Introduction

Diesel oil, including marine diesel oil, is a fuel for compression ignition engines and consists mainly of paraffinic, naphthenic and aromatic hydrocarbons separated from crude oil by distillation processes. It contains mainly carbon, hydrogen and sulfur. Diesel distillates have a much higher boiling point (180/350 °C) than the distillates from which petrol is made. The high sulfur content of these distillates is removed by hydrogen treatment through catalytic processes called hydrotreating. The composition and relative proportions of hydrocarbons in gas oils vary depending on the type of crude oil processed and the technological processes used to produce them1,2.

Microbial contamination is a serious problem in petroleum products, including automotive, aviation and marine fuels, transformer and engine oils, lubricants and oil emulsions. With the development of industry and wider transport, including by sea, it has become apparent that the problem of microbial contamination of fuels and lubricants is continuing and becoming more widespread. Studies have shown that the activity of microbial life can cause disturbance and even damage piston and jet engines3,4,5.

The adverse effects of microbial contamination in marine diesel oil have been well documented in numerous publications and reports6,7,8. Many products and processes are available to minimize these effects. However, of the 52 million barrels of diesel consumed daily worldwide, less than one percent is treated with an antimicrobial agent. This is because, to date, few operators of freight transport equipment are aware of the economic impact of uncontrolled microbial contamination and the need for appropriate measures to offset this contamination.

Microorganisms feed on the organic and inorganic particles in marine diesel oil. As a result, some species attack the fuel directly and thrive at the expense of the hydrocarbon and non-hydrocarbon components of the fuel. Biodegradation of the fuel, which encourages microbial growth, is a direct consequence of contamination. Good quality petroleum products must be clear and transparent, while microbial growth often contributes to changes in color, calorific value, pour point and flash point. In addition, studies9,10, including diesel oils, have shown the presence of sulfate-reducing bacteria in all tanks, which are responsible for pitting corrosion of the inner plates of the tanks. The above can be considered positive, but microbial contamination can also affect the composition of marine diesel oil, causing it to lose its physical and chemical properties, which determine its suitability for use, too quickly11,12.

Bacteria and fungi form biomass that accumulates at the fuel/water interface, on tank surfaces and filters. As the biomass rotates and metabolic waste and dead cells accumulate, they settle out as sludge, which accumulates at the bottom of the tank (Fig. 1). If enough sludge accumulates, its particles will be sucked up with the marine diesel oil. This can lead to clogging of filters and injector ports.

Figure1
figure 1

Contamination of the bottom of the reservoir with sludge, i.e. metabolic waste and dead bacterial and fungal cells13.

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It is only when biomass production has been inhibited and the filter life ‘extended’ that the existence of a pre-existing problem is recognized. Ignoring this problem can lead to dangerous events, such as uncontrolled engine shutdown due to lack of fuel supply. These are important enough reasons for the importance of pollution control. However, it is not always possible to do this directly, as the biofilm clogging the filters is often almost transparent. As a result, it usually goes unnoticed. A secondary, indirect effect of flow restriction is increased engine wear.

Microorganisms are found in all types of environments (e.g. water, soil, air) from which they enter crude oil and its products at different stages of extraction, distribution, storage and production14,15,16. The availability of nutrients—organic carbon compounds (hydrocarbons) and biogenic elements (nitrogen, phosphorus, sulfur)—is essential for the growth and development of microorganisms in fuels17,18,19. The problem of microbial contamination will be exacerbated by the increasing use of alternative fuels, including rapeseed oil ester diesel (biofuels), for environmental reasons. Compared to diesel, biodiesel and blends containing it are more susceptible to microbial degradation20,21 because it is a much better carbon source for microorganisms than diesel.

Considering the above, it was decided to study the effects of admixtures such as silver solution, colloidal nanosilver and effective microorganisms in liquid form and ceramic tubes with different percentages of them in marine diesel oil on its selected parameters and inhibition of bacteria and fungi. Microorganisms and silver compounds have been successfully used in other fields and are widely recognized as environmentally friendly. The proposed diesel additives represent an innovative solution not previously used in petroleum products, with a positive impact on the environment.

There are four basic aspects of diesel pollution prevention and control. These are22:

  • engineering,

  • monitoring,

  • maintenance,

  • purification.

Each of these aspects is important in effectively minimizing microbial contamination problems and consequently reducing the operating costs associated with these problems.

Fuel systems can be divided into components for storage, transport, purification and delivery to the engine. Storage takes place in tanks. Fuel tanks (service tanks, vehicle tanks, etc.) should be fitted with a drain plug at the lowest point. Service tanks should be installed in a way that allows easy drainage of water and sediment Configurations with conical bottoms are advantageous as they facilitate the concentration and drainage of sludge and water from the bottom. Tank vents should be fitted with filters to prevent particulate matter being drawn in during fuel discharge.

Monitoring is also important to indicate whether fuel systems are contaminated. If necessary, information can be provided to facilitate troubleshooting. These activities determine whether stored fuel has deteriorated beyond acceptable limits. In addition, monitoring the state of microbiological contamination of diesel oil allows preventive maintenance of fuel systems to be planned. This allows us to avoid unexpected failures during the exploitation of internal combustion engines23,24.

