Fuel Property Testing: Lubricity

Hannu Jääskeläinen

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Abstract: The lubricity of diesel fuel can be measured in vehicle tests, pump rig tests, or bench tests. The most important bench tests are the High Frequency Reciprocating Rig (HFRR) and the Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE). Since the various tests are based on different types of wear mechanisms, the correlation between methods is not always satisfactory.

Overview

The term lubricity is often defined as the ability of a lubricant—in this case diesel fuel—to minimize friction between and damage to surfaces in relative motion under load. Generally the tests used to evaluate diesel fuel lubricity try to create conditions of boundary lubrication. More specifically, test results that quantify a fuel’s lubricity are a measure of the fuel’s ability to minimize friction between and/or damage to surfaces in relative motion under boundary lubrication conditions.

Different types of methods have been developed to measure fuel lubricity:

Of the bench test methods, the HFRR is most commonly used to evaluate diesel fuels. The SLBOCLE was common in the 1990s but has seen little use since about 2005. Both methods are discussed in more detail in the following sections, and their main specifications are listed in Table 1. Care must be taken when interpreting the results of lubricity tests with any of these bench tests. They only reproduce a limited number of wear mechanisms that may affect diesel fuel systems. While the wear mechanisms they reproduce are generally important for diesel fuel systems, their relative importance in any particular fuel system is very much affected by fuel system design and operating conditions.

Table 1
Summary of Main Specifications of Different Bench Lubricity Test Methods
 ASTM D6078 SLBOCLEASTM D6079 HFRRISO 12156-1 HFRR
Parametermin. load where friction coefficient is ≥ 0.175wear scar on ballwear scar on ball
Fluid Temperature25°C25 or 60°C. 60°C preferred unless volatility or degradation is a problem60°C
Fluid Volume50 ml2 ml2 ml
Air25°C, 50% RH> 30% RHsee Figure 6
Load500 g - 5000 g200 g200 g
Duration60 s at each load increment75 min75 min
Ball:stationaryreciprocating, 50 Hz / 1 mm strokereciprocating, 50 Hz / 1 mm stroke
- diameter12.7 mm6 mm6 mm
- materialAISI E-52100AISI E-52100 chromium ally steelAISI E-52100
- finish5-10 EPRa < 0.05 µmRa < 0.05 µm
- hardnessRockwell hardness C 64-66Rockwell hardness C 58-66Rockwell hardness C 58-66
Ring/Disk:RingDisk, stationaryDisk, stationary
- speed525 rpm
- size49.2 mm10 mm10 mm
- materialSAE 8720AISO E-52100 chromium alloy steel, annealed. Turned lapped and polished.AISO E-52100 chromium alloy steel, annealed. Turned lapped and polished.
- finish0.04-0.15 µmRa < 0.02 µmRa < 0.02 µm
- hardnessRockwell hardness C 58-62Vickers “HV 30”: 190-210Vickers “HV 30”: 190-210
- velocity1.3 m/s constant0.1 m/s average, reciprocating0.1 m/s average, reciprocating
Fuelfuel is aerated
Contactcontact surface not submerged in fuelcontact surface is submergedcontact surface is submerged
Range of precision data1100-6200 g143-772 µm @ 25°C,
175-1000 µm @ 60°C
360 - 600 µm @ 60°C
Repeatability900 g62 µm @ 25°C,
80 µm @ 60°C
63 µm @ 60°C
Reproducibility1500 g127 µm @ 25°C,
136 µm @ 60°C
102 µm @ 60°C

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