Introduction

Nanotechnology concerns the use of devices and systems that exhibit new characteristics and properties related to matter with dimensions between 1 and 100 nm. Many fields and endeavor take benefits from the evolution of nanotechnologies, including electromechanical engineering, material science, physics, biology, chemistry, electronics, and computer science. The use of nanomaterials highlights several innovated industrial applications belonging to these fields with very high performance and high resistance to unpleasant surrounding environments, such as the rise or fall of temperatures. Looking to the applications in heat transfer field for example, studies carried out over the last few years have shown that the addition of nanoparticles in a fluid such as particles of copper oxide (CuO), copper (Cu), aluminum oxide (Al2O3), or carbon nanotubes in water for example could improve the heat transfer characteristics. In fact, nanoparticles added in fluid significantly modify the thermal conductivity, which leads to a remarkable improvement in convective transfers when these nanofluids are used, which is proved in the work of Choi et al. [1] who found in the beginning of this field that the effective thermal conductivity of the water–Al2O3 mixture increases by 20% for a volume concentration between 1 and 5% of Al2O3. In fact, Brownian agitation, linked to the nanometric size of the particles, minimizes the sedimentation problems encountered with larger particles. Therefore, the suspension of these nanoparticles in a fluid leads to interesting thermal characteristics compared to traditional fluids and undeniable advantages in improving heat transfer, Eastman et al. [2]. Thermal conductivity of nanofluids can be significantly higher than that of pure liquids. This high thermal conductivity indeed designates nanofluids as potential replacement of conventional fluids used in heat exchangers for example in order to improve their performance, Keblinski et al. [3]. Moreover, the addition of nanoparticles in a liquid increases its viscosity and therefore the pressure drops, Yang et al. [4]. However, the lack of stability over time of certain nanofluids can lead to agglomeration of the nanoparticles and a change in their thermal conductivity, Daungthongsuk et al. [5]. Remind that in particle suspension in fluids, different types of interactions between the particles themselves but also between the particles and the fluid govern the behavior of a suspension. Note that, these microscopic interactions have a very important macroscopic incidences such as the relative motion of particles compared to the liquid, which not only supports the diffusion of the particles in a suspension but also causes the regrouping, the arrangement of particles in aggregates even sedimentation and phase separation. The particles are also subjected to the action of gravity and buoyancy. Indeed, if the density or dimensions of particles increase, the sedimentation effect became faster. However, decreasing the scale to nano, nanofluids prevent the phenomenon of sedimentation since the thermal agitation can compensate the action of gravity. In fact, under a critical size of solid particles, the Brownian motion compensates the sedimentation. Conventionally, these nanoparticles have sizes, which do not exceed 50 nm, and their volume concentration φ does not exceed 10%. However, two families of nanofluids can be distinguished: those intended for thermal applications, for which φ remains less than 10% and those intended to present a magneto or electroactive behavior where φ can reach up to 30%. Mainly three groups of volume concentrations can be found in the literature: 0.1 to 1% for highly diluted nanofluids, up to 10% for diluted ones and > 10% for weakly diluted nanofluids. The nanoparticles commonly used consist of metallic or non-metallic materials and carbon nanotubes. In Fig. 1, Julien Chevalier [6] plotted the main families of nanofluids usually studied. Through this figure, the author emphasized the composition, size and volume concentration of common nanofluids. The base liquids generally used in the preparation of nanofluids are those of common use in heat transfer applications such as water, ethylene glycol and engine oil. Table 1 groups together a non-exhaustive list of combinations of nanoparticles and base fluids prepared by various research groups. Almost, by analyzing the references [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114] cited in the present review, one or more combinations were used. Each row of the table gives the types of base fluids which were used with the nanoparticle cited in this row.

Fig. 1
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Principal families of studied nanofluids [6]

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Table 1 Nanoparticle/fluid combination
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Several nanoparticles made from metallic/intermetallic elements such as Cu [7], Ni [8] or ceramic compounds such as MoS2 [9], Fe2O3 [10], Fe3O4 [11], CeO2 [12] and ZnO [13] are reported in the literature. A deep incite in this literature, several other base liquids are tested which can be selected from mixture of water and EG (W/EG), polyethylene glycol, diethylene glycol (DEG), vegetable oil [14], paraffin [15], coconut oil [16], engine oil [17], pump oil [18], gear oil [19] and kerosene [20]. The studied nanofluids as it is presented in the literature are made through a suspension using three phases drawn in Fig. 2; solid phase (nanoparticles), solid/ liquid interface and finally the liquid phase (base fluid).

