Trajectory-based combustion control for renewable fuels in free piston engines

Previously, the authors have developed an advanced combustion control, namely the trajectory-based combustion control, to further leverage the flexibility of free piston engine (FPE). With the assistance of this control method, the FPE enables optimization of both engine efficiency and emissions by implementing optimal piston trajectories. Extensive simulations have been conducted to prove the effectiveness of this combustion control on fossil fuels. In this paper, the investigation is extended to renewable fuels. Seven renewable fuels are considered herein including hydrogen, biogas, syngas, ethanol, dimethyl ether (DME), biodiesel, and Fischer-Tropsch fuel. The influences of both compression ratio (CR) and piston motion pattern between the two dead centers on the combustion process are considered in the study, which demonstrates the ultimate fuel flexibility and large tolerance of fuel impurity possessed by the FPE. In addition, the simulation results show that at a fixed CR, the thermal efficiency of the FPE can still be enhanced (5% in DME case) by varying the piston motion patterns alone. Furthermore, specific asymmetric piston trajectories are synthesized to further improve the engine thermal efficiency (8% in hydrogen case) and reduce the NOx emission simultaneously (around 70% reduction in hydrogen case). In other words, due to its ultimate fuel flexibility, large tolerance of fuel impurity, and controllable piston trajectory, the FPE, with the trajectory-based combustion control, enables a co-optimization of renewable fuels and engine operation.