The running-in process is a key stage for piston rings and cylinder walls to form a stable fit. It directly affects sealing performance, service life, wear characteristics, and the rate of carbon deposit formation.
1. Influence on Sealing Performance
The core function of piston rings is to seal the combustion chamber and control oil movement, and the running-in process lays the foundation for an effective seal:
Good running-in:Through gradual friction, the outer surface of the piston ring conforms to the curvature of the cylinder wall, and the clearance reaches the design standard. This effectively controls gas leakage, reduces oil consumption, and avoids issues such as “oil burning” and “power loss.”
Poor running-in: If the load is too high or lubrication is insufficient during running-in, the piston ring surface may develop “unflattened local high spots” or “steps from excessive wear,” resulting in a clearance out of spec. Combustion gases can leak into the crankcase, and oil can be carried into the combustion chamber. In severe cases, cylinder pressure drops and engine power decreases noticeably.
2. Influence on Wear and Service Life
The service life of piston rings largely depends on wear during the running-in stage:
Good running-in: Under low load and low speed, friction between the piston ring and cylinder wall is mainly “abrasive wear,” gradually reducing surface roughness and forming an even “run-in surface.” Wear becomes stable, leading into the “steady wear stage” with a lower wear rate, extending piston ring life.
Poor running-in: If rapid acceleration or overload occurs too early, “dry” or “boundary” friction may develop between the piston ring and cylinder wall, causing scratches, scoring or even cylinder scuffing. This reduces ring tension, shortens service life, and requires premature replacement.
3. Influence on Oil-Film Stability
The oil film between piston ring and cylinder wall is key to reducing friction and protecting surfaces, and the running-in process directly affects its stability:
Good running-in:The piston ring surface develops “oil-retention grooves,” which store and evenly distribute lubricating oil. Oil-film retention improves, and even at high speeds the film is less likely to break, preventing dry friction and overheating.
Poor running-in: If speed fluctuates widely during running-in, relative motion becomes unstable and the oil film can locally rupture, increasing boundary lubrication. Frictional heat may cause “tempering softening” of the ring, loss of tension, or even ring sticking.
4. Influence on Subsequent Carbon Deposits
Good running-in:Well-sealed piston rings effectively scrape excess oil off the cylinder wall, reducing the amount entering the combustion chamber and slowing carbon buildup. Less blow-by also means slower oil oxidation in the crankcase and less sludge clogging the ring grooves, preventing sticking.
Poor running-in: Large sealing gaps lead to “oil upflow” and “gas downflow,” accelerating carbon accumulation in the combustion chamber and in the ring grooves, which further worsens sealing and creates a vicious cycle of “carbon buildup → poor sealing → more carbon.” This can ultimately cause cylinder scuffing or even catastrophic engine failure.
