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Structural features: Design of the pressure relief ring
Position: The cylindrical surface above the sealing cone of the oil outlet valve (as shown in the figure), whose outer diameter is precisely matched with the inner diameter of the valve seat hole (the gap is only a few microns), forming a sliding seal pair.
Functional design:
The length and diameter of the pressure relief ring determine the pressure relief volume (the volume of fuel released when the ring cuts into the valve seat).
The surface of the ring is precisely ground to ensure the sealing and smooth sliding with the valve seat hole.

Working principle: Control the fuel passage and pressure in stages
1. At the beginning of fuel injection: the pressure relief ring makes way for the passage
Scenario: The injection pump plunger compresses the fuel upward, and when the oil pressure rises to overcome the spring force of the oil outlet valve, the oil outlet valve is lifted.
Process:
In the early stage of the oil outlet valve leaving the valve seat, the pressure relief ring has not completely separated from the valve seat hole. At this time, the fuel can only flow into the high-pressure oil pipe through the small gap between the ring and the valve seat hole, and the flow rate is limited.
As the outlet valve continues to move upward, the pressure relief ring belt completely detaches from the valve seat hole, the fuel passage is fully opened, the high-pressure fuel quickly enters the oil pipe, and the injector starts to spray.
Function:
The gradual opening of the ring belt avoids pressure fluctuations caused by instantaneous fuel impact, making the injection process smoother.
2. End of injection stage: the pressure relief ring belt cuts off the passage and relieves pressure
Scenario: The plunger of the injection pump reaches the end of the fuel supply, the oil pressure in the plunger chamber drops, and the outlet valve falls back under the action of the spring force.
Process:
Stage 1: Cutting off the passage
When the outlet valve falls back, the pressure relief ring belt first cuts into the valve seat hole. At this time, the ring belt and the valve seat hole are completely fitted, blocking the connection between the high-pressure oil pipe and the oil pump, and the fuel stops flowing into the oil pipe.
Stage 2: Instantaneous pressure relief
The outlet valve continues to fall (the sealing cone surface has not yet fitted). During the process of the pressure relief ring belt cutting into the valve seat hole, the ring belt occupies part of the space in the valve seat hole, resulting in an instantaneous increase in the volume in the high-pressure oil pipe (the increased volume is the pressure relief volume).
According to Boyle's law (ideal gas state equation), the increase in volume causes the oil pressure in the oil pipe to drop sharply (usually by 10-20MPa).
The oil pressure quickly drops below the injector opening pressure, ensuring that the injector needle valve is immediately closed and the injection is completely stopped.
The third stage: the sealing cone is seated
After the pressure relief ring belt completes the pressure relief, the oil outlet valve continues to fall, and the sealing cone surface fits the valve seat to form a complete seal to prevent the fuel from flowing back into the oil pump.
3. Key parameters: the influence of the pressure relief volume
Pressure relief volume = pressure relief ring belt cross-sectional area × ring belt length.
Too large volume: excessive pressure relief, the residual pressure in the oil pipe is too low, and the pressure is slowly built up during the next injection, which may cause delayed injection and poor atomization.
Too small volume: insufficient pressure relief, the pressure in the oil pipe is not completely reduced, which may cause secondary injection (after the injection is completed, the injector sprays again due to pressure fluctuations) or dripping.

Core function: avoid abnormal injection and ensure clean fuel cut-off
Prevent secondary injection
If there is no pressure relief ring, the pressure in the oil pipe may fluctuate due to the slow pressure relief of the plunger chamber at the end of injection, causing the injector to open again at the non-injection time (secondary injection), resulting in poor fuel atomization, incomplete combustion, and worsened emissions.
Eliminate dripping
After the pressure relief ring is relieved, the pressure in the oil pipe quickly drops below the injector closing pressure, ensuring that the injector needle valve quickly seats, avoiding continuous dripping of fuel after oil is stopped, polluting the combustion chamber and causing carbon deposition.
Stabilize residual pressure
Through the precise design of the pressure relief ring, the residual pressure in the oil pipe is controlled within a reasonable range (such as 10-20MPa), creating conditions for quickly establishing high pressure during the next injection, and ensuring the accuracy of injection timing and oil quantity.

Typical faults: the influence of pressure relief ring wear
Wear causes: impurities in the fuel (such as metal particles, colloid) have long-term erosion of the ring surface, or surface scratches and depressions caused by fuel corrosion.
Fault manifestations:
The decompression volume becomes larger, the pressure is excessively relieved, and the residual pressure is too low → injection delay, starting difficulty, and unstable idling speed.
The sealing performance is reduced, which may cause fuel reflux → large fluctuations in the oil pipe pressure, dripping or secondary injection, black smoke from the engine, and power reduction.
Maintenance points: Regularly check the oil outlet valve parts (after cleaning with diesel or special cleaning agent, check the wear marks of the ring belt by light transmission method), replace the original parts when necessary, and avoid using inferior fuel.