DE102011017486A1 - Operating method for a motor vehicle diesel engine with an emission control system - Google Patents

Abstract

The invention relates to an operating method for a motor vehicle diesel engine with an exhaust gas cleaning system comprising a three-way catalytic converter (34) and an SCR catalytic converter (36) arranged one behind the other in the flow direction of the exhaust gas. In the method according to the invention, the diesel engine (1) is in a first operating range (C, D, E, F), in which the SCR catalytic converter (34) falls below a predetermined minimum temperature, at least temporarily with an air-fuel ratio of approximately λ = 1 , 0 operated. In a second operating range (G), in which the SCR catalytic converter (34) exceeds the predeterminable minimum temperature, the diesel engine (1) is operated with an excess of air that is typical for normal diesel engine operation. According to the invention, to set the air-fuel ratio (λ) in the first operating range (C, D, E, F), an output signal correlating with a NOx concentration of the exhaust gas from an exhaust gas sensor arranged downstream of the three-way catalytic converter (34) is used.

Description

The invention relates to an operating method for a motor vehicle diesel engine with an exhaust gas purification system comprising in the flow direction of the exhaust gas arranged one behind the other a three-way catalyst and an SCR catalyst, wherein the diesel engine in a first operating range, in which the SCR catalyst a predetermined minimum temperature is at least temporarily operated with an air-fuel ratio of about λ = 1.0, and in a second operating range in which the SCR catalyst exceeds the predetermined minimum temperature, is operated with a typical for normal diesel engine operation excess air.

From the DE 10 2009 015 900 A1 a generic operating method is known, although it is not discussed how the non-typical for a diesel engine operation stoichiometric air-fuel ratio of about λ = 1.0 is set. This has proved to be difficult, in particular in the case of a comparatively slightly heated exhaust-gas cleaning system, since lambda sensors often used for this purpose provide corrupted measured values.

The object of the invention is therefore to provide an operating method which allows an accurate and reliable setting of a stoichiometric air-fuel ratio of about λ = 1.0, in particular at comparatively low heated exhaust gas purification system.

This object is achieved by an operating method with the features of claim 1. Thereafter, an output signal of an exhaust gas sensor arranged downstream of the three-way catalyst is used to adjust the air-fuel ratio in the first operating region. The invention is based on the finding that an NOx conversion effected by the three-way catalyst is sensitive to the air-fuel ratio, especially in a comparatively narrow range around the stoichiometric air-fuel ratio of λ = 1.0. As a result of the inventively used output signal of a downstream of the three-way catalyst arranged in the exhaust gas purification system exhaust gas sensor which provides an correlating with a NOx concentration of the exhaust gas output signal, thus a statement about the air-fuel ratio can be obtained and set this.

In this context, the λ-value characterizing the air / fuel ratio is, as usual, to be understood as a ratio of the amount of oxygen actually present in the combustion air / fuel mixture to the theoretically minimum required oxygen quantity for complete combustion of the fuel. A lean air-fuel mixture with an excess of air therefore has a λ-value of greater than one. A rich air-fuel mixture with a fuel surplus, however, has a λ-value of less than one. In the absence of oxygen sources or sinks in the exhaust system, the lambda value in the exhaust gas (exhaust gas λ) corresponds to the lambda value of the air-fuel mixture (combustion λ), with which the engine is operated. For simplicity's sake, therefore, only one λ value or shorter λ will be discussed below if no differentiation is required.

The three-way catalyst is a catalyst capable of removing both nitrogen oxides (NOx) and reducing exhaust gas constituents such as carbon monoxide (CO) and hydrocarbons (HC) from the exhaust gas within a narrow range of λ = 1.0 , Catalyst formulations with or without an oxygen storage capacity which perform this are known to the person skilled in the art, in particular from applications relating to the exhaust gas purification of gasoline engines, which is why this is not specifically dealt with here. The three-way catalyst may also be a classic (diesel) oxidation catalyst containing a catalyst material with the said three-way properties. As NOx are summarized at least the nitrogen oxides NO and NO ₂ understood.

The SCR catalyst is a catalyst which can selectively and continuously reduce NOx under oxidizing conditions, ie at λ> 1.0, by means of ammonia (NH ₃ ). Between the three-way catalyst and the SCR catalyst, a particulate filter is preferably arranged.

The exhaust gas sensor is preferably a NOx sensor. However, this does not preclude that this sensor can provide, in addition to the output signal correlating with the NOx concentration of the exhaust gas, one or more further output signals which correspond to the concentration of another exhaust gas component such as e.g. As oxygen or with an exhaust condition parameter such as the exhaust gas temperature correlate.

A transition from operation at λ = 1.0 to a normal diesel engine operation with excess air is provided when the SCR catalyst exceeds a predetermined minimum temperature. This minimum temperature is preferably a catalyst temperature which is typical for a predefinable NO x conversion and which corresponds to the so-called said light-off temperature correlates or corresponds to it. In this case, the catalyst temperature is measured directly or determined from an exhaust gas temperature determined or calculated before and / or after the catalytic converter, or equated to this.

