Energy-Exergy Analysis of Diesel Engine Fueled with Microalgae Biodiesel-Diesel Blend

: Renewable energy is getting more attention in recent times due to the rapid depletion of fossil fuel reserves. Production and consumption of biofuels derived from biomass has signiﬁcantly increased. In the present work, Spirulina microalgae have been chosen as feedstock for biodiesel production. Diesel and biodiesel were mixed in different volumetric ratios to prepare fuel blends (SBF 0 , SBF 20 , SBF 40 , SBF 60 , SBF 80 , and SBF 100 ). Energy and exergy analysis has been performed on a four-stroke, single-cylinder diesel engine. Experimentation was done under varying loads at 1500 RPM. The effect of multiple loads and blends was investigated for brake power (BP), cooling water losses (Q w ), exhaust gas losses (Q exh ), and unaccounted losses (Q un ). Pure diesel SBF 100 has the highest and lowest exergy efﬁciencies, respectively equaling roughly 31.65% and 29.75%. It has been observed that BP and Q w increase with the increase in load whereas Qexh and Qun show a decreasing trend. It was also observed that with an increase in blending, Qw increases while Qexh decreases. In the exergy analysis, it was observed that the exergy destruction rate has a maximum fraction of input exergy values of 46.01% and 46.29% for Diesel and SBF 20 respectively. The system engine sustainability index was in the range of 1.27 to 1.46, which is directly related to exergy efﬁciencies.


Introduction
As the world's energy requirements are rising and conventional sources of energy are becoming depleted, the world is looking for alternative sources to meet its energy demands [1]. The World Energy Investment 2022 report [2] says that the amount of money spent on energy around the world will go up by 8% in 2022. Many developed and developing countries want to become net zero emitters by the end of 2050. India is expected to reach this goal by the end of 2070. Energy security is attracting the attention of researchers these days, as dwindling fossil fuel stocks and rapid increases in energy demands due to growing population [3] have led the researchers to shift their focus towards renewable energy, even though crude oil has significant contribution in the transportation sector [4]. There are growing concerns about the concentration of GHGs in the atmosphere, which is increasing with the increase in population and anthropogenic activities, which have a direct influence on human health. Biofuel is a type of fuel that can be produced directly from natural sources such as plants, animals, municipal wastes etc. [5]. It can be produced from environmentally friendly sources of materials and is additionally referred to as "renewable energy". Crude oil and petroleum products will be in greater demand in a few years to match the demands of the growing population [6]. Due to their nonrenewable nature, crude oil reserves would be depleted as opposed to biodiesel, which can be obtained from trash, food, and non-food sources [7]. Medhat et al. [8] in their review article examine the significance of response surface methodology in the field of compression ignition engine performance and emission attributes using blends of diesel fuel, alternative fuels, and nanoparticle additives for obtaining the best outcome. ORC and ERC system for waste heat recovery from biodiesel-fueled diesel engines. To test the engine's exergy balance, López et al. [28] employed biodiesel made from oil produced from olive pomace to power a diesel engine. Two biodiesels made from petroleum diesel fuel and the methyl esters of soybean oil (SME) and yellow grease (YGME), and a 20% mix of each biodiesel with diesel fuel were used by Canakci et al. [29] for energy and exergy assessments of a diesel engine. The performance parameters of a multifuel diesel engine employing pure diesel and natural gas were studied theoretically [30] and empirically by Ramos et al. [31] in terms of both energy and exergy assessments. Using exergy and energy analysis, it is possible to distinguish between different types of combustion chambers such as hemispherical, trapezoidal and toroidal ones [32]. Because of its greater energy efficiency, biodiesel created with pumpkin seed oil was discovered to be a viable alternative to traditional diesel. Therefore, toroidal combustion chambers had superior engine characteristics than the other chambers. Chandra et al. [18] used Pithecellobium dulce seed oil (PDSO) for transesterification to produce methyl esters. As PDSO contains 2.12% free fatty acids (FFA), an alkaline-based potassium hydroxide (KOH) catalyst was utilized to produce methyl ester and found to be effective. At a 1:6 molar ratio, 60 • C reaction temperature, 0.8 weight percent catalyst, and a reaction time of 90 min, an optimum PDSOME yield of 93.2% is attained. Medhat et al. [33] investigated biodiesel that was trans-esterified from Scenedesmus obliquus algae, because pure biodiesel has drawbacks when used in engines. Compared to an exquisite biodiesel mix, it was discovered that CO 2 and Nox levels had increased while HC, CO, and O 2 levels had decreased. They concluded that adding pentane to a biodiesel blend considerably enhanced engine performance. The present work and literature comparison is represented in Table 1.
Although numerous research has looked at the energy and pollutant characteristics of alternative fuels, particularly biodiesels, there are very few studies on the effects of microalgae biodiesel and diesel binary blends on the direct-ignition CI engine under different loading conditions. We examined the quantity and quality of energy and exergy in a direct ignition diesel engine that was running with the microalgae biodiesel and diesel blends. Additionally, each fuel blend's sustainability index was determined.
Microalgae-based oil feedstock has an advantage over conventional biodiesel feedstock in that it does not add to the contentious "food vs. fuel" debate. The current investigation is to study the variation of loading conditions and blending ratios on the energetic and exergetic properties of microalgae biodiesel. The energy and exergy analysis for different blends and loading conditions of biodiesel will be worked on more by testing with a single cylinder diesel engine. The way biodiesel burns and what it puts out into the air was studied. The distribution of input fuel energy was observed and the analysis of sustainability was done to uncover the thermodynamics of irreversible processes. The outcomes of such a study could be very valuable to engineers, designers, and researchers in figuring out the best fuel mixtures and engine operating settings to produce more cost-effective and environmentally friendly operations.  Figure 1 illustrates a flowchart for production of microalgae biodiesel. Microalgae s extracted in the first step, followed by drying and dewatering in oven at 110 • C under atmospheric conditions (1 atm pressure, 27 • C). In the next step, oil is extracted by application of heat using a Soxhlet apparatus, keeping the reaction temperature about 65-70 • C using n-hexane as a solvent. The oil thus produced is utilized to produce biodiesel via a transesterification reaction of oil with methanol in the presence of catalyst. Biodiesel and glycerin are the outputs derived from transesterification reaction. A separating funnel was used to separate biodiesel and glycerin. The biodiesel produced was washed thoroughly with water to remove the remaining impurities. After water washing, the biodiesel was taken into a tank and utilized for research on diesel engines by blending it with pure diesel.

