Method for monitoring particle concentration in the exhaust gas of an internal combustion engine

Abstract

The invention relates to a method for monitoring a particle concentration in the exhaust gas of an internal combustion engine (110) during a measurement period using a resistive particle sensor (122). A measure for a permissible particle quantity is calculated using a complex model and compared with a measure for a measured particle quantity. The particle concentration in the exhaust gas is considered acceptable if the measure for the measured particle quantity is lower than the measure for the calculated permissible particle quantity. The invention provides that a chemical composition, in particular a hydrocarbon content, of the particles is taken into account using metrologically accessible parameters.

Description

State of the art

[0001] From EP 2 031 370 B1 of the applicant, a method for onboard diagnostics (OBD) of a particulate filter in the exhaust system of an internal combustion engine is already known by means of at least one resistive particulate sensor arranged downstream of the particulate filter in the exhaust system.It provides that, based on a predicted particle mass flow provided by an engine model, a limit filter model (which models what proportion of the predicted particle mass flow is retained in a limiting particle filter and what proportion passes through it and exits as the predicted particle mass flow limit), an adsorption model (which models the adsorption behavior of the particles present in the exhaust gas on the particle sensor, to which an exhaust gas volume flow, an exhaust gas temperature and a temperature of the particle sensor are supplied), and a conductivity model, a predicted signal change is determined, which, due to the use of the limit filter model, represents the signal change expected for a limiting particle filter, and that in a comparison stage the predicted signal change is compared with a measured signal change of the particle sensor.Furthermore, it stipulates that a defective particle filter is inferred if the measured signal change is higher than the predicted signal change, whereby the measured signal change of the particle sensor and / or the predicted signal change of the particle sensor is corrected taking into account influencing factors on cross-sensitivities of the particle sensor.

[0002] EP 2 031 370 B1 further discloses that the exhaust gas temperature, the temperature of the particle sensor and / or the exhaust gas volume flow and / or the influence of the gaseous exhaust gas components nitrogen oxides, hydrocarbons and / or water vapor are taken into account as influencing factors. Disclosure of the invention

[0003] The inventors have recognized that the electrical conductivity of particles located on a sensor element in the exhaust gas of an internal combustion engine depends not only on the mass of the deposited particles but also on their chemical composition, in particular on the hydrocarbon content of the particles.

[0004] For example, it was observed that the electrical conductance between two electrodes on a sensor element, isolated from each other, decreases when the sensor element is actively heated for a certain period of time by its electric heater. It was found that this is not a dependence of the conductance on the current temperature, as the decrease in conductance could still be observed after the sensor element cooled down. Instead, the observation should be interpreted as meaning that the heating altered the chemical composition of the particles, in particular reducing their hydrocarbon content, since hydrocarbons contained within the particles evaporated as a result of the temperature increase associated with heating the sensor element.

[0005] In order to reliably monitor a particle concentration in the exhaust gas of an internal combustion engine during a measurement period in which a permissible particle concentration in the exhaust gas is specified with time resolution, using a resistive particle sensor, even under boundary conditions where the chemical composition of the particles in the exhaust gas fluctuates, the invention provides to clean the sensor element of particles by burning them off before the start of the measurement period, to expose the sensor element to the exhaust gas during the measurement period so that particles are deposited on it, resulting in an electrically conductive connection between the two electrodes, and to measure the conductance of the electrical connection between the two electrodes during or at the end of the measurement period.In this process, a measure for a permissible particle quantity during or at the end of the measurement period is calculated from the time-resolved specified permissible particle concentration in the exhaust gas and from an accumulation model for the accumulation of particles on the particle sensor in the exhaust gas, and a measure for the measured particle quantity is determined from a measured conductance based on a conductivity model that takes into account the temperature of the particles and / or the particle sensor and / or the exhaust gas during or at the end of the measurement period.Furthermore, during or at the end of the measurement period, the measure for the measured particle quantity is compared with the calculated measure for the permissible particle quantity, and the particle concentration in the exhaust gas is assessed as acceptable if the measure for the measured particle quantity between the two electrodes is smaller than the measure for the calculated permissible particle quantity; and / or the particle concentration in the exhaust gas is assessed as unacceptable if the measure for the measured particle quantity between the two electrodes is larger than the measure for the calculated permissible particle quantity.

