AskPablo: The Tailpipe Mystery

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

15 responses

  1. Thanks Pablo. This is a great explanation of what for many people is an obscure concept. I hope you don’t mind if I borrow from this analysis when I talk to DriveNeutral members about CO2 emissions from their tailpipes and why they weigh so much. 12,000 lbs is a lot when you consider that CO2 is a lighter than air gas. Maybe next week we can talk about what “parts per million” in the atmosphere really means? I mean how many millions of parts are there anyway? Just kidding. Thanks for everything you do Pablo and keep up the good work.

  2. Good explanation but the myster continues, with irony. Your calcuations and those from other carbon offset calculators don’t include the effects of ethanol in gasoline and N2O formation in all vehicle catalytic converters. Why?
    In the case of ethanol, CO2 emissions that contribute to global warming would be reduced if it were included in calulators. The amount of ethanol in gasoline will increase each year thanks to implementation of a new Renewable Fuel Standard. The RFS started this year (2006) and will continue at least to 2012. It turns out, the RFS is the ONLY CO2 national emissions reduction program (although neither Alaska nor Hawaii are included unless they choose to opt in.). So, let’s give the feds the credit due them by taking credit for it in the calculations. The effect is rather small per vehicle–lowering CO2 emissions by very roughly 5%. But, it’s a start and it applies to EVERYONE who uses gasoline not just the relatively small number who decide to offset their emissions from driving.
    Now on to including N2O, called nitrous oxide (which is diffeent from NOx, NO or NO2, called oxides of nitrogen). N2O is formed in the catalyst from the nitrogen in the air used to burn the fuel. N2O is important becuase it is 23 times more potent in trapping heat in the atmosphere than an equivalent amount (mass) of CO2. The amount of N2O formed varies according to the type of catalyst. US EPA and IPCC estimates of the grams of N2O emissions per mile driven agree: roughly 0.06 to 0.3 grams of N2O are emitted per mile driven. You can do the arithmetic. You’ll find the CO2-eauivalent emissions of N2O are small compared to CO2 (don’t forget to include the factor of 23 so you can compare it to CO2 emissions).
    Including effects of the new RFS and N2O on equivalent CO2 emissions is about a wash. So, it seems that the new RFS will just about the offset the N2O emissions.
    This is somewhat ironic: the new RFS standard offsets the CO2-equivalent emissions that are the result of new (catalyst) technology, adopted to reduce VOC, NOx and CO tailpipe emissions (which form urban smog).

  3. Thank you for this great contribution Charlie! There are two reasons I did not include this in my column: 1) I was trying to keep it somewhat simple so that everyone could fully appreciate the answer, and not end up more confused, and 2) While I didn’t know the exact number, I knew that the good and the bad would essentially cancel each other out. There are other gasoline additives that I did not mention either, such as Methyl-tertiary-butyl-ether (MTBE) which was designed to make exhaust cleaner, but has ended up poisoning much of our water supply. Books could be written about the science of tailpipe emissions and I am not the one to do it, at least not right now. Thanks again!

  4. Pablo,
    I understand the need to keep things simple. But, the discussion has to be elevated if we’re to make any political progress. So, I would encourage that you err on the side of being current and clear rather than on simple and outdated. Further, a key point is that for the first time something is being done nationally to reduce CO2 emissions from cars that use gasoline. The RFS requires approximately a two-thirds increase in ethanol use through 2010. That’s nothing to overlook, especially when it means it’s also a one-third reduction in the iincremental growth in sales of gasoline manufactured by refiners over the same period. That hurts refiners economically, reduces the risks of global warming and gets us all of us on the right track! Refiners fought the RFS hard and lost. They’ve fought ethanol use in gasoline for more than 20 years. (Incidentally, there are adverse consequences to ethanol use in current vehicles–evaporative emissions on gasoline increase because of unexpected permeation, but that’s another story that will be remedied.)
    Now we can turn to improving vehicle designs, etc. to reduce CO2 emissions, which takes much longer to have its benefitical effects. We (globally) must stop adding 400-years worth of stored carbon to the atmosphere every year. Everything helps, even if incrementally its small and complicated to understand.

  5. Charlie, I think my column served its purpose of informing people on the basics, and this discussion forum is serving its purpose on advancing the discussion.
    The whole ethanol issue was far too big to include in this week’s column. Here is my quick take on it:
    Ethanol comes from factory-farmed corn, corn comes from Ammonium Nitrate fertilizer, which comes from Natural Gas using the Haber-Bosch Process. I have not had the time or resources to do a complete analysis of Ethanol myself but there are many people that claim that it requires more fossil fuel energy to make ethanol than you actually get out of it (and therefore more carbon emissions). I will stay neutral on that issue until I find the funding to see for myself. What do you think about it?

