Diesel Fuel - 101

In this article, we will investigate the source, preparation, and characteristics of diesel fuel, with the focus on those characteristics or improvements that constitute so-called "premium diesel fuel."

Premium diesel fuel refers to fuels having performance improvements for drivability, performance, engine durability, and special operating conditions (such as cold weather). Some typical formulations of premium diesel fuels will also be discussed. Discussions of chemistry will be kept mercifully brief.

Production of Diesel Fuel

Crude oil is converted into commercial products by separation, upgrading, and conversion processes. Distillation is the most widely used and important separation process. Crude oil is composed of thousands of components (chemicals) that range greatly in molecular size, shape, and elemental compositions. Well known petroleum products obtained by distillation and other refining processes include propane, naphtha or lighter fluid, aromatic solvents such as benzene and toluene, gasoline, diesel fuels, fuel oils, light machine oils, heavier oils, waxes, tars, creosote, and asphalt.

Undesirable components can sometimes be more easily removed, and the relative amounts of desired components can be enhanced, by upgrading processes such as catalytic hydrogenation (hydroprocessing). This process is also used to reduce the sulfur levels in today's diesel fuels by converting sulfur and its compounds to more easily separated chemicals.

Conversion processes such as thermal or catalytic cracking may be used to enhance the relative amounts of desired products, such as the gasoline and diesel fuel fractions, from crude oils that originally had lesser proportions of saleable products.

Diesel fuel accounts for a substantial use of petroleum, although much less than gasoline. For example, in 1995, about 25 billion gallons of diesel fuel were used in the US versus 119 billion gallons of gasoline and 23 billion gallons of jet fuel.

Worldwide use of diesel fuel is a good indicator of overall popularity of our favorite engine type: 187 billion barrels of diesel fuel were used worldwide in 1994. While this figure includes some off-road use of diesel fuel, in Europe and Asia the use of diesel fuel is nearly equivalent to gasoline usage.

There are several grades of diesel fuel in common use. In the US, #2 diesel is the most common, with #1 or a blend of the two grades being used in very cold weather. These fuels are similar to #2 and #1 fuel oils, and off-road diesel fuels are also similar. These non-highway product lines (off-road diesel fuels and fuel oils) may have higher sulfur content, and highway users may be subject to large fines for avoiding taxes levied on highway fuels, and for emissions non-compliance. A red dye is added to off-highway diesel fuels at a level such that five-fold dilution would still result in easily detectable levels of dye.

Diesel Engine Efficiency and Power Production

Large diesel engines such as those used in railroad locomotives and ocean-going ships, can have higher thermal efficiencies than automotive diesels, due to their lower and steadier speeds. For example, marine diesels may have three foot-diameter pistons, turn at only 60 to 200 rpm, and achieve up to 55% thermal efficiency even on heavy residual (not distillate) fuels.

Our "little" Turbo Diesels have engines of small displacement. They may turn near or over 5000 - 6,000 rpm, and they use swirl port technology to improve fuel-air mixing in the combustion chambers to improve efficiency at high engine speeds.

In contrast to diesels, the most efficient gasoline engines achieve only about 30% - 33% efficiencies, and then only at or near full-throttle where pumping losses are minimized.

Cummins and Bosch (the manufacturer of the engine's fuel pump) spend millions of dollars annually to improve these efficiencies while meeting emissions regulations. The possibilities for better power and fuel mileage with a special fuel blend must be balanced against cost and availability issues.

Those of us who are students of physics realize that to obtain a big horsepower increase without adding fuel, one would have to increase thermal efficiency tremendously in an already efficiently designed engine.

For a moment, lets explore the scientific principles involved in trying to improve engine efficiency so that significant power gains could be realized without adding more fuel, and with using conventional, readily available diesel fuel.

First, let's make the assumption that the basic engine stays the same, meaning the pistons, rings, bearings, reciprocating weight, etc., stay the same. Therefore, frictional losses will remain the same, and power will be "wasted" just to turn the engine. It much more power is to be made without adding more fuel, what might such power increases entail, and what else would you observe?

Second, black smoke indicates wasted fuel, so you would expect to see none of that.

