arcondicionado automotivo 1

8/14/2019 arcondicionado automotivo 1

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Air Conditioner in the Motor Vehicle

Fundamentals

Self-Study Programme 208

Service.

2 0 8


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Please always refer to the relevant Service Literaturefor all inspection, adjustment and repair instructions.Service literature.

The Self-Study Programmeis not a Workshop Manual!

New ImportantNote

Air conditioning systems have long ceased to beregarded as luxury equipment.Air conditioners have become a factor in active

safety, and today can almost be considered asan integral part of a vehicle's safetyspecification.

10 years ago, only about 10 percent ofall newly registered vehicles were fitted with anair conditioning system. By 1996, air conditionerswere being installed as standard in more thanone in four newly registered vehicles.

Customer demand for air conditioning is rising

continually.

The design of the refrigerant circuit of anair conditioner is identical in all vehicles.Air conditioner refrigerant circuits only vary in

respect of how they are adapted to meetrefrigeration requirements.

In this Self-Study Programme, you will familiariseyourself with the basic purpose and design of anair conditioner.You will learn the functions of the componentparts in the refrigeration process, the specialcharacteristics of the refrigerant and why airconditioners require special servicespecifications.

The component parts shown in the following SSPare common to most air conditioners.

Please note that the figures specifiedare given by way of example only.Depending on refrigeration requirements, theabsolute values vary from vehicle to vehicle.


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R134a

t" p

Table of Contents

The in-car climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Why air conditioning?

The physics of the cooling system . . . . . . . . . . . . . . . . . . . . . . . . 6Applied physics

The refrigerant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

The cooling system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12The principle of the refrigerant circuitRefrigerant circuit with expansion valveThe compressor

The mode of operation of the compressorMagnetic clutchThe condenserThe fluid container and drierExpansion valveExpansion valve new generationThe evaporatorRefrigerant circuit with restrictorThe restrictorThe collecting tank

System control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Components of the safety system

Cooling fan circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40Fan circuit for engine/condenser coolingRadiator fan control unit J293

Temperature control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42Manual controlAutomatic controlSystem overviewControl unit with operating and display unitThe main temperature sensors

Auxiliary signals for temperature controlPositioning motorAir ductingAir distributionAir recirculation mode

Technical service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64Safety precautionsGeneral information on function influencing factorsFault diagnosis through pressure testingFault diagnosis through self-diagnosis

Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72Key cooling system terminology


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4

The in-car climate

Why air conditioning?

People feel comfortable at a certain ambienttemperature and atmospheric humidity.

As a component part of active safety, the driver'swell-being is a key factor in driving ability.

The in-car climate has a direct bearing on thedriver, fatigue-free driving and driving safety.

A comfortable interior temperature is dependent

upon the prevailing ambient temperature andupon sufficient air flow:

Low ambient temperature, e.g. 20 oC Higher interior temperature 28 oCHigh air flow rate: 8 kg per min.

High ambient temperature, e.g. 40 oCLow interior temperature 23 oCHigh air flow rate: 10 kg per min.

Moderate ambient temperature, e.g. 10 oCLow interior temperature 21.5 oCLow air flow rate: 4 kg per min.

Even modern heating and ventilation systemshave difficulty maintaining a pleasant climateinside a vehicle at high ambient temperatures.Why?

In strong sunlight in particular, the heatedcabin air can only be exchanged for air withambient temperature.

In addition, the air temperature usually risesen route from the intake point to the air outlet.

Opening a window or sliding roof or setting ahigher fan speed for greater comfort willusually result in a draught and expose theoccupants to other nuisances such as noise,exhaust gases and pollen.

High levels of atmospheric humidity put the bodyunder considerably greater physical strain.

208_001

23 " C 42 " C

40 " C

28 " C 35 " C

24 " C

Temperatures in a mid-range passenger carwhere: driving time 1 h

ambient temperature 30 oCsunlight penetration into car

Areawith without

air conditioning

Head

Chest

Feet

air conditioning

0-10 10 20

20

22

26

28

24

-20

0

2

4

6

8

30 40 C"

C"

kg/min

208_043 Ambient temperature

I n t e r i o r

t e m p e r a

t u r e

A i r f l o w

r a

t e

Comfort curves


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5

A B C

Effects of an unfavourable vehicle interiortemperature on humans

Scientific studies conducted by the WHO (WorldHealth Organization) have shown that one'sability to concentrate and reactions are impairedwhen under stress.

Heat puts a strain on the body.

The best temperature for the driver is between20 and 22 oC.This is equivalent to climatic load A, the "comfortrange".

Strong sunlight can increase the interiortemperature by more than 15 oC above theambient temperature particularly in the headarea.This is where the effects of heat are mostdangerous.

The body temperature rises and the heart rate

increases.Heavier perspiration will typically occur, too.The brain is not receiving enough oxygen.Also refer to "climatic load range B".

Climatic loads in range C put an excessive strainon the body.Physicians specialising in traffic-related illnessesrefer to this condition as climatic stress.

Studies have shown that an increase intemperature from 25 to 35 oC reduces one'ssensory perception and powers of reasoning by20%. It has been estimated that this figure isequivalent to a blood alcohol concentration of0.5 millilitres alcohol level.

The air conditioner - a system which keeps the airtemperature at a level comfortable to humans,

as well as purifying and dehumidifying the air -was created in order to reduce or eliminatecompletely such stress.

With the help of an air conditioner it is possibleto produce at the air outlets a temperature whichis much lower than high outside airtemperatures.This is possible both when the vehicle is at astandstill and when it is in operation.

