Pre-chamber ignition has demonstrated capability to increase internal combustion engine in-cylinder burn rates and enable the use of low engine-out pollutant emission combustion strategies. In the present study, newly designed passive pre-chambers with different nozzle-hole patterns - that featured combinations of radial and axial nozzles - were experimentally investigated in an optically accessible, single-cylinder research engine. The pre-chambers analyzed had a narrow throat geometry to increase the velocity of the ejected jets. In addition to a conventional inductive spark igniter, a nanosecond spark ignition system that promotes faster early burn rates was also investigated. Time-resolved visualization of ignition and combustion processes was accomplished through high-speed hydroxyl radical (OH*) chemiluminescence imaging. Pressure was measured during the engine cycle in both the main chamber and pre-chamber to monitor respective combustion progress. Experimental heat release rates (HRR) calculated from the measured pressure profiles were used as inputs for two different GT-Power 1D simulations to evaluate the pre-chamber jet-exit momentum and penetration distance. The first simulation used both the calculated main-chamber and pre-chamber HRR, while the second used only the main chamber HRR with the pre-chamber HRR modeled. Results show discrepancies between the models mainly in the pressurization of the pre-chamber which in turn affected jet penetration rate and highlights the sensitivity of the simulation results to proper input selection. Experimental results further show increased pressurization, with an associated acceleration of jet penetration, when operating with nanosecond spark ignition systems regardless of the pre-chamber tip geometry used.
The mixed-mode engine combustion strategy where some combination of spark-assisted compression ignition (SACI) and pure advanced compression ignition (ACI) are used at part-load operation with exclusive spark-ignited (SI) combustion used for high power-density conditions has the potential to increase efficiency and decrease pollutant emissions. However, controlling combustion and switching between different modes of mixed-mode operation is inherently challenging. This chapter proposes to use ozone (O3)—a powerful oxidizing chemical agent—to maintain stable and knock-free combustion across the load-speed map. The impact of 0–50 ppm intake seeded O3 on performance, and emissions characteristics was explored in a single-cylinder, optically accessible, research engine operated under lean SACI conditions with two different in-cylinder conditions, (1) partially stratified (double injection—early and late injection) and (2) homogeneous (single early injection). O3 addition promotes end gas auto-ignition by enhancing the gasoline reactivity, which thereby enabled stable auto-ignition with less initial charge heating. Hence O3 addition could stabilize engine combustion relative to similar conditions without O3. The addition of ozone has been found to reduce specific fuel consumption by up to 9%, with an overall improvement in the combustion stability compared to similar conditions without O3. For the lowest loads, the effect of adding O3 was most substantial. Specific NOx emissions also dropped by up to 30% because a higher fraction of the fuel burned was due to auto-ignition of the end gas. Measurement of in-cylinder O3 concentrations using UV light absorption technique showed that rapid decomposition of O3 into molecular (O2) and atomic oxygen (O) concurred with the onset of low-temperature heat release (LTHR). The newly formed O from O3 decomposition initiated fuel hydrogen abstraction reactions responsible for early onset of LTHR. At the beginning of high-temperature heat release (HTHR), end gas temperatures ranged from 840 to 900 K, which is about 200 K cooler than those found in previous studies where intake charge heating or extensive retained residuals were used to preheat the charge. An included analysis indicates that in order to achieve optimal auto-ignition in our engine, the spark deflagration was needed to add 10–40 J of additional thermal energy to the end gas. We have leveraged these results to broaden our understanding of O3 addition to different load-speed conditions that we believe can facilitate multiple modes (SI, ACI, SACI, etc.) of combustion.
In-cylinder reforming of injected fuel during a negative valve overlap (NVO) recompression period can be used to optimize main-cycle combustion phasing for low-load low-temperature gasoline combustion (LTGC). The objective of this work is to examine the effects of reformate composition on main-cycle engine performance. An alternate-fire sequence was used to generate a common exhaust temperature and composition boundary condition for a cycle-of-interest, with performance metrics measured for these custom cycles. NVO reformate was also separately collected using a dump-valve apparatus and characterized by both gas chromatography (GC) and photoionization mass spectroscopy (PIMS). To facilitate gas sample analysis, sampling experiments were conducted using a five-component gasoline surrogate (iso-octane, n-heptane, ethanol, 1-hexene, and toluene) that matched the molecular composition, 50% boiling point, and ignition characteristics of the research gasoline. For the gasoline, it was found that an advance of the NVO start-of-injection (SOI) led to a corresponding advance in main-period combustion phasing as the combination of longer residence times and lower amounts of liquid spray piston impingement led to a greater degree of fuel decomposition. The effect was more pronounced as the fraction of total fuel injected in the NVO period increased. Main-period combustion phasing was also found to advance as the main-period fueling decreased. Slower kinetics for leaner mixtures were offset by a combination of increased bulk-gas temperature from higher charge specific heat ratios and increased fuel reactivity due to higher charge reformate fractions.
