Vehicle emissions - Pt. 2

Following on from Part 1 yesterday:

Did a bit of research into Bioethanol last night – its just the same as ethanol but using an organic feedstock. (Obvious really!) The combustion reaction is:

C2H5OH + 3O2 = 2CO2 + 3H2O

Which in terms of molecular weights looks like :

46.068 (ethanol) + 95.996 (oxygen) = 88.019 (carbon dioxide) + 54.046 (water)

So, 1 kg of ethanol will produce 1 x (88.019 / 46.068) = 1.911 kg of carbon dioxide on complete combustion. Given that the density of ethanol is 789 g/l, one litre of ethanol will give off 1508 g of CO2 when it burns, and a gallon (4.546 litres) will give off 6854 g of CO2. But as we commonly measure transport emissions in terms of “gCO2/km”, but think in terms of fuel economy in “mpg”, this needs to be divided by 1.6093 (ie 1.6093 km = 1 mile) to give my “magic number” – 4259, rounded to 4250

So, for pure Bioethanol, CO2 emissions in g/km = 4250 / mpg. Simple as that.

Which surprised me, as it is so much less than the figure for “petrol” at 6600 and more akin to LPG at 3250, suggesting that fuel economy should drop off markedly as bioethanol is blended in, but I also found an interesting snippet on the web that went: “By blending ethanol with gasoline we can also oxygenate the fuel mixture so it burns more completely and reduces polluting emissions.” (1)  And if it enables the petrol to burn more completely, perhaps it liberates more power from the mix, giving improved performance that offsets its lower carbon content.

But we don’t get pure bioethanol at the pumps - it’s normally E10 (10% bioethanol) or E85 (85% bioethanol). Simple mathematics gives the magic numbers for these blends as follows:

E10 Bioethanol – CO2 emissions in g/km = 6365 / mpg

E85 Bioethanol – CO2 emissions in g/km = 4600 / mpg 

So does blending bioethanol into a petrol mix actually maintain or improve fuel economy despite its lower carbon “chemistry”, and if it does, at what mix does the fuel economy start to drop off? I can feel a Part 3 to this story coming on … if I can ever find some info …

(1) http://www.esru.strath.ac.uk/EandE/Web_sites/02-03/biofuels/what_bioethanol.htm