Molecular Contribution of Fatty Acid Esters in Biodiesel Fueled CI Engines

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.

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, 12, 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, 24, 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 NO_X emission [14, 26, 27], are also contributed by these unsaturated FAEs; especially by the alkyl esters of oleic acid and linoleic acid. In specific, NO_X 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].

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.

2. Waste animal fat-oil (WaFO) biodiesel production

Waste animal fats and oil used in this study includes beef tallow, chicken fat, mutton suet and pork lard, and fish fat oil; and were rendered from wide variety of animal wastes. For instance, beef tallow was rendered from tannery fleshing and meat processing wastes; while, chicken fat, mutton suet and pork lard were rendered from their respective wastes collected from different slaughter house units. Besides, waste fish fat oil was directly procured from the leather tanneries associated with oil tanning process; in its existing form. Here, waste fats (tallow, suet, lard and chicken fat) were rendered from their respective wastes using dry rendering technique; which involved with autoclaving each waste individually, at 120°C and 2 bar pressure [30]. Post rendering, each fat was filtered to remove any solid residues, washed with distilled water to remove suspended residues; and was dehydrated to remove residual moisture content by heating at 110°C. Likewise, similar pre-treatment process was followed for waste fish fat oil. Following that, the pre-treated waste fats and oil were refined to remove phospholipids by means of degumming using orthophosphoric acid as explained by Srinivasan et al., [22, 30].

Post refining, tallow, suet, lard, chicken fat and fish fat oil were mixed in equivalent proportion; and was blended into a homogeneous feedstock (WaFO). The blended waste Fat-oil (WaFO) was esterified by refluxing it with ethanol and 1 wt.% of concentrated sulfuric acid (conc. H₂SO₄), in order to reduce its overall free fatty acid (FFA) content so as to avoid formation of soap during transesterification. For the production of WaFO biodiesel, the WaFO sample was transesterified by following the under mentioned reaction parameters: (i) oil to ethanol molar ratio: 1:8; (ii) catalyst concentration: 0.75 wt.% of potassium hydroxide (KOH); (iii) reaction temperature: 72°C; (iv) reaction time: 150 mins. Here, the volume of ethanol was calculated from the optimized molar ratio using a simple equation Eq. (1), which correlates the molecular weight and density of triglycerides and ethanol [30].

$$V_{\mathit{\text{alchol}}}=\frac{V_{\mathit{\text{sample}}}\ast m\ast {\rho }_{\mathit{TG}}\ast M_{\mathit{\text{alchol}}}}{[92.17-3+\left[3\left({\sum }_{i=1}^n{M}_{\mathit{FA}}\ast x_i)-17\right]\right]\ast {\rho }_{\mathit{\text{alchol}}}}$$E1

Completing the reaction, resultant mixture was decanted in a separating funnel for 24 h; when the residual glycerol got separated from the WaFO biodiesel and settled down at the bottom. Lastly, the separated biodiesel was washed with hot distilled water successively to remove residual ethanol and glycerol, soaps, and salts; and was dried at 110°C to remove moisture content from it.

3. Evaluation of fuel properties for WaFO and WaFO biodiesel

For the purpose of Characterization of fatty acids, the WaFO was processed into test sample as per the standard preparation technique [31], while, the WaFO biodiesel was tested directly, in a Gas chromatography-Mass Spectrometer (GC–MS); and was studied from their spectral data. Accordingly, WaFO reported oleic acid, palmitic acid, stearic acid as its dominant FAs; with their concentration as 35.41%, 24.24% and 16.15%, respectively. In the same way, WaFO biodiesel reported the ethyl esters of characterized dominant FAs, with their concentration as 35.63%, 27.73% and 18.34%, respectively. Summing up, WaFO Biodiesel was made up of 51.4% of saturated FAEs and 49.18% of unsaturated FAEs; and suggested that the resultant biodiesel was evenly balanced with both saturated and unsaturated FAEs, which reflected in its molecular formula (C₁₉H₃₇O₂).

Following that, the WaFO biodiesel was assessed for its fuel compatibility with neat diesel, and suitability in CI engines by evaluating its fuel properties in accordance with ASTM D6751 standards. To begin with, Density of WaFO biodiesel was measured using a simple hydrometer, as specified in ASTM D1298 method; and was reported to be 4.14% higher than neat diesel. Next up, ASTM D445 method was followed to measure the kinematic viscosity of WaFO biodiesel using a calibrated glass-viscosity tube, and was found to be 27.96% higher than neat diesel. Again, flash and fire point of WaFO biodiesel were reported to be 80 and 81OC higher than neat diesel, respectively; and were tested in Pensky Martens closed-cup apparatus as described in ASTM D93–16 method. And, cetane number of WaFO biodiesel, evaluated according to ASTM D613 method, was found to be 27.74% greater than neat diesel. Here, higher density, kinematic viscosity, flash point and cetane number for WaFO biodiesel than neat diesel were contributed by the long carbon chained FAEs like ethyl oleate, ethyl palmitate and ethyl stearate; yet, remained significantly lower due to the presence of unsaturated ethyl oleate in it [21, 25, 30, 32].

