Air Conditioning Compressors - Delphi

Air Conditioning Compressors

I n t r o d u c t i o n
Th e p a s s e n g e r c o m p a r t m e n t o f a modern
a u t o m o b i l e o ffe r s a ve r y d i ffe r e n t environ-
m e n t t o t h a t o f a ve h i c l e f r o m f i ft y years
a g o . As we l l a s t h e e r g o n o m i c a l l y designed
s e a t i n g a n d i n s t ru m e n t l ayo u t , t h e all round
v i s i b i l i t y a n d t h e e ff i c i e n t u s e o f space,
t o d ay ’s p a s s e n g e r h a s c o n t r o l ove r air tem-
Vi ew o f i n s t ru m e n t p a n e l a n d H VAC c o n t r ols
Th e ve r y h e a r t o f t h e a i r c o n d i tioning sys -
t e m i s t h e c o m p r e s s o r. I ts key t a sks are to
g e n e r a t e t h e r e q u i r e d flow o f r efrigerant
a r o u n d t h e s ys t e m a n d a t t h e same time
t o c o m p r e s s i t s u ff i c i e n tly t o raise its
t e m p e r a ture a b ove t h e a m bient in order
t h a t h e a t c a n b e r e j e c t e d f r o m t h e system.
Th e s e d e m a n d s m u s t b e m e t e fficiently over
ve r y w i d e r a n g e s o f c o mpressor s peed and
a m b i e n t c o n d i t i o n s . A t the same time ver y
d e m a n d i n g n o i s e g e n e r a t ion c o n s t raints and
ex p e c t a t i o n s o f d u r a b i l i t y have to be met.
A i r C o n d i t i o n i n g
C y c l e
I t i s s e l f ev i d e n t t h a t h e a t t e n d s to flow
f r o m high temperatu r e to low. If we wish to
p r ov i d e c o o l i n g a t a l o c a t i o n i n a h i g h tempe-
r a tu r e e nv i r o n m e n t t h e n we a r e e ffectively
a s k i n g h e a t t o f l ow f r o m l ow t e mperature
t o h i g h . To a ch i eve t h i s i t i s n e c essar y to
o p e r a t e a t h e r m o d y n a m i c c yc l e i n which a
perature, qualit y and movement in all corners
of the vehicle. The modern heating , ventila-
ting and air conditioning system (HVAC) can
ensure that thermal comfort and all round
visibilit y are delivered in environments that
range from the heat of Phoenix Arizona to
the coldest Scandinavian winter.
E x p a n d e d v i ew o f a m o d e r n
H VAC c o m p r e s s o r
refrigerant at a low temperature is used to
extract heat from the location to be cooled.
The refrigerant is subsequently compressed
until it becomes hot enough to be able to
reject heat to the ambient. After cooling to
near ambient temperature by the a mbient
air, it is expanded back to low pressure, a
process which generates further refrigerant
temperature reduction that becomes low
enough to be used for cooling at the re-
quired location. The cycle is complete. The

e q u i r e d c o o l i n g i s t h u s a ch i eve d b ut at the
ex p e n s e o f t h e wo r k n e cessar y t o drive the
c o m p r e s s o r.
heat rejected to ambient
expansion device
T h e A u t o m o t i v e
A i r C o n d i t i o n i n g
A p p l i c a t i o n
Th e a u t o m o t i ve a p p l i c ation places ver y
s p e c i a l d e m a n d s o n t h e air c o n ditioning sys-
t e m . A t y p i c a l ve h i c l e s ys t em has a similar
c o o l i n g c a p a c i t y t o t h a t r e q uired for the air
c o n d i t i o n i n g o f a s m a l l h o use d e spite the
va s t d i ffe r e n c e i n vo l u m e s t o b e c ooled. The
r e a s o n s fo r t h i s a r e t wo fold. Firstly cooling
d u t y p e r u n i t vo l u m e i s m u ch higher for the
ve h i c l e b e c a u s e h e a t t ransfe r c o efficients
b e t we e n h o t a m b i e n t a ir a n d t h e outside
s u r fa c e s a r e m u ch h i g h e r d u e t o movement
o f t h e ve h i c l e t h r o u g h t h e a i r. Secondly
t h e p r o p ortion o f t h e e nclosure consisting
o f g l a s s i s ve r y h i g h fo r t h e vehicle – a
fa c t o r t h a t m a ke s t h e e ffe c t o f direct solar
r a d i a t i o n h e a t i n g ve r y h i g h. O n t op of this
a p a r t i c u l a r l y d e m a n d i n g r e q u i r ement is to
c o o l t h e cabin ve r y r a p i d l y a ft e r t he vehicle
h a s b e e n s o a ke d i n a n a m bient t e mperature
o f 4 0 ° C or h i g h e r. A t t h e s t a r t o f the cool
d ow n t e mperatu r e s i n t he c a bin c an be as
h i g h a s 6 0 o r 7 0 ° C .
