<p>Two-stroke, opposed piston engines have gained recent attention for their improved thermal efficiency relative to the conventional inline or V-configuration. One advantage of two-stroke, opposed piston engines is a reduction in heat losses since there is no cylinder head. Another advantage is improved gas exchange via uniflow scavenging since the exhaust and intake ports may be located near bottom dead center of the exhaust and intake pistons, respectively. One challenge with the design of two-stroke engines is promoting turbulence within the cylinder. Turbulence is important for mixing air and fuel in the cylinder and for increasing flame speed during combustion. </p> <p>This work investigates the flow and turbulence through two-stroke, opposed piston engines using computational fluid dynamics (CFD). Specifically, the role of intake manifold and intake port geometry on turbulence within the cylinder was investigated by systematically modifying the engine geometry. Turbulence was then quantified using three metrics: circulation around the cylinder axis (swirl), circulation normal to the cylinder axis (tumble), and volume average turbulent kinetic energy (TKE) within the cylinder.</p> Increasing the swirl angle from 0 degrees to 10 degrees increased the in-cylinder swirl by a factor of 3. Increasing the swirl angle also increased the volume average TKE by a range of 7.6% to 36.5% across the three cylinders of the engine. A reverse tilt angle of 15 degrees increased tumble circulation near the piston face but decreased tumble circulation by a factor of 3 near the center of the cylinder. The next step for research on this would be to apply more geometric manipulations to the manifold of the swirl engine design to balance the mass flow rate for each port. Following the redesign of the manifold the next step is to perform a dynamic CFD test to verify the mass flow has been balanced under a dynamic scenario.

<div>A growing global population and improved standard of living in developing countries have resulted in an unprecedented increase in energy demand over the past several decades. While renewable energy sources are increasing, a huge portion of energy is still converted into useful work using heat engines. The combustion process in diesel and petrol engines releases carbon dioxide and other greenhouse gases as an unwanted side-effect of the energy conversion process. By improving the efficiency of internal combustion engines, more chemical energy stored in petroleum resources can be realized as useful work and, therefore, reduce global emissions of greenhouse gases. This research focused on improving the thermal efficiency of opposed-piston engines, which, unlike traditional reciprocating engines, do not use a cylinder head. The cylinder head is a major source of heat loss in reciprocating engines. Therefore, the opposed-piston engine has the potential to improve overall engine efficiency relative to inline or V-configuration engines.</div><div><br></div>The objective of this research project was to further improve the design of opposed-piston engines by using computational fluid dynamics (CFD) modeling to optimize the engine geometry. The CFD method investigated the effect of intake port geometry and exhaust piston lead angle on the scavenging process and in-cylinder turbulence. After the CFD data was analyzed, scavenging efficiency was found insensitive to transfer port geometry and exhaust piston lead angle with a maximum change of 0.61%. Trapping efficiency was altered exclusively by exhaust piston lead angle and changed from 18% to 26% as the lead angle was increased. The in-cylinder turbulence parameters of the engine (normalized swirl circulation, normalized tumble circulation, and normalized TKE) experienced more complex relationships. All turbulence parameters were sensitive to changing transfer port geometry and exhaust piston lead angle. Some examples of trends seen during the analysis include: an increase in normalized swirl circulation from 0.01 to 4.45 due to changes in swirl angle, a change in normalized tumble circulation from -28.52 to 21.11 as swirl angle increased, and an increase in normalized tumble circulation from 14.20 to 33.68 as exhaust piston lead angle was increased. Based on the present work, an optimum configuration was identified for a swirl angle of 15°, a tilt angle of 10°, and an exhaust piston lead angle of 20°. Future work includes expanding the numerical model’s domain to support a complete cylinder-port configuration, adding combustion products to the diffusivity equation in the UDF, and running additional test cases to describe the entire input space for the sensitivity analysis.<br>