Air Conditioning Compressors

Air Conditioning Compressors
Introduction
The passenger compartment of a modern
perature, qualit y and movement in all corners
a u t o m o b i l e o ffe r s a v e r y d i ffe r e n t e n v i r o n -
of the vehicle. The modern heating , ventila-
m e n t t o t h a t o f a v e h i c l e f r o m f i ft y y e a r s
t i n g a n d a i r c o n d i t i o n i n g s y s t e m ( H VAC ) c a n
ago. As well as the ergonomically designed
ensure that thermal comfort and all round
seating and instrument layout, the all round
visibilit y are delivered in environments that
visibility
range from the heat of Phoenix Arizona to
and
the
e ff i c i e n t
use
of
space,
t o d a y ’s p a s s e n g e r h a s c o n t r o l o v e r a i r t e m -
t h e c o l d e s t S c a n d i n a v i a n w i n t e r.
V i e w o f i n s t r u m e n t p a n e l a n d H VAC c o n t r o l s
The ver y heart of the air conditioning syst e m i s t h e c o m p r e s s o r. I t s k e y t a s k s a r e t o
generate the required flow of refrigerant
around the system and at the same time
to
compress
temperature
it
s u ff i c i e n t l y
above
the
to
ambient
raise
in
its
order
that heat can be rejected from the system.
T h e s e d e m a n d s m u s t b e m e t e ff i c i e n t l y o v e r
ver y wide ranges of compressor speed and
ambient conditions. At the same time ver y
demanding noise generation constraints and
expect ations of durabilit y have to be met.
Expanded view of a modern
H VAC c o m p r e s s o r
Air
refrigerant at a low temperature is used to
Conditioning
extract heat from the location to be cooled.
Cycle
The refrigerant is subsequently compressed
until it becomes hot enough to be able to
It is self evident that heat tends to flow
r e j e c t h e a t t o t h e a m b i e n t . A ft e r c o o l i n g t o
f r o m h i g h t e m p e r a t u r e t o l o w. I f w e w i s h t o
near ambient temperature by the ambient
provide cooling at a location in a high tempe-
a i r, i t i s e x p a n d e d b a ck t o l o w p r e s s u r e , a
r a t u r e e n v i r o n m e n t t h e n w e a r e e ffe c t i v e l y
p r o c e s s w h i ch g e n e r a t e s f u r t h e r r e f r i g e r a n t
asking heat to flow from low temperature
temperature
t o h i g h . To a ch i e v e t h i s i t i s n e c e s s a r y t o
enough to be used for cooling at the re-
o p e r a t e a t h e r m o d y n a m i c c y c l e i n w h i ch a
quired location. The cycle is complete. The
reduction
that
becomes
low
r e q u i r e d c o o l i n g i s t h u s a ch i e v e d b u t a t t h e
implication of this is that the compressor
expense of the work necessar y to drive the
d r i v e s h a ft m u s t p a s s o u t o f t h e c o m p r e s -
c o m p r e s s o r.
sor casing with the result ant potential for
refrigerant leakage. Hermetic compressors
a r e n o t p o s s i b l e a n d v e r y e ffe c t i v e s h a ft
heat rejected to ambient
seals must be used. A second implication
condenser
of the external drive is that the compressor
must be engine mounted so that lengths of
flexible hose must be introduced to accom-
expansion device
modate relative movement bet ween engine
compressor
a n d ch a s s i s m o u n t e d c o m p o n e n t s .
As with other air conditioning applications
evaporator
heat absorbed from cabin
s y s t e m e ff i c i e n c y i s v e r y i m p o r t a n t a n d t h e
automotive application is no exception. While
t h e ch a r a c t e r i s t i c s o f a l l t h e m a j o r c o m p o -
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 y c l e
nents in the system contribute to overall eff i c i e n c y a n y c o m p r e s s o r i n e ff i c i e n c i e s m u s t
The
Automotive
b e k e p t t o a m i n i m u m . A f u r t h e r fe a t u r e o f
the compressor that requires serious con-
Air
Conditioning
sideration is its intrinsic noise generation
- modern vehicle noise requirements mean
Application
that the compressor must be ver y quiet and
vibration free.
The
automotive
application
places
ver y
special demands on the air conditioning sys-
A n d f i n a l l y, t h r e e f u r t h e r r e q u i r e m e n t s o f
tem. A t ypical vehicle system has a similar
t h e c o m p r e s s o r, s p e c i f i c t o t h e a u t o m o t i v e
cooling capacit y to that required for the air
a p p l i c a t i o n , b u t w h i ch a p p l y e q u a l l y t o e v e r y
conditioning of a small house despite the
vehicle
v a s t d i ffe r e n c e i n v o l u m e s t o b e c o o l e d . T h e
straints on size, weight and cost.
component
–
are
stringent
con-
reasons for this are t wofold. Firstly cooling
d u t y p e r u n i t v o l u m e i s m u ch h i g h e r f o r t h e
v e h i c l e b e c a u s e h e a t t r a n s fe r c o e ff i c i e n t s
bet ween hot ambient air and the outside
s u r f a c e s a r e m u ch h i g h e r d u e t o m o v e m e n t
of
the
vehicle
through
the
a i r.