Biodiesel Production
Biodiesel can be produced by various methods such as micro-emulsification, transesterification, pyrolysis, etc. It has been found out through extensive literature survey that the transesterification process (TP) is the most effective method for biodiesel production as it has high conversion rates, economical, mild reaction conditions and the biodiesel obtained has properties closer to that of petroleum diesel, making it suitable for industrial production [2,5,7]. A wide variety of feedstocks could be utilized for the TP process for production of biodiesel as shown in Table 2, out of which the author has chosen Spirulina microalgae owing to the higher lipid contents present in them. Figure 1 shows the schematic process of oil extraction and biodiesel production.

Biodiesel Production
Biodiesel can be produced by various methods such as micro-emulsification, transesterification, pyrolysis, etc. It has been found out through extensive literature survey that the transesterification process (TP) is the most effective method for biodiesel production as it has high conversion rates, economical, mild reaction conditions and the biodiesel obtained has properties closer to that of petroleum diesel, making it suitable for industrial production [2,5,7]. A wide variety of feedstocks could be utilized for the TP process for production of biodiesel as shown in Table 2, out of which the author has chosen Spirulina microalgae owing to the higher lipid contents present in them. Figure 1 shows the schematic process of oil extraction and biodiesel production.

Oil Extraction from Dried Microalgae
The oil was extracted from dried Spirulina microalgae feedstock using a Soxhlet apparatus as shown in Figure 2 [22]. The dried microalgae is presented to a Soxhlet unit, placed on top of the round bottom flask containing n-hexane. A condenser has been put on top of the Soxhlet unit to condense the vapors of n-hexane. The system is heated with a rate of 5 watts per minute, causing vaporization of n-hexane, which rises through the Soxhlet unit to the condenser, becomes condensed, and falls back into the feedstock in the Soxhlet unit. This process is repeated several times until no more extraction of oil from the feedstock is possible.