Fig. 2
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Nanofluids suspension phases [21]

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To manufacture these nanofluids, a special attention in nanoparticles production is needed at the aim to obtain the nanometric sizes and to avoid the agglomeration or to plug the circuit. Nanoparticle manufacturing processes are numerous, they can be classified into two categories as listed in Yu et al. [22]. The first category is related to physical processes, such as mechanical grinding or inert-gas condensation technique. However, the second category concerns the chemical processes, such as laser pyrolysis, chemical precipitation, thermal spraying and chemical vapor deposition. There are two main methods to manufacture a nanofluid:

  • The one-step method consists in producing the nanoparticles in the fluid. This procedure is not widely used, but it helps to prevent agglomeration and oxidation of nanoparticles. An example of process consists on the solidification of the nanoparticles, which are initially introduced into the base fluid as gas phase [23]. The main drawback of this technique is that it is not appropriate for mass production [22].

  • The two-step method consists of first producing the nanoparticles and then dispersing them in the base fluid. To allow good dispersion, strong mechanical action using a rotary or ultrasonic agitator is often necessary in order to break up agglomerates. In addition, to avoid the agglomeration due to the attraction forces between the particles, we use forces of electrostatic repulsion by charging the surface of the particles through a pH adjustment. We can also use steric repulsive forces using molecules adsorbed or grafted on the surface. The main drawback of this technique is that there are a bad dispersion of the nanoparticles inside the fluid because of the clusters formed by nanoparticles during the preparation [22]. Despite this drawback, the two-step method is the most widely used method for the preparation of nanofluids, especially those based on nanotube carbon particles [24]. It has economic advantages and allows nanofluids to be prepared in large quantities due to the expanded industrial production of nanoparticles.

Figure 3 presents the annual evolution of the published papers related to the use of nanofluids in automotive systems. It is to be mentioned that although the number is still limited, there has been notable growth over the past five years.

Fig. 3
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Number of research articles published per year in the last 10 years, obtained from a search using the keywords (Nanofluid automobile) or (Nanofluid automotive) or (Nanofluid radiator) in Article Title, Abstract, Keywords for Scopus and in Topic for Web of Science

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The mastery of the manufacture of nanofluids reveals a wide range of applications in different areas. The main objective of the present paper is to report a literature review on the main studies that investigated the use of nanofluids in applications related to automotive during the recent years. The paper contains three main sections; the first deploys the use of nanofluids in a car radiator. The second reviews the main contribution on the addition of nanoparticles in the fuel or engine oil. Finally, the last section reviews the assessment of the wear effects of nanofluids on materials used in a car coolant system and in the car engine.

The potential of nanofluids in applications related to automotive systems

The evolution of technology in automobile industry and the growth of its customers’ needs, force the auto industry to look for innovative solutions that push up the performance of their vehicle in terms of engine reliability and fuel consumption with the main vision to remain competitive. In fact, to achieve highest performances, vehicle engines need coolants, lubricants and transmission fluids with high thermal conductivity. However, using conventional synthetic high-temperature heat transfer fluids limits the capability of vehicle’s components such as radiators, engines, gearboxes, heating, ventilation and air-conditioning systems. For coolant for examples, cars usually use ethylene glycol and water mixture as engine coolant, which have a poor heat transfer rates due to their lower thermal conductivity. This is the fact that gives the opportunity to scientists to test new fluids with high thermal conductivity such as nanofluids.

The use of nanofluids in car radiator

Car engines are often cooled by circulating a liquid called engine coolant through the engine block, where it is heated, then through a radiator where it loses heat to the atmosphere, and then returned to the engine. It is common to employ a water pump to force the engine coolant to circulate and an axial fan to force air through the radiator like it is represented in Fig. 4. The use of fluids with high thermal conductivity in a car radiator, such as nanofluids, allows the car engine to resist overheating due to friction between the different components, which increases the engine performances especially for sports cars that need more horsepower also for cars used in places with extreme weather conditions.

Fig. 4
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Car coolant system [37]

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Choi et al. [25] have introduced nanocoolant, applied in automotive since 2001, by dispersing metallic and oxide nanoparticles in ethylene glycol-based fluids. Authors claimed through experimental investigations that there is a remarkable enhancement of the thermal conductivity compared to conventional coolants, which is in total agreement with the discussions presented in the work of Maranville et al. [26] who used the same nanoparticles dispersed in water and ethylene glycol/water. Goldstein et al. [27] demonstrated that nanofluids enhance the thermal diffusivity of the radiator coolant. However, using a classic method of dispersion, a limitation of agglomeration and oxidation caused by metallic nanoparticles is denoted during these years. This problem is solved during the recent years by the evolution of methods used in manufacturing and dispersion of nanoparticles.

