Analysis and simulation of mobile air conditioning systemcoupled with engine cooling systemZhao-gang Qi *,Jiang-ping Chen,Zhi-jiu ChenInstitute of Refrigeration and Cryogenics,School of Mechanical Engineering,Shanghai Jiao Tong University,No.1954,Huashan Road,Shanghai 200030,PR ChinaReceived 19September 2005;received in revised form 28March 2006;accepted 8October 2006Available online 6December 2006AbstractMany components of the mobile air conditioning system and engine cooling system are closely interrelated and make up the vehicle climate control system.In the present paper,a vehicle climate control system model including air conditioning system and engine cooling system has been proposed under different operational conditions.All the components have been modeled on the basis of experimental data.Based on the commercial software,a computer simulation procedure of the vehicle climate control system has been developed.The performance of the vehicle climate control system is simulated,and the calculational data have good agreement with experimental data.Furthermore,the vehicle climate control simulation results have been compared with an individual air conditioning system and engine cooling system.The influences between the mobile air conditioning system and the engine cooling system are discussed.Ó2006Elsevier Ltd.All rights reserved.Keywords:Air conditioning system;Engine cooling system;Coupled analysis;Simulation;Comparison1.IntroductionA mobile air conditioning (MAC)system can supply drivers and passengers a safe and comfortable environ-ment.Perfect performance of the MAC is the target that automobile manufacturers pursue in the period of design and development.It is known very well that MAC can sup-ply cold capacity under summer operational conditions and waste heat of the engine is used to heat the passenger com-partment under winter operational conditions.For envi-ronmental factors,researches have been performed extensively to develop and improve the efficiencies of MAC and engine cooling systems.Heat exchangers are the research emphasis of MAC and engine cooling systems.A lot of correlations,experiments and models about vari-ous heat exchangers have been proposed.Chang and Wang [1,2]and Chang et al.[3]developed thermal characteristics correlations related to the geometrical parameters of heatexchangers with louvered fins.Their correlations have good agreement with their and previous experimental data in a wide range of Reynolds numbers based on louver pitch.Nowadays,many advanced technologies have been applied to enhance the performance of the heat exchangers of MAC and engine cooling systems.For engineers and researchers,the simulation procedure [4]of MAC and engine cooling systems can save test cost and manpower considerably.Raman Ali [5]developed a computer pro-gram for the MAC refrigerant circuit.The MAC included a condenser and an evaporator cooled by fans,a fixed power reciprocating compressor and a thermostatic expan-sion valve.The heat transfer processes of the condenser and evaporator were divided into three parts as liquid,two phase and gas phase.All the nonlinear algebraic equa-tions were solved by iterative procedures.Saiz Jabardo et al.[6]proposed a steady computer program for an auto-mobile air conditioning system.The authors implied that operational parameters such as compressor speed,return air temperature in the evaporator and condensing air tem-peratures have an obvious effect on the performance of a0196-8904/$-see front matter Ó2006Elsevier Ltd.All rights reserved.doi:10.1016/j.enconman.2006.10.005*Corresponding author.Tel.:+862162933242;fax:+862162632601.E-mail address:qizhaogang@ (Z.-g.Qi)./locate/enconmanEnergy Conversion and Management 48(2007)1176–1184MAC system.Calculational results deviated from the experimentally obtained results within a20%range,though most of them were within a10%range.Lee and Yoo[7] analyzed all the components under various operational conditions and proposed a MAC system model,which sim-ulated the performance of the integrated automobile air conditioning system very well.The component models were dependent on empirical correlations and previous proce-dures.For the engine cooling system,few literatures have been published because of commercial secrets.Bi et al.