Thermal air mass meter with low contamination sensitivity

Abstract

A hot-film air mass meter for measuring an air mass flow flowing in a main flow direction (122), in particular in the intake tract of an internal combustion engine, wherein the hot-film air mass meter comprises a sensor chip (110) with a chip surface over which the air mass flow can flow, wherein the chip surface comprises a measuring surface (114) and a main surface (112), wherein the sensor chip (110) has a thermal conductivity in the region of the measuring surface (114) that is at least one order of magnitude lower than in the region of the main surface (112), wherein conductor tracks (128) of a central hot-film air mass meter circuit are applied to the measuring surface (114), wherein the measuring surface (114) and a sensor region (136) substantially have the shape of rectangles (116, 138), wherein the longer sides of each rectangle (116, 138) are arranged substantially perpendicular to the main flow direction (122), characterized in thatthat the measuring surface (114) is larger by a factor of 3 to 4 than the sensor area (136) of the measuring surface (114) defined by the outer dimensions of the conductor tracks (128) of the central hot-film air mass meter circuit, wherein the conductor tracks (128) are composed of a central heating element (130) and two temperature sensors (132, 134), wherein a temperature sensor (132) is arranged upstream of the central heating element (130) and a temperature sensor (134) downstream, wherein the shorter side (124, 126) of the rectangle (116) of the measuring surface (114) is longer by a factor of 3 to 4 than the shorter side (144, 146) of the rectangle (138) of the sensor area (136), wherein the shorter side (144, 146) of the rectangle (138) of the sensor area (136) a length l, S in the range of 350 to 550 micrometers and particularly preferably 440 micrometers and the shorter side (124, 126) of the rectangle (116) of the measuring surface (114) has a length l Min the range of 1300 to 1700 micrometers and particularly preferably 1500 micrometers.

Description

[0001] The invention relates to a hot-film air mass meter for measuring an air mass flow flowing in a primary flow direction. Such hot-film air mass meters are used particularly in the intake tract of an internal combustion engine. In particular, the proposed hot-film air mass meter is suitable for measuring air mass flows with a flow velocity between 0 and 60 meters / s. State of the art

[0002] In many processes, for example in the fields of process engineering, chemistry, or mechanical engineering, a defined gas mass, especially an air mass, must be supplied. This particularly includes combustion processes that occur under controlled conditions. An important example is the combustion of fuel in automotive internal combustion engines, especially with subsequent catalytic exhaust gas purification. Various types of sensors are used to measure the air mass flow rate.

[0003] A sensor type known from the prior art is the so-called hot-film air mass meter (HFM), which is described in one embodiment, for example, in DE 196 01 791 A1. Such hot-film air mass meters typically employ a sensor chip having a thin sensor membrane, for example, a silicon sensor chip. Typically, at least one heating resistor is arranged on the sensor membrane, which is surrounded by two or more temperature measuring resistors (temperature sensors). In an air flow that is guided over the membrane, the temperature distribution changes, which in turn can be detected by the temperature measuring resistors. For example, an air mass flow can be determined from the resistance difference between the temperature measuring resistors. Various other variations of this sensor type are known from the prior art.

[0004] One problem with this type of sensor, known for example from DE 101 11 840 C2, is that contamination of the sensor chip can frequently occur, for example contamination by oil, other liquids or other types of impurities. 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. During operation of the internal combustion engine, oil can precipitate on the sensor chip and in particular on the sensor membrane. This oil precipitate can lead to an undesirable influence on the measurement signal from the sensor chip, particularly because an oil film on the surface of the sensor chip affects the thermal conductivity of the surface, which leads to falsification of the measurement signals. Oil contamination can also occur during or shortly after the internal combustion engine, e.g. a diesel engine, is switched off.This is particularly the case when, after the internal combustion engine is shut down, excess pressure in the crankcase dissipates via a crankcase ventilation system into the intake tract of the internal combustion engine. This carries oil vapor or oil mist along with it.

