DE102005057575B4 - Air mass meter with electrical oil separation and method for measuring an air mass flow of an internal combustion engine using such an air mass meter - Google Patents

Abstract

Hot-film air-mass meter (110) for measuring an air-mass flow, in particular in the intake tract (112) of an internal combustion engine, the hot-film air-mass meter (110) having a sensor surface (130) over which the air-mass flow can flow, the hot-film air-mass meter (110) having a field-generating device (118), which is configured , in order to generate an at least partially homogeneous electric field (150) through the air mass flow, and the field generating device (118) has a capacitor, in particular a plate capacitor (116), characterized in that the electrodes (144, 146) of the capacitor have at least approximately the same area and are designed in such a way that the air mass flow has already covered a drift distance (152) through the electric field (150) before it reaches the sensor surface (130), and that the electric field (150) has a post-drift distance (154) behind the Sensor surface (130) is extended.

Description

The invention relates to a hot-film air-mass meter for measuring an air-mass flow flowing in a main flow direction. Such hot-film air mass meters are used in particular in the intake tract of an internal combustion engine. Furthermore, the invention relates to a method for measuring an air mass flow.

State of the art

In many processes, for example in the field of process engineering, chemistry or mechanical engineering, a gas mass, in particular an air mass, must be supplied in a defined manner. These include, in particular, combustion processes that 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 exhaust gas cleaning. Various types of sensors are used to measure the air mass flow rate.

A sensor type known from the prior art is the so-called hot-film air mass meter (HFM), which is used, for example, in DE 196 01 791 A1 is described in one embodiment. In such hot-film air-mass meters, a sensor chip is usually used which has a thin sensor membrane, for example a silicon sensor chip. At least one heating resistor, which is surrounded by two or more temperature measuring resistors (temperature sensors), is typically arranged on the sensor membrane. The temperature distribution changes in an air flow that is guided over the membrane, which in turn can be detected by the temperature measuring resistors and can be evaluated by means of a control and evaluation circuit. For example, an air mass flow can be determined from a resistance difference of the temperature measurement resistors. Various other variants of this type of sensor are known from the prior art.

One example from DE 101 11 840 C2 A known problem with this type of sensor is that contamination of the sensor type can often 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 be deposited on the sensor chip and in particular on the sensor membrane. This oil precipitation can lead to an undesired influence on the measurement signal of the sensor chip, in particular since an oil film on the surface of the sensor chip affects the thermal conductivity of the surface, which leads to the measurement signals being falsified.

The oil contamination can also occur when the internal combustion engine, for example a diesel engine, is switched off or shortly after it is switched off. This is particularly the case when, after the internal combustion engine has been switched off, excess pressure in a crankcase is released via crankcase ventilation into the intake tract of the internal combustion engine (and thus, for example, also into the bypass channel of the hot-film air mass meter). Oil vapor or oil mist is often carried along. This oil mist is deposited at least partially on the sensor chip as a film of contamination and thus causes a considerable influence on the measurement signal, particularly when the internal combustion engine is switched on again.

the DE 101 11 840 C2 proposes a method of preventing contamination on a sensor chip using an auxiliary heater. The sensor chip has a sensor area and an additional heater arranged outside of the sensor area. This additional heater is electrically heated in such a way that thermal gradient vortices occur in the area of the additional heater, which lead to precipitation of the contamination of the flowing medium in the area of the additional heater away from the area of the sensor area.

The one in the DE 101 11 840 C2 However, the arrangement disclosed and the disclosed method are only conditionally suitable for solving the contamination problem, which, as described, is based on several effects. In particular, there is the problem that the oil deposits now occur predominantly away from the sensor area, but still on the sensor chip itself. If large amounts of contamination accumulate, they can be driven onto the sensor area, for example by air turbulence. Furthermore, the triggers in the DE 101 11 840 C2 The arrangement described also does not have the problem described above, that contamination forms in particular after the internal combustion engine has been switched off.

the DE 38 01 262 A1 discloses a device and a method for protecting air mass flow meters from contamination by foreign particles moving in an intake duct, in which the foreign particles are ionized and then deflected in a high-voltage field from the measuring element to be protected. Shielding grids and ions are used to ionize the particles sation grille, which are located at the entrance and exit of the intake port. A shielding tube, in which the measuring element is located, is used to deflect the particles. The shielding zone is located between the outer casing of the shielding tube and the inner casing of the intake duct.

