The working principle of the pressure limiting charging valve is: in most of the time, the pressure limiting valve is closed, only when the pressure of the working medium exceeds a certain value, the pressure limiting valve is opened and a part of the medium from the bypass to reduce the pressure in the system; when the pressure of the system reaches the lower limit, the pressure limiting valve is closed and the pressure of the system rises again. Such repeated adjustment, so that the medium pressure in the system is often at a reasonable level, to ensure that the fuel system, lubrication system, cooling system and hydraulic system work normally.