DE102019131829B3 - Process for exhaust aftertreatment of an internal combustion engine - Google Patents

Abstract

The invention relates to a method for exhaust gas aftertreatment of an internal combustion engine (10) with an air supply system (20) and an exhaust system (40) in which two SCR systems (92, 94) are arranged sequentially, and with a high-pressure exhaust gas recirculation (34), which connects the exhaust system (40) upstream of a turbine (44) of an exhaust gas turbocharger to the air supply system (20) downstream of a compressor (98) of the exhaust gas turbocharger (30) and a low-pressure exhaust gas recirculation (70), which connects the exhaust system (40) downstream of the first SCR system (92) and upstream of the second SCR system (94) with the air supply system (20) upstream of the compressor (98). It is provided that the internal combustion engine (10) can be operated in four different operating modes, the Exhaust gas recirculation via the high-pressure exhaust gas recirculation (34) and the low-pressure exhaust gas recirculation (70) is appropriately adapted to at least one SCR system (92, 94) r Conversion of nitrogen oxide emissions to bring ideal temperature. The first SCR system (92) is heated in a first operating mode of the internal combustion engine (10) and the heat is transferred from the first SCR system (92) to the second SCR system (94) in a second operating mode, in a third operating mode the first SCR system (92) is cooled and, in a fourth operating mode, both SCR systems (92, 94) are cooled.

Description

The invention relates to a method for exhaust gas aftertreatment of an internal combustion engine and an exhaust gas aftertreatment system for an internal combustion engine according to the preamble of the independent claims.

The current exhaust gas legislation and one that will become increasingly strict in the future place high demands on the engine-related raw emissions and the exhaust gas aftertreatment of internal combustion engines. The demands for a further decrease in consumption and the further tightening of the exhaust gas standards with regard to the permissible nitrogen oxide emissions represent a challenge for the engine developers. In gasoline engines, the exhaust gas cleaning is carried out in the known manner via a three-way catalytic converter and the three-way Upstream and downstream catalytic converters. Exhaust gas aftertreatment systems are currently used in diesel engines which have an oxidation catalytic converter or a NO _x storage catalytic converter, a catalytic converter for the selective catalytic reduction of nitrogen oxides (SCR catalytic converter) and a particle filter for separating out soot particles and possibly other catalysts. In order to meet the high requirements for minimum nitrogen oxide emissions, exhaust gas aftertreatment systems are known which have two SCR catalytic converters connected in series, each of the SCR catalytic converters being preceded by a metering element for metering in a reducing agent. A synthetic, aqueous urea solution is preferably used as the reducing agent, which is mixed with the hot exhaust gas flow in a mixing device upstream of the SCR catalytic converter. As a result of this mixing, the aqueous urea solution is heated, with the aqueous urea solution releasing ammonia in the exhaust gas duct. A commercially available, aqueous urea solution generally consists of 32.5% urea and 67.5% water.

From the EP 3 312 400 A1 an exhaust gas aftertreatment system for an internal combustion engine with an SCR catalytic converter and several bypasses in the exhaust system is known. A method for controlling the temperature of an SCR catalytic converter in the exhaust system of the internal combustion engine is described, the temperature at the inlet of the SCR catalytic converter being recorded and the bypasses being opened or closed accordingly in order to increase the exhaust gas temperature when entering the SCR catalytic converter Conversion of pollutants to bring optimal temperature.

The WO 2015/130211 A1 discloses a method for exhaust aftertreatment of an internal combustion engine with an exhaust aftertreatment system in which a diesel particulate filter is arranged in the flow direction of an exhaust gas flow through the exhaust aftertreatment system, a first SCR catalytic converter downstream of the diesel particulate filter and a second SCR catalytic converter further downstream.
A first additive to reduce NOx emissions is introduced into the exhaust gas flow via a first metering valve and converted by the first SCR catalyst downstream of the first metering valve, the soot particles being captured by the particle filter and nitrogen monoxide being oxidized to nitrogen dioxide by a catalytic coating . Furthermore, a second additive for reducing the NOx emissions is metered into the exhaust gas flow through a second metering valve downstream of the particulate filter and upstream of the second SCR catalytic converter.

From the WO 2015/092180 A2 a method for heating an SCR catalytic converter in the exhaust system of an internal combustion engine with an exhaust gas turbocharger is known, the exhaust system having a gas cleaning device connected upstream of the SCR catalytic converter, an exhaust gas recirculation line connecting the exhaust gas system of the internal combustion engine to the intake system, a branch circuit of the exhaust gas recirculation line which is in the Is able to take up part of the recirculated exhaust gas and introduce this part back into the exhaust gas circuit between the gas cleaning device and the SCR catalytic converter. It is provided that the method comprises a step in which the branching circuit is opened or closed according to the temperature of the gases entering the catalyst.

The DE 10 2017 206 425 A1 discloses an exhaust gas aftertreatment system for aftertreating exhaust gases from an internal combustion engine. The exhaust gas aftertreatment system comprises a catalytic converter for oxidizing the exhaust gas and / or a catalytic converter for storing nitrogen oxides, an inlet point for supplying a reducing agent, an SCR catalytic converter for the selective catalytic reduction of nitrogen oxides and a particle filter. It is provided that the particle filter is arranged after the SCR catalytic converter in the direction of flow and a second SCR catalytic converter and / or an ammonia slip catalytic converter is arranged after the particle filter in the flow direction.

DE 10 2016 118 309 A1 describes an exhaust gas aftertreatment system with an SCR particle filter and a downstream SCR catalytic converter. The system also includes low pressure exhaust gas recirculation and high pressure exhaust gas recirculation. When the exhaust gas temperature is low, the filling of the combustion chambers of the internal combustion engine is reduced in order to prevent the exhaust gas mass flow and thus the SCR components from cooling down.

