Expertise of oils

Density, color and contamination of oils

Density is the mass of a substance, enclosed in a unit of volume (kg / m3, g / cm3). The numerical value of the density is expressed by the ratio of the mass of any volume of matter to the mass of the same volume of water having a temperature of 4 C (the mass of 1 liter of water at 4 C is equal to 1 kg). The density of liquids is simply and accurately measured by a hydrometer (GOST 3900-85). Usually, the measurement temperature is also indicated next to the density value (the temperature may not be indicated if the density is determined at 20 ° C).

Sometimes the density of oil, like crude oil, is expressed in terms of API gravity? The ratio of degrees to the relative density d (the ratio of the densities of oil and water) is measured at a temperature of 15.6 C and is determined by the formula: API = (141.5 / d) - 131.5.

Density is required when converting oil volume to mass and vice versa. As the temperature rises, the density of oil products decreases and, the more, the lower the density. Density is not a determining factor in lubricating oils. However, by density, one can roughly judge the hydrocarbon composition of the oil, since paraffins have the lowest density, and naphthenic compounds have the highest density. The density of the operating oil determines the ingress of fuel into it. Density can help identify a particular oil when comparing multiple grades or brands.

Color and transparency

The quality and presentation of the oil is sometimes judged by its color and transparency. In most cases, with the exception of the use of solid extreme pressure additives (molybdenum disulfide, etc.), the transparency of the oil should be complete, without visible mechanical impurities. The color of the oil depends on the presence of dark resinous substances and on the properties of the oil from which the oil is made. There is no direct relationship between the color of the oil and the content of resinous substances, especially if the oil is produced from oil from different fields. By the color of the oil, one can only roughly judge the quality of its purification. Sometimes the color is an indicator of the presentation of the oil, as the buyer tends to judge the quality of the oil by its color. In most cases, the finished commercial oil does not stain and can be from light yellow to dark brown in color. This natural oil color is determined and expressed numerically according to the standards ISO 2049, ASTM D 1500, GOST 20284-74, etc. when comparing the color of the oil with a set of color standards, which are numbered from 0.0 (white) to 8.0 (very dark brown) through 0, 5 units (16 rooms in total).

In practice, some consumers try to determine the need to replace it by the color of the running engine oil. However, darkening of engine oil is natural and indicates that the oil is performing its detergent and dispersant functions. Thus, darkening of the oil cannot be taken as a basis for an indicator of reduced performance and the need for replacement. With some experience and in the presence of a calibration table, this method of resource estimation can still be applied.

Some oils take on bright colors. Automatic transmission fluids (ATFs) are colored red to distinguish them from other oils and to make leak detection easier. Two-stroke engine oils are colored green, blue or red to distinguish the fuel / oil mixture from pure fuel.

According to the old tradition, it is believed, especially in the United States, that a very high quality oil made from paraffinic oil should have a faint fluorescent green hue. For this reason, some manufacturers specifically add fluorescent dyes to the oil. If necessary, oils are tinted with oil-soluble organic dyes.

Mechanical contamination in oil (contamination) consists of solid particles that cause wear of parts and participate in the formation of deposits and sludge. Mechanical impurities are retained by the filter, however, particles less than 25-40 microns in size accumulate in the oil and participate in the wear process. Mechanical impurities in the oil are determined, most often, by filtering a gasoline solution (GOST 12275-66) or photometrically (GOST 24943-81). For preliminary assessment, it is convenient to determine in the light or on filter paper. To do this, the oil is heated to 50-60? C and 2-3 drops of oil are applied on filter paper, on which contamination is clearly visible. Pure oil gives an evenly colored stain. Drops can also be applied to clean glass.

One of the most important indicators of the quality of used oils is the presence of specific contaminants in their composition. If they accumulate too much, this leads to a deterioration in the dispersion properties. This, in turn, leads to the gradual formation of carbon compounds that build up on the surface of the engine.

To determine the state of working oils, the method of centrifugation or interaction with special solvents is most often used. The concentration of undissolved microparticles is determined by the sediment obtained as a result of the manipulations. An additional method for determining the degree of contamination concentration is coking. This parameter is formed based on the influence exerted by products that are not dissolved in benzene. These include soot and carbenes. An increase in the amount of deposits affects not only the properties of the oil, but also the optimal functioning of the engine and the entire piston complex. The parameters are easily monitored by the rate of sludge production and the resulting sediment in the centrifuge.

Water contamination (moisture) is the most undesirable contamination. Water enters the oil when contaminated from the outside: with dirt, during condensation in the crankcase of atmospheric moisture, during condensation of steam from the combustion products. Water can be in oil in free and dissolved form. Water dissolved in oil is an insignificant factor and does not significantly affect the properties. Dissolved water in non-motor oils is determined by its interaction with calcium hydride (GOST 7822-75). Free water can form an emulsion and thus significantly change the viscosity. It also interacts with additives such as zinc dithiophosphate and disrupts the oil's performance balance. This is why the free water content is strictly regulated.

