Uh... 6 = 20?

We've heard a number of times that when a gallon of gas is burned, it creates about 20 lbs of carbon dioxide (CO₂) exhaust.  But wait, you say, a gallon of gas only weighs about 6 lbs.  Doesn't that defy basic physics?  You know, the law of conservation of mass...

Okay, you're right.  In this case, 6 actually equals 19 (if you round down).  How?  Glad you asked...

When gasoline is burned, vaporized carbon atoms bond with oxygen in the air, making carbon dioxide (one carbon atom + two oxygen atoms).  Since carbon and oxygen have similar weight (and 1 + 2 does still make 3), the resulting carbon dioxide is roughly triple the weight of gasoline. 

I'm oversimplifying, so if you're interested in a detailed explanation, the World Resources Institute is a great place to start.  According to their calculations, burning a gallon of gas (6.25 lbs) produces 19.56 lbs of CO₂.  The Department of Energy also has a good simple explanation here.

If we continue using gasoline to fuel our vehicles, the best way to reduce CO₂ emissions is to simply get more miles out of each gallon used.

Comments

[Manu Sharma]

Thanks! I was thinking about this just yesterday. I figured that the rest must be air but was wondering where I'd find an explanation and what Google query would find it. :D

[Joe Willemssen]

Emission coefficients by fuel:

http://www.eia.doe.gov/oiaf/1605/factors.html

[Jim Beyer]

As Joe indicates, a short ton (2000 lbs) of coal emits 4900 lbs of CO2. That's 2.45 lbs of CO2 for every pound of coal burned.

The U.S. burned roughly 1100 million short tons of coal last year, or roughly 3.6 short tons per person, per year. That's 8.98 tons of CO2 released, or equivalent to that produced by 918 gallons of gasoline. For every man, woman, and child in the U.S.

So it's not just the car, its your hair dryer as well. And your TV, and your fridge, etc.

[Ken Fry]

Jim makes a good point. About 50% of our electricity is generated by coal. Another 20 percent of it is generated by various hydrocarbons, which create CO2 at about the same rate as gasoline. So add a couple more tons/person from that source.

Jim mentioned several electrical appliances. In some families, one such appliance might be an "electric" car, or a plug-in Prius -- the energy for which comes from the same coal and hydrocarbon sources.

I find it disconcerting that so many of the plug-in conversion kits for the Prius (and other hybrids) quote very high fuel mileage figures: typically 100mpg to 150 mpg. Take a long trip, and you'll find your mileage is just the same as it is in a standard Prius, if not lower due to the extra weight. The high mileage comes about only in the first 20 or so miles, when you're running off mainly coal and hydrocarbons. The marketers might just as well claim 1000 miles per gallon -- or infinite miles per gallon -- during that phase. Electricity is not free in either the monetary or environmental sense.