In contrast, calorific value of WaFO biodiesel was reported 11.1% lesser than neat diesel, upon tested inside a bomb calorimeter as per ASTM D240 method; and this reduction was clarified by its fuel bound oxygen molecules and absence of sulfur content, which fails to contribute a significant share towards its calorific value [33]. Looking into its chemical properties, saponification value and iodine vale of WaFO biodiesel was found to be 191.38 mg KOH/gm and 53.26 g I₂/100 gm, on account of its increased concentration of unsaturated FAEs. Meanwhile, the acid value of WaFO biodiesel was estimated as 0.11% by using ASTM D664 method, which acknowledged the effective conversion of FFAs and monoglycerides into fatty acid esters. Lastly, analytical data related to chemical composition of WaFO biodiesel stated its average molecular weight to be 35% higher than neat diesel; whose carbon and hydrogen content was estimated to be 9.92 and 12.06% lesser than the latter fuel. Moreover, WaFO biodiesel exhibited 10.75% of oxygen content available in it; and is regarded as an oxygenated biofuel in view of this fuel bound oxygen content. Table 1 summarizes the fuel properties of WaFO biodiesel and neat diesel evaluated as per ASTM standards, along with their permissible range and testing methods.

Properties	Diesel	WaFO biodiesel	ASTM standards	Permissible range
Density, kg/m^3	837 ± 7.6	871.68 ± 5.12	D1298	—
Specific gravity	0.84 ± 0.008	0.872 ± 0.006	D1298	0.86–0.90
Kinematic viscosity, mm^2/s	3.72 ± 0.24	4.76 ± 0.21	D445	1.90–6.0
Flash point, °C	64 ± 2.5	144 ± 2.6	D93-16	130 min
Fire point, °C	72 ± 2.2	153 ± 2.45	D93-16	—
Cloud point, °C	0 ± 1	1.5 ± 1	D2500	−3 to 12
Pour point, °C	−15 ± 1.5	−2.7 ± 1.5	D7346-15	−15 to 10
Cetane Number	50 ± 1.4	63.87 ± 1.6	D613	47 (min)
Calorific value, MJ/kg	42.6 ± 0.1	37.87 ± 0.1	D240	35 to 43
Saponification value, mg KOH	—	191.38 ± 1.1	D5558	—
Acid Value, %	—	0.11 ± 0.02	D664	0.80 max
Iodine value, g I2	—	53.26 ± 0.92	D5554	120 max
Carbon, wt.%	85.16 ± 1.14	76.71 ± 1.08	D5291	—
Hydrogen, wt.%	14.26 ± 0.75	12.54 ± 0.73	D5291	—
Oxygen, wt.%	0	10.75 ± 0.52	D5291	—
Sulfur, wt.%	9.87 ± 0.48	2.67 ± 0 .22	D5453	—
Phosphorus, wt.%	0.12 ± 0.02	0.001	D4951	—
Molecular formula	C16H28	C19H37O2	—	—
Molecular weight, g/mol	220.39	297.5	—	—

Table 1.

Fuel properties of WaFO biodiesel evaluated as per ASTM standards along with neat diesel and their permissible range.

4. Engine characteristics of WaFO and its dominant FAEs

The evaluation of performance, emission and combustion characteristics of WaFO biodiesel was carried out in a Kirloskar TV1 single cylinder CI engine equipped with in-built water cooling system, with Table 2 consolidates the product specifications of the test engine and flue gas analyzer used in this present study [15, 32]. Here, the parameters tested for this present study includes performance characteristics (specific fuel consumption and brake thermal efficiency), emission characteristics (mon- and di- oxides of carbon and nitrogen, unburnt Hydrocarbon emission, and exhaust gas temperature), and combustion characteristics (maximum in-cylinder pressure, ignition delay, heat release rate). For purpose of testing, two different types of samples have been used in this study and are named as follows: blend samples and ester samples. In specific, blend samples consist of B10, B20 and B30 samples, with 10%, 20% and 30% of biodiesel blended in neat diesel, respectively; and will be used for assessing the trends of biodiesel’s performance in engine. On the other hand, ester samples consist of characterized dominant FAEs, ethyl oleate, ethyl palmitate and ethyl stearate, blended in the concentration with respect to B20 blend; and are named as oleate blend, palmitate blend, stearate blend. For better understanding, the blending of blend and ester samples are represented in form of mathematical correlations Eqs. (2) and (3) [15]; and are used for calculating the volume of diesel and biodiesel/ester required for making the necessary blends.

