# Recent Advances in Preparation and Testing Methods of Engine-Based Nanolubricants: A State-of-the-Art Review

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## Abstract

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## 1. Introduction

_{2}[34]), and transition metal dichalcogenide (${\mathrm{MoS}}_{2}$ [35]).

## 2. Preparation Methods and Dispersion Stability of Nanolubricants

#### 2.1. Basic Concepts

#### 2.2. Preparation Methods of Nanolubricants

## 3. Tribological Performance of Nanolubricants

_{3})/rGO) as a new composite of nano additives. The composite of lanthanum hydroxide/reduced graphene oxide showed a substantial tribological performance under different conditions of load and working temperatures, as shown in Figure 8a.

## 4. Rheological and Thermophysical Properties of Engine Lubricant-Based Nanofluids

#### 4.1. Viscosity

#### Prediction Models for the Viscosity

#### 4.2. Heat Transfer Performance

#### 4.2.1. Flash Point and Pour Point

#### 4.2.2. Thermal Conductivity

#### 4.2.3. Prediction Models for the Thermal Conductivity

## 5. Fuel Economy and Emissions

## 6. Discussion

## 7. Future Directions and Challenges

#### 7.1. Molecular Dynamics Simulations

#### 7.2. Ionic Based Nano Lubricants

#### 7.3. Cost and Economics of Nanolubricants

## 8. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 3.**Preparation approaches and stability measurement techniques of nanolubricants mentioned in the literature.

**Figure 4.**Dispersion stability of CuO nanolubricant with different dispersing agents. (

**a**) UV–visible spectra, (

**b**) sedimentation analysis [38]. Adapted with permission.

**Figure 5.**Dispersion stability of hybrid ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$/TiO

_{2}nanolubricant. (

**a**) Electro-steric stabilization mechanism, (

**b**) sedimentation and UV analyses after 70 days [50]. Adapted with permission.

**Figure 7.**Tribological characteristics of multi-layer graphene engine-based nanolubricant. (

**a**) Friction coefficient reduction, (

**b**) wear reduction [99]. Adapted with permission.

**Figure 8.**Tribological performance of diesel engine oil with nano-lanthanum hydroxide/reduced graphene oxide composites. (

**a**) Reduction in coefficient of friction and wear volume, (

**b**) hypothesized wear mechanism of nanolubricant [62]. Adapted with permission.

**Figure 9.**Tribo-sintering mechanism of Cu/Gr of automobile engine nano-lubricant. (

**A**) SEM analysis of the tribo-lubricating layer (

**B**,

**C**), EDS mapping and spectrum of the tribo-film elements [63]. Adapted with open access permission.

**Figure 10.**Illustration of the contact model regarding SAE 5W-20 CuO nano-lubricant [119]. Adapted with permission.

**Figure 11.**Snapshots of molecular dynamic simulations regarding TiO

_{2}nanolubricant with PAO2 as base oil. (

**a**) Illustration of tribo-film mechanism with the addition of nano additives. (

**b**) Components of the simulation model [120]. Adapted with permission.

**Figure 12.**Tribo-sintering mechanism of ZrO

_{2}engine-based nanolubricants. (

**a**) SEM analysis, (

**b**) EDX anal-ysis [103]. Adapted with open access permission.

**Figure 13.**Effect of the shear rate on the non-Newtonian multigrade engine lubricants [124]. Adapted with permission.

**Figure 15.**Effect of the volumetric percentage of MWCNT/${\mathrm{SiO}}_{2}$ nano additive on the final viscosity behaviour of an engine oil [75]. Adapted with permission.

**Figure 16.**Flash and pour point of Mo${\mathrm{S}}_{2}$ and ZnO engine–based nanolubricants [61]. Adapted with open access permission.

**Figure 18.**Improvement in the thermal efficiency 5W-30 engine-based nanolubricant through hybrid Al

_{2}O

_{3}/TiO

_{2}. (

**a**) A 75% throttle valve opening, (

**b**) 100% throttle valve opening [25]. Adapted with permission.

**Figure 19.**Energy-saving revenue from employing graphene engine -based nanolubricant. (

**I**) Fuel consumption during New European Driving Cycle (NEDC). (

**II**) Exhaust emissions under various operating conditions [67]. Adapted with permission.

**Figure 20.**Effect of Al

_{2}O

_{3}and SiO

_{2}nanolubricants on engine performance [167]. (

**a**) Total fuel consumption. (

**b**) brake-specific fuel consumption. (

**c**) Brake thermal efficiency. Adapted with permission.

**Figure 21.**Enhancement of engine performance with nanolubrication. (

**a**) Total fuel consumption, (

**b**) brake thermal efficiency [59]. Adapted with permission.

**Figure 22.**Noise level reduction for engine-based nanolubricant [56].

**Figure 23.**Reduction in vibration acceleration, (

**a**) Longitudinal vibration, (

**b**) lateral vibration [56]. Adapted with permission.

**Figure 24.**Nano additives that have been used for engine lubricants as friction modifiers published during the last decade.

**Figure 26.**Molecular insight into the aggregation and dispersion behaviour of nanoparticles. (

**a**) PMF curve of nanoparticle when another particle approached, (

**b**,

**c**) aggregation number of nanoparticles with simulation time under different grafting conditions [194]. Adapted with permission.

**Figure 27.**Working methodology of ANN as a predictive artificial intelligence tool [204]. Adapted with open access permission.

