Oil Degradation Patterns in Diesel and Petrol Engines Observed in the Field—An Approach Applying Mass Spectrometry

: Engine oil degradation and tribological properties are strongly interrelated. Hence, understanding the chemical processes resulting in additive depletion and degradation products is necessary. In this study, in-service engine oils from petrol and diesel vehicles were analyzed with conventional and advanced methods (mass spectrometry). Additionally, the effect of the utilization proﬁle (short-vs. long-range) was studied. Petrol engine oils generally showed accelerated antioxidant and an-tiwear degradation and higher oxidation, especially in the case of a short-range utilization proﬁle, which can be attributed to the higher air-to-fuel ratio (more rich combustion) compared to diesel engines. A detailed overview of oxidation and nitration products, as well as degradation products resulting from zinc dialkyl dithiophosphate and boron ester antiwear additives, diphenylamine antioxidants and salicylate detergents is given. A side reaction between oxidation products (aromatic carboxylic acids) and the boron ester antiwear is highlighted. This reaction was only detected in the petrol engine oils, where the oxidation products were measured in a high abundance. However, no side reaction was found in the samples from the diesel vehicles, since there the aromatic carboxylic acids were largely absent due to lower oxidation.


Introduction
The European Parliament voted in favor of proposed amendments [1] to Regulation (EU) 2019/631 [2], which suggest a 100% reduction in CO 2 emissions in the automotive sector by 2035 through banning the sale of new passenger cars and new light-duty vehicles with internal combustion engines.In addition to the immense efforts put into the electrification of the European mobility and transportation sector, care should also be taken to investigate technologies that will allow us to decrease the emissions of used internal combustion engine vehicles.
The average passenger car age in the EU as of 2020 [3] is 11.8 years-with the two extremes of 6.7 years on average in Luxembourg and 17 years in Lithuania.Considering light-duty and heavy-duty vehicles, the average age increases to 11.9 and 13.8 years, respectively [3].Given the post-COVID economic environment burdened with the impact of the Russian-Ukrainian conflict and a looming energy crisis, the average age of the European vehicle fleet is expected to stagnate-if not rise-in the upcoming 5-10-year period.These factors also affect raw material prices-i.e., cobalt, nickel, and lithium-necessary for EV battery production [4].In other words, the internal combustion engine is expected to overstay its welcome.
The perception of biofuels and synthetic fuels ranges on a broad spectrum [5][6][7][8][9][10], since compared to battery electric and fuel cell electric vehicles they only offer a minor reduction in local harmful emissions.Nonetheless, considering a lifecycle point of view, implementing these technologies on used vehicles still contributes to a reduction in greenhouse gas (GHG) emission and end-of-life waste production.The key of utilizing alternative fuels lies in compatibility, which must be proved before widespread adoption.In addition to part compatibility-e.g., seals, fuel lines, pumps, etc.-a vital aspect of correct internal combustion engine operation is its possible impact on lubrication.Since engine oil dilution with fuel and the resulting degradation of the lubricant is a known and well-researched phenomenon [11][12][13][14][15], engine oil compatibility with alternative fuels must be regarded with great care during the introduction of new fuel formulations.
Highly detailed qualitative and quantitative information on the chemical composition of lubricants and their additives can be obtained using mass spectrometry (MS).To separate the complex mixture of base oil components and additives, mass spectrometry is frequently combined with chromatographic techniques, such as gas chromatography (GC) [16,17] or liquid chromatography (LC) [18,19], which typically involves increased time and more elaborate sample preparation.For direct comprehensive chemical analysis of lubricants by MS, ambient ionization techniques such as desorption electrospray ionization (DESI) [20,21], matrix-assisted laser desorption ionization (MALDI) [22], direct analysis in real time (DART) [23], and atmospheric solids analysis probe (ASAP) [24] are becoming more widespread and popular.Instruments with high resolution and mass accuracy allow additives to be identified without complex sample preparation or the use of standards, as demonstrated by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) [25] or dielectric barrier discharge ionization mass spectrometry (DBDI-MS) [26].
The presented study concludes a series of preliminary works, which aim to establish the groundwork of alternative fuels research in modern internal combustion engines through investigating engine oil dilution and degradation in the field, with currently available EN 228 [27] compatible gasoline and EN 590 diesel [28] fuels.In addition to already published conventional oil analysis results [29] and the results of principal component analysis of FTIR spectra [30], this article provides a detailed analysis of additive degradation and degradation products on the molecular level through mass spectrometry.A comparison with the corresponding conventional results is given, with the aim of uncovering the chemical structures responsible for commonly established degradation parameters.This could help understanding the differences in reaction pathways of fuel-induced oil degradation patterns and offers insights for formulating engine oils that can withstand alternative fuels.

Materials and Methods
A field test with various petrol-and diesel-fueled passenger cars was conducted utilizing 12 vehicles under real, on-road driving conditions, where oil samples through the dipstick pipes were taken at regular intervals.All participating vehicles were using the same fully synthetic SAE 0W-30 fresh oil (properties are displayed in Table 1).
The methodology and various oil analysis results are presented in detail in [29,30]; accordingly, here only a brief summary of the relevant points is given.

