Abstract:
The match with a measurement characteristic curve of a device for air flow measurement is disturbed by pulsations, soiling and poor flow behavior. An improvement in the measuring performance of the device is achieved by provisions, which are adapted to one another, for reducing these sources of trouble, according to which the flow cross section of an inlet conduit narrows in a flow direction in the inlet conduit toward a deflection conduit, and a peripheral face of a first portion of the deflection conduit is embodied in inclined fashion, and at least one outer face of a sensor carrier, together with a peripheral face, closer to an outlet conduit, of the inlet conduit, forms a flush transition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE 00/01850 filed on Jun. 7, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is directed to an improved mass flow measuring device for measuring the mass of a flowing medium . 
     2. Description of the Prior Art 
     A mass flow measuring device with a measurement conduit is already known (German Patent Disclosure DE 197 35 891 A1), which conduit accommodates a measuring element that is bathed by the inflowing medium there. The flowing medium flows from an inlet conduit first into a deflection conduit, which has a larger flow cross section than the inlet conduit and has a right-angled corner, so that there is an abrupt flow transition in the form of a shoulder toward the inlet conduit. Adjoining that, the medium flows from the deflection conduit, deflected by the corner, along the peripheral face of the deflection conduit into an outlet conduit adjoining it transversely, and leaves the outlet conduit through an outlet opening so that it can mix again with the medium flowing past the device. An inlet conduit longitudinal axis and an outlet conduit longitudinal axis are inclined by a predetermined angle relative to the longitudinal axis of the line, so that the inlet conduit has a region that is shaded from a primary flow direction. The measuring element is disposed in the shaded region of the measurement conduit, to prevent soiling and resultant defects of the measuring element. 
     Dirt particles that enter the inlet conduit along with the flowing medium can destroy the measuring element, if the dirt particles collide with it. Especially if micromechanical components, of the kind described in German Patent Disclosure DE 43 38 891 A1, for instance, are used as measuring elements, the dirt particles can strike a relatively thin diaphragm and permanently damage it. This can lead to increased wear of the measuring element and premature failure. Dirt particles that contain oil or grease can also become deposited on the measuring element, and especially on its diaphragm; they can act as adhesion promoters for solid particles, such as dust, and can permanently soil the measuring element. The soiling interferes with the thermal coupling between the measuring element and the flowing medium, causing a shift in a measurement characteristic curve that necessarily leads to measurement errors and thus incorrect triggering of the fuel injection valves. 
     From German Patent Disclosure DE 196 23 334 A1, it is known that the inlet conduit of such a device has a rectangular cross section; two side faces toward the chiplike measuring element are embodied as extending obliquely, resulting in a narrowing of the inlet conduit in the flow direction of the medium in the inlet conduit. A top face of the inlet conduit extending transversely to the side faces, from which top face the measuring element protrudes, and a bottom face of the inlet conduit opposite the top face, extend plane or parallel, with a constant spacing from one another. A device equipped with this kind of inlet conduit is also known from SAE Paper 950433 (International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 2, 1995, reprinted from: Electronic Engine Controls 1995 (SP-1082)). As can be seen from the sectional view in FIG. 7, top, on page 108 of this publication, the inlet conduit and the deflection/outlet conduit are essentially formed of two parts; a part hereinafter called the bottom part, together with the measuring element includes a side face, a top face, and a bottom face of the measurement conduit. Another part has only the second side face of the measurement conduit and thus forms a cap part. The bottom part and the cap part are made from plastic by plastic injection molding. The narrowing design of the side faces of the inlet conduit results in an increasing wall thickness in the flow direction. 
     In an internal combustion engine, opening and closing of the injection valves of the individual cylinders cause considerable fluctuations or pulsations in the flow, the severity of which depends on the intake frequency of the individual pistons and on the engine rpm. The flow pulsations propagate from the injection valves along the intake line to the measuring element in the inlet conduit and onward from there. The effect of the pulsations is that depending on their severity, because of thermal inertia and directional insensitivity of the measuring element, the measuring element produces a measurement result that can deviate considerably from the flow speed prevailing in the inlet conduit and from the resultant calculated intake air flow rate of the engine. The inlet conduit and the deflection/outlet conduits are adapted to one another in their dimensions such that when there is a pulsating flow in the intake line, the erroneous indication provided by the measuring element as a result of the flow fluctuations is minimal. Nevertheless, at high pulsation frequencies and a significant pulsation amplitude, flow and/or acoustical processes taking place in the deflection conduit can lead to an erroneous indication of the aspirated air flow rate. This erroneous indication arises especially because when there is a pulsating flow downstream of the measuring element at the shoulder between the outlet of the inlet conduit and the corner at the first portion of the deflection conduit, a pressure wave can occur, which is reflected from the peripheral face of the deflection conduit at the corner, so that feedback interferes with a measurement signal of the measuring element. 
