Introduction

Petroleum-derived fuels have powered road transport for over a century; however, their finite reserves and increasingly stringent emission regulations have necessitated the development of cleaner and renewable energy sources1. Among these, biofuels, particularly biodiesel, offer a promising and sustainable alternative to conventional petroleum-based fuels. Biodiesel can be produced from agricultural feedstocks or waste oils, significantly reducing net greenhouse gas emissions through a closed-loop carbon dioxide (CO₂) cycle, and can be utilized in existing compression ignition (CI) engines without major modifications2,3. Biodiesel consists of mono-alkyl esters produced primarily via base-catalyzed transesterification of triglycerides (such as vegetable oils or animal fats) with short-chain alcohols like methanol. This process yields a fuel that meets ASTM D6751 or EN 14,214 standards and glycerol as a valuable by-product4. Owing to its inherent oxygen content, biodiesel promotes more complete combustion, leading to significant reductions in particulate matter (PM), carbon monoxide (CO), and unburned hydrocarbons (HC). Several studies have identified the oxygen mass fraction as a key factor contributing to PM reduction5,6.

Beyond emission reduction, biodiesel offers additional advantages: it is sulphur-free, non-toxic, has a high flash point (> 100 °C), and biodegrades approximately four times faster than petroleum diesel, thereby reducing environmental risks associated with fuel spills7. Furthermore, life-cycle assessments have shown that biodiesel can achieve 50–90% lower net CO₂ emissions depending on the feedstock and production process. It also enhances fuel lubricity, extending engine component life, and supports rural economies by creating agricultural market opportunities8,9. Among the various biodiesel feedstocks, cottonseed oil methyl ester (CSOME) is particularly attractive because of its diesel-like physicochemical properties and its non-competitive relationship with food resources10. However, similar to other biodiesels, CSOME exhibits around 10% lower energy content, higher viscosity, and inferior cold-flow characteristics, which can negatively affect combustion efficiency and fuel atomization11,12.

To overcome these limitations, the use of nano-additives such as aluminum oxide (Al₂O₃) and cerium oxide (CeO₂) has gained significant attention13,14. When dispersed in diesel or biodiesel blends at particle sizes below 100 nm, these nanoparticles provide large reactive surface areas, catalyze in-cylinder oxidation, enhance soot oxidation, release lattice oxygen (oxygen buffering), and reduce ignition delay15,16. Prior to blending, the nanoparticles were characterized to confirm their morphology, size, and chemical composition. Scanning Electron Microscopy (SEM) was employed to examine particle shape, X-ray Diffraction (XRD) to identify crystalline phases, Fourier Transform Infrared Spectroscopy (FTIR) to detect functional groups, and Transmission Electron Microscopy (TEM) to analyze size distribution17. An effective additive must remain stably suspended in the fuel, resist shear forces, prevent abrasive wear, and deliver measurable performance and emission benefits without producing harmful by-products18,19. Previous research has demonstrated that CSOME-based blends reduce smoke emissions and maintain torque levels comparable to diesel, despite a 3–5% reduction in torque due to lower heating values20,21. This study therefore aims to comprehensively evaluate the synergistic effects of a dual-nanoparticle additive formulation (50 ppm Al₂O₃ + 50 ppm CeO₂) in B20 cottonseed oil biodiesel–diesel blends on engine performance and emissions. The investigation focuses on key parameters, including brake power (BP), brake torque (Tb), brake-specific fuel consumption (BSFC), and the emissions of CO, CO₂, HC, and O₂ across the full engine operating range (1154–3840 rpm). To ensure accurate and reproducible results, optimized ultrasonication protocols were applied to ensure uniform nanoparticle dispersion and mixture homogeneity22,23,24.

Literature review

Biodiesel derived from non-edible feedstocks such as Jatropha, Karanja, and cottonseed has garnered considerable attention as a sustainable alternative fuel for compression ignition (CI) engines. These biodiesels generally reduce emissions of carbon monoxide (CO), hydrocarbons (HC), and smoke opacity by 10–60%, owing to their inherent oxygen content that promotes more complete combustion. However, they often induce a slight increase in nitrogen oxides (NOₓ) emissions, typically around 2–4%, attributed to elevated in-cylinder temperatures during combustion25,26. Among the non-edible feedstocks, cottonseed oil methyl ester (CSOME) stands out due to its diesel-like properties and the advantage of not competing with food resources, making it a desirable option27. Nevertheless, CSOME’s approximately 10% lower energy density and higher viscosity impair fuel atomization and combustion efficiency, leading to a 3–5% reduction in torque and brake thermal efficiency (BTE) compared to conventional diesel28. Although the emission benefits of cottonseed oil methyl ester (CSOME) are well-documented, optimizing its performance with nano-additives under varying engine loads remains an area requiring further research29.

