A comprehensive review of the influences of nanoparticles as a fuel additive in an internal combustion engine (ICE)

Nanofluid is a colloidal mixture consisting of nano-sized particles dispersed in a liquid medium. It improves heat transfer properties and promotes high energy efficiency in a wide spectrum of engineering applications. In recent years, particularly in the automotive industry, the addition of nanofluid in diesel/biodiesel as an additive for ICE has become an attractive approach to promote enhanced combustion efficiency and emission reduction due to their superior thermophysical properties. Many researchers have previously demonstrated that the addition of nanoparticles in diesel/ biodiesel fuel improved the overall engine combustion characteristics. As a whole, this study aims to summarize the recent research findings related to the effect of nanoparticles on the fuel properties and engine combustion efficiency. Furthermore, different types of additive blended with varying fuel properties are also compared and discussed. Lastly, the advantages and prospects of using nanofluid as an additive fuel are summarized for future research opportunities.


Introduction
Global warming is expected to be the biggest challenge of the twenty-first century. The average temperature of the Earth increased by about 0.4-0.8°C in the past 10 decades [1,2]. Recently, the scientists from the Intergovernmental Panel on Climate Change have estimated that the average global temperatures will continue to rise by 1.4-5.8°C by the year 2100 [3]. Carbon dioxide (CO 2 ), water vapour, methane (CH 4 ), sulphur dioxide (SO 2 ), chlorofluorocarbon, and nitrogen dioxide (NO x ) are well known as the major contributors to the greenhouse effects.
Recently, many researchers have focused on developing a wide spectrum of renewable energies including oxygenated fuels, biofuel (n-butanol), fuel cell, and solar technologies to reduce the consumption of fossil fuels and control the emission of greenhouse gases (GHGs) to the atmosphere [4][5][6][7]. One of the most effective methods in controlling the emission of GHGs is to reduce the emission of CO 2 by reducing fossil fuel consumption [8][9][10][11]. CO 2 is directly related to the carbon content of the fuel and the amount of fuel consumption [11]. The production and combustion of transportation fuels also release CH 4 and nitrous oxide (N 2 O) other than CO 2 which contributed to the emission of GHGs. Other than using clean fuel alternatives, novel automotive engines with postcombustion emission control devices should be developed to reduce the GHG emissions and improve the efficiency of energy systems [12][13][14]. The application of biodiesel engine in transportation and power generation sectors has shown development in the past decades, and the latest research development trend is seeking a novel ICE with low emission [15][16][17], energy savings [18,19], and high-efficiency performance [20][21][22].
Biodiesel engines have an excellent reputation for low fuel consumption, high reliability, and high durability due to their high thermal brake performance, high compression ratio, and lower air-fuel mix [19,23,24]. However, both diesel and biodiesel fuels have their respective limitations in producing a higher NO x , which leads to poor combustion performance [25][26][27]. Thus, to overcome these limitations, the addition of fuel additives is gaining much attention to improve the oxidation characteristics of biodiesel. The blending of diesel/biodiesel with fuel additives could improve combustion performance and reduce GHG emissions effectively. Nanofluid is a potential fueladditive candidate and the pioneer researcher who suggested it was Choi [28]. Nanofluid is a two-phase colloidal mixture consisting of nano-sized particles (nanoparticles) dispersing in a base liquid. Nanoparticles are generally known as particles with sizes approximately between 1 and 100 nm. Rheological behaviour and thermophysical properties of the base fluid would be significantly affected when nanoparticles are dispersed into the base fluid.
This study presents recent research findings of the properties of nanoparticles as an additive in diesel/biodiesel fuels. The present study compares the combustion performance and emission characteristics of ICEs with different types of diesel/biodiesel-nanoparticle blends. Currently, metal oxides like cerium oxide (CeO 2 ), aluminium oxide (Al 2 O 3 ), copper oxide (CuO), silver oxide (Ag 2 O), iron oxide (Fe 2 O 3 ), titanium oxide (TiO 2 ), silicon (Si), zinc oxide (ZnO), and magnesium (Mg) and nonmetal oxides like carbon nanotubes (CNTs), multiwall CNTs (MWCNTs), graphene oxide (GO), etc. are among the most widely used fuel-additive nanoparticles in diesel/ biodiesel fuel.
The concentration of nanoparticles required for a stable dispersion is also considered in this article. It is well known that the use of different types of nanoparticles in different diesel/biodiesel mixtures can yield different results. For example, it will affect the engine performances, including brake-specific fuel consumption (BSFC), brake power (BP), brake heat efficiency (BTE), engine torque, and toxic gas emissions including CO, NO x , and particulate matter (PM). To the best of the author's knowledge, very few review papers were published on the fuel consumption and combustion performance of diesel/biodiesel with the addition of nanoparticle-based fuel additives in an ICE.
Hence, the authors aim to provide an extensive review by comparing the combustion, emission, and performance characteristics of the nanofuel additives diesel/ biodiesel blends. First, the renewable energies to control GHGs were briefed in the introduction section. This was followed by a brief review of the stability of nanoparticles in a base fluid. Third, reports the effect of different types of additives blended on the fuel properties and engine performance using the most widely used nanoparticles. That is, metal oxides, such as CeO 2 , Al 2 O 3 , CuO, Ag 2 O, Fe 2 O 3 , TiO 2 , Si, ZnO, and Mg, and non-metal oxides, such as CNT, MWCNT, and GO, were reviewed as potential additives in fuels. The research works of literature were sourced from Elsevier's' ScienceDirect, Google Scholar, and ISI Web of Science. For sourcing information, many keywords have been used. The keywords include nanoparticles, ICE, diesel/biodiesel, fuel properties, combustion efficiency, and emission control. In identifying the possible articles, more than two of the keywords were used. The select literature included peer-reviewed studies, which contributed significantly to the field of study. Elsevier's Mendeley reference management programme was also used to gather references from the selected articles. All of the findings obtained from these articles are presented in a tabular form. As a whole, this review article will help the researchers from the nanotechnology field and industrial engine manufacturers to gather a quick report on the emission issues and engine performance of different diesel/biodiesel fuel blends.

