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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).

Ambient gas density and fuel vaporization effects on the penetration and dispersion of diesel sprays were examined over a gas density range spanning nearly two orders of magnitude. This range included gas densities more than a factor of two higher than top-dead-center conditions in current technology heavy-duty diesel engines. The results show that ambient gas density has a significantly larger effect on spray penetration and a smaller effect on spray dispersion than has been previously reported. The increased dependence of penetration on gas density is shown to be the result of gas density effects on dispersion. In addition, the results show that vaporization decreases penetration and dispersion by as much as 20% relative to non-vaporizing sprays; however, the effects of vaporization decrease with increasing gas density.

The goal of this work was to better understand the relationship between diesel fuel composition and its ignition performance. Ignition delay measurements were made as a function of temperature in a constant-volume combustion bomb at simulated diesel engine conditions. The fuels studied were binary mixtures of pure compounds and for comparison Phillips Diesel Control Fuel. The fuels were tested with and without cetane improver additive. The results show that the mechanisms of fuel autoignition change with temperature and composition. Change points correspond well to the low-, intermediate-, and high-temperature regimes defined in classical hydrocarbon oxidation studies. Differences in ignition performance are discussed in terms of the production of effectively chain terminating stabilized free radicals. Cetane number improver additive enhanced the autoignition performance of all fuels.

Laser-induced incandescence (LII) has been explored as a diagnostic for qualitative two-dimensional imaging of the in-cylinder soot distribution in a diesel engine. Advantages of LII over elastic-scatter soot imaging techniques include no interfering signals from liquid fuel droplets, easy rejection of laser light scattered by in-cylinder surfaces, and the signal intensity being proportional to the soot volume fraction. LII images were obtained in a 2.3-liter, single cylinder, direct-injection diesel engine, modified for optical access. To minimize laser sheet and signal attenuation (which can affect almost any planar imaging technique applied to diesel engine combustion), a low-sooting fuel was used whose vaporization and combustion characteristics are typical of standard diesel fuels. Temporal and spatial sequences of LII images were made which show the extent of the soot distribution within the optically accessible portion the combusting spray plume.

This work presents the autoignition delay time characteristics of methane and natural gas under simulated diesel engine conditions. A constant-volume combustion vessel is used for the experiments. Results are presented for the pressure and temperature ranges of 5 to 55 atm and 600 to 1700 K, respectively. Comparisons are then made with autoignition data for methanol, ethanol, isooctane, and n-cetane. Three major trends are observed. First, there is little effect on the autoignition delay time of natural gas as the vessel pressure is increased from 5 to 55 atm. Second, there is a slight decrease in the autoignition delay time of methane-ethane gas mixtures as the concentration of ethane is increased. Third, the autoignition delay time of natural gas is strongly dependent on temperature and continually decreases with increasing temperature.

Methanol and ethanol are being considered as alternative fuels for diesel engines. One of the key concerns with using alcohol fuels in diesel engines is their poor ignition quality. This work presents the ignition characteristics of methanol and ethanol examined under simulated diesel engine conditions in a constant-volume combustion vessel. The ignition characteristics of isooctane and normal hexadecane (cetane) measured under the same conditions are also included for reference. Results show that to obtain ignition delays and rates-of-pressure-rise suitable for current diesel engine designs, methanol and ethanol require in-cylinder temperatures of about 1100 K at the time of injection. The results also show that the ignition delays of the alcohol fuels are independent of the chamber pressure and are unaffected by the presence of 10% by volume of water in the fuel.

The goal of this work was to investigate the ignition characteristics of several fuels and to try to determine why the cetane number accurately predicted ignition quality of some fuels while failing for others. The measurements were made under simulated diesel engine conditions in a constant-volume combustion bomb. The fuels were some of the same fuels tested in DI and IDI diesel engines by Needham and Doyle of Ricardo. Blends of the reference fuels used for cetane number rating were also tested for comparison. The results show that the cetane number, as currently defined, cannot provide a consistent and accurate measure of the ignition quality of fuels whose ignition characteristics depend on temperature (i.e., compression ratio) in a different manner than the reference fuels. An implication of this result is that the cold-start characteristics of a fuel cannot be determined from its cetane number if its ignition characteristics are not modeled by the reference fuels.