Third, since Bosch and Cummins have already optimized fuel atomization and fuel-air mixing, there is little opportunity for greatly improved combustion efficiency.

Fourth, rejected heat would have to be minimal or nonexistent because a much larger percentage of the fuel would have to be doing work, rather than being wasted as heat.

Therefore, not only would the water jacket remain cool, but the oil and the engine block would not get hot either. Such observations might be consistent with increasing the engine's power by 50-75% without adding any fuel. The problem with this hypothetical situation is that engine parts do absorb and transmit heat.

Our diesels stay relatively cool under light load because not too much heat is being produced and so relatively little is lost. The more fuel is used, the more heat is produced in the combustion chamber, and the more heat is rejected by absorption and subsequent dissipation through the cooling system and radiation of heat from the engine block, head, etc., into the atmosphere.

This situation is the cause of the famous thermostat/temperature gauge cycling issue in our '94-'98 12 valve Turbo Diesel engines with the big radiators Dodge provided. Under light load and cool ambient temperatures, little fuel is used, little waste heat is produced, and the engine block is a sufficiently efficient radiator to dissipate the waste heat without the coolant reaching the temperature where the thermostat would open.

Under slightly more load, a bit more waste heat is generated, and the coolant in the engine block and cylinder head does reach 181 degrees F so the thermostat "cracks" open. The engine gets a "sip" of cold water from the radiator, and the engine coolant temperature quickly cools below the opening temperature for the thermostat. The thermostat closes, and once again the coolant climbs from about 140 - 150 degrees toward the 181 degrees where the thermostat will open again. Under higher loads, more fuel is burned, more waste heat is produced, and the thermostat stays open.

We are forced by science to accept several facts. First, more input energy (in a diesel engine, this involves the combustion of fuel) is needed to make more power. Usually a greater amount of fuel is used, although fuel having a higher energy content is capable of providing some contribution for increasing power.

Second, the engine becomes less, not more, efficient at high speeds and power levels.

Third, some fuel energy will be "wasted" to overcome engine friction and to produce waste, rejected heat.

Once these principles are understood, we can begin to see why diesel engines that produce high power levels for their displacement (cubic inches) also produce heat and smoke: lower percentages of the fuel produce work at the very high relative power levels. Some of the fuel is making waste heat and smoke, but still a larger amount of fuel is doing work, so more overall work is done, and more power is generated.

Okay, what about using a fuel of higher energy content? Here we are restricted by fuel cost and availability, and engine strength. To address this question with real-world fuels, we will move into a bit of the chemistry of diesel fuel, its energy content, and its cetane number, while our attention is still riveted on two relevant issues - the production of power, and cost effective, abundant supply.

Some Simple Chemistry of Diesel Fuel

In their "simplest" forms, the diesel fuels are distillates of crude petroleum having specific boiling ranges - with minima and maxima around 350 and 700 degrees F, respectively. This boiling range includes chemical compounds having about ten to twenty-three carbons. This distillate boiling range includes aliphatic compounds that could commonly be related to waxes or oils, and aromatics, including benzene, toluene, and xylene derivatives. The highly unsaturated polynuclear aromatic hydrocarbons are also included in the boiling range, of which naphthalene and anthracene are simple, commonly known examples.

For a given number of carbons, the aliphati hydrocarbons contain more heating value (energy) than the aromatics, weight for weight. However, because of density differences, on a liquid volume basis, the aromatics have more internal energy content. Diesel fuel has about 12 - 14% more energy content (often measured in British Thermal Units, BTU) than gasoline, on a per volume basis.

As folks living in cold climates are well aware, another consideration in choosing constituents of the ideal diesel fuel is melting (freezing) point. Here one will observe that melting point generally increases with molecular weight. Large saturated hydrocarbon (paraffin) molecules are waxes (like candles are made of) while small ones are liquids (naphtha) or gases (propane). Molecular shape has a great influence on melting point. More highly branched chains usually melt at lower temperatures, because they don't fit as easily into a solid crystal lattice.

In simpler terms, molecules get closely packed on freezing, so molecules that pack easily freeze easier. You can get more soda straws (like straight-chain, or normal, hydrocarbons) into a small space, and more easily, than you can irregularly shaped items of the same overall mass.

