Page - 92 - in Hybrid Electric Vehicles

If the vehicle velocity is greater than the set value, the system changes to the parallel mode control.AparallelelectricassistcontrolstrategyisusedinthisresearchandisshowninFigure3. First, thedemandedspeedand torqueof theauxiliarypowerunit aredeterminedby theclutch modelaccordingto thepowerdemandof themotorPd. In theclutchmodel, thedecision logicof the clutch state sets theworking state of the clutch.When the clutch is engaged at theparallel modecontrol, therequestedtorqueof theAPU,whichequals theoutput torqueof theengine, is thesummationof thedrivingtorquedirectly transferredto thefinaldriveandthecharge torque that is transmitted to thegenerator. Themotorprovides the remainingdriving torque simulta- neously. Subsequently, the control parameters of the parallel mode are optimized across the overall engine’s working region. Same as the series mode control, the modified discharging/ charging power is calculated. Then, engine on/off state is decided by the clutch state and the SOC aswell as the requested vehicle velocity. At last, the requested torque and speed of the enginearedeterminedbasedontheenginestateandthepowerdemand. The requestedpower of the diesel engine is obtained by the series or parallel control strategy. Then,thepowerdemandoftheHESSPhesscanbecalculatedasthedifferencebetweenthepower demandofthemotorandtherequestpowerofthedieselengine.TheHESSenergymanagement strategy is in chargeof thedistributionof thepowerdemandbetween the lithium-ionbatteries and the supercapacitors. Thedetailed strategy shown inFigure 3 is a combination of the rule- basedcontrolandthe filter control. IfPhess ispositive, theHESSshouldoutputelectricpower to thepower line.First, analgorithmestimates themeandischargepowerduringthedrivingcycle anda low-pass filter outputs a filtereddischargepower of the batteries. Then, thedischarging powerdecisionblockcalculatestheoutputpowersofthebatteriesandthesupercapacitorsbased onthedesignedthresholdvaluesofSOCandthemeandischargepower. IfPhess isnegative, the motoroutputselectricpowertochargetheHESS.Thereby, thecontrolstrategyissimilarwiththe positive situation.Meanwhile, anSOCcorrectionalgorithmof the supercapacitors is employed basedonthevehiclevelocity for thedischargingprocess. 5.Resultsanddiscussion AprogramwasdevelopedusingMatlabandAdvisoraccording to themathematicalmodelof the coaxial power-split hybrid powertrain with an HESS. The designed simulation model combines the backward- and forward-facingmethods and can evaluate the drivability and economyofthecoaxialpower-splithybridpowertrainsystem. Inthis research,ahybridtransit buswitha totalmassof 15 tonmanufacturedbyHigerBusCompanyLimited is applied.The detailedparametersof thehybrid transitbusare listed inTable2. Using theCTBCDCdrivingcycle, theperformanceof thedesignedcoaxialpower-split hybrid powertrainwith anHESS is evaluated, and the results are shown inFigure 4. The target and available values of the vehicle speed are compared in Figure 4a. The target vehicle speed is defined by the CTBCDCdriving cycle. The available vehicle speed is determined using the establishedprogramandcan follow the target oneperfectly,whichmeans the coaxial power- split hybrid powertrain with an HESS can satisfy the requirements of the drivability completely. The evaluated power demand of the motor is shown as the solid profile of Figure 4b. As a contrast, the output power of the diesel engine is given as the dashed line. Hybrid Electric Vehicles92