Diesel Fuels
Diesel fuel is any fuel used in diesel engines. Chemical composition and cetane number. Boiling point and freezing point of representative diesel fuel hydrocarbons. Disadvantages of Diesel Fuel. Environment hazards of sulfur. Fuel value and price.
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Ministry of Education and Science of Ukraine
National Aviation University
Chemmotology Department
Individual work
On topic: “Diesel fuels”
Created by
Student of 206 group FLA
Novak Sergey
Kiev 2009
Contents
- Diesel fuel
- Chemical composition
- Boiling point and freezing point of representative diesel fuel hydrocarbons
- Cetane Number
- Reduction of sulfur emissions
- Refining
- Petroleum diesel
- Diesel engine
- Disadvantages of Diesel Fuel
- Environment hazards of sulfur
- Road hazard
- Synthetic diesel
- Biodiesel
- Transportation
- Use as car fuel
- Railroad
- Aircraft
- Other uses
- Fuel value and price
- Reference
Diesel fuel
Diesel fuel in general is any fuel used in diesel engines. The most common is a specific fractional distillate of petroleum fuel oil, but alternatives that are not derived from petroleum, such as biodiesel, biomass to liquid (BTL) or gas to liquid (GTL) diesel, are increasingly being developed and adopted. To distinguish these types, petroleum-derived diesel is increasingly called petrodiesel. Ultra-low sulfur diesel (ULSD) is a standard for defining diesel fuel with substantially lowered sulfur contents. As of 2007, almost every diesel fuel available in America and Europe is the ULSD type. In the UK, diesel is commonly abbreviated DERV, standing for Diesel Engined Road Vehicle (fuel).
Chemical composition
Petroleum-derived diesel is composed of about 75% saturated hydrocarbons (primarily paraffins including n, iso, and cycloparaffins), and 25% aromatic hydrocarbons (including naphthalenes and alkylbenzenes). The average chemical formula for common diesel fuel is C12H23, ranging approximately from C10H20 to C15H28.
Diesel fuel is a very complex mixture of thousands of individual compounds, most with carbon numbers between 10 and 22. Most of these compounds are members of the paraffinic, naphthenic, or aromatic class of hydrocarbons; each class has different chemical and physical properties. Different relative proportions of the three classes is one of the factors that make one diesel fuel different from another. The following discussion explains how properties of the three classes influence the properties of the whole fuel and affect its performance in a diesel engine.
diesel fuel chemical composition
Boiling point and freezing point of representative diesel fuel hydrocarbons
Boiling Points
For compounds in the same class, boiling point increases with carbon number. Forcompounds of the same carbon number, the order of increasing boiling point by class is isoparaffin, n-paraffin, naphthene, and aromatic. The boiling point difference (60° to80°C or 100° to 150°F) between isoparaffins and aromatics of the same carbon number is larger than the boiling point difference (about 20°C or 35°F) between compounds of the same class that differ by one carbon number. Thus, the compounds that boil at about 260°C (500°F), the middle of the diesel fuel boiling range, might be C12 aromatics, C13 naphthenes, C14 n-paraffin, and C15 isoparaffins.
Freezing Point
Freezing points (melting points) also increase with molecular weight, but they are strongly influenced by molecular shape. Molecules that fit more easily into a crystal structure have higher freezing points than other molecules. This explains the high melting points of n-paraffins and unsubstituted aromatics, compared to the melting points of isoparaffins and naphthenes of the same carbon number.
