It’s true: you burn a litre of diesel which weighs around 840g, but you end up with 2.64kg of carbon dioxide. Petrol is similar – 720g per litre, but 2.35kg of CO2. Why is this?
It’s related to what happens to the carbon in the fuel.
Diesel is about 86.2% carbon. That means there is approximately 720g per litre.
Petrol is about 87% carbon. That means there is approximately 640g per litre.
When the fuel is burned, the carbon combines with two oxygen molecules to form carbon dioxide. What causes the massive weight gain is the relative weights of the two elements.
Carbon is a molar weight of approximately 12, but oxygen has a molar weight of approximately 16, and there are two oxygen atoms for every carbon atom. Basically, you start with something that has a relative weight of 12, but end up with a product with a relative weight of 44! (12 + 16 + 16).
That gives us a simple formula we can use to determine how much CO2 is created from one litre of fuel being burned.
Petrol: amount of carbon in one litre (640g) / molar weight of carbon (12) * molar weight of CO2 (44) = 2.35kg.
Diesel: amount of carbon in one litre of diesel (720g) / molar weight of carbon (12) * molar weight of CO2 (44) = 2.64kg.
Those are the theoretical amounts. However, incomplete combustion causes some loss.
If you know how much fuel you used and how far you travelled you can use this carbon dioxide calculator to figure out how much CO2 you created while driving. The figures are close approximations because the quality of the fuel, the temperature and the completeness of combustion all affect the exact amount of CO2 created.
Vehicle manufacturers give an estimated grams per kilometre figure for new cars and this figure has been declining gradually over time, as you can see in the graph on page 17. If you look at a vehicle like a Toyota Corolla, the carbon dioxide output is given as 139g/km. This will vary in real-world driving, though, as it’s based on the ‘combined cycle’ used for giving standardised fuel economy figures.