Ase Lubricant | Nano-Additive | Preparation Method | Optimal Concentration in Terms of Stability | Dispersing Agents | Testing of Depression | Stable Condition/Duration | Processing Method/Processing Time | Reference | Year |
---|---|---|---|---|---|---|---|---|---|

Base as lubricant/PAO6 | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$$/{\mathrm{TiO}}_{2}$ | Two-step method | 0.1 wt.% | HDEHP | Sedimentation, Zeta potential, FTIR, UV | Stable/70 days | Ultrasonic dispersion/6 h Magnetic stirrer | [50] | 2021 |

SAE 5 W-30 | $\mathrm{ZnO}$$/{\mathrm{TiO}}_{2}$ | Two-step method | All concentrations | Oleic acid | Sedimentation | Stable/1 week | Ultrasonic dispersion/2 h Magnetic stirrer | [56] | 2021 |

SAE 20 W-40 | $\mathrm{MWCNT}/{\mathrm{TiO}}_{2}$ | Two-step method | 0.1 | - | Zeta potential | Stable/72 h | Ultrasonic dispersion/3 h | [57] | 2021 |

SAE 40 | ${\mathrm{SiO}}_{2}$ | Two-step method | All concentrations | - | Sedimentation | Stable/1 month | Ultrasonic dispersion/40 min Magnetic stirrer/30 min | [58] | 2021 |

SAE 30 | Graphite | 0.3 wt.% | 0.3 wt.% | Tween-80, Ethylene glycol | Sedimentation, zeta potential, | Stable/ 15 days | Ultrasonic dispersion/1.5 h | [59] | 2021 |

Base as lubricant/paroline oil | ${\mathrm{MoS}}_{2}$ | One-step method | All concentrations | Not reported | Sedimentation | Stable/10 days | Ultrasonic dispersion | [35] | 2020 |

Base as lubricant/HD 50 | Gr | Two-step method | 0.01 wt.% | Sodium dodecyl persulfate (SDS), Oleic acid (OA) | Sedimentation, FTIR, XRD | Stable/30 days | Magnetic stirrer/3 h, Ultrasonic dispersion/12 h | [6] | 2020 |

SAE 10W-40 | Gr-MS-Zn | One-step method | 0.5 wt.% | PEHA | Sedimentation | Stable/30 days | Not reported | [60] | 2020 |

SAE 10W-40 | ZnO/MWCNT | Two-step method | 0.25 wt.% | Oleic acid (OA) | Sedimentation | Stable/15 days | Ultrasonic dispersion/2 h | [30] | 2020 |

SAE 40 | $\mathrm{ZnO},\text{}{\mathrm{MoS}}_{2}$ | Two-step method | 0.1 wt.% | Not reported | Sedimentation | Stable-ZnO/6 days | Ultrasonic bath/45 min | [61] | 2020 |

SAE 5W-30 | MWCNT | Two-step method | 0.03 wt.% | Liqui Moly | Not reported | Stable | Not reported | [27] | 2020 |

Base as lubricant/PAO | CuO | One-step method | 0.1 wt.% | Toluene/OA, Hexane/OA, Et.Glycol/OA | Sedimentation, UV | Sable-Toluene/30 days | Magnetic stirrer/7 h | [38] | 2020 |

SAE 20W-50 | La(OH)3/rGO | One-step method | Not reported | Not reported | Sedimentation, XRD | Stable/28 days | Ultrasonic dispersion/1 h | [62] | 2020 |

SAE 5W-30 | Cu/Gr | Two-step method | 0.4 wt.% | Oleic acid (OA) | UV | Stable/11 days | Magnetic stirrer/4 h | [63] | 2019 |

Base as lubricant/hexadecane | GO | Two-step method | $10\text{}\mathrm{mg}\text{}{\mathrm{L}}^{-1}$ | Oleylamine | Sedimentation | Stable | Ultrasonic dispersion | [64] | 2019 |

SAE 5W-30 | GO | Two-step method | 0.1 wt.% | Dodecylamine, Ethanol, Xylene | UV, Sedimentation, XRD | Stable/32 Days | Ultrasonic dispersion/6 h | [65] | 2019 |

SAE 10W-40 | ${\mathrm{TiO}}_{2}$ | Two-step method | Not reported | Oleic acid (OA) | Sedimentation, Zeta sizer | Not stable | Magnetic stirrer/15 min Ultrasonic dispersion/20 min | [66] | 2018 |

SAE 5W-30 | Gr | Two-step method | 0.4 wt.% | Oleic acid (OA) | Sedimentation, UV | Stable/11 days | Magnetic stirrer/4 h | [67] | 2018 |

SAE 5W-30 | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$$/{\mathrm{TiO}}_{2}$ | Two-step method | 0.1 wt.% | Oleic acid (OA) | Not reported | Not reported | Magnetic stirrer/4 h | [68] | 2018 |

SAE 10W-40 | ZnO | Two-step method | 0.25–2 vol.% | Not reported | Sedimentation | Stable/72 h | Magnetic stirrer/2 h, Ultrasonic dispersion | [51] | 2017 |

Base as lubricant/PAO | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ | Two-step method | 1 wt.% | Oleic acid (OA) | Not reported | Not stable | Magnetic stirrer | [69] | 2017 |

SAE 5W-30 | ${\mathrm{MoS}}_{2}$ | Two-step method | - | Not reported | Not reported | Not reported | Magnetic stirrer, Ultrasonic dispersion | [70] | 2017 |

SAE 50 | ZnO | Two-step method | 0.125–1.5 vol.% | Not reported | Sedimentation | Stable/2 days | Ultrasonic dispersion/5 h | [71] | 2017 |

Base as lubricant/60SN | ZnO | Two-step method | 0.5 wt.% | Oleic acid (OA) | Sedimentation | Stable/12 h | Magnetic stirrer/20 min, Ultrasonic dispersion/30 min | [72] | 2017 |