Singh et al. [28] proved that the use of nanofluids with high thermal conductivity gives the possibility to automotive engineers to reduce 10% the frontal area of the car radiator which improve the aero-dynamism of the vehicle by reducing the air resistance which lead to a reduction in the fuel consumption. Delavari et al. [29] demonstrated that the use of nanofluids in a car radiator can lead to a gain of power needed for pumping. In fact, for a given heat transfer rate, the required base fluid flow rate is much higher than that denoted for nanofluids, which also reduce the fuel consumption. This is in complete agreement with the work of Peyghambarzadeh et al. [30] who proved that the overall heat transfer coefficient of the car radiator could be enhanced if the concentration of nanoparticles is increased especially for Fe2O3/water nanofluid, which enhances the car engine performance and decreases the fuel consumption. A 45% enhancement in the heat transfer efficiency of the car radiator was recorded in the work of Peyghambarzadeh et al. [31, 32] especially for low concentrations of Al2O3/water, Al2O3/EG in comparison with pure water, which is in contradiction with the author conclusions in their investigations on the use of Fe2O3/water nanofluid [30], which mark a question about the effect of nanoparticles concentrations and their types. Muhammad Ali et al. [33] proved the same conclusion with ZnO–water nanofluid. An enhancement up to 46% for 0.2% concentration of ZnO nanoparticles was recorded. A lowest enhancement percentage was recorded in the work of Naraki et al. [34] proving that the use of CuO/water nanofluid enhances the overall heat transfer coefficient up to 8% for a concentration of 0.4 vol%. Similarly in the work presented by  Leong et al. [35], the use of Cu nanoparticles dispersed in EG leads to the enhancement of the heat transfer by 3.8%. Vajjha et al. [36] proved that the friction factor and the convective heat transfer coefficient increase when the particle volumetric concentration of the Al2O3/EG, CuO/EG nanofluids increases. One can confirm that the highest concentrations of nanoparticles enhance the thermal efficiency of the car radiator. However, due to the increase of friction coefficient, several studies should be performed to investigate the negative effects of nanoparticles on the material wear and on the pumping power.

Analyzing the literature during the last few years, several types of nanoparticles have been dispersed in conventional coolant such as ethylene glycol, water and glycerol with the main purpose to test their performance in heat removal from a car engine. A summary of numerical and experimental studies on nanocoolant [38,39,40,41,42,43,44,45,46,47,48,49,50,51,52] is discussed in Table 2. It is remarked from this table that the thermal efficiency of nanofluids in a car radiator almost is the main purpose of investigations reported in this literature review. The most used nanoparticles in these investigations are the aluminum oxide (Al2O3), dispersed in water or ethylene glycol base fluids or a mixture between these two fluids with different percentages. The main factor analyzed is the effect of nanoparticles volume concentration. Some other factors are analyzed such as Reynolds number, the viscosity and the inlet temperature. The main common conclusions reveal the increasing effect on the thermal performance when all these parameters rise. Some contradictory conclusions about the effect of volume concentration on pumping power loss and the Nusselt number are denoted in this literature review, which need a deep analysis at the aim to found the best nanocoolant that can be safe used in a car radiator.

Table 2 Nanofluid applied on radiators
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Details of experimental studies are reported in Table 3, which describe the experimental setup, the main setting parameters of experiments such as the type of nanofluids, flow rate, temperature range, nanoparticle size and dispersion method and concentrations. Analyzing the experimental setups figured in Table 3, authors usually preferred to study nanocoolant using prototypes of car radiators plugged to a water pump (usually not specified if it is a car water pump working in the same real condition or not) that circulates nanofluids in a closed circuit equipped with a heater (as replacement of the heat generated by the car engine). The use of heaters instead of car engines differs considerably from actual working conditions, especially under extremely hot weather conditions. Ahmed et al. [45] and Palaniappan et al. [56] are the only authors who used a real car coolant circuit of a FIAT DOBLO 1.3 MJTD and HINO WO6D model (six-cylinder diesel engine). Analyzing the experimental results, presented in this literature review, it has been proved that nanoparticles concentrations, Reynolds number, and inlet temperature have an increasing effect on the thermal performances of the car radiator. The main drawback in these experimental studies is the absence of standardization of the experimental method and instrumentation, which lead to some lack of transparency in the determination of a precise enhancement percentage of the radiator performance under different type of nanofluids. However, the use of a real car coolant system that integrates a real car engine working in the real condition certainly leads to better results, where researchers can analyze with precision the performance of nanofluids and their effects on the working life of all engine components. The main idea is to collaborate with car manufacturer to test the best nanofluids on car prototypes used to validate the new car production series. In such tests, car manufacturers run these prototypes under extreme conditions to see the performance of each component of the car.

