The main changes in properties in a running engine occur for the following reasons:
high temperature and oxidizing effects;
mechanochemical transformations of oil components;
permanent accumulation:
conversion products of oil and its components;
fuel combustion products;
water;
wear products
contaminants in the form of dust, sand and dirt.
Oxidation.
In a running engine, hot oil constantly circulates and comes into contact with air, products of complete and incomplete combustion of fuel. Air oxygen accelerates oil oxidation. This process is faster in oils prone to foaming. The metal surfaces of the parts act as catalysts for the oil oxidation process. The oil heats up in contact with heated parts (primarily cylinders, pistons and valves), which greatly speeds up the process of oil oxidation. The result can be solid oxidation products (deposits).
The nature of the oil change in a running engine is influenced not only by the chemical transformations of the oil molecules, but also by the products of complete and incomplete combustion of the fuel, both in the cylinder itself and breaking through into the crankcase.
Effect of temperature on engine oil oxidation.
There are two types of engine temperature conditions:
operation of a fully warmed-up engine (main mode).
operation of an unheated engine (frequent car stops).
In the first case, there is high temperature mode of changing the properties of the oil in the engine, in the second - low temperature. There are many intermediate working conditions. When determining the level of oil quality, motor tests are carried out both in high and low temperatures.
Oxidation products and changes in engine oil characteristics.
acids (acides). The most significant products of oil oxidation are acids. They cause corrosion of metals, and alkaline additives are consumed to neutralize the resulting acids, as a result of which the dispersing and detergent properties deteriorate and the service life of the oil is reduced. An increase in the total acid number, TAN (totalacidnumber) is the main indicator of the formation of acids.
Carbon deposits in the engine (carbon deposits). A variety of carbon deposits form on the hot surfaces of engine parts, the composition and structure of which depend on the temperature of the metal and oil surfaces. There are three types of deposits:
soot,
varnish,
sludge.
It must be emphasized that the formation and accumulation of deposits on the surface of engine parts is the result of not only insufficient oxidative and thermal stability of the oil, but also its insufficient detergency. Therefore, engine wear and reduced oil life is a complex indicator of oil quality.
Nagar (varnish, carbondeposits) are products of thermal degradation and polymerization (crackingandpolymerization) of oil and fuel residues. It forms on very hot surfaces (450° - 950°C). Nagar has a characteristic black color, although it can sometimes be white, brown or other colors. The thickness of the deposit layer periodically changes - when there are a lot of deposits, heat removal worsens, the temperature of the upper layer of deposits rises and they burn out. Less deposits form in a warm engine running under load. According to the structure, deposits are monolithic, dense or loose.
Nagar has a negative effect on the operation and condition of the engine. Deposits in the piston grooves, around the rings, prevent their movement and pressing against the cylinder walls (jamming, sticking, ring sticking). As a result of jamming and difficulty in the movement of the rings, they do not press against the walls and do not provide compression in the cylinders, the engine power drops, gas breakthrough into the crankcase and oil consumption increase. Pressing the rings with deposits against the cylinder walls leads to excessive cylinder wear (excessive wear).
Cylinder wall polishing (borepolishing) - deposits on the top of the pistons (pistontopland) polish the inner walls of the cylinders. Polishing prevents the oil film from retaining and retaining on the walls and significantly accelerates the wear rate.
varnish (lacquer). A thin layer of brown to black, hard or sticky carbonaceous substance that forms on moderately heated surfaces due to the polymerization of a thin layer of oil in the presence of oxygen. The skirt and the inner surface of the piston, connecting rods and piston pins, valve stems and the lower parts of the cylinders are varnished. The varnish significantly impairs heat removal (especially of the piston), reduces the strength and persistence of the oil film on the cylinder walls.
Deposits in the combustion chamber (combustionchamberdeposits) are formed from carbon particles (coke), as a result of incomplete combustion of fuel and metal salts included in the composition of additives as a result of thermal decomposition of oil residues entering the chamber. These deposits heat up and cause premature ignition of the working mixture (before a spark appears). This kind of ignition is called preignition or preignition. This creates additional stresses in the engine (detonation), which leads to accelerated wear of the bearings and the crankshaft. In addition, individual parts of the engine overheat, power decreases, and fuel consumption increases.
Clogged spark plugs (sparkplugfouling). Deposits accumulated around the spark plug electrode close the spark gap, the spark becomes weak, and the ignition becomes irregular. As a result, engine power is reduced and fuel consumption is increased.