Secondly
the proportion of the enclosure consisting
of glass is ver y high for the vehicle – a
f a c t o r t h a t m a k e s t h e e ffe c t o f d i r e c t s o l a r
radiation heating ver y high. On top of this
a particularly demanding requirement is to
c o o l t h e c a b i n v e r y r a p i d l y a ft e r t h e v e h i c l e
has been soaked in an ambient temperature
o f 4 0 ° C o r h i g h e r. A t t h e s t a r t o f t h e c o o l
down temperatures in the cabin can be as
high as 60 or 70°C.
T h 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-
A n o t h e r s i g n i f i c a n t w a y i n w h i ch a u t o m o t i v e
s o r, i t i s o ft e n s u g g e s t e d t h a t a n e l e c t r i c a l l y
d r i v e n s y s t e m w o u l d p r o v i d e a m u ch b e tt e r
a i r c o n d i t i o n i n g d i ffe r s f r o m t h e d o m e s t i c
s o l u t i o n . T h e r e a s o n t h a t i t i s n o t fe a s i b l e
or commercial version is the question of
o n c u r r e n t v e h i c l e s i s i n s u ff i c i e n t e l e c t r i c a l
compressor drive. In the vehicle the com-
power is available and its application must
pressor is belt driven by the engine so that
await the widespread implement ation of high
independent
powered
control
over
the
compressor
and
e ff i c i e n t
integrated
starter/
speed is not possible. This obviously has
generators. The
significant implications for system control
electrical compressor would make real sense
and means that there can be calls for high
however are in hybrid and fuel cell vehicles
system
w h e r e s u ff i c i e n t e l e c t r i c a l p o w e r i s r e a d i l y
performance
at
times
when
the
c o m p r e s s o r s p e e d i s v e r y l o w. A n i m p o r t a n t
available.
applications
in
w h i ch
an
T h 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 i n e s c a n
testing
l e a d t o t h e s i t u a t i o n i n w h i ch i n s u ff i c i e n t r e -
necessar y
jected heat is available for comfort heating .
completed in time to meet the requirements
This fact has prompted the suggestion that
o f t h e E u r o p e a n l e g i s l a t i o n . I f R 74 4 b e c o m e s
the air conditioning system be operated in
t h e ch o s e n r o u t e t h e i m p l i c a t i o n s f o r c o m -
reverse as a heat pump to supplement the
p r e s s o r t e ch n o l o g y a r e v e r y f a r- r e a ch i n g . A i r
engine heat and the viability of the concept
c o n d i t i o n i n g s y s t e m s w i t h R 74 4 n e e d c o m -
has been demonstrated on a number of plat-
p r e s s o r s w i t h d i s p l a c e m e n t o f o n l y a f i ft h
forms. The implications for the compressor
o f t h a t r e q u i r e d f o r R 13 4 a b u t o p e r a t i n g a t
a r e t h a t t h e r e q u i r e d o p e r a t i n g l i fe c a n b e
m u ch h i g h e r p r e s s u r e s . D i s ch a r g e p r e s s u r e s
nearly doubled and the compressor ambient
o f 12 0 b a r a r e n o r m a l c o m p a r e d w i t h a t y p i -
s a fe o p e r a t i n g t e m p e r a t u r e r a n g e m u s t b e
c a l 18 b a r f o r a n R 13 4 a s y s t e m . A s c a n b e
extended downwards to minus 20°C.
imagined, these factors have big implications
for
the
and
then
implementation
manufacturing
compressor
capacity
layout
and
of
the
can
be
s t ru c tu r a l
strength requirements and also represent a
p a r t i c u l a r l y s e v e r e s h a ft s e a l ch a l l e n g e .
100
Delphi vehicle with Heat Pump supplement ar y heating
automotive
air
conditioning
industr y
0°C
40°C
80°C
120°C
160°C
200°C
10
R 13 4 a w a s c o m p l e t e f o r a l l v e h i c l e p r o d u c tion lines in the developed world. It soon
became apparent that this may not be the
end of the stor y and with increasing penetration of air conditioning in the European
market together with the demands of the
Ky o t o P r o t o c o l p r e s s u r e h a s b u i l t t o m a k e
160°C
0°C
2 y e a r s e a r l y, t h e ch a n g e o v e r f r o m R 12 t o
120°C
40°C
t h e 19 8 7 M o n t r e a l P r o t o c o l a n d b y 19 9 4 ,
80°C
responded pro-actively to the demands of
200°C
The
Issues
Pressure - MPa
Refrigerant
-40°C
1
100
200
300
400
500
Enthalpy - kJ/kg
600
700
Idealized pressure enthalpy diagram of CO2 cycle
Compressor
Te c h n o l o g i e s
ch a n g e s t o s t i l l f u r t h e r r e d u c e t h e p o t e n t i a l
contribution
to
global
of
air
warming .
conditioning
f o r v e h i c l e s i n t h e e a r l y 19 5 0 s . Fr o m t h e
b e g i n n i n g a n u m b e r o f d i ffe r e n t c o m p r e s s o r
European legislative process. It requires the
t e ch n o l o g i e s h a v e b e e n u s e d . T h e e a r l i e s t
e l i m i n a t i o n o f R 13 4 a f r o m a l l n e w m o d e l s
systems used piston compressors in various
f r o m 2 011 a n d f r o m a l l n e w v e h i c l e s b y 2 017.
c o n f i g u r a t i o n s . T h e Fr i g i d a i r e F 5 , a f i v e p i s -
The
are
ton wobble plate configuration, used in the
R 15 2 a w i t h a g l o b a l w a r m i n g p o t e n t i a l l e s s
earliest GM vehicles with air conditioning
t h a n 10 % o f t h a t o f R 13 4 a , o r t h e s o - c a l l e d
w a s f i r s t p r o d u c e d i n 19 5 6 – i t w e i g h e d i n a t
‘ n a t u r a l ’ r e f r i g e r a n t R 74 4 ( c a r b o n d i o x i d e ) .
