LUBRICATE ENGINE OIL CHARACTERIZATION BASED
ON SINGLE EXCITATION FLUORESCENCE SPECTROSCOPY
In the work the fluorescence spectroscopy based on fluorescence spectra for single excitation wavelength and excitation spectra for single emission wavelength were applied to expand the knowledge about the specific signatures of lubricate oil typically used in ship engine as a tool to oil quality assessment.
The fluorescence properties of oil are considered based on an exemplary lubricate-engine oil in two forms: fresh and used oil – exploited an adequate time. Based on the fluorescence spectra for single excitation wavelength and excitation spectra for single emission wavelength the changes of the spectra for both forms of oil are discussed and the peaks typical for both forms of oil were determined. Moreover the specified fluorescence signatures described by selected fluorescence spectrum for excitation wavelength 220 nm as a tool to distinguish the two forms of oil were determined. Additionally, there is discussed the use of total excitation spectrum as a tool to identification two forms of oil.
Keywords: fluorescence spectroscopy, fluorescence spectrum, excitation spectrum, lubricate oil, ship engine.
INTRODUCTION
The driving force of most modes of transport is generated by internal combustion engines whose proper functioning depend on quality of consumables – primarily on the parameters of lubricate oil. Greatest of lubricate oils are formed by crude oil processing plants. Crude oil and its derivatives (such lubricate oils and fuels) are highly complex mixture of aliphatic and mono- or poly-cyclic hydrocarbon compounds [7]. Taking into account the complex composition of oils a need to expand possibility of oil descriptions and characterisation arises. Due to that fact the wide spectrum of method depending on the oil application are in use. For example, if marine environment is taking into consideration, measurements carried out in these area are based on various methods such chromatography, Raman-, infrared- or visible spectroscopy [5, 6, 9, 10]. On the other hand, in our work we are interested in the specific indicators which allow to track exploitive quality of oil used in ship engine. In the transport sector methods for monitor oil properties after exploitation in combustion engines exists yet. Namely, the specific indicators of oils are based on the constituents of oil – especially water content, total acid number and total base number or viscosity vs. temperature [1].
Taking into account that lubricate oils contain aromatic components responsible for oil fluorescence in ultraviolet light it should be considered the fluorescence spectroscopy as a tool to expand oil signatures description in relation to the oil exploitation in the ship engine. So far fluorescence spectroscopy was used to description of lubricate oil used in ship engine based on excitation-emission spectra [3] or total synchronous fluorescence spectra [4, 8].
In below described analyses the quality of oil used in ship engine is analysed based on fluorescence spectra for single excitation wavelength and on excitation spectra for single emission wavelength. Using these measurements the specific spectra for both kinds of method were selected as favourable for description of considered kind of lubricate oil (used in ship engine).
1. MATERIAL AND METHOD
The exemplary lubricate oil Titan Truck Plus 15W40 (TTP 15W40) – in two forms: fresh oil (T) and used oil (UT) exploited 1344 hours in engine 3AL 25/30 Cegielski-Sulzer with the power 396 kW, was used [2]. To prepare dilutions of both forms of oil (concentration 100 mg/kg) n-hexane solvent with 96% purity was used. A detailed description of the sample preparation procedure has been described by the authors in the earlier paper [3].
To obtain single excitation fluorescence spectra of oil the excitation-emission fluorescence spectroscopy was applied. To measure excitation-emission spectra (EEMs) of oil spectrofluorometer Hitachi F-7000 FL was used. EEMs of oils were measured in 1×1 cm quartz cuvette. Excitation-emission spectra were performed for excitation wavelength from 200 nm to 340 nm with excitation sampling interval 5 nm, emission wavelength from 260 nm to 450 nm with emission sampling interval 5 nm. For other apparatus elements the individuals settings were applied respectively: excitation slit 5 nm, emission slit 5 nm, integration time 0.5 s and photomultiplier tube voltage 400 V. Measurement of lubricate oil diluted in n-hexane at a stabilised temperature 20º C were performed.
To obtain the real EEMs of lubricate oil each date of measured oil samples diluted in n-hexane were corrected by subtraction the spectra of pure n-hexane.
2. RESULTS AND DISCUSSION
The exemplary EEM spectrum for used lubricate oil is shown in Figure 1. The main spectroscopic information of EEM spectrum are fixed in the particular axis respectively: axis X describes excitation wavelengths, axis Y describes emission wavelengths and axis Z describes intensity of fluorescence. However for
location and determination the specific peaks for this kind of oil in both forms the EEMs spectra in two dimensional plane as a surface-contour visualisation were presented in Figure 2.
