Regan Brown
Diesel fuel has historically contained high levels of sulphur, which acted as a natural lubricant, helping to reduce friction and wear on engine components. However, due to environmental concerns, the push for cleaner-burning engines has resulted in a transition to ultra-low-sulphur diesel (ULSD). This change has presented significant challenges as low-sulphur diesel fuels lack the natural lubricity provided by sulphur, leaving engine components more vulnerable to wear and tear. To address this issue, various additives, such as fatty acid methyl esters derived from biofuels, have been explored to enhance lubricity and protect engine parts. The effectiveness of these additives is measured through tests like the High-Frequency Reciprocating Rig (HFRR) test, which evaluates the lubricating properties of diesel fuels. While some people suggest adding oil or other lubricants to diesel fuel, it is generally recommended to rely on fuel companies' lubricity additives in ULSD to ensure engine protection and performance.
Characteristics and Values of Diesel Fuel as a Lubricant
| Characteristics | Values |
|---|---|
| Lubricity | The ability of diesel fuel to reduce friction and prevent wear and tear on engine components like fuel injectors and pumps. |
| Environmental Impact | The push for environmentally safe fuels has led to a decrease in sulfur content in diesel, which has reduced the natural lubricity of the fuel. |
| Additives | Lubricity improvers or additives are used to enhance the lubricating properties of diesel fuel, such as fatty acid methyl esters derived from biofuels or commercial off-the-shelf additives. |
| Testing | Various tests like HFRR, SL BOCLE, and BOTD are used to evaluate the lubricity characteristics of diesel fuel and measure wear and tear. |
| Engine Protection | Adequate lubricity in diesel fuel is critical to preventing excessive wear and tear, reducing maintenance costs, and extending engine life. |
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What You'll Learn
The lubricating properties of diesel fuel
Diesel fuel has lubricating properties, and its lubricity is essential to the functioning of diesel engines. Lubricity is a measure of a fuel's ability to prevent or minimize wear and tear on engine components, such as fuel injectors and pumps, that come into contact with the fuel. These components are integral to the operation of a diesel engine, ensuring the precise delivery of fuel into the combustion chambers.
Historically, diesel fuels contained high levels of sulphur, which provided natural lubrication by reducing friction and wear. However, due to environmental concerns, there has been a push to reduce sulphur content in diesel fuel. This transition to ultra-low-sulphur diesel (ULSD) has resulted in a loss of natural lubricity, leaving engine components more vulnerable to wear and tear. The reduction in sulphur content has been correlated with increased wear rates in fuel injectors and pumps due to inadequate lubrication.
To address the lubricity challenges posed by low-sulphur diesel, fuel companies have introduced lubricity improvers or additives to restore and enhance the lubricating properties of the fuel. These additives include fatty acid methyl esters derived from biofuels, which form a protective layer on metal surfaces, similar to the function of sulphur. Biodiesel, in particular, has been identified as an excellent form of diesel fuel lubrication, improving the lubricity of ULSD by up to 30%. Other commercial lubricants, such as TC-W3 oil, have also been used by diesel engine owners to improve lubrication.
Various tests have been designed to measure the lubricity characteristics of diesel fuel, including bench tests like the High-Frequency Reciprocating Rig test (HFRR), the Scuffing Load Ball-On-Cylinder Lubricity Evaluator test (SL BOCLE), and the Ball-On-Three-Discs test (BOTD). These tests help evaluate and maintain fuel lubricity standards, ensuring that diesel fuels meet minimum lubricity requirements to protect engine components and optimize performance.
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The impact of sulphur on diesel lubrication
Sulphur, a derivative of the original crude oil source, is present in diesel fuel and contributes to air pollution and harmful corrosion in the engine after combustion. Sulphur levels in diesel fuel have decreased by 97% over the past few decades due to regulations aimed at reducing vehicle emissions. This reduction in sulphur content has resulted in a corresponding decrease in the natural lubricating properties of diesel fuel, leading to increased wear and tear on engine components.
The process of removing sulphur through hydro-processing replaces sulphur with hydrogen, improving fuel performance and reducing emissions. However, hydrogen's high reactivity also removes polar and aromatic compounds, reducing the fuel's inherent lubricating capability. As a result, engine components are more susceptible to issues such as shearing and micropitting, which accelerate the degradation of engine parts. Shearing occurs when the lubricating film between metal surfaces breaks down, increasing metal-on-metal contact and friction. Micropitting refers to the formation of small pits or indentations on metal surfaces due to repeated stress and inadequate lubrication.
To address the loss of lubricity in low-sulphur diesel, various additives have been explored. Biodiesel, for instance, has been shown to effectively restore lubricity and improve engine performance. Fatty acid methyl esters, derived from biofuels, have also been found to enhance the lubricating properties of ultralow-sulphur diesel fuel, reducing wear and friction.
While the reduction in sulphur content has positively impacted the environment and health, it has also introduced challenges related to storage, combustion, ignition, and lubricity. The International Council on Combustion Engines (CIMAC) has emphasized the importance of lubricity in engines running on low-sulphur fuel, and vessel managers are now responsible for ensuring adequate lubricity to mitigate increased emissions and potential engine failure.
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The use of additives to enhance lubrication
Diesel fuel has traditionally had high lubrication properties. However, the process of removing sulfur from diesel fuel to meet modern ultra-low-sulfur diesel (ULSD) requirements also strips it of the organic compounds responsible for lubrication. Lubricity is a measure of a diesel fuel's ability to prevent or minimize wear in the components that utilize the fuel as a lubricant.