Stability of nanoparticles in a base fluid
Generally, nanofluid has high surface energy owing to its large surface area, which tends to promote agglomeration and form micro-sized particles before deposition. For both scientific and practical applications, a stable and homogeneous nanofluid suspension is a crucial phenomenon. Stability plays a vital role in the production of nanofluid as it will affect its performance as a heat carrier and thermophysical properties. To promote a better stability of nanoparticles in a base fluid, many methods have been reported in the literature including ultrasonication, surface modification, the addition of surfactants, and pH control. From the literature, several previous studies investigated the effectiveness of ultrasonic dispersion in promoting and improving the stability of nanoparticles. A study by Hong et al. [29] reported that the stability of nanofluids can be enhanced with longer sonication time. From their results, it was revealed that prolonging the sonication time helps to reduce the agglomeration of particles. Amrollahi et al. [30] reported similar observations in their experiments. Both studies agreed that longer sonication times improved the stability of nanoparticles. In 2012, Ruan and Jacobi [31] reported that the ultrasonic method could effectively break down the particle agglomerates and promote a stable and better dispersion of nanoparticles in the base fluids. Based on their results, the thermal thermophysical properties of MWCNT-ethylene glycol nanofluid were found to be highly dependent on the sonification time.
Similar conceptual work was also carried out by Chung et al. [32]. By using two different ultrasonic dispersion methods of horn and bath, the effectiveness of both ultrasonic methods in dispersing the ZnO nanoparticles in the water medium was compared. From the findings, the ultrasonic horn method was found to be more effective in achieving a faster reduction rate, smaller colloidal particle size, and higher sedimentation rates. Unfortunately, the optimum sonication time varied for different types of nanoparticles and base fluids. A study on the homogeneous dispersion of nanoparticles in nanofluids was performed by Hwang et al. [33]. A stable colloidal mixture consisting of both CB and Ag nanoparticles was prepared from a two-step method using a stirrer, ultrasonic bath, ultrasonic disruptor, and high-pressure homogenizer. Sodium dodecyl sulfate (SDS) and oleic acid were used as the surfactants to improve the colloidal stability. From the results, the nanoparticles' colloidal prepared from the high-pressure homogenizer possessed the highest stability. With the use of a high-pressure homogenizer, the agglomerated particles can easily break off and separate.
Surfactants are the chemical compounds added to nanoparticles, which help to reduce the surface tension of the nanofluid and increase the absorption of particles. Some literature discussed the use of surfactants for a slower deposition rate; however, in some cases, the proper type of surfactants should be applied to the particles. In the literature, only a few types of surfactant such as hexadecyltrimethylammonium bromide/cetyl trimethyl ammonium bromide (CTAB) [34][35][36][37], sodium dodecylbenzene sulfonate (SDBS) [34,38,39], SDS [35,40,41], polyoxyethylene(10)nonylphenyl ether (TX-10) [34], polyvinyl chloride-polyvinyl pyrrolidone (PVP) [38,41,42], salt and oleic acid [33], dodecyltrimethylammonium bromide [39], gum Arabic [35,40], etc. were reported and used in different types of nanofluids. Li et al. [34] prepared the nanofluids from Cu-H 2 O with and without the addition of dispersant. Different types of surfactants including TX-10 (a non-ionic surfactant), CTAB (a cationic surfactant), and SDBS (an anionic surfactant) were used to modify the surface functionality of 0.1% Cu-H 2 O nanoparticles. The chemical structure of the surfactant can be seen in Figure 1. From the results, it was found out that the optimum concentrations of TX-10, CTAB, and SDBS in yielding better stability for 0.1% copper nano-suspension were 0.43%, 0.05%, and 0.07%, respectively. Li et al. [39] also reported similar observations in their experiments.
In 2018, Nema et al. [37] used CTAB to prepare Al 2 O 3 nanoparticles as a nanofuel additive. They performed the study in a dual-blend biodiesel-fuelled compressionignition (CI) engine using Al 2 O 3 nanoparticles. The Al 2 O 3 nanoparticles were found to be stable and well dispersed in the base fluid.
Sahooli et al. [42] incorporated a PVP surfactant to prepare a stable CuO/water nanofluid. The nanofluid was prepared from different pH values and PVP concentrations. Based on their findings, the CuO/water nanofluid had excellent colloidal stability with the optimum pH and PVP concentration of 8 and 0.095, respectively. Xia et al. [41] investigated the colloidal stability of Al 2 O 3 in deionized water using PVP. In their study, the effect of PVP and SDS on the thermal conductivity of the Al 2 O 3 / de-ionized water nanofluid was investigated and PVP demonstrated a better dispersion and stability performance than SDS. The optimal concentration ratio of surfactant mass fraction and particle volume fraction were found at the highest thermal conductivity of the nanofluid, where the ratio was partly associated with the particle size, and it decreased with the increase in particle volume fraction. The addition of surfactant is widely adopted to improve the dispersion of nanoparticles in a base fluid and to minimize the coagulation/agglomeration of particles. Figure 2 shows the influence of surfactants on the thermal conductivity of Al 2 O 3 /de-ionized water nanofluids with different particle sizes.
However, the addition of surfactants can cause some problems including the function of this method in improving the stability of nanofluids, which cannot be applied to nanofluids operating under high temperatures due to the possible bond breakdown between the surfactant and nanoparticles [43,44]. Chen et al. [45] studied the stability and thermal conductivity properties of CNT nanofluids stabilized by a cationic Gemini surfactant. From their finding, it was concluded that a high concentration of surfactants did not improve the thermal conductivity of nanofluids. Besides, the addition of surfactants increased the thermal resistance of the nanoparticle in the base fluids, which led to a poor thermal conductivity performance [43].

Effect of nanoparticles on fuel properties
The fuel properties are one of the significant factors that determine the quality of the fuel mixing and combustion process. Recently, the addition of nanoparticles has been considered as an advantageous approach in enhancing the fuel properties. Numerous researchers have tested fuel properties by adding different types of nanoparticles in various diesel/biodiesel fuels [37,[46][47][48][49][50][51][52][53][54][55][56]. Furthermore, the quality of the fuel mixing and combustion process was evaluated by studying their effects on different features including kinematic viscosity, caloric value, flash point, density, number of cetane, etc. Table 1 compares the physical properties of diesel/biodiesel blend with and without nano-additives. An experimental investigation of NO x reduction in a grapeseed oil biodiesel-fuelled CI engine was performed by Praveena et al. [46,47]. From their study, two different types of nano-additives, namely, CeO 2 and ZnO, were used to test the physical properties of the fuel under the American Society for Testing and Materials (ASTM) code. The calorific value of ZnO was found to be higher than that of CeO 2 . The low calorific values of CeO 2 caused an increase in BSFC. No significant effect was found in the density, pour, cloud points, and kinematic viscosity due to the addition of CeO 2 nanoparticles in the grapeseed oil biodiesel fuel. The experimental results were in good agreement with that obtained by Karthikeyan et al. [48] who also investigated the addition of CeO 2 (50 and 100 nm) nanoparticles in a grapeseed oil biodiesel fuel. However, there was a slight improvement in the flashpoint and calorific value between Praveena [46,47] and Karthikeyan [48]. As a result, the addition of CeO 2 nanoparticles' additive exhibited a significant improvement in the performance of and a reduction in harmful emissions as compared to the B20 (20% biodiesel + 80% diesel).
