Effects of EGR and Fe₃O₄ nanoadditives on performance of a variable compression ratio diesel engine fueled with tamarind biodiesel

Abstract

The rising demand for energy and the rapid depletion of fossil fuels underscore the need for sustainable alternatives that can ensure efficient engine operation while meeting emission regulations, such as biodiesel. Tamarind seed biodiesel has emerged as a renewable option; however, its tendency to increase nitrogen oxide (NOx) emissions restricts its large-scale application in the automotive industry. This study explored an innovative approach that integrates magnetite (Fe₃O₄) nanoparticle additives with exhaust gas recirculation (EGR) to enhance combustion, performance, and emission characteristics of a variable compression ratio (VCR) diesel engine. Experimental investigations were performed on a single-cylinder engine using TB20 (20% tamarind biodiesel and 80% diesel) and its nano-enhanced blends containing 50 ppm (TB20N50) and 100 ppm (TB20N100) of Fe₃O₄ under loads from 0% to 100% with EGR rates of 0%, 10%, and 20%. The results demonstrated that the addition of Fe₃O₄ nanoparticles enhanced the brake thermal efficiency by 2.61% and reduced the specific fuel consumption by 3.34% for TB20N100 relative to TB20. The combustion characteristics also improved, with the cylinder pressure and heat release rate increasing by 2.5–5% when nanoadditives were used. In terms of emissions, NOx levels were significantly reduced by 25.5–56.3% at higher EGR rates. CO emissions decreased by approximately 18% with nanoparticle addition, although the application of EGR led to a 20–40% increase in CO and HC emissions at high loads. Smoke opacity decreased by nearly 3% for the nanoblends but increased by up to 7% at higher EGR levels. The synergistic use of Fe₃O₄ nanoparticles and controlled EGR provides an effective strategy for optimizing biodiesel-fueled engines. This innovative approach demonstrates the potential of tamarind biodiesel nanoblends to balance efficiency and emissions, offering a sustainable pathway toward cleaner engine technologies.

Introduction

The continuous rise of the world population, along with growing energy needs, has spurred research into alternative and sustainable energy sources. In this regard, biodiesel made from renewable resources has emerged as a contender to offset the environmental effects of non-renewable fuel sources. Using biodiesel obtained from natural resources like jatropha, pongamia, castor oil, mahua oil, palm oil, waste cooking oil, etc., provides a viable avenue for decreasing the carbon footprint and minimizing environmental damage. Research has shown that blending biodiesel with diesel can significantly lower CO, hydrocarbon (HC) emissions, and particulate matter^1,2. This is because biodiesel contains oxygen, which helps to reduce emissions when mixed with diesel. The study also revealed several other eco-friendly alternative fuels³. In this particular investigation, tamarind seed oil was chosen as the biodiesel due to its wide availability and ease of extraction from the seed through various techniques. In-depth literature on the use of various biodiesels was provided.

Sai et al.⁴ combined rubber seed oil blends (10%, 20%, and 30%) with diesel, and their conclusions demonstrated that the greatest BTE and the lesser BSFC are seen under maximum load situations. CO and HC emissions progressively decrease as the blend ratio rises from B10 to B30. However, there was a significant increase in the level of NOx and CO₂. Ekrem et al.⁵ experimented to investigate the influence of 5%, 20%, and 70% concentrations of rapeseed biodiesel on the performance and pollution of diesel engines. The results indicated that blends with lower concentrations of biodiesel, precisely 20% and below, demonstrated better efficiency and produced less harmful emissions. Therefore, it was concluded that these blends could be a feasible alternative fuel option for unmodified diesel engines. Ashok et al.⁶ experimented on an engine using a type of biodiesel called Calophyllum inophyllum and its fuel blends of 30% and 60% concentration. The results showed that as the proportion of CIME biodiesel increased, the BTE decreased and BSFC increased. However, there was a decrease in emissions like HC, CO, and Smoke, while Nitrogen oxide emissions increased as the proportion of CIME biodiesel increased. Rahman et al.⁷ explored to carryout the performance test with Jatropha-based and Palm-based biodiesel blends. The study revealed that the blends resulted in lower levels of HC and CO pollutants but NOx emissions are improved than the diesel fuel. The study also found that the PB20 blend produced low CO and HC emissions. However, it had the highest NOx emissions. Vipul et al.⁸ studied using blends of mahua biodiesel and diesel, specifically M10, M20, and another blend represented as M100. The study found that a blend of 20% biodiesel and 80% diesel resulted in better engine performance and reduced emissions such as HC, CO, and smoke. However, the NOx was higher, likely due to the increased temperature inside the engine cylinder. Mithun et al.⁹ analysed the effects of castor biodiesel blends on a compression ignition engine. According to their findings, there is no noticeable difference between using diesel or any of the three biodiesel blends regarding engine performance. However, the braking thermal efficiency of the biodiesel blends is higher than that of diesel during full-load operation. This implies that lower blend concentrations can be used successfully to operate the engine, reducing the need for petro-diesel.

