Biodiesel can be used in diesel engines without significant modification of the engine prior to use because it has properties similar to those of petroleum diesel. Biodiesel, however, exhibits lower stability compared to petroleum diesel. Small differences in fuel properties such as component concentration or total acidity can lead to the formation of deposits which can reduce engine performance and increase maintenance requirements and costs. Thermo‐oxidative stressing was performed in two reactor systems in this study. For comparative purposes both sets of experiments were performed at 140oC. The systems used were a quartz crystal microbalance (QCM) in which oxygen was limited and open glass flasks under flowing air (unlimited oxygen). To simplify analysis, diesel model compound systems were used in which full boiling range diesel was replaced with single compounds representing the classes of compounds found in petroleum diesel. The model compounds were n‐hexadecane, tetralin and decalin. Fuel analysis was performed using gas chromatography (GC) with mass spectrometric (MS) and flame ionisation (FID) detection. Further analytical methods included Fourier transform infrared (FTIR) and ultraviolet‐visible (UV‐Vis) spectroscopy as well as electrospray ionisation‐mass spectrometry (ESI‐MS). This study represents the first application of QCM methodology to systems that contain fatty acid methyl esters (FAMEs). FAMEs with different degrees of unsaturation were investigated. The quantities of deposits formed were as follows: methyl linolenate (40.1 μg/cm2) and methyl linoleate (19.2 μg/cm2) when these were blended with diesel model compounds in a 20:80 ratio. These values are significantly higher than those typically reported for middle distillates. Increased polyunsaturation of the FAME led to increased deposit formation. Spectroscopic investigations revealed very small, if any, changes to bulk fuel composition during oxidation in a QCM. GC and ESI‐MS analysis demonstrated the formation of oxygenates but in small concentrations. Nonetheless these were sufficient to initiate the formation of deposits. More severe conditions used in open flask experiments led to greater quantities of, more types of, and more highly oxygenated products as seen by GC and, with respect to the latter, especially by ESI‐MS. FTIR spectroscopy revealed the inclusion of oxygen‐containing functional groups. Increasing deposit precursor molecular mass and polarity led to phase separation in these experiments. Colour and UV‐Vis changes were suggestive of the formation of conjugated systems. Examples of these species were identified by GC‐MS. The QCM was also useful in the exploration of fuel solvency. This study represents, as far as the author is aware, the first application of a QCM in this area. Fuels, with small quantities of biodiesel added, formed greater amounts of deposits than petroleum diesel model compounds or 100% biodiesel. Furthermore these systems consumed oxygen slightly faster. This points to a complex interaction between reactivity (faster oxygen uptake) and fuel polarity. This study has demonstrated the versatility of the QCM methodology for application to fuels beyond petroleum middle distillates. FAME molecules were observed to be highly oxygenated and formed highly oxygenated dimers and a small quantity of trimers/tetramers during oxidation. It was observed that under severe oxidative conditions (flowing oxygen) the types of high molecular species formed differed from those formed during constrained oxygen stressing in a QCM. Existing mechanisms for fuel oxidation were validated and in some cases extended. A possible soluble macromolecular oxidatively reactive species (SMORS), 1,4‐naphthoquinone, was identified from the oxidation of tetralin by GC‐MS and ESI‐MS. Previously unobserved species such as 1,2‐dicarboxylic acids, derived from tetralin and decalin, were reported and used to extend existing mechanistic schemes. The formation of higher molecular weight species that result from the interaction of alkenes (derived) from paraffin cracking with radicals such as the 1‐tetralyl radical are also explained.

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