A question for the nerds....
 

Bit boring maybe, but one I ponder on.

Now emissions testing on a vehicle is conducted under "controlled" conditions, and the CO2 is recorded and used across the board for that particular model. So the first part of the question is, whay sort of load is the car under, laiden weight etc?

The reason I ask is different engines respond in different ways, so a high torque engine would cope better with a bigger load than a smaller engine (I think)
So a 2.0 TDi would cope better and not increase its emissions as much as a 1.0 12v engine would under the same load.

So now apply the same to a bus. How is this tested, and how do its emissions change when carrying 80 odd people :wonder:

Not mega important, but just wondered how the figures change in reality.

Google is your friend. This article explains a lot of the testing mechanics for cars.

The figures are valid only if your driving style matches the test regime. Clarkson showed on Top Gear that a BMW cruising to keep up with a Prius (?) at full chat used less fuel.

Buses use a lot of stop/start driving so the standard car test would not work.

How on earth do they decide that the proportion of city driving is 36.8%, and not say 40% :eek:

Cheers mate :)

On the latter part, I would love to see an example of what the true emissions are say in the rush hour, slow moving traffic, stop start, a few tonnes of human cargo.. compared to a car with 1-2 people in it using stop start technology.

Then do the maths and see what the CO2 grams per mile is per person....

As for the figures... non rounded up numbers look far more impressive and thorough lol

The key thing IIRC is average fuel consumption figure. The CO2 emission level in weight depends solely on this. Every litre of petrol that is burnt emits around 2.3 kgs of CO2. Diesel produces around 2.7 kgs of CO2 per litre of fuel used. So a 400 bhp truck doing 100k miles per annum at 8 mpg puts out a serious weight of CO2.

IMHO the EC and others have over complicated the whole issue. dl

Really? A litre of fuel, approx 1KG, can release 2.3KG in CO2?

Cheers David, that sort of makes sense when put like that.
Only thing im unsure if is.... does the efficiency of an engine change when put under load?
When you overload a small car, and it struggles up the road, coughing and choking away, is it producing more CO2 due to it burning the fuel less efficiently?

Yes, it does - though probably not in the way you'd expect:

An engine at full throttle is more efficient than an engine on light throttle

...and...

big engines are more efficient than small ones.

Yes, you read that correctly, and you're right that everyday experience would seem to contradict both statements. However:

A car cruising along the road at a given speed requires the same amount of actual power regardless of the potential power capability of the engine. So, a small engined car will have its throttle more open more of the time than the big engined car, and that is why it requires less fuel.

Also, a big engine has less surface area per unit volume than a small one, which means it loses a smaller proportion of its heat of combustion through the cylinder walls. This means that, in terms of bhp released per unit of stored chemical energy in the fuel, it's actually able to be more efficient than a small engine. This goes some way toward making up for the fact that a big engine is typically run on a lighter throttle more of the time than a small one.

The heat loss issue is one major reason why very small car engines are sometimes made with only 3 cylinders, and why V6 and V8 engines tend to be made with correspondingly larger capacities. Small cylinders are inefficient. (That's also a reason why, given their size and weight, motorbikes tend to be so thirsty).

David is absolutely correct about CO2 released by burning petrol and diesel - it seems that not many people realise that fuel economy and CO2 emissions aren't just related, they're actually both measures of exactly the same thing. The amount of CO2 released is a property of the fuel, not the car, and you can work it out if you know how many carbon atoms there are in a litre of fuel.

All the carbon atoms that come out of a car via the tailpipe, went in through the fuel filler. It's that simple.

Cheers David and Andy, you have answered the main part of the question perfectly :)
Always nice to get one of those quandries out of your mind lol.

On to the next one hehe.

etruk (ecotec resources uk) - Calculating CO2 emissions

The CO2 weight thing is actually simple but I found it quite hard to accept in my mind! I don't think that the public have a clue about this and just think that their car is emitting a couple of balloon fulls of gas which can't harm anything :lol1:

Going back to my truck example, for instance. A 100k miles per annum truck at 8 mpg means 12,500 gallons of fuel = 56,750 litres used. This results in an annual CO2 output of 56,750 x 2.7 kgs of CO2 = 153,225 kgs which is 153 tonnes :eek:

But I bet if you confronted the average truck driver and stuck a lump of concrete weighing 153 tons in front of him and told him what it represened he would fall about laughing.

dl

Fair play David, I plead complete ignorance to such things, but am slowly becoming more aware of them. Not saying I will start hugging trees, but its always nice to have an understanding of things, all be it basic :)

I think if some of the publicity things were slightly more basic and factual, as the above replies have been, more people would have a better understanding of things, and be able to make an educated decision.

Cheers guys.

The fuel is combined with a huge weight of air when burnt (14.7:1), and all of the produce has to come out of the exhaust pipe (except for the little bit of carbon that gets stuck inside :)) So you just need to know the proportion of exhaust that is CO2 and you have the answer.

...i have just had a brainwave - why not base "carbon tax" on how much fuel that you use, as it is directly proportional to the mount of CO2 produced. Maybe just add a few pence to each litre (or rather, redirect fuel duty and VAT to "the greater cause").

That would mean that people who drive only a few miles per year don't get clobbered ('cos they aren't killing any penguins), new car buyers don't get hammered with doubled-up road tax, occasional drivers of older "high emission" cars don't get retrospectively screwed on their road tax and there will be no need to employ hundreds of thousands of people in non-jobs enforcing these useless new "rules" in their vast "funded by the state" offices as they await their gold-plated pensions ;)

mb

Exctly the sort of common sense approach that everyone except the government is crying out for.

As an alternative, how about tax that is proportional to the CO2 emissions? Why should a 100g/km car pay nothing yet is half as polluting as a 200g/km car that has to pay several hundred pounds :cuckoo: