E ﬀ ect of Nano-Graphene Oxide and n-Butanol Fuel Additives Blended with Diesel— Nigella sativa Biodiesel Fuel Emulsion on Diesel Engine Characteristics

: The present investigation uses a blend of Nigella sativa biodiesel, diesel, n-butanol, and graphene oxide nanoparticles to enhance the performance, combustion and symmetric characteristics and to reduce the emissions from the diesel engine of a modiﬁed common rail direct injection (CRDI). A symmetric toroidal-type combustion chamber and a six-hole solenoid fuel injector were used in the current investigation. The research aimed to study the e ﬀ ect of two fuel additives, n-butanol and synthesized asymmetric graphene oxide nanoparticles, in improving the fuel properties of Nigella sativa biodiesel (NSME25). The concentration of n-butanol (10%) from agglomerating in the base ﬂuid. The process was repeated for biodiesel, n-butanol and nanoﬂuid, and four di ﬀ erent stable and symmetric nanofuel mixtures were prepared by varying the graphene oxide (30, 60, 90 and 120 ppm). The nanofuel blend NSME25B10GO90 displayed an enhancement in the brake thermal e ﬃ ciency (BTE) and a reduction in brake-speciﬁc fuel consumption (BSFC) at maximum load due to high catalytic activity and the enhanced microexplosion phenomenon developed by graphene oxide nanoparticles. The heat release rate (HRR), in-cylinder temperature increased, while exhaust gas temperature (EGT) decreased. Smoke, hydrocarbon (HC), carbon monoxide (CO 2 ) and carbon monoxide (CO) emissions also fell, in a trade-o ﬀ with marginally increased NO x , for all nanofuel blends, compared with Nigella sativa biodiesel. The results obtained indicates that 90 ppm of graphene oxide nanoparticles and 10% n-butanol in Nigella sativa biodiesel are comparable with diesel fuel.


Introduction
The rapid depletion of fossil fuels has led to renewable sources of energy gaining considerable attention [1]. One of the well-known and widely used renewable source of energy is biodiesel; biodiesel can be produced by using wide-ranging number of oils without any diesel engine modifications [2,3]. The feedstocks for biodiesel are generally classified as edible and non-edible oils, animal fat and waste cooking oils (WCOs). Soybean, palm and rapeseed are mainly used as biodiesel feedstock, as well as canola and sunflower. However, in recent years, the utilization of the edible oils has led to several food vs. fuel debates [4,5]. The cost of feedstocks from edible sources constitute around 70-80% of the final fuel production price, the cost of the feedstocks can be reduced by utilization of non-edible biodiesel feedstocks [6]. The Indian sub-continent has a potential to supply abundant non-edible feedstocks due to the vast forest regions and uncropped marginal and waste lands. Hence, India has high potential due to its endless supply of non-edible feedstocks, which makes it possible to overcome the economic challenges [7][8][9][10].
Nigella sativa (the black seed), which is found in the Middle East and Europe but is native to south-western Asia, it belongs to the plant family Ranunculaceae. B20 is the standard and acceptable form of running fuel in standard CI engines without engine modifications based on extensive literature about the use of biodiesel in diesel engines [11]. The authors, however, attempted to study the combined and symmetric effect of CRDI diesel engine characteristics of B25 and fuel additives. Several advantages of biodiesel are overcome by few disadvantages, such as damage to rubber hoses, higher production costs, fuel filter clogging, and fuel gelling due to high viscosity during cold weather. Hence, these factors have led to lower demand for biodiesel [6,12]. The drawbacks of biodiesel in diesel engine can be overcome by addition of a few milligrams of nanofuel additives [1,13,14]. Researchers in this field have investigated the effect of adding metallic and carbon-based nano-additives to diesel-biodiesel fuel blends; several have reported an overall improvement in the performance of the diesel engine and emissions reduction due to the complete combustion of fuel [1][2][3]15,16] Due to their enhanced thermal and chemical properties and droplet combustion phenomena, asymmetric graphene oxide nanoparticles are more effective in reducing fuel emissions and improving performance than their metal-based counterparts, due to their enhanced thermal and chemical properties and droplet combustion behaviors and phenomenon [17]. The high exothermic heat release rate of graphene oxide NPs and the presence of oxygen functional groups, such as carboxylates, epoxides and hydroxides, promote chemically active spots resulting in overall fuel combustion [18]. The carbon depositions can also be addressed by addition of graphene oxide NPs, due to the presence of C, H and O atoms [19]. The majority of previous studies are based on the inclusion of metal-based nanoparticles Symmetry 2020, 12, 961 3 of 28 (such as aluminum oxide, zinc oxide, titanium oxide, cerium oxide, cobalt oxide, etc.) in the fuel blends. However, there is a lack of understanding about the combined effect of numerous high-energy-content carbon-based nanoparticles and alcohol additives in biodiesel blends. Hosseini et al. [20] investigated the effect on engine characteristics of carbon nanotubes (CNTs) (30,60 and 90 ppm) and waste cooking oil (WCO-B5 and B10). The authors reported an increase of 8.21% in the BTE, whereas CO, HC, NOx and smoke emissions decreased by 65.7%, 44.98%, 27.49% and 29.41%, respectively. The specific fuel consumption (SFC) also decreased by 7.12%, while torque and power increased significantly with the addition of nanoparticles in WCO biodiesel. Aram Heidari M et al. [14] reported the addition of nano-additives of graphene quantum dot (GQD) in ethanol-biodiesel blends improved the performance and emission characteristics of a CI engine. The GQD in proportions of 30 ppm were added to ethanol-biodiesel (B10 and E2, E4, E6 and E8) blends. The engine speeds were varied at constant loads, and the authors reported an increase in power by 28.18% and torque by 12.42%. SFC, CO and HC decreased by 14.35%, 29.54% and 31.12%, respectively, in comparison with D100 fuel. Manzoore Elahi M Soudagar et al. [3] investigated the effect of graphene oxide nanoparticles on daily scum oil methyl ester (DSOME) using three nano-biodiesel fuel blends (DSOME (B20) + graphene oxide NP (ppm) dosage. The fuel blends used in the investigations were DSOME2020, DSOME2040 and DSOME2060; the nanofuel blend DSOME2040 (40 ppm of graphene oxide NPs in B20) illustrated overall enhancement in performance, combustion, and emission characteristics due to high catalytic activity of graphene oxide NPs. S. Abdul Sheriff et al. [11] investigated the combined effect of multi-walled carbon nanotubes (MWCNTs) and CeO2 in lemon and orange peel oil biodiesel fuel blends in CI engine. B20 + (50 and 100 ppm); eight different fuel blends resulted in reductions in BSFC, HC, CO and NOx emissions and increases in BTE at 100% load due to fine atomization of fuel particles, leading to complete combustion. Enhancement of the engine characteristics was also achieved by replacing the conventional fuel injector and combustion chamber. Pasupathy et al. [21] studied the simulation on the effect of piston bowl geometry on engine characteristics in a DI turbocharged CI engine for heavy duty applications using STAR-CD. The simulation results showed that the toroidal bowl's extended lip enhances the friction and thus increases the A: F mixture and decreases soot and specific fuel consumption. B.R. Ramesh Bapu et al. [22] investigated the effect of Calophyllum inophyllum biodiesel on a single-cylinder, VCR diesel engine. The modified HCC piston bowl geometry demonstrated reductions in the HC, CO and smoke emissions due to high turbulence and enhancement in the A:F mixture compared to conventional HCC. The modification of the piston bowl geometry led to an enhancement in the overall engine operations using the B20 fuel blend. Table 1 shows the symmetric effect of graphene nano-additives combined with diesel and biodiesel fuel blends on diesel engine performance and emission characteristics. The current study is based on production of a much-less-explored, novel potential biodiesel feedstock, viz. Nigella sativa, and enhancement of the fuel properties of the Nigella sativa biodiesel through blending with graphene oxide nanoparticles and n-butanol fuel additives. The use of graphene oxide nanoparticles in Nigella sativa biodiesel improves the premixed combustion, resulting in a higher temperature in the combustion chamber. However, due to the high surface area of asymmetric graphene oxide nanoparticles, the thermal conductivity increases. In a single-cylinder common rail direct injection (CRDI) diesel engine with modified symmetric piston bowl geometry (Toroidal type) and a six-hole fuel injector, the effect of nanofuel blends are studied.
In this article, an attempt is made to study and analyze Nigella sativa methyl ester (NSME25) fuel properties with nanofuel and n-butanol blends (NSME25B10GO30, NSME25B10GO60, NSME25B10GO90 and NSME25B10GO120). A comparative analysis is carried out on the combined effect of NSME25 + n-butanol + graphene oxide fuel blends and NSME25 biodiesel fuel.

