The physicochemical and tribological studies of mineral and synthetic commercial engine oils have been carried out to investigate their performance variability and to propose generalized relationship among different physicochemical and performance parameters. Physicochemical parameters have been determined using standard test procedures proposed in ASTM and Indian Standards (BIS). The rheological parameters of these lubricants have been investigated to identify the flow behavior. The tribological performance in terms of their antifriction and antiwear properties has been studied using fourball tribotester. Correlation and regression analysis has been performed to ascertain relationship among physicochemical and tribological parameters and the causes of performance variability are highlighted. An empirical relation to calculate coefficient of friction as a function of physicochemical properties has been established using regression analysis. The developed relation has fair degree of reliability, as percentage of deviation is less than 20%.
Lubricants play a vital role in present day automotives. The engine oils in particular lubricate all the critical parts of IC engines. They not only reduce friction and wear between the moving parts but also dissipate frictional heat generated between the contacting parts of the engines [
The lubricants on the basis of their rheological behavior are characterized as Newtonian and nonNewtonian fluids. The fluids with molecular mass less than 1000 kg/mol show Newtonian behavior at low pressure and shear stress [
Tribology is the study of friction and wear of the machine parts. Lubricating oil forms a thin film between the surfaces that separates adjacent moving parts and minimizes the direct contact between them. As a result of this, the heat generated due to frictional heating decreases. Efficient lubrication aids in wear reduction, thus protecting the engine components from frequent failures. On the basis of the ratio of lubricant film thickness to the composite surface roughness of the contacting surfaces, different lubrication regimes ranging from boundary to hydrodynamic lubrication may occur. These lubrication regimes have dependency on the contact pressure and on the surface velocity of the surfaces in contact [
Interrelationship among various physicochemical and tribological parameters can be an effective tool to understand the behavior and performance variability of lubricants. Various attempts have been made to establish empirical relations among physicochemical parameters using mathematical/statistical techniques. In this context, variation in tribo performance of commercial engine oils was studied and correlations between tribological parameters like friction and wear with physicochemical properties were established [
A number of studies have been carried out in the past on determining and establishing the dependency among different lubricant parameters since Barus established a relation between viscosity and pressure by introducing pressure viscosity coefficient “
On the basis of literature review carried out it is observed that attempts have been made to develop dependencies among various characteristic properties of the lubricants. However, no comprehensive dependency in the form of empirical relations between the physicochemical properties and tribological performance of the engine oil exists. Hence, in the present work attempts have been made to investigate the relations between the physicochemical properties and the tribological performance of engine oils. The study has been performed on commercial engine oils and the characteristic properties pertaining to physicochemical, rheological, and tribological performance determined. The performance parameters were then correlated using correlation and regression analysis to establish dependency relations among them. The study will aid the lubrication and maintenance engineers in selecting appropriate parameters for the successful operations of the engines.
In this study, five different commercial engine oils coded as
Lubricants selected for the study.
Sl. number  Lubricant code  SAE grade  Base oil  Application 

1 

SAE40  Mineral  Diesel engine 
2 

SAE20W50  Mineral  Diesel/gasoline engine 
3 

SAE20W50  Mineral  Gasoline engine 
4 

SAE5W40  Synthetic  Diesel/gasoline engine 
5 

SAE5W40  Synthetic  Diesel/gasoline engine 
The selected lubricants have been characterized for their physicochemical properties, rheological behavior, and tribological performance. The physicochemical properties provide the basic qualitative information on the products selected while the rheological and tribological behaviors provide the information on performance of the lubricants. The TAN measures the presence of organic and strong inorganic acids in the oil and is an indicator of oil oxidation that may lead to corrosion of the components. TBN being the measure of basic components represents the ability of oil to neutralize acids produced in it during normal use. Similarly, sulphated ash represents the amount of metallic elements derived from the detergent and antiwear additives of the oil. The additive packages contain elements like calcium, magnesium, zinc, molybdenum, phosphorus, and so on that help in enhancing the performance of the engine oil.
The physicochemical properties such as density, viscosity, viscosity index, sulphated ash, total acid number (TAN), and total base number (TBN) have been determined using standard test procedures proposed in ASTM and Indian Standards (BIS). The metallic elements present in the additive package have been determined using the Inductively Coupled Plasma Atomic Emission Spectrometer (ICPAES), model: PS 3000 UV (DRE), Leeman Labs Inc. (USA).
The variation of rheological parameters (viscosity, shear stress, and torque) with temperature has been investigated using RHEOPLUS/32 MCR 302 from Anton Paar Austria. The rheometer capable of performing rheological studies in rotational or oscillatory mode consists of an EC motor with a torque range of 10–200 mNm. The experiments have been performed using concentric cylinder geometry as shown in Figure
Concentric cylindrical geometry of rheometer.
Tribological performance tests have been conducted on fourball tribotester (FBT) using the standard wear test procedure as mentioned in ASTM D: 4172B. The FBT used in the present study is shown in Figure
Fourball tribotester (FBT).
The experiments are performed on balls made up of AISI chrome alloy standard steel number E 52100, grade 25 EP (extra polish). The test conditions used are given in Table
Experimental test condition.
Parameter  Value 

