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ssp208-Air Conditioner in the Motor Vehicle 空调-1.pdf

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SSP208 AIR CONDITIONER IN THE MOTOR VEHICLE 空调
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1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0 -10 -20 -30 10 20 40 50 60 70 0 30 MPa C oTemperature PressureAir Conditioner in the Motor VehicleFundamentalsSelf-Study Programme 208Service.208 ?2 air air condition conditionPlease always refer to the relevant Service Literature for all inspection, adjustment and repair instructions. Service literature.The Self-Study Programme ?is not a Workshop Manual!New Important NoteAir conditioning systems have long ceased to be ?regarded as luxury equipment. Air conditioners have become a factor in active ?safety, and today can almost be considered as ?an integral part of a vehicle’s safety ?specification. 10 years ago, only about 10 percent of all newly registered vehicles were fitted with an ?air conditioning system. By 1996, air conditioners ?were being installed as standard in more than ?one in four newly registered vehicles. Customer demand for air conditioning is rising ?continually. The design of the refrigerant circuit of an air conditioner is identical in all vehicles. Air conditioner refrigerant circuits only vary in ?respect of how they are adapted to meet ?refrigeration requirements. In this Self-Study Programme, you will familiarise ?yourself with the basic purpose and design of an ?air conditioner. Y ou will learn the functions of the component ?parts in the refrigeration process, the special ?characteristics of the refrigerant and why air ?conditioners require special service ?specifications. The component parts shown in the following SSP ?are common to most air conditioners.Please note that the figures specified are given by way of example only. Depending on refrigeration requirements, the ?absolute values vary from vehicle to vehicle. ?3 ?MPa o C ?R 134a ? t pTable of ContentsThe in-car climate ?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Why air conditioning?The physics of the cooling system ?. . . . . . . . . . . . . . . . . . . . . . . . 6Applied physicsThe refrigerant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 The cooling system ?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12The principle of the refrigerant circuit Refrigerant circuit with expansion valve The compressor The mode of operation of the compressor Magnetic clutch The condenser The fluid container and drier Expansion valve Expansion valve — new generation The evaporator Refrigerant circuit with restrictor The restrictor The collecting tankSystem control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ?32Components of the safety systemCooling fan circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ?40Fan circuit for engine/condenser cooling Radiator fan control unit J293Temperature control ?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ?42Manual control Automatic control System overview Control unit with operating and display unit The main temperature sensors Auxiliary signals for temperature control Positioning motor Air ducting Air distribution Air recirculation modeTechnical service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ?64Safety precautions General information on function influencing factors Fault diagnosis through pressure testing Fault diagnosis through self-diagnosisInformation ?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72Key cooling system terminology ?4The in-car climateWhy air conditioning?People feel comfortable at a certain ambient ?temperature and atmospheric humidity. As a component part of active safety, the driver’s ?well-being is a key factor in driving ability. The in-car climate has a direct bearing on the ?driver, fatigue-free driving and driving safety. A comfortable interior temperature is dependent ?upon the prevailing ambient temperature and ?upon sufficient air flow: Low ambient temperature, e.g. —20 oCHigher 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 systems ?have difficulty maintaining a pleasant climate ?inside a vehicle at high ambient temperatures. ?Why?— In strong sunlight in particular, the heated ?cabin air can only be exchanged for air with ?ambient temperature. — In addition, the air temperature usually rises ?en route from the intake point to the air outlet. — Opening a window or sliding roof or setting a ?higher fan speed for greater comfort will ?usually result in a draught and expose the ?occupants to other nuisances such as noise, ?exhaust gases and pollen. High levels of atmospheric humidity put the body ?under considerably greater physical strain.208_001 ? 