Ang et al. [49] prepared the diesel fuel blends by using Al 2 O 3 , CNT, and SiO 2 nanoparticles with dosing levels of 25, 50, and 100 ppm. The physical fuel properties were tested with the engine loads of 0%, 25%, 50%, 75%, and 100% using ASTM code under a constant engine speed of 1,800 rpm. It was observed that the addition of Al 2 O 3 nanoparticles increased the density and calorific value of the blend with a viscosity reduction in DA25 (diesel fuel 1 kg + Al 2 O 3 25 mg). As expected, a significant increment in the viscosity was observed when the concentration of nanoparticles increased. On the contrary, the addition of CNT did not affect the fuel density substantially and showed a reduction in viscosity due to the lubricity of carbon atoms. Yet, the calorific value was found to be increased due to the higher carbon content. Meanwhile, the SiO 2 blends displayed a lower fuel density with no significant change in the calorific value as compared to diesel fuel with a lower viscosity characteristic.
In a different study, Rolvin et al. [52] prepared the diesel fuel blend from TiO 2 nanoparticles with a dosing level   [50] and Sahoo and Jain [51] also concluded that the nanoparticle additives possessed better fuel characteristics like flash point and calorific value. A lower BSFC was attributed to the higher calorific value [62].
From an experimental investigation performed by Sajin et al. [55], the influence of the size of ZnO nanoparticles on the physical properties of mango seed biodiesel was investigated. From the results, the biodiesel blended with 40 nm ZnO nanoparticles had a higher calorific value and cetane value and the addition of nanoparticles in the biodiesel promoted better combustion performance.

Effects of metal oxide
nanoparticles as additives in diesel/biodiesel fuel on the performance, combustion, and emission characteristics The main purposes of adding nanoparticles into the diesel/ biodiesel fuel are to promote a high surface-to-volume ratio and increase the number of reactive surfaces. It allows the nanoparticles to act as an effective chemical catalyst which improves the mixing pattern of fuel with air and the fuel combustion performance, subsequently leading to a fully combusted chemical catalyst.

CeO 2
CeO 2 can be served as an oxygen buffer to simultaneously induce the oxidation of hydrocarbons (HCs) and reduction of nitrogen oxide emission [66,67]. Several studies and reviews have explored the effects of CeO 2 nanofuel additives on the performance, combustion, and emission characteristics of CI engine. In 2015, Narasiman et al. [58] investigated the effects of CeO 2 nanoparticle's additive in diesel and sardine oil methyl ester (SOME). The mass fracture of nanoparticles used was 25 ppm. Throughout the experiments, a single four-stroke diesel engine was used at different loads under a constant speed. From the test, it was revealed that the nanoparticles could be used as an additive in diesel and biodiesel to induce complete fuel combustion and significantly improve the exhaust emissions.
Sathiyamoorthi et al. [59] investigated the performance, emission, and combustion characteristics of a single cylinder with two modified fuel blends: BN20 (biodiesel from neem oil) and CeO 2 nanoparticle's additive blended in BN20. Ultrasonicator was used to mix CeO 2 nano-additives with BN20 to promote better colloidal stability. The addition of nanoparticle additives promoted a higher BSFC and BTE as compared to the standard diesel fuel. The emissions of NO x , smoke, HC, and CO were found to significantly decrease after the nanoparticle's additive was added into the BN20 fuel. Experimental results also revealed that a higher amount of cylinder pressure and heat were released when CeO 2 nanoparticles were added into BN20 fuel. A significant study by Gharehghani et al. [60] included the effect of water and CeO 2 nanoparticles on engine performance, combustion, and emission characteristics of diesel/biodiesel fuels. The influence of water and nanoadditive in diesel/biodiesel fuel was found to improve the overall combustion quality. The experimental results were in good agreement with that obtained by Khalife et al. [68] and Mei et al. [69].
Sathiyamoorthi et al. [70] conducted a novel study to evaluate the performance, combustion, and emission characteristics of neat lemongrass oil biodiesel. The entire research was performed in the diesel engine using emulsified LGO25 (75% diesel volume and 25% lemongrass oil volume), a mixture of CeO 2 -emulsified LGO25, and a mixture of diethyl ether (DEE)-emulsified LGO25 with an exhaust gas recirculation (EGR). All of the nanofluid blends were compared to the standard diesel and LGO25 fuel. From the results, a significant improvement in NO x and smoke emission was observed in the mixture of DEE-emulsified LGO25 and EGR. The NO x and smoke emissions were found to be reduced by 30.72 and 11.2%, respectively. In the context of HC and CO emissions, both were reduced by 18.18 and 33.31%, respectively, whereas the BTE and BSFC increased by 2.87 and 10.8%, respectively. The combustion characteristics such as cylinder pressure and heat release rate increased by 4.46 and 3.29%, respectively, as compared to the emulsified LGO25. These findings are in line with those found in the literature and the nano-additive was demonstrated as an oxygen buffer to promote a better combustion and emission control efficiency [71].
Pandey et al. [72,73] investigated the effect of CeO 2 nanoparticles on the combustion performance and emission of Karanja oil biodiesel. From the results, Karanja oil biodiesel with 5 wt% of nano-additive demonstrated enhanced engine performance with a substantial reduction in particulate emissions including lower emissions of NO x (14-25%) and PM as compared to diesel fuel. Besides, the addition of nano-additive to Karanja oil biodiesel caused a slower heat release rate as compared to the standard diesel. It possessed a higher cetane number than diesel [66] and thus had a shorter delay in the ignition as compared to petrol. Apart from that, their results were in good agreement with that reported by Babu and Praneeth [74].