SAE 40 | MWCNT/CuO | Two-step method | 0.0625–1 vol.% | Not reported | Sedimentation | Stable/1 week | Magnetic stirrerUltrasonic dispersion | [73] | 2017 |

Base as lubricant/paraffin | ${\mathrm{MoS}}_{2}$ | Two-step method | 1 wt.% | ZDDP, Polyisobutylene amine succinimide (PIBS) | SEM | Stable | Ultrasonic dispersion/ 40 min | [74] | 2017 |

SAE 50 | $\mathrm{MWCNT}/{\mathrm{SiO}}_{2}$ | Two-step method | 0.0625–2 vol.% | Not reported | Sedimentation | Stable | Magnetic stirrer/2 h, Ultrasonic dispersion | [75] | 2016 |

SAE 5W-30 | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$$,{\mathrm{TiO}}_{2}$ | Two-step method | 0.25 wt.% | Oleic acid (OA) | UV, Zeta sizer | Stable/336 h | Magnetic stirrer/4 h | [76] | 2016 |

SAE 10W-40 | MWCNT/ZnO | Two-step method | 0.125–1 vol.% | Not reported | Sedimentation | Stable/1 week | Magnetic stirrer/2 h Ultrasonic dispersion/1 h | [77] | 2016 |

Base as lubricant/PAO | ZnO | One-step method | 0.2–1.5 wt.% | Oleic acid (OA) | FTIR | Stable | Magnetic stirrer | [78] | 2016 |

Base as lubricant/PAO | Gr | Two-step method | 0.01 wt.% | Span-80 | Sedimentation | Stable/4 weeks | Magnetic stirrer/10 min, ultrasonic dispersion/15 min | [79] | 2016 |

SAE 10W-40 | GO, Ag, GNP | Two-step method | 0.06–0.1 wt.% | Stearic amine, ethanol, SDS, glucose | Sedimentation, FTIR | Stable/2 weeks | Ultrasonic dispersion/5 h | [80] | 2016 |

SAE 10W-40 | GO | One- step method | $0.04\text{}\mathrm{mg}\text{}{\mathrm{mL}}^{-1}$ | Octadecylamine (ODA) | XRD, FTIR | Stable/30 days | Ultrasonic dispersion | [81] | 2015 |

Base as lubricant /500 N | ${\mathrm{MoS}}_{2}$ | One-step method | 0.58 wt.% | Sodium dodecyl persulfate (SDS), heptane | Sedimentation, UV | Stable/1 year | ultrasonic shaker/1 h | [82] | 2015 |

Base as lubricant/PAO | Cu/rGO | One-step method | 0.5–2 wt.% | Oleic acid (OA) | FTIR | Stable/ 7 days | Ultrasonic dispersion | [83] | 2015 |

SAE 40 | C | Two-step method | 0.01 vol.% | Not reported | Zeta sizer | Stable/14 days | Ultrasonic dispersion/ 1 h | [84] | 2014 |

Rapeseed oil/SAE20W40 | CuO | Two-step method | 0.5 wt.% | Not reported | UV | Stable/29 days | Rotary Shaker /5 h, ultrasonic dispersion/2 h | [85] | 2014 |

SAE 15W-40 | ${\mathrm{MoS}}_{2}$ | Two-step method | All concentrations | Span-80 | Sedimentation | Stable/14 days | High shear homogenizer/ 30 min | [86] | 2014 |

Base as lubricant/PAO | Gr | Two-step method | 0.5–1.5 wt.% | Oleic acid (OA) | FTIR | Stable/ 7 days | Magnetic stirrer/30 min | [87] | 2014 |

SAE 10W-40 | rGO | One-step method | $0.04\text{}\mathrm{mg}\text{}{\mathrm{mL}}^{-1}$ | Octadecylamine (ODA) | Sedimentation, XRD, zeta potential, FTIR | Stable/30 days | Ultrasonic dispersion | [88] | 2014 |

SAE 20W-50 | MWCNT | One-Step method | 0.1 wt.% | SOCL2, DMF, THF, dda | Sedimentation, XRD | Stable/720 h | Ultrasonic dispersion, Magnetic stirrer | [89] | 2013 |

SAE 20W-50 | MWCNT | Two-step method | 0.1 wt.% | Dodecylamine | Sedimentation | Stable/720 h | planetary ball mill/3 h | [90] | 2013 |

Base as lubricant/PAO | ${\mathrm{MoS}}_{2}$ | Two-step method | 3 wt.% | Benzethonium chloride | Sedimentation | Stable | Ultrasonic dispersion | [91] | 2012 |

500 W | GO | Two-step method | $0.025\text{}\mathrm{mg}\text{}{\mathrm{mL}}^{-1}$ | DMF | FTIR | Stable | Ultrasonic dispersion/1 h | [92] | 2011 |

Base Lubricant | Nano Additive | Reduction in COF % | Tribological Configuration | Testing Load/Speed | Testing Temperature °C | Concentration wt.% | Effective Concentration wt.% | Reference | Year |
---|---|---|---|---|---|---|---|---|---|

SAE-40 | ZnO | 5 | Pin-on-Disc | 20–75 N | Ambient Temperature | 0.1, 0.4, 0.7 | 0.4 | [102] | 2021 |

${\mathrm{MoS}}_{2}$ | 16 | 150 rpm | 0.7 | ||||||

SAE-30 | Graphite | 91.6 | Pin-on-Disc | 20–50 N 300 rpm | Ambient Temperature | 0.3 | 0.3 | [59] | 2021 |

