Alcohol as an Automotive Fuel
Alcohol has been used as a fuel for internal combustion engines since their invention. Reports on the use of alcohol as a motor fuel were published in 1907 and detailed research was conducted in the 1920s and 1930s. Historically, the level of interest in using alcohol as a motor fuel has followed cycles of fuel shortages and/or low feed-grain prices.
The properties of methyl, ethyl and butyl alcohol are compared with octane (high quality gasoline) and hexadecane (high quality diesel fuel) in Table 2. Note that octane and hexadecane (petroleum fuels) have higher boiling points, lower latent heats and are insoluble in water. The alcohols become more like petroleum fuels as their chemical weights increase.
Methyl alcohol has the lowest
combustion energy of all the fuels listed. However, it also has the lowest
stoichiometric or "chemically correct" air-fuel ratio. Therefore, an
Advantages of mixing alcohol with gasoline are that alcohol tends to increase the octane rating and reduce carbon monoxide emissions. However, alcohols may corrode certain materials used in engines.
Blending Alcohol and Gasoline
Mixing alcohol with gasoline produces gasohol. Advantages of fuel blends are that alcohol tends to increase the octane rating, which is particularly important in unleaded fuel, and reduce carbon monoxide (CO) emissions from the engine.
The primary disadvantage of mixing methanol and ethanol with gasoline is that under certain conditions these alcohols may separate from the gasoline. An engine adjusted to burn gasoline efficiently will produce less power from alcohol should it separate from the gasoline. Separation is caused by the polar nature of the alcohol molecules and their tendency to absorb water, also a polar substance.
Methanol is the most likely to separate, butyl alcohol the least likely. The tendency for separation increases as the temperature decreases, the quantity of water absorbed increases, and the quality of the gasoline decreases. The effect of using a blend of alcohol and gasoline in an engine adjusted for gasoline is to lean out the fuel mixture. This is illustrated in Figure 1 for an engine burning blends of ethanol and gasoline. A mixture of 10 percent ethanol in gasoline produced more power when the carburetor was adjusted for gasoline. Increasing the ethanol content to 25% reduced the power output. The test results in Figure 1 were obtained at an elevation of 5,000 feet where an engine adjusted to operate on gasoline is expected to run rich. The 10% blend produced a leaner and better air-fuel ratio; the 25% blend was too lean. Because of its higher stoichiometric air-fuel ratio, butyl alcohol can be mixed with gasoline in higher concentrations without affecting performance. Similarly, because of its low stoichiometric air-fuel ratio, only a small quantity of methyl alcohol can be mixed with gasoline without affecting performance. In other words, a fuel blend containing 20% methanol requires modification of the carburetor fuel jets to optimize power output, whereas a 20% blend of butyl alcohol does not. The effect of increasing the ethanol concentration in the fuel on exhaust emissions is shown in Figure 2. The primary effect of ethanol is to reduce the carbon monoixde (CO) emissions. It should be noted that the same effect was obtained using straight gasoline and carefully leaning the air-fuel ratio.
Alcohols as Fuel for Straight-Inline (SI) Engines
Alcohols have high knock resistance, which makes them excellent SI engine fuels. Their latent heat of vaporization is quite high, leading to excellent volumetric efficiency, but can also cause starting problems. Their calorific value is substantially lower than that of gasoline, which would lead to a larger fuel tank volume and also increased jet sizes in carburetors. Alchohols also have low air requirements for complete combustion, however this leads to practically the same mixture enthalpy content. Hence alcohol fueled SI engines can produce the same or even slightly higher power than gasoline.
Table 3 compares the power output and Best Mean Effective Pressure (BMEP) of gasoline and alcohol SI engines (DB 4.5 litre V8 engine). The higher the octane number of the alcohol allows a higher compression ratio to be used and the higher latent heat leads to higher charge induction, enabling the alcohol engine to produce approximately 10% higher power output.
Table 3: Comparison of Key Engine Factor with Ethanol and
Table 4 shows a comparison between the specific fuel consumption of the two fuels. On a volumetric basis, alcohol consumption is much higher due to its lower calorific value. On an energy basis, however, the superior efficiency of the alcohol engine is reflected in he significantly lower heat combustion rate.
Table 4: Comparison of Fuel Consumption with Ethanol and Gasoline
Alcohols and Diesel Engines
Alcohol has also been used in diesel engines. In this case, the alcohol may be blended with diesel fuel to produce diesohol, or the alcohol may be added to the air intake of the engine. A system for adding a mixture of ethanol and water to the air intake of a turbocharged diesel engine is commercially available. The primary function of the system is to cool the turbocharged air (using the latent heat), and thereby to increase the volumetric efficiency of the engine and produce more output power. A similar result can be obtained using an intercooler. Control of the quantity of alcohol added to the air intake may be difficult and could cause erratic engine operation and/or failure if a large quantity of alcohol was added to the air intake. Tests results using blends of ethanol in diesel fuel are shown in Figure 3. The engine used in these tests was naturally aspirated. As with gasohol, the primary effect of the ethanol was to lean the air-fuel mixture and produce more efficient combustion.
Methyl alcohol, because of its highly polar nature, does not mix with diesel fuel. Ethanol can be mixed with diesel fuel provided there is little water in the ethanol. A diesel engine normally will not operate on ethanol nor will ethanol provide lubrication for the fuel injection system.
Another problem with adding ethanol to diesel fuel is that the cetane number (ignition characteristic) may decrease below the level recommended by the engine manufacturer. Butyl alcohol can be mixed with diesel fuel in virtually any concentration. It does not separate as water is added or as the temperature is decreased. Further, butyl alcohol does not significantly change the cetane number of diesel fuel. In blends with diesel fuel, butyl alcohol tends to reduce the solidification temperature of the fuel at low temperatures.
A Brief Word on Corrosion
Alcohols may be corrosive to certain materials used in engines. Generally, methyl alcohol is the most corrosive and butyl alcohol is least corrosive. Alcohols also can cause injury or physical harm if not used properly. People who use alcohol in motor fuels should observe warning labels and follow precautions to avoid problems.
For internal combustion engines, ethanol has the greatest promise as substitutes for petroleum fuels among the alternatives available, at least until the affordable manufacture and effective usage of hydrogen becomes a reality. Alcohols are liquids at normal temperatures, can be manufactured from abundantly available raw materials and have desirable properties of engine fuels. Presently, the major hurdles include their relatively high cost of production and engine adaptation. Storage, transportation and handling without water is another important criterion for the successful use of alcohols as transporation fuels.
By Guest Author Krishnamachari Govindarajan of Petchem Consultants,
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