Effects of spray pattern and piston shape on in-cylinder flow in a two-cylinder spray-guided gasoline direct-injection optical engine

•Characteristics of in-cylinder flow in a two-cylinder optical spray-guided direct-injection engine.•Effects of spray pattern and piston shape on the turbulence enhancement.•Numerical analysis of the mixture formation for spray pattern and injection timing.•The in-cylinder flow can be intensified effectively in the asymmetrical spray pattern. In gasoline direct-injection engines, in-cylinder flow plays an important role in terms of fuel atomization, mixture, turbulence, combustion, and emission. The in-cylinder flow is affected by the air–fuel interaction and piston shapes, and it is required to understand and optimize these influences. In this study, visualization of in-cylinder flow was performed depending on the piston shape and spray pattern using particle image velocimetry measurements in a two-cylinder optical engine with high-compression ratio and spray-guided direct-injection. Effects of these parameters were evaluated quantitatively by calculating average flow speed, average tumble ratio, and average turbulent kinetic energy. The mixture was confirmed using numerical simulation. The piston shape had no significant effect on the structure and strength of the in-cylinder flow. The 7-hole injector with asymmetrical spray patter was more effective to enhance the turbulence than the 10-hole injector with symmetrical spray pattern. As the injection timing was delayed up to before top dead center 120°, the average turbulent kinetic energy after fuel injection increased by up to 574 % in the 7-hole injector. However, considering stratification and possible knock region, ideal mixtures were found in the injection timing range of before top dead center 270° to 210° in both injectors.