汽油(gasoline)
英国称petrol
从石油中得到的挥发和易燃的碳氢化合物的混合物，用作内燃机的燃料，也用作油料和脂肪的溶剂。由於汽油的燃烧能高，在化油器中容易与空气混合，且因原本大量供应而价格低廉，所以成为人们所乐用的汽车燃料。现在成本已大幅增加，除非有获补助。汽油最初是经由蒸馏产生的，後来为了从原油得到更多的汽油，设计了裂化过程，将大分子分裂为较小的分子。其他方法如异构化法︰将直链碳氢化合物转变为支链碳氢化合物。结果是产生数百种不同碳氢化合成的混合物。汽油的抗爆性，说明燃料蒸气在汽缸中燃烧太快，降低了效率。1930年代开始加入四乙基铅以延缓燃烧，但由於排出的燃烧产物中铅化合物具有毒性，在1980年代已不再继续使用。汽油的其他添加剂包括洗涤剂、防冻剂和抗氧化剂。20世纪中叶以後，汽油排放的废气已成为都市空气污染的主要污染源。人们开始寻求减少依赖汽油（一种非再生性资源）的方法，包括使用酒精-汽油混合燃料（gasohol），它是90%无铅汽油和10%乙醇的混合物。另外还发展了电瓶车。
English version：
gasoline
Mixture of volatile, flammable hydrocarbons derived from petroleum, used as fuel for internal-combustion engines (also as a solvent for oils and fats). Gasoline became the preferred automobile fuel because it releases so much energy when burned, it mixes readily with air in a carburetor, and initially was cheap due to a large supply. Costs have now increased greatly except where subsidized. Gasoline was first produced by distillation. Later processes increased the yield from crude oil by splitting large molecules into smaller ones. Still other methods, such as conversion of straight-chain hydrocarbons into their branched-chain isomers, followed. The result is a complex mixture of hundreds of hydrocarbons. A gasoline's octane number indicates its ability to resist knocking (which means combustion is too rapid) and can be altered by changing the proportions of certain components. Lead, once used to reduce knocking, has been banned as toxic. Other additives include detergents, antifreezes, and antioxidants. Since the mid-20th century gasoline fumes have been a major component of urban air pollution. Efforts to reduce dependence on gasoline, which is a nonrenewable resource, include use of “gasohol,” a 90:10 mix of gasoline and ethanol, and the development of electric automobiles.
