Influence of Dimethyl Carbonate and Dispersant Added Graphene Nanoplatelets in Diesel-Biodiesel Blends: Combustion, Performance, and Emission Characteristics of Diesel Engine

The aim of the current study is to investigate the combustion, performance, and emission characteristics of a diesel engine adopting graphene nanoplatelets and 10% v / v dimethyl carbonate as fuel additives in a 30% biodiesel and 70% diesel blend. The novel ﬁ ndings are documented in the subsequent sections. The surface modi ﬁ cation of graphene nanoplatelets using a lipophilic surfactant was used which gave highest stability in fuel samples which is a main distinctive in this research work. Nanofuels were prepared using 30, 60, and 90ppm concentrations of nanoparticles through ultrasonication. The behaviour of graphene nanoplatelet was characterized using ﬁ eld emission scanning gun-electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared, and X-ray di ﬀ raction. A diesel engine having uniform speed of 1500rpm was used for the experiment at various load conditions to assess the engine operating parameters for all the prepared samples, including baseline diesel. It was observed that the combustion characteristics were found to be greatly enhanced, such as cylinder pressure and heat release rate, increased by about 15.45% and 9.63%, respectively, for B30GNP60DMC10 sample than diesel at higher loads. Performance parameters such as brake thermal e ﬃ ciency (elevated by 8.98%) and brake-speci ﬁ c fuel consumption (diminished by 25.54%) have been signi ﬁ cantly analyzed and compared to diesel. While the emissions (such as hydrocarbons and carbon monoxide) were found to be reduced by 22.87% and 25.67%, respectively, for B30DMC10, the nitrous oxide and smoke opacity were also reduced by 9.57% and 12.4%, respectively, for the B30GNP60DMC10 sample. Hence, a combining operation of graphene nanoplatelets and dimethyl carbonate additives in a biodiesel blend presented great potential in terms of performance improvement and reduction in emission parameters in diesel engine.


Introduction
Accessibility and effective energy production are the primary concerns influencing every nation's financial circumstances. Compression ignition (CI) engines stand out among energy transformation devices due to specific outstanding capabilities, such as greater thermal efficiency, reduced brake fuel consumption, utilization of lean air-fuel ratios, stability, and lower operating costs in agricultural and road transportation [1][2][3]. However, due to the shortage of fossil fuel energy sources and the hazardous pollutants emitted by CI engines, the consumption of renewable/alternative fuels in compression ignition engines has been recommended globally. Researchers across the globe have now been focusing on developing methods for better efficiency and lower emissions in CI engines.
Biodiesel prepared from plant resources has become popular because of its favorable physicochemical properties, performance, combustion, and emission parameters [4]. Biodiesel may be combined with diesel and fueled for CI engines with or without engine modifications. Transesterification or alcoholization is the process in which oils and alcohol are combined with acids, bases, enzymes, or other types of catalysts to prepare biodiesel [5,6]. To reduce emissions from regular diesel fuel, utilization of biodiesel within acceptable boundaries, and in order to improve engine performance characteristics, many scholars have identified a variety of strategies such as engine modifications for complete combustion, exhaust gas reduction (EGR), and fuel reformulation techniques such as adding additives (alcohols/nanoadditives) to fuels, especially nanoadditives, which are referred to as nanofuels [7][8][9][10].
The combination of liquid (oxygenated alcohols) and solid (nanoparticles) fuel additives in biodiesel blends is gaining popularity in CI engines. Alcohol presents a challenge for fuel improvement in CI engines because of its higher autoignition temperature, poor lubrication abilities, and reduced cetane number [11]. At the same time, nanoparticles greatly influence the lowering of nitrogen oxide (NOx) and carbon monoxide (CO) emissions and improve engine performance. Several research studies have been conducted to explore the effects of oxygenated alcoholic fuel additives in CI engines. Among other oxygenated alcohol additives, the dimethyl carbonate (DMC) additive has a high oxygen concentration of approximately 53.3%. The interaction of the fuel with the oxygen present in the C-C bond enhances the combustion process, resulting in lower smoke emissions. The stability of liquid additives with biodiesel blends is a significant challenge. In this regard, DMC can be easily miscible in diesel fuel up to 15% vol. Still, ethanol and methanol are needed as effective surfactants/solvents to obtain a stable blend without separation [12]. Pan et al. and Elumalai et al. [13,14] investigated the effect of DMC (10% and 20% vol.) with diesel on diesel engine performance. DMC (10%) blend outperformed in terms of brake thermal efficiency (BTE), and when DMC (20%) is combined with diesel, soot emissions are reduced by 80%.
