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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.

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.

The relationship between combustion processes and engine-out soot was investigated in an optically accessible DI diesel engine using diethylene glycol diethyl ether (DGE) fuel, a viable diesel oxygenate. The high oxygen content of DGE enables operation without soot emissions at higher loads than with a hydrocarbon fuel. The high cetane number of DGE enables operation at charge-gas temperatures below those required for current diesel fuels, which may be advantageous for reducing NOx emissions. In-cylinder optical measurements of flame lift-off length and natural luminosity were obtained simultaneously with engine-out soot measurements while varying charge-gas density and temperature. The local mixture stoichiometry at the lift-off length was characterized by a parameter called the oxygen ratio that was estimated from the measured flame lift-off length using an entrainment correlation for non-reacting sprays.

The detailed mechanisms by which oxygenated diesel fuels reduce engine-out soot emissions are not well understood. The literature contains conflicting results as to whether a fuel's overall oxygen content is the only important parameter in determining its soot-reduction potential, or if oxygenate molecular structure or other variables also play significant roles. To begin to resolve this controversy, experiments were conducted at a 1200-rpm, moderate-load operating condition using a modern-technology, 4-stroke, heavy-duty DI diesel engine with optical access. Images of broadband natural luminosity (i.e., light emission without spectral filtering) from the combustion chamber, coupled with heat-release and efficiency analyses, are presented for three test-fuels. One test-fuel (denoted GE80) was oxygenated with tri-propylene glycol methyl ether; the second (denoted BM88) was oxygenated with di-butyl maleate. The overall oxygen contents of these two fuels were matched at 26% by weight.

The addition of oxygenates to diesel fuel can reduce particulate emissions, but the underlying chemical pathways for the reductions are not understood. While measurements of particulate matter (PM), unburned hydrocarbons (HC), and carbon monoxide (CO) are routine, determining the contribution of carbon atoms in the original fuel molecules to the formation of these undesired exhaust emissions has proven difficult. Using accelerator mass spectrometry (AMS) diagnostics, carbon atoms in a specific bond position in a specific fuel molecule can be labeled with carbon-14 (14C) and traced through the combustion event to determine whether they reside in PM, HC, CO, CO2, or other emission products. This knowledge of how specific molecular structures produce certain emissions can be used to refine chemical-kinetic combustion models and to optimize fuel composition to reduce undesired emissions.

This paper explores soot and soot-precursor formation characteristics of oxygenated fuels using experiments and numerical simulations under direct-injection diesel engine conditions. The paper strives to achieve four goals: 1)to introduce the “oxygen ratio” for accurate quantification of reactant-mixture stoichiometry for both oxygenated and non-oxygenated fuels; 2) to provide experimental results demonstrating that some oxygenates are more effective at reducing diesel soot than others; 3) to present results of numerical simulations showing that detailed chemical-kinetic models without complex fluid mechanics can capture some of the observed trends in the sooting tendencies of different oxygenated fuels; and 4) to provide further insight into the underlying mechanisms by which oxygenate structure and in-cylinder processes can affect soot formation in DI diesel engines. The oxygenates that were studied are di-butyl maleate (DBM) and tri-propylene glycol methyl ether (TPGME).

In-cylinder concentrations of nitric oxide (NO) in a diesel engine were studied using a laser-induced fluorescence (LIF) technique that employs two-photon excitation. Two-photon NO LIF images were acquired during the expansion and exhaust portions of the engine cycle providing useful NO fluorescence signal levels from 60° after top dead center through the end of the exhaust stroke. The engine was fueled with the oxygenated compound diethylene glycol diethyl ether to minimize soot within the combustion chamber. Results of the two-photon NO LIF technique from the exhaust portion of the cycle were compared with chemiluminescence NO exhaust-gas measurements over a range of engine loads from 1.4 to 16 bar gross indicated mean effective pressure. The overall trend of the two-photon NO LIF signal showed good qualitative agreement with the NO exhaust-gas measurements.

Fuel-injection schedules that use two injection events per cycle (“dual-injection” approaches) have the potential to simultaneously attenuate engine-out soot and NOx emissions. The extent to which these benefits are due to enhanced mixing, low-temperature combustion modes, altered combustion phasing, or other factors is not fully understood. A traditional single-injection, an early-injection-only, and two dual-injection cases are studied using a suite of imaging diagnostics including spray visualization, natural luminosity imaging, and planar laser-induced fluorescence (PLIF) imaging of nitric oxide (NO). These data, coupled with heat-release and efficiency analyses, are used to enhance understanding of the in-cylinder processes that lead to the observed emissions reductions.

The effect of oxygenate molecular structure on soot emissions from a DI diesel engine was examined using carbon-14 (14C) isotope tracing. Carbon atoms in three distinct chemical structures within the diesel oxygenate dibutyl maleate (DBM) were labeled with 14C. The 14C from the labeled DBM was then detected in engine-out particulate matter (PM), in-cylinder deposits, and CO2 emissions using accelerator mass spectrometry (AMS). The results indicate that molecular structure plays an important role in determining whether a specific carbon atom either does or does not form soot. Chemical-kinetic modeling results indicate that structures that produce CO2 directly from the fuel are less effective at reducing soot than structures that produce CO before producing CO2.

The influence of nitrogen dilution and charge-gas temperature on in-cylinder combustion processes and engine-out NOx and smoke emissions was investigated in an optically accessible heavy-duty DI diesel engine using a high-cetane-number, oxygenated fuel. Engine-out measurements of NOx and smoke emissions and in-cylinder images of natural luminosity were obtained for charge-gas oxygen concentrations from 9% to 21% and TDC charge-gas temperatures of 680 and 880 K. Charge-gas temperature was found to have a significant influence on engine-out NOx emissions, but NOx emissions levels less than 0.2 g/ihp-hr were achieved at both the 680 and 880 K charge-gas temperatures within the investigated range of oxygen concentrations. An indicated engine-out NOx emissions level of 0.09 g/ihp-hr at 18 bar IMEP was achieved using charge-gas dilution and 3.0 bar intake boost pressure.