Enhancement in Combustion, Performance, and Emission Characteristics of a Diesel Engine Fueled with Ce-ZnO Nanoparticle Additive Added to Soybean Biodiesel Blends

: This study considered the impacts of diesel–soybean biodiesel blends mixed with 3% cerium coated zinc oxide (Ce-ZnO) nanoparticles on the performance, emission, and combustion characteristics of a single cylinder diesel engine. The fuel blends were prepared using 25% soybean biodiesel in diesel (SBME25). Ce-ZnO nanoparticle additives were blended with SBME25 at 25, 50, and 75 ppm using the ultrasonication process with a surfactant (Span 80) at 2 vol.% to enhance the stability of the blend. A variable compression ratio engine operated at a 19.5:1 compression ratio (CR) using these blends resulted in an improvement in overall engine characteristics. With 50 ppm Ce-ZnO nanoparticle additive in SBME25 (SBME25Ce-ZnO50), the brake thermal e ﬃ ciency (BTE) and heat release rate (HRR) increased by 20.66%


Introduction
The ever increasing demand for fossil fuels in the transport and industrial segments have led to their rapid exhaustion [1,2].This has also resulted in increased environmental pollution, which has created a severe concern owing to numerous human health issues [3]. This has led to research on potential alternative fuels such as biodiesel. Biodiesel is a promising and eco-friendly fuel that can replace conventional fossil fuels, particularly petroleum-based diesel [4][5][6][7]. It is abundantly available and can fulfill the requirement of worldwide increasing energy demands [8][9][10]. There are several biodiesel feedstocks such as edible and non-edible oils, viz. palm, Jatropha curcas, Calophyllum inophyllum, Ceiba pentandra, waste cooking oils, and animal fats, viz. tallow and lard [11][12][13][14][15]. However, pure biodiesel has certain limitations to be used in compression ignition (CI) engines, such as high density, poor fuel atomization, cold flow properties, piston ring sticking, cold start problem, and a marginal reduction in the economy of the fuel [7,16,17]. These problems can be attenuated by the blending of biodiesel in diesel in a small percentage. However, that could result in a detrimental effect on some emissions, which can be mitigated by adding additives. The addition of additives results in enhanced performance and reduced exhaust emissions of a CI engine [18][19][20][21]. Current research focuses on the significance of fuel modification techniques through the use of additives for attaining improved engine performance and reduced emission characteristics.
Nanoparticles used as an additive to the biodiesels have delivered promising results such as improvement in performance and a considerable reduction in the emissions [9,22,23]. Nanoparticle additives (nanoadditives) are mainly known for reducing the emissions of a CI engine [9,[24][25][26]. Nanoadditives lower the CI engine emissions because the metallic particles react with water molecules, creating hydroxyl radical during combustion, which improves complete oxidation of soot particles by reacting with the carbon atom in the soot particles, thus reducing the oxidation temperature [27,28]. Fangsuwannarak et al. [29] concluded titanium oxide nanoparticles with biofuel constantly initiated complete combustion of hydrocarbons resulting in significant reductions in CO, HC, and NOx. Jung et al. [30] reported that use cerium oxide in diesel fuel results in a decrease in combustion lag, which reduces soot formation and complete combustion of fuel take place. Due to this, the combustion rate increases at the current intake. In general, cerium nanoparticles promote complete combustion in the diesel engine, thereby reducing the emissions of CO and HC, as well as lowering the BSFC [31]. This is due to metal base nanoparticles reacting with H 2 O and producing hydroxyl radicals, which enhances the soot oxidation reducing the combustion temperature [32]. Sajith et al. [33] experimented with a single cylinder CI engine. They concluded that CeO 2 nanoparticles emulsified at 20 and 80 ppm in Jatropha biodiesel (JME) reduced NOx and HC emissions up to a large extent by 30% and 40%, respectively, irrespective of a moderate rise in BTE by 1.5%. Although research has been done on nanoadditives in biodiesel, experimental evaluations of the performance parameters and pollutant emission on two nanoadditives in biodiesel are very few. To address this gap, this study focused on systematic infusion between two nanomaterials and analyzed its effect on engine emission and performance parameters in a single cylinder CI engine. Alagumalai [34] focused on the combustion of partially premixed charge in CI engine. The authors concluded that lemongrass (Cymbopogon flexuosus) oil is self-sufficient with proper combustion without distillation or not required any change in chemical composition. Basha and Anand [35] experimented on a single cylinder CI engine to analyze engine parameters, especially performance and combustion. The values of BTE for JME, JME with 5% water emulsion, and JME with 5% emulsion of water with 100 ppm of CNT were 24.80%, 26.34%, and 28.45%, respectively, and subsequently decreased emissions such as soot and NOx.
The purpose of the current investigation is to analyze the influence of 3% Ce coated ZnO nanoparticles and SBME25-diesel fuel blend at varying loads in a CI engine at a CR of 19.5. In the current study, different dosage levels (25,50, and 75 ppm) of Ce-ZnO nanoparticles were blended with SBME25 using a surfactant through the ultrasonication process. The loads were varied between 20% and 100%, and the effect of the selected fuel blends on engine performance, emission, and combustion were compared. Noted that the current study focused on improving the performance characteristics of SBME25 blend by adding Ce-ZnO nanoparticle. Diesel fuel was used only as a reference fuel.