In this figure are visible three specific maxima described by excitation wavelengths corresponding to the emission wavelengths signed as (Exmax/Emmax) for fresh and used oil respectively: 220/295, 235/340, 275/320; 220/295, 235/330, 280/330. The peaks were determined and described in details by authors in the earlier paper [3]. However, in the present case the essential aim is to obtain the fluorescence signatures of lubricate oil in both forms of oil (fresh and used one), respectively, described by the minimal base of fluorescence signatures as the possibility to describe lubricate oil based on define kind of oil. Therefore it is worth to analyse the emission spectra for single excitation wavelength for this kind of oil, which allow to describe the specific fluorescence points by fluorescence spectrum for single excitation wavelength. Based on presented surface-contour fluorescence spectra of oil in Figure 2 the specified fluorescence spectra for single excitation wavelengths were determined. The fluorescence spectra were selected from the whole EEM spectrum in three maximum points for emission wavelength for single excitation wavelengths 220 nm, 235 nm and 275 nm for both forms of this kind of oil signed as C, B, A in Figure 2 respectively. Determined fluorescence spectra for single excitation wavelength 220 nm – C, 235 nm – B and 275 nm – A are presented in Figure 3.
In this figure there is visible the difference in fluorescence intensity for both forms of oil. The intensity of fluorescence decreases after oil exploitation in ship engine. Moreover, in this figure the changing of fluorescence spectrum shape in relation to the excitation wavelength is observed. In the shape of fluorescence spectra are observed two maxima for the lower excitation wavelength 220 nm, however for higher excitation wavelength the fluorescence spectrum is described only by one fluorescence maximum at 320 nm corresponds to excitation wavelength 275 nm and at 340 nm corresponds to excitation wavelengths 235 nm.
Taking into account presented results one can conclude that fluorescence method based on fluorescence spectrum for single excitation wavelength probably allow to distinguish both forms of oil fresh and used one. This is clearly visible in fluorescence spectrum for excitation wavelength 220 nm, in which the significant difference in the shape for fresh and used oil manifests itself. For fresh oil in the shape of fluorescence spectra are observed two fluorescence maxima at 300 nm and 335 nm for the same intensity of fluorescence. In opposite for used oil, where the second maximum fluorescence is achieved at 335 nm, but decreases in relation to the same fluorescence maximum for fresh oil and for fluorescence maximum for lower emission wavelength 295 nm. The change in the shape of fluorescence spectrum can be taken into account as significant fluorescence signature to both forms of oil identification.
Fig. 1. Excitation-emission spectra in three dimensional plane of Titan Truck Plus 15W40 oil diluted in n-hexane for oil concentration c = 100 [mg/kg]
for used oil working 1344 h in ship engine [3]
Fig. 2. Excitation-emission spectra as a surface-contour visualization in two dimensional plane of Titan Truck Plus 15W40 oil diluted in n-hexane for oil concentration c = 100
[mg/kg] for fresh oil (left) and used oil (right) working 1344 h in ship engine
Moreover, important information are fixed in the shape of excitation spectra for single emission wavelength. Based on presented in Figure 2 surface-contour fluorescence spectra of oil both for fresh and used oil the specified excitation spectra for single emission wavelengths were determined. The excitation spectra were selected from the whole EEM spectrum in three maximum points for excitation wavelength for single emission wavelengths 295 nm, 320 nm, 330 nm
and 340 nm for both forms of this kind of oil signed as a, b, c and d in Figure 2 respectively. The determined excitation spectra for single emission wavelength 295 nm – a, 320 nm – b, 330 nm – c and 340 nm – d are presented in Figure 4.
In the figure there is visible the difference in the intensity of excitation spectra for both forms of oil. The intensity of excitation decreases after oil exploitation in an engine. Moreover, in this figure significant differences between excitation spectrum shape for various emission wavelength are observed. In the shape of fluorescence spectra are observed two maxima for each individual emission wavelength. Those results are not appropriate to distinguish both forms of oil.
Fig. 3. Fluorescence spectra of oil Titan Truck Plus 15W40 diluted in n-hexane for oil concentration c = 100 [mg/kg] for fresh oil (left) and used oil (right) for different excitation wavelengths, C – 220 nm, B – 235 nm, A – 275 nm
Fig. 4. Excitation spectra of oil Titan Truck Plus 15W40 diluted in n-hexane for oil concentration c = 100 [mg/kg] for fresh oil (left) and used oil (right) for different emission wavelengths, a – 295 nm, b – 320 nm, c – 330 nm
However, we can use the excitation spectra to distinguish different forms of oil taking into account the excitation fluorescence spectrum as a sum of intensities for every excitation wavelength. That procedure allows to determine the cumulative excitation spectrum.