Field Test under Real Driving Conditions
In detail, 3 diesel long-range, 3 petrol short-range and 6 petrol long-range drivetrains were studied, all utilizing an engine with 2.0 L total displacement and direct injection.Diesel engines were included in the fleet study as reference, to highlight the differences in primary influencing factors and oil aging pathways between engine types.Considering the recent decline in popularity of diesel passenger cars, the number of subjects with a certain engine type (i.e., petrol or diesel) was biased towards petrol variants.Petrol engines were prioritized since the market share of diesel has been steadily shrinking in recent years [31].Similarly, focus was laid on long-range utilization, as for short trips (city traffic), increasingly electric utilization is expected as hybrids and BEVs gain popularity.The vehicles were equipped with a GPS tracking system, which enabled a detailed analysis of utilization profiles.Based on the average speed [32] and average trip length [33], "short range" and "long range" groups were selected [29,30].Table 2 gives an overview of the vehicles, drivetrains, utilization profiles and total covered milage.From the available vehicle pool, 3 vehicles (the highest total mileage in the group) were selected for in-depth oil characterization (indicated in bold).Here, medium and high mileage samples from each vehicle were analyzed according to the methodology described in detail in Section 2.2.

Oil Condition Monitoring
Within this study, MS results were compared and correlated to conventional oil parameters.

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Accumulation of combustion-related degradation products; a.
Nitration at 1630 cm −1 Additionally, boron content of the used engine oil samples was determined via inductively coupled plasma-optical emission spectroscopy (ICP-OES, iCAP 7400 ICP-OES Duo, ThermoFisher, Waltham, MA, USA) after microwave-assisted nitric acid digestion.High-resolution mass spectrometry (MS) was utilized as an advanced analytical method for lubricants to gain information on additive depletion and degradation products on a molecular level.The analysis was carried out according to the method described in [34] via an LTQ Orbitrap XL hybrid tandem high-resolution mass spectrometer (Orbitrap-MS) (ThermoFisher Scientific, Bremen, Germany).After sample preparation, solutions were infused into the ion source where the analyte molecules underwent ionization.Here, ions with a single charge are generated, hence, the chemical composition of the molecules can be directly determined over the mass-to-charge (m/z) ratio.The identification of the composition is based on the fact that the precisely detected mass can only result from a distinct combination of atoms.Subsequently, fragmentation of the analyte ions was performed for accurate identification of the chemical structures.During fragmentation, certain bonds of the molecule break, which corresponds to the distinct structure, e.g., how atoms are linked with each other.The interpretation of these fragmentation mechanisms enables further elucidation of the exact structure.Generated fragment ions were detected with a high-resolution Orbitrap-MS detector.Table 3 gives an overview of the relevant measurement parameters.Data processing and evaluation were done using Xcalibur version 2.0.7 and Mass Frontier version 6.0 software (ThermoFisher Scientific, Bremen, Germany).The presented m/z values describe the mass-to-charge ratio of the detected species.Since all analyzed ions were single-charged species, m/z also describes the monoisotopic molecular masses of the detected compounds.It is noted that the ionization capability of chemical compounds is dependent on multiple parameters, such as the chemical structure, solvent, or matrix effects.Hence, a comparison of the obtained relative abundances of the individual ions is only possible between the same species found in the analyzed oil samples.The reported relative abundance is based on the total ion current (TIC).The highest measured peak intensity for a species is set to 100% of relative abundance.
During the analysis, care was given to ensure comparability of the reported relative abundances.All samples were analyzed via reliable automatized direct infusion in one session, to minimize the effect of eventual changes in the instrument sensitivity.Additionally, the instrument was calibrated before the measurement session with a calibration standard, which included both mass and TIC calibrations.
Furthermore, the exact position of alkyl sidechains might vary within the depicted molecules, although their presence can be detected accurately through the mass-to-charge ratio.The reported molecules are proposed structures in all cases, e.g., in the case of substituted aromatic molecules ortho-, meta-and para-isomers are all possible.

Characterization of the Fresh Oil
The respective degradation products of ZDDP and boron ester antiwear additives were of particular interest since wear protection is one of the most important indicators of the tribological performance of any given lubricant.With the depletion of antiwear additives, the tribofilm composition also changes [36] which usually results in higher wear rates [36].Complementarily, organic, and inorganic acids as well as selected oxidation and nitration products are presented.
Figure 1  reported relative abundance is based on the total ion current (TIC).The highest measured peak intensity for a species is set to 100% of relative abundance.
During the analysis, care was given to ensure comparability of the reported relative abundances.All samples were analyzed via reliable automatized direct infusion in one session, to minimize the effect of eventual changes in the instrument sensitivity.Additionally, the instrument was calibrated before the measurement session with a calibration standard, which included both mass and TIC calibrations.
Furthermore, the exact position of alkyl sidechains might vary within the depicted molecules, although their presence can be detected accurately through the mass-to-charge ratio.The reported molecules are proposed structures in all cases, e.g., in the case of substituted aromatic molecules ortho-, meta-and para-isomers are all possible.

Characterization of the Fresh Oil
The respective degradation products of ZDDP and boron ester antiwear additives were of particular interest since wear protection is one of the most important indicators of the tribological performance of any given lubricant.With the depletion of antiwear additives, the tribofilm composition also changes [36] which usually results in higher wear rates [36].Complementarily, organic, and inorganic acids as well as selected oxidation and nitration products are presented.

ZDDP Degradation in the Used Oils
Figure 2 gives an overview of the ZDDP degradation in the petrol and diesel vehicles, as observed in the field test via FTIR.The petrol vehicles generally exhibit a faster ZDDP degradation, which is especially accelerated during short-range utilization.This was attributed to the lower air-to-fuel ratio (AFR) in petrol engines compared to diesel counterparts, which results in a higher abundance of reactive, not completely combusted species, hence faster additive degradation [29].The AFR is even lower in the case of a short-range utilization profile, where the vehicles run predominantly colder, hence the engine control unit (ECU) has to inject extra fuel to compensate for the condensation on the cylinder walls [29].
Lubricants 2023, 11, x FOR PEER REVIEW 6 of 21 Figure 2 gives an overview of the ZDDP degradation in the petrol and diesel vehicles, as observed in the field test via FTIR.The petrol vehicles generally exhibit a faster ZDDP degradation, which is especially accelerated during short-range utilization.This was attributed to the lower air-to-fuel ratio (AFR) in petrol engines compared to diesel counterparts, which results in a higher abundance of reactive, not completely combusted species, hence faster additive degradation [29].The AFR is even lower in the case of a short-range utilization profile, where the vehicles run predominantly colder, hence the engine control unit (ECU) has to inject extra fuel to compensate for the condensation on the cylinder walls [29].All ZDDP species showed well-comparable degradation, hence the respective degradation pathways are demonstrated in the example of dipropyl dithiophosphate.
Figure 3a displays the abundance of dipropyl dithiophosphate (m/z 213.018), one of the main ZDDP components in the fresh and the used oils.The abundance of this molecule diminishes rapidly; it is detectable only in the first diesel used-oil sample (5300 km mileage).In this context, it is notable that even the short-range petrol engine oils show a close to complete depletion of the original ZDDP structure despite the relatively low mileage.Figure 3b presents the abundance of dipropyl thiophosphate (m/z 197.041),where a sulfur atom of the dipropyl dithiophosphate is substituted with oxygen, as well as dipropyl phosphate (m/z 181.063)where both sulfur atoms are substituted with oxygen.The longrange petrol and diesel engine oils show different patterns.In the diesel engine, the first sample (5300 km mileage) contains mostly dipropyl thiophosphate and some dipropyl phosphate.The subsequent sample (11,600 km) contains both dipropyl thiophosphate and dipropyl phosphate with a higher abundance.Comparatively, the long-range petrol engine oil samples contain practically no dipropyl thiophosphate, only in traces after 7200 km.Meanwhile, dipropyl phosphate is characterized by high abundance at both mileages.These findings suggest that ZDDP degradation is significantly faster in the long-range petrol engine, which relates to a faster formation of dipropyl phosphate.This corresponds well with the results of the conventional oil analysis (see Figure 2), where a significantly faster ZDDP degradation in the case of the petrol engines was found.The short-range petrol vehicle also shows expedited ZDDP degradation compared to the diesel engine, when the significantly lower mileage is considered.The initial short-range petrol engine oil sample with only 900 km mileage contains both dipropyl thiophosphate and dipropyl All ZDDP species showed well-comparable degradation, hence the respective degradation pathways are demonstrated in the example of dipropyl dithiophosphate.
