ASHRAE HVAC APPLICATIONS SI CH 10-2015 AUTOMOBILES.pdf

1、10.1CHAPTER 10AUTOMOBILESDesign Factors 10.1Air-Handling Subsystem 10.3Heating Subsystem 10.7Refrigeration Subsystem 10.7HERMAL systems in automobiles (HVAC, engine cooling,Ttransmission, power steering) have significant energy require-ments that can adversely affect vehicle performance. New and in-

2、novative approaches are required to provide the customer thedesired comfort in an energy-efficient way. In recent years, effi-ciency of the thermal systems has increased significantly com-pared to systems used in the early to mid-1990s. Providing thermalcomfort in an energy-efficient way has challen

3、ged the automotiveindustry to search for innovative approaches to thermal manage-ment. Hence, managing flows of heat, refrigerant, coolant, oil, andair is extremely important because it directly affects system perfor-mance under the full range of operating conditions. This createssignificant enginee

4、ring challenges in cabin and underhood thermalmanagement. Optimization of the components and the system isrequired to fully understand the components effects on the system.Thus, modeling the components and the system is essential for per-formance predictions. Simulation of thermal systems is becomin

5、gan essential tool in the development phase of projects. Durabilityand reliability are also important factors in design of these systems.Environmental control in modern automobiles usually consists ofone (or two for large cars, trucks, and sport utility vehicles) in-cabinair-handling unit that perfo

6、rms the following functions: (1) heating,(2) defrosting, (3) ventilation, and (4) cooling and dehumidifying (airconditioning). This unit is accompanied by an underhood vapor cyclecompressor, condenser, and expansion device. The basic system canbe divided into three subsystems: air handling, heating,

7、 and refriger-ation (cooling). All passenger cars sold in the United States mustmeet defroster requirements of the U.S. Department of Transporta-tion (DOT) Federal Motor Vehicle Safety Standard 103 (FMVSS), soventilation systems and heaters are included in the basic vehicledesign. The most common sy

8、stem today integrates the defroster,heater, and ventilation system. In the United States, the vast majorityof vehicles sold today are equipped with air conditioning as originalequipment.1. DESIGN FACTORSGeneral considerations for design include cabin indoor air quality(IAQ) and thermal comfort, ambi

9、ent temperatures and humidity,operational environment of components, airborne contaminants,vehicle and engine concessions, physical parameters, durability,electrical power consumption, cooling capacity, occupants, infiltra-tion, insulation, solar effect, vehicle usage profile, noise, and vibra-tion,

10、 as described in the following sections.Thermal Comfort and Indoor Air Quality (IAQ)ASHRAE Standard 55 provides information on the airflowvelocities and relative humidity required to provide thermal com-fort. Effective comfort cooling system design in cars must create airmovement in the vehicle, to

11、remove heat and occupants body efflu-ents and to control moisture build-up. Assuming an effectivetemperature of 22C with no solar load at 24C, 98% of people arecomfortable with zero air velocity over their body. If the tempera-ture increases to 27C, the same number of people are comfortablewith an a

12、ir velocity of 2.5 m/s. If panel vent outlets can deliver suf-ficient air velocity to the occupants, comfort can be reached at ahigher in-vehicle temperature than with low airflow (Figure 1).Several modeling manikins for predicting human physiologicalbehavior are described in Guan et al. (2003a, 200

13、3b, 2003c), Jones(2002a, 2002b), and Rough et al. (2005).During the increasingly common gridlock or stop-and-go condi-tions, tailpipe emissions can make outdoor air (OA) extremely pol-luted, and it is important to ensure that passengers exposures tothese gases do not exceed American Conference of Go

14、vernmentalIndustrial Hygienists (ACGIH 2010) short- or long-term exposurelimits.Tailpipe emissions include Nitrogen oxides (NOx), which include both nitric oxide (NO) andnitrogen dioxide (NO2), which always occur together (Pearson2001)Carbon monoxide (CO), which forms in the combustion chamberwhen o

15、xygen supply is insufficientHydrocarbons (HCs)Volatile organic compounds (VOCs)Diesel engines emit mainly NOxand HC, and gasoline enginesemit mainly CO and HC. Worldwide, road transportation accountsfor approximately 50% of NOxemissions, and gasoline-poweredvehicles alone account for 32% of HC emiss

16、ions in the United States(Pearson 2001).To limit passengers exposure to tailpipe emissions, the blowerunits air intake door can be switched from outdoor air mode to recir-culation mode during times of traffic congestion and likely poor OAquality (Mathur 2006). Once the vehicle is out of the traffic