In Figure 5 cumulative excitation spectrum for both forms of oil fresh (left) and used one (right) are presented. In this figure there is visible the difference in the intensity of cumulative excitation spectra both forms of oil. These spectra are characterised by two excitation maxima at 235 nm and 275 nm for both forms of oil. For fresh oil excitation maximum at 275 nm achieved the maximal value. The opposite situation in peaks position we observed for used oil, where the maximal value achieved peak at 235 nm for excitation wavelength while the second maximum at 275 nm for excitation wavelength decreases after oil exploitation in an engine.
Fig. 5.Total excitation spectra of oil in n-hexane for Titan Truck Plus 15W40 for oil concentration c = 100 [mg/kg] for fresh (left) and used oil (right)
working 1344 h in ship engine
Above results also leads to the conclusion that dividing the fluorescence intensity for 275 nm to intensity for 235 nm can constitute the index related to the lubricate oil overworking degree. In this case fresh oil is characterised by just defined wear indicator equal 1.05 whereas used oil (after 1344 h working in engine) has a value of 0.79. Following after such thinking the idea of the fluorometric oil quality monitor is imaginable which is consisted of two sources of monochromatic light (above results suggest that 235 and 275 nm) and the broadband light detector (measurer of the cumulative fluorescence).
Certainly worth examining to what extent the data obtained refer to other types of lube oil.
CONCLUSIONS
Presented study basing on fluorescence spectra and excitation spectra demonstrate possibility to distinguish lubricate oil in two forms fresh and after exploitation in combustion engine.
In the case when the fluorescence spectra for defined excitation wavelength are considered excitation wavelength at 220 nm is identified as the appropriate to identify two forms of oil due to the change of intensity of fluorescence at 295 nm and 335 nm for emission maximum for both forms of oil.
When the excitation spectra for defined emission wavelength are considered, there is worth to performed the totalizing procedure of excitation spectra for single emission wavelength. That approach has practical reference, namely corresponds with possible simplified fluorometer in which two filters (or lasers, in this studied case 235 nm and 270 nm) instead of exciting monochromator would be applied, whereas emitting light monochromator would be unnecessary, because fluorescence light can be captured by the broadband light detector.
ACKNOWLEDGEMENTS
This paper was partially supported by the Gdynia Maritime University grants No. DS/411/2015 and 06/BMN/M/2015.
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CHARAKTERYSTYKA SILNIKOWEGO OLEJU SMARNEGO NA PODSTAWIE WIDM FLUORESCENCJI
DLA POJEDYNCZEJ DŁUGOŚCI FALI WZBUDZENIA
Streszczenie
W pracy przedstawiono metodę spektroskopii fluorescencyjnej jako narzędzia do śledzenia jakości oleju silnikowego, które pozwoli na poszerzenie wiedzy odnośnie do typowych wskaźników różnicu-jących oleje. Analizę właściwości fluorescencyjnych silnikowego oleju smarnego przeprowadzono, wykorzystując dwa rodzaje widm: widma fluorescencji dla pojedynczej długości fali wzbudzenia oraz widma wzbudzenia dla pojedynczej długości fali emisji.
Fluorescencyjne właściwości oleju dyskutowane są w odniesieniu do przykładowego silnikowego oleju smarnego w dwóch formach, odpowiednio, oryginalnej oraz zużytej podczas pracy w silniku okrętowym w odpowiednim czasie pracy. Na podstawie widm fluorescencji, jak również widm wzbu-dzenia, dyskutowane są zmiany w kształcie widm w odniesieniu do obu form oleju oraz wyznaczone specyficzne piki dla obu form oleju. Ponadto wyznaczono charakterystyczne wskaźniki fluorescen-cyjne dla rozważanego rodzaju oleju z wykorzystaniem zarówno wyselekcjonowanego widma fluorescencji dla długości fali wzbudzenia 220 nm, jak i wyznaczonego całkowitego widma wzbu-dzenia jako narzędzia do rozróżniania obu form oleju.
Słowa kluczowe: fluorescencja, widmo fluorescencji, widmo wzbudzenia, olej smarny, silnik okrętowy.
