Combustion Characteristics and Cycle-to-Cycle Variations in a Diesel Generator Fueled with Used Frying Oil Biodiesel Blends

This paper investigates the combustion characteristics and cycle-to-cycle variations in a stationary direct-injection diesel generator fueled with biodiesel derived from used frying oil (UFO) blended with conventional diesel. Used frying oil methyl ester (UFOME), referred to as UB100, was produced through a single-step alkali transesterification process and blended with diesel fuel in varying proportions (UB10, UB20, UB30, UB40, and UB50). Experiments were conducted on a single-cylinder, naturally aspirated diesel engine operating at a constant speed of 1500 rpm under five different load conditions: 0%, 25%, 50%, 75%, and 90%. In-cylinder pressure data from 400 consecutive cycles were analyzed to determine combustion parameters such as peak cylinder pressure (PP), mean indicated pressure (MIP), start of combustion (SOC), location of peak pressure (LPP), and cycle-to-cycle variations, quantified by the coefficient of variation in MIP (COV-MIP). Key findings indicate that UB100 exhibited higher density and viscosity, a significantly higher flash point, a lower cetane number, and a lower heating value compared to regular diesel (RD). UB blends generally exhibited lower PP and MIP compared to RD. Higher biodiesel concentrations in blends (UB40, UB50) resulted in an advanced state of charge (SOC) and lower power loss per kilowatt (LPP), particularly at higher engine loads. While UB blends showed increased COV-MIP by 5-25% compared to diesel, all values remained well below the 10% threshold associated with drivability issues. In terms of performance, the analysis revealed that UB blends had lower air-fuel ratios (AFR) and higher brake-specific fuel consumption (BSFC). Regarding emissions, tests demonstrated that UB blends produced less carbon monoxide (CO) and nitrogen oxides (NOx) and also had lower exhaust gas temperatures (Texh) compared to diesel fuel. These results highlight the potential of used frying oil biodiesel as an alternative fuel for diesel generators, offering environmental benefits, though with some trade-offs in performance and combustion stability.