J.M. García-Oliver

Fundamental spray and combustion measurements of soy methyl-ester biodiesel

Although biodiesel has begun to penetrate the fuel market, its effect on injection processes, combustion and emission formation under diesel engine conditions remains somewhat unclear. Typical exhaust measurements from engines running biodiesel indicate that particulate matter, carbon monoxide and unburnt hydrocarbons are decreased, whereas nitrogen oxide emissions tend to be increased. However, these observations are the result of complex interactions between physical and chemical processes occurring in the combustion chamber, for which understanding is still needed. To characterize and decouple the physical and chemical influences of biodiesel on spray mixing, ignition, combustion and soot formation, a soy methyl-ester (SME) biodiesel is injected into a constant-volume combustion facility under diesel-like operating conditions. A range of optical diagnostics is performed, comparing biodiesel to a conventional #2 diesel at the same injection and ambient conditions. Schlieren high-speed imaging shows virtually the same vapour-phase penetration for the two fuels, while simultaneous Mie-scatter imaging shows that the maximum liquid-phase penetration of biodiesel is higher than diesel. Differences in the liquid-phase penetration are expected because of the different boiling-point temperatures of the two fuels. However, the different liquid-phase penetration does not affect overall mixing rate and downstream vapour-phase penetration because each fuel spray has similar momentum and spreading angle. For the biodiesel and diesel samples used in this study, the ignition delay and lift-off length are only slightly less for biodiesel compared to diesel, consistent with the fuel cetane number (51 for biodiesel, 46 for diesel). Because of the similarity in lift-off length, the differences in equivalence ratio distribution at the lift-off length are mainly affected by the oxygen content of the fuels. For biodiesel, the equivalence ratio is reduced, which, along with the fuel molecular structure and oxygen content, significantly affects soot formation downstream. Spatially resolved soot volume fraction measurements obtained by combining line-of-sight laser extinction measurements with planar laser-induced incandescence imaging show that the soot concentration can be reduced by an order of magnitude for biodiesel. These integrated measurements of spray mixing, combustion and quantitative soot concentration provide new validation data for the development of computational fluid dynamics spray, combustion and soot formation models suitable for the latest biofuels.

Combustion Recession after End of Injection in Diesel Sprays

This work contributes to the understanding of physical mechanisms that control flashback, or more appropriately combustion recession, in diesel-like sprays. Combustion recession is the process whereby a lifted flame retreats back towards the injector after end-of-injection under conditions that favor autoignition. The motivation for this study is that failure of combustion recession can result in unburned hydrocarbon emissions. A large dataset, comprising many fuels, injection pressures, ambient temperatures, ambient oxygen concentrations, ambient densities, and nozzle diameters is used to explore experimental trends for the behavior of combustion recession. Then, a reduced-order model, capable of modeling non-reacting and reacting conditions, is used to help interpret the experimental trends. Finally, the reduced-order model is used to predict how a controlled ramp-down rate-ofinjection can enhance the likelihood of combustion recession for conditions that would not normally exhibit combustion recession. In general, fuel, ambient conditions, and the spray rate-of-injection transient during the end-of-injection determine the success or failure of combustion recession. The likelihood of combustion recession increases for higher ambient temperatures and oxygen concentrations as well as for higher reactivity fuels. In the transition between high and low ambient temperature (or oxygen concentration), the behavior of combustion recession changes from spatially sequential ignition to separated, or isolated, ignition sites that eventually merge. In contradistinction to typical diesel ignition delay trends where the autoignition times are longer for increasing injection pressure, the time required for combustion recession increases with injection pressure.

A Progress Review on Soot Experiments and Modeling in the Engine Combustion Network (ECN)

The following individuals and funding agencies are acknowledged for their support. The authors from DTU acknowledge the Technical University of Denmark, Danish Strategic Research Council, and MAN Diesel & Turbo University of Wisconsin: Financial support provided by the Princeton Combustion Energy Frontier Research Center. ETH Zurich: Financial support from the Swiss Federal Office of Energy (grant no. SI/500818-01) and the Swiss Competence Center for Energy and Mobility (CCEM project “In-cylinder emission reduction”) is gratefully acknowledged. Argonne National Labs: Work was funded by U.S. DOE Office of Vehicle Technologies, Office of Energy Efficiency and Renewable Energy under Contract No. DE-AC02-06CH11357. We also gratefully acknowledge the computing resources provided on Fusion, a computing cluster operated by the Laboratory Computing Resource Center at Argonne National Laboratory. Sandia National Labs, Combustion Research Facility: Work was supported by the U.S. Department of Energy, Office of Vehicle Technologies. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DEAC04-94AL85000. Chris Carlen and Dave Cicone are gratefully acknowledged for technical assistance. The authors from ANL and SNL also wish to thank Gurpreet Singh and Leo Breton, program managers at U.S. DOE, for their support.

Experimental facility and methodology for systematic studies of cold startability in direct injection Diesel engines

Cold start at low temperatures in current direct injection (DI) Diesel engines is a problem which has not yet been properly solved and it becomes particularly critical with the current trend to reduce the engine compression ratio. Although it is clear that there are some key factors whose control leads to a proper cold start process, their individual relevance and relationships are not clearly understood. Thus, efforts on optimization of the cold start process are mainly based on a trial-and-error procedure in climatic chambers at low ambient temperature, with serious limitations in terms of measurement reliability during such a transient process, low repeatability and experimental cost. This paper presents a novel approach for an experimental facility capable of simulating real engine cold start, at room temperature and under well-controlled low speed and low temperature conditions. It is based on an optical single cylinder engine adapted to reproduce in-cylinder conditions representative of those of a real engine during start at cold ambient temperatures (of the order of −20 °C). Such conditions must be realistic, controlled and repeatable in order to perform systematic studies in the borderline between ignition success and misfiring. An analysis methodology, combining optical techniques and heat release analysis of individual cycles, has been applied.

