How much is diesel in your area?
27 May 2008, 03:04 PM
#32
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The fact that a diesel car does better mpg means you do still get more miles per unit pollution - but that's not the same as saying the cost per litre should also be less.
27 May 2008, 06:50 PM
#33
Andy,
If a petrol car is greener than a diesel, why on earth are the emissions of my diesel less than half of my Scoob? Your statement does not figure.
Even on a 2ltr Turbo Diesel compared to a 2ltr Turbo Petrol, the Diesel is greener by far is it not?
Regards,
Shaun
If a petrol car is greener than a diesel, why on earth are the emissions of my diesel less than half of my Scoob? Your statement does not figure.
Even on a 2ltr Turbo Diesel compared to a 2ltr Turbo Petrol, the Diesel is greener by far is it not?Regards,
Shaun
27 May 2008, 08:30 PM
#34
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A petrol car produces less CO2 per litre than a diesel, but it burns more litres per mile - so the total CO2 produced per mile is greater.
This is something which I don't think is always as clear as it could be: fuel economy and CO2 emissions are actually just two different ways of measuring exactly the same thing. All the carbon atoms that leave a car through the exhaust pipe originally entered through the fuel filler.
You can demonstrate that this is true just by looking at the advertised economy and CO2 figures for any car. For example, here are the stats for the current BMW 3-series range:
318i Petrol 142 (152) 47.9 (44.1)
320i Petrol 146 (156) 46.3 (43.5)
325i Petrol 170 (170) 39.8 (39.8)
330i Petrol 173 (173) 39.2 (39.2)
335i Petrol 218 (221) 31.0 (30.7)
318d Diesel 123 60.1
320d Diesel 128 (144) 58.9 (52.3)
325d Diesel 155 (169) 47.9 (44.1)
330d Diesel 160 (175) 46.3 (42.8)
335d Diesel 177 42.2
The first figure is the rated CO2 emissions in g/km (manual, with auto in brackets), and the second is the combined fuel consumption in mpg.
So, a little maths:
Picking the figures for the 318i manual petrol we have 142 g/km CO2
Multiply by 1.609 gives 228.49 g/mile.
The mpg figure is 47.9, divide by 4.55 gives 10.53 miles per litre.
Therefore, the total CO2 per litre of fuel burned is (amount of CO2 per mile) x (number of miles per litre) = 2405 g/l.
You'll get the same figure for any petrol car, because it's a physical property of the fuel, not the engine. For diesel I calculate about 2625 g/l, which seems a little bit low, but nevertheless the same regardless of the car.
This is something which I don't think is always as clear as it could be: fuel economy and CO2 emissions are actually just two different ways of measuring exactly the same thing. All the carbon atoms that leave a car through the exhaust pipe originally entered through the fuel filler.
You can demonstrate that this is true just by looking at the advertised economy and CO2 figures for any car. For example, here are the stats for the current BMW 3-series range:
318i Petrol 142 (152) 47.9 (44.1)
320i Petrol 146 (156) 46.3 (43.5)
325i Petrol 170 (170) 39.8 (39.8)
330i Petrol 173 (173) 39.2 (39.2)
335i Petrol 218 (221) 31.0 (30.7)
318d Diesel 123 60.1
320d Diesel 128 (144) 58.9 (52.3)
325d Diesel 155 (169) 47.9 (44.1)
330d Diesel 160 (175) 46.3 (42.8)
335d Diesel 177 42.2
The first figure is the rated CO2 emissions in g/km (manual, with auto in brackets), and the second is the combined fuel consumption in mpg.
So, a little maths:
Picking the figures for the 318i manual petrol we have 142 g/km CO2
Multiply by 1.609 gives 228.49 g/mile.
The mpg figure is 47.9, divide by 4.55 gives 10.53 miles per litre.
Therefore, the total CO2 per litre of fuel burned is (amount of CO2 per mile) x (number of miles per litre) = 2405 g/l.
You'll get the same figure for any petrol car, because it's a physical property of the fuel, not the engine. For diesel I calculate about 2625 g/l, which seems a little bit low, but nevertheless the same regardless of the car.
28 May 2008, 08:38 AM
#36
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Andy, interesting commentary which I agree with although one might bear in mind that diesels do chuck out some nasty particulates. Do you know why the EC regulations for CO2 weight testing is so complex? Since it bears a direct relationship with fuel consumption. I can't see why they don't just come up with a straightforward and standardised fuel consumption test and work out CO2 output from that. There must be a reason but I'm buggered if I can think what that is.
I'm a bit ambivalent about the damage that car emissions do but I suppose there are so many cars around that there should be some concern now that the balance of thinking is accepting that CO2 is harmful.
But let's put it into perspective. I was talking to a chap who has 13 Frigates to look after. Each one uses about 2.6m litres of marine diesel per year. Now that is a serious weight of CO2 - about 7000 tonnes - per year which is chucked out!! The Frigates use the same 2200 bhp td engines (x 4 per vessel) as used in the older Inter City trains, just thought you'd like to know that!!! dl
I'm a bit ambivalent about the damage that car emissions do but I suppose there are so many cars around that there should be some concern now that the balance of thinking is accepting that CO2 is harmful.
But let's put it into perspective. I was talking to a chap who has 13 Frigates to look after. Each one uses about 2.6m litres of marine diesel per year. Now that is a serious weight of CO2 - about 7000 tonnes - per year which is chucked out!! The Frigates use the same 2200 bhp td engines (x 4 per vessel) as used in the older Inter City trains, just thought you'd like to know that!!! dl
28 May 2008, 09:25 AM
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TBH I've no idea how it's done, or whether there are in fact two separate sets of measurements taken - my point was simply that fuel economy and CO2 emissions are, for a given fuel, exactly the same thing.
All that's changed is that, say, 10 years ago the politically fashionable worry was that one day the oil would run out, so people talked about mpg. Now it's fashionable to talk about carbon dioxide, so we talk about g/km. But because the fuel itself has remained the same, it's still exactly the same thing - all that's changed is the political emphasis.
The irony is that the oil will run out - I can't see anyone reasonably suggesting that massive reserves will be left in the ground forever - and that this will forcibly reduce carbon dioxide emissions considerably. The only real question is the timescale over which this will happen. I hope it's longer than my lifetime; the economic consequences will be catastrophic.
All that's changed is that, say, 10 years ago the politically fashionable worry was that one day the oil would run out, so people talked about mpg. Now it's fashionable to talk about carbon dioxide, so we talk about g/km. But because the fuel itself has remained the same, it's still exactly the same thing - all that's changed is the political emphasis.
The irony is that the oil will run out - I can't see anyone reasonably suggesting that massive reserves will be left in the ground forever - and that this will forcibly reduce carbon dioxide emissions considerably. The only real question is the timescale over which this will happen. I hope it's longer than my lifetime; the economic consequences will be catastrophic.
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