The particulate matter in diesel engine exhaust gases consist of extremely small particles of soot (unburned carbon) from the combustion process. In the diesel, atomised droplets of fuel are sprayed into hot, highly compressed air. Combustion is initiated in an initial premix phase, followed by diffusion combustion in which the atomised fuel droplets are sprayed into the pre-initiated combustion. The chemical basis form soot formation is provided by the formation of heavier hydrocarbons in momentarily rich spray zones of the fuel/air mixture in the combustion chamber. This occurs when insufficient air is present and the value of lambda is between about 0.25 and 0.5 (lambda = 1 represents the ideal stoichiometric mixture ratio).
The hydrocarbons formed in this manner condense and form cores for the soot particles. Unburned hydrocarbons and other substances condense on the soot and increase the mass of the particles (PM). The cores grow to form spheres (about 30 Nm in diameter) that can grow into clusters and chains, and can number up to one thousand per particle. This creates a distribution of particles of different sizes.
However, the soot particles become highly oxidised even during combustion in the cylinder. Modern engines equipped with modern combustion and high-pressure fuel-injection systems are highly efficient, resulting in the emission of extremely small quantities of particulates. Most diesels comply with current statutory requirements (and those for 2006) without the need for a particulate filter. In addition the diesel’s oxidising catalytic converter, which oxidises hydrocarbons and carbon monoxide exactly as in the petrol engine, converts a proportion of the hydrocarbons bound in a more volatile manner to the particles. Nevertheless, a small quantity of the dry soot leaves the engine.
Assessment of the health risks has been related to the quantity and local occurrence of the particles, to the polycyclic hydrocarbons and to particles less than 2.5µm in size.
The following figures specify the percentages by which the levels of controlled emissions for 2005 remain to be reduced, relative to the values applicable to diesel-driven cars in the base year that the particular statutory limit came into force in the EU:
CO (carbon monoxide) 1971-2005, 1% remaining
HC (hydrocarbons) 1971-2005, 4% remaining
NOx (nitrogen oxides) 1975-2005, 7% remaining
PM (particulate matter) 1992-2005, 10% remaining
In the case of particulates, the remaining 10% of the 1992 level is what is available to the car industry to work with. But what will be its significance overall?
Wind erosion, volcanoes, natural biomass fires and sea salt account for 75% of the world’s total particulates. Human activity adds a further 25%, mainly from building operations, quarrying, combustion of fossil and biomass fuels, and industrial processes, with a small proportion from socially useful activities, including sea and rail traffic, agriculture and forestry.
But is not road traffic by trucks and cars not the biggest contributor?
In fact, road traffic worldwide accounts for only 0.15% of particulate emissions! And diesel cars account for less that one-fifth of this tiny percentage. In other words, no more that 0.03% of all particles are emitted by diesel-driven cars. Particulates from traffic also include particles of all types from road surface, tyre and brake wear.
However, of all emission sources, the public focus is now primarily on the car. As technical project manager and diesel expert Jan-Erling Rydquist puts it: “It’s a bit like a man that loses something in the dark and looks for it under a street lamp somewhere else because the light is good there ….”
“Although this is not a motive or an excuse for doing nothing,” he stresses. “The car industry must continue its dedicated work to minimise diesel engine emissions. But we should be aware that it is now beginning to cost a great deal to achieve very little.”
Approximately three-quarters of all of the particulates in the Earth’s atmosphere are of natural origin. Traffic accounts for about 0.15% of the total and diesel-driven cars for about 0.03%. This means that carmakers are spending large amounts of money to solve an extremely small part of the problem. (P2005_2832)
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