**2. Tests of chemical stability research of model blends**

The induction period (micro method) based on the EN 16091 [24] standard was used to evaluate the chemical stability of model fuels (replacing petrol). In the micro method, induction period is defined as the time elapsed from the start of the study to the critical point 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 to assess the stability of commercial petrol.

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, are soluble resins.

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 recorded by changes in conductivity begins rapidly increase.

The tests conditions were as follows:

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 commer-

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

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 con-

further reactions leading to the formation of indole/phenalenes.

**2. Tests of chemical stability research of model blends**

The induction period (micro method) based on the EN 16091 [24] standard was used to evaluate the chemical stability of model fuels (replacing petrol). In the micro method, induction period is defined as the time elapsed from the start of the study to the critical point

cial diesel used in the United States.

4 Improvement Trends for Internal Combustion Engines

ventional conditions.

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

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

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

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

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

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 vented glass bottles. At set intervals, another bottle with the fuel was removed from the chamber and deposit content was determined.

Induction period of hydrocarbons and ethyl alcohol is shown in **Table 2**. Among the tested compounds, the induction period using micro method was determined for cyclohexane, cyclohexene and ethanol. In the case of other pure compounds, after 360 min (6 hours) of test, there was no pressure drop meeting the criteria of the critical point. For comparison, induction period of fuel fractions, which are used for composing petrol, was determined. The tests were performed for three refinery fractions—the cracked fractions comprising about 20% v/v of olefins and aromatics, alkylate consisting only of paraffins and reformate—which the major component was aromatic compounds. The test results indicate that, among petroleum fractions, the cracked fraction containing unsaturated hydrocarbons undergoes the fastest oxidation.

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The values of the induction period for model mixtures determined by a micro method are shown in **Table 3**. Comparatively, the results for the commercial petrol of 95 research octane numbers (95RON) and (98RON), to which deliberately introduced 3% v/v cyclohexene

**MP-1 MP-2 MP-3 MP-4 MP-5 95RON 95RON + OLEF 98RON 98RON + OLEF**

The MP-1 hydrocarbon fuel was chemically stable. The addition of cyclohexene, which, according to the literature [6], is easily oxidized, affects the chemical stability of the MP-1 model fuel. The higher the content of this compound, the shorter the induction period of MP-3 and MP-4 mixture. The effect of oxygen compound (ethanol) to this parameter was also

Not determ\* Not determ\* 91.5 88.75 Not determ\* 44.1 34.9 47.9 39.8

(OLEF), were presented in the article. Petrol 95 RON contained 5% v/v ethanol.

**Model petrol Commercial petrol**

**Components Induction period at 140°C, (EN 16091) min**

Heptane Not determined\* Isooctane Not determined\*

Toluene Not determined\*

**Table 2.** Induction period of pure hydrocarbons, ethanol and fuel fractions.

\*Recorded pressure drop was less than 10% after 360 min of the test.

**Table 3.** Induction period determined by micro methods according to PN-EN 16091.

Induction period, min

Cyclohexane 311.12

Cyclohexene 11.31 Ethanol 298.17 Cracked naphtha 33.5 Alkylate 63.5 Reformate 79.9 \*Recorded pressure drop was less than 10% after 360 min of the test.

## **2.1. Model blend replacing petrol: stability test results**

For the preparation of model petrol, the following hydrocarbons were used: n-heptane representing paraffins; isooctane, branched-chain alkanes; cyclohexane, cyclic alkane; toluene, aromatic compound; and cyclohexene, olefin. A non-hydrocarbon component—anhydrous ethanol—which is added to petrol to increase the octane number was also used. The organic peroxide (tertiary—butyl hydroperoxide TBHP) dosed in an amount of 50 mmol/l was used to prepare and simulate the oxidized fuel [27]. The two antioxidants, one being a mixture of sterically hindered phenols (AO-1) and the second—phenylenediamine (AO-2)—were used to improve the oxidation resistance of the selected compositions.

In addition, commercial petrol with research octane numbers 95 (95 RON) and 98 (98 RON) and fuel fractions used for petrol composing, i.e. cracked naphtha, alkylate and reformate were studied. Petrol 95 RON contained ethanol.

