Density is a fundamental physical property that can be used in conjunction with other properties to characterize both the light and heavy fractions of petroleum and petroleum prod- ucts. Accurate determination of the density of petroleum products is necessary for the conversion of measured vol- umes to volumes at the standard temperature of 15C (60F).
Density is a factor governing the quality of crude petroleum;
TABLE 5—Detailed Requirements for Fuel OilsA
Property
ASTM Test MethodB
No. 1 S500B
No. 1 S5000B
No. 2 S500B
No. 2 S5000B
Grade No. 4
(Light)B No. 4
No. 5 (Light)
No. 5
(Heavy) No. 6 Flash Point,C, min D93 –
Proc. A
38 38 38 38 38 . . . . . . . . . . . .
D93 – Proc. B
. . . . . . . . . . . . . . . 55 55 55 60
Water and sedi- ment, percent volume, max
D2709 0.05 0.05 0.05 0.05 . . . . . . . . . . . . . . .
D95 þ D473
. . . . . . . . . . . . (0.50)C (0.50)C (1.00)C (1.00)C (2.00)C
Distillation Temperature,C
D86
10 percent volume recovered, max
215 215 . . . . . .
90 percent vol- ume recovered, min
. . . . . . 282 282
90 percent vol- ume recovered, max
288 288 338 338
Kinematic viscosity at 40C, mm2/s
D445
min 1.3 1.3 1.9 1.9 1.9 >5.5 . . . . . . . . .
max 2.4 2.4 4.1 4.1 5.5 24.0D
Kinematic viscosity at 100C, mm2/s
D445
min . . . . . . . . . . . . . . . . . . 5.0 9.0 15.0
max . . . . . . . . . . . . . . . . . . 8.9D 14.9D 50.0D
Ramsbottom car- bon residue on 10% distillation residue percent mass, max
D524 0.15 0.15 0.35 0.35 . . . . . . . . . . . . . . .
Ash, percent mass, max
D482 . . . . . . . . . . . . 0.05 0.10 0.15 0.15 . . .
Sulfur, percent mass maxE
D129 . . . 0.50 . . . 0.50 . . . . . . . . . . . . . . .
D2622 0.05 0.05
however, it is an uncertain indication of petroleum product quality unless correlated with other properties.
API gravity (D1298) is another measure of the density of fuel that has been used for years. It is calculated with the fol- lowing formula:
API gravity; degẳ ð141:5=specific gravity 60=60Fị
131:5 ð1ị
As use of the metric (SI) system of units and measure becomes more common, use of API gravity decreases. How- ever, it is still commonly used to describe the density of crude oil and in fuel terminals, pipeline operations, and sim- ilar custody transfer situations. The primary benefit of using API gravity, aside from habit, is that it permits calculations, descriptions, and transactions using whole numbers instead of just decimals.
Ignition and Combustion Characteristics (Cetane Number)
Diesel engine performance is a function of compression ratio, injection timing, the manner in which fuel and air are mixed, and the resulting ignition delay or time from the start of injection to the beginning of combustion. The nature of the fuel is an important factor in reducing igni- tion delay. Physical characteristics such as viscosity, gravity,
and mid-boiling point are influential. Hydrocarbon compo- sition is also important as it affects both the physical and combustion characteristics of the fuel. Straight-chain paraf- fins ignite readily under compression, but branched-chain paraffins and aromatics react more slowly. The first widely used measure of ignition quality was the diesel index. The diesel index was calculated as:
Diesel Indexẳ ẵðAPI GravityịðAniline Pointị=100 ð2ị By the mid-1930s, it was determined that a better measure- ment of ignition quality was needed. The result was an engine test, ASTM D613, Standard Test Method for Ignition Quality of Diesel Fuels by the Cetane Method. This test involves operating a standard, single-cylinder, variable com- pression ratio engine using a specified fuel flow rate and time of injection (injection advance) for the fuel sample and each of two bracketing reference fuels of known cetane number. The engine compression ratio is adjusted for each fuel to produce a specified ignition delay, and the cetane number is calculated to the nearest tenth by interpolation of the compression ratio values.
The cetane number scale uses two primary reference fuels.
One,n-hexadecane (normal cetane), has excellent ignition qual- ities and, consequently, a very short ignition delay. This fuel was arbitrarily assigned a cetane number of 100. The second fuel, a-methylnaphthalene, has poor ignition qualities and was
TABLE 5—Detailed Requirements for Fuel OilsA(Continued)
Property
ASTM Test MethodB
No. 1 S500B
No. 1 S5000B
No. 2 S500B
No. 2 S5000B
Grade No. 4
(Light)B No. 4
No. 5 (Light)
No. 5
(Heavy) No. 6 Copper strip corro-
sion rating, max, 3 h at a minimum control tempera- ture of 50C
D130 No. 3 No. 3 No. 3 No. 3 . . . . . . . . . . . . . . .
