Effect of oxygenated fuel on combustion and emissions in a light-duty turbo diesel engine

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 low-sulfur 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 NOx 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 NOx 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 NOx and soot reduction through a combination of flame temperature decrease and suppression of soot precursors. The combined effects on thermal NO, 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/NOx 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.