Mineral oil was first produced commercially in the 18th and early 19th centuries, but this ubiquitous resource really established itself towards the end of the 19th century, following the drilling of Drake’s well in Titusville, Pennsylvania, USA in 1859. Production started at a similar time in Russia and Romania, and liquid fuels become readily available. Oil was initially refined for use as lamp oil. Inventors rapidly tried to find uses for the new liquid by-products of the lamp oil refining industry.
In 1892, Rudolph Diesel achieved initial success with an engine that bears his name. It did not require to burn coal dust for power. Diesel found that he could use one of the liquid by-products instead. The liquid fuel he eventually chose is now called diesel fuel.
Over time, the diesel engine has been adapted for use in trucks, buses, marine propulsion, railroad locomotive, automobiles, and industrial power plants. In fact, diesels are used almost anywhere efficient power is needed.
The diesel engine is the most efficient reciprocating internal combustion engine in use today. Mechanically, it is quite similar to the petrol/gasoline engine. The major distinguishing characteristic is the compression ignition principle.
In a spark ignition (or petrol/gasoline) engine (SI engine) a spark is generated at a predetermined point towards the end of the compression stroke to ignite a premixed homogeneous air fuel mixture (with an air-to-fuel ratio of approximately 15:1).
A diesel, or compression ignition engine (CI engine) operates on a heterogeneous charge of previously compressed air and a finely atomized spay of liquid fuel. The liquid fuel is injected into the engine cylinder towards the end of compression. After a suitably intensive mixing process with the hot air already in the cylinder, the self-ignition properties of the fuel cause combustion to be initiated from small nuclei. These spread rapidly so that complete combustion of all injected fuel, usually with air-fuel ratios in the 16-100:1 range, is ensured.
The mixing process is crucial to the operation of the diesel engine. It has received a great deal of attention from engine designers, which is reflected in a wide variety of combustion systems which may conveniently be grouped in two broad categories.
in which fuel is injected directly into the combustion chamber, and good mixing and combustion is achieved by using muliti-hole fuel injectors, high injection pressures & swirling of the intake air.
in which fuel is injected in turbulent air in a pre-chamber where ignition occurs and then burning air-fuel mixture enters the cylinder through a small throat, mixes with the remaining air to complete the combustion.
WHAT IS DIESEL FUEL?
Most liquid fuel is derived from crude oil. Crude oil, formed millions of years ago, is found in underground reservoirs. It is a mixture of hydrocarbon molecules and varies in color, composition and consistency.
Different oil-producing areas of the world yield significantly different varieties of crude oil. The amount of petrol, diesel, and other products produced varies geographically with the crude oils, refineries, and market demands.
Separating crude oil into various “fractions” or “cuts” begins at a distillation unit. These “fractions” include light ends like gasoene, middle distillates, and residuums. Diesel fuels in general are derived from middle distillates.
Middle distillate fuels are petroleum oils having a higher boiling range than gasolines. Finished diesel fuels consist of blends of middle distillate streams.
(Biodiesels have not been considered in this paper)
DIESEL FUEL MANUFACTURING
|In the early 1900s crude oil was divided into distinct products by use of a sample distillation process. Distillation separates the crude oil into fractions according to boiling point. Distillation works with crude oil because a hydrocarbon’s boiling point depends on its molecular size and shape.
Light hydrocarbons boil off right away and provide products such as butane and propane. Heavy hydrocarbons give us products such as asphalt. Gasoline, jet fuel, diesel fuel, fuel oil and lubricating oil base stocks can all be obtained by distillation. Products obtained by distillation without further processing, are referred to as straight-run products. The problem with early simple distillation processes was that there was no control over the end result; we got whatever was originally in the crude oil. It soon become apparent that simple distillation of crude oil couldn’t produce enough fuel (primarily gasoline) of sufficient quality to satisfy the rapidly growing demand.
Engineers and chemists tackled the problem and found that hydrocarbons with higher boiling points (that is larger hydrocarbons) could be broken down of “cracked” into lower boiling smaller hydrocarbons by subjecting them to very high temperature. This process was named Thermal Cracking. Starting in 1913, thermal cracking was used to increase fuel production. Eventually, it was discovered that substances called “catalysts” did a better job of cracking hydrocarbons, than did high temperatures. Catalysts speed up a chemical reaction without undergoing a permanent chemical change themselves. When large, heavy
hydrocarbons are broken into smaller, lighter hydrocarbons by the Catalytic Cracking process,
larger volumes of gasoline and other light products, that are in great demand, are produced.
