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rangefrom559.1 to584.9V.Thestablevoltagecharacteristicwillbebeneficial for theoperations of themotor and thegenerator.Although theoutput voltageof the supercapacitors varies in a largescope, it isstillwithintheallowablevoltageratiooftheDC/DCmodule.Thecurrentsofthe batteries and thesupercapacitors aredisplayed inFigure5e. The currentof the supercapacitors increaseswhile thecurrentof thebatteries ismuchsmallerandthedischarging/chargingrateof thebatteries is less than4C,which isveryhelpful for the lifeextensionof thebatteries. Figure 4. Analysis results of the system performance of the designed coaxial power-split hybrid powertrain with an HESSusingtheChineseTransitBusCityDrivingCycle: (a) comparisonof the targetandestimatedvehiclespeeds, (b) the driving power requested by the transit bus Pd and the requested power provided by the engine Pe, (c) the system operationmodewhere 0meanspure electric vehiclemodeand1 represents series hybridmodeand2 is parallel hybrid mode, (d) theenginespeed,and(e) theengine requested torque. Hybrid Electric Vehicles94