When Rudolph Diesel unveiled his new engine at the 1900 World's Fair, he made a point of demonstrating that it could be run on peanut oil. "Such oils may become, in the course of time, as important as petroleum and the coal tar products of the present time," he said. And so it has come to pass that US President George Bush has decreed that America must wean itself off oil with the help of biofuels made from corn, sugar cane and other suitable crops. At its simplest, the argument for biofuels is this: By growing crops to produce organic compounds that can be burnt in an engine, you are not adding to the overall levels of carbon dioxide in the atmosphere. The amount of CO2 that the fuel produces when burnt should balance the amount absorbed during the growth of the plants. However, many biofuel crops, such as corn, are grown with the help of fossil fuels in the form of fertilisers, pesticides and the petrol for farm equipment. One estimate is that corn needs 30 per cent more energy than the finished fuel it produces. Another problem is the land required to produce it. One estimate is that the grain needed to fill the petrol tank of a 4X4 with ethanol is sufficient to feed a person for a year.