Materials and Methods
This section describes the production of Nigella sativa methyl ester using the transesterification reaction, characterization of graphene oxide nanoparticles, test engine setup and physicochemical properties of the test fuels.

Preparation of Nigella sativa Methyl Ester
Nigella sativa belongs to the botanical family Ranunculaceae and is generally referred to as black seed, black cumin seed, kalonji and seed of blessing, with an oil content of more than 35% [4,29]. It is generally used in traditional medicines like Unani and Ayurveda, Nigella sativa is mentioned in Tibb-e-Nabwi (Prophetic Medicine) for its usage on a daily basis for its ability to cure diabetes, renal and gastric-related diseases and use as a bronchodilator, anti-microbial treatment, etc. [30]. It is cultivated globally, but abundantly available in south and southwest Asia. The feedstock yields (85.6-97.7%) of biodiesel and can be deemed one of the key biodiesel feedstocks in Asia due to its high yield and availability [4]. The plant grows 20-90 cm tall and its leaves are finely segregated, with narrow linear treaded segments. The flowering petals are usually 5-10 in number and are symmetric, blue, white, pale pink and purple colored [31,32]. The seeds are dicotyledonous, regulose tubercular, tiny (2-3.5 mm/1-2 mm) and asymmetric, black on the outside and white on the inside, aromatic and bitter tasting [30]. Figure 1 shows the Nigella sativa plant, flowers, and seeds. The current study is based on production of a much-less-explored, novel potential biodiesel feedstock, viz. Nigella sativa, and enhancement of the fuel properties of the Nigella sativa biodiesel through blending with graphene oxide nanoparticles and n-butanol fuel additives. The use of graphene oxide nanoparticles in Nigella sativa biodiesel improves the premixed combustion, resulting in a higher temperature in the combustion chamber. However, due to the high surface area of asymmetric graphene oxide nanoparticles, the thermal conductivity increases. In a single-cylinder common rail direct injection (CRDI) diesel engine with modified symmetric piston bowl geometry (Toroidal type) and a six-hole fuel injector, the effect of nanofuel blends are studied.
In this article, an attempt is made to study and analyze Nigella sativa methyl ester (NSME25) fuel properties with nanofuel and n-butanol blends (NSME25B10GO30, NSME25B10GO60, NSME25B10GO90 and NSME25B10GO120). A comparative analysis is carried out on the combined effect of NSME25 + n-butanol + graphene oxide fuel blends and NSME25 biodiesel fuel.

Materials and Methods
This section describes the production of Nigella sativa methyl ester using the transesterification reaction, characterization of graphene oxide nanoparticles, test engine setup and physicochemical properties of the test fuels.

Preparation of Nigella sativa Methyl Ester
Nigella sativa belongs to the botanical family Ranunculaceae and is generally referred to as black seed, black cumin seed, kalonji and seed of blessing, with an oil content of more than 35% [4,29]. It is generally used in traditional medicines like Unani and Ayurveda, Nigella sativa is mentioned in Tibbe-Nabwi (Prophetic Medicine) for its usage on a daily basis for its ability to cure diabetes, renal and gastric-related diseases and use as a bronchodilator, anti-microbial treatment, etc. [30]. It is cultivated globally, but abundantly available in south and southwest Asia. The feedstock yields (85.6%-97.7%) of biodiesel and can be deemed one of the key biodiesel feedstocks in Asia due to its high yield and availability [4]. The plant grows 20-90 cm tall and its leaves are finely segregated, with narrow linear treaded segments. The flowering petals are usually 5-10 in number and are symmetric, blue, white, pale pink and purple colored [31,32]. The seeds are dicotyledonous, regulose tubercular, tiny (2-3.5 mm/1-2 mm) and asymmetric, black on the outside and white on the inside, aromatic and bitter tasting [30]. Figure 1 shows the Nigella sativa plant, flowers, and seeds. Crude Nigella sativa oil was purchased with a purity of 98.65% from local markets in India. The FFA% of the crude Nigella sativa oil is higher than the vegetable oils, hence transesterification reaction is employed in the current study. The method of transesterification was adopted from the previous investigation conducted on the production of Nigella sativa biodiesel [4,29]. Figure 2 illustrates the transesterification reaction of Nigella sativa oil. Crude Nigella sativa oil was purchased with a purity of 98.65% from local markets in India. The FFA% of the crude Nigella sativa oil is higher than the vegetable oils, hence transesterification reaction is employed in the current study. The method of transesterification was adopted from the previous investigation conducted on the production of Nigella sativa biodiesel [4,29]. Figure 2 illustrates the transesterification reaction of Nigella sativa oil. In the esterification process, the crude oil is initially mixed with methanol (CH3OH-2:1 volume) in a reactor at 60 °C for 3 h at steady stirring at a speed of 750 rpm. An application of 98% concentrated sulfuric acid (H2SO4: 1% vol/vol oil) is used to reduce the excess fat content of the esterified mixture. Additionally, 37% concentrated hydrochloric acid (HCl: 1% vol/vol oil) is used in the esterification reaction. The methanol and the esterified oil form an upper and lower layer, respectively. A separating funnel was used to separate the esterified oil from the methanol, then a rotary evaporator was used to remove the hints of moisture from the esterified oil. Further, the transesterification reaction (alkaline catalysis) was used in this investigation: the mixture of esterified oil and methanol (4:1) by volume of reactor was mixed with Potassium hydroxide (1%) and stirred at a constant speed of 750 rpm at 60 °C for two hours. Later, the biodiesel forms an upper layer in a separating funnel and a lower layer of glycerol. The obtained trans esterified oil was washed constantly using warm water, and the obtained biodiesel was stirred in a magnetic stirrer for removal of excess water, methanol, and moisture content. To finish, the biodiesel was washed with sodium sulphate (Na₂SO₄) and a fine meshed filter paper used to eliminate any traces of impurities. The conversion output of Nigella sativa methyl ester through the transesterification reaction is 92.8% weight. The biodiesel output was in symmetry with the preceding literature [2,3]. Table 2 illustrates the Fatty acid composition of Nigella sativa methyl ester.  In the esterification process, the crude oil is initially mixed with methanol (CH 3 OH-2:1 volume) in a reactor at 60 • C for 3 h at steady stirring at a speed of 750 rpm. An application of 98% concentrated sulfuric acid (H 2 SO 4 : 1% vol/vol oil) is used to reduce the excess fat content of the esterified mixture. Additionally, 37% concentrated hydrochloric acid (HCl: 1% vol/vol oil) is used in the esterification reaction. The methanol and the esterified oil form an upper and lower layer, respectively. A separating funnel was used to separate the esterified oil from the methanol, then a rotary evaporator was used to remove the hints of moisture from the esterified oil. Further, the transesterification reaction (alkaline catalysis) was used in this investigation: the mixture of esterified oil and methanol (4:1) by volume of reactor was mixed with Potassium hydroxide (1%) and stirred at a constant speed of 750 rpm at 60 • C for two hours. Later, the biodiesel forms an upper layer in a separating funnel and a lower layer of glycerol. The obtained trans esterified oil was washed constantly using warm water, and the obtained biodiesel was stirred in a magnetic stirrer for removal of excess water, methanol, and moisture content. To finish, the biodiesel was washed with sodium sulphate (Na 2 SO 4 ) and a fine meshed filter paper used to eliminate any traces of impurities. The conversion output of Nigella sativa methyl ester through the transesterification reaction is 92.8% weight. The biodiesel output was in symmetry with the preceding literature [2,3]. Table 2 illustrates the Fatty acid composition of Nigella sativa methyl ester.

Properties and Characterization of Graphene Oxide Nanoparticles
The graphene oxide nanoparticles (GO) were prepared at lab scale in India; the method of preparation of graphene oxide nanoparticle additives is explained and is in symmetry with the Symmetry 2020, 12, 961 7 of 28 preceding research by Soudagar et al. [3]. The average size of the synthesized GO NPs was 23-27 nm (±0.5 nm). Figure 3 illustrates the XRD analysis of the graphene oxide nanoparticles.