Advanced automotive gasoline engines that leverage a combination of reduced heat transfer, throttling, and mechanical losses; shorter combustion durations; and higher compression and mixture specific heat ratios are needed to meet aggressive DOE VTP fuel economy and pollutant emission targets. Central challenges include poor combustion stability at low-power conditions when large amounts of charge dilution are introduced and high sensitivity of conventional inductive coil ignition systems to elevated charge motion and density for boosted high-load operation. For conventional spark ignited operation, novel low-temperature plasma (LTP) or pre-chamber based ignition systems can improve dilution tolerances while maintaining good performance characteristics at elevated charge densities. Moreover, these igniters can improve the control of advanced compression ignition (ACI) strategies for gasoline at low to moderate loads. The overarching research objective of the Gasoline Combustion Fundamentals project is to investigate phenomenological aspects related to enhanced ignition. The objective is accomplished through targeted experiments performed in a single-cylinder optically accessible research engine or an in-house developed optically accessible spark calorimeter (OASC). In situ optical diagnostics and ex situ gas sampling measurements are performed to elucidate important details of ignition and combustion processes. Measurements are further used to develop and validate complementary high-fidelity ignition simulations. The primary project audience is automotive manufacturers, Tier 1 suppliers, and technology startups—close cooperation has resulted in the development and execution of project objectives that address crucial mid- to long-range research challenges.
In-cylinder reforming of injected fuel during an auxiliary negative valve overlap (NVO) period can be used to optimize main-cycle auto-ignition phasing for low-load Low-Temperature Gasoline Combustion (LTGC), where highly dilute mixtures can lead to poor combustion stability. When mixed with fresh intake charge and fuel, these reformate streams can alter overall charge reactivity characteristics. The central issue remains large parasitic heat losses from the retention and compression of hot exhaust gases along with modest pumping losses that result from mixing hot NVO-period gases with the cooler intake charge. Accurate determination of total cycle energy utilization is complicated by the fact that NVO-period retained fuel energy is consumed during the subsequent main combustion period. For the present study, a full-cycle energy analysis was performed for a single-cylinder research engine undergoing LTGC with varying NVO auxiliary fueling rates and injection timing. A custom alternate-fire sequence with 9 pre-conditioning cycles was used to generate a common exhaust temperature and composition boundary condition for a cycle-of-interest, with performance metrics recorded for each custom cycle. The NVO-period reformate stream and main-period exhaust stream of the cycles-of-interest were separately collected, with sample analysis by gas chromatography used to identify the retained and exhausted fuel energy in the respective periods. To facilitate gas sample analysis, experiments were performed using a 5-component gasoline surrogate (iso-octane, n-heptane, ethanol, 1-hexene, and toluene) that matched the molecular composition, 50% boiling point, and ignition characteristics of a research gasoline. The highest total cycle thermodynamic efficiencies occurred when auxiliary injection timings were early enough to allow sufficient residence time for slow reforming reactions to take place, but late enough to prevent significant fuel spray crevice quench. Increasing the fraction of total fuel energy injected into the NVO-period was also found to increase total cycle thermal efficiencies, in part due to a modest reduction in NVO-period heat loss from a combination of fuel-spray charge cooling and endothermic fuel decomposition by pyrolysis. The effect was most pronounced at the lowest loads where larger charge mass reformate fractions increased overall specific heat ratios and main-period combustion phasing advanced closer to top dead center. These effects improved both expansion efficiency and combustion stability.
