Abstract A single-cylinder heavy-duty optical engine is used to characterize dual-fuel (DF) combustion. In experiments, methane is applied as the main fuel while directly injected pilot diesel ignites the premixed methane-air mixture close to the top-dead center (TDC). In the present study, diesel-methane DF combustion is analyzed as a function of (1) the methane equivalence ratio, (2) initial charge temperature, and (3) the quantity of pilot diesel. Experiments are conducted at 1400 rpm and a load of 9–10 bar IMEP, and DF combustion is visualized in the engine through Bowditch-designed optical access. Meanwhile, a high-speed camera records temporally resolved natural luminosity (NL) color images of the combustion event. The results of the study suggest that DF combustion based on the apparent heat release rate (HRR) data consists of three overlapping combustion stages, where the level of overlap depends on mixture fractions of both pilot-diesel and methane in the in-cylinder charge. The stages are identified by analyzing the second derivative of HRR data. The study revealed that during the first stage, most of the pilot diesel burns in the premixed mode, and that the ignition delay time (IDT) directly influences the burnt charge mixture fraction of pilot diesel and entrained premixed methane-air mixture. In addition, the first-stage combustion is visualized as initial flame kernels originating from pilot-diesel sprays. IDT is found to be especially sensitive to the methane equivalence ratio and initial charge temperature. Furthermore, the concentration of methane and the quantity of pilot diesel in the charge distinctively influence combustion duration trends.