Properties and Characterization of Graphene Oxide Nanoparticles
The graphene oxide nanoparticles (GO) were prepared at lab scale in India; the method of preparation of graphene oxide nanoparticle additives is explained and is in symmetry with the preceding research by Soudagar et al. [3]. The average size of the synthesized GO NPs was 23-27 nm (±0.5 nm). Figure 3 illustrates the XRD analysis of the graphene oxide nanoparticles. The X-ray diffraction spectrum (XRD) was used to confirm a successful GO synthesis by identifying their cellular structural units (d-spacing). The analysis was carried out using a Bruker D8 ADVANCE diffractometer with radiation Cu Kα (λ = 1.54060 Å). Figure 3 illustrates the XRD patterns. The XRD pattern of asymmetric graphene oxide exhibits both a prominent peak of the (001) plane at 2θ = 10.74° and a (100) plane diffraction peak at 2θ = 42.34°, confirming the successful GO synthesis [33]. The SEM images shown in Figure 4a,b, the SEM and EDX were analyzed with EDAX using the Scanning Electron Microscope XL 30 ESEM. Resolution: up to 2; magnification: up to 250,000x, Acc. Voltage: 30 kV. It is evident that GO NPs have a multiple lamellar layer structure, and the edges of individual sheets can be distinguished from the SEM images. The asymmetric films are stacked above each other and display wrinkled areas. It can also be noted that the edges of the GO sheets were thicker. This is because the functional groups containing oxygen were mainly combined at graphene oxides edges. From the EDX analysis in Figure 4c, it is evident that the GO contains around 61.13% at. C, 38.65% at. % O and 0.23% atom S. Hence, the graphene oxide contains sufficient carbon and oxygen which confirms successful synthesis of GO nanoparticles.
The TEM images are also illustrated in Figure Figure 4g shows the Highresolution transmission electron microscopy (HRTEM) pattern. The TEM and HRTEM study was performed using JEOL 2000 FX-II TEM with an ultra-thin Oxford Instruments EDS window system. The appearance of TEM images of GO NPs are multilayered and wrinkled. The HRTEM demonstrates the lattice fringes of graphene oxide nanoparticles [34,35]. The crystallographic structure of the graphene oxide was characterized by the SAED pattern. The previous research cited illustrates a single set of hexagonal diffraction patterns with sharp, symmetric and clear diffraction spots in the graphene oxide nanoparticles [35]. The X-ray diffraction spectrum (XRD) was used to confirm a successful GO synthesis by identifying their cellular structural units (d-spacing). The analysis was carried out using a Bruker D8 ADVANCE diffractometer with radiation Cu Kα (λ = 1.54060 Å). Figure 3 illustrates the XRD patterns. The XRD pattern of asymmetric graphene oxide exhibits both a prominent peak of the (001) plane at 2θ = 10.74 • and a (100) plane diffraction peak at 2θ = 42.34 • , confirming the successful GO synthesis [33]. The SEM images shown in Figure 4a,b, the SEM and EDX were analyzed with EDAX using the Scanning Electron Microscope XL 30 ESEM. Resolution: up to 2; magnification: up to 250,000×, Acc. Voltage: 30 kV. It is evident that GO NPs have a multiple lamellar layer structure, and the edges of individual sheets can be distinguished from the SEM images. The asymmetric films are stacked above each other and display wrinkled areas. It can also be noted that the edges of the GO sheets were thicker. This is because the functional groups containing oxygen were mainly combined at graphene oxides edges. From the EDX analysis in Figure 4c, it is evident that the GO contains around 61.13% at. C, 38.65% at. % O and 0.23% atom S. Hence, the graphene oxide contains sufficient carbon and oxygen which confirms successful synthesis of GO nanoparticles.
The TEM images are also illustrated in Figure Figure 4g shows the High-resolution transmission electron microscopy (HRTEM) pattern. The TEM and HRTEM study was performed using JEOL 2000 FX-II TEM with an ultra-thin Oxford Instruments EDS window system. The appearance of TEM images of GO NPs are multilayered and wrinkled. The HRTEM demonstrates the lattice fringes of graphene oxide nanoparticles [34,35]. The crystallographic structure of the graphene oxide was characterized by the SAED pattern. The previous research cited illustrates a single set of hexagonal diffraction patterns with sharp, symmetric and clear diffraction spots in the graphene oxide nanoparticles [35].  n-butyl alcohol or butanol is a primary alcohol consisting of a four-carbon chain structure and the chemical formula is C4H9OH. Its isomers of n-Butanol include tert-butanol, 2-butanol, and isobutanol. Figure 4h and Figure 4i illustrates the chemical structure of n-butanol and SDBS surfactant. Butanol belongs to the group of fusel alcohols in German language or bad liquor and is readily soluble in water. Fermentation of sugars and carbohydrates produces n-Butanol, and it is also found in several food products and beverages [36,37]. Butanol is essentially a petrochemical substance and (h) (i) n-butyl alcohol or butanol is a primary alcohol consisting of a four-carbon chain structure and the chemical formula is C 4 H 9 OH. Its isomers of n-Butanol include tert-butanol, 2-butanol, and iso-butanol. Figures 4h,i illustrates the chemical structure of n-butanol and SDBS surfactant. Butanol belongs to the group of fusel alcohols in German language or bad liquor and is readily soluble in water. Fermentation of sugars and carbohydrates produces n-Butanol, and it is also found in several food products and beverages [36,37]. Butanol is essentially a petrochemical substance and is made from propylene. n-butanol is used as a diesel and gasoline substitute. Butanol is also known for its use as a biofuel; 85 percent of butanol can be used in diesel and petrol-powered cars without engine modification, providing more energy than ethanol [38]. To reduce the soot emissions, butanol is added to diesel fuel [39]. In the current investigation, commercially available n-butanol and SDBS surfactant were procured from Sigma Aldrich. The addition of surfactant enhances the stability of NPs in aqueous suspensions to convert the surfaces of NPs from hydrophobic to hydrophilic and conversely to non-aqueous. It also reduces the possibility of surface tension, coagulation and coalescence and breaks the asymmetric bonds. They tend to position themselves and establish a degree of continuity between the NPs and the base fluid at the interface [1].

Test Engine Setup
In the current investigation, Kirloskar TV1 was assisted by a common rail direct injection system, four-stroke, water-cooled diesel engine. In the current world scenario, the fuel efficiency of diesel engines is increased by the application of CRDI; nowadays, CRDI technology is widely used in commercial vehicles [40]. In order to increase the fuel injection pressure to 900 bar, the regulatory pressure valve and the fuel injector are attached to the existing engine. The schematic representation of the CRDI engine test rig is illustrated in Figure 5. The engine runs at a constant speed of 1500 rpm; the CRDI engine's rated power output is 5.2 KW; it has a 900 bar injection pressure and 23 • BTDC injection timing. Several temperatures (water inlet and outlet temperature, exhaust gas temperature) were measured using thermocouples. The combustion chamber used in the current investigation is a toroidal type, replacing the hemispherical combustion chamber for better mixing of the A:F. A solenoid six-hole, 0.1-mm orifice diameter fuel injector at 900 bar injection pressure is used. Figure 6 illustrates the top and side views of the Toroidal combustion chamber (TCC) used in the current investigation.

Test Engine Setup
In the current investigation, Kirloskar TV1 was assisted by a common rail direct injection system, four-stroke, water-cooled diesel engine. In the current world scenario, the fuel efficiency of diesel engines is increased by the application of CRDI; nowadays, CRDI technology is widely used in commercial vehicles [40]. In order to increase the fuel injection pressure to 900 bar, the regulatory pressure valve and the fuel injector are attached to the existing engine. The schematic representation of the CRDI engine test rig is illustrated in Figure 5. The engine runs at a constant speed of 1500 rpm; the CRDI engine's rated power output is 5.2 KW; it has a 900 bar injection pressure and 23° BTDC injection timing. Several temperatures (water inlet and outlet temperature, exhaust gas temperature) were measured using thermocouples. The combustion chamber used in the current investigation is a toroidal type, replacing the hemispherical combustion chamber for better mixing of the A:F. A solenoid six-hole, 0.1-mm orifice diameter fuel injector at 900 bar injection pressure is used. Figure 6 illustrates the top and side views of the Toroidal combustion chamber (TCC) used in the current investigation.