Materials and Methods
The oil was first heated to 70 • C temperature. Esterification was then carried out with concentrated H 2 SO 4 and sodium methoxide in a circulating air oven for 60 min. The transesterification reaction was performed by blending NaOH at 6:1 M and heated at a temperature of 60 • C for 1 h. After that, a magnetic stirrer was utilized to simultaneously mix and heat the mixture for 30 min at 60 • C. The product of this reaction was then moved to a separating funnel for 24 h. Two separate layers of biodiesel and glycerol were seen. The glycerol, which was the lower layer, was removed. The top layer of biodiesel was washed with warm water to obtain pure biodiesel (SBME).
The fatty acid composition of soybean biodiesel (SBME) was estimated using gas chromatography-mass spectrometry (GCMS), Shimadzu, Japan (TQ 8030). Table 1 shows the GCMS analysis results; the free fatty acid (FFA) content for soybean biodiesel was validated from preceding research. The methyl ester was produced utilizing the transesterification of soybean oil [36][37][38]. Figure 1 illustrates the production of soybean biodiesel using transesterification reaction.  In the present study, a Kirloskar-make, variable CR (VCR), single cylinder, rated power 3.5 KW, water cooled, CI engine was used. Table 2 shows the VCR engine specifications, and Figure 2 shows the VCR test engine setup. The engine characteristics analyses were conducted at Apex innovation labs, India. For the measurement of emissions, a five-gas analyzer and smoke meter were utilized. A hemispherical combustion chamber (HCC), IT of 23.5 • BTDC, 220 bar injection pressure (IP), and a constant CR of 19.5 were employed. The Exhaust Gas Recirculation (EGR) rate was maintained at 3% (higher EGR percentage may reduce the pressure and increase smoke) at 27 • C. For comprehensive and reliable engine readings, Enginesoft software was used. In addition, data acquisition and the LabVIEW software (National Instruments, Austin, Texas, USA) was employed as a link among the PC and engine sensors for the measurement of the temperature (T) and load, air, and fuel flow. The HC, NOx, and CO emissions were measured by an AVL 473C exhaust gas analyzer and smoke emissions were measured by using AVL smoke meter. AVL 473C determines the emissions of CO and HC by infrared measurement method (non-dispersive infrared) and NO by electrochemical sensors.

Production of Ce Doped ZnO Nanoparticle Additives
In a typical synthesis procedure of 3% Ce doped ZnO, zinc acetate dihydrate along with cerium chloride nitrate heptahydrate (CeCl 3 .7H 2 O) with appropriate molar ratio were dissolved in water. Later, 5.027 g disodium succinate hexahydrate was added to the prepared solution, followed by stirring for 20 min using a magnetic stirrer. Then, to the above well-mixed solution, 20 mL of 0.02 M aq. NaOH solution were added dropwise. The resulting mixture was magnetically stirred for 30 min and then transferred to an autoclave of Teflon-lined stainless steel with a capacity of 200 mL, sealed, and held for 20 h at 120 • C. Subsequently, the autoclave automatically cooled to room temperature. The obtained precipitate was centrifuged washed and dried at 50 • C. Figure 3 illustrates synthesis of Ce doped ZnO nanoparticles.