Table 3 Experimental setups
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Effect of the volume concentration

Most of analyzed results summarized in Table 2 showed that nanocoolant could enhance the thermal performances of the car radiator with different degrees of efficiency considering the type of nanoparticles, concentrations, type of base fluid, temperature, etc. Many factors can affect the heat transfer coefficient, a key parameter analyzed by many researchers cited in [38,39,40,41,42,43,44,45,46,47,48,49,50,51,52]. In fact, Fig. 5 plots the best performance of nanofluids tested in these studies, the figure indicates the maximum of heat transfer enhancement measured for the optimal volume concentration. The best performance is recorded in the work of Moghaieb et al. [52] about 78.67% achieved at 1% volume concentration of c-Al2O3 nanoparticles suspended in pure water at a bulk temperature of 80 °C and flow velocity of 2 m/s. The main drawback of the use of Al2O3/Water in a car radiator is the type of materials of the car engine components, in fact authors precise that this type of nanofluid should be used to cool engine components made of cast iron. Much other type of nanofluids tested in [38, 40, 41, 44, 45, 47] and [54] can enhance the heat transfer in a car radiator between 45 and 55%. Selvam et al. [38] measured 51% enhancement for the highest volume concentration 0.5 vol % (of graphene nanoplatelets dispersed in water-ethylene glycol mixture) and the maximum mass flow rate (100 g/s) at 45 °C inlet temperature. A maximum pressure drop of 4.88 kPa at 35 °C for the same concentration and flow rate. In Jadar et al. [40, 41], the heat transfer enhancement reached 45% for 0.1 vol% of f-MWCNT dispersed in Water-EG mixture at 45 °C. Li et al. [44] recorded 53.81% for 0.5 vol% of silicon carbide (SiC) suspended in water/ethylene glycol mixture at 50 °C. TiO2-water nanocoolant is tested in the work of Ahmed et al. [45] where the heat performance enhancement reached 47% for a volume concentration 0.2%. The thermal conductivity measured increases remarkably, if the volume concentration increases from 0 to 0.35 vol% as it is denoted from Fig. 6. However, 0.5% TiO2 dispersed in 40% EG and 60% water at 45 °C enhances the heat transfer about 45.4% [47]. Awais et al. [54] measured 50% for 5% volume concentration of Al2O3 dispersed in pure water circulated in a uniform serpentine tube at 5L/min, which makes highest volume concentration of Al2O3 more than 1 vol% dispersed in water-based fluids not the optimal choice to obtain the best thermal performance in a car radiator as it is confirmed in the result of Moghaieb et al. [52]. Going back to Fig. 5, lower volume concentrations between 0.4 and 1% are the best choice to enhance the performance of the car coolant system. However, some discrepancies in the determination of the optimal nanoparticles volume concentration and the precise heat transfer enhancement are remarked through this literature review. The range of volume concentration above 2% is not enough investigated which need more intention in further studies at the aim to optimize the amount of nanoparticles dispersed in base coolants.

Fig. 5
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Effect of nanofluid type and concentration on heat transfer enhancement

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Fig. 6
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Thermal conductivity of TiO2 nanofluid at different volume fraction [45]

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Figures 7–9 are plotted using experimental data presented in the work of Delavari et al. [29] who tested the use of Al2O3 nanoparticles in water and ethylene glycol base fluids. Figure 7 plots the evolution of pumping power reduction in function of different volume concentrations. The increase in Al2O3 volume concentration up to 1% reduces the power needed to circulate the fluid in the coolant system. In fact, the required nanofluid flow rate is much smaller than that needed for base fluids, which is explained by the increase in viscosity when the nanoparticle concentration increases as it is remarked in Fig. 8. Kole et al. [59] proved through several experimental results that viscosity is a key factor that influences both the convective heat transfer and the pumping power. The pressure drop denoted in several experimental results found in the literature [60,61,62,63] trigs the need to increase the pumping power in a car radiator. In addition, dispersing nanoparticles in fluids increases the friction factor with the material of the coolant circuit [64,65,66], which limits the radiator performance. Therefore, a deep incite on the interaction between nanoparticles and materials of the coolant circuit is needed through more careful numerical and experimental investigations. The principal aim of these studies should be the determination of the optimum nanocoolant and the best pipes’s surface skin. For best results, such studies need experimental setups that use a real coolant system with a real car engine.

Fig. 7
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Effect of volume concentration on pumping power reduction

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Fig. 8
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Effect of volume concentration on viscosity

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Fig. 9
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Effect of volume concentration on Reynolds number

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Figure 9 plots the decreasing effect of nanoparticles volume concentration on Reynolds number Re, which is a result of the rise of viscosity when the nanoparticles volume concentration rises.