Pulsed nanosecond discharges (PND) can achieve ignition in internal combustion engines through enhanced reaction kinetics as a result of elevated electron energies without the associated increases in translational gas temperature that cause electrode erosion. Atomic oxygen (O), including its electronically excited states, is thought to be a key species in promoting low-temperature ignition. In this paper, high-voltage (17-24 kV peak) PND are examined in oxygen/nitrogen/carbon dioxide/water mixtures at engine-relevant densities (up to 9.1 kg/m3) through pressure-rise calorimetry and direct imaging of excited-state O-atom and molecular nitrogen (N2) in an optically accessible spark calorimeter, with the anode/cathode gap distance set to 5 mm or with an anode-only configuration (DC corona). The conversion efficiency of pulse electrical energy into thermal energy was measured for PND with secondary streamer breakdown (SSB) and similar low-temperature plasmas (LTP) without. The calorimetry measurements confirm that, similar to inductive spark discharges, SSB discharges promote ignition by increasing the local gas temperature. LTP discharges, on the other hand, had very little local gas heating, with electrical-to-thermal energy conversion efficiencies of ~1% at 9 bar. Instead, LTP discharges were found to generate substantial electronically-excited O-atom populations at lower pressures, but the observed image intensity decreased rapidly as the initial pressure was increased. The observed O-atom emission peaked ~20 ns after the start of the pulse and was concentrated near the anode and cathode tips, indicating that the presence of the cathode was beneficial for increasing radical production (although the likelihood of SSB increased). Decreasing oxygen and increasing carbon dioxide concentrations were found to reduce the observed image intensity, but had minimal impact on SSB probability and electrical-to-thermal conversion efficiency. The impact of changes in collisional quenching and the electron energy distribution on image intensity were evaluated.
High-voltage (20 kV peak), single-pulse, nanosecond, low-temperature plasma discharges were examined in nitrogen-diluted desiccated air (15.9% oxygen) with addition of 1%, 2%, and 3% carbon dioxide or water for a pin-to-pin electrode configuration in an optically accessible spark calorimeter at elevated density (2.9 kg/m3). Discharge behavior was characterized through pressure-rise calorimetry, direct imaging of excited-state atomic oxygen, and high-speed schlieren. Increasing carbon dioxide or water concentration led to an increased likelihood of surface discharges rather than the desired streamer discharge between the pin electrodes. For streamer discharges, carbon dioxide addition decreased the electrical-to-thermal conversion efficiency, while minimal impact was observed for water. Both carbon dioxide and water addition led to faster pressure rise rates. Carbon dioxide addition decreased excited state atomic oxygen signal, while water addition led to negligible changes. Finally, increased streamer branching was observed in the schlieren images when carbon dioxide or water was added to the gas mixture.
A computational fluid dynamics (CFD) analysis of a natural gas vehicle experie ncing a mechanical failure of a pressure relief device on a full CNG cylinder was completed to determine the resulting amount and location of flammable gas. The resulting overpressure if it were to ignite was also calculated. This study completes what is d iscussed in Ekoto et al. [1] which covers other related leak scenarios. We are not determining whether or not this is a credible release, rather just showing the result of a possible worst case scenario. The Sandia National Laboratories computational tool Netflow was used to calculate the leak velocity and temperature. The in - house CFD code Fuego was used to determine the flow of the leak into the maintenance garage. A maximum flammable mass of 35 kg collected along the roof of the garage. This would result in an overpressure that could do considerable damage if it were to ignite at the time of this maximum volume. It is up to the code committees to decide whet her this would be a credible leak, but if it were, there should be preventions to keep the flammable mass from igniting. Keywords: Natural Gas Vehicle Maintenance Facility, Pressure Relief Device Failure, CFD
Negative Valve Overlap (NVO) is a potential control strategy for enabling Low-Temperature Gasoline Combustion (LTGC) at low loads. While the thermal effects of NVO fueling on main combustion are well-understood, the chemical effects of NVO in-cylinder fuel reforming have not been extensively studied. The objective of this work is to examine the effects of fuel molecular structure on NVO fuel reforming using gas sampling and detailed speciation by gas chromatography. Engine gas samples were collected from a single-cylinder research engine at the end of the NVO period using a custom dump-valve apparatus. Six fuel components were studied at two injection timings: (1) iso-octane, (2) n-heptane, (3) ethanol, (4) 1-hexene, (5) cyclohexane, and (6) toluene. All fuel components were studied neat except for toluene - toluene was blended with 18.9% nheptane by liquid volume to increase the fuel reactivity. Additionally, a gasoline surrogate matching the broad molecular composition of RD587 gasoline was formulated using the chosen fuel palette and tested. The energy content of the injected fuel mass was kept constant for the sampled NVO cycle and the excess oxygen was relatively low (2.4%) compared to previous studies by the authors. The later injection timing studied resulted in useable recovered fuel energy near 70% and improved reformate yield of hydrogen and C1-C4 hydrocarbons compared to the earlier injection timing for all fuels except toluene/n-heptane. Analysis of the RD587 surrogate reformate compared to the individual component reformates suggests that fuel component interactions depend on injection timing, potentially through the in-cylinder equivalence ratio distribution.