SAE-40 | ${\mathrm{SiO}}_{2}$ | 18.46 | Pin-on-Disc | 120–180 N 120 rpm | Ambient Temperature | 0.1–1 vol.% | 0.1 vol.% | [58] | 2021 |

Base as lubricant—Group III | ${\mathrm{ZrO}}_{2}$ | 11 | Ball-on-Disc | 50–100 N 50 Hz | 100 | 0.1–0.6 | 0.4 | [103] | 2021 |

SAE 20 W-50 | hBN | 20.5 | Four-ball Tribometer | 392.5 N 1200 rpm | 75 | 0.025 vol.% | 0.025 vol.% | [104] | 2021 |

Base as lubricant—Group III | ZnO | 7.8 | Cylinder-on-flat | 500–530 Mpa 300 rpm | 100 | 0.33 | 0.33 | [15] | 2021 |

SAE 20W-50 | Boron nitride (BN) | 56 | Ball-on-Disc | 40–100 N 150 rpm | Ambient Temperature | 0.5 | 0.5 | [26] | 2020 |

Tungsten disulphide (WS2) | 37 | ||||||||

Graphene (Gr) | 2.5 | ||||||||

SAE 10W-40 | Gr-MS-Zn | 37 | Four-ball Tribometer | 392.5 N 1200 rpm | 75 | 0.025–0.1 | 0.05 | [60] | 2020 |

SAE 20W-50 | Nano-lanthanum hydroxide/reduced graphene oxide (LaOH3/rGO) | 16.7 | Ball-on-Disc | 1.96–5.88 N 0.1 m/s | 40–80 | 0.05, 0.1 | 0.1 | [62] | 2020 |

SAE 10W-30 | Hairy silica particles (HSP) | 15 | Ball-on-Three-plate testing module | 2 N 0.05 m/s | 25–100 | 0.1, 0.3, 0.5, 1 | 0.3 | [105] | 2020 |

SAE 10W-40 | ZnO/MWCNT | 32 | Ball-on-Disc | 35–55 N 5–15 Hz | Ambient Temperature | 0.25, 0.50, 0.75, and 1 | 0.25 | [30] | 2020 |

SAE 40 | ZnO | 5.9 | Pin-on-disc | 75 N | Ambient Temperature | 0.1, 0.4, and 0.7 | 0.4 | [61] | 2020 |

${\mathrm{MoS}}_{2}$ | 12 | 150 rpm | 0.7 | ||||||

Base as lubricant—Paroline oil | ${\mathrm{MoS}}_{2}$ | 64 | Ball-on-disc | 6 N 1.5 Hz | Ambient Temperature | 0.1, 0.2, 0.3 and 0.5 | 0.3 | [35] | 2020 |

Base as lubricant—Ionic liquid | Gr | 40 | Pin-on-disc | 0.5 N 0.01 m/s | Ambient Temperature | 0.5 | 0.5 | [106] | 2020 |

Diesel oil | ZnO | 5.86 | Pin-on-disc | 75 N | Ambient Temperature | 0.1, 0.4, 0.7 | 0.4 | [31] | 2020 |

Base as lubricant—PAO 40 | CuO | 24 | Ball-on-disc | 10 N 0.01 m/s | 50 | 0.5 | 0.5 | [32] | 2020 |

SAE 5W-30 | Gr | 40 | Ball-on-Plate | 10–50 N 3 mm/s | Ambient Temperature | 0.01, 0.05, 0.1 | 0.1 | [65] | 2019 |

SAE 5W-30 | Cu/Gr | 26–32 | Piston Ring/Cylinder Liner | 90–368 N 0.154–0.6 m/s | 100 | 0.03, 0.2, 0.4, and 0.6 | 0.4 | [63] | 2019 |

SN/GF-5 lubricant | C | 32 | piston ring/cylinder liner interface | 50–400 N 10 Hz | 100 | 1, 3 and 5 | 3 | [107] | 2019 |

SAE 5W-30 | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3},{\mathrm{TiO}}_{2}$ | 53 | piston ring/cylinder liner interface | 250 N 0.5 m/s | Ambient Temperature | 0.1 | 0.1 | [68] | 2018 |

SAE 5W-30 | Graphene (Gr) | 29–35 | Piston Ring/Cylinder Liner | 90–368 N 0.154–0.6 m/s | 70–90 | 0.03, 0.2, 0.4, 0.6 | 0.4 | [67] | 2018 |

SAE 10W-30 | Copper oxide (CuO) | 76 | Piston skirt-Liner Contact Tester | 2–9 N 200–300 rpm | Ambient Temperature | 0.005–0.01 | 0.008 | [108] | 2018 |

SAE 10W-30 | ${\mathrm{TiO}}_{2}$ | 86 | Pin-on-Disc | 40–60 N 1 m/s | Ambient Temperature | 0.3, 0.4, 0.5 | 0.5 | [34] | 2018 |

Base as lubricant—SN 500 | $\mathrm{Ni}-{\mathrm{MoS}}_{2}$ | 21 | Four-ball Tribometer | 392 N 1200 rpm | 75 | 0.1–0.5 | 0.5 | [97] | 2018 |

Base as lubricant—Mineral | Cu | 40–60 | Four-ball Tribometer, Pin-on-Disc | 392 N 1200 rpm | 40–100 | 0.3, 3 | 0.3 | [109] | 2018 |

Base as lubricant—SN 500 | Graphene oxide (GrO) | 33 | Four-ball Tribometer | 147 N 1200 rpm | Ambient Temperature | 0.04 | 0.04 | [110] | 2017 |

Base as lubricant—Liquid paraffin | ${\mathrm{MoS}}_{2}$ | 12.2–35.3 | Ball-on-Disk | 30 N 0.036 m/s | Ambient Temperature | 1 | 1 | [74] | 2017 |

