DE102011086148A1 - Method for operating resistive sensor in exhaust duct of internal combustion engine e.g. petrol engine, involves determining dew point end in exhaust duct from measured change in conductivity of resistive sensor - Google Patents

Abstract

The method involves configuring resistive sensor having a cross-sensitivity with respect to a water drop entry (50), resulting in a change in the sensor signal. A dew point end in an exhaust duct (30) is determined from a measured change in conductivity of the resistive sensor. The saturated or unsaturated water vapor atmosphere is determined with respect to high conductivity of the resistive sensor. An independent claim is included for a device for operating resistive sensor.

Description

State of the art

The invention relates to a method for operating a resistive sensor, wherein the sensor is arranged in the exhaust passage of an internal combustion engine and has a cross sensitivity to a water droplet entry, which leads to a change of the sensor signal.

The invention further relates to a corresponding device for carrying out the method according to the invention.

Combustion engines today use various exhaust gas sensors to be able to comply with current emission limit values or to monitor the system for faults. In gasoline engines standard lambda probes are used. In diesel engines both lambda probes and additionally also NO _x sensors, in SCR systems, are used. These exhaust gas sensors work with an ion-conductive ceramic, which has its conductivity characteristic only at high temperatures. The operation of such probes is only possible with a warm engine. When the engine is cold, ie before reaching the so-called dew point end, water droplets are contained in the exhaust gas, which destroy an exhaust gas temperature sensor heated to operating temperature or significantly adversely affect the measurement result of these sensors.

Usually, the dew point end in the exhaust system is currently being determined very laboriously empirically or model-based. All inaccuracies must be included in a "worst case" consideration in the time determined. In practice, this results in a significant time delay between the actual dew point end in the system and the calculated dew point end from which the release for the operation of the exhaust gas sensors is derived.

For passenger car emission measurements, the measuring cycle is generally started with a cold engine. Delayed switching on of the emissions control strategy based on the exhaust gas sensors is therefore accompanied by a significant deterioration in the emission in the cycle. Accordingly, a correct determination of the dew point end is critical.

This invention describes an operating strategy for a resistive sensor. Such sensors are known today as particle sensors and are used, for example, for monitoring the emission of soot from internal combustion engines and for on-board diagnostics (OBD), for example for monitoring the function of particulate filters. In this case, collecting, resistive particle sensors are known which evaluate a change in the electrical properties of an interdigital electrode structure due to particle deposits. Two or more electrodes can be provided, which preferably engage in one another like a comb. Due to an increasing number of particles attaching to the particle sensor, the electrodes are short-circuited, resulting in a decreasing electrical resistance with increasing particle deposition, a decreasing impedance, or a change in a parameter related to the resistance or the impedance, such as a voltage and / or current effect. For evaluation, a threshold value, for example a measuring current between the electrodes, is generally determined and the time until the threshold value is reached is used as a measure of the accumulated particle quantity. Alternatively, a signal change rate during the particle accumulation can be evaluated. If the particle sensor is fully loaded, the deposited particles are burned in a regeneration phase with the aid of a heating element integrated in the particle sensor.

Such a resistive particle sensor is in the DE 101 33 384 A1 described. The particle sensor is constructed of two intermeshing, comb-like electrodes which are at least partially covered by a catching sleeve. If particles are deposited from a gas flow at the particle sensor, this leads to an evaluable change in the impedance of the particle sensor, from which it is possible to infer the amount of deposited particles and thus the amount of entrained particles in the exhaust gas.

The DE 101 49 333 A1 describes a sensor device for measuring the humidity of gases, having a resistance measuring structure arranged on a substrate, wherein the measuring structure interacts with a soot layer and a temperature measuring device is provided. With this sensor device, the soot concentration in the exhaust gas of an internal combustion engine can also be determined.

The font DE 10 2009 001 064 A1 describes a method for determining a measure of the water droplet entry into an exhaust passage of an internal combustion engine using the output of a resistive particulate sensor. A threshold increase in the output signal is used as a trigger for determining the number of drops or a drop of water from the output signal.

