DE102016215386A1 - Method for optimizing NOx emissions in a combined exhaust aftertreatment system - Google Patents

Abstract

The invention relates to a method for optimizing NOx emissions in a combined exhaust aftertreatment system. This includes a NOx storage catalytic converter, an SCR catalytic converter and an exhaust gas recirculation system. The method comprises the following steps: A raw NOx emission (NOxRoh) is determined upstream of the combined exhaust aftertreatment system (40). Meanwhile, a measurement (41) of a NOx value (NOxBeh) is performed downstream of the combined exhaust aftertreatment system. Both NOx values (NOxRoh, NOxBeh) are compared (42) to determine a NOx setpoint (NOxSoll) (43). Furthermore, a cost function (50) is calculated from cost factors (45, 46, 47, 48, 49) and the NOx target value (NOx target) is optimized with the least possible cost by means of this cost function (50) (51).

Description

The present invention relates to a method for optimizing NOx emissions in a combined exhaust aftertreatment system. Furthermore, the present invention relates to a computer program that executes each step of the method according to the invention, when it runs on a computing device, as well as a machine-readable storage medium, which stores the computer program. Finally, the invention relates to an electronic control device which is set up to carry out the method according to the invention.

State of the art

Nowadays, several different systems are used for the post-treatment of exhaust gases of an internal combustion engine of a motor vehicle in order to reduce the emission of undesirable constituents of the exhaust gas. These systems include NOx storage catalytic converters, SCR catalysts (Selective Catalytic Reduction) and exhaust gas recirculation, which reduce the amount of nitrogen oxides (NOx) in the exhaust gas. The systems are operated independently of one another and the determination of the optimal operating points is usually determined manually by means of maps which map the engine operating points. Therefore, a setpoint for reducing the NOx emission is independently formed for each component.

In the SCR method, a urea-water solution (HWL), commercially known as AdBlue ^® introduced into the oxygen-rich exhaust gas. In the SCR catalyst, the HWL reacts to form ammonia, which then combines with the nitrogen oxides, resulting in water and nitrogen. In addition, with NOx storage catalysts, nitrogen oxides are stored in lean exhaust gas when an excess of air in the exhaust gas is present. The operating conditions of the internal combustion engine are briefly switched so that a lack of air in the exhaust gas, therefore rich exhaust prevails. Now the stored nitrogen oxides can be reduced to harmless nitrogen, which can then be ejected. In the exhaust gas recirculation parts of the treated exhaust gas are fed back into an intake pipe of the internal combustion engine, in which it mixes with the air / fuel mixture.

Disclosure of the invention

The method optimizes NOx emissions in a combined exhaust aftertreatment system. The combined exhaust aftertreatment system includes a NOx storage catalyst, an SCR catalyst, and exhaust gas recirculation. Of course, components may be added by a person skilled in the art, depending on the application and cost factor. An exhaust gas from an internal combustion engine connected to the combined exhaust aftertreatment system is passed through and treated in each of the two catalysts to reduce the amount of nitrogen oxides (NOx) in the exhaust gas. Exhaust gas recirculation is arranged to direct a portion of the exhaust gas into an intake passage of the internal combustion engine.

At the beginning of the process, a raw NOx emission is detected in the exhaust gas upstream of the combined exhaust aftertreatment system. Preferably, the raw NOx emission may be measured by a NOx sensor upstream of the combined exhaust aftertreatment system. In another aspect, the raw NOx emission may be modeled taking into account the operating condition of the internal combustion engine. Meanwhile, in another measurement, an NOx value is determined after the exhaust gas has passed through the exhaust aftertreatment system.

For the combined exhaust aftertreatment, a NOx target value is determined from a comparison between the raw NOx emission and the NOx value after exhaust aftertreatment. This indicates an amount of nitrogen that should be reduced. Preferably, the NOx target value is composed of target values for the exhaust gas recirculation, for the NOx storage catalytic converter and for the SCR catalytic converter, which are in a certain ratio. The setpoint values for the exhaust gas recirculation, for the NOx storage catalytic converter and for the SCR catalytic converter are particularly preferably calculated via their respective efficiency. This offers the advantage that current and / or future operating points are taken into account in the selection of the NOx target value.