SAE 5W-30 | ND | 36 | Block-on0ring | 30 kg 200 rpm | Ambient Temperature | 0.016 | 0.016 | [28] | 2017 |

Base as lubricant—60SN base oil | ZnO | 41 | Four-ball Tribometer | 500 N 1000 rpm | Ambient Temperature | 0.2, 0.5, 0.5, 0.8,and 1 | 0.5 | [72] | 2017 |

SAE 5W-30 | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3},{\mathrm{TiO}}_{2}$ | 48–50 | Piston Ring/Cylinder Liner | 30–250 N 50–800 rpm | 100 | 0.05, 0.1, 0.25, 0.5 | 0.25 | [96] | 2016 |

SAE 20W-50 | Gr | 21 | Four-ball Tribometer | 400 N 1200 rpm | 75 | 0.01 | 0.01 | [111] | 2016 |

SAE 5W-30 | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3},{\mathrm{TiO}}_{2}$ | 51 | piston ring/cylinder liner interface | 40–230 N 0.5–1.45 m/s | 100 | 0.05, 0.1, 0.25 and 0.5 | 0.1 | [76] | 2016 |

SAE 5 W- 30 | ${\text{}\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ | 35 | piston ring/cylinder liner interface | 185–340 | 60 | 0.25 | 0.25 | [112] | 2016 |

${\mathrm{TiO}}_{2}$ | 51 | 0.25–0.66 m/s | |||||||

Chevron Taro 30 DP 40 | Cu | 18.2 | Pin-on-disc | 0.1–180 mN | 25 | 0.3 vol.% | 3 vol.% | [113] | 2016 |

0.02 mm/s | 3 vol.% | ||||||||

Base as lubricant—Mineral | Fe–Carbon capsules | 8 | Block-on-ring | 650 N 1.65 m/s | Ambient Temperature | 0.01–0.1 | 0.07 | [114] | 2015 |

SAE 75 W- 85 | CuO | 14 | Four-ball Tribometer | 7000 N | Ambient Temperature | 0.5, 1 and 2 | 2 | [115] | 2015 |

${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ | - | 500 rpm | - | ||||||

SAE 40 | SWCNH | 12 | Ball-on-disc | 600 MPa 0.001–1.8 m/s | 25,40,60 and 80 | 0.005, 0.01 and 0.02 | 0.01 | [84] | 2014 |

SAE 10 | CuO | 18 | Four-ball Tribometer, Pin-on-Disc | 150 N 1420 rpm | Ambient Temperature | 0.5, 0.25 | All concentrations | [116] | 2013 |

Cu | 49 | 0.5 | |||||||

Fe | 39 | 0.5 | |||||||

Co | 20 | 0.5 | |||||||

Fe/Cu | 53 | 0.25/0.25 | |||||||

Fe/Co | 36 | 0.25/0.25 | |||||||

Co/Cu | 53 | 0.25/0.25 | |||||||

Base as lubricant—PAO 10 | ${\mathrm{MoS}}_{2}$ | 57 | Piston Skirt/Cylinder Liner | 250 N | 20–100 | 3 | 3 | [91] | 2012 |

BN | No improvement | 2 Hz | |||||||

SAE 15 W-40 | Cu | 37 | Ball-on-disc | 50 N 10–30 Hz | Ambient Temperature | 0.0125, 0.025, 0.0375 and 0.05 | 0.0375 | [117] | 2011 |

**Table 3.**A summary of the published literature of the viscosity behaviour for engine-based nano lubricants.

Base Lubricant | Nano Additive | Testing Temperature °C | Testing Shear Rate | Concentration | Viscosity Behaviour | Reference | Year |
---|---|---|---|---|---|---|---|

SAE 5W-30 | MgO | 5–55 | $666.5\text{\u2013}13,330\text{}{\mathrm{s}}^{-1}$ | 0.1–1.5 vol.% | non-Newtonian | [33] | 2020 |

SAE 50 | ZnO | 25–65 | $5\text{\u2013}55\text{}{\mathrm{s}}^{-1}$ | 0.125–1.5 vol.% | Newtonian | [130] | 2020 |

SAE 10W-40 | ZnO, MgO | 5–75 | 5–1000 rpm | 0.125–1.5 vol.% | Newtonian for both nanolubricants | [131] | 2019 |

Engine lubricant | $\mathrm{MWCNT}/\mathrm{Mg}{\left(\mathrm{OH}\right)}_{2}$ | 25–60 | 100–600 rpm | 0.125–1.5 vol.% | Newtonian | [132] | 2018 |

20W-50 | $\mathrm{MWCNT}/{\mathrm{SiO}}_{2}$ | 40–100 | $10\text{\u2013}70\text{}{\mathrm{s}}^{-1}$ | 0.05–1 vol.% | Newtonian | [133] | 2018 |

SAE 40 | MWCNT/MgO | 25–45 | 100–1000 rpm | 0.25–2 vol.% | non-Newtonian | [134] | 2018 |

SAE 10W-40 | ${\text{}\mathrm{ZrO}}_{2}$/MWCNT | 5–55 | $666.66\text{\u2013}11,999.97\text{}{\mathrm{s}}^{-1}$ | 0.05–1 vol.% | non-Newtonian | [135] | 2018 |

SAE 10W-40 | $\mathrm{MWCNT}/{\mathrm{SiO}}_{2}$ | 5–55 | $666.5\text{\u2013}11,997\text{}{\mathrm{s}}^{-1}$ | 0.05–1 vol.% | non-Newtonian | [136] | 2017 |

