DE102012011603A1 - Exhaust system and method for operating such - Google Patents

Abstract

The invention relates to an exhaust system (10) for a spark-ignition internal combustion engine (12), comprising an HC adsorber (22) and a catalytic converter (20) arranged downstream thereof, at least for the conversion of hydrocarbons (HC), and a method for operating an exhaust system ( 10). It is envisaged that downstream of the HC adsorber (22) and upstream of the catalytic converter (20) a burner device (30) operated with air and fuel or operable is arranged.

Description

The invention relates to an exhaust system for spark-ignited internal combustion engines (gasoline engines) and a method for operating such an exhaust system.

In an effort to reduce pollutant emissions from internal combustion engines, usually motor measures are taken to reduce the raw emissions. In order to effectively convert the non-completely avoidable raw emissions post-engine, catalysts and other emission control components are also installed in the exhaust systems. Catalysts comprise a carrier through which the exhaust gas can flow, for. As a ceramic monolith or metal support, with a, containing a catalytically active material coating. For example, oxidation catalysts are used especially in diesel engines, which convert specifically unburned hydrocarbons (HC) and carbon monoxide (CO). Reduction catalysts that convert nitrogen oxides (NO _X ) are used in diesel engines and gasoline engines. In addition, three-way catalysts are known which combine the function of oxidation and reduction catalysts and thus convert all three components catalytically and are used mainly in gasoline engines.

In principle, all catalysts require a specific minimum temperature, the so-called light-off or light-off temperature, at which they by definition have 50% of their maximum conversion performance. After a cold start of the engine, this temperature is usually not reached, so that if no further measures are taken, the emissions referred to as start emissions leave the exhaust system without conversion. In order to heat the catalysts quickly to their light-off temperature, often motor measures, such as late ignition or post fuel injection, taken.

A common measure to reduce the starting emissions is the close-coupled arrangement of relatively small volume precatalysts, which are also referred to as start catalysts. Due to their low volume and near-to-engine placement, pre-catalysts reach their light-off temperature relatively quickly and then convert most of the emissions until a downstream main catalytic converter reaches its operating temperature.

Furthermore, it is known to arrange HC adsorber upstream of an oxidation or three-way catalyst, which cache the HC emissions. Once the HC adsorber has reached a specific, coating-dependent desorption temperature, the stored hydrocarbons are desorbed. For these to be catalytically reacted on the downstream catalyst, it must have already reached its light-off temperature at the time of desorption. The problem is that the light-off temperatures of conventional catalyst coatings are often above the desorption temperatures of HC adsorbers, whereby a reliable conversion of hydrocarbons is difficult to ensure.

To remedy this, exhaust systems are known from the prior art, in which an annular HC adsorber is arranged upstream of an exhaust gas converter (for example, a three-way catalytic converter) (see 1 ; z. B. DE 103 50 516 A . US 5,315,824 ). In this case, the exhaust gas path is divided by an inner tube into an outer flow path, which has the HC ring adsorber, and a central flow path (bypass). By suitable adjusting means, the exhaust gas flow through the outer flow path through the HC adsorber can be passed or to bypass the same through the bypass. As long as the operating temperature and the HC adsorber have not yet reached its desorption temperature after an engine start of the main catalytic converter, the exhaust gas flow is conducted via the HC adsorber, which temporarily stores the hydrocarbons present in the exhaust gas. Before the HC adsorber reaches its desorption temperature, the exhaust stream is passed through the central flow path to bypass the HC adsorber. Once the downstream catalyst has reached its light-off temperature, in turn, a complete or partial passage of the exhaust gas through the HC adsorber, to discharge the hydrocarbons and transport into the main catalyst, where they are catalytically reacted. This arrangement allows a very good reduction of HC start emissions. However, the mechanism for diverting the exhaust stream (exhaust flap, etc.) and the production of annular catalyst carrier is relatively complex. In addition, this arrangement requires a complex management.

