When you burn a litre of diesel, which weighs around 840g, you get around 2.68kg of carbon dioxide (CO2). When you burn a litre of petrol, which weighs around 740g, you get around 2.31kg of carbon dioxide. Why is what comes out of the exhaust much heavier than the fuel itself?
The reason is that the process of burning the fuel grabs oxygen from the air used in combustion and combines it with carbon molecules to create CO2 which is a greenhouse gas.
Carbon molecules are quite light. If we talk in molar mass, the standard weight of carbon is 12.011g/mol. But in creating CO2, it grabs a couple of oxygen atoms and these weigh just under 16g/mol each. So, you’ve got your carbon (C = 12) and two oxygen (O2 = 32). Add those together and you get 44g/mol.
What does this mean? Well, every time you take 12g/mol out of the diesel, you create a product which is 44g/mol.
But diesel isn’t all carbon. Out of the 840g that one litre of diesel weighs, 720g is carbon (about 86.2%).
Almost all the carbon combines with oxygen and you can figure it out using this equation (with the figures conveniently rounded):
The weight of carbon in a litre of diesel (720) divided by the relative weight of carbon (12) multiplied by the relative weight of carbon dioxide (44).
724g / 12 * 44 = 2.65kg. That is close enough to the 2.68kg that is quoted as the amount of CO2 created when a litre of diesel is burned.
How much C02 is created when petrol is burned?
Petrol contains 87% carbon, or around 640 grams per litre. Using the same equation above, we have:
640 / 12 * 44 = 2.35kg of C02, which is close to the given average of 2.31kg.
The weight of petrol is not consistent and it depends on things like the standard temperature and pressure and the octane rating. It’s usually given as a figure between 0.7 and 0.78kg.