WO2017029074A1 - Method for function monitoring of an electrostatic soot sensor - Google Patents

Abstract

The invention relates to a method for function monitoring an electrostatic soot sensor, comprising a first electrode and a second electrode, wherein the first electrode and the second electrode are electrically insulated from one other by an insulating body and an electrical voltage can be applied between the first electrode and the second electrode, and wherein soot particles can enter a space between the first electrode and the second electrode via a gas flow. In order to provide a method for function monitoring an electrostatic soot sensor, with which a faulty soot sensor can be detected in a simple and safe manner, during a heating phase of the initially cold soot sensor, the measuring current flowing via water that has been deposited from the gas flow onto the insulating body as condensate is evaluated as an indicator for the functioning of the soot sensor, which measuring current is formed as a result of the applied voltage between the first electrode and the second electrode.

Description

description

Method for monitoring the function of an electrostatic soot sensor

The invention relates to a method for monitoring the function of an electrostatic soot sensor.

The reduction of exhaust emissions in motor vehicles is an important goal in the development of new motor vehicles. Therefore, combustion processes in internal combustion engines are thermodynamically optimized, so that the efficiency of the

Internal combustion engine is significantly improved. In the motor vehicle sector driving ^¬ diesel engines are increasingly being used, which have a very high efficiency in modern construction. The disadvantage of this combustion technique compared to optimized Otto engines, however, is a significantly increased emissions of soot. The soot is particularly carcinogenic due to the addition of polycyclic aromatics, which has already been reacted in various regulations. For example, exhaust emission standards with maximum limits for soot emissions were issued. In order to meet the exhaust emission standards nationwide for motor vehicles with diesel engines, there is a need to produce low-cost sensors that reliably measure the soot content in the exhaust stream of the motor vehicle.

The use of such soot sensors is used to measure the currently expelled soot, so that the engine management in a motor vehicle in a current driving situation information to reduce the emission values with regulatory adjustments. In addition, with the aid of the soot sensors active exhaust gas purification can be initiated by exhaust soot filters or exhaust gas recirculation to the internal combustion engine can take place. In the case of soot filtering regenerable filters are used, which filter out a significant portion of the carbon black content from the exhaust gas. Soot- Sensors for the detection of soot to monitor the function of the soot filters or to control their regeneration cycles. For this purpose, the soot filter, which is also referred to as a diesel particulate filter, may be preceded by a soot sensor and / or a soot sensor connected downstream.

The diesel particulate filter upstream sensor is used to increase the system security and to ensure a

Operation of the diesel particulate filter under optimum Bedingun ^¬ gene. Since this is highly dependent of the incorporated in the diesel particulate filter soot, is an accurate measurement of the particle concentration before the diesel particulate filter system, in particular the determination of a high particle concentration before the diesel particulate filter, of high importance.

A diesel particulate filter downstream soot sensor offers the ability to make an on-board diagnosis and also serves to ensure the correct operation of the exhaust aftertreatment system.

The prior art shows various approaches to Detekti ^¬ on of soot. A widely used approach in laboratories is the use of light scattering by the soot particles. This approach is suitable for complex Messge ^¬ devices. If it is attempted to use this as a mobile sensor system in the exhaust system, it must be noted that approaches for the realization of an optical sensor in a motor vehicle are associated with very high costs. Furthermore, there are unresolved problems regarding the pollution of the required optical windows by combustion exhaust gases.

DE 195 36 705 AI discloses a device for measuring soot particles, wherein an electric field between a through-flow of the gas flow from the cladding electrode and an inner electrode within this cladding electrode by applying a constant electrical DC voltage is generated and the charging current to maintain the constant DC voltage between the sheath electrode and inner electrode gemes ^¬ sen. Good measurement results are achieved in the context of the disclosure of DE 195 36 705 AI, when a DC voltage of 2000 to 3000 volts is used to generate the electric field.

In these electrostatic soot sensors, the current between the two electrodes changes depending on the soot concentration in the exhaust gas flow. However, the currents occurring here are relatively small and their current strength is of the order of nA. Therefore, the entire measuring arrangement for these electrostatic soot sensors must be made very high-impedance.

In some regulations and standards for the training of exhaust aftertreatment systems for motor vehicles is further determined that the functionality of the Rußson- sors used must be regularly monitored in the vehicle. This is to ensure that the soot sensor itself has no faulty function. In the case of the electrostatic soot sensors mentioned in the introduction, this is particularly difficult since the situation in which no measuring current flows via the electrostatic soot sensors can be based on the fact that the exhaust gas is extremely soot-poor at the time of the measurement or can therefore be caused by a Malfunction of the soot sensor, for example, a cable break, is present, which leads to the interruption of the measuring current and thus allows the conclusion of a low-carbon exhaust gas flow, although the exhaust gas flow can be highly soot polluted.

