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.

Experiments were conducted with a commercially available six-cylinder water-cooled turbocharged direct-injection diesel engine. The cylinder head was modified to permit access to the combustion chamber with an engine videoscope. The engine was operated with base diesel fuel (BP-15) and other blends, base diesel with 20 wt% biodiesel (B-20) and with 20wt% diglyme (O-20). A neat biodiesel (B-100) and a 95 wt% blend of diglyme with base diesel fuel (O-95) were also considered. These fuels were used for observing the effect of the fuel properties on injection timing, heat release, flame structure, and luminosity. All the tests were performed with the engine operated at light load (61 N m, 10 per cent of the rated load) and 1800 r/min. Visualization showed that the start of injection occurred 0.4° earlier with B-100 than with BP-15. B-100 showed the earliest start of injection among the fuels. An earlier start of injection was also observed with B-20 and O-20 blends compared with BP-15 fuel. Combustion analysis showed a lower premixed combustion heat release rate with the diglyme blends compared with the B-20, B-100, and BP-15. The highest premixed burn peak and the lowest premixed burn peak were observed with BP-15 and O-95 fuels respectively. It is difficult to distinguish between the spray flames of BP-15, B-20, B-100, and O-20. However, with much higher oxygen content in the O-95 fuel the natural luminosity of the flame was too faint for detection with the camera. The combination of combustion analysis and in-cylinder visualization employed in this study provides a unique opportunity to understand how oxygenates behave in a commercial engine.

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