[8] developed a simulation model of the cooling airflow for armored vehicle engines based on one dimensional tran-sient compressibleflow equations.Many influential factors had been taken into account in the model.In most litera-tures,the MAC and engine cooling systems are studied individually.In fact,many components of the MAC and engine cooling systems are closely interrelated to each other.With any change of vehicle speed,for example,the airflow through the condenser changes,affecting the whole air conditioning system including the vehicle compartment and engine cooling system.To avoid the above mentioned problems,an integrated system of mobile air conditioning system coupled with the engine cooling system,which is called the vehicle cli-mate control system,would be necessary.The targets of this study are to numerically analyze the influences of the engine cooling system on the mobile air conditioning sys-tem and to determine the main operational parameters affecting the vehicle climate control system performance. In the present paper,all the components of the vehicle cli-mate control system are analyzed based on the previous correlations and experimental data.A computer program consisting of the MAC and engine cooling systems is devel-oped to simulate the performance of the vehicle climate control system.2.Analysis of vehicle climate control systemIn the present study,a simulation model of the vehicle climate control system is to be constructed,which consists of a mobile air conditioning system and an engine cooling system.The mobile air conditioning system is mainly com-posed of a laminated evaporator,a parallelflow condenser, afixed displacement reciprocating compressor and an externally equalized thermostatic expansion valve.The engine cooling system is mainly composed of an engine,a serpentine type radiator and a tube in tube oil cooler,as schematically shown in Fig.1.In this vehicle climate con-NomenclatureA area(m2)c1,c2constant(dimensionless)c p specific heat(JkgÀ1KÀ1)f Fanning friction factor,dimensionlessf1,f2,f3correlation defined as Reference[3]F dflow depth(m)h specific enthalpy(J kgÀ1)D h specific enthalpy difference(J kgÀ1)j Colburn factor,dimensionlessL l louver length(m)L p louver pitch(m)g efficiency,dimensionlessd thickness(m)h louver angle,degreee heat exchanger effectiveness,dimensionless Subscriptsad adiabaticair air sidecom compressorcoolant coolant sidedis discharge of compressorffinficfictitious property of saturated air calculated at refrigerant’s temperature_m massflow rate(kg sÀ1)NTU number of transfer units,dimensionlessN compressor speed(rpm)P power(W)D P pressure drop(Pa)Q heat transfer rate(W)Re Lp Reynolds number based on louver pitchT temperature(K)D T temperature difference(K)T d tube width(m)T p tube pitch(m)U overall heat transfer coefficient(W mÀ2kÀ1) fri frictionalgrav gravitationalin inlet/insidelocal locallm mean logarithmic methodmax maximumme mechanicalmin minimummom momentumout outlet/outsideref refrigerant sidesuc suction of compressortot totalV compressor displacement(m3)v volumetricwet wet conditionGreek lettersq density(kg mÀ3)Z.-g.Qi et al./Energy Conversion and Management48(2007)1176–11841177trol system,the operational parameters that can affect the system performance are vehicle speed,air temperature and velocity at the inlet of the condenser and the air tem-perature,humidity and volumetric flow at the inlet of the evaporator.In order to reduce the complexity of the simulation models,the commercial software named KULI is applied to help the authors simulate the vehicle climate control sys-tem.All the models of the components in the vehicle cli-mate control system are proposed based on enormous experimental data.2.1.Heat exchanger model of MAC systemFor calculation of the heat transfer and pressure drop,the evaporator is divided into discrete area elements with their corresponding air and refrigerant mass flows [9,13,14].The changes of the variables of each element can be described by discrete differential equations.The equations are available from two energy balances (air side and refrigerant side).For the dry heat transfer rate of the air side,it is possible to use the following expression [10]:q tot ¼U ÁA ÁD T lm ð1Þwhere D T lm ¼T air ;in ÀT air ;outln T ref ÀT air ;outref air ;inð2Þand U is the global heat transfer coefficient evaluated at the mean properties of the element,which incorporates the influences of the heat transfer coefficients of the air side and refrigerant side and the tube and fin thermal resistances.The air side heat transfer and friction characteristics can be characterized by the j and f factors of a heat exchanger,respectively.The following correlation wassuggested by Chang and Wang [2]and Chang et al.[3]to obtain the air side heat transfer coefficient through the louver fin:j ¼1:18Re À0:505Lph 90 0:26F p L p À0:51T d L p À0:26L l L p0:82ÂT p L p À0:25d f L p À0:097ð3Þf ¼f 1Ãf 2Ãf 3ð4ÞThe correlations of the heat transfer coefficient of the refrigerant side are derived from Refs.[11,12].Because the air is cooled down in the evaporator and condensation can occur,it is necessary that a mass exchange should also be considered.It is based on an anal-ogy of the expression used in dry coils,while for wet coils,the mean logarithmic enthalpy difference is used instead.The heat transfer rate of a wet coil discrete element is given as follows:q wet ;tot ¼_m air ÁD h lm ð5Þwhere D h lm ¼h air ;in Àh air ;out ln h ref ;fic Àh air ;outref ;fic air ;inð6ÞThe pressure drop of the refrigerant side can be expressed in the following equation:D P tot ¼D P fri þD P mom þD P grav þD P localð7Þwhere D P grav =0because the refrigerant flow direction is horizontal.The detailed correlations of D P fri ,D P mom ,and D P local are derived from Ref.