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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Additive Manufacturing of Energy Harvesting Material System for Active Wireless MEMS Sensors

Combustion systems make up a large sum of the world’s yearly energy production despite advances in renewable energy sources. Precise control of pressure and temperature in combustion chambers allows for lower carbon emissions, and overall higher efficiency. Wireless temperature and pressure sensors are critical for the operation of combustion chambers. Energy harvesters have emerged as a viable way to power wireless sensors when heat and mechanical energy are available. Lithium niobate (LNB), barium titanate (BTO), and lead zirconate titanate (PZT) pyroelectric ceramics were studied and fabricated through additive manufacturing (AM) for use as energy harvesting structures in combustion environments. The electric power generation potential of LNB and PZT ceramics was studied by varying the temperature from 50 to 60 °C and by introducing cyclic compression loads of 2000 N amplitude. It was found that thermal energy conversion had the highest output of 500 nW and that combined thermal and mechanical conversion did not increase the harvesting potential because of the competing contributions of both effects. Powder bed and slurry extrusion AM methods were used to fabricate BTO and PZT ceramic structures. Optimization of the powder-based binder jetting method produced BTO ceramics with a relative density of 36.77% a piezoelectric coefficient of 153 pC/N. The density of the ceramics increased up to 56% when increasing the saturation of binder in the powder to because of liquid phase sintering. Finally, PZT structures were manufactured through AM and fin features were added to the design to enhance the heat transfer along the material. The harvested power density of the flat samples was 3.643 μW/cm 3 and of the finned samples was 3.034 μW/cm3. This research paves the ways for the development of self-powered wireless sensors in critical areas of operation such as combustion.

01 COAL, LIGNITE, AND PEAT↗
DOE OSTI2023

Alignment system and associated method

An alignment system includes an optical sensor device that may be inserted into a passage of a body that is located between a fuel injector and a combustion chamber of an engine cylinder. The body is shaped to transmit fuel ejected from a spray hole in a fuel injector into the combustion chamber of the engine cylinder. The system also includes a controller that may examine output of the optical sensor device and determine whether the passage of the body is aligned with the spray hole of the fuel injector based on the output from the optical sensor device. The controller may change a position of one or more of the body or the fuel injector responsive to determining that the passage of the body is not aligned with the spray hole of the fuel injector.

Richerson, Jay↗
DOE OSTI2020

Probing the Effects of NO x and SO x Impurities on Oxy-Fuel Combustion in Supercritical CO 2 : Shock Tube Experiments and Chemical Kinetic Modeling

The direct-fired supercritical carbon dioxide cycles are one of the most promising power generation methods in terms of their efficiency and environmental friendliness. Two important challenges in implementing these cycles are the high pressure (300 bar) and high CO 2 dilution (>80%) in the combustor. The design and development of supercritical oxy-combustors for natural gas require accurate reaction kinetic models to predict the combustion outcomes. The presence of a small amount of impurities in natural gas and other feed streams to oxy-combustors makes these predictions even more complex. During oxy-combustion, trace amounts of nitrogen present in the oxidizer is converted to NO x and gets into the combustion chamber along with the recirculated CO 2 . Similarly, natural gas can contain a trace amount of ammonia and sulfurous impurities that get converted to NO x and SO x and get back into the combustion chamber with recirculated CO 2 . In this work, a reaction model is developed for predicting the effect of impurities such as NO x and SO x on supercritical methane combustion. The base mechanism used in this work is GRI Mech 3.0. H 2 S combustion chemistry is obtained from Bongartz et al. while NOx chemistry is from Konnov. The reaction model is then optimized for a pressure range of 30–300 bar using high-pressure shock tube data from the literature. It is then validated with data obtained from the literature for methane combustion, H 2 S oxidation, and NO x effects on ignition delay. The effect of impurities on CH 4 combustion up to 16 atm is validated using NO x -doped methane studies obtained from the literature. In order to validate the model for high-pressure conditions, experiments are conducted at the UCF shock tube facility using natural gas identical mixtures with N 2 O as an impurity at ~100 bar. Current results show that there is a significant change in ignition delay with the presence of impurities. A comparison is made with experimental data using the developed model and predictions are found to be in good agreement. The model developed was used to study the effect of impurities on CO formation from sCO 2 combustors. It was found that NO x helps in reducing CO formation while the presence of H 2 S results in the formation of more CO. The reaction mechanism developed herein can also be used as a base mechanism to develop reduced mechanisms for use in CFD simulations.

