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What is diesel fuel?

What is diesel fuel?


What is diesel fuel?
Diesel fuel is principally a blend of petroleum-derived compounds called middle distillates (heavier than gasoline but lighter than lube oil) and may or may not contain additional additives. Other middle distillates include kerosene and No. 2 Heating Oil. Diesel fuel is designed to operate in a diesel engine where it is injected into the compressed, high-temperature air in the combustion chamber and ignites spontaneously. This differs from gasoline, which is ignited in a gasoline engine by the spark plugs.

What specification requirements of diesel fuel should concern me and why?
Cetane number (ignition quality), cleanliness, low-temperature operability and stability are the diesel fuel requirements of principal concern to you.

The cetane number is a measure of the ease with which the fuel is ignited in your engine. It is most significant in relation to low-temperature startability, warm-up and smooth, even combustion. The cetane number of the fuel should meet your engine manufacturer's minimum recommendation for the type of service. A cetane number higher than required does not materially improve engine efficiency or operation. Most engine manufacturers recommend diesel fuels with a cetane number of at least 40. Diesel fuels sold by reputable marketers meet or exceed this requirement.

Cleanliness refers to the absence of water and particulate contamination. This characteristic is important because dirt and water can plug fuel filters in your engine and cause severe damage to your fuel injection system because of the close tolerances within fuel pumps and injectors. All diesel engine manufacturers equip their engines with fuel filters to protect the fuel delivery system. You should replace these filters according to the manufacturer's recommendations. Some manufacturers also provide filters with drain valves and recommend periodic draining of any water that may accumulate from condensation and careless handling in storage or vehicle tanks.

Low-temperature operability is the ability of the fuel to flow and to be pumped through diesel fuel system filters without plugging at low temperatures. Filter plugging due to the presence of wax crystals in the fuel can be estimated by measuring the cloud point temperature or other low temperature fluidity and filterability tests.

Stability is the term used to describe a fuel's resistance to the formation of gums and insoluble oxidation products. Fuels with poor oxidation stability contain insoluble particles that can plug fuel filters. This may lead to decreased engine performance or engine stalling from fuel starvation.

How do I know which grade to use?
Most engines are designed to operate on ASTM No. 2-D grade, but some diesel engines in stop-and-go service require No. 1-D diesel fuels in order to perform satisfactorily. Follow the recommendations of the engine manufacturer and a reputable fuel supplier who recognize that some fuels may have special or additive-derived quality features.

What are some fuel-handling causes of poor diesel engine performance?
Contamination of fuel by water and dirt entering the fuel as a result of careless fuel handling may cause poor diesel engine performance. Extreme care must be exercised. Fuel-tank caps, dispensing nozzles and hoses should be kept clean to eliminate potential sources of contamination. Regularly removing water from storage tanks, vehicle fuel tanks, and filter bowls is important. Dry storage systems will reduce fuel emulsion problems, injection system corrosion and microbial growth.

Why do diesel engines smoke?
Diesel engine smoke is caused by incomplete combustion. White smoke is caused by tiny droplets of unburned fuel resulting from engine misfiring at low temperature. This smoke should disappear as the engine warms up. Black smoke could be caused by a faulty injector, insufficient air and overloading and/or over-fueling the engine. Blue-gray smoke is the result of burning lubricating oil and is an indication the engine is in poor mechanical condition.

Why was the sulfur content of diesel fuel reduced?
Low sulfur diesel fuels (less than 0.05 mass percent) are required by federal regulations for highway vehicles to ensure that these vehicles will meet emissions standards to improve the quality of the air we breathe. In California, other properties of vehicular diesel fuel are also controlled to help reduce smog.

Do Low Sulfur Diesel Fuels cause fuel system leaks?
When low sulfur diesel fuels were first introduced in some markets, there was a rash of fuel injector O-ring failures causing fuel system leaks. The failures were limited to older vehicles and the problems have not recurred.

