Analysis of the Effect of the Bore/Stroke (B/S) Ratio and Scavenge Port Angles on the Scavenging Process in a 2-Stroke Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) Engine

Abstract

In this study, a 2-stroke Boosted Uniflow Scavenged Direct Injection Gasoline (BUSDIG) engine was proposed and researched to achieve aggressive engine downsizing and downspeeding. Compared to the loop or cross scavenged 2-stroke engines, the BUSDIG engine can achieve excellent scavenging performance and be operated with higher boost pressure as well as the absence of air and fuel short-circuiting. As a fundamental engine geometric parameter, the bore/stroke (B/S) ratio would directly affect the scavenging process in the uniflow scavenged 2-stroke engine. In this paper, the three dimensional (3D) computational fluid dynamics (CFD) simulations were applied to investigate the scavenging process in the BUSDIG engine with different bore/stroke (B/S) ratios. Four bore and stroke (B/S) ratios of 0.66, 0.8, 1 and 1.3 were analyzed. The results indicate that a bigger B/S ratio leads to deteriorated swirl flow motion but better delivery ratio (DR), scavenging efficiency (SE) and charging efficiency (CE). In order to fulfil the potential of the BUSDIG engine with different B/S ratios, two key scavenge port angles, i.e. Axis Inclination Angle (AIA) and Swirl Orientation Angle (SOA), were varied from the baseline design (AIA=90⁰, SOA=20⁰) to study their effects on the scavenging process for each B/S ratio design. Overall, a larger AIA leads to lower swirl ratio (SR) but achieves better scavenge performance, which is much crucial for a large B/S ratio design. A small SOA design leads to noticeably lower SR but superior scavenging performances for a small B/S ratio design. An intermediate SOA, e.g. 10 and 20⁰, is preferred to improve the scavenging for a large B/S ratio design.