Density at 15C, kg/m3
D1298
min . . . . . . . . . . . . >876F . . . . . . . . . . . .
max 850 850 876 876 . . . . . . . . . . . . . . .
Pour PointC, maxG
D97 18 18 6 6 6 6 . . . . . . H
AIt is the intent of these classifications that failure to meet any requirement of a given grade does not automatically place an oil in the next lower grade unless in fact it meets all requirements of the lower grade. However, to meet special operating conditions, modifications of individual limiting requirements may be agreed on among the purchaser, seller, and manufacturer.
BUnder United States regulations, Grades No. 1 S5000, No. 1 S500, No. 2 S5000, No. 2 S500, and No. 4 (Light) are required by 40 CFR Part 80 to contain a sufficient amount of the dye Solvent Red 164 so its presence is visually apparent. At or beyond terminal storage tanks, they are required by 26 CFR Part 48 to contain the dye Solvent Red 164 at a concentration spectrally equivalent to 3.9 lb per thousand barrels of the solid dye standard Solvent Red 26.
CThe amount of water by distillation by Test Method D 95 plus the sediment by extraction by Test Method D 473 shall not exceed the value shown in the table. For Grade No. 6 fuel oil, the amount of sediment by extraction shall not exceed 0.50 mass percent, and a deduction in quantity shall be made for all water and sediment in excess of 1.0 mass percent.
DWhere low sulfur fuel oil is required, fuel oil falling in the viscosity range of a lower numbered grade down to and including No. 4 can be supplied by agreement between the purchaser and supplier. The viscosity range of the initial shipment shall be identified and advance notice shall be required when changing from one viscosity range to another. This notice shall be in sufficient time to permit the user to make the necessary adjustments.
EOther sulfur limits may apply in selected areas in the United States and in other countries.
FThis limit ensures a minimum heating value and also prevents misrepresentation and misapplication of this product as Grade No. 2.
GLower or higher pour points can be specified whenever required by conditions of storage or use. When a pour point less than18C is specified, the minimum viscosity at 40C for grade No. 2 shall be 1.7 mm2/s and the minimum 90% recovered temperature shall be waived.
HWhere low sulfur fuel oil is required, Grade No. 6 fuel oil will be classified as Low Pour (þ15C max) or High Pour (no max). Low Pour fuel oil should be used unless tanks and lines are heated.
assigned a cetane number of zero. Thea-methylnaphthalene was later replaced with heptamethylnonane, which was calibrated against the original fuels and assigned a cetane number of 15. The cetane number scale is now defined by the following equation for volumetric blends of the two primary reference materials:
Cetane Numberẳ%n-cetane
ỵ0:15ðpercent heptamethylnonaneị ð3ị In practice, the primary reference fuels are only used to cali- brate two secondary reference fuels. These are selected die- sel fuels of mixed hydrocarbon composition, which are designated as “T” and “U.” “T” fuel typically has a cetane number of approximately 75, while “U” fuel is usually in the low 20 cetane number range. Each set of “T” and “U” fuels are paired, and test engine calibrations define the cetane numbers for volumetric blends of these two secondary refer- ence fuels.
Higher cetane number fuels tend to lessen combustion noise, increase engine efficiency, increase power output, start easier (especially at low temperatures), reduce exhaust smoke, and reduce exhaust odor. In order to ensure accepta- ble cold weather performance, most modern diesel engines require a minimum cetane number of 40 [2] and this is the requirement in D975.
Cetane Index
Methods for calculating approximate cetane numbers were also developed for times when performing the engine test was not feasible. The two ASTM methods are:
• ASTM D976: Standard Test Method for Calculated Cetane Index of Distillate Fuels
• ASTM D4737: Standard Test Method for Calculated Cetane Index by Four Variable Equation
Both standards use fuel density and distillation values in their calculations. Standard D4737 is the more widely used method because it is newer and better represents diesel fuels currently in the market place. The following are among the limitations of these calculated cetane index methods.