23 ?C 42 ?C 40 ?C 28 ?C 35 ?C 24 ?CTemperatures in a mid-range passenger car where: driving time 1 h ambient temperature 30oC sunlight penetration into car Area with 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/min208_043Ambient temperature Interior temperature Air flow rate Comfort curves ?5 A B CEffects of an unfavourable vehicle interior ?temperature on humansScientific studies conducted by the WHO (World ?Health Organization) have shown that one’s ?ability to concentrate and reactions are impaired ?when under stress. ?Heat puts a strain on the body. ?The best temperature for the driver is between ?20 and 22 oC. This is equivalent to climatic load A, the “comfort ?range“. Strong sunlight can increase the interior ?temperature by more than 15 oC above the ?ambient temperature— particularly in the head ?area. This is where the effects of heat are most ?dangerous. 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 strain ?on the body. Physicians specialising in traffic-related illnesses ?refer to this condition as climatic stress. Studies have shown that an increase in ?temperature from 25 to 35 oC reduces one’s ?sensory perception and powers of reasoning by ?20%. It has been estimated that this figure is ?equivalent to a blood alcohol concentration of ?0.5 millilitres alcohol level. The air conditioner - a system which keeps the air ?temperature at a level comfortable to humans, ?as well as purifying and dehumidifying the air -— ?was created in order to reduce or eliminate ?completely such stress. With the help of an air conditioner it is possible ?to produce at the air outlets a temperature which ?is much lower than high outside air ?temperatures. ?This is possible both when the vehicle is at a ?standstill and when it is in operation. A technical side-effect of air conditioning is that ?the air is dehumidified and cleaned at the same ?time. However, this is just as important as the ?reduction in temperature. The pollen filter and activated charcoal filter also ?help to clean the air entering the vehicle. People with allergic illnesses benefit greatly from ?being able to breathe clean air.208_042Strain Transpiration Heart rate Body temperature Climatic load high moderate lowIn-vehicle air conditioning is - a real safety element - a functional accessory not only for expensive tastesComfort range MPa o C6Physics of the cooling systemApplied physicsLawsMany substances are known to exist in three ?aggregate states. Take water for example: solid — liquid — vapour. The principle of cooling follows this law. Even in ancient times there was a need for ?cooling. 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, thereby ?cooling them down. The ice melts as a result, assuming another ?aggregate state, namely that of a liquid (water). If the water is heated further, it will boil and ?evaporate. ?The water is now in the gaseous state. The gaseous substance can be converted back to ?a liquid by cooling it and will become a solid ?again if cooled further. ?This principle is applicable to almost all ?substances: — 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 substance ?to the colder substance. Air conditioners utilise the effects of heat ?exchange, a process in which a substance ?changes state at certain points. AES208_039 208_040 208_041Law Freezing point, Boiling pointIce — solid Ice — becomes a liquid when it absorbs ?heat Water — becomes a gas when it absorbs heat e.g. water becomes ice, e.g. water becomes steam ?7 ?MPa o CP P P P r r r r e e e e s s s s s s s s u u u u r r r r e e e e ? ?a a a a n n n n d d d d ? ?b b b b o o o o ii i i ll l l ii i i n n n n g g g g ? ?p p p p o o o o ii i i n n n n t t t tIf the pressure is changed using a liquid, the ?boiling point changes. ?All liquids behave similarly. Boiling point H2O/water = 100 oC Machine oil = 380 - 400 oC The lower the pressure, the lower the ?temperature at which water boils (evaporates). The evaporation process is also used in vehicle ?air conditioners. A substance with a low boiling point is used for ?this 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 the ?table always refer to an atmospheric pressure of ?0.1 MPa = 1 bar.) MPa 100 0,1 0,3 0,5 0,7 0,9 1,1 1,3 1,5 1,7 ?C 110 120 130 140 150 160 170 180 190 200 H 2 O 1 3 5 7 9 11 13 15 17 bar MPa bar -40 -20 0 20 40 60 80 100 0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 ?C R134a R12 0 5 10 15 20 25 30 35 40What does a vapour pressure curve tell us?We can draw the following conclusions from the vapour ?pressure curve for the two refrigerants R134a and R12 ?