[0005] The problem of contamination of the membrane or sensor surface is further exacerbated by thermodynamic effects. It is known that liquid droplets exhibiting a gradient in their surface tension experience a force in the direction of the higher surface tension. This leads to a movement of the droplet from low to high surface tension. In particular, this gradient can be caused by a temperature gradient on a surface to which the liquid droplet is applied. The temperature gradient and the resulting force typically move the droplet from a warmer area of ​​the surface to a cooler area. This effect is described, for example, in V.G. Levich, "Physicochemical Hydrodynamics," Prentice-Hall, NJ, 1962, pp. 373 and 380.

[0006] As described above, typical hot-film air mass meters are designed with a sensor membrane (e.g., a silicon membrane) with low thermal conductivity and a surrounding chip. During operation of the hot-film air mass meter, a temperature gradient typically builds up at the edges of the sensor membrane, i.e., at the boundary to the surrounding chip, and a corresponding liquid wall, for example, in the form of oil droplets, forms. This liquid wall can be completely or partially entrained by the air flow, allowing oil droplets to reach the sensor membrane and influence the measurement. Furthermore, the liquid wall runs along the edges of the membrane when the device is switched off, resulting in the associated reduction of the temperature gradient, allowing oil to flow onto the membrane.

[0007] Further air mass meters for measuring an air mass flow flowing in a main flow direction are known from the documents DE 199 45 168 A1, DE 199 19 398 A1, US 2004 / 0 090 305 A1, DE 100 63 070 A1, DE 102 15 954 A1, DE 103 43 793 A1 and WO 2004 / 106 863 A1. Advantages of the invention

[0008] A hot-film air mass meter is therefore proposed for measuring an air mass flow flowing in a primary flow direction, particularly in the intake tract of an internal combustion engine. This hot-film air mass meter avoids the disadvantages of prior art devices. The hot-film air mass meter was optimized using fluid mechanics calculations and corresponding experiments, particularly for measuring an air mass flow with a flow velocity between 0 and 60 m / s.

[0009] The hot-film air mass meter has a sensor chip with a chip surface over which the air mass flow can flow, wherein the chip surface has a measuring surface and a mainland surface, wherein the sensor chip has a thermal conductivity in the region of the measuring surface that is at least one order of magnitude lower than in the region of the mainland surface, wherein conductor tracks of a central hot-film air mass meter circuit are applied to the measuring surface, wherein the measuring surface and the sensor region essentially have the shape of rectangles, wherein the longer sides of each rectangle are arranged essentially perpendicular to the main flow direction.According to the invention, the measuring surface is designed to be larger by a factor of 3 to 4 than a sensor area of ​​the measuring surface defined by external dimensions of the conductor tracks of the central hot-film air mass meter circuit, wherein the conductor tracks are composed of a central heating element and two temperature sensors, wherein a temperature sensor is arranged upstream of the central heating element and a temperature sensor downstream, wherein the shorter side of the rectangle of the measuring surface is longer by a factor of 3 to 4 than the shorter side of the rectangle of the sensor area, wherein the shorter side of the rectangle of the sensor area has a length l. S in the range of 350 to 550 micrometers and particularly preferably 440 micrometers and the shorter side of the rectangle of the measuring surface has a length l M in the range of 1300 to 1700 micrometers and particularly preferably 1500 micrometers.

[0010] The hot-film air mass flow sensor comprises a sensor chip with a chip surface over which the air mass flow passes. For example, this can be a silicon chip, as described above. The chip surface, in turn, comprises a measuring surface and a land surface. In the area of ​​the measuring surface, the sensor chip exhibits a conductivity, particularly a transverse conductivity, that is at least one order of magnitude lower than in the area of ​​the land surface.