The invention is now based on the object of keeping the temperature of at least one SCR catalytic converter in an exhaust gas aftertreatment system with two SCR catalytic converters connected in series in the optimum operating range with regard to the selective, catalytic reduction of nitrogen oxides and thus further increasing the conversion of nitrogen oxides in exhaust gas aftertreatment improve.

• - Heizen des ersten SCR-Systems in einem ersten Betriebszustand des Verbrennungsmotors, wobei die Niederdruck-Abgasrückführung geöffnet und ein Abgaskanal der Abgasanlage stromabwärts der Verzweigung für die Niederdruck-Abgasrückführung durch eine Abgasklappe zumindest teilweise versperrt ist,
• - Heizen des zweiten SCR-Systems in einem zweiten Betriebszustand des Verbrennungsmotors durch konvektive Wärmeübertragung vom ersten SCR-System auf das zweite SCR-System, wobei die Niederdruck-Abgasrückführung geschlossen und die Abgasklappe geöffnet ist,
• - Kühlen des ersten SCR-Systems in einem dritten Betriebszustand des Verbrennungsmotors, wobei die Niederdruck-Abgasrückführung geöffnet und der Abgaskanal zumindest teilweise durch die Abgasklappe versperrt ist, wobei das zurückgeführte Abgas durch einen Abgasrückführungskühler in der Niederdruck-Abgasrückführung gekühlt wird (hierbei wird der Niederdruck-Abgasrückführungskühler aktiv von einem Kühlmedium, insbesondere vom Kühlmittelkreislauf des Verbrennungsmotors, gekühlt), und
• - Kühlen des ersten SCR-Systems und des zweiten SCR-Systems in einem vierten Betriebszustand des Verbrennungsmotors, wobei die Niederdruck-Abgasrückführung und die Abgasklappe geöffnet sind und ein über die Niederdruck-Abgasrückführung zurückgeführter Abgasstrom durch einen Abgasrückführungskühler in der Niederdruck-Abgasrückführung gekühlt wird.

According to the invention, this object is achieved by a method for exhaust gas aftertreatment of an internal combustion engine whose inlet is connected to an air supply system and whose outlet is connected to an exhaust system. A high-pressure exhaust gas recirculation is provided, which connects the exhaust system upstream of a turbine of an exhaust gas turbocharger with the air supply system downstream of a compressor of the exhaust gas turbocharger. Furthermore, a first SCR system close to the engine and a second SCR system are arranged in the exhaust system downstream of the turbine of the exhaust gas turbocharger and a second SCR system is arranged downstream of the first SCR system close to the engine. Downstream of the first SCR system and upstream of the second SCR system, a low-pressure exhaust gas recirculation branches off from an exhaust gas duct of the exhaust system, which connects the exhaust gas duct to an intake duct of the air supply system upstream of the compressor of the exhaust gas turbocharger, comprising the following steps, which are preferably in overrun mode of the internal combustion engine:

• - Heating the first SCR system in a first operating state of the internal combustion engine, wherein the low-pressure exhaust gas recirculation is open and an exhaust gas duct of the exhaust system downstream of the branch for the low-pressure exhaust gas recirculation is at least partially blocked by an exhaust gas flap,
• - Heating of the second SCR system in a second operating state of the internal combustion engine by convective heat transfer from the first SCR system to the second SCR system, the low-pressure exhaust gas recirculation being closed and the exhaust gas flap open,
• - Cooling of the first SCR system in a third operating state of the internal combustion engine, the low-pressure exhaust gas recirculation being opened and the exhaust gas duct being at least partially blocked by the exhaust gas flap, the recirculated exhaust gas being cooled by an exhaust gas recirculation cooler in the low-pressure exhaust gas recirculation (the low pressure is here Exhaust gas recirculation cooler actively cooled by a cooling medium, in particular by the coolant circuit of the internal combustion engine), and
• - Cooling of the first SCR system and the second SCR system in a fourth operating state of the internal combustion engine, the low-pressure exhaust gas recirculation and the exhaust gas flap being open and an exhaust gas flow recirculated via the low-pressure exhaust gas recirculation being cooled by an exhaust gas recirculation cooler in the low-pressure exhaust gas recirculation.

The features listed in the dependent claims allow advantageous improvements and non-trivial further developments of the method for exhaust gas aftertreatment specified in the independent claim.

In a preferred embodiment of the invention, it is provided that the exhaust gas temperature is determined and compared with a first threshold temperature, the first SCR system being heated at an exhaust gas temperature below the first threshold temperature.

In a further improvement of the method, it is provided that the exhaust gas temperature is determined and compared with a first threshold temperature and a second threshold temperature, with the second SCR system convectively by the at an exhaust gas temperature which is between the first threshold temperature and the second threshold temperature Waste heat from the first SCR system is heated.

In an advantageous embodiment of the method, it is provided that the exhaust gas temperature is determined and compared with a second threshold temperature and a third threshold temperature, the first SCR system by the third at an exhaust gas temperature which is between the second threshold temperature and the third threshold temperature Operating state of the internal combustion engine is cooled.

In a further improvement of the method it is provided that the exhaust gas temperature is determined and compared with a third threshold temperature, with the first SCR system and the second SCR system by the fourth operating state of the at an exhaust gas temperature which is above the third threshold temperature Internal combustion engine are cooled.

In a preferred embodiment of the method it is provided that the temperature of the first SCR system and the temperature of the second SCR system are determined, the temperature difference between the first SCR system and the second SCR system being determined and with a fourth threshold value is compared, with a temperature difference which is greater than the fourth threshold value, the heat convectively from the first SCR system to the second SCR system in the second operating state of the Internal combustion engine is transmitted to reduce the temperature difference between the two SCR systems.

