Air conditioning system operation

AIR CONDITIONING SYSTEM OPERATION

The air conditioning system provides the means of supplying cooled and dehumidified air into the interior of the vehicle. The cooling effect is obtained by drawing air, from the interior of the vehicle, through the matrix of an evaporator unit. The cooled and dehumidified air is then emitted from the centre and side vents of the fascia.

Sealed refrigerant

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

The sealed system comprises the following main components:

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

Refrigeration cycle

The compressor (1), 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 clutch action is controlled by the MEMS ECU which receives a temperature signal from a thermistor located on the exterior fins of the evaporator (5). The compressor is of the swashplate type having variable displacement.

Should the temperature at the evaporator (5) fall low enough for ice to begin to form on the fins, the thermistor signals the ECU which disengages the compressor clutch and also isolates the cooling fan relays. When the temperature at the evaporator (5) rises to above its threshold, the system is reactivated.

Should the system pressure become excessive or drop sufficiently to cause damage to the compressor (1) a trinary (triple) pressure switch (7), located in the high pressure line at the receiver drier, signals the ECU to disengage the compressor clutch.

When the air conditioning is switched off, the cooling fans are controlled by two thermostatic switches mounted in the top of the radiator. When the air conditioning is switched on, the cooling fans are also controlled by the trinary pressure switch. The cooling fans have two operating speeds, the higher speed being engaged if engine temperature rises excessively or if the air conditioning system pressure rises sufficiently to close the medium pressure switch of the trinary pressure switch.

From the compressor high pressure vaporised refrigerant (F1) passes to the condenser (2), which is mounted in front of the coolant radiator.

A cooling fan mounted on the exterior of the condenser operates continuously when the air conditioning is operating. The cooling fan cools the refrigerant vapour in the condenser sufficiently to form a high pressure slightly subcooled liquid (F2).

This liquid then passes to a receiver/drier (3) which performs three functions. It acts as a reservoir, moisture extractor (11) and by means of a sight glass (10) provides a method of determining the state of the refrigerant without breaking into the system.

From the receiver/drier (3) the moisture free refrigerant liquid (F3) passes through a thermostatic expansion valve (4). This valve, converts the liquid refrigerant to a low temperature, low pressure liquid vapour mixture (F4).

To prevent liquid passing through to the compressor, a capillary tube (6), attached to the outlet pipe of the evaporator (5) and connected to the thermostatic expansion valve (4), controls the amount that the valve opens and closes in relation to the temperature of the refrigerant vapour (F4) at the outlet.

The atomised refrigerant then passes through the evaporator (5). Fan assisted air (A2) passes through the matrix (A3) of the evaporator and is cooled by absorption due to the low temperature refrigerant extracting the heat from the air.

From the evaporator, low pressure slightly heated refrigerant (F5) passes to the compressor to complete the cycle.