什麼是汽油

英國稱petrol

從石油中得到的揮發和易燃的碳氫化合物的混合物，用作內燃機的燃料，也用作油料和脂肪的溶劑。由於汽油的燃燒能高，在化油器中容易與空氣混合，且因原本大量供應而價格低廉，所以成為人們所樂用的汽車燃料。現在成本已大幅增加，除非有獲補助。汽油最初是經由蒸餾產生的，後來為了從原油得到更多的汽油，設計了裂化過程，將大分子分裂為較小的分子。其他方法如異構化法︰將直鏈碳氫化合物轉變為支鏈碳氫化合物。結果是產生數百種不同碳氫化合成的混合物。汽油的抗爆性，説明燃料蒸氣在汽缸中燃燒太快，降低了效率。1930年代開始加入四乙基鉛以延緩燃燒，但由於排出的燃燒產物中鉛化合物具有毒性，在1980年代已不再繼續使用。汽油的其他添加劑包括洗滌劑、防凍劑和抗氧化劑。20世紀中葉以後，汽油排放的廢氣已成為都市空氣污染的主要污染源。人們開始尋求減少依賴汽油（一種非再生性資源）的方法，包括使用酒精-汽油混合燃料（gasohol），它是90%無鉛汽油和10%乙醇的混合物。另外還發展了電瓶車。

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.