Energy & Fuels↗
DOE OSTI2024

Experimental Combustion and Flame Characterization of a Chemical Looping-Based Oxidative Dehydrogenation Byproduct Fuel Mixture Containing High CO2 Dilution

Abstract This study investigates the combustion performance of a CO2-rich fuel mixture containing ethane and methane as active species using a constant volume combustion chamber. This fuel is obtained as a byproduct of a chemical looping-based oxidative dehydrogenation (Cl-ODH) process ethylene production. The byproduct gas mixture has 40.79% CO2, 39.49% ethane, and 4.88% methane by weight with other minor compounds. Using this fuel for energy extraction would improve the process efficiency of the ethane to ethylene conversion. After initial combustion modeling, the gas fuel mixture was reduced to just the major species: CO2, ethane, and methane. The mixture was then tested for flammability limits and combustion performance under spark-ignition conditions. Effects of ambient conditions like temperatures between 300 and 400 K with initial pressures from 1 to 10 bar were tested. The effects of stoichiometry were tested to understand flame velocities and heat release. The fuel mixture showed an overall reduced flame velocity compared to gasoline. Instability in combustion was believed to be caused by the dissociation of ethane under elevated conditions. At higher pressures, the flame produces lower cumulative heat release. Simulations were also performed using a model tuned to replicate the operations of the combustion chamber used in the experiments. Heat release and unburnt fuel mass data were calculated to identify the discrepancies in the combustion completeness at elevated pressures. The effects of CO2 quenching the flame coupled with the increased dissociation of the fuel species can lead to up to more than 75% of the fuel mixture being unburnt. Data from this study were used to modify a small-scale spark-ignition engine to use this fuel and produce usable energy.

Energy & Fuels↗
DOE OSTI2020

Efficiency of using nitrogen trifluoride as an oxidiser in a supersonic continuous-wave chemical HF laser

We report the results of an experimental study of possible ways to increase the efficiency of using NF{sub 3} nitrogen trifluoride as an oxidiser in an atomic fluorine generator of a supersonic continuous-wave chemical HF laser with a flat nozzle block corresponding to the nozzle – nozzle reagent mixing scheme. As such ways, we consider increasing the residence time of reagents in the combustion chamber and rising temperature in the chemical reaction zone. Comparison of specific energy characteristics of an HF laser operating with the use of the studied (NF{sub 3}) and ‘basic’ (F{sub 2}) oxidisers shows that the replacement of molecular fluorine with nitrogen trifluoride during the burning with deuterium in the combustion chamber of an atomic fluorine generator with allowance for the proposed measures leads to a reduction in the specific laser energy output by no more than 23 % – 24 % at high burning completeness. (paper)

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

System and method for control of compression in internal combustion engine via compression ratio and elastic piston

The present disclosure relates to a system for controlling ignition of an air/fuel mixture intake charge directed into an internal combustion engine. The system may have a longitudinally movable inner cylinder liner configured to fit within a cylinder wall portion of an internal combustion engine, and able to receive a piston of the engine therein. A portion of the inner cylinder liner defines an internal volume forming a combustion chamber, and the internal volume controls a compression ratio of the cylinder. The system also has a cylinder head assembly operatively associated with the inner cylinder liner and able to move linearly to cause longitudinal displacement of the inner cylinder liner relative to the cylinder wall portion. This enables the volume of the combustion chamber to be further varied, to thus further vary the compression ratio.

Flowers, Daniel L.↗
DOE OSTI2022

Investigation of Cycle-to-Cycle Variations in Internal Combustion Engine Using Proper Orthogonal Decomposition

The understanding, modeling and control of the cycle-to-cycle variation (CCV) in the modern internal combustion engine (ICE) is a key scientific challenge to achieve stable engine operation. High CCV in the engine combustion chamber may contribute to partial burn, misfire and knock, which adversely affects the engine performance and may potentially damage the engine. The objective of the current study is to leverage high-fidelity numerical simulations to improve the understanding of the causes of CCV. Using the massively parallel code, Nek5000, multi-cycle, wall-resolved large-eddy simulations (LES) were performed for the General Motors (GM), Transparent Combustion Chamber (TCC-III) optical engine under motored operating conditions. Further, the large-scale structures of the in-cylinder flow were investigated using a triple proper orthogonal decomposition (POD) technique to explore the characteristics of different parts of the flow and their contributions to CCV. The kinetic energy of the subset of flow structures were determined and correlated between the intake and compression strokes. The insights from the analysis of the large-scale flow structures will be used to assist the development of improved engine designs with reduced CCV and enhance the engine performance.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2022