Figure 3a displays the abundance of dipropyl dithiophosphate (m/z 213.018), one of the main ZDDP components in the fresh and the used oils.The abundance of this molecule diminishes rapidly; it is detectable only in the first diesel used-oil sample (5300 km mileage).In this context, it is notable that even the short-range petrol engine oils show a close to complete depletion of the original ZDDP structure despite the relatively low mileage.Figure 3b presents the abundance of dipropyl thiophosphate (m/z 197.041),where a sulfur atom of the dipropyl dithiophosphate is substituted with oxygen, as well as dipropyl phosphate (m/z 181.063)where both sulfur atoms are substituted with oxygen.The longrange petrol and diesel engine oils show different patterns.In the diesel engine, the first sample (5300 km mileage) contains mostly dipropyl thiophosphate and some dipropyl phosphate.The subsequent sample (11,600 km) contains both dipropyl thiophosphate and dipropyl phosphate with a higher abundance.Comparatively, the long-range petrol engine oil samples contain practically no dipropyl thiophosphate, only in traces after 7200 km.Meanwhile, dipropyl phosphate is characterized by high abundance at both mileages.These findings suggest that ZDDP degradation is significantly faster in the long-range petrol engine, which relates to a faster formation of dipropyl phosphate.This corresponds well with the results of the conventional oil analysis (see Figure 2), where a significantly faster ZDDP degradation in the case of the petrol engines was found.The short-range petrol vehicle also shows expedited ZDDP degradation compared to the diesel engine, when the significantly lower mileage is considered.The initial short-range petrol engine oil sample with only 900 km mileage contains both dipropyl thiophosphate and dipropyl phosphate in a higher abundance, similarly to the diesel engine oil after 11,600 km.This demonstrates how much faster the degradation process propagates.The subsequent short-range petrol sample (1750 km) contains only traces of dipropyl thiophosphate but significant amounts of dipropyl phosphate, similarly to the long-range petrol engine oils.
Lubricants 2023, 11, x FOR PEER REVIEW 7 of 21 phosphate in a higher abundance, similarly to the diesel engine oil after 11,600 km.This demonstrates how much faster the degradation process propagates.The subsequent short-range petrol sample (1750 km) contains only traces of dipropyl thiophosphate but significant amounts of dipropyl phosphate, similarly to the long-range petrol engine oils.Figure 3c shows the abundance of decyl sulfate (m/z 237.117) and decyl phosphate (m/z 237.126), both considered to be further ZDDP degradation products.The release of alkyl side chains (as olefins) during the later stages of ZDDP degradation was previously described in [37] as well.Decyl sulfate and decyl phosphate exhibit comparable trends in abundance to the already discussed ZDDP degradation products.The molecules show an increasing abundance with the mileage and are less pronounced in the diesel engine oil samples.It is also noteworthy that at this point the alkyl chains of the original ZDDP molecules change during utilization, as no decyl species were found in the original ZDDP composition.The short-range petrol samples display a high abundance of these degradation products, especially considering their low mileage.Banerji et al. proposed that hydroxyl groups contained in ethanol molecules could facilitate ZDDP degradation, especially alkyl ligand exchange between the oxygen and sulfur atoms [38].As petrol contains 5-10 v% ethanol depending on the product, this might give a possible explanation for the emergence of decyl sidechains in petrol vehicles.
Figure 3d compares the abundance of sulfuric acid (m/z 96.960) in the fresh and used oil samples.Sulfuric acid exhibits comparable behavior in all used oil samples, although the abundance seems to be somewhat higher in the long-range petrol samples, where the ZDDP degradation is most pronounced.It is noteworthy that the short-range petrol engine oil samples show a relatively high abundance of sulfuric acid despite their low total mileage.
It can be stated that the degradation of ZDDP is similar to prior field studies.In [34], it was also found that the ZDDP depletes rapidly in the first 5000-6000 km and the emergence of dialkyl thiophosphates followed by their rapid depletion was observed.Other degradation products were found in increasing amounts with mileage, especially dialkyl phosphates as well as phosphoric and sulfuric acid.In detail, the formation of sulfuric acid was visible after 8000-9000 km and the abundance of the species then remained at a high level [34].The presented results are comparable with this observation, where sulfuric acid can be detected in the used oil samples subsequent to the degradation of ZDDP but does not show a significant increase with the mileage.Follow-up tribological studies [36] revealed that once both dialkyl dithiophosphates and dialkyl thiophosphates are depleted, the tribofilms formed by the used oils contain less phosphorus and zinc, and the wear rate increases.