In a further preferred embodiment of the method it is provided that a switchover between the four operating modes of the internal combustion engine takes place in such a way that at least one of the SCR systems has a temperature between 250.degree. C. and 380.degree.

It is particularly preferred if a switchover between the four operating modes of the internal combustion engine takes place in such a way that both SCR systems have a temperature between 200 ° C. and 380 ° C., preferably between 250 ° C. and 350 ° C.

In a further improvement of the method it is provided that in the fourth operating mode of the internal combustion engine a high pressure exhaust gas recirculation valve is closed and the entire exhaust gas flow is passed through the first SCR system in order to effect a cooling of the first SCR system.

A further partial aspect of the invention relates to an internal combustion engine with an air supply system and an exhaust system, as well as with an engine control device which is set up to carry out such a method when a machine-readable program code is executed by the engine control device.

The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless stated otherwise in the individual case.

• 1 einen Verbrennungsmotor mit einem Luftversorgungssystem und einer Abgasanlage in einem ersten Betriebszustand, bei dem ein motornahes SCR-System beheizt wird,
• 2 den Verbrennungsmotor in einem zweiten Betriebszustand, in welchem ein SCR-System in Unterbodenlage durch die Abwärme des motornahen SCR-Systems beheizt wird,
• 3 den Verbrennungsmotor in einem dritten Betriebszustand, in welchem das motornahe SCR-System gekühlt wird,
• 4 den Verbrennungsmotor in einem vierten Betriebszustand, in welchem beide SCR-Systeme gekühlt werden,
• 5 die Konvertierung von Stickoxiden in Abhängigkeit von der Ammoniak-Beladung eines SCR-Katalysators und der Temperatur, und
• 6 die mögliche Ammoniak-Speichermenge eines SCR-Katalysators in Abhängigkeit von der Temperatur.

The invention is explained below in exemplary embodiments with reference to the accompanying drawings. The same components or components with the same function are provided with the same reference symbols in the figures. Show it:

• 1 an internal combustion engine with an air supply system and an exhaust system in a first operating state in which an SCR system close to the engine is heated,
• 2 the combustion engine in a second operating state in which an SCR system in the underbody position is heated by the waste heat from the SCR system close to the engine,
• 3 the internal combustion engine in a third operating state in which the SCR system close to the engine is cooled,
• 4th the internal combustion engine in a fourth operating state in which both SCR systems are cooled,
• 5 the conversion of nitrogen oxides as a function of the ammonia loading of an SCR catalytic converter and the temperature, and
• 6th the possible ammonia storage amount of an SCR catalytic converter as a function of the temperature.

1 shows the schematic representation of an internal combustion engine 10 . The internal combustion engine 10 is designed as a direct injection diesel engine. The internal combustion engine 10 has several combustion chambers 12 on. At the combustion chambers 12 each is a fuel injector 14th for injecting a fuel into the respective combustion chamber 12 arranged. The internal combustion engine 10 is with his inlet 16 with an air supply system 20th and with its outlet 18th with an exhaust system 40 connected. The internal combustion engine 10 further comprises a high pressure exhaust gas recirculation 34 with an exhaust gas recirculation line 38 and a high pressure EGR valve 36 , via which an exhaust gas from the internal combustion engine 10 from the outlet 18th to the inlet 16 can be traced back. At the combustion chambers 12 inlet valves and outlet valves are arranged with which a fluidic connection from the air supply system 20th to the combustion chambers 12 or from the combustion chambers 12 to the exhaust system 40 can be opened or closed.

The air supply system 20th includes an intake duct 22nd in which in the direction of flow of fresh air through the intake duct 22nd an air filter 24 , downstream of the air filter 24 an air mass meter 26th , in particular a hot film air mass meter, and downstream of the air mass meter 26th a compressor 98 of an exhaust gas turbocharger 30th are arranged. The air mass meter can do this 26th also in a filter housing of the air filter 24 be arranged so that the air filter 24 and the air mass meter 26th forms an assembly. Downstream of the compressor 98 is in the intake duct 22nd a throttle valve 32 arranged with which the the combustion chambers 12 of the internal combustion engine 10 supplied air volume can be controlled. There is also an intercooler 96 provided with which the through the compressor 98 compressed fresh air before entering the combustion chambers 12 Can be cooled to fill the combustion chambers 12 to improve.

The exhaust system 40 includes an exhaust duct 42 , in which in the flow direction of an exhaust gas of the internal combustion engine 10 through the exhaust duct 42 a turbine 44 of the exhaust gas turbocharger 30th is arranged, which the compressor 98 in the air supply system 20th drives over a shaft. The exhaust gas turbocharger 30th is preferably designed as an exhaust gas turbocharger with variable turbine geometry. To do this are a turbine wheel of the turbine 44 adjustable guide vanes upstream, via which the The exhaust gas flows onto the blades of the turbine 44 can be varied. Downstream of the turbine 44 The first component of exhaust gas aftertreatment is a first catalytic converter 46 , especially an oxidation catalyst 48 or a NOx storage catalytic converter 50 is arranged, which has an oxidation catalytic converter 48 includes. Downstream of the first catalyst 46 is a first SCR system 92 , which has a particle filter 54 with a coating 56 for the selective catalytic reduction of nitrogen oxides comprises, arranged. Downstream of the first catalyst 46 and upstream of the particulate filter 54 is a first metering valve 52 arranged, with which a reducing agent, in particular aqueous urea solution, into the exhaust duct 42 of the internal combustion engine 10 can be dosed.

Downstream of the particulate filter 54 is another SCR system 94 provided, which has a second SCR catalyst 60 includes, arranged. The second SCR catalytic converter 60 is an ammonia barrier catalyst 64 downstream, which is on the outlet side in the second SCR catalytic converter 60 is integrated or as a separate catalytic converter to the second SCR catalytic converter 60 is downstream.

