For a number of years now there has been an ever-growing list of governmental regulations to address concerns about the environment and dependency on foreign oil. Initially many of these regulations were focused on gasoline-powered automobiles and the fuel that powers them.
More recently there has been a growing list of regulations that require reduced emissions from diesel engines. In addition, regulations for cleaner diesel fuel have been adopted to both reduce emissions and to enable technologies that, while reducing emissions, require cleaner fuels to function properly. Recent concerns related to petroleum use and greenhouse gas emissions are also influencing state and federal policies and regulations.
The Clean Air Act has been the driving force for these changes. The first Clean Air Act was adopted in 1963 and was amended in 1967, 1970, 1977, and most recently in 1990. The first federal emissions requirements for diesel-powered vehicles were in 1971 (1969 in the case of California regulations). Since that time there has been ever-tightening diesel engine emission standards as well as requirements for cleaner diesel fuels. In fact, by the late 1990s, the emissions of a new model heavy-duty diesel truck were about 10 percent for particulate matter (PM) and 27 percent for oxides of nitrogen (NOx) compared to similar pre-control era diesel trucks. But additional emissions reductions were required. The most recent emissions reductions are very stringent and require more dramatic technological advances in emissions control technology. In order to operate properly, these more advanced technologies require diesel fuel with ultra-low sulfur levels.
Environmental issues are not the only thing driving changes in diesel fuel. Energy security also plays a role. Our growing dependence on foreign crude oil and imported transportation fuels has revived interest in renewable fuels including “biodiesel.” The Energy Policy Act of 2005 contained a “Renewable Fuels Standard” which requires that a growing amount of our transportation fuels must be renewable fuels such as biodiesel and ethanol. This has resulted in the increased use of biodiesel as a blend component of diesel fuel.
The ASTM standard for diesel fuels is “ASTM D 975 – Standard Specification for Diesel Fuel Oils.” This standard currently covers seven grades of diesel fuel oils. These grades include numbers 1-D (S15), 1-D (S500), 1-D (S5000), 2-D (S15), 2-D (S500), 2-D (S5000) and 4-D. The grades are listed in order of increasing density and viscosity. The parenthetic numbers such as (S15) refer to the maximum sulfur level for the grade. Thus 2-D (S15) refers to No. 2 diesel with a maximum of 15 parts/million (ppm) sulfur.
In addition to the property limits in ASTM D 975, numerous test methods are encompassed in the specification to accurately measure the specified properties. While the ASTM standards ensure acceptable fuel quality, some petroleum companies and pipeline operators may require more stringent standards. In addition, SAE International Surface Vehicle Standard J 313 – Diesel Fuels, provides diesel fuel quality guidelines and the Engine Manufacturers Association (EMA) also stipulates certain fuel quality parameters. Other countries may rely on ASTM D 975 while some, such as European countries and Japan, have their own standards which may vary slightly from the ASTM property limits.
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Cetane number is a measure of the ignition quality of the fuel. Cetane number affects combustion roughness. Consumers often think the cetane number is similar to the octane number for gasoline, but that is not the case. Octane is a measure of a spark ignition engine fuel’s (gasoline) ability to resist engine knock (pre-ignition from compression). Diesel cetane ratings work in the opposite direction. The higher the cetane rating, the more easily it ignites. Reaching desired cetane levels also limits the aromatic content of diesel fuel.
Diesel fuel cetane ratings are calculated by calibrating a fuel to a mixture of reference fuels in a specially designed Cooperative Fuel Research (CFR) engine.
Acquisition and operating costs for a CFR engine are expensive, and it is not the easiest test to perform. Various tests have been developed to calculate the cetane number from certain fuel properties. These tests usually involve some combination of fuel density and distillation properties. The two more commonly used cetane number estimate formulas are referred to as cetane indexes to distinguish their results from the engine test. The most common cetane indexes are ASTM D 976 and ASTM D 4737. There are other cetane index methods that incorporate various fuel properties, but they are not as widely used as the ASTM methods. One problem with cetane indexes is that they report the cetane index number of the fuel. If cetane improver additives have been used in the fuel, it will raise the cetane number of the fuel, but this will not be adequately reflected in the cetane index calculation.