[0011] This reduction in conductivity can be achieved in various ways. For example, as known from the prior art and described above, sensor chips can be used with a sensor membrane that is only a few µm thick. This utilizes the low thermal conductivity (approximately 0.026 W / m K) of the air surrounding the sensor membrane. Alternatively, porous regions can be created in the chip as a measuring region with a measuring surface facing the air mass flow, for example, by porosifying a silicon chip. In this way, measuring regions can be created that, due to the enclosed air cavities, produce transverse conductivities of 0.1 to 2 W / (m K), compared to a silicon substrate with a thermal conductivity of 156 W / (m K).

[0012] Conductor tracks of a central hot-film air mass meter circuit are applied to the measuring surface. For example, as described above, this could be a central heating element surrounded by two temperature sensors. Other geometries are also conceivable.

[0013] The production of the measuring surfaces, such as the membrane or the porous area, is complex and expensive in practice, particularly due to the use of corresponding semiconductor technology. Furthermore, the measuring surfaces are typically susceptible to failure and sensitive, as a membrane, for example, can be easily damaged. Accordingly, the measuring surfaces of conventional hot-film air mass meters are minimized in size, with the measuring surface being almost completely filled with the conductor tracks. This allows for optimal spatial utilization of the available measuring surface.

[0014] A fundamental idea of ​​the present invention, however, is the recognition that, as described above, liquid contamination occurs precisely at the transition between the measuring surface and the dry surface due to the temperature gradient that develops during operation. The closer this transition is to the conductor tracks of the central hot-film air mass meter circuit, the easier it is for contaminants, such as oil or other liquids, to penetrate from this transition onto the conductor tracks, thus causing a drift in the hot-film air mass meter's measurement signal.

[0015] A key idea is therefore to position this transition between the measuring surface and the dry surface as far as possible from the conductor tracks of the central hot-film air mass meter circuit. On the other hand, the resulting advantage of reduced contamination of the central hot-film air mass meter circuit is offset by considerable disadvantages. These disadvantages are primarily related to the fact that the enlarged measuring surface increases the sensitivity of the measuring surface and thus the susceptibility of the hot-film air mass meter to failure. Furthermore, this also reduces the space available on the chip surface for electronic leads and, if applicable, electronic switching elements. Furthermore, an excessively large enlargement of the measuring surface compared to the central hot-film air mass meter circuit can, under certain circumstances, result in the aforementioned cleaning effect being reversed.For example, the airflow can detach oil droplets at the transition between the measuring surface and the dry land surface, but after traveling a certain distance over the chip surface, they are pushed back onto the chip surface. Thus, even though the temperature gradient is sufficiently far away from the central hot-film air mass sensor circuit, oil droplets can still be carried back to the central air mass sensor circuit by the airflow.

[0016] The geometry of the sensor chip design must therefore be carefully optimized. Corresponding optimization calculations for a flow velocity between 0 and 60 m / s were performed. Accordingly, the hot-film air mass meter according to the invention has a three-part chip surface. In addition to the dry surface, the measuring surface is arranged on the sensor chip, as described above. The measuring surface, in turn, has a sensor area. This sensor area is defined by the external dimensions of the conductor tracks of the central hot-film air mass meter circuits. Leads to the conductor tracks are generally negligible and should not be considered. In many cases, for example, the sensor area has a rectangular geometry.Based on optimization calculations and experimental results, the sensor area of ​​the proposed hot-film air mass meter according to the invention is dimensioned such that the total measuring surface is a factor of 3 to 4 larger than the sensor area. This dimensioning minimizes the susceptibility of the hot-film air mass meter to contamination for the specified flow velocities. Thus, the signal drift of the hot-film air mass meter is minimized, leading to significantly improved measurement of air mass flows and thus improved engine control.

[0017] The described inventive design of the hot-film air mass meter can be advantageously developed in various ways. For various symmetry reasons, which can have a beneficial effect on the measuring electronics, it is advantageous if both the measuring surface and the sensor region are essentially rectangular. The longer sides of each rectangle are advantageously arranged perpendicular to the main flow direction. For example, the rectangle of the sensor region can be arranged essentially symmetrically within the rectangle of the measuring surface. For example, the rectangle of the measuring surface and the rectangle of the sensor region can have the same axis of symmetry, advantageously an axis of symmetry perpendicular to the main flow direction.

