Rajan Wilcox
Diesel fuel quality testing is a crucial process that ensures the reliability, efficiency, and longevity of diesel engines. By regularly testing diesel fuel, potential issues can be detected before they arise, preventing equipment damage and costly repairs. The presence of contaminants, such as water, microbes, and sludge, can lead to fuel degradation and harmful consequences for engine performance. Therefore, it is essential to test diesel fuel types 1 and 2 to maintain optimal engine functionality and avoid unexpected problems. This introduction will explore the significance of diesel fuel testing, the common contaminants, and the methods employed to ensure the quality and stability of diesel fuel.
| Characteristics | Values |
|---|---|
| Purpose | To ensure safe and usable fuel, maintaining peak performance and engine longevity |
| Testing for | Combustion, stability, and contaminants |
| Contaminants | Water, microbes, particulate, and sludge |
| ASTM D975 standards | Diesel fuel that flows easily at all temperatures, is clean, free from foreign contaminants and separated wax, ignites readily, and burns quietly, cleanly, and economically |
| Sulfur content | 15 ppm for S15 diesel fuels, 0.05% (500 ppm) for S500 diesel fuel oils, and 0.50% (5000 ppm) for S5000 diesel fuel oils |
| Viscosity | No. 2 diesel should have a viscosity between 1.9 and 4.1 mm2/s |
| Cetane index | Measures the "combustion quality" of diesel fuel |
| Oxidation test | Test #OX-20 will test for fuel oxidation, with results in under 20 minutes |
| Distillation test | The 10% recovery temperature reflects how well the fuel will start to vaporize, while the 90% recovery indicates the extent of complete vaporization |
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What You'll Learn
Testing for viscosity
Viscosity is an important parameter for diesel fuel. If the viscosity is too high, it can cause damage to the fuel pump and cam and follower wear due to higher pressure. If the viscosity is too low, it may lead to a lack of lubrication. Viscosity also influences the fuel delivery rate and the atomization of the fuel during injection.
Industry standards dictate that the typical temperatures at which viscosity should be measured are 40ºC and 100ºC. The measurement at 40ºC is useful for the early detection of oxidation, polymerisation, and thermal failure of the oil. It is also good for the detection of contaminants such as fuel and refrigerants, which will lower the viscosity. The measurement at 100ºC is better suited for detecting viscosity index improver shear down and is best for components that operate at high temperatures.
To perform a viscosity test, a preset volume of oil is added to a calibrated viscosity tube, which is immersed in a temperature-controlled bath, typically at either 40°C or 100°C. The oil is then allowed to flow, and the time it takes for the oil to pass by two regions on the tube is measured. By multiplying the measured time by the tube factor, the viscosity in centistoke (cSt) can be calculated.
Viscosity testing provides indications of oxidation and other thickening processes from oil degradation, fuel contamination in liquid-fueled engines, the addition of incorrect lubricants, and more. It is one of the few tests that can indicate a problem when the viscosity is too low or too high. Therefore, when evaluating the viscosity, it is important to consider other test results that might provide clues as to why the viscosity changed or remained unchanged.
When testing the viscosity of diesel fuel, it is important to consider the impact of temperature. Diesel fuel's viscosity increases as the temperature decreases, and it can even turn into a gel that cannot flow in fuel systems at very low temperatures. Special low-temperature diesel contains additives to keep it liquid at lower temperatures.
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Testing for water
Water in your diesel tanks can become a problem and result in major repairs if you don't test for it regularly. Water in your fuel will foster the growth of microbial bacteria, which can damage the walls of steel tanks if left unchecked. This growth can cause defects in engine and fuel components, resulting in costly repairs and equipment downtime.
It is quite normal to find water in your diesel fuel tanks, but there is a fine line when small amounts can turn into a large problem. The acceptable level of water in diesel fuel is no more than 200 mg/L or parts per million as per EN 590 standards.
There are several reasons why water may be inside your fuel tank:
- Condensation: As the daily temperature changes from hot to cold, the fuel tank draws in humid air that collects as water condensation on the inside walls of the tank, which then drops into your fuel.
- Poor Fuel Handling: Water can be accidentally added to diesel tanks during the filling process due to errors beyond your control. While fuel stations usually separate water from their diesel, it is still possible to transfer water into your tank from a contaminated pump.
- Tank Caps and Filler System Leaks: Leaks in filler caps or fuel filler systems can allow water and other contaminants to enter your tank.
There are a few ways to test for water in your diesel fuel:
- Water in Diesel Testing Kits: You can use water-in-diesel testing kits that quickly and safely indicate water in your fuel. These kits are fast, safe, and easy to use, providing results in just minutes. They can be used in the field or in the workshop without the need for an expensive laboratory analysis. The steps to use these kits are as follows:
- Place about 100 to 200 ml / 3.5 to 7 oz of fuel sample into a clear, clean, and dry plastic cup, container, or bottle.
- Pour the test agent into your fuel sample and shake/stir for 20 seconds to mix.
- Rest the sample for at least 5 minutes.
- View the sample from the bottom of the container. Pink/purple/red specks indicate water contamination at 20 PPM (parts per million) or higher.