Ananda et al. [75] demonstrated the emission reduction of an ethanol-gasoline blend using CeO 2 nanoparticles as a fuel additive. From the results, the thermal brake performance of nanoparticle mixtures was found to be improved. In the CO emission test, the CO 2 , HC, and NO x emissions reduced significantly with a substantial increment in O 2 concentration in all blends. From the combustion analysis, the gas pressure of nanoparticle blends was higher as compared to that of the pristine fuel.
Recently, Janakiraman et al. [76] examined the combustion performance and emissions of CeO 2 , ZrO 2 , and TiO 2 as nanoparticle additives. It was observed that the HC of TiO 2 , CeO 2 , and ZrO 2 blended with B20 (20% Garcinia gummi-gutta biodiesel + 80% diesel) were lower than the neat B20 by 6.39, 3.99, and 5.64%, respectively, under maximum load condition. Furthermore, B20 blend with TiO 2 had a better combustion efficiency, lower CO, unburned HC (UBHC), smoke, and diesel fuel combustion. The addition of nanoparticles into the blends was found to contribute a declining trend in the HC emissions due to the high availability of oxygen content and lower activation energy discharged by the nanoparticles [55,69,[77][78][79][80]. All the nanoparticle blends exhibited a lower brake specific energy consumption (BSEC) as compared to B100 by 23.42, 22.11, and 19.8%, under the maximum load condition. The reduction of fuel blends in BSEF is shown in Figure 3. The study from Senthil and Ramesh [80] evaluated the effect of ginger grass oil biodiesel on the performance, combustion, and emission characteristics using CeO 2 as a fuel additive. It was found out that the HC content increased with a higher concentration of ginger grass oil in the blends.
Thangavelu et al. [81] explored the potential of CeO 2 nanoparticles as a fuel additive in a four-stroke singlecylinder water-cooled compression ignition engine with the volumetric proportions of 5, 10, 15, and 20%. From the study, a significant reduction in the emission of HC by 3%, smoke by 7.7%, and CO by 1.33% was found as compared to that of the conventional diesel. It was concluded that B5D85 + CeO 2 (100 ppm) fuel blend had the best fuel ratio for a CI engine due to the high engine efficiency, low particle emission, and better combustion. Thus, it can be served as a suitable alternative fuel for the CI engine.
CeO 2 nanoparticles and iron dope (10 and 20% iron) were added into the waste cooking oil biodiesel-diesel blend in the study by Hawi et al. [82]. From the results, the NO x emission was reduced by 15.7% with no significant change in the HC emissions. At the same time, CO emission was reduced by 24.6% for B30 and 15.4% for B30 with nano-additives. From low to medium loads, a lower BSFC was found for the B30 fuel mixture with 10% FeCeO 2 nanoparticles and comparable to D100 at high loads. In contrast, BTE improved with an increase in engine load. Akram et al. [83] also conducted a study using waste cooking oil diesel/biodiesel. However, in their research, the emission reduction potential in CO, NO x , and UBHC was investigated using different concentrations of CeO 2 nanoparticles and Ce 0.5 Co 0.5 nanocomposite oxide under full engine load. It was found that CO, NO x , and UBHC emissions reduced by 18.27, 6.57, and 23.46%, respectively, when CeO 2 (100 ppm) was used. On the contrary, when Ce 0.5 Co 0.5 nanocomposite oxide (100 ppm) was used, CO, NO x , and UBHC emissions reduced by 24.18, 13.96, and 40.74%, respectively. It was observed that the addition of CeO 2 nanoparticles led to an increase in the viscosity index of the biodiesel owing to the high catalytic activity [84,85]. Table 2 summarizes the effect of CeO 2 nanoparticles in diesel/biodiesel fuel on the performance, combustion, and emission characteristics of the ICE with some undiscussed research works.

Al 2 O 3
In the following section, the effect of Al 2 O 3 nanoparticles as a fuel additive in diesel/biodiesel blend on the engine performance, combustion, and emission characteristics of the ICE are reviewed and discussed. Nassir and Shahad [87] attempted to examine the impact of Al 2 O 3 and TiO 2 nanoparticles on the combustion phasing of diesel fuel. The addition of nanoparticles improved the fuel properties. For instance, the cetane number enhanced from 51.6 to 54.3 with the addition of 150 ppm Al 2 O 3 nanoparticles. The effect of nanoparticles on the delay time and fractioning of heat release (premix and diffusion) was noticeable. From the results, the delay period decreased with higher nanoparticle concentration. The effect of Al 2 O 3 nanoparticles on viscosity, temperatures, and the cetane number was more prominent than the effect of TiO 2 . Nevertheless, TiO 2 nanoparticles contributed to the most significant reduction in the delay period, which can be attributed to the higher viscosity of Al 2 O 3 .
Sathiamurthi et al. [88] added 50 nm Al 2 O 3 nanoparticles into the diesel fuel and evaluated the combustion performance and emission characteristics of the diesel blend in a four-stroke, single-cylinder diesel engine under different load conditions and fuel blends (0.5 g and 1 g of 50 nm Al 2 O 3 nanoparticles were blended into 1 L of diesel fuel). The results obtained were similar to the study done by Venkatesan and Kadiresh [89] even though they used a smaller size of nanoparticles (40 nm) and different fuel rate (1 and 1.5 g/L). These results indicate that particle size has a significant effect on ignition and combustion. The smaller particle sizes showed a greater intensity of reaction. Furthermore, the addition of nanoparticle was found to improve the combustion characteristics and enhanced the surface to volume ratio. Thus, the burning efficiency of the test fuels was promoted as more diesel fuels reacted with the oxidizer. From the results, a significant increase in BTE and a considerable reduction in the NO x and UBHC contents of all loads were observed as compared to pure diesel fuel. This could be due to the improved combustion characteristics of nanoparticles and the enhanced air mixing ratio.
