Experimental Investigation of Gasoline Direct Injection (GDI) Sprays Using Diffused Back Illumination (DBI) and Structured Laser Illumination Planar Imaging (SLIPI) Techniques

Abstract

Abstract A large share of passenger vehicles are currently running on gasoline, which is considered the major contributor to carbon emissions from the transportation sector. In gasoline direct injection (GDI), the fuel is introduced directly into the combustion chamber in a spark ignition (SI) engine. GDI has the advantage of operating at a higher compression ratio due to the charge cooling effect, resulting in better fuel utilization and lower pollutant emissions. The spray formed from fuel injection under different engine running conditions governs the charge (air-fuel) characteristics. The present work focuses on experiments using a GDI system with a multi-hole injector for gasoline fuel sprays in a constant volume chamber. The saturation pressure of the fuel present within the nozzle increases with rising temperature (80 to 90°C) from heat transfer due to the placement of the injector directly into the combustion chamber. At part-load running conditions, when the heated fuel injected into an environment has a pressure sufficiently lower (achieved in early injection) than its saturation pressure, it attains the state of superheating and undergoes flash boiling. The abrupt vaporization of injected fuel results from flash boiling and affects spray characteristics. The ratio of chamber pressure to saturation pressure (Pch/Psat) indicates the propensity of flashing for a given super-heated operating condition. Macroscopic analysis of GDI sprays is reported in this study under flashing and non-flashing conditions through different experimental techniques to provide a comprehensive understanding. Experimental studies reported in this kind of literature typically adopt a specific technique to elucidate the details of spray evolution. A Diffused Back Illumination (DBI) imaging technique with a low-speed Nd: YAG laser has been carried out to analyse the liquid phase of the spray, such as penetration, cone angle, and width. Additionally, a novel Structured Laser Illumination Planar Imaging (SLIPI) technique has been adopted to identify the relatively dense spray regions by addressing the difficulty due to multiple scattering of light illuminating from surrounding scatterer media (droplets). SLIPI is used to suppress the effect of the surrounding light artefacts, helping explore the structure in a new dimension. A multi-plume spray is expected to have voids, dense, and dilute portions at different locations, affecting the fuel mixture, and interestingly, it appears after subduing the effects of multi-scattering from a modulated laser sheet using SLIPI. An LED-based spray illumination was also used to compare the DBI and SLIPI results to better understand the utility and effectiveness of different imaging techniques in a densely populated and highly transient spray phenomenon.