Page - 93 - in Hybrid Electric Vehicles

The designed coaxial power-split hybrid powertrainwith anHESS can recover the braking energy efficiently during the regenerative braking. When the vehicle is driving, the power demandof themotor increaseswith the vehicle velocity and the diesel enginewill start and providepart of thedriving energy if thedrivingpowerdemandof thevehicle is greater than thespecifiedvalueof theseriesmodecontrolor theparallelmodecontrol. The systemoperation state is shown inFigure 4c,where 0 represents theEVmode, 1means theseriescontrolmode,and2denotes theparallelcontrolmode. If thevehicledrivingpoweris greater than110kW, theoperationmode switches fromthe seriesmode to theparallelmode. Meanwhile, theEVmode canbe changed fromboth the series controlmode and theparallel controlmode.Theoutput speedandtorqueof thediesel engineareshowninFigure4dande, respectively. Because the power follower control is used for the series controlmode and the engine speed is proportional to the vehicle velocity at the parallel mode, the engine speed varied from1494 to 1842 r/minduring theworkingprocess. Furthermore, the engine output torquemaintainsatahigh levelof550.4–761.6Nmwhentheengine is running. The performance of the HESS for the coaxial power-split hybrid powertrain is displayed in Figure5.Theoutputpowerof theHESS isgiven inFigure5a.Theoutputpowersofeachof the energystoragedevicesaregiven inFigure5b,where thedashed line isused for the lithium-ion batteries, and thedashdotted line is used for the supercapacitors. InFigure 5, positive values representdischargingprocessandnegative isused forcharging.Theoutputpowerof theHESS varies with the power demand of the motor. The maximum and minimum output powers achieve104.8and 112.9kW,respectively.Theoutputpowerofthelithium-ionbatterieschanges from 50to50kWduringthedrivingcycle,whichmeanstheoutputcurrentof thebatteriescan be significantly decreased. The supercapacitors discharge or chargewith large powers for the high-powerworkingconditions tocompensate thepowerdifferencebetweenPhess andPbat.The SOCprofilesforthelithium-ionbatteriesandthesupercapacitorsareshowninFigure6c.During the charge sustainmode, the SOCof the batteriesmanifests a very small variationduring the cycle.Asacontrast, theSOCofthesupercapacitorsvarieswithinawiderangefrom0.395to0.99. The reason is that the capacity of the lithium-ion batteries is much greater than that of the supercapacitorsandtheoutputpowerofthelithium-ionbatteries isconstrainedtoasmallrange. Theoutputvoltagesof thebatteriesandthesupercapacitorsareshowninFigure5d.Theoutput voltageof the lithium-ionbatteries,whichequals thepowerlinevoltage,changes inaverysmall Parameter Value Unit Vehiclemassexcludingpack 12,930 kg Cargomass 2070 kg Dimensions 12 2.55 3 m m m Rollingresistancecoefficient 0.0094 – Aerodynamicdragcoefficient 0.70 – Vehicle frontal area 7.65 m2 Wheel radius 0.506 m Finalgear 5.833 – Table 2. Parameters of the transit bus. A Hybrid Energy Storage System for a Coaxial Power-Split Hybrid Powertrain http://dx.doi.org/10.5772/67756 93