Natural gas (NG) is attractive for heavy-duty (HD) engines for reasons of cost stability, emissions, and fuel security. NG cannot be reliably compression-ignited, but conventional gasoline 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 challenging in large-bore engines, where completing combustion in the available time is more difficult. Using two high-speed infrared (IR) cameras with borescopic access to one cylinder of an HD NG engine, the effect of ignition system on the early flame-kernel development and cycle-to-cycle variability (CCV) was investigated. Imaging in the IR yielded strong signals from water emission lines, which located the flame front and burned-gas regions and obviated image intensifiers. A 9.7-liter, six-cylinder engine was modified to enable exhaust-gas recirculation and to provide optical access. Three ignition technologies were studied: a conventional system delivering 65 mJ of energy to each spark, a high-energy conventional system delivering 140 mJ, and a Bosch Controlled Electronic Ignition (CEI) system. CEI uses electronics to extend the ignition event, yielding sparks up to 5 ms in duration with up to 300 mJ of energy. Air/fuel equivalence ratios, λ, as high as 1.6 (with minimum EGR) and EGR fractions as high as 23% (stoichiometric) were tested; ignition delay, engine-out emissions, fuel consumption and image-derived parameters were compared. In most lean or dilute cases, the 140-mJ system yielded the lowest CCV. The imagery provided information about the early stages of ignition and combustion, where pressure measurements are not reliable. Image-based metrics also revealed that early flame kernels located further from the head yielded better combustion, showing that borescopic IR imaging can provide guidance for future engine design.
Infrared Borescopic Evaluation of High-Energy and Long-Duration Ignition Systems for Lean/Dilute Combustion in Heavy-Duty Natural-Gas Engines
Sae Technical Papers
WCX World Congress Experience ; 2018
2018-04-03
Conference paper
English
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