**1. Introduction**

Use of biodiesel in CI engines for commercial and industrial purposes has been increasing steadily, very soon after various government policies stressed on the shift to renewable energy resources [1]. This paved progressive pathway for many

researchers to focus on improvising the performance of these biodiesel in CI engines; and also, simultaneously monitoring for controlled levels of exhaust emissions. Hence, numerous suggestions have been proposed for enhancing these engine characteristics which includes introducing blends [2, 3], adding fuel or chemical additives [4, 5], dual fuel mode [6, 7], fuel preheating [8]; and even modifying engine parameters like varying injection pressure, injection timing and introducing exhaust gas recirculation [9, 10]. However, operating engine on biodiesel blended with neat diesel is regarded as most economic and efficient technique; and in general, report increased rate of fuel consumption, carbon dioxide (CO2) and nitrogen oxide (NOx)emissions, and reduced thermal efficiencies and carbon monoxide (CO) emissions [11–13]. Though, many biodiesel report similar trends; variation in their results arises with differences in the fuel properties, besides their testing conditions. Hence, one requires fundamental understanding of various factors influencing the performance of biodiesel in CI engine; especially the role of fuel and its properties in deciding these parameters. In fact, these fuel properties are macroscopic entities; and in turn are governed by the chemical compounds available in it, along with their molecular properties. In other words, these parameters are primarily influenced by the fatty acid esters available in the biodiesel [14]; and supporting this, Srinivasan et al. [15] reported that engine characteristics of any biodiesel is an outcome of coordinated behavior of its FAE molecules in CI engine, especially contributed by its dominant FAEs [15].

However, one requires better understanding of these FAEs in order to study their influence and contribution in CI engine. To begin with, FAEs are the fundamental units of biodiesel, and are made up of commonly known fatty acids, in form of carboxylates at one end and an alkyl chain at its alcohol moiety bridged through an Ester functional group [16]. In general, palmitic acid, oleic acid, stearic acid, linoleic acid, linolenic acid and myristic acid are the most frequently reported fatty acids [17]; whilst, alcohol includes methanol, ethanol, isopropanol and butanol, etc. [18, 19]. Furthermore, palmitic acid and oleic acid are the most commonly occurring saturated and unsaturated FAs, respectively, followed by linoleic acid (unsaturated) and stearic acid (saturated) [20].

Moreover, these fatty acid esters collectively constitute to molecular structure of the biodiesel and contribute to the overall fuel properties of biodiesel based on their molecular properties [21, 22]. Moving further, long chain saturated FAEs, predominantly produced from saturated triglycerides report, increased kinematic viscosity, cetane number, calorific value in addition to reduced density than compared to their counterpart unsaturated FAEs. As a result, these FAEs has tendency to produce higher thermal efficiencies and reduced concentration of incomplete combustion products as a result of its complete oxidation [11, 21, 23–25]. On the other hand, unsaturated FAEs, with single or multiple unsaturated bonds in their FA moieties report lower cetane number and calorific value, thereby resulting in poor thermal efficiencies, in-cylinder pressure and heat release rates. Adding to this, high exhaust gas temperatures (EGTs) followed by increased concentration of NOX emission [14, 26, 27], are also contributed by these unsaturated FAEs; especially by the alkyl esters of oleic acid and linoleic acid. In specific, NOX emission of any biodiesel increases with addition of unsaturated bonds in their FA moiety; and is accounted by the increased adiabatic flame temperature upon combustion inside the cylinder [28]. Besides, adding an aliphatic (–CH2) group to the alcohol moiety simply enhanced the cetane number of the biodiesel; however, the concentration of particulate matter increased by two fold [29].

#### *Molecular Contribution of Fatty Acid Esters in Biodiesel Fueled CI Engines DOI: http://dx.doi.org/10.5772/intechopen.102956*

From these studies it is clearly evident that, FAEs have a significant role in deciding the overall engine characteristics of its biodiesel; yet, it lacks sufficient results necessary for explaining the contribution of FAEs, in case of a multiple feedstock based biodiesel. With these understandings of FAEs and necessity for this underdone work, this present chapter focus on studying the influence of dominant fatty acids esters in deciding the overall engine characteristics of a biodiesel produced from the homogeneous mixture of different waste animal fats and fish oil, blended in equivalent proportion.