A technical side-effect of air conditioning is thatthe air is dehumidified and cleaned at the sametime. However, this is just as important as thereduction in temperature.The pollen filter and activated charcoal filter alsohelp to clean the air entering the vehicle.People with allergic illnesses benefit greatly frombeing able to breathe clean air.

208_042

S t r a

i n

Transpiration

Heartrate

Body temperature

Climatic loadhighmoderatelow

In-vehicle air conditioning is

- a real safety element

- a functional accessory not only for expensive tastes

Comfort range


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Physics of the cooling system

Applied physics

Laws

Many substances are known to exist in threeaggregate states.

Take water for example: solid liquid vapour.The principle of cooling follows this law.

Even in ancient times there was a need forcooling. One of the first methods used to cool

foodstuffs was to store them in an icebox.

The ice (water in a solid aggregate state)absorbs the heat of the foodstuffs, therebycooling them down.

The ice melts as a result, assuming anotheraggregate state, namely that of a liquid (water).

If the water is heated further, it will boil andevaporate.

The water is now in the gaseous state.

The gaseous substance can be converted back toa liquid by cooling it and will become a solidagain if cooled further.This principle is applicable to almost allsubstances:

A substance absorbs heat when it is conver-ted from a liquid to a gas.

A substance gives off heat when it is conver-ted from a gas to a liquid.

Heat always flows from the warmer substanceto the colder substance.

Air conditioners utilise the effects of heatexchange, a process in which a substance

changes state at certain points.

A E S

208_039

208_040

208_041

Law

Freezing point,

Boiling point

Ice solid

Ice becomesa liquid when it absorbsheat

Water becomes

a gas when it absorbs heat

e.g. water becomes ice,

e.g. water becomes steam


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PPPPrrrreeee ssssssssuuuu rrrreeee aaaa nnnndddd bbbb oooo iiiilllliiiinnnngggg pppp oooo iiiinnnn tttt

If the pressure is changed using a liquid, theboiling point changes.All liquids behave similarly.

Boiling point H 2O/water = 100oC

Machine oil = 380 - 400 oC

The lower the pressure, the lower thetemperature at which water boils (evaporates).

The evaporation process is also used in vehicleair conditioners.A substance with a low boiling point is used forthis purpose.This substance is known as a refrigerant.Boiling point Refrigerant R12 29.8 oC

Refrigerant R134a 26.5 oC

(The boiling points specified for liquids in thetable always refer to an atmospheric pressure of0.1 MPa = 1 bar.)

M P a

100

0,1

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" C110 120 130 140 150 160 170 180 190 200

H2O

1

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5

7

9

11

13

15

17

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M P a

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-40 -20 0 20 40 60 80 100

0

0,5

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3,5

4,0

" C

R134a R12

0

5

10

15

20

25

30

35

40

What does a vapour pressure curve tell us?

We can draw the following conclusions from the vapourpressure curve for the two refrigerants R134a and R12(R12 is no longer used) and water:

At a constant pressure, the vapours become a liquidthrough temperature reduction (in the air conditionercircuit, this process takes place in the condenser =liquefier),

The refrigerant goes from a liquid state to a vapourstate through pressure reduction (in the air conditionercircuit, this process takes place in the evaporator).

208_006 208_005

Vapour pressure curveVapour pressure curve

Liquid

Gaseous

Liquid

Gaseous

Temperature Temperature

P r e s s u r e

P r e s s u r e


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Expansion

Evaporation

Temperature

Gaseous

P r e s s u r e

The refrigerant

Refrigerant RRRR111133334444aaaa Tetrafluorethanechem. formula CH 2F-CF3

a fluorocarbon (FFFFCCCC)environmentally friendly!

The refrigerant with a low boiling point usedfor vehicle air conditioners is a gas.

As a gas, it is invisible.As a vapour and as a liquid, it is colourless likewater.

Refrigerants may not be combined with eachother. Only the refrigerant specified for thesystem in question may be used.

Ordinance banning halogens

R134a

Vapour pressure curve of R134a

Liquid

208_050

With regard to vehicle air conditioners, the saleand filling of refrigerant R12 were banned inGermany with effect from 1995 and July 1998respectively.

In current automotive air conditioners, onlyrefrigerant R134a is used.

R134a a fluorocarbon contains no chlorine

atoms - unlike refrigerant R12 - which causedepletion of the ozone layer in the earth'satmosphere when they split.

The vapour pressure curves of R134a and R12are very similar.R134a has the same refrigeration capacity asR12.

It is possible to adapt air conditioners whichnow may no longer be filled with R12 to R134a

with a special conversion kit(Retrofit method).

The systems converted in this way are no lon-ger able to match their original refrigerationcapacity.

Depending on the pressure and temperatureconditions in the refrigerant circuit, therefrigerant will either be a gas or a liquid.

Refrigerant RRRR11112222 Dichlordifluormethane

chem. formula CCl 2F2

a chlorinated hydrocarbon (CC CCFFFFCCCC)harmful to the environment!


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R134a

60 60

200 240 280 320 360 400 440

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A

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kJ/kg

0 C 0 C

B

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16

20

40

b a r

State of refrigerant R134ain the cycle in an air conditioner

In addition to the vapour pressure curve, thecycle shows the change of state of the refrigerantunder pressure and temperature in addition tothe energy balance at which the refrigerantreturns to its original state.The diagram is an excerpt from the statediagram of refrigerant R134a for a vehicle airconditioner.Different absolute values are possible independence upon the demand of a vehicle typefor refrigeration capacity.

The energy content is a key factor in the designof an air conditioner.

It shows what energy is required (evaporatorheat, condenser heat) to achieve the intendedrefrigeration capacity.

Physical data of R134a:

Boiling point: 26.5 CFreezing point: 101.6 CCritical temperature: 100.6 CCritical pressure: 4.056 MPa

(40.56 bar)

A B Compression in the compressor, pressure and temperature rise,gaseous, high pressure, high temperature

B C Condensation process in the condenser, high pressure, temperature reduction,the liquid leaves the condenser slightly cooled

C D Expansion = sudden pressure relief, results in evaporation

D A Evaporation process (heat absorption) in the evaporator.

Transition path from vapour state to gaseous state (low pressure)

Temperature curve at point B

208_053 Energy content

P r e s s u r e

Temperature curveSaturated liquid

Temperature curveSaturated vapour

Critical point

(pressure/temperature)

P r e s s u r e

For a glossary refer to page 72.


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FKWR134a

FCKWR12

Refrigerants and ozone layer

Ozone protects the earth's surface againstUV radiation by absorbing a large proportion ofthese rays.UV rays split ozone (O 3) into an oxygenmolecule (O 2) and in an oxygen atom (O).Oxygen atoms and oxygen molecules from otherreactions combine again to form ozone.This process takes place in the ozonosphere, apart of the stratosphere at an altitude of between20 and 50 km.

Like R12, chlorine (Cl) is a constituent of a CFCrefrigerant .If handled improperly, the R12 molecule will riseup to the ozone layer since it is lighter than air.

UV radiation liberates a chlorine atom in theCFC, and this atom reacts with the ozone.In the process, the ozone decomposes leaving anoxygen molecule (O 2) and chlorine monoxide

(ClO), which then reacts again with oxygen andliberates chlorine (Cl). This cycle can repeat itselfas many as 100,000 times.

However, free oxygen molecules (O2) cannotabsorb UV radiation.

Refrigerants and global warming

The sunlight impinging upon the earth's surfaceis reflected in the form of infrared radiation.

However, trace gases most importantly CO 2 reflect these waves in the troposphere.This causes the earth's atmosphere to heat up, aphenomenon which is commonly referred to as"global warming". CFCs are heavily responsiblefor the increasing trace gas concentration.

1 kg of R12 has the same greenhouse effectas 4000 tons of CO 2.

R134a only makes a small contributionto global warming.Its ozone depletion potential is nil.

208_052

O z o n e

d e p

l e t i o n p o

t e n

t i a

l

Greenhouse potential

1

5

10

20

40

60

100

80

200

km

FCKW

ClO+ =

O

Cl

U V U VU V

U V U V

FCKW

T R O P O S P H R E

S T R A T O S P H R E

Cl

O 3

O 2

The refrigerant

Reaction between CFC and

ozone in the atmosphere(CFC = FCKW)

Global warming

208_051


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R134a

Important notes:

Do not store in the open (highly hygroscopic). Always keep oil tanks closed to protect them

against the ingress of moisture. Close openeddrums immediately.

Do not use old refrigerant. DDDDiiiisssspppp oooo sssseeee oooo ffff aaaa ssss ttttoooo xxxxiiiicccc wwwwaaaa sssstttteeee ....

Refrigerant may not be disposed of togetherwith engine oil or gear oil because of its che-mical properties.

50%10%

20%10%

10%

Distribution of oil quantity in the refrigerant circuit(roughly)The filling quantity of refrigerant varies according tothe design of the units used in a particular type ofvehicle.

Fluid container

Condenser

Evaporator

Refrigerant oil

A special oil the refrigerant oil free ofimpurities such as sulphur, wax and moisture isrequired to lubricate all the movable parts in theair conditioner.

The refrigerant oil must be compatible with therefrigerant itself, because some of the refrigerantoil mixes with the refrigerant in the refrigerantcircuit. In addition, the refrigerant oil must notattack the seals used in the system.

No other oils may be used, as they lead tocopper plating, the build-up of carbon depositsand the formation of residues which can causepremature wear and irreparable damage tomovable parts.

A special synthetic oil is used for the R134arefrigerant circuit. This oil may only be used forthis particular refrigerant, since it does not mixwith other refrigerants.

Also, the refrigerant oil can only be adapted to aspecific compressor type.

The refrigerant for R134a

Designation: PPPPAAAAGGGG = Polyalkylene glycol

Properties:

- high solubility in combination with

refrigerant- good lubrication properties- acid free- highly hygroscopic (water-attracting)- cannot be mixed with other oils

N.B.:

- must not in be used in older refrigerationsystemsfilled with refrigerant R12,since it is incompatible with R12

Suction hose

Compressor


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The cooling system

The principle of the refrigerant circuit

The cooling process and the technical conditi-ons

We know that:Too cool down an object, heat must be given off.A compression refrigeration system is used inmotor vehicles for this purpose. A refrigerantcirculates in the closed circuit, continuallyalternating changing from a liquid to a gas andvice versa. The refrigerant is:

compressed in the gaseous state, condensed through heat dissipation and evaporated through pressure reduction

and heat absorption.

Cool air is not produced, heat isextracted from the air flow in the vehicle.

Compressor

Low-pressure side

High-pressure side

Cooling airCondenser

The ccccoooo mmmmpppp rrrreeee ssssssssoooo rrrr induces cold, gaseous

refrigerant at a low pressure.

The refrigerant is compressed in thecompressor, causing it to heat up.The refrigerant is pumped into the circuit onthe high-pressure side.

In this phase, the refrigerantis in a gaseous state,has a high pressure anda high temperature.

208_071

How does this process work?


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Now in the gaseous state again, the refrigerant

emerges from the evaporator.The refrigerant is again drawn in by the compressorand passes through the cycle once again.Thus, the circuit is closed.

In this phase, the refrigerantis againgaseous,has a low pressureand a low temperature.

The refrigerant follows the short path to the

ccccoooo nnnndddd eeee nnnnsssseeee rrrr (liquefier).Heat is now extracted from the compressed, hotgas in the condenser by the air flowing through(headwind and fresh air blower).The refrigerant condenses and becomes a liquidwhen it reaches its melting point (pressure-dependent).

In this phase, the refrigerantis thereforein a liquid state,has a high pressureand a medium temperature.

The compressed liquid refrigerant continues toflow up to a narrowing. This narrowing can be inthe form of a restrictor or an expansion valve.Once the refrigerant reaches the narrowing, it isinjected into the evaporator causing its pressureto drop (low-pressure side).

Inside the eeee vvvvaaaa pppp oooo rrrraaaa ttttoooo rrrr, the injected liquidrefrigerant expands and evaporates. Theevaporation heat required for this purpose isextracted from warm fresh air which cools downwhen it passes through the evaporator fins.The temperature inside the vehicle is reduced toa pleasant level.

In this phase, the refrigerantis in a vapour state,under low pressure and

at low temperature.208_004

Warm fresh air

Evaporator

Valve

Cooled fresh air

208_073

208_072 208_074


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A

B

C

2

1 E

H

D

I

A B C D E F

G

ND

HD

H

ND

HD

I

The cooling system

Refrigerant circuit with expansion valve

Working pressure HD = High-pressureND = Low pressure

In technical documents such as WorkshopManuals, the components are represented in adiagrammatic form.

Pressures andtemperature inthe circuit(example)

208_032

21CCCCoooo mmmmpppp rrrr eeee ssssssssiiiioooo nnnn rrrr aaaa ttttiiiiooooat approx. 1.4 MPa (14 bar)Temperature approx. 65 oC

CCCCoooo nnnndddd eeee nnnnssssaaaa ttttiiiioooo nnnnPressure approx. 1.4 MPa (14 bar)Temperature reduction: 10 oC

The refrigerant circuit is activatedwhen the vehicle engine isrunning. For this purpose, thecompressor has a magneticclutch.

1 MPa = 10 barThe absolute values arealways vehiclespecific.Please observe theWorkshop Manual.


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F G

3

4

The refrigeration capacity of a vehicle airconditioner is dependent upon the car-specificinstallation conditions and the vehicle category(passenger cars, vans).

The components A to H exist in every circuit.Additional connections can be provided for

service work, temperature sensors, pressureswitches in the high- and low-pressure circuit andoil drain screws depending on the circuit designand requirements. The layout of componentswithin the circuit also differs from one vehicletype to another.Some systems have a damper before thecompressor in order to dampen refrigerantvibrations.

The components:

A Compressor with magnetic clutchB CondenserC Fluid container with drierD High-pressure switchE High-pressure service connectionF Expansion valveG EvaporatorH Low-pressure service connectionI Damper (vehicle-specific)

The pressures and temperatures in the circuit arealways dependent on momentary operatingstate. The specified values are intended as arough guideline only. They are reached after20 min. at an ambient temperature of 20 oC andat engine speeds of between 1500 and 2000rpm.

At 20 oC and when the engine is at a standstill , apressure of 0.47 MPa (4.7 bar) will build upinside the refrigerant circuit.

The components of the refrigerant circuit withexpansion valve will now be examined moreclosely (for details of the refrigerant circuit withrestrictor refer to page 28).

208_031

The refrigerant circuit may not beopened for safety reasons.If it is necessary to open therefrigerant circuit in order toperform repair work on thevehicle, the refrigerant must bedrawn off beforehand using asuitable service station.

3 4 1EEEExxxxpppp aaaa nnnnssssiiiioooo nnnnfrom approx. 1.4 MPa (14 bar) to approx. 0.12 MPa(1.2 bar), Temperature: from approx. 55 oC to 7 oC

EEEEvvvvaaaa pppp oooo rrrraaaa ttttiiiioooo nnnnPressure: approx. 0.12 MPa (1.2 bar)Temperature: approx. 7 oC

208_033

Legend

High pressure

Low pressure


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The cooling system

The compressorThe compressors used in vehicle air conditionersare oil-lubricated displacement compressors.They operate only when the air conditioner isswitched on, and this is controlled by means of amagnetic clutch.

The compressor increases the pressure of therefrigerant. The temperature of the refrigerantrises at the same time.

Were there to be no pressure increase, it would

not be possible for the refrigerant in the airconditioner to expand and therefore cool downsubsequently.

A special refrigerant oil is used for lubricatingthe compressor. About half of it remains in thecompressor while the other half is circulated withthe refrigerant.A pressure shut-off valve, which is usuallyattached to the compressor, protects the systemagainst excessively high pressures.

The compression process

The compressor draws in cold, gaseousrefrigerant through the evaporator under lowpressure .

It is "vital" for the compressor that therefrigerant be in a gaseous state, because liquidrefrigerant cannot be compressed and woulddestroy the compressor (in much the same wayas a water shock can damage an engine).

The compressor compresses the refrigerant andforces it towards the condenser as a hot gas onthe high-pressure side of refrigerant circuit.

The compressor therefore represents theinterface between the low-pressure and high-pressure sides of the refrigerant circuit.

208_028

208_045 Magnetic clutchCompressor


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17

Mode of operation of compressor

Compressors for air conditioners operateaccording to various principles:

Reciprocating compressors Coiled tube compressors Vane-cell compressors Wobbleplate compressors

Wobbleplate compressors will now be examinedin more detail.

