To meet the increasingly stringent demands of exhaust emissions legislation, vehicle manufacturers are taking appropriate measures to reduce untreated engine emissions and are providing suitable exhaust-gas aftertreatments. To effectively convert the inevitable untreated emissions in a post-engine process, catalytic converters coated with precious metal are installed in the exhaust system of the combustion engine. These catalytic converters and the exhaust gas must be at a minimum temperature level in order to convert the pollutants. Engine-related heating measures, such as retarding the ignition timing or operating the combustion engine substoichiometrically while simultaneously introducing secondary air, are used to bring the catalytic converter to an operating temperature as rapidly as possible following a cold start of the combustion engine. Electrical heating may be employed to rapidly and selectively heat the catalytic converter. Emissions may thereby be significantly reduced even in the cold-start phase.
Moreover, with the introduction of the EU6 stage legislation for gasoline engines, a limit value for a particle count has been mandated that, in many cases, necessitates the use of a gasoline particulate filter. Such a gasoline particulate filter is loaded with soot during vehicle operation. This gasoline particulate filter must be continuously or periodically regenerated to prevent an excessive increase in the exhaust gas back pressure. A high enough temperature level in conjunction with simultaneously present oxygen in the exhaust system of the gasoline engine is needed to thermally oxidize the soot retained in the gasoline particulate filter. This requires additional measures since today's gasoline engines are normally operated without excess oxygen at a stoichiometric air/fuel ratio (λ=1). Possible measures include increasing the temperature by adjusting the ignition timing, temporarily adjusting the gasoline engine toward lean, injecting secondary air into the exhaust system, for example, or a combination thereof. Until now, an ignition-timing retard has preferably been used in combination with an adjustment of the gasoline engine toward lean since this method does not require additional components and is able to supply a sufficient quantity of oxygen in most operating points of the gasoline engine.
The German Patent Application DE 10 2010 014 332 A1 discusses a method for thermally managing the exhaust system of a motor vehicle having a hybrid drive made up of a combustion engine and an electric motor. In this approach, the catalytic converter is initially electrically preheated with the aid of an electrically heatable catalytic converter and an injection of air, and then chemically heated by a combination of air injection into the exhaust duct and richness adjustment of the air-fuel mixture in the combustion engine. However, the disadvantage of such a method is the separation between the electrically heated catalytic converter and another catalytic converter that is to be heated and is to play a decisive role in transforming the pollutants.
The German Patent Application DE 10 2011 018 293 A1 describes a hybrid vehicle having a combustion engine and an electric motor, an electrically heatable catalytic converter and a second catalytic converter being mounted in the exhaust system. The electrically heatable catalytic converter is thereby used in a starting phase of the combustion engine to convert the harmful exhaust-gas components until the three-way catalytic converter reaches the light-off temperature thereof and is able to efficiently transform the exhaust-gas components.
From the World Patent Application WO 95/05531 A1, a combustion engine having an exhaust system is known where, in the first instance, an electrically heatable catalytic converter is configured downstream of an exhaust of the combustion engine, and a further three-way catalytic converter is configured downstream of the electrically heatable catalytic converter. The electrically heatable catalytic converter is thereby used, in fact, as a starter catalytic converter. To equalize the enrichment customarily present upon a cold starting of the combustion engine in order to improve the ignitability of the combustion mixture in the cold combustion chambers of the combustion engine, secondary air is injected into the exhaust duct upstream of the electrically heatable catalytic converter. The purpose is to facilitate the heating of the electrically heatable catalytic converter and to enhance an exhaust-gas aftertreatment.
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