It is important to prevent and protect petroleum products from microbiological contamination. Such protection can include physical methods, such as sedimentation, fuel filtration or thermal decontamination, and chemical methods. Physical methods are less disruptive to the environment, but unfortunately, their use is limited, partly because they cannot be used to decontaminate tanks, for example. For this reason, other means are used, such as biocides, i.e. compounds of synthetic or natural origin for the control of harmful organisms25,26.

Biocides are pesticides used, among other things, to control or reduce the growth of microorganisms in petroleum products. They should have a broad spectrum of activity against different microbial groups, be soluble in the aqueous and organic phases, be effective at low concentrations and efficient in use, have anti-corrosive properties, be compatible with different petroleum systems and not degrade at the operating temperature of lubricants. A distinction is made between fuel-soluble biocides for oil phase protection and water-soluble biocides for tank decontamination. Currently, the most commonly used biocides for the protection of petroleum products include isothiazolone and N-trihalomethylthionate biocides and various polymeric analogs of quaternary ammonium salts27,28. The structural formulae of these compounds are shown in Fig. 2.

Fig. 2
figure 2

Structural formulae of these compounds28: 1—2-propanol, 2—glutaraldehyde, 3—methyl isothiazolone (MIT), 4—chloromethylisothiazolinone (CMIT), 5—benzoic acid, 6—triclosan, 7—didecyldimethylammonium chloride (DDAC)28.

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Another current method of combating micro-organisms during fuel storage is the use of detergents that help to remove water, thereby reducing deposits such as rust. Such an additive is, for example, the Xbee fuel additive, which also reduces contamination in wet fuels. This is due to its ability to reduce the biomass that forms in the tank at the interface between the separated water and the fuel. When the amount of this biomass is reduced, the risk of corrosion and contamination is also reduced. This fuel additive removes water, minimizing the formation of inorganic sludge and slime in the tanks. Xbee enzymes are powerful natural surfactants and dispersants and also reduce the presence of fuel-degrading asphaltenes. Adding them to the fuel automatically eliminates all biological contaminants in the fuel and fuel system. All these living organisms are naturally burned with the fuel. In addition to antimicrobial agents and detergent additives, there are also simplified methods of determining the level of microorganisms in fuel using kits that contain a ready-made medium suitable for the growth of microorganisms. These can be used to monitor the microbiological status of the petroleum product, thereby preventing many accidents and allowing appropriate action to be taken to reduce microbial contamination in petroleum products29,30.

Despite growing evidence of the significant negative impact of microbial contamination on the operation of diesel engines and gas turbines, the use of antimicrobials is not widespread and most operators do not use them. One reason for this is the subtle nature of microbial contamination, which usually remains undetected until failure occurs. The physico-chemical properties of the fuel, the reliability of the fuel system and the life of the engine can all be significantly reduced as a direct and indirect result of microbial contamination. Consequently, the use of appropriate fuel preservatives can have a positive impact on economical and safe operation, ultimately reducing operating costs31,32.

Materials used in the research

The tests were carried out using marine diesel oil with the parameters shown in Table 1. MDO is a generic term referring to marine fuels, which are a mixture of various distillates and heavy fuel oil. Unlike diesel fuels used in land-based vehicles such as cars and trucks, marine fuels are not pure distillate. They additionally contain higher amounts of detergents, stabilizers, water dispersants and other important additives.

Table 1 Basic parameters of the test diesel33.
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The first of the measures added to marine diesel oil were effective microorganisms (EM), a complex of cultures of beneficial microorganisms found in nature, not genetically modified, which remain in a state of equilibrium, not only harmless to humans, animals and the environment, but actually necessary for their proper functioning. They are the smallest organisms on earth, specially selected and properly chosen. EMs are used in horticulture, environmental protection, medicine, industry and many other fields. Photosynthetic bacteria use the available conditions, e.g. CO2, and temperature, to produce useful biochemically active compounds from organic matter or toxic gases. Lactic acid bacteria slow down the growth of harmful bacteria. Yeast is another important part of the mix. The fermenting fungi break down organic matter and neutralize unpleasant odors. The EM principle is based solely on natural processes, they are not genetically modified and are completely environmentally friendly.

Effective Microorganisms are most commonly used in water treatment processes and in the treatment of wastewater and water bodies. The technology is also used in waste incinerators, with a clear effect on reducing dangerous dioxin emissions. In addition, the effect of heavy metals such as copper or silver in controlling bacteria, fungi and viruses is well known35,36.

Effective microorganisms in the form of ceramic tubes are a special type of clay inoculated with beneficial microflora (fermented clay), which is matured under natural conditions for several months and then fired at high temperatures under anaerobic conditions. They contain a mixture of beneficial micro-organisms, sugar cane molasses and restructured water. After firing, they take the form of a viscous liquid and, when cooled to ambient temperature, they become very hard and strongly binding. Ceramics fired using this method are characterized by lower porosity, higher density and greater hardness. Ceramics do not react chemically, do not burn and are harder than steel. The shape of the fired pieces or vessels is tailored to their future use, and the effective micro-organism content is 3% in each ceramic tube.

Effective Microorganisms® liquid was used for the test with the following composition: water 94%, Effective Microorganisms® (lactic acid bacteria, photosynthetic bacteria, saprophytic bacteria, yeast, fungi, and actinomycetes)—3%, molasses—3%. Greenland Technologia EM Sp. z o.o. is the exclusive licensee of EM Research Organization Inc. (EMRO) for Poland and Central and Eastern Europe. The Japanese company EMRO is also a licensor for product manufacturers and distributors in dozens of countries worldwide.

