19 has in this case an opening 20 for supplying the exhaust gas aftertreatment medium 9 more than one supply opening 20 could also be provided the exhaust gas aftertreatment medium 9 is introduced into the receiving space 19 through the supply opening 20 the pressure required for this purpose is generally applied by a pump 11 for example a membrane pump which pumps the exhaust gas aftertreatment medium 9 from a supply container 10 into the receiving space 19 of the metering device via a feed line 12 the pump 11 the feed line 12 and the supply container 10 are not crucial to the represented aspect of the exemplary embodiment graphically shown so they have been omitted from the drawings however in principle the volume quantity dispensing unit according to the invention comprises these components too the receiving space 19 opens in the exemplary embodiment into a fine atomiser nozzle 18 which can be closed and opened by the ram piston valve unit 13 the device consisting of the fine atomiser nozzle 18 and the ram piston valve unit 13 can thus depending on the position of displacement of the ram piston valve unit 13 relative to the fine atomiser nozzle 18 have different degrees of opening more than one fine atomiser nozzle 18 could also be provided also known are configurations of a volume quantity dispensing unit which do not have any fine atomiser nozzle at their end because the exhaust gas train is designed in such a way that optimum atomization is rendered superfluous the volume quantity dispensing unit is used to add the exhaust gas aftertreatment.