How much CO2 does a car contribute?

16 January 2011


If my car emits 218g per km, that equates to 11227g per gallon. A gallon of petrol weighs 3306g, were does all the extra mass come from? Air (inc CO2) is sucked through the engine in the ratio of 14 parts to one of fuel, so are we weighing CO2 that was already in the atmosphere.


Chris - Okay, well that's a really interesting question Alan, and I'm very glad that you raised it. So, the best way of approaching this is to think about the chemistry that's going on. So what's the chemical formula of the reaction in your engine between the fuel and the air. Well the fuel is a hydrocarbon and that means it's long chains of carbon atoms all glued together, and to make things simple, we'll assume that each carbon atom is associated with two hydrogen atoms because that's what a hydrocarbon chain looks like. So, CH2, the hydrocarbon, plus oxygen O2, and we're going to need two molecules of oxygen, is going to make a molecule of CO2 and a water molecule because the product of combustion is carbon dioxide and water. Now if we look at how much a molecule of the hydrocarbon weighs, for each CH2, that's 12 grams of carbon and H2 weighs 2. So we'll say that it's 14 grams for each mole or each group of molecules for the hydrocarbon, and the CO2 weighs 44. So therefore, the proportion of the products, CO2, that is the hydrocarbon to start with is 14 over 44. So if we ask Dave, he's got his calculator ready. Dave, what is 14 divided by 44, please?

Dave - Just below a third.

Chris - In other words, what comes out of the exhaust pipes is about three times the mass of the equivalent fuel that it took to make that in the first place. So, if we now go back to your original figures where you say that the gallon of the fuel weighs about 3,300 grams and that actually, after you've done a gallon's worth of mileage, you reckon you've produced 11,227 grams, what's the reason for the disparity? Well if we times the about 3 times 3,300, if you could just do this, Dave. So what does that come to?

Dave - It comes out as about 10,400 grams.

Chris - So therefore, we're almost in the right ballpark for the 11,000-ish that you quote in your car's figures. So actually, the reason for the disparity is that you forgot to take into account that oxygen weighs something and therefore, CO2 is actually a lot heavier than the original carbon that went in to make the fuel work in the first place?

Alan - What's coming out of the exhaust is not all that harmful then apart from the CO2.

Chris - Well you say apart from the CO2, but that's the bulk of what comes out unfortunately and if you look at the planet as a whole, burning fossil fuels has contributed an enormous amount. We put out something like 40 billion tons of carbon dioxide into the atmosphere every year. That's about 12 billion tons of raw carbon going into the atmosphere and this is fossilled carbon that was locked away, millions of years ago and turned into oil and coal. As a result, your contributing to the CO2 in the atmosphere which we think is linked to a rise in atmospheric temperatures because of greenhouse effect, and this will warm up the planet over time. The levels in the last 200 years have gone up by 30%, so you know, that's not bad for a single species to have altered the composition of the atmosphere of a whole planet by 30% for one gas in 200 years. It's pretty good going actually, in terms of an achievement from mankind, not necessarily a good one.

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