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The importance of oil and particle flow for exhaust valve wear - An experimental study
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribomaterial)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribomaterial)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribomaterial)
2014 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 69, p. 176-183Article in journal (Refereed) Published
Abstract [en]

Oil residue particles have a large impact in protecting the sealing surfaces of heavy duty exhaust valves. To increase the understanding of the mechanism behind this, the wear behavior of exhaust valves subjected to a flow of hot air with controlled amounts of oil particles of three different compositions has been investigated. Air flow without addition of oil was used as a reference. The degree and mechanisms of surface damage proved to be sensitive to the test parameters. Residues from the oils containing additives proved to form protective tribofilms, while the oil without additives promoted agglomeration of wear debris on the sealing surfaces. The dry reference showed severe wear with debris scattered over the surfaces.

Place, publisher, year, edition, pages
2014. Vol. 69, p. 176-183
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Engineering Science with specialization in Tribo Materials; Engineering Science with specialization in Materials Science
Identifiers
URN: urn:nbn:se:uu:diva-204467DOI: 10.1016/j.triboint.2013.09.009ISI: 000328183100020OAI: oai:DiVA.org:uu-204467DiVA, id: diva2:639151
Available from: 2013-08-06 Created: 2013-08-06 Last updated: 2018-02-22Bibliographically approved
In thesis
1. Combustion Valve Wear: A Tribological Study of Combustion Valve Sealing Interfaces
Open this publication in new window or tab >>Combustion Valve Wear: A Tribological Study of Combustion Valve Sealing Interfaces
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The exhaust valve system of combustion engines experiences a very complex contact situation of frequent impact involving micro sliding, high and varying temperatures, complex exhaust gas chemistry and possible particulates, etc. In addition, the tribological situation in the exhaust valve system is expected to become even worse due to strict future emission regulations, which will require enhanced combustion and cleaner fuels. This will substantially reduce the formation of combustion products that might ease the contact conditions by forming tribofilms on the contacting surfaces. The lack of protective films is expected to result in increased wear of the contact surfaces.

The aim of the work presented in this thesis has been to increase the tribological understanding of the valves. The wear that takes place in the valve sealing interface and how the change in operating conditions affects it have been studied. Such understanding will facilitate the development of future valve designs.

A test rig has been developed. It has a unique design with the ability to insert ppm amounts of media into a hot air flow, in order to simulate different environmental changes, e.g. varying amount and composition of combustion residue particles.

PVD coated valves were evaluated in a dry atmosphere. It was concluded that although some of the coatings showed potential, the substrate could not support the thin, hard coatings.

Investigations with an addition of different oils have been performed. Fully formulated oils proved to build up a protective oil residue tribofilm. This tribofilm has been in-depth analysed and proved to have similar composition and appearance as tribofilms found on low wear field tested valves. With a non-additivated oil, wear particles from the valve seat insert formed a wear particle tribofilm on top of the valve sealing surface. Without any oil the surfaces showed severe wear with wear particles spread over the surfaces.

The results presented give a hint about what to be expected in the future, when the engine oils are replaced with ash less oils with reduced amount of additives and the consumed amount of oil within the cylinders are reduced. 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. p. 57
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1058
Keywords
Combustion valves, wear, tribofilm, test rig, combustion residue
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-204636 (URN)978-91-554-8715-7 (ISBN)
Public defence
2013-09-20, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2013-08-30 Created: 2013-08-07 Last updated: 2018-01-11
2. 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
Keywords
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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Forsberg, PeterElo, RobinJacobson, Staffan

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