car tax and emmissions

[byootox]

Originally Posted by eternal optimist

1.51 what exactly?

‘Cars below 1.2 should be paying £30....’ do you mean cars emitting less than 120g of CO2 per km paying £30 tax?

The emissions test measures hydrocarbon emissions (parts per million) and CO emissions. They don’t measure CO2 emissions.

Apologies I thought you wanted to know what the numbers meant ?

[eternal optimist]

Originally Posted by byootox

Apologies I thought you wanted to know what the numbers meant ?

My question was directed to the OP who appeared from his first post to be unclear about how CO2 emissions and VED rates and MoT emissions testing worked. I’m interested in the OP’s point of view. Having opened the can of worms, the OP’s not been on this since post no. 7.

[SwedishBus]

Originally Posted by byootox

The value is essentially light absorption by the smoke the higher the value the thicker the smoke.
The M-1 part I believe is a molar value which is normally a value of a chemical or substance.
In this case smoke.
There's a bit more to the emissions testing than the little basic testers that are widely used at the mot stations I'm afraid.
That's why your ved/tax doesnt quite add up with what you pay.

The m-1 figure is actually a calculation of the opacity per metre (1/m), which is derived from the original methodology of homologation of a engine's free acceleration smoke when it is mounted on a testbed (it is done at the same time as the power and torque curves are homologated).

There were a couple of options for the smoke measurement, but most manufacturers used the Hartridge equipment, which is effectively a long tube with a light source and a light receiver at one end, a mirror at the other end. The opacity of the smoke affects how much light is lost between it being sent and received through the tube, and it dependent on a very finely balanced fresh air mixer flow-rate running through the apparatus, as well as precise exhaust feed pressure and temperature through the sample tubing. In short, it was a miracle that anything was derived from the measurements. The effective length of the measure tube was calculated for the light-path return journey, and the opacity expressed as a measure "per metre". The bigger the number, the "smokier" the engine. Decent manufacturers declared figures that were realistic, but within the boundaries of reasonableness, in order to account for future MoT's and the needs of their respective customers.