The composition of tested model petrol for ageing tests is shown in **Table 1**. Data concerning the major components of the commercial petrol in Europe presented in the publication [28] were used to determine the volume fractions of various hydrocarbons. The MP\_2 mixture was prepared by mixing a volume ratio of 95:5 MP\_1 petrol with anhydrous ethanol. Similarly, the MP\_3 and MP\_4 fuels were prepared, whereby cyclohexane was added instead of alcohol. The MP\_5 fuel consisted of 90% v/v MP\_1, 5% v/v ethyl alcohol and 5% v/v cyclohexene.


**Table 1.** Chemical composition of model petrol for ageing tests.

Induction period of hydrocarbons and ethyl alcohol is shown in **Table 2**. Among the tested compounds, the induction period using micro method was determined for cyclohexane, cyclohexene and ethanol. In the case of other pure compounds, after 360 min (6 hours) of test, there was no pressure drop meeting the criteria of the critical point. For comparison, induction period of fuel fractions, which are used for composing petrol, was determined. The tests were performed for three refinery fractions—the cracked fractions comprising about 20% v/v of olefins and aromatics, alkylate consisting only of paraffins and reformate—which the major component was aromatic compounds. The test results indicate that, among petroleum fractions, the cracked fraction containing unsaturated hydrocarbons undergoes the fastest oxidation.


**Table 2.** Induction period of pure hydrocarbons, ethanol and fuel fractions.

vented glass bottles. At set intervals, another bottle with the fuel was removed from the cham-

For the preparation of model petrol, the following hydrocarbons were used: n-heptane representing paraffins; isooctane, branched-chain alkanes; cyclohexane, cyclic alkane; toluene, aromatic compound; and cyclohexene, olefin. A non-hydrocarbon component—anhydrous ethanol—which is added to petrol to increase the octane number was also used. The organic peroxide (tertiary—butyl hydroperoxide TBHP) dosed in an amount of 50 mmol/l was used to prepare and simulate the oxidized fuel [27]. The two antioxidants, one being a mixture of sterically hindered phenols (AO-1) and the second—phenylenediamine (AO-2)—were used

In addition, commercial petrol with research octane numbers 95 (95 RON) and 98 (98 RON) and fuel fractions used for petrol composing, i.e. cracked naphtha, alkylate and reformate

The composition of tested model petrol for ageing tests is shown in **Table 1**. Data concerning the major components of the commercial petrol in Europe presented in the publication [28] were used to determine the volume fractions of various hydrocarbons. The MP\_2 mixture was prepared by mixing a volume ratio of 95:5 MP\_1 petrol with anhydrous ethanol. Similarly, the MP\_3 and MP\_4 fuels were prepared, whereby cyclohexane was added instead of alcohol. The MP\_5 fuel consisted of 90% v/v MP\_1, 5% v/v ethyl alcohol and 5%

MP\_1 – 14.5 36.5 11.7 37.3 – –

**Heptane Isooctane Cyclohexane Toluene Cyclohexene Ethanol**

13.8 34.8 11.1 35.4 – 5.0

13.8 34.8 11.1 35.4 5.0 –

13.5 34.0 10.8 34.7 7.0 –

13.0 32.9 10.5 33.6 5.0 5.0

ber and deposit content was determined.

6 Improvement Trends for Internal Combustion Engines

**2.1. Model blend replacing petrol: stability test results**

to improve the oxidation resistance of the selected compositions.

were studied. Petrol 95 RON contained ethanol.

**Symbol Composition Components (% v/v)**

**Table 1.** Chemical composition of model petrol for ageing tests.

v/v cyclohexene.

MP\_2 95% v/v MP\_1 5% v/v Ethanol

MP\_3 95%v/v MP\_1 5% v/v Cyclohexene

MP\_4 93%v/v MP\_1 7% v/v Cyclohexene

MP\_5 90%v/v MP\_1 5% v/v Cyclohexene 5% v/v Ethanol

The values of the induction period for model mixtures determined by a micro method are shown in **Table 3**. Comparatively, the results for the commercial petrol of 95 research octane numbers (95RON) and (98RON), to which deliberately introduced 3% v/v cyclohexene (OLEF), were presented in the article. Petrol 95 RON contained 5% v/v ethanol.


