This exptl. study elucidates the effects of hydrogen (H2) supplementation on the performance, combustion, and emission parameters of a waste cooking biodiesel (WCB)-fuelled compression ignition (CI) engine under variable fuel injection timings (FITs).Initially, a comprehensive trade-off study of methanol-to-oil ratios, reaction time and catalyst shares was performed to identify the optimal biodiesel production strategy, achieving a maximum yield of 97.60 %.The produced WCB was subsequently utilized in a water-cooled, single-cylinder CI engine operating at 8°-16° bTDC FITs.WCB100 exhibited suboptimal performance due to its high viscosity and d., resulting in delayed combustion and increased emissions.However, the integration of five H2 injection strategies significantly enhanced combustion behavior and reduced emissions, achieving reduction of up to 54.75 % in BSHC, 44.77 % in BSCO, and 43.62 % in BSPM.The findings highlight WCBH5 with a FIT of 12° bTDC as the optimal configuration for sustainable CI engine performance and remarkably reduced emissions.