Aggregating Skeptical Thought

CO2 Weight Translation

Because I’m a nerd I found this article, What is the weight ratio of CO2 released to fuel burned, over at Sciam interesting. It’s part of their Ask An Expert series.

Thus, the weight ratio of CO2 produced per octane molecule burned is 352/114, or roughly 3 to 1.

The only problem with this little factoid is that I can’t translate this into my reality (let’s assume I don’t understand chemistry so well, you know, for the sake of argument). So I googled this little factoid out of a page on the EPA’s website, Emission Facts: Average Carbon Dioxide Emissions resulting from Gasoline and Diesel Fuel.

CO2 emissions from a gallon of gasoline = 2,421 grams x 0.99 x (44/12) = 8,788 grams = 8.8 kg/gallon = 19.4 pounds/gallon

CO2 emissions from a gallon of diesel = 2,778 grams x 0.99 x (44/12) = 10,084 grams = 10.1 kg/gallon = 22.2 pounds/gallon

And before you think diesel is still evil remember that it is more fuel efficient than gasoline. So while you may be polluting more per gallon you are still going much further per gallon, I’ve read but can’t seem to find right now that using gasoline and diesel engines in comparable vehicles that the diesel equipped vehicle will get on average 10-15% in increased MPG over it’s gasoline counterpart.

Here’s some more goodies on converting gas/diesel to CO2 from wikipedia, including a write up of the new EPA CAFE standards for evaluating fuel efficiency in model year 2008 and newer vehicles.

I know Consumer Reports has been a big advocate in trying to change the CAFE rating process to more accurately reflect today’s driving conditions. Here’s a recent article on their blog regarding what it would really take to get the U.S. to 35 mpg. Keep at it guys.


Filed under: Environment, , , , , ,

6 Responses

  1. Tom says:

    If diesel is 10-15% more efficient in terms of MPG then it’s basically a wash, since the CO2 emissions are also a little more than 10% higher. I am unfamiliar with the specifics – is diesel more efficient just because it has a higher energy density? More reformed bonds from combustion would be consistent with more CO2 formed.

  2. Skeptigator says:

    It does basically come out in the wash as far as equivalent net emissions per mile, however there are 2 key factors regarding diesel

    1) Most diesel engines can run bio-diesel, although in some instances your fuel efficiency will go down but your CO2 emissions will drop more.

    2) Diesel engines in general last longer and therefore reduce other vehicle related pollution (like in the manufacture of vehicles themselves)

    I wasn’t too sure if the efficiencies with diesel were a function of the fuel or whether it’s the way the engine burns that fuel (or both).

    Here’s the answer (as best I understand remember I’m not a mechanic but I play on my blog):
    The answer is both, not only is diesel fuel more energy dense but the diesel engine output is higher. The following article goes into more detail that also gives more information on the pros and cons of diesel vs. gasoline.

    This was the best article I could find, but they site information from the Union of Concerned Scientists, a very good organization,

    Bottom line: Driving Less is a surefire way to lower your engines emissions, regardless of fuel type.

  3. Gordon Prince says:

    What about the idea of miles per pound when comparing diesel vs gasoline?

    Recently I came across a comment that the added miles/gallon of diesel fuel is largely due to diesel’s additional weight per gallon. Therefore there isn’t much of a reduction in “miles driven per barrel of crude oil refined” as a result of switching from gasoline to diesel. The diesel fuel is denser, has more energy in it and requires more crude oil to make a gallon of than gasoline does.

    Anyone know anything about this idea?

  4. Skeptigator says:

    I have honestly never heard that. I would figure that the difference in weight between diesel and gasoline would be negligible, but that’s just a gut reaction. I will look into this.

  5. ryan says:

    the VW Rabbit has a listed 22-29 mpg for the gasoline version.

    The Diesel version gets 42-49 mpg.

    That seems like more than ‘just a wash’ in favor of the Diesel version.

  6. Skeptigator says:

    Hey that’s really good gas mileage. i’m going to look into that

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