The oil filter is mainly composed of two parts: a filter element and a housing. There are also auxiliary components such as sealing ring, support spring and bypass valve. The functions of these auxiliary components are not small. For example, the function of the bypass valve is that the oil will flow into the engine through the bypass valve when the filter paper fails due to excessive impurities. Lubricate inside. However, what we see only from the appearance is only the whole of the oil filter, and the filter paper, bypass valve and other parts are invisible. That is to say, in extreme cases, even if the filter is clogged, it is necessary to ensure that the engine has sufficient oil supply to prevent the danger of dry grinding; especially at cold start, this is particularly important. The oil filter fully guarantees the above functions through the bypass valve and the check valve. However, often the inferior oil filter or the bypass valve is normally open (the opening pressure is very small), so that the dirty oil is not filtered to the engine lubrication part under normal conditions, or can not be opened, so that the cold start or oil filter The engine is dry-ground when blocked.