Threshold sensitivity study on spray–spray impingement under flexible injection strategy for fuel/air mixture evaluation

Abstract

To further improve the combustion and emissions performance of opposed-piston two-stroke (OP2S) engines, the implementation of a flexible injection strategy is crucial. The employed double injectors with different nozzle orifice diameters at different timings could make it easier to adjust the concentration stratification of the fuel–air mixture and better control the combustion characteristics. Despite the benefits, spray–spray impingement (SSI) is an unavoidable consequence of employing a dual direct injection system in OP2S engines. Therefore, this study aims to explore the influence of the flexible injection strategy on the SSI phenomenon. The results indicate that the impingement spray area and air entrainment are significantly reduced compared to those of the free spray. Additionally, the spray area is found to be proportional to t0.25 after impingement under synchronous injection. The observed phenomenon is attributed to the fact that SSI promotes droplet coalescence and suppresses the overall diffusion of the spray in the radial direction. Furthermore, the spray can be divided into dense core region, partial atomization region, and homogeneous mixture region by threshold sensitivity methodology to analyze spray spatial distribution and evaluate mixture formation more effectively. Finally, the injection strategy (Pinj = 180 MPa, ρamb = 50 kg/m3, ΔT = −800 μs) is selected to enhance atomization and mixing performance. This strategy is crucial for minimizing soot emissions and fuel consumption in the OP2S engines among the considered injection strategies.