DE102005028140B4 - Method for cleaning the measuring range of a hot-film air mass meter - Google Patents

Abstract

Method for measuring an air mass flow (142) flowing with a main flow direction (120) by means of a hot film air mass meter (110), in particular in the intake tract of an internal combustion engine, wherein the hot film air mass meter (110) has a sensor chip (130) with a measurement surface (132), wherein the measurement surface (132) with at least one heating element (144, 146, 148) is heated and during a start phase of the Heißfileinuftmassenmessers (110), the measuring surface (132) is heated asymmetrically, characterized by an additional phase of operation, wherein the operating phase of the starting phase is temporally downstream and wherein the measuring surface (132) is heated substantially symmetrically in the operating phase.

Description

The invention relates to a method for measuring an air mass flow, in particular in the intake tract of an internal combustion engine. In particular, the invention relates to a method for operating a Heißfileinuftmassenmessers having a sensor chip with a measuring surface.

State of the art

In many processes, for example in the field of process engineering, chemistry or mechanical engineering, defined gas mass, in particular an air mass, must be supplied. These include, in particular, combustion processes which take place under controlled conditions. An important example is the combustion of fuel in internal combustion engines of motor vehicles, in particular with subsequent catalytic emission control. Various types of sensors are used to measure the air mass flow rate.

One type of sensor known from the prior art is the so-called hot film air mass meter (HFM), which is used, for example, in US Pat DE 196 01 791 A1 in one embodiment. Such a hot-film air mass meter usually uses a sensor chip which has a thin sensor membrane, for example a silicon sensor chip. On the sensor membrane is typically arranged at least one heating resistor, which is surrounded by two or more temperature measuring resistors (temperature sensors). In an air flow, which is passed over the membrane, the temperature distribution changes, which in turn can be detected by the temperature measuring resistors. Thus, z. B. from the resistance difference of the temperature measuring resistors, an air mass flow can be determined. Various other variations of this type of sensor are known in the art.

An example from DE 101 11 840 C2 Known problem of this type of sensor is that often contaminations of the sensor chip may occur, for example, contamination by oil, other liquids or other types of contaminants. The sensor chip is usually used directly in the intake tract of the internal combustion engine or in a bypass to the intake tract of the internal combustion engine. In this case, oil may already precipitate on the sensor chip and in particular on the sensor membrane during operation of the internal combustion engine. This oil precipitate can lead to an unwanted measurement signal influencing of the sensor chip, in particular since an oil film acts on the surface of the sensor chip on the thermal conductivity of the surface, which leads to distortions of the measurement signals.

This oil contamination occurs in particular during or shortly after switching off the internal combustion engine, for example a diesel engine. After switching off the internal combustion engine, an overpressure present in a crankcase typically builds up in the intake tract of the internal combustion engine via a crankcase ventilation. In this case, oil vapor or oil mist is often entrained, which can be reflected in the intake tract and thus also arranged there (or arranged in a corresponding bypass) air mass meter as an oil film.

The problem of contamination of the membrane or the sensor surface is further exacerbated by thermodynamic effects. Thus, it is known that drops of liquid which have a gradient in their surface tension experience a force in the direction of the higher surface tension. This leads to a movement of the drop from low to high surface tension. In particular, this gradient may be caused by a temperature gradient on a surface to which the liquid droplet is applied. The drop is usually moved by the temperature gradient and the resulting force from a warmer area of the surface to a colder area of the surface. This effect is for example in V.G. Levich, "Physicochemical Hydrodynamics", Prentice-Hall, N.J., 1962, p. 384. This temperature gradient effect is also negatively noticeable when switching on a hot-film air mass meter. In the switched-off state of the hot-film air-mass meter, a liquid film, for example an oil film, usually collects on the surface of the sensor chip, in particular on the sensor membrane. When the hot-film air mass meter is turned on, the heating element usually arranged centrally on the sensor membrane is heated, whereby due to the effect described above, the oil film accumulated on the membrane is displaced upstream and downstream from the sensor membrane. As a result, an accumulation of an oil film (liquid wall) often forms on the boundary line between the membrane and the surrounding mainland of the sensor chip. This liquid wall can in turn be transported by the air mass flow to the surface of the sensor membrane and contaminate it again during operation. As a result, a drift of the measurement signal of the Heißfileinuftmassenmessers is caused during operation.