Compound Chemical Hydrocarbon Boiling Freezing Formula Class Point, °C/°F Point, ЦC/°F
|
Naphthalene |
C10H8 |
Aromatic |
218/424 |
80/176 |
|
|
Tetralin |
C10H12 |
Aromatic |
208/406 |
-35/-31 |
|
|
cis-Decalin |
C10H18 |
Naphthene |
196/385 |
-43/-45 |
|
|
1,3-Diethylbenzene |
C10H14 |
Aromatic |
181/358 |
-84/-119 |
|
|
n-Butylcyclohexane |
C10H20 |
Naphthene |
181/358 |
-75/-103 |
|
|
n-Pentylcyclopentane |
C10H20 |
Naphthene |
181/358 |
-83/-117 |
|
|
Decane |
C10H22 |
n-Paraffin |
174/345 |
-30/-22 |
|
|
Anthracene |
C14H10 |
Aromatic |
341/646 |
215/419 |
|
|
1-Pentylnaphthalene |
C15H18 |
Aromatic |
306/583 |
-24/-11 |
|
|
n-Nonylcyclohexane |
C15H30 |
Naphthene |
282/540 |
-10/14 |
|
|
n-Decylcyclopentane |
C15H30 |
Naphthene |
279/534 |
-22/-8 |
|
|
n-Pentadecane |
C15H32 |
n-Paraffin |
271/520 |
10/50 |
|
|
2-Methyltetradecane |
C15H32 |
Isoparaffin |
265/509 |
-8/18 |
|
|
1-Decylnaphthalene |
C20H28 |
Aromatic |
379/714 |
15/59 |
|
|
n-Tetradecylbenzene |
C20H34 |
Aromatic |
354/669 |
16/61 |
|
|
n-Tetradecylcyclohexane |
C20H40 |
Naphthene |
354/669 |
25/77 |
|
|
n-Pentadecylcyclopentane |
C20H40 |
Naphthene |
353/667 |
17/63 |
|
|
Eicosane |
C20H42 |
n-Paraffin |
344/651 |
36/97 |
|
|
2-Methylnonadecane |
C20H42 |
Isoparaffin |
339/642 |
18/64 |
Density
Table lists density and heat of combustion (heating value) for some representative diesel fuel hydrocarbons. For compounds of the same class, density increases with carbon number. For compounds with the same carbon number, the order of increasing density is paraffin, naphthene, and aromatic.
Net Heat of Net Heat of Hydrocarbon Carbon Density, Combustion, Combustion, Compound Class Number 20°C, g/cm3 25ЙC, kJ/kg 25ЙC, Btu/gal
|
Naphthalene |
Aromatic |
10 |
1.175 |
38,854 |
163,800 |
|
|
Tetralin |
Aromatic |
10 |
0.9695 |
40,524 |
140,960 |
|
|
1,3-Diethylbenzene |
Aromatic |
10 |
0.8639 |
41,384 |
128,270 |
|
|
n-Butylcyclohexane |
Naphthene |
10 |
0.7992 |
43,717 |
124,500 |
|
|
n-Pentylcyclopentane |
Naphthene |
10 |
0.7912 |
43,585 |
123,720 |
|
|
Decane |
n-Paraffin |
10 |
0.7301 |
44,236 |
115,880 |
|
|
2,2-Dimethyloctane |
Isoparaffin |
10 |
0.7245 |
44,145 |
114,750 |
|
|
Anthracene |
Aromatic |
14 |
1.251 |
38,412 |
172,410 |
|
|
n-Nonylbenzene |
Aromatic |
15 |
0.8558 |
42,147 |
129,410 |
|
|
n-Nonylcyclohexane |
Naphthene |
15 |
0.816 |
43,431 |
127,150 |
|
|
n-Decylcyclopentane |
Naphthene |
15 |
0.811 |
43,545 |
126,710 |
|
|
n-Pentadecane |
n-Paraffin |
15 |
0.7684 |
43,980 |
121,250 |
|
|
n-Tetradecylbenzene |
Aromatic |
20 |
0.8549 |
42,482 |
130,310 |
|
|
n-Tetradecylcyclohexane |
Naphthene |
20 |
0.825 |
43,445 |
128,590 |
|
|
n-Pentadecylcyclopentane |
Naphthene |
20 |
0.8213 |
43,524 |
128,260 |
|
|
Eicosane |
n-Paraffin |
20 |
0.7843 |
43,852 |
123,400 |
Cetane Number
Cetane number also varies systematically with hydrocarbon structure. Normal paraffins have high cetane numbers that increase with molecular weight. Isoparaffins have a wide range of cetane numbers, from about 10 to 80. Molecules with many short side chains have low cetane numbers; whereas those with one side chain of four or more carbons have high cetane numbers. Naphthenes generally have cetane numbers from 40 to 70. Higher molecular weight molecules with one long side chain have high cetane numbers; lower molecular weight molecules with short side chains have low cetane numbers. Aromatics have cetane numbers ranging from zero to 60. A molecule with a single aromatic ring with a long side chain will be in the upper part of this range; a molecule with a single ring with several short side chains will be in the lower part. Molecules with two or three aromatic rings fused together have cetane numbers below 20.
Reduction of sulfur emissions
In the past, diesel fuel contained higher quantities of sulfur. European emission standards and preferential taxation have forced oil refineries to dramatically reduce the level of sulfur in diesel fuels. In the United States, more stringent emission standards have been adopted with the transition to ULSD starting in 2006 and becoming mandatory on June 1, 2010 (see also diesel exhaust). U. S. diesel fuel typically also has a lower cetane number (a measure of ignition quality) than European diesel, resulting in worse cold weather performance and some increase in emissions.
