This experimental study comprehensively examines the performance and emission profiles of a single-cylinder, four-stroke, direct injection diesel engine operating on biodiesel blends (B10, B20, B30) produced from waste cooking oil via alkaline trans-esterification. The investigation addresses critical gaps in understanding biodiesel's practical viability as a drop-in fuel replacement amid depleting petroleum reserves and stringent emission norms. Tests conducted at constant 1500 rpm across brake loads of 0-9 kW reveal distinct trends: brake specific fuel consumption (BSFC) escalates progressively from 0.26 kg/kWh (baseline diesel, B0) to 0.32 kg/kWh (B30) at peak load, attributable to biodiesel's 12-15% lower calorific value (38 MJ/kg vs. 43 MJ/kg for diesel) and higher density/viscosity impeding volumetric efficiency. Brake thermal efficiency (BTE), however, peaks comparably at 31.8% for B20 versus 31.2% for B0 at 6 kW, reflecting enhanced combustion completeness from biodiesel's 11% oxygen content despite energy deficits. Emission outcomes underscore biodiesel's environmental merits: CO emissions drop 28-40% (1.2% to 0.55% vol at full load for B20), unburnt hydrocarbons (HC) reduce 45-55% (520 ppm to 115 ppm), and smoke opacity falls 35-40% (4.2 to 2.6 BSU), driven by oxygenated fuel promoting oxidation of incompletely burnt fractions. NOx rises 12-15% (1250 to 1400 ppm), linked to advanced ignition phasing, prolonged premixed combustion, and elevated adiabatic flame temperatures. Exhaust gas temperatures increase marginally (10-15°C), signalling hotter local combustion zones. B20 emerges optimal for unmodified engines, balancing performance penalties with substantial pollutant reductions.

diesel, diesel engine, biodiesel, emission, performance