Synthesis, stability, and emission analysis of magnetite nanoparticle-based biofuels

In recent years, the application of nanoadditives in biofuels is gaining much attention due to their increase in thermophysical properties such as high surface area, thermal conductivity, and mass diffusivity. However, lack of stability, high additive cost, and difficult recovery from engine exhaust are the high-priority and demanding characteristics, which may be chosen by many researchers. In this regard, the most promising nanoadditives are magnetite nanoparticles, having a high-specific area, strong magnetic response, control over the particle size and, most importantly, easy and rapid separation from exhaust gas by applying external magnetic bars. Moreover, it can be easily diluted into biodiesel, and thus, it can collect the advantages of biodiesel in water emulsion. From the literature survey, it is found that there is a lacuna in the synthesis and performance of magnetite nanofuels for internal combustion engine applications. Thus, the present study aims to epitomize the research findings related to the synthesis, characterization, stability, and properties of biodiesel/diesel-based fuels blended with magnetite nanoparticles and the influence of the magnetite nanofuels on engine performance. The study shows that the addition of nanoparticles to biodiesel has positive effects in reducing harmful emissions such as carbon black, smoke opacity and NOX, with improved thermal efficiency and fuel consumption.

Several researchers [8][9][10] have attempted to improve the fuel properties by introducing various nanoadditives into the fuel mixture. These nanoadditives have been shown to be helpful in improving combustion efficiency and reducing emission levels. Metaladded substances such as iron, copper, aluminum, silver, titanium, cerium, zinc, etc., have been analyzed completely for combustion behavior, engine performance, and emission characteristics. Most of the investigations have a general consensus that nanosized metal additives improve the catalytic activity during combustion, promote complete and efficient combustion, and ultimately lead to good overall engine performance. Despite the various advantages of incorporating nanoparticles into liquids, a major issue is the stability analysis of nanofluids and the arrival of nanoparticles into the environment. Some of the examinations have shown that settlement of nanoparticles decreases the fuel properties, which in turn decreases the engine characteristics and also increases the possibility of a clogged fuel injection system. To reduce the settling of nanoparticles in the base fuel, several techniques such as addition of surfactant, appropriate stirring, and proper selection of nanoparticles should be done [11][12][13][14].
The idea of adding magnetite nanoparticles is that it exhibits a fluid-magnetic coupling behavior where the fluid's position can be controlled by an applied magnetic field and also discharges extra heat over the span of the ignition interaction. Moreover, it can easily be diluted into biodiesel, and thus, it can collect the advantages of water-biodiesel emulsions. Therefore, it is well known that the researchers took advantage of utilizing magnetite nanoparticles in diesel and biodiesel mixtures and also the feature of gathering particles from the exhaust gas. To date, numerous review papers on the synthesis, characterization, and application of magnetite nanoparticles in various fields have been published. However, up to now, the published reviews do not highlight the details of reaction parameters in the synthesis and usage of nanoadditives with various fuels in IC engine applications. Most of the reviews only discuss the synthesis of nanoparticles in general without focusing specifically on the reaction parameters of magnetite nanoparticles. Hence, in this review, we will focus on the mechanism, process, and reaction parameters of two chemical synthetic methods, namely co-precipitation and solgel synthesis, and also the characterization and performance of magnetite nanoparticles blended biodiesel for I.C. engine applications.

Global magnetite nanoparticle market by applications
In recent times, the global market of magnetite nanoparticles has increased due to more usage of nanoparticles in various sectors such as electronics, bio-medical [15][16][17][18][19], energy, and waste treatment [20][21][22], which were shown in Figs. 1 and 2. According to a Marker Research Future Reports (MRFR) study, this market will grow at a CAGR of 10.8% by the end of 2026. At room temperature, these nanoparticles have super magnetic characteristics and can be controlled with a magnetic field. Because of the predicted growth in affordable healthcare plans, increased life expectancy, and an increase in the number of life-threatening chronic disorders, these nanoparticles have sparked significant interest in the biomedical application industry. Scientists are looking at ways to employ magnetite nanoparticles as a drug delivery agent and to improve cancer therapy efficacy. Furthermore, due to the rising penetration of items such as spintronics, nanowires, and quantum dots, these magnetite nanoparticles are becoming increasingly  pumpkin and lima beans. Mengmeng et al. [24] investigated the capacity of iron oxide nanoparticles to replace traditional Fe fertilizers as a fertilizer, and their findings indicated that Fe 2 O 3 nanoparticles increased root length, plant height, and biomass values in peanut plants. Roghayyeh et al. [25] explored the yield and growth of soybean when different iron oxide doses (0, 0.25, 0.5, 0.75, and 1 g/L) are used, and the outcomes revealed that 0.75 g/L enhanced leaf + pod dry weight and pod dry weight. The maximum grain yield was obtained when 0.5 g/L nano-iron oxide was used, which resulted in a 48% increase in grain production when compared to the control.

Catalytic convertor
A catalytic converter is a device that reduces the toxicity of internal combustion engine pollutants. A catalytic converter generates an atmosphere for a chemical process that transforms dangerous combustion byproducts into less toxic compounds. Zhiqing et al. [26] observe that Fe 2 O 3 catalyst exhibits excellent activity in the reduction of nitrogen oxides, and the author also mentions ANCF catalytic that can improve engine emission and combustion characteristics, thereby reducing energy and environmental problems caused by traditional fuel combustion. Syed et al. [27] created a low-cost three-way catalytic converter coated with aluminium oxide nanoparticles as a catalyst, and the author discovered that the conversion efficiencies of Al 2 O 3 -based catalytic converters are 99.5% for CO emissions and 92% for HC emissions, respectively. Songül et al. [28] created a catalytic converter with an Al 2 O 3 /SiO 2 /TiO 2 -coated catalyst that reduced exhaust emissions significantly, with a maximum reduction of 43.05% NO X emissions at 75% load, 56.84% HC, and CO emissions at 25% load and 66.7% at 25% load.

