[Sketch notes: a spring-loaded relief valve fitted in the cylinder head. It consists of a valve body, a valve disc/poppet seating on a seat, a spring holding the disc closed, a spring housing/loading screw, and a discharge passage. The valve is set to lift at a pressure above the normal maximum combustion pressure.]

1. Spring-loaded: the valve is held closed by a spring whose compression (set by a loading screw) determines the lifting pressure.
2. Set to lift at a pressure above the normal Pmax (e.g. about 10-15% above the maximum combustion pressure) so it does not lift in normal operation.
3. Large discharge area/passage so that when it lifts it can relieve the pressure quickly.
4. The valve disc and seat are of a material resistant to the hot combustion gases and to corrosion; the valve is designed to reseat cleanly.
5. A lifting lever/arrangement to test the valve manually.
6. The discharge is led to a safe place (often to the exhaust or to atmosphere via a pipe) so that the released gas does not endanger personnel.
7. It is fitted in the cylinder head (or on the cylinder cover) of each cylinder.

The cylinder relief valve protects the engine from damage due to excessive pressure in the cylinder. If the pressure rises above the set value (e.g. due to water in the cylinder causing hydraulic lock, over-fueling, a stuck injector, or a fault causing abnormal combustion), the relief valve lifts and releases the excess pressure, preventing the cylinder head, liner, piston, connecting rod and crankshaft from being over-stressed or damaged. It also gives a warning (by the noise of the gas escaping) that a fault exists.

Circumstances:

1. Water or liquid (e.g. from a leaking injector, water in the fuel, or a leaking cylinder head gasket) entering the cylinder causing a hydraulic lock - the pressure rises sharply as the piston compresses the incompressible liquid.
2. Over-fueling of the cylinder (faulty injector, stuck rack) giving excessive combustion pressure.
3. A fault in the fuel injection (e.g. injection too early, or a dribbling injector) causing abnormal pressure rise.
4. A stuck or leaking exhaust valve causing pressure build-up.
5. A scavenge fire or abnormal combustion.

Action to prevent damage:

• Immediately reduce the engine load/speed and investigate the cause; take the affected cylinder out of service if necessary (reduce its fuel feed).
• Check the fuel injection system (injector, pump, timing), the cylinder for water/liquid ingress, the exhaust valve, and the compression.
• If water is suspected, check the cooling water system and the fuel for water; drain and rectify.
• Do not continue running at high load until the cause is found and corrected; the relief valve lifting is a warning that must be acted upon to prevent serious engine damage.