[0006] Furthermore, for this purpose, in a first alternative within the conductivity model, a change in the chemical composition, in particular a hydrocarbon content, of the particles already deposited on the sensor element is taken into account using one of the following parameters: a temperature or temperature profile of the particles during the measurement period and / or a temperature or temperature profile of the particle sensor during the measurement period and / or a temperature or temperature profile of the exhaust gas during the measurement period and / or a quantity of heat introduced into the sensor element by electrical heating during the measurement period and / or a maximum heating power introduced into the sensor element by electrical heating during the measurement period, a quantity of heat introduced into the sensor element by the exhaust gas;Dynamic parameters of the operation of the internal combustion engine during the measurement period, from which the exhaust gas temperature during the measurement period can be determined.

[0007] Naturally, in a second alternative, changes in the chemical composition, particularly the hydrocarbon content, of particles already deposited on the sensor element can be taken into account not only within the conductivity model but also when calculating the permissible particle quantity. This could be done, for example, with an adjusted sign compared to the first alternative, as an inverse value, or similar. In both alternatives, a subsequent comparison of the measured particle quantity with the calculated permissible particle quantity, along with monitoring the particle concentration, will naturally yield the same result.

[0008] The sensor element can, for example, include an electric resistance heater. This can be used, for instance, to burn off any particles on the sensor element.

[0009] The method according to the invention provides for measuring the conductance of the electrical connection between the two electrodes during or at the end of the measurement period. Naturally, the conductance of the electrical connection between the two electrodes can also be measured during the measurement period, for example, continuously or periodically. For instance, the measurement period can be terminated when the measurement reaches a specific predetermined value. The criterion for this could be, for example, reaching a predetermined current flow (e.g., a current flow of 12 µA) between the electrodes of the sensor element at a voltage of 45 V applied between them.

[0010] The method according to the invention provides that a measure for a permissible particle quantity during or at the end of the measurement period is calculated from the time-resolved, predetermined permissible particle concentration in the exhaust gas and from an accumulation model for particle accumulation on the particle sensor in the exhaust gas. The accumulation model can, for example, take into account that the proportion of particles in the exhaust gas that accumulate on the sensor element depends on the exhaust gas flow velocity. It can also take into account that the flow velocity changes over time during the measurement period.

[0011] The deposition model can, for example, take into account that the proportion of particles in the exhaust gas that are deposited on the sensor element depends on the temperature of the exhaust gas and / or the sensor element, for example due to thermophoresis. It can also consider that these temperatures change over time during the measurement period.

[0012] The deposition model can, for example, take into account that the proportion of particles in the exhaust gas that are deposited on the sensor element depends on an electrical potential at which one or both electrodes are located, for example due to electrophoresis.

[0013] The method according to the invention provides that a measure of the measured particle quantity is determined from the measured conductivity based on a conductivity model that takes into account the temperature of the particles and / or the particle sensor and / or the exhaust gas during or at the end of the measurement period. The conductivity model can, for example, consider how the conductivity of particles increases with rising temperature at that time and calculate from the measured conductivity value at that time the conductivity that the collected particles would have at a given temperature at that time. Subsequently, starting from the conductivity that the collected particles would have at a given temperature at that time, the conductivity model can deduce the measure of the measured particle mass.In this sense, the conductivity model can interpret the temperature of the sensor element or the exhaust gas at that time, possibly after a correction, as the temperature of the particles at that time, especially if a direct measurement of the temperature of the particles at that time is not possible.

[0014] According to the invention, a change in the chemical composition, in particular the hydrocarbon content, of the particles already deposited on the sensor element is taken into account using one of the following parameters: a temperature or temperature profile of the particles during the measurement period and / or a temperature or temperature profile of the sensor element during the measurement period and / or a temperature or temperature profile of the exhaust gas during the measurement period and / or an amount of heat introduced into the sensor element by electrical heating during the measurement period and / or a maximum heating power introduced into the sensor element by electrical heating during the measurement period, an amount of heat introduced into the sensor element by the exhaust gas; dynamic parameters of the operation of the internal combustion engine during the measurement period, from which the exhaust gas temperature during the measurement period can be determined.

[0015] A temperature profile can, for example, include the results of a large number of temperature measurements taken during the measurement period, e.g., periodically.

[0016] The dynamic parameters of the operation of the internal combustion engine during the measurement period, from which the exhaust gas temperature during the measurement period can be determined, can be, for example, speed and / or torque, or include speed and / or torque.

