Particulates in a GDI Engine and Their Relation to Wall-Film and Mixing Quality 2022-01-0430

This paper investigates how particulates number PN is influenced by fuel wall-film, liner wetting, and the mixing quality for different start of injection timings (SOI). Both experimental data with PN measurements, endoscope images from a high-speed camera from a single-cylinder engine, and CFD simulations were used for the analysis. Engine geometry was a spray-guided system with 300 bar fuel pressure and with single injections. Data was captured for 2000 rpm / 9 bar IMEPn. The results show that fuel film on the piston was only found to significantly increase PN for over-advanced SOI (in our engine geometry, earlier than -310 CAD). This results in luminescence from diffusion burn on the piston surface, which strongly contributes to PN. For an SOI timing of -310 CAD, fuel film on piston reaches a maximum of 3% of the injected fuel, vaporizes, and no remaining fuel film is found at the time of ignition. Approximately 0.5-1% of the fuel ends up on the liner. Because of the slower evaporation, the liner film is exposed to scraping by the piston rings late in the compression stroke. For tested SOI timings of -310 CAD and later, all piston film evaporates before combustion, and the mixing quality starts dominating the PN formation. The mixing time has the strongest effect, leading to the reduction in PN with earlier SOI up until -310 CAD. Spray-tumble flow interactions are also shown to have appreciable effects on the mixing quality, and the usefulness of these interactions varies depending on the SOI.