Determining the Sensitivity of Fuel Lubricity Additive Concentration on Test Parameters and Contact Geometry, 08-R8513
Inclusive Dates: 01/05/15 – 05/05/15
Background — In diesel engines, the fuel pumps and fuel injectors are subjected to tremendous pressures, upward of 30,000 psi in modern systems. To generate and maintain this level of pressure, the internal pump and injector components are made to an exacting standard. The drawback is that the moving components are largely lubricated with only the diesel fuel. If the diesel fuel has insufficient lubricating quality, catastrophic wear and subsequent failures can occur. This work was undertaken in an attempt to improve the correlation of the high frequency reciprocating rig (HFRR) wear scar results with pump test results for a series of fuels with varying levels of lubricity additive.
Approach — It was known from the beginning of this work that the point contact generated by the ball-on-disk contact was likely the largest contributor to the insensitivity of the HFRR test. The first, and ultimately only, change to the standard test conditions was to replace the ball test specimen with a cylinder (pin) to generate a line of contact. The increase in surface area would allow the surface active molecules in the lubricity improver a chance to work from the very first stroke, and would give lower contact stress, pushing the lubricating regime towards the hydrodynamic and away from the adhesive wear conditions typically seen in poor HFRR results.
Accomplishments — A series of standard tests were done on the HFRR using fuel blended with a commercially available lubricity improver. Seven blends were made ranging from 0 to 400 parts per million (ppm) treat rate. The fuel was tested with no additive, and the wear scar measured just over 900 microns in mean diameter. At the 12.5ppm treat rate, a 1.2 percent decrease in wear scar was observed. At the 25ppm treat rate, a 7.1 percent decrease in wear scar was observed. When the standard ball (point contact) was changed to a pin (line contact), a dramatic change in additive response was observed. The fuel with no additive measured about 450 microns in mean width. At the 12.5ppm treat rate, a 20 percent decrease in wear scar was observed. At the 25ppm treat rate, a 48 percent decrease in wear scar was observed.