Several approaches have been explored to enhance the combustion efficiency, performance, and emission characteristics of biodiesel-fueled engines. Among these, the incorporation of nano-additives and the application of exhaust gas recirculation (EGR) have shown significant promise. Nano-additives improve atomization, act as combustion catalysts, and facilitate better fuel oxidation, while EGR effectively reduces nitrogen oxide (NOx) emissions by lowering combustion temperatures. A summary of recent studies is presented in Tables 1 and 2 below.

Table 1.

Effect of Nano-Additives on Biodiesel Engine Performance, Emissions, & Combustion.

Reference	Biodiesel Type	Nano-Additive	Performance Improvement	Emission Impact	Combustion Characteristics
Haiter et al.10	Waste cooking oil	CeO₂, ZnO, TiO₂	BTE ↑ 2.53%, BSEC ↓ 3.80%	CO ↓ 36.36%, HC ↓ 17.41%, Smoke ↓ 16.24%	Cylinder pressure ↓ 1.76%, HRR ↓ 7.59%
Debas et al.11	Cottonseed	CeO₂ + Al₂O₃	BSFC ↓ to 0.258 kg/kW·h	CO ↓ 6.7–23.2%, CO₂ ↓ 4.6–14.8%, HC ↓ 5.9–18.5%	Enhanced combustion due to the oxygen-storage effect
Samuel12	Castor seeds	CeO₂	↑	CO ↓ 12% (B5), HC ↓ 37.7% (B5); NOx ↑ 4.3% (B10)	Improved combustion due to catalytic activity
Seela et al.13	Mahua	Cerium oxide	BTE ↑ 1.8%, SFC ↓ 1%	CO ↓ 6–8%, HC ↓ 6–9%	Faster combustion, shorter ignition delay
Chinmoy et al.14	Mahua	Titanium oxide (TiO2)	—	CO ↓ 46.54%, HC ↓ 2.8%, NOx ↓ 2.3%	Cleaner combustion due to catalytic action
Duraisamy et al.15	Neem	Alumina (Al₂O₃)	BTE ↑ 7.2%, BSFC ↓ 6.7%	CO, HC, Smoke ↓	Higher in-cylinder pressure and HRR
Ameer et al.16	Waste plastic oil	Zinc oxide (ZnO)	BTE ↑ 2.747%	Smoke ↓ 11.86%, CO ↓ 5.7%, HC ↓ 28%, NOx ↓ 14.93%	Higher peak pressure and improved ignition
Sarah et al.17	Parsley biodiesel	Copper oxide (CuO)	BTE ↑ 33.8%, BSFC ↓ 3.28%	HC ↓ 1.6%, CO ↓ 5.7%, NOx ↓ 4.7%, CO₂ ↑ 2.15%	Faster flame propagation, higher HRR
Vikas et al.18	Waste cooking oil	Graphite & Graphene	BTE ↑ 8–10%	NOx ↓ 0.7–5%	In-cylinder pressure ↑ 0.5–2.5%
Pullagura et al.19	Tamarind oil methyl ester	TiO2	BTE ↑ 6.13%, BSFC ↓ 17.64%	CO ↓ 9.49%, HC ↓ 7.81%, NOx ↓ 6.53%	Cylinder Pressure ↑ 4.36%, HRR ↑ 10.6%
Bikkavolu et al.20	Tamarind oil methyl ester	TiO2	BTE ↑ 9.76%, BSFC ↓ 15.2%	CO ↓ 15.2%, HC ↓ 11.10%, NOx ↓ 9.06%	HRR ↑, CP ↑

Table 2.

Effect of EGR on Engine Performance, Emissions, and Combustion.

Reference	Fuel Type	EGR Condition	Performance Outcomes	Emission Outcomes	Combustion Outcomes
Praveen et al.21	Gasoline	0–20% EGR	Nano-additives improved BTE; 20% EGR slightly reduced BTE	EGR reduced NOx, but increased smoke, CO, and HC	Modified combustion due to exhaust dilution
Parvesh et al.22	Dual-fuel engine (biodiesel–gas mix)	Different EGR rates	EGR improved performance at low/medium loads; reduced at high loads	NOx reduced with higher EGR; CO, HC increased	Lower combustion temperature at higher EGR
Rajesh et al.23	Diesel/Biodiesel	10–30%	Higher EGR lowered BTE	NOx significantly reduced; CO and HC increased	Reduced heat release rate due to charge dilution
Masood et al.24	Cottonseed biodiesel blends	15–25%	BTE decreased at 25% EGR; BSFC increased	HC and CO increased; NOx sharply decreased	Combustion efficiency dropped with higher EGR
Saravanan et al.25	Diesel/Biodiesel	0–20%	Reduced BTE for all loads	Lower NOx for all EGR levels	EGR altered the heat-release profile and delayed the SOC

