Rudolf Diesel used vegetable oils in the early models of his unique engines, a century ago, and believed they would be as important to transportation and energy production as fossil fuels. With the development of biodiesel, that prediction is coming close to fruition. Biodiesel, though, is only one of the two most common types of biofuels. Biofuels are an alternative energy source to fossil fuels, which are considered the cause of global warming. One method of biofuel production involves crops high in sugar, such as sugar cane, or starch, such as corn; these are fermented to produce ethanol, which is blended with gasoline. The other widespread method of biofuel production involves plants high in vegetable oil, such as palm oil, soy, jatropha or castor bean; this oil is refined and added to diesel fuel, hence the name biodiesel. Pure biodiesel can be used by itself in some standard diesel engines, and as a blend with regular petroleum-based diesel fuel in most standard diesel engines built since the mid-1990s. The diesel engine is most prevalent today in the trucking industry, which is currently the main focus of biodiesel firms. Countries that encourage biodiesel use can theoretically make a significant reduction in their carbon emissions, and trucking firms can expect to save large sums of money on reduced fuel costs. Since diesel engines are already more fuel-efficient than their gasoline-burning counterparts, the viability of biodiesel could also spur greater use of the diesel engine in passenger cars. Biofuels are attractive energy sources for several reasons: They are renewable; their carbon emissions are theoretically offset by the carbon dioxide their plants absorbed while they were grown; and they reduce countries' energy dependence on fossil fuels. However, recent studies suggest the picture is not so rosy - or green, as it were. One major downside of large-scale biofuel production until now has been the creation of a crisis in the global agricultural market, as food crops such as corn and soy are diverted to fuel production. Another significant problem is that, instead of reducing carbon emissions, biofuels may actually increase them, since the energy required to produce biofuels is higher than that required to produce gasoline. Worse still, some countries have cut down rain forests and introduced other environmentally damaging methods in a scramble to make way for the profitable new energy crops. Plants such as jatropha and castor, though, offer the opportunity to produce biodiesel without competing for farmland or diverting food crops from the market. Assuming continued improvements in the efficiency of the fuel and in the methods of production, Rudolf Diesel very well could prove prophetic.