By the method according to the invention with operation at λ = 1.0 during a warm-up phase of the exhaust gas purification system is a high NOx reduction already in this critical phase of operation, in which the SCR catalyst shows no or at least no satisfactory NOx conversion possible. The NOx reduction takes place through the three-way catalyst. This is particularly advantageous for reducing the NOx emissions emitted following a cold start.

To set the air-fuel ratio in the first operating range, it is provided in an embodiment of the invention that from the correlating with the NOx concentration output of the exhaust gas sensor, a NOx-actual conversion of the three-way catalyst is determined and a motor supplied amount of air and / or fuel quantity are changed until the actual NOx-conversion at least approximately reaches a predefinable NOx target conversion. The determination of the actual NOx-conversion takes place by offsetting a raw NOx emission of the engine with the determined by means of the exhaust gas sensor NOx content in the exhaust gas downstream of the three-way catalyst. In this case, recourse is preferably made to stored characteristic curves which represent a raw NOx emission of the engine for the respective operating conditions. However, the raw NOx emission can also be determined metrologically by means of a suitable sensor upstream of the three-way catalyst. As a result of the λ-dependence of the NOx conversion of the three-way catalyst is a setting of a certain air-fuel ratio on the NOx conversion of the three-way catalyst specifically influencing parameters relating to air and / or fuel supply also enabled as the reverse Adjustment of a specific NOx-conversion by targeted influencing of the air-fuel ratio.

In a further embodiment of the invention, a target value for the air-fuel ratio lying within a predefinable target range is set in the first operating range. The target value is preferably in a narrow range around λ = 1.0 or an adjacent region. A range of about 0.95 <λ <1.05 is preferred. Target ranges with from 0.97 <λ <1.0 or 0.98 <λ <1.05 are particularly preferred. It is provided in a further embodiment that an oscillating within the target range target value for the air-fuel ratio is set. The oscillation frequency is preferably about 5 Hz to about 1 Hz. In this way, a good conversion performance of the three-way catalyst is ensured for both CO and HC and for NOx.

In a further embodiment of the invention, a pre-control value for the air-fuel ratio is set pilot-controlled by delivery of a required for a requested engine load fuel injection amount and at least approximately the amount of air required for their combustion and for setting the target value for the air-fuel ratio of the pilot value by at least one of the engine load At least approximately unaffected late post fuel injection is reduced. In this way, an accurate and smooth adjustment of the desired air-fuel ratio is possible. The setting of the air volume provided for the λ precontrol value is preferably carried out by setting a throttle valve arranged in the intake air line of the engine in conjunction with a charge pressure regulation and an inert gas or exhaust gas recirculation quantity setting to predeterminable pilot control values. The fuel injection quantity required for setting the engine load can take place via one or more pre-injections made before top dead center, via a main injection made at about top dead center and optionally one or more post-injections, in particular torque-effective. Provided is preferably at least one subsequent to the main injection post-injection. The precontrol value for the air-fuel ratio is preferably lean. Particularly preferred is a precontrol value of 1.05 <λ <1.20, in particular of 1.10 <λ <1.15. This is sufficient comparatively low, unproblematic and torque-neutral to be adjusted changes in order to achieve the target value of the air-fuel ratio. As a result of the known by measurement by means of air mass meter intake air quantity and the known injection quantity of the torque-effective fuel injections required to achieve the λ target value additional fuel Nacheinspritzmenge can be determined.

The at least one late post-injection used to set the exact λ target value is not or at most low-torque effective. For this purpose, this is preferably done at crank angles of greater than 80 ° after top dead center, wherein preferably the amount of air and inert gas supplied to the engine remains unchanged.

In a further embodiment of the invention, it is provided that the target range for the air-fuel ratio by predetermined NOx target conversion values of the three-way catalyst is predetermined, wherein for the assignment of air-fuel ratio values to target NOx conversion values to a stored NOx conversion characteristic is used which represents a dependency of the NOx conversion on the air-fuel ratio for the three-way catalyst. This characteristic is preferably determined in advance and stored in a control unit for controlling the engine operation and / or the operation of the exhaust gas purification system. It is preferred if NOx conversion characteristics are provided for a wide variety of conditions, so that the current conversion behavior of the three-way catalyst can always be considered as known. It is advantageous to estimate an aging occurring over time and, if appropriate, to adapt the NOx conversion characteristic to an aging-related change in the NOx conversion behavior. Also available in the control unit is preferably a map, which provides the current NOx raw emissions of the engine. On the basis of the thus-known NOx raw emissions of the engine and the measured by the exhaust gas sensor NOx concentration of the exhaust gas behind the three-way catalyst is the current NOx actual conversion of the three-way catalyst and thus from the NOx conversion characteristic of the current air-fuel ratio can be determined. By appropriate change or adjustment of the amount of air and / or fuel supplied to the engine, a desired air / fuel ratio can therefore be set by approximating the actual NOx / NOx conversion to the corresponding desired NOx conversion.

