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Today's automotive cooling systems are designed for the highest necessary heat rejection. This operating state is reached only during 3-5% of the operating time. Thus in most cases the thermal situation is subcritical and the engine is overcooled. Under these operating conditions a demand-responsive engine cooling system promises a high potential for saving energy, increased passenger comfort and environmental compatibility. To realize this potential new components and control strategies are needed. The present paper shows the potential of a demand-responsive cooling system. The requirements of the system and its components will be specified. The realization of this new cooling concept and experimental results regarding reduction of fuel consumption and also on improved driving comfort will be presented.

Today R134a is the worlds number one refrigerant for automotive air conditioning (A/C). Alternatives to be more environmentally friendly are so called ‘natural fluids’. There are options with water and ammonia in vapor compression and sorption systems, or nitrogen and helium as working fluids in different gas cooling cycles. The following options have already been investigated and tested more seriously: air as working fluid in Joule process systems; hydrocarbons as working fluid in vapor compression systems; carbon dioxide (CO2) transcritical vapor compression systems. These alternatives are either processes using flammable or toxic refrigerants or needing the development of totally new equipment because the type of process is new for mobile A/C or the pressure level is extremely increased compared with recent systems. Beside this there is also the chance to use the environmentally friendly CO2 as refrigerant together with other refrigerants.