Infrared Borescopic Characterization of Ignition and Combustion Variability in Heavy-Duty Natural-Gas Engines

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

Infrared Borescopic Evaluation of High-Energy and Long-Duration Ignition Systems for Lean/Dilute Combustion in Heavy-Duty Natural-Gas Engines

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.The information, data, or work presented herein was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DE-EE0007307. We also thank Dr. Hao Chen and Angela Wu for their help with software and James Elkins for engine-head modifications.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143838/1/2018-01-1149.pd

Time-resolved infrared imaging and spectroscopy for engine diagnostics

Molecular emissions in the infrared spectral region can provide access to a range of quantities that are of interest in internal- combustion engine research and development. Molecules; such as water, carbon dioxide, carbon monoxide, and hydrocarbons; provide the strongest signals in the range from 1.0 to 5.5 μm. We describe several imaging experiments that employed high-frame-rate infrared cameras to capture spectrally resolved and spectrally integrated signals from both optical and production engines. Spectrally resolved infrared emissions that were recorded at kHz rates (i.e., crank-angle steps) in an optically accessible, propane-fueled, single-cylinder engine are used to guide the development and validation of a radiative-emission model that is integrated into large-eddy simulations (not discussed in this paper). The emissions were dispersed with a spectrometer, and the spectra were recorded with an InSb camera. Clear spectral signatures from water and carbon dioxide were recorded, and the spectra reveal the evolution of combustion through each of 100 consecutive cycles for each engine run. Furthermore, at any wavelength of these spectra, cycle-to-cycle variation can be extracted readily. Cycle- to-cycle variation was of particular interest in a study of a production heavy-duty engine fueled by natural gas.The addition of two borescopes outfitted with high-frame-rate In- GaAs cameras enabled spectrally integrated measurements from 1.0-1.7 μm. The images allow cycle-resolved observations of ignition and flame growth. The intent of this work was to identify and quantify the impact of a range of ignition systems on lean and/or dilute operation limits from a combustion development and stability point of view.The information, data, or work presented herein was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Numbers DE-EE0007278 and DE-EE0007307. The University of Michigan provided Mr. Henrion with partial tuition and stipend support through the Rackham Merit Fellowship. Electro Optical Industries provided the integrating sphere and the blackbody source on loan. Mohammad Alzuabi supported experiments with the optical engine, while Justin Kern supported experiments with the production engine.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/149140/1/Sick_AVL_2018.pd

Infrared Borescopic Analysis of Ignition and Combustion Variability in a Heavy-Duty Natural-Gas Engine

Optical imaging diagnostics of combustion are most often performed in the visible spectral band, in part because camera technology is most mature in this region, but operating in the infrared (IR) provides a number of benefits. These benefits include access to emission lines of relevant chemical species (e.g. water, carbon dioxide, and carbon monoxide) and obviation of image intensifiers (avoiding reduced spatial resolution and increased cost). High-speed IR in-cylinder imaging and image processing were used to investigate the relationships between infrared images, quantitative image-derived metrics (e.g. location of the flame centroid), and measurements made with in-cylinder pressure transducers (e.g. coefficient of variation of mean effective pressure). A 9.7-liter, inline-six, natural-gas-fueled engine was modified to enable exhaust-gas recirculation (EGR) and provide borescopic optical access to one cylinder for two high-speed infrared cameras. A high-energy inductively coupled ignition system delivered 140 mJ of energy during each spark event. The engine was operated at 1000 rev/min and an indicated mean effective pressure of 6.8 bar over a range of air/fuel equivalence ratios, λ, (1 to 1.6) and EGR rates (2% to 23%). Strong emission lines of water are present in the sensitivity band of the cameras (1.0 to 1.7 μm) and can be used as a proxy for the flame front and burned-gas regions. Images were recorded every 5.5 degrees of crank angle (CAD); multiple measurements were interleaved to provide statistical information every 0.5 CAD. The greater cyclic variation resulting from lean/dilute operation is apparent in the images; the image-derived metrics measured early in the cycle correlate strongly with pressure-derived metrics measured later. Centroids calculated from the images show that flames farther from the head and spark plug yield better combustion, which is not evident in the pressure data.The information, data, or work presented herein was funded in part by the Office of Energy Efficiency and Renewable Energy (EERE), U.S. Department of Energy, under Award Number DE-EE0007307. The authors also thank Dr. Hao Chen and Angela Wu for their help with software, Dr. David L. Reuss for discussions of combustion and diagnostics, and James Elkins for engine-head modifications.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/143839/1/2018-01-0632.pd