     From German Patent Disclosure DE 197 41 031 A1, a measurement device with an inlet conduit is known, in which device, by the design of two walls of the inlet conduit, an acceleration of the flow in the inlet conduit can continue to be maintained; this acceleration is known to lead to a stabilization of the flow of the medium in the inlet conduit, especially at the inlet. 
     However, the known devices have at least two of the following disadvantages: 
     they do not offer adequate protection of the measuring element from dirt; 
     a flow around the sensor carrier and poor stabilization of the flow in the inlet conduit lead to scattering of the measurement signal; 
     narrowing of the inlet conduit in only one direction, or in other words two opposed side walls; 
     inadequate provisions, if any, for improved pulsation performance; 
     disadvantages in terms of production: the entire measuring device would have to be tilted for improved protection against dirt, with the resultant changes in the measurement stub into which the measurement device is inserted; and 
     because of the increasing wall thickness of the plastic, different cooling speeds occur along with accumulations of material, which can in particular cause sunken areas on the side faces of the measurement conduit and which, in planned mass production of the device, would cause more or less severe scattering of the attainable measurement precision of the devices. 
     SUMMARY OF THE INVENTION 
     The improved flow measuring device according to the invention has the advantage over the prior art that in a simple way, the measurement performance is improved by reducing systematic and static errors, such as pulsation of the flow, by reduced soiling, and by improved flow behavior of the medium specifically. 
     Characteristics of claims  2 - 7  and  21  have the advantage improved stabilization of the flow in the measurement conduit, improved protection from dirt particles, and improvement at in the pulsation behavior are acheived. 
     The sealing of the sensor carrier at the bypass cap, the narrowing, the streamlined embodiment of all four peripheral faces of the inlet conduit, and the generally S-shaped embodiment of the measurement conduit all stabilize the flow in the measurement conduit. 
     Because of the oblique front edges of the sensor carrier and because of transverse flow components resulting from the inclination of the inlet conduit at a tangent to the respective edge of the sensor carrier, liquid and solid contaminants are carried away during operation. The shaded region prevents further accumulation of dirt particles. A suitable embodiment of an edge of the bow of the measurement housing and of a side wall of the inlet opening contribute to reflecting dirt particles away from the inlet opening. 
     Erroneous indications that occur when pulsation frequencies are high are reduced by the provision that a protuberance is provided in a surrounding region of the outlet opening, and a peripheral face of a first portion of the deflection conduit is embodied as inclined toward the flow direction in the measurement conduit. A fluidic connection or communication with the outer flow in the intake line, provided in the deflection conduit and taking the form of an opening, reduces any residual interference with the pressure wave that may still exist in the deflection conduit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will be apparent from detailed description contained hereinbelow, taken with the drawings, in which: 
     FIG. 1 shows a device according to the invention for measuring air flow rates, in the built-in state; 
     FIG. 2 shows the inlet conduit, deflection conduit and outlet conduit in the measurement housing; 
     FIGS. 3 a ,  3   b  and  3   c  show a flush transition of the sensor carrier and the measurement conduit; 
     FIGS. 4,  4   a  and  5  each show a sectional view of FIG. 1; 
     FIG. 6 schematically shows the flow conditions on the upstream face end of the sensor carrier; 
     FIGS. 7 and 8 show further versions of the device of the invention; and 
     FIG. 9 shows various arrangements of the sensor carrier and measuring element. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 schematically shows how a device  1  according to the invention is built into a line  2  within which the medium to be measured flows. 
     The device  1  for air flow rate measurement comprises a measurement housing  6 , represented by a lower rectangle drawn in dashed lines, and a carrier part  7 , represented by an upper rectangle drawn in dashed lines, in which the evaluation electronics are for instance accommodated. The measurement housing  6  and the carrier part  7  have a common longitudinal axis  8 , which by way of example can also be the center axis. The device  1  is introduced, for instance in plug-in fashion, into a wall  5  of the line  2 . The wall  5  defines a flow cross section, in the center of which a center axis  4  extends in the direction of the flowing medium, parallel to the wall  5 . The direction of the flowing medium, hereinafter called the primary flow direction, is represented by corresponding arrows  3  and extends in this case from left to right. 