- Visual Inspection: One of the best ways to test for water is to look into your tanks with a flashlight. Diesel that has become contaminated with water is darker than usual. Clean diesel is clear and light-coloured in appearance. Dark or cloudy fuel can indicate the presence of water in the fuel system.
- Water Separator: Most diesel-powered vehicles and equipment have a water separator in the fuel system. You can check it yourself to ensure it is not gathering large amounts of water. Draining the water from the filter is usually all you need to do to remove the contamination.
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Testing for sulfur content
There are various methods to test sulfur content in diesel fuel. The ASTM method D2622 is a standard test method for quantifying sulfur concentration using wavelength dispersive X-ray fluorescence (WDXRF). This technique is highly sensitive, has a relatively straightforward sample preparation, and provides fast data acquisition. The practical limit of quantification for this method is 3 mg/kg of sulfur, depending on the equipment used. The ASTM method D6428 was previously used to test sulfur content but was later replaced due to inadequate precision.
Other test methods for sulfur content include wet chemistry, atomic spectroscopy, and various thermal combustion methods. The most common ASTM test methods for sulfur content include D 2622, D 5453, and D 7039. These tests are essential for complying with mandated fuel sulfur regulations, such as the 15 mg/kg limit for ultra-low sulfur diesel (ULSD) in North America and the European Union's 10 mg/kg limit for diesel and gasoline.
Regular testing of diesel fuel is essential to prevent issues with engine performance and stability. Poor-quality diesel can lead to inadequate performance and equipment damage, emphasising the importance of testing for sulfur content to ensure the fuel's quality and compatibility with engine requirements.
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Testing for oxidation
Testing diesel fuel quality is essential to ensure engine performance and longevity. Diesel fuel is prone to contamination by various impurities, including water and microbial growth, which can cause significant issues if left unchecked. One of the critical tests for diesel fuel is checking for oxidation, which starts to occur in fuel that is over 30 days old.
Oxidation of diesel fuel can lead to several problems, including darkening of the fuel, reduced efficiency, and, eventually, the formation of sludge and varnish that can clog fuel filters and corrode metal surfaces. Therefore, conducting oxidative stability tests is crucial to prevent fuel degradation and the issues that come with it.
One test for oxidation is the #OX-20 test, which is a simple and low-cost test that provides results in under 20 minutes. The test kit consists of an oxidation catalyst, test vials, and sampling pipettes. If the fuel is oxidized, the test will indicate a black reaction layer in the test vial. This test works on all diesel fuels, including #2 Diesel, Red Diesel, Maine Diesel, and Jet Fuels.
Another test for oxidation involves heating the fuel sample to high temperatures, typically around 1022 degrees Fahrenheit, causing most of the sample to evaporate. The remaining residue is then cooled and weighed to measure carbon levels. High carbon residue levels are undesirable as they can lead to combustion chamber deposits.
Additionally, testing for water content is crucial in preventing diesel oxidation. Water in diesel fuel can lead to microbial contamination, corrosion, and reduced fuel lubricity, all of which can cause engine performance issues and damage to fuel system components. Therefore, regular testing for water and other contaminants is essential to maintain the quality and stability of diesel fuel.
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Testing for distillation
The distillation test is used to determine the distillation characteristics of diesel fuel at atmospheric pressure. Test samples are classified into 'groups' based on their composition and the characteristics of expected volatility. In the case of diesel fuel, blends with up to 20% V/V biodiesel are included in Group 4. Belonging to a particular group defines the setup of the equipment to be used and the condenser temperature.
During the testing process, a 100 mL test portion is distilled under the specified conditions appropriate to the fuel group, and systematic observations of thermometer readings and volumes of condensate recovered are made. The introduction of new formulations in automotive fuels made necessary the adaptation of the standard in effect at the time of their appearance (ISO 3405:2000).
The test also establishes a clear definition of 'light distillates' and 'middle distillates'. Diesel fuel with blends of biodiesel belongs to the category of middle distillates.
The "distillation" standard is one of several tests used to address gasoline’s vaporization characteristics. Gasoline is metered in liquid form, through the fuel injectors (or carburetor), and mixed with air and atomized before entering the cylinders.
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Frequently asked questions
Diesel fuel testing is important to ensure the fuel is safe and usable. It also ensures complete combustion, maximising engine power and fuel efficiency, and helps to identify contaminants.
There are two main methods for testing diesel fuel: onsite and offsite in a specialised lab. Onsite tests are generally less expensive and can be done in 20 minutes using a fuel testing kit. Offsite lab tests are more expensive and less accurate, especially for microbial tests, as time delays between sampling and testing can cause microbes to proliferate.
ASTM D975 defines the minimum set standards for what diesel "fuel oil" should be. According to D975, an ideal diesel fuel flows easily at all temperatures, is clean and free from foreign contaminants and separated wax, ignites readily, and burns quietly, cleanly, and economically.
Diesel fuel testing checks combustion, stability, and contaminants by measuring cetane number, sulfur content, viscosity, distillation temperature, and the presence of water or sediment.