[0018] Perpendicular to the main flow direction, the sensor area should essentially fully utilize the measuring surface. This means, for example, that the sensor area perpendicular to the main flow direction has a maximum extension (e.g., the longer side of a rectangle bounding the sensor area) that is 80% to 100% of the maximum extension of the measuring surface (e.g., the longer side of the rectangle bounding the measuring surface).

[0019] If a rectangular measuring surface and a rectangular sensor area are used, it is advantageous if the shorter side of the rectangle of the measuring surface (i.e. parallel to the main flow direction) is larger by a factor of 3 to 4 than the shorter side of the rectangle of the sensor area (also parallel to the main flow direction). In particular, it has proven advantageous from a manufacturing point of view if the shorter side of the rectangle of the sensor area has a length in the range of 350 to 550 µm and particularly preferably 440 µm. The shorter side of the rectangle of the measuring surface advantageously has a length in the range of 1300 to 1700 µm and particularly preferably 1500 µm. Calculations and fluid mechanics considerations have shown an optimal distance between the sensor area (i.e. an upstream boundary of the sensor area) and the boundary of the measuring surface (i.e.an upstream boundary of the measuring surface towards the chip mainland) of approximately 540 micrometers, which is approximately fulfilled with the geometric lengths mentioned.

[0020] In the above-described embodiments of the invention, the measuring surface outside the sensor area is left largely unused. However, advantageous embodiments of the hot-film air mass meter are also conceivable in which this previously unused measuring surface is additionally utilized. For example, it can have at least one additional heating element and at least one additional temperature sensor in this area of ​​the measuring surface outside the sensor area. For example, this can be done by connecting the conductor tracks of the central hot-film air mass meter circuit in the sensor area of ​​the hot-film air mass meter to a control and evaluation circuit for controlling and evaluating the air mass flow measurement.The at least one additional heating element and the at least one additional temperature sensor, which are arranged on the measuring surface outside the sensor area, are advantageously connected to a temperature control circuit for setting and / or regulating a predetermined temperature in the area of ​​the at least one additional heating element.

[0021] In this way, for example, a constant temperature can be set on the measuring surface outside the sensor area at a defined distance from the sensor area, which temperature changes only insignificantly even during operation of the hot-film air mass meter due to the temperature control circuit. A major advantage of arranging at least one additional heating element on the measuring surface (and not on the chip mainland) is that the measuring surface has low thermal conductivity. This ensures that the heat applied to the measuring surface by the additional heating element does not immediately flow back into the surrounding sensor chip, but rather that a constant temperature can be maintained without significantly heating the surrounding sensor chip.

[0022] The adjustable, constant "temperature barrier" around the sensor area makes the hot-film air mass meter significantly less susceptible to temperature drift. In particular, the effect of a liquid barrier, which forms at the transition between the measuring surface and the chip's main body, is shielded from the actual sensor area. A slightly altered thermal conductivity at the edge of the measuring surface, triggered by a liquid film or liquid barrier accumulating there, thus has only a significantly prolonged effect on the temperature distribution in the sensor area. As a result, air mass measurement using the hot-film air mass meter according to the invention is considerably more reliable, less susceptible to interference, and subject to significantly less drift.

[0023] The sensor region is essentially rectangular in shape. The rectangle can have two sides arranged perpendicular to the main flow direction, wherein the at least one additional heating element can extend essentially parallel to the sides arranged perpendicular to the main flow direction. "Essentially" is understood to mean, for example, a deviation of less than 5°. For example, at least one first additional heating element and one first additional temperature sensor can be arranged "upstream" of the sensor region with respect to the main flow direction, and at least one second additional heating element and at least one second additional temperature sensor can be arranged downstream of the sensor region with respect to the main flow direction. In this way, the sensor region is shielded from interference from both sides.Alternatively or additionally, the at least one additional heating element can also be designed as a (fully closed or partially open) frame around the sensor area. This further increases the shielding effect. drawing

[0024] The invention is explained in more detail below with reference to the drawing.

