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

Sealed refrigerant

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 the following main components:

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

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 sport the system's requirements. The clutch action is normally controlled by a thermistor located at 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 to disengage the clutch and also isolates the cooling fans relays. When the temperature at the evaporator (5) rises to the control temperature, the system is reactivated.

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

On 1.6 models the switch (7) is a dual pressure switch and on 1.4 and 2.0 models, it is a trinary (triple) pressure switch, the third (medium pressure) switch providing a higher cooling fan speed when closed by system pressure.

The cooling fans are controlled by engine temperature when the air conditioning is not switched on. When the air conditioning is switched on, they are also controlled by air conditioning system pressure acting on the switch (7).

On 1.4 and 2.0 models, the fans can operates at two speeds, the higher speed being engaged if engine temperature or air conditioning system pressure rise above pre – determined figures. On 2.0 Turbo models, the fan speed is controlled by the MEMS ECU.

The fans on 1.6 models, operate at one speed.

From the compressor, which has an emergency high pressure relief valve (10) fitted, high pressure vaporised refrigerant (F1) passes to the condenser (2), which is mounted in front of the engine coolant radiator. Ram air(A1) passing through the condenser (2), supplemented by two cooling fans (8), mounted on the rear of the engine coolant radiator on 1.4 and 1.6 models and in front of the condenser on 2.0 models, cools the refrigerant vapour sufficiently to form a high pressure slightly subcooled liquid (F2). This liquid then passes to a receiver/drier (3) which fulfils three functions. It acts as a reservoir, moisture extractor (11) and by means of a sight glass (12) 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 low pressure high temperature refrigerant vapour (F4) at the outlet. The atomised refrigerant then passes through the evaporator (5). Fan blown air (A2) passes through the matrix (A3) of the evaporator and is cooled by absorption due to the low temperature refrigerant passing through the evaporator.

A thermistor is fitted in the airflow out of the evaporator to sense the temperature of the exterior fins. Should ice begin to form due to a too cold condition it will signal the electro – mechanical clutch on the compressor (1) to disengage.

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