Can somebody explain carbon footprints to me?

Discussion in 'Cars, Bikes 'n AFVs' started by jagman, Nov 15, 2009.

Welcome to the Army Rumour Service, ARRSE

The UK's largest and busiest UNofficial military website.

The heart of the site is the forum area, including:

  1. I posted this in another thread on the matter of carbon footprints-

    Further browsing of the stastics give the figure of 262g/km exhaust emmisions and fuel consumption of 9.2 litres per 100km

    That means (on these figures) my Discovery burns 83 grams of fuel per km.

    How does my car burn 83 grams of fuel yet produce 262 grams of CO2 emmisions per km?
    I know the CO2 is a gas but it is measured in weight on the emmision figures.

    I'm genuinely puzzled, in truth I couldn't care less how much CO2 it produces. All it does is feed the plants!
    I'm being taxed on my usage based on these figures and I would like to know what they are supposed to mean.
    Any of the boffins here on Arrse able to explain it to me please?
  2. I just released 2 grey squirrels into my back garden. Their storing of nuts,acorns etc for the winter inevitably leads to some trees being planted which cancels out my carbon footprint. Simples :D
  3. I don't have carbon footprints................I drive everywwhere!
  4. Yeh me too. Explains why I have a very green lawn, everytime I fire the Discovery up it feeds the grass next to it :D
  5. My bold
    Unless somebody can explain to me how burning 83 grams of fuel produces 262 grams of waste I cannot see any other explanation other than it being an outright lie.
  6. The products of combustion ALWAYS weigh more than the fuel as the weight of oxygen used in the process has to be added.
    I'll be more specific after I dig out my very old notes.

    Think of hydrogen,(lighter than air) combines with oxygen to produce water, (very much heavier than air).

    Edit: It's not that simple as diesel is a hydrocarbon, the hydrogen part combines with oxygen to produce water, the carbon part forms the carbon dioxide.

    Some info.
  7. Cheers Engineer, I shall read and inwardly digest.
    I struggle to see how the waste product can be over three times the quantity of the fuel burnt but I'll see if I can get my head around it!
  8. That link gives a figure of 2778grams of carbon per US gallon of diesel
    I believe a US gallon is 3.7 litres? Which equates to a litre of diesel having 750 grams of carbon in it?
    A litre of diesel weighs in at 900 grams or so
    That means 5/6ths of the diesel used in an engine is waste product?

    Or am I being way to simplistic here?
  9. Look, unless China shuts down our efforts are nothing.............. except to take more of our money in tax!
  10. Somewhat. The carbon footprint of a product also includes all the processes that deliver it to its point of use. So in the case of diesel, as you're the end user, you are responsible for its extraction, refining and distribution, plus the heating and lighting of the producers offices et al.
  11. I thought it was the marks the chimney sweep left on the Axminster?
  12. Presumably as an average. Should I not get a discount for living close to the refinery? :D
    The figures do state that the emmisons calculation is based entirely on what physically comes out of the exhaust though.
    The figures on the "whatgreencar" link in the 1st post state that for every litre of diesel used (weighing 83grams) 262 grams of CO2 come out of the back of my car.
    I shall ask by very bright nephew with a chemistry degree to explain it to me but he shall probably just smile at me and tell me I'm a bit dim :)
  13. Is there a definition of 'Carbon' given on the website? If it's just the element carbon, then you're probably right - if it's Carbon Dioxide (CO2), the weight of the carbon atom will be just under 1/3 of the total weight of the molecule. 1750/5800=~30.17% (edit: this will be less, because not all the fuel is going to become carbon - energy will be released, etc). If they're fiddling the numbers so output is given in terms of CO2, plus they're incorporating the cost of refining, plus variations in MPG (eg, city driving consuming more fuel than cruising on a freeway), it might just come to that number.

    Or perhaps I'm being overly optimistic on the ability of government agencies to be entirely truthful.

    Edit to fix figure
  14. A litre of diesel does not weigh 83 grams - a litre of water weighs 1 kilo (by definition - I forget which way) - diesel is lighter with a specific gravity of 0.820 to 0.950 - so you are missing a zero.

    You got the weights right there ...

    The purpose of a diesel engine is to convert the carbon and hydrogen content of the complex mix of hydrocarbons to CO2 and water, releasing energy. Therefore 750 grams of carbon per litre should produce (ideally) about 2750g of CO2. It won't, because not all of the carbon will burn and some will only burn to CO.
  15. I think you should also take the hydrogen content of the fuel into acount but it's not that simple as the exhaust carbon has been combined with oxygen. Water is also a by-product.

    I believe modern engines are at somewhere around 35-40% effeicient as regards fuel in/energy out.

    I've not been involved with engine calculations for a while (40 years, much has changed), bear this in mind whilst reading my statements. :)

    Some Wiki bumph.