CC BY
18
https://doi.org/10.29013/AJT-21-7.8-29-35
Sagin Sergii Victorovych, Doctor of Technical Science, Head of Department National University Odessa Maritime Academy
Odessa, Ukraine E-mail: [email protected] Stoliaryk Tymur Olexandrovich, postgraduate student National University Odessa Maritime Academy
Odessa, Ukraine E-mail: [email protected]
COMPARATIVE ASSESSMENT OF MARINE DIESEL ENGINE OILS
Abstract. Analysis of structural characteristics (thickness of boundary lubricating layer and degree of ordering of its molecules) of motor oils used in marine diesel engines is carried out. It is experimentally proved that increase in thickness of boundary lubricating layer and degree of ordering of its molecules provides decrease in wear of parts of marine diesel engines.
Keywords: marine diesel engines, engine oil, boundary lubricating layer, wear of parts
Internal combustion engines (diesels) are the it [4]. Modern marine two-stroke diesels (used as
most common thermal engines used in transport [1]. Diesels are the sources of mechanical power for road, rail, sea and river transport as well as agricultural and mobile (small or portable) machinery [2]. The engines using energy of storage batteries, appeared in last 3...5 years, can satisfy needs of only small part of automotive technique. Furthermore, operational power of such engines does not exceed 250.300 W, their duration of autonomous operation is 8.10 hours. Therefore today they aren't expected to be widely used even on heavy-duty vehicles, and even more so on railway (which exploitation power reaches 10000 ... 12000 kW), river (with power-plant capacity up to 20000 ... 25000 kW) and sea (with the total power of main and auxiliary diesel engines up to 100000 ... 120000 kW) transport [3].
Oil system is one of systems ensuring operation of diesel engines. Increase of power and dimensions of a diesel engine (cylinder diameter and piston stroke) increases complexity of construction of the oil system and increases number of auxiliaries included in
main engines - transfer their power to the propeller and ensure marine vessels movement) are equipped with two oil systems. One of them (lubricator system) feeds oil into the cylinder and provides lubrication of the cylinder group (piston rings and cylinder bushing) [5]. The second (circulation) feeds oil to the diesel engine bearings (crosshead, crankcase and flywheel). The main task of both lubricator and circulating system is to prevent contact interactions of the diesel engine parts by creating a lubricating layer and providing hydrodynamic or boundary regime of lubrication of the friction surfaces. Just under these conditions the minimum wear of the diesel parts and the minimum losses of its power are provided [6].
Marine diesel engine oils are disperse systems. Their performance in thin lubricating layers differs from volumetric properties and is determined by orientational ordering of molecules which promotes occurrence of additional wedging pressure between contact surfaces [7; 8]. The parameters that qualitatively and quantitatively characterize the structure
of lubricating layers are the degree of orientational ordering of molecules in the boundary layer S and the layer thickness dS [9; 10].
The research task was to determine the influence of structural characteristics of boundary lubricating layers of engine oils on the wear of parts of marine diesel engines.
The tests were conducted with engine oils Cas-trol CL 40 and Texaco Special HT 40. These oils were used in the lubrication system of marine diesel engine 6S60ME-C9 MAN-Diesel&Turbo with the following main characteristics:
Diesel type - two-stroke, crosshead; Nominal power - N =14280 kW;
nom '
Nominal rpm - n =105 min 1;
nom '
number of cylinders - 6;
cylinder diameter- d = 0.6 m;
piston storke - s = 2.4 m.
Both grades of oil are recommended by the manufacturer as the main grades during the operation of the diesel engine.
Initially, the structural characteristics of engine oils were investigated [11].
One of the methods for determining the characteristics ofboundary lubricating layer (degree ofordering of molecules and thickness) is the optical method of dichroism absorption [12; 13]. Its implementation is possible with the setup shown in (Fig. 1) [14; 15]
Figure 1. Schematic of the setup for determining the structural characteristics of a boundary lubricating layer: 1 - lens; 2 - light source; 3 - polarizer; 4 - wedge-shaped cuvette; 5 - oil; 6 - photoelectronic device; 7 - computer
From the light source 2 the light beam was focused by the lens 1 and in parallel beam followed through the polarizer 3 into the volume of investigated oil 5. A wedge-shaped cuvette 4 made ofpolished quartz glass was used to carry out the boundary layer thickness scanning procedure. The volume of the cuvette was filled with oil whose molecules formed a boundary layer with an ordered molecular structure near the quartz surface. During the experiment the cuvette was moved in the direction perpendicular to the incident light. Light intensity, passing through cuvette 4, was recorded by photoelectronic device 6 and transmitted to personal computer 7 [16].
Intensity of light I, passing through a layer of oil with thickness d, was determined according to Bou-guer's law by the expression
ln I = ln I0 - nd
or
D = nd,
where I is the intensity of light passing through a cuvette filled with oil;;
I0 - is the intensity of the light passing through the air-filled cuvette (without oil);
y - is the light absorption coefficient of the oil in the cuvette;
D = ln 1/ln 10 is the optical density of oil in the cuvette.
