PRESSURE RELIEF VALVE
The pressure relief valve shown in the figure. controls the oil pressure at the pre-set value demanded by the system. The valve is normally integral with the pump assembly and protects the system from excessive pressure. It is usually a spring-loaded plate-type valve, and can on some engines provide adjustment of pressure setting.
It is more usual to find a pressure relief valve that varies the pressure with engine speed or breather pressure. These valves are usually adjustable but usually only effect the max speed oil pressure see Figure.
This type of valve uses the oil system breather pressure and an adjustable spring to balance the oil pressure in the main oil feed line to the engine bearings.
Consider Fig With the engine running, the breather pressure plus the spring push the sliding valve to the left and restrict the pump spill back to return. This is balanced by the pressure from the main feed line trying to move the slide valve to the right. Should the pressure in the main feed line fall, the breather pressure and spring will move the slide valve further to the left and restrict the oil spill still further. This will allow more oil to flow to the system, and the oil pressure in the main feed line will increase. The slide valve will then move to the right, and the oil spill to the return will be controlled by the main feed line pressure balancing the spring and breather pressure.