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Exhaust emissions legislation for diesel engines generally limits only the mass of emitted particulate matter. This limitation reflects the concerns and measurement technology at the time the legislation was drafted. However, evolving diesel particulate filter (DPF) systems offer the potential for reductions in the mass and more importantly, the number of particles emitted from diesel exhausts. Particulate filters require frequent cleaning or regeneration of accumulated soot, if the engine is to continue to operate satisfactorily. Exothermic reactions during regeneration can lead to severe thermal gradients in the filter system resulting in damage. Fuel additives have been evaluated to show significant reductions in light off temperature which allow frequent small regeneration events to occur, under mild operating conditions.

There is mounting worldwide concern over the health effects of airborne ultra-fine particles. Of greatest concern are the risks due to the cancer-inducing properties of these particles and the aggravation of existing respiratory diseases by the ultra-fine (i.e. <2.5 micron) fraction. This disquiet has already resulted in legislation, regulations and other measures, either mandated or proposed, in the industrialised world to severely restrict particulate emissions from diesel-fuelled automotive transport. Emissions of particles from both new and existing vehicles have been addressed. With the rapid growth anticipated in some developing countries they to will need to address this problem. This paper outlines some alternative solutions to the problem, ranging from alternative power sources, alternative fuels, alternative engine technologies and after-treatment strategies. It also outlines what is required to implement these different solutions.

Four vehicles were chosen to cover a range of engine technologies. These vehicles were fitted with diesel particulate filters (DPFs) of differing technology. Three of the vehicles have been driven on the road using an additised fuel to demonstrate totally passive operation of the DPF. As part of this programme all three vehicles underwent regulated emissions testing to demonstrate that there was no deterioration in emissions during the programme. Additionally a light commercial vehicle was tested to demonstrate the effect on emissions of the combination of additised fuel and the DPF. The performance of the DPFs during on-road use has already been reported; this paper therefore concentrates on discussion of the results of the emissions testing.

Fuel-borne catalysts are now an accepted means of aiding the self-regeneration of diesel particulate filters (DPFs). In the past it has been possible to assess the effect of these fuel additives by investigating the temperature at which the filter reaches a pressure drop equilibrium. Under these temperature conditions, the particulate matter is oxidised at the same rate as it is being deposited and there is thus no change in pressure drop across the filter. This technique adequately demonstrates the oxidation temperature of the carbon in the presence of the catalyst. However, it is now well known that such fuel additives also influence the low temperature oxidation of particulate bound hydrocarbons. This phenomenon is not detected by the filter equilibrium technique.

Interest has been growing in many countries in the potential use of diesel particulate filters (DPF). This type of after treatment technology has been shown to make very significant reductions in both the mass of particulate emitted in diesel exhaust gas, and also in the number of fine particulates, which have been linked in recent years with concerns for human health. Work carried out during a development programme investigating the capability of fuel soluble metallic additives to assist DPF regeneration, indicated superior performance from a novel combination of metals in fuel soluble form. Earlier work showed that a fuel soluble combination of organo-metallic additives based on sodium and strontium gave very effective regeneration characteristics, and was capable of burning out carbon at temperatures from about 160°C.

Diesel particulate filter (DPF) are well known as a developing form of exhaust after-treatment for compression ignition engines. Subjected to extensive testing in experimental form, DPFs have yet to achieve widespread application in regular use on production road vehicles, despite their potential for delivering reductions of typically 90% in diesel exhaust particulate emissions. Tests have shown that different additives are effective in enhancing performance in a range of DPF types, and on engines of different configurations. Efforts have been made to correlate performance with engine operating regime, by linking soot particulate condition to the frequency of regeneration. A performance index has been developed to try to predict regeneration characteristics with additive treated fuel. The work has shown that there are engine operating conditions producing soot which is less likely to burn off in the DPF.