Hydrocarbon Fuels - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

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Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture36/36_1.htm[6/15/2012 3:09:52 PM] Module8:Engine Fuels and Their Effects on Emissions Lecture 36:Hydrocarbon Fuels and Quality Requirements FUELS AND EFFECTS ON ENGINE EMISSIONS The Lecture Contains: Transport Fuels and Quality Requirements Fuel Hydrocarbons and Other Components Paraffins Cycloparaffins Olefins Aromatics Alcohols and Ethers GENERAL FUEL QUALTY REQUIREMENTS Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture36/36_2.htm[6/15/2012 3:09:52 PM] Module8:Engine Fuels and Their Effects on Emissions Lecture 36:Hydrocarbon Fuels and Quality Requirements TRANSPORT FUELS AND QUALITY REQUIREMENTS Petroleum crude is presently the main source of automotive fuels although alternative fuels like natural gas, LPG are also being used in large numbers of road vehicles in some countries. The renewable fuels like ethyl alcohol and biodiesel too are being used in the form of blends with the petroleum derived gasoline and diesel fuels. Petroleum crude contains over 25000 compounds mainly the hydrocarbons composed of hydrogen and carbon elements. A number of these compounds also contain sulphur in varying amounts and, nitrogen and oxygen in small quantities. The commercial liquid engine fuels are the mixtures of a few hundred different hydrocarbons. Typically, the petroleum transport fuels contain about 85 - 86 percent carbon and 14 – 15 percent hydrogen by mass. Sulphur containing compounds are also present in significant amounts. In this module at first chemical composition and key characteristics of petroleum derived fuels and how they influence engine combustion and emissions are covered. Later, the most promising alternative fuels are also discussed. Fuel Hydrocarbons and Other Components Hydrocarbons are broadly grouped into paraffins, cycloparaffins (naphthenes), olefins and aromatics. The olefins are not present in the crude petroleum but are generated during processing and refining of crude when the practical transport fuels are being produced. Paraffins The paraffins are saturated hydrocarbons which are also known as alkanes. The carbon atoms have single bond between them. The large alkane molecules are either straight chain or branched chain. The empirical formula for the paraffin family is CnH2n+2. Methane (CH4) is the first member of this family, the higher members being ethane (C2H6), propane (C3H8), ……n-heptane, n-octane, isooctane (C8H18), n- hexadecane (n-cetane) and so on. The straight chain paraffins are called normal- (n-) and branched chain are called iso- paraffins. Structures of some paraffins are given below. There are several isomers of isooctane depending upon the position of the branches. The isooctane that is most commonly referred to is 2-2-4 trimethyl pentane having five carbon atoms in straight chain and three methyl groups in 2, 2 and 4 carbon atom positions. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture36/36_3.htm[6/15/2012 3:09:53 PM] radical such as methyl-, ethyl-, propyl- etc. Some examples of aromatics having side chains attached to ring are toluene (methylbenzene), ethyl benzene, xylene (dimethyl benzene) etc. Many aromatic hydrocarbons have two or more aromatic rings such as naphthalene, anthracene, benzo(a)pyrene. These compounds are known as polycyclic aromatic hydrocarbons (PAH). Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture36/36_4.htm[6/15/2012 3:09:53 PM] Module8:Engine Fuels and Their Effects on Emissions Lecture 36:Hydrocarbon Fuels and Quality Requirements Alcohols and Ethers Alcohols as engine fuel have been of interest for a long time. In the hydrocarbons when a hydrogen atom is substituted with hydroxyl radical (OH), alcohols are formed. Among the alcohols, methyl alcohol (CH3OH) and ethyl alcohol (C2H5OH) are considered as alternative fuels. Ethers, such as di- methyl ether (DME) has been investigated as diesel engine fuel and methyl teriary butyl ether (MTBE) is being used as a high octane blending component in gasoline. General Fuel Quality Requirements Many properties of the fuels influence engine performance and emissions. The most important properties of liquid fuels, gasoline and diesel most of which are specified in the fuel quality standards are given in Table 8.1 Table 8.1 Important Characteristics of Liquid Petroleum Fuels Gasoline Diesel Distillation Ignition quality (Cetane index, Cetane number) Reid vapour pressure Distillation (volatility), 90% boiling point Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture36/36_4.htm[6/15/2012 3:09:53 PM] Heat of combustion* Heat of combustion* Density Density Antiknock quality (Research and Motor octane number) Viscosity Oxidation stability Aromatic content Gum content, mg/100ml Sulphur content Lead content Storage stability Sulphur Injection system cleanliness Benzene Sediments Sediments * Heat of combustion is not specified in the fuel quality standards