[13].The correlations above are the basis of the simulation,and the calculational data will be corrected according to the specific type and geometry of the heat exchanger and the experimentaldata.Fig.1.Mobile air conditioning system coupled with engine cooling system.1178Z.-g.Qi et al./Energy Conversion and Management 48(2007)1176–1184The mathematical model for the condenser is very simi-lar to that of the evaporator.The functions and techniques used in the evaporator are adapted to the condenser.How-ever,the formulas for the inner heat transfer coefficients are quite different.pressor modelThe compressor model is essentially constructed by the characteristics curves.A lot of volumetric efficiency and isentropic efficiency curves were obtained from experi-ments.The massflow rate of refrigerant in the compression process is derived from the following equation:_m ref¼g vÁq sucÁnÁV disð8Þwhere g v is obtained from the characteristic curves of vol-umetric efficiency.The compressor power is obtained fromP com¼_m refÁðh dis;adÀh sucÞgmeð9ÞTo simplify the compressor model,the mechanical effi-ciency of the compressor is described as the following equa-tion based on the tested compressor:gme¼c1þc2lnð_m=_m maxÞð10Þwhere c1=0.8728and c2=0.1777.2.3.Expansion device modelFig.2shows the characteristic curve of the expansion valve.During normal operational conditions,the exter-nally equalized thermostatic expansion valve keeps the superheating degree at the exit of the evaporatorfixed at 5°C.On the basis of an adiabatic throttling,it follows that no change of the enthalpy of the refrigerant takes place in the expansion valve.The massflow equation is drawn from the Bernoulli equation and the adiabatic assumption as follows:_m ref¼A minÁffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2q minðp inÀp outÞpð11ÞThe analysis algorithms provide at any time that the characteristic curve for the superheat temperature is observed.The pressure drop of the expansion valve is not specially modeled.The pressure drop of the expansion valve results from the equalization process of the MAC circuit.2.4.Engine modelThe engine represents the most important heat source of a vehicle,and therefore,it is essential that the model of the engine should predict the heatflows precisely.The com-plexity of the combustion process as an initial heat source, the heatflows in the engine structure and the heat transfers tofluids and the surrounding air make the set up of this kind of model a very difficult task.In this study,the engine simulation model contains all relevant heat sources and heat transfer areas to calculate the heat impact on the coolant circuit,the oil circuit, and the engine structure(Fig.3)[14].The model contains three heat sources,one for the heatflow related to the coolant circuit,one for the heatflow to the oil circuit and one for the friction.The measurement of the heat flow in the cabin heater can be used to adjust the heat flow to the coolant because the heater will represent the main heat sink as long as the thermostat directs noflow to the radiator.The heatflow of the engine to the cool-ant,depending on engine speed and load,can be inte-grated in the simulation model using a heatflow map based on measured data.Similarly,utilizing the heat transfer data in the engine oil cooler,the heatflow to the oil circuit can be determined.The oil temperature and the oil viscosity are important for calculation of the friction loss.The difference in the heating behavior of the oil and the coolant leads to a var-iable heatflow from the oil to thecoolant.Fig.2.Characteristic curve of externally equalized thermostatic expansion valve.Z.-g.Qi et al./Energy Conversion and Management48(2007)1176–118411792.5.Heat exchangers of engine cooling systemThere is no phase change happening in the heat exchangers used in the engine cooling system.Pressure drops in the radiator and oil cooler are negligible.The models of the two heat exchangers are described using the e-NTU method[15].The heat transfer performance of the heat exchangers is given by the following equation:q¼ð_mc pÞminÁðT coolant;inÀT air;inÞð12Þ2.6.Other modelsOther additional components in the MAC and engine cooling systems are modeled based on the experimental data of the KULI models database[16,17].The effects of connective tubes and receiver dryer in the vehicle climate control system are considered negligible.2.7.Uncertainty analysisThe system model is composed of a number of compo-nents,each of which is modeled on the basis of much experimental data.Each component is modeled individu-ally and compared with the experimental data.These comparable data derive from steady state experiments. The inaccuracy of the component models is shown in Table1.Each component model brings an inaccuracy into the system model.The uncertainty of the predictions of the complete model is estimated by the method sug-gested by Moffat[18].The average uncertainties of the evaporator capacity of the MAC and the coolant temper-ature at the exit of the