Effect of Reynolds number

It is proved in several research papers such as [67,68,69,70,71,72] that the Reynolds number is a key parameter that influences the heat transfer coefficient of nanofluids. Authors preferred the turbulent flow regime at the aim to avoid sedimentation and agglomeration of nanoparticles, which improve the performance of a nanocoolant circuit. Chavan et al. [67] proved that the increase in turbulence improves considerably the heat transfer of both conventional fluids and nanofluids. Ali et al. [68] denoted a remarkable increase in the heat transfer coefficient by increasing the Reynolds number from 20,000 to 40,000, which proves the advantage of turbulence in enhancement of nanofluids performances. Karimi et al. [69] used the laminar flow regime (Re: 350–1060) in vertical and horizontal radiators, they confirmed that Reynolds number push up the Nusselt number and pressure drop in a nanocoolant circuit. Akash et al. [70] used also the laminar flow regime (Re: 300–1300), they linked the increasing effect of Reynolds number with the type of nanoparticiles dispersed in base fluids. In fact, authors confirmed that the overall heat transfer coefficient could be slightly increased with Reynolds number for copper and MWCNT nanofluids. However, the values of heat transfer coefficient do not change in the case of aluminum nanofluids, which makes some contradictions with the results of many other researchers such as Kumar et al. [71] who tested Al2O3/water nanofluid. In fact, in this work, authors confirmed that the increase in the flow rate increases considerably the heat transfer rate, which is in correlation with the Reynolds number. Toh et al. [72] showed in their work that volumetric concentration and Reynolds number enhance the Nusselt number. The best enhancement percentage is calculated for 0.5 vol.% of GnP and a Reynolds number equal 2000.

Figures 10 and 11 plot samples of results found in the literature [45, 56] that clearly show the positive effect of Reynolds number on heat transfer coefficient and the pumping power needed in a car radiator.

Fig. 10
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a Average of heat transfer coefficient as a function of Reynolds numbers [45], b overall of heat transfer coefficient as a function of Reynolds numbers on fly ash nanofluid [56]

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Fig. 11
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Influence of the Reynolds number (fly ash coolant) on pumping power of radiator [56]

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The main downside denoted in this literature is that the effect of Reynolds number on the heat transfer enhancement has different percentages from an investigation to another, which makes difficult to determine the best choice between any two nanofluids and to find which one exhibits more heat transfer characteristics.

Effect of the inlet temperature

The inlet temperature has an influence on the thermal performance of a car radiator, but it is less important compared to the effect of nanoparticles volume concentration and Reynolds number. In fact, if the inlet temperature increases, the viscosity of nanofluid decreases, which is clear in Fig. 12 plotted in the work of Li et al. [44] also proved in the review of Zhao et al. [73]. The decrease in viscosity helps the Brownian motion and the interaction between nanoparticles. Several investigations proved that the rise of temperature enhances the thermal conductivity of the nanofluid, but at certain limit, the rise of inlet temperature decreases the thermal performance of the radiator. Sharma et al. [74]. proved that the increase in inlet temperature of Al/water nanofluids enhances the thermal conductivity of the car radiator. However, the temperature can affect the friction factor, and at a certain value, the performance of the radiator will decrease, which is in complete agreement with the work of Sumanth et al. [75] who investigated the use of carboxyl graphene nanoplatelets/EG-water nanocoolant. However, in the work of Muhamed Ali et al. [33] and Ali et al. [68], authors remarked that the inlet temperature weakly influences the heat transfer rate in the case of ZnO/water and MgO/water. Samira et al. [62] noted that the increase in inlet temperature helps to reduce the pressure drop in a CuO/water coolant circuit which improves the radiator performance. Using the same nanofluid Naraki et al. [34] proved that the overall heat transfer coefficient decreases when the inlet temperature increases from 50 to 80 °C. However, the results of Li et al. [44] plotted in Fig. 13 in the case of SiC/EG-water nanocoolant proved that by increasing the temperature from 10 to 50 °C, the thermal conductivity increases remarkably. In the work of Tijani et al. [76], a numerical simulation studying the distribution of heat transfer across the surface of a radiator with flat tubes and louvered fins is presented. In this investigation, authors plotted the temperature profile, which proved that the heat is transferred via conduction and convection to the walls of the flat tube and to the fins.

Fig. 12
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Effect of temperature on viscosity of SiC nanofluid [44]

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Fig. 13
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Thermal conductivity of the nanofluid at different temperatures [44]

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Analyzing such literature review, it can be noticed that it is difficult to confirm one conclusion about the effect of temperature on the thermal conductivity of nanofluids, which is due to a variable influence of temperature on the thermophysical properties of nanofluids.

From the cost point of view, almost it can be noted that the manufacturing and maintenance of the radiator cost approximately 20 percent from the whole cost of the car engine. Through the use of nanofluids in the radiator, the performance of this key subsystem is remarkably increased. However, the design of this system can be improved to be smaller or integrating less components needed for cooling or circulating the nanocoolant. In fact, reducing the size or simplifying the design of the radiator can decrease the manufacturing and also the maintenance cost of the cooling system equipment. Compared to classic coolant, it is not costly to adjust the pH and add surfactant for the nanofluids to increase the heat transfer performance of the car radiator.

