Molecular weight
The alcohols are named accordingly to the basic molecules of hydrocarbon which derives from them: methanol (CH3OH); ethanol (C2H5OH); propanol (C3H7OH); and butanol (C4H9OH), although menthol and ethanol are the most frequently used as fuel because of their distinguishing properties. Theoretically, any of the organic molecules of the alcohol family can be used as a fuel. The list is somehow more extensive; however, only two of the alcohols are technically and economically suitable as fuels for internal combustion engines. These alcohols are those of the simplest molecular structure, i.e., methanol (CH3OH) and ethanol (C2H5OH) (Table A-11).
Methanol is produced by a variety of process, the most common are as follows: distillation of wood; distillation of coal; natural gas; and crude oil gas. Ethanol is produced mainly from biomass transformation, or bioconversion. It can also be produced by synthesis from crude oil or mineral coal.
In those countries with large territorial areas, ethanol has been the renewable fuel choice to replace gasoline. The reason is the fact that alcohol is a renewable source of energy. Currently, ethanol is produced from sugar beets and from molasses - a typical yield is 15 to 20 gal of ethanol per ton of sugar cane. Other crops can be used for the production of ethanol. Corn, for example, can yield approximately 75 gal liters of alcohol.
Ethanol (ethyl alcohol, CH3CH2OH), also referred to as bioethanol, is a clear, colorless liquid with a characteristic, agreeable odor. Currently, the production of ethanol by fermentation of corn-derived carbohydrates is the main technology used to produce liquid fuels from biomass resources.
Ethanol can be blended with gasoline to create E85, a blend of 85% ethanol and 15% gasoline. Fuel with higher concentrations of ethanol (E95) and pure bioethanol (E100) has been used successfully in Brazil. More widespread practice has been to add up to 20% to gasoline (E20, also called gasohol) to avoid engine changes. E100-fueled and M100-fueled vehicles have difficulty starting in cold weather, but this is not a problem for E85 and M85 vehicles because of the presence of gasoline.
Ethanol has a higher octane number (108), broader flammability limit, higher flame speed, and a higher heat of vaporization than gasoline. These properties allow for a higher compression ratio, shorter burn time, and leaner burn engine, which lead to theoretical efficiency advantages over gasoline in an internal combustion engine. On the other hand, the disadvantages of ethanol include its lower energy density than gasoline, corrosiveness, low flame luminosity, lower vapor pressure, miscibility with water, and toxicity to ecosystems.
The alcohols mix in all proportions with water due to the polar nature of the hydroxyl (OH) group. Low volatility is indicated by high boiling point and high flash point. Alcohols burn with no luminous flame, and methanol produces almost no soot, but the tendency to produce soot increases with molecular weight.
See also: Alcohols – Production, Ethanol, Methanol.
Alcohols – Combustion
There are some important differences in the combustion characteristics of alcohols and hydrocarbon derivatives. Alcohols have higher flame speeds and extended flammability limits. Also, alcohols produce a great number of product moles per mole of fuel burnt; therefore, higher pressure is achieved.
The alcohols mix in all proportions with water due to the polar nature of OH group. Low volatility is indicated by high boiling point and high flash point. Alcohols burn with no luminous flame and produce almost no soot, especially methanol. The tendency to soot formation increases with molecular weight.
Combustion of alcohol in presence of air can be initiated by an intensive source of localized energy, such as a flame or a spark, and also, the mixture can be ignited by application of energy by means of heat and pressure, such as happens in the compression stroke of a piston engine. The energy of the mixture reaches a level