Oil Extraction from Dried Microalgae
The oil was extracted from dried Spirulina microalgae feedstock using a Soxhlet apparatus as shown in Figure 2 [22]. The dried microalgae is presented to a Soxhlet unit, placed on top of the round bottom flask containing n-hexane. A condenser has been put on top of the Soxhlet unit to condense the vapors of n-hexane. The system is heated with a rate of 5 watts per minute, causing vaporization of n-hexane, which rises through the Soxhlet unit to the condenser, becomes condensed, and falls back into the feedstock in the Soxhlet unit. This process is repeated several times until no more extraction of oil from the feedstock is possible.

Conversion of Oil to Biodiesel
Biodiesel can be made from edible and nonedible oils. Feedstock yield determines the feasibility of employing a certain feedstock for mass production, which can power the automobile sector. In the transesterification process, the fatty acid triglycerides in any type of oil or other fats are changed into fatty acid methyl esters (FAME) by reacting with ethanol in the presence of an NaOH catalyst. The transesterification process of microalgae oil to make biodiesel includes the following steps: first, before the production of biodiesel, the acid value was determined (found less than 2%) [40] to avoid soap formation in the succeeding step. The microalgae oil was preheated to 60 • C in a 500 mL beaker. After that, oil and ethanol (molar ratio 1:6) and NaOH (0.6 wt% oil) were taken in a three-neck flat bottom flask. This flask was placed over hot plate magnetic stirrer at 60-64 • C and agitated at 500 rpm for 120 min. The processed oil was placed into a conical separating funnel. After a 24 h reaction period, two layers of the treated mixture were formed. Biodiesel, having a lower density, floats on top while higher-density glycerol sits at the bottom of the separating funnel. The two layers were separated through the tap provided in separating funnel. After that, the biodiesel was washed using 50 to 70% distilled water to get rid of the impurities, excess alcohol and unreacted catalyst. This washed sample was then placed again in separating funnel and the water was drained off through the tap. After that, the biodiesel was taken in a beaker and heated to over 100 • C for 25 to 30 min to remove the moisture from the biodiesel. The biodiesel formed was then blended with petro-diesel fuel in 20%, 40%,60%, and 80% volume fractions respectively. SBF 20 , SBF 40 , SBF 60 and SBF 80 are the names of blended fuel biodiesels made from microalgae oil. The fatty acid composition of the produced biodiesel is shown in Table 3.

Test Fuel for Engine
As test fuels, diesel and Spirulina algae biodiesel mixes were used. The baseline fuel was pure diesel with no additives. Spirulina algae biodiesel was mixed with diesel in various volume fractions to form SBF 20 , SBF 40 , SBF 60 , SBF 80 , and SBF 100 as test fuels. It is possible to use algae biodiesel as a replacement to lessen the added strain on diesel. Algal biodiesel's main advantage is that it lowers greenhouse gas emissions [10][11][12][13]. Table 4 lists the essential chemical characteristics and other fuel characteristics of the used fuels. Spirulina algae biodiesel fuel blends were created in accordance with the American Society for Testing Materials (ASTM) standard based on volume to test the engine while preserving the essential characteristics within permissible limits [41]. Algae oil biodiesel was measured and combined with diesel fuel in varied ratios of 20, 40, 60, and 80.

Research Engine
Four-stroke internal combustion diesel engines were used in the experiment. The test engine is shown in Figure 3 of the schematic diagrams and Table 5 below gives a technical description of the engine. An experimental setup included dynamometers, fuel tanks, fuel control valves, data acquisition systems (DAQ), exhaust gas analyzers, and sensors. A stability test was done on the engine to make sure that data recording would be as accurate Appl. Sci. 2023, 13, 1857 8 of 23 as possible. The engine was started under no-load conditions, running at a steady speed of 1500 rpm, which continued for 25 min to get it stabilized. The performance parameters were calculated after stable operating conditions were achieved in engine on application of different loads. The engine was cooled by letting water flow through the cooling jacket.