The use of nanofluids in transmission oil and fuel

Nanofluids used in transmission

During the last few years, many researchers have introduced the suspension of nanoparticles in oil-based fluids. Several type of nanoparticles and oils have been tested looking for the best thermal conductivity and dynamic viscosity, which leads to a high thermal efficiency needed in lubricant applications such as car engine oil and gearbox oils.

About ten years ago, Mohammadi et al. [77] investigated the suspension of Al2O3 and CuO in engine oil. They concluded that the thermal conductivity increased with the increase in concentration. The maximum enhancement calculated is 5% for alumina and 8% for copper oxide at 2% volume concentration. Similar conclusion has been remarked in the work of Vasheghani et al. [78] who tested the suspension of alumina and aluminum nitride in engine oil. Authors denoted that aluminum nitride engine oil nanofluid has the maximum thermal conductivity (75.23% enhancement at 3%) especially for the smaller size of nanoparticles. The main downside in this result is the high volume concentration of nanoparticles that can lead to some problems of performance in the engine components. Ettefaghi et al. [79] used MWCNTs-engine oil nanofluid, which enhanced the thermal conductivity about 27% at 0.5 vol%. Adding the zinc oxide with MWCNTs, Dinesh et al. [80] remarked an enhancement of the friction coefficient, wear resistance added to the enhancement of the thermophysical proprieties such as the flash point, viscosity and thermal conductivity. However, it is proved in the work of Rehman et al. [81] that single carbon nanotubes (SWCNTs) dispersed in engine oil gives higher skin friction and Nusselt number compared to multiwalled carbon nanotubes (MWCNTs) which is due to higher thermal conductivity and density. In addition, it is reported in these researches that nanoparticles enhance the performance of lubricant, which decreases the friction between the engine components and leads to a reduction in wear and material damage that increase the car engine performance and durability. Recently, many papers [82,83,84,85,86,87,88,89,90,91,92,93,94,95] have investigated the suspension of MWCNT, Al2O3, Fe2O3, MgO, ZnO, Cr2AlC, MoS2-WS2, Ni-MoS2 in engine and gearbox oils. The main contributions of these papers are reported in Table 4. Overall, authors agreed that the thermal conductivity of nanolubricant is enhanced when the temperature and volume concentration of nanoparticles increase. Promising results have been remarked in the suspension of alumina and carbon nanotube with lower volume concentration. Especially in the reduction in friction coefficient, and the prevention of wear and damage of mechanical components.

Table 4 Nanofluid applied on car engine as fuel or coolant
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Analyzing this literature review, one can note that it remains difficult to define until now the optimal value of nanoparticles volume concentration that should be dispersed in engine and gearbox oils. In fact, the heat transfer enhancement reached good values under low and high volume concentrations, which is related to the type and size of nanoparticles. The major factor that can help in further research is that nanolubricant should have an optimal value of viscosity that ensure the best lubricant film between components at the aim to reduce friction, which prevent the material wear, and damage. Noting that for a best performance of the car engine in higher temperature and during cold starts, a reduction in viscosity of lubricant is needed. Certainly, a deep investigation is needed to determine the best volume concentration of nanoparticles. In fact, researchers need to determine the best range of viscosity (which is positively influenced by the increase in volume concentration as seen in Fig. 14) for the best engine performances.

Fig. 14
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Viscosity enhancement in function of nanoparticles volume concentration [89]

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Nanofluids used in fuel

Nanofluids are not widely tested in fuel addition compared to their application in the car radiator or in engine and gear oils. However, the addition of metal-based nanofluids in fuel attracted the intention of many researchers. The main purpose in their investigations is the reduction in fuel consumption and gases’ emissions. The main new researches [96,97,98,99,100,101] performed during the last few years are reported in the last section of Table 4. Some tests on the addition of nanoparticles in biofuels (mustard oil methyl ester [97], AC BDD [98], Jatropha biodiesel [99], orange peel oil biodiesel [100], honge oil methyl ester [101]) have been reported also in this table. The main conclusions denoted from these researches and some previous ones such as [102,103,104,105,106,107] are that nano-added particles reduce the engine outflows generated by biodiesel such as NOx, SO2, CO, CO, HCs and smoke emissions. Many nanoparticles promote more oxygen, which help the combustion process in the engine.

For instance, during the combustion process of diesel fuel, more hydrogen can be produced by dispersing aluminum nanoparticles that help the decomposition of water. Aluminum nanofluids mixed with diesel fuel could enhance the combustion heat and reduce smoke and nitrous oxide from the engine emissions. Related to some type of nanoparticles, some researchers remarked a dramatic drop in NOx and SO2 emissions, and a remarkable increase in CO emission and smoke opacity such as the work of Sarvestani et al. [96] who tested Fe3O4 nanoparticles. A new detailed review on the effects of nano-additives on toxicity and exhaust emissions is presented in the work of Norhafana et al. [108].

