EXHAUST GAS SYSTEM AND METHOD FOR OPERATING SUCH A SYSTEM

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of German Patent Application No. 10 2012 011 603.9, filed Jun. 12, 2012, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to an exhaust gas system for spark-ignition internal combustion engines (Otto engines) as well as to a method for operating such an exhaust gas system.

In an effort towards reducing emissions from internal combustion engines, engine-related measures are normally taken for purposes of reducing raw emissions. Moreover, catalytic converters and other exhaust emission control components are installed in exhaust gas systems so that raw emissions that nevertheless cannot be completely avoided can be effectively converted downstream from the engine. Catalytic converters comprise a substrate, for example, a ceramic monolith or metal substrate with a coating containing a catalytically active material, through which the exhaust gas can flow. For instance, oxidation catalytic converters, which systematically convert unburned hydrocarbons (HC) as well as carbon monoxide (CO), are often employed, especially in the case of diesel engines. Reduction catalytic converters, which convert nitrogen oxides (NO_x), are used for diesel engines and Otto engines. Moreover, three-way catalytic converters are known that combine the function of oxidation and reduction catalytic converters, thus catalytically converting all three components; these catalytic converters are employed mainly for Otto engines.

Fundamentally, all catalytic converters require a specific minimum temperature, the so-called light-off or operating temperature, at which, by definition, they have reached 50% of their maximum catalytic conversion capacity.

This temperature is normally not yet reached after a cold start of the engine, so that, if no other measures are taken, the emissions referred to as cold-start emissions leave the exhaust gas system without being converted. In order to quickly heat up catalytic converters to their light-off temperature, engine-related measures are often adopted such as retarded ignition timing or fuel post-injection.

A widespread measure for reducing the cold-start emissions consists of installing relatively small-volume preconverters near the engine, which are also known as cold-start catalytic converters. Owing to their small volume and their positioning near the engine, preconverters reach their light-off temperature relatively quickly and then take over the task of converting a large portion of the emissions until a main catalytic converter located downstream has also reached its operating temperature.

It is likewise a known procedure to arrange hydrocarbon (HC) adsorbers upstream from an oxidation or three-way catalytic converter in order to buffer the hydrocarbon emissions. Once the HC adsorber has reached a specific, coating-dependent desorption temperature, the buffered hydrocarbons are desorbed. In order for these hydrocarbons to be catalytically converted in the downstream catalytic converter, the latter has to already have reached its light-off temperature at the time of the desorption. A problematic aspect here is that the light-off temperatures of conventional catalytic converter coatings are often above the desorption temperatures of HC adsorbers, as a result of which it is difficult to ensure a reliable conversion of the hydrocarbons.

In order to provide a remedy, the state of the art discloses exhaust gas systems in which a ring-shaped HC adsorber is installed upstream from an exhaust gas converter (e.g. a three-way catalytic converter) (see FIG. 1; for example, German Patent Application No. DE 103 50 516 A, U.S. Pat. No. 5,315, 824). In this context, the exhaust gas path is divided by an inner tube into an external flow path containing the HC ring adsorber, and a central flow path (bypass). Suitable actuating means can then direct the exhaust gas stream through the external flow path through the HC adsorber or else through the bypass, thereby circumventing the HC adsorber. As long as the main catalytic converter has not yet reached its operating temperature and the HC adsorber has not yet reached its desorption temperature after the engine has been started, the exhaust gas stream is conveyed through the HC adsorber which then buffers the hydrocarbons that are present in the exhaust gas. Before the HC adsorber reaches its desorption temperature, the exhaust gas stream is conveyed through the central flow path in order to bypass the HC adsorber. Once the downstream catalytic converter has reached its light-off temperature, the exhaust gas is conveyed partially or completely through the HC adsorber once again in order to remove the hydrocarbons and transport them into the main catalytic converter. This arrangement accounts for a very good reduction of the cold-start hydrocarbon emissions. However, the mechanical elements for diverting the exhaust gas stream (exhaust gas flap, etc.) are complex and the manufacture of ring-shaped catalyst substrates is relatively demanding. Moreover, this arrangement calls for complicated management.

German Patent Application No. DE 10 2010 027 984 A1 discloses an exhaust gas system for a diesel engine that comprises, in this order, a diesel-oxidation preconverter near the engine, a diesel oxidation main catalytic converter, a diesel particulate filter as well as a selective catalytic reduction (SCR) catalyst. A fuel injector is arranged upstream from the main catalytic converter in order to heat it up, and a burner device operated with air and fuel is installed upstream from the fuel injector.

SUMMARY OF THE INVENTION

The objective of the present invention is to put forward an exhaust gas system with which low hydrocarbon emissions can be ensured, which has a simpler structure and which entails easier management than the known devices that have ring-shaped HC adsorbers.

This objective is achieved by means of an exhaust gas system, especially for spark-ignition internal combustion engines, as well as by means of a method for operating such a system, both having the features of the independent claims. Other advantageous embodiments of the invention are the subject matter of the subordinate claims.

The exhaust gas system according to the invention for spark-ignition internal combustion engines (Otto engines) comprises an HC adsorber as well as, downstream from it, a catalytic converter designed to convert (oxidize) hydrocarbons (HC). Moreover, the exhaust gas system comprises a burner device which is operated or can be operated with air and fuel and which is installed downstream from the HC adsorber and upstream from the catalytic converter.

After the engine has been started, the burner installed between the HC adsorber and the catalytic converter makes it possible to very quickly heat up the catalytic converter to a temperature that allows an early catalytic conversion of hydrocarbons. In particular, this allows the catalytic converter to reach its light-off temperature before the upstream HC adsorber reaches its desorption temperature. Therefore, at the point in time when the hydrocarbon desorption from the adsorber is beginning, the downstream catalytic converter is already operational and ensures a reliable conversion of the desorbed hydrocarbons. Moreover, the exhaust gas system according to the invention is characterized by a structure that is greatly simplified in comparison to the above-mentioned concepts employing ring-shaped HC adsorbers since there is no need for constructive measures to divert the exhaust gas stream such as, for instance, exhaust gas flaps. Furthermore, the exhaust gas system according to the invention does not comprise any moving parts, and this prolongs its service life. Finally, the exhaust gas system according to the invention is characterized by a simplified control and monitoring within the scope of on-board diagnostics (OBD).

The catalytic converter located downstream from the HC adsorber is configured to catalytically convert, namely, to oxidize, at least hydrocarbons (HC). Preferably, it is configured as a three-way catalytic converter, so that, aside from oxidizing hydrocarbons, it also oxidizes carbon monoxide (CO) and additionally reduces nitrogen dioxide (NO_x). In a special embodiment of the invention, the main catalytic converter is configured as a so-called four-way catalytic converter, that is to say, it not only has its three-way function but also a particulate-filter function. Due to the three-way catalytic function, all of the relevant and statutorily restricted gaseous emissions are catalytically converted. Moreover, the optional particulate-filter function also ensures that the particulate constituents of exhaust gas that are increasingly becoming the focus of attention, also in Otto engines, are also held back. In this context, the regeneration of the particulate filter that is necessary from time to time can be carried out by the upstream burner device.

The HC adsorber is preferably configured as a full-flow adsorber. This refers here to the fact that the HC adsorber cannot be circumvented by a bypass line, especially not by a central bypass of the type found in ring-shaped HC adsorbers according to the state of the art. Instead, the substrate element of the HC adsorber (except for its flow channels) are configured so as to be virtually solid, for example, as a full cylinder with a circular or flattened cross section. On the one hand, the configuration of the HC adsorber as a full-flow adsorber simplifies its production process and improves its long-tem stability. On the other hand, the construction of the exhaust gas system overall is simplified.