Research Engine
Four-stroke internal combustion diesel engines were used in the experiment. The test engine is shown in Figure 3 of the schematic diagrams and Table 5 below gives a technical description of the engine. An experimental setup included dynamometers, fuel tanks, fuel control valves, data acquisition systems (DAQ), exhaust gas analyzers, and sensors. A stability test was done on the engine to make sure that data recording would be as accurate as possible. The engine was started under no-load conditions, running at a steady speed of 1500 rpm, which continued for 25 min to get it stabilized. The performance parameters were calculated after stable operating conditions were achieved in engine on application of different loads. The engine was cooled by letting water flow through the cooling jacket.

Energy Calculations
Energy and exergy analysis is important for the design aspect of the engines as it can minimize the losses when using energy and exergy balance data [42]. This analysis deals with the first and second laws of thermodynamics on the IC Engine [43]. For the analysis, the engine was considered as a control volume under steady-state conditions as shown in Figure 4; due to limitations of the engine setup, some assumptions were made, which allow the author to simplify the thermodynamic calculations, and are as follows: • The air-fuel mixture for combustion and exhaust gases were supposed as ideal gases.

•
The total energy produced by the fuel-air mixture was considered to be the energy input to the control volume; however, some part of it leaves as exhaust and other losses.  For energy and exergy analysis, the chemical reaction represented in Equatio was used to determine the emission characteristics data at the experimental setup.
In the present study, the energy-exergy analysis includes finding major losses w the system, such as water cooling losses (Qw), exhaust gas losses (Qexh), along with o miscellaneous losses (Qun). These analyses were done based on law of conservatio mass and energy as represented in Equations (2) and (3), respectively.
where min is the mass flow rate at the inlet and mout is the mass flow rate at outpu indicates the net heat input and h the specific enthalpy, respectively. Energy balance ory states that to calculate the heat lost from the engine to cooling water and heat ca For energy and exergy analysis, the chemical reaction represented in Equation (1) was used to determine the emission characteristics data at the experimental setup.
In the present study, the energy-exergy analysis includes finding major losses within the system, such as water cooling losses (Q w ), exhaust gas losses (Q exh ), along with other miscellaneous losses (Q un ). These analyses were done based on law of conservation of mass and energy as represented in Equations (2) and (3), respectively.
where m in is the mass flow rate at the inlet and m out is the mass flow rate at output. Q indicates the net heat input and h the specific enthalpy, respectively. Energy balance theory states that to calculate the heat lost from the engine to cooling water and heat carried by exhaust gases, the coolant inlet and outlet temperatures were needed. The flow rate of water was measured using a rotameter [44]. Net energy flow in (Q in ), the power developed by the engine (BP), water cooling losses (Q w ), exhaust gas losses (Q exh ), along with other miscellaneous losses (Q un ) were calculated by using the following Equations (4)-(8) The energy analysis is governed by the second law of thermodynamics, the principle of conservation of mass and the principle of conservation of energy. So, in the second part of the analysis, we started the exergy analysis of the IC engine by defining the exergy balance. The ratio of work done (BP) produced at the output shaft to the input fuel energy(Q in ) is termed as the piston thermal efficiency of the engine control volume, as described in Equation (9).

Exergy Calculations
The maximum amount of useful work that can be obtained from any process dealing with two thermal reservoirs, the equation for exergy balance for the selected engine control volume, is represented in Equations (10) and (11).
Ex air + Ex f uel = E(m out × ∈ out ) + Ex heat + Ex work + Ex dest The terms Ex air , Ex f uel represent the exergy transfer rate for air intake and fuel, respectively, whereas Ex heat , Ex work and Ex dest represent the exergy rate heat transfer from the source to the control volume to the environment, the exergy rate developed by the shaft power that is equivalent to the brake power calculated and the exergy rate of destruction, which is a measure of exergy destroyed in the control volume [45], and is the result of several factors such as friction and combustion. These exergy terms were calculated by Equations (12)- (19).
Ex air = m air × c p * air × (T air,i − T amb ) − T amb ln T air,i T amb (13) ∈ chem = RT amb ln( y i y r ) (19) η ex = Ex work Ex in (20) η ex is called exergetic efficiency and is characterized as the ratio of exergetic work output to the total exergy input to the system [46] as mentioned in Equation (20), which is based on the second law of thermodynamics and gives a more accurate calculation for the performance of the system.

