Dear Naia,
I know that some variations on that theme are out there. I have done a few myself. The stated question is really too broad for a concise (or useful) answer. "Petroleum" is a catch-all term for a great many hydrocarbons. Their particular processing and use as fuels is complex and extensive. Further, in terms of environmental impact, one must also consider the resources required to GET the ethanol.
I think that the real question is whether bio-ethanol is a better vehicle fuel than gasoline (or diesel). The short answer to that is simply the specific energy in a particular volume. Volume, because that is how it is carried in a vehicle, and energy because that is the only comparison. How that energy is used or wasted is a different topic. Therefore, looking at the specific energy of ethanol and gasoline we find:
Gasoline has an energy density of about 45 megajoules per kilogram (MJ/kg).
Ethanol has an enerrgy density of 23.4 – 26.8 megajoules per kilogram (MJ/kg).
Here is one resource: Energy Density of Gasoline - The Physics Factbook
"Petroleum" is the clear winner.
NOW if you want to consider ecological and environmental impacts, that is quite a bit different (and more complex) task. A "full life-cycle cost" of the product and environmental impacts really needs to be assessed. Again, the short answer is that ethanol may not be nearly as benign a fuel source as some imagine, and economically (including the energy density) it is never in the running. If you are burning a hydrocarbon you get the same post-combustion results per energy unit no matter where you got the starting materials.
Best regards,
Steven