Downstream of the particulate filter 54 and upstream of the second SCR catalyst 60 is a branch 58 provided on which an exhaust gas recirculation duct 72 a low-pressure exhaust gas recirculation 70 from the exhaust duct 42 branches off. Downstream of the branch 58 and upstream of the second SCR catalyst 60 is in the exhaust duct 42 a second metering valve 62 arranged, with which a reducing agent for selective, catalytic reduction in the exhaust duct 42 can be introduced. Furthermore, it is downstream of the branch, in particular downstream of the second SCR catalytic converter 60 an exhaust flap 66 arranged with which the exhaust gas back pressure in the exhaust duct 42 and thus that of the low-pressure exhaust gas recirculation 70 recirculated exhaust gas flow can be controlled. In an alternative embodiment, the exhaust flap 66 right after the branch 58 and before the second metering valve 62 be positioned. Further is downstream of the branch 58 and upstream of the second SCR catalyst 60 a second metering valve 62 provided with which the reducing agent for the selective, catalytic reduction of nitrogen oxides upstream of the second SCR catalyst 60 in the exhaust duct 42 can be dosed. The dosing valves 52 , 62 are connected via reducing agent lines to a storage container in which the reducing agent is stored.

The low pressure exhaust gas recirculation 70 has a filter on the inlet side 78 which prevents the penetration of particles from the exhaust gas flow into the low-pressure exhaust gas recirculation. Furthermore, are in the low pressure exhaust gas recirculation 70 a low pressure exhaust gas recirculation valve 74 and an exhaust gas recirculation cooler 76 arranged, with which the recirculated exhaust gas flow can be cooled before it reaches a confluence 28 in the intake duct 22nd is introduced and mixed there with the fresh air flow.

Downstream of the turbine 44 of the exhaust gas turbocharger 30th is on the exhaust duct 42 a first temperature sensor 80 arranged. Other temperature sensors 82 , 84 , 86 , 88 , 90 are in particular before and after the particle filter 54 , upstream of the second SCR catalyst 60 , between the second SCR catalytic converter 60 and ammonia barrier catalyst 64 as well as downstream of the ammonia barrier catalyst 64 and upstream of the exhaust flap 66 arranged to the exhaust gas temperature T _EG at different positions of the exhaust system 40 and thus the most accurate possible estimation of the component temperatures of the two SCR catalytic converters 56 , 60 to enable. However, a computational estimate of component temperatures is also possible, which can lead to the omission of individual temperature sensors in the exhaust duct.

The internal combustion engine 10 is with an engine control unit 68 connected, which via signal lines not shown with the temperature sensors 80 , 82 , 84 , 86 , 88 , 90 , the dosing elements 52 , 62 and the exhaust gas recirculation valves 36 , 74 connected is. Furthermore, the control unit with the injectors 14th connected to the injection timing of the fuel as well as the amount of it in the combustion chambers 12 to control injected fuel. In addition, the engine control unit 68 the position of the throttle valve 32 as well as the exhaust flap 66 can be varied.

The SCR systems 92 , 94 are due to the low exhaust gas temperatures T _EG due to the high efficiency of the diesel engine 10 Optimized to function optimally with low exhaust gas output and as soon as possible after a cold start. When driving down cold or after starting the engine with a cold exhaust system, the SCR system close to the engine takes over 92 the dosage of the reducing agent and the associated first SCR catalytic converter 56 the conversion of the NOx emission. The first SCR catalytic converter is due to its close proximity to the engine 56 Warm up and operational much faster than the second SCR catalytic converter 60 in the underbody position. As the temperature of the second SCR catalyst rises 60 In the underbody position, the dosing of the reducing agent of this twin dosing concept is increasingly directed to the second dosing valve 62 switched. Dosing in the subsoil position is advantageous in that it is downstream of the branch 58 of the low-pressure exhaust gas recirculation duct 72 from the exhaust duct 42 takes place. On the one hand, the dosing quantity required in the underbody position is lower, since the mass flow is a percentage of the part of the low pressure exhaust gas recirculation 70 recirculated exhaust gas flow is reduced, on the other hand, no reducing agent is via the low-pressure exhaust gas recirculation line 72 back in the internal combustion engine 10 returned. In addition, the second SCR system can 94 also during regeneration of the diesel particulate filter 54 actively reduce nitrogen oxides.

The conversion behavior and the maximum possible conversion rate depend on the temperature of the SCR catalytic converter and the amount of NH3 stored. As in 5 shown, a maximum NOx conversion can only be achieved at a sufficiently high temperature. Outside of an optimal temperature _{operating window} T _opt , the conversion decreases significantly both at too low a temperature and at too high a temperature.

Another factor affecting the conversion of the SCR catalytic converters 56 , 60 in the two SCR systems 92 , 94 influenced is the one in the SCR catalytic converter 56 , 60 Stored ammonia (NH3 -) storage amount. The conversion increases over the entire temperature range if a higher fill level can be ensured on the SCR catalytic converters. The conversion is significantly worse when the ammonia reservoirs are less full and less reducing agent is available for reducing the nitrogen oxides. The possible ammonia level of the SCR catalytic converters 56 , 60 depends on the respective catalyst volume and essentially on the temperature. Up to a temperature of approx. 250 ° C, large amounts of ammonia can get on the SCR catalytic converters 56 , 60 stored and made available for the reduction of NOx emissions in the form of an ammonia fill level. As the temperature rises, the storage capacity of the SCR catalytic converter decreases 56 , 60 strongly. The possible ammonia level m _NH3 as a function of the temperature of the SCR catalytic converter 56 , 60 is in 6th shown. This applies both to the first SCR catalytic converter located close to the engine 56 as well as for the second SCR catalytic converter located in the underbody 60 . The second decisive parameter for optimal NOx conversion is the temperature of the exhaust gas T _EG and the resulting component temperature of the SCR catalytic converter 56 , 60 .