Although extensive literature indicates that exhaust gas recirculation (EGR) and nanoadditives are predominantly investigated as independent strategies, with limited attention given to their combined and interactive effects on diesel engine performance. While EGR effectively reduces NOx emissions, it often leads to reduced combustion efficiency and performance deterioration, whereas nanoadditives such as Fe₃O₄ enhance combustion through improved catalytic activity. However, the ability of nanoadditives to compensate for EGR-induced performance losses remains insufficiently explored, particularly in conjunction with waste-derived, non-edible biodiesel feedstocks such as tamarind seed oil. Despite the growing emphasis on sustainable fuels, the integration of waste seed biodiesel, EGR, and nanoadditives has not been systematically investigated. This clearly defines the research gap as the absence of a comprehensive framework that captures the interaction between emission-control techniques and combustion-enhancement strategies using sustainable fuel sources. To address this gap, the present study develops a systematic experimental investigation into the combined effects of EGR and Fe₃O₄ nanoadditives on a diesel engine fueled with tamarind biodiesel derived from waste seeds. The novelty of this work lies in its integrated and multi-dimensional approach, which simultaneously incorporates waste-to-energy fuel utilization, emission reduction via EGR, and combustion enhancement through nanoadditives. Unlike conventional studies focusing on isolated parameters, this research elucidates the synergistic and compensatory interactions between EGR-induced combustion suppression and nanoparticle-assisted combustion improvement. Furthermore, the use of tamarind seed biodiesel enhances the sustainability and economic relevance of the study. Overall, the work provides new insights into optimizing diesel engine performance through the combined application of advanced combustion strategies and sustainable fuel resources, thereby establishing its originality and scientific contribution.

Materials and methods

Preparation of fuel samples

In the present study, tamarind seed oil is used and obtained from a commercial oil shop in Chennai, India, to produce tamarind biodiesel (TB) through transesterification. The process consists of three stages. In the first step, raw oil is heated to 45 °C, then a combination of 0.1 L/L 99% pure methanol and 1 mL/L 95% pure sulfuric acid (H2SO4) is mixed to dissolve the solid fats and remove the glycerin. After being heated for 90 min, the mixture can settle for two hours. In the second stage, sodium hydroxide (6.5 g NaOH) is added to 0.25 L of methanol to produce sodium methoxide, which is then added to the heated mixture to neutralize the sulfuric acid. After heating the mixture to 60 °C, it is allowed to settle for eight hours, enabling the separation of glycerin from the oil. In the third stage, the oil is mixed with one-third of the volume of water, and the mixture is washed several times to eliminate the salt present. Finally, the solution is heated to above 100 °C to evaporate any remaining water and conserve the biodiesel. The tamarind biodiesel is blended with diesel to produce the TB20 blend, which contains 20% (200 ml) biodiesel and 80% (800 ml) diesel. The whole process is illustrated in Fig. 1.

Fig. 1.

Various steps involved in the preparation of tamarind biodiesel.

Addition of Fe₃O₄ nanoparticles to the TB20 blend

During the experimentation process, Fe₃O₄ nanoparticles were purchased from the “Platonic Nanotech” supplier, and the properties of nanoparticles are listed in Table 3. These nanoparticles were then incorporated into TB20 at 50 and 100 mg/l using probe sonicator to ensure a homogeneous solution. These nanoparticles were added and sonicated at 20 kHz for 30 min to produce stable nano additive fuels. The nano additive fuels obtained were designated as TB20N50 and TB20N100. The approach used to create Fe₃O₄ nano-blended fuel is demonstrated in Fig. 2. These nano-blended fuels were evaluated in the VCR engine under different loading conditions.

Table 3.

Properties of Fe₃O₄ nanoscale particles.

Properties	Value
Physical appearance
Color	Dark brown
	CAS NO.: 1317-61-9
Form	Powder
Purity	99.50%
Average Particle Size	30–50 nm
Morphology	Spherical
SSA	40–60 m2/g
Bulk Density	0.85 g/ cm3
True density	5.8–5.1 g/cm3
Molecular Weight	231.53 g/mol

Fig. 2.

Preparation of Fe₃O₄ nanoparticles-based nano-fuels.

