1. Viscosity and viscosity temperature performance

When a liquid flows under external forces, internal friction occurs between molecules, and this physical quantity is called viscosity coefficient or internal friction coefficient, abbreviated as viscosity. The viscosity of engine oil changes with temperature, and as the temperature increases, the viscosity decreases; The decrease in temperature leads to an increase in viscosity, and the degree of change in this relationship is called the viscosity temperature performance of engine oil. The smaller the change in viscosity with temperature, the better its viscosity performance; On the contrary, it is worse. Viscosity and viscosity temperature performance are important usage indicators of internal combustion engine oil, and are the basis for oil grade classification. The choice of viscosity is very important. For lubrication, it is required that the engine oil has a suitable viscosity and can form a sufficiently thick oil film on the friction surface; For cooling and cleaning, it is required to use some oils with very low viscosity; In order to seal, it is also required to use oil with higher dryness. Therefore, the selection of viscosity should pay attention to the following points:

1) Maximum viscosity at start-up

2) Can maintain the minimum viscosity of the oil film

3) Good adhesive temperature performance

2. Good low-temperature performance

The low-temperature performance of internal combustion engine oil includes low-temperature starting performance and low-temperature pumping performance. The low-temperature starting performance is related to the low temperature of internal combustion engine oil, while the freezing point has little effect on the low-temperature starting performance of engine oil. The freezing point mainly affects the low-temperature pumping performance of internal combustion engine oil. This is because some internal combustion engine oil can start the engine at low temperatures, but the pump cannot supply oil in a timely and normal manner, providing appropriate lubrication for the moving parts of the engine, resulting in serious wear and increased noise of the moving parts.

3. Appropriate freezing point

The temperature at which the engine oil cools to a point where it cannot flow at all is called the freezing point of the engine oil. It is an indicator to ensure the fluidity and filtration of engine oil at low temperatures. Usually, high viscosity engine oil also has a high freezing point. The freezing point of commonly used engine oil on diesel engines is generally between 0-20 ℃.

To measure the fluidity of engine oil at low temperatures, the freezing point is often used. Internal combustion engine oil with a high freezing point has poor low-temperature flowability. When used at a low temperature, it can reduce or even interrupt fuel supply, causing wear and tear on the parts, and in severe cases, damage to the parts. Therefore, for safety reasons, it is generally hoped that the freezing point of engine oil will be about 5-7 ℃ lower than the average minimum temperature during use.

4. Good oiliness

The ability of engine oil to maintain a tight and firm oil film on a metal surface is called the oiliness of engine oil, sometimes also known as lubricity. The quality of oil directly affects the reliable lubrication of engine machinery and avoids wear of parts. Otherwise, when the engine load increases, the strength of the oil film on the lubricated metal surface cannot withstand high pressure and is destroyed, resulting in friction, causing wear and abrasion on the friction surface of the machine parts, and even sintering phenomenon.

5. Good cleanliness and dispersibility

Good internal combustion engine oil can suspend the gel like substances and carbon deposits generated by oxidation in the oil, making them less likely to deposit on mechanical parts. And it can also clean the sediment that has already been deposited on the parts, suspend it in the oil, and then remove it through the filter in the circulation of the internal combustion engine oil to maintain the cleanliness of the parts, thereby reducing the tendency of paint film and carbon deposition.

6. Good foam resistance

Internal combustion engine oil in the crankcase is easy to produce foam and even fill the crankcase due to the intense agitation and splash lubrication of the crankshaft. In addition to affecting the oil pump pressure, it is not conducive to lubrication and wear machinery, it will also waste internal combustion engine oil, accelerate the oxidation and deterioration of engine oil quality, and shorten the trial period of internal combustion engine oil.

7. Acid value and corrosiveness

The acidity value indicates the amount of acidic substances in the engine oil. Acid value is the number of milligrams of potassium hydroxide (KOH) required to neutralize the acidic substances contained in 1g of engine oil. Acidic substances generally originate from organic acids formed during the processing of engine oil or from oxidation and deterioration during use. Engine oil contains acidic substances that have a corrosive effect on diesel engine components, which is more severe at high temperatures. Therefore, it must be limited. According to national standards, the corrosiveness of engine oil is evaluated by its corrosiveness. That is, the lead sheet is placed at a temperature of 140 ℃ and subjected to intermittent action of engine oil and air for 10 hours, resulting in a weight loss of lead sheet (g/m) 2） To evaluate.

8. Residual carbon and ash content

The amount of charcoal and ash in engine oil is evaluated by the percentage it contains, and the lower the better.