Browse Publications Technical Papers 2020-01-0291
2020-04-14

Combined experimental/numerical study of the soot formation process in a gasoline direct-injection spray in the presence of laser-induced plasma ignition 2020-01-0291

Gasoline engines with direct-injection systems operated in a stratified mode provide improved engine efficiency. However, in such mode the particulate emissions are increased compared to conventional spark engine due to the limited mixing between air and fuel, which results in locally rich burn zones where soot is formed. Eliminating these rich combustion events requires detailed understanding of the complex physical phenomena occurring in an engine, including a very high Reynolds number two-phase flow with a complex chemistry. This study proposed a combined experimental/numerical work to better understand the soot formation process when varying the local fuel-air mixture at the spark-ignition location. Combustion issued from an eight-hole, direct-injection spray was experimentally studied in a constant-volume pre-burn combustion vessel using simultaneous high-speed diffused backlit imaging (DBI) and OH* chemiluminescence. DBI has been employed to observe the liquid-phase of the spray and to investigate the soot formation and oxidation taking place during the combustion, while OH* chemiluminescence was used to track the high-temperature ignition and flame. In the current study, the fuel-air mixture was ignited with a plasma induced by a single-shot 10-Hz (7-ns pulse) Nd:YAG laser (at 1064 mn), which allows for precisely controlling the ignition location in space and time. Systematic experimental studies were performed by varying ignition timing relative to the end of injection, thus varying the local mixing at the ignition location, from non-sooting to high-sooting regimes. These results have been used as a database for the numerical simulation. Reynolds-averaged Navier–Stokes (RANS) simulations were performed to assess mixing and combustion. First, non-reactive spray simulations were developed, guided by experimental liquid penetration and width measurements for various nozzle rotation angles. Second, a reactive case, where the laser-induced plasma ignition was modeled, has been used to provide more information on the relation between local air-fuel mixing, at the ignition location and throughout the charge, on soot production.

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