How can I avoid having filter plugging problems?
Fuel filters can plug from a variety of causes. In the winter, small amounts of water or using fuel with inadequate low-temperature fluidity can cause filter plugging. At other times, common dirt and fuel distribution debris can cause filter plugging. And, if a fuel storage facility is not properly maintained, bacteria and algae can grow in the fuel and cause filter problems. With the exception of the fuel with inadequate low-temperature fluidity, all of these problems can normally be avoided by keeping the fuel storage system clean and dry. You should be sure to drain water accumulation frequently. That will obviously avoid icing problems in the Winter but will also minimize bacterial growth. Be sure to use dispenser filters when filling the vehicle tank.

How does water get into diesel fuel and what problems can it cause?
Water gets into diesel fuel storage and vehicle tanks in several ways – by condensation of humid air, during transportation from refineries to service stations, by leakage through faulty fill pipes or vents and by careless handling. Water can cause injector nozzle and pump corrosion, microorganism growth and fuel filter plugging with materials resulting from the corrosion or microbial growth. Both vehicle and storage tanks should be checked frequently for water and drained or pumped out as necessary. In extreme cases, biocides may be required to control microorganism growth. In cold northern winters, ice formation in fuels containing water creates severe fuel line and filter plugging problems. Regularly removing the water is the most effective means of preventing this problem; however, small quantities of alcohol may be used on an emergency basis to prevent fuel line and filter freeze-ups.

What does the heat energy or BTU content of a diesel fuel mean?
The energy content of diesel fuel is defined by the amount of energy stored in one gallon. The higher the fuel's heat of combustion, the more power will be derived from each gallon of fuel consumed. Energy content is measured in British Thermal Units (BTU) per gallon and is related to the fuel's specific gravity and the temperature range at which it vaporizes. The BTU content per unit volume increases as the specific gravity increases. The BTU content of a unit volume (gallon or liter) of diesel fuel is higher than that of gasoline, and the BTU content of No. 2-D diesel fuel is generally higher than that of No.1-D diesel fuel. The BTU content of No. 2-D diesel fuel is typically about 130,000 BTU/Gal.

What factors affect power/fuel economy?
Engine design is the most important factor leading to power and fuel economy. However, fuels with a higher density like our Low Sulfur Diesel No. 2 and Exxon Diesel Extra will provide improved power and fuel economy compared to fuels with Diesel Fuel No. 1 blended into them. Exxon Diesel Extra, because of its detergents, maintains fuel injector cleanliness, which prevents much of the degradation in engine performance experienced with non-additized fuels.

What is low-temperature operability?
Low-temperature operability is a diesel fuel’s ability to perform at low temperatures. All petroleum distillate products contain waxy materials, which, at low temperatures, can crystallize and plug fuel filters. The cloud point of the fuel measures the first appearance of the waxes, although filter plugging will not typically occur until the ambient temperature is 5° to 10°F below the cloud point. There are several approaches, both operational and fuel-related, to ensure proper operability in cold weather.

Vehicle-related approaches include the use of fuel heaters, keeping the engine running, parking the vehicle in heated areas, etc. Each of these is designed to keep the fuel warm, above the point where waxing occurs. But each of these approaches has obvious drawbacks, as well.

The low-temperature operability of the fuel can be adjusted in several ways, as well. Blending a No. 2 fuel with No. 1 can lower its cloud point; this is probably the most common approach to dealing with winter operability. But, the use of Diesel Fuel No. 1 reduces power and fuel economy, and often is more expensive, so minimizing the amount of No. 1 Fuel in the blend is an important consideration. Another approach to reduce the filter plugging incidence is to use wax-modifying additives. These additives can give operability benefits equivalent to No. 1 Fuel blending without the power and fuel economy losses.

Note that, especially early in the winter season, water buildup in vehicle tanks, when it freezes, can also plug fuel filters or fuel lines. If you experience filter plugging problems when the ambient temperatures are between 10° and 30°F, the most likely cause is water, not the fuel itself.

