Reasons for the reduction of engine oil base number

Q: After 3500 kilometers, the engine oil (gasoline engine) was tested and the base number was found to have dropped to 1.2 mgKOH/g. At the same time, the amount of oil in the engine oil also decreased. Why did the base number decrease?

A: The base number (BN value) is a test indicator of engine oil (for automobiles, marine gasoline/diesel engines, and gas engines). The combustion process, oil oxidation, and chemical reactions of the engine oil will generate acidic substances, and the alkaline substances in the engine oil can play a neutralizing role. It is generally believed that the higher the BN value, the stronger the ability of the engine oil to neutralize acidic substances. The BN value of newly purchased engine oil is generally between 5 and 15. With use, the oil is oxidized and contaminated, and the BN value will decrease. The rate of decline is related to the amount of acidic substances contained in the engine oil.

Through oil testing, we can determine the BN value of the engine oil. By tracking the BN value, we can understand how long the service life of the engine oil is left. Once the BN value is found to fall below 3, it is considered to be low and you should prepare to change the engine oil.

Causes of reduced BN value:

As mentioned earlier, in addition to oil oxidation, poor fuel quality can also lead to reduced BN value. These two factors are the most common causes of reduced BN value. If the fuel used is of low quality and has a high sulfur content, the engine will produce relatively more sulfides during the combustion process, producing sulfuric acid, which will increase the acidity of the oil and reduce the BN value after entering the oil. The oil will also produce acidic substances during the oxidation process. If the oxidation and deterioration rate is fast, or the oil's resistance to high-temperature oxidation is not good, it will also cause the BN value to decrease. Therefore, the rapid reduction of the BN value is related to the rapid deterioration of the oil, but it may also be caused by fuel factors. Judging the antioxidant capacity of the oil by simply relying on the BN value may not necessarily get an accurate result.

For this case, the BN value dropped to 1.2 in more than 3,000 kilometers. This value is relatively low. There must be an abnormal situation, which is likely to be a quality problem of the oil. If inferior oil is used, the use of additives will not help and cannot improve the antioxidant capacity of the oil.

In addition to fuel quality and engine oil quality, driving conditions, whether the engine oil is kept clean during use, and other factors will also cause differences in the decrease in BN value. For vehicles with relatively poor working conditions, the rate of decrease in BN value will be different for the same engine oil. In addition, fuel dilution will also cause the BN value of the engine oil to decrease. Fuel dilution refers to gasoline or diesel entering the engine oil.

If you want to rule out the possibility of fuel dilution, you can perform a flash point test on the engine oil. Fuel will cause the flash point of the engine oil to decrease. If the fuel content reaches 2%, you should pay attention. In addition to testing the flash point, you can also test the engine oil viscosity. Fuel dilution will cause the oil viscosity to decrease.