Equipment Type

Diesel Fuel Oils and Analysis

The diesel fuel market is a continuously evolving business. Understanding the fuel
market changes that have occurred over the past few years can be overwhelming. This article
summarizes diesel fuel specifications and diesel fuel analysis.

September 23, 2009

Introduction to Biodiesel & Diesel Fuel Grades

Note: Within the grades mentioned below, several other grades are specified, mainly attributable to the different sulfur grades. As an example, No.2 diesel fuel oil has three grades—S15, S500, and S5000—that reference the maximum allowable sulfur content for that grade where the maximum sulfur content is 15 ppm, 500 ppm, and 5000 ppm respectively. The majority of diesel fuel oil produced in the U.S. today is refined to meet No. 2-D S15 requirements. (Visit for emissions management strategies.)

• ASTM D975 Standard Specification for Diesel Fuel Oils specifies light middle (No.1 diesel), middle (No. 2 diesel), and heavy (No. 4 diesel) distillate fuel oils for diesel engines. Industry focus is generally on No. 2 diesel, as it is the more commonly used general purpose diesel fuel oil. However, it is not unusual for No. 1 diesel (which has better low-temperature properties) to be blended with No. 2 for improved low-temperature operability in cold climates or during winter months. This fuel is sometimes referred to as winter blend. As of October 2008, ASTM D975 allows up to 5 percent by volume of biodiesel, as long as the blended fuel still meets the requirements outlined in Table 1 of D975. ASTM D975 also requires that the biodiesel used for blending meet the specification of ASTM D6751, which is the standard specification for B100 to be used as a blend stock for middle distillate fuels.

• ASTM D6751 Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels specifies that biodiesel be used as a blend stock with middle distillate fuel to create biodiesel blends. It is important to understand that biodiesel is most commonly blended with middle distillate fuel to produce biodiesel blends, as opposed to biodiesel (B100) being used directly as a stand alone fuel. D6751 is to be used only for the specification of the biodiesel blend stock.

• ASTM D7467 Standard Specification for Diesel Fuel, Biodiesel Blends (B6 to B20) specifies blends of middle distillate fuel oil with biodiesel (B100). Blends are referenced by the amount of biodiesel content in them, which is stated as BXX, whereas XX is the concentration of biodiesel. These fuels are generally used in the same applications as the diesel fuel oils specified in ASTM D975, but these biodiesel blends require their own specification. With the introduction of biodiesel into middle distillate fuel, new parameters need to be monitored to evaluate any effects on fuel quality that could be resultant of the biodiesel component. While most of the parameters for monitoring both diesel fuel and biodiesel blends are the same, there are some differences.

• ASTM D396 Standard Specification for Fuel Oils specifies several grades of fuel oil for use in commercial and industrial burners. Similar to D975, D396 specifies light middle, middle, and heavy distillate fuel oils; however, D396 also includes residual fuel oils grade No.5 and No.6 where a No.5 light and heavy are both specified.

• ISO 8217 Fuel Standard, marine distillate fuels and for marine residual fuels, specifies four marine distillate fuel grades and ten marine residual fuel grades.

Key Parameters for Evaluating Middle Distillate Fuel Oils

Appearance: Most middle distillate fuel specifications contain a workmanship statement specifying that the fuel is to be visually free of undissolved water, sediment, and suspended matter. Middle distillate fuel should be bright and clear with no visible sediment or free water. Any haziness to the diesel fuel at room temperature is indicative of emulsified water, which occurs when the fuel’s water content is above the water saturation point. This haze will generally begin to develop at around 150 ppm and can be used as a first warning sign that excessive water contamination is present in the fuel.

Low Temperature Operability: Similar to water freezing, fuel will gel up and eventually solidify at cold temperatures. However, unlike tap water, which has a known freeze point with very little variance depending on the source, middle distillate fuels from different sources can potentially have a wide variance in cold temperature performance. This is caused by differences in crude oil, refining processes, and additives. The cloud point is the temperature at which the fuel begins to display a wax haze or cloud caused by wax formation precipitating out of solution. Use of fuel at or below its cloud point may cause fuel starvation operability issues due to the wax crystals triggering fuel system plugging and/or inadequate flow. The low temperature operability of your fuel can be measured by methods such as cloud point, cold filter plugging point (CFPP), and low temperature flow test (LTFT).

40°C Viscosity: Viscosity is a measure of the fuel’s resistance to flow. If the fuel’s viscosity is too low, fuel injection pump and injector leakage can occur. Proper viscosity affects the fuel’s ability to lubricate fuel system components and properly atomize during injection. Poor viscosity can lead to increased wear and poor engine performance. 

Particulate Contamination: Particulate contamination analysis measures the cleanliness of fuel by determining the total particulate amount. High levels of particulate contamination can lead to increased filter plugging potential, causing increased costs associated with premature filter plugging. High levels of particulate can also lead to accelerated levels of fuel system wear and injector failures, especially with modern high pressure common rail fuel injection systems, as any hard particles can cause abrasive wear to injectors at the high pressures employed.

Water Content: There will always be some level of water in middle distillate fuel; however, most quality diesel fuel possesses water content less than 100 ppm and is not a concern. Once the water content approaches 150 ppm to 200 ppm, the first alarm level is triggered. Keeping the water content in check can prevent several water related problems. Water contamination can corrode fuel system components and lead to increased wear. Too much water impairs the fuel from properly lubricating, especially when enough water is present to create emulsions. Water issues are even more of a concern with use of ultralow sulfur and biodiesel blends, as water removal can be more difficult. Microorganisms require water to grow and, since most microbial growth occurs at the fuel water interface, keeping fuel systems dry will greatly reduce the likelihood of microbial contamination and its related problems. Monitor your fuel and storage tank’s water content to make certain your fuel system is dry.

