300Tdi Discovery
AIR CONDITIONING
3
DESCRIPTION AND 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.
A sealed system, charged with Refrigerant R134a,
together with a blower unit, blend unit and control
system combine to achieve the cooled air condition.
For air conditioning air distribution system.
See
HEATING AND VENTILATION, Description and
operation, Heating and ventilation unit
The air conditioning system comprises five major
units:
1. An engine-mounted compressor.
2. A condenser mounted in front of the radiator.
3. A receiver/drier unit located in front of the
condenser.
4. Thermostatic expansion valve mounted above
the evaporator.
5. An evaporator unit mounted in front of the heater
matrix.
NOTE: Vehicles fitted with rear air
conditioning have an additional
evaporator/blower motor assembly located
behind the LH rear compartment lower trim panel.
These units are interconnected by hoses and pipes
carrying Refrigerant R134a, the evaporator is linked
into the vehicle ventilation system.
Refrigeration cycle
1. Compressor
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 normally controlled by a thermostat located
at the evaporator (5). The compressor is of the
swashplate type having fixed displacement.
Should the temperature at the evaporator (5) fall low
enough for ice to begin to form on the fins, the
thermostat disengages the clutch and also isolates the
cooling fans relays. When the temperature at the
evaporator (5) rises to the control temperature, the
clutch is re-engaged.
Should the system pressure become excessive or
drop sufficiently to cause damage to the compressor
(1) a dual pressure switch (7), located in the high
pressure line, signals the relay unit to disengage the
clutch. The compressor also has an emergency high
pressure relief valve (9) fitted.
The cooling fans are controlled by engine temperature
when the air conditioning is not switched on.
2. Condenser
From the compressor, hot 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 2 cooling fans (8) mounted in front
of the condenser, cools the refrigerant vapour
sufficiently to form a high pressure slightly subcooled
liquid (F2).
3. Receiver/drier
This liquid then passes to a receiver/drier (3) which
fulfils two functions. It acts as a reservoir and moisture
extractor (11).
A sight glass (10), in the high pressure line, provides a
method of determining the state of the refrigerant
without breaking into the system.
4. Expansion valve
From the receiver/drier (3) the moisture free high
pressure liquid refrigerant (F3) passes through a
thermostatic expansion valve (4). A severe pressure
drop occurs across the valve and as the refrigerant
enters the evaporator space at a temperature of
approximately -5
°
C it boils and vaporises.