1. They are not applicable to fuels containing additives for raising cetane number.
2. They are not applicable to pure hydrocarbons, synthetic fuels, alkylates, or coal tar products.
3. They are not applicable to biodiesel or blends of biodie- sel with petroleum diesel.
Volatility/Distillation
The distillation characteristics of a diesel fuel exert a great influence on its performance. Two methods are commonly used to measure distillation characteristics:
• ASTM D86: Standard Test Method for Distillation of Petroleum Products
• ASTM D2887: Standard Test Method for Boiling Range Distribution of Petroleum Fractions by Gas Chromatography Method D86 is the specified method, in D975, for measuring distillation characteristics. Figure 3 is a plot of distillation data for a single, typical diesel fuel using both methods. It is obvious from this plot that the two methods can give quite different results. For most fuels, results like these are typical, with more deviation at the beginning and end of the distillation and less deviation at the center. For all fuel grades in D975, Test Method D2887 can be used as an alternate. Results from Test Method D2887 shall be reported as “Predicted D86” results by application of the
correlation in Appendix X5 of Test Method D2887 to con- vert the values.
In case of dispute, Test Method D86 is the referee method. Grade No. 4-D does not have distillation require- ments. The average volatility requirements of diesel fuels vary with engine speed, size, and design. However, fuels having too-low volatility tend to reduce power output and fuel econ- omy through poor atomization, while those having too-high volatility may reduce power output and fuel economy through vapor lock in the fuel system or inadequate droplet penetration from the nozzle. In general, the distillation range should be as low as possible without adversely affecting the flash point, burning quality, heat content, or viscosity of the fuel. If the 10 % point is too high, poor starting may result.
An excessive boiling range from 10 to 50 % evaporated may increase warm-up time. A low 50 % point is desirable to mini- mize smoke and odor. Low 90 % and end points tend to ensure low carbon residues and minimum crankcase dilution.
The temperature for 50 % evaporation, known as the mid-boiling point, is usually taken as an overall indication of the fuel distillation characteristics when a single numerical value is used alone. For example, in high-speed engines a 50
% point above 302C might cause smoke formation, give rise to objectionable odor, cause lubricating oil contamination, and promote engine deposits. At the other extreme, a fuel with excessively low 50 % point would have too low a viscos- ity and heat content per unit volume. Therefore, a 50 % point in the range of 232 to 280C is desirable for the major- ity of higher speed type diesel engines. This temperature range usually is broadened for larger, slower speed engines.
For the above reasons, some points on the distillation curve are considered more important and are included in fuel specifications more often. ASTM D975 contains only a limit on the 90 % point. Other specifications include require- ments for initial boiling point (more so for gasoline), 10 %, 50 %, and, to a lesser degree, 95 % and final boiling point.
Viscosity
The method for measuring viscosity of diesel fuel is ASTM D445: Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and the Calculation of Dynamic Viscosity). The unit of measurement for this
Fig. 3—Distillation curves for a typical No. 2 diesel fuel.
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method is squared millimeters per second (mm2/s), also known as centistokes (cSt). This is the most widely used unit of measurement currently in use in the United States. Other units used in the past, and occasionally still used in the pres- ent, include Saybolt Universal Seconds (SUS) and centipoise.
For some engines it is advantageous to specify a mini- mum viscosity because of power loss due to injection pump and injector leakage. Maximum viscosity, on the other hand, is limited by considerations involved in engine design and size, and the characteristics of the injection system. Fuel vis- cosity exerts a strong influence on the shape of fuel spray.
High viscosities can cause poor atomization, large droplets, and high-spray jet penetration. With high viscosities, the jet tends to be a solid stream instead of a spray of small drop- lets. As a result, the fuel is not distributed in, or mixed with, the air required for burning. This results in poor com- bustion, accompanied by loss of power and economy. In small engines, the fuel spray may impinge on the cylinder walls, washing away the lubricating oil film and causing dilution of the crankcase oil. Such a condition contributes to excessive wear.
Low fuel viscosities result in a spray that is too soft and does not penetrate far enough in the combustion chamber for good mixing. Combustion is impaired and power output and economy are decreased. Low viscosity can lead to exces- sive leakage past the injection pump plunger. Fuel metering becomes inaccurate and engine efficiency is reduced.
Fuel viscosity requirements for high-speed engines range from 1.8 to 5.8 cSt at 40C. Usually the lower viscosity limit is established to prevent leakage in worn fuel injection equipment as well as to supply lubrication for injection sys- tem components in certain types of engines. During opera- tion at low-atmospheric temperatures, the viscosity limit sometimes is reduced to 1.4 cSt at 40C to obtain increased volatility and sufficiently low-pour point. Fuels having viscos- ities greater than 5.8 cSt at 40C usually are limited in appli- cation to the slower-speed engines. The very viscous fuels commonly used in large stationary and marine engines nor- mally require preheating for proper pumping, injection, and atomization.
