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

Abstract

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

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

“…One approach to reliably ignite leaner and more EGR-dilute mixtures is to employ an advanced ignition system, which is the goal of the project of which this work is part. The first results of the impact of igniter choice on the combustion of lean/dilute natural-gas mixtures resulting from the project are described elsewhere [13].…”

“…CCV is important to understand, because it can lead to engine damage and the variations in engine output torque that result from it affect drivability [10,11,12,8]. In a companion paper, we apply the image-based diagnostics to study the effect of different ignition systems on flame development and CCV in this engine [13].…”

Thermal Energy and Luminosity Characterization of an Advanced Ignition System Using a Non-Intrusive Methodology in an Optically Accessible Calorimeter

“…Predominantly, these analyses are performed exploiting high-speed cameras, whose output frames permit not only luminosity emission to be assessed but also 2D-spatial information to be maintained [24,25]. Another technique to collect discharge or flame luminosity consists of placing an optical sensor-a photodetector-inside the combustion chamber, to provide luminous emission magnitude of certain area but losing the spatial information [26,27].…”

Flammability and Explosion Characteristics of Methane in Oxygen-Reduced Air and Its Application in Air Injection IOR Process

“…In addition, a method of using over-oxygen air to improve the overall performance is adopted in diesel engines, which is not designed according to the gas volume of complete combustion; thus, the exhaust gas has a high oxygen content. As for the ignition in NG engines, forced ignition is required, which is especially difficult to be ignited in large bore engines, and good flammability is necessary for applications . Natural gas engines utilize spark ignition (SI) for premixed fuel–air mixtures.…”

“…As for the ignition in NG engines, forced ignition is required, which is especially difficult to be ignited in large bore engines, and good flammability is necessary for applications. 24 Natural gas engines utilize spark ignition (SI) for premixed fuel−air mixtures. Furthermore, heat rejection in an SI engine is 30% higher than that of a diesel engine, which accelerates the accumulation of high temperatures and the occurrence of explosion accidents.…”