condenser
evaporator
S ch e m a t i c o f a i r c o n d i t i o n i n g c yc l e
compressor
heat absorbed from cabin
A n o t h e r s i g n i f i c a n t way i n w h ich a utomotive
a i r c o n d i t i o n i n g d i ffe r s from t h e domestic
o r c o m m e r c i a l ve r s i o n is t h e q u estion of
c o m p r e s s o r d r i ve . I n t he ve h icle the com-
p r e s s o r i s b e l t d r i ve n by t h e e n g i ne so that
i n d e p e n d e n t c o n t r o l over t h e c ompressor
s p e e d i s n o t p o s s i b l e . This o bv i ously has
s i g n i f i c a n t i m p l i c a t i o n s fo r sys t e m control
a n d m e a n s t h a t t h e r e c an b e calls for high
s ys t e m perfo r m a n c e a t t imes when the
c o m p r e s s o r s p e e d i s ve r y l ow. A n import ant
implication of this is that the compressor
drive shaft must pass out of the compres-
sor casing with the result ant potential for
refrigerant leakage. Hermetic compressors
are not possible and ver y effective shaft
seals must be used. A second implication
of the external drive is that the compressor
must be engine mounted so that lengths of
flexible hose must be introduced to accom-
modate relative movement bet ween engine
and chassis mounted components.
As with other air conditioning applications
system efficiency is ver y import ant and the
automotive application is no exception. While
the characteristics of all the major compo-
nents in the system contribute to overall ef-
ficiency any compressor inefficiencies must
be kept to a minimum. A further feature of
the compressor that requires serious con-
sideration is its intrinsic noise generation
- modern vehicle noise requirements mean
that the compressor must be ver y quiet and
vibration free.
And finally, three further requirements of
the compressor, specific to the automotive
application, but which apply equally to ever y
vehicle component – are stringent con-
straints on size, weight and cost.
Th e ch a l l e n g e o f c o m p r e s s o r p a ck a g i n g
As a result of all these constraints deriving
from the use of an engine driven compres-
sor, it is often suggested that an electrically
driven system would provide a much better
solution. The reason that it is not feasible
on current vehicles is insufficient electrical
power is available and its application must
await the widespread implement ation of high
powered and efficient integrated st arter/
generators. The applications in which an
electrical compressor would make real sense
however are in hybrid and fuel cell vehicles
where sufficient electrical power is readily
available.

Th e h i g h e ff i c i e n c y o f m o d e r n e n g ines can
l e a d t o t h e s i tu a t i o n i n which i n s u ff icient re-
j e c t e d h e a t i s ava i l a b l e fo r comfo r t heating .
Th i s fa c t h a s p r o m p t e d t h e s u g g estion that
t h e a i r c onditioning s ys tem b e o p erated in
r eve r s e a s a h e a t p u m p t o s u p p l e ment the
e n g i n e h e a t a n d t h e v i a b i lit y o f the concept
h a s b e e n d e m o n s t r a t e d o n a n u m b er of plat-
fo r m s . Th e i m p l i c a t i o n s fo r t h e c o mpressor
a r e t h a t t h e r e q u i r e d o perating life can be
n e a r l y d o u b l e d a n d t h e c ompressor ambient
s a fe o p e r a t i n g t e m p e r a tu r e r a n g e must be
ex t e n d e d d ow nwa r d s t o minus 20°C.
