AskPablo: The Tailpipe Mystery

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This week on AskPablo I address the question: “Why does burning one kg of gasoline in my car’s engine result in more than 1 kg of tailpipe emissions?” Well, you could dust off your college chemistry textbook and figure it out yourself, or you could leave the headache to me.


OK, here it goes… Our petrochemical fuels are essentially hydrocarbon chains (a chain of carbon atoms, with two hydrogen atoms for each carbon atom, plus one on each end). One such “alkane” molecule is Octane, or C8H18, which has a molecular weight of 114 g/mol (one mol is equal to 6.02 x 10^23 molecules). Let’s assume, for my sake, that gasoline consists solely of Octane.
220px-Fire_triangle.svg.png Fuel makes up one critical part of the fire triangle (fuel, oxygen, heat), a car engine adds the other two, oxygen from the air intake, and heat from the spark plug spark (without one of these, no fire). When combustion occurs a chemical reaction releases heat (which expands the air in the engine and provides the energy that moves the car) and breaks up the C8H18. Since the broken up C and H atoms are lonely they grab hold of the O’s and form new molecules. These include C02 (carbon dioxide) and H20 (water vapor). In the absence of enough oxygen CO (carbon monoxide) forms, and sometimes Nitrogen from the intake air forms NOx (any number of different nitrogen oxides).
So, one 114 g/mol Octane molecule and 25 O’s (roughly 13 O2’s, because they use the buddy-system, weighing 32 g/mol each) join forces to make 8 CO2 molecules (44 g/mol) and 9 H20 molecules (34 g/mol). So let’s see how it all adds up per kg… One kg of Octane contains 8.77 mols (1000g/kg divided by 114g/mol). And from what I said earlier we know that this 1 kg of Octane results in 70.16 CO2 molecules (8.77 x 8), or 3,087g (70.16 x 44). We also know that the reaction makes 78.93 H20 molecules (8.77 x 9), or 2,683.6 g (78.93 x 34). So the total tailpipe emissions per 1 kg amount to around 5,770g (3,087g + 2,683g), of course the H20 isn’t really of much concern. The CO2 is worrisome though, since it is a major contributor to global climate change. So now you know where the numbers (3.087 kg of CO2 per kg of fuel) come from that are used to determine your carbon emissions by carbon calculators such as the one used by DriveNeutral.
Let’s see how much my car emits per year, based on these numbers… My car averages 30 miles/gallon, or 12.75 km/l. The density of Octane is .703 kg/l, or 1.4225 l/kg. This means that I travel 18.14 km/kg (12.75 km/l x 1.4225 l/kg). If I drive 18,000 miles per year, or 28,968 km (let’s round up to 30,000 km), I would need to buy 1,654 kg of fuel (30,000 km / 18.14 km/kg). Based on the factor that I derived above this amounts to carbon dioxide emissions of 5,105 kg (1,654 kg x 3.087 kg/kg), or roughly 5.1 metric tons. The DriveNeutral calculator returned a remarkably similar 11,640 lbs, or 5,279 kg (the result is different because we assumed earlier that gasoline is made up entirely of Octane, which is not true), putting me in Tier Two. Offsetting my carbon emissions for the year cost me only $40 (about the price of a tank of gas) and I got my pretty new DriveNeutral certification decal in the mail on Friday…
Having trouble grasping exactly what 5.1 metric tons of carbon dioxide looks like? Well, the average party balloon can hold around 1 cubic foot of gas. CO2 weighs about 11g per cubic foot, so my annual emissions would fit in just over 464,000 party balloons (imagine 25 ballons popping out of your tailpipe every mile, or one every 211 feet). Still too ambiguous? Fourteen cars-worth of annual tailpipe CO2 emissions would fill the Hindenburg. And just like the passengers of the Hindenburg, we can expect a fiery downfall if we don’t do something substantial about global climate change soon.
Pablo Päster, MBA
Sustainability Engineer
pablo(dot)paster(at)gmail(dot)com