Porsche P-80 2005, Руководство пользователя

Страница: 14 / 108

Page 1.8
Climate Control Systems Diagnosis & Repair
Air Conditioning Basics
Compressor
The compressor is an engine-driven refrigerant pump. It
compresses the refrigerant gas to raise both temperature
and pressure. The compressor also moves refrigerant
through the system.
All Porsche Sports Cars and Cayenne A/C compressors
are variable displacement swash plate designs. Fixed
displacement compressors were used on 911 Carrera
(993) and earlier models.
The compressor is belt driven. On Sports Cars an electro-
magnetic clutch is built into the compressor pulley to
engage and disengage the drive. On Cayenne, there is no
clutch. The compressor shaft turns at all times with the
engine. When no A/C is desired, compressor displacement
can be reduced to nearly 0%.
Compressor piston downstroke draws in low pressure
refrigerant gas from the evaporator. On the upstroke, the
refrigerant is compressed, raising both pressure and
temperature. Refrigerant pressure increases from approx.
0.6-1.7 bar (9-25 psi) to approx. 6-25 bar (87-363 psi).
The compressed refrigerant gas is pushed out of the
compressor to the condenser. The compressor can raise
the pressure of the refrigerant because it is pumping
against a restriction created by the expansion valve. The
system high side contains high pressure refrigerant, and
includes the compressor outlet, condenser, receiver-dryer
and expansion valve inlet. The flow of refrigerant gas into
and out of the compressor is controlled by reed valves.
Reed valves are flat plates of spring material located in the
cylinder head above each piston. They bend one way only
and allow flow in one direction. They open and close auto-
matically in response to pressure changes as the pistons
travel up and down.
The reeds are designed to pump refrigerant gas only, and
the compressor pistons have little clearance at the top of
their stroke. Liquid refrigerant must be kept out of the
compressor as it can hydraulic lock and damage it.
Fixed Displacement Compressors
Fixed displacement compressors have a fixed piston
stroke and always pump the same volume of refrigerant
for each revolution of the compressor shaft. The piston
stroke never varies and the compressor's displacement
never changes.
All A/C compressors must have sufficient capacity for
good cooling under high heat load conditions and low
engine speeds. Since fixed displacement compressors
cannot vary the amount that they pump, under conditions
with high engine speed and low heat demands, these
compressors have much greater capacity than is required.
Energy can be wasted pumping this excess refrigerant.
Also, the compressor clutch may cycle ON and OFF more
frequently than is desired.
Notes: