Causes of Piston Crown Burning and Deterioration

1. Stresses on the Piston
   • Due to Gas Pressure: During firing, the piston crown is subjected to both compressive and tensile stresses. The top portion of the crown experiences compressive stress, whereas the bottom surface of the crown is subjected to tensile forces.
   • Due to Inertia Forces: At Top Dead Centre (TDC), inertia forces tend to cause tensile stresses on the piston surface. At Bottom Dead Centre (BDC), inertia forces combined with the acceleration of the piston subject the surface to compressive stresses.
   • Thermal Stresses: During combustion, the piston crown is suddenly and cyclically exposed to very high temperatures, resulting in severe thermal stresses.
2. Burning and Cracking of Piston Crown
   • Fatigue Failure: Continuous cyclic stresses arising from inertia forces, gas pressure, and thermal stresses eventually lead to fatigue failure, cracking, and burning of the piston crown.
   • Hot Corrosion: Heavy Fuel Oil combustion produces very high temperatures. Hot corrosion occurs on the piston crown surface, particularly aggravated when the Vanadium to Sodium ratio in the fuel is about 3:1. This results in rapid burning and material loss from the crown.
   • Improper Cooling: Inadequate cooling of the piston causes the crown temperature to rise steadily, leading to overheating and cracking.
   • Improper Fuel Injection: Faulty fuel injection such as excessive fuel delivery, defective nozzle operation, or enlarged nozzle holes can cause:
     • Excessive pressures and temperatures.
     • High penetration leading to impingement on the piston crown.
     • Both conditions promote cracking and localised burning.
   • Cooling Arrangements for the Piston Crown: To counter overheating and burning, pistons are provided with positive circulation of coolant:
     • Shaker Method: Lubricating oil (L.O.) enters the piston and is splashed onto the underside of the crown due to inertia when the piston moves down. On the upward stroke, the oil drains out and is replaced by fresh oil.
     • Jet Method: Coolant is pumped under pressure through a concentric pipe and sprayed continuously on the underside of the piston crown. It is then drained through another pipe.
   • Design Consideration
     • Slight Taper in Piston Crown: Due to high temperatures, the crown expands more at the periphery (where temperature is maximum) compared to the centre. To compensate for this uneven expansion, the crown is manufactured with a slight taper.

Possible Reasons for Complete Burn Out of Piston Crown

• Continuous operation with poor cooling.
• Use of fuel containing high vanadium and sodium content.
• Prolonged faulty fuel injection causing impingement.
• Excessive thermal and mechanical stresses not addressed.
• Ignoring early signs of cracking or hot spots on the crown.

Steps to Correct the Situation

• Immediate Action: Stop the engine at the earliest safe opportunity to prevent further damage.
• Inspection: Open up the unit, inspect piston crown, liner, and fuel injection equipment.
• Rectification: Replace the burnt piston crown or complete piston assembly. Ensure proper alignment and cooling system function.
• Preventive Measures:
  • Maintain effective piston cooling arrangements.
  • Use proper grade of fuel and maintain additives to control vanadium/sodium ratio.
  • Overhaul and maintain fuel injectors regularly to avoid faulty spray patterns and penetration.
  • Monitor exhaust temperatures and piston cooling temperatures to detect abnormalities early.