Engine Top End
Cracking into Valve Head Failure
FEDERAL-MOGUL speaks about the various problems associated with valve heads as a result of modern automotive technology.
Today’s downsized, turbocharged engines place enormous stresses on many engine components. One component that can suffer from these stresses more than others is the valve. Some of the most common failures and their causes are listed below.
VALVE HEAD CHORDAL FAILURE
A classic valve head chordal failure is one where the head breaks along the B-X line as indicated in fig. 1. The failure can also cause extensive secondary damage to the piston, cylinder head and even the cylinder bore.
Chordal failure is usually caused by fatigue because of stresses on the valve from gas loading and high valve head temperatures that the valve head material is not strong enough to cope with.
More common in engines under development than in service, valve head chordal failures could be caused by:
· Excessive load on the engine
· Poor finish on the under-head radius
· Damage marks on the under-head radius
VALVE HEAD TULIPING
The head is cupped downwards in a tulip shape, creating a poor seal that leads to power loss. This can be due to conditions in the engine, engine over-speed, the design or materials of the valve head – or any combination of the causes.
Once tuliping starts to occur, the valve clearance would disappear. Valve tuliping would affect the engine’s performance in respect of poor idle, hesitation and possible misfire. If not spotted in time, tuliping can lead to chordal failure.
RADIAL RIM CRACKING
Often known as thermal fatigue, radial rim cracking can occur on inlet or exhaust valve heads. The crack starts in the rim of the valve and travels inwards to the centre. Sometimes the crack turns 90° across the valve head and back to the valve seating face. As a result, a triangular-shaped piece of the valve head will break off as a result. Other times, the crack will split to opposite directions, causing two triangular shaped pieces to break off.
These pieces will typically disappear by the time a mechanic examines the engine causing it to be misdiagnosed as a chordal failure. However, the causes are vastly different.
Typical causes include:
· Extreme temperature fluctuations because of excessive temperature and pressure in the combustion chamber
· Continual engine overloading followed by abrupt unloading
· Poor temperature distribution across the valve and abnormal deflection of the valve head due to excessive combustion pressures
· The rim or peripheral land of the valve head being too thin, with sharp edges – usually due to too frequent reclaiming or regrinding
BACK-OF-FACE BURN THROUGH
Sometimes an exhaust valve will burn a hole through the valve head, at the back of the seating face. The failure would have started with a rim crack, and then escaping combustion gases would have rapidly eroded the valve head material.