Preparation of Fuel Blend
In the current investigation, a two-step process for the preparation of nanofluids is used; the graphene oxide nanoparticles are steadily dispersed in the SDBS surfactant and distilled water using a probe sonicator, which is also known as the indirect mode of sonication.
The oxygenated additive, n-butanol, is blended with biodiesel at a constant volume of 10% during the ultasonication process of NPs and fuel. The fuel blends are dosed with graphene oxide nanoparticles at 30, 60, 90 and 120 mg/L dosage levels using a magnetic stirrer for 25 min, 1 h using an ultrasonicator bath and 20 min using a probe sonicator at a frequency of 15-20 Hz per three seconds. Table 3 illustrates the composition of test fuel blends.
The fuel blend NSME25B10GO30 is prepared by blending NSME25 with 30 ppm graphene oxide NPs and 15 ppm of SDBS surfactant. For the NSME25B10GO60 blend, NSME25 is blended with 60 ppm graphene oxide NPs and 25 ppm SDBS surfactant. For NSME25B10GO90 and NSME25B10GO120 fuel, NSME25 biodiesel is blended with 90 ppm and 120 ppm of graphene oxide NPs and 35 ppm and 45 ppm of SDBS surfactant, respectively. For preparation of nanofluids, the ultrasonic waves are supplied to a combination of distilled water (5 mL), graphene nanoparticles and SDBS surfactant using a probe sonicator at a frequency of 20 KHz for 20-30 min for each nanofluid blend. The sonication breaks the intermolecular and symmetric interactions which prevents the agglomeration. Later, the NSME25 biodiesel is blended with the nanofluids, the blend is stirred using a magnetic stirrer at 60 • C to remove the water and moisture content for 15 min, a bath-type sonicator is used for blending the different sets of nanofuel blends for one hour for each blend, and, further, the nanoparticle stability is increased by sonicating in a probe sonicator. These steps ensure synthesis of stable nanofuel blends. Figure 7 shows the steps and methodology adopted in the study on nanofuel blends. In the current investigation, a two-step process for the preparation of nanofluids is used; the graphene oxide nanoparticles are steadily dispersed in the SDBS surfactant and distilled water using a probe sonicator, which is also known as the indirect mode of sonication.
The oxygenated additive, n-butanol, is blended with biodiesel at a constant volume of 10% during the ultasonication process of NPs and fuel. The fuel blends are dosed with graphene oxide nanoparticles at 30, 60, 90 and 120 mg/L dosage levels using a magnetic stirrer for 25 min, 1 h using an ultrasonicator bath and 20 minutes using a probe sonicator at a frequency of 15-20 Hz per three seconds. Table 3 illustrates the composition of test fuel blends.
The fuel blend NSME25B10GO30 is prepared by blending NSME25 with 30 ppm graphene oxide NPs and 15 ppm of SDBS surfactant. For the NSME25B10GO60 blend, NSME25 is blended with 60 ppm graphene oxide NPs and 25 ppm SDBS surfactant. For NSME25B10GO90 and NSME25B10GO120 fuel, NSME25 biodiesel is blended with 90 ppm and 120 ppm of graphene oxide NPs and 35 ppm and 45 ppm of SDBS surfactant, respectively. For preparation of nanofluids, the ultrasonic waves are supplied to a combination of distilled water (5 mL), graphene nanoparticles and SDBS surfactant using a probe sonicator at a frequency of 20 KHz for 20-30 min for each nanofluid blend. The sonication breaks the intermolecular and symmetric interactions which prevents the agglomeration. Later, the NSME25 biodiesel is blended with the nanofluids, the blend is stirred using a magnetic stirrer at 60 °C to remove the water and moisture content for 15 min, a bath-type sonicator is used for blending the different sets of nanofuel blends for one hour for each blend, and, further, the nanoparticle stability is increased by sonicating in a probe sonicator. These steps ensure synthesis of stable nanofuel blends. Figure 7 shows the steps and methodology adopted in the study on nanofuel blends.  Figure 7. Methodology adopted in the study on nanofuel blends.

Physicochemical Properties of Fuel Blends
The prepared fuel blends are compared with diesel and Nigella sativa biodiesel using ASTM D6751-15c standards. NSME25 fuel blend's calorific value is the lowest amongst all the fuel blends, and also showed the highest density and viscosity. The addition of n-butanol to the Nigella sativa biodiesel showed higher miscibility and stability rates. Furthermore, the addition of n-butanol and graphene oxide NPs to the fuel samples reduces the ID period due to enhancement of the combustion phenomenon and reductions in the combustion duration, density, viscosity, and emissions. Table 4 demonstrates the physicochemical properties of fuel blends in accordance with the ASTM standards.
The overall cost for experimentation and synthesis of nano-fuel blends, which includes the engine operating cost, transportation, electricity, and labor, was around $210.64 ( The overall cost for experimentation and synthesis of nano-fuel blends, which includes the engine operating cost, transportation, electricity, and labor, was around $210.64 ( ₹ 15,771.88, conversion from USD to INR, April, 2020). The use of nanoparticles in the biodiesel fuel marginally raises the fuel cost by around 2.56%. Nevertheless, graphene oxide nano-additives and n-butanol increase the overall fuel characteristics, which overcomes the cost setting.

Uncertainty Analysis of Expected Errors
Uncertainty analysis is the systematic and symmetric set of procedures followed for calculation of errors in experimental data. The inaccuracies occur due to the error in electronic and mechanical components, environmental factors, and human miscalculations [41,42]. The information is gathered at standard conditions, and the specifications and details of all the components are already available. Measurement errors which come from several different sources are classified into bias and precision errors; throughout the experimentation, the bias errors remain constant. The measurement uncertainty is estimated as the experiments are carried out. The uncertainty measurements using standard parameters are explained in this section.
The propagation of uncertainty for parameters that have been determined on the factors depending on two or more independent parameters is determined using Equation (1) where, Uy is uncertainty; y is testing value; x1, x2...., xn are evaluated parameters, and the uncertainty of emissions is Uy = .
The overall uncertainty of the parameters of engine performance, emission, and combustion is calculated using Equation (2).
The overall uncertainty for the measured parameters is ± 1.12, which is within the permissible limit and in symmetry with previously reported studies [2,3].  April, 2020). The use of nanoparticles in the biodiesel fuel marginally raises the fuel cost by around 2.56%. Nevertheless, graphene oxide nano-additives and n-butanol increase the overall fuel characteristics, which overcomes the cost setting.

Uncertainty Analysis of Expected Errors
Uncertainty analysis is the systematic and symmetric set of procedures followed for calculation of errors in experimental data. The inaccuracies occur due to the error in electronic and mechanical components, environmental factors, and human miscalculations [41,42]. The information is gathered at standard conditions, and the specifications and details of all the components are already available. Measurement errors which come from several different sources are classified into bias and precision errors; throughout the experimentation, the bias errors remain constant. The measurement uncertainty is estimated as the experiments are carried out. The uncertainty measurements using standard parameters are explained in this section.
The propagation of uncertainty for parameters that have been determined on the factors depending on two or more independent parameters is determined using Equation (1), where, U y is uncertainty; y is testing value; x 1 , x 2 , . . . , x n are evaluated parameters, and the uncertainty of emissions is U y = Resolution Range . The overall uncertainty of the parameters of engine performance, emission, and combustion is calculated using Equation (2). Table 5 also illustrates the uncertainty and accuracy levels of calculated engine parameters.
The overall uncertainty for the measured parameters is ±1.12, which is within the permissible limit and in symmetry with previously reported studies [2,3]. The HRR is negative during the ID period due to the cooling effect produced by the vaporization of the fuel and the loss of heat at the cylinder walls. The increase in HRR for all nanofuel mixtures is due to an improved cetane number and a lower ID period, which helps in increasing the engine efficiency [42][43][44][45]. Figure 8 shows the variation of HRR and crank angle for test fuels at maximum load. The HRR is negative during the ID period due to the cooling effect produced by the vaporization of the fuel and the loss of heat at the cylinder walls. The increase in HRR for all nanofuel mixtures is due to an improved cetane number and a lower ID period, which helps in increasing the engine efficiency [42][43][44][45]. Figure 8 shows the variation of HRR and crank angle for test fuels at maximum load. Owing to higher molecular weight and lower burning velocity, the HRR for the NSME25 blend is lower than the other fuel blends [46]. The enhancement in HRR for the all nanofuel blends is due to improved surface-area-to-volume ratio, high ignition and improved fuel properties and thermal conductivity [47,48]. This results in an increase in the peak pressure and, hence, the HRR is also increased. The maximum peaks observed for NSME25B10 GO (30, 60, 90 and 120 mg/L) were 65.25, 75.72, 90.54 and 87.223 J/°CA, while the peaks observed for diesel and NSME25 were 91.997 and 60.813 J/°CA. An enhancement of 29.727 J/°CA in HRR was observed with the addition of 90 ppm of GO and 10% volume n-butanol to NSME25 when compared with neat NSME25.

