What is Biodiesel?
Biodiesel is not simply vegetable oil. It is possible to run a vehicle using unmodified vegetable oil (indeed this is referred to as SVO, or Straight Vegetable Oil), but it will entail serious modification of the engine. It will be necessary to replace the injectors and glow plugs with ones that are more suited to heavier oil, plus a heater will be needed as vegetable oil has a higher temperature at which it starts to gel (by this, we mean that as it gets colder, it starts to thicken, so that it won't flow through your fuel system properly). It is also possible to mix SVO with ordinary petro-diesel, but again, the higher viscosity becomes a problem.
With biodiesel, we have none of these drawbacks; biodiesel is a fuel in its own right. If made correctly, it has all the characteristics of petrodiesel; pour it in your fuel tank and off you go. So, what exactly is it? Biodiesel is a light-to-dark yellow liquid with a viscosity slightly less than that of water (around 0.88g/cm3). It is immiscible with water, has a low flash-point (around 150oC), making it generally non-flammable and, if uncontaminated with initial reagents, can be considered non-toxic.