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Despite the known benefits of direct injection (DI) spark ignition (SI) engines, port fuel injection (PFI) remains a highly relevant injection concept, especially for cost-sensitive market segments. Since particulate number (PN) emissions limits can be expected also for PFI SI engines in future emission legislations, it is necessary to understand the soot formation mechanisms and possible countermeasures. Several experimental studies demonstrated an advantage for PFI SI engines in terms of PN emissions compared to DI. In this paper an extended focus on higher engine loads for future test cycles or real driving emissions testing (RDE) is applied. The combination of operating parameter studies and optical analysis by high-speed video endoscopy on a four-cylinder turbocharged SI engine allows for a profound understanding of relevant soot formation mechanisms.

A major source for soot particle formation in Gasoline-Direct-Injection (GDI) engines are fuel-rich zones near walls as a result of wall wetting during injection. To address this problem, a thorough understanding of the wall film formation and evaporation processes is necessary. The wall temperature before, during and after fuel impingement is an important parameter in this respect, but is not easily measured using conventional methods. In this work, a recently developed laser-based phosphor thermography technique is implemented for investigations of spray-induced surface cooling. This spatially and temporally resolved method can provide surface temperature measurements on the wetted side of the surface without being affected by the fuel-film. Zinc oxide (ZnO) particles, dispersed in a chemical binder, were deposited onto a thin steel plate obtaining a coating thickness of 17 μm after annealing.

BMW VALVETRONIC technology is able to maintain the most important measures to reduce emissions. The further optimized charge movement created by VALVETRONIC stabilizes the combustion in the catalyst heating mode with extremely retarded ignition timing. When the engine is warm the high residual gas tolerance ensures very low Engine-Out NOx emissions and at the same time a low level of hydrocarbons. The atomization of fuel droplets due to high flow velocity in the valve gap area leads to improved mixture formation and reduced wall wetting. Engine-Out HC emissions in a cold engine are therefore reduced. Combined, the emission measures achieve robust and efficient emission control. In combination with additional after-treatment like secondary air system and catalysts using high cell density VALVETRONIC engines form an excellent base for SULEV emission regulations without neglecting the typical BMW claim of efficient dynamics.