CO2 ratings and fuel efficiency don't add up.
Can anyone explain to me how a vehicle with better fuel efficiency can have a poorer carbon dioxide rating and hence higher road tax than another using the same fuel but returning a lower mpg? This doesn't seem to make sense.
The chemistry is that a given fuel type contains a fixed amount of carbon per litre, and 99.9% of that carbon can basically go only one place, out the exhaust pipe as carbon dioxide. A tiny amount may go into the oil or get trapped in the cat or silencer, but that is inconsequential. Carbon dioxide always has two oxygen atoms per carbon atom. Thus there doesn't seem to be much room for variation here, a vehicle that burns one gallon of a given fuel grade in doing 30 miles under standard test track conditions MUST, by the laws of chemistry, release the same amount of carbon dioxide as another likewise doing 30 miles from a gallon of the same fuel.
Moreover, a 60mpg car will only process half the amount of carbon atoms during the same test, and so MUST have a better CO2 rating. Or, so you would think.
Thus it would stand to reason that for a given fuel, there should be NO difference between CO2/Tax ratings and mpg ratings. So, why are there such differences, often large differences, between models?
The only way I understand your question is to assume you are comparing petrol with diesel.Am I right?
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29-01-2015, 10:24 PM
(This post was last modified: 29-01-2015, 10:25 PM by ntm1275.)
The CO2 reading are probably calculated at a set RPM as you would find in a MOT test
Quote:Quote from http://www.motuk.co.uk
Quote:The engine must be at its normal idle speed and operating temperature when checking exhaust emissions.
That means that from that test, manufacturers can tailor their cars to have specific CO2 emissions at the given rpm
Add a Cat and emissions are lowered, at a Particle Filter (diesels) and the emissions are lowed again
They don't set the CO2 over the whole of the rev range just at the set idle speed
My 110 HP XTR has more BHP and Torque that a 90HP model, but because it has the Particle Filter, the emissions are lower meaning it is in a lower car tax bracket
But I do agree with what you are trying to say, but it would be very difficult for a manufacturer to provide CO2 emissions details over a full rev range taking in to account of everybody's different driving habits, so that is why it is set at a fixed rpm point
(29-01-2015, 10:24 PM)ntm1275 Wrote: The CO2 reading are probably calculated at a set RPM as you would find in a MOT test
Hmm, I think you are confusing smoke or carbon monoxide emissions (actual pollution as in cough, cough) with CO2. The MoT test does not cover CO2 ratings anyway.
CO2 ratings, and hence road tax banding, are based on the notional amount of carbon dioxide 'greenhouse gas' the car emits per mile driven at a typical constant speed. Clearly that could not be assessed at idling speed or with an unloaded engine, it could only be measured under realistic road conditions. I presume
it is based on test track results. The question is, why do these differ from fuel efficiency ratings?
29-01-2015, 11:32 PM
(This post was last modified: 29-01-2015, 11:56 PM by ntm1275.)
I understand what you are saying and I am probably confusing the figures, but in my own case, my lingo has a DPF and so my CO2 is supposed to be lower and so my tax is lower
Does a DPF reduce CO2?
That is a question I don't know the answer to
If the answer is no, then they must be using a calculation other than just CO2 to determine a cars tax band
As far as I know, the only difference between my 110HP model and a 70/90HP is that it uses a VNT turbo and a different ecu map and the DPF of course
I've tried searching for "how are vehicle co2 emission calculated", but there doesn't appear to be this information available
If you are correct that the CO2 rating are based on "notional amount of carbon dioxide 'greenhouse gas' the car emits per mile driven at a typical constant speed", that doesn't take in to account all driving conditions
A heavy footed person is probably never going to achieve the published figures as is a person who spends a lot of their time stuck in traffic jams
I suspect the figures are probably calculated as a best possible scenario, so that the car manufactures can sell more cars based on their better 'green' credentials than their competitors
The mpg figures for my car say that Extra Urban motoring can get me about 60mpg which is virtually impossible to get unless I was living in a completely flat place with minimal traffic and travelling at 40mph or less - I've tried it and have never got above 58mpg over a short distance. As soon as I start stretching the miles travelled, I encounter a hill and the mpg drops to just over 50mpg then goes back up to about 55mpg goes down the other side. Eventually the ups and down start to average out the more miles I do and settle about 52ish mpg.
This then brings me back to what I said earlier "I suspect the figures are probably calculated as a best possible scenario, so that the car manufactures can sell more cars based on their better 'green' credentials than their competitors"
In the end, I'm happy to have a 100HP model with DPF if it means I have cheaper car tax
(29-01-2015, 09:43 PM)ron Wrote: The only way I understand your question is to assume you are comparing petrol with diesel.Am I right?
No, I'm saying that it doesn't make sense for any two vehicles that use the same fuel and return the same mpg to have a different CO2 rating.
There is no such thing as perfect Stoichiometric combustion, not all the Carbon is converted into CO2, some becomes CO, some into solid particles of Carbon and various other Carbon compounds.
Expect future VED banding to be based on a combination of CO2. NOx and CO.
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Although, the amounts of CO and particulates in exhaust are of the order of a fraction of a percent by volume. Therefore that still leaves 99% of the carbon to exit as CO2, does it not? Diesels in any case burn with excess oxygen except at full throttle, so in cruising almost all carbon will be used up. Most modern petrol engines burn stoichiometric except when accelerating, and maintain that to within quite fine tolerances. So in both cases a cruise test should show all fuel consumed.
Agree that it would be more sensible to base VED on actual pollution that causes health or environment problems (asthma, smog, etc) but the EU Commission has a bee in its bunnet about global warming. (Regardless of whether that is a real concern or not, and I'm not entirely sure it is, especially as the met guys reckon temperatures stopped rising a long while back.)
What do you expect if the government decide it
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