Investigation of Charge Mixing and Stratified Fuel Distribution in a DISI Engine Using Rayleigh Scattering and Numerical Simulations

Kapitel i bok, 2022

The stratified fuel distribution and early flame development in a firing spray-guided direct-injection spark-ignition (DISI) engine are characterized applying optical diagnostics.
The goal is to compare effects of single and double injections on the stratified air–fuel mixing and early flame development. Vaporized in-cylinder fuel distributions resulting from both single and double injections before, during and after ignition are selectively visualized applying Rayleigh scattering. Reynolds-averaged Navier–Stokes (RANS) simulations are performed to facilitate interpretation of the obtained experimental data. Two hypotheses are tested. First, injecting the fuel as a closely coupled double injections can improve mixing. Second, the better mixing putatively associated with double injections is mainly due to either a longer mixing time or higher mixing rate (driven by turbulence generated by the injections). The optical investigation of the in-cylinder fuel distributions and early flame propagation corroborated the better mixing, showing that double injections are associated with more evenly distributed fuel, fewer local areas with high fuel concentrations, faster initial flame spread and more even flame propagation (more circular flame spreading). The results from both the experiments and the simulations support the hypothesis that delivering fuel in closely coupled double injections results in better mixing than corresponding single injections. According to the simulations, the improved mixing stems from the longer time available for mixing of the air and fuel in double injection events, which has stronger effects than the higher computed peak bulk mixing rate for single injections.