# Compatibility of Different Automotive Elastomers in Paraffinic Diesel Fuel
## 2 Materials and Methods  
The study of the impact of test fuels on rubber seals involved the injection system components specified below. They can be found in a common rail (CR) injection diesel engine: (i) CR injector overflow nozzle O-ring 3.2 \(\times\) 1.6--acrylonitrile-butadiene rubber (NBR) and (ii) CR injector overflow nozzle O-ring 3.2 \(\times\) 1.6--fluoroelastomer (FKM). The aforementioned elastomers have been selected because they are widely used in fuel supply systems, primarily in the fuel lines and O-rings. NBR is a product of butadiene and acrylonitrile copolymerization, wherein increasing the acrylonitrile content simultaneously increases resistance to fuel impact. In contrast, FKM is a copolymerization of monomers, the molecules of which contain fluorine. It exhibits high mechanical strength and resistance to damage, extreme temperatures and aggressive substances, including aromatic hydrocarbons. NBR and FKM rubbers are good as seals, since they maintain their shape when exposed to working fluids and are resistant to chemical substances [42]. The following fuel samples were selected:  
* Conventional diesel fuel, produced from crude oil without FAME--DF,
* Conventional diesel fuel, produced from crude oil with 7%(\(v/v\)) FAME--B7,
* Paraffinic diesel fuel, produced through synthesis from natural gas, by means of the gas to liquid process--GTL. B7 is a commercial fuel that is widely available at service stations. Diesel is also a fuel available at service stations. Moreover, it is primarily used by the armed forces, whereas GTL was produced from natural gas through the Fischer Tropsch synthesis. Synthetic fuel production process involves three key stages. These are synthesis gas production--an organic material is converted into synthesis gas through the gasification process; synthesis--synthesis gas is converted into liquid hydrocarbons with the use of steam and a catalyst, which leads to obtaining synthetic crude oil; and finally, hydrocracking and isomerization--synthetic raw material composed mainly of n-paraffins is converted to isoparaffins and separated through distillation into individual fuel fractions (diesel, jet), depending on carbon atom content [43]. The physico-chemical properties of fuel samples were investigated and are shown in Table 1.  
It should be noted that conventional and paraffinic diesel fuels are subject to different standards wherein their properties are specified. Paraffinic fuels are characterized by very good low-temperature and self-ignition properties, which result from the high content of paraffin hydrocarbons. The fact that there are only trace amounts of sulfur in the fuel positively impacts the environment during the combustion process. As already mentioned, GTL fuel does not contain aromatic hydrocarbons at all. Although they are characterized by good calorific values, their absence has a positive impact on unburned hydrocarbon and soot emissions during the combustion process. The properties of selected rubbers in terms of changes in mass, volume and hardness (IRHD scale) were tested in order to determine the impact of prepared fuels on selected O-rings. The following fuel-rubber interaction conditions were selected: (i) interaction time--168 h (7 days), room temperature (20 \({}^{\circ}\)C) and (iii) 100 mL of tested fuel in a closed, glass container. Percentage of mass change was calculated from:  
\[\Delta M=\frac{(M_{2}-M_{1})}{M_{1}}100\ [\%] \tag{1}\]  
where \(M_{1}\) is the initial O-ring mass in air [g] and \(M_{2}\) is the O-ring mass in air after immersion [g].