Hydrocracking is similar to catalytic cracking, but takes place in a hydrogen atmosphere. It can crack hydrocarbons that resist being cracked by catalysis alone. The two processes work as a team.
is a variation of a thermal cracking unit that is applied to the heaviest crude streams not suitable for catalytic cracking. Cracking units form new, upgraded lighter hydrocarbons from heavier fractions, and solid coke.
Other units further separate and condense these newly formed hydrocarbon molecules into finished petroleum products, thus maximizing production of quality fuel from each barrel of crude oil.
produces home-heating oils and diesel fuels through the interaction of the “middle distillates” from the crude oil with hydrogen and catalyst. Hydrotreating results in products having lower sulfur level. In a modern, full integrated refinery, diesel fuel components are obtained from the crude distillation units and/or one or more of these processes.
Diesel is generally simpler to refine from petroleum than gasoline, and contains hydrocarbons having a boiling point in the range of 180–360°C.
Other processing units enhance the yield of high-grade products from the crude. The various fuel stocks are blended into gasolines, jet fuels, and fuel oils. Some components (not wanted in the fuel) are removed from the crude, such as sulfur. These components, in themselves, have commercial value in industry and agriculture.
Many countries, organizations & industries have developed standards for diesel fuels. The ASTM classification of diesel fuels, ASTM D 975, defines three grades of fuel. It is intended as a statement of permissible limits for significant fuel properties used for classifying the wide variety of commercially available diesel fuels, in accordance with their service application.
The No. 1-D grade fuel comprises volatile fuel oils from kerosene to the intermediate distillates. They are applicable for use in high-speed engines in services involving frequent and wide variations in loads and speeds, and also for use in cases where low ambient temperatures are encountered.
The No. 2-D grade fuel includes distillate gas oils of lower volatility. These fuels are for use in high-speed engines in services involving high loads and uniform speeds, or in engines not requiring fuels have the higher volatility or other properties specified for Grade No.1-D. Most automotive-and truck-type diesel engines use No. 2-D fuel.
The No. 4 D grade fuel covers the more viscous distillates and blends of these distillates with residual fuel oils. These are applicable for use in low-and medium-speed engines in services involving sustained loads at substantially constant speed. Large stationary power generation, and marine diesel engines are primary users of No.4-D fuels. Boiler fuels are defined by ASTM D 396 standards. Marine Distillate and Residual fuels defined by ISO 8217 standards. Diesels are also classified as Ultra Low Sulfur Diesel; Low Sulfur Diesel; No. 2 Diesel; Motor Vehicle Diesel Fuel; Non-Road Diesel Fuel; Locomotive/Marine Diesel Fuel
Because of efficiency, diesel engines have become popular for use in many areas. They are built in many shapes and sizes ranging from very large engines in Ships to smaller engines in cars, trucks, and buses.For Every different use of the diesel engine, there is an appropriate grade of diesel fuel to satisfy the engine’s needs.
The physical and chemical properties that affect engine performance are outlined below. While all of the properties listed are important in designing a diesel fuel, certain ones are more critical, and these will be discussed in details.
When fuel is injected into the combustion chamber of a diesel engine, ignition does not occur immediately. The interval between the beginning of injection and autoignition of the fuel is called the ignition delay period. The duration of the delay period is a function of engine design, operating conditions, and hydrocarbon composition of the fuel. If the delay is too long, the engine may be hard to start, and when the accumulated fuel does ignite, the rate of energy release is so great that is causes engine roughness or diesel knock. If the delay is short, combustion is even, and the engine runs smoothly.
|Several performance factors may be influenced by the ignition quality of the fuel. They are cold starting, warm-up, combustion roughness, acceleration, and exhaust smoke density.
The ignition quality of diesel fuel is expressed in terms of cetane number. This scale is based on the ignition characteristics of two hydrocarbons; normal cetane, and alpha-methylnaphthalene. Normal cetane was arbitrarily assigned a cetane number of 100 because it has a short delay period and ignites readily. Alpha-methylnaphthalene was assigned a cetane number of zero because it has a long delay period and, therefore, poor ignition quality.