In case of layer structural heterogeneity (for example, presence of ordered structure of oil molecules near the surface) the dependency D - f (d) is not
linear and is characterized by the presence of a break point (Fig. 2, a), which allows to calculate the value of molecule ordering of boundary layer S and to determine its thickness d„.
a) b)
Figure 2. Results of experiment:a) dependence of optical density D (relative units) on oil layer thickness d (mkm); b) degree of ordering of molecules in the boundary layer S; 1 - Texaco Special HT 40 oil; 2 - Castrol CL 40 oil
The higher the slope angle of the dependency D - f (d), the higher the value of degree of ordering of molecules of the boundary layer S [17; 18]. Maximum S and minimum S values of the degree of
max min &
ordering of molecules in the boundary layer of the studied oils are shown in (Fig. 2, b). The break point D - f (d) corresponds to transition of bulk phase into boundary lubricating layer with well-ordered molecules and determines thickness of boundary layer ds [19; 20].
Further tests were carried out on a MAN-Diesel&Turbo 6S60ME-C9 marine diesel engine. Texaco Special HT 40 and Castrol CL 40 engine oils were used for the lubrication of diesel cylinders [21]. Principle diagram of the lubricator system of the diesel engine is shown in (Fig. 3) [22; 23].
The two engine oil tanks (positions 1 and 2) provided the opportunity to use different oils for
different groups of cylinders. The experiments were carried out during an ocean crossing of the vessel of 11.12 days duration only on steady-state diesel operation modes. Constant load on the diesel engine was determined by constant crankshaft speed and constant cyclic fuel supply. The 6S60ME-C9 MAN-Diesel&Turbo marine diesel engine was operated with the same fuel grade RMC350 (0.37% sulphur content) throughout the duration of the experiment. Castrol CL 40 oil was supplied to cylinders 1 to 3, and Texaco Special HT 40 to cylinders 4 to 6.
Cylinder group of diesel engines is an object of constant technical condition monitoring [24]. Visual inspections of cylinder bushings are not always possible. First of all, it is connected with a period of continuous work of the main engines of sea ships (for example, duration of ocean passages can reach 20 ... 30 days), and also with the big expenses for
their performance [25]. Therefore, indirect methods are used to diagnose technical condition of cylinder bushings. The most widespread and accessible for conditions of a sea vessel is definition of residual alkaline number (BN) and quantity of metal impurities in oil.
Figure 3. Schematic diagram of the lubrication system of a marine diesel engine 6S60ME-C9 MAN-Diesel&Turbo:1, 2 - oil tank; 3 - flow meter; 4 - controller; 5, 6 - oil tank; 7 - diesel engine
Oil sampling and its further analysis was carried out in Cylinder Scrape-Down Oil Analysis technical laboratory every 20 hours of diesel operation in accordance with recommended technology and sequence [4, 5]. At that, a different value of cylinder oil feed was set for each diesel engine cylinder. Cylinder oil feeding deviation did not exceed 5% of average value. Spent oil was sampled from under-piston space of each cyl-
inder with subsequent determination of the following components content in the oil in the ship's technical laboratory: PQI (Particle Quantity Index), Fe (iron), V (Vanadium), Ni (Nickel), Si (Silicon) and BN. The results are summarised in (fig. 4).
The following conclusions can be drawn from the research.
Engine oils, which are used in marine diesel engines, belong to disperse systems. Fine oil layers (which separate friction surfaces and provide hy-drodynamic or boundary regime of lubrication) are characterised by the ordered structure of molecules. A qualitative parameter of this structure is the degree of molecular ordering, a quantitative parameter is the thickness of ordered (boundary) lubricating layer. The degree of molecular order of the boundary layer and its thickness can be determined by optical dichroism absorption method. For Texaco Special HT 40 and Castrol CL 40 motor oils (used in marine diesel engine oil systems), the thickness of the ordered (boundary) lubricating layer is 13.6 mkm and 15.4 mkm respectively. The degree of molecular ordering in the boundary layer for Texaco Special HT 40 engine oil is in the range of 0.66...0.73, for Castrol CL 40 engine oil it is in the range of0.78.0.83.
Structural ordering of boundary layers of engine oils is connected with mechanical losses of energy of marine diesel engines and wear of details which are lubricated by these oils (for example, the cylinder group). It is expedient to estimate wear of details of the cylinder group by quantity of mechanical impurity which gets into engine oil and by residual alkaline number of engine oil.
By means of the experimental researches executed on ship diesel engine 6S60ME-C9 MAN-Diesel&Turbo it has been established, that the engine oils which structure of boundary layers is characterised by greater ordering of molecules and greater thickness of a boundary layer, provide smaller value of wear of details of diesel engine. For Cas-trol CL 40 engine oil (with layer thickness of 15.4 mkm) there is a 7.5 to 8.3 times reduction of iron
impurities, 3.5 to 3.8 times reduction of vanadium impurities in comparison with Texaco Special HT impurities and 1.6 to 1.8 times reduction of nickel 40 engine oil (with layer thickness of 13.6 mkm).
Figure 4. BN (base number), Fe (Iron), V (Vanadium), PQI (Particle Quantity Index), Ni (Nickel), Si (Silicon) values in waste oil samples from 6S60ME-C9 MAN-Diesel&Turbo marine diesel engine: 1 - when using Texaco Special HT 40 engine oil; 2 - when using Castrol CL 40 engine oil
When evaluating the performance of engine oils, diesel parts and contribute to a reduction in their
it is also necessary to consider their structural char- mechanical losses.
acteristics, which ensure a reduction in the wear of
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