The pamphlet DE 195 36 705 A1 discloses a particle measuring method and a device for the quantitative determination of the electrically conductive or charged particles contained in a gas flow. An electric field is generated between a jacket electrode through which the gas stream flows and an inner electrode within this jacket electrode by applying a constant electrical DC voltage, and the charging current is measured to maintain the constant DC voltage between the jacket electrode and the inner electrode.

the DE 24 45 004 A1 discloses a method and a device for determining the dust content in flowing gases, in which the gas flow is passed between a pointed electrode and a second electrode. A voltage is applied between the electrodes so that a corona discharge occurs between the electrodes, with the difference between the current flowing through the first and second electrodes being used as a measure of the dust content of the air.

the DE 40 22 207 A1 shows a device for burning off a hot wire or hot film of an air mass meter, in which a current flow through an electrical resistance is used to reach a burning-off temperature.

the DE 102 53 691 A1 discloses an air flow meter with a dust collector, which includes an ionization area located upstream of a sensor surface of the actual measuring element and a collection area. The ionization region has a pair of electrodes facing each other and between which an electric field is created. The dust is ionized by means of the electrodes. The collection area features magnets and a collector sheet. The magnets create a magnetic field that spirals the previously ionized dust and directs it outwards until the dust settles on the collector blade.

Advantages of the Invention

A hot-film air mass meter for measuring an air mass flow, in particular in the intake tract of an internal combustion engine, is therefore proposed, which avoids the disadvantages of the devices known from the prior art. The hot-film air mass meter can be used universally, but is particularly optimized for measuring an air mass flow with a flow rate between 0 and 60 m/s. Furthermore, a method for operating a hot-film air-mass meter according to the invention is proposed.

In principle, the hot-film air-mass meter can include all techniques and variants of hot-film air-mass meters known to those skilled in the art. For example, the hot-film air mass meter can include a sensor chip, for example a sensor chip with a sensor membrane, as is shown in DE 196 01 791 A1 is described. However, other embodiments of hot-film air-mass meters are also possible. The hot-film air-mass meter has a sensor surface over which the air-mass flow can flow. For example, as described above, this can be the surface of a sensor membrane.

A basic idea of the present invention is to avoid oil deposits and other contaminations on the sensor surface by using electrical effects. For this purpose, the hot-film air-mass meter has a field-generating device that is designed to generate a homogeneous electric field through the air-mass flow, which is preferably at least partially homogeneous.

This electric field can influence the oil contamination of the sensor surface in different ways. It should be noted that the air mass flow usually contains an oil mist (aerosol) with tiny oil droplets, which are carried along with the air mass flow. This effect can occur both during operation of the internal combustion engine and, as described above, after the internal combustion engine has been switched off. In many cases, these oil droplets are electrically charged. This electrical charging occurs, for example, through molecular friction, wall impacts with the wall of a flow channel, for example, through ambient radiation or through interaction with other oil droplets. In addition, the charging of the oil droplets can be intensified by an electric field, since in this case already charged oil droplets are accelerated, collide with other droplets and thus generate new charges in a kind of "avalanche effect".

The oil particles charged in this way are deflected in the electric field according to their polarity. In this way, the oil droplets can be kept away from the sensor surface by electrical deflection.

A further effect can then be that oil droplets are polarized in the electric field without a net charging taking place. In this way, each oil droplet becomes an electric dipole. Since the positively charged part of a first oil droplet and the negative part of a second oil droplet attract each other, two such polarized oil droplets can easily fuse (coagulate) with each other. In this way, the smallest oil droplets, which are carried along by the air mass flow, form large, heavy droplets, which then fall out due to the force of gravity acting on them. In this way, too, the oil mist can be kept away from the sensor surface, in particular if the sensor surface is arranged vertically when the hot-film air-mass meter is in operation.