The turning motion of the input shaft is convertedto an axial motion (= piston stroke) by means ofthe wobbleplate.Depending on compressor type, between 3 and10 pistons can be centred around the input shaft.A suction/pressure valve is assigned to eachpiston.These valves open/close automatically in rhythmwith the working stroke.An air conditioner is rated for the max. speed of

the compressor.However, the compressor output is dependent onengine rpm.Compressor rpm differences of between 0 and6000 rpm can occur.This affects evaporator filling as well asthe cooling capacity of the air conditioner.Controlled-output compressors with a variabledisplacement were developed in order to adaptcompressor output to different engine speeds,ambient temperatures or driver-selected interiortemperatures.

Compressor output is adapted by adjusting theangle of the wobbleplate.

In constant-displacement compressors,compressor output is adapted to the demand forrefrigeration by switching the compressor on andoff periodically via the magnetic clutch.

Wobbleplate compressor (self-regulating)Angle of wobbleplate variableDisplacement variable

Wobbleplate compressor (non-self-regulating)Angle of wobbleplate constant

Displacement constant

208_027

208_046

Piston

Wobbleplate

Input shaft

Piston Wobbleplate

Suction/ pressurevalve


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18

CCCChhhhaaaa mmmmbbbb eeee rrrr pppp rrrreeee ssssssssuuuu rrrreeee is dependent upon thehigh and low pressures acting upon the

regulating valve and by the calibrated restrictorbore.

High pressure, low pressure and chamberpressure are equal when the air conditioner isoff.The springs before and after the wobbleplate setit to a delivery rate of about 40%.The advantage of output control isthat it eliminates compressor cut-in shock, whichoften manifests itself in a jolt while driving.

The cooling system

The turning motion of the input shaft istransmitted to the drive hub and converted to

axial motion of the piston via the wobbleplate.The wobbleplate is located longitudinally in aslide rail.

The piston stroke and the delivery rate aredefined by the inclination of the wobbleplate.

IIIInnnncccclllliiiinnnnaaaa ttttiiiioooo nnnn dependent on the chamberpressure and hence the pressure conditions atthe base and top of the piston.The inclination is supported by springs locatedbefore and after the wobbleplate.

208_047 Upper side

Piston

Lower side

Chamber pressure Drive hub

Wobbleplate

Springs

Slide railRegulating valveCalibratedrestrictor bore

High pressure

Low pressure

All control positions between upper stop (100 %)and the lower stop (approx. 5 %) are adapted to the required delivery rateby altering the chamber pressure.The compressor is on continuous duty during the control cycle!

Input shaft

The self-regulating compressorruns continously in air condition mode

Control range of compressor


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19

High delivery rate for high cooling capacity - low chamber pressure

The high and low pressures are relatively high.

Bellows 2 is compressed by the high pressure .

Bellows 1 is also compressed by the relativelyhigh low pressure.

Regulating valve opens. Chamber pressure isreduced via the low-pressure side.

208_048

Bellows 1Bellows 2

High pressure Low pressure

Chamber pressure

Spring 1 Spring 2

The combined force resulting from the low

pressure acting upon the upper sides of thepiston and the force of spring 1 is greater thanthe combined force resulting from the cham-ber pressure acting upon the lower sides ofthe piston and the force of spring 2.

Inclination of wobbleplateincreases= large stroke with high delivery rate

Restrictor bore

Regulating valve

Chamber pressure


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20

The cooling system

The high and low pressures are relatively low.

Bellows 2 opens out.

Bellows 1 also opens out as a result of therelatively low pressure.

Regulating valve closes.The low-pressure side is closed against thechamber pressure.

Chamber pressure rises via the calibrated

restrictor bore.

Chamber pressure

Low pressureHigh pressure

Low delivery rate and low cooling capacity - high chamber pressure

208_049

Bellows 1Bellows 2

Spring 1 Spring 2

Restrictor bore

The combined force resulting from the low

pressure acting upon the upper side of thepiston and the force of spring is greater thanthe combined force resulting from the cham-ber pressure acting upon the lower sides ofthe piston plus the force of spring 2.

Inclination of wobbleplatedecreases= small stroke with low delivery rate.

Regulating valve

Chamber pressure


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21

Magnetic clutch

The drivetrain is connected between thecompressor and vehicle engine while the engineis running by means of the magnetic clutch.

Design

The clutch comprises

Belt pulley with bearing Spring plate with hub

Magnetic coil

The hub of the spring plate is permanentlymounted the compressor input shaft. The beltpulley is mounted in a pivot bearing on thehousing of the compressor at the shaft output.The magnetic coil is permanently connected tothe compressor housing. There is an open spaceA between the spring plate and the belt pulley.

Function

The vehicle engine drives the belt pulley (Arrow)by means of the ribbed V-belt.The belt pulley follows on freely when thecompressor is switched off.

When the compressor is connected, voltage ispresent at the magnetic coil. A magnetic forcefield is created. This force field draws the springplate towards the rotating belt pulley (the openspace A is bridged) and makes a positiveconnection between the belt pulley and the inputshaft of the compressor.The compressor runs on.The compressor runs on until the electrical circuitto the magnetic coil is opened.The spring plate is then retracted by the beltpulley by means of springs.The belt pulley again runs without driving thecompressor shaft.

A

Schematic diagram of clutch switched off

Schematic diagram of clutch switched on

For compressor switch-on and switch-off

conditions refer to Air conditioningfunction control.

Spring platewith hub

Belt pulley with bearing

Compressorhousing,

CompressorInput shaft

Force flow

208_002

208_003

Magnetic coil


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23

The fluid container and drier

In the refrigerant circuit with expansion valve, thefluid container serves as a refrigerant expansiontank and reservoir.

