DoE-Based Numerical Optimization of Intake and Exhaust Port Geometry of a Small Opposed-Piston 2-Stroke (OP2S) Hydrogen Engine

The future potential of an opposed-piston two-stroke (OP2S) engine has attracted the attention of researchers worldwide as it offers a high thermal efficiency and power-to-weight ratio with a simple engine configuration. This engine can be used with low-carbon fuels and hydrogen to reduce greenhouse gas emissions. However, the two-stroke operation has always been limited by its low scavenging efficiency and short-circuit of fresh charge. The current work is focused on optimizing scavenging efficiency and short-circuit in a small 200 cc single-cylinder OP2S SI engine using 3-D computational fluid dynamic (CFD) simulations. The effect of four parameters, namely, area of intake ports, area of exhaust ports, and angular orientations of intake ports (swirl and tilt) on scavenging efficiency and short-circuit, has been assessed and optimized. A Latin-hypercube based Design of Experiments (DoE) methodology is used to sample the design space spanning over a range of four parameters. A response surface is generated using the Kriging method, and the geometry of intake and exhaust ports have been optimized for maximum scavenging efficiency and minimum short-circuit using a genetic algorithm on the response surface. The results show that the scavenging efficiency improves with the increase in exhaust port area, but it also increases the short circuit of fresh air. The Intake port swirl angle significantly impacts scavenging efficiency and short-circuit. The current optimization process achieved a scavenging efficiency of 85% (percentage of the total mass in the cylinder) and a short circuit of 12% (percentage of trapped fresh air). Apart from the geometric parameters, the effect of intake boost pressure and the engine speed on scavenging efficiency and short-circuit has also been evaluated.