Basic Knowledge About Piston Ring

1. What impact does piston ring failure have on diesel engine operation?

A: Excessive piston ring wear leads to a loss of sealing performance, resulting in air leakage during compression. This reduces the amount of air supplied for combustion. Excessive leakage worsens combustion within the cylinder, increasing the average cylinder temperature and thus increasing thermal stress on components in contact with the combustion gases. Exhaust valves are also prone to burning out.

Air leakage during the working stroke reduces diesel engine power and damages the lubricating oil film on the cylinder walls, accelerating wear between the piston rings and cylinder walls. Severe leakage can even cause the scavenging air box to ignite. For cylindrical piston engines, leakage contaminates the crankcase oil, accelerating its deterioration and aging, and may even cause oil mist in the crankcase, potentially leading to a crankcase explosion. Therefore, maintaining piston rings in good technical condition is crucial.

2. What is "break-in"? How are worn piston rings inspected and replaced?

A: Before a diesel engine is put into normal operation, the piston rings and cylinder liners must undergo proper break-in under low load to ensure a proper seal between the piston rings and cylinder wall against the high-temperature, high-pressure combustion gases. This is called "break-in operation."

Before the piston is removed from the cylinder and cleaned, check the piston rings' movement and carbon buildup in the ring grooves, and measure their top and bottom clearances. Then remove the piston rings. After cleaning, measure the ring thickness and compare it to the previous measurement to further determine the degree of circumferential wear and loss of elasticity. Place each ring sequentially in the least worn part of the cylinder or in a spare cylinder liner and check the overlap clearance. If the piston ring overlap clearance has reached the wear limit (especially the top two rings), they should be replaced.

If only the top two rings have reached the wear limit, while the other rings have not, they should not be used in the top two ring grooves for an extended period. This is especially important if the cylinder liner is already worn. This not only weakens the ring's elasticity but also creates a larger leakage channel at the joint, disrupting the oil film's travel and accelerating wear. Furthermore, although the worn ring surface is polished, a hardened layer remains, leading to poor fit between the piston ring and cylinder liner after repositioning and hindering proper break-in. Poor break-in can cause cylinder scoring. For low-boost diesel engines, in the absence of new rings, the top two rings can be replaced with lower piston rings with suitable joint clearance, but the included angles at the top and bottom ends of the rings must be rounded before installation.

Experience shows that new piston rings break in faster with the cylinder liner surface than piston rings that have already been broken in on other cylinders. Therefore, replacing the top two piston rings with brand new ones is reasonable.