What are most common wear faults of 2-stroke engines exhaust valves?
Exhaust valve components wear observed in different forms :
- Adhesion and abrasive wear could be found in stem sealing area
- Deposits and dent marks
The deposits formed during combustion from the fuel and LO contaminants (sulphur, vanadium, sodium).
- Low-temperature corrosion
Surfaces exposed to the temperature below the dew point of sulfurous acid suffer from condensed acid. Acid origins from sulfur content of fuel and water mist content of fuel and air. Most frequent targets of low temperature corrosion are: valve housing, bottom piece, valve guide and valve spindle stem.
- High-temperature corrosion
High-temperature corrosion caused by the presence of Sulfur, Vanadium and Sodium in the fuel. Hard salt deposits produced during the combustion process accumulated on the seat surfaces of the valve spindle. It leads to dent marks caused by each closing stroke of the valve spindle. This can lead to crucial micro gas channels growing over the time. Because of the high temperature, the salt deposits reach their melting point, then the “cobble stone corrosion” starts at the seat and finally leads to blow-byes.
- Low cycle fatigue
The valve spindle gets the maximum thermal stress especially during the warm-up period. Most of fatigue problems are at the valve spindles. During warm-up the temperature at the center of the valve bottom and the seat reaches the maximum difference. Under regular operating conditions the temperature distribution is much lower. The mechanical load of a valve spindle is nearly constant between warmup and regular operation.
There are two typical failure reasons. One is a crack that starts in the under head radius. After the micro cracks get to a critical length, the speed of the crack growth increases. A catastrophic failure follows, with parts of the spindle dropping down into the combustion chamber