Kirloskar engine TV 1 specifications		AVL DI GAS 444 N (five gas analyzer)
Type: four stroke, single cylinder water cooled		Measurement	Resolution
Rated power	5.2 kW	CO [0–15% Vol]	0.0001% Vol
Rated speed	1500 rpm	HC [0–20000 ppm Vol]	1 ppm/10 ppm
Bore diameter (D)	87.5 mm	CO2 [0–20% Vol]	0.1% Vol
Stroke (L)	110 mm	O2 [0–25% Vol]	0.01% Vol
Compression ratio	17.5:1	NOX [0–6000 ppm Vol]	1 ppm Vol

Table 2.

Product specification of test engine and flue gas analyzer [15, 32].

$$\text{Amount of diesel}\left(\mathrm{ml}\right)={\mathrm{V}}_{\text{overall}}\ast \left\{1-\left[{\mathrm{\phi }}_{\mathrm{E}}\ast {\mathrm{\psi }}_{\mathrm{B}}\right]\right\}$$E2

$$\text{Amount of biodiesel}\left(\mathrm{ml}\right)={\mathrm{V}}_{\text{overall}}\ast \left\{{\mathrm{\phi }}_{\mathrm{E}}\ast {\mathrm{\psi }}_{\mathrm{B}}\right\}$$E3

Here, B20 blend was identified as ideal proportion for understanding the influence of FAEs in deciding the engine characteristics of WaFO biodiesel; and was acknowledged due to the increased performance of any biodiesel at their 20% blend [34]. In addition, blending ester samples reduced the technical challenges associated with low temperature crystallization and increased viscosity, besides their cost. Table 3 reports the overall engine characteristics of WaFO biodiesel blends, along with neat diesel averaged over their engine loads. For ensuring accuracy in results, all the experimental runs were performed in triplicates and are reported in form of mean ± standard error, wherever applicable.

Parameters	Unit	Diesel sample	B10 blend	B20 blend	B30 blend
Pmax	Bar	51.6 ± 1.24	56.8 ± 1.19	58 ± 1.22	59.4 ± 1.32
iHRR	kJ/m^3.deg	56.3 ± 1.58	62.28 ± 1.62	63.1 ± 1.72	64.4 ± 1.64
ID	°CA	19.2 ± 0.52	16.8 ± 0.57	16.2 ± 0.56	15.6 ± 0.6
SFC	kg/kW-hr	0.31 ± 0.02	0.36 ± 0.03	0.38 ± 0.02	0.4 ± 0.02
BTE	%	32.3 ± 0.47	30 ± 0.51	28.1 ± 0.49	27.3 ± 0.48
CO Emission	%	0.25 ± 0.03	0.17 ± 0.02	0.15 ± 0.04	0.13 ± 0.03
CO2 Emission	%	5.8 ± 0.57	6.8 ± 0.59	7.3 ± 0.62	7.7 ± 0.61
NOX Emission	PPM	581.3 ± 10.3	703.8 ± 11.4	734.9 ± 11.2	771.1 ± 10.6
HC Emission	PPM	57.8 ± 2.5	49.6 ± 3.11	52.4 ± 2.97	55 ± 3.2
EGT	°C	208.8 ± 5.16	252.2 ± 6.07	263.8 ± 4.27	273.6 ± 7.12

Table 3.

Engine characteristics of blend samples, averaged over the engine load.

4.1 Combustion characteristics

4.1.1 Maximum/peak In-cylinder pressure (Pmax)

In general, in-cylinder pressure inside the cylinder signifies the degree of homogenous mixing of injected fuel with air, and helps in enhancing the rate of combustion. From Table 3 and Figure 1, both blend and ester samples reported higher in-cylinder pressure against neat diesel sample owing to their higher cetane number, which shortened their ignition delay (ID), thereby allowing them to get combusted using their fuel bound oxygen content [32, 35, 36]. Accordingly, B10 blend reported 10.33%, B20 blend reported 12.64% and B30 blend reported 15.46%, higher peak in-cylinder pressure than compared to neat diesel. Likewise, stearate blend reported 3.27%, palmitate blend reported 5.1% and oleate blend reported 6.75%, higher peak in-cylinder pressure than compared to neat diesel.

Figure 1.

Maximum in-cylinder pressure of WaFO B20 blend and ester samples.