1 8 k g ! T h e R 4 c o m p r e s s o r w i t h a s c o t ch y o k e
R 15 2 a
ch a r a c t e r i s t i c s
m e ch a n i s m w a s f i r s t p r o d u c e d b y H a r r i s o n
v e r y s i m i l a r t o t h o s e o f R 13 4 a a n d w i l l b e
( n o w D e l p h i ) i n 19 74 a n d r e m a i n e d i n p r o -
directly
d u c t i o n u n t i l 19 9 5 . I t w a s 19 0 m m i n o v e r a l l
considered
alternatives
thermodynamic
usable
t e ch n o l o g y.
Its
with
Directive
Air conditioning began to become available
recently completed its progress through the
has
new
systems
has
currently
A
current
potential
draw
compressor
b a ck
is
a
diameter and weighed 9kg .
degree of flammability that has made the
industr y reluctant to consider it. More rec e n t l y o t h e r l a r g e ch e m i c a l c o m p a n i e s h a v e
announced the potential availabilit y of new
a l t e r n a t i v e s . L i tt l e i s k n o w n a b o u t t h e m a t
this st age but it is assumed that they too
would
function
with
current
compressor
t e ch n o l o g y. T h e r e m u s t b e a q u e s t i o n a s t o
whether
the
necessar y
development
and
An early
c o m p r e s s o r,
the Delphi R4
The
piston
Early compressors were all fixed displace-
compressor soon moved to a double acting
p r e fe r r e d
configuration
for
a
m e n t a n d s y s t e m c o n t r o l w a s a ch i e v e d b y
s w a s h - p l a t e d e s i g n o f w h i ch t h e H a r r i s o n
s w i t ch i n g
(Delphi) HD6 is t ypical. The fixed swash-
means
plate is held bet ween the opposing ends
contained
of the double piston by a pair of shoes and
As consideration began to be given to the
t wo steel balls.
application
the
of
an
compressor
within
of
on
and
o ff
e l e c t r o - m e ch a n i c a l
air
the
compressor
conditioning
to
by
c l u t ch
p u l l e y.
smaller
vehicles with smaller engines it was found
that the significant and sudden increase in
engine load that resulted from compressor
s w i t ch i n g
caused
problems
for
the
small
e n g i n e w i t h r e s u l t a n t i s s u e s o f d r i v a b i l i t y.
A
compressor
with
variable
displacement
o ffe r e d t h e s o l u t i o n t o t h i s p r o b l e m . I n 19 8 5
Harrison (Delphi) were the first to go into
series production with a variable compress o r, t h e V 5 , a w o b b l e p l a t e d e s i g n w i t h f i v e
pistons.
Section through an early fixed swash-plate
c o m p r e s s o r, t h e D e l p h i H D 6
A second positive displacement configurat i o n u s e d i s t h e s l i d i n g v a n e . I t o ffe r s a d vant ages of compactness and low cost but
c a r r i e s t h e p e n a l t y o f v e r y p o o r e ff i c i e n c y a t
high pressure ratios.
The other main configuration is the scroll
c o m p r e s s o r i n w h i ch o n e s c r o l l w i t h a n i n volute surface orbits inside a fixed second
scroll of the same shape. It can be seen
from the figure that the contact points of
the scrolls enclose a cavit y of reducing size
(shaded green) that moves from the outside
to the centre as the one scroll orbits inside
T h e m e ch a n i s m o f a v a r i a b l e w o b b l e p l a t e c o m p r e s s o r
t h e o t h e r. T h e r e s u l t i s a n i n d u c t i o n o f g a s
at the peripher y of the scroll and deliver y
Although scroll, vane and piston compres-
of
sors are all still in use in automotive applica-
compressed
compressors
but
represent
gas
are
a
at
ver y
the
centre. These
e ff i c i e n t
significant
and
quiet
ch a l l e n g e
for
tions, the piston compressor is dominant.
Where
the
combination
of
e ff i c i e n c y
and
manufacture and hence for cost. They have
controllabilit y is the key requirement, piston
the disadvant age that although a variable
c o m p r e s s o r s o ffe r t h e b e s t s o l u t i o n .
version of this compressor is possible, the
s u p e r i o r e ff i c i e n c y a d v a n t a g e i s l o s t .
