Lyle M. Pickett

The influence of charge dilution and injection timing on low-temperature diesel combustion and emissions

The effects of charge dilution on low-temperature diesel combustion and emissions were investigated in a small-bore 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 unbumed 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.

Comparison of Diesel Spray Combustion in Different High-Temperature, High-Pressure Facilities

Diesel spray experiments at controlled high-temperature and high-pressure conditions offer the potential for an improved understanding of diesel combustion, and for the development of more accurate CFD models that will ultimately be used to improve engine design. Several spray chamber facilities capable of high-temperature, high-pressure conditions typical of engine combustion have been developed, but uncertainties about their operation exist because of the uniqueness of each facility. For the IMEM meeting, we describe results from comparative studies using constant-volume vessels at Sandia National Laboratories and IFP. Targeting the same ambient gas conditions (900 K, 60 bar, 22.8 kg/m{sup 3}, 15% oxygen) and sharing the same injector (common rail, 1500 bar, KS1.5/86 nozzle, 0.090 mm orifice diameter, n-dodecane, 363 K), we describe detailed measurements of the temperature and pressure boundary conditions at each facility, followed by observations of spray penetration, ignition, and combustion using high-speed imaging. Performing experiments at the same high-temperature, high-pressure operating conditions is an objective of the Engine Combustion Network (http://www.ca.sandia.gov/ECN/), which seeks to leverage the research capabilities and advanced diagnostics of all participants in the ECN. We expect that this effort will generate a high-quality dataset to be used for advanced computational model development at engine conditions.

Soot Formation in Diesel Fuel Jets Near the Lift-Off Length

Abstract Soot formation in the region downstream of the lift-off length of diesel fuel jets was investigated in an optically accessible constant-volume combustion vessel under quiescent-type diesel engine conditions. Planar laser-induced incandescence and line-of-sight laser extinction were used to determine the location of the first soot formation during mixing-controlled combustion. OH chemiluminescence imaging was used to determine the location of high-heat-release reactions relative to the soot-forming region. The primary parameters varied in the experiments were the sooting propensity of the fuel and the amount of fuel-air premixing that occurs upstream of the lift-off length. The fuels considered in order of increasing sooting propensity were: an oxygenated fuel blend (T70), a blend of diesel cetane-number reference fuels (CN80), and a #2 diesel fuel (D2). Fuel-air mixing upstream of the lift-off length was varied by changing ambient gas and injector conditions, which varied either the lift-off length or the air entrainment rate into the fuel jet relative to the fuel injection rate. Results show that soot formation starts at a finite distance downstream of the lift-off length and that the spatial location of soot formation depends on the fuel type and operating conditions. The distance from the lift-off length to the location of the first soot formation increases as the fuel sooting propensity decreases (i.e. in the order D2 < CN80 < T70). At the baseline operating conditions, the most upstream soot formation occurs at the edges of the jet for D2 and CN80, while for T70 the soot formation is confined to the jet central region. When conditions are varied to produce enhanced fuel-air mixing upstream of the lift-off length in D2 fuel jets, the initial soot formation shifts towards the fuel jet centre and eventually no soot is formed. For all experimental conditions, the observed location of soot formation relative to the heat-release location (lift-off) suggests that soot formation occurs in a mixture of combustion products originating from partially premixed reactions and a diffusion flame. The results also imply that soot precursor formation rates depend strongly on fuel type in the region between the lift-off length and the first soot formation.

Evaluation of the equivalence ratio-temperature region of diesel soot precursor formation using a two-stage Lagrangian model

Abstract The two-stage Lagrangian (TSL) reacting-jet model of Broadwell and Lutz is applied to n-heptane fuel jets to understand soot formation at diesel engine operating conditions. The model employs a diffusion-flame reactor and homogeneous core reactor with jet entrainment rates determined by empirical correlations. Detailed chemical kinetics, consisting of 696 species and 3224 reactions, are used for predictions of n-heptane oxidation and soot precursor formation up to seven-ring polycyclic aromatic hydrocarbons. Boundary conditions are based on realistic diesel operating conditions, mixing rates, and flame lift-off lengths. TSL soot precursor simulations are compared with closed-reactor (Senkin) predictions over a range of temperatures and equivalence ratios. Results show that the equivalence ratio-temperature region of soot precursor formation varies from the closed-reactor predictions and depends upon parameters such as ambient oxygen concentration, injection pressure, nozzle orifice size, and flame lift-off. The lack of a unique equivalence ratio-temperature region for soot precursor formation implies that the soot formation process depends upon the equivalence ratio-temperature path followed during jet mixing, and the residence time along the path.

Orifice Diameter Effects on Diesel Fuel Jet Flame Structure

The effects of orifice diameter on several aspects ofdiesel fuel jet flame structure were investigated in a constant-volume combustion vessel under heavy-duty direct-injection (DI) diesel engine conditions using Phillips research grade #2 diesel fuel and orifice diameters ranging from 45 μm to 180 μm. The overall flame structure was visualized with time-averaged OH chemiluminescence and soot luminosity images acquired during the quasi-steady portion of the diesel combustion event that occurs after the transient premixed burn is completed and the flame length is established. The lift-off length, defined as the farthest upstream location of high-temperature combustion, and the flame length were determined from the OH chemiluminescence images. In addition, relative changes in the amount of soot formed for various conditions were determined from the soot incandescence images. Combined with previous investigations of liquid-phase fuel penetration and spray development, the results show that air entrainment upstream of the lift-off length (relative to the amount of fuel injected) is very sensitive to orifice diameter As orifice diameter decreases, the relative air entrainment upstream of the lift-off length increases significantly. The increased relative air entrainment results in a reduced overall average equivalence ratio in the fuel jet at the lift-off length and reduced soot luminosity downstream of the lift-off length. The reduced soot luminosity indicates that the amount of soot formed relative to the amount of fuel injected decreases with orifice diameter. The flame lengths determined from the images agree well with gas jet theory for momentum-driven nonpremixed turbulent flames.