Temperature-Following Thermal Barrier Coatings for High Efficiency Engines (Final Report)

HRL Laboratories and General Motors (GM), pursued this effort to increase the efficiency of internal combustion engines (ICEs) by developing and implementing temperature-following thermal barrier coatings (TBCs) to decrease heat loss from the combustion chamber. HRL developed microsphere TBCs based on hollow high-temperature alloy spheres with average diameter of 50 μm and wall thickness ~1 μm (microspheres) that achieve unprecedented low thermal conductivity and heat capacity (10X lower than the state of the art) while offering exceptional environmental and mechanical resistance.> In current ICEs, approximately 29% of the fuel’s energy is lost to the cooling system and about 22% goes into moving the car. By limiting heat losses from the combustion chamber with insulating coatings, fuel energy can be redirected into additional piston work and into the exhaust stream. The additional energy that goes into the exhaust stream can be turned into useful work through the use of an exhaust energy recovery device, such as turbocompounding and/or through driving a turbocharger to increase the power density of the engine allowing downsizing. Insulating coatings for piston crowns have been pursued in the past, but previous materials—typically ceramics—exhibited low thermal conductivity but retained high heat capacity. Such materials reduce heat losses but stabilize at a high surface temperature. The high surface temperature heats the incoming gases, which lowers volumetric efficiency and increases propensity for knock, resulting in degraded engine performance. This program developed an innovative new material that combines low thermal conductivity with low heat capacity. These unique properties allow it to follow rapid changes in gas temperature during each combustion cycle. A metallic microsphere TBC has been demonstrated that exhibits increased surface temperature during the combustion period, resulting in reduced heat transfer losses, while still returning to a low surface temperature during the gas exchange period. A 10X lower thermal conductivity and heat capacity than state-of-the-art thermal barrier coatings has been demonstrated. These unprecedented thermal properties are achieved through the coating architecture, which consists of closed pores on the microscale and exhibits a total porosity of 90% to 95%. By selecting a high-temperature nickel alloy as the coating material, the ductility and strength of metals can be harnessed to achieve much better durability and damage tolerance than brittle ceramic coatings can achieve.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2023

Insert device for fuel injection

An insert device includes a body having an upper body portion configured to couple with a cylinder head of an engine cylinder and a lower body portion extending from the upper body portion toward a combustion chamber of the engine cylinder. The body includes an interior surface extending around a central volume positioned to receive liquid fuel from a fuel injector. The body includes gas inlet channels and fuel-and-gas mixture outlet channels. The gas inlet channels direct gas into the central volume where the gas mixes with the liquid fuel to form the fuel-and-gas mixture. The fuel-and-gas mixture outlet channels direct the mixture into the combustion chamber. The interior surface includes concave surface portions between the inlet channels and the outlet channels along a center axis of the body that are shaped to direct the gas into the central volume toward the liquid fuel in the central volume.

Heher, Brett↗
DOE OSTI2024

System for multiple burns from a rocket motor

A system for multiple burns from a solid fuel rocket motor may extinguish rocket fuel after the rocket has been ignited. The motor may be extinguished via rapid decompression of the combustion chamber. The fuel may then be reignited by a suitable igniter, and the process of extinguishing and reigniting may be repeated, enabling multi-burn maneuvers. A decompressive extinguishing plug nozzle may extinguish solid rocket fuel after the rocket has been ignited and/or keep a rocket in a disarmed (zero thrust) state until the rocket is to be armed. The nozzle may include a plug that mostly impedes the opening of the nozzle and an outer cowl that is movable to rapidly decompress the combustion chamber. This rapid decompression extinguishes the solid rocket fuel. In some aspects, the fuel can be reignited and extinguished multiple times.