(R12 is no longer used) and water: — ? At a constant pressure, the vapours become a liquid ?through ?temperature reduction (in the air conditioner ?circuit, this process takes place in the condenser = ?liquefier), — ? The refrigerant goes from a liquid state to a vapour ?state through pressure reduction (in the air conditioner ?circuit, this process takes place in the evaporator).208_006 208_005Vapour pressure curve Vapour pressure curve Liquid Gaseous Liquid Gaseous Temperature Temperature Pressure Pressure R 134a8 ?0 -10 -20 -30 10 20 40 50 60 70 C o 30 bar 16 14 12 10 8 6 4 2 0Expansion Evaporation Temperature Gaseous PressureThe refrigerantRefrigerant R R R R1 1 1 13 3 3 34 4 4 4a a a a— Tetrafluorethane chem. formula CH2F-CF3 a fluorocarbon (F F F FC C C C) environmentally friendly! The refrigerant with a low boiling point used for vehicle air conditioners is a gas. ?As a gas, it is invisible. As a vapour and as a liquid, it is colourless like ?water. Refrigerants may not be combined with each ?other. Only the refrigerant specified for the ?system in question may be used.Ordinance banning halogens R134aVapour pressure curve of R134aLiquid208_050With regard to vehicle air conditioners, the sale ?and filling of refrigerant R12 were banned in ?Germany with effect from 1995 and July 1998 ?respectively. In current automotive air conditioners, only ?refrigerant R134a is used. — R134a — a fluorocarbon contains no chlorine ?atoms - unlike refrigerant R12 - which cause ?depletion of the ozone layer in the earth’s ?atmosphere when they split. — The vapour pressure curves of R134a and R12 ?are very similar. R134a has the same refrigeration capacity as ?R12. It is possible to adapt air conditioners which ?now 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 refrigeration ?capacity. Depending on the pressure and temperature conditions in the refrigerant circuit, the ?refrigerant will either be a gas or a liquid. Refrigerant R R R R1 1 1 12 2 2 2— Dichlordifluormethane chem. formula CCl2F2a chlorinated hydrocarbon (C C C CF F F FC C C C) harmful to the environment! ?9 ?R 134a ?60 60 200 240 280 320 360 400 440 0,3 0,4 0,6 0,8 0,2 1,0 1,6 2,0 4,0 10 20 30 40 50 70 80 90 10 20 30 40 50 70 80 85 90 D C A MPa kJ/kg 0 C 0 C B 3 4 6 8 2 10 16 20 40 barState of refrigerant R134a ?in the cycle in an air conditionerIn addition to the vapour pressure curve, the ?cycle shows the change of state of the refrigerant ?under pressure and temperature in addition to ?the energy balance at which the refrigerant ?returns to its original state. The diagram is an excerpt from the state ?diagram of refrigerant R134a for a vehicle air ?conditioner. ?Different absolute values are possible in ?dependence upon the demand of a vehicle type ?for refrigeration capacity. The energy content is a key factor in the design ?of an air conditioner. It shows what energy is required (evaporator ?heat, condenser heat) to achieve the intended ?refrigeration capacity.Physical data of R134a:Boiling point: —26.5 ?C Freezing point: —101.6 ?C Critical temperature: 100.6 ?C Critical 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 B208_053Energy content Pressure Temperature curve ?Saturated liquid Temperature curve ?Saturated vapour Critical point (pressure/temperature) Pressure For a glossary refer to page 72. R 134a10 ?0 0 1 3 2 1 4 FKW R134a FCKW R12Refrigerants and ozone layerOzone protects the earth’s surface against ?UV radiation by absorbing a large proportion of ?these rays. UV rays split ozone (O3) into an oxygen ?molecule (O2) and in an oxygen atom (O). Oxygen atoms and oxygen molecules from other ?reactions combine again to form ozone. This process takes place in the ozonosphere, a ?part of the stratosphere at an altitude of between ?20 and 50 km. Like R12, chlorine (Cl) is a constituent of a CFC ?refrigerant . If handled improperly, the R12 molecule will rise ?up to the ozone layer— since it is lighter than air—. UV radiation liberates a chlorine atom in the ?CFC, and this atom reacts with the ozone. In the process, the ozone decomposes leaving an ?oxygen molecule (O2) and chlorine monoxide ?(ClO), which then reacts again with oxygen and ?liberates chlorine (Cl). This cycle can repeat itself ?as many as 100,000 times.However, free oxygen molecules (O2) cannot ?absorb UV radiation. Refrigerants and global warmingThe sunlight imping

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