Methods for preparing magnetite nanoparticles
Different magnetite nanoparticles can be synthesized by different techniques like coprecipitation [29][30][31][32][33], hydrothermal [34], thermal decomposition [35][36][37], and sol-gel [38,39]. These technologies have the potential to be beneficial in the preparation of various forms of Fe 3 O 4 . The co-precipitation and sol-gel methods are the most extensively utilized for the synthesis of nanoparticles. In these methods, the desired shape and size of nanopowder are obtained by considering pH, reaction temperature, reaction time, and concentration of the initial solution. The co-precipitation method is the most wellknown and widely used chemical method for producing nanoscale Fe 3 O 4 [40,41]. The process requires mixing of iron(III) and iron(II) particles in a strong basic solution at ambient temperature or high temperature at a molar ratio of 1:2 [42,43]. The molecular size and shape of the magnetite nanoparticle can be determined via changing the proportion of iron salts (like chlorides, sulfates, nitrates), reaction temperature, pH value, medium ionic strength, and other reaction parameters (for example, stirring speed, basic solution-dropping speed) [44][45][46]. This section summarizes the two most common methods for producing magnetite nanoparticles, namely co-precipitation and sol-gel techniques, as shown in Fig. 3.

Co-precipitation method
The co-precipitation technique is a strategy for the synthesis of magnetite nanoparticles which is easy to do with an achievement rate of 96 to 99.9% [47]. Iron oxide nanoparticles are prepared using this technique by adding Fe(II) and Fe(III) aqueous solutions to the base solution under anaerobic conditions at atmospheric or high temperatures [48]. However, certain factors such as salt type, reaction temperature, solution pH, Fe(II): Fe(III) proportion, and ionic strength of the mixture all have a distinct influence on the molecular size and shape of the appealing Fe 2 O 3 nanoparticles [49]. The chemical formation of magnetite nanoparticles by co-precipitation methods is shown in Eq. 1 [50].
Kumar et al. [51] utilize two solutions of ferric nitrate salt and sodium hydroxide in a 1:2 mole proportion with distilled water that are thoroughly mixed and sonicated for 60 min. The completed homogeneous solution is then calcined up to 350 o C to take out moisture and achieve crystallization. The particle's average diameter was computed and viewed as about 30 nm. Wu et al. [48] utilized ferrous sulphate hexahydrate as the starting material to create magnetite nanoparticles by co-precipitation. The molar proportion of Fe 3+ and Fe 2+ in the FeCl 3 arrangement was changed in accordance with 1  Tables 1 and 2 show a brief summary of magnetite nanoparticle synthesis using an ultrasonicator and mechanical stirrer.

Sol-gel method
One more excellent synthetic methodology for creating magnetite nanoparticles is the sol-gel strategy [81,82]. It includes changing over discrete particles (sol) into a threedimensional polymeric organization by hydrolysis of precursors into a stable colloidal arrangement (gel). The gelation cycle can be reversible (for electrostatic contacts or hydrogen bonds) or irreversible (for covalent bonds) depending on the nature of interactions between colloidal particles [83]. The gel is then dried or warmed to create the nanomaterial [84]. Equations 2 and 3 [85] show the chemical formation of magnetite nanoparticles using sol-gel methods.
Shaker et al. [86] dissolved Fe(NO 3 ) 3 and C 2 H 6 O 2 in the appropriate concentrations and agitated for 120 min at 400°C. The produced mixture was then warmed to 800°C to generate brown gel. After that, the gel was matured at room temperature for  Table 3 shows a brief summary of magnetite nanoparticle synthesis using the sol-gel method.

Characterization of magnetite nanoparticles
Once the magnetite nanoparticles were synthesized, they needed to be characterized in order to evaluate their shape, molecule size, size distribution, composition, and magnetic characteristics. In this section, we will go through the different techniques for determining the physical and chemical characteristics of these nanoparticles, which were shown in Figs. 4, 5, and 6 [85][86][87][88][89][90][91][92][93][94][95][96][97][98]105]. The scanning electron microscopy (SEM) method gives a direct assessment of nanotubes arrangement and size. Veeradate et al. [84] utilize SEM to examine the morphology of nanoparticles. The SEM images show that the majority of the nanoparticles have a circular shape with a molecule size of 100nm, which is obtained from the ablated laser energies. Besides, it was seen that the molecule size developed with an increase in laser energy. Syed et al. [106] utilize electron microscopy to investigate the morphological and surface characterization of iron oxide nanoparticles. It was noticed that spherical shaped iron oxide nanoparticles were obtained and the diameter dispersion ranged from 24 to 65nm. Nouri et al. [104] utilized transmission electron microscopy (TEM) and SEM to look at the size and form of nanoparticles, confirming that the size of the nanoparticles was 20-40nm. Shafii et al. [102] noticed the size and shape of nanoparticles utilizing TEM examination. The average molecule diameter is around 10nm, which is consistent with expectations. Kulkarni et al. [101]   utilize a coprecipitation strategy to make magnetite nanoparticles and were characterized using X-ray diffraction (XRD) examination and scanning electron microscopy. According to these two techniques, homogenous-sized magnetite nanoparticles with a spherical form may be created. The SEM of magnetite nanoparticles shows a spherical structure with a diameter of around 10-15 nm, which agrees with the XRD information. The X-ray diffraction technique is utilized to find out the oxygen content, purity, and crystalline structure [98]. Suhel et al. [87] examined X-ray patterns with a 0.05 step size at 2ɵ angle (20 -70 o ), revealing that the patterns of produced magnetite nanoparticles had a cubic structure. Moreover, magnetite nanoparticles were viewed as being of high purity and crystalline in this review. Following that, TEM is utilized to determine the shape and molecule size of magnetite nanoparticles. The two magnifications of the TEM images revealed that the molecules have a spherical shape. The author also shows the molecular size distribution histogram plotted from the diameters of 70 nanoparticles. According to the histogram, the mean crystal size is 1.81nm with a standard deviation of 1.8nm. On the other hand, Debye Scherer's formula gives an average crystal size of 18.21 nm. This difference in crystal size shows a variation in size from 11 to 20nm. P. OU et al. [99] synthesized magnetite nanoparticles using a solvothermal technique and analyzed them using SEM at 200 and 250°C for 24 h, respectively. The powder is comprised of homogeneous spherical nanoparticles with a diameter of around 12 nm on average. When the solvothermal temperature was raised to 250°C, however, significant variations in the diameter of magnetite crystallites with a size of roughly 53nm were noticed.