A crucial aspect that must be ensured when operating an exhaust gas aftertreatment system is the avoidance of ammonia slip downstream of the flow through the exhaust duct 42 last SCR catalytic converter 60 . Since the storage capacity of ammonia decreases sharply with increasing temperature, the SCR catalytic converter can 56 , 60 no longer be loaded with a high amount of ammonia with increasing temperature. The additional risk of thermally induced ammonia slip via temperature peaks from a dynamic or high-load operating point of the internal combustion engine 10 make it necessary that a sufficiently high safety margin to the maximum filling level is ensured in dynamic operation so that dynamic exhaust gas temperature peaks do not cause ammonia slip. This temperature-dependent storage capacity is one of the reasons why, in dynamic operation, the exhaust gas aftertreatment system can increasingly be operated with lower fill levels as the temperature increases.

In real, dynamic driving operation, there is rarely an optimal temperature level for both SCR catalytic converters 56 , 60 a. Process-related arises due to the heat losses from the exhaust system 40 a downstream temperature gradient in the exhaust system 40 a. For the SCR system close to the engine 92 When driving, the temperature level is higher than for the second SCR system 94 in the underbody position. At high driving speeds and correspondingly high exhaust gas mass flows and high SCR catalytic converter temperature, the storage capacity of ammonia on the SCR catalytic converters decreases 56 , 60 so that for the selective reduction of nitrogen oxides, the maximum available catalyst volume or the maximum ammonia storage capacity of the SCR catalyst is no longer available 56 , 60 can be used. At temperatures> 350 ° C, the conversion performance of the SCR catalytic converter also decreases 56 , 60 from. Due to the arrangement of the first SCR catalytic converter close to the engine 56 is it too hot for optimal conversion.

Even if the temperature is too low, the conversion decreases significantly. On the one hand, this is due to the kinetics of the SCR catalytic converters 56 , 60 ongoing reactions and, on the other hand, because the mixture preparation of the metered urea-water solution and the subsequent hydrolysis and thermolysis of the urea-water solution to ammonia take increasingly longer with decreasing temperature. The second SCR catalytic converter is particularly important because it is installed far from the engine 60 in real driving of a motor vehicle often too cold for an optimal conversion.
The following cases arise in real driving, for which the temperature gradient in the exhaust system is achieved by means of exhaust gas heat transfer (Exhaust Aftertreatment Heat Transfer EAHT) 40 can be made uniform:

Case 1: Immediately after starting the engine, the first SCR system close to the engine must 92 be heated as quickly as possible to above the light-off temperature. Here, EAHT can be used to heat the close-coupled particulate filter 54 of the first SCR system 92 accelerated and the active catalyst volume increased. The close-coupled particle filter is here 54 as a particle filter 54 with SCR coating 56 executed.

Case 2: In many operating points it is on the first SCR catalytic converter close to the engine 56 too hot and on the second SCR catalyst 60 too cold to be able to ensure the best possible NOx conversion. The basic idea behind the SCR-EAHT is the transfer of heat from the first SCR system close to the engine 92 to the second SCR system that is too cold 94 in the underbody position. Using the EAHT process, the in the exhaust system 40 Stored thermal energy evenly on both SCR catalysts 56 , 60 be distributed.

Case 3: SCR-EAHT also only allows cooling of the first particulate filter close to the engine 54 with SCR coating 56 without influencing the heat balance of the second SCR catalytic converter 60 . The particle filter is used for this 54 via that in the low-pressure exhaust gas recirculation 70 integrated exhaust gas recirculation cooler 76 cooled without exhaust gas or exhaust air passing through the second SCR catalytic converter remote from the engine 60 flows and cools it.

Case 4: During very long periods of high-load operation, it can happen that the temperature on both SCR components 56 , 60 is too high. In this case EAHT can use the excess heat from both SCR catalytic converters 56 , 60 discharged and the process for cooling the two SCR catalysts 56 , 60 be used.

In normal operation with diesel engines 10 the throttle valve in overrun phases 32 closed to cool the exhaust system 40 to prevent. The invention provides for an adaptation of the application of the internal combustion engine 10 in the overrun phases to an SCR system 92 , 94 to be able to operate as efficiently as possible by using the heat capacities without compromising the efficiency of the internal combustion engine 10 affect negatively.

In 1 is a first operating state of the internal combustion engine 10 shown in which the close-coupled particle filter 54 with the SCR coating 56 is heated in an overrun phase of the motor vehicle. Immediately after starting the combustion engine 10 or in very long low-load phases, the primary task of exhaust gas temperature management is the SCR catalytic converter close to the engine 56 to bring it to its operating temperature. In order for the NOx emissions emitted after the engine start or in the low load phases to be converted, both the exhaust gas temperature required for the metering release and the particle filter must be reached as quickly as possible 54 must be heated to a minimum temperature level so that on the SCR coating 56 of the close-coupled particulate filter 54 NOx emissions can be reduced accordingly.

The effectiveness of the NOx conversion depends on several factors: First, the temperature must be in front of the first SCR catalytic converter 56 be correspondingly high so that the dosed urea-water solution evaporates and can be made available to the reduction reaction as NH3 by means of hydrolysis and thermolysis. Second, the component temperature of the particulate filter must 54 with the SCR coating 56 be sufficiently high in the front flow area, above the light-off temperature, so that the SCR coating 56 of the particle filter 54 catalytically supports the reduction reaction. Thirdly, the efficiency or the overall degree of conversion depends on the total active catalyst volume available. Only the area (the volume) of the first SCR catalytic converter is active here 56 which is sufficiently hot. The purpose of the SCR heating method here is to control the active volume of the first SCR catalytic converter that takes part in the reduction reaction 56 by adding the first SCR system close to the engine 92 in the overrun phases of the internal combustion engine 10 an additional heat flow is made available, by means of which the SCR coating 56 of the particle filter 54 can be heated faster over the entire catalyst volume.