D e l p h i ve h i c l e w i t h H e a t Pu m p s u p p l e m e n t a r y h e a t i n g
R e f r i g e r a n t I s s u e s
Th e a u t o m o t i ve a i r c o n ditioning industr y
r e s p o n d e d p r o - a c t i ve l y t o t h e d emands of
t h e 19 8 7 M o n t r e a l Pr o tocol a n d by 1994,
2 ye a r s e a r l y, t h e ch a n g e ove r f r o m R12 to
R 13 4 a wa s c o m p l e t e fo r a ll ve h icle produc-
t i o n l i n e s i n t h e d eve l oped wo rld. It soon
b e c a m e a p p a r e n t t h a t t h is m ay n ot be the
e n d o f t h e s t o r y a n d w i th i ncreasing pene-
t r a t i o n o f a i r c o n d i t i o n i n g i n t h e European
m a r ke t t ogether w i t h t he d e mands of the
Kyo t o Pr o t o c o l p r e s s u r e has b u ilt to make
ch a n g e s t o s t i l l f u r t h e r r e d uce the potential
c o n t r i b u t i o n o f a i r c o nditioning systems
t o g l o b a l wa r m i n g . A n ew D i r ective has
r e c e n t l y c o m p l e t e d i t s p r o g r ess t h r ough the
E u r o p e a n l e g i s l a t i ve p r o cess. I t r e q uires the
e l i m i n a t i o n o f R 13 4 a f r om a ll n ew models
f r o m 2 011 a n d f r o m a l l n ew ve h icles by 2017.
Th e c u rrently c o n s i d e r ed alternatives are
R 15 2 a w i th a g l o b a l wa rming p o t e ntial less
t h a n 10 % o f t h a t o f R 13 4 a, o r t h e so-called
‘ n a tu r a l ’ r e f r i g e r a n t R 744 ( carbon dioxide).
R 15 2 a h a s t h e r m o d y n amic characteristics
ve r y s i m i l a r t o t h o s e o f R 13 4a a nd will be
d i r e c t l y u s a b l e w i t h current c ompressor
t e ch n o l o g y. I t s p o t e n t i a l d r aw back is a
d e g r e e o f f l a m m a b i l i t y that h as made the
i n d u s t r y reluct a n t t o c onsider i t. More re-
c e n t l y o t h e r l a r g e ch e m i c al c o m panies have
a n n o u n c e d t h e p o t e n t i a l ava ilabilit y of new
a l t e r n a t i ve s . L i tt l e i s k n ow n a b o u t them at
t h i s s t a g e b u t i t i s a s s u m e d t hat they too
wo u l d f u n c t i o n w i t h c u rrent c ompressor
t e ch n o l o g y. Th e r e m u s t b e a q u estion as to
w h e t h e r t h e n e c e s s a r y d eve l o pment and
testing and then implement ation of the
necessar y manufacturing capacit y can be
completed in time to meet the requirements
of the European legislation. If R744 becomes
the chosen route the implications for com-
pressor technology are ver y far-reaching . Air
conditioning systems with R744 need com-
pressors with displacement of only a fifth
of that required for R134a but operating at
much higher pressures. Discharge pressures
of 120 bar are normal compared with a t ypi-
cal 18 bar for an R134a system. As can be
imagined, these factors have big implications
for the compressor layout and structural
strength requirements and also represent a
particularly severe shaft seal challenge.
Pressure - MPa
100
10
1
100
-40°C
0°C
200
40°C
80°C 120°C 160°C 200°C
0°C
40°C
300 400
Enthalpy - kJ/kg
500
I d e a l i ze d p r e s s u r e e n t h a l py d i a g r a m o f C O 2 c yc l e
C o m p r e s s o r
Te c h n o l o g i e s
Air conditioning began to become available
for vehicles in the early 1950s. From the
beginning a number of different compressor
technologies have been used. The earliest
systems used piston compressors in various
configurations. The Frigidaire F5, a five pis-
ton wobble plate configuration, used in the
earliest GM vehicles with air conditioning
was first produced in 1956 – it weighed in at
18kg! The R4 compressor with a scotch yoke
mechanism was first produced by Harrison
(now Delphi) in 1974 and remained in pro-
duction until 1995. It was 190mm in overall
diameter and weighed 9kg .
A n e a r l y
c o m p r e s s o r,
t h e D e l p h i R 4
80°C
120°C
600
160°C
200°C
700

Th e p r e fe rred c o n f i g u r ation fo r a piston
c o m p r e s s o r s o o n m ove d t o a d o u ble acting
s wa s h - p l a t e d e s i g n o f w h ich t h e Harrison
( D e l p h i ) H D 6 i s t y p i c a l . Th e fixe d swash-
p l a t e i s h e l d b e t we e n t h e o p p osing ends
o f t h e d o u b l e p i s t o n by a pair o f s hoes and
t wo s t e e l balls.
