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In previous studies on a single-cylinder IDI diesel engine and a V-8 DI turbo diesel engine, significant reductions in particulate matter emissions were observed with the blends of glycol ethers in diesel fuel. In this study, experiments on the effects of oxygenated fuels on emissions and combustion were performed in a 4-cylinder TDI diesel engine. A blend of 20 wt % monoglyme and 80 wt % diglyme, referred to as CETANER, has been examined as a diesel reformulating agent. Blend ratios were considered to provide approximately 2, 4, and 6 wt % oxygen to lowsulfur diesel fuel. Gaseous and particulate emission measurements, as well as heat release rate analysis, have been used to address how emissions and combustion scale with increasing weight percent oxygen in the fuel. The results demonstrate that the oxygenated fuel provides significant reduction in particulate matter with a small penalty on NO x emission, especially at high load. This oxygenated fuel effect may result from an enhanced concentration of oxygen atoms in the over-rich mixture thereby contributing to soot suppression and thermal NO x formation through a shift to a leaner mixture. Low load results imply that the combined effect of relatively high exhaust gas recirculation (EGR) ratio and oxygen addition contributes to both NO x and soot reduction through a combination of flame temperature decrease and suppression of soot precursors. The combined effects on thermal NO x reduction at low load appear to be confirmed by heat release analysis, which indicates a small reduction in premixed burn peak and in-cylinder pressure. The slight reduction in HC and CO emissions under most conditions indicates an improvement of combustion efficiency with the use of oxygen addition. This result also represents the potential of diesel reformulation coupled with high EGR ratio for a better particulate/NO x tradeoff. Particulate morphology, as seen in transmission electron microscopy (TEM) micrographs, shows that enhanced oxidation of unburned hydrocarbon due to oxygen addition leaves a less agglomerated particulate structure especially at low mode, leading to a higher number density of smaller particles and a lower particulate mass.

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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, NO x 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 NO x 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.

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