It has proven to be particularly advantageous if, in a further embodiment of the invention from time to time in the second operating range, an adaptation of the target range for the air-fuel ratio is made, with a current actual NOx-conversion of the three-way catalyst is determined and further, a correction value for a currently set target value of the air-fuel ratio is determined such that the target value of the air-fuel ratio corrected with the correction value corresponds to an air-fuel ratio to be assigned to the NOx actual conversion by the NOx conversion characteristic. Based on the determined NOx conversion and the stored NOx conversion characteristic curve, it is thus checked whether a lambda value to be set also corresponds to the λ value to be expected according to the characteristic curve. Deviations are compensated by a λ offset correction. In this case, the correction values are preferably written in a characteristic map to be used for all relevant operating conditions of the first operating range for the λ setting. As a result of the adaptation made according to the invention, an effective compensation of drifting and / or aging phenomena is made possible. A setting of an air-fuel ratio of about 1.0 made in connection with a subsequent engine start-up is thus made possible in an improved and more accurate manner. It is particularly advantageous if the adaptation is carried out directly or at least for a short time before a thermal regeneration of a particle filter to be carried out. For such a reduced air-fuel ratio and an increased exhaust gas temperature are adjusted anyway, so that on the one hand reliable operation of the exhaust gas sensor is ensured and on the other hand practically no additional increased fuel consumption results.

In a further embodiment of the invention, the diesel engine is operated in the first operating range within a particular first temperature range for the three-way catalyst with a Heizbrennverfahren with excess air. This allows a particularly rapid warming of the three-way catalyst allows. In this Heizbrennverfahren different fuel injection conditions are set in particular from normal operation, with which higher exhaust gas temperatures can be generated compared to the normal operation. The temperature of the three-way catalyst is preferably an exhaust gas temperature measured in front of or behind the three-way catalyst, or a temperature determined in the catalyst bed itself, as representative.

A further acceleration of the heating of the emission control system can be achieved if in a further embodiment of the invention in the first operating range within a particular second temperature range for the three-way catalyst, a three-way catalytic converter upstream electrical heating element is energized. As a heating element here is preferably a so-called E-Kat in question, which is connected as a disk-shaped, optionally catalytically coated and electrically heatable metal support element immediately upstream of the three-way catalyst. It is particularly preferred if, in a further embodiment of the invention, a starting of the electric heating element is started before performing an engine start. This low-emission engine operation can be achieved very quickly or

the pollutant emission during warm-up be kept particularly low. It is advantageous if, during the starting process itself, when electrical energy is required to crank the engine from a starter, the heating is temporarily deactivated.

Further advantages, features and details of the invention will become apparent from the following description of preferred embodiments and from the drawings. The features and combinations of features mentioned above in the description as well as the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figures are not only indicated in the respective figures Combination but also in other combinations or in isolation, without departing from the scope of the invention.

The figures show in:

1 1 is a schematic schematic diagram of a motor vehicle diesel engine with an advantageous embodiment of an exhaust gas purification system connected thereto, in which the method according to the invention can be used;

2 a diagram with a schematic representation of operating areas in which different operating conditions are provided and

3 a diagram with an exemplary NOx conversion curve of the three-way catalyst.

1 shows a schematic schematic diagram of an advantageous embodiment of a motor vehicle diesel engine 1 with connected emission control system, in which the method explained in more detail below can be used. The diesel engine 1 has in this case a two-stage supercharging and a two-stage exhaust gas recirculation and includes an engine block 2 with working cylinders 3 with unspecified combustion chambers, the working cylinders 3 or their respective combustion chamber by means of a high-pressure pump 4 Fuel is supplied. About an air supply system 5 can work cylinders 3 or the respective combustion chamber combustion air are supplied and an exhaust tract 6 will exhaust from the working cylinders 3 dissipated. In the air supply system 5 are an air filter 7 , a first compressor 10 one as a high pressure turbocharger 11 trained first exhaust gas turbocharger, a second compressor 8th one as a low-pressure exhaust gas turbocharger 9 trained second exhaust gas turbocharger, a charge air cooler 12 and a throttle 13 arranged. In the exhaust tract 6 are starting from the engine block 2 in the flow direction of the exhaust gas, a high-pressure exhaust gas turbocharger 11 associated first turbine 14 , a low-pressure exhaust gas turbocharger 9 associated second turbine 15 and an emission control system 16 arranged. The emission control system 16 in this case has a particle filter 35 for filtering particles from the exhaust gas and an upstream three-way catalyst 34 on. Further, an unillustrated electric heating element is immediately before the three-way catalyst 34 intended. The three-way catalyst 34 is preferably formed as a so-called diesel oxidation catalyst with three-way catalyst function, in particular with metal foil carrier body. Also, the electric heating element is preferably designed as a coated metal foil carrier body (so-called E-Kat). The particle filter 35 may be formed in sintered metal or as a wall-flowed filter unit in honeycomb construction. Preferably, for the particulate filter 35 a catalytic coating, for example provided with an oxidation-catalytically active material and / or with an SCR catalyst material.