Upon comparing ester samples with Biodiesel (B20) blend, oleate blend reported minimal variation in peak in-cylinder pressure by 5.52%, followed by palmitate blend and stearate blend reporting 6.72% and 8.3%, respectively. Here, the reduced peak pressure for palmitate and stearate blend signifies their early start of combustion (SOC) citing their shortened ID, besides their reduced concentration. In contrast, oleate blend reported marginal reduction in peak pressure, citing its unsaturation, which reduced its cetane number and prolonged its ID. This prolonged time delay accumulated a significant amount of fuel during premixed burn phase, and got combusted using the available fuel bound oxygen during the diffusion combustion phase [37]. Correlating this, presence of saturated FAEs (ethyl palmitate and ethyl stearate) in WaFO biodiesel initiated the early SOC during the premixed combustion phase, because of their higher cetane number; and provided sufficient activation energy for initiating the combustion of unsaturated FAEs (ethyl oleate, etc.) during the controlled combustion phase. Moreover, in-cylinder pressure increased with engine load for both blend and ester samples, considering the increasing amount of fuel combusted, intending to meet the energy demand of the engine.

4.1.2 Instantaneous heat release rate (iHRR)

More often, heat release rate curve briefs out about the time line of the combustion stroke, indicating the Start Of Injection (SOI), Ignition Delay (ID), Start of Combustion (SOC); and ultimately, the amount of heat released during the combustion of fuel [38]. From Table 3 and Figure 2, both blend and ester samples happened to report higher iHRR than neat diesel citing their early initiation of combustion and its prolonged duration, which provided adequate time for the accumulated low volatile fuel to undergo combustion during both premixed phase and diffusion combustion phase [39]. In addition, fuel bound oxygen played a crucial role in ensuring the complete oxidation of these FAEs in blend and ester samples. Comparatively, B10 blend reported 11.36%, B20 blend reported 12.97% and B30 blend reported 15.47%, higher heat release rate than compared to neat diesel. In like manner, stearate blend reported 3.75%, palmitate blend reported 5.73%, and oleate blend reported 6.82%, higher heat release rate than compared to neat diesel.

Figure 2.

Instantaneous heat release rate of WaFO B20 blend and ester samples.

Relative to Biodiesel (B20) blend, oleate blend reported minimal variation in iHRR (by 5.39%), followed by palmitate blend (6.41%) and stearate blend (8.12%), respectively. From above comparison, it was evident that HRR of oleate blend remained higher owing to its unsaturation content, resulting in prolonged ID and reduced premixed combustion phase; helping the accumulated low volatile fuel to oxidize completely using its fuel bound oxygen during the diffusion combustion phase. In contrast, palmitate blend exhibited higher iHRR because of its saturation content, which required less activation energy, and minimal ID; thereby initiating early combustion and providing enough energy for the progressing combustion. Similar trend was reported for stearate blend; however, it remained lower than all other ester samples due to the reduced concentration of ethyl stearate in the diesel blend. Collectively, it can be inferred that saturated FAEs (ethyl palmitate and ethyl stearate) were responsible for the activities during the premixed combustion phase, especially the early ignition of WaFO biodiesel. Following this, unsaturated FAEs (ethyl oleate) were found to be playing crucial role in enhancing the overall HRR through their delayed combustion during diffusion combustion phase, thereby liberating high amount of heat energy. Like Pmax, HRR also increased with engine load for both blend and ester samples, considering the increasing amount of fuel combusted, in order to meet the energy demand of the engine.

4.1.3 Ignition delay (ID)

Ignition delay of the fuel signifies the delay period noted between the SOI and SOC; and is always represented in terms of crank shaft angle. From Table 3 and Figure 3, both blend and ester samples reported reduced ID due to their high cetane number; and played a significant role in initiating the combustion well before the neat diesel. As a matter of fact, this ID is widely influenced by both physical and chemical delay; but is predominantly influenced by chemical delay [40]. Accordingly, variation in ID between neat diesel and B10 blend, B20 blend and B30 blend were found to be 2.4°, 3° and 3.6° CA BTDC, lower than the former. In the same manner, variation in ID between neat diesel and oleate, stearate and palmitate blend were reported to be 0.4°, 1° and 1.2° CA BTDC, lower than the diesel sample.

Figure 3.

Ignition delay of WaFO B20 blend and ester samples.