From the JP S60-13 1469 A is a method of measuring an air mass flow flowing with a main flow direction by means of a Heißfileinuftmassenmessers known in which the Heißfileinuftmassenmesser having a sensor chip with a measuring surface, wherein the measuring surface is heated with at least one heating element and the measuring surface is due to their eccentric position on the sensor chip is basically asymmetrically heated.

Advantages of the invention

Accordingly, a method is proposed for measuring an air mass flow flowing with a main flow direction by means of a hot film air mass meter, in particular for use in the intake tract of an internal combustion engine, which avoids the disadvantages of the methods known from the prior art. A basic idea of the present invention is the recognition that the above-described liquid wall at the downstream boundary line and thus at the downstream transition between hot and cold surface, is carried along by the air mass flow and transported away. This liquid wall therefore no longer influences the measuring signal permanently. The liquid wall located upstream on the boundary line, on the other hand, influences the measuring signal due to a changed thermal mass and possibly changed flow. Furthermore, due to the air mass flow, in particular when switching off the hot film air mass meter, this liquid wall can reach the measuring surface of the sensor chip and thus lead to a drift of the measuring signal.

Thus, a basic idea of the method according to the invention is to change the heating of the air mass meter during the switching on of the hot film air mass meter in relation to methods known from the prior art, so that accumulated liquid can be transported almost completely downstream. Thus, the upstream liquid wall is degraded in a startup phase of the hot film air mass meter. The drift of the sensor signal of the Heißfileinuftmassenmessers is thereby significantly reduced, and significantly increases the reliability of the measurement signal of the Heißfileinuftmassenmessers. A key idea is to heat the measuring surface substantially symmetrically in an operating phase, which is downstream of a start phase of the hot-film air mass meter, in which the measuring surface is heated asymmetrically.

As mentioned above, the hot-film air mass meter should have a sensor chip with a measuring surface. The measurement surface may be, for example, the surface of a sensor membrane, wherein the air mass flow flows substantially parallel over this surface. In addition to the design as a sensor membrane, however, other configurations are possible. An essential point here is that the sensor chip in the region of the measurement surface has a significantly reduced transverse thermal conductivity than in the main surface of the sensor chip surrounding the measurement surface. For example, the transverse thermal conductivity in the region of the measurement surface may be at least an order of magnitude lower than in the surrounding mainland. In addition to the design as a sensor membrane, such as a silicon membrane, which in DE 196 01 791 A1 described, other embodiments are possible, for example, an embodiment in the form of a porous material having a low thermal conductivity. At least one heating element, which can heat the measuring surface, is arranged on the measuring surface. By "on the surface" is also to be understood when this at least one heating element is arranged slightly below this surface, for example, is covered by additional protective layers.

Due to the low thermal conductivity of the measuring surface causes a strong temperature gradient forms when heating the at least one heating element at a boundary line between the Meßobertläche and the surrounding mainland. According to the invention, the measuring surface is heated asymmetrically by the at least one heating element. This asymmetric heating advantageously takes place exclusively in a starting phase of the hot-film air mass meter, that is, for example, immediately after switching on the internal combustion engine. In addition, however, an asymmetrical heating of the measuring surface can also take place in a later phase, for example an operating phase.

