# Lubrication Performance of Engine Commercial Oils with Different Performance Levels: The Effect of Engine Synthetic Oil Aging on Piston Ring Tribology under Real Engine Conditions

^{*}

## Abstract

**:**

## 1. Introduction

_{2}emission levels and to improve engine efficiency. Cars, motor vehicles, and trucks are the main sources of CO

_{2}emissions. Low engine speeds during the cold start-up period lead to some significant energy loss through friction in city driving [7]. This is convenient, according to the New European Driving Cycle, who say that a large portion (60–80%) of engine emissions occur during their cold start-up period. As a result, the majority of industrial groups are attempting to reduce mechanical energy loss and to find ways to improve the performance of tribocontacts. A significant portion of engine friction losses—in the order of 35–45%—are caused by the piston system. Engine oils designed with piston and ring-pack assembly have a crucial role in the reduction of friction, wear, and fuel consumption in internal combustion engines [8,9].

## 2. Experimental Details and Methods

#### 2.1. Description of the Capillary Tube Viscometer

#### 2.2. Method of Viscosity Measurement with EH105 Viscometer

#### 2.3. Uncertainty Analysis of Viscosity Measurement

- Uncertainty in the measurement of pressure, ${u}_{\Delta p}$
- Uncertainty in the measurement of flow rate, ${u}_{q}$
- Uncertainty in the measurement of volume, ${u}_{v}$
- Uncertainty in the measurement of time, ${u}_{t}$
- Uncertainty from the radius of capillary tube, ${u}_{r}$
- Uncertainty from the length of capillary tube, ${u}_{l}$

#### 2.4. CFD Quasi-Static Analysis of Piston Ring-Liner Conjunction

_{cr}is the crank-pin radius, ω is the rotational crankshaft speed, ϕ is the crank angle, and λ

_{CR}is the control ratio. The ring twist is not accounted for. The in-cylinder pressure, p

_{c}, and the outlet pressure, p

_{out}(from crankcase), values are assumed as inputs, depending on the direction of the piston motion.

_{T}and the gas force F

_{G}should be equal to the hydrodynamic reaction, W

_{h}, due to the lubricant film in the ring-liner conjunction, and the load, due to asperities, W

_{c}. At every crankshaft position, ring balance should take place according to the following expression:

_{5/2}(λ) is expressed as a relation of the Stribeck oil film parameter $\lambda =\frac{h\left(x,y\right)}{\sigma}$ [27]. The limits between various regimes of lubrication are used to indicate the changes at contact, and these regimes are depicted in the minimum film thickness curve.

_{c}is also defined as [26]:

_{2}(λ) is:

^{3}. The fluid region is meshed with finite volumes. The CFD procedure is specified by Zavos and Nikolakopoulos [12] in more detail. The current analysis assumes an isothermal mixed lubrication model, where the thermal gradient in the contact is negligible. Therefore, the lubricant density and viscosity were evaluated with the pressure variation according to Roelands [30], as follow:

## 3. Results and Discussion

#### 3.1. Validity of Viscosity Measurement

#### 3.2. Viscosity Measurements for Commercial Engine Oils

#### 3.3. CFD Model Validation

#### 3.4. The Effect of Synthetic Oil Aging on Piston Ring Tribology

## 4. Conclusions

- The variation of the dynamic viscosity for the engine oil SAE10W40 is more than 19% at 25 °C and more than 28.5% at 75 °C.
- The minimum film thickness has insignificant variation through the crankshaft angles for the fresh and aged SAE10W40 oil at 40 °C, whereas very high variation appears as temperature increases. In practical terms, there is a significant reduction in the order of 13.5%, for example in 300 degrees, due to the lubricant’s viscosity temperature variations for 2 MPa test pressure at 75 °C.
- The variation of total ring friction is almost 7.8% at the TDC between fresh and aged oil. The smaller value concerns aged oil, due its slight viscosity variation at 40 °C. Instead, boundary friction is higher, 19.25% at 75 °C, for aged oil than for fresh oil. This means that aged oil significantly influences in engine friction losses and contact wear. The impact of oil aging on ring and liner wear is the next plan of this work.

