Q6 (16 Marks) Engine Operation & Maintenance
MEKM • Written Exam

With reference to piston rings:

(a) State reasons for breakage.

(b) How maintenance and engine operation could minimize breakage?

(c) Explain the recent developments in piston rings to minimize breakage.

Appeared In: Jan 2020

Verified Model Answer (Text Solution)

Structured for DG Shipping MEO Class II examination scoring criteria.

Exam Ready
Part (a)

Reason for piston ring breakage:

  • Excessive wear in the cylinder liner leads to increased piston ring movement, both radially and axially. This fluctuating motion can cause tilting and eventual breakage of the rings.
  • If the piston ring does not exert sufficient pressure on the liner, gas pressure can penetrate between the ring and liner, collapsing the ring into the groove and causing breakage.
  • Ridge formation near scavenge pockets can create stress concentrations at the piston ring's radial edge, promoting fracture.
  • Jamming or sticking of rings caused by excessive carbon deposits, often due to improper combustion or inadequate cleaning during maintenance.
  • Excessive wear in the piston ring grooves causes the rings to impact the groove walls during operation, leading to hammering and eventual breakage.
  • Inadequate cylinder lubrication results in overheating and increased friction, weakening the rings and causing breakage.
  • Acidic corrosion and high-temperature corrosion weaken the ring material, predisposing them to fracture.
  • Excessive engine loading can cause the rings to deform beyond their elastic limit, leading to collapse and breakage.
  • Using low-quality or non-manufacturer-specified rings compromises material strength and durability, increasing the risk of breakage.
  • Improper installation during ring renewal can lead to misalignment, increased stress, and premature failure.
Part (b)

Minimizing Breakage through Maintenance and Engine Operation:

Maintenance practice:

  • Perform routine inspections and overhauls of pistons, piston rings, and cylinder liners as per the PMS schedule.
  • During overhauls, ensure piston rings and grooves are thoroughly cleaned and all necessary clearances are measured to verify proper fit.
  • Reuse piston rings only if measurements indicate they are within the safe operational limits until the next overhaul.
  • Regularly maintain the fuel injection systems to prevent improper combustion and minimize stress on piston rings.
  • Ensure that piston rings and liners are free of marks, scratches, or other signs of wear during scavenge inspections.
  • During overhaul, install piston rings with proper tools and techniques, ensuring free movement of rings in their grooves.
  • A proper running-in procedure after installing new pistons and rings helps to ensure correct seating and minimizes initial wear.

Engine Operation:

  • Maintaining adequate cylinder oil lubrication minimizes friction and heat generation.
  • Maintain appropriate cooling of the cylinder liner and piston to avoid thermal stresses.
  • Use properly treated fuel oil and ensure correct operation of fuel pumps, injectors, and Variable Injection Timing (VIT) systems.
  • Maintaining correct combustion parameters minimizes improper combustion and reduces carbon deposits.
  • Keep air filters clean to avoid the ingress of dust and abrasive particles into the engine.
  • Avoid overloading the engine, which can stress the piston rings and cause failure.
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