The object of the invention is to determine the dew point end in the exhaust system of an internal combustion engine precisely and safely and moreover to provide measures to prevent or reduce condensation of the components in the exhaust system.

It is a further object of the invention to provide a device suitable for carrying out the method.

Disclosure of the invention

The object of the method is solved by the features of claims 1 to 6.

The inventive method provides that from a measured change in the conductivity of the resistive sensor, a dew point end is determined in the exhaust duct, wherein in a saturated or supersaturated steam atmosphere compared to an unsaturated water vapor atmosphere comparatively high conductivity of the resistive sensor is measured. A dew point end can therefore be detected precisely and reliably, as this is clearly noticeable in a signal change in the conductivity measurement. Current methods, as mentioned above, in which a dew point end, for example, model estimated, can be replaced with this method. Since exhaust gas sensors can go into operation via the dew point end for the emission control strategy, rapid detection of the dew point end also has a directly positive effect on a lower exhaust gas emission of the internal combustion engine.

An advantageous operating strategy for this resistive sensor provides that a condensation of the resistive sensor is actively prevented or reduced by means of a heating element arranged on or in the resistive sensor. Water droplets that have settled on the surface of the sensor element can be evaporated. In addition, therefore, other exhaust gas sensors in the exhaust duct, which are arranged downstream of the resistive sensor or in its immediate vicinity, can be protected from an otherwise damaging moisture in the form of condensed water.

This sensor heating also allows a distinction between condensed water, which is carried in the exhaust gas and between water, which was previously condensed on the surface. For this purpose, it is provided that after the start of the internal combustion engine condensed moisture is evaporated by means of the heating element.

Due to the above-described characteristics of the conductivity as a function of the degree of saturation of the water vapor, therefore, a successful elimination of the condensation or elimination of the condensed moisture can be detected via a decrease in the conductivity of the resistive sensor. Water, which had previously condensed and evaporated by means of the heating element, is first noticeable in a decrease in the conductivity. A subsequently measurable rise in conductivity is due to moisture in the exhaust gas. The feature for a dew-point end can be determined via a conductivity drop below a minimum value. This feature is independent of whether a heater is used. However, a heater can contribute to a faster determination of the dew point end.

In a further variant for the operating strategy, it may be provided that the resistive sensor, before detecting the dew-point end, i. even during the determination of the incoming moisture, with reduced power and / or periodically heated. This has an advantageous effect if initially a relatively large amount of moisture is contained in the exhaust gas, which leads to a condensation on the sensor, since the dew point end can then be detected more quickly. Otherwise, the condensed moisture would have to be removed after reaching the dew point end only via the exhaust gas.

Due to its sensitivity to moisture entry, it has proven to be particularly advantageous if a particle sensor for determining the soot concentration in the exhaust gas or for determining a soot load is used as the resistive sensor. Such particle sensors, as mentioned above, are already installed in the exhaust system of diesel engines today and are therefore designed to be robust enough to be used as a sensor for a precise determination of the dew point end in the exhaust passage of an internal combustion engine can.

A preferred use of a resistive sensor and application of the operating strategies described above provides for use in the exhaust gas duct of a gasoline-powered internal combustion engine, in which usually no particle sensor is previously provided. Thus, a possible condensation on the one hand detected and on the other hand appropriate countermeasures are initiated, so that other sensors in the exhaust duct, e.g. Lambda sensors, can be protected and emission control can be started earlier.

The object concerning the device is achieved in that the resistive sensor has a heating element and is electrically connected to a control unit, wherein the control unit has devices, such as comparators and memory units, for carrying out the method as described above, wherein the conductivity of the resistive sensor with the control unit evaluable and the heating element with the control unit is at least temporarily controllable. The functionality can also be implemented in a higher-level engine control.