Tars, sludge, resinous deposits (sludge) (resins, sludge, sludge deposits) in the engine, sludge is formed as a result of:
oxidation and other transformations of oil and its components;
accumulation in the oil of fuel or decomposition products and incomplete combustion;
water.
Resinous substances are formed in oil as a result of its oxidative transformations (crosslinking of oxidized molecules) and polymerization of oxidation products and incomplete combustion of fuel. The formation of resins increases when the engine is not warm enough. Products of incomplete combustion of fuel break into the crankcase during prolonged idling or in stop-start mode. At high temperatures and intensive engine operation, the fuel burns more completely. To reduce tar formation and engine oils, dispersant additives are introduced that prevent coagulation and precipitation of resins. Resins, carbon particles, water vapor, heavy fuel fractions, acids and other compounds condense, coagulate into larger particles and form sludge in the oil, the so-called. black sludge.
Sludge (sludge) is a suspension and emulsion in oil of insoluble solids and resinous substances from brown to black. Composition of crankcase sludge:
oil 50-70%
water 5-15%
products of oil oxidation and incomplete combustion of fuel, solid particles - the rest.
Depending on the temperature of the engine and oil, the processes of sludge formation are somewhat different. Distinguish between low temperature and high temperature
Low temperature sludge (low temperature sludge). It is formed when breakthrough gases containing fuel and water residues interact with oil in the crankcase. In a cold engine, water and fuel evaporate more slowly, which contributes to the formation of an emulsion, which subsequently turns into sludge. The formation of sludge in the crankcase (sludgeinthesump) is the cause of:
increase in viscosity (thickening) of the oil (viscosityincrease);
clogging of the channels of the lubrication system (blockingofoilways);
violation of the oil supply (oilstarvation).
The formation of sludge in the rocker box is the cause of insufficient ventilation of this box (foulairventing). The resulting sludge is soft, friable, but when heated (during a long trip) becomes hard and brittle.
high temperature sludge (high temperature sludge). It is formed as a result of the combination of oxidized oil molecules under the influence of high temperature. An increase in the molecular weight of the oil leads to an increase in viscosity.
In a diesel engine, sludge formation and an increase in oil viscosity are caused by the accumulation of soot. Soot formation is facilitated by engine overload and an increase in the fat content of the working mixture.
additive consumption. Consumption, the operation of additives is the determining process of reducing the oil resource. The most important engine oil additives - detergents, dispersants and neutralizers - are used to neutralize acidic compounds, are retained in filters (together with oxidation products) and decompose at high temperatures. The consumption of additives can be indirectly judged by a decrease in the total base number TBN. The acidity of the oil increases due to the formation of acid oxidation products of the oil itself and sulfur-containing products of fuel combustion. They react with additives, the alkalinity of the oil gradually decreases, which leads to a deterioration in the detergent and dispersant properties of the oil.
The effect of increasing power and forcing the engine. The antioxidant and detergent properties of the oil are especially important when boosting engines. Gasoline engines are boosted by increasing the compression ratio and crankshaft speed, while diesel engines are boosted by increasing effective pressure (mainly with turbocharging) and crankshaft speed. With an increase in the crankshaft speed by 100 rpm or with an increase in effective pressure by 0.03 MPa, the piston temperature increases by 3°C. When forcing engines, their mass is usually reduced, which leads to an increase in mechanical and thermal loads on parts.
FLUSHING THE ENGINE.
During the operation of the car, even when using high-quality motor oils, harmful carbon deposits inevitably form on the internal surfaces of the engine and the channels of the lubrication system. When changing oil, some old used engine oil also inevitably remains in the internal cavities of the engine. Therefore, if fresh engine oil is poured immediately after draining the engine used without prior flushing, the detergent additives of the newly filled oil will immediately begin to actively dissolve all these deposits and contaminants remaining in the engine, which in turn can lead to a number of extremely negative consequences: in particular , to partial clogging of the oil filter and, accordingly, to a decrease in the efficiency of its operation, as well as to premature operation of the additive package and the loss of cleaning properties of fresh engine oil. All this has the most detrimental effect on the engine resource and its power characteristics. Today, the need to flush the lubrication system when changing engine oil is quite obvious, no one doubts and does not need any additional justification. In the combustion chamber of a gasoline engine, where the fuel-air mixture enters, it ignites, completely or partially combusts, resulting in carbon deposits. In addition, the products of incomplete combustion of fuel are the cause of the formation of varnish deposits on the internal surfaces of the engine. Further, most of the combustion products leave through the exhaust system, however, a small part of the gases breaks into the crankcase and, accordingly, comes into contact with the engine oil. In this case, the oil is oxidized and diluted, hardly soluble oxidation products are formed, which, in turn, additionally contribute to the formation of sludge and other deposits. In diesel engines, in addition, sulfur enters the combustion chamber with the fuel. As a result of oxidative reactions of sulfur, during the combustion of the fuel-air mixture, harmful deposits are formed, which result in corrosion and engine wear. Carbon deposits formed on the internal surfaces, channels of the lubrication system and engine parts lead not only to a deterioration in heat removal, but also to a noticeable decrease in the adhesion of oil to friction surfaces, which, accordingly, worsens the retention of the oil film on engine parts in friction units.