In another study, Yilmaz et al. [15] concluded that the addition of propanol, n-butanol, and 1-pentanol (10 vol.%) into waste oil biodiesel resulted in improved performance and reduced emissions including nitrogen emission. Similar study was conducted by Atmanli [16] in a turbocharged diesel engine operated with 2-ethylhexyl nitrate as a cetane improver with 70% diesel, hazelnut oil of 20%, and nbutanol of 10% by volume. The results revealed that the addition of ethylhexyl improved fuel characteristics along with cold flow properties and cetane number. Moreover, the addition of n-butanol or 1-pentanol in the blend improved the overall performance while reducing emissions significantly. In a recent study, Pullagura et al. [17] concluded that the addition of aluminum oxide nanoadditives (at a concentration of 50, 75, and 100 ppm) in n-butanol and diesel blend enhanced performance parameters (BTE is improved by 7.31% and brake-specific fuel consumption (BSFC) is reduced by 20.23%). Further, the emissions such as CO, unburnt hydrocarbons (UHC), and NOx were decreased by 22.04, 21.07, and 14.47%, respectively.
Micron-sized nanoadditives could result in a cluster in the fuel sample, thus causing sedimentation. This issue could be resolved by adding nanoadditives ranging from 1 to 100 nm [1]. Many scholars used metal oxide nanoadditives, such as aluminum, iron, copper, silver, cobalt, and boron, in biodiesel blends to increase the performance of the CI engine [18]. As represented in Figure 1(a), the momentum of fuel spray jet is enhanced further, resulting in intense secondary atomization [18].
Several researchers have employed metal nanoparticles/ metal oxides as additives for better engine performance [9]. While improving performance, these additives emit toxic materials into the environment as combustion residues resulting in connected side effects, which is a significant gap in research. As a result, instead of metal-based nanoparticles, organic materials such as carbon allotropes were found to eliminate the issues of hazardous emissions. Graphene nanoplatelets (GNP) are a well-known catalyst in various chemical processes, mainly when scattered in fuels. They can enhance combustion and lower pollutants.
Soudagar et al. [19] studied the influence of graphene oxide nanoparticles on diesel engine performance when combined with a biodiesel-diesel mixture containing the sodium dodecyl sulfate surfactant. The nanofuel mixtures were stable at a mass fraction of 1 : 4. The DSOME2040 blend's combustion and performance parameters were improved, and emissions were reduced, leading to the conclusion that the biodiesel-diesel blend with graphene oxide nanoparticles was an adequate sustainable fuel for improving overall performance and lowering emission parameters. Yusuf et al. [20] studied how different dosage levels of octanol and multiwall carbon nanotubes (MWCNT) are combined with cotton seed biodiesel-diesel blend (B20) for diesel engine characteristics. The findings of the experiments demonstrated that the inclusion of octanol lowered BSFC and that the performance and fuel consumption are further stabilized when MWCNT nanoparticles are incorporated, although there is no change in emissions. Soudagar et al. [21] investigated the impacts on common rail direct injection (CRDI) diesel engines with waste plastic oil, n-butanol, hybrid nanoparticles such as Al 2 O 3 and TiO 2 , and diesel blends. The use of plastic oil, n-butanol, and hybrid nanoparticles considerably reduced the emission findings, and it was also recommended that waste plastic be prohibited, alternative fuels be supported, and plastic manufacture be minimized.