It is an object of the present invention to provide a method for monitoring the function of an electrostatic soot sensor, a ^fault-¬ exemplary soot sensor can be detected with a simple and secure manner. The object is achieved by the features of the independent Ansprü ^¬ che.

Characterized in that during the heating phase of cold at the beginning of the soot sensor, the current flowing through the deposited from the gas stream on the insulation ^¬ body as condensate water measured ^¬ stream which forms as a result of the applied voltage between the first electrode and the second electrode, as Indi ^¬ kator is evaluated for the operability of the soot sensor, the functionality of the soot sensor can be determined based on a highly reproducible process at regular intervals during operation of the vehicle. This is an important measure to ensure the func ^¬ tion capability of the entire exhaust aftertreatment system and thus to protect the environment from contamination.

In a development of the invention, the functionality of the soot sensor is recognized when the measuring current flowing between the first electrode and the second electrode follows a predetermined diagnostic model. The measuring current is very characteristic in egg ^¬ nem error-free soot sensor, when observed in the warm-up phase of the soot sensor. This cha ^¬ teristic curve can be easily stored as a diagnostic model for In ^¬ play in the motor controller, or other electronic evaluation unit. It is conceivable to determine the Di ^¬ agnosemodel with a guaranteed error-free soot sensor before the actual monitoring function of the soot sensor and store the once determined values in an electronic memory in the vehicle or to choose a mathematical diagnosis model based on other data, such as outdoor temperature and Humidity, recalculating the diagnostic model at each function monitoring of the soot sensor.

Characterized in that during a heating phase of the initially cold soot sensor flowing over from the gas stream on the insulating body as condensate water flowing measuring current, due to the applied voltage between the first Formed electrode and the guard electrode, is evaluated as an indicator of the operability of the soot sensor, the functionality of the soot sensor can also be determined on the basis of an excellent reproducible process at regular intervals during operation of the vehicle. This is also an important measure to ensure the functionality of the entire exhaust aftertreatment system and thus to protect the environment from contamination. In one embodiment of the invention, the functionality of the soot sensor is detected when the measuring current flowing between the first electrode and the guard electrode follows a predetermined diagnostic model. Here, too, the measuring ^¬ current is very characteristic in a faultless soot sensor, when observed in the warm-up phase of the soot sensor. This characteristic curve can also be easily stored as a diagnostic model, for example in the engine control or another electronic evaluation unit. It is conceivable to determine the diagnostic model with a guaranteed defect-free soot sensor before the actual function monitoring of the soot sensor and to store the once determined values in an electronic memory in the vehicle or to select a mathematical diagnosis model based on further data, such as outside temperature and humidity, the diagnostic semodel at each remote monitoring of the soot sensor re ^¬ be counted.

It may be advantageous if the voltage at the guard electrode is at a higher potential than the voltage at the second electrode.

In a development of the invention, the insulating body consists of a ceramic material. In the following, the present invention will be explained with reference to the accompanying drawings and preferred embodiments. These embodiments include Soot sensors for use in a motor vehicle. Show it :

FIG. 1 shows a soot sensor,

Figure 2 shows a soot sensor according to the invention

 Method can be operated

FIG. 3 shows a further soot sensor which can be operated according to the method according to the invention,

FIG. 4 shows current-time curves generated by two soot sensors operated by the method according to the invention;

5 shows a current-time curve which was generated by an operated with the OF INVENTION ^¬ to the invention method soot sensor. FIG. 1 shows a soot sensor 1. The soot sensor 1 consists of a first electrode 2, which is arranged in the interior of a second electrode 3. Between the first electrode 2 and the second electrode 3 is the exhaust gas of the combus ^¬ tion motor, in which soot particles 4 are included. The concentration of the soot particles 4 in the exhaust gas should be measured by the soot sensor. In other words, it can be said that the soot content in the exhaust gas flow 17 should be determined with the soot sensor. For this purpose, a measuring voltage is applied by the voltage supply 6 between the first electrode 2 and the second electrode 3. The first electrode 2 is electrically insulated from the second electrode 3 by means of the insulating body 5. The insulating body 5 may be constructed, for example, as a disk of a ceramic material. Furthermore, it can be seen in FIG. 1 that between the voltage supply and the second electrode 3 there is connected an ohmic resistor 7, which is designed to be of high resistance, in order to measure the relatively small currents that occur due to the Soot particles 4 between the first electrode 2 and the second electrode 3 to form. The measurement of these currents is carried out by the current measuring element 8, which is connected to an evaluation ^¬ electronics 9. Such soot sensors 1 are used for on-board diagnosis in motor vehicles with diesel engines.