Reasons for the formation of deposits and soot in the engine
The use of high-quality oils does not eliminate the problem of coking, since deposits and deposits can form in the engine for reasons not related to the quality of fuels and lubricants:
1. Engine overheating . As a result of regular overheating, the oil ages faster, loses viscosity and forms polymer deposits in the grooves under the piston rings, on the walls of the combustion chamber, lubrication system and other parts.
2. Operation at low temperatures . The water vapor formed during the combustion of fuel reacts with cold oil, which leads to the formation of sludge in the crankcase.
3. Urban mode of operation . Short trips and traffic jams. With such operation, the engine does not reach normal operation, and as a result, carbonization of the cylinder-piston group begins.
4. Untimely oil change leads to a sharp increase in deposits arising as a result of its aging processes.
5. Turbocharger wear , as a result of which hot exhaust gases begin to enter the oil, and the properties of the oil change.
6. Antifreeze getting into the crankcase when the cooling system is depressurized, which changes the properties of the oil and initiates the processes of its polymerization.
7. Poor quality fuel . With incomplete combustion of fuel, part of it enters the crankcase through the rings and accelerates the aging process of the oil.
8. The formation of excess soot due to weak compression or late fuel injection in diesel engines.
When distilling oil with a low content of sulfur compounds, diesel fuels with high chemical stability are obtained. Such fuels retain their qualities for a long time (more than 5 years of storage).
After the use of such fuel in a diesel engine, carbon deposits and tarry deposits appear. The reason for this is incomplete evaporation and poor atomization of diesel fuel inside the cylinders due to the high viscosity of the fuel with a heavy fractional composition. In addition, the presence of mechanical impurities in diesel fuel is the cause of carbon formation.
Consequently, the presence of sulfur, actual tars, ash (non-combustible impurities) in the fuel and the tendency of such fuel to carbon formation determine the dynamics of carbon deposits accumulation, which is characterized by coke number, i.e. the ability of the fuel to form a carbonaceous residue during high-temperature (more than 800 ... 900? C) decomposition of the fuel without air access.
The carbonaceous residue or mineral residue is ash, i.e. non-combustible impurity that increases carbon formation. In addition, ash entering the engine oil causes accelerated wear of internal combustion engine parts. Therefore, the amount of ash is limited to a norm of not more than 0.01%. Thus, the following factors are the cause of the formation of carbonaceous residue:
1) insufficient depth of fuel purification from tar-asphalten compounds;
2) increased viscosity of diesel fuel;
3) heavy fractional composition of the fuel.
Also, the tendency of diesel fuel to soot is characterized by the content of actual resins in it, i.e. impurities remaining after cleaning the basic distillers. The actual resins cause gumming of the fuel, due to the presence of unsaturated hydrocarbons in the fuel, the amount of which is judged by the iodine number.
The iodine number is an indicator of unsaturated hydrocarbons (olefins) in diesel fuel, numerically equal to the number of grams of iodine added to unsaturated hydrocarbons contained in 100 g of fuel.
Usually, unsaturated hydrocarbons (olefins) react with iodine. That is, the more unsaturated hydrocarbons in the fuel, the more iodine reacts. Normal is such an amount of unsaturated hydrocarbons that react with iodine not exceeding 6 g of iodine per 100 g of winter or summer diesel fuel.
The more actual resins in diesel fuel, the higher its tendency to carbon formation. Therefore, the content of actual resins should not exceed:
for winter diesel fuel - 30 mg per 100 ml;
For summer DT - 60 mg per 100 ml.
The tendency of diesel fuel to varnish formation is estimated by the content of varnish in mg per 100 ml of fuel. To do this, the fuel is evaporated in a special lacquer at a temperature of 250?