Piston
Compressors
The compressor has the t ask of inducing gas
at a low suction pressure and delivering it at
a h i g h e r d i s ch a r g e p r e s s u r e . I t a ch i e v e s t h i s
by using reed valves that operate automatically to control the fluid flow into and out of
the cylinders. Starting with the piston at top
d e a d c e n t r e t h e p r e s s u r e i n t h e s m a l l c l e a ra n c e v o l u m e w i l l b e c l o s e t o t h e d i s ch a r g e
pressure as the piston has just delivered its
ch a r g e . A s t h e p i s t o n r e c e d e s a n d t h e p r e s sure in the clearance volume falls, the deliver y r e e d v a l v e c l o s e s a u t o m a t i c a l l y, d r i v e n b y
Illustration of Scroll compression process
t h e d i s ch a r g e p r e s s u r e . S i n c e t h e c l e a r a n c e
v o 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 p s q u i ck l y
T h e s e c o n d m e a s u r e o f c o m p r e s s o r p e r f o r-
until it is below the suction pressure. At
m a n c e q u a l i t y i s t h e i s e n t r o p i c e ff i c i e n c y.
this st age the int ake valve opens and the
It is given by the ratio of the theoretical
further
minimum work required to compress the gas
movement
of
the
piston
induces
f r e s h ch a r g e .
f r o m s u c t i o n t o d i s ch a r g e p r e s s u r e r e l a t i v e
to the work actually done. The theoretical
minimum is based on an idealized ‘isentro-
25
p i c ’ p r o c e s s i n w h i ch n o t u r b u l e n c e g e n e r a -
Cylinder Pressure - bar
discharge pressure P d
tion
20
or
viscosity
e ffe c t s
o c c u r. T h e
third
measure of compressor performance qualit y
15
i s t h e m e ch a n i c a l e ff i c i e n c y. T h i s i s t h e r a t i o
of the work actually done on the gas relative
10
to the measured power input to the comp r e s s o r s h a ft . T h e d i ffe r e n c e b e t w e e n t h e s e
5
t wo figures is due to friction.
suction pressure P s
5
0
10
15
20
Cylinder volume - cm 3
Indicator diagram for swash-plate compressor
presses the gas induced into the cylinder
u n t i l i t r e a ch e s t h e d i s ch a r g e p r e s s u r e . A t
this st age the deliver y valve is opened autom a t i c a l l y b y t h e p r e s s u r e d i ffe r e n t i a l a n d
t h e c o m p r e s s e d ch a r g e i s d e l i v e r e d .
T h e r e a r e s e v e r a l k e y fe a t u r e s o f t h i s o v e rall process import ant for performance and
e ff i c i e n c y. T h e c l e a r a n c e v o l u m e m u s t b e a s
s m a l l a s p o s s i b l e w i t h o u t r i s k i n g i n t e r fe rence bet ween the piston and the cylinder
h e a d a s a r e s u l t o f d i ffe r e n t i a l t h e r m a l e x p a n s i o n o r s t r e s s e ffe c t s . T h e c o n s e q u e n c e s
of finite clearance volume are principally
by
the
compressor
Piston
Compressors
A ft e r b o tt o m d e a d c e n t r e t h e p i s t o n c o m -
indicated
Va r i a b l e
volumetric
e ff i c i e n c y w h i ch i s t h e r a t i o o f t h e v o l u m e
of delivered gas measured at the suction
pressure relative to volume displaced by the
p i s t o n . T h e d y n a m i c r e s p o n s e ch a r a c t e r i s -
As st ated above, Harrison (now Delphi) was
the first to go into series production with a
v a r i a b l e c o m p r e s s o r. T h e w o b b l e p l a t e t h a t
drives the pistons with this design is free
to tilt so that the piston stroke, and hence
d i s p l a c e m e n t c a n b e ch a n g e d . T h e w o b b l e
plate
m e ch a n i s m
allows
ver y
short
and
compact pistons to be used. The ball-ended
connecting rods are swaged into the pistons
a n d w o b b l e p l a t e w h i ch r e s u l t s i n s t r o n g ,
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 acting do not all lie along the piston axes. The
result is that higher vibrational harmonics
c a n b e g e n e r a t e d w h i ch m a k e s m a n a g e m e n t
o f c o m p r e s s o r n o i s e m o r e o f a ch a l l e n g e
than with later generations of swash-plate
c o m p r e s s o r. N o n e t h e l e s s t h e v a l u e o f t h i s
tics of the reed valves are also import ant.
configuration is illustrated by the fact that it
Vo l u m e t r i c e ff i c i e n c y i s a l s o i m p a c t e d b y t h e
is still produced at a rate of several million
quantity of gas that leaks past the pistons
units per year worldwide for inst allation in
t o t h e c r a n k c a s e , w h i ch i s d e t e r m i n e d b y
n e w v e h i c l e s a s o r i g i n a l e q u i p m e n t , o ffe r i n g
piston sealing and clearances.
a good compromise bet ween cost and the
functionalit y of a variable displacement comp r e s s o r.
Efficiency
vo l u m e t r i c η v
isentropic ηs
m e ch a n i c a l η m
Definition
d e l i ve r e d vo l u m e
d i s p l a c e d vo l u m e
The next generation of variable compress o r s u s e d a s w a s h - p l a t e m e ch a n i s m , l a r g e l y
because of its advant ages of low vibration
i d e a l m i n i m u m wo r k n e e d e d
and
wo r k d o n e t o c o m p r e s s g a s
compressor displacement is controlled can
wo r k d o n e o n c o m p r e s s e d g a s
wo r k i n p u t t o c o m p r e s s o r s h a ft
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
noise. The
m e ch a n i s m
by
w h i ch
the
b e d e s c r i b e d i n p r i n c i p l e r e l a t i v e l y s i m p l y,
but in fact the actual displacement of the
compressor in any given situation results
from a quite complex interaction of forces.
is accelerated upwards over the half of the
swash-plate furthest from its centre of rot ation and correspondingly decelerated over
the half nearest to the centre of rotation.
