ENGINE Chapter 7. Lubrication System

ENGINE 

Chapter 7.

Lubrication System

1. The Role of Engine Oil

The main role of the engine oil is to move the dynamic system such as the piston in the cylinder and the crankshaft smoothly. To do so, the oil forms a oil film at the metal surface to reduce the friction between the metal surfaces. At the same time, the engine oil can secure the combusted gas not to leak to the crankcase, cools the piston and valve, and reduce the shock transmitted from the piston to the crankshaft as well as clean the engine inside.

As the lubrication is already mentioned in the section of 『Journal Bearing』, here, we will explain the other roles of the oil. The first is the sealing role in which the engine oil between the piston ring and cylinder prevents the leaking the compressed air or combustion air.

Oil jet

The cylinder head and the piston crown directly contacts with the hot combusted gas. The cylinder head is cooled not only by the cooling water but also by the oil at the dynamic system. The heat of piston crown is transmitted to the cylinder wall via the piston ring. Also the piston cools down by spraying the engine oil.

Another important role of the oil is to reduce the shock. The combustion force is very strong up to several tons weight force. This force is transmitted from the piston to piston pin, to Connecting rod, to crank pin and to crankshaft. In that process, the oil absorbes shock at the piston pin and the crank pin.

Except that, the engine oil can wipe out the foreign materials such as carbon particle generated by burned oil and metal particles from the wear of metal surfaces.

The oil also prevent inside of the engine from being rusted by chemicals from the combustion.

2. Lubrication Method

In old style engine, the lubrication method was so called the Splash type in which the big end of connecting rod hit and sprayed the oil contained in the oil pan under the connecting rod. Nowadays, the lubrication oil is sent to necessary portion by the oil pump and collects to the oil pan. According to the oil circulation method, there are the dry-sump type and the wet-sump type. The lubrication device consists of the oil pan containing oil, the oil filter purifying the oil, the oil pump sending the oil to each portion.

The wet-sump is equipped in most car. The oil contained in the oil pan is filtered big foreign materials using the oil strainer consisting of steel meshes and sent to the oil filter by oil pump to eliminate any tiny foreign materials. There is a oil gallery in the engine block, it is a oil passage for each dynamic parts such as crankshaft, cylinder head, connecting rod and cylinder wall.

Wet-sump type

Dry-sump type

The oil is returned to the oil pan from the piston, connecting rod, crankshaft and cylinder head. When the car is turned rapidly or accelerated or decelerated abruptly, the oil in the oil pan is leaned one side so it can not be pumped well. Some engines have a separator, a kind of partition in the oil pan to prevent the oil from leaning. For the special engine for racing car, the Scavenging pump takes the oil and air together, and the oil and air are separated by a separator and then oil is contained to the separated oil tank. This is the dry-sump type.

As the dry-sump method does not contain the oil in the pan, the oil pan can be thinner. So, the lower part of engine is smaller and the engine is designed to have lower weight center. However, the device should be complexed. It is applied only to the special case for equipping the opposed engine. Some engine equips a semi-dry-sump type having the oil pump for collecting the oil in the oil pan.

3. Parts of Lubricant System

The 3 main parts consisting of the lubricant system are oil pump, oil filter and the oil cooler cooling the heated oil.

There are many kinds in the oil pump taking up the oil in the oil pan. The passenger car uses a gear pump combined with toothed wheel and linked to the crankshaft directly.

The gear pump comprises of a driven gear in the pump body and the drive gear linked with the tooth of the driven gear. When the drive gear is rotating, the driven gear is also rotating. However, the centers of each gear are departing each other so the oil between them is pumped from the inlet port to outlet port. According to the tooth shape, there is an Involute type, the Trochoidal type, and so on.

Gear type oil pump

The oil sending amount by the oil pump is proportional to the engine rpm. At the high rotation of the engine, the oil pressure is too high. At the low temperature, the degree of viscosity of oil is increased so the oil pressure is too high. The device for maintaining the oil pressure should be equipped. It is called the pressure regulator or the relief valve.

The oil filter purifying the oil from the carbon or metal particles. A folded filter paper is inserted into the cleaner case. There are two types for the replacement the element type and the catridge type. The cartridge type is replaced the filter with case, the element type is replaced only filter element when the filter is replaced.

The proper working temperature of the engine oil is about 80℃. If the temperature is too low, the friction becomes high because of high viscosity. If the temperature is too high, the oil pressure is lowered so lubricating ability will be degraded and also  the oil in the high performance engine can be easily deteriorated. So the oil cooler is required for the the high performance engine.

The kinds of oil cooler are divided into the water type and the air type. The water type oil cooler maintains the oil temperature using the engine cooling water, and the air type oil cooler uses the running winds.

The air type has simpler device, but the cooling efficiency is lower than the water type. The water type is more complicated, but it ensures the more stable cooling efficiency.

4. Engine Oil

As we have mentioned, the engine oil works for reducing the wear, cooling the piston and the cylinder head, sealing the gap between the piston and cylinder, releasing the shock, cleaning the engine inside, preventing the knocking and so on.

The required characteristics of the engine oil are as follows,

- Proper viscosity at working condition

- Good lubricant performance

- High heat and corrosion resistances

- Anti-bubble

The most important characteristic is the viscosity. Therefore, the engine oil is classified by two aspects, the viscosity or the quality.

The commercial engine oil is sold in case of 1ℓ, 4ℓ or 20ℓ. At the case, there are name of manufacturer, brand name and oil name with the viscosity class and the quality class.

In the viscosity classification, according to the standard by SAE(Society Automotive Engineers), the lower viscosity has lower number and higher viscosity has higher number. For cold weather, letter “W” is added. For example, certain classification like that the number 30 is for general purpose, and the number 20 is for winter, is the single grade using one number system only. Another classification like that a range is represented such as 5W-30 or 10W-30, is the multi grade. In this case, by comparing the 5W-30 with the 10W-30, the 5W-30 has lower viscosity than 10W-30 at low temperature, but has higher viscosity at the high temperature.

Generally, when the temperature is increased, the viscosity of oil will be decreased. To indicate how the viscosity is changed, the viscosity index is used. If the viscosity is not easily changed, then the viscosity index of the oil is high. The higher viscosity index is easier to use.

In the quality classification, the standard by the API(American Petroleum Institute) is used. For gasoline engine, the letter “S” followed from other alphabet letters is assigned. For the diesel engine, the letter “C” followed from other alphabet letters is assigned. For example, letters from SD to SG are assigned for the gasoline engine.

The kind of engine oil and replacing period are decided by type of engine, driving condition and ambient temperature, so please refer to the manual carefully to select the engine oil. The replacing running time is about 10,000km for SD, and 15,000km for SE, and SF 15,000km for gasoline engine roughly. For turbo engine, the engine oil should be replaced at every 5,000km running time because the driving condition is very tough. The maintenance intervals for each engines are varies, refer the manuals for the each engine.