Driver Knowledge Tests

Why does burning 1 litre of fuel create over 2kg of carbon dioxide?

One litre of petrol weighs around 740g but produces around 2.31kg of carbon dioxide when burned. A litre of diesel, which weighs around 840g, produces even more – 2.68kg of carbon dioxide. This CO2 is created in the combustion process in your engine and it’s a potent greenhouse gas.

The amount of CO2 you create while driving is related to your fuel economy. The more fuel you use per kilometre, the more carbon dioxide is produced per kilometre, and this figure is often given by car manufacturers. For example, a Ford Ranger produces between 218 and 230g/km using the given combined fuel economy of 8.3-8.8 litres per 100km.

To understand why the total carbon dioxide produced is heavier than the original litre of fuel, we have to look at the chemistry: diesel and petrol are full of carbon, but carbon is quite light. When the fuel burns, the carbon combines with two oxygen molecules (both of which are heavier than the carbon) to create carbon dioxide. Each oxygen molecule weighs about 25% more than a carbon molecule and there are two of them!

To put it into number perspectives, if carbon weighs 12, two molecules of oxygen weigh 32!

What proportion of diesel and petrol are carbon, though? With diesel, about 720g of the 840g that one litre weighs is carbon. With petrol, around 640g of the 740g that one litre weighs is carbon.

Petrol: approximately 87% carbon (640g)

Diesel: approximately 86.2% carbon (720g)

There’s an easy formula we can use to provide the total approximate weight of carbon dioxide produced:

Weight of carbon in one litre of fuel divided by the molar weight of carbon multiplied by the molar weight of carbon dioxide.

Diesel: 724g/12*44=2.65kg.

Petrol: 640g/12*44=2.35kg

There are variances in the actual amount due to variables in the fuel, incomplete combustion, the octane rating of the petrol, etc, but those figures give you the approximate amount of carbon dioxide created when you burn a litre of petrol or diesel.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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