Sustainable MgO Nanocatalyst Additives for Boosting Performance and Mitigating Emissions of Used Cooking Oil Biodiesel–Diesel Blends in Compression Ignition Engines

Abstract

With conventional fuels dwindling and emissions rising, there is a necessity to develop and assess innovative substitute fuel for compression ignition (CI) engines. This study investigates the potential of magnesium oxide (MgO) nanoparticles as a sustainable additive to enhance the performance and reduce emissions of used cooking oil (UCO) biodiesel–diesel blends in CI engines. MgO nanoparticles were biosynthesized using Citrus aurantium peel extract, offering an environmentally friendly production method. A single-cylinder CI engine was used to test the performance of diesel fuel (B0), a 20% biodiesel blend (B20), and B20 blends with 30 ppm (B20M30) and 60 ppm (B20M60) MgO nanoparticles. Engine performance parameters (brake thermal efficiency (BTE), brake-specific fuel consumption (BSFC), and exhaust gas temperature (EGT)) and emission characteristics (CO, NO_x, unburnt hydrocarbons (HCs), and smoke opacity) were measured. The B20M60 blend showed a 2.38% reduction in BSFC and a 3.38% increase in BTE compared to B20, with significant reductions in unburnt HC, CO, and smoke opacity. However, NO_x emissions increased by 6.57%. The green synthesis method enhances sustainability, offering a promising pathway for cleaner and more efficient CI engine operation using UCO biodiesel, demonstrating the effectiveness of MgO nanoparticles.

1. Introduction

Fossil fuels continued to dominate the global energy mix, meeting approximately 80% of primary energy demand and contributing to global carbon dioxide emissions [1]. This dependency has far-reaching consequences, including adverse health impacts, environmental degradation, and geopolitical tensions arising from uneven distribution and control of fossil fuel reserves. To address these challenges, there is a pressing need to transition toward sustainable and renewable energy sources. Among numerous alternatives, biofuels appeared as promising solutions due to their renewable nature, capability to reduce greenhouse gas emissions, and compatibility with existing energy infrastructure [2]. Biomass–oil-based biodiesel has gained significant attention as a substitute for petroleum diesel in compression ignition (CI) engines. First-generation biodiesel is produced from edible sources (or oil), posing intense competition with the food industry and raising ethical and economic concerns over food security [3]. This challenge has driven a shift toward non-edible sources for second-generation biodiesel production. The third-generation biodiesel sourced from microalgae and waste cooking oil is under active investigation and holds promise for commercial scalability [4].

Biodiesel is produced via transesterification between oils or fats and alcohol under a catalytic environment. It received significant consideration due to its environmental and performance benefits. It contains 11% more oxygen by weight than conventional diesel, which allows for more complete combustion. This minimizes carbon monoxide and hydrocarbon (HC) emissions and soot particles, making it a cleaner fuel option. The higher cetane number of biodiesel improves ignition quality, resulting in a safer flash point (150 °C) compared to conventional diesel. These characteristics make biodiesel a promising environmentally friendly fuel [5]. However, biodiesel also presents challenges limiting its widespread adoption in fuel infrastructure. These include a lower heating value and reduced brake thermal efficiency, affecting engine performance. Biodiesel suffers from poor cold flow properties, leading to potential fuel clogging in colder climates. Additionally, biodiesel produces increased nitrogen oxide (NO_x) emissions, which can contribute to air pollution. Its shelf life is generally shorter due to oxidative instability. As a result, biodiesel has largely been limited to blending with diesel at ratios of up to 20%, reducing its overall performance potential [6,7]. Despite these drawbacks, biodiesel remains a carbon-neutral fuel, as the carbon dioxide (CO₂) emitted from combustion is counterbalanced by CO₂ absorbed by the feedstock during its growth. This makes biodiesel a more sustainable substitute for mineral diesel, helping to mitigate the impact of fossil fuel dependence and greenhouse gas emissions [8].

One promising source of biodiesel is used cooking oil (UCO), a waste product generated from domestic cooking activities. Used cooking oil is a cheap and readily available resource and finds application in industries such as plastics, pet food, soaps, cosmetics, lubricants, and cleaning products [9,10]. Recycling used cooking oil into biodiesel helps address multiple challenges, including waste management and pollution reduction, while also partially replacing petroleum-based fuels. To further promote this process, the FSSAI launched the Repurpose UCO program, aiming to collect and transform UCO into biodiesel, thereby supporting the country’s goal of achieving a 20% biodiesel blend by 2030 [11,12]. To augment the performance of biodiesel as an engine fuel, fuel additives are commonly used. The issues such as low BTE, increased fuel intake, and elevated exhaust emissions are often addressed. One of the most promising additives in this regard is nanoparticles. Due to their unique properties, including high surface-to-volume ratios, enhanced dispersion capabilities, improved air–fuel mixing, and inherent oxygen content, nanoparticles have been shown to improve combustion characteristics. Studies have demonstrated that incorporating nanoparticles in biodiesel can result in shorter ignition delays, higher combustion enthalpy, and faster heat release rates, leading to improved engine performance and reduced emissions [13,14,15]. However, the production of nanoparticles itself includes several challenges. The nanoparticle synthesis is a costly and time-consuming process that requires precise control over particle size, shape, and porosity. Poor regulation of these factors can lead to instability in the fuel and inefficient nanoparticle dispersion. Therefore, optimizing the synthesis process and ensuring consistent nanoparticle characteristics are essential for maximizing the benefits of nanoparticles in biodiesel [16,17].