Out DE 10 2010 027 984 A1 For example, an exhaust system for a diesel internal combustion engine is known, which in this order comprises a close-coupled diesel oxidation precatalyst, a main diesel oxidation catalyst, a diesel particulate filter and an SCR catalyst. For heating the main diesel oxidation catalyst, a fuel injector is arranged upstream of it and preceded by a burner device operated with air and fuel.

The present invention has for its object to provide an exhaust system, are ensured with the low HC emissions and at the same time over known Devices with annular HC adsorbers has a simpler structure and requires a simplified management.

This object is achieved by an exhaust system, in particular for spark-ignited internal combustion engines, and by a method for operating such having the features of the independent claims. Further advantageous embodiments of the invention are the subject of the other dependent claims.

The exhaust system according to the invention for spark-ignition internal combustion engines (gasoline engines). comprises a HC adsorber and a downstream of this, at least for the conversion (oxidation) of hydrocarbons HC catalyst formed. Furthermore, the exhaust system comprises a downstream of the HC adsorber and upstream of the catalyst with air and fuel operated or operable burner device.

The burner installed between the HC adsorber and the catalyst makes it possible to raise the catalyst to a temperature level very quickly after engine start, which enables an early conversion of hydrocarbons. In particular, it is achieved in this way that the catalyst reaches its light-off temperature before the upstream HC adsorber reaches its desorption temperature. At the time of incipient HC desorption from the adsorber, the downstream catalyst is thus already ready for use and ensures safe conversion of the desorbed hydrocarbons. Furthermore, the exhaust system according to the invention is characterized by a structure which is significantly simplified compared to the concepts discussed at the beginning with annular HC adsorbers, since design measures for deflecting the exhaust gas flow, for example exhaust gas flaps, are eliminated. In addition, the exhaust system according to the invention comprises no moving components, whereby their life is extended. Finally, the exhaust system according to the invention is characterized by a simplified control and monitoring by means of on-board diagnostics (OBD).

The downstream of the HC adsorber catalyst is designed to at least catalytically convert hydrocarbons HC, namely to oxidize. Preferably, it is designed as a three-way catalyst so that it also oxidizes carbon monoxide CO in addition to the oxidation of hydrocarbons and additionally reduces nitrogen oxides NO _x . In a specific embodiment of the invention, the main catalytic converter is designed as a so-called four-way catalytic converter, that is to say it has a particle filter function in addition to its three-way function. The three-way catalytic function converts all relevant and legally limited gaseous pollutant components. In addition, the optional particulate filter function ensures that the particulate components of the exhaust gas, which are increasingly coming into focus in gasoline engines, are retained. In the process, the regeneration of the particle filter, which becomes necessary from time to time, is made possible by the upstream burner device.

The HC adsorber is preferably designed as Vollstromadsorber. This is understood in the present case that the HC adsorber is bypassed by no bypass line, in particular no central bypass as in the annular HC adsorbers of the prior art is available. Rather, the carrier body of the HC adsorber (apart from its flow channels) quasi solid, for example, as a solid cylinder with a circular or flattened cross-section. The formation of the HC adsorber as Vollstromadsorber on the one hand leads to a simplification of its production process and to increase its long-term stability. On the other hand, the construction of the exhaust system is simplified as a whole.

Optionally, upstream of the HC adsorber, a precatalyst formed at least for the conversion of hydrocarbons can be arranged at a position close to the engine as possible. For example, this can be arranged directly on a manifold outlet or even within the manifold pipes. The precatalyst performs the function of a starting catalyst, that is, it takes over the conversion of pollutants, immediately after an engine start, when the downstream exhaust gas components have not yet reached their light-off temperature. Due to the arrangement close to the engine and the usually smaller catalyst volume, an early heating of the precatalyst is achieved. As well as the downstream catalyst (main catalyst), but independent of this, the precatalyst is preferably designed as a three-way catalyst. In one development of this embodiment, it additionally has a particle filter function, that is to say it is designed as a so-called four-way catalytic converter.