**Table 3.** Induction period determined by micro methods according to PN-EN 16091.

The MP-1 hydrocarbon fuel was chemically stable. The addition of cyclohexene, which, according to the literature [6], is easily oxidized, affects the chemical stability of the MP-1 model fuel. The higher the content of this compound, the shorter the induction period of MP-3 and MP-4 mixture. The effect of oxygen compound (ethanol) to this parameter was also checked. Ethanol did not deteriorate stability of the MP-1 fuel, moreover positively affected the stability of the fuel comprising cyclohexene—the MP-3 mixture broke after 91.5 min, and for the mixture containing both cyclohexene and ethanol (MP-5), after 6 hours (360 min) of the test decrease in oxygen pressure in the system was less than criterial 10%. In the case of commercial fuels, tested samples were characterized by a much shorter induction period than the model fuel. The addition of di-olefins (OLEF) in the amount of 3% v/v caused a sharp decline in the value of this parameter.

Two antioxidants were used in order to improve the oxidation resistance of the MP-3 sample. One of them, defined as AO-1, comprises a mixture of mono-*tert*-butylphenol, di-*tert*-butylphenol and tri-*tert*-butylphenol as active substance; the second (AO-2) comprises *N*,*N′*-di-secbutyl-p-phenylenediamine as the active ingredient. The effect of antioxidants was evaluated by the induction period using the micro method. The activity of antioxidants in petroleum fractions was checked. The results, given in **Table 6**, clearly show that in the tested systems,

**+100 mg AO-1/kg** 

**+25 mg AO-2/1 litre** 

**+ TBHP+25 mg/l** 

**AO-2**

**+43 mg AO-2/1 litre** 

**+ TBHP+43 mg/l** 

**AO-2**

**MP-3l**

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**MP-3**

**MP-3**

Alkylate 63.5 73.5 86.8 205 Not determined\* Reformate 79.9 114.4 149.4 Not determined\* Not determined\*

The induction period of the MP-3 mixture with the addition of antioxidants, for which conditions of oxidized fuel were simulated by the addition of TBHP, was also studied. The tests were performed in two variants: (1) antioxidant and peroxide were added to the model mixture, and (2) an antioxidant was introduced to oxidized by peroxide sample. Similar results were obtained, regardless of the order of addition of the additive and peroxide. **Table 7** con-

Antioxidants, both phenolic and amine, advantageously improved chemical stability of the fuel. However, their effectiveness was influenced by the presence of superoxide radicals, which is confirmed by the test results shown in the table above. It can be assumed that antioxidants introduced into the fuel, which is already occurring processes associated with oxida-

**AO-1**

Induction period, min 22.3 22.5 23.5 32.1 36.0

**Table 7.** Induction period of MP-3 model mixture containing antioxidant additive and TBHP.

**+ TBHP+100 mg/kg** 

Diesel fuel, same as petrol, is a mixture of many hydrocarbons of various structural features. The following hydrocarbon groups are in the oil: n-alkanes, iso-alkanes, cycloalkanes, ole-

the addition of an amine is more effective.

**Without additives**

**Induction period, min**

**MP-3**

\*Recorded pressure drop was less than 10% after 360 min of the test.

**Model fuel MP-3**

**+ TBHP + TBHP+60 mg/kg AO-1**

**+60 mg AO-1/1 kg** 

MP-3 91.5 Not determined\* – Not determined\* – Cracked naphtha 33.5 48.7 67.3 60.3 87.3

**Table 6.** Induction period of MP-3 model mixture and petroleum fractions with addition of antioxidants.

tains the induction period for option 2.

tion, do not fulfil their protective function.

**2.2. Model blend replacing diesel oil: stability test results**

The obtained results of the induction period were confirmed by the tests of potential resins according to ASTM D 873, as shown in **Table 4**. As a result of the oxidation, the insoluble deposits have formed only in the case of MP-3 mixture (containing cyclo-olefin). The presence of soluble deposits has been found for all tested mixtures; the lowest value was observed for the base composition containing neither cyclohexene nor ethanol. Comparatively the oxidation of fuel samples available on the market was carried out.


