Air conditioning system operation

AIR CONDITIONING SYSTEM OPERATION

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 the conditioned air being supplied is controlled by a variable speed blower.

A sealed system, charged with Refrigerant R134a, together with a blower unit, blend unit and control system combine to achieve the cooled air condition.

The sealed system comprises of the following main components:

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

Refrigeration cycle

The compressor is driven by a belt from the crankshaft pulley. The compressor pressurises and circulates the refrigerant through the system. Mounted on the compressor, an electro-mechanical clutch maintains the correct temperature and pressure of the refrigerant by engaging or disengaging the compressor to support the system requirements. Operation of the clutch is controlled by the engine control module (ECM), the trinary (triple) pressure switch and a thermostatic switch located on the exterior fins of the evaporator.

If the temperature at the evaporator falls low enough for ice to form on the fins, the thermostatic switch will open, breaking the circuit to the clutch and releasing the drive on the compressor. The ECM detects that the air conditioning system is not operating and switches off the condenser cooling fan. When the temperature at the evaporator rises sufficiently for the thermostatic switch to close, the air conditioning system will be restarted.

If the system pressure becomes excessive or drops sufficiently to cause damage to the compressor, a trinary pressure switch located in the high pressure line at the receiver drier will break the electrical circuit to the compressor clutch, releasing drive from the compressor.

The two cooling fans are controlled by the ECM dependent upon engine coolant temperature and air conditioning system pressure. With the air conditioning system switched off, only the inboard (radiator) cooling fan is operational with the ECM controlling the 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 ECM and operate at low speed. As the pressure of the air conditioning refrigerant rises above the threshold of the medium pressure setting on the trinary pressure switch, the cooling fans are connected in parallel and operate at high speed.

If engine coolant temperature continues to rise with both cooling fans running at high speed, the ECM will break the circuit to the compressor clutch releasing drive to the compressor. This reduces the load on the engine whilst maintaining a high cooling fan speed to lower the engine coolant temperature. Once engine coolant temperature has dropped sufficiently the air conditioning system will be reinstated.

From the compressor, which has an emergency high pressure relief valve fitted, high pressure vaporised refrigerant is passed to the condenser which is mounted in front of the radiator. Ram air passing through the condenser, supplemented by the cooling fans, cools the refrigerant vapour in the condenser sufficiently to form a high pressure slightly subcooled liquid.

This subcooled liquid then passes to the receiver drier which extracts moisture from the refrigerant as well as acting as a reservoir.