The field generating device has one or more capacitors, in particular a plate capacitor, although the plates of the plate capacitor do not necessarily have to be parallel. The electrodes of the at least one capacitor have at least approximately the same area. In this case, deviations of 50%, preferably not more than 10% and particularly preferably not more than 5% can be tolerated in order to generate a preferably at least partially homogeneous electric field. The electrodes are designed in such a way that the air mass flow has already covered a drift distance through the electric field before it reaches the sensor surface, and that the electric field is extended by a post-drift distance behind the sensor surface. If the hot-film air-mass meter has a flow channel, in particular a bypass channel, the electrodes of the capacitor can be introduced into the flow channel. These can be separate electrodes, for example in the form of metal layers introduced into the flow channel. Alternatively, the walls of the flow channel itself can also be provided with a metallization, or the flow channel itself can have a metallic component, in particular an at least partially metallic cover and/or base part. A plurality of capacitors can also be arranged in series, for example a capacitor in front of the sensor chip and a capacitor after the sensor chip.

In many cases, the air mass flow will flow with a main flow direction. If a flow duct is used, for example a bypass duct, the term "main flow direction" is to be understood locally in each case, with the term "main flow direction" being understood to mean the main transport direction of the air mass flow at the respective location in the flow duct. Local turbulence should be neglected. It is particularly preferred if the electric field penetrates the air mass flow essentially perpendicular to the main flow direction. In this way, the deflection effect described above is used particularly well. The electric field preferably runs horizontally when the hot-film air-mass meter is installed, for example in the intake tract of the internal combustion engine when the internal combustion engine is horizontal. In this way, precipitation of coagulated droplets can be increased.

In order to support the effects of deflection and precipitation described above (in addition to which other electrical effects can also have an effect), it is preferred if the air mass flow has a drift distance of at least 1 mm, in particular at least 3 mm and particularly preferably at least 5 mm, before reaching the sensor surface travels in the electric field. This can be done, for example, by appropriately dimensioning the capacitor described. In particular, for example, if a flow channel (for example a bypass channel) is used, the entire section of the bypass channel can be provided with a capacitor, for example by metalizing the cover and base of the bypass channel. It is particularly preferred if the preferably at least partially homogeneous electric field has a field strength in a range from 1 kV/m to 5 MV/m, in particular 10 kV/m to 2.5 MV/m and particularly preferably at approximately 0.4 MV/m. In practical tests, these field strengths have proven to be particularly effective and yet technically easy to implement and manage.

As described above, the contamination effect occurs in particular after the internal combustion engine has been switched off, when air mixed with oil mist expands in the crankcase and diffuses through a crankcase ventilation opening into the intake tract. Accordingly, the hot-film air-mass meter according to the invention can also be operated in different ways. On the one hand, the field generating device can generate an electric field during the entire operation or only during partial phases of the operation of the internal combustion engine and in this way continuously prevent contamination of the sensor surface by utilizing the described effects. However, alternatively or additionally, it is also preferred to use the field-generating device after the internal combustion engine has been switched off. Accordingly, a method is proposed in which the field generating device generates an electric field after the internal combustion engine has been switched off for a predetermined run-on period. Basically any run-on time can be used. In particular, overrun times of between 3 seconds and 10 minutes, preferably between 5 seconds and 5 minutes and particularly preferably between 10 seconds and 3 minutes have proven useful. In this way, contamination of the sensor surface caused by diffusion of oil mist when the internal combustion engine is at a standstill can be avoided or greatly reduced. It should also be pointed out that the term “main flow direction” used above should be interpreted broadly when the internal combustion engine is at a standstill, since in this case scenarios can occur in which oil mist flows “from both directions” to the sensor surface. Therefore, the drift path described above can be arranged, for example, “before” and/or “after” the sensor surface, ie, with reference to the main flow direction during operation of the internal combustion engine, upstream or also downstream of the sensor surface. Different electric fields can also be provided, for example an electric field “downstream” of the sensor surface, which is only switched on after the internal combustion engine has been switched off, in order to prevent oil mist from diffusing back against the main flow direction during operation of the internal combustion engine after the internal combustion engine has been switched off, or to prevent the effects to reduce the contamination by this back-diffusion.

