Why do you have to change the filter element when changing the oil?

Today, someone asked me if I could change the oil filter without changing it. I told him very clearly: the oil filter must be replaced along with the oil, and the original parts or genuine parts with guaranteed quality must be replaced. Don't be greedy for cheap replacement of inferior oil filters. To some extent, the oil filter is more important than the oil, and the oil filter can also affect the oil replacement cycle.

Why make the topic so serious? Because many car owners and friends pay special attention to the quality of oil when maintaining their cars, they don't pay attention to the oil filter, and even think that it is dispensable and can be replaced. In fact, this is a serious misunderstanding of oil filters, and I don't know the harm of inferior oil filters. I've ever encountered a case where the engine was eventually damaged due to abnormal oil pressure caused by the use of inferior oil filter. Although it is rare, it does exist.

I believe everyone knows the function of oil filter. As a certain amount of metal debris, carbon deposit, colloid and other impurities will be produced during the working process of the engine, their mixing in the engine oil will reduce the lubricating performance of the engine oil and affect the service life of the engine. The function of the oil filter is to filter these impurities in the oil to keep the standby oil clean, so that the circulating oil can be purified before being sent to the surface of moving parts. Ensure good lubrication of friction surface and prolong its service life. If the oil is compared to the blood of the engine, the oil filter can be called "the kidney of the engine", and its importance can be seen from this metaphor.

What many people don't know is that there are actually three kinds of oil filters on the engine. The first is the oil filter, which is installed in front of the oil pump in the oil pan, and can prevent large particles of impurities from entering the oil pump and causing abnormal wear of the oil pump; The second is the coarse oil filter, which is installed between the oil pump and the main oil passage to filter out the impurities with larger particle size in the oil, and plays the most important filtering role. The machine filter that we replace during maintenance refers to it; The third type is the fine oil filter, which is connected in parallel in the main oil passage and can filter small impurities in the oil. Generally, it is only equipped on large diesel engines, but not on cars. Most of them are centrifugal filters.

The structure of the oil filter is shown in the following figure, in which the filter paper is the main component for filtering the oil. It is a special microporous filter paper, which can filter impurities with larger particle size in engine oil, and has less flow resistance to engine oil. In order to ensure the maximum filtering area in the minimum volume and improve the rigidity of the filter element, it is generally folded into a paper sector and a corrugated shape.

It should be noted that the oil filter has a very important index, that is, the dirt tolerance, which refers to the ability of the oil filter to filter and contain impurities in the oil. It is directly related to the amount of filter paper used in oil filter. Generally, the larger the filter, the more filter paper, the greater the dirt capacity and the longer it can be used. Nowadays, in order to save the cost, the general oil filters of automobiles are made smaller, and the number of filter paper inside is limited, which can only ensure that they can filter during the oil replacement cycle. If the oil replacement cycle is exceeded, the oil filter will be blocked. Especially in some areas with harsh environment and heavy dust, there are more impurities in the oil and the oil filter is more likely to be blocked. Therefore, in these areas, the oil filter must be replaced at the same time to shorten the replacement cycle of the oil.

There are also two very important valves on the oil filter-check valve and bypass valve. Check valve is a one-way valve, its function is to prevent the oil from flowing back to the oil pan after the engine is turned off, to ensure that the oil pressure is directly formed after the engine is started, and to minimize the dry friction time of the engine; Bypass valve is a kind of overflow valve, which is used to transport oil in case of filter paper blockage, so as to prevent the engine from losing lubrication. The opening pressure of the bypass valve is precisely calculated, and if it is too large or too small, the lubrication performance of the engine will be deteriorated.

What happens if these two valves fail to function? For example, if the opening pressure of the bypass valve is too low, under the normal oil pressure, the bypass valve will already be opened, so that the oil directly enters the main oil passage without being filtered, which is equivalent to the fact that the oil filter doesn't work; However, if the pressure of the bypass valve is too high, and the filter paper is not opened when it is blocked due to failure, the oil will not enter the main oil passage, and the engine will lose lubrication, which may even lead to cylinder pulling and tile melting. Let's look at the check valve. If there is no check valve in the oil filter, or the check valve is poorly sealed, the oil will flow back to the oil pan after parking, and the oil pressure will build up slowly when the cold car starts, resulting in a long dry friction time of the engine. As we all know, the main engine wear is in the cold start period. If this period is not good, it will undoubtedly aggravate this wear and shorten the life of the engine.

Some fake oil filters, mainly because the functions of these two valves do not meet the requirements, and the opening pressure is too low or too high. In addition, in order to save costs, they use inferior filter paper, which has low filtration accuracy, and the amount of filter paper is also small, and the amount of dirt is insufficient, so the filter paper will soon be blocked. At this time, if the bypass valve is opened, the oil will enter the main oil passage to participate in lubrication without filtration; If it is not turned on, the engine will lose lubrication. Therefore, the consequences of using fake or inferior oil filters are very serious.