Gasoline

Without a doubt, the most popular automotive fuel currently is gasoline. Gasoline is not a chemically unique substance—its composition has been continuously improved since its introduction, and it is also adjusted seasonally. It is a mixture of more than 500 components dominated by hydrocarbons with 3 to 12 carbon atoms. Most are branched (see discussion of the difference between octane and isooctane in the next section). The two main characteristics of gasoline are the following.

3.5.1 Heat of Combustion

Since the composition of gasoline is variable, its heat of combustion is not a fixed quantity. One may as well use the values for heptane or octane («45MJ/kg) as a representative higher heat of combustion.

3.5.2 Antiknock Characteristics

As far as energy content is concerned, gasoline has a decisive advantage over alcohol. However, there is no point in using a high-energy fuel if this leads to a low engine efficiency. As discussed previously, the efficiency is determined in part by the compression ratio, which, if too high, causes knocking (see next section). Alcohols tolerate substantially higher compressions than most gasolines and therefore lead to greater engine efficiencies. This somewhat compensates for the lower specific energy of the alcohols. Gasolines with better antiknock characteristics (higher octane rating) do not necessarily have higher energy content; in fact, they tend to have lower energy. It makes sense to use the gasoline with the lowest possible octane rating (i.e., the cheapest) compatible with the engine being fueled. On the other hand, cheaper gasolines, independent of their octane rating, may cause gum formation and other deposits in the engine and may result in more exhaust pollution.

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