Analysis of metal content in the lubricating oils used in truck diesel engines

The lubricating oil of an internal combustion engine (I.C.E.) performs several functions with a major contribution to the reliability of the system, which involves proper operation during the operating period. Changing the engine oil early or late could generate additional costs and/or damage the engine’s performance. Thus, it becomes necessary to periodically analyse the oil used in the engine. In this work, the authors propose to analyse the metal content of the used engine oil and, based on the results obtained, to issue maintenance strategies. Samples taken from a number of 43 DAF trucks equipped with the same type of engine, MX - 11 330 H2, EURO 6, which use the same assortment of 10W30 oil, were analysed.


Introduction
Lubricating oils are decisive in reducing the process of friction between an engine moving element and serve to diminish their wear in the period of technical systems operation, which surely conduct to increase total efficiency, along with energy and fuel economy.Engine oils are the most universal of all lubricants and are commonly used for gasoline and diesel engine vehicles.It is well known that engine oils are composed of the base oil and a maximum of 20% additives [1].The base oils can be mineral oils, obtained in the petroleum distillation process, or synthetic oils.
In the period of engine operation, different situations/changes can occur, which facilitate the appearance of contamination, gradually leading to the diminishing of oil quality and its properties.In conclusion of that, the specialists in the domain recommend technical overhaul and the lubricating agent need to be eliminated from the engine and replaced with a fresh one.
The oils used in powered vehicles can intermediate the appearance of a big environmental problem, especially when is about logistics of the waste disposal.It is interesting that this process is based on the reality that the engine oil is found in small quantities and utilized by a huge number of car owners which are in distinctives sites.Thus, according to [1], in Europe, the quantity of engine oils is considered large being between 1.7 and 3.5 million tons and varies every year.For that, it can be said that these quantities of used engine oils imply important environmental and economic consequences.
The chemical composition of used oils is based on important factors, such as the base oil and additives used, the conditions to which the oil was subjected during the engine's operation.Thus, the analysis of the variation of the physico-chemical properties of the engine oil during its use is a subject of interest for the reliability of vehicles.Used oils may contain, among other elements, hydrocarbons, water, glycol and heavy metals [2][3][4].
High concentration of heavy metals can be present in engine oils and this fact implies many discussions and therefore interests.The biggest issue is highlighted when it is about the adverse effects on the human health and environment.There are different sources of metals in used oils, first it is well known that they can be native parts of crude oil and from the additives [5].Secondly, metals can appear from the action of wear and corrosion of the metal surfaces in contact during operating period and also from the deterioration process of used oil, like oxidation process, additives transformation in solid residues, etc.
It's well known the possibility that the engine oil to have microscopic metal shavings on occasion and it is a truth that their appearances should be impossible to see with the naked eye.If the metal shavings in the oil have dimensions that make them easily visible, they become very dangerous for the engine.
During the cold start of the engine, it is known that the lubrication is deficient and particles resulting from the dry friction of the engine parts may appear in the oil.However, the accumulation of metal shavings that happens unexpectedly can reveal a sign of wear inside the engine [3].
So, without appropriate lubrication, the engine metal elements begin to rub producing wear, allowing shavings to consume from the parts.The next step is that these metal shavings are left in the oil and this process can lead to significant issues as they are circulated through the engine [6].The metals which can be found in engine oil are iron, copper, bronze or brass, zinc, nickel, molybdenum or chromium, manganese, aluminium, etc. [7,8].
This paper aims to complete with one step the research gap in the domain by analysing the concentrations of selected heavy metals (Cu, Fe, Zn) in used oils from the same diesel engine which equips trucks of different mileage and different year of manufacture (2016 -2021).Moreover, another objective of this paper is to make preliminary diagnose of internal combustion engines to emit preventive maintenance strategies in the period of operation.
The conclusions of the study can also help to make an important and effective decision regarding the timing of the technical overhaul, but also regarding the waste oil management strategies [9].