In the present case is downstream of the particulate filter 35 an SCR catalyst 36 in the exhaust tract 6 arranged. The SCR catalyst 36 is capable of reducing nitrogen oxides (NOx) with, in particular, ammonia as a selective reducing agent. To enrich the exhaust gas with ammonia upstream of the SCR catalyst 36 an adding device 38 which ammonia or a reducing agent capable of splitting off ammonia, such as, for example, urea-water solution, is introduced into the exhaust gas tract 6 can inject. To improve uniform distribution, a downstream mixer, not separately shown, in the exhaust gas tract 6 be arranged. In the case of a particle filter coated with an SCR catalyst 35 the addition device may also be upstream of the three-way catalyst 34 or between the three-way catalyst 34 and the particulate filter 35 be provided or it may be provided there an additional NH ₃ -Zugabestelle.

Downstream of the second compressor 8th branches the high-pressure exhaust gas turbocharger 11 bypass compressor bypass 18 in which a compressor bypass valve 19 is arranged, so that by means of the second compressor 8th compressed fresh air or a fresh air-exhaust mixture, depending on an operating condition of the engine 1 and a resulting position of the compressor bypass valve 19 the first compressor 10 can happen to a greater or lesser extent. In this way is a boost pressure of the engine 1 adjustable or at low engine speeds 1 in which the high pressure turbocharger 11 due to a low exhaust pressure is not yet operable, the first compressor 10 over the compressor bypass 18 bypassed.

In the exhaust tract 6 are also bypasses 20 . 21 arranged, which each have a turbine 14 . 15 deal, namely a first turbine bypass 20 in which a first turbine bypass valve 22 is arranged and a second turbine bypass 21 in which a second turbine bypass valve 23 is arranged. At low engine speeds 1 and consequent low exhaust pressure is the high pressure exhaust gas turbocharger 11 not yet operable, therefore, in this operating state is the first turbine bypass valve 22 be controlled such that an exhaust gas mass flow through the first turbine bypass 20 at the first turbine 14 is passable and so completely to drive the second turbine 15 the low-pressure exhaust gas turbocharger 9 is usable.

At very high engine speeds 1 is the one on the turbines 14 . 15 the exhaust gas turbocharger 9 . 11 acting exhaust pressure high, causing them to reach high speeds. This results in a high compressor capacity of the compressor 8th . 10 the exhaust gas turbocharger 9 . 11 and thereby a high boost pressure of the fresh air exhaust gas mixture. However, this may not exceed a predetermined value, so that upon reaching this predetermined value, one or both turbine bypasses 20 . 21 can be used as a so-called wastegate. Here are the turbine bypass valves 22 . 23 be controlled such that they partially open, for example, whereby a part of the exhaust gas mass flow to the turbines 14 . 15 passable and thereby the on the turbines 14 . 15 acting and this driving exhaust pressure can be reduced. This results in a lower compression of the compressor 8th . 10 the exhaust gas turbocharger 9 . 11 compressed gas, ie a lower boost pressure.

By means of this arrangement of the low-pressure exhaust gas turbocharger 9 and the high-pressure exhaust gas turbocharger 11 is a power of the engine 1 can be optimized in different speed ranges and a respective optimal boost pressure can be provided. As a result, in particular, a so-called turbo lag, ie a lack of or low boost pressure and the resulting low power of such an engine 1 In low speed ranges preventable or at least significantly reduce this problem and thus, for example, a driving behavior and a fuel consumption of a by this engine 1 driven vehicle optimized.

Downstream of the particulate filter 35 and upstream of the adding device 38 , ie on a low pressure side of the exhaust tract 6 , branches off the exhaust tract 6 a low pressure exhaust gas recirculation (EGR) line 24 off, upstream of the second compressor 8th the low-pressure exhaust gas turbocharger 9 and downstream of the air filter 7 back into the air supply system 5 empties. By means of a in the exhaust tract 6 arranged Abgasaufstauklappe 17 is the amount or proportion of the low-pressure exhaust gas recirculation line 24 recycled exhaust gas influenced. Although present behind the branch point of the low-pressure exhaust gas recirculation line 24 shown arranged, the exhaust gas flap 17 also behind the SCR catalyst 36 be arranged.

In the low-pressure EGR line 24 is downstream of the branch of the exhaust tract 6 seen in the flow direction of a low-pressure EGR mass flow, a low-pressure EGR cooler 25 and a low pressure EGR valve 26 arranged. Optionally, the cooling of the low pressure EGR mass flow may be elimination of the low pressure EGR cooler 25 take over the used pipe lengths or pipe designs. The cooling of the low pressure EGR mass flow ensures that at the compressors 8th . 10 In exhaust gas recirculation mode no inadmissibly high temperatures occur.

