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.