Furthermore, at temperatures above 650°C, these nanoplatelets contribute to combustion and dissolve carbon dioxide shown in Figure 1(b). GNP exhibits higher nitromethane rates and higher thermal conductivity, thereby improving 2 International Journal of Energy Research heat transfer rates than Al 2 O 3 and amorphous silicon dioxide (SiO 2 ). Further study on GNP is needed to fill the gaps remaining in the experimental studies. It is suggested that biofuels improve combustion, performance, and emissions parameters over a broad surface area with high intrinsic mobility and higher stability and homogeneity. According to recent studies, it is concluded that by employing nanoparticles and oxygenated alcohols as additives in biodiesel-diesel blends, the overall performance is improved while significantly lowering exhaust emissions in CI engines. Hence, this study is mainly aimed at investigating the synergistic effect of graphene platelet nanoparticles and oxygenated DMC alcohol in biodiesel-diesel blends on diesel engine performance.
1.1. Motivation and Objective of the Study. According to the research investigations, when both nanoparticles and oxygenated alcohols were employed as additives in biodieseldiesel blends, the performance parameters were improved, and exhaust pollutants were significantly reduced in CI engines. A few studies are available with metal nanoparticles that are toxic in nature and combined with alcohol additives in the presence of biodiesel-diesel blends. Moreover, the usage of the combined effect of carbon-based GNP, which are nontoxic and biodegradable, and higher-level alcohols with 30% of Sterculia foetida seed oil methyl ester (SFOME)-70% of diesel (B30) has not yet been studied in the literature. Hence, the current study is aimed at investigating the combined effect of GNP and DMC in the B30 sample on the stability of fuel blends and engine combustion, performance, and emission parameters as well. Nevertheless, this study also focuses on the stability of the nanoalcoholic-biodiesel-diesel blends.

Materials and Methodology
2.1. Materials. The high oil content of Sterculia foetida oil makes it the most attractive nonedible biodiesel feedstocks available. These seeds were gathered in the Visakhapatnam district of Arakuveli. The heart-shaped Sterculia foetida fruits contain around 15-20 seeds [22], which are white when young but become black when ripe and contain 50-60% oil. Molekula bioKEMIX Limited, Hyderabad, India, provided the solvent methanol (99.5%), n-hexane (99%), and KOH pallets (85%) required for biodiesel production.
For oil extraction, Sterculia foetida seeds were washed, sun-dried, and pulverized in an expeller. When 5% (v/v) hexane is added to 1000 g of raw oil and heated at 80°C for 30 minutes with steady stirring, the oil is refined and degummed. The hexane was evaporated while the impurities and sediments were removed using a funnel during the heating process. Because of its minimal free fatty acid concentration, the base-catalyzed/alkali-catalyzed transesterification technique was adopted. 885 g of Sterculia oil and 1.5% KOH catalyst (on a weight basis) were mixed together in a 2 L round bottom flask, agitated at 500 rpm, and heated at 60°C for an hour using a heating mantle [23][24][25][26][27][28]. Finally, gravity separation in a separating funnel after six hours was done to get methyl ester at the top of the funnel and glycerol phase at the bottom. It was washed and dried numerous times with distilled water to purify the methyl ester phase. The highest SFOME conversion was found to be 97.45%. Figure 2 depicts the experimental stages required in biodiesel production.  3 International Journal of Energy Research the surface modification technique. The mixture is sonicated for 15 minutes, and the solvent is allowed to evaporate, leaving surface-modified GNP. Several trials of tests were carried out before arriving at an optimal ratio, i.e., a 1 : 1 mixture ratio of GNP and dispersant. This optimized ratio is used to prepare the remaining samples. The stability analysis performed is explained in the following sections. For a closer examination, a high-resolution transmission electron microscopy (HR-TEM) study was conducted using JEOL; JEM 2100F, FEG TEM 200 kV gives a bettermagnified image of the GNP sample, as shown in Figure 3(b). The expansion of the lattice plane and the presence of the graphene nanosheets were clearly observed in the above images. In addition, GNPs with five layers of graphene and a thickness of around 6 nm were revealed in wellmagnified images, which are in accordance with the specifications of the manufacturer, as given in Table 1. Figure 4 shows the chemical nature of GNP to distinguish the functional groups of the surface-modified nanoplatelets, which were analyzed using Fourier transform infrared (FTIR) spectroscopy at a wavenumber of 500 cm -1 to 4000 cm -1 . Six major absorption peaks were noticed in the FTIR spectra of GNP. An    International Journal of Energy Research absorption peak at 3436.96 cm -1 represents the existence of a hydroxyl group on the surface of GNP. Figure 5 illustrates the X-ray diffraction (XRD) configurations of the GNP in which the diffraction peaks are obtained at 2θ = 26:41°, 43.4°, and 54.19°, which were attributed to the crystal planes of (002), (100), and (004) correspondingly. The above results were in agreement with the findings of Radhika et al. [29].

