Fatty acid methyl esters (FAME) are the most frequently used substituent to fulfill the legally obliged addition of biofuels to fossil diesel aiming the reduction of greenhouse gas emissions in the transportation sector. Due to the chemical structure of FAME, the storage stability is limited and oxidation reactions can occur under certain circumstances, which can cause difficulties or incompatibilities in engine application. Not only from an ecological or economical point of view but also an ethical one, maximum utilization of the biological raw materials is required. In particular, FAME are classified as a first-generation biofuel and competes with food. Therefore, it is by all means necessary to avoid the biological raw material becoming unusable for its initial purpose due to a chemical deterioration, which in turn would lead to a needless waste of resources.

Raw materials for FAME, natural fats and oils represent a major category of renewable resources and consist of medium to long-chained fatty acids. These fatty acids, which occur in fats and oils, can be distinguished between saturated and mono- or polyunsaturated fatty acids. The distinctive feature between these two compound classes is the existence, quantity and position of carbon double bonds in the fatty acid molecule. These carbon double bonds increase the reactivity of fatty acids and supply an attacking position for radicals.

In the course of the presented research activities, a rapeseed methyl ester (RME) blend (B20) and pure RME were artificially aged with the goal to describe their changes in physicochemical properties. Furthermore, the formation of so-called secondary oxidation products could be reconstructed for pure RME. During the thermochemically induced, artificial aging process of pure RME, almost all – 97% – of the unsaturated fatty acid methyl esters decomposed. The formed secondary oxidation products were segregated and identified as oxiranes/epoxides by gas chromatography/ mass spectrometry (GC/MS). Two reaction paths could be introduced for the formation of the dominating species oxiraneoctanacid-3-ocytl-methylester (cis- and trans-type). The first path explains the formation as the autooxidation and decomposition of hydroperoxides of oleic acid to alkoxyl radicals, and the second way is the direct epoxidation of carbon double bonds via peroxy acid. Furthermore, smaller amounts of other secondary oxidation products could be detected from carboxyl, oxo, epoxy and ether groups.

Consecutively, the influence of aged FAME blends to the spray behavior during injection, the internal combustion and emissions, as well as endurance tests for provoking injector cocking and the exhaust gas recirculation compatibility, were investigated. The aging stages of the RME blend did not show distinct tendencies regarding internal combustion and emissions. Arguably, the aging stage only plays a minor role in internal combustion with as-good–as-new injectors without any coke depositions. In full-load operation, the utilization of the RME blend generally presented a deficit in maximum power, which is explainable by the lower calorific value of FAME compared to conventional diesel. The aging stage did not significantly influence the maximum power output. Over the course of an endurance test, it could be demonstrated that seriously aged RME blends had a major influence on injector coking. After 50 hours of operation, significant coke depositions were already recognizable around the injector holes. After 200 hours, these depositions resulted in insufficient combustion, with frequent misfires. The results of the combustion analysis are supported by the emissions; hence the presence of species that are products of incomplete combustion increased extremely during the endurance test.

The knowledge about occurring reactions supports the selection of additive agents to improve oxidation stability. In the final test series, the effect of different phenolic antioxidants could be proved. Generally, they cause a significant rise in oxidation stability for pure FAME as well as for FAME blends. The antioxidants butylated hydroxyl toluene, butylated hydroxyl anisole, propyl gallate and tert-butylhydroquinone show the potential to improve the storage capability of FAME blends significantly, and these antioxidants do not have any negative impacts on combustion or on emissions.


Deterioration of Fatty Acid Methyl Esters \u2013 Mechanisms, Consequences and Countermeasures

Division / Sponsors: 
Internal Combustion Engine Division (ICED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ICEF/ICEF2020/volume/84034

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