Uncertainty Analysis
The uncertainty analysis is the set of errors obtained during the experimentation process and other external factors. In addition, the uncertainties in the measured values may develop because of errors in the measuring instruments, human errors in measurement of the experimental data and computations errors, and slightly affected by the environmental factors. The ideal conditions are utilized to gather the data. To calculate the uncertainty in the obtained results, the method described by Moffat et al. [39] was used in the current investigation. The propagation of errors was studied by plotting error bars for each reading and taking the average of six experimental readings. Table 3 demonstrates the uncertainty analysis of several parameters. Table 3. Uncertainty analysis of the obtained results.

Preparation and Physicochemical Properties of Nanoadditive Fuel Blends
In the current investigation, sorbitan monooleate (Span 80) 2 vol.% surfactant, which is highly dispersible and stable in the base fluid, was blended with 25% diesel-soybean biodiesel blend. For the ultrasonication process, methods discussed in previous studies done by Soudagar et al. [40,41] and Gavhane et al. [22] were used. The blending was carried using the ultrasonication process. Initially, the blend of diesel, span 80, Ce-ZnO nanoparticles (dosage level: 25, 50, and 75 mg/L), and biodiesel was stirred and heated at 60 • C using a magnetic stirrer to remove any water traces followed by bath sonication for an agitation period of 60 min. Figure 4 illustrates preparation of nano fuel blends using ultrasonication.   Table 4 demonstrates the properties of test fuels.

Results and Discussion
In this section, the performance characteristics, such as BTE and BSFC; emission characteristics such as HC, NOx, smoke, and CO; and combustion characteristics, such as HRR and ID are evaluated for nano fuel blends at constant speed of 1500 rpm, IT of 23.5 • BTDC, IP of 220 bar, and CR of 19.5. Figure 6 illustrates stepwise analysis of synthesized Ce-ZnO nanoadditives on the engine characteristics of VCR engine.