The voltage applied between the first electrode 2 and the second electrode 3 is relatively high in order to obtain useful measuring currents. Such a voltage is between 100 V and 3 kV and is therefore relatively expensive to control.

In particular, water deposits on the first electrode 2 and the second electrode 3, as well as on the insulating body 5 can lead to a complete falsification of the soot measurement. Therefore, the soot sensor 1 must be completely dried before the soot measurement, which is signaled by the so-called dew point ^¬ release.

FIG. 2 shows a soot sensor 1 with a first electrode 2 and a second electrode 3. The first electrode 2 is electrically insulated from the second electrode 3 by an insulating body 5, and an electrical voltage is applied between the first electrode 2 and the second electrode 3 generated by the electric power supply 6.

Soot particles 4, which are transported in an exhaust gas flow from an internal combustion engine through an exhaust pipe, can penetrate into the soot sensor 1 integrated in the exhaust gas system.

The soot particles 4 enter an electric field, which forms due to the applied electrical voltage in the space between the first electrode 2 and the second electrode 3 ^¬ . In order to produce a good measurable electric current between the first electrode 2 and the second electrode 3, 3 elements 15 are on the surface of the first electrode 2 and / or on the surface of the second electrode Concentration of the electric field strength formed. In this example, the first electrode 2 is formed as a rod-shaped Ge ^¬ windestange, wherein the elements 15 are formed to Konzentra ^¬ tion of the electric field strength through the threads, between which triangular tips are formed. At these tips of the electric field is concen ^¬ riert, whereby the electric field strength in the area of Spit ^¬ zen is very high. The large increase in the electric field strength in the area of the peaks can exceed the breakdown field strength of the gas in the area. When exceeding the breakdown field strength of the gas elec tric ^¬ charged particles are formed which are accelerated in the direction of entge ^¬ gengesetzten electrode and as a result of impact ionization to a lavinenartigen formation of charge carriers lead. If these charge carrier slaves a

Electrode surface is reached, a very high flow is measur ^¬ bar, which can be well evaluated and which is proportional to the number of charged particles in the exhaust gas. However, FIG. 2 also shows an ohmic resistor 7, which is advantageous in order to be able to measure the electrical measuring current with the evaluation electronics 9, which flows between the first electrode 2 and the second electrode 3. In towards ^¬ can be seen from a protective cap 10 in Figure 2, which is used for targeted guidance of the exhaust gas flow 17 through the soot sensor. 1 The exhaust gases can penetrate 11 into the soot sensor 1, where between the first electrode 2 and second electrode 3 of the soot content in the From ^¬ gas can be measured for example by a first voltage ^¬ Publ. Thereafter, the exhaust gas flow 17 leaves the soot sensor 1 through the second opening 12 formed in the second electrode 3, and is returned to the main exhaust gas flow via the third port 13.

In the insulation body 5, a guard electrode 16 can be detected. With the guard electrode 16 may be of about the niederge- from the gas stream on the insulating body 5 as condensate during a heating phase of cold ^¬ at the beginning of the soot sensor 1 water flowing measuring current, which forms due to the applied voltage between the first electrode 2 and the guard electrode 16, are evaluated as an indicator of the Funktionsfä ^¬ ability of the soot sensor 1.

The measuring current is driven by the voltage at the first electrode 2 (in this example 1 kV) across the moist insulating body to the guard electrode 16, which in this example is slightly biased at 0.5 V against the ground potential GND at the second electrode 3. The characteristic curve of the measuring current on the drying time and the heating up of the soot sensor can be evaluated with a diagnostic model ^¬ to and in accordance the course of the measuring current with the diagnostic model, the accuracy soot sensor loading can correct.

However, it is also conceivable to use a current measurement between the first electrode 2 and the second electrode 3, ie between 1 kV and the ground potential GND, in order to check the functionality of the soot sensor 1. Again, the cold and fully functional soot sensor 1 during the heating phase shows a characteristic measuring current that can be measured and compared with a diagnostic model.