Table 4 presents the significant physicochemical properties of TB20 and nano additive blends. The physicochemical features of TB20 and nano additive blends are compared with diesel. The calorific values of the nano additive-based TB20 blend have been improved with 41500.1 and 41694.5 kJ/kg for 50 and 100 ppm, respectively. The increased calorific value implies that the fuel has greater energy per unit mass or volume²⁶. As a result, when the fuel is burned, it produces greater heat, which may be used for a variety of purposes. Furthermore, the concentration of the nano addition increases the fuel’s density, flash, and fire point, making it safer to handle, store, and transport fuel by lowering the risk of accidental igniting and evaporative losses. The kinematic viscosity of the fuel also increases with the addition of the nano additive, which enhances lubrication and reduces friction and wear on essential engine components, specifically in high-pressure fuel injection systems. Furthermore, the improved fuel atomization during injection leads to more efficient combustion, enhancing fuel efficiency and reducing emissions.

Table 4.

Fuel properties of various samples.

Characteristics	ASTM D6751	Diesel	TB100	TB20	TB20N50	TB20N100
Kinematic Viscosity @40°C in cSt	1.9-6	2.557	7.27	3.18	3.21	3.23
Flash Point °C	> 130	45	159	44	45	46
Fire Point °C		52	163	53	54	56
Calorific Value kJ/kg	-	42,500	8.70	41,402.9	41,500.1	41,694.5
Density in kg/m3	860–900	830	884	851	852	854

Experimental setup

The test setup for the diesel engine comprises a four-stroke, single-cylinder, direct-injection engine manufactured by Kirloskar, as depicted in Figs. 3 and 4. This engine is water-cooled, has a 5.2 kW power rating at 1500 rpm, and is equipped with a Rotameter (40–400 LPH) for engine cooling and calorimeter cooling (25–250 LPH) enhancement. A combination of RTD, PT 100, and K-type thermocouples are used to measure engine water and exhaust temperatures. Piezoelectric sensors measure combustion and fuel inlet pressures with a range of 5000 PSI. The setup also includes a fuel tank with a capacity of 15 L, an airbox, a manometer, and a fuel burette in the control panel, along with a Denso fuel pump and injector nozzle at a standard injection pressure of 220 bar. The Yokogawa fuel flow transmitter is also included in the setup. The engine is coupled with a water-cooled SAJ eddy current dynamometer for load measurement, and a strain gauge-type load cell determines the load while a rotary encoder measures the speed. All sensors are connected to the data acquisition system. Emissions are evaluated using an AVL Digas-444 portable five-gas analyzer and an AVL437C smoke meter. The analyzer probe is placed into the engine’s exhaust pipeline, and readings are recorded accordingly. Details regarding the test engine specifications for this study are described in Table 5.

Fig. 3.

Experimental setup coupled with EGR system.

Fig. 4.

Laboratory VCR engine with EGR system.

Table 5.

Kirloskar engine TV1 specifications.

Engine type	1 cylinder, 4 stroke, VCR Engine (Computerized)
Rated power	5.2 kW at 1500 rpm
Bore and Stroke	87.5 mm and 110 mm
Compression ratio	17.5:1
Loading	Eddy Current Dynamometer
Load sensor	Load cell, type strain gauge
Temperature sensor	Type RTD, PT100, and Thermocouple, Type K
Fuel flow transmitter	DP transmitter, Range 0–500 mm WC
Airflow transmitter	Pressure transmitter, Range (-) 250 mm WC
Rotameter	Engine cooling 40-400 LPH; Calorimeter 25‐250 LPH
Software	Engine soft
Data acquisition device	NI USB-6210, 16‐bit, 250kS/s.

EGR setup

The Exhaust Gas Recirculation (EGR) system is a highly effective and widely used method for controlling NOx emissions from engine exhausts. This system consists of a complex piping structure connected to the exhaust manifold. The control valves on the piping structure adjust the concentration of EGR supplied to the engine cylinder, as shown in Fig. 5. During the suction stroke, a specific proportion of exhaust gases is mixed with fresh air and delivered to the engine cylinder. The percentage of EGR supplied to the engine cylinder is determined by calculating the amount of exhaust gases that need recirculation. The EGR system recirculates exhaust gases back into the engine cylinder, diluting the incoming air-fuel mixture. This lowers the combustion temperature and reduces NOx output substantially. The amount of EGR supplied to the engine cylinder can be precisely controlled using the EGR valve, which can be actuated electronically or pneumatically. This technique effectively reduces NOx emissions and makes it an important technology in the automotive industry. It is also essential for meeting emissions standards and regulations set by various governments and agencies. However, using EGR can also lead to challenges such as increased particulate matter emissions, reduced fuel efficiency, and increased engine complexity. Nonetheless, EGR remains an essential technology for mitigating the environmental impact of internal combustion engines. The percentage of EGR is determined by calculating the below

Fig. 5.

EGR piping arrangement of the experimental setup.

