The Lubricity Problem in Low-Sulfur Fuels
Historically, sulfur compounds naturally present in diesel provided incidental lubrication to fuel pump and injector components operating in boundary contact. Deep hydrodesulfurization (HDS) to produce Ultra-Low Sulfur Diesel (ULSD, ≤10 ppm S) and Low Sulfur Marine Gas Oil removes these polar compounds, along with aromatic and nitrogen compounds that also contributed lubricity, leaving a fuel significantly more damaging to fuel injection equipment.
High-pressure common rail diesel injection systems (2,000+ bar) and aviation fuel system components rely on the fuel itself to lubricate closely-fitted metal surfaces operating in boundary contact. Insufficient lubricity causes premature wear of fuel pump plungers, barrel-and-plunger assemblies, and injector needle/seat surfaces, leading to leakage, poor injection quality, reduced power, and component failure.
Key test: The High-Frequency Reciprocating Rig (HFRR) test (EN ISO 12156-1) measures wear scar diameter (WSD) of a steel ball sliding against a steel disk in the fuel under controlled conditions. EN 590 limits HFRR WSD to ≤ 460 μm, a limit many ULSD base stocks fail without lubricity additive treatment.
How Lubricity Improvers Work
Lubricity improvers are polar molecules, typically fatty acid derivatives, acid esters, or amide-based chemistries often derived from vegetable or animal oils, with a strong affinity for metal surfaces. The polar head group adsorbs onto the metal surface, with the hydrocarbon tail oriented toward the fuel phase. This forms a thin molecular film providing boundary lubrication between sliding metal surfaces, reducing friction and wear even under very thin film conditions. Activity is highly fuel-specific: HFRR performance in one base stock does not reliably predict performance in another.
Specification Requirements by Fuel Type
| Fuel | Lubricity Standard | Typical Treat Rate |
|---|---|---|
| ULSD / Low-S Diesel | EN 590: ≤ 460 μm HFRR WSD at 60°C | 50–300 mg/kg; lower treat rates with high-efficacy fatty acid-based products |
| Marine Gas Oil (MGO) | ISO 8217:2017: ≤ 520 μm HFRR | 100–400 mg/kg depending on base fuel lubricity |
| Aviation Turbine Fuel | BOCLE / SLBOCLE test per DEF STAN 91-091 / ASTM D1655; DCI-6A provides both lubricity and corrosion protection | Per specification limits; aviation products must appear on approved product lists |
| Biodiesel Blends | FAME has high inherent lubricity; lubricity additive may not be needed in B5+ blends | Evaluate base ULSD lubricity before FAME addition; dose as required to meet specification |