Advantageously, the asymmetrical heating of the measurement surface during the start-up phase is configured in such a way that regions of the measurement surface upstream of the main flow direction of the air mass flow are heated to a greater extent than regions of the measurement surface located downstream. For example, the measurement surface may have an inflow side and an outflow side, wherein the measurement surface is heated such that a temperature maximum of the temperature of the measurement surface is arranged closer to the inflow side than to the outflow side. However, other temperature distributions are conceivable, for example temperature distributions, in which, instead of a maximum in the temperature distribution, a center of gravity of the temperature distribution (for example an integral) is arranged closer to the inflow side than to the outflow side.

This inventive asymmetric embodiment of the heating of the measurement surface is a thermokapillar induced Fluid displacement is controlled from hot to cold so that fluid is largely forced into non-critical areas for the function of the hot film air mass meter. This is in contrast to methods known in the art in which liquid is usually forced to approximately equal parts into critical and non-critical areas. According to the invention, on the other hand, an increased heating takes place near the upstream region of the boundary line in the starting phase. The resulting temperature gradient displaces the liquid film over the downstream boundary line. After this displacement process in the start phase, the sensor can then continue to be operated normally, for example symmetrically, as known from the prior art. Alternatively, a faster and more targeted transport mechanism away from the measuring surface can be realized by a further local and temporal staggering of heating. This principle is z. B. used in the field of microreactors, such as in AA Darhuber et al., "Microfluidic actuation by modulation of surface stresses", Applied Physics Letters, 82 (4), January 2003 , described.

The measuring surface may, for example, have at least two temperature sensors, wherein the at least two temperature sensors may also be used as additional heating elements in addition to a main heating element. This corresponds to the usual structure of a sensor chip, in which a main heating element is surrounded symmetrically to the flow direction of two such temperature sensors. Usually, both the main heating element and the temperature sensor are designed as strip conductors on a surface of the sensor chip. According to the invention, these temperature sensors can also be used as additional heating elements. In this case, for example, in the starting phase, a first temperature sensor, which is arranged upstream, are heated more strongly than at least one second temperature sensor, which is arranged downstream. In particular, the downstream at least one second temperature sensor may remain unheated during the starting phase.

As described above, the starting phase can also be subdivided into a plurality of individual phases, in which the measuring surface of the sensor chip is heated differently. For example, the heating power and the temperature distribution on the measuring surface can be staggered during the starting phase. For example, the heating power in the at least two individual phases can be reduced in stages over time. Thus, for example, a specifically adjustable temporal and spatial "walking" of a temperature maximum on the measuring surface can be realized.

In addition to a temporal graduation, a spatial graduation can be made. Advantageously, this spatial staggering takes place such that in the main flow direction, the measuring surface is heated with decreasing heating power. For this purpose, the measuring surface can have, for example, at least three heating elements, which are heated in a staggered manner in the direction of the main flow direction, each with reduced heating power.

Overall, the inventive method leads to a signal drift of the signals of the Heißfileinuftmassenmessers by impurities with oil, other liquids or by dirt deposits, which are promoted in particular by oil as a primer, is greatly reduced. The method according to the invention can be implemented, for example, on commercially available hot-film air-mass meters corresponding to the state of the art. Structural changes are thus not necessarily required, which makes the method directly usable Thus, the reliability of already in use hot-film air mass meter can be significantly improved.

list of figures

Reference to the drawings, the invention will be explained in more detail below.

• 1 eine perspektivische Teildarstellung eines Heißfilmluftmassenmessers zum Einsatz im Ansaugtrakt einer Verbrennungskraftmaschine;
• 2A einen Ausschnitt eines bekannten Sensorchips mit einer Sensormembran, welche von Öltröpfchen benetzt ist;
• 2B die Sensormembran des Sensorchips gemäß 2A bei herkömmlichem Betrieb; und
• 3 die Sensormembran des Sensorchips gemäß 2A bei Betrieb mit einer Ausführungsform des erfindungsgemäßen Verfahrens.

