David Lock

I read that burning a litre of petrol produces about 2.35 kgs of CO2. For diesel it is 2.66 kgs. I checked another source with similar figures.

Are there any chemists or fuel technologists on here that can confirm these figures and explain them please?

I just can't get my head around them as it just seems so much. Thanks, dl

LG John

This was raised before in relation to CO2 produced from air-traffic. You might pick it up in a search. Some chemist geek did some fancy maths to prove that the CO2 produced by X weight of fuel could actually weigh more than X weight of that fuel. Seems a bit odd to me but I know no better. Peronally I'd not worry about it - global warming is happening, we are not doing about, adapt........... or die

TopBanana

It's because the carbon in the fuel combines with oxygen from the air. Oxygen weighs quite a bit (14g/mol in fact)

Fart Man

I suppose that for every carbon atom of the fuel (petrol and diesel being chains of hydrocarbons; hydrogen and carbon), it has to combine with 2atoms of oxygen when it is burnt, so twice as much oxygen is needed per atom of carbon.

Now engines don't breath oxygen, they breath air, which already has loads of CO2 in it, combine it all together and you end up producing more weight in CO2 than the weight of the fuel itself.


Did that make sense?

TopBanana

They don't count the CO2 the engine sucks in

Fart Man

Oh well, thats one government conspiracy quashed - but it wouldn't suprise me one bit if they did

How about the CO2 the driver emits too?

David Lock

Thanks for replies. I do have mild concern about pollution and I do like to try and understand the basic figures.

I thought a family car would just chuck out a few odd pounds of CO2 in a year. In fact I reckon a 30 mpg car doing 30,000 miles a year would emit just over 10 tonnes of CO2, which is about 9.9 more tonnes than I would have guessed

Fart Man

Now, I wonder if you could somehow adjust the burning process and inject Helium and somehow get that to combine with the CO2 to make it lighter (HeCO2 ? )to get in a lower tax bracket

speedking

The section titled "Example calculation" here shows the principal involved.

One C(12g.mol-1) combines with two O(16) to give one CO2 (44). If you burnt a kilogram of carbon you would get 44/12 = 3.7kg of CO2. The extra is the combined oxygen from the air.

warrenm2

However that reaction doesnt actually happen in an engine. The basic formula would be 2C8H18 + 24O2 -> 16CO2 + 18H2O

Some of the mass of the fuel leaves as water vapour

David Lock

So do your more detailed figures make a significant difference to the CO2 emission weights I have quoted?

Is your formula for petrol or diesel?

dl

dnc

As above
1 mol petrol (octane c8h18) burns to give 8 x mols carbon dioxide. Taking density of petrol at 0.8 g/ml, 1 litre=0.8 kg. This then gives 2.47 kg CO2. Difference between this and figure quoted will be due to some carbon monoxide, CO, being produced by incomplete combustion (at expense of CO2) and also that petrol will not be pure octane but will have other similar hydrocarbs present. Also my figure of 0.8 for density might be 0.75 - 0.85 and this will give a different figure.

From this and your mpg figure you can work out you g/km CO2 emissions

HTH
DNC

andys

Hey less of the chemistry geek I am a proper IT one now

warrenm2

Petrol - or more accurately - iso-octane

dnc

Us Chemists have a looooooooong way to catch up with you IT boys in the art of geekness
DNC

BTW if anyone is interested I think Diesel is cetane C10H24 so gives off 10 x CO2...........................no, OK!!

Prasius

iTrader: (1)

How much CO2 does a human produce then?

Just might make an interesting comparsion.