17、jam, theThe preparation of this chapter is assigned to TC 9.3, Transportation AirConditioning.Fig. 1 Comfort as Function of Air Velocity(Atkinson 2000. Reprinted with permission fromSAE Paper 2000-01-1273. Copyright 2000 SAE International.)10.2 2015 ASHRAE HandbookHVAC Applications (SI)mode door can

18、 be switched back to outdoor air mode (Mathur2007a).Carbon dioxide (CO2) from passengers exhalations can alsobuild up in the cabin, especially in low-body-leakage vehicles, sothe vehicles AC system should not be operated in recirculationmode for extended periods. This issue becomes critical when sev

19、-eral occupants are in a vehicle that has 100% return air in recircu-lation mode. A timed strategy is recommended for recirculation;after the set time (e.g., 30 min) elapses, the mode automaticallychanges to outdoor to reduce CO2levels in the cabin. A CO2sensorcan be installed to monitor levels in t

20、he cabin, and automaticallyswitch to OA mode when set levels are exceeded (Mathur 2007b,2008, 2009a, 2009b).Relative humidity also affects cabin IAQ. Too high a levelaffects occupant comfort and can lead to condensation and foggingon windows. A relative humidity sensor can detect excessive humid-ity

21、 and intervene.See the section on Controls under Air-Handling Subsystem formore information on cabin IAQ.Cooling Load FactorsOccupancy. Occupancy per unit volume is high in automotiveapplications. The air conditioner (and auxiliary evaporators and sys-tems) must be matched to the intended vehicle oc

22、cupancy. Infiltration. Like buildings, automobiles are not completelysealed: wiring harnesses, fasteners, and many other items must pen-etrate the cabin. Infiltration varies with relative wind/vehicle veloc-ity. Unlike buildings, automobiles are intended to create a relativewind speed, and engines m

23、ay emit gases other than air. Body sealingand body relief vents (also known as the drafter) are part of air-conditioning design for automobiles. Occasionally, sealing beyondthat required for dust, noise, and draft control is required. By design, vehicles are allowed to have controlled body leakageth

24、at allows air movement in the vehicle to provide comfort to the pas-sengers. This also helps control moisture build-up and the occupantsperceived comfort level. However, excessive body leakage results inloss of heating and cooling performance. Vehicle body leakage char-acteristics typically are sign

25、ificantly different in dynamic conditionscompared to static conditions. Air can leak from the vehicles doors,windows, door handles, and trunk seals (uncontrolled exit points);drafters allow a controlled exit for air from the cabin, and should beself-closing to prevent inflow when the body pressure i

26、s negative withrespect to the exterior pressure. According to the Society of Automo-tive Engineers (SAE) Standard J638, infiltration of untreated air intothe passenger compartment through all controlled and uncontrolledexit points should not exceed 0.165 m3/s at a cabin pressure of0.25 kPa (Atkinson

27、 2000). However, each vehicle has different bodyleakage characteristics. Some vehicles have two drafters inside thetrunk on either side, and some have only one.Insulation. Because of cost and mass considerations, insulationis seldom added to reduce thermal load; insulation for sound con-trol is gene

28、rally considered adequate. Additional dashboard andfloor thermal insulation helps reduce cooling load. Some new vehi-cles have insulated HVAC ducts to reduce heat gain during coolingand heat loss during heating. Typical interior maximum tempera-tures are 93C above mufflers and catalytic converters,

29、50C forother floor areas, 63C for dash and toe board, and 43C for sidesand top.Solar Effects. The following four solar effects add to the coolingload:Vertical. Maximum intensity occurs at or near noon. Solar heatgain through all glass surface area normal to the incident light isa substantial fractio

30、n of the cooling load.Horizontal and reflected radiation. Intensity is significantlyless, but the glass area is large enough to merit consideration.Surface heating. Surface temperature is a function of the solarenergy absorbed, the vehicles interior and exterior colors, inte-rior and ambient tempera

31、tures, and the automobiles velocity.Vehicle colors and glazing. The vehicles interior and exteriorcolors, along with the window glazing surfaces (clear or tinted),strongly affect vehicle soak temperature. Breathing-level temper-atures after a 1 h soak can be 22 to 33 K higher than ambient, withinter

32、nal surfaces being 28 to 55 K above ambient (Atkinson2000).Ambient Temperatures and Humidity. Several ambient tem-peratures need to be considered. Heaters are evaluated for perfor-mance at temperatures from 40 to 21C. Air-conditioning systemsare evaluated from 4 to 45C, although ambient temperatures