Optionally, a preconverter configured to convert at least hydrocarbons can be arranged upstream from the HC adsorber as close as possible to the engine. For instance, it can be arranged directly on a manifold outlet or even inside the manifold pipes. The preconverter fulfills the function of a cold-start catalytic converter, that is to say, it takes over the conversion of emissions immediately after the engine has been started, at a time when the downstream exhaust gas components have not yet reached their light-off temperature. The arrangement near the engine and the normally smaller volume of the catalytic converter make it possible to heat up the preconverter at an early point in time. Like the downstream catalytic converter (main catalytic converter)—although independently of it—the preconverter is preferably also configured as a three-way catalytic converter. In a refinement of this embodiment, it additionally has a particulate-filter function, that is to say, it is configured as a so-called four-way catalytic converter.

In an alternative embodiment of the invention, the exhaust gas system does not comprise a preconverter. Due to the fact that the catalytic converter located downstream from the HC adsorber can be rapidly heated up to its light-off temperature, and due to the hydrocarbon-adsorbing function, effective hydrocarbon conversion is possible shortly after the cold start of the engine, so that a preconverter can be dispensed with. This especially translates into cost savings.

Advantageously, the catalytic converter as well as the burner device are arranged at a position underneath the exhaust gas system. The advantage of this position is the relatively large installation space available on the underbody of a vehicle, which affords a large space to accommodate the catalytic converter itself as well as its burner device and associated components. Since the catalytic converter is not dependent on being heated up by the hot exhaust gas—which would otherwise preclude the installation of catalytic converters on the underbody—this advantageous arrangement at a distance from the engine is possible.

According to another embodiment of the invention, the exhaust gas system also comprises a control unit for controlling the operation as well as the heating output of the burner device. For this purpose, the control unit is designed to control the air mass flow and/or the fuel mass flow to the burner device.

The invention also relates to a method for operating an exhaust gas system according to the invention, whereby, at the time of or after the start of the internal combustion engine, the burner device is started and kept in operation until at least 90%, especially at least 95% and preferably at least 98% of the hydrocarbons adsorbed by the HC adsorber has desorbed, or else until the catalytic converter downstream from the HC adsorber has reached a prescribed temperature threshold, especially its light-off temperature. The rapid heating up of the catalytic converter downstream from the HC adsorber ensures that the former reaches its light-off temperature very quickly. Therefore, a reliable catalytic conversion is attained already at the point in time when the hydrocarbon desorption begins.

According to a preferred embodiment of the method, the burner device continues to operate for a certain period of time after the above-mentioned switch-off criteria have been met. This embodiment prevents the catalytic converter from cooling off again due to the relatively cold exhaust gas after the burner device has been switched off. The low exhaust gas temperatures after the engine start are caused especially by the still-cold components of the cold exhaust gas system, which therefore act as heat sinks. The prescribed time period can be selected, for example, in such a way that a certain minimum exhaust gas temperature is reached that prevents the catalytic converter from cooling off again.

In a preferred embodiment of the invention, the heating output of the burner device is regulated via the amount of fuel fed to the burner device. In this process, the amount of fuel is pre-regulated as a function of the volume of air fed in. This embodiment takes into consideration the fact that the volume of air fed to the burner device can also be influenced by fluctuations in the exhaust gas counter-pressure in the exhaust gas channel. Therefore, in order to attain the desired combustion-air ratio (lambda) in the burner device, the amount of fuel to be fed to the burner device is determined as a function of the air mass flow momentarily being fed into the burner device, and it is then fed into the burner device. In this manner, it is possible to always ensure the desired combustion-air ratio and thus the desired heating output, for example, a constant heating output. In another configuration, a lambda sensor is installed in the exhaust gas channel downstream from the burner device, so that the pre-regulation described above can be supplemented by controlling the lambda value.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in greater detail below on the basis of the embodiments. The following is shown:

FIG. 1: schematically, an exhaust gas system according to the state of the art;

FIG. 2: schematically, an exhaust gas system according to the present invention;

FIG. 3: the time curves of the temperatures as well as of the hydrocarbon concentrations at different places in an exhaust gas system according to the invention, measured on a test bench.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows an overview of an exhaust gas system 10′ of an internal combustion engine 12 according to the state of the art.

