**1. Introduction**

Chemical stability of fuel, in terms of resistance to changes during storage and distribution, is an important parameter determining its quality. Processes deteriorating the fuel properties include, among others, oxidation. The oxidation products are chemically active substances and undergo further transformations to non-volatile macromolecular substance (resin), and the particulates precipitate in the form of deposits/sediments. Fuels, even under stable storage

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

conditions, can undergo autoxidation, where the hydrocarbon molecules react with atmospheric oxygen or with themselves [1].

phenalenes and indoles to indole/phenalene salt complexes, described by Marshman and Davia [8]. An acidic environment is conducive to the reactions. Phenalenes are formed as a result of the oxidation of active olefins, and indoles are a natural component of the fuel. The organic acids (necessary to catalyze the reaction) are typically presented in the fuel components or are formed by oxidation of the mercaptans to sulfonic acids. Li et al. [10] conducted research on the thermo-oxidative stability of aviation fuel. Oxidation processes were followed by determination of hydroperoxides and FTIR spectral analysis. The results confirmed the theory presented by Zabarnick [11] that the fuel oxidation occurs according to the mechanism of free radical chain reactions in which the hydroperoxides can be considered as intermediates for subsequent reactions of hydrocarbon autoxidation. During the tests it was observed that the fuel that contains more polar components is more readily oxidized. Normally, the

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The complexity of the hydrocarbon fuel oil composition is a difficult in-depth analysis of the degradation process of the fuel components and consequently the finished fuel which is a mixture of hundreds of compounds. The chemical composition of the fuel determines the physicochemical properties and influences on the engine operating characteristics [16]. Commercial fuels, depending on a producer, may differ from each other in terms of composition and physicochemical properties. Fuel oxidation and its stability issues were studied by numerous scientists for simple systems using pure hydrocarbons, representative for each fuel type or model mixtures. A key decision about the composition of the model fuel is a selection of the individual compounds—most often pure components representing a hydrocarbon groups (it is assumed that their chemical interactions are typical of other similar chemical

Gernigon et al. [17] conducted a study aimed at recognition of the oxidation mechanisms of aviation fuel. The research was performed for model mixtures containing such mono-compounds as n-dodecane, 1,3-diisopropylbenzene, cyclohexylbenzene and 2,2,4,4,6,8,8-heptamethylnonane in characteristic proportions of the natural composition of the fuel. Studies of the decomposition kinetics of hydrocarbons showed that after 48 hours, test diminished from 40 to 80% of each of mono-compounds. For all hydrocarbons, two stages of degradation were recorded: the first is the rapid decrease in the hydrocarbon content during the first hour of the experiment; a second step, after approx. 8 hours, is characterized by slow 'consumption' of hydrocarbon and has an almost linear behaviour. IR spectrum analysis indicated that the new structures are formed, because with increasing time, the degradation of characteristic band intensity, for the compounds formed by the oxidation (OH, C-O, C=O), increases. The

antioxidants used during the tests slowed down the hydrocarbons degradation.

Li et al. [10] used n-cetane of 99% purity as a reference fuel for spectroscopic studies of oxidized fuels. Natelson et al. [18] conducted a comparative study of aviation fuel and diesel reactivity with respect to mixtures of chemical compounds that have been selected as a model fuel (surrogate of diesel and aviation fuel). The studies described herein have focused on three substitutes, from three hydrocarbon groups (paraffins, naphthenes, aromatics), which are in the aircraft fuel and diesel. In order to maintain the small number of elements for modelling, one component from each of these groups was selected. N-decane (C10H22) was the

presence of polar components is the main factor of instability.

structures) are selected for the composing process.

There is an abundance of literature on fuel instability [2–13]. Nevertheless, reactions related with the degradation of the fuel are not well researched, so it is difficult to predict the rate and direction of changes of the fuel properties during storage and distribution. In addition, the diversity of the composition of hydrocarbon in commercial fuel species resulting from the variety types of processed crude oil and refinery technologies makes it difficult to determine the mechanisms of these changes. There are many theories describing the mechanisms of oxidation of petroleum products [14].