Combustion-Related Degradation in the Used Oils (Oxidation, Nitration, and AO Depletion)
Figure 4 displays the oxidation measured by FTIR in the whole field data matrix.The increase in oxidation is slightly slower in diesel vehicles compared to long-range petrol cars.In the case of the short-range petrol drivetrains, oxidation is further elevated.These differences once again can be attributed to the lower AFR in petrol engines [29].Figure 5a-d gives an overview of the identified oxidation products in the engine oil samples.Aromatic dicarboxylic diacids, e.g., m/z 165.019 (Figure 5a) as well as aromatic carboxylic acids e.g., salicylic acid (m/z 137.024) and methylated salicylic acid (m/z 151.040) were identified (Figure 5b,c, respectively).Additionally, oleic acid, an aliphatic carboxylic acid, was found (Figure 5d).All four mentioned organic acids are considered oxidation products, as they were not detected in the fresh oil.The abundance of all these degradation products seems to increase over mileage.Short-range petrol and long-range diesel samples display comparable low abundance of all oxidation products, which is in agreement with the FTIR evaluation which indicated similar oxidation values of the oils (see Figure 4).This is especially remarkable in regard of their considerably different mileages.
Long-range petrol vehicles show a significantly higher abundance.This is again corroborated by FTIR results (see Figure 4), where faster oxidation in the case of the petrol vehicles was highlighted.As discussed in detail in [29], the faster oxidation in petrol drivetrains can once again be attributed to the lower AFR compared to diesel vehicles.
Figure 6 gives an overview of the nitration of engine oils determined by FTIR.Similarly to oxidation, nitration is also slower in diesel engines compared to petrol ones, and the shortrange utilization profile once again results in a more rapid nitration product accumulation.
Lubricants 2023, 11, x FOR PEER REVIEW 11 of 21 Figure 6 gives an overview of the nitration of engine oils determined by FTIR.Similarly to oxidation, nitration is also slower in diesel engines compared to petrol ones, and the short-range utilization profile once again results in a more rapid nitration product accumulation.Figure 7a-c displays the abundance of various identified nitration products (m/z 168.032; m/z 154.015) and nitric acid (m/z 61.989) in the oil samples.The organic nitration products are aromatic, also containing oxo and hydroxyl groups.Previous investigations of used petrol engine oils [35] suggested similar aromatic structures as well.However, the aliphatic/organophosphate-based structures that were reported in [35] were not detected in this field study.The diesel engine oil only contains trace amounts of organic nitration products, whereas the petrol engine oils exhibit a higher abundance, most notably in the long-range petrol engine oil.This corresponds to the results obtained by conventional oil analysis (see Figure 6), where a considerably higher nitration in the case of the long-range petrol engines was found.
Figure 7c shows the abundance of nitric acid as a combustion by-product [34].Similarly to the discussed oxidation and nitration products, nitric acid also exhibits a lower abundance in the diesel engine oil than in the long-range petrol samples.The amount of nitric acid is comparable, even slightly lower than that of the short-range petrol engine oil, although the mileage of the diesel vehicle is almost seven times higher.Figure 7a-c displays the abundance of various identified nitration products (m/z 168.032; m/z 154.015) and nitric acid (m/z 61.989) in the oil samples.The organic nitration products are aromatic, also containing oxo and hydroxyl groups.Previous investigations of used petrol engine oils [35] suggested similar aromatic structures as well.However, the aliphatic/organophosphate-based structures that were reported in [35] were not detected in this field study.The diesel engine oil only contains trace amounts of organic nitration products, whereas the petrol engine oils exhibit a higher abundance, most notably in the long-range petrol engine oil.This corresponds to the results obtained by conventional oil analysis (see Figure 6), where a considerably higher nitration in the case of the long-range petrol engines was found.
Figure 7c shows the abundance of nitric acid as a combustion by-product [34].Similarly to the discussed oxidation and nitration products, nitric acid also exhibits a lower abundance in the diesel engine oil than in the long-range petrol samples.The amount of nitric acid is comparable, even slightly lower than that of the short-range petrol engine oil, although the mileage of the diesel vehicle is almost seven times higher.