In contrast to the devices known from the prior art for reducing or avoiding oil contamination on the sensor surface, the hot-film air mass meter according to the invention and the method described for operating the same are distinguished by numerous advantages. One advantage is, for example, that oil deposits are avoided overall on the sensor surface by precipitating oil before it reaches the sensor surface and/or by keeping it away from the sensor surface. There is therefore also no accumulation of oil in the vicinity of the sensor surface, which could then be carried onto the sensor surface by air turbulence. The system is thus characterized by increased robustness and susceptibility to faults.

Furthermore, the described hot-film air-mass flow meter has a comparatively simple structure. No additional control mechanisms are required, for example to maintain certain temperature ranges on the sensor surface, as is the case, for example, with the DE 101 11 840 C1 the case is. Furthermore, no microsystem-technical changes to the sensor surface itself are required, ie additional microstructures on a sensor chip, for example. Existing sensor chips can thus continue to be used simply by adding a field generating device. As a rule, it is not necessary to change the lithographic processes or the layout of sensor chips.

The field generating device can be implemented by simply introducing electrodes into a flow channel. As described above, metal foils can be used, for example, or walls of a flow channel can also be metallized. For this purpose, cost-effective and reliable large-scale production processes known to those skilled in the art can be used, for example injection molding processes for producing a flow channel, followed by standard metallization processes. For example, there are numerous techniques for subsequently metallizing injection-molded components, in particular with the aid of metallizable plastics and/or subsequently making the plastics metallizable by applying, for example, metal seeds and subsequent metallization, for example in a chemical metallization bath. Such techniques are known to those skilled in the art. Structured metallization can also be applied to plastics, for example using the methods known from the technology of injection-molded printed circuit boards (molded interconnect devices, MID) (e.g. hot embossing, laser structuring, etc.). In this way, the hot-film air-mass meter described can be manufactured with minor modifications to already known types of hot-film air-mass meter using a cost-effective and reliable manufacturing method.

character list

• 1 einen in einem Ansaugtrakt einer Verbrennungskraftmaschine eingesetzten Heißfilmluftmassenmesser;
• 2 einen Schnitt durch einen Bypasskanal eines Heißfilmluftmassenmessers mit einem in den Bypasskanal hineinragenden Chipträger;
• 3 einen Schnitt durch den Bypasskanal eines Ausführungsbeispiels eines Heißfilmluftmassenmessers mit Schnittebene senkrecht zur Schnittebene gemäß 2; und
• 4 ein Deckelteil eines Heißfilmluftmassenmessers zum Aufsetzen auf den Bypasskanal gemäß 2.

The invention is explained in more detail below with reference to the drawing. It shows:

• 1 a hot film air mass meter used in an intake tract of an internal combustion engine;
• 2 a section through a bypass channel of a hot-film air mass meter with a chip carrier protruding into the bypass channel;
• 3 according to a section through the bypass channel of an embodiment of a hot-film air mass meter with a section plane perpendicular to the section plane 2 ; and
• 4 according to FIG 2 .

exemplary embodiments

In 1 an example of a hot-film air-mass meter 110 is shown, which is used in the intake tract 112 of an internal combustion engine. Such hot-film air-mass meters 110 are commercially available and are state of the art. The hot-film air mass meter 110 is designed to detect the direction of flow of an exhaust gas stream in the case of a pulsating flow and is designed for load detection in internal combustion engines with gasoline or diesel fuel injection. The installation of the hot-film air-mass meter 110 usually takes place between an air filter and a throttle device and usually takes place as a preassembled assembly in the form of a plug-in sensor, which has a measuring housing 114 .

In the 2 until 4 an exemplary embodiment of a hot-film air-mass meter 110 according to the invention is shown, in which a plate capacitor 116 is used as field-generating device 118 . while showing 2 the hot-film air-mass meter 110 with the meter housing 114 open, the bypass channel 120 being visible. The measuring housing 114 thus includes an electronics area 122 in which the (in 2 not shown) control and evaluation electronics of the hot-film air mass meter 110 is housed, as well as a flow area 124, in which the bypass channel 120 is accommodated. As in 1 shown, the flow area 124 protrudes into the intake tract 112 of the internal combustion engine. A chip carrier 126 with a sensor chip 128 inserted therein protrudes from the electronics area 122 into the bypass channel 120. The chip carrier 126 can, for example, be a metallic chip carrier 126, for example in the form of a sheet metal part. In another embodiment, the chip carrier 126 can also be a plastic part, for example a sensor nose that is injection-molded onto a printed circuit board accommodated in the electronics area 122 and protrudes into the bypass channel 120 . The sensor chip 128 is let into a depression in the chip carrier 126 so that the air mass flow passing through the bypass duct 120 flows over its sensor surface 130 .