In recent years, metal oxide nanoparticles have garnered significant attention as fuel additives to enhance combustion efficiency and reduce emissions in CI engines. Oxides of metals such as aluminum oxide (Al₂O₃) [18,19], cerium oxide (CeO₂) [20], titanium oxide (TiO₂) [21], ferric oxide (Fe₂O₃) [22], copper oxide (CuO) [23], and tungsten oxide (WO₃) [24] have been explored for their ability to improve engine performance when added to fuels. These nanoparticles provide additional oxygen in the combustion chamber, facilitating the oxidation of incomplete combustion byproducts, which in turn improves heat release rate, combustion efficiency, and overall engine performance. For example, cerium oxide (CeO₂) nanoparticles act as catalysts during combustion, promoting the oxidation of CO and HC while enhancing combustion efficiency. A study showed that CeO₂ nanoparticles contributed to a reduction in BSFC, NO_x emissions, and smoke opacity by 2.5%, 15.7%, and 34.7%, respectively [25]. Sarma et al. [26] show that incorporating TiO₂ nanoparticles in fuel caused a remarkable reduction in CO, HC, and NO_x emissions by 46.56%, 28.4%, and 2.3%, respectively. Additionally, the use of CuO₂ nanoparticles in biodiesel-fueled engines has shown improvements in combustion, enhanced performance characteristics, and a reduction in exhaust emissions [27,28,29]. These findings underline the potential of metal oxide nanoparticles in improving fuel efficiency and reducing harmful emissions in CI engines. Al₂O₃ nanoparticles have also been extensively studied as fuel additives, and their performance benefits have been attributed to their high surface-to-volume ratio as well as improved thermal conductivity. The nanoparticles act as chemical catalysts, accelerating the combustion process and improving the fuel burning rate. The addition of Al₂O₃ nanoparticles has been found to reduce CO and unburnt HC emissions while increasing the NO_x formation, thus suggesting that careful optimization of the nanoparticle concentration is crucial for maximizing the environmental benefits of this approach [30,31,32,33].

The benefits of metal oxide nanoparticles as fuel additives have been widely investigated; however, consumption of MgO nanoparticles in biodiesel–diesel blends is rarely performed [34], particularly with UCO biodiesel, which remains underexplored. MgO nanoparticles have recently gained popularity due to their unique properties, including high thermal conductivity, strong catalytic activity, and low toxicity. These properties make MgO nanoparticles an ideal candidate for enhancing fuel combustion in CI engines [18]. The mechanism by which MgO nanoparticles improve engine performance lies in their ability to act as combustion catalysts. MgO facilitates the breakdown of hydrocarbon chains during combustion, promoting complete oxidation and reducing HC emissions. Furthermore, MgO’s high oxygen storage capacity, which makes it store and release oxygen, not only enhances air–fuel mixing but also ensures efficient combustion, which results in inferior CO emissions and improved thermal efficiency [35,36]. In addition to enhancing combustion, MgO nanoparticles contribute to reducing smoke opacity by accelerating the oxidation of soot particles within the combustion chamber. This capability further improves the environmental performance of biodiesel-fueled engines [35]. Despite the promising potential of MgO nanoparticles, the synthesis method plays a pivotal role in determining their physicochemical properties, including size, morphology, and surface area, all of which directly affect their performance as fuel additives. Traditional chemical synthesis methods, although effective, can be costly and environmentally damaging. In contrast, green synthesis techniques, which utilize plant extracts, algae, or microorganisms, have emerged as sustainable and environmentally friendly routes. These techniques offer a way to produce nanoparticles with enhanced properties while minimizing environmental impact.

This study adopts a green synthesis approach to produce MgO nanoparticles using Citrus aurantium (bitter orange) peel extract as a reducing and stabilizing agent. Bioactive compounds in peel extract, such as flavonoids, alkaloids, and phenolic acids, not only reduce metal ions to nanoparticles but also provide a stable coating that prevents agglomeration and enhances dispersion within the biodiesel blend. This method remains cost-effective and aligns well with sustainability principles by reducing the reliance on conventional chemical methods and offering a more environmentally friendly alternative. The significance of this research lies in its novel approach to addressing the limitations of biodiesel, particularly the challenges associated with engine performance and emissions. By incorporating biosynthesized MgO nanoparticles as additives in biodiesel blends, this study demonstrates improvements in engine performance, including reduced BSFC, enhanced BTE, and reductions in detrimental emissions such as CO, unburnt HCs, and smoke opacity. These results highlight MgO nanoparticles’ capability to improve the practicality of biodiesel as an alternative fuel while overcoming some of its inherent drawbacks, as identified in recent studies. Furthermore, the use of green synthesis via Citrus aurantium peel extract offers a scalable, eco-friendly method for nanoparticle production, providing an added layer of sustainability to the entire process. The findings of this research not only contribute to the advancement of biodiesel technology but also revealed MgO nanoparticles as a viable solution for improving engine performance and reducing emissions in CI engines.

2. Results and Discussion

2.1. Experimental Setup and Measurement Technology

The schematic representation of the engine test rig along with the mass flow and data flow configurations are depicted in Figure 1. To ensure the reliability and accuracy of experimental results, uncertainty analysis was conducted, as presented in

Table 1. Uncertainty analysis of measured parameters in the experimentation.

