Effects of Lubricant Additives on Auto-Ignition under a Hot Co-Flow Atmosphere

Pre-ignition may lead to an extreme knock (super-knock or mega-knock) which will impose a severe negative influence on the engine performance and service life, thus limiting the development of downsizing gasoline direct injection (GDI) engine. More and more studies reveal that the auto-ignition of lubricants is the potential source for pre-ignition. However, pre-ignition is complicated to study on the engine test bench. In this paper, a convenient test method is applied to investigate the influence of lubricants metal-additives on pre-ignition. 8 groups of lubricants are injected into a hot co-flow atmosphere which generated by a burner. A single-hole nozzle injector with a diameter of 0.2 mm at 20 MPa injection pressure is utilized for lubricants' injection and spray atomization. The ignition delays of lubricants with different additives of calcium, ZDDP (Zinc Dialkyl Dithiophosphates) and magnesium content under the hot co-flow atmosphere are recorded with a high-speed camera. The experiments are carried out at one atmospheric pressure and the co-flow temperature varies from 1123 K to 1223 K. The result shows that the ignition delays of lubricants decline sharply with the increase of co-flow temperature in the whole temperature range. There is one critical temperature about 1173K in this study. Under this temperature, effects of calcium content on the auto-ignition delay are significant; over this temperature, its effect is much smaller and almost no difference. Lubricants with higher content of ZDDP present a longer ignition delay over the entire temperature range. And the experimental result also indicates that the ignition delay is not sensitive to the magnesium content.