S e c t i o n t h r o u g h a n e a r l y f i xe d s wa s h-plate
c o m p r e s s o r, t h e D e l p h i H D 6
A s e c o n d p o s i t i ve d i s p l a c ement c onfigura-
t i o n u s e d i s t h e s l i d i n g va n e . I t o ffers ad-
va n t a g e s o f c o m p a c t n e ss a n d l ow cost but
c a rries t h e p e n a l t y o f ve r y p o o r e fficiency at
h i g h p r e s s u r e r a t i o s .
Th e o t h e r m a i n c o n f i g u r a tion i s t he scroll
c o m p r e s s o r i n w h i ch o n e s c r oll w i th an in-
vo l u t e s u r fa c e o r b i t s i n side a fixe d second
s c r o l l o f t h e s a m e s h a pe. I t can be seen
f r o m t h e f i g u r e t h a t t h e c o n t a c t points of
t h e scrolls enclose a cav it y of reducing size
( s h a d e d green) that move s from the outside
t o t h e c e n t r e a s t h e o n e scroll o r b i ts inside
t h e o t h e r. Th e r e s u l t i s a n i n d u c tion of gas
a t t h e p e r i p h e r y o f t h e scroll a n d deliver y
o f c o m p r e s s e d g a s a t t h e c e n t r e. These
c o m p r e s s o r s a r e ve r y e ff icient a nd quiet
b u t r e p r esent a s i g n i f icant ch allenge for
m a n u fa c tu r e a n d h e n c e fo r c o s t. They have
t h e d i s a d va n t a g e t h a t a l t h o u g h a variable
ve r s i o n o f t h i s c o m p r e s sor i s p o ssible, the
s u p e r i o r e ff i c i e n c y a d va n t a g e is lost.
I l l u s t r a t i o n o f Scroll compression process
Early compressors were all fixed displace-
ment and system control was achieved by
switching the compressor on and off by
means of an electro-mechanical clutch
cont ained within the compressor pulley.
As consideration began to be given to the
application of air conditioning to smaller
vehicles with smaller engines it was found
that the significant and sudden increase in
engine load that resulted from compressor
switching caused problems for the small
engine with result ant issues of drivabilit y.
A compressor with variable displacement
offered the solution to this problem. In 1985
Harrison (Delphi) were the first to go into
series production with a variable compres-
sor, the V5, a wobble plate design with five
pistons.
Th e m e ch a n i s m o f a va r i a b l e wo b b l e p l a t e c o m p r e s s o r
Although scroll, vane and piston compres-
sors are all still in use in automotive applica-
tions, the piston compressor is dominant.
Where the combination of efficiency and
controllabilit y is the key requirement, piston
compressors offer the best solution.
P i s t o n C o m p r e s s o r s
The compressor has the t ask of inducing gas
at a low suction pressure and delivering it at
a higher discharge pressure. It achieves this
by using reed valves that operate automa-
tically to control the fluid flow into and out of
the cylinders. St arting with the piston at top
dead centre the pressure in the small clear-
ance volume will be close to the discharge
pressure as the piston has just delivered its
charge. As the piston recedes and the pres-
sure in the clearance volume falls, the delive-
r y reed valve closes automatically, driven by
the discharge pressure. Since the clearance

vo l u m e i s s m a l l , t h e p r e s s u r e d r o ps quickly
u n t i l i t i s b e l ow t h e s u c tion p r e ssure. At
t h i s s t a g e t h e i n t a ke valve o p e n s and the
f u r t h e r m ove m e n t o f t h e p iston induces
f r e s h ch a r g e .
Cylinder Pressure - bar
25
20
15
10
5
0
suction pressure P s
discharge pressure P d
5 10
15 20
Cylinder volume - cm 3
I n d i c a t o r d i a g r a m fo r s wa s h - p l a t e c ompressor
A ft e r b o ttom d e a d c e n t re t h e p iston com-
p r e s s e s t h e g a s i n d u c e d i n t o t h e cylinder
u n t i l i t r e a ch e s t h e d i s charge p r essure. At
t h i s s t a g e t h e d e l i ve r y valve i s o p e ned auto-
m a t i c a l l y by t h e p r e s s u r e d i ffe r e ntial and
t h e c o m p r e s s e d ch a r g e is delive r ed.