Cloud Point
All diesel fuels contain dissolved paraffin wax. As the temper- ature of the fuel decreases, so does the solubility of the wax in the fuel. At some point wax crystals will begin to precipi- tate. If enough wax precipitates, the crystals can block fuel flow through screens, filters, and other restricted passages in the fuel system. The temperature at which the wax precipita- tion occurs depends on the origin, type, refining, and boiling range of the fuel. This temperature is known as the cloud point of the fuel. As the cloud point goes up, the suitability of the fuel for low-temperature operation decreases. The cloud point of the fuel can be measured by the following methods:
• ASTM D2500: Standard Test Method for Cloud Point of Petroleum Oils
The following methods are variations of D2500, including both automatic and automated methods.
• ASTM D3117: Standard Test Method for Wax Appear- ance Point of Distillate Fuels
• ASTM D5771: Standard Test Method for Cloud Point of Petroleum Products (Optical Detection Stepped Cooling Method)
• ASTM D5772: Standard Test Method for Cloud Point of Petroleum Products (Linear Cooling Rate Method)
• ASTM D5773: Standard Test Method for Cloud Point of Petroleum Products (Constant Cooling Rate Method) For all grades of fuel listed in D975, any of these test methods may be used to measure cloud point. Method D2500 is the specified method and the others are considered alternates. In case of dispute, Test Method D2500 is the ref- eree method.
Pour Point
Before a fuel can be burned in an engine, it must first be pumped from the fuel tank. The lowest temperature at which a fuel can be pumped is known as the pour point of the fuel. Although pour point is not specified in D975, test methods for measuring pour point include:
• ASTM D97: Standard Test Method for Pour Point of Petroleum Oils
The following methods are variations of D97, including both automatic and automated methods.
• ASTM D5949: Standard Test Method for Pour Point of Petroleum Products (Automatic Pressure Pulsing Method)
• ASTM D5950: Standard Test Method for Pour Point of Petroleum Products (Automatic Tilt Method)
• ASTM D5985: Standard Test Method for Pour Point of Petroleum Products (Rotational Method)
The pour point should be considered only a guide to the lowest temperature at which a fuel can be used. Some fuels, especially waxy fuels, will still flow at temperatures below their tested cloud point. In general, pour points are from 3 to 6C below the cloud point for a given fuel; how- ever, it is not uncommon for the difference to be as much as 11C.
For any given fuel, there will be no wax precipitation problems at temperatures above the cloud point. At tempera- tures below the pour point, it is highly unlikely that the fuel will give satisfactory performance. It is not unusual to obtain satisfactory engine performance with a fuel at ambient tem- peratures between the cloud point and pour point. The degree of performance and the temperature depend on the engine, the vehicle design, and the fuel system configuration.
Vehicles and fuel systems with small diameter lines, constric- tions, small porosity strainers and filters, and fuel lines exposed to ambient temperatures or wind will tend toward poorer performance. Systems with insulation, supplemental heaters, or large sections of the fuel system in close proximity to engine heat can probably expect better performance at lower temperatures [1].
Low-Temperature Flow Test
As discussed earlier, the mere presence of wax crystals in a fuel does not guarantee the fuel will plug filters or other fuel system components. The tendency of a fuel to plug screens and filters at low temperatures is a dynamic property dependent on the size and shape of the wax crystals. [Vehi- cle fuel system design is also a factor.] For this reason, numerous dynamic tests for low-temperature operability have been developed. ASTM standardized one such test in 1985, ASTM D4539: Standard Test Method for Filterability of Diesel Fuels by Low-Temperature Flow Test (LTFT).
The LTFT was designed to yield results indicative of the low-temperature flow performance of the test fuel in some diesel vehicles.
Cold Filter Plugging Point
The Cold Filter Plugging Point (CFPP) was developed for use in Europe. The method is published by ISO as EN 116–IP 309. It is similar to the LTFT with two exceptions:
(1) The fuel is cooled by immersion in a constant tempera- ture bath, making the cooling rate nonlinear but compara- tively much more rapid (about 40C per hour); (2) The CFPP is the temperature of the sample when 20 mL of the fuel first fails to pass through a wire mesh in less than 60 seconds.
While the CFPP is the preferred method in Europe and is used in several European specifications, it appears to over- estimate the benefit of using some additives, most especially for vehicles manufactured in North America [4,5].