The former therefore exerts greater torque
and the net result is a tendency to upstroke
w h i ch i s p r o p o r t i o n a l t o t h e s q u a r e o f t h e
piston speed and thus becomes significant
at higher compressor speeds.
Va r i a b l e s w a s h - p l a t e c o m p r e s s o r s h o w i n g m e ch a n i s m
T h e s w a s h - p l a t e h a s a l i n k a g e m e ch a n i s m
t h a t a l l o w s i t t o r o t a t e t h u s ch a n g i n g i t s a n gle and with it, the piston displacement. The
linkage is designed so that the clearance
volume remains as const ant and small as
possible
over
the
full
swash-plate
angle
range from full stroke to minimum stroke.
This swash-plates freedom to rot ate means
that under any given operating conditions
it will t ake up a position that is determined by the balance of all the forces acting
on it that tend to cause it to rotate about
its centre of rot ation. The first element of
complexit y is that the centre of rot ation is
n o t f i x e d a s i t s h i ft s l o c a t i o n s l i g h t l y w i t h
the swash-plate angle. It lies fairly near to
the swash-plate connection with the link but
A second torque generating force that is a
similar function of compressor speed comes
f r o m c e n t r i f u g a l a ffe c t s a c t i n g o n b o t h t h e
l i n k m e ch a n i s m a n d t h e s w a s h - p l a t e i t s e l f. I f
one considers the t wo halves of the swashplate nearest and most remote from the centre of rotation it is clear that the centrifugal
forces of the more remote half generate the
greater torque and the net result will be a
t e n d e n c y t o d e s t r o k e t h e c o m p r e s s o r.
Fu r t h e r
forces
generating
rotation
torque
c a n c o m e f r o m t h e f i tt i n g o f u p s t r o k e a n d / o r
downstroke springs. These can be inst alled
o n t h e s h a ft t o a s s i s t t h e u p s t r o k e / d o w n s t r o ke p r o c e s s u n d e r s o m e c o m p r e s s o r o p e r a t i n g c o n d i t i o n s a n d i f f i tt e d w i l l o b v i o u s l y
contribute to the overall balance of torques.
its exact locus is a function of the linkage
H e r e t h e e ffe c t w i l l b e a s i m p l e f u n c t i o n o f
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 n d o f t h e
the swash-plate angle.
swash-plate angle. The most obvious force
that comes into play is the gas pressure
T h e f i n a l c o n t r i b u t o r t o t h e b a l a n c e o f t o r-
acting on the piston crown. Estimation of
ques is that generated on the underside
i t s a v e r a g e e ffe c t , h o w e v e r i s c o m p l e x a s
of the pistons by the crank case pressure.
the cylinder pressure varies greatly around
Although the crank case pressure remains
the cycle. At the same time its line of action
const ant around the cycle its line of action
i s c o n t i n u a l l y ch a n g i n g a n d h e n c e s o i s t h e
around the plate must be integrated to ob-
e ffe c t i v e t o r q u e i t e x e r t s a b o u t t h e c e n t r e
t ain an average net torque. It is the abilit y to
of rot ation. This torque will obviously tend
control this pressure that allows the balance
t o u p s t r o k e t h e c o m p r e s s o r.
t o b e s h i ft e d a n d t h u s t h e s w a s h - p l a t e a n g l e
to be controlled. Crank case pressure is con-
A less obvious source of torque generation
trolled with bleeds from both suction and
on the swash-plate is the inertia of the pis-
high side. The small suction bleed is usually
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 a t e d a n d
permanent and the high side bleed is con-
decelerated as the compressor is operated.
trolled by means of a valve. If the high side
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 c t o f t h e
bleed is closed the suction bleed pumps the
forces
deceleration
crankcase down to suction pressure with the
c a n c e l e a ch o t h e r o u t b u t i n f a c t t h e p i s t o n
result that the compressor operates at full
for
acceleration
and
Mechanism
E f fe c t
Comment
Cylinder pressure
Upstroke
Integrated around swash-plate
Piston inertia
Upstroke
Significant at high speed
Centrifugal forces
Destroke
Significant at high speed
Springs
Upstroke/destroke
W h e r e f i tt e d
Crank case pressure
Destroke
Adjusted for control
Ta b l e o f s w a 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
Th e u s e o f ex t e r n a l c o n t r o l g i ve s a n ex t r a
high, the high pressure bleed can be opened
d e g r e e o f f l ex i b i l i t y. P n e u m a t i c c o n t r o l m a i n -
and the crank case pressure increases. At
tains
some st age it will be high enough for the
t e m p e r a t u r e s r e m a i n a fe w d e g r e e s a b o v e
balance of torques to destroke the compres-
0°C and any need for higher temperatures
s o r. T h e c o n t r o l h a s b e e n a ch i e v e d .
t h a n t h i s a r e a ch i e v e d b y m i x i n g w i t h w a r m
Swash-Plate Angle - radians
s t r o k e . I f t h e d i s ch a r g e p r e s s u r e g e t s t o o
eva p o r a t o r
pressure
so
that
a i r- o ff
a m b i e n t o r r e c i r c u l a t e d a i r. A s a r e s u l t m o r e
0,4
Compressor speed: 10 0 0 min -1
Suction pressure: 3 bar
Cur ves marked with head
pressure in bar.
