Page - 100 - in Hybrid Electric Vehicles

6.Conclusions Inthisstudy, thesystemperformanceofacoaxialpower-splithybridpowertrainwithanHESS for transit bus applicationwas investigated. First, a system topologywas designed. Then, a mathematical model was established and an energy management strategy was developed. Finally, the energy efficiency of the hybrid powertrain systemwas evaluated byMatlab and Advisor.Themainconclusionsaresummarizedas follows: 1. Comparedwith theresultsofacoaxialpower-splithybridpowertrainwithsupercapacitors as energy storagedevice, the equivalent fuel consumptionof thedesigned coaxial power- splithybridpowertrainwithanHESSisabithigher(24.43vs.20.46L/100km).Thereasonis that theenergycapacityof thesupercapacitors in thisstudyismuchsmaller thanthatof the hybridpowertrainwithonly supercapacitors.Thus, theamountof the recovered regenera- tiveenergy is smaller than thatof thehybridpowertrainwithonlysupercapacitors.Onthe other hand, theDC/DC converter and the battery pack also have some losses during the working processes. As a result, the energy efficiency of the coaxial power-split hybrid powertrainwithanHESSis lower thanthatof thehybridpowertrainwithsupercapacitors. Becausewehavenodataabout thecoaxialpower-split hybridpowertrainwithonlybatte- ries,aquantitativecomparisoncannotbegivenhere.Generally, ifonlybatteriesareusedfor the energy storage system, a large sizewill be used and the costwill increase a lot. The energy efficiency of such a systemwill be a trade-off between cost and lifetime of the batteries. Nevertheless, the results of this study show that the coaxial power-split hybrid powertrainwithanHESShasaverygoodenergyefficiencycomparedwithaconventional powertrainof thesamelevel. 2. Because the designedHESS has a smaller size andweight and lower price than thatwith only supercapacitors, and the lithium-ionbatteries of theHESS canoperate at an averaged current and thus have a longer life cycle, theHESS ismore suitable for hybrid transit bus application. 3. The power line voltage of theHESS ismore stable than thatwith only supercapacitors. Thiswillbebeneficial for theoperationof theaccessoriessuchastheairconditioneror the in-vehicle infotainment systemduring thedrivingprocess. Nomenclature A vehicle frontal area (m2) C capacitance (F) CD aerodynamicdragcoefficient F tractive force (N) f rollingresistancecoefficient g standardgravitationalacceleration (9.8067m/s2) i current (A) J inertia (kg m2) Hybrid Electric Vehicles100