Abstract
Diesel engines and gasoline direct injection (GDI) engines both produce soot due to incomplete combustion of the fuel and some enters the lubricant where it accumulates between drain intervals, promoting wear of rubbing engine components. Currently the most favoured mechanism for this wear is that the anti-wear additives present in engine oils, primarily zinc dialkyldithiophosphates (ZDDPs), react very rapidly with rubbing surfaces to form relatively soft reaction products. These are easily abraded by soot, resulting in a corrosive-abrasive wear mechanism. This study has explored the impact of engine oil dispersant additives on this type of wear using combinations of dispersant, ZDDP and carbon black, a soot surrogate. It has been found that both the concentration and type of dispersant are critical in influencing wear. With most dispersants studied, wear becomes very high over an intermediate dispersant concentration range of ca 0.1–0.4 wt% N, with both lower and higher dispersant levels showing much less wear. However a few dispersants appear able to suppress high wear by ZDDP and carbon black over the whole concentration range. A series of experiments have been carried out to determine the origin of this behaviour and it is believed that high levels of dispersant, and, for a few dispersants, all concentration levels, protect the iron sulphide tribofilm initially formed by ZDDP from abrasion by carbon black.
Highlights
High levels of soot in engine lubricants are frequently reported to induce high wear rates of engine components
The main evidence for this is that when both zinc dialkyldithiophosphate (ZDDP) and the soot surrogate carbon black (CB) are present together in a lubricant, the combination can lead to much higher wear than if either zinc dialkyldithiophosphates (ZDDPs) or CB is absent
It is generally accepted that CB, being harder than most ZDDP films, abrades the tribofilm formed by ZDDP very rapidly and that the resulting loss of ferrous material is responsible for the high wear often seen when both ZDDP and CB are present in a lubricant [19, 21]
Summary
High levels of soot in engine lubricants are frequently reported to induce high wear rates of engine components. Other mechanisms suggested by different studies were that soot can induce enhanced oil degradation [7], metal reduction from anti-wear F e3O4 to pro-wear FeO [8], lubricant starvation [9,10,11] and abrasion, either of the rubbing surfaces [12, 13] or anti-wear film [4, 12,13,14,15,16,17,18,19,20,21]. In 2010, Olomolehin et al investigated the influence of CB and other nanoparticles on wear in anti-wear additivecontaining model lubricants [19] They found that when CB was added to an oil containing ZDDP, the combination gave much more wear than when no ZDDP was present; i.e. ZDDP became pro-wear. They suggested a corrosiveabrasive mechanism in which the CB removes the initial
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