Temperature characteristics of oils show critical points of oil operation - high temperature and low temperature.

High temperature performance:
- flash point
- ignition temperature

Low temperature characteristics:
- pour point
- equilibrium (stable) pour point
- cloud point

The limiting operating temperatures of engine and transmission oil in a cold state are determined by the change in viscosity, on devices that simulate real operating conditions.

The flash point is the lowest temperature at which vapors of the heated oil product form such a mixture with the ambient air that flashes from an open fire, but quickly goes out due to insufficient evaporation. With further heating, the fire point is reached, upon reaching which the oil burns for at least 5 s (GOST 4333-48).

The flash point of an oil is almost always listed in the specification sheet. It is related to the fractional composition of the oil and the molecular structure of the base components and is important for several reasons. First, it is an indicator of the fire hazard of the oil, so a higher flash point is preferable. Secondly, it shows the presence of volatile fractions in the oil, which evaporate faster in a running engine (oil consumption for waste). Thirdly, when analyzing the operating oil, the dilution of the oil with the fuel is easily determined by the decrease in the flash point. Combined with a decrease in oil viscosity, a decrease in flash point serves as a signal for troubleshooting the ignition or fuel supply system.

The flash point of oil is determined by two methods: in an open and in a closed crucible. The open flash method is called the Cleveland Open Cup COC method (ISO 2592, ASTM D 92, GOST 6356-75), the closed cup method is called the Pensky-Martens Cup method (ISO 2719, ASTM D 93, GOST 6356-75). Usually, the numerical values found by these two methods differ by about 20 ° C. For oils, the Cleveland open crucible method (SOS) is most often used, and for fuel, the Pensky Martens closed crucible (PMC) method. In practice, the flash point of oil is sometimes determined by the closed crucible method.

The pour point or pour point is the lowest temperature at which the oil still has the ability to flow. According to foreign standards, the pour point is the temperature that is 3 C higher than the actual solidification temperature - at which the oil is stationary for 5 seconds.

The pour point only indicates the possibility of overflowing oil (for example, from a container), without resorting to preheating. There is no unambiguous relationship between the pour point of an oil and its cold starting properties. The pour point must necessarily be below the temperature at which the pumpability is determined according to the SAE J 300 classification.

Mineral oil is a multicomponent system, the solidification of which is a complex multistage process that depends on the interaction of individual components, their mutual dissolution, etc. In mineral oil, with decreasing temperature, paraffin crystals first of all nucleate and grow. With the appearance of small crystals, the oil becomes cloudy and this temperature is called the cloud point. Subsequently, paraffin crystals grow, join, stick together and ultimately form a crystalline framework, the oil becomes immobile, jelly-like. Thus, the pour point is actually the gelling temperature. Between the crystal framework, the oil still remains liquid and upon shaking or stirring, the fluidity of the entire mass of the oil can be partially restored. Such a solidification process, as a specific crystallization process, depends on the cooling rate and on the thermal and mechanical history of the oil (low-temperature regime, intensity and duration of forced flow, in the time interval until the pour point changes). Therefore, when determining this temperature, strict adherence to the prescribed cooling and liquid holding procedure is required.

The US military department, for military transport oils, demanded the determination of the so-called equilibrium (stable) pour point.

A low pour point is important for winter and multigrade oils. When starting a cold engine or at the beginning of movement with an unheated engine, engine oil at the very first moment of its operation should enter the narrowest and most distant places of friction. Therefore, the pour point must be below the minimum expected ambient temperature.

The pour point often serves as an indicator of the maximum minimum temperature for filling, pouring and, in part, for operating the oil. Therefore, it is included in the list of typical characteristics of oils and hydraulic fluids for motor vehicles. The minimum operating temperature for engine oils, according to the SAE J300 APR97 specification, is determined by the low temperature characteristics of viscosity and pumpability.

Characteristics of transmission oil

Parts and assemblies of the car are lubricated with special gear oils, which have a number of features associated with the operating conditions of the mechanisms themselves. These features include:

• high specific load at the point of engagement;
• boundary friction, which occurs especially at the start of work;
• high sliding speed of the teeth;
• runaway of working temperatures.

At the same time, special requirements are applied to such oils, namely, that they are aimed at reducing wear of parts, heat dissipation, reducing friction force, absence of corrosion aggression, the ability to protect the transmission from shock loading and, in addition, should not foam. The following types are divided:

• with mild anti-wear additives or no additives at all - used in gears with low loads and low operating temperatures;
• with anti-scuffing additives - used in hypoid gears;
• for hydrostatic and hydromechanical transmissions.

In addition, it is customary to divide transmission oils according to their use in climatic conditions - they can be summer, winter, all-season, arctic, northern. Distinguishes between their viscosity and temperature properties.