     FIG. 2 shows the measurement housing  6  with a measurement conduit  40  and the carrier part  7 , without a cap  49  that closes the measurement conduit  40 . The measurement conduit  40  is formed by a bottom part  42  and a cap  49  (FIG.  3 ). The primary flow direction  3  of the medium is represented by arrows. The measurement conduit comprises an inlet conduit  13 , a deflection conduit  15 , which in turn splits into a first part  16  and second part  17 , and an outlet conduit  19 . The flow directions  25 ,  26  in the inlet conduit  13  and outlet conduit  19  are also represented by arrows. The center line  23  of the inlet conduit is curved in this case, since the peripheral faces  35  of the inlet conduit are embodied in streamlined fashion. The center line  22  of the outlet conduit in this case is a straight line. 
     In the front or upstream region  39  of the measurement conduit  40  upstream of an inlet opening  11  through which the medium flows in, a flow obstacle  24  is provided, which brings about a defined flow separation that is operative in the measurement conduit. This is described in further detail in German Patent Disclosure DE 44 41 874 A1 and is meant to be part of the present disclosure. 
     A bow  69  of the measurement housing  6  is shaped such that solid or liquid particles are reflected away from the inlet opening  11 . To that end, the bow  69  is inclined counter to the carrier part  7 . 
     A face  34 , drawn in dashed lines and extending parallel to the primary flow direction  3 , together with the peripheral face of the inlet conduit toward the carrier part  7 , forms a shaded or lee region  33 , into which only a few if any dirt particles or liquids enter. 
     In the first part  16  of the deflection conduit  15 , a peripheral face  20  is inclined counter to the primary flow direction  3  by an angle δ. The angle δ can be in the range from about 30-60° and ideally is about 45°. The influence of this embodiment is described in further detail in German Patent Disclosure DE 196 23 334 A1 and is meant to be part of the present disclosure. The peripheral face  20  has a depth tr (FIG. 4) and a width br extending perpendicular to it which is equivalent to at least ⅔ the width b of the inlet opening  11  of the inlet conduit  13 . Perpendicular to the width br, the peripheral face  20  has a depth tr that is approximately equivalent to the depth t of the inlet conduit  13  perpendicular to its width b at the inlet opening  11 . However, it is also possible to embody the peripheral face  20  with a depth tr that is somewhat less than the depth t of the inlet opening  11  of the inlet conduit  13 . Adjoining the peripheral face  20 , the wall of the first portion  16  extends approximately in the direction of the longitudinal axis  8 . 
     An opening  18  that establishes a communication with a medium that bathes the device  1  is provided in the second portion  17  of the deflection conduit  15 . There can also be more than one opening. The opening or openings can also be located only in the first part or in the first part  16  and second part  17  of the deflection conduit  15 . The opening or openings can be located on the side walls  41  and/or can lead to a lower outer face  21  of the measurement housing  6  of the device  1  having the measurement conduit  40 , in order to establish the communication with the line  2 . The outlet opening  12  is located at the end of the outlet conduit  19 , and its face forms an angle χ with the primary flow direction  3 , through which the medium leaves the measurement conduit again. The outlet opening  12  has a larger cross section than the outlet conduit  19 , and as a result the pulsation behavior is improved. 
     The sensor carrier  9  protrudes into the inlet conduit  13  and in this example protrudes in part into a recess  38 , which is provided in the peripheral face  27  of the inlet conduit  13  closer to the outlet conduit  19 . A partition  52  located on a cap  49  (FIG. 3) that closes the inlet conduit  13 , deflection conduit  15  and outlet conduit  19 , forms a flush transition  50  with a part of a side, toward the cap  49  and forming an outer face, of the sensor carrier  9  and engages the inside of the recess  38  in such a way that it continues the peripheral face  27  in the region of the recess  38 , so that no flow flowing around the sensor carrier  9  will occur here. 
     The measuring element  10  is accommodated in the sensor carrier  9  and is appropriately located in the shaded region  33 . The layout of a measuring element  10  of this kind is adequately known to one skilled in the art, for instance from German Patent Disclosure DE 195 24 634 A1, whose disclosure is meant to be part of the present patent application. In some regions between walls of the measurement conduit  40  and outer faces of the measurement housing  6 , indentations  53  are provided in the bottom part  42  of the measurement housing; in some parts they bring about a constant wall thickness, and in others a reduction in the wall thickness, of the peripheral faces of the measurement conduit  40 . 
