Refrigerant Circuit, Function - GF83.40-P-2008FGA

Model 213 

up to model year 2021 

with code 1U6 (R744 refrigerant) 

Schematic diagram of the refrigerant circuit 

Courtesy of MERCEDES-BENZ USA

Courtesy of MERCEDES-BENZ USA

Function requirements, general 

• Engine running, refrigerant compressor switched on
• Climate control switched on

Refrigerant circuit in general 

The A/C system is operated with natural refrigerant R744 carbon dioxide (CO₂ ).

CO₂ is usually available in gaseous form. It can be compressed below a temperature of 31°C to become a colorless fluid. At room temperature a pressure of approx. 60 bar would be required for this. In solid form, CO₂ is used as so-called dry ice.

Because of the physical properties of CO₂ it is not possible to define at exactly which sections of the refrigerant circuit the refrigerant is gaseous or liquid. The refrigerant can be gaseous and liquid at various points. This depends essentially on the pressure and temperature ratios in the system and thus on each load case.

The refrigerant flow rate is controlled by the climate control control unit (N22/1) by actuating the refrigerant compressor magnetic clutch (A9/7y2), the refrigerant compressor closed-loop control valve (A9/7y1) and the CO₂ expansion valve.

The refrigeration circuit basically consists of the following components:

• Refrigerant compressor
• Gas cooler
• Battery with heat exchanger
• CO₂ expansion valve
• Evaporator

The individual components of the refrigerant circuit are interconnected through hoses and pipelines and form a closed system.

The refrigerant circuit consists of the following functional sections:

• Refrigerant feed 
• Refrigerant return 

The separation points are the refrigerant compressor and the CO 2 expansion valve.

The refrigerant circuit is equipped with pressure limiting elements and several sensors to monitor various parameters. The connections for the individual components of the refrigerant circuit are located outside the passenger compartment or they are encapsulated. The CO₂ concentration level in the vehicle interior is also monitored.

Refrigerant feed 

The refrigerant compressor sucks in the gaseous refrigerant from the battery with heat exchanger, compresses it and routes it into the gas cooler. The refrigerant has a temperature of 0 to 30°C upstream of the refrigerant compressor. This is increased by the compression to 30 to 160°C. The pressure increases from 30 to 50 bar to 50 to 130 bar. The compressed hot refrigerant is cooled in the gas cooler by the blow-by air or the outside air drawn in by the fan motor (M4/7) to 5 to 55°C. The refrigerant then flows into the battery with heat exchanger. In the inner heat exchanger, the refrigerant is cooled again to 5 to 35°C. The battery is used for storing the refrigerant, and is fed by refrigerant return line. The heat exchanger is used for boosting the system's efficiency. The refrigerant then flows on to the CO₂ expansion valve. The refrigerant under high pressure is then relieved and vaporized in the evaporator.

Refrigerant return 

The refrigerant is relieved in the evaporator and consequently changes its aggregation state to gaseous, if it was liquid beforehand. The required evaporation heat is removed from the air blowing by at the evaporator fins, cooling it down in the process. The cooled air is led to the vehicle interior via the electric blower motor (A32m1). If a certain evaporator temperature is dropped below, the climate-control control unit switches off the refrigerant compressor. The refrigerant has a temperature of 0 to 15°C at the evaporator outlet. The refrigerant pressure has dropped to 30 to 50 bar after evaporation. The gaseous refrigerant flows further through the battery with heat exchanger, and it is then sucked in and compressed by the refrigerant compressor.