Guarnitauto

Gaskets: failure analysis

DETONATION

Definition

Detonation is a combustion which propagetes through fuel/air mixture with high speed. This causes an instantaneus combustion with generation of high temperature and pressure therefore the kuock. Detonation is important particulary in engines running on petrol (Eight cicle) more than burning oil (Diesel cicle).

Detonation causes low engine performance and high temperature in combustion chamber therefore damaging exposed surfaces (Piston head perforation, exhaust valve burning, grommet fusion, etc.).

Causes on eight cycle engine

• Excessive compression ratio
• Over advanced timing
• Presence of overhead surface in combustion chamber (Hot Spots)
• Use of low octane fuel
• Incorrect air/fuel ratio
• Incorrect spark plugs
• Engine overheating
• Low speed
• High load

Recommendation

• Use only the correct spark plugs for the particular engine
• Use the correct fuel for the particular engine
• Insure the cooling system is efficient
• Adjust the timing to that declared by manufactures
• Chemically remove combustion and oil residue from combustion chambreand piston
• Insure the correct air/fuel ratio

Effects

Detonation demages head cylinder gasket like overheading. Gasket result burned where detonation has occured and beading rings became inflamed surface on.

INCORRECT TORQUE

Definition

Incorrect torque of cylinder head causes low seal of cylinder head gasket, water and oil lose and efficiency's engine decrease.

Causes

• Reuse of used cylinder head bolts
• Cylinder head bolts low torque
• Cylinder head bolts and/or hole dirty sufficiently lubricate
• Damaged thread
• Incorrect calibration of torque wrench
• Incorrect use of torque wrench

Recommendation

• Used cylinder head bolts must be replaced
• Before assembling clean and lighly lubricate threads and surfaces
• Don't lubricate holes and threads too much
• Follow manufacturer's instructions about assembly (torque value, tightening angle, tightening cylinder head bolts order)
• Always use a torque wrench by angle grip, never use extensions and, if necessary, always use the shortest socket possible

Effects

Gasket surface is without compression traces and the tickness is about that one of new gasket. Motion of gasket caused by low tightening can break beading rings.The low seal cause leakage of combustion gas therefore surface gasket burning.Operational defects arise during the first hours of operating.

OVERHEATING

Definition

Overheating produce hadening of cylinder head gasket material and an irreversible lose of their elasticity. Therefore the seal decrease because the gasket doesn't shape to the relevant movement between head and cylinder block.

Causes

• Low coolant level
• Excessive corrusion or incrostation in engine or radiator
• Cooling system component failure
• Incorrect antifreeze/water mix

Recommendation

• Ensure coolant level is correct and check for special level coolant refill procedures.
• Verify all cooling system component are working properly. After verification start the engine and insure that all cooling system are working properly at operating temperature with regards to coolant fan.
• Verify tha lack of excessive incrustation or corrosion in the cooling system because this reduces cooling efficiency for 2 causes:
  • Reduction of coolant system cross-section and consequent coolant fluid flow
  • reduction of head exchange coefficent because 3 mm of incrustation will reduces it about 35%
• Employ only the correct coolant mix as specified by manufactures

Effects

Cylinder head gaskets become hard, inflessible, carbonised and with surface cracked. Beding rings become inflamed on surface.