M_INT= Intake mixture; m_EGR= mass flow rate of exhaust gas recirculated.

Experimental procedures

The tests were carried out to evaluate the performance of TB20 and nano blends of TB20 under varying loading conditions. The load was adjusted by manipulating the load regulator on the setup control panel. To ensure the accuracy and stability of the results, engine data for each set were recorded immediately after applying the load. In the first experimentation stage, TB20 was subjected to varying EGR rates of 0%, 10%, and 20% at different loads. The engine was allowed to reach a steady-state condition, and parameters such as mass flow rate of fuel, air flow rate, cylinder pressure, CO, NOx, etc., were recorded. The data obtained from this stage was used to analyze the impact of EGR rates on engine performance, combustion characteristics, and emissions. In the second stage, nano blends such as TB20N50 and TB20N100 fuels were tested using the same procedure. The engine performance, combustion characteristics, and emissions were assessed, and data was recorded for each fuel type to ensure accuracy and stability. This stage aimed to evaluate the performance of the nanoblends and determine how they compared to the performance of TB20 under similar loading conditions. Overall, the experiments were designed to provide detailed insights into the performance of TB20 and nano blends of TB20 under various loading conditions. The results obtained from these experiments could be used to improve the design and performance of engines.

Error analysis and uncertainties

The gap between the actual value and the observed value is known as an error. Instruments, human observation, and environmentally influenced factors are the major causes of errors. Analyzing errors is crucial for accurate test results. Uncertainty analysis is particularly beneficial for complex experiments because they can discover the most prevalent mistakes and, from there, recommend a suitable error correction to give test data that is more dependable. Using the same tests over and over again may help cut down on errors. Each performance variable error is estimated, and the test apparatus’s overall uncertainty is appraised in Table 6 below.

Table 6.

Accuracy and uncertainty of instruments.

Instrument	Parameter	Range	Resolution	Uncertainty
AVL437C smoke meter	Smoke	0-100%	0.1%	± 0.15%
AVL Digas-444 portable five-gas analyzer	CO	0–15% vol	0.01% vol	± 1.5%
	CO2	0–20% vol	0.01% vol	± 0.15%
	HC	0-30000 ppm	10 ppm	± 0.15%
	O2	0–25% vol	0.01% Vol	± 0.5%
	NOX	0-5000 ppm	10 ppm	± 1%
AVL Angle encoder	Crank angle	0-7200	± 1%	± 0.15%
AVL Pressure transducer	Pressure	0-100 bar	± 0.1 bar	± 1%
Data acquisition system	Combustion characteristics	16 bit	± 0.01 bit	± 0.01%

To evaluate the overall uncertainty of the variables, Eq. (1) was utilized.

1

Where U_Y = overall uncertainty, Y = uncertainty function; u = dimensions factor; u₁, u₂, u₃, and u_n = individual variables:

The experimental test’s overall level of uncertainty is calculated by

Results and discussion

This section discusses a research study that examines a diesel engine’s performance, combustion characteristics, and emissions. The study analyses the impact of using nano additive blends with compression ratios of 16, 17.5, and 19 on different engine parameters, including in-cylinder pressure, heat release rate, brake thermal efficiency (BTE), specific fuel consumption (SFC), NOx, HC, CO, and smoke emissions. The engine was operated at a consistent speed of 1500 rpm, injection timing (IT) of 23⁰ bTDC (before top dead center), and five varying loads. Table 6 provides a summary of the factors that were considered and utilized in this investigation.

Performance characteristics

Brake thermal efficiency

Figure 6 illustrates the variation of brake thermal efficiency (BTE) with engine load for diesel, TB20, and Fe₃O₄ nanoparticle-blended fuels. For all test fuels, BTE increases with increasing engine load due to improved combustion efficiency and a reduction in the proportion of heat losses relative to useful work output. At full load, TB20 exhibits a 6.62% lower BTE than diesel, primarily due to its lower calorific value, which results in reduced energy release.

Fig. 6.

Variation of BTE against engine load for various rates of EGR.

The addition of Fe₃O₄ nanoparticles enhances the BTE of TB20 blends. The maximum BTE of 31.04% observed for TB20N100 represents a 2.61% improvement over TB20. This improvement is attributed to the catalytic activity of nanoparticles, which accelerates oxidation reactions, along with their high surface area and superior thermal conductivity, promoting rapid heat transfer and more complete combustion. However the application of exhaust gas recirculation (EGR) results in a reduction in BTE for all fuels. At full load, BTE reductions of 0.38%, 0.187%, and 0.279% were observed with 10% EGR, and 4.57%, 3.79%, and 3.25% with 20% EGR for TB20, TB20N50, and TB20N100, respectively, compared to non-EGR conditions. This decline is mainly due to oxygen dilution and reduced combustion temperature, which slow down the combustion process. These trends are consistent with reported findings for biodiesel-fueled engines under varying EGR conditions^27,28.