Optionally, longer-term events that affect the reduction of nitrogen oxide content can be taken into account via boundary conditions when determining the NOx target value. These include, for example, a scheduled regeneration of a particulate filter and / or an in-vehicle diagnosis. As a result, the NOx setpoint can be determined more accurately.

According to a further step of the method, a cost function is calculated from cost factors of the individual components of the exhaust aftertreatment. Finally, the NOx setpoint is optimized by means of the cost function so that the lowest possible total costs arise during operation. In this case, the ratio of the setpoint values of the exhaust gas recirculation, the NOx storage catalytic converter and the SCR catalytic converter is preferably optimized via the cost function, the costs for the exhaust gas recirculation, the NOx storage catalytic converter and the SCR catalytic converter particularly preferably being considered separately and in the cost function be combined. As a result, the optimization is done dynamically taking into account the cost factors and the operating conditions.

The cost factors preferably include dynamic influencing factors. These include, among other things, a CO ₂ emission and a particle emission of the internal combustion engine, via whose operating parameters either a CO ₂ emission or a NO x emission is favored. Furthermore, the dynamic impact factors include aging of the system, as well as a fuel level and a reductant level of the SCR catalyst that directly affect the treatment of the NOx. If the fuel level is low, the ratio of the set points of the exhaust gas recirculation, the NOx storage catalytic converter and the SCR catalytic converter can be shifted in favor of the SCR catalytic converter in order to minimize the fuel consumption due to the regeneration of the NOx catalytic converter. On the other hand, nitrogen can be increasingly stored in the NOx storage catalyst when the reducing agent level is low.

Another aspect of the method involves the inclusion of multiple exhaust aftertreatment strategies in the optimization of the NOx setpoint. This has the consequence that the optimization of the NOx target value is calculated several times and from this an optimization function can be mapped. This offers the advantage that an optimal exhaust aftertreatment strategy for the currently existing operating conditions and cost factors can be found. Optionally, a future exhaust aftertreatment strategy may be predicted by taking route information into account. Since different routes require different operating conditions, such as highway and city traffic, the exhaust aftertreatment strategies may be pre-set to suit the various operating conditions.

In the further course of the method, a check of the NOx value after the combined exhaust aftertreatment can be carried out. Based on this NOx value, the NOx setpoint can be dynamically adjusted.

The computer program is set up to perform each step of the method, in particular when it is performed on a computing device or controller. It allows the implementation of the method in a conventional electronic control unit without having to make any structural changes. For this purpose it is stored on the machine-readable storage medium.

By loading the computer program on a conventional electronic control unit, the electronic control unit according to the invention is obtained, which is arranged to control the positioning of the crankshaft by means of the method according to the invention.

Brief description of the drawings

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

1 schematically shows a combined exhaust aftertreatment system, which can be controlled by means of an embodiment of the method according to the invention.

2 shows a flowchart of an embodiment of the method according to the invention.

Embodiment of the invention

In 1 shows an exhaust system 1 that with an internal combustion engine 2 connected is. The internal combustion engine generates exhaust gas through the exhaust line 1 is dissipated. Furthermore, the 1 a combined exhaust aftertreatment system 10 which is in the exhaust system 1 is arranged and adapted to the exhaust gas in the exhaust system 1 to treat. This includes the combined exhaust aftertreatment system 10 a NOx storage catalyst 11 , an SCR catalyst 12 and an exhaust gas recirculation 13 , as a result of which nitrogen oxides (NOx) in the exhaust gas are reduced. The exact modes of operation of these components will not be described in detail herein in order to avoid departing from the main focus of this invention.

The exhaust gas flows out of the internal combustion engine 2 through the exhaust system 1 in the combined exhaust aftertreatment system 10 , where the nitrogen oxide content through the NOx storage catalyst 11 and an SCR catalyst 12 is reduced. About the exhaust gas recirculation 13 Part of the treated exhaust gas is returned to the internal combustion engine 2 guided. This is controlled by an exhaust gas recirculation valve 14 that at the exhaust gas recirculation 13 and the exhaust system 1 is arranged. The remainder of the exhaust gas leaves the exhaust aftertreatment system 10 over the exhaust system 1 ,

It should be noted that the components NOx storage catalyst 11 , SCR catalyst 12 and exhaust gas recirculation valve 14 not necessarily in specified order in the exhaust system 1 must be arranged. In particular, the exhaust gas recirculation valve 14 may in another embodiment (not shown) also upstream of the NOx storage catalyst 11 be arranged.

