Vehicular Fuels

Motor Fuels

Fuels consists primarily of hydrocarbon compounds, Paraffin, Olefin, Naphthene & Aromatics and contains additives that are determined by the specific use of the fuel. Non hydrocarbon compounds such as sulfur & sulfur compounds are also found.

Crude Petroleum Oil

Crude oil is a mixture of an almost infinite number of HC compounds, ranging from light gases of simple chemical structure to heavy tar like liquids and waxes of complex chemical structure.The oil as it comes from the ground also contains various amounts of Sulfur, oxygen, Nitrogen, Sand & WaterThe percentage of Carbon varying generally from 83% to 87% & that of Hydrogen from 11% to 14%.

Refining Crude

Separation process exploits the fact that various components in crude oil have different boiling points. When crude oil is heated, following are produced.

1- gases evolved (methane, ethane, propane & butane)

2- Vapors are released that condensed to form light distillate

3- Gasoline

4- Kerosene

5- Middle distillate

6- Residue (lub.oil, wax, bitumen)

Refining Processes

Some of the refining processes are:

Distillation (Separation process)

Cracking (Conversion Process)

Polymerization (Conversion Process)

Alkylation (Conversion Process)

Isomerization (Conversion Process)

Refining Processes

Cracking:: Cracking is the process of breaking large molecular components into more useful components of smaller molecular weight

Distillation: Distillation is used to separate the mixtures into single components or smaller ranges of components. Generally, the larger the molecular weight of a component, the higher is its boiling temperature. Low boiling temperature components (smaller molecular weights) are used for solvents and fuels (gasoline), while high boiling temperature components with their large molecular weights are used for tar and asphalt or returned to the refining process for further cracking.

Fuel (Gasoline) requirement for SI Engine

A small percentage of components that vaporize (boil) at low temperature is needed to assure the starting of a cold engine; fuel must vaporize before it can burn. How ever, too much of this front-end volatility can cause problems when the fuel vaporizes too quickly. Volumetric efficiency of the engine will be reduced if fuel vapor replaces air too early in the intake system. Another serious problem this can cause is vapor lock, which occurs when fuel vaporizes in the fuel supply lines or in the carburetor in the hot engine compartment. When this happens, the supply of fuel is cut off and the engine stops. A large percent of fuel should be vaporized at the normal intake system temperature during the short time of the intake process. To maximize volumetric efficiency, some of the fuel should not vaporize until late into the compression stroke and even into the start of combustion. This is why some high-molecular-weight components are included in gasoline mixtures. If too much of this high-end volatility is included in the gasoline, however, some of the fuel never gets vaporized and ends up as exhaust pollution or condenses on the cylinder walls and dilutes the lubricating oil.

Average Distillation Ranges

(Combustion-Engine Fuels)

Gasoline-Description

One way that is sometimes used to describe a gasoline is to use three temperatures: the temperature at which 10% is vaporized, at which 50% is vaporized, and at which 90% is vaporized. The gasoline in Fig. could therefore be classified as 57-81-103°C.

Chemical Structure The Carbon & Hydrogen combine in different proportions to form a variety of hydrocarbons

The Carbon to Hydrogen ratio and their ratio of bonding determine the energy characteristics of hydrocarbon fuel and affect the combustion processes

Depending upon the number of Carbon & Hydrogen the products are classified in to different groups.

A saturated hydrocarbon molecule will have no double or triple carbon-to carbon bonds and will have a maximum number of hydrogen atoms.

An unsaturated molecule will have double or triple carbon-to-carbon bonds.

Basic Families of Hydrocarbons

Family General Formula

Molecular Structure

Saturated/

Unsaturated

Stability

Paraffin (Alkanes)

CnH2n+2 Chain Saturated Stable

Olefin CnH2n Chain Unsaturated Unstable

Naphthene CnH2n Ring Saturated Stable

Aromatic CnH2n-6 Ring Un Saturated More Stable, than other unsaturated compounds

The paraffin family (alkane) in turn can be subdivided into normal parrafin hydrocarbons, which have a straight or open-chain structure with one bond between each atom such as heptane and isomers, which have the same number of C and H atoms and the same molecular mass but a different structure, such as the three isomers of normal heptane. More complicated paraffin hydrocarbons may have a greater number of isomers. The difference in the structure of the molecules results in different physical properties and reaction characteristics.