Th e r e a r e s eve r a l key fe a tu r e s o f this ove -
r a l l p r o c e s s i m p o r t a n t for p e r fo rmance and
e ff i c i e n c y. Th e c l e a r a n c e volume must be as
s m a l l a s p o s s i b l e w i t h o u t risking interfe -
r e n c e b e t we e n t h e p i s t on a n d t h e cylinder
h e a d a s a r e s u l t o f d i ffe r e n tial t h e r mal expan-
s i o n o r s t r e s s e ffe c t s . Th e c o nsequences
o f f i n i t e c l e a r a n c e vo l u m e a r e p rincipally
i n d i c a t e d by t h e c o m p r essor volumetric
e ff i c i e n c y w h i ch i s t h e ratio o f t he volume
o f d e l i ve red g a s m e a s u red a t t h e suction
p r e s s u r e relative t o vo l ume displaced by the
p i s t o n . Th e d y n a m i c r e s p o nse characteris-
t i c s o f t h e r e e d va l ve s are a lso i mport ant.
Vo l u m e t r i c e ff i c i e n c y i s a lso impacted by the
q u a n t i t y o f g a s t h a t l e a k s p ast t he pistons
t o t h e c r a n k c a s e , w h i ch is d e t e rmined by
p i s t o n s e a l i n g a n d c l e a r a nces.
E ff i c i e n c y D e f i n i t i o n
volumetric η v
isentropic η s
mechanical η m
Ta b l e o f c o m p r e s s o r e ff i c i e n c i e s
delivered volume
displaced volume
ideal minimum work needed
work done to compress gas
work done on compressed gas
work input to compressor shaft
The second measure of compressor perfor-
mance qualit y is the isentropic efficiency.
It is given by the ratio of the theoretical
minimum work required to compress the gas
from suction to discharge pressure relative
to the work actually done. The theoretical
minimum is based on an idealized ‘isentro-
pic’ process in which no turbulence genera-
tion or viscosit y effects occur. The third
measure of compressor performance qualit y
is the mechanical efficiency. This is the ratio
of the work actually done on the gas relative
to the measured power input to the com-
pressor shaft. The difference bet ween these
t wo figures is due to friction.
V a r i a b l e P i s t o n
C o m p r e s s o r s
As st ated above, Harrison (now Delphi) was
the first to go into series production with a
variable compressor. The wobble plate that
drives the pistons with this design is free
to tilt so that the piston stroke, and hence
displacement can be changed. The wobble
plate mechanism allows ver y short and
compact pistons to be used. The ball-ended
connecting rods are swaged into the pistons
and wobble plate which results in strong ,
low friction and ver y simple joints. The fact
that the connecting rods are tilted slightly
during compression as a result of the wobble
plate movement means that the forces act-
ing do not all lie along the piston axes. The
result is that higher vibrational harmonics
can be generated which makes management
of compressor noise more of a challenge
than with later generations of swash-plate
compressor. Nonetheless the value of this
configuration is illustrated by the fact that it
is still produced at a rate of several million
units per year worldwide for inst allation in
new vehicles as original equipment, offering
a good compromise bet ween cost and the
functionalit y of a variable displacement com-
pressor.
The next generation of variable compres-
sors used a swash-plate mechanism, largely
because of its advant ages of low vibration
and noise. The mechanism by which the
compressor displacement is controlled can
be described in principle relatively simply,
but in fact the actual displacement of the
compressor in any given situation results
from a quite complex interaction of forces.

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

s t r o ke . I f t h e d i s ch a r g e p r e s s u r e gets too
h i g h , t h e h i g h p r e s s u r e b l e e d c a n b e opened
a n d t h e c r a n k c a s e p r e s s u r e i n c r eases. At
s o m e s t a g e i t w i l l b e h i g h e n o u g h for the
b a l a n c e o f t o r q u e s t o d e s t r o ke t h e compres-
s o r. Th e c o n t r o l h a s b e e n a ch i eve d .
Swash-Plate Angle - radians
0,4
0,3
0,2
0,1
0,0
0 2 4 6 8 10 12 14 16
Crank Case Pressure - bar
C o m p r e s s o r control cur ve s
Th e h i g h p r e s s u r e c o n t rol va lve c an be an
a u t o m a t i c , s p r i n g o p e r a t e d va lve o r an elec-
t r i c a l l y s w i t ch e d a n d e l e c t r o nically control-
l e d va l ve . Th e fo r m e r c o nfiguration is known
a s p n e u m a t i c c o n t r o l , t h e latt e r is called
ex t e r n a l c o n t r o l .
