The effects of charge dilution on low-temperature diesel combustion and emissions were investigated in a smallbore single-cylinder diesel engine over a wide range of injection timing. The fresh air was diluted with additional N 2 and CO 2 , simulating 0 to 65% exhaust gas recirculation in an engine. Diluting the intake charge lowers the flame temperature T due to the reactant being replaced by inert gases with increased heat capacity. In addition, charge dilution is anticipated to influence the local charge equivalence ratio φ prior to ignition due to the lower O 2 concentration and longer ignition delay periods. By influencing both φ and T, charge dilution impacts the path representing the progress of the combustion process in the φ-T plane, and offers the potential of avoiding both soot and NO x formation.In-cylinder pressure measurements, exhaust-gas emissions, and imaging of combustion luminosity were performed to clarify the path of the combustion process and the effects of charge dilution and injection timing on combustion and fuel conversion efficiency. Based on the findings, a postulated combustion process in the φ-T plane is presented for different dilution levels and injection timings. Although the ignition delay increased with high dilution and early injection, the heat release analysis indicated that a large portion of the combustion and emissions formation processes was still dominated by the mixing-controlled phase rather than the premixed phase. Because of the incomplete premixing, and the need to mix a greater volume of charge with unburned or partially-burned fuel to complete combustion, the diluted mixtures increased CO emissions. Injecting the fuel at earlier timings to extend the ignition delay helped alleviate this problem, but did not eliminate it. Fuel conversion efficiencies calculated for each dilution level and start of injection provide guidance as to the appropriate combustion phasing and practical levels of charge dilution for this low-temperature diesel combustion regime.

Shadowgraph/schlieren imaging techniques have often been used for flow visualization of reacting and nonreacting systems. In this paper we show that high-speed shadowgraph visualization in a high-pressure chamber can also be used to identify cool-flame and hightemperature combustion regions of diesel sprays, thereby providing insight into the time sequence of diesel ignition and combustion. When coupled to simultaneous high-speed Mie-scatter imaging, chemiluminescence imaging, pressure measurement, and spatially-integrated jet luminosity measurements by photodiode, the shadowgraph visualization provides further information about spray penetration after vaporization, spatial location of ignition and hightemperature combustion, and inactive combustion regions where problematic unburned hydrocarbons exist. Examples of the joint application of high-speed diagnostics include transient non-reacting and reacting injections, as well as multiple injections. Shadowgraph and schlieren image processing steps required to account for variations of refractive index within the hightemperature combustion vessel gases are also shown.

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An n-dodecane spray in temperature and pressure conditions typical of diesel engines, known as Spray A, is modelled by the transported probability density function (TPDF) method coupled with a time-dependent Reynoldsaveraged k − turbulence model and a Lagrangian discrete phase model of the liquid spray. To establish a baseline for comparisons, non-reacting cases are first studied. Good results are obtained for the vapour penetration, the mean and variance of fuel mixture fraction, and velocity profiles, with variations in ambient density and injection pressure. These comparisons are more extensive than previous studies due to new experimental data being available. Reacting cases are then investigated for a number of ambient * Corresponding author, Phone number: +1 (630) 252-5237, Present address:conditions and injection parameters, employing a reduced chemical kinetic model. The chemical mechanism incorporates an OH* sub-mechanism (Hall and Petersen, Int. J. Chem. Kinet. 38, 2006, pp. 714-724) which enables a direct comparison with experimental measurements of the lift-off length that are based on OH* chemiluminescence. To assess the importance of interactions between turbulence and chemistry, the results from the PDF model are compared to the measurements and to those from a well-mixed model that ignores turbulent fluctuations. Variations of ambient temperature, ambient oxygen concentration, ambient density, and injection pressure are considered. In all cases the PDF model with the EMST mixing model and C φ = 1.5 shows an excellent agreement with the experimental lift-off length and presents improved results compared with the well-mixed model.Ignition delay is however over-predicted by both the PDF method and wellmixed models. Available shock tube data suggests that this may be due to the chemical kinetic model over-predicting ignition delay at higher pressures.

Nerva, J.; Genzale, CL.; Kook, S.; García Oliver, JM.; Pickett, LM. (2013) soy methyl-ester biodiesel is injected into a constant-volume combustion facility. A range of optical diagnostics is performed, comparing biodiesel to a conventional #2 diesel at the same injection and ambient conditions. Schlieren imaging shows virtually the same vapor-phase penetration for the two fuels, while simultaneous Mie-scatter imaging shows that the maximum liquid-phase penetration of biodiesel is higher than diesel due to their different boiling-point temperatures. But the different liquid-phase penetration does not affect overall mixing rate and downstream vapor-phase penetration because each fuel spray has similar momentum and spreading angle. The ignition delay and lift-off length are only slightly less for biodiesel compared to diesel, consistent with the CN. Because of the similarity in lift-off length, the differences in equivalence ratio distribution at the lift-off length are mainly affected by the fuels oxygen content. 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 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 CFD spray, combustion, and soot formation models suitable for the latest biofuels. AbstractThough biodiesel has begun to penetrate the fuel market, its effect on injection processes, combustion, and emissions formation under diesel engine conditions remains somewhat unclear. Typical tailpipe measurements from engines running biodiesel indicate that PM, CO and UHC are decreased, whereas NO X 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 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 vapor-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. But the different liquid-phase penetration does not affect overall mixing rate and downstream vapor-phase penetration because each...

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