SAE 10W-40 | ZnO | 5–55 | $666.5\text{}\mathrm{and}\text{}11,997\text{}{\mathrm{s}}^{-1}$ | 0.25–2 vol.% | Newtonian | [51] | 2017 |

SAE 50 | ${\mathrm{TiO}}_{2}$ | 25–50 | $666.5\text{\u2013}9331\text{}{\mathrm{s}}^{-1}$ | 0.125–1.5 vol.% | Newtonian/non-Newtonian | [137] | 2017 |

Engine lubricant | Cu | 40–100 | $5\text{\u2013}40\text{}{\mathrm{s}}^{-1}$ | 0.2–1 wt.% | Newtonian/non-Newtonian | [138] | 2017 |

SAE 50 | MWCNT/MgO | 20–50 | $670\text{\u2013}8700\text{}{\mathrm{s}}^{-1}$ | 0.0625–1 vol.% | non-Newtonian | [139] | 2017 |

SAE 40 | MWCNT/ZnO | 25–60 | 1333–13,333 rpm | 0.05–1 vol.% | Newtonian | [140] | 2017 |

SAE 40 | $\mathrm{MWCNT}/{\mathrm{SiO}}_{2}$ | 25–50 | 100–500 rpm | 0.625–2 vol.% | Newtonian/non-Newtonian | [75] | 2016 |

SAE 40 | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$/MWCNT | 25–50 | $1333\text{\u2013}13,333\text{}{\mathrm{s}}^{-1}$ | 0.0625–1 vol.% | Newtonian | [127] | 2016 |

SAE 40 | $\mathrm{MWCNT}/{\mathrm{SiO}}_{2}$ | 25–60 | $667\text{\u2013}6667\text{}{\mathrm{s}}^{-1}$ | 0.0625–1 vol.% | Newtonian | [141] | 2016 |

SAE 10W-40 | MWCNT/ZnO | 5–55 | 5–1000 rpm | 0.125–1 vol.% | Newtonian | [77] | 2016 |

SAE 50 | MWCNT/MgO | 25–50 | 100–700 rpm | 0.25–2 | Newtonian | [128] | 2016 |

SAE 20W-50 | CuO | 5–70 | $10\text{\u2013}70\text{}{\mathrm{s}}^{-1}$ | 0.2–6 wt.% | Newtonian | [126] | 2015 |

**Table 4.**A summary of the published literature of the viscosity improvement for engine-based nanolubricants.

Base Lubricant | Nano Additive | Testing Temperature °C | Concentration | Effective Concentration | Most Viscosity Refinement | Reference | Year |
---|---|---|---|---|---|---|---|

SAE 40 | $\mathrm{ZnO}\text{}\mathrm{and}\text{}\mathrm{Mo}{\mathrm{S}}_{2}$ | 40–100 | 0.1–0.7 wt.% | 0.7 wt.% | Increase by 9.57%/MOS_{2}, 10.12%/ZnO, at 100 °C | [102] | 2021 |

SAE 20W-40 | $\mathrm{Si}{\mathrm{O}}_{2}$ | 50–90 | 0.3–1.5 wt.% | 0.3 wt.% | Increase by 99%, at 90 °C | [143] | 2021 |

SAE 50 | ZnO | 25–65 | 0.125–1.5 vol.% | 1.5 vol.% | Increase by 25.3% | [130] | 2020 |

SAE 40 | $\mathrm{ZnO}\text{}\mathrm{and}\text{}\mathrm{Mo}{\mathrm{S}}_{2}$ | 40–100 | 0.1–0.7 wt.% | 0.7 wt.% | Increase by 9.58%/MOS_{2}, 10.14%/ZnO, at 100 °C | [61] | 2020 |

SAE 5W-30 | MgO | 5–55 | 0.1–1.5 vol.% | 1.5 vol.% | Increase by 25% at 15 °C | [33] | 2020 |

Engine lubricant | MgO and ZnO | 5–55 | 0.125–1 vol.% | 1.5 vol.% for both nanolubricants | Increase by 124.3%/ZnO, 75%/MgO, at 55 °C | [131] | 2019 |

SAE 10W-40 | $\mathrm{MWCNT}/\mathrm{Zr}{\mathrm{O}}_{2}$ | 5–55 | 0.05–1 vol.% | 1 vol.% | Increase by 31% at 55 °C | [135] | 2018 |

Engine lubricant | MWCNT/Mg | 25–60 | 0.25–2 vol% | 2 vol% | Increase by 60% at 60 °C | [132] | 2018 |

SAE 20W-50 | °C $\mathrm{MWCNT}/\mathrm{Si}{\mathrm{O}}_{2}$ | 40–100 | 0.05–1 vol.% | 1 vol.% | Increase by 171% at 100 °C | [133] | 2018 |

Engine lubricant | Cu | 4–100 | 0.2–1 wt.% | 1 wt.% | Increase by 37% at 40 °C | [138] | 2017 |

SAE 50 | MWCNT/MgO | 25–50 | 0.0625–1 vol.% | 1 vol.% | Decrease by 75% at 50 °C | [139] | 2017 |

SAE 40 | MWCNT/CuO | 25–50 | 0.0625–1 vol.% | 1 vol.% | Increase by 29.47% at 30 °C | [73] | 2017 |

SAE 40 | MWCNT/ZnO | 25–60 | 0.05–1 vol.% | 1 vol% | Increase by 33.3% at 40 °C | [140] | 2017 |

SAE 40 | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$/MWCNT | 25–50 | 0.0625–1 vol.% | 1 vol.% | Increase by 46% at 35 °C | [127] | 2016 |

