Effects of diesel fuel combustion-modifier additives on In-cylinder soot formation in a heavy-duty Dl diesel engine.

Based on a phenomenological model of diesel combustion and pollutant-formation processes, a number of fuel additives that could potentially reduce in-cylinder soot formation by altering combustion chemistry have been identified. These fuel additives, or ''combustion modifiers'', included ethanol and ethylene glycol dimethyl ether, polyethylene glycol dinitrate (a cetane improver), succinimide (a dispersant), as well as nitromethane and another nitro-compound mixture. To better understand the chemical and physical mechanisms by which these combustion modifiers may affect soot formation in diesel engines, in-cylinder soot and diffusion flame lift-off were measured, using an optically-accessible, heavy-duty, direct-injection diesel engine. A line-of-sight laser extinction diagnostic was employed to measure the relative soot concentration within the diesel jets (''jetsoot'') as well as the rates of deposition of soot on the piston bowl-rim (''wall-soot''). An OH chemiluminescence imaging technique was utilized to measure the lift-off lengths of the diesel diffusion flames so that fresh oxygen entrainment rates could be compared among the fuels. Measurements were obtained at two operating conditions, using blends of a base commercial diesel fuel with various combinations of the fuel additives. The ethanol additive, at 10% by mass, reduced jet-soot by up to 15%, and reduced wall-soot by 30-40%. The other fuel additives also affected in-cylinder soot, but unlike the ethanol blends, changes in in-cylinder soot could be attributed solely to differences in the ignition delay. No statistically-significant differences in the diesel flame lift-off lengths were observed among any of the fuel additive formulations at the operating conditions examined in this study. Accordingly, the observed differences in in-cylinder soot among the fuel formulations cannot be attributed to differences in fresh oxygen entrainment upstream of the soot-formation zones after ignition

Spatio-Temporal Progression of Two-Stage Autoignition for Diesel Sprays in a Low-Reactivity Ambient: n-Heptane Pilot-Ignited Premixed Natural Gas

[EN] The spatial and temporal locations of autoignition depend on fuel chemistry and the temperature, pressure, and mixing trajectories in the fuel jets. Dual-fuel systems can provide insight into fuel-chemistry aspects through variation of the proportions of fuels with different reactivities, and engine operating condition variations can provide information on physical effects. In this context, the spatial and temporal progression of two-stage autoignition of a diesel-fuel surrogate, n-heptane, in a lean-premixed charge of synthetic natural gas (NG) and air is imaged in an optically accessible heavy-duty diesel engine. The lean-premixed charge of NG is prepared by fumigation upstream of the engine intake manifold. Optical diagnostics include: infrared (IR) imaging for quantifying both the in-cylinder NG concentration and the pilot-jet penetration rate and spreading angle, high-speed cool-flame chemiluminescence imaging as an indicator of low-temperature heat release (LTHR), and high-speed OH* chemiluminescence imaging as an indicator high-temperature heat release (HTHR). To aid interpretation of the experimental observations, zero-dimensional chemical kinetics simulations provide further understanding of the underlying interplay between the physical and chemical processes of mixing (pilot fuel-jet entrainment) and autoignition (two-stage ignition chemistry). Increasing the premixed NG concentration prolongs the ignition delay of the pilot fuel and increases the combustion duration. Due to the relatively short pilot-fuel injections utilized, the transient increase in entrainment near the end of injection (entrainment wave) plays an important role in mixing. To achieve desired combustion characteristics, i.e., ignition and combustion timing (e.g., for combustion phasing) and location (e.g., for reducing wall heat-transfer or tailoring charge stratification), injection parameters can be suitably selected to yield the necessary mixing trajectories that potentially help offset changes in fuel ignition chemistry, which could be a valuable tool for combustion design.This research was sponsored by the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE). Optical engine experiments were conducted at the Combustion Research Facility of Sandia National Laboratories in Livermore, CA. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration (NNSA) under contract DE-NA0003525. We gratefully acknowledge the contributions of Keith Penney and Dave Cicone for their assistance in developing research tools and maintaining the optical engine.Rajasegar, R.; Niki, Y.; García-Oliver, JM.; Li, Z.; Musculus, M. (2021). Spatio-Temporal Progression of Two-Stage Autoignition for Diesel Sprays in a Low-Reactivity Ambient: n-Heptane Pilot-Ignited Premixed Natural Gas. SAE International. 1-16. https://doi.org/10.4271/2021-01-052511

