# Oxygenated Diesel Fuels and Their Effect on PM Emissions
## 1 Introduction
Similar issues are related to the research using air with a higher than atmospheric oxygen content (e.g., [30; 31; 32; 33]), which led to a locally leaner air-fuel mixture. However, the significant difference is that in oxygenated compounds, oxygen is present in a bonded form, most often with a very strong oxygen-carbon bond. One of the first studies explaining the mechanism of the influence of oxygenated compounds on PM emission was presented by [PERSON] and [PERSON] [34]. Their research included the analysis of oxygenates' influence on the process of soot precursors formation in a diffusion flame. A stationary diffusion flame generated in a counter-flow burner was used in this experiment. The fuel stream consisted of propane and nitrogen, with the oxidant stream composed of a mixture of oxygen and atmospheric air. The overall composition of the mixture was rich (\(\lambda\approx 0.55\)). Before being introduced into the flame, the oxygenated component was vaporized by heating in a bath. Two oxygenated compounds were used in the research: dimethyl carbonate and ethanol. In preliminary tests, it was found that within the flame, the concentration of fuel, i.e., propane, drops sharply, while intermediate products such as ethylene, methane, ethane, acetylene and propylene appear. The presence of n-butane, n-hexane, propyne and benzene was also found. The addition of 10% \(v/v\) of dimethyl carbonate to the fuel stream resulted in a significant reduction of intermediate products content in the flame, including soot precursors: acetylene by 15% and benzene by 27%. Increasing the content of dimethyl carbonate to 15% resulted in an even greater reduction in the content of acetylene and benzene, by 28% and 50%, respectively. By introducing ethanol into the fuel, a reduction in the concentration of acetylene and benzene was also achieved, but it was clearly lower than with the addition of dimethyl carbonate. In further work [34], [PERSON] and [PERSON] analyzed the potential mechanisms of reducing acetylene concentration in a flame in the presence of oxygenated compounds. Flame temperature change was indicated as a possible mechanism of this phenomenon. The adiabatic propane flame temperature is actually lower in the presence of oxygen compounds. However, the lowest flame temperature was found for ethanol, and nonetheless a clearly higher concentration of acetylene than for dimethyl carbonate was observed. This indicates that the flame temperature is not critical to the acetylene concentration. The reduction of the carbon content in fuel, which only occurs with the addition of ethanol, is also not of significant importance; the dimethyl carbonate molecule contains three carbon atoms, as does the propane molecule. An important factor in reducing the concentration of acetylene is the oxygen content in the fuel. The concentration of acetylene in the flame decreases approximately linearly with increasing oxygen content in the fuel. According to [PERSON] and [PERSON] [34], the reduction of acetylene concentration takes place due to the presence of additional oxygen atoms in the areas of the rich mixture within the flame. The pyrolysis of oxygenated compounds in this area leads to the emission of O atoms and OH radicals. They then react with acetylene and other propane pyrolysis products, promoting their oxidation. The O atoms and OH radicals compete with the H atoms, which play a major role in the fuel pyrolysis process, thus changing the propane pyrolysis process towards lower acetylene production. The amount of acetylene produced presumably also depends on the number of C-C bonds of the fuel components. It is worth noting that dimethyl carbonate does not contain such bonds at all, while there is one such bond in the ethanol molecule and two in propane. Similar to those presented above, the results of research on the reduction of soot precursor concentrations were obtained by [PERSON] et al. [35] by examining ethylene flame (rich mixture, \(\lambda\approx 0.4\)). The oxygenated component used was dimethyloxmethane.