Preparation of magnetite nanofuels
To make a homogeneous magnetite nanofuel, techniques like ultra-sonication, pH adjustment, and magnetic stirring are performed [107]. To improve the thermal agitation of these nanoparticles, numerous ultra-sonication procedures, including probe-type and bath-type, are applied [108]. Magnetic stirring is best for lower concentrations of magnetite nanoparticles in the vessels containing fluids driven by a rotating fixed electromagnet or a couple of turning electromagnets attached underneath the mixing device. The PH modification approach depends on changing the pH value of nanofluids in order to improve the mixing dispersion. In practice, a combination of one of the aforementioned dispersion methods (i.e., ultra-sonication, magnetic stirring, and pH modification) is commonly used to provide improved efficiency and stability. Suhel et al. [87] have prepared magnetite nanofuel by mixing 20% chicken fat methyl ester (CFME) and 80% pure diesel with different dosages of 50, 100, and 150 ppm. In this experiment, the author uses the titanium horn test ultrasonicator for the suspension of magnetite nanoparticles for 45 min at 40 o C with a frequency of 20 kHz. This sonicator scatters the particles equally, which prevents agglomeration [53] create magnetic nanofuel from 98.7% algal oil methyl ester (CROME), 1% Fe 3 O 4 , and 0.3% [(CH 3 ) 3  A Highly crystalline and average particle size were 18.21nm [103]. B Highly crystalline and average particle size were 19.4nm [48]. C Average particle sizes were 2.02nm, 5.58nm, and 8.35nm [82]. D Crystalline and average particle sizes were 16nm [54]  nanoparticles are suspended by an ultrasonic agitator with continual agitation for an hour. The ferrofluid-altered base fuel was kept in a glass container with a stopper at room temperature for 1 month to assess the phase change characteristics. Syed et al. [106] also prepare the magnetic nanofuel by considering 1 l of diesel and afterward adding 0.025g and 0.05 g of iron(II, III) oxide nanoparticles to achieve dosage levels of 25 ppm and 50 ppm, individually. The mixture was stirred for 30 min in an ultrasonic shaker to make a uniform distribution. Yuvarajan et al. [109] utilized Mahua oil methyl ester (MOME), magnetite nanoparticles, and surfactant in a beaker and stirred with an ultrasonic agitator for 60 min. Nouri et al. [104] have used pure diesel with 30, 60, and 90 ppm of magnetite nanoparticles in the solution kept in an ultrasonic bath at a 40-kHz frequency and a temperature of 50°C. The following 2 days, there was some nanoparticle collection. Prior to its utilization in the engine, the fuel mix was homogenized by exposing it to ultrasonic waves. Figure 7 shows the preparation of nanofuel using an ultrasonicator.

Stability analysis of magnetite nanofuels
The dispersion of various nanoparticles in liquid fuels offers huge advantages in burning applications. Many methodologies for assessing the stability of nanofluids have been shown in Fig. 8. The simplest one is the sedimentation method [110,111], where the weight or the sediment volume of the nanoparticles in the nanofluid under external force shows the indication of the stability of the characteristic nanofluid. With respect to the stability of nanofluids, authors such as Kuo et al. [112] have assessed that the size of the molecules in the colloid was small and of less weight. This reduced the occurrence of sedimentation and the chance of the molecules settling, which made for a more stable nanofluid. Yu et al. [11] have utilized a particular instrument to notice the variance in the concentration or size of the molecule size with settling time. Nanofluids are viewed as stable when the molecule size or concentration of particles remains constant. Sediment imaging captured by an in vitro nanofluid camera is a more typical method for assessing nanofluid stability. According to Saxena et al. [113], the long-term stability of nanofuel suspension is dependent on different rules, and the most critical of which are nanoparticle size, dose, and the synthesis process. In general, nanoparticles have less weight, which minimizes sedimentation and makes the nanofuel more stable. For settling techniques, a long observation time is the default. Therefore, a centrifugation method is performed to evaluate the stability of the nanofluid. For the sedimentation method, a long observation time is the defect. Therefore, a centrifugation method was performed to evaluate the stability of the nanofluid. Singh et al. [114] synthesized the silver nanofluid by reducing AgNO 3 with polyvinyl pyrrolidone (PVP) as a stabilizing agent, and the stability was tested by a centrifugation method. In this, the produced nanofluids have been shown to be stable for a month in a fixed state, and this process undergoes centrifugation at 3000 rpm for over 10 h. Another way of evaluating the stability of nanofluids is zeta possible examination, which involves studying their electrophoretic behavior [115,116]. According to the stabilization theory [117], if the zeta potential has a large absolute value, the electrostatic repulsions between the particles increase, bringing out good suspension stability [118]. A value of zeta potential of 20 or 25 mV can be considered as the criterion value for isolating a weakly charged surface from a strongly charged surface, and an absolute value of zeta potential of 40 to 60 mV is accepted to be stable and that a value of more than 60 mV has excellent stability [119][120][121][122]. Said et al. [123] utilized a zeta-seizer to decide the size of the nanoparticles in the base fluid as well as the value of the zeta potential. Following 30 days, a nanofluid with a high zeta capacity of 41.8 mV was created. Venkatachalapathy et al. [124] have shown that the electro-kinetic characteristics of nanofluids impact their stability. To find out the stability of the nanofluid, a zeta potential test was performed utilizing the zetasizer, giving a value of +31.4 mV, confirming the nanofluid stability. The purpose of the UV-Vis spectrophotometer [125,126] and turbidity meter methods is to analyze the stability of the different nanofluid concentrations. UV spectral analysis is used to find the extinction of light passing through a sample. It is a valuable tool for identifying, characterization, and studying nanomaterials due to their sensitivity to optical properties. Turbidity is also a versatile optical method similar to UV spectral analysis, used for monitoring the growth of suspended nanoparticles. Turbidity describes the clarity of a material due to the presence of particulates. The UV-Vis spectrometer determines the absorption of liquids at wavelengths from 200 to 900nm and is used to examine different dispersions in liquids [127]. Gad et al. [128] studied the UV-Vis spectroscopy of nanoparticles dispersed in diesel fuel at a concentration of 30ppm for 30 days. The wavelength of 222nm was determined to have the highest absorption.
The UV absorption was raised by raising the wavelength until it arrived at its maximum wavelength of 222 nm, after which it decreased. A turbidity meter is also another way of assessing the stability of nanofluids. In this, a light source is used to examine the nanofuel. Over an eight-week period, Balamurugan et al. [129] observed optical density decreases to 8 Nephelo turbidity units (NTU) for all samples except nanoparticles added at a rate of 10 ppm. The value of optical density value approaches 13 NTU for nanoparticles added to a sample of 10 ppm because equilibrium is formed between the surfactant monomers of the particle and the solute, resulting in effective stabilization. Seela et al. [130] study the suspension lifetime of nanosized multiwall carbon nanotubes and aluminium oxide in Jatropha methyl ester blends B20, B50, and B70. The nanofuel stability rate was determined through turbidity investigation. In more than an 18-day time frame, the rate of stability for multiwall carbon nanotubes and aluminium oxide was found to be 83.3 and 87.03%, respectively. With the 100 ppm concentration of multi-walled carbon nanotubes (MWCNT) and Al 2 O 3 nanobiodiesel mixes, there was a significant decline in suspension.