The cetane number of diesel fuel is the percentage by volume of normal cetane in a blend with alpha-methylnaphthalene, which matches the ignition quality of the fuel when compared in a single-cylinder laboratory test engine. A high cetane number indicates good ignition quality (short delay period), and low cetane number indicates poor ignition quality (long delay period).
The sulfur content of a diesel fuel depends on the crude oil source and the subsequent refining and treatment steps it undergoes. Environmental concerns have led to a sulfur content regulation. Sulfur oxides can cause damages to buildings, and irritation to the respiratory system.
There are two sulfur oxides: sulfur dioxide, S₂, and sulfur trioxide, S₃. The latter is normally collected as sulfuric acid since it reacts very rapidly with water vapor in the exhaust. Both are produced by the combustion of sulfur in the fuel. Sulfur oxides emissions are reduced by reducing the sulfur in the fuel.
Sulfur content of diesel fuels is also limited by regulations directed at exhaust emission control. Diesel particulate emissions (smoke and exhaust particles too small to be seen) tend to be higher with higher sulfur fuels. Particulate emissions can be reduced by reducing fuel sulfur. The lower sulfur levels are achieved by hydrotreating.
During the combustion process the temperature of the main body of gas is high, but in the vicinity of the cylinder walls the gas is cooled by the metal surfaces, and condensation of water will occur if the wall temperature is low enough. Condensation temperature (dew point) varies with the gas in the cylinder and the sulfur content of the fuel. At times it can be higher than the temperature of the cylinder walls. When this happens, sulfur oxides will go into solution to form corrosive acids-including sulfuric acid-which attack metal parts causing corrosive wear and contribution to engine deposits.
This deleterious effect can be controlled by the use of lubricating oils with alkaline additives adequate to neutralize the acid condensate, and by keeping the internal surface temperature of the cylinder walls as much above the dew point as possible. Even so, for any given oil, the higher the sulfur content of the fuel, the greater the corrosive wear
Certain properties, such as volatility, gravity, cetane number, heating value, and cloud and pour points are interrelationship. Low volatility, straight run fuels usually have lower APR gravity; but have higher viscosity, cetane number, heating valve, and cloud and pour points than do fuels of higher volatility from similar crude sources. Since diesel fuels are composed of complex mixtures of many hydrocarbon compounds, the influence of hydrocarbon composition is also reflected in the interrelationships.
The heat of combustion of a diesel fuel, is a measure of the amount of energy available to produce work. Average heating value of diesel is 43.1 MJ/kg. A knowledge of this value is essential when considering the thermal efficiency of power generating equipment. In general, a diesel fuel having a higher volumetric heating value will produce more power, or provide better fuel economy, than a fuel of lower volumetric heating value. However, the normal variation in heat of combustion of diesel fuels is usually less than 5%. In practice, the effect of variations in heating value is not as important as other factors, such as differences in inlet air temperatures or altitudes.
Volatility of a diesel fuel is normally measured by ASTM D 86. While volatility has no direct affect on power or economy, less volatile (higher boiling) fuels normally have a higher heating value and thus performance is indirectly affected. Starting and warm-up are better with higher front-end volatility, and deposit formation, wear, and exhaust smoke are increased, in some engines, by higher end points.
The density of diesel is about 0.832 kg/l, about 12% more than ethanol-free petrol (gasoline), which has a density of about 0.745 kg/l . About 86.1% of the fuel mass is carbon, and when burned, it offers a net heating value of 43.1 MJ/kg as opposed to 43.2 MJ/kg for gasoline. However, due to the higher density, diesel offers a higher volumetric energy density at 35.86 MJ/L vs. 32.18 MJ/L for petrol(gasoline), some 11% higher, which should be considered when comparing the fuel efficiency by volume. The CO2 emissions from diesel are 73.25 g/MJ, just slightly lower than for gasoline at 73.38 g/MJ.).
Viscosity is an important physical property of a diesel fuel affecting injector lubrication and fuel atomization. It is measured by ASTM D 445.