Diesel engines remain a primary power source, but environmental concerns and petroleum depletion have driven research toward sustainable alternatives. Biodiesel–diesel blends are widely studied for their eco-friendly properties, with additives and predictive models enhancing combustion and reducing emission. Hydrogen-assisted blends have been shown to improve flame propagation and fuel oxidation, leading to lower emissions30. Similarly, machine learning predictions for nanoparticle-enhanced pongamia biodiesel demonstrated up to 9.8% higher brake thermal efficiency (BTE) and 18.7% lower brake-specific fuel consumption (BSFC) compared to standard B20 blends31. These studies highlight the potential of advanced additives and data-driven strategies to optimize engine performance and reduce environmental impact. Collectively, they provide a strong foundation for ongoing research into sustainable and efficient diesel fuel technologies. Regarding biodiesel production, base-catalysed transesterification continues to be the predominant method for CSOME synthesis, typically employing methanol with sodium or potassium hydroxide catalysts at molar ratios of 6:1 to 10:1 and reaction temperatures between 55 and 65 °C, yielding conversion efficiencies exceeding 95%32. Although acid pretreatment and supercritical transesterification techniques enable the use of feedstocks with high free fatty acid content, their industrial scalability is limited due to high cost and process complexity33. Recent research has focused on improving catalyst recovery and valorising waste oils to enhance sustainability33,34; however, integrating nano-additives during biodiesel production, especially for CSOME, remains an underexplored area.

Nano-additives have emerged as promising agents to improve emission profiles and engine performance synergistically. Cerium oxide (CeO₂), for example, functions as an oxygen buffer and catalyst, reducing CO and HC emissions by up to 80% and 60%, respectively, while also lowering NOₓ emissions by approximately 45% through catalytic oxidation processes35. Alumina (Al₂O₃) nanoparticles enhance fuel–air mixing via micro-explosive combustion effects, resulting in NOₓ reductions of around 35% when blended with CSOME fuels36. Specifically, CeO₂ addition to CSOME blends has been reported to decrease smoke emissions by 27% and reduce brake-specific fuel consumption (BSFC) by 9.3% at 2400 rpm37. Conversely, Al₂O₃ at 50 ppm concentration in B20 cottonseed biodiesel blends improved BTE by 8.5% but showed limited control over HC and CO emissions38. Hybrid nano-additive systems, such as CeO₂ combined with multi-walled carbon nanotubes (MWCNTs), have achieved soot reductions nearing 48%, even though with increased complexity and cost39. Although many studies have shown that nano-additives like CeO₂ and Al₂O₃ can improve biodiesel performance, there is very little research on how they work together in cottonseed biodiesel (CSOME). Studies using only one additive have shown drawbacks—for example, CeO₂ can increase NOx emissions, while Al₂O₃ may not reduce hydrocarbons (HC) or carbon monoxide (CO) effectively. Moreover, there has been limited investigation into the combined effects of Al₂O₃ and CeO₂ in CSOME blends at concentrations between 50 and 100 ppm, inconsistent nanoparticle dispersion techniques affecting reproducibility, and scarce data on the long-term impacts of nano-additives on engine wear and durability40. This study fills these gaps by evaluating a B20 CSOME blend containing individual nanoparticles (50 ppm Al₂O₃ or 50 ppm CeO₂) and a hybrid combination (50 ppm Al₂O₃ + 50 ppm CeO₂) to determine whether the hybrid additive can integrate the advantages of both nanoparticles to improve engine performance and lower emissions.

