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Ethanol Fuel Debate

posted Apr 24, 2015, 4:13 PM by Dimitrios Simitas   [ updated Apr 24, 2015, 4:16 PM ]

Date: October 27, 2012
Just a "rant" about Ethanol fuels ...

Per litre it has 21.2 MJ of energy. Since I realize that most here are American I'll use both systems, you end up with 80.25 MJ/US Gal.
  • Gasoline with a Octane of 91? 34.8MJ/L or 131.73MJ/Gal.
  • So you end up with 90/10 (90% gasoline and 10% Ethanol) having 33.7MJ/L or 127.56MJ/Gal.
  • That newish E85 they are pushing? 25.2MJ/L or 95.39MJ/Gal.
Car engines have a maximum theoretical efficiency of 37%. Most new "fuel" efficient engines are only at 20%.

So on say a Harley with it's 48mi/Gal. One Gallon of Regular fuel Octane 91, and assuming a 20% efficiency, means to drive 48 miles you need 26.35MJ's of energy, this is more or less a "absolute", in that you can't suddenly make the bike use only 1MJ of energy to do the same distance, regardless of what fuel you use. Since Harley says "under ideal laboratory conditions", we all know real world fuel economy will be worse then listed, but that 26.35MJ's of energy can be considered a minimum energy we'd need to move the bike.

If you use 90/10 you'd need 

131.73MJ/Gal / 127.56MJ/Gal = 1.03 Gallons of fuel.


131.73MJ/Gal / 95.39MJ/Gal = 1.38 Gallons of Fuel.

Now as for "pollution" to get us to where we are going. 

When you burn Gasoline you end up with this equation:

2 C8H18 + 25 O2 → 16 CO2 + 18 H2O


C2H5OH + 3 O2 → 2 CO2 + 3 H2O 

The molar mass of Gasoline is ~0.75g/cm^3 (100oz/gal) while for Ethanol it is ~0.79g/cm^3 (105.4oz/gal)

So in one Gallon of Gasoline you have 2835g of gasoline.

In one Gallon of 90/10 you have 2552g of gasoline and 299g of ethanol.

In one Gallon of E85 you have up to 425g of gasoline and 2456g of ethanol.

The molar mass of gasoline is ~114g/mol, while Ethanol is ~46g/mol.

  • So in regular gasoline you have 24.9 mol of gasoline. 
  • 90/10 you have 22.4 mol of gasoline and 6.5 mol of ethanol. 
  • In E85 you have 3.7 mol of gasoline and 53.4 mol of ethanol. 

As per the above chemical equations, you end up with eight times the CO2 in moles then you have in gasoline, and two times that of Ethanol.
  • So regular Gasoline has a easy 199.2 mol of CO2.
  • The 90/10 stuff has 192.2 mol of CO2.
  • And E85 has 136.4 mol of CO2.
So that looks like Ethanol has a "good" trade off in CO2 emissions. Which is what everyone talks about. And perhaps they are right. But remember my fuel efficiency conversions I mentioned above?
  • Regular Gasoline simply produces 199.2 mol of CO2 to get us 48mi down the road.
  • The 90/10 stuff now requires 198 mol of CO2 to get us to 48mi down the road.
  • And finally the E85 stuff now requires 188 mol of CO2 to get us the same 48mi down the road as the other two.
So while Ethanol costs us more at the pump then crude based gasoline, even with the lower production of CO2, we still end up within 6% of the same amount of CO2 being released in the air. Which is more then eaten up due to efficiency losses caused by dirty air filters, oil changes, tune ups etc.