The exhaust gas system 10′ comprises an exhaust gas channel 16 in which a preconverter 18 is arranged close to the engine and it fulfills the function of a cold-start catalytic converter. Downstream from the preconverter 18, especially in an underbody position, there is another catalytic converter 20 (main catalytic converter) that is designed specifically to catalytically convert hydrocarbons. Upstream from the main catalytic converter 20, a ring-shaped HC adsorber 22 is provided which is mounted on an inner tube 24. This creates a bypass 26 in the central section of the HC adsorber 22 as well as of the inner tube 24. In order to convey the exhaust gas stream selectively either via the HC adsorber 22 or via the bypass 26, actuating means 28 are present which have, for instance, a pivoting or rotating exhaust gas flap installed on or in the inner tube 24. FIG. 1 shows neither the actuators for the actuating means 28 such as, for example, an electric motor, nor the control means.

The exhaust gas system 10′ shown in FIG. 1 has the following function: after the internal combustion engine 12 has been started, first of all, the exhaust gas flap 28 is set in such a way that the bypass 26 is closed and the exhaust gas stream is conveyed through the HC adsorber 22. During this phase, the hydrocarbon emissions of the internal combustion engine 12 are buffered in the HC adsorber 22. Shortly before the HC adsorber has reached its desorption temperature, the bypass 26 is opened, so that the exhaust gas stream bypasses the HC adsorber 22. In the meantime, the HC adsorber 22 is heated up by the exhaust gas stream. As soon as the downstream catalytic converter 20 has reached its operating temperature, the exhaust gas flap 28 is closed once again so that the hot exhaust gas removes the desorbed hydrocarbons from the HC adsorber 22 and conveys them to the downstream catalytic converter 20, where they are catalytically converted.

Even though the exhaust gas system 10′ shown in FIG. 1 brings about an effective reduction of cold-start hydrocarbon emissions, the construction and the regulation are relatively complicated.

FIG. 2 shows a schematic overview of an exhaust gas system 10 according to the present invention. Components corresponding to those of FIG. 1 are designated by the same reference numerals.

The internal combustion engine 12 is a spark-ignition internal combustion engine (Otto engine) typically comprising several cylinders 14.

The exhaust gas system 10 comprises an exhaust gas channel 16 in which an HC adsorber 22 is arranged, preferably in an underbody position. The HC adsorber 22 is configured as a full-flow adsorber, that is to say, it does not have a bypass, so that the entire exhaust gas stream always flows through it. For instance, it is designed as a full cylinder with a circular or oval cross section.

Downstream from the HC adsorber 22, there is a main catalytic converter 20, likewise in an underbody position. The catalytic converter 20 is preferably configured as a three-way catalytic converter for the conversion of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO_x). For this purpose, the catalytic coating of the catalytic converter 20 contains catalytically active noble metals such as platinum, palladium and/or rhodium, preferably a combination of platinum and rhodium or palladium and rhodium. Optionally, the catalytic converter 20 can also have a particulate-filter function. Towards this end, the catalytic converter 20 can have flow channels that are opened and closed alternatingly, so that the exhaust gas is forced to pass through the porous side walls of the flow channels. In this process, particulate constituents of the exhaust gas are held back in the walls. Otto particulate filters can fundamentally have a similar structure to diesel particulate filters.

Optionally, the exhaust gas system 10 can also comprise a preconverter 18 that takes over the function of a cold-start catalytic converter and, for this purpose, is arranged in the closest possible position to the engine Like the main catalytic converter 20, the preconverter 18 is preferably configured as a three-way catalytic converter, optionally with an additional particulate-filter function.

