Lube-Tech •
Friction modifier (FM) additives are widely used in low viscosity passenger car engine lubricants (these are usually Molybdenum based FMs, or organic FMs, usually based on glycerol mono-oleate or oleyl-amide, or both [25]).
•
Stop-start systems are now widely used to stop excessive idling in passenger cars and heavy-duty trucks. For city-type, stop-start driving, these systems can usually result in a 10-15% reduction in fuel consumption.
•
Improved surface finishing – in some applications, such as motorsport, bearing surfaces for example are superfinished, to give a much lower surface roughness than normal engineering surfaces. However, the cost of improved surface finishing will add to the manufacturing cost of a machine, so may not be cost-effective for mass production.
•
Hard coatings, either ceramic or based on diamond like coatings (DLC) are increasingly being used for high pressure contacts, such as valve tappets (in passenger cars), to reduce both friction and wear.
•
In addition, interesting research is ongoing into the possibility of superlubricity [26-28], whereby the friction coefficient between moving surfaces could potentially be reduced by a factor of ten, or more. Such research is being pursued in tribology laboratories around the world, but has not yet been widely commercialised.
Conclusions
In conclusion, the improved prediction of mixed/ boundary friction is becoming of increasing importance as energy efficiency of machines is being improved and lower viscosity lubricants are being used.
There is recent experimental data that suggests commonly used models significantly underestimate the amount of mixed/boundary friction in machines, and a new, simple-to-use, model has been proposed
36 LUBE MAGAZINE NO.181 JUNE 2024 tonnes.
It would be very useful to extend this analysis to other sectors, such as heavy-duty transport (including mining vehicles, shipping, trains, buses, and heavy duty freight) and also to industry. Some studies have looked at the overall impact of tribology on these various sectors [23,24] but have not explicitly reported results for mixed/boundary friction separately.
It is anticipated that tribologists and lubrication engineers will be researching ways to reduce mixed/ boundary friction in machines for many years to come, and significant reductions in costs and CO2
emissions
could potentially be possible if superlubricity research currently ongoing in laboratories can be more widely commercialied.
References [1] R.I. Taylor, “Tribology and Energy Efficiency: From Molecules to Lubricated Contacts to Complete Machines”, Faraday Discussions, 156(1), pp 361-382, 2012
[2] P.J. Blau, “On the Nature of Running-In”, Tribology International, 38(11-12), pp 1007-1012, 2005
[3] Y-R. Jeng, Z-W. Lin & S-H. Shyu, “Changes of Surface Topography During Running-In Process”, J. Trib., 126(3), pp 620-625, 2004
[4] M.M. Khonsari, S. Ghatrehsamani & S. Akbarzadeh, “On the Running-In Nature of Metallic Tribo-Components: A Review”, Wear, 474-475, 203871, 2021
PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.152 page 6
which is much better agreement with experimental data.
The cost to overcome mixed/boundary friction has been quantified for passenger car engines, and worldwide, it has been estimated that, annually, at least $100 billion is spent (mainly on energy) to overcome mixed/boundary friction, and that the associated CO2
emissions are at least 150 million
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