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Protective Tribofilms on Combustion Engine Valves
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Ångström Tribomaterials Group)ORCID iD: 0000-0003-1274-4974
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 [en]
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: urn:nbn:se:uu:diva-342549ISBN: 978-91-513-0243-0 (print)OAI: oai:DiVA.org:uu-342549DiVA, id: diva2:1184646
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
List of papers
1. The importance of oil and particle flow for exhaust valve wear - An experimental study
Open this publication in new window or tab >>The importance of oil and particle flow for exhaust valve wear - An experimental study
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.

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:nbn:se:uu:diva-204467 (URN)10.1016/j.triboint.2013.09.009 (DOI)000328183100020 ()
Available from: 2013-08-06 Created: 2013-08-06 Last updated: 2018-02-22Bibliographically approved
2. Formation and breakdown of oil residue tribofilms protecting the valves of diesel engines
Open this publication in new window or tab >>Formation and breakdown of oil residue tribofilms protecting the valves of diesel engines
2015 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 330-331, p. 193-198Article in journal (Refereed) Published
Abstract [en]

The contacting surfaces of modern valve systems experience a complex contact situation with repeated micro sliding at high temperatures and pressures. The wear rate of the surfaces has to be extremely low to fulfill the high demands on engine efficiency during its entire life-time-wear will cause valve recession and thus make the combustion less efficient. In addition to this, legislation requires reduced particulates in the emissions, which leads to aggravating conditions for the valves. Studies of field samples from well-functioning engines have shown that a protective tribofilm is formed on the contacting surfaces of the valve. This tribofilm is primarily built up from combustion residues of the engine oil and fuel, making its composition sensitive to their additives. Since the low wear rate is dependent on the formation of a tribofilm based on oil residues, while future legislation will demand even cleaner emissions, a deepened understanding about the formation and dynamics of these tribofilms is needed. How quickly are they formed, how quickly do they wear, do they require constant supply of "building material", can they efficiently protect the surfaces also when substantially less building material is available? In the present study, the formation and breakdown mechanisms of this type of protective tribofilms have been investigated in a specifically designed valve rig. This rig uses real engine components and allows controlled addition of oil mist (in the present case from a fully formulated engine oil) into a hot air stream, passing the operating valve. Four phases were identified in the tribofilm dynamics. In the first-formation-phase, oil residue particles become trapped on the sealing surfaces of the valve, and then smeared out between the closing surfaces to form a covering tribofilm. In the second-equilibrium-phase, the tribofilm coverage is stable, meaning that the addition of new particles is balanced by wear of the film. Two types of films form, one thick carbon-based film and one thinner additive-based film. If the supply of oil is cut off, the third-breakdown-phase commences. Here the carbon-based film is quickly removed while the additive-based film keeps protecting the valve surface for numerous closing cycles. When also this film become worn through, the final phase-wear of exposed valve material -commences, involving severe wear and oxidization. Interestingly, it was found that the breakdown was slower if the equilibrium phase was longer, indicating that the additive-based tribofilm becomes more durable by being more worked.

Keywords
Internal combustion engines, Valve, Wear protection, Lubricant additives, Tribofilm, Test rig
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-260161 (URN)10.1016/j.wear.2015.01.066 (DOI)000357438000022 ()
Available from: 2015-08-17 Created: 2015-08-17 Last updated: 2018-02-22
3. Surface texturing to promote formation of protective tribofilms on combustion engine valves
Open this publication in new window or tab >>Surface texturing to promote formation of protective tribofilms on combustion engine valves
2018 (English)In: Proceedings of the Institution of mechanical engineers. Part J, journal of engineering tribology, ISSN 1350-6501, E-ISSN 2041-305X, Vol. 232, no 1, p. 54-61Article in journal (Refereed) Published
Abstract [en]

In a combustion engine, the valve system controls the flow of gases in to and out of the combustion chamber.The contacting surfaces experience a harsh tribological situation with high temperatures, high speed impacts, corrosiveenvironment and high closing forces causing micro sliding in the interface. The components have to endure in the rangeof hundreds of millions to a billion operational cycles, resulting in extreme demands on low wear rate. Such low wearrates can be accomplished by the protective action from tribofilms forming from oil residues, avoiding a pure metal-tometalcontact. Such tribofilms are found on well-functioning engine valves from a variety of engines, but some stationarygas engines experience problems with wear occurring seemingly randomly at normal running conditions. For somereason, the tribofilm has not protected the surfaces sufficiently, causing wear. One way to combat the random behaviourcould be to promote robust function of the protective tribofilms by texturing the valve sealing surfaces to improve thecapture and storage of oil residues. By stabilising the supply in this way, the damage from periods with low access totribofilm forming material could be reduced. The present work demonstrates that turning of the valve seat inserts,creating valleys perpendicular to the sliding motion, can be developed into a useful solution. The amount and localisationof tribofilms became more predictable and stable than without the texture, leading to reduced component wear. Thevalleys should not be too wide, since this increased the amount of exposed metal if the tribofilm flaked off. When havingthe same width, the deeper valleys showed less flaking off of the tribofilm.

Keywords
Tribofilm formation, combustion engine valve, surface texture, rig testing, scanning electron microscopy
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-335323 (URN)10.1177/1350650117739738 (DOI)000419852700006 ()
Available from: 2017-12-04 Created: 2017-12-04 Last updated: 2018-02-22Bibliographically approved
4. Wear protective capacity of tribofilms formed on combustion engine valves with different surface textures
Open this publication in new window or tab >>Wear protective capacity of tribofilms formed on combustion engine valves with different surface textures
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.

Keywords
Combustion engine valves, Surface texturing, Protective tribofilms, Rig testing
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-329132 (URN)10.1016/j.wear.2016.12.060 (DOI)000403902000057 ()
Available from: 2017-10-10 Created: 2017-10-10 Last updated: 2018-02-22
5. Tribofilm formation on combustion engine valves ­– influence of additive content in the oil
Open this publication in new window or tab >>Tribofilm formation on combustion engine valves ­– influence of additive content in the oil
(English)Manuscript (preprint) (Other academic)
National Category
Materials Engineering
Identifiers
urn:nbn:se:uu:diva-342487 (URN)
Available from: 2018-02-21 Created: 2018-02-21 Last updated: 2018-02-22

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