Kirloskar Engine Test Rig		AVL Digas 444N and AVL 437C Smoke Meter
Measurement	Uncertainty	Measurement	Uncertainty
Engine load	$\pm 0.5\%$	CO (0–15% volume)	$\pm 0.1\%$
Engine power	$\pm 0.5\%$	HC (0–20,000 ppm)	$\pm 0.3\%$
Engine speed	$\pm 0.5\%$	NOx (0–6000 ppm)	$\pm 0.1\%$
BTE	$\pm 0.6\%$	Smoke opacity (0–100 in %)	$\pm 0.8\%$
BSFC	$\pm 1.0\%$
Total Uncertainty = square root of addition of squares of discrete uncertainties $\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{U}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{y}=\sqrt{({0.5}^2+{0.5}^2+{0.5}^2+{0.6}^2+{1.0}^2+{0.1}^2+{0.3}^2+{0.1}^2+{0.8}^2)}=\pm 1.6911\%$

, to account for contributions from both stochastic and fixed errors during calibration and observation. Such an analysis is crucial for validating experimental outcomes and minimizing data discrepancies. The study employs an engine load cell setup that integrates a 3.5 kW single-cylinder, four-stroke, water-cooled CI engine of Kirloskar make, coupled with a piezoelectric load cell for precise load measurement. The test rig is equipped with dual-fuel capability, enabling flexible testing scenarios, and incorporates EngineSoft software for automated data acquisition and real-time observation recording. All experimental trials were conducted under controlled conditions, maintaining 1500 rpm engine speed with variable engine loads to simulate real-world operational scenarios. Emission parameters, a critical focus of the study, were measured using advanced analytical instruments, including the AVL Digas 444N gas analyzer for gaseous emissions and the AVL 437C smoke meter for quantifying particulate emissions. This precise instrumentation ensures the accurate capture of emission characteristics, which are essential for assessing the environmental impact of fuel blends.

The fuel samples tested include pure diesel, a 20% biodiesel blend in diesel (B20), and two variants of the B20 blend infused with MgO nanoparticle 30 ppm (B20M30) and 60 ppm (B20M60) concentrations, respectively. The inclusion of MgO nanoparticles is hypothesized to improve combustion efficiency and diminish emissions because of their catalytic and thermophysical properties. The choice of a 20% biodiesel blend (B20) is supported by prior research, which identifies B20 as an optimal composition for achieving a balance between engine performance and emission control. For instance, studies have demonstrated that B20 offers a substantial decrease in particulate matter and hydrocarbon emissions compared to pure diesel, without substantial loss in engine efficiency [37].

The investigation systematically evaluates the performance metrics, including BTE, BSFC, and EGT, alongside emission parameters under various steady-state and loading conditions. By comparing these metrics across the fuel samples, the study provides insights into the effects of MgO nanoparticles on combustion dynamics, thermal efficiency, and emissions. The MgO nanoparticle-doped blends, particularly B20M60, are expected to exhibit superior thermal stability, improved atomization of fuel, and enhanced oxidation reactions, which collectively contribute to better combustion and reduced emissions.

2.2. Performance Parameters

The performance metrics in this study are discussed here, using the diesel fuel and UCO biodiesel blend B20 as a basis for comparison.

2.2.1. Brake Thermal Efficiency (BTE)

The BTE represents the efficiency of converting chemical energy of fuel into mechanical power at the engine shaft, serving as a critical indicator of engine performance. A higher BTE signifies more effective utilization of the fuel’s energy for mechanical work. Figure 2 shows variation in BTE with engine load for diesel, B20 biodiesel blends, and nanoparticle-dispersed biodiesel fuels. Diesel typically exhibits higher BTE because of its superior energy content and combustion characteristics. However, biodiesel blends, particularly those infused with nanoparticles, show remarkable potential for improving BTE. B20 biodiesel blends containing MgO nanoparticles demonstrate a significant enhancement in BTE, with the B20M60 blend achieving a peak BTE of 24.92% under maximum engine load. This development is primarily credited to the multifaceted role of nanoparticles in the combustion process [38]. MgO nanoparticles act as efficient combustion catalysts, promoting more complete oxidation of the fuel. Their high surface area facilitates enhanced interaction with the air–fuel mixture, accelerating chemical reactions during combustion. This catalytic effect reduces the presence of unburned hydrocarbons and other incomplete combustion byproducts, thereby increasing overall thermal efficiency.

Furthermore, MgO nanoparticles show higher surface activity and oxygen-carrying capacity, enriching the combustion chamber with additional oxygen [39,40]. This is particularly advantageous in fuel-rich conditions where oxygen availability may be limited. The supplemental oxygen aids in achieving more efficient combustion, resulting in higher energy release and improved thermal efficiency [40]. Additionally, the incorporation of nanoparticles modifies the physical properties of biodiesel, i.e., viscosity and surface tension, leading to better atomization of the fuel spray. Enhanced atomization produces finer fuel droplets and a more homogeneous air–fuel mixture, ensuring optimal combustion and boosting BTE. The concentration of nanoparticles in biodiesel blends directly enhances BTE because of their dual role as oxygen additives and combustion catalysts [41].

