Cycle-resolved visualization of pre-ignition and abnormal combustion phenomena in a GDI engine

Highlights: • Pre-ignition and knocking were investigated in a GDI engine. • Main and secondary flames are correlated with abnormal combustion. • Secondary ignitions were recorded both pre- and post-spark timing. • Secondary flames close to the cylinder wall were related to more intense knock. • Fuel deposits in the combustion chamber play a decisive role in abnormal combustion. - Abstract: Downsized spark ignition engines are highly efficient but promote abnormal combustion phenomena like pre-ignition and knocking. This limits the maximum load of SI engines and consequently the scale of downsizing. In this paper, cycle resolved visualization and thermodynamic analysis were used to investigate abnormal combustion phenomena such as pre-ignition at localized surfaces and due to moving hot-particles, as well as post-spark discharge secondary flames near the walls and knock. Tests were performed on an optically accessible single cylinder 'wall guided' GDI engine equipped with self-lubricant rings in order to avoid oil residuals in the combustion chamber and to exclude them as cause of pre-ignitions. Cycle resolved visualization allowed obtaining information on hot-particles trajectory and provided insight into the morphology and speed of flame fronts (both spark-ignited and secondary ones). Optical data were correlated to in-cylinder pressure and rate of heat release curves. Secondary flames were detected both in normal and knocking cycles. Under heavy knocking conditions, when secondary flames were identified, their occurrence was correlated to the onset of knock; more advanced start of pressure oscillations corresponded to prior evidence of non-spark ignited sites near the walls. A distribution map of secondary flames showed higher probability of occurrence near the cylinder walls and towards the injector location, suggesting that fuel deposits play a major role in the initiation of abnormal combustion phenomena.