Amongst ester samples compared with B20 biodiesel blend, oleate blend reported 2.6° CA BTDC, stearate blend reported 2° CA BTDC, and palmitate blend reported 1.8° CA BTDC, higher ID. It follows that, both palmitate and stearate blends exhibited shortened ID owing to their higher cetane number because of their higher saturation. Yet, higher delay period than B20 (biodiesel) blend was explained by the reduced availability of ethyl palmitate and ethyl stearate in their blend samples. On contrary, oleate blend reported longer ID than other ester samples due to their low cetane number, accounting its unsaturation and increased availability; besides its high viscosity. Eventually, WaFO biodiesel reported shortened ID because of its saturated FAEs (ethyl palmitate and ethyl stearate) which exhibited early SOC, and initiated the combustion of their unsaturated counterparts. Adding to this, the unsaturated FAEs (ethyl oleate) themselves had higher CN than diesel, which allowed it to initiate early SOC. Here, ID of test samples reduced with increasing engine load, citing the increased availability of fuel. Especially, both blend and ester samples reported lower ID in view of more amount of fuel injected, which indirectly signified increased cetane number.

4.2 Performance characteristics

4.2.1 Specific fuel consumption (SFC)

In general, Specific fuel consumption reports about the fuel requirement of the engine, for producing 1 unit of power [41, 42]. From Table 3 and Figure 4, it can be noted that diesel sample reported lowest SFC amongst all test samples owing to its superior calorific value, and low density. As well, absence of long to very long carbon chained molecules in the diesel simply reduced its viscosity, which enhanced its rate of atomization and vaporization. Supporting this, B10 blend reported 18.77%, B20 blend reported 23.20% and B30 blend reported 29.87%, higher SFC than neat diesel; whereas, ester samples reported higher SFC by 5.8%, 9.66%, 13.67% for stearate blend, palmitate blend and oleate blend, respectively.

Figure 4.

Specific fuel consumption rate of WaFO B20 blend and ester samples.

In comparison with B20 (biodiesel) blend, oleate blend reported 7.71%, palmitate blend reported 10.97%, and stearate blend reported 14.10%, lower SFC. Here, oleate blend reported highest SFC amongst other ester samples owing to its unsaturation, resulting in reduced calorific value, which demanded more fuel to meet the energy equivalence demand. Besides, increased density and kinematic viscosity favored poor atomization and vaporization, thereby leading to poor combustion; and again demanded surplus fuel to satisfy the energy demand. Meanwhile, both stearate blend and palmitate blend reported low SFC due to their slightly higher calorific value, which helped in deriving maximum heat energy output. Inspite of long carbon chains contributing to their increased density and viscosity, these samples reported low rate of fuel consumption citing their reduced availability in the blend sample and superior calorific value of diesel, itself. Summing up, both saturated (ethyl palmitate and ethyl stearate), and unsaturated FAEs (ethyl oleate) are responsible for the increased SFC of WaFO biodiesel, accounting their long carbon chains. Also, unsaturation in the WaFO biodiesel had negative impact on its overall calorific value, thus consuming more fuel to produce the equivalent work. Oddly, trend of SFC curve reduced with increasing engine load for all test samples, suggesting that the brake power increased along with engine load [43].

4.2.2 Brake thermal efficiency (BTE)

In common practice, the capability of the engine to produce actual mechanical work output by converting the stored chemical energy in the fuel is signified by its brake thermal efficiency; and correlates brake power with the fuel power [32]. From Table 3 and Figure 5, compared with neat diesel, lower BTE was reported for B10 blend by 10.29%, B20 blend by 13.05% and B30 blend by 15.5%; and, for stearate blend by 2.47%, palmitate blend by 4.68%, and oleate blend by 7.62%. Here, high BTE for diesel, inspite of low cetane number, was explained by its superior calorific value and low volatility, which allowed it to undergo complete combustion especially during its diffusion combustion phase; inspite of its lack of fuel bound oxygen content [44].

Figure 5.

Brake thermal efficiencies of WaFO B20 blend and ester samples.

Compared to B20 (biodiesel) blend, oleate blend reported 6.24%, palmitate blend reported 9.65%, and stearate blend reported 12.22%, higher BTE. Here, stearate blend exhibited highest BTE amidst other ester samples because of its increased calorific value; and reduced availability of ethyl stearate in the blend sample, which had a significant effect on its resultant viscosity. Moreover, shortened ID of ethyl stearate provided it sufficient time to get combusted during premixed combustion phase, and supply sufficient energy for the accumulated diesel to get combusted rapidly during the diffusion combustion phase; thereby resulting adequate amount of heat energy. Likewise, palmitate blend also reported similar phenomenon; however, reduced BTE was explained by its increased concentration than stearate blend. Unlike this, oleate blend reported lowest BTE amongst ester samples citing its unsaturation, inferior calorific value, and increased rate of viscosity; hence, requiring more amount of fuel for energy equivalence. However, higher BTE than B20 blend was explained by the reduced availability of ethyl oleate in the blend sample, and its efficacy to undergo complete oxidation using its fuel bound oxygen. Comparing these results, it can be inferred that saturated FAEs (ethyl palmitate and ethyl stearate) initiated combustion during the premixed phase, and provided sufficient activation energy for initiating the combustion of unsaturated FAEs (ethyl oleate) during the diffusion combustion phase. Besides, in view of early SOC due to shorted ID, FAEs in WaFO reported early ignition and underwent complete oxidation using its fuel bound oxygen content; thus, reporting similar BTE like neat diesel. Again, BTE of all test samples increased with engine load, considering the increasing amount of fuel combusted, in order to meet the energy demand of the engine [45].