What should I do in the winter to adjust for the cold temperatures?
We recommend that you purchase a diesel fuel that has been winterized for your area by mixing it with Diesel Fuel No. 1 or low-temperature fluidity improver additives. Non-winterized diesel fuel will not generally cause problems as long as temperatures are at or above 10°F. The addition of about 15% to 20% Diesel Fuel No. 1 to Diesel Fuel No. 2 will reduce the cloud point of the fuel by about 5°F.

We offer winterized product in a majority of markets that experience severe weather conditions. If temperatures fall well below norms for the local area or you will be driving much farther north, additional Diesel Fuel No. 1 blending is recommended.

Why shouldn’t I just use Diesel Fuel No. 1?
While Diesel Fuel No. 1 has an advantage in low temperature operability, there are some disadvantages, as well. The energy content of Diesel Fuel No. 1 is about 95% that of Diesel Fuel No. 2 and will provide a correspondingly lower fuel economy. Diesel Fuel No. 1 is also lower in viscosity and provides less lubrication for the fuel pump and fuel distributor.

What are the differences among diesel fuels, heating oils and kerosenes?
Diesel fuels, heating oils and kerosenes are all products distilled from crude oil. While they are similar, there are some critical specification and legal differences among the products that normally prohibits using them interchangeably. These differences come about because their respective applications differ; diesel fuels are used in internal combustion engines, heating oils in oil burners and kerosenes are used in lamps and non-vented heaters. In addition to these differences, the products are subject to different tax laws. A few of the differences are highlighted below.

Diesel fuels have cetane specifications to assess combustion properties in diesel engines. They are also exposed to outdoor temperatures and relatively fine filtration. The most common measure for low-temperature fluidity is the cloud point. Neither heating oils nor kerosines require cetane number or cloud point specs. In the United States, heating oils and kerosenes are not taxed as are over-the-road diesel fuels. To distinguish them, with a few exceptions, the IRS requires that they be dyed dark red.

How long can I store diesel fuel?
If you keep it clean, cool and dry, diesel fuel can be stored 6 months to 1 year without significant quality degradation. Storage for longer periods can be accomplished through use of periodic filtrations and addition of fuel stabilizers and biocides.

What effect does blending used lubricating oil into diesel fuel have on engine performance and fuel quality?
In general, this practice is not recommended. It may adversely affect fuel quality features and could lead to fuel system and piston deposits, increased exhaust emissions and fuel-filter plugging. This practice may also result in the diesel fuel being out of compliance with state or federal regulations or other specifications. If you wish to pursue this issue further you should consult your engine manufacturer regarding the blending of used lubricating oils into diesel fuel.

Does diesel fuel color affect performance?
No. There is no relationship between natural diesel fuel color and such desirable diesel fuel qualities as heat content, viscosity, cloud point, cetane number or distillation range. Diesel fuel color varies with the crude source, refinery methods and the use of dyes. However, if the fuel color darkens appreciably during storage, this could indicate oxidation and/or contamination from dirt, water, or other sources, which can cause operational problems.

What is flash point?
The flash point of a fuel is the temperature at which vapors formed above the surface of the liquid fuel will ignite when exposed to an open flame under prescribed laboratory test conditions. Flash point has a negligible effect on engine performance but can be a significant fire hazard in the handling and storage of fuel. A low flash point temperature may indicate contamination of the diesel fuel with gasoline or other volatile materials such as alcohols.

What is diesel fuel lubricity?
Diesel fuel lubricity is a measure of diesel fuel's ability to reduce wear on contacting metal surfaces found in fuel pumps and injectors. In the case of diesel engines, fuel pumps and fuel injectors are lubricated by the fuel, so lubricity is a measure of a diesel fuel's ability to prevent wear in these parts.

 

Source: Exxon