SECTOR FOCUS: TRIBOLOGY
Engine tribology: a thorny way from lab to field
Boris Zhmud, Ph.D., Assoc. Prof., FRSC, BIZOL Germany GmbH
The term tribology was coined by Peter Jost in 1966 in a report where he highlighted the cost of friction, wear and corrosion to the UK economy. However, the roots of tribology stretch back to ancient times. Historical records point to the use of lime slurries, vegatable oils and animal fats by Egyptian pyramid builders to ease moving large pieces of rocks. The first recorded definition of the coefficient of friction dates back to Leonardo da Vinci: it is 500 years old and still valid!
Since friction leads to energy losses, heat generation, and wear, tribology has always been an integral part of mechanical engineering, spreading to automotive, industrial and energy generation sectors. However, due to a growing focus on energy efficiency and component longevity, tribological research has been rapidly intensifying over the past decades.
Unfortunately, there exists a certain gap, both in attitude and competence, between university researchers and lube industry professionals as their willingness to venture out for new technologies. A narrow-sighted one-component-at-a-time optimization approach still prevails at many OEMs due to their internal R&D organization, and “academic headcount” does not always help streamline the development process. I have seen many examples – presented at conferences and published in papers – proving that the one-component optimisation is often nothing less than misleading. If you study the piston ring-cylinder bore friction in order to optimise the honing structure of the bore by using a reciprocating friction and wear tester, you will discover that a smoother bore produces less friction. So, a temptation may arise to go for a smoother finish.
20 LUBE MAGAZINE NO.154 DECEMBER 2019
Tried - it doesn’t work. As soon as Rpk falls below 0.4 to 0.6 um, the scuffing issue pops up in the firing engine tests. Hence, a “scientific” honing structure optimisation is nothing less than misleading. Similarly, an SRV measurement may show that some exotic nano-additive brings 50% reduction in friction, and then none is found in the firing engine tests [1] - and such examples are abundant.
The following diagram shows a relationship between practical relevance and cost of different tribological tests. Hence, basic tests may be inexpensive, but often also utterly useless. However, such tests are broadly used for marketing purposes, where the impression factor outruns the truth. In general, it is fairly easy to find materials or additives excelling in a given tribological test – there are tons of papers competing with each other in how low one can go in terms of friction and wear. It is much more challenging to identify materials and additives that can be used by industry. Cost, health safety, supply situation and compatibility with a myriad of other materials already in use play a huge role here. This shows that, only complete system tests can be viewed as relevant. As engineers always say, Field experience is the King!
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