**Table 4.** Content of potential resins in the model mixtures according to ASTM D 873.

Induction period of model petrol and the commercial petrol, where the oxidation effect was simulated by the addition of TBHP as a source of radicals, was examined using micro method mixtures. The obtained results are shown below (**Table 5**):


**Table 5.** Induction period of model fuels and petrol with TBHP determined by the micro method.

The addition of peroxide to the model mixture resulted in a deterioration of oxidation resistance of samples containing the cyclic olefin (MP-3, MP-5). The samples without the addition of cyclohexene did not 'break' during the test run for 6 hours. In contrast, commercial fuels with intentionally introduced TBHP are characterized by significantly lower induction period when compared to fuel without the addition of this substance.

Two antioxidants were used in order to improve the oxidation resistance of the MP-3 sample. One of them, defined as AO-1, comprises a mixture of mono-*tert*-butylphenol, di-*tert*-butylphenol and tri-*tert*-butylphenol as active substance; the second (AO-2) comprises *N*,*N′*-di-secbutyl-p-phenylenediamine as the active ingredient. The effect of antioxidants was evaluated by the induction period using the micro method. The activity of antioxidants in petroleum fractions was checked. The results, given in **Table 6**, clearly show that in the tested systems, the addition of an amine is more effective.

checked. Ethanol did not deteriorate stability of the MP-1 fuel, moreover positively affected the stability of the fuel comprising cyclohexene—the MP-3 mixture broke after 91.5 min, and for the mixture containing both cyclohexene and ethanol (MP-5), after 6 hours (360 min) of the test decrease in oxygen pressure in the system was less than criterial 10%. In the case of commercial fuels, tested samples were characterized by a much shorter induction period than the model fuel. The addition of di-olefins (OLEF) in the amount of 3% v/v caused a sharp

The obtained results of the induction period were confirmed by the tests of potential resins according to ASTM D 873, as shown in **Table 4**. As a result of the oxidation, the insoluble deposits have formed only in the case of MP-3 mixture (containing cyclo-olefin). The presence of soluble deposits has been found for all tested mixtures; the lowest value was observed for the base composition containing neither cyclohexene nor ethanol. Comparatively the oxida-

Induction period of model petrol and the commercial petrol, where the oxidation effect was simulated by the addition of TBHP as a source of radicals, was examined using micro method

> **MP-5 + TBHP**

Not determ\* Not determ\* 22.3 52 44.1 17.5 47.9 26.2

**95RON 95RON**

**+ TBHP**

**98RON 98RON**

**+ TBHP**

Soluble gum mg/100 ml 0.1 0.3 0.3 0.2 31.9 33.9 22.3 25.5 Insoluble gum 0.0 0.0 0.4 0.0 0.2 1.1 0.3 1.2 Potential gum 0.1 0.3 0.7 0.2 32.1 35.0 22.6 26.7

**MP-1 MP-2 MP-3 MP-5 95RON 95RON + OLEF 98RON 98RON + OLEF**

The addition of peroxide to the model mixture resulted in a deterioration of oxidation resistance of samples containing the cyclic olefin (MP-3, MP-5). The samples without the addition of cyclohexene did not 'break' during the test run for 6 hours. In contrast, commercial fuels with intentionally introduced TBHP are characterized by significantly lower induction period

decline in the value of this parameter.

8 Improvement Trends for Internal Combustion Engines

tion of fuel samples available on the market was carried out.

**Parameter Unit Model petrol Commercial petrol**

**Table 4.** Content of potential resins in the model mixtures according to ASTM D 873.

**Parameter Model petrol Commercial petrol**

**MP-3 + TBHP**

**Table 5.** Induction period of model fuels and petrol with TBHP determined by the micro method.

mixtures. The obtained results are shown below (**Table 5**):

**MP-2 + TBHP**

\*Recorded pressure drop was less than 10% after 360 min of the test.

**MP-1 + TBHP**

Induction period, min

when compared to fuel without the addition of this substance.


**Table 6.** Induction period of MP-3 model mixture and petroleum fractions with addition of antioxidants.

The induction period of the MP-3 mixture with the addition of antioxidants, for which conditions of oxidized fuel were simulated by the addition of TBHP, was also studied. The tests were performed in two variants: (1) antioxidant and peroxide were added to the model mixture, and (2) an antioxidant was introduced to oxidized by peroxide sample. Similar results were obtained, regardless of the order of addition of the additive and peroxide. **Table 7** contains the induction period for option 2.