Figure 8 displays the depletion of aminic AOs during the field test.The degradation of the AOs is analogous to the oxidation values.Petrol vehicles show a faster oxidative degradation, characterized by more pronounced oxidation (see Figure 4) and, hence, AO depletion, while diesels seem to be slower in this aspect.Short-range utilization in the case of petrol engines accelerates the AO degradation further, which can again be attributed to a lower AFR caused by the cold operation of those engines [29].However, aminic AOs are not completely depleted in any of the investigated vehicles.Figure 8 displays the depletion of aminic AOs during the field test.The degradation of the AOs is analogous to the oxidation values.Petrol vehicles show a faster oxidative degradation, characterized by more pronounced oxidation (see Figure 4) and, hence, AO depletion, while diesels seem to be slower in this aspect.Short-range utilization in the case of petrol engines accelerates the AO degradation further, which can again be attributed to a lower AFR caused by the cold operation of those engines [29].However, aminic AOs are not completely depleted in any of the investigated vehicles.Figure 8 displays the depletion of aminic AOs during the field test.The degradation of the AOs is analogous to the oxidation values.Petrol vehicles show a faster oxidative degradation, characterized by more pronounced oxidation (see Figure 4) and, hence, AO depletion, while diesels seem to be slower in this aspect.Short-range utilization in the case of petrol engines accelerates the AO degradation further, which can again be attributed to a lower AFR caused by the cold operation of those engines [29].However, aminic AOs are not completely depleted in any of the investigated vehicles.Figure 9a shows the antioxidant diphenylamine with a nonyl sidechain substituent on one of the phenyl groups (m/z 296.237).A chemically similar substance containing a  Figure 9a shows the antioxidant diphenylamine with a nonyl sidechain substituent on one of the phenyl groups (m/z 296.237).A chemically similar substance containing a nonyl sidechain on both phenyl groups (m/z 422.378), which showed comparable tendencies regarding abundance, was also found and is depicted in Figure 9b.The amount of aminic antioxidant is considerably reduced in all oil samples, but not completely depleted.The reduction in abundance is more pronounced with higher mileages, by trend.This confirms the findings of the FTIR (see Figure 8), which revealed the faster depletion of the aminic antioxidants over the mileage, but detectable residual amounts at the end of the field test.The short-range petrol engine oils show a comparable abundance with the other oil samples, despite their mileage being significantly lower, which indicates increased chemical stress due to the short-range utilization profile (lower AFR [29]).Figure 9c shows a degradation product of the aminic antioxidant (m/z 528.418).This and further chemically analogous species are only present in the petrol engine oils, and are especially abundant in the long-range petrol engine oils.Since these structures were not present in the diesel engine oils with a similar abundance, it can be assumed that different reaction pathways are pronounced in the various engine types and that this specific degradation product is characteristic of petrol engines.
The reduction in abundance is more pronounced with higher mileages, by trend.This confirms the findings of the FTIR (see Figure 8), which revealed the faster depletion of the aminic antioxidants over the mileage, but detectable residual amounts at the end of the field test.The short-range petrol engine oils show a comparable abundance with the other oil samples, despite their mileage being significantly lower, which indicates increased chemical stress due to the short-range utilization profile (lower AFR [29]).Figure 9c shows a degradation product of the aminic antioxidant (m/z 528.418).This and further chemically analogous species are only present in the petrol engine oils, and are especially abundant in the long-range petrol engine oils.Since these structures were not present in the diesel engine oils with a similar abundance, it can be assumed that different reaction pathways are pronounced in the various engine types and that this specific degradation product is characteristic of petrol engines.
Phenolic AOs were also detected in the fresh engine oil, e.g., di-tert-butyl-hydroquinone (m/z 221.153).The detailed analysis of phenolic AOs was not possible via the applied Orbitrap-MS method, as these species are impacted by ionization effects, especially when ZDDP is present in the engine oils.For an accurate determination of phenolic Aos, other techniques such as GC-MS would be required, as described in [39], which were not in the focus of the current investigation.Phenolic AOs were also detected in the fresh engine oil, e.g., di-tert-butyl-hydroquinone (m/z 221.153).The detailed analysis of phenolic AOs was not possible via the applied Orbitrap-MS method, as these species are impacted by ionization effects, especially when ZDDP is present in the engine oils.For an accurate determination of phenolic Aos, other techniques such as GC-MS would be required, as described in [39], which were not in the focus of the current investigation.

