Comparison of the Impact of Intake Oxygen Enrichment and Fuel Oxygenation on Diesel Combustion and Emissions

Abstract

Among continuing efforts to develop low-emission combustion engines, oxygen-enhanced combustion has long been considered a promising approach. A number of investigations have focused on the effects of oxygen addition on soot formation and oxidation by using various oxygen introduction techniques, such as blending different oxygen-containing fuels or direct oxygen addition into the intake air stream. The present study of oxygen addition was performed on a Volkswagen 1.9 L “TDI” turbodiesel engine to investigate and compare the relative effect of two oxygen addition methods on diesel emission and combustion: oxygen enrichment of the intake air and oxygenation of the fuel. The oxygen enrichment was accomplished by connecting an oxygen generator to the intake air surge tank, while fuel oxygenation was accomplished using two compounds with different cetane number and molecular structure. The key observations are that both intake oxygen enrichment and fuel oxygenation via linear structure oxygenated molecules are effective for reduction of diesel particulate matter, yielding even greater reductions in PM emissions than for fuel oxygenation via ring-structured oxygenated molecules. However, NOx emissions are greatly increased with intake oxygen enrichment, owing to either increased availability of atomic oxygen or attainment of a higher temperature during leaner combustion, which enhances the kinetics for thermal NOx formation. Comparison between the addition of two substantially different oxygenated fuels, a mixture of glycol ethers and 1,3-dioxolane, has also shed light on the mechanisms of soot reduction via oxygen addition. With their linear structure, the glycol ethers were shown to be far more effective for soot reduction than an equivalent oxygen addition via dioxolane, which has a ring structure, despite no significant difference in heat release rate.