Pushing the Boundaries of the HFRR: Impact of Increased Test Severity on Wear
The continued drive for efficiency led to many changes in FIE technologies, materials and pressures. Modern high pressure common rail pumps reach significantly higher pressures, with 2200 bar available today and pressures up to 3000 bar discussed in the industry. Alongside these hardware changes there have been significant changes in the diesel fuel, with a continued move towards more highly refined fuels, and the introduction of highly paraffinic sources from Fischer-Tropsch processes and hydrogenated vegetable oils.
Despite these changes there have been no widespread field issues since the introduction of HFRR specifications. The objective of this work was to understand the flexibility of the HFRR test by investigating the impact of increased test severity on wear. The main variable investigated was the effect of load, which was varied from 2-10N. Other variables investigated were frequency, test duration and stroke length. Additionally, preliminary studies were conducted using phosphate coated specimens more in line with those expected in current and future FIE systems.
HFRR tests utilising a more highly loaded contact exhibit an increased mean wear scar. Trends observed are similar to those under standard conditions (2N), with differentiation between untreated fuels with poor lubricity and those that are additised. Certain fuels exhibit unusual friction coefficient behaviour, with temporary periods of significantly elevated friction. This is attributed to micro-seizure; however under additised conditions it was not observed. One interesting observation was a correlation between wear scar pattern and additive technology. Ester based lubricity additives give a striated wear scar, whereas acid technologies produce a stippled pattern, likely due to low level corrosion, a known possibility with this type of chemistry. The relatively soft nature of the phosphate coatings used meant that under the highly loaded contact conditions of the HFRR they quickly wore through; however the presence of additives minimised this wear and prolonged coating lifetimes. Further work indicated that other fuel additives such as cetane improver can impact lubricity performance, and may require different lubricity improver treat rates in order to meet specifications.
In summary, increasing contact severity within the HFRR increases the level of wear in the system, with some evidence of micro-seizure. However, the use of additives under these conditions can reduce the wear to acceptable levels and prevents micro-seizure events. The trends and discrimination between conventional and higher load conditions remains similar. This indicates that modifications to the HFRR test method are unlikely to provide a means of defining how to achieve additional protection for future more severe conditions and the current test method remains suitable.