If the engine is the heart of a car, then the filter is the lungs of the car. As one of the three filters (oil filter, fuel filter, and air filter), oil filter is of great significance for achieving engine performance and durability functions.

The significance of the filter bypass valve
The indicators for evaluating the performance of oil filters are not only the filtering effect of the filter, but also the pressure loss of the filter on the system. A bypass valve must be installed on the full flow engine oil filter, while diesel filters do not need to be installed. The performance of the bypass valve has a significant impact on the filtering effect of the oil filter and the pressure loss of the system.

If the structural design and manufacturing of the filter bypass valve are unreasonable, the specific manifestations are:
1. The opening pressure of the bypass valve is too low, and the sealing of the bypass valve is not good, resulting in poor filtering effect of the filter element and even failure of the filter element;
2. The size of the filter bypass valve port is too small and the designed opening pressure is too high, resulting in excessive system pressure loss and ultimately affecting engine performance. The fundamental reason for this problem is that the filter bypass valve has not been properly designed and calculated.

The working principle of the filter bypass valve
The oil filter bypass valve is normally closed. When the pressure drop of the filter element is too high, sufficient oil (even if this part of the oil is unfiltered) is supplied to the various friction pairs of the engine through transportation or splashing to provide lubrication and cooling, and to carry away heat and iron debris impurities. When the engine starts in a cold state (with high oil viscosity) or when the filter element is blocked and the flow rate is difficult to meet the engine requirements, the filter bypass valve opens to meet the working needs.
The bypass valve is connected in parallel with the filter in the lubrication system. There are two forms of arrangement: one is inside the filter, bonded with the filter element to form a closed surface, and the valve body is spot welded on this surface. When the pressure difference between the inside and outside of the valve core exceeds the design pressure, it opens; Another type is on the assembly, located on an interface that is connected to the oil inlet, and the oil discharge port is connected to the oil outlet port.

Opening pressure of the filter bypass valve
The opening pressure of the bypass valve should be determined according to needs or the requirements of the supporting factory, and should meet the filter life pressure requirements.
The opening pressure of the bypass valve should not be too small. If the opening pressure is too low, unfiltered oil entering the main oil passage will increase, and impurities are highly likely to accelerate the wear of moving parts as the oil enters the friction pair.
The original resistance of the filter is generally between a few KPa to 25KPa, some of which are relaxed to 30kPa, while the filter life pressure drop is 70KPa. When the pressure drop is lower than the life pressure drop, the bypass valve should generally not be opened. Unless the viscosity of the cold start motor is too high, temporary opening is allowed. So the bypass valve is designed to open, and the pressure drop should not be lower than 70kPa during the filter life.
  
But the opening pressure of the bypass valve should not be too high.
1. Opening the pressure drop is based on not damaging the filter element. The strength of the filter element is determined by the material of the filter paper, folding structure, etc. It can generally withstand a pressure drop of no more than 300KPa~400KPa. If the pressure exceeds, the filter element will be compressed or even blocked.
2. The opening pressure drop is too high, and the main oil supply pressure is too low. When the cold start or filter element is blocked by impurities, if the opening pressure is too high, the oil cannot be delivered to the moving parts of the diesel engine in a timely manner, and the main oil duct pressure drops. When the oil supply pressure in the oil duct remains too low, serious accidents such as burning of bearings and seizing of the crankshaft may occur. In order to ensure the normal operation of the lubrication system, the resistance should not exceed about 200KPa when passing through the rated flow rate.