Our industry is aware that particulates and aerosols (smoke) from diesel engine exhausts present a health risk and have been leaders in research, testing and exhaust treatment.

Particles less than 10µm, known as PM10, are most readily respirable and therefore can do the most damage in the lungs. In more recent years we have become increasingly concerned about the smaller fractions, PM2.5 (2.5µm) and ultra-fine particles (or nano-particles) in the range 0.1 to 1µm in diameter.

We have always known that there was no 'safe' level (threshold value) for the long term exposure to particles and that long term exposure contributes to respiratory and cardiovascular disease. Some epidemiological evidence suggests that concentrations less than 5-10 µg/m3 may have little effect in that they approach 'background' levels. The World "target" level of 3µg/m3, which has been adopted in (Draft) AS3584-2001, Diesel Engine Systems for Underground Coal Mines, meets "best practice".

There is evidence to suggest that minimising the overall mass of particles ingested minimises the risks of adverse health effects.

New research indicates that the overall number of ingested particles may be as important as is the mass of those particles. This is significant. When one considers that the mass of one particle with a diameter of 3µm is equal to the mass of 1 million particles with a diameter of 0.03µm and that 40% of the total particle numbers fall into the nano-particle range, then particle count and size can be seen to be very significant in the management of health risks.

Research indicates that a typical particle breakdown by count, measured in the atmosphere is as follows. (For particles that are the product of engine combustion.)

There is evidence to suggest that the chemical composition of inhaled particles is not important. The toxicity of nano particles results from their small size rather than their composition. It is suggested that particles form free radicals in the lungs causing inflammation. (MacNee & Donaldson 1999). Experimental evidence suggest that the smallest particles >0.1µg may cause more inflammation in the periphery of the lungs (alveolar) than does larger particles.

The ratio of volatiles to carbon vary very considerably with engine type and operating load.

In the absence of carbonaceous material, and in the presence of sulphates and/or metals, (~1nm) and low volatility condensable materials, nucleation particles can be formed. This may be due to thermal desorption of low volatility materials at high exhaust temperatures.

Volatiles would seem to condense preferentially on carbon nuclei whenever such nuclei are present in sufficient concentrations.

It is probable that such particles may remain suspended in the atmosphere for many weeks particularly at low relative humidities.

Those engines which have been perceived to be "clean" in that there is little or no visible smoke in the exhaust have been demonstrated to be little different from those which may be observed to produce black smoke. The difference lies in the particle size.

Of significance is that modern directly injected gasoline engines demonstrate a particle count little better than conventional directly injected diesel engines. It is worthy of note that conventional diesel engines fitted with a catalysed trap produce fewer particles than do LPG powered spark ignition engines.

One of the more remarkable discoveries from recent research is that the total number of particles and the size of those particles appears to be relatively unaffected by engine load, excess air and duty cycle. The particle count rises modestly with increasing fuel injection.

The variation across diesel engine types indicates a spread of results in the order of 2.5 orders of magnitude.

There is no discernible correlation between gaseous emissions and the count of particles.

Particle mass/flow and carbon dioxide do have a correlation. When an engine is tuned to minimise NOX emissions, then there is generally a compensatory and proportional rise in CO. The increase in particle count under such tuning is almost expediential.

How can the emission of nano-particles be minimised ?

- Efficient engines [kwh/Kg (of fuel)]. The issue with all particulates is the long term exposure level of personnel. The more efficient the engine, the less the exposure.

- Low sulphur fuel. There is a strong correlation between sulphur in fuel and particulate mass/flow and particle count. The best treatment methods mandate low sulfur (less than 30ppm).

- Catalytic converters with filtration technology. Regenerating converter/filter systems offer reductions of up to 90% in carbon monoxide and hydrocarbons, 10% reductions in NOX and 80% reduction in particulates (measured mass/flow) or 55% in particulates (measured by count).

- Water-based Exhaust Conditioners (scrubbers). Remove up to 30% of particles when measured as mass/flow and 55% when measured by count.

Sources

J Anderson & B Wedekind (DP01/0515) Particulate Research Program (1998-2001); A Little, (1998), Fuel Qualities; MacNee & Johnston (1999), Particulate Air Pollution; Maricq, Chase Podsiadlik & Vogt (SAE1999-01-1461);Exhaust particle size distributions; Pope & Dockery (1999) Epidemiology of Particle Effects.