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Thegeneratoroperatesonlyasmallpartof the timeanditsoutputpowerchangesmainly from 24.38 to104.8kW.Thecurrentof thegeneratorvarieswith theoutputpowerbecause thepower linevoltage isstable. To evaluate the potential of fuel saving for the coaxial power-split hybrid powertrainwith an HESS, 12 prototypes of hybrid transit buswere built and applied to a practical city routes in Ningbo,ZhejiangProvince.A totalmileage of over 40,000 km for eachhybrid transit buswas achieved,andtheaverage fuel consumption isapproximately24.53L/100km,which is listed in Table3.Usingtheestablishedmathematicalmodelandtheanalysisprogram, theestimatedfuel consumption of the coaxial power-split hybrid powertrain with an HESS is 24.43 L/100 km, which is also listed inTable3. TheSOCdifferenceof the lithium-ionbatteriesbetween thestart andendpoints is0.0034.Thepracticaldrivingroutes inNingboaredifferentwith theCTBCDC drivingcycle.The start/stop frequency isdecreasedcomparedwith theCTBCDCdrivingcycle. Therefore, the improvement of fuel efficiency is a bit lower. On the other hand, the ambient temperature and the total vehicleweight varying during the practical driving conditions also Figure 6. Results of themotor and the generator: (a) the input powerof themotor (a positive value is used for driving processandanegativeone is for regenerativebraking), (b) theoutputcurrentof themotor, (c) therequestedpowerof the generator, and (d) theoutput currentof thegenerator. Hybrid Electric Vehicles96