In the low-pressure EGR line 24 can be upstream of the low-pressure EGR cooler 25 an unillustrated second SCR catalyst may be provided. This makes it possible to reduce nitrogen oxide and / or ammonia or oxygen which may be present in the recirculated exhaust gas. This in turn deposits or corrosion phenomena are avoided or reduced and it is an improved process in the combustion chambers of the engine 1 successful fuel combustion allows. In addition, the second SCR catalytic converter can assume a filter function, so that at least comparatively coarse particles are removed from the exhaust gas recirculated via the low-pressure path. Further, upstream and / or downstream of the three-way catalyst 34 or of the particulate filter 35 one or more further exhaust aftertreatment effective cleaning components, such as another oxidation catalyst, an SCR catalyst and / or a nitrogen oxide storage catalyst in the exhaust system 6 be arranged, which is not shown separately. In particular, it is preferred if downstream of the SCR catalyst 36 an oxidation-catalytically active exhaust gas purification component is arranged, by means of which an ammonia slip of the SCR catalyst 36 can be removed from the exhaust.

Upstream of the turbine 14 the high-pressure exhaust gas turbocharger 11 , ie on a high pressure side of the exhaust tract 6 , branches off an exhaust manifold 33 the exhaust tract 6 a high pressure EGR pipe 27 off, the downstream of the throttle 13 into the air supply system 5 empties. By means of this high-pressure EGR line 27 is a high pressure EGR mass flow via a high pressure EGR valve 28 into the air supply system 5 be conducted. In the illustrated embodiment, in the high pressure EGR conduit 27 a high pressure EGR cooler 29 arranged, which optionally with the low pressure EGR cooler 25 can be structurally and / or functionally united. Optionally, however, a cooling of the high-pressure EGR mass flow, for example, over a tube length of the high-pressure EGR line 27 respectively. For the low pressure EGR cooler 25 and / or the high pressure EGR cooler 29 bypass lines may be provided, in particular with adjusting means for variable throughput adjustment, which is not shown separately.

The illustrated diesel engine 1 thus has an exhaust gas recirculation, in the exhaust gas upstream of the turbine 14 the high-pressure exhaust gas turbocharger 11 via a corresponding high-pressure path as well as downstream of the exhaust gas purification unit 16 via a corresponding low-pressure path to the exhaust tract 6 is removable and, if necessary after Cool down, upstream of the compressor 8th the low-pressure exhaust gas turbocharger 9 and downstream of the throttle 13 of the air supply system 5 and thus the combustion chambers 3 can be fed. The motor 1 is optionally operable without exhaust gas recirculation, with high-pressure exhaust gas recirculation or low-pressure exhaust gas recirculation or simultaneously with high-pressure exhaust gas recirculation and low-pressure exhaust gas recirculation with variable exhaust gas recirculation quantities. Thus, the combustion chambers 3 a combustion gas with a variable within wide limits exhaust gas recirculation rate with variable low pressure component and variable high pressure component fed. An adjustment of an exhaust gas recirculation amount, ie the recirculated exhaust gas mass flow and thus the EGR rate, by means of the Abgasaufstauklappe 17 and / or the low pressure EGR valve 26 and by means of the high pressure EGR valve 28 as adjusting means, whereby the low pressure portion and the high pressure portion of the total recirculated exhaust gas are also adjustable within wide limits. This achieves a total of clean exhaust gas recirculation mass flows, a better cooling of the exhaust gas recirculation mass flows, avoids sooting the exhaust gas recirculation cooler 25 . 29 and allows a good mixing of the exhaust gas recirculation mass flows with fresh air in the air supply system 5 , High exhaust gas recirculation rates are possible and it is a homogeneous or at least partially homogeneous operation of the internal combustion engine 1 possible.

The exhaust flap 17 and the low pressure EGR valve 26 In the present case, these are actuators of an exhaust gas recirculation control designed as a feedforward control. Both the low pressure EGR valve 26 as well as the exhaust gas flap 17 are preferably continuously adjustable. With the help of the exhaust gas flap 17 and the low pressure EGR valve 26 in front of the compressor 8th is the low pressure component of the entire exhaust gas recirculation mass flow adjustable and the latter thus also influenced. As long as a sufficient pressure gradient for promoting the low-pressure exhaust gas recirculation mass flow is present, this is initially exclusively via the low-pressure EGR valve 26 adjustable. If this is no longer the case, there is also the exhaust gas flap 17 slightly adjustable to the pressure drop across the low-pressure EGR valve 26 to increase. In this case, a very good mixing of the low-pressure Abbgasrückführungsmassenstroms is ensured with the fresh air. Another advantage is, among other things, that the exhaust gas recirculated via the low-pressure path is clean and virtually free of pulsation. In addition, an increased compressor capacity is available, since at a high low-pressure fraction recirculated exhaust gas, a comparatively high exhaust gas mass flow through the turbines 14 . 15 is conductive. As the recirculated exhaust gas to the compressors 8th . 10 through the powerful intercooler 12 is conductive, the temperature of the fresh air and exhaust gas comprehensive combustion gas can also be kept relatively cold. The internal combustion engine 1 Depending on requirements, it can be operated with either high-pressure exhaust gas recirculation or low-pressure exhaust gas recirculation or both.

