DE102017200089B3 - Motor vehicle with exhaust gas turbocharger and SCR exhaust aftertreatment and method for its operation - Google Patents

Abstract

A method for operating a motor vehicle having a first SCR exhaust aftertreatment device (5, 6) upstream of a branch to a low-pressure exhaust gas recirculation (4). The motor vehicle (1) has a hybrid-electric drive, and downstream of the branch to the low-pressure exhaust gas recirculation (4), a second SCR exhaust aftertreatment device (7) is arranged in the exhaust tract. Ammonia for the SCR exhaust aftertreatment is supplied or generated upstream of the branch to the low pressure exhaust gas recirculation (4). During driving, at least the current ammonia loading of the first SCR exhaust aftertreatment device (5, 6) is continuously determined and also checked whether the current propulsion requirement for the vehicle corresponds to a combination of temperature and volume flow in the exhaust system, which is expected to ammonia desorption in the first SCR exhaust aftertreatment device (5, 6) leads. As long as this is the case, the current combination of torque and speed of the internal combustion engine (1) using the hybrid electric drive to maintain the current propulsion request is changed so that either the operating conditions of the first SCR exhaust aftertreatment device (5, 6) below the desorption Limits remain or the first SCR exhaust aftertreatment device (5, 6) desorbed ammonia accelerates, while in the meantime switching from low-pressure exhaust gas recirculation (4) to high-pressure exhaust gas recirculation (2).

Description

The invention relates to a method for operating a motor vehicle with an internal combustion engine with an exhaust gas turbocharger and a first SCR exhaust aftertreatment device upstream of a branch to a low-pressure exhaust gas recirculation and also relates to a corresponding motor vehicle according to the preambles of the independent claims.

Such a method and motor vehicle are from the US 2014/0165560 A1 known. The low-pressure exhaust gas recirculation line contains a filter which converts any residual ammonia in the recirculated exhaust gas into harmless substances, so that no ammonia is returned to the internal combustion engine. As a result, however, the ammonia is not completely used for nitrogen oxide reduction, and the urea carried in the vehicle must be refueled accordingly more frequently.

Internal combustion engines often generate significant amounts of nitrogen oxides (NOx) during operation. Especially in diesel and petrol engines used in motor vehicles, the amounts of nitrogen oxide in the exhaust gas are generally above the permissible limits, so that an exhaust aftertreatment to reduce the NOx emissions is necessary. In many engines, the reduction of nitrogen oxides by the non-oxidized constituents contained in the exhaust gas, namely by carbon monoxide (CO) and unburned hydrocarbons (HC), using a three-way catalyst. In particular, in diesel and gasoline lean-burn engines, however, this method is not available due to the small amounts of non-oxidized exhaust gas constituents. In the case of lean-burn engines, a NOx storage catalytic converter (hereinafter referred to as LNT for the English Lean NOx Trap) is used in a widespread method, which absorbs and stores the nitrogen oxides contained in the exhaust gas of the internal combustion engine. From time to time, regeneration of the LNT occurs, for example, for which excess fuel is generated in the exhaust gas conducted by the LNT.

A disadvantage of LNTs is the limited temperature window in which sufficient NOx conversion efficiency can be achieved. Above a maximum LNT operating temperature, the NOx conversion efficiency becomes inefficient, or the catalyst even undergoes unintentional aging and deterioration of the catalyst properties.

For nitrogen oxide reduction even at higher exhaust gas temperatures than the maximum LNT operating temperature SCR exhaust aftertreatments are suitable, d. H. with Selective Catalytic Reaction (SCR). SCR exhaust aftertreatment devices are z. B. SCR catalysts and SCR coated diesel particulate filter, abbreviated SDPF.

For selective catalytic reduction, ammonia (NH 3) is admixed to the exhaust gas, in the case of motor vehicle internal combustion engines in the form of an aqueous urea solution called AdBlue, which is evaporated after the injection and decomposed into ammonia and other substances. The ammonia is stored in the SCR exhaust aftertreatment device and converts the nitrogen oxides contained in the exhaust gas, ideally in nitrogen.