2.2.2. Brake-Specific Fuel Consumption (BSFC)

BSFC is a critical parameter in evaluating engine performance, defined as the ratio of fuel consumption rate to the brake power produced. It is conveyed in units of grams per kilowatt-hour (g/kWh) and serves as an indicator of an engine’s fuel efficiency. Several factors influence BSFC, including the specific gravity, viscosity, and calorific value of the fuel utilized. Figure 3 illustrates the relationship between BSFC, engine load, and nanoparticle concentrations. In this context, diesel fuel exhibits a lower BSFC than the B20 blend. This difference may be credited to diesel’s higher calorific value and lower density relative to the B20 blend. The calorific value, or quantity of energy released during combustion, directly impacts the energy output and efficiency of the engine. Higher calorific values result in more energy per unit of fuel, thereby reducing the BSFC.

The MgO nanoparticles in the fuel blend have been shown to reduce fuel consumption and enhance combustion efficiency. This development is primarily because of catalytic activity of metal oxide nanoparticles, which facilitate more complete combustion processes. The higher oxygen content in MgO nanoparticles promotes oxidation reactions, leading to more efficient conversion of fuel into energy [42]. This phenomenon aligns with established findings [43,44] regarding the role of metal oxide additives in combustion enhancement. Specifically, the B20M30 and B20M60 blends demonstrate a decrease in BSFC by 1.47% and 2.38%, respectively, at peak load compared to the B20 blend. This reduction indicates that the addition of MgO nanoparticles not only compensates for the lower calorific value of the B20 blend but also contributes to improved fuel efficiency. The enhanced combustion efficiency resulting from the catalytic action of MgO nanoparticles leads to a more effective utilization of the fuel’s energy content, thereby reducing the overall fuel consumption for a given power output.

2.2.3. Exhaust Gas Temperature

The EGT is a key parameter for assessing engine performance, particularly in terms of heat rejection and thermal efficiency. Higher EGT values often reflect increased heat loss to the surroundings, leading to lower thermal efficiency. Figure 4 illustrates the variation in EGT for pure diesel, biodiesel blends (B20), and nanoparticle-doped B20 blends across various engine loads. As depicted in Figure 4, the EGT rises proportionally with increasing engine load for all tested fuels due to greater fuel consumption and higher combustion temperatures under elevated loads. However, the inclusion of nanoparticles in B20 blends marginally mitigates EGT levels. The B20M30 blend (biodiesel blend with 30 ppm of nanoparticles) achieved a 0.71% reduction in EGT compared to B20, while the B20M60 blend (60 ppm of nanoparticles) demonstrated a 1.42% rise in EGT. These variations can be attributed to the nanoparticles’ catalytic properties, which enhance combustion efficiency.

Nanoparticles promote faster and more complete combustion by reducing ignition delay and improving the fuel–air mixing process. Their high thermal conductivity facilitates better heat transfer, minimizing localized hot spots and preventing excessive heat loss to the exhaust gases. Moreover, the improved combustion process ensures that more of the energy content of the fuel is converted into useful work, thereby reducing waste heat. Figure 4 also indicates that though pure diesel and B20 blends exhibit higher EGT values across all engine loads, the inclusion of nanoparticles in B20 blends consistently displays lower EGT values, underscoring the beneficial effects of nanoparticles in optimizing thermal efficiency [45].

2.3. Emission Parameters

The study meticulously evaluates brake-specific emission parameters, including unburnt HC, CO, NO_x, and smoke emissions from all test fuels. These parameters are crucial indicators of combustion efficiency, fuel quality, and environmental impact.

2.3.1. Brake-Specific Unburned Hydrocarbons (HCs)

The release of unburned HCs is a complex phenomenon affected by insufficient oxygen availability, heterogeneous air–fuel mixing, reduced in-cylinder temperatures, and shorter mixing times during combustion. These factors collectively result in incomplete combustion, leading to a loss of fuel, reduced thermal efficiency, and increased emissions [46]. The higher HC emissions in diesel engines are primarily due to the limited oxygen and inefficient atomization of the fuel during combustion. The B20 biodiesel blend demonstrates significantly lower unburnt HC emissions compared to diesel fuel. This decrease is credited to the biodiesel’s inherently higher cetane number, which makes faster ignition, and its intrinsic oxygen content, which facilitates more complete combustion. The addition of metal oxide nanoparticles improves combustion efficiency by enhancing atomization, accelerating flame propagation, and reducing ignition delay, resulting in complete combustion and lower HC emissions [47].

Experimental results reveal that the B20 blend, along with MgO-doped variants (B20M30 and B20M60), achieves substantial reductions in HC emissions (Figure 5a). Specifically, the B20 blend shows an 11.43% decrease in HC emissions at peak load compared to pure diesel. With the inclusion of MgO nanoparticles, the reductions are even more pronounced, with B20M30 and B20M60 blends showing decreases of 38.26% and 43.49%, respectively, under identical operating conditions. The catalytic activity of MgO nanoparticles facilitates enhanced oxidation of unburnt hydrocarbons, thereby further improving combustion efficiency [48].

2.3.2. Brake-Specific Carbon Monoxide (CO)

CO is formed when the air–fuel mixture lacks sufficient oxygen or combustion temperatures are suboptimal. Its emissions depend on factors like air–fuel ratio, engine conditions, fuel spray, and wall quenching. At low engine loads, lower cylinder pressures and temperatures increase CO emissions, while higher loads improve combustion, reducing emissions. Biodiesel’s higher oxygen content plays a pivotal role in mitigating CO emissions. The oxygen molecules within the biodiesel structure enhance CO₂ formation via CO oxidation in the combustion process [49,50]. Additionally, the addition of MgO nanoparticles in B20 blend further reduces CO emissions across all engine loading conditions. The catalytic properties of MgO nanoparticles contribute to improved fuel atomization, disruption of fuel homogeneity, and enhanced oxidation reactions during combustion, which collectively minimize CO formation [51].

