# Oxygenated Diesel Fuels and Their Effect on PM Emissions
## 1 Introduction
That is, in about 90% of the cases, an oxygen atom with a single bond to adjacent carbon atoms, i.e., at positions B and D, eventually fuses to the carbon at position B, directly forming CO\({}_{2}\). Other studies using the labeled atoms method are described in [51]. In these tests, the research engine was fueled by mixtures of diesel fuel and ethanol containing from 9% to 37% of this alcohol. Analysis of the obtained particulate matter showed that the carbon contained in the ethanol also participates in the formation of PM. However, the probability of soot formation is 50% lower for ethanol carbon atoms than for diesel fuel carbon atoms. [PERSON] et al. [52] presents the results of modeling soot formation during the combustion of n-heptane with the addition of oxygenated compounds such as dimethoxmethane and ethanol. As shown in this study, the addition of oxygenates causes significant drops in peak concentrations of soot precursors in the rich mixture kinetic combustion zone, which in turn inhibits the formation of aromatic compounds and, as a result, reduces the intensity of soot formation. The concentration of acetylene and benzene decreases approximately linearly with increasing oxygen content in the fuel, while the concentration of propargyl begins to decrease significantly after exceeding 20% of oxygen in the fuel. According to model studies in [52], ethanol is slightly more effective than dimethoxmethane in reducing acetylene and benzene concentrations. For the same amount of oxygen in the fuel, but introduced with the addition of ethanol, greater decreases in the concentration  
Figure 2: Nomenclature of the positions of carbon atoms in the dibutyl maleate molecule. of soot in the diffusion flame were noted. At lower oxygen concentrations, the soot concentration decreased linearly with increasing oxygen content; after exceeding about 15% of oxygen, its influence became weaker. The combustion was soot-free when the fuel contained approximately 35% of oxygen. Oxygenated compounds are less likely to form soot precursors than conventional hydrocarbons, which results, inter alia, from the fact that the C-O bond is stronger than the C-C bond. However, an additional and probably more important factor is the increase in the concentrations of radicals such as O, OH and HCO in the combustion chamber resulting from the introduction of oxygenates. The increased supply of these radicals influences the soot formation process in two ways. First, high concentrations of O and OH promote the oxidation of carbon atoms in the flame, which reduces the amount of carbon that can form soot precursors. The formation of large amounts of HCO works similarly as it is quickly converted to CO or CO\({}_{2}\) in a single reaction. Second, an increased concentration of radicals, in particular OH, in the soot formation region downstream of the rich mixture combustion zone inhibits soot nucleation by oxidizing aromatics and limiting the development of PAHs. The aforementioned slightly better effectiveness of ethanol than that of dimethoxymethane in reducing the concentration of soot is probably due to the fact that significantly more OH radicals are formed with the addition of ethanol. For dimethoxymethane, more HCO radicals are formed, which also reduces the concentration of soot precursors, but they are not as effective in the subsequent suppression of the expansion of aromatic compounds [52]. As already mentioned, the formation of PM depends on the specific local temperature and the local air-fuel ratio (\(\lambda\) coefficient). The soot formation does not occur at too low a temperature and requires the production of precursors. Soot precursors oxidize at higher temperatures, and therefore the soot formation process is limited by a temperature range of about 1500-2500 K. Due to its high heat of vaporization, N-decanol was used as a fuel by [PERSON] et al. [53] and lowered the local temperatures in the reaction zone, reducing the amount of soot released.