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

[EN] 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, this study focuses 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 mu s to 760 mu s 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.This research was sponsored in part by the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE). Optical engine experiments were conducted at the Combustion Research Facility of Sandia National Laboratories in Livermore, CA. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration (NNSA) under contract DE-NA0003525.Niki, Y.; Rajasegar, R.; Li, Z.; Musculus, MP.; García-Oliver, JM.; Takasaki, K. (2022). Verification of diesel spray ignition phenomenon in dual-fuel diesel-piloted premixed natural gas engine. International Journal of Engine Research. 23(2):180-197. https://doi.org/10.1177/146808742098306018019723

An improved entrainment rate measurement method for transient jets from 10 KHZ particle image velocimetry.

International audienceOne strategy to reduce soot formation in compression–ignition engines is extending the ignition delay to provide more time for mixing. However, vapor–fuel concentration measurements have shown that near-injector mixtures become too lean to achieve complete combustion, leading to a relative increase in unburned hydrocarbon emissions. One potential contributor to over-leaning is an "entrainment wave," which is a transient increase in local entrainment after the end of injection. Although an entrainment wave can be predicted by a one-dimensional (1D) free-jet model, no previous measurements at diesel injection conditions have demonstrated conclusively its existence, nor has its magnitude been verified. Using particle image velocimetry (PIV) in the ambient gases, we measure entrained gas velocity through a diesel jet boundary before, during, and after the injection. The entrainment calculation depends on the definition of the jet boundary, here newly proposed based on the minimum of the radial coordinate and the radial velocity (rνr). Unlike previous formulations, the method is robust even in the presence of axial flow gradients in the ambient gases. Prior to the end of injection, the measured entrainment rates that agree well with non-reacting steady gas-jet behavior, as well as with the 1D free-jet model. After end of injection, the local entrainment rate temporarily increases by a factor of 2, which is similar to the factor 2.5 increase predicted by the 1D model. However, the entrainment wave is more broadly distributed in the experimental data, likely due to confinement and/or other real-jet processes absent in the 1D model

Mechanisms of Post-Injection Soot-Reduction Revealed by Visible and Diffused Back-Illumination Soot Extinction Imaging

Small closely-coupled post injections of fuel in diesel engines are known to reduce engine-out soot emissions, but the relative roles of various underlying in-cylinder mechanisms have not been established. Furthermore, the efficacy of soot reduction is not universal, and depends in unclear ways on operating conditions and injection schedule, among other factors. Consequently, designing engine hardware and operating strategies to fully realize the potential of post-injections is limited by this lack of understanding. Following previous work, several different post-injection schedules are investigated using a single-cylinder 2.34 L heavy-duty optical engine equipped with a Delphi DFI 1.5 light-duty injector. In this configuration, adding a closely-coupled post injection with sufficiently short injection duration can increase the load without increasing soot emissions. With increasing post-injection duration, the plateau in soot emissions eventually turns upward until the post-injection increases engine-out soot above that for a single-injection strategy at the same load and main injection timing. To gain more insight into in-cylinder processes affecting soot with post-injections, a new optical diagnostic technique is utilized. Diffused back-illumination imaging (DBI) of soot extinction has previously been used in a high-pressure constant volume vessel, but has not yet been reported in the literature for heavy-duty engines. The DBI setup developed for this experiment enables quantitative 2-dimensional (2D) line-of-sight optical thickness (KL) measurements from soot extinction with a temporal resolution of 42 kHz. The high temporal resolution and relatively large field of view (FoV) quantifies the evolution of in-cylinder soot for roughly the downstream half of one diesel jet of the multi-hole injector throughout each cycle. The DBI imaging reveals that at these operating conditions, when the post injection is sufficiently short, the majority of the soot in the post injection is oxidized, thus allowing for increased load with similar soot emissions compared to a single-injection condition. A transient increase in entrainment that occurs after the end of injection (the "entrainment wave") is a candidate explanation for the observed completeness of post-injection soot oxidation. Additionally, semi-quantitative comparisons of soot KL and natural luminosity (NL) trends reveal decreasing KL accompanied by increasing NL. This observation is consistent with an increase in post-injection soot temperature after the end of the post injection, which may further aid oxidation