HYDROCARBON FUELS

Paraffins CnH2n+2

Heptane,, CC77HH1616

H H H H H H H | | | | | | |

H—C—C—C—C—C—C—C—H

| | | | | | | H H H H H H H

Paraffins

Isomer of normal heptane

H H H

H C H H H H | | | | | |

2-Methyl Hexane, H—C—C—C—C—C—C—H

| | | | | | H H H H H H

The number indicates the position of the carbon atom to which the methyl group is attached.

Paraffins

Isomer of normal heptane

H H H

H C H H H | | | | |

2,2 Dimethyl Pentane, H—C—C—C—C—C—H

| | | | | H C H H H

H H H

Paraffins

Paraffins

The olefins (alkenes), CnH2n, are also straight or open-chain hydrocarbons but have two atoms of hydrogen fewer per molecule with one carbon-carbon double bond.

H H H H H H H H | | | | | | | | C8H16 H—C—C—C—C=C—C—C—C—H

| | | | | | H H H H H H

The open-chain hydrocarbons, paraffins and olefins, are classed jointly as aliphatics.

Olefins (CnH2n ) suffix-ene

Olefins (CnH2n ) suffix-ene

The diolefins, CnH2n-2, are unsaturated hydrocarbons with two double-bonded molecules.

H H H H | | | |

1,5-Hexadien, C6H10 C=C—C—C—C=C | | | | | |

H H H H H H

The olefins and diolefins are unsaturated compounds and therefore unstable; the carbon atoms with a double bond can attach themselves to and absorb additional atoms of hydrogen, oxygen, or sulphur if such are present.

Diolefins (CnH2n-2 ) suffix-diene

Diolefins (CnH2n-2 ) suffix-diene

The naphthenes (Cycloparaffins/cycloalkanes), have the same chemical formula, CnH2n, but are closed-chain hydrocarbons and there are no double bonds.

H H H H H H | |

H—C—C—C—C—H | |H—C—C—C—C—H | |

H H H H H H

Cyclo-Octane, C8H16

Cycloparaffins (CnH2n )

Cycloparaffins (CnH2n )

The alkynes, CnH2n-2, are unsaturated hydrocarbons with a triple carbon-carbon bond. The best known member of the family is acetylene (C2H2).

H— C C —H

Alkynes (CnH2n-2 )

The aromatics, CnH2n-6, are hydrocarbons with carbon-carbon double-bonds internal to a ring structure. The most common aromatic is benzene.

The double bonds alternate in position between the carbon atoms. This makes the molecule hard to break and, as a result, aromatics are desirable in gasoline since they increase the octane number. Aromatics are undesirable components of diesel fuels.

Aromatics (CnH2n-6 )

Aromatics (CnH2n-6 )

Aromatics generally make good gasoline fuel components, with some exceptions due to exhaust pollution. They have high densities in the liquid state and thus have high energy content per unit volume. Aromatics have high solvency characteristics, and care must be used in material selection for the fuel delivery system (e.g., they will dissolve or swell some gasket materials). Aromatics will dissolve a greater amount of water than other hydrocarbons and thus can create fuel line freezing problems when the temperature is lowered and some of the water comes out of solution. Aromatic make poor CI engine fuel.

At atmospheric conditions, hydrocarbon molecules with a low number of carbon atoms, 1 to 4, are gases. Hydrocarbons with 5 to 15 carbon atoms are more or less volatile light oils, and those with 16 to 26 carbon atoms are referred to as heavy oils.

Commercial fuels and lubricating oils are mixtures of many kinds of hydrocarbons in various proportions.

Another group of hydrocarbons consists of methyl, CH4O,

ethyl, C2H6O, and butyl, C4H9OH, alcohols. These are not

true hydrocarbons, since each contains oxygen in the molecule.

HYDROCARBON FUELS: General Remarks

Alcohols

Alcohols are similar to paraffins with one of the hydrogen atoms replacedWith hydroxyl radical OH. The most common alcohols are:

Nitromethane (CH3NO2) is formed from a paraffinic hydrocarbon by replacing a hydrogen atom with a NO2 group. It has twice the bound oxygen and can combust without air. At ambient temperature, it is a liquid, and is widely used as a drag racing fuel.

