Introduction

Improving lubrication properties is an essential economic parameter in overall energy consumption of mechanical systems. Nowadays, all of the lubricants with mineral or synthetic base need additives to gain intended properties. Nano-lubricants are a new level of lubricants that are prepared by dispersion of particles of nano-sized in the base oil. Using chemical additives with unique properties causes an improvement in physical, chemical, and mechanical properties of the base oils [1,2,3]. As an example, the addition of nanoparticles to base oil improves its thermal characteristics. Enhancement of heat transfer is affected by Brownian motions and inter-particle forces [4,5,6]. Viscosity changes need to be considered along with the enhancement of heat transfer in nanofluid compared to the base fluid, since it is an important parameter in the evaluation of needed power for pumping in thermal systems. Addition of nanoparticles causes a change in the rheological behavior of the base oil. Researchers have lots of studies on rheological properties of nanofluids [7,8,9,10,11,12,13,14,15,16,17,18]. For example, Krishnakumar et al. [19] have studied rheological properties of Al2O3- rthylene glycol nanofluid and showed that despite the behavior of the base fluid, nanofluid shows shear-thinning behavior in low temperatures and shear rates. Hemmat and Abbasian Arani [20] have shown that carbon nanotubes considerably affect the non-Newtonian behavior of nano-oil containing MWCNT-TiO2. Also, they showed that by increasing the concentration, the shear-thinning behavior of nano-oil will be intensified. Sepiani et al. [21] have investigated ZnO/SAE50 nano-oil viscosity in φ = 0−1% in different temperatures and shear rates. They proved that the viscosity of this nano-oil increases by 12% with increasing the amount of ZnO nanoparticles. In another research, Nadoushan et al. [22] have predicted the rheological behavior of SiO2-MWCNT/10W40 hybrid nano-lubricant using an artificial neural network. Soltani and Akbari [23] have investigated the effects of temperature and solid volume fraction on the viscosity of MgO-MWCNT/EG hybrid nanofluid. In this research that was done in φ = 0−1% and T = 30−60 ˚C, the maximum viscosity enhancement was 168% in φ = 1%. Hemmat esfe et al. [24] have shown that maximum viscosity enhancement in alumina-engine oil nano-lubricant was 132% compared to the base oil. Table 1 gives a brief summary of researches accomplished on nano-lubricants viscosity.

Table 1 A brief summary of studies accomplished on nano-lubricants viscosity in different temperatures and concentrations
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Furthermore, applications of nano-lubricants are very wide due to their abilities for improving surface properties and engine efficiency and decreasing maintenance and costs of systems. Researchers have synthesized different nanomaterials from metallic, organics, and minerals for application in nano-lubricant production with improved tribological properties [34,35,36,37,38,39,40,41,42,43]. Using nanoparticles causes a reduction of wear and friction coefficient in oil. There are some mechanisms for this favor behavior of nanoparticles in oil. These mechanisms are as follows:

  • Rolling effect of nanoparticles improves relative motion of contacted surfaces [44,45,46]

  • These nanoparticles have repairing effect on surfaces in a way that it fills surface perforations and results in an improvement in roughness of the surface [47,48,49].

  • These nanoparticles create a strong protective film between the surfaces to ease their lubrication [50, 51].

Lee et al. [52] have investigated lubrication properties of nano-oil with dispersed fullerene nanoparticles with disk on disk tribometer. Results of this experiment, which was carried out in different exerted forces and solid volume fractions, have shown that the presence of these nanoparticles decreases friction and wear. Nasiri Khozani et al. [53] stated that using nanoparticles in tractors engine decreases wear in cylinder, shaft, and bearings by 68%. Table 2 gives a brief summary of researches on tribological properties of nano-lubricants.

Table 2 A brief summary of studies accomplished on tribological properties of nano-lubricants
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Some of the researchers have studied other important factors of engine oil including flash point and pour point. Ahmadi et al. [59] measured flash point and pour point of nano-oils containing MWCNT nanoparticles with three different concentrations. They observed that the flash point and pour point increase by 13% and 3.3%, respectively, with a mass percentage of 0.2%. Moreover, these properties decrease with further increase in concentration. In another research, Ahmadi et al. [60] dispersed CuO nanoparticles in engine oil. They noticed that flash point has enhanced by 7.9% in a concentration of 0.1% compared with the base oil.

