Protective Tribofilms on Combustion Engine Valves

Sammanfattning: 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.