Conclusions:
1) When a diesel engine runs on sour fuel, hard deposits and varnish deposits are formed that are difficult to remove, which causes wear on engine parts when it runs at low temperatures.
2) The carbonization of the fuel also leads to the formation of carbon deposits and varnish formation, as a result of which piston ring jamming can occur.
3) Due to the presence of mercapt sulfur particles in the fuel, during the oxidation of the fuel, resins are formed, which, in combination with resins formed from olefins and even the actual resins that are in diesel fuel, varnish films are deposited on the nozzle needles, which eventually causes the needles to freeze inside the nozzles.
4) Multifunctional additives and their influence on the properties of diesel fuels.
Improving the properties of diesel fuel is achieved by introducing multifunctional additives into their composition, such as:
Depressor;
· Increasing cetane number;
· Antioxidant;
· Detergent-dispersing;
Reducing the smoke of exhaust gases, etc.
Anti-smoke additives grades MST-15, ADP-2056, EFAP-6 at a concentration of 0.2…0.3 make it possible to reduce exhaust smoke by 40…50% and reduce soot content.
Zinc naphthenate grade anti-corrosion additive at a concentration of 0.25 ... 0.3%, added to engine oil, effectively neutralizes the destructive effect of acids.
To increase the cetane number of diesel fuel, improve its starting properties, additives are used: thionitrates RNSO; isopropyl nitrates; peroxide RCH 2 ONO at a concentration of 0.2 ... 0.25%.
Depressant additives - copolymers of ethylene and vinyl acetate with a concentration of 0.001 ... 2.0% are used to lower the pour point. They cover with a monomolecular layer of microcrystals of hardening paraffins, prevent their enlargement and precipitation.
Antioxidant additives at a concentration of 0.001 ... 0.1% increase the thermal-oxidative resistance of fuels.
Anti-corrosion additives at a concentration of 0.0008 ... 0.005% reduce the corrosive aggressiveness of diesel fuels.
Biocidal additives at a concentration of 0.005 ... 0.5%, which suppress the reproduction of microorganisms in the fuel.
Multifunctional additives, consisting of depressant, detergent and anti-smoke components, which not only expand the low-temperature properties of fuels, but also reduce the toxicity of exhaust gases. For example, the introduction of ADDP additive into diesel fuel in the amount of 0.05...0.3% reduces the pour point of the fuel by 20...25%, while the filterability temperature decreases by 10...12? C, smoke - by 20...55? C, and carbon formation - by 50 ... 60%.
Thus, the introduction of various additives and additives into diesel fuel significantly improves its performance properties.
EFFECT OF TEMPERATURE ON DEPOSITS IN THE ENGINE
Study of deposits in automobile engines.
One of the reserves for increasing the operational reliability of internal combustion engines is the reduction of deposits of deposits, varnishes and sediments on the surfaces of their parts in contact with engine oil. Their formation is based on the aging processes of oils (oxidation of hydrocarbons that make up the oil base). The decisive influence on the processes of oil oxidation in engines, the formation of deposits and the efficiency of the internal combustion engine as a whole is exerted by the thermal regime of heat-loaded parts.
Key words: temperature, piston, cylinder, motor oil, deposits, soot, varnish, performance, reliability.
Deposits on the surfaces of internal combustion engine parts are divided into three main types - deposits, varnishes and sediments (sludge).
Nagar - solid carbonaceous substances deposited during engine operation on the surfaces of the combustion chamber (CC). At the same time, carbon deposits mainly depend on temperature conditions, even with the same composition of the mixture and the same design of engine parts. Nagar has a very significant effect on the combustion process of the air-fuel mixture in the engine and on the durability of its operation. Almost all types of abnormal combustion (knock combustion, glow ignition, and others) are accompanied by one or another effect of soot on the surfaces of the parts that form the combustion chamber.
Lacquer is a product of the change (oxidation) of thin oil films that spread and cover the parts of the cylinder-piston group (CPG) of the engine under the influence of high temperatures. The greatest harm to internal combustion engines is caused by varnish formation in the area of the piston rings, causing the processes of their coking (occurrence with loss of mobility). Lacquers, deposited on the surfaces of the piston in contact with oil, disrupt proper heat transfer through the piston, impair heat removal from it.
The amount of precipitation (sludge) formed in the internal combustion engine is decisively influenced by the quality of engine oil, the temperature regime of parts, the design features of the engine and operating conditions. Deposits of this type are most typical for winter operation conditions, they are intensified with frequent starts and stops of the engine.
