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Wear protective capacity of tribofilms formed on combustion engine valves with different surface textures
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.ORCID iD: 0000-0003-1274-4974
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.ORCID iD: 0000-0002-3955-5746
2017 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 376-377, p. 1429-1436Article in journal (Refereed) Published
Abstract [en]

The valve system controls the flow of gases into and out of the cylinders of combustion engines. As the combustion chamber is repeatedly filled with air and emptied from combustion residues, the contacting surfaces of the valve and valve seat insert (VSI) are exposed to high temperatures, high speed impacts, corrosive environment and high closing forces. Since the expected wear life of the components is hundreds of millions of operational cycles, the wear rate has to be extremely low. Previous studies have shown that this is accomplished by the continual formation of protective tribofilms on the surfaces, formed from oil additive residues. For some stationary gas engines, seemingly random occurrence of severe wear, i.e. without correlation to special running conditions, calls for an improved understanding of the wear protective abilities of these tribofilms. Further, the potential of making the protection more robust by using simple textures on sealing surfaces is investigated. Real valves and VSI's were therefore run in a dedicated test rig, previously shown capable of forming tribofilms similar to those found in real engine valves. Two surface textures, turned or ground, were tested. After a running in phase where fully covering tribofilms were formed, the tests were continued without adding the oil residues needed for replenishing the tribofilm. The flow of oil residues was either completely stopped or only intermittently open. The resulting wear was monitored during testing and after finishing the tests remaining tribofilms were analysed with SEM and EDS. The severity of the wear was graded and the presence of tribofilm and localization of wear was correlated. The endurance of the tribofilms was surprisingly high for both textures. Typically the tribofilms exhibited better wear resistance than the underlying metals. Once fully formed, the tribofilms can therefore endure for a long time without addition of new tribofilm forming material. Both textures showed the first sign of wear after roughly the same time and then followed the same progression, when the flow of oil residues was cut. With the intermittent oil residue supply, both textures performed even better. Especially the turned texture showed only slow, mild wear.

Place, publisher, year, edition, pages
2017. Vol. 376-377, p. 1429-1436
Keyword [en]
Combustion engine valves, Surface texturing, Protective tribofilms, Rig testing
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:uu:diva-329132DOI: 10.1016/j.wear.2016.12.060ISI: 000403902000057OAI: oai:DiVA.org:uu-329132DiVA, id: diva2:1148161
Available from: 2017-10-10 Created: 2017-10-10 Last updated: 2018-02-22
In thesis
1. Protective Tribofilms on Combustion Engine Valves
Open this publication in new window or tab >>Protective Tribofilms on Combustion Engine Valves
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Inside the complex machinery of modern heavy-duty engines, the sealing surfaces of the valve and valve seat insert have to endure. Right next to the combustion, temperatures are high and high pressure deforms the components, causing a small relative motion in the interface. The wear rate of the surfaces has to be extremely low; in total every valve opens and closes up to a billion times. The minimal wear rate is achieved thanks to the formation of protective tribofilms on the surfaces, originating from oil residues that reach the surfaces - even though these are not intentionally lubricated. The increasing demands on service life, fuel efficiency and clean combustion, lead to changes that may harm the formation of tribofilms, which would lead to dramatically reduced service lives of the valves. This calls for an improved understanding of the formation of tribofilms and how their protective effects can be promoted.

The best protective effect is provided by tribofilms formed from engine oil additives. This is not a typical lubricating effect, but protection by formation and replenishment of a solid coating. Oils without additives cannot form solid films that offer the same protection. Tribofilms are formed from oil residue particles that land, agglomerate and so gradually cover the surfaces. Once covered, the surfaces stay protected relatively long also if no new residues reach the surface. In fact, the tribofilms have a higher wear resistance than do the component surfaces. If the tribofilms become worn off, the underlying surfaces wear quickly, but as long as new residues reach the surfaces, the tribofilms can rebuild and maintain the wear protection indefinitely.

This tribofilm formation and endurance can be promoted by texturing the surfaces.  A texture can improve the amount of oil residues captured and their surface coverage, reducing random occurrence of wear and the demand for new residues to maintain the tribofilm. The tribofilm formation is also affected by the additive content of the engine oil, where especially high sulfur content is found to promote tribofilm coverage. A custom engine oil with high additive content could be used for efficient tribofilm formation during running-in of engines.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 83
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1635
Keyword
Internal combustion engine, valve, sealing surface, tribofilm, oil residue, test rig
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-342549 (URN)978-91-513-0243-0 (ISBN)
Public defence
2018-04-13, Polhemsalen, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2018-03-21 Created: 2018-02-22 Last updated: 2018-04-24

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Elo, RobinHeinrichs, JannicaJacobson, Staffan

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