[0025] It shows: Fig. 1A shows a prior art design of a measuring surface of a sensor chip; Fig. 1B shows a first embodiment of a sensor chip according to the invention; and Fig. 2 a second embodiment of an inventive design of a sensor chip of a hot-film air mass meter. Examples of implementation

[0026] In Fig. 1A shows a prior art design of a sensor chip 110 (shown only in outline) of a hot-film air mass meter. The sensor chip 110 can be used, for example, in the intake tract of an internal combustion engine or in a bypass duct to the intake tract of an internal combustion engine. Such devices are known, for example, from DE 196 01 791 A1. The sensor chip according to the design in Fig. 1A shows a chip mainland with a mainland surface 112 in the plane of the drawing (only partially shown). In this embodiment, it is assumed that the sensor chip 110 is a silicon sensor chip.

[0027] Furthermore, the sensor chip 110 has a measuring area with a measuring surface 114 in the plane of the drawing. In this exemplary embodiment, the measuring surface 114 is designed in the form of a rectangle 116, which has longer sides L M118, 120 perpendicular to a main flow direction 122 of an air mass flow. The shorter sides I M of the rectangle 116 are designated by the reference numerals 124, 126 and are arranged parallel to the main flow direction 122.

[0028] The sensor chip 110 has a thermal conductivity in the area of ​​the measuring surface 114 of 0.1 to 2 W / (m K), compared to the surrounding land area at 156 W / (m K). This can be achieved by porosifying the silicon in the area of ​​the measuring surface 114. Alternatively, a sensor membrane with a thermal conductivity comparable to that of the ambient air of 0.026 W / m K can be used.

[0029] Conductor tracks of a central hot-film air mass meter circuit 128 are arranged in the area of ​​the measuring surface 114. These conductor tracks 128 consist of a central heating element 130 and two temperature sensors 132, 134. A temperature sensor 132 is arranged upstream of the central heating element 130 and a temperature sensor 134 downstream. The outer dimensions of the conductor tracks 128 define a sensor region 136 on the measuring surface 114. In this exemplary embodiment, this sensor region 136 is also configured in the form of a rectangle 138, which has the longer sides 140, 142 and shorter sides 144, 146. The connection-side shorter side 144 of the rectangle 138 lies on the connection-side shorter side 124 of the rectangle 116 of the measuring surface. The side lengths of the rectangle 138 of the sensor area 136 are in Fig. 1A with L S and l S designated.

[0030] In the embodiment corresponding to the state of the art according to Fig. 1A, the conductor tracks 128 of the central HFM circuit extend almost to the outer rectangle 116 of the measuring surface 114. Typically, the longer sides 118, 120 of the rectangle 116 have a length L M of approximately 1600 µm and the shorter sides 124, 126 of the rectangle 116 have a length of l M = 450-500 µm. The rectangle 138 of the sensor area 136 is only slightly smaller, where, for example, L S approximately 0.9 to 0.95 × L M is and l S approximately 0.7 × l m is.

[0031] Furthermore, Fig. 1A also illustrates the problem of the accumulation of oil droplets 148 along the rectangle 116 of the measuring surface 114. These oil droplets 148 are thus arranged in the immediate vicinity of the conductor tracks 128. A slight external force, for example from the air mass flow, causes oil droplets 148 to reach the conductor tracks 128. Furthermore, the accumulation of oil droplets 148 also causes a change in the thermal conductivity of the sensor chip 110 in the region of the edge of the rectangle 116 of the measuring surface 114. In particular, the oil droplets 148 can increase the conductivity at the transition between the measuring surface 114 and the dry surface 112. This has a significant influence on the temperature distribution on the measuring surface 114. Furthermore, the oil droplets 148 often act as an adhesion promoter for dust and soot. In addition, in many cases, an "oil wall" with a height of approximately30 micrometers in the area of ​​the edge of rectangle 116 of the measuring surface, which leads to air turbulence in this area that only settles down after a certain distance. This leads to further distortion of the measurement signal. Thermal effects and flow effects, both caused by the oil droplets 148, often interact and together lead to a change in the measurement signal.

