Evaluations of Scavenge Port Designs for a Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) Engine by 3D CFD Simulations

摘要

The 2-stroke engine has great potential for aggressive engine downsizing due to its double firing frequency which allows lower indicated mean effective pressure (IMEP) and peak in-cylinder pressure with the same output toque compared to the 4-stroke engine. With the aid of new engine technologies, e.g. direct injection, boost and variable valve trains, the drawbacks of traditional 2-stroke engine, e.g. low durability and high emissions, can be resolved in a Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) engine. Compared to the loop-flow or cross-flow engines, the BUSDIG engine, where intake ports are integrated to the cylinder liner and controlled by the movement of piston top while exhaust valves are placed in the cylinder head, can achieve excellent scavenging performance and be operated with high boost. In order to fulfil the potential of the BUSDIG engine, various scavenge ports were designed with different scavenge port number (SPN), Axis Inclination Angle (AIA) and Swirl Orientation Angle (SOA), and their effects were evaluated by three dimensional (3D) computational fluid dynamics (CFD) under different intake pressures and engine speeds. The scavenging process was analyzed by its delivery ratio (DR), trapping efficiency (TE), scavenging efficiency (SE) and charging efficiency (CE). In addition, the in-cylinder flow motions, which play important roles in controlling the charge mixing and combustion process, were studied for different scavenge port designs. Finally, the vertical position of scavenge ports, which determines the scavenge port opening (SPO) timing, the scavenge port height (SPH), and the exhaust valve opening (EVO) timings were varied to investigate their impacts on the scavenging performance and in-cylinder flow motions.