Empirical data indicate that the B20M30 and B20M60 fuel blends achieve significant reductions in CO emissions, with reductions of 29.87% and 35.4%, respectively, compared to pure diesel (Figure 5b). These reductions are credited to the synergistic effects of biodiesel’s oxygen content and the catalytic action of MgO nanoparticles. The findings are consistent with Prabu et al. [52], who reported enhanced fuel evaporation and atomization due to nanoparticle doping, causing complete combustion and lower CO emissions. Furthermore, the nanoparticles ensure a higher CO oxidation by reducing the quenching effect observed at low-temperature regions of the combustion chamber such as near the cylinder walls.

2.4. Brake-Specific Oxides of Nitrogen

NO_x emissions are a significant concern for diesel engines due to the ideal conditions they create for NO_x formation: high cylinder temperatures and pressures. Biodiesel, with its high oxygen content, has been shown to increase NO_x formation in some cases [53]. Adding nanoparticles to biodiesel–diesel blends mitigated this effect. Nanoparticles serve as combustion catalysts, enhancing fuel oxidation and boosting the combustion process. They also improve the thermal conductivity of the fuel, helping to reduce cylinder temperatures. Lower temperatures result in fewer hot spots, which are areas in the combustion chamber where localized temperatures are high enough to encourage NO_x formation. The study in question demonstrated that biodiesel–diesel blends doped with nanoparticles produced lower NO_x emissions compared to pure diesel [54]. Specifically, B20M30 and B20M60 blends reduced NO_x emissions by 1.71% and 2.33%, respectively, compared to pure diesel (Figure 6a). The MgO nanoparticles, in particular, contribute to the reduction in NO_x emissions due to their oxygen storage properties. This feature enables a complete combustion process, minimizing the formation of hot spots. Additionally, MgO nanoparticles act as reduction catalysts, breaking down large hydrocarbons and increasing the formation of hydroxyl radicals in the combustion chamber, which further aid in reducing NO_x emissions.

Smoke Opacity

Smoke opacity in diesel engine exhaust is a measure of the light absorbed by the exhaust gases and is indicative of incomplete combustion, often caused by fuel-rich mixture zones in the combustion chamber. Such zones, where fuel is not fully combusted, contribute to higher smoke levels [55]. Smoke formation is a major concern for diesel engines, as it is not only a visible pollutant but also an indicator of inefficient combustion [56]. The study showed that biodiesel blends, including B20, B20M30, and B20M60, exhibited lower smoke opacity than pure diesel, particularly at peak load conditions (Figure 6b). This decrease is credited to the higher oxygen percentage in biodiesel, which helps promote complete combustion, thus reducing the formation of soot and particulate matter. Furthermore, adding MgO nanoparticles to the biodiesel blends further decreases smoke opacity. This is because MgO nanoparticles act as combustion catalysts, improving the fuel’s combustion efficiency. Additionally, the nanoparticles provide a better surface-to-volume ratio, which enhances their catalytic effects and reduces the formation of particulate matter. Specifically, the B20M30 and B20M60 blend reduced smoke opacity by 21.88% and 39.29%, respectively, compared to pure diesel. This highlights the potential of nanoparticle-doped biodiesel blends to reduce engine emissions, particularly in terms of smoke opacity [57].

Table 2. Summary of engine performability and emission characteristics for test fuels at peak load conditions.

Parameter	Pure Diesel	B20	B20M30	B20M60
BTE (%)	26.8	24.1	24.68	24.92
BSFC (kg/kWh)	0.3072	0.3289	0.3241	0.3212
EGT (°C)	386	419	398	392
HC (g/kWh)	0.156	0.140	0.113	0.108
CO (g/kWh)	6.41	5.0	4.87	4.66
NOx (g/kWh)	6.728	6.848	6.615	6.535
Smoke Intensity (%)	82	79	75	78

details the influence of doping MgO nanoparticles into a UCO biodiesel blend (B20) on engine performance and emissions under peak load conditions. The study findings highlight how the inclusion of nanoparticles can enhance the efficiency of the engine while simultaneously reducing harmful emissions. The B20M30 and B20M60 blends showed improvements in both NO_x emissions and smoke opacity, demonstrating the potential benefits of using nanoparticle-doped biodiesel blends in modern diesel engines for better environmental and operational performance.

The performance and emission parameters of diesel engines powered by diesel fuel were compared to MgO-doped B20 fuels (B20M30 and B20M60). It shows a closer engine performance and a significant reduction in emission levels when compared to other research outcomes. Most of the existing literature outcomes reduce exhaust emissions by marginal amounts with the optimal addition of nanoparticles in the biofuels. A similar trend also was observed in the current study, as illustrated in

Table 3. Comparison of present work experimental outcomes with the reviewed literature.