4.3 Emission characteristics

4.3.1 Carbon monoxide (CO) emission

In general, CO emission is considered as secondary by-product during combustion; and its presence in exhaust gas signifies incomplete combustion of fuel inside engine cylinder. Infact, CO emission arises in case of poor atomization, improper air-fuel mixing, deprived oxygen content, insufficient time for completion of combustion, and even engine’s operating conditions; in addition to fuel’s molecular properties like unsaturation, C/H ratio, and even aromaticity [46]. From Table 3 and Figure 6, both blend and ester samples reported lower CO emission against neat diesel because of their fuel bound oxygen content, which was responsible for the completion of their oxidation; and, leaving behind only a small portion of partially combusted CO emissions. Relatively, CO emission remained reduced for B10 blend by 35.66%, B20 blend by 45.76% and B30 blend by 52.04%; and, for palmitate blend by 10.52%, stearate blend by 22.31%, than compared to neat diesel. In contrast, oleate blend reported higher CO emission (by 15%), than compared to neat diesel sample.

Figure 6.

Carbon monoxide emission of WaFO B20 blend and ester samples.

As compared with B20 (biodiesel) blend, stearate blend reported 45.86%, palmitate blend reported 70.69%, and oleate blend reported 122.45%, higher CO emission. Here, both palmitate and stearate blends reported higher CO emissions; and was explained by their reduced availability and long carbon chained molecules, inspite of their shortened ID and fuel bound oxygen content. Furthermore, oleate blend reported highest CO emission amongst other test samples on account of its unsaturated double bond in its FA moieties [40, 47]. Besides, delayed combustion encouraged the rapid combustion of accumulated fuel during diffusion combustion phase, thereby increasing the CO concentration. Summarizing this, WaFO biodiesel reported reduced CO emission in view of its fuel bound oxygen molecules in their FAEs; yet, it reported significant traces of CO due to its unsaturated FAEs (ethyl oleate). To be noted, saturated FAEs (ethyl palmitate and ethyl stearate) ensured complete oxidation of WaFO biodiesel by providing sufficient activation energy for its unsaturated counterparts. Again, CO emissions of both blend and ester samples increased along with engine load, and were explained by the increasing amount of fuel injected into the cylinder to meet the energy demand of the engine.

4.3.2 Carbon dioxide (CO₂) emission

Unlike CO emission, CO₂ emission is considered as the primary product during combustion; and its presence in exhaust gas signifies the completion of fuel’s combustion inside the engine cylinder. Again, concentration of CO₂ emission is influenced by the fuel’s molecular properties like unsaturation, C/H ratio, and even aromaticity; besides the operating condition of the engine [46]. From Table 3 and Figure 7, both blend and ester samples reported higher CO₂ emission than diesel sample citing the presence of their fuel bound oxygen molecules and their higher cetane number; which prolonged its combustion duration for their complete oxidation. Equally important, higher concentration of CO₂ emission was also contributed by the long carbon chains in their FAE molecules. Accordingly, B10 blend reported 23.59%, B20 blend reported 35.76% and B30 blend reported 45.58%, higher CO₂ emission than compared to neat diesel. Likewise, stearate and palmitate blends reported increased CO₂ emission by 6.23% and 15.41% higher CO₂ emission, respectively; meanwhile, oleate blend reported lower CO₂ emission by 6.97%.

Figure 7.

Carbon dioxide emission of WaFO B20 blend and ester samples.