  6. Pablo,
    My comments addressed two points: calculators used by carbon offset providers and education of the public about remedying causes of global warming (your readers).
    My question about accounting for ethanol-use in gasoline on CO2 emissions in CO2 calcuators still stands. Why isn’t it included? Shouldn’t it be? That the reduction is relatively small shouldn’t matter to the calculator but it does to the driver. Using ethanol lowers the CO2 emissions that contribute to global warming. Ethanol is used nationally and more is certain to be used. The amount used is known: its prescribed by federal law. All it takes to account for ethanol in the calculators is a simple change in the emission factor. Shouldn’t drivers who want to offset their emissions receive that benefit? (I mentioned N2O as a point of information and to add irony. Those emissions are dropping as catalysts improve–by 80% in California vehicles.)
    I understand and appreciate your wish to inform people on the basics. That’s helpful. I just ask that you use the opportunity to get them current and to push the offset providers to be the same.
    On the use of ethanol. While you claim neutrality, you only cite the negaives of opponents. I’m not an ethanol promoter, especially made from corn. While I bellieve the best work shows that it is net beneficial, the federal subsidies for corn-based ethanol have and continue to introduce market distortions, which, contrary to corn-grower claims, may delay the path to cellulosic-based ethanol. If cellulosic-ethanol becomes commercial, corn-based ethanol will have faclitiated the development of infrastructure and scale. I look forward to your piece.

  7. Charlie, I certainly agree with you on the importance of being accurate in measuring emissions and taking into account all factors. But for carbon offset services like DriveNeutral ethanol may present a computational problem. Since different states have varying levels of ethanol in their fuel you would have to ask not only where a driver lives, but also which state they get their gas in. You could also suggest that each driver needs to log their fuel economy, rather than using national averages, since fuel economy is dependant on driving style and traffic conditions. At under $10 per metric ton of CO2 most customers of carbon offset services are not going to mind “overpaying” by 2% ($0.20) because they would rather offset a bit too much than not enough. When carbon offsetting becomes required I would be fully in support of complete emissions accounting, or even a tailpipe sensor in every car.

  8. Pablo,
    It’s not nearly as complicated as you suggest. The buyer does nothing. The change in calculator is very simple. The buyer simply is assured that best-estimation practices are being followed.
    Here’s why: The RFS is a national fuel standard. So just like calculators now use average fuel economies for model years and vehicle type when drivers don’t know them, the calculator can use the (national average) federal RFS set and enforeced by law. (Better than fuel economy standards, which are not enforced, right?) That RFS average is 2.78% for 2006, which grows to 5.10% by 2010, according to EPA estimates. Because the emission factor in the caculator would be reduced by 2.78% for 2006 and later by up to 5.10% that’s the difference in emissions. What the price is and how the tiers are set are all topics that are irrelevant to doing the calcs (incidentally not measurements as you say in your first sentence)correctly.
    You’re probably right that a customer doesn’t care about a few cents here or there, or about buying too many credits. If they did, they would not like the coarse tier structure that TerraPass and Drive Neutral have, which has some paying too little and some too much depending on where one draws the line. And by the way, you can’t be serious about putting sensors on cars to measure emissions or be serious about laws that require vehicle offsets. Just think for a moment about the administrative burden, the problems of enforcement, etc. Calculating emissions is much, much more accurate and a lot less expensive than measuring, recording and reporting the result to some place or someone. All the fuel used in some period goes to carbon (old carbon that is rather than carbon used to make the fuel–think ethanol), which is where this discussion started. So, let’s end this where we began. If you think its all too complicated, so be it. I have to ask this: isn’t loading all the new car data and corresponding mileage figures into the calculator each year more complicated than changing the emissions factor one time each year?

  9. ok the numbers are there.
    but, what about the fact that one netflix customer is getting 18-25, 3 mile trips in a month
    and a store renter is only taking 4-8 a month
    netflix renters are conditioned to continuously expect a new disk every other day.
    while picking up and dropping off a store rental, you are probably on your way to and from another destination anyway.
    we live in a walking town, and while some people drive, many of our customers walk to rent. i’d like to see how netflix is making a fat lazy cheap society, and still, really making no profound contributions to the communities they infiltrate.
    thanks to netflix, hundreds of mom&pops go under. and this is exactly the mission of netflix and blockbuster.

  10. Dear Charlie,
    You would like to be so perfer.
    May I suggest you make a clear calculation concerning the CO2 emission for a medium passager car, light commercial vehicule,a truck, a boat = a tanker and a plane.(take the most polupar models/types of these tranport)
    info per miles or per gallons
    for car it could be petrol/gaz of Diesel
    thank you

  11. Dear Charlie,
    You would like to be so perfect.
    May I suggest you make a clear calculation concerning the CO2 emission for a medium passager car, light commercial vehicule,a truck, a boat = a tanker and a plane.(take the most polupar models/types of these tranport)
    info per miles or per gallons
    for car it could be petrol/gaz of Diesel
    thank you

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