the resulting decrease in pressure and partial boiling
lowers its temperature to its new boiling point. As the
refrigerant flows through the evaporator, passenger
compartment air passes over the outside surface of the
evaporator coils. As it boils, the refrigerant absorbs
heat from the air and thus cools the passenger
The heat from the passenger
compartment is absorbed by the boiling refrigerant and
hidden in the vapor. The refrigeration cycle is now
under way. The following functions must be done to
complete the refrigeration cycle:
1. Disposing of the heat in the vapor
2. Converting the vapor back to liquid for reuse
3. Returning of the liquid to the starting point in the
The compressor and condenser (fig. 7-35) perform
these functions. The compressor pumps the refrigerant
vapor (containing the hidden heat) out of the
evaporator and suction accumulator drier, then forces
it under high pressure into the condenser which is
located in the outside air stream at the front of the
vehicle. The increased pressure in the condenser raises
the refrigerant condensation or saturation temperature
to a point higher than that of the outside air. As the heat
transfers from the hot vapor to the cooler air, the
refrigerant condenses back to a liquid. The liquid
under high pressure now returns through the liquid line
to the fixed orifice tube for reuse.
It may seem difficult to understand how heat can
be transferred from a comparatively cooler vehicle
passenger compartment to the hot outside air. The
answer lies in the difference between the refrigerant
pressure that exists in the evaporator and the pressure
that exists in the condenser. In the evaporator, the
compressor suction reduces the pressure and the
boiling point below the temperature of the passenger
compartment. Thus heat transfers from the passenger
compartment to the boiling refrigerant.
condenser, the compressor raises the condensation
point above the temperature of the outside air. Thus
the heat transfers from the condensing refrigerant to
the outside air.
The fixed orifice tube and the
compressor simply create pressure conditions that
permit the laws of nature to function.
There are three basic types of air-conditioning
compressors in general use in automotive applications.
Each of these uses a reciprocating (back-and-forth
motion) piston arrangementtwo-cylinder
reciprocating, swash plate, and scotch yoke. Most
automotive compressors are semihermetic.
Two-cylinder compressors (fig. 7-36) usually
contain two pistons in a parallel V-type configuration.
The pistons are attached to a connecting rod, which is
driven by the crankshaft. The crankshaft is connected
to the compressor clutch assembly, which is driven by
an engine belt.
Reed valves generally are used to
control the intake and exhaust of the refrigerant gas
during the pumping operation. These compressors are
usually constructed of die cast aluminum.
In the swash plate or "wobble plate" compressor
(fig. 7-37), the piston motion is parallel to the
Figure 7-36.Two-cylinder reciprocating compressor.
Figure 7-37.Five-cylinder swash plate compressor.