Dallmann, Nicholas↗
DOE OSTI2020

Pyrolysis of Radioactive Spent Resins in the PRIME Installation - 20190

Montair and Belgoprocess developed the pilot PRIME (Pyrolysis Resins In Mobile Electric) Installation for treatment of spent resins in one single reactor without pre-treatment and without intermixing of other waste, allowing for easy traceability and isotopic characterization of the end product. Both low level waste (LLW, < 2 mSv/h) and medium level waste (MLW, > 2 mSv/h) can be treated in the installation. The PRIME pilot installation is designed to treat 2 batches of 100 liter of saturated ion exchange resins over a period of 24 hours. A dosing unit with screw feeder is used for the dosing of the saturated ion exchange resins to the pyrolysis reactor. This completely new developed innovative pyrolysis reactor contains a special design mechanical stirrer and can operate at a temperature of up to 600 deg. C under inert conditions. As a result, the organic molecules will decompose at an accelerated rate until only the mineralized material remains. The lower temperature and the absence of oxygen also prevent the breakdown of ion exchange resins into (powdery) ash, which is a considerable advantage during further handling. Gases that are released during drying and pyrolysis are treated in an also newly developed thermal oxidizer, which uses an innovative, electrically heated design instead of a traditional natural gas- or fuel oil secondary combustion chamber design. After passing through the thermal oxidizer, the gases are cooled down to saturation temperature and treated in the wet scrubbing system. An extraction fan keeps the system under negative pressure to prevent the possibility of any off-gas leakage out of the system. A series of tests were performed successfully with wet cationic resins, anionic resins and a mixture of both. The results of these tests are described in the presentation. For intellectual property protection, the system and process of pyrolysis of organic waste and the system of thermal oxidation of pyrolysis gases described in this paper are patent pending. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2020

Ducted Fuel Injection vs. Conventional Diesel Combustion: Extending the Load Range in an Optical Engine with a Four-Orifice Fuel Injector

Ducted fuel injection (DFI) is a technique to attenuate soot formation in compression ignition engines relative to conventional diesel combustion (CDC). The concept is to inject fuel through a small tube inside the combustion chamber to reduce equivalence ratios in the autoignition zone relative to CDC. DFI has been studied at loads as high as 8.5 bar gross indicated mean effective pressure (IMEP g ) and as low as 2.5 bar IMEP g using a four-orifice fuel injector. Across previous studies, DFI has been shown to attenuate soot emissions, increase NO x emissions (at constant charge dilution), and slightly decrease fuel conversion efficiencies for most tested points. This study expands on the previous work by testing 1.1 bar IMEPg (low-load/idle) conditions and 10 bar IMEPg (higher-load) conditions with the same four-orifice fuel injector, as well as examining potential causes of the degradations in NO x emissions and fuel conversion efficiencies. DFI and CDC are directly compared at each operating point in the study. At the low-load condition, the intake charge dilution was swept to elucidate the soot and NO x performance of DFI. The low-load range is important because it is the target of impending, more-stringent emissions regulations, and DFI is shown to be a potentially effective approach for helping to meet these regulations. The results also indicate that DFI likely has slightly decreased fuel conversion efficiencies relative to CDC. We find that the increase in NO x emissions with DFI is likely due to longer charge gas residence times at higher temperatures, which arise from shorter combustion durations and advanced combustion phasing relative to CDC.

42 ENGINEERING↗
DOE OSTI2020

Enhancing the efficiency benefit of thermal barrier coatings for homogeneous charge compression ignition engines through application of a low-k oxide

Prior experiments reported by the authors have proven the hypothesis that achieving a dynamic temperature swing on the combustion chamber surface will lead to improved thermal and combustion efficiencies of the homogeneous charge compression ignition engine. A thin layer of yttria-stabilized zirconia, roughly 150 μm, was plasma sprayed on the piston top. It led to markedly advanced ignition and heat release in the gasoline homogeneous charge compression ignition engine, accompanied with reduced unburnt hydrocarbon and carbon monoxide emissions, improved combustion efficiency, and a higher thermal efficiency. A related computational study highlighted the critical role of coating thermal conductivity in achieving a desired dynamic response; hence, the second phase of experimental investigations focused on introducing structured porosity in the yttria-stabilized zirconia coating, as a means of reducing effective conductivity. Indeed, additional incremental improvements were observed, as well as limitations related to adverse effects of the surface roughness and the fuel interactions with the surface roughness and open pores. Erosion can also be a problem in a direct injection engine. Therefore, the third round of investigations focused on a material with a natively low conductivity (low-k), sprayed on the top of an Al piston in a relatively dense form, and in a way that yields a smooth surface. The objective was to capitalize on the low conductivity, while avoiding the pitfalls accompanying high-porosity formulations. The heat-storage capacity was limited by keeping the thickness relatively low. The results verify the paramount importance of thermal conductivity in the context of high “temperature swing” behavior and indicate a potential to improve the homogeneous charge compression ignition engine’s combustion efficiency roughly 1.5%, with the overall indicated efficiency improvement on the order of 5%, on a relative basis. In addition, the low-k oxide thermal barrier applied to the piston extended significantly the low-load homogeneous charge compression ignition operability limit.