Load  40 kgf 
Temperature  75°C 
Speed  1200 rpm 
Test duration  1 hr 
Postexperimental investigations on the used test specimens were performed to investigate the mode and mechanism of wear. Further, the capability of additives to form boundary layers on the test surface was investigated using scanning electron microscopy (SEM) using FESEM from FEI Netherlands model Quanta 200F fitted with EDX system.
The results for the measurements carried out on the physicochemical properties of the lubricants are given in Table
Physicochemical properties of the lubricants.
Sl. number  Characteristics  Lubricant name  





 

Density at 15°C (g cm^{−3})  0.8711  0.8910  0.8695  0.8655  0.8526 

Kinematic viscosity (mm^{2}/s) @ 40°C  123.06  166.71  154.93  83.68  79.82 

Kinematic viscosity (mm^{2}/s) @ 100°C  14.17  17.75  17.93  13.28  13.05 

Viscosity index (VI)  115  117  118  162  166 

TAN (mg KOH/g)  0.44  1.93  0.93  2.13  2.00 

TBN (mg KOH/g)  11.16  11.09  9.65  14.41  14.25 

Sulphated ash %wt  1.06  0.77  0.93  0.80  1.10 
It is evident from Table
The results for the trace metal analysis are given in Table
Trace metal analysis.
Sl. number  Lubricant code  Element (mg/l)  

Zn  Mo  P  


549.10  36.60  512.30 


977.10  93.30  893.50 


724.60  50.00  677.60 


907.10  1.00  857.90 


924.60  <1.00  877.90 
The variation in dynamic viscosity with temperature is shown in Figure
Variation of viscosity with temperature.
The variation of viscosity with temperature for the selected lubricants with the help of curve fitting technique is found to obey Reynolds’ equation [
The variation of shear stress/shear rate is shown in Figure
Variation of shear stress with shear rate for the lubricants.
Utilizing the experimental data represented in Figure
Power law index of the lubricants.
Sl. number  Lubricant code  Power law index 



0.9967 


0.9969 


0.9916 


0.9940 


0.9998 
The variation of viscosity with shear rate is shown in Figure
Variation of viscosity with shear rate for the lubricants.
The tribological performance of lubricants is defined in terms of their friction and wear behavior.
Figure
Friction coefficient versus time in seconds for
The wear scars as observed on the ball test specimens are shown in Figure
Wear scar diameters for the lubricants (a)
For a better comparison of test results, the coefficient of friction and WSD is tabulated in Table
Tribological performance of lubricants.
Sl. number  Lubricant code  Coefficient of friction  Average wear scar diameter (mm) 



0.1429  0.710 


0.1155  0.746 


0.1416  0.676 


0.0890  0.391 


0.0881  0.446 
Figure
SEM/EDX micrographs for the lubricants (a)
The EDX analysis for the specimens reveals the presence of elements like zinc, sulphur, phosphorus, and so on, which signifies that a thin boundary layer of lubricant is formed on the steel surfaces. The boundary films formed with the help of extreme pressure additives help in protecting the surfaces from further damage.
Correlation analysis predicts the association between two or more variables and infers the strength of the relationship among them. The value of correlation coefficient “
Correlation coefficient matrix for lubricant properties (physicochemical and tribological).



VI  TAN  TBN  Sulphated 
Zn  Mo  P  COF  WSD  


1 

0.73  0.73  0.08  0.59  −0.68  0.08 

0.01  0.41 



1 
.91 
0.90 

0.91 

−0.15 

0.22 



0.73  .91 
1 
0.76  −0.34 
0.85  −0.46 
0.02 

−0.04 


VI  −0.73  −0.90  −0.76  1 
0.71 

0.13  0.53  −0.83  0.60 


TAN  −0.08 

−0.34 
0.71  1 
0.70  0.33 

−0.20 



TBN  −0.59  −0.91  −0.85  0.94  0.70  1 
0.08  0.50  −0.76  0.56 


Sulphated 
−0.68 

−0.46 
0.13  0.33  0.08  1 
0.51  −0.46  −0.47  −0.31  −0.45 
Zn  0.08  −0.15  0.02  0.53 