Different amounts of refrigerant are pumpedthrough the circuit when operating conditionssuch as the thermal load on the evaporator andcondenser and compressor rpm are variable.

The fluid container is integrated in the circuit in

order to compensate for these fluctuations.

The drier binds chemically moisture which hasentered the refrigerant circuit during installation.The drier can absorb between 6 and 12 g ofwater, depending on type. The amount of waterthat can be absorbed is temperature-dependent.The amount of water absorbed increases as thetemperature drops. Abraded material from thecompressor, dirt arising from installation workand similar is also deposited.

Function

The liquid refrigerant coming from the condenserenters the container at the side. The refrigerant iscollected in the container, then it flows throughthe drier and along the riser to the expansionvalve in an uninterrupted flow containing nobubbles.

The fluid container is replaced everytime the refrigerant circuit is opened.The fluid container must be kept closedas long as possible prior to installationin order to minimise absorption ofmoisture from the ambient air in thedrier.

Filterstrainer

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208_025

Fromcondenser

Drier

Toexpansion valve


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24

The cooling system

Expansion valve

The expansion valve is the point where therefrigerant in the evaporator expands and coolsdown. It forms the interface between the high-pressure side and low-pressure side of therefrigerant circuit.

The expansion valve is used to regulate therefrigerant flow to the evaporator independence upon the temperature of therefrigerant vapour at the evaporator outlet.

No more refrigerant than is necessary tomaintain a steady refrigerating climate in theevaporator is expanded in the evaporator.

The closed control loop

The refrigerant flow is controlled by theexpansion valve in dependence upontemperature.

When the temperature of the refrigerantleaving the evaporator rises, the refrigerant inthe thermostat expands. The flow rate of therefrigerant to the evaporator at the globevalve increases.

When the temperature of the refrigerantleaving the evaporator drops, the refrigerantvolume in the thermostat decreases. The flowrate to the evaporator at the globe valve isreduced.

There are three forces at play in the thermostaticexpansion valve:

1. The pressure in the sensor line is dependenton the temperature of the superheatedrefrigerant. This pressures acts upon themembrane as an opening force (P F).

2. The evaporator pressure (P Sa) acts upon

the membrane in the opposite direction.

3. The pressure exerted by the regulatingspring (PFe) acts in the same direction as theevaporator pressure.

The expansion valves are set.Their settings may not be altered.Do not bend the sensor line, because itis filled with a special gas.

208_022

208_015

PF

PSa

PFe

Regulating spring

Globe valve

From condenser(high pressure)

To compressor (lowpressure)

Membranes

Thermostat with sensor lineand refrigerant

To evaporator(low pressure)

From evaporator(low pressure)


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25

Expansion valve new generation

This expansion valve is also positioned betweenthe high-pressure side and low-pressure side ofthe refrigerant circuit directly upstream of theevaporator.

The expansion valve is heat-controlled. It has acontrol unit with a thermal head and a globevalve.The thermal head on one side of the membranehas a special gas filling. The other side isconnected to the evaporator outlet (low pressure)via pressure-equalising holes.The globe valve is push rod actuated.The pressure of the special gas, and thereforealso the amount of refrigerant injected, isdependent upon the temperature on the low-

pressure side.

The expansion valve is always fitted with thermalinsulation.

Fitting the valve without thermalinsulation will alter the set controlcharacteristic.

208_017

To compressor(low pressure)

Membranes

Pressure-equalising

holeFrom condenser(high pressure)

Evaporator outlet(low pressure)

Push rod

To evaporator(low pressure)

Globe valveRegulating spring

Thermal head withspecial gas filling


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26

The cooling system

The valve opening ratio is dependent upon thetemperature at the evaporator outlet (lowpressure).Pressure equalisation is controlled.

When the temperature of therefrigerant at the evaporator outlet

drops, thepressure (p b) in the thermal head drops

The cross-section of the globe valve,

and therefore also the flow rate toevaporator, will again be reduced.

208_018

208_019

208_020

208_021

A increase in cooling load increases thetemperature at the evaporator outlet

the pressure (p a ) of the gas filling inthermal head to rise

The globe valve cross-section isenlarged

via diaphragms and the push rod.

Refrigerant flows to evaporator andabsorbs heat at the transition from high

pressure to low pressure

Heat is extracted from the air flowingthrough the evaporator

pb

pa

causing


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27

The evaporator

The evaporator operates according to the sameprinciple as a heat exchanger.

It is an integral part of the air conditioner in theheater box. When the air conditioner is switchedon, heat is extracted from the air which flowsthrough the fins of the cold evaporator. This air iscooled, dried and cleaned in the process.

Function

The refrigerant released by the expansion valveexpands in the evaporator, cooling theevaporator down considerably.

The refrigerant becomes a gas (boiling point).

When the refrigerant in the evaporator boils, thetemperatures are well below the freezing pointof water.

The refrigerant extracts the heat required forevaporation from its surroundings which is theair flowing through the evaporator in this case.

This air is channeled into the passenger cabin ina cooled" state.

Moisture in the cooled air collects at theevaporator in the places where the airtemperature drops below the dewpointtemperature, i.e. it condenses. Condensationwater is produced.

The air is dried.This improves the climate and air quality insidethe vehicle noticeably.Deposits of matter suspended in the air build upat the evaporator in addition to moisture.The evaporator also "purifies" the air.

Pools of water below a stationaryvehicle (condensation) are therefore notan indication of a fault.

