????6. 丙烷

????保養容易、排放減少，這兩個優點已使輕型車隊（警車和校車）以及大家熟知的肯沃思公司（Kenworth）和彼得比爾特公司（Peterbilt）的重載卡車都用上了丙烷。目前公路上奔馳的丙烷汽車已有27萬輛。丙烷也叫液化石油氣（LPG），是天然氣加工和原油提純的副產品。盡管擁有較高辛烷值和燃燒清潔的特性，丙烷的成本卻比汽油還要便宜三分之一。不過它必須用壓力罐儲存，同時丙烷加注基礎設施也很少。

????6. Propane

????Ease of maintenance and reduced emissions have spurred the use of propane in light-vehicle fleets (police cars and school buses) as well as in heavy-duty trucks with such familiar nameplates as Kenworth and Peterbilt. More than 270,000 propane vehicles are now on the road. Also known as liquefied petroleum gas (LPG), propane is produced as a by-product of natural gas processing and crude oil refining. Despite a high octane rating and clean burning properties, propane costs about one-third less than gasoline. But it must be stored in a pressurized tank, and the propane refueling infrastructure is limited.