A fundamental investigation of oil additives on pre-ignition in a high pressure combustion chamber

The irregular combustion phenomenon of low speed pre-ignition (LSPI) is still an important issue in modern internal combustion engine development and research, despite numerous publications looking into different directions such as the influence of injection strategy, scavenging, engine oil and fuel properties, and solid particulate matter as a potential ignition source.

As the occurrence of pre-ignition is heavily dependent on the test engine, a pressure chamber test bench was set up to enable experiments to be carried out under constant and reproducible conditions. Pressure and temperature can be set in such a way that ambient conditions similar to those in the combustion chamber can be created.

The first step was to investigate whether a single droplet of oil in the combustionchamber can be the trigger for pre-ignition. The ignition delay times of pure PAO6 oiland PAO6 oil with different calcium and magnesium contents have been investigated.As it is quite conceivable that oil could accumulate in the combustion chamberand remain there for several operating cycles, different surface materials were alsoexamined in order to investigate possible catalytic effects. ... mehrTherefore, in a second stepa collector for oil was developed with which it was possible to record the ignitiontemperature of the oil in the constant-volume chamber. This collector had to beoptimized to be able to hold a very small amount of oil, heat up quickly and be coatedwith different materials.

The oils were then examined with regard to their auto-ignition temperature atdifferent chamber pressures and temperatures. In addition to metallic, ceramic andnon-metallic materials, which have a chemically inert and possibly catalytic effect,materials that occur in the combustion chamber were also used as surfaces.

This research has shown that the common mechanisms of action found in theliterature can be refuted. A single oil droplet moving freely in the combustion chambercannot be the trigger for pre-ignition due to the excessively long ignition delay time.In addition, catalytic effects between the oil and a surface made of gold, chromiumnitride, graphite and aluminium oxide could not be observed. This is shown by theself-ignition temperature of the oil, which could not be reduced by the surfaces.