Enhancing mixture formation and combustion performance of loop-scavenged two-stroke GDI engines through flow-guiding piston

Two-stroke gasoline engines are favored for unmanned aerial vehicles (UAVs) due to their high power density. Although gasoline direct injection (GDI) effectively improves fuel economy by eliminating fuel short-circuiting losses, its inherently short mixing duration poses challenges to mixture preparation. To address this, this study proposes a novel flow-guiding piston specifically designed for a loop-scavenged two-stroke architecture. Three-dimensional CFD simulations are performed using CONVERGE under lean-burn conditions (equivalence ratio of 0.85) to systematically investigate the interaction between the spray and the flow-guiding piston within a loop-scavenging architecture, with its performance evaluated against a baseline flat-top piston. Results show that the guiding dimple effectively regulates the flow field, confining the vortex axis and high turbulent kinetic energy (TKE) regions within the cavity at ignition. The flow-guiding piston requires relatively small inter-jet angles and spray orientation toward the dimple to fully exploit its guiding effect. Unlike the relatively homogeneous mixture of the flat-top baseline, the flow-guiding piston achieves pronounced mixture stratification at ignition, maintaining a local equivalence ratio of approximately 1.15 near the spark plug and a stratification index (SI) of 0.59. This effectively enables stratified combustion, which substantially reduces wall heat transfer losses and yields a 1.37% improvement in indicated thermal efficiency (ITE). Benefiting from the locally enriched mixture and high TKE within the dimple, the flow-guiding piston achieves faster initial flame propagation and lower cyclic variations. These findings demonstrate the superiority of the flow-guiding piston for lean-burn two-stroke GDI engines.