Can I ask a really stupid question?
My car pumps out 184g CO2 per KM
It will do about 900km on a tank of lets say 70 litres of diesel.
now a gramme of CO2 weighs the same as anyother gramme so 184g x 900 km = 165.6 kg
70 litres of diesel - I think a litre of diesel is about 0.85kg - so therefore my 70l of DERV weighs about 60kg
While I realise that adding another oxygen moecule to things will add weight, can it really add so much? Including the PMs and other gasses emitted, it seems that my 60kg of fuel when burnt will emit about 200kg of by-product. Can this be true? Can some intelligent person explain?
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Hydrocarbon fuel of any type contains very little oxygen. As the name suggests it is made of mostly hydrogen, carbon, sulphur and, nitrogen.
The oxygen will come almost solely from the air. Most of the carbon from the fuel will be converted to CO2, but a small amount will be CO (carbon monoxide) and unburnt hydrocarbon.
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I agree it at first sight looks odd- so much weight obtained from so little, and all the free energy as well.
But think back to school physics days - did you do the experiment of weighing some magnesium ribbon in a crucible, burning it, then re-weighing? The residue weighed more than the original foil, again because of the addition of oxygen from the air.
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Remember also that oxygen (atomic number 8) is a bigger atom than carbon (6), and that CO2 is mostly oxygen anyway. So your total CO2 output ought to be 44/12 x the mass of carbon in your fuel tank, which will itself be a lot less than the total mass of the fuel, because all those hydrogen atoms contribute to the mass too, but end up as water in the exhaust gas.
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The Carbon content of the CO2 is approx 27% by weight.
27% of 166 Kg is approx 45 Kg.
Chemical Formula for Diesel is approx C12H26, which means approx 85% of Diesel is Carbon by weight, so 60kg has 51kg Carbon.
I guess the other 6kg comes off as CO, soot and partially burnt Diesel products or is due to between the CO2 per km figure and the litres/km value not relating to the same test conditions.
The 184g figure is presumably the govt test figure so I guess you should use the official consumption values that go with it (is this the 900km with 70 litres) IF they are derived from the same test (possibly not?).
I sure Mr Cruncher can tackle this is greater detail.
But yes the Oxgen molecule does add alot of weight...
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thanks Saltrampen. For a while there I thought that I had quite a lot of phlogiston in my tank.
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"derived from the same test"
Which is what, exactly? I had a quick Google just now, but found very little that explains what method is used to obtain the official figures. Is is just an extrapolation from the combined fuel consumption result?
I did stumble upon this item from Auto Express that confirms that the emissions vary wildly with driving method (as you would expect) and I imagine that manufacturers have quickly become adept at presenting their vehicles in a favourable light...
www.autoexpress.co.uk/videos/featuresvideos/208833...l
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Is is just an extrapolation from the combined fuel consumption result?
It's the other way round - emissions are measured and then fuel consumption is calculated from the CO2 emissions. So there's always a direct correlation between emissions and MPG.
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so as the quoted mpg is basically unattainable, the CO2 rating is also unattainable.
if some cars are over doing the "real world" mpg by 30% (there was a thread about it somewhere) their 160g or whatever it might be is actually more like 208g?
so basically like mpg, it is all a bit of a con
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Put a large balloon over the exhaust pipe, drive for one mile, then tie it up and weigh it. Compare with the weight of a balloon filled with oxygen.
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Put a large balloon over the exhaust pipe drive for one mile then tie it up and weigh it. Compare with the weight of a balloon filled with oxygen.
Or air?
Incidentally, oxygen is denser than air, which is why those balloons won't fly off. If you blow it up by mouth it will contain more water vapor (which is lighter than air), and far more CO2 (which is denser).
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so as the quoted mpg is basically unattainable the CO2 rating is also unattainable.
That's exactly the point of the AutoExpress article, linked above.
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It's the other way round - emissions are measured and then fuel consumption is calculated from the CO2 emissions. So there's always a direct correlation between emissions and MPG.
Exhaust gases are allowed up to 4.5% (highly toxic) Carbon Monoxide. I'm not sure the chemistry involved with this, but according to New Scientist (15 years ago!
www.newscientist.com/article/mg13618471.400-mot-te...l ) , average CO emissions were about 2% of exhaust gases. Clearly then if you are emitting say 123g of CO2 and 1.5% of your exhaust gases are CO, if you increase CO2 and decrease CO2, putting CO up to say 4%, while not reducing total exhaust gases, your car is miraculously sub-120g, and far more marketable.
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Which all highlights the lunacy of trying to set fixed limits for a variable quantity. The simplest method, surely, is to remove all taxes except the one on fuel?
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Permitted CO emissions are 1.0g/km for petrol, 0.5g/km for diesel, under current Euro IV. If that 4.5% figure ever applied it must have been years ago. The limits are now absolute, not percentages.
Certainly, in theory you might just be able to nudge your car under a 120g/km threshold by increasing your HC and CO output, but given the respective molecular weights and limits, I don't think you would be able to change your CO2 figure by as much as 1g/km.
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Oh, and for what it's worth, they do include CO and HC measurements in the fuel efficiency calculation. They don't just compute it from the CO2.
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But 'permitted emissions' are nonsensical unless set at a level that cannot be exceeded. Saying an engine *can* produce 120g/km is very different from saying that it can produce no more than 120g/km. It's daft to set an arbitrary limit on something that varies in normal use!
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Saying an engine *can* produce 120g/km is very different from saying that it can produce no more than 120g/km. It's daft to set an arbitrary limit on something that varies in normal use!
It's very silly.
My sister is a right-on ecofascist vegetarian. She lives in Norfolk and needed to get to Woodford (East London) from Norwich.
The day she was going the train from Colchester to Chelmsford was out so she needed to get a two trains and a replacement coach, plus a tube, which took over three hours. This with her boyfriend.
They own something like a Vauxhall Astra. The stated reason for the journey was that she was worried about her CO2 output. When I pointed out the several trains and half-empty coach would produce more CO2 than her car, she said that the train was going anyway. Funnily enough she doesn't accept that argument when it comes to my long-haul flights to Thailand. (not to mention the train journey was significantly more expensive)
Anyway, these CO2 numbers were based on average outputs, but I think the proper approach is to drive at 1500rpm in top gear (say 50mph) along the motorway to further reduce emissions.
Meanwhile Red Ken is going to allow anyone with a sub 120g car to congest London for free soon. So get your Mini Cooper D, drive it like an idiot, emit as much CO2 as a 4x4, and you won't pay a penny.
FWIW, the ecofriendly alternative to off-peak train services (which are often underutilised) is probably a network of long-distance taxis, as they have in parts of Asia. Pack five people in a car with a small engine and you'll be much less polluting than a train.
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