recording car emissions (cragcentral #241)

Because the manufacturer has an incentive to lie, and because cars characteristics change as they age, and because it’s easy to measure properly.

Brgds
Tony

andy_ross

website team CRAGger Glasgow Leamington

I would suggest the simple answer to that, Robin, is that it is a
decision each new CRAG needs to take. I am pretty sure (and you can
check on the files pages) that the four operational CRAGs are using
the mileage method due to its simplicity and verifiability.

If new CRAGs (Wokingham, Islington, etc) wish to try the fuel
counting approach, then why not? The drivers will always have their
MoTs to go back to if fuel counting is too much of an ‘ask’ or leads
to arguments about verification at the end of the year.

It would be great if someone(or some people) had the discipline and
enthusiasm to keep a record of all their visits to the petrol
station. We could then compare the results of the two approaches – another good article here, Guy?

I suspect that the typical differential between the two approaches,
for an annual mileage of 10000 miles, is not likely to be more than
about 450kg of CO2. If these emissions are all in excess of the
driver’s carbon ration it would cost him or her about £18 in carbon
debt in a group adopting 4p per kgCO2. This is less than a tankful
of petrol! So let’s not get too worried at this stage about getting
it absolutely exactly right.

The beauty of CRAGs is that we will learn as we go and can refine
our methods as their current shortcomings become apparent.

Andy

robinsmith3

website team CRAGger Islington Leeds Wokingham WSP

Andy, perhaps I should have been a bit clearer. I hear you on the
simplicity versus equity part. But I want to KNOW FOR SURE one methods
merits over another well in advance of any adoption beyond CRAG.
(sorry about the caps)

Its implied then that volume has the equity and miles has the
practical merit.

Yep I agree with you Robin, the best measure is fuel volume since the CO2 measures are derived from fuel consumption figures which are very variable. I do not think it worth being specific about the car involved as Tom suggested since I suspect that variation is simply due to the variation in how the fuel consumption and therefore CO2 emmissions was measured. Coincidentally, both Tom and I came out with a figure for diesel of arounf 2.7 Kg CO2 per litre, which is the very figure that Andy provided in his CO2 calculations spreadsheet! Nice to get some confirmation of the figures.

Good to hear about the 60-65 mph optimum speed for fuel economy as I am finding about the same. If it was purely down to physics I believe that the faster you go the more fuel you use as it takes more energy to move an object quickly than slowly (the reason why I can walk a mile without noticing it but collapse after running a mile). But, the gearing of a car has a large influence upon this – hence driving smoothly at around 60 mph is more fuel efficient that driving at 30 mph. The only problem with driving at 60mph on motorways is that it can cause problems as lorries try to overtake.

Peter.

Having previously been a firm supporter of the recording mileage to
establish car-derived emissions – for its simplicity – I am fast
becoming convinced that fuel consumption is what we should be
measuring. Maybe we should be strongly suggesing that CRAGs adopt this
as soon as is practical?

I think that counting fuel consumption is not really that labour
intensive, especially if you get into the habit of filling up round
numbers of litres i.e. 10 litres of fuel rather than £10’s worth. We
could produce a pro-forma pocket sized card as a resource for CRAGs to
hand out to their members, similar to that used in crazy golf! Every
time you filled up you made a note of the date and the number of
litres…no real hassle!

I see this issue on a par with current CRAG rules for green energy
suppliers, biodiesel or public transport – where the economics
strongly encourages this behaviour.

The current adoption of recording mileage discourages car use but not
the use of an inefficent car e.g. 20-30 mpg. Whereas measuring fuel
consumption would force CRAGers to look at their MPG and if necessary
change to a more efficient and therefore greener car e.g. 45-60 mpg.

I say this from the point of view of a being a driver of an old F-Reg
banger. I have to admit I’m not sure what its current MPG is – I’ll have to check over the course of this week.

All the best,
Nick

tom

CRAGger Cotswolds Wychwood

I agree (following Tony’s chemistry lesson, gratefully received) that
counting fuel volume is preferable because it’s more accurate and
because it encourages economic driving and the use of efficient cars.
I’m sure Tony is right that the variation I found in
emissions:consumption is to do with counting or rounding errors,
rather than a meaningful indication of engine performance.

Remember to ask for a VAT receipt when you buy your fuel, because
this records details of date and the litres you bought. I stick mine
in my glove compartment so I can write them up at a later date.

Lastly, I concur that driving style has a dramatic effect on
consumption figures. I’ve started keeping to 60mph on motorways – this weekend I averaged 60mpg in my diesel estate, loaded with family
and associated clobber. Interestingly this exceeds the manufacturer’s
figures for extra urban consumption. It’s surprising how little the
journey time is increased (my upper limit used to be 80mph).

Many cars built in the last ten years will have a simple computer
which shows your current and average consumption figures; this is a
useful tool for tweaking your driving style to improve efficiency.

Tom

Hi Tom,

Do you have any links to further info about the differing levels of CO2 emissions for different models of cars? Are there any published lists of this info? I had always wondered about this.

