Air conditioning system operation

AIR CONDITIONING SYSTEM OPERATION

General

The air conditioning system provides the means of supplying cooled and dehumidified, fresh or recirculated air to the interior of the vehicle. The cooling effect is obtained by blowing air through the matrix of an evaporator unit and when required, mixing that air with heated air by means of the heater distribution and blend unit, to provide the conditions required inside the vehicle. The volume of conditioned air being supplied is controlled by a variable speed blower.

The sealed system is charged with refrigerant and together with a blower unit, blend unit and control system combine to achieve the cooled air condition. The system comprises the following main components:

1. Compressor 2. Condenser 3. Receiver/drier 4. Thermostatic expansion valve 5. Evaporator

Compressor

The compressor, belt driven from the crankshaft pulley, pressurises and circulates the refrigerant through the system. Mounted on the compressor, an electro-mechanical clutch maintains the correct temperature and pressure by engaging or disengaging to support the system’s requirements. The operation of the clutch is controlled by the engines MEMS ECM, the trinary (triple) pressure switch and a thermistor located in the evaporator fins.

Should the temperature at the evaporator fall low enough for ice to begin to form on the fins, the thermistor signals the ECM to disengage the clutch and also isolates the cooling fans relays. When the temperature at the evaporator rises to the control temperature, the system is reactivated.

Should the system pressure become excessive, above 27 bar or drop sufficiently, below 2.0 bar to cause damage to the compressor the trinary pressure switch, located in the high pressure line, signals the ECM to disengage the compressor clutch.

Cooling fans

The two cooling fans are controlled by the MEMS ECM dependent on engine coolant temperature and air conditioning system pressure. With the air conditioning system switched off, only the radiator (LH) fan is operational with the MEMS ECM controlling fan speed according to engine temperature.

With the air conditioning system switched on, both the radiator and condenser cooling fans are connected in series by the MEMS ECM to operate at slow speed. As pressure of the air conditioning refrigerant rises above the medium pressure setting of 19 bar on the trinary switch, the cooling fans are connected in parallel and operate at fast speed. Should the pressure continue to increase, an emergency high pressure relief valve in the compressor will open.

Condenser

From the compressor, high pressure vaporised refrigerant passes to the condenser, which is mounted in front of the engine coolant radiator. Ram air passing through the condenser, supplemented by forced air from two cooling fans mounted on the rear of the engine coolant radiator, cools the refrigerant vapour sufficiently to form a high pressure slightly subcooled liquid refrigerant.