Leaner lifted-flame combustion (LLFC) is a mixing-controlled combustion strategy for compression-ignition (CI) engines that does not produce soot because the equivalence ratio at the lift-off length is less than or equal to approximately two. In addition to completely preventing soot formation, LLFC can simultaneously control emissions of nitrogen oxides because it is tolerant to the use of exhaust-gas recirculation for lowering in-cylinder temperatures. Experiments were conducted in a heavy-duty CI engine that has been modified to provide optical access to the combustion chamber, to study whether LLFC is facilitated by an oxygenated fuel blend (T50) comprising a 1:1 mixture by volume of tri-propylene glycol mono-methyl ether with an ultra-low-sulfur #2 diesel emissions-certification fuel (CFA). Results from the T50 experiments are compared against baseline results using the CFA fuel without the oxygenate. Experimental measurements include crank-angle-resolved natural luminosity and chemiluminescence imaging. Dilution effects were studied by adding nitrogen and carbon dioxide to the intake charge. Initial experiments with a 2-hole fuel-injector tip achieved LLFC at low loads with the T50 fuel, and elucidated the most important operating parameters necessary to achieve LLFC. The strategy was then extended to more moderate loads by employing a 6-hole injector tip, where lowering the intake-manifold temperature, reducing the coolant temperature, and retarding the start-ofcombustion timing resulted in sustained LLFC at both 21% and 16% intake-oxygen mole fractions at loads greater than 5 bar gross indicated mean effective pressure. In contrast to the results with T50, LLFC was not achieved under any of the test conditions with CFA.
Natural luminosity and chemiluminescence imaging diagnostics were employed to investigate if a 50/50 blend by volume of tripropylene-glycol monomethyl ether (TPGME) and ultra-low sulfur #2 diesel certification fuel (CF) could enable leaner-lifted flame combustion (LLFC), a non-sooting mode of mixing-controlled combustion associated with equivalence ratios below approximately 2. The experiments were performed in a single-cylinder heavy-duty optical compression-ignition engine at three injection pressures and three dilution levels. Results indicate that TPGME addition effectively eliminated engine-out smoke emissions by curtailing soot production and/or increasing soot oxidation during and after the end of fuel injection. TPGME greatly reduced soot luminosity when compared with neat CF, but did not enable LLFC because the equivalence ratios at the lift-off length, φ(H), never reached the non-sooting limit and incandescence from hot soot within the combustion chambered remained visible. Concerning other engine-out emissions, injection pressure influenced the effects of TPGME addition on NOx emissions. HC and CO emissions were higher compared to the baseline fuel, likely due to the lower net heat of combustion of TPGME and the need to limit fuel-injection duration for valid optical measurements.
Dumitrescu, Cosmin E.; Polonowski, Christopher J.; Fisher, Brian T.; Lilik, Gregory K.; Mueller, Charles J.
In this study, elastic scattering was employed to investigate diesel fuel property effects on the liquid length (i.e., the maximum extent of in-cylinder liquid-phase fuel penetration) using select research fuels: an ultralow-sulfur #2 diesel emissions-certification fuel (CF) and four of the Coordinating Research Council (CRC) Fuels for Advanced Combustion Engines (FACE) diesel fuels (F1, F2, F6, and F8). The experiments were performed in a single-cylinder heavy-duty optical compression-ignition engine under time-varying, noncombusting conditions to minimize the influence of chemical heat release on the liquid-length measurement. The FACE diesel fuel and CF liquid lengths under combusting conditions were also predicted using Siebers scaling law and pressure data from previous work using the same fuels at similar in-cylinder conditions. The objective was to observe if the liquid length under noncombusting or combusting conditions provides additional insights into the relationships among the main fuel properties (i.e., cetane number (CN), the 90 vol % distillation recovery temperature (T90), and aromatic content) and smoke emissions. Results suggest that liquid-length values are best correlated to fuel distillation characteristics measured with ASTM D2887 (simulated distillation method). This work also studied the relationship between liquid length and lift-off length, H (i.e., distance from the fuel-injector orifice exit to the position where the standing premixed autoignition zone stabilizes during mixing-controlled combustion). Two possible cases were identified based on the relative magnitudes of liquid length under combusting conditions (Lc) and H. The low-CN fuels are representative of the first case, Lc < H, in which the fuel is always fully vaporized at H. The high-CN fuels are mostly representative of the second case, Lc ≥ H, in which there is still liquid fuel at H. Lc ≥ H would suggest higher smoke emissions, but there is not enough evidence in this work to support a compounding effect of a longer liquid length on top of the aromatic-content effect on smoke emissions for fuels with similar CN, supporting previous findings in the literature that lift-off length plays a more important role than liquid-length on diesel combustion. At the same time, the experimental results suggest a decrease in the fuel-jet spreading angle, i.e., a decrease in the entrainment rate into the jet at and downstream of H, under combusting conditions, that is not accounted for in the model used to predict the values of ø(H). As a result, Lc may be of interest for accurate predictions of ø(H), especially for combustion strategies designed to lower in-cylinder soot by operating near or below the nonsooting ø(H)-value (i.e., ø(H) - 2).
Ducted fuel injection (DFI) is a technique for lowering emissions (primarily soot emissions) from high-efficiency compression-ignition (CI) engines, as well as other devices employing the direct injection of fuel into a combustion chamber. The DFI concept was inspired by the cleaner burn that is created by premixing fuel and air in the tube of a Bunsen burner, which was created to reduce soot production common in burners of the period as stated by Kohn [American Chemical Society, 1949].
Lilik, Gregory K.; Mueller, Charles J.; Dumitrescu, Cosmin E.; Gehrke, Christopher R.