Upon comparing ester samples with Biodiesel (B20) blend, palmitate blend reported 12.33%, stearate blend reported 18.72%, and oleate blend reported 28.32%, lower CO₂ emission. As a matter of fact, both palmitate and stearate blends reported higher CO₂ concentration than oleate blend on account of their reduced availability and saturation, which improvised their overall effectiveness of combustion. Especially, palmitate blend reported its CO₂ emission closer to B20 blend, stating its higher concentration than stearate blend; and its ability to initiate early combustion, thereby providing sufficient time for the accumulated diesel to combust completely. Meanwhile, oleate blend reported lowest CO₂ emission amongst all test samples on account of its unsaturation and increased availability, which reduced the effectivity of atomization thereby combusting poorly [48]. In addition, delayed SOC allowed it to combust rapidly which hindered its complete oxidation, thereby forming incomplete combustion products. Summing up, higher concentration of CO₂ emission for WaFO biodiesel, inspite of its unsaturation was explained by the presence of its saturated FAEs (ethyl palmitate and ethyl stearate), which initiated early SOC and ensured the progress of combustion of the unsaturated FAEs (ethyl oleate). Again, CO₂ emissions increased along with engine load, and were also explained by the increasing amount of fuel injected into the cylinder to meet the energy demand of the engine.

4.3.3 Nitrogen oxide (NO_X) emission

Often, NO_X emission in exhaust gas is also regarded as secondary by-product during combustion; however, it arises when engine reports high operating temperatures, especially high exhaust gas temperatures. In relevance to that, NO_X emissions due to high EGTs are explained by higher cetane number and fuel bound oxygen content inducing prolonged combustion; besides the viscosity of fuel [30, 32]. From Table 3 and Figure 8, both blend and ester samples reported higher NOx emission due to their shortened ID, and increased viscosity; which increased the overall duration of combustion, and liberate sufficient heat energy fairly enough for producing NO_X emission. In addition, calorific value of these test samples also contributed to this harmful emission. Supporting this, NO_X emission was increased by 22.17% for B10 blend, 28.2% for B20 blend and 36.06% for B30 blend; and 6.55% for stearate blend, 11.8% for palmitate blend, and 17.3% for oleate blend, than compared to neat diesel.

Figure 8.

Nitrogen oxide emission of WaFO B20 blend and ester samples.

In comparison with B20 (biodiesel) blend, oleate blend reported 8.53%, palmitate reported 12.66% and stearate blend reported 16.57%, lower NO_X emission. In specific, palmitic and stearate blend reported lower NO_X emission than B20 blend signifying their early SOC due to shortened ID; and provided sufficient activation energy for initiating the combustion of diesel during diffusion combustion phase. Yet, these samples reported reduced NO_X emission because of their volatility. On the other hand, oleate blend exhibited higher NO_X emission owing to its increased availability, high viscosity, and reduced cetane number which led to its accumulation in event of its delayed SOC. Besides, rapid combustion of this accumulated fuel liberated high temperature inside the cylinder, and produced high NO_X emission. Outlining these results, higher NO_X emission of WaFO biodiesel was influenced by its unsaturated FAEs (ethyl oleate), which liberated very high temperatures inside the cylinder, thereby forming high NO_X emissions. Interestingly, saturated FAEs (ethyl palmitate and ethyl stearate) also liberated very high temperatures during premixed phase, besides contributing activation energies to unsaturated FAEs, thus favoring NO_X formation. Like other emissions, NO_X emissions of both blend and ester samples increased with engine load on account of more fuel being combusted inside the engine to meet the energy demand, thereby delivering their equivalent work and heat.

4.3.4 Exhaust gas temperatures (EGT)

EGT from the engine defines the progress of combustion inside the cylinder; and is dependent on the engine’s operating conditions and properties of fuel used. Conventionally, fuel reporting delayed SOC, with prolonged duration exhibits higher EGTs; and these high temperatures contribute to NOx emissions [49, 50]. From Table 3 and Figure 9, both blend and ester samples exhibited higher EGTs accounting their higher cetane number and fuel bound oxygen content; which favored higher rate of combustion and liberated large amount of heat. Moreover, viscosity and calorific value of these samples also contributed to their high EGTs. Supporting this, B10 blend reported 20.97%, B20 blend reported 26.49% and B30 blend reported 31.52%; and stearate blend reported 6.1%, palmitate blend reported 10.62%, and oleate blend reported 15.46%, higher EGTs than compared to neat diesel.

Figure 9.

Exhaust gas temperature of WaFO B20 blend and ester samples.

Amongst ester samples compared with B20 biodiesel blend, oleate blend reported 8.48%, palmitate blend reported 12.23% and stearate blend reported 15.76%, lower exhaust gas temperature. Especially, palmitate and stearate blends combusted earlier due to shortened ID, which forced the highly volatile, accumulated diesel to combust rapidly, and limiting the heat generation and EGT. In case of oleate blend, low cetane number allowed it to undergo prolonged combustion, and assisted the diesel for combustion during diffused combustion and after burning phase; thus liberating large amount of heat and increase its EGT. On the whole, WaFO biodiesel with significant amount of unsaturated FAEs (ethyl oleate) exhibited prolonged combustion accompanied with high rate of combustion using their fuel bound oxygen molecules; and liberated high EGTs [29]. Meanwhile, saturated FAEs (ethyl palmitate and ethyl stearate) contributed to a minimal amount to EGT, accounting their early ignition and supplying of activation energy to the unsaturated FAEs; thus contributing minimal to EGTs. Like NOx emissions, EGT of both blend and ester samples increased with engine load on account of more fuel being combusted inside the engine to meet the energy demand, thereby delivering their equivalent work and heat.