[0032] In the upper area of Fig. 1A shows a temperature distribution parallel to the main flow direction 128 on the measuring surface 114. It is assumed that the central heating element 130 is heated to a temperature T max The surrounding chip mainland with the mainland surface 112 has an ambient temperature T0. The curves 150, 152 in the upper area of Fig. 1A denotes the temperature distribution along the main flow direction 122 on the measuring surface 114, specifically in the case without the accumulation of oil droplets 148 (curve 150, solid line) or in the case with the accumulation of oil droplets 148 (curve 152, dashed line). It can be clearly seen that, as a result of the increased thermal conductivity and / or due to the above-described flow effects caused by the oil droplets 148, the temperature in the area of ​​the temperature sensors 132, 134 drops. Accordingly, a temperature difference of the magnitude ΔT messlower temperature is measured than in the case of a measurement free of oil droplets. This has negative effects in several respects. One effect is that a lower measured temperature generally leads to a larger relative measurement error. Another effect is that a fluctuation in the contamination by oil droplets 148 also leads to a fluctuation in the temperature drop of the ΔT mess This in turn leads to a drift in the signal of the hot-film air mass meter.

[0033] In Fig. 1B, in contrast, shows an embodiment of a sensor chip 110 according to the invention. In principle, the design of the sensor chip 110 is similar to the design according to the prior art embodiment in Fig. 1A. However, in Fig. 1B according to the invention, the dimensioning of the rectangles 116 and 138 of the measuring surface 114 and the sensor area 136 differs from the representation according to Fig. 1A. In this embodiment, the rectangle 138 of the sensor area 136 has a shorter side length l S of 440 µm, whereas the shorter side length of the rectangle 116 of the measuring area 114 has a length of l M = 1500 µm. The dimensions of the longer side lengths of the rectangles 116, 138 are L M = 1800 µm and L S = 1600 µm. This means that the area of ​​the rectangle 116 of the measuring surface 114 in this preferred embodiment is larger by a factor of 3.8 than the area of ​​the rectangle 138 of the sensor area 136. For the shorter sides l m , l S results in a ratio of 3.4, for the longer sides L M , L S a ratio of 1.1.

[0034] As can be seen from the approximate scale representation of the oil droplets 148 in Fig. As can be seen from Figure 1B, in this exemplary embodiment according to the invention, the oil droplets 148 at the transition between the measuring area 114 and the land surface 112 are considerably farther away from the sensor area 136 and thus from the conductor tracks 128. "Considerably farther" can be understood, for example, to mean that the spacing between the oil droplets 148 and the conductor tracks 128 exceeds the diameter of the oil droplets 148 by several times. However, since discrete oil droplets 148 do not always form, but rather, for example, continuous liquid films or liquid barriers, this definition is not applicable in all cases.

[0035] Furthermore, Fig. 1B, the upper part again shows a temperature profile when the central heating element 130 is heated as intended. Again, the solid curve 150 describes the case without oil contamination, whereas the dashed curve 152 describes the temperature profile when contaminated by oil droplets 148. Even at first glance, it is clear that the curves 150, 152 differ only slightly in this example. This means that in the area of ​​the temperature sensors 132, 134, the measured value difference ΔT mess between a temperature measurement without oil contamination and a measurement with oil contamination is considerably lower than in the state-of-the-art case according to Fig. 1A. This allows the measured value differences to be reduced by approximately 80 to 95%, depending on the operating point. This means that the temperatures measured by the temperature sensors 132, 134 are higher overall than in the case of Fig. 1A, which reduces the relative measurement errors overall. Furthermore, contamination-related drift of the hot-film air mass meter signals is also significantly reduced. Thus, an arrangement according to Fig. 1B significantly improves the reliability of the hot-film air mass sensor's measurement signals and their long-term stability. Signal drift is also significantly reduced.

