Effect of Low-Lubricity Fuels on Diesel Injection Pumps - Part II:Laborator Evaluation 920824

This paper is the second of two that describe the effects of low-lubricity fuels on diesel injection pump performance. The first paper describes the primary failure mechanisms and wear processes in a number of failed pumps removed from both military and civilian vehicles that had been operated on Jet A-1 and diesel fuels. However, the multitude of unregulated parameters in practical operation renders quantitative comparison between different fuels and pump combinations impractical. This paper describes the degradation in pump performance and the wear processes associated with fuels of varying lubricity in the well-defined environment of a pump test stand. The test methodology concentrates on those areas previously demonstrated to be most susceptible to wear. The results indicate that pump durability is reduced by highly refined low-viscosity fuels, but may be successfully counteracted by either improved metallurgy or lubricity additives. The measured wear rate from the full-scale pump stand tests is compared with results from a lubricity measurement technique commonly used in aviation. Based on this comparison, initial criteria for the minimum fuel lubricity requirements of the injection system are suggested.

MANY FUELS PROVIDE a limited range of contact conditions in which successful lubrication is possible. Fuel systems are designed to reflect these needs; however, seemingly minor changes in fuel composition or equipment design may significantly alter component durability. The Part I Field Performance paper (1)* described post-failure disassembly and examination of failed pumps returned from the field. However, in many instances ultimate failure was promoted by a number of causes, and the effects of fuel lubricity could not be isolated.

The surface protection provided by a fuel is not a unique characteristic, but rather is highly dependent on the test environment and mechanical configuration. The selection of a laboratory test to accurately, yet rapidly, simulate field conditions is necessarily a compromise among competing variables. The fuel pump and injection components perform under a variety of contact geometries, pressures, and velocities to cover lubrication conditions from boundary to fully developed hydrodynamic film. However, preliminary tests and calculations reported in Reference 1 indicated that pump seizure was not primarily due to the decreased viscosity of the aviation fuels. As a result, the test series was designed to highlight the effects of oxidative/corrosive wear.