**3. Conclusions**

The model fuel as a petrol substitute composed of four hydrocarbons (heptane, isooctane, cyclohexane, toluene) was chemically stable at elevated temperature and oxygen pressure. The addition of a reactive olefin, i.e. cyclohexene caused a noticeable reduction in oxidation resistance determined by an induction period—the higher the content of this compound in the fuel, the lower the chemical resistance. The results of the induction period were confirmed by the study of potential resin content—as a result of oxidation, most deposits were formed in fuels with cyclohexene. Ethanol introduced in an amount of 5% v/v to the model fuel does not have a negative effect on the induction period, whereas the fuel with a cyclic olefin improved the value of this parameter. Antioxidants (phenolic and amine) added to the fuel with cyclohexene improved effectively oxidation resistance. The test of induction period of model fuels, where oxidation was simulated by the addition of TBHP as a source of reactive radicals, showed deterioration of the parameter sample cyclohexene.

Comparative studies conducted using commercial petrol have shown that the model fuel reflects behaviour of real fuel to a limited extent. The main drawback is the lack of a 'breaking' of MP-1 model fuel designating the induction period of accelerated oxidation tests in microscale. It can be assumed that in the model, fuel lacked representative olefin susceptible to oxidizing agents but less reactive than cyclohexane.

The diesel fuel was replaced by the model mixtures, which were prepared from three to four compounds, representative for hydrocarbon groups presented in commercial fuel. Applied hydrocarbons are characterized by a high resistance to oxidation (the induction period was over 60 min). Fatty acid methyl esters, which have been used in studies as components from renewable sources, were the least resistant to oxidation. The IR spectrophotometric analysis of individual hydrocarbons and model mixtures showed that oxidized products with carbonyl or hydroxyl binding (carboxylic acids, aldehydes, ketones) were formed as a result of oxidation.

During long-term ageing tests at elevated temperature, the M0 and M7 model mixtures are characterized by high stability, and the induction period varied slightly with time. Only for the M7 mixture was an increase in the amount of deposits after 6 months of storage, which probably was related with the presence of fatty acid methyl esters in the studied system.

In the case of long-term ageing test of commercial A and B fuels, an increase in the amount of deposits according to ASTM D5304 after 4 months of storage was observed. In contrast, the oxidative stability study using EN ISO 12205 method showed a fourfold and sevenfold increase in amount of deposits, respectively, for A and B samples, indicating that the degradation proceeds more quickly in the fuels with the higher FAME. The oxidation stability of A and B samples decreased with increasing time of storage.

Based on the obtained results, it can be concluded that the model blends of diesel fuel of the composition shown in **Table 9** may be used to predict the oxidative stability of diesel fuels using the method of the induction period EN 16091, and for the samples containing fatty acid methyl esters, the EN 15751 method can also be used. However, model mixtures cannot be used to predict the rate of degradation of diesel fuel expressed as the amount of filterable sediments and resins. The model mixtures, during long-term ageing process, generate a small amount of sediments constant at the same level.