Cetane number requirements of an engine will vary depending on engine size, speed and load variations, starting conditions and atmospheric conditions.
Since a diesel engine ignites the fuel without a spark, proper cetane levels are very important. The air/fuel mixture is ignited by the combination of compression and heating of the air due to compression. The fuel is injected into the cylinder at the precise time ignition is desired to optimize performance, economy and emissions.
While gasoline engines time the spark to ignite the fuel, a diesel engine controls ignition by the injection of the fuel using either mechanical injectors or, more recently, by electronically controlled fuel distributors and individual injectors. This also necessitates much higher fuel pressures to overcome the pressure in the combustion chamber during the compression stroke. More simply put, in a spark ignition engine the amount of air is changed to control speed and power, while in the diesel engine the amount of air remains constant while the amount of fuel is varied. Diesel engines can operate at very lean mixtures when idling (e.g., 80:1) or move to richer mixtures during high load conditions (e.g., 20:1).
Given the operating conditions, it is easy to see why cetane level is important. In addition to improving fuel combustion, increasing cetane level also tends to reduce emissions of nitrogen oxides (NOx) and particulate matter (PM). These emissions tend to be more pronounced when starting with lower cetane number fuels. Increasing the cetane number value above that required for a given engine may not, however, improve engine performance. Some tests have shown that excessively high cetane number fuels may cause smoking (higher PM emissions).
The minimum cetane number for diesel fuel (Grades No. 1 and 2) is 40. The fuel should also meet a minimum cetane index of 40 or, alternatively, contain no more than 35 volume percent aromatics. Some manufacturers may recommend higher cetane number fuels.
Results of Inadequate Cetane Number
- Poor Ignition Quality
- Long Ignition Delay
- Abnormal Combustion
- Abnormally High Combustion Pressure
- Potential Uneven Thrust on Piston / Cylinder
- Louder Engine Knock
- Excessive Engine Knock & Smoke at Cold Start
The viscosity of diesel fuel is an important property that impacts the performance of fuel injection systems. Some injection pumps can experience excessive wear and power loss due to injector or pump leakage if viscosity is too low. If fuel viscosity is too high, it may cause too much pump resistance, filter damage and adversely affect fuel spray patterns.
In general, fuels with low viscosity tend to have poorer lubrication properties. ASTM D 975 requires a kinematic viscosity range of 1.9 minimum to 4.1 maximum mm2/S at 40ºC, for No. 2 diesel fuels (note that the term mm2/S replaces the former term of centistokes [cst]).
Unlike spark-ignition engines, the power and economy of diesel engines are comparatively insensitive to fuel volatility. There is some indirect impact in that less volatile fuels have higher heating values (energy content). Conversely fuels with higher front-end volatility tend to improve starting and warm-up performance and reduce smoke.
Ideal fuel volatility requirements will vary based on engine size and design, speed and load conditions, and atmospheric conditions. As an example, more volatile fuels may provide better performance for fluctuating loads and speeds such as those experienced by trucks and buses. ASTM D 975 only sets a minimum/maximum range for the temperature at which 90 percent of the fuel will evaporate. This is referred to as T90, and the range for No. 2 grades of diesel fuel is 282ºC to 338ºC. This limits the level of high boiling point components that could lead to increased engine deposits.
A carbon residue test is performed to approximate the engine deposit-forming tendency of diesel fuels. In the ASTM specification, this is referred to as the “Ramsbottom Carbon Residue on 10 mass percent Distillation Residue.” This number is limited to a maximum of 0.35 mass percent for No. 2 diesel.
Engine wear and deposits can vary due to the sulfur content of the fuel. Today the greater concern is the impact that sulfur could have on emission control devices. As such, sulfur limits are now set by the U.S. Environmental Protection Agency (EPA), and those limits have been incorporated into ASTM D 975. For No. 2 grade low sulfur diesel, the limit is a maximum of 0.05 percent mass (500 ppm) and, for ultra-low sulfur diesel, it is 15 parts per million (ppm) maximum.