0,3
0,2
down to the low temperature than is really
n e c e s s a r y. Ve r y s i g n i f i c a n t e n e r g y s a v i n g s
can be made by using the external control
15
12
9
compressor work is done to cool the air
to manage the system capacit y so that only
s u ff i c i e n t c o o l i n g i s p r o v i d e d t o m e e t t h e
0,1
real need so that significant reheating of
0,0
0
2
4
6
8
10
12
Crank Case Pressure - bar
14
16
the comfort air is not needed.
Compressor control cur ves
The high pressure control valve can be an
automatic, spring operated valve or an elect r i c a l l y s w i t ch e d a n d e l e c t r o n i c a l l y c o n t r o l led valve. The former configuration is known
a s p n e u m a t i c c o n t r o l , t h e l a tt e r i s c a l l e d
external control.
The
dominant
Section through electronically
controlled compressor valve
element
of
system
control
is the thermal expansion valve that acts to
This potential for energy saving is the most
maint ain a given degree of superheat of the
impor tant aspect of ex ternal electronic con -
refrigerant
the
trol but it also of fers other advantages such
evaporator pressure becomes too low and
as de - stroking at system shut down so that
the risk of evaporator freezing occurs, the
the engine load felt by the engine at system
pneumatic control valve comes into opera-
star t- up can be gradually increased – known
tion to reduce the compressor stroke and
a s “s o f t s t a r t ”. A f u r t h e r a d v a n t a g e i s t h e a b i l -
maint ain the required evaporator pressure.
it y to exercise a degree of control over cabin
The
humidit y and avoid excessive dr yness that
leaving
automatic
the
control
e v a p o r a t o r.
m e ch a n i s m
If
comes
f r o m t h e b e l l o w s w h i ch i s u n d e r p e r m a n e n t
can occur with simple pneumatic control.
vacuum on its inside and exposed to the
this
Fixed compressors have electromagnetically
p r e s s u r e d i ffe r e n t i a l f a l l s t o o l o w d u e t o
o p e r a t e d c l u t ch e s t o a l l o w s y s t e m c o n t r o l
low suction pressure the bellows expand
b y s w i t ch i n g t h e c o m p r e s s o r d r i v e o n a n d
to drive the needle to open the connecting
o ff a s r e q u i r e d . Va r i a b l e c o m p r e s s o r s h a v e
passage
and
t r a d i t i o n a l l y r e t a i n e d t h e c l u t ch t o b e a b l e
crank case. The resulting increase in crank
t o t u r n t h e s y s t e m o ff w h e n n o t r e q u i r e d o r
case pressure leads to a reduction in stroke
s h o u l d s y s t e m c o n d i t i o n s a p p r o a ch u n s a fe
with the consequential reduction in refri-
operating levels for any reason. A more recent
gerant mass flow rate and increase in suc-
i n n o v a t i o n i s t h e i n t r o d u c t i o n o f c l u t ch l e s s
tion pressure. As the evaporator pressure
compressors. It is made possible by the
increases
drives
implement ation of external compressor con-
the needle to close the connecting passage
trol. Here the compressor can be destroked
b e t w e e n d i s ch a r g e p r e s s u r e a n d c r a n k c a s e .
t o s u ch a n e x t e n t t h a t i t n o l o n g e r p u m p s
The crank case pressure falls as a result of
and
the suction bleed and the compressor stroke
dissipation
increases.
d e s t r o k e d e n e r g y c o n s u m p t i o n i s o n l y 15 0 W
suction
pressure
bet ween
the
on
the
outside.
d i s ch a r g e
pressure
If
pressure
d i ffe r e n t i a l
can
be
permanently
of
significant
driven
without
e n e r g y.
Ty p i c a l
a t 3 0 0 0 r e v / m i n . Ve r y s m a l l s t r o k e c a n b e
r e l a t i v e l y e a s i l y a ch i e v e d a n d t h e ch a l l e n g e
is to activate upstroke again when required.
Section through
pneumatic compressor
control valve
The
advant ages
are
principally
p a ck a g i n g
and weight although cost savings are also
possible.
A
Compressor
Range
trend for both increasing sophistication and
c o s t g o i n g f r o m l e ft t o r i g h t a n d i n c r e a s i n g
O n e f r e q u e n t l y h e a r s a n e e d ex p r e s s e d w i t h
d i s p l a c e m e n t g o i n g f r o m t o p t o b o tt o m .
p h r a s e s s u ch a s : ” I wa n t t h e m o s t e ff i c i e n t
a n d l i g h t e s t c o m p o n e n t p o s s i b l e .” I n r e a l i t y
The SP range of fixed swash-plate compressors
h oweve r, i t i s n o t p o s s i b l e t o o p t i m i s e a g a i n s t
each with five double acting pistons represents
t wo i n d e p e n d e n t p a r a m e t e r s a t t h e s a m e t i m e
the lowest cost solution for applications that
- t h e m o s t e ff i c i e n t p o s s i b l e w i l l n o t n e c e s s a -
can tolerate the implications of a cycling clutch.
r i l y b e t h e l i g h t e s t a n d v i c e - ve r s a ( o t h e r t h a n
The double piston configuration means that
by c o i n c i d e n c e ) . W h i l e o p t i m i s a t i o n a g a i n s t a
in effect there are a tot al of ten compression
single parameter is possible, the significance
cavities which yields ver y low levels of pres-
o f t h e s e c o n d c a n o n l y t h e n b e ex p r e s s e d a s
sure fluctuation. This feature combined with the
a l i m i t o r a c o n s t r a i n t . “ I wa n t t h e m o s t e ff i -
smoothness of the swash-plate drive results in
c i e n t c o m p o n e n t p o s s i b l e we i g h i n g l e s s t h a n
an inherently quiet compressor.
