N atural gas (NG) is attractive for heavy-duty (HD) engines for reasons of cost stability, emissions, and fuel security. NG cannot be reliably compressionignited, 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
O ptical 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). Highspeed 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.
A new optical diagnostic technique for burned gas temperature measurements was introduced using visible thermally-excited fluorescence of strontium monohydroxide (SrOH). The technique is a significant improvement over previously developed alkali metal-based techniques in that it requires only one tracer substance, strontium acetylacetonate in ethanol, compared to two or three alkali metal precursors. Combustion of the precursor forms 1.5 to 12 ppm SrOH, depending on equivalence ratio, and thermal excitation leads to visible light emission. For the purpose of temperature measurements, multiple emission bands were spectrally resolved and recorded with an intensified camera coupled to a spectrometer during experiments in an optical spark-ignited direct-injected engine. A hybrid experimental and computational approach was taken to validate the feasibility of SrOH based temperature measurements as well as to determine missing spectroscopic information. To that end, emission spectra were recorded for equivalence ratios ranging from lean to rich, and GT Power simulations provided a calibration base for burned gas temperatures. The optical engine was operated with early injection leading to near-homogenous mixing prior to ignition. It was confirmed that the measured data follow relationships consistent with a Boltzmann distribution of excited states populations. Furthermore, the experimentally determined energy difference between the excited [Formula: see text] and [Formula: see text] states of SrOH that form the basis of the temperature evaluation was found to be in good agreement with literature data. The calibrated analysis model then allowed to process spectra from over 400 single engine cycles from 10 runs with four different operating conditions to determine instantaneous burned gas temperatures. Average temperatures ranged from 1759 to 2490 K with standard deviations of 46–122 K. Variations were higher for more marginal fuel conditions. Self-absorption of the emission signals was characterized and would lead to a temperature error of no more than 0.5%.
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