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Energy management strategy is very important for hydraulic hybrid vehicles to improve fuel economy. The rule-based energy management strategies are widely used in engineering practice due to their simplicity and practicality. However, their performances differ a lot from different parameters and control actions. A rule-based energy management strategy is designed in this paper to realize real-time control of a novel hydraulic hybrid vehicle, and a control parameter selection method based on dynamic programming is proposed to optimize its performance. Firstly, the simulation model of the hydraulic hybrid vehicle is built and validated by the data tested from prototype experimental platform. Based on the simulation model, the optimization method of dynamic programming is used to find the global optimal solution of the engine control for the UDDS drive cycle. Then, the engine control parameters of the rule-based energy management strategy are selected according to the engine control trajectory of the global optimal solution. The simulation results show that the 100 km fuel consumption of the proposed rule-based energy management strategy is 12.7L, which is very close to the global optimal value of 12.4L and is suboptimal.

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In order to achieve higher-energy conversion efficiency, a free-piston engine with an improved four-stroke thermodynamic cycle is investigated in this paper. This cycle is optimized according to the variable strokes feature and is characterized by the short intake stroke, the complete expansion stroke, the external pressurization, and the intercooling. The development of a four-stroke free-piston engine system simulation model was described, and the effects of the cycle on the system performances were qualitatively analyzed. According to the experiment of the prototype, the generating efficiency of 33.4% can be achieved when the system is fueled with gasoline and the output power is significantly increased from 1.62 to 2.68 kW. The simulation and experiment results are analyzed in detail, giving insight into the performances of the system. Studies show that the energy-saving and environmental protection performances of the system can be significantly promoted by using the improved thermodynamic cycle.