CFD Simulation of Metal and Optical Configuration of a Heavy-Duty CI Engine Converted to SI Natural Gas. Part 1: Combustion Behavior

Internal combustion engines with optical access (a.k.a. optical engines) provide additional information in the quest for understanding the fundamental in-cylinder combustion phenomena. However, most optical engines have flat bowl-in-piston combustion chamber to optimize the visualization process, which is different, for example, from the traditional re-entrant bowl in compression ignition engines. A conventional heavy-duty direct-injection compression ignition engine was converted to spark ignition operation by replacing the fuel injector with a spark plug in both optical and metal setups to investigate the effect of the bowl geometry on flame propagation. Experimental data from steady-state lean-burn conditions was used to develop and validate a 3D CFD model of the engine. Numerical simulation results show that flame propagation in the radial direction was similar for both combustion chambers despite their different geometries. However, there were differences in the late combustion behavior. As a result, the similar flame propagation inside the optical engine suggests that such engines are best used to investigate flame inception and early flame propagation inside heavy-duty CI engines converted to natural gas spark ignition operation.