Efficiency Coupling Analysis and Optimal Control of Powertrain of Parallel Hybrid Electric Vehicles Under Engine-only Mode

Powertrain of parallel hybrid electric vehicle (PHEV) is composed of a battery, a drive motor, an engine, an automatic transmission, and several other key components. The efficiency characteristics of these components are coupled with each other. To achieve optimal overall efficiency of the system, the coupling relationships between the efficiency of different components need to be analyzed. The control parameters that determine the overall efficiency under different working modes need to be identified and should be optimized to achieve maximum system efficiency. In this paper, PHEV equipped with continuously variable transmission is studied. First, the efficiency characteristics of key components in the system were analyzed; efficiency models of these key components were established; and relationships between efficiency, control parameters, and state parameters were revealed. Furthermore, the coupling relationships between the efficiency of different components in the power transfer path were analyzed under the engine-only mode. The relationships between fuel consumption, state parameters, and control parameters of the system were deduced. According to the analysis results, power requirement and vehicle speed were selected as state parameters; and CVT ratio and engine torque were selected as control parameters. An objective function and constraints were established to achieve minimize fuel consumption, and the system optimization issues were defined. According to the characteristics of the optimal equation, an algorithm based on simulated annealing was proposed to solve the optimization equation. The relationships between CVT ratio, engine torque, and state parameters of the system were obtained to achieve minimum fuel consumption. Simulations and tests were carried out under a standard driving cycle and fixed speeds. Results indicate that the efficiency of CVT has a great influence on the overall efficiency of the system. The engine efficiency is reduced by using the optimal control law, but the efficiency of CVT increased more and the overall efficiency of the system is increased. Under driving cycle where the power requirement is fixed, fuel consumption under the proposed control algorithm reduced by about 1%-2% compared with the traditional control algorithm, which only consider reaching the highest engine efficiency. © 2019, Editorial Department of China Journal of Highway and Transport. All right reserved.