is not assigned a connecting device since the exhaust gas 6 flows through this idling module 5 even when the first connecting device 8 and the second connecting device 9 are in a first position preventing flow through the first module 3 and the second module 4 in particular for large volume internal combustion engines for example of locomotives of water borne vehicles such as ships or boats in particular and of stationary installations it is advantageous for the idling module 5 to be adapted to the exhaust gas situation during idling phases by way of example in the case of a switching locomotive the internal combustion engine is in idling mode for a very large part of its operating time and it is therefore useful to adapt to idling conditions moreover large volume internal combustion engines have a very low idling speed and very low flow velocities and consequently low reynolds numbers in idling mode if the exhaust gas 6 were to also flow through both the idling module 5 as well as the first module 3 and the second module 4 for exhaust gas treatment in idling mode the result would be a very low reynolds number of the exhaust gas flow in all of the modules 3 4 5 this would lead to more of a laminar flow which is generally undesirable in modules for exhaust gas treatment if for example open particulate filters are included within the modules 3 4 5 however laminar flow through these particulate filters is undesirable fig2 diagrammatically depicts a portion of.