Abstract
The reduction of friction and wear losses in boundary lubrication regime of a piston ring-cylinder liner tribo-system has always been a challenge for engine and lubricant manufacturers. One way is to use lubricant additives, which can form boundary film quickly and reduce the direct contact between asperities. This article focuses on the assessment of boundary film forming behavior of two phosphonium-based ionic liquids (ILs) as additives in engine-aged lubricant to further improve its film forming capabilities and hence reduce friction and wear of contacting surfaces. A reciprocating piston ring segment-on-flat coupon under fully flooded lubrication conditions at room temperature (approx. 25 °C) was employed. The trihexyltetradecyl phosphonium bis(2-ethylhexyl) phosphate and trihexyltetradecyl phosphonium bis(2,4,4-tri-methylpentyl) phosphinate ionic liquids were used as additives in 6 vol. % quantity. Benchmark tests were conducted using fully formulated new lubricant of same grade (with and without ILs). Results revealed that the addition of phosphonium ILs to engine-aged lubricant led to quicker initiation of boundary film forming process. In addition, friction and wear performance of engine-aged lubricant improved by the addition of both ILs and these mixtures outperformed the fresh fully formulated oil. Chemical analysis showed higher concentration of phosphorus element on the worn surface indicating presence of ILs in the formed tribofilms.
Highlights
Improved fuel economy and lower emissions demand for higher energy-conserving engine oils and better fuel-efficient vehicles [1]
The stability of mixtures containing non-polar engine oil and polar ionic liquids (ILs) is very important for overall performance, both during the testing stage and in real applications
Using ILs as an additive in fully formulated lubricants can be a challenge since it may lead to an unstable emulsion
Summary
Improved fuel economy and lower emissions demand for higher energy-conserving engine oils and better fuel-efficient vehicles [1]. Different engine components may experience different and/or more than one lubrication regime during operation [1]. 40%–50% of total frictional loss accounts to the sliding contact of piston rings against the cylinder liner surface alone [1,2]. The larger part of the cylinder liner—the piston ring interaction area during each piston stroke—experiences. Frictional losses in this region are attributed to traction produced by shearing of pressurized lubricant film at the contact interface. The use of low viscosity lubricants can result in low viscous shear and less frictional losses can be achieved [3]
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