CFD Simulation of Metal and Optical Configuration of a Heavy-Duty CI Engine Converted to SI Natural Gas. Part 2: In-Cylinder Flow and Emissions 2019-01-0003

Internal combustion diesel engines with optical access (a.k.a. optical engines) increase the fundamental understanding of combustion phenomena. However, optical access requirements result in most optical engines having a different in-cylinder geometry compared with the conventional diesel engine, such as a flat bowl-in-piston combustion chamber. This study investigated the effect of the bowl geometry on the flow motion and emissions inside a conventional heavy-duty direct-injection diesel engine that can operate in both metal and optical-access configurations. This engine was converted to natural-gas spark-ignition operation by replacing the fuel injector with a spark plug and adding a low-pressure gas injector in the intake manifold for fuel delivery, then operated at steady-state lean-burn conditions. A 3D CFD model based on the experimental data predicted that the different bowl geometry did not significantly affect in-cylinder emissions distribution. In addition, while in-cylinder flow motion was similar for both engine configurations, the different combustion chamber geometry affected the combustion-induced flow motion. Similar turbulence-generating mechanisms for engines with or without optical access show promise for optical investigations of cold-flow turbulence measurements representative of heavy-duty diesel engines converted to natural-gas spark-ignition operation.