FIG. 2 also shows a turbine 32 of an exhaust gas turbocharger (not shown here). The exhaust gas turbine 32 is preferably situated in a position upstream from the preconverter 18.

According to the invention, a burner device 30 is arranged between the HC adsorber 22 and the main catalytic converter 20. The burner device 30 has an air-conveying means (30) (not shown here) which supplies the burner device 30 with combustion air from the environment. This air-conveying means can comprise, for example, a conventional secondary air pump. Furthermore, the burner device 30 has fuel-conveying means (likewise not shown here) which supply fuel to the burner device 30. In particular, the burner device 30 is supplied with the same fuel as the internal combustion engine 12. In this manner, the fuel-conveying means of the burner device 30 comprises, for example, a fuel line leading from the fuel tank to the burner device 30, a fuel pump and a fuel injector.

The exhaust gas system 10 according to the invention also comprises a control unit 34 to regulate the burner device 30. In particular, the control unit 34 regulates the air-conveying means (not shown in FIG. 2) which feeds combustion air to the burner device 30, and it also regulates the fuel-conveying means (likewise not shown) which feeds fuel to the burner device 30.

FIG. 2 does not show a sensor system of the exhaust gas system 10, which normally comprises a lambda sensor that is located near the engine and that is arranged, for instance, upstream from the preconverter 18 and that serves to control the lambda value of the internal combustion engine 12 in a generally known manner. Moreover, the sensor system can comprise another lambda sensor that is located downstream from the burner device 30 and that serves to control the air-fuel mixture that is fed to the burner device 30. Moreover, there can be several temperature sensors in the exhaust gas system 10, for example, upstream from, on or downstream from the main catalytic converter 20, which serve to ascertain the temperature of the catalytic converter 20. Furthermore, additional temperature sensors can be provided upstream from, on and/or downstream from the HC adsorber 22, which make it possible to determine the temperature of the adsorber. In addition, hydrocarbon sensors can also be provided, especially downstream from the HC adsorber 22 and/or downstream from the catalytic converter 20. The signals from the temperature sensors, the signals from the lambda sensor located downstream from the burner device 30 as well as the signals from the hydrocarbon sensors are all fed to the control unit 34. The latter has an algorithm in a computer-readable format to regulate the burner device 30. The control unit 34 can also have several stored characteristic maps.

The exhaust gas system 10 shown in FIG. 2 has the following function: immediately with the cold start of the internal combustion engine 12, once the minimum rotational speed has been reached, the control unit 34 starts the burner device 30. For this purpose, it regulates the air and fuel-conveying means accordingly. During this phase, the burner device 30 is preferably operated at the highest possible heating output that is permissible for the components, especially the main catalytic converter 20. In a preferred embodiment of the invention, the burner device 30 is operated at a constant high heating output, for instance, at least 10 kW, preferably at least 15 kW and especially preferably about 20 kW. In order to display this target output, the control unit 34 ascertains the momentary air mass flow that is being fed to the burner device 30, for example, by means of an air mass flow meter. As a function of the air mass flow r ate, the control unit 34 ascertains the fuel mass flow that is to be fed in and that is needed to display the desired lambda value. For this purpose, the control unit 34 can access, for instance, a stored characteristic map that displays the fuel mass flow rate as a function of the desired lambda value as well as of the momentary air mass flow rate. Subsequently, the control unit 34 regulates the fuel-conveying means—especially the timing of the opening of the injector—in such a way that the ascertained fuel amount is fed to the burner device 30.

