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The most cost-effective way to reduce the level of diesel particulate emissions is to retrofit exhaust aftertreatment devices. While diesel oxidation catalysts will reduce the mass of particles emitted, they will not significantly reduce the number of ultrafine particles, that are considered the most harmful to health. Diesel Particulate Filters (DPFs) are therefore considered the most effective retrofit devices. One obstacle to the widespread adoption of DPFs is that many DPF technologies require low sulphur fuel. Using a Fuel Borne Catalyst (FBC) to facilitate regeneration of the DPF allows a sulphur tolerant DPF system to be produced.

Diesel fuel distilled from crude oil should contain no greater than trace amounts of sodium. However, fuel specifications do not include sodium; there is a limit of five parts per million for the amount of sodium plus potassium in fatty acid methyl esters (FAME) used as biodiesel. Sodium compounds are often used as the catalyst for the esterification process for producing FAME and sodium hydroxide is now commonly used in the refining process to produce ultra-low sulphur diesel (ULSD) fuel from crude oil. Good housekeeping should ensure that sodium is not present in the finished fuel. A finished fuel should not only be free of sodium but should also contain a diesel fuel additive package to ensures the fuel meets the quality standards introduced to provide reliable operation, along with the longevity of the fuel supply infrastructure and the diesel engines that ultimately burn this fuel.

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.

Work was carried out on three passenger cars and a light truck. The test vehicles were chosen to cover a range of engine technologies. Different DPF technologies were also employed. The programme showed that an improved fuel additive based on the combination of iron and strontium compounds would allow all four vehicles to be successfully operated under a wide range of conditions. The three passenger cars were driven over the road for considerable distances. Regeneration of the DPF was successfully achieved under normal operating conditions in all the vehicles without recourse to use of additional heaters, fuel injection or other technique to assist regeneration. Fuel additive treat rate was low, suggesting that long-term operation without significant ash accumulation in the DPF could be achieved.

The need to meet the US 2007 emissions legislation has necessitated a change in Diesel engine technology, particularly to the fuel injection equipment (FIE). At the same time as these engine technology changes, legislation has dictated a reduction in fuel sulphur levels and there has also been increased use of fatty acid methyl esters (FAME) or biodiesel as a fuel blending component. The combination of changes to the engine and the fuel has apparently led to a sharp rise in the number of reports of field problems resulting from deposits within the FIE. The problem is usually manifested as a significant loss of power or the engine failing to start. These symptoms are often due to deposits to be found within the fuel injectors or to severe fouling of the fuel filter. The characteristics of the deposits found within different parts of the fuel system can be noticeably different.

Trapping diesel particulates is effective in reducing both the number and the mass of fine particulate emissions from diesel engines, but unless the accumulated soot can be burned out or regenerated periodically, the vehicle to which the trap is fitted will cease to function after a relatively short time. A programme of work with soot traps using a low treat rate iron-strontium organo-metallic fuel additive to assist and secure regeneration has been carried out. As part of this programme, an advanced specification diesel engine passenger car equipped with a diesel particulate filter (DPF), was operated on roads in the UK for approximately 18 months, during which time the vehicle covered over 50,000 km After completion of 50,000 km on roads, the vehicle was operated on a chassis dynamometer to increase the distance covered with a DPF more rapidly to a final total of 80,000 km.

Transport Refrigeration Units (TRU) powered by small diesel engines emit high PM and cause locally high PM levels. The concomitant health risks spurred efforts to devise a cost-effective curtailment of these emissions. Diesel particulate filters (DPF) of ceramic honeycomb construction very efficiently trap PM emissions, even ultrafines in the lung penetrating size range of below 300 nm. A fuel borne catalyst (FBC) can facilitate trap regeneration, by lowering the exhaust temperature requirements, but cannot alone guarantee reliable regeneration under all operating conditions of the TRU. A Swiss development team together with industrial partners therefore developed a fully automatic active regeneration system for the California Air Resources Board.