The Multiple Criteria Assessment on the selected vegetable oils for the lubricating engine

Vegetable oils are considered to be an alternative and environmentally friendly resource of lubricants. Suitable for almost all industrial applications. They are obtained from natural raw materials, which are renewable and non-toxic to humans, living organisms, and the environment. Chemically, green oils are triacylglycerols with varying fatty acids compositions depending on the plant or seed, the crop, the growing conditions and the season, etc. The conducted research concerning matching the proper of type of vegetable oil to lubrication of the engine. The main objectives of engine oil have been drawn to the attention. To project of assessment has selected four roles of engine oil: lubrication, cooling, corrosion protection, and others (including oxidative stability, ash content, saponification value, and foam property). Each objective is described by a couple of detailed parameters of lubricants (for example viscosity index, coefficient of friction, wear scar diameter, pour point, cloud point, melting point, smoke point, flash point, iodine value, thermal conductivity, specific heat capacity, density, water content, acid number, anti-corrosion properties, etc.). In the evaluation process, AHP (Analytic Hierarchy Process) methods were used to determine the weights of individual criteria. Everything creates multiple criteria assessment of green oil. As a result of the multi-criteria assessment, most high scores have soybean oil, linseed oil, cottonseed oil, and olive.


Introduction
Lubricating oils are commonly used in automotive, industrial, process, and marine applications. In order to reduce consumption of energy by means of minimizing friction and wear of moving surfaces, cooling of elements, and extend the service life of mechanical devices [1]. In general, lubricants are composed of base oils and adjusted additives. Currently, a lot of bases are mineral oils derived from crude oil. However, the crude reserve is depleting and the environmental concern about the damaging impact of mineral oil is growing. There is a diligent demand for promising alternatives for mineral base stocks, which may be vegetable oils [2].
The use of lubricants derived from natural raw (seeds, plants) has been in practice for many years. However, this idea was neglected due to the discovery of the aforementioned crude oil and the availability of low-cost mineral oils. Green oils are a quite good solution for commercial lubricants, since they retain a lot of technical properties of mineral oils.
The most important attributes of natural lubricants are non-toxicity, non-mutagenic, biodegradability. They have good lubricity properties, high lubricity index, and high flash point, etc. [3,4].
They are recommended for lubrication of the engine: soybean, olive, and coconut oil [1,5]. However, there is no broad explanation for such choices. This paper used multicriteria assessment to select the best green oil from 9 popular in Poland types of bio-oils (rapeseed oil, castor oil, soybean oil, sunflower oil, linseed oil, palm oil coconut oil, and olive). On the basis of the main roles of lubricants in the engine, and selected 24 detailed parameters.

Vegetable oil
The vegetable oils are called green oils, natural oils, bio-oils, plant oils, or natural esters [6]. Bio-lubricant is defined as a lubricant obtained from natural raw materials, which are renewable and non-toxic for humans, living organisms, and environment. From a technical point of view, natural esters display very good physicochemical properties, such as high viscosity indexes and flash points as well as good resistance to shear [1].
Chemically, plant oils are completely different from commercial mineral or synthetic oils. The major element of plant oils (approximately 98%) is triacylglycerols with varying fatty acids composition depending on the plant or seed, the crop, the growing conditions and the season, etc. The rest components of green oils are diacylglycerols, monoacylglycerols, fatty acids, sterols, and tocopherols. Most of these minor elements are deleted in processing, and some are valuable by products [4,7].
The variety of plant oils allows them to be used in different industrial applications, including as biofuels, transmission fluids, metal casting, printing, inks, paints, coatings, disinfectants, greases, hydraulic fluids, etc.In table 1 shows the selected popular type of green oils with possible industrial applications. Table 1. Selected vegetable oils developed for industry applications [1,5].

No.
Type of oil Application

Olive Engine oils, Automotive lubricants
According to the authors of the cited table 1 [1,5] for engine applications, the best choice may be soybean, olive, coconut oil. However, there is no broad explanation for such choices. In the next part of the paper, the multiple criteria assessment of vegetable oils is based on the most important objectives of engine oil.
However, the use of green oil to make lubricants is problematic, since most connect with the food chain, thereby causing speculation regarding the costs of natural oils and producing social imbalances and strong growth prices [1].

ENGINE OIL
Lubricating oil plays a key role in engines by diminishing friction and wear of moving pieces, inhibiting corrosion of the engine system, improving sealing, and cooling the engine [8]. In engine systems, lubricants must operate under variable pressure and temperature conditions and have to be stable to prolonged exposure to contaminating acids, which can cause progressive deterioration of the lubricant [1,9].
In general, a good lubricant should have high viscosity index (VI), high flash point, low pour point, high oxidation, and thermal stability, etc., as shown in Fig. 1.