Preparation of Nanofuel Blends.
Commercial diesel was fueled as a benchmark in CI engine. The second sample was B30 (30% Sterculia foetida seed oil biodiesel and 70% diesel). The third sample includes 10% of DMC by volume in a B30 blend termed B30DMC10, which was prepared with no nanoadditives. The remaining three blends consist of a fixed amount of 10% DMC by volume along with three dosages of 30, 60, and 90 ppm graphene nanoplatelets which were mixed in B30 blend and denoted by B30GNP30DMC10, B30GNP60DMC10, and B30GNP90DMC10, respectively. The biodiesel samples were mixed with nanoplatelets using bath sonification, an ultrasonication technique. The physicochemical properties of biodiesel and its blend samples are tabulated in Table 2.
2.5. Stability Test Analysis. The stability of nanoparticles in liquid fuel blends plays a major role in order to achieve complete combustion. A dispersion becomes an important feature in such scenarios since it avoids separation, agglomeration, and sedimentation of nanoparticles. The nanoparticle stability of B30 was determined with a spectrophotometer and analyzed as to transmittance in the current study. The test lasted 30 days, with readings acquired from day 1 to day 20 in a variation of 10 days. The change in transmittance with wavelength is illustrated in Figures 6(a)-6(c). The transmittance range was studied by a comparable method for all of the test fuel blends, as can be shown. Regardless of test fuel, the transmittance range was slightly enhanced on days 10 and 20 compared to day 1. The reduced transmittance significantly implies strong stability of nanoplatelets in the B30 throughout a 20-day period. No light can pass through the fuel sample because the nanoparticles are opaque. As a result, the fuel blend exhibits a lower light transmittance (i.e., higher light absorption), confirming good stability.

Experimental Setup
A 4-stroke, 1-cylinder CI engine was involved in this investigation which is capable of producing the rated power capacity of 5.2 kW and operated at 1500 rpm with variable engine loads using a dynamometer. The specifications of the engine setup are given in Table 3, and a schematic view and a pictorial representation of the engine setup are illustrated in Figures 7(a) and 7(b). Before operating the engine with prepared samples, the engine was steadily run for 30 min to achieve uniform temperature for the engine components. Then, diesel fuel is used to collect baseline/reference results. Later, the fuel blends such as B30, B30DMC10, B30GNP30DMC10, B30GNP60DMC10, and B30GNP90DMC10 were fueled to find the engine performance, combustion, and emission characteristics and correlated with referenced fuel. The experiments were repeated five times with the aforementioned blends to ensure the consistency of findings at various doses. The emission data were collected using an AVL gas analyzer. Before attempting to take readings, all sensors were verified and attached to an analog-todigital converter (ADC) and connected to a computer. The combustion analysis was obtained by a computer employing the integrated engine soft program. At the same time, the total uncertainty of experiments is shown in Table 4.