Note that, some mechanical problems should be analyzed when using nanofluids in fuel. In fact, some mechanical defects can happen in the injection system (injectors, pump, and pipes) for example which usually have a high maintenance cost. Corrosion problems should be also analyzed certainly in the case of use of water-based additives, a major problem causing a material wear and damage leading to more mechanical defects with highest maintenance cost.

Furthermore, regarding the nanofluids capability in reducing the fuel consumption and the combustion efficiency, the cost will be considerably reduced. Certainly, less emission rate, less NOx production and less fuel consumption are key parameters that can be considered in the application of nanofuels. Capable fluids have promising attractions regarding above-mentioned considerations.

Wear effects of nanofluids on materials of the car coolant system

The present section reports the main contributions of few researches found in the literature about how nanofluids react with the radiator material, or with all the other components of the cooling system of the car engine such as the pump, the pipes, the engine block and the cylinder head. In fact, few studies are related to the eventual reactions between nanofluids and radiator’s materials. The procedure used on these tests is based on the calculation of the mass loss under different impact angles and fluid velocity. Celata et al. [109] investigated the effects of nanofluids flow on metal (copper, stainless steel, aluminum) surfaces. The action of several nanoparticles such as Al2O3, SiC, TiO2 and ZrO2 has been analyzed. Authors remarked that stainless steel has the best resistance to nanofluids flow; however, the aluminum is the weakest material especially in the case of Al2O3 nanoparticles addition. From the observation of the effects of the nanoparticles on pump gears, it is denoted that Al2O3 nanoparticles caused the most serious damage as it is presented in Fig. 15, while TiO2 nanoparticles remain the less effect.

Fig. 15
figure 15

Abrasion of the PTFE gear: new gear (a), using TiO2 (b) and using Al2O3 (c) [109]

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Bubbico et al. [110] proved through experimental investigations using the same materials and the same nanoparticles that the material damage is caused by chemical corrosion rather than by mechanical erosion, which can be solved by maintaining the pH of suspension within the passivation range, at this time aluminum material can resist to the nanofluid flow reactions. Aktaruzzaman [111] has taken a measurement of the normalized Ra roughness for 3003-T3 aluminum after a several hours of treatment with Al2O3/distilled water nanofluid (2 vol% and 10.7 m/s jet speed). The result plotted in Fig. 16 marks a decrease in the surface state caused by the action of nanofluid flow. In fact, the normalized roughness is increased by 50% compared to the action of flow without alumina nanoparticles. A.M Mohammed [112] proved that the suspension of Cu in a distilled water decreases the cavitation phenomenon. Properties of nanomaterials and their effects on the erosion–corrosion behavior due to the cavitation phenomenon cause pipe wall erosion. Gandham et al. [113] conducted a study of corrosion resistance in automotive coolant system, measured in terms of mass loss of materials. Based on this study, authors recommended the use of oxidized MWCNTs in automotive systems, while silver and Al2O3 nanoparticles produce a higher wear rate than the base fluid. Xian et al. [114] studied the erosion–corrosion of an aluminum impeller of a water pump subjected by nanofluid (GnP/Water-EG) flow. It was observed that nanocoolant favors the erosion–corrosion, which increases the wear of the impeller as denoted in Fig. 17. However, no remarkable difference in the corrosion effect is found between base coolant and nanocoolant.

Fig. 16
figure 16

Effect of Al2O3/water nanofluid on surface roughness for 3003-T3 aluminum [111]

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Fig. 17
figure 17

Views of impeller for base coolant (a), (b) and nanocoolant (c), (d). [114]

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It is very important in these types of investigations to analyze a wide range of nanocoolant by varying the volume concentrations, the flow rate, temperature, etc., at the aim to understand which type of nanocoolant causes higher wear rate. It is necessary to analyze the wear of all the coolant system components, certainly those having the highest maintenance cost. The initiation of further researches to assess wear and damage of materials in the engine bloc and cylinder heads is very important, which is needed for better mastery of the technology of nanocoolant and which type leads to the best thermal performances without damaging the principal components of a coolant system such as the pump, the radiator, the engine bloc, the cylinders.

Discussions

Analyzing the previous sections, it is clear that nanofluids applied as coolant in a car radiator demonstrated better efficiency due to their higher thermal conductivity compared to conventional coolants. Nanocoolant pushed up the performance of the radiator at higher level and opened the possibility in future to improve the design of radiators to be smaller and lighter by integrating less components needed for cooling or circulating the nanocoolant, which will decrease the manufacturing and maintenance costs of this car subsystem and leading to better fuel consuption and gase’s emissions. Analyzing carefully the different studies presented in Sect. 2.1, a classification of nanocoolant is drawn in Fig. 18 in terms of order of thermal performance. This classification is based on different comparisons presented in [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76].