The urea dosing must correspond to the current nitrogen oxide emission of the internal combustion engine and therefore depends on the current operating conditions of the engine. If the dosage is too low, too little nitrogen oxide is reduced to comply with the pollutant limits. If the dosage is too high, excess ammonia enters the exhaust gas, which is called NH3 slip. This must be prevented because ammonia is harmful to health, even in very small concentrations leads to an odor nuisance and may affect downstream exhaust aftertreatment devices.

Exhaust aftertreatment devices are also known which contain both an LNT and an SCR catalyst, e.g. B. from the DE 10 2010 010 039 B4 , If at high exhaust temperatures the LNT is no longer efficient enough, excess NOx is converted to the SCR catalyst by urea injection.

From the DE 10 2015 204 093 A1 is a hybrid drive for a motor vehicle is known whose SCR catalyst is regulated so that a temperature of the flowing exhaust gas does not exceed a predetermined limit temperature.

The DE 10 2011 107 692 B3 discloses an exhaust gas purification system having both low and high pressure exhaust gas recirculation, exhaust turbocharger, first SCR exhaust aftertreatment device upstream of a low pressure exhaust gas recirculation branch, second SCR exhaust aftertreatment device downstream of the low pressure exhaust gas recirculation branch.

The DE 10 2012 204 352 A1 discloses a drive device with an internal combustion engine and a further drive unit, in which it is checked whether the internal combustion engine is in a favorable for exhaust aftertreatment operating point.

The invention is based on the object of being able to operate an internal combustion engine with exhaust-gas turbocharger and SCR exhaust-gas treatment for as long as possible with low-pressure exhaust gas recirculation without having to accept an excess of ammonia consumption due to the low-pressure exhaust gas recirculation.

This object is achieved by a method and a motor vehicle having the features of the independent patent claims.

Advantageous developments of the invention are specified in the dependent claims.

According to the invention, a generic motor vehicle is a hybrid electric vehicle and downstream of the branch to the low-pressure exhaust gas recirculation, a second SCR exhaust aftertreatment device is arranged in the exhaust tract, wherein ammonia is supplied or generated for SCR exhaust aftertreatment upstream of the branch to the low-pressure exhaust gas recirculation.

According to the invention, while driving, at least the current ammonia loading of the first SCR exhaust aftertreatment device is continuously determined and it is also checked whether the current propulsion requirement for the vehicle corresponds to a combination of temperature and volume flow in the exhaust tract, which is expected to ammonia desorption in the first SCR exhaust aftertreatment device leads. As long as this is the case, the current engine torque and speed combination using the hybrid electric drive to maintain the current propulsion request is changed so that either the operating conditions of the first SCR exhaust aftertreatment device remain below the desorption limits or the first SCR Exhaust after-treatment device accelerates ammonia desorbed, while being switched from low-pressure exhaust gas recirculation to high-pressure exhaust gas recirculation.

This hybrid drive assisted SCR exhaust aftertreatment strategy maximizes the time that the engine is operated with low pressure exhaust gas recirculation, optimizing the benefits of low pressure exhaust gas recirculation with minimal ammonia to engine feedback.

In a preferred embodiment of the invention, when it is determined that the current propulsion demand corresponds to a combination of temperature and volumetric flow that is expected to result in undesirable ammonia desorption in the first SCR exhaust aftertreatment device, the low pressure exhaust gas recirculation is maintained for some time the actual combination of torque and speed of the internal combustion engine is adjusted so that the operating conditions of the first SCR catalyst or SDPF remain below its desorption limits.

If, after expiration of the predetermined time, it is still determined that the current propulsion demand corresponds to a combination of temperature and volumetric flow that is expected to result in undesirable ammonia desorption in the first SCR exhaust after-treatment device, in a preferred embodiment of the invention, the current combination of torque and engine speed is adjusted so that the ammonia stored in the first SCR exhaust aftertreatment device is desorbed faster than before or as fast as possible during which the engine is operated with high pressure exhaust gas recirculation.

