Ask Pablo: Abracadabra in the Tailpipe


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Dear Pablo: I heard that for every gallon of gasoline that I put in my car I emit 20 pounds of CO2 emissions. How can this be?It is certainly counter-intuitive that a gallon of gasoline, which weighs 5.87 pounds, would result in CO2 emissions that weigh almost four times as much but the answer lies is simple chemistry. Gasoline is a complex mixture of hydrocarbons, meaning chains of carbon atoms, each of which is holding two hydrogen atoms.

Geek Speak:

In any alkane hydrocarbon (methane, ethane, propane, octane, etc.) the number of hydrogen atoms is always two times the number of carbon atoms, plus two. Methane, which only has one carbon atom is CH4, while Octane, which has eight carbon atoms is C8H18. The bond between the atoms is where the energy is stored and so a more complex hydrocarbon has more energy per mol but, because carbon atoms weigh more than hydrogen, the ratio between carbon and hydrogen makes the energy per kg higher for methane.

Combustion, which is a coming-together of fuel, an oxidizer (oxygen), and an ignition source, converts the fuel's chemical potential energy into heat. In the process the carbon and hydrogen atoms go their separate ways, mostly reconnecting with the oxygen to form carbon dioxide (CO2) and water vapor (H2O).

So, if the fuel is being turned into carbon dioxide and water, shouldn't the carbon dioxide weigh half as much as the fuel?

The answer lies in the oxydizer. An oxygen atom weighs 16 grams per mol, while a carbon atom weighs only 12 grams per mol and a hydrogen atom weighs in at only 1 gram per mol. When the carbon atoms give up their light hydrogen atoms during combustion they combine with two of the heavier oxygen atoms. This is where the weight comes from.

What is the exact conversion factor from gasoline to CO2?

Gasoline is made up of various hydrocarbons but, on average, we can assume that it is just like octane. Octane (C8H18) weighs 114 grams per mol. Its eight carbon atoms will join with 16 oxygen atoms to make eight CO2 molecules, at 44 grams per mol (total resulting weight: 352 grams). So every 114 grams of gasoline become 352 grams of CO2, a multiplier of 3.09, turning a gallon of gasoline (5.87 pounds) into 18.14 pounds of carbon dioxide.

What other factors need to be considered?

Of course carbon dioxide is not the only greenhouse gas that automobile engines emit. Small amounts of methane and nitrous oxide (N2O) are also created during combustion. Though small in volume, these gases are not insignificant because of their potency; methane is 23 times and nitrous oxide is 310 times more harmful to the climate than carbon dioxide. The creation of these gases depends highly on the conditions inside the engine. Since each engine varies, so do these emissions. The EPA estimates that nitrous oxide emissions from highway vehicles with a 3-way catalyst are 0.029 grams per km, or about 174 kg per year. Pablo Päster answers readers' questions every week. Got something you want to ask Pablo? Write to: Pablo(at) Additional Resources on Fuel Economy: No Wonder Fuel Economy is Stagnant, Cars Ballooned Up Since 1980 Obama Announces New Fuel Economy Standards (35.5 MPG in 2016) 2009 Fuel Economy Guide: Toyota Prius First, Lamborghini MurciĆ©lago Last