Fig. 3 shows a soot sensor 1, which is adapted to be by means of the inventive method for Taupunktfreigabe Betrie ^¬ ben. The soot sensor 1 has a first electrode 2 and a second electrode 3. The soot particles can with the exhaust gas flow 17 through a first opening 11, which in the

Protective cap 10 is formed to penetrate into the interior of the Rußsen ^¬ sors. The soot particles enter the space between the first electrode 2 and the second Elek ^¬ trode 3. As long as the exhaust gas sensor 1 is cold, which z. B. after restarting the internal combustion engine will be the case, who ^¬ the water molecules present in the gas stream 17 as condensate in the entire soot sensor 1 deposited. This water condensate lies, inter alia, on the insulating body 5, wherein an electrically conductive connection between the first electrode 2 and the second electrode 3 is formed. By means of the voltage ^¬ supply 6 can be connected to the first electrode 2, a high voltage of z. B. 1000 V are created. The second electrode 3 is preferably kept at ground potential GND. Due to the condensate, which has formed on the surface of the insulating body 5, a large current through the insulation ^¬ body 5 begins to flow. This current, which flows due to the condensate on the insulating body between the first electrode 2 and the second electrode 3, shows a characteristic course over time until complete drying of the soot sensor 1. The current flowing through the insulating body 5 due to the aqueous condensate , makes a soot measurement with the wetted soot sensor impossible. Nevertheless, this current can be observed to determine the operability of the soot sensor. Until namely the soot sensor 1 due to the hot exhaust gas 17 heated up in such a way that the entire watery condensate removed from the interior of the soot sensor, is a characteristic curve of the Messtromes can bobachtet ^¬ the. For current measurement, the system shown in Fig. 3 on current measuring elements 8, which are connected to the transmitter 9. This evaluation 9 can record and evaluate the sensor currents shown in Figures 4 and 5. Another way to determine the functionality of the soot sensor 1, the measurement of the current flowing between the first electrode 2 of the soot sensor and the guard electrode 16. The guard electrode 16 is integrated in this example in the ceramic insulation body 5 of the soot sensor 1. The insulating body 5 does not necessarily consist of a ceramic. It is also conceivable, for. B. use a heat-resistant plastic or other insulating materials such. B. materials from the element carbon in a corresponding crystal lattice structure, which leads to a high isolation, or silicon materials. The voltage at the guard electrode 16 in this example is 0.5 V higher than the ground potential GND, which at the second electrode 3 is applied. The measurement of the current that flows through the insulating body 5 as a result of the aqueous condensate, now takes place between the guard electrode 16 and the first electrode 2. Again, a characteristic current that is at least one power of ten above the measuring current in the dried operation of the soot sensor 1, observed, while on the insulating body is an aqueous condensate before ^¬ hands. Once this aqueous condensate is dried off from the insulation ^¬ body 5 and from the interior of the soot sensor 1, the current through the insulating body suddenly collapses, after which the soot sensor 1 can accommodate the measuring operation.

This situation is illustrated by a current-time curve in FIG. The current I in A is plotted on the ordinate of the diagram in FIG. 4 and the time t in s on the abscissa. It can be seen two curves, the first cure ^¬ ve 19, the current signal from a first soot sensor represents, and the second curve 20 represents the current signal by a second soot sensor. Both soot sensors are installed in the exhaust system of a motor vehicle. The soot sensors are located at different locations in the exhaust line of the soot sensor. After the start of the internal combustion engine, the current signal 19 of the first soot sensor rises relatively steeply, wherein at point B a maximum is reached. From time 0 to time B at about 500 sec, a high amount of condensate is introduced through the exhaust gas flow in the first soot sensor 1, whereby a high current flows through the insulating body 5 of the first soot sensor. At time 500 sec, that is, at time B, however, the first soot sensor is so far heated by the exhaust gas flow that the complete drying of the condensate in Rußsen ^¬ sors is done. This can be clearly recognized by the steep drop of the current signal around point B. After point B at 500 sec, the soot sensor only shows a very small current signal which changes over time, which is due to the measurement of the soot particles in the exhaust gas flow 17. A similar picture is shown for the second soot sensor, whose current signal 20 is shown in the second curve. The second soot sensor is placed in the exhaust line at a higher distance from the engine than the first soot sensor, whereby the heating of the second soot sensor takes much longer time. From time 0 s, in which the cold engine was started, the current signal 20 of the second soot sensor increases relatively steeply up to a time ^¬ point C, which is approximately at 700 s. At this time C er ^¬ follows the drying of the second soot sensor, which is due to the steep drop in the current signal 20 of the second soot sensor in the range of 700 to 800 s after the cold start of the engine. After drying of the second soot sensor, the measuring phase of the soot sensor can also be initiated here. For comparison, FIG. 4 likewise shows the temperature curve 18 in the exhaust gas line of the internal combustion engine. It can be seen that the temperature in the exhaust system rises sharply after the cold start of the internal combustion engine and reaches a maximum after about 800 seconds and then settles to a constant value.

