Analysis of the impact of exhaust valve profile on the scavenging and combustion process in a 2-stroke Boosted Uniflow Scavenged Gasoline (BUSDIG) engine

Abstract

In order to take advantage of the 2-stroke cycle operation but avoid the drawbacks of conventional ported 2-stroke engine designs, a novel 2-stroke Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) engine was proposed and designed to achieve aggressive engine down-sizing and down-speeding. The uniflow gas scavenging minimises the air short-circuiting through multiple intake ports and exhaust valves in the cylinder head. The intake ports in the BUSDIG engine are integrated to the cylinder liner and designed to create desirable large-scale flow motion. The exhaust valves in the cylinder head are equipped with a variable valve timing actuator and a cam profile switching device. At part-load operation, a low lift exhaust cam profile can be adopted to effectively retain a part of hot residual gas from previous cycle in the cylinder and achieve the hybrid spark ignition (SI) - controlled auto-ignition (CAI) combustion. In order to understand the impact of the low exhaust valve lift profile and the opening timing on the scavenging process and combustion performances, the three-dimensional (3D) computational fluid dynamics (CFD) simulations are performed with a validated SI-CAI hybrid combustion model. It is found that the exhaust valve opening (EVO) timing of the low lift design can be used to effectively control the mass of the trapped residual gas and hence the subsequent scavenging process and intake charge mass. The CAI combustion can be achieved with an appropriate residual gas fraction (RGF) by adjusting the EVO timing. The current work demonstrates the potential of using a low exhaust valve lift design to achieve efficient clean combustion in the 2-stroke BUSDIG engine. The results would be useful for the development of efficient and high power density engines for a range of applications.