Th e d o m i n a n t e l e m e n t o f sys t e m control
i s t h e t h e r m a l ex p a n s i o n valve t h at acts to
m a i n t a i n a give n degree of superheat of the
r e f r i g e r a n t l e av i n g t h e eva p o r a t or. If the
eva p o r a t o r p r e s s u r e b e comes t o o low and
t h e r i s k of eva p o r a t o r f reezing o ccurs, the
p n e u m a t i c c o n t r o l va l ve c o m es i n to opera-
t i o n t o r educe t h e c o mpressor s troke and
m a i n t a i n t h e r e q u i r e d eva p o rator pressure.
Th e a u t o m a t i c c o n t r o l m echanism comes
f r o m t h e b e l l ow s w h i ch i s u n d e r permanent
va c u u m on i t s i n s i d e a nd ex p o s ed to the
s u c t i o n p r e s s u r e o n t he o u t side. If this
p r e s s u r e d i ffe r e n t i a l fa l ls t o o l ow due to
l ow s u c t i o n p r e s s u r e t h e b ellow s expand
t o d r i ve t h e n e e d l e t o open t h e c onnecting
p a s s a g e b e t we e n d i s ch a r g e p r e ssure and
c r a n k c a s e . Th e r e s u l t i n g i n c r ease in crank
c a s e p r e s s u r e l e a d s t o a r e d uction in stroke
w i t h t h e c o n s e q u e n t i a l r e d uction in refri-
g e r a n t m a s s f l ow r a t e a n d i ncrease in suc-
t i o n p r e s s u r e . As t h e eva p o r a t o r pressure
i n c r e a s e s t h e p r e s s u r e diffe r e n tial drives
t h e n e e d l e t o c l o s e t h e c o n n ecting passage
b e t we e n d i s ch a r g e p r e s s u r e a n d c rank case.
Th e c r a n k c a s e p r e s s u r e falls a s a result of
t h e s u c t i o n b l e e d a n d t h e c ompressor stroke
i n c r e a s e s .
9
12 15
Compressor speed: 1000 min -1
Suction pressure: 3 bar
Cur ves marked with head
pressure in bar.
S e c t i o n t h r o u g h
p n e u m a t i c c o m p r e s s o r
c o n t r o l va l ve
The use of external control gives an extra
degree of flexibility. Pneumatic control main-
tains evaporator pressure so that air-off
temperatures remain a few degrees above
0°C and any need for higher temperatures
than this are achieved by mixing with warm
ambient or recirculated air. As a result more
compressor work is done to cool the air
down to the low temperature than is really
necessar y. Ver y significant energy savings
can be made by using the external control
to manage the system capacit y so that only
sufficient cooling is provided to meet the
real need so that significant reheating of
the comfort air is not needed.
T his p ote ntial fo r e n e rg y s av i n g i s t h e m o s t
i m p o r t a nt a s p e c t of ex te r n a l e l e c t ronic c o n -
t ro l b u t i t a l s o of fe r s other a d v a nt a g e s such
a s d e - s t roking a t s y s te m s hut d ow n s o t h a t
t h e e n g i n e l o a d fe l t by t h e e n g i n e a t s y s te m
s t a r t - u p c a n b e g r a d u a lly i n c re a s e d – k n ow n
a s “s of t s t a r t”. A fur t h e r a d v a nt a g e i s t h e a b i l -
i t y to exe rcise a d e g re e of c o ntro l over c a b i n
humidit y a n d avoid exc e s s i ve d r y n e s s t h a t
c a n o c c u r w i t h simple p n e u m a t i c c o ntro l .
Fixed compressors have electromagnetically
operated clutches to allow system control
by switching the compressor drive on and
off as required. Variable compressors have
traditionally ret ained the clutch to be able
to turn the system off when not required or
should system conditions approach unsafe
operating levels for any reason. A more recent
innovation is the introduction of clutchless
compressors. It is made possible by the
implement ation of external compressor con-
trol. Here the compressor can be destroked
to such an extent that it no longer pumps
and can be permanently driven without
dissipation of significant energy. Typical
destroked energy consumption is only 150W
at 30 0 0rev/min. Ver y small stroke can be
relatively easily achieved and the challenge
is to activate upstroke again when required.
The advant ages are principally packaging
and weight although cost savings are also
possible.