Techniques for improving the stability of nanofluids
Various researchers have performed different investigations to increase the stability of nanofluids, such as ultrasonic oscillators [131,132], addition of surfactants [133,134], and pH control [135,136] to accomplish optimal blending of the nanoparticles in the liquid fuel [137]. For example, there are three strategies of fluid dispersion technology; for example, medium control, mechanical control, and agent control. For molecule dispersion, the mechanical control utilizes the ultrasonic vibrator, disintegrator, and electromagnetic agitators [138].

Ultrasonic vibration
An ultrasonicator is a well-known physical methodology for dispersing agglomerated nanoparticles in the base fluid. Many examinations utilize a probe and a bath sonicator to equally scatter the nanoparticles [139][140][141]. Typically, ultrasonication is performed at different powers and frequencies for varying lengths of time. The duration required for sonication might be impacted by the molecule size and shape, the mixing proportion of nanoparticles, and the production process [11].
Hong et al. [142] show that longer sonication times can improve the stability of nanofluids. It was also revealed that increasing the sonication period helps to prevent the agglomeration of the particles. Amrollahi et al. [143] also reported similar examinations that showed longer sonication periods enhanced nanoparticle stability. Chung et al. [144] use the bath and horn sonicator for the dispersion of ZnO nanoparticles in water, in which the horn type of sonicator is more effective in terms of quicker reduction rates and greater settling rates. El-Seesy et al. [145] use an ultrasonic bath for 90 min to disperse SiO 2 nanoparticles in distilled water. The silica nanofluids remained stable for 72 h without any visible settling [132]. Longo et al. [146] have used aluminium oxide and titanium oxide nanoparticles for the nanofluid preparation. The nanoparticles were mechanically stirred in the first phase and afterward sonicated at 25 kHz. The author's observer that sonication improved the efficiency of dispersion compared with mechanical stirring and that the two nanofluids were shown to be stable for a period of 1 month. Parametthanuwat et al. [147] used an ultrasonic bath at a frequency of 43 kHz for 3 h to make Ag nanofluids. The author observed that the nanoparticles were stable for as long as 48 h. Therefore, the ultrasonicator is a fundamental methodology since its objective is to decompose clusters by utilizing the sound energy of a sonicator at different frequency levels and durations for nanofluids. In addition, the molecule size and shape, mixing ratio, concentration, particle preparation process, and base fluid type all impact the best sonication conditions [148].
Vivek et al. [155] have utilized cetyltrimethyl ammonium bromide surfactant for the preparation of Al 2 O 3 nanoparticles blended nanofuel, and the fuel was tested on a compression ignition engine. By the use of CTAB surfactant, the nanoblended fuel organizes and controls particles and prevents deposition. Zhai et al. [156] used sodium dodecyl sulphate as a surfactant to improve the Al 2 O 3 +H 2 O nanofuel stability. Tiwari et al. [157] utilized a surfactant called cetyltrimethyl ammonium bromide for stability improvement and scattering of nanofluids without changing their thermo-physical characteristics. Xia et al. [153] investigated the two surfactants, namely sodium dodecyl sulphate and PVP, in Al 2 O 3 added nanofuel for dispersibility and stability. In this examination, PVP demonstrated preferred dispersibility by enhancing stability over sodium dodecyl sulphate at surfactant concentrations of 0.5, 1, and 2%. One reason for nonionic PVP's superior performance, according to the author, is its relatively long alkyl chain. Kakati et al. [158] used 0.03% sodium dodecyl sulphate to create an Al 2 O 3 + water nanofluid with a volume centralization of 0.1 to 0.8% at temperatures ranging from 10 to 50 o C. Accordingly, the nanofluid is stable for 4-5 days, though without surfactant nanoparticles, it sediments after 1 h of production.