It shows:

• 1 a partial perspective view of a hot film air mass meter for use in the intake of an internal combustion engine;
• 2A a section of a known sensor chip with a sensor membrane, which is wetted by oil droplets;
• 2 B the sensor membrane of the sensor chip according to 2A in conventional operation; and
• 3 the sensor membrane of the sensor chip according to 2A when operating with an embodiment of the method according to the invention.

In 1 is a perspective view of a Heißfileinuftmassenmesser 110 shown. The hot film air mass meter has a housing 112 made of plastic, which is designed as a plug-in sensor. The housing 112 is divided into an electronics area 114 and a bypass area 116 with a bypass channel 118 , The hot film air mass meter 110 is usually used in an intake tract of an internal combustion engine that the bypass area 116 protrudes into the intake of the internal combustion engine. For example, a hot film air mass meter 110 be used between a throttle and an air filter of a suction. Of the Air mass flow is not measured directly in the intake, but in the bypass channel 118 , The air flows in a main flow direction 120 (However, a current reversal is possible) through the bypass channel 118 , In the electronics area 114 is a carrier element 122 recorded, which is a sensor nose 124 having. The sensor nose 124 protrudes through an opening 126 from the electronics sector 114 in the bypass channel 118 , The sensor nose 124 has a recess 128 into which a sensor chip 130 is used. The sensor chip 130 , which is, for example, a sensor chip according to the in DE 196 0 1791 A1 described embodiment, has an active area in the form of a rectangular measuring surface 132 on. The longer side of this rectangle is perpendicular to the main flow direction 120 in the bypass channel 118 arranged. The sensor nose 124 also has a leading edge 134 on, which in the mainstream direction 120 opposite side of the sensor nose 124 is arranged.

Not shown in 1 are on the carrier element 122 arranged electronic components for driving and evaluation of the sensor chip 130 as well as corresponding connections between this electronic control and the sensor chip 130 ,

In 2A is a sensor chip 130 for use in a hot film air mass meter 110 as shown in 1 shown. With this sensor chip 130 For example, it is a sensor chip according to the in DE 196 01 791 A1 described embodiment with a sensor membrane, which has a measuring surface 132 in the form of a membrane surface. The measuring surface is of a rectangular shape and has a boundary line 136 on. As described above, this boundary line separates 136 thus those within the boundary line 136 lying measuring surface 132 , which has a low thermal conductivity, from the surrounding mainland 138 from. On the measurement surface 132 are three trace loops 140 arranged, which is substantially perpendicular to the main flow direction 120 of the air mass flow 142 extend. The air mass flow 142 overflows the surface of the sensor chip 130 essentially parallel. The tracks 140 are preferably configured similar, but are in operation of the Heißfileinuftmassenmessers 110 used differently. This is how the middle of the conductor loop loops 140 during operation of the hot film air mass meter 110 as the main heating element 144 whereas the upstream trace loop 146 and the downstream trace loop 148 can be used either as a temperature sensor or as additional heating elements. When used as a temperature sensor, the conductor track loops 146 and 148 used as temperature measuring resistors.

The state in 2A shows the sensor chip 130 when the hot film air mass meter is switched off 110 , It is shown that the entire measuring surface 132 of oil droplets 150 or even covered by a continuous liquid layer (for example, an oil layer). Of course, the surrounding mainland is also 138 wholly or partially burdened by a corresponding oil contamination, which in 2A but not shown.

In 2 B is the measurement surface 132 of the sensor chip 130 as shown in 2A after putting the hot film air mass meter into operation 110 shown. To the surrounding mainland 138 was omitted in this presentation. This is above the measurement surface 132 schematically the profile of the temperature profile of the temperature T along the main flow direction 120 on the measuring surface 132 shown.

In this in 2 B illustrated conventional operation of the Heißfileinuftmassenmessers 110 becomes the main heating element 144 heated, whereas the conductor loops 146 and 148 not heated. The conductor loops 146 and 148 are used in this operation only as a temperature sensor.