4.3.5 Unburnt hydrocarbon (HC) emission

Unburnt hydrocarbons in exhaust gas signifies the inability of the fuel to get completely combusted near the cylinder wall, owing to reduced flame temperatures near the fuel-rich zones and poor combustion kinetics and quenched flame [51]. From Table 3 and Figure 10, both blend and ester samples displayed reduced HC emission in event of complete oxidation using their fuel bound oxygen content. Adding to this, these oxygen molecules helped in liberating high flame temperatures, and propagated throughout the cylinder and combusted unburnt hydrocarbons. Unfortunately, neat diesel reported traces of HC emission as a consequence of its rapid combustion, owing to its high volatility which reduced the adiabatic flame temperature near the cylinder walls. In comparison with diesel blend, B10 blend reported 17.75%, B20 blend reported 10.91% and B30 blend reported 5.89%; and oleate blend reported 23.71%, palmitate blend reported 27.15%, and stearate blend reported 34.72%, lower HC emissions.

Figure 10.

Hydrocarbon emission of WaFO B20 blend and ester samples.

Relatively, stearate blend reported lowest HC emission (by 27.22%), followed by palmitate blend (18.54%) and oleate blend (14.66%), than compared to B20 (biodiesel) blend. Supporting this, palmitate and stearate blends exhibited lower HC emission, and was clearly evident that presence of oxygen in these samples reduced their HC emission. Explaining this, these saturated FAEs initiated early combustion and provided sufficient temperature inside the cylinder for ensuring complete oxidation of diesel [27]. Whilst, oleate blend reported higher HC emission than other ester samples because of its increased availability and unsaturation; which resulted in poor atomization and vaporization, and reduced the effectiveness of combustion (i.e. in complete combustion) for the liquid droplets present at the localized zone with reduced flame temperatures. Consolidating these results, it can be concluded that WaFO biodiesel combusted completely using its fuel bound oxygen content. Interestingly, unsaturated FAEs (ethyl oleate) in WaFO biodiesel reduced its rate of atomization, thus forming micro fuel droplets; however, they were combusted by the heat energy supplied by the saturated FAEs (ethyl palmitate and ethyl stearate). Here, HC emission of test samples increased with engine load; yet, HC emission of blend and ester samples remained lower than diesel sample even at higher loads due to high engine temperatures, besides their high cylinder pressures [52].

5. Conclusions

Thus, this present chapter strongly concludes that the overall engine characteristics of a biodiesel is contributed by its FAEs; and are influenced by their molecular properties including their chain length and unsaturation. Accordingly, engine characteristics, which includes their performance, combustion and emission characteristics of WaFO biodiesel were influenced by its dominant FAEs, and following were the key conclusions deduced from the above study:

1. Ethyl palmitate and ethyl stearate were identified as dominant saturated FAEs, which were responsible for initiating early combustion due to their higher cetane number, and contributing to higher efficiencies owing to their high calorific values. On the other hand, ethyl oleate was characterized as the dominant unsaturated FAEs, and was acknowledged for prolonging the combustion duration due to its unsaturation and need for high activation energy.

2. High cylinder pressure and heat release rate were explained by early SOC during premixed phase by saturated FAEs, which initiating the combustion of unsaturated FAEs using fuel bound oxygen, and liberated large amount of heat and temperature.

3. Inspite of complete oxidation of both saturated and unsaturated FAEs, biodiesel reported reduced thermal efficiencies and increased fuel consumption rate in view of their inferior calorific value than neat diesel. In addition, slightly higher density and viscosity also setback biodiesel’s overall performance in engine.

4. Increased concentration of completely combusted products and reduced concentration of incompletely combusted products were acknowledged by the fuel bound oxygen content of biodiesel. Overall emission characteristics of biodiesel were improved by prolonged combustion of unsaturated FAEs, which was further improvised by saturated FAEs.

Based on these conclusions, it is again evident that FAEs decide the overall engine characteristics of their biodiesel; and this work can be used as a preliminary guideline for deciding the idle feedstock for producing biodiesel, which meets the requirement of the engine’s output and application.