The invention will be explained in more detail below with reference to an embodiment shown in FIGS. Show it:

1 schematically an executed as a particle sensor resistive sensor in an exploded view and

2 the technical environment of the invention in a schematic representation.

1 shows a schematic representation of a particle sensor 10 executed resistive sensor according to the prior art in an exploded view.

On insulating support layers 11 For example, of alumina, are a first electrode 12 and a second electrode 13 applied. The electrodes 12 . 13 are designed in the form of two interdigital, interlocking comb electrodes. At the front ends of the electrodes 12 . 13 are a first connection 12.1 and a second connection 13 .1 provided, via which the electrodes 12 . 13 for power supply and for carrying out the measurement with a control unit, not shown 40 (please refer 2 ) can be connected. In addition, in the example shown, between the insulating support layers 11 a heating element 14 integrated, which via the connections 14.1 . 14.2 with the control unit 40 connected is.

Will such a particle sensor 10 operated in a particle-carrying gas stream, for example in an exhaust passage of a diesel engine, so the particles are stored from the gas stream to the particle sensor 10 from. In the case of the diesel engine, the particles are soot particles 16 with a corresponding electrical conductivity. The deposition rate of the soot particles depends on this 16 to the particle sensor 10 in addition to the particle concentration in the exhaust, inter alia, from the voltage which, at the electrodes 12 . 13 is applied. The applied voltage generates an electric field which is due to electrically charged soot particles 16 and on soot particles 16 exerts a corresponding attraction with a dipole charge. By suitable choice of the electrodes 12 . 13 Therefore, the deposition rate of the soot particles can be applied 16 to be influenced.

In the exemplary embodiment, the electrodes are 12 . 13 and the uppermost insulating support layer 11 on which are the electrodes 12 . 13 located, with a protective layer 15 overdrawn. This optional protective layer 15 protects the electrodes 12 . 13 at the mostly prevailing high operating temperatures of the particle sensor 10 before corrosion.

Such particle sensors 10 have a significant cross-sensitivity to moisture, which is condensed in particular on the surface, and can therefore be used advantageously for a determination of a dew point end.

2 shows in a highly simplified schematic representation of the technical environment of the invention. Shown is an internal combustion engine 20 in the exhaust duct 30 an exhaust gas flow is guided, wherein the exhaust gas volume flow is a water droplet entry 50 , for example, at a dew point below the exhaust duct 30 , may have. In the exhaust duct 30 is a particle sensor in the example shown 10 as he according to the in 1 illustrated embodiment, arranged. That in the particle sensor 10 integrated heating element 14 serves in particular for the regeneration of the particle sensor 10 in which the particle sensor 10 heated to several hundred ° C (typically up to 800 ° C). For operation of the particle sensor 10 this is electrically connected to a control unit 40 connected, on the one hand, the signals of the particle sensor 10 can be evaluated by this and the other the heating element 14 is controllable for regeneration. The control unit 40 can be an integral part of a higher-level engine control.

The inventive method is based on the fact that a water entry, which in the combustion in the internal combustion engine 20 arises, is conductive. This conductivity is measurable as long as the internal combustion engine 20 is not yet warm and the water component of the exhaust gas condenses into droplets. The current in this case is typically between 50 μA and 100 μA (applies to pure water). In the transition to a warm engine, the water in the exhaust stream becomes gaseous, which corresponds to a dew point end, and contributes to no measurable conductivity more.

A resistive sensor, such as a resistive particle sensor 10 , Can therefore be used to the non-gaseous water content of the exhaust gas of the internal combustion engine 20 to eat. If harmful water droplets are still in the exhaust gas for the operation of exhaust gas sensors, then the resistive sensor measures a finite resistance. After reaching the dew point end, there is only gaseous water in the exhaust gas and thus no measurable conductivity more. From the measured change in conductivity can be concluded directly on the dew point end.

