The free-piston engine generator (FPEG) is a novel power generation device with an estimated brake efficiency (energy contained in the fuel that is transformed into useful work) of up to 46%, compared to the 25-35% reported in conventional reciprocating engines. This paper seeks to address a major challenge in the development of new and complex technologies-how do we effectively communicate and understand the influence of key design parameters on its operating performance? In this paper, the

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... G is described using a simple numerical model, a model which is reduced to a forced mass-spring vibration system under external excitation, enabling all the major input parameters to be decoupled. It proved that the engine piston position as a function of time and output power could be predicted directly from the input parameters with acceptable accuracy. The influence of the key FPEG design parameters on the piston oscillation characteristics and electric power output can be characterised with respect to one another and summarised. Key design parameters include piston mass, compression stroke length, piston cross sectional area, and electric load. Compared with previous and more complex numerical models, the presented methods can be used to simply describe the sensitivity of key design parameters on the FPEG performance. It will provide useful general guidance for the FPEG hardware design process. √ Ls Electric power output, P e [W] P e = 8 kv Kt Hu p0 πCv T0 AFR CR γ−1 2 ·A 2 Piston area, A [m] P e ∝ A 2 Coefficient of electric load force [N/(m·s −1 )] P e ∝ 1 kv Energies 2017, 10, 486 9 of 14 Simulation Results and Discussion When the design parameters change from a wide range from −50% to 50%, compared with the reference value listed in Table 1 , i.e., without considering its physical feasibility, the influence on the design target is simulated. During the simulation, the engine was assumed to be operated at stoichiometric air-fuel ratio, and a wide open throttle (WOT) was applied. Other operational parameters in the model remain unchanged throughout the simulation. The data in Figure 3 presents how the design parameters affect the piston amplitude. The influence of the piston cross-sectional area, moving mass, and the compression stroke length on the piston amplitude are identical, generally linear, and monotonic. Any change on these design parameters will lead to charge changes in piston TDC and compression ratio (CR). For the prototype with an engine effective stroke length of 35 mm, as adopted in this research, a TDC variation of ±1% of the stroke length would be equivalent to 0.35 mm and would produce a CR variation of approximately ±1.0. The electric load plays a very important role during the matching design process and, in principle, can be changed in real-time during operation. From Figure 3 , it is observed that the electric load is a more effective parameter for increasing the piston amplitude, and the piston amplitude or piston TDC will be improved significantly by reducing the electric load.