Abstract
The heating-up time of the passenger compartment has to be short even in vehicles with modern direct injection engines. An efficient supplementary heating system is required to meet this need. A prototype CO2 air conditioning system was tested in heat pump mode. The heating capacity was between 2 and 6 kW at ambient temperatures of +5°C and –5°C and the relative humidity was 80%. Frost formation on the exterior heat exchanger surface was the limiting factor of the system. The formation of frost could be delayed by controlling the evaporation pressure with a variable displacement compressor. The air-flow through the exterior heat exchanger was blocked after 15-20 minutes of operation.