Although soot-formation processes in diesel engines have been well characterized during the mixing-controlled burn, little is known about the distribution of soot throughout the combustion chamber after the end of appreciable heat release during the expansion and exhaust strokes. Hence, the laser-induced incandescence (LII) diagnostic was developed to visualize the distribution of soot within an optically accessible single-cylinder direct-injection diesel engine during this period. The developed LII diagnostic is semi-quantitative; i.e., if certain conditions (listed in the Appendix) are true, it accurately captures spatial and temporal trends in the in-cylinder soot field. The diagnostic features a vertically oriented and vertically propagating laser sheet that can be translated across the combustion chamber, where “vertical” refers to a direction parallel to the axis of the cylinder bore. The diagnostic allows soot visualization in almost the entire region above the piston bowl late in the cycle (until the piston descends below the imaged field of view). It also enables estimation of the total in-cylinder soot mass as a function of crank angle. These attributes of the diagnostic allow it to provide unique insights into the production, oxidation, and distribution of soot within the combustion chamber. This manuscript reports on the development of the diagnostic and results from its initial application, in which 21-, 18-, and 16-mol% intake-oxygen conditions were examined.
An optically accessible heavy-duty diesel engine was used to investigate the impact of methyl decanoate (MD) on combustion and emissions. Specific goals of the study were to produce experimental data for validating engine combustion models using MD (a biodiesel surrogate), as well as to determine if MD could enable soot-free leaner-lifted flame combustion (LLFC), a mode of mixing-controlled combustion associated with equivalence ratios below approximately 2. An ultralow sulfur diesel certification fuel (CF) was used as the baseline fuel, and experiments were conducted at two fuel-injection pressures with three levels of charge-gas dilution; start of combustion and duration of fuel injection were held constant. In addition to conventional pressure-based and engine-out emissions measurements, exhaust laser-induced incandescence, in-cylinder natural luminosity, and in-cylinder chemiluminescence diagnostics were used to provide detailed insight into combustion processes. Results indicate that MD effectively eliminated soot emissions but that soot formation still occurred in-cylinder, with equivalence ratios at the flame lift-off length in excess of approximately 3. Nevertheless, the oxygen content of MD sufficiently limited soot formation and promoted soot oxidation such that very little soot remained at exhaust-valve open. Nitrogen oxides (NOx) emissions for MD relative to CF showed different trends depending on fuel-injection pressure, with distinct fuel effects influencing NOx formation depending on engine operating condition. Hydrocarbon (HC) and CO emissions were higher for MD compared to CF and corresponded to lower fuel-conversion and combustion efficiencies. These differences were attributed to the lower-load conditions of MD, resulting from its lower energy density and the need to limit fuel-injection duration to obtain valid lift-off length measurements.
A state-of-the-art, grid-convergent simulation methodology was applied to three-dimensional calculations of a single-cylinder optical engine. A mesh resolution study on a sector-based version of the engine geometry further verified the RANS-based cell size recommendations previously presented by Senecal et al. (“Grid Convergent Spray Models for Internal Combustion Engine CFD Simulations,” ASME Paper No. ICEF2012-92043). Convergence of cylinder pressure, flame lift-off length, and emissions was achieved for an adaptive mesh refinement cell size of 0.35 mm. Furthermore, full geometry simulations, using mesh settings derived from the grid convergence study, resulted in excellent agreement with measurements of cylinder pressure, heat release rate, and NOx emissions. On the other hand, the full geometry simulations indicated that the flame lift-off length is not converged at 0.35 mm for jets not aligned with the computational mesh. Further simulations suggested that the flame lift-off lengths for both the nonaligned and aligned jets appear to be converged at 0.175 mm. With this increased mesh resolution, both the trends and magnitudes in flame lift-off length were well predicted with the current simulation methodology. Good agreement between the overall predicted flame behavior and the available chemiluminescence measurements was also achieved. Our present study indicates that cell size requirements for accurate prediction of full geometry flame lift-off lengths may be stricter than those for global combustion behavior. This may be important when accurate soot predictions are required.
The accurate quantification and control of mixture stoichiometry is critical in many applications using new combustion strategies and fuels (e.g., homogeneous charge compression ignition, gasoline direct injection, and oxygenated fuels). The parameter typically used to quantify mixture stoichiometry (i.e., the proximity of a reactant mixture to its stoichiometric condition) is the equivalence ratio, /gf. The traditional definition of /gf is based on the relative amounts of fuel and oxidizer molecules in a mixture. This definition provides an accurate measure of mixture stoichiometry when the fuel molecule does not contain oxidizer elements and when the oxidizer molecule does not contain fuel elements. However, the traditional definition of /gf leads to problems when the fuel molecule contains an oxidizer element, as is the case when an oxygenated fuel is used, or once reactions have started and the fuel has begun to oxidize. The problems arise because an oxidizer element in a fuel molecule is counted as part of the fuel, even though it acts as an oxidizer. Similarly, if an oxidizer molecule contains fuel elements, the fuel elements in the oxidizer molecule are misleadingly lumped in with the oxidizer in the traditional definition of /gf. In either case, use of the traditional definition of /gf to quantify the mixture stoichiometry can lead to significant errors. This paper introduces the oxygen equivalence ratio, /gf/gV, a parameter that properly characterizes the instantaneous mixture stoichiometry for a broader class of reactant mixtures than does /gf. Because it is an instantaneous measure of mixture stoichiometry,/gf/gV can be used to track the time-evolution of stoichiometry as a reaction progresses. The relationship between /gf/gV and /gf is shown. Errors are involved when the traditional definition of /gf is used as a measure of mixture stoichiometry with fuels that contain oxidizer elements or oxidizers that contain fuel elements; /gf/gV is used to quantify these errors. Proper usage of /gf/gV is discussed, and /gf/gV is used to interpret results in a practical example.