With the in the particle sensor 10 integrated heating element 14 can actively be made possible an evaporation of water, which is condensed on the surface of the sensor element. The sensor heater enables the distinction of the condensed water that is carried in the exhaust gas, and the water that was previously possibly condensed on the sensor surface. For this purpose, after the start of the engine, the possibly condensed during the shutdown water by means of the heating element 14 evaporated. The feature for successful evaporation is via the decaying conductivity at the particle sensor 10 to investigate. A subsequently measurable rise in conductivity is due to moisture in the exhaust gas. The feature for a dew point end shall be determined by a conductivity drop below a minimum value. This feature is independent of whether a heater is used. However, a heater can contribute to a faster determination of the dew point. Optionally, the sensor can still be heated weakly or periodically during the determination of the newly arriving moisture before the dew point end.

3 shows an example in a history diagram 60 the conductivity curve and the heating curve for operating cases, as described above, wherein in the upper part of the history diagram 60 the conductivity 61 depending on the time 63 and in the lower part the heating power 62 depending on the time 63 is applied. Shown in the upper part is a conductivity without heating 64 , a conductivity curve with a heating course 1 65 after a cold start and a course of conductivity with a heating course 2 66 , In the lower part are the corresponding Heizverläufe 1 and 2 67 . 68 shown, wherein the heating 2 68 after an initial high heat output after the cold start in contrast to the heating process 1 67 is reduced to a residual calorific value in the reheating phase. In the course of conductivity, these different heating strategies express on the one hand different absolute levels of conductivity after the heating phase directly after the cold start and on the other hand a different decay of the conductivity below a threshold value for the dew point end 69 after the start time, decrease saturation 73 , A dew-point end without heating 70 will be detected late. In the heating process 1 67 becomes the dew point end after heating 1 71 reached earlier. In the heating process 2 68 becomes the dew point end after heating process 2 72 achieved in no time.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10133384 A1 [0007]
• DE 10149333 A1 [0008]
• DE 102009001064 A1 [0009]

Claims (8)

Method for operating a resistive sensor, wherein the sensor in the exhaust gas channel ( 30 ) an internal combustion engine ( 20 ) and a cross-sensitivity to a water droplet entry ( 50 ), which leads to a change of the sensor signal, characterized in that from a measured change in the conductivity of the resistive sensor, a dew point end in the exhaust gas channel ( 30 ) is determined, wherein in a saturated or supersaturated water vapor atmosphere compared to an unsaturated steam atmosphere comparatively high conductivity of the resistive sensor is measured. Method according to claim 1, characterized in that by means of a heating element arranged on or in the resistive sensor ( 14 ) actively prevents condensation of the resistive sensor or is reduced. Method according to one of claims 1 or 2, characterized in that after the start of the internal combustion engine ( 20 ) condensed moisture by means of the heating element ( 14 ) is evaporated. Method according to one of claims 2 or 3, characterized in that a successful elimination of the condensation or elimination of the condensed moisture is detected via a decrease in the conductivity of the resistive sensor, wherein the feature for a dew-point over a conductivity drop below a minimum value is determined. Method according to one of claims 1 to 4, characterized in that the resistive sensor is heated prior to detecting the dew point end with reduced power and / or periodically. Method according to one of claims 1 to 5, characterized in that as a resistive sensor, a particle sensor ( 10 ) is used to determine the soot concentration in the exhaust gas or to determine a soot load. Use of a resistive sensor and application of the operating strategies according to one of Claims 1 to 6 in the exhaust gas duct ( 30 ) of a gasoline-powered internal combustion engine ( 20 ). Device for operating a resistive sensor, wherein the sensor in the exhaust gas channel ( 30 ) an internal combustion engine ( 20 ) and a cross-sensitivity to a water droplet entry ( 50 ), which leads to a change in the sensor signal, wherein the resistive sensor is a heating element ( 14 ) and with a control unit ( 40 ) is electrically connected, characterized in that the control unit ( 40 ) Comprises means for carrying out the method according to claims 1 to 6, wherein the conductivity of the resistive sensor with the control unit ( 40 ) and the heating element with the control unit ( 40 ) is at least temporarily controllable.

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