     FIG. 3 shows two examples of how a flush transition  50  between an outer face of the sensor carrier  9  and a peripheral face  54  of the inlet conduit  13  is achieved. The drawing shows a section taken along the longitudinal axis  8 . In the first example, FIG. 3 a ), there is no recess in the peripheral face  54  of the inlet conduit  13 . Between a face end  47  of the sensor carrier  9  and a peripheral face  54  of the inlet conduit  13  closer to the outlet conduit  19 , there is a sealing means  48 , which fills the gap  56  that may be present because of tolerances and thus forms the flush transition  50 , so that no flow underneath takes place there. Alternatively, the sealing means  48  can also be applied around the sensor carrier  9  at the level of the face end  47 , or in other words around the gap  56  that is present because of tolerances. The gap  56  is thus closed and forms the flush transition  50  in such a way that no flow underneath occurs there. 
     In FIG. 3 b ), a recess  38  is present in the peripheral face  54  of the inlet conduit  13  closer to the outlet conduit  19 , and the sensor carrier  9  protrudes with its face end  47  into this recess. The partition  52 , located on the cap  49  that closes the inlet conduit  13 , deflection conduit  15  and outlet conduit  19 , engages the inside of the recess  38  in such a way that it continues the streamlined peripheral face  35  of the inlet conduit  13  in the region  27  of the recess  38 . Located between a face end of the partition  52  and a side of the sensor carrier  9  that forms an outer face toward the cap  49 , there is a sealing means  48 , which fills the gap  56  that might be present because of tolerances and thus forms the flush transition  50 . Alternatively, the sealing means  48  can also be applied around the sensor carrier  9  at the level of the peripheral face  54 , or in other words around the gap  56  that exists because of tolerances. The gap  56  is thus closed and forms the flush transition  50  in such a way that no flow underneath occurs there. A sealing means  48  is also, but not necessarily, located between the sensor carrier  9  and a peripheral face, farther away from the measuring element  10 , in the recess  38  of the inlet conduit  13 . 
     FIG. 3 a , at  44  shows an instance where the sealing has been carried out by means of welding, such as by ultrasound or laser, wherein the sensor carrier  9  has been joined to the opposite side of the inlet conduit. 
     FIG. 4 shows a section taken along the line IV—IV in FIG. 2, including the cap  49  that extends through the shaded region  33 . 
     The inlet conduit  13  of the device  1  has a blocklike shape and extends along an inlet conduit center line  23  extending centrally in the inlet conduit  13  from an inlet opening  11 , which for example has a rectangular cross section, to an outlet opening  14 , which for instance also has a rectangular cross section. The device  1  is built into the line  2  preferably in such a way that a perpendicular projection of the inlet conduit center line  23  in the direction of the center line  4  onto a plane that is perpendicular to the longitudinal axis  8  extends parallel to the center line  4 . However, it is also possible, as indicated in FIG. 4 a  to install the device  1  in an installed position rotated about the longitudinal axis  8 , so that the line  55 , which represents the axis of the device  1 , and the center line  4  form an angle γ of a few degrees. 
     A receptacle  57  for the measuring element  10  is recessed out of the sensor carrier  9  on one side. The measuring element  10  and the two side faces  58 , extending approximately parallel to the center line  23  of the inlet conduit, of the sensor carrier  9  are thus bathed by the medium. 
     The side faces  73 ,  74  of the measurement conduit  40  extend obliquely to a plane  75  defined by the center line  23  of the measuring element and by the longitudinal axis  8 , and with it they form an acute angle, so that viewed in the primary flow direction  3 , the inlet conduit  13  narrows axially and then discharges with its smallest cross section at the outlet opening  14  into a first portion  16  of the deflection conduit  15 . 
     The narrowing has the effect that in the region of the measuring element  10 , a parallel flow that is as unimpeded and as uniform as possible can prevail. To avoid flow separations in the region of the inlet opening  11 , the inlet opening  11  of the inlet conduit  13  has a rounded edge  78 , shown in FIG.  5 . 
     The measuring element  10  is disposed in the receptacle  57  downstream at the narrowest point of the inlet conduit  13  or upstream of the outlet opening  14  in the inlet conduit  13 . 
     The deflection conduit  15 , put together from the first portion  16  and second portion  17 , preferably has a rectangular cross section, which is approximately equivalent to the cross-sectional area of the inlet opening  11  of the inlet conduit  13 , so that the flow cross section abruptly increases at a shoulder  76  at the outlet opening  14  between the inlet conduit  13  and the deflection conduit  15 . 