In an alternative embodiment of the invention, the exhaust system does not comprise a pre-catalyst. Due to the rapid heating of the HC adsorber downstream catalyst to its light-off temperature and the HC Adsorberfunktion an effective HC conversion is possible quickly after engine cold start, so that can be dispensed with a pre-catalyst. This results in particular cost advantages.

Advantageously, the catalyst and the burner device is arranged at an underfloor position of the exhaust system. Advantage of this position is the comparatively large space in the underfloor region of a vehicle, which offers a large design space for the catalyst itself and its burner device and associated components. Since the catalyst is not dependent on heating by the hot exhaust gas, which often prohibits an underbody position of catalysts, this advantageous remote motor arrangement is made possible.

According to a further embodiment of the invention, the exhaust system further comprises a control device for controlling an operation and a heating power of the burner device. For this purpose, the control device is set up to control an air mass flow and / or fuel mass flow supplied to the burner device.

The invention further relates to a method for operating an exhaust system according to the invention, wherein with or after a start of the internal combustion engine, the burner device is started and operated until the adsorbed by the HC adsorber hydrocarbons at least 90%, in particular at least 95% and preferably at least are desorbed to 98%, or until the HC adsorber downstream catalyst has reached a predetermined temperature threshold, in particular its light-off temperature. Due to the rapid heating of the HC adsorber downstream catalyst, it is ensured that this reaches its light-off temperature very quickly. Thus, a reliable conversion is achieved already at the time of incipient HC desorption.

According to a preferred embodiment of the method, the burner device is still operated after fulfillment of the shutdown criteria described above for a predetermined period of time. This configuration prevents the catalyst from cooling down again by still comparatively cold exhaust gas after switching off the burner device. The low exhaust gas temperatures after engine start are caused in particular by the still cold components of the cold exhaust system, which thus act as heat sinks.

The predetermined period of time, for example, be chosen so that a certain minimum exhaust gas temperature is reached, which prevents re-cooling of the catalyst.

In a preferred embodiment of the invention, the heating power of the burner device is controlled by the amount of fuel supplied to the burner device. In this case, the amount of fuel is piloted depending on the amount of air supplied. This embodiment takes into account the fact that due to fluctuating exhaust back pressures in the exhaust duct and the amount of air supplied to the burner device can be influenced. In order thus to represent a desired combustion air ratio (lambda) in the burner device, the fuel quantity to be supplied is determined as a function of the air mass flow currently supplied to the burner device and fed to the burner device. In this way, always a desired combustion air ratio and thus a desired heat output can be ensured, for example, a constant heat output. In a further embodiment, a lambda probe is arranged in the intake duct downstream of the burner device, so that the feedforward control described above can be supplemented by a lambda control.

The invention will be explained in more detail with reference to embodiments. Show it:

1 schematically an exhaust system according to the prior art;

2 schematically an exhaust system according to the present invention and

3 Time profiles of the temperatures and the HC concentrations at various points of an exhaust system according to the invention in a test bench measurement.

1 shows a schematic overview of an exhaust system 10 ' an internal combustion engine 12 according to the prior art.

The exhaust system 10 ' includes an exhaust passage 16 , in which at a near-engine position, a precatalyst 18 is arranged, which fulfills the function of a starting catalyst. Downstream of the precatalyst 18 , especially at an underbody position, is another catalyst 20 (Main catalyst), which is designed in particular for the catalytic conversion of hydrocarbons. Upstream of the main catalyst 20 is a ring-shaped HC adsorber 22 provided on an inner tube 24 is stored. In the central section of the HC adsorber 22 and the inner tube 24 thus becomes a bypass 26 educated. To the exhaust gas flow either via the HC adsorber 22 or through the bypass 26 to guide are adjusting means 28 present, for example, comprise a pivotable or rotatable exhaust flap, the or in the inner tube 24 is arranged. Not shown in 1 are actuators for the actuating means 28 , For example, an electric motor, and control means.

