Biodiesel
Biodiesel is similar to diesel, but has differences akin to those between petrol and ethanol. For instance, biodiesel has a higher cetane rating (45-60 compared to 45-50 for crude-oil-derived diesel) and it acts as a cleaning agent to get rid of dirt and deposits. It has been argued that it only becomes economically feasible above oil prices of $80 (£40 or €60 as of late February, 2007) per barrel. This does however depend on locality, economic situation, government stance on biodiesel and a host of other factors- and it has been proven to be viable at much lower costs in some countries. Also, it yields about 10% less energy than ordinary diesel. Analogous to the use of higher compression ratios used for engines burning higher octane alcohols and petrol in spark-ignition engines, taking advantage of biodiesel's high cetane rating can potentially overcome the energy deficit compared to ordinary Number 2 diesel.
Read more about this topic: Liquid Fuels