Effects of oxygenates on soot processes in DI diesel engines: Experiments and numerical simulations

摘要

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 incylinder 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). Experiments were conducted in a constant-volume combustion vessel and a single-cylinder DI diesel engine, each with extensive optical access. Numerical simulations were conducted using detailed chemical-kinetic mechanisms for combustion of the oxygenated fuels in a homogenous-reactor configuration. Both the experimental and the numerical approaches showed that fuels containing the TPGME oxygenate are more effective at reducing soot than those containing the DBM oxygenate. Detailed chemical-kinetic analysis showed that over 30% of the oxygen in DBM is unavailable for eliminating soot precursors. Fuel oxygenation and enhanced charge-gas entrainment are investigated as in-cylinder, sootreduction strategies. Fuel oxygenation to a given mixture stoichiometry with either TPGME or DBM is found to be more effective at reducing soot than enhancing the entrainment of oxygen from the charge gases. The two strategies can be used together to achieve non-sooting combustion. The results suggest that jet-wall and multiple- jet interactions in an engine could produce regions that are conducive to soot formation, providing an extra challenge for soot reduction as non-sooting conditions are approached.