In addition, there are two NOx sensors 30 and 31 in the exhaust system 1 arranged. A first NOx sensor 30 is between the internal combustion engine 2 and the exhaust aftertreatment system 10 is arranged and is set up, a NOx raw emission NOxRoh the internal combustion engine 2 to eat. A second NOx sensor 31 is downstream of the exhaust aftertreatment system 10 arranged there and measures there a NOx NOxBeh of the treated exhaust gas. The two NOx sensors 30 and 31 are with a control unit 3 connected and provide this information about the NOx content of the exhaust gas. In addition, the controller 3 set up, the combined exhaust aftertreatment system 10 and the internal combustion engine 2 to control. W = NOxBeh / NOxRoh

2 shows a flowchart of an embodiment of the method according to the invention. The first NOx sensor 30 performs a measurement 40 the NOx raw emission NOxRoh, that of the internal combustion engine 2 is ejected. As a result, the second NOx sensor leads 31 also a measurement 41 the NOx value NOxBeh of the exhaust gas after the exhaust aftertreatment, downstream of the combined exhaust aftertreatment system 10 , by. Both NOx values NOxRoh and NOxBeh are sent to the control unit 3 which performs a comparison 42 of the two NOx values NOxRoh and NOxBeh. About the comparison 42 the two NOx values NOxRoh and NOxBeh, a NOx target value NOxSoll is determined 43 , Longer-term events 44 , such as a scheduled regeneration of a particulate filter at 500 to 1000 km and / or an in-vehicle diagnosis at 100 km, are considered boundary conditions in the determination 43 of the NOx target value NOxSoll considered.

An efficiency W _{EGR of} the exhaust gas recirculation 13 , an efficiency W _{NSC of} the NOx storage catalyst 11 and an efficiency W _{SCR of} the SCR catalyst 12 Depend on operating points of the internal combustion engine and can be calculated via this. NOxSoll = a · AGRsoll + b · NSCsoll + c · SCRsoll (Formula 1)

The NOx target value NOxSoll is, as shown in formula 1, from the target value AGRsoll for the exhaust gas recirculation 13 , the setpoint NSCsetpoint for the NOx storage catalytic converter 11 and the setpoint SCRsoll for the SCR catalyst 12 composed. The setpoints AGRset, NSCsetpoint and SCRsetpoint are related to one another via the factors a, b and c. The setpoint values AGRsetpoint, NSCsetpoint and SCRsetpoint are dependent on the respective efficiency W _AGR , W _NSC or W _SCR , whereby current and / or future operating points are taken into account. There are many possibilities to distribute the NOx setpoint NOxSoll via the factors a, b and c to the setpoints AGRsoll, NSCsoll and SCRsoll. In the method according to the invention a cost-optimized solution is shown.

Dynamic influencing factors, such as a CO ₂ emission 45 , a particle emission 46 , an aging 47 of the system, as well as a fuel level 48 and a reductant level 49 be considered cost factors in a cost function 50 displayed. This is the sum of the costs for exhaust gas recirculation 13 , for the NOx storage catalyst 11 and for the SCR catalyst 12 Combined exhaust aftertreatment 10 from. In a further step 51 is the NOx setpoint NOxSoll by means of the cost function 50 optimized so that it can be realized with the lowest possible cost. More specifically, the factors a, b and c are adjusted so that the resulting cost function - and accordingly the total cost - becomes minimal.

For this purpose, different exhaust aftertreatment strategies 52 applied and the optimization 51 of the NOx target value NOxSoll is repeatedly performed until finally by comparison 53 an optimum NOx target NOxSoll is found. Finally, this is in an optimization function 54 displayed.

