A free piston linear alternator (FPLA) can convert the fuel's chemical energy to electrical energy efficiently via a linear alternator. Since the piston motion in the FPLA is not constrained, there are significant cyclic variabilities in the FPLA's operation; thus, advanced control methods are essential for prolonged stable operation. In this paper, an FPLA with a high-stiffness helix spring is proposed, called a spring-assisted FPLA. The spring can work as a flywheel to stabilize the engine operation and reduce the cyclic variabilities. The steady-state thermodynamic performance of the engine is studied. Homogeneous charge compression ignition (HCCI) is used for combustion due to its high thermal efficiency and low exhaust emissions. This research is the first focusing on spring-assisted HCCI FPLA with a fully functional system-level model, with accurate predictions of combustion, gas exchange, friction, etc. with each submodel validated. A zero-dimensional system-level model is built in MATLAB/Simulink with a predictive combustion model developed by experimental data on a crank-slider engine. Since the thermodynamics and piston dynamics are highly coupled in FPLAs, a decoupling algorithm called Piston Motion Generator (PMG) is developed to remove the need for controls over the model predictions so that the current study can focus on the thermodynamic performance. A parametric sweep is performed with changing compression ratio, scavenging efficiency, and spring energy ratio beta, i.e., spring stiffness. The simulation results show that the free piston motion is distorted with higher acceleration near top dead center (TDC), especially with low energy ratio springs, which can significantly reduce the heat transfer losses. Low energy ratio springs can also delay the ignition timing and enable the use of a low octane rating fuel with high compression ratio to achieve high thermal efficiency. As a result, the maximum brake efficiency (electrical conversion efficiency) and thermal efficiency are 41.1% and 46.7%, respectively, achieved with a high compression ratio, high scavenging efficiency, and low spring energy ratio beta.