Multi-dimensional numerical simulation of scavenge flow in a small two-stroke engine

Abstract

Computational study of scavenging flow in a typical small vehicle engine is accomplished using multi-dimensional moving-mesh unsteady and quasi-steady models. A representative two-stroke 5 port engine was selected and sectioned to measure the geometrical features of the transfer ports. The commercial codes Auto CAD and Pro- Engineer was used to model the cylinder and ports. The commercial code Star-CD was used to analyze the unsteady gas exchange process within the engine combustion chamber soon after combustion. Pressure boundary conditions were used to represent crankcase conditions and exhaust tail pipe conditions at the scavenge port inlets and the exhaust port exit respectively. The simulation was stated at top-dead-centre (TDC) after combustion, assuming no exhaust gas remained in the engine from the previous cycle. For moving-mesh calculation, a cycle simulation is performed in two stages, separated by a theoretical constant-volume combustion calculation. The TDC calculation was initialized using a preliminary simulation staring at the bottom-dead-centre (BDC) assuming no exhaust gas remained in the cylinder at the start of simulation at Parallel computing was used to run the simulation. Both quasi-steady and transient simulations indicate that typical multi-port inlet placement and timing allow significant short-circuiting of mixture, consistent with emissions observations. Residual gas is well scavenged from the hemi-spherical head cylinder. Backflow of combustion gasses into the transfer ports were observed upon opening of the transfer ports which indicate an inadequate blow down period. The inlet angles of the mixture from transfer ports are oriented upwards towards the dome of the cylinder and are satisfactory. The relative positioning and orientation of the scavenge ports promote good mixing and helps to reduce the inflow velocities, though considerable short-circuiting is still present in the engine.