ASTM D 975 includes a flashpoint requirement. This is not related directly to engine performance. The flashpoint is controlled to meet safety requirements for fuel handling and storage. The flashpoint is the lowest fuel temperature at which the vapor above a fuel sample will momentarily ignite under the prescribed test conditions. For No. 2 diesel grades, the flashpoint is a minimum of 52ºC.
Low Temperature Operability
The cloud point of a diesel fuel is the temperature at which the amount of precipitated wax crystals becomes large enough to make the fuel appear cloudy or hazy. Wax may form because normal paraffins occur naturally in diesel fuel. As the temperature of the fuel is lowered, these paraffins become less soluble in the fuel and precipitate out as wax crystals. In some fuel systems, cloud point can indicate the onset of fuel-filter plugging. Although ASTM D 975 provides a test method for determining cloud point, it does not set a specific temperature. This is because it is impractical to set low temperature properties for all ambient temperatures. Also, depending on equipment design and operating conditions, satisfactory operation may be achieved even below the cloud point. Cloud point and other low temperature operability limits such as low temperature filterability, cold filter plugging point, and pour point are generally specified by con- tract between the fuel supplier and fuel purchaser, who can best determine the necessary limit based on intended use and anticipated climate.
Pour point is the lowest temperature at which the fuel will flow and is used to predict the low- est temperature at which the fuel can be pumped. As mentioned above, other tests include the “Filterability of Diesel Fuels by Low Temperature Flow Test” (LTFT) and the “Cold Filter Plugging Point” (CFPP) test. One or more of these can help predict a diesel fuel’s low temperature operability properties.
Abrasive solids or soluble metallic soaps may be present in diesel fuel. These ash-forming mate- rials can result in injector and fuel pump wear, as well as piston and ring wear, in the case of abrasive solids, and engine deposits may also increase. The primary concern with soluble soaps is their contribution to engine deposits. ASTM D 975 sets a maximum limit of 0.01 mass percent ash content for both No. 1 and No. 2 diesel fuels.
A copper strip corrosion limit (under specified conditions) is used to predict possible problems with copper, brass or bronze fuel system components.
Water and Sediment
Because diesel fuel moves through various pipelines and tanks, and in some cases is moved by waterborne vessels, the potential exists for water and sediment to contaminate the fuel. Water and sediment contamination can contribute to filter plugging and fuel injection system wear. These contaminants may also lead to increased corrosion. The ASTM limit for water and sediment in diesel fuel is a maximum of 0.05 percent by volume.
Diesel fuel lubricity is a very important property, since the diesel fuel injection system relies on the fuel to lubricate moving parts. As with low-viscosity fuels, if lubricating properties are inadequate, it will lead to increased wear on injectors and pumps.
In years past, naturally occurring lubricity agents in diesel fuel provided adequate protection. More recently, certain refinery processes such as those used to comply with new low sulfur and aromatics requirements, tend to remove these naturally occurring materials. Lubricity additives are often required to avoid catastrophic fuel pump or injector failures. As little as one tank full of poor lubricity fuels can cause such catastrophic failures.
Current test methods for assessing fuel lubricity continue to be improved. Work on diesel fuel lubricity and the best test procedure to measure wear is ongoing within several organizations, including ASTM. The two most common test methods are the “Scuffing Load Ball-on Cylinder Lubricity Evaluator” (SLBOCLE) test and the “High Frequency Reciprocating Rig” (HFRR) test. The current version of ASTM D 975 specifies the HFRR test method, and the current requirement is a maximum of 520 microns at 60ºC (measurement of wear).
The actual composition of diesel fuel can differ among refineries, or even between batches produced at one refinery, because of differences in crude oil inputs and other factors. Each component used tends to have somewhat different properties and interrelationships with properties of the fuel to which it is added. This is why the ASTM specifications focus on the performance properties of the fuel rather than exact composition.
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