石油發(fā)動機燃料的價格和二氧化碳（CO2）排放的關(guān)注正在引起人們對可能替代物的關(guān)注。2008年，經(jīng)通脹因素調(diào)整后，世界石油價格創(chuàng)下歷史新高，延續(xù)了數(shù)年來的價格上漲格局。從傳統(tǒng)原油中提取的石油產(chǎn)品占美國最終使用能源供應(yīng)量的50%以上，占美國運輸部門全部能源使用量的95%以上。來自石油消費的排放量占全國二氧化碳排放量的44%，其中約33%的國家由運輸燃料使用造成的二氧化碳排放（EIA，2007年A）。近期和中期（約10-20年）通?？紤]的替代運輸燃料選擇包括生物質(zhì)燃料（如乙醇、生物柴油）和非傳統(tǒng)化石燃料液體燃料，這些燃料來源于重油、油砂、油頁巖和煤液化。

Both the price of petroleum motor fuels and concerns regarding emissions of carbon dioxide (CO2) are driving attention to possible substitutes. In 2008, the world price of oil reached record highs after being adjusted for inflation, continuing a pattern of price increases over several years. Petroleum products derived from conventional crude oil constitute more than 50 percent of end-use energy deliveries in the United States and more than 95 percent of all energy used in the U.S. transportation sector. Emissions from the consumption of petroleum account for 44 percent of the nation’s CO2 emissions, with approximately 33 percent of national CO2 emissions resulting from transportation-fuel use (EIA, 2007a). Commonly considered alternative transportation-fuel options for the near and medium terms (roughly 10–20 years) include both biomass-based fuels (e.g., ethanol, biodiesel) and unconventional fossil-based liquid fuels derived from such sources as heavy oils, oil sands, oil shale, and coal liquefaction.