[0017] The invention takes into account changes in the chemical composition, particularly the hydrocarbon content, of particles already deposited on the sensor element. This can be done within the conductivity model in a self-explanatory manner by evaluating the actual mass of the collected particles based on their chemical composition for the measured conductivity.

[0018] Additionally or alternatively, this can also be taken into account when calculating the permissible particle quantity. The calculated permissible particle quantity is then corrected so that it subsequently indicates the value of a particle quantity that, for a given chemical composition (e.g., pure carbon without hydrocarbons), has the same electrical conductivity as the calculated permissible particle quantity before correction for the actual chemical composition, for example, the actual hydrocarbon content.

[0019] The change in the chemical composition of the particles can be taken into account in such a way that the value of a decrease in the hydrocarbon content of the particles is greater the larger the value of the hydrocarbon content and / or the higher the size taken into account.

[0020] Additionally, it may be provided that within the conductivity model and / or in the calculation of the permissible particle quantity, the chemical composition, in particular a hydrocarbon content, of particles is already taken into account at the time of their deposition on the sensor element, based on one of the following parameters: a temperature of the particles at the time of their deposition on the sensor element and / or a temperature profile of the sensor element at the time of the deposition of the particles on the sensor element, a temperature of the exhaust gas at the time of the deposition of the particles on the sensor element; dynamic parameters of the operation of the internal combustion engine at the time of the deposition of the particles on the sensor element, from which the exhaust gas temperature at the time of the deposition of the particles on the sensor element can be determined.

[0021] It may be provided that the value of the hydrocarbon content of the particles at the time of their deposition on the sensor element is taken into account less the higher the size taken into account at the time of deposition of the particles on the sensor element.

[0022] Additionally, it may be provided that within the conductivity model and / or in the calculation of the permissible particle quantity, the chemical composition, in particular the hydrocarbon content, of particles at the end of the measurement period is taken into account using one of the following parameters: a maximum temperature of the particles in the time interval between the time of deposition of the particles on the sensor element and the end of the measurement period; and / or a maximum temperature of the sensor element in the time interval between the time of deposition of the particles on the sensor element and the end of the measurement period;a maximum temperature of the exhaust gas in the time interval between the time of deposition of the particles on the sensor element and the time of the end of the measurement period: maximum values ​​of dynamic parameters of the operation of the internal combustion engine in the time interval between the time of deposition of the particles on the sensor element and the time of the end of the measurement period, wherein the exhaust gas temperature in the time interval between the time of its deposition on the sensor element and the time of the end of the measurement period can be determined from the dynamic parameters of the operation of the internal combustion engine;

[0023] It can also always be stipulated that the value of the hydrocarbon content of the particles is given less weight the larger the size being considered.

[0024] An important application of the method according to the invention aims at diagnosing a particulate filter located upstream of the particulate sensor in the exhaust gas of an internal combustion engine. In this context, the permissible particulate concentration in the exhaust gas is determined by an engine model for the internal combustion engine and by a limit filter model for a particulate filter that is just barely considered to be functioning correctly. Depending on the arrangement in a vehicle, the output values ​​of the engine model can represent the input values ​​of the limit filter model for the particulate filter.

[0025] In this application, the particulate filter in the exhaust gas is rated as OK if the particle concentration in the exhaust gas is rated as OK. Additionally or alternatively, the particulate filter in the exhaust gas is rated as not OK if the particle concentration in the exhaust gas is rated as not OK.

[0026] Another application of the method aims at diagnosing the internal combustion engine, in whose exhaust gas the particulate sensor is located. The particulate sensor can, for example, be positioned upstream of a particulate filter, thus being exposed to the raw emissions of the internal combustion engine. However, it can also be located in an exhaust section of the internal combustion engine where no particulate filter is present. In this context, the permissible particulate concentration in the exhaust gas from an engine model is directly determined for the internal combustion engine. This is based on an engine model that predicts the emission of particulate matter at the level considered just within acceptable limits.

[0027] In this application, the internal combustion engine is rated as OK if the particle concentration in the exhaust gas is rated as OK. Additionally or alternatively, the internal combustion engine is rated as not OK if the particle concentration in the exhaust gas is rated as not OK.