Characterization of Nanoparticle
XRD technique was employed to analyze the purity and phase identification of the 3% Ce-ZnO nanostructures. Figure [42,43]. The appearance of no cerium-related peak suggests that the cerium goes into ZnO lattice, and there is no change in the ZnO crystal structure upon doping. Debye-Scherer formula calculated the average size of 3% Ce-ZnO in crystallite. The average crystallite size of 3% Ce-ZnO nanostructures was found to be 12.16 nm. This decrease in 3% Ce-ZnO crystallite size is primarily due to the development of Ce-O-Zn on the sample's surface, which might, to some degree, impede the growth of ZnO nanoparticles. The room temperature optical absorption spectra of 3% Ce-ZnO is shown in Figure 7b. The absorption of lattices can provide details on the atomic vibrations involved. ZnO optical absorption edge typically was measured at 360 nm. The ZnO absorption edge clearly shifts with cerium doping (440 nm) towards the blue shift. Three-percent Ce-ZnO surface morphology was studied using FESEM. Figure 7c displays an as-synthesized sample FESEM image, which confirms the formation of 3% Ce-ZnO nanostructures. Figure 7c shows a large number of irregular counter-like nanostructures ranging 50-80 nm in size. The elemental composition of 3% Ce-ZnO (Figure 7d) was confirmed by energy dispersive spectroscopy (EDS). For the 3% Ce-ZnO, the chemical composition of Ce, Zn, and O atomic ratios was found to be 2.58, 58.90, and 38.52, respectively. The non-existence of peaks in relation to any other impurity element indicates that only Ce, Zn, and O are composed of the synthesized material. Microexplosion is caused by heterogeneous nucleation, where nucleation occurs at the droplet surface. Once the nanoparticles encased in water droplet biodiesel emulsion are exposed to high pressure and temperature environment in the engine cylinder, the water droplets will absorb the heat rapidly due to the lower boiling point of water in comparison to the biodiesel. This effect results in the explosion of water droplets through the surrounding oil layers known as microexplosion. Accordingly, secondary fuel droplets of very fine size are formed, which in turn evaporate quickly. Therefore, the formation of secondary droplets in the combustion chamber enhances the fuel-air mixing in the presence of potential NPs. Fuel blends with nanoadditives lead to enhanced microexplosion phenomenon, which results in reduction in the level of toxic pollutants in the exhaust gases and improvement in combustion rate with comparison with the biodiesel without additives [35,44]. Figure 8 shows the microexplosion phenomenon.  Figure 9a shows the variation of BTE with load for different test fuels, and the VCR engine was run at a constant speed and a CR of 19.5. A surge in BTE was observed with the rising loads; the highest BTE value was observed for a load of 80% [22,41]. The properties of the SBME25 fuel blend indicated lower calorific value and inferior cold flow properties. The engine operation showed lower BTE value for SBME25 fuel blend at all loads. At full load, the BTE for SBME25, SBME25Ce-ZnO25, SBME25Ce-ZnO50, and SBME25Ce-ZnO75 were 24.2%, 25.9%, 29.2%, and 27%, respectively. The findings reveal that the addition of Ce-ZnO nanoparticles in the Soybean biodiesel increases the BTE at all loads. The Ce-ZnO nanoparticles in the fuel blends encourage the microexplosion phenomenon, which results in comprehensive combustion of fuel particles [46,47]. The fuel blend SBME25Ce-ZnO50 demonstrated the highest BTE value, and the BTE increased by 20.66% and 11.5% for 75 ppm of Ce-ZnO when compared to SBME25. A previous study by Mujtaba et al. [8] showed an increase in BTE by on average 8.01% and 5.49% for dimethyl carbonate (DMC) and titanium dioxide (TiO 2 ) additive with B30 blend, respectively, compared to b30 blends without additives at full load operation. Thus, addition of nanoparticle was found to be improve the BTE. Figure 9b demonstrates BSFC with load variation for different fuel blends. The BSFC is the ratio of the rate of fuel consumed and the amount of BP generated. The loads were varied, and the readings were drawn at maximum load. The SBME25 fuel blend illustrated the highest BSFC due to lower calorific value and poor cold flow properties, which results in incomplete fuel combustion. The BSFC for the blends with the Ce-ZnO nanoparticles illustrated lower emissions owing to higher O 2 , which assists incomplete fuel combustion and enhances the microexplosion phenomenon [22,33]. The fuel blends SBME25Ce-ZnO25, SBME25Ce-ZnO50, and SBME25Ce-ZnO75 reduced the BSFC by 9.1%, 21.81%, and 15.75%, respectively, compared to SBME25. The higher CR of 19.5 and higher surface to area ratio of Ce-ZnO nanoparticles aided in thorough combustion of fuel. Previous study by Mujtaba et al. [8] showed an a reduction in BSFC reduction by 6.76% and 1.45% for B30 + TiO 2 and B30 + DMC blends in comparison with unblended B30. Thus, addition of nanoparticle was found to improve the BSFC.

CO Emission
CO is primarily generated because of the improper combustion of carbon in the H-C chain. According to the United States Environmental Protection Agency (EPA), about 95% of all CO emissions are emitted from motor vehicle exhaust [48]. Figure 10 sows the variation of CO with load for different fuel blends. The Ce-ZnO nanoparticles encourage thorough combustion of the fuel particles in comparison with the neat biodiesel fuel blends. Hence, the blends with Ce-ZnO nanoparticles illustrated the lowest CO emissions as a result of complete fuel combustion amongst all the fuels caused by the existence of fuel borne O 2 molecules and surface reactive area (Ce-ZnO), which lead to high mean combustion chamber temperature and also high pressure due to CR 19.5. The CO emissions for SBME25, SBME25Ce-ZnO25, SBME25Ce-ZnO50, and SBME25Ce-ZnO75 are 0.3, 0.28, 0.21, and 0.25% vol. respectively. The emissions reduced by 6.6%, 30%, and 16.5% for concentrations of 25, 50, and 75 ppm, respectively, for the Ce-ZnO nanoparticles in biodiesel compared to soybean biodiesel. Örs et al. [49] found that addition of TiO 2 in B20 resulted in 10.83% and 25.56% CO reduction compared to B20 and petroleum diesel, respectively. A similar observation was reported by Saxena et al. [50].