By means of a preferably provided, the intercooler 12 immediate intercooler bypass 30 in the air supply system 5 , is a mocking of the intercooler 12 preventable. The risk of so-called sooting is, for example, when a water vapor and possibly containing gas mixture in the charge air cooler 12 is cooled below the dew point and condensation occurs.

It is preferably provided that the entire fresh air-exhaust gas mixture or only a part thereof via the charge air cooler bypass 30 which is upstream of the intercooler 12 branches off, on the intercooler 12 can be routed past, thereby passing through the intercooler 12 is not cool and therefore the temperature does not fall below the dew point. To ensure that the fresh air-exhaust gas mixture, if necessary, ie at high temperatures of the fresh air-exhaust gas mixture, continue by means of the intercooler 12 is effectively coolable, is downstream of the compressor 8th . 10 and upstream of the intercooler 12 in the air supply system 5 a temperature sensor 31 arranged so that when reaching a predetermined temperature in the charge air cooler bypass 30 arranged intercooler bypass valve 32 can be controlled accordingly and then this intercooler bypass valve 32 for example, completely opens or completely closes or partially opens in another embodiment.

For optimal operation of the engine 1 and the exhaust aftertreatment system 16 are preferably other sensors in the exhaust system 6 as well as in the air supply system 5 provided, which is not shown in detail for clarity. There may be temperature and / or pressure sensors on the output side of the exhaust manifold 33 in the turbine bypasses 20 . 21 , input and / or output side or within the combination of three-way catalyst, preferably designed as a compact unit 34 and particle filters 35 Emission control module, input and / or output side of the SCR catalyst 36 , input and / or output side of the air filter 7 , input and output side of the compressor 8th . 10 , in the exhaust gas recirculation lines 24 . 27 and optionally be arranged at other locations to detect the temperature and pressure conditions. Preferably, an air mass flow sensor is further downstream of the air filter 7 provided to detect the fresh air mass flow. Furthermore, exhaust gas sensors are preferably in the exhaust gas tract 6 , such as a lambda probe in exhaust manifold 33 and before and / or after the three-way catalyst 34 or the particle filter 35 arranged, provided. Above all, according to the invention, an exhaust gas sensor, preferably designed as a NOx sensor, is arranged between the particle filter and the adding device 38 or the branch of the low-pressure EGR line 24 intended. The NOx sensor may emit an output signal which correlates with a NOx concentration of the exhaust gas and optionally further output signals, in particular an output signal correlating with the exhaust gas λ. In addition, a NOx sensor also not shown separately on the output side of the SCR catalytic converter 36 be provided.

The signals of the existing sensors are processed by a control unit, not shown, which based on the signals and stored characteristics and maps operating conditions of the engine 1 in general, especially in the exhaust tract 6 and in the air supply system 5 determine and controlled by controlling actuators and / or set regulated. In particular, exhaust gas recirculation mass flows in the low and high pressure paths and a load condition of the engine 1 in relation to torque or medium pressure and speed determined or adjustable. Furthermore, fuel injection parameters such as number of fuel injections per duty cycle and their injection pressure, duration and time are adjustable.

Hereinafter, referring to in the diagram of 2 characterized operating areas the operating method according to the invention described in more detail. In this case, the operating ranges denoted by A to G are defined by values for a temperature T as well as for an engine load M related to a rated load, for example given as effective mean pressure p _me . The temperature T is an immediately behind the three-way catalyst 34 occurring temperature in the exhaust tract 6 , which preferably detected by means of a temperature sensor and as authoritative for the temperature of the three-way catalyst 34 is seen. In the following discussion, it will be understood, by way of example, without limitation of generality, that the operating ranges designated A through G are in alphabetical order, starting from an engine cold-start at engine temperatures 1 and the emission control system 16 of 30 ° C or lower. It is further assumed that apart from the operating range G, a temperature of the SCR catalyst 36 or one immediately before or after the SCR catalyst 36 measurable exhaust gas temperature 200 ° C has not yet been exceeded.

