Abstract
Hybrid electric vehicles equipped with continuously variable transmission show dramatic improvements in fuel economy and driving performance because they can continuously adjust the operating points of the power source. This article proposes an optimal control strategy for continuously variable transmission–based hybrid electric vehicles with a pre-transmission parallel configuration. To explore the fuel-saving potential of the given configuration, a ‘control-oriented’ quasi-static vehicle model is built, and dynamic programming is adopted to determine the optimal torque split factor and continuously variable transmission speed ratio. However, a single-criterion cost function will lead to undesirable drivability problems. To tackle this problem, the main factors affecting the driving performance of a continuously variable transmission–based hybrid electric vehicle are studied. On that basis, a multicriterion cost function is proposed by introducing drivability constraints. By varying the weighting factors, the trade-off between fuel economy and drivability can be evaluated under a predetermined driving cycle. To validate the effectiveness of the proposed method, simulation experiments are performed under four different driving cycles, and the results indicate that the proposed method greatly enhanced the drivability without significantly increasing fuel consumption. Compared to a single-criterion cost function, the use of multiple criteria is more representative of real-world driving behaviour and thus provides better reference solutions to evaluate suboptimal online controllers.
Highlights
Both academic researchers and car manufacturers are continuously attempting to improve the fuel consumption of hybrid electric vehicles (HEVs)
To avoid the ‘Curse of Dimensionality’ when implementing the algorithm, the battery state of charge (SOC) is chosen as the only state variable and the global minimum of fuel consumption is solved by optimizing the torque split factor and continuously variable transmission (CVT) speed ratio
A single-criterion cost function will lead to excessive engine on/off events and violent fluctuations in the CVT speed ratio signal, which will cause a sharp change of the vehicle velocity
Summary
Both academic researchers and car manufacturers are continuously attempting to improve the fuel consumption of hybrid electric vehicles (HEVs). Advances in Mechanical Engineering one of the major challenges because it greatly impacts the fuel economy by controlling the CVT speed ratio and power distributions among power sources. To address this challenge, a series of important theories and methods have been developed, which can be divided into two categories: heuristic methods and optimization methods. Strategies focused on fuel optimization for a CVT-based HEV can lead to excessive engine on/off events and violent fluctuations of the CVT speed ratio. Two penalty terms were chosen to constrain excessive engine events and the rate of change of the CVT speed ratio. According to Bellman’s26 principle of optimality, the DP algorithm needs to backward compute the cost-togo function Jk(xik) at every node in the state grid
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