NOx Emission
NOx is generated from the reaction between two gases, viz. N 2 and O 2 , during the combustion process, usually at high temperatures [51]. The nitrogen oxide emissions for the catalysts are generally higher than the neat fuel blends. Figure 11 illustrates the variation of NOx and load for various fuel blends. At full load, the emissions of NOx produced for the nano fuel blends SBME25Ce-ZnO25, SBME25Ce-ZnO50, and SBME25Ce-ZnO75 are 490.8, 525.15, and 588.71 ppm, respectively. The rise in the concentrations of Ce-ZnO nanoparticles in SBME25 fuel blends increases the NOx due to the increase in the combustion chamber temperature and excess oxygen atoms due to oxygen donating nanoparticles. The NOx rises by 11.46% for 25 ppm of Ce-ZnO in the SBME25 fuel blend compared to SBME25.

Smoke Emission
Residual carbon atoms in exhaust gas are known as soot or smoke opacity. Fuel atomization and lack of oxygen are the two key elements that affect smoke emissions in CI engines [52,53]. In Figure 12, the smoke opacity rises with a rise in the engine loading conditions due to the rich A:F mixture in the combustion chamber. The neat soybean biodiesel (SBME25) presents higher smoke due to incomplete fuel combustion at all loading conditions. Adding Ce-ZnO nanoparticles in the test fuel reduced the smoke considerably. The smoke opacity for fuel blend SBME25Ce-ZnO50 was similar to D100 at all loads. The addition of a higher dosage level of Ce-ZnO nanoparticles slightly increases the smoke emissions due to rich air and fuel ratio. The smoke opacity for fuel blends SBME25Ce-ZnO25, SBME25Ce-ZnO50, and SBME25Ce-ZnO75 at maximum load are 55.71, 49.2 and 51.4 HSU, and the reduction in smoke was 8.2%, 18.7%, and 15.3% when contrasted with the soybean biodiesel (B25) fuel blend, respectively. This is caused by an enhancement in the microexplosion phenomenon, which enables complete fuel combustion.   Figure 13 describes HC emissions vs. load variation for neat SBME25 and nano fuel blends. HC emissions are comprised of unburned particles of fuel because of the lower temperature close to the cylinder wall [53]. The HC emissions can be lowered by facilitating appropriate residence time for the oxidation phenomenon, increasing the combustion chamber (CC) temperature, and combining the HC with oxidizing gases. The presence of nanoparticles in the biodiesel blends lowers the HC owing to higher CC temperature. The neat SBME25 fuel blends increase the HC emissions due to the cooling effect, while, due to rapid and effective combustion provided by the Ce-ZnO nanoparticles, the HC emissions reduce. The HC emissions for the fuel blends SBME25Ce-ZnO25, SBME25Ce-ZnO50, and SBME25Ce-ZnO75 are reduced by 10%, 21.5%, and 19.1%, respectively, compared to SBME25 fuel blend.