Basha [90] also studied the combustion performance of diesel fuel blend with Al 2 O 3 nanoparticles. The result of the analysis revealed that a significant increase in BTE and a marginal reduction in harmful pollutants including NO x , CO, and smoke were observed with the diesel fuel blend in contrast to the pure diesel fuel. The addition of Al 2 O 3 nanoparticles in diesel fuels boosted the combustion efficiency of the diesel engine. However, many research works are still ongoing to capture potential nanoparticles released from the exhaust to avoid environmental pollution. Kao et al. [91] conducted an experimental investigation in a single-cylinder horizontal diesel engine using Al 2 O 3 additive diesel fuel with different percentages of water (3-6%). A significant reduction in the BSFC and harmful pollutants was observed. The addition of Al 2 O 3 nanoparticles to the diesel fuel provided a large surface area for interaction between water molecules and   [80] Ginger grass oil CeO 2 30 ppm Engine performance: Higher BTE and lower BSFC Harmful emission: Lower CO 2 , HC, and NO x emission. All samples have lower CO emission except for B10, which was higher at lower load and then decreased at maximum load 17 Thangavelu et al. [81] Waste tyre oil CeO 2 100 ppm Engine performance: Improvement in BTE fuel particles. Due to the high surface activity of the water molecules, hydrogen atom decomposes from the water during combustion which promotes complete combustion.
Gumus et al. [50] carried out a study to investigate the influence of Al 2 O 3 and CuO nanoparticles in diesel fuel. The nanoparticle fuel blends were prepared from a concentration level of 50 ppm before comparing the performance, combustion, and emission characteristics of the blends with standard diesel fuel. The testing was conducted between 1,200 and 3,600 rpm with an interval of 250 rpm. Based on their findings, the BSFC of nanoparticle fuel blend was lower than that of diesel fuel. The BSFC of CuO and Al 2 O 3 additives decreased by up to 0.5 and 1.2%, respectively. With the addition of Al 2 O 3 to pure diesel, CO, HC, and NO x emissions reduced up to 11, 13, and 6%, respectively. On the contrary, the CO, HC, and NO x emissions of CuO additive reduced up to 5, 8, and 2%, respectively. The presence of excess oxygen and nanoparticles in diesel fuels improved the fuel properties and combustion efficiency, which led to lower BSFC and harmful emissions [92,93].
In another study, Ang et al. [49] demonstrated a significant reduction in BSFC and higher BTE in the diesel fuel blends with Al 2 O 3 , CNT, and SiO 2 nanoparticles. As compared to Al 2 O 3 , and SiO 2 blend, the CNT blend exhibited the highest emission reduction of 19.8% with a 100 ppm of CNT blends with pure diesel (DC100) under 25% engine load. Similarly, the CNT blend also had the highest reduction in BTE. From the findings of Selvan et al. [88], CNT blends also demonstrated an improvement in BSFC. These results can be ascribed to the higher calorific value of CNT which led to a substantial reduction in BSFC and BTE. However, the findings are in contrast with the results obtained by Raju et al. [94] who investigated the effect of different dosing levels of Al 2 O 3 and CNT nanoparticles in novel tamarind seed methyl ester (TSME) biodiesel for diesel engine applications. As compared to other samples, the TSME biodiesel with a dosing level of 60 ppm Al 2 O 3 nanoparticles exhibited better diesel engine characteristics with a lower BSFC value. This can be explained by the oxygen content of Al 2 O 3 in TSME biodiesel. Besides, the addition of nanoparticles showed a remarkable improvement in exhaust emissions except for NO x . The NO x emission was found to be higher in the fuel blend of Al 2 O 3 nanoparticles with a dosing level of 60 ppm and lower than the B20 fuel blends. The most significant observation from this study was the engine combustion characteristics of TSME biodiesel with nano-additive demonstrated a remarkable improvement in exhaust emission as compared with B20 fuel blends.
Balasubramanian et al. [95] investigated the influence of Al 2 O 3 nanoparticles in lemongrass oil biodiesel in a single-cylinder diesel engine. Different concentrations of nanoparticle additive (10, 20, and 30 ppm) were prepared and blended in the lemongrass oil biodiesel using an ultrasonicator. The best engine combustion performance and exhaust emissions were obtained under the dosing level of 20 ppm. For the same concentration level of Al 2 O 3 nanoparticles, BTE was significantly improved by 11.5% as compared to the neat biodiesel. All emissions like HC, CO, NO x , and smoke emission decreased by 40, 6, 31, and 39%, respectively, as compared to the neat biodiesel. It is worth noting that the addition of Al 2 O 3 nanoparticle in the lemongrass oil biodiesel improved engine combustion performance and reduced the harmful emission owing to the lower NO x emission and better combustion characteristics. Recently, Soudagar et al. [96] studied the potential use of Al 2 O 3 nanoparticles as a fuel additive in Honge oil methyl ester (HOME20) biodiesel using different dosing levels of 20, 40, and 60 ppm. Based on their experiments, HOME20 fuel blend with a nanoparticle concentration level of 40 ppm demonstrated a better engine combustion performance as compared to that of 20 and 60 ppm. For the dosing level of 40 ppm, BTE was found to be increased by 10.57% with a reduction in BSFC by 11.65%. Furthermore, HC, CO, and smoke emission reduced by 26.72, 48.43, and 22.84%, respectively. Conversely, the NO x emission increased by 11.27%. Table 3 summarized all of the above-mentioned experimental studies with some other undiscussed research works.

TiO 2
Recently, Kumar et al. [103] investigated the influence of TiO 2 nanoparticles on the performance, emission, and combustion characteristics of waste orange peel oil biodiesel with the dosing levels between 50 and 100 ppm. From the findings, the addition of nanoparticles with the dosing levels of 50 and 100 ppm promoted the BTE for about 1.4% and 3.0%, respectively, under maximum break power. However, pure diesel fuel showed maximum efficiency as compared with other testing samples. Also, they observed a significant reduction in smoke (24.2%), NO x (9.7%), CO (18.4%), and HC (16.0%) for the sample with a dosing level of 100 ppm. Overall, orange oil biodiesel nano-emulsions (OOMEs) with TiO 2 additive had better engine combustion performance as compared to neat biodiesel due to the improvement in the cylinder peak pressure, heat release rate, and combustion emissions. Furthermore, TiO 2 nano-additives produce hydrogen from water, which can be attributed to its catalytic photoelectric effect [104,105] and its ability to activate molecular bonds in the water-diesel emulsion [106].
