# Oxygenated Diesel Fuels and Their Effect on PM Emissions
## 1 Introduction
It is very important to properly prepare the air-fuel mixture, which can be influenced by appropriate selection of injection timing, injection pressure, and multiple and auxiliary injections. For example, higher injection pressure leads to better atomization and better air-fuel mixture homogeneity, which results in finer combustion characteristics leading to PM reduction [12]. A significant reduction in PM emissions is achieved through the use of particulate filters, which, however, must be regenerated. Lowering the soot burning temperature is achieved by using appropriate catalytic layers in filters or catalytic fuel additives. For about 30 years, the limits of PM emissions from vehicles have been regularly reduced [13; 14; 15]. Vehicles of higher emission classes are assumed to emit less PM, with studies showing that they are also less harmful [16]. The type and properties of the fuel used have a very important role in reducing emissions [17; 18; 19; 20; 21]. The type of fuel used also affects the toxicity of PM emitted [22]. In the case of conventional diesel fuel, parameters such as density, viscosity, cetane number, PAH content, distillation and sulfur content are important for PM emissions [17; 23]. However, the possibilities of modifying these parameters have already been used to a large extent and their further modification is very limited. For example, it is difficult to further reduce the sulfur content once it is close to zero. Therefore, in order to use the potential of fuels to reduce the toxicity of diesel exhaust gases to an even greater extent, unconventional modifications of their properties are necessary. One of the promising directions of this type of activity is the use of oxygenated fuels, i.e., the introduction of oxygen carrier components (oxygenated compounds) into the composition of diesel fuels. Due to the specificity of combustion in a diesel engine, i.e., non-homogeneity of the air-fuel mixture and local oxygen deficiencies favoring the formation of PM, the emission benefits associated with the use of oxygenated fuels in diesel engines should be significant. This paper is devoted to the issue of the influence of oxygenated fuels on PM emissions from diesel engines. The main objective of this study is the experimental verification of the influence of oxygenated compounds addition to diesel fuel on PM emissions under dynamic driving tests. These tests correspond well to the conditions of actual car operation and provide an indication of the most favorable oxygenates' variants. The results of our research are discussed against the background of the works of other researchers. An additional objective of this study is to collect and analyze views on the mechanism of the influence of oxygenated compounds on PM (soot) formation in diesel engines. What distinguishes this study is, on the one hand, the fact that as many as 12 oxygenated compounds from 4 chemical groups were tested. On the other hand, emissions tests were performed under dynamic cycles on a chassis dynamometer, i.e., conditions close to the actual operation, which allows for a reliable inference about the possible environmental benefits resulting from oxygenated fuels application in practice. The Mechanism of the Influence of Fuel Oxygenates on the Process of Soot Formation in Diesel Engines  
An increased interest in oxygenated fuels since the mid-1990s and the significant reduction in black smoke and PM emissions via the use of oxygenated compounds [24; 25; 26; 27; 28; 29], found in research undertaken at that time, prompted researchers to seek a deeper understanding of the mechanism of the influence of these compounds on the soot formation process in diesel engines. This mechanism is related to the oxygen contained in the molecules of oxygenated compounds, but there were many unknowns--for example, at which stage of the formation of particulate matter and how the interaction of oxygenates takes place, whether oxygen introduced with any oxygenated compound is equally effective, etc.