Effect of Fuel Additives on the In-Cylinder Temperature of the Engine
The mean value of cylinder temperature of combusted and unburned gases in the combustion chamber that occurs during a cycle is known as in-cylinder or mean cylinder temperature. The gases in the cylinder are a symmetric blend of combusted and unburned air and fuel. The temperature in the cylinder determines the fuel's combustion reaction rate, and the desired value should be closer to the adiabatic flame temperature (Ta) [49]. Figure 9 demonstrates the variation of heat release rate and crank angle for test fuels at maximum load. Owing to higher molecular weight and lower burning velocity, the HRR for the NSME25 blend is lower than the other fuel blends [46]. The enhancement in HRR for the all nanofuel blends is due to improved surface-area-to-volume ratio, high ignition and improved fuel properties and thermal conductivity [47,48]. This results in an increase in the peak pressure and, hence, the HRR is also increased. The maximum peaks observed for NSME25B10 GO (30, 60, 90 and 120 mg/L) were 65.25, 75.72, 90.54 and 87.223 J/ • CA, while the peaks observed for diesel and NSME25 were 91.997 and 60.813 J/ • CA. An enhancement of 29.727 J/ • CA in HRR was observed with the addition of 90 ppm of GO and 10% volume n-butanol to NSME25 when compared with neat NSME25.

Effect of Fuel Additives on the In-Cylinder Temperature of the Engine
The mean value of cylinder temperature of combusted and unburned gases in the combustion chamber that occurs during a cycle is known as in-cylinder or mean cylinder temperature. The gases in the cylinder are a symmetric blend of combusted and unburned air and fuel. The temperature in the cylinder determines the fuel's combustion reaction rate, and the desired value should be closer to the adiabatic flame temperature (Ta) [49]. Figure 9 demonstrates the variation of heat release rate and crank angle for test fuels at maximum load. The adiabatic temperature is achieved when there is no loss in thermal energy and the adiabatic state is reached. It is observed that there is a 15°ATDC distance in the injector where flame reaches the wall of the cylinder [49,50]. At this point, maximum pressure is reached, although the combustion process is incomplete and the combustion at the boundary of the combustion chamber continues a further few °CA [50]. Initially, the CRDI engine was operated on diesel fuel; diesel showed a maximum in-cylinder temperature of 973. 51

In-Cylinder Temperature
Crank angle (°CA) Figure 9. The variation of heat release rate and crank angle for test fuels at 100% load.
The adiabatic temperature is achieved when there is no loss in thermal energy and the adiabatic state is reached. It is observed that there is a 15 • ATDC distance in the injector where flame reaches the wall of the cylinder [49,50]. At this point, maximum pressure is reached, although the combustion process is incomplete and the combustion at the boundary of the combustion chamber continues a further few • CA [50]. Initially, the CRDI engine was operated on diesel fuel; diesel showed a maximum in-cylinder temperature of 973.51 • C to 970.963 • C from 422 • CA to 460 • CA. The nanofuel blend, NSME25B10GO90, illustrated a mean cylinder temperature of 851.63 • C to 852.94 • C at 418 • CA to 460 • CA. Meanwhile, the mean cylinder temperature of all the nanofuel mixes at dosage levels of 30, 60 and 120 mg/L showed an increase in the temperature of the in cylinder. Due to high viscosity and density, NSME25 however reduced the mean cylinder temperature.

The Effect of Fuel Additives on Exhaust Gas Temperature (EGT)
The variation of the exhaust gas temperature (EGT) and engine brake power is shown in Figure 10 for different fuel blends. An increasing trend in EGT was observed for increasing loads. Lowest EGT (395 • C) was observed for diesel fuel and highest EGT (480 • C) was observed for Nigella biodiesel at maximum load; 90 ppm of graphene oxide NPs with 10% n-butanol additive illustrated a very close range of values of EGT with diesel at all brake powers, the EGT at maximum load is (413 • C). Nigella biodiesel combined with n-butanol 10% and 30 ppm of graphene illustrated slightly lower EGT compared with NSME25.
This indicates the reduction in EGT with the presence of nano-additives. The GO nanoparticles have higher thermal conductivity than the NSME25, which helps in faster heat transfer. The biodiesel blends exhibits improper combustion in the engine due to their viscous nature and lower calorific value. This leads to an increase in heat in the combustion chamber due to many unburnt gases during the process of the combustion phenomenon and temperature rise. Lower concentrations of graphene oxide nanoparticles showed higher EGT, while higher content of nanoparticles reduced the EGT.

The Effect of Fuel Additives on Cylinder Pressure (CP)
The burnt fuel fraction determines the peak cylinder pressure in a diesel engine during the initial combustion time (pre-mixed stage of burning). The CP is a function of the crank angle from the end of the compression stroke and in the early portion of the power stroke. The amount of fuel burnt during the uncontrolled combustion process is directly related to the engine cylinder pressure production. Figure 11 shows the variation in the crank angle of the cylinder pressure at full load. The addition of GO NPs and n-butanol to Nigella sativa biodiesel leads to production of higher oxygen and improves the cetane content, and the large volume-to-asymmetric-surface-area ratio result in an increase in the cylinder pressure. The NSME25 illustrated the lowest CP due to high viscosity; however, addition of 90 mg/L of GO NPs and 10% n-butanol increased the CP. A reasonable increase was observed for all the nanofuel blends when compared with NSME25.

The Effect of Fuel Additives on Cylinder Pressure (CP)
The burnt fuel fraction determines the peak cylinder pressure in a diesel engine during the initial combustion time (pre-mixed stage of burning). The CP is a function of the crank angle from the end of the compression stroke and in the early portion of the power stroke. The amount of fuel burnt during the uncontrolled combustion process is directly related to the engine cylinder pressure production. Figure 11 shows the variation in the crank angle of the cylinder pressure at full load. The addition of GO NPs and n-butanol to Nigella sativa biodiesel leads to production of higher oxygen and improves the cetane content, and the large volume-to-asymmetric-surface-area ratio result in an increase in the cylinder pressure. The NSME25 illustrated the lowest CP due to high viscosity; however, addition of 90 mg/L of GO NPs and 10% n-butanol increased the CP. A reasonable increase was observed for all the nanofuel blends when compared with NSME25.
of the compression stroke and in the early portion of the power stroke. The amount of fuel burnt during the uncontrolled combustion process is directly related to the engine cylinder pressure production. Figure 11 shows the variation in the crank angle of the cylinder pressure at full load. The addition of GO NPs and n-butanol to Nigella sativa biodiesel leads to production of higher oxygen and improves the cetane content, and the large volume-to-asymmetric-surface-area ratio result in an increase in the cylinder pressure. The NSME25 illustrated the lowest CP due to high viscosity; however, addition of 90 mg/L of GO NPs and 10% n-butanol increased the CP. A reasonable increase was observed for all the nanofuel blends when compared with NSME25. Figure 11. Variation of cylinder pressure and crank angle with maximum load for all test fuels.

