How to calculate emissions from gasoline

Part One - Formula that is basis for gasoline emissions

Hey everybody. It’s the Energy Chef. Today’s recipe we will be looking at how to calculate emissions from the combustion of gasoline. This is the technical chemistry behind emissions from C8H18 aka octane aka gasoline. Our format will be in Q&A form.

Remember gasoline that when combusted is converted to energy (i.e. joules). So let’s understand the relationship between energy usage and emissions with gasoline.

Energy Calculation:

Q: How many kg of gasoline is needed to generate 1 GJ?

It’s helpful to know the combustion reaction equation of how one mole of gasoline, C8H18 creates 5.5 megajoules:

C8H18 (l) + 12.5 O2 (g) —> 8 CO2 + 9 H2O (g) + 5.5 MJ

Step 1: Let’s determine the atomic mass of a mole of gasoline (octane) to see how much is weighs. That is 12 grams per mol of carbon and 1 gram per mol of hydrogen gives you 114 grams per mol of gasoline.

12 g / mol of carbon x 8 mol of carbon + 1 g / mol of hydrogen x 18 mol of hydrogen = 96 + 18 = 114 grams per mol of octane

Step 2: So 114 g of octane generates 5.5 MJ. How much does a kg generate?

114 g C8H18 / 5.5 MJ x 10^3 MJ / 1 GJ x 1 kg / 1000 g = 20.7 kg of gasoline

A: 20.7 kg of gasoline (octane) generates 1 GJ

Now that we know how to determine the mass of gasoline used to generate 1 GJ, let’s see how we determine the emissions from combusting that gasoline.

Emissions Calculation:

Q: How many kg of CO2 are emitted from generating 1 GJ from gasoline?

So from the combustion reaction equation above we know that 8 moles of carbon dioxide (CO2) and from that we can calculate the mass of CO2 from that reaction.

Step 1: Let’s get the mass of those 8 moles of carbon.

12 g per mol of carbon + 2 x 16 g per mol of oxygen = 44 g per mol of CO2

8 mol of CO2 x 44 g = 352 g of CO2 per mol of octane combusted

Step 2: Let’s figure out the amount of CO2 grossing up the 5.5 MJ equation to 1 GJ.

Because 8 mol of CO2 is produced from 1 mol of octane when combusted to generate 5.5 MJ of energy, you can say that the ratio is 8 mol of CO2 per 5.5 MJ from gasoline combustion.

352 g of CO2 (i.e. 8 mol of CO2) / 5.5 MJ x 10^3 MJ / 1 GJ x 1 kg / 1000 g = 64 kg of CO2 per GJ from gasoline combustion

A: 64 kg of CO2 is emitted when combusting gasoline (octane) to generate 1 GJ of energy.

There you have it. The technical equations behind emissions from gasoline. To learn how this compares to miles in your car and how the EPA and other governing bodies approach emissions. Check out my next blog post continuing this example to make comparisons.

Energy Chef