In the 1 illustrated exhaust system 10 ' shows the following function. After a start of the internal combustion engine 12 , first, the exhaust flap 28 placed so that the bypass 26 closed and the exhaust stream through the HC adsorber 22 is directed. In this phase, the HC emissions of the Internal combustion engine 12 in the HC adsorber 22 saved. Just before the HC adsorber has reached its desorption temperature, the bypass becomes 26 opened so that the exhaust stream the HC adsorber 22 bypasses. Meanwhile, there is a heating of the HC adsorber 22 through the exhaust gas flow instead. Once the downstream catalyst 20 has reached its operating temperature, the exhaust flap 28 closed again so that the hot exhaust gas desorbs the hydrocarbons from the HC adsorber 22 and the downstream catalyst ( 20 ), where they are catalytically converted.

In the 1 illustrated exhaust system 10 ' While leading to effective reduction of HC startup emissions, the design and control is relatively expensive.

2 shows a schematic overview of an exhaust system 10 according to the present invention. Corresponding components are here with the same reference numerals as in 1 designated.

The internal combustion engine 12 is a spark-ignited internal combustion engine (gasoline engine) with typically a plurality of cylinders 14 ,

The exhaust system 10 includes an exhaust passage 16 in which preferably at an underbody position, an HC adsorber 22 is arranged. The HC adsorber 22 is designed as Vollstromadsorber, that is, it does not include a bypass and is thus always flows through the entire exhaust stream. For example, it has the shape of a solid cylinder with a circular or oval cross-section.

Downstream of the HC adsorber 22 is a major catalyst 20 also arranged at an underbody position. The catalyst 20 is preferably designed as a three-way catalyst for the conversion of hydrocarbons HC, carbon monoxide CO and nitrogen oxides NO _X. For this purpose, contains the catalytic coating of the catalyst 20 catalytically active noble metals such as platinum, palladium and / or rhodium, preferably a combination of platinum and rhodium or palladium and rhodium. Optionally, the catalyst 20 furthermore have a particle filter function. For this purpose, the catalyst 20 have mutually closed and open flow channels, so that the exhaust gas is forced to penetrate porous side walls of the flow channels. In this case, particulate components of the exhaust gas are retained in the walls. Ottopartikelfilter can have a fundamentally similar structure as diesel particulate filter.

Optionally, the exhaust system 10 also a precatalyst 18 include, which takes over the function of a starting catalyst and is arranged for this purpose at a position close to the engine as possible. The precatalyst 18 is the same as the main catalyst 20 preferably designed as a three-way catalyst with optional additional particulate filter function.

In 2 is also a turbine 32 a not shown exhaust gas turbocharger shown. The exhaust gas turbine 32 is preferably located at a position upstream of the precatalyst 18 ,

According to the invention is between the HC adsorber 22 and the main catalyst 20 a burner device 30 arranged. The burner device 30 has not shown air conveyor, which the burner device 30 supply with combustion air from the environment. These air conveying means may comprise, for example, a conventional secondary air pump. Furthermore, the burner device 30 also not shown fuel conveying means, which the burner device 30 provide fuel. In particular, the burner device 30 supplied with the same fuel as the internal combustion engine 12 , Thus, the fuel delivery means comprise the burner device 30 for example, one from the fuel tank to the burner device 30 leading fuel line, a fuel pump and a fuel injector.