[0028] The measurement phase can, in principle, include phases in which the internal combustion engine is active, i.e., producing exhaust gases, as well as phases in which the internal combustion engine is inactive, i.e., not producing exhaust gases, so-called engine shutdown phases. For example, a measurement phase can include one or more engine shutdown phases, whereby, after the engine shutdown phase or after all engine shutdown phases, the measurement phase includes at least one phase in which the internal combustion engine is active again, i.e., generates forces through combustion that cause the movement of a part of the internal combustion engine and / or a vehicle part and / or the vehicle itself.

[0029] On the one hand, such engine shutdown phases can be rather short in duration. After these phases, the inventive method can be continued without further ado once the internal combustion engine is producing exhaust gas again.

[0030] On the other hand, such engine shutdown phases can also be rather long. In this case, there is a risk that the sensor element and the particles located between its electrodes will become damp, i.e., that water will adhere to them and, during or after the engine shutdown phase, represent an additional cause of conductivity between the electrodes of the sensor element. It can then no longer be assumed that the measured conductivity is simply due to particle adhesion on the sensor element.

[0031] In a further development of the invention, in the case where the measurement phase includes at least one engine shutdown phase in which the internal combustion engine is switched off so that it does not produce any exhaust gas, it can be provided that an evaluation is carried out on the basis of predetermined criteria as to whether drying of the sensor element by active heating of the sensor element is necessary after the engine shutdown phase.

[0032] Following such an engine shutdown phase, a change in the chemical composition, in particular a hydrocarbon content, of particles that were already deposited on the sensor element before the engine shutdown phase can be taken into account within the conductivity model and / or in the calculation of the measure of the permissible particle quantity, whereby the change is particularly attributable to the drying or moistening of the sensor element that took place during the engine shutdown phase.

[0033] For example, a change in the chemical composition, in particular a hydrocarbon content, of particles that are already deposited on the sensor element before the engine shutdown phase and before drying can be taken into account using one of the following parameters: a temperature profile of the particles during drying and / or a temperature profile of the sensor element during drying and / or a quantity of heat introduced into the sensor element by electrical heating during the measurement period and / or a maximum heating power introduced into the sensor element by electrical heating during the measurement period.

[0034] Exemplary embodiments of the invention are explained below with reference to the drawing. The drawing shows: The Fig. 1 by way of example an environment in which the present invention can be carried out; The Fig. 2 by way of example a sensor element of a particle sensor as it can be used in the method according to the invention; The Fig. 3 a block diagram of a process flow underlying the invention for monitoring a particle filter; The Fig. 4. A time course of several quantities.

[0035] In Fig. Figure 1 shows an exemplary environment in which the present invention can be implemented. A vehicle is schematically depicted and designated collectively by reference numeral 100. The vehicle 100 comprises an internal combustion engine 110, for example a diesel engine, which includes six cylinders with reciprocating pistons, as indicated in the drawing. The vehicle 100 further comprises an exhaust system 120, which includes a catalyst 124 and a particulate filter 122, as well as a processing unit 130, which is configured to control the internal combustion engine 110 and the exhaust system 120 and is connected to them via a data transmission link. Furthermore, in the illustrated example, the processing unit 130 is connected via a data transmission link to sensors 121, 123, 127, which detect operating parameters of the internal combustion engine 110 and / or the exhaust system 120. A particulate sensor 127 is arranged downstream of the particulate filter 122, which is configured to detect particles present in the exhaust gas passing through the exhaust system 120.

[0036] It is understood that additional sensors may be present that are not shown. The exhaust system 120 may also include further cleaning components, such as additional particulate filters and / or additional catalytic converters, which are not shown here for the sake of simplicity. Variations in the order of the components are also possible within certain limits.

[0037] In the example shown here, the computing unit 130 includes a data storage unit 132, in which, for example, calculation instructions and / or parameters (e.g., threshold values, characteristic values ​​of the internal combustion engine 110 and / or the exhaust system 120, etc.) can be stored.

[0038] The internal combustion engine 110 drives wheels 140 to move the vehicle 100, and can also be driven by the wheels 140 in certain operating phases (e.g. so-called overrun mode).

[0039] The Fig. Figure 2 shows an example of a sensor element 10 of a particle sensor 127, as it can be installed in the exhaust system 120 of a vehicle 100 downstream of a diesel particulate filter 122 in the method according to the invention, see also Fig. 1.