Diesel fuels with extremely low viscosities may not provide sufficient lubrication for the closely-fit pumps and injector plungers. They can promote abnormal wear and cause injector leakage and dribbling leading to loss of power or smoke problems.
Since viscosity influences the size of the fuel droplets. It governs the degree of atomization and penetration of the fuel spray. These are major factors in obtaining sufficient mixing of fuel and air essential for proper combustion. If the viscosity is too high, the fuel droplets will be too large for proper mixing and poor combustion will result. Also, the droplets may strike the relatively cold cylinder wall and fail to burn. If the viscosity is too low, the fuel spray will not travel across the combustion chamber and the poor mixing will result in improper combustion. Poor combustion results in loss of power and excessive exhaust smoke.
SMOKE AND PARTICULATES
It has long been recognized that the presence of smoke in diesel engine exhaust is an indication of poor combustion, usually as the result of some malfunction or maladjustment. Public opinion has reacted against diesel exhaust on account of unpleasant odor and highly visible smoke. Therefore, most industrialized countries have introduced regulations to control smoke emission from diesel-powered vehicles.
Smoke may be defined as particles, either solid or liquid (aerosols), suspended in the exhaust gases, which obstruct, reflect, or refract light. Diesel engine exhaust smoke can be categorized under two headings.
Blue /White in appearance under direct illumination, and consisting of a mixture of fuel and lubricating oil particles in an unburned, partly burned, or cracked state.
Black (or Grey) in appearance, and consisting of solid particles of carbon from otherwise complete combustion of fuel.
White smoke is usually a result of too low a temperature in the combustion chamber during the fuel injection period. Droplet size is about 1.3µm. This can occur as a transient condition during the starting period, in low ambient temperatures, or at high altitude, disappearing as the engine warms up. Under these conditions, a higher cetane fuel or a more volatile fuel will tend to promote better combustion and reduce smoke. On the other hand, white smoke could result from too late fuel injection or may be an indication of a design fault. Any operating variable (i.e. jacket temperature, inlet air temperature, etc.), that increases compression temperature or reduces ignition delay will reduce white smoke.
The blue component derives mainly from an excess of lubricating oil in the combustion chamber, resulting from deterioration of piston ring sealing, or valve guide wear, and is thus an indication of a need for mechanical overhaul. However, unburned fuel can also appear as blue smoke if the droplet size is around 0.5µm.
Black smoke is produced at or near full load, if fuel in excess of the maximum designed value is injected, or if the air intake is restricted. Diesel engines are usually rated according to the maximum horsepower developed at the “smoke limit”. At a certain speed, a definite amount of air enters the cylinder. This amount of air is sufficient to produce complete combustion of a given quantity of fuel, depending on the turbulence in the cylinder. If more fuel is injected, overloading the engine beyond the rated horsepower, there will not be sufficient air for complete combustion and black smoke will result.
Such smoke consists essentially of carbon particles or coagulates of a wide range of sizes, ranging from 0.02µm up to over 1.0µm in diameter. This size distribution will depend on the type of combustion system, which will also affect the onset of smoke emissions as fuel input quantity is increased. In general, direct injection systems show a rather gradual increase in exhaust visibility with increasing fueling, while indirect injection systems tend to have a critical fueling level above which smoke emissions increase very rapidly.
Any variable that increases the amount of fuel injected, reduces the amount of air taken into the cylinder, or impairs the quality of the fuel injection process itself will increase the tendency to produce black exhaust smoke. These include such items as sticking or malfunctioning injectors which result in poor atomization, low injection pressure, improper timing, restricted air flow to the cylinder from clogged air filters, or high altitude of malfunctioning Roots blowers or turbo-chargers.
The fuel variables that can affect black smoke are gravity, viscosity, and cetane number. An engine may smoke when a fuel of lower API gravity is used without readjusting the injection system. This is an over-fueling problem that occurs because injectors meter fuel on a volume basis and low API gravity fuels have more heating value, and therefore, less fuel is required for equal power, equal air utilization, and equal smoke.
Increasing viscosity can also cause overfueling by reducing the leakage in the injector pump, thus allowing more fuel to be injected into the cylinder.