SAE 50 | MWCNT/MgO | 25–50 | 0.25–2 vol.% | 2 vol% | Increase by 65% at 40 °C | [128] | 2016 |

SAE 40 | $\mathrm{MWCNT}/\mathrm{Si}{\mathrm{O}}_{2}$ | 25–60 | 0.0625–1 vol.% | 1 vol.% | Increase by 37.4% at 60 °C | [141] | 2016 |

SAE 40 | MWCNT/SiO2 | 25–50 | 0.0625–2 vol.% | 0.5 vol.% | Increase by 1.7% at 40 °C | [75] | 2016 |

SAE 10W-40 | MWCNT/ZnO | 5–55 | 0.125–1 vol.% | 1 vol.% | Increase by 55% at 55 °C | [77] | 2016 |

Reference | Year | Base Lubricant | Nano Additive | Proposed Model | Accuracy of the Model |
---|---|---|---|---|---|

[129] | 2018 | SAE 5W-50 | MWCNT/ZnO | $\begin{array}{ll}\frac{{\mu}_{nf}}{{\mu}_{bf}}=0.866& +\text{}0.802\phi +8.38\left({10}^{-3}*T\right)\\ & -\left(7.86*{10}^{-6}*\dot{\gamma}\right)\\ & -\left(5.83*{10}^{-4}*\phi *T\right)\\ & -\left(6.26*{10}^{-7}*T*\dot{\gamma}\right)-\left(1.081*{\phi}^{2}\right)\\ & -\left(2.44*{10}^{-4}*{T}^{2}\right)\\ & +\left(2.83*{10}^{-9}*{\dot{\gamma}}^{2}\right)\\ & +\left(1.26*{10}^{-8}*{T}^{2}*\dot{\gamma}\right)+\left(0.572*{\phi}^{3}\right)\\ & +\left(1.63*{10}^{-6}*{T}^{3}\right)\end{array}$ | R2 = 0.9715 |

[133] | 2018 | SAE 20W-50 | $\mathrm{MWCNT}/\mathrm{Si}{\mathrm{O}}_{2}$ | $\begin{array}{c}\frac{{\mu}_{nf}}{{\mu}_{bf}}=0.09422-[(\frac{T}{\phi}{)}^{2}+\\ 0.100556{T}^{0.8827}{\phi}^{0.3148}\left]\mathrm{exp}\right(72,474.75{\phi}^{3.7951})\end{array}$ | Margin of deviation < 1% |

[140] | 2017 | SAE 40 | MWCNT/ZnO | $\frac{{\mu}_{nf}}{{\mu}_{bf}}=A+B\phi +C{\phi}^{2}+D{\phi}^{3}$ | Maximum error = 2% |

[147] | 2017 | SAE 10W-40 | $\mathrm{Ti}{\mathrm{O}}_{2}$/MWCNT | $\frac{{\mu}_{nf}}{{\mu}_{bf}}={a}_{0}+{a}_{1}\phi +{a}_{2}{\phi}^{2}+{a}_{3}{\phi}^{3}$ | Margin of deviation = 1.1% |

[51] | 2017 | SAE 10W-40 | ZnO | $\frac{{\mu}_{nf}}{{\mu}_{bf}}={a}_{0}+{a}_{1}\phi +{a}_{2}{\phi}^{2}+{a}_{3}\phi \mathrm{ln}\left(\phi \right)$ | Margin of deviation < 1% |

[73] | 2017 | SAE 40 | MWCNT/CuO | $\frac{{\mu}_{nf}}{{\mu}_{bf}}={a}_{0}+{a}_{1}\phi \mathrm{exp}\left(\phi \right)+{a}_{2}{\phi}^{2}+{a}_{3}{\phi}^{3}$ | - |

[128] | 2016 | SAE 50 | MWCNT/MgO | ${\mu}_{nf}=328,201\times {T}^{-2.053}\times {\phi}^{0.09359}$ | Maximum error = 8% |

[148] | 2016 | SAE 40 | $\mathrm{MWCNT}/\mathrm{Si}{\mathrm{O}}_{2}$ | $\frac{{\mu}_{nf}}{{\mu}_{bf}}=0.00337+\mathrm{exp}(0.07731{\phi}^{1.452})$ | Margin of deviation = 4% |

[75] | 2016 | SAE 40 | $\mathrm{MWCNT}/\mathrm{Si}{\mathrm{O}}_{2}$ | $\frac{{\mu}_{nf}}{{\mu}_{bf}}={a}_{0}+{a}_{1}\phi +{a}_{2}{\phi}^{2}+{a}_{3}{\phi}^{3}$ | Margin of deviation = 1.2% |

[127] | 2016 | SAE 40 | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$/MWCNT | $\begin{array}{ll}\frac{{\mu}_{nf}}{{\mu}_{bf}}=1.123& +0.3251\phi -0.08994T+0.002552{T}^{2}\\ & -0.00002386*{T}^{3}+0.9695{(\frac{T}{\phi})}^{0.01719}\end{array}$ | Margin of deviation = 2% |

Base Lubricant | Nano Additive | Testing Temperature °C | Concentration | Effective Concentration | Improvement in Thermal Conductivity | Reference | Year |
---|---|---|---|---|---|---|---|

SAE 20 W-40 | $\mathrm{MWCNT}/{\mathrm{TiO}}_{2}$ | 20–70 | 0.1–0.8 vol.% | 0.8 vol.% | Increase by 24.42% at 70 °C | [57] | 2021 |

SAE 50 | ZnO | 25–55 | 0.125–1.5 vol.% | 1.5 vol.% | Increase by 8.74% at 55 °C | [151] | 2019 |