radiator are6.5%and7.8%at a steady condition,respectively. Fig.3.Engine simulation model.Table1The average inaccuracy of each component at steady condition Components InaccuracyEvaporator 5.2%(evaporator capacity),6.8%(pressure drop) Condenser 4.8%(condenser capacity),7.6%(pressure drop) Compressor7.2%(discharge pressure),10.2%(power) Expansion device 2.0%(quality at the exit of expansion device) Engine9.8%(combustion heat),torque(12.6%) Radiator 5.4%(heat output)Oil cooler9.0%(heat output)Fans 3.8%(volumeflow rate ofair)Fig.4.Schematic diagram of environmental simulation equipment of MAC.Table2Some main components’geometriesComponents Style GeometryEvaporator Laminated125mm·250mm·90mm Condenser Parallelflow600mm·455mm·20mm Compressor Fix displacement Displacement:120(cm3) ExpansionvalveThermostatic expansionvalveSaginomiya1.5ton ofrefrigerationEngine Displacement:1.8L Radiator Serpentine600mm·400mm·30mm Oil cooler Tube in tube60mm·80mm·120mm1180Z.-g.Qi et al./Energy Conversion and Management48(2007)1176–11843.Simulation results and discussion 3.1.Experimental validationIn order to validate the accuracy of the vehicle climate control system simulation models,validation experiments were conducted.Fig.4shows the Environmental Simula-tion Laboratory schematic diagram of the validation exper-iments.Some main components’geometries are shown in Table 2.The amount of superheat at the exit of the evapo-rator is set to 5°C,and the subcooling at the exit of the condenser is fixed at 5°C.The experiments were organized according to the automobile industry standard [19].Fig.5shows the simulation and the experimental condition.The precision of the measured parameters is shown in Table 3.The refrigerant used in the experiments is R-134a.In order for the proposed system to be satisfactory,the capacity to cool the passenger compartment must be offered.In general,the direct measurement of evaporator capacity is very complicated.We often calculate the evap-orator capacity using experimental data.Some errors will be brought into the results.In engineering,the cooling curve is usually used to indicate the satisfactoriness of the system.Fig.6shows the calculational results of the cooling curve compared with the experimental data.During the first 15min,the simulation data have good agreement with the experimental data.After that time,the experimental data is about 2–3°C higher than the calculational data because the ornaments in the passenger compartment have a large heat capacity,which is not reflected in the simula-tion models,and the temperature change will usually respond in a few minutes.It is well established that this dif-ference of experimental and calculational data is acceptable in engineering.The compassion between simulation results and experi-mental data of the coolant temperature at the exit of the radiator is shown in Fig.7.It shows that during all the sim-ulation and experimental conditions,the two results are mostly coincident in a wide test range,and the maximum error is about 5%.The two figures show that the total sim-ulation model of the vehicle climate control system is avail-able in performance analysis,and the calculational results have an adequate accuracy.3.2.Effects of engine cooling system on MACThe simulation results of the individual MAC and MAC coupled with engine cooling system were compared.Fig.8describes the variability of the evaporator capacity in the two different systems.It shows that the evaporator capacity of the vehicle climate control system is lower than that of the individual MAC system because heat from theengineFig.5.Simulation and experimental conditions.Table 3The precision of experimental parameters Items Scale Precision Vehicle speed 0–200(km h À1)±0.1(km h À1)Environmental temperature À30°C to 60°C ±1(°C)Relative humidity 15–95%±5%Sunlight power0–1100(W m À2)The fluctuation of the temperatureon the top of vehicle is ±3(°C)Air velocity0–140(km h À1)±0.5(km h À1)Thermocouple (K type)±0.1(°C)Pressure transducer0–18(bar)±0.1(bar)parison of cooling curve vs.test time.Z.-g.Qi et al./Energy Conversion and Management 48(2007)1176–11841181compartment is conducted to the passenger compartment through the vehicle body,which increases the heat duty.Fig.9shows the variability of power consumption of the compressor in the two different systems.It presents that compressor power of the vehicle climate control system is greater than that in the individual MAC during the whole simulation period.It is considered that the engine opera-tional status in the engine cooling system influences the compressor operation via the viscous clutch.These phe-nomena are particularly obvious during the low speed,gra-dient and idle status.Fig.10shows the effect of the engine cooling system on the coefficient of performance (COP)of the mobile air con-ditioning system.The engine cooling system results in the COP of the mobile air conditioning system being decreasedclearly during the entire simulation time,especially during the low vehicle speed and idle status.The maximum