An investigation of diesel soot formation processes using micro-orifices

Soot formation processes of diesel fuel jets were investigated in a constant-volume combustion vessel under heavy-duty, direct-injection (DI) diesel engine conditions using orifice diameters as small as 50 μ m. Soot was measured with line-of-sight laser extinction and planar laser-induced incandescence techniques, and flame liftoff lengths were determined with time-averaged OH chemiluminescence imaging. Results show that as fuel-air mixing upstream of the liftoff length increases, the amount of soot measured within a fuel jet decreases. When the cross-sectional average equivalence ratio at the liftoff length decreases to a value less than approximately 2, soot is no longer formed within the fuel jet. The soot measurements provide direct proof of the link between soot formation and mixing of fuel and air upstream of the liftoff length previously observed using total soot luminosity measurements. The non-sooting conditions were achieved with the 50 μ m micro-orifice at an ambient gas temperature and density of 1000 K and 14,8 kg/m 3 and ambient oxygen concentrations between 21% and 10%. The temperature and density are typical of DI diesel in-cylinder conditions. The lack of soot for the lower oxygen concentration conditions, which have substantially lower flame temperatures, suggests that NO x and soot can potentially be simultaneously reduced with small orifices and exhaust-gas recirculation.

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.

Fuel Effects on Soot Processes of Fuel Jets at DI Diesel Conditions

The effects of fuel composition on soot processes in diesel fuel jets were studied in an optically-accessible constant-volume combustion vessel at experimental conditions typical of a Dl diesel. Four fuel blends used in recent engine studies were investigated, including three oxygenates and one diesel reference fuel: (1) T70, a fuel blend containing the oxygenate tetraethoxy-propane; (2) BM88, a fuel blend containing the oxygenate dibutyl-maleate; (3) GE80, a fuel blend containing the oxygenate tri-propylene-glycol-methyl-ether and (4) CN80, a diesel reference fuel composed of an n-hexadecane and heptamethyl-nonane mixture. Measurements of the soot distribution along the axis of quasi-steady fuel jets were performed using laser extinction and planar laser-induced incandescence (PLII) and were compared to previous results using a #2 diesel fuel (D2). In addition to the soot measurements, lift-off length and ignition delay measurements were performed for an extensive range of ambient gas temperatures and densities. Flame lift-off lengths were used in the interpretation and analysis of the soot measurements. Lift-off lengths, ignition delays and soot levels for these fuel blends follow similar trends with respect to ambient temperature or density established using D2 fuel. With increasing ambient temperature or density, lift-off length and ignition delay decrease and peak soot levels in a fuel jet increase. The increase in peak soot level is linear with respect to temperature and non-linear with respect to ambient density. Although following established trends with temperature or density, at a given experimental condition there is significant variation in lift-off length, ignition delay, and soot level for each fuel blend. The soot level in decreasing order with respect to fuel composition is: D2 > CN80 > BM88 > T70 > GE80. The distance from the injector to the region of first soot formation has an inverse relationship to the sooting propensity given above. That is, the first-soot distance is longest for GE80 and shortest for D2. The order in sooting tendency is found at either fixed ambient and injector operating conditions or at equivalent fuel-oxygen mixtures at the jet lift-off length, confirming that fuel molecular structure effects are important to the soot processes at diesel conditions. Differences in soot level with respect to fuel composition are quantified at many experimental conditions and axial positions of the fuel jet.

Ignition, soot formation, and end-of-combustion transients in diesel combustion under high-EGR conditions

The ignition, soot formation, and end of combustion transients of n-heptane and #2 diesel jets were investigated in an optically accessible constant-volume combustion vessel under high exhaust-gas recirculation (EGR) environments. A wide range of EGR levels were simulated by systematically decreasing the ambient oxygen concentration from 21 to 8 per cent, while holding other experimental conditions constant. Characteristics of the effect of EGR on the ignition transient include: development of a cool flame early after injection for all EGR levels, an increase in the premixed-burn (high-temperature combustion) ignition delay inversely proportional to ambient oxygen concentration, ([O2]−1), and lower apparent heat-release rates during the premixed-burn with increasing EGR. The timing of soot formation is strongly dependent upon EGR, and the time between ignition and the first soot formation increases with decreasing ambient oxygen concentration. Soot-forming fuel jets are shown to become soot-free at high-EGR conditions by reducing the injection duration to be less than the soot formation time, but longer than the ignition delay time (negative ignition dwell). While past studies show success in reducing soot formation when the injection duration is less than the ignition delay (positive ignition dwell), this result shows that high EGR can suppress soot formation even with negative ignition dwell, thereby permitting higher-load operation by using longer injection durations. At the end of injection, increasing EGR presents difficulties in completing combustion because of the lower ambient oxygen concentration. Despite eventually reaching the same pressure rise (i.e., combustion efficiency) more time is required for the higher EGR conditions to mix fuel with sufficient oxygen to complete combustion.

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.