A sustainable development is essential for the future of the planet. Therefore, the technology must satisfy current needs without compromising the ability of future generations to meet their own needs. Hence, this work assesses the stability conditions of a free piston engine powering a linear permanent magnetic generator. This kind of machine uses renewable fuels and has greater energy efficiency. The numerical model used to do it englobe three physical domains: electromagnetism, dynamics, and thermodynamics. The proposed machine works in its resonance frequency. Therefore, a sensitivity analysis was played out, interpreting how the three variables influence the engine cycle stability: the inlet pressure, air–fuel ratio and the associated mechanical spring stiffness. It was shown that when working in resonance the machine is more robust and less influenced by the thermodynamics conditions.