[0036] In Fig. 2 shows a particularly preferred further development of a sensor chip 110 of a hot-film air mass meter. The sensor chip 110 is essentially configured as in the exemplary embodiment according to Fig. 1B. This means, in particular, that the sensor chip 110 in turn has a measuring surface 114 with a sensor region 136 with conductor tracks 128 of a central HFM circuit. The dimensions of the rectangles 116 of the measuring surface 114 and 138 of the sensor region 136 are in this embodiment according to Fig. 2 identical to the embodiment according to Fig. 1B.

[0037] In addition to the conductor tracks 128 of the central HFM circuit in the sensor area 136, however, in this embodiment according to Fig. Two additional conductor tracks are arranged on the measuring surface 114 outside the sensor area 136. Thus, the measuring surface 114 further comprises two additional heating elements 154, 156 and two additional temperature sensors 158, 160. The additional heating elements 154, 156 and the additional temperature sensors 158, 160 are arranged essentially parallel to the conductor tracks 128 of the central HFM circuit. However, perpendicular to the main flow direction, these additional heating elements 154, 156 and the additional temperature sensors 158, 160 extend beyond the conductor tracks 128 almost to the shorter side 126 of the rectangle 116, facing away from the connection side.

[0038] As in Fig. As symbolically shown in Figure 2, the conductor tracks 128 of the central HFM circuit are connected to a control and evaluation circuit 162 for controlling and evaluating the air mass flow measurement. The additional heating elements 154, 156 and the additional temperature sensors 158, 160, on the other hand, are connected to a temperature control circuit 164.

[0039] In the upper part of the Fig. 2 is analogous to Fig. 1B, the temperature profile on the measuring surface 114 of the sensor chip 110 during a measurement of the air mass flow is again shown. Here, the dashed curve 152 indicates the case of a measurement with contamination by oil droplets 148, whereas the solid line 150 indicates a contamination-free measurement. The temperature control circuit 164 is operated such that, by means of the additional heating elements 154, 156, the temperature of the additional temperature sensors 158, 160 is kept constant at a predetermined value T fix Ideally, this constant temperature T fix above the temperature that would be reached at the location of the additional temperature sensors 158, 160 if the additional heating elements 154, 156 were switched off and only the central heating element 130 were operating. This ensures that the temperature can be controlled to the value T fix solely by the additional heating elements 154, 156.

[0040] As the temperature curve in the upper part of the Fig. 2 shows, the contamination by oil droplets 148 now only affects the temperature profile upstream of the upstream temperature sensor 158 and downstream of the downstream temperature sensor 160 (cf. the profile of curves 152, 154). The measuring temperature T mess at the location of the temperature sensors 132, 134 is hardly affected by oil contamination. Thus, by this inventive design of the sensor chip 110, not only is the oil contamination 158 displaced to an area far away from the sensor area 136, but a "temperature barrier" is also created around the sensor area 136 by the additional heating elements 154, 156 and the additional temperature sensors 158, 160. This has the effect that the air mass measurement is practically no longer affected by oil contamination. Thus, by the inventive design according to Fig. 2 Contamination influences and signal drift due to contamination with oil droplets 148 are almost excluded.

[0041] Finally, it should be noted that the design of the sensor chips 110 according to the embodiments in the Fig. 1B and Fig. 2 are symmetrical to a line of symmetry 166. This line of symmetry 166 is arranged perpendicular to the main flow direction 122 of the air mass flow. Such a symmetrical arrangement of the conductor tracks 128 and the additional heating elements 154, 156, as well as the additional temperature sensors 158, 160, significantly facilitates the evaluation of the measurement signals of the hot-film air mass meter. In this case, a final correction of the measurement signals due to asymmetry-related artifacts can be omitted. This facilitates the evaluation. Of course, asymmetric designs of sensor chips 110 and measuring surfaces 114 are also conceivable.

