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Pounds of CO2 per gallon of gas

Each gallon of gas produces about 14 pounds of carbon dioxide. Over 12,000 miles (the distance an average American will drive in a year), a 25 mpg car will produce 3.4 tons of carbon dioxide.

Case study: The Babcock School

With all the different kinds of cars and SUVs out there, we figured it would be tough to create a representative average. Instead, we calculated how much CO2 is released per gallon of gas. With that information in hand, we just needed to figure out how many miles Babcock's fleet drove in a year to know how much carbon dioxide is produced by students, faculty, and staff driving back and forth to school.

- Jack Clayton

1. Carbon dioxide produced by one gallon of gas

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Here's a little chemistry to see how much CO2 is released when you burn one gallon of gas in your car. Cars combust a mixture of gasoline and air to drive pistons and turn their drive shaft. The drive shaft turns the wheels.

Gasoline is a mixture of different hydrocarbons, but it is composed mostly of octane. The percent of octane present is given in the octane rating of the fuel. Octane's chemical formula is C8H18, and it combines with atmospheric oxygen (O2) to produce carbon dioxide (CO2), water (H2O), and energy. This combustion reaction is the same reaction our body uses to generate energy from food. Other elements in the air (such as nitrogen and carbon dioxide) are burned as well. They create other molecules like nitrates, but they are ignored in our calculations.

Here's our chemical reaction so far: C8H18 + O2 -> CO2 + H2O .

We just need to balance it so that the number of elements that enter our reaction exit our reaction. Unbalanced, the formula has 8 carbon atoms (C), 18 hydrogen atoms (H), and 2 oxygen atoms (O) creating 1 carbon atom, 3 oxygen atoms, and 2 hydrogen atoms. Atoms can't appear or disappear! We need to balance the equation by saying how many molecules of each element entered and exited. We do that by putting coefficients in front of each molecule in our equation that, when multiplied by the number of atoms in the coefficient's associated molecule, create the same number of atoms entering and exiting. Balancing the combustion reaction yields, 2C8H18 + 25O2 -> 16CO2 + 18H2O. 16 carbon atoms, 36 hydrogen atoms, and 50 oxygen atoms combine to create 16 carbon atoms, 36 hydrogen atoms, and 50 oxygen atoms. When you look at the coefficients, our combustion reaction takes in two octane molecules for every 16 carbon dioxide molecules it creates. That means for every kerosene molecule, 8 carbon dioxide molecules are created. Scaling up, for every gram of kerosene consumed, 8 grams of carbon dioxide are created.

That's all the chemistry we need. We just need to know the density of octane so we can move from a measure of volume like gallons to a measure of mass like grams. The density of octane is 0.803 grams/milliliter or 3,039.69 grams/gallon.

1

Gallon of fuel (a mixture of hydro-
carbons) burned

>

0.93

Gallons of octane consumed (1)

>

2,826.91

Grams of octane consumed (2)

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14.1

Pounds of CO2 produced per gallon consumed (3)

  1. 1 gallon gas * 93% gallon octane / 1 gallon gas
  2. 0.93 gallons octane * 3,039.69 grams octane / gallon
  3. 2,826.91 grams octane * 8 grams CO2 / 1 gram octane * 1 kilogram / 1,000 grams * 0.625 pounds / kilogram

There you have it. To the best of our ability, on average...

one gallon of gas produces 14 pounds of carbon dioxide.

Case Study: Babcock Graduate School of Management

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The Babcock School is home to nearly 700 students, faculty, and staff most of whom drive between school and home several times a week. To figure out how much carbon dioxide all that driving produces, we first need to figure out how many miles Babcock's student, staff, and faculty drive in a year.

Exhibit A Total miles driven by Babcock students, staff, and faculty between home and school in a year

Units marked in columns

Group Group size No. back-and-forths
to school each week
No. weeks
driving to school
Average one-way
trip length (miles)
Total miles in a year
Full-time students
160.0 (1)
8.0 (3)
32.0 (6)
5.0 (10)
409,600
All WP students
400.0 (2)
3.5 (4)
46.0 (7)
10.0 (11)
1,288,000
Faculty
40.0
7.0 (5)
40.0 (8)
7.0 (12)
156,800
Staff
60.0
7.0 (5)
48.0 (9)
7.0 (12)
282,240
TOTAL        
2,136,640
  1. Two classes with around 80 students each
  2. Four programs with two classes of 50 students each
  3. Four days of classes and four meetings or events each week at school
  4. Two nights of classes and 1.5 meetings each week at school
  5. Five working days plus two lunches off campus
  6. Two 16 week semesters
  7. 52 weeks less 6 weeks of vacation
  8. 52 weeks less 4 weeks of PTO, 2 weeks of official holidays, and 6 weeks researching at home
  9. 52 weeks less 2 weeks of PTO taken and 2 weeks of official holidays
  10. Most full-time students live in apartments and houses relatively close to campus
  11. Most WP students live farther away or travel greater distances from work
  12. Faculty and staff live farther away from campus in surrounding cities

We're halfway there. We know that students, faculy, and staff drive around 2,136,640 miles each year. Now we just need to figure out the average fuel efficiency of Babcock's fleet of cars and SUVs to figure out how many gallons of gas all that driving consumes.

Exhibit B Average fuel efficiency of all student, faculty, and staff cars

Units marked in columns

Fuel Efficiency
(miles/gallon)
as a % of all student,
staff, and faculty vehicles
Weighted fuel
economy component
Cars
30
50.0%
15
SUVs and trucks
18
50.0%
9
TOTAL     24
Source: Several quick parking lot surveys, plans are in the works to launch a survey for better data.

Now we know how far students, staff, and faculty drive each year, and we know the average fuel efficiency of all those cars. Putting it all together yields...

89,027

Gallons of gas consumed in a year (1)

>

14.1

Pounds of CO2 per gallon of gas consumed

>

1,257,947

Pounds of CO2 (629 tons) from Babcock's cars

  1. 2,136,640 miles * 1 gallon / 24 miles

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