Generally, it is assumed that the fuels are oxidized by a radical mechanism, whereby the hydrocarbons are involved in radical chain reactions in the following sequence: initiation, propagation, branching and chain termination. The initiation reaction known as the initiation of an oxidative chain reaction begins with an attack of oxygen molecules from the air on the C-H bond in the hydrocarbon molecules. As a result, alkyl radicals (the radicals react with oxygen to hydroperoxides, which are aggressive oxidants) are formed. Decomposition products of peroxides are active alkoxy and hydroxyl radicals, which in the following reaction steps detach another hydrogen atoms from the hydrocarbons (propagation phase and chain branching). The last phase of hydrocarbon oxidation is termination, which is the completion of the reaction, where due to the recombination of hydrocarbon and peroxide, free radicals become deactivated creating non-radical products, for example, alcohols, ketones and acids, which can undergo further transformations to macromolecular substances. Among others, Denisov [15] described in detail oxidation of hydrocarbons in the liquid phase. However, we still do not know how an oxidation chain, in the absence of oxidation promoters such as sunlight, a source of free radicals and metal ions, is formed.

It is considered that the chemical structure of hydrocarbons has an influence on the rate of oxidation processes of petrol. The studies presented in the article [5] have shown that in petrol the tendency to produce resins depends on the amount and type of unsaturated hydrocarbons. In the case of petrol fractions from the cracking process, the presence of active radicals and trace amounts of metal ions originating from the catalyst was found. Pereira et al. [6] proved that not all of the olefins presented in the fuel are transformed equally to resins. Among tested olefins, those that formed the secondary allyl radicals (2,4-hexadiene and cyclohexene) had the largest share in the formation of macromolecular substances. These compounds are classified, respectively, to the unsaturated hydrocarbons with conjugated bonds and cyclic olefins. The considerations regarding to the stability of allyl and alkyl radicals and autoxidation reaction mechanism confirming the radical chain reactions were presented. Research of effect of ethanol and copper presence on the stability of petrol [7] showed that the addition of alcohol does not deteriorate fuel stability, whereas copper in the system is undesirable because it accelerates the oxidation processes.

However, in the case of diesel fuels, chemical instability is caused by the presence in the fuel compounds containing nitrogen and sulphur, reactive olefins and organic acids which are precursors of macromolecular structures having a limited solubility. One of the wellrecognized mechanisms generating insoluble precipitates in the diesel fuel is conversion of phenalenes and indoles to indole/phenalene salt complexes, described by Marshman and Davia [8]. An acidic environment is conducive to the reactions. Phenalenes are formed as a result of the oxidation of active olefins, and indoles are a natural component of the fuel. The organic acids (necessary to catalyze the reaction) are typically presented in the fuel components or are formed by oxidation of the mercaptans to sulfonic acids. Li et al. [10] conducted research on the thermo-oxidative stability of aviation fuel. Oxidation processes were followed by determination of hydroperoxides and FTIR spectral analysis. The results confirmed the theory presented by Zabarnick [11] that the fuel oxidation occurs according to the mechanism of free radical chain reactions in which the hydroperoxides can be considered as intermediates for subsequent reactions of hydrocarbon autoxidation. During the tests it was observed that the fuel that contains more polar components is more readily oxidized. Normally, the presence of polar components is the main factor of instability.

conditions, can undergo autoxidation, where the hydrocarbon molecules react with atmo-

There is an abundance of literature on fuel instability [2–13]. Nevertheless, reactions related with the degradation of the fuel are not well researched, so it is difficult to predict the rate and direction of changes of the fuel properties during storage and distribution. In addition, the diversity of the composition of hydrocarbon in commercial fuel species resulting from the variety types of processed crude oil and refinery technologies makes it difficult to determine the mechanisms of these changes. There are many theories describing the mechanisms of

Generally, it is assumed that the fuels are oxidized by a radical mechanism, whereby the hydrocarbons are involved in radical chain reactions in the following sequence: initiation, propagation, branching and chain termination. The initiation reaction known as the initiation of an oxidative chain reaction begins with an attack of oxygen molecules from the air on the C-H bond in the hydrocarbon molecules. As a result, alkyl radicals (the radicals react with oxygen to hydroperoxides, which are aggressive oxidants) are formed. Decomposition products of peroxides are active alkoxy and hydroxyl radicals, which in the following reaction steps detach another hydrogen atoms from the hydrocarbons (propagation phase and chain branching). The last phase of hydrocarbon oxidation is termination, which is the completion of the reaction, where due to the recombination of hydrocarbon and peroxide, free radicals become deactivated creating non-radical products, for example, alcohols, ketones and acids, which can undergo further transformations to macromolecular substances. Among others, Denisov [15] described in detail oxidation of hydrocarbons in the liquid phase. However, we still do not know how an oxidation chain, in the absence of oxidation promoters such as