With reference numeral 132 are in 2 Diffusion paths of the oil mist in the air mass flow drawn. These diffusion paths 132 are the paths that the oil mist travels from the intake tract 112 to the sensor chip 128 when the internal combustion engine is switched off. During operation of the internal combustion engine, however, the air mass flow flows in the region of sensor chip 128 with a main flow direction through bypass channel 120, with the main flow direction in 2 is symbolically denoted by the arrow 134. In order to ensure that the sensor surface 130 is overflown, the chip carrier 126 is equipped with a leading edge 136 upstream.

The measuring housing 114 is shown in the perspective view according to FIG 2 viewed from above in open condition. In 4 a cover part 138 is shown in perspective, which after folding about a vertical axis onto the measuring housing 114 in 2 can be placed to close the bypass channel 120. The cover part 138 then forms an integral part of the measuring housing 114. The delimiting structures 140 of the bypass channel 120 can be clearly seen in the cover part. Furthermore, it can be seen that the walls of the bypass channel 120 do not run parallel in all areas, but that the flow can be guided by local narrowing or widening of the bypass channel 120 . For example, a convergence ramp 142 is formed in the area of the bypass channel 120 into which the chip carrier 126 protrudes, so that the flow cross section is narrowed in this area.

In 3 a section through the area of the bypass channel 120 in the area of the convergence ramp 142 is shown. The cutting line runs in 2 horizontally through the chip carrier 126, and the viewing direction is in 2 from below, parallel to the plane of the drawing. The configuration of the field generating device 118 is also particularly clear in this representation. It can be seen that this field generating device 118 is composed in this exemplary embodiment of a first electrode 144 on the cover part 138 and a second electrode 146 on a bottom part 148 of the measuring housing 114. If the two electrodes 144, 146 are charged differently, then between these an electric field 150, which in 3 is represented symbolically. In this exemplary embodiment, the electric field is essentially homogeneous and deviates from homogeneity only in the edge regions. With typical channel widths between 2 and 10 mm, voltages of up to approx. 5 kV were used in tests. When using non-metallic chip carriers 126, for example in the form of injection molded components, the field lines in the area of the chip carrier 126 are only slightly affected. However, other configurations are also possible, in particular configurations with metallic chip carriers 126.

In this exemplary embodiment, the two electrodes 144 , 146 are produced by simply applying conductive lacquer to the channel walls of the bypass channel 120 . This is by synopsis of the 2 and 4 recognizable. In 2 the second electrode 146, which is applied to the bottom part 148 of the measuring housing 114, is visible. In the perspective view according to 2 this second electrode 146 is partially covered by the chip carrier 126 . In 4 the first electrode 144, which is applied to the cover part 138, can be seen. By folding cover part 138 onto measuring housing 114, first electrode 144 and second electrode 146 form the in 3 shown capacitor structure.

How out 3 As can be seen, the electric field 150 does not begin directly at the leading edge 136 of the chip carrier 126, but the electrodes 144, 146 are designed in such a way that the air mass flow has already covered a drift distance 152 through the electric field 150 before it reaches the sensor surface 130. This ensures that the above-described effects of the precipitation of coagulated oil droplets or the deflection of charged oil droplets can have a sufficient effect before the air mass flow reaches sensor surface 130 . In this illustration, the precipitation of oil droplets takes place according to 3 , which shows the viewing direction from below, perpendicularly out of the plane of the drawing. In this case, the electric field 150 is approximately horizontal, although deviations from the horizontal (preferably by no more than 20°) can also be tolerated.

Furthermore, in 3 to recognize that the electric field 150 does not end immediately after the sensor surface 130, but only by a post-drift distance 154 behind the sensor surface 130. For example, this post-drift distance 154 can be approximately as long as the drift distance 152. This post-drift distance 154 ensures that oil vapors flowing counter to the main flow direction 134 even after the internal combustion engine has been switched off (see the diffusion paths 132 in 2 ), what a 3 flow from the right, cannot or only insignificantly contaminate the sensor chip. In the embodiment according to the 2 until 4 the drift section 152 is selected to be longer than the post-drift section 154. Typical lengths of the drift sections 152, 154 are in the range from one to fifty millimeters.