Most researchers have discussed the rheological and tribological properties of nano-lubricants with different nanoparticles and base oils. In this paper, for the first time, despite the fact that the effects of SiO2 nanoparticles on different properties of SAE40 engine oil (pour point, flash point, rheological, and tribological properties) have been investigated experimentally, the anti-wear characteristic of this nano-lubricant has been evaluated by using it into diesel engines in real condition, and the abrasive analysis has been performed. The purpose of using SiO2 nanoparticles is easy-suspending of these nanoparticles as they are oil-friendly. Moreover, the spherical shape of SiO2 nanoparticles is crucial matter in order to improve the tribological properties of engine oil. Hence, these factors are critical ones to choose suitable nanoparticles in engine oils.

Preparation of nanofluid

The samples used in this study vary from 0% to 1%, and the concentrations have been chosen to have meaningful changes.

Materials

Materials used in preparation of this nano-oil are:

  1. (1)

    SAE40 engine oil (base fluid) has been produced by Pars Oil Company, which is a suitable lubricant for diesel engines. The properties of this lubricant are presented in Table 3, which are taken up from the specification of the SAE40 engine oil.

  2. (2)

    SiO2 nanoparticles prepared by AEROSIL Company have been used. The properties of these nanoparticles are presented in Table 4.

Table 3 Properties of SAE40 engine oil
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Table 4 Properties of SiO2 nanoparticle
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To determine the morphology and structure of nanoparticles, XRD and SEM analyses are performed and the results are presented in Figs. 1 and 2. As can be seen in XRD pattern, these nanoparticles do not have crystalline structure and are amorphous.

Fig. 1
figure 1

SEM images of SiO2 nanoparticles

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Fig. 2
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XRD analysis of SiO2 nanoparticles

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Nanoparticle dispersion

Nano-lubricants used in this research are prepared using a two-step method. Two-step method is the fastest and most economical way of nanofluids production. A digital scale, magnetic stirrer, and ultrasonic homogenizer are used in preparation of this nano-lubricant. First, SiO2 nanoparticles are added to the base oil and mixed with magnetic stirrer for 30 min. Then, the agglomerations are broken in solution using ultrasonic waves for 40 min with power of 200 W. Then, nanoparticles are completely dissolved in base oil in a way that there is no evidence of sedimentation or change in color of the solution after one month. Figure 3 shows the nano-lubricant samples after one month.

Fig. 3
figure 3

Samples after 1-month

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Specifications of measuring equipment and experimental conditions

Viscometer

Viscosity measurement of the samples is performed using cone and plate viscometer CAP2000 + manufactured by Brookfield Co (USA) with accuracy of \(\pm 2\mathrm{\%}\). The specifications and operational conditions of this device are listed in Table 5.

Table 5 Specifications of CAP2000 + viscometer
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Nano-lubricants viscosity is measured in six solid volume fractions (0%, 0.1%, 0.25%, 0.5%, 0.75%, and 1%), temperature in the range of 25˚C–65 ˚C, and shear rates up to 13,000 1/s. Figure 4 shows CAP2000 + viscometer.

Fig. 4
figure 4

CAP2000 + viscometer

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Pin-on-disk tribometer

In order to measure tribological properties of nano-lubricant such as friction coefficient and wear rate, a pin-on-disk tribometer is used according to the ASTM G99. Description and operating conditions of this equipment are presented in Table 6.

Table 6 Specifications of pin-on-disk tribometer
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These experiments are carried out for this nano-lubricant with solid volume fractions of 0%, 0.01%, 0.025, 0.05%, and 0.1% under two different loads of 120 N and 180 N at room temperature. Time for each experiment is 1 h, and speed of disk rotation is 120 rpm. The disk is made from CK45 steel, and it is replaced with a new one after each measurement. Figure 5 shows a schematic of the test procedure.

Fig. 5
figure 5

Pin-on-disk tribometer

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Diesel engine

In order to compare the quality of prepared nano-lubricant with the base oil, two similar one-cylinder diesel engines manufactured by KAMA Motors Company are used. Specifications and operational conditions of two engines are presented in Table 7.