decrease of COP is up to 10%.It is considered to be due to the decrease of evaporator capacity and the increase of the compressor work simultaneously during a wide opera-tional conditioning range.The effect of vehicle speed on the performance of the mobile air conditioning system is shown Fig.11.When the vehicle speed is changed from 20km/h to 40km/h,the cooling capacity increases up to 13%,and the compres-sor power increases up to 23%at the same time,but the COP of the MAC decreases sharply.It is considered to be due to the fact that the rate of increase in the compres-sor power becomes larger than the rate of increase in the evaporator capacity.The evaporator capacity will keep steady as the vehicle speed is higher than 40km/h.Inotherparison of COP (individual MAC and MAC coupled with engine coolingsystem).parison of coolant temperature at the exit of radiator vs.testtime.parison of evaporator capacity (individual MAC and MAC coupled with engine coolingsystem).parison of compressor power (individual MAC and MAC coupled with engine cooling system).1182Z.-g.Qi et al./Energy Conversion and Management 48(2007)1176–1184words,the mobile air conditioning system can maintain a better cooling performance in a wide vehicle speed range.3.3.Effect of MAC on the engine cooling systemFig.12shows the comparison of the heat output of the radiator between the vehicle climate control system and the individual MAC.The heat output of the radiator is higher than that of the individual MAC system throughout the whole simulation time.It is the result of the exit air temper-ature of the condenser being higher than the environmental air temperature,which decreases the temperature differenceof heat transfer on the air side of the radiator.The maxi-mum difference in heat output of the radiator between the two different systems is about 3kW.Fig.13shows the effect of air temperature at the exit of the condenser on the heat output of the radiator at the vehicle speed of 120km/h.It shows that the heat output of the radiator decreases with the increase of the exit air temperature of the condenser.At the worst condition,the heat output of the radiator decreases about 1.5kW.Fig.14shows the effect of air temperature at the exit of the condenser on the temperatures of the coolant,oilFig.11.Effect of vehicle speed on the performance of air conditioningsystem.parison of heat output of radiator (individual MAC and MAC coupled with coolingsystem).Fig.13.Effect of air temperature at the exit of condenser on the heat output of radiator.Z.-g.Qi et al./Energy Conversion and Management 48(2007)1176–11841183and air in the engine cooling system at the vehicle speed of 120km/h.Most temperatures in the engine cooling system will increase with the increase of air temperature at the exit of the condenser.The coolant temperature at the exit of the radiator reaches 102°C when the air tem-perature at the exit of the condenser is higher than the environmental temperature up to 11–14°C,which exceeds the temperature within which the engine can work normally.This condition should be avoided in the design period and improvement process of the mobile air conditioning system and engine cooling system.4.ConclusionsBased on commercial software,a simulation model of the mobile air conditioning system coupled with the engine cooling system is developed.The vehicle climate control system mainly contains a laminated evaporator,a parallel flow condenser,a fixed displacement reciprocat-ing compressor,an externally equalized thermostatic expansion valve,an engine,a serpentine type radiator and a tube in tube oil cooler.The models of the compo-nents are based on great amounts of experimental data.Then,validation experiments are performed at the envi-ronmental simulation laboratory,and the experimental results are compared with the simulation results.The comparative results show that the simulation model of the vehicle climate control system is available in engineer-ing and has a good accuracy.The following conclusions are drawn from the perfor-mance simulation and analysis of the vehicle climate con-trol system:1.The engine cooling system affects the performance of the mobile air conditioning system considerably.The simu-lation results show the engine cooling system results in the COP of the mobile air conditioning system decreas-ing clearly during the entire simulation time,especially during the low vehicle speed and idle status.The maxi-mum decrease of COP is up to 10%.2.Changes of heat duty of the mobile air conditioning sys-tem result in high air temperature at the exit of the con-denser,reducing the driving potential for heat transfer from the coolant to air,which induces the heat output of the radiator to decrease sharply.The maximum differ-ence of heat output of the radiator between the engine cooling systems and the vehicle climate control system is about 3kW.References[1]Chang YJ,Wang CC.Air side performance of brazed aluminum heat exchangers.J Enhanc Heat 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