Materials and methods

The materials used in this study included a range of feedstocks, chemicals, nano-additives, and laboratory equipment essential for biodiesel production and engine performance testing. Cottonseed oil (Gossypium arboreum L.), belonging to the Malvaceae family, was sourced from Addis Mojo Oil Factory (Ethiopia) and selected as the primary feedstock owing to its high triglyceride content, wide availability as an agricultural byproduct, and favourable properties compared with other vegetable oils such as soybean and palm oil. Commercial petroleum diesel fuel was obtained from Oil Libya, Adama, Ethiopia, and used for blending with the cottonseed oil methyl ester (CSOME).

Biodiesel production process

For the transesterification process, analytical-grade methanol (99.8% purity) was used as the alcohol, sodium hydroxide (NaOH) was the base catalyst, and distilled water was used for washing and purification. Al₂O₃ and CeO₂ nanoparticles, obtained with 99.9% purity, were of nanoscale particle sizes 30–50 nm for Al₂O₃ and 20–30 nm for CeO₂, with favourable density and surface area characteristics. Their specifications are summarized in Table 1.

Table 1 Nanoparticle Specifications.
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The cottonseed oil methyl ester (CSOME) was produced through an alkali-catalyzed transesterification process. In this process, vegetable oils or animal fats react with alcohol (such as methanol) in the presence of a catalyst(NaOH) to produce Fatty Acid Methyl Esters (FAME), commonly known as biodiesel, with glycerin as a by-product, as illustrated in Fig. 1. The resulting FAME mixture is then purified to remove contaminants such as catalyst residues, methanol, and water.

$$RCOO\prime + R\prime \prime OH R\prime OH + RCOOR\prime \prime$$
Fig. 1
Fig. 1
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Transesterification reactions with methanol41,42.

RCOOR’ indicates an ester, R”OH an alcohol, R’OH another alcohol (glycerol),

RCOOR” is an ester mixture and acts as a catalyst.

The detailed production steps are described in the following sections. The procedure began with a pre-treatment step in which the cottonseed oil was titrated to determine its free fatty acid (FFA) content, measured at 1.72 mg KOH/g. This value confirmed the oil’s suitability for a single-step transesterification process without acid pre-treatment43,44. For the transesterification reaction, 500 mL of cottonseed oil was mixed with the methoxide solution, prepared by dissolving 5 g of sodium hydroxide (NaOH) in 100 mL of methanol. The mixture was stirred at 1000 rpm and maintained at 60 °C for 60 min to ensure complete reaction. After the reaction, the mixture was allowed to settle for 24 h to facilitate phase separation, during which glycerol formed at the bottom and was separated from the upper CSOME layer, as illustrated in Fig. 2.

Fig. 2
Fig. 2
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Transesterification process flow steps: (1) Oil preheating, (2) Methoxide preparation, (3) Reaction, (4) Glycerol separation, (5) Biodiesel washing and drying.

Purification of the reaction products

After separating glycerin from the reaction mixture, the produced fatty acid methyl esters (FAME) still contained impurities such as residual methanol, NaOH, and glycerin. These contaminants negatively affect biodiesel quality by increasing the cloud and pour points and lowering the flash point45,46. Therefore, a purification process was essential to meet the required standards, such as ASTM D6751 and EN 1421447. The biodiesel was purified by washing with warm distilled water at 60 °C to remove residual impurities. The cottonseed oil methyl ester (CSOME) was placed in a separating funnel, and half the volume of heated water was added to wash the ester. The mixture was allowed to settle, and the contaminated water was removed by opening the funnel’s valve. This process was repeated five times until the wash water appeared clear. The washing continued for 24 h while maintaining the temperature at 60 °C to ensure thorough removal of contaminants. A high amount of soap and impurities was observed during the first wash, settling at the bottom of the separator. Finally, the purified CSOME was dried by heating it to 130 °C to eliminate any remaining moisture content, as illustrated in Fig. 3.

Fig. 3
Fig. 3
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Cleaning process.

Preparation of diesel–biodiesel blends and nanoparticles

B20 fuel, consisting of 20% cottonseed oil methyl ester (COME) biodiesel and 80% petro-diesel, was used as the base blend for performance testing. To enhance fuel characteristics, aluminum oxide (Al₂O₃) and cerium oxide (CeO₂) nanoparticles were incorporated into the B20 blend at a concentration of 50 ppm each. The nanoparticles were dispersed using an ultrasonic cleaner operating at 120 W and 40 kHz for 40 min, with cetyl trimethyl ammonium bromide (CTAB) added as a surfactant to improve nanoparticle stability and dispersion. Three nano-fuel blends were prepared: B20A50 (B20 with 50 ppm Al₂O₃), B20C50 (B20 with 50 ppm CeO₂), and B20A50C50 (B20 with both 50 ppm Al₂O₃ and 50 ppm CeO₂). The blending process used standard laboratory equipment, including Beakers, an Ultrasonic cleaner, a Digital balance, a Hot plate with magnetic stirrer, Cerium oxide nano particle, Aluminum oxide nanoparticles, Cetyle trimethyl ammonium bromide (CTAB), petrodiesel, and prepared cottonseed oil biodiesel40, as shown in Fig. 4.

Fig. 4
Fig. 4
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The equipment and materials used for diesel–biodiesel-nanoparticle blending.

Fuel samples were prepared through a systematic procedure. First, the B20 blend was prepared by mixing 800 ml of diesel with 200 ml of biodiesel (20% by volume) in a 1-L bottle, followed by vigorous shaking for at least two minutes to ensure uniform blending. For nanoparticle dispersion, 1000 ml of the B20 blend was measured into a beaker. Specific nanoparticle-surfactant combinations were added: 0.05 g Al₂O₃ with 0.05 g CTAB for B20 + Al₂O₃ (50 ppm), 0.05 g CeO₂ with 0.05 g CTAB for B20 + CeO₂ (50 ppm), and a combination of 0.05 g Al₂O₃, 0.05 g CeO₂, and 0.05 g CTAB each for B20 + Al₂O₃ + CeO₂ (50 ppm each). Each mixture was stirred for at least 15 min and then poured into the remaining 900 ml of the respective B20 fuel bottle. To ensure thorough homogenization, all fuel blends (B20 and nanoparticle-modified B20) were processed in an ultrasonic cleaner at 40 kHz and 40 °C for 40 min, as shown in Fig. 5. Finally, the cleaning process switches off the ultrasonic cleaner using the “power on/off” key and disconnects the power supply. The final prepared fuel samples, showing the five main blends and their corresponding similar products, are shown in Fig. 6.

Fig. 5
Fig. 5
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Ultrasonic cleaning process.

Fig. 6
Fig. 6
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Prepared fuel samples for the test.

Determination of fuel properties

The physicochemical properties of B20 and its nanoparticle-enriched blends were evaluated following ASTM standards18, and the results are summarized in Table 2. All tested fuels complied with the required standards for diesel–biodiesel blends. Density values (0.8435–0.8447 g/ml) were consistent with diesel limits (0.82–0.86), ensuring reliable injection performance. The flash points of the blends (85–87 °C) were lower than neat biodiesel (185 °C) but well above the diesel standard (ASTM D975, ≥ 52 °C), confirming safe handling and storage [ASTM D975; ASTM D6751]48,49. The cloud point remained nearly constant at ≈2 °C, within the acceptable biodiesel range (− 3  to 12 °C). Importantly, the kinematic viscosity decreased with nanoparticle addition, from 4.428 cSt (B20) to 3.890 cSt (B20 + CeO₂), improving flow and atomization while staying within ASTM limits (1.9–6.0 cSt)50,51. The calorific value consistently increased with the addition of nanoparticles. While the absolute gains are relatively small, statistical analysis confirmed that the increases for the B20 + Al₂O₃ (p < 0.05), B20 + CeO₂ (p < 0.05), and B20 + Al₂O₃ + CeO₂ (p < 0.01) blends are statistically significant compared to the base B20 fuel, indicating that these improvements are genuine and exceed the measurement uncertainty. Overall, the ternary blend (B20 + Al₂O₃ + CeO₂) showed the most balanced improvement in fuel properties, making it the most suitable candidate for diesel engine applications52,53,54.

Table 2 Physicochemical properties of prepared fuels51.
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Experimental setup and procedure

Engine performance and emission characteristics were evaluated using the TBMC3-02 computer-controlled test bench, which houses a single-cylinder, four-stroke, air-cooled diesel engine manufactured by Edibon, as detailed in Table 3.

Table 3 Specification of TBM3-02 test bench for single-cylinder diesel engine55.
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The TBMC3-02 test bench, comprised six core components: (1) Diesel engine, (2) Asynchronous motor dynamometer for torque/power measurement and load application, (3) fuel tank, (4) SCADA control system with real-time data acquisition, (5) Computer, and (6) Exhaust gas analyzer for monitoring emissions, as shown in Fig. 7. The engine performance and emission tests were conducted under steady-state conditions. All measurements were taken at a constant full load (100% of maximum torque) while the engine speed was varied. The specific speeds investigated were 1154, 1250, 1500, 2000, 2400, 3000, 3600, and 3840 rpm. At each stabilized speed and load point, data for performance parameters and emissions were recorded.

Fig. 7
Fig. 7
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Experimental setup of TBM3-02 test bench for single-cylinder diesel engine.

The test bench was equipped with an integrated Supervisory Control and Data Acquisition (SCADA) system, EDIBON SCADA Software (ESS), Version 2.1 (https://www.edibon.com/en/computer-controlled-test-bench-for-2-2-kw-engines), which enabled real-time monitoring, control, and data collection. The SCADA-based control and monitoring system comprised five core components: (1) Main software operations, (2) Sensor displays, (3) Actuator controls, (4) Channel selection and other plot parameters, and (5) Real-time graphics display, as shown in Fig. 8.

Fig. 8
Fig. 8
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SCADA main screen.

Test fuels

The experiments were then conducted included five samples: pure petro-diesel (B0), a baseline biodiesel blend (B20: 20% cottonseed oil methyl ester (CSOME) + 80% diesel), and three nanoparticle-enhanced variants, B20 + 50 ppm Al₂O₃ (B20A50), B20 + 50 ppm CeO₂ (B20C50), and B20 + 50 ppm Al₂O₃ + 50 ppm CeO₂ (B20A50C50).

Preparation

Before testing, the lubricating oil level was checked, the fuel tank was filled with the respective sample, and electrical connections were established. The SCADA software was then initialized, and the engine was allowed to idle for 10 min to reach thermal stabilization. The full experimental setup is illustrated in Fig. 9.

Fig. 9
Fig. 9
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Diagrammatic experimental setup.

Performance testing

Performance testing was done by gradually applying incremental loads using the dynamometer while monitoring engine parameters through SCADA (Fig. 3.23). At each load step, brake power (BP), brake torque (Tb), mass fuel consumption (mf), and brake-specific fuel consumption (BSFC) were calculated using Equations:

Brake power (BP)

Is the engine’s usable output at the crankshaft, measured with a power absorption device. It is calculated using the equation:

$${\text{T}}_{{\text{b}}} = \frac{{{\text{WL}} \times 2{\pi N}}}{60}{ }\left( {\text{W}} \right)$$
(1)

where; Pb = Brake power in W, WL = torque in N·m, and N = Engine speed in rpm.

BrakeTorque(Tb)

Represents the usable torque available at the engine’s flywheel, was measured for each test fuel across various engine speeds using the TBMC-3 test bench.

$${\text{T}}_{{\text{b}}} = \frac{{60{\text{P}}_{{\text{b}}} }}{{2{\pi N}}}{ }\left( {{\text{N}}.{\text{m}}} \right)$$
(2)

where; Tb = engine brake torque in N.m, Pb = engine brake power in kW, and N = engine speed in rpm.

Mass fuel consumption (mf)

Was measured by recording the time taken to consume a known fuel volume. The mass of fuel was then calculated by multiplying the volume by fuel density, using the following equation:

$$m_{f } = v_{f} x _{f} g/s$$
(3)

where; vf = the volume flow rate of the fuel in ml/min, and \(_{{\text{f}}}\) = density of fuel in g/ml.

Brake-specific fuel consumption (BSFC)

Is the fuel consumed per hour per unit of brake power. It was determined at various operating conditions using the following equation:

$${\text{BSFC}} = \frac{{{\text{m}}_{{\text{f}}} * 3600}}{{{\text{P}}_{{\text{b}}} }}{\text{kg}}/{\text{kW}}.{\text{hr}}$$
(4)

Where, mf is the mass of fuel consumed in kg/s, and BP is the brake power in kW.

Emission testing

Once the engine achieved thermal stability, the same load sequence was applied while maintaining constant speed at each load level. The TBMC_AGE analyzer continuously measured the exhaust gas concentrations of CO, CO₂, HC, and O₂, and all data were logged through SCADA. This procedure was repeated for all fuel samples, B0, B20, B20 + 50 ppm Al₂O₃, B20 + 50 ppm CeO₂, and B20 + 50 ppm Al₂O₃ + 50 ppm CeO₂, under different loads and speeds to allow consistent and comparable evaluation of engine emissions.

Result and discussion

The study examined the effects of aluminum oxide (Al₂O₃) and cerium oxide (CeO₂) nanoparticles on the emission and performance characteristics of a compression ignition (CI) engine. The nanoparticles improve fuel atomization and evaporation, generating finer fuel droplets and better air–fuel mixing, which ensures uniform and stable combustion. Their catalytic activity accelerates oxidation reactions, leading to the complete conversion of carbon monoxide (CO) and unburned hydrocarbons (HC) into carbon dioxide (CO₂), while also increasing the heat release rate and reducing ignition delay, thereby improving thermal efficiency. Furthermore, CeO₂ acts as an oxygen carrier, storing and releasing oxygen during combustion to maintain an optimal oxygen balance in the cylinder, facilitating more complete fuel oxidation and minimizing harmful emissions. Test fuels included petroleum diesel (B0), B20 biodiesel derived from cottonseed oil, and nanoparticle-enhanced B20 blends. Emission analysis considered carbon monoxide (CO), carbon dioxide (CO₂), hydrocarbons (HC), and oxygen (O₂) concentrations, while performance evaluation focused on brake power (BP), brake torque (BT), and brake specific fuel consumption (BSFC).

Performance characteristics

Brake power

Brake power exhibited a clear increasing trend with engine speed for all tested fuel samples, attaining a peak value at 3600 rpm before experiencing a slight decline at higher speeds, as illustrated in Fig. 10. Among the fuels tested, the B20 biodiesel blend enhanced with a combination of 50 ppm Al₂O₃ and 50 ppm CeO₂ nanoparticles demonstrated the highest brake power, reaching 3.56 kW at 3600 rpm. This value represents a significant improvement compared to conventional diesel (B0), which delivered a maximum brake power of 3.01 kW, and the unmodified B20 blend, which achieved 2.89 kW at the same engine speed. The observed enhancement in brake power can be attributed to the improved combustion characteristics provided by the synergistic effect of the Al₂O₃ and CeO₂ nanoparticles, which promote better atomization, faster oxidation, and more complete fuel burning under high-speed operating conditions.

Fig. 10
Fig. 10
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Brake power at different engine speeds.

Brake torque

The engine brake torque exhibited a distinct variation with engine speed for all tested fuels, initially increasing as the speed rose, reaching a maximum at 2400 rpm, and subsequently decreasing at higher speeds, as illustrated in Fig. 11. The peak torque values were observed consistently at 2400 rpm across all fuel types, indicating the engine’s optimal torque-producing condition. Specifically, conventional diesel (B0) achieved a maximum torque of 8.5 Nm, while the unmodified B20 blend produced 8.1 Nm. The addition of nanoparticles enhanced the torque values, with B20 + 50 ppm Al₂O₃ reaching 8.2 Nm and B20 + 50 ppm CeO₂ achieving 8.4 Nm. The highest torque was recorded for the B20 blend containing a combination of 50 ppm Al₂O₃ and 50 ppm CeO₂, which reached 8.6 Nm at 2400 rpm. This improvement in torque can be attributed to the catalytic effect of the metal oxide nanoparticles, which promote more efficient combustion, improve fuel–air mixing, and enhance energy conversion within the cylinder, particularly at mid-range engine speeds.

Fig. 11
Fig. 11
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Engine speed VS brake power.

Brake-specific fuel consumption (BSFC)

Brake specific fuel consumption (BSFC) demonstrated a consistent decreasing trend with increasing engine speed for all tested fuels. This behavior is attributed to the engine generating higher power outputs at elevated speeds without a proportional increase in fuel consumption, reflecting improved energy utilization56. Among the fuels examined, the B20 biodiesel blend exhibited the highest BSFC values, primarily due to its lower calorific value and higher density compared to conventional diesel, which necessitated a greater fuel mass to achieve the same power output. The incorporation of metal oxide nanoparticles, specifically Al₂O₃ and CeO₂, enhanced the combustion characteristics of the B20 blend, leading to a notable reduction in BSFC across the tested speed range. The most significant improvement was observed for the B20 blend containing a combination of 50 ppm Al₂O₃ and 50 ppm CeO₂, which achieved the lowest BSFC of 0.258 kg/kW.h at 3840 rpm. This reduction indicates that the nanoparticles facilitated more complete and efficient fuel oxidation, improved atomization, and enhanced thermal energy conversion within the combustion chamber, thereby allowing the engine to deliver higher power output with lower fuel consumption, as illustrated in Fig. 12.

Fig. 12
Fig. 12
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The variation of brake-specific fuel consumption with engine speed.

Emission characteristics

CO emission

Carbon monoxide (CO) emissions were significantly reduced with the incorporation of nanoparticles into the B20 biodiesel blend. Among the tested fuels, the B20 + Al₂O₃ + CeO₂ blend exhibited the lowest CO emissions, measuring 0.075%, which corresponds to a 23.2% reduction relative to conventional diesel (B0). This substantial decrease can be attributed to the catalytic oxidation properties of Al₂O₃ and CeO₂ nanoparticles, which enhance the conversion of carbon monoxide (CO) to carbon dioxide (CO₂) during combustion, thereby promoting more complete fuel oxidation57. In terms of average emission reductions across the tested fuels, the unmodified B20 blend achieved a 6.7% decrease, B20 + Al₂O₃ reduced CO emissions by 11.2%, B20 + CeO₂ by 9.7%, and the combined B20 + Al₂O₃ + CeO₂ blend attained the highest reduction of 23.2%, as illustrated in Fig. 13. These results highlight the synergistic effect of the two nanoparticles, which not only improve combustion efficiency but also contribute to lower pollutant formation, demonstrating their effectiveness in mitigating CO emissions from biodiesel–diesel blends.

Fig. 13
Fig. 13
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Carbon monoxide (CO) emissions at different engine speeds.

CO2 emission

Carbon dioxide (CO₂) emissions also decreased with the addition of nanoparticles to the B20 biodiesel blend. The B20 + Al₂O₃ + CeO₂ combination exhibited the lowest recorded CO₂ concentration of 2.04% at 1154 rpm. In general, biodiesel blends produced lower CO₂ emissions than conventional diesel (B0), primarily due to their lower carbon-to-hydrogen ratio, which results in reduced carbon content per unit of fuel58. At 1554 rpm, the measured CO₂ emissions were 2.87% for B0, 2.64% for B20, 2.72% for B20 + Al₂O₃, 2.51% for B20 + CeO₂, and 2.51% for B20 + Al₂O₃ + CeO₂, as illustrated in Fig. 14. When averaged across the tested speed range, the reductions relative to B0 were 4.6% for B20, 8.1% for B20 + Al₂O₃, 8.8% for B20 + CeO₂, and 14.8% for the combined B20 + Al₂O₃ + CeO₂ blend. The observed decrease in CO₂ emissions can be attributed to the enhanced combustion efficiency provided by the nanoparticles, which promote more complete oxidation of the fuel and reduce the formation of excess carbon-containing exhaust gases. These results indicate that the synergistic effect of Al₂O₃ and CeO₂ nanoparticles not only improves engine performance but also contributes to mitigating greenhouse gas emissions from biodiesel–diesel blends.

Fig. 14
Fig. 14
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Variation of Carbon dioxide (CO2) emissions with engine speed.

Hydrocarbon (HC) emission

Hydrocarbon (HC) emissions were significantly reduced with the addition of nanoparticles to the B20 biodiesel blend. Among the fuels tested, the B20 + Al₂O₃ + CeO₂ combination exhibited the lowest HC emissions, measuring 15 ppm at 3840 rpm. Elevated HC emissions are typically associated with the presence of fuel-rich zones within the combustion chamber and incomplete combustion processes59. The experimental results demonstrated that all cottonseed biodiesel blends, including B20, B20 + Al₂O₃, B20 + CeO₂, and B20 + Al₂O₃ + CeO₂, produced lower HC emissions compared to conventional diesel (B0), as depicted in Fig. 15. The reduction in HC emissions can be attributed to the catalytic and thermal effects of Al₂O₃ and CeO₂ nanoparticles, which enhance fuel atomization, improve air–fuel mixing, accelerate flame propagation, and promote more complete combustion. Consequently, the use of these nanoparticles not only improves engine performance but also effectively mitigates unburned hydrocarbon emissions, highlighting their potential for cleaner and more efficient operation of biodiesel–diesel blends.

Fig. 15
Fig. 15
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Variation of hydrocarbon emissions with engine speed.

Oxygen (O2) emission

The oxygen (O₂) concentration in the exhaust gases was consistently higher for all biodiesel blends compared to conventional diesel (B0). This increase is primarily due to the inherent oxygen content of biodiesel fuels and is further enhanced by the catalytic effects of Al₂O₃ and CeO₂ nanoparticles, rather than being indicative of incomplete combustion. At 1554 rpm, the measured O₂ concentrations were 16.51% for B0, 17.63% for B20, 17.74% for B20 + 50 ppm Al₂O₃, 17.83% for B20 + 50 ppm CeO₂, and 17.97% for the B20 blend containing a combination of 50 ppm Al₂O₃ and 50 ppm CeO₂, as illustrated in Fig. 16. The elevated O₂ levels observed in the biodiesel and nanoparticle-enhanced blends reflect the additional oxygen supplied by the fuel molecules and the improved combustion environment created by the nanoparticles, which promote more complete and efficient oxidation of the fuel. Overall, these results indicate that the higher exhaust oxygen content is a beneficial effect of biodiesel and nanoparticle additives, supporting enhanced combustion processes rather than signaling any decrease in engine efficiency.

Fig. 16
Fig. 16
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Variation of oxygen (O2) with engine speed.

Comparison of the current study with recent work

This study was compared with previous literature based on the type of biodiesel used, engine operating conditions (speed and load), the nanoparticles or additives applied, and the resulting performance and emission outcomes. The comparison is summarized in Table 4.

Table 4 Critical summary of recent literature on nano-enhanced biodiesel.
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Conclusion

This study investigated the effects of cottonseed biodiesel (B20) and its blends with aluminum oxide (Al₂O₃) and cerium oxide (CeO₂) nanoparticles on the performance and emissions of a single-cylinder CI engine. The brake power of diesel (B0) was 3.02 kW, while B20 alone produced 2.89 kW. Adding nanoparticles improved performance: B20 + 50 ppm Al₂O₃ reached 3.21 kW, B20 + 50 ppm CeO₂ reached 3.35 kW, and the combined B20 + 50 ppm Al₂O₃ + 50 ppm CeO₂ achieved the highest brake power of 3.56 kW. For brake torque, B0 recorded 8.5 Nm, B20 produced 8.1 Nm, and the nanoparticle blends improved it to 8.2 Nm with Al₂O₃, 8.4 Nm with CeO₂, and 8.6 Nm with the combined blend. The brake specific fuel consumption (BSFC) was highest for B20 due to its lower energy content, while the combined nanoparticle blend had the lowest BSFC of 0.258 kg/kW.h, showing better fuel efficiency.

All biodiesel blends reduced emissions compared to diesel. The combined B20 + Al₂O₃ + CeO₂ blend achieved the largest reductions: 23.2% in CO, 14.8% in CO₂, and 18.5% in HC, while also increasing exhaust oxygen levels to 17.97%, indicating more complete combustion. These improvements are due to the multifunctional role of the nanoparticles: they act as catalysts to enhance fuel oxidation, improve fuel atomization and air–fuel mixing, accelerate flame propagation, and, in the case of CeO₂, supply extra oxygen during combustion. Overall, adding Al₂O₃ and CeO₂ nanoparticles significantly enhances the performance and emission characteristics of B20 biodiesel without any engine modifications, offering a cleaner and more efficient fuel option.

Future work

This study has two main limitations: it was conducted on a single-cylinder research engine, and NOx emissions were not measured, which may affect the assessment of emission trade-offs. Future research should address these limitations.

Scalability

The findings should be validated on multi-cylinder diesel engines under realistic operating conditions to evaluate scalability, performance, and durability in real-world applications.

NOx reduction

Further studies should focus on optimizing the type and dosage of nanoparticles and investigating their combined effect with techniques such as exhaust gas recirculation (EGR) to specifically reduce NOx emissions from biodiesel–diesel blends.