Fig. 18
figure 18

Classification of nanocoolant in term of thermal performance

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In the application of nanofluid in a car radiator, common findings can be highlighted as follows:

  • The thermal performance of the car radiator increases when the nanoparticle volume concentration, Reynolds number, the viscosity, and the inlet temperature increase. Commonly, the overall heat transfer coefficient of the car radiator is enhanced at higher nanoparticles concentrations.

  • The use of nanofluids with high thermal conductivity gives the possibility to automotive engineers to reduce the exchange area of the car radiator which leads to the reduction in the fuel consumption by improving the aerodynamic effects of the car. By using nanocoolants, the design of the radiator can be improved to be smaller, lighter and integrating less components needed for cooling fluids, which will reduce the manufacturing and maintenance costs of this car subsystem.

    Some limitations noted can be cited as follows:

  • The use of classic method of dispersion can cause a problem of agglomeration and oxidation induced by metallic nanoparticles. However, the turbulent flow regime is preferred to avoid sedimentation and agglomeration of nanoparticles and thus enhances the performances

  • The use of nanofluids as coolant causes more pressure drop, which is explained by the increase in viscosity which requires more pumping power and limits the radiator performances.

  • Some contradictory conclusions especially about the effect of nanofluids volume fraction on pumping power loss and the Nusselt number are encountered, which requires further investigation at the aim to find the best nanocoolant that can be safe used in a car radiator.

Figure 19 plots a classification of several nanofluids tested as lubricant in a car engine or a car transmission system. This classification is based on the different studies presented in [77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95]. It is remarked that hybrid nanofluid gives better performance to these system by reducing friction and improving the heat transfer coefficient. Generally, the viscosity nanolubricants should have an optimal value which guaranties the best lubricant film between components in order to reduce friction, which prevents the material wear and helps the lubricant flow.

Fig. 19
figure 19

Classification of nanofluids tested in car engine and transmission

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The tests of nanoparticles suspension in fuels remain few, as said previously in Sect. 2.2.2. However, one can note that TiO2 nanoparticle added in biofuel seems to be the best in terms of toxic gases emissions and in terms of engine performance enhancement. Nevertheless, some mechanical problems should be analyzed when using nanofluids in fuel. In fact, some mechanical defects can happen in the injection system (injectors, pump, and pipes) for example, which usually have a high maintenance cost.

During the last few months, some findings have been published about new nanofluid fuel. Ao et al. [115] mixed kerosene and nano-aluminum (n-Al) particles coated with polydopamine (PDA) at the aim to improve the stability of combustion. Promising results have been presented proving that n-Al/kerosene coated with PDA is the better choice in terms of combustion stability compared to uncoated n-AL/kerosene and other PDA-coated nanofluids. In similar investigation, Gao et al. [116] studied the combustion of n-Al/CuO kerosene fuels coated with PDA. Suozhu et al. [117] noted an enhancement of combustion in engines operated with methanol mixed with CeO2 nanoparticles. The NOx and smoke emissions decreased by 70.9% and 90.3%, respectively, compared with the diesel mode. In fact, the environmental impact of the combustion of nanoparticles mixed in fuel is a key parameter that needs further development in this field.

Conclusions

The present paper reported a literature review on the use of nanofluids in applications related to automotive during the recent years. Applications of several nanofluids in car radiator, engine, transmission systems and fuel mixture are comprehensively reviewed in the different sections. As novelty, the paper highlighted new tested nanofluids with critics of their efficiency, their wear effects on components of the car engine, their environmental impact in terms of gases emissions when nanoparticles are added to fuels. Based on deep analysis of vast number of available references, a classification of nanofluids within their efficiency to enhance the performance of these subystems is drawn. For engine cooling system, nanofluids can be used (1) as coolant in the car radiator. Due to the high thermal conductivity of nanoparticles, the heat transfer coefficient in the system can be enhanced with variable percentage in function of the type and the volume concentration. The main drawback is that nanocoolants cause more pressure drop, which is explained by the increase in viscosity which requires more pumping power and limits the radiator performance, which pushed the majority of researchers to use low nanoparticle volume concentrations. Some discrepancies in the determination of the optimal value of volume fraction and the precise heat transfer enhancement is remarked through this literature review. In addition, the use of classic methods of dispersion causes a limitation of agglomeration and oxidation impelled by metallic nanoparticles. (2) Dispersed in engine or gearboxes oils, several nanoparticles lead to a reduction in friction coefficient and prevent wear and damage of the mechanical components. Hybrid nanolubricant such as Al2O3-MWCNT/oil presented the best performances as coolant. It is to be noted that, further detailed studies are required to determine the optimal values of viscosity that guaranties the best lubricant film between components. Finally, nanoparticles mixed with fuel such as diesel, biodiesel and kerosene could enhance the combustion heat, reduce smoke and NOx from the engine emissions which improve the environmental impact of fuels. Current research on nanofuels is still at its initial steps and needs further development.