In engines which are sensitive to cetane number, the tendency toward black smoke is greater as cetane number increases. The short delay period of a high cetane number fuel assures that some raw fuel is sprayed into an established flame where the atmosphere is too rich for complete combustion.
In addition to visible smoke, all combustion engines emit particles which are not visible, yet can be trapped on filters. These particles, along with visible smoke, are described as particulates. Diesel engines produce high amounts of these particulates. Because particulates contribute to visible atmospheric haze and because they are suspected of being hazards to health, authorities in various countries have introduced regulations which limit the amount which diesel engines can emit.
Among the factors which contribute to particulates are engine characteristics such as injection timing, in cylinder turbulence, fuel spray from and structure, and crevice volumes within the combustion chamber (which lead to flame quenching and incomplete combustion). Beyond cetane and viscosity effects on smoke, which can be corrected by engine design, fuel properties such as sulfur and aromatics content have been shown to contribute in a fundamental way to the formation of these particulates.
LOW TEMPERATURE OPERABILITY
Engines are expected to start with a minimum of delay, even in the coldest of climates, without recourse to such crude devices as holding a burning rag in front of the air intake. Low temperature operability requires that consideration is given to:
Fuel system design
Normal equipment protection for cold weather operation,
Type of operation,
Use of low improver additives,
The area in which the fuel will be used, and
Any unusual weather of operating conditions.
Was formation has been the major low temperature operability problem plaguing diesel operators for years. All middle distillate fuels contain paraffin wax, a crystalline mixture of straight chain or normal hydrocarbons, melting in the approximate range of 40 to 60C. this paraffin wax occurs naturally in the crude oil from which fuel oils are distilled. The wax content varies greatly depending on the crude oil from which the fuel is produced and on the processing used.
There is a strong relationship between temperature and wax solubility. As fuel cools, a temperature is reached at which the fuel becomes saturated with wax, and the wax begins to precipitate out of solution. The temperature at which the solution is saturated is the cloud point. Cloud point indicates the onset of filter plugging with some fuel system designs and expressed in increment of 1C.
If the fuel is cooled below the cloud point, more wax precipitates. About 3 to 8C below the cloud point (for fuels which do not contain a pour point depressant additive) the fuel becomes so thick it will no longer flow. This point is called for pour point, and is expressed in increments of 3C.
Since diesel-powered equipment is frequently used at temperatures low enough to cause wax to precipitate, a number of techniques have been devised to prevent the wax from causing problems of plugging fuel screens, lines, filters, etc., and preventing fuel flow to the engine. Vehicles designed for low temperature operation are usually equipped with heated fuel tanks, insulated fuel lines, heated fuel filters and other mechanisms to warm the fuel so that the wax does not precipitate.The most common method of preventing wax problem is to dilute heavier, higher wax content fuels with a lighter, lower wax content fuel.
Another problem facing diesel operators is ice. Free water (not dissolved) in the fuel can freeze at low temperatures and the resulting ice crystals can plug fuel filters causing fuel starvation. Free water in vehicle fuel tanks usually comes from the bulk storage tanks, or by condensation or precipitation of dissolved water. Care should be taken to keep fuel storage tanks dry.
Condensation occurs when the air in the fuel tank cools down during a shutdown period. This can be avoided by topping off fuel tanks before shutdown to reduce the volume of the air space above the fuel.
Dissolved water comes out of solution as fuel cools. For example, as fuel cools from 4 to -29C the solubility of water in the fuel reduces by 70 percent. Therefore, fuel pumped from a relatively warm underground tank which may sit in a vehicle overnight in subzero temperatures could precipitate some free water. While this source of free water is almost negligible, and generally not enough to cause a problem, it may be enough to form a visible haze.
Diesel fuels are essentially sterilized by the high temperatures encountered in the refining process. However, they become contaminated soon after leaving the refinery by microorganisms. These microorganisms, primarily bacteria and fungi, exist rather harmlessly in moisture-free fuel, passing through fuel systems without causing any problems.
However, in the presence of water, these microorganisms begin to metabolize (grow and reproduce). The rate of metabolism depends on how well the environment suits the particular microorganism’s needs. The problem is more severe in warm climates.
