Scuffing transition diagrams for heavy duty diesel fuel injector materials in ultra low-sulfur fuel-lubricated environment

In order to meet stricter emissions requirements, advanced heavy duty diesel fuel injection systems will be required to resist scuffing at higher pressures, temperatures and in ultra low-sulfur fuels. Using a 3D friction-scuffing mapping technique, scuffing transition diagrams were generated for diesel fuel injector materials to exemplify the effects of surface finish and sliding velocity on scuffing characteristics in a fuel-lubricated environment. On-highway #2 diesel fuel and Jet A aviation fuel were selected as a baseline and a surrogate of ultra low-sulfur fuel, respectively. Two material combinations, self-mated AISI 52100 steel and MgO partially stabilized zirconia (TTZ) against 52100 steel, were tested at three composite roughness levels. Scuffing generally initiated at the low-sliding-velocity stroke ends, which had the worst lubrication condition. Materials survived longer in the diesel fuel than in the aviation fuel. TTZ/steel showed higher scuffing resistance than steel/steel. Among the three roughness levels, self-mated steel exhibited best scuffing resistance at the intermediate level, while TTZ against steel seemed to favor the roughest level. Surface morphology examination (optical and scanning electron microscopy) and chemical analysis (energy dispersive spectroscopy and scanning auger microprobe) were conducted to investigate the wear modes and tribochemical film compositions. A mixture of abrasive wear, adhesive wear, and plastic deformation has been observed. The worn surfaces in #2 diesel fuel presented higher non-carbide carbon concentration and a thinner iron oxide-rich layer than those in Jet A fuel.