The second additive mixed into the marine diesel oil was silver. Silver is an element that has long been used for protective and medicinal purposes. Silver is also used in filters and to purify air and water from micro-organisms in confined spaces such as aircraft. The silver ionization process has helped to enhance the disinfectant properties of this element. Silver ionization, like copper ionization, has found application in the elimination of certain bacteria. Ionised forms of copper and silver are able to penetrate biofilms in plumbing systems and reduce the growth of micro-organisms in them. Studies have been conducted on the antibacterial effects of copper and silver ions in hospital water. It was found that in water samples enriched with copper and silver ions, there was a 30% reduction in bacteria and fungi compared to water that did not contain these elements37.

Silver in solution and colloidal nanosilver were used in the study. Colloidal silver (nanosilver, colargol) is a raw material in the form of an aqueous solution formed by combining crushed silver with protein, water or gelatine. Some of the silver particles are so small that they are called nanoparticles—they are less than 100 nm (nm) in size. Colloidal silver is an aqueous colloid of silver nanoparticles measuring 5/15 nm, present as non-ionic metal particles. The silver nanoparticles are suspended and dissolved in demineralized water and do not bind to the water molecules. Due to the same electrical charge, the silver nanoparticles repel each other, keeping them in constant motion and floating in the demineralized water. Non-ionic colloidal silver is produced by dissolving a silver electrode in water under the influence of electricity. This produces nanoparticles that do not react chemically. Non-ionic colloidal silver is characterized by a darker yellow color because the silver particles dispersed in the water block light of a specific wavelength, which is around 400 nm.

Figure 3 shows a transmission electron microscope (TEM) image of silver nanoparticles and an example of the size distribution of silver nanoparticles38.

Fig. 3
figure 3

Transmission electron microscope (TEM) image of silver nanoparticles (a) and particle size distribution (b) of Ag-NPs synthesized under optimal conditions38.

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The size distribution of silver nanoparticles shown above was performed using an ultrasonic synthesis technique using honey as a reducing and encapsulating agent. These results can be extended to other precious metals such as gold, palladium and copper, offering various additional applications from medicine to industry.

Colloidal silver consists of 80% silver particles and 20% silver ions. The determining factors for colloidal silver are the content of silver particles and their total active surface area. In colloidal silver, the particles form a colloid and there is no protein substrate.

Silver particles are tens of times smaller than viruses and thousands of times smaller than bacteria. This allows them to penetrate microorganisms and cause their death39. Ionic colloidal silver: is transparent like water and contains 90% silver ions and only about 10% silver particles. Because 90% of the particles are silver ions, a more appropriate name is ‘silver solution’. It is formed by chemical processes and is missing an electron. This makes it capable of undergoing chemical reactions40,41.

The silver solution (transparent), sold under the name Silver Klar Ag+, was developed on the basis of demineralized water and ionic silver (25 ppm). This product was manufactured by Naoliwieni, a company based in Rosanow (Poland). Colloidal silver (amber), on the other hand, available under the name ARGENTUM200, was developed on the basis of demineralized water and contains colloidal non-ionic silver at a concentration of 25 ppm. This product was manufactured by Aura Herbals Sp. z o.o. in Sopot.

We do not know the results of studies on the use of such additives and their effect on marine diesel oil parameters and on slowing down microbial contamination. Therefore, the use of these additives is an innovative solution that could have a positive impact on reducing the growth of harmful bacteria and fungi in the marine diesel oil.

Further research is needed to demonstrate the impact of these additives on the combustion process in the engine and on the wear of its components and to confirm the results obtained in real operating conditions.

The research stand and methodology

Prior to testing, fuel tanks were prepared with four different additives, including effective micro-organisms in liquid form and in ceramic tube form, as well as silver in solution form (ionic silver) and colloidal nanosilver (non-ionic silver).

The additives were blended with diesel at concentrations of 2% and 5% in 20 dm3 tanks. These concentrations were chosen because other biocomponents used in fuels are present at around 6%, which provided a reference point for the selection of concentrations for the study. Additives containing effective micro-organisms have not yet been used in petroleum products and, as the research is at an early stage, we do not yet have full knowledge of their effects on fuel properties and engine performance at higher concentrations. Therefore, we felt that it was safer to reduce the concentration from 5 to 2% in the initial phase of the study, rather than increasing it to, for example, 8%. In addition, a higher amount of additives is associated with higher costs, which may make their use uneconomical.

The article presents studies of selected parameters of pure marine diesel oil in comparison with marine diesel oil mixed with ecological additives (effective microorganisms, silver solution, colloidal silver), including determination of flash point, water content and acid number, as well as density and kinematic viscosity. These tests were carried out in the UMG laboratories in Gdynia, while the marine diesel oil was also subjected to microbiological tests, but due to the lack of suitable equipment, these were outsourced to an external company42 dealing with such matters.

The flash point is defined as the lowest temperature at which fuels floating above a liquid (under standardized conditions, where the atmospheric pressure is 101.3 kPa) are sufficient to form a flammable mixture with air, ignited by a flame or an electrical spark. This is a mixture in which the application of an ignition source will momentarily ignite the vapors above the surface of the liquid being tested. Importantly, the flash point characterizes the ability of the fuel to depend on both the thermal state of the fuel itself and the thermal state of the environment.

