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 four 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). Four ignition technologies were studied: a conventional system delivering 65 mJ of energy to each spark, a high-energy conventional system delivering 140 mJ, a Bosch Controlled Electronic Ignition (CEI) system, which uses electronics to extend the duration and the energy of the ignition event, and a high-frequency corona system (BorgWarner EcoFlash). 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.
The high-speed infrared borescopic imaging diagnostic developed here is shown to be an excellent method to accurately identify small flame kernels without the need of image intensifiers, comparable to intensified OH* imaging but with reduced experimental complexity. The results acquired from the production engine under varying air/fuel equivalence ratios and EGR rates uniquely demonstrate that stretched and wrinkled early flame kernels have a great advantage over spherical flames to complete combustion faster, and unlike conventional igniters, corona ignition system produces such flame kernels repeatably without heavy reliance on the flow and compositional conditions of the mixture.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149912/1/mazaci_1.pd