The Effect of Fuel Additives on Ignition Delay (ID) Period
The ID occurs in the combustion chamber mainly due two attributes, the chemical and the physical delay. The chemical delay is due to pre-combustion reactions in the mixture of fuel and air. The physical delay is due to the vaporization, atomization and mixing of the fuel-air mixture [48]. Figure 12 shows a declining trend in the ignition delay period with an increase in brake power. The chamber combustion temperature rises with brake power, resulting in physical delay attributed to

The Effect of Fuel Additives on Ignition Delay (ID) Period
The ID occurs in the combustion chamber mainly due two attributes, the chemical and the physical delay. The chemical delay is due to pre-combustion reactions in the mixture of fuel and air. The physical delay is due to the vaporization, atomization and mixing of the fuel-air mixture [48]. Figure 12 shows a declining trend in the ignition delay period with an increase in brake power. The chamber combustion temperature rises with brake power, resulting in physical delay attributed to ignition delay. Additionally, the ignition delay is reduced, due to lower fuel viscosity, density and bulk modulus [51]. ignition delay. Additionally, the ignition delay is reduced, due to lower fuel viscosity, density and bulk modulus [51]. Nigella biodiesel (NSME25) illustrated poor fuel properties; hence, the delay is maximum, while 90 ppm of GO NPs with 10% n-butanol additive mixed with biodiesel has closer values to diesel due to better combustion characteristics. NSME25B10GO90 exhibited 8.11 °CA compared with NSME25 ID (10.2 °CA); the increase in the ID period for NMSE25 might be due to higher viscosity rate. The ignition delay reduces the brake power of all mixtures due to the higher engine temperature, which causes faster and symmetric combustion of fuel. [15]. Additionally, the reduction in the ID is due to enhanced A:F mixing, which results in an improved combustion due to secondary atomization and micro-explosion. The ID decreases with an increase in the load; a difference of 4.45 °CA is observed for diesel fuel from minimum to maximum loading conditions. Additionally, the ID period of diesel and NSME25B10GO90 fuel blends was comparable at all the load conditions. Nigella biodiesel (NSME25) illustrated poor fuel properties; hence, the delay is maximum, while 90 ppm of GO NPs with 10% n-butanol additive mixed with biodiesel has closer values to diesel due to better combustion characteristics. NSME25B10GO90 exhibited 8.11 • CA compared with NSME25 ID (10.2 • CA); the increase in the ID period for NMSE25 might be due to higher viscosity rate. The ignition delay reduces the brake power of all mixtures due to the higher engine temperature, which causes faster and symmetric combustion of fuel. [15]. Additionally, the reduction in the ID is due to enhanced A:F mixing, which results in an improved combustion due to secondary atomization and micro-explosion. The ID decreases with an increase in the load; a difference of 4.45 • CA is observed for diesel fuel from minimum to maximum loading conditions. Additionally, the ID period of diesel and NSME25B10GO90 fuel blends was comparable at all the load conditions. BTE is the BP (Brake Power) of an engine as an application of thermal energy input from the fuel; it is the measure of the conversation of heat energy from fuel to mechanical power. Figure 13 shows the variance of BTE with BP at constant load; the results showed that the CRDI diesel engine's BTE increased at all graphene oxide NP dosage levels. Compared with the neat diese-biodiesel fuel blends, the oxides of carbon-based nanoparticles facilitate complete combustion of the fuel charge. Graphene oxide serves as an oxygen buffer and thus increases the BTE [24]. As the dosage of graphene NPs in the fuel blends increases, the BTE increases. An increase of 18.37 % was observed in the BTE with the addition of 90 ppm of GO NPs and 10% n-butanol additives in Nigella sativa biodiesel at maximum load compared to the neat NSME25. However, the BTE decreases significantly at higher concentrations of asymmetric GO NPs (120 ppm), leading to an increase in viscosity and density of the fuel blend. The BTE values for the NSME25B10GO90 nanofuel blends were also comparable with diesel. The increase in BTE for all the nanofuel mixtures at specific loads is due to the enhanced symmetric micro-explosion phenomenon and higher catalytic activity of NSME25 GO NPs [24].

The Effect of Additives on Brake-Specific Fuel Consumption (BSFC)
BSFC is a diesel engine's efficiency to completely combust the fuel charge and generate shaft power. This is the ratio of the fuel consumed to the power generated. The ignition delay is highly dependent on the cetane number of the fuel blend, which determines the duration of the lag. Therefore, cetane number is a critical diesel fuel property which has a major impact on the CD. The atomization of fuel is also inversely proportional to the kinematic viscosity and density of [52]. Figure 14 shows the variation of BSFC and loads for fuel blends at different BP. NSME25 fuel blends' high viscosity and density decrease the fuel atomization process; thus, adding n-butanol to the fuel decreases density and viscosity and increases the calorific value and cetane number of the fuel blends by supplying more oxygen for complete combustion, resulting in a high degree of vaporization [53][54][55][56][57]. The high catalytic activity and high reactive surface area of the GO NPs also enhances the micro-explosion phenomenon [25]. Hence, the addition of additives to Nigella sativa biodiesel enhances the fuel combustion process, resulting in complete combustion, which reduces the fuel consumption. The BSFCs observed for the NSME25B10 GO (30, 60, 90 and 120 ppm) were 325, 310, 260 and 285 g/kWh, respectively, compared to 345 g/kWh for NSME25 and 248 g/kWh for D100 at maximum load.

The Effect of Additives on Brake-Specific Fuel Consumption (BSFC)
BSFC is a diesel engine's efficiency to completely combust the fuel charge and generate shaft power. This is the ratio of the fuel consumed to the power generated. The ignition delay is highly dependent on the cetane number of the fuel blend, which determines the duration of the lag. Therefore, cetane number is a critical diesel fuel property which has a major impact on the CD. The atomization of fuel is also inversely proportional to the kinematic viscosity and density of [52]. Figure 14 shows the variation of BSFC and loads for fuel blends at different BP. NSME25 fuel blends' high viscosity and density decrease the fuel atomization process; thus, adding n-butanol to the fuel decreases density and viscosity and increases the calorific value and cetane number of the fuel blends by supplying more oxygen for complete combustion, resulting in a high degree of vaporization [53][54][55][56][57]. The high catalytic activity and high reactive surface area of the GO NPs also enhances the micro-explosion phenomenon [25]. Hence, the addition of additives to Nigella sativa biodiesel enhances the fuel combustion process, resulting in complete combustion, which reduces the fuel consumption. The BSFCs observed for the NSME25B10 GO (30, 60, 90 and 120 ppm) were 325, 310, 260 and 285 g/kWh, respectively, compared to 345 g/kWh for NSME25 and 248 g/kWh for D100 at maximum load.
atomization of fuel is also inversely proportional to the kinematic viscosity and density of [52]. Figure 14 shows the variation of BSFC and loads for fuel blends at different BP. NSME25 fuel blends' high viscosity and density decrease the fuel atomization process; thus, adding n-butanol to the fuel decreases density and viscosity and increases the calorific value and cetane number of the fuel blends by supplying more oxygen for complete combustion, resulting in a high degree of vaporization [53][54][55][56][57]. The high catalytic activity and high reactive surface area of the GO NPs also enhances the micro-explosion phenomenon [25]. Hence, the addition of additives to Nigella sativa biodiesel enhances the fuel combustion process, resulting in complete combustion, which reduces the fuel consumption. The BSFCs observed for the NSME25B10 GO (30, 60, 90 and 120 ppm) were 325, 310, 260 and 285 g/kWh, respectively, compared to 345 g/kWh for NSME25 and 248 g/kWh for D100 at maximum load.

The Effect of Graphene Oxide Nanoparticle and n-Butanol in Nigella sativa Biodiesel on the Emissions of the CRDI Engine
The general equation for combustion of hydrocarbons is represented by Equation (3).

The Effect of Graphene Oxide Nanoparticle and n-Butanol in Nigella sativa Biodiesel on the Emissions of the CRDI Engine
The general equation for combustion of hydrocarbons is represented by Equation (3).
The carbon (C) and hydrogen (H) atoms react to form CO 2 and H 2 O; a lack of oxygen molecules during fuel combustion leads to incomplete combustion. The exhaust gas content after the combustion process is described in Figure 15. CxHy + (x + y/4)O2→xCO2 + y/2 H2O The carbon (C) and hydrogen (H) atoms react to form CO2 and H2O; a lack of oxygen molecules during fuel combustion leads to incomplete combustion. The exhaust gas content after the combustion process is described in Figure 15. The incomplete combustion of fuel charge in the combustion chamber, with partial and asymmetric oxidation, contributes to carbon monoxide emissions. CO is produced during instantaneous acceleration and the engine starting phase, where there is necessity of a rich A:F mixtures. Hence, hydrocarbons, due to a shortage of oxygen molecules, convert the carbon dioxide to carbon monoxide. Figure 16 illustrates the variation of CO emissions and BP for test fuels. The incomplete combustion of fuel charge in the combustion chamber, with partial and asymmetric oxidation, contributes to carbon monoxide emissions. CO is produced during instantaneous acceleration and the engine starting phase, where there is necessity of a rich A:F mixtures. Hence, hydrocarbons, due to a shortage of oxygen molecules, convert the carbon dioxide to carbon monoxide. Figure 16 illustrates the variation of CO emissions and BP for test fuels.