In boundary lubrication, asperities (rough spots) on sliding surfaces are just touching, but the lubricant still supports most of the load. The fuel's effectiveness as a boundary lubricant is dictated by its chemistry. Diesel fuel molecules with polar groups will adhere to the metal surfaces, while the non-polar portion of these molecules will occupy the space between the surfaces. These non-polar "tails" effectively trap additional lubricating medium to reduce the degree of contact, thereby protecting the surfaces from wear.
To enhance the lubricity of diesel fuel, lubricity agents are added at the fuel "rack," where the fuel is dispensed. These agents are often a small amount of biodiesel, typically 1% or less. However, mistakes can happen when adding these agents, and some diesel fuel may have an inherently low lubricity quality. This can be corrected with lubricity additives, which are designed to increase the fuel's lubricity and protect the fuel injection system.
When choosing a lubricity improver additive, it is important to look for data indicating that the additive manufacturer has tested the fuel dosed at the recommended concentration with their additive. The wear-scar size or improvement when compared to untreated fuel should be measured, and scientific lab results are preferred over anecdotal evidence. One should also ensure that the additive does not contain an acid-based dimer or monoacid-based lubricity enhancer, as these can react with metal cations such as calcium and sodium to form "soaps," which can clog filters and cause injection system deposits.
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Testing methods for diesel lubricity
Lubricity is defined as the ability of a lubricant to minimize friction between and damage to surfaces in relative motion under load. Testing methods for diesel lubricity have been developed to create conditions of boundary lubrication. The results quantify a fuel's lubricity by measuring its ability to minimize friction and damage under boundary lubrication conditions.
The High-Frequency Reciprocating Rig (HFRR) is a commonly used testing tool for evaluating diesel fuel lubricity. It places a load on a bearing for 90 minutes, using the fuel as a lubricant. The size of the wear scars on the bearing is then measured, with smaller scars indicating better lubricity. The ASTM standard for the maximum size of these scars is 520 microns.
The Ball-on-Cylinder Lubricity Evaluator (BOCLE) is another test used to measure the lubrication properties of aviation turbine fuels. It is particularly useful for assessing the effects of fuels and additives on oxidative wear, an important factor in aviation fuel systems.
The Scuffing Load Ball-on-Cylinder Lubricity Evaluator (SLBOCLE) was developed in the 1990s in response to fuel system failures due to reduced sulfur content in diesel fuels. This method has similarities to the BOCLE test but with modifications to make it less sensitive to viscosity effects.
Vehicle tests are also used to evaluate diesel fuel lubricity. In this test, a vehicle is operated on the fuel for a specified length of time or distance, after which the fuel system components are examined for wear. This test is advantageous as it closely represents real-world conditions and can measure all possible wear-related failures.
It is important to note that the methods and equipment used to evaluate diesel fuel lubrication are often complicated and expensive. Consumers should be cautious of companies claiming lubricity without proper testing to back up their assertions.
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$84.49
The role of diesel lubrication in engine performance and longevity
Diesel fuel has historically contained high levels of sulphur, which provided necessary lubrication for critical engine components. Sulphur's lubricating function helped to reduce friction and wear, particularly on sliding contacts. However, due to its detrimental impact on emissions, there has been a push to reduce sulphur content in diesel fuel. This transition to ultra-low-sulphur diesel (ULSD) has made maintaining sufficient lubricity more challenging as low-sulphur diesel fuels lack the natural lubricity of their predecessors.
The reduction in sulphur content has been correlated with increased wear rates in fuel injectors and pumps due to inadequate lubrication. Engine components that rely solely on the fuel for lubrication, such as rotary/distributor type fuel pumps, are especially vulnerable to wear and tear. This has led to the exploration of various additives to enhance lubricity in low-sulphur diesel fuels.
One effective lubricating additive is fatty acid methyl esters, derived from biofuels. These additives form a protective layer on metal surfaces, similar to the function of sulphur. Biodiesel, in particular, has been identified as an excellent form of diesel-fuel lubrication. Studies have shown that even a small percentage of biodiesel added to diesel fuel can significantly improve its lubricating properties.
Additionally, off-the-shelf lubricant additives are available for purchase, and these can be beneficial for engine lubrication as well. It is important to note that while diesel fuel has some lubricating properties, it does not provide sufficient lubrication for all engine components. Therefore, diesel engines also rely on engine oil for additional lubrication and cooling.
In summary, diesel lubrication plays a crucial role in engine performance and longevity. Adequate lubricity in diesel fuel helps prevent excessive wear and tear on critical engine components, reducing maintenance costs and extending engine life. With the transition to low-sulphur diesel, the use of lubricity additives has become essential to ensure the proper functioning and durability of diesel engines.
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Frequently asked questions
Yes, diesel fuel is a lubricant. Diesel fuel molecules with polar groups will adhere to metal surfaces, while the non-polar portion of these molecules will occupy the space between the surfaces.
Historically, diesel fuels contained high levels of sulphur, which provided necessary lubrication for critical components. However, due to environmental concerns, the amount of sulphur in diesel fuel has been reduced, leading to a decrease in the natural lubricity of the fuel.
Various tests have been designed to measure the lubricity characteristics of diesel fuel, including the High-Frequency Reciprocating Rig test (HFRR), the Scuffing Load Ball-On-Cylinder Lubricity Evaluator test (SL BOCLE), and the Ball-On-Three-Discs test (BOTD).
Yes, you can add lubrication to diesel fuel. Commercial lubricants can be purchased and added to the fuel, and this will not harm your engine. Additionally, biofuels have been identified as effective lubricating additives, with some studies showing that a 1% addition of biodiesel can enhance the lubricity of ultra-low-sulphur diesel by 30%.