The exhaust system according to the invention 10 further comprises a control device 34 for controlling the burner device 30 , In particular, the control device controls 34 in the 2 Not shown air conveyor for supplying combustion air to the burner device 30 and also not shown fuel delivery means for supplying fuel to the burner device 30 ,

Not in 2 shown is a sensor of the exhaust system 10 , which usually includes a lambda sensor close to the engine, which, for example, upstream of the precatalyst 18 is arranged and in a known manner the lambda control of the internal combustion engine 12 serves. Furthermore, the sensor system can be another, downstream of the burner device 30 arranged lambda probe, which the control of that of the burner device 30 supplied air-fuel mixture of the burner device 30 serves. In addition, in the exhaust system 10 be arranged different temperature sensors, for example, before, on and / or after the main catalyst 20 which determines the temperature of the catalyst 20 serve. In addition, other temperature sensors before, on and / or after the HC adsorber 22 be provided, which allow to determine the Adsorbertemperatur. In addition, HC sensors may also be provided, in particular downstream of the HC adsorber 22 and / or downstream of the catalyst 20 , The signals of the temperature sensors, the signals of the burner means 30 downstream lambda probe and the signals of the HC sensors go into the control device 34 one. This has an algorithm for controlling the burner device 30 in computer readable form. In addition, the control device 34 have different stored maps.

In the 2 illustrated exhaust system 10 has the following function. Immediately with a cold start of the engine 12 when reaching a minimum speed, the controller starts 34 the burner device 30 , For this purpose, it controls the air and fuel conveying accordingly. Preferably, the burner device 30 operated in this phase with the highest possible heating power, which with the components and in particular the main catalyst 20 is compatible. In a preferred embodiment of the invention, the burner device 30 operated with a constant high heating power, for example with at least 10 kW, preferably at least 15 kW and more preferably with about 20 kW. To represent this target performance, the controller determines 34 the current, the burner device 30 supplied air mass flow, for example, with an air mass meter. Depending on the air mass flow determines the controller 34 the fuel mass flow to be supplied, which is required to represent the desired lambda. For this purpose, the control device 34 For example, access a stored map that represents the fuel mass flow as a function of the desired lambda and the current air mass flow. Then controls the controller 34 the fuel delivery, in particular opening times of the injector so that the determined amount of fuel to the burner device 30 is supplied.

The burner device 30 is operated at least until the in the HC adsorber 22 stored hydrocarbons are at least 90% desorbed and / or until the main catalyst 20 a predetermined temperature threshold, for example, has reached its light-off temperature. The time of sufficient HC desorption from the HC adsorber 22 can be detected for example by means of a downstream HC sensor. Furthermore, the desorption over the temperature of the HC adsorber 22 be determined, which in turn can be measured with temperature sensors or modeled by calculation. Finally, the HC desorption from the HC adsorber 22 can also be modeled using a time function. The temperature of the main catalyst 20 can also be measured by the temperature sensors or via suitable calculation models depending on an operating point of the internal combustion engine 12 be determined. Preferably, the temperature of the catalyst 20 but determined via temperature sensors. If at least one of the aforementioned shutdown criteria is met, the burner operation is preferably extended for a predetermined period of time. This is to ensure that the temperature of the main catalyst 20 through the initially still cool exhaust gas after switching off the burner device 30 instantly sinks again.

3 shows temporal temperature and HC curves during operation of an exhaust system according to the invention according to 2 ,

In the upper part of the 3 is the vehicle _speed V _Fzg following a standard _cycle as well as the period of operation of the burner _device 30 shown. In the lower part of the 3 is the temperature in the main catalyst T _HK to see the exhaust gas temperature before the HC adsorber T _{before Ad} and the exhaust gas temperature downstream of the HC adsorber T _{to Ad} . Further, the HC concentration is shown downstream of the HC adsorber HC _{to Ad} and the HC concentration downstream of the main catalyst HC _{to HK} .

In the illustrated embodiment, a start of the operation of the burner device takes place 30 with engine start. By the burner operation is a very rapid increase in the temperature of the main catalyst T _HK . However, only after a considerable delay does a significant increase in the exhaust gas temperature take place before the HC adsorber T _{before Ad} , since the thermal energy of the exhaust gas initially for the heating of the HC adsorber 22 upstream exhaust system is consumed. Even slower is the increase in the exhaust gas temperature behind the HC adsorber T _{after Ad} . The actual adsorber temperature is typically in the range between the courses T _{before Ad} and T _{after Ad} .