The exhaust aftertreatment strategies 52 can do this via route information 55 be predicted, because at different track conditions different exhaust aftertreatment strategies 52 are preferably used. In one embodiment, a prediction of the future exhaust aftertreatment strategies may be made 52 calculated using an "electronic horizon". In another embodiment, the future exhaust aftertreatment strategy 52 Extrapolated over past operating parameters, such as speed, load, etc.

Finally, a permanent control 56 of the NOx value NOxBeh of the exhaust gas after the exhaust aftertreatment. The NOx target value is then dynamically adjusted based on this of the NOx value NOxBeh of the exhaust gas after exhaust aftertreatment.

Claims (12)

Method for optimizing NOx emission in a combined exhaust aftertreatment system ( 10 ) comprising a NOx storage catalyst ( 11 ), an SCR catalyst ( 12 ) and an exhaust gas recirculation ( 13 ), comprising the following steps: - identification ( 40 ) of a raw NOx emission (NOxRoh) upstream of the combined exhaust aftertreatment system ( 10 ); - Measurement ( 41 ) of an NOx value (NOxBeh) downstream of the combined exhaust aftertreatment system ( 10 ); - Comparison ( 42 ) between the raw NOx emission (NOxRoh) and the NOx value (NOxBeh) after exhaust aftertreatment to determine a NOx target value (NOx target) ( 43 ); - Calculation of a cost function ( 50 ) from cost factors ( 45 . 46 . 47 . 48 . 49 ); - Optimization ( 51 ) of the NOx target value (NOxSoll) with as low a cost as possible, by means of the cost function ( 50 ). Method according to claim 1, characterized in that the optimization ( 51 ) of the NOx target value (NOxSoll) via a ratio of the target value (AGRsoll) of the exhaust gas recirculation ( 13 ), the desired value (NCRsoll) of the NOx storage catalytic converter and the desired value (SCRsoll) of the SCR catalytic converter ( 11 ) of the combined exhaust aftertreatment system ( 10 ) he follows. A method according to claim 2, characterized in that the desired value (AGRsoll) the exhaust gas recirculation via an efficiency (W _AGR ) of the exhaust gas recirculation, the target value (NCRsoll) of the NOx storage catalytic converter via an efficiency (W _NSC ) of the NOx storage catalytic converter and the setpoint (SCRsoll ) of the SCR catalyst ( 11 ) on an efficiency (W _SCR ) of the SCR catalyst ( 11 ) is determined. Method according to one of the preceding claims, characterized in that the determination ( 40 ) of the raw NOx emission (NOxRoh) via a measurement of a first NOx sensor ( 30 ) upstream of the combined exhaust aftertreatment system ( 10 ) he follows. Method according to claim 1 or 2, characterized in that the determination ( 40 ) of the raw NOx emission (NOxRoh) via modeling for one with the combined exhaust aftertreatment system ( 1 ) connected internal combustion engine ( 2 ) is realized. Method according to one of the preceding claims, characterized in that the cost factors include the following: - CO ₂ emission ( 45 ) of an internal combustion engine ( 2 ); - Particle emission ( 46 ) of an internal combustion engine ( 2 ), - aging ( 47 ) of the exhaust aftertreatment system ( 10 ), - fuel level ( 48 ); - Reducing agent level ( 49 ) of the SCR catalyst ( 12 ). Method according to one of the preceding claims, characterized in that a plurality of exhaust gas aftertreatment strategies ( 52 ) are included in the calculation, whereby the optimization ( 51 ) of the NOx target value (NOxSoll) is calculated several times until a comparison ( 53 ) an optimal NOx target value is found (NOx target) and from this an optimization function ( 54 ) is mapped. Method according to one of the preceding claims, characterized in that route information ( 55 ) are taken into account for a future exhaust aftertreatment strategy ( 52 ) to predict. Method according to one of the preceding claims, characterized in that a control ( 56 ) of the NOx value (NOxBeh) after the combined exhaust aftertreatment is performed and based on which the NOx target value (NOxSoll) is dynamically adjusted. A computer program adapted to perform each step of the method of any one of claims 1 to 9. Machine-readable storage medium on which a computer program according to claim 10 is stored. Electronic control unit ( 3 ) arranged to exhaust NOx by a method according to any one of claims 1 to 9 in a combined exhaust aftertreatment system ( 10 ) to optimize.

Images