Thermal overstress / Mechanical Overstress

Breakage - Thermal Overstress

Failure caused by wide variations of temperature and pressure within the combustion chamber.

Results: Valve head breaks, along a chord of its circles, by the under head radius (the fracture starts with a fissure in the radius).

 

Cause: Very high pressure and temperatures in the combustion chamber. These problems are mainly associated with the exhaust valves.

 

Contributing Factors:

a.)    Engine Overspeed. Valve Float

b.)    Weak valve springs.

c.)     High seating speed.

d.)    Abnormal combustion

e.)    Incorrect Fuel.

 

 

Result: A radial crack inward from the margin. If the fissure advances, the head will break.

 

Cause: Thermal fatigue due to high temperatures and unequal temperatures in different zones of the valve head.

 

Contributing Factors:

a.)    Thermal shock.

b.)    Engine Overload

c.)     Excessive combustion temperature and pressure

d.)    Weak valve springs

e.)    Seating velocity too high

 

 

 


Mechanical Overstress

Failure caused by mechanical origin. This includes wear and breakage, which has nothing to do with the combustion chamber environment.

Result: Valve head to stem breakage.

 

Cause: Repeated stem stress.

 

Contributing Factors:

a.)    Weak springs. Valve Float

b.)    Engine over speed

c.)     Seating velocity too high due to excessive lash

d.)    Incorrect valve train clearance

e.)    Engine over speed, especially on valves with rectangular keeper grooves.

 

 

 

Result: Valve stem breakage in the keeper groove area.

 

Cause: Material fatigue due to mechanical hardening on the surface. It occurs on the contact zone between grooves and keepers.

 

Contributing Factors:

a.)    Defective Keeper assembly

b.)    Keeper groove damages

c.)     Valve float

d.)    Excessive valve train clearance

e.)    Engine over speed, especially on valves with rectangular keeper grooves

 

 

 

 


Breakage

Impact Failure:

 


Characteristic appearance of an impact failure.

 

Note: Breakage by impact is a sudden failure

 

Example: Piston / Valve collision.

 

 

Fatigue Failure

 


Characteristic appearance of a fatigue failure

 

Note: Breakage by material fatigue happens after thousands of cycles, starting in a small defect and advancing until breakage occurs.

 

 

 


Valve Burning

Result: Valve face burnt and torched

Cause: Excessive localized heat in the valve head, distortion and seat leakage (poor seating).

Contributing factors:
a) Lack of stem to guide clearance.
b) Worn valve guide and/or misalignment of valve stem and guide.
c) Pre-ignition (lean air fuel mixture, incorrect fuel).
d) Improper compression ratio.
e) Defective cooling system.
f) Incorrect lash adjustment.
g) Excessive carbon build up on valve.

 

 

 


VALVE face pitting

Result: Pitted valve and valve seal faces.

Cause: Solid particles pressed between valve face and valve seat.

Contributing factors:
a) excessive oil consumption (through piston rings, valve guides and valve stem seals).
b) Abnormal combustion.
c) Long idle periods.
d) Thermostat malfunction (below normal engine temperature).