The Linear Joule Engine Generator is an energy conversion system made up from a linear expander, a linear compressor, and a linear alternator. It is adaptable to variable renewable energy fuel sources, e.g. biogases, biofuels, hydrogen and ammonia, etc. In this paper, an investigation on the system dynamics and thermodynamic characteristics under different operating conditions is presented. Real time adjustable parameters were identified, i.e. the system pressure, the valve timings, and the coefficient of electric resistance force. Their influence on the indicated power of the expander, the electric power output from the linear alternator and the energy conversion efficiency are scrutinised using a validated numerical model. In order to achieve stable operation of the system, each parameter is controlled within a practical range, and optimised to maximise the electricity generation efficiency. The system pressure was proved to be the most effective parameter to alter the system power output. The indicated power of the expander with the existing dimensions can reach up to 11.0 kW by adjusting the system pressure, and it cannot exceed 8.0 kW by just tuning valve timings or optimising the coefficient of electric resistance force. The coefficient of electric resistance force is found to be the most influential parameter to maximise the electricity generation efficiency up to 80%.