It is considered that the chemical structure of hydrocarbons has an influence on the rate of oxidation processes of petrol. The studies presented in the article [5] have shown that in petrol the tendency to produce resins depends on the amount and type of unsaturated hydrocarbons. In the case of petrol fractions from the cracking process, the presence of active radicals and trace amounts of metal ions originating from the catalyst was found. Pereira et al. [6] proved that not all of the olefins presented in the fuel are transformed equally to resins. Among tested olefins, those that formed the secondary allyl radicals (2,4-hexadiene and cyclohexene) had the largest share in the formation of macromolecular substances. These compounds are classified, respectively, to the unsaturated hydrocarbons with conjugated bonds and cyclic olefins. The considerations regarding to the stability of allyl and alkyl radicals and autoxidation reaction mechanism confirming the radical chain reactions were presented. Research of effect of ethanol and copper presence on the stability of petrol [7] showed that the addition of alcohol does not deteriorate fuel stability, whereas copper in the system is undesirable because it

However, in the case of diesel fuels, chemical instability is caused by the presence in the fuel compounds containing nitrogen and sulphur, reactive olefins and organic acids which are precursors of macromolecular structures having a limited solubility. One of the wellrecognized mechanisms generating insoluble precipitates in the diesel fuel is conversion of

spheric oxygen or with themselves [1].

2 Improvement Trends for Internal Combustion Engines

oxidation of petroleum products [14].

accelerates the oxidation processes.

sunlight, a source of free radicals and metal ions, is formed.

The complexity of the hydrocarbon fuel oil composition is a difficult in-depth analysis of the degradation process of the fuel components and consequently the finished fuel which is a mixture of hundreds of compounds. The chemical composition of the fuel determines the physicochemical properties and influences on the engine operating characteristics [16]. Commercial fuels, depending on a producer, may differ from each other in terms of composition and physicochemical properties. Fuel oxidation and its stability issues were studied by numerous scientists for simple systems using pure hydrocarbons, representative for each fuel type or model mixtures. A key decision about the composition of the model fuel is a selection of the individual compounds—most often pure components representing a hydrocarbon groups (it is assumed that their chemical interactions are typical of other similar chemical structures) are selected for the composing process.

Gernigon et al. [17] conducted a study aimed at recognition of the oxidation mechanisms of aviation fuel. The research was performed for model mixtures containing such mono-compounds as n-dodecane, 1,3-diisopropylbenzene, cyclohexylbenzene and 2,2,4,4,6,8,8-heptamethylnonane in characteristic proportions of the natural composition of the fuel. Studies of the decomposition kinetics of hydrocarbons showed that after 48 hours, test diminished from 40 to 80% of each of mono-compounds. For all hydrocarbons, two stages of degradation were recorded: the first is the rapid decrease in the hydrocarbon content during the first hour of the experiment; a second step, after approx. 8 hours, is characterized by slow 'consumption' of hydrocarbon and has an almost linear behaviour. IR spectrum analysis indicated that the new structures are formed, because with increasing time, the degradation of characteristic band intensity, for the compounds formed by the oxidation (OH, C-O, C=O), increases. The antioxidants used during the tests slowed down the hydrocarbons degradation.

Li et al. [10] used n-cetane of 99% purity as a reference fuel for spectroscopic studies of oxidized fuels. Natelson et al. [18] conducted a comparative study of aviation fuel and diesel reactivity with respect to mixtures of chemical compounds that have been selected as a model fuel (surrogate of diesel and aviation fuel). The studies described herein have focused on three substitutes, from three hydrocarbon groups (paraffins, naphthenes, aromatics), which are in the aircraft fuel and diesel. In order to maintain the small number of elements for modelling, one component from each of these groups was selected. N-decane (C10H22) was the representative of paraffins, n-butyl cyclohexane (C10H20) was representative of naphthenes, and n-butylbenzene (C10H14) was a representative of aromatics. The surrogates have been used in respective proportions, depending on the structure and key features of the real fuel. The surrogate reactivity was compared to the average of ordinary aviation fuel and commercial diesel used in the United States.

determined by the pressure drop of the test system by 10% compared to the maximum pressure registered during oxidation. The fuel ageing process was performed at 140°C, the initial oxygen pressure at 500 kPa and the amount of sample at 5 ml. The principle of measurement in the above-mentioned norm is similar to the principle used in the Standard ISO 7536 [19]. The micro method is less labour intensive than the ISO 7536 standard, which normally is used