Preferably, a similar adjustment of torque and speed of the internal combustion engine is made with switching to high-pressure exhaust gas recirculation, if the current ammonia load of the second SCR exhaust aftertreatment device is below a lower threshold.

If it is determined that the current ammonia load of the first SCR exhaust aftertreatment device falls below a lower temperature and flow rate dependent threshold, the current engine torque and speed combination may be reset and the engine may be again operated with low pressure exhaust gas recirculation.

The first SCR exhaust aftertreatment device is preferably or includes an SCR catalyst and / or an SDPF.

Upstream of the first SCR exhaust aftertreatment device, a urea injector or other device for supplying or producing ammonia is preferably arranged.

The first SCR exhaust aftertreatment device is preferably an LNT or contains such.

The second SCR exhaust after-treatment device is preferably an active SCR catalyst or contains one.

• 1 einen Abgastrakt eines Verbrennungsmotors in einem Kraftfahrzeug; und
• 2 ein Flussdiagramm zur Erläuterung eines Betriebs des Kraftfahrzeugs.

The following is a description of embodiments with reference to the drawings. Show:

• 1 an exhaust tract of an internal combustion engine in a motor vehicle; and
• 2 a flowchart for explaining an operation of the motor vehicle.

1 schematically shows an exhaust tract of an internal combustion engine 1 with high-pressure exhaust gas recirculation 2 in front of a turbine 3 an exhaust gas turbocharger. The internal combustion engine 1 forms a part of the powertrain of a hybrid electric vehicle and has a combined low pressure / high pressure exhaust gas recirculation and an exhaust aftertreatment device with a plurality of spatially separated SCR exhaust aftertreatment devices.

In the embodiment shown, the exhaust aftertreatment device contains within the low-pressure branch of the exhaust gas recirculation, that is, upstream of a branch for the low-pressure exhaust gas recirculation 4 , an LNT 5 which may also be combined with a DOC (Diesel Oxidation Catalyst), and also a first SCR catalyst or SDPF 6 ,

• - Er kann in einem anderen Temperaturfenster als der erste SCR-Katalysator oder der SDPF arbeiten, so dass in einem breiteren Temperaturfenster NOx-Umwandlung stattfinden kann.
• - Er kann in einem niedrigeren Temperaturfenster als der erste SCR-Katalysator oder der SDPF arbeiten, was den Alterungseinfluss eines mit der Zeit erhöhten Ammoniak-Schlupfes des ersten SCR-Katalysators oder des SDPF vermindert.
• - Das Volumen des ersten, wegen der Niederdruck-Abgasrückführung relativ nahe am Verbrennungsmotor angeordneten ersten SCR-Katalysators ist häufig durch bauliche Umstände beschränkt, und dies gilt noch häufiger für einen SDPF, einen Dieselpartikelfilter mit SCR-beschichtetem Filtersubstrat.

Downstream of the branch for the low-pressure exhaust gas recirculation 4 the exhaust aftertreatment device includes a second, passive SCR catalyst 7 , Downstream of the LNT 5 and upstream of the first SCR catalyst or SDPF 6 there is a urea injector 8th , and downstream of the second SCR catalyst 7 there is a NOx sensor 9 in the exhaust stream. The second SCR catalyst 7 which can be advantageously added in the form of a small underbody SCR catalyst offers the following advantages:

• It can operate in a different temperature window than the first SCR catalyst or the SDPF, so that NOx conversion can take place in a wider temperature window.
• It can operate in a lower temperature window than the first SCR catalyst or SDPF, reducing the aging effect of increased ammonia slip of the first SCR catalyst or SDPF over time.
• The volume of the first SCR catalytic converter, which is located relatively close to the internal combustion engine due to the low-pressure exhaust gas recirculation, is often restricted by structural circumstances, and this is more frequently the case for an SDPF, a diesel particulate filter with SCR-coated filter substrate.

• - Rückgeführtes Ammoniak wird nicht zur NOx-Umwandlung in dem zweiten SCR-Katalysator 7 genutzt.
• - Es hat sich gezeigt, dass rückgeführtes Ammoniak im Verbrennungsmotor zu NOx verbrannt wird. Daher muss zusätzliches Ammoniak injiziert werden, um auch das zusätzlich erzeugte NOx aus dem Abgas zu entfernen.