The thermal state of the internal combustion engine has a decisive influence on the processes of formation of various types of deposits, the strength characteristics of the materials of the parts, the output effective indicators of the engines, and the wear processes of the surfaces of the parts. In this regard, it is necessary to know the threshold temperatures of the CPG parts, at least at characteristic points, the excess of which leads to the previously indicated negative consequences.
It is advisable to analyze the temperature state of the parts of the ICE CPG according to the temperature values at characteristic points, the location of which is shown in Fig. 1 . The temperatures at these points should be taken into account in the production, testing and development of engines to optimize the design of parts, when choosing engine oils, when comparing the thermal states of various engines, and when solving a number of other technical problems of designing and operating internal combustion engines.
Rice. Fig. 1. Characteristic points of the cylinder and piston of the internal combustion engine during the analysis of their temperature state for diesel (a) and gasoline (b) engines
These values have critical levels:
1. The maximum temperature value at point 1 (in diesel engines - at the edge of the CS, in gasoline engines - in the center of the piston bottom) should not exceed 350C (for a short time, 380C) for all aluminum alloys commercially used in automotive engine building, otherwise the edges of the CS in the diesel engines and, often, burnout of pistons in gasoline engines. In addition, the high temperatures of the firing surface of the piston bottom cause the formation of deposits of high hardness on this surface. In the practice of engine building, this critical temperature value can be increased by adding silicon, beryllium, zirconium, titanium and other elements to the piston alloy.
The prevention of exceeding critical temperatures at this point, as well as in the volumes of internal combustion engine parts, is also ensured by optimizing their shapes and proper organization of cooling. Exceeding the temperatures of CPG engine parts of acceptable values is usually the main limiting factor for forcing them in terms of power. For temperature levels, a certain margin should be kept, taking into account possible extreme operating conditions.
2. The critical temperature value at point 2 of the piston - above the upper compression ring (VKK) - 250 ... 260C (short-term, up to 290C). When this value is exceeded, all mass engine oils coke (intense varnish formation occurs), which leads to “occlusion” of piston rings, that is, the loss of their mobility, and as a result, to a significant decrease in compression, an increase in engine oil consumption, etc.
3. The maximum temperature limit at point 3 of the piston (the point is located symmetrically along the cross section of the piston head on its inner side) is 220C. At higher temperatures, intense varnish formation occurs on the inner surface of the piston. Lacquer deposits, in turn, are a powerful thermal barrier that prevents heat removal through the oil. This automatically leads to an increase in temperatures in the entire volume of the piston, and hence on the surface of the cylinder mirror.
4. The maximum allowable temperature value at point 4 (located on the surface of the cylinder, opposite the stop point of the VCC at TDC) is 200C. When it is exceeded, the engine oil liquefies, which leads to a loss of stability in the formation of an oil film on the cylinder mirror and “dry” friction of the rings on the mirror. This causes an intensification of molecular mechanical wear of CPG parts. On the other hand, it is known that the reduced temperature of the cylinder walls (below the dew point of the exhaust gases) contributes to the acceleration of their corrosion-mechanical wear. The mixture formation also deteriorates and the rate of combustion of the air-fuel mixture decreases, which reduces the efficiency and economy of the engine, causing an increase in the toxicity of exhaust gases. It should also be noted that at significantly lower temperatures of the piston and cylinder, condensed water vapor penetrating into the crankcase oil causes intense coagulation of impurities and hydrolysis of additives with the formation of precipitation - "sludge". These deposits, contaminating oil channels, oil sump nets, oil filters, significantly disrupt the normal operation of the lubrication system.
The intensity of the processes of formation of carbon deposits, varnishes and deposits on the surfaces of internal combustion engine parts is significantly affected by the aging of motor oils during their operation. The aging of oils consists in the accumulation of impurities (including water), changes in their physical and chemical properties, and oxidation of hydrocarbons.
The change in the fractional composition of pure filled oil as the engine is running is mainly caused by reasons that change the composition of its oil base and the percentage of additives for individual components (paraffin, aromatic, naphthenic).
These include:
processes of thermal decomposition of oil in areas of overheating (for example, in valve bushings, areas of the upper piston rings, on the surfaces of the upper chords of the cylinder mirror). Such processes lead to the oxidation of the lightest fractions of the oil base or even their partial boiling off;
adding to the hydrocarbons the base of unevaporated fuel, which enters the crankcase oil sump through the piston seal zone during the initial periods of starts (or with a sharp increase in the fuel supply to the cylinders to accelerate the vehicle);
water entering the oil pan or oil sump of the engine, which is formed during the combustion of fuel in the COP of cylinders.
