Characterization of Thermal Stability of Synthetic and Semi-Synthetic Engine Oils

Anand Tripathi,Ravikrishnan Vinu

doi:10.3390/lubricants3010054

Abstract

Engine oils undergo oxidative degradation and wears out during service. Hence it is important to characterize ageing of engine oils at different simulated conditions to evaluate the performance of existing oils and also design new formulations. This work focuses on characterizing the thermo-oxidative degradation of synthetic and semi-synthetic engine oils aged at 120, 149 and 200 °C. Apparent activation energy of decomposition of aged oils evaluated using the isoconversional Kissinger-Akahira-Sunose technique was used as a thermal stability marker. The temporal variation of stability at different ageing temperatures was corroborated with kinematic viscosity, oxidation, sulfation and nitration indices, total base number, antiwear additive content and molecular structure of the organic species present in the oils. At the lowest temperature employed, synthetic oil underwent higher rate of oxidation, while semi-synthetic oil was stable for longer time periods. At higher temperatures, the initial rate of change of average apparent activation energy of synthetic oil correlated well with a similar variation in oxidation number. A mixture of long chain linear, branched, and cyclic hydrocarbons were observed when semi-synthetic oil was degraded at higher temperatures.

Highlights

Engine oils are carefully engineered for use in today’s sophisticated engines by blending with various additives
As engine oils are designed with a mixture of base oil and performance augmented additives such as friction modifiers, antioxidants, dispersants and detergents, and viscosity index improvers [1,2,3], it is necessary to understand the thermal stability of engine oils under various operating conditions in order to evaluate the performance of specific additive’s package and to design better additive formulations
Samples were collected at different time intervals during degradation to understand the kinetics of ageing using thermogravimetric analyzer (TGA)

Summary

Introduction

Engine oils are carefully engineered for use in today’s sophisticated engines by blending with various additives. Oils are prone to thermal and oxidative degradation during service in internal combustion engines as they are exposed to extremely high temperatures The degree of degradation depends on engine conditions, cycle time and duration of its use. Some of the desirable characteristics of lubricants include reduction of friction, high boiling point, optimum viscosity and thermal stability, corrosion retardation and resistance to oxidation. As engine oils are designed with a mixture of base oil and performance augmented additives such as friction modifiers, antioxidants, dispersants and detergents, and viscosity index improvers [1,2,3], it is necessary to understand the thermal stability of engine oils under various operating conditions in order to evaluate the performance of specific additive’s package and to design better additive formulations

Methods

Results

Conclusion