**Table 7.** Induction period of MP-3 model mixture containing antioxidant additive and TBHP.

Antioxidants, both phenolic and amine, advantageously improved chemical stability of the fuel. However, their effectiveness was influenced by the presence of superoxide radicals, which is confirmed by the test results shown in the table above. It can be assumed that antioxidants introduced into the fuel, which is already occurring processes associated with oxidation, do not fulfil their protective function.

#### **2.2. Model blend replacing diesel oil: stability test results**

Diesel fuel, same as petrol, is a mixture of many hydrocarbons of various structural features. The following hydrocarbon groups are in the oil: n-alkanes, iso-alkanes, cycloalkanes, olefins, aromatics and polyaromatics. Most of hydrocarbons have 15–17 carbon atoms in the molecule, the least hydrocarbons containing less than 9 and more than 23 carbon atoms in the molecule. Diesel fuel usually contains 40–50% by weight of paraffins, 20–30% by weight of cycloparaffins, 5–15% by weights of aromatics, 2–7% by weight of olefins and up to 7% by weight of fatty acid methyl esters (FAME). A disadvantage of esters is their low resistance to oxidation. In contrast to the 'pure' diesel fuel, esters added to the fuel make it more susceptible to oxidation or autoxidation during prolonged storage.

trum was obtained for the oxidized n-butyl cyclohexane, with the difference that was only one characteristic carbonyl peak. In the spectrum of the n-butyl benzene, changes were observed only in a range of the carbonyl band, and compounds with hydroxyl group were not formed. In the spectra of fatty acid methyl esters, there were no changes within the carbonyl band (the band is characteristic for esters), whereas area under the hydroxyl peak significantly increased.

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The model mixtures (the composition is given in **Table 9**) were prepared for the tests of resistance to oxidation. Two mixtures were selected: the first mixture composed exclusively of hydrocarbons (M0) and the second one composed of hydrocarbons enriched with biocompo-

 **Figure 1.** Comparison of the n-cetane before oxidation IR spectra (solid line) and after oxidation (dashed line).

**Components Model fuel M0, %(v/v) Model fuel M7, %(v/v)**

n-Cetane 50 46.5 n-Butyl cyclohexane 28 26.0 n-Butyl benzene 22 20.5 FAME 0 7.0

nent—fatty acid methyl esters (M7).

**Table 9.** Composition of the model blends.

In the present study, n-hexadecane (n-cetane), n-butyl cyclohexane, n-butyl benzene and fatty acid methyl esters were used to compose the model diesel fuel.

The first stage of the study included accelerated oxidation tests of hydrocarbons and fatty acid methyl esters. An induction period examination (according to EN 16091 standard) has been used for this purpose. The analysis of possible changes in the quality of oxidized samples of monocompounds on the basis of the IR spectrum was also conducted. The induction period results are shown in **Table 8**.


**Table 8.** Induction period of pure ingredients and FAME.

For all the tested substances, there was a 10% decrease in oxygen pressure in the measuring chamber (on this basis the induction period was determined). N-butyl cyclohexane (saturated cyclic hydrocarbon) was the least resistant to oxidation at elevated temperature. N-butyl benzene (aromatic hydrocarbon with an aliphatic side chain) was the most stable compound. Among the four tested components, fatty acid methyl esters demonstrated the lowest resistance to oxidation. The changes that occurred as a result of accelerated oxidation of each model hydrocarbons were visible in IR spectrum. Products of hydrocarbons oxidation occurred in two spectral ranges. For a hydroxyl group, a broad band was from 3600 to 3300 cm−1 (with a maximum of about 3500 cm−1), whereas for the carbonyl group, bands were in the range from 1750 to 1650 cm−1 (characteristic for oxidation products such as carboxylic acids, aldehydes, esters, ketones). Each position of the bands may undergo some shifts depending on the structure of the compound and the proximity of other groups. **Figure 1** shows an example of one of the tested hydrocarbon (n-cetane) spectra, before and after accelerated oxidation.

In the spectrum of n-cetane after oxidation, there are three peaks at a wavelength of 1710 cm−1, 1714 cm−1 (area A) and 3558 cm−1 (area B). These peaks are not present in the spectrum of the n-cetane before oxidation. Peaks at 1710 cm−1 and 1714 cm−1 are characteristic for carbonyl groups (C═O) and are evidence of carbonyl compound formation in the n-cetane, while the third band at 3558 cm−1 indicates the formation of hydroxy compounds (─OH). A similar spectrum was obtained for the oxidized n-butyl cyclohexane, with the difference that was only one characteristic carbonyl peak. In the spectrum of the n-butyl benzene, changes were observed only in a range of the carbonyl band, and compounds with hydroxyl group were not formed. In the spectra of fatty acid methyl esters, there were no changes within the carbonyl band (the band is characteristic for esters), whereas area under the hydroxyl peak significantly increased.

