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Metal based fuel additives were first studied as smoke suppressants and cetane improvers . Several metals, including Ba, Ca, Fe, Ce, and Mn, have been found effective in lowering the amount of soot formed during combustion in both diesel and SI engines. This effect was explained by a combination of the following three mechanisms:
The most effective smoke suppressants, Ba and Ca, are believed to work primarily through the first two mechanisms. Metallic additives could also reduce PM mass and other emissions, and improve fuel economy. However, as engine technology has evolved to produce far lower levels of PM emissions the inclusion of metals which are not destroyed by the oxidation process can add more to the PM mass than they can reduce the carbonaceous PM mass.
Since the late 1990s, the main interest in metal based fuel additives is related to diesel particulate filters (DPFs), as opposed to reducing engine-out emissions. There are two major reasons for this shift of focus: (1) as noted above there is a diminishing emission reduction effect from the use of additives in modern diesel engines, and (2) growing health concerns related to metallic particulate emissions. In old technology diesel engines, using metallic fuel additives (e.g., 10 to 100 ppm of Fe, Ce) could reduce engine-out PM emissions by as much as 30-40% and improve fuel economy by as much as 10%.
However, in more recent diesel engines (say US 2004 or Euro III onwards)—just as it was the case with the effect of fuel properties—the emission reduction and fuel economy improvement are typically small, hardly justifying the use of additives. If additives are used, the metal will tend to be emitted from the engine in the form of metal oxide (ash) nanoparticles. Even if the mass of such emission is low, the particle numbers can be very high, making them a possible public health concern. Therefore, environmental authorities usually remain cautious about the use of metallic fuel additives, unless a highly efficient particulate filter is used, which can capture the metal ash particles.
Fuel Borne Catalysts. Numerous metal additives have been investigated as soot oxidation catalysts to facilitate regeneration in DPF systems—common examples include iron, cerium, strontium, copper, as well as platinum. Fuel additives used to facilitate DPF regeneration are usually known as fuel borne catalysts (FBCs). As the FBC is combusted in the engine cylinder, its metal component leaves the combustion chamber in the form of the corresponding metal oxide or other inorganic compound (e.g. sulfate). These compounds can form particles of their own or can be incorporated into diesel particulates. After being collected in the DPF, the catalytic metal is distributed throughout the diesel particulate phase and can thus effectively catalyze the oxidation of these predominantly carbon based particles. If the catalyst is incorporated directly into the DPF as in a catalyzed filter then soot/catalyst contact only takes place on the filter surface, the use of the FBC provides better contact between the catalyst and the carbon/soot particles. This is probably why FBC regenerated filters regenerate at somewhat lower temperatures than catalyzed filters. The intimate contact of the catalyst and the bulk of the trapped carbon particles will also explain why the regeneration of the trapped material is usually far more rapid than with a catalyzed filter . The first chart of Figure 1 shows an attempt to force a regeneration in a catalyzed filter, fitted to a passenger car, by high speed driving, while the second chart shows a passive regeneration in the same vehicle which has been driven with FBC added to the fuel.
In these figures the lighter color shows the pressure and temperature recorded on a 5 second basis with the darker line showing the 1 hour rolling average. With the catalyzed filter and untreated fuel the DPF reached an unacceptable exhaust backpressure and the driver tried to force a regeneration by driving at high speed. As can be seen from the figure on the left this resulted in significant bouts of pre-DPF temperatures in excess of 400°C and even longer bouts when the temperature was above the 250°C threshold when soot oxidation should be occurring via the NO2 route that will be discussed below. When the car was driven with an FBC added to the fuel the backpressure tended to rise in a similar manner but on only the second occasion that the pre-DPF temperature exceeded 400°C (just after 15 hr in the chart) a spontaneous passive regeneration is triggered and there is a complete regeneration of the DPF within only a few minutes.
Even though FBC/DPF systems are more likely to regenerate under low temperature regimes, they are unlikely to provide passive filter regeneration in all operating regimes, especially in light-duty applications. As illustrated in the following examples of filter systems, commercial applications require “quasi-active” or “passive-active” approaches, where additive-induced regeneration is additionally supported by certain engine management measures in order to periodically increase exhaust gas temperature.