After an engine cold start is the emission control system 16 still cold and only warms up by heat absorption due to flow with exhaust gas more or less elevated temperature. In the characterized by a temperature T <150 ° C and the entire load range engine load M operating area A is used for rapid heating of the emission control system 16 and in particular the three-way catalyst 34 the immediate input side of the three-way catalyst 34 arranged electrical heating element energized. The energization can be started with the beginning of a self-sufficient motor self-running. However, it is preferred if the energization is started before the engine is started. As a trigger for this purpose can be provided a detection of a door lock operation or a driver seat occupancy or a buckle lock. Furthermore, after the engine has started, the engine 1 Typically operated with excess air, but with a special Heizbrennverfahren, which has an increased exhaust gas temperature compared to the normal diesel engine operation. In Heizbrennverfahren the main fuel injection is shifted to about 3 ° KWnOT to 7 ° KWnOT (control start) late and reduced the main injection quantity in favor of the amount of post-injection. In addition, a reduction of the injection pressure can be provided. Furthermore, one or two pilot injections are provided before top dead center. Characteristically, incomplete post-injection incineration is avoided to avoid increased HC / CO emissions due to temperature-induced lack of activity of the three-way catalyst 34 would result.

However, such a post-injection, preferably at crank angles of the activation start of more than 80 ° nOT, takes place in addition to the measures provided in the operating region A in the operating region B. As in 2 illustrated, the operating range B is characterized by a temperature in the range 150 ° C <T <250 ° C and the entire load range covering engine load M. In this operating range is already a more or less strong HC conversion of the three-way catalyst 34 allows. As a result of the released heat of reaction, this heats up quickly and the activity therefore also increases rapidly.

Upon further heating of the three-way catalyst 34 to a temperature range of 250 ° C <T <350 ° C, depending on the engine load M operating ranges C and D reached. In this case, the operating range C is additionally characterized by an engine load M of less than 20% and the operating range D by an engine load M of more than 20% of the rated load. In the operating areas C and D, the energization of the electric heating element preferably remains active. As a result of the advanced heating of the three-way catalyst 34 this is operational in the operating ranges C and D in relation to its three-way function and the motor 1 will be on converted an operation with a combustion λ of about 1.0. In this case, a target value for the air-fuel ratio λ, which oscillates in a target range between a lower and an upper limit value, is set. In the operating range C, a combustion λ of 0.97 and in the operating range D of 0.98 is provided as the lower limit. The upper limits are set to 1.0 and 1.05 respectively in operating range C and D respectively. As a result of the somewhat lower mean value of the combustion λ in the operating region C, a CH ₄ or N ₂ O emission is advantageously reduced there.

The procedure provided according to the invention for setting the combustion λ of approximately 1.0 will be explained in more detail below. For this purpose, an in 3 schematically represented NOx conversion curve of the three-way catalyst 34 Referenced. This is merely an example of a stored for the relevant operating range λ-dependence of the NOx conversion of the three-way catalyst 34 again. By way of example, a λ target range Δλ in of 0.98 <λ <1.05 is entered. In the present case, this corresponds to a target range for the NOx conversion between 43% and 94%. For adjusting the λ target value in the λ target range Δλ, a pilot value for the air-fuel ratio is initially in the range between λ = 1.10 and λ = 1.15 set. For this purpose, the air supply quantity and the exhaust gas recirculation quantity become operating point-dependent pilot control values by actuating the throttle valve 13 , Adjusting means for the turbocharger 9 . 11 as well as the EGR valves 26 . 28 set and set the required for the λ pre-control amount of fuel over the sum of pilot injection, main injection and subsequent post-injection. Typically, the throttle valve is closed at values between 70% and 95%, a charge pressure flap at values between 5% to 45% and a wastegate at values between 25% and 45%. The high pressure EGR valve is preferred 28 completely closed and the exhaust gas recirculation amount by pressing the low-pressure exhaust gas recirculation valve 26 and the exhaust gas flap 17 set. By settling a calculated pilot quantity of a late, non-torque-effective post-injection at a crank angle of> 80 ° nOT, enrichment to achieve the λ target value takes place. Its exact value is calculated by calculating the actual NOx conversion of the three-way catalyst 34 and recourse to the NOx sales curve. In order to calculate the actual NOx-conversion, the NOx-emission of the engine, which is stored according to the characteristic map, is determined by the exhaust gas sensor behind the three-way catalytic converter 34 metrologically determined NOx concentration of the exhaust gas charged. By gradually increasing or decreasing the amount of fuel injected late, an increase or decrease in the measured NOx conversion is effected in such a way that it assumes a desired value within the target range and therefore the λ target value lies within the λ target range. In this case, there is an oscillating sweeping of the λ target range with a frequency between 1 and 5 Hz.

An improvement in the accuracy of the Nacheinspritzmengen setting is made possible by an adaptation, which takes place from time to time, preferably just before a requested thermal particle filter regeneration. It is based on an operation of the engine 1 With excess air, analogously to the procedure described above, a λ value is set which should correspond to a predefinable λ target value in the λ target range Δλ. By determining the NOx actual conversion and using the NOx conversion curve, the associated λ actual value of the air-fuel ratio is determined. If deviations from the λ target value are detected, then a correction value is determined in such a way that its addition to the λ target value yields the λ actual value. By this adaptation, the accuracy of a later setting of the air-fuel ratio of about λ = 1.0 is improved.