This is done in the overrun phases of the internal combustion engine 10 the low-pressure exhaust gas recirculation 70 used to generate additional hot air or a correspondingly high heat flow through the particle filter 54 the first SCR system close to the engine 92 respectively. In the case of an engine overrun phase, the low-pressure exhaust gas recirculation valve is used for this purpose 74 open. Optionally, a bypass valve for the exhaust gas recirculation cooler can also be activated at the same time 76 in the low pressure exhaust gas recirculation 70 in order not to additionally cool the exhaust air as well as the control of the throttle valve 32 of the air supply system 20th to set a correspondingly high flushing gradient and to circulate the draw air. The effect of the forced air heating can also be reinforced by closing the exhaust flap at the same time 66 . The exhaust air is here both in the combustion engine 10 itself is heated by the compression and the hot combustion chamber walls and also absorbs heat in the hot manifold and turbo charger 30th and provides this heat flow to the particle filter 54 with SCR coating 56 available in the overrun phases. The overrun air mass flow and the resulting heat flow can be controlled by the exhaust flap actuators 66 , Low pressure exhaust gas recirculation valve 74 , Bypass of the Exhaust gas recirculation cooler 76 as well as the throttle valve 32 be managed.
The additional heat flow made available in the overrun phases leads to faster heating of the SCR catalytic converter volume. The recirculated air process is terminated as soon as the resulting heat flow no longer has a positive effect on heating the catalyst volume.

This first operating mode of the internal combustion engine 10 is advantageously used when both the first SCR catalytic converter close to the engine 56 of the close-coupled SCR system 92 in the temperature range <200 ° C and the second SCR catalytic converter 60 of the SCR system remote from the engine 94 is in the temperature range <200 ° C. This is mainly the case after a cold start when the goal of the operating strategy is the SCR system close to the engine 92 to be heated up as quickly as possible and ready for operation. Advantageously, the first 10 minutes after the internal combustion engine has been cold started for this purpose 10 used. The first SCR system close to the engine 92 and the second SCR system 94 both are not ready for operation during a cold run. In particular, the components in the exhaust area, manifold and turbocharger that get hot quickly 30th and the heat stored in these can be used in this first operating mode to control the SCR system close to the engine 92 to heat faster above the light-off temperature. This first operating mode remains active until the metering valve 52 for the SCR system close to the engine 92 has exceeded a temperature release threshold. This should advantageously be in the region of 180 ° C.

In 2 is a second operating state of the internal combustion engine 10 shown. In this second operating state, the heat from the SCR system close to the engine 92 on the second SCR system 94 transfer. This is always the case when the SCR catalytic converter is close to the engine 56 too hot for optimal conversion and the second SCR catalytic converter 60 too cold for optimal conversion. This is done using the throttle 32 of the air supply system 20th no longer completely closed in overrun phases, but remains or is opened to a certain extent. As a result, a large and hot exhaust gas or air mass flow is present even in overrun phases, which is a second SCR catalytic converter arranged downstream in the underbody of the vehicle 60 can be made available and used. The air is no longer produced by the internal combustion of the internal combustion engine 10 warmed up. Due to the large heat capacities of the internal combustion engine 10 , the combustion chamber walls and the exhaust system 40 to the SCR catalytic converter close to the engine 56 the draw air is heated and kept at a sufficiently high temperature level. The resulting heat flow becomes the second SCR catalytic converter 60 heated and tempered. The exhaust gas temperature or the theoretically usable exhaust gas heat flow to the second SCR catalytic converter 60 is made available via the position of the throttle valve 32 in the intake duct 22nd and the control duty cycle of the high pressure exhaust gas recirculation valve 36 regulated. From the control positions of the throttle valve actuators 32 and high pressure exhaust gas recirculation valve 36 the exhaust gas mass flow and temperature for warming up, heating and keeping the second SCR catalyst warm 60 . About the temperature sensors 86 , 88 , 90 downstream of the particulate filter 54 with SCR coating 56 as well as via the throttle valve 32 and via the high-pressure exhaust gas recirculation 34 recirculated exhaust gas mass flow is the temperature of the exhaust gas mass flow in the overrun phase upstream of the second SCR system 94 regulated such that the temperature upstream of the second SCR system 94 always greater than the component temperature of the second SCR catalytic converter 60 , 64 is, so that there is always a positive temperature gradient for the necessary heat transfer from the hot exhaust system components located upstream to the catalytic converters 60 , 64 of the second SCR system 94 adjusts. The low pressure exhaust gas recirculation valve 74 and the associated exhaust gas recirculation cooler 76 are in this second operating state of the internal combustion engine 10 to heat up the second SCR catalytic converter 60 not active in the underbody position.

This second operating mode is advantageously used when the first SCR catalytic converter close to the engine is located 56 of the close-coupled SCR system 92 in the temperature range> 300 ° C and the second SCR catalytic converter 60 is in the temperature range <300 ° C.

In 3 is a third operating state of the internal combustion engine 10 shown. The first SCR catalytic converter close to the engine is activated in this third operating state 56 chilled. The Exhaust gas recirculation cooler 76 in the low pressure exhaust gas recirculation 70 activated to during the overrun phase of the internal combustion engine 10 via the exhaust gas recirculation cooler 76 Heat from the first SCR system close to the engine 92 via the low-pressure exhaust gas recirculation 70 to the cooling system of the internal combustion engine 10 submit. The low pressure exhaust gas recirculation 70 can be used in overrun mode to only activate the first SCR catalytic converter close to the engine 56 to flow through and to cool without the temperature and the heat content of the second SCR catalyst 60 to influence. The exhaust flap is used for this 66 closed and the exhaust air during an overrun phase via the low-pressure exhaust gas recirculation 70 and the intake duct 22nd as well as the internal combustion engine 10 pumped in a circle. The heat is generated by the first SCR catalytic converter near the engine 56 recorded and via the exhaust gas recirculation cooler 76 in the low pressure exhaust gas recirculation 70 as well as the intercooler 96 delivered to the engine cooling circuit.