PH control (surface chemical effect)
One more technique for making the nanofluid more stable is to change the pH of the solution. Wen [159] used an acid treatment on carbon nanotubes (CNT) nanofluid and showed that the suspension was moderately stable. Graphene nanoplatelets (GNP) nanoparticles were acid-treated by suspending them in a 1:3 mixture of HNO 3 and H 2 SO 4 [160]. After that, the mixture is washed with DI water and dried in an oven. Finally, the GNP solution is mixed with the Ag(NO 3 )OH solution, which has a shelf life of 60 days. Lee et al. [127] have investigated Al 2 O 3 nanoparticle added nanofluids with various pH levels. From the test information shows that at pH 1.7 nanofluids, the agglomerated molecule size drops by 18% and at pH 7.66, the agglomerated molecule size increases by 51% due to a decrease in electric repulsion force. When alumina is lowered into water, ions of the hydroxyl group form on the surface. The pH allows the necessary processes to take place. The hydroxyl groups react with the positively charged H + in water at lower pH values. Most significantly, keeping the pH of the solution in the optimal range is critical since maintaining acidic and basic solutions is unsafe for personnel and the workplace. Moreover, applying these kinds of soluble and acidic solutions in industries might cause surface corrosion [161]. Many investigations used sonication and dispersant addition to achieve stability, while others utilized surface functionalization and pH modification techniques. However, the majority of research focuses on short-term stability, which can be challenging when applied in practical applications and also in commercialization. Subsequently, more research is expected to evaluate the impacts of different factors on long-term stability.

Effect of magnetite nanoparticles on improved fuel properties
One of the important aspects of determining the fuel quality and engine performance is the fuel characteristics. Recently, the addition of nanoparticles has been considered to be a beneficial strategy for improving fuel characteristics. Several studies [155,[162][163][164][165][166][167][168][169][170][171][172] investigated fuel characteristics by incorporating various types of nanoparticles into various diesel/biodiesel blended fuels. In addition, the quality of the blended fuel and engine performance was evaluated by examining their effects on several characteristics such as density, flash point, kinematic viscosity, caloric value, and cetane number. According to Mukul et al. [173], increasing the dosing of nanoadditives causes increases in the calorific value of the fuel. During combustion processes, the nanoparticles operate as catalysts and have a good influence on ignition qualities. Furthermore, these nanoparticles have a high specific surface area and have superior heat transport capabilities, thereby having a higher calorific value. The particles enhance momentum density, which increases fuel injection velocity into the combustion chamber and hence improves engine performance. Table 4 shows the fuel characteristics of a blended diesel and biodiesel with magnetite nanoparticles in different concentrations.
Shiva Kumar et al. [51] have assessed the impact of Pongamia methyl ester blends (B10, B20, and B30) with 0.5, 1%, and 1.5% ferrofluid on the fuel properties, engine performance, and emission characteristics of a diesel engine. The results showed that the density, kinematic viscosity, and cetane number increased slightly, but there was no significant change in flash point and calorific value. Yuvarajan et al. [177] examined that adding 1.3% magnetite to the rice bran oil methyl ester results in a 3.15% increase in kinematic viscosity and a 2°C rise in flash point and water content by 0.02% increase. Other properties, such as density and calorific value, improve by 2.96% and 2.61%, respectively.  Venkata et al. [174] also examine the fuel properties by adding 1% of ferrofluid to rice bran oil. This increases kinematic viscosity by 3.56%, flash point by 3 o C, water content by 0.02%, and other properties like density, calorific value, and cetane index are increased by 5.45%, 2.60%, and 2.97% individually. This is due to its huge surface area. Ferrofluid has highly significant catalytic activity, which prompts improvements in these characteristics. Karthikeyan et al. [53] found that the addition of ferrofluid changed all of the characteristics. When magnetite is added to the Caulerpa Racemosa Oil, methyl ester properties like density, flash point, kinematic viscosity, calorific value, and cetane index are improved, which also increases the water content by 0.02%. The water content increases somewhat since ferrofluid contain a significant amount of water. Vinoothan et al. [176] have examined the improvement of fuel characteristics of B20 blend biodiesel from waste cooking oil with the concentrations of 25, 50, 75, and 100 ppm magnetite nanoparticles. The outcomes reveal that when the concentration of nanoadditive increases, fuel properties like calorific value, thickness, and viscosity are increased. According to the adding of magnetite nanoparticles to diesel raised the viscosity, calorific value, flash point, and cetane number. Dinesh et al. [109] investigated the addition of 1% magnetite to Mahua oil methyl ester and noticed a significant decrease in viscosity, as well as a rise in cetane number, calorific value, and water content by 0.05%. This is because of the presence of water in ferrofluid. The purpose of adding magnetite nanoparticles to diesel/biodiesel blends is to improve engine performance by increasing fuel characteristics such as density, flash point, calorific value, and cetane number.