Hereinafter, referring again to the in 2 illustrated diagram illustrates the settings of the values provided operating ranges. In this case, upon further heating of the three-way catalyst 34 with an output measured temperature of 350 ° C <T <450 ° C and an engine load M <20% of the operating range E reached. With the exception of the deactivated heating element, the settings made here correspond to those of operating region C. In this case analogous to operating region C, the laughing gas and / or methane formation at the three-way catalytic converter is a bit fatter than the operating region D. 34 largely avoidable. In the operating range F which adjoins higher temperatures up to T = 650 ° C. or higher engine loads M, in addition to the deactivated heating element, the settings of the operating range D are provided. Above a temperature of T = 650 ° C in the operating range G, the engine 1 normal, ie operated with a typical for diesel engine operation excess air, since it is assumed that the SCR catalyst 34 has exceeded a temperature of at least 180 ° C and is ready for use.

It is understood that the above-described operation areas A to G need not necessarily be taken in their alphabetical order. Depending on the driving mode and associated with it depending on the exhaust gas temperature and exhaust gas mass flow, the operating ranges A to G can be achieved rather alternately and over time.

In the context of the procedure described, it is furthermore preferably provided that an aging-related loss of revenue of the three-way catalyst occurs over time 34 estimate. In correlation to the determined aging, the temperature limits which delimit the operating ranges A to G are raised. This is provided in particular for the temperature limits between the operating ranges A and B as well as B and C or D.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 102009015900 A1 [0002]

Claims (10)

Operating method for a motor vehicle diesel engine with an exhaust gas purification system comprising, in the flow direction of the exhaust gas, one behind the other a three-way catalyst ( 34 ) and an SCR catalyst ( 36 ), in which the diesel engine ( 1 ) in a first operating range (C, D, E, F) in which the SCR catalyst ( 34 ) is below a predetermined minimum temperature, at least temporarily with an air-fuel ratio of about λ = 1.0 is operated, and in a second operating range (G), in which the SCR catalyst ( 34 ) exceeds the predetermined minimum temperature is operated with a typical for normal diesel engine operation excess air, characterized in that for adjusting the air-fuel ratio (λ) in the first operating range (C, D, E, F) correlates with a NOx concentration of the exhaust gas Output signal from downstream of the three-way catalyst ( 34 ) arranged exhaust gas sensor is used. A method according to claim 1, characterized in that for setting the air-fuel ratio (λ) in the first operating range (C, D, E, F) from the correlating with the NOx concentration output of the exhaust gas sensor, a NOx-actual conversion of the three Pathway catalyst ( 34 ) and a motor ( 1 ) supplied amount of air and / or fuel quantity are changed until the actual NOx-conversion reaches a predetermined target NOx-conversion at least approximately. A method according to claim 1 or 2, characterized in that in the first operating range (C, D, E, F) within a predetermined target range (Δλ) lying target value for the air-fuel ratio (λ) is set. A method according to claim 3, characterized in that within the target area (Δλ) oscillating target value for the air-fuel ratio (λ) is set. A method according to claim 3 or 4, characterized in that a precontrol value for the air-fuel ratio (λ) is adjusted by delivering a required for a requested engine load (M) fuel injection amount and at least approximately required for their combustion air quantity and setting the target value for the air-fuel ratio (λ), the pre-control value is reduced by at least one late fuel post-injection which leaves the engine load (M) at least approximately uninfluenced. Method according to one of claims 3 to 5, characterized in that the target range (Δλ) for the air-fuel ratio (λ) by assigned desired NOx conversion values of the three-way catalyst ( 34 ), wherein for the assignment of air-fuel ratio values (λ) to desired NOx conversion values a stored NOx-conversion characteristic curve is used which determines a dependence of the NOx-conversion on the air-fuel ratio (λ) for the three-way Catalyst ( 34 ) A method according to claim 6, characterized in that from time to time in the second operating range, an adaptation of the target range (Δλ) for the air-fuel ratio (λ) is carried out, wherein a current actual NOx-conversion of the three-way catalyst ( 34 Further, a correction value for a currently set target value of the air-fuel ratio (λ) is determined so that the target value of the air-fuel ratio (λ) corrected with the correction value corresponds to an air-fuel ratio (λ) represented by the NOx Turnover characteristic is to be assigned to the actual NOx sales. Method according to one of claims 1 to 7, characterized in that in the first operating range (C, D, E, F) within a particular first temperature range for the three-way catalyst ( 34 ) the diesel engine ( 1 ) is operated with a Heizbrennverfahren with excess air. Method according to one of claims 1 to 8, characterized in that in the first operating range (C, D, E, F) within a particular second temperature range for the three-way catalyst ( 34 ) to the three-way catalyst ( 34 ) upstream electrical heating element is energized. A method according to claim 9, characterized in that a starting of the electric heating element is started before performing an engine start.

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