Air Conditioning Compressors - Delphi

Va r i a b l e s wa sh-plate compressor show i n g m e ch a n i s m
Th e s wa s h - p l a t e h a s a l i n k a g e m echanism
t h a t a l l ow s i t t o r o t a t e t h u s ch a n g i ng its an-
g l e a n d w i t h i t , t h e p i s t o n d i s p l a c e ment. The
l i n k a g e i s d e s i g n e d s o t h a t t h e clearance
vo l u m e r e m a i n s a s c o n s t a n t a n d small as
p o s s i b l e ove r t h e f u l l s wa s h - p l a te angle
r a n g e f r o m f u l l s t r o ke t o m i n i m u m stroke.
Th i s s wa s h - p l a t e s f r e e d o m t o r o t a te means
t h a t u n d e r a ny g i ve n o p e r a t i n g c onditions
i t w i l l t a ke u p a p o s i t i o n t h a t i s determi-
n e d by t h e b a l a n c e o f a l l t h e fo r c es acting
o n i t t h a t t e n d t o c a u s e i t t o r o t a te about
i t s c e n t r e o f r o t a t i o n . Th e f i r s t e l ement of
c o m p l ex i t y i s t h a t t h e c e n t r e o f r o t ation is
n o t f i xe d a s i t s h i ft s l o c a t i o n s l i g htly with
t h e s wa s h - p l a t e a n g l e . I t l i e s fa i r l y near to
t h e s wa s h - p l a t e c o n n e c t i o n w i t h t h e link but
i t s ex a c t l o c u s i s a f u n c t i o n o f t h e linkage
m e ch a n i s m l o c a t i o n , d i m e n s i o n s a nd of the
s wa s h - p l a t e a n g l e . Th e m o s t o bv i o us force
t h a t c o m e s i n t o p l ay i s t h e g a s pressure
a c t i n g o n t h e p i s t o n c r ow n . E s t i mation of
i t s ave r a ge e ffe c t , h oweve r i s c o mplex as
t h e c y l i n d e r p r e s s u r e va r i e s g r e a t l y around
t h e c yc l e . A t t h e s a m e t i m e i t s l i n e of action
i s c o n t i n ually ch a n g i n g a n d h e n c e so is the
e ffe c t i ve t o r q u e i t exe r t s a b o u t t he centre
o f r o t a t i o n . Th i s t o r q u e w i l l o bv i o usly tend
t o u p s t r o ke t h e c o m p r e s s o r.
A l e s s o bv i o u s s o u r c e o f t o r q u e g eneration
o n t h e s wa s h - p l a t e i s t h e i n e r t i a o f the pis-
t o n s w h i ch a r e r e p e a t e d l y a c c e l e r ated and
d e c e l e r a t e d a s t h e c o m p r e s s o r i s operated.
I t m i g h t b e t h o u g h t t h a t t h e e ffe ct of the
fo r c e s fo r a c c e l e r a t i o n a n d d e c eleration
c a n c e l e a ch o t h e r o u t b u t i n fa c t t he piston
Ta b l e o f s wa s h - p l a t e t o r q u e g e n e r a t i n g m e ch a n i s m s
is accelerated upwards over the half of the
swash-plate furthest from its centre of rot a-
tion and correspondingly decelerated over
the half nearest to the centre of rot ation.
The former therefore exerts greater torque
and the net result is a tendency to upstroke
which is proportional to the square of the
piston speed and thus becomes significant
at higher compressor speeds.
A second torque generating force that is a
similar function of compressor speed comes
from centrifugal affects acting on both the
link mechanism and the swash-plate itself. If
one considers the t wo halves of the swash-
plate nearest and most remote from the cen-
tre of rot ation it is clear that the centrifugal
forces of the more remote half generate the
greater torque and the net result will be a
tendency to destroke the compressor.
Further forces generating rot ation torque
can come from the fitting of upstroke and/or
downstroke springs. These can be inst alled
on the shaft to assist the upstroke /down-
stroke process under some compressor opera-
ting conditions and if fitted will obviously
contribute to the overall balance of torques.
Here the effect will be a simple function of
the swash-plate angle.
M e ch a n i s m E ffe c t Comment
The final contributor to the balance of tor-
ques is that generated on the underside
of the pistons by the crank case pressure.
Although the crank case pressure remains
const ant around the cycle its line of action
around the plate must be integrated to ob-
t ain an average net torque. It is the abilit y to
control this pressure that allows the balance
to be shifted and thus the swash-plate angle
to be controlled. Crank case pressure is con-
trolled with bleeds from both suction and
high side. The small suction bleed is usually
permanent and the high side bleed is con-
trolled by means of a valve. If the high side
bleed is closed the suction bleed pumps the
crankcase down to suction pressure with the
result that the compressor operates at full
Cy l i n d e r p r e s s u r e U p s t r oke Integrated around swash-plate
P i s t o n i n e r t i a U p s t r oke Significant at high speed
C e n t r i f u gal fo r c e s D e s t r oke Significant at high speed
S p r i n g s U p s t r oke/destroke Where fitted
C r a n k c a s e p r e s s u r e D e s t r oke Adjusted for control