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The subject addressed by this work, currently discussed in Europe following an European Commission inquiry, is the evaluation of the possibility to prevent the MAC (Mobile Air Conditioning) use below 18°C and its benefits in terms of CO2 emissions saving. This strategy, while providing an uncertain fuel consumption saving, has to be faced with basic safety and cabin comfort conditions. The OEMs (Original Equipment Manufacturers) may evaluate to address these concerns by controlling the cabin absolute humidity content. In order to maintain safety it should be acceptable to turn the AC on based on other inputs, such as air distribution modes (defrost or floor/defrost), windshield wiper usage, rear defroster usage, etc. FGA (FIAT Group Automobiles) exploited our proprietary prediction tool to assessing the yearly fuel efficiency that can be achieved in real use by means of the testing results of representative vehicles.

In the framework of the activities for the selection of an alternate refrigerant to comply with the R-134a ban, FIAT chose to develop a carbon dioxide (R-744) MAC system on a vehicle representing the FIAT portfolio. The vehicle model was specifically selected to deal with two critical parameters in the low cost intermediate segment: a cabin compartment of comfortable size and a tiny engine compartment. The design target was therefore a MAC system featuring both a considerable cooling capacity and efficient packaging. The prototype was built with the most up to date version of R-744 components available, allowing the evaluation of advanced technology, resulting in meaningful results. General system and component specific issues are mentioned to explain the necessity of using additional components with respect to the typical R-134a MAC system (i.e. internal heat exchanger, accumulator, expansion device and externally controlled compressor).

The European f-Gas Directive phases out HFC-134a from Mobile Air Conditioning systems (MACs) in new vehicles by 2017. In the US pending California and USEPA regulations have incentives to phase out HFC-134a earlier than 2017. As a result industry is striving to transform all global markets to a single new refrigerant in order to simplify global marketing. One of the global tools to help evaluate alternatives during this transition is the global LCCP (Life Cycle Climate Performance) and the development of the GREENMAC- LCCP© model. This model has become the global standard to measure the Life Cycle Analysis (LCA) greenhouse emissions of any proposed alternative refrigerant for MACs starting from bench test results and supporting the car manufacturer choice of the best suitable alternative refrigerant from an environmental perspective.

The current A/C system performance prediction codes usually describe the TXV behaviour in terms of SH dependence on the evaporation pressure. The data currently issued by TXV suppliers give the maximum outlet evaporator pressure values at 0 °C and 10 °C outlet evaporator temperature (Fig. 1).

Following the development of new technologies in Vehicle Thermal Management aiming to both enhancing the MAC System efficiency and reducing the thermal load to be managed, a prediction tool based on the AMEsim platform was developed at Advanced PD EMEA. This tool is dedicated to predict the effect of the implementation of sensors monitoring both the relative humidity and the carbon dioxide (CO2) concentration (taking into account passengers' generated moisture and CO2). This model implemented with the usual comfort inputs (CO2 and RH acceptable ranges) considers the system variables influencing the comfort and predicts the increase of both RH and CO2 concentration in the cabin compartment in any driving cycle depending on the number of occupants.