Fundamental insights on ignition and combustion of natural gas in an active fueled pre-chamber spark-ignition system

[EN] Pre-chamber spark-ignition (PCSI), either fueled or non-fueled, is a leading concept with the potential to enable diesel-like efficiency in medium-duty (MD) and heavy-duty (HD) natural gas (NG) engines. However, the inadequate scientific base and simulation tools to describe/predict the underlying processes governing PCSI systems is one of the key barriers to market penetration of PCSI for MD/HD NG engines. To this end, experiments were performed in a heavy-duty, optical, single-cylinder engine fitted with an active fueled PCSI module. The spatial and temporal progress of ignition and subsequent combustion of lean-burn natural gas using PCSI system were studied using optical diagnostic imaging and heat release analysis based on main-chamber and pre-chamber pressure measurements. Optical diagnostics involving simultaneous infrared (IR) and high-speed (30 kfps) broadband and filtered OH* chemiluminescence imaging are used to probe the combustion process. Following the early pressure rise in the pre-chamber, IR imaging reveals initial ejection of unburnt fuel-air mixture from the pre chamber into the main-chamber. Following this, the pre-chamber gas jets exhibit chemical activity in the vicinity of the pre-chamber region followed by a delayed spread in OH* chemiluminescence, as they continue to penetrate further into the main-chamber. The OH* signal progress radially until the pre-chamber jets merge, which sets up the limit to a first stage, jet-momentum driven, mixing-controlled (temperature field) premixed combustion. This is then followed by the subsequent deceleration of the pre-chamber jets, caused by the decrease in the driving pressure difference (AP) as well as charge entrainment, resulting in a flame front evolution, where mixing is not the only driver. Chemical-kinetic calculations probe the possibility of flame propagation or sequential auto-ignition in the second stage of combustion. Finally, key phenomenological features are then summarized so as to provide fundamental insights on the complex underlying fluid-mechanical and chemical-kinetic processes that govern the ignition and subsequent combustion of natural gas near lean-limits in high-efficiency lean-burn natural gas engines employing PCSI system.This research was sponsored by the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) . Optical engine experiments were conducted at the Combustion Re-search Facility of Sandia National Laboratories in Livermore, CA. Sandia National Laboratories is a multi-mission laboratory man-aged and operated by National Technology and Engineering So-lutions of Sandia, LLC., a wholly owned subsidiary of Honey-well International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration (NNSA) under contract DE-NA0003525. We gratefully acknowledge the contributions of Keith Penney and Dave Cicone for their assistance in developing research tools and maintaining the optical engine. Jose M. Garcia-Oliver ac-knowledges the support of the Generalitat Valenciana government in Spain through Grant #Best/2019/176 during his scientific visit to the Combustion Research Facility.Rajasegar, R.; Niki, Y.; García-Oliver, JM.; Li, Z.; Musculus, MP. (2021). Fundamental insights on ignition and combustion of natural gas in an active fueled pre-chamber spark-ignition system. Combustion and Flame. 232:1-20. https://doi.org/10.1016/j.combustflame.2021.111561S12023