Result and Discussion
In the present study, the biodiesel produced from microalgae biomass was utilized as fuel in a research CI engine to measure the energy-exergy characteristics of the engine. Six different fuel blends were operated at four loading levels (25,50,75, and 100% as 0.92 kW, 1.96 kW, 2.91 kW and 3.7 kW). The engine rpm (1500 RPM) and compression ratio (17.5) were kept constant.  Figure 5 shows the variation in the energy distribution of different biodiesel blends with pure diesel. Power output (BP), heat lost to the exhaust gases (Qexh), heat carried away by the cooling water (Qw) and other unaccounted losses (Qun) were the major contributors in the energy analysis. The percentage variation of BP developed compared with the pure diesel was lower by 0.76%, 1.35%, 1.94%, 2.67% and 3.64% for SBF20, SBF40, SBF60, SBF80, and SBF100, respectively, which is mainly because the higher biodiesel density and lower biodiesel LHV relative to diesel fuel values are connected to the use of biodiesel and its blends. In other words, when the engine is operating at the same speed, the mass flow is enhanced as compared to the same volumetric flow of fuel provided by the injection pump, which leads to higher fuel consumption and lowers the power developed [47,48]. The watercooling loss was increased by 1.28%, 2.67%, 4.21%, 6.59%, and 8.69% for SBF20, SBF40, SBF60, SBF80, and SBF100, respectively. This is because biodiesel fuel has a large amount of oxygen, which encourages full combustion and raises the temperature within the cylinder [49], causing loss of power increases by cooling water [50]. The exhaust losses were decreased [27] by 0.35%, 0.80%,1.35%,2.34%, and 3.39% for SBF20, SBF40, SBF60, SBF80, and SBF100, respectively, due to the higher oxygen content present, which promotes the proper combus- The percentage variation of BP developed compared with the pure diesel was lower by 0.76%, 1.35%, 1.94%, 2.67% and 3.64% for SBF 20 , SBF 40 , SBF 60 , SBF 80 , and SBF 100, respectively, which is mainly because the higher biodiesel density and lower biodiesel LHV relative to diesel fuel values are connected to the use of biodiesel and its blends. In other words, when the engine is operating at the same speed, the mass flow is enhanced as compared to the same volumetric flow of fuel provided by the injection pump, which leads to higher fuel consumption and lowers the power developed [47,48]. The water-cooling loss was increased by 1.28%, 2.67%, 4.21%, 6.59%, and 8.69% for SBF 20 , SBF 40 , SBF 60 , SBF 80 , and SBF 100 , respectively. This is because biodiesel fuel has a large amount of oxygen, which encourages full combustion and raises the temperature within the cylinder [49], causing loss of power increases by cooling water [50]. The exhaust losses were decreased [27] by 0.35%, 0.80%,1.35%,2.34%, and 3.39% for SBF 20 , SBF 40 , SBF 60 , SBF 80 , and SBF 100 , respectively, due to the higher oxygen content present, which promotes the proper combustion and hence reduction the exhaust gases temperature [51]. As a result, the unaccounted losses were found to increase with the blends with high microalgae concentrations, and were observed as 2.11%,4.01%, 5.98%, 8.59%, and 11.31% for SBF 20 , SBF 40 , SBF 60 , SBF 80 , and SBF 100 , respectively, when compared to pure diesel [34]. Figures 6-11 represent the variation of fuel energy contribution for diesel and blends of biodiesel and diesel blend. It was observed that the percentage share of BP and Qw increases with the increase in load whereas Qexh and Qun decrease with the increase in load for diesel and all biodiesel-diesel fuel blends. it was observed that the total input energy converted to useful output was 33.12%, heat loss by cooling water was 24.02%, heat loss by exhaust gases was 19.57%, and unaccounted losses were 23.27% for pure diesel at full loading conditions of 3.7 kw [52] It was observed in Figures 6 and 7 that the percentage increases the amount of heat lost via water cooling by 2.57%,2.03%,1.85%, and 1.28% for SBF 20 as compared to diesel for 0.92 kW, 1.96 kW, 2.92 kw and 3.7 kw loading conditions, respectively; the reason behind this is that biodiesel contains more oxygen so there is complete combustion, which increases the cylinder temperature and leads to higher losses [53,54]. Similarly, the exhaust gas losses were decreased by 1.73%, 0.929%, 0.8%, and 0.35%, respectively, for SBF 40             Similar results were observed for SBF20, SBF40, SBF60, SBF80, and SBF100. From Figures  8-11, SBF40 shows percentage increases in cooling water losses of 5.71%, 4.13%, 3.49%, and 2.67%, whereas the percentage decreases in exhaust gases loss were 4.66%, 2.01%, 1.3% and 0.8%, respectively, for 0.92 kW, 1.96 kW, 2.92 kW and 3.7 kW loading conditions compared to pure diesel, as described in Figure 8. From Figure 9, it was observed that, with an increase of biodiesel proportion in the blend, for SBF60, 8.4%, 6.3%, 5.96 and 4.22% water-cooling losses and 7.13%, 3.22%, 2.73%, and 1.36% exhaust losses were observed for an incremental loading condition of 25%. The losses were found to be higher for a higher blend ratio [55].

