Influences of intake ports and pent-roof structures on the flow characteristics of a poppet-valved two-stroke gasoline engine

Abstract

In order to minimize the short-circuiting of the intake charge in a poppet-valved two-stroke engine, a ''step structure'' on the intake side of the pent-roof was employed to shroud the bulk flow at the upper periphery of the intake valves at low valve lifts. Several intake-port geometries and pent-roof angles were evaluated on their charging and reversed tumble generating abilities using a steady flow test rig built in computational fluid dynamics, among which top-entry ports and relatively small pent-roof angle were selected. The scavenging processes of the two-stroke engine at high load and 3000 r/min condition with different intake valve seat heights were simulated and evaluated. The local flow patterns and composition distribution were also studied. The results show that lowering the intake valve seat height achieves higher delivery ratio and charging efficiency, but marginally better scavenging efficiency and slightly lower trapping efficiency. The scavenging process experiences a slow displacement period due to low flow momentum for the high valve seat cases and flow interaction at the exhaust exits for the low valve seat cases. Reversed tumble is the dominant flow pattern during the scavenging process. The lower the valve seat, the higher the tumble ratio in the early stage and at the end of the scavenging period. For all valve seat configurations, the least scavenged regions are located near the engine head wall and between the intake and the exhaust valves, while the regions rich in fresh charge are located along the major path of the reversed tumble. Low intake valve seat height is effective in increasing the overall in-cylinder air proportion. Further reducing it can result in higher fresh charge proportion below the exhaust valves and near the head wall.

2-Stroke Scavenging in Conventional and Minimally-Modified 4-Stroke Engines for Heavy Duty Applications at Low to Medium Speeds

“…The experimental jockey structure in Figure 2c results to α ≈ 120 • . As is schematically shown in Figure 2e and closely related to the vertical intake port design [11], fresh gas then descends down to the piston (in this phase near BDC) and U-turns toward the outlet region (loop scavenging), thereby more effectively replacing the cylinder's volume before entering the outlet. The directional nozzle, formed by the valve disk and jockey and effective in the last possible placement of the flow, can enforce a comparably intense tumbling, as shown below.…”

“…A potentially attractive concept for a qualitatively advantageous 2-stroke scavenging in an almost normal 4-stroke design is presented via a modified intake valve ( Figure 2) which, by itself, induces a directed flow instead of changing the complete cylinder head and intake geometry [8][9][10][11] or the combustion chamber [12,13]. It must be noted that a modified inlet valve for 2-stroke operation is not a new idea but was already treated by many and mostly outdated patent applications, including a very related invention from Jeong et al [17] in 1996.…”

“…It should be noted that a fixed guiding structure within the intake [8][9][10][11] (instead of a jockey) cannot perform in the same way; the annular gap of the radially re-directing valve disk would still be fully opened toward the very close outlet and premature losses would still occur [10,11], as is also simulated below.…”

“…The issue of poor-quality scavenging was gradually relieved with modified geometries for the cylinder head and the piston. These geometries, e.g., smaller inlet valves in a vertical intake port [8][9][10][11] and/or edges in the combustion chamber and piston crown [12,13] or stepped pistons [14] are, however, not very attractive in terms of (i) considerable re-engineering with relatively higher engine designs and (ii) a potentially affected combustion (due to rugged and increased inner surfaces of the combustion chamber).…”

Scavenging Ports’ Optimal Design of a Two-Stroke Small Aeroengine Based on the Benson/Bradham Model

“…Mattarelli et al reported a CFD study on a two-stroke Opposed Piston High Speed Direct Injection Diesel Engine (OPHSDI) and analyzed the influence of the offset between the crankshafts by performing 3D CFD simulations [20]. Li et al investigated the influences of intake ports and pent-roof structures on the flow characteristics of a poppet-valved two-stroke gasoline engine to minimize the short-circuiting of the intake charge [21]. He et al emphasized the significant effects of exhaust back pressure, porting timing, and intake port layout on the scavenging quality and trapped air mass in cylinder by transient CFD simulation, including blow-down and scavenging [22].…”