As in the upper area of the 2 B is shown in this operation of the Heißfileinuftmassenmessers 110 one with respect to the main flow direction 120 essentially symmetrical temperature distribution over the measuring surface 132 one. By "substantially" is to be understood that slight asymmetries in the temperature distribution can occur, which by the air mass flow 142 are conditional. This slight asymmetry effect, which here in 2 B is neglected, is just the effect on which the measurement of the air mass flow 142 in hot film air mass meter 110 based.

With the temperature T ₀ is in the illustration according to 2 B the ambient temperature of the mainland 138 designated. This ambient temperature is typically in the operation of the hot film air mass meter 110 at about 20 ° C. The main heating element 144 is heated in such a way that the peak temperature T _max in the operation of the Heißfileinuftmassenmessers 110 is at about 120 ° to 180 ° above the ambient temperature T ₀ . Other temperature settings are possible. Furthermore, the in 2 B Idealized illustrated in the upper part of the temperature profile. In practice, exactly linear temperature profiles and sharp edges will only occur comparatively seldom.

Furthermore, in 2 B represented as the oil droplets 150 at the in 2 B shown symmetrical temperature operation of the sensor chip 130 evenly from the measuring surface 132 be displaced. This forms both along the upstream front boundary line 152 as well as along the downstream rear boundary line 154 which are both sections of the boundary line 136 form, a liquid wall 156 respectively. 158 out. In this case, there is in particular the upstream liquid wall 156 the danger of this by the air mass flow 142 on the measuring surface 132 will be carried. The downstream liquid wall 158 however, the air mass flow 142 from the surface of the sensor chip 130 away. This differentiates the front boundary line 152 and the rear boundary line 154 in their properties, since every liquid wall 156 . 158 causes a change in thermal conductivity and heat capacity.

In 3 however, is the operation according to the invention of a hot-film air mass meter 110 analogous to the representation in 2 B shown. Again, only the measurement surface is 132 shown as well as in the upper part of the 3 the temperature distribution over this measuring surface. It shows 3 the operation of the hot film air mass meter 110 in an initial starting phase. For example, this starting phase may have a duration of from 0.2 to 10 seconds, preferably between 1 second and 6 seconds and particularly preferably of about 3 seconds. After this in 3 The starting phase shown can be the Heißfileinuftmassenmesser 110 be operated in an operating phase, for example according to the in 2 B illustrated operation.

As in 3 is shown in this inventive embodiment of the method, the upstream track loop 146 not used as a temperature sensor, but is heated by applying a corresponding current and thus used as an additional heating element. The main heating element 144 and the downstream trace loop 148 In contrast, in this preferred embodiment of the method according to the invention, no current is applied. As in the upper part of the 3 Thus, an asymmetric temperature profile is formed, whose maximum T _max to the front boundary line 152 is postponed. Advantageously, this temperature maximum becomes as close as possible to the front boundary line 152 introduced. The in 3 represented maximum temperature T _max of in 2 B correspond to maximum temperature T _max . However, it can also be for this starting phase compared to in 2 B used maximum temperature T _max increased maximum temperature or decreased maximum temperature can be used. Particularly preferably, the maximum temperature in the starting phase is above the maximum temperature in the 2 B shown operating phase.

In the 3 illustrated inventive asymmetric temperature distribution on the measuring surface 132 causes in the area of the front boundary line 152 a significantly stronger temperature gradient (in this illustration directed to the left) forms than in comparison to the rear boundary line 154 (pointing to the right there). Accordingly, the thermo capillary induced fluid displacement is at the front boundary 152 considerably more trained than at the rear boundary line 154 , This in conjunction with the air mass flow 142 causes the upstream liquid wall 156 in this operation of the hot film air mass meter 110 is designed considerably smaller than the downstream liquid wall 158 , It should be noted that "liquid bulk" does not necessarily mean a continuous liquid line, but that individual droplets may well fall under this term.