Combustion Characteristics
4.1. In-Cylinder Pressure. The pressure vs. crank angle for various samples at higher load conditions is shown in Figure 8(a). The rate of combustion is majorly affected by the fuel sample burnt in the initial phase/premixed zone of combustion, which affects the peak pressure in the CI engine. The high peak pressure for biofuel and DMC blends is owing to the higher oxygen concentration in biofuels and DMC alcohol [30]. The in-cylinder pressure (ICP) for B30GNP30DMC10, B30GNP60DMC10, and B30GNP90DMC10 blends was 63.21 bar, 69.98 bar, and 66.76 bar, respectively. The ICP improved significantly when surface-modified GNP was dispersed in a B30 and 10% DMC blend, owing to the ability of nanoparticles to enhance the heat uniformly in the entire combustion cycle. Another factor that contributed to the increased peak pressure was DMC and GNP additives, which act as oxygenated additives along with an improved surface-tovolume ratio and higher calorific value of GNP nanoadditives, thus facilitating an effective combustion process. Similar findings are observed in a few studies [31,32]. In a research study, the higher peak pressure for DMCincluded biodiesel-diesel blend is attributed to a lower calorific value, and the blend with a lower calorific value requires more fuel during combustion [33]. In another     Figure 8(b) illustrates the HRR vs. crank angle for standard diesel and other samples at maximum load conditions. The HRR for B30 and DMC blends is higher than for conventional diesel, which was attributed to their higher calorific value [35,36]. However, when surface-modified GNP was dispersed in B30 mixed with 10% DMC, the HRR increased more than in the other samples. This HRR was improved by 9.63% for the B30GNP60DMC10 sample over diesel due to increased calorific value, shorter delay period, and catalytic effect. Nonetheless, the improved combustion characteristics of GNP and DMC are owing to the ability to contribute more oxygen atoms from their molecular structures in the combustion process and a higher surface-to-volume ratio of GNPs. In an investigation, it was revealed that the addition of DMC reduced density, viscosity, and rich oxygen HRR was improved [37,38]. Devarajan [34] studied the engine performance, combustion, and emission characteristics of a diesel engine when fueled with a DMC-biodieseldiesel blend. The HRR for the BD80DMC20 blend is the highest, owing to the greater oxygen concentration and improved cetane number, which results in improved combustion and a shorter ignition delay period. As a result, HRR increases.
where dQ total /dθ indicates the HRR, γ sp:heat represents the adiabatic index, P cyl specifies the cylinder pressure (Pas-cal), dQ w /dθ is the heat transfer rate from the gases to the cylinder wall, and V is the volume of combustion space. Figure 9(a) [39]. Finally, the BTE was enhanced by incorporating GNPs and DMC into the B30 blend at different concentration levels of 30, 60, and 90 ppm, as well as a 10% constant volume of DMC when compared to the B30 blend. Among all of these blends, the maximum BTE is improved by 33.78% for the B30GNP60DMC10 blend, which is 8.98% better than the B30 blend at maximum load. Higher oxygen content in biofuels and DMC, better atomization, improved latent heat of vaporization, and a larger surface area to volume ratio were the major parameters to contribute to improved BTE. In a study, biodiesel prepared from waste cooking oil with GNPs and carbon nanotubes showed similar results [24]. Pullagura et al. [40] explored that the addition of nanoparticles and DMC additives in a biodiesel-diesel blend when fueled in a diesel engine. The outcomes revealed that the BTE is improved for nanoparticles and DMC-added biodiesel-diesel blend, which is owing to the higher thermal conductivity of nanoadditives, and the oxygenated alcohol component lowered viscosity and enhanced the blend's self-ignition temperature. As a result, BTE is improved. The results were correlated with a few journal articles [25,35,41]. Figure 9(b) depicts the effects of BSFC vs. load for different fuel samples. The BSFC for the B30 blend is higher International Journal of Energy Research than standard diesel owing to lower calorific value and higher viscosity [42]. At max load, BSFC was reduced to 0.277 kg/kWh for B30GNP60DMC10 and 0.282 kg/kWh for the B30 blend. Thus, the addition of GNP and DMC results in a lower BSFC compared to the B30 sample, which was attributed to higher oxidation content in the alcohol and a higher heat transfer coefficient in the GNP due to their higher thermal conductivity, which improves combustion rate. In a study, the addition of 120 ppm of GNP and 10% of DMC in a B30 blend reduced BSFC due to lower density, higher calorific value, and oxygen content [43,44]. Soudagar et al. [45] reported that the oxygenated ternary blends showed a promising reduction in BSFC which is attributed to reducing viscosity and density, thus improving spray characteristics, and further combustion properties are enhanced. Other studies found similar results [11,35].