Methodology and experimental setup
The determination of the metals content in the oil used in diesel truck engines is carried out by X-ray fluorescence (XRF) spectrometry.This is a non-destructive analytical technique used for the quantitative and qualitative analysis of the composition of materials.Briefly, this analysis consists of irradiating a sample (solid or liquid) with high-energy X-rays.When an atom is hit with such an X-ray, an electron on an inner layer (K or L) is displaced.The stability of the atom will be restored by filling the vacant place with an electron coming from a higher energy layer.Following this relocation of the electron, its energy decreases by releasing a fluorescent X-ray (fluorescent light).The energy of this ray is, obviously, equal to the energy difference between the two states of the electron.The basis of XRF analysis is, in fact, the measurement of this energy [10].The equipment used in this research is Spectro Midex M which is shown in figure 1, a multifunctional X-ray micro fluorescence (XRMF) analysis system.Samples in solid or liquid form are very small and are not destroyed by analysis, as stated previously.The detector of the device measures the X-photons emitted from the sample under analysis, these being counted with a impulses counter.The number of impulses is entered into the data processing system of the device under the name intensity I.The value of the intensity thus measured is associated with the concentration of the respective element in the sample [11] .
The spectra resulting from the analysis of the sample show a graphic representation of the intensity peaks.Each peak indicates an element, and the height of the peak indicates the concentration of the element in the analyzed sample [10].Taking into account the above, 43 samples of used engine oil collected under similar conditions from DAF trucks that have the same type of engine with characteristics presented in table 1 were analyzed.All the trucks were manufactured between years 2016 and 2021.

Engine type
Diesel, EGR, DPF, SCR/Paccar MX-11330 H2 Cylinder capacity 10800 cm 3   Engine power@speed 449 HP@1600 rpm Emissions standards EURO 6 Mileage 100000 -800000 km Type of the engine oil DAF Xtreme FE 10W30 (approved by the manufacturer) Among the component elements detected in the analyzed samples, in this work we focused on Fe, Cu and Zn.Iron is part of the alloys from which the cylinders, the crankshaft, are made and, through their wear, iron particles can be found in the engine oil.Iron can also come from the corrosion of parts in contact with water.Water can get into the engine through various ways, such as: the fuel, the environment, from the cooling system through leaks.Copper particles can get into the engine oil through the wear of bearings and heat exchangers.Also, copper is used as a lubrication additive in systems subject to severe demands.Zinc is found in EP (extreme pressure) type additives, used in lubricants intended for systems that work under extreme pressure conditions.It is also used in bearing alloys and is a common material for metal coatings [13].
A qualitative analysis was made of the resulting spectra for the 43 samples comparing with the spectrum of the unused oil.Considering the relationship between the number of impulses seen in the spectrum and the concentration of the component in the analyzed sample, as presented above, we calculated the difference between the number of impulses for each component studied in the sample (used oil) and the unused oil, noting with DIF_Fe, DIF_Cu respectively DIF_Zn.Afterwards, the average value was determined for the samples coming from trucks with similar mileage.

