So What Gordon (is a mooron)

In message , Warwick writes

How would you propose measuring CO2 emissions in the MOT centre? You can't just sit there revving the engine with a probe up the exhaust, you need a test which accounts for weight, aerodynamics and gearing.

You could probably get an approximation using a rolling road, the car's weight and the manufacturer's original drag data, but if Saxo boy has lowered it, fitted wider tyres and the spoiler off a Klingon warship that's going to be well out.

Or, the tax could go on fuel where it precisely relates to CO2 emitted.

The rolling road will get wider tyres, though not spoilers.

(Resists the temptation to ask which class of Klingon warship has anything close to a spoiler)

Minus unburnt hydrocarbons, and evaporation.

It would pick up a different rolling radius and it might show a switch from low rolling resistance eco-tyres to something with a little grip. It won't pick up the increase in frontal area (and therefore aerodynamic drag) from wider tyres.

:O)

Yeah, but you can probably assume that those are first order terms. (So that CO2out = K.FuelIn where K is a constant).

I suspect these are fairly minor. Unless you make major body shape alterations, you are going to be pretty close if you assume that it's stock - you're not going to change the drag coefficient of a Citroen C1, to a Hummer H2.

But this level of testing would be insane with the current levels of VED. The only sort-of-logical solution to this that I can see is to drastically encourage people to buy more economic cars, by pro-rating the fuel duty. Under 100g/Km, 0 duty, average fuel use cars stay the same, and high fuel use cars go up.

Tyres, yes. Overall, well, possibly enough change to go up a VED band. Modern cars seem very sensitive to anything which messes with the aerodynamics.

The point is really that you can't practically measure this stuff in an MOT. It would just be too expensive, and is ultimately pointless when fuel tax is so much better a representation of actual emissions.

Thank you. And everyone further down the thread who added in the extra info too. It was speculation and I wasn't aware of how the initial tests were done on current cars.

Judging from the replies, I gather that weeks of different driving conditions are tested on tracks with a car full of test equipment to get the figure.

Extrapolating on, I suppose that that wasn't done until recently and it isn't in the goverment's or the manufacturer's interest to do those tests for older cars and especially not to repeat them every 5 years or so especially considering how varied a life any given model would have had after only a few years.

So the MOT emissions test is just a guide to ensure that there's nothing badly wrong with the cat/MAF/gaskets/timing etc?

Warwick

Yes. It checks for badly malfunctioning engines - oil clouds coming out of the exhaust, and things that make the catalyst not work, as well as catalyst function. Basically, the criteria are "Is it much worse emissions (NoX, CO, ...) wise than it should be", and "Will anything fall off tomorrow".

The MOT doesn't really answer much more than that.