If the crankcase ventilation system operates efficiently enough, and the crankcase walls are heated to 90-95°C, water does not condense on them and is removed to the atmosphere by the crankcase ventilation system. If the temperature of the crankcase walls is significantly lowered, then the water that has entered the oil will take part in its oxidation processes. The amount of condensed water in this case can be quite significant. Even if we assume that only 2% of gases can break through all the compression rings of the cylinder, then 2 kg of water will be pumped through the crankcase of an engine with a working volume of 2-2.5 liters for every 1000 km of run. Suppose that 95% of the water is removed by the crankcase ventilation system, then after a run of 5000 km, about 0.5 liters of H2O will fall on 4.0 liters of engine oil. This water, when the engine is running, is converted by an antioxidant additive contained in engine oil into impurities - coke and ash.
For the reasons stated earlier, it is necessary to keep the temperature of the crankcase walls sufficiently high during engine operation, and, if necessary, to use dry sump lubrication systems with a separate oil tank.
It should be noted that measures that slow down the processes of changing the composition of the oil base significantly slow down the formation of carbon deposits, varnish and deposits, and also reduce the wear intensity of the main parts of automobile engines.
The fractional and chemical composition of oils can vary over a fairly wide range.
limits under the influence of various factors:
the nature of the raw material, depending on the field, the properties of the oil well;
features of the technology for the manufacture of motor oils;
features of transportation and duration of storage of oils.
For a preliminary assessment of the properties of petroleum products, various laboratory methods are used: determination of the distillation curve, flash points, turbidity and solidification, assessment of oxidizability in media with different aggressiveness, etc.
The aging of automotive engine oil is based on the processes of oxidation, decomposition and polymerization of hydrocarbons, which are accompanied by the processes of oil contamination with various impurities (soot, dust, metal particles, water, fuel, etc.). Aging processes significantly change the physical and chemical properties of the oil, lead to the appearance of various oxidation and wear products in it, and worsen its performance. There are the following types of oil oxidation in engines: in a thick layer - in the oil pan or in the oil tank; in a thin layer - on the surfaces of hot metal parts; in a foggy (drip) state - in the crankcase, valve box, etc. In this case, the oxidation of oil in a thick layer gives precipitation in the form of sludge, and in a thin layer - in the form of varnish.
The oxidation of hydrocarbons is subject to the theory of peroxides by A.N. Bach and K.O. Engler, supplemented by P.N. Chernozhukov and S.E. Crane. Oxidation of hydrocarbons, in particular, in ICE engine oils, can proceed in two main directions, shown in Fig. 2, the results of oxidation for which are different. In this case, the result of oxidation in the first direction is acidic products (acids, hydroxy acids, estolides and asphaltogenic acids), which form precipitation at low temperatures; the result of oxidation in the second direction are neutral products (carbenes, carboids, asphaltenes and resins), from which either varnishes or deposits are formed in various proportions at elevated temperatures.
Rice. 2. Pathways for the oxidation of hydrocarbons in a petroleum product (for example, in engine oil for internal combustion engines)
In the processes of oil aging, the role of water that enters the oil during the condensation of its vapors from crankcase gases or in other ways is very significant. As a result, emulsions are formed, which subsequently enhance the oxidative polymerization of oil molecules. The interaction of hydroxy acids and other products of oil oxidation with oil-in-water emulsions causes increased formation of deposits (sludge) in the engine.
In turn, the resulting sludge particles, if they are not neutralized by the additive, serve as catalysis centers and accelerate the decomposition of the oil that has not yet been oxidized. If at the same time the engine oil is not replaced in a timely manner, the oxidation process will proceed as a chain reaction with an increasing speed, with all the ensuing consequences.
The decisive influence on the formation of deposits, varnishes and deposits on the surfaces of internal combustion engine parts in contact with engine oil is their thermal state. In turn, the design features of engines, their operating conditions, operating modes, etc. determine the thermal state of the engines and thus affect the formation of deposits.
No less important influence on the formation of deposits in the internal combustion engine is exerted by the characteristics of the engine oil used. For each specific engine, it is important that the oil recommended by the manufacturer corresponds to the temperature of the surfaces of the parts in contact with it.