Engine	Biodiesel (Blend Ratio)	Additive	Results of the Study
			BTE	BSFC	HC	CO	NOx	Smoke	Reference
1-cylinder, DI, WC, CI engine	Used Cooking oil 20%	NA	NA	+3.2%	−50%	−21.4%	13.5%	−34.7%	[7]
1-cylinder, DI, WC, CI engine	Microalgae biodiesel, (10%)	NA	Lower	+3.2%	−5.37%	−3.1%	1.18%	−5.45%	[58]
1-cylinder, DI, WC, CI engine	Microalgae biodiesel, (10%)	NA	+3.2%	−12.41%	−8.8%	2.21%	−9.43%	39
1-cylinder, DI, WC, CI engine	Karanja biodiesel 45%	Di-Tetra-Butyl-Phenol, (DTBP) 10%	−0.2%	+0.3 kg/kWhr	−4.9%	−7.6%	+1.8%	−3.2%	[59]
1-cylinder, DI, WC, CI engine	Karanja biodiesel 45%	1-Pentadecanol 10%	+0.4%	+0.3 kg/kWhr	−3.3%	−4.9%	+3.1%	−4.9%
4-cylinder, DI, WC, CI engine	Biodiesel 20%	Butanol, 5%	−6%	Higher than diesel	Slight rise in HC	−50%	−2%	−15%	[60]
1-cylinder, DI, WC, CI engine	Tamarind oil methyl ester 20%	TiO2 50 ppm	+6.13%	−17.64%	−7.81%	−9.49%	−6.53%	NA	[61]
1-cylinder, DI, AC, CI engine	Mahua oil	Copper oxide, 100 ppm	+0.9%	−2.1 g/kWh	−5.6%	−4.9%	−3.9%	−2.8%	[62]
2-cylinder, DI, WC, CI engine	Mustard oil biodiesel	Aluminum oxide, 100 ppm	NA	NA	−2.2%	−4.3%	−4.8%	−3.4%	[63]
1-cylinder, DI, AC, CI engine	Neat palm stearin biodiesel	Silver oxide, 10 ppm	+2.4%	−2.7%	−8.8%	−11.9%	−14.4%	NA	[64]
1-cylinder, DI, AC, CI engine	Neem biodiesel	Carbon nano tubes, 100 ppm	NA	NA	−6.7%	−5.9%	−9.2%	−7.8%	[65]
1-cylinder, DI, WC, CI engine	Waste cooking oil biodiesel 20%	Magnesium oxide, 30 ppm	−7.01%	5.23%	−38.3%	−29.9%	+1.7%	−28.13%	Present work
1-cylinder, DI, WC, CI engine	Waste cooking oil biodiesel 20%	Magnesium oxide, 60 ppm	−7.84%	4.37%	−43.5%	−35.4%	+2.33%	−46.43%	Present work

.

3. Materials, Methods, and Characterization

The study utilizes high-purity reagents, including 98% pure magnesium nitrate (Mg(NO₃)₂), and Grade I deionized water, all manufactured by Loba Chemie Pvt. Ltd. Mumbai, India, and procured through Sudarshan Scientific laboratories, Nandgaon, Nashik, India, ensuring consistency and reliability in experimental procedures. The Citrus aurantium (sour orange) fruit peels, used as a key biological substrate for biosynthesis, ae sourced fresh from a local market to maintain their natural properties. The REMI make 1 LPH magnetic stirrer with hot plate is used for extraction and synthesis.

3.1. Preparation of Aqueous Extract from Citrus Aurantium Fruit Peel

Figure 7 outlines the detailed procedure for preparing an aqueous extract of Citrus aurantium fruit peels, which serves as a vital component in nanoparticle biosynthesis. Initially, the fruit peels were meticulously cut into small, uniform pieces and thoroughly washed twice with distilled water to eliminate surface impurities and residues. The cleaned peels were then air-dried to remove excess moisture. A total of 10 g of Citrus aurantium fruit peels were weighed and powdered using an electric blender. The powdered fruit peels were then heated in 100 cc of distilled water and swirled with a magnetic stirrer for around 30 min. This process led to the formation of a light brown-colored extract, indicative of the release of bioactive compounds such as flavonoids, phenolics, and other organic molecules crucial for nanoparticle formation. The resulting extract was carefully filtered (Whatman filter paper No. 1, Cytiva Whatman, Piscataway, NJ, USA) to remove any solid residues, ensuring a clear and uniform solution. Finally, the filtrate was stored at 4 °C to preserve its chemical integrity and bioactivity for subsequent experimental procedures. This environmentally friendly and straightforward approach highlights the potential of natural substrates in green synthesis methodologies [36].

3.2. Synthesis of MgO Particles

MgO nanoparticles were synthesized through a green approach using Citrus aurantium (bitter orange) fruit peel extract. Initially, 100 mL of the extract (a reducing and stabilizing agent due to its rich composition of flavonoids, phenolic acids, and alkaloids) was slowly added dropwise to 50 mL of a 0.1 M magnesium nitrate (Mg(NO₃)₂) solution. The mixture was stirred continuously at 60 °C for 12 h, as illustrated in Figure 8. This prolonged stirring facilitated the uniform interaction of the phytochemicals in the extract with the magnesium ions (Mg²⁺), leading to the formation of a stable nanoparticle precursor. The resulting dispersion was later centrifuged at 4000 rpm for 30 min, separating the precipitate, which was subsequently filtered, washed thoroughly with deionized (DI) water (to remove any unreacted ions or impurities), and dried in oven at 70 °C for 2 h. To achieve pure MgO nanoparticles, the dried precipitate was subjected to calcination in a muffle furnace at 400 °C for 3 h, during which the organic components decomposed, leaving behind highly crystalline MgO nanoparticles. The sample was allowed to cool overnight to room temperature before further characterization.