In accordance with the manufacturers’ recommendations34, these agents were added to the diesel four weeks before the test, creating eight test samples. The resulting mixtures were mixed regularly using a hand-held homogenizer (morning and afternoon) to prevent delamination. In addition, they were stored in a ventilated room with constant humidity and temperature. Immediately before the tests, no loss of stability was observed in the mixtures that constituted the test samples.

Fuel ignition point

The ignition temperature of the fuel is particularly important in compression ignition engines. If the ignition temperature is too high, the performance, efficiency and economy of the combustion engine can be reduced. Another negative effect can be an increase in the emission of toxic substances into the environment, i.e. a negative impact on the surrounding nature.

If the temperature is too low, an explosion of oil vapor can occur when fuel is poured from the dispenser into the tank, a theoretically safe, standard activity. Careful determination of the ignition temperature of the fuel by the supplier is important and can determine fire safety43.

The determination of the flash point of pure marine diesel oil and with additives has been carried out using an automatic flash point apparatus in a closed crucible (Fig. 4). The advantages of the apparatus are the speed and precision of the determination, the patented technology for heating and cooling from − 25 °C to + 420 °C (in one apparatus) and the determination meth ods used according to the standards: ASTM D6450 and D 7094.

Fig. 4
figure 4

Apparatus for testing the flash point in a closed cup—EraFLASH44.

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This pparatus can measure the flash point of all types of fuels such as diesel, petrol, biofuels, solvents, odors and flavors, paints, varnishes, residual fuels, marine fuels, tars, asphalts and solids. It can also be used to determine the degree of fuel dissolution in engine oil. It is also easy and convenient to use in the field. The small sample volume: 1 ml for ASTM D 6450 or 2 ml for ASTM D 7094 method and the closed crucible during the test ensure the highest level of safety. The sample is heated from above in a closed measuring chamber. An electric arc is used for ignition. This instrument does not use an open flame or electric filament. The ignition temperature is measured as the point at which the gas pressure in the measuring chamber rises rapidly. Using small samples reduces the cost of collecting, storing and disposing of samples and beakers, and makes the instrument easier to clean. With regard to the testing of the samples used, the graphs and analyses were carried out in accordance with ASTM D7094.

The water content of the fuel was also examined. Fuel always contains a certain percentage of water. However, efforts are made to keep it within acceptable limits, well below the saturation point (dissolved water, not free water). Engine manufacturers specify that the fuel should contain no water at the intake. Saturation points range from about 50 ppm to 1800 ppm, depending on the temperature of the petroleum fuel and its biofuel content. As shown in Fig. 5, biodiesel can hold significantly more water at saturation than its petroleum counterpart. However, blending biodiesel with petroleum diesel does not result in a mathematically proportional moisture content. The mixture will hold less in solution than the sum of the parts, which means that free water precipitation can occur when they are mixed.

Fig. 5
figure 5

Water saturation points of petro-diesel and bio-diesel45.

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A Cou-LoAquaMax KF automatic titrator was used to test the water content of marine diesel oil with and without additives using the Karl Fischer titration method45. The Karl Fischer titration (according to ASTM D6304). Method is a laboratory test that has been used since 1935 to determine the water content of a liquid sample. The test is very accurate and requires only a small sample. It can detect even small amounts of dissolved water, up to about 50 ppm in marine diesel oil. Water content below and above saturation (dissolved and free water) can be measured. This device (Fig. 6) uses a coulometric titration technique to provide a reproducible and reliable measurement of water content from 1 ppm upwards.

Fig. 6
figure 6

Tested oil with solvent and iodine on Automatic Cou-Lo AquaMax46.

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It uses a technique for the precise production of iodine by electrolysis, which takes place in a highly sealed measuring vessel with the lowest degree of external moisture transfer or “drift”. Selected parameters of the Cou-LoAquaMax KF automatic titrator are shown in Table 2.

Table 2 Selected parameters of the Cou-Lo AquaMax KF automatic titrator46.
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The measuring system contains a platinum charge-generating electrode, which is responsible for the precise, optimal and stoichiometric release of iodine into the system (with a ceramic membrane) according to the relationship: a charge of 10.71 coulombs generated in the system releases an amount of iodine that balances the content of 1 mg of water. The AquaMAX KF operates according to the principles of coulometry and determines the water content of the sample by coulometric titration to the end point.

In this apparatus, 1 mol of water reacts with 1 mol of iodine. This means that 1 mg of water (0.001 g) is equivalent to 10.71 electrical coulombs (1C = 1A × 1 s). The instrument determines the water content of the test sample by measuring the total amount of charge generated as the sum of the electrolysis current required to create the required amount of iodine in the system to bind the water contained in the test material46.

The acid number was measured using a TitroMatic 2S universal kit, equipped with two burettes with a Defined Endpoint (DEP) sensor, which allows titration to a defined endpoint (EP). The Titrator has a built-in function to automatically check the correct dosage of the interchangeable burette, according to the guidelines of the UNE-EN ISO 8655 standard.

