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Research Papers: Mixed and Boundary Lubrication

A Boundary Lubrication Friction Model Sensitive to Detailed Engine Oil Formulation in an Automotive Cam/Follower Interface

[+] Author and Article Information
Rupesh Roshan, Martin Priest, Anne Neville, Ardian Morina, Chris P. Warrens

Xin Xia

 Institute of Engineering Thermofluids, Surfaces and Interfaces, School of Mechanical Engineering, University of Leeds, Leeds, LS2 9JT, UKpennyxx@gmail.com

Marc J. Payne

Castrol Limited, Technology Centre, Whitchurch Hill, Pangbourne, Reading, RG8 7QR, UKmarc.payne@uk.bp.com

J. Tribol 133(4), 042101 (Oct 10, 2011) (9 pages) doi:10.1115/1.4004880 History: Received January 17, 2011; Revised August 12, 2011; Published October 10, 2011; Online October 10, 2011

Theoretical studies have shown that in severe operating conditions, valve train friction losses are significant and have an adverse effect on fuel efficiency. However, recent studies have shown that existing valve train friction models do not reliably predict friction in boundary and mixed lubrication conditions and are not sensitive to lubricant chemistry. In these conditions, the friction losses depend on the tribological performance of tribofilms formed as a result of surface–lubricant additive interactions. In this study, key tribological parameters were extracted from a direct acting tappet type Ford Zetec SE (Sigma) valve train, and controlled experiments were performed in a block-on-ring tribometer under conditions representative of boundary lubrication in a cam and follower contact. Friction was recorded for the tribofilms formed by molybdenum dithiocarbamate (MoDTC), zinc dialkyldithiophosphate (ZDDP), detergent (calcium sulfonate), and dispersant (polyisobutylene succinimide) additives in an ester-containing synthetic polyalphaolefin (PAO) base oil on AISI E52100 steel components. A multiple linear regression technique was used to obtain a friction model in boundary lubrication from the friction data taken from the block-on-ring tribometer tests. The model was developed empirically as a function of the ZDDP, MoDTC, detergent, and dispersant concentration in the oil and the temperature and sliding speed. The resulting friction model is sensitive to lubricant chemistry in boundary lubrication. The tribofilm friction model showed sensitivity to the ZDDP–MoDTC, MoDTC–dispersant, MoDTC–speed, ZDDP–temperature, detergent–temperature, and detergent–speed interactions. Friction decreases with an increase in the temperature for all ZDDP/MoDTC ratios, and oils containing detergent and dispersant showed high friction due to antagonistic interactions between MoDTC–detergent and MoDTC–dispersant additive combinations.

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Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Block-on-ring tribometer and (b) schematic block-on-ring configuration, from Ref. [23]

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Figure 2

Experimental friction values versus predicted coefficient of friction

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Figure 3

Friction coefficient as a function of temperature for model oils containing (a) MoDTC only, (b) variable wt. % of ZDDP and MoDTC, (c) 0.72 wt. % ZDDP and variable wt. % MoDTC/detergent, and (d) 2 wt. % detergent and variable wt. % MoDTC/dispersant. Other factors are taken as constant at a maximum Hertzian pressure of 570 MPa, a sliding speed of 0.7 m/s, and a composite surface roughness of 0.2 μ m, and the λ ratio is less than 0.6.

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Figure 4

Friction coefficient as a function of sliding speed for model oils containing (a) MoDTC only, (b) variable wt. % ZDDP and MoDTC, (c) 0.72 wt. % ZDDP and variable wt. % MoDTC/detergent, and (d) 2 wt. % detergent and variable wt. % MoDTC/dispersant. Other factors are taken as constant at a maximum Hertzian pressure of 570 MPa, a temperature of 100 °C, and a composite surface roughness of 0.2 μm, and the λ ratio is less than 0.2.

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