This does make the carbon counting for car emissions even more complicated and could make it a real headache for some CRAGers. It does beg the question that given that volume of fuel is not a reliable indicator of emissions, how would the government deal with this issue in a national carbon rationing scheme, especially for those citizens with more than one car.

All the best,
Nick

Hi Nick,

Do you have any links to further info about the differing levels of
CO2 emissions for different models of cars? Are there any published
lists of this info? I had always wondered about this.

The Vehicle Certification Agency is a pretty good resource. Go to
http://www.vcacarfueldata.org.uk/search/search.asp to search for a
particular model of car. Any advance on the Bentley Azure A4’s heinous
495 grams CO2/km?

Olly

I have had a play with the data provided in the VCA source provided below by Olly. Both it and car adverts only provide one value for CO2 emmissions but they provide 3 values for fuel efficiency – one for urban driving, one for extra-urban and one for combined. I took 3 example cars from different manufacturers (Audi, Ford and Vauxhall) and used the fuel efficiency data to work backwards to determine how they had calculated the CO2 emissons figures. It seems that they are taking a standard figure for diesel fuel of around 2.7 Kg per litre and using this to calculate the emissions based upon the combined fuel efficiency (it comes out as 2.71 for the data provided for an A4, 2.64 for a Focus and 2.69 for a Vectra). This suggests that there is a direct link between CO2 emmissions and fuel volume – which seems to make sense since the carbon in the fuel has to go somewhere). Consequently the best way to determine emissions and cars is to measure fuel used since this is fixed and is not dependent upon efficiency, speed of driving, etc. We could determine an average figure for petrol and diesel using the above technique to use in a carbon calculator, but I would prefer to find a definitive source such as the DTI which usually publishes such data.

Sorry if you got bored with that, but I enjoyed it. I will go away and calm down now.

Peter.

tom

CRAGger Cotswolds Wychwood

Hi Peter

I’ve also been furiously checking the VCA figures… there is some
variation in CO2 / fuel across different vehicles, but less than I’d
thought. Here’s a range of diesel cars:

VW Touareg: 2700
Skoda Fabia: 2696
BMW 1 Series diesel: 2679
Ford C-Max: 2645
Toyota Land Cruiser: 2630
Mercedes A-Class: 2629
London Black Cab: 2629
Smart four: 2625

I’m dividing the combined metric consumption by 100, and multiplying
it by the g/km figure.

I’d say that this variation is just enough to be worthwhile taking
into account for rationing purposes. What do you think?

Best wishes

Tom

Quite. The C atoms in the fuel either end up as C, CO, CmHn or CO2 in the atmosphere or as gunge in the engine, and the CO and CmHn soon get oxidised in the air to CO2. There’s nothing else that can happen to them. The volume of C – soot – emitted isnt going to be very high and if it isnt emitted the engine wont work for long as it’ll get clogged. Both petrol and diesel are almost exactly (CH2)n though diesel is denser so has more (c10%) C atoms per litre.

Why bother using theory which may be wrong especially for an aging car and which the manufacturer has no great incentive to get right, when it’s very easy to measure actual use.
The difference in quoted figures found by Tom are almost certainly errors of measurement, else density and C/H ratio of different samples of fuel might vary slightly – but nothing to do with the car.

Rgds, Tony.

Hi Tom,

I am interested in what you said about CO2 emmissions not being linked to fuel efficiency – I had thought (maybe incorrectly) that a certain volume of fuel contained a certain mass of carbon and so would release a known mass of CO2, irrespective of how efficiently it was burned. A certain volume of fuel may translate into varying milage depending upon vehicle efficiency, but does it not always lead to the same mass of CO2?

Peter.

I’m not an expert but ‘particulates’ are often unburnt hydrocarbons which more efficient burning would actually have reduced, rather like in a really efficient woodstove. They used to talk of engines burning lean when hot and rich (with choke) when cold when the fuel to air ratio was much greater. This obviously has an effect on how much fuel is combined with how much oxygen to make how much CO2.

There’s been a scandal recently, can’t remember where reported, on car official CO2 and milepergallon figures. They are done on a sort of rolling road with simulated conditions which bear little relation to real life. Many are a serious underestimate of the consumption achieved in real life. Driving predictively so as to need less braking and light on the right foot so as to accelerate gently and only as necessary can have a great effect on mpg as can speeds at which distance is covered. Doubling speed quadruples energy use as a rule of thumb. So fuel consumed is a better measure than just engine size and mileage. In the long term average efficiency figures for a particular car and driver could be arrived at. Wouldn’t it be great if they became a subject of competition to see who can use least fuel.

Duncan

robinsmith3

website team CRAGger Islington Leeds Wokingham WSP

These are brilliant data points thank you.

May I be so bold as to ask about your driving behavious. I assume good given you are a cragger. So am I correct to assume you are “light” footed?

Robin.

andy_ross

website team CRAGger Glasgow Leamington

Great example of carbon visibility. The car tax emissions figure is a really useful thing to able to point out at CRAG kick-off meetings. Thanks, Peter. I only wish the electricity and gas companies would carbon label our bills to make it household carbon counting easier. Something to mention in our comments on the Climate Change Bill perhaps?