referring now to the figures of the drawings in detail and first , particularly , to fig1 thereof , there is seen a diagrammatic view of a structure of an exhaust system 1 with an exhaust - gas recirculation system 12 and a combustion engine 2 . the exhaust - gas recirculation system 12 is formed with two lines , that is to say it includes a first line 19 and a second line 20 . an exhaust - gas cooler 14 is disposed in the first line 19 , bringing about cooling of a partial stream of exhaust gas flowing through it , in particular at high temperatures of the exhaust gas . regulation of a direction of flow 10 in the exhaust - gas recirculation system 12 takes place through the use of a valve 11 , which is preferably configured as a rotating double valve . a drive 21 of the valve 11 is connected to a control unit 22 , so that pre - determinable partial streams of exhaust gas can be produced . in principle , a fuel - air mixture is fed to the combustion engine 2 through an air - intake pipe 24 and throttle valves 26 . in this respect , it is specifically in the cold - starting phase of the combustion engine 2 with an exhaust - gas recirculation system 12 , that coking deposits occur on the feed pipes 24 of the fuel - air mixture with the returned exhaust gas , in particular in the vicinity of the valve 11 . furthermore , the throttle valves 26 , valves in combustion chambers or spaces 25 or other regions of feed pipes 23 which at least for a time confine only a relatively small free flow cross section , are affected in particular . the combustion of the fuel - air mixture takes place in the combustion chambers 25 of the combustion engine 2 as shown . exhaust gases containing , for example , hydrocarbons , carbon monoxides , carbon dioxides and nitrogen oxides , are produced and subsequently passed on through an exhaust pipe 3 . harmful substances contained in the exhaust gas are partly catalytically converted in a downstream emission control system before they are released into the environment . illustrated components , namely a filter 16 , an adsorber 18 and a catalytic converter 17 , for example , serve that purpose . the illustrated device has two reaction chambers or spaces 4 for catalytic oxidation and / or cracking of long - chain hydrocarbons , in which a catalyst 5 ( shown in fig2 and 3 ) is applied to an inert support in the reaction chambers 4 . the reaction chamber 4 itself can be cooled , in addition or alternatively to the exhaust gas cooler 14 . the reaction chambers 4 are disposed downstream of the combustion engine 2 , and accordingly the fluid flowing through the reaction chambers 4 is directed toward the combustion chamber 2 in the direction of flow 10 . since the catalyst 5 has such a high acidity , the long - chain hydrocarbons are already converted at temperatures lower than 100 ° c . in the illustrated exemplary embodiment , the reaction chambers 4 are disposed in both exhaust - gas recirculation pipes 13 of the two - line exhaust - gas recirculation system 12 , in which the fluid flowing through is a partial stream of exhaust gas to be returned to the combustion engine 2 . fig2 diagrammatically and perspectively shows an embodiment of the reaction chamber 4 in which a catalyst 5 is applied to a honeycomb body 7 . the honeycomb body 7 is formed as a separate component in the reaction chamber 4 . the metallic honeycomb body 7 with passages 9 through which the fluid can flow is formed by at least partly structured sheet metal layers 8 . the reaction chamber 4 has a volume 15 of about 0 . 5 liters ( meaning the outer overall volume with wall structures and passages 9 ) and is disposed near a connection between the exhaust - gas recirculation system 12 ( shown in fig1 ) and the air - intake pipe 24 ( shown in fig1 ), so that with any desired setting of the valve 11 ( shown in fig1 ) the long - chain hydrocarbons are always cracked or catalytically converted in the entire partial stream of exhaust gas to be returned or fed back . fig3 diagrammatically shows the structure of a coating of the reaction chamber 4 . the fragmentary view shows a piece of a sheet metal layer 8 of the honeycomb body 7 on which the coating has been applied with high temperature resistance . the coating has an extremely fissured surface , as is characteristic of zeolites . the coating is doped with a catalyst 5 and includes aluminum silicate 6 . in the present case , the catalyst 5 is activated aluminum silicate 6 , and it additionally includes constituents of boron trifluoride and / or antinomy pentafluoride . the cracking of the hydrocarbons of the fuel also has the effect of making it better accessible and of its chemical energy content being able to be better utilized . this also results in a reduction in the specific carbon consumption . with the aid of this method , the long - chain hydrocarbons of the gasoline , such as for example decane , can for the first time be transformed in the vehicle into short - chain alkanes and , with additional dehydration , also into alkenes , which leads to an improved octane number of the gasoline .