Research regarding higher efficiency engines and renewable energy has lead to HCCI engine technology as a viable option with the ability to utilize a variety of fuels. With a larger focus on environmental effects the ability of HCCI engines to produce low levels of NOx and potentially other combustion products is another attractive feature of the technology. Biomass gas as a renewable primary fuel is becoming more predominant regarding internal combustion engine research. The simulated fuel in this study replicates compositions derived from real-world gasification processes; the focus in this work corresponds to fuel composition variations and their effects regarding combustion phasing and performance. There are three biomass gas fuel compositions investigated in this study. All compositions consisted of combustibles of CH₄, CO, and H₂ accompanied by CO₂ then balanced with N₂. The CH₄ and CO₂ constituents of each fuel mixture are held constant at 2% and 5% respectively. The H₂ content varies from 10% to 20% and the CO ranges from 15% to 25%; the H₂:CO ratio varies between 0.40 and 1.33. All compositions are balanced with N₂. The engine speed was held constant at 1500 rpm with variations in both equivalence ratio and intake temperature. All results are derived from in-cylinder cycle-resolved pressure measurements. Combustion phasing was quantified through CA10 or SOC and CA10-90 (crank angle corresponding to 10% of the heat release and 10-90% of heat release). Analysis of IMEP and indicated efficiency will be reported on to quantify the useful work produced.