In this research oxygen enrichment, gasoline pilot port injection, and delayed time of 50% cumulative heat release (CA50) are evaluated to expand the range for stable and safe combustion of a lean-burning biogas-fueled HCCI engine. A 4-cylinder 1.9 L Volkswagen TDI engine was modified to run in HCCI mode at 1800 rpm, and boost pressures and charge heating are used to promote autoignition of the biogas-in-air mixture at desired combustion timings. A typical biogas composition of 60% CH4 and 40% CO2 in a volumetric basis was simulated by controlling the CH4 and CO2 flow rates. A range of 2-2.2 bar absolute intake pressure and 473-483 K initial charge temperatures allowed HCCI operation. At lowest equivalence ratio (0.25) excessive cycle-to-cycle variations were observed and at highest equivalence ratio (0.4) unacceptable ringing intensities were observed. To reduce cycle-to-cycle variability at low equivalence ratios, two strategies were used in enhancing the autoignition behavior of biogas: (1) oxygen enrichment of the inducted charge, and (2) gasoline pilot port injection. To increase gross Indicated Mean Effective Pressure (IMEPg) without excessive ringing intensities at high equivalence ratios, delayed CA50 was used. Oxygen enrichment increased cycle-to-cycle variability and total hydrocarbon emissions because of decreased burning rates and delayed CA50. Gasoline pilot port injection lowered cycle-to-cycle variability, CO and THC emissions, and increased IMEPg at low loads. Higher IMEPg was achieved with high equivalence ratios (above 0.4) and ringing intensities were kept within acceptable limits using delayed CA50, however NOx emission was increased also. HCCI combustion of biogas enables high overall efficiency, and the strategies explored in this research allow higher power output, and more stabilized combustion. (C) 2012 Elsevier Ltd. All rights reserved.