In another recent study, Senthil et al. [107] evaluated the influence of two different dosing levels of TiO 2 nanoparticles on the emission behaviour of diesel fuel in four-stroke, single-cylinder, CI engine. Under the dosing levels of 50 and 100 ppm, the addition of TiO 2 nanoparticles improved the calorific values by 0.4 and 0.68%, respectively. Likewise, the flashpoint of the biodiesel was also enhanced by 4.4 and 6.67% under the dosing level of 50 and 100 ppm, respectively. The addition of TiO 2 to diesel fuel also caused a significant reduction in CO, HC, NO x , and smoke emissions. These findings are consistent with the conclusions of previous studies [52,62,108].
Karthikeyan and Viswanath [109] studied the effect of TiO 2 nanoparticle on the combustion performance and emission characteristics of tamanu biodiesel in a two-cylinder diesel engine under a constant speed of 2,000 rpm. All emissions such as HC, CO, NO x , and smoke were found to be lower. The concentration level of 100 ppm produced the best combustion performance among other concentrations.
The influence of TiO 2 nanoparticles in Calophyllum inophyllum biodiesel on the properties of fuel combustion features, engine performance, and emissions was studied by Praveen et al. [110]. They found that the kinematic viscosity, calorific value, and cetane number of the biodiesel increased with the addition of TiO 2 nanoparticles as compared to the biodiesel. Higher cetane numbers improved the BTE owing to the higher oxygen quantity.
The NO x and HC emissions reduced with the addition of nanoparticles as compared to the neat biodiesel. Similar findings were also reported by the previous researcher who studied the influence of TiO 2 nanoparticles in Calophyllum inophyllum biodiesel [62].
Nanthagopal et al. [62] carried out the experiment using two different types of nanoparticles, namely, TiO 2 and ZnO; 50 and 100 ppm of each nanoparticle were blended into the biodiesel using an ultrasonicator. It was reported that a higher BTE was observed in the biodiesel with the addition of TiO 2 and ZnO nanoparticles as compared to pure biodiesel. For biodiesel fuel doped with 100 ppm of TiO 2 nanoparticles, BTE increased by 17% as compared to that of pure biodiesel. Balasubramanian et al. [111] also observed a similar result when Mimusops elengi methyl ester (MEME) biodiesel was doped with TiO 2 nanoparticle. This effect was due to the function of TiO 2 nanoparticles as oxygen buffers and fuel boosters, which resulted in complete combustion and higher thermal efficiency.
El-Seesy et al. [112] carried out the experimental investigations to evaluate the combustion performance and emission characteristics of different fuel mixtures with varying percentages of n-hexane (5, 10, and 15% by volume) and dosing levels of TiO 2 nanoparticles (25 and 50 ppm). All of the mixtures were tested in a diesel engine under different engine loads and at a constant speed of 2,000 rpm. It was found out that by adding TiO 2 nanoparticles in jojoba biodiesel-diesel-n-hexane mixture J30D5H (30% JME + 65% D100 + 5% n-hexane), the BTE increased up to 15% as compared to the J30D5H fuel blend. A significant reduction in BSFC up to 12% was also observed in the J30D5H nanoparticle's fuel blend. This can be credited to the beneficial presence of the nanoparticles which enhanced the combustion and mixing process. Furthermore, the NO x emission was observed to be higher when the nanoparticles were blended with J30D5H biodiesel. Surprisingly, the addition of n-hexane and npentane enhanced the BTE and reduced the harmful emissions [113]. Table 4 summarizes the effect of TiO 2 nanoparticles on the performance, combustion, and emission characteristics Sundararajan and Anand [115] Plastic diesel oil TiO 2 20 ppm Engine performance: Higher BTE and lower BSFC. Improved combustion efficiency Harmful emission: Lower NO x emissions 10 El-Seesy et al. [112] J30D5H blended biodiesel TiO 2 25 and 50 ppm Engine performance: BSFC reduced by 12%, while BTE increased by 15% Harmful emission: CO and UHC emissions decreased by 20 and 50%, respectively. However, there was an increase in NO x by 15% 11 Örs et al. [116] Waste cooking oil biodiesel TiO 2 -Engine performance: Increased the maximum cylinder pressure and heat release rate values. Higher BTE. Lower BSFC Harmful emission: Lower CO, HC, and smoke opacity emission. Higher CO 2 and NO emissions 12 Kandasamy and Jabaraj [117] Cottonseed oil methyl ester TiO 2 20 and 40 ppm Engine performance: Improved combustion efficiency Harmful emission: Lower exhaust gas emissions 13 Nithya et al. [64] Canola biodiesel TiO 2 300 ppm Engine performance: -Harmful emission: NO x reduced by 5%. CO, HC, and smoke opacity emissions reduced to 32, 30, and 52%, respectively 14 Pandian et al. [118] Mustard oil biodiesel TiO 2 100, 200, and 300 ppm Engine performance: -Harmful emission: Significant reduction in HC, CO, NO x, and smoke emission. The dosing level of 300 ppm produced the best performance 15 Prabhu et al. [119] Neem oil biodiesel TiO 2 250 and 500 ppm Engine performance: BTE increased, and BSFC decreased Harmful emission: Lower CO, HC, and smoke emissions. However, a slight increase was observed in NO x for the dosing level of 250 ppm as compared with B20 and 500 ppm. Overall, 250 ppm produced the best performance of diesel/biodiesel fuel with some other undiscussed research works.

Other metal oxides nanoparticles
The effects of other metal oxide nanoparticles as an additive on the performance, combustion, and emission characteristics in the ICE are presented in this section and listed in Table 5. Recently, Mehregan and Moghiman [120] added 25 and 50 ppm Mn 2 O 3 and Co 3 O 4 , respectively, into the waste frying oil biodiesel which contained 20% wastes frying oil biodiesel and 80% ultralow sulphur diesel and urea-selective catalytic reduction (SCR) system. A reduction in NO x emissions was reported from the nanoparticle fuel blend. Co 3 O 4 with the dosing level of 50 ppm demonstrated remarkable thermal properties.
Recently Vedagiri et al. [121] investigated the influence of different open burning chamber geometries on the performance, combustion, and emission of CI engines by adding ZnO nanoparticles into grapeseed oil methyl ester (GOME) biodiesel as a fuel additive. With the addition of ZnO nanoparticles, the BTE was found to be increased from 28.17 to 29.3%, and BSFC decreased slightly from 0.3258 to 0.3128 kg/kWh. A significant reduction in NO x , HC, and CO emissions was also observed. Overall, the toroidal combustion chamber produced a better combustion performance as compared to the other combustion chambers.