Refining
Petroleum diesel, also called petrodiesel, or fossil diesel is produced from the fractional distillation of crude oil between 200°C (392°F) and 350°C (662°F) at atmospheric pressure, resulting in a mixture of carbon chains that typically contain between 8 and 21 carbon atoms per molecule.
Petroleum diesel
Petroleum diesel, also called petrodiesel, or fossil diesel is produced from the fractional distillation of crude oil between 200°C (392°F) and 350°C (662°F) at atmospheric pressure, resulting in a mixture of carbon chains that typically contain between 8 and 21 carbon atoms per molecule.
Diesel engine
Diesel engines have long been the workhorse of industry. Favored for their high torque output, durability, exceptional fuel economyand ability to provide power under a wide range of conditions, diesels are the dominant engines used in applications such as trucking, construction, farming, and mining. They are also extensively used for stationary power generation and marine propulsion and in passenger vehicles in many regions of the world.
Diesel engines are not used widely in light-duty vehicles in the United States primarily because they do not meet U. S. emissions standards. However, because of significant improvements in diesel engine performance, injection technology, and exhaust aftertreatment devices, particulate matter and nitrogen oxides emissions have been reduced such that diesels are poised to achieve future emissions standards.
Diesel engines are similar to gasoline engines in many ways. Both are internal combustion engines and most versions of them use a four-stroke cycle. There are four fundamental differences:
The conventional gasoline engine injects fuel into the air as it is drawn into a cylinder. The diesel engine draws air into a cylinder and injects fuel after the air has been compressed. For a discussion about the Direct Injection Spark Ignition engine, please see the companion publication Motor Gasoline Technical Review.
The gasoline engine ignites the fuel-air mixture with a spark. The diesel engine relies on high temperature alone for ignition. Diesel engines are often referred to as compressionignition engines because this high temperature is the result of compressing air above the piston as it travels upward.
The power output of a gasoline engine is controlled by a throttle, which varies the amount of fuel-air mixture drawn into a cylinder. A diesel engine does not throttle the intake air. It controls the power output by varying the amount of fuel injected into the air, thereby, varying the fuel-air ratio. This is one of the primary reasons that diesel engines are more fuel efficient than spark-ignition gasoline engines.
A conventional gasoline engine runs stoichiometrically - the fuel-air ratio is fixed so that there is just enough air to burn all the fuel. A diesel engine runs lean - there is always more air than is needed to burn the fuel. The main advantage of a diesel engine is its high thermal efficiency.2 Diesel engines can achieve thermal efficiencies in excess of 50 percent. The best conventional gasoline engines are approximately from 30 to 33 percent efficient, and then only at wide throttle openings. As a result, diesel engines have better fuel economy than gasoline engines.
Disadvantages of Diesel Fuel
Diesel engines have no use for glow plugs for ignition, unlike gasoline engines, because the fuel is pumped directly into the cylinder, causing it to react (burn) when it encouters oxygen, thus producing power.
Yet a major drawback of this occurs in the winter. Diesel fuel viscosity increases when the temperature decreases, usually between - 15 degrees celsius (5 degree F) and - 19 degrees celsius (-2.2 degrees F). This was a major problem on older diesel engines, and made engines very difficult to start in the colder months of the year, but this is usually counteracted by plugging in an engine heater, or a block heater.
Another problem is the rare runaway failure. Since diesel engines do not require a spark to achieve ignition, they can sustain power as long as diesel fuel is supplied to the cylinder. Fuel is typically supplied via a fuel pump, and if the pump gets stuck in the "open" position, the supply of fuel cannot be regulated, and the engine will "runaway", incapable of being shut down.
Modifications to engine design, and improvements to fuel pump construction has also limited the likelihood of an engine runaway.
Environment hazards of sulfur
High levels of sulfur in diesel are harmful for the environment because they prevent the use of catalytic diesel particulate filters to control diesel particulate emissions, as well as more advanced technologies, such as nitrogen oxide (NOx) adsorbers (still under development), to reduce emissions. Moreover, sulfur in the fuel is oxidized during combustion, producing sulfur dioxide and sulfur trioxide, that in presence of water rapidly convert to sulfuric acid, one of the chemical processes that results in acid rain. However, the process for lowering sulfur also reduces the lubricity of the fuel, meaning that additives must be put into the fuel to help lubricate engines. Biodiesel and biodiesel/petrodiesel blends, with their higher lubricity levels, are increasingly being utilized as an alternative. The U. S. annual consumption of diesel fuel in 2006 was about 190 billion litres (42 billion imperial gallons or 50 billion US gallons).