Effect of magnetite nanoparticles on engine performance
Based on the findings of this study, several experiments show that adding magnetite nanoparticles to diesel or biodiesel fuel improves the thermo-physical properties [181][182][183], thereby increasing the calorific value and activating the atomization of droplets of the test fuel, which results in shorter ignition delay time and complete combustion [184]. Due to the ignition delay being shorter, more fuel is consumed in the pre-mixed combustion stage during the early-on part of the expansion stroke, resulting in less brake-specific fuel consumption (BSFC) [185]. Another aspect that may play a role is the high concentration of oxygen in additive mixes, which promotes better combustion and reduced fuel consumption [182][183][184][185][186]. Vinoothan et al. [176] noticed that the presence of magnetite nanoparticles in the fuel blend improves break thermal efficiency (BTE). At full load, the 100ppm B20 blend had the highest brake thermal efficiency (BTE) (i.e., 1.49% higher) and the lowest BSFC. Finally, it was concluded that 100ppm magnetite nanoparticles in the B20 blend of waste cooking oil methyl ester is the optimum blend for improving CI engine performance. Shafii et al. [102] examined the impacts of 0.4 and 0.8% concentrations of water-based ferrofluid on diesel fuel in a 2200 rpm operated 4-stroke diesel engine. According to the findings, 0.4% ferrofluid lowers the BSFC by 3.23-6.45% while increasing it by 3.33-6.89%, BTE, whereas 0.8% ferrofluid lowers the BSFC by 5.06-10.85% while increasing it by 5.33-12.17%. Yuvarajan et al. [121] investigate the effects of adding magnetite to rice bran oil methyl esters in a constant speed diesel engine. From the results, it was shown that performance was improved by 4.27% and decreased by 5.17% BSFC. This is due to increased oxygen accessibility, which improves air-fuel mixing during combustion. Ferro fluid additionally has positive combustion properties, such as high temperatures of combustion and rapid energy release rates. Sayed et al. [106] use an air-cooled four-stroke diesel engine with a 300 bar injection pressure. The outcomes reveal that the 25ppm nanoblended fuel gives almost the same BSFC as diesel fuel, while 50 ppm fuel shows a significant decline of roughly 9% when compared to different samples. But in the case of brake thermal efficiency, 50ppm gives around 2% increment. This is because of the nanoparticles' large surface area and significant chemical reactivity, which lead to an improvement in the ignition effectiveness of the nanoparticle blended diesel fuel.
Shiva Kumar et al. [51] have examined the B20 blend of pongamia methyl ester with ferrofluid with different proportions. From the outcomes, it was shown that a higher value of BSFC and a lower value of BTE were found for biodiesel-diesel blend when compared with diesel due to viscosity and lower calorific value. When adding ferrofluid to the B20 blend, there is an improvement in performance, and the blend with 1% additive gives the least BSFC and maximum BTE. Dinesh et al. [109] used 14-nm size magnetite nanoparticles in Mahua Oil Methyl Ester, which showed an increase in BTE of about 5.27% and a decrease in BSFC of about 5.37%. This is due to the fact that nanosized ferro-additives have a shorter igniting delay [180]. Sarath et al. [187] examined the impacts of 4%, 8%, and 12% ferrofluid on diesel and showed that there were decreases in BSFC by 6.06-9.09%, 8.00-15.15%, and 10.52-18.18%. Similarly, BTE rises by 6.21-9.10%, 9.6-16.69%, and 11.07-21.43%, respectively. Based on the observations, it was concluded that ferrofluid added to diesel fuel has a significant effect on performance. Ranaware et al. [188] examined the cerium oxide and ferrofluid nanoadditives to the diesel on a 4 stroke DI variable compression ratio engine as an experiment. From the results, it was shown that ferrofluid added to diesel fuel increases up to 12% BTE and decreases up to 11% BSFC relative to diesel fuel. Ameer et al. [175] explore the impacts of magnetite nanoparticles in chicken fat biodiesel blend with hydrogen induction on the DICI engine. From the investigational results, 30% chicken fat biodiesel blend (CFB30) blend added magnetite nanoparticle shows a 7.49% improvement in BTE and a 22.86% decrease in BSFC compared with the CFB30 blend. When hydrogen injection of 10 lpm was introduced, there was a 5.08% increase in BTE and a 14.17% decrease in BSFC. Table 5 summarizes the engine performance with various concentrations of magnetite nanoparticles in the diesel/biodiesel blend.

Cylinder pressure
Combustion characteristics are important features that reflect the engine's combustion efficiency and have a significant impact on engine performance and emissions. To evaluate this, two key parameters are typically used, i.e., heat release rate (HRR) and pressure rise rate (PRR). Figure 9 illustrates the effect of nano-added diesel or biodiesel on the timing of P max inside the combustion chamber. Figure 9A [106] shows that the pressure starts increasing significantly from 7° before TDC for 50ppm nanoblended diesel and has a peak pressure of 73.70 bars. This is due to the high surface areas of nanoparticles, which increase their chemical reactivity, which in turn reduces the ignition delay. Figure 9B [191] shows the impact of Al 2 O 3 and Fe 3 O 4 nanoparticles on in-cylinder pressure, in which both the nanoblends have increased cylinder pressure compared with the biodiesel blend. Figure 9C [ 180] shows the variation of cylinder gas pressure when the engine was fueled with 20% pongamia methyl ester with 50,100 ppm nanoparticles and shows that the iron oxide nanoparticles added blends have a higher peak pressure. Figure 9D [109] shows the pressure variation when nanoblends with surfactant are used. It shows that the lower peak pressure of the nanoblend is due to the viscosity of the nanoblend being higher, causing atomization and mixing of fuel with air to be uneven, resulting in longer breakup length, lower dispersion rate, and increased ignition delay. Figure 9E, F [100] shows that pressure increases significantly from 7° before TDC for 40 ppm iron oxide nanoblend and 8° before TDC for 40 ppm aluminium oxide nanoblend fuel. Further, there are no major changes in the dosage level of nanoparticles. Figure 9G, H [104] examines the rate of pressure rise at 1800 rpm under 50% and full engine load, revealing increases of 0.7%, 1.2%, and 3.1% at 50% engine load, and 1.7%, 2.8%, and 3.6% at full load for diesel+ 30ppm Al 2 O 3 + Fe 2 O 3 nanoblend, 90ppm Al 2 O 3 blend, and finally, 90ppm Fe. Figure 10 depicts the fluctuation of heat release rate with crank angle of different nanoparticle doses. The heat release rate was found to be typically enhanced with the inclusion of nanoparticles, as shown in Fig. 10A [106] According to the author's findings, the greatest amount of heat released rate for 50 ppm nanoblend was 168.468 kJ/m 3 c owing to the fuel's faster combustion and shorter ignition latency. The degree of fuel-air mixing and uniform burning may have improved as a result of a shorter ignition delay. The addition of Al 2 O 3 and Fe 3 O 4 nanoparticles to biodiesel increased the rate of heat release, as shown in Fig. 10B [191]. From the author's finding, the maximum amount of heat release rate for 80ppm Fe 3 O 4 nanoblend is 114.82 kJ/m 3 c and for 120ppm there is a slight decrease, but when compared with the biodiesel blend, the HRR for all the nanoblends is increased. Figure 10C [180] shows the variation of HRR when the engine was fueled with 50,100 ppm of 20% pongamia methyl ester, which shows that the heat release rate was observed for iron oxide nanoparticles added blends. Figure 10D, E [100] depicts the 20% biodiesel blend with 40ppm and 80ppm nanoparticles, which shows that the HRR is 124.747, 132.828, and 137.938 kJ/m 3 c for the 20% biodiesel blend, 40ppm Fe 2 O 3 blend, and 40ppm Al 2 O 3 blend, and finally, it was stated that increasing the nanoparticle dosage level tends to enhance the heat release rate. Figure 10F [109] depicts the change in HRR    the latter part for MOME and nanoblend in comparison to diesel. Figure 10G, H [104] shows the maximum heat release rate of the engine increased by 3.4%, 9.4%, and 15.2% at full load. Furthermore, the maximum HRR for 50% load was 6.8%, 11%, and 13.9% for diesel+, 30ppm Al 2 O 3 + Fe 2 O 3 nanoblend, 90ppm Al 2 O 3 blend, and 90ppm Fe 2 O 3 blend.