The Effect of Fuel Additives on Carbon Monoxide (CO) Emissions
The incomplete combustion of fuel charge in the combustion chamber, with partial and asymmetric oxidation, contributes to carbon monoxide emissions. CO is produced during instantaneous acceleration and the engine starting phase, where there is necessity of a rich A:F mixtures. Hence, hydrocarbons, due to a shortage of oxygen molecules, convert the carbon dioxide to carbon monoxide. Figure 16 illustrates the variation of CO emissions and BP for test fuels. The diesel fuel showed the lowest CO emissions, due to low viscosity, high calorific value and high heat release rate, which led to complete fuel combustion [23,59]. Meanwhile, NSME25 illustrated the highest CO emissions at all loads, owing to the rich A:F mixture, shortage of oxygen atoms in the combustion chamber, high density and viscosity and poor rate of combustion. The extra oxygen atoms in the combustion chamber are supplied through the addition of butanol, which acts as an oxygenated additive, and, for rapid combustion [57], the calorific value of the fuel is increased through addition of asymmetric graphene oxide nanoparticles. The graphene oxide and butanol improve the combustion characteristics of NSME25, by enhancing the oxidation, combustion rate, and calorific value and sustaining a desired combustion chamber temperature. The NSME25B10 + GO (90, 120, 60 and 30) reduced the CO emissions by 50.15%, 29.925%, 9.84% and 4.56% compared to The diesel fuel showed the lowest CO emissions, due to low viscosity, high calorific value and high heat release rate, which led to complete fuel combustion [23,59]. Meanwhile, NSME25 illustrated the highest CO emissions at all loads, owing to the rich A:F mixture, shortage of oxygen atoms in the combustion chamber, high density and viscosity and poor rate of combustion. The extra oxygen atoms in the combustion chamber are supplied through the addition of butanol, which acts as an oxygenated additive, and, for rapid combustion [57], the calorific value of the fuel is increased through addition of asymmetric graphene oxide nanoparticles. The graphene oxide and butanol improve the combustion characteristics of NSME25, by enhancing the oxidation, combustion rate, and calorific value and sustaining a desired combustion chamber temperature. The NSME25B10 + GO (90, 120, 60 and 30) reduced the CO emissions by 50.15%, 29.925%, 9.84% and 4.56% compared to NSME25. Additionally, the CO emissions for 90 ppm of GO in NSME25 were comparable to diesel fuel at all loads. Hence, the addition of GO NPs and n-butanol fulfils the deficient oxygen atoms, which convert the CO emissions to CO 2 , therefore precluding the toxic emissions [1,3,7,54].

The Effect of Fuel Additives on Hydrocarbon (HC) Emissions
The activation energy in the graphene oxide NPs in the fuel blends combusts the carbon particles within the combustion chamber at cylinder wall temperature, minimizing HC emissions for all nanofuel blends [51,60]. Figure 17 illustrates the variation of hydrocarbon emissions and brake power for test fuel blends.

The Effect of Fuel Additives on Hydrocarbon (HC) Emissions
The activation energy in the graphene oxide NPs in the fuel blends combusts the carbon particles within the combustion chamber at cylinder wall temperature, minimizing HC emissions for all nanofuel blends [51,60]. Figure 17 illustrates the variation of hydrocarbon emissions and brake power for test fuel blends. The mixture of Nigella sativa biodiesel, 10% butanol and different dosage levels of GO NPs illustrated an overall reduction in the HC emissions compared to Nigella sativa methyl ester (NSME25). The fuel blend NSME25B10GO90 showed the lowest HC emissions compared 30, 60 and 120 ppm of GO NPs. The HC emissions observed for 90 ppm at of GO NPs and 10% butanol at maximum load were 0.0058 g/kWh; the emission was also 0.0002 g/kWh less than neat diesel and 0.2742 g/kWh lower than NSME25; the reduction in HC emission for NSME25B10GO90 fuel blend was around 3.44% and 48.72% less than diesel and NSME25, respectively. This is due to high surface catalytic activity inside the combustion chamber, resulting in acceleration of the combustion of asymmetric fuel particles with air [3,48,60].

The Effect of Fuel Additives on Nitrogen Oxide (NOx) Emissions
The temperature of oxygen and its concentration in the combustion chamber are the major contributors to NOx formation [1,61,62]. The variation of NOx emissions and brake power for test fuel blends is shown in Figure 18. NOx emissions from NSME25 and NSME25B10 (GO-30, 60, 90 and 120 ppm) are higher than from diesel. The possibility of high NOx emissions is due to the presence of excess oxygen molecules, the oxygen buffering additive, 10% butanol and the various dosage levels of GO NPs, which supply more oxygen atoms. Other explanations are the probable increase in the peak cylinder pressure, which results in high NOx emissions. The nanofuel blend NSME25B10GO120 emits the highest NOx emissions due to high percentage of oxygen-donating GO NPs and the presence of butanol; the emissions for NSME25B10GO (120 and 90) are 885 ppm and 854 ppm, while diesel and NSME25 emitted 800 ppm and 815 ppm, respectively.

Effects of Fuel Additives on Carbon Dioxide (CO2) Emissions
Compared to the B20 fuel blend, the addition of graphene oxide nanoparticles to Nigella sativa biodiesel reduces CO2 emissions. At higher loading conditions (80% and 100% load), the CO2 emissions increased due to an increase in unburned HC. The complete combustion of hydrocarbons produces CO2 and H2O (vapor). Figure 19 shows the variation in CO2 emissions and the brake power of test fuel blends. NOx emissions from NSME25 and NSME25B10 (GO-30, 60, 90 and 120 ppm) are higher than from diesel. The possibility of high NOx emissions is due to the presence of excess oxygen molecules, the oxygen buffering additive, 10% butanol and the various dosage levels of GO NPs, which supply more oxygen atoms. Other explanations are the probable increase in the peak cylinder pressure, which results in high NOx emissions. The nanofuel blend NSME25B10GO120 emits the highest NOx emissions due to high percentage of oxygen-donating GO NPs and the presence of butanol; the emissions for NSME25B10GO (120 and 90) are 885 ppm and 854 ppm, while diesel and NSME25 emitted 800 ppm and 815 ppm, respectively.

Effects of Fuel Additives on Carbon Dioxide (CO 2 ) Emissions
Compared to the B20 fuel blend, the addition of graphene oxide nanoparticles to Nigella sativa biodiesel reduces CO 2 emissions. At higher loading conditions (80% and 100% load), the CO 2 emissions increased due to an increase in unburned HC. The complete combustion of hydrocarbons produces CO 2 and H 2 O (vapor). Figure 19 shows the variation in CO 2 emissions and the brake power of test fuel blends.
Compared to the B20 fuel blend, the addition of graphene oxide nanoparticles to Nigella sativa biodiesel reduces CO2 emissions. At higher loading conditions (80% and 100% load), the CO2 emissions increased due to an increase in unburned HC. The complete combustion of hydrocarbons produces CO2 and H2O (vapor). Figure 19 shows the variation in CO2 emissions and the brake power of test fuel blends. Adding 10% n-butanol to the blend of fuel increases the combustion process at lower engine and combustion temperatures [48,54]. At higher loads and combustion rates producing more CO2 emissions, the CO2 emissions for the NSME25B10GO90 fuel blend were comparable with the diesel fuel, indicating complete fuel combustion due to improved oxidation. At 100% load, the CO2 Adding 10% n-butanol to the blend of fuel increases the combustion process at lower engine and combustion temperatures [48,54]. At higher loads and combustion rates producing more CO 2 emissions, the CO 2 emissions for the NSME25B10GO90 fuel blend were comparable with the diesel fuel, indicating complete fuel combustion due to improved oxidation. At 100% load, the CO 2 emissions for NSME25B10GO90, NSME25B10GO120, NSME25B10GO60 and NSME25B10GO30 nanofuel blends were reduced compared to NSME25. The Nigella sativa biodiesel (NSME25) produced the highest CO 2 emissions due to the incomplete combustion of this fuel blend.

