# Oxygenated Diesel Fuels and Their Effect on PM Emissions
## 1 Introduction
A decrease in the concentrations of C2-C4 intermediates and a very significant decrease in C5-C10 products were observed. Confirmation of the reduction of soot precursors by oxygenates was also obtained by modeling the kinetics of chemical reactions occurring during the combustion of oxygenated fuels [36; 37]. Particular attention should be paid to the extensive work by [PERSON] et al. [38]. They based their modeling of kinetics on the combustion model in a diesel engine developed by [PERSON] [39]. According to this model, the self-ignited fuel is not fully oxidized due to insufficient oxygen availability, resulting in the presence of partial oxidation products such as CO, H\({}_{2}\) and simple hydrocarbon products, including acetylene, ethylene, propylene and others, that in effect lead to soot formation. Due to the very complex composition of conventional diesel fuel, [PERSON] adopted n-heptane as a diesel surrogate in his modeling work. The modeling covered self-ignition and combustion of a pre-mixed, rich mixture, with the initial simulation conditions: T = 770 K, p = 10 MPa and \(\lambda\approx 0.33\). After a short period of auto-ignition delay, the temperature rapidly increased to about 1600 K and the products of this combustion phase were released. The described research showed that about 44% of the carbon contained in the fuel and participating in this combustion phase is converted into unsaturated soot precursors, mainly ethylene, acetylene, propylene and propyne. These products form simple aromatic compounds and then soot. In the analysis of the mechanism of the influence of oxygenated compounds, the identification of products formed after the ignition of a rich mixture which increase and reduce the emission of soot plays an important role. The soot formation kinetics, which are described in the works of [PERSON] et al. (e.g., [40; 41]), are that simple aromatic and polycyclic hydrocarbons, such as benzene, toluene, naphthalene, pyrene and styrene, are formed from simple unsaturated hydrocarbons, such as: acetylene, ethylene, propylene, allene, propyne and cyclopentadiene, and also from hydrocarbon radicals such as propargyl, allyl, methyl allyl and cyclopentadienyl. The authors of [42; 43] described changes in the concentrations of acetylene, ethylene and propargyl inside the diffusion flame caused by adding oxygenated compounds to the fuel. Calculations made by [PERSON]'s team [38] for mixtures containing oxygenates showed a reduction in the content of soot precursors in the combustion products of a rich mixture, proportional to the amount of oxygen in the fuel. At the same time, an increase in the amount of CO and CO\({}_{2}\) was noted, whereas oxygen in this phase arose mainly in the form of CO and H\({}_{2}\)O. The combustion temperature also increased due to the composition of the mixture closer to the stoichiometric one. The concentration of the soot precursors dropped to a very low level for 30-40% oxygen content in fuel. The calculation of PAH concentration in rich mixture combustion products using the model developed by [PERSON] et al. [44] showed that the concentration of PAHs also decreased to a very low level with a fuel oxygen content of about 30%. [PERSON] and [PERSON] showedthat soot formation ceases when the local excess-air coefficient \(\lambda\) at ignition exceeds the value of 0.5 [45]. Detailed analysis of the ignition kinetics of oxygenated compounds shows that most of the oxygen atoms initially contained in the molecules of these compounds react directly to produce CO. The strong bond of CO is not broken during ignition; hence, the bonded C atom is not involved in the formation of soot. For example, for the simplest oxygenated compound, methanol, the reaction has the following course [46]:  
\[\mathrm{CH_{3}OH}\rightarrow\mathrm{CH_{2}OH}\rightarrow\mathrm{CH_{2}O} \rightarrow\mathrm{HCO}\rightarrow\mathrm{CO}. \tag{1}\]  
For more complex oxygenates, the reactions are more complicated.