Table 7 Specifications of diesel engine
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To compare the performance of nano-oil and base oil, these two samples have been tested in operational condition of diesel engines. The operational times of both engines are thirty hours dividing to ten intervals. After thirty hours, both engine oils are drained according to the standard conditions. Analysis of abrasive particles is performed for both samples. Results are discussed in the following section.

Results and discussion

Rheological behavior

In order to investigate the rheological behavior of this nano-lubricant, Fig. 6 has been presented which shows viscosity changes versus shear rate in different solid volume fractions. As shown in this figure, in temperatures higher than T = 45 ˚C, the slope of the lines is close to zero in all different concentrations. Deviation from zero slopes of lines is increased by increasing solid volume fraction. The pseudoplastic behavior can be perceived, since viscosity is decreased with increasing shear rate. The maximum deviation of the slope belongs to φ = 1%.

Fig. 6
figure 6

Viscosity versus shear rate

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Figure 7 shows shear stress changes versus shear rate in different solid volume fractions and temperatures. This figure also presents one of the methods of investigating the rheological behavior of the fluids. The deviation from the straight line shows nanofluid behavior tendency to non-Newtonian fluid behavior. The presence of nanoparticles causes to not have a linear trend between the shear stress and shear rate.

Fig. 7
figure 7

Shear stress versus shear rate

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Different theoretical and experimental models for nanofluids viscosity have already been proposed by researchers. A brief summary of these models is presented in Table 8. Moreover, the errors of experimental results with these models have been presented by Eq. 1. These values obtained in volume fraction of 0.1%, and two temperatures of 35˚C and 55 ˚C. As can be seen, the mentioned models do not consider the temperature and therefore cannot predict the viscosity of nano-lubricant accurately.

Table 8 Important models for viscosity of nanofluids
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$${\text{Error}}(\%)=\frac{{\mu }_{\text{exp}}-{\mu }_{\text{theory}}}{{\mu }_{\text{theory}}}*100$$
(1)

Figure 8 shows a comparison between those obtained from previously mentioned models and experimental viscosity data in different solid volume fractions at T = 45˚C and shear rate of 5332 s-1. It can be seen that Einstein, Bachelor, and Wang models have a good approximation of nano-oil viscosity at concentrations less than 0.5%. More deviation is evident in higher concentrations, as these models do not consider the effects of temperature on the viscosity of nanofluids. Nevertheless, Wang’s model has higher accuracy between these models with experimental results. So, the presentation of a new experimental model is essential for having a more accurate prediction of viscosity for nano-lubricant containing SiO2 nanoparticles which are presented in the following sections.

Fig. 8
figure 8

Comparison between theoretical models and experimental results at 45 °C

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It is expected that fluid viscosity decreases with increasing temperature due to the reduction in intermolecular forces. Figure 9 shows the variations of nano-lubricant viscosity with temperature. It is observed that temperature increment causes a similar reduction in base oil and nano-oil viscosities in all solid volume fractions. According to this figure, the slope of the curve is variable in different temperatures and it decreases with increasing temperature.

Fig. 9
figure 9

Viscosity versus temperature

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Figure 10 shows relative viscosity changes in different temperatures and solid volume fractions, in the shear rate of 5332 (s-1). It is obvious that the viscosity of nano-lubricant increases with increasing concentration. Also, relative viscosity in φ = 0.25% has experienced a sudden drop in all temperatures. According to these observations, φ = 0.25% would be a proper choice for industrial applications due to the minor changes in its viscosity. On the other hand, a sudden drop can be observed in Fig. 10(b) in T = 35 ˚C and solid volume fractions of 0.1%, 0.25%, 0.5%, and 0.75%, and after that, it increases with increasing temperature. So, T = 35 ˚C is a specific temperature in different applications of this nanofluid.

Fig. 10
figure 10

a Relative viscosity versus solid volume fractions, b relative viscosity versus temperature in shear rate of 5332 s-1

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Figure 11 shows the power-law index versus solid volume fraction of nano-oil in different temperatures. According to Eq. 2, if n = 1, then the nanofluid behavior is Newtonian, if n < 1, it has pseudoplastic behavior, and if n > 1, it shows dilatant behavior. It can be observed in this figure that n < 1 in all solid volume fractions and nano-oil has pseudoplastic behavior. Also, values of n decrease with increasing temperature. The minimum value of n, which is about 0.8, corresponds to the φ = 0.25% and T = 65 ˚C. Hence, it should be careful in using this concentration in lubrication to not pass the allowable viscosity index.