Heat Release Rate
The critical parameters influencing of HRR are the energy content, cetane number (CN), and viscosity of the fuel [54]. The combustion phenomenon begins early because of the enhanced ignition characteristics of Ce-ZnO nanoparticles in the SBME25 fuel blends. Figure 14 shows the behavior of HRR at different crank angles.
The HRR is calculated by applying the 1st law-single zone model shown in the Equation (1), where dQ n dθ is the heat release rate, "T m " is the mean in-cylinder temperature, dQ w dθ is the heat transfer rate from the gases to the cylinder wall, P is the cylinder pressure, and V is the volume of combustion chamber. The specific heat γ h is illustrated in Equation (2), The HRR for SBME25 fuel is lower in comparison with nano fuel blends as a result of higher molecular weight and lower burning velocity. In addition, CR 19.5 facilitates an improvement in the HRR as a result of higher pressure and combustion rate resulting in complete fuel combustion leading to elevated energy yield. The peak HRR values observed for the fuel blends SBME25Ce-ZnO25, SBME25Ce-ZnO50, and SBME25Ce-ZnO75 are 62.1, 68.5, and 65.7 • CA, respectively. The HRR increases by 7.06%, 18.1%, and 13.2% for Ce-ZnO nanoparticles concentration of 25, 50, and 75 ppm in comparison with the neat SBME25. The Ce-ZnO nanoparticles in the SBME25 fuel assist in better fuel atomization and supply higher oxygen content for complete fuel combustion [55,56].

Ignition Delay
ID period depends on the fuel type and their percentage in the CC. The factors accountable for regulating the length of the ID period of a diesel engine are the temperature of the charge throughout compression stoke, energy release during the phase of pre-ignition, and heat transfer or heat loss to the surroundings. This delay period consists of the physical delay, wherein atomization, vaporization, and mixing of air fuel occur, and chemical delay attributed to pre-combustion reactions. The ID is higher at lower loadings conditions and rises gradually at elevated loads due to enhanced compression and adequate A:F mixture. Figure 15 illustrates the variation of HRR at different crank angles. At full load, the ID values for the fuel blends SBME25Ce-ZnO25, SBME25Ce-ZnO50, and SBME25Ce-ZnO75 are 9.15, 8.5 and 8.7 • CA, and the reductions compared to the fuel blend SBME25 are 5.1%, 11.20% and 9.12%, respectively.

Conclusions
This paper reports the effect of different dosage Ce-ZnO nanoparticles (25,50, and 75 ppm) and SBME25-diesel-span80 surfactant fuel blends on VCR CI engine characteristics at a CR 19.5, IT 23 • BTDC, 1500 rpm, and varying loads. The diesel data are presented as reference only. The subsequent conclusions are derived based on the results achieved.

1.
The physicochemical properties, for instance the calorific value and CN increase, and cold flow properties were enhanced by including Ce-ZnO nanoparticles in SBME25 fuel. In addition, the viscosity and density of all the nano fuel blend slightly reduced due to the addition of a 2% span80 surfactant compared to SBME25.

2.
The laboratory synthesis of Ce-coated ZnO nanoparticles and soybean biodiesel considerably reduced the procurement cost of commercially available biodiesel and nanoparticles. 3.
The ultrasonication process and addition of span80 enabled steady dispersal of Ce-ZnO nanoparticles in the biodiesel fuel blends.

4.
The fuel blend SBME25Ce-ZnO50 illustrated a comprehensive enhancement in engine characteristics. At CR 19.5, Ce-ZnO nanoparticles in SBME25 enhanced the performance characteristics; 50 ppm of Ce-ZnO in SBME25 improved BTE by 20.6% and lowered the specific fuel consumption by 21.8% in comparison with SBME25 fuel blend. 5.
The ternary fuel blends illustrated lower emissions; for SBME25Ce-ZnO50, the CO, HC, and smoke reduced by 30%, 21.5%, and 18.7% compared to SBME25 fuel blends, respectively, because of improved microexplosion and oxygen content, resulting in complete fuel combustion. 6.
The combustion characteristics improved with the addition of Ce-ZnO in the SBME25 fuel blend: the HRR enhanced by 18.1% and the ID reduced by 11.2%. 7.
SBME25Ce-ZnO50 showed best results for most criteria compared to other blends as well as SBME25 fuel operation. 8.
The combustion, performance, and emission characteristics of SBME25Ce-ZnO50 operation were slightly worse than those of diesel operation.
The findings confirm that the Ce-ZnO nanoparticles in SBME25 at CR 19.5 improve the overall engine characteristics of the VCR engine compared to SBME25 operation.