1 kg .”
The V5 range of wobble plate variable compresIt is generally recognised that the automo-
sors has already been described as the original
t i ve i n d u s t r y s o u r c e s a g a i n s t t h e s i n g l e o p t i -
Delphi variable compressor. It has seen con-
misation parameter – cost. While there are
siderable refinement over the years and whilst
ve r y
w h i ch
it cannot compete with the CVC swash-plate
t h ey h ave t o o p e r a t e – b o t h a p p l i c a t i o n s a n d
range for quietness and efficiency it represents
m a r ke t b a s e d c o n s t r a i n t s – t h e s e c a n o n l y b e
a ver y cost-effective introduction to variable
ex p r e s s e d a s l i m i t i n g va l u e s . Th e t a r g e t w i l l
compressor technology. The dual PTFE piston
a l ways b e t o a c q u i r e t h e p r o d u c t t h a t m e e t s
rings contribute to low blow-by and good oil
t h e n e e d s o f t h e a p p l i c a t i o n a t t h e l owe s t
retention.
m a ny
other
constraints
within
p o s s i b l e c o s t . S i n c e t h e n e e d s ex p r e s s e d a s
c o n s t r a i n t s va r y g r e a t l y, d e p e n d i n g u p o n a p p l i-
The Compact Variable Compressor (CVC) range
c a t i o n , a s u p p l i e r n e e d s t o o ffe r a n ex t e n s i ve
of compressors uses swash-plate technology
r a n g e o f c o m p r e s s o r s o ffe r i n g a s p e c t ru m o f
to ensure that a/c system generated noise and
b a l a n c e s b e t we e n s o p h i s t i c a t i o n a n d c o s t i n
vibration is kept to a minimum. Internally and
o r d e r t o s t a n d a ch a n c e o f m e e t i n g t h e n e e d s
externally controlled versions are available.
o f a s i g n i f i c a n t p r o p o r t i o n o f t h e m a r ke t . Th i s
w i l l ex t e n d f r o m s i m p l e f i xe d c o m p r e s s o r s t o
The externally controlled range of CVC compres-
m e e t t h e m o s t s t r a i g h t fo r wa r d a p p l i c a t i o n s t o
sors are offered with and without clutch. In the
e l e c t r o n i c a l l y c o n t r o l l e d va r i a b l e c o m p r e s s o r s
clutchless compressor the abilit y to upstroke
w h e r e p e r fo r m a n c e , a c c u r a c y a n d e ff i c i e n c y
again after destroke to a minimum that is only 1
p l ay a m o r e i m p o r t a n t r o l e .
or 2% of maximum stroke is ensured by the ver y
tight tolerances of the piston and cylinder bore
com-
dimensions. They ensure ver y low blow-by which
p r e s s o r p r o d u c t r a n g e f o r R 13 4 a i s i n d i c a t e d
helps maint ain low parasitic losses and ensure
b e l o w.
that upstroke can be achieved when required.
As
means
of
illustration
Compressors
the
are
Delphi
shown
with
the
Fixed
Va r i a b l e
Wo b b l e P l a t e
Increasing
displacement
5 C VC 12 0 i
12 3 V 5 i
SP08
13 2 V 5 i
S P 10
14 4 V 5 i
S P 13
15 1 V 5 i
S P 15
15 6 V 5 i
S P 17
Swash-Plate
6 C VC 12 5 e
6 C VC 13 5 e
6 C VC 13 5 c
6 C VC 14 0 i
6 C VC 14 0 e
6 C VC 14 0 c
6 C VC 16 0 i
6 C VC 16 0 e
6 C VC 16 0 c
7 C VC 16 5 i
17 9 V 7 e
5 C VC 12 0 e
6 C VC 12 5 c
6 C VC 13 5 i
15 6 V 5 e
SP21
5 C VC 12 0 c
6 C VC 12 5 i
7 C VC 16 5 c
7 C VC 1 8 5 i
7 C VC 1 8 5 c
7 C VC 16 5 e
7 C VC 1 8 5 e
Increasing sophistication and cost trend
SP
- fixed displacement
### - displacement in cc
#
##
- d i s p l a c e m e n t i n c c / 10
V
- wobble plate
C VC - v a r i a b l e s w a s h - p l a t e
c
- c l u t ch l e s s
#
- number of cylinders
### - max displacement in cc
e
- external control
i
- internal control
e
- external control
D e l p h i e n g i n e - d r i v e n R 13 4 a c o m p r e s s o r r a n g e
- number of pistons
i
- internal control
I t c a n b e s e e n t h a t t h e C VC r a n g e i s v e r y
will be bet ween 7 and 9. The reason for this
comprehensive.
