# Oxygenated Diesel Fuels and Their Effect on PM Emissions
## 1 Introduction
Figure 1 shows one of the variants of decomposition of the tripropylene glycol monomethyl ether molecule. The products of this process are: two formaldehyde molecules, two propylene molecules, one acetaldehyde molecule and one OH radical [38]. Three of the four oxygen atoms are attached to carbon atoms, which hence do not participate in the formation of soot. The OH radical participates in the oxidation of soot precursors. The structure of the oxygenated compound, for example, single or double bonds between carbon and oxygen atoms, or the distribution of oxygen atoms in the molecule, may influence its effectiveness in reducing PM emissions. As a result of the research of [PERSON] et al. [47; 48], it was found that ester-type oxygenates are less effective in soot inhibition than ether-type compounds. The modeling of the kinetics of chemical reactions described in [49] showed that dibutyl maleate is less effective in the reduction of soot formation because of the direct formation of CO\({}_{2}\) by some of the oxygen atoms in its molecule. The preventive effect of oxygen could extend to two carbon atoms if the same amount of oxygen were used to form two CO molecules. Additionally, it is unfavorable for the dibutyl maleate that the two carbon atoms in its molecule, connected by a double bond, directly form the soot precursor acetylene. In [PERSON] et al. [50], the above-mentioned observations have been verified by accelerated mass spectroscopy. This method allows for the labeling of individual carbon atoms in the molecule, and then experimental tracking of their fate in the combustion process. For the purposes of this experiment, three batches of dibutyl maleate were synthesized, and in each of them, carbon atoms in the positions indicated in Figure 2 were substituted with isotope \({}^{14}\)C. Due to the very low content of the isotope \({}^{14}\)C in petroleum products (\({}^{14}\)C/C \(<10^{-15}\)), most of this isotope in the analyzed combustion products (PM, CO\({}_{2}\)) came from labeled atoms. Figure 1: One of the variants of the tripropylene glycol monomethyl ether molecule decomposition in the process of its ignition. If all oxygen atoms in the fuel molecule react according to the same scheme during combustion, the proportion of individually labeled atoms in the combustion products should be the same as in the fuel. The results presented in [50] indicate that this is not the case with dibutyl maleate. Carbon atoms in the dibutyl maleate molecule in the 1,4-positions of the maleate group (B) are not present at all in the composition of particulate matter but are entirely present in the form of CO\({}_{2}\) in exhaust gas. This confirms the results presented in [48; 49] in which the carbon-oxygen double bond in the ester molecule is not broken during combustion. Carbon atoms in the dibutyl maleate molecule in the 2,3 positions in the maleate group (A), despite the presence of a double bond, show the same tendency to soot formation as the average carbon atom contained in the tested fuel. This indicates that unsaturated hydrocarbon bonds do not necessarily favor more soot formation if sufficient oxygen is available. In the analyzed case, the tendency of the carbon double bond to produce soot is inhibited by the adjacent oxygen atoms bound to the carbon atoms in position B. The connection of carbon atoms with oxygen atoms means for the adjacent carbon atoms, not linked to oxygen atoms, a better availability of gaseous oxygen present in the air-fuel mixture. Carbon atoms in the dibutyl maleate molecule in the 1 positions of the butyl group (D), assuming the best use of the oxygen contained in the molecule of this compound during combustion, should remain connected with the neighboring oxygen atoms, first forming CO, which is later, already under lean conditions, oxidized to CO\({}_{2}\). It transpires that the carbon atoms in the D positions form PM only to a slightly lesser extent than the average carbon atom contained in the fuel.