That in the 2 until 4 The exemplary embodiment of the hot-film air-mass meter 110 according to the invention that is described represents only one of several possibilities for realizing the principle according to the invention. The electrodes 144, 146 can be electrically contacted in a simple manner, for example by means of conductor tracks applied to the measuring housing 114, and can be connected to a corresponding voltage source. Alternatively, complete metallization of the walls of the bypass channel 120 is also possible, so that in this case the drift section 152 or the post-drift section 154 encompasses the entire bypass channel 120 . Furthermore, separate electrodes, separate from the cover part 138 and base part 148 , can also be introduced into the bypass channel 140 so that, for example, the electric field 150 does not extend over the entire width of the bypass channel 120 . In this way it is possible to work with lower voltages. Furthermore, it is also conceivable to use other electrode geometries, for example other geometric shapes of the electrodes 144, 146. The chip carrier 126, which, as described above, can also be metallic, can itself be used as an electrode.

As described above, it is particularly preferred if the hot-film air-mass meter 110 according to the invention continues to be operated for a specific run-on time after the internal combustion engine has been switched off. This prevents oil mist reaching the sensor chip 128 via the diffusion paths 132 from being able to contaminate it when the internal combustion engine is at a standstill. This ensures, in particular, that even after the internal combustion engine is switched on again, the hot-film air-mass meter 110 is again available almost immediately for corresponding measurements and thus for engine control.

Claims (8)

Hot-film air-mass meter (110) for measuring an air-mass flow, in particular in the intake tract (112) of an internal combustion engine, the hot-film air-mass meter (110) having a sensor surface (130) over which the air-mass flow can flow, the hot-film air-mass meter (110) having a field-generating device (118), which is configured , in order to generate an at least partially homogeneous electric field (150) through the air mass flow, and the field generating device (118) has a capacitor, in particular a plate capacitor (116), characterized in that the electrodes (144, 146) of the capacitor have at least approximately the same area and are designed in such a way that the air mass flow has already covered a drift distance (152) through the electric field (150) before it reaches the sensor surface (130), and that the electric field (150) has a post-drift distance (154) behind the Sensor surface (130) is extended. Hot-film air-mass meter (110) according to the preceding claim, characterized by at least one flow channel, in particular a bypass channel (120), the electrodes (144, 146) being a metal layer introduced into the flow channel and/or a metallization applied to the walls of the flow channel and/or a metallic component of the flow channel, in particular a metallic component of a cover part (138) and/or a base part (148) of the flow channel. Hot-film air-mass meter (110) according to one of the preceding claims, characterized in that the air-mass flow flows in a main flow direction (134), the electric field (150) penetrating the air-mass flow essentially perpendicularly to the main flow direction (134). Hot-film air-mass meter (110) according to one of the preceding claims, characterized in that the electric field (150) runs essentially horizontally when the hot-film air-mass meter (110) is installed. Hot-film air-mass meter (110) according to one of the preceding claims, characterized in that the electric field (150) is designed in such a way that the air-mass flow has a drift distance (152) of at least 1 mm, in particular of at least 3 mm and particularly preferably of at least 5 mm in the electric field (150). Hot-film air-mass meter (110) according to one of the preceding claims, characterized in that the at least partially homogeneous electric field (150) has a field strength in a range from 1 kV/m to 5 MV/m, in particular 10 kV/m to 2 .5 MV/m and most preferably at approximately 0.4 MV/m. Method for measuring an air mass flow of an internal combustion engine, in particular in the intake tract of the internal combustion engine, using a hot-film air mass meter (110) according to one of the preceding claims, characterized in that the field generating device (118) generates an electric field (150) for a predetermined after-run period after the internal combustion engine has been switched off generated. Method according to the preceding claim, characterized in that the run-on time is between 3 seconds and 10 minutes, in particular between 5 seconds and 5 minutes and particularly preferably between 10 seconds and 3 minutes.

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