Refrigerant returnline (gaseous state)

Refrigerant supplyline (vapour state)

208_030

208_029

Tubular evaporator


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28

2

1

AB

C

D

H

IA B C D

E

FGHI

ND

HD

21

The cooling system

Refrigerant circuit with restrictor

Working pressure HD = high pressureND = low pressure

Schematic diagram of a refrigerant circuit withrestrictor

1 MPa = 10 bar

Pressures and

temperaturesin the circuit

CCCCoooo mmmmpppp rrrreeee ssssssssiiiioooo nnnn rrrraaaa ttttiiiiooooMax. pressure 2 MPa (20 bar)Max. temperature 70 oC

CCCCoooo nnnn dddd eeee nnnn ssssaaaa ttttiiiioooo nnnnMax. pressure 2 MPa (20 bar)Temperature reduction: approx. 10 oC

208_034


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29

34

E

F

G

208_007

In contrast to the circuit with expansion valve, theliquid refrigerant is injected into the evaporatorthrough a restrictor.

In restrictor-regulated air conditioners, acollecting tank is fitted at the low-pressure side in

place of the fluid container on the high-pressureside.

This collecting tank serves as a reservoir andprotects the compressor (fluid shock). Also referto page 31.

The component parts:

A Compressor with magnetic clutchB Low-pressure switchC CondenserD High pressure service connectionE RestrictorF EvaporatorG Low-pressure switchH Low-pressure service connectionI Collecting tank

EEEExxxxpppp aaaa nnnnssssiiiioooo nnnnfrom 2 MPa (20 bar) to > 0.15 MPa (1.5 bar)Temperature: from 60 oC to > 4 oC

EEEEvvvvaaaa pppp oooo rrrr aaaa ttttiiiioooo nnnnMax. pressure > 0.15 MPa (1.5 bar)Temperature: > 4 oC

All other components are identical to those usedin the circuit with expansion valve.

Additional connections for service work orsensors for monitoring functions can beintegrated in the circuit, depending on circuit

design and necessity.

The pressures and temperatures are dependentupon the momentary operating state of therefrigerant circuit. The specified values areachieved after a specific period depending onthe ambient temperature (refer to WorkshopManual).

3 4 1208_033

Legend

High pressure

Low pressure


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30

The cooling system

The restrictor

The restrictor is a narrowing in the refrigerantcircuit located directly upstream of theevaporator. This narrowing restricts the flow ofthe refrigerant.

The refrigerant is warm under high pressureupstream of the restrictor.

The pressure of the refrigerant drops rapidilywhen it passes the restrictor.

The refrigerant is cold at low pressure.

The restrictor is therefore the interface betweenthe high-pressure and low-pressure sides of therefrigerant circuit. A seal ensures that therefrigerant only passes the restrictor at thenarrowing.

Tasks

To determine the flow rate of the refrigerantby means of a calibrated bore. The amount ofrefrigerant which can flow through this bore islimited by the pressure in the refrigerantcircuit.

To maintain the pressure on the high-pressureside of the refrigerant circuit and keep therefrigerant in a liquid state when thecompressor is running.

The pressure in the restrictor drops. Therefrigerant cools down before it enters theevaporator through partial evaporation.

Atomisation of the refrigerant.

The restrictor has a dirt strainer upstream of thenarrowing.A strainer for atomising the refrigerant before itreaches the evaporator is located downstreamthe narrowing.

208_035

208_016

To evaporator

Dirt strainer

0-ring, seals the high-pressureside off from the low-pressureside

Strainer for atomising therefrigerant

Calibrated bore

Please note the installation position!The arrow on the restrictor points to theevaporator.


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31

The collecting tank

There is a collecting tank in the low-pressure partof air conditioners with restrictor. This tank isinstalled in a warm location in the enginecompartment (post-evaporation).

It serves as an equalising vessel and reservoir forthe refrigerant and refrigerant oil and alsoprotects the compressor.

The gaseous refrigerant coming from the

evaporator enters the tank. If there are traces ofmoisture in the refrigerant, they are bound in theintegrated drier.

The refrigerant collects in the upper part of theplastic cap and is certain to be in a gaseous statewhen it is drawn in by the compressor throughthe U-tube.

As a result, the compressor draws in gaseousrefrigerant only, and no liquid droplets.

Protection of the compressor against damage isthereby ensured.

The refrigerant collects at the base of thecollecting tank.

The refrigerant drawn in by the compressorabsorbs refrigerant through a hole in the U-tube.

A filter strainer prevents the ingress of impurerefrigerant through this hole.

208_036

208_037

The collecting tank must be kept closedas long as possible prior to installation(leave the sealing plugs on theconnections) in order to minimisemoisture absorption from the ambientair in the drier.

Intake point for gaseous refrigerant

Fromevaporator

Tocompressor

Drier

U-tube

Hole for refrigerantFilter strainer

Plastic cap


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" p

32

System control

An air conditioner will only function if all systemcomponents are working properly. Failure of oneof these components could cause the workingpressures to change. In this case, it is not possibleto rule out consequential damage to the systemand the engine. To avoid this, there aremonitoring devices in the refrigerant circuit.

A control unit processes the signals from themonitoring devices and controls the periodic

switch-off and switch-on of the compressor andthe speed of the fan. This ensures that thepressure level in the refrigerant circuit alwaysadapts itself to the normal values.In systems equipped with an open-loopcompressor, the signals from the monitoringdevice are also used for adaptation to demandfor cooling.(switch-on and switch-off the air conditioner inaccordance with demand for refrigeration.Icing of the evaporator is avoided at the sametime.)The diagram shows the basic layout of the airconditioner.

1

2

3

4

5

6

7

8

910

208_054

The air conditioner does not necessarilyhave to have all the components shownin the diagram. Neither do thesecomponents have to be connected inthis way.