The microbiological testing of marine diesel oil was carried out using the standard practice for the determination of live bacteria and fungi in liquid fuels—the ASTM 6974-3 filtration and culture method. This practice includes a membrane filter (MF) method for the detection and quantification of heterotrophic bacteria (HPC) and fungi in liquid fuels with kinematic viscosity ≤ 24 mm2/s at ambient temperature. The ability of individual microorganisms to form colonies on specific growth media depends on the taxonomy and physiological state of the microorganisms to be enumerated, the chemical composition of the growth medium and the incubation conditions. Therefore, test results should not be interpreted as absolute values. Rather, they should be used as part of diagnostic or condition monitoring activities that include other test parameters, according to Guide D 6469.

The microbiological study determined the number of aerobic microorganisms, i.e. bacteria and fungi (incubation temperature 25 °C) in the fuels. The test was carried out in two volume variants (1 replicate for each volume) with optimization. Optimization means that the test was carried out in two stages, first from the larger volume and, after reading the result, from the selected smaller volume. During the optimization of the test in the second and third stages, 1 ml of sample was sieved in each case and additional sample dilutions were used. The test set-up consisted of a class II laminar flow chamber and a membrane filtration kit. Samples were aseptically mixed prior to each test step. In Stage I of the study, 5 samples were subjected to filtration after aseptic mixing − 10 ml of sample (due to the lower density of the sample).

In stage II, due to the nature of the samples (density), the membrane filtration technique was preceded by the classical tenfold dilution method as part of an optimization aimed at obtaining more accurate results. Membrane filters with a pore size of 0.45 μm were washed with Tween 20 wash solution, followed by Ringer’s solution. Samples obtained in this way were percolated in duplicate on separate filters for a total of 4 and 5 ml and incubated at 25 °C, reading the results after 72 h and after a further 5 days. The final result is given as the number of bacteria/fungi in 1 dm3 of fuel, assuming that each colony develops from one cell.

Membrane filters are placed on a culture medium consisting of TSA nutrient agar (tryptone-soy-casein agar) and MEA or fungal alternative agar (SABOURODA) to provide culture conditions for bacteria and fungi, respectively.

In stage III, the classical technique of tenfold dilution with Tween 20 liquid was used. The results are given as the mean value of stages I-III, including countable values42.

Results and discussion

The article presents a study of selected parameters of pure marine diesel oil compared to diesel blended with environmentally friendly additives, including the determination of flash point, water content and acid number. The marine diesel oil was also subjected to microbiological tests.

Statistical analysis was performed at the 99% confidence level. The results obtained for fuel consumption without additives and with additives at different concentrations at the α = 0.01 significance level did not show any significant errors as they fell within the lower and upper confidence intervals.

The test results are shown in Figs. 7, 8, 9, 10, 11, 12 and Tables 3 and 4. The following description has been used:

  • for marine diesel oil without additives—pure DO,

  • for marine diesel oil with the addition of effective microorganisms in liquid—DO with EM fluid,

  • for marine diesel oil with the addition of effective microorganisms in the form of ceramic tubes—DO with EM cer,

  • for marine diesel oil with addition of silver solution—DO with SS,

  • for marine diesel oil with the addition of colloidal nanosilver—DO with CN.

Fig. 7
figure 7

Flash point of pure diesel oil and diesel oil with EM in liquid form, EM ceramic tubes, silver solution and colloidal nanosilver for 2% concentration.

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Fig. 8
figure 8

Flash point of pure diesel oil and diesel oil with EM in liquid form, EM ceramic tubes, silver solution and colloidal nanosilver for 5% concentration.

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Fig. 9
figure 9

Water content of pure diesel oil and diesel oil with EM in liqiud form, EM ceramic tubes silver solution and colloidal nanosilver for 2% concentration.

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Fig. 10
figure 10

Water content of pure diesel oil and diesel oil with EM in liqiud form, EM ceramic tubes, silver solution and colloidal nanosilver for 5% concentration.

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Fig. 11
figure 11

Kinematic viscosity and density of pure diesel oil and diesel oil with EM in liqiud form, EM ceramic tubes, silver solution and colloidal nanosilver for 2% concentration.

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Fig. 12
figure 12

Kinematic viscosity and density of pure diesel oil and diesel oil with EM in liqiud form, EM ceramic tubes, silver solution and colloidal nanosilver for 5% concentration.

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Table 3 Comparison of kinematic viscosity of oil without and with various additives (2%).
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Table 4 Comparison of kinematic viscosity of oil without and with various additives (5%).
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Ignition temperature (flash point) of marine diesel oil

In order to analyse the ignition temperature of the oil (MDO) for each sample, the test results are presented in the form of graphs for a concentration of 2% in Fig. 7 and for a concentration of 5% in Fig. 8. These graphs show the changes in ignition temperature values of pure marine diesel oil and after the addition of effective microorganisms in liquid form and in the form of ceramic tubes. In addition, silver solution and colloidal nanosilver were added to the diesel in the same proportions as the effective microorganisms. Each sample of pure oil and oil with additives was tested three times. However, the article presents the average result for each case.