Specific fuel consumption

Figure 7 shows the variation of specific fuel consumption (SFC) with engine load for diesel, TB20, and Fe₃O₄ nanoparticle-blended fuels. For all fuels, SFC decreases with increasing load due to improved combustion efficiency and higher brake power output. At full load, TB20 exhibits 9.03% higher SFC than diesel, mainly due to its lower calorific value, which necessitates a higher fuel flow rate to produce equivalent power output. Furthermore, poorer atomization characteristics of biodiesel also contribute to inefficient fuel utilization.

Fig. 7.

Variation of SFC against engine load for various rates of EGR.

The incorporation of Fe₃O₄ nanoparticles reduces SFC by 2.47% and 3.34% for TB20N50 and TB20N100 respectively. This improvement is attributed to enhanced combustion efficiency resulting from catalytic activity and better fuel–air interaction facilitated by nanoparticles. However the application of EGR increases SFC for all fuels. At full load with 10% EGR, the SFC values for TB20, TB20N50, and TB20N100 are 0.2885, 0.281, and 0.2789 kg/kWh, respectively, while at 20% EGR, they increase to 0.3006, 0.2911, and 0.2872 kg/kWh. This increase is primarily due to oxygen dilution and incomplete combustion caused by the recirculated exhaust gases. These observations are consistent with the findings reported by Miya et al.²⁴.

Emission characteristics

CO emissions

Figure 8 presents the variation in CO emissions with engine load. CO emissions are mainly formed due to incomplete combustion under fuel-rich or oxygen-deficient conditions. At full load, TB20 produced higher CO emissions than diesel owing to poorer atomization and the formation of locally rich fuel zones, which hindered the complete oxidation of carbon.

Fig. 8.

Variation of CO against engine load for various rates of EGR.

The addition of Fe₃O₄ nanoparticles significantly reduced the CO emissions by 18.40% and 18.82% for TB20N50 and TB20N100, respectively. This reduction is attributed to the catalytic oxidation effect of the nanoparticles, which enhances the conversion of CO into CO₂, along with improved fuel–air mixing that minimizes rich combustion regions. The application of EGR increased the CO emissions for all the fuels. The CO emission increases by 19.3%, 34.2%, and 34.7% with 10% EGR, and 29.7%, 41.3%, and 41.2% with 20% EGR for TB20, TB20N50, and TB20N100, respectively compared with non-EGR,. This increase was due to oxygen dilution and lower combustion temperatures, which led to incomplete oxidation. These trends are in good agreement with the findings reported by Praveen et al.²¹.

HC emissions

Figure 9 shows the variation of HC emissions for all test fuels. At full load, TB20 exhibits a 10% reduction in HC emissions compared to diesel, primarily due to its inherent oxygen content, which enhances the oxidation of unburnt hydrocarbons. Additionally, the higher cetane number of biodiesel leads to shorter ignition delay, reducing fuel accumulation and improving combustion completeness.

Fig. 9.

Variation of HC against engine load for various rates of EGR.

The addition of Fe₃O₄ nanoparticles results in a slight increase in HC emissions (3–6%). This may be attributed to localized quenching effects and possible nanoparticle agglomeration, which can create non-uniform combustion regions and marginally increase unburnt hydrocarbons. The application of EGR further increases HC emissions. Compared to zero EGR, HC emissions increase by 2% and 11% with 10% EGR, and by 13% and 33% with 20% EGR for TB20N50 and TB20N100, respectively. This increase is mainly due to oxygen dilution and reduced combustion temperature, leading to incomplete oxidation of hydrocarbons. These trends are consistent with the findings reported by Saravanan et al.²⁵.

NOx emissions

Figure 10 illustrates the variation of NOₓ emissions with engine load. NOₓ formation is strongly dependent on combustion temperature and oxygen availability. At full load, TB20 produces approximately 5.1% higher NOₓ emissions than diesel due to its oxygenated nature, which promotes higher in-cylinder temperatures and enhances thermal NOₓ formation.

Fig. 10.

Variation of NOx against engine load for various rates of EGR.

The addition of Fe₃O₄ nanoparticles reduces NOₓ emissions by 4.54% and 4.71% for TB20N50 and TB20N100, respectively. This reduction is attributed to improved heat distribution and controlled combustion, which help in reducing localized peak temperatures. The application of EGR significantly reduces NOₓ emissions. Compared to non-EGR conditions, NOₓ decreases by 25.5%, 31.9%, and 32.1% with 10% EGR, and by 35.8%, 43.8%, and 56.3% with 20% EGR for TB20, TB20N50, and TB20N100, respectively. This reduction is mainly due to oxygen dilution and lower combustion temperatures. These trends are consistent with the findings reported by Ramakrishna et al.²⁹.

