Fuel Property Testing: Lubricity

Hannu Jääskeläinen

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• Overview
• ASTM D6079/D7688 HFRR
• ISO 12156-1 HFRR
• ASTM D6078 SLBOCLE
• Correlation Between Methods
• Effect of Additives

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:

• Vehicle tests. In a vehicle test [1241], the vehicle is operated on the fuel for a specified length of time or a specified distance. The fuel system components can then be disassembled and examined for wear. This test has the advantage of being the most representative of real-world conditions and can measure all possible wear-related failures not just those associated with boundary lubrication. Tests of this nature are, however, very expensive and time consuming and do not lend themselves to testing a large number of fuel combinations.
• Pump rig tests. An alternative to the vehicle test is a pump rig test [1243][1244][1520][1549][1522][1523]. In a pump rig test, a fuel injection pump is mounted on a test stand and is driven by an electric motor. Fuel is circulated through the pump for a specified time. The pump and any other equipment attached to it can then be disassembled and examined for wear and other deleterious effects. This test has the advantage of being less costly than a full vehicle test while maintaining the ability to test for wear-related failures beyond those associated with boundary lubrication. It is still time consuming and expensive to operate. One test can require as much as 500-1000 hours of test time. Pump rig tests are often necessary to evaluate the effectiveness of much simpler bench tests. ASTM D6898 was one version of such a test that used a Stanadyne Model DB4427-4782 rotary injection pump. Such pumps are no longer in common usage and D6898 was withdrawn in 2021.
• Bench tests. A number of bench tests that try to recreate boundary lubrication conditions similar to those found in fuel injection equipment have been developed to allow rapid and relatively inexpensive measurements of fuel lubricity:
  • The Ball-on-Cylinder Lubricity Evaluator (BOCLE) was developed for aviation jet fuels. It continues to be used for this application. It is particularly useful for measuring the effects of fuels and additives on oxidative wear—an important wear mechanism in aviation fuel systems. It is standardized as ASTM D5001.
  • The Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE) was developed in the mid-1990s in response to diesel fuel system failures resulting from the introduction of low sulfur diesel fuels. It is similar to the BOCLE test but with modifications to make it less sensitive to oxidative wear and more sensitive to adhesive scuffing.
  • The High Frequency Reciprocating Rig (HFRR) was also developed in the 1990s to make it useful for evaluating diesel fuel lubricity. It can produce a wide range of wear mechanisms depending on the fuel being tested.

Of the bench test methods, the HFRR is most commonly used to evaluate diesel fuels. The SLBOCLE was common in the 1990s but saw little use after about 2005 and its ASTM method (D6078) was withdrawn in 2021. 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.

The Ball on Three Disks (BOTD) lubricity evaluator, developed by Falex Corp., received some attention for use with diesel fuels starting in the 1990s. It was intended as a compact and more economical version of another method, the Ball on Three Seats (BOTS) apparatus. The BOTD used three flat disks with point contact on a loaded and rotating ball while the BOTS used three conforming seats with a larger contact area but that were difficult to manufacture with the required tolerance. Standardization work on the BOTD evaluator was started by ASTM but never resulted in a final standard being published [575][6674][6675][6680].

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