[0040] The sensor element 10 has a first support layer 12 and a second support layer 18, each of which can be based on a ceramic film and, for example, consist of zirconium oxide. A heating element 14 is integrated between the support layers 12. This heating element can be connected via contacts 16 to a control unit 26 or a suitable voltage source and serves to burn off any deposited particles, such as particulate matter, from the sensor element 10. An insulating layer 72c, 72d, which can be made of Al₂O₃ using printing technology, is arranged between the heating element 14 and the support layers 12 and 18.

[0041] On the second support layer 18, an insulating layer 72e, which is, for example, printed from Al2O3, is arranged, and on its surface 90, a structure consisting of two electrodes 20, 22 is arranged. For example, the electrodes 20, 22 are designed as interdigital electrodes, so that they interlock in a comb-like manner as shown. The electrodes 20, 22 can be connected to an electronic control unit 26 via contacts 24. Leads between the electrodes 20, 22 and the contacts 24 can be covered by an insulating layer 72f, which can be printed from Al2O3.

[0042] In the Fig. 2 Below the first support layer 12, a temperature measuring element 71 is arranged, which can also be connected to the electronic control unit 26. Immediately above and below the temperature measuring element 71, an insulating layer 72a, 72b is arranged, which can be made, for example, of Al2O3 using printing technology.

[0043] The particle sensor 127 can further comprise a housing which is located in the Fig. The sensor element 10 shown in section 2 is not shown to simplify the explanation of the structure. For example, the housing can be designed as a retaining sleeve which has an opening in an area located above the electrodes 20, 22 and serves to calm a gas flow flowing in the exhaust system.

[0044] The basic operating principle of sensor element 10 according to the Fig. 2 consists of the following. If 90 electrically conductive particles are present on the surface, the electrical conductance between the two electrodes 20, 22 increases after some time. This conductance can be measured by applying a voltage to the electrical contacts and detecting the resulting current flow.

[0045] To regenerate the sensor element 10, the accumulated particles can be burned off after a certain time using the heating element 14.

[0046] The Fig. Figure 3 shows a block diagram of a process flow underlying the invention for monitoring a particulate filter of an internal combustion engine.

[0047] An engine model 201 determines the current particle emission of the internal combustion engine based on input variables currently measured on an internal combustion engine 110, for example speed, torque and possibly lambda, and based on predefined mathematical functions or characteristic maps.

[0048] The current particle emission of the internal combustion engine 110 is the input variable of a limit filter model 202. The limit filter model 202 models the quantity or proportion of particles that pass through a particle filter 122 and are considered acceptable. A particle filter 122 with a lower particle permeability than represented by the limit filter model is also considered acceptable. Conversely, a particle filter 122 with a higher particle permeability than represented by the limit filter model is identified as a fault. Therefore, in this example, the limit filter model always determines a currently permissible (or, in terms of the limit value of the limit filter model, a maximum permissible) particle concentration in the exhaust gas for a region downstream of the particle filter 122.

[0049] Furthermore, the measurement of the permissible particle deposition rate downstream of the particulate filter 122 by a particle sensor 127 is modeled using a deposition model 203. This takes into account that only a certain proportion of the particles that have passed through the particulate filter 122 deposition on the particle sensor 127. The size of this proportion depends, for example, on the exhaust gas velocity, the exhaust gas temperature, and the temperature of the sensor element 10. The values ​​of these quantities can result from measurements of the instantaneous values ​​or can also be calculated by models, for example, from an exhaust gas temperature model 204 and / or a volume flow model 205 for the exhaust gas. If necessary, further measured or calculated influencing factors are included in the deposition model 203.

[0050] The permissible particle deposition rate can be calculated in any unit, for example as the reciprocal of the trigger time. The trigger time is the time after which the particle sensor should be considered loaded if deposition continues unchanged at the specified particle deposition rate.

[0051] The permissible particle quantity is derived from the permissible particle deposition rate by temporal integration (203S). The integration can start, for example, at a point in time shortly after sensor regeneration. The upper limit of the integration can be, for example, at the current time.

[0052] If the reciprocal trigger time is chosen as the unit of particle deposition rate, then the permissible particle quantity indicates the proportion of the currently calculated permissible particle quantity to the particle quantity at which, for example, the particle sensor (see below) should be triggered. This proportion can also be denoted as R.

[0053] A particle sensor 127 located in the exhaust gas stream (see also Fig. 1) On the other hand, it provides measured values ​​that indicate a certain conductance between its electrodes 20, 22 (see Fig. 1) represent. For example, a voltage of, say, 45 V can be applied between electrodes 20 and 22, and a current flow that develops after some time, which is, for example, in the range of a few µA, can be measured. The current flow then represents the conductance directly or after calculation, for example, after division by the applied voltage.

