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Discussion Starter #1 (Edited)
On November 12th, the Wall Street Journal published a section titled "Guide to Going Green". In that section, there was an article about automotive carbon emissions - I'm paraphrasing here, but it basicly said: "A typical automobile that gets 23.9 miles per gallon, generates 0.93 pounds of carbon per mile".

Now, I thought about that, and the numbers seemed out of whack. I did a little mental arithmetic - at 25mpg, the car would be burning 1/25 = .04 gallon of gas per mile. If .04 gallon of gas contains almost a pound of carbon, then a full gallon would contain almost 25 pounds. But, even if gas were 100% carbon (and it's not - it has more elements in it), if a gallon weighs 25 pounds, then my 5 gallon gas can would weigh 125 pounds when full - and, I know it's not that heavy!

So, I sharpened my pencil, did the math more precisely, and sent the following message to the WSJ:

"The example refers to a car that gets 23.9 miles/gallon, which works out to .042 gallons per mile. So, if .042 gallons of gasoline produces .93 pounds of carbon, there would have to be 22.22 pounds of carbon in every gallon of gasoline (23.9 mi/gal X .93 #/mi = 22.22 #/gal). But a quick web search shows that a gallon of gasoline weights only 5.8 to 6.5 pounds. So, even if gasoline was 100% carbon, our hypothetical 23.9 MPG car could only emit .24 to .27 pounds of carbon per mile."

However, the reply that I got back from the WSJ made even less sense than their original quote:

"Below is the correction we ran on Nov. 17, 2007.

An automobile generates 0.93 pound of carbon dioxide per mile, according to the Environmental Protection Agency, so a car driven 93 miles generates 86.5 pounds of carbon dioxide, assuming the car gets a typical 23.9 miles per gallon. In Monday's Energy Journal Report, an article about how consumers can make an environmental difference incorrectly said such a car driven 93 miles generates about 100 pounds of carbon dioxide."

Am I crazy, or are the EPA and WSJ? I'm pretty sure that my math is sound, and that I am correct in assuming that mass is conserved in a simple chemical reaction like burning gasoline.

Mine isn't a political question - no, I don't like/respect the EPA any more than you do, and yes, the WSJ's editorial page can be pretty "out there". But, are my math and reasoning OK?
 

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But Mad North-Northwest
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I'm sitting in a meeting right now so I can't do the detailed math, but what you're missing is the weight of the oxygen pulled from the air. The reaction is:

Fuel + O2 -> CO2 + water

So some of the mass of CO2 comes from the fuel going in and the rest comes from the air. If you model gasoline as C8H18, it would be

2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O

assuming I'm thinking straight and doing my mass balance right.
 

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Ideal conditions have the engine burning 14.7 parts air to one part fuel by mass. In the above mass balance you have to remember each of those carbons get together with 2 oxygens, who are heavier than the carbon.
 

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But Mad North-Northwest
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Okay, I can do a little math here. That works out to 228g fuel in yields 704g of CO2 out. So on a mass basis you get about 3X the CO2 out compared to the mass of fuel you put in (again, assuming I'm doing my assumptions and calcs correctly).

I'll leave it to you to use this to check the WSJ's numbers.
 

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On November 12th, the Wall Street Journal published a section titled "Guide to Going Green". In that section, there was an article about automotive carbon emissions - I'm paraphrasing here, but it basicly said: "A typical automobile that gets 23.9 miles per gallon, generates 0.93 pounds of carbon dioxide per mile".

Now, I thought about that, and the numbers seemed out of whack. I did a little mental arithmetic - at 25mpg, the car would be burning 1/25 = .04 gallon of gas per mile. If .04 gallon of gas contains almost a pound of carbon, then a full gallon would contain almost 25 pounds. But, even if gas were 100% carbon (and it's not - it has more elements in it), if a gallon weighs 25 pounds, then my 5 gallon gas can would weigh 125 pounds when full - and, I know it's not that heavy!

So, I sharpened my pencil, did the math more precisely, and sent the following message to the WSJ:

"The example refers to a car that gets 23.9 miles/gallon, which works out to .042 gallons per mile. So, if .042 gallons of gasoline produces .93 pounds of carbon, there would have to be 22.22 pounds of carbon in every gallon of gasoline (23.9 mi/gal X .93 #/mi = 22.22 #/gal). But a quick web search shows that a gallon of gasoline weights only 5.8 to 6.5 pounds. So, even if gasoline was 100% carbon, our hypothetical 23.9 MPG car could only emit .24 to .27 pounds of carbon per mile."