Analysing the results obtained, Fig. 7 shows that for pure oil the ignition temperature is 60.3 °C. On the other hand, the addition of effective microorganisms in liquid form to the marine diesel oil in the amount of 2% of the total tank volume of 20 dm3 increases the ignition temperature to 62.2 °C, which is the highest value obtained compared to other additives, while effective microorganisms in the form of ceramics cause an increase in ignition temperature of 1 °C compared to marine diesel oil without additives. In addition, the addition of silver in the form of colloidal nanosilver also raises the flash point to 61.6 °C. The lowest temperature obtained after the use of ionic silver was 60 °C, which is closest to the value for pure marine diesel oil, so this additive is least likely to have a negative effect on engine operating conditions. As mentioned above, too high a temperature will reduce the efficiency and economy of engine operation and increase toxic emissions to the environment, so effective microorganisms in liquid form are least suitable for use with marine diesel oil.

It follows that effective micro-organisms in the form of ceramics and ionic silver mixed with marine diesel oil do not adversely affect the flash point of marine diesel oil and can therefore be added to new oil. The situation is different for microorganisms in liquid form and colloidal nanosilver. In this case, the flash point of diesel is higher than that of pure diesel without these additives, and there may be a risk of diesel vapor explosion.

Figure 8 shows the flash point for pure marine diesel oil and with the addition of microorganisms and silver at a concentration of 5% of the total tank volume. Better results were obtained at the 5% concentration compared to the 2% concentration of additives.

For the effective microorganisms in both liquid and ceramic form, as well as for colloidal silver, the ignition temperature is 61 °C, which is 0.7 °C higher than the result obtained for pure diesel oil. The best result was obtained for ionic silver, which is 0.2 °C higher compared to pure diesel. This value is closest to the flash point value for pure marine diesel oil, which means that the ionic silver additive has virtually no effect on the change in the flash point of marine diesel oil, so in this respect it will be the best choice as an ecological additive that has an effect on reducing microbial contamination.

Acid number of marine diesel oil

Acid number is an important parameter for fuels. For example, according to EN 14214 and ASTM D6751, the acid number of fuel should be less than 0.5 mg KOH/g. All samples tested for acid number showed a value of less than 0.5 KOH/g, meaning they are safe for fuel systems in terms of preventing corrosion of fuel systems and engine components.

Water content in marine diesel oil

The results of the water content tests are presented as graphs in Figs. 9 and 10. These graphs show an analysis of the effect of each additive on the amount of water in the diesel oil samples. Each sample of pure oil and oil with additives was tested three times. However, this article presents the average result for each case.

Analyzing the results, it can be seen in Fig. 9 that the water content of the oil without additives is 0.0066%. The addition of 2% of effective microorganisms and silver increases the water content of marine diesel oil. After the addition of liquid effective microorganisms, the water content was 0.0102%, in the form of ceramics − 0.0068%, while for silver solution − 0.0123%, and for colloidal silver lower value was obtained − 0.0107%. All the additives, except effective microorganisms in ceramic form, have a higher water content than pure diesel, which is because the above additives are mainly composed of water. For this reason, we observe higher water contents in marine diesel oil, with very similar values for effective microorganisms in liquid form and colloidal silver, with the highest content obtained for ionic silver and the lowest content, which is within the statistical error, obtained for effective microorganisms in ceramic form.

After adding 5% (Fig. 10) marine diesel oil additives, the following water contents were obtained in marine diesel oil with liquid effective microorganisms − 0.0109%, EM in the form of ceramics − 0.0078%, silver solution 0.0145% and colloidal silver − 0.0113%. The trend of increasing water content in marine diesel oil is similar to that obtained for a 2% concentration of additives and correspondingly higher due to the higher percentage concentration of marine diesel oil additives. In this case, the water content is also the highest for silver solution and the lowest for EM in ceramic form, while the water content for liquid EM and colloidal silver are also similar to each other.

When analyzing the water content as a percentage, it was EM in ceramics that gave the closest result to pure diesel, i.e. the most neutral effect on this parameter. However, when studying the effects of these additives on fuel consumption and exhaust gas composition, it was the silver solution that proved to be the best additive, causing the highest water content in marine diesel oil. Thanks to this additive, fuel consumption dropped and the level of toxic compounds in the exhaust gas decreased.

Kinematic viscosity of marine diesel oil and its density

The subsequent test results are presented as graphs in Figs. 11 and 12. These graphs show the effect of each additive on the kinematic viscosity of the diesel samples. Each diesel sample was tested at a temperature ranging from 2 to 63 °C, as this was the maximum temperature to which the heater working with the viscometer could heat the oil. The kinematic viscosity up to 100 °C was then determined using a program based on ASTM D341 interpolation47. To observe changes in viscosity over a wider temperature range, the viscosity was tabulated to describe the parameters of the oil at 40 °C and 100 °C. A density curve for the new oil was also plotted over the same range. This density allowed the kinematic viscosity to be determined.

In addition, for better clarity, the results obtained are also presented in Tables 3 and 4, with the addition of the results obtained at − 10 °C and − 5 °C to illustrate the parameters that occur at negative temperatures.

