Abstract >> A free piston linear engine could be operated under HCCI combustion due to its variable compressionratios. To obtain HCCI combustion, the free piston linear engine needs a high compression ratio to achieve auto-ignition of the fuel/air mixture. In this study, an idea for obtaining a high compression ratio using the transitionfrom SI combustion to HCCI combustion was proposed. The fuel used in this study is hydrogen, which is consideredto be an environmentally friendly fuel. Besides, the effects of key parameters such as equivalence ratio (ϕ), load resistance (R L ) and intake temperature (T in ) on the SI-HCCI transition were numerically investigated. The simulationresults show that the SI-HCCI transition is successful without any significant reduction of in-cylinder pressure as the intake temperature is increased from T in =300K (SI mode) to T in =450K (HCCI mode), while the load resistanceand equivalence ratio are retained respectively at R L =120Ω and ϕ=0.6 in both SI mode and HCCI mode.