Air Conditioning

The system runs a vapour-compression cycle with refrigerant R-1234yf in most new cars (older vehicles use R-134a, which has a much higher global-warming potential). The compressor raises low-pressure vapour (roughly 2–3 bar gauge in operation) to high pressure (roughly 10–20 bar depending on ambient temperature), and its temperature rises with it. Engine-driven compressors are swash-plate types, either cycled by an electromagnetic clutch or with internally controlled variable displacement; electric and hybrid vehicles use high-voltage scroll compressors independent of engine speed.

In the condenser the superheated vapour rejects heat to ambient air, condenses and is often subcooled. A receiver-drier (with a thermostatic expansion valve) or an accumulator (with a fixed orifice tube) stores liquid and removes moisture, which would otherwise freeze at the expansion device or form acids. The expansion device throttles the liquid to low pressure: it flashes partly to vapour and its saturation temperature falls to just above 0 °C. In the evaporator it boils, absorbing latent heat from the cabin air; the expansion valve modulates flow to keep a small superheat at the evaporator outlet so only vapour returns to the compressor. An evaporator temperature sensor prevents icing.

Efficiency is expressed as coefficient of performance, COP = Q̇_evaporator / W_compressor, typically around 2–3 for car systems: 1 kW of compressor work moves 2–3 kW of heat. The HVAC module mixes air from the evaporator and the heater core via blend doors, so the evaporator can dehumidify while the heater re-warms the air. Many electric vehicles reverse the cycle as a heat pump, extracting heat from outside air or drivetrain coolant to warm the cabin at a COP above 1, which significantly reduces the range penalty of winter heating compared with resistive heaters.