The delicate balance of tribofilm formation on combustion engine valves
The surfaces of almost any tribological system will become modified during use. These modifications may involve deformation hardening, phase transformations, oxidation, and more. In many systems, transfer of material from the counter surface, reaction with lubricant additives or pick up of material from loose particles add even more complexity to the outermost surface layers, i.e. the tribofilms. The tribofilms typically have very strong impact on the friction and wear performance of the system.
The valve/valve seat insert system of diesel engines constitutes one particular system were the presence of tribofilms has been shown to be decisive for the wear life. Recent studies have revealed the thickness, the layered structure and the composition of tribofilms formed on the sealing surfaces of the valves. Typically, the films are gradually built-up from oil additive residues that pass through the valve. Some of these microscopic particles land on the sealing surfaces, become squeezed when the valve closes, and then gradually become sheared out to form a relatively coherent, covering, layered structure.
The balance between forming and wearing off this film is delicate. If it becomes too thin (or not form at all) the wear rate due to direct contact between the surfaces become high. If it grows too thick, it runs the risk of local flaking or wear. Such local loss of the film leads to the formation of channels where the high-pressure combustion gasses can leak out while the valve is closed. This local leakage leads to loss of power, but more importantly also to a high risk of catastrophic wear, due to local overheating.
The increasingly stricter regulation on cleaner exhausts on many markets has required transfer to cleaner oils and fuels, also with strongly reduced amounts of ash forming additives. This changes result in less formation of tribofilms, which often brings about wear problems. Moreover, the differences in oil and fuel qualities between different regions of the world have increased, which put severe demands on the sealing systems to work properly even under the very diverse conditions ranging from almost no film formation to formation of thick black layers.
The presentation will first focus on the importance of tribofilms, starting by presenting the composition and structure of tribofilms formed in real truck engines as investigated with the SEM and analytical techniques. Next, the focus will be on the tribological properties of the tribofilm, as well as the dynamics of its growth and destruction, as investigated in a dedicated valve rig.
3rd European Friction, Wear and Wear Protection Karlsruhe, Germany, May 2014