**Abstract**

This present chapter set one's sight on understanding the contribution of fatty acid ester (FAE) molecules in deciding the performance, emission, and combustion characteristics of their biodiesel in CI engine. For this purpose, both produced waste animal fat-oil (WaFO) biodiesel and their characterized FAEs, blended in calculated proportions with neat diesel were tested individually under same testing conditions. Preliminary findings confirmed the significant contribution of FAEs in deciding the overall engine characteristics of WaFO biodiesel; and were influenced by their fundamental molecular properties like chain length, and degree of unsaturation. Superior combustion characteristics were accounted by early initiation of combustion by saturated FAEs; followed by prolonged combustion of unsaturated FAEs using fuel bound oxygen content. Meanwhile, mixed performance characteristics were explained by its long chained saturated and unsaturated FAEs, which imparted their higher density and viscosity, and reduced calorific value than neat diesel. Emission characteristics reported reduced CO and HC emission, and increased CO2 and NOX emissions citing the equally balanced concentration of both long chained saturated and unsaturated FAEs, which favored complete combustion using its oxygen molecules. Besides assessing engine characteristics, WaFO biodiesel was evaluated for its fuel properties as per ASTM standards, along with neat diesel.

**Keywords:** biodiesel combustion, fatty acid esters, cetane number, degree of unsaturation, carbon chain length
