Recently, there has been a lot of focus on environmental pollution and rapid reduction in fossil fuel resources [
1]. Worldwide, this has become an important challenge, and several techniques for reducing the negative consequences of fossil fuel emissions have been proposed [
2]. Researchers are continuously working to find solutions to replace conventional fossil-based diesel fuel [
3]. Fossil fuels generate a considerable amount of pollutant gases, which have caused global warming, as well as negative impacts on human health and other living beings [
4]. The combustion of fossil fuels produces harmful pollution such as nitrogen oxides (NOx), hydrocarbons (HC), sulphur dioxide (SO
2), carbon monoxide (CO), particulate matter (PM), and carbon dioxide (CO
2) [
5]. Human toxicity has been established for several hazardous contaminants. In 2007–2020, the combustion of fossil fuels resulted in the release of 4.1 million metric tonnes of CO
2 globally [
6]. Countries in the EU (European Union) alone consume nearly 25.7% of energy in residential sectors responsible for huge CO
2 emission [
7]. Renewable fuels, such as biodiesel, have been proposed to replace petroleum fossil fuels to reduce global CO
2 emissions. Biodiesel is non-toxic and environmentally beneficial [
8]. Furthermore, when compared with fossil fuels, biodiesel fuel can cut CO
2 emissions by about 78.45% [
8]. Biodiesel is an alternative sustainable source, which can be produced from plant seed oils, animal fats, and other long-chain fatty acid containing substances such as waste cooking oil [
9]. In addition, biodiesels are also used as an additive to improve the lubricity property of fossil diesel [
9]. Saturated (SFA), monounsaturated (MUFA), and polyunsaturated (PUFA) fatty acids are the three types of fatty acids (FAs) found in biodiesel [
10]. The SFAs are those that have single (C–C) bonds, while USFAs contain double (C=C) bonds [
10]. The unsaturated fatty acids (USFAs) are classified into two types: MUFAs with only one (C=C) double bond and PUFAs with more than two (C=C) double bonds [
11]. As the number of double bonds increases, the melting temperature decreases, hence USFAs have a lower melting point than SFAs [
10]. Chemical reactivity increases as the amount of double bonds increases; thus, USFAs are more chemically reactive than SFAs [
10]. Puhan et al. (2010) studied the impact of three biodiesels with distinct molecular architectures on combustion and emission parameters in a stationary diesel engine. Linseed, jatropha, and coconut oils were used to make the biodiesels. They found that, as the degree of unsaturation of biodiesels increased, the ignition delay (ID) and exhaust gas temperatures increased [
12]. They found that, owing to significant NOx emissions and low thermal efficiency, linseed biodiesel is not suitable for use in diesel engines [
12]. Sochonborn et al. (2009) examined the effect of fatty acid methyl-ester combustion behavior in a single-cylinder research engine [
13]. They reported that the carbon chain structure of the methyl ester had a significant impact on NOx emissions [
13]. Kruczynski (2013) investigated Camelina biodiesel–diesel blends in a direct injection diesel engine [
14]. They revealed that, as the composition of biodiesel increased, the ignition delay and heat release decreased [
14]. They observed that, when the biodiesel percentage in the blend increased, NOx, HC, CO, CO
2, and smoke emissions increased [
14]. Fuel properties directly influence the fuel combustion and emission characteristics. Zhang et al. (2009) investigated the premixed combustion behavior of four carbon methyl esters in a research engine [
15]. They examined the impact of low-temperature fuel oxidation on exhaust gases at different compression ratios [
15]. The authors reported that, owing to the existence of (C=C) double bonds in the fatty acid carbon chain, the tested fuels exhibited different ignition behaviors [
15].
Selvam and Nagrajan (2013) evaluated the combustion characteristics of higher SFA biodiesel fuel [
16]. Pongamia, rice bran, sunflower, and palm oil were used to make the biodiesels. They observed that biodiesel with a high SFA has a higher cetane number, which improves the fuel’s combustion efficiency [
16]. Furthermore, they determined that, the higher the SFA level in biodiesel, the lower the NOx formation. Most of the studies found in the literature were focused on biodiesel–diesel blends, biodiesel–alcohol blends, and biodiesel–diesel with additives, whereas, very few studies were found investigating the combustion characteristics of 100% biodiesels (neat biodiesel) based on their fatty acids’ compositions. In addition, there is a clear gap in the literature on how the three levels of biodiesel saturation (highly saturated, mono-unsaturated, and poly-unsaturated) affect the combustion and emission characteristics. Furthermore, studies reported in the literature were performed in various types of diesel engines (with different operating parameters such as compression ratios, injection timings, and so on). When testing the neat biodiesels with different saturation levels in the same engine (i.e., same operating parameters of the engine), it is essential to investigate the combustion and emission characteristics of neat biodiesel fuels. Hence, the aim of this study is to examine the combustion and emission behavior of the highly saturated, mono-unsaturated, and poly-unsaturated methyl esters in the same diesel engine. Furthermore, a parameter, known as the degree of unsaturation (DU) (product of mono and poly unsaturation), is used. The combustion characteristics of biodiesel will also be examined in this study, which are dependent on the percentage of DU. A single-cylinder stationary variable compression ratio diesel engine was use for the biodiesel test with varied saturation levels. The objectives of this investigation are as follows: (i) to produce biodiesels with various levels of saturation; (ii) the measurement and comparison of the biodiesels’ fuel properties based on their saturation levels; (iii) to further understand the behavior of FAME compositions, investigating the combustion and emission characteristics of neat biodiesels under the same engine operating conditions; and (iv) to study the effect of degree of unsaturation on combustion and emission characteristics. Six biodiesel fuels were used based on their fatty acids’ profile: coconut, palm, castor, karanja, jatropha, and waste cooking oil biodiesel. These six biodiesel samples are divided into two groups: (i) group I, fuels containing very high saturated, mono-unsaturated, and poly-unsaturated methyl esters; (ii) group II, fuels containing low saturated, mono-unsaturated, and poly-unsaturated methyl esters. Finally, the combustion and emission qualities of biodiesels will be compared in order to determine the correlations between the various saturation levels.