However, the reply that I got back from the WSJ made even less sense than their original quote:

"Below is the correction we ran on Nov. 17, 2007.

An automobile generates 0.93 pound of carbon dioxide per mile, according to the Environmental Protection Agency, so a car driven 93 miles generates 86.5 pounds of carbon dioxide, assuming the car gets a typical 23.9 miles per gallon. In Monday's Energy Journal Report, an article about how consumers can make an environmental difference incorrectly said such a car driven 93 miles generates about 100 pounds of carbon dioxide."

Am I crazy, or are the EPA and WSJ? I'm pretty sure that my math is sound, and that I am correct in assuming that mass is conserved in a simple chemical reaction like burning gasoline.

Mine isn't a political question - no, I don't like/respect the EPA any more than you do, and yes, the WSJ's editorial page can be pretty "out there". But, are my math and reasoning OK?
 

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Discussion Starter #6
Yes, yes - After typing in my message, I went outside, did some gardening, pondered the problem, and the answer hit me. I came back inside, and saw that Alfasoon & Gubi had hit upon the same error: the EPA was talking about "carbon dioxide", but some journalism major at the WSJ just typed in "carbon". So sure, the extra mass comes from the oxygen in the air.

A gallon of gas that weighs about 6#, when combined 1:15 by weight with air, results in a combination that weighs 96#, so the exhaust product could easily contain 22# of CO2.

Thanks all!
 

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Whew!!

And for a minute there I thought that both the WSJ AND theEPA were wrong:eek:
Perish the thought:p
 

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Bleah!

If the 1:15 ratio matters, it means that you are counting the carbon dioxide taken IN the intake and then pumped back out the tailpipe as part of the car's emission.

The only Carbon ADDED is from the fuel, so air ratio doesn't matter. You just take the carbons, tear them apart, add O's and make CO2 plus other stuff we are not counting.

C=12, O=16. 12 lb of fuel* plus 16 * 2 lbs of oxygen equals 44 lbs of CO2. 12 lbs of fuel is about 2 gallons, good for 48 miles in the hypothetical car, so 44/48 = 0.92 lbs of CO2 per mile added by the car burning gas is about right. And 73% of that is oxygen from the air that was already there. 27% or about 110 gm per mile is carbon.

* actually there is a lot of hydrogen in the fuel too, about as many H's as C's, so it takes about 13 lbs of fuel to get 12 lbs of carbon.

R
 

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OK, smart guys - I have four old, stinky cars that I drive every weekend (say 60 miles on average/ week - rotating them evenly). They stink, they eat 91 Octane (or 100 for the 911) and I drive them hard. Assume 12 miles/ gallon for the 911, and the typical rate you use for the Alfa, same for the 356 (Maybe 20 miles/ gallon?).

How many carbon offsets do I have to buy if I want to go green?

Please provide detailed calculations and assumptions:D
 

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Bleah!

If the 1:15 ratio matters, it means that you are counting the carbon dioxide taken IN the intake and then pumped back out the tailpipe as part of the car's emission.

The only Carbon ADDED is from the fuel, so air ratio doesn't matter. You just take the carbons, tear them apart, add O's and make CO2 plus other stuff we are not counting.

C=12, O=16. 12 lb of fuel* plus 16 * 2 lbs of oxygen equals 44 lbs of CO2. 12 lbs of fuel is about 2 gallons, good for 48 miles in the hypothetical car, so 44/48 = 0.92 lbs of CO2 per mile added by the car burning gas is about right. And 73% of that is oxygen from the air that was already there. 27% or about 110 gm per mile is carbon.

* actually there is a lot of hydrogen in the fuel too, about as many H's as C's, so it takes about 13 lbs of fuel to get 12 lbs of carbon.

R
Good notes, racer.

Yeah, EPA guy mistakenly called CO2 Carbon, technically wrong. But, does the CO2 emitted by EPA include CO2 already in the air sucked in for the combustion, or is it NEWLY GENERATED CO2 EXCLUDING the CO2 already present used in the intake?? Then, that CO2 should actually be SUBTRACTED from the gross CO2 product coming from the exhaust, get it?

Joe
 
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