Ethanol percentage

Figure 3.10 Addition of ethanol to gasoline results in a mixture with higher octane rating.

octane rating, Om, of gasohol (gas/alcohol mixture) can be calculated from the octane rating, Og, of the original gasoline and from the blending octane value, B, of the alcohol:

where x is the ratio of the additive volume to that of the gasoline. Depending on the initial quality of the fuel, the blending octane value of ethanol can be as high as 160. Methanol has a B of 130, although, when used alone, its rating is only 106. Gasohol can achieve high octane ratings without the use of lead and with only moderate addition of cyclic hydrocarbons. Thus, gasohol brings substantial public health advantages.^

Since 1516, Brazil has been the world's leading sugarcane producer. The widely fluctuating international price of sugar prompted Brazil to develop gasohol as a means of disposing of excess production. In years when the price was low, the alcohol percentage in Brazilian gasoline was high (typically, 24%). When sugar prices were high, much less ethanol found its way into automotive fuel (say, 5%). Starting in the 1970s, Brazil decided to sell pure (hydrated) alcohol as fuel for its fleet of specially designed cars, thus achieving a certain independence from the importation of oil.

Alcohol is more than an additive; it is itself a fuel. However, its energy content is lower than that of gasoline (Table 3.6). Per unit volume, ethanol contains only 71% of the energy of heptane, the main constituent of gasoline. Nevertheless, Brazilian alcohol-driven cars (using gasoline-free ethanol) have a kilometrage that approaches that of gasoline engines. This is due to the higher efficiency of the high-compression ethanol motors. Because water can be mixed with alcohol—inviting the "stretching" of the fuel sold at refueling stations—Brazilian pumps are equipped with densit-ometers permitting the consumer to check on the quality of the product.

Table 3.6 Properties of Two Important Alcohols Compared with Heptane and Octane (Higher heats of combustion for fuels at 25 C)

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