Boron Ester Antiwear Additive Degradation in the Used Oils
Figure 10 displays the boron content in the engine oils during the field study determined by ICP-OES.Boron shows a similar behavior to the previously discussed additives.The depletion is slower in diesel vehicles, and especially accelerated in the short-range petrol engines.Among other processes, boron depletion is caused by hydrolysis of the borate ester additives, with the subsequent formation of volatile boric acid which is removed with the exhaust gases [40].borate ester additives, with the subsequent formation of volatile boric acid which is removed with the exhaust gases [40].Figure 11a shows an identified borate ester species which is considered an antiwear additive.Multiple sidechains are detectable; additionally to the displayed tetradecyl group (C14; m/z 405.281), hexadecyl (C16; m/z 433.31) and octadecyl (C18; m/z 461.340) groups are present (one sidechain per aromatic ring is present; the exact position of the alkyl sidechain on the aromatic ring is not known, see 2.2).All identified species exhibit similar trends in abundance regardless of the length of the specific alkyl sidechain.As displayed, the original borate ester additive depletes rapidly during utilization, as it is not detectable in the used engine oils, except in the diesel samples with 5300 km mileage.Figure 11b shows the identified reaction products, where various combinations of the alkyl sidechains are present, namely C14/C14 (m/z 675.479),C14/C16 (m/z 703.510),C16/C16 (m/z 731.542),C16/C18 (m/z 759.573) and C18/C18 (m/z 787.604), collectively referred to as "long-sidechain" reaction products.These species are present in the fresh oil only in a low amount, then accumulate in the used oil samples with comparable abundance in all six oils, regardless of the engine fueling or utilization profile.During utilization in all engines, a new borate ester was formed with another aromatic compound.Since this species also contains C14, C16 and C18 sidechains, it can be assumed that these reaction products originate either through the "dimerization" of the original additive molecules, or through a reaction with the salicylate detergents, which also contain C14, C16 and C18 alkyl sidechains (see Figure 1).An accurate description of the reaction pathway would be possible using isotope tracing [41] but was not considered for this study.
As shown in Figure 11d, further similar species were identified, where a notable difference is that the alkyl sidechain on the second aromatic ring (depicted on the right side) is either a methyl group (C1) or missing completely (C0).The discovered species with a short or missing sidechain on the second aromatic ring are: C14/C0 (m/z 479.260),C14/C1 (m/z 493.276),C16/C0 (m/z 507.291),C16/C1 (m/z 521.307),C18/C0 (m/z 535.322) and C18/C1 (m/z 549.388), referred to as "short-sidechain" reaction products.The degradation products presented in Figure 11d are mostly present in the petrol engine oils in a higher concentration, and are especially abundant in the long-range oil samples, but also detectable in the short-range oil samples in traces.Compared to the abundance of the structure presented in Figure 11b, containing two long alkyl side chains, it is visible that different Figure 11a shows an identified borate ester species which is considered an antiwear additive.Multiple sidechains are detectable; additionally to the displayed tetradecyl group (C14; m/z 405.281), hexadecyl (C16; m/z 433.31) and octadecyl (C18; m/z 461.340) groups are present (one sidechain per aromatic ring is present; the exact position of the alkyl sidechain on the aromatic ring is not known, see 2.2).All identified species exhibit similar trends in abundance regardless of the length of the specific alkyl sidechain.As displayed, the original borate ester additive depletes rapidly during utilization, as it is not detectable in the used engine oils, except in the diesel samples with 5300 km mileage.Figure 11b shows the identified reaction products, where various combinations of the alkyl sidechains are present, namely C14/C14 (m/z 675.479),C14/C16 (m/z 703.510),C16/C16 (m/z 731.542),C16/C18 (m/z 759.573) and C18/C18 (m/z 787.604), collectively referred to as "long-sidechain" reaction products.These species are present in the fresh oil only in a low amount, then accumulate in the used oil samples with comparable abundance in all six oils, regardless of the engine fueling or utilization profile.During utilization in all engines, a new borate ester was formed with another aromatic compound.Since this species also contains C14, C16 and C18 sidechains, it can be assumed that these reaction products originate either through the "dimerization" of the original additive molecules, or through a reaction with the salicylate detergents, which also contain C14, C16 and C18 alkyl sidechains (see Figure 1).An accurate description of the reaction pathway would be possible using isotope tracing [41] but was not considered for this study.
As shown in Figure 11d, further similar species were identified, where a notable difference is that the alkyl sidechain on the second aromatic ring (depicted on the right side) is either a methyl group (C1) or missing completely (C0).The discovered species with a short or missing sidechain on the second aromatic ring are: C14/C0 (m/z 479.260),C14/C1 (m/z 493.276),C16/C0 (m/z 507.291),C16/C1 (m/z 521.307),C18/C0 (m/z 535.322) and C18/C1 (m/z 549.388), referred to as "short-sidechain" reaction products.The degradation products presented in Figure 11d are mostly present in the petrol engine oils in a higher concentration, and are especially abundant in the long-range oil samples, but also detectable in the short-range oil samples in traces.Compared to the abundance of the structure presented in Figure 11b, containing two long alkyl side chains, it is visible that different reaction partners have to be involved, as the relative abundances show completely different trends despite the similarity of the structures.
abundant in the long-range petrol engine oil, corresponding to the highest oxidation.These reaction products display a smaller abundance in the short-range petrol engines and only traces in the diesel samples, going along with the low abundances of the degraded borate esters with C0 and C1 sidechains.Hence, it is most likely that recombination with oxidation products is responsible for the origin of the structures presented in Figure 11d.Figure 11c displays two identified oxidation products, (salicylic acid and methylated salicylic acid).Regardless of whether the reaction products originate from "dimerization" of two borate esters or reaction with salicylates, it seems likely that a side reaction with the presented oxidation products can also occur.The relative abundances of the oxidation products also support this theory, as salicylic acid and 5-methyl salicylic acid are most abundant in the long-range petrol engine oil, corresponding to the highest oxidation.These reaction products display a smaller abundance in the short-range petrol engines and only traces in the diesel samples, going along with the low abundances of the degraded borate esters with C0 and C1 sidechains.Hence, it is most likely that recombination with oxidation products is responsible for the origin of the structures presented in Figure 11d.
Figure 12 gives an overview of all boron ester species in the long-range petrol oil samples.In the position on the second ("right") aromatic ring, either the original alkyl sidechains (C14, C16, C18) or the two displayed variants (C0, C1) were found; no other species could be identified.It also has to be mentioned that the measured counts of any given degradation product with short sidechains are lower compared to the long-sidechain counterparts, as shown in Figure 12.The individual counts are influenced by the abundance and ionization capability of the respective species.In this case, the ionization capability can be assumed to be comparable since the structures are very similar.However, for clear evidence, ionization studies would have to be performed.Nevertheless, it can be assumed that the long-sidechain species represents the main reaction pathway, which occurs in both diesel and petrol engines in comparable abundance, while the C0 and C1 species are the result of a side reaction unique to the petrol samples.Figure 12 gives an overview of all boron ester species in the long-range petrol oil samples.In the position on the second ("right") aromatic ring, either the original alkyl sidechains (C14, C16, C18) or the two displayed variants (C0, C1) were found; no other species could be identified.It also has to be mentioned that the measured counts of any given degradation product with short sidechains are lower compared to the longsidechain counterparts, as shown in Figure 12.The individual counts are influenced by the abundance and ionization capability of the respective species.In this case, the ionization capability can be assumed to be comparable since the structures are very similar.However, for clear evidence, ionization studies would have to be performed.Nevertheless, it can be assumed that the long-sidechain species represents the main reaction pathway, which occurs in both diesel and petrol engines in comparable abundance, while the C0 and C1 species are the result of a side reaction unique to the petrol samples.