Results and discussion
Relying on the methodology presented, the analysis was made for the iron content presented in figure 3. Figure 3a shows DIF_Fe, and figure 3b shows the average DIF_Fe by mileage categories.
Figure 3b shows that the average iron content decreases with the increase in the number of kilometers traveled by the truck, i.e., with the total mileage of the truck.In our opinion, when the truck is new, that is, it has low mileage, the grinding of the parts is intense, the particles of material that break due to the friction of the moving parts are increased.A slight increase is observed for trucks with a mileage of over 700,000 km.In the category of trucks with a mileage of 700,000 km, there are two samples (Sno_30 and Sno_33) that exceed the average for this segment of trucks (fig 3a).In order to have a better idea of the trend regarding the iron content in the samples, when calculating the average value of DIF_Fe, the two samples were eliminated.
The high iron content in the two samples leads to an increase in viscosity and sediment level, which is in agreement with the results regarding the analysis of the physico-chemical properties of the same samples of used motor oil presented by the authors in the work published in [4].
Figure 4 shows the spectra of the two samples overlapped on the spectrum of fresh engine oil in which the high Fe content is clearly highlighted.
For the trucks from which the Sno_30 and Sno_33 samples were collected, it is recommended to check the oil level (the amount in the engine).The degree of wear of parts such as: segments, cylinders, bearings must also be checked.Figure 5 shows the copper content.In Fig. 5b, which shows the DIF_Cu average by mileage category, it can be seen that the Cu content, as in the case of iron, decreases with the increase in mileage.The explanation can be that in the first hundred thousand kilometers traveled, the engine is in run-in period.From figure 5a showing DIF_Cu, it can be seen that the Sno_30 sample has an increased copper content.In the calculation of the average value of DIF_Cu, this value was excluded (as in the case of iron).Therefore, the maintenance recommendations previously made for the truck from which this sample was collected become mandatory.Figure 6 shows the zinc content.In fig.6b, which shows the average value of DIF_Zn by mileage category, it can be seen that the content of Zn, as in the case of iron and copper, is decreasing with the increase of mileage.The explanation can be that in the first hundred thousand kilometers traveled, the engine is in run-in period.For trucks with high mileage, over 500,000 km, the zinc content begins to increase slightly.The degree of wear of parts such as: segments, cylinders, bearings make their deterioration process accelerate.
From figure 6a, which shows DIF_Zn, it is also noticeable that the sample Sno_30 shows an increased content of zinc.In the calculation of the average value of DIF_Zn, this value was excluded.The maintenance recommendations made previously for the truck from which this sample was collected are kept.
Figure 6 shows that some samples were excluded, because negative DIF_Zn values were obtained, which makes us believe that there were measurement errors for these cases.Although, the work does not propose to study the manganese contamination of the samples, nevertheless, out of the 43 samples, manganese was found in three samples: Sno_2, Sno_5, Sno_30.The first two are in the 100,000 km category, the third in the 700,000 km category.Manganese can come from the intense abrasion process during break-in (Sno_2, Sno_5) or for sample Sno_30 it can be caused by insufficient lubrication (small amount of oil, considerable oil loss), or from the addition of an engine oil that contains an additive on manganese base.Figure 7

Conclusions
It is well known the close connection between the reliability of the vehicle engine and the lubrication performance, the subject being of major interest both for manufacturers and for users, especially in the case of vehicle fleet owners.
Thus, vehicle manufacturers recommend precise periods of technical overhaul.Only that, in the period between technical overhaul, the conditions of vehicle operation, vary widely and sometimes unexpectedly and can affect engine oil quality in different ways.From here, two directions of discussion and analysis can be distinguished: on the one hand, the oil deteriorates rapidly, and compliance with the overhaul period can lead to early wear of the engine with considerable economic implications.On the other hand, the oil is not damaged, and it can be used even after the overhaul period, with economic consequences, as well, but also in terms of the environment, i.e. reducing the amount of waste.Considering what has been said, an analysis of the oil used in the engine can lead to the right decisions.
Metal contamination of engine oils during operation can occur for various reasons and can produce major negative effects on the engine.Determining the content of iron, copper and zinc can lead to a quick diagnosis of the engine.These metals are part of the alloys from which different engine components are made, but also of the composition of the various lubricant additives.
From the analysis presented in this paper, it can be seen that the oil from engines with a mileage of less than 100,000 km, which are therefore in the break-in period, have a higher metal content than in any other category of mileage.It has been observed that the amount of metals in the oil decreases with the increase in engine speed.The oils that come out of this trend lead to the idea that the engines that worked with these oils have problems, so they should be diagnosed and put in a maintenance process.It must be checked if the amount of oil in the engine is within the prescribed limits, what is the degree of wear of the parts (cylinders, segments, bearings, valves, etc.), if the operator has not topped up with another type of oil that has different basic elements, especially additives.
Consequently, this paper demonstrates that it is possible to diagnose engines by analyzing the physico-chemical properties of the oil used in the engine and issuing effective maintenance strategies.Moreover, the results of this paper regarding contamination with metals are in agreement with the results obtained in the paper [3,4] regarding the physical properties of the same oil samples.

Figure 4 .
Figure 4. Spectra of the Sno_30 and Sno_33 samples overlapped on the spectrum of fresh engine oil.

Figure 7 .
shows the spectra of samples a) Sno_2 and b) Sno_5 overlapped on the spectrum of fresh oil.Spectra of samples a) Sno_2 and b) Sno_5.