0.50  −0.51  1 
0.03  0.99 

−0.42 
Mo 




0.20  0.76  −0.46  0.03  1 
−0.04  0.55 

P  0.01  −0.22  −0.04  0.60 

0.56  −0.47  0.99  −0.04  1 

−0.49 
COF  0.41 





−0.31 

0.55 

1 
0.82 
WSD 






−0.45 

−0.35  0.65  0.82 
1 
On examining the correlation coefficients of physicochemical and tribological properties, it is observed that the kinematic viscosity at 40°C has a positive correlation coefficient of 0.83 indicating that density affects viscosity directly. Positive correlation of 0.92 between metal additive Mo and density and of 0.95 between Mo and kinematic viscosity at 40°C shows that Mo affects the density and kinematic viscosity of the lubricant positively. Very high positive correlation coefficient of 0.94 between VI and TBN is a clear indicator that more neutralization of acid produced improves the VI of oil, thus prolonging the operational life. Trace metals Zn and P have very high value of correlation coefficients 0.96 and 0.98, respectively, with TAN indicating that though they improve the performance of the oil, yet they cause increase in lubricant acidity. This subsequently leads to increase in friction as interaction between the surfaces enhances oxidation and oxides in general get adsorbed on the surface [
Regression analysis has been performed for estimating the causal relationships for coefficient of friction and WSD with the physicochemical characteristic properties. Linear regression is the technique used for establishing causal relationship between a dependent variable and two or more independent variables. This helps to establish a relationship between the parameters of interest. The dependent variable, coefficient of friction (
Data for regression analysis.
Sl. number  COF ( 
Density, ( 
Kinematic viscosity, 
TAN 

1  0.1429  0.8711  123.06  0.44 
2  0.1155  0.8910  166.71  1.93 
3  0.1416  0.8695  154.93  0.93 
4  0.0890  0.8655  83.68  2.13 
5  0.0881  0.8526  79.82  2 
The firstorder multiple regression model was implemented on the data given in Table
Analysis of variance (ANOVA).
Source  df  SS  MS 

Significance 

Regression  3  0.003263  0.001088  10.8076  0.005098 
Residual  7  0.000705  0.000101  
Total  10  0.003968 
Inference for multiple regression was later drawn by fitting a linear equation to the observed data. The least square fit was assumed and the line residuals were determined. The test statistics, that is, the ratio of slope and standard deviation in each observation, is given in Table
Inference in linear regression.
Coefficient  Standard error 




Intercept  0.085467  0.2280  0.3748  0.7189 
Density @ 15°C ( 
0.033305  0.2740  0.1215  0.9067 
Kinematic viscosity @ 40°C ( 
0.000241  0.0001  1.7749  0.1192 
TAN 

0.0064 

0.01189 
After determining the coefficient of intercepts and independent variables the regression equation is written in the linear form as
Significance
In the present study, experimental investigations have been carried out to study the performance variability and establish a correlation between the characteristic properties of engine oils. The experiments have been performed to investigate the physicochemical, rheological, and tribological properties of mono and multigrade engine oils of different API performance standards. Thus, on the basis of the investigations made, the following broad conclusions are drawn:
The commercial engine oils are nearly similar in their physicochemical characteristics. However, the synthetic lubricants possess high VI and TBN and higher concentrations of additives as compared to mineral based oils.
The rheological behavior of lubricants reveals that the variation of viscosity with temperature for the tested engine oils obeys Reynolds’ equation. The lubricants describe nonNewtonian shear thinning behavior with the power law index values close to 0.99.
The tribological performance of lubricants reveals that the synthetic base lubricant possesses superior antifriction and antiwear properties than the mineral base lubricants. The coefficient of friction varies from 0.0881 to 0.1429 for the tested lubricants. Similarly the wear scar diameter varies from 0.391 mm to 0.746 mm for the tested lubricants The tribo performance of the lubricants is predominantly influenced by the viscosity and the additives present.
The worn out surfaces reveal that the synthetic base lubricants result in less surface distress while the contemporary mineral base lubricants show rigorous scuffing. All the lubricants are capable of forming thin boundary film on the steel surfaces.
The correlation analysis reveals that the friction and wear behavior of lubricants is influenced by their viscosity. The viscosity in turn is influenced by density, TAN, and TBN values. Moreover, the TAN and TBN are influenced by the concentrations of the trace metals present in the additives used.
An empirical relation correlating friction, viscosity, density, and TAN values of the lubricants is given by
Total acid number
Total base number
Inductively coupled plasma emission spectrometer
Fourball tribotester
Wear scar diameter
Extreme pressure
Scanning electron microscope
Viscosity index
Dynamic viscosity
Absolute temperature
Power law index
Coefficient of friction
Density at 15°C
Kinematic viscosity at 40°C
Kinematic viscosity at 100°C
Coefficient of multiple correlations
Coefficient of determination
Sum of squares
Degree of freedom
Mean squared
Mean squared error
Residual sum of squares
Total sum of squares.
The authors declare that there are no conflicts of interest regarding the publication of this paper.