Influence of compression ratio on the combustion and emission behavior of ultrasonically synthesized animal-fat biodiesel

Abstract

Animal-fat biodiesel is biodegradable, nontoxic, and offers advantages such as improved lubricity and higher cetane number when blended with diesel, while its typical drawbacks such as high viscosity and poor cold-flow behavior can be mitigated through blending and moderate engine optimization. In this study, a 20% animal-fat biodiesel blend (AFB20) was produced from slaughterhouse waste using basecatalyzed transesterification enhanced by a 20 kHz ultrasonic horn, which reduced reaction time from 3 hours (magnetic stirring) to 10 minutes. The ultrasonically synthesized AFB20 was tested in a single-cylinder, four-stroke, variable-compression-ratio diesel engine at compression ratios from 16.5:1 to 19.5:1. Operation at 19.5:1 was most favorable; brake specific fuel consumption improved by 6.7% relative to 16.5:1, and brake thermal efficiency increased to 35% due to enhanced atomization. Peak cylinder pressure reached 72.76 bar, and maximum heat release rate was 30.98 J/°CA, indicating shorter ignition delay. Unburnt hydrocarbons decreased by ∼ 20%, and smoke opacity reduced significantly because of better oxidation, while NO_x increased moderately by about 9.5%. Overall, AFB20 operated at CR 19.5:1 demonstrated improved efficiency, lower fuel consumption, and cleaner combustion with only a moderate increase in NO_x emissions, highlighting its potential as a sustainable biodiesel blend for existing stationary and agricultural diesel engines without requiring hardware modification.