Detecting the onset of localized scuffing in a pin-on-twin fuel-lubricated test for heavy-duty diesel fuel injectors

Scuffing, usually considered to result from lubrication failure, severely limits the life of heavy-duty diesel fuel injectors. A new method is introduced to detect the onset of localized scuffing and monitor scuffing propagation in a fuel-lubricated 'pin-on-twin' reciprocating tribosystem. Tests were conducted on annealed and hardened AISI 52100 steel lubricated by on-highway #2 diesel fuel and ultra-low-sulphur Jet A aviation fuel. Unlike most current reciprocating tests that use changes in the nominal (averaged) friction to detect scuffing, this study analyses the detailed friction traces of individual strokes. The correlation between the change in the friction traces and the onset and progression of scuffing has been validated by examining the surface morphology of wear scars. This method was then applied to study the scuffing characteristics under various test conditions. Generally, scuffing initiated earlier and propagated more rapidly the higher the load, the lower the reciprocating frequency, and the poorer the lubricity of the fuel. The stroke ends, where direction reversal occurred, were especially vulnerable to scuffing due to their low sliding velocity.