Optimal Design And Mode Shift Control Of Multi-mode Hybrid Electric Vehicles

This thesis focuses on the optimal design and control of the multi-mode hybrid electric vehicles(HEVs),which mostly uses the planetary gear(PG)sets to couple the engine,two motor/generators(MGs)and the vehicle together.Clutches can be added between the nodes of PGs.By switching the clutch states,different operating modes,like Electronic Continuous Variable Transmission(ECVT)modes,parallel modes and series modes,can be achieved in the same powertrain.The existence of multiple operating modes makes it possible to achieve better fuel economy and launching performance than any single-mode hybrid powertrain.The dynamics,operating modes,energy management system,topology search,optimal design and mode shift optimization of the multi-mode EV will be investigated in sequence.The main contribution of the thesis are as follows:1)The dynamic model of the multi-mode HEV is built up firstly,including a quasi-static engine model,a quasi-static motor/generator(MG)model,an equivalent circuit based battery model,a driver model and the vehicle longitudinal dynamics.In addition,the dynamics of the power-split device is modeled by a proposed method,called automated modeling.By following the rules of the automated modeling,the system base matrix,the torque transition matrix and the acceleration transition matrix are constructed in sequence.Then,the system characteristic matrix of the powertrain which governs the relationship between the angular acceleration of the powertrain devices and the torques is extracted from the state-space representation of the dynamic model.2)The dynamics of all possible operating modes for the multi-mode HEV is derived based on the proposed automated modeling and deep first search.The number of the unique mode is calculated for both double-and triple-PG hybrid powertrain.Based on the correlation coefficients of the system characteristic matrix,a binary tree is created to classify all modes into 14 different mode types,including some common mode types,i.e.,series mode,compound-split mode(2DoF),input-split mode,output-split mode,EV mode(2MGs,1DoF)and etc.3)The performance of each mode is analyzed from two perspectives,energy efficiency and launching performance.Firstly,a Global Normalized Efficiency Factor(GNEF)is proposed to evaluate the efficiency of the hybrid driving modes by normalizing the energy flows between the engine and vehicle output shaft.In addition,a parameter,referred as Driving Cycle Weighted Efficiency(DCWE)which takes the working conditions of the modes into consideration is defined to evaluate the overall efficiency of each mode.Besides,the launching,climbing and towing performance is evaluated by formulating an maximum problem to calculate the maximum output torque of each mode.4)An instantaneous optimization method,called Normalized Efficiency Maximum Strategy(NEMS)is presented to optimize the power of powertrain components in both hybrid and EV modes.By combining NEMS with dynamic programming(DP),a near-optimal energy management strategy,NEMS+ is proposed where DP decides the mode shift signal and NEMS optimizes the energy distribution.The comparison study between DP and NEMS+show that NEMS+ is up to 100,000 times faster than DP while achieving similar performance.5)Two topology search methods are presented in this thesis from the perspectives of search accuracy and search efficiency.Exhaustive search is designed for double-PG hybrid powertrain,while mode combination method is proposed for triple-PG.For the case study with triple-PG,mode combination method was found to be able to identify more superior designs than the shrink exhaustive search method while is computationally much more efficient.However,the selection of the scope of modes imposes a constraint to narrow the design space,which affects the computation time needed and the optimality of the final identified designs.If no constraint is imposed,the proposed mode combination method will revert back to exhaustive search and identify all valid designs.The case study of the exhaustive search improves the all-electric range,fuel economy and 0 to 100km/h acceleration time of Volt2016 in 3.57%,4.01%and 3.61%respectively,while mode combination improve the fuel economy and 0 to 100km/h acceleration time of GM Silverado Hybrid in 2.58%and 14.89%,respectively.6)This paper presents a comparative study of hybrid powertrains with different numbers of PG sets,which we term configurations.The analysis of different configuration types is investigated both qualitatively and quantitatively.In the qualitative analysis,the performances of operating modes for different configurations are compared,in terms of mode number,normalized efficiency,and maximum output torque.The quantitative approach compares the designs of different configurations;the fuel economy and acceleration performance of all superior designs are evaluated to make the comparison iconic.The results show that triple-PG hybrids do not have significant fuel economy improvement compared with double-PG hybrids,but they achieve a dramatic improvement in acceleration performance,which can be beneficial for sport utility vehicles(SUVs),light trucks,and buses.For cost consideration,it is suggested that passenger cars adopt double-PG hybrid powertrains.7)Two novel frameworks combining the optimal topology selection,component sizing,and control optimization are presented and compared.One approach is nested optimization which searches through the whole design space exhaustively.The second approach is called enhanced iterative optimization,which executes the topology optimization and component sizing alternately.A case study shows that the later method can converge to the global optimal design generated from the nested optimization,and is much more computationally efficient.8)A forward design methodology of the mode shift map for the multi-mode HEV is proposed based on the energy efficiency,maximum output torque and energy changes during the mode shift.By combining the mode shift map with Equivalent Consumption Minimum Strategy(ECMS),a real-time coordinated control strategy is presented by optimizing the energy distribution while maintaining smooth mode shift.