 **Figure 1.** Comparison of the n-cetane before oxidation IR spectra (solid line) and after oxidation (dashed line).

The model mixtures (the composition is given in **Table 9**) were prepared for the tests of resistance to oxidation. Two mixtures were selected: the first mixture composed exclusively of hydrocarbons (M0) and the second one composed of hydrocarbons enriched with biocomponent—fatty acid methyl esters (M7).


**Table 9.** Composition of the model blends.

fins, aromatics and polyaromatics. Most of hydrocarbons have 15–17 carbon atoms in the molecule, the least hydrocarbons containing less than 9 and more than 23 carbon atoms in the molecule. Diesel fuel usually contains 40–50% by weight of paraffins, 20–30% by weight of cycloparaffins, 5–15% by weights of aromatics, 2–7% by weight of olefins and up to 7% by weight of fatty acid methyl esters (FAME). A disadvantage of esters is their low resistance to oxidation. In contrast to the 'pure' diesel fuel, esters added to the fuel make it more suscep-

In the present study, n-hexadecane (n-cetane), n-butyl cyclohexane, n-butyl benzene and fatty

The first stage of the study included accelerated oxidation tests of hydrocarbons and fatty acid methyl esters. An induction period examination (according to EN 16091 standard) has been used for this purpose. The analysis of possible changes in the quality of oxidized samples of monocompounds on the basis of the IR spectrum was also conducted. The induction period results are

**Component Induction period at 140°C (EN 16091) min**

For all the tested substances, there was a 10% decrease in oxygen pressure in the measuring chamber (on this basis the induction period was determined). N-butyl cyclohexane (saturated cyclic hydrocarbon) was the least resistant to oxidation at elevated temperature. N-butyl benzene (aromatic hydrocarbon with an aliphatic side chain) was the most stable compound. Among the four tested components, fatty acid methyl esters demonstrated the lowest resistance to oxidation. The changes that occurred as a result of accelerated oxidation of each model hydrocarbons were visible in IR spectrum. Products of hydrocarbons oxidation occurred in two spectral ranges. For a hydroxyl group, a broad band was from 3600 to 3300 cm−1 (with a maximum of about 3500 cm−1), whereas for the carbonyl group, bands were in the range from 1750 to 1650 cm−1 (characteristic for oxidation products such as carboxylic acids, aldehydes, esters, ketones). Each position of the bands may undergo some shifts depending on the structure of the compound and the proximity of other groups. **Figure 1** shows an example of one

of the tested hydrocarbon (n-cetane) spectra, before and after accelerated oxidation.

In the spectrum of n-cetane after oxidation, there are three peaks at a wavelength of 1710 cm−1, 1714 cm−1 (area A) and 3558 cm−1 (area B). These peaks are not present in the spectrum of the n-cetane before oxidation. Peaks at 1710 cm−1 and 1714 cm−1 are characteristic for carbonyl groups (C═O) and are evidence of carbonyl compound formation in the n-cetane, while the third band at 3558 cm−1 indicates the formation of hydroxy compounds (─OH). A similar spec-

tible to oxidation or autoxidation during prolonged storage.

10 Improvement Trends for Internal Combustion Engines

acid methyl esters were used to compose the model diesel fuel.

n-Cetane 80.57 n-Butyl cyclohexane 61.83 n-Butyl benzene 175.55 FAME 27.33

**Table 8.** Induction period of pure ingredients and FAME.

shown in **Table 8**.

The test of accelerated oxidation according to the previously described EN 16091 standard (PetroOxy test) was used for the mixture oxidation. In the case of the M7 mixture (containing FAME), additionally the oxidative stability examination according to EN 15751 (Rancimat test) was performed. The results are shown in **Table 10**.

*2.2.1. Long-term ageing test*

mixtures. The results are shown in **Table 11**.