Fig. 11
figure 11

Power-law index versus solid volume fraction

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$$\tau =m{\dot{\gamma }}^{\text{n}}$$
(2)

Figure 12 shows the consistency index of nano-oil versus solid volume fraction at different temperatures. The maximum value of this index corresponds to T = 25 ˚C, and it decreases with increasing temperature. Increasing nanoparticle concentration in the base oil results in an increase in the consistency index except, at T = 65 ˚C.

Fig. 12
figure 12

Consistency index versus solid volume fraction

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The exact values of n and m are presented in Table 9.

Table 9 n and m values for different volume fractions and temperatures
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An accurate correlation is proposed based on experimental data for prediction of viscosity changes due to solid volume fraction and temperature. It is demonstrated in Eq. 3 and is obtained through the ANOVA method.

$$\frac{{\mu }_{\text{nf}}}{{\mu }_{\text{bf}}}=1.312617-0.40743\varphi -0.02201T-1.0867\varphi .T+6637.082{\varphi }^{2}+0.000532{T}^{2}+0.012826\varphi .{T}^{2}-298214{\varphi }^{3}-0.0000038{T}^{3}$$
(3)

The results of ANOVA analysis for the model of viscosity are presented in Table 10. F value equal to 16.321 shows the considerable effectiveness of regression model. Pvalue of this model is less than 0.05 that shows the significance of model parameters. If the p value of any parameter is higher than 0.05 that parameter would be insignificant. All the insignificant parameters are omitted from the model.

Table 10 ANOVA results for relative viscosity
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Figure 13 shows a comparison between experimental data and those obtained from the proposed correlation at different temperatures. Little difference between these two data groups indicates the high accuracy of the proposed correlation for the viscosity prediction of this nano-lubricant.

Fig. 13
figure 13

Comparison between experimental data and correlation outputs

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Tribological behavior

Figures 14 and 15 show the variations of friction coefficient with different solid volume fractions and under loads of 120 N and 180 N. In these figures, it is shown that increasing solid volume fraction of SiO2 nanoparticles causes an improvement in lubrication and reduction in friction coefficient. But this reduction does not continue with increasing concentration, since the excessive addition of nanoparticles has an opposite effect on the friction coefficient. The main reason is nanoparticles stick and creating agglomerations in the engine oil in higher concentrations. Furthermore, considering the descending trend of friction coefficient versus time in all solid volume fractions, it can be concluded that this nano-lubricant has a good mending and polishing effect.

Fig. 14
figure 14

Friction coefficient in different solid volume fractions at 120 N load

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Fig. 15
figure 15

Friction coefficient in different solid volume fractions at 180 N load

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Figure 16 shows changes in the average friction coefficient in different solid volume fractions under two different loads. The minimum friction coefficient is recorded in φ = 0.1%. In this volume fraction, the friction coefficient decreased by 18.46% and 16.21% under 120 N and 180 N of load, respectively, compared with the base oil.

Fig. 16
figure 16

Average friction coefficient versus different solid volume fractions

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In addition to the measurement of friction coefficient, the amount of decrease in mass of disks is measured with an accurate scale after each test. The disks are washed completely after each test to remove the remaining oil, and then, the amount of decrease in mass is reported. Equation 3 is used to calculate the wear rate:

$${W}_{\text{r}}=\frac{\Delta m}{\rho *l*F}$$
(4)

where \({\rho }_{\text{Disk}}=8.908 \, {g} \, {{mm}^{-3}}\)

Figure 17 shows changes in the wear rate of different samples. It can be seen that increasing solid volume fraction reduces the wear rate. The maximum reduction in wear rate, while using nano-lubricant as compared with the base oil is about 50% and is occurred at φ = 0.1% and F = 120 N.