42 ENGINEERING↗
DOE OSTI2023

Bio-fuel effects and seasonal fuel property variations related to abnormal combustion occurrences observed in the field

Since 2015, the automotive industry has regularly observed sharp increases in customer complaints of poor driveability and increased warranty claims during the months of September and October. These persistent seasonal complaint increases have been tracked to occur from increased abnormal combustion events that coincide with the seasonal transition from summer to winter fuel volatility, beginning on September 15th annually. The Reid Vapor Pressure (RVP) of gasoline during this seasonal transition has been found to be the primary property attributed to this annual issue. It’s hypothesized that an increase in volatile butanes and pentanes from advanced petroleum recovery techniques and hydrofracking production has hastened the transition from summer to winter fuel volatility in the marketplace, rather than the gradual increase of the RVP over time as historically intended. At worst, mid-winter fuel, with an RVP ranging from 12 to 16 psi, depending on location in the U.S., enters the market while ambient temperatures can still be above average for the winter season. This combination of high volatility fuel with unseasonably warm weather will increase fuel system flash boiling, injector spray cavitation, and spray collapse inside the engine’s combustion chamber, ultimately linking fuel property variations to abnormal engine combustion processes. This CRADA program will investigate these known knowledge gaps by coupling with two national laboratories, Oak Ridge National Laboratory, and the National Renewable Energy Laboratory to employ both unique tools and expertise throughout the industry to provide the insight, knowledge, and data critical to understand the observed interactions. Overall findings of this work are that fuel wall retention increased SPI proportionally, and fuels with an increased distillation (i.e. energy fraction) above the wall temperature increased fuel retention. Likewise fuels with increased enthalpy of vaporization (HoV) increased fuel retention proportional to the temperature reduction from the HoV (~13K reduction in this work). Thus, fuel with increased HoV and increased fuel distillation above the linear temperature also exhibited increased SPI rates. Fuel with high RVP did exhibit increased fuel spray collapse form spray imaging measurements, but in SPI testing the high volatility of these fuels at the tested conditions did not necessarily increase SPI rates despite exhibiting increased spray collapse in injector imaging and increased spray-wall impingement measured in fired engine testing. It was also found that spray imaging showed collapse was possible at throttled conditions, but this was not relevant to high boost pressures relevant to SPI conditions, however, under transient load and or speed encountered in real-world operation fuel spray collapse could affect fuel retention and real-world SPI that were not probed in the automated high load test sequence of this work.

09 BIOMASS FUELS↗
DOE OSTI2023

Method for designing a combustion system with reduced environmentally-harmful emissions

A method for designing a combustion system which emits less of at least one environmentally-harmful emission is presented. In a describing step, an injector which introduces a fuel into a combustion chamber is described via a CFD code. In a modeling step, combustion kinetics of the fuel are modeled via a pre-processing code as the fuel mixes and reacts with an oxidizer. In a first selecting step, at least one primary scalar is derived during the modeling of the combustion kinetics. In a performing step, a table look-up is performed to obtain at least one data from a look-up database based on the primary scalar. In a second selecting step, at least one secondary scalar is selected in addition to the primary scalar(s). In a specifying step, at least one chemical pathway of formation or destruction for the secondary scalar is specified via a chemistry manager wherein the secondary scalar is representative of the environmentally-harmful emission(s) of the chemical pathway(s). In a utilizing step, the data is utilized to evaluate the chemical pathway(s) to quantify the environmentally-harmful emission(s). In an identifying step, an improvement to the combustion system is identified which reduces the environmentally-harmful emission(s).

Zambon, Andrea C.↗
DOE OSTI2024

Implementation of Surface Tension on a Reacting Flow Solver, PeleLM: Preprint

In liquid rocket engines, the fuel is supplied to the combustion chamber in the liquid state though injectors. Such fuel undergoes atomization, vaporization, and combustion processes. To design reliable and efficient injectors, it is required to understand the full processes. This research is part of an effort to develop a full atomization-vaporization-combustion solver from first principles. As an initial step to tackle the atomization process, a multiphase flow solver is under development. For the development, a library of the volume of fluid scheme for multiphase, IRL is coupled with a reacting Navier-Stokes equation solver, PeleLM. Furthermore, as the surface tension has considerable effects on spray breakup. surface tension is implemented in the momentum equation using the continuum surface force model and the improved height function technique.

