Abstract
In this study, effects of intake scavenge port designs and exhaust valve opening profiles were studied on the scavenging process in a newly proposed two-stroke boosted uniflow scavenged direct injection gasoline engine by detailed three-dimensional engine simulations. As the most important geometric parameters, the axis inclination angle and swirl orientation angle of scavenge ports, as shown in Figure 1, were investigated and optimized for best scavenging performances at first. With the optimal axis inclination angle of 90° and swirl orientation angle of 20°, various combinations of scavenge port opening timing, exhaust valve opening duration and exhaust valve opening timing were then analysed. Four distinct scavenging periods, that is, early backflow period, backflow scavenging period, main scavenging period and post backflow period, were identified and their impacts on the in-cylinder flow motions and scavenging performances were investigated. The results show that the optimal scavenging performance can be achieved with a higher delivery ratio, charging efficiency and scavenging efficiency when the post backflow is just avoided by tuning the difference between the closing timings of scavenge ports and exhaust valves (Δ close) and the overlap between the opening profiles of scavenge ports and exhaust valves (Δ overlap) for a specific exhaust valve opening duration. A longer exhaust valve opening duration can be used to further improve the scavenging performances. In addition, the difference between the opening timings of scavenge ports and exhaust valves (Δ open) can be increased to improve scavenging efficiency. The Δ close also shows strong positive correlation with in-cylinder swirl ratio and negative correlation with tumble ratio. The results presented in this study provide the fundamental knowledge of the scavenging process in the uniflow scavenged two-stroke engine and assist the design of scavenge ports and valve strategies to optimize in-cylinder flow motion and scavenge performances in the two-stroke boosted uniflow scavenged direct injection gasoline engine with a variable valve actuation system for exhaust valves.
Highlights
The engine down-sizing and down-speeding technologies show great potential to improve the fuel consumption of the automotive engine
The exhaust valve opening duration (ED) was fixed at 126 °CA and the exhaust valve opening (EVO) timing is fixed at 117 °CA
The 3D computational fluid dynamics (CFD) simulations were performed to understand the effect of scavenge port designs and EVO profiles on the in-cylinder flow motions and scavenge performances in a two-stroke boosted uniflow scavenged direct injection gasoline (BUSDIG) engine
Summary
The engine down-sizing and down-speeding technologies show great potential to improve the fuel consumption of the automotive engine. The three-dimensional (3D) CFD simulations were performed to understand the effects of scavenge port angles, that is, AIA and SOA, the SPO timings and EVO profiles on the incylinder flow motions and scavenge performances in the proposed boosted uniflow scavenged direct injection gasoline (BUSDIG) engine.[27,28] The analysis of the scavenging process with different AIA and SOA was carried out at first to determine the optimal scavenge port angles for the BUSDIG engine. Based on the optimal design of scavenge port angles, the simulations with various combinations of SPO timing, exhaust valve opening duration (ED) and EVO timing were performed to understand their impacts on in-cylinder flow motions and scavenge performances. The mesh sensitivity study performed in our previous work[28] indicated that the mesh with an average grid size of 1.6 mm was able to reproduce the same results as finer mesh and was adopted in this study
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