[Sketch notes: The relief valve is fitted in the cylinder head. It consists of: (1) a valve body screwed into the cylinder head; (2) a valve disc/poppet which seats on a seat in the body; (3) a spring which holds the disc closed, the spring being compressed by a loading screw/nut in a spring housing; (4) a discharge passage leading from the valve to a safe outlet (often to the exhaust or to atmosphere); (5) a lifting lever/arrangement for manual testing.]

The valve is a spring-loaded safety valve. The valve disc is held on its seat by the spring. The set (lifting) pressure is adjusted by the loading screw which compresses the spring. When the cylinder pressure exceeds the set pressure, the upward force on the disc overcomes the spring force and the valve lifts, allowing the excess gas to escape through the discharge passage, relieving the pressure. When the pressure falls below the reseat pressure, the spring closes the valve. The valve is set to lift at a pressure above the normal maximum combustion pressure (Pmax) so it does not operate in normal running. The discharge is led to a safe location to protect personnel. A lifting lever allows the valve to be tested manually.

1. Water or liquid in the cylinder (hydraulic lock) - e.g. from a leaking injector, water in the fuel, a leaking cylinder head gasket, or water from the cooling system - causing a sharp pressure rise as the piston compresses the incompressible liquid.
2. Over-fueling of the cylinder (faulty injector, stuck fuel rack, or excessive fuel) giving an abnormally high combustion pressure.
3. Faulty fuel injection timing (injection too early) or a dribbling injector causing an abnormal pressure rise.
4. A stuck or leaking exhaust valve causing pressure build-up in the cylinder.
5. A scavenge fire or abnormal combustion raising the pressure.
6. A fault in the compression (e.g. a stuck ring or a foreign object) causing a pressure spike.

In each case the relief valve lifts to protect the engine from over-pressure damage, and the cause must be investigated and rectified.