**Induction period EN 16091, min**

**Storage time, day Model fuel M0 Model fuel M7**

**Table 11.** Results of model fuels storage stability test at elevated temperature.

**Deposit content ASTM D 5304, mg/100 ml**

 110.90 0.6 92.58 38.2 0.9 109.45 0.4 91.90 38.0 0.7 110.25 0.6 91.25 37.7 1.1 108.83 0.8 90.84 37.8 1.5 108.30 0.7 88.55 37.3 2.8 107.15 1.0 88.05 37.1 2.6 105.52 0.9 86.62 36.4 3.5

The M0 and M7 model mixtures were stored for 6 months. Every 30 days, measurements of oxidation stability (according to EN 16091 and EN 15751) have been performed, and the amount of sediment has been determined (according to ASTM D5304) for the batch of stored

> **Induction period EN 16091, min**

**Oxidation stability EN 15751, hr**

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**Deposit content ASTM D5304, mg/100 ml**

The M0 and M7 model mixtures had a high stability during conducting long-term ageing test. Induction period measured by PetroOxy test varied slightly for both samples M0 and M7. At the end of the test, threefold increase of the total sediment for the M7 sample was observed. The M0 mixture (without FAME) contained the amount of sludge at a similar level. The M7 mixture had high oxidation stability, 38.2 hours, which decreased slightly during 180 days of

Two samples of commercial diesel fuel containing various amounts of fatty acid methyl esters were examined. The sample A contained 1% by weight of FAME, while sample B, 7% by weight of FAME. The samples were stored at 45°C for 6 months in a dark place and at 30-day intervals were collected for testing in order to determine the amount of deposits formed during fuel storage (ASTM D 5304 test) and determine the induction period (EN 16091 test, EN

In addition to the above-mentioned tests, the oxidation stability test (according to EN ISO 12205) was performed. The test was performed at the beginning and after 180 days of storage

resins, 27 g/m3

resins were produced while for the same sample after 6 months of storage,

total.

total sludge including 4 g/m3

filterable

*2.2.1.1. Model diesel oil*

storage to 36.4 hours.

the samples A and B.

sludge and 3 g/m3

25 g/m3

*2.2.1.2. Diesel oil with various content of FAME*

15751 test). The results are shown in **Table 12**.

filterable sludge and 2 g/m<sup>3</sup>

At the beginning of the study of the A sample, 7 g/m<sup>3</sup>


**Table 10.** Results of the oxidative stability of the blends of M0 and M7.

For the both studied mixtures, there was a 10% decrease in oxygen pressure in the measuring chamber; therefore, the induction period has been determined. The M7 mixture, due to the presence of methyl esters, was less resistant to oxidation than M0 blend. Comparative analysis of the IR spectra showed that in the M0 mixture after oxidation carbonyl compounds with characteristic bands 1715 cm−1 and 1743 cm−1, shown in **Figure 2** within D area and wide, expanding bands characteristic for hydroxyl groups (C area). In the spectrum of oxidized M7 mixture containing fatty acids methyl esters are not substantially changed within the carbonyl band, because in this spectral range lies peak characteristic for esters. There was however a slight increase of the area of 1743 cm−1 the carbonyl band with a typical inflexion point of the peak at 1743 cm−1, which indicates the formation of oxidation products. In **Figure 2** the spectra of the M0 mixture before and after oxidation are shown.

 **Figure 2.** Comparison of the M0 mixture before oxidation (solid line) and after oxidation (dashed line) IR spectra.

#### *2.2.1. Long-term ageing test*

#### *2.2.1.1. Model diesel oil*

The test of accelerated oxidation according to the previously described EN 16091 standard (PetroOxy test) was used for the mixture oxidation. In the case of the M7 mixture (containing FAME), additionally the oxidative stability examination according to EN 15751 (Rancimat

**Model fuel Oxidative stability (EN 15751), h Induction period (EN 16091), min**

For the both studied mixtures, there was a 10% decrease in oxygen pressure in the measuring chamber; therefore, the induction period has been determined. The M7 mixture, due to the presence of methyl esters, was less resistant to oxidation than M0 blend. Comparative analysis of the IR spectra showed that in the M0 mixture after oxidation carbonyl compounds with characteristic bands 1715 cm−1 and 1743 cm−1, shown in **Figure 2** within D area and wide, expanding bands characteristic for hydroxyl groups (C area). In the spectrum of oxidized M7 mixture containing fatty acids methyl esters are not substantially changed within the carbonyl band, because in this spectral range lies peak characteristic for esters. There was however a slight increase of the area of 1743 cm−1 the carbonyl band with a typical inflexion point of the peak at 1743 cm−1, which indicates the formation of oxidation products. In **Figure 2** the spectra

 **Figure 2.** Comparison of the M0 mixture before oxidation (solid line) and after oxidation (dashed line) IR spectra.

test) was performed. The results are shown in **Table 10**.