Effect of Fuel Additives on Smoke Emissions
Graphene oxide nanoparticles have a large asymmetric surface area of180 m 2 /g, and therefore act as an activated reaction site for air and fuel molecules [3]. Figure 20 shows variations in smoke emissions and brake power for fuel test blends. The higher thermal conductivity of graphene oxide NPs (5000 W/mK) allows the nanoparticles to transfer sufficient heat to individual molecules of the combustion product placed on the additive surface and thus enhances the oxidation of carbonaceous particles. Additionally, the inherent properties of Butanol (10%), such as higher oxygen content, lower viscosity and lower BP lead to enhanced combustion characteristics through improved atomization, reducing the smoke emissions [48].
Symmetry 2020, 12, x FOR PEER REVIEW 24 of 31 emissions for NSME25B10GO90, NSME25B10GO120, NSME25B10GO60 and NSME25B10GO30 nanofuel blends were reduced compared to NSME25. The Nigella sativa biodiesel (NSME25) produced the highest CO2 emissions due to the incomplete combustion of this fuel blend.

Effect of Fuel Additives on Smoke Emissions
Graphene oxide nanoparticles have a large asymmetric surface area of180 m 2 /g, and therefore act as an activated reaction site for air and fuel molecules [3]. Figure 20 shows variations in smoke emissions and brake power for fuel test blends. The higher thermal conductivity of graphene oxide NPs (5000 W/mK) allows the nanoparticles to transfer sufficient heat to individual molecules of the combustion product placed on the additive surface and thus enhances the oxidation of carbonaceous particles. Additionally, the inherent properties of Butanol (10%), such as higher oxygen content, lower viscosity and lower BP lead to enhanced combustion characteristics through improved atomization, reducing the smoke emissions [48]. At maximum loads, the smoke emissions increase due to a large quantity of smoke entering the combustion chamber. For maximum load, the smoke emissions for NSME25B10GO90 and NSME25B10GO120 nanofuel blends were 31.68% (53.85 HSU) and 22.54% (57.55 HSU) lower when compared with NSME25 (70.55 HSU); fuel and smoke emissions for NSME25B10GO90 were comparable to neat diesel fuel. The lower ID period of the nanofuel blends also resulted in a faster premixed combustion phase, which caused a reduction in smoke emissions. At maximum loads, the smoke emissions increase due to a large quantity of smoke entering the combustion chamber. For maximum load, the smoke emissions for NSME25B10GO90 and NSME25B10GO120 nanofuel blends were 31.68% (53.85 HSU) and 22.54% (57.55 HSU) lower when compared with NSME25 (70.55 HSU); fuel and smoke emissions for NSME25B10GO90 were comparable to neat diesel fuel. The lower ID period of the nanofuel blends also resulted in a faster premixed combustion phase, which caused a reduction in smoke emissions.

Response Surface Methodology of Nanofuel Blends
In Figure 21, the fuel blends (1 to 6) represent (1) pure diesel, (2) NSME, (3) NSME25B10GO30, (4) NSME25B10GO60, (5) NSME25B10GO90, and (6) NSME25B10GO120. The contour plot of BTE shows that, for BP = 0, BTE = 0, for all the blends. As the BP increases, the region of BTE slowly increases and the asymmetric region of maximum BTE is observed in the zone of BP 4 and 5. Figure 21a indicates that the BTE increases with the increase in BP, but the maximum zone is observed for neat diesel and blends with 120 ppm of GO. For pure Nigella oil, the BTE is lowest of all. In Figure 21d, the smoke emissions for all blends are depicted. The BP influences the increase in smoke emission from the engine. At BP < 3 the smoke emission is minimal. Only for pure Nigella oil at BP>5 is the zone of maximum emission observed. The presence of GO nanoparticles with a 120-ppm concentration leads the emissions to reduce. This zone starts with the addition of GO nanoparticles and ends at a minimal point at the right top corner. Tables 6 and 7 illustrate the regression equations in uncoded units and the model summary.  For pure Nigella oil, the BTE is lowest of all. In Figure 21d, the smoke emissions for all blends are depicted. The BP influences the increase in smoke emission from the engine. At BP < 3 the smoke emission is minimal. Only for pure Nigella oil at BP>5 is the zone of maximum emission observed. The presence of GO nanoparticles with a 120-ppm concentration leads the emissions to reduce. This zone starts with the addition of GO nanoparticles and ends at a minimal point at the right top corner. Tables 6 and 7 illustrate the regression equations in uncoded units and the model summary. In Figure 21c the CO emission is quite clear and indicates that the lowest CO emissions are for either diesel at the lowest BP or for Nigella oil with 90 and 120 ppm of GO nanoparticles. The maximum zone of CO emissions is at BP > 5, and for oil with lower concentrations (30 to 60 ppm) of NPs. The zone shown in yellow has covers a larger area and shows an acceptable amount of CO emissions at BP ranging from 0 to 4 kW. Hence, the CO emissions can be observed to be at the safest point for BP 0 to 4 kW and all fuel blends, while pure diesel and oil with 120 ppm at minimum load are best. It can also be said that, for high BP loads, the addition of less quantity of nanoparticles does not have any positive side effect; rather, the emission character of the diesel engine deteriorates. n-butanol also combusts very easily compared with biodiesel due to the high cetane number; thus, it is the most suitable oxygenated liquid fuel additive [59,63,64].

Conclusions
Experimental investigations were conducted on the CRDI engine; the toroidal combustion chamber and a six-hole fuel injector were used in this investigation. An oxygenated additive, n-butanol (10%), was added to reduce the viscosity and increase the cetane number of the biodiesel and graphene oxide nanoparticles; this increases the micro-explosion phenomenon and acts as a catalyst. In this section, the effects of Nigella sativa biodiesel (NSME25) blended with diesel, n-butanol, and asymmetric graphene oxide nanoparticles on modified CRDI engine are summarized:

1.
The NSME25, n-butanol and graphene oxide nanoparticle emulsion fuel illustrated an enhanced BTE value compared to the NSME (B25) fuel blend; the BTE of NSME25B10GO90 was nearly equivalent to diesel fuel. For NSME25B10GO90 fuel, the BTE increased by 18.37% compared to the NSME25. The enhancement in the BTE for all nanofuel blends is due to the catalytic effect of GO NPs.

2.
In addition, the BSFC values for all nanofuel blends with n-butanol decreased due to complete fuel combustion and an improved A: F mixing ratio; this is due to excess oxygen atoms in the combustion chamber and higher in-cylinder temperature.

3.
The toroidal combustion chamber demonstrated better A:F mixing due to an enhanced swirling motion and six-hole fuel injector at an injection pressure of 900 bar; this led to an instantaneous combustion, which improved the heat release rate and cylinder pressure.

4.
The NOx emissions for the NSME25, n-butanol and GO NPs fuel blend increased due to surplus oxygen molecules present in the combustion chamber and high combustion chamber temperature; lower NOx emissions were observed for NSME25 and diesel fuel. Meanwhile, the NSME25B10GO120 emitted maximum NOx emissions at all loads.

5.
The addition of GO NPs to the Nigella sativa reduced all the other noxious emissions (CO 2 , HC, smoke, and CO). At maximum load, NSME25B10GO90 fuel blend smoke, HC and CO emissions decreased by 31.68%, 48.571% and 50.15% compared with NSME25 fuel blend. 6.
The enhancement of the BTE increased the EGT for diesel and nanofuel blend, leading to rapid combustion of fuel blends, and lowered the ignition delay at maximum load shortly after the SOC period.
The experimental investigation recommends that the addition of high concentrations of graphene oxide nanoparticles, 90 and 120 ppm and 10% n-butanol in Nigella sativa biodiesel improves the overall CRDI engine characteristics, in symmetry with preceding research. Additionally, the toroidal combustion chamber and six-hole fuel injector nozzle with an orifice diameter of 0.1 mm increases the swirl motion and high-pressure impingement of fuel spray for a rapid combustion of fuel. Acknowledgments: We acknowledge the lab facilities, guidance, consistent encouragement, and support provided by Manzoore Elahi M Soudagar, R Harish Kumar, and N R Banapumath.

Conflicts of Interest:
The authors declare no conflict of interest.

Nomenclature NPs
Nanoparticles NSME25 with 10% n-butanol and 90 ppm GO NPs