Basic Knowledge About Piston

1. Why should the hardness of piston rings be slightly higher than that of the cylinder wall surface?

A: During the reciprocating motion of the piston, the piston rings are constantly subjected to friction, while the cylinder wall surface intermittently rubs against the piston rings. Therefore, the wear per unit area of the piston rings is much higher than that of the cylinder wall. Furthermore, when the piston rings are near top dead center, they are frequently in contact with high-temperature combustion gases, operating under much harsher conditions than the cylinder liner. To extend their lifespan while minimizing cylinder liner wear, the piston ring hardness should be slightly higher than that of the cylinder wall.

However, piston rings that are too hard are also undesirable. Excessive hardness will accelerate cylinder liner wear, and the metal shavings from the cylinder liner mixing with the cylinder lubricating oil will further accelerate the wear between the piston rings and the cylinder liner.

2. What inspections should be performed when replacing piston rings?

A: Visual inspection of piston rings: Observe the outer surface of the rings for cracks, pinholes, looseness, etc. If any of these defects are present, the rings should not be used. If the surface has rust or scratches, it must be carefully removed with a fitter's tools before use.

Ring and Ring Groove Inspection: Use a feeler gauge to measure the clearance between the piston ring and the ring groove around the circumference. Move the piston ring and measure at multiple points for accuracy. If the clearance is too small, the ring size is incorrect, and the ring cannot be used.

Check Ring Groove Depth and Radial Width: Generally, the piston ring groove depth should be 0.5–1.0 mm greater than the piston ring radial width. Under no circumstances should it protrude from the groove.

Check Ring Groove Shape: After cleaning the ring groove, check the upper and lower surfaces for scratches and steps. If any are found, carefully repair them.

Piston Ring Thickness Measurement: For the old ring, measure the thickness at three equally spaced points on the circumference, starting from the opposite side of the piston ring joint.

Piston Ring Break-in: After replacing the piston rings, a break-in period should be ensured at low speed to prevent cylinder scoring. Follow the break-in procedure specified in the instruction manual. At the beginning of the break-in period, increase the cylinder oil level and then gradually decrease it to normal.

3. What are the causes of piston ring breakage?

A: The causes of piston ring breakage are listed below:

* **Insufficient interlocking clearance:** During operation, a smaller interlocking clearance is preferable for airtightness. However, due to the different temperatures of each ring, the interlocking clearance of the first and second rings should be slightly larger. If the clearance is too small, the piston ring will expand due to heat, and the interlocking clearance will become jammed, causing it to break on the opposite side of the interlock. This can even lead to cylinder scoring or seizure.

* **Breakage due to carbon buildup bending:** When the carbon buildup on the lower surface of the ring groove is still soft, the piston ring can still move and maintain airtightness. When the ring movement becomes sluggish, the lubricating oil mixes with metal powder and, under the influence of leaking combustion gases, forms hard carbon deposits. At this time, the piston ring will be subjected to alternating bending forces as shown in Figure 37, leading to breakage. Each time the piston ring moves, it breaks segment by segment due to bending, eventually breaking into many small segments. In this case, the ring groove often wears into a wavy shape.

Impact fracture: Wear forms a step on the top of the cylinder liner. When the piston reaches top dead center, the first ring often contacts this step and breaks due to impact. Therefore, this step must be removed during cylinder reassembly.

Ring groove wear causing ring torsion and bending, leading to fracture: During reciprocating motion, the piston ring continuously impacts the groove surface, wearing it into a trumpet shape. Due to poor ring surface support, the piston ring is subjected to additional torsional and bending forces, resulting in fracture. To prevent ring groove wear, flame hardening, chrome plating, and hard alloy welding are sometimes used. Anti-wear rings are also sometimes installed; these are very thin and must be prevented from breaking within the groove.

Ring breakage due to the inlet/outlet port: The elasticity of the piston ring is generally strongest at the joint. Because of the gas flow, the joint is hotter than other parts. Under these conditions, the ring opening can break easily if it slightly catches the intake/exhaust port. Furthermore, the ribs between the intake and exhaust ports are the weakest in strength and prone to thermal deformation. Besides the air inlet needing to be well-rounded, the joint of the ring on older models also needs to be rounded.

6. The ring material does not meet requirements.

7. The ring's machining process is poor.