H |

H—C—NO2 | H

Nitromethane (CH3NO2)

Family PROPERTIES

Paraffin

(Alkanes)

The alkanes are stable in storage, clean burning & don't attack the usual gaskets or metals. Since they have the maximum possible amount of hydrogen, so they have the highest heating values per unit mass of hydrogen & due to lowest densities having lowest heating values per unit volume. The paraffins are practically insoluble in water but are soluble in other hydrocarbons. (High H---- high SE) & Storage Stability

Olefin

(Alkenes)

Because of the free bond, the olefins are chemically active. They may unite with oxygen to form an undesirable residue, gumSpecification limit the olefin contents in jet fuel. Actual olefin concentration are about 1 to 3 %--- Storage Instability

Naphthene

(Cyclanes)

In general, paraffins & Naphthenes comprise from 75% to 95% of most jet fuel.--- Storage Stability

Aromatics Aromatics have the highest densities of the hydrocarbons and therefore have the highest heating value per unit volume. And the lowest per unit mass. Aromatics are stable in storage, smoky in burning. Jet specification limit the aromatic contents. The aromatics may dissolve as much as 6 gal water per 100,000 gal fuel, and when the fuel temp. decreases, the water comes out of solution leading to troubles such as freezing. Concentration of aromatics is limited to 25%.- Attack seals & diaphragms--- (High C---- Low SE)

Summary

Fuel Additives Compounds are added to the fuel in very small amount to provide special or

improved quantities. These are1– Anti-knock additives2– Anti-oxidants (to prevent the formation of gums, caused by oxidation of fuel)3– Static dissipater: (reduce the hazardous effects of static electricity generated

by movement of fuel through modern high flow rate fuel transfer system.4– Corrosion Inhibitors: 5—Anti-Icing additives: ( To reduce the freezing point of water precipitated from

fuels due to cooling at high altitudes and prevent the formation of ice crystals which restrict the flow of fuel to the engine.

6– Metal de-activators To suppress the catalytic effect which some metals, particularly copper , have on fuel oxidation.

7– Biocide additives: ( to combat microbiological growths in fuel)8– Thermal Stability Improvers: ( to Inhibit deposit formation in the high

temperature areas of the aircraft fuel system.

Fuel PropertiesRelative density (defines the energy contents in fuel tank)

Vapor pressure

Flashpoint ( lowest temp at which liquid gives off enough to form a flammable mixture)

Volatility point (can be assessed by, flash point, dist. range &vap. pres)

Viscosity

Surface Tension

Freezing point

Specific heat

Calorific value

SIT

Flammability limit

Properties of Fuels

Properties of FuelsProperties/Fuel Propane Nat. Gas Hydrogen Methanol Ethanol Gasoline

Molecular wt. 44.10 18.10 2.015 32.04 46.07 ~110Vapor pressure (KPa) at 38•C

32 17 62~90

Boiling Point ( ̊C) at 1bar -160 -253 65 78 30~225

Enthalpy of vaporization

hfg (KJ/kg)at 298K1215 850 310

Lower heating Value,

mass (MJ/Kgfuel)46.4 50.0 120 19.9 26.8 44.5

Lower heating Value,

volume (MJ/Lfuel)8.1 at 15C, 22MPa

15.7 21.1 32.9

Lower heating Value,

Stoichiometric (MJ/Kgfuel)2.98 2.92 3.52 3.09 3.00 2.96

O.N (research) 112 120 106 112 111 90~98O.N (motor) 97 120 91 92 80~90St. A/F ratio (mass) 15.58 17.12 34.13 6.43 8.94 15.04Vapor Flammability Limit (% Vol.)

5.3~15 5~75 5.5~26 3.5~26 0.6~8

Adiabatic Flame Temp. (K)

2268 2227 2383 2151 2197 2266

Routine Fuel Tests (lab)API-Gravity & Specific GravityVapor Pressure (D323)Flash & Fire point ASTM Distillations (D86)Color- Jet fuel grades vary from water white to light yellowViscosityCloud & Pour PointSulfur Test (D90)Gum Test (D381, D525)Smoke point (D1322)Freezing Point (D2386Doctors test

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