Exergy Analysis
Exergy analysis may help design more effective energy systems by reducing the system's irreversibilities. For use with diesel fuel, the exergies related to input energy, generated power, heat loss from exhaust gases, other heat loss and system irreversibilities or destruction of input energy were assessed during the exergy study [24,27,50]. The differences in thermal and exergy efficiency for test fuels with 1500 RPM speed, 17.5 CR and full load are shown in Figure 12 Exergy analysis may help design more effective energy systems by reducing the system's irreversibilities. For use with diesel fuel, the exergies related to input energy, generated power, heat loss from exhaust gases, other heat loss and system irreversibilities or destruction of input energy were assessed during the exergy study [24,27,50]. The differences in thermal and exergy efficiency for test fuels with 1500 RPM speed, 17.5 CR and full load are shown in Figure 12  The experimental engine's exergy efficiency and thermal efficiency both followed the same pattern for all test fuel blends. Exergy efficiency has a lower value under identical circumstances than thermal efficiency. Between these two there was a 3-6% difference. Low energy effectiveness was brought on by the conversion of just a small quantity of work exergy and the destruction of most of the provided fuel exergy, which is also called inlet exergy rate [30,58], by the fuel. Diesel fuel has a higher value of both efficiencies as compared to other test fuels because of the higher calorific content of diesel [59]. When the amount of biodiesel in the mixture is raised, the exergy flow is (slightly) decreased as a result of increased fuel consumption. Due to the fuel's chemical makeup and lower calorific content, this has occurred.
The exergy analysis depicted in Figure 13 shows that for pure diesel, brake power exergy was 31.06%, exergy exhaust rate was 18.35%, exergy destruction was 46.14% and exergy rate by heat transfer was 4.43%. For other fuels, the brake power was found to have a declining trend and decreased by 30.79%, 30.57%, 30.35%, 30.09%, and 29.75% for SBF20, SBF40, SBF60, SBF80, and SBF100 respectively. This result shows that biodiesel blends lead to a reduction in power output due to their lower calorific value [60]. Exhaust exergy and exergy transfer by heat were following an increasing trend. Exhaust gas exergy flow rate increased as a result of the fuel mixture, presumably as a result of incomplete combustion brought on by the addition of more long-chain hydrocarbons to the fuel, and was observed to increase by 18.35%, 18.4%, 18.46%, 18.54%, 18.7%, and 18.86% for exergy exhaust The experimental engine's exergy efficiency and thermal efficiency both followed the same pattern for all test fuel blends. Exergy efficiency has a lower value under identical circumstances than thermal efficiency. Between these two there was a 3-6% difference. Low energy effectiveness was brought on by the conversion of just a small quantity of work exergy and the destruction of most of the provided fuel exergy, which is also called inlet exergy rate [30,58], by the fuel. Diesel fuel has a higher value of both efficiencies as compared to other test fuels because of the higher calorific content of diesel [59]. When the amount of biodiesel in the mixture is raised, the exergy flow is (slightly) decreased as a result of increased fuel consumption. Due to the fuel's chemical makeup and lower calorific content, this has occurred.