Claims

[1] Hot-film air mass meter for measuring an air mass flow flowing in a main flow direction (122), in particular in the intake tract of an internal combustion engine, wherein the hot-film air mass meter has a sensor chip (110) with a chip surface over which the air mass flow can flow, wherein the chip surface has a measuring surface (114) and a main surface (112), wherein the sensor chip (110) has a thermal conductivity in the region of the measuring surface (114) that is at least one order of magnitude lower than in the region of the main surface (112), wherein conductor tracks (128) of a central hot-film air mass meter circuit are applied to the measuring surface (114), wherein the measuring surface (114) and a sensor region (136) have substantially the shape of rectangles (116, 138), wherein the longer sides of each rectangle (116, 138) are arranged substantially perpendicular to the main flow direction (122), characterized bythat the measuring surface (114) is larger by a factor of 3 to 4 than the sensor area (136) of the measuring surface (114) defined by the outer dimensions of the conductor tracks (128) of the central hot-film air mass meter circuit, wherein the conductor tracks (128) are composed of a central heating element (130) and two temperature sensors (132, 134), wherein a temperature sensor (132) is arranged upstream of the central heating element (130) and a temperature sensor (134) downstream, wherein the shorter side (124, 126) of the rectangle (116) of the measuring surface (114) is longer by a factor of 3 to 4 than the shorter side (144, 146) of the rectangle (138) of the sensor area (136), wherein the shorter side (144, 146) of the rectangle (138) of the sensor area (136) a length l S in the range of 350 to 550 micrometers and particularly preferably 440 micrometers and the shorter side (124, 126) of the rectangle (116) of the measuring surface (114) has a length l Min the range of 1300 to 1700 micrometers and particularly preferably 1500 micrometers. [2] Hot film air mass meter according to the preceding claim, characterized by that the rectangle (138) of the sensor area (136) is arranged substantially symmetrically with respect to an axis of symmetry (166) perpendicular to the main flow direction (122) in the rectangle (116) of the measuring surface (114). [3] Hot film air mass meter according to one of the preceding claims, characterized by that the sensor area (136) perpendicular to the main flow direction (122) has a maximum extension L S from 80% to 100% of the maximum extension L M the measuring surface (114) perpendicular to the main flow direction (122). [4] Hot-film air mass meter according to one of the preceding claims, additionally comprising at least one additional heating element (154, 156) and at least one additional temperature sensor (158, 160), wherein the at least one additional heating element (154, 156) and the at least one additional temperature sensor (158, 160) are arranged on the measuring surface (114) outside the sensor area (136). [5] Hot-film air mass meter according to the preceding claim, wherein the conductor tracks (128) of the central hot-film air mass meter circuit are connected to a control and evaluation circuit (162) for controlling and evaluating the measurement of the air mass flow, characterized by that the at least one additional heating element (154, 156) and the at least one additional temperature sensor (158, 160) are provided with a temperature control circuit (164) for setting and / or regulating a predetermined temperature T fix are connected. [6] Hot-film air mass meter according to one of the two preceding claims, wherein the sensor area (136) is designed substantially in the form of a rectangle (138), wherein the rectangle (138) has two sides (140, 142) arranged perpendicular to the main flow direction (122), characterized by that the at least one additional heating element (154, 156) extends substantially parallel to the sides (140, 142) arranged perpendicular to the main flow direction (122). [7] Hot film air mass meter according to one of the three preceding claims, characterized bythat at least one first additional heating element (154) and at least one first additional temperature sensor (158) are arranged upstream of the sensor region (136) with respect to the main flow direction (122), and that at least one second additional heating element (156) and at least one second additional temperature sensor (160) are arranged downstream of the sensor region (136) with respect to the main flow direction (122).

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