At engine start (in 3 at about 8 seconds) is a rapid increase in HC concentration both after the HC adsorber 22 as well as after the catalyst 20 to observe (curves HC _{to Ad} and HC _{to HK} ). These are due to the increased raw HC emissions of the internal combustion engine 12 due during engine start. The effect of the HC adsorber 22 is shown by the different HC concentrations after HC adsorber and after catalyst 20 , In the phase of HC storage heats the burner device 30 the main catalyst 20 so that this at the time of turning off the burner 30 has reached its light-off temperature. The desorption of hydrocarbons begins at about the second 50 , after switching off the burner. The desorbed hydrocarbons become on the main catalyst 20 reacted, indicating the HC concentration after the catalyst 20 can be read.

The measurements show that by means of the exhaust system according to the invention 10 an effective HC conversion can be achieved by the main catalyst, so that can be dispensed with a complicated construction using a ring-shaped HC adsorber and corresponding exhaust gas control means.

LIST OF REFERENCE NUMBERS

10

Exhaust system according to the invention

10 '

Exhaust system according to the prior art

12

Internal combustion engine

14

cylinder

16

exhaust duct

18

precatalyzer

20

Catalyst / main catalyst

22

HC adsorber

24

inner tube

26

central flow path / bypass

28

Actuating means / exhaust flap

30

burner means

32

exhaust turbine

34

control device

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10350516 A [0006]
• US 5315824 [0006]
• DE 102010027984 A1 [0007]

Claims (10)

Exhaust system ( 10 ) for a spark-ignition internal combustion engine ( 12 ) comprising an HC adsorber ( 22 ) and a downstream of this, at least for the conversion of hydrocarbons (HC) formed catalyst ( 20 ), characterized in that downstream of the HC adsorber ( 22 ) and upstream of the catalyst ( 20 ) an air and fuel operated or operable burner device ( 30 ) is arranged. Exhaust system ( 10 ) according to claim 1, characterized in that the HC adsorber ( 22 ) is designed as Vollstromadsorber. Exhaust system ( 10 ) according to claim 1 or 2, characterized in that upstream of the HC adsorber ( 22 ) a close-coupled precatalyst ( 18 ) is arranged. Exhaust system ( 10 ) according to one of the preceding claims, characterized in that the catalyst ( 20 ) and / or the precatalyst ( 18 ) is designed as a three-way catalyst or are. Exhaust system ( 10 ) according to claim 4, characterized in that the catalyst ( 20 ) and / or the precatalyst ( 18 ) is designed as a three-way catalyst with a particle filter function or are. Exhaust system ( 10 ) according to one of the preceding claims, characterized in that the catalyst ( 20 ) and the burner device ( 30 ) at an underfloor position of the exhaust system ( 10 ) are arranged. Exhaust system ( 10 ) according to one of the preceding claims, characterized by a control device ( 34 ) for controlling an operation of the burner device ( 30 ). Method for operating an exhaust system ( 10 ) according to one of claims 1 to 7, wherein with or after a start of the internal combustion engine ( 12 ) the burner device ( 30 ) is started and at least until operated by the HC adsorber ( 22 ) adsorbed hydrocarbons are at least 90% desorbed and / or until the catalyst ( 20 ) has reached a predetermined temperature threshold. A method according to claim 8, characterized in that after desorption of at least 90% of the hydrocarbons adsorbed by the HC adsorber and / or after reaching the predetermined temperature threshold (TS_UK) of the catalyst ( 20 ) the burner device ( 30 ) is still operated for a predetermined period of time. A method according to claim 8 of 9, characterized in that a heating power of the burner device ( 30 ) over the burner device ( 30 ) is controlled, wherein the amount of fuel in dependence on one of the burner device ( 30 ) is precontrolled air mass flow supplied.

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