Abstract The practice of the green renewable fuels in diesel engines is increasing concerning the progressive deterioration of the green environment and the scarcity of the fossil fuels. In these aspects, several experiments in dual fuel mode (DFM) at different global fuel–air equivalence ratio (Φglobal), intake charge preheating and loads have been carried out considering biogas as the inducted renewable fuel. The Φglobal was varied from 0.30 to 0.89 from part to higher loads with biogas flow rate (BFR) of 0.67–3.99 kg/h. The maximum diesel replacements (DR) and biogas energy share (BES) are found to be 92.49% and 97.55%, respectively. However, at higher Φglobal, the brake thermal efficiency (BTE) reduces drastically. Although with preheating, there is an increment of BTE by 5.72% and 2.60% at loads of 4.36 N·m and 21.78 N·m, respectively. At lower Φglobal in DFM and with preheating, the diesel like trends of BTE is achieved. Overall combustion behaviour deteriorates at higher Φglobal. However, it significantly improved with the preheating and at controlled Φglobal. Higher cycle-to-cycle variation of cylinder peak pressure (CPP) is noticed at higher Φglobal. With preheating, at part and higher loads the reduction of CO of 29.41% and 65.49% are estimated. At higher load (21.78 N·m) and with preheating a reduction of 53.33% of HC is noticed. The drastic reduction of NOx is observed with the increment of Φglobal. The superior performance is achieved at each of the tested loads at the optimum Φglobal and with preheating.