Biodiesel a diesel substitute fuel made from biomass, which means that it is inherently renewable and it, in itself, contributes nothing to carbon-dioxide loading of the atmosphere.
Biodiesel commonly uses soybean or canola oil as its base, but animal fat or recycled cooking oil can also be used. Diesel-powered vehicles require no modification at all to run on 20% biodiesel mixed with regular diesel (known as B20).
Thus any diesel-powered truck or bus is, potentially, already an alternative-fueled vehicle! Since biodiesel is not a fossil fuel, it can cut greenhouse-gas emissions as well as ordinary pollutants (particularly soot) by displacing petroleum diesel fuel. B20 can be stored and dispensed in exactly the same manner as petroleum diesel fuel.
The energy balance of soy biodiesel is quite positive: approximately 3.2 units of usable energy are obtained per unit of energy expended in the fuel production.