This inventive embodiment of the method according to 3 So, that causes oil droplets 150 by targeted utilization of thermocapillar-induced fluid displacements from the critical front boundary line 152 towards the less critical rear boundary line 154 have been displaced. From this back border line 154 out can the oil droplets 150 then from the air mass flow 142 from the sensor chip 130 be removed. Overall, the oil contamination of the measuring surface 132 greatly reduced by this operation of the method according to the invention. As described above, following the in 3 shown start phase follow an operating phase, which may be configured, for example, as in 2 B shown.

Alternatively or additionally, other heating elements on the measuring surface 132 or even the surrounding mainland 138 are used, which in turn are operated asymmetrically heated in a startup phase. In this way, for example, a staggered temperature distribution can be set. Overall, this makes it possible to further improve the cleaning effect of the method according to the invention.

Furthermore, the method can also be designed staggered in time, for example by the in 3 shown starting phase is divided into several individual phases. For example, the individual phases according to the in 3 be configured illustrated method, but differ in the maximum temperature T _max . Thus, for example, at the beginning of the starting phase, a high maximum temperature T _max can be used, which is gradually reduced in subsequent individual phases, until finally in the operating phase, z. B. as shown in FIG 2 B , is redirected. In this way, the method according to the invention can be additionally optimized by time staggering.

LIST OF REFERENCE NUMBERS

110

Hot film air mass meter

112

casing

114

electronics sector

116

bypass area

118

bypass channel

120

Main flow direction

122

support element

124

sensor nose

126

opening

128

deepening

130

sensor chip

132

measuring surface

134

leading edge

136

boundary line

138

Mainland

140

conductor tracks

142

Air mass flow

144

main heating

146

upstream track loop

148

downstream trace loop

150

oil droplets

152

front boundary line

154

rear boundary line

156

Fluid wall, upstream

158

Fluid wall, downstream

Claims (7)

Method for measuring an air mass flow (142) flowing with a main flow direction (120) by means of a hot film air mass meter (110), in particular in the intake tract of an internal combustion engine, wherein the hot film air mass meter (110) has a sensor chip (130) with a measurement surface (132), wherein the measurement surface (132) with at least one heating element (144, 146, 148) is heated and during a start phase of the Heißfileinuftmassenmessers (110), the measuring surface (132) is heated asymmetrically, characterized by an additional phase of operation, wherein the operating phase of the starting phase is temporally downstream and wherein the measuring surface (132) is heated substantially symmetrically in the operating phase. Method according to the preceding claim, wherein the measuring surface (132) has an upstream side (152) and a downstream downstream side (154) with respect to the main flow direction, characterized in that the measuring surface (132) is heated in the starting phase such that a Temperature maximum of the temperature of the measuring surface (132) closer to the inflow side (152) than to the downstream side (154) is arranged. Method according to one of the preceding claims, wherein the measuring surface (132) has at least one main heating element (144) and at least two temperature sensors (146, 148), wherein the at least two temperature sensors (146, 148) can also be used as additional heating elements (146, 148) are, wherein at least one first temperature sensor (146) with respect to the main flow direction (120) relative to at least one second temperature sensor (148) disposed upstream, characterized in that during the starting phase, the at least one first temperature sensor (146) is heated more than the at least a second temperature sensor (148). Method according to the preceding claim, characterized in that the at least one second temperature sensor (148) is not heated. Method according to one of the preceding claims, characterized in that the starting phase has at least two individual phases, wherein the measuring surface (132) is heated differently in the at least two individual phases. Method according to the preceding claim, wherein the heating powers of the heating elements (144, 146, 148) are reduced in a staggered manner in the at least two individual phases. Method according to one of the preceding claims, wherein the measuring surface (132) has at least three heating elements (144, 146, 148), characterized in that the at least three heating elements (144, 146, 148) in the starting phase with respect to the main flow direction (120) spatially staggered with in the main flow direction (120) decreasing heating power to be heated.

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