The formation of MgO nanoparticles can be attributed to the reduction of magnesium nitrate by the bioactive compounds present in the fruit peel extract. During the synthesis, the magnesium ions (Mg²⁺) in the solution interact with reducing agents in the extract, forming magnesium hydroxide (Mg(OH)₂) as an intermediate product. Upon calcination, Mg(OH)₂ undergoes thermal decomposition to yield MgO nanoparticles, as illustrated in the following chemical reactions:

Reduction and Precipitation Reaction:

Mg(NO₃)₂ + Bioactives → Mg(OH)₂ (precipitate) + Byproducts

(1)

Thermal Decomposition of Magnesium Hydroxide:

$$\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\stackrel{400 °\mathrm{C}}{\to }\mathrm{MgO}+{\mathrm{H}}_2\mathrm{O}$$

(2)

This eco-friendly synthesis approach not only minimizes the use of toxic chemicals but also leverages the renewable and abundant Citrus aurantium fruit peel waste, aligning with sustainable and green chemistry principles.

3.3. Characterization of MgO Particles

The synthesized MgO particles were thoroughly characterized using various techniques. JEOL JSM-7600F SEM (make: JEOL, Tokyo, Japan) and Tecnai G2 F30 TEM (make: FEI, Hillsboro, OR, USA) were employed to examine their surface morphology, revealing details about particle size, shape, and distribution. The optical absorption was examined using a Cary 60 UV–Vis spectrophotometer (make: Agilent Technologies, Santa Clara, CA, USA) to determine the bandgap energy and light absorption efficiency of MgO particles. FTIR, conducted with a Bruker Alpha-II spectrometer (make: Bruker, Madison, WI, USA), identified functional groups on the MgO surface, providing insights into its chemical composition. The Bruker D8 Advance (make: Bruker, Madison, WI, USA) was used to determine the crystalline phases, lattice constants, and crystallinity, confirming phase purity and structural properties. These analyses provided a comprehensive understanding of the MgO particles’ characteristics.

The crystal structure of MgO nanoparticles was thoroughly examined using a Bruker D8 X-ray diffraction (XRD) device (make: Bruker, Madison, WI, USA), which scanned the samples at a consistent rate of 2° per min. Figure 3 shows that the XRD analysis revealed distinct and notable diffraction peaks at 32.2°, 35°, 36.7°, and 48.18°, corresponding to the (220), (311), (111), and (012) crystal planes of MgO, respectively [66]. These peaks, characterized by their sharpness and intensity, signify a highly crystalline material with well-ordered structures. Such sharp and intense peaks are indicative of the successful synthesis and the high degree of crystallinity of the MgO nanoparticles. The XRD results, illustrated in Figure 9, depict a series of sharp peak patterns, which underscore that the green synthesis method utilizing Citrus aurantium fruit peel extract has successfully produced highly crystalline MgO nanoparticles. This suggests that the method not only yields high-quality nanomaterials but also promotes sustainability in nanomaterial production, as it harnesses natural, environmentally friendly sources for the synthesis.

The synthesized MgO nanoparticles were characterized in the range of 200 to 800 nm on a Cary 60 (Agilent Technologies, Santa Clara, CA, USA) single-cuvette, double-beam UV–Vis spectrometer. A distinct absorption peak was observed at 290 nm, as shown in Figure 10, which is a clear indication of the formation of MgO nanoparticles via green synthesis. This result aligns well with earlier research findings [67,68], confirming the successful synthesis of MgO nanoparticles. To further analyze the optical properties, the absorption data were converted into the bandgap energy using the Tauc plot method. The calculated bandgap was found to be approximately 4.3 eV, consistent with the known values for MgO nanoparticles [69,70]. This high bandgap value indicates that the material possesses excellent optical transparency and minimal absorption in the visible range, making it suitable for various applications, including enhancing biodiesel–diesel blend activity in CI engines. Optical properties of MgO nanoparticles contribute to their ability to act as photocatalysts under UV light, which can promote the breakdown of organic impurities in the biodiesel–diesel blend, ensuring cleaner combustion. Additionally, nanoparticles can diminish carbon deposit formation and other harmful emissions by facilitating more efficient fuel oxidation. These optical benefits not only improve the combustion efficiency of CI engines but also contribute to a reduced environmental impact by lowering the levels of particulate matter and greenhouse gas emissions. This underscores the capability of MgO nanoparticles as a sustainable and effective additive in biodiesel–diesel blends.

