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haveagreat influenceonthefuelconsumption.Herein, theexperimentalresultonlygivesavery coarse and average evaluation of the fuel efficiency.Moreover, the fuel consumption of a con- ventional powertrain using a YC6G270 diesel engine and a five-gear manual transmission is computedbasedonthesamevehicleparametersas thehybrid transitbus.The totalmassof the bus with the conventional powertrain is 15,000 kg, which is the same as that of the coaxial power-split hybridpowertrainwithanHESS.The result is 36.33L/100km.Comparedwith the results of the conventional powertrain, the fuel consumption of the coaxial power-split hybrid powertrainwithanHESScanbedecreasedsignificantlybyabout32.5%. Fromtheviewpointofenergyefficiency,thereasonforsuchagreatfuelreductioncanbeexplained. Figure7 shows the effective thermal efficiencymapof theYC6J220diesel engineobtainedonan engine test bench. The engine’s working points estimated by the analysis program are also displayed.TheOOLlineoftheseriescontrolmodeisrepresentedbythethicksolidlineinFigure7. As a contrast, the effective thermal efficiencymapof theYC6G270diesel engine for the conven- tionalpowertrainandthecorrespondingworkingpointsaregiven inFigure8. It canbeseen that the engineworking points of the coaxial power-split hybrid powertrainwith anHESS are very close to the regionhaving thepeakefficiencyand their thermal efficiencies aregreater than40%. However, theworkingpointsof theconventionalpowertrain showninFigure8will changewith the vehicle velocity, resulting in a very wide distribution from the idle speed to the full load. Therefore, many working points of the conventional powertrain locate in the low-efficiency regions, leadingtoalowefficiencyof theentirepowertrainsystem. The energy efficiencymap of the PMSM obtained on amotor test bench and the relative working points for the CTBCDC driving cycle are given in Figure 9. A large part of the working points situates close to the peak-efficiency region. The efficiencies ofmost of the workingpoints arehigher than93.4%except for the low-speedand small-load regions. The energy-weighted average efficiency of the motor during the CTBCDC driving cycle is 91.92%.Because themotor isconnectedtothe finaldrivewithouta transmission,aparticular design of the PMSM can ensure that the motor efficiency is high enough for low-speed workingconditions.Theenergyefficiencymapof thePMSGmeasuredonamotor testbench and the corresponding working points for the CTBCDC driving cycle are displayed in Figure 10. The efficiencies of the PMSG are found to be between 92 and 93% during the CTBCDC driving cycle, and the energy-weighted average efficiency is 92.55%, which approaches thepeakefficiencyof thePMSG Powertrain Fuel consumption (l/100km) Energyreductiona (%) Conventionalpowertrainbus 36.33 Coaxialpower-splithybridbus 24.43b 32.76 24.53c 32.48 aRelative to theconventionalbusequippedwithaYC6G270diesel engine. bAnalysis resultof thehybrid transitbusequippedwithaYC6J220diesel engine. cTest resultof thehybrid transitbusequippedwithaYC6J220diesel engine. Table 3. Results of fuel consumption. A Hybrid Energy Storage System for a Coaxial Power-Split Hybrid Powertrain http://dx.doi.org/10.5772/67756 97