Emission Characteristics
6.1. CO. Carbon monoxide (CO) pollution is produced when complete combustion is not achieved due to a lack of oxygen in the combustion chamber, which is also affected by engine temperature, fuel penetration, injection time, and injection pressure [46]. Figure 10(a) shows that the CO emission increased for all tested fuels at different loads. CO emission was reduced when a B30 blend was used instead of diesel oil due to increased oxygen content, which aids in complete combustion. Furthermore, CO emission was significantly lower for B30DMC10 compared to the B30 blend due to excess oxygen in alcohol and a higher cetane number value. Nevertheless, the B30GNP60DMC10 blend has a 25.67% reduction in CO emissions. At a higher load, this is attributed to the increased surface energy of GNP, the presence of C-C bonds, and oxygen content in the DMC additives. In a study, it was concluded that the CO was decreased with a higher concentration of GNP and lower content of DMC due to an improved ID period [44]. In a study, alcoholic ter-nary blends resulted in a maximum reduction in CO emissions which is attributed to higher oxygen content and modified molecular structure, thus facilitating all CO emissions to get converted into CO 2 emissions [47]. A few articles show reduced CO results [35,43,46]. 6.2. CO 2 . The variation of carbon dioxide (CO 2 ) emissions for tested fuel blends is shown in Figure 10(b). The improvement of CO 2 emissions could be a positive outcome since the complete combustion results in higher CO 2 levels. The CO 2 levels were noticed to be improved with the addition of GNP and DMC10 in the B30 blend for all increased loads. The improved CO 2 is observed as 5.04% for the B30GMP60DMC10 blend, which is improved by 13.2% at a higher load when compared to diesel. The synergistic effect of GNPs and DMC resulted in better combustion and led to producing higher CO 2 emissions at maximum loads. In addition, the GNPs act as a catalyst, and DMC acts as oxygenated fuel and improves atomization properties. In a study, the hotter combustion environment promotes higher CO 2 [9,26].
6.3. UHC. The major reason for producing unburned hydrocarbon (UHC) emission is poor atomization, due to which complete combustion may not be taken place. Figure 11(a) represents UHC emissions for different tested fuel samples in which UHC emissions are linearly increasing due to additional fuel supply with increasing load. It is also observed from the results that the B30 blend shows less UHC emission than normal diesel due to more oxygen in biodiesel which enhances the rate of combustion, thus decreasing UHC emission [48]. In addition to this, HC emissions are reduced for all GNP and DMC-added B30 blends. There is a considerable reduction in HC emission by 11.62%, 22.87%, and 15.45% for B30GNP30DMC10, B30GNP60DM10, and B30GNP90DMC10, respectively. This is due to the fact that the inclusion of GNPs in the B30 blend lowers the carbon combustion activation temperature for effective combustion, secondary atomization, and enhanced catalytic effect. As a result, HC emissions are reduced [49]. In a study, the reduction in UHC was reported due to the addition of DMC as an additive that promotes complete combustion [13,14]. Mujtaba et al. [50] explored that the HC emissions are emitted due to incomplete combustion, and all ternary fuel blends resulted in lower HC emissions owing to higher cetane number and oxygen content, which reduces HC emissions compared to biodiesel-diesel blends.