## Author Contributions

## Funding

## Conflicts of Interest

## Nomenclature

A | nominal contact area |

A_{c} | asperity contact area |

b | ring face-width |

d_{gap} | ring end gap |

E’ | equivalent (reduced) modulus of elasticity |

E_{ring} | Young’s modulus of elasticity of the ring |

E_{l} | Young’s modulus of elasticity of the liner |

F | applied ring load |

f_{tot} | total friction |

f_{fl} | viscous friction |

f_{b} | boundary friction |

F_{G} | combustion gas force |

F_{T} | ring tension force |

F_{5/2}, F_{2} | statistical functions |

h | lubricant film thickness |

h_{min} | minimum film thickness with time |

h_{s} | ring axial profile |

I_{ring} | second moment of ring area |

p_{h} | hydrodynamic pressure |

p_{el} | ring elastic pressure due to fitment |

p_{bk} | ring back gas pressure acting behind the inner ring rim |

p_{c} | combustion pressure |

p_{out} | outlet pressure at the ring conjunction |

p_{atm} | ambient pressure |

r_{cr} | Crank-pin radius |

r_{o} | nominal bore radius |

t | time |

w | ring radial width |

W_{c} | load share of asperities |

W_{h} | load carried by the lubricant film |

X | parameter for load balance criterion |

Greek symbols | |

a* | modified pressure–viscosity coefficient |

δ_{p} | local contact deformation |

ζ | surface density of asperity peaks |

κ | average asperity tip radius |

λ | Stribeck oil film parameter |

λ_{CR} | control ratio |

μ | lubricant dynamic viscosity |

μ_{ο} | ambient dynamic viscosity |

ρ | lubricant density |

ρ_{ο} | lubricant density at atmospheric pressure and ambient temperature |

σ | root mean square roughness value of examined tribo pair |

ς | coefficient parameter of boundary shear strength |

τ | viscous shear stress |

τ_{ο} | Eyring shear stress of the lubricant film |

υ_{ring} | ring sliding velocity |

ϕ | crank angle |

χ | step for minimum film thickness loop |

ω | rotational crankshaft speed |

Abbreviations | |

BDC | Bottom Dead Center |

CFD | Computational Fluid Dynamics |

NEDC | New European Drive Cycle |

RS | Root Square |

TDC | Top Dead Center |

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**Figure 10.**Comparison of minimum film thickness between the current analysis (blue line) and published work [15].

**Figure 12.**Variations of minimum film thickness for fresh and aged synthetic engine oil SAE 10W40 at 40 °C.

**Figure 13.**Variations of total ring friction for fresh and aged synthetic engine oil SAE 10W40 at 40 °C.

**Figure 14.**Variations of minimum film thickness for fresh and aged synthetic engine oil SAE 10W40 at 75 °C.

**Figure 15.**Variations of total ring friction for fresh and aged synthetic engine oil SAE 10W40 at 75 °C.

Parameter ( ${\mathit{x}}_{\mathit{i}}$) | Value ( ${\mathit{u}}_{{\mathit{x}}_{\mathit{i}}}$) |
---|---|

${\Delta}_{p}$ | $0.5\text{}\mathrm{mm}$ |

$v$ | $0.005\text{}\mathrm{mL}$ |

$t$ | $0.01\text{}\mathrm{s}$ |

$r$ | $0.001\text{}\mathrm{mm}$ |

$l$ | $0.5\text{}\mathrm{mm}$ |

Temperature | Standard Viscosity (μPas) | Experimental Viscosity (μPas) | Percentage Error |
---|---|---|---|

25 °C | 890.3 | 839 ± 0.7 | 5.75% |

40 °C | 652.6 | 645.1 ± 0.5 | 1.1462% |

Properties | AWS-100 | SAE 10W40 | SAE 30 |
---|---|---|---|

Density (kg/m^{3}) @ 40 °C | 850 | 865 | 878 |

Viscosity (Pa·s) @ 40 °C | 0.085 | 0.079 | 0.075 |

Flash Point (°C) | 220 | 230 | 229 |

Viscosity (Pa·s) @ 110 °C | 0.0157 | 0.0139 | 0.0117 |

SAE 10W40 (Fresh) | SAE 10W40 (100 h Aged) | ||
---|---|---|---|

Coefficients | $\mathsf{\alpha}=1.455$ $\mathrm{b}=-0.06874$ $\mathrm{c}=0.00421$ $\mathrm{d}=0.02497$ | Coefficients | $\alpha =0.750$ $b=-0.05078$ $c=5.008\times {10}^{-17}$ $d=0.4318$ |

Goodness of fit | $SSE:$ 1.103 × 10^{−11}$R-square:1$ | Goodness of fit | $SSE:$ 1.11 × 10^{−5}$R-square:0.9994$ |

Parameter | Value | Unit |
---|---|---|

nominal cylinder diameter | 0.0524 | m |

Crank-pin radius | 0.025 | m |

rod length | 0.096 | m |

ring face width | 0.0005 | m |

ring width | 0.003 | m |

Piston-ring end gap | 0.0015 | m |

ring material | Chromium plated | ── |

Young’s modulus of elasticity for ring | 276 | GPa |

ring Poisson’s ratio | 0.21 | ── |

cylinder block material | Aluminum | ── |

Young’s modulus of elasticity for cylinder | 70 | GPa |

cylinder Poisson’s ratio | 0.33 | ── |

roughness parameter | 0.04 | ── |

asperity slope | 0.0015 | ── |

ring roughness | 0.2 | μm |

cylinder | 0.15 | μm |

curvature height | 3 | μm |

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Nikolakopoulos, P.G.; Mavroudis, S.; Zavos, A.
Lubrication Performance of Engine Commercial Oils with Different Performance Levels: The Effect of Engine Synthetic Oil Aging on Piston Ring Tribology under Real Engine Conditions. *Lubricants* **2018**, *6*, 90.
https://doi.org/10.3390/lubricants6040090

**AMA Style**

Nikolakopoulos PG, Mavroudis S, Zavos A.
Lubrication Performance of Engine Commercial Oils with Different Performance Levels: The Effect of Engine Synthetic Oil Aging on Piston Ring Tribology under Real Engine Conditions. *Lubricants*. 2018; 6(4):90.
https://doi.org/10.3390/lubricants6040090

**Chicago/Turabian Style**

Nikolakopoulos, Pantelis G., Stamatis Mavroudis, and Anastasios Zavos.
2018. "Lubrication Performance of Engine Commercial Oils with Different Performance Levels: The Effect of Engine Synthetic Oil Aging on Piston Ring Tribology under Real Engine Conditions" *Lubricants* 6, no. 4: 90.
https://doi.org/10.3390/lubricants6040090