The diagram shows the system control of asimple manual air conditioner.

1 Air conditioner switch2 Pressure relief valve at compressor

3 Radiator fan4 Air conditioner pressure switch5 Coolant temperature sender6 Radiator fan thermo switch7 Evaporator temperature sender8 Fresh air blower9 Engine control unit

10 Magnetic clutch

Air conditioner control unit(and/or radiator fancontrol unit,depending on system type)


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33

t" p

Colour codes:PositiveNegativeInput signal

Output signalBidirectional signal

3015

31X

31

t 1 t 2

p 2

-t"

31

J 257J 257

M

A

S

18F

p 1

V 7101J

153G

J 32

62G

129F

E 35

301J

25N

3015

31X

A BatteryE35 Air conditioner switchF18 Radiator fan thermo switch

t1 = 95oC

t2 = 103oC

F129 Pressure switch for air conditionerP1 = 0.2 MPa (2 bar)/3.2 MPa (32 bar)P2 = 1.6 MPa (16 bar)

G62 Coolant temperature sender

G153 Evaporator temperature senderJ32 Air conditioner relayJ101 Radiator fan 2nd speed relayJ257 Mono-Motronic control unitJ301 Air conditioner control unitN25 Air conditioner magnetic clutch

V7 Radiator fanS Fuse

208_055

In the new-generation air conditioners,there is a high pressure sender in place

of the pressure switch for air conditioner(refer to page 36).

Simple functional example showing how thecompressor (via magnetic clutch N25) and radiatorfan are switched on and off.


34/37


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35

t" p

To monitor and/or limit the pressure conditions inthe closed refrigerant circuit, high-pressure and

low-pressure switches are installed on the highpressure side.

If unacceptable pressures build up inside thesystem, the compressor will be switched off viathe magnetic clutch.

The pressure switch can be directly integrated inthe line or attached to the fluid container.

Pressure switch F129 is a 3-way combinationswitch for:

safeguarding the cooling air flow (fan circuit) safeguarding the pressure conditions.

The pressure switch operates in the followingconditions:

it switches off the magnetic clutch via the air

conditioner control unit at an excess pressureof approx. 2.4 to 3.2 MPa (24 to 32 bar). Thisexcess pressure can be caused by a dirtycondenser, for example.

it switches off the magnetic clutch via the airconditioner control unit when the pressure

drops below a minimum value (0.2 MPa/ 2bar) . This can be caused by loss ofrefrigerant, for example.

it switches the fan one speed higher at 1.6MPa (16 bar) excess pressure. As a result,condenser performance is optimised.

Pressure switch F129

208_057

p > 3,2 MPa =

p < 0,2 MPa =

p > 1,6 MPa =

208_058

208_060

208_059


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" p

36

A new generation of senders for monitoringthe refrigerant circuit.

Air conditioner pressure switch F129 has beenreplaced by an electronic pressure sensor.The evaluation electronics in the air conditio-ner and engine control units have been adap-ted accordingly.

Like pressure switch F129, the high pressuresender is integrated in the high pressure line.

It registers the refrigerant pressure and convertsthe physical quantity of pressure to an electricalsignal.Unlike the air conditioner pressure switch, thesender registers not only the defined pressurethresholds, it also monitors the refrigerantpressure throughout the working cycle.

These signals indicate the load being exerted onthe engine by the air conditioner and thepressure conditions in the refrigerant circuit. Thenext higher stage of the cooling fan and themagnetic clutch of the compressor are activated

and deactivated via the radiator fan control unit.

If the radiator fan control unit fails to detect anysignals, then the compressor will be switched offfor safety reasons.

The idling speed of the engine can beadapted exactly to the power consumption ofa specific compressor.

The radiator fan speed activation and

deactivation cycles are staggered with a shorttime delay.As a result, changes in the speed of thecooling fan are barely perceptible at idlingspeed. This enhances comfort especially invehicles with engines with low power outputs.

The fault in the high pressure sender is stored tothe fault memory of the ee ee nnnngggg iiiinnnneeee eeee lllleeee ccccttttrrrroooo nnnn iiiiccccssss .

System control

High pressure sender G65

Signal utilisationin the engine control unitin the radiator fan control unit

Substitute function

Advantages

Self-diagnosis fault message

208_062

e.g.: 00819 high pressure sender G65Signal too low


37/37

t" p

B

A2V/Div 5ms/D

0

Test voltage

At low pressure

Function of high pressure sender

Pulse-widthmodulated

signalMicroprocessor

Voltage

The microprocessor of the high pressure sender

outputs a small pulse width at low pressures.

Pulse width signals are generated at a frequencyof 50 Hz per second.This is equivalent to a period durationof 20 ms = 100%.

At a low pressure of 0.14 MPa (1.4 bar), the pulsewidth is 2.6 ms.This is equivalent to 13% of the period duration.

Silicon crystal(resistance)

208_063

208_064

Period duration 20 ms

Pulse width 2.6 ms

Pulse width signal

The refrigerant pressure is applied to a siliconcrystal. Depending on the pressure level, the

crystal will be more or less deformed.

The silicon crystal, together with amicroprocessor, is integrated in the sensor andsupplied with voltage.

One of the properties of the silicon crystal is thatits electrical resistance changes when it isdeformed. Depending on the pressurecharacteristic, a test voltage picked off at thesilicon crystal also changes as a result.

The test voltage is conducted to themicroprocessor and converted to a pulse-widthmodulated signal (A = pulse width, B = signaldistance).

At a low pressure, the crystal undergoes minimaldeformation".The voltage applied is therefore only opposed toa low electrical resistance.

The voltage change is small.

208_109

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