Engine lubrication system

4.9.1 Purpose of the lubrication system

Engine contains a large number of parts that rub against each other. They are all metal, and they all need lubrication, because they heat up and, as a result, can jam. Therefore, the engine is equipped with lubrication system: with channels (lines), with a sump and with an oil pump. A simplified diagram of the lubrication system is shown in Figure 4.38.

In addition to lubrication, the oil also acts as a cooler for hot rubbing engine parts. That is why, special sprayers are often installed in diesel, and sometimes in gasoline engines. These sprayers are aimed at the lower parts of the pistons, but more on that later.

Figure 4.38 Simplified diagram of lubrication system.

4.9.2 Oil pump

The name of this device indicates its purpose. An oil pump is required for pumping engine oil from the oil sump, which is located at the very bottom of the engine, to all rubbing parts through special oil channels.

Gear-type pumps with external or internal gearing are used for this purpose. Pumps of the first type are now very rarely installed on passenger cars due to their size, therefore, we will consider the type of pump, which is the most relevant today – gear-type pump with internal gearing, an example of which can be seen in Figure 4.39.

Figure 4.39 Gear-type oil pump with internal gearing.

The oil pump is usually driven from the crankshaft by a chain, belt or gear, depending on the type of timing mechanism, or directly mounted on the crankshaft The work of the pump lies in the fact that when rotating, the small gear rolls over the large gear, carrying the engine oil along with it. Through the channels the pump under pressure transfers oil to the rubbing parts.

4.9.3 Pressure reducing valve

The pressure reducing valve serves to limit the oil pressure in the oil passages of the lubrication system. The oil pressure can rise when the engine speed is too high, or when the oil is too thick, for example in a cold engine. The pressure reducing valve is usually installed in the pump casing. It is a ball, which is pressed by a spring. While the oil pressure is normal - the ball is pressed tightly against the spring. When the pressure starts rising excessively, the ball moves, compressing the spring, thus opening the bypass channel through which the oil from the sump through the pump again flows into the sump.

4.9.5 Oil radiator 

Having learned that during operation all engine parts get very hot, you could assume that the oil that lubricates these very parts also heats up, reaching decent temperatures. In case of severe overheating, the engine oil starts rapidly losing its properties. This can have quite disastrous consequences for the engine.

Note
When the engine is running, the temperature of the engine oil should not rise too much to avoid a drop in its viscosity.

Oil cooler is installed, which is sometimes similar to the radiator of the cooling system (see figure 4.33), in order to maintain the temperature of the engine oil within the most efficient range.

In case of air cooling, a tubular-type oil cooler, which is included in the oil line, shall be placed in front of the radiator of the engine water cooling system. If there is no oil cooler, the oil is cooled in the oil sump. The sump itself, in this case, may have additional ribs for increasing the area of contact between the oil and the sump.

Note
In case if the design assumes liquid cooling of the oil, then it is called a heat exchanger, not a radiator (schematically, such a heat exchanger can be seen in Figure 4.32).

Note
Water-cooled radiator provides not only cooling of the oil during operation in severe conditions, but also its rapid warming up when the engine is started.

4.9.6 Oil pan, crankcase

Oil pan is most often a pressed part, which looks like a bowl or kitchen baking pan. This is the container, which contains the engine oil. From there it is fed through the oil receiver (Figure 4.38) to all rubbing parts and flows down there after lubricating these parts. The dipstick, with the help of which the engine oil level is measured, is described the "Maintenance" chapter. This dipstick, or rather its thin plate with applied marks, is inserted precisely into the oil pan.

Note
The engine oil level is often checked for a cold engine.

Note
Oil shall be poured into the sump to a certain level, which must be maintained during engine operation. In case if the crankcase is overfilled, oil is excessively sprayed on the cylinder walls and can enter the combustion chambers. This will increase the carbon formation in them. Foaming of the oil is also possible. It can lead to a significant drop in pressure in the system and, if not stopped in time, to engine failure. It is also obvious that a lack of oil in the system can lead to the so-called oil starvation, which is why there are frequent cases of cranking of the liners in the main bearings of the crankshaft.

The crankcase is the largest body part of the engine. It can be cast together with the cylinder block or as a separate piece bolted to the block.

4.9.7 Crankcase ventilation

Most modern cars are equipped with forced ventilation systems for crankcase gases. Such a system usually includes valves and pipes that connect the crankcase cavity to the intake manifold.

The crankcase ventilation itself is extremely important for the normal operation of the engine. The fact is that one way or another, the exhaust gases enter the engine crankcase through the gaps of the piston group. Gases are also created when engine oil comes into contact with hot engine parts. The escaping exhaust gases dilute the engine oil while acting on it. This leads to a decrease in service life and a loss of efficiency. Depending on the operating mode of the engine, gases that are trapped inside the crankcase can sharply increase the excess pressure. This will lead to the squeezing out of the sealing cuffs (oil seals) and gaskets. This is why manufacturers install electronically controlled valves, which are responsible for crankcase ventilation.

4.9.8 Oil used for lubrication

Oils of mineral (now rare), semi-synthetic and synthetic origin are used for the lubrication of engines.

Special additives (special chemical compounds) are added to oil in order to improve its quality. These additives increase the lubricity of the oil, make its viscosity more stable, lower the pour point, reduce the oxidizing effect of oil. Additives in the oil also help washing away resinous deposits from the gaps of rubbing parts, etc.

Depending on the season and climatic conditions, oils of different viscosities should be used for lubricating the engine. In winter, the viscosity should be less, because oil with a high viscosity at low temperatures will thicken and in a cold engine it will not penetrate well into the gaps of rubbing parts. It will also be difficult to fill in oil and start a cold engine.

In summer, the viscosity of the oil should be higher, because oil with a low viscosity at elevated temperatures becomes even more liquid and does not provide normal engine lubrication. However, all-season engine oils are commonly used nowadays.

4.9.8.a Viscosity of oils according to SAE classification

Below we will consider the designation of the viscosity of oils according to the SAE (Society of Automotive Engineers) classification.

This designation has two numbers that are separated by the letter W. This letter indicates that the oil is all-season. In this case, the first digit indicates the minimum negative temperature at which the engine crankshaft can be rotated. Thus, 0W40 oil should be pumped from –35 °С, 15W40 - from –20 °С. The second digit determines the viscosity of the oil at a temperature of 100 °C, or more precisely, not the viscosity itself, but the permissible range of its change. Thus, for "30" the viscosity at 100 °C can vary in the range from 9.3 to 12.5 cSt (centistokes are the units of viscosity measurement); for "40" - from 12.5 to 16.5 cSt, and for " 50 "- from 16.3 to 21.9 cSt. That is, the kinematic viscosity within the permissible range can vary by 10 ... 15%.

4.9.8.b API viscosities

In parallel with the SAE classification, which characterizes the viscosity of engine oil, there is an API (American Petroleum Institute) classification, which determines its applicability for a particular engine.

The oil brand includes a two-letter index. The first index determines the type of engine: S (Service Station) - gasoline engines and C (Commercial) - diesel engines. The second index (A, B, C, D, E, F, G, H, J, L, M) indicates the level of performance. The brand of oil can be fractional, then from the point of view of application it is universal - for gasoline and diesel engines.