Recent advances in emission control technology have led to the introduction of the so-called "clean" diesel engine. Through a combination of refined fuel injection systems and exhaust filtration devices (trap oxidizers), it has become possible to reduce dramatically the visible black smoke associated with diesel operation. Edmonton's newest diesel buses often no longer leave a visible smoke trail as they accelerate from bus stops and move along the street.
But are these new diesel buses really as "clean" as the term "clean diesel" tends to imply?
Analysis of the exhaust from "clean" diesel buses reveals that the nitrogen oxide emissions are similar to levels from conventional diesel, in some cases even higher. It also shows that the amount of particulate--the black 'soot' you see behind conventional diesels--is less when measured by weight. There are doubtless some benefits for the environment and public health associated with these reductions.
Diesel exhaust has been found to contain over 41 different toxins, many of which form part of the particulate. The same toxins are present in "clean diesel". Because "clean" diesel particulate is so much finer than that from conventional diesel engines, these toxins have an easier time entering our bodies. There is no safe level of exposure. German researchers insist the toughest diesel emission standards are not tough enough.
One "clean" diesel bus produces more emissions in its average lifetime than over 110 automobiles. Additionally, no reduction in urban noise pollution is achieved with "clean" diesel.
There is recent evidence that the devices intended to clean up vehicle exhausts are backfiring on the environment. Microscopic particles from the heavy metals used in the manufacture of catalyst type exhaust filters (i.e. diesel trap oxidizers and catalytic converters on gasoline engines) have been discovered embedded in the ice in Greenland. These metals can also accumulate in plants and animals and enter the food chain. Scientists say the release of heavy metals from exhaust treatment devices is a global problem that stands to threaten human health.
The importance of environmental factors in stimulating ridership and investment in public transit is growing. "Clean" diesel buses pale in comparison to electric trolleys in terms of their attractiveness as an environmental solution.