Ignition delay
The time duration between the initiation of fuel injection and the beginning of combustion is referred to as the ignition delay period. Figure 11A [128] illustrates the addition of alumina nanoparticles to diesel, which was found to reduce ignition delay due to improved combustion of the air-fuel combination by increasing the surface area to volume ratio. Dose concentrations of 20, 30, and 40ppm of Al 2 O 3 nanoparticles added to diesel fuel resulted in 11%, 25%, and 37% reductions in ignition delay, compared to crude diesel at full load. Figure 11B, C [192] shows a 5.47% and 0.99% reduction in delay duration for TiO 2 and Al 2 O 3 added diesel of 25 ppm at full load, respectively. As seen in Fig. 11D [193], the addition of iron oxide nanoparticles to the ternary fuel reduced the delay period as compared to pure diesel. The shorter delay period is attributable to improved atomization due to the high surface tension and calorific value of the blend. Furthermore, the existence of iron oxide nanoparticles and pentanol, both of which tend to raise the latent heat of fuel blends, impedes the synthesis of carbon chain molecules.

Cylinder temperature
Figure 12A [128] illustrates the effect of aluminium nanoadditive in pure diesel on cylinder temperatures, indicating that adding Al 2 O 3 additives to diesel fuel reduces ignition delay, increases peak cylinder pressure, and also improves HRR, which results in a rise in cylinder temperature. Peak cylinder temperatures at full load for diesel, 20, 30, and 40 ppm nanoadditives are 1470, 1510, 1570, and 1620K, respectively. Figure 12B [ 194] depicts the waste cooking oil biodiesel with TiO 2 nanoadditive and shows the mean cylinder temperatures were to be 1270, 1000, 1030, 1080, and 1160 K for diesel, B20, 25ppm, 50ppm, and 100ppm of nanoblends, respectively. The aluminium nanoadditive in 20% calophyllum inophyllum biodiesel enhances the mean cylinder gas temperature, as seen in Fig. 12C [195]. Peak mean cylinder gas temperatures at full load are 1475, 1457, 1496, 1405, and 1416°C for diesel, 20% CIB, 40ppm, 20% CIB+ 20% EGR, and 40ppm+20% CIB + 20% EGR fuel samples, respectively. Figure 12D [ 196] depicts the change of in-cylinder temperature on a CRDI engine. According to the graph, the mean cylinder temperature of all nanofuel blends at dose rates of 30, 60, and 120 mg/L revealed a rise in the temperature of the cylinder.

Combustion duration
The presence of nanoparticles causes a micro-explosion of the fuel droplets, which accelerates fuel air mixing and evaporation, which shortens the length of the combustion. Figure 13A [198] shows a 20% microalgae blend with 0.1 and 0.2% (vol./vol.) Al 2 O 3 nanofluid, with combustion durations of 67° CA, 58° CA, and 47° CA, respectively. According to the graph, the Al 2 O 3 nanoblend has a shorter combustion time than the 20% microalgae blend. This is due to the Al 2 O 3 nanoblend's decreased density and viscosity, which causes the improved combustion. Figure 13D [ 199] illustrates the combustion duration of each fuel under various operating conditions. According to the graph, at high speeds, the combustion duration of fuel is somewhat shorter than that at low speeds since the mixing speed of fuel and air is quicker, and therefore, the combustion speed rises. The burning time of the 100ppm nanoblend fell by 6.74% and 7.30% rpm at 1000 and 1800 rotational speeds, respectively.