S e c t i o n t h r o u g h e l e c t r o n i c a l l y
c o n t r o l l e d c o m p r e s s o r va l ve

A C o m p r e s s o r R a n g e
One frequently hears a need expressed with
phrases such as: ”I want the most efficient
and lightest component possible.” In reality
however, it is not possible to optimise against
two independent parameters at the same time
- the most efficient possible will not necessa-
rily be the lightest and vice-versa (other than
by coincidence). While optimisation against a
single parameter is possible, the significance
of the second can only then be expressed as
a limit or a constraint. “I want the most effi-
cient component possible weighing less than
1kg.”
It is generally recognised that the automo-
tive industr y sources against the single opti-
misation parameter – cost. While there are
ver y many other constraints within which
they have to operate – both applications and
market based constraints – these can only be
expressed as limiting values. The target will
always be to acquire the product that meets
the needs of the application at the lowest
possible cost. Since the needs expressed as
constraints var y greatly, depending upon appli-
cation, a supplier needs to offer an extensive
range of compressors offering a spectrum of
balances between sophistication and cost in
order to stand a chance of meeting the needs
of a significant proportion of the market. This
will extend from simple fixed compressors to
meet the most straightfor ward applications to
electronically controlled variable compressors
where performance, accuracy and efficiency
play a more important role.
As means of illustration the Delphi com-
pressor product range for R134a is indicated
below. Compressors are shown with the
I n c r e a s i n g
d i s p l a c e m e n t
S P 0 8
F i x e d
S P10
S P13
S P15
S P17
S P21
SP - fixed displacement
## - displacement in cc/10
Wo b b l e Plate
12 3 V 5 i
13 2 V 5 i
14 4 V 5i
15 1 V 5i
15 6V5i
15 6V5e
D e l p h i e n g i n e - d r i ve n R 13 4 a c o m p r e s s o r r a n g e
trend for both increasing sophistication and
cost going from left to right and increasing
displacement going from top to bottom.
The SP range of fixed swash-plate compressors
each with five double acting pistons represents
the lowest cost solution for applications that
can tolerate the implications of a cycling clutch.
The double piston configuration means that
in effect there are a total of ten compression
cavities which yields very low levels of pres-
sure fluctuation. This feature combined with the
smoothness of the swash-plate drive results in
an inherently quiet compressor.
The V5 range of wobble plate variable compres-
sors has already been described as the original
Delphi variable compressor. It has seen con-
siderable refinement over the years and whilst
it cannot compete with the CVC swash-plate
range for quietness and efficiency it represents
a very cost-effective introduction to variable
compressor technology. The dual PTFE piston
rings contribute to low blow-by and good oil
retention.
The Compact Variable Compressor (CVC) range
of compressors uses swash-plate technology
to ensure that a/c system generated noise and
vibration is kept to a minimum. Internally and
externally controlled versions are available.
The externally controlled range of CVC compres-
sors are offered with and without clutch. In the
clutchless compressor the ability to upstroke
again after destroke to a minimum that is only 1
or 2% of maximum stroke is ensured by the very
tight tolerances of the piston and cylinder bore
dimensions. They ensure very low blow-by which
helps maintain low parasitic losses and ensure
that upstroke can be achieved when required.
Variable
Swash-Plate
I n c r e a sing sophistication and cost trend
### - displacement in cc
V - wobble plate
179V7e
# - number of cylinders
i - internal control
e - external control
5CVC120i
6CVC125i
6CVC135i
6CVC140i
6CVC160i
7CVC165i
7CVC185i
# - number of pistons
5CVC120c
CVC - variable swash-plate
6CVC125c
6CVC135c
### - max displacement in cc
6CVC140c
6CVC160c
7CVC165c
7CVC185c
5CVC120e
6CVC125e
6CVC135e
6CVC140e
i - internal control
c - clutchless
6CVC160e
7CVC165e
e - external control
7CVC185e

I t c a n b e s e e n t h a t t h e C VC range is ver y
c o m p r e h e n s i ve . N o t s how n h e r e is the
f u r t h e r f l ex i b i l i t y o ffe r ed by a ver y wide
r a n g e o f p o s s i b l e p i p ing c o n f igurations
w h i ch t o g e t h e r c a rr y t h e implication that a
l a r g e n u m b e r o f d i ffe r e nt c ompressor con-
f i g u r a t i o n s m u s t p a s s d ow n t h e p roduction
l i n e s . I t h a s b e e n c o n c l u d e d by s ome that
t h i s c o m plex i t y i s l i ke l y t o have a negative
e ffe c t o n p r o d u c t q u a l i t y w h e n i n fact the
o p p o s i t e is t ru e . To m a n a g e t h e c omplexit y
a s ys t e m k n ow n a s R F I D – radio f requency
i d e n t i f i c a t i o n – i s u s e d i n w hich each individ-
u a l c o m p ressor c a rries a ch i p w i t h it down
t h e p r o d u c t i o n l i n e . D u ring t h e passage
d ow n the line the ch i p reads and verifies all
t h e i n fo r m a t i o n a b o u t t he i n dividual compo-
n e n t s a s t h ey a r e a s s e m b l e d (manufacturing
d a t e s , d i m e n s i o n s , e t c ) a n d c ommunicates
i t b a ck t o a c e n t r a l c o m puter fo r filing . This
d a t a o n e a ch i n d i v i d u a l c ompressor is then
ava i l a b l e fo r l a t e r a c c e s s s h o uld there be
p r o b l e m s f u r t h e r d ow n t h e l i n e o r problems
f r o m t h e f i e l d . Th e ava i l a b ilit y o f a ll this com-
p r e h e n s i ve a n d d e t a i l e d d a t a c o n t ributes to
t h e e s t a b l i s h m e n t a n d maintenance of the
ve r y h i g h l eve l s o f r e l i a b ilit y r e q u i red by the
a p p l i c a t i o n .
