Analysis of scavenge port designs and exhaust valve profiles on the in-cylinder flow and scavenging performance in a two-stroke boosted uniflow scavenged direct injection gasoline engine

Abstract

© The author(s) and/or their employer(s), 2017. In this study, effects of intake scavenge port designs and exhaust valve opening profiles were studied on the scavenging process in a newly proposed 2-stroke boosted uniflow scavenged direct injection gasoline (BUSDIG) engine by detailed three dimensional engine simulations. As the most important geometric parameters, the axis inclination angle (AIA) and swirl orientation angle (SOA) of scavenge ports, as shown in Figure 1, were investigated and optimized for best scavenging performances at first. With the optimal AIA of 90⁰ and SOA of 20⁰, various combinations of scavenge port opening (SPO) timing, exhaust valve opening duration (ED) and exhaust valve opening (EVO) timing were then analysed. Four distinct scavenging periods, i.e. early backflow period (EB), backflow scavenging period (BS), main scavenging period (MS) and post backflow period (PB), 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 (DR), charging efficiency (CE) and scavenging efficiency (SE) when the post backflow is just avoided by tuning the difference between the closing timings of scavenge ports and exhaust valves (Δclose) and the overlap between the opening profiles of scavenge ports and exhaust valves (Δoverlap) for a specific ED. A longer ED can be used to further improve the scavenging performances. In addition, the difference between the opening timings of scavenge ports and exhaust valves (Δopen) can be increased to improve SE. The Δclose also shows strong positive correlation with in-cylinder swirl ratio (SR) and negative correlation with tumble ratio (TR). The results presented in this study provide the fundamental knowledge of the scavenging process in the uniflow scavenged 2-stroke engine and assist the design of scavenge ports and valve strategies to optimize in-cylinder flow motion and scavenge performances in the 2-stroke BUSDIG engine with a variable valve actuation system for exhaust valves.