Dual-fuel technology is suggested as a solution for effectively utilizing alternative fuel types in the near future. Charge air mixed methane combined with a compression ignition engine utilizing a small diesel pilot injection seems to form a worthwhile compromise between good engine efficiency and low emission outcome. Problems concerning dual-fuel technology profitableness seems to be related to fully control the combustion in relation to lean conditions. Lean operating conditions solves the problems concerning pumping losses, but brings challenges in controlling the slow heat release of the premixed methane-air mixture. In the present work, a single cylinder 'free parameter' diesel engine was adapted for dual-fuel (diesel-methane) usage. A parameter study related to lambda window widening possibilities was carried out. The main variables studied were the diesel pilot injection timing and pressure, diesel substitution rate and the relation between combustion characteristics and charge air temperature. Diesel pilot injection parameter optimization seems to affect less the combustion quality compared to the benefits from increasing the charge mass reactivity through preheating the charge air. Improvements in combustion quality were noted with preheated charge air in the lean condition tests (λ= 2.0-2.2) with diesel substitution rates E% = 0-70%. Remarkable combustion efficiency gain was experienced in the misfire limit in a form of premixed mixture ignition in the end gas region (PREMIER).