Microbial Contamination: Two of the most common problems with microbial contamination are the increased potential for filter plugging and the corrosion of fuel system components. The waste created by microbial growth creates a potential cause of premature filter plugging. Because microbial contamination requires water, if there is a known water contamination issue currently or in the past in a fuel system, a check for microbial contamination is recommended. Proper fuel storage tank housekeeping is a chief preventative tool for microbial contamination and its associated problems. Periodic inspection of tank bottoms and dispensers is suggested, and water should be removed from tanks regularly. As part of a good housekeeping program, periodic tank bottom samples should be taken for monitoring.

Cetane Number/Index: Cetane index is used to estimate the natural cetane number of diesel fuel. Cetane index is calculated using a four-variable equation that uses the density of the fuel and the 10%, 50%, and 90% recovery temperatures determined by distillation. Note: Cetane index is not affected by cetane number improvers. Cetane number is determined using a specific test engine. As cetane number test engines are scarce and expensive, the cetane index is commonly used as a routine method for estimating the cetane number. Cetane number is a measure of the ignition quality of diesel fuel. Cetane number is essentially a measure of a fuel's ignition delay: the time period between the start of injection and the actual start of fuel combustion. Generally, fuels with higher cetane numbers provide a shorter ignition delay period than fuels with lower cetane numbers. Diesel fuel that possesses good ignition quality will provide good cold start performance.

Flash Point: Flash point measures the temperature at which vapors on the surface of the fuel will ignite when exposed to a flame. Flash point is generally specified for legal and safety concerns. When the flash point does not meet the minimum specification, there is indication that the product is contaminated with a more volatile product. If enough higher volatile contamination material is present, the cetane will likely be adversely affected, and the fuel will provide poor diesel engine performance.

Sulfur: Sulfur limits have been driven by emissions standards more than operability concerns. On-road diesel has been mandated to contain a maximum sulfur limit of 15 ppm. Use of non-S15 (ultralow sulfur) diesel fuel can poison catalysts used in advanced emission control devices, resulting in increased maintenance costs. Use of non-S15 diesel fuel can also potentially shorten drain intervals with CJ-4 formulated oils, as the higher sulfur fuels can create more acidic byproducts that are forced to be neutralized by the alkalinity reserve of the oil. This faster depletion of the alkalinity reserve can lead to a shortened remaining useful life of the oil and subsequent drain interval.

Lubricity: Fuel is also required to lubricate most fuel system components. Premature failures and increases in maintenance costs can occur when using fuel that does not possess sufficient lubricity. Fuel with insufficient lubricity may cause increased wear, while fuel with sufficient lubricity should provide reduced wear and longer component life.

Distillation: Distillation is a measure of the boiling range and fractional cut of diesel fuel. It is a measurement of volatility and quality. Engine designs dictate the required volatility requirements. Acceptable fuel volatility is needed to have good engine performance. As previously stated, the 10%, 50%, and 90% recovery temperatures determined by distillation, along with density, are used to calculate the cetane index.

Ash Percentage (%): Ash percentage (%) is a determination of the amount of ash-forming material present. Ash content is a general indicator of inorganic material in the fuel. Inorganic material is generally present as abrasive solids and soluble metallic soaps, both of which can be harmful to diesel fuel systems and lead to increased fuel filter plugging potential and injector wear and/or deposits.

Carbon Residue: Carbon residue is a measure of the coking tendency of the fuel. It can be used to estimate the carbon-depositing potential of fuel.

For Users of Middle Distillate Fuels

Aged fuel: In general, diesel fuel oils have adequate storage stability for normal use. For longterm storage of fuel (more than 12 months), a fuel monitoring program is an important part of a long-term storage program. Periodic sampling and analysis will provide the means by which to make important decisions regarding aged fuel, its storage, and its suitability for continued use. High temperature stability analysis can be used to determine the relative oxidative and thermal stability of the fuel.

Modern High Pressure Common Rail (HPCR) fuel injection systems: MHPCR systems are more adversely affected by particulate due to the high pressure and tighter clearances employed. Therefore, MHPCR systems require cleaner fuel than previously required. Injector deposit issues can occur with fuel that does not have good high temperature stability. In general, modern fuel systems are designed to return the non-injected portion of the fuel back to the tank. This returned fuel has been highly pressurized and heated, which promotes polymerization and breakdown of the fuel, thereby creating carbon particulate. As more fuel is pressurized, heated, and recirculated, the amount of this carbon particulate generated in the system increases, leading to possible increases in injector failure and filter plugging.

Fuel Filter Plugging: In the past few years, our laboratory group has experienced a rise in inquiries regarding problems with prematurely plugged fuel filters. Inquiries have originated from both users of conventional middle distillate fuel and from those using biodiesel blends. Users in the biodiesel blends group indicated that some of the systems had used biodiesel blends for several years while others just recently made the transition to biodiesel blends. Increased fuel filter plugging potential can be associated with water and microbial contamination, poor fuel thermal stability, high levels of particulate, fuel solvency, and poor fuel system housekeeping.

D975, “Standard Specification for Diesel Fuel Oils,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA.
D396, “Standard Specification for Fuel Oils,” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA.
D6751, “Standard Specification for Biodiesel Fuel Blend Stock (B100) for Middle Distillate Fuels.” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA.
D7467, “Standard Specification for Diesel Fuel, Biodiesel Blends (B6 to B20).” Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA.

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