The problem is a sharp Intensification of the health risks in all urban agglomerations, due to rapid increase in motorized traffic during the past 50 years. Fine particles in the alveoli intruding range are “Toxic substance Number 1”. These ultrafine solid particles in the size range < 500 nm are exclusively anthropogenic, resulting from technical combustion, I.e. from stationary combustion e.g. building heating, but above all from the combustion engines. All engines produce these fine particles, both as soot and also ultrafine metal oxides from abrasion and lube oil additives. However, the heterogeneous combustion processes in Diesel engines deliver the highest concentrations. Despite intense engineering efforts. the emissions of the number of nanoparticles from new IC-engines are not diminished. Indeed, deterioration is observed, due to emission of smaller particles than from the older models. The famous 6-Cities US Study (Dockery 1993) first showed the strong impact of the ultrafine particle content on the residents’ mortality, Including the carcinogenic and cardiovascular consequences. The WHO has quantified the total effect for several countries. The statistics for Switzerland reveal the magnitude of this risk, which is now at least 6-fold higher than the risk of traffic accidents. This is representative for all other industrialized countries.
The two lower Figures illustrate that mainly the very fine particles Intrude Into the lung and from the lung are transported to other body organs. This penetration contrasts with the response to particles > 1 µ. Those larger particles are efficiently Intercepted and expelled, thanks to the respiratory cleansing mechanism, which has naturally evolved. Clearly the particle size Is a major factor and, of course, the particle solubility. Hence, the modem approach focuses on the solid particles in the alveoli-intruding size range. The WHO classifies these particle emissions, since 1988 as carcinogenic. This fact has prompted environmental legislation, in many countries, stipulating the minimizing of such emissions using the best available technology. Because there is no safe threshold for carcinogenic substances. That requirement defines the engineering task: determine which exhaust-gas after-treatment Is the “best available technology” and decide how widely it should be employed to curtail the combustion engine emissions. This task comprises both new engines and also the presently deployed engines. The extension to existing engines, I.e. retrofitting, bounds the technology choice and is very demanding on the application technique.
Historically, legislation is confined to rules for new vehicles. The emission legislation ameliorates the recognized health risks through progressively more stringent emission limits. This tightening exhaust-gas legislation is a big challenge to the engine manufacturers that has required expensive engineering. Figure 3 Illustrates the advancement of exhaust-gas legislation for utility road vehicles and for off-road vehicles (construction machines, tractors, etc.) in Europe.
These particle limiting values are defined as the total particle mass (PM, g/l<kWh). This metric Is inadequate for EUR04/EUR05 because their PM is approaching the detection limits. The question also arises, whether the PM metric represents the significance of particle size described above. Ultrafine particles in the 100 nm range have a very small mass, approx. g/ particle. In the mix of larger and smaller particles. The modem engines indeed attain the legislated limits but continue to emit high concentrations of extremely toxic ultrafine particles, which are preventable. Nevertheless note that this modern technology also has disadvantages. The platinum coated oxidation catalytic converters, in PM·Cat, can substantially increase the undesirable N02. And the SCR technology can cause ammoniac slip and release other toxins, which are not yet sufficiently Investigated. These are secondary emissions, which must be prevented (testing mandatory in Swltzer1and and in USA) when introducing new technologies. Numerous Investigations In past decades have researched methods to curtail the particle emissions from Diesel engines. The following methods were investigated:
- Oxidation catalytic converters
- Fuel additives
- Sulfur-free fuel
- Bio fuels
- Water I Diesel emulsions
- Improved lubrication oils
- Optimizing the combustion, the fuel injection and the supercharging
Many methods can diminish the mass PM, comprising the solids and condensates. The ultrafine particles are at the most decreased by one order of magnitude. It is Insufficient compared to good particle filters that can diminish ultrafine particles by 3 to 5 orders of magnitude. The efficiency of this fitter is higher for smaller particle sizes. That Is a desirable feature in view of their toxicity. This effect is physically plausible because of the diffusion behavior of ultrafine particles. The particle content of the exhaust-gas is thus diminished below the particle content of the environment. Besides the soot particles, all heavy metal particles from abrasion and lube oil are intercepted, too. Also filtered are mineral particles and fiber fragments, e.g. from the mufflers. This outstanding attribute of modem particle fitters Is sustained over tong time periods. There are no Indications of deterioration due to aging. All particle filters not only curtail the solid particles. They also curtail the concentration of the polycyclic aromatic hydrocarbons, which adhere to the particles. Many fitters, when catalytic coated, also diminish CO and HC. Some particle fitters Increase the N02 content of the exhaust-gas. Selecting the appropriate finer regeneration technology can prevent the undesirable N02 effect.
Costs and Benefit/Cost ratio
Figure 4 shows the specific Installation costs in CHF pro kW, It is based on present market prices, mainly influenced by deployment on construction site machines.
High costs persist. The slope of the curve Indicates the unfavorable circumstances for smaller machines, which suffer very high specific costs. If the cost Impact were formulated as cost/kg soot disposed, then the curve would be almost horizontal. The data for filler fitted ex-factory, both trucks and car manufacturing, shows the possible economies. The retrofitting costs should not be underestimated. Usually, much customized retrofitting is necessary, which is expensive. Further cost reductions will result, when larger unit volumes rationalize retrofit kits that lower fitting expenses. Nevertheless, the heterogeneity of construction machines would keep costs high. More promising are lower costs for utility vehicles. These benefit from more uniformly in vehicle types, deployment range and the placement situation. Thes
e high costs are macro-economically justified when there are comparable financial benefits In saving medical expenditure. The following table Intend to quantify the cost I benefit ratio, for various deployment situations, I.e. the cost to prevent 1 kg of emitted soot comparing the use of high efficiency wall now filters for HCV to partial flow filters for LDV as well as for off-road/construction machines.
Clearly. the Cost/Benefit for retrofitting high efficiency wall now fitters at HDV at apparently high costs is much more cost efficient than the use of partial flow filters as they are proposed for retrofitting LDV and the Cost/Benefit for off-road and construction equipment where filter retrofit cost is again higher Is very affordable for large and for small engines ~use of the high emissions from these small engines. Moreover, the deployment of partial flow (open) filters on cars has a very poor cost/benefit ratio, i.e. such products should be definitely avoided. A useful benchmark, for evaluating these cost/benefit ratios, Is the WHO value for the so-called external health costs proposing 280 E per kg soot.