Figure 11 shows the FTIR spectrum of MgO nanoparticles over 400 to 4000 cm⁻¹. The distinct peaks observed around 3700 cm⁻¹, 3410 cm⁻¹, 1480 cm⁻¹, and 680 cm⁻¹ correspond to various functional group vibrations. The 3700 cm⁻¹ peak is attributed to O−H stretching vibrations, suggesting the presence of hydroxyl groups (−OH) over nanoparticle surfaces. Similarly, the wide peak at 3410 cm⁻¹ represents water and hydroxyl (−OH) groups bending vibrations, attached to the particle surfaces. These functional groups play a significant role in enhancing surface reactivity and nanoparticle distribution in liquid media. The 1480 cm⁻¹ peak agrees with water molecules bending vibrations, while the 679.8 cm⁻¹ peak is allocated to Mg−O−Mg bonds stretching vibration, confirming the structural identity of MgO nanoparticles [71]. The endothermic peaks were observed at approximately 1600 cm⁻¹, 1100 cm⁻¹, and 870 cm⁻¹. The 1600 cm⁻¹ peak resembles weak stretching vibrations of water molecules, likely retained during the green synthesis process. The 1100 cm⁻¹ peak is attributed to C−H stretching vibrations, while the one at 870 cm⁻¹ represents C−H bending vibrations. These peaks indicate the presence of organic compounds originating from the Citrus aurantium fruit peel extract used in the green synthesis method. Moreover, the Mg−O stretching vibrations observed in the range of 550–670 cm⁻¹ confirm the successful formation of MgO nanoparticles. These findings collectively validate the effective capsulation and stabilization of MgO nanoparticles through the bioactive compounds of Citrus aurantium fruit peel extract [72].

The identified functional groups in MgO nanoparticles offer several benefits when applied to biodiesel–diesel blends in CI engines. Hydroxyl (−OH) groups enhance the oxidative stability of biodiesel, minimizing the formation of polymers and deposits during combustion. This improved stability contributes to cleaner combustion, reducing the buildup of carbon deposits on engine components and extending engine life. These oxygen donors facilitate more complete combustion of the biodiesel–diesel blend, reducing emissions of unburned hydrocarbons (UHCs) and particulate matter (PM). Additionally, the MgO nanoparticles serve as a combustion catalyst, promoting the oxidation of fuel molecules and improving thermal efficiency. Therefore, the presence of these functional groups and the catalytic properties of MgO nanoparticles make them a promising additive for biodiesel–diesel blends in CI engines.

The SEM analysis provided detailed insights into shape, size, and nanoparticle distribution. Figure 12a illustrates the formation of MgO nanoparticles, which are observed as clusters of aggregated nanoparticles. These clusters exhibit an average particle diameter ranging from approximately 100 nm, with the individual particles displaying a distinct cuboid-like shape. This morphology aligns well with the expected size range and geometric characteristics of MgO nanoparticles, highlighting the successful synthesis of the material. Figure 12b presents a high-resolution SEM image of the MgO nanoparticles, further corroborating their structural uniformity. The higher magnification reveals particles with a diameter predominantly in the range of 80 to 120 nm, supporting the conclusion that the synthesis yielded nanoparticles with consistent and well-defined dimensions. This observation underscores the precision of the synthesis process, ensuring the reliability of the resulting nanomaterial for potential applications.

The morphological and structural characterizations of MgO nanoparticles were also performed using transmission electron microscopy (TEM)(make: JEOL, Tokyo, Japan). Figure 12c presents the TEM micrograph of MgO nanoparticles at a scale bar of 20 nm. The image reveals the uniform distribution of nearly cubic MgO nanoparticles with a median particle size of 80 to 100 nm. The observed particle size is in close promise with SEM-estimated size, confirming the reliability of synthesis method in producing MgO nanoparticles with uniform morphology. Figure 12d displays an HRTEM image of a single MgO nanoparticle at a scale bar of 2 nm. The well-resolved lattice fringes indicate the high crystallinity of the nanoparticles. The periodic arrangement of atoms confirms crystalline MgO particles, agreeing well with XRD analysis. The inset of Figure 12d represents the Fast Fourier Transform (FFT) pattern of the HRTEM image, demonstrating a well-defined diffraction pattern. The calculated interplanar spacing (d-spacing) from the FFT analysis is 0.45 nm, which corresponds to the characteristic lattice planes of cubic MgO. Figure 12e displays the EDX spectrum of the MgO nanoparticles. The elemental composition analysis confirms that magnesium (Mg) and oxygen (O) are the primary constituents, with no significant impurities detected. The quantitative elemental composition analysis reveals the weight percentages of Mg (46.4%) and O (53.6%), which closely match the expected stoichiometric ratio for MgO. This near-stoichiometric ratio provides strong evidence for the synthesis of highly pure MgO nanoparticles, free from significant impurities. The compositional purity and structural uniformity observed through SEM and EDX analyses affirm the successful fabrication of MgO nanoparticles, paving the way for their utilization in various scientific and industrial applications. Figure 12f illustrates the SAED pattern of MgO nanoparticles. The presence of sharp concentric rings with bright diffraction spots confirms the crystalline nature of MgO nanoparticles. The diffraction rings correspond to the characteristic crystallographic planes of cubic MgO, further validating the results obtained from XRD and HRTEM analyses.

4. Conclusions

The present work showed that MgO nanoparticles as a nanocatalyst-doped biodiesel blend (B20M60) are a sustainable alternative to diesel-fueled engines. The results demonstrate that B20M60 outperforms all other tested fuels, showing higher BTE under peak load conditions. Compared to pure diesel, the B20M60 blend significantly reduces unburnt HC emissions by 43.49%, CO emissions by 35.4%, and smoke opacity by 46.43%. Additionally, B20M60 does show a slight 4.14% decrease in NO_x emissions, which is otherwise a common outcome due to biodiesel’s higher oxygen content. The addition of MgO nanoparticles enhances combustion by promoting more complete oxidation and reducing soot formation. Despite a 1.4% rise in EGT, overall benefits (reduced emissions and improved fuel efficiency) of the B20M60 blend suggest that it is a viable fuel for CI engines and support sustainable energy practices.