Engine oil | ZnO, MgO | 15–55 | 0.125–1.5 vol.% | 1.5 vol.% for both nanoparticles | Increase by 28%/ZnO, 32%/MgO, at 55 °C | [131] | 2019 |

Engine oil | ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$/MWCNT | 25–50 | 0.125–1.5 vol.% | 1.5 vol.% | Increase by 45% at 50 °C | [157] | 2018 |

SAE 10W-40 | MWCNT/ZnO | 15–55 | 0.125 0 1 vol.% | 1 vol.% | Increase by 40% at 55 °C | [158] | 2018 |

SAE 5 W-50 | $\mathrm{Mg}{\left(\mathrm{OH}\right)}_{2}$ | 25–60 | 0.25–2 vol.% | 2 vol.% | Increase by 50% at 60 °C | [132] | 2018 |

SAE 50 | MWCNT/MgO | 25–50 | 0.25–2 vol.% | 2 vol.% | Increase by 62% at 50 °C | [128] | 2016 |

Engine oil | Cu | 40–100 | 0.2–1 wt.% | 1 wt.% | Increase by 37% at 100 °C | [138] | 2016 |

SAE 20W-50 | Gr | 10–180 | 0.01 wt.% | 0.01 wt.% | Increase by 23% at 80 °C | [111] | 2016 |

SAE 20W-50 | CuO | 20 | 0.1–0.5 wt.% | 0.1 wt.% | Increase by 3% | [159] | 2013 |

**Table 7.**A summary of the proposed models for estimating the thermal conductivity of the engine-based nanolubricants.

Reference | Year | Base Lubricant | Nano Additive | Proposed Model | Accuracy of the Model |
---|---|---|---|---|---|

[151] | 2019 | SAE 50 | ZnO | $\frac{{K}_{nf}}{{K}_{bf}}=(0.0055$ ${T}^{0.632}{\phi}^{0.831})+0.964$ | Margin of deviation < 1% |

[157] | 2018 | Engine oil | $\mathrm{MWCNT}/{\text{}\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ | ${K}_{nf}=0.1534+1.1193$ $\phi $ + 0.00026 T | Maximum error = 2% |

[132] | 2018 | SAE 5W-50 | $\mathrm{MWCNT}/\mathrm{Mg}{\left(\mathrm{OH}\right)}_{2}$ | ${K}_{nf}=0.159+1.1112$ $\phi $ + 0.003 T | Maximum error = 2% |

[128] | 2016 | SAE 50 | MWCNT/MgO | ${K}_{nf}=0.162+0.691\phi +0.00051T$ | Maximum error = 3% |

Reference | Year | Substantial Findings | Studied Property |
---|---|---|---|

[194] | 2020 | Addressing of effective method to quantify aggregation/dispersion range through molecular simulation | Dispersion Stability |

[195] | 2021 | MD simulations stated that the transformation from dispersion state to the reversible state is related to the Lennard–Jones(LJ) particle number | |

[190] | 2020 | Providing a facile method to produce high dispersion stability of GO suspensions. MD simulations indicated that the used surface agents could form a high reaggregation barrier on the surface of nanoparticles | |

[196] | 2021 | By increasing the receiving heat flux of nanofluid, the thermal conductivity is improved. | Rheological and Thermophysical Characteristics |

[191] | 2014 | Discussing the hypotheses behind the flow of nanofluids using near and main flow models | |

[192] | 2021 | The common hypotheses of the ball-bearing lubrication mechanism have been confirmed through the MD simulations, which verified the rolling motion of nanoparticles. | Tribological Characteristics |

[197] | 2014 | A clear investigation was concluded about the effect of sliding velocity and load capacity on the formation of the nano tribo-films at the contacted surfaces | |

[198] | 2020 | Frictional heating and anti-wear properties of the friction pair is controlled through the existence of nanoparticles | |

[199] | 2020 | Simulations of MD clarified that Gr nano additives could form a thick layer of tribo-film that can help in reducing the coefficient of friction and friction force | |

[200] | 2014 | Nanofluids have a higher transition pressure than the base fluid, with an excellent load-carrying capacity | |

[201] | 2020 | Theoretical guidance was implemented for the lubrication mechanism of MOS2 nanoparticles | |

[193] | 2015 | Confirmation of the ball-bearing lubrication mechanism of nanoparticles under mild velocities and loads | |

[189] | 2015 | The load-carrying capacity of nanofluid is improved regarding the base oil before the rupture of the lubricant film | |

[120] | 2018 | Atomistic simulations confirmed the mending mechanism of nanolubricants through which nanoparticles fill in valleys of the sliding asperities |

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**MDPI and ACS Style**

Akl, S.; Elsoudy, S.; Abdel-Rehim, A.A.; Salem, S.; Ellis, M.
Recent Advances in Preparation and Testing Methods of Engine-Based Nanolubricants: A State-of-the-Art Review. *Lubricants* **2021**, *9*, 85.
https://doi.org/10.3390/lubricants9090085

**AMA Style**

Akl S, Elsoudy S, Abdel-Rehim AA, Salem S, Ellis M.
Recent Advances in Preparation and Testing Methods of Engine-Based Nanolubricants: A State-of-the-Art Review. *Lubricants*. 2021; 9(9):85.
https://doi.org/10.3390/lubricants9090085

**Chicago/Turabian Style**

Akl, Sayed, Sherif Elsoudy, Ahmed A. Abdel-Rehim, Serag Salem, and Mark Ellis.
2021. "Recent Advances in Preparation and Testing Methods of Engine-Based Nanolubricants: A State-of-the-Art Review" *Lubricants* 9, no. 9: 85.
https://doi.org/10.3390/lubricants9090085