     FIG. 5 shows a section taken along the line V—V in FIG. 2, but without a sensor carrier  9 , and with a front region  39  that is located upstream of the inlet opening  11 . A side wall  77  of the inlet conduit  13  has an edge  78  in the front region  39 . This edge is chamfered in such a way that oncoming particles, such as dirt or liquids, are reflected away from the inlet opening  11 . The narrowing of the inlet conduit  13  by the side face  73  can also be seen. The opposed side face to the side face  73  is formed by the cap  49  (FIG.  3 ). The recess  38  is located in the peripheral face of the inlet conduit  13  that is closer to the outlet conduit  19 . The shoulder  76  has a height of 1 mm, for example, and could be reduced, compared to the precursor model of the device  1 , by narrowing all the peripheral faces of the inlet conduit  13 , in order to avoid, greater wall thicknesses and the attendant production problems. 
     FIG. 6 shows a schematic illustration of the flow conditions at an upstream face end  81  of the sensor carrier  9 , which is beveled there by at least one bladelike transverse side  81 , with the flow components  51 , which is located in the oblique face  81 , and  59  of the flow direction  25  in the inlet conduit  13 . The transverse flow component  51  exerts a force that is oriented upward, in terms of FIG. 6, on dirt particles that adhere to the oblique face  81 . This effect is familiar to one skilled in the art from German Patent Disclosure DE 197 35 891 A1 and is meant to be part of this present disclosure. 
     FIGS. 7 and 8 show further exemplary embodiments of the device  1  of the invention. Elements already described are provided with the same reference numerals. A tear-off edge  62  in FIG. 7 can be sharp-edged or can have a very small radius of curvature. In both cases, a protuberance  60  protrudes past a respective upstream end  63 , in terms of the primary flow direction  3 , of the outlet opening  12 . In other words, a plane  64  extending perpendicular to the primary flow direction  3  of the line  2  and touching the tear-off edge intersects the outlet opening  12 . The protuberance  60  preferably has a substantially triangular cross-sectional contour; one corner of the triangular cross-sectional contour forms the tear-off edge  62 , and a further corner of the triangular cross-sectional contour coincides with the upstream end  63  of the outlet opening  12 , in terms of the primary flow direction  3 . 
     In FIG. 8, a further exemplary embodiment of the device  1  of the invention is shown, in which the protuberance  60  is disposed in a surrounding region  68  of the outlet opening  12  remote from the primary flow direction  3 . The protuberance  60  here is shaped in undulating fashion and is rounded in an end region  66  toward the primary flow direction  3 . The protuberance  60  is curved steadily and in the downstream region  65  in terms of the primary flow direction  3 , it merges with a plane  21 , without forming any edges. When the protuberance is provided upstream of the outlet opening, the pulsation error is shifted in the direction of an underindication, and the pulsation error occurring as a systematic measurement error is compensated for. Conversely, if the protuberance is disposed downstream of the outlet opening  12  in the primary flow direction  3 , the pulsation error is shifted in the direction of an excess indication. The result in the region of the protuberance is a relatively slight turbulence in the flow, and the protuberance presents a relatively slight flow resistance to the primary flow in the line  2 . A backpressure is built up in the end region  66  of the protuberance  60  and makes the flow through the measurement conduit  40  more difficult. In the case of a reverse flow in the line  2  counter to the primary flow direction  3 , this backpressure counteracts a flow through the measurement conduit  40  in the reverse flow direction. 
     FIG. 9 shows various arrangements of the sensor carrier  9  and measuring element  10  inside the measurement housing  6  that is drawn with dashed lines. In FIG. 9 a ), the sensor carrier  9  is disposed as in FIG. 2, for instance: A longitudinal axis  8  of the sensor carrier  9  is perpendicular to the primary flow direction  3 , and a longitudinal axis  45  of the measuring element  10  extends parallel to the longitudinal axis  8 . In FIG. 9 a ), however, the measuring element  10  is disposed with its longitudinal axis  45  inclined by an angle φ in the sensor carrier  9  compared to the longitudinal axis  8 . 
     In FIG. 9 b ), the longitudinal axis  46  of the sensor carrier  9  is inclined by an angle E from the longitudinal axis  8 . A longitudinal axis of the measuring element  10  extends parallel to the longitudinal axis  8 . With these arrangements, the behavior in terms of the oncoming flow and the flow around the measuring element  10  and the sensor carrier  9  can be improved still further. 
     The foregoing relates to 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.