in order to increase cabin space and simultaneously increase the amount of engine cooling which can be effected by natural air drafts which occur during forward running of the vehicle , the engine radiator is disposed on a forward portion of the vehicle roof and arranged to receive coolant vapor generated in the engine coolant jacket . the coolant is condensed and returned to the coolant jacket via a level sensor controlled pump in a manner which maintain the structure of the engine subject to high heat flux securely immersed in coolant . when the engine is stopped the negative pressure which develops due to the condensation of the vapor is used to induct coolant from a reservoir in a manner to fill the system and obviate any negative pressure which would otherwise tend to induct contamination atmospheric air thereinto . in the event that some non - condensible matter collects in the system , a purge function is provided which pumps excess coolant into the coolant jacket and radiator and flushes out any air or the like each time the engine is started with the coolant temperature below a predetermined level .