Up to this point in the discussion, it would seem that the ideal diesel fuel would contain larger molecules (more dense, more heating value), with a lot of branching and aromaticity for low freezing points and high energy content per volume. However, the burning process is also critical, and with this issue a new consideration must be introduced into this discussion, the cetane number of the fuel.

Cetane Number

The cetane number is a measure of a fuel's tendency to ignite in the absence of an ignition source (flame or spark). In other words, a high cetane number is assigned to a fuel having a quick ignition, and a low number to a fuel that easily ignites sluggishly. This index is essentially the opposite of the octane rating, where a high octane fuel resists spontaneous ignition, and therefore does not tend to "knock."

The cetane number of 100 was arbitrarily assigned to "cetane" or n-hexadecane, a straight-chain aliphatic chemical compound that burns very well, in the single-cylinder test diesel engine. Originally, the poorly burning 1-methylnaphthalene was assigned a cetane number of zero for testing, but in 1962 it was replaced for testing purposes by heptamethylnonane (also called "isocetane"), a highly branched aliphatic hydrocarbon that has a cetane number of fifteen. Both of these compounds belong to the class sometimes called "paraffins."

In general, normal (straight-chain) saturated hydrocarbons have high cetane numbers, and these numbers increase with the size (molecular weight, or number of carbons) of the compound. Cetane number decreases with "branching" of the chain, or with closing the chain into one or more rings. This trend was noted above with the dramatic reduction in cetane number from 100 to 15 by branching alone, on two isomeric molecules (that is, having the same number of carbons and hydrogens). Compounds with aromatic rings have relatively low cetane ratings, and those with two or more fused aromatic rings (like naphthalene and anthracene) have very low cetane numbers. The low cetane numbers of these aromatic compounds tell us that they won't burn easily or well, although their internal energy content per volume is high.

Now it is beginning to be clear that good diesel fuel is more complicated to produce than simply using the crude residue from gasoline production, and the cost of diesel fuel has at least some relationship to the difficulty of making it. High cetane numbers promote better burning, but the compounds that improve cetane numbers also have poor cold weather characteristics - they tend to crystallize (freeze) out of the mixture as wax. Branched molecules and aromatics burn less well, but may have better cloud point and pour point low temperature characteristics (that is, they will become cloudy at a lower temperature, and can be poured out of a bottle at lower temperatures).

On a per-gallon basis, aromatic content may improve energy content, but poor burning characteristics may favor the use of better and cleaner burning aliphatic hydrocarbons. When it gets cold outside, resistance to freezing or forming a gel is more important than a slight difference in energy content, although greater energy content can translate into better fuel economy.

Achieving the proper balance of constituents is made more difficult by the fact that the crude petroleum feedstock usually contains a chemical mixture very different from what is desired.

Conversion into marketable fuel requires the processes briefly mentioned at the beginning of this article - upgrading and conversion processes, and separation, usually through distillation. The conversion processes of removing sulfur by hydrotreating (a type of catalytic hydrogenation and a separation process) is expensive, but needed to meet EPA on-road diesel sulfur limits.

* The producer has to balance lubricity (improved by the sulfur and nitrogen containing compounds naturally occurring in diesel fuel) against the regulated sulfur maximum content.

Overall, the production of good quality, standard diesel fuel from the crude oils available today is neither easy nor cheap.

Premium Diesel Fuel

Right up front, we need to understand that the concept of premium diesel fuel is different from that of premium gasoline. Premium and mid-grades of gasoline are designed primarily to avoid knocking in high compression engines, that is, they have higher octane ratings. The premium grade of diesel fuel, on the other hand, usually involves more properties than simply an enhancement of the cetane number. Some of these other properties are detergency, stability, cold weather performance, lubricity, and fuel system protection.

Additives to improve each of these properties add from one to several tenths of a cent to the cost per gallon of the fuel. The additives are typically added in parts per billion (nanogram per gram) concentration ranges. At least some of these additives will be present in standard grade diesel fuel as required for the climate and type of use for which the fuel is intended.