The exergy analysis depicted in Figure 13 shows that for pure diesel, brake power exergy was 31.06%, exergy exhaust rate was 18.35%, exergy destruction was 46.14% and exergy rate by heat transfer was 4.43%. For other fuels, the brake power was found to have a declining trend and decreased by 30.79%, 30.57%, 30.35%, 30.09%, and 29.75% for SBF 20 , SBF 40 , SBF 60 , SBF 80 , and SBF 100 respectively. This result shows that biodiesel blends lead to a reduction in power output due to their lower calorific value [60]. Exhaust exergy and exergy transfer by heat were following an increasing trend. Exhaust gas exergy flow rate increased as a result of the fuel mixture, presumably as a result of incomplete combustion brought on by the addition of more long-chain hydrocarbons to the fuel, and was observed to increase by 18.35%, 18.4%, 18.46%, 18.54%, 18.7%, and 18.86% for exergy exhaust rate and by 4.43%, 4.5%, 4.56%,4.62%, 4.69% and 4.77% for the exergy rate transfer, for SBF20, SBF40, SBF60, SBF80, and SBF100, respectively. Most input energy is lost during combustion, and only a tiny portion of it can be transformed into exergetic work during an engine cycle. The major portion of input exergy lost is in the form of exergy destruction [23,59], which was observed as an increasing trend with the increase of blend ratio, was found to be 46.14%, 46.29%, 46.39%, 46.47%, 46.51%, and 46.61% for SBF 20 , SBF 40 , SBF 60 , SBF 80 , and SBF 100 respectively. The exergy analysis shows that engine operation was more sustainable for diesel fuel than other biodiesel blends [24,61]. SBF 20 shows the closest result to diesel fuel. SBF 20 biodiesel blends have good potential for working in a diesel engine. [23,59], which was observed as an increasing trend with the increase of blend ratio, was found to be 46.14%, 46.29%, 46.39%, 46.47%, 46.51%, and 46.61% for SBF20, SBF40, SBF60, SBF80, and SBF100 respectively. The exergy analysis shows that engine operation was more sustainable for diesel fuel than other biodiesel blends [24,61]. SBF20 shows the closest result to diesel fuel. SBF20 biodiesel blends have good potential for working in a diesel engine. Figure 13. Exergy balance of Diesel and biodiesel blends at 1500 RPM and full loading conditions. Figure 14 depicts the sustainability index (SI) parameter for the diesel system used with different fuel blends under four different loads. The engine's energy efficiency has a direct impact on the sustainability score [58]. As a result, the operational parameters' impacts under the same operating circumstances were comparable to their effects on the exergy efficiency. Since the environmental effect and the sustainability index of a process are mutually exclusive [62], and were examined for all fuel sources, the diesel engine sustainability index increased at 1500 revolutions per minute. Then, during the engine tests taken into consideration, the diesel engine's sustainability index varied from 1.27 to 1.46 [63]. When the results were calculated based on the fuel type, it became clear that the engine running on diesel fuel was more environmentally friendly than the engine running on biodiesel blends [64].  Figure 14 depicts the sustainability index (SI) parameter for the diesel system used with different fuel blends under four different loads. The engine's energy efficiency has a direct impact on the sustainability score [58]. As a result, the operational parameters' impacts under the same operating circumstances were comparable to their effects on the exergy efficiency. Since the environmental effect and the sustainability index of a process are mutually exclusive [62], and were examined for all fuel sources, the diesel engine sustainability index increased at 1500 revolutions per minute. Then, during the engine tests taken into consideration, the diesel engine's sustainability index varied from 1.27 to 1.46 [63]. When the results were calculated based on the fuel type, it became clear that the engine running on diesel fuel was more environmentally friendly than the engine running on biodiesel blends [64].