For next-generation engines that operate using low-temperature gasoline combustion (LTGC) modes, a major issue remains poor combustion stability at low-loads. Negative valve overlap (NVO) enables enhanced main combustion control through modified valve timings to retain combustion residuals along with a small fuel injection that partially reacts during the recompression. While the thermal effects of NVO fueling on main combustion are well understood, the chemical effects of NVO reactions are less certain, especially oxygen-deficient reactions where fuel pyrolysis dominates. To better understand NVO period chemistry details, comprehensive speciation of engine samples collected at the end of the NVO cycle was performed by photoionization mass spectroscopy (PIMS) using synchrotron generated vacuum-ultraviolet light. Two operating conditions were explored: 1) a fuel lean condition with a short NVO fuel injection and a relatively high amount of excess oxygen in the NVO cycle (7%), and 2) a fuel-rich condition with a longer NVO fuel injection and low amount of NVO-cycle excess oxygen (4%). Samples were collected by a custom dump-valve apparatus from a direct injection, single-cylinder, automotive research engine operating under low-load LTGC and fueled by either isooctane or an 88-octane research certification gasoline. Samples were stored in heated stainless steel cylinders and transported to the Lawrence Berkeley National Laboratory Advanced Light Source for analysis using a Sandia National Laboratories flame sampling apparatus. For all isooctane fueled conditions, NVO cycle sample speciation from the PIMS measurements agreed well with previously reported GC sample measurements if the sum total of all isomer constituents from the PIMS measurements were considered. PIMS data, however, provides richer speciation information that is useful for validation of computational modeling approaches. The PIMS data also revealed that certain species for the GC diagnostic were either misidentified during the calibration process or not identified at all. Examples of unidentified species include several classes of oxygenates (e.g., ketenes, aldehydes, and simple alcohols) and simple aromatics (e.g., benzene and toluene). For the gasoline fueled NVO cycles, performance characteristics were well matched to corresponding isooctane fueled NVO cycles. However, significant PIMS cross-talk from a wide range of gasoline components restricted the sampling analysis to a handful of species. Nonetheless, it was confirmed that for fuel-lean NVO operation there was a comparable increase in acetylene with NVO injection timing retard that is attributed to the prevalence of locally-rich, piston-surface pool fires caused by fuel spray impingement.
Most experimental investigations of underexpanded hydrogen jets have been limited to circular nozzles in an attempt to better understand the fundamental jet-exit flow physics and model this behaviour with pseudo source models. However, realistic compressed storage leak exit geometries are not always expected to be circular. In the present study, jet dispersion characteristics from rectangular slot nozzles with aspect ratios from 2 to 8 were investigated and compared with an equivalent circular nozzle. Schlieren imaging was used to observe the jet-exit shock structure while quantitative Planar Laser Rayleigh Scattering was used to measure downstream dispersion characteristics. These results provide physical insight and much needed model validation data for model development.
Radiative heat fluxes from small to medium-scale hydrogen jet flames (<10 m) compare favorably to theoretical predictions provided the product species thermal emittance and optical flame thickness are corrected for. However, recent heat flux measurements from two large-scale horizontally orientated hydrogen flames (17.4 and 45.9 m respectively) revealed that current methods underpredicted the flame radiant fraction by 40% or more. Newly developed weighted source flame radiation models have demonstrated substantial improvement in the heat flux predictions, particularly in the near-field, and allow for a sensible way to correct potential ground surface reflective irradiance. These updated methods are still constrained by the fact that the flame is assumed to have a linear trajectory despite buoyancy effects that can result in significant flame deformation. The current paper discusses a method to predict flame centerline trajectories via a one-dimensional flame integral model, which enables optimized placement of source emitters for weighted multi-source heat flux prediction methods. Flame shape prediction from choked releases was evaluated against flame envelope imaging and found to depend heavily on the notional nozzle model formulation used to compute the density weighted effective nozzle diameter. Nonetheless, substantial improvement in the prediction of downstream radiative heat flux values occurred when emitter placement was corrected by the flame integral model, regardless of the notional nozzle model formulation used.
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