Summary
An overview of the obtained results is presented in Tables 4 and 5.

Summary
An overview of the obtained results is presented in Tables 4 and 5.

Conclusions
The effect of fueling (petrol vs. diesel) and utilization profile (short-range vs. longrange) on engine oil degradation was investigated.The MS analysis was focused on commonly utilized oil additives, namely antioxidants, antiwear additives and detergents.Furthermore, established degradation products, such as oxidation and nitration species, were analyzed.
Petrol engine oils generally showed higher oxidative degradation compared to diesel engine oils, which was further accelerated in the case of a short-range utilization profile.This was visible in the higher abundance of both aliphatic and aromatic organic acids as well as nitration products.Additionally, antioxidant and antiwear additive depletion was faster in the petrol engine oils, especially in the case of the short-range utilization profile considering the low mileage.Regarding ZDDP degradation, differences between petrol and diesel engine oils were highlighted, where petrol engines displayed a more advanced depletion of the original additives as well as a higher abundance of various degradation products.The short-range petrol engines showed expedited additive degradation in multiple cases, e.g., borate ester and ZDDP.These additives exhibited close to complete depletion as well as a high abundance of various corresponding degradation products, such as alkyl phosphates and organic acids, being found, despite the relatively short mileage.
For petrol engines, differences in the borate ester antiwear additive degradation were also detected, possibly related to the mentioned higher abundance of various organic acids.In the case of petrol engine oils, a side reaction between borate esters and aromatic carboxylic acids (oxidation products) was indicated, which was not present in diesel engine oils.This highlights the high interrelation of chemical degradation in the complex systems of internal combustion engine oils and how differences in one degradation process can influence another reaction on the molecular level, simply by supplying further compatible reactants.Similar variations in the degradation of aminic antioxidants were also shown.
Concludingly, it can be stated the presented study provides a valuable input in understanding oil degradation in internal combustion engines, especially when it comes to variations in fueling or utilization profile.This approach can be used to understand oil-fuel interactions in the future, especially in cases where alternative combustion fuels are used.In the case of synthetic hydrocarbons, biologically-sourced fuels and carbon-free alternatives, such as hydrogen or ammonia, new degradation mechanisms and reaction pathways are expected.For example, when ammonia is utilized as a fuel for diesel combustion, an increase in TBN is possible due to the alkalinity of the fuel [42].Such interactions were previously not reported, which highlights why oil-condition monitoring techniques have to evolve together with ICE fuels.Based on an MS approach, appropriate countermeasures to degradation mechanisms, such as oil optimization, can be established enabling prolonged oil service life, and hence more economical and sustainable lubrication.

Figure 1 .
Figure 1.Overview of the additives present in the fresh oil.

Figure 1 .
Figure 1.Overview of the additives present in the fresh oil.

Figure 2 .
Figure 2. ZDDP evaluated by FTIR.Samples selected for MS indicated by full markers.

Figure 2 .
Figure 2. ZDDP evaluated by FTIR.Samples selected for MS indicated by full markers.

Lubricants 2023 , 21 Figure 4 .
Figure 4. Oxidation evaluated by FTIR.Samples selected for MS indicated by full markers.

Figure 6 .
Figure 6.Nitration evaluated by FTIR.Samples selected for MS indicated by full markers.

Figure 6 .
Figure 6.Nitration evaluated by FTIR.Samples selected for MS indicated by full markers.

Figure 8 .
Figure 8. Residual amine AO evaluated by FTIR.Samples selected for MS indicated by full markers.

Figure 8 .
Figure 8. Residual amine AO evaluated by FTIR.Samples selected for MS indicated by full markers.

Figure 8 .
Figure 8. Residual amine AO evaluated by FTIR.Samples selected for MS indicated by full markers.

Figure 10 Figure 9 .
Figure10displays the boron content in the engine oils during the field study determined by ICP-OES.Boron shows a similar behavior to the previously discussed additives.The depletion is slower in diesel vehicles, and especially accelerated in the short-range petrol engines.Among other processes, boron depletion is caused by hydrolysis of the

Figure 10 .
Figure 10.Boron content evaluated by ICP-OES.Samples selected for MS indicated by full markers.

Figure 10 .
Figure 10.Boron content evaluated by ICP-OES.Samples selected for MS indicated by full markers.

Figure 11 .
Figure 11.Borate ester and respective degradation products: (a) abundance of a fresh borate ester, (b) abundance of degraded borate esters, long sidechains (c) abundance of oxidation products, (d) abundance of degraded borate esters, short sidechains.

Figure 11 .
Figure 11.Borate ester and respective degradation products: (a) abundance of a fresh borate ester, (b) abundance of degraded borate esters, long sidechains (c) abundance of oxidation products, (d) abundance of degraded borate esters, short sidechains.

Figure 12 .
Figure 12.Borate ester degradation products in the long-range petrol samples.All detected configurations: only C0, C1, C14, C16 and C18 sidechains are present, other configurations are largely absent.

Figure 12 .
Figure 12.Borate ester degradation products in the long-range petrol samples.All detected configurations: only C0, C1, C14, C16 and C18 sidechains are present, other configurations are largely absent.

Table 2 .
Vehicle parameters of the conducted field test.Vehicles selected for mass spectrometry (MS) analysis are indicated in bold.

Table 4 .
Summary of MS results-Additives.Not detected or present in traces only; +, ++ and +++: Present in low, moderate, and high abundance.

Table 5 .
Summary of MS results-Degradation products.Not detected or present in traces only; +, ++ and +++: Present in low, moderate, and high abundance.