Road hazard
Petrodiesel spilled on a road will stay there until washed away by sufficiently heavy rain, whereas gasoline will quickly evaporate. After the light fractions have evaporated, a greasy slick is left on the road which can destabilize moving vehicles. Diesel spills severely reduce tire grip and traction, and have been implicated in many accidents. The loss of traction is similar to that encountered on black ice. Diesel slicks are especially dangerous for two-wheeled vehicles such as motorcycles.
Synthetic diesel
Wood, hemp, straw, corn, garbage, food scraps, and sewage-sludge may be dried and gasified to synthesis gas. After purification the Fischer-Tropsch process is used to produce synthetic diesel. This means that synthetic diesel oil may be one route to biomass based diesel oil. Such processes are often called biomass-to-liquids or BTL.
Synthetic diesel may also be produced out of natural gas in the gas-to-liquid (GTL) process or out of coal in the coal-to-liquid (CTL) process. Such synthetic diesel has 30% lower particulate emissions than conventional diesel (US - California).
Biodiesel
Biodiesel can be obtained from vegetable oil (vegidiesel/vegifuel), or animal fats (bio-lipids), using transesterification. Biodiesel is a non-fossil fuel, cleaner burning alternative to petrodiesel. It can also be mixed with petrodiesel in any amount in some modern engines, but some manufacturers strongly recommend against such use. Biodiesel has a higher gel point than petrodiesel, but is comparable to diesel. This can be overcome by using a biodiesel/petrodiesel blend, or by installing a fuel heater, but this is only necessary during the colder months. A small fraction of biodiesel can be used as an additive in low-sulfur formulations of diesel to increase the lubricity lost when the sulfur is removed. In the event of fuel spills, biodiesel is easily washed away with ordinary water and is nontoxic compared to other fuels.
Biodiesel can be produced using kits. Certain kits allow for processing of used vegetable oil that can be run in any conventional diesel motor with modifications. The necessary modification is the replacement of fuel lines from the intake and motor and all affected rubber fittings in injection and feeding pumps a. s. o (in vehicles manufactured before 1993). This is because biodiesel is an effective solvent and will replace softeners within unsuitable rubber with itself over time. Synthetic gaskets for fittings and hoses prevent this.
Chemically, most biodiesel consists of alkyl (usually methyl) esters instead of the alkanes and aromatic hydrocarbons of petroleum derived diesel. However, biodiesel has combustion properties very similar to petrodiesel, including combustion energy and cetane ratings. Paraffin biodiesel also exists. Due to the purity of the source, it has a higher quality than petrodiesel does.
Transportation
Diesel fuel is widely used in most types of transportation. The gasoline-powered passenger automobile is the major exception.
Unlike petroleum ether and liquefied petroleum gas engines, diesel engines do not use high voltage spark ignition (spark plugs). An engine running on diesel compresses the air inside the cylinder to high pressures and temperatures (compression ratios from 15: 1 to 21: 1 are common); the diesel is generally injected directly into the cylinder near the end of the compression stroke. The high temperatures inside the cylinder cause the diesel fuel to react with the oxygen in the mix (burn or oxidize), heating and expanding the burning mixture in order to convert the thermal/pressure difference into mechanical work; i. e., to move the piston. (Glow plugs are used to assist starting the engine to preheat cylinders to reach a minimum operating temperature.) High compression ratios and throttleless operation generally result in diesel engines being more efficient than many spark-ignited engines.
This efficiency and its lower flammability and explosivity than gasoline are the main reasons for military use of diesel in armoured fighting vehicles like tanks and trucks. Engines running on diesel also provide more torque and are less likely to stall as they are controlled by a mechanical or electronic governor.
A disadvantage of diesel as a vehicle fuel in some climates, compared to gasoline or other petroleum derived fuels, is that its viscosity increases quickly as the fuel's temperature decreases, turning into a non-flowing gel at temperatures as high as - 19°C (-2.2°F) or - 15°C (5°F), which can't be pumped by regular fuel pumps. Special low temperature diesel contains additives that keep it in a more liquid state at lower temperatures, yet starting a diesel engine in very cold weather may still pose considerable difficulties.
Another rare disadvantage of diesel engines compared to petrol/gasoline engines is the possibility of runaway failure. Since diesel engines do not require spark ignition, they can sustain operation as long as diesel fuel is supplied. Fuel is typically supplied via a fuel pump. If the pump breaks down in an "open" position, the supply of fuel will be unrestricted and the engine will runaway and risk terminal failure.