Cetane Improvements

Until about thirty years ago, American diesel fuel would be characterized as "above average" and it was a simple distillate from crude oil. With the problems in obtaining adequate quantities and high quality raw materials (crude oil), fuel quality has decreased worldwide. For example, the average cetane number in 1966 was 50. The average cetane number declined to 48 during the first half of the 1910's, and dropped to about 46 by the late 1970's. In the mid-1980's, the average cetane number was in the range of 44 - 45, and by the first half of the 1990's it had dropped further, to 44.

*** Today, cetane numbers of around" 40 - 42" are all too common.

Improving the cetane number of diesel fuel is an important goal, especially for cold weather starting and general winter operation. Diesel fuel ignition and completeness of burning directly affect engine performance and fuel economy. If the fuel does not start burning promptly, accumulated fuel in the cylinder can experience multiple ignition points throughout the air/fuel mixture with rapid pressure rise, inefficient combustion, and increased smoking. Improving the cetane number of the fuel helps to keep the burning and the pressure rise consistent and even throughout the cylinder.

Since cetane number is a property of the fuel itself, it can be increased only by adding a high quality fuel component or by including an additive such as 2-ethylhexyinitrate. Additives do not provide cetane improvements in a linear relationship to their amount. That is, a small amount of additive will give some gain, but twice as much additive will give less than double that gain. It turns out that about three to six cetane numbers of improvement is the practical limit to the cetane number increase available from additives.

Detergents

Detergent additives are intended to remove gummy deposits from the fuel injectors, and to prevent any further formation of such deposits. Lacquers and gums impede fuel spray patterns, and prevent the free movement of close tolerance injection system parts. Detergent additives today are usually made from non-ash forming amines and amides.

Fuel Stabilizers and Fuel System Protectants

These additives retard the natural degradation of diesel fuel during storage. Typical classes of fuel stabilizers include de-emulsifiers to minimize water suspension in the fuel, sludge retardants, dispersants to minimize build-up of deposits, and antioxidants to minimize the formation of gums and deposits. Dispersants are typically polymers or surfactant amines that also reduce the fuel surface tension for easier atomization.

Antioxidants have gained in importance with low sulfur fuels, because these fuels tend to form peroxides during long-term fuel degradation. Peroxides attack elastomers (synthetic "rubber") in the fuel system. Other fuel system-related additives may be included, such as anticorrosive surfactants and rust inhibitors. Metal de-activators inhibit the decomposition of fuel through catalysis by copper or other trace metals. In high humidity climates, biocides may be added to prevent the proliferation of microorganisms at water-fuel interfaces.

Cold Weather Performance Additives

Within this basic category are three types of cold-flow improvers: pour point depressants, cloud point reducers, and flow improvers.

The pour point of the fuel is the lowest temperature at which the fuel will still flow. In the context of diesel fuel, the pour point is mostly due to paraffins forming waxes, usually before other compounds in the fuel freeze. Pour point depressants modify the formation of wax crystals, both their size and shape.

Cloud point reducers perform a related function, since they lower the temperature where paraffins begin to haze or form cloudiness. This haze is the onset of freezing or crystallization. Such additives allow the refiner to use a higher molecular weight (heavier) cut or distillate without flow problems in cold weather. This heavier cut will generally have a higher energy content, potentially improving fuel mileage for the consumer.

Other flow improvers assist in cold filter-ability as well as improving the cloud point of the fuel. They block the growth of crystals in the fuel so it will flow through the filter media.

De-emulsifiers and glycol ether de-icers are added primarily to facilitate the use of pipeline and storage systems that might have condensed or accumulated water in them.

Lubricity Improvers

Fuel lubricity is important in all diesel fuel injection systems, many of the internal components are lubricated by fuel.

Bosch VE and VP44 pumps, used from 1989-1993 and from 1998.5 to present, respectively, are lubricated by fuel only. The Bosch P7100 pump, used from 1994 to the first half of 1998, uses engine oil to lubricate some of the pump, but fuel lubricity is still significant. When the US EPA required lowering sulfur content of diesel fuels, there was widespread concern about lubricity. It turned out that if the fuel was not excessively desulfurized the lubricity was usually sufficient, or could be made so with an additive. Hence, lubricity should be monitored closely by refiners.