the
uncertainty is that the appropriate balance
f u r t h e r f l e x i b i l i t y o ffe r e d b y a v e r y w i d e
bet ween cost and noise/durabilit y has not
range
yet been est ablished.
of
Not
possible
shown
piping
here
is
configurations
w h i ch t o g e t h e r c a r r y t h e i m p l i c a t i o n t h a t a
l a r g e n u m b e r o f d i ffe r e n t c o m p r e s s o r c o n figurations must pass down the production
lines. It has been concluded by some that
this complexit y is likely to 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 f a c t t h e
o p p o s i t e i s t r u e . To m a n a g e t h e c o m p l e x i t y
a system known as RFID – radio frequency
i d e n t i f i c a t i o n – i s u s e d i n w h i ch e a ch i n d i v i d u a l c o m p r e s s o r c a r r i e s a ch i p w i t h i t d o w n
the
production
line.
During
the
passage
Delphi CO2 compressor
d o w n t h e l i n e t h e ch i p r e a d s a n d v e r i f i e s a l l
It can be seen that as a result of the ver y
the information about the individual compo-
d i ffe r e n t f l u i d p r o p e r t i e s , t h e d i s p l a c e m e n t s
nents as they are assembled (manufacturing
o ffe r e d a r e v e r y m u ch l o w e r t h a n f o r R 13 4 a .
dates, dimensions, etc) and communicates
T h e r a n g e fe a t u r e s s h a ft s e a l i n g b y f a c e s e a l
i t b a ck t o a c e n t r a l c o m p u t e r f o r f i l i n g . T h i s
a n d i n t e r n a l o i l s e p a r a t o r. T h e n e e d f o r t h e
d a t a o n e a ch i n d i v i d u a l c o m p r e s s o r i s t h e n
available for later access should there be
problems further down the line or problems
from the field. The availabilit y of all this comprehensive and det ailed dat a contributes to
the establishment and maintenance of the
ver y high levels of reliabilit y required by the
application.
l a tt e r c o m e s f r o m t h e s i g n i f i c a n t e v a p o r a t o r
performance degradation that occurs with
CO2 if excess oil is allowed to circulate.
T h e s e c o n d n e w t e ch n o l o g y c o n c e r n s c o m pressors
for
future
powertrains,
whether
hybrid or fuel cell – the electrically driven
c o m p r e s s o r. U n d e r d e v e l o p m e n t i s a f a m i l y
o f s c r o l l c o m p r e s s o r s d r i v e n b y 3 15 v o l t
brushless electric motors. Scroll displacements are 28 and 38cc for cooling capacities
N e w Te c h n o l o g i e s
o f 5 . 6 a n d 7. 1 k W r e s p e c t i v e l y. T h e m o t o r s
In recent years a serious interest in the
are cooled using the low temperature re-
u s e o f R 74 4 a s r e f r i g e r a n t i n a u t o m o t i v e
frigerant vapour from the evaporator with
air
a r e s u l t a n t m o t o r e ff i c i e n c y o f 9 4 % . T h e
conditioning
rated.
systems
European
has
legislation
been
has
gene-
recently
been passed that will force the industr y
t o m o v e a w a y f r o m R 13 4 a a n d R 74 4 h a s
inverter is cooled by conduction from the
s a m e r e f r i g e r a n t a n d i t t o o h a s e ff i c i e n c y i n
the 94 to 98% range.
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. A s a r e s u l t
Delphi is developing a range of swash-plate
compressors
to
meet
this
need.
Current
protot ypes have five pistons and maximum
displacement of 30cc although the number
3
1
2
of pistons in the final product range is still
under development. Both fixed and variable
v e r s i o n s o f d i s p l a c e m e n t s o f 15 , 2 1 a n d
30cc are planned. It is envisaged that for
low
displacement
the
number
of
pistons
w i l l b e b e t w e e n 5 a n d 7, a t t h e h i g h e n d i t
Section through Delphi Electric Compressor
1 Scroll Compressor
2 Pe r m a n e n t M a g n e t M o t o r
3 Inverter and Integrated Electronics
Conclusion
T h i s p a p e r h a s a i m e d to p re s e n t t h e te ch n i c a l re q u i re m e n t s o f t h e c o m p re s s o r fo r u s e i n
a u to m o t i ve 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 s a n d to s h ow h ow t h e d i ve r s e re q u i re m e n t s o f t h e
a p p l i c a t i o n m e a n t h a t a s u p p l i e r m u s t h ave ava i l a b l e a ra n ge o f te ch n o l o g i e s a n d s p e c i f i c a t i o n s to b e a b l e to m e e t t h e s e n e e d s . I t go e s o n to s h ow h ow t h e D e l p h i c o m p re s s o r
p ro d u c t ra n ge i s d e s i g n e d to g i ve c o m p re h e n s i ve c ove ra ge a n d f i n i s h e s w i t h c o n s i d e ra t i o n
o f c o m p re s s o r s fo r d e ve l o p m e n t a l te ch n o l o g i e s t h a t a re l i ke l y to b e c o m e s i g n i f i c a n t i n t h e
re l a t i ve l y n e a r f u tu re .
Delphi Thermal Systems European Headquarters
Avenue de Luxembourg
L - 4940 Bascharage
Grand-Duché de Luxembourg
Tél : +352 50 18 1
Fax : +352 50 18 48 00
w w w . d e l p h i . c o m
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