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How can I properly estimate CO2 emissions for a given travel distance (of a private vehicle)?

I found the following formula, but I'm not sure that it's correct:

kgCO2 = Average fuel consumption per 100 km * kgCO2 per liter of fuel * travel distance / 100 km

For example, if a vehicle consumes around 5 l/km, traveling distance is 35km and this is a diesel vehicle, then the result would be:

5 * 2.9 * 35 / 100 = 5.075 kgCO2

(I don't know if 2.9 if a correct value of kgCO2 per liter of diesel).

Moreover, I want to consider a vehicle occupancy factor (for instance, 1 passenger would refer to 25%, 2 passengers would refer to 50%, etc.) that also somehow impacts CO2 emissions.

Of course, many other factors (e.g. car production year) might be additionally considered. However, I'm searching for a relatively simple (though quite accurate) formula that would take into account fuel type (diesel, petrol, gas), vehicle occupancy factor, travel distance and average fuel consumption per 100km.

Divin
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  • This is not really a maintenance & repair question. Having said that the equation you provided might be accurate enough depending on the purpose. Diesel cars in general have lower CO2 emissions due to better fuel economy. – rana Aug 28 '15 at 18:44
  • I'm voting to close this question as off-topic because it doesn't meet the guidelines in the [help] – Move More Comments Link To Top Aug 31 '15 at 17:02

1 Answers1

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From the chemical side, the answer is quite simple.

When burning 12g of carbon, this carbon takes 32g of oxygen from the air and so forms 44g of CO2. The reason is simple: One carbon atom weights 12 units, one oxygen atom 16 units. So a CO2 molecule weights 12+2*16=44 units.

So, if you know how much carbon is in one liter / gallon / kilogram of fuel, you can calculate how much CO2 is released when you burn it.

Example:

Ethanol: A molecule of C2H6O weights 2*12+6*1+1*16=46 units and contains 2*12=24 units of carbon. So, 1kg of ethanol contains 0.52kg carbon, which turns to 1.91kg of CO2. The density of ethanol is 0.789kg/l, so 1 liter ethanol generates 1.51kg CO2.

Diesel (and gasoline) are mixtures of different substances, so it's hard to calculate the amount of carbon inside without knowing the composition of that fuel. So, I found a lower value for diesel on Wikipedia:

Diesel:     2.62kg CO2 per liter of fuel
Gasoline:   2.32kg CO2 per liter of fuel
Ethanol:    1.51kg CO2 per liter of fuel
LPG:     1.8-2.0kg CO2 per liter of fuel

Note: LPG is a mixture of butan and propan, but the ratio can vary heavily.

The more important question is:

How much fuel do you actually burn?

We all know the mileage stated by the manufacturers have nothing to do with reality. Highway consumes less than city, mountainous routes more than flat routes. Having more people / luggage in a car does not consume much more on the highway, but in the city / mountains, it does. Weather, altitude and driver behaviour have effects, too.
The age of the car doesn't matter (except it has a generally worse mileage). An old engine may spit more unburnt/partially burnt fuel, but that's negligible compared to the total amount of burnt fuel.

Finally, your mileage can vary a lot by outer circumstances.

So, the most accurate value can be obtained if you exactly know how much fuel you burned:

kgCO2 = kgCO2perLiter * litersBurnt

The next best value can be calculated from your experience what your average mileage is, and then you can use exactly your formula.

If you want to split the CO2 among the people in the car, do it. It depends on what you want to express with your calculation.

sweber
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