Analysing the results, Fig. 11 shows that the kinematic viscosity of pure diesel is 2.35 mm2/s at 40 °C and 1.03 mm2/s at 100 °C. The addition of 2% effective microorganisms and silver to diesel shows a similar trend. As for the values obtained, at 40 °C the closest value to the oil without additives was obtained for the effective microorganisms in liquid form, i.e. 2.22 mm2/s, while the highest value was obtained after the addition of microorganisms in ceramic form, which was as high as 2.46 mm2/s. A similar value to the diesel-EM blend in ceramics was obtained for colloidal nanosilver at 2.48 mm2/s and a lower value for the silver solution at 2.41 mm2/s. At 100 °C the trend is similar, with a higher kinematic viscosity value than that obtained for pure diesel for each additive except EM in liquid. From the results shown in the table, the kinematic viscosity for the diesel-EM mixture in liquid is 0.99 mm2/s, while that for pure oil is 1.03 mm2/s. As for the temperature of 40 °C, at 100 °C the highest kinematic viscosity was also obtained for EM in ceramics, at 1.11 mm2/s. These are the only additives that do not contain water in their composition, so such a result is probably a consequence of the composition of EM in ceramics. For the silver solution and colloidal nanosilver, kinematic viscosities of − 1.07 mm2/s and 1.12 mm2/s respectively were obtained.

At − 10 °C it can be observed that for pure marine diesel oil the kinematic viscosity is 9.25 mm2/s. A lower viscosity value was obtained after using liquid effective microorganisms, which is 9.18 mm2/s, and the lowest viscosity was obtained after mixing marine diesel oil with silver solution. Much higher viscosity values were obtained when fuel was mixed with EM in ceramics and colloidal nanosilver, which were 9.46 mm2/s and 9.52 mm2/s respectively. On the other hand, the lowest value was obtained with ionic silver, i.e. 9.15 mm2/s. From the point of view of the atomisation of the injected fuel in the combustion chamber, the lower the viscosity, the more favourable the conditions for the formation of the fuel–air mixture, i.e. the use of ionic silver has the most favourable effect on the parameters of marine diesel oil.

The results obtained for a 5% additive concentration are similar to those obtained for a lower concentration and are correspondingly higher due to the higher proportion of additives in the blend (Fig. 12). In any case, the kinematic viscosity at temperatures of 40 °C and 100 °C is significantly higher than that obtained for pure marine diesel oil. The curves for the higher 5% concentration are also very close. At 40 °C a viscosity value of 2.49 mm2/s was obtained for EM in liquid and 2.61 mm2/s for EM in ceramic, and again this is the highest value obtained compared to the other additives. One of the higher values is also shown by the addition of colloidal nanosilver, which is 2.57 mm2/s, a slightly lower value was obtained for the silver solution, which is 2.46 mm2/s. The situation is similar at 100 °C, i.e. the highest viscosity was obtained for EM in ceramic with 1.14 mm2/s, followed by the addition of colloidal nanosilver, which gave a viscosity of 1.15 mm2/s. The addition of EM in liquid and silver solution increased the kinematic viscosity to values of 1.12 mm2/s and 1.07 mm2/s respectively.

For the 2% concentration of additives, the effect of the additives on the kinematic viscosity at − 10 °C was also checked. The situation is similar to that for the lower concentration, i.e. the lowest viscosity value was obtained for the silver solution, i.e. 9.6 mm2/s, followed by 9.93 mm2/s for the effective liquid microorganisms. The highest viscosity values were obtained for the effective microorganisms in form ceramic form and colloidal nanosilver, which were 10.16 mm2/s and 10.29 mm2/s respectively. From the results presented, it can be concluded that the use of ionic silver at a concentration of 2% is the most favourable from the point of view of viscosity.

Additives containing mainly water in their composition, i.e. liquid EM, silver solution and colloidal nanosilver, cause a smaller increase in viscosity than the EM ceramic additive. The addition of silver solution and effective microorganisms, those based on water, will reduce viscosity slightly, but not enough to have a catastrophic effect on engine components. In order for the engine to operate correctly, the fuel must be atomized as finely as possible, while at the same time remaining homogeneous. This makes it much easier for the fuel to evaporate, which is essential for uniform combustion. Many factors influence the quality of diesel atomization, but the viscosity of the fuel plays an important role.

Users of vehicles with diesel engines attach great importance to the so-called engine culture, which is significantly influenced by the uniformity of the load on the individual cylinders. This depends, among other things, on the dose of fuel per cycle and the quality of its atomization. This is influenced by the viscosity of the diesel fuel. The PN-EN 590 standard indicates that the most favorable viscosity values are those between 2.8 mm2/s and 8 mm2/s at 20 °C. From the results presented, it can be seen that all the blends meet this condition. This is very important because excessive fuel viscosity causes it to spray in droplets that are too large. Then their heaviness makes it difficult to distribute them evenly and limits the level of evaporation. Unused fuel will be deposited as carbon build-up on the walls of the engine compartment and piston heads, reducing engine performance.

It would also be detrimental if the viscosity of the marine diesel oil was too low, as this would affect the injection and combustion process when the injection system is adapted to the engine.

Microbiological tests

In addition to tribological testing of marine diesel oil, samples were subjected to microbiological testing to determine the presence of live bacteria and fungi in liquid fuels according to ASTM D 6974-3. After filtering a certain volume of diesel fuel samples through a membrane filter, conditions are created that are suitable for the culture of bacteria and fungi. The results obtained after culture are summarised in Table 5.

