Abstract
Natural gas (NG) is attractive for heavy-duty (HD) engines for reasons of cost stability, emissions, and fuel security. NG requires forced ignition, but conventional gasoline-engine ignition systems are not optimized for NG and are challenged to ignite mixtures that are lean or diluted with exhaust-gas recirculation (EGR). NG ignition is particularly difficult in large-bore engines, where it is more challenging to complete combustion in the time available. High-speed infrared (IR) in-cylinder imaging and image-derived quantitative metrics were used to compare two ignition systems in terms of the early flame-kernel development and cycle-to-cycle variability (CCV) in a heavy-duty, natural-gas-fueled engine that had been modified to enable exhaust-gas recirculation and to provide optical access via borescopes. Imaging in the near IR and short-wavelength IR yielded strong signals from the water emission lines, which acted as a proxy for flame front and burned-gas regions while obviating image intensification (which can reduce spatial resolution). The ignition systems studied were a conventional system and a high-frequency corona system. The air/fuel mixtures investigated included stoichiometric without dilution and lean with EGR. The corona system produced five separate elongated, irregularly shaped, nonequilibrium-plasma streamers, leading to immediate formation of five spatially distinct wrinkled flame kernels around each streamer. Compared to the conventional spark ignition, which produces a single flame kernel that exhibits an initial laminar growth regime before wrinkling, corona ignition's early achievement of higher flame surface areas significantly shortened the ignition delay, resulting in reduced overall combustion duration and CCV for each mixture. Additionally, although the lean, dilute mixture produced higher CCV than the stoichiometric, minimally diluted mixture with both igniters, the mixtures ignited by the corona system suffered less than those ignited by the conventional system. Image-based measurements of CCV agreed with those based on in-cylinder pressure.
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