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The refrigerant cycle in conventional omnibus HVAC systems has a significant influence on fuel consumption and, as a result, on vehicle emissions. The additional emissions resulting from the use of the air conditioning system are called indirect emissions. In addition, there are so-called direct emissions from the air conditioning system caused by unintended leakage of refrigerant. A reduction in indirect emissions can be achieved, for instance, by adjusting the capacity of the refrigerant compressor. A reduction in direct emissions can be achieved by so-called alternative or natural refrigerants. To investigate approaches to reducing direct and indirect emissions, a total vehicle simulation model of a coach with detailed HVAC systems was developed with full implementation in Modelica. For this total vehicle simulation of a coach with a detailed HVAC system, a refrigerant cycle based on the natural refrigerant CO2 (R-744) was modeled and validated. In addition, an efficient control strategy was developed by adjusting the capacity of the refrigerant compressor to cover the actual cooling capacity demand and save fuel. Based on virtual driving test scenarios, the fuel saving potential of the developed compressor capacity control strategy is investigated to determine average annual fuel savings.

Investigation of fuel reduction potential of a capacity controlled HVAC system for buses using virtual test drives