Use as car fuel
Diesel-powered cars generally have a better fuel economy than equivalent gasoline engines and produce less greenhouse gas emission. Their greater economy is due to the higher energy per-litre content of diesel fuel and the intrinsic efficiency of the diesel engine. While petrodiesel's higher density results in higher greenhouse gas emissions per litre compared to gasoline, the 20-40% better fuel economy achieved by modern diesel-engined automobiles offsets the higher per-litre emissions of greenhouse gases, and a diesel-powered vehicle emits 10-20 percent less greenhouse gas than comparable gasoline vehicles. Biodiesel-powered diesel engines offer substantially improved emission reductions compared to petro-diesel or gasoline-powered engines, while retaining most of the fuel economy advantages over conventional gasoline-powered automobiles. However, the increased compression ratios mean that there are increased emissions of oxides of nitrogen (NOx) from diesel engines. This is compounded by biological nitrogen in biodiesel to make NOx emissions the main drawback of diesel versus gasoline engines.
Railroad
Diesel displaced coal and fuel oil for steam power vehicles in the latter half of the 20th century, and is now used almost exclusively for combustion engine of self-powered rail vehicles (locomotives and railcars).
Aircraft
The first diesel-powered flight of a fixed wing aircraft took place on the evening of September 18, 1928, at the Packard Motor Company proving grounds at Utica, Michigan, with Captain Lionel M. Woolson and Walter Lees at the controls (the first "official" test flight was taken the next morning). The engine was designed for Packard by Woolson and the aircraft was a Stinson SM1B, X7654. Later that year, Charles Lindbergh flew the same aircraft. In 1929 it was flown 621 miles (999 km) non-stop from Detroit to Langley, Virginia (near Washington, D. C.). This aircraft is now owned by Greg Herrick and is at the Golden Wings Flying Museum nearby Minneapolis, Minnesota. In 1931, Walter Lees and Fredrick Brossy set the non-stop flight record flying a Bellanca powered by a Packard diesel for 84 hours and 32 minutes. The Hindenburg rigid airship was powered by four 16-cylinder diesel engines, each with approximately 1, 200 horsepower (890 kW) available in bursts, and 850 horsepower (630 kW) available for cruising. Modern diesel engines for propellor-driven aircraft are manufactured by Thielert Aircraft Engines and SMA. These engines can run on Jet A fuel, which is similar in composition to automotive diesel and cheaper and more plentiful than the 100 octane low-lead gasoline (avgas) used by the majority of the piston-engine aircraft fleet.
The most-produced aviation diesel engine in history has been the Junkers Jumo 205, which, along with its similar developments from the Junkers Motorenwerke, had approximately 1000 examples of the unique opposed piston, two-stroke design power plant built in the 1930s leading into World War II in Germany.
Other uses
Poor quality (high sulfur) diesel fuel has been used as a palladium extraction agent for the liquid-liquid extraction of this metal from nitric acid mixtures. Such use has been proposed as a means of separating the fission product palladium from PUREX raffinate which comes from used nuclear fuel. In this system of solvent extraction, the hydrocarbons of the diesel act as the diluent while the dialkyl sulfides act as the extractant. This extraction operates by a solvation mechanism. So far, neither a pilot plant nor full scale plant has been constructed to recover palladium, rhodium or ruthenium from nuclear wastes created by the use of nuclear fuel.
Fuel value and price
The density of petroleum diesel is about 0.85 kg/l (7.09 lb/US gal), about 18% more than petrol (gasoline), which has a density of about 0.72 kg/l (6.01 lb/US gal). When burnt, diesel typically releases about 38.6 MJ/l (138,700 BTU/US gal), whereas gasoline releases 34.9 MJ/l (125,000 BTU/US gal), 10% less by energy density, but 45.41 MJ/kg and 48.47 MJ/kg, 6.7% more by specific energy. Diesel is generally simpler to refine from petroleum than gasoline. 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 recent changes in fuel quality regulations, additional refining is required to remove sulfur which contributes to a sometimes higher cost. In many parts of the United States and throughout the United Kingdom and Australia diesel may be higher priced than petrol. Reasons for higher priced diesel include the shutdown of some refineries in the Gulf of Mexico, diversion of mass refining capacity to gasoline production, and a recent transfer to ULSD, which causes infrastructural complications.
Reference
1. “Diesel Fuels Technical Review”. John Bacha, John Freel, Andy Gibbs. © 2007 Chevron Corporation.
2. http://suite101.com
3. http://wikipedia.org
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