KEY POINTS:
Dirty diesel is getting a good face-wash, but needs more scrubbing behind the ears before winning the Auckland Regional Council's approval for road use.
Although diesel engines are inherently more fuel-efficient than their petrol counterparts, and therefore spew out less globally warming carbon dioxide, they pose greater immediate risks to human health by leaving higher levels of nitrogen oxides and fine sooty particles called particulates in the air we breathe.
Nitrogen oxides contribute to smog, and the council blames particulates - mainly from diesel vehicles but also from domestic wood-burners - for almost 400 premature deaths a year among Aucklanders with lung or heart problems.
That has left it refusing to switch the bulk of its vehicle fleet from petrol to diesel until at least 2009, when emission limits for the two fuel types will start becoming better aligned.
Even though their maximum particulate emissions must be reduced fivefold by then, to the same five parts per million to be permitted for petrol motors, new diesel engines will still be allowed to chug out three times more nitrogen oxides until 2014.
Auckland's environmental watchdog felt harshly treated in February, when the Green Party gave it the worst score of the country's 14 regional councils for the fuel inefficiency of its vehicle fleet.
The Greens calculated that the Auckland council could reduce carbon dioxide emissions by 134 tonnes a year simply by replacing its 107-strong fleet with more fuel-efficient vehicles.
Green MP Keith Locke said the party suspected the council's poor score was caused by its purchase of vehicles when diesel had a high sulphur content, blamed for unacceptable air pollution in Auckland.
"This is no longer the case, and the ARC has the chance to significantly upgrade its fuel efficiency by buying diesel when it replaces them," he said.
It is true that the sulphur content of diesel, a major source of particulates, has been cut substantially since the council declared war on polluting vehicles in 2000 with its Operation Smoky phone-in campaign.
The maximum permitted level has been reduced from 3000 parts per million to 50 ppm since then, and is due to be cut to 10 ppm in 2009.
But regional council air quality scientist Dr Gerda Kuschel said particulate emissions from diesel vehicles were still typically 10 times higher than from petrol motors.
She said her organisation was under pressure to cut Auckland's overall particulate pollution by 58 per cent by 2013 to meet national environmental requirements.
Council chairman Mike Lee accused the Greens of "ignoring the human health impacts of diesels" in its focus on greenhouse gas emissions, and pointed to his organisation's tree-planting efforts towards carbon neutrality.
Although the council has 15 diesel vehicles, these are mainly for park duties outside the region's main population centres.
It also has four petrol-electric hybrid vehicles but, like many other fleet operators, appears deterred from large numbers by their comparatively high prices.
Edward Rowe, a spokesman for European vehicle importer Ateco, said no engine type was free of emissions and deciding whether to use petrol or diesel meant having to "choose your poison".
But he said there was an international consensus that carbon dioxide was the world's biggest pollution headache, given its high contribution to global warming, and the only way to reduce its emissions was to burn less fossil fuel.
That put diesel ahead in the battle, since a modern engine burning that fuel type used 40 per cent of the energy content on propelling a vehicle forward compared with what he said was an efficiency rating of just 25 per cent from a typical petrol motor.
