Re: "...any visible smoke..."
I looked up the pollution standard for heavy diesel vehicles.
That was a UK specific issue. UK was the only country in Europe which disallowed diesel fumigation (*). In the pre-DPF days fumigating a diesel with small injection of LPG at the air intake manifold was the only way to reduce soot by several orders of magnitude. This technique is now out of fashion for automotive - it is replaced by DPF. It will probably not pass today reqs too - it makes the burn much cleaner resulting in significantly higher NO2 emissions. It is still used for motorboats as well as other places where DPF is not suitable.
Unfortunately, in addition to reducing soot, it also increases power output and reduces diesel fuel consumption. As a result it suffered the fate of anything which could endanger the fuel excise revenue. Sleazy Tony Bliar and Gordon Brownpants did not make compromises on anything like that. Anything which would significantly reduce excise revenue was sabotaged by any means possible. So UK pretended it did not exist while most of Eu large city public transport (f.e all of Milan, most German cities, etc) ran fumigated.
This is the root cause of the ridiculous soot standard for city buses, etc ~15-20 years ago. It was there in order to be able to pretend that fumigation is unnecessary.
(*)Aka white diesel, disel bianco, etc
Biting the hand that feeds IT