[0054] A conductivity model 206 is used to calculate a measure of the particle quantity present at the particle sensor 127, or between the electrodes. In addition to conductance, the particle sensor 127 can also provide a temperature value as input, for example, the temperature of the sensor element 10. It can be taken into account that the conductance of a given particle quantity between the electrodes 20, 22 is higher the hotter the particles are. The temperature of the particles, on the other hand, can be approximated by the temperature of the sensor element 10.

[0055] At the appropriate time, a comparison can be made between the measure for the particle quantity and the measure for the permissible particle quantity. For this purpose, a comparator 208 is activated, for example, when the value of the measure for the permissible particle quantity has reached a predefined value, such as a trigger quantity. If the reciprocal of the trigger time is chosen for the unit of particle deposition rate, the trigger quantity is given by the dimensionless number 1.

[0056] The trigger quantity can be determined, for example, by the quantity of particles between electrodes 20, 22 of the particle sensor, which under certain conditions, for example at 20°C, is associated with a conductance of 12 µA / 45V.

[0057] The result of the comparison is crucial in determining whether the particulate filter is rated as satisfactory or unsatisfactory: The particulate filter is rated as satisfactory if the measured particle quantity is less than the calculated permissible particle quantity. Otherwise, in this example, it is rated as unsatisfactory.

[0058] For example, this comparison can be made at the point when the calculated measure for the permissible particle quantity reaches a value of 12 µA. If the measured particle quantity is then greater than 12 µA, the particle filter is rated as defective; however, if the measured particle quantity is not greater than 12 µA at this point, the particle filter is rated as functioning correctly. The measurement period can be considered or ended at the time of the comparison. Alternatively, it can also be continued.

[0059] The Fig. Figure 4 shows a time course of several quantities within the context of an embodiment of the present invention.

[0060] Part A) shows the maximum permissible particle quantity in units of R (see above). Time t0 is immediately after regeneration of the sensor element; therefore, no particles have yet been deposited on the sensor element, and the permissible particle quantity is accordingly set to, for example, 0 by the model.

[0061] Part B) shows the temperature of the sensor element in degrees Celsius. It can be assumed to be a good approximation that the temperature of the sensor element corresponds to the temperature of the particles. Furthermore, the temperature of the sensor element follows the temperature of the exhaust gas, albeit with some inertia. At time t0, the temperature of the sensor element in this example is 300 °C.

[0062] Part C) shows the mass-related hydrocarbon content of the deposited particles. Since the temperature of the sensor element is relatively high at time t0 (300 °C), the hydrocarbon content of the particles deposited in the interval [t0, t1] is comparatively small, 5% in this example (see the dashed line in Part C) between times t0 and t1).

[0063] At times t1 and t2, the temperature of sensor element 10 is only 100 °C. Consequently, the hydrocarbon content of the particle deposited on sensor element 10 at these times is higher, namely 15% (see dashed line in part C) between times t1 and t2.

[0064] In the interval between time points t2 and t3, the temperature of sensor element 10 rises again to up to 200 °C. Consequently, the hydrocarbon content of the particles deposited on sensor element 10 at these times is again lower, namely 7.5% (see dashed line in part C) between time points t2 and t3.

[0065] The increased temperature of sensor element 10 in the interval between times t2 and t3 not only results in a lower concentration of hydrocarbons in the particles deposited in this interval, but also in a decrease in the concentration of particles deposited on the sensor element in the interval between times t1 and t2. The model assumes that this reduction is reliably equivalent to the concentration of particles deposited on the sensor element in the interval between times t2 and t3. In Part C), this is represented by the solid line between times t1 and t2.

[0066] In the interval between times t3, t4, and t5, the temperature of sensor element 10 decreases again. The hydrocarbon content of the particles deposited at these times is therefore comparatively high, as shown by the dashed lines. Since the particles do not undergo any further heating during the depicted time period, the hydrocarbon content of the particles deposited at these times does not decrease further. The same applies to the particles deposited between times t0 and t1. This is indicated in the diagram by a solid line at the same level as the dashed line in these intervals.