Statistical Analysis
According to the ANOVA results as represented in Tables 6 and 7, the Model F-value of 41,217.33 implies the model is significant. There is only a 0.01% chance that an F-value this large could occur due to noise. p-values less than 0.0500 indicate model terms are significant. In this case A, B, AB, A 2 , B 2 , A 2 B, AB 2 , and A 3 (A stands for load and B stands for blend ratio) are significant model terms, where the load has the biggest impact on BP, followed by the blend ratio. The p-value reflects this, and the inference made is within sizable error bounds. The surface plot of BP (percentage of energy) versus blend and load (percentage) is shown in Figure. It demonstrates that the maximum BP is observed at the highest load; the surface plot can be used to observe the relative variation of BP, and load. The lack-of-fit F-value of 0.04 implies that the lack-of-fit is not significant relative to the pure error. There is a 99.98% chance that a lack-of-fit F-value this large could occur due to noise. A non-significant lack of fit is good. Adeq. Precision measures the signal to noise ratio. A ratio greater than four is desirable. Our ratio of 562.754 indicates an adequate signal. This model can be used to navigate the design space.

Statistical Analysis
According to the ANOVA results as represented in Table 6 and 7, the Model F-value of 41,217.33 implies the model is significant. There is only a 0.01% chance that an F-value this large could occur due to noise. p-values less than 0.0500 indicate model terms are significant. In this case A, B, AB, A², B², A²B, AB², and A³ (A stands for load and B stands for blend ratio) are significant model terms, where the load has the biggest impact on BP, followed by the blend ratio. The p-value reflects this, and the inference made is within sizable error bounds. The surface plot of BP (percentage of energy) versus blend and load (percentage) is shown in Figure. It demonstrates that the maximum BP is observed at the highest load; the surface plot can be used to observe the relative variation of BP, and load. The lack-of-fit F-value of 0.04 implies that the lack-of-fit is not significant relative to the pure error. There is a 99.98% chance that a lack-of-fit F-value this large could occur due to noise. A non-significant lack of fit is good. Adeq. Precision measures the signal to noise ratio. A ratio greater than four is desirable. Our ratio of 562.754 indicates an adequate signal. This model can be used to navigate the design space.

Discussion
According to the findings of the property tests, biodiesels have lower calorific values than diesel and lower kinematic viscosity, density, and cetane number. The energy-exergy study revealed that when the volume fraction of biodiesel fuel increases relative to diesel, the heat loss via cooling water increases. This is because biodiesel fuel has a higher oxygen concentration, which causes better combustion, a rise in cylinder temperature and an increase in heat loss via cooling water.
The primary reason for the system's inefficiency is the exergy being destroyed by irreversible processes, mostly burning. Other factors which affect the exergy destruction include energy losses from heat transfer and exhaust gas energy losses. By devoting further research, we may be able to determine how the fuel energy can be utilized by the engine more efficiently based on these variables.

Conclusions and Future Scope
The research presents an energy and exergy analysis of diesel and biodiesel blends. Transesterification was used to create the biodiesel in consideration of its compatibility. The engine was operated at 1500 RPM and 17.5 CR for diesel and different blends under four loading conditions.

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The energy-exergy analysis and SI value of a direct-injected diesel engine have been significantly impacted by the characteristics of biodiesel.

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With a minor exception, the results for all blends were in good agreement with the pure diesel.

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The deviation of exergy efficiency and thermal efficiency was observed as 5.09 and 5.71% for Diesel and SBF 20 respectively. • Maximum destruction exergy was observed at the 100% loading condition with SBF 100 fuel blend, at 46.60%, whereas maximum power output was for diesel and SBF 20 , with 31.06% and 30.79% of total fuel energy input.

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The sustainability index was found in the range of 1.27 to 1.45.

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The increase in engine load increases the sustainability index and exergy efficiency in all fuel blends.

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With the use of exergy analysis along with energy analysis, we were able to obtain findings that were more accurate and realistic.
It was shown that the evaluated biodiesels provide competitive energetic performance with diesel. The best alternative in terms of using renewable resources and reducing exhaust is biodiesel. Therefore, biodiesel may be seen as a future fuel that will become widely used when petroleum is overexploited to the point of no return. Assessment of energy and exergy distribution is important when evaluating the performance of a thermal engine operated with different types of fuels or under different operational systems, because it can give the researcher a clear picture of the energy conversion processes and recommend the best way to reduce emissions, energy losses, or performance optimization. The appropriate alternative fuel can be chosen for future studies by looking at the impact of energy losses on the sustainability index.