Fig. 5 shows a similar situation as Fig. 4 with a single soot sensor, which is arranged in the exhaust line of the internal combustion engine. In the upper diagram the temperature T is indicated on the ordinate, in the lower diagram the current I in A is indicated on the ordinate. Both abscissas show the time t in s. It can again be seen that after the restart of the internal combustion engine, an increase of the current signal 19 takes place. Here, however, it should be noted that initially only a very small current is to be measured up to a time of about 100 seconds, since obviously relatively little condensate has formed in the interior of the soot sensor 1, which then after the point A at about 200 s significantly changed, which can be seen by a significant increase in the current signal 19, to the point B. At about 400 s, the signal rises and then collapses abruptly, indicating that the soot sensor in

Dried inside, since the temperature of the exhaust gas stream has heated the soot sensor so that no further condensate sat more inside the soot sensor 1 can hold. At time B at about 400 s, the release of the fault-free soot sensor can take place and the soot sensor changes into the measuring phase. For comparison, the temperature over time is shown in the upper diagram of FIG. The temperature in the gas line from ^¬ rises after the cold start of the engine continuously, until after about 200 seconds to set a constant value of the exhaust gas temperature. The diagnostic model with which the actual current profile in the drying phase of the soot sensor after starting the Ver ^¬ combustion engine is compared can be determined on the one hand with a guaranteed error-free sensor and stored in a memory unit, but it can also be created as a mathematical diagnostic model , is recalculated with reference to ver ^¬ various parameters of the profile of the current of a fault-free soot sensor. If fact ^¬ plural measured current during the drying phase the soot ^¬ sensors differs from that according to the diagnostic model expected measuring current over a certain tolerance addition, the Rußsen ^¬ sor is classified as defective, which, for example, with an entry in the memory of the on-board diagnostics Unit can be doku ^¬ mentiert and / or can lead to an error message in the driver's display.

A mathematical diagnostic model can be described as follows:

mCond ⁼

I Guard ⁼ f ^ ¹ Cond ^ Gas Sensor) m _Co nd is the mass of the condensed water m _Ga s is the mass of the passing exhaust gas T _Gas Sensor is the temperature of the soot sensor on the inner

Surface . With these values, for example, the measuring current at the guard electrode I _Gu ar _d can be calculated.

Claims

Method for monitoring the function of an electrostatic soot sensor (1) with a first electrode (2) and a second electrode (3), the first electrode (2) and the second electrode (3) are electrically insulated from each other by an ^insulation body (5) and between the first electrode (2) and the second electrode (3) an electrical voltage can be applied and soot particles (4) with a gas flow into a space between the first electrode (2) and the second electrode (3) can reach, characterized in that during a heating phase of the initially cold soot sensor (1) the measuring current flowing over water that has been ^precipitated as condensate from the gas stream on the insulating body (5), which is due to the applied voltage between the first electrode (2) and the second electrode (3) is ^evaluated as an indicator of the functionality of the soot sensor (1). Method for monitoring the function of an electrostatic soot sensor (1) according to claim 1, characterized in that the functionality of the soot sensor (1) is recognized when the ^measuring current flowing between the first electrode (2) and the second electrode (3) follows a predetermined diagnostic model . Method for monitoring the function of an electrostatic soot sensor (1) with a first electrode (2) and a second electrode (3), the first electrode (2) and the second electrode (3) being electrically insulated from one another by an ^insulating body (5). and wherein the soot sensor (1) has a guard electrode (16) which is formed in and/or on the insulating body (5) and wherein an electrical voltage can be applied between the first electrode (2) and the guard electrode (16). is and soot particles (4) can reach ^a space between the first electrode (2) and the second electrode (3) with a gas stream, characterized in that during a heating phase of the initially cold soot sensor (1) the over from the Gas flow on the insulating body (5), water precipitated as condensate, measuring current, which is formed as a result of the applied voltage between the first electrode (2) and the guard electrode (16), is ^evaluated as an ^indicator for the functionality of the soot sensor (1). becomes. Method for monitoring the function of an electrostatic soot sensor (1) according to claim 3, characterized in that the functionality of the soot sensor (1) is recognized when the ^measuring current flowing between the first electrode (2) and the guard electrode (16) follows a predetermined diagnostic model. Method for monitoring the function of an electrostatic soot sensor (1) according to claim 3 or 4, so that the voltage at the guard electrode (16) is at a higher potential than the voltage at the second electrode (3). Method for monitoring the function of an electrostatic soot sensor (1) according to one of the preceding claims, characterized in that the insulation body (5) consists of a ceramic material.