6.4. NOx. Figure 11(b) depicts the nitrogen oxide (NOx) emissions versus loads for various tested samples. As more fuel is consumed at a max load, the average gas temperature of the engine rises, resulting in higher NOx formation. Furthermore, the presence of oxygen in biodiesel blends shortens the delay period. As a result, the residence time of high temperatures is increased. Similar trends were observed in another study [51]. The samples B30GNP30DMC10, B30GNP60DMC10, and B30GNP90DMC10 illustrated a significant decrease in NOx emissions of 8.75%, 9.57%, and 7.45%, respectively, compared to that of B30. The inclusion of GNP improves homogenization and boosts chemical reactivity, as well as increases the delay period in the premixed zone, thus resulting in lower NOx emissions [49,51]. Nevertheless, the addition of DMC reduces the cylinder temperature during the combustion process. Pullagura et al. and Yilmaz et al. [15,52] showed a substantial decrease in NOx emissions by including GNPs in 11 International Journal of Energy Research jatropha biodiesel. Devarajan [34] reported that the NOx emissions are reduced by 2.2 to 4.5% by the addition of DMC additive with a concentration of 10 and 20% in biodiesel. The NOx emission is reduced due to oxygenated alcohol which reduced ignition delay and accelerated combustion and decreases NOx emissions.

Smoke Opacity.
Smoke opacity vs. load for the tested fuels is presented in Figure 11(c). B30 blend has lower smoke than diesel oil which is attributed to extra oxygen molecules, which leads to uniform combustion [53]. Furthermore, the smoke opacity of B30 surface modified with GNP and DMC was found to be lower than B30DMC10, which is due to high surface-to-volume ratio, better mixture strength, breaking of fuel jet into fine particles, atomization, evaporation characteristics, and oxygen content in the DMC. Hence, smoke opacity is reduced. The maximum reduction in smoke opacity is 12.4% for the B30GNP60DMC10 blend, and similar results were noticed in a few articles [35,54]. Ganesan et al. [55] investigated the use of the DMC alcohol addition in biodiesel-diesel blends, where DMC works as an oxygenated buffer, donating excess oxygen and promoting combustion. Smoke emissions are decreased as a result.

Conclusion
The current study is aimed at investigating the influence of different GNP doses and DMC alcohol (10% vol.) in Sterculia foetida oil methyl ester at a variety of loads and constant speeds to evaluate the diesel engine performance, emission, and combustion parameters. Biodiesel blend B30 was prepared as well as two distinct fuel additives, i.e., a fixed amount of DMC (10%), and different concentrations of GNPs (30, 60, and 90 ppm) were combined with the B30 blend. Subsequent conclusions were noted from the above study as the most promising ratio of graphene to QPAN was 1 : 1, where GNPs are more stable and homogeneous in biodiesel. The biodiesel blend infused with DMC and surface-modified GNPs exhibited a considerable increase in BTE and BSFC.
(i) QPAN 80 dispersants with surface modification improved the stability of GNPs in the B30 blend (ii) The combustion characteristics, such as CP and HRR, were enhanced by adding GNPs and DMC in B30. At max load, the peak CP and maximum HRR have been increased by 15.45% and 9.63%, respectively, for B30DMC10 included with 60 ppm GNPs when correlated with diesel fuel (iii) A major decrease in BSFC by 25.54% and the highest BTE of 8.98% were detected for B30GNP60DMC10 blend in performance characteristics (iv) A substantial decline in CO and UHC was noted for graphene nanofuel samples, followed by B30DMC10. A considerable reduction of 25.67% and 22.87% in CO and UHC emissions was obtained for oxygenated fuel blend B30GNP60DMC10, respectively, compared to B30 fuel (v) In contrast, a remarkable decrement in NOx and smoke opacity by 9.57% and 12.4% was observed for the B30GNP60DMC10 blend in comparison with B30. Therefore, GNP and DMC together demonstrated promising ability for operating diesel engines (vi) Finally, the surface-modified GNPs at a concentration level of 60 ppm and 10% of DMC in the B30 blend improved the overall engine performance and reduced emissions substantially. Hence, the blend B30GNP60DMC10 is suggested to be used directly as a potential substitute for diesel fuel in CI engines without any modifications This study can be extended further with another type of higher alcohols with carbon allotrope nanoadditives in biodiesel-diesel blends with variable injection timing/injection pressures for improvement in engine performance and emission characteristics.

Data Availability
The data is available in the manuscript.

Conflicts of Interest
The authors declare that they have no conflict of interest.