N e w Te c h n o l o g i e s
I n r e c e n t ye a r s a s e r i ous i n t e r est in the
u s e o f R 74 4 a s r e f r i g e rant i n a u tomotive
a i r c o n d i tioning s ys t e m s has b een gene-
r a t e d . E uropean l e g i s l a tion h as recently
b e e n p a s s e d t h a t w i l l fo r c e t h e industr y
t o m ove away f r o m R 13 4a a n d R744 has
b e c o m e t h e l e a d i n g c o n t e n d e r. As a result
D e l p h i i s d eve l o p i n g a r a n g e o f s wash-plate
c o m p r e s s o r s t o m e e t t his n e e d. Current
p r o t o t y p e s h ave f i ve p i s tons a n d maximum
d i s p l a c e m e n t o f 3 0 c c a l though t h e number
o f p i s t o n s i n t h e f i n a l p roduct r a nge is still
u n d e r d eve l o p m e n t . Bo t h f ixe d a n d variable
ve r s i o n s o f d i s p l a c e m e nts o f 15 , 21 and
3 0 c c a r e p l a n n e d . I t i s e nvisaged that for
l ow d i s p l a c e m e n t t h e n umber o f pistons
w i l l b e b e t we e n 5 a n d 7, a t t h e high end it
C o n c l u s i o n
will be bet ween 7 and 9. The reason for this
uncert aint y is that the appropriate balance
bet ween cost and noise/durabilit y has not
yet been est ablished.
D e l p h i C O 2 c o m p r e s s o r
It can be seen that as a result of the ver y
different fluid properties, the displacements
offered are ver y much lower than for R134a.
The range features shaft sealing by face seal
and internal oil separator. The need for the
latter comes from the significant evaporator
performance degradation that occurs with
CO 2 if excess oil is allowed to circulate.
The second new technology concerns com-
pressors for future powertrains, whether
hybrid or fuel cell – the electrically driven
compressor. Under development is a family
of scroll compressors driven by 315 volt
brushless electric motors. Scroll displace-
ments are 28 and 38cc for cooling capacities
of 5.6 and 7.1 kW respectively. The motors
are cooled using the low temperature re-
frigerant vapour from the evaporator with
a result ant motor efficiency of 94%. The
inverter is cooled by conduction from the
same refrigerant and it too has efficiency in
the 94 to 98% range.
1
S e c t i o n t h r o u g h D e l p h i E l e c t r i c C o m p r e s s o r
1 S c r o l l C o m p r e s s o r
2 Pe r m a n e n t M a g n e t M o t o r
3 I nve r t e r a n d I n t e g r a t e d E l e c t r o n i c s
This paper has aimed to present the technical requirements of the compressor for use in
automotive air conditioning applications and to show how the diverse requirements of the
application mean that a supplier must have available a range of technologies and specifi-
cations to be able to meet these needs. It goes on to show how the Delphi compressor
product range is designed to give comprehensive coverage and finishes with consideration
of compressors for developmental technologies that are likely to become significant in the
relatively near future.
2
3

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About Delphi Multi-national: Delphi conducts its business operations through various subsidiaries and has headquarter in Troy Michigan USA, Paris, Tok yo and São Paulo, Brazil.