However, low pressure exhaust gas recirculation also carries part of upstream of the first SCR catalyst or SDPF 6 injected ammonia to the internal combustion engine 1 back, and this has two unfavorable effects:

• - Returned ammonia does not become NOx conversion in the second SCR catalyst 7 used.
• - It has been shown that recycled ammonia is burned in the internal combustion engine to NOx. Therefore, additional ammonia must be injected to also remove the additionally generated NOx from the exhaust gas.

The amount of ammonia that can be stored in an SCR catalyst without significant ammonia slip is due to the balance between adsorption and desorption reactions. The higher the temperature, the more the desorption manifests itself over the adsorption. Therefore, the maximum possible ammonia loading of an active SCR catalyst generally decreases with temperature. In addition, ammonia slip increases with volumetric flow, but is too dynamic to adequately control ammonia loading.

Due to the structural limitations of volume restriction of the first, relatively close to the internal combustion engine arranged SCR catalyst or SDPF 6 ammonia slip is likely to occur therein, especially with increasing SCR catalyst or SDPF aging 6 , Given the limited volume allows the second, z. B. under the underbody of the motor vehicle arranged SCR catalyst 7 to achieve the necessary overall NOx conversion efficiency.

In the case of low-pressure exhaust gas recirculation, the ammonia concentration in the exhaust gas between two upstream and downstream of a branch for the low-pressure exhaust gas recirculation SCR exhaust aftertreatment devices is a problem, as explained above.

When catalytic operation changes to higher temperatures and / or flow rates during a drive cycle, downstream of the first SCR catalyst or SDPF 6 quite high ammonia concentrations occur. Is the internal combustion engine 1 at the same time in a mode with low-pressure exhaust gas recirculation, quite high amounts of ammonia can be recycled. This feedback must be minimized.

One way to reduce ammonia recycle would be to switch to high pressure exhaust gas recirculation as long as ammonia desorption occurs in the first SCR catalyst or SDPF. This could happen once a certain concentration has been reached, or better based on one expected ammonia slip, which is obtained by evaluating the current engine operating conditions and derived from expected operating conditions. Nevertheless, ammonia desorption could sometimes take place, and the benefits of low pressure exhaust gas recirculation would be lost.

By contrast, the benefits of low pressure exhaust gas recirculation are largely maintained when using the hybrid electric drive to control ammonia desorption and migration from the first closer to the engine SCR catalyst or SDPF 6 to the second SCR catalyst further away from the engine 7 to control in an effective manner while the degree of utilization of low pressure exhaust gas recirculation 4 is maximized.

For this purpose, use is made of the fact that hybrid-electric drives make it possible to control temperatures and / or volume flows through the exhaust gas tract by controlling the torque and / or rotational speed of the internal combustion engine. In any case, this is possible within the limits as torques and / or rotational speeds of the internal combustion engine deviating from a current propulsion request can be compensated for by the hybrid drive or electric motors by the electric machine (s) either supplying missing power or converting more power of the internal combustion engine into electric power ,

Using the hybrid drive undesirable ammonia desorption during low pressure exhaust gas recirculation is counteracted as follows, wherein 2 Reference is made to:

In a first step S1, taking into account the current ammonia loading of the first SCR catalyst or SDPF 6 , which can be determined by any method known in the art, checks whether the driver's current propulsion requirement (or in the case of an autonomous motor vehicle of a computer program) corresponds to a combination of temperature and volumetric flow rate in the exhaust tract that results in undesirable ammonia desorption in the first SCR catalyst or SDPF 6 would lead.

In a second step S2, it is first assumed that the danger of ammonia desorption will exist only for a short time, and accordingly the combination of torque and speed of the internal combustion engine 1 adjusted (which in this case a reduction of torque and speed of the engine 1 and generating the power difference by the electric machine means) that the operating conditions of the first SCR catalyst or SDPF 6 remain below its desorption limits, so that the low-pressure exhaust gas recirculation can be maintained without the risk of ammonia desorption.

