Abstract
Abstract To extend drain intervals and improve efficiency, new engine oils with increased dispersant concentration and reduced viscosity are required. Low viscosity engine oils can increase the prevalence of boundary friction at low temperature and increase its severity at higher temperatures. As a result, combinations of organic and inorganic friction modifiers (FM) will be used to reduce boundary friction across a range of temperatures, also preventing damage to vehicle catalysts. This paper presents an experimental case study of such a new generation of fully formulated engine lubricants with varying concentrations of polyisobutylene succinimide dispersant, organic, and inorganic FM. Representative conditions pertaining to those encountered at the top dead center reversal of the piston compression ring-cylinder liner contact are created, and the generated friction measured through use of a sliding-strip tribometry. Subsequently, X-ray photoelectron spectroscopy (XPS) is used to determine the composition of the formed surface tribofilms in order to explain the observed frictional characteristics. The key interactions and frictional behavior of the dispersant and friction modifiers are highlighted across a range of operating temperatures.
Highlights
Improving energy efficiency of internal combustion (IC) engines is a key intermediate term challenge for the automotive industry
This paper presents a case study demonstrating the mutual interactions in lubricant formulations that contains high dispersant concentrations and varying Inorganic Friction Modifier (IFM) (MoDTC) and Organic Friction Modifier (OFM) (Amide) combinations
The coefficient of friction increases with surface temperature due to the formation of a ZDDP-based tribofilm indicated by the Zn2p and P2p elements shown in the X-ray Photoelectron Spectroscopy (XPS) results measured after the final friction measurements conducted at 140oC
Summary
Improving energy efficiency of internal combustion (IC) engines is a key intermediate term challenge for the automotive industry. Martin et al [17] investigated the role of nitrogen in tribochemical reactions between a polyisobutylene succinimide (PIBS) dispersant and zinc dithiophosphate under boundary regime lubrication They formed tribofilms on an AISI 52100 steel substrate with surface roughness of 0.02 μm (Ra) using a reciprocating friction tester operating at 60°C surface temperature and in the presence of a dispersant with 1wt% concentration. Miklozic et al [28] generated a MoDTC derived tribofilm using a high frequency reciprocating rig and characterised it for elemental composition through use of Raman spectroscopy and at nanoscale by using AFM They found that friction reducing characteristics of a molybdenum disulphide film, formed on the surface depended on the concentration of MoDTC in the solution and on the operating temperature. Petroleum ether 40-60 evaporates quickly and it is unlikely that a residue would remain on the surface
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