The growth of a large colony of microorganisms in a fuel system can cause several problems. The first and usually most obvious is fuel filter plugging with a greenish-black or brown slime, frequently accompanied by a foul odor. This slimy, string-like colony can also plug sharp bends in fuel lines, fuel meters and other restrictions. The second problem these microorganisms can cause is corrosion due to the acid byproducts some of them produce. It is also possible, if the microorganisms pass through the fuel filter, that they will form deposits and cause damage in the fuel pumps and injectors.
Growth of these microorganisms can be prevented. Since all metabolic processes of an organism are conducted in an aqueous environment, denying the microorganisms access to water will inhibit growth rates, preventing the development of large, troublesome colonies. Therefore, the first and most important step in prevention is to keep fuel systems dry.
Only when microbial contamination is a recurring problem is a microbiocide needed to chemically sterilize the fuel and/or the water. There are two general classes of biocides fuel-soluble and water-soluble.
Fuel-soluble biocides are best suited for treating fuel which are to pass through several storage steps in the distribution process. A fuel-soluble biocide injected into the fuel early in the distribution system is carried with the fuel through the entire downstream system, effectively sterilizing the fuel until usage.
Water-soluble biocides are more economical for use in treating one step in a fuel distribution system, such as the end-user’s storage tank. The water-soluble biocides, since they are insoluble in fuel, stay where they are placed until the water bottoms are pumped from the tank. Special disposal procedures must be employed with water bottom containing biocides.
The presence of more sulfur and nitrogen and higher molecular weight compounds associated with a higher end point makes diesel fuel more prone to oxidative attack in storage and to thermal degradation in use, than gasoline.
Most fuels are adequately stabilized during the sulfur removal process, which inhibits sediment forming radicals. However, the use of unstable cracked stocks (non hydrofined) in diesel fuel has created the need for stabilizing additives. Marine fuels that are sometimes blended from different stocks at the point of delivery, are more prone to instability, causing sedimentation.
Storage stability is the ability of a fuel to remain in storage over extended periods of time without appreciable deterioration as measured by gum formation and the deposition of insolubles.
Diesel fuel specifications generally include a limit on sediment formation as a control on its storage stability. ASTM D 2274, Oxidation Stability of Distillate Fuel Oil (Accelerated Method) subjects a fuel to an accelerated oxidizing process at an elevated temperature (95C) in the presence of oxygen. ASTM D 4625, Distillate Fuel Storage Stability at 43C (110F) simulates long-terms stability by subjecting the fuel to storage at 43C (110F) for periods of 0-24 weeks (usually about 12 weeks).
Thermal stability is the ability of a fuel to withstand relatively high fuel system temperatures for short periods of time without cocking and fouling fuel lines, filters, or nozzles.
Fuel system deposits are controlled through the use of detergent and disperant type additives.
Commercial diesel fuels may contain a variety of additives that can enhance or impart certain desirable properties. Diesel fuel additives are classified by their function in the accompanying table .
In temperate countries the price of diesel traditionally rises during colder months as demand for heating oil rises, which is refined in much the same way. Because of changes in fuel quality regulations, such as the move towrds ultra-low-sulphur diesel ( ULSD), additional refining is required to remove sulfur, which contributes to a sometimes higher cost. In some countries diesel may be priced higher than petrol.
In some countriestaxes on diesel fuel are higher than on heating oil due to fuel ta, and in those areas, heating oil is marked with dyes and trace chemicals to prevent and detect tax evasion. Similarly, "untaxed" diesel is available in some countries for use primarily in agricultural applications, such as fuel for tractors.This untaxed diesel are also dyed. In India, taxes on diesel fuel are lower than on petrol, as the majority of the transportation for grains and other essential commodities across the country runs on diesel.
In some countries, diesel fuel is taxed lower than petrol (gasoline), but the annual vehicle tax, based on engine displacement, is higher for diesel vehicles than for petrol vehicles. This gives an advantage to vehicles that travel longer distances. Due to a rise in oil prices from about 2009, the advantage point at which using a Diesel vehicle vs a petrol(gasolene) vehicle started to drop, causing more people opting to buy a diesel car where they would have opted for a gasoline car a few years ago. Such an increased interest in diesel has resulted in slow but steady "dieseling" of the automobile fleet in the countries affected, sparking concerns in certain authorities about the harmful effects of diesel pollution.