Cleanliness
Diesel fuel cleanliness can mean many things to many peo- ple. It is safe to say that most users would consider any fuel that is visually free of water, sediment, and suspended mat- ter to be a clean fuel. Indeed, this is the cleanliness (work- manship) requirement stated in D975. However, it is known that microscopic particulates in the fuel can lead to prob- lems just as serious as the visible contaminants. The three most common methods of measuring cleanliness of diesel fuel are as follows:
• ASTM D2709: Standard Test Method for Water and Sediment in Middle Distillate Fuels by Centrifuge
• ASTM D4860: Standard Test Method for Free Water and Particulate Contamination in Mid-Distillate Fuels (Clear and Bright Numerical Rating)
• ASTM D6217: Standard Test Method for Particulate Con- tamination in Middle Distillate Fuels by Laboratory Filtration
D2709 is the method currently specified in D975. It is used to measure the amount of visible water, sediment, and suspended matter. This method gives no effective measure- ment of the presence or amount of microscopic particulates.
Many would consider the level of contamination sufficient to produce readable results in this test to be gross contamina- tion. However, this is an extremely sensitive test for contami- nation. The human eye is capable of seeing very small macroscopic particles, and the presence of one or two such particles could be considered a failure. In practice, the per- son conducting the test must exercise judgment based on experience and the requirements of the end use for the fuel.
D4860 was developed to give a means to quantify the level of cleanliness of a fuel. As with D2709, D4860 only pro- vides a valid measurement of visible contamination. The ben- efit of this method is that the measurement is now an objective measure of cleanliness. While this test does provide a quantitative result, it is far less sensitive than D2709.
D6217 could be considered a combination of the best fea- tures of the two previous methods. It is sensitive to small amounts of particles and is even capable of detecting micro- scopic particulates. It is also a quantitative measure of the cleanliness of the fuel. The quantitative measure is in the form of milligrams of particulate (contamination) per liter of fuel.
Currently there is no consensus standard with a specification limit for D6217. Many user specifications, including most fed- eral and military diesel fuel specifications, include of limit of 10 mg/L for the results of this analysis. Most users have found fuel that meets this limit to give satisfactory performance in the vehicle.
The two methods for filterability of diesel fuels are as follows:
• ASTM D2068: Standard Test Method for Filter Plugging Tendency of Distillate Fuel Oils
• ASTM D6426: Standard Test Method for Determining Filterability of Distillate Fuel Oils
The primary weakness of visual and gravimetric methods is that there is no generally accepted correlation between the results of the test and the performance of the fuel in a vehicle fuel system. That is, how long could the vehicle operate on that fuel before the fuel filter plugs or the water separator fails? The British Royal Navy first developed D2068 as a dynamic test of the cleanliness of fuel for shipboard gas tur- bine engines [6]. The test was designed around the specific requirements of gas turbine–powered ships in the British Royal Navy. The most important requirement being that the fuel filters had a nominal porosity of 1 mm. As such, a glass fiber laboratory filter with pore size of l mm is used in the test. Over the years, the test apparatus was upgraded, making it more automated.
D6426 is a modification of Method D2068. The first dif- ference between the two methods is the pump used in each.
D2068 uses a piston pump, whereas D6426 uses a peristaltic pump. The second difference is the filter. D2068 uses a 13-mm-diameter, 1-mm pore size filter. D6426 uses a specially constructed specimen. The specimen is called an F-cell Filter Unit. It is a disposable, precalibrated assembly consisting of a shell and plug containing a 25-mm-diameter nylon mem- brane filter of nominal 5.0-mm pore size, nominal 60 % porosity, with a 17.7-mm2effective filtering area.
Despite the differences in equipment, both D2068 and D6426 have the same pass/fail criteria. A fuel fails the test if the pressure drop across the filter reaches 105 kPa (15 psi) before 300 mL of test fuel passes through the filter.
ASTM D4176, Standard Test Method for Free Water and Particulate Contamination in Distillate Fuels (Visual Inspection Procedures), is widely used in field situations for examining fuel for gas turbines, both marine and industrial gas turbines.
Stability
For the purposes of this discussion, fuel stabilityis defined as the resistance of the fuel to physical and chemical changes brought about by the interaction of the fuel with its environment.
There are three types of stability usually of concern for diesel fuel. They are thermal, oxidative, and storage. Each of these will be discussed separately.
THERMAL STABILITY
Thermal stability is the resistance of the fuel to change caused by thermal stress (elevated temperature). The ASTM method is D6468, Standard Test Method for High Tempera- ture Stability of Distillate Fuels. Arguably, this test is the most often used method to monitor/predict fuel stability.
The primary reason for its popularity is the short test time and simple equipment requirements. This has made the 150C test, in one form or another, especially popular with many pipeline companies and others with the need to moni- tor the quality of fuel but to do it rapidly. No quantitative relationship exists between pad ratings and the gravimetric mass of filterable insolubles formed during the test. Addi- tional information on the interpretation of results is found in Appendix XI of the test method.