ASHRAE HVAC APPLICATIONS IP 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 71F with no solar load at 75F, 98% of people arecomfortable with zero air velocity over their body. If the tempera-ture increases to 81F, the same number of people are comfortablewith an a

12、ir velocity of 500 fpm. 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 Applicationsmode door can be s

18、witched 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-eral

19、 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 the ca

20、bin, 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 and

21、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 occupan

22、cy. 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 may em

23、it 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 leakagethat al

24、lows air movement in the vehicle to provide comfort to thepassengers. This also helps control moisture build-up and the occu-pants perceived comfort level. However, excessive body leakageresults in loss of heating and cooling performance. Vehicle bodyleakage characteristics typically are significant

25、ly different in dy-namic conditions compared to static conditions. Air can leak fromthe vehicles doors, windows, door handles, and trunk seals (uncon-trolled exit points); drafters allow a controlled exit for air from thecabin, and should be self-closing to prevent inflow when the bodypressure is ne

26、gative with respect to the exterior pressure. Accordingto the Society of Automotive Engineers (SAE) Standard J638, infil-tration of untreated air into the passenger compartment through allcontrolled and uncontrolled exit points should not exceed 350 cfmat a cabin pressure of 1 in. of water (Atkinson

27、 2000). However, eachvehicle has different body leakage characteristics. Some vehicleshave two drafters inside the trunk on either side, and some have onlyone.Insulation. Because of cost and weight considerations, insula-tion is seldom added to reduce thermal load; insulation for soundcontrol is gen

28、erally 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 200F above mufflers and catalytic converters

29、, 120F forother floor areas, 145F for dash and toe board, and 110F 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 fr

30、action 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 te

31、mperatures, 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 40 to 60F higher than ambient, withi

32、nternal surfaces being 50 to 100F 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 70F. Air-conditioning systemsare evaluated from 40 to 110F, although ambient temper

33、aturesabove 125F are occasionally encountered. The load on the air-con-ditioning system is also a function of ambient humidity (at most testconditions, this latent load is around 30% of the total). Typicaldesign points follow the combinations of ambient temperature andhumidities of higher probabilit

34、y, starting at around 90% rh at 90Fand with 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 unoccup

35、ied times when the vehicle is soaked in the sun. Interiortemperatures as high as 190F are regularly recorded after soaks inthe desert southwestern United States. Achieving a comfortable inte-rior temperature after a hot soak is usually one of the design condi-tions for most vehicle manufacturers.Ope

36、rational Environment of ComponentsUnderhood components may be exposed to very severe environ-ments. Typical maximum temperatures can reach 250F. The driveto achieve more fuel-efficient automobiles has reduced availablespace under the vehicle hood to a minimum. This crowding exposesmany components to

37、 temperatures 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 10 to 25F (Mathur 2005a)

38、. Asimilar effect is found during idle when air from the engine com-partment is reentrained into the air flowing through the condenser(Mathur 2005b). Air temperatures as high as 160Fhave beenencountered on parts of a vehicles condenser during operation witha tailwind in ambient temperatures as low a

39、s 100F. 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 7F andcabin temperatures of 2 to 6F) and a reduction in head pressures(30 to 77 psi) ha

40、ve been obtained. Recirculation of hot engine com-partment air was reduced from 52.2F over ambient (base case) toapproximately 27F over ambient. Further details are provided inthe section on Vehicle Front-End Design.Airborne Contaminants and VentilationNormal airborne contaminants include bacteria,

41、pollutants,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

42、in virtually 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 coldd

43、ays).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 deman

44、d a highlevel 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

45、 the 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 compre

46、ssorsmust provide the required cooling while compressor speed varieswith the vehicle condition rather than the load requirements. Vehicleengine speeds can vary from 500 to 8000 rpm.Physical Parameters, Access, and DurabilityDurability of vehicle systems is extremely important. Hours ofoperation are

47、short compared to commercial systems (160,000miles at 40 mph = 4000 h), but the shock, vibration, corrosion, andother extreme conditions the vehicle receives or produces must notcause a malfunction or failure. Automotive systems have someunique physical parameters, such as engine motion, proximity t

48、ocomponents causing adverse environments, and durability require-ments, that are different from stationary systems. Relative to the restof the vehicle, the engine moves both fore and aft because of inertia,and in rotation because of torque; this action is referred to as enginerock. Fore and aft move

49、ment may be as much as 0.5 in.; rotationalmovements at the compressor may be more than 0.75 in. from accel-eration and 0.5 in. from deceleration when the length to center ofrotation is considered. Additionally, the need for components to sur-vive bumper impacts of up to 5 mph 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-