height function↗
DOE OSTI2020

Ducted Fuel Injection versus Conventional Diesel Combustion: An Operating-Parameter Sensitivity Study Conducted in an Optical Engine with a Four-Orifice Fuel Injector

Ducted fuel injection (DFI) has been shown to attenuate engine-out soot emissions from diesel engines. The concept is to inject fuel through a small tube within the combustion chamber to enable lower equivalence ratios at the autoignition zone, relative to conventional diesel combustion. Previous experiments have demonstrated that DFI enables significant soot attenuation relative to conventional diesel combustion for a small set of operating conditions at relatively low engine loads. This is the first study to compare DFI to conventional diesel combustion over a wide range of operating conditions and at higher loads (up to 8.5 bar gross indicated mean effective pressure) with a four-orifice fuel injector. This study compares DFI to conventional diesel combustion through sweeps of intakeoxygen mole fraction (XO 2 ), injection duration, intake pressure, start of combustion (SOC) timing, fuel-injection pressure, and intake temperature. DFI is shown to curtail engine-out soot emissions at all tested conditions. Under certain conditions, DFI can attenuate engine-out soot by over a factor of 100. In addition to producing significantly lower engine-out soot emissions, DFI enables the engine to be operated at low-NO x conditions that are not feasible with conventional diesel combustion due to high soot emissions.

42 ENGINEERING↗
DOE OSTI2020

Verification of diesel spray ignition phenomenon in dual-fuel diesel-piloted premixed natural gas engine

Dual-fuel (DF) engines, in which premixed natural gas and air in an open-type combustion chamber is ignited by diesel-fuel pilot sprays, have been more popular for marine use than pre-chamber spark ignition (PCSI) engines because of their superior durability. However, control of ignition and combustion in DF engines is more difficult than in PCSI engines. In this context, here we focus on the ignition stability of n-heptane pilot-fuel jets injected into a compressed premixed charge of natural gas and air at low-load conditions. To aid understanding of the experimental data, chemical-kinetics simulations were carried out in a simplified engine-environment that provided insight into the chemical effects of methane (CH4) on pilot-fuel ignition. The simulations reveal that CH4 has an effect on both stages of n-heptane autoignition: the small, first-stage, cool-flame-type, low-temperature ignition (LTI) and the larger, second-stage, high-temperature ignition (HTI). As the ratio of pilot-fuel to CH4 entrained into the spray decreases, the initial oxidization of CH4 consumes the OH radicals produced by pilot-fuel decomposition during LTI, thereby inhibiting its progression to HTI. Using imaging diagnostics, the spatial and temporal progression of LTI and HTI in DF combustion are measured in a heavy-duty optical engine, and the imaging data are analyzed to understand the cause of severe fluctuations in ignition timing and combustion completeness at low-load conditions. Images of cool-flame and hydroxyl radical (OH*) chemiluminescence serve as indicators of LTI and HTI, respectively. The cycle-to-cycle and spatial variation in ignition extracted from the imaging data are used as key metrics of comparison. The imaging data indicate that the local concentration of the pilot-fuel and the richness of the surrounding natural-gas air mixture are important for LTI and HTI, but in different ways. In particular, higher injection pressures and shorter injection durations increase the mixing rate, leading to lower concentrations of pilot-fuel more quickly, which can inhibit HTI even as LTI remains relatively robust. Decreasing the injection pressure from 80 MPa to 40 MPa and increasing the injection duration from 500 µs to 760 µs maintained constant pilot-fuel mass, while promoting robust transition from LTI to HTI by effectively slowing the mixing rate. This allows enough residence time for the OH radicals, produced by the two-stage ignition chemistry of the pilot-fuel, to accelerate the transition from LTI to HTI before being consumed by CH4 oxidation. Thus from a practical perspective, for a premixed natural gas fuel–air equivalence-ratio, it is possible to improve the “stability” of the combustion process by solely manipulating the pilot-fuel injection parameters while maintaining constant mass of injected pilot-fuel. This allows for tailoring mixing trajectories to offset changes in fuel ignition chemistry, so as to promote a robust transition from LTI to HTI by changing the balance between the local concentration of the pilot-fuel and richness of the premixed natural gas and air. This could prove to be a valuable tool for combustion design to improve fuel efficiency or reduce noise or perhaps even reduce heat-transfer losses by locating early combustion away from in-cylinder walls.

42 ENGINEERING↗