Smoke opacity

Figure 11 shows the variation of smoke opacity with engine load. Smoke formation is associated with incomplete combustion and the presence of fuel-rich zones. At full load, TB20 exhibits higher smoke opacity (69.3%), which is 8.94% greater than diesel due to inferior atomization and richer fuel mixtures.

Fig. 11.

Variation of smoke opacity against engine load for various rates of EGR.

The addition of Fe₃O₄ nanoparticles reduces smoke opacity by 2.21% and 2.97% for TB20N50 and TB20N100, respectively. This reduction is attributed to improved oxygen availability and enhanced combustion efficiency promoted by nanoparticles. The application of EGR increases smoke emissions. Compared to zero EGR, smoke opacity increases by 0.8%, 0.7%, and 1.1% with 10% EGR, and by 6.4%, 7.3%, and 6.6% with 20% EGR for TB20, TB20N50, and TB20N100, respectively. This increase is due to charge dilution and reduced air–fuel mixing quality, leading to incomplete combustion. These trends are consistent with the findings reported by Dinesha et al.³⁰.

Combustion characteristics

Cylinder pressure

Figure 12 illustrates the variation of in-cylinder pressure with crank angle at full load. Cylinder pressure is a direct indicator of combustion intensity and heat release. Diesel exhibits the highest peak cylinder pressure of 69.95 bar, which is 3.68% higher than TB20 due to its better combustion characteristics and higher calorific value.

Fig. 12.

Variation of cylinder pressure against engine crank angle for various rates of EGR.

The addition of Fe₃O₄ nanoparticles increases the peak cylinder pressure to 67.47 bar and 68.19 bar for TB20N50 and TB20N100, respectively. This increase is attributed to enhanced combustion rates and improved oxidation facilitated by nanoparticles. The application of EGR reduces peak cylinder pressure due to charge dilution and reduced combustion intensity. Pressure decreases by 1.01% and 1.92% with 10% EGR, and by 1.35% and 1.49% with 20% EGR for TB20N50 and TB20N100, respectively. These trends are consistent with reductions in cylinder pressure reported by Praveena et al.³¹ for grapeseed biodiesel nano-emulsion fuels.

Heat release rate

Figure 13 shows the variation of heat release rate (HRR) with crank angle at full load. HRR reflects the rate of energy release during combustion. Diesel exhibits the highest peak HRR of 43.3 J/°CA, which is 9.26% higher than TB20 due to its shorter ignition delay and rapid combustion.

Fig. 13.

Variation of Heat release rate against engine crank angle for various rates of EGR.

The addition of Fe₃O₄ nanoparticles improves HRR by 3.10% and 5.21% for TB20N50 and TB20N100, respectively. This enhancement is attributed to reduced ignition delay and accelerated combustion reactions due to the catalytic activity and high surface area of nanoparticles. The application of EGR results in a slight reduction in HRR. At full load, HRR decreases to 40.43 and 41.21 J/°CA with 10% EGR, and to 40.12 and 40.97 J/°CA with 20% EGR for TB20N50 and TB20N100, respectively. This reduction is due to oxygen dilution and the presence of inert gases, which slow down the combustion process. These trends are consistent with observations reported for TiO₂ nanoadditive biodiesel fuels under EGR conditions³².

Conclusion

This study evaluated the influence of Fe₃O₄ nanoparticle addition and varying EGR rates on the performance, combustion, and emission characteristics of a single-cylinder diesel engine fueled with tamarind biodiesel (TB20). The main observations are:

• The addition of Fe₃O₄ nanoparticles in TB20 showed modest improvements in brake thermal efficiency (2.23% for TB20N50 and 2.61% for TB20N100) and reductions in specific fuel consumption (2.47% and 3.34%, respectively) under full load. The application of 10% and 20% EGR slightly reduced these gains, with performance in some metrics remaining comparable to baseline TB20.

• Cylinder pressure and heat release rate showed small improvements with nanoparticle addition, but EGR slightly diminished these effects. Peak cylinder pressures at 0%, 10%, and 20% EGR were 68.19, 66.91, and 67.19 bar, with corresponding HRR values of 41.34, 41.21, and 40.97 J/°CA.

• CO and smoke emissions generally decreased with Fe₃O₄ nanoblends, while HC emissions showed slight increases. NOx emissions were reduced by 31–56% with nanoparticle addition, and EGR further lowered NOx levels.

• Lower EGR rates provided a better compromise between performance retention and NOx reduction.