Fig. 17
figure 17

Wear rate versus solid volume fraction

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The uncertainty of friction coefficient has been calculated using Eq. 4 [67].

$${\omega }_{\text{R}}={\left[{\left(\frac{\partial R}{\partial {X}_{1}}{\omega }_{1}\right)}^{2}+{\left(\frac{\partial R}{\partial {X}_{2}}{\omega }_{2}\right)}^{2}+\dots {\left(\frac{\partial R}{\partial {X}_{\text{n}}}{\omega }_{\text{n}}\right)}^{2}\right]}^\frac{1}{2}$$
(5)

In this equation \({\omega }_{\text{R}}\) is the absolute uncertainty, and R is the function of independent variables (X1,X2,…,Xn). According to Eqs. 5 and 6, the uncertainty of friction coefficient can be calculated by each parameter’s uncertainty.

$${C}_{\text{f}}=\frac{F}{N}$$
(6)
$$\frac{{\omega }_{{C}_{\text{f}}}}{{C}_{\text{f}}}={\left[{\left(\frac{{\omega }_{\text{F}}}{F}\right)}^{2}+{\left(\frac{{\omega }_{\text{N}}}{N}\right)}^{2}\right]}^\frac{1}{2}$$
(7)

The value of \(\frac{{\omega }_{{C}_{\text{f}}}}{{C}_{\text{f}}}\) is the relative uncertainty of friction coefficient. The uncertainty of each parameter is 2%, and hence, the friction coefficient uncertainty is 2.82%.

To demonstrate the effect of nanoparticles on the lubrication property of the oil on the pin-on-disk tribometer, SEM images are obtained from the worn path created on the disks which can be seen in Fig. 18.

Fig. 18
figure 18

SEM images of abrasion path on the disks in the presence of a: base oil b: base oil + 0.1%SiO2

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Since minimum friction coefficient and wear rate, in loads of 120 N and 180 N, occur in φ = 0.1%, it can be concluded that using this nano-lubricant with φ = 0.1% can have a considerable effect on the quality of engine oil and also on the operation of internal combustion engines. So, other properties of the oil like flash point and pour point are measured for the base fluid and SiO2/SAE40 in φ = 0.1% and are reported in Fig. 19.

Fig. 19
figure 19

Flash point and pour point versus solid volume fraction

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The flash point of oil determines an upper limit for its operating temperature. As the flash point is higher, it will be possible for the oil to operate in higher temperatures. As shown in this figure, the addition of SiO2 nanoparticles to the base oil in φ = 0.1% causes an increase of 3.8% in the flash point of the nano-oil as compared to the base oil. However, there is no change in the pour point by using the nanoparticles. Accordingly, it can be concluded that the addition of nanoparticles to the base oil improves the operational temperature range of the oil.

Oil test in engine

Since the friction coefficient and the flash point of 0.1%SiO2 /SAE40 are optimized, this concentration is chosen for comparing these two oils in engine. After thirty hours of operation, both engine oils are drained and abrasive particle analysis is performed. The analysis is performed according to the ASTM D6595, and the results are depicted in Fig. 20. In this experiment, abrasive particles like Fe, Cr, Al, Cu, and Pb are measured. It can be observed that the presence of nanoparticles in the oil has reduced the amount of abrasive particles in nano-oil compared to the base oil. This improves the lubrication and reduces the abrasion of the cylinder wall and piston. These results strongly confirm the anti-wear property of 0.1%SiO2 /SAE40 nano-lubricant compared with base oil.

Fig. 20
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Abrasive particle analysis

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Conclusions

In this study, a comprehensive investigation and analysis of important factors of a nano-lubricant including viscosity, coefficient of friction, pour point, and flash point was carried out. The effects of temperature, shear rate, and solid volume fraction on these parameters were studied. The main results of this experimental study are listed as follows:

  • The viscosity has increased with increasing nanoparticle concentration and decreasing temperature. But, two exceptions were observed in nano-lubricant behavior; one in φ = 0.25% in all temperatures and the other in T = 35 ˚C in all solid volume fractions.

  • Investigation on rheological behavior using the power-law method has shown that the value of the power-law index was less than unity for all samples, which shows pseudoplastic behavior for this nano-lubricant.

  • Mean value of friction coefficient in different solid volume fractions and exerted loads indicates that the minimum friction coefficient was recorded in φ = 0.1%. Moreover, the addition of SiO2 nanoparticles to the base oil in φ = 0.1% causes a 3.8% increase in flash point as compared with the base oil.

  • Measuring the amounts of different elements in nano-lubricant and base oil showed that these amounts in nano-lubricant with φ = 0.1% were lower than the base oil. This implies that nano-lubricants can reduce wear between parts of piston and cylinder in internal combustion engines.