Table of Contents

ABSTRACT

This study addresses the need for sustainable diesel alternatives by evaluating the combustion stability of biodiesels from waste and non-edible sources. The research focuses on mitigating efficiency losses from cyclic variability, which can lead to power loss, reduced fuel efficiency, and higher emissions. Used frying oil (UB) and jatropha (JB) biodiesel blends (10-50%) were analyzed in a single-cylinder, direct-injection diesel engine at 1500 rpm under various loads (0-90%). Key quantitative findings reveal that JB blends advanced the start of combustion (SOC) by up to 2°CA at high loads, while UB blends delayed SOC at lower concentrations but advanced it at higher blends (UB40, UB50). Peak pressures (PP) increased with biodiesel concentration; JB blends yielded slightly higher PP (up to 3.9%) and UB blends slightly lower PP (down to 3.8%) than diesel. All blends operated smoothly, with maximum pressure rise rates (MPRR) remaining within the 4–7.5 bar/°CA limit, ensuring noise-free operation. Cyclic variability, measured by coefficients of variation (COV), decreased with increasing load, with COV-MIP (2.4-5.24%) and COV-PP (<2%) indicating stable combustion. Notably, COV-MPRR (12-20%) was higher, reflecting greater cycle-to-cycle variability in pressure rise rates, though JB blends exhibited reduced variability at high loads. These results provide quantitative evidence that both used frying oil and jatropha biodiesel blends can serve as viable drop-in alternatives to diesel, offering a path to improved combustion stability without requiring engine modifications. Keywords: coefficient of variation, combustion characteristics, combustion stability, cyclic variability, non-edible feedstock, transesterification, waste-derived biodiesel