Schlieren Methodology for the Analysis of Transient Diesel Flame Evolution

This work was partially funded by the Spanish Ministry of Education and Science through the “LES METHODS FOR THE SIMULATION OF MULTIPHASE SPRAYS” project (ENE2010-18542). Mr. Francisco J. Briceno wishes to acknowledge financial support through a PhD studies grant (AP2008-02231) also sponsored by the Spanish Ministry of Education and Science

Effects of Intake Pressure on Particle Size and Number Emissions from Premixed Diesel Low Temperature Combustion

In this study, 10 premixed diesel low-temperature combustion engine operating conditions were chosen based on engine intake pressure (1.2–1.6 bar), intake oxygen concentration (10%, 11%, and 12%), and injection timing (−24° after top dead centre in all test conditions). At each intake oxygen concentration, the effects of intake pressure on combustion parameters and emission measurements (carbon monoxide, hydrocarbons, nitrogen oxides, particulate matter mass concentration, and particle size distributions) were analyzed. Although increased intake pressure resulted in higher in-cylinder charge air density that improved fuel/air premixing and late-cycle oxidation quality, higher intake pressure also advanced the start of combustion and thereby decreased the time available for fuel and air premixing. But even with the decrease in premixing time available before start of combustion, increased intake pressure caused significant decreases in carbon monoxide, hydrocarbon, particulate matter mass, and particle number emissions. Particle size distribution measurements allowed greater understanding of how higher intake pressure decreased the particulate matter mass concentrations with respect to particle size. To further investigate the experimental results, a zero-dimensional engine heat release code was used to analyze combustion temperatures, and a one-dimensional free spray model was used to estimate the relative levels of liquid fuel spray impingement on the piston surface and maximum local equivalence ratio at start of combustion for each test case. Therefore, though the premixing time was shortened by higher intake pressures, the decreased emissions were understood by combined effects of enhanced fuel and air premixing quality and improved late-cycle oxidation near the end of combustion.

Evaluation of combustion models based on tabulated chemistry and presumed probability density function approach for diesel spray simulation

Two combustion models of different complexity have been implemented in a RANS solver in the CFD platform OpenFOAM®. Both models rely on the flame prolongation of ILDM (FPI) method, which allows the use of detailed chemistry mechanisms at relatively low computational costs. The homogeneous auto-ignition (HAI) model, directly using the chemical data from the FPI tabulation, does not take into account subgrid turbulence-chemistry interaction. Therefore, the second, more advanced model combines the FPI method with a presumed conditional moment approach. This auto-ignition-presumed conditional moment (AI-PCM) model accounts for the fluctuations of the mixture fraction and the progress variable caused by the turbulent flow. Both models have been evaluated by means of a parametric study of a single diesel spray at varying initial temperatures and oxygen concentration levels. The results obtained with the CFD models have been compared with experimental data from the engine combustion network (ECN). The comparison of the two models demonstrates the important role of the subgrid turbulence-chemistry interaction on the accuracy of the auto-ignition process and the diesel flame structure, as indicated by the agreement of the AI-PCM predictions with the measured data.

Study of Air Flow Interaction with Pilot Injections in a Diesel Engine by Means of PIV Measurements

The support of the Spanish Ministry of Economy and Competitiveness (TRA2014-58870-R,) is greatly acknowledged through HiReCo project.

An Experimental Study on Diesel Spray Injection into a Non-Quiescent Chamber

This work was partially funded by the Government of Spain through COMEFF Project (TRA2014-59483-R). In addition, the authors acknowledge that some equipment used in this work has been partially supported by FEDER project funds (FEDER-ICTS-2012-06), framed in the operational program of unique scientific and technical infrastructure of the Ministry of Science and Innovation of Spain. The authors want also to express their gratitude to CONVERGENT SCIENCE Inc for their kind support for this research.

Optical study on characteristics of non-reacting and reacting diesel spray with different strategies of split injection

Even though studies on split-injection strategies have been published in recent years, there are still many remaining questions about how the first injection affects the mixing and combustion processes of the second one by changing the dwell time between both injection events or by the first injection quantity. In this article, split-injection diesel sprays with different injection strategies are investigated. Visualization of n-dodecane sprays was carried out under both non-reacting and reacting operating conditions in an optically accessible two-stroke engine equipped with a single-hole diesel injector. High-speed Schlieren imaging was applied to visualize the spray geometry development, while diffused background-illumination extinction imaging was applied to quantify the instantaneous soot production (net result of soot formation and oxidation). For non-reacting conditions, it was found that the vapor phase of second injection penetrates faster with a shorter dwell time and independently of the duration of the first injection. This could be explained in terms of one-dimensional spray model results, which provided information on the local mixing and momentum state within the flow. Under reacting conditions, interaction between the second injection and combustion recession of the first injection is observed, resulting in shorter ignition delay and lift-off compared to the first injection. However, soot production behaves differently with different injection strategies. The maximum instantaneous soot mass produced by the second injection increases with a shorter dwell time and with longer first injection duration.