Table 5 Summary of microbiological test results.
Full size table

It can be observed that each of the additives effected reducing the microbial contamination of marine diesel oil. For pure diesel, a value of 1.1 × 106jtk/1 dm3 was obtained for bacteria and 7.3 × 103jtk/1 dm3. For each type of blend, a value of less than 1 × 102 jtk/1 dm3 was obtained for bacteria, which is 10,000 times less than for pure diesel. As for the number of fungi in the mixtures, we also observed a 73-fold decrease for marine diesel oil mixed with effective microorganisms in liquid and ceramic form, 48-fold less was recorded after the use of non-ionic silver, while the best result was obtained with ionic silver, where this decrease is greater than the 73-fold obtained with effective microorganisms.

The problem of microbial contamination is most common in tanks that are stored for long periods of time. The concentrations of microorganisms found in fuels used in high-pressure engines vary widely. The microorganisms most commonly found in fuels such as diesel and biodiesel include molds of the genus Cladosporium sp., Pseudomonasaeruginosa bacteria and sulfate-reducing bacteria such as Desulfovibrio.

The microbiological study shows that the fuel containing ionic silver was the least degraded, as there were the least bacteria and molds in it. As a result, the physical and chemical parameters of the fuel changed the least and there was the least microbiological contamination (sludge) in the fuel.

In carrying out the study, the authors encountered some difficulties that could affect the results obtained. After reviewing the literature dealing with the issue of reducing microbial contamination in petroleum products, the authors concluded that the proposed diesel admixtures are a new solution not previously used in petroleum products. Therefore, there are difficulties in making a comparative analysis with similar studies. Nonetheless, the authors have undertaken research into the use of these additives as eco-friendly, as there are other areas in which they are successfully used. These additives have a positive impact on environmental protection and the performance of the compression ignition engine.

Another problem is the testing equipment available. For example, each sample of pure diesel and oil with additives was tested at a temperature between 2 and 63 °C, that is, within the range of the viscosity meter used. Due to this limitation, kinematic viscosity up to 100° C was determined using a program based on ASTM D341 interpolation. Another limitation that may affect the test results obtained is the method of mixing the resulting sample. This activity was carried out twice for several minutes in a manual manner. In future studies, it would be worthwhile to use continuous mixing with a homogenizer. This will guarantee the stability of the solution over a long time interval.

The authors in a publication48 presented the results of a study on the effects of the same additives on selected compression-ignition engine performance parameters, which showed no negative effects, and even the addition of a silver solution improved, positively influencing the composition of some exhaust gas components and fuel consumption.

Therefore, further research is planned for the practical implication of fuel with these additives to confirm the results obtained under real-world conditions. In addition, the mechanisms of additive effects, i.e. the combination of two or three additives that show the most beneficial effect on the properties of the diesel under study, would also need to be investigated.

Conclusion

The addition of effective microorganisms and silver to marine diesel oil affects the kinematic viscosity of the oils tested at both 40 °C and 100 °C. The trend of the changes shown is similar for both 2% and 5% concentrations. The kinematic viscosity increases compared to the values obtained for oil without additives and increases as the additive concentration increases. The closest value to the oil without additives was obtained for effective liquid microorganisms, while the highest value was obtained after the addition of microorganisms in ceramic form and a concentration of 5% (up to about 11%). These are the only additives that do not contain water in their composition, so likely such a result is directly related to the composition of EM in ceramic form. A similar value of change in kinematic viscosity shown for a mixture of marine diesel oil with EM in ceramics was obtained for colloidal nanosilver and a lower value for silver solution. In conclusion, despite the effect of the additives used on increasing the kinematic viscosity of MDO, the changes in this parameter are not large enough to affect the injection and combustion process of this fuel.

From the results obtained it can be concluded that the additives used in MDO do not have a significant effect on its flash point. The increase in ignition temperature was only from 0.1 °C to about 2 °C compared to MDO without additives. From the above it can be concluded that the additives do not limit the effective fuel supply and combustion of the engine.

Based on the results of the study, it can be concluded that the most favourable effect on diesel viscosity is achieved by using ionic silver at a concentration of 2%. This is because, from the point of view of the atomization of the injected fuel in the combustion chamber, the lower the viscosity, the more favorable the conditions for the formation of a fuel–air mixture.

It should also be noted that additives containing mainly water in their composition, i.e. liquid EM, silver solution and colloidal nanosilver, could potentially have adverse effects of a corrosive nature on selected engine components. However, all samples were also subjected to acid number tests. Values of less than 0.5 KOH/g were obtained, i.e. values that can be considered safe from a potential corrosion hazard point of view.

The fuel was also subjected to microbiological testing. It was found that each of the additives had a positive effect in reducing the microbial contamination of the marine diesel oil. For each type of blend, significantly lower levels of bacteria and fungi were found in the oil. The bacterial content was up to 10,000 times lower and the fungal content several times lower.

Considering the physical and chemical parameters of marine diesel oil and its microbiological purity, the addition of ionic silver is very beneficial.

In conclusion, the best additive will be a silver solution with a concentration of 2% in MDO. Then MDO has the best physico-chemical parameters and this mixture has the least development of microbial contamination. The above would need to be confirmed in further studies. Therefore, further research is planned for the practical implication of fuel with these additives to confirm the results obtained under real-world conditions. In addition, it would also be necessary to investigate the mechanisms of additive effects, i.e. the combination of two or three additives that show the most beneficial effect on the properties of the diesel under study. In addition, it would be necessary to study the effect of the additive on the combustion process in the engine and the wear of its components.