Effect of magnetite nanoparticles on emission characteristics
The major challenge of the twentyfirst century is likely to be global climate change, and it has risen to 1.4-5.8°C by the year 2100. Pollutants like carbon dioxide (CO 2 ), water vapor, methane (CH 4 ), sulphur dioxide (SO 2 ), and nitrogen dioxide (NO X ) are the major contributors to climate change. Researchers have concentrated on creating a wide range of renewable energy sources to minimize these emissions into the environment, including oxygenated fuels, biofuels, fuel cells, and solar technologies. Because of their strong thermal braking performance, high compression ratio, and reduced air-fuel mix, the use of fossil fuel in diesel engines has an outstanding reputation for low fuel consumption, high dependability, and high durability. However, both diesel and biodiesel fuels have their respective limitations in producing higher NO X , which leads to poor combustion performance. Thus, to overcome these limitations, the addition of fuel additives like magnetite nanoparticles is gaining much attention to improve the oxidation characteristics of biodiesel since magnetite nanoparticles have an excess of oxygen that promotes better combustion and lower fuel usage. This section provided a critical review of the most recent research papers on the effect of nanofuel additives on diesel engine emissions. Table 6 summarized the influences of magnetite nanoparticles in various fuel blends on emission characteristics with various concentrations. Karthikeyan et al. [53] explored the impacts of ferrofluid mixed with Caulerpa racemosa oil methyl ester as a test fuel. The base fuel is treated with 1% ferrofluid and 0.3% surfactant (CH 3 ) 3 NOH). It was revealed from the test that harmful pollutants such as HC, CO, NO X , SO 2 , and smoke were drastically reduced when compared to diesel. Venkata et al. [174] investigated a four-stroke single chamber, air-cooled diesel engine with magnetite nano-added rice bran oil methyl ester blend. From the experimentation, there is a decrease of 19 ppm, 0.011%, and 93 ppm in HC, CO, and NO X , and also 9% of smoke opacity is decreased. Yuvarajan et al. [178] have also examined the rice bran oil methyl ester with magnetite nano in a two-chamber CI engine. From their outcomes,    [51] have utilized the B20 blend of pongamia biodiesel with ferrofluid on a Kirloskar TV1 engine at various loads, keeping 1500 rpm as a steady speed. When the ferrofluid was added to the blend, CO and HC decreased by 35.8% and 22.9%, respectively, at full load. At last, it was concluded that the 1% ferrofluid of the B20 blend gives maximum efficiency and fewer emissions compared to all the other blends. Sarath et al. [187] explored the impacts of 4%, 8%, and 12% ferrofluid in diesel fuel at an engine speed of 15,000 rpm. From the outcomes, it was shown that emissions like nitrogen oxides and particulate matter have a decreasing trend while other emissions such as HC and CO show an increasing trend when the concentration of ferrofluid content of the blend increases. Ranaware et al. [188] have shown the impact of utilizing various nanoadditives like cerium oxide and ferrofluid in diesel fuel. From the observations, it was shown that adding 0.4 and 0.8 water-based ferrofluid to diesel fuel not only reduces NO X emissions but also slightly increases CO emissions. Syed [191] changed the dosing amount, i.e., from 40 to 120 ppm nanoparticles in a MME20 fuel blend. The results show that 80 ppm of both nanoadditives gives the best emission characteristics. Similarly, an 80 ppm Al 2 O 3 nanoblend shows better combustion characteristics when compared with magnetite nanoparticles. Ameer et al. [175] researched the impacts of magnetite nanoparticles in chicken fat biodiesel blend, i.e., CFB30, and also hydrogen induction on a DICI engine. From the outcomes, it was shown that for 100ppm nanoblend the decrease in NO X , smoke, UHC, and CO was by 15.72%, 15.64%, 22.27%, and 0.119, respectively, relating to the CFB30 blend. Similarly, injecting 10 lpm of hydrogen into the fuel causes the outflows like smoke, UHC, and CO to decrease by 11.57%, 14.7%, and 0.037 (vol.), while slightly increasing NO X emissions. Based on the findings, it is determined that CFB30 nanoblend fuel combined with 10 lpm hydrogen enhances direct injection compression ignition (DICI) engine performance overall.

Conclusions
This review covers a brief overview of magnetite nanoparticle-based additives with a focus on the synthesis of nanoparticles, preparation and stability of nanofuels, and application of magnetite nanoblends in the diesel engine were analyzed thoroughly.
➢ The two chemical synthesis methods have been shown to be straightforward and cost-effective. The co-precipitation method by ultrasonic waves and magnetic fieldassisted has shown excellent properties, and the effect of their binary combination on CCP synthesis was investigated and optimized for large-scale preparation of mag- netite nanoparticles. The sol-gel method with varying annealing temperatures indicated that the different sized nanoparticles were obtained. ➢ Different techniques such as sedimentation, zeta potential testing, UV-Vis spectrophotometer, and turbidity meter are used to study the stability of nanofluids. ➢ Adding magnetite nanoparticles to diesel and biodiesel causes them to increase the calorific value and cetane number, improving combustion characteristics such as ignition delay time, heat release rate, and rate of pressure rise. The other properties like density, kinematic viscosity, and flash point are improved. ➢ The incorporation of magnetite nanoparticles into biodiesel or diesel allows for the release of high heat and a faster rate of energy release during the combustion process. As a result, there was an increase in BTE and braking power, but BSFC tended to decrease. ➢ The addition of magnetite nanoparticles to biodiesel/diesel reduces CO, HC, NO X emissions, and smoke opacity significantly. These favorable results were achieved as a result of the high oxygen concentration and catalytic oxidation.

Challenges
Magnetite nanoparticles are increasingly being used in IC engines to improve efficiency, combustion, and pollution. However, there are several problems and obstacles associated with using nanoparticles in real-world applications. As a result, new study possibilities for scientific writers and researchers all around the world have emerged.
➢ Even though the synthesis of magnetite nanoparticles is less costly than traditional techniques and is highly recommended for large-scale production, an adequate selection of mechanisms, process parameters, and equipment is required for the entire process to be completed, which adds to the overall cost. As a result, the high cost may limit its potential industrial applicability. ➢ The dispersion nanoparticles, which create a clogging effect mostly owing to agglomeration, have a substantial impact on the stability and thermo-physical characteristics of nanofluids. As a result, uniform dispersion of nanoparticles into the base fluid is a key step in creating excellent, homogeneous, and stable nanofluids. Instability can be reduced by using proper preparation procedures and enhancing mechanisms. ➢ The widespread and increased use of nanoparticles not only improves performance but also raises safety and health issues due to their discharge from exhaust gases. Due to their small size, they may readily enter biological systems and cause harm to the lungs, brain, and other internal organs. As a result, trapping nanoparticles from exhaust gas is highly recommended. Magnetite nanoparticles demonstrated minimal toxicity and the ability to be gathered by a magnetic bar, making them acceptable diesel fuel modifiers.

Future directions
Future studies on magnetite nanoparticle synthesis will add to the body of knowledge about the different parameters that influence the synthesis process and advanced characterization approaches for effective future applications. As a result, adopting both cost-effective and environmentally friendly approaches is always critical in avoiding difficulties such as expensive materials, instrument rust, health concerns, and environmental pollution. Furthermore, enhancing the stability and thermal properties of nanofluids would lessen the challenges associated with their utilization in practical applications. The following are some of the prospective directions for magnetite nanoblend research: ➢ The stability of magnetite nanoblends is a major challenge that needs to be further investigated, mainly considering the effects of nanoparticle nature and morphology, ultra-sonication time, particle-particle interaction, base fluid-particle interaction, temperature, and time. ➢ Investigations into the thermophysical properties of nanoblends are to be continued as they are largely affected by the dispersed nanoparticles in base fluids, which provide a clogging effect mainly due to agglomeration. ➢ Additional research is required on the performance and emission characteristics of test fuels under various operating situations, such as injection timing, engine speed, and compression ratio.