The aromatic content of the fuel is also reduced by hydrotreating (the process used to remove sulfur). In a few engines, fuel system seals swelled and worked better with highly aromatic fuels, but they tended to shrink and leak when the engines were switched to low sulfur, low aromatic fuels.

Conclusions

In summary, diesel fuel is a complex petroleum product, and numerous additives can be used to improve its performance for special applications, neither special fuels, fuel additives, nor purported engine efficiency modifications can be expected to provide more than moderate improvements over properly formulated, good quality diesel fuel that is blended for the climate.

Premium diesel fuel may be a viable alternative for those who prefer to use some sort of additives in their fuel. On the other hand, most commercial diesel fuels are formulated with those additives that are actually necessary for the needs of the area where the fuel is sold and intended for use.

The cetane number is perhaps the characteristic most associated with premium diesel fuel, as an analogy to premium gasoline with higher octane ratings. A cetane improver can provide a significant improvement in cold weather performance.

*** Fuel having a high "natural" cetane number obtained through high energy paraffin content may provide better fuel mileage than lower energy fuel or fuel with poor natural burning characteristics plus a cetane improver, if ambient temperatures are adequate to prevent gelling of the high paraffin fuel.

In many situations, the cost, weather compatibility, and/or availability of high-paraffin diesel fuel may not be reasonable; so we may be stuck with the basic grade of fuel that is available.

For better performance, we may have to rely on additives. Diesel fuel additives can be obtained by means of packaged additive protectants such as Power Service, AMSOIL, or a refiner's commercial "premium" diesel fuel like Amoco Premier or PowerBlend.

By Joe Donnelly - Las Vegas, Nevada

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Diesel Fuel Terms

Sulfur Content - Affects wear, deposits, and particulate emissions. Diesel fuels contain varying amounts of various sulfur compounds which increase oil acidity. Legislation has reduced the sulfur content of highway fuel to 0.05% by weight. Off road fuel has an average of 0.29% sulfur by weight.

Cetane Number - A measure of the starting and warm-up characteristics of a fuel. In cold weather or in service with prolonged low loads, a higher cetane number is desirable. Legislation dictates the Cetane index should be 40 or above.

Aromatic Content - By definition, aromatic content is characterized by the presence of the benzene family in hydrocarbon compounds that occur naturally in the refining of diesel fuel. In the chemical make up of fuel, the heavier aromatic compounds of toluene, xylene, and naphthalene are also present. Limiting these aromatic compounds has the effect of reducing burning temperature and thus NOx formation.

Cloud & Pour Point - Affect low-temperature operation. The cloud point of the fuel is the temperature at which crystals of paraffin wax first appear. Crystals can be detected by a cloudiness of the fuel. These crystals cause filters to plug.

API Gravity - Related to heat content, affecting power and economy. Gravity is an indication of the energy content of the fuel. A fuel with a high density (low API gravity) contains more BTU's per gallon than a fuel with a low density (higher API gravity).

Ash - Measures inorganic residues - The small amount of non-combustable metallic material found in almost all petroleum products is commonly called ash. Ash content should not exceed 0.02 mass percent.

Water & Sediment - Affect the life of fuel filters and injectors. The amount of water and solid debris in the fuel is generally classified as water and sediment. It is good practice to filter fuel while it is being put into the fuel tank. More water vapor condenses in partially filled tanks due to tank breathing caused by temperature changes. Filter elements, fuel screens in the fill pump, and fuel inlet connections on injectors must be cleaned or replaced when they become dirty. These screens and filters, in performing intended function, will become clogged when using a poor or dirty fuel and will need to be changed more often. Water and sediments should not exceed 0.1 volume percent.

Viscosity - Affects injector lubrication and atomization. The injector system works most effectively when the fuel has the proper "body" or viscosity. Fuels that meet the requirements of 1-D or 2-D diesel fuels are satisfactory with Cummins fuel systems.

Carbon Residue - Measures residue in fuel - can influence combustion. The tendency of a diesel fuel to form carbon deposits in an engine can be estimated by various tests to determine the carbon residue after 90% of the fuel has been evaporated.
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