MULTIPLECRITERIA ASSESMENT OF VEGATABEL OIL
According to the division presented in point 3, criteria have been designed on the basis of which selected engine oils will be assessed (table 2). Each of the functions has been assigned appropriate detailed parameters that characterize the selected roles. In the evaluation process, AHP (Analytic Hierarchy Process) methods were used to determine the weights of individual criteria [10].

Lubrication
The basic role of engine oil is to lubricate the surface of engine moving parts, which are in friction. It thus reduces friction, which tends to increase wear and tear of engine elements. In order to evaluate this role have been used: viscosity index, viscosity at 40 and 100 degrees Celsius, coefficient of friction, wear scar diameter, pour point, cloud point, melting point, smoke point, flash point, and iodine value (table 3). Indicator of the number of unsaturation in the triglyceride.This parameter gives an indication of green oil ability to dry in the presence of oxygen in the air. The possible range of IV [11]: IV < 110 -non-drying oil, 110 < IV < 150 -semi-drying oil, IV > 150 -drying oil.

Cooling
The energy is lost through combustion, and the friction between parts causes the engine temperature to rise. The lubrication provided by the engine oil helps to release the heat through the lubrication system. It supports the coolant, which only cools certain elements of the engine. The parameters describing the ability to cool are thermal conductivity, specific heat capacity, viscosity at the lowest known temperature, pour point, density (table 4).

Corrosion Protection
Fuel combustion generates corrosive acid that can damage the metal surfaces in the engine.
With the special additives added to lubricants, corrosion is delayed. Over time, and in contact with oxygen, engine oil may oxidize and no longer play its corrosion inhibiting role. In order to evaluate this role have been used: water content, acid number, anti-corrosion properties, iodine value (table 5). This parameter gives an indication of green oil ability to dry in the presence of oxygen in the air.

Others
This group listed four parameters. Surface tension, describing the ability to build foam. Next oxidation stability, ash content and saponification value -specific number for vegetable oils. A higher value for a given oil specifies a larger amount of smaller molecular weight fatty acids [14].

24.
Oxidation stability, 110°C h 0,5247 Oxidative stability depends on the vegetable oil composition and the conditions the sample is subjected to. Resulting in chemical, and physical properties changes [15].
Based on the detailed calculations, the results for each objective are listed in table 7 and 8. The sum of the results from the 4 roles gives the overall score for the multiple criteria assessment. Analyzing the obtained scores, soybean oil has the best lubrication properties. Castor oil turned out to be top for cooling. Linseed oil will protect the engine from corrosion. Coconut oil is most appropriate in terms of oxidative stability and ash content.  According to the multi-criteria assessment of green oils (table 7 and table 8), soybean oil with a score of 7.31, the best will fulfill the function of engine oil. The second-place has a linseed oil with a score of 6.399. The last place was taken by two cottonseed oil and olive, with very similar results (5.768 and 5.701 respectively). The general scores of mentioned four green lubricants are marked with gray.
Palm oil received the lowest score, 2.588. Coconut oil (presented in table 1 as a possible solution for engine oil) could have obtained much better results, but a high pour point causes a strong reduction in the overall rating. The high value of the pour point is crucial for the engine starting in winter conditions.

CONCLUSION
The crude reserve is depleting and the environmental concern about the damaging impact of mineral oil is growing. There is a diligent demand for alternatives for mineral base stocks, which may be vegetable oils. There are many types of vegetable oils, and they differ in chemical composition. The question remains, which of these a lot of types will be the most suitable for engine lubrication.
The paper shows a multiple criteria assessment of selected popular green oils. This assessment paid attention to four main roles of engine oil, they are lubrication, cooling, corrosion protection, and others (including oxidative stability, ash content, saponification value, and foam property). The author's assessment contains 24 detailed parameters of oils. The based on the collected values of parameters from own and other authors' research. Evaluated nine popular oils: rapeseed oil, castor oil, soybean oil, sunflower oil, linseed oil, palm oil coconut oil, and olive.
As a result of multi-criteria assessment it turned out that the most high scores have soybean oil, linseed oil, cottonseed oil and olive. The soybean and olive oils were known as prospective engine oils. The quite good result of linseed and cottonseed oil is surprising. Moreover, they have inedible sources. It is important due to social imbalances involved with deficiencies of food. Therefore, it is recommended a make wide own research usability this type of oils (linseed oil and cottonseed oil) to application for lubrication of the engine. It is also will allow to grade quality of prepared multi-criteria assessment.
This investigation is significant in the present context of increasing global environmental pollution and decreasing crude sources. The crucial benefits of green lubricants are their high biodegradability, renewable resource, and low toxicity. On the other hand, they are more expensive than mineral oils and some of them have edible sources. However, it should be research conducted possibility of replace the traditional mineral oils with bio-based oil. Especially need to pay attention to two aspects, economy and ecology.