[0067] It has been observed that particles with a high hydrocarbon content exhibit reduced conductivity when their hydrocarbon content is reduced by temperature. Within the scope of the present invention, this reduced conductivity occurs at the actual particle sensor 127, for example, through a reduced current flowing between electrodes 21 and 22 when a certain voltage is applied.

[0068] To compensate for this influence, an additional weighting factor is introduced when determining the permissible particle mass; this factor is higher the higher the hydrocarbon content of the particles adhering to the sensor element 10. In the Fig. 4. This can be seen from the fact that in part A), the gradient of the increase in the permissible particle mass in the individual time intervals is greater the higher the hydrocarbon content of the particles is in these time intervals. As can be seen exemplified in the time interval [t1, t2], the hydrocarbon content of the particles at the time of their attachment to the assortment is not necessarily relevant, since this content can decrease again at a later time if the temperature of the sensor element 10 reaches a temperature that is higher than the temperature of the sensor element 10 at the time of the particles' attachment to the sensor element 10.

[0069] In other words, in this example, each temperature of sensor element 10 is assigned a certain hydrocarbon content to the particles. The hydrocarbon content of the particles attached to sensor element 10 is then assumed to be that corresponding to the highest temperature the sensor element reaches at the time of particle attachment or later until the end of the measurement period. This reflects the idea that while hydrocarbons in the particles can evaporate when the particle or sensor element is heated, they do not return when the particles or sensor element cool down again.

[0070] Of course, more precise and therefore more complex approaches than the one described above are also possible. For example, it can be taken into account that the reduced hydrocarbon content of the particles, corresponding to a certain elevated temperature, is only actually reached over longer periods. It can also be considered that if the elevated temperature acts on the particles for only a short period, the hydrocarbon content will not be reduced to this value within that short time. Such dynamic effects can be described, for example, using rate equations, such as the Arrhenius equation.

[0071] Of course, alternative approaches are also possible for modeling the chemical composition of the particles or the resulting electrical properties (e.g., specific conductivity). According to one such approach, average temperatures of the particles, the sensor element, and / or the exhaust gas can be calculated during the measurement period, for example, as moving averages over the measurement period. Optionally, the temperatures included in the average calculation can be weighted by the time-resolved permissible particle concentration in the exhaust gas. The assumed chemical composition of the particles can be determined during or at the end of the measurement period as a function of such average values.

[0072] Other approaches may be based on the use of neural networks.

[0073] In the example shown, the conductivity of the deposited particles is compensated for due to their hydrocarbon content within the calculation of the permissible particle quantity. Alternatively, it is of course also possible to perform the correction analogously within the conductivity model 206. The actual deposited particle mass is then inferred from the conductivity actually measured at the sensor element 10, irrespective of the particles' chemical composition. The comparison of the measured particle quantity with the calculated permissible particle quantity by the comparator 208 (see [reference]) is performed by the method. Fig. 3) naturally leads to the same result in both cases. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] EP 2 031 370 B1 [0001, 0002]