The burner device 30 is operated at least until 90% of the hydrocarbons buffered in the HC adsorber 22 has been desorbed and/or until the main catalytic converter 20 has reached a prescribed temperature threshold, for instance, its light-off temperature. The point in time of sufficient hydrocarbon desorption from the HC adsorber 22 can be detected, for example, by means of a hydrocarbon sensor installed downstream. Moreover, the desorption can be determined on the basis of the temperature of the HC adsorber 22, whereby said temperature can once again be measured by means of temperature sensors or else determined by means of mathematical modeling. Finally, the hydrocarbon desorption from the HC adsorber 22 can also be modeled using a time function. The temperature of the main catalytic converter 20 can likewise be measured by means of the temperature sensors or else determined by means of suitable mathematical models as a function of the operating point of the internal combustion engine 12. Preferably, however, the temperature of the catalytic converter 20 is determined by means of temperature sensors. Once at least one of the above-mentioned switch-off criteria has been met, the operation of the burner is preferably prolonged for a prescribed period of time. This is supposed to ensure that the temperature of the main catalytic converter 20 immediately drops after the burner device 30 has been switched off due to the initially still cold exhaust gas.

FIG. 3 shows the time curves of the temperature and of the hydrocarbon concentrations during the operation of an exhaust gas system according to the invention as shown in FIG. 2.

The upper section of FIG. 3 shows the vehicle speed V_Fzgfollowing a standard cycle as well as the period of operation of the burner device 30. The lower section of FIG. 3 shows the main catalytic converter temperature T_HK, the exhaust gas temperature upstream from the HC adsorber T_vorAdas well as the exhaust gas temperature downstream from the HC adsorber T_nachAdLikewise shown are the hydrocarbon concentration downstream from the adsorber HC_nachAdas well as the hydrocarbon concentration downstream from the main catalytic converter HC_nachHK.

In the embodiment shown, the operation of the burner device 30 begins when the engine is started. The operation of the burner accounts for a very fast rise in the main catalytic converter temperature T_HK. In contrast, only after a considerable delay does a significant rise occur in the exhaust gas temperature upstream from the HC adsorber T_vorAdsince the thermal energy of the exhaust gas is first consumed in order to heat up the exhaust gas system located upstream from the HC adsorber 22. The rise in the exhaust gas temperature downstream from the HC adsorber T_nachAdis even slower. The actual adsorber temperature is typically between the curves T_vorAdand T_nachAd.

When the engine is started (after about 8 seconds in FIG. 3), a rapid rise in the hydrocarbon concentration downstream from the HC adsorber 22 as well as downstream from the catalytic converter 20 can be observed (HC_nachAdand HC_nachHKcurves). This is due to the elevated raw hydrocarbon emissions of the internal combustion engine 12 while the engine is being started. The effect of the HC adsorber 22 can be seen on the basis of the different hydrocarbon concentrations downstream from the HC adsorber and downstream from the catalytic converter 20. During the phase while the hydrocarbon is being buffered, the burner device 30 heats up the main catalytic converter 20, so that it has reached its light-off temperature at the point in time when the burner 30 is switched off. The desorption of the hydrocarbons starts about 50 seconds after the burner has been switched off. The desorbed hydrocarbons are converted in the main catalytic converter 20, and this can be seen from the hydrocarbon concentration downstream from the catalytic converter 20.

The measurements show that the exhaust gas system 10 according to the invention makes it possible to attain an effective conversion of the hydrocarbons using the main catalytic converter, so that there is no need for a complex construction that employs a ring-shaped HC adsorber and corresponding exhaust gas means.

LIST OF REFERENCE NUMERALS

10 exhaust gas system according to the invention

10′ exhaust gas system according to the state of the art

12 internal combustion engine

14 cylinder

16 exhaust gas channel

18 preconverter

20 catalytic converter/main catalytic converter

22 HC adsorber

24 inner tube

26 central flow path/bypass

28 actuating means/exhaust gas flap

30 burner device

32 exhaust gas turbine

34 control unit

EXHAUST GAS SYSTEM AND METHOD FOR OPERATING SUCH A SYSTEM

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CPC

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Priority Claims (1)