Porsche P-80 2005, Manual

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Air Conditioning Basics
Climate Control Systems Diagnosis & Repair
Page 1.9
Variable Displacement Compressors
Porsche vehicles today use a variable displacement
compressor. Seven axial pistons are arranged
concentrically around the central compressor shaft. A
smooth disk is attached to the compressor shaft. Slipper
shoes follow the disk's oscillating motion and the angle of
the disk transmits reciprocating forces to the pistons.
Variable displacement compressors can vary the piston
stroke and can change output volume. The longer the
piston stroke, the greater the displacement and output.
The stroke is determined by the angle of the swash plate
on the compressor shaft. Variable displacement compres-
sors change the swash plate angle to match the
refrigerant flow rate to the heat load on the system.
Sports Cars – Variable Displacement Compressor,
Low Displacement Position
Sports Cars – Variable Displacement Compressor,
High Displacement Position
Variable displacement compressors are more efficient
than fixed displacement compressors because they pump
only enough refrigerant to achieve the desired cooling.
Energy is not wasted pumping more refrigerant than is
necessary. Also, clutch cycling is eliminated and engine
idling is improved with variable displacement
compressors.
Heat Load
The compressor senses system refrigerant pressures to
determine heat load and automatically adjust the volume
of refrigerant pumped for optimum heat transfer. The
output of the compressor varies between 5% and 100%.
Compressor displacement is 160 cm
3 at 100% output.
Low Heat Load - 5% Output
The conditions are met when engine r.p.m. is consistently
high, e.g. expressway driving, and/or temperature is low.
The suction pressure is relatively low and the control valve
is open. The angle of the rotating disk is at its lowest and
so delivery is at its minimum.
High Heat Load - 100% Output
The conditions are met when engine r.p.m. is consistently
low and/or temperature is high. The suction pressure is
high and closes the control valve. The angle of the rotating
disk is at its greatest and so delivery is at its maximum.
Notes: