Influence of CeO₂ nanoparticle addition on engine performance, combustion, and emissions of ethiopian podocarpus falcatus biodiesel

Abstract

The identification of region-specific, non-edible biodiesel feedstocks is critical for expanding sustainable fuel use without competing with food resources. This study reports the first comprehensive evaluation of engine performance, combustion behavior, and emissions of Ethiopian Podocarpus falcatus biodiesel enhanced with low-dose cerium oxide (CeO₂) nanoparticles in a compression ignition engine. Biodiesel–diesel blends containing 10–30% biodiesel was tested with and without 80 ppm CeO₂. Increasing biodiesel content alone reduced brake thermal efficiency from 54.7% for diesel to 49.2% for B30 and decreased brake power from 4.44 to 3.86 kW at full load. The addition of CeO₂ effectively mitigated these penalties, increasing brake thermal efficiency by up to 11.7% and restoring brake power by 3–10% across all blends, while brake specific fuel consumption decreased by 54–56% relative to untreated fuels under identical conditions. Combustion analysis showed higher peak cylinder pressure up to 71.2 bar, enhanced premixed heat-release rate reaching 130.2 J per °CA, advanced combustion phasing by 2–3 °CA, shortened ignition delay by about 2 °CA, and reduced combustion duration by 3–4 °CA. Emission measurements indicated substantial reductions in carbon monoxide by 7.5–16.7%, unburned hydrocarbons by up to 70.2%, and smoke opacity by 9–10%, accompanied by a moderate increase in NOx emissions of approximately 7% at full load due to higher combustion temperatures. Overall, the results demonstrate that low-dose CeO₂ nanoparticles can effectively offset biodiesel-induced performance losses while enhancing combustion and emission characteristics, highlighting Podocarpus falcatus as a promising and sustainable biodiesel feedstock for high-efficiency compression ignition engine operation without hardware modification.

Introduction

Compression ignition (CI) engines continue to play a central role in transportation, agricultural mechanization, and decentralized power generation because of their high thermal efficiency, durability, and reliability across diverse operating conditions¹. However, their extensive use contributes substantially to nitrogen oxides (NOₓ), carbon monoxide (CO), unburned hydrocarbons (HC), and particulate matter (PM), posing significant environmental and public-health concerns. These challenges are especially acute in developing regions where aging engine fleets remain predominant and advanced emission-control technologies are limited. As global emission standards tighten, fuel-based mitigation strategies that reduce pollutants without major modifications to existing compression-ignition engines have become increasingly important^1,2.

The nomenclature used in this study is summarized in Table 1.

Table 1.

Nomenclature and abbreviations for PF biodiesel–diesel blends with CeO₂.

Symbol	Description
B0	100% diesel (neat diesel)
B10	10% biodiesel + 90% diesel
B20	20% biodiesel + 80% diesel
B30	30% biodiesel + 70% diesel
B0 + 80 ppm CeO₂	100% diesel + 80 ppm cerium oxide nanoparticles
B10 + 80 ppm CeO₂	10% biodiesel + 90% diesel + 80 ppm cerium oxide nanoparticles
B20 + 80 ppm CeO₂	20% biodiesel + 80% diesel + 80 ppm cerium oxide nanoparticles
B30 + 80 ppm CeO₂	30% biodiesel + 70% diesel + 80 ppm cerium oxide nanoparticles
BSFC	Brake specific fuel consumption (kg/kWh)
BTE	Brake thermal efficiency (%)
BP	Brake power (kW)
CA	Crank angle (°)
CA50	Crank angle corresponding to 50% heat release (°CA)
CD	Combustion duration (°CA)
CeO₂	Cerium oxide nanoparticles
CI engine	Compression ignition engine
CO	Carbon monoxide (%)
HC	Hydrocarbons (ppm)
UHC	Unburned hydrocarbon emissions (ppm)
HRR	Heat release rate (J/°CA)
ID	Ignition delay (°CA)
NOx	Nitrogen oxides (ppm)
Pcyl	Cylinder pressure (bar)
CaO	Calcium oxide powder
PF	Podocarpus falcatus
PFME	Podocarpus falcatus Methyl ester

Biodiesel represents one of the most practical and technically mature alternatives to petroleum diesel. Owing to its inherent oxygen content, it typically promotes more complete combustion and reduces CO and HC emissions. Its biodegradability, low toxicity, and favorable safety characteristics further reinforce its environmental advantages³ .Nonetheless, the long-term scalability of biodiesel is constrained by feedstock costs, availability, and competition with food resources issues that have historically hindered edible-oil-based biodiesel programs⁴. Consequently, research attention has shifted toward non-edible, underutilized, and region-specific oil sources that can be cultivated on marginal land without affecting food security. Extensive studies have examined the performance and emission characteristics of compression-ignition engines fueled with biodiesel from various feedstocks⁵. These investigations consistently show that biodiesel properties viscosity, cetane number, oxygen content, and fatty-acid composition strongly influence combustion phasing, efficiency, and pollutant formation. Most biodiesel blends exhibit decreased CO, HC, and particulate emissions due to enhanced oxidation, while NOₓ trends often vary with molecular saturation and in-cylinder temperature behavior^6,7. Recent studies have shown that non-edible biodiesel blends significantly influence cylinder pressure, heat release rate, and ignition delay relative to diesel fuel⁸.

Parallel research has explored metallic and metal-oxide nanoparticles including aluminum oxide (Al₂O₃), cerium oxide (CeO₂), titanium dioxide (TiO₂), and copper oxide (CuO) as combustion enhancers to alleviate biodiesel-related performance limitations⁶. For instance, Tuan et al.⁹ investigated CeO₂ nanoparticles in biodiesel-diesel blends and reported that 30-50 ppm additions significantly improved engine performance while reducing CO, HC, and smoke emissions. Krishnaiah et al.¹⁰ found that 50 ppm CuO nanoparticles enhanced mahua biodiesel–diesel blends, improving thermal efficiency, reducing fuel consumption, and lowering CO, HC, and smoke emissions, with only a small increase in NOₓ. Zeleke et al.¹¹ showed that titanium dioxide and ethanol reduced fuel consumption and emissions in castor biodiesel blends. Dessie et al.¹² reported that combined Al₂O₃ and CeO₂ nanoparticles in diesel–cottonseed biodiesel reduced CO, CO₂, and HC while increasing exhaust oxygen, indicating more complete combustion. Similar studies by Kaushik et al.¹³ found that Al₂O₃ nanoparticles in diesel–neem biodiesel improved performance and reduced emissions.Mahendran et al.¹⁴, Tamrat et al.¹⁵, and Akram et al.¹⁶ demonstrated that CeO₂ nanoparticles enhanced brake thermal efficiency, lowered fuel consumption, and reduced emissions across different biodiesel feedstocks. Akansu et al.¹⁷ ,Gad et al.¹⁸ ,Anish et al.¹⁹, Santosh et al.²⁰, and Das et al.²¹ further confirmed that metallic and metal-oxide nanoparticles improve combustion, engine performance, and emission characteristics in various biodiesel blends.

Cerium oxide nanoparticles have long been investigated as combustion enhancers in biodiesel–diesel blends because of their catalytic oxidation behavior, oxygen storage and release capability, and high thermal stability. Across a wide range of studies, these properties are associated with improved oxidation kinetics, enhanced in-cylinder heat transfer, and suppression of carbon monoxide, unburned hydrocarbons, and soot. As a result, CeO₂ has become one of the most frequently examined metal oxide nano-additives in biodiesel research, particularly for improving combustion efficiency and moderating emission formation. Building on these fundamental attributes, experimental investigations and comprehensive reviews have shown that nano-additives can partially offset the inherent performance penalties of biodiesel–diesel blends. Reported outcomes typically include small gains in brake thermal efficiency, moderate reductions in brake specific fuel consumption, and incremental shifts in combustion phasing. Emission improvements are often evident for CO, HC, and smoke, although they are frequently accompanied by trade-offs such as increased NOₓ formation. Importantly, these benefits are highly sensitive to operating conditions, nanoparticle dosage, and fuel properties, indicating that the effectiveness of CeO₂ is not universal but context dependent^3,4,21.

As the literature has expanded, attention has turned to non-edible biodiesel feedstocks, where low-dose nano-additives have been shown to restore engine performance while maintaining acceptable emission balances when fuel stability is properly controlled. Nevertheless, most of these studies emphasize generalized performance and emission trends rather than detailed combustion-resolved behavior^20–22. Moreover, the overwhelming focus remains on conventional biodiesel sources, leaving limited understanding of how feedstock-specific molecular composition and physicochemical characteristics influence nano-additive interactions during combustion.

Within this context, Podocarpus falcatus biodiesel represents a distinct and largely unexplored fuel. Derived from a high-oil, non-edible species widely distributed in the Ethiopian highlands, it possesses a unique chemical profile that may interact differently with CeO₂ nanoparticles. This gap in knowledge is particularly relevant given Ethiopia’s dependence on imported diesel and the availability of indigenous oil-bearing plants that grow on non-arable land while contributing to soil stabilization and watershed protection. Despite its potential for region-specific and sustainable biofuel development, the combustion behavior of Podocarpus falcatus biodiesel, especially under CeO₂-assisted conditions, has not been reported.

Against this background, the present study is positioned to address both feedstock-level and performance-level gaps by providing the first engine-based evaluation of CeO₂-enhanced Podocarpus falcatus biodiesel. Using a single-cylinder compression ignition engine, the work systematically examines the influence of 80 ppm CeO₂ nanoparticles on engine performance, combustion characteristics including cylinder pressure, heat release rate, ignition delay, and combustion phasing, and regulated emissions. Through this chronological progression from established nano-additive principles to unexplored feedstock application, the study offers a mechanistic and regionally relevant assessment of CeO₂ effectiveness in biodiesel-fueled engines.

Finally, it is important to clarify the distinct roles of CeO₂ considered in this framework. In biodiesel production, CeO₂ participates as a heterogeneous catalyst that facilitates transesterification through surface redox activity. In contrast, when introduced into the fuel at low concentration, it functions as a combustion additive, promoting oxidation reactions, enhancing oxygen transfer, and supporting micro-explosion phenomena. These mechanisms are independent and operate at different stages, underscoring the need to evaluate CeO₂ specifically under engine combustion conditions, as undertaken in this study.

Materials and methods

Feedstock collection and oil extraction

Podocarpus falcatus seeds were collected from the Sidama Region, Ethiopia. The seeds were manually cleaned to remove adhering dust and foreign materials and subsequently air-dried at ambient conditions for one week to reduce the moisture content to approximately 8 wt%. To achieve uniform dehydration while avoiding thermal degradation of oil constituents, the seeds were further oven-dried at 60 °C for 24 h prior to oil extraction. Oil was extracted from Podocarpus seeds using a hydraulic press. Using whole seeds, including the husk, the oil yield was 0.42 v/w (42%), corresponding to 420 mL/kg or approximately 17.6 L from 42 kg of dry seeds. When only the dehulled kernels were used, the yield increased to 0.51 v/w (51%), corresponding to 510 mL/kg or approximately 21.4 L from the same seed mass. The increase in yield demonstrates that husk removal significantly improves oil recovery, as the husk contributes minimal oil. These values are consistent with reported literature ranges for Podocarpus kernel extraction, with maximum yields approaching 58% under optimal conditions. Reporting both whole-seed and kernel-only yields provides a comprehensive assessment of oil extraction efficiency and practical applicability for hydraulic pressing methods^23,24.

Chemicals and catalysts used

Calcium oxide (CaO, purity 99.9%) and cerium oxide (CeO₂) nanoparticles with an average particle size of 30–50 nm were procured from Ultra Nanotech, India. Analytical-grade methanol (≥99.8%) was obtained from a local supplier and used without further purification. Anhydrous sodium sulfate (Na₂SO₄) was employed for final biodiesel drying, while Whatman No. 541 filter paper was used for product filtration as shown in Table 2.

Table2.

Physicochemical properties of CeO₂ nanoparticles.

S.No	Property	Value
1	Chemical name	Cerium (IV) oxide (CeO₂)
2	Chemical composition	Cerium ≈ 81.4%; Oxygen ≈ 18.6%
3	Molecular weight	172.11 g/mol
4	Purity	99.9%
5	Particle size	30–50 nm
6	Colour	Pale yellow (as observed)
7	Thermal conductivity	~12 W·m⁻1·K⁻1 (bulk)
8	Density	7.22 g·cm⁻3
9	Melting point	2400–2600 °C
10	Boiling point	~3500 °C
11	Crystal structure	Cubic fluorite (Fm3̅m)
12	Specific surface area	20–60 m2·g⁻1

CeO₂ was employed in two distinct roles in this study. First, as part of the heterogeneous CaO-CeO₂ catalyst, it facilitated the transesterification of Podocarpus falcatus oil, improving biodiesel yield and catalyst stability. Second, at a low concentration of 80 ppm, CeO₂ was added directly to biodiesel–diesel blends as a combustion additive, enhancing oxidation, reducing brake-specific fuel consumption, and improving overall engine performance. These two functions are separate: the first affects biodiesel production, while the second improves fuel combustion characteristics^6,16.

Catalyst characterization

The structural and morphological characteristics of the synthesized catalysts, including pure CaO, pure CeO₂, and CaO-CeO₂ composites with 70/30 and 80/20 wt% ratios, were investigated using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD measurements were performed in a 2θ configuration using Cu Kα radiation (λ = 0.15406 nm) in continuous scan mode, with a counting time of 0.4 s per step, a step size of 0.02°, and a scanning speed of 3° min⁻¹ over a 2θ range of 10°–80°. The average crystallite size (D) was calculated from the most intense diffraction peaks using the Debye-Scherrer equation²⁵:

where K is the shape factor (0.9) λ, is the X-ray wavelength, β, is the full width at half maximum (FWHM) of the diffraction peak in radians, and θ is the Bragg angle. SEM analysis was performed using a VEGA3 TESCAN microscope at an accelerating voltage of 20 kV, a working distance of 27.8 mm, and a view field of 4.15 mm, with representative micrographs captured at 100 µm magnification to evaluate particle morphology, dispersion, and surface uniformity across all samples.

Preparation of biodiesel and blends

Biodiesel was synthesized from Podocarpus falcatus seed oil via transesterification using a heterogeneous CaO-CeO₂ composite catalyst. Catalysts with compositions of 20 wt% CeO₂-80 wt% CaO and 30 wt% CeO₂-70 wt% CaO were employed during biodiesel production. The reaction was carried out in a 1 L batch reactor, in which preheated oil was reacted with methanol at a molar ratio of 12:1 in the presence of 2 wt% catalyst. The process was conducted at a controlled temperature of 65 °C for 1.52 h under continuous mechanical stirring at 600 rpm.After completion, the reaction mixture was allowed to settle, yielding biodiesel and glycerol phases. The biodiesel layer was subsequently purified by warm distilled-water washing, dried over anhydrous Na₂SO₄, filtered (Whatman No. 541), and stored at ambient temperature for further characterization. From the purified biodiesel, fuel blends containing 10%, 20%, and 30% biodiesel (B10, B20, and B30) were prepared by volumetrically mixing the required biodiesel fraction with commercial diesel until homogeneity was achieved. The overall production cycle of biodiesel from podocaprus seed oil shown below Fig. 1.

Fig. 1.

Overall biodiesel production process from seed collection to biodiesel synthesis.

To improve physicochemical and combustion characteristics, CeO₂ nanoparticles were incorporated into each blend at a dosage of 80 ppm. The nanoparticles were accurately weighed and dispersed using ultrasonication (110 W, 35 kHz, 30 min), a method widely recognized for deagglomerating particle clusters and restoring nanoscale dispersion. The resulting nano-enhanced blends (B10+80 ppm, B20+80 ppm, and B30+80 ppm) exhibited good stability, confirmed through periodic visual inspection of sealed samples stored under controlled conditions, with no observable sedimentation or clustering throughout the test period.

The dispersion stability of CeO₂-treated fuel blends was initially evaluated through visual observation under static storage conditions, where no visible sedimentation or phase separation was detected over the test duration. Although no quantitative stability metric was employed in the present work, previous studies have shown that low-dose CeO₂ nanoparticles with ultrafine particle size and appropriate mixing protocols exhibit adequate short-term stability in hydrocarbon and biodiesel fuels. Based on these literature findings and the absence of visible settling during the experimental period, the prepared nano-fuel blends were considered sufficiently stable for engine testing.

Experimental setup and engine testing procedure

The experimental investigations were conducted on a single-cylinder, four-stroke, water-cooled, CT110 direct-injection compression ignition engine coupled to an eddy-current dynamometer. The engine was operated at a constant speed of 2500 rpm, while the load was varied from 20% to full load to evaluate performance, combustion, and emission characteristics under different operating conditions. The detailed specifications of the engine and measuring instruments are provided in Table 3. Fuel consumption was measured using a calibrated burette and electronic timer, while brake torque and brake power were obtained directly from the dynamometer system. Intake air flow was monitored using a standard air box fitted with an orifice meter²¹. Cooling water flow rates and exhaust gas temperatures were continuously monitored to ensure stable operating conditions throughout the experiments. In-cylinder pressure data were acquired using a piezoelectric pressure transducer mounted flush with the cylinder head and synchronized with a crank angle encoder having a resolution of 1°CA. The pressure signals were recorded over 100 consecutive engine cycles and ensemble-averaged to minimize cyclic variability. Apparent heat release rate, ignition delay, combustion duration, and CA50 were calculated from the averaged pressure–crank angle data using a first-law-based combustion analysis²². Exhaust emissions, including carbon monoxide (CO), unburned hydrocarbons (HC), nitrogen oxides (NOₓ), and carbon dioxide (CO₂), were measured using a calibrated Kane Auto plus five exhaust gas analyzer, while smoke opacity was determined using an AVL smoke meter. Prior to each test, the analyzers were zeroed and spanned according to the manufacturer’s specifications to ensure measurement accuracy. All tests were initially performed using neat diesel fuel to establish baseline data. Subsequently, biodiesel blends (B10, B20, and B30), both with and without 80 ppm CeO₂ nanoparticle addition, were evaluated under identical operating conditions. The CeO₂ concentration of 80 ppm was selected based on previous experimental evidence identifying the 75-100 ppm range as optimal for diesel-biodiesel compression ignition engine. Lower concentrations (<50 ppm) generally yield insufficient catalytic activity, whereas higher concentrations may cause nanoparticle agglomeration, compromised fuel atomization, and potential NOₓ escalation due to excess oxygen availability. Thus, 80 ppm represents a balanced and scientifically justified dosage for achieving performance gains without inducing undesirable emission penalties^13,26.Before each fuel change, the fuel line was flushed, and the engine was allowed to stabilize to ensure repeatability and eliminate fuel cross-contamination. Each experimental point was repeated three times, and the average values were reported to ensure data reliability. All reported percentage improvements and reductions are calculated relative to the corresponding baseline fuel at identical engine speed and load conditions to ensure consistent comparison. the schematic layout and the actual experimental setup are shown in Fig. 2 and 3, respectively.

Table 3.

Test engine specifications.

Item	Specification
Manufacturer	Kubota (Osaka, Japan)
Model	EA300-E2-NB1
Engine type	four-stroke, single-cylinder, direct-injection (DI), compression ignition engine
Cooling type	Water-cooled
Fuel	Diesel
Crank radius to connecting-rod length ratio	0.25
Displacement	309 cm3
Bore	75 mm
Stroke	70 mm
Rated output	5.1 kW at 3000 rpm
Lubricating oil capacity	1.3 L
Noise level	95 dB(A)
Compression ratio	23:1
Injection timing	23° crank angle (CA) before TDC

Fig. 2.

Schematic layout of the experimental setup.

Fig. 3.

Actual single-cylinder compression ignition engine (GUNT CT110 Test Stand).

Engine performance, combustion, and emission measurements

Brake power (BP), brake thermal efficiency (BTE), and brake-specific fuel consumption (BSFC) were computed using standard engine performance relations^27,28:

where T is torque (N·m), N engine speed (rpm), is the fuel mass flow rate, and CV the lower value

In-cylinder pressure was measured using a piezoelectric pressure transducer synchronized with a high-resolution crank-angle encoder (1° CA). The pressure–crank angle data were used to derive the apparent heat release rate (HRR), ignition delay, and combustion duration. The apparent HRR was calculated from the first law of thermodynamics^2,29:

A constant specific heat ratio (γ = 1.35) was assumed for heat-release rate calculations, following common practice in diesel engine combustion analysis. Although γ varies with temperature and gas composition, the selected value represents a reasonable average for diesel and oxygenated fuel combustion under typical engine operating conditions. Previous combustion studies have demonstrated that using a constant γ introduces minimal relative error when comparative analysis is the primary objective¹⁵.

The instantaneous cylinder volume was obtained from the geometric relation:

Where, V_c is clearance volume, R crank radius, R_c is compression ratio, and γ is specific heat-ratio, fixed at 1.35 during the combustion phase. Each cycle comprised 720 data points (1° CA resolution). Pressure derivatives were computed using finite-difference methods. A Savitzky-Golay filter (11° CA window, third-order polynomial) was applied to reduce noise while preserving peak fidelity. HRR data represent the mean of 50 consecutive cycles, with uncertainties expressed as ±1 standard deviation. Top dead center (TDC) was referenced at 360° CA.

Gaseous emissions were quantified using a Kane Auto Plus 5 Portable Gas Analyzer, calibrated prior to each test according to the manufacturer’s protocol. The analyzer measures CO, CO₂, HC, and NOₓ with high-resolution electrochemical and NDIR sensors, ensuring accurate capture of transient exhaust chemistry. Smoke opacity was recorded using a standard optical-type smoke meter. All emission signals were logged through the integrated data-acquisition system for synchronized comparison with combustion and performance parameters^20,22.

Uncertainty analysis

Experimental measurements are inherently subject to uncertainties arising from instrument precision, calibration limitations, environmental conditions, and data acquisition procedures. To quantify the reliability of the measured and derived parameters, an uncertainty analysis was carried out following standard experimental practice. The overall uncertainty of derived parameters was calculated using the root-sum-square (RSS) method³⁰:

Here, U_R denotes the overall uncertainty of the calculated parameter, while Uxi refers to the uncertainty associated with each individual measured variable contributing to it. The uncertainty analysis was based on the accuracy specifications provided by the manufacturers of the measuring instruments used in this study. Since the same instruments and measurement procedures were employed at all engine operating conditions, the estimated uncertainty limits are applicable across the entire load range. The accuracy specifications of the main instruments used are summarized in Table 4 below.

Table 4.

Measuring instruments, ranges, and reported uncertainties for engine performance and exhaust emission parameters.

Measured parameter	Instrument / sensor	Measuring range	Reported uncertainty (Accuracy)
Carbon monoxide (CO)	Gas analyzer (Kane auto plus)	0–10 % vol	±5% of reading or ±0.06 % vol
Carbon dioxide (CO₂)	Gas analyzer (Kane auto plus)	0–16 % vol	±5% of reading or ±0.5 % vol
Hydrocarbons (HC, ppm propane eq.)	Gas analyzer (Kane auto plus)	0–20,000 ppm	±5% of reading or ±12 ppm
Nitric oxide / total NOₓ	Gas analyzer (Kane auto plus)	0–5000 ppm	±5% of reading or ±25 ppm
Oxygen (O₂)	Gas analyzer (Kane auto plus)	0–21 % vol	±5% of reading or ±0.1 % vol
Exhaust gas temperature	Thermocouple / EGT probe	Up to 1000 °C	±1% of reading
Smoke opacity	Compatible smoke meter	Full instrument range	Typically, ±1% of reading
Cylinder pressure	Pressure transducer (Piezoelectric)	0–250 bar	±0.79% of full scale
Engine speed	CT110 tachometer	0–3000 rpm	±0.5% of reading (typical for CT110)
Brake torque	CT110 torque Sensor	0–15 N·m	±0.5% of full scale (typical for CT110)

Result and discussion

Fuel properties of Podocarpus falcatus biodiesel blends

The physicochemical properties of Podocarpus falcatus biodiesel and its B10–B30 blends are summarized in Table 5. These properties reveal monotonic and physically consistent trends, demonstrating the potential of Podocarpus blends as sustainable diesel alternatives.

Table 5.

Physicochemical properties of Podocarpus falcatus biodiesel blends.

Properties	ASTM Standard	Diesel (B0)	B10	B20	B30	B100
Lower heating value (MJ kg⁻1)	ASTM D240	43.00	42.91	42.74	42.57	41.30
Density at 20 °C (kg m⁻3)	ASTM D4052	830	846.3	831.63	832.44	834.07
Kinematic viscosity at 40 °C (mm2 s⁻1)	ASTM D445	2.73	4.43	2.9	2.98	3.15
Cetane number	ASTM D613	48	50.89	48.29	48.43	48.72
Flash point (°C)	ASTM D93	58	175	69.7	75.55	87.25
Pour point (°C)	ASTM D97	-33	-28.2	-24.7	-22.7	-9
Cloud point (°C)	ASTM D2500	-2.5	-3	-3.4	-3.9	-7
Acid value (mg KOH/g)	ASTM D664	0.12	0.15	0.18	0.21	0.41

Minor non-monotonic variations in density and kinematic viscosity observed at intermediate blend ratios are primarily associated with blending interactions and experimental uncertainty rather than true compositional inconsistency. At low to medium biodiesel fractions, the combined effects of biodiesel proportion, trace moisture, and measurement resolution can produce small departures from strictly linear trends. Similar behavior has been widely reported for biodiesel–diesel blends and is generally regarded as acceptable within standard uncertainty limits for fuel property measurements.

As the biodiesel content increases, density, viscosity, and cetane number show an overall rising trend, which enhances ignition quality and supports stable fuel injection. Neat Podocarpus falcatus biodiesel exhibits a lower heating value of 41.2 MJ kg⁻¹, maintaining energy content close to that of petroleum diesel. A substantial increase in flash point from 58 °C to 248 °C indicates improved handling and storage safety, while the shift in pour point from −33 °C to −9 °C reflects reduced but still acceptable cold-flow performance. Taken together, these property trends indicate that B10–B30 Podocarpus falcatus blends provide a favorable balance between efficiency, safety, and engine compatibility, supporting their potential as practical drop-in alternatives to conventional diesel fuel^31,32.

FTIR analysis of PF Oil and PFME

Fig. 4 shows the FTIR spectra of PF Oil and PFME in the range 4000–500 cm⁻¹. Both samples exhibit strong bands at 3000–2800 cm⁻¹, attributed to aliphatic C–H stretching, confirming the preservation of long hydrocarbon chains essential for fuel energy content. A prominent absorption near 1743 cm⁻¹ in PF Oil corresponds to carbonyl (C=O) vibrations, which becomes sharper and more intense in PFME, indicating successful esterification. The appearance of a distinct C–O stretching band at ~1172 cm⁻¹ in PFME further confirms methyl ester formation³³.Bending vibrations of –CH₂/–CH₃ groups are observed at 1457–1442 cm⁻¹, while characteristic –CF₂ bending bands at ~717–725 cm⁻¹ appear in both spectra, demonstrating the structural stability of fluorinated groups after conversion. Overall, the FTIR results confirm effective transformation of PF Oil into PFME while retaining hydrocarbon and fluorinated frameworks, supporting its suitability for advanced fuel and energy applications due to improved chemical stability and combustion-related properties^31,33.

Fig. 4.

FTIR spectra of PF oil and PFME.

Heterogenous catalyst characterization

The X-ray diffraction analysis of CaO-CeO₂ composites reveals well-defined patterns with high crystallinity for both compositions. Sharper and more intense reflections are obtained from the 80 wt% sample in comparison with the 70 wt% composition, evidently showing enhanced crystallinity as a function of increasing CaO content^9,34.

Dominant peaks which can be indexed to cubic fluorite CeO₂ ((111), (200), (220), and (311) planes at about 28.6°, 33.1°, 47.5°, and 56.3° respectively) aobserved. meanwhile, other reflections demonstrate the existence of a secondary phase assigned to crystalline CaO powder. The pure CeO₂ reference sample presents a crystallite size of about 31.4 nm, consistent with previous works^11,21.Notably, CaO was applied as a commercial bulk powder rather than a nanosized material.The well-defined peaks across all patterns indicate that the incorporation of CaO into the CeO₂ lattice does not disrupt the fluorite structure, and the composites retain nanoscale features crucial for catalytic activity.These structural characteristics are directly linked to catalytic performance as shown Fig. 5^35,36. The more intense, sharper reflections seen for the 80 wt% CaO-CeO₂ catalyst imply a greater density of well-ordered domains, which in turn provides more basic sites available for interaction with reactants. In contrast, Slightly broader peaks are observed for the 70 wt% composition consistent with reduced surface exposure and hence catalytic efficiency.Basicity is improved by the presence of dispersed CaO along with CeO₂ and isolated O^2- anions. More crystallite size-less surface area in the 80 wt% sample promotes better molecular contact and interaction strength with the reactants interacting. Hence, this explains more catalytic performance for the 80 wt% CaO-CeO² composite, highlighting the critical role of composition and structural order in optimizing heterogeneous catalyst function^18,37,38.The SEM micrographs of the CaO-CeO₂ composites and their individual components provide a clear visualization of surface morphology and particle dispersion at a microscale level as shown Fig. 6. The 70 wt% CaO-CeO₂ composite (Fig. 6a) exhibits a relatively heterogeneous surface, with areas of particle agglomeration interspersed with more loosely packed domains. In contrast, the 80 wt% CaO-CeO₂ composite (Fig. 6b) displays a noticeably more uniform texture, with finer and more evenly distributed particles, suggesting that a higher CaO content promotes better dispersion and reduced local clustering. This structural uniformity likely contributes to enhanced exposure of active sites and improved catalytic accessibility, correlating with the superior performance observed for the 80 wt% CaO-CeO₂ composition in transesterification reactions²¹.

Fig. 5.

XRD patterns of fresh and heterogenous CaO-CeO₂ catalysts.

Fig. 6.

SEM image of CaO, CeO₂, and CaO-CeO₂ composite catalysts with 70 wt% and 80 wt% CaO loading at magnifications (100 µm) (a) 70 wt% CaO-CeO₂ (100 µm), (b) 80 wt% CaO-CeO₂ (100 µm), (c) CaO(100 µm) (d) CeO₂ (100 µm).

The pure components further contextualize these observations. The CaO powder (Fig. 6c) shows a coarse and irregular morphology, consistent with its bulk powder form, highlighting its tendency toward aggregation and limited surface area in isolation. CeO₂ (Fig.6d), on the other hand, demonstrates a more granular and evenly distributed microstructure, indicative of intrinsic nanoscale features and a relatively high surface area. When combined in the composites, these contrasting morphologies appear to synergistically optimize surface texture: the CeO₂ provides a stable dispersive matrix while CaO contributes to the density of basic sites. Overall, the SEM analysis underscores how compositional tuning influences particle distribution, surface uniformity, and ultimately the catalytic efficiency of CaO-CeO₂ heterogeneous catalysts¹⁴.

Engine performance parameters

The engine performance was assessed for all fuel blends, both with and without 80 ppm CeO₂ nanoparticles, at a constant speed of 2500 rpm across a range of loads from 20% to full engine load. This allowed for a comparative analysis of the influence of nanoparticle addition on key performance parameters under varying operating conditions.

Brake therml efficency(BTE)

The brake thermal efficiency (BTE) of biodiesel blends across engine loads is presented for pure blends (Fig. 7a) and CeO₂-added blends (Fig.7b). For the pure blends, efficiency increases with load, with B0 achieving the highest BTE (54.7% at full load) and B30 the lowest (49.2%), reflecting the effect of increasing biodiesel content. The addition of 80 ppm CeO₂ nanoparticles consistently enhances BTE, most notably for B0 + 80 ppm CeO₂ (61.1%), followed by B10 + 80 ppm CeO₂ (59.3%), B20 + 80 ppm CeO₂ (55.7%), and B30 + 80 ppm CeO₂ (52.3%), indicating improved combustion due to catalytic activity and faster oxidation. Gains are modest at low loads but become substantial at higher loads, demonstrating the synergistic effect of CeO₂ and engine stress. The plotting order for CeO₂-added blends clearly distinguishes each blend from its base fuel, facilitating direct visual comparison with the pure blends^15,19.

Fig. 7.

Effect of biodiesel blending and 80 ppm CeO₂ nanoparticle addition on performance.

Brake power (BP)

Brake power (BP) increased steadily with engine load for all fuels, with neat diesel (B0) consistently producing the highest output (Fig.7c) for pure blends. At full load, B0 reached 4.44 kW, whereas B10, B20, and B30 yielded 4.16 kW, 3.95 kW, and 3.86 kW, corresponding to reductions of 6.4 %, 4.5 %, and 2.4 % relative to the diesel baseline. The addition of 80 ppm CeO₂ markedly enhanced performance across all blends (Fig.7d). B0 +80 ppm CeO₂ achieved 4.88 kW, an increase of 10 % over B0, while B10 +80 ppm CeO₂, B20 +80 ppm CeO₂, and B30 +80 ppm CeO₂ reached 4.45 kW, 4.15 kW, and 3.98 kW, representing 7 %, 5 %, and 3 % improvements over their respective base fuels. The CeO₂-treated curves remain consistently above the untreated blends with nearly parallel slopes, indicating enhanced combustion efficiency without affecting the mechanical load response. These results corroborate previous studies showing that cerium-based additives accelerate oxidation and improve energy release, demonstrating that low-dose CeO₂ nanoparticles effectively restore and elevate brake power in both diesel and biodiesel fuels^11,39.

Brake specific fuel consumption(BSFC)

The variation of brake specific fuel consumption (BSFC) with engine load is presented for pure blends (Fig.7e) and for blends with 80 ppm CeO₂ nanoparticles (Fig.7f). Across all pure blends, BSFC decreases steadily with increasing load, reflecting improved combustion efficiency at higher engine operation. Among the base fuels, B0 exhibits the lowest BSFC, while B30 shows the highest, indicating that higher biodiesel content slightly reduces thermal efficiency due to its lower calorific value. The addition of 80 ppm CeO₂ systematically lowers BSFC for all blends. Specifically, B0 +80 ppm CeO₂ decreases from 0.468 kg/kWh to 0.205 kg/kWh, a 56.2% reduction, whereas B30 + 80 ppm CeO₂ drops from 0.507 kg/kWh to 0.232 kg/kWh, a 54.3% reduction. The relative impact of CeO₂ is slightly more pronounced in lower biodiesel blends; for instance, at partial loads, B0 +80 ppm CeO₂ shows an improvement of 1.5-2%, whereas B30 + 80 ppm CeO₂ exhibits a 1.3–1.8% improvement over B30. These results indicate that CeO₂ nanoparticles facilitate more complete oxidation of the fuel, with the effect being proportionally larger in diesel-rich blends due to higher volatility and better mixing characteristics. Overall, the addition of CeO₂ leads to a consistent reduction in BSFC, highlighting its potential as an effective combustion catalyst for biodiesel–diesel blends^13,35.

Combustion characteristics

Cyminder pressure(Pcyl)

Fig.8 illustrates the variation of cylinder pressure (Pcyl) with crank angle at full load and 2500 rpm, with emphasis on its magnitude and crank angle location for each fuel. For the pure blends, neat diesel B0 exhibits the highest Pcyl of approximately 70.0 bar at about 368°CA, occurring shortly after top dead center due to its favorable volatility and rapid premixed combustion. As the biodiesel proportion increases, a progressive reduction in Pcyl and a slight retardation in its occurrence are observed as shown in Fig. 8a. B10 attains a Pcyl of around 57.9 bar at approximately 370°CA, followed by B20 at about 49.4 bar near 371°CA, while B30 records the lowest Pcyl of roughly 42.7 bar at around 372°CA. This trend reflects the combined influence of lower heating value, higher viscosity, and slower evaporation characteristics of biodiesel, which suppress premixed combustion intensity and delay pressure development^35,40.

Fig. 8.

Cylinder pressure variation with crank angle at full load and 2500 rpm.

The addition of 80 ppm CeO₂ nanoparticles results in a consistent enhancement of Pcyl and an advancement of its crank angle location for all fuels. Diesel with CeO₂ reaches the highest Pcyl of approximately 71.2 bar at about 366°CA, indicating intensified and earlier combustion relative to neat diesel as described in Fig. 8b. Correspondingly, Pcyl increases to around 61.2 bar at approximately 368°CA for B10+ 80 ppm CeO₂, 53.3 bar at nearly 369°CA for B20+ 80 ppm CeO₂, and 46.4 bar at about 370°CA for B30+ 80 ppm CeO₂. Compared with the respective pure blends, the pressure development for CeO₂ doped fuels shifts closer to top dead center, highlighting the catalytic oxygen buffering and redox activity of cerium oxide, which accelerates fuel oxidation and strengthens early-stage combustion. Overall, the results demonstrate that CeO₂ nanoparticle addition effectively mitigates the weakened and delayed Pcyl characteristics associated with higher biodiesel blends, bringing their combustion behavior closer to that of conventional diesel operation under full load conditions^37,40.

Apparent heat release rate (HRR)

Fig.9 presents the apparent heat release rate (HRR) as a function of crank angle at full load and 2500 rpm, highlighting both magnitude and crank angle of the peak for each fuel. For the pure blends, neat diesel B0 exhibits the highest premixed combustion peak of approximately 119.5 J/°CA at around 389°CA. As the biodiesel fraction increases, the peak HRR decreases progressively: B10 reaches about 105.5 J/°CA at 389°CA, B20 approximately 90.1 J/°CA at 389°CA, and B30 around 76.8 J/°CA at the same crank angle. The early-stage low temperature reaction shows slightly higher HRR for diesel compared with the biodiesel blends, reflecting its lower viscosity and higher volatility, which promote faster vaporization and pre-combustion reactions as illustrated in Fig.9a. The reduction in peak HRR with increasing biodiesel content is primarily due to the higher cetane number and oxygenated nature of biodiesel, which shorten ignition delay and reduce the fraction of fuel participating in rapid premixed combustion, shifting overall combustion toward a more diffusion-controlled process^40,41.

Fig. 9.

Heat release rate with crank angle at full load and 2500 rpm.

Upon addition of 80 ppm CeO₂ nanoparticles, the premixed combustion peaks increase substantially for all blends and shift slightly closer to TDC. Diesel with CeO₂ (B0+80 ppm CeO₂) reaches a maximum HRR of approximately 130.2 J/°CA at 387°CA, while B10+80 ppm CeO₂ achieves 112.6 J/°CA at 388°CA. B20+ 80 ppm CeO₂ records a peak of 97.4 J/°CA at 388°CA, and B30+ 80 ppm CeO₂ reaches 84.3 J/°CA at 388°CA as shown in Fig.9b. The enhancement in HRR and slight advancement in its crank angle are attributed to the catalytic redox activity and oxygen buffering effect of CeO₂, which accelerate fuel oxidation, improve air–fuel mixing, and increase the fraction of fuel participating in the premixed phase. These results demonstrate that while increasing biodiesel fraction moderates the intensity of premixed combustion, CeO₂ nanoparticle addition effectively restores and intensifies early-stage heat release, optimizing combustion phasing and aligning with trends observed in previous diesel–biodiesel studies^2,37.

Ignition delay (ID)

Fig.10 presents the ignition delay (ID) in crank angle degrees (°CA) for diesel (B0) and biodiesel blends (B10, B20, B30) with and without 80 ppm CeO₂ nanoparticles, highlighting the observed trends. Ignition delay progressively increases with higher biodiesel content, with B0 exhibiting the shortest ID of 7°CA and B30 the longest at 13°CA.

Fig. 10.

Ignition delay of diesel–biodiesel blends with and without 80 ppm CeO₂.

This behavior is attributed to the elevated viscosity and lower volatility of biodiesel, which impede fuel atomization and vaporization, thereby delaying ignition. The higher oxygen content and reduced calorific value of biodiesel further contribute to slower ignition. In contrast, the inclusion of CeO₂ nanoparticles systematically shortens the ID for all blends; for example, B0 with CeO₂ reduces from 7°CA to 5°CA, while B30 with CeO₂ decreases from 13°CA to 11°CA, reflecting enhanced early-stage oxidation and improved combustion phasing^30,42.

Combustion duration (CD)

Fig.11 presents the combustion duration (CD) expressed in crank angle degrees at 100 percent load and 2500 rpm for the tested biodiesel blends B0, B10, B20, and B30, evaluated under neat fuel conditions and with the addition of 80 ppm CeO₂ nanoparticles. Without nanoparticle supplementation, combustion duration increases progressively as the biodiesel fraction rises. Neat diesel B0 records the minimum combustion duration of around 68 °CA, while B10, B20, and B30 exhibit higher values of nearly 70, 72, and 74 °CA, respectively. This trend is primarily associated with the physicochemical characteristics of biodiesel fuels, including their relatively higher viscosity and reduced volatility, which adversely affect spray breakup and evaporation processes. Consequently, the combustion phase extends as the proportion of biodiesel in the blend increases^37,42,43.

Fig. 11.

Combustion duration of diesel–biodiesel blends with and without 80 ppm CeO₂.

The incorporation of 80 ppm CeO₂ nanoparticles leads to a consistent shortening of combustion duration for all blends. Specifically, the combustion duration of B0 decreases from about 68 to 65 °CA, whereas B10, B20, and B30 reduce to approximately 68, 70, and 72 °CA, respectively. This improvement is linked to the catalytic behavior of CeO₂ nanoparticles, which enhance oxidation reactions during combustion. Owing to their oxygen buffering capability, these nanoparticles facilitate improved fuel oxidation, strengthen fuel-air interactions, and accelerate the completion of the combustion process. As a result, the overall combustion duration is reduced despite the higher biodiesel content. The observed behavior confirms that while increasing biodiesel concentration tends to prolong combustion, CeO₂ nanoparticles effectively mitigate this effect by improving combustion dynamics and promoting faster energy release^7,42.

CA50 for biodiesel blends with and without CeO₂

Fig.12 depicts the evolution of CA50, representing the crank angle at which 50 percent of the total heat release occurs, for biodiesel blends B0, B10, B20, and B30 operating at 100 percent load and 2500 rpm, both without and with 80 ppm CeO₂ nanoparticles. In the absence of nanoparticles, a clear retardation of CA50 is observed as the biodiesel proportion increases. Neat diesel B0 exhibits the earliest CA50 at around 365 °CA, whereas B10, B20, and B30 progressively shift to later crank angles of approximately 369, 371, and 375 °CA, respectively. This delayed combustion phasing is primarily governed by the intrinsic fuel properties of biodiesel, including elevated viscosity and reduced volatility, which tend to lengthen ignition delay and moderate the premixed combustion rate. Consequently, a greater fraction of the fuel energy is released during the later stages of combustion, pushing CA50 toward a retarded position^5,17.

Fig. 12.

CA50 for B0–B30 blends with and without 80 ppm CeO₂.

The introduction of 80 ppm CeO₂ nanoparticles induces a systematic advancement of CA50 across all tested blends. Specifically, the CA50 of B0 advances from about 365 to 361 °CA, while B10, B20, and B30 shift earlier to nearly 364, 367, and 370 °CA, respectively. This favorable shift can be ascribed to the pronounced catalytic activity of CeO₂ nanoparticles, which intensify oxidation reactions and hasten the heat release process. Their oxygen buffering capability enhances fuel reactivity and promotes more effective fuel–air interactions, resulting in improved combustion phasing. The persistent advancement of CA50 with nanoparticle addition underscores the capability of CeO₂ to fine tune the timing of energy release, thereby supporting enhanced thermal efficiency and more desirable in cylinder combustion behavior^15,36.

The recovery of brake thermal efficiency and brake power observed with CeO₂ addition can be mechanistically attributed to the oxygen buffering and redox cycling behavior of cerium oxide nanoparticles. CeO₂ undergoes reversible Ce⁴⁺/Ce³⁺ transitions, allowing temporary storage and controlled release of oxygen during combustion. This buffering effect enhances oxidation in locally fuel-rich regions during the premixed phase, promoting faster early heat release and advancing combustion phasing. At the same time, the catalytic activity of CeO₂ improves fuel–air interaction and supports radical generation, which shortens ignition delay and increases the premixed heat release rate. Together, these mechanisms accelerate energy conversion, reduce combustion losses, and account for the improved combustion phasing and efficiency observed in CeO₂-treated biodiesel blends.

Engine emission characteristics

Carbon monooxide emission(CO)

CO emission profiles for the biodiesel blends are presented for pure blends (Fig.13a) and for blends with 80 ppm CeO₂ nanoparticles (Fig.13b) For the pure blends, CO steadily decreases with increasing engine load, with B0 exhibiting the highest CO levels and B30 the lowest, reflecting the natural oxygen content of biodiesel. The addition of 80 ppm CeO₂ shifts the emission trends downward across all blends. B0 shows the most pronounced reduction, ranging from 16.7% at 20% load to 7.5% at full load, followed by B10 (3.8-9.5%), B20 (2.4-10.7%), and B30 (2.4-9.1%). These results indicate that CeO₂ is most effective in diesel-rich blends, particularly under lower loads where incomplete oxidation is more likely. Across all blends, CeO₂ consistently reduces CO emissions, demonstrating its role as an effective oxidation promoter for diesel–biodiesel mixtures^15,20.

Fig. 13.

Effect of biodiesel blending and 80 ppm CeO₂ Nanoparticle addition on emissions.

Unburned hydrocarbon emissions (UHC)

The variation of unburned hydrocarbon (UHC) emissions with engine load is shown for pure biodiesel blends (Fig.13c) and for blends with 80 ppm CeO₂ nanoparticles (Fig.13d). For the pure blends, UHC emissions decrease progressively with increasing load, indicating improved fuel vaporization and more complete combustion at higher operating conditions. Among the base fuels, B0 exhibits the highest UHC emissions at partial loads, while B30 shows the lowest, reflecting the enhanced oxidation associated with higher biodiesel content; for example, at 20% load, UHC decreases from 97 ppm for B0 to 89 ppm for B30. The addition of 80 ppm CeO₂ further suppresses UHC emissions across all blends, with B0 + 80 ppm CeO₂ decreasing from 94 ppm at 20% load to 28 ppm at full load, corresponding to a 70.2% reduction relative to the base fuel. The impact of CeO₂ is more pronounced in diesel-rich blends, where its catalytic activity promotes oxidation under conditions prone to incomplete combustion. Overall, CeO₂ nanoparticles act synergistically with biodiesel oxygenation to systematically reduce UHC emissions across the entire load range^39,40.

NOx emission

The variation of NOx emissions with engine load is shown for pure biodiesel blends (Fig.13e) and for blends with 80 ppm CeO₂ nanoparticles (Fig.13f). For the pure blends, NOx emissions increase progressively with load, reflecting higher in-cylinder temperatures and increased oxygen availability at elevated operating conditions¹¹. Among the base fuels, B0 exhibits the lowest NOx levels, while B30 shows the highest, indicating that increasing biodiesel content promotes nitrogen oxidation due to its inherent oxygen content and slightly advanced combustion characteristics; at full load, NOx increases from 58 ppm for B0 to 168 ppm for B30. The addition of 80 ppm CeO₂ further elevates NOx emissions across all blends, with B0 +80 ppm CeO₂ rising from 55 ppm at 20% load to 145 ppm at full load, and B30 + 80 ppm CeO₂ reaching 180 ppm at 100% load¹². This increase is attributed to enhanced combustion intensity and higher peak temperatures induced by CeO₂, which accelerate thermal NOx formation. The relative increment is more pronounced at higher biodiesel fractions, suggesting that CeO₂ amplifies the existing oxygen- and temperature-driven NOx formation pathways. Overall, while CeO₂ improves fuel oxidation and combustion efficiency, its influence on NOx emissions highlights the need for complementary mitigation strategies in biodiesel-diesel blends^15,41.

The observed increase in NOₓ emissions with CeO₂ addition can be attributed to enhanced combustion efficiency, higher in-cylinder temperatures, and increased availability of oxygen during the premixed combustion phase. While this trade-off is common in oxygenated and nano-additive fuels, several mitigation strategies may be employed in practical applications, including exhaust gas recirculation, injection timing optimization, and compatibility with after-treatment systems such as selective catalytic reduction. These approaches could offset NOₓ increases while preserving the combustion and efficiency benefits observed in the present study.

Smoke opacity

The variation of smoke opacity with engine load is presented for pure biodiesel–diesel blends (Fig.13g). and for blends with 80 ppm CeO₂ nanoparticles (Fig.13h). For the pure blends, smoke opacity generally decreases with increasing load up to 80% and shows a slight rise at full load for B0 and B10, reflecting the combined effects of fuel atomization, combustion temperature, and particulate formation at high engine demand. Increasing biodiesel content consistently lowers smoke opacity due to the higher inherent oxygen content, which promotes more complete combustion. The addition of 80 ppm CeO₂ further suppresses smoke formation across all blends. Quantitatively, B0 +80 ppm CeO₂ decreases from 11% at 20% load to 12.5% at full load, while B30 + 80 ppm CeO₂ decreases from 8% to 9.2%, corresponding to an overall reduction of approximately 9-10% relative to the respective pure blends. The lowest smoke opacity is observed for B30 +80 ppm CeO₂ at intermediate loads, highlighting the synergistic effect of biodiesel oxygenation and nanoparticle-assisted soot oxidation. Overall, CeO₂ acts as an effective combustion catalyst, enhancing oxidative pathways and promoting more complete fuel burnout, with a proportionally stronger impact in diesel-rich blends where inherent oxygen availability is lower^16,42,43.

Effect of biodiesel blending and CeO₂ addition on engine performance and emissions at full load

Fig. 14 illustrates the rate of change in performance and emission parameters of biodiesel blends with and without 80 ppm CeO₂ at full load. Biodiesel blends exhibited a progressive reduction in brake power (BP) and brake thermal efficiency (BTE) relative to diesel, with BP decreasing by 10.45%, 15.93%, and 20.27% for B10, B20, and B30, respectively, while BTE declined marginally by 1.23%, 2.46%, and 3.69%. Correspondingly, brake specific fuel consumption (BSFC) increased by 3.81%, 8.10%, and 13.33% for these blends^39,44.

Fig. 14.

Rate of change in performance and emission parameter at full load.

The addition of 80 ppm CeO₂ significantly mitigated these penalties, with BP increasing by 10.00% for B0+80 ppm CeO₂ and smaller reductions for CeO₂-added blends, BTE improving by up to 1.64%, and BSFC decreasing by 2.38% relative to diesel. Emissions of CO, unburned hydrocarbons (HC), and smoke opacity decreased markedly with biodiesel fraction (CO: 17.5-47.5%; HC: 20.0-43.3%; Smoke: 11.1-25.9%), while NOx increased modestly (5.41-13.51%). CeO₂ addition further enhanced emission performance, with CO and HC reductions reaching 50.0% and 56.7% for B30+80 ppm CeO₂, smoke opacity decreasing up to 31.9%, and NOx increasing moderately by 21.62%, demonstrating the dual effect of catalytic nanoparticle addition in improving combustion efficiency and emission control. These results collectively highlight the potential of CeO₂-enriched biodiesel blends to optimize engine performance while substantially reducing harmful emissions^39,44,46.

The simultaneous reduction of carbon monoxide, unburned hydrocarbons, and smoke opacity, accompanied by a moderate increase in NOₓ emissions, reflects a combustion regime characterized by enhanced oxidation kinetics and elevated in-cylinder temperatures. Similar emission trade-offs have been reported in hydrogen-carrier-assisted and additive-enhanced biodiesel combustion systems, where accelerated flame development and increased local temperature promote soot and CO oxidation while favoring thermal NOₓ formation. Studies involving thermally regulated hydrogen release and hydrogen-enriched biodiesel–diesel dual-fuel operation further demonstrate that improved premixed combustion and shortened ignition delay lead to analogous emission behavior. These parallels reinforce the interpretation that CeO₂ nanoparticles act primarily by intensifying early-stage combustion rather than altering late-cycle diffusion-controlled burning.

Comparative study of podocarpus falcatus biodiesel with other biodiesel feedstocks

While many biodiesel feedstocks (Jatropha, Pongamia, Soybean, WCO, Palm) show trade-offs reducing CO, HC, and smoke but slightly compromising performance or increasing BSFC Podocarpus biodiesel with CeO₂ nanoparticles restores engine performance almost to diesel levels while simultaneously reducing CO, UHC, and smoke significantly. CeO₂ addition provides a synergistic advantage, compensating for the typical performance penalties of biodiesel blends observed in other feedstocks. Podocarpus blends also demonstrate better BTE recovery, lower BSFC, and moderated NOx increase compared to most other feedstocks, making it more balanced across both performance and emissions^14,45,47.