Claims

[1] Method for monitoring a particle concentration in the exhaust gas of an internal combustion engine (110) during a measurement period using a resistive particle sensor (127), wherein a permissible particle concentration in the exhaust gas is specified with time resolution during the measurement period, wherein the particle sensor (127) has a sensor element (10) with two electrodes (20, 22) that are insulated from each other, wherein the sensor element (10) is cleaned of particles by burning before the start of the measurement period, wherein the sensor element (10) is exposed to the exhaust gas during the measurement period so that particles are deposited on it, resulting in an electrically conductive connection between the two electrodes (20, 22), wherein the conductance of the electrical connection between the two electrodes (20, 22) is measured during the measurement period or at the end of the measurement period,wherein a measure for the permissible particle quantity is calculated from the time-resolved specified permissible particle concentration in the exhaust gas and from an accumulation model for the accumulation of particles on the particle sensor (127) in the exhaust gas during or at the end of the measurement period, wherein a measure for the measured particle quantity is determined from the measured conductance based on a conductivity model that takes into account the temperature of the particles and / or the particle sensor (127) and / or the exhaust gas during or at the end of the measurement period, wherein the measure for the measured particle quantity is compared with the calculated measure for the permissible particle quantity during or at the end of the measurement period, and the particle concentration in the exhaust gas is considered acceptable if the measure for the measured particle quantity between the two electrodes (20,22) is smaller than the calculated measure for the permissible particle quantity; and / or the particle concentration in the exhaust gas is assessed as not acceptable if the measure for the measured particle quantity between the two electrodes (20, 22) is larger than the calculated measure for the permissible particle quantity, , characterized by, that within the conductivity model and / or in the calculation of the measure for the permissible particle quantity, a change in the chemical composition, in particular a hydrocarbon content, of particles already deposited on the sensor element is taken into account by means of one of the following quantities: a temperature or temperature profile of the particles during the measurement period and / or a temperature or temperature profile of the sensor element (10) during the measurement period and / or a temperature or temperature profile of the exhaust gas during the measurement period and / or a quantity of heat introduced into the sensor element (10) by electrical heating during the measurement period and / or a maximum heating power introduced into the sensor element (10) by electrical heating during the measurement period, a quantity of heat introduced into the sensor element (10) by the exhaust gas;dynamic parameters of the operation of the internal combustion engine (110) during the measurement period, from which the exhaust gas temperature during the measurement period can be determined.; [2] Method according to claim 1, characterized by, that within the conductivity model and / or in the calculation of the measure for the permissible particle quantity, the chemical composition, in particular a hydrocarbon content, of particles at the time of their deposition on the sensor element (10) is taken into account on the basis of one of the following parameters: a temperature of the particles at the time of their deposition on the sensor element (10) and / or a temperature of the sensor element (10) at the time of deposition of the particles on the sensor element (10), a temperature of the exhaust gas at the time of deposition of the particles on the sensor element (10); dynamic parameters of the operation of the internal combustion engine at the time of deposition of the particles on the sensor element (10), from which the exhaust gas temperature at the time of deposition of the particles on the sensor element (10) can be determined. [3] Method according to claim 1 or 2, characterized by, that within the conductivity model and / or in the calculation of the measure for the permissible particle quantity, the chemical composition, in particular a hydrocarbon content, of the particles at the time of the end of the measurement period is taken into account using one of the following parameters: a maximum temperature of the particles in the time interval between the time of deposition of the particles on the sensor element (10) and the time of the end of the measurement period; and / or a maximum temperature of the sensor element (10) in the time interval between the time of deposition of the particles on the sensor element (10) and the time of the end of the measurement period;a maximum temperature of the exhaust gas in the time interval between the time of deposition of the particles on the sensor element (10) and the time of the end of the measurement period: Maximum values ​​of dynamic parameters of the operation of the internal combustion engine (110) in the time interval between the time of deposition of the particles on the sensor element (10) and the time of the end of the measurement period, wherein the exhaust gas temperature in the time interval between the time of its deposition on the sensor element (10) and the time of the end of the measurement period can be determined from the dynamic parameters of the operation of the internal combustion engine (110). [4] A method according to any of the preceding claims, wherein a particulate filter (122) is arranged upstream of the particulate sensor (127) in the exhaust gas of the internal combustion engine (110), and wherein the measure for the permissible particulate concentration in the exhaust gas is determined from an engine model for the internal combustion engine (110) and based on a limit filter model for a particulate filter (122) that is just barely acceptable, and wherein the particulate filter (122) located in the exhaust gas is considered acceptable if the particulate concentration in the exhaust gas is considered acceptable; (110) and / or wherein the particulate filter (122) located in the exhaust gas is considered unacceptable if the particulate concentration in the exhaust gas is considered unacceptable. [5] Method according to any of the preceding claims, wherein the measurement phase includes at least one engine shutdown phase in which the internal combustion engine (110) is switched off so that it does not produce any exhaust gas. [6] Method according to the preceding claim, wherein an assessment is carried out on the basis of predetermined criteria as to whether drying of the sensor element (10) by active heating of the sensor element (10) is required following the motor shutdown phase. [7] Method according to the preceding claim, wherein, in the case where it is assessed that drying of the sensor element (10) by active heating is required following the motor shutdown phase, drying of the sensor element (10) following the motor shutdown phase is carried out by active heating of the sensor element (10) during a drying phase which is included in the measurement period. [8] Method according to the preceding claim, wherein a change in the chemical composition, in particular a hydrocarbon content, of particles that are already deposited on the sensor element (10) before the engine shutdown phase and before drying is taken into account by means of one of the following parameters: a temperature or temperature profile of the particles during drying and / or a temperature or temperature profile of the sensor element (10) during drying and / or a quantity of heat introduced into the sensor element (10) by electrical heating during the measurement period and / or a maximum heating power introduced into the sensor element (10) by electrical heating during drying.

Images