In this paper, the relationship between the temperatures of the surfaces of the pistons of the engines ZMZ-402.10 and ZMZ-5234.10 and the processes of formation of carbon deposits and varnishes on them was analyzed, and sedimentation was assessed on the surfaces of the crankcase and valve cover of the engines when using the engine oil M 63 / 12G1 recommended by the manufacturer .
To study the dependence of the quantitative characteristics of deposits in engines on their thermal state and operating conditions, various methods can be used, for example, L-4 (England), 344-T (USA), PZV (USSR), etc. . In particular, according to the 344-T method, which is a US regulatory document, the condition of a “clean” unworn engine is rated at 0 points; the condition of an extremely worn and polluted engine - 10 points. A similar method for assessing varnish formation on piston surfaces is the domestic ELV method (authors - K.K. Papok, A.P. Zarubin, A.V. Vipper), the color scale of which has points from 0 (no varnish deposits) to 6 (maximum deposits varnish). To recalculate the points of the ELV scale into points of the 344-T method, the readings of the first one must be increased by one and a half times. The specified method is similar to the domestic method of negative assessment of deposits of the All-Russian Research Institute of Oil and Gas (10 point scale).
For experimental studies, 10 ZMZ-402.10 and ZMZ-5234.10 engines were used. Experiments to study the processes of deposit formation were carried out jointly with the laboratories for testing cars and trucks UKER GAZ on engine stands. During the tests, among other things, the flow rates of air and fuel, the pressure and temperature of the exhaust gases, the temperature of the oil and the coolant were monitored. At the same time, the following modes were maintained on the stands: the crankshaft speed corresponding to the maximum power (100% of the load), and, alternately, for 3.5 hours - 70% of the load, 50% of the load, 40% of the load, 25% of the load and no load (with closed throttle valves), i.e. experiments were carried out on the load characteristics of engines. At the same time, the temperature of the coolant was kept in the range of 90...92C, the temperature of the oil in the main oil line was 90...95C. After that, the engines were disassembled and the necessary measurements were made.
Preliminary studies were carried out to change the physico-chemical parameters of motor oils during testing of ZMZ-402.10 engines as part of GAZ-3110 vehicles at the UKER GAZ test site. At the same time, the following conditions are met: the average technical speed is 30 ... 32 km / h, the ambient temperature is 18 ... 26C, the mileage is up to 5000 km. As a result of the tests, it was obtained that with an increase in vehicle mileage (engine operation time), the amount of mechanical impurities and water in engine oils, its coke number and ash content increased, and other changes occurred, which is presented in Table. 1
Carbon formation on the surfaces of the piston bottoms of the ZMZ-5234.10 engines was characterized by the data presented in fig. 3 (for engines ZMZ-402.10 the results are similar). From the analysis of the figure, it follows that with an increase in the temperature of the piston bottoms from 100 to 300С, the thickness (existence zone) of carbon deposits decreased from 0.45 ... 0.50 to 0.10 ... engines. The hardness of the soot increased from 0.5 to 4.0...4.5 points due to the sintering of soot at high temperatures.
Rice. 3. Dependences of carbon formation on the surfaces of the piston bottoms of ZMZ-5234.10 engines on their temperatures:
a - soot thickness; b - soot hardness;
the symbols show the averaged experimental values
Evaluation of varnish deposits on the side surfaces of the pistons and their internal (non-working) surfaces was also carried out on a ten-point scale, according to the 344-T method used in all leading research institutions in the country.
Data on varnish formation on the surfaces of engine pistons are presented in fig. 4 (the results for the studied brands of engines are the same). The test modes are indicated earlier and correspond to the modes in the studies of carbon formation on parts.
From the analysis of the figure, it follows that the varnish formation on the surfaces of engine pistons unambiguously increases with an increase in the temperatures of their surfaces. The intensity of varnish formation is affected not only by an increase in the temperature of the surfaces of parts, but also by the duration of its action, i.e. the duration of the engines. In this case, however, the processes of varnish formation on the working (rubbing) surfaces of the pistons slow down significantly compared to the internal (non-working) surfaces, due to the erasing of the varnish layer as a result of friction.
Rice. 4. Dependences of varnish deposits on the surfaces of pistons of ZMZ-5234.10 engines on their temperatures:
a - internal surfaces; b - side surfaces; the symbols show the averaged experimental values
Nagar and varnish formation on the surfaces of parts is significantly intensified when oils of groups "B" and "C" are used, which is confirmed by a number of studies carried out by the authors on similar and other types of automobile engines.