Amirabedia et al. [122] and Amirabedi et al. [123] blended two different types of nanoparticles into the spark-ignition (SI) gasoline along with ethanol. An ultrasonicator was used to blend 10 ppm Mn 2 O 3 and 20 ppm Co 3 O 4 nanoparticles into the gasoline-ethanol fuel. From the results, it was observed that higher BP values were obtained with increasing dosing levels of nanoparticles. For the dosing levels of 10 and 20 ppm Mn 2 O 3 , the BP values increased by 14.38 and 19.56%, respectively. Meanwhile, the BP values of 10 and 20 ppm Co 3 O 4 increased by 7.96 and 11.5%, respectively. Furthermore, a reduction in CO and UHC emissions was observed with an increase in CO 2 when ethanol was mixed with the nanoparticles. From the results, the gasoline-ethanol fuel blend with 20 ppm Mn 2 O 3 nanoparticles produced the best combustion and emission performances. These findings are consistent with the previous study by Ananda et al. [75], where the addition of ethanol and nanoparticle additives in gasoline fuel improved engine performance and exhaust emissions.
Devarajan et al. [53] investigated the effect of different mass fractions of AgO nano-additive (5 and 10 ppm) and particle sizes (10 and 20 nm) on the performance, emission, and combustion behaviour of palm stearin biodiesel. At peak load conditions, a higher BTE value and a low BSFC emission were obtained from the biodiesel fuel blend with nanoparticles with a particle size of 20 nm and a concentration level of 10 ppm. The addition of AgO nanoparticles to the biodiesel reduced the harmful emission. It was observed that the biodiesel fuel with 10 ppm of 20 nm AgO nanoparticles exhibited the best engine performance. As compared to biodiesel, the in-cylinder pressure reduced by 2.2% and the net heat release rate value improved by 4.7%.
Tamilvanan et al. [124] investigated the performance, combustion, and emission behaviours of Calophyllum inophyllum seed oil (CISO) biodiesel with 30 ppm Cu additives in a single-cylinder diesel engine under varying loads. At all loads, a higher BTE value was obtained from the biodiesel blend with Cu as compared to that without additive. However, the BTE value was slightly lower than diesel. The reduction in BTE value in biodiesel can be ascribed to the lower heat of combustion. The addition of Cu nanoparticles in biodiesel fuel blends increased the combustion properties of the engine and reduced the emissions significantly.
Venu and Appavu [125] added Zr nanoparticles into the jatropha biodiesel for the CI engine. It was pointed out that the BSFC value of jatropha biodiesel with Zr nanoparticles was the lowest, and the BTE value was the highest as compared to diesel and biodiesel without Zr nanoparticles. A significant reduction in HC, CO, and smoke emissions was observed with a slight increase in NO x and CO 2 emissions. Some similarities can be found between this work and the work of Yogaraj et al. [126] who used Ag-TiO 2 nanoparticle as the fuel additive. By adding the additives, incomplete combustion led to lower BSFC, CO 2 , and HC emissions. However, a higher NO x emission was observed in all cases.
The effect of adding CuO nano-additives to the Neochloris oleoabundans methyl ester diesel (NOMED) blend was investigated by Kalaimurugan et al. [127]. A month later, the same authors published the work using RuO 2 nanoparticles, whose fuel properties were very similar to that of CuO nanoparticles [128] except for the difference in the cetane index value. The cetane index of RuO 2 and CuO was 52 and 55, respectively. The fuel test was performed using different concentration levels between 25 and 100 ppm under varying load conditions. From the results, both 100 ppm nanoparticles promoted a lower BTE, BSFC, and exhaust emissions.
Srinidhi et al. [129] studied the CI performance of NiO nanoparticles in Azadirachta indica biodiesel at different injection timings and dosing levels from 25 to 100 ppm.
Three types of injection timing were prepared including 23, 19, and 27°bTDC (before top dead centre). As a result, the increase in fuel injection timing and the presence of nanoparticles improved the overall engine performance and reduced the release of harmful emissions from the engine.
The blending of CuO nanoparticles in pongamia biodiesel fuel has undergone different engine operation characteristics in a single-stroke four-cylinder engine. By using a 10-mL Span80 surfactant and an ultrasonicator, 50 and 100 ppm CuO nanoparticles were mixed with the pongamia biodiesel to obtain different biodiesel blends. For the dosing level of 100 ppm, the engine performance and maximum emission reduction enhanced [54]. Higher cetane number and oxygen content improved the BTE value with a reduction in HC, CO, and smoke emissions [130][131][132][133].
In one of the most recent works, Soudagar et al. [134] investigated the influences of GO nanoparticles on the performance and emissions of a CI engine fuelled with dairy scum oil (DSOME) biodiesel using dosing levels of 20, 40, and 60 ppm under constant speed and varying BP and load conditions. GO is known as a nanoparticle with ultrahigh strength, good hydrophilicity, and dispersibility [147]. From the results, it was revealed that the net heat release and BTE values improved with an increasing amount of GO nano-additives in the fuel blends. Furthermore, the addition of GO nanoparticles in the biodiesel enhanced engine performance and emission characteristics. El-Seesy et al. [139,143] showed similar results on the performance and emissions of CI engine fuelled by Jatropha methyl ester biodiesel nanoparticle blend with GO nanoparticles. The addition of GO nanoparticles in the base fuel enhanced the combustion characteristics and reduced the exhaust emissions including CO, NO x , SO 2 , CO 2 , and smoke. It also improved the overall engine performance parameters such as BTE and BP with a reduction in the BSFC values (as shown in Figure 4). Such superior performance can be attributed to the high surface area to volume ratio of the mixture, which promoted a better fuelair mixing pattern during the combustion process.
Mei et al. [135] compared the combustion and emission of a standard rail diesel blended with non-metallic and metallic oxide nanoparticles. CNT and MoO 3 nanoparticles were used as the fuel additives in the study. All CNT diesel blend and MoO 3 diesel blend were found to achieve high efficiencies in fuel economy, combustion, and emissions as compared to pure diesel. This is because CNT nanoparticles possessed excellent thermal conductivity and surface deficits [148], while MoO 3 owned superior catalytic oxidation performance. Based on the findings, it was observed that CNT offered superior combustion efficiency and better ability to reduce exhaust emissions as compared to MoO 3 diesel blend.