33、 above52C are occasionally encountered. The load on the air-condition-ing system is also a function of ambient humidity (at most test con-ditions, this latent load is around 30% of the total). Typical designpoints follow the combinations of ambient temperature and humid-ities of higher probability,

34、starting at around 90% rh at 32C andwith decreasing humidity as temperature increases.Because the system is an integral part of the vehicle, the effects ofvehicle-generated local heating must be considered. For interiorcomponents, the design high temperature is usually encountered dur-ing unoccupied

35、 times when the vehicle is soaked in the sun. Interiortemperatures as high as 90C are regularly recorded after soaks in thedesert southwestern United States. Achieving a comfortable interiortemperature after a hot soak is usually one of the design conditionsfor most vehicle manufacturers.Operational

36、 Environment of ComponentsUnderhood components may be exposed to very severe environ-ments. Typical maximum temperatures can reach 120C. The driveto achieve more fuel-efficient automobiles has reduced availablespace under the vehicle hood to a minimum. This crowding exposesmany components to tempera

37、tures approaching that of exhaust sys-tem components. Heat from the vehicle also adds to the coolingloads that the air-conditioning system must handle. During idle,heat convected off the hood can raise the temperature of air enteringthe air inlet plenum by as much as 6 to 14 K (Mathur 2005a). A sim-

38、ilar effect is found during idle when air from the engine compart-ment is reentrained into the air flowing through the condenser(Mathur 2005b). Air temperatures as high as 70C have beenencountered on parts of a vehicles condenser during operation witha tailwind in ambient temperatures as low as 38C.

39、 Typically, frontair management is improved by using air guides and seals to preventair bypassing either the condenser or radiator at idle. Significantimprovements in vent outlet temperatures (a maximum of 4 K andcabin temperatures of 1 to 3 K) and a reduction in head pressures(200 to 530 kPa) have

40、been obtained. Recirculation of hot enginecompartment air was reduced from 29 K over ambient (base case) toapproximately 15 K over ambient. Further details are provided inthe section on Vehicle Front-End Design.Airborne Contaminants and VentilationNormal airborne contaminants include bacteria, pollu

41、tants,vapors from vehicle fluids, and corrosive agents (Mathur 2006).Exposure to these must also be considered when selecting materialsfor seals and heat exchangers. Incorporating particulate and/or car-bon filters to enhance interior air quality (IAQ) is becoming com-mon. Air-handling systems in vi

42、rtually all vehicles can exceed theventilation recommendations for buildings and public transporta-tion in ASHRAE Standard 62.1. However, the driver has completecontrol of the HVAC system in the vehicle, and can reduce cabin air-flow to virtually zero when desired (e.g., before warm-up on colddays).

43、Automobiles 10.3Power Consumption and AvailabilityMany aspects of vehicle performance have a significant effect onvehicular HVAC systems. Modern vehicles have a huge variety ofelectric-powered systems. The need to power these systems whilemaintaining fuel efficiency leads manufacturers to demand a h

44、ighlevel of efficiency in electrical power usage. On some vehicles, elec-trical power use is monitored and reduced during times of minimalavailability. The mass of the HVAC system is also closely controlledto maintain fuel efficiency and for ride or handling characteristics.The power source for the

45、compressor is the vehicles engine. Atengagement, the need to accelerate the rotational mass as well aspump the refrigerant can double the engine torque. This suddensurge must not be perceptible to the driver, and is controlled throughcareful calibration of the engine controls. Automotive compressors

46、must provide the required cooling while compressor speed varieswith the vehicle condition rather than the load requirements. Vehicleengine speeds can vary from 8.3 to 100 rev/s.Physical Parameters, Access, and DurabilityDurability of vehicle systems is extremely important. Hours ofoperation are shor

47、t compared to commercial systems (260 000 km at65 km/h = 4000 h), but the shock, vibration, corrosion, and otherextreme conditions the vehicle receives or produces must not causea malfunction or failure. Automotive systems have some uniquephysical parameters, such as engine motion, proximity to comp

48、o-nents causing adverse environments, and durability requirements,that are different from stationary systems. Relative to the rest of thevehicle, the engine moves both fore and aft because of inertia, andin rotation because of torque; this action is referred to as enginerock. Fore and aft movement m

49、ay be as much as 13 mm; rotationalmovements at the compressor may be more than 19 mm from accel-eration and 13 mm from deceleration when the length to center ofrotation is considered. Additionally, the need for components to sur-vive bumper impacts of up to 8 km/h leads to additional clearanceand strength requirements. Vehicle components may also beexposed to many different types of chemicals, such as road salt, oil,hydraulic fluid (brakes and power steering), and engine coolant.Automobiles also increasingly incorporate electrical and elec-tronic components a