If the danger of ammonia desorption persists for a long time, in which the electric machine is the internal combustion engine 1 supported, the hybrid drive will reach the limit of battery capacity. If this situation persists, the opposite of what was done in the second step S2 is done.

Namely, in a third step S3, the combination of torque and rotational speed of the internal combustion engine is set in the other direction (which in this case means an increase in torque and rotational speed of the internal combustion engine and absorption of the power difference by the electric machine). As a result, the ammonia is very fast, preferably as fast as possible from the first SCR catalyst or SDPF 6 desorbed and the second SCR catalyst 7 fed. As long as this lasts, the low-pressure exhaust gas recirculation is closed and becomes the internal combustion engine 1 with high-pressure exhaust gas recirculation 2 operated.

The freedom, torque and speed of the internal combustion engine 1 of a hybrid electric vehicle to vary within certain limits, it allows to shorten the described desorption phase. After the necessary ammonia desorption from the first SCR catalyst or SDPF 6 In other words, if the current ammonia load remains below the desorption limit for the current operating conditions, the internal combustion engine becomes 1 again with low-pressure exhaust gas recirculation 4 operated, and it returns to step S1.

Desirable ammonia desorption may also be effected when it is determined in a step S4 that the current ammonia loading of the second SCR catalyst 7 is below a lower threshold and the second SCR catalyst 7 should be loaded with ammonia again. For this purpose, in step S5, a similar strategy as in step S3 with short-time switching to high-pressure exhaust gas recirculation and with short-term ammonia injection can be used, after which the method begins again with step S1.

• - Alternativ zu Urea-Injektion könnte Injektion von gasförmigem Ammoniak angewendet werden.
• - Alternativ oder zusätzlich zu dem Urea-Injektor 8 könnte der LNT 5 verwendet werden, um Ammoniak zu erzeugen. Dazu muss möglicherweise ein Verdampfer oder externer Kraftstoffinjektor für effiziente Steuerung des Spülens des LNT 5 installiert werden.
• - Stromaufwärts der Abzweigung zu der Niederdruck-Abgasrückführung 4 könnte sich nur ein aktiver LNT 5 (und ggf. DOC), aber kein SCR-Katalysator oder SDPF 6 befinden, wobei eine spezielle Spülstrategie des LNT 5 verwendet wird, um darin Ammoniak für den SCR-Katalysator 7 stromabwärts der Abzweigung zu der Niederdruck-Abgasrückführung 4 zu erzeugen.

With reference to 2 The method described is also applicable to exhaust tract designs which differ in one or more of the following points from those described in US Pat 1 different arrangement shown:

• - Alternatively to urea injection, injection of gaseous ammonia could be used.
• - Alternatively or in addition to the urea injector 8th could the LNT 5 used to generate ammonia. This may require the installation of an evaporator or external fuel injector for efficient control of the flushing of the LNT 5.
• - Upstream of the branch to the low pressure exhaust gas recirculation 4 could only be an active LNT 5 (and possibly DOC), but no SCR catalyst or SDPF 6 with a special flushing strategy of the LNT 5 is used to ammonia for the SCR catalyst 7 downstream of the branch to the low pressure exhaust gas recirculation 4 to create.

LIST OF REFERENCE NUMBERS

1

internal combustion engine

2

High-pressure exhaust gas recirculation

3

turbine

4

Low-pressure exhaust gas recirculation

5

LNT

6

SCR catalyst or SDPF

7

second passive SCR catalyst

8th

Urea Injector

9

NOx sensor

S1-S5

steps

Claims (10)