Sivathanu and Anantham [137] studied the capability of MWCNT as a fuel additive in improving the performance, emission, and combustion behaviour of diesel engine fuelled with waste fishing oil. As compared with neat biodiesel fuel, the addition of MWCNT in biodiesel promoted a lower exhaust emission and a shorter ignition delay. Under 100% load, BTE increased by 3.83%. On the contrary, the BSFC value by 3.87% with the presence of MWCNT. The findings also showed a substantial decrease in engine exhaust emissions including CO, UHC, NO, and smoke by 25, 9.09, 5.25, and 14.81%, respectively. At the same time, a slight increase by 17.39% in CO 2 emission was also observed. These results are in good agreement with the findings of Sulochana and Bhatti [138] and El-Seesy et al. [146].
Sandeep et al. [140] studied the ability of CNT as a fuel additive in improving diesel engine performance using HOME. As compared to pure biodiesel at full load, the addition of CNT enhanced the BTE value by 2.24% and reduced the BSFC by 20.68%. Besides, a remarkable reduction in harmful emissions was also observed.
Basha [141] investigated the combustion performance of pristine biodiesel and biodiesel emulsions blended with CNT and DEE in IC engines. The experimental results showed that the biodiesel emulsion fuel with CNT and DEE showed better efficiency, emission, and combustion characteristics in contrast to pure diesel and biodiesel. The BTE value of CNT + DEE fuel increased by 3.5%. Besides, NO x and smoke emissions were reduced by 445 ppm and 35%, respectively, when compared with the pure diesel. The study also found out that the addition of 50 ppm of CNT and 50 mL of DEE to emulsified biodiesel fuel could reduce the delay in ignition. This is owing to higher cetane number of DEE.
Hosseini et al. [142] investigated the performance and emission characteristics of mixed diesel B5 and B10 with the addition of CNT. Different concentrations of 30, 60, and 90 ppm CNT nanoparticles were added into diesel. The findings revealed that the power, BTE, and EGT of all CNT fuel blends improved by 3.67, 8.12, and 5.57%, respectively. At the same time, biodiesel blends with CNT nanoparticle additive exhibited a significant reduction in BSFC. In terms of emission characteristics, it was found out that the CO, UHC, and soot emission of the diesel biodiesel mixture reduced with an increase in NO x emission.
Hariram et al. [145] added MWCNT nanoparticles into the Jojoba biodiesel and the addition of 50, 100, and 150 ppm MWCNT nanoparticles reduced the ignition delay period. The BTE values of all concentration levels were found to be increased, while the BSFC reduced. The concentration level of 100 ppm MWCNT nanoparticles produced better combustion characteristics and engine performance as well as low harmful emissions.
El-Seesy et al. [146] studied the effect of different concentrations of MWCNT (1-50 ppm) into the jojoba methyl ester diesel under different load conditions and engine speeds. The experimental analysis revealed that the addition of MWCNT nanoparticles improved the performance and emission characteristics. The BTE value increased upon the addition of MWCNT nanoparticle up to 16% in the biodiesel blend. At the same time, the BSFC performance decreased by 15%. By adding the MWCNTs into the biodiesel blend, the emissions of NO x , UHC, and CO significantly reduced. With the nanoparticle's concentration of 20 ppm, the NO x , CO, and UHC emissions Table 6: Summary of non-metal oxides nanoparticles as an additive in diesel/biodiesel fuel

Authors
Base fuel Nanoparticles Mass fraction of nanoparticles decreased by 35%, 50%, and 60%, respectively Overall, the concentration level of 50 ppm produced better combustion characteristics and engine performance as well as low harmful emissions.

Conclusion
In this article, a recent review of the effects of nanofluid as a fuel additive in diesel/biodiesel is presented. The application of nanofluid biodiesel blend in the ICE can serve as a potential approach in reducing GHG emissions and improving engine efficiency. Many previous studies demonstrated that nanoparticles can be used to enhance the fuel properties, engine performance, fuel combustion, and exhaust emission. Fuel properties are one of the most significant factors that determine the engine performance and combustion quality. The summary of this review article is listed as follows.
-The addition of nanoparticles plays a vital role in promoting better combustion quality. With the addition of nanoparticles to the diesel/biodiesel, it can improve the fuel properties such as kinematic viscosity, caloric value, flash point, density, and cetane number, leading to complete combustion. The increase in BTE is due to the catalytic activity and improvement in fuel properties. In contrast, a low calorific value will increase BSFC. In short, a higher calorific value will promote a lower BSFC and higher BTE. -Fuel properties such as kinematic viscosity, caloric value, flash point, density, and cetane number also depend on the type of biodiesel, type of nanoparticle and its size, and the amount of dosing level. This may have a varied effect on the combustion and emission characteristics. -Increasing the dosing level of nanoparticles can greatly enhance the engine performance and emission characteristics, excessive number of nanoparticles will lead to unburned fuel-air mixture during the combustion process. -The addition of ethanol and nanoparticles in gasoline fuel can also improve engine performance and exhaust emissions. -Among the above nanoparticles, aluminium is the best metal candidate, while CNT is used for non-metal nanoparticles. This is owing to their presence of excess oxygen quantity, and the positive effects of nanoparticles on the fuel properties (i.e. highest calorific value) of diesel/biodiesel blended could reportedly increase in combustion efficiency, leading to reduced BSFC and harmful emissions.
As a whole, the addition of nanoparticles in diesel/ biodiesel plays a significant role in improving the fuel properties and enhancing the performance of the CI engine as well as reducing the exhaust emissions.

Current challenges and recommendations
From authors' review, most of the results are favourable, because the addition of nanoparticles in the diesel/biodiesel can enhance the performance of CI engine and reduce the harmful emissions (HC, CO, smoke, and NO x emissions) causing the global air pollution. This literature review reveals that most of the experiments conducted showed a remarkable improvement in producing a lower exhaust emission. Moreover, there are contradictory observations by researchers regarding the engine performance such as BTE, BSFC, peak pressure, and emissions characteristics such as HC, CO, smoke, and NO x emissions as noted in the literature. Among them, few researchers reported that NO x emission release was higher with the addition of additives. The discrepancy occurred due to the different types of biodiesel, types of nanoparticles and its size, and also its volume concentration. The particle size is one of the important key factors that affect the combustion quality. Further study is therefore needed to overcome this discrepancy by considering the source of vegetable oils and different types of nanoparticles and its size by varying the dosing level of nanoparticles.