12 Improvement Trends for Internal Combustion Engines

**Table 10.** Results of the oxidative stability of the blends of M0 and M7.

M0 – 110.90 M7 38.20 92.58

of the M0 mixture before and after oxidation are shown.

The M0 and M7 model mixtures were stored for 6 months. Every 30 days, measurements of oxidation stability (according to EN 16091 and EN 15751) have been performed, and the amount of sediment has been determined (according to ASTM D5304) for the batch of stored mixtures. The results are shown in **Table 11**.


**Table 11.** Results of model fuels storage stability test at elevated temperature.

The M0 and M7 model mixtures had a high stability during conducting long-term ageing test. Induction period measured by PetroOxy test varied slightly for both samples M0 and M7. At the end of the test, threefold increase of the total sediment for the M7 sample was observed. The M0 mixture (without FAME) contained the amount of sludge at a similar level. The M7 mixture had high oxidation stability, 38.2 hours, which decreased slightly during 180 days of storage to 36.4 hours.

## *2.2.1.2. Diesel oil with various content of FAME*

Two samples of commercial diesel fuel containing various amounts of fatty acid methyl esters were examined. The sample A contained 1% by weight of FAME, while sample B, 7% by weight of FAME. The samples were stored at 45°C for 6 months in a dark place and at 30-day intervals were collected for testing in order to determine the amount of deposits formed during fuel storage (ASTM D 5304 test) and determine the induction period (EN 16091 test, EN 15751 test). The results are shown in **Table 12**.

In addition to the above-mentioned tests, the oxidation stability test (according to EN ISO 12205) was performed. The test was performed at the beginning and after 180 days of storage the samples A and B.

At the beginning of the study of the A sample, 7 g/m<sup>3</sup> total sludge including 4 g/m3 filterable sludge and 3 g/m3 resins were produced while for the same sample after 6 months of storage, 25 g/m3 filterable sludge and 2 g/m<sup>3</sup> resins, 27 g/m3 total.


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

Study of Stability Changes of Model Fuel Blends

http://dx.doi.org/10.5772/67056

15

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

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

Automotive Industry Institute, Liquid Fuels and Bio-Economy Department, Warsaw, Poland

[1] Czarnocka J., Matuszewska A., Odziemkowska M. Autoxidation of Fuels During Storage. In: Biernat K., editor. Storage Stability of Fuels. InTech; 2015. pp. 157–188. DOI:

Małgorzata Odziemkowska, Joanna Czarnocka\* and Katarzyna Wawryniuk

10.5772/5979 Available from: http://dx.doi.org/10.5772/59807

oxidizing agents but less reactive than cyclohexane.

and B samples decreased with increasing time of storage.

amount of sediments constant at the same level.

\*Address all correspondence to: j.czarnocka@pimot.eu

**Author details**

**References**

**Table 12.** Results of commercial diesel oil storage stability test at elevated temperature.

The same for the B sample, at the beginning of the test, the amount of filterable sludge was 7 g/m<sup>3</sup> , and 1 g/m3 resins, 8 g/m3 total while for the same sample after 6 months of storage, 56.2 g/ m3 filterable sludge and 2 g/m<sup>3</sup> resins, 58.2 g/m3 total. The presented data show that with the increase of the sample storage period, the total amount of deposits increases several times. The increase of sediments was mainly caused by the formation of filterable deposits, while the amount of resins did not change.

After 6 months of fuel storage, induction period determined by EN 16091 was lower than at the beginning of the study, for both samples A and B. For A sample the induction period dropped from 41.9 to 32.8 min and for the B sample from 31.1 to 26.6 min. The induction period measured by Rancimat test for B sample containing 7.2% FAME was constant during 180 days of storage fuel.