In the comparative assessment, qualitative descriptors such as high, moderate, or balanced performance are applied based on the combined behavior of brake thermal efficiency, brake specific fuel consumption, emission reduction levels, and combustion stability relative to baseline fuels. These terms are used to represent overall performance balance rather than superiority in any single parameter.

Previous studies on cerium oxide nanoparticles as combustion enhancers in biodiesel–diesel blends generally report modest benefits that are highly dependent on feedstock type and operating conditions as shown in Table 6. Typical findings include brake thermal efficiency gains of about 2–6%, brake specific fuel consumption reductions of 5–15%, peak cylinder pressure increases below 5%, and reductions in unburned hydrocarbons and carbon monoxide that rarely exceed 40–50%, often with limited or inconsistent changes in combustion phasing^5,8,9. Against this established background, the present results show markedly stronger effects when low-dose CeO₂ is combined with Podocarpus falcatus biodiesel. At full load, brake thermal efficiency increases by up to 11.7%, brake power is recovered by 3–10% across blends, and brake specific fuel consumption decreases by more than 54% compared with untreated fuels. Combustion behavior is significantly enhanced, with peak cylinder pressure reaching 71.2 bar, premixed heat release rate exceeding 130 J per degree crank angle, ignition delay shortened by about 2°CA, and combustion duration reduced by up to 4°CA. Emission improvements are similarly pronounced, including unburned hydrocarbon reductions of up to 70% along with consistent decreases in carbon monoxide and smoke opacity. These results exceed the performance ranges commonly reported for CeO₂-assisted biodiesel systems and point to a strong synergistic interaction between CeO₂ nanoparticles and the specific molecular and combustion characteristics of Podocarpus falcatus biodiesel, establishing clear feedstock-specific and performance-level novelty.

Table 6.

Comparative trends of podocarpus falcatus biodiesel with other feedstocks.

Feedstock	Performance (BP/BTE/BSFC)	Combustion (HRR/ID)	Emissions (CO/UHC/Smoke/NOx)	Overall balance	Reference
Podocarpus + CeO₂	BP/BTE near diesel; BSFC reduced	Advanced Pcyl, improved HRR, shorter ID	CO/UHC/Smoke ↓, NOx moderately ↑	High - balanced performance and emissions	Current study
Jatropha	Slight BP/BTE loss; BSFC slightly higher	Moderate HRR reduction	CO/HC ↓, NOx ↑	Moderate - emissions improved; performance slightly reduced	46
Pongamia (Karanja)	BTE lower than diesel; BSFC higher	HRR slightly lower	CO/UHC ↓, NOx ↑	Moderate - similar trade-offs	47
Soybean	Slightly lower BP/BTE	HRR mildly lower	CO/HC ↓, NOx slightly ↑	Moderate - emissions improved, minor performance loss	22
Waste cooking oil (WCO)	BP/BTE lower at higher blends	Slower combustion	CO/HC ↓, NOx ↑	Moderate - emissions improved but BP loss noticeable	29
Palm	Moderate BTE; BSFC tends to increase	HRR slightly delayed	CO/UHC/Smoke ↓, NOx ↑	Moderate - emissions improved; performance moderate	48
Castor + CeO₂	Moderate BP and BTE improvements for B10: BTE ↑); BSFC reduced (↓)	Enhanced combustion efficiency, shorter ignition delay	CO ↓, HC ↓, NOx slight ↑	Moderate - good performance -emission trade-off	15

Uncertainty results

To assess the reliability of the experimental results, the uncertainties associated with the measured and derived parameters were quantified. For engine performance, the uncertainties in brake power, brake thermal efficiency, and brake-specific fuel consumption were ±1.2%, ±2.4%, and ±2.0%, respectively³⁰. These values are significantly lower than the variations observed among different biodiesel blends and CeO₂ additive conditions. For combustion characteristics, the uncertainties in peak cylinder pressure and heat release rate were estimated to be ±1.9% and ±2.2%, respectively. The changes in combustion behavior with increasing biodiesel content and nanoparticle addition exceeded these uncertainty limits. Regarding emissions, the maximum uncertainties were ±3.3% for CO, ±3.0% for HC, ±2.8% for NOₓ, and ±2.5% for smoke opacity¹⁷. The observed reductions in CO, HC, and smoke emissions, as well as the variations in NOₓ emissions, were substantially greater than the corresponding uncertainty margins. Overall, since the observed differences in performance, combustion, and emission characteristics consistently exceed the associated uncertainties, the experimental trends and comparative analyses presented in this study are considered reliable and statistically meaningful^17,30,38.

While individual measurement uncertainties arise from pressure transducers, crank angle resolution, and fuel flow measurements, their influence on derived combustion parameters such as CA50, premixed heat release fraction, and combustion duration is primarily associated with cycle-to-cycle pressure variation and numerical differentiation during heat release analysis. Because identical instrumentation, data acquisition settings, and analysis procedures were applied to both treated and untreated fuel blends, the relative comparison of combustion parameters remains robust despite absolute uncertainty propagation. Consequently, the observed shifts in combustion phasing and heat release characteristics can be attributed with confidence to CeO₂ addition rather than experimental variability.

Conclusions and future perspective

The experimental investigation confirms that Podocarpus falcatus biodiesel blends (B10–B30) possess physicochemical properties compatible with compression ignition engines, exhibiting systematic trends in viscosity, density, cetane number, and flash point that support stable engine operation. FTIR analysis verified successful conversion of PF oil to methyl ester, while catalyst characterization showed that the CaO-CeO₂ heterogeneous catalyst, particularly with higher CaO loading, exhibits favorable crystallinity and surface characteristics for enhanced catalytic activity. Engine tests revealed that increasing biodiesel fraction led to moderate reductions in brake power and brake thermal efficiency and slightly higher brake-specific fuel consumption due to lower heating value and higher viscosity. The addition of 80 ppm CeO₂ nanoparticles effectively mitigated these penalties, improving thermal efficiency, restoring brake power, and reducing fuel consumption, particularly at higher loads. Combustion analysis indicated that biodiesel blends showed lower peak cylinder pressure and slightly delayed combustion phasing; however, CeO₂ addition advanced ignition, increased peak pressure and heat release rate, shortened combustion duration, and shifted CA50 closer to top dead center. Emission results demonstrated significant reductions in CO, unburned hydrocarbons, and smoke opacity, further enhanced by CeO₂, while NOₓ increased moderately but remained within manageable limits.

Overall, CeO₂-enriched Podocarpus falcatus biodiesel blends present a viable and sustainable alternative fuel for compression ignition engines without major hardware modifications. The uncertainty analysis confirms that the observed trends exceed experimental error limits, validating result reliability. Future research should focus on spray characterization and mixture formation, long-term engine durability and deposit formation, and assessment of potential environmental and health impacts of nanoparticle use. Integration of optimized nano-additive dosing with appropriate after-treatment strategies offers a practical pathway toward safe and sustainable deployment.

Author contributions

Biru Birhanu: Conceptualization, Methodology, Data collection, Analysis, Writing-original draft, Writing-review & editing. Devendra Deshmukh: Conceptualization, Methodology, Data collection, Analysis, Validation, Supervision, Writing-review & editing. Tigabu Haileleul Yemane: Data collection, Investigation, Experimental support.

Funding

The author has not disclosed any funding.

Data availability

Data are available from the corresponding authors on reasonable request.

Declarations

Competing of interest

The authors declare no competing interests.

Ethical Approval

This study did not involve human participants or animal experiments and therefore did not require ethical approval.