A systematic increase in varnish deposits on the internal (non-working) surfaces of the pistons causes a decrease in heat removal to the crankcase oil with an increase in engine operating time. This causes, for example, a gradual increase in the level of the thermal state of the engines as the operating time approaches the oil change at the next TO-2 of the car.
The formation of sediments (sludge) from motor oils occurs to the greatest extent on the surfaces of the crankcase and valve cover. The results of studies of sedimentation in ZMZ-5234.10 engines are shown in fig. 5 (for engines ZMZ-402.10 the results are similar). Deposit formation on the surfaces of the previously mentioned parts was evaluated depending on their temperatures, for the measurement of which thermocouples were mounted (welded by capacitor welding): on the surfaces of the crankcase, 5 pieces for each engine, on the surfaces of valve covers, 3 pieces.
As follows from Fig. 5, with an increase in the temperatures of the surfaces of engine parts, sedimentation on them decreases due to a decrease in the water content in the crankcase oil, which does not contradict the results of previous experiments by other researchers. In all engines, sedimentation on the surfaces of crankcase parts turned out to be greater than on the surfaces of valve covers.
On motor oils of forcing groups "B" and "C", sedimentation on ICE parts in contact with engine oil occurs more intensively than on oils of forcing groups "G", which is confirmed by a number of studies.
In this work, deposits on the cylinder mirrors during operation of engines with the most modern oils were not studied, however, we can confidently assume that for the engines under study they will be no more than when they are operated with lower quality oils.
The results obtained on the relationship between temperature changes in the main parts of engines ZMZ-402.10 and ZMZ-5234.10 (pistons, cylinders, valve covers and oil crankcases) and the amount of deposits made it possible to identify patterns in the processes of formation of deposits, varnishes and deposits on the surfaces of these parts. To do this, the results were approximated by functional dependencies by the least squares method and are presented in Figs. 3-5. The obtained regularities of the processes of formation of deposits on the surfaces of parts of automobile carburetor engines should be taken into account and used by designers and engineering and technical workers involved in fine-tuning and operation of internal combustion engines.
The car engine works with the greatest efficiency only under certain conditions. The optimal temperature regime of heat-loaded parts is one of such conditions and provides high technical characteristics of the engine with a simultaneous decrease in wear and deposits and, consequently, an increase in its reliability.
The optimal thermal state of the internal combustion engine is characterized by the optimal temperatures of the surfaces of their heat-loaded parts. Analyzing the studies of the processes of formation of deposits on the parts of the studied ZMZ carburetor engines and similar studies on gasoline engines, it is possible to determine with a sufficient degree of accuracy the intervals of optimal and dangerous temperatures for the surfaces of parts of this class of engines. The information obtained is presented in Table. 2.
At temperatures of engine parts in a dangerous low-temperature zone, the thickness of soot on the surfaces of the parts forming the combustion chamber increases, which leads to the detonation combustion of air-fuel mixtures, and also at low temperatures of the surfaces of engine parts, the amount of precipitation from engine oils increases on them. All this disrupts the normal operation of the engines. In turn, deposits lead to a redistribution of heat flows passing through the pistons and an increase in piston temperatures at critical points - in the center of the fire surface of the piston bottom and in the VKK groove. The temperature field of the ZMZ-5234.10 engine piston, taking into account the deposits of deposits and varnishes on its surfaces, is shown in Fig. 7.
The problem of heat conduction by the finite element method was solved with the first-class GU obtained by thermometering the piston in the rated power mode during bench tests of the engine. Thermoelectric experiments were carried out with the same piston, for which preliminary studies of the temperature state were carried out without taking into account deposits. The experiments were carried out under identical conditions. Previously, the engine worked on the stand for more than 80 hours, after which the stabilization of deposits and varnishes begins. As a result, the temperature in the center of the piston bottom increased by 24°C, in the zone of the VPC groove - by 26°C in comparison with the piston model without deposits. The temperature value of the piston surface above the VCC 238°C is included in the hazardous high-temperature zone (Table 2). Close to the dangerous high temperature zone and the temperature value at the center of the piston crown.
At the design and development stage of engines, the effect of carbon deposits on the heat-receiving surfaces of pistons and varnishes on their surfaces in contact with engine oil is taken into account extremely rarely. This circumstance, together with the operation of engines as part of a vehicle under increased thermal loads, increases the likelihood of failures - piston burnout, piston ring coking, etc.
N.A. Kuzmin, V.V. Zelentsov, I.O. Donato
Nizhny Novgorod State Technical University. R.E. Alekseeva, Moscow-Nizhny Novgorod Motorway Administration