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The chemical stability of selected petrol model mixtures was also evaluated by the ASTM D 873 method [26]. It determines the fuel potential to create resins and sediments. In this method, petrol is oxidized by 4 hours at 100°C in oxygen atmosphere at a pressure of 690–705 kPa. The test result is given as the contents of potential resins (the total amount of insoluble and soluble resins). Insoluble resins—precipitate adjacent to the glass wall of the test cell, from which the oxidized fuel was removed, precipitation and soluble resins—are determined by weight gain of test cell after the study, compared to the mass of clean test cell before the examination. The oxidation products present in the fuel after ageing in dissolved form in the oxidized fuel or as deposits adhering to the walls of the test cell, soluble in toluene-acetone,

Four most commonly used methods of testing the degree of diesel oils and their surrogate degradation were selected. All of them are methods of accelerated ageing. Since one of the characteristics of the fuel ageing process is the deposit formation, two of this type of tests were selected for the research, including ASTM D 5304 [21] and EN ISO 12205 [22], where the amount of insoluble filterable sediments precipitated during the test and retained on the filter and the resin adhering to the walls of the test cell is determined. At the same time, to verify the theory of the possibility of the fuel degradation without producing deposits, it was decided on two another oxidation stability tests EN 16091 [24] and EN 15751 [23], where the result is presented in minutes or hours. The EN 15751 test is dedicated to a fuel containing at least 2% v/v of fatty acids methyl esters, and the induction period measured by this method is the time from the start of measurement to the moment, where the formation of oxidation products

ASTM D 5304—temperature, 90°C; oxygen atmosphere at a pressure of 800 kPa; time, 16 hours

Spectrophotometric analysis of model samples or their components, before and after accelerated ageing by PetroOxy test, was carried out using a Magna 750 FT-IR spectrophotometer (Nicolet). The spectra measurements were performed in KBr cuvette having a thickness of

Long-term ageing test was carried out to determine the oxidative stability of the fuel during the long-term storage. The fuel samples were placed in a thermostatic chamber at 45°C in

EN 16091 (called PetroOxy test)—temperature, 140°C; oxygen pressure of 700 kPa

recorded by changes in conductivity begins rapidly increase.

EN ISO 12205—temperature, 95°C; oxygen barbotage; time, 16 hours

EN 15751 (called Rancimat test)—temperature, 110°C; air flow

The tests conditions were as follows:

0.065 mm in the range of 4000 to 400 cm−1.

to assess the stability of commercial petrol.

are soluble resins.

The authors of following research conducted the study on the stability of diesel fuel during long-term storage and the results presented in the work [1]. In the case of petrol, after 6-month storage, in the IR spectra, slight change in the chemical composition was found. The oxidation-promoting substances, i.e. cyclic olefins and hydroperoxide intentionally added to the stored fuel, slightly influenced on intensification of transformation process. The study of induction period and potential resins content showed the potential adverse changes in petrol stability, which were poorly correlated with changes demonstrated by IR spectroscopy. However, in diesel fuel samples after 3–4 months of storage, there was a significant increase in the content of sediments. The IR spectra confirmed the changes in the chemical composition of the fuel. In the stored samples appeared degradation products of hydrocarbons, containing functional groups characteristic for acids, aldehydes, ketones and phenols, conducive to further reactions leading to the formation of indole/phenalenes.

Aforementioned studies were carried out for commercial fuels containing hundreds of chemical components of different elemental compositions and structures. Standard tests determining the petrol stability which include induction period [19] and resins content [20] are laborious and time-consuming. Also, the tests do not allow for early detection of changes in the fuel stability, which are visible on the IR spectra. The situation is similar in the assessment of stability of middle distillates; the scientists have much more methods to evaluate this parameter [21–24]. The authors have attempted to assess the stability of petrol using a quick and simple method of induction period at the micro level. To eliminate the influence of a large number of compounds presented in petrol and the interactions between them, the study was carried out for the model mixture replacing commercial petrol. Furthermore it was checked how the presence of the chemically unstable component (cyclohexene) changes the oxidation stability of model petrol. The induction period for the model mixture with the addition of oxygenated compound (ethanol) that improves the octane number was also determined. The studies relating to the stability was conducted for the model fuel replacing diesel. Due to the fact that during storage, middle distillates more rapidly degraded than petrol fractions, in addition to conventional ageing tests to assess the stability of the fuel at the time of storage, an accelerated ageing test at 45°C based on ASTM D 4625 [25] was conducted. The increased temperature helps to speed up the reactions occurring in the fuel during storage under conventional conditions.
