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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.

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A four-stroke free-piston engine with internal and external irreversibilities of nite combustion rate of the fuel, heat transfer and friction is investigated in this paper. Under the condition of the xed fuel consumption per cycle, the optimal piston motion trajectory for maximizing the net work output of di erent cases is derived by applying optimal control theory. Considering the path constraints and boundary conditions of this free-piston Miller cycle, a Gauss pseudospectral method (GPM) is presented for solving optimal motion trajectory. The results show that the optimal piston trajectory improves the e ciency by more than 10% and also suitably reduces the heat transfer losses compared to the conventional piston motion. By optimizing the piston motion around the top dead center (TDC), the in-cylinder gas pressure and temperature have a remarkable improvement while the heat transfer losses have a suitable reduction. The e ects of other parameters, such as combustion duration and the frictional coe cient on the piston trajectory, are also investigated. It is shown that optimizing the piston motion trajectory is a good approach for the free-piston engine to improve the e ciency and it can provide guidelines for the optimal control of the practical free-piston engine.

The direct yaw moment control can effectively enhance the yaw stability of the vehicle under extreme conditions, which has become one of the essential technologies for the distributed driving electric bus. Due to the features of a large mass and high center of gravity of the bus, lateral instability is more likely to occur under extreme driving conditions. To reduce the uncertainty and interference in the yaw movement process of the bus, this paper targets the instability caused by the coupling problem between the sideslip angle and yaw rate. An adaptive fuzzy sliding mode control is proposed to execute direct yaw moment control. The weight coefficient of the sideslip angle and the yaw rate is adjusted via fuzzy control in real time. The optimal direct yaw moment is finally obtained. A distribution method based on the vertical load proportion is adopted for the allocation of four motors’ torque. Under three typical working conditions, a joint simulation test was carried out. The simulation results demonstrate that the raised method decreases the amplitude of the sideslip angle by 20.90%, 12.75%, and 23.67% and the yaw rate is 8.62%, 6.89%, and 9.28%, respectively. The chattering and sudden changes in the additional yaw moment are also lessened. The control strategy can realize the control target, which effectively strengthens the yaw stability of the bus.