Method for operating a motor vehicle having an internal combustion engine (1) with an exhaust gas turbocharger (3) and a first SCR exhaust aftertreatment device (5, 6) upstream of a branch to a low-pressure exhaust gas recirculation (4), characterized in that the motor vehicle has a hybrid electric drive ; downstream of the branch to the low-pressure exhaust gas recirculation (4) a second SCR exhaust aftertreatment device (7) is arranged in the exhaust tract; Ammonia is supplied or generated for SCR exhaust aftertreatment upstream of the branch to the low pressure exhaust gas recirculation (4); and that during driving at least the current ammonia loading of the first SCR exhaust aftertreatment device (5, 6) is determined and also checked whether the current propulsion requirement for the vehicle corresponds to a combination of temperature and flow in the exhaust system, the expected to ammonia desorption in the first SCR exhaust aftertreatment device (5, 6), wherein, as long as this is the case, the current combination of torque and speed of the internal combustion engine (1) is changed using the hybrid electric drive to maintain the current propulsion request such that either the Operating conditions of the first SCR exhaust aftertreatment device (5, 6) remain below the desorption limits or that the first SCR exhaust aftertreatment device (5, 6) desorbed desirably ammonia, while switched from low-pressure exhaust gas recirculation (4) to high-pressure exhaust gas recirculation (2) becomes. Method according to Claim 1 characterized in that, when it is determined that the current propulsion demand corresponds to a combination of temperature and volumetric flow that is expected to result in undesirable ammonia desorption in the first SCR exhaust aftertreatment device (5, 6), the low pressure exhaust gas recirculation (4 ) is maintained for a time during which the current combination of torque and speed of the internal combustion engine (1) is adjusted so that the operating conditions of the first SCR catalyst or SDPF (6) remain below its desorption limits. Method according to Claim 1 or 2 characterized in that, if the current propulsion demand, possibly after expiration of the predetermined time, corresponds to a combination of temperature and volumetric flow which leads, as expected, to undesired ammonia desorption in the first SCR exhaust aftertreatment device (5, 6), the current combination of Torque and speed of the internal combustion engine (1) is set so that the ammonia stored in the first SCR exhaust aftertreatment device (5, 6) is desorbed faster than before or as fast as possible, while the internal combustion engine (1) is operated with high-pressure exhaust gas recirculation , Method according to one of the preceding claims, characterized in that the current ammonia load of the second SCR exhaust aftertreatment device (7) is continuously determined while driving and compared with a lower threshold, wherein when the ammonia loading of the second SCR exhaust aftertreatment device ( 7) is below the lower threshold, the current combination of torque and speed of the internal combustion engine (1) is adjusted to desorb the ammonia stored in the first SCR exhaust aftertreatment device (5, 6) faster than before or as fast as possible, while the internal combustion engine (1) is operated with high-pressure exhaust gas recirculation. Method according to Claim 3 or 4 CHARACTERIZED IN THAT , when it is determined that the current ammonia load of the first SCR exhaust aftertreatment device (5, 6) falls below a lower threshold, the combination of torque and speed of the internal combustion engine (1) is reset and the engine ( 1) again with low-pressure exhaust gas recirculation (4) is operated. Motor vehicle with an internal combustion engine (1) with an exhaust gas turbocharger (3) and a first SCR exhaust aftertreatment device (5, 6) upstream of a branch to a low-pressure exhaust gas recirculation (4), characterized in that the motor vehicle is a hybrid electric vehicle; downstream of the branch to the low-pressure exhaust gas recirculation (4) a second SCR exhaust aftertreatment device (7) is arranged in the exhaust tract; upstream of the branch to the low-pressure exhaust gas recirculation (4), means (8) for supplying or producing ammonia for the SCR exhaust aftertreatment is arranged; and the motor vehicle is set up to carry out the method steps specified in one of the preceding claims. Motor vehicle after Claim 6 , characterized in that the first SCR exhaust aftertreatment device (5, 6) comprises an SCR catalyst and / or a SDPF. Motor vehicle after Claim 6 or 7 , characterized in that upstream of the first SCR exhaust aftertreatment device (5, 6) an apparatus (8) for supply or production of ammonia is arranged. Motor vehicle according to one of Claims 6 to 8th , characterized in that the first SCR exhaust aftertreatment device (5, 6) comprises an LNT. Motor vehicle according to one of Claims 6 to 9 , characterized in that the second SCR exhaust aftertreatment device (7) comprises an active SCR catalyst.

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