Changes in fuel compositions and crude sources over the past twenty years have led to fuels with lower lubricating properties. This is compounded by the fact that these critical components are operating at higher pressure. Key moving components such as pumps and injectors rely solely on the fuel for providing lubrication.
There are several common test methods for testing lubricity in distillate fuels; Ball-on-Cylinder Lubricity Evaluator (BOCLE), Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE), and High Frequency Reciprocating Rig (HFRR). Test methods ASTM D6079/D7688 (ISO 12156/IP 450) for ‘Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig’ (HFRR) is most commonly used to evaluate diesel fuels. The test measures the size of a wear scar in millimeters created on a chromium alloy steel ball while under load on a stationary chromium alloy steel disk reciprocating at a very high frequency for 75 minutes. HFRR is designed to evaluate boundary lubrication properties of a fuel, with the smaller the wear scar the better.
Diesel fuel requirements are standardized by ASTM Specification D975 and requires that a fuel would produce a wear scar of no greater than 520 microns. While the common limit in many regions is a maximum scar diameter of 520 mm, the Engine Manufacturers Association had requested a limit of no greater than 460 microns, typical of the pre-ULSD fuels. Most experts agree that a 520 micron standard is adequate. In Europe the EN 590 uses a more stringent wear scar size limit of 460 mm, and the World Wide Fuel Charter even recommends a maximum wear scar of 400 mm.
Removing sulfur from diesel fuel eliminates compounds that provide lubricating properties. This is offset by utilizing lubricity additives to restore fuel lubricity to a suitable level. Lubricity improver additives and the blending of fatty acid methyl ester (FAME) into diesel fuel will improve lubricity. Fuel additives minimize friction and damage to surfaces moving in relative motion under high velocity and pressure. The HFRR test measures the comparative effectiveness of these additives and additive treat rates for a given fuel. Worth noting when using aftermarket fuel additives is that some additives, including cetane improvers, can compete with the lubricity additives, thus reducing lubricity.
ALS Tribology has the capability to test lubricity in diesel fuels by HFRR. This can be helpful when troubleshooting a fuel lubricity performance problem, system component failure, or additives and treat rates effectiveness. Feel free to contact you ALS Tribology representative for further information.
David Doyle, CLS, OMA I, OMA II