This third operating state of the internal combustion engine 10 is advantageously used when both the first SCR catalytic converter 56 of the close-coupled SCR system 92 in the temperature range> 350 ° C and the second SCR catalytic converter 60 of the SCR system remote from the engine 94 are in the temperature range> 300 ° C. The prerequisite for this third operating mode is that the first SCR catalytic converter is close to the engine 56 too hot and the second SCR catalytic converter 60 works in the optimal temperature range. It is just the cooling of the particle filter 54 with SCR coating 56 desirable without affecting the second SCR catalytic converter 60 . Cooling and releasing excess heat from the exhaust system into the internal combustion engine's cooling system 10 only makes sense if all components of the exhaust system 40 have reached their optimal operating range and a further increase in temperature leads to a deterioration in the efficiency of the overall system.

In 4th is a fourth operating state of the internal combustion engine 10 shown. This is after long-term full load operation or immediately after regeneration of the particle filter 54 the temperature level of both SCR catalytic converters 56 , 60 too high for optimal operation. Temperatures that are too high> 500 ° C lead to poorer conversion behavior; Aging effects can also occur from 700 ° C. Both SCR catalytic converters can 56 , 60 be cooled. Both exhaust gas recirculation valves are used for this purpose 36 , 74 closed, the throttle valve 32 in the air supply system 20th of the internal combustion engine 10 regulated open and the exhaust system 40 cooled with draw air.

This fourth operating state is advantageously used when both the first SCR catalytic converter close to the engine is located 56 of the close-coupled SCR system 92 in the temperature range> 350 ° C and the second SCR catalytic converter 60 of the SCR system remote from the engine 94 are also in the temperature range> 350 ° C and simultaneous cooling of both components is required. Here, the high pressure exhaust gas recirculation valve is in overrun mode 36 closed. The complete exhaust gas mass flow of the combustion engine 10 is thus over the first SCR catalytic converter 56 directed and cools this and the downstream second SCR catalyst 60 . In addition, the control of the low-pressure exhaust gas recirculation valve 74 the mass flow and the associated cooling capacity for both SCR catalytic converters 56 , 60 regulated separately. By activating the low-pressure exhaust gas recirculation valve 74 becomes a controllable part of the total mass flow upstream of the second SCR catalytic converter 60 back to the internal combustion engine 10 guided. Through the low-pressure exhaust gas recirculation 70 can thus for the first SCR catalytic converter 56 higher cooling capacities can be realized. The required cooling capacities for both SCR catalytic converters 56 , 60 can therefore be regulated separately.

1. 1. Prinzipbedingt ist das Temperaturniveau des Abgases aufgrund der Wärmeverluste in der Abgasanlage für motorfernere Bauteile deutlich geringer.
2. 2. Zusätzlich ist der Wärmestrom, der sich aus dem Temperaturniveau und zusätzlich dem Abgasmassenstrom ergibt, für das motornahe erste SCR-System 92 deutlich größer. Das motornahe erste SCR-System 92 wird vom gesamten Abgasmassenstrom des Verbrennungsmotors 10 durchströmt. Der Wärmestrom für das zweite SCR-System 94 ist um den Anteil der Niederdruck-Abgasrückführung 70 geringer, da dieser Anteil des Massenstroms stromabwärts des ersten SCR-System 92 über die Niederdruck-Abgasrückführung 70 wieder dem Verbrennungsmotor 10 zugeführt wird und nicht durch die SCR-Katalysatoren 60, 64 des zweiten SCR-Systems 94 strömt.

Under real driving conditions, the SCR system close to the engine will be reflected in particular at higher engine loads and speeds 92 Set a significantly higher temperature level than on the second SCR system remote from the engine 94 . There are two reasons for this:

1. 1. Due to the principle, the temperature level of the exhaust gas is significantly lower for components further away from the engine due to the heat losses in the exhaust system.
2. 2. In addition, the heat flow, which results from the temperature level and also the exhaust gas mass flow, is for the first SCR system close to the engine 92 significantly larger. The first SCR system close to the engine 92 is derived from the entire exhaust gas mass flow of the combustion engine 10 flows through. The heat flow for the second SCR system 94 is the proportion of low-pressure exhaust gas recirculation 70 lower, since this portion of the mass flow is downstream of the first SCR system 92 via the low-pressure exhaust gas recirculation 70 back to the internal combustion engine 10 is supplied and not through the SCR catalytic converters 60 , 64 of the second SCR system 94 flows.

The best possible conversion for the overall system results when both SCR systems 92 , 94 can be operated at the optimum operating temperature.

List of reference symbols

10

Internal combustion engine

12

Combustion chamber

14th

Fuel injector

16

inlet

18th

Outlet

20th

Air supply system

22nd

Intake duct

24

Air filter

26th

Air mass meter

28

Confluence

30th

Exhaust gas turbocharger

32

throttle

34

High pressure exhaust gas recirculation

36

High pressure exhaust gas recirculation valve

38

Exhaust gas recirculation line

40

Exhaust system

42

Exhaust duct

44

turbine

46

first catalyst

48

Oxidation catalyst

50

NOx storage catalytic converter

52

Dosing valve

54

Particle filter

56

SCR coating / first SCR catalytic converter

58

branch

60

second SCR catalytic converter

62

second metering valve

64

Ammonia barrier catalyst

66

Exhaust flap

68

Control unit

70

Low pressure exhaust gas recirculation

72

Exhaust gas recirculation duct

74

Low pressure exhaust gas recirculation valve

76

Exhaust gas recirculation cooler

78

filter

80

first temperature sensor

82

second temperature sensor

84

third temperature sensor

86

fourth temperature sensor

88

fifth temperature sensor

90

sixth temperature sensor

92

close-coupled SCR system

94

Underbody SCR system

96

Intercooler

98

compressor

Claims (10)

Method for exhaust gas aftertreatment of an internal combustion engine (10), the inlet (16) of which is connected to an air supply system (20) and the outlet (18) of which is connected to an exhaust system (40), with a high-pressure exhaust gas recirculation being provided, which the exhaust system (40) is upstream a turbine (44) of an exhaust gas turbocharger with the air supply system (20) downstream of a compressor (98) of the exhaust gas turbocharger (30), with a first SCR system close to the engine in the exhaust system (40) downstream of the turbine (44) of the exhaust gas turbocharger (30) (92) and downstream of the first SCR system (92) close to the engine, a second SCR system (94) is arranged, with a low-pressure exhaust gas recirculation system (94) downstream of the first SCR system (92) and upstream of the second SCR system (94). 70) branches off from an exhaust duct (42) of the exhaust system (40), which connects the exhaust duct (42) to an intake duct (22) of the air supply system (20) upstream of the compressor (98) of the exhaust gas turbocharger (30) indet, comprising the following steps: - Heating of the first SCR system (92) in a first operating state of the internal combustion engine (10), with the low-pressure exhaust gas recirculation (70) open and an exhaust gas duct (42) of the exhaust system downstream of the branch (58) for the low-pressure exhaust gas recirculation (70 ) is at least partially blocked by an exhaust flap (66), - Heating the second SCR system (94) in a second operating state of the internal combustion engine (10) by convective heat transfer from the first SCR system (92) to the second SCR system (94), the low-pressure exhaust gas recirculation (70) being closed and the exhaust flap (66) is open, - Cooling of the first SCR system (92) in a third operating state of the internal combustion engine (10), the low-pressure exhaust gas recirculation (70) being open and the exhaust gas duct (42) being at least partially blocked by the exhaust flap (66), the recirculated exhaust gas is cooled by an exhaust gas recirculation cooler (76) in the low-pressure exhaust gas recirculation (70), - Cooling of the first SCR system (92) and the second SCR system (94) in a fourth operating state of the internal combustion engine (10), wherein the low-pressure exhaust gas recirculation (70) and the exhaust gas flap (66) are open and a low pressure Exhaust gas recirculation (70) recirculated exhaust gas flow is cooled by an exhaust gas recirculation cooler (76) in the low-pressure exhaust gas recirculation (70). Procedure according to Claim 1 , Characterized in that the exhaust gas temperature (T _EC) is determined and is compared with a first threshold temperature (T _S1), wherein the first at an exhaust gas temperature (T _EG) is below the first threshold temperature (T _S1) SCR system is heated. Procedure according to Claim 1 or 2 , characterized in that the exhaust gas temperature (T _EG ) is determined and compared with a first threshold temperature (T _S1 ) and a second threshold temperature (T _S2 ), with an exhaust gas temperature (T _EG ) which is between the first threshold temperature (T _S1 ) and the second threshold temperature (T _S2 ), the second SCR system (94) is convectively heated by the waste heat of the first SCR system (92). Method according to one of the Claims 1 to 3 , characterized in that the exhaust gas temperature (T _EG ) is determined and compared with a second threshold temperature (T _S2 ) and a third threshold temperature (T _S3 ), with an exhaust gas temperature (T _EG ) which is between the second threshold temperature (T _S2 ) and the third threshold temperature (T _S3 ), the first SCR system (92) is cooled by the third operating state of the internal combustion engine (10). Method according to one of the Claims 1 to 4th , Characterized in that the exhaust gas temperature (T _EC) is determined and is compared with a third threshold temperature (T _S3), wherein, the first at an exhaust gas temperature (T _EG), which is above the third threshold temperature (T _S3) SCR system (92) and the second SCR system (94) are cooled by the fourth operating state of the internal combustion engine (10). Method according to one of the Claims 1 to 5 , characterized in that the temperature (T _SCR1 ) of the first SCR system (92) and the temperature (T _SCR2 ) of the second SCR system (94) are determined, the temperature difference (ΔT) between the first SCR system ( 92) and the second SCR (94) is determined and is compared with a fourth threshold value (.DELTA.T _S), at a temperature difference (.DELTA.T) which is greater than the fourth threshold value (.DELTA.T _S), the heat convection from the first SCR system (92) is transferred to the second SCR system (94) in the second operating state of the internal combustion engine (10) in order to reduce the temperature difference (ΔT) between the two SCR systems (92, 94). Method according to one of the Claims 1 to 6th , characterized in that switching between the four operating modes of the internal combustion engine (10) takes place in such a way that at least one of the SCR systems (92, 94) has a temperature between 200 ° C and 380 ° C, preferably between 220 ° C and 350 ° C. Procedure according to Claim 7 , characterized in that switching between the four operating modes of the internal combustion engine (10) takes place in such a way that both SCR systems (92, 94) have a temperature between 200 ° C and 380 ° C, preferably between 220 ° C and 350 ° C, exhibit. Method according to one of the Claims 1 to 8th , characterized in that in the fourth operating mode of the internal combustion engine (10) a high-pressure exhaust gas recirculation valve (36) is closed and the entire exhaust gas flow is passed through the first SCR system (92) in order to cool the first SCR system (92) to effect. Internal combustion engine (10) with an air supply system (20) and an exhaust system (40), as well as with an engine control unit (68) which is set up to implement a method according to one of the Claims 1 to 9 to be performed when a machine-readable program code is executed by the engine control unit (68).

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