a beveled section 41 that is in the lee relative to the
 primary flow direction 4 of the flow line 2. As a result of the deflection
 conduit 40, when there are highly pulsating flows, only slight measurement
 error occurs even if there is a relatively strong reverse flow component.
 The exemplary embodiment shown in FIG. 2 also differs from the exemplary
 embodiment shown in FIG. 1 in that the introduction of the medium that is
 contaminated more strongly with dirt particles into the bypass conduit 22
 and of the medium contaminated less or only negligibly with dirt particles
 in the measuring conduit 20, is effected not by means of a curved portion
 24 but instead by radially offsetting the measuring conduit 20 from the
 inlet opening 12 relative to the longitudinal axis 3 of the line 2. Since
 the flight path of the dirt particles is oriented substantially parallel
 to the longitudinal axis 3 of the line 2, only relatively few dirt
 particles reach the measuring conduit 20, when the dividing point 15 is
 located outside the projection of the inlet opening 12, parallel to the
 longitudinal axis 3 of the flow line 2. In this exemplary embodiment, the
 partition 27 is streamlined in shape, in order to prevent flow separations
 and to present the least possible flow resistance to the flowing medium.
 FIG. 3 shows a third exemplary embodiment of the measuring device of the
 invention. In FIG. 3 as well, elements already described or corresponding
 to those already described are provided with the same reference numerals,
 to make it easier to find them. In a way similar to the exemplary
 embodiment shown in FIG. 1, the flow conduit 10 branches in the primary
 flow direction 19 into the measuring conduit 20, in which the measuring
 element 21 is disposed, and the bypass conduit 22.
 The measuring conduit 20 and the first bypass conduit 22 are curved
 contrary to one another upstream of the dividing point 15 in the primary
 flow direction 19, so that the bypass conduit 22 takes a relatively short
 course to the second outlet opening 14 on the trailing end 28 of the
 measuring device 1. In the manner already described, the measuring conduit
 20 adjoins an inner region and the bypass conduit 22 adjoins a peripheral
 region of the curved portion 24, so that because of the centrifugal force
 acting on the dirt particles, these particles predominantly invade the
 bypass conduit 22 and do not reach the measuring element 21. The curvature
 of the curved portion 24 continues in the measuring conduit 20, and the
 measuring conduit 20 together with the curved portion 24 forms a loop,
 extending from the inlet opening 19 to the first outlet opening 13, that
 forms an angle of approximately 360.degree..
 The measuring element 21 in this exemplary embodiment is approached by a
 flow counter to the primary flow direction 4 in the line 2, and the
 measuring conduit 20 is continued in a deflection and then returned back
 into the flow line 2 at the outlet opening 13 in approximately the same
 axial position relative to the inlet opening 12, but laterally offset in
 accordance with the width of the flow conduit 10. The result is a
 virtually symmetrical design of the measuring conduit 20, as a result of
 which the mass of the flowing medium detected by the measuring device 1 of
 the invention is largely independent of the flow direction. This is
 especially important with highly pulsating flows that have a relatively
 major reverse flow, component, as is the case for instance in the intake
 line of an internal combustion engine. However, any flow phenomena that
 may occur in the vicinity of the sensor can lead to an intentionally
 asymmetrical embodiment of the measuring conduit 20, without sacrificing
 the advantages of a major reverse flow component.
 FIG. 4 shows a fourth exemplary embodiment of the measuring device 1 of the
 invention. Once again, already described or equivalent elements are
 provided with the same reference numerals, so that in this respect
 repetition is unnecessary.
 The exemplary embodiment shown in FIG. 4 is largely equivalent to the
 exemplary embodiment already described in conjunction with FIG. 3. A
 special feature is that not only a first dividing point 15 is provided, at
 which the flow conduit 10 branches in the primary flow direction 19 into
 the measuring conduit 20 and a first bypass conduit 21 that discharges at
 the outlet opening 14 and bypasses the measuring element 21. In addition,
 a second dividing point 50 is provided between the measuring element 21
 and the first outlet opening 13; at this second dividing point, the flow
 conduit 10 branches counter to the primary flow direction 19 into the
 measuring conduit 20, in which the measuring element 21 is disposed, and a
 second bypass conduit 51 that bypasses the measuring element 21. In the
 exemplary embodiment shown, the second bypass conduit 51 discharges into
 the line 2 at a third outlet opening 52, at a side face of the measuring
 device 1 that is substantially parallel to the longitudinal axis 3 of the
 line 2. The second bypass conduit 51 likewise leads over a relatively
 short path to the associated outlet opening 52. The short paths of the two
 bypass conduits 21 and 51 prevent the dirt particles from becoming
 deposited there. Also provided in the exemplary embodiment shown in FIG. 1
 is a second curved portion 53, which is disposed between the first outlet
 opening 13 and the second dividing point 50. The measuring conduit 20 here
 adjoins an inner region 54 with a relatively small radius of curvature of
 the second curved portion 53 counter to the primary flow direction 19,
 while the second bypass conduit 51 adjoins a peripheral region 55 with a
 relatively great radius of curvature of the second curved portion 53
 counter to the primary flow direction 19.
 The purpose of the second dividing point 50 and the second bypass conduit
 51 is to assure, even in the presence of highly pulsating flows with a
 relatively major reverse flow component counter to the primary flow
 direction 19, that in the same way as at the first dividing point 15, dirt
 particles entering through the second outlet opening 13 during the reverse
 flow will not reach the measuring conduit 20 but will reach the second
 bypass conduit 51 instead, because of centrifugal force, and will emerge
 from the third outlet opening 52, bypassing the measuring element 21. In
 this way, even during the reverse flow, dirt particles briefly flowing
 counter to the primary flow direction 19 are reliably kept away from the
 measuring element 21.
 The first curved portion 24 and the second curved portion 53 of the flow
 conduit 10 are preferably embodied substantially symmetrically. In
 addition, the first curved portion 24, the measuring conduit 20 and the
 second curved portion 53 preferably combine to form a loop that encloses
 an angle of approximately 360.degree..
 The invention is not limited to the exemplary embodiments shown. The
 measuring conduit 20 and the bypass conduits 22 and 52 can also be
 embodied in some other way if that is appropriate for a particular
 application. Optionally it may be advantageous to provide the third outlet
 opening 52 on the trailing end 28 of the measuring device as well, and to
 lengthen the second bypass conduit 51 accordingly. The measuring device of
 the invention is suitable for measuring the mass of flowing media in both
 gaseous and liquid form.
 The foregoing relates to a preferred exemplary embodiments of the
 invention, it being understood that other variants and embodiments thereof
 are possible within the spirit and scope of the invention, the latter
 being defined by the appended claims.