Overall, the study demonstrates that Fe₃O₄ nanoparticle-enhanced TB20 can improve engine performance and reduce emissions under the tested conditions, with the magnitude of benefits dependent on nanoparticle concentration, EGR rate, and engine load. These findings highlight the potential of Fe₃O₄ nanoadditives for cleaner and more efficient biodiesel combustion, without overgeneralizing their effectiveness.

Future scope

• Explore other metal oxide nanoparticles (e.g., cerium oxide, aluminum oxide) to further optimize performance and emissions.

• Study long-term engine durability and wear characteristics using nanoparticle-enhanced biodiesel blends.

• Optimize nanoparticle concentrations and EGR rates using advanced computational tools or machine learning models.

• Evaluate the economic and environmental feasibility of large-scale application of biodiesel nanoblends in diesel engines.

Cost-effectiveness

The Table 7 presents a structured and quantitative evaluation of cost-effectiveness by integrating input costs, performance improvements, and economic returns. The addition of 100 ppm Fe₃O₄ nanoparticles introduces a modest incremental cost of ₹0.50 per liter, while the experimentally observed 2.61% improvement in brake thermal efficiency results in a fuel saving of 0.026 L per liter.

Table 7.

Engine parameters.

Factors Considered	Parameters Employed
Engine	Variable Compression Ratio (VCR)
Injection Pressure (IP)	220 bars
Injection timing (IT)	230 bTDC
Speed	1500 rpm (Constant)
Compression Ratio (CR)	17.5
EGR Dosages	0%, 10%, 20%
Fuel	Diesel, TB20, TB20N50, TB20N100

This efficiency gain translates into a gross saving of ₹2.60 per liter, yielding a net benefit of ₹2.10 per liter after accounting for nanoparticle and processing costs. When scaled to 1000 L, the net saving reaches ₹2,100, demonstrating practical economic viability. The benefit–cost ratio greater than 4 confirms that the performance advantages significantly outweigh the additional investment, providing a robust and analytically supported justification for the use of Fe₃O₄ nanoparticles in biodiesel blends.

Table 8.

Detailed Cost–Effectiveness Analysis of 100 ppm Fe₃O₄ Nanoparticles in TB20 Blend.

Category	Parameter	Value	Unit	Remarks
Input (Cost)	Nanoparticle dosage	100 ppm	(0.1 g/L)	Optimized concentration
	Fe₃O₄ market price	4,200	₹/kg	Commercial cost
	Material cost	0.42	₹/L	Based on 0.1 g/L
	Processing cost	0.08	₹/L	Ultrasonication
	Total incremental cost	0.50	₹/L	Combined cost
Performance	BTE improvement	2.61	%	Experimental result
	Fuel saving	0.026	L/L	Due to efficiency gain
Economic Output	Diesel price	100	₹/L	Reference value
	Gross saving	2.60	₹/L	From fuel reduction
	Net saving	2.10	₹/L	After cost deduction
	Net saving (1000 L)	2,100	₹	Scaled benefit
	Benefit–cost ratio	~ 4.2	–	Economic feasibility

Abbreviations

EGR

Exhaust gas recirculation BTE: Brake-Thermal efficiency

SFC

Specific-Fuel Consumption CeO₂:Cerium Oxide

ZnO

Zinc Oxide CO: Carbon Monoxide

HC

Hydrocarbons NOx: Nitrogen oxide

TB20

20% Tamarind biodiesel

TB20N50

20% Tamarind biodiesel + 50ppm Fe₃O₄

TB20N100

20% Tamarind biodiesel + 100ppm Fe₃O₄

Author contributions

M. Srinivasarao – Conceptualization of the study, experimental design, and data acquisition. Ch. Srinivasarao – Fuel preparation, addition of Fe₃O₄ nanoparticles, and analysis of fuel properties. A. Swarna Kumari – Literature review, methodology refinement, and data validation. Joga Rao Bikkavolu – Engine testing supervision, performance analysis, and interpretation of combustion results. Gandhi Pullagura – Emission measurement, analysis, and graphical presentation of results. Debabrata Barik – Drafting, critical revision of the manuscript, and highlighting innovation and application aspects. Ayyar Dinesh – Support in nanoparticle characterization and preparation of visual illustrations. Saravanan Rajendran – Statistical analysis, interpretation of emission trends, and strengthening of discussion. N. Ashok – Overall supervision, coordination among authors, and final approval of the manuscript.

Funding

This research received no external funding.

Data availability

The data supporting the findings of this study are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Informed Consent

Not applicable. No human subjects were involved in this study.

Consent to Participate

Not applicable. The study did not involve any human or animal subjects requiring ethical approval.

Consent to Publish

Not applicable. No identifiable data from individuals was used in this study.

Institutional Review Board Statement

Not applicable. The study did not involve humans or animals.